JP7740596B2 - Steel plates, components, and their manufacturing methods - Google Patents

Description

The present invention relates to steel plates, components made from the steel plates, and methods for manufacturing them.

From the perspective of protecting the global environment, improving the fuel efficiency of automobiles has become an important issue. As a result, there has been a growing movement to reduce the weight of automobile bodies by increasing the strength and thinning of the steel sheets used in automobile components.

In addition, societal demands for improved automobile collision safety are becoming ever stronger. Therefore, there is a demand for the development of steel sheets that not only have high strength but also have excellent impact resistance in the event of a collision while the automobile is in motion (hereinafter simply referred to as impact resistance). In particular, from the perspective of corrosion prevention performance for the vehicle body, steel sheets used as raw materials for automobile components are often zinc-plated. Therefore, there is a demand for the development of zinc-plated steel sheets that not only have high strength but also have excellent impact resistance.

As an example of a steel sheet that can be used as a material for such automobile parts, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet having a thickness of 0.6 to 5.0 mm and a plating layer on the surface of the steel sheet, the steel sheet structure containing a ferrite phase of 40 to 90% by volume and a retained austenite phase of 3 to 25% by volume, the retained austenite phase having a solute carbon content of 0.70 to 1.00%, an average particle size of 2.0 μm or less, an average distance between particles of 0.1 to 5.0 μm, a decarburized layer thickness in the steel sheet surface layer of 0.01 to 10.0 μm, an average particle size of oxides contained in the steel sheet surface layer of 30 to 120 nm, and an average density of 1.0 × ¹⁰ particles/m ^{The present} invention discloses a high-strength hot-dip galvanized steel sheet that has excellent mechanical cutting properties while ensuring high strength of a maximum tensile strength of 900 MPa or more, characterized in that the coefficient of hardening (n value) at the time of plastic deformation of 3 to 7% is 0.080 or more on average, and that has high ductility.

Patent Document 2 also discloses a high-strength hot-dip galvanized steel sheet with excellent delayed fracture resistance, characterized in that it has, by volume, 40 to 90% ferrite phase and 5% or less retained austenite phase, with the proportion of unrecrystallized ferrite in the entire ferrite phase being 50% or less, in volume fraction; the grain size ratio, which is the value obtained by dividing the average grain size of the ferrite phase crystal grains in the rolling direction by the average grain size in the sheet width direction, is 0.75 to 1.33; the length ratio, which is the value obtained by dividing the average length of the island-like dispersed hard structure in the rolling direction by the average length in the sheet width direction, is 0.75 to 1.33; and the average aspect ratio of inclusions is 5.0 or less.

Patent Document 3 also discloses a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having good elongation and bendability, which are characterized by comprising a steel sheet and a hot-dip galvanized layer, the steel sheet including a base material and a decarburized ferrite layer, a structure at a depth of ¼ of the sheet thickness of the steel sheet containing 5.0 vol% or more tempered martensite and 0.5 vol% or more but less than 7.0 vol% retained austenite, with the balance mainly consisting of 4 to 70 vol% ferrite and bainite, a part or all of the tempered martensite and the retained austenite forming M-A, the decarburized ferrite layer containing 120% or more ferrite with respect to the content of ferrite at a depth of ¼ of the sheet thickness, an average ferrite grain size of 20 μm or less, a thickness of ⁵ μm or more and 200 μm or less, 1.0 vol% or more tempered martensite, and a number density of 0.01 particles/μm2 or more, and methods for manufacturing the same.

Patent Document 4 also discloses a high-strength hot-dip galvanized steel sheet with excellent workability and a low YR, characterized by a chemical composition containing, by mass, 0.05-0.3% C, 0.01-2.5% Si, 0.5-3.5% Mn, 0.003-0.100% P, 0.02% or less S, 0.010-1.5% Al, and 0.007% or less N, with the balance being Fe and unavoidable impurities, and a microstructure containing, by area ratio, 20-87% ferrite, 3-10% martensite and retained austenite in total, and 10-60% tempered martensite, with a high TS-El balance, excellent stretch flangeability, and a low YR.

Patent No. 5354135 Patent No. 5352793 Patent No. 6536294 Patent No. 5256689

In recent years, the practical use of steel plates with a tensile strength (hereinafter referred to as TS) of 780 MPa or higher has been increasing for automobile impact energy absorption components, such as front side members and rear side members.

In other words, improving the yield stress YS (hereinafter also referred to as YS) is effective in increasing the energy absorbed during impact (hereinafter also referred to as impact absorption energy) so as to obtain excellent fracture resistance characteristics during a vehicle collision. However, increasing the TS and YS of a steel sheet generally reduces press formability, particularly properties such as ductility, hole expandability, and bendability. Therefore, when applying such steel sheets with increased TS and YS to the aforementioned automotive impact energy absorption components, press forming becomes difficult, and variations during forming reduce yield. In particular, reduced press formability at the edges of the steel sheet can lead to end-face cracks in the actual component.

Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet in which ductility is improved by the generation of retained austenite inside the steel sheet and mechanical cutting properties are improved by the formation of a decarburized layer on the surface of the steel sheet. However, no consideration is given at all to the improvement in bendability and fracture resistance at the time of vehicle collision due to the formation of a soft surface layer (decarburized layer), and the press formability of the steel sheet end portion.
Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet in which the main structure inside the steel sheet is soft ferrite and the amount of unrecrystallized ferrite is limited to a small amount, thereby improving ductility, and the delayed fracture resistance and its anisotropy are improved by forming a decarburized layer in the surface layer of the steel sheet. However, no consideration is given at all to the improvement in bendability and fracture resistance in the event of a vehicle collision, which are achieved by forming a surface soft layer (decarburized layer), and the press formability of the steel sheet end portion.
Patent Document 3 discloses a hot-dip galvanized steel sheet and a galvannealed steel sheet in which ductility is improved by the generation of M-A inside the steel sheet and bendability is improved by the formation of a soft layer (decarburized ferrite layer) on the surface layer of the steel sheet, but does not take into consideration at all the press formability of the steel sheet end portion.
Patent Document 4 discloses a high-strength hot-dip galvanized steel sheet in which both ductility, which is the press formability inside the steel sheet, and stretch flangeability, which is the press formability at the end of the steel sheet, are improved. However, no consideration is given at all to improving bendability by forming a soft surface layer (decarburized layer) or improving fracture resistance during a vehicle collision.
From the above, it cannot be said that the steel plates disclosed in Patent Documents 1 to 4 have a TS of 780 MPa or more, a high YS, excellent press formability inside the steel plate (bendability and stretch formability of the steel plate), excellent press formability at the steel plate end (bendability of the steel plate end (shear cross section)), and fracture resistance properties at the time of collision (bending fracture properties and axial crush properties).

The present invention has been developed in view of the above-described current situation, and aims to provide a steel sheet having a tensile strength TS of 780 MPa or more and less than 1180 MPa, a high yield stress YS, excellent press formability inside the steel sheet (bendability and stretch formability of the steel sheet), and excellent press formability at the steel sheet end (bendability of the steel sheet end (shear cross section)), and a method for manufacturing the same.
Another object of the present invention is to provide a member made from the above steel plate and a method for manufacturing the same.

The steel sheet referred to here also includes zinc-plated steel sheet, which is either hot-dip galvanized steel sheet (hereinafter also referred to as GI) or alloyed hot-dip galvanized steel sheet (hereinafter also referred to as GA).

Here, the tensile strength TS is measured by a tensile test in accordance with JIS Z 2241 (2011).
Furthermore, having a high yield stress YS, excellent press formability inside the steel sheet (bendability and stretch formability of the steel sheet), and excellent press formability at the steel sheet end (bendability of the steel sheet end (shear cross section)) means satisfying the following:
A high yield stress YS means that the YS measured in a tensile test in accordance with JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on the TS measured in the tensile test.
(A) When 780 MPa ≦ TS < 980 MPa, 550 MPa ≦ YS
(B) When 980 MPa ≦ TS < 1180 MPa, 700 MPa ≦ YS

Furthermore, the steel plate has excellent bendability when a 90-degree V-bend test is performed in accordance with JIS Z 2248 (2022) with a bending radius of 0.5 mm, and the length of the cracks that propagate along the bend ridges formed other than at the ends of the bend ridges (crack length other than at the V-bend end surface) is 200 μm or less,
A close contact bending test is performed, and the spacer plate thickness at the crack limit where cracks of 0.5 mm or more do not occur along the bending ridge is 3.0 mm or less;
This means that when a close contact bending test is performed with a 3.0 mm spacer, the depth of the crack that progresses in the plate thickness direction at the bending ridge subjected to compressive stress (close contact bending internal crack depth) is 200 μm or less, and when a close contact bending + perpendicular 90 degree V bending test is performed, and the bending radius at which cracks of 0.5 mm or more do not occur along the bending ridge is defined as the crack limit bending radius (handkerchief bending boundary bending radius), the crack limit bending radius is 5.0 mm or less.
Detailed measurement methods for the 90-degree V-bend test with a bending radius of 0.5 mm, the close bending test, and the close bending + perpendicular 90-degree V-bend test are as described in the examples below.

Furthermore, excellent internal stretch formability of the steel sheet means excellent ductility, and refers to the total elongation (El) measured in a tensile test in accordance with JIS Z 2241 (2011) satisfying the following formula (A) or (B) depending on the TS measured in the tensile test.
(A) When 780 MPa ≦ TS < 980 MPa, 17.0% ≦ El
(B) 980 MPa ≦ TS < 1180 MPa, 11.0% ≦ El

Furthermore, excellent bendability at the end of the steel plate (shear cross section) means that, in a 90-degree V-bend test in accordance with JIS Z 2248 (2022) with a bending radius of 0.5 mm, the crack length propagating from the end of the bend ridge toward the ridge (V-bend end face crack length) is 200 μm or less.

The present inventors have conducted extensive research to achieve the above object.
As a result, the component composition of the base steel sheet of the steel sheet is appropriately adjusted, and the base steel sheet of the steel sheet has a surface soft layer whose Vickers hardness is 84% or less of the Vickers hardness at a quarter-thickness position of the sheet thickness, and the surface soft layer satisfies the following formula (1):
The tissue in the superficial soft layer is
Ferrite area ratio: 50.0% or more and 100.0% or less,
Among the structures other than ferrite, the area ratio of fresh martensite divided by the total area ratio of bainite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less,
The structure at 1/4 of the thickness of the base steel sheet is as follows:
an area ratio of ferrite: 76.5% or less (including 0.0%); a total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): 20.0% or more and 90.0% or less; a volume ratio of retained austenite: 3.5% or more and 10.0% or less; and an area ratio of fresh martensite: 10.0% or less (including 0.0%);
It has been found that a steel plate can be obtained which has a tensile strength of 780 MPa or more and less than 1180 MPa, a high yield stress YS, excellent press formability inside the steel plate (bendability and stretch formability of the steel plate), and excellent press formability at the steel plate end (bendability of the steel plate end (shear cross section)).
20≦X≦120-3800×[Sb]-1900×[Sn]...(1)
In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.

The present invention was completed based on the above findings and further research.

That is, the gist and configuration of the present invention are as follows.
[1] In mass%,
C: 0.050% or more and 0.400% or less,
Si: 0.20% or more and 3.00% or less,
Mn: 1.00% or more and less than 3.50%;
P: 0.001% or more and 0.100% or less,
S: 0.0001% or more and 0.0200% or less,
Al: 0.005% or more and 2.000% or less,
N: 0.0100% or less,
Sb: 0.200% or less (including 0%), and Sn: 0.200% or less (including 0%)
and the balance being Fe and unavoidable impurities,
The steel sheet has a soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at a position 1/4 of the sheet thickness from the surface of the base steel sheet,
The surface soft layer satisfies the following formula (1):
The structure in the superficial soft layer is
The area ratio of ferrite is 50.0% or more and 100.0% or less,
When the area ratio of ferrite is less than 100.0%, the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less;
The structure at a 1/4 position of the plate thickness of the base steel plate is
The area ratio of ferrite is 76.5% or less (including 0.0%),
The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is 20.0% or more and 90.0% or less,
The area ratio of retained austenite is 3.5% or more and 10.0% or less,
The area fraction of fresh martensite is 10.0% or less (including 0.0%),
A steel plate having a tensile strength of 780 MPa or more and less than 1180 MPa.
20≦X≦120-3800×[Sb]-1900×[Sn]...(1)
In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.
[2] The component composition further includes, in mass%,
Nb: 0.200% or less,
Ti: 0.200% or less,
V: 0.200% or less,
B: 0.0100% or less,
Cr: 1.000% or less,
Ni: 1.000% or less,
Mo: 1.000% or less,
Cu: 1.000% or less,
Ta: 0.100% or less,
W: 0.500% or less,
Mg: 0.0200% or less,
Zn: 0.0200% or less,
Co: 0.0200% or less,
Zr: 0.1000% or less,
Ca: 0.0200% or less,
Se: 0.0200% or less,
Te: 0.0200% or less,
Ge: 0.0200% or less,
As: 0.0500% or less,
Sr: 0.0200% or less,
Cs: 0.0200% or less,
Hf: 0.0200% or less,
Pb: 0.0200% or less,
The steel sheet according to [1], containing at least one selected from Bi: 0.0200% or less and REM: 0.0200% or less.
[3] The steel sheet according to [1] or [2], wherein one or both sides of the base steel sheet have a plating layer, and the plating layer is a hot-dip galvanized layer.
[4] The steel sheet according to [1] or [2], wherein one or both sides of the base steel sheet have a plating layer, and the plating layer is a galvannealed layer.
[5] A member made using the steel plate according to any one of [1] to [4].
[6] A hot rolling process in which a steel slab having the component composition according to [1] or [2] is hot rolled to form a hot-rolled steel sheet;
After the hot rolling step, a pickling step of pickling the hot-rolled steel sheet;
a cold rolling step of cold rolling the steel sheet after the pickling step at a rolling reduction of 20% or more and 80% or less;
An annealing step in which the steel sheet after the cold rolling step is heated and annealed at an annealing temperature of Ac1 (°C) or higher and 900°C or lower, an annealing time of 20 seconds or longer, and an atmosphere with a dew point of -10°C or higher under conditions that satisfy formula (2) and formula (3);
a cooling step of cooling the steel sheet after the annealing step to a cooling stop temperature of 100°C or higher and 300°C or lower;
a first holding step of reheating the steel sheet after the cooling step to a reheating holding temperature range of 370°C or higher and 460°C or lower and holding the temperature for 10 seconds or longer;
a surface strain introducing step of applying a tension of 2.0 kgf/mm2 or ^more to the steel sheet after the first holding step in the reheating holding temperature range;
A second holding step of holding the steel sheet after the surface strain introduction step at 300 ° C. or higher and 460 ° C. or lower for 10 seconds or longer.
Steel plate manufacturing method.
2400≦Y≦20000...Formula (2)
Y=[{(T-Ac1)×t1}/2}]+{(T-Ac1)×t2}...(3)
Here, in formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing step, t2 is the annealing time (s), and Ac1 is Ac1 (°C).
[7] The method for producing a steel sheet according to [6], further comprising a hot-dip galvanizing step of performing hot-dip galvanizing on the steel sheet after the annealing step to form a hot-dip galvanized layer.
[8] The method for producing a steel sheet according to [6], further comprising a galvannealed hot-dip galvanizing step of subjecting the steel sheet to a galvannealed hot-dip galvanizing treatment after the annealing step to form a galvannealed hot-dip galvanized layer.
[9] A method for manufacturing a component, comprising a step of subjecting the steel plate according to any one of [1] to [4] to at least one of forming and joining to form a component.

According to the present invention, a steel sheet can be obtained which has a tensile strength TS of 780 MPa or more and less than 1180, a high yield stress YS, excellent press formability inside the steel sheet (bendability and stretch formability of the steel sheet), and excellent press formability at the steel sheet end (bendability of the steel sheet end (shear cross section)).
Furthermore, members made from the steel plate of the present invention have high strength and can be used extremely advantageously as impact energy absorbing members for automobiles.

FIG. 1 shows an example of a tissue image taken by SEM used for tissue identification. FIG. 2(a) is a schematic diagram of the sample after being bent at a 90-degree V-shape, and FIG. 2(b) is a diagram of the sample shown in FIG. 2(a) viewed in the Z direction (negative direction). 3-1(a) and (b) are schematic diagrams for explaining end surface cracking in 90-degree V-bending. Figure 3-2(a) is a schematic diagram for explaining a method for measuring the crack length of an end face crack that occurs in a 90-degree V-bend, and Figure 3-2(b) is a diagram showing an example of a profile waveform used to measure the crack length. Figure 4-1(a) is a diagram for explaining a method for measuring the spacer plate thickness at the crack limit when performing a close contact bending test, and Figure 4-1(b) is a diagram for explaining a method for measuring the crack depth that progresses in the plate thickness direction at the bending ridge portion that is subjected to compressive stress when performing a close contact bending test. Figure 4-2(c) is a diagram for explaining a method for cutting out an observation cross section for measuring the depth of cracks that progress in the thickness direction at the bend ridgeline that has been subjected to compressive stress when a close contact bending test is performed, and Figure 4-2(d) is a diagram for explaining a method for measuring the depth of cracks that progress in the thickness direction at the bend ridgeline that has been subjected to compressive stress in the above observation cross section.

The present invention is described based on the following embodiments.

[1. steel plate]
The steel sheet of the present invention has a base steel sheet having a component composition containing, in mass%, C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.005% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (inclusive), and Sn: 0.200% or less (inclusive), with the balance being Fe and unavoidable impurities;
The steel sheet has a soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at a position 1/4 of the sheet thickness from the surface of the base steel sheet,
The surface soft layer satisfies the following formula (1):
The tissue in the superficial soft layer is
The area ratio of ferrite is 50.0% or more and 100.0% or less,
When the area fraction of ferrite is less than 100.0%, the value obtained by dividing the area fraction of fresh martensite by the total area fraction of bainite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less;
The structure at 1/4 of the thickness of the base steel sheet is as follows:
The area ratio of ferrite is 76.5% or less (including 0.0%),
The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is 20.0% or more and 90.0% or less,
The area ratio of retained austenite is 3.5% or more and 10.0% or less,
The area fraction of fresh martensite is 10.0% or less (including 0.0%),
The tensile strength is 780 MPa or more and less than 1180 MPa, and the steel sheet has a high yield stress YS, excellent press formability inside the steel sheet (bendability and stretch formability of the steel sheet), and excellent press formability at the steel sheet end (bendability of the steel sheet end (shear cross section)).
20≦X≦120-3800×[Sb]-1900×[Sn]...(1)
In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.

First, the chemical composition of the base steel sheet of the steel sheet according to one embodiment of the present invention will be described. Note that the units for the chemical compositions are all "mass %", but hereinafter, unless otherwise specified, they will be simply expressed as "%".

C: 0.050% or more and 0.400% or less C is an element that is effective in generating appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite, and retained austenite to ensure a TS of 780 MPa or more and less than 1180 MPa and a high YS. Here, if the C content is less than 0.050%, the area ratio of ferrite increases, making it difficult to achieve a TS of 780 MPa or more. This also leads to a decrease in YS.
On the other hand, if the C content exceeds 0.400%, the area fraction of fresh martensite increases excessively, making it difficult to achieve a TS of less than 1180 MPa. Furthermore, fresh martensite becomes the starting point for void formation during a 90° V-bend test, a close bending test, and a close bending + orthogonal 90° V-bend test, making it impossible to achieve the desired bendability of the steel sheet and the bendability of the sheared edge. Furthermore, the area fraction of retained austenite and the amount of solute C in the retained austenite increase excessively. Furthermore, the hardness of fresh martensite formed by the deformation-induced transformation of retained austenite during shearing increases significantly, promoting the subsequent formation of voids and crack propagation, making it more difficult to achieve the desired bendability of the sheared edge.
Therefore, the C content is set to 0.050% or more and 0.400% or less. The C content is preferably 0.100% or more. The C content is preferably 0.300% or less. The C content is more preferably 0.200% or less.

Si: 0.20% or more and 3.00% or less Si suppresses the formation of carbides during cooling after annealing and promotes the formation of retained austenite. That is, Si is an element that affects the area fraction of retained austenite. Here, if the Si content is less than 0.20%, the area fraction of retained austenite decreases, and ductility decreases.
On the other hand, if the Si content exceeds 3.00%, the area ratio of ferrite increases excessively, and the C concentration in austenite during annealing increases excessively, making it impossible to achieve the desired bendability at the sheared edge.
Therefore, the Si content is set to 0.20% or more and 3.00% or less, preferably 2.00% or less, and more preferably 1.50% or less.
The Si content is preferably 0.50% or more.

Mn: 1.00% or more and less than 3.50% Mn is an element that adjusts the area ratio of bainitic ferrite, tempered martensite, etc. Here, if the Mn content is less than 1.00%, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 780 MPa or more. This also leads to a decrease in YS.
On the other hand, if the Mn content is 3.50% or more, the martensitic transformation start temperature Ms (hereinafter also referred to simply as the Ms point or Ms) decreases, and the amount of martensite formed during the cooling process decreases. As a result, the amount of martensite formed during final cooling increases, and the martensite formed at that time is not sufficiently tempered, resulting in an increase in the area fraction of hard fresh martensite. Fresh martensite serves as the starting point for void formation during a 90-degree V-bend test, a close-contact bending test, and a close-contact bending + orthogonal 90-degree V-bend test. If the area fraction of fresh martensite exceeds 10.0%, the desired bendability of the steel sheet and the bendability of the sheared edge cannot be achieved.
Therefore, the Mn content is set to 1.00% or more and less than 3.50%, preferably 2.00% or more, and preferably 3.00% or less.

P: 0.001% or more and 0.100% or less P is an element that has a solid solution strengthening effect and increases the TS and YS of the steel sheet. To obtain this effect, the P content is set to 0.001% or more.
On the other hand, if the P content exceeds 0.100%, P segregates at the prior austenite grain boundaries and embrittles the grain boundaries, which increases the amount of voids generated after shearing the steel sheet, making it impossible to achieve the desired bendability at the sheared edge.
Therefore, the P content is set to 0.001% or more and 0.100% or less, and preferably 0.003% or more.
The P content is preferably 0.030% or less, more preferably 0.010% or less, and even more preferably 0.005% or less.

S: 0.0001% or more and 0.0200% or less S exists as sulfides in steel. In particular, if the S content exceeds 0.0200%, the amount of voids generated increases after shearing the steel sheet, making it impossible to achieve the desired bendability of the sheared edge.
Therefore, the S content is set to 0.0200% or less, preferably 0.0080% or less, and more preferably 0.0050% or less.
Due to constraints on production technology, the S content is set to 0.0001% or more, preferably 0.0003% or more, and more preferably 0.0005% or more.

Al: 0.005% or more and 2.000% or less Al suppresses the formation of carbides during cooling after annealing and promotes the formation of retained austenite. That is, Al is an element that affects the area ratio of retained austenite. To achieve these effects, the Al content is set to 0.005% or more.
On the other hand, if the Al content exceeds 2.000%, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 780 MPa or more. This also leads to a decrease in YS. In addition, the C concentration in austenite during annealing increases excessively, making it impossible to achieve the desired bendability at the sheared edge.
Therefore, the Al content is set to 0.005% or more and 2.000% or less, and preferably 0.010% or more.
The Al content is preferably 1.000% or less, more preferably 0.100% or less, and further preferably 0.050% or less.

N: 0.0100% or less N exists as nitrides in steel. In particular, if the N content exceeds 0.0100%, the amount of voids generated increases after shearing the steel sheet, making it impossible to achieve the desired bendability of the sheared edge.
Therefore, the N content is set to 0.0100% or less, and preferably 0.0050% or less.
Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more.

Sb: 0.200% or less (including 0%)
Sb is a useful element that segregates on the steel sheet surface during annealing to improve platability and chemical conversion treatability. Therefore, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.005% or more. The Sb content is more preferably 0.007% or more, and even more preferably 0.009% or more.
On the other hand, if the Sb content exceeds 0.200%, the effect of improving platability and chemical conversion treatability will saturate, and there is a risk of deterioration in the press formability (bendability within the steel sheet) and crack propagation resistance within the steel sheet. Therefore, when Sb is contained, the Sb content is set to 0.200% or less. The Sb content is more preferably 0.020% or less. Further preferably, it is 0.018% or less. The Sb content is more preferably 0.016% or less, and further preferably 0.014% or less.

Sn: 0.200% or less (including 0%)
Like Sb, Sn is a useful element that segregates on the steel sheet surface during annealing to improve platability and chemical conversion treatability. Therefore, the Sn content is preferably 0.002% or more. The Sn content is more preferably 0.005% or more. The Sn content is more preferably 0.007% or more, and even more preferably 0.009% or more.
On the other hand, if the Sn content exceeds 0.200%, the effect of improving galvanizability and chemical conversion treatability will saturate, and there is a risk of deterioration in the press formability (bendability) and crack propagation resistance of the steel sheet interior. Therefore, when Sn is contained, the Sn content must be 0.200% or less. The Sn content is more preferably 0.020% or less, and even more preferably 0.016% or less. The Sn content is more preferably 0.014% or less, and even more preferably 0.012% or less.

The basic chemical composition of the base steel sheet of a steel sheet according to one embodiment of the present invention has been described above. The base steel sheet of a steel sheet according to one embodiment of the present invention contains the above basic chemical components, with the balance other than the above basic chemical components comprising Fe (iron) and unavoidable impurities. Here, it is preferable that the base steel sheet of a steel sheet according to one embodiment of the present invention has a chemical composition containing the above basic chemical components, with the balance consisting of Fe and unavoidable impurities.

In addition to the basic components described above, the base steel sheet of a steel sheet according to one embodiment of the present invention may contain at least one selected from the optional components listed below. The effects of the present invention can be achieved as long as the optional components listed below are contained in amounts up to the upper limit amounts shown below, so no lower limit is set. If the optional elements listed below are contained in amounts below the preferred lower limit values described below, the elements are considered to be included as unavoidable impurities.

Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less , Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.100 At least one selected from the group consisting of 0% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less

Nb: 0.200% or less Nb forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more. The Nb content is more preferably 0.010% or more, and even more preferably 0.020% or more.
On the other hand, if the Nb content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points of cracks during a 90-degree V-bend test, a close bending test, and a close bending + perpendicular 90-degree V-bend test, and the desired bendability of the steel sheet and bendability of the shear edge may not be achieved. Therefore, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.

Ti: 0.200% or less Like Nb, Ti forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more. The Ti content is more preferably 0.010% or more.
On the other hand, if the Ti content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for cracks during a 90-degree V-bend test, a close bending test, and a close bending + perpendicular 90-degree V-bend test, and the desired bendability of the steel sheet and the bendability of the sheared edge may not be achieved. Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less. The Ti content is more preferably 0.050% or less, and even more preferably 0.030% or less.

V: 0.200% or less Like Nb and Ti, V forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. The V content is further preferably 0.010% or more, and even more preferably 0.020% or more.
On the other hand, if the V content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points of cracks during a 90-degree V-bend test, a close bending test, and a close bending + perpendicular 90-degree V-bend test, and the desired bendability of the steel sheet and bendability of the sheared edge may not be achieved. Therefore, when V is contained, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.

B: 0.0100% or less B is an element that segregates at austenite grain boundaries to improve hardenability. B is also an element that suppresses the formation and grain growth of ferrite during cooling after annealing. To achieve this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. The B content is further preferably 0.0005% or more, and even more preferably 0.0007% or more.
On the other hand, if the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling. Furthermore, after the steel sheet is subjected to shearing, the amount of voids generated increases, and the desired bendability of the sheared edge may not be achieved.
Therefore, when B is contained, the B content is preferably 0.0100% or less, more preferably 0.0050% or less, and more preferably 0.0020% or less.

Cr: 1.000% or less Cr is an element that improves hardenability, so the addition of Cr produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To achieve this effect, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.010% or more. The Cr content is further preferably 0.030% or more, and even more preferably 0.040% or more.
On the other hand, if the Cr content exceeds 1.000%, the area ratio of hard fresh martensite increases excessively, and fresh martensite becomes the origin of void formation in the 90-degree V-bend test, the close bending test, and the close bending + perpendicular 90-degree V-bend test. As a result, the desired bendability of the steel sheet and the bendability of the sheared edge may not be achieved. Therefore, when Cr is contained, the Cr content is preferably 1.000% or less. Furthermore, the Cr content is more preferably 0.800% or less, and even more preferably 0.700% or less. The Cr content is more preferably 0.100% or less, and even more preferably 0.080% or less.

Ni: 1.000% or less Ni is an element that improves hardenability, so the addition of Ni produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To achieve this effect, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. The Ni content is even more preferably 0.040% or more, and even more preferably 0.060% or more.
On the other hand, if the Ni content exceeds 1.000%, the area fraction of fresh martensite increases excessively, and fresh martensite becomes the origin of void generation in the 90-degree V-bend test, the close bend test, and the close bend + orthogonal 90-degree V-bend test. As a result, there is a risk that the desired bendability of the steel sheet and the bendability of the sheared edge may not be achieved. Therefore, when Ni is contained, the Ni content is preferably 1.000% or less. The Ni content is more preferably 0.800% or less.
The Ni content is more preferably 0.600% or less, even more preferably 0.400% or less, and more preferably 0.200% or less.

Mo: 1.000% or less Mo is an element that improves hardenability, so the addition of Mo produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To achieve this effect, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.030% or more.
On the other hand, if the Mo content exceeds 1.000%, the area fraction of fresh martensite increases excessively, and fresh martensite becomes the origin of void formation in the 90-degree V-bend test, the close bending test, and the close bending + perpendicular 90-degree V-bend test. As a result, the desired bendability and bendability of the sheared edge of the steel sheet may not be achieved. Therefore, when Mo is contained, the Mo content is preferably 1.000% or less. The Mo content is more preferably 0.500% or less, even more preferably 0.450% or less, and even more preferably 0.400% or less. The Mo content is more preferably 0.350% or less, and even more preferably 0.300% or less. The Mo content is more preferably 0.100% or less, and even more preferably 0.080% or less.

Cu: 1.000% or less Cu is an element that improves hardenability, so the addition of Cu produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To achieve this effect, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.008% or more, and even more preferably 0.010% or more. The Cu content is more preferably 0.020% or more. The Cu content is even more preferably 0.050% or more, and even more preferably 0.100% or more.
On the other hand, if the Cu content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and large amounts of coarse precipitates and inclusions may be formed. In such cases, the fresh martensite and coarse precipitates and inclusions may become the starting points for void formation during a 90-degree V-bend test, a close bending test, and a close bending + orthogonal 90-degree V-bend test, which may prevent the desired bendability of the steel sheet and bendability of the sheared edge surface from being achieved. Therefore, when Cu is contained, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.

Ta: 0.100% or less Like Ti, Nb, and V, Ta increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. In addition, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta)(C, N). This suppresses coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS and YS. To achieve this effect, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.002% or more, and even more preferably 0.004% or more.
On the other hand, if the Ta content exceeds 0.100%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for void formation during a 90-degree V-bend test, a close bend test, and a close bend + perpendicular 90-degree V-bend test. As a result, the desired bendability of the steel sheet and the bendability of the sheared edge may not be achieved. Therefore, when Ta is contained, the Ta content is preferably 0.100% or less.
The Ta content is more preferably 0.090% or less, even more preferably 0.080% or less, more preferably 0.050% or less, and even more preferably 0.020% or less.

W: 0.500% or less W is an element that improves hardenability, and the addition of W produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To achieve this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.020% or more.
On the other hand, if the W content exceeds 0.500%, the area fraction of hard fresh martensite increases excessively, and fresh martensite becomes the origin of void formation in the 90-degree V-bend test, the close bending test, and the close bending + perpendicular 90-degree V-bend test. As a result, the desired bendability of the steel sheet and the bendability of the sheared edge may not be achieved. Therefore, when W is contained, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, and even more preferably 0.400% or less. The W content is even more preferably 0.300% or less. The W content is more preferably 0.100% or less, and even more preferably 0.050% or less.

Mg: 0.0200% or less Mg is an element that is effective in spheroidizing the shape of inclusions such as sulfides and oxides and improving the bendability of the sheared end face. To achieve this effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more, and even more preferably 0.0010% or more. The Mg content is more preferably 0.0020% or more, and even more preferably 0.0030% or more.
On the other hand, if the Mg content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for void formation during a 90-degree V-bend test, a close bending test, and a close bending + perpendicular 90-degree V-bend test. As a result, the desired bendability and bendability of the sheared edge of the steel sheet may not be achieved. Therefore, when Mg is added, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Mg content is more preferably 0.0100% or less, and even more preferably 0.0080% or less.

Zn: 0.0200% or less Zn is an element that is effective in making the shape of inclusions spherical and improving the bendability of the sheared end surface. To achieve this effect, the Zn content is preferably 0.0010% or more. The Zn content is more preferably 0.0020% or more, and even more preferably 0.0030% or more.
On the other hand, if the Zn content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for void formation during a 90-degree V-bend test, a close bending test, and a close bending + perpendicular 90-degree V-bend test. As a result, the desired bendability and bendability of the sheared edge of the steel sheet may not be achieved. Therefore, when Zn is contained, the Zn content is preferably 0.0200% or less. The Zn content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Zn content is more preferably 0.0100% or less, and even more preferably 0.0080% or less.

Co: 0.0200% or less Like Zn, Co is an element that is effective in spheroidizing the shape of inclusions and improving the bendability of the sheared end face. To achieve this effect, the Co content is preferably 0.0010% or more. The Co content is more preferably 0.0020% or more, and even more preferably 0.0030% or more. The Co content is more preferably 0.0050% or more.
On the other hand, if the Co content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for void formation during a 90-degree V-bend test, a close bending test, and a close bending + perpendicular 90-degree V-bend test. As a result, the desired bendability of the steel sheet and bendability of the sheared edge may not be achieved. Therefore, when Co is contained, the Co content is preferably 0.0200% or less. The Co content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

Zr: 0.1000% or less Zr, like Zn and Co, is an element that is effective in making inclusions spheroidal and improving the bendability of the sheared end surface. To obtain such effects, the Zr content is preferably 0.0010% or more.
On the other hand, if the Zr content exceeds 0.1000%, a large amount of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting point for void formation during a 90-degree V-bend test, a close bend test, and a close bend + perpendicular 90-degree V-bend test. As a result, the desired bendability and bendability of the sheared edge of the steel sheet may not be achieved. Therefore, when Zr is added, the Zr content is preferably 0.1000% or less.
The Zr content is more preferably 0.0300% or less, further preferably 0.0100% or less, and more preferably 0.0050% or less.

Ca: 0.0200% or less Ca exists as inclusions in steel. Here, if the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be formed. In such cases, the coarse precipitates and inclusions become the starting points for void formation during a 90-degree V-bend test, a close-contact bending test, and a close-contact bending + orthogonal 90-degree V-bend test. As a result, the desired bendability and bendability of the sheared edge of the steel sheet may not be achieved. Therefore, when Ca is contained, the Ca content is preferably 0.0200% or less.
The Ca content is preferably 0.0020% or less. Although the lower limit of the Ca content is not particularly limited, the Ca content is preferably 0.0005% or more. Furthermore, due to constraints on production technology, the Ca content is more preferably 0.0010% or more.

Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, REM: 0.0200% or less. Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are all effective elements for improving the bendability of the sheared edge. To achieve this effect, the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are each preferably 0.0001% or more.
On the other hand, when the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM exceed 0.0200% each, and/or when the content of As exceeds 0.0500%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for void formation during a 90-degree V-bend test, a close bending test, and a close bending + orthogonal 90-degree V-bend test, which may prevent the desired bendability and bendability of the sheared edge of the steel sheet from being achieved. Therefore, when at least one of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM is contained, the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM are preferably each 0.0200% or less. When As is contained, the As content is preferably 0.0500% or less.

The Se content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Se content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Se content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Te content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Te content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Te content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Ge content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Ge content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Ge content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The As content is more preferably 0.0010% or more, and even more preferably 0.0015% or more. The As content is more preferably 0.0100% or more, and even more preferably 0.0150% or more. The As content is more preferably 0.0400% or less, and even more preferably 0.0300% or less.
The Sr content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Sr content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Sr content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Cs content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Cs content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Cs content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Hf content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Hf content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Hf content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Pb content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Pb content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Pb content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Bi content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Bi content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Bi content is more preferably 0.0100% or less, and even more preferably 0.0050% or less.
The REM content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The REM content is more preferably 0.0010% or more, and even more preferably 0.0030% or more. The REM content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The REM content is more preferably 0.0100% or less.
In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanoids from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM concentration in the present invention refers to the total content of one or more elements selected from the above-mentioned REMs.
The REM is not particularly limited, but is preferably at least one of Sc, Y, Ce and La.

That is, the base steel sheet of the steel sheet of the present invention contains, in mass%, C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (including 0%), and Sn: 0.200% or less (including 0%), and optionally Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, and Mo: 1.000% or less. and at least one selected from Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less, with the balance being Fe and unavoidable impurities.

Steel structure (structure at 1/4 of the plate thickness of the base steel plate)
Next, the steel structure of the steel plate according to one embodiment of the present invention will be described.
In the structure at the 1/4 position of the plate thickness of the base steel plate, the area ratio of ferrite is 76.5% or less (including 0.0%), the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is 20.0% or more and 90.0% or less, the area ratio of retained austenite is 3.5% or more and 10.0% or less, and the area ratio of fresh martensite is 10.0% or less (including 0.0%).
The reasons for each limitation will be explained below.

Ferrite area ratio: 76.5% or less (including 0.0%)
Soft ferrite is a phase that improves ductility. However, an excessive increase in the area fraction of ferrite makes it difficult to achieve a TS of 780 MPa or more. This also leads to a decrease in YS. In addition, the C concentration in austenite during annealing increases excessively, making it impossible to achieve the desired bendability at the sheared edge. Therefore, the area fraction of ferrite is set to 76.5% or less. The area fraction of ferrite is preferably 60.0% or less.
The lower limit of the area ratio of ferrite is not particularly limited and may be 0.0%. The area ratio of ferrite may be 5.0% or more, or 10.0% or more.

Total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): 20.0% or more and 90.0% or less. Bainitic ferrite and tempered martensite have intermediate hardness between soft ferrite and hard fresh martensite, and are important phases for ensuring good bendability of steel sheets and bendability of sheared edges. Bainitic ferrite is also a useful phase for obtaining an appropriate amount of retained austenite by utilizing the diffusion of C from bainitic ferrite to untransformed austenite. Tempered martensite is effective in improving TS. Therefore, the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is set to 20.0% or more, preferably 30.0% or more.
On the other hand, if the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) increases excessively, ductility decreases. Therefore, the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is set to 90.0% or less. The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is preferably 87.0% or less. It is more preferable that the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is 80.0% or less.
Bainitic ferrite is upper bainite with little carbide that is produced in a relatively high temperature range.

Area fraction of retained austenite: 3.5% or more and 10.0% or less In order to obtain good ductility, the area fraction of retained austenite is set to 3.5% or more. The area fraction of retained austenite is preferably more than 3.5%.
On the other hand, if the volume fraction of retained austenite increases excessively, fresh martensite formed by stress-induced transformation during shearing becomes the starting point for void formation, making it impossible to achieve the desired bendability of the sheared edge. Therefore, the area fraction of retained austenite is set to 10.0% or less. The area fraction of retained austenite is preferably 9.0% or less, and more preferably 8.0% or less.

Area ratio of fresh martensite: 10.0% or less (including 0.0%)
If the area fraction of fresh martensite increases excessively, the fresh martensite may become the origin of void generation in a 90-degree V-bend test, a close bending test, and a close bending + orthogonal 90-degree V-bend test, and the desired bendability of the steel sheet and bendability of the sheared edge may not be achieved.
From the viewpoint of ensuring good bendability of the steel sheet and bendability of the sheared edge, the area ratio of fresh martensite is set to 10.0% or less, and preferably 5.0% or less.
The lower limit of the area ratio of fresh martensite is not particularly limited, and may be 0.0%.
Note that fresh martensite is martensite that has been quenched (not tempered).

The area ratio of the remaining structure other than the above is preferably 10.0% or less. The area ratio of the remaining structure is more preferably 7.0% or less, and even more preferably 5.0% or less. The area ratio of the remaining structure may also be 0.0%.
The remaining structure is not particularly limited, and examples thereof include carbides such as lower bainite, pearlite, cementite, etc. The type of the remaining structure can be confirmed by observation using, for example, a scanning electron microscope (SEM).

Soft surface layer The base steel sheet of the steel sheet according to one embodiment of the present invention preferably has a soft surface layer on the surface of the base steel sheet. The soft surface layer contributes to suppressing the propagation of bending cracks during press forming, further improving the bendability of the steel sheet. The soft surface layer refers to a decarburized layer, and is a surface region having a Vickers hardness of 84% or less of the Vickers hardness of the cross section at 1/4 of the sheet thickness.
In order to obtain the effect of improving the bendability of the steel sheet, the thickness of the surface soft layer is set to 20 μm or more, and the thickness of the surface soft layer is set to 120 μm or less.
The Vickers hardness is measured based on JIS Z 2244-1 (2020) at a load of 10 gf.

Surface soft layer thickness (X): 20≦X≦120−3800×[Sb]−1900×[Sn] ... (1)
In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.
The soft surface layer in the present invention refers to a region in which the Vickers hardness is 84% or less of the Vickers hardness at a position 1/4 of the sheet thickness from the surface of the base steel sheet. The soft surface layer thickness (X) must satisfy the formula (1).
If the thickness (X) of the surface soft layer is less than 20 μm, the desired bendability intended by the present invention may not be obtained.
On the other hand, if the surface soft layer thickness (X) exceeds (120-3800×[Sb]-1900×[Sn]) μm, it is not possible to achieve both high strength and excellent press formability as intended by the present invention. Therefore, the surface soft layer thickness (X) is specified to be 20 μm or more and (120-3800×[Sb]-1900×[Sn]) μm or less.
In the present invention, as described above, Sb and Sn are added as needed to improve plating properties and chemical conversion treatability, but when Sb and Sn are added, the surface segregation of these elements described above reduces the allowable upper limit of the soft surface layer thickness (X) that affects bending cracks. For this reason, the upper limit of the soft surface layer that provides good bendability is (120 - 3800 × [Sb] - 1900 × [Sn]) μm.
The thickness of the surface soft layer is preferably 25 μm or more, and more preferably 30 μm or more.
The thickness of the surface soft layer is preferably 100 μm or less, and more preferably 90 μm or less.

Steel structure in the soft surface layer Ferrite area ratio: 50.0% or more and 100.0% or less When subjected to bending, the surface layer is deformed more than the interior. Therefore, voids are more likely to form in the surface layer. In the present invention, by controlling the amount of ferrite in the soft surface layer to 50.0% or more, voids that serve as crack initiation points are less likely to form in the surface layer, and crack propagation is suppressed. The area ratio of ferrite in the soft surface layer is preferably 60.0% or more.
The area ratio of ferrite may be 100.0%. The area ratio of ferrite may be 99.9% or less, 95.0% or less, or 90.0% or less.

[0023] Value obtained by dividing the area fraction of fresh martensite by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite): 0.5 or less. If the area fraction of fresh martensite in the soft surface layer is excessively increased, the fresh martensite becomes the origin of void generation in a 90-degree V-bend test, a close bending test, and a close bending + orthogonal 90-degree V-bend test, making it impossible to achieve the desired bendability of the steel sheet. From the viewpoint of ensuring good bendability of the steel sheet and bendability of the sheared edge, when the area fraction of ferrite is less than 100.0%, the value obtained by dividing the area fraction of martensite in the soft surface layer by the area fraction of hard phases other than ferrite is set to 0.5 or less.
Here, the hard phase other than ferrite refers to bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite).
The lower limit of the value obtained by dividing the area ratio of martensite in the soft surface layer by the area ratio of the hard phase other than ferrite is not particularly limited, and may be 0.00.

For example, by controlling the tension during the surface strain introduction step in the manufacturing method described below, the area ratio of fresh martensite in the surface soft layer divided by the area ratio of hard phases other than ferrite can be suppressed to 0.5 or less. By applying a tension of 2.0 kgf/ ^mm2 or more once or more after the first holding step, untransformed austenite undergoes strain-induced transformation to become fresh martensite, which is then tempered during the subsequent second holding step, finally becoming tempered martensite.

Here, the area ratios of ferrite, bainitic ferrite, tempered martensite, and hard phase (hard second phase (retained austenite + fresh martensite)) in the 1/4 position of the sheet thickness of the base steel sheet and in the soft surface layer are measured as follows: The structure of the soft surface layer is measured at a position 1/2 of the thickness of the soft surface layer.
A sample is cut out from the base steel sheet so that the plate thickness cross section (L cross section) parallel to the rolling direction of the base steel sheet serves as the observation surface. The observation surface of the sample is then mirror-polished using diamond paste. The observation surface of the sample is then finish-polished using colloidal silica, and then etched with 3 vol. % nital to reveal the structure.
Then, using a scanning electron microscope (SEM), under conditions of an acceleration voltage of 15 kV and a magnification of 5000x, three fields of view of 25.6 μm × 17.6 μm were photographed at the outermost layer position (a position half the thickness of the surface soft layer) and at a position one-quarter of the plate thickness within a range of ±100 μm. At the outermost layer position, the zinc plating layer was excluded and the internal oxide layer was included.
From the obtained structural image (see FIG. 1), ferrite, bainitic ferrite, tempered martensite, and other hard phases (hard second phases (retained austenite + fresh martensite)) are identified as follows.

Ferrite: A black region with a blocky shape. It contains almost no iron-based carbides. However, if iron-based carbides are contained, the area of the ferrite includes the area of the iron-based carbides. The same applies to bainitic ferrite and tempered martensite, which will be described later.
Bainitic ferrite: This is a region that is black to dark gray in color and has a massive or amorphous shape. It contains no iron-based carbides or a relatively small amount of them.
Tempered martensite: A gray region with an amorphous shape. It also contains a relatively large number of iron-based carbides.
Hard second phase (retained austenite + fresh martensite): This is a region that is white to light gray in color and has an amorphous form. It does not contain iron-based carbides. If the size is relatively large, the color gradually darkens as it moves away from the interface with other structures, and the interior may be dark gray.
Carbides: These are white areas that appear as dots or lines and are included in tempered martensite, bainitic ferrite, and ferrite.
Remaining structure: The above-mentioned lower bainite, pearlite, inner oxides, etc. are included, and the forms thereof are as known.

Next, the area of each phase identified in the structural image is calculated using the following method. An evenly spaced 20 x 20 grid is placed over an area of 25.6 μm x 19.2 μm in actual length on the 5000x magnification SEM image, and the area fraction of ferrite, bainitic ferrite, tempered martensite, and other hard phases (hard second phases) is investigated using the point counting method, which counts the number of points on each phase. The area fraction is calculated as the average of three area fractions determined on separate 5000x magnification SEM images.

The area ratio of retained austenite is measured as follows.
The base steel sheet is mechanically ground in the thickness direction (depth direction) to a position corresponding to one-quarter of the thickness, and then chemically polished with oxalic acid to obtain an observation surface. The observation surface is then observed by X-ray diffraction. MoKα rays are used as incident X-rays, and the ratios of the diffraction intensities of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensities of the (200), (211), and (220) planes of bcc iron are calculated. The volume fraction of retained austenite is calculated from the ratio of the diffraction intensities of each plane. The retained austenite is then considered to be three-dimensionally homogeneous, and the volume fraction of retained austenite is taken as the area fraction of retained austenite.

The area ratio of fresh martensite is determined by subtracting the area ratio of retained austenite from the area ratio of the hard second phase determined as described above.
[Area fraction of fresh martensite (%)] = [Area fraction of hard second phase (%)] - [Area fraction of retained austenite (%)]

The area ratio of the remaining structure is determined by subtracting the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of other hard phases (hard second phases) determined as described above from 100.0%.
[Area ratio of remaining structure (%)] = 100.0 - [Area ratio of ferrite (%)] - [Area ratio of bainitic ferrite (%)] - [Area ratio of tempered martensite (%)] - [Area ratio of hard secondary phase (%)]

Next, the mechanical properties of the steel plate according to one embodiment of the present invention will be described.

Tensile Strength (TS): 780 MPa or More and Less than 1180 MPa The tensile strength TS of a steel plate according to one embodiment of the present invention is 780 MPa or more and less than 1180 MPa.
The predetermined yield stress (YS), yield ratio (YR), stretch formability (total elongation (El)) inside the steel plate, bendability of the steel plate, and bendability of the sheared end surface of the steel plate according to one embodiment of the present invention are as described above.
The ratio YR (yield ratio) of the yield stress YS to the tensile strength TS preferably satisfies 0.70≦YR.

The tensile strength (TS), yield ratio (YR), yield stress (YS), and total elongation (El) are measured by a tensile test in accordance with JIS Z 2241 (2011), which is described later in the examples. The bendability of the steel sheet is measured by a close bend test and a close bend + perpendicular 90-degree V-bend test, which are described later in the examples. The bendability of the sheared edge is measured by a 90-degree V-bend test, which is described later in the examples.

Plated layer (hot-dip galvanized layer, alloyed hot-dip galvanized layer)
A steel sheet according to one embodiment of the present invention may have a plating layer formed on the base steel sheet (on the surface of the base steel sheet), and this plating layer may be provided on only one surface of the base steel sheet, or on both surfaces.

The plating layer (galvanized layer) referred to here refers to a plating layer whose main component is Zn (Zn content of 50.0% or more), and examples include hot-dip galvanized layers and alloyed hot-dip galvanized layers.

The hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0% or less by mass of Fe, and 0.001% to 1.0% by mass of Al. The hot-dip galvanized layer may also optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0% to 3.5% by mass. The Fe content of the hot-dip galvanized layer is more preferably less than 7.0% by mass. The remainder, other than the above elements, is unavoidable impurities.

The galvannealed layer is preferably composed of, for example, 20% by mass or less Fe and 0.001% by mass or more and 1.0% by mass or less Al. The galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0% by mass or more and 3.5% by mass or less. The Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more. The Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less. The remainder other than the above elements is unavoidable impurities.

In addition, the coating weight of the plating layer (zinc plating layer) per side is not particularly limited, but is preferably 20 g/ ^m2 or more. Also, the coating weight of the plating layer (zinc plating layer) per side is preferably 80 g/ ^m2 or less.

The coating weight of the plating layer (zinc plating layer) is measured as follows.
That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (Ivit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass % aqueous hydrochloric acid solution. Next, a steel sheet (galvanized steel sheet) to be used as a test material is immersed in the treatment solution to dissolve the plating layer (galvanized layer). The mass loss of the test material before and after dissolution is measured, and this value is divided by the surface area of the base steel sheet (the surface area of the part that was covered with the plating) to calculate the plating coverage (g/ ^m2 ).

The thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more, more preferably 0.6 mm or more, and even more preferably 0.8 mm or more. Furthermore, the thickness of the steel plate is preferably 2.3 mm or less, more preferably 1.6 mm or less, and even more preferably 1.2 mm or less.

[2. Steel Sheet Manufacturing Method]
Next, a method for manufacturing a steel sheet according to one embodiment of the present invention will be described.

A method for producing a steel sheet according to one embodiment of the present invention includes a hot rolling step of hot rolling a steel slab having the above-mentioned component composition to obtain a hot-rolled steel sheet;
After the hot rolling step, a pickling step of pickling the hot-rolled steel sheet;
a cold rolling step of cold rolling the steel sheet after the pickling step at a rolling reduction of 20% or more and 80% or less;
An annealing step in which the steel sheet after the cold rolling step is heated and annealed at an annealing temperature of Ac ₁ point (°C) or higher and 900°C or lower, an annealing time of 20 seconds or longer, and an atmosphere with a dew point of -10°C or higher, under conditions that satisfy formula (2) and formula (3);
a cooling step of cooling the steel sheet after the annealing step to a cooling stop temperature of 100°C or higher and 300°C or lower;
a first holding step of reheating the steel sheet after the cooling step to a reheating holding temperature range of 370°C or higher and 460°C or lower and holding the temperature for 10 seconds or longer;
a surface strain introducing step of applying a tension of 2.0 kgf/ ^mm2 or more to the steel sheet after the first holding step in a reheating first holding temperature range;
and a second holding step of holding the steel sheet after the surface strain introducing step at 300°C or higher and 460°C or lower for 10 seconds or longer.
2400≦Y≦20000...Formula (2)
Y=[{(T-Ac1)×t1}/2}]+{(T-Ac1)×t2}...(3)
Here, in formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing step, t2 is the annealing time (s), and Ac1 is Ac1 (°C).
Ac1 (°C): 727.0 - 32.7 × [%C] + 14.9 × [%Si] + 2.0 × [%Mn], where [%C] is the C content of the steel plate (steel slab), [%Si] is the Si content of the steel plate (steel slab), and [%Mn] is the Mn content of the steel plate (steel slab).
Unless otherwise specified, the above temperatures refer to the surface temperatures of the steel slab and steel plate.

First, a steel slab having the above-mentioned chemical composition is prepared. For example, a steel material is melted to produce molten steel having the above-mentioned chemical composition. The melting method is not particularly limited, and known melting methods such as converter melting and electric furnace melting can be used. The resulting molten steel is then solidified to produce a steel slab. The method for obtaining a steel slab from molten steel is not particularly limited, and for example, continuous casting, ingot casting, thin slab casting, etc. can be used. Continuous casting is preferred from the perspective of preventing macrosegregation.

[Hot rolling process]
The steel slab is then hot rolled to form a hot rolled steel sheet.
Hot rolling may be performed using an energy-saving process, such as direct rolling (a method in which a steel slab is charged into a heating furnace as a hot strip without being cooled to room temperature, and then hot-rolled) or direct rolling (a method in which a steel slab is briefly kept at a constant temperature and then immediately rolled).

The hot rolling conditions are not particularly limited, and the hot rolling can be carried out under the following conditions, for example.
That is, the steel slab is once cooled to room temperature, and then reheated and rolled. The slab heating temperature (reheating temperature) is preferably 1100°C or higher from the viewpoint of dissolving carbides and reducing the rolling load. Furthermore, in order to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or lower. The slab heating temperature is based on the temperature of the steel slab surface.

The steel slab is then subjected to rough rolling according to conventional methods to produce a rough-rolled plate (hereinafter also referred to as a sheet bar). The sheet bar is then subjected to finish rolling to produce a hot-rolled steel plate. If the slab heating temperature is low, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling to prevent problems during finish rolling. The finish rolling temperature is preferably 800°C or higher to reduce the rolling load. Furthermore, if the reduction ratio is high in the unrecrystallized austenite state, abnormal structures elongated in the rolling direction may develop, potentially reducing the workability of the annealed sheet. Furthermore, by setting the finish rolling temperature to 800°C or higher, the steel structure at the hot-rolled steel plate stage and, ultimately, the steel structure of the final product, tend to be uniform. A non-uniform steel structure tends to reduce bendability. On the other hand, a finish rolling temperature above 950°C increases the amount of oxide (scale) formation. As a result, the interface between the base steel and the oxide may become rough, which may deteriorate the surface quality of the steel sheet after pickling and cold rolling. Furthermore, the crystal grains may become coarse, which may cause a decrease in the strength and bendability of the steel sheet. For these reasons, the finish rolling temperature is preferably set to a range of 800°C or higher. Furthermore, the finish rolling temperature is preferably set to a range of 950°C or lower.

After finish rolling, the hot-rolled steel sheet is coiled. The coiling temperature is preferably 450°C or higher. It is also preferable that the coiling temperature be 750°C or lower.

Note that the sheet bars may be joined together during hot rolling and continuous finish rolling may be performed. The sheet bars may also be temporarily wound before finish rolling. Furthermore, in order to reduce the rolling load during hot rolling, part or all of the finish rolling may be performed as lubricated rolling. Performing lubricated rolling is also effective from the viewpoint of uniformity of the steel sheet shape and material properties. The coefficient of friction during lubricated rolling is preferably in the range of 0.10 to 0.25.
In the hot rolling process (hot rolling process) which includes rough rolling and finish rolling, steel slabs are generally made into sheet bars by rough rolling and then made into hot rolled steel sheets by finish rolling. However, depending on the mill capacity, etc., such division is not important and it is not a problem as long as the specified size is achieved.

[Acid washing process]
After the hot rolling process, the hot-rolled steel sheet is pickled. Pickling can remove oxides from the steel sheet surface, ensuring good chemical conversion treatability and plating quality. Pickling may be performed once or multiple times. There are no particular restrictions on the pickling conditions, and conventional methods may be used.

[Cold rolling process]
The cold rolling is carried out by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.
The reduction rate (cumulative reduction rate) of cold rolling is not particularly limited, but is set to 20% or more and 80% or less. If the reduction rate of cold rolling is less than 20%, the steel structure is likely to become coarse and non-uniform in the annealing step, and there is a risk that the TS and bendability of the final product will decrease.
On the other hand, if the reduction ratio in cold rolling exceeds 80%, the steel sheet is likely to have a defective shape and the coating weight may become non-uniform.
Furthermore, the cold-rolled steel sheet obtained after cold rolling may be optionally subjected to pickling.

[Annealing process]
Next, in one embodiment of the present invention, after the cold rolling step, the steel sheet obtained as described above is heated and annealed in an atmosphere having an annealing temperature of Ac1 (°C) or higher and 900°C or lower, an annealing time of 20 seconds or longer, and a dew point (annealing dew point) of -10°C or higher. Note that the number of annealing steps may be two or more, but one step is preferred from the viewpoint of energy efficiency.

Annealing temperature: Ac1 (°C) or higher and 900°C or lower If the annealing temperature is lower than the Ac1 point (°C), the proportion of austenite generated during heating in the two-phase region of ferrite and austenite becomes insufficient, resulting in an excessive increase in the area fraction of ferrite after annealing, and a decrease in TS and YS.
On the other hand, if the annealing temperature exceeds 900°C, excessive grain growth of austenite occurs, the Ms point rises, a large amount of tempered martensite containing carbides is formed, it becomes difficult to obtain retained austenite with an area fraction of 3.5% or more, and ductility decreases.
Therefore, the annealing temperature is set to be equal to or higher than the Ac1 point (° C.) and equal to or lower than 900° C. The annealing temperature is preferably equal to or lower than 880° C. The annealing temperature is the maximum temperature (soaking temperature) reached in the annealing step.

Ac1 (°C) is calculated by the following formula:
Ac1 (°C) = 727.0-32.7×[%C]+14.9×[%Si]+2.0×[%Mn]
Here, [%C] is the C content of the steel plate (steel slab), [%Si] is the Si content of the steel plate (steel slab), and [%Mn] is the Mn content of the steel plate (steel slab).

Annealing time (soaking time): 20 seconds or more If the annealing time is less than 20 seconds, the austenite generation rate during heating in the two-phase region of ferrite and austenite becomes insufficient. As a result, the area ratio of ferrite increases excessively after annealing, resulting in a decrease in TS and YS. In addition, the C concentration in austenite increases excessively during annealing, making it impossible to achieve the desired bendability of the sheared edge. Furthermore, it is impossible to form a soft surface layer thickness of 20 μm or more during annealing, making it impossible to achieve the desired bendability of the steel sheet. Therefore, the annealing time is set to 20 seconds or more. The annealing time is preferably 40 seconds or more.
The annealing time (soaking time) refers to the holding time in a temperature range of (annealing temperature - 40°C) or more and (annealing temperature - 40°C) or less. In other words, the annealing time includes not only the holding time at the annealing temperature but also the residence time in a temperature range of (annealing temperature - 40°C) or more and (annealing temperature - 40°C) or less during heating and cooling before and after reaching the annealing temperature.

2400≦Y≦20000...Formula (2)
Y=[{(T-Ac1)×t1}/2}]+{(T-Ac1)×t2}...(3)
Here, in formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing step, t2 is the annealing time (s), and Ac1 is Ac1 (°C).
In the present invention, it is necessary to perform annealing under annealing conditions that satisfy formulas (2) and (3). If Y in formula (3) is less than 2400, the soft surface layer defined in the present invention will be less than 20 μm. On the other hand, if Y exceeds 20,000, the soft surface layer defined in the present invention will be more than (120 - 3,800 × [Sb] - 1,900 × [Sn]) μm. Therefore, Y in formula (3) is set to be 2,400 or more and 20,000 or less.
t1 is preferably 30 seconds or more, and t1 is preferably 80 seconds or less.

Dew point of the atmosphere (annealing atmosphere) in the annealing step (annealing dew point): -10°C or higher In one embodiment of the present invention, the dew point of the atmosphere (annealing atmosphere) in the annealing step is preferably -10°C or higher. By performing annealing with the dew point of the annealing atmosphere in the annealing step set to -10°C or higher, the decarburization reaction is promoted and a deeper soft surface layer can be formed. The dew point of the annealing atmosphere in the annealing step is preferably -5°C or higher, more preferably 0°C or higher, and even more preferably +10°C or higher. There is no particular upper limit for the dew point of the annealing atmosphere in the annealing step, but due to constraints in production technology, the dew point of the annealing atmosphere in the annealing step is preferably 30°C or lower.

[Isothermal holding process (preferred requirements)]
After the annealing step, during cooling from the annealing temperature, an isothermal holding step may be performed, if necessary, at 400°C or more and 600°C or less (hereinafter also referred to as an isothermal holding temperature range) for less than 80 seconds in order to promote bainite transformation.
In the isothermal holding step, bainitic ferrite is generated, and C diffuses from the generated bainitic ferrite to untransformed austenite adjacent to the bainitic ferrite, thereby ensuring a predetermined area ratio of retained austenite and improving elongation.

Isothermal holding temperature range: 400°C or higher and 600°C or lower If the isothermal holding temperature is lower than 400°C, lower bainite and martensite containing a large amount of carbide are generated, which suppresses the diffusion of C into untransformed austenite, and there is a risk that the area ratio of the specified amount of retained austenite cannot be secured.
On the other hand, if the isothermal holding temperature exceeds 600°C, untransformed austenite may transform into pearlite, making it difficult to ensure TS and ductility. Therefore, the isothermal holding temperature is preferably set to 400°C or higher and 600°C or lower.

Holding time in isothermal holding temperature range: less than 80 seconds If the holding time in the isothermal holding temperature range is 80 seconds or more, the area ratio of bainitic ferrite will increase excessively, and the C concentration in the untransformed austenite will increase excessively, which may make it difficult to achieve the desired bendability of the sheared edge. Therefore, it is preferable that the holding time in the isothermal holding temperature range be less than 80 seconds.

[Cooling process (first cooling process)]
Next, in the cooling step, the steel sheet after the annealing step is cooled to a cooling stop temperature of 100° C. or more and 300° C. or less. The average cooling rate is preferably 10° C./s or more and 50° C./s or less, and the dew point of the atmosphere is preferably −20° C. or less.

Cooling stop temperature: 100°C or higher and 300°C or lower Average cooling rate: 10°C/s or higher and 50°C/s or lower Atmospheric dew point: -20°C or lower (preferred conditions)
In the cooling step, the steel sheet after the annealing step is cooled to a cooling stop temperature of 100°C or higher and 300°C or lower.
At this time, the cooling start temperature can be Ac1 (°C) or more and 900°C or less, and can be 400°C or more and 600°C or less when an isothermal holding step is performed.
The cooling step is necessary to control the area fractions of tempered martensite and retained austenite, which are formed in the subsequent first holding step (reheating and holding step), within predetermined ranges. If the cooling stop temperature is less than 100°C, the untransformed austenite present in the steel during the cooling step will be almost entirely transformed into martensite. This ultimately results in an excessive increase in the area fraction of tempered martensite, making it difficult to obtain an area fraction of retained austenite of 3.5% or more, and reducing ductility.
On the other hand, if the cooling stop temperature exceeds 300°C, the area ratio of tempered martensite decreases and the area ratio of fresh martensite increases. As a result, fresh martensite becomes the origin of void generation in the 90° V-bend test, the close bending test, and the close bending + orthogonal 90° V-bend test, and the desired bendability of the steel sheet and the bendability of the sheared edge cannot be achieved. Therefore, the cooling stop temperature is set to 100°C or higher and 300°C or lower. The cooling stop temperature is preferably 120°C or higher. Furthermore, the cooling stop temperature is preferably 280°C or lower.
The average cooling rate during this cooling step is preferably 10°C/s or more. Furthermore, the average cooling rate during this cooling step is preferably 50°C/s or less. The metal phase defined in the present invention can be obtained by this cooling step. Here, if the average cooling rate is less than 10°C/s, the amount of untransformed austenite that is completely transformed into martensite during the cooling step increases, making it difficult to finally obtain retained austenite in an area fraction of 3.5% or more, and ductility may decrease. On the other hand, if the average cooling rate exceeds 50°C/s, self-relaxation during martensite transformation is suppressed, and the sheet shape may deteriorate.
Furthermore, the dew point of the atmosphere in this cooling step is preferably -20°C or lower. If the dew point of the atmosphere exceeds -20°C, the thickness of the soft surface layer in the in-plane direction of the steel sheet will vary greatly, and the tensile strength specified in the present invention may not be obtained. For these reasons, the dew point of the atmosphere in this cooling step is preferably -20°C or lower.
The average cooling rate (°C/s) is obtained by dividing the difference between the cooling start temperature (°C) and the cooling stop temperature (°C) in the cooling step by the cooling time (s).

[First holding process (first reheating holding process)]
Next, in the first holding step (first reheating holding step), the steel sheet is reheated to a temperature range of 370°C or more and 460°C or less (also referred to as a reheating holding temperature range, but hereinafter also referred to as a first reheating holding temperature range to distinguish it from the reheating holding temperature range of the second holding step), and held for 10 seconds or more.

Reheating holding temperature (first reheating holding temperature): 370°C or higher and 460°C or lower In the reheating holding step, C is concentrated in the austenite remaining after the cooling step, thereby reducing the area ratio of fresh martensite in the final structure while ensuring a predetermined amount of area ratio of retained austenite.
If the reheating temperature (first reheating temperature) is less than 370°C, the concentration of C in the austenite remaining after the cooling step is insufficient, making it difficult to obtain retained austenite in an area ratio of 3.5% or more, and ductility is reduced.
On the other hand, if the reheating temperature (first reheating temperature) exceeds 460°C, C will be excessively concentrated in the untransformed austenite, and the untransformed austenite in the surface layer will not undergo stress-induced transformation in the surface strain introduction step described below, but will instead become retained austenite or fresh martensite. Furthermore, in the soft surface layer, the value obtained by dividing the area fraction of fresh martensite by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite will exceed 0.5. Therefore, the reheating temperature (first reheating temperature) is set to 370°C or higher and 460°C or lower.

Holding time in the reheating holding temperature range (first reheating holding temperature range): 10 seconds or more If the holding time in the reheating holding temperature range is less than 10 seconds, C concentration in the austenite remaining after the cooling step will be insufficient, making it difficult to obtain retained austenite with an area fraction of 3.5% or more, and ductility may decrease. Therefore, the holding time in the first reheating holding temperature range is set to 10 seconds or more.

[Surface strain introduction process]
In the surface layer strain introduction step, a tension of 2.0 kgf/mm ² or more is applied between the first holding step (reheating and holding step) and the second holding step, thereby introducing strain into the surface layer.
By applying a tension of 2.0 kgf/ ^mm2 or more once or more, untransformed austenite in the surface layer structure of the steel sheet undergoes strain-induced transformation to martensite, which then transforms into tempered martensite in the subsequent second holding step, resulting in the desired bendability of the steel sheet.
Here, the tension is obtained by dividing the sum of the loads (kgf) of the load cells on the left and right of the roll through which the steel plate passes while in contact with it by the cross-sectional area of the steel plate (= plate thickness (mm) × plate width (mm)) (mm ² ). The load cells must be arranged parallel to the tension direction.
The load cells are preferably positioned 200 mm from both ends of the roll, and the length of the roll body is preferably 1500 to 2500 mm.
Furthermore, this tension is preferably 2.2 kgf/ ^mm2 or more, more preferably 2.4 kgf/ ^mm2 or more. Furthermore, this tension is preferably 15.0 kgf/ ^mm2 or less, more preferably 10.0 kgf/ ^mm2 or less. Here, the unit of tension can be converted from kgf/ ^mm2 to N/ ^mm2 , with 1 kgf/ ^mm2 being ^equivalent to 9.8 N/mm2.

[Second holding step]
Next, in the second holding step, the steel sheet is held at 300° C. or higher and 460° C. or lower for 10 seconds or longer.
The term "holding" as used herein also includes cooling (slow cooling) within a range of 300°C or higher and 460°C or lower for 10 seconds or longer.

Second holding temperature (reheating holding temperature range (second reheating holding temperature range)): 300°C or higher and 460°C or lower In the second holding step, the martensite formed in the surface layer in the surface layer strain introduction step is tempered. As a result, the value obtained by dividing the area ratio of fresh martensite in the surface layer by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) becomes 0.5 or lower, and the desired bendability of the steel sheet is obtained.
If the second holding temperature is less than 300°C, the martensite formed in the surface layer in the surface strain introduction process is not tempered, and the value obtained by dividing the area ratio of fresh martensite in the surface layer by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite exceeds 0.5.
On the other hand, if the second holding temperature exceeds 460°C, the retained austenite inside the steel sheet decomposes, and the desired El cannot be obtained.
Therefore, the second holding temperature (second reheating holding temperature range) is set to 300°C or higher and 460°C or lower.

Holding time at second holding temperature (second reheating holding temperature range): 10 seconds or more If the holding time at the second holding temperature (reheating holding temperature range: 300°C or more and 460°C or less) is less than 10 seconds, the martensite formed in the surface layer in the surface strain introduction step will not be sufficiently tempered, and the value obtained by dividing the area ratio of fresh martensite in the surface layer by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite will exceed 0.5. Therefore, the holding time in the reheating holding temperature range is set to 10 seconds or more.

[Plating process (hot-dip galvanizing process, galvannealed hot-dip galvanizing process)]
Next, in the plating step, the steel sheet is subjected to a galvanizing treatment to obtain a galvanized steel sheet. Examples of the galvanizing treatment include a hot-dip galvanizing treatment and a galvannealing treatment.
The galvanizing treatment in the plating step is performed after the annealing step. The galvanizing treatment may be performed, for example, during the cooling step, during the first holding step, after the first holding step and before the surface layer strain introducing step, after the surface layer strain introducing step and before the second holding step, during the second holding step, or after the second holding step.

In the case of hot-dip galvanizing, it is preferable to immerse the steel sheet in a galvanizing bath (hot-dip galvanizing bath) at a temperature of 440°C to 500°C, and then adjust the coating weight by gas wiping or other methods. The hot-dip galvanizing bath is not particularly limited as long as it provides the above-mentioned zinc coating layer composition. However, it is preferable to use a plating bath with an Al content of 0.10% by mass to 0.23% by mass, with the balance consisting of Zn and unavoidable impurities.

In the case of alloying hot-dip galvanizing treatment, it is preferable to perform the hot-dip galvanizing treatment as described above, and then heat the hot-dip galvanized steel sheet to an alloying temperature of 450°C or higher and 600°C or lower to perform alloying treatment.
If the alloying temperature is less than 450° C., the Zn—Fe alloying rate will be slow, and alloying may become difficult.
On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite transforms into pearlite, resulting in a decrease in ductility. The alloying temperature is more preferably 480°C or higher. The alloying temperature is more preferably 550°C or lower.

The coating weight of both the hot-dip galvanized steel sheet (GI) and the galvannealed steel sheet (GA) is preferably 20 g/ ^m2 or more per side. The coating weight of both the hot-dip galvanized steel sheet (GI) and the galvannealed steel sheet (GA) is preferably 80 g/ ^m2 or less per side. The coating weight can be adjusted by gas wiping or the like.

[Second cooling step (preferable condition)]
The steel sheet is then preferably cooled to a second cooling stop temperature of 50°C or less.

Second cooling stop temperature: 50° C. or less The cooling conditions for the final cooling step are not particularly limited and may be any conventional method. Examples of cooling methods that can be used include gas jet cooling, mist cooling, roll cooling, water cooling, and air cooling.
From the viewpoint of preventing surface oxidation, the material is preferably cooled to 50° C. or less, more preferably to room temperature. The average cooling rate is preferably, for example, 1° C./sec or more and 50° C./sec or less.

The steel sheet obtained as described above may be further subjected to temper rolling. If the temper rolling reduction exceeds 2.00%, the yield stress increases, which may result in a decrease in dimensional accuracy when the steel sheet is formed into a component. Therefore, the temper rolling reduction is preferably 2.00% or less. While there is no particular lower limit to the temper rolling reduction, it is preferably 0.05% or more from the perspective of productivity. Temper rolling may be performed on an apparatus continuous with the annealing apparatus used to perform the above-mentioned processes (online), or may be performed on an apparatus discontinuous with the annealing apparatus used to perform the processes (offline). The number of temper rolling passes may be one, two, or more. Rolling using a leveler or the like may also be used as long as it can impart an elongation rate equivalent to that of temper rolling.

Conditions other than those mentioned above are not particularly limited and may be performed in accordance with conventional methods. From the standpoint of productivity, it is preferable to carry out the above-mentioned series of processes, such as annealing, hot-dip galvanizing, and zinc plating alloying, in a continuous galvanizing line (CGL). After hot-dip galvanizing, wiping can be performed to adjust the coating weight. Plating conditions other than those mentioned above may be in accordance with conventional hot-dip galvanizing methods.

[3. Components]
Next, a member according to one embodiment of the present invention will be described.
A member according to one embodiment of the present invention is a member made using (using as a raw material) the above-mentioned steel plate. For example, the raw material steel plate is subjected to at least one of forming and joining to form the member.
Here, the above-mentioned steel sheet has a TS of 780 MPa or more and less than 1180 MPa, a high YS, excellent press formability inside the steel sheet (bendability and stretch formability of the steel sheet), and excellent press formability at the steel sheet end (bendability of the steel sheet end (shear cross section)). Therefore, a member according to one embodiment of the present invention has high strength and excellent press formability. Therefore, a member according to one embodiment of the present invention is particularly preferably applied to impact energy absorbing members used in the automotive field.

[4. Manufacturing method of member]
Next, a method for manufacturing a member according to one embodiment of the present invention will be described.
A method for manufacturing a component according to one embodiment of the present invention includes a step of subjecting the above-mentioned steel plate (e.g., a steel plate manufactured by the above-mentioned steel plate manufacturing method) to at least one of forming processing and joining processing to form a component.
Here, the molding method is not particularly limited, and for example, a general processing method such as press working can be used. The joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, rivet joining, caulking joining, etc. The molding conditions and joining conditions are not particularly limited, and may be in accordance with ordinary methods.

Steel materials having the chemical compositions shown in Table 1 (the balance being Fe and unavoidable impurities) were melted in a converter and then cast into steel slabs by continuous casting. In Table 1, "-" indicates the content of unavoidable impurities, which is treated as 0 (zero).
The calculated transformation point Ac1 (°C) shown in Table 1 is calculated by the following formula:
Ac1 point (°C) = 727.0-32.7×[%C]+14.9×[%Si]+2.0×[%Mn]
Here, [%C] is the C content of the steel plate (steel slab), [%Si] is the Si content of the steel plate (steel slab), and [%Mn] is the Mn content of the steel plate (steel slab).

The obtained steel slab was heated to 1200°C, and after heating, the steel slab was subjected to hot rolling consisting of rough rolling and finish rolling at a finish rolling temperature of 900°C to obtain a hot-rolled steel sheet. Next, the obtained hot-rolled steel sheet was subjected to pickling and cold rolling (reduction rate: 50%) to obtain a cold-rolled steel sheet having a thickness shown in Table 3. Next, the obtained cold-rolled steel sheet was subjected to treatments in an annealing step, an isothermal holding step, a cooling step, a first holding step (reheating holding step), a surface strain introduction step, and a second holding step under the conditions shown in Table 2, and if necessary, a treatment in a plating step (hot-dip galvanizing step or galvannealed hot-dip galvanizing step) to obtain a steel sheet.
The plating step was performed after the first holding step and before the surface strain introduction step in Nos. 1, 5 to 9, 11, 14 to 16, 21, 23 to 31, 33 to 36, 38, 40, 44, 45, 47, 51 to 56, 59, 61, 63, 64, 66, and 67, and during the cooling step in Nos. 3, 4, 10, 12, 13, 17 to 20, 22, 32, 37, 39, 41 to 43, 46, 48 to 50, 57, 58, 60, 62, 65, and 68.

Here, the plating process involved hot-dip galvanizing or galvannealed hot-dip galvanizing to obtain hot-dip galvanized steel sheet (hereinafter also referred to as GI) or galvannealed hot-dip galvanized steel sheet (hereinafter also referred to as GA). In Table 2, the type of plating process is also indicated as "GI" or "GA." Steel sheets that have not undergone either hot-dip galvanizing or galvannealed hot-dip galvanizing are indicated as "CR." In Table 2, for CR steel sheets or GI steel sheets, the alloying temperature is indicated as - because no alloying process is performed.

The galvanizing bath temperature was 470°C for both GI and GA production.
The zinc plating coverage ^was 45 to 72 g/m2 per side when producing GI, and 45 g/ ^m2 per side when producing GA.
The composition of the finally obtained plating layer (galvanized layer) of the steel sheet was as follows: GI: 0.1 to 1.0 mass% Fe, 0.2 to 0.33 mass% Al, and the balance being Zn and unavoidable impurities; GA: 8.0 to 12.0 mass% Fe, 0.1 to 0.23 mass% Al, and the balance being Zn and unavoidable impurities.
In addition, the plating layers (galvanized layers) were formed on both sides of the base steel sheets in all cases.

Using the obtained steel sheet, the steel structure of the base steel sheet was identified using the method described above. The measurement results are shown in Table 3. As shown in Figure 1, F is ferrite, BF is bainitic ferrite, TM is tempered martensite, RA is retained austenite, and FM is fresh martensite. Also in Table 3, LB is lower bainite, and θ is carbide.

The method for measuring the surface soft layer is as follows. A thickness cross section (L cross section) parallel to the rolling direction of the steel plate was smoothed by wet polishing, and then measurements were made using a Vickers hardness tester at 1 μm intervals from a position 1 μm from the surface of the steel plate in the thickness direction to a position 100 μm in the thickness direction with a load of 10 gf (9.8 × 10 ^-2 N). Measurements were then made at 20 μm intervals to the center of the plate thickness. The region where the Vickers hardness has decreased to 84% or less compared to the hardness at 1/4 of the plate thickness is defined as the soft layer (surface soft layer), and the thickness of this region in the thickness direction is defined as the thickness of the soft layer.

The structure of the soft surface layer was identified at a position halfway through the thickness of the soft surface layer using the same method as used to identify the steel structure of the base steel plate.

In addition, tensile tests, 90-degree V-bend tests, contact bending tests, and contact bending + orthogonal 90-degree V-bend tests were conducted according to the following procedures, and the tensile strength (TS), yield stress (YS), yield ratio (YR), total elongation (El), bendability of the steel plate, and bendability of the sheared edge were evaluated according to the following criteria.

・TS (tensile strength)
◯ (Pass): 780 MPa or more and less than 1180 MPa × (Fail): Less than 780 MPa or 1180 MPa or more

・YS (yield stress)
〇 (Pass):
(A) When 780 MPa ≦ TS < 980 MPa, 550 MPa ≦ YS
(B) When 980 MPa ≦ TS < 1180 MPa, 700 MPa ≦ YS
× (fail):
(A) When 780 MPa ≦ TS < 980 MPa, 550 MPa > YS
(B) When 980 MPa ≦ TS < 1180 MPa, 700 MPa > YS

・El (stretch formability inside the steel plate)
〇 (Pass):
(A) When 780 MPa ≦ TS < 980 MPa, 17.0% ≦ El
(B) 980 MPa ≦ TS < 1180 MPa, 11.0% ≦ El
× (fail):
(A) When 780 MPa ≦ TS < 980 MPa, 17.0% > El
(B) 980 MPa ≦ TS < 1180 MPa, 11.0% > El

- A 90-degree V-bend test was conducted with a bending radius of 0.5 mm, and the length of the crack that propagates along the bend ridge formed other than at the end of the bend ridge (crack length other than at the V-bend end face) (bendability of steel plate)
○ (Pass): The crack length is 200 μm or less except on the V-bend end surface. × (Fail): The crack length is more than 200 μm except on the V-bend end surface.

- A 90-degree V-bend test was conducted with a bending radius of 0.5 mm, and the crack length that propagated from the end of the bend ridge toward the ridge (V-bend end crack length) (bendability of the steel plate end (shear cross section))
◯ (Pass): The length of the crack at the V-bend end is 200 μm or less. × (Fail): The length of the crack at the V-bend end is more than 200 μm.

- Conduct a close contact bending test and determine the spacer plate thickness at which cracks of 0.5 mm or more do not occur along the bending ridge (close contact bending boundary spacer plate thickness) (bendability of steel plate)
○ (Pass): The thickness of the spacer at the boundary between the tightly bent sections is 3.0 mm or less. × (Fail): The thickness of the spacer at the boundary between the tightly bent sections is more than 3.0 mm.

- Conduct a close contact bending test using a 3.0 mm spacer to measure the depth of the crack that develops in the thickness direction at the bending ridge line subjected to compressive stress (close contact bending internal crack depth) (bendability of steel plate)
○ (Pass): The depth of the internal cracks in the tightly bent specimen is 200 μm or less. × (Fail): The depth of the internal cracks in the tightly bent specimen is more than 200 μm.

- Conduct a close bending + perpendicular 90-degree V-bending test and determine the bending radius at which cracks of 0.5 mm or more do not occur along the bending ridge (handkerchief bending boundary bending radius) (bendability of steel plate)
◯ (Pass): The handkerchief bending boundary bending radius is 5.0 mm or less. × (Fail): The handkerchief bending boundary bending radius is more than 5.0 mm.

(1) Tensile Test The tensile test was conducted in accordance with JIS Z 2241 (2011). That is, a JIS No. 5 test piece was taken from the obtained steel sheet at a quarter position of the coil width so that the longitudinal direction was perpendicular to the rolling direction of the base steel sheet. Using the taken test piece, a tensile test was conducted at a crosshead speed of 10 mm/min, and TS, YS, YR, and El were measured. The results are shown in Table 4.

(2) 90° V-bend test: A rectangular test piece measuring 100 mm C (C direction: the direction perpendicular to the rolling direction of the steel sheet) x 30 mm L (L direction: the direction along the rolling direction) was taken from the quarter-coil width position of the obtained steel sheet. The 100 mm long end face was sheared, and the sheared state (without machining to remove burrs) was bent so that the burrs were on the outer periphery of the bend. The shear clearance was 15% and the rake angle was 0°. V-bending was performed using an autograph manufactured by Shimadzu Corporation, with a punch bending radius of R = 0.5 mm, a punch bending angle of 90°, a punch stroke speed of 30 mm/min, a pressing load of 10 tons, and a pressing time of 5 seconds, resulting in an L-direction bending (bending ridge length: 30 mmL).

An example of a sample after the 90-degree V-bend test with a bending radius of 0.5 mm is shown in Figure 2. Figure 2(b) is an overhead view of the sample viewed from the Z direction shown in Figure 2(a). If the section extending from the bend apex along the steel plate surface and extending 5 mm in the C direction (2.5 mm on both sides from the bend apex) is defined as the bend ridgeline, then the section (area o) extending 5 mm in the L direction from the very end of the bend ridgeline is defined as the bend ridgeline end. The crack length Y1 propagating from the bend ridgeline end in the ridgeline direction (L direction), and the crack length Y2 propagating in the L direction along the bend ridgeline formed other than the bend ridgeline end, are measured using the following methods.

After the 90° V-bend test with a bending radius of 0.5 mm, the length of the crack propagating from the end of the bent ridge in the ridge direction was measured as follows.
Figure 3-1(a) shows the crack at the end of the bending ridge of a sample after a V-bend test. When measuring the length of a crack at the center of the bending ridge, it is common to observe the plate surface (plane b) from the Z direction. Since the sample after an actual V-bend test has a saddle shape as shown in Figure 3-1(b), plane b is significantly deformed, reducing the accuracy of measuring the crack length and potentially making it impossible to accurately evaluate the bendability of the sheared edge. In the present invention, accurate measurements can be achieved by using the following measurement method. Note that the symbol y in Figure 3-1(a) corresponds to the symbol Y1 (crack length Y1) in Figure 2(b).
The shear plane a of the bent sample after a 90-degree V-bend test with a bending radius of 0.5 mm was placed facing up, and the end of the bend ridge was photographed at 40x magnification using a one-shot 3D shape measuring instrument (Keyence, VR6000 series or newer models). The obtained height data was analyzed using the analysis software provided with the one-shot 3D shape measuring instrument. As shown in Figure 3-2(a), an arc-shaped measurement line i was drawn as close as possible to the outside of the bend that was subjected to tensile stress, in line with the bend ridge. An example of the obtained profile waveform j is shown in Figure 3-2(b). The length of each crack (y1 + y2)/2 was calculated using the measurement tool in the software, and the length of the longest crack was taken as the crack length propagating from the end of the bend ridge toward the ridge after a 90-degree V-bend test with a bending radius of 0.5 mm.

After the above 90-degree V-bend test with a bending radius of 0.5 mm, the length of the cracks propagating along the bend ridge formed other than at the end of the bend ridge was measured by visual observation using a stereomicroscope at 25x magnification.

(3) Contact Bending Test A 60 mmC x 30 mmL test piece was taken from the 1/4 coil width position of the obtained steel plate. After finishing both end faces of the 60 mm length by grinding, a primary bending process (U-bending) was performed to prepare a test piece for contact bending. The U-bending was performed using a hydraulic bending tester with a punch bending radius of R = 5.0 mm, a stroke speed of 10 mm/s, and C-direction bending (bending ridge length: 30 mmL), so that no cracks occurred in any of the test materials. Next, contact bending was performed on the test piece after U-bending. The contact bending was performed using a hydraulic bending tester. As shown in Figure 4-1(a), a spacer q (plate thickness Z1) was inserted as necessary, and the stroke speed was 10 mm/min, the pressing load was 10 ton, and the pressing time was 3 seconds, so that the bending ridge of the test piece after U-bending was perpendicular to the pressing direction.
In samples subjected to a tight bending test, if the area of width Z2 (Z2 = (2t + Z1) × π/2, where t is the sample thickness) from the bend apex along the steel plate surface on both sides in the circumferential direction (C direction) is defined as the outside of the bend ridgeline, the minimum spacer plate thickness at which the L-direction length Y3 of a crack extending in the L direction outside the bend ridgeline is less than 0.5 mm (no cracks of 0.5 mm or more occur) was defined as the critical spacer plate thickness for cracking. The crack length outside the bend ridgeline was measured by visual observation using a stereomicroscope at 25x magnification.

Furthermore, after a 3.0 mm spacer contact bending test (a 3.0 mm spacer thickness bending test), when the compressive stress-stricken side (the inner surface of the sample) was subjected to a width Z3 (9 mm) (see region s) on both sides of the circumferential direction (direction C) from the bending apex along the steel plate surface on the inside of the bending ridge, as shown in FIG. 4-1(b), the crack depth propagating in the plate thickness direction on the inside of the bending ridge was measured as follows.
As shown in FIG. 4-2(c), a specimen was cut out from the sample after the above-mentioned close bending so that the cross section k at the 1/2 position in the L direction was the observation surface.
Next, the observation surface of the sample was mirror-polished using diamond paste. Then, using a SEM (Scanning Electron Microscope), under conditions of an acceleration voltage of 15 kV and a magnification of 50 times, a field of view of 2560.0 μm × 1920.0 μm (m in FIG. 4-2(d)) was photographed at the position m in FIG. 4-2(d) which is the bending apex of the observation surface of the sample, and the entire crack was observed. In the obtained image of the crack, the distance X between the start point and the end point of the crack was taken as the crack depth. This X
A close contact bending test was carried out using a 3.0 mm spacer, and the crack depth was evaluated as the depth of cracks that developed in the thickness direction at the bend ridge line subjected to compressive stress.

(4) Close Bending + Perpendicular 90-Degree V-Bend Test Using the sample after the close bending test with a 3.0 mm spacer, a V-bend test in the L direction (perpendicular 90-degree V-bend test) was performed. When the r region (Z2 region in the figure) in Figure 4-1(a) after V-bending is the bend ridgeline after handkerchief bending, the bend radius at which cracks extending in the L direction at the bend ridgeline do not occur and which are 0.5 mm or longer in the L direction was defined as the crack limit bend radius (handkerchief bend boundary bend radius). The crack length of the bend ridgeline was measured by visual observation using a stereomicroscope at 25x magnification.

In Tables 1 to 4, the underlined parts indicate values outside the appropriate range of the present invention.
As shown in Table 4, in all of the inventive examples, the tensile strength (TS), yield stress (YS), and total elongation (El) were acceptable, and the crack length other than at the V-bend end face, the crack length at the V-bend end face, the thickness of the spacer for tight bending, the depth of the crack inside the tight bending, and the bending radius at the boundary of the handkerchief bending were all within the specified range.
On the other hand, in the comparative examples, at least one of the tensile strength (TS), yield stress (YS), total elongation (El), crack length other than at the V-bend end face, crack length at the V-bend end face, tight bending spacer thickness, tight bending internal crack depth, and handkerchief bending boundary bending radius was insufficient.

F Ferrite FM Fresh martensite RA Retained austenite BF Bainitic ferrite TM Tempered martensite

Claims (13)

In mass%,
C: 0.050% or more and 0.400% or less,
Si: 0.20% or more and 3.00% or less,
Mn: 1.00% or more and less than 3.50%;
P: 0.001% or more and 0.100% or less,
S: 0.0001% or more and 0.0200% or less,
Al: 0.005% or more and 2.000% or less,
N: 0.0100% or less,
Sb: 0.200% or less (including 0%), and Sn: 0.200% or less (including 0%)
and the balance being Fe and unavoidable impurities,
The steel sheet has a soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at a position 1/4 of the sheet thickness from the surface of the base steel sheet,
The surface soft layer satisfies the following formula (1):
The structure in the superficial soft layer is
The area ratio of ferrite is 50.0% or more and 100.0% or less,
When the area ratio of ferrite is less than 100.0%, the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less;
The structure at a 1/4 position of the plate thickness of the base steel plate is
The area ratio of ferrite is 76.5% or less (including 0.0%),
The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is 20.0% or more and 90.0% or less,
The area ratio of retained austenite is 3.5% or more and 10.0% or less,
The area fraction of fresh martensite is 10.0% or less (including 0.0%),
A steel plate having a tensile strength of 780 MPa or more and less than 1180 MPa.
20≦X≦120-3800×[Sb]-1900×[Sn]...(1)
In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively. The component composition further includes, in mass %,
Nb: 0.200% or less,
Ti: 0.200% or less,
V: 0.200% or less,
B: 0.0100% or less,
Cr: 1.000% or less,
Ni: 1.000% or less,
Mo: 1.000% or less,
Cu: 1.000% or less,
Ta: 0.100% or less,
W: 0.500% or less,
Mg: 0.0200% or less,
Zn: 0.0200% or less,
Co: 0.0200% or less,
Zr: 0.1000% or less,
Ca: 0.0200% or less,
Se: 0.0200% or less,
Te: 0.0200% or less,
Ge: 0.0200% or less,
As: 0.0500% or less,
Sr: 0.0200% or less,
Cs: 0.0200% or less,
Hf: 0.0200% or less,
Pb: 0.0200% or less,
The steel sheet according to claim 1, further comprising at least one selected from Bi: 0.0200% or less and REM: 0.0200% or less. The steel sheet according to claim 1 or 2, wherein one or both sides of the base steel sheet have a plating layer, the plating layer being a hot-dip galvanized layer. The steel sheet according to claim 1 or 2, wherein one or both sides of the base steel sheet have a plating layer, the plating layer being a galvannealed layer. A component made using the steel plate described in claim 1 or 2. A component made using the steel plate described in claim 3. A component made using the steel plate described in claim 4. A hot rolling step of hot rolling a steel slab having the component composition according to claim 1 or 2 to obtain a hot-rolled steel sheet;
After the hot rolling step, a pickling step of pickling the hot-rolled steel sheet;
a cold rolling step of cold rolling the steel sheet after the pickling step at a rolling reduction of 20% or more and 80% or less;
An annealing step in which the steel sheet after the cold rolling step is heated and annealed at an annealing temperature of Ac1 (°C) or higher and 900°C or lower, an annealing time of 20 seconds or longer, and an atmosphere with a dew point of -10°C or higher under conditions that satisfy formula (2) and formula (3);
a cooling step of cooling the steel sheet after the annealing step to a cooling stop temperature of 100°C or higher and 300°C or lower;
a first holding step of reheating the steel sheet after the cooling step to a reheating holding temperature range of 370°C or higher and 460°C or lower and holding the temperature for 10 seconds or longer;
a surface strain introducing step of applying a tension of 2.0 kgf/mm2 or ^more to the steel sheet after the first holding step in the reheating holding temperature range;
A second holding step of holding the steel sheet after the surface strain introduction step at 300 ° C. or higher and 460 ° C. or lower for 10 seconds or longer.
a base steel sheet having the above-described composition; and a surface soft layer having a Vickers hardness of 84% or less of the Vickers hardness at a position 1/4 of the sheet thickness from the surface of the base steel sheet,
The surface soft layer satisfies the following formula (1):
The structure in the superficial soft layer is
The area ratio of ferrite is 50.0% or more and 100.0% or less,
When the area ratio of ferrite is less than 100.0%, the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less;
The structure at a 1/4 position of the plate thickness of the base steel plate is
The area ratio of ferrite is 76.5% or less (including 0.0%),
The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is 20.0% or more and 90.0% or less,
The area ratio of retained austenite is 3.5% or more and 10.0% or less,
The area fraction of fresh martensite is 10.0% or less (including 0.0%),
A method for producing a steel plate having a tensile strength of 780 MPa or more and less than 1180 MPa .
20≦X≦120-3800×[Sb]-1900×[Sn]...(1)
In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.
2400≦Y≦20000 (2)
Y=[{(T-Ac1)×t1}/2}]+{(T-Ac1)×t2}...(3)
Here, in formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing step, t2 is the annealing time (s), and Ac1 is Ac1 (°C). The method for manufacturing a steel sheet according to claim 8, further comprising a hot-dip galvanizing step in which the steel sheet is subjected to a hot-dip galvanizing treatment to form a hot-dip galvanized layer after the annealing step. The method for manufacturing a steel sheet according to claim 8, further comprising a galvannealing step of subjecting the steel sheet to a galvannealing treatment after the annealing step to form a galvannealed layer. A method for manufacturing a component, comprising the step of subjecting the steel plate according to claim 1 or 2 to at least one of forming and joining to form the component. A method for manufacturing a component, comprising the step of subjecting the steel plate described in claim 3 to at least one of forming and joining to form a component. A method for manufacturing a component, comprising the step of subjecting the steel plate described in claim 4 to at least one of forming and joining to form the component.