JP7804249B2 - Steel plates and parts - Google Patents

Description

The present disclosure relates to steel sheets and components.
This application claims priority based on Japanese Patent Application No. 2024-006002, filed on January 18, 2024, the contents of which are incorporated herein by reference.

In recent years, efforts have been made to reduce the weight of automobile and machine parts. Designing parts with optimal shapes to ensure rigidity makes it possible to reduce the weight of automobile and machine parts. Furthermore, for blank-formed parts such as press-formed parts, weight can be reduced by reducing the thickness of the part material.

However, if one attempts to maintain the strength properties of parts, such as static fracture strength and yield strength, while reducing plate thickness, it becomes necessary to use high-strength materials. In particular, the use of higher-strength steel plates is beginning to be considered for automotive suspension parts such as lower arms, trail links, and knuckles. These automotive suspension parts are manufactured by subjecting steel plates to burring, stretch flange forming, bending, and other processes. Therefore, steel plates used for these automotive suspension parts are required to have excellent formability, particularly ductility and hole expandability.

It is also known that the stronger the steel plate, the more likely it is that cracks will occur from the inside of the bend during bending (hereafter, cracks that occur on the inside of the bend during bending will be referred to as inside-bend cracks). The mechanism of inside-bend cracks is thought to be as follows: During bending, compressive stress is generated on the inside of the bend. Initially, the entire inside of the bend deforms uniformly as the bending progresses, but as the amount of deformation increases, the deformation cannot be borne by the uniform deformation alone, and strain concentrates in localized areas, causing the deformation to progress (the generation of shear deformation bands). As these shear deformation bands continue to grow, cracks initiate and grow from the inside surface of the bend along the shear bands.

The reason that internal cracking is more likely to occur as the strength of steel plate increases is thought to be that the reduced work hardening ability associated with higher strength makes it difficult for uniform deformation to occur, making it more likely that uneven deformation will occur, resulting in the formation of shear deformation bands early in processing (or under loose processing conditions). Automotive suspension parts are processed in a variety of ways, and steel plate is often bent. Internal cracking during processing of steel plate reduces the durability and crashworthiness of the part, so it is undesirable for internal cracking to occur. Therefore, steel plate, particularly for use in automotive suspension parts, is required to be able to suppress the occurrence of internal cracking, i.e., to have excellent resistance to internal cracking.

For example, Patent Document 1 discloses a high-strength hot-rolled steel sheet with excellent punchability, characterized in that it has a bainite phase area ratio of more than 95% throughout the thickness direction, and the average grain size of the bainite phase in the region from the surface to a position 1/4 of the thickness in the thickness direction is 5 μm or less in a thickness section parallel to the rolling direction and 4 μm or less in a thickness section perpendicular to the rolling direction, and further has a structure in which there are 7 or fewer crystal grains elongated in the rolling direction with an average aspect ratio of 5 or more in a region whose width in the thickness direction is 1/10 of the thickness centered at the center of the thickness, and has a tensile strength TS of 780 MPa or more.

Japanese Patent Application Publication No. 2012-62562

However, Patent Document 1 does not take into account internal cracks during bending.

This disclosure has been made in consideration of the above-mentioned problems, and aims to provide a steel plate having high strength, as well as excellent ductility, hole expansion ability, and resistance to internal bending cracking, and a part made using the same.

The gist of the present invention is as follows.
[1] Chemical composition, in mass%,
C: 0.045-0.120%,
Si: 0-3.00%,
Mn: 1.20-3.00%,
Ti: 0.020 to 0.180%,
Al: 0.010-0.400%,
P: 0 to 0.080%,
S: 0 to 0.0100%,
N: 0 to 0.0050%,
O: 0 to 0.0100%,
Nb: 0 to 0.100%,
V: 0 to 1.000%,
Cu: 0 to 1.000%,
Cr: 0-2.000%,
Mo: 0-3.000%,
Ni: 0-1.000%,
B: 0 to 0.0100%,
Ca: 0-0.0500%,
Mg: 0 to 0.050%,
REM: 0-0.1000%,
Bi: 0-0.100%,
Ta: 0-0.100%,
Zr: 0 to 0.500%,
Co: 0-3.000%,
Zn: 0-0.200%,
W: 0-0.200%,
Sb: 0 to 0.500%,
As: 0 to 0.050%,
Sn: 0 to 0.050%, and
The balance is Fe and impurities.
In the region from the surface to a depth of 1/8 of the plate thickness to a depth of 3/8 of the plate thickness from the surface,
The average aspect ratio of the prior austenite grains , as expressed by the following formula, is 3.00 to 5.50,
The area ratio of bainite is 70 to 95%,
The area ratio of martensite is 5 to 30%,
a ratio of the sum of the amount of Ti and the amount of Nb analyzed as an electrolytic extraction residue at a depth of 1/60 of the sheet thickness from the surface to the sum of the amount of Ti and the amount of Nb analyzed as an electrolytic extraction residue at a depth of 1/4 of the sheet thickness from the surface to 1.5 or more.
where ri is the "major axis/minor axis" of the i-th prior austenite grain, and Ai is the area of the i-th prior austenite grain.
[2] The chemical composition is, in mass%,
Nb: 0.001 to 0.100%,
V: 0.001 to 1.000%,
Cu: 0.001 to 1.000%,
Cr: 0.001-2.000%,
Mo: 0.001-3.000%,
Ni: 0.001 to 1.000%,
B: 0.0001 to 0.0100%,
Ca: 0.0001-0.0500%,
Mg: 0.001-0.050%,
REM: 0.0001-0.1000%,
Bi: 0.001-0.100%,
Ta: 0.001 to 0.100%,
Zr: 0.001 to 0.500%, Co: 0.001 to 3.000%,
Zn: 0.001-0.200%,
W: 0.001-0.200%,
Sb: 0.001 to 0.500%,
The steel sheet according to [1], characterized in that it contains one or more of As: 0.001 to 0.050% and Sn: 0.001 to 0.050%.
[3] The steel plate according to [1] or [2], characterized in that the plate thickness is 1.2 to 8.0 mm.
[4] The steel plate according to any one of [1] to [3], characterized in that the tensile strength is 980 MPa or more.
[5] A part made of the steel sheet according to any one of [1] to [4].

The above aspects of the present disclosure make it possible to provide steel plates having high strength, as well as excellent ductility, hole expansion properties, and resistance to internal bending cracks, as well as parts made from such steel plates.

FIG. 10 is a diagram for explaining a bending test.

The inventors have discovered that by controlling the average aspect ratio of prior austenite grains in the internal region, it is possible to achieve both excellent ductility and hole expandability in high-strength steel sheets.

The inventors have also discovered that the resistance to internal bending cracking of a steel sheet can be improved by increasing the total amount of Ti and Nb in the surface layer of the steel sheet to a level greater than the total amount of Ti and Nb in the interior of the steel sheet. The inventors speculate that this is due to the following reasons.

The C and Ti (and Nb if Nb is included) contained in the steel plate are consumed as coarse precipitates. It has been discovered that consuming C can increase the ductility of the metal structure of the surface layer. It has therefore been discovered that consuming C, Ti, and Nb as coarse precipitates in the surface layer of the steel plate can reduce the strength of the surface layer of the steel plate and increase the ductility. In this way, it has been discovered that increasing the ductility of the surface layer of the steel plate can improve the internal bending crack resistance of the steel plate.

The inventors also discovered that the above-mentioned steel sheet can be manufactured by strictly controlling the temperature history, particularly from rough rolling to finish rolling.

A steel sheet according to one embodiment of the present disclosure (hereinafter, sometimes referred to as the steel sheet according to this embodiment) will be described. However, this disclosure is not limited to the configuration disclosed in this embodiment, and various modifications are possible within the scope of the spirit of this disclosure.

The individual constituent elements of the present disclosure will be described in detail below. First, the reasons for limiting the chemical composition of the steel sheet according to the present embodiment will be described.
Below, numerical ranges defined by "to" include the lower and upper limits. Numerical values indicated as "less than" or "greater than" are not included in the numerical range. In the following description, percentages relating to chemical compositions are mass % unless otherwise specified.

The steel sheet according to this embodiment has a chemical composition, in mass%, of C: 0.045-0.120%, Si: 0-3.00%, Mn: 1.20-3.00%, Ti: 0.020-0.180%, Al: 0.010-0.400%, P: 0-0.080%, S: 0-0.0100%, N: 0-0.0050%, O: 0-0.0100%, Nb: 0-0.100%, V: 0-1.000%, Cu: 0-1.000%, Cr: 0-2.000%, Mo: 0- 3.000%, Ni: 0-1.000%, B: 0-0.0100%, Ca: 0-0.0500%, Mg: 0-0.050%, REM: 0-0.1000%, Bi: 0-0.100%, Ta: 0-0.100%, Zr: 0-0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0-0.500%, As: 0-0.050%, Sn: 0-0.050%, and the balance: Fe and impurities. Each element will be described in detail below.

C: 0.045-0.120%
C is an important element for improving the strength of steel sheet. To obtain the desired strength, the C content is set to 0.045% or more. The C content is preferably 0.050% or more, 0.055% or more, 0.060% or more, or 0.065% or more.
On the other hand, if the C content exceeds 0.120%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the C content is set to 0.120% or less. The C content is preferably 0.100% or less, and more preferably 0.900% or less.

Si: 0-3.00%
Si is an element that has the effect of suppressing the formation of carbides during ferrite transformation and improving the toughness of the steel sheet. Since Si may not be contained, the Si content may be 0%. In order to reliably obtain the above effect, the Si content is preferably 0.10% or more. The Si content is more preferably 0.20% or more, or 0.50% or more.
On the other hand, if the Si content exceeds 3.00%, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.50% or less, 2.00% or less, 1.50% or less, or 1.00% or less.

Mn: 1.20-3.00%
Mn is an element effective in improving the hardenability and strength of the steel sheet by solid solution strengthening. To obtain the desired strength, the Mn content is set to 1.20% or more. The Mn content is preferably 1.30% or more, 1.50% or more, or 1.70% or more.
On the other hand, if the Mn content exceeds 3.00%, MnS, which adversely affects the ductility of the steel sheet, is likely to be formed. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.70% or less, 2.50% or less, or 2.30% or less.

Ti: 0.020-0.180%
Ti precipitates in steel as carbides or nitrides, refining the metal structure through a pinning effect and increasing the strength and yield ratio of the steel sheet through precipitation strengthening. If the Ti content is less than 0.020%, the strength of the steel sheet decreases. Therefore, the Ti content is set to 0.020% or more. The Ti content is preferably 0.050% or more, 0.080% or more, or 0.100% or more.
On the other hand, if the Ti content exceeds 0.180%, recrystallization is significantly inhibited, making it impossible to favorably control the average aspect ratio of the prior austenite grains, and the hole expandability of the steel sheet deteriorates. Therefore, the Ti content is set to 0.180% or less. The Ti content is preferably 0.160% or less, 0.150% or less, or 0.130% or less.

Al: 0.010-0.400%
Al has the effect of improving the quality of steel by deoxidation and also has the effect of controlling ferrite transformation. Furthermore, if the Al content is less than 0.010%, the hole expandability of the steel sheet deteriorates. Therefore, the Al content is set to 0.010% or more. The Al content is preferably 0.020% or more, 0.050% or more, or 0.100% or more.
On the other hand, if the Al content exceeds 0.400%, cluster-like precipitates of alumina are generated, which increases the cracking susceptibility of the slab and makes the slab difficult to handle. Therefore, the Al content is set to 0.400% or less. The Al content is preferably 0.300% or less, 0.250% or less, or 0.200% or less.

P: 0-0.080%
P is an element that affects the weldability of steel sheets. In particular, if the P content exceeds 0.080%, the weldability of the steel sheets deteriorates significantly. Furthermore, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, the P content is set to 0.080% or less. The P content is preferably 0.040% or less, 0.020% or less, or 0.010% or less.
The P content may be 0%. From the viewpoint of refining costs, the P content may be 0.001% or more.

S: 0~0.0100%
S is an element that affects the hole expandability of steel sheets. In particular, if the S content exceeds 0.0100%, a large amount of inclusions such as MnS, which are harmful to the hole expandability of steel sheets, are generated. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less.
The S content may be 0%. From the viewpoint of refining costs, the S content may be 0.0001% or more, or 0.0010% or more.

N: 0 to 0.0050%
N is an element that combines with Ti to form Ti nitrides. In particular, if the N content exceeds 0.0050%, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, the N content is set to 0.0050% or less. The N content is preferably 0.0040% or less, or 0.0030% or less.
The N content may be 0%. From the viewpoint of refining costs, the N content may be 0.0001% or more, or 0.0010% or more.

O: 0 to 0.0100%
O is an element that, when contained in large amounts in steel, forms coarse oxides that become fracture initiation sites, causing brittle fracture and hydrogen-induced cracking. If the O content exceeds 0.0100%, brittle fracture and hydrogen-induced cracking are more likely to occur. In addition, the hole expandability of the steel sheet deteriorates. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0035% or less.
Since O may not be contained, the O content may be 0%. In order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be 0.0005% or more, or 0.0010% or more.

Nb: 0-0.100%
Nb has the effect of increasing the strength of the steel sheet by refining the crystal grain size of the steel sheet and precipitation strengthening of NbC. The Nb content may be 0%, but to obtain this effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the Nb content exceeds 0.100%, the above effect saturates. Also, the hole expandability of the steel sheet deteriorates. Therefore, even when Nb is contained, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less, 0.060% or less.

V: 0~1.000%
V has the effect of increasing the strength of the steel sheet by strengthening through precipitation, grain refinement strengthening by suppressing ferrite grain growth, and dislocation strengthening by suppressing recrystallization. The V content may be 0%, but to obtain these effects, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the V content is excessive, a large amount of carbonitrides will precipitate, deteriorating the ductility and hole expandability of the steel sheet. Therefore, the V content is set to 1.000% or less. The V content is preferably 0.800% or less, 0.600% or less, 0.400% or less, or 0.200% or less.

Cu: 0-1.000%
Cu exists in the form of fine particles in steel and has the effect of increasing the strength of the steel sheet. The Cu content may be 0%, but to obtain this effect, the Cu content is preferably 0.001% or more. The Cu content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the Cu content is excessive, the weldability of the steel sheet deteriorates. Therefore, the Cu content is set to 1.000% or less. The Cu content is preferably 0.800% or less, 0.600% or less, 0.400% or less, or 0.200% or less.

Cr: 0-2.000%
Cr is an element effective in improving the strength of steel sheets. The Cr content may be 0%, but to obtain this effect, the Cr content is preferably 0.001% or more. The Cr content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the Cr content is excessive, the ductility and hole expandability of the steel sheet will deteriorate. Therefore, the Cr content is set to 2.000% or less. The Cr content is preferably 1.500% or less, 1.200% or less, 1.000% or less, 0.600% or less, or 0.300% or less.

Mo: 0-3.000%
Mo is an element effective for strengthening ferrite precipitation. The Mo content may be 0%, but to obtain this effect, the Mo content is preferably 0.001% or more. The Mo content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the Mo content is excessive, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, the Mo content is set to 3.000% or less. The Mo content is preferably 2.500% or less, 2.000% or less, 1.500% or less, 1.000% or less, 0.500% or less, or 0.300% or less.

Ni: 0-1.000%
Ni has the effect of suppressing phase transformation at high temperatures and increasing the strength of the steel sheet. The Ni content may be 0%, but to obtain this effect, the Ni content is preferably 0.001% or more. The Ni content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the Ni content is excessive, the weldability of the steel sheet deteriorates. Therefore, the Ni content is set to 1.000% or less, and preferably 0.500% or less, 0.300% or less, or 0.150% or less.

B: 0-0.0100%
B has the effect of suppressing phase transformation at high temperatures and increasing the strength of the steel sheet. The B content may be 0%, but to obtain this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0005% or more, and more preferably 0.0010% or more.
On the other hand, if the B content is excessive, B precipitates are formed, reducing the strength of the steel sheet. Therefore, the B content is set to 0.0100% or less, and preferably 0.0080% or less, and 0.0050% or less.

Ca: 0-0.0500%
Ca has the effect of dispersing a large number of fine oxides during deoxidation of molten steel and refining the structure of the steel sheet. Ca also fixes S in the steel as spherical CaS, suppresses the formation of elongated inclusions such as MnS, and improves the hole expandability of the steel sheet. The Ca content may be 0%, but to obtain these effects, the Ca content is preferably 0.0001% or more. The Ca content is more preferably 0.0005% or more, or 0.0010% or more.
On the other hand, even if the Ca content exceeds 0.0500%, the above effect saturates. Therefore, the Ca content is set to 0.0500% or less. The Ca content is preferably 0.0300% or less, 0.0200% or less, 0.0150% or less, or 0.0100% or less.

Mg: 0-0.050%
Mg has the effect of adjusting the shape of inclusions in steel to a preferred shape, thereby increasing the yield ratio of the steel sheet. The Mg content may be 0%, but to obtain this effect, the Mg content is preferably 0.001% or more. The Mg content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the Mg content exceeds 0.050%, excessive inclusions are formed in the steel, resulting in a decrease in the yield ratio of the steel sheet. Therefore, the Mg content is set to 0.050% or less. The Mg content is preferably 0.040% or less, more preferably 0.030% or less.

REM: 0~0.1000%
REM has the effect of increasing the yield ratio of steel sheets by adjusting the shape of inclusions in steel to a preferred shape. The REM content may be 0%, but to obtain this effect, the REM content is preferably 0.0001% or more. The REM content is more preferably 0.0005% or more, and more preferably 0.0010% or more.
On the other hand, if the REM content exceeds 0.1000%, excessive inclusions are formed in the steel, resulting in a decrease in the yield ratio of the steel sheet. Therefore, the REM content is set to 0.1000% or less. The REM content is preferably 0.0800% or less, 0.0600% or less, 0.0300% or less, or 0.0100% or less.
Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the content of REM refers to the total content of these elements. In the case of lanthanoids, they are industrially added in the form of misch metal.

Bi: 0~0.100%
Bi has the effect of increasing the yield ratio of the steel sheet by refining the solidification structure. The Bi content may be 0%, but to obtain this effect, the Bi content is preferably 0.001% or more. The Bi content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the Bi content exceeds 0.100%, the effect of the above action saturates, which is economically undesirable. Therefore, the Bi content is set to 0.100% or less. The Bi content is preferably 0.080% or less, 0.060% or less, or 0.040% or less.

Ta: 0~0.100%
Ta, like V, has the effect of increasing the strength of the steel sheet by forming fine carbides in the steel. The Ta content may be 0%, but in order to obtain this effect, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.005% or more, more preferably 0.010% or more.
On the other hand, if the Ta content exceeds 0.100%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the Ta content is set to 0.100% or less. The Ta content is preferably 0.080% or less, more preferably 0.050% or less.

Zr: 0-0.500%
Zr has the effect of increasing the strength of the steel sheet by solid solution strengthening. The Zr content may be 0%, but to obtain this effect, the Zr content is preferably 0.001% or more. The Zr content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the Zr content exceeds 0.500%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the Zr content is set to 0.500% or less. The Zr content is preferably 0.300% or less, more preferably 0.100% or less.

Co: 0-3.000%
Co has the effect of increasing the strength of the steel sheet by solid solution strengthening. The Co content may be 0%, but to obtain this effect, the Co content is preferably 0.001% or more. The Co content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the Co content exceeds 3.000%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the Co content is set to 3.000% or less. The Co content is preferably 1.000% or less, and more preferably 0.500% or less.

Zn: 0-0.200%
Zn has the effect of increasing the strength of the steel sheet through solid solution strengthening. The Zn content may be 0%, but to obtain this effect, the Zn content is preferably 0.001% or more. The Zn content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the Zn content exceeds 0.200%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the Zn content is set to 0.200% or less. The Zn content is preferably 0.150% or less, more preferably 0.100% or less.

W: 0-0.200%
W has the effect of increasing the strength of the steel sheet by solid solution strengthening. The W content may be 0%, but to obtain this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the W content exceeds 0.200%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the W content is set to 0.200% or less. The W content is preferably 0.150% or less, more preferably 0.100% or less.

Sb: 0-0.500%
Sb has the effect of suppressing the generation of oxides that serve as fracture initiation sites, thereby improving the ductility and hole expandability of the steel sheet. The Sb content may be 0%, but to obtain this effect, the Sb content is preferably 0.001% or more. The Sb content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, even if Sb is contained in a large amount, the above effect saturates, so the Sb content is set to 0.500% or less, preferably 0.300% or less, and more preferably 0.100% or less.

As: 0~0.050%
As has the effect of reducing the austenite single-phase temperature, thereby refining prior austenite grains and improving the hole expandability of the steel sheet. The As content may be 0%, but to obtain this effect, the As content is preferably 0.001% or more. The As content is more preferably 0.005% or more, more preferably 0.010% or more.
On the other hand, since the above effects are saturated even when As is contained in a large amount, the As content is set to 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less.

Sn: 0-0.050%
Sn has the effect of suppressing the generation of oxides that serve as fracture initiation sites, thereby improving the ductility and hole expandability of the steel sheet. The Sn content may be 0%, but to obtain this effect, the Sn content is preferably 0.001% or more. The Sn content is more preferably 0.005% or more, and more preferably 0.010% or more.
On the other hand, even if Sn is contained in a large amount, the above effect saturates, so the Sn content is set to 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less.

The steel sheet according to this embodiment contains the above chemical components, with the remainder consisting of Fe and impurities. In this embodiment, impurities refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment, and/or substances that are tolerated to the extent that they do not adversely affect the properties of the steel sheet according to this embodiment.

The chemical composition of the steel sheet described above is analyzed using a spark discharge optical emission spectrometer, etc. C and S are analyzed using a combustion-infrared absorption method, N is analyzed using an inert gas fusion-thermal conductivity method, and O is analyzed using an inert gas fusion-non-dispersive infrared absorption method.
When the steel sheet has a plating layer or a coating film on the surface, the plating layer or the coating film is removed by mechanical grinding or the like as necessary before analyzing the chemical composition.

Next, the metal structure of the steel plate according to this embodiment will be described.
In the steel plate according to the present embodiment, in a region from the surface to a depth of 1/8 of the plate thickness to a depth of 3/8 of the plate thickness from the surface, the average aspect ratio of prior austenite grains is 3.00 to 5.50, the area fraction of bainite is 70 to 95%, and the area fraction of martensite is 5 to 30%, and the ratio of the sum of the amount of Ti and the amount of Nb analyzed as electrolytic extraction residue at a depth of 1/60 of the plate thickness from the surface to the sum of the amount of Ti and the amount of Nb analyzed as electrolytic extraction residue at a depth of 1/4 of the plate thickness from the surface is 1.5 or more.

In this embodiment, the region from 1/8 of the plate thickness depth from the surface to 3/8 of the plate thickness depth from the surface refers to the region starting from 1/8 of the plate thickness depth from the surface and ending at 3/8 of the plate thickness depth from the surface. The metal structure in this region is specified because the metal structure in this region represents a typical metal structure of a steel plate.

When the steel sheet has a plating layer, a coating film, or the like on its surface, the surface here refers to the interface between the steel sheet and the plating layer, the coating film, or the like.
The interface between the steel sheet and the plating layer, coating, etc. is identified using a BSE image (specifically, a COMPO image (BSE compositional image)) obtained by the following method. A sample is cut out so that the cross section of the steel sheet through its thickness can be observed. The cut out sample is mechanically polished and then mirror-finished. A scanning microscope is used to observe an area of 40,000 ^μm2 or more at 400x magnification. When the cross section is observed using a BSE image (specifically, a COMPO image), a clear difference in contrast is confirmed between the plating layer, coating, etc. and the base steel (steel sheet). Therefore, the position where the contrast changes from the outermost surface can be identified as the interface between the steel sheet and the plating layer, coating, etc. When the parts described below have a plating layer, coating, etc., the interface is identified using a similar method.

Average aspect ratio of prior austenite grains: 3.00 to 5.50
If the average aspect ratio of the prior austenite grains in the inner region is less than 3.00, the ductility of the steel sheet will deteriorate. Therefore, the average aspect ratio of the prior austenite grains in the region from the surface to a depth of 1/8 of the sheet thickness to a depth of 3/8 of the sheet thickness from the surface is set to 3.00 or more. The average aspect ratio of the prior austenite grains in the above region is preferably 3.40 or more, 3.60 or more, or 3.80 or more.
On the other hand, if the average aspect ratio of the prior austenite grains in the above region exceeds 5.50, the hole expandability of the steel sheet deteriorates. Therefore, the average aspect ratio of the prior austenite grains in the inner region is set to 5.50 or less. The average aspect ratio of the prior austenite grains in the inner region is preferably 5.30 or less, 5.00 or less, or 4.50 or less.

The aspect ratio of a prior austenite grain is the value obtained by dividing the major axis of the prior austenite grain by the minor axis, and is a value of 1.00 or more.

The average value of the aspect ratio of the prior austenite grains is measured by the following method.
It is preferable to collect a sample at a position sufficiently far from the widthwise end of the steel plate, for example, at a quarter-width position from the end face in the widthwise direction of the steel plate (a position 1/4 of the width from the end face in the widthwise direction), so that the metal structure of the thickness cross section (thickness direction x rolling direction cross section) can be observed. The size of the sample depends on the measuring device, but for example, it may be a rectangular parallelepiped measuring the full thickness in the thickness direction, 15 mm in the rolling direction, and 10 mm in the width direction. The method for revealing prior austenite grains is the "Bechet-Beaujard method using corrosion with a saturated aqueous solution of picric acid" in Appendix JA.2 of JIS G 0551:2020. Grains that appear black due to corrosion are identified as prior austenite grains. The observation surface revealing the prior austenite grains is observed using an optical microscope, and eight fields of view, each measuring 200 μm in the thickness direction and 600 μm or more in the rolling direction, are photographed within a region from ⅛ of the plate thickness depth from the surface to ⅜ of the plate thickness depth from the surface. The ratio of the major axis to the minor axis measured for each prior austenite grain is calculated from the photographed structure photographs, and the ratio is weighted by the area of each prior austenite grain to calculate an average value, thereby obtaining an average value of the aspect ratios of the prior austenite grains. For example, if the "major axis/minor axis" of a certain prior austenite grain G1 is r1 and its area is A1, and the "major axis/minor axis" of another prior austenite grain G2 is r2 and its area is A2, the average value of the aspect ratios of the two prior austenite grains is calculated as "(A1 × r1 + A2 × r2) / (A1 + A2)."
The general formula can be expressed by the following formula: where ri is the "major axis/minor axis" of the i-th prior austenite grain, and Ai is the area of the i-th prior austenite grain.

If the above-mentioned method does not sufficiently reveal the prior austenite grains, the prior austenite grains are identified using the reconstruction method described in "Studies toward improving the accuracy of the reconstruction method for the austenite structure of steel" (Hata Kengo, Wakita Masayuki, Fujiwara Tomoya, Kono Kaori, Nippon Steel Sumitomo Metal Technical Report No. 404 (2016), pp. 24-30), and the average aspect ratio of the prior austenite grains is calculated.

If prior austenite grains with a circle equivalent diameter of less than 2 μm are present, they are excluded from the above measurements. This is because prior austenite grains with a circle equivalent diameter of less than 2 μm do not adversely affect the properties of the steel sheet according to this embodiment.

Unless the rolling direction is known in advance, the rolling direction of the steel sheet is determined by the following method.
A test piece is taken from any position at least 50 mm away from the end of the steel plate so that the thickness cross section can be observed. The thickness cross section of the taken test piece is mirror-polished and then observed using an optical microscope at magnifications of 100x, 200x, 500x, and 1000x. An appropriate magnification at which the dimensions of the inclusions can be measured is selected depending on the size of the inclusions. The observation range is 500 μm or more in width and across the entire thickness of the plate, and areas with dark brightness are determined to be inclusions. Observation may be performed from multiple fields of view. Next, using the thickness cross section initially observed by the above method as a reference, a plane parallel to a plane rotated in 5° increments in the range of 0° to 180° around the thickness direction is observed using the same method as above. The average length of the major axes of the multiple inclusions in each cross section is calculated for each cross section. The cross section with the largest average value of the major axis length of the inclusions obtained is identified. The direction parallel to the major axis of the inclusions in that cross section is determined to be the rolling direction.
The rolling direction of the part is also determined in a similar manner.

Area ratio of bainite: 70 to 95%
Bainite is a structure consisting of fine crystal grains and carbides. If the area fraction of bainite is less than 70%, the desired ductility cannot be obtained in the steel plate. Therefore, the area fraction of bainite is set to 70% or more. The area fraction of bainite is preferably 75% or more, or 80% or more.
On the other hand, if the area fraction of bainite exceeds 95%, the steel plate cannot obtain the desired strength. Therefore, the area fraction of bainite is set to 95% or less. The area fraction of bainite is preferably 93% or less, 90% or less, or 87% or less.

Area ratio of martensite: 5 to 30%
Martensite is a structure that increases the strength of a steel sheet. If the area fraction of martensite is less than 5%, the desired strength cannot be obtained. Therefore, the area fraction of martensite is set to 5% or more. The area fraction of martensite is preferably 7% or more, 10% or more, or 15% or more.
On the other hand, if the area fraction of martensite exceeds 30%, the desired ductility cannot be obtained. Therefore, the area fraction of martensite is set to 30% or less. The area fraction of martensite is preferably 25% or less, more preferably 20% or less.

Residual tissue: 0-5%
The metal structure of the internal region of the steel plate according to this embodiment may contain ferrite and pearlite as a remaining structure in addition to bainite and martensite. The area ratio of the remaining structure may be 0 to 5%. The area ratio of the remaining structure may be 3% or less, 2% or less, or 1% or less, or there may be no remaining structure, i.e., the area ratio of the remaining structure may be 0%.
The total area ratio of bainite and martensite may be 95 to 100%, or may be 97% or more, 98% or more, 99% or more, or 100%.

The area ratios of bainite, martensite and the remaining structure are measured by the following method.
A test piece is taken from the steel plate so that the metal structure can be observed at the 1/4 position of the plate thickness (in the range from the surface to the 1/8 position of the plate thickness in the plate thickness direction). The plate thickness cross section of the test piece is mirror-polished and LePera etched, and then an FE-SEM (thermal field emission scanning electron microscope, JEOL JSM-7001F) is used to observe a region of 200 μm (in the plate thickness direction) × 600 μm (in the direction perpendicular to the plate thickness direction) at the 1/4 position of the plate thickness, and image analysis is performed.

In Repela corrosion, martensite and retained austenite are not corroded. Therefore, by calculating the area fraction of the uncorroded areas (i.e., the white areas), the total area fraction of martensite and retained austenite is obtained.

The area fraction of retained austenite can be obtained by X-ray diffraction.
A test piece taken from the steel plate is ground from the plate surface to a 1/4 position in the plate thickness direction (a range from a position 1/8 of the plate thickness to a position 3/8 of the plate thickness from the surface in the plate thickness direction), and the exposed surface is used as the observation surface. This observation surface is mirror-polished and then finished by electrolytic polishing. For the observation surface, the integrated intensity of a total of five peaks, α (200), α (211), γ (200), γ (220), and γ (311), is determined using a Rigaku RINT-2500 and Mo-Kα, and the volume fraction of retained austenite is calculated using the intensity averaging method. This volume fraction of retained austenite is considered to be the area fraction of retained austenite.

The total area fraction of martensite is obtained by subtracting the area fraction of retained austenite obtained by X-ray diffraction from the sum of the area fractions of martensite and retained austenite obtained by observation using the Fe-SEM described above. If the total area fraction of martensite is calculated to be a negative value, the total area fraction of martensite is considered to be 0%.

The area ratio of pearlite is obtained by the following method.
For the same region (200 μm × 600 μm) as that used to determine the area ratios of martensite and retained austenite, only the corroded layer is removed by polishing and the specimen is mirror-finished, and then etched using a nital solution. Observation is then performed using an FE-SEM, and image analysis is performed.
The area where cementite and ferrite are arranged in a lamellar shape is determined as pearlite, and the area ratio of this area is calculated to obtain the area ratio of pearlite.

The area ratio of ferrite is obtained by the following method. The following operation is performed on regions other than the region determined to be pearlite by the above method.
The same region (200 μm × 600 μm) as used to determine the area fractions of martensite and retained austenite was subjected to colloidal polishing or electrolytic polishing, and then crystal orientation information was obtained by electron backscatter diffraction at measurement intervals of 0.2 μm. For the measurements, an EBSD analysis device consisting of a thermal field emission scanning electron microscope (JEOL JSM-7200F) and an EBSD detector (EDAX Velocity® ultra-high speed EBSD detector) was used. The vacuum level inside the device was 9.6 × 10 Pa or less, the acceleration voltage was 25 kV, and the probe current level was 16.

The following analysis is performed using the obtained crystal orientation information using version 7 or later of OIM Analysis (registered trademark) manufactured by EDAX/TSL Solution. Measurement points with a crystal orientation difference of 15° or more are considered to be grain boundaries, and the area surrounded by these grain boundaries is considered to be crystal grains. Next, the difference in crystal orientation between all measurement points within a crystal grain is calculated, and the average of these differences is calculated to obtain the GAM value (Grain Average Misorientation value) of that crystal grain. Crystal grains with a GAM value of 0.5° or less are considered to be ferrite, and their area ratio is calculated to obtain the ferrite area ratio.

The area fraction of bainite is obtained by subtracting the area fractions of martensite, retained austenite, pearlite, and ferrite obtained above from 100%. If the area fraction of bainite is calculated to be a negative value, the area fraction of bainite is set to 0%.
In this embodiment, the area ratio of the metallographic structure is calculated by image analysis using an FE-SEM, X-ray diffraction, and EBSD analysis, and therefore the total of the individual structures may not be 100%. In such cases, the area ratio of each structure is corrected so that the total becomes 100%. For example, if the total of the area ratios of each structure is 103%, the area ratio of each structure is corrected by multiplying it by "100/103".

The observation conditions for the FE-SEM are as follows:
Electron gun type: Thermal emission type Current irradiation number: 9
WD (working distance): 10mm
Acceleration voltage: 20 kV
Objective aperture number: 4
Number of pixels: 5120 x 3840

The ratio of the sum of the Ti and Nb amounts analyzed as electrolytic extraction residue at a depth of 1/60 of the plate thickness from the surface to the sum of the Ti and Nb amounts analyzed as electrolytic extraction residue at a depth of 1/4 of the plate thickness from the surface: 1.5 or more The present inventors have investigated a method for improving internal bending crack resistance while ensuring high strength and excellent ductility and hole expandability in a steel plate. As a result of focusing on the Ti and Nb amounts in the surface layer and the interior of the steel plate, the present inventors have found that the internal bending crack resistance of the steel plate can be improved by increasing the Ti and Nb amounts in the surface layer of the steel plate compared to the Ti and Nb amounts in the interior of the steel plate. Specifically, it has been found that by setting the ratio (ER 1/60 /ER _{1/4 ) of the sum of the Ti and Nb amounts analyzed as electrolytic extraction residue at a depth of 1/60 of the plate thickness from the surface (ER 1/60} ) to the sum of the Ti and Nb amounts analyzed as electrolytic extraction residue at a depth _{of 1/4 of the plate thickness from the surface (ER 1/4 ) to} be 1.5 or more, it is possible to improve the internal bending crack resistance of the steel plate while ensuring the above-mentioned properties.

If ER _1/60 /ER _1/4 is less than 1.5, the resistance to internal bending cracking of the steel plate cannot be improved. Therefore, ER _1/60 /ER _1/4 is set to 1.5 or more. _{ER 1/60} /ER _1/4 is preferably 1.8 or more or 2.0 or more.
There is no particular upper limit to ER _1/60 /ER _1/4 , but ER _1/60 /ER _1/4 may be 5.0 or less, 4.0 or less, or 3.5 or less.

The sum of the amounts of Ti and Nb analyzed as electrowinning residue at a depth of 1/60 of the plate thickness from the surface (ER _1/60 ) and the sum of the amounts of Ti and Nb analyzed as electrowinning residue at a depth of 1/4 of the plate thickness from the surface (ER _1/4 ) are measured by the following method.
Test pieces of a predetermined size are collected from the steel plate. The test pieces are mechanically polished to expose a location at a depth of 1/60 of the plate thickness -2.5 μm from the surface of the steel plate, and a location at a depth of 1/4 of the plate thickness -2.5 μm from the surface of the steel plate. If necessary, protect the surfaces other than the exposed surface with resin or the like. The exposed surface of the test piece is subjected to anodic dissolution at a current density of 20 mA/ ^cm2 using a 10% by volume acetylacetone-1% by mass tetramethylammonium chloride methanol solution as the electrolyte, to a depth of 5.0 μm, to extract carbonitrides and nitrides as residue. The extracted residue is then acid-decomposed, followed by ICP (inductively coupled plasma) emission spectrometry to measure the masses of Ti and Nb in the residue. The masses of Ti and Nb in the residue are divided by the dissolved amount of the test piece to determine the total amount of Ti and Nb present as carbonitrides and nitrides. This gives the total amount of Ti and Nb analyzed as electrolytic extraction residue.

Tensile strength (TS): 980 MPa or more The steel sheet according to this embodiment may have a tensile strength of 980 MPa or more. The tensile strength is more preferably 1000 MPa or more. By setting the tensile strength to 980 MPa or more, the steel sheet can be used for any part without being limited to specific parts, and can contribute greatly to reducing the weight of the vehicle body.
There is no particular need to set an upper limit to the tensile strength, but from the viewpoint of suppressing die wear, it may be set to 1500 MPa or less, 1300 MPa or less, or 1250 MPa or less.

Total elongation (El): 10.0% or more Hole expansion ratio (λ): 50% or more The steel sheet according to this embodiment may have a total elongation (total elongation at break) of 10.0% or more, and a hole expansion ratio of 50% or more. The total elongation is preferably 12.0% or more or 13.0% or more. The hole expansion ratio is preferably 55% or more, 60% or more, or 70% or more.

The tensile strength and total elongation are evaluated by conducting a tensile test in accordance with JIS Z 2241:2022. The test specimen is a No. 5 test specimen of JIS Z 2241:2022. The tensile test specimen is preferably taken from a position 1/4 width from the end in the sheet width direction (a position 1/4 of the sheet width from the end in the sheet width direction), with the direction perpendicular to the rolling direction being the longitudinal direction.
When measuring the tensile strength of a part, if it is not possible to take a No. 5 test piece from the part because the part is small in size or has a complex shape, a small rectangular piece having a parallel portion of any width may be taken and a tensile test may be performed, and the tensile strength may be determined from the maximum test force and the original cross-sectional area of the parallel portion.
The hole expansion ratio is measured by performing a hole expansion test in accordance with JIS Z 2256:2020.

Resistance to cracking within bending: crack length within bending is 50 μm or less The steel plate according to this embodiment may have a crack length within bending of 50 μm or less, as determined by the following evaluation.
For example, a rectangular test piece measuring 100 mm (rolling direction) x 50 mm (perpendicular to the rolling direction) is cut from a half-width position in the width direction of the steel plate (a position half the width from the end in the width direction) to obtain a bending test piece. In accordance with the V-block method of JIS Z 2248:2020, as shown in Figure 1, the sampled test piece is subjected to a bending test using a presser with an interior angle of 60° and a V-block, so that the bending axis is perpendicular to the rolling direction (C-axis bending, i.e., L-bending). The radius R of the tip of the presser is set such that the ratio R/t of the radius R to the plate thickness t is 0.517. The length of a crack occurring within the bend in a cross section perpendicular to the bending axis is measured to obtain the crack length within the bend. If the crack length within the bend is 50 μm or less, it can be determined that the steel plate has excellent resistance to internal bending cracking.
The length of the crack is measured at the center of the plate width, on the inner side of the bent portion at the center of the bent portion, on a plane perpendicular to the bending axis and perpendicular to the plate surface. Specifically, the test piece after bending is cut at the above plane, the cross section is mirror-polished, and the cracks are observed under an optical microscope, and the length of the crack observed on the inner side of the bent portion of the test piece is measured.

The thickness of the steel plate according to this embodiment is not particularly limited, but may be 1.2 to 8.0 mm. If the thickness of the steel plate is less than 1.2 mm, it may be difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the steel plate according to this embodiment may be 1.2 mm or more. Preferably, it is 1.4 mm or more.
On the other hand, if the plate thickness exceeds 8.0 mm, it may be difficult to obtain the above-mentioned metal structure after hot rolling. Therefore, the plate thickness may be 8.0 mm or less, and preferably 6.0 mm or less, 4.8 mm or less, or 3.6 mm or less.

The steel sheet according to this embodiment, having the above-described chemical composition and metallographic structure, may be provided with a plating layer on the surface to provide a surface-treated steel sheet for the purpose of improving corrosion resistance, etc. The plating layer may be an electroplated layer or a hot-dip plated layer. Examples of electroplated layers include electrogalvanized plating and electrolytic Zn—Ni alloy plating. Examples of hot-dip plated layers include hot-dip galvanized plating, alloyed hot-dip galvanized plating, hot-dip aluminum plating, hot-dip Zn—Al alloy plating, hot-dip Zn—Al—Mg alloy plating, and hot-dip Zn—Al—Mg—Si alloy plating. The coating weight is not particularly limited and may be the same as conventional coating weights.
Furthermore, it is also possible to further enhance corrosion resistance by carrying out an appropriate chemical conversion treatment after plating (for example, by applying a silicate-based chromium-free chemical conversion treatment solution and drying it).

The steel sheet according to the present embodiment has high strength, excellent ductility and hole expandability, and is thought to have excellent crash resistance properties due to suppression of internal bending cracks, and is therefore suitable for use in parts, particularly automobile parts, including lower arms, trail links, knuckles, and other automobile suspension parts.
The steel sheet according to this embodiment may be a hot-rolled steel sheet.

Parts manufactured using the steel plate according to this embodiment have the same chemical composition as the steel plate described above. Furthermore, parts may contain a mixture of processed and unprocessed parts. The unprocessed parts have the same metallurgical structure as the steel plate described above. Processed parts generally have the same metallurgical structure as the steel plate described above, but if heavily processed, they may not have the metallurgical structure described above. Therefore, when measuring the metallurgical structure of a part, measurements are taken of unprocessed parts. If there are no unprocessed parts, measurements are taken of parts that have not been heavily processed. Unprocessed or heavily processed parts refer to, for example, flat parts of the part and parts that avoid parts that have been punched, hole-expanded, bent, etc. As an example, in the case of the above-mentioned part, a test piece is taken from the flat, largest-area part near the center of gravity and examined.

Next, a preferred method for manufacturing the steel plate according to this embodiment will be described. The manufacturing method described below allows for stable manufacturing of the steel plate according to this embodiment. Note that the temperature of the slab and the temperature of the steel plate in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel plate, and are measured using a radiation thermometer.

A preferred method for manufacturing a steel sheet according to this embodiment is as follows:
Rough rolling is started in a temperature range of 1050 to 1200 ° C., and the surface temperature rise width ΔT from 1 second to 10 seconds after the end of rough rolling is set to 150 ° C. or more,
In the finish rolling, the total reduction rate below 1050 ° C is 30 to 50%,
After the finish rolling is completed, the steel sheet is cooled to a temperature range of 500 to 680 ° C. at an average cooling rate of 30 ° C./s or more, and in this temperature range, the steel sheet is slowly cooled at an average cooling rate of 20 ° C./s or less for 2.0 seconds or more,
In the temperature range from the slow cooling completion temperature to 200°C, cooling is carried out at an average cooling rate of 30°C/s or more.
Each step will be described below.

The slab to be subjected to rough rolling is not particularly limited except for the chemical composition described above. For example, a slab can be produced by melting molten steel of the above chemical composition using a converter or electric furnace, and then using a continuous casting method. Instead of continuous casting, an ingot casting method, thin slab casting method, etc. may also be used. When heating the slab before rough rolling, the heating temperature should be in the range of 1100 to 1300°C.

Rough rolling starts in a temperature range of 1050 to 1200 ° C., and the surface temperature rise width ΔT from 1 second after the end of rough rolling to 10 seconds after the end of rough rolling (= "surface temperature 10 seconds after the end of rough rolling" - "surface temperature 1 second after the end of rough rolling") is preferably 150 ° C. or more.
The present inventors have found that in order to favorably control the ratio (ER 1/60 /ER 1/4 ) of the total amount of Ti and Nb analyzed as electrolytic extraction residue at a depth of 1/60 of the plate thickness from the surface to the total amount of Ti and Nb analyzed as electrolytic extraction residue at a depth _{of 1/4 of} the plate thickness from the surface, it is effective to strictly control the temperature history from the start of rough rolling to the start of finish rolling. Specifically, the present inventors have found that it is effective to increase the temperature difference between the surface layer and the interior after the start of rough rolling.

After rough rolling begins in the temperature range of 1050 to 1200 ° C., when the surface of the rough bar (slab) during rough rolling is cooled by contact with the rolling roll, cooling water from the rolling roll, or descaling (injection of high-pressure water), the surface temperature of the steel plate rises due to reheating from the inside after rough rolling is completed. Therefore, in this embodiment, the temperature rise width ΔT of the surface temperature from 1 second to 10 seconds after the end of rough rolling is set to 150 ° C. or more. In order to achieve a temperature rise width ΔT of 150 ° C. or more, a person skilled in the art can achieve a ΔT of 150 ° C. or more, which is a condition that is normally unattainable, by shortening the interpass time and performing multiple rough rollings in a short period of time, or by injecting as much cooling water as possible. The fact that the temperature 10 seconds after the end of rough rolling is 150 ° C. or more higher than the temperature 1 second after the end of rough rolling indicates that the temperature has risen sufficiently due to reheating from the inside, and that the temperature difference between the surface and the inside has been increased by cooling. By setting the temperature rise width ΔT of the surface temperature from 1 second to 10 seconds after the end of rough rolling to 150° C. or more, ER _1/60 /ER _1/4 can be preferably controlled.

If the rough bar (slab) is not cooled sufficiently during rough rolling, the temperature 10 seconds after the end of rough rolling may not be increased by 150 ° C or more compared to the temperature 1 second after the end of rough rolling. As a result, a large amount of coarse precipitates may precipitate inside, making it impossible to preferably control ER _1/60 /ER _1/4 . In addition, if the bar is cooled excessively, the temperature of the rough bar (slab) may drop too much and finish rolling may not be performed sufficiently (for example, the total reduction rate of finish rolling at less than 1050 ° C may not be 30 to 50%).

The temperatures 1 second and 10 seconds after the end of rough rolling are measured using a radiation thermometer installed between the roughing mill and the finishing mill.

In finish rolling, the total reduction rate below 1050°C is set to 30-50%. By setting the total reduction rate in the temperature range below 1050°C to 30-50%, it is possible to favorably control the average aspect ratio of prior austenite grains in the region from 1/8 of the plate thickness depth from the surface to 3/8 of the plate thickness depth from the surface.

The total rolling reduction in the temperature range below 1050°C can be expressed by (1-t1/ _t0 ) x 100 (%), where _t0 is the entry thickness of the first rolling in the temperature range below 1050° _C and _t1 is the exit thickness of the last rolling in the temperature range below 1050°C.

After finish rolling is complete, it is preferable to cool the material to a temperature range of 500-680°C at an average cooling rate of 30°C/s or more, and then perform slow cooling in this temperature range at an average cooling rate of 20°C/s or less for 2.0 seconds or more. By starting slow cooling in the temperature range of 500-680°C and continuing slow cooling in this temperature range for 2.0 seconds or more, the desired amount of bainite can be obtained.

In this embodiment, the average cooling rate is the temperature difference between the start and end points of the set range divided by the elapsed time from the start point to the end point.

After the slow cooling is complete, it is preferable to cool the steel sheet so that the average cooling rate in the temperature range from the slow cooling completion temperature to 200°C is 30°C/s or more. By cooling under these conditions, the desired amount of martensite can be obtained. Cooling should be carried out to a temperature range of 200°C or less, and may be, for example, 100°C or less. After cooling, the steel sheet is wound into a coil.

Next, the effects of one aspect of the present disclosure will be explained in more detail using examples. However, the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to these examples. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the present disclosure and achieve the objectives of the present disclosure.

Steels having the chemical compositions shown in Tables 1 and 2 were melted and continuously cast into slabs with thicknesses of 240 to 300 mm. Blank cells in the tables indicate that the element in question was below the detection limit.
Using the obtained slabs, steel plates shown in Tables 4A and 4B were obtained under the production conditions shown in Tables 3A and 3B. The thickness of the obtained steel plates was 1.2 to 8.0 mm.
Finish rolling was started in a temperature range of 1000 to 1150° C., and cooling was started in a temperature range of 850 to 1050° C. In addition, cooling was performed at an average cooling rate of 30° C./s or more up to the slow cooling start temperature, and the average cooling rate of the slow cooling was 20° C./s or less.

The items in the table indicate the following:
ΔT: The temperature rise of the surface temperature from 1 second after the end of rough rolling to 10 seconds after the end of rough rolling ("Surface temperature 10 seconds after the end of rough rolling" - "Surface temperature 1 second after the end of rough rolling")
Total reduction rate: total reduction rate in finish rolling below 1050°C Average cooling rate: average cooling rate in the temperature range from the slow cooling completion temperature to 200°C

For the obtained steel plates, the average aspect ratio and metal structure of the prior austenite grains in the region from the surface to a depth of 1/8 of the plate thickness to a depth of 3/8 of the plate thickness, the ratio (ER 1/60 /ER 1/4 ) of the total amount of Ti and Nb analyzed as electrolytic extraction residue at a depth of 1/60 of the plate thickness from the surface to the total amount of Ti and Nb analyzed as electrolytic extraction residue at a depth of 1/4 of the plate thickness from _{the surface} , tensile strength (TS), total elongation (EL), hole expansion ratio (λ), and crack length (L) in bending were determined by the above-mentioned methods.
In the examples, the rolling direction was known in advance, so the above-mentioned determination of the rolling direction was not performed.

In the metal structure in the region from the surface of the steel plate to a depth of 1/8 of the plate thickness to a depth of 3/8 of the plate thickness from the surface, in addition to the bainite and martensite shown in the table, there were also cases where ferrite and pearlite were included as the remaining structure.
The measurement results obtained are shown in Tables 4A and 4B.

If the tensile strength was 980 MPa or higher, the steel plate was deemed to have high strength and passed the test. On the other hand, if the tensile strength was less than 980 MPa, the steel plate was deemed to not have high strength and failed the test.

If the total elongation was 10.0% or more, the steel sheet was deemed to have excellent ductility and was judged to have passed. On the other hand, if the total elongation was less than 10.0%, the steel sheet was deemed to have poor ductility and was judged to have failed.

If the hole expansion ratio was 50% or more, the steel sheet was deemed to have excellent hole expandability and was judged to have passed. On the other hand, if the hole expansion ratio was less than 50%, the steel sheet was deemed to have poor hole expandability and was judged to have failed.

If the crack length within the bend was 50 μm or less, the steel plate was deemed to have excellent resistance to internal bending cracking and was judged to have passed. On the other hand, if the crack length within the bend was more than 50 μm, the steel plate was deemed to have poor resistance to internal bending cracking and was judged to have failed.

Looking at Tables 1 to 4B, it can be seen that the steel plates according to the present invention have high strength as well as excellent ductility, hole expansion ability and resistance to internal bending cracking.

In addition, for all examples, lower arms (components) were manufactured by press working. The flat portion of the lower arm was evaluated using the same method as described above. The measurement and evaluation results were the same as those shown in Tables 4A and 4B.

The above aspects of the present disclosure make it possible to provide steel plates having high strength, as well as excellent ductility, hole expansion properties, and resistance to internal bending cracks, as well as parts made from such steel plates.

Claims (5)

The chemical composition, in mass%, is
C: 0.045-0.120%,
Si: 0-3.00%,
Mn: 1.20-3.00%,
Ti: 0.020 to 0.180%,
Al: 0.010-0.400%,
P: 0 to 0.080%,
S: 0 to 0.0100%,
N: 0 to 0.0050%,
O: 0 to 0.0100%,
Nb: 0 to 0.100%,
V: 0 to 1.000%,
Cu: 0 to 1.000%,
Cr: 0-2.000%,
Mo: 0-3.000%,
Ni: 0-1.000%,
B: 0 to 0.0100%,
Ca: 0-0.0500%,
Mg: 0 to 0.050%,
REM: 0-0.1000%,
Bi: 0-0.100%,
Ta: 0-0.100%,
Zr: 0 to 0.500%,
Co: 0-3.000%,
Zn: 0-0.200%,
W: 0-0.200%,
Sb: 0 to 0.500%,
As: 0 to 0.050%,
Sn: 0 to 0.050%, and
The balance is Fe and impurities.
In the region from the surface to a depth of 1/8 of the plate thickness to a depth of 3/8 of the plate thickness from the surface,
The average aspect ratio of the prior austenite grains , as expressed by the following formula, is 3.00 to 5.50,
The area ratio of bainite is 70 to 95%,
The area ratio of martensite is 5 to 30%,
a ratio of the sum of the amount of Ti and the amount of Nb analyzed as an electrolytic extraction residue at a depth of 1/60 of the sheet thickness from the surface to the sum of the amount of Ti and the amount of Nb analyzed as an electrolytic extraction residue at a depth of 1/4 of the sheet thickness from the surface to 1.5 or more.

where ri is the "major axis/minor axis" of the i-th prior austenite grain, and Ai is the area of the i-th prior austenite grain. The chemical composition is, in mass %,
Nb: 0.001 to 0.100%,
V: 0.001 to 1.000%,
Cu: 0.001 to 1.000%,
Cr: 0.001-2.000%,
Mo: 0.001-3.000%,
Ni: 0.001 to 1.000%,
B: 0.0001 to 0.0100%,
Ca: 0.0001-0.0500%,
Mg: 0.001-0.050%,
REM: 0.0001-0.1000%,
Bi: 0.001-0.100%,
Ta: 0.001 to 0.100%,
Zr: 0.001 to 0.500%,
Co: 0.001 to 3.000%,
Zn: 0.001-0.200%,
W: 0.001-0.200%,
Sb: 0.001 to 0.500%,
The steel plate according to claim 1, further comprising at least one of As: 0.001 to 0.050% and Sn: 0.001 to 0.050%. The steel plate according to claim 1 or 2, characterized in that the plate thickness is 1.2 to 8.0 mm. The steel plate according to claim 1 or 2, characterized in that it has a tensile strength of 980 MPa or more. An automotive part made from the steel sheet according to claim 1 or 2.