JP7688288B2 - hot rolled steel plate - Google Patents

Description

The present invention relates to a hot-rolled steel sheet. Specifically, the present invention relates to a hot-rolled steel sheet that is used by being formed into various shapes by press working or the like, and in particular to a hot-rolled steel sheet that has high strength and exhibits little deterioration in crack propagation arrestability after plastic deformation.
This application claims priority based on Japanese Patent Application No. 2021-113549, filed in Japan on July 8, 2021, the contents of which are incorporated herein by reference.

In recent years, efforts to reduce carbon dioxide emissions have been made in many fields from the perspective of protecting the global environment. Automobile manufacturers are also actively developing technologies to reduce the weight of vehicles in order to improve fuel efficiency. However, reducing the weight of vehicles is not an easy task, as emphasis is also placed on improving crashworthiness to ensure the safety of passengers.

In order to reduce greenhouse gas emissions by reducing the weight of vehicle bodies, the use of high-strength steel plates to make components thinner is being considered. For this reason, there is a strong demand for steel plates that combine high strength with excellent formability, and several technologies have been proposed to meet these demands. On the other hand, Non-Patent Document 1 describes that as the strength of steel plates increases, plastic deformation becomes more difficult and crack propagation arrestability generally decreases.

Furthermore, heavily processed areas such as bent sections undergo large plastic deformation during press forming, and as a result of work hardening and an increase in strength, the crack arrestability decreases further and press cracks can occur in areas that experience large plastic deformation. Deterioration of crack arrestability after plastic deformation has traditionally been an issue with thick plate materials used for ships and structural steel, but with the trend toward higher strength in recent years, it has also become necessary to consider this issue when forming hot-rolled steel sheets, which are used in automobiles.

Regarding technology for improving toughness after plastic deformation, for example, Patent Document 1 discloses a steel plate for large structures that has excellent crack propagation arrest properties after plastic deformation by strictly controlling impurity elements and setting the ferrite grain size in the surface layer to 3 μm or less.

Patent Document 2 discloses a large structural steel plate that has excellent crack propagation arrest properties after plastic deformation by having a structure containing ferrite grains with an aspect ratio of 2 or more and a short axis diameter of 5 μm or less, and subgrains in the ferrite crystals that have a circular equivalent diameter of 3 μm or less.

Japanese Patent No. 3499085 Japanese Patent No. 3467767

Yuzo Takahashi, Osamu Kono, Kosaku Ushioda, Shuji Aihara: Iron and Steel, 99, (2013), 4, 312-321

The technologies disclosed in Patent Documents 1 and 2 are both technologies related to steel plates for large structures, and are not intended for hot-rolled steel plates. In addition, both have a structural design that is primarily ferrite, and the strength of the steel plates is 450 to 700 MPa, so it may be difficult to apply the technologies disclosed in Patent Documents 1 and 2 to high-strength hot-rolled steel plates of 980 MPa or more that are primarily composed of bainite or martensite.

The present invention has been made in consideration of the above-mentioned problems of the conventional technology, and aims to provide a hot-rolled steel sheet that has high strength and exhibits little deterioration in crack propagation arrestability after plastic deformation.

In view of the above-mentioned problems, the inventors have conducted extensive research into the relationship between the chemical composition and metal structure of hot-rolled steel sheets and their mechanical properties, and have obtained the following findings (a) to (e) and completed the present invention.

(a) In order to obtain excellent tensile (maximum) strength, it is preferable to utilize a hard structure. In other words, it is preferable to include martensite or bainite in the metal structure.

(b) However, since hard structures have poor crack propagation arresting properties, simply having a metal structure that is primarily composed of these structures cannot ensure excellent crack propagation arresting properties.

(c) Furthermore, plastic deformation causes work hardening, which further reduces the crack propagation arrestability after plastic deformation.

(d) In order to provide a high-strength hot-rolled steel sheet with excellent crack arrestability and to suppress a deterioration of the crack arrestability after plastic deformation, it is effective to obtain a fine structure in which the crack propagation path is bent after plastic deformation.
Specifically, making the fracture surface units fine after plastic deformation is effective in suppressing the deterioration of the crack propagation arrestability after plastic deformation.

(e) In order to refine the fracture surface units after plastic deformation, it is effective to control the heating conditions of the slab, the hot rolling conditions, and the cooling conditions after hot rolling. This makes it possible to create fine austenite grains that have a large orientation difference between the structures generated during the bcc transformation, and to refine the fracture surface units after plastic deformation.

The gist of the present invention made based on the above findings is as follows.
(1) A hot-rolled steel sheet according to one embodiment of the present invention has a chemical composition, in mass%,
C: 0.040-0.400%,
Si: 0.05-3.00%,
Mn: 1.00-4.00%,
sol. Al: 0.001 to 0.500%,
P: 0.100% or less,
S: 0.0300% or less,
N: 0.1000% or less,
O: 0.0100% or less,
Ti: 0 to 1.000%,
V: 0 to 1.000%,
Nb: 0 to 1.000%,
Cu: 0-2.00%,
Cr: 0-2.00%,
Mo: 0-1.00%,
Ni: 0-2.00%,
B: 0 to 0.0100%,
Ca: 0-0.0200%,
Mg: 0 to 0.0200%,
REM: 0-0.1000%,
Bi: 0 to 0.020%,
One or more of Zr, Co, Zn and W: 0 to 1.00% in total; and Sn: 0 to 0.05%;
The balance is Fe and impurities,
In a cross section parallel to the rolling direction, the metal structure at a depth of 1/4 of the plate thickness from the surface (a region from 1/8 depth to 3/8 depth from the surface) and at the center position in the plate width direction is
The retained austenite is less than 3.00% by area percent,
The Rcf value, which indicates the ratio of the average fracture surface unit before and after plastic deformation, expressed as the ratio (Cf1/Cf2) of the Cf1 value, which is the average fracture surface unit before plastic deformation, to the Cf2 value, which is the average fracture surface unit after plastic deformation in which a 10% compressive prestrain is applied, is 2.00 or more;
The tensile strength is 980 MPa or more.
(2) The hot-rolled steel sheet according to (1) above, wherein the chemical composition is, in mass%,
Ti: 0.010 to 1.000%,
V: 0.010-1.000%,
Nb: 0.010-1.000%,
Cu: 0.01-2.00%,
Cr: 0.01-2.00%,
Mo: 0.01-1.00%,
Ni: 0.02-2.00%,
B: 0.0001 to 0.0100%,
Ca: 0.0005-0.0200%,
Mg: 0.0005-0.0200%,
REM: 0.0005 to 0.1000%, and Bi: 0.0005 to 0.020%
may contain one or more selected from the group consisting of:
(3) In the hot-rolled steel sheet according to (1) or (2) above, the metal structure may be, in area percentage, 15.00 to 60.00% ferrite and 40.00 to 85.00% martensite.
(4) In the hot-rolled steel sheet according to (1) or (2) above, the metal structure may have, in terms of area percentage, bainite of 50.00% or more.
(5) In the hot-rolled steel sheet according to (1) or (2) above, the metal structure may have martensite content of more than 85.00% by area percentage.

According to the above-described aspect of the present invention, it is possible to obtain a hot-rolled steel sheet having high strength and little deterioration in crack propagation arrestability after plastic deformation.
The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for automobile parts, machine structural parts, and further building parts.

The chemical composition and metal structure of the hot-rolled steel sheet according to this embodiment will be described in more detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the present invention.

The numerical ranges listed below with "~" include the lower and upper limits. Numerical values marked "less than" or "more than" are not included in the numerical range. In the following description, percentages relating to the chemical composition of steel plate are mass percentages unless otherwise specified.

1. Chemical composition The hot-rolled steel sheet according to this embodiment contains, by mass%, C: 0.040 to 0.400%, Si: 0.05 to 3.00%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and the balance: Fe and impurities. Each element will be described in detail below.

(1-1) C: 0.040-0.400%
C increases the fraction of the hard phase and lowers the transformation point of the hard phase, thereby increasing the strength of the hot-rolled steel sheet. If the C content is less than 0.040%, it is difficult to obtain the desired strength. Therefore, the C content is set to 0.040% or more. The C content is preferably 0.060% or more, more preferably 0.070% or more, and even more preferably 0.080% or more.
On the other hand, if the C content exceeds 0.400%, a large amount of carbides are generated in the structure, which promotes the generation of internal defects during plastic deformation and increases the deterioration of crack propagation arrestability after plastic deformation. As a result, the desired Rcf value cannot be obtained. Therefore, the C content is set to 0.400% or less. The C content is preferably 0.300% or less, more preferably 0.250% or less, and even more preferably 0.150% or less.

(1-2) Si: 0.05-3.00%
Si has the effect of increasing the strength of the hot-rolled steel sheet by solid solution strengthening at room temperature, and the effect of improving the toughness of the hot-rolled steel sheet by solid solution softening at low temperatures. In addition, Si has the effect of improving the soundness of the steel by deoxidization (suppressing the occurrence of defects such as blowholes in the steel). If the Si content is less than 0.05%, the above effects cannot be obtained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.50% or more, more preferably 0.80% or more.
However, if the Si content exceeds 3.00%, the surface quality and chemical conversion treatability of the steel sheet, as well as its ductility and weldability, are significantly deteriorated, and the surface energy of fracture is reduced. This makes it easier for cracks to occur and propagate during plastic deformation, and the crack propagation arrestability after plastic deformation is greatly deteriorated. As a result, the desired Rcf value cannot be obtained. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.70% or less, more preferably 2.50% or less.

(1-3) Mn: 1.00-4.00%
Mn has the effect of suppressing ferrite transformation to increase the strength of the hot-rolled steel sheet, and the effect of softening in solid solution at low temperatures to improve the toughness of the hot-rolled steel sheet. If the Mn content is less than 1.00%, the desired tensile strength cannot be obtained. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.30% or more, and more preferably 1.50% or more.
On the other hand, if the Mn content exceeds 4.00%, the effect of decreasing the surface energy of fracture becomes large, which makes it easier for cracks to occur and propagate during plastic deformation, and the crack propagation arrestability after plastic deformation becomes more deteriorated. As a result, the desired Rcf value cannot be obtained. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.70% or less, more preferably 3.50% or less.

(1-4) sol. Al: 0.001-0.500%
Like Si, Al has the effect of deoxidizing steel to improve its soundness, and also exhibits solid solution softening at low temperatures and has the effect of increasing the toughness of hot-rolled steel sheets. If the sol. Al content is less than 0.001%, the above-mentioned effects cannot be obtained. Also, if the sol. Al content is less than 0.001%, the desired Rcf value cannot be obtained. Therefore, the sol. Al content is set to 0.001% or more. The sol. Al content is preferably 0.010% or more.
On the other hand, if the sol. Al content exceeds 0.500%, the above effects are saturated and it is economically undesirable, so the sol. Al content is set to 0.500% or less, preferably 0.300% or less, and more preferably 0.100% or less.
Incidentally, sol. Al means acid-soluble Al, and indicates solute Al present in the steel in a solid solution state.

(1-5) P: 0.100% or less P is an element that is generally contained as an impurity, but it also has the effect of increasing the strength of the hot-rolled steel sheet by solid solution strengthening. Therefore, P may be actively contained, but P is an element that is easily segregated, and if the P content exceeds 0.100%, the decrease in grain boundary strength due to grain boundary segregation becomes significant, and grain boundary fracture is likely to occur. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.030% or less.
There is no particular need to specify a lower limit for the P content, but from the viewpoint of refining costs, it is preferable to set the lower limit to 0.001% or more.

(1-6) S: 0.0300% or less S is an element contained as an impurity, and forms sulfide-based inclusions in steel to promote crack generation. If the S content exceeds 0.0300%, crack generation during plastic deformation becomes significant, and the crack propagation stopping property after plastic deformation is significantly reduced. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0050% or less.
There is no particular need to specify a lower limit for the S content, but from the viewpoint of refining costs, it is preferably set to 0.0001% or more.

(1-7) N: 0.1000% or less N is an element contained in steel as an impurity, and has the effect of promoting the generation of cracks originating from the impurities. If the N content exceeds 0.1000%, the generation of cracks during plastic deformation becomes significant, and the crack propagation stopping property after plastic deformation is significantly reduced. Therefore, the N content is set to 0.1000% or less. The N content is preferably 0.0800% or less, more preferably 0.0700% or less, and even more preferably 0.0100% or less.
The lower limit of the N content does not need to be particularly specified, but may be 0.0001% or more. When one or more of Ti, Nb, and V are contained to make the metal structure finer, the N content is preferably 0.0010% or more, and more preferably 0.0020% or more, in order to promote the precipitation of carbonitrides.

(1-8) O: 0.0100% or less If the O content in steel is large, it forms coarse oxides that become the starting point of fracture, causing brittle fracture and hydrogen-induced cracking. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, and more preferably 0.0050% or less.
In order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be set to 0.0005% or more, or 0.0010% or more.

The balance of the chemical composition of the hot-rolled steel sheet according to this embodiment may be 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 hot-rolled steel sheet according to this embodiment.

The hot-rolled steel sheet according to this embodiment may contain the following elements as optional elements in place of a portion of Fe. When no optional elements are contained, the lower limit of the content is 0%. The optional elements are described in detail below.

(1-9) Ti: 0.010 to 1.000%, Nb: 0.010 to 1.000%, and V: 0.010 to 1.000%
Ti, Nb and V are elements that finely precipitate in steel as carbides and nitrides, improving the strength of the steel through precipitation strengthening. Therefore, one or more of these elements may be contained. In order to obtain this effect more reliably, it is preferable that the contents of Ti, Nb and V are each 0.010% or more. It is not necessary to contain all of Ti, Nb and V, and it is sufficient that the content of any one of them is 0.010% or more. The contents of Ti, Nb and V are each preferably 0.060% or more, more preferably 0.080% or more.
On the other hand, if the content of any one of Ti, Nb and V exceeds 1.000%, the workability of the hot-rolled steel sheet deteriorates. Therefore, the contents of Ti, Nb and V are each set to 1.000% or less, preferably 0.800% or less, and more preferably 0.500% or less.

(1-10) Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 1.00%, Ni: 0.02 to 2.00%, and B: 0.0001 to 0.0100%
Cu, Cr, Mo, Ni and B all have the effect of increasing the hardenability of the hot-rolled steel sheet. Furthermore, when Cu is contained, Ni has the effect of effectively suppressing grain boundary cracking of the slab caused by Cu. Therefore, one or more of these elements may be contained.

As mentioned above, Cu has the effect of increasing the hardenability of hot-rolled steel sheets. In order to obtain the above-mentioned effect more reliably, the Cu content is preferably 0.01% or more, and more preferably 0.05% or more. However, if the Cu content exceeds 2.00%, grain boundary cracking of the slab may occur. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.50% or less, more preferably 1.00% or less.

As mentioned above, Cr has the effect of increasing the hardenability of hot-rolled steel sheet. In order to obtain the above-mentioned effect more reliably, the Cr content is preferably 0.01% or more, and more preferably 0.05% or more. However, if the Cr content exceeds 2.00%, the chemical conversion treatability of the hot-rolled steel sheet is significantly reduced. Therefore, the Cr content is set to 2.00% or less.

As mentioned above, Mo has the effect of increasing the hardenability of hot-rolled steel sheets and of precipitating in the steel as carbides to increase the strength of the hot-rolled steel sheets. In order to obtain the above-mentioned effects more reliably, the Mo content is preferably 0.01% or more, and more preferably 0.02% or more. However, even if the Mo content exceeds 1.00%, the effects of the above-mentioned actions are saturated and it is not economically preferable. Therefore, the Mo content is set to 1.00% or less. The Mo content is preferably 0.50% or less, more preferably 0.20% or less.

As mentioned above, Ni has the effect of increasing the hardenability of hot-rolled steel sheet. In addition, when Cu is contained, Ni has the effect of effectively suppressing grain boundary cracking of the slab caused by Cu. In order to obtain the above-mentioned effect more reliably, it is preferable that the Ni content be 0.02% or more. Since Ni is an expensive element, it is economically undesirable to contain a large amount of it. Therefore, the Ni content is set to 2.00% or less.

As mentioned above, B has the effect of increasing the hardenability of hot-rolled steel sheet. To obtain the effect of this action more reliably, the B content is preferably 0.0001% or more, and more preferably 0.0002% or more. However, if the B content exceeds 0.0100%, the formability of the hot-rolled steel sheet is significantly reduced, so the B content is set to 0.0100% or less. The B content is preferably set to 0.0050% or less.

(1-11) Ca: 0.0005 to 0.0200%, Mg: 0.0005 to 0.0200%, REM: 0.0005 to 0.1000%, and Bi: 0.0005 to 0.020%
Ca, Mg and REM all have the effect of improving the crack propagation stopping property of the hot-rolled steel sheet by adjusting the shape of the inclusions in the steel to a preferred shape. Bi also has the effect of improving the crack propagation stopping property of the hot-rolled steel sheet by refining the solidification structure. Therefore, one or more of these elements may be contained. In order to obtain the effect of the above action more reliably, it is preferable that any one or more of Ca, Mg, REM and Bi is 0.0005% or more. However, if the Ca content or Mg content exceeds 0.0200%, or the REM content exceeds 0.1000%, inclusions are excessively generated in the steel, which may actually reduce the crack propagation stopping property of the hot-rolled steel sheet. Also, even if the Bi content exceeds 0.020%, the effect of the above action is saturated, which is not economically preferable. Therefore, the Ca content and Mg content are set to 0.0200% or less, the REM content is set to 0.1000% or less, and the Bi content is set to 0.020% or less. The Bi content is preferably 0.010% or less.
Here, REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids, and the content of the REM refers to the total content of these elements. In the case of lanthanoids, they are industrially added in the form of misch metal.

(1-12) One or more of Zr, Co, Zn and W: 0 to 1.00% in total, and Sn: 0 to 0.05%
Regarding Zr, Co, Zn, and W, the inventors have confirmed that the effect of the hot-rolled steel sheet according to the present embodiment is not impaired even if these elements are contained in a total amount of 1.00% or less. Therefore, one or more of Zr, Co, Zn, and W may be contained in a total amount of 1.00% or less.
The inventors have confirmed that the effect of the hot-rolled steel sheet according to the present embodiment is not impaired even if a small amount of Sn is contained. However, since a large amount of Sn may cause defects during hot rolling, the Sn content is set to 0.05% or less.

The chemical composition of the above-mentioned hot-rolled steel sheet may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Sol. Al may be measured by ICP-AES using the filtrate after thermally decomposing the sample with acid. C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method.

2. Metallographic Structure of Hot-Rolled Steel Sheet Next, the metallographic structure of the hot-rolled steel sheet according to this embodiment will be described.
The hot-rolled steel sheet according to this embodiment has a metal structure in which the amount of retained austenite is less than 3.00% by area, and the Rcf value, which indicates the average ratio of fracture surface units before and after plastic deformation, is 2.0 or more.

Therefore, the hot-rolled steel sheet according to this embodiment can obtain high strength and excellent crack propagation arrestability even after plastic deformation.
In this embodiment, the structure fraction and Rcf value of the metal structure at a 1/4 depth of the sheet thickness from the surface (a region from 1/8 depth to 3/8 depth from the surface) and at the center position in the sheet width direction in a cross section parallel to the rolling direction are specified because the metal structure at this position shows a typical metal structure of a steel sheet.

(2-1) Area ratio of retained austenite: less than 3.00% Retained austenite is a metal structure that exists as fcc even at room temperature. The retained austenite has carbon concentrated in the surrounding structure, and may become a crack initiation point by transforming into hard martensite during plastic deformation. If the area ratio of retained austenite is 3.00% or more, the above action becomes apparent, and the crack propagation stopping property after plastic deformation is significantly reduced. Therefore, the area ratio of retained austenite is less than 3.00%. The area ratio of retained austenite is preferably 2.00% or less, less than 1.50% or less than 1.00%, and more preferably less than 1.00% or less than 0.50%. Since the amount of retained austenite is preferably as small as possible, the area ratio of retained austenite may be 0.00%.

Methods for measuring the area ratio of retained austenite include X-ray diffraction, EBSP (Electron Backscatter Diffraction Pattern) analysis, magnetic measurement, etc. In this embodiment, the area ratio of retained austenite is measured by X-ray diffraction.

In the measurement of the area ratio of retained austenite by X-ray diffraction in this embodiment, first, in a cross section parallel to the rolling direction at a depth of 1/4 of the thickness of the hot-rolled steel sheet (the region from 1/8 depth from the surface to 3/8 depth from the surface) and at the center position in the sheet width direction, Co-Kα radiation is used to determine the integrated intensity of a total of seven peaks, α(110), α(200), α(211), γ(111), γ(200), and γ(220), and the area ratio of retained austenite is obtained by calculating using the intensity averaging method.

The metal structure of the heat-rolled steel sheet according to this embodiment may contain ferrite, martensite, bainite, and pearlite in addition to retained austenite.
The area ratio of ferrite may be 60.00% or less, 50.00% or less, or 45.00% or less. The area ratio of ferrite may be 0.00% or more, or 0.05% or more.
The area ratio of martensite may be 100.00% or less, or 99.00% or less, or 0.00% or more, 1.00% or more, or 1.50% or more.
The area fraction of bainite may be 100.00% or less or 96.00% or less. Also, the area fraction of bainite may be 0.00% or more or 0.01% or more.
The area ratios of the retained austenite, ferrite, bainite, and martensite described above are area ratios that apply to all three types of steel (DP steel, bainite steel, and martensitic steel) described below.
In the following, preferred area ratios of each structure in each of the three steel types (DP steel, bainite steel, and martensitic steel) will be described.

(2-2) DP steel When the metal structure is composed of less than 3.00% area fraction of retained austenite, ferrite, martensite, and a small amount of remaining structure, it is preferable that the area fraction of ferrite is 15.00 to 60.00%, the area fraction of martensite is 40.00 to 85.00%, and the area fraction of the remaining structure is less than 45.00%. By setting the area fraction of each structure as described above, it is possible to improve strength and ductility in a well-balanced manner.
Each tissue in this embodiment will be described below.

Ferrite area ratio: 15.00 to 60.00%
Ferrite is a structure that is formed when fcc transforms to bcc at a relatively high temperature. Ferrite has a high work hardening rate, and therefore acts to improve the strength-ductility balance of hot-rolled steel sheets. By setting the area ratio of ferrite to 15.00% or more, the above-mentioned effects can be sufficiently obtained. Therefore, it is preferable that the area ratio of ferrite is 15.00% or more. The area ratio of ferrite is more preferably 20.00% or more, 25.00% or more, or 30.00% or more.
On the other hand, since ferrite has low strength, if the area ratio is excessive, the desired tensile strength may not be obtained. Therefore, the area ratio of ferrite is preferably 60.00% or less. The area ratio of ferrite is more preferably 55.00% or less, and even more preferably 50.00% or less.

Area ratio of martensite: 40.00 to 85.00%
Martensite is a structure that is generated when fcc transforms to bcc at a relatively low temperature. Martensite is a structure consisting of fine crystal grains with high dislocation density, and has the effect of increasing the strength of the hot-rolled steel sheet. By setting the area ratio of martensite to 40.00% or more, the above-mentioned effect can be sufficiently obtained. Therefore, it is preferable that the area ratio of martensite is 40.00% or more. The area ratio of martensite is preferably 50.00% or more.
On the other hand, martensite has poor ductility, and if the area ratio is excessive, the ductility of the hot-rolled steel sheet may decrease. Therefore, the area ratio of martensite is preferably 85.00% or less. The area ratio of martensite is more preferably 80.00% or less, and even more preferably 75.00% or less or 70.00% or less.

Area ratio of remaining structure: less than 45.00% In this embodiment, the remaining structure may include pearlite and bainite in a total amount of less than 45.00%. The area ratio of the remaining structure may be 10.00% or less or 5.00% or less. The remaining structure may not be included, and the area ratio may be 0.00% in total.

(2-3) Bainite steel When the metal structure is composed of less than 3.00% area fraction of retained austenite, bainite and a small amount of remaining structure, it is preferable that the area fraction of bainite is 50.00% or more and the area fraction of the remaining structure is less than 50.00%. By setting the area fraction of each structure as described above, it is possible to simultaneously increase the strength, ductility and hole expandability.
Each tissue in this embodiment will be described below.

Area ratio of bainite: 50.00% or more Bainite is a structure that is generated when fcc transforms to bcc at low temperatures. Bainite is a structure consisting of fine crystal grains and carbides, and has the effect of improving the strength, ductility, and hole expandability of hot-rolled steel sheets in a balanced manner. By setting the area ratio of bainite to 50.00% or more, the above-mentioned effects can be fully obtained. Therefore, the area ratio of bainite is preferably set to 50.00% or more. The area ratio of bainite is more preferably 80.00% or more, 85.00% or more, or 90.00% or more.
Although there is no particular upper limit, it may be set to 100.00% or less.

Remainder structure: less than 50.00% In this embodiment, the remainder structure may include pearlite, ferrite, and martensite in a total amount of less than 50.00%. The area ratio of the remainder structure is more preferably 20.00% or less, 15.00% or less, or 10.00% or less. The remainder structure may not be included, and the area ratio may be 0.00% in total.

(2-4) Martensitic steel When the metal structure is composed of less than 3.00% area ratio of retained austenite, martensite, and a small amount of remaining structure, it is preferable that the total area ratio of martensite is 85.00% or more and the area ratio of the remaining structure is less than 15.00%. By setting the area ratio of each structure as above, it is possible to simultaneously increase the strength and hole expandability.
Each tissue in this embodiment will be described below.

Total area ratio of martensite: over 85.00% As described above, martensite has the effect of increasing the strength of the hot-rolled steel sheet. In addition, since the crystal orientation of the structure of martensite is random, it has the effect of increasing the hole expandability of the hot-rolled steel sheet. By setting the area ratio of martensite to over 85.00%, the effect of the above action can be fully obtained. Therefore, it is preferable that the area ratio of martensite is over 85.00%. The area ratio of martensite is more preferably 90.00% or more, 93.00% or more, or 95.00% or more. The upper limit is not particularly specified, but it may be 100.00% or less.

Remainder structure: less than 15.00% In this embodiment, the remainder structure may include pearlite, ferrite, and bainite in a total amount of less than 15.00%. The area ratio of the remainder structure is more preferably 10.00% or less, 7.00% or less, or 5.00% or less. The remainder structure may not be included, and the area ratio may be 0.00% in total.

(2-5) Pearlite area ratio: less than 5.00% Pearlite is a lamellar metal structure in which cementite is precipitated in layers between ferrite, and is a softer metal structure than bainite or martensite. If the pearlite area ratio is 5.00% or more, carbon is consumed by the cementite contained in the pearlite, the strength of martensite and bainite decreases, and a tensile strength of 980 MPa or more may not be obtained. Therefore, in any of the above embodiments, the pearlite area ratio may be less than 5.00%. The pearlite area ratio is more preferably 3.00% or less. In order to improve the stretch flangeability of the hot-rolled steel sheet, it is preferable to reduce the pearlite area ratio as much as possible, and it is even more preferable that the pearlite area ratio is 0.00%.
The area ratio of pearlite referred to here is an area ratio that applies to all of the above-mentioned three types of steel (DP steel, bainitic steel, and martensitic steel).

The structure other than the retained austenite is measured by the following method.
The area ratios of ferrite and pearlite are measured by the following method. A sample is taken from a cross section parallel to the rolling direction so that it can be observed at 1/4 depth of the sheet thickness from the surface (a region from 1/8 depth to 3/8 depth from the surface) and at the center position in the sheet width direction. The cross section of this sample parallel to the rolling direction is mirror-finished and polished for 8 minutes at room temperature with colloidal silica that does not contain an alkaline solution to remove strain introduced into the surface layer of the sample.

At any position in the longitudinal direction of the sample cross section, a region of 50 μm in length, 1/4 of the plate thickness from the surface (a region from 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface), and at the center position in the plate width direction is measured by electron backscatter diffraction at measurement intervals of 0.1 μm to obtain crystal orientation information. The number of measurement points is at least 500. For the measurement, an EBSD device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the EBSD device is 9.6×10 ⁻⁵ Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62.

Furthermore, a backscattered electron image is taken in the same field of view. First, the crystal grains in which ferrite and cementite are precipitated in layers are identified from the backscattered electron image, and the area ratio of the crystal grains is calculated to obtain the area ratio of pearlite. Then, for the crystal grains other than those determined to be pearlite, the obtained crystal orientation information is used to determine the areas with a Grain Average Misorientation value of 1.0° or less as ferrite using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. The area ratio of the areas determined to be ferrite is obtained.

The same observation surface as in the above measurement is polished, then subjected to nital etching, and at least three 30 μm x 30 μm regions at a position 1/4 of the plate thickness from the surface (region 1/8 depth from the surface in the plate thickness direction to 3/8 depth from the surface in the plate thickness direction) are observed using an optical microscope and a scanning electron microscope (SEM). The area ratio of bainite is obtained by performing image analysis on the microstructural photograph obtained from this microstructural observation. The same observation positions are then subjected to Lepera etching, and microstructural observation is performed using an optical microscope and a scanning electron microscope, and the area ratio of martensite is calculated by performing image analysis on the obtained microstructural photograph.

In the above-mentioned structure observation, each structure is identified by the following method.
Martensite has a high dislocation density and has a substructure such as blocks and packets within the grains, so it can be distinguished from other metal structures by electron channeling contrast images taken with a scanning electron microscope.
Bainite is defined as a structure that is a collection of lath-shaped crystal grains and does not contain Fe-based carbides with a major axis of 20 nm or more, and is not martensite, and also as a structure that contains Fe-based carbides with a major axis of 20 nm or more and has a single variant, i.e., is elongated in the same direction. Here, Fe-based carbides elongated in the same direction refer to those in which the difference in the elongation direction of the Fe-based carbides is within 5°.
The rolling direction of the hot-rolled steel sheet is determined by the following method.
First, a test piece is taken so that the thickness cross section of the hot-rolled steel sheet can be observed. The thickness cross section of the taken test piece is mirror-polished and then observed using an optical microscope. The observation range is the entire thickness of the sheet, and the direction parallel to the elongation direction of the crystal grains is determined to be the rolling direction.

Rcf value indicating the ratio of the average fracture surface unit before and after plastic deformation: 2.00 or more Generally, the strength increases due to the introduction of dislocations during plastic deformation, so the crack propagation arrestability of the hot-rolled steel sheet after plastic deformation decreases. Since cracks are formed on cleavage fracture surfaces called fracture surface units, it is important to make the fracture surface units fine and bend the propagation path in order to suppress crack propagation. In this embodiment, the Rcf (Ratio of cleavage facet) value indicating the ratio of the fracture surface units before and after plastic deformation is controlled to suppress the decrease in the crack propagation arrestability after plastic deformation.

The Rcf value represents the ratio of the average fracture surface unit before and after plastic deformation, and is expressed by the following formula using the Cf1 value, which is the average fracture surface unit before plastic deformation, and the Cf2 value, which is the average fracture surface unit after plastic deformation.

Rcf=Cf1/Cf2

In hot-rolled steel sheets that have undergone plastic deformation, the strength increases due to work hardening caused by the introduction of dislocations, and the crack arrestability decreases. On the other hand, the crack arrestability is also affected by the fracture surface unit; the finer the fracture surface unit, the more the propagation path is curved and the more the crack propagation is suppressed. Therefore, in order to maintain high crack arrestability even after plastic deformation, it is necessary to increase the Rcf value.

If the Rcf value is less than 2.00, the effect of increasing the strength due to the introduction of dislocations is greater than the effect of the refinement of the fracture surface unit, so it is presumed that the crack propagation arrestability is reduced. Therefore, the Rcf value is set to 2.00 or more. It is preferably 2.20 or more, and more preferably 2.30 or more.
Since a higher Rcf value is more preferable, the upper limit is not particularly specified, but it may be 5.00 or less, 4.00 or less, or 3.00 or less.

The Cf1 and Cf2 values can be obtained by the following method.
In this embodiment, in order to calculate the Cf1 value and the Cf2 value, it is necessary to cause brittle fracture. As a test method for causing brittle fracture, for example, in accordance with JIS Z 2242:2018, a V-notch test piece of 2.5 mm subsize is prepared in the direction in which the width direction (C direction) of the hot-rolled steel sheet is the longitudinal direction of the test piece, and a Charpy impact test is performed at -196 ° C. For hot-rolled steel sheets with a thickness of less than 2.5 mm, the test may be performed at the full thickness.
The rolling direction of the hot-rolled steel sheet is determined by the above-mentioned method, and the direction perpendicular to the rolling direction is determined as the width direction of the hot-rolled steel sheet.

The region of the SEM image taken to calculate the Cf1 and Cf2 values is a 1/4 depth position of the sheet thickness from the surface of the steel sheet (a region from 1/8 depth of the sheet thickness from the surface to 3/8 depth of the sheet thickness from the surface) in a cross section parallel to the rolling direction, and a central position in the sheet width direction. To take the SEM image, a SU-6600 Schottky electron gun manufactured by Hitachi High-Technologies Corporation is used, the emitter is tungsten, and the acceleration voltage is 1.5 kV in a vacuum of 9.6×10 ⁻⁵ Pa or less. The magnification of the image is 1000 times, and the number of fields of view is 3 or more.

In the SEM images taken, ductile fracture areas called tear ridges are captured as bright contrast. The area surrounded by the tear ridges is considered to be one cleavage facet, and the circular equivalent diameter is calculated from the area of each cleavage facet, which is used as the fracture surface unit for each cleavage facet. From the fracture surface units obtained, the area average diameter weighted by the area of each cleavage facet is calculated, which is used as the average fracture surface unit.

The above-mentioned process is performed on the hot-rolled steel sheet before plastic deformation and on the hot-rolled steel sheet after plastic deformation to calculate the Cf1 value and the Cf2 value, respectively.
For the Charpy impact test after plastic deformation, a JIS No. 5 tensile test specimen is prepared with the width direction (C direction) of the hot-rolled steel plate oriented in the longitudinal direction of the test specimen, and a compressive pre-strain of 10% is imparted to the steel material in the longitudinal direction of the test specimen, after which various test specimens are taken.

Standard deviation of Mn concentration: 0.60 mass% or less In the hot-rolled steel sheet according to the present embodiment, the standard deviation of the Mn concentration at 1/4 depth of the sheet thickness from the surface (region from 1/8 depth to 3/8 depth from the surface) and at the center position in the sheet width direction may be 0.60 mass% or less. This makes it possible to suppress the development of a region in which Mn is locally concentrated and fracture energy is reduced, and to further suppress the occurrence of local cracks during plastic deformation and the deterioration of crack propagation arrestability after plastic deformation.

The standard deviation of the Mn concentration is preferably 0.50% by mass or less, and more preferably 0.47% by mass or less. From the viewpoint of suppressing the decrease in fracture energy, it is desirable for the lower limit of the standard deviation of the Mn concentration to be as small as possible, but due to the constraints of the manufacturing process, the practical lower limit is 0.10% by mass.

After mirror polishing the cross section (L cross section) parallel to the rolling direction of the hot-rolled steel sheet, the standard deviation of the Mn concentration is measured by measuring the 1/4 depth of the sheet thickness from the surface of the steel sheet (the region from 1/8 depth of the sheet thickness from the surface to 3/8 depth of the sheet thickness from the surface) and the center position in the sheet width direction with an electron probe microanalyzer (EPMA). The measurement conditions are an acceleration voltage of 15 kV and a magnification of 5000 times, and a distribution image is measured in a range of 20 μm in the sample sheet thickness direction (the region from 1/8 depth to 3/8 depth from the surface). More specifically, the measurement interval is 0.1 μm, and the Mn concentration is measured at 40,000 or more points. Next, the standard deviation is calculated based on the Mn concentrations obtained from all measurement points to obtain the standard deviation of the Mn concentration.

3. Tensile properties Tensile properties (tensile strength) among the mechanical properties of hot-rolled steel sheets are evaluated in accordance with JIS Z 2241:2011. The test specimen is a No. 5 test specimen of JIS Z 2241:2011. The tensile test specimen is taken from a quarter portion from the end in the sheet width direction, and the direction perpendicular to the rolling direction is the longitudinal direction.

The hot-rolled steel sheet according to this embodiment has a tensile (maximum) strength of 980 MPa or more. Preferably, it is 1000 MPa or more. If the tensile strength is less than 980 MPa, the applicable parts are limited, and the contribution to reducing the weight of the vehicle body is small. There is no need to set an upper limit, but from the viewpoint of suppressing die wear, it may be 1780 MPa or less, 1500 MPa or less, or 1300 MPa or less.

4. Sheet thickness The sheet thickness of the hot-rolled steel sheet according to this embodiment is not particularly limited, but may be 1.20 to 8.00 mm. If the sheet thickness of the hot-rolled steel sheet is less than 1.20 mm, it may be difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the sheet thickness of the hot-rolled steel sheet according to this embodiment may be 1.20 mm or more. It is preferably 1.40 mm or more. On the other hand, if the sheet thickness exceeds 8.00 mm, the influence of the standard deviation of the Mn concentration becomes significant, and it may be difficult to obtain the desired Rcf value. Therefore, the sheet thickness may be 8.00 mm or less. It is preferably 6.00 mm or less or 3.00 mm or less.

5. Others (5-1) Plating layer The hot-rolled steel sheet according to the present embodiment having the above-mentioned chemical composition and metal structure may be provided with a plating layer on the surface for the purpose of improving corrosion resistance, etc., to form a surface-treated steel sheet. The plating layer may be an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include electrogalvanizing and electrogalvanizing Zn-Ni alloy plating. Examples of the hot-dip plating layer include hot-dip galvanizing, alloyed hot-dip galvanizing, 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 in the past. In addition, it is also possible to further improve the corrosion resistance by performing an appropriate chemical conversion treatment (for example, application of a silicate-based chromium-free chemical conversion treatment solution and drying) after plating.

6. Manufacturing Conditions A suitable manufacturing method for the hot-rolled steel sheet according to this embodiment having the above-mentioned chemical composition and metal structure is as follows.

In order to obtain the hot-rolled steel sheet according to this embodiment, it is effective to heat the slab under specified conditions, then hot roll it, accelerate cool it to a specified temperature range, and then slowly cool it as necessary, controlling the cooling history until coiling.

In a preferred method for producing a hot-rolled steel sheet according to this embodiment, the following steps (1) to (7) are sequentially performed. Note that the temperature of the slab and the temperature of the steel sheet in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet.
(1) The slab is heated and held at a temperature of 1100° C. or higher for 6000 seconds or more. It is more preferable that the slab is held at a temperature of 700 to 850° C. for 900 seconds or more during heating, and then further heated and held at a temperature of 1100° C. or higher for 6000 seconds or more.
(2) Hot rolling is performed in the temperature range of 850 to 1100°C, resulting in a total thickness reduction of 90% or more.
(3) The starting temperature of hot rolling is 850°C or higher and less than 930°C, the rolling temperatures from the first stage to the stage two before the final stage of hot rolling are 850°C or higher and less than 950°C, and the rolling reduction ratio of the rolling is less than 30%.
(4) The rolling temperature in the final stage of hot rolling and the stage before the final stage is 930°C or higher and less than 1010°C, the rolling reduction is 50% or higher, and the rolling completion temperature is 950°C or higher and less than 1010°C.
(5) Within 1.0 second after the completion of hot rolling, the steel sheet is cooled at an average cooling rate of 50°C/second or more, and the cooling start temperature is set to 850°C or more and less than 960°C.
(6) Cool to a temperature range of 600 to 730°C at an average cooling rate of 50°C/s or more, and then perform slow cooling at an average cooling rate of less than 5°C/s for 2.0 seconds or more in the temperature range of 600 to 730°C. After that, cool to a temperature range of 350°C or less at an average cooling rate of 50°C/s or more.
The slow cooling does not have to be performed. In the case where the slow cooling is not performed, the material is cooled to a temperature range of 350° C. or lower at an average cooling rate of 50° C./s or higher.
(7) Winding is performed in a temperature range of 350°C or less.

(6-1) Slab, slab temperature and holding time when used for hot rolling The slab used for hot rolling can be a slab obtained by continuous casting or a slab obtained by casting and blooming, and if necessary, a slab that has been subjected to hot working or cold working can be used. The slab used for hot rolling is preferably heated and held at a temperature range of 1100°C or higher for 6000 seconds or more. In addition, when held at 1100°C or higher, the steel sheet temperature may be fluctuated or may be constant in a temperature range of 1100°C or higher. By holding at a temperature range of 1100°C or higher for 6000 seconds or more, the austenite grains during slab heating can be made uniform. By making the austenite grains uniform, the recrystallization of austenite in the first stage of hot rolling (the first stage to the second stage before the final stage of hot rolling) described later can be suppressed, and as a result, a desired Rcf value can be obtained. If the holding temperature is less than 1100°C or the holding time is less than 6000 seconds, it becomes difficult to make the austenite grains uniform, and the recrystallization of austenite in the stage prior to the hot rolling described below cannot be suppressed. As a result, the desired Rcf value may not be obtained.

In addition, when heating the slab, it may be held in the temperature range of 700 to 850 ° C for 900 seconds or more, and then further heated and held in the temperature range of 1100 ° C or more for 6000 seconds or more. When held in the temperature range of 700 to 850 ° C, the steel plate temperature may be varied in this temperature range or may be constant. In the austenite transformation in the temperature range of 700 to 850 ° C, Mn is distributed between ferrite and austenite, and by lengthening the transformation time, Mn can be diffused in the ferrite region. This eliminates Mn microsegregation unevenly distributed in the slab and significantly reduces the standard deviation of Mn concentration. If the standard deviation of Mn concentration is large, Mn is locally concentrated and regions with reduced fracture energy develop, promoting crack generation during plastic deformation, and the desired Rcf value may not be obtained.

For hot rolling, it is preferable to use a reverse mill or a tandem mill as a multi-pass rolling. In particular, from the viewpoint of industrial productivity and the stress load on the steel sheet during rolling, it is more preferable to use a tandem mill for at least the final two stages of hot rolling.

(6-2) Reduction rate of hot rolling: Total thickness reduction of 90% or more in the temperature range of 850 to 1100 ° C. By performing hot rolling such that the total thickness reduction is 90% or more in the temperature range of 850 to 1100 ° C., mainly recrystallized austenite grains are refined and the accumulation of strain energy in the non-recrystallized austenite grains is promoted. Then, the recrystallization of austenite is promoted and the atomic diffusion of Mn is promoted, and the standard deviation of the Mn concentration can be reduced. As a result, crack generation during plastic deformation is promoted and a desired Rcf value can be obtained. Therefore, it is preferable to perform hot rolling such that the total thickness reduction is 90% or more in the temperature range of 850 to 1100 ° C. If the total reduction rate in the temperature range of 850 to 1100 ° C. is less than 90%, the standard deviation of the Mn concentration becomes high, and it is not possible to suppress the development of the area where Mn is locally concentrated and the fracture energy is reduced, and the crack generation during plastic deformation may be promoted. This may result in the desired Rcf value not being obtainable.

The thickness reduction in the temperature range of 850 to 1100°C can be expressed as {(t0 - t1)/t0} x 100 (%), where t0 is the entrance thickness before the first rolling in this temperature range, and t1 is the exit thickness after the final stage of rolling in this temperature range.

(6-3) Starting temperature of hot rolling: 850°C or more and less than 930°C, rolling temperature of the first stage to the second stage before the last stage of hot rolling: 850°C or more and less than 950°C, reduction ratio of the rolling: less than 30% It is preferable that the starting temperature of hot rolling is 850°C or more and less than 930°C, the rolling temperature of the first stage to the second stage before the last stage of hot rolling is 850°C or more and less than 950°C, and the reduction ratio of the first stage to the second stage before the last stage of hot rolling is less than 30%. By setting the starting temperature of hot rolling to a relatively low temperature and performing the first stage of hot rolling at a low temperature and a low reduction ratio, it is possible to suppress recrystallization in the first stage of hot rolling and accumulate strain in the austenite grains. As a result, unrecrystallized austenite with a high dislocation density in the grains can be maintained until the second stage of rolling. This makes it possible to obtain a desired Rcf value. If the rolling start temperature is 930°C or higher, if the rolling temperature from the first stage to the second stage before the final stage of hot rolling is 950°C or higher, or if the rolling reduction rate of the rolling is 30% or higher, it may be impossible to suppress austenite recrystallization in the first stage to the second stage before the final stage of hot rolling, and as a result, it may be impossible to obtain a desired Rcf value. Also, if the hot rolling start temperature is less than 850°C, or if the rolling temperature from the first stage to the second stage before the final stage of hot rolling is less than 850°C, it becomes difficult to set the rolling temperature in the final stage and the stage before the final stage to 930°C or higher, and as a result, it may be impossible to obtain a desired Rcf value.

(6-4) Rolling temperature in the final stage of hot rolling and the stage before the final stage: 930°C or more, less than 1010°C, reduction ratio of the rolling: 50% or more, rolling completion temperature: 950°C or more, less than 1010°C. It is preferable that the rolling temperature in the final stage of hot rolling and the stage before the final stage is 930°C or more, less than 1010°C, the reduction ratio of the final stage and the stage before the final stage is 50% or more, and the rolling completion temperature (temperature after rolling in the final stage) is 950°C or more. By setting the reduction ratio in the final stage of hot rolling and the stage before the final stage to 50% or more and the rolling completion temperature to 950°C or more, it is possible to promote the recrystallization of austenite in the latter stage of rolling. By causing recrystallization in the latter stage of rolling for crystal grains that have a high dislocation density due to the suppression of recrystallization in the former stage of rolling, the orientation difference between the structures generated from the recrystallized austenite becomes large. This allows the Rcf value to be increased, and the desired Rcf value to be obtained. It is presumed that between structures with a large orientation difference, the crack propagation stopping property of the interface is improved by the occurrence of crystal rotation during plastic deformation, and the Rcf value is improved. If the rolling temperature of the final stage of hot rolling and the stage before the final stage is less than 930°C, the rolling reduction of the final stage and the stage before the final stage is less than 50%, or the rolling completion temperature is less than 950°C, the recrystallization of austenite is insufficient, and the desired Rcf value may not be obtained.

In addition, by setting the rolling temperature in the final stage of hot rolling, the rolling temperature in the stage before the final stage, and the rolling completion temperature to less than 1010°C, the coarsening of the austenite grain size can be suppressed, thereby making the structure finer. This suppresses the occurrence of cracks during plastic deformation, and increases the Rcf value.

(6-5) Average cooling rate within 1.0 second after completion of hot rolling: 50 ° C./sec or more, cooling start temperature: 850 ° C. or more, less than 960 ° C. In order to suppress the growth of austenite grains refined by hot rolling, it is preferable to cool at an average cooling rate of 50 ° C./sec or more within 1.0 second after completion of hot rolling, and to set the cooling start temperature to 850 ° C. or more and less than 960 ° C. In order to perform cooling at an average cooling rate of 50 ° C./sec or more within 1.0 second after completion of hot rolling, cooling with a large average cooling rate may be performed immediately after completion of hot rolling, for example, by spraying cooling water on the steel sheet surface. By setting the cooling start temperature to 850 ° C. or more and less than 960 ° C., and cooling at an average cooling rate of 50 ° C./sec or more within 1.0 second after completion of hot rolling, the austenite grains and the structure generated thereafter can also be refined. This suppresses the occurrence of cracks during plastic deformation, and increases the Rcf value.

The cooling start temperature referred to here means the temperature immediately before cooling at an average cooling rate of 50° C./sec or more, for example, the temperature immediately before cooling water is sprayed onto the steel sheet surface.
The average cooling rate refers to the value obtained by dividing the temperature drop of the steel plate from the start of accelerated cooling (when the steel plate is introduced into the cooling equipment) to the completion of accelerated cooling (when the steel plate is removed from the cooling equipment) by the time required from the start of accelerated cooling to the completion of accelerated cooling.

(6-6) Cool to a temperature range of 600 to 730° C. at an average cooling rate of 50° C./s or more, and then perform slow cooling at an average cooling rate of less than 5° C./s for 2.0 seconds or more in the temperature range of 600 to 730° C. After that, cool to a temperature range of 350° C. or less at an average cooling rate of 50° C./s or more.
The slow cooling does not have to be performed. In the case where the slow cooling is not performed, the material is cooled to a temperature range of 350° C. or lower at an average cooling rate of 50° C./s or higher.

After the cooling, the steel is cooled to a temperature range of 600-730°C at an average cooling rate of 50°C/sec or more, which suppresses the formation of low-strength ferrite and pearlite. This improves the strength of the hot-rolled steel sheet.

During cooling, slow cooling at an average cooling rate of less than 5°C/s is performed for 2.0 seconds or more in the temperature range of 600 to 730°C, so that bainite can be sufficiently precipitated. This refines the structure, so that the strength and crack propagation stopping property of the hot-rolled steel sheet can be compatible. The average cooling rate here refers to the value obtained by dividing the temperature drop of the steel sheet from the cooling stop temperature of accelerated cooling to the stop temperature of slow cooling by the time required from the stop of accelerated cooling to the stop of slow cooling.
The above-mentioned DP steel can be stably manufactured by performing slow cooling in the high temperature range (660 to 730°C) of the temperature range of 600 to 730°C. The above-mentioned bainite steel can be stably manufactured by performing slow cooling in the low temperature range (600°C or higher and lower than 660°C) of the temperature range of 600 to 730°C.

In order to suppress the area ratio of pearlite and obtain a desired tensile strength, it is preferable that the average cooling rate from the cooling stop temperature of the slow cooling to the coiling temperature is 50° C./sec or more, which makes it possible to harden the matrix structure.
The average cooling rate referred to here means a value obtained by dividing the temperature drop width of the steel sheet from the cooling stop temperature of slow cooling, in which the average cooling rate is less than 5°C/s, to the coiling temperature, by the time required from the stop of slow cooling, in which the average cooling rate is less than 5°C/s, to coiling.

The upper limit of the time for slow cooling is determined by the equipment layout, but should generally be less than 10.0 seconds. There is no particular lower limit on the average cooling rate for slow cooling, but since raising the temperature without cooling involves a large investment in equipment, it may be set to 0°C/s or more.

It is not necessary to perform slow cooling. By performing accelerated cooling to a temperature range of 350°C or less at an average cooling rate of 50°C/sec or more without performing slow cooling, it is possible to promote the formation of martensite while suppressing the formation of ferrite and pearlite, which have low strength. This makes the structure finer, and the martensitic steel described above can be stably produced, and it is possible to achieve both strength and crack propagation arrestability of the hot-rolled steel sheet.
The average cooling rate referred to here means a value obtained by dividing the temperature drop of the steel plate from the start of accelerated cooling (when the steel plate is introduced into the cooling equipment) to the completion of accelerated cooling (when the steel plate is removed from the cooling equipment) by the time required from the start of accelerated cooling to the completion of accelerated cooling.

There is no particular upper limit for the cooling rate, but a faster cooling rate requires larger cooling equipment, which increases equipment costs. For this reason, taking equipment costs into consideration, a rate of 300°C/sec or less is preferable.

(6-7) Coiling temperature: 350° C. or less The coiling temperature is set to 350° C. or less. By setting the coiling temperature to 350° C. or less, the amount of precipitation of iron carbide can be reduced, and the variation in hardness distribution in the hard phase can be reduced. As a result, the number of crack initiation points and propagation paths can be reduced, the occurrence of cracks during plastic deformation can be suppressed, and the desired Rcf value can be obtained.

Next, the effect of one aspect of the present invention will be explained in more detail using an example. However, the conditions in the example are an example of conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to this example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

Steels having the chemical compositions shown in Tables 1 and 2 were melted and continuously cast into slabs having thicknesses of 240 to 300 mm. The resulting slabs were used to obtain hot-rolled steel sheets shown in Tables 5A and 5B under the manufacturing conditions shown in Tables 3A to 4.
Martensitic steel was obtained by cooling to a temperature range of 350° C. or less at a desired average cooling rate without slow cooling. In the example where slow cooling was performed, DP steel was obtained by performing slow cooling in the high temperature range (660 to 730° C.) of the temperature range of 600 to 730° C., and bainitic steel was obtained by performing slow cooling in the low temperature range (600° C. or more and less than 660° C.) of the temperature range of 600 to 730° C.

The area ratio of the metal structure, the Rcf value, the standard deviation of the Mn concentration, and the tensile strength TS of the obtained hot-rolled steel sheets were determined by the above-mentioned methods. The obtained measurement results are shown in Tables 5A and 5B.

Method for evaluating the properties of hot-rolled steel sheets (1) Tensile properties When the tensile strength TS was 980 MPa or more, the hot-rolled steel sheets were judged to be high-strength and passed the test. On the other hand, when the tensile strength TS was less than 980 MPa, the hot-rolled steel sheets were judged to be not high-strength and passed the test.

(2) Crack Propagation Arrest Property The crack propagation arrest property was evaluated by a Charpy impact test. In accordance with JIS Z 2242:2018, a V-notch test piece with a subsize of 2.5 mm was prepared in such a way that the width direction (C direction) of the hot-rolled steel sheet was the longitudinal direction of the test piece, and a Charpy impact test was performed at −196° C. For hot-rolled steel sheets with a thickness of less than 2.5 mm, the test was performed at the full thickness.

In addition, JIS No. 5B tensile test specimens were prepared with the width direction (C direction) of the hot-rolled steel plate as the longitudinal direction of the test specimen, and the steel was given a compressive pre-strain of 10% in the longitudinal direction of the test specimen, after which the above-mentioned V-notch test specimens were taken. Charpy impact tests were carried out on these test specimens at -196°C using the method described above to obtain the absorbed energy after plastic deformation.

If the reduction rate of absorbed energy after plastic deformation (("Absorbed energy before plastic deformation" - "Absorbed energy after plastic deformation") / "Absorbed energy before plastic deformation") was 30.00% or less, the deterioration of the crack propagation arrestability before and after plastic deformation was small and the specimen was judged to have passed. On the other hand, if the reduction rate of absorbed energy after plastic deformation was more than 30.00%, the deterioration of the crack propagation arrestability before and after plastic deformation was large and the specimen was judged to have failed.

From Tables 5A and 5B, it can be seen that the hot-rolled steel sheets according to the examples of the present invention have high strength and show little deterioration in crack propagation arrestability after plastic deformation. On the other hand, it can be seen that the hot-rolled steel sheets according to the comparative examples do not have one or more of the above characteristics.

Claims (5)

The chemical composition, in mass%, is
C: 0.040-0.400%,
Si: 0.05-3.00%,
Mn: 1.00-4.00%,
sol. Al: 0.001 to 0.500%,
P: 0.100% or less,
S: 0.0300% or less,
N: 0.1000% or less,
O: 0.0100% or less,
Ti: 0 to 1.000%,
V: 0 to 1.000%,
Nb: 0 to 1.000%,
Cu: 0-2.00%,
Cr: 0-2.00%,
Mo: 0-1.00%,
Ni: 0-2.00%,
B: 0 to 0.0100%,
Ca: 0-0.0200%,
Mg: 0 to 0.0200%,
REM: 0-0.1000%,
Bi: 0 to 0.020%,
One or more of Zr, Co, Zn and W: 0 to 1.00% in total; and Sn: 0 to 0.05%;
The balance is Fe and impurities,
In a cross section parallel to the rolling direction, the metal structure at a depth of 1/4 of the plate thickness from the surface (a region from 1/8 depth to 3/8 depth from the surface) and at the center position in the plate width direction is
The retained austenite is less than 3.00% by area percent,
The Rcf value, which indicates the ratio of the average fracture surface unit before and after plastic deformation, expressed as the ratio (Cf1/Cf2) of the Cf1 value, which is the average fracture surface unit before plastic deformation, to the Cf2 value, which is the average fracture surface unit after plastic deformation in which a 10% compressive prestrain is applied, is 2.00 or more;
A hot-rolled steel sheet having a tensile strength of 980 MPa or more. The chemical composition, in mass%,
Ti: 0.010 to 1.000%,
V: 0.010-1.000%,
Nb: 0.010-1.000%,
Cu: 0.01-2.00%,
Cr: 0.01-2.00%,
Mo: 0.01-1.00%,
Ni: 0.02-2.00%,
B: 0.0001 to 0.0100%,
Ca: 0.0005-0.0200%,
Mg: 0.0005-0.0200%,
REM: 0.0005 to 0.1000%, and Bi: 0.0005 to 0.020%
The hot-rolled steel sheet according to claim 1, further comprising one or more selected from the group consisting of: The hot-rolled steel sheet according to claim 1 or 2, wherein the metal structure is, in area percentage, 15.00 to 60.00% ferrite and 40.00 to 85.00% martensite. The hot-rolled steel sheet according to claim 1 or 2, wherein the metal structure is 50.00% or more bainite by area percentage. The hot-rolled steel sheet according to claim 1 or 2, in which the metal structure is more than 85.00% martensite by area percentage.