WO2023218732A1

WO2023218732A1 - Steel sheet, member, and methods for producing same

Info

Publication number: WO2023218732A1
Application number: PCT/JP2023/006926
Authority: WO
Inventors: 芳怡王; 由康川崎; 達也中垣内
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-05-11
Filing date: 2023-02-27
Publication date: 2023-11-16
Also published as: JPWO2023218732A1

Abstract

Provided are: a steel sheet that has a high YS and a TS of at least 1180 MPa, excellent press formability (ductility, hole expandability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial crushing characteristics) when being crushed; a member; and methods for producing the steel sheet and the member.　According to the present invention, a base steel sheet has a predetermined component composition. In the steel structure of the base steel sheet: bainitic ferrite, tempered martensite, residual austenite, and fresh martensite are contained in predetermined ranges; the concentration of carbon in the residual austenite and the density of carbides in the tempered martensite are in predetermined ranges; the amount of diffusible hydrogen is at most 0.50 ppm by mass; and, when a V-VDA bending test is conducted up to the maximum load point, the length of a crack in an L cross-section is at most 400 μm, and the amount of change between before and after processing in the particle size of bainitic ferrite in the thickness direction in a predetermined region of VDA bending is at most 5.0.

Description

Steel plates, members and their manufacturing methods

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

In recent years, improving the fuel efficiency of automobiles has become an important issue from the perspective of preserving the global environment. Therefore, there has been an active movement to reduce the weight of automobile bodies by increasing the strength and thinning of steel plates, which are the raw materials for automobile parts.

In addition, social demands for improved collision safety of automobiles are becoming even higher. Therefore, it is desired to develop a steel plate that not only has high strength but also has excellent impact resistance properties (hereinafter simply referred to as impact resistance properties) in the event of a collision while an automobile is running.

For example, in Patent Document 1, as a steel plate that is a material for such automobile parts, C is 0.04 to 0.22%, Si is 1.0% or less, and Mn is 3.0%. % or less, P is 0.05% or less, S is 0.01% or less, Al is 0.01-0.1%, and N is 0.001-0.005%, with the balance being Fe and unavoidable impurities. It is composed of a ferrite phase as a main phase and a martensite phase as a second phase, and the maximum grain size of the martensite phase is 2 μm or less and its area ratio is 5% or more. A high-strength steel plate with excellent stretch flangeability and collision resistance characteristics is disclosed.

Furthermore, Patent Document 2 describes a cold-rolled steel sheet whose surface layer has been polished to a thickness of 0.1 μm or more and which is pre-plated with Ni at 0.2 g/m2 ^or more and 2.0 g/m2 or ^less . A hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface, in mass %, C: 0.05% or more and 0.4% or less, Si: 0.01% or more and 3.0% or less, Mn: 0.1% or more, 3.0% or less, P: 0.04% or less, S: 0.05% or less, N: 0.01% or less, Al: 0.01% or more, 2.0% or less, Contains Si + Al > 0.5%, the remainder consists of Fe and unavoidable impurities, the microstructure contains 40% or more of ferrite as the main phase in volume fraction, and 8% or more of retained austenite, as specified below. Containing two or more types of martensite [3] of three types of martensite [1], [2], and [3], 1% or more of bainite, and 0 to 10% of pearlite, and containing the three types of martensite [1], [2], and [3] are volume fractions, respectively: martensite [1]: 0% or more, 50% or less, martensite [2]: 0% or more, less than 20%, martensite [3] : 1% or more and 30% or less, and has a hot-dip galvanized layer containing less than 7% Fe, with the remainder consisting of Zn, Al and inevitable impurities, and has a tensile strength TS (MPa), Plating adhesion characterized by having a total elongation rate EL (%) and a hole expansion rate λ (%) of TS x EL of 18000 MPa % or more, TS x λ of 35000 MPa % or more, and a tensile strength of 980 MPa or more. High-strength hot-dip galvanized steel sheet with excellent formability (martensite [1]: C concentration (CM1) is less than 0.8%, hardness Hv1 is Hv1/(-982.1×CM1 ² +1676×CM1+189) ≦0.60, martensite [2]: C concentration (CM2) is 0.8% or more, hardness Hv2 is Hv2/(-982.1×CM2 ² +1676×CM2+189)≦0.60, martensite [3]: It is disclosed that the C concentration (CM3) is 0.8% or more and the hardness Hv3 is Hv3/(-982.1×CM3 ² +1676×CM3+189)≧0.80.

Furthermore, in Patent Document 3, in mass %, C: 0.15% or more and 0.25% or less, Si: 0.50% or more and 2.5% or less, Mn: 2.3% or more and 4.0% or less. , P: 0.100% or less, S: 0.02% or less, Al: 0.01% or more and 2.5% or less, with the balance consisting of Fe and unavoidable impurities. Martensite phase: 30% or more and 73% or less, ferrite phase: 25% or more and 68% or less, retained austenite phase: 2% or more and 20% or less, other phases: 10% or less (including 0%), and The other phases include martensitic phase: 3% or less (including 0%), bainitic ferrite phase: less than 5% (including 0%), and the average grain size of the tempered martensitic phase is 8 μm. The following discloses a high-strength hot-dip galvanized steel sheet having a steel sheet structure in which the amount of C in the retained austenite phase is less than 0.7% by mass.

Patent No. 3887235 Patent No. 5953693 Patent No. 6052472

Incidentally, the current situation is that steel plates with a tensile strength (hereinafter also referred to as TS) of 590 MPa class are only used for impact energy absorbing members of automobiles, such as front side members and rear side members.

That is, improving the yield stress (hereinafter also referred to as YS) is effective in increasing the absorbed energy during impact (hereinafter also referred to as impact absorbed energy). However, when the TS and YS of a steel sheet are increased, press formability, particularly properties such as ductility, hole expandability, and bendability are generally reduced. Therefore, if we assume that a steel plate with high TS and YS is to be applied to the above-mentioned impact energy absorbing member of an automobile, it will not only be difficult to press-form, but also the member will be difficult to perform in an axial crush test simulating a crash test. In other words, the actual impact absorption energy is not as high as expected from the value of YS. Therefore, at present, the impact energy absorbing member described above is limited to steel plates having a TS of 590 MPa class.

In fact, the steel sheets disclosed in Patent Documents 1 to 3 also have a TS of 1180 MPa or more, a high YS, excellent press formability (ductility, hole expandability, and bendability), and rupture resistance during crushing. (bending rupture characteristics and axial crushing characteristics).

The present invention was developed in view of the above-mentioned current situation, and has a tensile strength TS of 1180 MPa or more, a high yield stress YS, and excellent press formability (ductility, hole expandability, and bendability). It is an object of the present invention to provide a steel plate having fracture resistance properties (bending fracture properties and axial crush properties) during crushing, together with an advantageous manufacturing method thereof.
Another object of the present invention is to provide a member made of the above-mentioned steel plate and a method for manufacturing the same.

The steel sheet referred to herein also includes a galvanized steel sheet, and the galvanized steel sheet is a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or an alloyed hot-dip galvanized steel sheet (hereinafter also referred to as GA).

Here, the tensile strength TS is measured by a tensile test based on JIS Z 2241 (2011).
In addition, having a high yield stress YS, excellent press formability (ductility, hole expandability, and bendability), and fracture resistance during crushing (bending fracture properties and axial crush properties) means that the following are satisfied. Point.
High yield stress YS means that YS measured in a tensile test based on JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on the TS measured in the tensile test. refers to doing.
(A) When 1180MPa≦TS<1320MPa, 750MPa≦YS
(B) If 1320MPa≦TS, 850MPa≦YS

Furthermore, having excellent ductility means that the total elongation (El) measured in a tensile test based on JIS Z 2241 (2011) is the following (A) or (B) depending on the TS measured in the tensile test. ) refers to satisfying the formula.
(A) When 1180MPa≦TS<1320MPa, 12.0%≦El
(B) When 1320MPa≦TS, 10.0%≦El

In addition, "excellent hole expansion property" refers to a critical hole expansion rate (λ) of 30% or more measured in a hole expansion test based on JIS Z 2256 (2020).

In addition, "excellent bendability" means that the bending angle (α) at the maximum load measured in a bending test based on the VDA standard (VDA238-100) specified by the German Automobile Manufacturers Association is 80° or more. Point.

Furthermore, the term "excellent bending and breaking properties" means that the stroke at maximum load (S _Fmax ) measured by the V-VDA bending test is 26.0 mm or more.

In addition, having excellent axial crushing properties means that the fracture (appearance crack) after the axial crushing test is R= 5.0mm, refers to 3 or less locations within a 200mm area.

The above El, λ, and α are characteristics that indicate the ease of forming a steel plate during press forming. On the other hand, the V-VDA bending test is a test that simulates the deformation and fracture behavior of the bending ridge line part in a collision test, and the stroke at the maximum load (S _Fmax ) measured in the V-VDA bending test This is a characteristic that indicates shadyness.

Now, the present inventors have made extensive studies to achieve the above object.
As a result, the composition of the base steel plate of the steel plate was adjusted appropriately, and the steel structure of the base steel plate of the steel plate was such that the area ratio of bainitic ferrite was 3.0% or more and 20.0% or less, and tempered martensite. Area ratio (excluding retained austenite): 40.0% to 90.0%, area ratio of retained austenite: more than 3.0% to 15.0%, carbon concentration in retained austenite: 0.60% by mass 1.30% by mass or less, the area ratio of fresh martensite: 10.0% or less (including 0.0%), and the density of carbides in tempered martensite is 8.0 pieces/μm ² or less The base steel sheet has a diffusible hydrogen content of 0.50 mass ppm or less, and the V-VDA bending test is performed up to the maximum load point, and the L In the cross section, the crack length is 400 μm or less, and furthermore, from each position of the start line that extends from the bending apex on the outside of the VDA bending to a position of 50 μm in the plate thickness direction, on both sides of the start line in the vertical direction Regarding the average grain size in the plate thickness direction of bainitic ferrite in the regions formed up to 50 μm in each region, the ratio of the average grain size before processing to the average grain size after processing (change amount before and after processing: By setting the average particle size (nm) before processing/average particle size (nm) after processing to 5.0 or less, the tensile strength TS: 1180 MPa or more, high YS, and excellent press forming can be achieved. It was discovered that a steel plate having good properties (ductility, hole expandability, and bendability) and fracture resistance upon crushing (bending fracture properties and axial crush properties) can be obtained.
The present invention was completed based on the above findings and further studies.

That is, the gist of the present invention is as follows.
[1] A steel plate comprising a base steel plate, the base steel plate comprising:
In mass%,
C: 0.050% or more and 0.400% or less,
Si: more than 0.75% and 3.00% or less,
Mn: 2.00% or more and less than 3.50%,
P: 0.001% or more and 0.100% or less,
S: 0.0001% or more and 0.0200% or less,
Contains Al: 0.010% or more and 2.000% or less and N: 0.0100% or less, with the remainder consisting of Fe and inevitable impurities,
The base steel plate is
Area ratio of bainitic ferrite: 3.0% or more and 20.0% or less,
Area ratio of tempered martensite: 40.0% or more and 90.0% or less,
Area ratio of retained austenite: more than 3.0% and less than 15.0%,
Carbon concentration in retained austenite: 0.60% by mass or more and 1.30% by mass or less,
Fresh martensite area ratio: 10.0% or less,
Density of carbides in tempered martensite: 8.0 pieces/μm ² or less,
It has a steel structure that is
The amount of diffusible hydrogen in the base steel sheet is 0.50 mass ppm or less,
Furthermore, a V-VDA bending test was performed up to the maximum load point,
In the L section,
The length of the crack is 400 μm or less,
Furthermore, in a region formed at a position of up to 50 μm on each side of the start line in the vertical direction from each position of the start line, which starts from the bending apex on the outside of the VDA bending and extends up to a position of 50 μm in the plate thickness direction,
Regarding the average grain size in the plate thickness direction of bainitic ferrite, the ratio of the average grain size before processing to the average grain size after processing is 5.0 or less,
A steel plate having a tensile strength of 1180 MPa or more.
[2] The composition of the base steel sheet further includes, in mass%,
Nb: 0.200% or less,
Ti: 0.200% or less,
V: 0.200% or less,
B: 0.0100% or less,
Cr: 1.000% or less,
Ni: 1.000% or less,
Mo: 1.000% or less,
Sb: 0.200% or less,
Sn: 0.200% or less,
Cu: 1.000% or less,
Ta: 0.100% or less,
W: 0.500% or less,
Mg: 0.0200% or less,
Zn: 0.0200% or less,
Co: 0.0200% or less,
Zr: 0.1000% or less,
Ca: 0.0200% or less,
Se: 0.0200% or less,
Te: 0.0200% or less,
Ge: 0.0200% or less,
As: 0.0500% or less,
Sr: 0.0200% or less,
Cs: 0.0200% or less,
Hf: 0.0200% or less,
Pb: 0.0200% or less,
The steel plate according to [1] above, containing at least one selected from Bi: 0.0200% or less and REM: 0.0200% or less.
[3] The steel sheet according to [1] or [2], wherein one or both surfaces of the steel sheet include a galvanized layer as the outermost layer.
[4] When the base steel plate has an area of 200 μm or less in the thickness direction from the surface of the base steel plate as the surface layer,
The surface layer has a soft surface layer whose Vickers hardness is 85% or less with respect to the Vickers hardness at the 1/4 position of the plate thickness,
Nano hardness of 300 points or more in a 50 μm x 50 μm area of the plate surface at 1/4 position and 1/2 depth in the plate thickness direction of the surface soft layer from the surface of the base steel plate, respectively. When measuring,
The proportion of measurements where the nano-hardness of the plate surface at 1/4 of the depth in the thickness direction of the soft surface layer from the surface of the base steel sheet is 7.0 GPa or more is 1/4 of the depth in the thickness direction of the soft surface layer. 0.10 or less for the total number of measurements at 4 positions,
Furthermore, the standard deviation σ of the nano-hardness of the plate surface at a position 1/4 of the thickness direction depth of the surface soft layer from the base steel plate surface is 1.8 GPa or less,
Furthermore, any one of [1] to [3] above, wherein the standard deviation σ of nano-hardness of the plate surface at a position 1/2 the thickness direction depth of the surface soft layer from the base steel plate surface is 2.2 GPa or less. Steel plate described in Crab.
[5] The steel plate according to any one of [1] to [4], which has a metal plating layer formed on the base steel plate on one or both sides of the steel plate.
[6] A member using the steel plate according to any one of [1] to [5] above.
[7] A hot rolling process of hot rolling a steel slab having the composition described in [1] or [2] above to obtain a hot rolled steel plate;
A pickling step of pickling the hot rolled steel sheet;
An annealing step of annealing the steel plate after the pickling step at an annealing temperature of (Ac ₁ +0.4×(Ac ₃ −Ac ₁ ))° C. or higher and 900° C. or lower, and an annealing time of 20 seconds or more;
A first cooling step of cooling the steel plate after the annealing step to a first cooling stop temperature of 400° C. or higher and 600° C. or lower;
Cooling the steel plate after the first cooling step to a second cooling stop temperature of 100 ° C. or more and 300 ° C. or less at an average cooling rate of 25.0 ° C. / seconds or less,
During the cooling, a tension of 2.0 kgf/mm ² or more is applied to the steel plate at least once in a temperature range of 300 ° C. or higher and 450 ° C. or lower,
after that,
A process of applying the steel plate to a roll having a diameter of 500 mm or more and 1500 mm or less for 4 passes or more while contacting the roll for 1/4 turn per pass, and applying the steel plate to a roll of 500 mm or more and 1500 mm or less in diameter per pass for 1/4 pass. A second cooling step in which a process is performed in which two or more passes are applied while contacting for two rounds;
The steel plate after the second cooling step is heated to a tempering temperature range of more than 300°C and 500°C or less, and is heated in the temperature range for a tempering time of 20 seconds or more and 900 seconds or less, and During the reheating treatment, a reheating step in which the carbide control parameter CP shown by the following formula (1) is set to 10,000 or more and 15,000 or less,
Alternatively, the method for producing a steel plate further includes a cold rolling process of cold rolling the steel plate after the pickling process and before the annealing process to obtain a cold rolled steel plate.
CP=(T+273)×(k+1.2×logt)...Formula (1)
Here, T: tempering temperature (°C), k: material constant depending on C content, t: tempering time (seconds),
k=-6×C _M +17.8,
_CM : Carbon content (mass%) in martensite produced in the second cooling step.
[8] The steel plate according to [7], which includes a galvanizing process of subjecting the steel plate after the first cooling process and before the second cooling process to form a galvanized layer on the steel plate. manufacturing method.
[9] The method for manufacturing a steel plate according to [7] or [8], wherein the annealing in the annealing step is performed in an atmosphere with a dew point of −30° C. or higher.
[10] [7] to [9] above, including a metal plating step of applying metal plating to form a metal plating layer on one or both sides of the steel plate after the pickling step and before the annealing step. A method for producing a steel plate according to any one of the above.
[11] A method for producing a member, comprising the step of subjecting the steel plate according to any one of [1] to [5] to at least one of forming and joining to produce a member.

According to the present invention, the tensile strength TS is 1180 MPa or more, the high yield stress YS, the excellent press formability (ductility, hole expandability, and bendability), and the rupture resistance property at the time of crushing (bending rupture A steel plate having the following properties is obtained.
In addition, the member made of the steel plate of the present invention has high strength and excellent impact resistance, so it can be extremely advantageously applied to impact energy absorbing members of automobiles.

FIG. 1 is a tissue image taken by SEM to explain tissue identification. FIG. 2-1(a) is a diagram for explaining the V-bending process (primary bending process) in the V-VDA bending test of the example. FIG. 2-1(b) is a diagram for explaining VDA bending (secondary bending) in the V-VDA bending test of the example. FIG. 2-2(c) is a perspective view showing a test piece subjected to V-bending (primary bending) in V-VDA. FIG. 2-2(d) is a perspective view showing a test piece subjected to VDA bending (secondary bending) in V-VDA. FIG. 2-3(e) is a perspective view showing a test piece subjected to VDA bending (secondary bending) in V-VDA and an L cross-sectional observation surface. Figure 2-3(f) shows the change in grain size in the thickness direction of bainitic ferrite before and after processing on the L cross-section observation surface of a test piece subjected to VDA bending (secondary bending) in V-VDA. FIG. 3 is a cross-sectional view showing locations where quantities are measured. FIG. 2-4 is a schematic diagram for explaining the AB area. FIG. 3 is a schematic diagram of the stroke-load curve obtained when performing the V-VDA test. FIG. 4 is a tissue image taken by SEM to explain the measurement of the length of a crack specified in the present invention (Example No. 36 of the present invention). FIG. 5(a) is a structure image taken by SEM to explain the method for measuring the grain size of bainitic ferrite before deformation by processing specified in the present invention (invention example No. 35). FIG. 5(b) is a structure image taken by SEM to explain the method for measuring the grain size of bainitic ferrite after deformation due to processing specified in the present invention (invention example No. 35). FIG. 6-1(a) is a front view of a test member made by spot welding a hat-shaped member and a steel plate, which was manufactured for the axial crush test of the example. FIG. 6-1(b) is a perspective view of the test member shown in FIG. 6-1(a). FIG. 6-2(c) is a schematic diagram for explaining the axial crush test of the example.

The present invention will be explained based on the following embodiments.

[1. steel plate]
The steel plate of the present invention is a steel plate comprising a base steel plate, in which the base steel plate includes, in mass %, C: 0.050% or more and 0.400% or less, Si: more than 0.75% and 3.00% or less, Mn : 2.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.010% or more and 2.000% or less and N: 0.0100% or less, with the remainder consisting of Fe and unavoidable impurities, and the base steel sheet has a bainitic ferrite area ratio of 3.0% or more and 20.0% or less, Area ratio of tempered martensite: 40.0% to 90.0%, area ratio of retained austenite: more than 3.0% to 15.0%, carbon concentration in retained austenite: 0.60% by mass or more1. It has a steel structure of 30% by mass or less, area ratio of fresh martensite: 10.0% or less, density of carbides in tempered martensite: 8.0 pieces/μm ² or less, and diffusible hydrogen of the base steel sheet. The amount is 0.50 mass ppm or less, and the V-VDA bending test is performed up to the maximum load point, and the crack length is 400 μm or less in the L section, and the bending apex on the outside of the VDA bending is 0.50 mass ppm or less. The average grain size of bainitic ferrite in the thickness direction is determined from each position of the starting line that extends up to 50 μm in the thickness direction from Regarding the diameter, the ratio of the average particle size before processing to the average particle size after processing is 5.0 or less, and the tensile strength is 1180 MPa or more.
The steel plate may have a galvanized layer as the outermost layer on one or both sides of the steel plate. The steel sheet having a galvanized layer may be a galvanized steel sheet.

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

C: 0.050% or more and 0.400% or less C produces appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite, and retained austenite to ensure tensile strength TS of 1180 MPa or more and high YS. It is an effective element for Here, if the C content is less than 0.050%, the area ratio of ferrite increases, making it difficult to increase the TS to 1180 MPa or more. It also causes a decrease in YS.
On the other hand, when the C content exceeds 0.400%, the carbon concentration in retained austenite increases excessively. Therefore, when a steel plate is subjected to a punching process in a hole expansion test or a V-bending process in a V-VDA test, the hardness of fresh martensite generated by process-induced transformation increases significantly, and the subsequent formation of voids and Crack propagation is promoted and the desired λ and S _Fmax cannot be achieved.
Therefore, the C content is set to 0.050% or more and 0.400% or less. The C content is preferably 0.100% or more. Further, the C content is preferably 0.300% or less.

Si: More than 0.75% and 3.00% or less Si suppresses the formation of carbides during cooling and holding after annealing and promotes the formation of retained austenite. That is, Si is an element that affects the volume fraction of retained austenite and the carbon concentration in retained austenite. Here, if the Si content is 0.75% or less, the volume fraction of retained austenite decreases and ductility decreases.
On the other hand, when the Si content exceeds 3.00%, the area ratio of ferrite increases excessively, making it difficult to increase the TS to 1180 MPa or more. It also causes a decrease in YS. In addition, the carbon concentration in the austenite during annealing increases too much and the desired λ and _SFmax cannot be achieved.
Therefore, the Si content is set to more than 0.75% and 3.00% or less. The Si content is preferably 2.00% or less.

Mn: 2.00% or more and less than 3.50% Mn is an element that adjusts the area ratio of bainitic ferrite, tempered martensite, and the like. Here, if the Mn content is less than 2.00%, the area ratio of ferrite increases excessively, making it difficult to increase the TS to 1180 MPa or more. It also causes a decrease in YS.
On the other hand, when the Mn content is 3.50% or more, the martensite transformation start temperature Ms (hereinafter also simply referred to as the Ms point or Ms) decreases, and the martensite generated in the second cooling step decreases. As a result, martensite generated during final cooling increases, martensite generated at that time is not sufficiently tempered, and the area ratio of hard fresh martensite increases. Fresh martensite becomes the starting point of void generation during the hole expansion test, VDA bending test, or V-VDA bending test, and if the area ratio of fresh martensite exceeds 10%, the desired λ, α, and S _Fmax cannot be achieved. .
Therefore, the Mn content is set to 2.00% or more and less than 3.50%. The Mn content is preferably 2.50% or more. Further, the Mn content is preferably 3.20% or less.

P: 0.001% or more and 0.100% or less P is an element that has a solid solution strengthening effect and increases the TS and YS of the steel sheet. In order to obtain such an effect, the P content is set to 0.001% or more. On the other hand, when the P content exceeds 0.100%, P segregates at prior austenite grain boundaries and embrittles the grain boundaries. Therefore, after punching a steel plate or V-bending in a V-VDA bending test, the amount of voids increases, making it impossible to achieve the desired λ and S _Fmax .
Therefore, the P content is set to 0.001% or more and 0.100% or less. The P content is preferably 0.030% or less.

S: 0.0001% or more and 0.0200% or less S exists as a sulfide in steel. In particular, when the S content exceeds 0.0200%, the amount of voids increases after punching a steel plate or V-bending in a V-VDA bending test, and the desired λ and S Unable to achieve _Fmax .
Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0080% or less. Note that the lower limit of the S content is set to 0.0001% or more due to constraints on production technology.

Al: 0.010% or more and 2.000% or less Al suppresses the formation of carbides during cooling and holding after annealing, and also promotes the formation of retained austenite. That is, Al is an element that affects the volume fraction of retained austenite and the carbon concentration in retained austenite. In order to obtain such an effect, the Al content is set to 0.010% or more.
On the other hand, when the Al content exceeds 2.000%, the area ratio of ferrite increases excessively, making it difficult to increase the TS to 1180 MPa or more. It also causes a decrease in YS. In addition, the C concentration in the austenite during annealing increases too much, making it impossible to achieve the desired λ and _SFmax .
Therefore, the Al content is set to 0.010% or more and 2.000% or less. Al content is preferably 0.015% or more. Further, the Al content is preferably 1.000% or less.

N: 0.0100% or less N exists as a nitride in steel. In particular, when the N content exceeds 0.0100%, the amount of voids generated increases after punching a steel plate or V-bending in a V-VDA bending test, and the desired λ and S Unable to achieve _Fmax .
Therefore, the N content is set to 0.0100% or less. Further, the N content is preferably 0.0050% or less. Note that, although the lower limit of the N content is not particularly specified, it is preferable that the N content is 0.0005% or more due to constraints on production technology.

The basic component composition of the base steel plate of the steel plate according to one embodiment of the present invention has been described above. The base steel plate of the steel plate according to one embodiment of the present invention contains the above basic components, and the remainder other than the above basic components is Fe. (iron) and unavoidable impurities. Here, it is preferable that the base steel sheet of the steel sheet according to one embodiment of the present invention contains the above-mentioned basic components, with the remainder consisting of Fe and inevitable impurities.

In addition to the above-mentioned basic components, the base steel sheet of the steel sheet according to an embodiment of the present invention may contain at least one selected from the following optional components. Note that the effects of the present invention can be obtained for the optional components shown below as long as they are contained in amounts below the upper limit shown below, so there is no particular lower limit set. In addition, when the following arbitrary elements are contained below the preferable lower limit value mentioned later, the said elements shall be contained as an unavoidable impurity.

Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0. 0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% Below, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, At least one type selected from Bi: 0.0200% or less and REM: 0.0200% or less

Nb: 0.200% or less Nb increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. In order to obtain such an effect, it is preferable that the Nb content is 0.001% or more. The Nb content is more preferably 0.005% or more. On the other hand, if the Nb content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be produced. In such a case, coarse precipitates and inclusions become the starting point of cracks during the hole expansion test, VDA bending test, or V-VDA bending test, and the desired λ, α, and S _Fmax may not be achieved. Therefore, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.

Ti: 0.200% or less Like Nb, Ti increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. In order to obtain such an effect, it is preferable that the Ti content is 0.001% or more. The Ti content is more preferably 0.005% or more. On the other hand, if the Ti content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be generated. In such a case, coarse precipitates and inclusions become the starting point of cracks during the hole expansion test, VDA bending test, or V-VDA bending test, and the desired λ, α, and S _Fmax may not be achieved. Therefore, when containing Ti, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.

V: 0.200% or less Like Nb and Ti, V increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. In order to obtain such an effect, it is preferable that the V content is 0.001% or more. The V content is more preferably 0.005% or more. The V content is more preferably 0.010% or more, and even more preferably 0.030% or more. On the other hand, when the V content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be generated. In such a case, coarse precipitates and inclusions become the starting point of cracks during the hole expansion test, VDA bending test, or V-VDA bending test, and the desired λ, α, and S _Fmax may not be achieved. Therefore, when V is contained, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.

B: 0.0100% or less B is an element that improves hardenability by segregating at austenite grain boundaries. Further, B is an element that suppresses the formation of ferrite and grain growth during cooling after annealing. In order to obtain such an effect, it is preferable that the B content is 0.0001% or more. The B content is more preferably 0.0002% or more.
The B content is more preferably 0.0005% or more, and even more preferably 0.0007% or more.
On the other hand, if the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling. Further, after punching a steel plate or V-bending in a V-VDA bending test, the amount of voids generated increases, and there is a possibility that the desired λ and S _Fmax cannot be achieved.
Therefore, when B is included, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less.

Cr: 1.000% or less Since Cr is an element that increases hardenability, a large amount of tempered martensite is generated by adding Cr, and a TS of 1180 MPa or more and a high YS can be ensured. In order to obtain such effects, the Cr content is preferably 0.0005% or more. Further, the Cr content is more preferably 0.010% or more.
The Cr content is more preferably 0.030% or more, and even more preferably 0.050% or more.
On the other hand, when the Cr content exceeds 1.000%, the area ratio of hard fresh martensite increases excessively, and fresh martensite becomes the starting point for void formation in the hole expansion test, VDA bending test, or V-VDA bending test. Therefore, there is a possibility that the desired λ, α, and S _Fmax cannot be achieved. Therefore, when Cr is contained, the Cr content is preferably 1.000% or less. Further, the Cr content is more preferably 0.800% or less, still more preferably 0.700% or less.

Ni: 1.000% or less Ni is an element that improves hardenability, so adding Ni generates a large amount of tempered martensite, ensuring a TS of 1180 MPa or more and a high YS. In order to obtain such an effect, it is preferable that the Ni content be 0.005% or more. The Ni content is more preferably 0.020% or more. The Ni content is more preferably 0.040% or more, and even more preferably 0.060% or more.
On the other hand, when the Ni content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and fresh martensite becomes the starting point for void formation in the hole expansion test, VDA bending test, or V-VDA bending test. , the desired λ, α, and S _Fmax may not be achieved. Therefore, when Ni is contained, the Ni content is preferably 1.000% or less. The Ni content is more preferably 0.800% or less.
The Ni content is more preferably 0.600% or less, and even more preferably 0.400% or less.

Mo: 1.000% or less Mo is an element that improves hardenability, so adding Mo generates a large amount of tempered martensite, ensuring a TS of 1180 MPa or more and a high YS. In order to obtain such an effect, it is preferable that the Mo content is 0.010% or more. Mo content is more preferably 0.030% or more.
On the other hand, when the Mo content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and fresh martensite becomes the starting point for void formation in the hole expansion test, VDA bending test, or V-VDA bending test. There is a possibility that the desired λ, α and S _Fmax cannot be achieved. Therefore, when Mo is contained, the Mo content is preferably 1.000% or less. The Mo content is more preferably 0.500% or less, still more preferably 0.450% or less, even more preferably 0.400% or less. The Mo content is more preferably 0.350% or less, and even more preferably 0.300% or less.

Sb: 0.200% or less Sb is an effective element for suppressing the diffusion of C near the surface of the steel sheet during annealing and controlling the formation of a soft layer near the surface of the steel sheet. If the soft layer increases excessively near the surface of the steel sheet, it becomes difficult to increase the TS to 1180 MPa or more. It also causes a decrease in YS. Therefore, it is preferable that the Sb content is 0.002% or more. The Sb content is more preferably 0.005% or more.
On the other hand, when the Sb content exceeds 0.200%, a soft layer is not formed near the surface of the steel sheet, which may lead to a decrease in hole expandability and bendability. Therefore, when Sb is contained, the Sb content is preferably 0.200% or less. The Sb content is more preferably 0.020% or less.

Sn: 0.200% or less Like Sb, Sn is an effective element for suppressing the diffusion of C near the surface of the steel sheet during annealing and controlling the formation of a soft layer near the surface of the steel sheet. If the soft layer increases excessively near the surface of the steel sheet, it becomes difficult to increase the TS to 1180 MPa or more. It also causes a decrease in YS. Therefore, it is preferable that the Sn content is 0.002% or more. The Sn content is more preferably 0.005% or more.
On the other hand, if the Sn content exceeds 0.200%, a soft layer will not be formed near the surface of the steel sheet, which may lead to a decrease in hole expandability and bendability. Therefore, when Sn is contained, the Sn content is preferably 0.200% or less. The Sn content is more preferably 0.020% or less.

Cu: 1.000% or less Cu is an element that improves hardenability, so adding Cu generates a large amount of tempered martensite, ensuring a TS of 1180 MPa or more and a high YS. In order to obtain such an effect, it is preferable that the Cu content is 0.005% or more. The Cu content is more preferably 0.008% or more, and even more preferably 0.010% or more. The Cu content is more preferably 0.020% or more.
On the other hand, when the Cu content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and a large amount of coarse precipitates and inclusions may be generated. In such cases, fresh martensite and coarse precipitates and inclusions become starting points for voids and cracks during the hole expansion test, VDA bending test, or V-VDA bending _test . There is a possibility that this cannot be achieved. Therefore, when Cu is contained, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.

Ta: 0.100% or less Like Ti, Nb, and V, Ta increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. In addition, Ta is partially dissolved in Nb carbides and Nb carbonitrides to form composite precipitates such as (Nb, Ta) (C, N). This suppresses coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS and YS. In order to obtain such an effect, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.002% or more, and even more preferably 0.004% or more.
On the other hand, if the Ta content exceeds 0.100%, large amounts of coarse precipitates and inclusions may be produced. In such a case, coarse precipitates and inclusions become the starting point of cracks during the hole expansion test, VDA bending test, or V-VDA bending test, and there is a possibility that the desired λ, α, and S _Fmax cannot be achieved. Therefore, when Ta is contained, the Ta content is preferably 0.100% or less.
The Ta content is more preferably 0.090% or less, and even more preferably 0.080% or less.

W: 0.500% or less W is an element that improves hardenability, so the addition of W generates a large amount of tempered martensite, ensuring a TS of 1180 MPa or more and a high YS. In order to obtain such an effect, it is preferable that the W content is 0.001% or more. The W content is more preferably 0.030% or more.
On the other hand, when the W content exceeds 0.500%, the area ratio of hard fresh martensite increases excessively, and fresh martensite becomes the starting point for void formation in the hole expansion test, VDA bending test, or V-VDA bending test. Therefore, there is a possibility that the desired λ, α, and S _Fmax cannot be achieved. Therefore, when W is contained, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, still more preferably 0.400% or less. It is even more preferable that the W content is 0.300% or less.

Mg: 0.0200% or less Mg is an effective element for making inclusions such as sulfides and oxides spheroidal and improving the hole expandability of the steel sheet. In order to obtain such an effect, it is preferable that the Mg content is 0.0001% or more. The Mg content is more preferably 0.0005% or more, and even more preferably 0.0010% or more.
On the other hand, if the Mg content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such a case, coarse precipitates and inclusions become the starting point of cracks during the hole expansion test, VDA bending test, or V-VDA bending test, and there is a possibility that the desired λ, α, and S _Fmax cannot be achieved. Therefore, when Mg is contained, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

Zn: 0.0200% or less Zn is an effective element for spheroidizing the shape of inclusions and improving the hole expandability of the steel sheet. In order to obtain such an effect, the Zn content is preferably 0.0010% or more. The Zn content is more preferably 0.0020% or more, and even more preferably 0.0030% or more.
On the other hand, if the Zn content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, coarse precipitates and inclusions may become starting points for cracks during the hole expansion test, VDA bending test, or V-VDA bending test, and the desired λ, α, and S _Fmax may not be achieved. Therefore, when Zn is contained, the Zn content is preferably 0.0200% or less. The Zn content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

Co: 0.0200% or less Co, like Zn, is an effective element for spheroidizing the shape of inclusions and improving the hole expandability of the steel sheet. In order to obtain such an effect, the Co content is preferably 0.0010% or more. The Co content is more preferably 0.0020% or more, and even more preferably 0.0030% or more.
On the other hand, if the Co content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such a case, coarse precipitates and inclusions become the starting point of cracks during the hole expansion test, VDA bending test, or V-VDA bending test, and there is a possibility that the desired λ, α, and S _Fmax cannot be achieved. Therefore, when Co is contained, the Co content is preferably 0.0200% or less. The Co content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

Zr: 0.1000% or less Zr, like Zn and Co, is an effective element for making the shape of inclusions spherical and improving the hole expandability of the steel sheet. In order to obtain such an effect, the Zr content is preferably 0.0010% or more. On the other hand, if the Zr content exceeds 0.1000%, large amounts of coarse precipitates and inclusions may be formed. In such a case, coarse precipitates and inclusions become the starting point of cracks during the hole expansion test, VDA bending test, or V-VDA bending test, and the desired λ, α, and S _Fmax may not be achieved. Therefore, when containing Zr, the Zr content is preferably 0.1000% or less.
The Zr content is more preferably 0.0300% or less, and even more preferably 0.0100% or less.

Ca: 0.0200% or less Ca exists as inclusions in steel. Here, if the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be generated. In such a case, coarse precipitates and inclusions become the starting point of cracks during the hole expansion test, VDA bending test, or V-VDA bending test, and there is a possibility that the desired λ, α, and S _Fmax cannot be achieved. Therefore, when Ca is contained, the Ca content is preferably 0.0200% or less. The Ca content is preferably 0.0020% or less.
The Ca content is more preferably 0.0019% or less, and even more preferably 0.0018% or less.
Note that the lower limit of the Ca content is not particularly limited, but the Ca content is preferably 0.0005% or more. Furthermore, due to production technology constraints, the Ca content is more preferably 0.0010% or more.

Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, REM: 0.0200% or less Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all is also an effective element for improving the hole expandability of steel sheets. In order to obtain such an effect, it is preferable that the content of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is each 0.0001% or more.
On the other hand, when the content of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM exceeds 0.0200%, and when the content of As exceeds 0.0500%, coarse A large amount of precipitates and inclusions may be generated. In such cases, coarse precipitates and inclusions may become starting points for cracks during hole expansion tests, VDA bending tests, or V-VDA bending tests, and there is a possibility that desired λ, α, and S _Fmax cannot be achieved. Therefore, when containing at least one of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and The content of REM is preferably 0.0200% or less, and the content of As is preferably 0.0500% or less.
The Se content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Se content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Te content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Te content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Ge content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Ge content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
As for As content, it is more preferred that it is 0.0010% or more, and it is still more preferred that it is 0.0015% or more. As for As content, it is more preferred that it is 0.0400% or less, and it is still more preferred that it is 0.0300% or less.
The Sr content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Sr content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Cs content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Cs content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Hf content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Hf content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Pb content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Pb content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Bi content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Bi content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The REM content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The REM content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
In addition, REM as used in the present invention refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. Refers to lanthanoids. The REM concentration in the present invention is the total content of one or more elements selected from the above-mentioned REMs.
REM is not particularly limited, but preferably Sc, Y, Ce, and La.

That is, the base steel plate of the steel plate according to an embodiment of the present invention has, in mass %, C: 0.050% or more and 0.400% or less, Si: more than 0.75% and 3.00% or less, and Mn: 2.00. % or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.010% or more and 2.000% or less, and N: 0 .0100% or less, optionally Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0. 0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% Hereinafter, a component composition containing at least one selected from Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less, with the remainder being Fe and unavoidable impurities. have

Steel Structure Next, the steel structure of the base steel plate of the steel plate according to one embodiment of the present invention will be described.
The steel structure of the base steel sheet of the steel sheet according to an embodiment of the present invention has an area ratio of bainitic ferrite of 3.0% to 20.0% and an area ratio of tempered martensite (excluding retained austenite) of 40. 0% or more and 90.0% or less, volume fraction of retained austenite: more than 3.0% and 15.0% or less, carbon concentration in retained austenite: 0.60% by mass or more and 1.30% by mass or less, fresh martensite Area ratio: 10.0% or less (including 0.0%), the density of carbides in the tempered martensite is 8.0 pieces/μm ² or less, and the amount of diffusible hydrogen in the base steel sheet is 0.50 mass ppm or less, and furthermore, the V-VDA bending test was performed up to the maximum load point, and the crack length was 400 μm or less in the L cross section, and furthermore, the crack length was 400 μm or less in the VDA bending, and Regarding the average grain size in the plate thickness direction of bainitic ferrite in the regions formed up to 50 μm on both sides of the start line in the vertical direction from each position of the start line that exists up to 50 μm in the process, The ratio of the average particle size before processing to the average particle size is 5.0 or less.

The reasons for each limitation will be explained below.

Area ratio of bainitic ferrite: 3.0% or more and 20.0% or less Bainitic ferrite has an intermediate hardness compared to soft ferrite and hard fresh martensite, and has good hole expandability. , is an important phase to ensure bendability, bending rupture properties and axial crush properties. Bainitic ferrite is also a useful phase for obtaining an appropriate amount of retained austenite area ratio and carbon concentration in retained austenite by utilizing the diffusion of C from bainitic ferrite to untransformed austenite. be. Therefore, the area ratio of bainitic ferrite is set to 3.0% or more.
The area ratio of bainitic ferrite is preferably 5.0% or more, more preferably 8.0% or more.
On the other hand, when the area ratio of bainitic ferrite increases excessively, the strength decreases and the area ratio of retained austenite exceeds a predetermined amount. Therefore, the area ratio of bainitic ferrite is set to 20.0% or less.
The area ratio of bainitic ferrite is preferably 18.0% or less, more preferably 15.0% or less.
Note that bainitic ferrite is upper bainitic bainite that is generated in a relatively high temperature range and has few carbides.

Area ratio of tempered martensite (excluding retained austenite): 40.0% or more and 90.0% or less Tempered martensite has an intermediate hardness compared to soft ferrite and hard fresh martensite. It is an important phase for ensuring good hole expandability, bendability, bending rupture properties, and axial crushing properties. Furthermore, tempered martensite is effective in improving TS. Therefore, the area ratio of tempered martensite is set to 40.0% or more. The area ratio of tempered martensite is preferably 60.0% or more. On the other hand, when the area ratio of tempered martensite increases excessively, ductility decreases. Therefore, the area ratio of tempered martensite is 90.0% or less.
The area ratio of tempered martensite is preferably 85.0% or less, more preferably 80.0% or less.

Area ratio of retained austenite: more than 3.0% and not more than 15.0% From the viewpoint of obtaining good ductility, the area ratio of retained austenite is set to be more than 3.0%. The area ratio of retained austenite is preferably 5.0% or more. On the other hand, if the area ratio of retained austenite increases excessively, fresh martensite generated by deformation-induced transformation occurs when a steel plate undergoes punching in a hole expansion test or V-bending in a V-VDA test. This becomes a starting point for void generation, making it impossible to achieve the desired λ and S _Fmax . Therefore, the area ratio of retained austenite is set to 15.0% or less. The area ratio of retained austenite is preferably 12.0% or less, more preferably 10.0% or less.

For example, the area ratio of retained austenite can be suppressed to 15.0% or less by controlling the tension during the second cooling step in the manufacturing method described below. After the first cooling process (in the case of galvanizing, after galvanizing (if necessary, after alloying)), a tension of 2.0 kgf/ ^mm2 or more is applied at least once, and then the steel plate is is applied for 4 passes or more while contacting a roll with a diameter of 500 mm or more and 1500 mm or less for 1/4 rotation per pass, and a process of applying the steel plate to a roll with a diameter of 500 mm or more and 1500 mm or less per pass for 1/2 rotation of the roll. By performing the treatment for two or more passes while contacting each other for a few minutes, the unstable residual austenite undergoes deformation-induced transformation and becomes fresh martensite, which is then tempered during cooling and finally becomes tempered martensite. .

Carbon concentration in retained austenite: 0.60% by mass or more and 1.30% by mass or less The carbon concentration in retained austenite is an index that affects the stability with which retained austenite transforms into martensite during deformation. If the carbon concentration in the retained austenite is less than 0.60% by mass, the retained austenite is unstable and deformation-induced martensitic transformation occurs after stress is applied but before plastic deformation occurs, making it impossible to obtain the required elongation. On the other hand, if the carbon concentration in the retained austenite exceeds 1.30% by mass, fresh carbon is generated from the retained austenite when the steel plate is punched in the hole expansion test or when it is subjected to V-bending in the V-VDA test. The hardness of martensite increases significantly, the formation and connection of voids is promoted, and the desired λ and _SFmax cannot be achieved. Therefore, the carbon concentration in the retained austenite is set to 0.60% by mass or more and 1.30% by mass or less. The carbon concentration in the retained austenite is preferably 0.80% by mass or more. Further, the carbon concentration in the retained austenite is preferably 1.20% by mass or less.

Fresh martensite area ratio: 10.0% or less (including 0.0%)
If the area ratio of fresh martensite increases excessively, fresh martensite becomes the starting point for void formation in the hole expansion test, VDA bending test, or V-VDA bending test, making it impossible to achieve the desired λ, α, and S _Fmax . Furthermore, as the fresh martensite area ratio increases, the amount of diffusible hydrogen in the steel sheet increases, and the hole expandability and bendability further decrease. From the viewpoint of ensuring good hole expandability and bendability, the area ratio of fresh martensite is 10.0% or less, preferably 5.0% or less. Note that the lower limit of the area ratio of fresh martensite is not particularly limited, and may be 0.0%.
Note that fresh martensite is martensite that is still quenched (not tempered).

Density of carbides in tempered martensite (number density): 8.0 pieces/μm ² or less In the present invention, if the density of carbides in tempered martensite exceeds 8.0 pieces/μm ² , hole expansion test, VDA In the bending test or the V-VDA bending test, the increase in the number of voids due to carbides promotes crack initiation and propagation, and as a result, the desired λ, α and S _Fmax cannot be achieved. Therefore, the density of carbides in tempered martensite is set to 8.0 carbides/μm ² or less. The density of carbides in the tempered martensite is preferably 7.0 pieces/μm ² or less, more preferably 6.0 pieces/μm ² or less.
Although the lower limit is not particularly limited, the density of carbides in the tempered martensite is preferably 1.0 pieces/μm ² or more, more preferably 2.0 pieces/μm ² or more.

Note that examples of residual structures other than those mentioned above include carbides such as ferrite, lower bainite, pearlite, and cementite. In order to ensure a TS of 1180 MPa or more and a high YS, the area ratio of pearlite is preferably 5.0% or less.
The type of residual tissue can be confirmed, for example, by observation using a scanning electron microscope (SEM).

The area ratio of bainitic ferrite, tempered martensite, and hard second phase (retained austenite + fresh martensite) is measured as follows at a position of 1/4 of the thickness of the base steel plate.
That is, a sample is cut out from the base steel plate so that the plate thickness cross section parallel to the rolling direction of the base steel plate serves as the observation surface. Next, the observation surface of the sample is mirror-polished using diamond paste. Then, after final polishing the observation surface of the sample using colloidal silica, 3vol. Etch with % nital to reveal the tissue.
Then, three fields of view of 25.6 μm×17.6 μm of the observation surface of the sample are photographed using an SEM (Scanning Electron Microscope) under the conditions of acceleration voltage: 15 kV and magnification: 5000 times.
From the obtained structure image, bainitic ferrite, tempered martensite, hard second phase (retained austenite + fresh martensite), and residual structure are identified as follows.

Bainitic ferrite: A black to dark gray region, with a lumpy or irregular shape. Also, it does not contain iron-based carbides or contains relatively few iron-based carbides.
Tempered martensite: A gray area with an amorphous shape. It also contains a relatively large number of iron-based carbides.
Hard second phase (retained austenite + fresh martensite): This is a white to light gray region with an amorphous shape. Also, it does not contain iron-based carbides. Note that when the size is relatively large, the color becomes gradually darker as it moves away from the interface with other tissues, and the inside may take on a dark gray color.
Ferrite: A black region with a block-like shape. In addition, it contains almost no iron-based carbide. However, when iron-based carbide is included, the area of the iron-based carbide is also included in the area of ferrite. The same applies to the aforementioned bainitic ferrite and tempered martensite.
Cementite: A white region with a dotted or linear shape. Enclosed in tempered martensite, bainitic ferrite, and ferrite.
Lower bainite, pearlite, etc.: Their forms are known.

Next, the region of each phase identified in the tissue image is calculated using the following method. On the SEM image with a magnification of 5,000 times, a 20 x 20 grid with equal spacing is placed over an area of actual length 23.1 μm x 17.6 μm, and bainitic processing is performed using a point counting method that counts the number of points on each phase. Calculate the area percentages of ferrite, tempered martensite, and hard second phase. The area ratio is the average value of three area ratios obtained from separate SEM images with a magnification of 5000 times.

Moreover, the area ratio of retained austenite is measured as follows.
That is, the base steel plate is mechanically ground in the thickness direction (depth direction) to a position of 1/4 of the plate thickness, and then chemically polished with oxalic acid to form an observation surface. Then, the observation surface is observed by X-ray diffraction. MoKα rays were used for the incident X-rays, and the diffraction intensity of the (200), (211) and (220) planes of BCC iron was compared with the (200), (220) and (311) planes of FCC iron (austenite). The ratio of the diffraction intensities of each surface is determined, and the volume fraction of retained austenite is calculated from the ratio of the diffraction intensities of each surface. Then, assuming that the retained austenite is three-dimensionally homogeneous, the volume fraction of the retained austenite is defined as the area fraction of the retained austenite.

Regarding the distribution of carbon concentration in the retained austenite, the lattice constant of the retained austenite is determined using the diffraction peak of the (220) plane of FCC iron (austenite) measured by the above-mentioned X-ray diffraction method. Next, the carbon concentration in the retained austenite is determined from the following formula.
Cγ=((A-(3.572+0.0012×[Mn%]-0.00157×[Si%]+0.0056×[Al%]))/0.033
Here, A: lattice constant of retained austenite,
Cγ: carbon concentration in retained austenite; [Mn%], [Si%], [Al%]: content (mass%) of Mn, Si, and Al in the steel sheet, respectively.

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

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

The density of carbides in tempered martensite is measured as follows.
Tempered martensite and carbide are extracted by color from the SEM microstructure image used for the above-mentioned microstructure fraction measurement by hand painting, and an image of only the tempered martensite or carbide is obtained. Here, carbides with a diameter (circular equivalent diameter) of 100 nm or more are targeted. Thereafter, the area of the tempered martensite and the number of carbides in the tempered martensite are determined using open source ImageJ. The value obtained by dividing the number of carbides in tempered martensite by the area of the tempered martensite is the density of carbides in tempered martensite, and is averaged by randomly extracting 10 tempered martensite from separate SEM images. Let the value be the density of carbides in tempered martensite.
In addition, regarding one carbide, in the SEM image, a granular region whose outer periphery is surrounded by tempered martensite and is integrally formed without interruption is measured as one.

Amount of diffusible hydrogen: 0.50 mass ppm or less From the viewpoint of obtaining better hole expandability and bendability, the amount of diffusible hydrogen in the base steel sheet is preferably 0.50 mass ppm or less. Further, the amount of diffusible hydrogen in the base steel sheet is more preferably 0.30 mass ppm or less. Note that the lower limit of the amount of diffusible hydrogen in the base steel sheet is not particularly specified, and may be 0 mass ppm. Further, due to constraints on production technology, the amount of diffusible hydrogen in the base steel sheet is more preferably 0.01 mass ppm or more.

Here, the amount of diffusible hydrogen in the base steel sheet is measured as follows.
A test piece with a length of 30 mm and a width of 5 mm is taken from a steel plate, and if a galvanized layer is formed on the steel plate, the galvanized layer is removed with alkali. Next, the amount of hydrogen released from the test piece is measured by temperature programmed desorption analysis. Specifically, the test piece is continuously heated from room temperature (-5 to 55°C) to 300°C at a heating rate of 200°C/h, and then cooled to room temperature. At this time, the amount of hydrogen released from the test piece (cumulative amount of hydrogen) is measured in the temperature range from room temperature to 210° C. during the continuous heating. Then, the measured amount of hydrogen is divided by the mass of the test piece (the test piece after removing the galvanized layer and before continuous heating), and the value converted to mass ppm is taken as the amount of diffusible hydrogen in the base steel sheet. It is preferable to measure the amount of diffusible hydrogen after the production of the steel sheet is completed. It is more preferable to measure the amount of hydrogen within one week after the completion of manufacturing the steel sheet.
In addition, the room temperature should be within the range of local temperature changes over a one-year period, taking into account production in various countries around the world. Generally, the temperature is preferably in the range of 10 to 50°C.

For products (components) after forming or joining steel plates, test pieces are cut out from the product in a general usage environment and the amount of diffusible hydrogen in the base steel plate is measured in the same manner as above. is measured, and if the value is 0.50 mass ppm or less, it can be considered that the amount of diffusible hydrogen in the base steel sheet of the steel sheet at the raw material stage before forming or joining processing is also 0.50 mass ppm or less.

Soft Surface Layer The base steel sheet of the steel plate according to one embodiment of the present invention preferably has a soft surface layer on the surface of the base steel sheet. The soft surface layer contributes to suppressing the propagation of bending cracks during press molding and car body collisions, further improving the bending fracture resistance. Note that the surface soft layer means a decarburized layer, and is a surface layer region having a Vickers hardness of 85% or less of the Vickers hardness of the cross section at the 1/4 thickness position.
Here, the surface soft layer is formed in an area of 200 μm or less in the thickness direction from the surface of the base steel sheet. The area where the surface soft layer is formed is preferably 150 μm or less, more preferably 120 μm or less in the thickness direction from the base steel plate surface. Although the lower limit of the thickness of the soft surface layer is not particularly determined, the thickness of the soft surface layer is preferably 8 μm or more, more preferably 11 μm or more. Moreover, the surface soft layer preferably has a thickness of 30 μm or more, more preferably 40 μm or more.
Further, the position of 1/4 of the thickness of the base steel plate at which the Vickers hardness is measured is a non-surface soft layer (a layer that does not satisfy the hardness conditions of the surface soft layer defined in the present invention).
Vickers hardness is measured based on JIS Z 2244-1 (2020) with a load of 10 gf.

Nano-hardness of surface soft layer 300 points in an area of 50 μm x 50 μm on the plate surface at 1/4 position and 1/2 depth in the thickness direction of the surface soft layer from the surface of the base steel plate, respectively. When the above nanohardness was measured, the proportion of measurements where the nanohardness of the plate surface at 1/4 of the depth in the thickness direction of the surface soft layer from the surface of the base steel sheet was 7.0 GPa or more was determined by the thickness of the surface soft layer. 0.10 or less for all measurements at 1/4 position of depth In the present invention, in order to obtain excellent bending properties during press forming and excellent bending rupture properties during collision, it is necessary to When nanohardness was measured at 300 points or more in an area of 50 μm x 50 μm on the board surface at 1/4 depth in the thickness direction and 1/2 depth in the thickness direction of the surface soft layer, The proportion of measurements where the nanohardness of the plate surface at 1/4 of the depth in the thickness direction of the surface soft layer from the steel plate surface is 7.0 GPa or more is It is preferable that it is 0.10 or less with respect to the number of measurements. If the ratio of nanohardness of 7.0 GPa or more is 0.10 or less, it means that the ratio of hard structures (such as martensite), inclusions, etc. is small; It becomes possible to further suppress the generation and connection of voids during press molding and collision, as well as the propagation of cracks, resulting in excellent R/t, α, and _SFmax .

The standard deviation σ of the nano-hardness of the plate surface at a position 1/4 of the depth in the thickness direction of the surface soft layer from the steel plate surface is 1.8 GPa or less, and The standard deviation σ of the nanohardness of the plate surface at the 1/2 position is 2.2 GPa or less. In the present invention, in order to obtain excellent bendability during press forming and excellent bending rupture properties during collision, it is necessary to The standard deviation σ of the nano-hardness of the plate surface at 1/4 of the depth in the thickness direction of the soft layer is 1.8 GPa or less, and furthermore, the standard deviation σ of the nanohardness of the plate surface at 1/4 of the depth in the thickness direction of the surface soft layer from the steel plate surface is 1/2 of the depth in the thickness direction of the surface soft layer. It is preferable that the standard deviation σ of the nanohardness of the plate surface is 2.2 GPa or less. The standard deviation σ of the nano-hardness of the plate surface at a position 1/4 of the depth in the thickness direction of the surface soft layer from the steel plate surface is 1.8 GPa or less, and If the standard deviation σ of the nanohardness of the plate surface at the 1/2 position is 2.2 GPa or less, it means that the difference in the structure hardness in the micro region is small. It becomes possible to further suppress the generation and connection of voids and the propagation of cracks during a collision, resulting in excellent R/t, α, and _SFmax .
The preferred range of the standard deviation σ of the nano-hardness of the plate surface at a position 1/4 of the thickness direction depth of the surface soft layer from the base steel plate surface is preferably 1.7 GPa or less. The standard deviation σ of the nanohardness of the plate surface at a position of 1/4 of the thickness direction depth of the surface soft layer from the base steel plate surface is more preferably 1.3 GPa or less. Although the lower limit is not particularly limited, the standard deviation σ of the nanohardness of the plate surface at a position 1/4 of the thickness direction depth of the surface soft layer from the base steel plate surface may be 0.5 GPa or more.
A more preferable range of the standard deviation σ of the nano-hardness of the plate surface at 1/2 the thickness direction depth of the surface soft layer from the base steel plate surface is 2.1 GPa or less. The standard deviation σ of the nanohardness of the plate surface at a position 1/2 the thickness direction depth of the surface soft layer from the base steel plate surface is more preferably 1.7 GPa or less. Although the lower limit is not particularly limited, the standard deviation σ of the nanohardness of the plate surface at 1/2 the thickness direction depth of the surface soft layer from the base steel plate surface may be 0.6 GPa or more.

Here, the nanohardness of the plate surface at the 1/4 position and 1/2 position of the depth in the thickness direction is the hardness measured by the following method.
If a plating layer is formed, after peeling off the plating layer, mechanical polishing is performed from the surface of the base steel sheet to a position 1/4 of the depth in the thickness direction of the surface soft layer - 5 μm, and the base steel sheet is polished. Buff polishing with diamond and alumina is performed from the surface to 1/4 of the depth in the thickness direction of the surface soft layer, and further polishing with colloidal silica is performed. Here, the plating layer to be peeled off is a galvanized layer if a galvanized layer is formed, a metal plating layer if a metal plating layer is formed, and a galvanized layer and a metal plating layer. If it is formed, it is a zinc plating layer and a metal plating layer.
Nanohardness is measured using Hysitron's tribo-950 with a Berkovich-shaped diamond indenter under the conditions of load: 500 μN, measurement area: 50 μm×50 μm, and dot spacing: 2 μm.
Next, mechanical polishing, buff polishing with diamond and alumina, and colloidal silica polishing are performed to 1/2 the depth of the surface soft layer in the thickness direction. Nanohardness is measured using Hysitron's tribo-950 with a Berkovich-shaped diamond indenter under the conditions of load: 500 μN, measurement area: 50 μm×50 μm, and dot spacing: 2 μm.
Nanohardness is measured at 300 or more points at a position of 1/4 of the depth in the thickness direction, and nanohardness at 300 or more points is also measured at a position of 1/2 of the depth in the thickness direction.
For example, when the soft surface layer thickness is 100 μm, the 1/4 position is 25 μm from the surface of the soft surface layer, and the 1/2 position is 50 μm from the surface of the soft surface layer. Nanohardness is measured at 300 or more points at this 25 μm position, and nanohardness at 300 or more points is also measured at the 50 μm position.

Metal plating layer (first plating layer)
Furthermore, the steel sheet according to an embodiment of the present invention has a metal plating layer (first plating layer, pre-plating layer) (in addition, a metal plating layer (first plating layer) on one or both surfaces of the base steel sheet). , a hot-dip galvanized layer, and an alloyed hot-dip galvanized layer (excluding the galvanized layer). The metal plating layer is preferably a metal electroplating layer, and below, the metal electroplating layer will be explained as an example.
By forming the metal electroplating layer on the surface of the steel sheet, the metal electroplating layer on the outermost layer contributes to suppressing the occurrence of bending cracks during press forming and when a vehicle body collides, so that the bending rupture resistance is further improved.
In the present invention, by setting the dew point to more than -5°C, the thickness of the soft layer can be increased, and the axial crushing properties can be made very excellent. In this regard, in the present invention, by having a metal plating layer, the dew point can be set to −5° C. or less, and even if the soft layer thickness is small, the same axial crushing characteristics as when the soft layer thickness is large can be obtained.

The metal species of the metal electroplating layer include Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Rt, Any of Au, Hg, Ti, Pb, and Bi may be used, but Fe is more preferable. In the following description, an Fe-based electroplated layer will be explained as an example, but the following conditions for Fe can be similarly adopted for other metal types.

The amount of the Fe-based electroplated layer deposited is more than 0 g/m ² , preferably 2.0 g/m ² or more. The upper limit of the amount of the Fe-based electroplated layer per side is not particularly limited, but from the viewpoint of cost, it is preferable that the amount of the Fe-based electroplated layer applied per side is 60 g/m ² or less. The amount of the Fe-based electroplated layer deposited is preferably 50 g/m ² or less, more preferably 40 g/m ² or less, and even more preferably 30 g/m ² or less.

The adhesion amount of the Fe-based electroplating layer is measured as follows. A sample with a size of 10 x 15 mm is taken from a Fe-based electroplated steel plate and embedded in resin to form a cross-sectional embedded sample. Three arbitrary points on the same cross section were observed using a scanning electron microscope (SEM) at an accelerating voltage of 15 kV and a magnification of 2,000 to 10,000 times depending on the thickness of the Fe-based plating layer. By multiplying the average value by the specific gravity of iron, it is converted into the amount of Fe-based electroplating layer deposited on one side.

In addition to pure Fe, Fe-based electroplating layers include Fe-B alloy, Fe-C alloy, Fe-P alloy, Fe-N alloy, Fe-O alloy, Fe-Ni alloy, Fe-Mn alloy, Fe- An alloy plating layer such as Mo alloy or Fe-W alloy can be used. The composition of the Fe-based electroplated layer is not particularly limited, but 1 selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co. Alternatively, it is preferable that the composition contains two or more elements in a total of 10% by mass or less, with the remainder consisting of Fe and unavoidable impurities. By controlling the total amount of elements other than Fe to 10% by mass or less, a decrease in electrolytic efficiency can be prevented and an Fe-based electroplated layer can be formed at low cost. In the case of Fe--C alloy, the C content is preferably 0.08% by mass or less.

It is more preferable to have a soft surface layer under the Fe-based electroplated layer, whereby the bending resistance to breakage can be significantly improved. When having an Fe-based electroplated layer, measure the Vickers hardness distribution from the interface between the Fe-based electroplated layer and the base steel sheet in the thickness direction using the method described above, and evaluate the depth of the surface soft layer in the thickness direction. do.

A V-VDA bending test was performed up to the maximum load point, and the crack length in the L cross section was 400 μm or less.In the present invention, the above-mentioned steel plate with a crack length exceeding 400 μm is susceptible to the formation and propagation of voids in the steel plate structure. The bending resistance to breakage decreases quickly. Therefore, the above crack length is set to 400 μm or less. The above crack length is preferably 300 μm or less, more preferably 200 μm or less. Although the lower limit is not particularly limited, this value may be 0 μm.

The V-VDA bending test was performed up to the maximum load point, and on the L cross section, from each position of the starting line that existed up to a position of 50 μm in the plate thickness direction, starting from the bending apex on the outside of the VDA bending, on both sides of the starting line in the vertical direction Regarding the average grain size in the plate thickness direction of bainitic ferrite, the ratio of the average grain size before processing to the average grain size after processing (the average grain size before and after processing ): 5.0 or less Next, the amount of change in the grain size in the plate thickness direction of the above-mentioned bainitic ferrite before and after processing will be explained.
In FIG. 1, the symbol BF indicates bainitic ferrite, the symbol F indicates ferrite, and the symbol TM indicates tempered martensite. Moreover, in FIG. 1, θ(TM) indicates a carbide in tempered martensite, H1 indicates a hard second phase, and X1(BF) indicates an island-like second phase in bainitic ferrite.
As shown in FIG. 1, island-like retained austenite is formed inside the bainitic ferrite BF in the steel sheet structure due to carbon distribution. When the bainitic ferrite BF is deformed by processing, voids are likely to occur at the boundaries between the bainitic ferrite BF and the hard fresh martensite generated by the processing-induced transformation between the bainitic ferrite BF and the island-like residual austenite. If the amount of change in the average grain size in the thickness direction of the bainitic ferrite BF before and after processing exceeds 5.0, the bainitic ferrite BF will be subjected to tensile stress in the rolling direction, resulting in an increase in the number of voids. The initiation and propagation of cracks is promoted, resulting in a decrease in bending rupture resistance. Therefore, the amount of change in the average grain size in the plate thickness direction of the bainitic ferrite before and after processing is set to be 5.0 or less. The amount of change is preferably 4.8 or less, more preferably 4.5 or less.
In addition, if the amount of change in the average grain size of bainitic ferrite before and after processing is less than 0.2, in bainitic ferrite subjected to compressive stress, island-like residual austenite inside the bainitic ferrite will be formed. There is a possibility that voids are likely to be generated at the boundaries of hard fresh martensite generated by deformation-induced transformation, and the bending rupture resistance may deteriorate. Therefore, the amount of change in the average grain size of the bainitic ferrite in the plate thickness direction before and after processing is preferably 0.2 or more. The amount of change is preferably 0.3 or more, more preferably 0.5 or more.

The above V-VDA bending test is conducted as follows.
A 60 mm x 65 mm test piece is taken from the obtained steel plate by shearing. Here, the 60 mm side is parallel to the rolling (L) direction. A test piece is prepared by performing a 90° bending process (primary bending process) in the rolling (L) direction with the width (C) direction as an axis at a radius of curvature/plate thickness of 4.2. In the 90° bending process (primary bending process), as shown in FIG. 2-1(a), a punch B1 is pushed into a steel plate placed on a die A1 having a V groove to obtain a test piece T1. Next, as shown in FIG. 2-1(b), the punch B2 is pushed into the test piece T1 placed on the support roll A2 so that the bending direction is perpendicular to the rolling direction. (secondary bending). In FIGS. 2-1(a) and 2-1(b), the symbol D1 indicates the width (C) direction, and the symbol D2 indicates the rolling (L) direction.
FIG. 3 shows a schematic diagram of the stroke-load curve obtained when performing the V-VDA test. The V-VDA test was performed up to the maximum load point P, and then, when the load reached 94.9 to 99.9% of the maximum load (see symbol R in Figure 3), the sample was unloaded and subjected to V-VDA bending. This will be used as an evaluation sample in the test.

In the V-VDA bending test, a test piece T1 obtained by subjecting a steel plate to V-bending (primary bending) is shown in FIG. 2-2(c). Further, a test piece T2 obtained by subjecting the test piece T1 to VDA bending (secondary bending) is shown in FIG. 2-2(d). The position indicated by the broken line in test piece T2 in Figure 2-2(d) is the above-mentioned V-bending ridgeline, and the position indicated by the broken line in test piece T1 in Figure 2-2(c) before VDA bending. corresponds to the position. In the present invention, the overlapping region of the V-bending ridgeline and the VDA-bending ridgeline is the center position of the VDA bending apex and the broken line shown as a in FIG. 2-2(d).
Specifically, the V-bending ridgeline portion refers to a region extending in the width direction from the V-bending corner (apex) to 5 mm on both sides. Further, the area other than the V-bending ridgeline portion is a V-bending flat portion.
Further, the VDA bending ridgeline portion refers to an area extending 5 mm on both sides from the VDA bending corner (apex) that is subjected to VDA bending and extends in the rolling direction.
The positional relationship between the L cross section AL of the V bending ridgeline part and the VDA bending ridgeline part and the test piece T2 is shown by the dotted line in FIG. 2-3(e). FIG. 2-3(f) shows the L cross section AL when the D2 direction is perpendicular to the paper surface and the D1 direction is parallel to the paper surface.

The V-VDA bending test is performed up to the maximum load point, and the length of the crack in the L cross section (hereinafter also referred to as the AL plane) in the overlapping region of the V bending ridgeline and the VDA bending ridgeline is determined as follows.
A sample is cut out from the base steel plate so that the AL surface of the steel plate subjected to the V-VDA bending test up to the maximum load point becomes the observation surface. Next, the observation surface of the sample is mirror-polished using diamond paste. Then, after final polishing the observation surface of the sample using colloidal silica, 3vol. Etch with % nital to reveal the tissue.
Then, using an SEM (Scanning Electron Microscope), the axis of symmetry of the AL plane was perpendicular to the bending apex of the observation surface of the sample under the conditions of accelerating voltage: 15 kV and magnification: 200 times. A field of view of 25.6 μm x 17.6 μm is photographed to observe the entire appearance of the crack.
In the obtained crack image, the vertical distance between the crack starting point and ending point is defined as the crack length. Figure 4 shows an example of an image of a crack that was actually measured. In FIG. 4, the symbol D2 indicates the rolling (L) direction, and the symbol D4 indicates the plate thickness direction. Further, the symbol L indicates the length of the crack.

Next, a V-VDA bending test was performed up to the maximum load point, and in the L cross section in the overlapping region of the V bending ridgeline and the VDA bending ridgeline, the bending peak was 0 to 50 μm from the steel plate surface on the outside of the VDA bending, and The amount of change in the average grain size in the plate thickness direction of bainitic ferrite before and after processing in the area 50 μm left and right from the center (AB area shown by the dotted line in Figure 2-3 (f), hereinafter also referred to as AB area) Explain the measurement method.
First, the AB area will be explained with reference to FIG. 2-4. FIG. 2-4 is a schematic diagram for explaining the AB area. As shown in _FIG . 2-4, the AB region starts from the bending apex t ₀ on the outside of VDA bending and extends from each position of the starting line L ₀ that extends to a position of 50 μm in the plate thickness direction. This refers to a region formed at a distance of up to 50 μm on each side in the vertical direction.
As a method for measuring the above change, first, after performing the above V-VDA bending test up to the maximum load point, a sample with the AL surface as the observation surface (hereinafter also referred to as the sample after deformation) and the above steel sheet structure were used. Using the sample used to measure the area ratio (hereinafter also referred to as the sample before deformation), 25. Five visual fields of 6 μm x 17.6 μm are photographed for each sample. In the sample after deformation, the inside of the AB region is photographed to observe the bainitic ferrite that has undergone processing and deformed (hereinafter also referred to as bainitic ferrite after deformation). In the sample before deformation, a photograph is taken from the surface of the base steel plate to a position where the plate thickness is 50 μm, and undeformed bainitic ferrite (hereinafter also referred to as bainitic ferrite before deformation) is observed.
From the obtained tissue image, 10 pieces of bainitic ferrite after deformation and 10 pieces of bainitic ferrite before deformation were randomly extracted by hand painting, and the longest point in the thickness direction of each bainitic ferrite was Particle size.
The grain sizes in the thickness direction of the obtained 10 bainitic ferrites after deformation and 10 bainitic ferrites before deformation were averaged, and the average grain size in the thickness direction of the bainitic ferrite before deformation was calculated. The value obtained by dividing the diameter by the average grain size in the thickness direction of the bainitic ferrite after deformation (ratio of the average grain size before processing to the average grain size after processing: average grain size before processing (nm) / after processing Let the average grain size (nm) of bainitic ferrite be the amount of change in the average grain size in the thickness direction of the bainitic ferrite before and after processing.
FIG. 5 shows an example of images of bainitic ferrite before deformation and bainitic ferrite after deformation. In FIG. 5, the symbol BF1 indicates bainitic ferrite before deformation, and the symbol BF2 indicates bainitic ferrite after deformation.
Regarding one piece of bainitic ferrite, in the SEM image, a granular region whose outer periphery is surrounded by other structures and is integrally formed without interruption is measured as one piece.

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

Tensile strength (TS): 1180 MPa or more The tensile strength TS of the steel plate according to one embodiment of the present invention is 1180 MPa or more. Although the upper limit is not particularly defined, the tensile strength TS is preferably less than 1470 MPa.
In addition, the yield stress (YS), total elongation (El), critical hole expansion rate (λ), critical bending angle (α) in the VDA bending test, and critical bending angle (α) in the V-VDA bending test of the steel plate according to an embodiment of the present invention are The reference value of the stroke at maximum load (S _Fmax ) and the presence or absence of axial crush fracture are as described above.

Moreover, tensile strength (TS), yield stress (YS), and total elongation (El) are measured by a tensile test based on JIS Z 2241 (2011), which will be described later in Examples. The critical hole expansion rate (λ) is measured by a hole expansion test based on JIS Z 2256 (2020), which will be described later in Examples. The limit bending angle (α) in the VDA bending test is measured by the VDA bending test in accordance with VDA238-100, which will be described later in Examples. The stroke at maximum load (S _Fmax ) in the V-VDA bending test is measured by the V-VDA bending test described later in the Examples. The presence or absence of axial crush fracture is determined by the axial crush test described later in Examples.

Galvanized layer (second plating layer)
The steel sheet according to an embodiment of the present invention has a galvanized layer formed on the base steel sheet (on the surface of the base steel sheet or on the surface of the metal plating layer if a metal plating layer is formed) as the outermost layer. Alternatively, this galvanized layer may be provided only on one surface of the base steel sheet, or may be provided on both surfaces.
That is, the steel sheet of the present invention has a base steel plate, and a second plating layer (galvanized layer) may be formed on the base steel plate, and also has a base steel plate, and may have a metal plating layer on the base steel plate. The layer (first plating layer (excluding the second plating layer of the galvanized layer)) and the second plating layer (zinc plating layer) may be formed in this order.
The steel sheet having a galvanized layer may be a galvanized steel sheet.

Note that the galvanized layer here refers to a plating layer containing Zn as a main component (Zn content is 50.0% or more), and includes, for example, a hot-dip galvanized layer and an alloyed hot-dip galvanized layer.

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

Further, the alloyed hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0% by mass or less of Fe, and 0.001% by mass or more and 1.0% by mass or less of Al. Additionally, the alloyed hot-dip galvanized layer may optionally be selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM. One or more types of elements may be contained in a total amount of 0.0% by mass or more and 3.5% by mass or less. The Fe content of the alloyed hot-dip galvanized layer is more preferably 7.0% by mass or more, and still more preferably 8.0% by mass or more. Further, the Fe content of the alloyed hot-dip galvanized layer is more preferably 15.0% by mass or less, still more preferably 12.0% by mass or less. Note that the remainder other than the above elements are unavoidable impurities.

In addition, the amount of plating deposited on one side of the galvanized layer is not particularly limited, but is preferably 20 g/m ² or more. Further, the amount of plating deposited on one side of the galvanized layer is preferably 80 g/m ² or less.

In addition, the plating adhesion amount of the galvanized layer is measured as follows.
That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (Ivit 700BK (registered trademark) manufactured by Asahi Chemical Co., Ltd.) to 1 L of a 10% by mass hydrochloric acid aqueous solution. Next, a steel plate serving as a test material is immersed in the treatment liquid to dissolve the galvanized layer. Then, by measuring the amount of mass loss of the test material before and after melting, and dividing that value by the surface area of the base steel sheet (the surface area of the part covered with plating), the amount of plating coating (g/m ² ) is calculated.

The thickness of the steel plate according to an embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more, more preferably 0.6 mm or more.
The plate thickness is more preferably over 0.8 mm. The plate thickness is more preferably 0.9 mm or more. The plate thickness is more preferably 1.0 mm or more. The plate thickness is more preferably 1.2 mm or more.
Further, the thickness of the steel plate is preferably 3.5 mm or less. The plate thickness is more preferably 2.3 mm or less.
Further, the width of the steel plate of the present invention is not particularly limited, but is preferably 500 mm or more, more preferably 750 mm or more. Further, the width of the steel plate is preferably 1600 mm or less, more preferably 1450 mm or less.

[2. Manufacturing method of steel plate]
Next, a method for manufacturing a steel plate according to an embodiment of the present invention will be described.

A method for producing a steel plate according to an embodiment of the present invention includes a hot rolling process in which a steel slab having the above-mentioned composition is hot rolled to produce a hot rolled steel plate, and a pickling process in which the hot rolled steel plate is pickled. and an annealing step in which the steel plate after the pickling step is annealed at an annealing temperature of (Ac ₁ +0.4×(Ac ₃ −Ac ₁ ))° C. or higher and 900° C. or lower, and an annealing time of 20 seconds or more; A first cooling step of cooling the steel plate after the annealing step to a first cooling stop temperature of 400°C or more and 600°C or less, and a second cooling stop temperature of 100°C or more and 300°C or less of the steel plate after the first cooling step. The steel plate is cooled at an average cooling rate of 25.0°C/sec or less, and during cooling, a tension of 2.0kgf/mm2 ^or more is applied to the steel plate at least once in a temperature range of 300°C or more and 450°C or less. Thereafter, the steel plate is applied with a roll having a diameter of 500 mm or more and 1500 mm or less for 4 passes or more while contacting the roll for 1/4 of the roll per pass, and the steel plate is applied to a roll 1 of rolls with a diameter of 500 mm or more and 1500 mm or less per pass. /A second cooling step in which the steel plate is subjected to two passes or more while being in contact for two turns, and the steel plate after the second cooling step is heated to a tempering temperature range of more than 300°C and less than 500°C, and A reheating step in which a reheating treatment is performed in the above temperature range for a tempering time of 20 seconds or more and 900 seconds or less, and during the reheating treatment, the carbide control parameter CP shown by the following formula (1) is set to 10,000 or more and 15,000 or less. or further includes a cold rolling process of cold rolling the steel plate after the pickling process and before the annealing process to obtain a cold rolled steel plate.
CP=(T+273)×(k+1.2×logt)...Formula (1)
Here, T: tempering temperature (°C), k: material constant depending on C content, t: tempering time (seconds),
k=-6×C _M +17.8,
_CM : Carbon content (mass%) in martensite produced in the second cooling step.
In addition, each temperature mentioned above means the surface temperature of a steel slab and a steel plate unless otherwise specified.

First, a steel slab having the above-mentioned composition is prepared. For example, a steel material is melted to obtain molten steel having the above-mentioned composition. The melting method is not particularly limited, and known melting methods such as converter furnace melting and electric furnace melting can be used. Then, the obtained molten steel is solidified to form a steel slab. The method for obtaining a steel slab from molten steel is not particularly limited, and for example, a continuous casting method, an ingot forming method, a thin slab casting method, etc. can be used. From the viewpoint of preventing macro segregation, continuous casting is preferred.

[Hot rolling process]
Then, in the hot rolling process, the steel slab is hot rolled to form a hot rolled steel plate.
Hot rolling may be performed by applying an energy saving process. Energy-saving processes include direct rolling (a method in which the steel slab is charged into a heating furnace as hot pieces without being cooled to room temperature and hot rolled) or direct rolling (a method in which the steel slab is subjected to a slight heat retention process). A method in which rolling is carried out immediately afterwards) can be mentioned.

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

Next, the steel slab is subjected to rough rolling according to a conventional method to obtain a rough rolled plate (hereinafter also referred to as a sheet bar). Next, the sheet bar is subjected to finish rolling to obtain a hot rolled steel plate. In addition, when the slab heating temperature is set to be low, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling, from the viewpoint of preventing troubles during finish rolling. The finish rolling temperature is preferably 800° C. or higher in order to reduce the rolling load. Furthermore, if the rolling reduction ratio of austenite in a non-recrystallized state increases, an abnormal structure elongated in the rolling direction may develop, which may reduce the workability of the annealed sheet. Furthermore, by setting the finish rolling temperature to 800° C. or higher, the steel structure in the hot-rolled steel sheet stage and, by extension, the steel structure in the final product tend to be uniform. Note that when the steel structure becomes non-uniform, bendability tends to decrease.
On the other hand, when the finish rolling temperature exceeds 950°C, the amount of oxide (scale) produced increases. As a result, the interface between the base iron and the oxide becomes rough, and the surface quality of the steel sheet after pickling and cold rolling may deteriorate. Furthermore, the coarse grains may cause a decrease in the strength and bendability of the steel sheet. Therefore, the finish rolling temperature is preferably in the range of 950°C or higher. From the above, the finish rolling temperature is preferably in the range of 800°C or higher and 950°C or higher.

After finish rolling, the hot rolled steel sheet is wound up. The winding temperature is preferably 450°C or higher. Further, the winding temperature is preferably 750°C or less.

Note that the sheet bars may be joined together during hot rolling and finish rolling may be performed continuously. Further, the sheet bar may be wound up once before finishing rolling. Further, in order to reduce the rolling load during hot rolling, part or all of the finish rolling may be performed as lubricated rolling. Performing lubricated rolling is also effective from the viewpoint of uniformizing the shape of the steel sheet and uniforming the material quality. Note that the friction coefficient during lubricated rolling is preferably in the range of 0.10 or more and 0.25 or less.
In a hot rolling process (hot rolling process) including rough rolling and finish rolling, a steel slab generally becomes a sheet bar during rough rolling and becomes a hot rolled steel plate through finish rolling. However, depending on the mill capacity, etc., there is no problem with such division as long as it is a predetermined size.

[Acid washing process]
The hot rolled steel sheet after the hot rolling process is pickled. By pickling, oxides on the surface of the steel sheet can be removed, ensuring good chemical conversion treatment properties and plating quality. Note that the pickling may be performed only once, or may be performed in multiple steps. The pickling conditions are not particularly limited, and any conventional method may be used.

[Cold rolling process]
Then, if necessary, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. Cold rolling is performed, for example, by multi-pass rolling that requires two or more passes, such as tandem multi-stand rolling or reverse rolling.
The reduction rate (cumulative reduction rate) of cold rolling is not particularly limited, but is preferably 20% or more. Moreover, it is preferable that the reduction ratio of cold rolling is 80% or less. If the rolling reduction ratio in cold rolling is less than 20%, the steel structure tends to become coarse and non-uniform in the annealing process, and there is a risk that the TS and bendability of the final product will deteriorate. On the other hand, if the rolling reduction ratio in cold rolling exceeds 80%, the steel sheet tends to be defective in shape, and the amount of zinc plating deposited may become uneven.
Further, optionally, the cold rolled steel sheet obtained after cold rolling may be pickled.

[Metal plating (metal electroplating, first plating) process]
In one embodiment of the present invention, one side of the steel sheet after the hot rolling process (after the pickling process, or if cold rolling is performed, after the cold rolling process after the pickling process) and before the annealing process or It may include a first plating step of applying metal plating to both surfaces to form a metal plating layer (first plating layer).
For example, the surface of the hot-rolled steel sheet or cold-rolled steel sheet obtained as described above may be subjected to a metal electroplating treatment to obtain a pre-annealed metal electroplated steel sheet in which a pre-annealed metal electroplating layer is formed on at least one side. . Note that the metal plating mentioned here excludes zinc plating (secondary plating).
The metal electroplating method is not particularly limited, but as described above, it is preferable that the metal electroplating layer is formed on the base steel sheet, so it is preferable to perform the metal electroplating process.
For example, in the Fe-based electroplating bath, a sulfuric acid bath, a hydrochloric acid bath, or a mixture of both can be used. Furthermore, the amount of deposited metal electroplating layer before annealing can be adjusted by adjusting the current application time and the like. Note that "pre-annealed metal electroplated steel sheet" means that the metal electroplated layer has not undergone an annealing process, and refers to a hot rolled steel sheet before metal electroplating, a pickled sheet after hot rolling, or a cold rolled steel sheet that has been annealed in advance. This does not exclude such aspects.

Here, the metal species of the electroplating layer include Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Any of Rt, Au, Hg, Ti, Pb, and Bi may be used, but since Fe is more preferable, the manufacturing method of Fe-based electroplating will be described below. Conditions in electroplating systems can be similarly adopted.

The Fe ion content in the Fe-based electroplating bath before the start of current application is preferably 0.5 mol/L or more as Fe ²⁺ . If the Fe ion content in the Fe-based electroplating bath is 0.5 mol/L or more as Fe ²⁺ , a sufficient amount of Fe deposition can be obtained. Further, in order to obtain a sufficient amount of Fe deposited, it is preferable that the Fe ion content in the Fe-based electroplating bath before the start of current application is 2.0 mol/L or less.

In addition, the Fe-based electroplating bath contains Fe ions and at least one selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co. It can contain one kind of element. The total content of these elements in the Fe-based electroplating bath is preferably such that the total content of these elements in the Fe-based electroplated layer before annealing is 10% by mass or less. Note that the metal element may be contained as a metal ion, and the non-metal element may be contained as a part of boric acid, phosphoric acid, nitric acid, organic acid, or the like. Further, the iron sulfate plating solution may contain a conductivity aid such as sodium sulfate or potassium sulfate, a chelating agent, or a pH buffer.

Other conditions for the Fe-based electroplating bath are not particularly limited either. The temperature of the Fe-based electroplating solution is preferably 30° C. or higher, and preferably 85° C. or lower, in view of constant temperature retention. Although the pH of the Fe-based electroplating bath is not particularly specified, it is preferably 1.0 or higher from the viewpoint of preventing a decrease in current efficiency due to hydrogen generation, and considering the electrical conductivity of the Fe-based electroplating bath, .0 or less is preferable. The current density is preferably 10 A/dm ² or more from the viewpoint of productivity, and preferably 150 A/dm ² or less from the viewpoint of facilitating control of the amount of Fe-based electroplated layer deposited. The plate passing speed is preferably 5 mpm or more from the viewpoint of productivity, and preferably 150 mpm or less from the viewpoint of stably controlling the amount of adhesion.

Note that as treatments before performing the Fe-based electroplating treatment, degreasing treatment and water washing can be performed to clean the steel plate surface, and furthermore, pickling treatment and water washing can be performed to activate the steel plate surface. Following these pre-treatments, Fe-based electroplating treatment is performed. The methods of degreasing and washing with water are not particularly limited, and ordinary methods can be used. In the pickling treatment, various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof can be used. Among these, sulfuric acid, hydrochloric acid, or a mixture thereof is preferred. Although the acid concentration is not particularly defined, it is preferably about 1 to 20 mass% in consideration of the ability to remove an oxide film and the prevention of rough skin (surface defects) due to overacid washing. Further, the pickling treatment liquid may contain an antifoaming agent, a pickling accelerator, a pickling inhibitor, and the like.

[Annealing process]
Then, in one embodiment of the present invention, after the pickling process (in the case of performing cold rolling, after the cold rolling process, in the case of performing metal plating to form a metal plating layer (first plating layer), the metal plating If cold rolling and metal plating are performed after the (first plating) process, the steel plate obtained as described above is annealed at annealing temperature: (Ac ₁ +0.4) ×(Ac ₃ −Ac ₁ )) Annealing is performed at a temperature of 900° C. or higher and an annealing time of 20 seconds or longer. The number of times of annealing may be two or more times, but from the viewpoint of energy efficiency, one time is preferable.

Annealing temperature: (Ac ₁ + 0.4 x (Ac ₃ - Ac ₁ )) ℃ or more and 900 ℃ or less When the annealing temperature is less than (Ac ₁ + 0.4 x (Ac ₃ - Ac ₁ )) ℃, ferrite and austenite The rate of austenite formation during heating in the two-phase region becomes insufficient. Therefore, the area ratio of ferrite increases excessively after annealing, and YS decreases. In addition, the C concentration in the austenite during annealing may increase too much and the desired λ and _SFmax may not be achieved. Furthermore, it becomes difficult to increase the TS to 1180 MPa or more.
On the other hand, when the annealing temperature exceeds 900°C, austenite grain growth occurs excessively, the _MS point rises, a large amount of tempered martensite containing carbides is generated, and retained austenite exceeding 3.0% can be obtained. becomes difficult and ductility decreases. Therefore, the annealing temperature is set to (Ac ₁ +0.4×(Ac ₃ −Ac ₁ ))°C or more and 900°C or less. The annealing temperature is preferably 880°C or lower. The annealing temperature is more preferably 870°C or lower. Further, the annealing temperature is preferably (Ac ₁ +0.5×(Ac ₃ −Ac ₁ ))°C or higher, more preferably (Ac ₁ +0.6×(Ac ₃ −Ac ₁ ))°C or higher. .
Note that the annealing temperature is the highest temperature reached in the annealing step.

Ac ₁ point (℃) and Ac ₃ point (℃) are calculated by the following formula:
Ac ₁ point (°C) = 727.0-32.7×[%C]+14.9×[%Si]+2.0×[%Mn]
Ac ₃ points (°C) = 912.0-230×[%C]+31.6×[%Si]-20.4×[%Mn]
Here, [%C]: C content (mass %), [%Si]: Si content (mass %), and [%Mn]: Mn content (mass %).

Annealing time: 20 seconds or more When the annealing time is less than 20 seconds, the proportion of austenite produced during heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite increases excessively after annealing, and YS decreases. In addition, the C concentration in the austenite during annealing increases too much, making it impossible to achieve the desired λ and _SFmax . Furthermore, it becomes difficult to increase the TS to 1180 MPa or more. Therefore, the annealing time is set to 20 seconds or more. The annealing time is preferably 30 seconds or more, more preferably 50 seconds or more. Although the upper limit of the annealing time is not particularly limited, the annealing time is preferably 900 seconds or less, more preferably 800 seconds or less. The annealing time is more preferably 300 seconds or less, and even more preferably 220 seconds or less.
Note that the annealing time is the holding time in a temperature range of (annealing temperature -40° C.) or higher and lower than the annealing temperature. That is, in addition to the holding time at the annealing temperature, the annealing time also includes the residence time in the temperature range from (annealing temperature -40°C) to below the annealing temperature during heating and cooling before and after reaching the annealing temperature.

Dew point of annealing atmosphere in annealing step: −30° C. or higher In an embodiment of the present invention, the dew point of the atmosphere in an annealing step (annealing atmosphere) is preferably −30° C. By performing annealing with the dew point of the annealing atmosphere at -30° C. or higher in the annealing step, the decarburization reaction is promoted and a deeper soft surface layer can be formed. The dew point of the annealing atmosphere in the annealing step is more preferably -25°C or higher, even more preferably -15°C or higher, and most preferably higher than -5°C.
Although there is no particular upper limit for the dew point of the annealing atmosphere in the annealing process, in order to suitably prevent oxidation of the surface of the Fe-based electroplated layer and improve plating adhesion when providing the galvanized layer, the annealing atmosphere in the annealing process should be set. The dew point is preferably 30°C or lower.

[First cooling process]
Next, the steel plate annealed as described above is cooled to a first cooling stop temperature of 400°C or more and 600°C or less.

First cooling stop temperature: 400°C or more and 600°C or less When the first cooling stop temperature becomes less than 400°C, the area ratio of bainitic ferrite increases excessively, the retained austenite volume ratio exceeds a predetermined amount, and the desired λ and S _Fmax cannot be achieved. On the other hand, when the first cooling stop temperature exceeds 600° C., the area ratio of pearlite increases, so that the strength may decrease. Therefore, the first cooling stop temperature is set to 400°C or more and 600°C or less. The first cooling stop temperature is preferably 460°C or higher. Further, the first cooling stop temperature is preferably 550°C or lower.

[Holding process (preferred requirements)]
After the first cooling step, if necessary, a holding step may be performed in which the steel plate is held in a temperature range of 400° C. or higher and 600° C. or lower (hereinafter also referred to as a holding temperature range) for less than 80 seconds. Here, the holding temperature range may be the first cooling stop temperature described above.

Holding time in holding temperature range: less than 80 seconds In the holding step, bainitic ferrite is generated and C diffuses from the generated bainitic ferrite to untransformed austenite adjacent to the bainitic ferrite. . As a result, a predetermined amount of area ratio of retained austenite is ensured.
Here, if the holding time in the holding temperature range is 80 seconds or more, the area ratio of bainitic ferrite may increase excessively, and YS may decrease. Further, excessive diffusion of C from bainitic ferrite to untransformed austenite occurs, and the area ratio of retained austenite exceeds 15.0%, which may make it impossible to achieve desired λ and S _Fmax . Therefore, the holding time in the holding temperature range is preferably less than 80 seconds. The holding time in the holding temperature range is more preferably less than 60 seconds. Note that the retention time in the retention temperature range does not include the residence time in the temperature range after zinc plating treatment is performed in the plating process.

[Zinc plating process (second plating process)]
After the first cooling step (or after the holding step if a holding step is performed), the steel plate may be subjected to galvanizing treatment. A galvanized steel sheet can be obtained by performing galvanizing treatment. Examples of the galvanizing treatment include hot dip galvanizing and alloyed galvanizing.

In the case of hot-dip galvanizing, it is preferable to immerse the steel sheet in a galvanizing bath at a temperature of 440° C. or higher and 500° C. or lower, and then adjust the coating amount by gas wiping or the like. The hot-dip galvanizing bath is not particularly limited as long as it has the composition of the galvanized layer described above, but for example, the bath has an Al content of 0.10% by mass or more, and the remainder consists of Zn and inevitable impurities. It is preferable to use a plating bath having the same composition. The above Al content is preferably 0.23% by mass or less.

Further, in the case of alloyed galvanizing treatment, it is preferable that after performing hot-dip galvanizing treatment as described above, the galvanized steel sheet is heated to an alloying temperature of 450° C. or higher to perform alloying treatment. The above alloying temperature is preferably 600°C or less.
If the alloying temperature is less than 450°C, the Zn--Fe alloying rate will be slow and alloying may become difficult. On the other hand, when the alloying temperature exceeds 600° C., untransformed austenite transforms to pearlite, making it difficult to increase the TS to 1180 MPa or more, resulting in a decrease in ductility. Note that the alloying temperature is more preferably 470°C or higher. Further, the alloying temperature is more preferably 570°C or lower.

Further, it is preferable that the coating weight of both the hot-dip galvanized steel sheet (GI) and the alloyed hot-dip galvanized steel sheet (GA) be 20 g/m ² or more per side. Further, the amount of plating deposited on one side of the galvanized layer is preferably 80 g/m ² or less. Note that the amount of plating deposited can be adjusted by gas wiping or the like.

[Second cooling process]
Then, the steel plate after the first cooling step is cooled to a second cooling stop temperature of 100°C or more and 300°C or less at an average cooling rate of 25.0°C/second or less.

Second cooling stop temperature: 100°C or more and 300°C or less The second cooling step is for controlling the area ratio of tempered martensite and the area ratio of retained austenite generated in the subsequent reheating process within a predetermined range. This is a necessary process. Here, when the second cooling stop temperature is less than 100° C., almost all of the untransformed austenite present in the steel is transformed into martensite in the second cooling step. As a result, the area ratio of tempered martensite ultimately increases excessively, making it difficult to obtain retained austenite of more than 3.0%, and ductility decreases. On the other hand, when the second cooling stop temperature exceeds 300°C, the area ratio of tempered martensite decreases and the area ratio of fresh martensite increases. Furthermore, the amount of diffusible hydrogen in the steel sheet may also increase. As a result, the desired λ and S _Fmax cannot be achieved. Furthermore, there are cases where the desired α cannot be obtained. Therefore, the second cooling stop temperature is set to 100°C or more and 300°C or less. The second cooling stop temperature is preferably 120°C or higher. Moreover, the first cooling stop temperature is preferably 280° C. or lower.

Average cooling rate in the second cooling process: 25.0°C/sec or less If the cooling rate in the second cooling process exceeds 25.0°C/sec, fine carbides will be generated, and the density of carbides in the tempered martensite will increase. becomes more than a predetermined amount. As a result, the desired λ and S _Fmax cannot be achieved. Furthermore, there are cases where the desired α cannot be obtained. Therefore, the average cooling rate in the second cooling step is 25.0° C./second or less.
Here, the average cooling rate can be calculated by "(cooling start temperature (°C) - second cooling stop temperature (°C)"/cooling time (s)).

In the present invention, when the steel plate is cooled to the second cooling stop temperature, a tension of 2.0 kgf/mm ² or more is applied at least once in a temperature range of 300° C. or higher and 450° C. or lower.
Then, the steel plate after applying the above tension is applied for 4 passes or more while being brought into contact with a roll having a diameter of 500 mm or more and 1500 mm or less per pass for 1/4 rotation of the roll, and the steel plate is A process is performed in which the coating is applied for two or more passes while contacting a roll of 500 mm or more and 1500 mm or less for 1/2 revolution of the roll.
As mentioned above, by applying a tension of 2.0 kgf/mm ² or more to the steel plate once or more and applying a specified number of passes, residual austenite excessively generated in the steel plate structure undergoes deformation-induced transformation. and becomes martensite, which then becomes tempered martensite during subsequent cooling. As a result, desired λ and S _Fmax can be obtained.

The number of passes applied to the steel plate while contacting with the roll for 1/4 of the roll is preferably 5 passes or more, more preferably 6 passes or more.
Although the upper limit is not particularly limited, the number of passes applied to the steel plate while contacting with the roll for 1/4 rotation is preferably 12 passes or less, more preferably 10 passes or less.
The number of passes applied to the steel plate while contacting with the roll for 1/2 rotation is preferably 3 passes or more, more preferably 4 passes or more.
Although the upper limit is not particularly limited, the number of passes applied to the steel plate while contacting with the roll for 1/2 rotation is preferably 6 passes or less, more preferably 5 passes or less.

Here, the tension is obtained by dividing the total value of the loads (kgf) of the load cells on the left and right sides of the roll by the cross-sectional area of the steel plate (=plate thickness (mm) x plate width (mm)) (mm ² ). Note that the load cell must be placed parallel to the tension direction.
Here, the load cell is preferably arranged at a position 200 mm from both ends of the roll. Moreover, it is preferable that the body length of the roll used is 1500 mm or more. Moreover, it is preferable that the body length of the roll used is 2500 mm or less.
Further, this tension is preferably 2.2 kgf/mm ² or more, more preferably 2.4 kgf/mm ² or more.
Further, this tension is preferably 15.0 kgf/mm ² or less, more preferably 10.0 kgf/mm ² or less. This tension is more preferably 7.0 kgf/mm ² or less, even more preferably 4.0 kgf/mm ² or less.

Regarding applying tension more than once, for example, applying tension twice means first applying tension of 2.0 kgf/mm ² or more once, and then applying tension of 2.0 kgf/mm ^{2 or more} . This means applying a second tension of 2.0 kgf/mm ² or more after the tension becomes less than 2.0 kgf/mm 2 . Also, applying tension three times means that first, a tension of 2.0 kgf/mm ² or more is applied once, and then, after the tension becomes less than 2.0 kgf/mm 2, a second application of 2.0 kgf/mm ^{2 or} more is applied. This refers to applying a tension of 2.0 kgf/mm ² or more, and then applying a third tension of 2.0 kgf/mm ² or more after the tension becomes less than 2.0 kgf/mm ² .

[Reheating process]
Then, the steel plate is reheated to a temperature range of more than 300°C and not more than 500°C (hereinafter also referred to as a reheating temperature range), and the steel plate is held in a temperature range of more than 300°C and not more than 500°C for 20 seconds to 900 seconds. do.
This tempers the martensite present in the steel at the end of the second cooling step. Further, by diffusing C dissolved in supersaturated solid solution in martensite into untransformed austenite, austenite that is stable at room temperature, that is, residual austenite is generated.

Reheating temperature (tempering temperature): over 300°C and below 500°C If the reheating temperature (tempering temperature) is below 300°C, the martensite present in the steel at the end of the second cooling process will not be sufficiently tempered. In addition to the excessive increase in fresh martensite, the coarsening of carbides in tempered martensite does not progress sufficiently, and the density of carbides in tempered martensite exceeds a predetermined amount, resulting in λ, α and S _Fmax cannot be achieved. Furthermore, hydrogen contained in the base steel sheet is not sufficiently released to the outside, and the amount of diffusible hydrogen in the base steel sheet increases. This further reduces the hole expandability and bendability.
On the other hand, if the reheating temperature (tempering temperature) exceeds 500°C, the martensite present in the steel will be excessively tempered at the end of the second cooling process, making it difficult to increase the TS to 1180 MPa or higher. . Moreover, since the untransformed austenite present in the steel at the end of the second cooling step decomposes as carbide (pearlite), ductility decreases. Furthermore, hydrogen contained in the base steel sheet may not be sufficiently released to the outside, and the amount of diffusible hydrogen in the base steel sheet may increase. This reduces the hole expandability. Therefore, the reheating temperature is set to be higher than 300°C and lower than 500°C. The reheating temperature is the highest temperature reached in the reheating step. The reheating temperature is preferably 340°C or higher, more preferably 360°C or higher. The reheating temperature is preferably 460°C or lower, more preferably 440°C or lower.

Holding time in the reheating temperature range (tempering time): 20 seconds or more and 900 seconds or less If the holding time in the reheating temperature range (tempering time) is less than 20 seconds, it will be present in the steel at the end of the second cooling process. Tempering of martensite does not proceed sufficiently, and fresh martensite increases excessively. Further, the coarsening of carbides in the tempered martensite may not proceed sufficiently, and the density of the carbides in the tempered martensite may exceed a predetermined amount. As a result, desired λ, α and S _Fmax cannot be achieved. Furthermore, hydrogen contained in the base steel sheet is not sufficiently released to the outside, and the amount of diffusible hydrogen in the base steel sheet increases. This further reduces the hole expandability and bendability.
On the other hand, if the holding time (tempering time) in the reheating temperature range exceeds 900 seconds, the martensite present in the steel will be excessively tempered at the end of the second cooling process, so the TS should be set to 1180 MPa or more. Things become difficult. Moreover, since the untransformed austenite present in the steel at the end of the second cooling step decomposes as carbide (pearlite), ductility decreases. Therefore, the holding time in the reheating temperature range is set to 20 seconds or more and 900 seconds or less. The holding time is preferably 30 seconds or more, more preferably 40 seconds or more. The holding time is preferably 500 seconds or less, more preferably 100 seconds or less.
The holding time in the reheating temperature range includes not only the holding time at the reheating temperature but also the residence time in the temperature range during heating and cooling before and after reaching the reheating temperature.

Carbide control parameter CP during reheating: 10,000 or more and 15,000 or less If the carbide control parameter CP during reheating becomes less than 10,000, the martensite present in the steel at the end of the second cooling process will not be sufficiently tempered. In addition to the excessive increase in fresh martensite, the coarsening of carbides in tempered martensite does not progress sufficiently, and the density of carbides in tempered martensite exceeds a predetermined amount, resulting in the desired λ, α and S _Fmax cannot be achieved. Furthermore, hydrogen contained in the base steel sheet is not sufficiently released to the outside, and the amount of diffusible hydrogen in the base steel sheet increases. This further reduces the hole expandability and bendability.
On the other hand, if the carbide control parameter CP during reheating exceeds 15,000, the martensite present in the steel will be excessively tempered at the end of the second cooling process, making it difficult to increase the TS to 1,180 MPa or more. . Moreover, since the untransformed austenite present in the steel at the end of the second cooling step decomposes as carbide (pearlite), ductility decreases. Therefore, the carbide control parameter CP during reheating is set to 10,000 or more and 15,000 or less.
The carbide control parameter CP during reheating is preferably 11,000 or more, more preferably 12,000 or more. The carbide control parameter CP during reheating is preferably 14,500 or less, more preferably 14,000 or less.

The carbide control parameters during reheating are calculated by the following formula:
CP=(T+273)×(k+1.2×logt)...Formula (1)
Here, CP: carbide control parameter, T: tempering temperature (°C), k: material constant depending on C content, t: tempering time (seconds), and 1.2 in the term of 1.2×logt. is a correction coefficient (preset correction coefficient) that takes into consideration the cooling time after the reheating step.
Also, the material constant k is calculated using the following formula:
k=-6×C _M +17.8
Here, C _M is the amount of carbon (% by mass) in martensite produced in the second cooling step.
Note that the amount of carbon in martensite produced in the second cooling step can be measured as follows.
First, the amount of carbon in each phase immediately before the second cooling step satisfies the following relationship.
V _γ1 ×C _γ1 +V _F ×C _F +V _BF ×C _BF =C _T ...Formula (2)
V _γ1 =1-V _F -V _BF ...Formula (3)
Here, V _γ1 and C _γ1 are the area ratio (%) of untransformed austenite immediately before the second cooling step and the carbon concentration (mass %) in the untransformed austenite,
V _F and C _F are the area ratio (%) of ferrite immediately before the second cooling step and the carbon concentration (mass %) in the ferrite,
V _BF and C _BF are the area ratio (%) of the bainitic ferrite immediately before the second cooling step and the carbon concentration (mass %) in the bainitic ferrite,
C _T is the carbon concentration (mass %) in the steel (immediately before the second cooling step).
Furthermore, the area ratio of ferrite V _F (%) and the area ratio of bainitic ferrite V _BF (%) immediately before the second cooling process are the area ratio of ferrite in the final structure (the steel structure of the steel sheet finally obtained). (%) and the area ratio (%) of bainitic ferrite. Further, the carbon concentration C _F (mass %) in ferrite and the carbon concentration C _BF (mass %) in bainitic ferrite can be zero.
Therefore, equation (2) is
C _T =V _γ1 ×C _γ1 +V _F ×0 (zero) +V _BF ×0 (zero) = V _γ1 ×C _γ1 ...Equation (2-2)
becomes.
Also, from equations (2-2) and (3), C _T = (1-V _F - V _BF ) x C _γ1 , so C _γ1 = C _T / (1- V _F - V _BF ) It can be done. Furthermore, since the transformation from γ1 (untransformed austenite) to martensite in the second cooling step is a transformation that does not involve diffusion of C, _CM = C _γ1 , that is, _CM = C _γ1 = C _T / (1-V _F -V _BF ).

The cooling conditions after holding in the reheating temperature range are not particularly limited, and any conventional method may be used. As the cooling method, for example, gas jet cooling, mist cooling, roll cooling, water cooling, air cooling, etc. can be applied. In addition, from the viewpoint of preventing surface oxidation, it is preferable to cool down to 50° C. or lower after holding in the reheating temperature range, and more preferably to about room temperature. The average cooling rate during cooling after holding in the reheating temperature range is preferably, for example, 1° C./second or more and 50° C./second or less.

Additionally, the steel plate obtained as described above may be further subjected to temper rolling. If the reduction ratio in temper rolling exceeds 2.00%, the yield stress will increase, and there is a risk that the dimensional accuracy when forming the steel plate into a member will decrease. Therefore, the reduction ratio in temper rolling is preferably 2.00% or less. Note that the lower limit of the rolling reduction in skin pass rolling is not particularly limited, but from the viewpoint of productivity, it is preferably 0.05% or more. In addition, skin pass rolling may be performed on a device that is continuous with the annealing device for performing each process mentioned above (online), or on a device that is discontinuous with the annealing device for performing each process (offline). You may go. Further, the number of times of temper rolling may be one, or two or more times. Note that rolling with a leveler or the like may be used as long as it can provide an elongation rate equivalent to that of temper rolling.

Conditions other than the above are not particularly limited and may be according to conventional methods.

[3. Element]
Next, a member according to an embodiment of the present invention will be explained.
A member according to an embodiment of the present invention is a member made of (made of) the above-mentioned steel plate. For example, a steel plate as a raw material is subjected to at least one of forming and bonding to form a member.
Here, the above-mentioned steel plate has a TS of 1180 MPa or more, a high YS, excellent press formability (ductility, hole expandability, and bendability), and fracture resistance during crushing (bending fracture properties and axial crushing properties). Therefore, the member according to one embodiment of the present invention has high strength and excellent impact resistance. Therefore, a member according to an embodiment of the present invention is particularly suitable for application as an impact energy absorbing member used in the automotive field.

[4. Manufacturing method of parts]
Next, a method for manufacturing a member according to an embodiment of the present invention will be described.
A method for manufacturing a member according to an embodiment of the present invention includes the step of subjecting the above steel plate (for example, a steel plate manufactured by the above steel plate manufacturing method) to at least one of a forming process and a joining process to form a member. .
Here, the molding method is not particularly limited, and for example, a general processing method such as press working can be used. Further, the joining method is not particularly limited, and for example, common welding such as spot welding, laser welding, arc welding, riveting joining, caulking joining, etc. can be used. Note that the molding conditions and bonding conditions are not particularly limited, and conventional methods may be followed.

A steel material having the component composition shown in Table 1 (the remainder being Fe and unavoidable impurities) was melted in a converter and made into a steel slab by continuous casting. In Table 1, - indicates the content at the inevitable impurity level.
The calculated transformation points Ac ₁ point (°C) and Ac ₃ point (°C) shown in Table 1 are calculated by the following formula.
Ac ₁ point (°C) = 727.0-32.7×[%C]+14.9×[%Si]+2.0×[%Mn]
Ac ₃ points (°C) = 912.0-230×[%C]+31.6×[%Si]-20.4×[%Mn]
Here, [%C]: C content (mass %), [%Si]: Si content (mass %), and [%Mn]: Mn content (mass %).

The obtained steel slab was heated to 1200°C, and after heating, the steel slab was subjected to hot rolling consisting of rough rolling and finish rolling at a finishing rolling temperature of 900°C to obtain a hot rolled steel plate. Then, the obtained hot rolled steel sheet No. 1~No. 61, No. 64~No. 78, No. 84~No. 98, No. 104~No. No. 109 was pickled and cold rolled (reduction ratio: 50%) to obtain cold rolled steel sheets having the thicknesses shown in Table 3, Table 6, and Table 9. Moreover, No. of the obtained hot-rolled steel sheet. 62~No. 63, No. 79~No. 83, No. 99~No. 103 was pickled to obtain hot rolled steel sheets (white skin) having the thicknesses shown in Table 3, Table 6, and Table 9.
Then, the obtained cold-rolled steel sheet or hot-rolled steel sheet (white skin) is subjected to treatments in an annealing process, a first cooling process, a holding process, a galvanizing process, a second cooling process, and a reheating process under the conditions shown in Table 2. Also, under the conditions shown in Tables 5 and 8, the first plating process (metal plating process), annealing process, first cooling process, holding process, second plating process (zinc plating process), second cooling process and A steel plate (galvanized steel plate) was obtained by processing in a reheating step.
In addition, No. in Table 5 and Table 8. 64~No. The presence or absence of the first plating process (metal plating process) and the type of plating when performing the treatment in the metal plating process are shown for the steel plate No. 109. No. in Table 6 and Table 9. 64~No. The thickness of the soft surface layer, the amount of metal plating deposited, and the hardness distribution of the soft surface layer are shown for steel plate No. 109.

Here, in the galvanizing process, hot-dip galvanizing treatment or alloyed galvanizing treatment was performed to obtain a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or an alloyed hot-dip galvanized steel sheet (hereinafter also referred to as GA). In addition, in Tables 2, 5, and 8, the types of plating processes are also indicated as "GI" and "GA." In the case of the GI steel sheets in Tables 2, 5, and 8, the alloying temperature is indicated as - because no alloying treatment is performed. In addition, in Table 8, cold-rolled steel sheets obtained without galvanizing are indicated as "CR," and hot-rolled steel sheets are obtained without galvanizing in the galvanizing process. ``HR'' is displayed for those that are.

The zinc plating bath temperature was 470° C. in both GI and GA production.
The amount of zinc plating deposited was 45 to 72 g/m ² per side when manufacturing GI, and 45 g/m ² per side when manufacturing GA.
The composition of the galvanized layer of the finally obtained galvanized steel sheet is, in GI, Fe: 0.1 to 1.0% by mass, Al: 0.2 to 0.33% by mass, and the remainder were Zn and unavoidable impurities. Furthermore, GA contained Fe: 8.0 to 12.0% by mass, Al: 0.1 to 0.23% by mass, and the remainder was Zn and inevitable impurities.
Further, all galvanized layers were formed on both sides of the base steel sheet.

In Tables 2, 5, and 8, the number of passes 1 means that the steel plate is applied with an average tension of 2.0 kgf/mm ² or more at least once in a temperature range of 300°C or more and 450°C or less during the second cooling process. is the number of passes in which the steel plate is applied to a roll with a diameter of 500 mm or more and 1500 mm or less per pass while contacting the roll for 1/4 rotation, and the number of passes 2 means that the steel plate is then applied to a roll with a diameter of 500 mm or more and 1500 mm or less per pass. This is the number of passes applied while contacting the roll for 1/2 revolution.

Using the obtained steel plate, the steel structure of the base steel plate was identified and the amount of diffusible hydrogen was measured in the manner described above. The measurement results are shown in Table 3, Table 6, and Table 9. In Tables 3, 6, and 9, BF is bainitic ferrite, TM is tempered martensite, RA is retained austenite, FM is fresh martensite, LB is lower bainite, P is pearlite, and θ is carbide. Further, C _M is the amount of carbon in martensite produced in the second cooling step, C _γ is the amount of carbon in retained austenite, and ρ _C is the density of carbides in the tempered martensite.

The method for measuring the surface soft layer is as follows. After smoothing the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate by wet polishing, the plate thickness was measured from the steel plate surface using a Vickers hardness tester at a load of 10gf based on JIS Z 2244-1 (2020). Measurements were performed at 1 μm intervals from a position of 1 μm in the direction to a position of 100 μm in the plate thickness direction. Thereafter, measurements were taken at intervals of 20 μm up to the center of the plate thickness. The area where the hardness has decreased to 85% or less compared to the hardness at 1/4 of the plate thickness is defined as a soft layer (surface soft layer), and the thickness of this area in the plate thickness direction is defined as the thickness of the soft layer. .

In addition, tensile tests, hole expansion tests, VDA bending tests, V-VDA bending tests, and axial crushing tests were conducted according to the following procedures, and the tensile strength (TS), yield stress (YS), total elongation (El), critical hole expansion rate (λ), critical bending angle (α) in the VDA bending test, stroke at maximum load (S _Fmax ) in the V-VDA bending test, and presence or absence of axial crush fracture were evaluated.

・TS
〇 (Pass): 1180MPa or more × (Fail): Less than 1180MPa

・YS
〇(Passed):
(A) When 1180MPa≦TS<1320MPa, 750MPa≦YS
(B) If 1320MPa≦TS, 850MPa≦YS
× (fail):
(A) If 1180MPa≦TS<1320MPa, 750MPa>YS
(B) If 1320MPa≦TS, 850MPa>YS

・El
〇(Passed):
(A) When 1180MPa≦TS<1320MPa, 12.0%≦El
(B) When 1320MPa≦TS, 10.0%≦El
× (fail):
(A) When 1180MPa≦TS<1320MPa, 12.0%>El
(B) When 1320MPa≦TS, 10.0%>El

・λ
〇(Pass): 30% or more ×(Fail): Less than 30%

・α
〇 (Pass): 80° or more × (Fail): Less than 80°

・S _Fmax
〇(Pass): 26.0mm or more ×(Fail): Less than 26.0mm

・Presence or absence of axial crush fracture A (pass): No cracks were observed in the sample after the axial crush test B (pass): Cracks were observed in two or less places in the sample after the axial crush test C (pass): Axial crush Cracks were observed in 3 or less places in the sample after the test. D (Fail): Cracks were observed in 4 or more places in the sample after the axial crush test, or the sample after the axial crush test was broken.

(1) Tensile test The tensile test was conducted in accordance with JIS Z 2241 (2011). That is, a JIS No. 5 test piece was taken from the obtained steel plate so that the longitudinal direction was perpendicular to the rolling direction of the base steel plate. Using the sampled test piece, a tensile test was conducted at a crosshead speed of 10 mm/min, and TS, YS, and El were measured. The results are shown in Table 4, Table 7, and Table 10.

(2) Hole expansion test The hole expansion test was conducted in accordance with JIS Z 2256 (2020). That is, a 100 mm x 100 mm test piece was taken from the obtained steel plate by shearing. A hole with a diameter of 10 mm was punched into the test piece with a clearance of 12.5%. Next, using a die with an inner diameter of 75 mm, a wrinkle pressing force of 9 tons (88.26 kN) was applied around the hole, and a conical punch with an apex angle of 60° was pushed into the hole to perform a test at the crack generation limit (at the time of crack generation). The diameter of the hole in the piece was measured. Then, the critical hole expansion rate: λ (%) was determined using the following formula. Note that λ is an index for evaluating stretch flangeability. The results are shown in Table 4, Table 7, and Table 10.
λ (%) = {(D _f - D ₀ )/D ₀ }×100
here,
D _f : Diameter of hole in test piece at the time of crack occurrence (mm)
D ₀ : Diameter of hole in initial test piece (mm)
It is.

(3) VDA bending test The VDA bending test was conducted in accordance with the VDA standard (VDA238-100) stipulated by the German Automobile Industry Association.
Specifically, a 70 mm x 60 mm test piece was taken from the obtained steel plate by shearing. Here, the 60 mm side is parallel to the rolling (L) direction.
The test piece was subjected to a VDA bending test under the following conditions.
Test method: Roll support, punch pushing Roll diameter: φ30mm
Punch tip R: 0.4mm
Distance between rolls: (plate thickness x 2) + 0.5mm
Stroke speed: 20mm/min
Bending direction: Direction perpendicular to rolling (C) At this time, the outer bending angle of the central part of the plate-shaped specimen when the load F from the push bending jig from above is maximum is the limit bending angle (°) Measure as. Let α (°) be the average value of the limit bending angle at the maximum load when the above VDA bending test is carried out three times. The results are shown in Table 4, Table 7, and Table 10.

(4) V-VDA bending test (V bending + orthogonal VDA bending test)
The V-VDA bending test was conducted as follows.
A 60 mm x 65 mm test piece was taken from the obtained steel plate by shearing. Here, the 60 mm side is parallel to the rolling (L) direction. A test piece was prepared by performing 90° bending (primary bending) in the rolling (L) direction with the width (C) direction as the axis at a radius of curvature/plate thickness of 4.2. In the 90° bending process (primary bending process), as shown in Figure 2-1(a), a punch B1 was pushed into a steel plate placed on a die A1 having a V groove to obtain a test piece T1. . Next, as shown in FIG. 2-1(b), the punch B2 is pushed into the test piece T1 placed on the support roll A2 so that the bending direction is perpendicular to the rolling direction. Secondary bending process) was applied. In FIGS. 2-1(a) and 2-1(b), the symbol D1 indicates the width (C) direction, and the symbol D2 indicates the rolling (L) direction.
The V-bending conditions in the V-VDA bending test (V-bending + orthogonal VDA bending test) are as follows.
Test method: die support, punch press molding load: 10t
Test speed: 30mm/min
Holding time: 5s
Bending direction: rolling (L) direction VDA bending conditions in the V-VDA bending test are as follows.
Test method: Roll support, punch pushing Roll diameter: φ30mm
Punch tip R: 0.4mm
Distance between rolls: (plate thickness x 2) + 0.5mm
Stroke speed: 20mm/min
Test piece size: 60mm x 60mm
Bending direction: rolling perpendicular (C) direction In the stroke-load curve obtained when performing the above VDA bending, the stroke at the maximum load is determined. The average value of the stroke at the maximum load when performing the above V-VDA bending test three times is defined as S _Fmax (mm). The results are shown in Table 4, Table 7, and Table 10.

(5) Axial crush test A 160 mm x 200 mm test piece was taken from the obtained steel plate by shearing. Here, the 160 mm side is parallel to the rolling (L) direction. Using a mold with a punch shoulder radius of 5.0 mm and a die shoulder radius of 5.0 mm, the molding process (bending process) was performed to a depth of 40 mm. A hat-shaped member 10 shown in 6-1(b) was manufactured. Further, a steel plate used as a material for the hat-shaped member was separately cut into a size of 80 mm x 200 mm. Next, the cut steel plate 20 and the hat-shaped member 10 were spot welded to produce a test member 30 as shown in FIGS. 6-1(a) and 6-1(b). FIG. 6-1(a) is a front view of a test member 30 produced by spot welding the hat-shaped member 10 and the steel plate 20. FIG. 6-1(b) is a perspective view of the test member 30. As shown in FIG. 6-1(b), the spot welds 40 were positioned so that the distance between the end of the steel plate and the weld was 10 mm, and the distance between the welds was 45 mm. Next, as shown in FIG. 6-2(c), the test member 30 was joined to the base plate 50 by TIG welding to prepare a sample for the axial crush test. Next, the impactor 60 was made to collide with the produced sample for the axial crush test at a constant velocity of 10 mm/min, and the sample for the axial crush test was crushed by 70 mm. As shown in FIG. 6-2(c), the crushing direction D3 was parallel to the longitudinal direction of the test member 30.
The samples after crushing were evaluated in the manner described above, and the results are shown in Tables 4, 7, and 10.

The VDA bending test, V-VDA bending test, and axial crushing test on steel plates with a thickness of more than 1.2 mm were all conducted on steel plates with a plate thickness of 1.2 mm, taking into consideration the influence of the plate thickness. Steel plates with a thickness of more than 1.2 mm were ground on one side to a thickness of 1.2 mm. Since the bendability of the steel plate surface may be affected by the grinding process, in the VDA bending test, the ground surface is placed on the inside of the bend (the side that contacts the punch), and in the V-VDA bending test, the ground surface is placed on the outside of the bend during the V-bending test. (the side that contacts the die), and then subjected to the VDA bending test with the ground surface bent inside (the side that contacts the punch).
On the other hand, in the VDA bending test, V-VDA bending test, and axial crushing test of steel plates with a thickness of 1.2 mm or less, the effects of the plate thickness were small, so the tests were conducted without grinding.

Furthermore, "*1" and "*2" in Table 4, Table 7, and Table 10 indicate that the V-VDA bending test was performed up to the maximum load point, and the results were formed at the L cross section of the V bending ridgeline and the VDA bending ridgeline. The length of the crack, 50 μm from the surface of the steel plate on the outside of VDA bending, and the area of 50 μm left and right centered on the bending apex of VDA bending (from the bending apex on the outside of VDA bending as the starting point to a position of 50 μm in the plate thickness direction) This is the amount of change in grain size in the plate thickness direction of bainitic ferrite before and after processing in regions formed at positions up to 50 μm on both sides of the starting line in the vertical direction from each position of the existing starting line.

<Nano hardness measurement>
In order to obtain excellent bendability during press forming and excellent bending rupture properties during collision, it is necessary to place the base material at a position of 1/4 of the depth in the thickness direction and 1/2 of the depth in the thickness direction of the surface soft layer from the surface layer of the substrate. When nanohardness was measured at 300 or more points in a 50 μm x 50 μm area of the plate surface at each position, the nanohardness of the plate surface at a position 1/4 of the thickness direction depth of the surface soft layer from the base steel plate surface was It is more preferable that the number of measurements of 7.0 GPa or more is 0.10 or less with respect to the total number of measurements at 1/4 position of the depth in the plate thickness direction. If the ratio of nanohardness of 7.0 GPa or more is 0.10 or less, it means that the ratio of hard structures (such as martensite) and inclusions is small. It became possible to further suppress the generation and connection of voids and the propagation of cracks during press molding and collision, and excellent R/t and SFmax were obtained.

In the case of zinc plating, after peeling off the plating layer, mechanical polishing is performed from the surface of the base steel sheet to 1/4 of the depth in the thickness direction of the soft surface layer -5 μm, and the depth in the thickness direction of the surface soft layer is removed from the surface of the base steel sheet. After buffing with diamond and alumina to the 1/4th position, colloidal silica polishing was performed. Using Hysitron's tribo-950, with a Berkovich-shaped diamond indenter,
Load: 500μN
Measurement area: 50μm x 50μm
Dot spacing: 2μm
Nanohardness was measured at a total of 512 points under these conditions.
In addition, the above-mentioned plating layer that peels off is a galvanized layer if a galvanized layer is formed, a metal plating layer if a metal plating layer is formed, and a galvanized layer and a metal plating layer. When a plating layer is formed, it is a galvanized layer and a metal plating layer.

Next, mechanical polishing, buff polishing with diamond and alumina, and colloidal silica polishing were performed to 1/2 the depth of the surface soft layer in the thickness direction. Using Hysitron's tribo-950, with a Berkovich-shaped diamond indenter,
Load: 500μN
Measurement area: 50μm x 50μm
Dot spacing: 2μm
Nanohardness was measured at a total of 512 points under these conditions.

In Tables 1 to 10, the underlined portions indicate outside the appropriate range of the present invention.
As shown in Table 4, Table 7, and Table 10, in each of the invention examples, tensile strength (TS), yield stress (YS), total elongation (El), critical hole expansion ratio (λ), VDA bending test The limit bending angle (α) in the V-VDA bending test and the maximum load stroke (S _Fmax ) in the V-VDA bending test all passed, and there was no fracture in the axial crushing test.
On the other hand, in the comparative example, tensile strength (TS), yield stress (YS), total elongation (El), critical hole expansion rate (λ), critical bending angle (α) in VDA bending test, and V-VDA bending test At least one of the stroke at maximum load (S _Fmax ) and the presence or absence of fracture in the axial crush test were insufficient.
In addition, in Tables 5 to 10, when the dew point is in the range of -30°C or more and -5°C or less, the soft layer thickness is less than 11 μm, and the judgment of fracture (appearance cracking) in the axial crush test is "B" or "C". However, even if the soft layer thickness is less than 11 μm, if it has a metal plating layer, the rupture (appearance cracking) in the axial crush test was evaluated as "A".

In addition, the members obtained by forming or bonding the steel sheets of the present invention have tensile strength (TS), yield stress (YS), total elongation (El), limit The hole expansion rate (λ), the limit bending angle (α) in the VDA bending test, and the stroke at maximum load in the V-VDA bending test (S _Fmax ) all have the excellent characteristics featured in the present invention. It was found that there was no breakage in the axial crushing test, and that it had the excellent properties featured in the present invention.

10 Hat-shaped member 20 Galvanized steel plate 30 Test member 40 Spot weld 50 Base plate 60 Impactor A1 Die A2 Support roll B1 Punch B2 Punch D1 Width (C) direction D2 Rolling (L) direction D3 Crushing direction D4 Thickness direction T1 Test piece T2 Test piece P Maximum load point R Area where the stroke increases from the maximum load point and the load is 94.9 to 99.9% of the maximum load AB Overlapping area of the V bending ridge line part and the VDA bending ridge line part 0 to 50 μm from the surface of the steel plate on the outside of VDA bending in the L cross section at , and 50 μm to the left and right around the bending apex of VDA bending AL L cross section in the overlapping region of the V bending ridgeline part and the VDA bending ridgeline part F Ferrite BF Bainitic ferrite BF1 Bainitic ferrite before deformation BF2 Bainitic ferrite after deformation TM Tempered martensite θ Carbide H1 Hard second phase X1 Island-like second phase

Claims

A steel plate comprising a base steel plate, the base steel plate comprising:
In mass%,
C: 0.050% or more and 0.400% or less,
Si: more than 0.75% and 3.00% or less,
Mn: 2.00% or more and less than 3.50%,
P: 0.001% or more and 0.100% or less,
S: 0.0001% or more and 0.0200% or less,
Contains Al: 0.010% or more and 2.000% or less and N: 0.0100% or less, with the remainder consisting of Fe and inevitable impurities,
The base steel plate is
Area ratio of bainitic ferrite: 3.0% or more and 20.0% or less,
Area ratio of tempered martensite: 40.0% or more and 90.0% or less,
Area ratio of retained austenite: more than 3.0% and less than 15.0%,
Carbon concentration in retained austenite: 0.60% by mass or more and 1.30% by mass or less,
Fresh martensite area ratio: 10.0% or less,
Density of carbides in tempered martensite: 8.0 pieces/μm 2 or less,
It has a steel structure that is
The amount of diffusible hydrogen in the base steel sheet is 0.50 mass ppm or less,
Furthermore, a V-VDA bending test was performed up to the maximum load point,
In the L section,
The length of the crack is 400 μm or less,
Furthermore, in a region formed at a position of up to 50 μm on each side of the start line in the vertical direction from each position of the start line, which starts from the bending apex on the outside of the VDA bending and extends up to a position of 50 μm in the plate thickness direction,
Regarding the average grain size in the plate thickness direction of bainitic ferrite, the ratio of the average grain size before processing to the average grain size after processing is 5.0 or less,
A steel plate having a tensile strength of 1180 MPa or more.
The composition of the base steel sheet further includes, in mass%,
Nb: 0.200% or less,
Ti: 0.200% or less,
V: 0.200% or less,
B: 0.0100% or less,
Cr: 1.000% or less,
Ni: 1.000% or less,
Mo: 1.000% or less,
Sb: 0.200% or less,
Sn: 0.200% or less,
Cu: 1.000% or less,
Ta: 0.100% or less,
W: 0.500% or less,
Mg: 0.0200% or less,
Zn: 0.0200% or less,
Co: 0.0200% or less,
Zr: 0.1000% or less,
Ca: 0.0200% or less,
Se: 0.0200% or less,
Te: 0.0200% or less,
Ge: 0.0200% or less,
As: 0.0500% or less,
Sr: 0.0200% or less,
Cs: 0.0200% or less,
Hf: 0.0200% or less,
Pb: 0.0200% or less,
The steel plate according to claim 1, containing at least one selected from Bi: 0.0200% or less and REM: 0.0200% or less.
The steel plate according to claim 1 or 2, comprising a galvanized layer as the outermost layer on one or both sides of the steel plate.
When the base steel plate has an area of 200 μm or less in the thickness direction from the surface of the base steel plate as the surface layer,
The surface layer has a soft surface layer whose Vickers hardness is 85% or less with respect to the Vickers hardness at the 1/4 position of the plate thickness,
Nano hardness of 300 points or more in a 50 μm x 50 μm area of the plate surface at 1/4 position and 1/2 depth in the plate thickness direction of the surface soft layer from the surface of the base steel plate, respectively. When measuring
The proportion of measurements where the nano-hardness of the plate surface at 1/4 of the depth in the thickness direction of the soft surface layer from the surface of the base steel sheet is 7.0 GPa or more is 1/4 of the depth in the thickness direction of the soft surface layer. 0.10 or less for the total number of measurements at 4 positions,
Furthermore, the standard deviation σ of the nano-hardness of the plate surface at a position 1/4 of the thickness direction depth of the surface soft layer from the base steel plate surface is 1.8 GPa or less,
Further, according to any one of claims 1 to 3, the standard deviation σ of nanohardness of the plate surface at a position 1/2 the depth in the plate thickness direction of the surface soft layer from the surface of the base steel plate is 2.2 GPa or less. steel plate.
The steel plate according to any one of claims 1 to 4, having a metal plating layer formed on the base steel plate on one or both sides of the steel plate.
A member made of the steel plate according to any one of claims 1 to 5.
A hot rolling step of hot rolling a steel slab having the composition according to claim 1 or 2 to obtain a hot rolled steel plate;
A pickling step of pickling the hot rolled steel sheet;
An annealing step of annealing the steel plate after the pickling step at an annealing temperature of (Ac 1 +0.4×(Ac 3 −Ac 1 ))° C. or higher and 900° C. or lower, and an annealing time of 20 seconds or more;
A first cooling step of cooling the steel plate after the annealing step to a first cooling stop temperature of 400° C. or higher and 600° C. or lower;
Cooling the steel plate after the first cooling step to a second cooling stop temperature of 100 ° C. or more and 300 ° C. or less at an average cooling rate of 25.0 ° C. / seconds or less,
During the cooling, a tension of 2.0 kgf/mm 2 or more is applied to the steel plate at least once in a temperature range of 300 ° C. or higher and 450 ° C. or lower,
after that,
A process of applying the steel plate to a roll having a diameter of 500 mm or more and 1500 mm or less for 4 passes or more while contacting the roll for 1/4 turn per pass, and applying the steel plate to a roll of 500 mm or more and 1500 mm or less in diameter per pass for 1/4 pass. A second cooling step in which a process is performed in which two or more passes are applied while contacting for two rounds;
The steel plate after the second cooling step is heated to a tempering temperature range of more than 300°C and 500°C or less, and is heated in the temperature range for a tempering time of 20 seconds or more and 900 seconds or less, and During the reheating treatment, a reheating step in which the carbide control parameter CP shown by the following formula (1) is set to 10,000 or more and 15,000 or less,
Alternatively, the method for manufacturing a steel plate further includes a cold rolling process of cold rolling the steel plate after the pickling process and before the annealing process to obtain a cold rolled steel plate.
CP=(T+273)×(k+1.2×logt)...Formula (1)
Here, T: tempering temperature (°C), k: material constant depending on C content, t: tempering time (seconds),
k=-6×C M +17.8,
CM : Carbon content (mass%) in martensite produced in the second cooling step.
The method for manufacturing a steel sheet according to claim 7, comprising a galvanizing step of performing galvanization on the steel sheet after the first cooling step and before the second cooling step to form a galvanized layer on the steel sheet.
The method for manufacturing a steel plate according to claim 7 or 8, wherein the annealing in the annealing step is performed in an atmosphere with a dew point of −30° C. or higher.
After the pickling step and before the annealing step, a metal plating step is performed to form a metal plating layer on one or both surfaces of the steel plate, according to any one of claims 7 to 9. Method of manufacturing steel plates.
A method for producing a member, the method comprising the step of subjecting the steel plate according to any one of claims 1 to 5 to at least one of forming and joining to produce a member.