WO2023218731A1

WO2023218731A1 - Steel sheet, member, and method for producing same

Info

Publication number: WO2023218731A1
Application number: PCT/JP2023/006925
Authority: WO
Inventors: 由康川崎; 悠佑和田; 秀和南; 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

Abstract

The purpose of the present invention is to achieve a TS of at least 1180 MPa, high YS and YR, excellent press formability (ductility, hole expandability, and bendability), and rupture resistance at impact (flexural rupture properties and axial crush properties).　A base steel plate is made to have a predetermined chemical composition. In a steel microstructure at 1/4 of the thickness of the base steel plate, the amounts of ferrite, fresh martensite, retained austenite, bainite, tempered bainite, and tempered martensite are within predetermined ranges, the average grain size of island-like fresh martensite and island-like retained austenite in bainite grains and tempered bainite grains is 2.00 μm or less, the average grain size of carbides in the bainite grains and the tempered bainite grains is 500 nm or less, and the number density of carbides having a grain size of at least 300 nm in the bainite grains and the tempered bainite grains is 3.0/μm² or less. The amount of diffusible hydrogen contained in the base steel plate is 0.50 ppm by mass or less.

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.

2. Description of the Related Art Progress is being made in increasing the strength of steel plates for automobiles, with the aim of reducing _CO2 emissions by improving fuel efficiency by making the steel plates used in automobile bodies thinner and lighter, and improving collision safety. Additionally, new laws and regulations are being introduced one after another. Therefore, for the purpose of increasing the strength of the car body, high-strength steel plates are used for the main structural members and reinforcing members (hereinafter also referred to as automobile frame structural members) that are assembled into the frame of the car cabin. The number of applications of high-strength steel plates of 780 MPa or higher is increasing. Furthermore, high-strength steel plates used for automobile frame structural members and the like are required to have high member strength when press-formed. In order to increase the strength of parts, it is effective, for example, to increase the yield ratio (hereinafter also simply referred to as YR), which is the value obtained by dividing the yield stress (hereinafter also simply referred to as YS) of the steel plate by TS. As a result, the impact absorption energy (hereinafter also simply referred to as impact absorption energy) at the time of a car collision increases. Further, among automobile frame structural members, for example, a crash box has a bent portion. Therefore, from the viewpoint of press formability, it is preferable to use a steel plate having high bendability for such parts. In addition, from the viewpoint of anticorrosion performance of the car body, steel plates that are used as raw materials for automobile parts are often galvanized. Therefore, it is desired to develop a hot-dip galvanized steel sheet that not only has high strength but also has excellent press formability and impact resistance.

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.

Further, Patent Document 4 discloses an alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer on the surface of the steel sheet, in which the steel sheet has a carbon content of 0.03% or more and 0.35% or less in mass %. , Si: 0.005% or more and 2.0% or less, Mn: 1.0% or more and 4.0% or less, P: 0.0004% or more and 0.1% or less, S: 0.02% or less, sol. It has a chemical composition consisting of Al: 0.0002% or more and 2.0% or less, N: 0.01% or less, and the balance is Fe and impurities, and is stretched in the rolling direction at a depth of 50 μm from the surface of the steel plate. The average spacing in the direction perpendicular to the rolling direction of the enriched regions where Mn and/or Si are concentrated is 1000 μm or less, and the number density of cracks with a depth of 3 μm or more and 10 μm or less on the surface of the steel sheet is 3 pieces/mm or more and 1000 pieces/mm or less, and contains bainite: 60% or more, retained austenite: 1% or more, martensite: 1% or more, and ferrite: 2% or more and less than 20%, and The alloyed hot-dip galvanized steel sheet has a steel structure in which the average distance between the ultrahard phases, which is the average value of the closest distance between martensite and retained austenite, is 20 μm or less, and the alloyed hot-dip galvanized steel sheet has a tensile strength (TS) of 780 MPa or more. An alloyed hot-dip galvanized steel sheet is disclosed that is characterized by mechanical properties.

Patent No. 3887235 Patent No. 5953693 Patent No. 6052472 Patent No. 5699764

Incidentally, steel plates with a tensile strength TS (hereinafter sometimes simply referred to as TS) exceeding 590 MPa are increasingly being used in automobile frame members such as center pillars, but they are not suitable for front side members and rear side members. Currently, the impact energy absorbing members of typical automobiles are limited to steel plates with a TS of 590 MPa.

In other words, in order to increase the absorbed energy at the time of a collision (hereinafter also referred to as shock absorption energy), the yield stress YS (hereinafter sometimes simply referred to as YS) and the yield ratio YR (hereinafter simply referred to as YR) are ) is effective. However, when the YS and YR 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, the current situation is that steel plates with a TS of 590 MPa are used as the above-mentioned impact energy absorbing members.

In fact, the steel sheets disclosed in Patent Documents 1 to 4 also have a TS of 1180 MPa or more, high YS and YR, excellent press formability (ductility, hole expandability, and bendability), and impact resistance. It cannot be said that it has rupture properties (bending rupture properties and axial crush properties).

The present invention was developed in view of the above-mentioned current situation, and has a tensile strength TS of 1180 MPa or more, high yield stress YS, high yield ratio YR, and excellent press formability (ductility, hole expansion). The object of the present invention is to provide a steel plate having good strength (flexural strength and bendability) and fracture resistance upon collision (bending fracture properties and axial crushing properties), and a method for manufacturing the same.
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.

Note that the steel sheet referred to here 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). .

Moreover, here, the tensile strength TS is measured by a tensile test based on JIS Z 2241 (2011).
In addition, high yield stress YS and yield ratio YR means that YS measured in a tensile test based on JIS Z 2241 (2011) is either (A) or Indicates that formula (B) is satisfied.
(A) When 1180MPa≦TS<1320MPa, 750MPa≦YS and 0.64≦YR
(B) When 1320MPa≦TS, 850MPa≦YS and 0.64≦YR

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, 8.0%≦El
(B) When 1320MPa≦TS, 7.0%≦El

In addition, "excellent hole expansion property" refers to a critical hole expansion rate (λ) of 25% or more measured in a hole expansion test based on JIS Z 2256 (2020).

In addition, excellent bendability means that R (limit bending radius)/t (plate thickness) measured in the V-bending test based on JIS Z 2248 (2014) is either (A) or ( B) Refers to satisfying the formula.
(A) When 1180MPa≦TS<1320MPa, 2.5≧R/t
(B) If 1320MPa≦TS, 3.0≧R/t

In addition, "excellent axial crushing properties" means that the critical spacer thickness (ST) in the U-bending + close-contact bending test satisfies the following formula (A) or (B) depending on TS.
(A) When 1180MPa≦TS<1320MPa, 5.5mm≧ST
(B) If 1320MPa≦TS, 6.0mm≧ST

Furthermore, having excellent axial crushing characteristics means that the stroke at maximum load (SFmax) measured in the V-bending + orthogonal VDA bending test satisfies the following formula (A) or (B) depending on the TS. Point.
(A) When 1180MPa≦TS<1320MPa, 25.5mm≦SFmax
(B) If 1320MPa≦TS, 24.5mm≦SFmax

Furthermore, having excellent axial crushing properties means that the fracture (appearance cracking) after the axial crushing test is within the range of R = 5.0 mm and 200 mm at the lower two bending ridges in Figure 4(b) (Figure 4(a) ), (b), refer to area Cx) in one or less locations.

Also, having excellent bending rupture properties means that the critical spacer thickness (ST) in the above U-bending + close-contact bending test satisfies the above formula (A) or (B) depending on the TS, and It means that the stroke at maximum load (SFmax) measured in the bending + orthogonal VDA bending test satisfies the above formula (A) or (B) depending on the TS.

The above El (ductility), λ (stretch flangeability), and R/t (bendability) are characteristics that indicate the ease of forming a steel plate during press forming (the degree of freedom in forming for press forming without cracking). It is. On the other hand, the U-bending + close bending test is a test that simulates the deformation and fracture behavior of the vertical wall part in a collision test, and the critical spacer thickness (ST) measured in the U-bending + close bending test is It is an index showing the resistance to cracking (impact resistance properties for absorbing impact energy without breaking) of steel plates and components of automobile bodies.
In addition, the V-bending + orthogonal 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 (SFmax) at the maximum load measured in the V-bending + orthogonal VDA bending test is the energy This is an index that shows how hard the absorbent member is to crack.

In order to achieve the above-described problems, the present inventors have made extensive studies and have obtained the following knowledge.
(1) By controlling the area ratio of ferrite to less than 20.0%, the area ratio of bainite and tempered bainite to more than 10.0%, and the area ratio of tempered martensite to 30.0% or more using predetermined components. , it is possible to secure a TS of 1180 MPa or more.
(2) By controlling the area ratio of ferrite to less than 20.0%, the area ratio of bainite and tempered bainite to more than 10.0%, and the area ratio of tempered martensite to 30.0% or more using a predetermined component. , high YS and YR can be secured.
(3) By controlling the area ratio of bainite and tempered bainite to more than 10.0% with a predetermined component, El, an index of ductility that is correlated with stretch formability, which is one mode of press formability, can be improved. Improvements can be made.
(4) By controlling the area ratio of fresh martensite to 15.0% or less and the area ratio of retained austenite to 3.0% or less using a predetermined component, isolated hard islands within bainite grains and tempered bainite grains are formed. The average grain size of the second phase (martensite + retained austenite) is 2.00 μm or less, and the number density of carbides with a grain size of 300 nm or more in bainite grains and tempered bainite grains is 3.0 pieces/μm. By setting it to ² or less, it is possible to improve λ, which is an index of hole expandability that is correlated with stretch flangeability, which is one mode of press formability.
(5) With a predetermined component, the area ratio of fresh martensite is controlled to 15.0% or less, and the average crystal grain size of isolated island martensite in bainite grains and tempered bainite grains is 2.00 μm or less. Furthermore, by setting the number density of carbides with a grain size of 300 nm or more in bainite grains and tempered bainite grains to 3.0 pieces/ ^μm2 or less, the index of bendability, which is one mode of press formability. A certain improvement in R/t can be achieved.
(6) Si: By controlling the area ratio of retained austenite to 3.0% or less with 0.75% by mass or less and a predetermined component, processing induction of retained austenite that occurs during primary processing such as punching and press forming is induced. It is possible to suppress the generation of hard fresh martensite generated by transformation, and to suppress the generation of voids and the propagation of cracks in subsequent tests. Furthermore, the area ratio of fresh martensite is controlled to 15.0% or less, and the average grain size of isolated island-like hard second phases (martensite + retained austenite) in bainite grains and tempered bainite grains is 2.0%. By setting the number density of carbides with a grain size of 300 nm or more in bainite grains and tempered bainite grains to 3.0 pieces/μm ^or less, the durability of steel plates and components of automobile bodies in the event of a collision is improved. The critical spacer thickness (ST) measured in a U-bending + close bending test that simulates the deformation and fracture behavior of the vertical wall part in a collision test, which is an index of impact properties, and the deformation of the bending ridge line part in the collision test and It is possible to improve the stroke at maximum load (SFmax) measured by a V-bending + orthogonal VDA bending test that simulates fracture behavior.

The present disclosure has been made based on the above findings. That is, the gist of the present disclosure is as follows.
[1] A steel plate comprising a base steel plate, the base steel plate comprising:
In mass%,
C: 0.030% or more and 0.250% or less,
Si: 0.01% or more and 0.75% or less,
Mn: 2.00% or more and less than 3.50%,
P: 0.001% or more and 0.100% or less,
S: 0.0200% or less,
Al: 0.010% or more and 2.000% or less,
N: 0.0100% or less,
, with the remainder consisting of Fe and unavoidable impurities;
As the structure at the 1/4 plate thickness position of the base steel plate,
Ferrite area ratio: less than 20.0%,
Fresh martensite area ratio: 15.0% or less,
Area ratio of retained austenite: 3.0% or less,
Area ratio of bainite and tempered bainite: more than 10.0% and not more than 70.0%,
Area ratio of tempered martensite: 30.0% or more and 80.0% or less,
Furthermore, the average crystal grain size of island-like fresh martensite and island-like retained austenite in the bainite grains and tempered bainite grains is 2.00 μm or less,
The average crystal grain size of carbides in bainite grains and tempered bainite grains is 500 nm or less,
Furthermore, a steel structure in which the number density of carbides with a grain size of 300 nm or more in bainite grains and tempered bainite grains is 3.0 pieces/μm ² or less,
has
A steel plate, wherein the amount of diffusible hydrogen contained in the base steel plate is 0.50 mass ppm or less, and the tensile strength is 1180 MPa or more.
[2] The component composition further includes, in mass%,
Nb: 0.200% or less,
Ti: 0.200% or less,
V: 0.200% or less,
B: 0.0100% or less,
Cr: 1.000% or less,
Ni: 1.000% or less,
Mo: 1.000% or less,
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 element 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,
Further, any one of [1] to [3] above, wherein the standard deviation σ of the 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 steel slab having the composition described in [1] or [2] above,
A hot rolling process in which hot rolling is performed at a finish rolling temperature of 820°C or higher to obtain a hot rolled steel plate;
An annealing step in which the steel plate after the hot rolling step is annealed at an annealing temperature of (Ac ₁ + (Ac ₃ - Ac ₁ )×5/8)° C. or higher and 950° C. or lower and an annealing time of 20 seconds or more. ,
After the annealing step, a first cooling step of cooling to a temperature range of 300° C. or higher and 550° C. or lower;
After the first cooling step, an intermediate holding step of holding at an intermediate holding temperature of 300° C. or more and 550° C. or less and a holding time of 20 seconds or more;
Applying a tension of 2.0 kgf/mm ² or more to the steel plate after the intermediate holding step in a temperature range of 300 ° C. or higher and 450 ° C. or lower,
After that, the steel plate is passed through 5 passes or more while contacting a roll with a diameter of 500 mm or more and 1500 mm or less for 1/4 rotation of the roll per pass,
Then, a second cooling step of cooling to a cooling stop temperature of less than 300°C;
After the second cooling step, the steel plate is reheated to a temperature range from the cooling stop temperature to 440° C. and held for 20 seconds or more, or further after the hot rolling step, and A method for manufacturing a steel plate, comprising a cold rolling process, in which the steel plate before the annealing process is subjected to cold rolling at a rolling reduction of 20% or more and 80% or less to obtain a cold rolled steel plate.
[8] The steel sheet according to [7], which includes a galvanizing step of subjecting the steel sheet after the intermediate holding step and before the second cooling step to form a galvanized layer on the steel sheet. Production method.
[9] The method for producing a galvanized steel sheet 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] After the hot rolling step and before the annealing step, the above [7] to [9] includes a metal plating step of applying metal plating to form a metal plating layer on one or both sides of the steel sheet. ] The 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, high yield stress YS and yield ratio YR, excellent press formability (ductility, hole expandability, and bendability), and rupture resistance at the time of collision. A steel plate having the following properties (bending fracture properties and axial crush properties) is obtained.
In addition, members made of the steel plate of the present invention have high strength, excellent press formability and impact resistance, and therefore can be extremely advantageously applied to automobile frame members, impact energy absorbing members, etc. Can be done.

FIG. 1 is an example of a SEM image of the present invention (Example No. 13 of the present invention). FIG. 2A is a diagram for explaining the U-bending process (primary bending process) in the U-bending + close contact bending test of the example. FIG. 2(b) is a diagram for explaining the close bending process (secondary bending process) in the U-bending + close bending test of the example. FIG. 3A is a diagram for explaining the V-bending process (primary bending process) in the V-bending + orthogonal VDA bending test of the example. FIG. 3(b) is a diagram for explaining the orthogonal VDA bending process (secondary bending process) in the V-bending + orthogonal VDA bending test of the example. FIG. 4(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. 4(b) is a perspective view of the test member shown in FIG. 4(a). FIG. 4(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: 0.030% or more and 0.250% or less, Si: 0.01% or more and 0.75% or less, and Mn. : 2.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0. 0100% or less, with the balance consisting of Fe and unavoidable impurities, and the structure at the 1/4th thickness position of the base steel plate has a ferrite area ratio of less than 20.0%, and fresh martensite. area ratio of retained austenite: 3.0% or less; area ratio of bainite and tempered bainite: more than 10.0% and 70.0% or less; area ratio: 30.0% or more and 80.0% or less, and furthermore, the average crystal grain size of island-like fresh martensite and island-like retained austenite in the bainite grains and tempered bainite grains is 2.00 μm or less. , the average crystal grain size of carbides in bainite grains and tempered bainite grains is 500 nm or less, and the number density of carbides with a grain size of 300 nm or more in bainite grains and tempered bainite grains is 3.0 pieces/μm ² or less, the amount of diffusible hydrogen contained in the base steel sheet is 0.50 mass ppm 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 "mass %", but hereinafter, unless otherwise specified, they will simply be expressed as "%".

C: 0.030% or more and 0.250% or less C is an effective element for generating an appropriate amount of tempered martensite, bainite, tempered bainite, etc., and ensuring a TS of 1180 MPa or more, high YS, and high YR. It is. Here, if the C content is less than 0.030%, 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 and YR.
On the other hand, when the C content exceeds 0.250%, the area ratio of fresh martensite increases, TS becomes excessively high, and El decreases. Furthermore, the area ratio of fresh martensite increases, the bendability in the V-bending test decreases, and the desired R/t (press formability) cannot be obtained. Furthermore, the area ratio of retained austenite increases when the steel plate undergoes punching in the hole expansion test, U-bending in the U-bending + close bending test, or V-bending in the V-bending + orthogonal VDA test. During the test, hard fresh martensite is generated due to deformation-induced transformation of retained austenite, and in subsequent tests, void formation and crack propagation occur, resulting in desired λ (press formability) and ST (impact fracture resistance) and SFmax (fracture resistance at the time of collision) cannot be obtained. Therefore, the C content is set to 0.030% or more and 0.250% or less. The C content is preferably 0.080% or more. Further, the C content is preferably 0.160% or less.

Si: 0.01% or more and 0.75% or less Si promotes ferrite transformation during annealing and during the cooling process after annealing. That is, Si is an element that affects the area ratio of ferrite. Here, if the Si content is less than 0.01%, the area ratio of ferrite decreases and ductility decreases.
On the other hand, when the Si content exceeds 0.75%, the volume fraction of retained austenite increases, and when the steel plate is subjected to punching in the hole expansion test or U-bending in the U-bending + contact bending test, Alternatively, when V-bending is performed in a V-bending + orthogonal VDA test, hard fresh martensite is generated due to deformation-induced transformation of retained austenite, and in subsequent tests, voids are formed and cracks grow, resulting in the desired λ , ST and SFmax cannot be obtained. Therefore, the Si content is set to 0.01% or more and 0.75% or less. The Si content is preferably 0.10% or more. Further, the Si content is preferably 0.70% or less.

Mn: 2.00% or more and less than 3.50% Mn is an element that adjusts the area ratio of tempered martensite, bainite, and further tempered bainite. Here, if the Mn content is less than 2.00%, the area ratio of ferrite increases and it becomes difficult to make the TS 1180 MPa or more. It also causes a decrease in YS and YR.
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 first cooling step decreases. As a result, the amount of fresh martensite generated in the second cooling process increases, and the fresh martensite is not sufficiently tempered in the subsequent reheating process, resulting in an increase in the area ratio of fresh martensite and the bending property of the V-bending test. decreases, and the desired R/t cannot be obtained. Therefore, the Mn content is set to 2.00% or more and less than 3.50%. The Mn content is preferably 2.30% or more. Further, the Mn content is preferably 3.30% 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, during the V-bending test, voids are generated and cracks grow along the prior austenite grain boundaries, making it impossible to obtain the desired R/t. In addition, when a steel plate is punched in a hole expansion test, when it is subjected to U bending in a U bending + close bending test, or when it is subjected to V bending in a V bending + orthogonal VDA test, prior austenite grain boundaries Void generation and crack propagation occur along the curve, making it impossible to obtain the desired λ, ST, and SFmax. 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: 0.0200% or less S exists as a sulfide in steel. In particular, when the S content exceeds 0.0200%, voids are generated and cracks propagate starting from the sulfides during the V-bending test, making it impossible to obtain the desired R/t. In addition, when a steel plate is punched in a hole expansion test, when it is subjected to a U-bending process in a U-bending + close bending test, or when it is subjected to a V-bending process in a V-bending + orthogonal VDA test, the sulfides are Void generation and crack growth occur at the starting point, making it impossible to obtain the desired λ, ST, and SFmax. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0080% or less.
Note that although the lower limit of the S content is not particularly specified, it is preferable that the S content is 0.0001% or more due to constraints on production technology.

Al: 0.010% or more and 2.000% or less Al promotes ferrite transformation during annealing and during the cooling process after annealing. That is, Al is an element that affects the area ratio of ferrite. Here, if the Al content is less than 0.010%, the area ratio of ferrite decreases and ductility decreases.
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 and YR. 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, if the N content exceeds 0.0100%, voids are generated and cracks propagate starting from the nitride during the V-bending test, making it impossible to obtain the desired R/t. In addition, when the steel plate was punched in the hole expansion test, when it was subjected to U bending in the U bending + close contact bending test, or when it was subjected to V bending in the V bending + orthogonal VDA test, the above nitrides were Voids are generated and cracks grow at the starting point, making it impossible to obtain the desired λ, ST, and SFmax. 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, due to constraints on production technology, the N content is preferably 0.0005% or more.

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 no lower limit is set in particular. 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, YS, and YR 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 formed. In such cases, coarse precipitates and inclusions become starting points for voids and cracks during hole expansion tests, V-bending tests, U-bending + close bending tests, or V-bending + orthogonal VDA bending tests. Desired λ, R/t, ST and SFmax may not be obtained. 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 Similar to Nb, Ti increases TS, YS, and YR 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 cases, coarse precipitates and inclusions become starting points for voids and cracks during hole expansion tests, V-bending tests, U-bending + close bending tests, or V-bending + orthogonal VDA bending tests. Desired λ, R/t, ST and SFmax may not be obtained. 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 cases, coarse precipitates and inclusions become starting points for voids and cracks during hole expansion tests, V-bending tests, U-bending + close bending tests, or V-bending + orthogonal VDA bending tests. Desired λ, R/t, ST and SFmax may not be obtained. 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 controls the generation and grain growth of ferrite 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. In addition, since the internal crack becomes the starting point of the crack during the hole expansion test, V-bending test, U-bending + close bending test, or V-bending + orthogonal VDA bending test, the desired λ, R/t, ST and SFmax may not be obtained. 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 improves hardenability, addition of Cr generates an appropriate amount of tempered martensite, thereby increasing TS, YS, and YR. 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.
Cr is more preferably 0.030% or more, and even more preferably 0.050% or more.
On the other hand, if the Cr content exceeds 1.000%, the area ratio of fresh martensite increases, hole expandability and V-bending test bendability decrease, and the desired λ and R/t may not be obtained. There is. 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, and the addition of Ni produces a large amount of tempered martensite, thereby increasing TS, YS, and YR. 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, if the Ni content exceeds 1.000%, the area ratio of fresh martensite increases, hole expandability and V-bending test bendability decrease, and the desired λ and R/t may not be obtained. There is. 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, and the addition of Mo generates a large amount of tempered martensite, thereby increasing TS, YS, and YR. 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, the hole expandability and the bendability in the V-bending test decrease, and the desired λ and R/t may not be obtained. There is. 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, and even more preferably 0.400% or less. The Mo content is more preferably 0.350% or less, and even more preferably 0.300% or less.

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 may be difficult to increase the TS to 1180 MPa or more. Furthermore, there is a possibility that YS will be lowered. 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 λ, R/t, ST, and SFmax. 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 may be difficult to increase the TS to 1180 MPa or more. Furthermore, there is a possibility that YS will be lowered. 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 cause a decrease in λ, R/t, ST, and SFmax. 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, and the addition of Cu generates a large amount of tempered martensite, thereby increasing TS, YS, and YR. 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, if the Cu content exceeds 1.000%, the area ratio of fresh martensite may increase excessively. Further, a large amount of coarse precipitates and inclusions may be generated. In such cases, excessively generated fresh martensite and coarse precipitates and inclusions may cause voids during hole expansion tests, V-bending tests, U-bending + close bending tests, or V-bending + orthogonal VDA bending tests. In addition, the desired λ, R/t, ST and SFmax may not be obtained because the cracks become the starting point of cracks. 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, YS, and YR by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. let 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 cases, excessively coarse precipitates and inclusions become starting points for voids and cracks during hole expansion tests, V-bending tests, U-bending + close bending tests, or V-bending + orthogonal VDA bending tests. , desired λ, R/t, ST and SFmax may not be obtained. 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, and the addition of W generates a large amount of tempered martensite, thereby increasing TS, YS, and YR. 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 fresh martensite increases, hole expandability and V-bending test bendability decrease, and the desired λ and R/t may not be obtained. There is. 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 and bendability 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 cases, excessively coarse precipitates and inclusions become starting points for voids and cracks during hole expansion tests, V-bending tests, U-bending + close bending tests, or V-bending + orthogonal VDA bending tests. , desired λ, R/t, ST and SFmax may not be obtained. 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 and bendability 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, excessively coarse precipitates and inclusions become starting points for voids and cracks during hole expansion tests, V-bending tests, U-bending + close bending tests, or V-bending + orthogonal VDA bending tests. , desired λ, R/t, ST and SFmax may not be obtained. 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 and bendability 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 cases, excessively coarse precipitates and inclusions become starting points for voids and cracks during hole expansion tests, V-bending tests, U-bending + close bending tests, or V-bending + orthogonal VDA bending tests. , desired λ, R/t, ST and SFmax may not be obtained. 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 spheroidizing the shape of inclusions and improving the hole expandability and bendability 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%, excessively coarse precipitates and inclusions may occur during the hole expansion test, V-bending test, U-bending + close bending test, or V-bending + During the orthogonal VDA bending test, the desired λ, R/t, ST and SFmax may not be obtained because it becomes a starting point for voids and cracks. Therefore, when Zr is contained, 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 cases, excessively coarse precipitates and inclusions become starting points for voids and cracks during hole expansion tests, V-bending tests, U-bending + close bending tests, or V-bending + orthogonal VDA bending tests. , desired λ, R/t, ST and SFmax may not be obtained. 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, and 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 and bendability 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, if the content of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM exceeds 0.0200% each, or if the content of As exceeds 0.0500% each, coarse A large amount of precipitates and inclusions may be generated. In such cases, excessively coarse precipitates and inclusions become starting points for voids and cracks during hole expansion tests, V-bending tests, U-bending + close bending tests, and V-bending + orthogonal VDA bending tests. λ, R/t, ST and SFmax may not be obtained. 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 REM content is preferably 0.0200% or less, and the As content is preferably 0.0500% or less.
The Se content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Se content is more preferably 0.0180% 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. Bi is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
REM is more preferably 0.0005% or more, and even more preferably 0.0008% or more. REM 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 is preferably Sc, Y, Ce, or La.

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.

Ferrite area ratio: less than 20.0% (including 0.0%)
If the area ratio of ferrite increases excessively, it becomes difficult to increase the TS to 1180 MPa or more. It also causes a decrease in YS and YR. Therefore, the area ratio of ferrite is less than 20.0% (including 0.0%). Further, the area ratio of ferrite is preferably 15.0% or less.

Fresh martensite area ratio: 15.0% or less (including 0.0%)
In the present invention, if the area ratio of fresh martensite increases excessively, fresh martensite becomes the starting point for void generation during hole expansion in the hole expansion test or bending in the V-bending test, so that the desired λ and R/ t cannot be obtained. Therefore, the area ratio of fresh martensite is set to 15.0% or less. Further, the area ratio of fresh martensite is preferably 10.0% or less.
Note that the lower limit of the area ratio of fresh martensite is not particularly limited, and may be 0.0%. The fresh martensite referred to here is martensite that is still quenched (not tempered). Furthermore, the fresh martensite referred to herein also includes (isolated) island-like fresh martensite within bainite grains and tempered bainite grains, which will be described later.

Area ratio of retained austenite: 3.0% or less (including 0.0%)
In the present invention, when the area ratio of retained austenite increases excessively, when the steel plate is punched in the hole expansion test, when the steel plate is subjected to U bending in the U-bending + close bending test, or when the steel plate is subjected to U-bending in the V-bending + orthogonal VDA test, When subjected to V-bending, hard fresh martensite is generated due to deformation-induced transformation of retained austenite, and in subsequent tests, voids are formed and cracks grow, and the desired λ, ST, and SFmax are not obtained. do not have. Therefore, the area ratio of retained austenite is set to 3.0% or less. The area ratio of retained austenite is preferably 2.5% or less, more preferably 2.0% or less.
The lower limit of the area ratio of retained austenite is not particularly limited, but is preferably 0.1% or more, more preferably 0.2% or more.
The retained austenite herein also includes (isolated) island-like retained austenite within bainite grains and tempered bainite grains, which will be described later.

Here, during the second cooling step in the manufacturing method described below, a tension of 2.0 kgf/mm ² or more is applied to the steel plate in a temperature range of 300°C or more and 450°C or less, and then the steel plate is heated to a diameter of 500 mm or more per pass. By passing the steel plate through 5 passes or more while contacting a roll of 1500 mm or less for 1/4 of the roll, untransformed austenite undergoes deformation-induced transformation and becomes fresh martensite, and in the subsequent reheating process, the fresh martensite is transformed into fresh martensite. By controlling the area ratio of fresh martensite to 15.0% or less and the area ratio of retained austenite to 3.0% or less after tempering, the desired area ratio of tempered martensite can be secured. It becomes possible.

Area ratio of bainite and tempered bainite: more than 10.0% and not more than 70.0% Bainite (B) is a structure generated in the first cooling step and intermediate holding step, as shown in FIG. Moreover, the tempered bainite (BT) referred to here is a structure in which the bainite generated in the reheating process has been tempered, as shown in FIG. In FIG. 1, F: ferrite, M: martensite, RA: retained austenite, TM: tempered martensite, θ: carbide.
When the area ratio of bainite and tempered bainite is 10.0% or less, it becomes difficult to ensure good ductility, that is, to obtain a desired El. Therefore, the area ratio of bainite and tempered bainite is made to be more than 10.0%.
On the other hand, if the area ratio of bainite and tempered bainite increases excessively to more than 70.0%, it becomes difficult to secure a TS of 1180 MPa or more. Therefore, the area ratio of bainite and tempered bainite is 70.0% or less. Further, the area ratio of bainite and tempered bainite is preferably 15.0% or more. Further, the area ratio of bainite and tempered bainite is preferably 65.0% or less.

Area ratio of tempered martensite: 30.0% or more and 80.0% or less Tempered martensite is a structure obtained in a reheating process. Here, the hard second phase (fresh martensite + retained austenite) is an effective structure for securing the desired TS, but since it is a structure that promotes the generation of voids and the propagation of cracks during press forming and collision, The area ratio of martensite must be 15.0% or less, and the volume ratio of retained austenite must be 3.0% or less. In particular, during the second cooling step in the manufacturing method described below, a tension of 2.0 kgf/mm2 ^or more is applied in a temperature range of 300°C or more and 450°C or less, and then the steel plate is By passing 5 or more passes while contacting the roll for 1/4 rotation of the roll, untransformed austenite undergoes deformation-induced transformation and becomes fresh martensite, and in the subsequent reheating process, the fresh martensite is tempered and tempered. It becomes martensite. That is, the above-mentioned tempered martensite has a structure necessary to obtain desired λ, R/t, ST and SFmax. Therefore, the area ratio of tempered martensite is set to 30.0% or more. The area ratio of tempered martensite is preferably 35.0% or more.
On the other hand, when the area ratio of tempered martensite increases too much, the desired area ratio of bainite and tempered bainite cannot be obtained, and it becomes difficult to ensure good ductility, that is, to obtain the desired El. Therefore, the area ratio of tempered martensite is 70.0% or less. The area ratio of tempered martensite is preferably 60.0% or less.

Average grain size of island-like fresh martensite and island-like retained austenite in bainite grains and tempered bainite grains: 2.00 μm or less In the present invention, isolated island-like fresh martensite in bainite grains and tempered bainite grains When the average grain size of the isolated island-like retained austenite is small, it is possible to further suppress the generation of voids while ensuring a TS of 1180 MPa or more, and to obtain better λ, R/t, ST and SFmax. Therefore, the average crystal grain size of isolated island-like fresh martensite and isolated island-like retained austenite in bainite grains and tempered bainite grains is set to 2.00 μm or less.
In the present invention, the average crystal grain size of isolated island-like fresh martensite in bainite grains and tempered bainite grains and isolated island-like retained austenite is the same as that of island-like fresh martensite in bainite grains and tempered bainite grains. and the average grain size of the island-like retained austenite. That is, in the present invention, the average crystal grain size of island-like fresh martensite and island-like retained austenite in bainite grains and tempered bainite grains is set to 2.00 μm or less.
Further, the average crystal grain size of the island-like fresh martensite and the island-like retained austenite in the bainite grains and in the tempered bainite grains is preferably 1.00 μm or less.
Although the lower limit is not particularly limited, the average crystal grain size of island-like fresh martensite and island-like retained austenite in bainite grains and tempered bainite grains is preferably 0.10 μm or more, more preferably 0.20 μm or more. be.

Average grain size of carbides in bainite grains and tempered bainite grains: 500 nm or less In the present invention, when the average grain size of carbides in bainite grains and tempered bainite grains is small, while securing a TS of 1180 MPa or more, The generation of voids can be further suppressed, and better λ, R/t, ST and SFmax can be obtained. Therefore, the average crystal grain size of carbides within the bainite grains and within the tempered bainite grains is set to 500 nm or less. Note that the average crystal grain size of carbides within the bainite grains and within the tempered bainite grains is preferably 300 nm or less.
Although the lower limit is not particularly limited, the average crystal grain size of carbides in bainite grains and tempered bainite grains is preferably 50 nm or more, more preferably 80 nm or more.

Number density of carbides with a grain size of 300 nm or more in bainite grains and tempered bainite grains: 3.0 pieces/μm ² or less In the present invention, the number of carbides with a grain size of 300 nm or more in bainite grains and tempered bainite grains When the density is small, while ensuring a TS of 1180 MPa or more, the generation of voids can be further suppressed, and better λ, R/t, ST and SFmax can be obtained. Therefore, the number density of carbides with a grain size of 300 nm or more in bainite grains and tempered bainite grains is set to 3.0 pieces/μm ² or less. The number density of carbides having a grain size of 300 nm or more in the bainite grains and tempered bainite grains is preferably 2.5 carbides/μm ² or less.
Although the lower limit is not particularly limited, the number density of carbides within the bainite grains and within the tempered bainite grains is preferably 0.2 pieces/μm ² or more, more preferably 0.5 pieces/μm ² or more.

Note that the area ratio of the remaining structures other than the aforementioned ferrite, fresh martensite, retained austenite, bainite, tempered bainite, and tempered martensite is preferably 10.0% or less. The area ratio of the remaining tissue is more preferably 5.0% or less. Further, the area ratio of the remaining tissue may be 0.0%.

The residual structure is not particularly limited, and examples thereof include unrecrystallized ferrite and pearlite. The type of residual tissue can be confirmed, for example, by observation using a scanning electron microscope (SEM).

Here, the area ratio of ferrite, bainite, tempered bainite, tempered martensite, and hard second phase (fresh martensite + retained austenite) is measured as follows at the 1/4 thickness position of the base steel plate.
That is, the sample is cut out so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate serves as the observation surface. Next, the observation surface of the sample is polished with diamond paste, and then final polished with alumina. Next, the observation surface of the sample was exposed to 3 vol. % nital to reveal the tissue. Next, the observation position is set at 1/4 of the thickness of the steel plate, and 5 fields of view are observed using an SEM at a magnification of 3000 times. From the obtained structure image, the area of each constituent structure (ferrite, bainite, tempered bainite, tempered martensite, and hard second phase (fresh martensite + retained austenite)) was measured using Adobe Photoshop from Adobe Systems. The area ratio divided by is calculated for five fields of view, and these values are averaged to determine the area ratio of each tissue.
Ferrite: A black region with a block-like shape. In addition, it contains almost no carbide. Furthermore, isolated island-like fresh martensite and isolated island-like retained austenite within the ferrite grains are not included in the area ratio of ferrite.
Bainite and tempered bainite: A black to dark gray area, with a lumpy or irregular shape. It also contains a relatively small amount of carbide.
Tempered martensite: A gray area with an amorphous shape. It also contains a relatively large number of 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 carbide.
Carbide: A white region with a dotted or linear shape. It is included in bainite, tempered bainite, and tempered martensite.
Residual structure: Examples include the above-mentioned unrecrystallized ferrite and pearlite, and their forms are known.

From the SEM image used for the above-mentioned structure fraction measurement, isolated island-like fresh martensite and isolated island-like retained austenite inside bainite grains and tempered bainite grains were extracted by hand, and the images were extracted using open source ImageJ. , the average grain size of isolated island-like fresh martensite and isolated island-like retained austenite in bainite grains and tempered bainite grains is determined.
The above average grain size is determined by dividing the total area of island-like fresh martensite and island-like retained austenite by the number of island-like fresh martensite and island-like retained austenite to find the average area, and then dividing the above average area into a circle. The equivalent circle diameter obtained by dividing by the periodicity π and multiplying the square root by 2 is defined as the average crystal grain size.
Regarding an isolated island-like fresh martensite or an isolated island-like retained austenite, in the SEM image, the outer periphery is surrounded by bainite and/or tempered bainite, and the island-like region is formed integrally without interruption. Measure as one piece.
In addition, from the SEM image used for the above-mentioned structure fraction measurement, only the carbides inside the bainite grains and tempered bainite grains were extracted by hand, and using open source ImageJ, the inside of the bainite grains and tempered bainite grains were extracted. The average grain size of carbides and the number density of carbides with a grain size of 300 nm or more among the carbides in the bainite grains and tempered bainite grains are determined.
The above average grain size is calculated by dividing the total area of carbides by the number of carbides to find the average area, dividing the above average area by pi, and multiplying the square root of the circle by 2. The equivalent diameter is taken as the average grain size.
In addition, regarding one carbide, in the SEM image, an island-like region whose outer periphery is surrounded by bainite and/or tempered bainite and is integrally formed without interruption is measured as one.

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.

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 (%)]

In addition, the area ratio of the residual structure is obtained by subtracting the area ratio of ferrite, the area ratio of bainite and tempered bainite, the area ratio of tempered martensite, and the area ratio of the hard second phase obtained as described above from 100.0%. Find by.
[Area ratio of residual structure (%)] = 100.0 - [Area ratio of ferrite (%)] - [Area ratio of bainite and tempered bainite (%)] - [Area ratio of tempered martensite (%)] - [Hard second phase area ratio (%)]

Amount of diffusible hydrogen (in steel) contained in the base steel sheet: 0.50 mass ppm or less When the amount of diffusible hydrogen in the steel sheet exceeds 0.50 mass ppm, the desired λ, R/t, ST and SFmax I can't get it.
Note that the amount of diffusible hydrogen in the steel sheet is preferably 0.25 mass ppm or less. Further, although the lower limit of the amount of diffusible hydrogen in the steel sheet is not particularly specified, it is preferable that the amount of diffusible hydrogen in the steel sheet is 0.01 mass ppm or more due to constraints on production technology.
Note that the base steel plate for measuring the amount of diffusible hydrogen may be a high-strength steel plate before plating treatment, or a base steel plate of high-strength galvanized steel sheet after galvanizing treatment and before processing. In addition, it may be a base steel plate of a steel plate that has been subjected to processes such as punching and stretch flange forming after galvanizing, or it may be a base part of a product manufactured by welding the processed steel plate. I don't mind.

Here, the method for measuring the amount of diffusible hydrogen in a steel sheet is as follows. A test piece with a length of 30 mm and a width of 5 mm is taken, and if a galvanized layer is formed on the steel sheet, the hot-dip galvanized layer or the alloyed hot-dip galvanized layer is removed with alkali. Thereafter, the amount of hydrogen released from the test piece is measured by temperature programmed desorption analysis. Specifically, after continuously heating from room temperature (-5 to 55°C) to 300°C at a heating rate of 200°C/h, cooling to room temperature, and measuring the cumulative amount of hydrogen released from the test piece from room temperature to 210°C. The amount is measured and taken as the amount of diffusible hydrogen in the steel sheet. It is preferable to measure the amount of diffusible hydrogen after the production of the steel sheet is completed. In addition, it is more preferable to measure the amount of hydrogen within one week after the completion of manufacturing the steel plate.
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.

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 surface soft layer is not particularly determined, it is preferably 7 μm or more, and more preferably more than 14 μm. 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 the total number of measurements at 1/4 position of the directional depth In the present invention, in order to obtain excellent bendability during press forming and excellent bending rupture characteristics 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 percentage of measurements where the nanohardness of the sheet surface at 1/4 of the depth in the thickness direction of the surface soft layer from the steel sheet 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 (martensite, etc.), 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 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 microstructure hardness in the micro region is small, and it is difficult to prevent the formation and connection of voids during press forming and collision. It becomes possible to further suppress the propagation of cracks, and excellent R/t and SFmax can be obtained.
Further, a preferable range of 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 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 a position 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.
First, 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 thickness direction depth of the surface soft layer - 5 μm, and the base steel plate is Buff polishing with diamond and alumina is performed from the surface to 1/4 of the depth in the board thickness direction of the surface soft layer, and further polishing is performed with colloidal silica. 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.
Further, mechanical polishing is performed to 1/2 the depth in the thickness direction of the surface soft layer, buff polishing with diamond and alumina, and further colloidal silica polishing. Then, the 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) (note that the metal plating layer (first plating layer) is , 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 a 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 -20°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, even if the dew point is −20° C. or lower and 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.

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 of the steel plate according to one embodiment of the present invention is 1180 MPa or more.
In addition, the yield stress (YS), yield ratio (YR), total elongation (El), critical hole expansion rate (λ), critical spacer thickness in U bending + close contact bending test ( The reference value of the stroke at maximum load (SFmax) in the ST) and V-bending + orthogonal VDA bending tests, and the presence or absence of fracture (appearance cracking) in the axial crushing test are as described above.

In addition, tensile strength (TS), yield stress (YS), yield ratio (YR), and total elongation (El) are measured by a tensile test in accordance with JIS Z 2241 (2011) 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 critical spacer thickness (ST) is measured by the U-bending + close-contact bending test described later in Examples. The stroke (SFmax) at maximum load in the V-bending + orthogonal VDA bending test is measured by the V-bending + orthogonal VDA bending test described later in the Examples. The presence or absence of fracture (appearance cracking) in the axial crushing test is determined by the axial crushing test described later in Examples.

Galvanized layer (second plating layer)
A 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. The steel sheet having a galvanized layer may be a galvanized steel sheet.
That is, the steel sheet of the present invention has a base steel plate, and a second plating layer (a galvanized layer, an aluminum plating layer, etc.) may be formed on the base steel plate. A metal plating layer (a first plating layer (excluding the second plating layer of the galvanized layer)) and a second plating layer (a zinc plating layer, an aluminum plating layer, etc.) may be formed in this order on the base 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.

Note that 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.
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.
The method for manufacturing a steel plate of the present invention includes a hot rolling process in which a steel slab having the above-mentioned composition is hot rolled at a finish rolling temperature of 820°C or higher to obtain a hot rolled steel plate; An annealing process in which the steel plate after the rolling process is annealed at an annealing temperature of (Ac ₁ + (Ac ₃ - Ac ₁ )×5/8)°C or higher and 950°C or lower and an annealing time of 20 seconds or more; After that, a first cooling step of cooling to a temperature range of 300° C. or more and 550° C. or less, and after the first cooling step, holding under the conditions of intermediate holding temperature: 300° C. or more and 550° C. or less, holding time: 20 seconds or more, After the intermediate holding step, a tension of 2.0 kgf/mm ² or more is applied to the steel plate in a temperature range of 300°C or more and 450°C or less, and then the steel plate is heated to a diameter of 500 mm or more and 1500 mm per pass. A second cooling process in which the steel plate is passed through 5 passes or more while being in contact with the following roll for 1/4 rotation of the roll, and then cooled to a cooling stop temperature of less than 300°C. After the second cooling process, the steel plate is cooled from room temperature to A reheating step of cooling to a cooling stop temperature of less than 300 ° C., then reheating to a temperature range from the cooling stop temperature to 440 ° C. and holding for 20 seconds or more, or further after a hot rolling step, The method also includes a cold rolling process in which the steel plate before the annealing process is subjected to cold rolling at a rolling reduction of 20% or more and 80% or less to obtain a cold rolled steel plate.

In the present invention, the method of melting the steel material (steel slab) is not particularly limited, and any known melting method such as a converter or an electric furnace is suitable. Moreover, in order to prevent macro segregation, the steel slab (slab) is preferably manufactured by a continuous casting method, but it is also possible to manufacture it by an ingot method, a thin slab casting method, or the like. In addition to the conventional method of manufacturing a steel slab, cooling it to room temperature and then heating it again, there are also methods in which the steel slab is charged into a heating furnace as a hot piece without being cooled to room temperature, or it is slightly heat-retained. Energy-saving processes such as direct rolling and direct rolling, which involve rolling immediately after rolling, can also be applied without problems.

(Hot rolling process)
When heating the slab, the slab heating 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 the temperature of the slab surface. In addition, slabs are roughly rolled into sheet bars under normal conditions, but if the heating temperature is lower, from the perspective of preventing trouble during hot rolling, a bar heater etc. is used to roll the slabs into sheets before finishing rolling. Preferably, the bar is heated.

Finish rolling temperature: 820°C or higher Finish rolling increases the rolling load and the reduction rate in the unrecrystallized state of austenite, which develops an abnormal structure that is elongated in the rolling direction, resulting in poor ductility and holes in the final material. Decreases spreadability and bendability. For this reason, the finish rolling temperature is set to 820°C or higher. The finish rolling temperature is preferably 830°C or higher, more preferably 850°C or higher. Further, the finish rolling temperature is preferably 1080°C or lower, more preferably 1050°C or lower.

Furthermore, the coiling temperature after hot rolling is not particularly limited, but it is necessary to consider the case where the ductility, hole expandability, and bendability of the final material are reduced. For this reason, the coiling temperature after hot rolling is preferably 300°C or higher. Further, the coiling temperature after hot rolling is preferably 700°C or less.

Note that the rough rolled plates may be joined together during hot rolling and finish rolling may be performed continuously. Alternatively, the rough rolled plate may be wound up once. 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 uniformity of the shape of the steel sheet and uniformity of material quality. Note that the friction coefficient during lubricated rolling is preferably 0.10 or more. Further, the friction coefficient during lubricated rolling is preferably 0.25 or less.

(pickling process)
The hot rolled steel sheet produced as described above may be pickled. Since pickling can remove oxides on the surface of the steel sheet, it can be carried out to ensure good chemical conversion treatment properties and plating quality in the final high-strength steel sheet. Moreover, pickling may be carried out once or may be divided into multiple times.

(cold rolling process)
The hot-rolled pickled plate or hot-rolled steel plate obtained as described above is subjected to cold rolling, if necessary. When cold rolling is performed, the pickled plate may be cold rolled after hot rolling, or cold rolling may be performed after heat treatment. Further, optionally, the cold rolled steel sheet obtained after cold rolling may be pickled.
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.

If necessary, cold rolling reduction rate: 20% or more and 80% or less When cold rolling is performed, the cold rolling reduction rate (cumulative reduction rate) is not particularly limited, but should be 20% or more and 80% or less. It is preferable. 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. Therefore, the reduction ratio in cold rolling is preferably 20% or more. 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. Therefore, the reduction ratio in cold rolling is preferably 80% or less.

(Metal plating (metal electroplating, first plating) process)
In one embodiment of the present invention, metal plating is applied to one or both sides of the steel plate after the hot rolling process (or after the cold rolling process if cold rolling is performed) and before the annealing process. It may include a first plating step of forming a 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 Fe is more preferable. Although Fe-based electroplating will be described below as an example, the following conditions for Fe-based electroplating can be similarly adopted for other metal-based electroplating.

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. .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 concentration of acid is not particularly specified, it is preferably 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)
In one embodiment of the present invention, after the hot rolling process (if cold rolling is performed, after the cold rolling process, if metal plating is performed to form a metal plating layer (first plating layer), metal plating is performed. If cold rolling and metal plating are performed after the process, after the metal plating process), the annealing temperature for the steel plate is: (Ac ₁ + (Ac ₃ - Ac ₁ ) x 5/8) ℃ or more and 950 ℃ or less, Holding time: Includes an annealing step of 20 seconds or more.

Annealing temperature: (Ac ₁ + (Ac ₃ - Ac ₁ ) x 5/8) ℃ or more and 950 ℃ or less When the annealing temperature is less than (Ac ₁ + (Ac ₃ - Ac ₁ ) x 5/8) ℃, ferrite and The rate of austenite formation during heating in the austenite two-phase region becomes insufficient. Therefore, the area ratio of ferrite increases excessively after annealing, making it impossible to obtain desired TS, YS, and YR.
On the other hand, when the annealing temperature exceeds 950°C, the grain size of austenite becomes coarse, and finally the average crystal grain size of isolated island-like fresh martensite and isolated island-like residual austenite in bainite grains and tempered bainite grains. exceeds 2.00 μm, making it difficult to obtain good λ, R/t, ST, and SFmax.
Therefore, the annealing temperature is set to be at least (Ac ₁ +(Ac ₃ -Ac ₁ )×5/8)°C and at most 950°C. The annealing temperature is preferably 900°C or lower. Note that the annealing temperature is the highest temperature reached in the annealing step.

Ac ₁ point (°C) and Ac ₃ point (°C) can be calculated using 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, making it impossible to obtain TS, YS, and YR. 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.
The number of times of annealing may be two or more times, but from the viewpoint of energy efficiency, one time is preferable.

Dew point of annealing process atmosphere (annealing atmosphere): −30° C. or higher In an embodiment of the present invention, it is preferable that the dew point of the annealing step atmosphere (annealing atmosphere) is −30° C. or higher. By performing annealing at a dew point of the annealing atmosphere of −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, still more preferably higher than -20°C, even more preferably -15°C or higher, and most preferably -5°C or higher.
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. The dew point of the annealing atmosphere in the annealing step is more preferably 25°C or lower, and even more preferably 20°C or lower.

(First cooling process)
In the present invention, after the annealing step, the first cooling stop temperature is set to 300° C. or more and 550° C. or less, and includes a first cooling step of cooling to the first cooling stop temperature.

First cooling stop temperature: 300°C or more and 550°C or less When the first cooling stop temperature is less than 300°C or more than 550°C, the area ratio of bainite and tempered bainite becomes 10.0% or less, ensuring good ductility, i.e. , it becomes difficult to obtain the desired El.
Further, the average crystal grain size of isolated island-like fresh martensite and isolated island-like retained austenite in the bainite grains and tempered bainite grains is more than 2.00 μm. Further, the average crystal grain size of carbides in bainite grains and tempered bainite grains is more than 500 nm, and the number density of carbides with a grain size of 300 nm or more in bainite grains and tempered bainite grains is 3.0 pieces/μm. It may exceed ² in some cases. This makes it difficult to obtain good λ, R/t, ST and SFmax. Therefore, in the present invention, after the annealing step, the first cooling stop temperature is set to 300° C. or more and 550° C. or less, and the material is cooled to this first cooling stop temperature.

(Intermediate holding process)
Intermediate holding temperature: 300°C or more and 550°C or less, intermediate holding time: 20 seconds or more In the present invention, after the first cooling step, in the intermediate holding step, intermediate holding temperature: 300°C or more and 550°C or less, holding time: 20 seconds or more Retention is performed under the following conditions.
When the intermediate holding temperature is less than 300°C or more than 550°C, or when the holding time (intermediate holding time) is 20 seconds or more, the area ratio of bainite and tempered bainite becomes 10.0% or less, ensuring good ductility. That is, it becomes difficult to obtain the desired El. Further, the average crystal grain size of isolated island-like fresh martensite and isolated island-like retained austenite in the bainite grains and tempered bainite grains is more than 2.00 μm. Further, the average crystal grain size of carbides in bainite grains and tempered bainite grains is more than 500 nm, and the number density of carbides with a grain size of 300 nm or more in bainite grains and tempered bainite grains is 3.0 pieces/μm. It may exceed ² in some cases. This makes it difficult to obtain good λ, R/t, ST and SFmax.
Therefore, in the present invention, in the intermediate holding step, holding is performed under conditions of intermediate holding temperature: 300° C. or higher and 550° C. or lower, and holding time (intermediate holding time): 20 seconds or more.

(Zinc plating process (second plating process))
In the present invention, the steel plate may be galvanized after the intermediate holding step. 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 to perform the hot-dip galvanizing treatment in the manner described above, and then heat the hot-dip galvanized steel sheet to an alloying temperature of 450° C. or higher to perform the 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 speed becomes slow and alloying may become difficult. On the other hand, when the alloying temperature exceeds 600°C, untransformed austenite transforms into pearlite, making it difficult to make the TS 1180 MPa or higher. Note that the alloying temperature is more preferably 500°C or higher, and still more preferably 510°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)
In the present invention, after the intermediate holding step (or after the galvanizing step in the case of a galvanizing step), a tension of 2.0 kgf/mm ² or more is applied to the steel plate in a temperature range of 300°C or higher and 450°C or lower. Then, the steel plate is passed through 5 passes or more while being in contact with a roll having a diameter of 500 mm or more and 1500 mm or less for 1/4 rotation of the roll per pass, and then cooled to a cooling stop temperature (second cooling stop temperature) of less than 300 ° C. A second cooling step is included.

Tension applied in a temperature range of 300°C to 450°C: 2.0 kgf/mm ² or more In the present invention, by applying a tension of 2.0 kgf/mm ^{2 or more} to the steel plate one or more times as described above, , the majority of austenite becomes martensite through deformation (stress/strain)-induced transformation, and then undergoes tempering in the reheating process, so the area ratio of fresh martensite in the final structure can be reduced, and the appropriate amount of tempered martensite can be reduced. Can be secured. Moreover, the amount of austenite immediately after the second cooling process can be reduced, and the volume fraction of retained austenite in the final structure can be reduced. As a result, desired λ, R/t, ST and SFmax are obtained.
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.
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.

The number of passes in which the steel plate is passed through a roll having a diameter of 500 mm or more and 1500 mm or less per pass while contacting the roll for 1/4 of the roll: 5 passes or more In the present invention, the steel plate is passed through a roll having a diameter of 500 mm or more and 1500 mm or less per pass. By passing the steel plate through 5 or more passes while contacting the roll for 1/4 rotation, most of the austenite becomes martensite due to deformation (stress/strain)-induced transformation, and is then tempered in the reheating process, resulting in a final structure. The area ratio of fresh martensite can be reduced, and an appropriate amount of tempered martensite can be secured. Further, the amount of austenite immediately after the second cooling step can be reduced, and the volume fraction of retained austenite in the final structure can be reduced. As a result, desired λ, R/t, ST and SFmax are obtained.
The number of passes is preferably 6 passes or more, more preferably 7 passes or more.
Although the upper limit is not particularly limited, the number of passes is preferably 10 passes or less, more preferably 9 passes or less.

Second cooling stop temperature: less than 300°C The cooling conditions for the second cooling step are not particularly limited, and may be according to a conventional method. As the cooling method, for example, gas jet cooling, mist cooling, roll cooling, water cooling, air cooling, etc. can be applied. By setting the second cooling stop temperature to less than 300°C, an appropriate amount of austenite transforms into martensite and is then tempered in the reheating process, which reduces the area ratio of fresh martensite in the final structure. , it is possible to secure an appropriate amount of tempered martensite. Further, the amount of austenite immediately after the second cooling step can be reduced, and the volume fraction of retained austenite in the final structure can be reduced. As a result, desired λ, R/t, ST and SFmax are obtained. In addition, from the viewpoint of preventing surface oxidation, it is preferable to cool to 250° C. or lower. The lower limit is not particularly limited, but it is preferably room temperature (-5°C or higher and 55°C or lower). It is preferable that the average cooling rate is, for example, 1° C./second or more. Further, the average cooling rate is preferably 50° C./second or less. Here, the average cooling rate (°C/s) is calculated from (cooling start temperature (°C)−cooling stop temperature (°C))/cooling time (s).

(Reheating process)
After the second cooling step, as a reheating step, the steel plate is reheated to a temperature range from the cooling stop temperature (second cooling stop temperature) to 440° C. and held for 20 seconds or more.

Reheating temperature: Temperature range above the above cooling stop temperature (second cooling stop temperature) and below 440°C Reheating holding time: 20 seconds or more In the present invention, reheating to above the cooling stop temperature (second cooling stop temperature) By holding the temperature for 20 seconds or more, diffusible hydrogen in the steel is released. Furthermore, the area ratio of fresh martensite in the final structure can be reduced, and an appropriate amount of tempered martensite can be secured. Moreover, the amount of austenite immediately after the second cooling process can be reduced, and the volume fraction of retained austenite in the final structure can be reduced. As a result, desired λ, R/t, ST and SFmax are obtained.
On the other hand, when the reheating temperature exceeds 440° C., some of the zinc plating melts and adheres to the roll when galvanizing is performed, making it impossible to obtain a uniformly galvanized hot-dip galvanized steel sheet. Moreover, when the reheating holding time is less than 20 seconds, the desired amount of diffusible hydrogen in the steel is not released.
Therefore, in the present invention, the temperature is reheated to a temperature range from the second cooling stop temperature to 440° C. and held for 20 seconds or more.
The reheating temperature is preferably 40°C or higher, more preferably 160°C or higher.
Further, the reheating temperature is preferably 420°C or lower, more preferably 320°C or lower.
The reheating holding time is preferably 25 seconds or more, more preferably 30 seconds or more.
Further, the reheating holding time is preferably 300 seconds or less, more preferably 200 seconds or less.

Additionally, the steel plate obtained as described above may be further subjected to temper rolling. When the rolling reduction ratio in temper rolling exceeds 2.00%, yield stress increases and dimensional accuracy when forming a steel plate into a member may 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 equipment that is continuous with the annealing equipment for performing each process mentioned above (online), or on equipment that is discontinuous with the annealing equipment 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 for other manufacturing methods are not particularly limited, but from the viewpoint of productivity, a series of treatments such as annealing, hot-dip galvanizing, and alloying treatment of galvanizing are performed on a CGL (Continuous Galvanizing Line), which is a hot-dip galvanizing line. It is preferable to carry out the process using Line). After hot-dip galvanizing, wiping can be performed to adjust the coating weight. Note that the conditions for plating and the like other than the above-mentioned conditions can be based on a conventional method for hot-dip galvanizing.

[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 a forming process and a bonding process to produce a member.
Here, the above steel plate has a TS: 1180 MPa or more, high YS and YR, excellent press formability (ductility, hole expandability, and bendability), and rupture resistance properties at the time of collision (bending rupture 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 a 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 forming processing and joining processing to produce a member. have
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, rivet 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 were calculated using 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, [%Si]: Si content, and [%Mn]: Mn content.

The obtained steel slab was heated to 1200° C., and after heating, the steel slab was subjected to rough rolling and hot rolling to obtain a hot rolled steel plate. Then, the obtained hot rolled steel sheet No. 1~No. 56, No. 60~No. 83, No. 92~No. 106, No. 112~No. No. 117 was pickled and cold rolled to obtain cold rolled steel sheets having the thicknesses shown in Table 3, Table 5, and Table 7. Moreover, No. of the obtained hot-rolled steel sheet. 57~No. 59, No. 84~No. 91, No. 107~No. No. 111 was pickled to obtain hot rolled steel sheets (white skin) having the thicknesses shown in Table 3, Table 5, and Table 7.
Then, the obtained cold-rolled steel sheet or hot-rolled steel sheet (white skin) is subjected to an annealing process, a first cooling process, an intermediate holding process, a galvanizing process, a second cooling process, and a reheating process under the conditions shown in Table 2. process,
In addition, under the conditions shown in Table 4, the first plating process (metal plating process), annealing process, first cooling process, intermediate holding process, second plating process (zinc plating process), second cooling process, and reheating process were performed. The treatment was carried out to obtain a steel plate (galvanized steel plate).
Further, under the conditions shown in Table 6, the first plating step (metal plating step), annealing step, first cooling step, intermediate holding step, second cooling step, and reheating step were performed to obtain a steel plate.

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 Table 2, the types of plating processes are also indicated as "GI" and "GA". In Tables 2 and 4, the alloying temperature is indicated as - because no alloying treatment is performed in the case of GI steel sheets. Further, in Table 6, no galvanizing treatment was performed, and the sheets are indicated as CR (cold rolled steel sheet (no plating)) or HR (hot rolled steel sheet (no plating)).

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.
In addition, the composition of the galvanized layer of the finally obtained hot-dip galvanized steel sheet contains, in GI, Fe: 0.1 to 1.0 mass%, Al: 0.2 to 0.33 mass%, The remainder was 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.
In addition, all galvanized layers were formed on both sides of the base steel sheet.

Using the obtained steel plate, the steel structure of the base steel plate was identified in the manner described above. The measurement results are shown in Table 3, Table 5, and Table 7. In Tables 3, 5, and 7, F is ferrite, M is martensite, RA is retained austenite, B and BT are bainite and tempered bainite, TM is tempered martensite, P is pearlite, θ is carbide, and F' is It is unrecrystallized ferrite.

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. .

The underlined portions in Tables 1 to 7 indicate outside the appropriate range of the present invention.
In addition, tensile tests, hole expansion tests, V-bending tests, U-bending + close-contact bending tests, V-bending + orthogonal VDA bending tests, and axial crushing tests were conducted according to the following procedures, and the tensile strength (TS) was determined according to the following criteria. , yield stress (YS), yield ratio (YR), total elongation (El), critical hole expansion rate (λ), R/t in V-bending test, critical spacer thickness in U-bending + close bending test ( ST), the stroke at maximum load (SFmax) measured in the V-bending + orthogonal VDA bending test, and the presence or absence of fracture (appearance cracking) in the axial crushing test 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

・YR
〇(Pass): 0.64≦YR
× (fail): 0.64>YR

・El
〇(Passed):
(A) When 1180MPa≦TS<1320MPa, 8.0%≦El
(B) When 1320MPa≦TS, 7.0%≦El
× (fail):
(A) When 1180MPa≦TS<1320MPa, 8.0%>El
(B) When 1320MPa≦TS, 7.0%>El

・λ
〇 (Pass): 25% or more × (Fail): Less than 25%

・R/t
〇(Passed):
(A) When 1180MPa≦TS<1320MPa, 2.5≧R/t
(B) If 1320MPa≦TS, 3.0≧R/t
× (fail):
(A) When 1180MPa≦TS<1320MPa, 2.5<R/t
(B) If 1320MPa≦TS, 3.0<R/t

・ST
〇(Passed):
(A) When 1180MPa≦TS<1320MPa, 5.5mm≧ST
(B) If 1320MPa≦TS, 6.0mm≧ST
× (fail):
(A) When 1180MPa≦TS<1320MPa, 5.5mm<ST
(B) When 1320MPa≦TS, 6.0mm<ST

・SFmax
〇(Passed):
(A) When 1180MPa≦TS<1320MPa, 25.5mm≦SFmax
(B) 1320MPa≦TS, 24.5mm≦SFmax
× (fail):
(A) When 1180MPa≦TS<1320MPa, 25.5mm>SFmax
(B) When 1320MPa≦TS, 24.5mm>SFmax

- Presence or absence of axial crushing fracture (external appearance cracking) ◎ (Pass): No external appearance cracking was observed in the sample after the axial crushing test.
〇 (Pass): One or less appearance cracks were observed in the sample after the axial crush test. × (Fail): Two or more appearance cracks were observed in the sample after the axial crush test.

(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, YR, and El were measured. The results are shown in Table 3, Table 5, and Table 7.

(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 (when a crack occurs). 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 3, Table 5, and Table 7.
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) V bending test The V (90°) bending test was conducted in accordance with JIS Z 2248 (2014).
A 100 mm x 35 mm test piece was taken from the obtained steel plate by shearing and end face grinding. Here, the 100 mm side is parallel to the width (C) direction.
Bending radius R: Changes at 0.5mm pitch Test method: Die support, punch press molding load: 10 tons
Test speed: 30mm/min
Holding time: 5s
Bending direction: Evaluation was performed three times in the direction perpendicular to rolling (C), and R/t was calculated by dividing the minimum bending radius (limit bending radius) R without cracking by the plate thickness t. Furthermore, using a Leica stereomicroscope, cracks with a length of 200 μm or more at 25x magnification were determined to be cracks. Note that R/t is an index for evaluating bendability of press formability. The results are shown in Table 3, Table 5, and Table 7.

(4) U-bending + close-contact bending test The U-bending + close-contact bending test was conducted as follows.
A 60 mm x 30 mm test piece was taken from the obtained steel plate by shearing and end face grinding. Here, the 60 mm side is parallel to the width (C) direction. A test piece was prepared by U-bending (primary bending) in the width (C) direction with the rolling (L) direction as the axis at a radius of curvature/plate thickness of 4.2. In the U-bending process (primary bending process), as shown in FIG. 2(a), a punch B1 was pushed into a steel plate placed on a roll A1 to obtain a test piece T1. Next, as shown in FIG. 2(b), the test piece T1 placed on the lower mold A2 was subjected to close bending (secondary bending) by crushing it with the upper mold B2. In FIG. 2(a), D1 indicates the width (C) direction, and D2 indicates the rolling (L) direction. Note that a spacer S, which will be described later, was inserted between the test pieces.

The U-bending conditions in the U-bending + close contact bending test are as follows.
Test method: Roll support, punch pushing Punch tip R: 5.0mm
Clearance between roll and punch: plate thickness + 0.1mm
Stroke speed: 10mm/min
Bending direction: rolling perpendicular (C) direction The conditions for close bending in the U-bending + close bending test are as follows.
Spacer thickness: Changes at 0.5mm pitch Test method: Die support, punch press molding load: 10 tons
Test speed: 10mm/min
Holding time: 5s
Bending direction: rolling right angle (C) direction

The above U-bending + close-contact bending test was performed three times, and the limit spacer thickness (ST) was determined when no cracking occurred all three times. Furthermore, using a Leica stereomicroscope, cracks with a length of 200 μm or more at 25x magnification were determined to be cracks. Note that ST serves as an index for evaluating the fracture resistance at the time of a collision (the fracture resistance of the vertical wall portion in an axial crush test). The results are shown in Table 3, Table 5, and Table 7.

(5) V-bending + orthogonal VDA bending test The V-bending + orthogonal VDA bending test is performed as follows.
A test piece of 60 mm x 65 mm was taken from the obtained steel plate by shearing and end face grinding. 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 FIG. 3(a), a punch B3 was pressed into a steel plate placed on a die A3 having a V-groove to obtain a test piece T1. Next, as shown in FIG. 3(b), the punch B4 is pushed into the test piece T1 placed on the support roll A4 so that the bending direction is perpendicular to the rolling direction (secondary bending). bending process). In FIGS. 3(a) and 3(b), D1 indicates the width (C) direction, and D2 indicates the rolling (L) direction.

The V-bending conditions in the V-bending + orthogonal VDA bending test are as follows.
Test method: die support, punch press molding load: 10 tons
Test speed: 30mm/min
Holding time: 5s
Bending direction: rolling (L) direction

The VDA bending conditions in the V-bending + orthogonal 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 right angle (C) direction

In the stroke-load curve obtained when performing the above VDA bending, determine the stroke at the maximum load. The average value of the stroke at the maximum load when the V-bending + orthogonal VDA bending test was performed three times was defined as SFmax (mm). Note that SFmax is an index for evaluating the fracture resistance at the time of a collision (the fracture resistance of a bending ridgeline portion in an axial crush test). The results are shown in Table 3, Table 5, and Table 7.

(6) 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 b) was produced. Further, a steel plate used as a material for the hat-shaped member was separately cut into a size of 80 mm x 100 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. 4(a) and 4(b). FIG. 4A is a front view of a test member 30 produced by spot welding the hat-shaped member 10 and the steel plate 20. FIG. 4(b) is a perspective view of the test member 30. As shown in FIG. 4(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. 4(c), the test member 30 was joined to the base plate 50 by TIG welding to prepare a sample for an axial crush test. Next, the impactor 60 was made to collide at a constant velocity of 10 mm/min to the produced sample for the axial crush test, and the sample for the axial crush test was crushed by 70 mm. As shown in FIG. 4(c), the crushing direction D3 was parallel to the longitudinal direction of the test member 30. The results are shown in Table 3, Table 5, and Table 7.

U-bending + close bending tests, V-bending + orthogonal VDA bending tests, and axial crushing tests on steel plates with a thickness of over 1.2 mm were all conducted on steel plates with a thickness of 1.2 mm, taking into account 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 U-bending + close bending bending test, the ground surface is the inside of the bend (valley side), and in the V-bending + orthogonal VDA bending test, the ground surface is was set as the outside of the bend (peak side), and the ground surface was set as the inside of the bend (valley side) during the subsequent VDA bending test. On the other hand, in the U-bending + close bending test, V-bending + orthogonal 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.

<Nano hardness measurement>
In order to obtain excellent bending properties during press forming and excellent bending rupture properties during collision, it is necessary to set the position from the surface layer of the base material to the surface soft layer at 1/4 of the depth in the thickness direction and 1/2 of the depth in the thickness direction. 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 this example, when plating is applied, after the plating layer is peeled off, mechanical polishing is performed from the surface of the base steel sheet to a depth of 1/4 of the thickness direction of the surface soft layer -5 μm. After buffing with diamond and alumina to 1/4 of the depth of the layer in the thickness direction, 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.

Next, mechanical polishing, buff polishing with diamond and alumina, and colloidal silica polishing were performed to a depth of 1/2 of the thickness of the surface soft layer. 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 the following conditions.

As shown in Table 3, Table 5, and Table 7, the invention examples all have tensile strength (TS), yield stress (YS), yield ratio (YR), total elongation (El), and critical hole expansion rate ( λ), R/t in V-bending test, critical spacer thickness (ST) in U-bending + close bending test, and stroke at maximum load measured in V-bending + orthogonal VDA bending test (SFmax) All of the tests passed, and there was no breakage (appearance cracking) in the axial crush test.
On the other hand, in the comparative example, tensile strength (TS), yield stress (YS), yield ratio (YR), total elongation (El), critical hole expansion rate (λ), R/t in V-bending test, and U-bending + Critical spacer thickness (ST) in close bending test, stroke at maximum load (SFmax) measured in V-bending + orthogonal VDA bending test, and presence or absence of fracture (appearance cracking) in axial crushing test. One wasn't enough.
In addition, in Tables 5 and 7, when the dew point is in the range of -30°C or more and -20°C or less, the soft layer thickness of the surface layer is 14 μm or less, and the judgment of fracture (appearance cracking) in the axial crush test is "○". However, even if the surface soft layer thickness was 14 μm or less, if the metal plating layer was present, the rupture (appearance cracking) in the axial crush test was evaluated as “◎”.

In addition, the members obtained by forming or joining the steel sheets of the present invention example have tensile strength (TS), yield stress (YS), yield ratio (YR), Elongation (El), critical hole expansion rate (λ), R/t in V-bending test, critical spacer thickness (ST) in U-bending + close bending test, and measured in V-bending + orthogonal VDA bending test All of the strokes (SFmax) at the maximum load applied have the excellent characteristics characterized by the present invention, and there is no breakage (appearance cracking) in the axial crush test, and the excellent characteristics characterized by the present invention. I understand.

10 Hat-shaped member 20 Steel plate 30 Test member 40 Spot weld 50 Base plate 60 Impactor A1 Die A2 Support roll A3 Die A4 Support roll B1 Punch B2 Punch B3 Punch B4 Punch D1 Width (C) direction D2 Rolling (L) direction D3 Crushing direction S Spacer T1 Test piece F Ferrite M Martensite RA Retained austenite B Bainite BT Tempered bainite TM Tempered martensite θ Carbide

According to the present invention, TS: 1180 MPa or more, high YS and YR, excellent press formability (ductility, hole expandability, and bendability), and rupture resistance properties at the time of collision (bending rupture properties and axial It becomes possible to manufacture steel plates and members with high crushing properties). Further, by applying the steel plates and members obtained according to the method of the present invention to, for example, automobile structural members, it is possible to improve fuel efficiency by reducing the weight of the vehicle body, and the industrial value thereof is extremely large.

Claims

A steel plate comprising a base steel plate, the base steel plate comprising:
In mass%,
C: 0.030% or more and 0.250% or less,
Si: 0.01% or more and 0.75% or less,
Mn: 2.00% or more and less than 3.50%,
P: 0.001% or more and 0.100% or less,
S: 0.0200% or less,
Al: 0.010% or more and 2.000% or less,
N: 0.0100% or less,
, with the remainder consisting of Fe and unavoidable impurities;
As the structure at the 1/4 plate thickness position of the base steel plate,
Ferrite area ratio: less than 20.0%,
Fresh martensite area ratio: 15.0% or less,
Area ratio of retained austenite: 3.0% or less,
Area ratio of bainite and tempered bainite: more than 10.0% and not more than 70.0%,
Area ratio of tempered martensite: 30.0% or more and 80.0% or less,
Furthermore, the average crystal grain size of island-like fresh martensite and island-like retained austenite in the bainite grains and tempered bainite grains is 2.00 μm or less,
The average crystal grain size of carbides in bainite grains and tempered bainite grains is 500 nm or less,
Furthermore, a steel structure in which the number density of carbides with a grain size of 300 nm or more in bainite grains and tempered bainite grains is 3.0 pieces/μm 2 or less,
has
A steel plate, wherein the amount of diffusible hydrogen contained in the base steel plate is 0.50 mass ppm or less, and the tensile strength is 1180 MPa or more.
The component composition further includes, in mass%,
Nb: 0.200% or less,
Ti: 0.200% or less,
V: 0.200% or less,
B: 0.0100% or less,
Cr: 1.000% or less,
Ni: 1.000% or less,
Mo: 1.000% or less,
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 element 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 steel slab having the composition according to claim 1 or 2,
A hot rolling process in which hot rolling is performed at a finish rolling temperature of 820°C or higher to obtain a hot rolled steel plate;
An annealing step in which the steel plate after the hot rolling step is annealed at an annealing temperature of (Ac 1 + (Ac 3 - Ac 1 )×5/8)° C. or higher and 950° C. or lower and an annealing time of 20 seconds or more. ,
After the annealing step, a first cooling step of cooling to a temperature range of 300° C. or higher and 550° C. or lower;
After the first cooling step, an intermediate holding step of holding at an intermediate holding temperature of 300° C. or more and 550° C. or less and a holding time of 20 seconds or more;
Applying a tension of 2.0 kgf/mm 2 or more to the steel plate after the intermediate holding step in a temperature range of 300 ° C. or higher and 450 ° C. or lower,
After that, the steel plate is passed through 5 passes or more while contacting a roll with a diameter of 500 mm or more and 1500 mm or less for 1/4 rotation of the roll per pass,
Then, a second cooling step of cooling to a cooling stop temperature of less than 300°C;
After the second cooling step, the steel plate is reheated to a temperature range from the cooling stop temperature to 440° C. and held for 20 seconds or more, or further after the hot rolling step, and A method for manufacturing a steel plate, comprising a cold rolling process, in which the steel plate before the annealing process is subjected to cold rolling at a rolling reduction of 20% or more and 80% or less to obtain a cold rolled steel plate.
The method for manufacturing a steel sheet according to claim 7, comprising a galvanizing step of subjecting the steel sheet after the intermediate holding 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 hot rolling step and before the annealing step, the method includes a metal plating step of applying metal plating to one or both surfaces of the steel sheet to form a metal plating layer. manufacturing method of steel plate.
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.