Steel plates, components, and their manufacturing methods

Abstract

The present invention provides: a steel sheet having 590 MPa or greater TS, high YS, excellent press formability (ductility, hole expandability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial crushing characteristics) in a collision; a member; and a method for producing these.　A base steel sheet has a predetermined component composition, wherein when nano-hardness is measured at 300 or more points in a 50 µm x 50 µm region of a sheet plane located at the 1 / 4 depth position from the base steel sheet surface in a sheet thickness direction of a surface soft layer, the percentage of the points where the nano-hardness is 7.0 GPa or higher is 0.10 or less, a sheet plane located at the 1 / 4 depth position from the base steel sheet surface in the sheet thickness direction of the surface soft layer has a nano-hardness standard deviation σ of 1.8 GPa or lower, and a sheet plane located at the 1 / 2 depth position from the base steel sheet surface in the sheet thickness direction of the surface soft layer has a nano-hardness standard deviation σ of 2.2 GPa or lower.

Description

[Technical Field]

[0001] The present invention relates to a steel plate, a member made from the steel plate, and a method for manufacturing the same. [Background technology]

[0002] Strengthening of automotive steel sheets has been promoted to achieve both improved fuel economy and CO2 emissions through the reduction of steel sheet thickness and weight used in automobile bodies, while also improving collision safety. Furthermore, new regulations are being introduced one after another. Therefore, to increase vehicle body strength, the application of high-strength steel sheets to major structural and reinforcing components (hereinafter referred to as automotive frame structural components) assembled to the frame of an automobile cabin is increasing. Furthermore, high-strength steel sheets used in automotive frame structural components are required to have high component strength when press-formed. To increase component strength, for example, increasing the yield stress (hereinafter referred to simply as YS) of the steel sheet is effective. This increases the impact absorption energy (hereinafter referred to simply as impact absorption energy) during a vehicle collision. Furthermore, among automotive frame structural components, for example, crash boxes have bent sections. Therefore, from the viewpoint of press formability, it is preferable to use steel sheets with high bendability for such components. Furthermore, from the viewpoint of corrosion prevention performance of the vehicle body, steel sheets used as the raw material for automotive components are often zinc-plated. Therefore, there is a demand for the development of hot-dip galvanized steel sheets that not only have high strength but also excellent press formability and impact resistance.

[0003] As a steel sheet that can be used as a raw material for such automotive parts, for example, Patent Document 1 discloses a high-strength steel sheet with excellent stretch flangeability and crash resistance, which has a component composition that contains, expressed by mass%, 0.04 to 0.22% C, 1.0% or less Si, 3.0% or less Mn, 0.05% or less P, 0.01% or less S, 0.01 to 0.1% Al, and 0.001 to 0.005% N, with the balance being Fe and unavoidable impurities, and which is composed of a ferrite phase as a main phase and a martensite phase as a second phase, with the maximum grain size of the martensite phase being 2 μm or less and the area fraction of the martensite phase being 5% or more.

[0004] In addition, Patent Document 2 discloses a method for forming a steel sheet by grinding a surface layer to a thickness of 0.1 μm or more and then disposing Ni at 0.2 g / m 2 More than 2.0g / m 2The following is a hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of a pre-galvanized cold-rolled steel sheet, containing, by 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 and 3.0% or less, P: 0.04% or less, S: 0.05% or less, N: 0.01% or less, Al: 0.01% or more and 2.0% or less, with Si + Al > 0.5%, the balance being Fe and unavoidable impurities, and the microstructure contains, by volume fraction, ferrite as the main phase at 40% or more, retained austenite at 8% or more, two or more types of martensite [1] [2] [3] specified below, including martensite [3], 1% or more of bainite, and 0 to 10% of pearlite, and the three types of martensite [1] [2] [3] specified below, A high-strength hot-dip galvanized steel sheet with excellent coating adhesion and formability, characterized in that the steel sheet has a volume fraction of martensite [1]: 0% or more and 50% or less, martensite [2]: 0% or more and less than 20%, and martensite [3]: 1% or more and 30% or less, respectively, and has a hot-dip galvanized layer containing less than 7% Fe and the balance consisting of Zn, Al, and unavoidable impurities on the surface thereof, and the tensile strength TS (MPa), total elongation EL (%), and hole expansion ratio λ (%) are such that TS × EL is 18,000 MPa·% or more, TS × λ is 35,000 MPa·% or more, and the tensile strength is 980 MPa or more. (Martensite [1]: C concentration (CM1) is less than 0.8%, and hardness Hv1 is Hv1 / (-982.1 × CM1) 2 +1676×CM1+189)≦0.60, martensite [2]: C concentration (CM2) is 0.8% or more, and hardness Hv2 is Hv2 / (-982.1×CM2 2 +1676×CM2+189)≦0.60, martensite [3]: C concentration (CM3) is 0.8% or more, and hardness Hv3 is Hv3 / (-982.1×CM3 2 +1676×CM3+189)≧0.80 is disclosed.

[0005] Patent Document 3 discloses a high-strength hot-dip galvanized steel sheet having a composition consisting of, in mass%, C: 0.15% to 0.25%, Si: 0.50% to 2.5%, Mn: 2.3% to 4.0%, P: 0.100% or less, S: 0.02% or less, Al: 0.01% to 2.5%, and the balance being Fe and unavoidable impurities, and having a steel sheet structure in which, in area percentages, the tempered martensite phase is 30% to 73%, the ferrite phase is 25% to 68%, the retained austenite phase is 2% to 20%, and other phases are 10% or less (including 0%), and the other phases are 3% or less (including 0%) of the martensite phase and less than 5% (including 0%) of the bainitic ferrite phase, the tempered martensite phase has an average crystal grain size of 8 μm or less, and the retained austenite phase has a C content of less than 0.7% by mass.

[0006] Patent Document 4 also describes a galvannealed steel sheet having a galvannealed layer on the surface of the steel sheet, the steel sheet having a chemical composition consisting of, in mass %, C: 0.03% to 0.35%, Si: 0.005% to 2.0%, Mn: 1.0% to 4.0%, P: 0.0004% to 0.1%, S: 0.02% or less, sol.Al: 0.0002% to 2.0%, N: 0.01% or less, the balance being Fe and impurities, and an average spacing of enriched portions, which is the average spacing in the direction perpendicular to the rolling direction of enriched portions where Mn and / or Si are enriched and extended in the rolling direction at a depth of 50 μm from the surface of the steel sheet, is 1 The steel sheet has a steel structure in which the surface area is 3000 μm or less, the number density of cracks at the surface of the steel sheet having a depth of 3 μm or more and 10 μm or less is 3 / mm or more and 1000 / mm or less, the steel sheet contains, in area percentages, 60% or more bainite, 1% or more retained austenite, 1% or more martensite, and 2% or more but less than 20% ferrite, and the mean spacing of ultra-hard phases, which is the average value of the nearest neighbor distances of martensite and retained austenite, is 20 μm or less, and the galvannealed steel sheet has mechanical properties such as a tensile strength (TS) of 780 MPa or more. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 3887235 [Patent Document 2] Patent No. 5953693 [Patent Document 3] Patent No. 6052472 [Patent Document 4] Patent No. 5699764 Summary of the Invention [Problem to be solved by the invention]

[0008] Incidentally, the use of steel sheets with a tensile strength TS (hereinafter sometimes simply referred to as TS) exceeding 590 MPa is increasing in automotive frame components such as center pillars, but the current situation is that the use of steel sheets with a TS of 590 MPa or so is limited to automotive impact energy absorbing components such as front side members and rear side members.

[0009] That is, improving the yield stress YS (hereinafter sometimes referred to simply as YS) is effective in increasing the absorbed energy during a collision (hereinafter also referred to as impact absorption energy). However, increasing the YS of a steel sheet generally results in a decrease in press formability, particularly in properties such as ductility, hole expandability, and bendability. Therefore, when applying a steel sheet with such increased TS and YS to the aforementioned automotive impact energy absorption components, not only does press forming become difficult, but the component cracks in an axial crushing test simulating a crash test. In other words, the actual impact absorption energy is not as high as expected from the YS value. As such, there is room for improvement in the conventional technology.

[0010] In fact, the steel sheets disclosed in Patent Documents 1 to 4 cannot be said to have high YS and YR, excellent press formability (ductility, hole expandability, and bendability), and fracture resistance during collision (bending fracture properties and axial crush properties).

[0011] The present invention was developed in view of the above-mentioned current situation, and aims to provide a steel sheet having a tensile strength TS of 590 MPa or more, a high yield stress YS, excellent press formability (ductility, hole expandability, and bendability), and fracture resistance during collision (flexural fracture properties and axial crush properties), and a method for manufacturing the same. Another object of the present invention is to provide a member made from the above steel plate and a method for manufacturing the same.

[0012] The tensile strength TS is measured by a tensile test in accordance with JIS Z 2241. Furthermore, a high yield stress YS means that the yield stress (YS) measured in a tensile test conforming to JIS Z 2241 satisfies the following formulas (A) to (F) according to the TS measured in the tensile test. (A) When 590MPa≦TS<780MPa, 350MPa≦YS (B) When 780MPa≦TS<980MPa, 400MPa≦YS (C) When 980MPa≦TS<1180MPa, 550MPa≦YS (D) When 1180MPa≦TS<1320MPa, 750MPa≦YS (E) When 1320MPa≦TS<1470MPa, 900MPa≦YS (F) When 1470MPa≦TS, 1050MPa≦YS

[0013] Furthermore, excellent ductility means that the total elongation (El) measured in a tensile test conforming to JIS Z 2241 satisfies the following formulas (A) to (F) according to the TS measured in the tensile test. (A) When 590MPa≦TS<780MPa, 24.0%≦El (B) When 780MPa≦TS<980MPa, 18.0%≦El (C) 980MPa≦TS<1180MPa, 11.0%≦El (D) When 1180MPa≦TS<1320MPa, 8.0%≦El (E) When 1320MPa≦TS<1470MPa, 7.5%≦El (F) When 1470MPa≦TS, 7.0%≦El

[0014] Furthermore, excellent hole expandability means that the limiting hole expanding ratio (λ) measured in a hole expanding test in accordance with JIS Z 2256 is 20% or more.

[0015] Furthermore, excellent bendability means that R (limit bending radius) / t (plate thickness) measured in a V-bend test in accordance with JIS Z 2248 satisfies the following formulas (A) to (F) according to TS. (A) When 590MPa≦TS<780MPa, 1.0≧R / t (B) When 780MPa≦TS<980MPa, 2.0≧R / t (C) When 980MPa≦TS<1180MPa, 2.5≧R / t (D) When 1180MPa≦TS<1320MPa, 3.0≧R / t (E) When 1320MPa≦TS<1470MPa, 3.5≧R / t (F) When 1470MPa≦TS, 4.0≧R / t

[0016] Furthermore, excellent bending fracture properties means that the stroke (SFmax) at maximum load measured in a V-bending + orthogonal VDA bending test satisfies the following formulas (A) to (F) depending on TS. (A) When 590MPa≦TS<780MPa, 29.0mm≦SFmax (B) When 780MPa≦TS<980MPa, 27.5mm≦SFmax (C) When 980MPa≦TS<1180MPa, 27.0mm≦SFmax (D) When 1180MPa≦TS<1320MPa, 26.0mm≦SFmax (E) When 1320MPa≦TS<1470MPa, 24.5mm≦SFmax (F) When 1470MPa≦TS, 24.0mm≦SFmax

[0017] Furthermore, excellent axial crushing properties means that there are no fractures (visual cracks) in the sample after the axial crushing test, or there is only one visual crack in the sample after the axial crushing test.

[0018] The aforementioned El (ductility), λ (stretch flangeability), and R / t (bendability) are properties that indicate the ease with which a steel sheet can be formed during press forming (the degree of freedom in press forming to prevent cracking). On the other hand, the V-bend + orthogonal VDA bend test is a test that simulates the deformation and fracture behavior of the bend ridge in a crash test, and the stroke at maximum load (SFmax) measured in the V-bend + orthogonal VDA bend test is an index that indicates the resistance to cracking of an energy absorption component. [Means for solving the problem]

[0019] The present inventors have conducted extensive research to achieve the above-mentioned object, and as a result have obtained the following findings. When the nanohardness of a steel sheet with a specified composition is measured at 300 or more points in a 50 μm × 50 μm area on the sheet surface at a position 1 / 4 of the way to the depth in the sheet thickness direction of the soft surface layer from the surface of the steel sheet (the surface of the base (underlying) steel sheet), the proportion of the nanohardness of 7.0 GPa or more is 0.10 or less, and further, the standard deviation σ of the nanohardness of the sheet surface at a position 1 / 4 of the way to the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet is 1.8 GPa or less, and further, the nanohardness of the sheet surface at a position 1 / 2 of the way to the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet is 1.8 GPa or less. When the standard deviation σ of the nanohardness of the sheet surface at the 1 / 2 position in the sheet thickness direction of the soft surface layer is measured in the same manner as at the 1 / 4 position, it was found that if the standard deviation σ of the nanohardness of the sheet surface at the 1 / 2 position in the sheet thickness direction is 2.2 GPa or less, it is possible to improve R / t, which is an index of bendability, which is one mode of press formability, and to improve the stroke at maximum load (SFmax), which is an index of the impact resistance properties of steel sheets and components of automobile bodies during a collision, measured in a V-bend + orthogonal VDA bend test that simulates the deformation and fracture behavior of the bend ridge in a crash test.

[0020] The present disclosure has been made based on the above findings. That is, the gist of the present disclosure is as follows. [1] In mass%, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 3.00% or less, Mn: 0.30% or more and less than 10.00% P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.005% or more and 2.000% or less, N: 0.0100% or less, The base steel sheet has a chemical composition containing: a base steel sheet containing a material having a composition in which Ceq represented by the following formula (1) satisfies 0.30% or more and 0.85% or less, and the balance consisting of Fe and unavoidable impurities; a surface soft layer having a Vickers hardness of 85% or less of the Vickers hardness at a 1 / 4 position in the sheet thickness of the base steel sheet is formed in a region of 200 μm or less from the surface of the base steel sheet in the sheet thickness direction, When nano hardness was measured at 300 points or more in a 50 μm × 50 μm region on the sheet surface at a 1 / 4 position and a 1 / 2 position of the sheet thickness direction depth of the soft surface layer from the surface of the base steel sheet, the ratio of the number of measurements of nano-hardness of 7.0 GPa or more on the sheet surface at a position of 1 / 4 of the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet is 0.10 or less with respect to the total number of measurements, Furthermore, the standard deviation σ of the nanohardness of the sheet surface at a position 1 / 4 of the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet is 1.8 GPa or less, Furthermore, the standard deviation σ of the nano-hardness of the sheet surface at a position halfway from the surface of the base steel sheet to the depth in the sheet thickness direction of the soft surface layer is 2.2 GPa or less. Ceq=[%C]+[%Mn] / 6+[%Si] / 24+[%Ni] / 40+[%Cr] / 5+[%Mo] / 4+[%V] / 14...Equation (1) Here, [%C] is the C content, [%Mn] is the Mn content, [%Si] is the Si content, [%Ni] is the Ni content, [%Cr] is the Cr content, [%Mo] is the Mo content, and [%V] is the V content, and if no V is contained, it is 0 (zero). [2] The total area ratio of ferrite and bainite present in the soft surface layer is 20% or more, The steel sheet according to [1] above, wherein a value obtained by dividing the area ratio of martensite present in the soft surface layer by the sum of the area ratios of martensite and tempered martensite present in the soft surface layer is 0.45 or less. [3] 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, Bi:0.0200% or less, REM: 0.0200% or less The steel sheet according to [1] or [2] above, containing at least one element selected from the following: [4] The steel sheet according to any one of the above [1] to [3], wherein the base steel sheet has a metal plating layer as a first plating layer on one or both surfaces of the base steel sheet. [5] The steel sheet according to any one of the above [1] to [4], which has a metal plating layer as a second plating layer on the outermost surface of one or both surfaces of the steel sheet. [6] The steel sheet according to any one of the above [1] to [5], wherein the depth of surface irregularities of a test piece after 90° V-bending is such that R / t, obtained by dividing the limit bending radius R by the sheet thickness t, is 4.5 or more and 5.0 or less, is 20.0 μm or less. [7] A member made using the steel sheet according to any one of the above [1] to [6]. [8] A hot rolling process in which a steel slab having the composition described in [1] or [3] is hot rolled; An annealing process in which the obtained steel sheet is annealed under the conditions of an annealing temperature of Ac1 point or higher and 950 ° C or lower, an annealing time of 10 seconds or longer, and a dew point of −30 ° C or higher in an atmosphere; After the annealing step, the obtained steel sheet is cooled in a temperature range from Ac1 point to 450 ° C. under an atmosphere with a dew point of −30 ° C. or less at an average cooling rate of 8 ° C. / second or more; a second cooling step of cooling the steel plate obtained after the first cooling step at an average cooling rate of less than 8°C / sec in a temperature range from less than 450°C to 300°C. [9] The method for producing a steel sheet according to [8], further comprising a cold rolling step of cold rolling the steel sheet after the hot rolling step and before the annealing step.

[10] A method for producing a steel sheet according to [8] or [9], comprising a first plating step of applying metal plating to one or both sides of the steel sheet after the hot rolling step and before the annealing step to form a first plating layer.

[11] The method for producing a steel sheet according to any one of the above [8] to

[10] , further comprising a second plating step of plating the steel sheet after the second cooling step.

[12] The method for manufacturing a steel sheet according to any one of the above [8] to

[11] , further comprising a reheating and holding step of cooling the steel sheet obtained after the second cooling step to a cooling stop temperature from 250°C or lower to room temperature, reheating the steel sheet to a reheating temperature range from (the cooling stop temperature + 50°C) to 450°C, and holding the steel sheet in the reheating temperature range for 10 seconds or more.

[13] A method for manufacturing a component, comprising the step of subjecting the steel plate according to any one of the above [1] to [6] to at least one of forming and joining to form the component. [Effects of the Invention]

[0021] According to the present invention, a steel sheet can be obtained which has a tensile strength TS of 590 MPa or more, a high yield stress YS, excellent press formability (ductility, hole expandability, and bendability), and fracture resistance during collision (axial crushing properties). Furthermore, members made from the steel plate of the present invention have high strength, excellent press formability and impact resistance, and can therefore be extremely advantageously applied to automotive frame members and impact energy absorbing members, etc. [Brief explanation of the drawings]

[0022] [Figure 1] 10 is a cross-sectional photograph for explaining the depth of the surface irregularities. [Figure 2] 1A is a diagram illustrating the V-bending (primary bending) process in the V-bending + orthogonal VDA bending test of the example, and FIG. 1B is a diagram illustrating the orthogonal VDA bending (secondary bending) process in the V-bending + orthogonal VDA bending test of the example. [Figure 3-1] 1A is a front view of a test member manufactured for an axial crushing test of an example, in which a hat-shaped member and a steel plate are spot-welded together, and FIG. 1B is a perspective view of the test member shown in FIG. 1A. [Figure 3-2] 1C is a schematic diagram for explaining an axial crushing test of the example. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention will be described based on the following embodiments.

[0024] [1. Steel plate] The steel sheet of the present invention comprises, in mass%, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 3.00% or less, Mn: 0.30% or more and less than 10.00%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.005% or more and 2.000% or less, The steel sheet has a base steel sheet having a chemical composition in which Ceq expressed by the following formula (1) is 0.30% or more and 0.85% or less, with the balance consisting of Fe and unavoidable impurities, and a surface soft layer having a Vickers hardness of 85% or less of the Vickers hardness at the 1 / 4 position in the sheet thickness direction is formed in an area of ​​200 μm or less from the surface of the base steel sheet in the sheet thickness direction, and the surface soft layer has a Vickers hardness of 85% or less of the Vickers hardness at the 1 / 4 position in the sheet thickness direction and ...2 position in the sheet thickness direction from the surface of the base steel sheet. When the nanohardness is measured at 300 or more points in an area of ​​100 μm x 50 μm, the ratio of the number of measurements where the nanohardness is 7.0 GPa or more on the sheet surface at a position 1 / 4 of the way from the surface of the base steel sheet to the depth in the sheet thickness direction of the soft surface layer is 0.10 or less relative to the total number of measurements, and further, the standard deviation σ of the nanohardness on the sheet surface at a position 1 / 4 of the way from the surface of the base steel sheet to the depth in the sheet thickness direction of the soft surface layer is 1.8 GPa or less, and further, the standard deviation σ of the nanohardness on the sheet surface at a position 1 / 2 of the way from the surface of the base steel sheet to the depth in the sheet thickness direction of the soft surface layer is 2.2 GPa or less. Ceq=[%C]+[%Mn] / 6+[%Si] / 24+[%Ni] / 40+[%Cr] / 5+[%Mo] / 4+[%V] / 14...Equation (1) Here, [%C] is the C content, [%Mn] is the Mn content, [%Si] is the Si content, [%Ni] is the Ni content, [%Cr] is the Cr content, [%Mo] is the Mo content, and [%V] is the V content, and if no V is contained, it is 0 (zero).

[0025] Ingredient composition First, the chemical composition of the base steel sheet of the steel sheet according to one embodiment of the present invention will be described. Note that the units for the chemical compositions are all "mass %", but hereinafter, unless otherwise specified, they will be simply expressed as "%". Furthermore, in the present invention, it is specified that the steel sheet has a base steel sheet, and a plating layer (a metal plating layer (first plating layer), a zinc plating layer such as a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer or an electrogalvanized layer, or a plating layer (second plating layer) such as a hot-dip aluminum plating layer) on the base steel sheet is described below, but this plating layer is a suitable constituent element of the steel sheet, and the steel sheet does not necessarily have to have this plating layer.

[0026] C: 0.030% or more and 0.500% or less Carbon (C) is an effective element for ensuring high TS and YS by generating appropriate amounts of tempered martensite and bainite. Here, a C content of less than 0.030% increases the area fraction of ferrite, making it difficult to achieve the desired TS. It also reduces YS. On the other hand, a C content of more than 0.500% increases the area fraction of martensite, resulting in an excessively high TS, which prevents the desired El (press formability (ductility)) and R / t (press formability (bendability)). Furthermore, the volume fraction (area fraction) of retained austenite increases. When the steel sheet is punched in a hole expansion test or V-bended in a V-bend + orthogonal VDA test, hard martensite is generated by the deformation-induced transformation of retained austenite. This leads to void formation and crack propagation during subsequent testing, resulting in the failure to achieve the desired λ (press formability (hole expandability)) and SFmax (crash fracture resistance (bending fracture resistance)). Therefore, the C content is set to 0.030% or more and 0.500% or less, preferably 0.050% or more, and more preferably 0.300% or less.

[0027] Si: 0.01% or more and 3.00% or less Si promotes ferrite transformation during annealing and the cooling process after annealing. In other words, Si is an element that affects the area fraction of ferrite. Here, if the Si content is less than 0.01%, the area fraction of ferrite decreases, resulting in reduced ductility. On the other hand, if the Si content exceeds 3.00%, the volume fraction of retained austenite increases. When the steel sheet is punched in a hole expansion test or V-bended in a V-bending + orthogonal VDA test, hard martensite is generated due to the deformation-induced transformation of retained austenite. This leads to the generation of voids and crack propagation in subsequent tests, preventing the desired λ and SFmax from being achieved. Therefore, the Si content is set to 0.01% or more and 3.00% or less. The Si content is preferably 0.10% or more. The Si content is also preferably 2.0% or less.

[0028] Mn: 0.30% or more and less than 10.00% Mn is an element that adjusts the area ratio of tempered martensite, bainite, etc. Here, if the Mn content is less than 0.30%, the area ratio of ferrite increases, making it difficult to ensure the desired TS. It also leads to a decrease in YS. On the other hand, if the Mn content is 10.00% or more, the formation and increase of ε-martensite, which is an embrittlement phase, prevents the desired El (press formability (ductility)), R / t (press formability (bendability)), and SFmax (fracture resistance during collision (bending fracture properties)) from being obtained. Therefore, the Mn content is set to 0.30% or more and less than 10.00%. The Mn content is preferably 1.00% or more. Furthermore, the Mn content is preferably 3.50% or less.

[0029] P: 0.001% or more and 0.100% or less P has a solid-solution strengthening effect and increases the TS and YS of steel sheets. To achieve this effect, the P content is set to 0.001% or more. On the other hand, if the P content exceeds 0.100%, P segregates at prior austenite grain boundaries, embrittling the grain boundaries. As a result, during V-bending tests, voids are generated and cracks propagate along the prior austenite grain boundaries, preventing the desired R / t. Furthermore, when the steel sheet is punched in a hole-expansion test or V-bended in a V-bending + orthogonal VDA test, voids are generated and cracks propagate along the prior austenite grain boundaries, preventing the desired λ 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.

[0030] S: 0.0200% or less S exists in steel as sulfides. In particular, if the S content exceeds 0.0200%, voids will form and cracks will propagate from the sulfides during a V-bend test, preventing the desired R / t. Furthermore, when the steel sheet is punched in a hole-expanding test or V-bended in a V-bend + orthogonal VDA test, voids will form and cracks will propagate from the sulfides, preventing the desired λ and SFmax. Therefore, the S content is set to 0.0200% or less. The S content is preferably set to 0.0080% or less. While there is no particular lower limit for the S content, it is preferable that the S content be 0.0001% or more due to production technology constraints.

[0031] Al: 0.005% or more and 2.000% or less Al promotes ferrite transformation during annealing and in the cooling process after annealing. That is, Al is an element that affects the area fraction of ferrite. Here, if the Al content is less than 0.005%, the area fraction of ferrite decreases, and ductility decreases. On the other hand, if the Al content exceeds 2.000%, the area fraction of ferrite increases excessively, making it difficult to ensure the desired TS. It also leads to a decrease in YS. Therefore, the Al content is set to 0.005% or more and 2.000% or less. The Al content is preferably 0.010% or more, more preferably 0.015% or more. Furthermore, the Al content is preferably 1.000% or less.

[0032] N: 0.0100% or less N exists in steel as nitrides. In particular, if the N content exceeds 0.0100%, voids will form and cracks will propagate from the nitrides during a V-bend test, preventing the desired R / t. Furthermore, when the steel sheet is punched in a hole-expanding test or V-bended in a V-bend + orthogonal VDA test, voids will form and cracks will propagate from the nitrides, preventing the desired λ and SFmax. Therefore, the N content is set to 0.0100% or less. Furthermore, the N content is preferably 0.0050% or less. While there is no particular lower limit for the N content, a N content of 0.0005% or more is preferred due to production technology constraints.

[0033] Ceq: 0.30% or more and 0.85% or less Ceq is an index for ensuring high TS and high YS in order to generate appropriate amounts of martensite, retained austenite, tempered martensite, etc. Here, if Ceq is less than 0.30%, it becomes difficult to generate martensite, retained austenite, or tempered martensite at the 1 / 4 thickness position, making it difficult to ensure the desired TS and YS. On the other hand, if Ceq exceeds 0.85%, the area fraction of martensite increases, TS becomes excessively high, and the desired El (press formability (ductility)) and R / t (press formability (bendability)) cannot be obtained. Furthermore, when the volume fraction of retained austenite increases and the steel plate is punched in the hole expansion test or V-bended in the V-bend + orthogonal VDA test, hard martensite is generated by the deformation-induced transformation of the retained austenite, which causes voids to form and cracks to grow in subsequent tests, preventing the desired λ (press formability (hole expansion ability)) and SFmax (fracture resistance during collision (bending fracture properties)). Therefore, Ceq is set to 0.30% or more and 0.85% or less. Ceq is preferably 0.35% or more. Also, Ceq is preferably 0.80% or less. Ceq is calculated using the following formula (1). Ceq=[%C]+[%Mn] / 6+[%Si] / 24+[%Ni] / 40+[%Cr] / 5+[%Mo] / 4+[%V] / 14...Equation (1) Here, [%C] is the C content, [%Mn] is the Mn content, [%Si] is the Si content, [%Ni] is the Ni content, [%Cr] is the Cr content, [%Mo] is the Mo content, and [%V] is the V content, and if no V is contained, it is 0 (zero).

[0034] The basic chemical composition of the base steel sheet of the steel sheet according to one embodiment of the present invention has been described above, but the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present invention has a chemical composition containing the basic chemical components with the balance other than the basic chemical components including Fe (iron) and unavoidable impurities. Here, it is preferable that the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present invention has a chemical composition containing the basic chemical components with the balance consisting of Fe and unavoidable impurities.

[0035] In addition to the basic components, the base steel sheet of a steel sheet according to one embodiment of the present invention may contain at least one selected from the optional components shown below. Note that the effects of the present invention can be obtained so long as the optional components shown below are contained in amounts up to the upper limit amounts shown below, so no lower limit is particularly set. Note that when the optional elements listed below are contained in amounts less than the preferred lower limit values ​​described below, the elements are considered to be included as inevitable impurities.

[0036] 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 %, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and at least one selected from REM: 0.0200% or less

[0037] Nb: 0.200% or less Nb increases TS, YS, and YR by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. To achieve this effect, the Nb content is preferably 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, the coarse precipitates and inclusions may become the origin of voids and cracks during hole expansion tests, V-bend tests, and V-bend + orthogonal VDA bend tests, making it difficult to achieve the desired λ, R / t, and SFmax. Therefore, when Nb is added, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.

[0038] Ti: 0.200% or less Like Nb, Ti increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. To achieve this effect, the Ti content is preferably 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 formed. In such cases, the coarse precipitates and inclusions may become the origin of voids and cracks during hole expansion tests, V-bend tests, and V-bend + orthogonal VDA bend tests, making it difficult to obtain the desired λ, R / t, and SFmax. Therefore, when Ti is added, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.

[0039] V:0.200% or less Like Nb and Ti, V forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. The V content is further preferably 0.030% or more. On the other hand, if the V content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the origin of voids and cracks during hole expansion tests, V-bend tests, and V-bend + orthogonal VDA bend tests, making it difficult to obtain the desired λ, R / t, and SFmax. Therefore, when V is added, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.

[0040] B: 0.0100% or less B is an element that segregates at austenite grain boundaries to improve hardenability. B is also an element that controls the formation and grain growth of ferrite during cooling after annealing. To achieve this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. The B content is further preferably 0.0010% or more. On the other hand, if the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling. Furthermore, the internal cracks may become the starting points for cracks during hole expansion tests, V-bend tests, and V-bend + orthogonal VDA bend tests, so the desired λ, R / t, and SFmax may not be obtained. Therefore, when B is added, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less.

[0041] Cr:1.000% or less Cr is an element that improves hardenability, and its addition generates an appropriate amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.010% or more. The Cr content is even more preferably 0.030% or more, and even more preferably 0.200% or more. On the other hand, if the Cr content exceeds 1.000%, the area fraction of martensite increases, which reduces hole expandability and bendability in V-bend tests, making it difficult to achieve the desired λ and R / t. Therefore, when Cr is added, the Cr content is preferably 1.000% or less. The Cr content is more preferably 0.700% or less, and even more preferably 0.600% or less.

[0042] Ni: 1.000% or less Ni is an element that improves hardenability, and the addition of Ni results in the formation of a large amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. The Ni content is further preferably 0.080% or more, and even more preferably 0.100% or more. On the other hand, if the Ni content exceeds 1.000%, the area ratio of martensite increases, the hole expandability and bendability in a V-bend test decrease, and the desired λ and R / t may not be obtained. 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 further preferably 0.600% or less, and even more preferably 0.400% or less.

[0043] Mo: 1.000% or less Mo is an element that improves hardenability, and the addition of Mo results in the formation of a large amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.030% or more. On the other hand, if the Mo content exceeds 1.000%, the area fraction of martensite increases, which reduces the hole expandability and bendability in V-bend tests, and the desired λ and R / t may not be obtained. Therefore, when Mo is added, the Mo content is preferably 1.000% or less. Furthermore, the Mo content is more preferably 0.500% or less. The Mo content is more preferably 0.450% or less, and even more preferably 0.400% or less.

[0044] Sb: 0.200% or less Sb is an element that is effective in suppressing the diffusion of C near the steel sheet surface during annealing and adjusting the thickness of the soft layer formed near the steel sheet surface. The Sb content is preferably 0.002% or more, and more preferably 0.004% or more. On the other hand, if the Sb content exceeds 0.200%, a soft layer is not formed near the steel sheet surface, which may result in a decrease in λ, R / t, and SFmax. Therefore, when Sb is added, the Sb content is preferably 0.200% or less. The Sb content is more preferably 0.020% or less.

[0045] Sn: 0.200% or less Sn is an element that is effective in suppressing the diffusion of C near the steel sheet surface during annealing and adjusting the thickness of the soft layer formed near the steel sheet surface. The Sn content is preferably 0.002% or more, and more preferably 0.004% or more. On the other hand, if the Sn content exceeds 0.200%, a soft layer is not formed near the steel sheet surface, which may result in a decrease in λ, R / t, 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.

[0046] Cu:1.000% or less Cu is an element that improves hardenability, and the addition of Cu results in the formation of a large amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.020% or more. The Cu content is further preferably 0.080% or more, and even more preferably 0.150% or more. On the other hand, if the Cu content exceeds 1.000%, the area fraction of martensite may increase excessively. Furthermore, large amounts of coarse precipitates and inclusions may be formed. In such cases, the excessively formed martensite and coarse precipitates and inclusions may become the origin of voids and cracks during hole expansion tests, V-bend tests, and V-bend + orthogonal VDA bend tests, making it difficult to obtain the desired λ, R / t, and SFmax. Therefore, when Cu is added, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.

[0047] Ta:0.100% or less Ta, like Ti, Nb, and V, increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. Additionally, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta)(C, N). This suppresses coarsening of precipitates and stabilizes precipitation strengthening, further improving TS and YS. To achieve this effect, the Ta content is preferably 0.001% or more. On the other hand, if the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may form. In such cases, excessively coarse precipitates and inclusions may become the initiation points for voids and cracks during hole expansion tests, V-bend tests, and V-bend + orthogonal VDA bend tests, preventing the desired λ, R / t, and SFmax from being achieved. Therefore, if Ta is added, the Ta content is preferably 0.100% or less. The Ta content is more preferably 0.080% or less, and even more preferably 0.020% or less.

[0048] W: 0.500% or less W is an element that improves hardenability, and the addition of W results in the formation of a large amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.030% or more. On the other hand, if the W content exceeds 0.500%, the area fraction of martensite increases, which reduces hole expandability and bendability in V-bend tests, and the desired λ and R / t may not be obtained. Therefore, when W is added, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, and even more preferably 0.400% or less. The W content is more preferably 0.150% or less, and even more preferably 0.080% or less.

[0049] Mg: 0.0200% or less Mg is an element that effectively spheroidizes inclusions such as sulfides and oxides and improves the hole expandability and bendability of steel sheets. To achieve this effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more, and even more preferably 0.0020% or more. On the other hand, if the Mg content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions may become the origin of voids and cracks during hole expansion tests, V-bend tests, and V-bend + orthogonal VDA bend tests, making it difficult to obtain the desired λ, R / t, and SFmax. Therefore, when Mg is added, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0150% or less, and even more preferably 0.0100% or less.

[0050] Zn: 0.0200% or less Zn is an element that effectively spheroidizes the shape of inclusions and improves the hole expandability and bendability of steel sheets. To achieve this effect, the Zn content is preferably 0.0010% or more. The Zn content is more preferably 0.0005% or more, and even more preferably 0.0020% or more. On the other hand, if the Zn content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions may become the origin of voids and cracks during hole expansion tests, V-bend tests, and V-bend + orthogonal VDA bend tests, making it difficult to obtain the desired λ, R / t, and SFmax. Therefore, when Zn is added, the Zn content is preferably 0.0200% or less. The Zn content is more preferably 0.0150% or less, and even more preferably 0.0100% or less.

[0051] Co:0.0200% or less Like Zn, Co is an element that effectively spheroidizes the shape of inclusions and improves the hole expandability and bendability of steel sheets. To achieve this effect, the Co content is preferably 0.0010% or more. The Co content is more preferably 0.0010% or more, and even more preferably 0.0020% or more. On the other hand, if the Co content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions may become the starting points for voids and cracks during hole expansion tests, V-bend tests, and V-bend + orthogonal VDA bend tests, making it difficult to obtain the desired λ, R / t, and SFmax. Therefore, when Co is added, the Co content is preferably 0.0200% or less.

[0052] Zr: 0.1000% or less Like Zn and Co, Zr is an element that is effective in making inclusions spheroidal and improving the hole expandability and bendability of steel sheets. To obtain such effects, 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 become the origin of voids and cracks during hole expansion tests, V-bend tests, and V-bend + orthogonal VDA bend tests, making it difficult to obtain the desired λ, R / t, and SFmax. Therefore, when Zr is added, the Zr content is preferably 0.1000% or less. The Zr content is more preferably 0.0150% or less, and even more preferably 0.0100% or less.

[0053] Ca:0.0200% or less Ca exists as inclusions in steel. If the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions may become the origin of voids and cracks during hole expansion tests, V-bend tests, and V-bend + orthogonal VDA bend tests, making it difficult to obtain the desired λ, R / t, and SFmax. Therefore, if Ca is added, the Ca content is preferably 0.0200% or less. The Ca content is preferably 0.0080% or less. Although there is no particular lower limit for the Ca content, the Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more due to constraints on production technology.

[0054] Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, REM: 0.0200% or less Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all elements effective for improving the hole expandability and bendability of steel sheets. To obtain these effects, the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are each preferably 0.0001% or more. The contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are more preferably 0.0020% or more. On the other hand, if the content of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM exceeds 0.0200%, or if the content of As exceeds 0.0500%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions may become the origin of voids and cracks during hole expansion tests, V-bend tests, and V-bend + orthogonal VDA bend tests, making it difficult to achieve the desired λ, R / t, and SFmax. Therefore, when at least one of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is contained, it is preferable that the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are each 0.0200% or less, and the As content is 0.0500% or less. The contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are more preferably 0.0120% or less. In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM concentration in the present invention refers to the total content of one or more elements selected from the above-mentioned REM. The REM is not particularly limited, but is preferably Sc, Y, Ce, or La.

[0055] surface soft layer Next, the steel structure of the base steel sheet according to one embodiment of the present invention will be described. A steel sheet according to one embodiment of the present invention has a soft surface layer on the surface of the base steel sheet (in the case of hot-dip galvanized steel sheet, galvannealed steel sheet, electrogalvanized steel sheet, and other metal-plated steel sheets, this soft surface layer is located on the base (underlying) of the various platings). The soft surface layer contributes to suppressing the propagation of bending cracks during press forming and vehicle collisions, thereby further improving bending fracture resistance. The soft surface layer refers to a decarburized layer, and is a surface region having a Vickers hardness of 85% or less of the Vickers hardness of a cross section (plane parallel to the steel sheet surface) at 1 / 4 of the sheet thickness of the base steel sheet. Here, the soft surface layer is formed in a region of 200 μm or less from the surface of the base steel sheet in the sheet thickness direction. The region where the soft surface layer is formed is preferably 150 μm or less, more preferably 120 μm or less, from the surface of the base steel sheet in the sheet thickness direction. The region where the soft surface layer is formed is preferably 30 μm or more, more preferably 40 μm or more, from the surface of the base steel sheet in the sheet thickness direction. The 1 / 4 sheet thickness position of the base steel sheet where 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 specified in the present invention). Vickers hardness is measured based on JIS Z 2244-1 (2020) at a load of 10 gf.

[0056] Hardness of the soft surface layer When nano hardness is measured at 300 or more points in a 50 μm x 50 μm area on the sheet surface at 1 / 4 of the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet and at 1 / 2 of the depth in the sheet thickness direction, the ratio of measurements where the nano hardness is 7.0 GPa or more on the sheet surface at 1 / 4 of the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet is 0.10 or less of the total number of measurements In the present invention, to obtain excellent bendability during press forming and excellent bending fracture properties during collision, when the nanohardness is measured at 300 or more points in a 50 μm × 50 μm region on the sheet surface at a position 1 / 4 of the way from the surface of the base steel sheet to the depth in the sheet thickness direction of the soft surface layer, the proportion of nanohardnesses of 7.0 GPa or more on the sheet surface at a position 1 / 4 of the way from the surface of the base steel sheet to the depth in the sheet thickness direction of the soft surface layer must be 0.10 or less relative to the total number of measurements (total number of measurements at a position 1 / 4 of the way from the surface of the base steel sheet to the depth in the sheet thickness direction). If the proportion of nanohardnesses of 7.0 GPa or more is 0.10 or less, it means that the proportion of hard structures (such as martensite) and inclusions is small, which makes it possible to further suppress the generation and coalescence of voids and crack propagation in hard structures (such as martensite) and inclusions during press forming and collision, resulting in excellent R / t and SFmax. The proportion of the sheet surface at a position 1 / 4 of the way from the surface of the base steel sheet to the depth in the sheet thickness direction of the soft surface layer where the nano-hardness is 7.0 GPa or more is preferably 0.08 or less, more preferably 0.07 or less, relative to the total number of measurements. The lower limit is not particularly limited, but may be 0.01 or more.

[0057] The standard deviation σ of the nano-hardness of the sheet surface at a position 1 / 4 of the way from the steel sheet surface to the depth in the sheet thickness direction of the soft surface layer is 1.8 GPa or less, and further, the standard deviation σ of the nano-hardness of the sheet surface at a position 1 / 2 of the way from the steel sheet surface to the depth in the sheet thickness direction of the soft surface layer is 2.2 GPa or less In the present invention, to obtain excellent bendability during press forming and excellent bending fracture properties during collision, the standard deviation σ of the nanohardness of the sheet surface at a position ¼ of the depth in the sheet thickness direction of the soft surface layer from the steel sheet surface must be 1.8 GPa or less, and further, the standard deviation σ of the nanohardness of the sheet surface at a position ½ of the depth in the sheet thickness direction of the soft surface layer from the steel sheet surface must be 2.2 GPa or less. When the standard deviation σ of the nanohardness of the sheet surface at a position ¼ of the depth in the sheet thickness direction of the soft surface layer from the steel sheet surface is 1.8 GPa or less, and further, the standard deviation σ of the nanohardness of the sheet surface at a position ½ of the depth in the sheet thickness direction of the soft surface layer from the steel sheet surface is 2.2 GPa or less, this means that the difference in microstructural hardness in the microregion is small, making it possible to further suppress the generation and coalescence of voids and the propagation of cracks during press forming and collision, and achieving excellent R / t and SFmax. Furthermore, the standard deviation σ of the nanohardness of the sheet surface at a position ¼ of the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet is preferably 1.7 GPa or less. The standard deviation σ of the nanohardness of the sheet surface at a position ¼ of the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet is more preferably 1.3 GPa or less. Although there is no particular lower limit, the standard deviation σ of the nanohardness of the sheet surface at a position ¼ of the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet may be 0.5 GPa or more. A preferred range of the standard deviation σ of the nanohardness of the sheet surface at a position halfway from the surface of the base steel sheet into the soft surface layer in the sheet thickness direction is 2.1 GPa or less. The standard deviation σ of the nanohardness of the sheet surface at a position halfway from the surface of the base steel sheet into the soft surface layer in the sheet thickness direction is more preferably 1.7 GPa or less. Although there is no particular lower limit, the standard deviation σ of the nanohardness of the sheet surface at a position halfway from the surface of the base steel sheet into the soft surface layer in the sheet thickness direction may be 0.6 GPa or more.

[0058] Here, the nano-hardness of the plate surface at the 1 / 4 and 1 / 2 positions in the depth direction of the plate is the hardness measured by the following method. First, if a plating layer is present, remove it. Then, mechanical polishing is performed from the surface of the base steel sheet to a depth of 5 μm, 1 / 4 of the way down the thickness of the soft surface layer. Then, buffing is performed with diamond and alumina from the surface of the base steel sheet to a depth of 1 / 4 of the way down the thickness of the soft surface layer, followed by colloidal silica polishing. Using a Hysitron Tribo-950 with a Berkovich-shaped diamond indenter, nanohardness is measured under the following conditions: load: 500 μN, measurement area: 50 μm x 50 μm, and impact spacing: 2 μm. The soft surface layer is mechanically polished to half the depth in the plate thickness direction, then buffed with diamond and alumina, and finally polished with colloidal silica.Then, using a Hysitron tribo-950, the nanohardness is measured with a Berkovich-shaped diamond indenter under the following conditions: load: 500 μN, measurement area: 50 μm x 50 μm, and impact spacing: 2 μm. Nano hardness is measured at 300 or more points at a position 1 / 4 of the way through the thickness of the plate, and also at 300 or more points at a position 1 / 2 of the way through the thickness of the plate. For example, if the thickness of the surface soft layer is 100 μm, the 1 / 4 position is 25 μm from the surface of the surface soft layer, and the 1 / 2 position is 50 μm from the surface of the surface soft layer. At this 25 μm position, nano-hardness is measured at 300 or more points, and also at the 50 μm position, nano-hardness is measured at 300 or more points.

[0059] Depth of surface irregularities of test piece after 90° V bending where R / t (limiting bending radius R divided by plate thickness t) is 4.5 to 5.0: 20.0 μm or less Controlling the surface irregularities of the steel sheet is effective for further improving SFmax, a feature of the present invention. Specifically, the method for evaluating the surface irregularities of the steel sheet is as follows. Because concave portions of the surface irregularities of the steel sheet are likely to be the initiation points for bending cracks, if the depth of the surface irregularities of a test piece after 90° V bending, where R / t (the critical bending radius R divided by the sheet thickness t) is 4.5 to 5.0, exceeds 20.0 μm, the desired R / t and SFmax may not be obtained under specified conditions. Here, the metal plating layer (first plating layer (a plating layer formed by plating in the first plating step (metal plating step)) mainly containing Fe or Ni contains internal oxides present in the outermost layer, reducing the depth of the surface irregularities of a test piece after 90° V bending, where R / t (the critical bending radius R divided by the sheet thickness t) is 4.5 to 5.0, thereby achieving better R / t and SFmax. This surface irregularity depth is preferably 12.0 μm or less, more preferably 9.0 μm or less. The lower limit is not particularly limited, but may be 0.1 μm or more, or 0.3 μm or more. The depth of the surface irregularities on the bent cross section was determined by taking five 1500x SEM images of the bent cross section, drawing reference lines at the maximum height of the surface protrusions and the maximum depth of the microcrack depressions within an 83 μm range in the horizontal direction of the photograph, determining the shortest distance between them, and averaging the five SEM images. The average value of the shortest distances thus determined was taken as the depth of the surface irregularities. Here, the above-mentioned evaluation conditions for evaluating the depth of the surface irregularities are different from the conditions for measurement in accordance with JIS Z 2248 when evaluating bendability, i.e., the evaluation criteria for determining whether or not cracks occur after bending.

[0060] The total area ratio of ferrite and bainite in the soft surface layer: 20% or more, and (area ratio of martensite present in the soft surface layer) / (total area ratio of martensite and tempered martensite present in the soft surface layer): 0.45 or less When a soft surface layer is present, the origin of void generation and connection, as well as crack propagation, during press forming and collision is the boundary between the soft phases ferrite and bainite in the soft surface layer and the hard phases martensite and tempered martensite. When the total area ratio of ferrite and bainite present in the soft surface layer is increased and further the value obtained by dividing the area ratio of martensite present in the soft surface layer by the total area ratio of martensite and tempered martensite present in the soft surface layer is small, cracking during press forming and collision can be suppressed, and excellent R / t and SFmax can be obtained. If the total area fraction of ferrite and bainite present in the soft surface layer is less than 20%, and / or if the value obtained by dividing the area fraction of martensite present in the soft surface layer by the total area fraction of martensite and tempered martensite present in the soft surface layer exceeds 0.45, the generation and coalescence of voids and the propagation of cracks during press forming and impact may not be suppressed, and the desired R / t and SFmax may not be obtained. Therefore, it is preferable that the total area fraction of ferrite and bainite present in the soft surface layer is 20% or more, and the value obtained by dividing the area fraction of martensite present in the soft surface layer by the total area fraction of martensite and tempered martensite present in the soft surface layer is 0.45 or less, more preferably 0.40 or less. Furthermore, the lower limit of the value obtained by dividing the area ratio of martensite present in the soft surface layer by the sum of the area ratios of martensite and tempered martensite present in the soft surface layer is not particularly limited, and may be 0.00. The upper limit of the sum of the area ratios of ferrite and bainite present in the soft surface layer is not particularly limited, and may be 100%.

[0061] Steel structure at 1 / 4 of the plate thickness The area ratio of martensite is preferably 50.0% or less. If the area fraction of martensite increases excessively, it may become the starting point for void generation during the hole expansion process in the hole expansion test or the bending process in the V-bend test, making it possible that the desired λ and R / t cannot be obtained. Therefore, the area fraction of martensite is preferably 50.0% or less. Furthermore, the area fraction of martensite is more preferably 45.0% or less. The lower limit of the area fraction of martensite is not particularly limited and may be 0.0%. The martensite referred to here is as-quenched (untempered) martensite.

[0062] The volume fraction of retained austenite is preferably 20.0% or less. If the volume fraction of retained austenite increases excessively, when the steel plate is punched in a hole expansion test or V-bended in a V-bend + orthogonal VDA test, hard martensite is generated by the deformation-induced transformation of the retained austenite, which may cause voids to form and cracks to grow in subsequent tests, making it impossible to obtain the desired λ and SFmax. Therefore, it is preferable that the volume fraction of retained austenite be 20.0% or less. Furthermore, tempered martensite is a metal phase that can impart strength and workability to the steel sheet of the present invention, and therefore the area ratio of tempered martensite can be appropriately set to 100% or less.

[0063] The remaining structure other than martensite, retained austenite, and tempered martensite is not particularly limited, and examples thereof include ferrite, unrecrystallized ferrite, pearlite, bainite, and ε-martensite. The type of the remaining structure can be confirmed by observation using, for example, an SEM.

[0064] Here, the area ratios of ferrite, bainite, tempered martensite, and hard second phase (martensite + retained austenite) are measured at a 1 / 4 position in the sheet thickness direction of the base steel sheet as follows.

[0065] Specifically, a sample is cut out so that the observation surface is the thickness cross section (L cross section) parallel to the rolling direction of the steel sheet. The observation surface of the sample is then polished with diamond paste, and then finish-polished using alumina. The observation surface of the sample is then etched with 3 vol.% nital to reveal the structure. The observation position is then set at 1 / 4 of the thickness of the base steel sheet, and five fields of view are observed at 3000x magnification using an SEM. From the obtained structure images, Adobe Photoshop by Adobe Systems Inc. is used to calculate the area ratios of each constituent structure (ferrite, bainite, tempered martensite, and hard second phase (martensite + retained austenite)) divided by the measured area for the five fields of view, and these values ​​are averaged to determine the area ratio of each structure. Ferrite: A black region with a blocky shape. It contains almost no carbides. Also, isolated islands of martensite and isolated islands of retained austenite within ferrite grains are not included in the area ratio of ferrite. Bainite: This is a region that is black to dark gray in color and has a massive or amorphous shape. It also contains relatively small amounts of carbides. Tempered martensite: This is a gray area with an amorphous shape. It also contains a relatively large number of carbides. Hard second phase (retained austenite + martensite): This is a white to light gray region with an amorphous morphology and does not contain carbides. Carbides: These are white areas that appear as dots or lines. They are included in bainite and tempered martensite. Remaining structure: In addition to the above-mentioned ferrite and bainite, examples include pearlite, cementite, unrecrystallized ferrite, ε martensite, etc., and the forms of these are well known.

[0066] The volume fraction of retained austenite is measured as follows. Specifically, the base steel sheet is mechanically ground in the thickness direction (depth direction) to a position one-quarter of the thickness, and then chemically polished with oxalic acid to obtain the observation surface. The observation surface is then observed using X-ray diffraction. MoKα radiation is used as the incident X-ray, and the ratio of the diffraction intensities of the (200), (211), and (220) planes of fcc iron (austenite) to those of the (200), (220), and (311) planes of bcc iron is calculated. The volume fraction of retained austenite is then calculated from the ratio of the diffraction intensities of each plane. The retained austenite is then considered to be three-dimensionally homogeneous, and the volume fraction of retained austenite is taken as the area fraction of retained austenite. The area ratio of martensite is determined by subtracting the area ratio of retained austenite from the area ratio of the hard second phase determined as described above. [Area fraction of martensite (%)] = [Area fraction of hard second phase (%)] - [Area fraction of retained austenite (%)] The area ratio of the remaining structure is determined by subtracting the area ratio of ferrite, the area ratio of bainite, the area ratio of tempered martensite, and the area ratio of the hard second phase determined as described above from 100.0%. [Area fraction of remaining structure (%)] = 100.0 - [Area fraction of ferrite (%)] - [Area fraction of bainite (%)] - [Area fraction of tempered martensite (%)] - [Area fraction of hard second phase (%)]

[0067] The structure present in the soft surface layer (ferrite, bainite, martensite, tempered martensite) is measured in the same manner as the structure at the 1 / 4 position in the thickness direction of the base steel plate, except that the observation position is changed to the 1 / 4 position in the thickness direction of the soft surface layer instead of the 1 / 4 position in the thickness direction of the base steel plate.

[0068] Next, the mechanical properties of the steel plate according to one embodiment of the present invention will be described. Tensile strength (TS): 590 MPa or more The tensile strength TS of the steel plate according to one embodiment of the present invention is 590 MPa or more. The yield stress (YS), total elongation (El), limiting hole expansion ratio (λ), limiting bending radius / plate thickness (R / t), stroke at maximum load in a V-bending + orthogonal VDA bending test (SFmax), and the presence or absence of fracture (visual cracks) in an axial crushing test of a steel plate according to one embodiment of the present invention are as described above. The tensile strength (TS), yield stress (YS), and total elongation (El) are measured by a tensile test conforming to JIS Z 2241, which will be described later in the examples. The limiting hole expansion ratio (λ) is measured by a hole expansion test conforming to JIS Z 2256, which will be described later in the examples. The limiting bend radius / plate thickness (R / t) is measured by a V-bend test conforming to JIS Z 2248, which will be described later in the examples. The stroke at maximum load (SFmax) in a V-bend + orthogonal VDA bend test is measured by a V-bend + orthogonal VDA bend test, which will be described later in the examples. The presence or absence of fracture (visual cracks) in an axial crush test is measured by an axial crush test, which will be described later in the examples.

[0069] First plating layer The steel sheet according to one embodiment of the present invention preferably has a metal plating layer (first plating layer, pre-plating layer) (note that the metal plating layer (first plating layer) excludes a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, a zinc plating layer of an electrogalvanized layer, and a hot-dip aluminum plating layer) on one or both surfaces of the base steel sheet. The metal plating layer is preferably a metal electroplated layer, and the following description will be given taking a metal electroplated layer as an example. By forming a metal electroplated layer on the surface of the steel sheet, the outermost metal electroplated layer contributes to suppressing the occurrence of bending cracks during press forming and vehicle collisions, thereby further improving bending fracture resistance.

[0070] 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, Pt The metal may be any of Au, Hg, Ti, Pb, and Bi, but Fe is more preferable. The following description will be given using an Fe-based electroplated layer as an example, but the following conditions for Fe can also be applied to other metals.

[0071] The adhesion weight of the Fe-based electroplating layer is 0 g / m 2 More than 2.0 g / m 2 Although there is no particular upper limit to the amount of coating of the Fe-based electroplated layer per side, from the viewpoint of cost, the amount of coating of the Fe-based electroplated layer per side is set to 60 g / m or more. 2 The coating weight of the Fe-based electroplated layer is preferably 50 g / m or less. 2 More preferably, it is 40 g / m or less. 2 More preferably, it is 30 g / m or less. 2 The following applies.

[0072] The deposition weight of the Fe-based electroplated layer is measured as follows: A 10 x 15 mm sample is taken from the Fe-based electroplated steel sheet and embedded in resin to create a cross-section embedded sample. Three randomly selected locations on the cross-section are 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. The average thickness of the three fields of view is multiplied by the iron density to convert it into the deposition weight of the Fe-based plating layer per side.

[0073] Examples of suitable Fe-based electroplating layers include pure Fe, Fe-B alloys, Fe-C alloys, Fe-P alloys, Fe-N alloys, Fe-O alloys, Fe-Ni alloys, Fe-Mn alloys, Fe-Mo alloys, and Fe-W alloys. While the composition of the Fe-based electroplating layer is not particularly limited, it is preferred that the composition contain a total of 10% by mass or less of one or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, with the remainder consisting of Fe and unavoidable impurities. Limiting the total amount of elements other than Fe to 10% by mass or less prevents a decrease in electrolysis efficiency and allows for the formation of Fe-based electroplating layers at low cost. In the case of Fe-C alloys, the C content is preferably 0.08% by mass or less.

[0074] Second plating layer The steel sheet according to one embodiment of the present invention may have a plating layer (a second plating layer such as a zinc plating layer or an aluminum plating layer) on the outermost surface of one or both sides of the steel sheet, and this plating layer may be a hot-dip galvanized layer, a galvannealed layer, an electrogalvanized layer, a hot-dip aluminum plating layer, or the like. The plating layer of a hot-dip galvanized steel sheet having a hot-dip galvanized layer formed on the surface of the steel sheet, a galvannealed hot-dip galvanized steel sheet having a hot-dip galvanized layer formed on the surface of the steel sheet, an electrogalvanized steel sheet having an electrogalvanized layer formed on the surface of the steel sheet, and a plated steel sheet having another metal plating layer (such as an aluminum plating layer) formed on the surface of the steel sheet may be formed only on one surface of 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 on the surface of the metal plating layer), or may be formed on both surfaces. That is, the steel sheet of the present invention may have a base steel sheet on which a second plating layer (a zinc plating layer, an aluminum plating layer, etc.) is formed, or may have a base steel sheet on which a metal plating layer (a first plating layer (excluding a second plating layer such as a zinc plating layer, an aluminum plating layer, etc.)) and a second plating layer (a zinc plating layer, an aluminum plating layer, etc.) are formed in this order on the base steel sheet.

[0075] The plating layer of the hot-dip galvanized steel sheet, galvannealed hot-dip galvanized steel sheet, and electro-galvanized steel sheet referred to here refers to a plating layer containing Zn (zinc) as the main component (Zn content of 50.0 mass % or more). The plating layer of an aluminum-plated steel sheet refers to a plating layer containing Al (aluminum) as the main component (Al content of 50.0 mass % or more).

[0076] Here, the hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0 mass% or less of Fe, and 0.001 mass% to 1.0 mass% or less of Al. The hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 3.5 mass%. The Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The remainder other than the above elements is unavoidable impurities.

[0077] The galvannealed layer is preferably composed of, for example, 20% by mass or less of Fe and 0.001% by mass to 1.0% by mass or less of Al. The galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0% by mass to 3.5% by mass. The Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more. The Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less. The remainder other than the above elements is unavoidable impurities.

[0078] In addition, the coating weight of the zinc plating layer per side is not particularly limited, but is preferably 20 g / m 2 The coating weight of the zinc plating layer on one side is preferably 80 g / m or more. 2 It is preferable to do the following:

[0079] The coating weight of the zinc coating layer of the hot-dip galvanized steel sheet and the galvannealed steel sheet is measured as follows. That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (Ivit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass % aqueous solution of hydrochloric acid. Next, a hot-dip galvanized steel sheet to be used as a test material is immersed in the treatment solution to dissolve the zinc coating layer. The mass loss of the test material before and after dissolution is measured, and this value is divided by the surface area of ​​the base steel sheet (the surface area of ​​the part that was coated with the coating) to determine the coating weight (g / m 2 ) is calculated.

[0080] The thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more. The thickness is preferably 3.5 mm or less, and more preferably 2.3 mm or less. The thickness is more preferably 0.8 mm or more, even more preferably 1.0 mm or more, and even more preferably 1.2 mm or more.

[0081] [2. Steel plate manufacturing method] Next, a method for manufacturing a steel sheet according to one embodiment of the present invention will be described. The method for producing a steel sheet of the present invention includes a hot rolling step of hot rolling a steel slab having the aforementioned chemical composition; an annealing step of annealing the resulting steel sheet under conditions of an annealing temperature of not less than Ac1 point and not more than 950°C, an annealing time of not less than 10 seconds, and an atmosphere with a dew point of not less than -30°C; a first cooling step of cooling the resulting steel sheet after the annealing step at an average cooling rate of not less than 8°C / second in a temperature range from Ac1 point to 450°C in an atmosphere with a dew point of not more than -30°C; and a second cooling step of cooling the resulting steel sheet after the first cooling step at an average cooling rate of less than 8°C / second in a temperature range from below 450°C to 300°C.

[0082] In the present invention, the method for melting the steel material (steel slab) is not particularly limited, and any of the known melting methods, such as converters and electric furnaces, is suitable. Furthermore, the steel slab is preferably produced by a continuous casting method to prevent macrosegregation, but it can also be produced by an ingot casting method, thin slab casting method, or the like. Furthermore, in addition to the conventional method in which the steel slab is cooled to room temperature after production and then reheated, energy-saving processes such as direct rolling or direct rolling in which the slab is charged as a hot slab into a heating furnace without being cooled to room temperature, or is immediately rolled after a short period of heat retention, can also be applied without any problems.

[0083] (Hot rolling process) When heating the slab, the slab heating temperature is preferably 1100°C or higher from the viewpoints of dissolving carbides, reducing the rolling load, and improving the uniformity of the formation of the soft surface layer, which is a feature of the present invention. Furthermore, the slab heating temperature is preferably 1300°C or lower to prevent an increase in scale loss. Setting the slab heating temperature to 1300°C or lower also prevents the formation of the soft surface layer, which is a feature of the present invention, from becoming excessively large, making it possible to stably obtain the strength, which is a feature of the present invention. The slab heating temperature is the temperature of the slab surface. The slab is made into a sheet bar by rough rolling under normal conditions, but if the heating temperature is set low, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling in order to prevent problems during hot rolling.

[0084] The finish rolling temperature is preferably 820°C or higher because an increase in the rolling load and a high reduction rate in the unrecrystallized austenite state can lead to the development of an abnormal structure elongated in the rolling direction, which can reduce the ductility, hole expandability, and bendability of the final material. Furthermore, the coiling temperature after hot rolling is preferably 300°C or higher because there is a concern that the ductility, hole expandability, and bendability of the final material may be reduced. Furthermore, the coiling temperature is preferably 700°C or lower. During hot rolling, the coiling temperature is preferably 400°C or higher from the viewpoint of improving the uniformity of the formation of the soft surface layer characterized by the present invention. The coiling temperature is preferably 650°C or lower. If the coiling temperature is lower than 400°C, it may be difficult to stably form the soft surface layer within the range specified by the present invention. If the coiling temperature exceeds 650°C, the soft surface layer characterized by the present invention may be formed too much, making it difficult to stably obtain the strength characterized by the present invention. Therefore, the coiling temperature is more preferably 400°C or higher. The coiling temperature is preferably 650°C or lower.

[0085] During hot rolling, the rough-rolled sheets may be joined together and continuously finished. The rough-rolled sheets may also be wound up once. In order to reduce the rolling load during hot rolling, some or all of the finish rolling may be performed as lubricated rolling. Performing lubricated rolling is also effective from the viewpoint of uniforming the shape and material properties of the steel sheet. The friction coefficient during lubricated rolling is preferably 0.10 or more. The friction coefficient during lubricated rolling is preferably 0.25 or less.

[0086] (pickling process) The hot-rolled steel sheet thus produced may be subjected to pickling. Pickling can remove oxides from the steel sheet surface, and therefore can be performed to ensure good chemical conversion treatability and plating quality in the final high-strength steel sheet product. Pickling may be performed once or multiple times.

[0087] (Cold rolling process) The hot-rolled pickled sheet or hot-rolled steel sheet obtained as described above is subjected to cold rolling as necessary. When cold rolling is performed, the pickled sheet may be subjected to cold rolling as it is after hot rolling, or may be subjected to heat treatment and then cold rolling. Optionally, the cold-rolled steel sheet obtained after cold rolling may be subjected to pickling. The cold rolling is carried out by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.

[0088] Cold rolling reduction: 20% to 80% When cold rolling is performed, the reduction ratio (cumulative reduction ratio) of the cold rolling is not particularly limited, but is preferably 20% or more. Furthermore, the reduction ratio of the cold rolling is preferably 80% or less. If the reduction ratio of the cold rolling is less than 20%, the steel structure is likely to become coarse and non-uniform during the annealing process, which may result in reduced TS and bendability in the final product. On the other hand, if the reduction ratio of the cold rolling exceeds 80%, the steel sheet is likely to have poor shape and the coating weight of zinc plating or other coatings may become non-uniform.

[0089] (Metal plating (metal electroplating, first plating) process) In one embodiment of the present invention, the method may include a first plating step in which metal plating is applied to one or both sides of the steel sheet after the hot rolling step (after the cold rolling step if cold rolling is performed) and before the annealing step to form a first plating layer. For example, a pre-annealed metal electroplated steel sheet may be obtained by subjecting the surface of the cold-rolled steel sheet obtained as described above to a metal plating treatment (first plating treatment) such as metal electroplating treatment, thereby forming a pre-annealed metal electroplated layer (first plating layer) on at least one surface. The metal plating referred to here excludes second plating such as zinc plating or aluminum plating. While the metal electroplating method is not particularly limited, as described above, it is preferable to form a metal electroplated layer on the base steel sheet, and therefore metal electroplating treatment is preferred. For example, an Fe-based electroplating bath may be a sulfuric acid bath, a hydrochloric acid bath, or a mixture of both. The coating weight of the pre-annealed metal electroplated layer can be adjusted by, for example, the current application time. The pre-annealed metal electroplated steel sheet means a steel sheet whose metal electroplated layer has not undergone an annealing process, and does not exclude hot-rolled steel sheets before metal electroplating treatment, pickled steel sheets after hot rolling, or cold-rolled steel sheets that have been annealed in advance.

[0090] 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, Pt Although any of Au, Hg, Ti, Pb, and Bi may be used, Fe is more preferable, and therefore the manufacturing method for Fe-based electroplating will be described below.

[0091] The Fe ion content in the Fe-based electroplating bath before the start of current application is 2+ The content of Fe ions in the Fe-based electroplating bath is preferably 0.5 mol / L or more. 2+ A sufficient amount of Fe can be deposited if the Fe ion content in the Fe-based electroplating bath before the start of current application is 0.5 mol / L or more. In order to obtain a sufficient amount of Fe, the Fe ion content in the Fe-based electroplating bath before the start of current application is preferably 2.0 mol / L or less.

[0092] The Fe-based electroplating bath may contain Fe ions and at least one element selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co. The total content of these elements in the Fe-based electroplating bath is preferably 10 mass% or less in the Fe-based electroplated layer before annealing. Metal elements may be contained as metal ions, and nonmetal elements may be contained as part of boric acid, phosphoric acid, nitric acid, organic acids, etc. The iron sulfate plating solution may also contain conductivity enhancers such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.

[0093] Other conditions for the Fe-based electroplating bath are not particularly limited. The temperature of the Fe-based electroplating solution is preferably 30°C or higher, and preferably 85°C or lower, in consideration of maintaining a constant temperature. The pH of the Fe-based electroplating bath is not particularly specified, but is preferably 1.0 or higher in order to prevent a decrease in current efficiency due to hydrogen generation, and is preferably 3.0 or lower in consideration of the electrical conductivity of the Fe-based electroplating bath. The current density is 10 A / dm from the viewpoint of productivity. 2 It is preferable that the current is 150 A / dm or more, and from the viewpoint of facilitating control of the coating weight of the Fe-based electroplating layer, 2The sheet threading speed is preferably 5 mpm or more from the viewpoint of productivity, and is preferably 150 mpm or less from the viewpoint of stable control of the adhesion amount.

[0094] Prior to the Fe-based electroplating treatment, the steel sheet may be subjected to a degreasing treatment and water rinsing to clean the surface, and further to a pickling treatment and water rinsing to activate the surface. These pretreatments are followed by the Fe-based electroplating treatment. The degreasing and water rinsing methods are not particularly limited, and conventional methods can be used. Various acids can be used in the pickling treatment, such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof. Among these, sulfuric acid, hydrochloric acid, or mixtures thereof are preferred. The acid concentration is not particularly limited, but a range of 1 to 20 mass% is preferred, taking into consideration the ability to remove oxide films and the prevention of surface roughness (surface defects) due to excessive pickling. The pickling treatment solution may also contain an antifoaming agent, a pickling accelerator, a pickling inhibitor, etc.

[0095] (Annealing process) In one embodiment of the present invention, after the hot rolling step (after the cold rolling step if cold rolling is performed, or after the first plating step if a first plating treatment for forming a first plating layer is performed), an annealing step is included in which the steel sheet is annealed under conditions of an annealing temperature of not less than the Ac1 point and not more than 950°C, an annealing time of not less than 10 seconds, and an atmosphere with a dew point of not less than -30°C.

[0096] Annealing temperature: Ac1 point (℃) or higher and 950℃ or lower If the annealing temperature is lower than the Ac1 point (°C), the proportion of austenite generated during annealing will be insufficient. As a result, the area ratio of ferrite at the 1 / 4 sheet thickness position after annealing will increase excessively, and the desired TS and YS will not be obtained. On the other hand, if the annealing temperature exceeds 950°C, the austenite single phase region will be reached, making it difficult to form a soft surface layer. Therefore, the annealing temperature is set to be equal to or higher than the Ac1 point (°C) and equal to or lower than 950°C. The annealing temperature is preferably equal to or higher than the Ac1 point + 10°C. Also, it is preferably equal to or lower than 900°C. More preferably, it is equal to or lower than 840°C. The annealing temperature is the maximum temperature reached in the annealing process.

[0097] Annealing time: 10 seconds or more If the annealing time is less than 10 seconds, it becomes difficult to form a soft surface layer. Therefore, the annealing time is set to 10 seconds or more. The upper limit of the annealing time is not particularly limited, but it is preferably set to 900 seconds or less. The annealing time is the holding time in a temperature range of (annealing temperature - 30°C) or more and (annealing temperature - 30°C) or less. In other words, the annealing time includes not only the holding time at the annealing temperature but also the residence time in a temperature range of (annealing temperature - 30°C) or more and (annealing temperature - 30°C) or less during heating and cooling before and after reaching the annealing temperature. The number of annealing steps may be two or more, but one step is preferred from the viewpoint of energy efficiency.

[0098] Dew point of the annealing process atmosphere (annealing atmosphere): -30°C or higher The dew point of the atmosphere in the annealing step (annealing atmosphere) must be −30°C or higher. By performing the annealing step at a dew point of −30°C or higher, the decarburization reaction is promoted, and the soft surface layer is formed deeper. As a result, when the nanohardness is measured at 300 or more points in a 50 μm × 50 μm region on the sheet surface at a position ¼ of the way down the sheet thickness direction of the soft surface layer from the surface of the base steel sheet, the proportion of nanohardness values ​​of 7.0 GPa or higher is 0.10 or lower. The annealing atmosphere in the annealing step is preferably −15°C or higher, more preferably −5°C or higher. While there is no particular upper limit for the dew point of the annealing atmosphere in the annealing step, it is preferable that the dew point of the annealing atmosphere in the annealing step be 30°C or lower in order to effectively prevent oxidation of the surface of the Fe-based electroplated layer and to ensure good plating adhesion when a zinc-plated layer is formed.

[0099] (First cooling process) In the temperature range from Ac1 point to 450°C, the dew point is -30°C or less, and the average cooling rate (first average cooling rate) is 8°C / sec or more. In the present invention, by increasing the area ratio of ferrite at a position 1 / 4 of the way down in the thickness direction of the soft surface layer from the surface of the base steel sheet and at a position 1 / 2 of the way down in the thickness direction of the soft surface layer from the surface of the base steel sheet, the standard deviation σ of the nanohardness of the sheet surface at a position 1 / 4 of the way down in the thickness direction of the soft surface layer from the surface of the base steel sheet is 1.8 GPa or less, and further, the standard deviation σ of the nanohardness of the sheet surface at a position 1 / 2 of the way down in the thickness direction of the soft surface layer from the surface of the base steel sheet is 2.2 GPa or less. In this regard, if the average cooling rate in the temperature range from Ac1 point to 450°C in the first cooling step after the annealing step is less than 8°C / sec, the nano-hardness cannot be adjusted to the above range. Therefore, in the present invention, the average cooling rate in the temperature range from Ac1 point to 450°C in the first cooling step after the annealing step is set to 8°C / sec or more. It is also necessary to perform cooling in the temperature range from the Ac1 point to 450°C under conditions in an atmosphere with a dew point of -30°C or lower. If the dew point is higher than -30°C, the soft surface layer on the steel sheet surface will be formed non-uniformly within the steel sheet surface, which will likely result in cases where the standard deviation σ of the nanohardness of the sheet surface at a position 1 / 4 of the thickness direction depth of the soft surface layer from the surface of the base steel sheet is 1.8 GPa or lower, and the standard deviation σ of the nanohardness of the sheet surface at a position 1 / 2 of the thickness direction depth of the soft surface layer from the surface of the base steel sheet is 2.2 GPa or lower, as specified in the present invention. There is no particular restriction on the lower limit of the dew point, but it is preferably -55°C or higher due to equipment restrictions. C1 It is preferable that the cooling in the temperature range from 0°C to 450°C is carried out in an atmosphere with a dew point of -32°C or lower. Here, the first average cooling rate (°C / sec) can be calculated by cooling start temperature (Ac1 point)-cooling end temperature (450°C) / cooling time (sec) in the first cooling step. The Ac1 point (°C) can be calculated using the following formula: Ac1 point (℃)=727.0-32.7×[%C]+14.9×[%Si]+2.0×[%Mn] Here, [%C] is the C content, [%Si] is the Si content, and [%Mn] is the Mn content.

[0100] (Second cooling process) Average cooling rate in the temperature range from below 450°C to 300°C (second average cooling rate): less than 8°C / sec In the present invention, by reducing the area ratio of hard phases of martensite and retained austenite at a position 1 / 4 of the way from the surface of the base steel sheet to the depth in the sheet thickness direction of the soft surface layer and at a position 1 / 2 of the way from the surface of the base steel sheet to the depth in the sheet thickness direction of the soft surface layer, the standard deviation σ of the nanohardness of the sheet surface at a position 1 / 4 of the way from the surface of the base steel sheet to the depth in the sheet thickness direction of the soft surface layer is 1.8 GPa or less, and further, the standard deviation σ of the nanohardness of the sheet surface at a position 1 / 2 of the way from the surface of the base steel sheet to the depth in the sheet thickness direction of the soft surface layer is 2.2 GPa or less. In this regard, if the average cooling rate in the temperature range from less than 450°C to 300°C in the second cooling step after the annealing step is 8°C / second or more, the nano-hardness cannot be adjusted to the above-mentioned range. Therefore, in the present invention, the average cooling rate in the temperature range from less than 450°C to 300°C in the second cooling step after the annealing step is set to less than 8°C / second. Here, the second average cooling rate (°C / sec) can be calculated by the following formula: cooling start temperature (less than 450°C)-cooling end temperature (300°C) / cooling time (sec) in the second cooling step.

[0101] (Plating process (second plating process)) In one embodiment of the present invention, a second plating step may be included in which the steel sheet after the second cooling step is plated to form a second plating layer. The plating step (second plating step) is preferably a zinc plating step or a hot-dip aluminum plating treatment, and examples of the zinc plating treatment in the zinc plating step include hot-dip galvanizing treatment, galvannealing treatment, and electrogalvanizing treatment.

[0102] In the case of hot-dip galvanizing, it is preferable to immerse the steel sheet in a galvanizing bath at 440°C or higher, and then adjust the coating weight by gas wiping or the like. In the above treatment, it is preferable to immerse the steel sheet in a galvanizing bath at 500°C or lower. The hot-dip galvanizing bath is not particularly limited as long as it provides the above-mentioned composition of the galvanized layer, but it is preferable to use a plating bath with a composition containing 0.10 mass% or more of Al, with the balance consisting of Zn and unavoidable impurities. The above Al content is preferably 0.23 mass% or less.

[0103] Furthermore, in the case of a galvannealing treatment, it is preferable to perform an alloying treatment by heating the steel sheet having a galvanized layer (hot-dip galvanized steel sheet) to an alloying temperature of 450°C or higher after performing the hot-dip galvanizing treatment as described above. The alloying temperature is preferably 600°C or lower. If the alloying temperature is lower than 450°C, the Zn-Fe alloying rate will be slow, and alloying may become difficult. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite will transform to pearlite, making it difficult to achieve a TS of 590 MPa or higher. The alloying temperature is more preferably 510°C or higher. Furthermore, the alloying temperature is more preferably 570°C or lower.

[0104] In addition, the coating weight of steel sheets with a hot-dip galvanized layer (galvannealed steel sheets) (GI) and steel sheets with a galvannealed layer (galvannealed steel sheets) (GA) is 20 g / m per side. 2 The coating weight of the zinc plating layer on one side is preferably 80 g / m or more. 2 It is preferable that the plating thickness is set to the following: The plating thickness can be adjusted by gas wiping or the like.

[0105] When hot-dip aluminum plating, which is a specific example of other metal plating treatments, is performed, the cold-rolled sheet obtained by the cold-rolled sheet annealing is immersed in an aluminum plating bath at 660°C or higher to perform the hot-dip aluminum plating treatment, and then the coating weight is adjusted by gas wiping, etc. The aluminum plating bath is preferably set at 730°C or lower.

[0106] Furthermore, when electrogalvanizing is performed, although there are no particular limitations, the thickness of the coating is preferably 2 μm or more, and more preferably 15 μm or less.

[0107] (Reheating and holding process) After the second cooling step (or after the second plating step if a second plating treatment is performed), the obtained steel sheet may be cooled to a cooling stop temperature of 250°C or lower to room temperature in a reheating and holding step, as necessary. This causes a portion of the untransformed austenite in the soft surface layer to transform to martensite. Furthermore, in the reheating and holding step, the steel sheet may be reheated to a reheating temperature range of (cooling stop temperature + 50°C) to 450°C and held in the reheating temperature range for 10 seconds or more. This tempers the martensite in the soft surface layer, reducing the area ratio of martensite in the soft surface layer and increasing the area ratio of tempered martensite in the soft surface layer. Furthermore, the steel structure at the 1 / 4 sheet thickness position of the base steel sheet is also tempered by the reheating and holding, increasing YS and λ. Here, the room temperature is preferably 1° C. or higher, and more preferably 50° C. or lower, and even more preferably 29° C. or lower.

[0108] Cooling methods that can be used after the second cooling step or after the reheating and holding step include, for example, gas jet cooling, mist cooling, roll cooling, water cooling, and air cooling. From the viewpoint of preventing surface oxidation, cooling to 250°C or less is preferred. The average cooling rate is preferably, for example, 1°C / sec or more and 50°C / sec or less.

[0109] The steel sheet obtained as described above may further be subjected to temper rolling. If the temper rolling reduction exceeds 2.00%, the yield stress increases, which may result in a decrease in dimensional accuracy when the steel sheet is formed into a component. Therefore, the temper rolling reduction is preferably 2.00% or less. The lower limit of the temper rolling reduction is not particularly limited, but is preferably 0.05% or more from the viewpoint of productivity. Temper rolling may be performed on an apparatus continuous with the annealing apparatus for carrying out the above-mentioned steps (online), or may be performed on an apparatus discontinuous with the annealing apparatus for carrying out the steps (offline). Temper rolling may be performed once or twice or more times. As long as an elongation rate equivalent to that of temper rolling can be imparted, rolling using a leveler or the like may be performed.

[0110] Other manufacturing method conditions are not particularly limited, but from the viewpoint of productivity, it is preferable that the above-mentioned series of processes such as annealing, hot-dip galvanizing, and galvanizing alloying be carried out in a continuous galvanizing line (CGL). After hot-dip galvanizing, wiping can be performed to adjust the coating weight. Note that plating conditions other than those described above can be based on conventional hot-dip galvanizing methods.

[0111] [3. Materials] Next, a member according to one embodiment of the present invention will be described. A member according to one embodiment of the present invention is a member made using (using as a raw material) the above-mentioned steel plate. For example, the raw material steel plate is subjected to at least one of forming and joining to form the member. Here, the steel sheet has a TS of 590 MPa or more, a high YS, excellent press formability (ductility, hole expandability, and bendability), and fracture resistance characteristics (axial crushing characteristics) during a collision. Therefore, a member according to one embodiment of the present invention has high strength, excellent press formability, and excellent impact resistance. Therefore, a member according to one embodiment of the present invention is particularly suitable for application to impact energy absorbing members used in the automotive field.

[0112] [4. Manufacturing methods for components] Next, a method for manufacturing a member according to one embodiment of the present invention will be described. A method for manufacturing a component according to one embodiment of the present invention includes a step of subjecting the above-described steel plate to at least one of forming and joining to form a component. Here, the molding method is not particularly limited, and for example, a general processing method such as press working can be used. The joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, rivet joining, caulking joining, etc. The molding conditions and joining conditions are not particularly limited, and may be in accordance with ordinary methods. [Example]

[0113] Steel materials having the chemical compositions shown in Table 1 (the balance being Fe and unavoidable impurities) were melted in a converter and formed into steel slabs by continuous casting. In Table 1, "-" indicates the content of unavoidable impurities. The calculated transformation point Ac1 shown in Table 1 is calculated using the following formula. Ac1 point (℃)=727.0-32.7×[%C]+14.9×[%Si]+2.0×[%Mn] Here, [%C] is the C content, [%Si] is the Si content, and [%Mn] is the Mn content.

[0114] The obtained steel slabs were heated to 1200°C, and after heating, the steel slabs were subjected to rough rolling and hot rolling, with the hot rolling coiling temperature set to 500°C, to produce hot-rolled steel sheets. Next, Nos. 1 to 96 and 107 to 146 of the obtained hot-rolled steel sheets were subjected to pickling and cold rolling to produce cold-rolled steel sheets with the thicknesses shown in Tables 2 to 4. Furthermore, Nos. 97 to 106 of the obtained hot-rolled steel sheets were subjected to pickling to obtain hot-rolled steel sheets (white skin) with the thicknesses shown in Table 3. Next, the obtained cold-rolled steel sheets or hot-rolled steel sheets (white skin) were subjected to treatments in a first plating step (metal plating step), an annealing step, a first cooling step, a second cooling step, and a plating step (second plating step) under the conditions shown in Tables 2 to 4 to obtain steel sheets. Note that some of the steel sheets were not subjected to treatment in the first plating step (metal plating step) and / or treatment in the plating step (second plating step). Further, for Nos. 107 to 110, 112 to 115, 117 to 120, 122 to 125, and 127 to 142, treatment in a reheating and holding step was carried out after the second plating step to obtain steel sheets. Here, in the second plating step, a part of the hot-rolled steel sheet (white skin) or cold-rolled steel sheet was subjected to a hot-dip galvanizing treatment or a galvannealed treatment to obtain a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or a galvannealed steel sheet (hereinafter also referred to as GA). In addition, some of the hot-rolled steel sheets (white skin) or cold-rolled steel sheets were subjected to electrogalvanization (bath temperature: room temperature) to obtain electrolytic galvanized steel sheets (hereinafter also referred to as EG). In addition, some of the hot-rolled steel sheets (white skin) or cold-rolled steel sheets were subjected to hot-dip aluminum plating (bath temperature: 680°C) to obtain hot-dip aluminum-plated steel sheets (hereinafter also referred to as Al). In Tables 2 to 4, the type of second plating process is also indicated as "GI," "GA," "EG," or "Al." Those that were not subjected to the second plating process are indicated as "CR."

[0115] The galvanizing bath temperature was set to 460°C for both GI and GA production. When manufacturing GI, the amount of zinc plating applied is 45 to 75 g / m per side. 2 (Double-sided plating) When manufacturing GA, the thickness is 40 to 65 g / m per side 2 (Double-sided plating). The composition of the zinc coating layer of the finally obtained steel sheets was as follows: GI: 0.1-1.0 mass% Fe, 0.20-0.33 mass% Al, and the balance being Zn and unavoidable impurities; GA: 7.0-12.0 mass% Fe, 0.10-0.23 mass% Al, and the balance being Zn and unavoidable impurities. In addition, the hot-dip galvanized layer, alloyed hot-dip galvanized layer, electrogalvanized layer and other metal plated layer (corresponding to the hot-dip aluminum plated layer in the examples) were all formed on both sides of the base steel sheet.

[0116] Using the obtained steel sheets, the steel structure at the 1 / 4 position in the sheet thickness direction (base steel sheet) was identified using the method described above. The measurement results are shown in Tables 2 to 4. In Tables 2 to 4, F is ferrite, M is martensite, RA is retained austenite, B is bainite, TM is tempered martensite, P is pearlite, θ is carbide, F' is unrecrystallized ferrite, and εM is ε martensite. For Nos. 107 to 146 shown in Table 4, the steel structure in the soft surface layer was identified in the same manner as described above.

[0117] The underlined parts in the table indicate values ​​outside the appropriate range of the present invention.

[0118] In addition, tensile tests, hole expansion tests, V-bending tests, V-bending + orthogonal VDA bending tests, and axial crushing tests were performed according to the following procedures, and the tensile strength (TS), yield stress (YS), total elongation (El), limiting hole expansion ratio (λ), R / t in the V-bending test, stroke at maximum load (SFmax) measured in the V-bending + orthogonal VDA bending test, and the presence or absence of fracture (visual cracks) in the axial crushing test were evaluated according to the following criteria.

[0119] ·TS 〇(Pass): 590MPa or more × (Fail): Less than 590 MPa

[0120] YS 〇(Pass): (A) When 590MPa≦TS<780MPa, 350MPa≦YS (B) When 780MPa≦TS<980MPa, 400MPa≦YS (C) When 980MPa≦TS<1180MPa, 550MPa≦YS (D) When 1180MPa≦TS<1320MPa, 750MPa≦YS (E) When 1320MPa≦TS<1470MPa, 900MPa≦YS (F) When 1470MPa≦TS, 1050MPa≦YS ×(Fail): If 590MPa≦TS<780MPa, 350MPa>YS If 780MPa≦TS<980MPa, 400MPa>YS When 980MPa≦TS<1180MPa, 550MPa>YS If 1180MPa≦TS<1320MPa, 750MPa>YS When 1320MPa≦TS<1470MPa, 900MPa>YS If 1470MPa≦TS, 1050MPa>YS

[0121] El 〇(Pass): (A) When 590MPa≦TS<780MPa, 24.0%≦El (B) When 780MPa≦TS<980MPa, 18.0%≦El (C) 980MPa≦TS<1180MPa, 11.0%≦El (D) When 1180MPa≦TS<1320MPa, 8.0%≦El (E) When 1320MPa≦TS<1470MPa, 7.5%≦El (F) When 1470MPa≦TS, 7.0%≦El ×(Fail): When 590MPa≦TS<780MPa, 24.0%>El When 780MPa≦TS<980MPa, 18.0%>El When 980MPa≦TS<1180MPa, 11.0%>El When 1180MPa≦TS<1320MPa, 8.0%>El When 1320MPa≦TS<1470MPa, 7.5%>El When 1470MPa≦TS, 7.0%>El

[0122] λ 〇(Pass): 20% or more × (Fail): Less than 20%

[0123] R / t 〇(Pass): (A) When 590MPa≦TS<780MPa, 1.0≧R / t (B) When 780MPa≦TS<980MPa, 2.0≧R / t (C) When 980MPa≦TS<1180MPa, 2.5≧R / t (D) When 1180MPa≦TS<1320MPa, 3.0≧R / t (E) When 1320MPa≦TS<1470MPa, 3.5≧R / t (F) When 1470MPa≦TS, 4.0≧R / t ×(Fail): 1.0 when 590MPa≦TS<780MPa <R / t 2.0 when 780MPa≦TS<980MPa <R / t 2.5 when 980MPa≦TS<1180MPa <R / t 3.0 when 1180MPa≦TS<1320MPa <R / t 3.5 when 1320MPa≦TS<1470MPa <R / t 4.0 when 1470MPa≦TS <R / t

[0124] SFmax 〇(Pass) (A) When 590MPa≦TS<780MPa, 29.0mm≦SFmax (B) When 780MPa≦TS<980MPa, 27.5mm≦SFmax (C) When 980MPa≦TS<1180MPa, 27.0mm≦SFmax (D) When 1180MPa≦TS<1320MPa, 26.0mm≦SFmax (E) When 1320MPa≦TS<1470MPa, 24.5mm≦SFmax (F) When 1470MPa≦TS, 24.0mm≦SFmax ×(Fail) When 590MPa≦TS<780MPa, 29.0mm>SFmax When 780MPa≦TS<980MPa, 27.5mm>SFmax When 980MPa≦TS<1180MPa, 27.0mm>SFmax When 1180MPa≦TS<1320MPa, 26.0mm>SFmax When 1320MPa≦TS<1470MPa, 24.5mm>SFmax When 1470MPa≦TS, 24.0mm>SFmax

[0125] Presence or absence of axial crushing fracture (external cracks) ⊚ (Pass): No external cracks were observed in the sample after the axial crushing test. ○ (Pass): One or less external cracks were observed on the sample after the axial crushing test. × (Fail): Two or more external cracks were observed on the sample after the axial crushing test.

[0126] (1) Tensile test Tensile tests were conducted in accordance with JIS Z 2241. Specifically, JIS No. 5 test pieces were taken from the obtained steel sheets so that the longitudinal direction was perpendicular to the rolling direction of the base steel sheet. Using the taken test pieces, tensile tests were conducted at a crosshead speed of 10 mm / min, and TS, YS, and El were measured. The results are also shown in Tables 2 to 4.

[0127] (2) Hole expansion test The hole expansion test was conducted in accordance with JIS Z 2256. Specifically, 100 mm × 100 mm test pieces were cut from the obtained steel sheets by shearing. A 10 mm diameter hole was punched into the test piece with a clearance of 12.5%. Next, a blank holder force of 9 ton (88.26 kN) was applied around the hole using a die with an inner diameter of 75 mm. In this state, a conical punch with an apex angle of 60° was pressed into the hole, and the diameter of the hole in the test piece at the crack initiation limit (when a crack occurred) was measured. The critical hole expansion ratio λ (%) was then calculated using the following formula. λ is an index for evaluating stretch flangeability. The results are also shown in Tables 2 to 4. λ(%)={(D f -D0) / D0}×100 where: D f : diameter of the hole in the test piece when the crack occurred (mm) D0: Initial diameter of the hole in the test piece (mm) is.

[0128] (3) V-bend test The V (90°) bending test is performed in accordance with JIS Z 2248. Test pieces measuring 100 mm x 35 mm were taken from the obtained steel plates by shearing and edge grinding, with the 100 mm side parallel to the width (C) direction. Bending radius R: Varies in 0.5mm increments Test method: Die support, punch push Molding load: 10ton Test speed: 30 mm / min Holding time: 5s Bending direction: perpendicular to rolling (C) direction The evaluation was carried out three times, and R / t was calculated by dividing the minimum bending radius (critical bending radius) R at which cracks did not appear by the plate thickness t. Using a Leica stereo microscope at 25x magnification, cracks with a length of 200 μm or more were judged to be cracks. R / t is an index for evaluating the bendability of press formability. The results are also shown in Tables 2 to 4.

[0129] In addition, a 90° V-bend test was performed in which R / t, calculated by dividing the critical bending radius R by the plate thickness t, was between 4.5 and 5.0. Five SEM images of the bent cross section were taken at 1500x magnification, as shown in Figure 1. Reference lines were drawn at the maximum height of the surface convexities and the maximum depth of the microcrack depressions within an 83 μm range in the horizontal direction of the photograph, and the shortest distance between these lines was determined and averaged over the five SEM images. The average value of the shortest distances thus determined was evaluated as the depth of surface irregularities.

[0130] (4) V-bend + orthogonal VDA bend test The V-bend + orthogonal VDA bend test is performed as follows. From the obtained steel sheets, 60 mm × 65 mm test specimens were obtained by shearing and edge grinding. Here, the 60 mm side was parallel to the rolling (L) direction. The test specimens were prepared by bending the steel sheets 90° (primary bending) in the rolling (L) direction with the width (C) direction as the axis, with a curvature radius / thickness of 4.2. For the 90° bending (primary bending), as shown in Figure 2(a), punch B1 was pressed into the steel sheet placed on a V-grooved die A1 to obtain test specimen T1. Next, as shown in Figure 2(b), test specimen T1 was placed on support roll A2, and punch B2 was pressed into the test specimen T1 so that the bending direction was perpendicular to the rolling direction, performing orthogonal bending (secondary bending). In Figures 2(a) and 2(b), D1 indicates the width (C) direction, and D2 indicates the rolling (L) direction.

[0131] The V-bending conditions for the V-bending + orthogonal VDA bending test are as follows: Test method: Die support, punch push Molding load: 10ton Test speed: 30 mm / min Holding time: 5s Bending direction: Rolling (L) direction

[0132] The VDA bending conditions for the V-bending + orthogonal VDA bending test are as follows: Test method: Roll support, punch indentation Roll diameter: φ30mm Punch tip R: 0.4 mm Distance between rolls: (plate thickness x 2) + 0.5 mm Stroke speed: 20mm / min Test piece size: 60mm x 60mm Bending direction: perpendicular to rolling (C) direction

[0133] The stroke at maximum load was determined from the stroke-load curve obtained when the VDA bending test was performed. The V bending + orthogonal VDA bending test was performed three times, and the average value of the stroke at maximum load was taken as SFmax (mm). SFmax is an index for evaluating fracture resistance during a collision (fracture resistance of the bend ridge in an axial crushing test). The results are also shown in Tables 2 to 4.

[0134] (5) Axial crushing test A 160 mm × 200 mm test piece was cut from the obtained steel plate by shearing. Here, the 160 mm side was parallel to the rolling (L) direction. Using a die with a punch shoulder radius of 5.0 mm and a die shoulder radius of 5.0 mm, the hat-shaped component 10 shown in FIGS. 3-1(a) and 3-1(b) was fabricated by forming (bending) the hat-shaped component to a depth of 40 mm. The steel plate used as the raw material for the hat-shaped component was separately cut to a size of 80 mm × 200 mm. The cut-out steel plate 20 and the hat-shaped component 10 were then spot-welded to fabricate the test component 30 shown in FIGS. 3-1(a) and 3-1(b). FIG. 3-1(a) is a front view of the test component 30 fabricated by spot-welding the hat-shaped component 10 and the steel plate 20. FIG. 3-1(b) is a perspective view of the test component 30. The position of the spot weld 40 is as shown in FIG. 3-1(b), with the edge of the steel plate and the weld 10 mm apart, and the distance between the welds is 45 The spacing between the test pieces was set to 1 / 2 mm. Next, as shown in FIG. 3-2(c), the test piece 30 was joined to the base plate 50 by TIG welding to prepare a sample for the axial crush test. Next, an impactor 60 was caused to collide with the prepared sample for the axial crush test at a constant velocity of 10 mm / min, and the sample for the axial crush test was crushed by 70 mm. As shown in FIG. 3-2(c), the crush direction D3 was set parallel to the longitudinal direction of the test piece 30. The results are also shown in Tables 2 to 4.

[0135] The V-bend test, V-bend + orthogonal VDA bend test, and axial crush test for 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 effect of plate thickness.Steel plates with a thickness of over 1.2 mm were ground on one side to make the plate thickness 1.2 mm. Because grinding may affect the bendability of the steel plate surface, the ground surface was placed on the inside of the bend (valley side) in the V-bend test, and in the V-bend + orthogonal VDA bend test, the ground surface was placed on the outside of the bend (peak side) during the V-bend test, and then the ground surface was placed on the inside of the bend (valley side) during the subsequent VDA bend test.

[0136] <Nano hardness measurement> To obtain excellent bendability during press forming and excellent bending fracture properties during collision, when the nanohardness is measured at 300 or more points in a 50 μm × 50 μm region on the sheet surface at a position 1 / 4 of the way down from the surface of the base steel sheet to the depth in the sheet thickness direction of the soft surface layer, the proportion of the nanohardness of 7.0 GPa or more on the sheet surface at a position 1 / 4 of the way down from the surface of the base steel sheet to the depth in the sheet thickness direction of the soft surface layer is 0.10 or less relative to the total number of measurements. When the proportion of the nanohardness of 7.0 GPa or more is 0.10 or less, it means that the proportion of hard structures (martensite, etc.), inclusions, etc. is small, and it is possible to further suppress the generation and connection of voids and crack propagation in the hard structures (martensite, etc.) and inclusions during press forming and collision, resulting in excellent R / t and SFmax.

[0137] After the plating was removed, the steel sheet was mechanically polished from the surface to 5 μm below the 1 / 4 depth of the soft surface layer in the thickness direction, and then buffed with diamond and alumina from the surface to 1 / 4 depth of the soft surface layer in the thickness direction, followed by colloidal silica polishing. Using a Hysitron Tribo-950, a Berkovich-shaped diamond indenter was used. Load: 500μN Measurement area: 50μm x 50μm Dot spacing: 2μm The nano-hardness was measured at a total of 512 points under the above conditions.

[0138] Next, the soft surface layer was mechanically polished to a depth of 1 / 2 of the thickness, buffed with diamond and alumina, and polished with colloidal silica. Load: 500μN Measurement area: 50μm x 50μm Dot spacing: 2μm The nano-hardness was measured at a total of 512 points under the above conditions.

[0139] The soft surface layer was measured as follows. A thickness cross section (L cross section) parallel to the rolling direction of the steel plate was smoothed by wet polishing, and then measurements were made at 1 μm intervals using a Vickers hardness tester in accordance with JIS Z 2244-1 (2020) at a load of 10 gf, from a position 1 μm from the surface of the steel plate in the thickness direction to a position 100 μm in the thickness direction. Measurements were then made at 20 μm intervals up to the center of the plate thickness. The region where the hardness has decreased to 85% or less compared to the hardness at the 1 / 4 position in the plate thickness is defined as the soft layer (soft surface layer), and the thickness of this region in the thickness direction is defined as the soft layer thickness.

[0140] [Table 1]

[0141] [Table 2]

[0142] [Table 3]

[0143] [Table 4]

[0144] As shown in Tables 2 to 4, all of the inventive examples passed the tensile strength (TS), yield stress (YS), total elongation (El), limiting hole expansion ratio (λ), R / t in the V-bend test, and stroke at maximum load (SFmax) measured in the V-bend + orthogonal VDA bend test, and there was no fracture (visual cracking) in the axial crush test.

[0145] On the other hand, in the comparative examples, at least one of the following was insufficient: tensile strength (TS), yield stress (YS), total elongation (El), limiting hole expansion ratio (λ), R / t in the V-bend test, stroke at maximum load (SFmax) measured in the V-bend + orthogonal VDA bend test, and the presence or absence of fracture (visual cracking) in the axial crush test.

[0146] As described above, it was found that the steel sheet of the present invention has high strength, and also has excellent press formability and impact resistance.

[0147] Furthermore, it was found that the steel plates of the present invention were used to produce members obtained by forming, joining, and further by forming and joining, and that because the steel plates of the present invention have high strength and excellent press formability and impact resistance, they also have high strength and excellent press formability and impact resistance, similar to the steel plates of the present invention. [Industrial Applicability]

[0148] According to the present invention, it is possible to produce steel plates and members that have a TS of 590 MPa or more, a high YS, excellent press formability (ductility, hole expandability, and bendability), and fracture resistance properties in collisions (flexural fracture properties and axial crush properties). Furthermore, 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 reduce the vehicle body weight and thereby improve fuel efficiency, and the industrial value of the steel plates and members is extremely great.

Claims

1. In mass%, C: 0.030% or more and 0.500% or less, Si: 0.10% or more and 2.0% or less, Mn: 1.00% or more and 3.50% or less, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.005% or more and 2.000% or less, N: 0.0100% or less, The composition contains: a Ceq represented by the following formula (1) of 0.30% or more and 0.85% or less; and the balance being Fe and unavoidable impurities; a base steel sheet having a steel structure at a 1 / 4 position in the sheet thickness direction, the steel structure consisting of martensite, retained austenite, tempered martensite and a remaining structure, wherein the area ratio of martensite is 50.0% or less, the volume ratio of retained austenite is 20.0% or less, and the area ratio of tempered martensite is 0.9 to 99.5%, a surface soft layer having a Vickers hardness of 85% or less of the Vickers hardness at a quarter-thickness position of the base steel sheet is formed in a region of 200 μm or less from the surface of the base steel sheet in the thickness direction, When nano-hardness was measured at 300 or more points in a 50 μm×50 μm region on the sheet surface at a ¼ position and a ½ position of the sheet thickness direction depth of the surface soft layer from the surface of the base steel sheet, the ratio of the number of measurements in which the nano-hardness of the sheet surface at a position of 1 / 4 of the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet is 7.0 GPa or more to the total number of measurements is 0.10 or less, Furthermore, the standard deviation σ of the nano-hardness of the sheet surface at a position of ¼ of the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet is 1.8 GPa or less, Furthermore, the steel sheet has a standard deviation σ of nano-hardness of the sheet surface at a position half the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet of 2.2 GPa or less. Ceq=[%C]+[%Mn] / 6+[%Si] / 24+[%Ni] / 40+[%Cr] / 5+[%Mo] / 4+[%V] / 14...Formula (1) Here, [%C] is the C content, [%Mn] is the Mn content, [%Si] is the Si content, [%Ni] is the Ni content, [%Cr] is the Cr content, [%Mo] is the Mo content, and [%V] is the V content, and if no V is contained, the value is 0 (zero).

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, Bi: 0.0200% or less, REM: 0.0200% or less The steel sheet according to claim 1, further comprising at least one element selected from the group consisting of:

3. The steel plate according to claim 1, which satisfies one or more selected from the following (A), (B), (C), and (D): (A) the total area ratio of ferrite and bainite present in the soft surface layer is 20% or more, The value obtained by dividing the area ratio of martensite present in the soft surface layer by the sum of the area ratios of martensite and tempered martensite present in the soft surface layer is 0.45 or less. (B) The base steel sheet has an Fe-based electroplated layer or a Ni-based electroplated layer as a first plating layer on one or both surfaces thereof. (C) The steel sheet has a metal plating layer as a second plating layer on the outermost surface of one or both sides. (D) The depth of surface irregularities of the test piece after 90° V-bending, in which R / t, obtained by dividing the limit bending radius R by the plate thickness t, is 4.5 or more and 5.0 or less, is 20.0 μm or less.

4. The steel sheet according to claim 2, which satisfies one or more of the following (A), (B), (C), and (D): (A) the total area ratio of ferrite and bainite present in the soft surface layer is 20% or more, The value obtained by dividing the area ratio of martensite present in the soft surface layer by the sum of the area ratios of martensite and tempered martensite present in the soft surface layer is 0.45 or less. (B) The base steel sheet has an Fe-based electroplated layer or a Ni-based electroplated layer as a first plating layer on one or both surfaces thereof. (C) The steel sheet has a metal plating layer as a second plating layer on the outermost surface of one or both sides. (D) The depth of surface irregularities of the test piece after 90° V-bending, in which R / t, obtained by dividing the limit bending radius R by the plate thickness t, is 4.5 or more and 5.0 or less, is 20.0 μm or less.

5. A member made using the steel plate according to any one of claims 1 to 4.

6. A hot rolling step of hot rolling a steel slab having the component composition according to claim 1 or 2; The obtained steel sheet was subjected to annealing at an annealing temperature of Ac １ an annealing step of annealing under the conditions of a temperature of 950°C or higher, an annealing time of 10 seconds or more, and a dew point of -30°C or higher; After the annealing step, the obtained steel sheet was １ a first cooling step of cooling the mixture at an average cooling rate of 8°C / second or more in a temperature range from 0°C to 450°C under an atmosphere with a dew point of -30°C or less; a second cooling step of cooling the steel plate obtained after the first cooling step at an average cooling rate of less than 8°C / sec in a temperature range from less than 450°C to 300°C, Alternatively, the steel sheet further comprises a base steel sheet having a steel structure at a quarter-thickness position of the sheet that satisfies one or more of the following (a), (b), (c), and (d), which is composed of martensite, retained austenite, tempered martensite, and a remaining structure, and in which the area fraction of martensite is 50.0% or less, the volume fraction of retained austenite is 20.0% or less, and the area fraction of tempered martensite is 0.9 to 99.5%, A surface soft layer having a Vickers hardness of 85% or less of the Vickers hardness at a position 1 / 4 of the sheet thickness of the base steel sheet is formed in a region of 200 μm or less from the surface of the base steel sheet in the sheet thickness direction, and when nano-hardness is measured at 300 points or more in a 50 μm×50 μm region of the sheet surface at each of a position 1 / 4 of the sheet thickness depth and a position 1 / 2 of the sheet thickness depth of the surface soft layer from the surface of the base steel sheet, a ratio of the number of measurements in which the nano-hardness of the sheet surface at a position 1 / 4 of the way to the depth in the sheet thickness direction of the soft surface layer is 7.0 GPa or more to the total number of measurements is 0.10 or less, and further, the standard deviation σ of the nano-hardness of the sheet surface at a position 1 / 4 of the way to the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet is 1.8 GPa or less, and further, the standard deviation σ of the nano-hardness of the sheet surface at a position 1 / 2 of the way to the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet is 2.2 GPa or less. (a) A cold rolling step of cold rolling the steel sheet after the hot rolling step and before the annealing step is included. (b) A first plating step is included in which Fe-based electroplating or Ni-based electroplating is applied to one or both sides of the steel sheet after the hot rolling step and before the annealing step to form a first plating layer. (c) A second plating step of plating the steel sheet after the second cooling step to form a second plating layer. (d) if the plating treatment in the second plating step is not performed, after the second cooling step, and When the plating treatment in the second plating step is performed, a zinc alloy plating treatment is performed as the plating treatment, and after the second plating step, The method includes a reheating and holding step of cooling the obtained steel sheet to a cooling stop temperature from 250°C or lower to room temperature, reheating the steel sheet to a reheating temperature range from (the cooling stop temperature + 50°C) to 450°C, and holding the steel sheet in the reheating temperature range for 10 seconds or more.

7. A method for manufacturing a component, comprising a step of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming and joining to form a component.

Images