Steel sheet, steel member, and method for producing steel sheet

Abstract

This steel sheet has a prescribed chemical composition and is such that the microstructure at a quarter-thickness position, which is a position at one quarter of the sheet thickness in the sheet thickness direction from the surface, comprises, by area%, 3-10% of retained austenite, 0-10% of ferrite, 0-5% of pearlite, and 75-97% in total of martensite and bainite. In a surface layer part, which is at a position 50-100 μm from the surface in the sheet thickness direction, the area ratio of prior austenite grains containing therein one or more residual austenite grains having a long-side length of 0.5 μm or greater and an aspect ratio of 2.0 or greater is 30% or greater.

Description

Steel plate, steel member, and method for manufacturing steel plate

[0001] The present invention relates to a steel plate, a steel member, and a method for manufacturing a steel plate. This application claims priority to Japanese Patent Application No. 2024-053238, filed on March 28, 2024, the contents of which are incorporated herein by reference.

[0002] In recent years, improvements in automobile fuel efficiency have been required in light of greenhouse gas emission regulations associated with global warming countermeasures. To reduce the weight of vehicle bodies and ensure collision safety, the application of high-strength steel sheets to automobile parts has been expanding. Recently, there has been a growing need for ultra-high-strength steel sheets with a tensile strength of 980 MPa or more. Meanwhile, to convert steel sheets into automobile parts (components), press forming, welding, and the like are performed. Therefore, steel sheets used for automobile parts are required not only for strength but also for various workability requirements during part forming, such as press formability and weldability. For example, from the viewpoint of press formability, steel sheets are often required to have excellent elongation (total elongation in a tensile test: EL). However, as the strength of steel sheets increases, the total elongation (EL) tends to decrease, making it difficult to simultaneously ensure high levels of strength and total elongation. In response to this, for example, as shown in Patent Documents 1 to 4, TRIP (Transformation Induced Plasticity) steel sheets are known that utilize the transformation-induced plasticity of retained austenite to achieve both high strength and workability.

[0003] However, in recent years, there has been a demand for further suppression of fracture when subjected to axial crushing deformation after being formed into a component such as a collision deformation component, by improving the properties of the steel sheet itself. However, although Patent Documents 1 to 4 take into consideration the formability of the steel sheet, there is room for further study regarding the occurrence of fracture when subjected to axial crushing deformation after being formed into a component.

[0004] International Publication No. 2013 / 051238 Japanese Patent Application Publication No. 2006-104532 Japanese Patent Application Publication No. 2007-262494 International Publication No. 2018 / 179386

[0005] As described above, there has been room for further study in the past regarding the prevention of fracture when a steel plate is subjected to axial crushing deformation after being formed into a component such as a collision-deformable component. Therefore, an object of the present invention is to provide a steel plate that has high strength and excellent elongation and is capable of preventing fracture of the component when it is subjected to axial crushing deformation after being formed into the component, a method for manufacturing the same, and a steel component that is obtained using this steel plate and is capable of preventing fracture when it is subjected to axial crushing deformation.

[0006] The inventors investigated whether fracture during axial crushing deformation of a component could be suppressed by the structure (chemical composition, microstructure, etc.) of the steel plate. As a result, they found that in steel having a microstructure mainly composed of bainite and / or martensite and containing retained austenite, fracture can be suppressed by increasing the proportion of prior austenite grains in the surface layer that contain a predetermined amount of retained austenite inside.

[0007] Furthermore, steel plates are formed into components by processing, and the components are further welded (e.g., spot welded) to form separate components as needed. Even in such welded components, the welded portion and HAZ portion account for a small proportion of the component, so if the other portions (non-welded portions) have a structure capable of suppressing component fracture, the effect of suppressing component fracture can be achieved. However, the HAZ portion may be altered by the heat of welding, making it more susceptible to fracture. Therefore, a structure capable of suppressing component fracture in the HAZ portion can achieve a more preferable effect of suppressing component fracture. Therefore, the inventors also investigated the suppression of fracture during axial crush deformation at spot welds of components. As a result, they found that fracture in the HAZ portion of a component can be suppressed by controlling the concentration distribution of Mn and Si at the stage of the steel plate used as the base material.

[0008] The present invention has been made in view of the above findings. The gist of the present invention is as follows. [1] A steel sheet according to one embodiment of the present invention has, in mass %, C: 0.08 to 0.20%, Si: 0.50 to 1.80%, Mn: 2.00 to 3.50%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0050%, N: 0.0100% or less, and O: 0.0100% or less. Bottom, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, Ta: 0-1.00%, S n: 0-1.00%, Sb: 0-0.50%, Nb: 0-0.200%, V: 0-1.00%, As: 0-0.10%, Zn: 0-1.000%, Ca: 0-0.0100%, The steel sheet has a chemical composition consisting of Mg: 0-0.0100%, Zr: 0-0.0100%, Bi: 0-0.0100%, REM: 0-0.0100%, Hf: 0-0.0100%, and the balance: Fe and impurities. The microstructure at a quarter-thickness position, which is a position from the surface to one-quarter of the plate thickness in the plate thickness direction, consists, in area %, of 3-10% retained austenite, 0-10% ferrite, 0-5% pearlite, and 75-97% martensite and bainite in total. In a surface layer portion located 50-100 μm from the surface in the plate thickness direction, the area ratio of prior austenite grains containing one or more retained austenite grains with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more is 30% or more. [2] The steel sheet according to [1] may have a standard deviation σ of 0.80 or less of MS values ​​calculated by formula (1) from the Mn concentration [Mn] and the Si concentration [Si] in mass % at each of a plurality of measurement points measured by EPMA in a 35 μm × 50 μm region in the range of ⅛ to ⅜ thickness centered at the ¼ thickness position on a cross section parallel to the sheet thickness direction and the rolling direction: MS = [Mn] + (⅔) × [Si] ... (1) [3] The steel sheet according to [1] or [2] may have a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer on the surface.[4] A steel member according to another aspect of the present invention has a steel plate including a non-processed portion, and the non-processed portion contains, in mass %, C: 0.08 to 0.20%, Si: 0.50 to 1.80%, Mn: 2.00 to 3.50%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0050%, N: 0.010 0% or less, O: 0.0100% or less, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 1.00%, W: 0 to 1.00%, Ta: 0-1.00%, Sn: 0-1.00%, Sb: 0-0.50%, Nb: 0-0.200%, V: 0-1.00%, As: 0-0.10%, Zn: 0-1.000%, Ca: 0- The non-machined portion has a chemical composition consisting of 0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, Bi: 0-0.0100%, REM: 0-0.0100%, Hf: 0-0.0100%, and the balance: Fe and impurities, and the microstructure at a 1 / 4 thickness position, which is a position from the surface to 1 / 4 of the thickness in the thickness direction, of the non-machined portion is composed, in area %, of retained austenite: 3-10%, ferrite: 0-10%, pearlite: 0-5%, and a total of 75-97% martensite and bainite, and in a surface layer portion located 50-100 μm from the surface in the thickness direction, the area ratio of prior austenite grains containing one or more retained austenite grains with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more is 30% or more. [5] The steel member according to [4] includes a steel plate having a spot weld, a HAZ around the spot weld, and a non-welded portion other than the spot weld and the HAZ, and the non-processed portion may be present in the non-welded portion. [6] The steel member according to [4] or [5] may have, in the non-processed portion, a standard deviation σ of 0.80 or less of MS values ​​calculated by formula (1) from the Mn concentration [Mn] and the Si concentration [Si] at each of a plurality of measurement points measured by EPMA in a 35 μm × 50 μm region in the range of ⅛ to ⅜ thickness centered at the ¼ thickness position.MS=[Mn]+(2 / 3)×[Si] (1) [7] The steel member according to any one of [4] to [6] may have a zinc-plated layer or a galvannealed hot-dip galvannealed layer on the surface of the steel sheet. [8] A method for producing a steel sheet according to another aspect of the present invention is a method for producing a steel sheet according to [1], comprising: a continuous casting step of obtaining a slab having the chemical composition according to [1] by continuous casting; a hot rolling step of heating the slab to a heating temperature, subjecting it to hot rolling including rough rolling and finish rolling, cooling it to a coiling temperature, and then coiling it at the coiling temperature to obtain a hot-rolled steel sheet; and a cold rolling step of, as necessary, subjecting the hot-rolled steel sheet to pickling and cold rolling at a cumulative reduction of 30 to 75% to obtain a cold-rolled steel sheet. and a heat treatment process for performing heat treatment on the hot-rolled steel sheet or the cold-rolled steel sheet, wherein in the hot-rolling process, the heating temperature of the slab is 1200°C or higher, and when the number of passes of the finish rolling is n, where the first pass is the first pass and the final pass is the n-th pass, the inlet temperature of the n-2th pass is 950°C or higher and the outlet temperature of the n-th pass is 900°C or higher, and in the finish rolling, a reduction rate in one pass of more than 25% is performed at least once, and the inter-pass times between the n-th pass and the n-1th pass, and between the n-1th pass and the n-th pass, are 0.2 to 1.0 seconds, the time from the completion of the nth pass to the start of the cooling is 1.0 to 3.0 seconds, the coiling temperature is 200 to 550°C, and in the cooling to the coiling temperature, the average cooling rate between 600 and 750°C is 20°C / s or more, the heat treatment step has a heating step, a cooling step, and a holding step, and in the heating step, The cold-rolled steel sheet is heated to a first temperature range of Ac3-20°C to 950°C and held in the first temperature range for 1 to 1000 seconds, and in the cooling process, the hot-rolled steel sheet or the cold-rolled steel sheet after the heating process is cooled to a second temperature range of Ms-200°C to Ms-50°C at a cooling rate of 10°C / s or more between 500 and 650°C and an average cooling rate of 20°C / s or more between Ms-50 and Ms°C, and in the holding process, after the cooling process, the steel sheet is held at 360 to 480°C for 10 to 600 seconds.[9] In the method for producing a steel sheet according to [8], in the continuous casting step, casting is performed so as to satisfy the following formula (2), and in the rough rolling, rolling may be performed at least three times with a reduction ratio exceeding 20% ​​while the steel sheet temperature is 1050°C or higher. Here, τ, τ, and T in the formula (2) and the formula (3) S (τ), T C (τ), T L , f L , M L , M δ , S L , S δ , D δ Mn , D δ Si and T respectively indicate the following: C (τ) is calculated by the above formula (3). τ: elapsed time from the start of casting in seconds τ1: T C (τ) is the solidification completion temperature T in units of K δ Time T S (τ): slab surface temperature at time τ in K units T C (τ): Estimated slab internal temperature at time τ in K. L : solidification start temperature f in K L :Temperature T C (τ) Equilibrium volume fraction of the liquid phase at L : Temperature T in mass% C Equilibrium Mn concentration in the liquid phase at (τ) M δ : Temperature T in mass% C (τ) Equilibrium Mn concentration of δ at S L : Temperature T in mass% C Equilibrium Si concentration in the liquid phase at (τ) S δ : Temperature T in mass% C Equilibrium Si concentration of δ at (τ) D δ Mn : Unit is m 2 / sec, the temperature T C Diffusion coefficient of Mn in the δ phase at (τ) D δ Si: Unit is m 2 / sec, the temperature T C

[10] The method for producing a steel sheet according to [8] or [9] may further include a plating step of forming a galvanized layer on the surface of the hot-rolled steel sheet or the cold-rolled steel sheet during the cooling step of the heat treatment step, between the cooling step and the holding step, during the holding step, or after the holding step.

[11] The method for producing a steel sheet according to

[10] may further include an alloying step of alloying the hot-dip galvanized layer to form an alloyed hot-dip galvanized layer after the plating step.

[0009] According to the above aspects of the present invention, it is possible to provide a steel plate that has high strength and excellent elongation and that can suppress fracture of a component when it is subjected to axial crushing deformation after being formed into the component, a method for manufacturing the same, and a steel component that is obtained using the steel plate and that can suppress fracture when it is subjected to axial crushing deformation.

[0010] 1 is a schematic diagram for explaining the procedure for carrying out a component axial crushing test, showing an example of the shape of a member having a closed cross-sectional structure;

[0011] A steel plate according to one embodiment of the present invention (steel plate according to this embodiment), a steel member obtained using the steel plate (steel member according to this embodiment), and methods for manufacturing them will be described. In this embodiment, a position of 1 / 4 of the plate thickness from the surface of the steel plate in the plate thickness direction will be described as the 1 / 4 thickness position. In the case of a steel member, a position of 1 / 4 of the thickness from the surface of the steel member in the thickness direction of the steel member (for example, the plate thickness direction of the steel plate constituting the steel member) will be described as the 1 / 4 thickness position. In addition, a position (range) of 50 to 100 μm from the surface of the steel plate in the plate thickness direction, and a position (range) of 50 to 100 μm from the surface of the steel member in the thickness direction (for example, the plate thickness direction of the steel plate constituting the steel member) will be described as the respective surface layer portions.

[0012] <Steel Sheet> A steel sheet according to this embodiment has a predetermined chemical composition, and a microstructure at a quarter thickness position is composed of, in area %, 3 to 10% retained austenite, 0 to 10% ferrite, 0 to 5% pearlite, and at least one of martensite and bainite: 75 to 97% in total, and in a surface layer portion, an area ratio of prior austenite grains containing one or more retained austenite grains having a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more is 30% or more. The steel sheet according to this embodiment may be a hot-rolled steel sheet or a cold-rolled steel sheet, and may further have an electrogalvanized layer, a hot-dip galvanized layer, or a galvannealed layer formed on the surface (i.e., it may be an electrogalvanized steel sheet (EG), a hot-dip galvanized steel sheet (GI), or a galvannealed steel sheet (GA)). When the steel sheet has a plating layer (electrogalvanized layer, hot-dip galvanized layer, or galvannealed hot-dip galvanized layer) (when the steel sheet is a plated steel sheet having a base material and a plating layer formed on the surface of the base material), the surface that serves as the reference for the surface layer portion and the 1 / 4 thickness position is the surface of the base material excluding the plating layer. In addition, the chemical composition of the steel sheet is also the chemical composition of the base material excluding the plating layer. Each of these will be explained below.

[0013] (Chemical Composition) The reasons for limiting the chemical composition of the steel sheet according to this embodiment will be explained. "%" relating to the content of each element constituting the chemical composition means "mass %" unless otherwise specified.

[0014] C: 0.08 to 0.20% Carbon (C) is an essential element that contributes to the formation of martensite and bainite, which contribute to increasing the strength of steel sheets. If the C content is less than 0.08%, the desired microstructure cannot be obtained, and sufficient tensile strength cannot be obtained. Therefore, the C content is set to 0.08% or more. The C content is preferably 0.10% or more. On the other hand, if the C content exceeds 0.20%, the toughness of the material decreases, and the fracture resistance during axial crushing deformation deteriorates. In addition, the weld metal zone becomes embrittled, making fracture at the weld more likely to occur. Therefore, the C content is set to 0.20% or less. The C content is preferably 0.18% or less, and more preferably 0.15% or less. That is, the C content is 0.08 to 0.20%, preferably 0.10 to 0.15%, for example.

[0015] Si: 0.50 to 1.80% Si (silicon) is a solid-solution strengthening element and is effective in increasing the strength of steel sheets. It also increases the amount of retained austenite by suppressing the formation of iron carbides. To achieve this effect, the Si content is set to 0.50% or more. The Si content is preferably 0.80% or more. On the other hand, excessive Si content significantly deteriorates the chemical conversion treatability of the steel sheet and its wettability with hot-dip galvanizing. It also reduces the toughness of the material and deteriorates its fracture resistance during axial crushing deformation. Furthermore, it embrittles the weld metal zone, making fracture at the weld more likely to occur. Therefore, the Si content is set to 1.80% or less. The Si content is preferably 1.60% or less, more preferably 1.40% or less. That is, the Si content is 0.50 to 1.80%, preferably 0.80 to 1.40%, for example.

[0016] Mn: 2.00 to 3.50% Mn (manganese) is a strong austenite-stabilizing element and is effective in improving the hardenability of steel plate. To improve hardenability and increase the area ratio of martensite and bainite, the Mn content is set to 2.00% or more. The Mn content is preferably 2.20% or more. On the other hand, excessive Mn content reduces the toughness of the material and deteriorates the fracture resistance during axial crushing deformation. Furthermore, the weld melt zone becomes embrittled, making fracture at the weld more likely to occur. Therefore, the Mn content is set to 3.50% or less. The Mn content is preferably 3.20% or less, and more preferably 3.00% or less. That is, the Mn content is 2.00 to 3.50%, and preferably, for example, 2.20 to 3.00%.

[0017] P: 0.050% or less P (phosphorus) is a solid solution strengthening element and is effective in increasing the strength of steel sheets, but excessive content of P deteriorates weldability and toughness. Therefore, the P content is set to 0.050% or less. The P content is preferably 0.045% or less, 0.035% or less, or 0.020% or less. The P content may be 0%, but excessive reduction of the P content increases the cost of dephosphorization, so from an economical standpoint, the lower limit is preferably set to 0.001%.

[0018] S: 0.0100% or less S (sulfur) is an element contained as an impurity and forms MnS in steel, which deteriorates toughness and hole expandability. Therefore, the S content is set to 0.0100% or less as a range in which the deterioration of toughness and hole expandability is not significant. The S content is preferably 0.0050% or less, 0.0040% or less, or 0.0030% or less. The S content may be 0%, but if the S content is reduced too much, the desulfurization cost will be high, so from an economical standpoint, the lower limit is preferably set to 0.0001%.

[0019] Al: 0.001 to 1.000% At least 0.001% of Al (aluminum) is contained to deoxidize the steel. The Al content may be 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, excessive Al content not only saturates the effect and leads to unnecessary cost increases, but also raises the transformation temperature of the steel, increasing the load during hot rolling and resulting in reduced mechanical properties of the steel sheet. Therefore, the Al content is set to 1.000% or less. The Al content may also be 0.800% or less, 0.600% or less, or 0.300% or less.

[0020] Ti: 0.001 to 0.100% Ti (titanium) is an element that fixes solute N in steel as TiN and suppresses the formation of BN. To achieve this effect, the Ti content is set to 0.001% or more. The Ti content is preferably 0.005% or more. On the other hand, if the Ti content is excessive, Ti carbide is formed in excess, reducing toughness. Therefore, the Ti content is set to 0.100% or less. The Ti content is preferably set to 0.080% or less. In other words, the Ti content is 0.001 to 0.100%, and preferably, for example, 0.005 to 0.080%.

[0021] B: 0.0005 to 0.0050% B (boron) is an element that segregates at austenite grain boundaries to improve the hardenability of steel and reduce the area ratio of ferrite. To achieve this effect, the B content is set to 0.0005% or more. The B content is more preferably set to 0.0008% or more. On the other hand, if the B content exceeds 0.0050%, borides are formed, thereby losing the above effect and reducing hot workability. Therefore, the B content is set to 0.0050% or less. The B content is preferably 0.0035% or less. That is, the B content is 0.0005 to 0.0050%, and preferably, for example, 0.0008 to 0.0035%.

[0022] N: 0.0100% or less N (nitrogen) is an element contained as an impurity, and if its content is high, it may form coarse nitrides in the steel, deteriorating bendability and hole expandability. Therefore, the N content is limited to 0.0100% or less. The N content is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. The N content may be 0%, but if the N content is reduced too much, the cost of denitrification will be high, so from an economical standpoint, the lower limit is preferably set to 0.0001%.

[0023] O: 0.0100% or less O (oxygen) is an element contained as an impurity, and if its content is high, it may form coarse oxides in the steel, deteriorating bendability and hole expandability. Therefore, the O content is limited to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. The O content may be 0%, but from the viewpoint of manufacturing costs, the lower limit is preferably set to 0.0001%.

[0024] The steel sheet according to this embodiment may contain the above elements, with the balance being Fe and impurities. However, for the purpose of improving various properties, it may further contain one or more elements (optional elements) selected from the following: Cr, Mo, Cu, Ni, Co, W, Ta, Sn, Sb, Nb, V, As, Zn, Ca, Mg, Zr, Hf, Bi, and REM. Since the optional elements do not need to be contained, the lower limit is 0%.

[0025] Cr: 0 to 1.00% Mo: 0 to 1.00% Cu: 0 to 1.00% Ni: 0 to 1.00% Co: 0 to 1.00% W: 0 to 1.00% Ta: 0 to 1.00% Sn: 0 to 1.00% Sb: 0 to 0.50% Nb: 0 to 0.200% V: 0 to 1.00% As: 0 to 0.10% Zn: 0 to 1.000% Cr (chromium), Mo (molybdenum), Cu (copper), Ni (nickel), Co (cobalt), W (tungsten), Ta (tantalum), Sn (tin), Sb (antimony), Nb (niobium), V (vanadium), As (arsenic), and Zn (zinc) are all elements effective in increasing the strength of steel plate, and one or more of these elements may be contained as necessary. The content of each element may be 0.001% or more, 0.005% or more, or 0.010% or more. On the other hand, if these elements are contained in excessive amounts, the effect saturates and the cost increases. Therefore, when these elements are contained, the contents of Cr, Mo, Cu, Ni, Co, W, Ta, Sn, V, and Zn are each 1.000% or less, Sb is 0.50% or less, Nb is 0.200% or less, and As is 0.10% or less. The contents of Cr, Mo, Cu, Ni, Co, W, Ta, Sn, Sb, V, and Zn are preferably 0.40% or less, more preferably 0.30% or less, and even more preferably 0.20% or less.

[0026] Ca: 0-0.0100% Mg: 0-0.0100% Zr: 0-0.0100% Bi: 0-0.0100% REM: 0-0.0100% Ca (calcium), Mg (magnesium), Zr (zirconium), and REM (rare earth elements) are elements that contribute to the fine dispersion of inclusions in steel, and Bi (bismuth) is an element that reduces the microsegregation of substitutional alloy elements such as Mn and Si in steel. Each of these elements contributes to improving the bendability of the steel sheet. Therefore, they may be added as needed. To achieve the above effects, it is preferable to add 0.0001% or more, and more preferably 0.0010% or more, of one or more elements selected from Ca, Mg, Bi, Zr, and REM. On the other hand, excessive addition of these elements deteriorates elongation. Therefore, the contents of Ca, Mg, Bi, Zr, and REM are all set to 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0060% or less.

[0027] Here, REM refers to a total of 17 elements, including Sc, Y, and lanthanoids, and the REM content means the total content of these elements. Lanthanoids are industrially added in the form of misch metal.

[0028] Hf: 0 to 0.0100% Hf is effective as a deoxidizing element. Therefore, it may be contained. On the other hand, if the Hf content exceeds 0.0100%, the HAZ toughness deteriorates, so the Hf content is set to 0.0100% or less. The Hf content is preferably 0.0080% or less, and more preferably 0.0050% or less.

[0029] As described above, the steel sheet according to this embodiment may contain C, Si, Mn, P, S, Al, Ti, B, N, and O, with the balance being Fe and impurities, and may further contain one or more elements (optional elements). The impurities refer to components that are mixed in from raw materials such as ore and scrap or due to other factors when industrially producing steel, and are acceptable within a range that does not adversely affect the properties.

[0030] The chemical composition of the steel plate according to this embodiment may be measured by a common method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) on chips in accordance with JIS G 1201:2014. In this case, the chemical composition is the average content across the entire plate thickness. C and S, which are difficult to measure using ICP-AES, may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method. The analytical sample is taken so as to obtain the average chemical composition across the entire plate thickness, as described in JIS G 0417:1999. Specifically, the analytical sample is taken from the 1 / 4 thickness position, avoiding the widthwise ends of the steel plate.

[0031] In the steel sheet according to this embodiment, the average Mn and Si contents of the entire steel sheet are set within the above-mentioned ranges, and the standard deviation σ of the MS value calculated by formula (1) from the Mn concentration [Mn] and Si concentration [Si] in mass % at each of multiple measurement points measured by EPMA is preferably 0.80 or less. MS = [Mn] + (2 / 3) × [Si] (1) A small standard deviation σ of the MS value suppresses fracture in the HAZ when a component having spot welds undergoes axial crushing deformation after forming. The reason why fracture is more likely to occur when the standard deviation σ of the MS value is large is presumed to be due to the following reasons: The HAZ temperature rises due to the heat of welding during welding. The HAZ region has a region where the temperature reaches the two-phase region (Ac1 to Ac3). Furthermore, because the cooling rate of the weld is extremely fast, the austenite formed at this time becomes as-quenched martensite. That is, in this region, the microstructure is a composite structure containing ferrite and martensite. Such a structure is brittle and prone to fracture during axial crushing deformation. In particular, the greater the proportion of martensite containing a large amount of Si and Mn, the greater the embrittlement. It is thought that the smaller the standard deviation σ of the MS value of the steel sheet, the smaller the proportion of martensite containing a large amount of Si and Mn in the HAZ, which reduces fracture.

[0032] The standard deviation of MS values ​​is determined using the following method. The concentration distribution of Si and Mn is measured using an FE-EPMA (electron probe microanalyzer). In a cross section parallel to the thickness direction, the Si concentration and Mn concentration are measured at 0.2 μm intervals in a 35 μm x 50 μm area in the range of 1 / 8 to 3 / 8 of the thickness from the surface of the steel sheet in the thickness direction (range of 1 / 8 to 3 / 8 thickness), centered at the 1 / 4 thickness position. The Mn concentration [Mn] and Si concentration [Si] at each measurement position are used to calculate [Mn] + (2 / 3) × [Si]. By performing the above analysis for four fields of view, the value of [Mn] + (2 / 3) × [Si] is obtained at approximately 175,000 points. The standard deviation σ is calculated using this population. For example, a JEOL JXA-8530F FE-EPMA can be used, with an acceleration voltage of 15 kV. The characteristic X-ray spectroscopy method is wavelength dispersive. The analyzing crystal can be selected from an appropriate material depending on the element being analyzed, for example, LiF for Mn and TAP for Si. The output Si and Mn concentrations are values ​​converted to mass% from the detected intensity of characteristic X-rays using the program attached to the JXA-8530, but this is subject to calibration using standard materials.

[0033] (Microstructure) [1 / 4 thickness position] In the steel sheet according to this embodiment, the microstructure (metal structure) at the 1 / 4 thickness position, which is a position at 1 / 4 of the sheet thickness from the surface in the sheet thickness direction, has the following structures (phases). Hereinafter, the proportion of each structure is an area ratio.

[0034] Retained austenite: 3 to 10% Retained austenite is a structure that contributes to improving elongation through the TRIP effect. Therefore, the area ratio of retained austenite is set to 3% or more. On the other hand, if the area ratio of retained austenite is excessive, the grain size of the retained austenite becomes large. Such retained austenite with a large grain size becomes coarse and hard martensite after deformation due to forming, etc. In this case, cracks are more likely to start, and bendability deteriorates. For this reason, the area ratio of retained austenite is set to 10% or less.

[0035] Ferrite: 0 to 10% In order to suppress fracture during axial crushing deformation after forming into a component, a uniform structure with small differences in hardness between structures is preferable. Because ferrite is a soft structure (phase), it is difficult to obtain high strength if the microstructure is mainly composed of ferrite. Therefore, in the steel plate according to this embodiment, the microstructure is mainly composed of martensite and bainite, as described below. Therefore, the area fraction of ferrite is set to 10% or less. The area fraction of ferrite is preferably small, preferably 5% or less, more preferably 3% or less, and may be 0%.

[0036] Pearlite: 0 to 5% Pearlite is a brittle structure that acts as a fracture origin, degrading the local ductility of the steel sheet. Therefore, its area ratio is set to 5% or less. The area ratio of pearlite is preferably 3% or less, more preferably 2% or less, and may be 0%.

[0037] Martensite and bainite: 75 to 97% in total Martensite and bainite are structures effective in increasing the strength of the steel sheet. As described above, a structure as uniform as possible is necessary to suppress fracture during axial crushing deformation after forming into a component. Therefore, in the steel sheet according to this embodiment, the remaining components other than retained austenite, ferrite, and pearlite are martensite and / or bainite. The total area ratio of martensite and bainite is 75 to 97%. Preferably, the total area ratio of martensite and bainite is 80 to 97%, 85 to 97%, or 90 to 97%. Both martensite and bainite have lath-shaped structures, and in the steel sheet according to this embodiment, there is no need to particularly specify the area ratio of each (one may be 0%). Here, martensite includes so-called fresh martensite and tempered martensite.

[0038] The area ratio of each structure (phase) in the microstructure (metal structure) at the 1 / 4 thickness position of the steel sheet according to this embodiment is measured as follows. The structure fraction (area ratio) is evaluated using a secondary electron image taken using an FE-SEM and X-ray diffraction. The FE-SEM may be, for example, a JSM-7200F manufactured by JEOL. First, a sample is taken from a thickness cross section (cross section parallel to the thickness direction) of the steel sheet parallel to the rolling direction of the steel sheet at a position at least 50 mm away from the end in the width direction, with the thickness cross section as the observation surface. The observation surface is mechanically polished to a mirror finish, and then etched using a nital solution. If the end in the width direction has already been cut and the width position of the steel sheet to be used as the base material is unknown, a sample may be taken from a position at least 50 mm away from the end in the width direction of the steel sheet to be used as the base material. Next, a total of 2.0 × 10 －９ m ２ A secondary electron image is taken for the above area. From the obtained secondary electron image, the area fractions of ferrite and pearlite are measured and regarded as the area ratios. There is no need to set an upper limit for the field area, but the larger the area, the greater the number of steps required for point counting. Therefore, a maximum of 1.0 × 10 －８ m ２ The following can be used as a guideline: The magnification is 5000 times, and the field of view area is 2.0 × 10 －９ m ２The number of photographs taken is equal to or greater than the number required for the microstructure identification. When identifying the microstructure, regions where cementite is precipitated in a lamellar form are judged to be pearlite. Regions with low brightness and no visible substructure are judged to be ferrite. Regions that do not fit any of the above criteria are judged to be bainite, martensite, or austenite (retained austenite). The area fractions of bainite and martensite can be determined by subtracting the area fraction of austenite measured by the X-ray diffraction method described below from the area fraction of the region judged to be bainite, martensite, or austenite. The area fractions of each microstructure are calculated using a point counting method. The point counting intervals are 2 μm both vertically and horizontally. The area fraction of retained austenite is measured using the X-ray diffraction method. Specifically, the steel plate is mechanically polished and chemically polished to remove the area from the surface to a depth of 1 / 4 in the thickness direction. The polished sample was then subjected to MoKα1 radiation as characteristic X-rays, and the structural fraction of retained austenite was calculated from the integrated intensity ratio of the (200), (211) diffraction peaks of the bcc phase and the (200), (220), and (311) diffraction peaks of the fcc phase. This was used to calculate the volume fraction of retained austenite. In the steel sheet according to this embodiment, the volume fraction and the area fraction are considered to be equal, and the resulting volume fraction of retained austenite is used as the area fraction of retained austenite. The rolling direction is self-evident when the raw material is a coil or when the rolling direction is recorded. For full-width cut sheet samples, the rolling direction can be determined from the dimensions if the width is known. Furthermore, if at least one width edge remains, those skilled in the art can easily determine the width direction and rolling direction from the condition of the end face (presence or absence of edge drop, presence or absence of plating). If the above information is lost, the rolling direction can be identified using the following method. The Z-plane of the sheet (the plane parallel to both the longitudinal and transverse directions of the sheet) is polished to a quarter-thickness position and mirror-polished, after which a Mn concentration map of a 1000 μm × 1000 μm area is obtained using an EPMA. When the solidification segregation of Mn is measured as streaks, the longitudinal direction of the streaks is determined to be the rolling direction. Furthermore, even when the steel sheet is processed into a part, the rolling direction can be determined using the above method for a weakly processed part of the part (for example, a flat part that has received relatively little processing).

[0039] [Surface Layer Portion] In the steel sheet according to this embodiment, the surface layer portion has an area ratio of 30% or more of prior austenite grains containing at least one retained austenite grain with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more. By setting the area ratio of prior austenite grains containing at least one retained austenite grain with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more to 30% or more, fracture during axial crushing deformation is suppressed. Retained austenite contributes to an increase in the amount of work hardening by transforming into hard martensite through stress-induced transformation. A decrease in the proportion of prior austenite grains containing retained austenite means that the retained austenite is unevenly present in specific prior austenite grains. When such a structure is subjected to axial crushing deformation, work hardening due to stress-induced transformation occurs non-uniformly, resulting in non-uniform inter-structure hardness, making the structure prone to fracture during axial crushing deformation. Therefore, the proportion of crystal grains containing (encompassing) the specified retained austenite grains among the prior austenite grains is increased. Specifically, the area ratio of prior austenite grains containing one or more of the specified retained austenite grains described above is set to 30% or more. An area ratio of less than 30% does not provide sufficient effects. The area ratio is preferably 50% or more, more preferably 60% or more. The upper limit of the area ratio is not specified, and it may be 100%. Furthermore, in this embodiment, only the distribution of retained austenite with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more is targeted. Retained austenite with an aspect ratio of less than 2.0 is particularly unstable to processing and is likely to undergo processing-induced transformation during the part forming stage before axial crushing deformation, so it does not affect axial crushing deformation. Therefore, it is excluded from the target. Furthermore, minute retained austenite with a long side length of less than 0.5 μm is excluded from the target because it is difficult to accurately detect using the measurement method described below.

[0040] The reason for specifying the surface layer (50 to 100 μm from the surface) is that in axial crushing deformation, the steel plate is subjected to bending deformation, which causes strain to concentrate in the region close to the surface, making the structure close to the surface important. However, in the range less than 50 μm from the surface, accurate structure analysis using the method described below may be difficult due to the influence of surface sagging. For this reason, the structure in the above range is specified.

[0041] The area ratio of prior austenite grains containing a specified amount of retained austenite is measured using the following method. A sample is taken from a cross section of the steel sheet parallel to the rolling direction and thickness direction, at a position at least 50 mm away from the widthwise end, as the observation surface. The observation surface is mechanically polished to a mirror finish, and then electrolytically polished to remove the processed layer. If the widthwise end has already been cut and the width position of the steel sheet to be used as the raw material is unknown, a sample can be taken from a position at least 50 mm away from the widthwise end of the steel sheet to be used as the raw material. Next, using FE-SEM, EBSD analysis is performed to analyze the crystal structure by EBSD, with a measurement step of 0.05 μm, over a range of 50 μm in the thickness direction and 100 μm in the rolling direction, from a position 50 to 100 μm from the surface. For example, a JSM-7200F manufactured by JEOL Ltd. can be used as the FE-SEM. The data obtained by the EBSD analysis method is analyzed, for example, using "OIM Analysis 6.0" manufactured by TSL, to obtain a map of FCC iron, i.e., retained austenite. Furthermore, in order to reveal the prior austenite grain boundaries, the sample after the crystal structure analysis by EBSD is etched with an etchant made by adding a saturated aqueous solution of picric acid and a sodium dodecylbenzenesulfonate etchant. A secondary electron image is taken using an FE-SEM in the same field of view as that used for the crystal structure analysis by EBSD. In order to photograph the same field of view, a Vickers indentation or the like may be added in advance as a marker. The areas imaged as bright contrast in the secondary electron image are prior austenite grain boundaries. The prior austenite grain boundaries thus obtained are superimposed on the map of retained austenite obtained by the EBSD method, and the area of ​​prior austenite grains containing retained austenite with an aspect ratio of 2.0 or more and a long side length of 0.5 μm or more is determined. The total area of ​​these prior austenite grains is calculated based on the measured field of view area (5000 μm ２) to calculate the area ratio of prior austenite grains in the surface layer portion that contain one or more retained austenite grains with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more. Here, the aspect ratio of retained austenite is defined as the ratio of the long side to the short side. The long side of retained austenite is defined as the length of a line segment when the retained austenite is cut in the direction in which the line segment length is longest. The short side is defined as the length of a line segment when the retained austenite is cut in the direction in which the line segment length is shortest. Furthermore, "containing retained austenite inside" includes not only a state in which retained austenite is present inside prior austenite grains, but also a state in which the periphery of the retained austenite is partially shared with the prior austenite grain boundary.

[0042] (Mechanical Properties) The steel sheet according to this embodiment is targeted to have a tensile strength (TS) of 980 MPa or more, which contributes to reducing the weight of automobile bodies. While there is no upper limit to the tensile strength, a high tensile strength may result in reduced formability, so the tensile strength may be set to 2000 MPa or less. The elongation (EL) is also targeted to be 10.0% or more. The tensile strength (TS) and elongation (EL) are determined by taking a JIS No. 5 tensile test specimen from the steel sheet in a direction perpendicular to the rolling direction and conducting a tensile test in accordance with JIS Z 2241:2022. If it is difficult to determine the rolling direction, the tensile test may be conducted in any direction. Furthermore, if it is difficult to take a JIS No. 5 tensile test specimen, a JIS No. 13B tensile test specimen may be used. If this is also difficult, any small test specimen having a shape similar to the JIS No. 13B tensile test specimen may be used. In this case, the gauge length may also be changed in accordance with the scale of the test piece. For example, if the test piece has a 1 / 2 similar shape, the gauge length may be set to 25 mm.

[0043] (Thickness) The thickness of the steel plate according to this embodiment is not limited, but is preferably 0.4 to 3.0 mm from the viewpoint of achieving both weight reduction of the automobile body and suppression of fracture during crushing deformation as a component.

[0044] (Zinc Plated Layer) (Galvannealed Hot-Dip Galvanized Layer) The steel sheet according to this embodiment may have a zinc plated layer on its surface (it may have a base steel sheet and a zinc plated layer formed on the surface of the base steel sheet). Having a zinc plated layer improves corrosion resistance. Automotive steel sheets may not be thinned below a certain thickness even if they are strengthened due to concerns about corrosion-induced perforation. One of the purposes of strengthening steel sheets is to reduce weight by thinning them, so even if a high-strength steel sheet is developed, its application is limited if its corrosion resistance is low. To solve these problems, a highly corrosion-resistant zinc plated layer may be formed on the surface of the steel sheet. The zinc plated layer may be an electrogalvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer formed by alloying a hot-dip galvanized layer. A hot-dip galvanized layer is preferable from the standpoint of cost, and an alloyed hot-dip galvanized layer is preferable because it provides excellent weldability and paintability due to the incorporation of Fe into the hot-dip galvanized layer through the alloying process. Furthermore, an upper layer of plating may be applied on the galvanized layer (electrogalvanized layer, hot-dip galvanized layer, or galvannealed layer) for the purpose of improving paintability and weldability. Furthermore, in the cold-rolled steel sheet according to this embodiment, various treatments, such as chromate treatment, phosphate treatment, lubricity improvement treatment, and weldability improvement treatment, may be applied on the galvanized layer. The galvanized layer and galvannealed layer may be composed of Zn and Fe, but may also contain elements other than Zn and Fe, such as Al, Mg, and Si. The coating weight of the galvanized layer is not particularly limited and may be a general coating weight. A general coating weight for automotive applications is, for example, 20 to 100 g / m per side. ２ is.

[0045] <Steel Member> The steel member according to this embodiment is a steel member obtained by forming the steel plate according to this embodiment into a predetermined shape, or a steel member obtained by joining another steel material to the steel plate by spot welding after forming. The steel member obtained by forming (processing) the steel plate into a predetermined shape has a steel plate including a processed portion and a non-processed portion. Furthermore, the steel member obtained by joining another steel material to the steel plate by spot welding has a steel plate including a spot weld, a HAZ formed around the spot weld and affected by the heat of the spot welding, and a non-welded portion other than the spot weld and the HAZ. Here, the non-processed portion is a flat portion of the steel member that is the thickest part of the flat portion. The processed portion is a portion of the flat portion of the steel member that is thinner than the surrounding area or has a certain curvature. Furthermore, the non-welded portion is a portion other than the spot weld and the HAZ affected by the heat of the spot welding. The steel member includes the above-described steel plate. The steel member may be made of the above-described steel plate. Furthermore, the surface of the steel sheet constituting the steel member may have a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer. Examples of the steel member include automobile parts such as front side members, rear side members, and side sills.

[0046] The steel member according to this embodiment (before spot welding) has the following characteristics in the non-processed portion: Furthermore, the steel member according to this embodiment (after spot welding) has the following characteristics in the non-processed portion of the non-welded portion:

[0047] (Non-processed portion and non-welded portion (non-processed portion in the case of a steel member before spot welding)) The non-welded portion is not affected by spot welding, and therefore has the same chemical composition as the steel plate according to this embodiment. Furthermore, although the microstructure changes due to forming, the non-processed portion of the non-welded portion that is not subject to processing strain is the same as the steel plate according to this embodiment. That is, in the non-processed and non-welded portion (non-processed portion in the case of a steel member before spot welding, the same applies below), the following contents are contained in mass %: C: 0.08 to 0.20%, Si: 0.50 to 1.80%, Mn: 2.00 to 3.50%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00% %, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, Ta: 0-1.00%, Sn: 0-1.00%, Sb: 0-0.50%, Nb: 0-0.200%, V: 0-1.00%, As: 0-0.10%, Zn: 0-1.000%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, Bi: 0-0.0100%, REM: 0-0.0100%, Hf: 0-0.0100%, and the balance: Fe and impurities. Furthermore, in the unprocessed and unwelded portions, the microstructure at a quarter-thickness position, which is a position from the surface in the thickness direction (for example, the thickness of the steel plate when composed of steel plate), consists of, in area %, 3-10% retained austenite, 0-10% ferrite, 0-5% pearlite, and a total of 75-97% martensite and bainite, and in a surface layer portion located 50-100 μm from the surface in the thickness direction, the area ratio of prior austenite grains containing one or more retained austenite grains with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more is 30% or more. By having such a chemical composition and microstructure, fracture during axial crushing deformation is suppressed.

[0048] Since the chemical composition of the unprocessed and unwelded portions is basically unchanged from that of the steel plate according to this embodiment, the standard deviation σ of the MS value calculated by formula (1) from the Mn concentration [Mn] and the Si concentration [Si] in mass % at each of a plurality of measurement points measured by EPMA in a 35 μm × 50 μm region in the range of ⅛ to ⅜ thickness centered at the ¼ thickness position of the cross section in the thickness direction (in the thickness direction of the steel plate if the steel plate is included) may be 0.80 or less: MS = [Mn] + (⅔) × [Si] (1)

[0049] If the standard deviation σ of the MS values ​​in the unprocessed and unwelded portions is 0.80 or less, fracture in the HAZ is suppressed when the steel member is subjected to axial crushing deformation after spot welding. Also, if the standard deviation σ of the MS values ​​in the unprocessed and unwelded portions is 0.80 or less in a steel member having spot welds, fracture in the HAZ is suppressed when the steel member is subjected to axial crushing deformation.

[0050] Furthermore, the mechanical properties and thickness may be similar to those of the steel sheet according to the present embodiment described above.

[0051] The other steel plate joined by spot welding may be the steel plate according to this embodiment, or may not be the steel plate according to this embodiment. If the other steel plate is not the steel plate according to this embodiment, it is sufficient that the portion obtained from the steel plate according to this embodiment satisfies the above. In other words, it is sufficient that a part of the steel member satisfies the above.

[0052] (Spot Welds) There are no limitations on the spot welds, and they may be spot welds formed under normal welding conditions.

[0053] The chemical composition, microstructure, and standard deviations of the Mn and Si concentration distributions of the unprocessed and unwelded parts can be determined in the same manner as at the steel plate stage. Regarding mechanical properties, if a JIS No. 5 tensile test piece can be taken from the unprocessed and unwelded parts of the component, they can be determined in the same manner as at the steel plate stage, but if it is not possible to take a JIS No. 5 tensile test piece, as mentioned above, the tensile strength of a JIS No. 13B tensile test piece or a small test piece with a similar shape to the JIS No. 13B tensile test piece should be evaluated.

[0054] <Manufacturing Method> The steel sheet and steel member according to this embodiment can achieve the effects described above regardless of the manufacturing method. However, the following method is preferred because it allows stable manufacturing. The steel sheet according to this embodiment can be obtained by a manufacturing method including the following steps (I) to (IV): (I) a continuous casting step of obtaining a slab having a predetermined chemical composition by continuous casting; (II) a hot rolling step of heating the slab to a heating temperature, hot rolling including rough rolling and finish rolling, cooling to a coiling temperature, and then coiling at the coiling temperature to obtain a hot-rolled steel sheet; (III) a cold rolling step of, as necessary, pickling the hot-rolled steel sheet and cold rolling with a cumulative reduction of 30 to 75% to obtain a cold-rolled steel sheet; and (IV) a heat treatment step of heat treating the hot-rolled steel sheet or the cold-rolled steel sheet. Furthermore, in the method for producing a steel sheet according to this embodiment, when cold rolling is performed, one or both of the following steps may be optionally further carried out between the hot rolling step and the cold rolling step: (II') a pickling step of pickling the hot-rolled steel sheet; (II'') a hot-rolled sheet heat treatment step of heat treating the hot-rolled steel sheet. When the steel sheet according to this embodiment is to be a plated steel sheet, one or both of the following steps may be further carried out on the steel sheet (hot-rolled steel sheet or cold-rolled steel sheet) obtained by the above steps: (V) a plating step of forming a galvanized layer on the surface of the steel sheet; (VI) an alloying step of converting the hot-dip galvanized layer into an alloyed hot-dip galvanized layer. Furthermore, the steel member according to this embodiment can be obtained by further subjecting the steel sheet (hot-rolled steel sheet or cold-rolled steel sheet) obtained by the above steps (I) to (IV) or the steel sheet (galvanized steel sheet) obtained by the above steps (I) to (V) or (I) to (VI) to the following steps (VII), or (VII) and (VIII). (VII) A forming step of forming the steel sheet according to the present embodiment into a predetermined shape. (VIII) A joining step of joining the steel sheet after the forming step to another steel sheet by spot welding. Preferred conditions for each step will be described.

[0055] (Continuous Casting Step) In the continuous casting step, a slab having the same chemical composition as the steel sheet according to the present embodiment is obtained by continuous casting.

[0056] In the case where the standard deviation σ of the MS value calculated from the Mn concentration [Mn] and the Si concentration [Si] by the formula (1) is to be 0.80 or less in the steel sheet obtained through the subsequent process, it is preferable to perform casting in the continuous casting process so as to satisfy the following formula (2). C (τ) is calculated by equation (3).

[0057]

[0058]

[0059] Here, τ, τ, and T in the formula (2) and the formula (3) S (τ), T C (τ), T L , f L , M L , M δ , S L , S δ , D δ Mn , D δ Si τ: Elapsed time from the start of casting [seconds] τ1: T C (τ) is the solidification completion temperature T in units of K δ Time to reach [seconds] T S (τ): Slab surface temperature at time τ [K] T C (τ): Estimated slab internal temperature (center temperature) at time τ [K] T L : Solidification start temperature [K] f L :Temperature T C (τ) Equilibrium volume fraction of the liquid phase at L :Temperature T C Equilibrium Mn concentration in the liquid phase at (τ) [mass%] M δ :Temperature T C Equilibrium Mn concentration [mass%] of δ at (τ) S L :Temperature T C Equilibrium Si concentration in the liquid phase at (τ) [mass%] S δ :Temperature T C Equilibrium Si concentration [mass%] of δ at (τ) D δ Mn :Temperature T CThe diffusion coefficient of Mn in the δ phase at (τ) [m 2 / sec] D δ Si :Temperature T C The diffusion coefficient of Si in the δ phase at (τ) [m 2 / second]

[0060] When continuously casting a slab from molten steel, the molten steel is initially liquid but gradually solidifies from the surface until it completely solidifies at a certain temperature (i.e., the solidification completion temperature). In this regard, Equation (2) defines the temperature range (partially solidified temperature range) in which the liquid and solid phases coexist before the molten steel completely solidifies. In this temperature range, as solidification progresses from the surface of the molten steel, alloying elements are distributed (extracted) from the solid phase to the liquid phase, resulting in the alloying elements concentrating in the liquid phase. This concentration of alloying elements increases the standard deviation σ of the MS values. More specifically, the greater the partitioning of Mn and Si between the solid and liquid phases, the more likely it is that the final product will have locally high and low MS values. Therefore, in the temperature range in which the liquid and solid phases coexist during continuous casting, it is important to sufficiently suppress the partitioning of Mn and Si between the solid and liquid phases. In this regard, the inventors have found that satisfying the formulas (2) and (3), particularly controlling the value calculated by the left side of formula (2) to less than 0.0100, can suppress the distribution of Mn and Si between the solid and liquid phases, and can significantly reduce the standard deviation σ of the MS values. Here, τ = 0 in formula (2) means the start of casting, and τ is the temperature at which the internal temperature (center temperature) of the slab reaches the solidification completion temperature T δ Therefore, formula (2) can be understood as the time (seconds) when the distribution of Mn from the solid phase to the liquid phase is integrated over time. Therefore, the smaller the value of the left side of formula (2), the more the distribution of Mn and Si from the solid phase to the liquid phase is suppressed. The smaller the value of the left side of formula (2) is, the more preferable it is, and specifically, it is preferably 0.0100 or less, more preferably 0.0050 or less, and even more preferably 0.0030 or less. T in formulas (2) and (3) L、 T Ｓ、 f L , M L , M δ , SL , S δ、 D δ Mn , D δ Si is a value determined by the temperature and the chemical composition of the molten steel. C (τ) is the internal temperature (center temperature) of the slab at time τ and is estimated from the surface temperature of the slab using equation (3). The surface temperature of the slab is measured at intervals of one second. If it is difficult to measure the surface temperature at one second intervals, values ​​at one second intervals may be obtained by linear interpolation between measurement points. If the measurement interval is shorter than one second, data may be thinned out to form one second intervals. Therefore, a person skilled in the art can control the value calculated by the left side of equation (2) within a desired range by appropriately selecting the chemical composition of the molten steel and the temperature history during continuous casting. T L , T δ , f L , M L , M δ , S L , S δ can be obtained by performing a phase diagram calculation for the target steel composition using, for example, commercially available thermodynamic calculation software "Thermo-Calc 2022b" (Thermo-Calc Software Inc.). The phases to be calculated are the LIQUID phase and the BCC_A2 phase, and the other phases are SUSPEND. Equilibrium calculations are performed in one axis mode with temperature as a variable. The calculation temperature range should include a temperature range in which the number of moles of the LIQUID phase is 0 to 1, for example, a range of 1400 to 1600°C. The number of step divisions is set so that the calculation results are in 1°C increments. For example, if the calculation temperature range is 200°C, there are 200 divisions. The step method is set to Normal. The pressure is set to 100,000 Pa, the system size is set to 1 Mol, and global minimization is enabled. After the calculation, the Mn concentration and Si concentration in the LQUID phase and the Mn concentration and Si concentration in the BCC_A2 phase were output as variables of temperature, and the Mn concentration and Si concentration at each temperature were calculated. L , S L , M δ , S δThe volumes of the LQUID phase and the BCC_A2 phase are output as variables of temperature, and the value obtained by dividing the volume of the LQUID phase by the sum of the volumes of the LQUID phase and the BCC_A2 phase is used as f at each temperature. L The number of moles of the LQUID phase is output, and the lowest temperature at which the number of moles becomes 1 is called T Ｌ The highest temperature at which the number of moles becomes 0 is T δ Also, D δ Mn , D δ Si The values ​​calculated by the following formulas (4) and (5) are used.

[0061]

[0062]

[0063] (Hot rolling process) In the hot rolling process, the slab is heated to a heating temperature, hot-rolled including rough rolling and finish rolling, cooled to a coiling temperature, and then coiled at the coiling temperature to obtain a hot-rolled steel sheet. In the hot rolling process, the heating temperature of the slab is 1200°C or higher. If the heating temperature is lower than 1200°C, the diffusion of alloy elements becomes insufficient. There is no upper limit for the heating temperature, but it is preferably 1350°C or lower in order to suppress a decrease in yield due to scale-out. Furthermore, in order to increase the proportion of prior austenite grains containing retained austenite grains, where the number of passes of finish rolling is n, the first pass is the first pass, and the final pass is the nth pass, the inlet temperature of the n-2th pass (i.e., the third pass counting from the final pass) is 950°C or higher, the outlet temperature (steel sheet surface temperature) of the nth pass (final pass) is 900°C or higher, and a reduction ratio in one pass exceeding 25% is performed at least once in finish rolling. Furthermore, the inter-pass times between the n-2th pass and the n-1th pass, and between the n-1th pass and the nth pass, are each 0.2 to 1.0 seconds, and the time from the completion of the nth pass to the start of cooling is 1.0 to 3.0 seconds. There are no upper limits for the inlet temperature and the outlet temperature, but the inlet temperature and the outlet temperature may be 1100°C or lower. This increases the number of nucleation sites for phase transformation, resulting in a refined hot-rolled sheet structure, resulting in a uniform dispersion of cementite, which serves as a source of retained austenite. If one or more of the following parameters are outside the above ranges: the inlet temperature of the n-2th pass (i.e., the third pass from the end), the outlet temperature of the nth pass, the number of reductions exceeding 25% in one pass, the inter-pass time between the final three passes, and the time from the completion of the final pass to the start of cooling, cementite will not be sufficiently uniformly dispersed. If cementite is present unevenly, the distribution of C in austenite will become uneven during heating in the subsequent heat treatment process, resulting in the formation of locally low-C austenite regions. The martensite or bainite transformed from such austenite will have a low C concentration, and austenite will no longer remain (contain retained austenite) within the prior austenite grains. In the hot-rolling process and subsequent processes, the controlled temperatures are all surface temperatures of the steel sheet, unless otherwise specified.

[0064] Furthermore, in the cooling after the completion of the final pass (nth pass), the steel is cooled to a coiling temperature of 200 to 550°C so that the average cooling rate between 600 and 750°C is 20°C / s or more. This results in a microstructure mainly composed of martensite and bainite, with cementite uniformly dispersed. If the average cooling rate between 600 and 750°C is less than 20°C / s or the coiling temperature exceeds 550°C, structures other than martensite and bainite, such as ferrite and pearlite, are generated in large amounts, and cementite does not uniformly disperse. The coiling temperature is preferably 540°C or less. If the coiling temperature is less than 200°C, uneven cooling may occur, which may cause deformation of the hot-rolled coil and hinder productivity. Furthermore, the upper limit of the average cooling rate does not need to be particularly limited, but from an operational standpoint, it may be 200°C / s or less.

[0065] Furthermore, in the case where the standard deviation σ of the MS value is set to 0.80 or less in the steel sheet obtained through the subsequent process, it is preferable to perform at least three or more rolling (rolling reduction) processes with a reduction ratio exceeding 20% ​​in a state where the steel sheet temperature is 1050 ° C. or higher. The upper limit of the number of rolling processes exceeding 20% ​​is not limited, but may be seven or less. The upper limit of the rough rolling temperature is not particularly limited, but may be 1250 ° C. or less from an operational standpoint.

[0066] (Pickling step) In the pickling step, the hot-rolled steel sheet is pickled. The pickling method may be a conventional method. The pickling step may not be performed. Furthermore, skin-pass rolling may be performed to correct the shape of the hot-rolled coil and improve the pickling properties.

[0067] (Hot-rolled sheet heat treatment process) In order to reduce the load on the cold rolling mill, the hot-rolled sheet may be subjected to heat treatment to soften it before cold rolling. When heat treatment is performed, if the maximum temperature reached is less than 400°C, softening may not proceed sufficiently. Furthermore, if the temperature exceeds 650°C, cementite becomes coarse, which delays the reverse transformation in the heat treatment process, and the desired microstructure may not be obtained. Therefore, the maximum temperature reached is preferably 400 to 650°C. The residence time may be approximately 60 seconds to 40 hours. The above heat treatment may be performed before or after pickling.

[0068] (Cold Rolling Step) In the cold rolling step, the hot-rolled steel sheet is pickled and cold-rolled with a cumulative reduction of 30 to 75% to obtain a cold-rolled steel sheet. However, the cold rolling step is not always necessary. If the reduction exceeds 75%, the load on the equipment increases, resulting in unnecessary increases in costs. On the other hand, if the reduction is less than 30%, the roughness of the rolling rolls is not sufficiently transferred to the steel sheet surface, resulting in inferior surface roughness compared to when the reduction exceeds 30%.

[0069] (Heat Treatment Process) In the heat treatment process, heat treatment is performed on a hot-rolled steel sheet (when a cold rolling process has not been performed) or a cold-rolled steel sheet (when a cold rolling process has been performed) (collectively sometimes simply referred to as steel sheet). The heat treatment process includes a heating process, a cooling process, and a holding process. In the heating process, the hot-rolled steel sheet or cold-rolled steel sheet is heated to a first temperature range of Ac3-20°C to 950°C and held at that temperature range for 1 to 1,000 seconds. In the cooling process, the hot-rolled steel sheet or cold-rolled steel sheet after the heating process is cooled to a second temperature range of Ms-200°C to Ms-50°C at an average cooling rate of 10°C / s or more between 500 and 650°C, and 20°C / s or more between Ms-50 and Ms°C. In the holding process, after the cooling process, the steel sheet is held at 360 to 480°C for 10 to 600 seconds. If the first temperature range is below Ac3-20°C or the holding time in the first temperature range is less than 1 second, austenite transformation does not occur sufficiently, and the desired microstructure cannot be obtained in the steel sheet after the final process. On the other hand, if the first temperature range is above 950°C or the holding time is above 1000 seconds, the austenite grain size increases excessively, and toughness decreases. Furthermore, by maintaining the above-mentioned condition range during the cooling process, the partitioning of C from a phase already transformed to martensite or bainite to austenite during the transformation to bainite or martensite is suppressed. If the cooling conditions are outside the above-mentioned range, the partitioning of C progresses. As the partitioning of C progresses, the proportion of prior austenite grains containing retained austenite decreases. There are no upper limits on the average cooling rate between 500 and 650°C and between Ms-50 and Ms°C, but from an operational standpoint, it may be set to 200°C / s or less. Furthermore, during the holding process, carbon is concentrated in the austenite to stabilize it (austempering) in order to obtain the desired retained austenite content. If the reheating temperature is less than 360°C or 480°C or higher, or if the holding time is less than 10 seconds, carbon is not sufficiently concentrated in the austenite, and the proportion of untransformed austenite that remains at room temperature and becomes retained austenite during the subsequent cooling process to room temperature decreases. On the other hand, if the holding time exceeds 600 seconds, austenite decomposes into cementite, and the desired retained austenite content cannot be obtained.During the holding process, if the temperature of the steel sheet after the cooling process is less than 360°C, it can be heated as necessary and then held in the above temperature range. Even if heating is performed, it should not be heated to a temperature range exceeding 480°C. Here, Ac3 and Ms (transformation point) (°C) can be calculated using the following formulas: Ac3 = 912 - 230.5 x [C] + 31.6 x [Si] - 20.4 x [Mn] - 39.8 x [Cu] - 18.1 x [Ni] - 14.8 x [Cr] + 16.8 x [Mo] + 100 x [Al] Ms = 561 - 474 x [C] - 33 x [Mn] - 17 x [Cr] - 17 x [Ni] - 21 x [Mo] - 7.5 x [Si] + 10 x [Co] In the formulas, [element] represents the mass% content of each element in the steel sheet.

[0070] (Plating Process) When forming an electrogalvanized layer on a steel sheet, it may be carried out by a known method. Furthermore, when forming a hot-dip galvanized layer on the surface of a steel sheet, a known plating process may be carried out in which the steel sheet is immersed in a hot-dip galvanizing bath, pulled out, and the coating weight is adjusted by wiping. The plating process may be carried out at any stage. For example, as long as the conditions for the above-mentioned heat treatment process are satisfied, the plating process may be carried out during the cooling process of the heat treatment process, between the cooling process and the holding process, during the holding process, or after the holding process.

[0071] (Alloying Step) When the hot-dip galvanized layer is to be an alloyed hot-dip galvanized layer, an alloying step may be performed after the plating step to alloy the hot-dip galvanized layer. The alloying step may be performed at any time after the plating step. For example, as long as the conditions for the heat treatment step described above are satisfied, the alloying step may be performed after the plating step and during the cooling step of the heat treatment step, between the cooling step and the holding step, during the holding step, or after the holding step. However, when the plating step or the alloying step is performed, the conditions for the heat treatment step described above are satisfied, even when the holding time at a predetermined temperature in these steps is included. That is, for example, when the plating step or the alloying step is performed during the cooling step of the heat treatment step, the average cooling rate is controlled to be 10°C / s or more between 500°C and 650°C, and 20°C / s or more between Ms-50°C and Ms°C, even when the plating step or the alloying step is included. Furthermore, for example, when the plating step or the alloying step is performed under conditions where the temperature of the steel sheet is 360 to 480°C between the cooling step and the holding step, or after the holding step, the holding time in the holding step and the time for the steel sheet to reach 360 to 480°C in the plating step and the alloying step are controlled so as not to exceed 600 seconds.

[0072] (Forming process) In the forming process, the steel sheet according to the present embodiment obtained through the above-described process is formed into a predetermined shape as required. The forming method and the formed shape are not limited. For example, the steel sheet may be an automotive frame member having a closed cross section perpendicular to the longitudinal direction, such as a front side member, a rear side member, or a side sill.

[0073] (Joining Process) For example, to obtain the above-described closed cross-section structure, the steel plate after the forming process is spot-welded (resistance spot welding) to be joined to another steel plate. The conditions for spot welding are not limited, but a nugget diameter of 3√t to 6√t (t: plate thickness) is desirable. The welding current value, current pattern, pressure, welding electrodes, etc. may be selected so as to obtain the desired nugget diameter. The spacing between spot welds may be approximately 15 to 50 mm.

[0074] Slabs having the chemical compositions shown in Table 1 were obtained by continuous casting. Continuous casting was performed so that the value of the left side of the formula (2) above was as shown in Table 2A. The slabs were then heated to the slab heating temperature shown in Table 2A, and hot rolling, including rough rolling and finish rolling, was performed as shown in Table 2A. Here, the R1 inlet temperature in the table is the inlet temperature at the n-2 pass (the third pass counting from the final pass), R1 is the reduction rate at the n-2 pass, R2 is the reduction rate at the n-1 pass, R3 is the reduction rate at the n pass (final pass), the R3 outlet temperature is the outlet temperature at the n pass, t1 is the interpass time between the n-2 pass and the n-1 pass, t2 is the interpass time between the n-1 pass and the n pass, and t3 is the time from the completion of the n pass to the start of cooling. After hot rolling, the steel sheets were cooled to the coiling temperature shown in Table 2A so that the average cooling rate from 600 to 750°C was as shown in Table 2A. These steel sheets were then pickled and cold-rolled with the reduction (cumulative reduction) shown in Table 2B. All sheets had a thickness of 1.2 mm after cold rolling. The cold-rolled steel sheets were heat-treated under the conditions shown in Table 2B. The holding time during the heating process in the heat treatment step in the table refers to the time during which the steel sheet remained at Ac3-20°C or higher. However, for No. 24, this refers to the holding time at the maximum temperature reached (±10°C). Furthermore, the holding time during the holding process in the heat treatment step in the table refers to the time during which the steel sheet remained at the holding temperature ±10°C. In some examples, a galvanized layer (electrogalvanized layer or hot-dip galvanized layer) was formed on the surface. In some examples where a hot-dip galvanized layer was formed, the hot-dip galvanized layer was alloyed to form an alloyed hot-dip galvanized layer. The electrogalvanized layer was formed after the heat treatment step and cooling to room temperature. The hot-dip galvanized layer was formed by reheating or cooling the steel sheet after the holding step to 460°C and then immersing it in a hot-dip galvanized bath. The immersion time was 3 seconds. The alloying treatment was performed by immersing the steel sheet in the hot-dip galvanized bath, heating it to the temperature shown in Table 2, and holding it for 20 seconds. In the product type column in the table, HR is an example of a hot-rolled steel sheet on which no galvanized layer was formed, CR is an example of a cold-rolled steel sheet on which no galvanized layer was formed, EG is an example on which an electrogalvanized layer was formed, GI is an example on which a galvannealed layer was formed, and GA is an example on which an alloyed hot-dip galvanized layer was formed.

[0075] For the obtained steel sheets, the microstructure at the 1 / 4 thickness position, the area ratio of prior austenite grains in the surface layer containing at least one retained austenite grain with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more, and the standard deviation of MS values ​​in the range of 1 / 8 to 3 / 8 thickness centered at the 1 / 4 thickness position were determined using the methods described above. The FE-EPMA used was JXA-8530F manufactured by JEOL, with an acceleration voltage of 15 kV. The data obtained by the EBSD method was analyzed using "OIM Analysis 6.0" manufactured by TSL. The results are shown in Table 3.

[0076] Furthermore, No. 5 tensile test pieces according to JIS Z 2241:2022 were taken from the obtained steel sheets, with the longitudinal direction being perpendicular to the rolling direction, and these test pieces were subjected to tensile tests in accordance with JIS Z2241:2022 to measure tensile strength (TS) and elongation (EL). If TS was 980 MPa or more and EL was 10.0% or more, it was determined that the steel sheets had high strength and excellent elongation.

[0077] Furthermore, using the obtained steel plate as a raw material, a hat-shaped formed body having a bent ridge of R = 5 mm and a flat plate serving as a backing plate were cut and joined by spot welding to create a component with a closed cross-section structure having the shape shown in Figure 1A. As shown in Figure 1B, the spacing between weld points was 15 mm, and the welding current was set to a value such that the diameter of the molten nugget was 5.5 times √t (t: plate thickness). Other spot welding conditions were as follows: Welding electrode: tip diameter φ6 mm, tip curvature radius R = 40 mm, Cr-Cu electrode Power source: 50 Hz single-phase AC Pressurizing force: 400 kgf Welding time: 20 cyc Hold time: 5 cyc

[0078] The chemical composition, metal structure and mechanical properties of the non-welded and non-processed parts of the obtained steel members were the same as those of the raw material.

[0079] A flat impactor was collided with the resulting steel member from the upper end, as shown in Figure 2, with the lower end fully restrained. The impactor weighed 334 kg, and the impact speed was 11.1 m / s. The test specimens were observed after the impact, and the presence or absence of cracks was visually confirmed in the general parts (non-welded parts) and welded parts (HAZ parts). Those in which no cracks were found were rated "○: GOOD", and those in which cracks were found were rated "×: BAD". The results are shown in Table 3.

[0080]

[0081]

[0082]

[0083]

[0084] As can be seen from Tables 1 to 3, steel members (which had similar characteristics to the steel members in non-welded and non-processed areas) obtained using steel plates having a predetermined chemical composition, a microstructure at the quarter thickness position, and an area ratio of prior austenite grains in the surface layer of 30% or more, each of which contained at least one retained austenite grain with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more, exhibited excellent fracture resistance during axial crushing deformation. Furthermore, when the standard deviation σ of the MS values ​​was 0.80 or less, the steel members also exhibited excellent fracture resistance during axial crushing deformation in the HAZ.

[0085] According to the present invention, it is possible to provide a steel plate having high strength and excellent elongation, and capable of suppressing fracture of the component when subjected to axial crushing deformation after being formed into the component, a method for manufacturing the same, and a steel component obtained using the steel plate, capable of suppressing fracture when subjected to axial crushing deformation. Therefore, the present invention has a high industrial applicability.

Claims

1. In mass%, C: 0.08 to 0.20%, Si: 0.50 to 1.80%, Mn: 2.00 to 3.50%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 1.00%, W: 0 to 1.00%, Ta: 0 to 1.00%, Sn: 0 to 1.00%, The steel sheet has a chemical composition consisting of Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0 to 1.00%, As: 0 to 0.10%, Zn: 0 to 1.000%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Bi: 0 to 0.0100%, REM: 0 to 0.0100%, Hf: 0 to 0.0100%, and the balance: Fe and impurities, and the microstructure at a 1 / 4 thickness position, which is a position from the surface to 1 / 4 of the sheet thickness in the sheet thickness direction, consists, in area %, of retained austenite: 3 to 10%, ferrite: 0 to 10%, pearlite: 0 to 5%, and martensite and bainite: 75 to 97% in total, in a surface layer portion located 50 to 100 μm from the surface in the sheet thickness direction, an area ratio of prior austenite grains containing one or more retained austenite grains with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more is 30% or more.

2. The steel sheet according to claim 1, wherein the standard deviation σ of MS values ​​calculated by formula (1) from the Mn concentration [Mn] and the Si concentration [Si] in mass % at each of a plurality of measurement points measured by EPMA in a 35 μm × 50 μm region in the range of 1 / 8 to 3 / 8 thickness centered at the 1 / 4 thickness position on the cross section parallel to the sheet thickness direction and rolling direction is 0.80 or less. MS = [Mn] + (2 / 3) × [Si] ... (1) 3. The steel sheet according to claim 1 or 2, characterized in that the surface has a hot-dip galvanized layer or a hot-dip galvannealed layer.

4. A steel plate including a non-machined portion, wherein the non-machined portion contains, in mass%, C: 0.08 to 0.20%, Si: 0.50 to 1.80%, Mn: 2.00 to 3.50%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 1.00%, W: 0 to 1.00%, Ta: 0 to 1.00%, The non-machined portion has a chemical composition consisting of Sn: 0 to 1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0 to 1.00%, As: 0 to 0.10%, Zn: 0 to 1.000%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Bi: 0 to 0.0100%, REM: 0 to 0.0100%, Hf: 0 to 0.0100%, and the balance: Fe and impurities, and the microstructure of the non-machined portion at a 1 / 4 thickness position, which is a position from the surface to 1 / 4 of the thickness in the thickness direction, is, in area %, retained austenite: 3 to 10%, ferrite: 0 to 10%, pearlite: 0 to 5%, and a steel member comprising 75 to 97% martensite and bainite in total, and in a surface layer portion located 50 to 100 μm from the surface in the thickness direction, an area ratio of prior austenite grains containing one or more retained austenite grains having a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more is 30% or more.

5. A steel member according to claim 4, comprising a steel plate having a spot weld, a HAZ around the spot weld, and a non-welded portion other than the spot weld and the HAZ, wherein the non-machined portion is present in the non-welded portion.

6. The steel member according to claim 4 or 5, wherein the standard deviation σ of MS values ​​calculated by formula (1) from the Mn concentration [Mn] and the Si concentration [Si] at each of a plurality of measurement points measured by EPMA in a 35 μm × 50 μm region in the range of 1 / 8 to 3 / 8 thickness centered at the 1 / 4 thickness position in the non-machined portion is 0.80 or less: MS = [Mn] + (2 / 3) × [Si] (1) 7. The steel member according to claim 4 or 5, wherein the surface of the steel plate has a zinc-plated layer or a galvannealed hot-dip galvannealed layer.

8. A method for producing a steel sheet as set forth in claim 1, comprising: a continuous casting step of obtaining a slab having the chemical composition as set forth in claim 1 by continuous casting; a hot rolling step of heating the slab to a heating temperature, performing hot rolling including rough rolling and finish rolling, and cooling to a coiling temperature, and then coiling at the coiling temperature to obtain a hot-rolled steel sheet; a cold rolling step of, as necessary, subjecting the hot-rolled steel sheet to pickling and cold rolling at a cumulative reduction of 30 to 75% to obtain a cold-rolled steel sheet; and a heat treatment step of heat treating the hot-rolled steel sheet or the cold-rolled steel sheet, wherein in the hot rolling step, the heating temperature of the slab is 1200°C or higher, and when the number of passes of the finish rolling is n, with the first pass being the first pass and the final pass being the n-th pass, the inlet temperature of the (n-2)th pass is 950°C or higher and the outlet temperature of the n-th pass is 900°C or higher, the finish rolling is performed at least once with a reduction rate of more than 25% in one pass, the inter-pass times between the n-2 pass and the n-1 pass, and between the n-1 pass and the n pass are 0.2 to 1.0 seconds, the time from the completion of the n pass to the start of the cooling is 1.0 to 3.0 seconds, the coiling temperature is 200 to 550°C, and in the cooling to the coiling temperature, an average cooling rate between 600 and 750°C is 20°C / s or more, the heat treatment step has a heating step, a cooling step, and a holding step, and in the heating step, the hot-rolled steel sheet or the cold-rolled steel sheet is heated to a first temperature range of Ac3-20°C to 950°C and held in the first temperature range for 1 to 1000 seconds, in the cooling step, the hot-rolled steel sheet or the cold-rolled steel sheet after the heating step is cooled to a second temperature range of Ms-200°C to Ms-50°C at a cooling rate of 10°C / s or more between 500 and 650°C and an average cooling rate of 20°C / s or more between Ms-50 and Ms°C; and in the holding step, after the cooling step, the steel sheet is held at 360 to 480°C for 10 to 600 seconds.

9. A method for producing a steel plate according to claim 8, characterized in that in the continuous casting step, casting is carried out so as to satisfy the following formula (2): Here, τ, τ, and T in the formula (2) and the formula (3) S (τ), T C (τ), T L , f L , M L , M δ , S L , S δ , D δ Mn , D δ Si and T respectively indicate the following: C (τ) is calculated by the above formula (3). τ: elapsed time from the start of casting in seconds τ1: T C (τ) is the solidification completion temperature T in units of K δ Time T S (τ): slab surface temperature at time τ in K units T C (τ): Estimated slab internal temperature at time τ in K. L : solidification start temperature f in K L :Temperature T C (τ) Equilibrium volume fraction of the liquid phase at L : Temperature T in mass% C Equilibrium Mn concentration in the liquid phase at (τ) M δ : Temperature T in mass% C (τ) Equilibrium Mn concentration of δ at S L : Temperature T in mass% C Equilibrium Si concentration in the liquid phase at (τ) S δ : Temperature T in mass% C Equilibrium Si concentration of δ at (τ) D δ Mn : Unit is m 2 / sec, the temperature T C Diffusion coefficient of Mn in the δ phase at (τ) D δ Si : Unit is m 2 / sec, the temperature T C Diffusion coefficient of Si in the δ phase at (τ) 10. The method for manufacturing a steel sheet according to claim 8 or 9, further comprising a plating step of forming a zinc plating layer on the surface of the hot-rolled steel sheet or the cold-rolled steel sheet during the cooling step of the heat treatment step, between the cooling step and the holding step, during the holding step, or after the holding step.

11. The method for producing a steel sheet according to claim 10, wherein the galvanized layer is a hot-dip galvanized layer, and further comprising, after the plating step, an alloying step of alloying the hot-dip galvanized layer to form an alloyed hot-dip galvanized layer.

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