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

Abstract

This steel sheet has a prescribed chemical composition, wherein: the microstructure at a 1 / 4 thickness position that is the position at 1 / 4 the plate thickness in the plate thickness direction from the surface, is composed of, in area%, retained austenite: 3-10%, ferrite: 0-10%, pearlite: 0-5%, and martensite and bainite: 75-95% in total; and, in a cross section parallel to the plate thickness direction, when a region measuring 80 μm × 80 μm centered at the 1 / 4 thickness position in the plate thickness direction and measuring 80 μm in a direction perpendicular to the plate thickness direction is divided into 16 parts each measuring 20 μm × 20 μm, and the area ratio of retained austenite in each divided region is determined, σ1 / γave is 0.20 or less, where σ1 is the standard deviation and γave is the average of the area ratio of retained austenite in each divided region.

Description

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

[0001] This application claims priority to Japanese Patent Application No. 2024-053452, 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 still room for further study on 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 suppression 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 suppressing 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 the steel plate and is capable of suppressing fracture when it is subjected to axial crushing deformation.

[0006] The present inventors investigated whether fracture during axial crushing deformation of a component can be suppressed by the structure (chemical composition, microstructure, etc.) of the steel plate. As a result, they found that fracture can be suppressed by controlling the distribution of retained austenite in steel having a microstructure that is mainly composed of bainite and / or martensite and contains retained austenite.

[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 aspect of the present invention comprises, 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%, M o: 0-1.00%, Cu: 0-1.00%, 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: 0-1.0000%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, A steel sheet has a chemical composition consisting of 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 of, in area %, retained austenite: 3 to 10%, ferrite: 0 to 10%, pearlite: 0 to 5%, and martensite and bainite: 75 to 97% in total, and in a cross section parallel to the sheet thickness direction, an 80 μm × 80 μm range centered at the 1 / 4 thickness position in the sheet thickness direction and 80 μm in a direction perpendicular to the sheet thickness direction is divided into 16 20 μm × 20 μm regions, and when the area ratio of the retained austenite in each divided region is determined, the standard deviation of the area ratio of the retained austenite in each divided region is σ １ , the average is γ ａｖｅ Then, σ １ / γ ａｖｅ [2] The steel sheet according to [1] has a 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 ⅛ to ⅜ thickness centered at the ¼ thickness position of the cross section parallel to the sheet thickness direction. ２may be 0.80 or less. MS = [Mn] + (2 / 3) × [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 sheet including an unprocessed portion, wherein the unprocessed 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, and O: 0.0100%. Below, 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%, Sn: 0-1.00% , Sb: 0-0.50%, Nb: 0-0.200%, V: 0-1.00%, As: 0-0.10%, Zn: 0-1.0000%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0 The non-machined portion has a chemical composition consisting of: 0.0100%, Bi: 0-0.0100%, REM: 0-0.0100%, Hf: 0-0.0100%, and the balance: Fe and impurities; 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, consists, in area %, of retained austenite: 3-10%, ferrite: 0-10%, pearlite: 0-5%, and martensite and bainite: 75-97% in total; in a cross section parallel to the thickness direction, an 80 μm x 80 μm range centered at the 1 / 4 thickness position in the thickness direction and 80 μm in the direction perpendicular to the thickness direction is divided into 16 20 μm x 20 μm regions, and the area ratio of the retained austenite in each divided region is determined; the standard deviation of the area ratio of the retained austenite in each divided region is σ １ , the average is γ ａｖｅ Then, σ １ / γ ａｖｅis 0.20 or less. [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] has a 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-processed portion. ２may be 0.80 or less. MS=[Mn]+(2 / 3)×[Si] (1) [7] The steel member according to any one of [4] to [6] may have a galvanized layer or a galvannealed layer on the surface of the steel plate.[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, performing hot rolling including rough rolling and finish rolling, cooling the slab to a coiling temperature, and then coiling the slab at the coiling temperature to obtain a hot-rolled steel sheet; a cold rolling step of, as necessary, performing pickling on the hot-rolled steel sheet and cold rolling at a cumulative reduction rate of 30 to 75% to obtain a cold-rolled steel sheet; and a heat treatment step of performing heat treatment on the hot-rolled steel sheet or the cold-rolled steel sheet. In the hot rolling step, the heating temperature of the slab is 1200°C or higher, and the number of passes of the finish rolling is n, where n is the number of passes, and 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 of more than 25% in one pass is performed at least once, and the inter-pass times between the n-2th pass and the n-1th pass and between the n-1th pass and the n-th pass are 0.2 to 1.0 seconds, and the time from the completion of the nth pass to the start of the cooling is 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 includes a heating step, a first cooling step, a first holding step, a second cooling step, and a second holding step, and in the heating step, the hot-rolled steel sheet or the cold-rolled steel sheet is heated to a temperature range of Ac3-20°C to 950°C and held in the temperature range for 1 to 1000 seconds, and in the first cooling step, the hot-rolled steel sheet or the cold-rolled steel sheet after the heating step is cooled to an average cooling rate between 650°C to 550°C In the first holding step, the hot-rolled steel sheet or the cold-rolled steel sheet after the first cooling step is held in the first temperature range for 20 to 100 seconds, in the second cooling step, the hot-rolled steel sheet or the cold-rolled steel sheet after the first holding step is cooled to 200°C or less, and in the second holding step, the hot-rolled steel sheet or the cold-rolled steel sheet after the second cooling step is heated to a second temperature range of 200 to 420°C and held in the second temperature range so as to satisfy the following formulas (2) and (3): Here, t and t in the formula (2) and the formula (3) ｆ、 T 、 T ｍａｘ t indicates the elapsed time [seconds]. ｆ : Residence end time [seconds] T: Temperature at time t [K] T ｍａｘ [9] In the method for producing a steel plate described in [8], in the rough rolling, the slab may be subjected to three or more passes of reduction with a reduction ratio exceeding 20% ​​while the slab is in a state of being at 1050°C or higher, and in the continuous casting step, casting may be performed so as to satisfy the following formulas (4) and (5): Here, τ, τ, and T in the formula (4) and the formula (5) 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 (5). 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% CEquilibrium 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 surface of the cold-rolled steel sheet during the first cooling step or the second cooling step of the heat treatment step, between the first cooling step or the second cooling step and the first holding step or the second holding step, during the first holding step or the second holding step, or after the first holding step or the second holding step.

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

[10] may further include an alloying step of alloying the galvanized layer to form an alloyed 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] 1A and 1B are schematic diagrams for explaining the procedure for conducting a component axial crushing test, showing examples of the shape of a member with a closed cross-sectional structure, and showing examples of welds.

[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 the same 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.

[0012] <Steel Plate> The steel plate according to the present embodiment has a predetermined chemical composition, and a microstructure at a quarter thickness position is composed of, in area %, retained austenite: 3 to 10%, ferrite: 0 to 10%, pearlite: 0 to 5%, and martensite and bainite: 75 to 97% in total. In a cross section parallel to the thickness direction, an 80 μm×80 μm range having an 80 μm center at the quarter thickness position in the thickness direction and 80 μm in the direction perpendicular to the thickness direction is divided into 16 20 μm×20 μm regions, and the area ratio of the retained austenite in each divided region is calculated. The standard deviation of the area ratio of the retained austenite in each divided region is σ １ , the average is γ ａｖｅ Then, σ １ / γ ａｖｅ is 0.20 or less. 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 (an electrogalvanized layer, a hot-dip galvanized layer, or a galvannealed 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 serving as the reference for the quarter-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, if Al is contained in excess, the effect saturates, resulting in unnecessary increases in costs, and it may also raise 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: 0-1.00% Mo: 0-1.00% Cu: 0-1.00% Ni: 0-1.00% Co: 0-1.00% W: 0-1.00% Ta: 0-1.00% Sn: 0-1.00% Sb: 0-0.50% Nb: 0-0.200% V: 0~1.00% As: 0~0.10% Zn: 0~1.0000% 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 sheets, and one or more of these elements may be contained as needed. The content of each element may be 0.001% or more, 0.005% or more, or 0.010% or more. However, excessive inclusion of these elements saturates the effect and increases costs. Therefore, when these elements are contained, the contents of Cr, Mo, Cu, Ni, Co, W, Ta, Sn, and V are each 1.00% or less, the Zn content is 1.0000% or less, the Sb content is 0.50% or less, the Nb content is 0.200% or less, and the As content is 0.10% or less. The contents of Cr, Mo, Cu, Ni, Co, W, Ta, Sn, Sb, and V are preferably 0.40% or less, more preferably 0.30% or less, and even more preferably 0.20% or less. The Zn content is preferably 0.4000% or less, more preferably 0.3000% or less, and even more preferably 0.20000% 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 content and Si content of the entire steel sheet are set to the above ranges, and the standard deviation σ (σ in this embodiment) of the MS values ​​calculated by formula (1) from the Mn concentration [Mn] and Si concentration [Si] in mass% at each of a plurality of measurement points measured by EPMA is ２ ) is preferably 0.80 or less. MS=[Mn]+(2 / 3)×[Si] (1) Standard deviation σ of MS values ２ If the standard deviation of the MS value is small, fracture at the HAZ is suppressed when a component having spot welds is subjected to axial crushing deformation after forming. ２ The reason why fracture is more likely to occur when σ is large is presumed to be due to the following reasons: When welding, the HAZ portion is heated by the heat of welding. In the HAZ portion, there is a region where the temperature reached is the two-phase region (Ac1 to Ac3). Furthermore, because the cooling rate of the weld is extremely high, the austenite formed at this time becomes as-quenched martensite. In other words, in this region, the microstructure becomes a composite structure containing ferrite and martensite. Such a structure is brittle, so fracture is more likely to occur in axial crushing deformation. In this case, the greater the proportion of martensite containing a large amount of Si and Mn, the greater the embrittlement. The standard deviation σ of the MS value of the steel plate ２It is believed that the smaller the σ is, the smaller the proportion of martensite containing a large amount of Si and Mn in the HAZ, and therefore fracture is suppressed.

[0032] The standard deviation of the MS values ​​is determined by 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 measurement 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 thickness position to 3 / 8 thickness position), centered at the 1 / 4 thickness position, and [Mn] + (2 / 3) × [Si] is calculated from the Mn concentration [Mn] and Si concentration [Si] at each measurement position. By performing the above analysis for four fields of view, the value of [Mn] + (2 / 3) × [Si] is obtained at approximately 175,000 points. This is used as the population to calculate the standard deviation σ (σ ２ ) is calculated. For example, a JEOL JXA-8530F can be used as the FE-EPMA, 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 can be used for Mn, and TAP for Si. The output Si concentration and Mn concentration are values ​​converted to mass% from the detected intensity of the characteristic X-rays using a program attached to the JXA-8530, but this assumes that calibration has been performed 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) 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. 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] In the steel sheet according to the present embodiment, in a cross section parallel to the sheet thickness direction, an area of ​​80 μm×80 μm, which is 80 μm centered at the 1 / 4 thickness position in the sheet thickness direction and 80 μm in the direction perpendicular to the sheet thickness direction, is divided into 16 areas of 20 μm×20 μm, and the area ratio of the retained austenite in each divided area is calculated. The standard deviation of the area ratio of the retained austenite in each divided area is defined as σ １ , the average is γ ａｖｅ Then, σ １ / γ ａｖｅ The residual austenite is transformed into hard martensite by strain-induced transformation, thereby contributing to an increase in the amount of work hardening. １ / γ ａｖｅ The increase in σ means that the distribution of retained austenite becomes more uneven. When such a structure is subjected to axial crushing deformation, work hardening due to strain-induced transformation occurs non-uniformly, resulting in non-uniform hardness between the structures, making it prone to fracture during axial crushing deformation. １ / γ ａｖｅ is reduced to 0.20 or less. １ / γ ａｖｅ is preferably 0.15 or less, more preferably 0.10 or less.

[0040] σ １ / γ ａｖｅis measured by the following method. First, a sample is taken from a position at least 50 mm away from the end in the width direction, with the thickness cross section of the steel sheet as the observation surface. The cross section is preferably parallel to the rolling direction. The observation surface is mechanically polished and mirror-finished, and then electrolytically polished. Next, crystal structure and orientation analysis is performed by SEM-EBSD on an 80 μm x 80 μm area, which is 80 μm centered at 1 / 4 thickness of the steel sheet in the thickness direction on the observation surface and 80 μm in the direction perpendicular to the thickness direction. For example, a JSM-7200F manufactured by JEOL may be used as the FE-SEM. For example, "OIM Analysis 6.0" manufactured by TSL may be used to analyze data obtained by the EBSD method. The step distance (step) is 0.1 μm. Regions with an FCC crystal structure are considered to be retained austenite. The obtained 80 μm×80 μm area is divided into 16 20 μm×20 μm areas, and the area ratio of retained austenite in each divided area is measured. １ , average value γ ａｖｅ Find σ １ / γ ａｖｅ Calculate.

[0041] (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.

[0042] (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.

[0043] (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.

[0044] <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 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 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-mentioned steel plate. The steel member may be made of the above-mentioned steel plate. Furthermore, the surface of the steel plate constituting the steel member may have a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer. The steel member according to this embodiment (before spot welding) has the following characteristics in the non-processed portion. Examples of applications of the steel member include automotive parts such as front side members, rear side members, and side sills.

[0045] 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:

[0046] (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 unprocessed and unwelded portions, the mass % is: 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%, C It has a chemical composition consisting of: 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: 0-1.0000%, 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. In addition, in the non-processed and non-welded portions, the microstructure at a 1 / 4 thickness position, which is a position from the surface to 1 / 4 of the thickness (for example, the thickness of the steel plate constituting the member) in the thickness direction, is composed of, in area %, retained austenite: 3 to 10%, ferrite: 0 to 10%, pearlite: 0 to 5%, and martensite and bainite: 75 to 97% in total. In a cross section parallel to the plate thickness direction, an 80 μm x 80 μm range, which is 80 μm centered on the 1 / 4 thickness position in the thickness direction and 80 μm in the direction perpendicular to the plate thickness direction, is divided into 16 20 μm x 20 μm regions, and the area ratio of the retained austenite in each divided region is calculated. The standard deviation of the area ratio of the retained austenite in each divided region is σ １ , the average is γ ａｖｅ Then, σ １ / γ ａｖｅ The chemical composition and microstructure described above suppress fracture during axial crushing deformation.

[0047] The chemical composition of the non-processed and non-welded portions is basically unchanged from that of the steel plate according to this embodiment, and therefore the standard deviation σ of the 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 of the cross section in the thickness direction (in the thickness direction of the steel plate when the steel plate is included) is ２ may be 0.80 or less. MS=[Mn]+(2 / 3)×[Si] (1)

[0048] In the non-processed and non-welded parts, the standard deviation σ of the MS values ​​(σ ２ If the standard deviation σ of the MS value in the non-processed and non-welded portion of a steel member having a spot weld is 0.80 or less, fracture in the HAZ portion is suppressed when the steel member is subjected to axial crushing deformation after spot welding. ２ When the tensile strength is 0.80 or less, fracture in the HAZ is suppressed when subjected to axial crushing deformation.

[0049] Furthermore, the mechanical properties and thickness may be similar to those of the steel sheet according to the present embodiment described above.

[0050] 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.

[0051] (Spot Welds) There are no limitations on the spot welds, and they may be spot welds formed under normal welding conditions.

[0052] Chemical composition, microstructure, and σ of unprocessed and unwelded parts １ / γ ａｖｅThe standard deviation of the concentration distribution of Mn, Si, and Mn can be determined in the same manner as at the steel plate stage. Regarding the mechanical properties, if a JIS No. 5 tensile test piece can be taken from an unprocessed and unwelded part 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 described 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 is evaluated.

[0053] <Manufacturing Method> The steel sheet according to this embodiment and the steel member according to this embodiment can achieve the effects as long as they have the above-mentioned characteristics, regardless of the manufacturing method. However, the following method is preferable 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 in which a slab having a predetermined chemical composition is obtained by continuous casting; (II) a hot rolling step in which 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; (III) a cold rolling step in which, as necessary, the hot-rolled steel sheet is pickled and cold-rolled at a cumulative reduction of 30 to 75% to obtain a cold-rolled steel sheet; and (IV) a heat treatment step in which heat treatment is performed on 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 performed 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 performed 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; and (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.

[0054] (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.

[0055] 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 (4). C (τ) is calculated by equation (5).

[0056]

[0057]

[0058] Here, τ, τ, and T in the formula (4) and the formula (5) 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]

[0059] 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 (4) 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 (4) and (5), particularly controlling the value calculated by the left side of formula (4) 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 (4) 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 This means the time (seconds) at which the temperature reaches 0°C. Therefore, equation (4) can be understood as the time integral of the distribution of Mn from the solid phase to the liquid phase. Therefore, the smaller the value of the left side of equation (4), 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 equation (4) 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 equations (4) and (5) 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 (5). 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 (4) 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 (6) and (7) are used.

[0060] (Hot Rolling Process) In the hot rolling process, a 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. The upper limit of the heating temperature is not limited, but 1350°C or lower is preferable to suppress a decrease in yield due to scale-out. Furthermore, in order to homogenize the distribution of retained austenite in the final structure, when the number of passes of finish rolling is n, and the first pass is the first pass and the final pass is the n-th pass, the inlet temperature of the n-2th pass (i.e., the third pass from the end) is 950°C or higher, and the outlet temperature (steel sheet surface temperature) of the n-th pass (final pass) is 900°C or higher, and a reduction ratio of more than 25% per pass is performed at least once in the finish rolling. Furthermore, 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 set to 0.2 to 1.0 seconds, and the time from the completion of the n pass to the start of cooling is set to 1.0 to 3.0 seconds. The upper limits of the inlet and outlet temperatures are not limited, but the inlet and outlet temperatures may be 1100°C or lower. This increases the number of nucleation sites for phase transformation, resulting in a refined hot-rolled sheet structure and uniform dispersion of cementite, which serves as a source of retained austenite. If one or more of the inlet temperature of the n-2 pass (i.e., the third pass from the end), the outlet temperature of the n pass, the number of reductions in one pass exceeding a 25% reduction ratio, the inter-pass time between the final three passes, and the time from the completion of the final pass to the start of cooling are outside the above ranges, cementite will not be sufficiently uniformly dispersed. If cementite is present non-uniformly, the distribution of C in austenite becomes non-uniform during heating in the subsequent heat treatment process, resulting in the formation of austenite regions with locally low C concentrations. The martensite or bainite transformed from such austenite will have a low C concentration, resulting in a non-uniform distribution of retained austenite in the final structure. In the hot rolling process and subsequent processes, the controlled temperatures are all surface temperatures of the steel sheet unless otherwise specified.

[0061] 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, many structures other than martensite and bainite, such as ferrite and pearlite, are generated, 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. There is no need to particularly limit the upper limit of the average cooling rate between 600 and 750°C, but from an operational standpoint, it may be 200°C / s or less.

[0062] In addition, in the steel sheet obtained through the subsequent process, the standard deviation σ of the MS value ２ When the rough rolling temperature is set to 0.80 or less, it is preferable to perform at least three or more 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 times of rolling exceeding 20% ​​is not limited, but it may be set to seven or less. The upper limit of the rough rolling temperature is not particularly limited, but from an operational viewpoint, it may be set to 1250°C or less.

[0063] (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.

[0064] (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.

[0065] (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%.

[0066] (Heat Treatment Step) In the heat treatment step, a hot-rolled steel sheet (when a cold rolling step has not been performed) or a cold-rolled steel sheet (when a cold rolling step has been performed) (collectively sometimes simply referred to as a steel sheet) is subjected to heat treatment. The heat treatment step includes a heating step, a first cooling step, a first holding step, a second cooling step, and a second holding step. The microstructure can be controlled by the heat treatment.

[0067] (Heating Process) In the heating process, the hot-rolled steel sheet or cold-rolled steel sheet is heated to a temperature range of Ac3-20°C to 950°C and held at this temperature range for 1 to 1000 seconds. If the heating temperature (maximum heating temperature) is less than Ac3-20°C or the holding time is less than 1 second, austenite transformation does not occur sufficiently, and the specified microstructure cannot be obtained in the steel sheet after the final process. On the other hand, if the heating temperature exceeds 950°C or the holding time exceeds 1000 seconds, the austenite grain size increases excessively and toughness decreases.

[0068] (First Cooling Process) (First Holding Process) In the first cooling process, the hot-rolled or cold-rolled steel sheet after the heating process is cooled to a first cooling stop temperature so that the average cooling rate between 650 and 550°C is 10°C / s or more. The first cooling stop temperature at this time is set to be within the range of Ms-50°C to 550°C (first temperature region). Thereafter, in the first holding process, the hot-rolled or cold-rolled steel sheet after the first cooling process is held in the first temperature region for 20 to 100 seconds. In this process, it is sufficient for the steel sheet to remain in the temperature region of Ms-50°C to 550°C for 20 to 100 seconds, and it is not necessarily required to hold it at a constant temperature. Holding in the first temperature region partially promotes bainite transformation and forms a C-enriched region. This C-enriched region contributes to uniform distribution of retained austenite. If the average cooling rate between 650 and 550°C in the first cooling process is less than 10°C / s, a large amount of ferrite will be formed during cooling, making it impossible to obtain the desired microstructure. The upper limit does not need to be particularly limited, but from an operational standpoint, it may be set to 200°C / s or less. Furthermore, if the cooling stop temperature is less than Ms-50°C, which is lower than the first temperature range, martensite will be tempered in the subsequent plating process, resulting in a non-uniform distribution of retained austenite. On the other hand, if the cooling stop temperature exceeds 550°C, ferrite will be formed, making it difficult to obtain the desired microstructure. Furthermore, if the holding time in the first temperature range (the time the steel sheet dwells in the temperature range from Ms-50°C to 550°C) is less than 20 seconds, the effect of partially promoting bainite transformation will be insufficient, resulting in a non-uniform distribution of retained austenite. On the other hand, if the holding time exceeds 100 seconds, excessive bainite transformation will progress, resulting in a non-uniform distribution of retained austenite. 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] is the mass% content of each element in the steel plate.

[0069] (Second Cooling Process) (Second Holding Process) In the second cooling process, the hot-rolled or cold-rolled steel sheet after the first holding process is cooled to 200°C or below. The cooling rate at this time is not limited. Thereafter, in the second holding process, the hot-rolled or cold-rolled steel sheet after the second cooling process is heated to a second temperature range of 200 to 420°C and held in the second temperature range so as to satisfy the following formulas (2) and (3). In this process, untransformed austenite is stabilized and retained austenite is secured. If the cooling stop temperature exceeds 200°C, the generation of martensite in the steel sheet (i.e., the distribution of untransformed austenite) becomes non-uniform, resulting in σ １ / γ ａｖｅ is outside the desired range. There is no particular restriction on the lower limit of the cooling stop temperature, but it is practically difficult to set it below room temperature (20°C). Equation (2) is a parameter related to the stability of untransformed austenite. If this value does not satisfy the desired range, the stabilization of austenite will be insufficient, and a portion of the untransformed austenite will transform to martensite during cooling to room temperature, causing the volume fraction of retained austenite to not satisfy the desired range. If the second holding temperature is lower than 200°C or higher than 420°C, or if the middle part of Equation (2) is 9500 or less or 13500 or more, the amount of retained austenite will be below the predetermined value.

[0070]

[0071]

[0072] Here, t and t in the formulas (2) and (3) ｆ、 T 、 T ｍａｘ t indicates the elapsed time [seconds]. ｆ : Residence end time [seconds] T: Temperature at time t [K] T ｍａｘ : Maximum temperature reached during retention [K]

[0073] (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 of the above-mentioned heat treatment process are satisfied, the plating process may be carried out during the cooling process (first cooling process or second cooling process), between the cooling process and the holding process (first holding process or second holding process), during the holding process, or after the holding process.

[0074] (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. For example, when the plating step or the alloying step is performed between the cooling step and the holding step or after the holding step under conditions where the temperature of the steel sheet is 360 to 480°C, 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.

[0075] (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.

[0076] (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.

[0077] 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 (4) 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. The thickness of each sheet after cold rolling was 1.2 mm. The cold-rolled steel sheets were heat treated under the conditions shown in Table 2B. That is, the steel sheets were heated to the heating temperature shown in Table 2B and held there (heating step), then cooled to the cooling stop temperature (first cooling stop temperature) at the average cooling rate shown in Table 2B (first cooling step) and held at that temperature (first holding step) (here, the cooling stop temperature of the first cooling step = the holding temperature of the first holding step). Thereafter, cooling was performed at the average cooling rate shown in Table 2B (second cooling step). Thereafter, the steel sheets were further heated and held at that temperature (second holding step). The holding time in the heating step of the heat treatment step in the table is the time the steel sheets stayed at Ac3-20°C or higher. However, for No. 24 indicates the holding time at the maximum temperature (±10°C). The holding time in the holding step of the heat treatment step in the table indicates 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 in which 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 first holding step to 460°C and immersing it in a hot-dip galvanized bath. The immersion time was 3 seconds.The alloying treatment was carried out by immersing the specimen in a hot-dip galvanizing bath, heating the specimen to the temperature shown in Table 2, and maintaining the temperature for 20 seconds. In the column for product type in the table, CR indicates an example in which a galvanized layer was not formed, EG indicates an example in which an electrogalvanized layer was formed, GI indicates an example in which a hot-dip galvanized layer was formed, and GA indicates an example in which an alloyed hot-dip galvanized layer was formed.

[0078] The microstructure at the 1 / 4 thickness position of the obtained steel sheet, σ １ / γ ａｖｅ , the standard deviation σ of the MS values ​​in the range of 1 / 8 to 3 / 8 thickness centered at the 1 / 4 thickness position ２ was determined by the method described above. The FE-EPMA used was a JXA-8530F manufactured by JEOL, with an acceleration voltage of 15 kV. Data obtained by the EBSD method was analyzed using "OIM Analysis 6.0" manufactured by TSL. The results are shown in Table 3.

[0079] 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.

[0080] 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

[0081] 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.

[0082] A flat impactor was collided with the resulting steel member from the upper end (other end) while the lower end (one end) was fully restrained, as shown in Figure 2. The impactor weighed 334 kg, and the impact speed was 11.1 m / s. After the impact, the test specimen was observed, and the general parts (non-welded parts) and welded parts (HAZ parts) were visually checked for the presence or absence of cracks. Those that did not find any cracks were rated "○: GOOD", and those that found any cracks were rated "×: BAD". The results are shown in Table 3.

[0083]

[0084]

[0085]

[0086]

[0087] As can be seen from Tables 1 to 3, the alloys have a predetermined chemical composition and a microstructure at the quarter thickness position, and σ １ / γ ａｖｅ The steel members obtained using the steel plates having a standard deviation of MS value of 0.20 or less (which had the same characteristics as the steel plates in the non-welded and non-processed portions) had excellent fracture resistance properties during axial crushing deformation. ２ When the value of the tensile strength was 0.80 or less, the fracture resistance characteristics during axial crushing deformation in the HAZ were also excellent.

[0088] 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 1.0000%, 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 the cross section parallel to the thickness direction, an 80 μm×80 μm range having an 80 μm center at the 1 / 4 thickness position in the thickness direction and an 80 μm range in the direction perpendicular to the thickness direction is divided into 16 20 μm×20 μm regions, and the area ratio of the retained austenite in each divided region is calculated. The standard deviation of the area ratio of the retained austenite in each divided region is defined as σ １ , the average is γ ａｖｅ Then, σ １ / γ ａｖｅ is 0.20 or less.

2. 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 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 of the cross section parallel to the plate thickness direction. ２ The steel sheet according to claim 1, wherein the value of Mn 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 1.0000%, 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 martensite and bainite: 75 to 97% in total, and in a cross section parallel to the thickness direction, an 80 μm×80 μm range having an 80 μm center at the ¼ thickness position in the thickness direction and an 80 μm range in a direction perpendicular to the thickness direction is divided into 16 20 μm×20 μm regions, and when an area ratio of retained austenite in each divided region is determined, the standard deviation of the area ratio of retained austenite in each divided region is σ １ , the average is γ ａｖｅ Then, σ １ / γ ａｖｅ A steel member characterized in that:

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. In the non-processed portion, the standard deviation σ of the 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 ２ The steel member according to claim 4 or 5, wherein the formula MS=[Mn]+(2 / 3)×[Si] (1) is 0.80 or less.

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 ratio in one pass exceeding 25%, 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 first cooling step, a first holding step, a second cooling step, and a second holding step, and in the heating step, the hot-rolled steel sheet or the cold-rolled steel sheet is heated to a temperature range of Ac3-20°C to 950°C and held in the temperature range for 1 to 1000 seconds, In the first cooling step, the hot-rolled steel sheet or the cold-rolled steel sheet after the heating step is cooled to a first cooling stop temperature in a first temperature range of Ms-50°C to 550°C so that the average cooling rate from 650°C to 550°C is 10°C / s or more; in the first holding step, the hot-rolled steel sheet or the cold-rolled steel sheet after the first cooling step is held in the first temperature range for 20 to 100 seconds; in the second cooling step, the hot-rolled steel sheet or the cold-rolled steel sheet after the first holding step is cooled to 200°C or less;in the second holding step, the hot-rolled steel sheet or the cold-rolled steel sheet after the second cooling step is heated to a second temperature range of 200 to 420°C, and held in the second temperature range so as to satisfy the following formulas (2) and (3): Here, t and t in the formula (2) and the formula (3) ｆ、 T 、 T ｍａｘ t indicates the elapsed time [seconds]. ｆ : Residence end time [seconds] T: Temperature at time t [K] T ｍａｘ : Maximum temperature reached during retention [K] 9. A method for producing a steel plate according to claim 8, characterized in that the rough rolling involves performing three or more passes of reduction with a reduction rate exceeding 20% ​​while the slab is at 1050°C or higher, and the continuous casting step involves casting so as to satisfy the following formulas (4) and (5): Here, τ, τ, and T in the formula (4) and the formula (5) 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 (5). 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 surface of the cold-rolled steel sheet during the first cooling step or the second cooling step of the heat treatment step, between the first cooling step or the second cooling step and the first holding step or the second holding step, during the first holding step or the second holding step, or after the first holding step or the second 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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