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

Abstract

This steel plate has a predetermined chemical composition, and the microstructure at a 1 / 4 thickness position, which is a position from the surface in the plate thickness direction, is composed of, in area %, 3 to 10% retained austenite, 0 to 10% ferrite, 0 to 5% pearlite, and a total of 75 to 97% martensite and bainite, and in a surface layer portion located 50 to 100 μm from the surface in the plate thickness direction, the area ratio of prior austenite grains containing one or more retained austenite grains with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more is 30% or more.

Description

[Technical Field]

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

[0002] In recent years, there has been a demand for improved fuel efficiency in automobiles in light of greenhouse gas emission regulations as part of measures to combat global warming. To reduce the weight of automobile bodies and ensure collision safety, the use of high-strength steel sheets in automobile parts is becoming more widespread. Recently, there has been an increasing need for ultra-high-strength steel sheets with a tensile strength of 980 MPa or more. On the other hand, to make steel sheets into automobile parts (components), press forming, welding, etc. are performed. Therefore, steel sheets used for automobile parts are required to have not only strength but also various workability required for 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 tensile tests: 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, as shown in Patent Documents 1 to 4, for example, 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 components such as collision deformation components, by improving the properties of the steel sheet itself. However, although Patent Documents 1 to 4 take into consideration the formability of the steel sheet, there is room for further study regarding the occurrence of fracture when subjected to axial crushing deformation after being formed into a member. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2013 / 051238 [Patent Document 2] Japanese Patent Application Publication No. 2006-104532 [Patent Document 3] Japanese Patent Application Publication No. 2007-262494 [Patent Document 4] International Publication No. 2018 / 179386 Summary of the Invention [Problem to be solved by the invention]

[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 deformation component. Therefore, an object of the present invention is to provide a steel plate that has high strength and excellent elongation and that can suppress 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 that can suppress fracture when it is subjected to axial crushing deformation. [Means for solving the problem]

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

[0007] Furthermore, steel plates are formed into components through processing, and the components are then further welded (spot welded, etc.) to form separate components as needed. Even in such welded components, the welded zones and HAZs account for a small proportion of the component, so as long as the other parts (non-welded parts) have a structure that can prevent the component from breaking, the effect of preventing the component from breaking can be achieved. However, the heat of welding can cause deterioration in the HAZ, making it more susceptible to fracture. Therefore, by having a structure that can prevent fracture of the component even in the HAZ, a more preferable effect of preventing fracture of the component can be achieved. Therefore, the inventors also investigated the prevention of fracture during axial crush deformation at spot welds of components, and discovered that fracture at the HAZ of components can be prevented 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 light of the above findings. [1] A steel sheet according to one embodiment of the present invention comprises, by mass%, C: 0.08 to 0.20%, Si: 0.50 to 1.80%, Mn: 2.00 to 3.50%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0050%, N: 0.0100% or less, and O: 0.0100% or less. Bottom, Cr:0~1.00%, Mo:0~1.00%, Cu:0~1.00%, Ni:0~1.00%, Co:0~1.00%, W:0~1.00%, Ta:0~1.00%, S n:0~1.00%, Sb:0~0.50%, Nb:0~0.200%, V:0~1.00%, As:0~0.10%, Zn:0~1.000%, Ca:0~0.0100%, The steel sheet has a chemical composition consisting of Mg: 0-0.0100%, Zr: 0-0.0100%, Bi: 0-0.0100%, REM: 0-0.0100%, Hf: 0-0.0100%, and the balance: Fe and impurities. The microstructure at a 1 / 4 thickness position, which is a position from the surface to 1 / 4 of the sheet thickness in the sheet thickness direction, is composed, in area %, of retained austenite: 3-10%, ferrite: 0-10%, pearlite: 0-5%, and a total of 75-97% martensite and bainite. In a surface layer portion located 50-100 μm from the surface in the sheet thickness direction, the area fraction of prior austenite grains containing one or more retained austenite grains with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more is 30% or more. [2] In the steel sheet described in [1], the standard deviation σ of MS values ​​calculated by formula (1) from the Mn concentration [Mn] and the Si concentration [Si] in mass% at each of a plurality of measurement points measured by EPMA in a 35 μm × 50 μm region in the range of 1 / 8 to 3 / 8 thickness centered at the 1 / 4 thickness position on a cross section parallel to the sheet thickness direction and the rolling direction may 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 layer on the surface. [4] A steel member according to another aspect of the present invention has a steel plate including a non-machined portion, and 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: 0.010 0% or less, O: 0.0100% or less, 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:0~1.000%, Ca:0~ The unprocessed portion has a chemical composition consisting of 0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, Bi: 0-0.0100%, REM: 0-0.0100%, Hf: 0-0.0100%, and the balance: Fe and impurities, and the microstructure at a 1 / 4 thickness position, which is a position from the surface to 1 / 4 of the thickness in the thickness direction, is composed, in area %, of retained austenite: 3-10%, ferrite: 0-10%, pearlite: 0-5%, and a total of 75-97% martensite and bainite, and in a surface layer portion located 50-100 μm from the surface in the thickness direction, the area fraction of prior austenite grains containing one or more retained austenite grains with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more is 30% or more. [5] The steel member described in [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] In the steel member according to [4] or [5], in the non-machined portion, 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 area in the range of 1 / 8 to 3 / 8 thickness centered at the 1 / 4 thickness position may 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; and a cold rolling step of, if necessary, performing pickling and cold rolling at a cumulative reduction rate of 30 to 75% on the hot-rolled steel sheet 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 the number of passes of the finish rolling is n, where the first pass is the first pass and the final pass is the n-th pass, the inlet temperature of the n-2th pass is 950°C or higher and the outlet temperature of the n-th pass is 900°C or higher, and in the finish rolling, a reduction rate in one pass of more than 25% is performed at least once, and the n-2th pass is the inter-pass times between the n-th pass and the n-1th pass, and between the n-1th pass and the n-th pass, are 0.2 to 1.0 seconds, the time from the completion of the nth pass to the start of the cooling is 1.0 to 3.0 seconds, the coiling temperature is 200 to 550°C, and in the cooling to the coiling temperature, the average cooling rate between 600 and 750°C is 20°C / s or more, the heat treatment step has a heating step, a cooling step, and a holding step, and in the heating step, The cold-rolled steel sheet is heated to a first temperature range of Ac3-20°C to 950°C and held in the first temperature range for 1 to 1000 seconds, and in the cooling process, the hot-rolled steel sheet or the cold-rolled steel sheet after the heating process is cooled to a second temperature range of Ms-200°C to Ms-50°C at a cooling rate of 10°C / s or more between 500 and 650°C and an average cooling rate of 20°C / s or more between Ms-50 and Ms°C, and in the holding process, after the cooling process, the steel sheet is held at 360 to 480°C for 10 to 600 seconds. [9] [8] The method for producing a steel sheet according to the present invention may be such that, in the continuous casting step, casting is performed so as to satisfy the following formula (2), and, in the rough rolling, rolling is performed at least three times with a reduction rate exceeding 20% ​​while the steel sheet temperature is 1050°C or higher.

number

number

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

[11]

[10] The method for producing a steel sheet may further include an alloying step of alloying the hot-dip galvanized layer after the plating step to form an alloyed hot-dip galvanized layer. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0010] [Figure 1A] FIG. 1 is a schematic diagram for explaining the procedure for conducting a component axial crushing test, showing an example of the shape of a member having a closed cross-sectional structure. [Figure 1B] FIG. 1 is a schematic diagram for explaining the procedure for carrying out a component axial crushing test, showing an example of a welded portion. DETAILED DESCRIPTION OF THE INVENTION

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

[0012] <Steel plate> The steel sheet according to this embodiment has a predetermined chemical composition, and the microstructure at the 1 / 4 thickness position is composed of, in area %, 3 to 10% retained austenite, 0 to 10% ferrite, 0 to 5% pearlite, and at least one of martensite and bainite: a total of 75 to 97%, and in the surface layer portion, the area ratio of prior austenite grains containing one or more retained austenite grains with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more is 30% or more. 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 a steel sheet has a plating layer (electrogalvanized layer, hot-dip galvanized layer, or galvannealed hot-dip galvanized layer) (when the steel sheet is a plated steel sheet having a base material and a plating layer formed on the surface of the base material), the surface that serves as the reference for the surface layer and the 1 / 4 thickness position is the surface of the base material excluding the plating layer. In addition, the chemical composition of the steel sheet is 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 below. Unless otherwise specified, "%" relating to the content of each element constituting the chemical composition means "mass %."

[0014] C: 0.08 to 0.20% C (carbon) 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%, it is not possible to obtain the desired structure, 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 becomes embrittled, making fracture more likely to occur at the weld. 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%, and preferably, for example, 0.10 to 0.15%.

[0015] Si: 0.50 to 1.80% Silicon (Si) 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, deteriorating its fracture resistance during axial crushing deformation. Furthermore, the weld metal zone becomes embrittled, making fractures 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, and more preferably 1.40% or less. That is, the Si content is 0.50 to 1.80%, and preferably, for example, 0.80 to 1.40%.

[0016] Mn: 2.00 to 3.50% Manganese (Mn) is a strong austenite stabilizing element and is an element that is effective in improving the hardenability of steel sheet. In order 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 metal becomes embrittled, making fractures more likely to occur at the weld. 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 set to 0.045% or less, 0.035% or less, or 0.020% or less. The P content may be 0%, but reducing the P content too much 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 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 excessively reducing the S content increases the desulfurization cost, 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 aluminum (Al) is added to deoxidize the steel. The Al content may be 0.005% or more, 0.010% or more, or 0.020% or more. However, excessive Al content not only saturates the effect and leads to unnecessary increases in costs, but also raises the transformation temperature of the steel, increasing the load during hot rolling and potentially 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 dissolved N in steel as TiN and suppresses the formation of BN. To obtain this effect, the Ti content is set to 0.001% or more, and preferably 0.005% or more. On the other hand, if the Ti content is excessive, Ti carbides are formed in excess, resulting in a decrease in toughness. Therefore, the Ti content is set to 0.100% or less, and the Ti content is preferably set to 0.080% or less. That is, 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% Boron (B) 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%, the above effects are lost due to the formation of borides, and the hot workability is also reduced. 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 can form coarse nitrides in the steel, which can deteriorate 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, which can deteriorate bendability and hole expandability. Therefore, the O content is limited to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. The O content may be 0%, but from the viewpoint of manufacturing costs, the lower limit is preferably set to 0.0001%.

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

[0025] Cr: 0 to 1.00% Mo: 0 to 1.00% Cu: 0 to 1.00% Ni: 0 to 1.00% Co: 0 to 1.00% W: 0 to 1.00% Ta: 0 to 1.00% Sn: 0 to 1.00% Sb: 0 to 0.50% Nb: 0 to 0.200% V: 0 to 1.00% As: 0 to 0.10% Zn: 0 to 1.000% Cr (chromium), Mo (molybdenum), Cu (copper), Ni (nickel), Co (cobalt), W (tungsten), Ta (tantalum), Sn (tin), Sb (antimony), Nb (niobium), V (vanadium), As (arsenic), and Zn (zinc) are all elements effective in increasing the strength of steel 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. On the other hand, if these elements are contained in excess, the effects will saturate and costs will increase. Therefore, if these elements are contained, the contents of Cr, Mo, Cu, Ni, Co, W, Ta, Sn, V, and Zn should each be 1.000% or less, Sb should be 0.50% or less, Nb should be 0.200% or less, and As should be 0.10% or less. The contents of Cr, Mo, Cu, Ni, Co, W, Ta, Sn, Sb, V, and Zn are preferably 0.40% or less, more preferably 0.30% or less, and even more preferably 0.20% or less.

[0026] Ca: 0 to 0.0100% Mg: 0 to 0.0100% Zr: 0 to 0.0100% Bi: 0 to 0.0100% REM: 0 to 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 alloying elements such as Mn and Si in steel. Each of these elements contributes to improving the bendability of steel sheet. Therefore, they may be added as needed. In order to obtain the above effects, it is preferable to contain at least one selected from Ca, Mg, Bi, Zr and REM in an amount of at least 0.0001%, and more preferably at least 0.0010%. On the other hand, excessive content of these elements deteriorates elongation. Therefore, the contents of Ca, Mg, Bi, Zr, and REM are all set to 0.0100% or less. The contents of Ca, Mg, Bi, Zr, and REM are all preferably set to 0.0080% or less, and more preferably 0.0060% or less.

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

[0028] Hf: 0 to 0.0100% Hf is effective as a deoxidizing element, and therefore 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 sheet 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) of chips in accordance with JIS G 1201:2014. In this case, the chemical composition is the average content across the entire sheet thickness. C and S, which are difficult to measure using ICP-AES, may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method. The analytical sample is taken so as to obtain the average chemical composition across the entire thickness of the steel plate, as described in JIS G 0417: 1999. Specifically, the analytical sample is taken from the 1 / 4 thickness position, avoiding the ends in the width direction of the steel plate.

[0031] In the steel sheet according to this embodiment, it is preferable that the average Mn content and Si content of the entire steel sheet are within the above-mentioned ranges, and that the standard deviation σ of the MS value calculated by formula (1) from the Mn concentration [Mn] and Si concentration [Si] in mass% at each of multiple measurement points measured by EPMA is 0.80 or less. MS = [Mn] + (2 / 3) × [Si] (1) If the standard deviation σ 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. The reason why breakage is more likely to occur when the standard deviation σ of the MS value is large is presumed to be as follows. During welding, the HAZ temperature rises due to the heat of welding. Within the HAZ, there is a region where the temperature reaches 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. This type of structure is brittle, so fractures are more likely to occur during axial crushing deformation. In particular, the greater the proportion of martensite containing a large amount of Si and Mn, the greater the embrittlement. It is thought that the smaller the standard deviation σ of the MS value of a steel plate, the smaller the proportion of martensite containing a large amount of Si and Mn in the HAZ, which reduces fractures.

[0032] The standard deviation of the MS values ​​is determined by the following method. The concentration distribution of Si and Mn was measured using an electron probe microanalyzer (FE-EPMA). In a cross section parallel to the thickness direction, the Si and Mn concentrations were measured at 0.2 μm intervals in a 35 μm x 50 μm area in the range of 1 / 8 to 3 / 8 of the thickness from the surface of the steel plate in the thickness direction (range of 1 / 8 to 3 / 8 thickness), centered at the 1 / 4 thickness position. The Mn concentration [Mn] and Si concentration [Si] at each measurement position were used to calculate [Mn] + (2 / 3) × [Si]. By performing the above analysis for four fields of view, the value of [Mn] + (2 / 3) × [Si] was obtained at approximately 175,000 points. This was used as the population to calculate the standard deviation σ. The FE-EPMA used can be, for example, a JEOL JXA-8530F, with an acceleration voltage of 15 kV. The spectral method for characteristic X-rays is wavelength dispersive. The analyzing crystal can be selected appropriately depending on the element being analyzed; for example, LiF can be used for Mn, and TAP for Si. The output Si and Mn concentrations are values ​​converted to mass% from the detected intensity of characteristic X-rays using the program included with 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 the position from the surface to 1 / 4 of the sheet thickness 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% The retained austenite is a structure that contributes to improving elongation due to the TRIP effect, and therefore the area ratio of the retained austenite is set to 3% or more. On the other hand, if the area fraction of retained austenite is excessive, the grain size of the retained austenite becomes large. Such large-grain retained austenite becomes coarse and hard martensite after deformation by forming, etc. In this case, cracks are more likely to start and bendability deteriorates. For this reason, the area fraction of retained austenite is set to 10% or less.

[0035] Ferrite: 0 to 10% To prevent 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 ferrite. Therefore, in the steel sheet according to this embodiment, the microstructure is mainly composed of martensite and bainite, as described below. Therefore, the area ratio of ferrite is set to 10% or less. The area ratio of ferrite is preferably small, preferably 5% or less, more preferably 3% or less, and may be 0%.

[0036] Perlite: 0-5% Pearlite is a brittle structure that acts as a fracture origin, deteriorating 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: 75-97% in total Martensite and bainite are effective structures for increasing the strength of steel sheets. As described above, in order to suppress fracture during axial crushing deformation after forming into a component, it is necessary to make the structure as uniform as possible. Therefore, in the steel sheet according to this embodiment, the remaining structures 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 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 specify the area ratio of each (one of them 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 plate according to this embodiment is measured as follows. The structure fraction (area fraction) is evaluated using secondary electron images taken using an FE-SEM and X-ray diffraction. The FE-SEM used may be, for example, a JEOL JSM-7200F. First, a sample is taken from a thickness cross section (a cross section parallel to the thickness direction) of the steel sheet parallel to the rolling direction of the steel sheet at a position at least 50 mm away from the end in the width direction as the observation surface. The observation surface is mechanically polished to a mirror finish, and then etched using a nital solution. If the end in the width direction has already been cut and the width position of the steel sheet to be used as the raw material is unknown, a sample may be taken from a position at least 50 mm away from the end in the width direction of the steel sheet to be used as the raw material. Next, a total of 2.0 × 10 -9 m 2 A secondary electron image is taken for the above area. From the obtained secondary electron image, the area fractions of ferrite and pearlite are measured and regarded as the area ratio. There is no need to set an upper limit for the field of view area, but the larger the area, the greater the man-hours required for point counting. Therefore, a maximum of 1.0 x 10 -8 m 2 The following should be used as a guideline: the magnification should be 5000x and the field of view area should be 2.0 x 10 -9 m 2 Take the above number of photos. When identifying the structure, 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 into any of the above categories 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 regions judged to be bainite, martensite, or austenite. The area fractions of each structure are calculated using the point counting method. The point counting intervals are 2 μm both vertically and horizontally. The area fraction of retained austenite is measured by X-ray diffraction. That is, the steel plate is mechanically polished and chemically polished to remove a portion of the steel plate from the plate surface to a depth of 1 / 4 in the plate thickness direction. Then, the polished sample is subjected to MoKα1 radiation as characteristic X-rays. The structural fraction of retained austenite is calculated from the integrated intensity ratio of the diffraction peaks of (200), (211) of the bcc phase and (200), (220), and (311) of the fcc phase, and this is taken as the volume fraction of retained austenite. In the steel plate according to this embodiment, the volume fraction and the area fraction are considered to be equal, and the obtained volume fraction of retained austenite is taken as the area fraction of retained austenite. The rolling direction is self-evident when the 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 surface (presence or absence of edge drop, presence or absence of plating). If the above information is lost, the rolling direction can be determined using the following method. The Z-plane of the sheet (a surface parallel to both the longitudinal and transverse directions of the sheet) is polished to a quarter-thickness position and mirror-polished. An Mn concentration map of a 1000 μm × 1000 μm area is then obtained using an EPMA. When Mn solidification segregation is measured as streaks, the longitudinal direction of the streaks is determined to be the rolling direction. Furthermore, even when steel sheets are processed into parts, the rolling direction can be determined using the above method for lightly processed parts of the part (e.g., flat parts that have received relatively little processing).

[0039] [Surface layer] In the steel sheet according to this embodiment, the surface layer portion has an area ratio of 30% or more of prior austenite grains containing one or more retained austenite grains with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more. By setting the area ratio of prior austenite grains containing at least one retained austenite grain with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more to 30% or more, fracture during axial crushing deformation is suppressed. Retained austenite contributes to an increase in the amount of work hardening by transforming into hard martensite through stress-induced transformation. A decrease in the proportion of prior austenite grains containing retained austenite means that the retained austenite is unevenly distributed in specific prior austenite grains. When such a structure is subjected to axial crushing deformation, work hardening due to stress-induced transformation occurs non-uniformly, resulting in non-uniform hardness between the structures, making it prone to fracture during axial crushing deformation. Therefore, the proportion of crystal grains containing (containing) the specified retained austenite grains among the prior austenite grains is increased. Specifically, the area ratio of prior austenite grains containing one or more of the specified retained austenite grains as described above is set to 30% or more. If the area ratio is less than 30%, sufficient effects cannot be obtained. The area ratio is preferably 50% or more, more preferably 60% or more. There is no specified upper limit to the area ratio, and it may be 100%. In this embodiment, only the distribution of retained austenite with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more is considered. Retained austenite with an aspect ratio of less than 2.0 is particularly unstable to processing and is likely to undergo processing-induced transformation during the part forming stage before axial crushing deformation, so it does not affect axial crushing deformation. Therefore, it is excluded from the scope of the present invention. Furthermore, minute retained austenite with a long side length of less than 0.5 μm is excluded from the scope of the present invention because it is difficult to accurately detect it using the measurement method described below.

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

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

[0042] (mechanical properties) The steel sheet according to this embodiment is targeted to have a tensile strength (TS) of 980 MPa or more, which is a strength that contributes to reducing the weight of automobile bodies. There is no upper limit to the tensile strength, but if the tensile strength is too high, there is a risk of reduced formability, so the tensile strength may be set to 2000 MPa or less. In addition, the elongation (EL) is targeted to be 10.0% or more. Tensile strength (TS) and elongation (EL) are determined by taking a JIS No. 5 tensile test piece from the steel plate 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 can be conducted in any direction. If it is difficult to take a JIS No. 5 tensile test piece, a JIS No. 13B tensile test piece may also be used. If this is also difficult, any small test piece that is similar in shape to a JIS No. 13B tensile test piece may also be used. In this case, the gauge length can also be changed to match the scale of the test piece; for example, if a 1 / 2 similar shape is used, the gauge length should be 25 mm.

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

[0044] (Zinc plating layer) (galvannealed 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. For automotive steel sheets, there are cases where they cannot be thinned below a certain thickness even if they are made stronger due to concerns about holes due to corrosion. One of the purposes of increasing the strength of steel sheets is to reduce weight by making them thinner, so even if a high-strength steel sheet is developed, its application locations will be limited if its corrosion resistance is low. As a method for solving these problems, a highly corrosion-resistant zinc-plated layer may be formed on the surface of the steel sheet. The galvanized layer may be an electrogalvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer obtained by alloying a hot-dip galvanized layer. A hot-dip galvanized layer is preferable from the viewpoint of cost, and an alloyed hot-dip galvanized layer is preferable because it can provide excellent weldability and paintability due to the incorporation of Fe into the hot-dip galvanized layer by the alloying treatment. Furthermore, an upper layer of plating may be applied to a galvanized layer (electrogalvanized layer, hot-dip galvanized layer, or alloyed hot-dip galvanized 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 to the hot-dip galvanized layer. The galvanized layer and the galvannealed layer may be composed of Zn and Fe, but may also contain elements such as Al, Mg, and Si in addition to Zn and Fe. The coating weight of the galvanized layer is not particularly limited and may be a general coating weight. A general coating weight for automotive applications is, for example, 20 to 100 g / m per side. 2 is.

[0045] <Steel parts> 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 forming the steel plate and then joining it to another steel material by spot welding. A steel member obtained by forming (processing) into a predetermined shape has a steel plate including a processed portion and a non-processed portion. In addition, a steel member obtained by joining another steel member by spot welding has a steel plate including a spot weld, a HAZ portion 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 portion. Here, the non-machined portion is a flat portion of the steel member that is the thickest part of the flat portion, and the machined portion is a flat portion of the steel member that is thinner than the surrounding area or has a certain curvature. The non-welded portion is a portion other than the spot welded portion and the HAZ portion that has been affected by the heat of the spot weld. 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. Examples of applications of steel members for automobile parts include front side members, rear side members, and side sills.

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

[0047] (Non-processed and non-welded parts (non-processed parts if they are steel parts before spot welding)) The non-welded portions are not affected by spot welding and therefore have the same chemical composition as the steel plate according to this embodiment. Furthermore, although the microstructure changes due to forming, the non-processed portions of the non-welded portions that are not subject to processing strain are the same as the steel plate according to this embodiment. That is, in unprocessed and unwelded parts (unprocessed parts in the case of steel members before spot welding, the same applies below), the following contents are used by mass: C: 0.08 to 0.20%, Si: 0.50 to 1.80%, Mn: 2.00 to 3.50%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00% %, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, Ta: 0-1.00%, Sn: 0-1.00%, Sb: 0-0.50%, Nb: 0-0.200%, V: 0-1.00%, As: 0-0.10%, Zn: 0-1.000%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, Bi: 0-0.0100%, REM: 0-0.0100%, Hf: 0-0.0100%, and the balance: Fe and impurities. In addition, in the unprocessed and unwelded parts, the microstructure at a 1 / 4 thickness position, which is a position from the surface in the thickness direction (for example, the thickness of the steel plate if the steel plate is composed of steel plate), is composed of, in area %, 3 to 10% retained austenite, 0 to 10% ferrite, 0 to 5% pearlite, and a total of 75 to 97% martensite and bainite, and in the surface layer portion located 50 to 100 μm from the surface in the thickness direction, the area ratio of prior austenite grains containing one or more retained austenite grains with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more is 30% or more. By having such a chemical composition and microstructure, fracture during axial crushing deformation is suppressed.

[0048] The chemical composition of the non-processed and non-welded parts 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 area in the range of 1 / 8 to 3 / 8 thickness centered at the 1 / 4 thickness position on the cross section in the thickness direction (in the thickness direction of the steel plate if the steel plate is included) may be 0.80 or less. MS = [Mn] + (2 / 3) × [Si] (1)

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

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

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

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

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

[0054] <Manufacturing method> The steel plate and the steel member according to the present embodiment can obtain the effects as long as they have the above-mentioned characteristics, regardless of the manufacturing method. However, the following method is preferred because it allows stable manufacturing. The steel sheet according to this embodiment is obtained by a manufacturing method including the following steps (I) to (IV). (I) a continuous casting step for obtaining a slab having a predetermined chemical composition by continuous casting; (II) 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; (III) a cold rolling step of, as necessary, subjecting the hot-rolled steel sheet to pickling and cold rolling at a cumulative reduction rate of 30 to 75% to obtain a cold-rolled steel sheet; (IV) A heat treatment step of subjecting the hot-rolled steel sheet or the cold-rolled steel sheet to heat treatment. In addition, in the method for manufacturing 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 a plated steel sheet, the steel sheet (hot-rolled steel sheet or cold-rolled steel sheet) obtained by the above steps may be further subjected to one or both of the following steps. (V) A plating process in which a zinc plating layer is formed on the surface of steel sheets. (VI) An alloying process in which the hot-dip galvanized layer is converted into an alloyed hot-dip galvanized layer. 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 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 plate according to this embodiment into a predetermined shape. (VIII) A joining process in which the steel plate after the forming process is joined to another steel plate by spot welding. The preferred conditions for each step will be explained below.

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

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

[0057]

number

[0058]

number

[0059] Here, τ, τ1, and T in the formula (2) and the formula (3) S (τ), T C (τ), T L , f L , M L , M δ , S L , S δ , D δ Mn , D δ Si indicates the following, respectively. τ: Time elapsed since the start of casting [seconds] τ1:T C (τ) is the solidification completion temperature T in K δ Time it took 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 CEquilibrium volume fraction of the liquid phase at (τ) M L :Temperature T C Equilibrium Mn concentration in the liquid phase at (τ) [mass%] M δ :Temperature T C Equilibrium Mn concentration of δ at (τ) [mass%] S L :Temperature T C Equilibrium Si concentration in the liquid phase at (τ) [mass%] S δ :Temperature T C Equilibrium Si concentration of δ at (τ) [mass%] D δ Mn :Temperature T C (τ) Diffusion coefficient of Mn in the δ phase [m 2 / sec] D δ Si :Temperature T C (τ) is the diffusion coefficient of Si in the δ phase [m 2 / sec]

[0060] When continuously casting slabs from molten steel, the molten steel is initially liquid. However, it gradually solidifies from the surface until it completely solidifies at a certain temperature (i.e., the end-of-solidification temperature). Equation (2) defines the temperature range (partially solidified state) where 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 partitioned (extracted) from the solid phase to the liquid phase, resulting in the concentration of alloying elements in the liquid phase. This concentration of alloying elements increases the standard deviation σ of the MS value. More specifically, the greater the partitioning of Mn and Si between the solid and liquid phases, the more likely the final product will have locally high and low MS values. Therefore, it is important to adequately suppress the partitioning of Mn and Si between the solid and liquid phases in the temperature range where the liquid and solid phases coexist during continuous casting. In this regard, the present inventors have found that by satisfying formulas (2) and (3), in particular by controlling the value calculated by the left side of formula (2) to be less than 0.0100, the distribution of Mn and Si between the solid and liquid phases can be suppressed, and the standard deviation σ of the MS values ​​can be significantly reduced. Here, τ = 0 in equation (2) means the start of casting, and τ1 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) when the temperature reaches 1000 K. Therefore, equation (2) 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 (2), the more the distribution of Mn and Si from the solid phase to the liquid phase is suppressed. The smaller the value of the left side of equation (2) is, the more preferable it is, and specifically, it is preferably 0.0100 or less, and more preferably 0.0050 or less, and even more preferably 0.0030 or less. T in equations (2) and (3) L、 T S、 f L , M L , M δ , S L , S δ、 D δ Mn , D δ Si is a value determined by the temperature and the chemical composition of the molten steel. C(τ) is the internal temperature (center temperature) of the slab at time τ and is estimated from the surface temperature of the slab using equation (3). The surface temperature of the slab is measured at 1-second intervals. If it is difficult to measure the surface temperature at 1-second intervals, values ​​at 1-second intervals may be obtained by linear interpolation between measurement points. If the measurement interval is shorter than 1 second, data may be reshaped to 1-second intervals by thinning out the data. Therefore, a person skilled in the art can control the value calculated by the left side of equation (2) within a desired range by appropriately selecting the chemical composition of the molten steel and the temperature history during continuous casting. T L ,T δ ,f L ,M L ,M δ , S L , S δ can be obtained by performing a phase diagram calculation for the target steel composition using, for example, the 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 other phases are set to suspended. The equilibrium calculation is performed in one-axis mode with temperature as a variable. The calculation temperature range should include the 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 and Si concentrations in the liquid phase and the BCC_A2 phase are output as temperature variables, and the values ​​are used to calculate the Mn and Si concentrations at each temperature. L , S L , M δ , S δ The volumes of the LIQUID phase and the BCC_A2 phase are output as variables of temperature, and the volume of the LIQUID phase divided by the sum of the volumes of the LIQUID phase and the BCC_A2 phase is used as the f L The number of moles of the liquid phase is output, and the lowest temperature at which the number of moles becomes 1 is called T LThe highest temperature at which the number of moles becomes 0 is T δ Also, D δ Mn , D δ Si The values ​​calculated using the following formulas (4) and (5) are used.

[0061]

number

[0062]

number

[0063] (Hot rolling process) In the hot rolling process, the slab is heated to a heating temperature, subjected to hot rolling 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 set to 1200°C or higher. If the heating temperature is lower than 1200°C, the diffusion of the alloying elements will be insufficient. There is no upper limit to the heating temperature, but it is preferably 1350°C or lower to prevent a decrease in yield due to scale-out. Furthermore, to increase the proportion of prior austenite grains containing retained austenite grains, the number of passes in finish rolling is defined as n, with the first pass being the first pass and the final pass being the nth pass. The inlet temperature of the n-2th pass (i.e., the third pass counting from the final pass) is set to 950°C or higher, and the outlet temperature (steel sheet surface temperature) of the nth pass (final pass) is set to 900°C or higher. In finish rolling, a reduction ratio of more than 25% per pass is performed at least once. The inter-pass times between the n-2th pass and the n-1st pass, and between the n-1st pass and the nth pass, are set to 0.2 to 1.0 seconds, and the time from the completion of the nth 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 finer structure of the hot-rolled sheet, which allows cementite, which is the source of retained austenite, to be uniformly dispersed. If one or more of the following conditions are outside the above ranges: the inlet temperature of the n-2th pass (i.e., the third pass from the end), the outlet temperature of the nth pass, the number of reductions exceeding 25% in one pass, the interpass time between the final three passes, and the time from the end of the final pass to the start of cooling, cementite will not be dispersed uniformly. If cementite is present non-uniformly, the distribution of carbon in austenite will become non-uniform during heating in the subsequent heat treatment process, resulting in the formation of locally low-carbon austenite regions. The martensite or bainite transformed from such austenite will have a low carbon concentration, and austenite will no longer remain (contain retained austenite) within the prior austenite grains. In the hot rolling process and subsequent processes, the controlled temperatures are all surface temperatures of the steel sheet unless otherwise specified.

[0064] In addition, 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 that is primarily composed of martensite and bainite, with cementite being uniformly dispersed. If the average cooling rate between 600 and 750°C is less than 20°C / s, or if the coiling temperature exceeds 550°C, structures other than martensite and bainite, such as ferrite and pearlite, are produced in large quantities, and cementite is not uniformly dispersed. The coiling temperature is preferably 540°C or lower. Furthermore, if the coiling temperature is less than 200°C, uneven cooling occurs, which may cause the shape of the hot-rolled coil to be distorted, possibly hindering productivity. The upper limit of the average cooling rate does not need to be particularly limited, but from the viewpoint of operation, it may be set to 200° C. / s or less.

[0065] Furthermore, in order to make the standard deviation σ of the MS values ​​in the steel sheet obtained through the subsequent process 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 during rough rolling. The upper limit of the number of rolling processes exceeding 20% ​​is not limited, but it may be seven or less. The upper limit of the temperature for rough rolling is not particularly limited, but from an operational standpoint, it may be 1250°C or less.

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

[0067] (Hot-rolled sheet heat treatment process) In order to reduce the load on the cold rolling mill, the hot-rolled sheet may be subjected to a 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 reverse transformation during 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 heat treatment may be performed before or after pickling.

[0068] (Cold rolling process) In the cold rolling process, 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 process is not always necessary. If the reduction exceeds 75%, the load on the equipment increases, resulting in unnecessary increases in costs. On the other hand, if the reduction is less than 30%, the roughness of the rolling rolls is not sufficiently transferred to the steel sheet surface, resulting in inferior surface roughness compared to when the reduction exceeds 30%.

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

[0070] (Plating process) When forming an electrogalvanized layer on a steel sheet, a known method may be used. When forming a hot-dip galvanized layer on the surface of a steel sheet, a known plating process may be performed 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 performed at any stage. For example, as long as the conditions for the heat treatment process described above are satisfied, the plating process may be performed during the cooling process of the heat treatment process, between the cooling process and the holding process, during the holding process, or after the holding process.

[0071] (Alloying process) When the hot-dip galvanized layer is to be an alloyed hot-dip galvanized layer, an alloying step of alloying the hot-dip galvanized layer may be carried out after the plating step. The alloying step may be carried out 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 carried out 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 a plating step or an alloying step is carried out, the steps are carried out so as to satisfy the above conditions for the heat treatment step, even when the holding time at the predetermined temperature in these steps is included. That is, for example, when the plating process or alloying process is performed during the cooling process of the heat treatment process, even including the plating process and alloying process, the average cooling rate is controlled to be 10°C / s or more between 500 and 650°C up to the second temperature range of Ms-200°C to Ms-50°C, and 20°C / s or more between Ms-50 and Ms°C. Furthermore, for example, when the plating process or alloying process is performed under conditions where the temperature of the steel sheet is 360 to 480°C between the cooling process and the holding process, or after the holding process, the holding time in the holding process and the time for the steel sheet to reach 360 to 480°C in the plating process and alloying process are controlled so as not to exceed 600 seconds.

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

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

[0074] By continuous casting, slabs having the chemical compositions shown in Table 1 were obtained. During continuous casting, casting was carried out so that the value of the left side of the above formula (2) was as shown in Table 2A. Thereafter, the slabs were heated to the slab heating temperature shown in Table 2A, and hot rolling including rough rolling and finish rolling was carried out 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 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 sheet was 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. After pickling, the steel sheets were cold-rolled at 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 subjected to heat treatment under the conditions shown in Table 2B. The holding time in the heating process of the heat treatment process in the table is the time the steel sheet stayed at Ac3-20°C or higher. However, for No. 24, it is the holding time at the maximum temperature reached (±10°C). Also, the holding time in the holding process of the heat treatment process in the table is the time the steel sheet stayed at the holding temperature ±10°C. In some examples, a zinc plating 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 process and cooling to room temperature. The hot-dip galvanized layer was formed by reheating or cooling the steel sheet after the holding process to 460°C and then immersing it in a hot-dip galvanized bath. The immersion time was 3 seconds. The alloying treatment was performed by immersing the steel sheet in the hot-dip galvanized bath, heating it to the temperature listed in Table 2, and holding it for 20 seconds. In the product type column in the table, HR is an example of a hot-rolled steel sheet that did not have a zinc-plated layer, CR is an example of a cold-rolled steel sheet that did not have a zinc-plated layer, EG is an example of a sheet that had an electro-galvanized layer, GI is an example of a sheet that had a hot-dip galvanized layer, and GA is an example of a sheet that had an alloyed hot-dip galvanized layer.

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

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

[0077] Furthermore, using the obtained steel plate as the raw material, a hat-shaped formed body with a bent ridge of R = 5 mm and a flat plate that would serve as the backing plate were cut and joined by spot welding to create a closed cross-section structure component with the shape shown in Figure 1A. As shown in Figure 1B, the spacing between the weld points was 15 mm, and the welding current was set so 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 supply: 50Hz single-phase AC Pressure: 400kgf Energizing time: 20cyc Hold time: 5 cycles

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

[0079] With the resulting steel member fully restrained at the bottom, a flat impactor was collided with it from the top, 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) were visually checked for cracks. Those that did not find any cracks were rated "○: GOOD", and those that found cracks were rated "×: BAD". The results are shown in Table 3.

[0080] [Table 1]

[0081] [Table 2A]

[0082] [Table 2B]

[0083] [Table 3]

[0084] As can be seen from Tables 1 to 3, steel members (which had the same characteristics as the steel plate in non-welded and non-processed parts) obtained using steel plates having a predetermined chemical composition and a microstructure at the 1 / 4 thickness position, and in which the area ratio of prior austenite grains in the surface layer, which contained at least one retained austenite grain with a long side length of 0.5 μm or more and an aspect ratio of 2.0 or more, was 30% or more, had excellent fracture resistance properties during axial crushing deformation. Furthermore, when the standard deviation σ of the MS value was 0.80 or less, the fracture resistance properties during axial crushing deformation in the HAZ were also excellent. [Industrial Applicability]

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

Claims

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

2. The standard deviation σ of the MS value calculated by formula (1) from the Mn concentration [Mn] and the Si concentration [Si] in mass% at each measurement point 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 and the rolling direction is 0.80 or less. The steel sheet according to claim 1 , characterized in that MS=[Mn]+(2 / 3)×[Si]...(1)

3. The surface has a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer. The steel sheet according to claim 1 or 2, characterized in that

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

5. The steel plate includes a spot weld, a HAZ around the spot weld, and a non-welded portion other than the spot weld and the HAZ, The non-processed portion is present in the non-welded portion. The steel member according to claim 4 , characterized in that it is

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

7. The surface of the steel sheet has a zinc-plated layer or a galvannealed layer, The steel member according to claim 4 or 5, characterized in that it is

8. The method for producing a steel sheet according to claim 1, a continuous casting step for obtaining a slab having the chemical composition according to 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, 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, if necessary, subjecting the hot-rolled steel sheet to pickling and cold rolling at a cumulative reduction rate of 30 to 75% to obtain a cold-rolled steel sheet; a heat treatment step of performing heat treatment on the hot-rolled steel sheet or the cold-rolled steel sheet; and In the hot rolling step, The heating temperature of the slab is 1200°C or higher, When the number of passes of the finish rolling is n, the first pass is the first pass, and the final pass is 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, In the finish rolling, a rolling reduction rate of more than 25% in one pass is performed at least once, 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 nth pass to the start of the cooling is 1.0 to 3.0 seconds; The coiling temperature is 200 to 550°C, 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 cooling step, and a holding step, In the heating process, the hot-rolled steel sheet or the cold-rolled steel sheet is heated to a first temperature range of Ac3-20 ° C to 950 ° C and held in the first temperature range for 1 to 1000 seconds, In the cooling process, the hot-rolled steel sheet or the cold-rolled steel sheet after the heating process is cooled to a second temperature range of Ms-200°C to Ms-50°C at a cooling rate of 10°C / s or more between 500 and 650°C and an average cooling rate of 20°C / s or more between Ms-50 and Ms°C, In the holding step, after the cooling step, the material is held at 360 to 480°C for 10 to 600 seconds. A method for manufacturing a steel sheet, comprising:

9. In the continuous casting step, casting is performed so as to satisfy the following formula (2): In the rough rolling, rolling is performed at least three times with a reduction ratio exceeding 20% ​​at a steel sheet temperature of 1050°C or higher. The method for manufacturing a steel sheet according to claim 8 . [Equation 1] [Equation 2] Here, τ and τ in the formula (2) and the formula (3) 1 , T S (τ), T C (τ), T L , f L , M L , M δ , S L , S δ , D δ Mn , D δ Si and T respectively indicate the following: C (τ) is calculated by the above formula (3). τ: Time elapsed since the start of casting, in seconds τ 1 : T in seconds C (τ) is the solidification completion temperature T in units of K δ The time it takes to reach T S (τ): Slab surface temperature at time τ in K T C (τ): Estimated internal slab temperature at time τ in K T L : solidification start temperature in K f L :Temperature T C Equilibrium volume fraction of the liquid phase at (τ) M L : Temperature T in mass% C Equilibrium Mn concentration in the liquid phase at (τ) M δ : Temperature T in mass% C Equilibrium Mn concentration in the δ phase at (τ) 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 the δ phase 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 further includes a plating step of forming a zinc plating layer on the surface of the hot-rolled steel sheet or the cold-rolled steel sheet during the cooling step of the heat treatment step, between the cooling step and the holding step, during the holding step, or after the holding step. The method for manufacturing a steel sheet according to claim 8 or 9.

11. The zinc-plated layer is a hot-dip galvanized layer, Further, after the plating step, an alloying step is provided in which the hot-dip galvanized layer is alloyed to form an alloyed hot-dip galvanized layer. The method for manufacturing a steel sheet according to claim 10.

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