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 to 1 / 4 of the plate thickness in the plate 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 plate thickness direction, an 80 μm × 80 μm range, which is 80 μm centered at the 1 / 4 thickness position in the plate thickness direction and 80 μm in a direction perpendicular to the plate 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 defined as σ1 and the average is defined as γ ave Then, σ1 / γ ave is less than 0.20.

Description

Technical Field

[0001] The present invention relates to a steel sheet, a steel member, and a method for manufacturing a steel sheet. This application claims priority based on Japanese Patent Application No. 2024-053452 filed in Japan on March 28, 2024, and incorporates its content herein by reference.

Background Art

[0002] In recent years, from the perspective of regulations on greenhouse gas emissions associated with measures to combat global warming, improvement of the fuel efficiency of automobiles has been demanded. In order to reduce the weight of the vehicle body and ensure collision safety, the application of high-strength steel sheets to automotive parts is increasingly expanding. Recently, the need for ultra-high-strength steel sheets with a tensile strength of 980 MPa or more has been increasing. On the other hand, in order to use a steel sheet as an automotive part (member), press forming, welding, etc. are performed. Therefore, for the steel sheet used for automotive parts, not only strength but also various workabilities required during part forming, such as press formability and weldability, are required. For example, from the perspective of press formability, the steel sheet often requires excellent elongation (total elongation in a tensile test: EL), etc. However, with the increase in the strength of the steel sheet, the total elongation: EL tends to decrease, and it becomes difficult to ensure both strength and total elongation at a high level simultaneously. In contrast, for example, as shown in Patent Documents 1 to 4, a TRIP steel sheet (Transformation Induced Plasticity) that utilizes the transformation-induced plasticity of retained austenite and achieves both high strength and workability is known.

[0003] However, in recent years, there has been a demand for further suppression of fracture when the member such as a collision deformation member is subjected to axial crushing deformation after being formed. This requires improvement of the characteristics of the steel sheet itself to further suppress fracture. However, in Patent Documents 1 to 4, although the formability of the steel sheet is considered, there is still room for further examination regarding the occurrence of fracture when the member is subjected to axial crushing deformation after being formed.

Prior Art Documents

Patent Document

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0005] As described above, conventionally, there has been room for further study regarding the suppression of fracture when a steel plate is formed into a member such as a collision deformation member and then subjected to axial crushing deformation. Therefore, an object of the present invention is to provide a steel plate, a method for manufacturing the same, and a steel member obtained by using this steel plate, which have high strength and excellent elongation and can suppress fracture of the member when subjected to axial crushing deformation after being formed into a member.

Means for Solving the Problems

[0006] The present inventors examined whether fracture during axial crushing deformation of a member can be suppressed by the configuration (chemical composition, microstructure, etc.) of the steel plate. As a result, it has been found that fracture is suppressed by controlling the distribution state of retained austenite in steel having a microstructure mainly composed of bainite and / or martensite and containing retained austenite.

[0007] In addition, the steel plate is formed into a member by processing, and the member is further joined to another member by welding (such as spot welding) as required. Even for such a member after welding, since the proportion of the welded portion and the HAZ portion in the member is small, if the non-welded portion has a configuration capable of suppressing breakage of the member, an effect of suppressing breakage of the member can be obtained. However, the HAZ portion may be altered by the heat of welding and may be prone to breakage. Therefore, by having a configuration capable of suppressing breakage of the member even in the HAZ portion, a more preferable effect of suppressing breakage of the member can be obtained. Therefore, the inventors of the present invention also studied the suppression of breakage during axial crushing deformation at the spot welded portion of the member. As a result, it was found that by controlling the concentration distribution of Mn and Si at the stage of the steel plate used as the material, breakage at the HAZ portion of the member can be suppressed.

[0008] The present invention has been made in view of the above findings. The gist of the present invention is as follows. [1] The steel sheet according to one aspect of the present invention has, in mass %, C: 0.08 to 0.20%, Si: 0.50 to 1.80%, Mn: 2.00 to 3.50%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0050%, N: 0.0100% or less, 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%, 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 has a microstructure at the 1 / 4 thickness position, which is 1 / 4 of the plate thickness from the surface in the plate thickness direction, consisting of, in area %, retained austenite: 3 to 10%, ferrite: 0 to 10%, pearlite: 0 to 5%, and martensite and bainite: a total of 75 to 97%. In the cross section parallel to the plate thickness direction, an 80 μm × 80 μm range, which is 80 μm in the plate thickness direction centered on the 1 / 4 thickness position and 80 μm in the direction perpendicular to the plate thickness direction, is divided into 16 parts of 20 μm × 20 μm. When the area ratio of retained austenite is determined in each divided region, the standard deviation of the area ratio of retained austenite in each of the divided regions is σ1, and the average is γ ave When it is ave σ1 / γ [2] The steel sheet according to [1] may have a standard deviation σ2 of the MS value calculated by formula (1) from the concentration [Mn] of Mn and the concentration [Si] of Si in mass % at each measurement point of a plurality of measurement points measured by EPMA in a region of 35 μm × 50 μm in the range of 1 / 8 to 3 / 8 of the plate thickness centered on the 1 / 4 thickness position of the cross section parallel to the plate thickness direction, which is 0.80 or less. MS = [Mn] + (2 / 3) × [Si] ···(1) The steel sheet according to [3], [1] or [2] may have a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface. [4] A steel member according to another aspect of the present invention has a steel sheet including a non-processed portion, and the non-processed portion has, in mass%, C: 0.08 to 0.20%, Si: 0.50 to 1.80%, Mn: 2.00 to 3.50%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0050%, N: 0.0100% or less, 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%, 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 the 1 / 4 thickness position, which is the position of 1 / 4 of the thickness in the thickness direction from the surface of the non-processed portion, has, 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, which is 80 μm in the thickness direction centered on the 1 / 4 thickness position and 80 μm in the direction perpendicular to the thickness direction, is divided into 16 parts of 20 μm × 20 μm, and when the area ratio of retained austenite is determined in each divided region, the standard deviation of the area ratio of retained austenite in each divided region is σ1, and the average is γ ave When it is ave σ1 / γ [5] The steel member according to [4] includes a steel sheet having a spot weld portion, a HAZ portion around the spot weld portion, and a non-welded portion other than the spot weld portion and the HAZ portion, and the non-processed portion may be present in the non-welded portion. The steel member according to [6], [4], or [5] may have a standard deviation σ2 of the MS value calculated by formula (1) at each of a plurality of measurement points measured by EPMA in a region of 35 μm × 50 μm in the range of 1 / 8 to 3 / 8 of the thickness centered on the 1 / 4 thickness position in the non-processed portion, where the MS value is calculated from the Mn concentration [Mn] and the Si concentration [Si] at each measurement point. MS = [Mn] + (2 / 3) × [Si] ···(1) [7] The steel member according to any one of [4] to [6] may have a zinc plating layer or an alloyed hot-dip zinc plating layer on the surface of the steel sheet. [8] The manufacturing method of the steel sheet according to another aspect of the present invention is the manufacturing method of the steel sheet described in [1], comprising a continuous casting step of obtaining a slab having the chemical composition described in [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 to a coiling temperature, and then coiling at the coiling temperature to obtain a hot-rolled steel sheet, an optional cold rolling step of performing pickling and cold rolling with 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 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. When the number of passes of the finish rolling is n, the first pass is the first pass, and the final pass is the nth pass, the entry temperature of the (n - 2)th pass is 950 °C or higher, the exit temperature of the nth pass is 900 °C or higher. In the finish rolling, rolling with a reduction rate exceeding 25% per pass is performed at least once. The time between each pass between the (n - 2)th pass and the (n - 1)th pass, and between the (n - 1)th pass and the nth pass is 0.2 to 1.0 second. 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, the average cooling rate between 600 and 750 °C is 20 °C / s or higher. The heat treatment step includes a heating process, a first cooling process, a first holding process, a second cooling process, and a second holding process. In the heating process, 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 this temperature range for 1 to 1000 seconds. In the first cooling process, the hot-rolled steel sheet or the cold-rolled steel sheet after the heating process is cooled to a first cooling stop temperature in a first temperature range of Ms - 50 °C to 550 °C such that the average cooling rate between 650 °C and 550 °C is 10 °C / s or higher. In the first holding process, the hot-rolled steel sheet or the cold-rolled steel sheet after the first cooling process is held in the first temperature range for 20 to 100 seconds. In the second cooling process, the hot-rolled steel sheet or the cold-rolled steel sheet after the first holding process is cooled to 200 °C or lower. In the second holding process, the hot-rolled steel sheet or the 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). [Number] [Number] Here, in the formulas (2) and (3), t and t f、 T 、 T max respectively represent the following. t: Elapsed time [seconds] t f : End time of residence [seconds] T: Temperature at time t [K] T max : Maximum temperature reached during residence [K] In the method for manufacturing a steel plate described in [9] [8], in the rough rolling, the slab is in a state of 1050 °C or higher, and rolling with a reduction ratio exceeding 20% is performed in 3 or more passes. In the continuous casting process, casting may be performed so as to satisfy the following formulas (4) and (5). [Number] [Number] Here, in the formulas (4) and (5), τ, τ1, T S (τ), T C (τ), T L , f L , M L , M δ , S L , S δ , D δ Mn , D δ Si respectively represent the following. T C (τ) is obtained by the formula (5). τ: Elapsed time from the start of casting in seconds τ1: T in seconds C (τ) is the time when the solidification completion temperature T in units of K δ is reached T S (τ): Slab surface temperature at time τ in units of K TC (τ): The estimated internal temperature of the slab at time τ, with the unit of K T L : The solidification start temperature, with the unit of K f L : The equilibrium volume fraction of the liquid phase at temperature T C (τ) M L : The equilibrium Mn concentration in the liquid phase at temperature T, with the unit of mass% C (τ) M δ : The equilibrium Mn concentration of δ at temperature T, with the unit of mass% C (τ) S L : The equilibrium Si concentration in the liquid phase at temperature T, with the unit of mass% C (τ) S δ : The equilibrium Si concentration of δ at temperature T, with the unit of mass% C (τ) D δ Mn : With the unit of m 2 / s, the diffusion coefficient of Mn in the δ phase at temperature T C (τ) D δ Si : With the unit of m 2 / s, the diffusion coefficient of Si in the δ phase at temperature T C (τ) The method for manufacturing a steel sheet according to

[10] [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 first cooling process or the second cooling process in the heat treatment step, between the first cooling process or the second cooling process and the first holding process or the second holding process, during the first holding process or the second holding process, or after the first holding process or the second holding process. The method for manufacturing a steel sheet according to

[11]

[10] is such that the zinc plating layer is a hot-dip zinc plating layer, and may further include an alloying step of alloying the hot-dip zinc plating layer to form an alloyed hot-dip zinc plating layer after the plating step.

Advantages of the Invention

[0009] According to the above aspect of the present invention, it is possible to provide a steel plate, a method for manufacturing the same, and a steel member obtained by using the steel plate, which have high strength and excellent elongation and can suppress breakage of the member when it undergoes axial crushing deformation after being formed into a member.

Brief Description of the Drawings

[0010]

Figure 1A

Figure 1B

Embodiments for Carrying Out the Invention

[0011] A steel plate according to an embodiment of the present invention (the steel plate according to this embodiment), a steel member obtained by using the steel plate (the steel member according to this embodiment), and a manufacturing method thereof will be described. In this embodiment, for the steel plate, the position at 1 / 4 of the plate thickness in the plate thickness direction from the surface is described as the 1 / 4 thickness position. In the case of a steel member, the position at 1 / 4 of the thickness in the thickness direction of the steel member from the surface (for example, the plate thickness direction of the steel plate constituting the steel member) is described as the 1 / 4 thickness position.

[0012] <Steel plate> The steel plate according to this embodiment has a predetermined chemical composition, and the microstructure at the 1 / 4 thickness position consists of, in area %, retained austenite: 3 to 10%, ferrite: 0 to 10%, pearlite: 0 to 5%, and martensite and bainite: a total of 75 to 97%. In a cross-section parallel to the plate thickness direction, an 80 μm × 80 μm range, which is 80 μm in the plate thickness direction centered on the 1 / 4 thickness position and 80 μm in the direction perpendicular to the plate thickness direction, is divided into 16 parts of 20 μm × 20 μm. When the area ratio of retained austenite is obtained in each divided region, the standard deviation of the area ratio of retained austenite in each of the above divided regions is σ1, and the average is γ ave When it is set as, σ1 / γ aveis 0.20 or less. The steel sheet according to the present embodiment may be a hot-rolled steel sheet or a cold-rolled steel sheet, and further, an electro-galvanized layer, a hot-dip galvanized layer or an alloyed hot-dip galvanized layer may be formed on the surface (that is, it may be an electro-galvanized steel sheet (EG), a hot-dip galvanized steel sheet (GI), or an alloyed hot-dip galvanized steel sheet (GA)). When the steel sheet has a plating layer (electro-galvanized layer, hot-dip galvanized layer or alloyed hot-dip galvanized layer) (when it is a plated steel sheet having a plating layer formed on the surface of the base material part and the base material part), the surface serving as the reference at the 1 / 4 thickness position is the surface of the base material part excluding the plating layer. Also, the chemical composition of the steel sheet is the chemical composition of the base material part excluding the plating layer. Hereinafter, each will be described.

[0013] (Chemical composition) The reason for limiting the chemical composition of the steel sheet according to the present embodiment will be described. “%” regarding the content of each element constituting the chemical composition means “mass %” unless otherwise specified.

[0014] C: 0.08 to 0.20% C (carbon) is an essential element that contributes to the formation of martensite and bainite that contribute to the high strength of the steel sheet. If the C content is less than 0.08%, a predetermined structure 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, when the C content exceeds 0.20%, the toughness of the material decreases and the fracture resistance characteristics during axial crushing deformation deteriorate. Also, since the weld metal part becomes brittle, fracture at the welded part is 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%, and preferably, for example, 0.10 to 0.15%.

[0015] Si: 0.50 to 1.80% Si (silicon) is a solid solution strengthening element and is effective for increasing the strength of the steel sheet. It is also an element that increases the amount of retained austenite by suppressing the formation of iron carbide. To obtain this effect, the Si content should be 0.50% or more. The Si content is preferably 0.80% or more. On the other hand, when the Si content becomes excessive, the chemical conversion treatment property of the steel sheet and the wettability with hot-dip galvanizing deteriorate significantly. Also, the toughness of the material decreases, and the fracture resistance characteristics during axial crushing deformation deteriorate. Further, since the weld metal part becomes brittle, fracture at the welded part is 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 - 1.80%, preferably, for example, 0.80 - 1.40%.

[0016] Mn: 2.00 - 3.50% Mn (manganese) is a strong austenite stabilizing element and is effective for improving the hardenability of the steel sheet. To enhance the 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, when the Mn content is excessively contained, the toughness of the material decreases, and the fracture resistance characteristics during axial crushing deformation deteriorate. Also, since the weld metal part becomes brittle, fracture at the welded part is likely to occur. Therefore, the Mn content is set to 3.50% or less. The Mn content is preferably 3.20% or less, more preferably 3.00% or less. That is, the Mn content is 2.00 - 3.50%, preferably, for example, 2.20 - 3.00%.

[0017] P: 0.050% or less P (phosphorus) is a solid solution strengthening element and is effective for increasing the strength of the steel sheet. However, if it is contained in excess, it is an element that deteriorates weldability and toughness. Therefore, the P content should be 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 to extremely reduce the P content, the cost of dephosphorization increases. Therefore, from the perspective of economy, it is preferable to set the lower limit at 0.001%.

[0018] S: 0.0100% or less S (sulfur) is an element contained as an impurity and is an element that forms MnS in the steel and deteriorates toughness and hole expansion properties. Therefore, the S content is set to 0.0100% or less within the range where the deterioration of toughness and hole expansion properties 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 to extremely reduce the S content, the cost of desulfurization increases. Therefore, from the perspective of economy, it is preferable to set the lower limit at 0.0001%.

[0019] Al: 0.001 - 1.000% Al (aluminum) is contained at least 0.001% for deoxidation of 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, not only does the effect saturate and only cause a cost increase, but it also raises the transformation temperature of the steel and increases the load during hot rolling, and as a result, the mechanical properties of the steel sheet may be deteriorated. Therefore, the Al content should be 1.000% or less. The Al content may be 0.800% or less, 0.600% or less, or 0.300% or less.

[0020] Ti: 0.001 - 0.100% Ti (titanium) is an element that fixes the dissolved N in the steel as TiN and suppresses the formation of BN. To obtain this effect, the Ti content should be 0.001% or more. The Ti content is preferably 0.005% or more. On the one hand, when the Ti content becomes excessive, excessive Ti carbide is generated, resulting in a decrease in toughness. Therefore, the Ti content should be 0.100% or less. It is preferably 0.080% or less. That is, the Ti content is 0.001 to 0.100%, preferably, for example, 0.005 to 0.080%.

[0021] B: 0.0005 to 0.0050% B (boron) is an element that has the effect of increasing the hardenability of steel and reducing the ferrite area ratio by segregating at the austenite grain boundaries. To obtain this effect, the B content should be 0.0005% or more. It is more preferably 0.0008% or more. On the other hand, when the B content exceeds 0.0050%, the above effect is lost by forming borides, and the hot workability deteriorates. Therefore, the B content should be 0.0050% or less. The B content is preferably 0.0035% or less. That is, the B content is 0.0005 to 0.0050%, preferably, for example, 0.0008 to 0.0035%.

[0022] N: 0.0100% or less N (nitrogen) is an element contained as an impurity. When its content is high, coarse nitrides may be formed in the steel, deteriorating the bendability and hole expansion property. 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 to extremely reduce the N content, the denitrification cost becomes high. Therefore, from the perspective of economy, it is preferable to set the lower limit 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 the bendability and hole expansion properties. 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 perspective of manufacturing cost, it is preferable to set the lower limit at 0.0001%.

[0024] The steel sheet according to this embodiment contains the above elements, and the balance may be Fe and impurities. However, for the purpose of improving various properties, one or more elements (optional elements) selected from Cr, Mo, Cu, Ni, Co, W, Ta, Sn, Sb, Nb, V, As, Zn, Ca, Mg, Zr, Hf, Bi, and REM shown below may be further contained. Since it is not necessary to contain the optional elements, 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 1.0000% 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 for strengthening the steel sheet, and one or more of these elements may be contained as necessary. The content of each element may be 0.001% or more, 0.005% or more, or 0.010% or more. On the one hand, excessive inclusion of these elements will lead to saturation of the effects and an increase in cost. Therefore, when including them, the content of Cr, Mo, Cu, Ni, Co, W, Ta, Sn, V should be 1.00% or less respectively, the content of Zn should be 1.0000% or less, the content of Sb should be 0.50% or less, the Nb content should be 0.200% or less, and the As content should be 0.10% or less. The content of Cr, Mo, Cu, Ni, Co, W, Ta, Sn, Sb, and V is 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. These elements each contribute to the improvement of the bendability of the steel plate. Therefore, they may be included as necessary. To obtain the above effects, it is preferable to include 0.0001% or more, and more preferably 0.0010% or more, of one or more selected from Ca, Mg, Bi, Zr, and REM respectively. On the other hand, excessive inclusion of these elements will deteriorate the elongation. Therefore, the content of Ca, Mg, Bi, Zr, and REM should all be 0.0100% or less. The content of Ca, Mg, Bi, Zr, and REM is preferably all 0.0080% or less, and more preferably 0.0060% or less.

[0027] Here, REM refers to the 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 mischmetal.

[0028] Hf: 0 to 0.0100% Hf is effective as a deoxidizing element. Therefore, it may be contained. On the other hand, when the content of Hf exceeds 0.0100%, the HAZ toughness deteriorates. Therefore, the content of Hf 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 the present embodiment contains C, Si, Mn, P, S, Al, Ti, B, N, and O, and the balance may be Fe and impurities. Further, it may contain one or more elements (optional elements). Impurities mean components that are mixed from raw materials such as ore and scrap or other factors during the industrial production of steel materials and are allowed within a range that does not adversely affect the properties.

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

[0031] In the steel sheet according to this embodiment, after setting the Mn content and Si content in the overall average of the steel sheet within the above ranges, the standard deviation σ (σ2 in this embodiment) of the MS value calculated by the formula (1) from the Mn concentration [Mn] and Si concentration [Si] in mass% at each measurement point of a plurality of measurement points measured by EPMA is preferably 0.80 or less. MS = [Mn] + (2 / 3)×[Si] ···(1) When the standard deviation σ2 of the MS value is small, when the member having the spot welded portion is subjected to axial crushing deformation after forming, fracture in the HAZ portion is suppressed. It is presumed that the reason why fracture is likely to occur when the standard deviation σ2 of the MS value is large is as follows. The temperature of the HAZ portion rises due to the heat of welding during welding. In the HAZ portion, there will be a region where the reached temperature is in the two-phase region (Ac1~Ac3). Also, since the cooling rate of the welded portion is extremely high, the austenite generated at this time becomes martensite as it is quenched. That is, in this region, the microstructure becomes a composite structure containing ferrite and martensite. Such a structure is brittle, so fracture is likely to occur during axial crushing deformation. At this time, especially, the higher the proportion of martensite containing a large amount of Si and Mn, the more embrittled it becomes. The smaller the standard deviation σ2 of the MS value of the steel sheet, the smaller the proportion of such martensite containing a large amount of Si and Mn in the HAZ portion, so it is considered that fracture is suppressed.

[0032] The standard deviation of the MS value is obtained by the following method. The concentration distributions of Si and Mn are measured using FE-EPMA (electron probe microanalyzer). In a cross-section parallel to the plate thickness direction, in the range of 1 / 8 to 3 / 8 of the plate thickness in the plate thickness direction from the surface of the steel sheet centered on the 1 / 4 thickness position (the range from the 1 / 8 thickness position to the 3 / 8 thickness position), for a region of 35μm×50μm, the Si concentration and Mn concentration are measured at a measurement interval of 0.2μm, and [Mn] + (2 / 3)×[Si] is obtained from the Mn concentration [Mn] and Si concentration [Si] at each measurement position. By performing the above analysis in 4 fields of view, the value of [Mn] + (2 / 3)×[Si] is obtained at about 175,000 points. Using this as the population, the standard deviation σ (σ2) is calculated. For FE-EPMA, for example, JXA-8530F manufactured by JEOL can be used, and the acceleration voltage can be set to 15 kV. The spectral method for characteristic X-rays shall be wavelength dispersive type. As for the spectral crystal, an appropriate material may be selected according to the element to be analyzed. For example, LiF can be used for Mn, and TAP can be used for Si. The output Si concentration and Mn concentration are values converted from the detected intensity of characteristic X-rays to mass% by the program attached to JXA-8530, on the premise that calibration with a standard substance has been performed.

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

[0034] Retained austenite: 3 to 10% Retained austenite is a structure that contributes to the improvement of elongation by the TRIP effect. Therefore, the area ratio of retained austenite shall be 3% or more. On the other hand, when the area ratio of retained austenite becomes excessive, the grain size of retained austenite becomes large. Such retained austenite with a large grain size becomes coarse and hard martensite after deformation by forming or the like. In this case, the starting point of cracking is likely to occur, and the bendability deteriorates. Therefore, the area ratio of retained austenite shall be 10% or less.

[0035] Ferrite: 0 to 10% In order to suppress fracture during axial crushing deformation after forming into a member, it is preferable to have a uniform structure with a small hardness difference between structures. Since ferrite is a soft structure (phase), it is difficult to obtain high strength when the microstructure is mainly composed of ferrite. Therefore, in the steel sheet according to the present embodiment, in the microstructure, martensite and bainite as described below are the main components. Therefore, the area ratio of ferrite shall be 10% or less. The smaller the area ratio of ferrite, the better, preferably 5% or less, more preferably 3% or less, and even 0% is acceptable.

[0036] Pearlite: 0 - 5% Pearlite is a brittle structure and serves as a starting point for fracture, thus deteriorating the local ductility of the steel plate. 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 even be 0%.

[0037] Martensite and bainite: 75 - 97% in total Martensite and bainite are structures effective for increasing the strength of the steel plate. As described above, in order to suppress fracture during axial crushing deformation after forming the member, it is necessary to make the structure as uniform as possible. Therefore, in the steel plate according to this embodiment, except for retained austenite, ferrite, and pearlite, it is martensite and / or bainite. The total area ratio of martensite and bainite is 75 - 97%. Preferably, the total area ratio of martensite and bainite is 80 - 97%, 85 - 97%, or 90 - 97%. Both martensite and bainite are lath-like structures, and in the steel plate according to this embodiment, it is not particularly necessary to define 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 (metallic structure) at the 1 / 4 thickness position of the steel plate according to this embodiment is measured as follows. The fraction (area ratio) of the structure is evaluated by the secondary electron image taken using FE-SEM and X-ray diffraction method. For FE-SEM, for example, JSM-7200F manufactured by JEOL Ltd. may be used. First, a sample is taken with a plate thickness cross-section (cross-section parallel to the plate thickness direction) parallel to the rolling direction of the steel plate, and the cross-section at a position at least 50 mm away from the end in the width direction is used as the observation surface. After mechanically polishing the observation surface to a mirror finish, etching is performed using nital solution. Next, in one or more observation fields in the range from 1 / 8 thickness position to 3 / 8 thickness position centered on the 1 / 4 thickness position from the surface of the steel plate on the observation surface, a total of 2.0×10 -9 m2 Take a secondary electron image of the above area. From the obtained secondary electron image, measure the area fractions of ferrite and pearlite respectively, and regard them as the area ratios. Although there is no need to determine the upper limit value of the visual field area, the larger the area, the greater the man-hours required for point counting. Therefore, it is advisable to use 1.0×10 -8 m 2 or less as a guide. The magnification is set to 5000 times, and take pictures of the number of sheets with a visual field area of 2.0×10 -9 m 2 or more. When identifying the structure, determine the area where cementite is precipitated in a lamellar shape as pearlite. Also, determine the area with low brightness and no lower structure visible as ferrite. Determine the area that does not fall into any of the above as bainite or martensite or austenite (retained austenite). By subtracting the area ratio of austenite measured by the X-ray diffraction method described later from the area ratio of the area determined as bainite or martensite or austenite, the area ratios of bainite and martensite can be obtained. Calculate the area ratio of each by the point counting method to obtain the area ratio of each structure. The interval for point counting is 2μm both vertically and horizontally. Measure the area ratio of retained austenite by the X-ray diffraction method. That is, remove the area from the plate surface of the steel plate to a depth of 1 / 4 of the plate thickness in the plate thickness direction by mechanical polishing and chemical polishing. Then, calculate the tissue fraction of retained austenite from the integrated intensity ratios of the diffraction peaks of (200), (211) of the bcc phase and (200), (220), (311) of the fcc phase obtained using the MoKα1 line as the characteristic X-ray for the polished sample, and regard this as the volume fraction of retained austenite. In the steel plate according to this embodiment, it is regarded that the volume fraction and the area fraction are equal, and the obtained volume fraction of retained austenite is taken as the area ratio of retained austenite. The rolling direction is obvious when the material is in coil form or when the rolling direction is recorded. In the case of a full-width cut plate sample, if the width is known, the rolling direction can be determined from the dimensions. Also, when at least one side width edge remains, a person skilled in the art can easily determine the width direction and the rolling direction from the state of the end face (presence or absence of edge drop, presence or absence of plating). When the above information is lost, the rolling direction is specified by the following method. Polish the Z plane of the plate (a plane parallel to both the longitudinal direction and the width direction of the plate) to the 1 / 4 thickness position, finish it with mirror polishing, and then obtain an Mn concentration map of a 1000μm×1000μm region by EPMA. When the solidification segregation of Mn is measured in streaks, the longitudinal direction of the streak pattern is determined as the rolling direction. Also, even when the steel plate is processed into a part, the rolling direction of the weakly processed part of the part (for example, a flat part that has not been relatively processed) can be determined using the above method.

[0039] In the steel plate according to the present embodiment, also, in a cross section parallel to the plate thickness direction, an 80μm×80μm range centered on the 1 / 4 thickness position in the plate thickness direction and 80μm in the direction perpendicular to the plate thickness direction is divided into 16 equal parts of 20μm×20μm, and when the area ratio of retained austenite is determined in each divided region, the standard deviation of the area ratio of retained austenite in each of the divided regions is σ1, and the average is γ ave When set as ave σ1 / γ Retained austenite contributes to an increase in the work hardening amount by transforming into hard martensite by work-induced transformation. That σ1 / γ ave becomes large means that the deviation of the distribution of retained austenite becomes large. Such a structure, when subjected to processing by axial crushing deformation, work hardening by work-induced transformation occurs unevenly, resulting in uneven hardness between tissues, so it is likely to break during axial crushing deformation. Therefore, make σ1 / γ ave smaller and set it to 0.20 or less. σ1 / γ ave is preferably 0.15 or less, more preferably 0.10 or less.

[0040] σ1 / γ aveIt is measured by the following method. First, a sample is taken with a cross-section of the steel plate thickness as the observation surface from a position at least 50 mm away from the end in the width direction. The cross-section is preferably parallel to the rolling direction. After mechanically polishing the observation surface to a mirror finish, electrolytic polishing is performed. Next, crystal structure and orientation analysis are carried out by the SEM-EBSD method for an area of 80 μm × 80 μm centered on the 1 / 4 thickness position of the steel plate in the thickness direction of the observation surface and 80 μm in the direction perpendicular to the thickness direction. For FE-SEM, for example, JSM-7200F manufactured by JEOL may be used. For the analysis of the data obtained by the EBSD method, for example, "OIM Analysis 6.0" manufactured by TSL can be used. Also, the scoring interval (step) is set to 0.1 μm. The region with an FCC crystal structure is regarded as retained austenite. The obtained 80 μm × 80 μm is divided into 16 parts of 20 μm × 20 μm, and the area ratio of retained austenite in each divided region is measured. For the 16 specimens thus obtained, the standard deviation σ1 and the average value γ ave are obtained, and σ1 / γ ave is calculated.

[0041] (Mechanical properties) The steel plate according to this embodiment aims to have a tensile strength (TS) of 980 MPa or more as the strength contributing to the weight reduction of the automobile body. The upper limit of the tensile strength is not limited, but if the tensile strength is too high, the formability may decrease, so the tensile strength may be set to 2000 MPa or less. Also, the elongation (EL) aims to be 10.0% or more. The tensile strength (TS) and the elongation (EL) are obtained by taking a JIS No. 5 tensile test piece from the steel plate in a direction perpendicular to the rolling direction and performing a tensile test in accordance with JIS Z 2241:2022. When it is difficult to specify the rolling direction, the tensile test may be performed in an arbitrary direction. Also, if it is difficult to take a JIS No. 5 tensile test piece, a JIS No. 13B tensile test piece may be used. If that is also difficult, an arbitrary small test piece similar to the JIS No. 13B tensile test piece may be used. In that case, the gauge length may also be changed according to the scale of the test piece. For example, when it is a 1 / 2 similar shape, the gauge length may be set to 25 mm.

[0042] (Thickness of the steel sheet) Although the thickness of the steel sheet according to the present embodiment is not limited, from the viewpoint of achieving both weight reduction of the automobile body and suppression of fracture during the crushing deformation as a member, 0.4 to 3.0 mm is preferable.

[0043] (Zinc plating layer) (Alloyed hot-dip zinc plating layer) The steel sheet according to the present embodiment may have a zinc plating layer on the surface (it may have a zinc plating layer formed on the surface of the base steel sheet and the base steel sheet). By having a zinc plating layer, the corrosion resistance is improved. In the case of a steel sheet for automobiles, if there is a concern about perforation due to corrosion, even if the strength is increased, it may not be possible to reduce the thickness below a certain thickness. One of the purposes of increasing the strength of the steel sheet is weight reduction by reducing the thickness. Therefore, even if a high-strength steel sheet is developed, if the corrosion resistance is low, the applicable parts are limited. As a method for solving these problems, a zinc plating layer with high corrosion resistance may be formed on the surface of the steel sheet. The zinc plating layer may be an electro-galvanized layer, a hot-dip zinc plating layer, or an alloyed hot-dip zinc plating layer in which the hot-dip zinc plating layer is alloyed. The hot-dip zinc plating layer is preferable in terms of cost. In the alloyed hot-dip zinc plating layer, since Fe is incorporated into the hot-dip zinc plating layer by the alloying treatment, excellent weldability and paintability can be obtained, so it is preferable. In addition, upper-layer plating may be performed on the zinc plating layer (electro-galvanized layer, hot-dip zinc plating layer, alloyed hot-dip zinc plating layer) for the purpose of improving the paintability and weldability. Further, in the cold-rolled steel sheet according to the present embodiment, various treatments, for example, chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment, etc. may be performed on the hot-dip zinc plating layer. The zinc plating layer and the alloyed hot-dip zinc plating 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. Further, the adhesion amount of the zinc plating layer is not particularly limited and may be a general adhesion amount. A general adhesion amount in the case of being used for automobile applications is, for example, 20 to 100 g / m per side 2 is.

[0044] <Steel member> The steel member according to the present embodiment is a steel member obtained by forming the steel plate according to the present embodiment into a predetermined shape, or a steel member obtained by further joining with another steel material by spot welding after forming. The steel member obtained by forming (processing) into a predetermined shape has a steel plate including a processed portion and an unprocessed portion. In addition, the steel member obtained by joining with another steel material by spot welding has a steel plate including a spot welding portion, a HAZ portion formed around the spot welding portion and affected by the heat of spot welding, and a non-welded portion other than the spot welding portion and the HAZ portion. Here, the unprocessed portion is a flat portion in the steel member and is the thickest portion among the flat portions. The processed portion is a portion in the steel member that is thinner than the surrounding portion or has a certain curvature among the flat portions. In addition, the non-welded portion is a portion other than the spot welding portion and the HAZ portion affected by the heat of spot welding. The steel member includes the above steel plate. The steel member may be made of the above steel plate. Further, the surface of the steel plate constituting the steel member may have a hot-dip galvanized layer or an alloyed hot-dip galvanized layer. The steel member (before spot welding) according to the present embodiment has the following characteristics in the unprocessed portion. If the steel member is an automotive part, examples of its application include to a front side member, a rear side member, a side sill, etc.

[0045] The steel member (before spot welding) according to the present embodiment has the following characteristics in the unprocessed portion. In addition, the steel member (after spot welding) according to the present embodiment has the following characteristics in the unprocessed portion of the non-welded portion.

[0046] (Unprocessed portion and non-welded portion (unprocessed portion if it is a steel member before spot welding)) Since the non-welded part is not affected by spot welding, its chemical composition is the same as that of the steel sheet according to this embodiment. Also, although the microstructure changes due to forming, the non-worked part of the non-welded part where no working strain is introduced is the same as the steel sheet according to this embodiment. That is, in the non-worked and non-welded part, in terms of 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%, 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, has a chemical composition. Also, in the non-worked and non-welded part, the microstructure at the 1 / 4 thickness position, which is 1 / 4 of the thickness (for example, the thickness of the steel sheet constituting the member) in the thickness direction from the surface, in terms of area%, retained austenite: 3 to 10%, ferrite: 0 to 10%, pearlite: 0 to 5%, and martensite and bainite: a total of 75 to 97%. In a cross-section parallel to the plate thickness direction, an 80 μm × 80 μm range 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 parts of 20 μm × 20 μm, and when the area ratio of retained austenite is obtained in each divided region, the standard deviation of the area ratio of retained austenite in each divided region is σ1, and the average is γ ave When it is ave σ1 / γ is 0.20 or less. By having such a chemical composition and microstructure, fracture during axial crushing deformation is suppressed.

[0047] 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 σ2 of the MS value calculated by formula (1) from the Mn concentration [Mn] and the Si concentration [Si] in mass % at each of a plurality of measurement points measured by EPMA in a 35 μm × 50 μm region in the range of 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)

[0048] In unprocessed and unwelded areas, if the standard deviation σ (σ2) of the MS value is 0.80 or less, fracture in the HAZ is suppressed when subjected to axial crushing deformation after spot welding. Furthermore, in a steel member having spot welds, if the standard deviation σ2 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.

[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 σ1 / γ of unprocessed and unwelded parts aveThe standard deviation of the concentration distribution of Mn and Si 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 non-machined and non-welded part of the component, it 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.

[0053] <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 process in which a slab having a predetermined chemical composition is obtained 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 in which, if necessary, the hot-rolled steel sheet is pickled and cold-rolled 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.

[0054] (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.

[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): Here, T in equation (4) C (τ) is calculated by equation (5).

[0056]

number

[0057]

number

[0058] Here, τ, τ1, 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 δSi respectively represent the following: τ: Elapsed time from the start of casting [seconds] τ1: T C (τ) is the time [seconds] when the solidification completion temperature T δ in units of K is reached 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) is the equilibrium volume fraction of the liquid phase M L : Temperature T C (τ) is the equilibrium Mn concentration in the liquid phase [mass%] M δ : Temperature T C (τ) is the equilibrium Mn concentration of δ [mass%] S L : Temperature T C (τ) is the equilibrium Si concentration in the liquid phase [mass%] S δ : Temperature T C (τ) is the equilibrium Si concentration of δ [mass%] D δ Mn : Temperature T C (τ) is the diffusion coefficient of Mn in the δ phase [m 2 / s] D δ Si : Temperature T C (τ) is the diffusion coefficient of Si in the δ phase [m 2 / s]

[0059] When continuously casting a slab from molten steel, the molten steel is initially liquid, but gradually solidifies from the surface and completely solidifies at a certain temperature (i.e., the solidification completion temperature). In this regard, Equation (4) defines the temperature range where the liquid phase and the solid phase coexist before the molten steel is completely solidified (the temperature range in the partially solidified state). In this temperature range, as solidification progresses from the surface of the molten steel Subsequently, since alloying elements are distributed (discharged) from the solid phase to the liquid phase, the alloying elements become concentrated in the liquid phase at this stage. Such concentration of alloying elements is a factor that increases the standard deviation σ of the MS value. More specifically, the greater the distribution of Mn and Si between the solid phase and the liquid phase, the more regions with high and low MS values will be locally generated in the final product. Therefore, in the temperature range where the liquid phase and the solid phase coexist during continuous casting, it is important to sufficiently suppress the distribution of Mn and Si between the solid phase and the liquid phase. In this regard, the inventors have found that by satisfying equations (4) and (5), particularly by controlling the value obtained by the left side of equation (4) to be less than 0.0100, the distribution of Mn and Si between the solid phase and the liquid phase can be suppressed, and the standard deviation σ of the MS value can be significantly reduced. Here, τ = 0 in equation (4) means the start of casting, and τ1 means the time (seconds) when the internal temperature (center temperature) of the slab reaches the solidification completion temperature T δ This is why equation (4) can be understood as integrating the distribution of Mn from the solid phase to the liquid phase over time. 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), the more preferable it is. Specifically, it is preferably 0.0100 or less, and more preferably 0.0050 or less, 0.0030 or less. T in equations (4) and (5) L、 T S、 f L 、M L 、M δ 、S L 、S δ、 D δ Mn 、D δ Si are values determined by the temperature and the chemical composition of the molten steel. T 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 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 (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, 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 Land set the highest temperature at which the number of moles becomes 0 as T δ Let it be so. Also, D δ Mn For D δ Si values obtained by the following formulas (6) and (7) are used.

Number

Number

[0060] (Hot rolling process) In the hot rolling process, the slab is heated to the heating temperature, hot rolling including rough rolling and finish rolling is performed, cooled to the 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 less than 1200°C, the diffusion of alloying elements becomes insufficient. The upper limit of the heating temperature is not limited, but it is preferably 1350°C or lower in order to suppress the reduction in yield due to scale out. Also, in order to homogenize the distribution of retained austenite in the final structure, when the number of passes of finish rolling is n, the first pass is the first pass, and the final pass is the nth pass, the entry side temperature of the (n - 2)th pass (that is, the third pass counted from the end) is 950°C or higher, the exit side temperature (steel sheet surface temperature) of the nth pass (final pass) is 900°C or higher, and in finish rolling, rolling with a reduction rate exceeding 25% in one pass is performed at least once or more. Also, the time between each pass between the (n - 2)th pass and the (n - 1)th pass, and between the (n - 1)th pass and the nth pass is 0.2 to 1.0 seconds, and the time from the completion of the nth pass to the start of cooling is 1.0 to 3.0 seconds. The upper limits of the entry side temperature and the exit side temperature are not limited, but the entry side temperature and the exit side temperature may be 1100°C or lower. As a result, the number of nucleation sites for phase transformation increases, so the hot rolled plate structure is refined, and cementite, which is the source of retained austenite, is 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 carbon distribution 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, 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] 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, hindering productivity. There is no particular need to set an upper limit to the average cooling rate between 600 and 750°C, but from an operational standpoint, it may be set to 200°C / s or less.

[0062] Furthermore, in order to make the standard deviation σ2 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 (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.

[0063] (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.

[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 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.

[0065] (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%.

[0066] (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) (sometimes collectively referred to simply as a steel sheet) is subjected to heat treatment. The heat treatment process 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 in 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 desired 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, resulting in a decrease in toughness.

[0068] ((first cooling process)) ((first retention process)) In the first cooling step, the hot-rolled or cold-rolled steel sheet after the heating step 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 is set to be within the range of Ms-50°C to 550°C (first temperature region). Then, in the first holding step, the hot-rolled or cold-rolled steel sheet after the first cooling step is held in a first temperature range for 20 to 100 seconds. In this step, the steel sheet only needs to be held in a temperature range of Ms-50°C to 550°C for 20 to 100 seconds, and does not necessarily need to be held at a constant temperature. Holding in the first temperature region partially promotes bainite transformation and forms a C-enriched region, which contributes to the uniform distribution of retained austenite. If the average cooling rate between 650 and 550°C in the first cooling step is less than 10°C / s, a large amount of ferrite will be formed during cooling, making it impossible to obtain the desired microstructure. There is no need to particularly limit the upper limit, but from an operational standpoint, it may be set to 200°C / s or less. If the cooling stop temperature is lower than Ms-50°C, which is lower than the first temperature range, the 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 is higher than 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 is in the temperature range of Ms-50°C to 550°C) is less than 20 seconds, the effect of partially promoting the bainite transformation is insufficient, resulting in a non-uniform distribution of the retained austenite.On the other hand, if the holding time exceeds 100 seconds, the bainite transformation progresses excessively, resulting in a non-uniform distribution of the retained austenite. 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 %.

[0069] ((Second cooling process)) ((Second retention process)) In the second cooling step, the hot-rolled steel sheet or cold-rolled steel sheet after the first holding step is cooled to 200° C. or less. The cooling rate at this time is not limited. Then, in the second holding step, the hot-rolled steel sheet or 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). In this step, the untransformed austenite is stabilized to ensure the presence of retained austenite. If the cooling stop temperature exceeds 200°C, the formation of martensite in the steel sheet (i.e., the distribution of untransformed austenite) becomes uneven, and the σ1 / γ ave The lower limit of the cooling stop temperature is not particularly limited, 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 fall outside 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]

number

[0071]

number

[0072] Here, t and t in the above formulas (2) and (3) f、 T 、 T max indicates the following, respectively. t: elapsed time [seconds] t f : Stay 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 adjusted by wiping. The plating process may be carried out at any stage. For example, as long as the conditions for the heat treatment process described above 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 process) When the hot-dip galvanized layer is made into an alloyed hot-dip galvanized layer, an alloying process for alloying the hot-dip galvanized layer may be performed after the plating process. The alloying process may be performed at any time after the plating process. For example, as long as each condition of the above heat treatment process is satisfied, it can be performed after the plating process and 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. However, when performing the plating process and the alloying process, even including the holding time at a predetermined temperature by these processes, it is performed so as to satisfy the conditions of the above heat treatment process. For example, when performing the plating process or the alloying process between the cooling process and the holding process, or after the holding process under the condition that the temperature of the steel sheet is 360 to 480 °C, the holding time in the holding process and the time during which the steel sheet reaches 360 to 480 °C in the plating process and the alloying process are combined and 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 processes is formed into a predetermined shape according to requirements. The forming method and the shape to be formed are not limited. For example, it is an automotive skeletal member such as a front side member, a rear side member, or a side sill, in which the cross section perpendicular to the longitudinal direction is a closed cross section as represented.

[0076] (Joining process) For example, in order to obtain a closed cross-section structure as described above, spot welding (resistance spot welding) is performed on the steel sheet after the forming process to join it to another steel sheet. The conditions of spot welding are not limited, but a nugget diameter of 3√t to 6√t (t: plate thickness) is desirable. Also, the welding current value, energization pattern, pressing force, welding electrodes used, etc. may be selected so as to obtain the nugget diameter. Also, the interval between spot welding points may be about 15 to 50 mm.

Example

[0077] By continuous casting, a slab having the chemical composition shown in Table 1 was obtained. In continuous casting, casting was performed such that the value on the left side of the above formula (4) was as shown in Table 2A. Thereafter, the slab was 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 temperature on the inlet side of R1 in the table is the inlet side temperature in the (n - 2)-th pass (the third pass counted from the final pass), R1 is the reduction ratio in the (n - 2)-th pass, R2 is the reduction ratio in the (n - 1)-th pass, R3 is the reduction ratio in the n-th pass (final pass), the temperature on the outlet side of R3 is the outlet side temperature in the n-th pass, t1 is the inter-pass time between the (n - 2)-th pass and the (n - 1)-th pass, t2 is the inter-pass time between the (n - 1)-th pass and the n-th pass, and t3 is the time from the completion of the n-th pass to the start of cooling. After hot rolling, the steel plate was cooled to the coiling temperature shown in Table 2A such that the average cooling rate was as shown in Table 2A in the range of 600 to 750 °C. For this steel plate, after pickling, cold rolling with the reduction ratio (cumulative reduction ratio) shown in Table 2B was performed. The thickness of the steel plate after cold rolling was all set to 1.2 mm. The steel plate after cold rolling was heat-treated under the conditions shown in Table 2B. That is, it was heated to the heating temperature shown in Table 2B and held (during the heating process), then cooled to the cooling stop temperature (first cooling stop temperature) at the average cooling rate shown in Table 2B (first cooling process), and held at that temperature (first holding process) (here, the cooling stop temperature in the first cooling process = the holding temperature in the first holding process). Thereafter, cooling was performed at the average cooling rate shown in Table 2B (second cooling process). Further, thereafter, it was heated and held at that temperature (second holding process). In the table, the holding time in the heating process of the heat treatment process is the time during which the steel plate stays at Ac3 - 20 °C or higher. However, for No. 24, it is the holding time at the maximum reaching temperature (±10 °C). Also, in the table, the holding time in the holding process of the heat treatment process is the time during which the steel plate stays at the holding temperature ±10 °C. For some examples, a zinc plating layer (electro-galvanized layer or hot-dip galvanized layer) was formed on the surface. In some examples where a hot-dip galvanized layer was formed, the hot-dip galvanized layer was alloyed to form an alloyed hot-dip galvanized layer. The formation of the electro-galvanized layer was carried out after cooling to room temperature following the heat treatment process. The formation of the hot-dip galvanized layer was performed by reheating or cooling the steel sheet after the first holding process to 460 °C and then immersing it in a hot-dip galvanizing bath. The immersion time was set to 3 seconds. The alloying treatment was carried out by heating to the temperature described in Table 2 after immersion in the hot-dip galvanizing bath and holding for 20 seconds. In the column of product type in the table, CR is an example where no galvanized layer was formed, EG is an example where an electro-galvanized layer was formed, GI is an example where a hot-dip galvanized layer was formed, and GA is an example where an alloyed hot-dip galvanized layer was formed.

[0078] Regarding the obtained steel sheet, the microstructure at the 1 / 4 thickness position, σ1 / γ ave and the standard deviation σ2 of the MS value in the range of 1 / 8 to 3 / 8 thickness centered on the 1 / 4 thickness position were determined by the above method. For FE-EPMA, JXA-8530F of JEOL was used, and the acceleration voltage was 15 kV. For the analysis of the data obtained by the EBSD method, "OIM Analysis 6.0" manufactured by TSL was used. The results are shown in Table 3.

[0079] Also, regarding the obtained steel sheet, a No. 5 tensile test piece of JIS Z 2241:2022 with the longitudinal direction being the direction perpendicular to the rolling direction was sampled, and a tensile test in accordance with JIS Z2241:2022 was carried out using this test piece, and the tensile strength (TS) and elongation (EL) were measured. If TS was 980 MPa or more and EL was 10.0% or more, it was judged to have high strength and excellent elongation.

[0080] Furthermore, using the obtained steel sheet as a material, a hat-shaped molded body having a bending ridge line with R = 5 mm and a flat plate serving as a back plate were sampled and joined by spot welding to create a member having a closed cross-section structure with the shape shown in Fig. 1A. At that time, as shown in Fig. 1B, the interval between the weld spots was set to 15 mm, and the welding current value was set to the current value at which 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: Single-phase AC 50Hz Pressing force: 400 kgf Electrical conduction time: 20 cyc Holding time: 5 cyc

[0081] In the obtained steel member, the chemical composition, metal structure, and mechanical properties of the non-welded and non-processed parts were the same as those of the raw material.

[0082] As shown in Fig. 2, for the obtained steel member, with the lower end side (one end side) completely restrained, a flat impacter was collided from the upper end side (the other end side). The weight of the impacter was 334 kg, and the speed at the time of collision was 11.1 m / s. After the collision, the test piece was observed, and for the general part (non-welded part) and the welded part (HAZ part), the presence or absence of cracks was visually confirmed. Those without cracks were marked as "○: GOOD", and those with cracks were marked as "×: BAD". The results are shown in Table 3.

[0083]

Table 1

[0084]

Table 2A

[0085]

Table 2B

[0086]

Table 3

[0087] As can be seen from Tables 1 to 3, the steel member obtained using a steel plate having a predetermined chemical composition, a microstructure at the 1 / 4 thickness position, and σ1 / γ ave of 0.20 or less (the non-welded and non-processed parts had the same characteristics as the steel plate) had excellent fracture resistance characteristics during axial crushing deformation. In addition, when the standard deviation σ2 of the MS value is 0.80 or less, the fracture resistance characteristics during axial crushing deformation in the HAZ portion were also excellent.

Industrial Applicability

[0088] According to the present invention, it is possible to provide a steel plate, a method for manufacturing the same, and a steel member obtained by using the steel plate, which have high strength and excellent elongation and can suppress fracture of the member when the member is axially crushed after being formed. Therefore, the present invention has high industrial applicability.

Claims

1. 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 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 1.0000%, 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, having a chemical composition consisting of the microstructure at the 1 / 4 thickness position, which is the position 1 / 4 of the plate thickness in the plate thickness direction from the surface, is, by area%, retained austenite: 3 to 10%, [[ID=3�]]ferrite: 0 to 10%, pearlite: 0 to 5%, and [[ID= ​ In a cross-section parallel to the plate thickness direction, a range of 80 μm × 80 μm, which is 80 μm in the plate thickness direction centered on the 1 / 4 thickness position and 80 μm in the direction perpendicular to the plate thickness direction, is divided into 16 parts of 20 μm × 20 μm. When the area ratio of retained austenite is obtained in each divided region, the standard deviation of the area ratio of retained austenite in each divided region is σ １ , and the average is γ ａｖｅ . When taking them as such, σ １ / γ ａｖｅ is 0.20 or less. ​ ​ The standard deviation σ of the MS value calculated by formula (1) from the concentration [Mn] of Mn and the concentration [Si] of Si in mass% at each measurement point of a plurality of measurement points measured by EPMA in a region of 35 μm × 50 μm in the range of 1 / 8 to 3 / 8 of the thickness centered on the 1 / 4 thickness position of the cross section parallel to the plate thickness direction ２ [[ID= ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Bi: 0 to 0.0100%, REM: 0 to 0.0100%, Hf: 0 to 0.0100%, and the balance: Fe and impurities, having a chemical composition consisting of wherein the microstructure at the 1 / 4 thickness position, which is the position of 1 / 4 of the thickness in the thickness direction from the surface of the non-processed part, 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 consisting of In a cross-section parallel to the thickness direction, when a range of 80 μm × 80 μm, which is 80 μm with respect to the 1 / 4 thickness position centered in the thickness direction and 80 μm in the direction perpendicular to the thickness direction, is divided into 16 parts of 20 μm × 20 μm and the area ratio of retained austenite is determined in each divided region, the standard deviation of the area ratio of retained austenite in each of the divided regions is σ １ , and the average is γ ａｖｅ . When taking them as such, σ １ / γ ａｖｅ is 0.20 or less. a steel member, characterized in that.

5. including a steel plate having a spot weld portion, a HAZ portion around the spot weld portion, and a non-welded portion other than the spot weld portion and the HAZ portion, wherein the non-processed part exists in the non-welded portion, the steel member according to claim 4, characterized in that.

6. In the unprocessed 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 measurement point of a plurality of measurement points measured by EPMA in a region of 35 μm × 50 μm in the range of 1 / 8 to 3 / 8 of the thickness centered on the 1 / 4 thickness position ２ is 0.80 or less the steel member according to claim 4 or 5, characterized in that. MS = [Mn] + (2 / 3) × [Si]... (1)

7. having a zinc plating layer or an alloyed hot-dip zinc plating layer on the surface of the steel plate, the steel member according to claim 4 or 5, characterized in that.

8. a method for manufacturing the steel plate according to claim 1, comprising: a continuous casting step of 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 to a coiling temperature, and then coiling at the coiling temperature to obtain a hot-rolled steel plate; an optional cold rolling step of performing pickling and cold rolling with a cumulative reduction rate of 30 to 75% on the hot-rolled steel plate to obtain a cold-rolled steel plate; a heat treatment step of performing heat treatment on the hot-rolled steel plate or the cold-rolled steel plate; having 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 last pass is the nth pass, the entry temperature of the (n - 2)th pass is 950°C or higher, and the exit temperature of the nth pass is 900°C or higher; in the finish rolling, rolling with a reduction rate exceeding 25% in one pass is performed at least once; the time between each pass between the (n - 2)th pass and the (n - 1)th pass, and between the (n - 1)th pass and the nth pass is 0.2 to 1.0 second; 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, the average cooling rate between 600 and 750°C is 20°C / s or higher. The heat treatment process includes a heating process, a first cooling process, a first holding process, a second cooling process, and a second holding process. In the heating process, 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 at this temperature range for 1 to 1000 seconds. In the first cooling process, after the heating process, the hot-rolled steel sheet or the cold-rolled steel sheet is cooled to a first cooling stop temperature in a first temperature range of Ms - 50°C to 550°C such that the average cooling rate from 650°C to 550°C is 10°C / s or more. In the first holding process, after the first cooling process, the hot-rolled steel sheet or the cold-rolled steel sheet is held at the first temperature range for 20 to 100 seconds. In the second cooling process, after the first holding process, the hot-rolled steel sheet or the cold-rolled steel sheet is cooled to 200°C or lower. In the second holding process, after the second cooling process, the hot-rolled steel sheet or the cold-rolled steel sheet is heated to a second temperature range of 200°C to 420°C and held in the second temperature range to satisfy the following formulas (2) and (3). A method for manufacturing a steel sheet, characterized by the above. 【Number 1】 【Number 2】 Here, in the formula (2) and the formula (3), t, t ｆ、 T 、 T ｍａｘ respectively represent the following. t: Elapsed time [seconds] t ｆ : End time of residence [seconds] T: Temperature at time t [K] T ｍａｘ : Maximum temperature reached during residence [K]

9. In the rough rolling, while the slab is in a state of 1050°C or higher, rolling with a reduction rate exceeding 20% is performed in 3 or more passes. In the continuous casting process, casting is performed to satisfy the following formulas (4) and (5). A method for manufacturing a steel sheet according to claim 8, characterized by the above. [Number 3] [Number 4] Here, in the formula (4) and the formula (5), τ, τ 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 respectively represent the following. T C (τ) is obtained by the formula (5). τ: Elapsed time from the start of casting, with the unit of seconds τ 1 : T in seconds C (τ) is the time when the solidification completion temperature T δ in units of K is reached T S T(τ): The slab surface temperature at time τ, with the unit of K T C T(τ): Estimated internal temperature of the slab at time τ, with the unit of K T L : Solidification start temperature in units of K f L : Temperature T C Equilibrium volume fraction of the liquid phase at M L : The equilibrium Mn concentration in the liquid phase at temperature T C (τ) in mass % units M δ : The equilibrium Mn concentration of δ at temperature T C (τ) in mass % units S L : Equilibrium Si concentration in the liquid phase at temperature T C (τ) in mass % S δ : Equilibrium Si concentration of δ at temperature T C (τ) with the unit being mass% D δ Mn : The unit is m 2 / s, the diffusion coefficient of Mn in the δ phase at temperature T C (τ) D δ Si : The unit is m 2 / s, and the diffusion coefficient of Si in the δ phase at temperature T C (τ)

10. Furthermore, during the first cooling process or the second cooling process of the heat treatment process, between the first cooling process or the second cooling process and the first holding process or the second holding process, during the first holding process or the second holding process, or after the first holding process or the second holding process, a plating process for forming a zinc plating layer on the surface of the hot-rolled steel sheet or the surface of the cold-rolled steel sheet is provided. A method for manufacturing a steel sheet according to claim 8 or 9, characterized by the above.

11. The zinc plating layer is a hot-dip zinc plating layer. Furthermore, after the plating process, an alloying process for alloying the hot-dip zinc plating layer to form an alloyed hot-dip zinc plating layer is provided. A method for manufacturing a steel sheet according to claim 10, characterized by the above.

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