Steel plate, its manufacturing method, and parts

Abstract

This steel plate has a predetermined chemical composition, and the microstructure at a 1 / 4 depth position contains, in area ratios, 80% or more martensite, 0 to 15% total of ferrite, bainite, and pearlite, and 0 to 10% retained austenite; the microstructure of a surface layer portion contains, in area ratios, 60% or more total of ferrite, bainite, and pearlite, and 0 to 40% total of martensite and retained austenite; the average C content in a range from a position 10 μm from the surface along the plate thickness direction to a position 20 μm from the surface along the plate thickness direction is 0.085 mass% or less; the maximum C content in a range from the surface to a position 30 μm along the plate thickness direction is 0.130 mass% or less; the tensile strength is 1470 MPa or more; and the VDA bend angle is 75° or more.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet, a manufacturing method thereof, and a part. This application claims priority based on Japanese Patent Application No. 2024-069146, filed on April 22, 2024, the contents of which are incorporated herein by reference. [Background technology]

[0002] In today's highly specialized industrial technology fields, the materials used in each field are required to have specialized and advanced performance. In particular, with regard to automotive steel sheets, there has been a significant increase in demand for thin-wall, highly formable, high-tensile steel sheets to reduce the weight of automobile bodies and improve fuel efficiency, due to considerations of the global environment. Furthermore, among automotive components, collision components require not only high strength but also excellent bendability in order to absorb energy during a collision. Furthermore, in recent years, high-strength hot-dip galvanized steel sheets and high-strength alloyed hot-dip galvanized steel sheets, which have a zinc plating layer on the surface of the steel sheet, have also been used to ensure sufficient corrosion resistance of vehicle bodies and parts.

[0003] For example, Patent Document 1 discloses a hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet having a strength of 980 MPa or more, which are excellent in coatability, balance between strength and ductility, workability such as bendability and hole expandability, and delayed fracture resistance, and a method for manufacturing the same.

[0004] However, there are challenges with the use of high-strength galvanized and galvannealed steel sheets for automotive parts. Resistance spot welding is primarily used for processes such as assembling automobile bodies and installing parts. Resistance spot welding involves clamping overlapping base materials between the tips of properly shaped electrodes, concentrating current and pressure on a relatively small area to apply localized heat. However, when galvanized steel sheets (hot-dip galvanized, electrogalvanized, or galvannealed) are resistance-welded for the assembly of car bodies and / or parts, a type of cracking called liquid metal embrittlement (LME) can occur in the spot welds. LME cracking occurs when the heat generated during resistance spot welding melts the zinc in the galvanized layer, penetrating the grain boundaries of the steel sheet structure at the weld, and applying tensile stress to the resulting cracks. The conditions for cracking to occur are that molten zinc comes into contact with solid steel sheet during welding, and that tensile stress (strain) acts at that point. The higher the strength of the steel sheet, the greater the susceptibility to LME cracking tends to be. Since LME cracking reduces the strength of spot welds, it is important to suppress LME cracking (improve LME resistance).

[0005] However, Patent Document 1 does not disclose a steel sheet having a tensile strength of 1470 MPa or more and excellent bendability, nor does it consider any measures against LME cracking.

[0006] To address the above issues, technologies have been proposed to improve the LME resistance of galvanized steel sheets during spot welding. For example, Patent Document 2 discloses a steel sheet in which, in a cross-sectional structure cut in the width direction perpendicular to the rolling direction, the block diameter is specified in a first depth region of 1 to 10 μm from the surface, a second depth region of 10 to 60 μm from the surface, and a third depth region of 60 μm to ¼ of the sheet thickness from the surface. Patent Document 2 shows that by using a three-layer structure in which the block diameter is controlled to be inclined from the surface layer of the plate thickness toward the central layer of the plate thickness, even during spot welding, the block diameter is large when deformation occurs, and the softer layer (second layer) bears the strain, making it possible to suppress excessive increases in strain in the outermost layer (first layer), and thereby suppressing the occurrence of spot weld LME cracks.

[0007] However, Patent Document 2 does not take into consideration bendability, and it is believed that there is room for improvement in terms of bendability. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2016 / 111275 [Patent Document 2] International Publication No. 2021 / 251276 Summary of the Invention [Problem to be solved by the invention]

[0009] As described above, conventionally, no steel sheet has been proposed that has high strength and excellent bendability and that can further suppress LME cracking during spot welding. Therefore, an object of the present invention is to provide a steel sheet that has high strength and excellent bendability and can suppress LME cracking during spot welding, a method for manufacturing the same, and a part that includes the steel sheet. [Means for solving the problem]

[0010] The present inventors have investigated methods for improving strength and bendability while simultaneously suppressing LME cracking during spot welding, and have obtained the following findings. (A) In order to obtain high strength, it is effective to control the chemical composition and the microstructure at the 1 / 4 depth position. (B) By providing a soft layer within a certain range from the surface, bendability is improved. (C) The soft layer can be formed by decarburization. (D) In ​​the soft layer, the amount of decarburization is increased more than usual, and the average C concentration and the maximum C concentration are set within a predetermined range, thereby making it possible to suppress the occurrence of LME cracking. (E) In order to obtain a soft layer having the above effects, it is more effective to accelerate the diffusion of C in γ and promote decarburization than under general conditions.

[0011] The present invention has been made in light of the above findings. [1] A steel sheet according to one embodiment of the present invention has a chemical composition, in mass%, of C: 0.18 to 0.30%, Si: 0.50 to 2.00%, Mn: 2.4 to 3.5%, Al: 0.001 to 0.100%, B: 0.0001 to 0.0050%, Nb: 0.035 to 0.100%, P: 0.015% or less, S: 0.0030% or less, N: 0.0200% or less, O: 0.0030% or less, Ti: 0 to 0.050%, Cr: 0 to 1.000%, Mo: 0-0.500%, W: 0-0.500%, Co: 0-0.500%, V: 0-0.100%, Ta: 0-0.100%, Sn: 0-0.050%, Sb: 0-0.050%, As: 0-0.050%, Ni: 0-1.000%, Cu: 0-1.000%, Ca: 0-0.050%, Zr: 0-0.050%, Mg: 0-0.050%, REM: 0-0.100%, and the balance: Fe and impurities. When a position 1 / 4 of the sheet thickness from the surface is defined as a 1 / 4 depth position and the range from the surface to 30 μm is defined as a surface layer portion, the microstructure at the 1 / 4 depth position contains, in area ratios, 80% or more of martensite, 0 to 15% of a total of ferrite, bainite, and pearlite, and 0 to 10% of retained austenite, and the microstructure of the surface layer portion contains, in area ratios, 60% or more of a total of ferrite, bainite, and pearlite and 0 to 40% of a total of martensite and retained austenite, the average C content in the range from 10 μm from the surface along the sheet thickness direction to 20 μm from the surface along the sheet thickness direction is 0.085 mass% or less, and the maximum C content in the range from the surface to 30 μm along the sheet thickness direction is 0.130 mass% or less, the tensile strength is 1470 MPa or more, and the VDA bend angle is 75° or more. [2] The steel sheet according to [1] has a chemical composition, in mass%, of Ti: 0.001 to 0.050%, Cr: 0.001 to 1.000%, Mo: 0.010 to 0.500%, W: 0.001 to 0.500%, Co: 0.010 to 0.500%, V: 0.001 to 0.100%, Ta: 0.001 to 0.100%, Sn: 0.001 to 0.050%, S It may contain one or more of b: 0.001 to 0.050%, As: 0.001 to 0.050%, Ni: 0.010 to 1.000%, Cu: 0.001 to 1.000%, Ca: 0.001 to 0.050%, Zr: 0.001 to 0.050%, Mg: 0.0001 to 0.050%, and REM: 0.001 to 0.100%. [3] The steel sheet according to [1] or [2] may have a hot-dip galvanized layer on the surface. [4] In the steel sheet according to [3], the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer. [5] A method for producing a steel sheet according to another aspect of the present invention comprises: [1] A method for producing a steel sheet according to the present invention, [1] The method includes a heating step of heating a slab having the chemical composition described above to a heating temperature of 1180°C or higher, a hot rolling step of hot-rolling the slab after the heating step to obtain a hot-rolled steel sheet, a coiling step of starting cooling of the hot-rolled steel sheet at 800°C or higher, cooling the hot-rolled steel sheet to a coiling temperature at an average cooling rate of 50°C / s or higher, and coiling the hot-rolled steel sheet at the coiling temperature of 400°C or lower, a cold rolling step of cold-rolling the hot-rolled steel sheet after the coiling step under conditions where the sheet thickness reduction rate is 5 to 50%, to obtain a cold-rolled steel sheet, and an annealing step of annealing the cold-rolled steel sheet. The hot rolling process includes rough rolling, which converts the slab into the hot-rolled steel sheet, and finish rolling, which further hot-rolls the hot-rolled steel sheet after the rough rolling by multiple passes. In the finish rolling, the surface temperature of the hot-rolled steel sheet before the start of the final three passes is set to 900°C or less, and the reduction rate in each of the final three passes is set to 30% or more. In the annealing process, the cold-rolled steel sheet is heated to 840°C at an average heating rate of 5°C / s or more, and is held at a temperature range of 840 to 900°C for 60 seconds or more in an atmosphere with a dew point of -15 to 10°C. [6] A component according to another embodiment of the present invention includes the steel plate according to [1] or [2]. [7] In the part described in [6], the steel sheet may have a hot-dip galvanized layer on the surface. [ 8 In the component described in [7], the hot-dip galvanized layer may be a galvannealed layer. [Effects of the Invention]

[0012] According to the above aspects of the present invention, it is possible to provide a steel sheet that has high strength and excellent bendability and can suppress LME cracking during spot welding, a method for manufacturing the same, and a part including the steel sheet. DETAILED DESCRIPTION OF THE INVENTION

[0013] A steel sheet according to one embodiment of the present invention (the steel sheet according to this embodiment), a part including the steel sheet, and methods for manufacturing the same will be described. In this embodiment, the position of 1 / 4 of the plate thickness from the surface along the plate thickness direction is defined as the 1 / 4 depth position, and the range from the surface to 30 μm is defined as the surface layer. Here, the reference surface for the 1 / 4 depth position and the surface layer portion is the surface of the base steel sheet excluding the plating layer when the steel sheet has a plating layer (when the steel sheet is a plated steel sheet having a base steel sheet and a plating layer).

[0014] <Steel plate> The steel sheet according to this embodiment has a predetermined chemical composition, and the microstructure at a 1 / 4 depth position contains, in area ratios, 80% or more of martensite, 0 to 15% of a total of ferrite, bainite, and pearlite, and 0 to 10% of retained austenite; the microstructure of a surface layer portion contains, in area ratios, 60% or more of a total of ferrite, bainite, and pearlite, and 0 to 40% of a total of martensite and retained austenite; the average C content in a range from 10 μm from the surface along the thickness direction to 20 μm from the surface along the thickness direction is 0.085 mass% or less; the maximum C content (maximum C concentration) in a range from the surface to 30 μm along the thickness direction is 0.130 mass% or less; the tensile strength is 1470 MPa or more; and the VDA bend angle is 75° or more. Each of these will be explained below.

[0015] [Chemical composition] The chemical composition of the steel sheet according to this embodiment will be described below. Unless otherwise specified, "%" indicating the content of each element in the chemical composition always means mass %.

[0016] (C: 0.18 to 0.30%) C is an essential element for increasing the strength of steel sheets. If the C content is less than 0.18%, sufficient tensile strength cannot be obtained. Therefore, the C content is set to 0.18% or more. The C content is preferably 0.23% or more. On the other hand, if the C content exceeds 0.30%, the weldability and bendability deteriorate. Therefore, the C content is set to 0.30% or less. From the viewpoint of suppressing deterioration of press formability and weldability, the C content is preferably 0.25% or less.

[0017] (Si: 0.50 to 2.00%) Si is a solid solution strengthening element and is an element that is effective in increasing the strength of steel sheets. To obtain the above effect, the Si content is set to 0.50% or more, and preferably 0.65% or more. On the other hand, excessive Si content may cause embrittlement of the steel sheet, resulting in reduced manufacturability and workability. Therefore, the Si content is set to 2.00% or less, and preferably 1.50% or less.

[0018] (Mn: 2.4 to 3.5%) Mn has the effect of improving the hardenability of steel and is an effective element for obtaining a microstructure mainly composed of martensite. If the Mn content is less than 2.4%, it becomes difficult to obtain the above effect sufficiently. Therefore, the Mn content is set to 2.4% or more. The Mn content is preferably 2.6% or more. On the other hand, if the Mn content exceeds 3.5%, the formation of α (ferrite) in the surface layer of the steel sheet is suppressed, and the desired bendability cannot be obtained. Therefore, the Mn content is set to 3.5% or less. The Mn content is preferably 3.0% or less.

[0019] (Al: 0.001 to 0.100%) Al is an element that has a deoxidizing effect on steel. To obtain the above effect, the Al content is set to 0.001% or more, and preferably 0.005% or more. On the other hand, if Al is added excessively, the effect saturates, leading to an increase in costs, and the transformation temperature of the steel rises, increasing the load during hot rolling and significantly impairing the flatness of the hot-rolled steel sheet, making it difficult to perform the subsequent cold rolling process. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.090% or less.

[0020] (B: 0.0001 to 0.0050%) B is an element that suppresses the formation of ferrite and pearlite during the cooling process from austenite and promotes the formation of low-temperature transformation structures such as martensite. B is also an element that is beneficial for increasing the strength of steel sheets. To achieve the above effects, the B content is set to 0.0001% or more. On the other hand, if the B content exceeds 0.0050%, coarse B oxides and borides that can become the starting points for voids during press forming may form in the steel, which may deteriorate the workability of the steel sheet. For this reason, the B content is set to 0.0050% or less.

[0021] (Nb: 0.035 to 0.100%) Nb is an element effective in suppressing recrystallization. In the steel sheet according to this embodiment, Nb suppresses recrystallization during rolling and the annealing process, thereby increasing the dislocation density of the structure before reverse transformation. This allows lattice defects to exist near the surface even after γ (austenite) transformation, increases the diffusion rate of C in γ during annealing, and promotes decarburization, forming a decarburized layer that is less susceptible to LME cracking. Therefore, to achieve the above effects, the Nb content is set to 0.035% or more. On the other hand, if the Nb content exceeds 0.100%, a large number of coarse Nb carbides that become the starting points for voids during press forming may precipitate, deteriorating the workability of the steel sheet. For this reason, the Nb content is set to 0.100% or less.

[0022] (P:0.015% or less) P is an element that segregates at grain boundaries, embrittling steel and deteriorating bendability. Therefore, the lower the P content, the better; even 0% is acceptable. However, taking into consideration the time and cost required to remove P, the P content is set to 0.015% or less. The P content is preferably 0.013% or less, and more preferably 0.010% or less.

[0023] (S :0.0030% or less) S is an element that forms sulfide-based inclusions and deteriorates bendability. Therefore, the lower the S content, the better; even 0% is acceptable. However, taking into consideration the time and cost required to remove S, the S content is set to 0.0030% or less. The S content is preferably 0.0010% or less.

[0024] (N: 0.0200% or less) N is an element that forms coarse nitrides in steel sheets and deteriorates the bendability and hole expandability of the steel sheets. If the N content exceeds 0.0200%, the deterioration becomes significant, so the N content is set to 0.0200% or less. The N content is preferably 0.0060% or less or 0.0050% or less. On the other hand, the N content may be 0%, but if the N content is less than 0.0001%, the manufacturing cost increases significantly. Therefore, the N content may be 0.0001% or more, or 0.0005% or more.

[0025] (O:0.0030% or less) O is an element that forms coarse oxides in steel and deteriorates bendability and hole expandability. If the O content exceeds 0.0030%, the deterioration of the above properties becomes significant. For this reason, the O content is set to 0.0030% or less. The O content is preferably 0.0020% or less. A low O content is preferable, and 0% is acceptable, but an O content of less than 0.0001% leads to excessive cost increases and is not economically preferable. For this reason, the O content may be set to 0.0001% or more. The O content may be set to 0.0010% or more.

[0026] The steel sheet according to this embodiment may contain the above elements with the balance being Fe and impurities. On the other hand, the steel sheet according to this embodiment may further contain one or more elements (optional elements) selected from the following: Ti, Cr, Mo, W, Co, V, Ta, Sn, Sb, As, Ni, Cu, Ca, Zr, Mg, and REM. Since the optional elements do not have to be contained, the lower limit is 0%.

[0027] (Ti: 0 to 0.050%) Ti is an element that is effective in controlling the morphology of carbides. Therefore, Ti may be contained. To obtain the above effects, the Ti content is preferably 0.001% or more. On the other hand, if the Ti content exceeds 0.050%, coarse Ti oxides or Ti carbonitrides may be present in the steel, which may reduce the workability of the steel sheet. Therefore, if Ti is contained, the Ti content is set to 0.050% or less.

[0028] (Cr: 0 to 1.000%) Cr is an element that improves hardenability and contributes to increasing the strength of the steel sheet. Therefore, Cr may be contained. To obtain the above effects, the Cr content is preferably 0.001% or more. On the other hand, if the Cr content exceeds 1.000%, Cr may segregate in the center of the steel sheet, forming coarse Cr carbides, which may reduce cold formability. Therefore, if Cr is contained, the Cr content is set to 1.000% or less. The Cr content is preferably 0.800% or less or 0.600% or less.

[0029] (Mo: 0 to 0.500%) Mo is an element that is effective in increasing the strength of steel sheets, and therefore Mo may be contained. To obtain the above effect, the Mo content is preferably 0.010% or more. On the other hand, if the Mo content exceeds 0.500%, the cost increases and coarse Mo carbides are formed, which may reduce the cold workability of the steel sheet. Therefore, if Mo is contained, the Mo content is set to 0.500% or less. The Mo content is preferably 0.400% or less or 0.300% or less.

[0030] (W:0~0.500%) W is a carbide-forming element and is an element effective in increasing the strength of steel sheets. Therefore, W may be contained. To obtain the above effects, the W content is preferably 0.001% or more. The W content is more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if the W content is too high, not only will the effect saturate but the cost will also increase. Therefore, if W is contained, the W content is set to 0.500% or less. The W content is preferably 0.400% or less, and more preferably 0.300% or less.

[0031] (Co: 0 to 0.500%) Co is an element that is effective in increasing the strength of steel sheets, and therefore Co may be contained. To obtain the above effects, the Co content is preferably 0.010% or more, and more preferably 0.050% or more. On the other hand, if the Co content is too high, the ductility of the steel sheet may decrease, resulting in decreased hole expandability and bendability. Therefore, if Co is contained, the Co content is set to 0.500% or less. The Co content is preferably 0.400% or less or 0.300% or less.

[0032] (V: 0 to 0.100%) V is an element that is effective in controlling the morphology of carbides and is also effective in improving the toughness of steel sheets. Therefore, V may be contained. To obtain the above effects, the V content is preferably 0.001% or more. On the other hand, if the V content exceeds 0.100%, a large number of fine V carbides precipitate, which increases the strength of the steel sheet but significantly reduces ductility and workability. Therefore, if V is contained, the V content is set to 0.100% or less. The V content is preferably 0.080% or less.

[0033] (Ta: 0 to 0.100%) Ta is an element that is effective in controlling the morphology of carbides and improving the strength of the steel sheet. Therefore, Ta may be added. To obtain the above effects, the Ta content is preferably 0.001% or more. On the other hand, if the Ta content is too high, a large number of fine Ta carbides will precipitate, which may reduce the ductility, hole expandability, and bendability of the steel sheet. Therefore, if Ta is contained, the Ta content is set to 0.100% or less. The Ta content is preferably 0.020% or less, and more preferably 0.011% or less.

[0034] (Sn: 0 to 0.050%) Sn is an element that can be contained in steel sheets when scrap is used as the raw material for the steel sheets. Furthermore, Sn is an element that may cause a decrease in the hole expandability and bendability of the steel sheets due to the embrittlement of ferrite. Therefore, the lower the Sn content, the better. The Sn content is set to 0.050% or less, and preferably 0.040% or less. The Sn content may be 0%, but reducing the Sn content to less than 0.001% would result in an excessive increase in refining costs, so the Sn content may be set to 0.001% or more.

[0035] (Sb: 0 to 0.050%) Like Sn, Sb is an element that can be contained in steel sheets when scrap is used as a raw material for the steel sheets. Sb is an element that strongly segregates at grain boundaries and may cause embrittlement of the grain boundaries, a decrease in ductility, and even a decrease in hole expandability and bendability. Therefore, the lower the Sb content, the better. The Sb content is set to 0.050% or less, and preferably 0.040% or less. The Sb content may be 0%, but reducing the Sb content to less than 0.001% results in an excessive increase in refining costs, so the Sb content may be set to 0.001% or more.

[0036] (As: 0 to 0.050%) Like Sn and Sb, As is an element that can be contained in steel sheets when scrap is used as the raw material for the steel sheets. As is an element that strongly segregates at grain boundaries and may cause a decrease in hole expandability and bendability. Therefore, the lower the As content, the better. The As content is set to 0.050% or less, and preferably 0.040% or less. The As content may be 0%, but reducing the As content to less than 0.001% results in an excessive increase in refining costs, so the As content may be set to 0.001% or more.

[0037] (Ni: 0 to 1.000%) Ni is an element that is effective in improving the strength of steel sheets. Therefore, Ni may be contained. To obtain the above effect, the Ni content is preferably 0.001% or more, and more preferably 0.010% or more. On the other hand, if the Ni content is too high, the ductility of the steel sheet may decrease, resulting in decreased hole expandability and bendability. Therefore, when Ni is contained, the Ni content is set to 1.000% or less. The Ni content is preferably 0.600% or less, and more preferably 0.300% or less.

[0038] (Cu: 0 to 1.000%) Cu is an element that contributes to improving the strength of the steel sheet. Therefore, Cu may be contained. To obtain the above effects, the Cu content is preferably 0.001% or more. On the other hand, if the Cu content is too high, red shortness may occur, which may reduce productivity during hot rolling. Furthermore, the formation of coarse inclusions may also reduce hole expandability and bendability. Therefore, if Cu is contained, the Cu content should be 1.000% or less. The Cu content is preferably 0.700% or less or 0.600% or less, and more preferably 0.300% or less.

[0039] (Ca: 0 to 0.050%) Ca is an element that is effective in controlling the morphology of sulfides even in small amounts. Therefore, Ca may be contained. To obtain the above effects, the Ca content is preferably 0.001% or more. On the other hand, if the Ca content is too high, coarse Ca oxides may be generated. These Ca oxides become the starting point for cracks during cold forming, so if the Ca content is too high, hole expandability and bendability may deteriorate. Therefore, if Ca is contained, the Ca content is set to 0.050% or less. The Ca content is preferably 0.030% or less.

[0040] (Zr: 0 to 0.050%) Zr is an element that is effective in controlling the morphology of sulfides even in small amounts. Therefore, Zr may be contained. To obtain the above effects, the Zr content is preferably 0.001% or more. On the other hand, if the Zr content is too high, coarse Zr oxides may be generated, which may reduce the hole expandability and bendability. Therefore, if Zr is contained, the Zr content is set to 0.050% or less. The Zr content is preferably 0.040% or less.

[0041] (Mg: 0 to 0.050%) Mg is an element that controls the morphology of sulfides and oxides and contributes to improving the bendability of the steel sheet. Therefore, Mg may be added. To obtain the above effects, the Mg content is preferably 0.0001% or more. On the other hand, if the Mg content is too high, coarse inclusions may form, which may reduce the hole expandability and bendability. Therefore, if Mg is contained, the Mg content is set to 0.050% or less, and preferably 0.040% or less.

[0042] (REM: 0 to 0.100%) REM is an element that effectively controls the morphology of sulfides even when contained in trace amounts. Therefore, REM may be contained in steel. To obtain the above effect, the REM content is preferably 0.001% or more. On the other hand, if the REM content is too high, coarse REM oxides are generated, which may reduce workability, fracture resistance, hole expandability, and bendability. Therefore, if REM is contained, the REM content is set to 0.100% or less. The REM content is preferably 0.080% or less or 0.060% or less. Here, REM stands for Rare Earth Metal, a collective term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). "REM content" refers to the total content of these rare earth elements.

[0043] (balance: Fe and impurities) As described above, the chemical composition of the steel sheet according to this embodiment may include C, Si, Mn, Al, B, Nb, P, S, N, O, with the balance being Fe and impurities, or may include C, Si, Mn, Al, B, Nb, P, S, N, O, and one or more optional elements, with the balance being Fe and impurities. Here, the term "impurities" refers to elements that are mixed in during industrial steel production due to various factors in raw materials such as ores and scraps, and in the manufacturing process, and whose presence is permitted to the extent that they do not impair the properties of the steel sheet according to the present embodiment. The term also includes elements that are not intentionally added to the steel sheet.

[0044] The chemical composition of the steel sheet according to this embodiment may be measured by a general method. For example, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) can be used for measurement. However, elements that are difficult to measure with ICP-AES can be measured by other methods. For example, C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-non-dispersive infrared absorption method. The sample for measurement is taken from around the 1 / 4 depth position. When the chemical composition of the slab is known, the chemical composition of the slab may be used as the chemical composition of the steel sheet according to this embodiment.

[0045] [Microstructure at 1 / 4 depth] Next, the microstructure (metal structure) at the 1 / 4 depth position of the steel plate according to this embodiment will be described. In the description of the microstructure of the steel plate according to this embodiment, the structure fraction is expressed as an area fraction. Therefore, unless otherwise specified, "%" represents "area %".

[0046] (Martensite: 80% or more) Martensite is a hard structure, which contributes to improving tensile strength. To obtain a tensile strength of 1470 MPa or more, the area ratio of martensite is set to 80% or more. The area ratio of martensite is preferably 85% or more, and more preferably 90% or more. The area ratio of martensite may be 100%. In this embodiment, "martensite" refers to fresh martensite and tempered martensite. Fresh martensite is martensite that does not contain iron carbide. Furthermore, "tempered martensite" is martensite that contains iron carbide.

[0047] (Total of ferrite, bainite, and pearlite: 0-15%) Ferrite is a soft phase that forms during intercritical annealing or slow cooling after annealing. When ferrite is mixed with a hard phase such as martensite, it improves the ductility of steel sheets. However, to achieve a certain level of strength, the area fraction of ferrite must be limited. Generally, bainite is a phase that is formed by holding the material at 400 to 550°C for a certain period of time during the cooling process after holding it at the annealing temperature. Bainite is softer than martensite and has the effect of improving ductility, but in order to achieve a certain level of strength, its area fraction must be limited, just like ferrite. Pearlite is a structure that contains cementite within its structure and consumes carbon (C) in steel, which contributes to improving strength. Therefore, if the area ratio of pearlite is excessive, the strength of the steel sheet decreases. Therefore, the total area ratio of ferrite, bainite, and pearlite is set to 15% or less. These structures (phases) do not need to be included, so the lower limit of the area ratio is 0%.

[0048] (Residual austenite: 0 to 10%) The retained austenite is a structure that contributes to improving elongation through transformation induced plasticity (TRIP). The steel sheet according to this embodiment does not need to contain retained austenite (the area fraction may be 0%), but in order to obtain excellent elongation, it is preferable that the area fraction of retained austenite is 5% or more. On the other hand, if the area fraction of retained austenite is excessive, the grain size of the retained austenite becomes large. Such retained austenite with a large grain size becomes coarse and hard martensite after deformation. In this case, cracks are more likely to start, and bendability deteriorates. Therefore, the area fraction of retained austenite is set to 10% or less. The area fraction of retained austenite is preferably 5% or less.

[0049] To identify each structure and calculate its area ratio, first determine the area that should be retained austenite in a specified observation area, and then identify ferrite, bainite, martensite, or pearlite in the same observation area. It is desirable to take samples from within 100 mm from the edge in the width direction, but if the original material is small and this is difficult, take samples from the center of the material. Specifically, the area fraction of retained austenite is measured as follows. After mirror-finishing the observation surface, colloidal silica polishing or electrolytic polishing are used to polish the observation surface. Using an EBSD (Electron Backscatter Diffraction) (EBSD) attached to a scanning electron microscope, diffracted electrons are measured in a 100 μm × 100 μm square area, centered at 1 / 4 depth of the steel sheet, at 0.2 μm intervals (grid arrangement) in both the thickness and rolling directions. The resulting pseudo-Kikuchi patterns are analyzed to identify the crystal orientation and crystal system. The incident electron acceleration voltage is 15 kV. The sample preparation conditions are within the range recommended by the Japan Society for Materials Science's standard, "Crystal Orientation Measurement Standard for Materials Evaluation by Electron Backscatter Diffraction (EBSD) Method." The measurement points detected as FCC phase in the measurement data are considered retained austenite, and the ratio of the number of measurement points detected as FCC phase to the total number of measurement points is considered the area fraction of retained austenite. The area ratios of ferrite, bainite, martensite, and pearlite can be measured by the following method. Specifically, the same square area as that used for the observation of retained austenite above is etched with nital solution and photographed (magnification: 5000x) with a field emission scanning electron microscope (FE-SEM). From the obtained secondary electron image of the structure, a point counting method is used at intervals of 2 μm (grid arrangement), and the ratio to the total number of measurement points for each structure is taken as the area ratio. The observation area is set to 1200 μm to minimize variations depending on the observation location. 2 If the area of ​​one field of view is insufficient, the area should be 1200 μm or more. 2 It is sufficient to measure multiple fields of view so that the observation area is equal to or larger than 2500 μm. 2 The following may also be used.

[0050] In this embodiment, the rolling direction is determined by the following method. The Z-plane of the plate (the plane parallel to both the longitudinal and transverse directions of the plate) is polished to a depth of 1 / 4 and then mirror-polished. An Mn concentration map of a 500 μm × 500 μm area is then obtained using an EPMA. When the solidification segregation of Mn is measured as streaks, the longitudinal direction of the streaks is determined to be the rolling direction. When the rolling direction of a steel sheet is known in advance, the rolling direction of the steel sheet may be determined without using the above-mentioned determination method. Furthermore, even when a steel sheet is processed into a part, the rolling direction can be determined using the above-mentioned method for a lightly processed part of the part (for example, a flat part that has received relatively little processing).

[0051] Each structure is judged as follows: Ferrite has a granular or needle-like shape and does not contain iron-based carbides inside. Bainite has a lath-like shape (lath structure) and is a region where iron-based carbides with a major axis of 20 nm or more exist inside the lath structure and the carbides elongate in the same direction. Fresh martensite has a lath-like shape (lath structure) and is a region where iron-based carbides with a major axis of 20 nm or more exist inside the lath structure and the carbides elongate in multiple different directions. Furthermore, the region where ferrite and cementite are in a lamellar shape is called pearlite. When identifying ferrite, bainite, martensite, and pearlite, the above-mentioned visual identification is not performed on areas that have been determined to be retained austenite in the preceding identification of retained austenite. However, the area ratio is calculated as a ratio to the total number of measurement points in the observation range, including the retained austenite area.

[0052] If the total area ratio of each texture obtained by the above evaluation method is different from 100%, the area ratio of each texture is multiplied by 100 / (total area ratio of each texture).

[0053] [Microstructure of the surface layer] The microstructure of the surface layer has a particularly large effect on bendability and LME resistance (suppression of LME cracking).

[0054] (Total of ferrite, bainite, and pearlite: 60% or more) Ferrite, bainite, and pearlite contribute to improving the bendability of the steel sheet. Therefore, the total area ratio of these is set to 60% or more. The total area ratio of ferrite, bainite, and pearlite is preferably 80% or more. The upper limit of these area ratios is not limited and may be 100%.

[0055] (Total of martensite and retained austenite: 0-40%) If the total area ratio of one or both of martensite and retained austenite exceeds 40%, the bendability deteriorates, so the total area ratio is set to 40% or less.

[0056] The identification and calculation of the area ratio of each structure (phase) in the surface layer can be done in the same way as the calculation of the area ratio of each structure at the 1 / 4 depth position, except for the measurement position. In the case of the surface layer, the measurement position is a square area from the surface of the steel plate to a depth of 30 μm (a square area with one side at the surface, 30 μm in the plate thickness direction × 30 μm in the rolling direction).

[0057] [Average C content in the range from 10 μm from the surface along the thickness direction to 20 μm from the surface along the thickness direction] [Maximum carbon content in the range from the surface to 30 μm along the thickness direction] If the C content is high, LME cracking is more likely to occur. Therefore, in the steel sheet according to this embodiment, a soft layer (decarburized layer) is formed to reduce the C content near the surface. Specifically, the average C content in the range from a position 10 μm from the surface along the sheet thickness direction to a position 20 μm from the surface along the sheet thickness direction is set to 0.085 mass% or less. There is no need to set a lower limit for the average C content, but it may be 0.001 mass% or more. Furthermore, even if the average C content is below a certain level, if there are locations with a high C content (concentration) near the surface, LME cracking is more likely to occur during processes in which the steel sheet is exposed to high temperatures, such as welding, due to the influence of C diffusing from C enrichment points near the surface toward the surface. Therefore, the maximum C content near the surface is reduced. Specifically, the maximum C content in the range from the surface to a position 30 μm in the sheet thickness direction is set to 0.130 mass% or less. There is no need to set a lower limit for the maximum C content, but it may be 0.001 mass% or more. It is known that high dew-point annealing can form a decarburized layer near the surface, but in steel sheets having a C content of 0.18 mass% or more at the 1 / 4 depth position, ordinary high dew-point annealing alone is not sufficient to form a decarburized layer having the above-mentioned average and maximum C contents. Therefore, when producing a steel sheet according to this embodiment, the content of Nb, which suppresses recrystallization during rolling, is increased, and the production conditions, particularly the conditions in the hot rolling and annealing steps, are controlled as described below.

[0058] The average C content in the range from 10 μm from the surface along the thickness direction to 20 μm from the surface along the thickness direction, and the maximum C content in the range from the surface to 30 μm along the thickness direction, are obtained by performing glow discharge optical emission spectroscopy (GDS) and measuring the C element concentration profile from the surface to a specified position. When measuring, a sample is taken from the same position as in the case of microstructure observation, and measurements are performed on this sample so that the depth of the measurement range in the plate thickness direction is 200 μm or more, and so that there are 10,000 or more measurement points up to a depth of 200 μm. When the steel sheet has a surface plating layer (hot-dip galvanized layer, galvannealed hot-dip galvanized layer, etc.), in the case of a plated steel sheet, the surface position of the steel sheet is defined as follows. Specifically, the Mn concentration profile was measured by GDS from the surface of the steel sheet, including the coating layer, to a position 200 μm in the sheet thickness direction (a position 200 μm deep). The position where the Mn content is half the average value of the Mn content in the 190-200 μm depth range was defined as the surface (most superficial position). The C profile was determined by measuring the C element concentration profile from the data used in the Mn measurement, with the most superficial position defined above as the origin. The average C content in the range from 10 μm in the sheet thickness direction to 20 μm in the sheet thickness direction from the surface, and the maximum C content in the range from 30 μm in the sheet thickness direction from the surface were calculated. However, the entire measurement value was corrected so that the average C content in the 190-200 μm depth range matched the C content at the 1 / 4 depth position.

[0059] [Tensile strength] The steel sheet according to this embodiment has a tensile strength (TS) of 1470 MPa or more, which is a strength that contributes to weight reduction and impact resistance of automobile bodies. There is no upper limit to the tensile strength, and the tensile strength may be 1650 MPa or less in order to ensure sufficient bendability.

[0060] [VDA bending angle] In the steel plate according to this embodiment, the VDA bend angle is set to 75° or more to suppress fracture during a collision. The VDA bend angle can be determined by a bending test in accordance with VDA (German Association of the Automotive Industry) standard 238-100. Due to the nature of the test, the VDA bend angle is set to 180° or less.

[0061] Plate Thickness Although there are no limitations on the thickness of the steel plate according to this embodiment, if the thickness is too large, it becomes difficult to obtain the effect of reducing the weight of the vehicle body, so the thickness is preferably 2.3 mm or less, more preferably 1.6 mm or less, and even more preferably 1.2 mm or less. Furthermore, if the plate thickness is too thin, it becomes difficult to control the plate thickness, so the plate thickness may be set to 0.8 mm or more, or 1.0 mm or more.

[0062] [Plating layer] The steel sheet according to this embodiment may have a zinc-plated layer on the surface. Providing a plated layer on the surface improves corrosion resistance. Even if automotive steel sheets are strengthened, they may not be able to be thinned below a certain thickness due to concerns about holes due to corrosion. One of the purposes of strengthening steel sheets is to reduce weight by thinning them, so even if a steel sheet is developed, its application areas will be limited if its corrosion resistance is low. One possible method for solving these problems is to apply a highly corrosion-resistant coating, such as hot-dip galvanizing, to the steel sheet. The steel sheet according to this embodiment is capable of hot-dip galvanizing because the steel sheet composition is controlled as described above. The hot-dip galvanized layer may be an alloyed hot-dip galvanized layer.

[0063] <Parts> The part according to this embodiment can be obtained by cutting the steel plate according to this embodiment to a predetermined size as needed, processing it into a predetermined shape by pressing or the like as needed, and joining it to other members by welding or the like as needed. Therefore, the component according to this embodiment includes, at least in part, the steel plate according to this embodiment having the above-described characteristics.

[0064] A sample is taken from the steel material constituting the part according to this embodiment, and measurements are made in the same manner as for the steel plate according to this embodiment. If the results show that the chemical composition, the microstructure at a 1 / 4 depth position, the microstructure of the surface layer, the average C content in the range from 10 μm from the surface along the thickness direction to 20 μm from the surface along the thickness direction, the maximum C content in the range from the surface along the thickness direction to 30 μm, the tensile strength, and the VDA bending angle are equivalent to those of the steel plate according to this embodiment, then it can be said that the part according to this embodiment includes the steel plate according to this embodiment.

[0065] <Manufacturing method> The steel sheet according to this embodiment can achieve the above-described effects regardless of the manufacturing method, but is preferably manufactured by a manufacturing method including the following steps, since it can be manufactured stably. (I) a heating step in which a slab having a predetermined chemical composition is heated to a heating temperature of 1180°C or higher; (II) a hot rolling step of hot rolling the slab after the heating step to obtain a hot-rolled steel sheet; (III) a coiling step of cooling the hot-rolled steel sheet and coiling it at a coiling temperature of 400°C or less; (IV) a cold rolling step of cold-rolling the hot-rolled steel sheet after the coiling step under conditions such that the sheet thickness reduction rate is 5 to 50% to obtain a cold-rolled steel sheet; (V) An annealing step of annealing the cold-rolled steel sheet. Furthermore, the part according to this embodiment can be manufactured by subjecting the steel sheet according to this embodiment obtained as described above to the following steps. (VI) a processing step of forming the steel plate according to the present embodiment into a predetermined size and / or shape, if necessary; (VII) A welding step of joining the steel plate according to this embodiment to another steel material by welding, as necessary. Each of these will be explained below. For conditions and steps not described, known conditions can be applied.

[0066] [Heating process] In the heating step, the slab having a predetermined chemical composition obtained through the casting step is heated to a heating temperature of 1180°C or higher. If the heating temperature is less than 1180°C, the NbC formed in the casting process cannot be brought into solution, and the effect of Nb in suppressing recrystallization in the subsequent hot rolling process cannot be obtained. The heating temperature is preferably 1220°C or higher. There is no upper limit to the heating temperature, but the heating temperature may be 1350° C. or less in terms of fuel costs.

[0067] The chemical composition of the slab to be subjected to the heating process does not change substantially during the manufacturing process except for a certain range from the surface that becomes the decarburized layer, for example, at a 1 / 4 depth position. Therefore, the chemical composition of the slab may be made equivalent to the chemical composition at a 1 / 4 depth position of the target steel plate.

[0068] [Hot rolling process] In the hot rolling process, the slab after the heating process is hot-rolled to obtain a hot-rolled steel sheet. The hot rolling process includes rough rolling, which converts the slab into a hot-rolled steel sheet, and finish rolling, which further hot-rolls the hot-rolled steel sheet after rough rolling by multiple passes. In the hot rolling process, in finish rolling, the surface temperature of the hot-rolled steel sheet before the start of the final three passes is set to 820°C or higher and 900°C or lower, and the reduction rate (sheet thickness reduction rate) of each of the final three passes (in the case of n-pass rolling, the three passes are the n-2 pass, the n-1 pass, and the n pass) is set to 30% or higher. It is preferable that the finish rolling be performed continuously using a rolling mill having multiple stands. Dislocations are introduced into the austenite by the rolling. In the steel sheet according to this embodiment, recrystallization is suppressed by Nb, so the introduced dislocations are carried over to subsequent processes. If the surface temperature of the steel sheet before the start of the final three passes of finish rolling (for example, before the start of the fifth pass in the case of seven passes) exceeds 900°C, or if any of the rolling reductions in the final three passes is less than 30%, sufficient dislocations cannot be introduced. There is no lower limit to the rolling start temperature, but a temperature of 820°C or higher is preferred to suppress the formation of a soft phase in the surface layer. Preferably, the surface temperature of the hot-rolled steel sheet before the start of the final three passes is 860°C or less. Also, preferably, the reduction ratio in each of the final three passes is 35% or more. There is no upper limit to the reduction ratio in the final three passes, but in terms of equipment load, the reduction ratio in each of the final three passes may be 50% or less.

[0069] [Winding process] In the coiling process, the hot-rolled steel sheet after the hot rolling process is coiled at a coiling temperature of 400°C or less. In order to prevent the formation of a soft structure in the surface layer, water cooling is performed after the end of rolling, and cooling is started at 800°C or higher, with the average cooling rate from the start of cooling to the coiling temperature (cooling stop temperature) being 50°C / s or more. The cooling start temperature may be 850°C or less. Furthermore, in terms of facility capacity, the average cooling rate from the start of cooling to the coiling temperature (cooling stop temperature) may be 200°C / s or less. By performing the above cooling and coiling at a low temperature, a large amount of lattice defects can be introduced into the austenite after reverse transformation. If the average cooling rate is slow or the coiling temperature exceeds 400°C, sufficient lattice defects (dislocations and grain boundaries) will not remain. From the viewpoint of lattice defects, a lower coiling temperature is preferable. However, if the coiling temperature is less than 250°C, the steel sheet will become excessively hardened, and there is a risk of the steel sheet breaking during cold rolling. Therefore, the coiling temperature is preferably 250°C or higher.

[0070] [Cold rolling process] In the cold rolling step, the hot rolled steel sheet after the coiling step is cold rolled under conditions where the sheet thickness reduction rate (rolling reduction rate) is 5 to 50% to obtain a cold rolled steel sheet. If the thickness reduction rate exceeds 50%, the dislocation density is reduced by recrystallization during the annealing process. In this case, the desired decarburized layer cannot be obtained. Therefore, the thickness reduction rate is set to 50% or less. From the viewpoint of suppressing fracture during cold rolling, the sheet may be reheated to 200°C or less before cold rolling. If the sheet is reheated to a temperature above 200°C, the dislocation density decreases and the desired decarburized layer cannot be obtained. On the other hand, if the thickness reduction rate is too low, thickness variation during hot rolling becomes a problem, so the thickness reduction rate is set to 5% or more.

[0071] [Annealing process] In the annealing process, the cold-rolled steel sheet is heated to 840°C at an average heating rate of 5°C / s or more, and then annealed at an annealing temperature in the range of 840 to 900°C for 60 seconds or more in an atmosphere with a dew point of -15 to 10°C. This annealing process results in a desired decarburized layer. By annealing steel sheet under the above conditions, which contains a certain amount of lattice defects due to control in the previous process, decarburization is promoted more than usual, and the desired decarburized layer is obtained. If the dew point is below -15°C or the holding time is less than 60 seconds, the decarburization is insufficient. Also, if the annealing temperature is below 840°C, the strength may be insufficient. If the annealing temperature is above 900°C, the austenite grains become coarse, and the amount of soft ferrite formed in the surface layer is insufficient. If the average heating rate up to 840°C is less than 5°C / s, recrystallization will proceed during heating, the number of lattice defects introduced by hot rolling will decrease, and decarburization will not proceed sufficiently. The average heating rate is not limited as long as it is 5°C / s or more, but it is preferably 50°C / s or less from the viewpoint of temperature controllability when the maximum temperature is reached. When the dew point exceeds 10°C, oxides of elements other than C are formed on the surface, inhibiting the decarburization reaction, forming a C-enriched layer on the surface, and increasing the maximum C content near the surface. The upper limit of the holding time is not limited, but may be 200 seconds or less from the viewpoint of productivity. After the maximum temperature is reached, quenching and tempering are performed to obtain a martensite structure, and an isothermal holding process is performed to obtain ferrite, bainite, and pearlite structures.

[0072] [Plating process] When a plating layer is formed on the surface of the steel sheet, a plating step may be further carried out. The plating step may be carried out during the cooling after the above-mentioned holding in the annealing step, or may be carried out after once cooling to room temperature. When forming a hot-dip galvanized layer on the surface as a plating layer and manufacturing a cold-rolled steel sheet (hot-dip galvanized steel sheet) having a hot-dip galvanized layer, the steel sheet may be immersed in a plating bath at an equivalent temperature while the steel sheet temperature is above 425°C and below 600°C to perform hot-dip galvanization.

[0073] [Alloying process] When manufacturing a cold-rolled steel sheet having a galvannealed layer on its surface (galvannealed steel sheet), after forming the galvannealed layer on the surface of the steel sheet in the above-mentioned plating step, the hot-dip galvannealed layer may be converted into a galvannealed layer by, for example, performing an alloying heat treatment by heating to more than 450°C and less than 600°C. The thermal history associated with the plating step and the alloying step is included in the thermal history to be controlled in the annealing step if it is performed during cooling in the annealing step, but is not included in the thermal history to be controlled in the annealing step if it is performed after cooling to room temperature in the annealing step.

[0074] [Processing process] When processing is performed, the steel plate according to this embodiment is formed into a predetermined size and / or shape in the processing step. The processing method may be a known method, for example, shearing, punching, drilling, bending, pressing, stretching, or the like to process into a predetermined size and / or shape.

[0075] [Welding process] When the welding step is performed, the steel plate according to the present embodiment (including that which has undergone a processing step) and another steel material are joined by welding such as spot welding. There are no limitations on the welding conditions, and known conditions may be applied. [Example]

[0076] Slabs having the chemical compositions shown in Tables 1-1 and 1-2 were obtained by continuous casting. These slabs were heated to 1250°C and then subjected to hot rolling including rough rolling and finish rolling. In finish rolling, the surface temperature of the hot-rolled steel sheet before the start of the final three passes in a rolling mill with multiple stands and the reduction ratios in each of the final three passes were as shown in Tables 2-1 and 2-2. After hot rolling, the hot-rolled steel sheet was water-cooled from a temperature of 800°C or higher at an average cooling rate of 50°C / s or higher from the start of cooling to the coiling temperature, and was coiled at the coiling temperatures shown in Tables 2-1 and 2-2. The hot-rolled steel sheets after the coiling process were cold-rolled at the thickness reduction rates shown in Tables 2-1 and 2-2 to obtain cold-rolled steel sheets with thicknesses of 0.8 to 1.6 mm. The cold-rolled steel sheets after cold rolling were annealed under the conditions shown in Tables 2-1 and 2-2. In some examples, hot-dip galvanization was performed during cooling after holding at the annealing temperature in the annealing step to form a hot-dip galvanized layer on the surface. Further, in some of these examples, alloying was performed to form the hot-dip galvanized layer as an alloyed hot-dip galvanized layer.

[0077] The area ratio of the microstructure at the 1 / 4 depth position and in the surface layer, and the C concentration distribution in the surface layer were evaluated as described above for the obtained steel sheets. In addition, the tensile strength, bendability, and LME resistance during spot welding were evaluated as follows.

[0078] [Tensile strength] The tensile strength (TS) was determined by taking a JIS No. 5 tensile test piece from the steel sheet in a direction perpendicular to the rolling direction and conducting a tensile test on this test piece in accordance with JIS Z 2241: 2022. The method for determining the rolling direction was as described above.

[0079] [Bendability] A bending test was conducted in accordance with VDA (German Association of the Automotive Industry) 238-100 to evaluate the VDA bending angle. The limiting bending angle was evaluated when the bending ridgeline was parallel to the rolling direction. A VDA bending angle of 75° or more was considered to have excellent bendability.

[0080] [LME resistance] The obtained steel sheet was placed on top of two commercially available GA steel sheets with a tensile strength of 980 MPa, and spot welding was carried out at a current value and current duration that resulted in a nugget diameter of 5 mm. At this time, the angle (strike angle) between the spot welding electrode and the plate was 5 degrees, and the clearance was 0.3 mm. The cross section of the center of the nugget after spot welding was observed to determine whether or not there was a crack in the HAZ, and if there was a crack, the length of the crack in the HAZ was measured. The crack length in the HAZ was defined as the straight line connecting the crack start point on the sheet surface to the end point of the crack on the observed cross section. The above spot welding and cross-sectional observation were performed on five samples for each test number. Samples with a maximum crack length of 30 μm or more were judged to have poor LME resistance (NG), and samples with no cracks or a maximum crack length of less than 30 μm were judged to have excellent LME resistance (OK).

[0081] [Table 1-1]

[0082] [Table 1-2]

[0083] [Table 2-1]

[0084] [Table 2-2]

[0085] [Table 3-1]

[0086] [Table 3-2]

[0087] [Table 4-1]

[0088] [Table 4-2]

[0089] According to Tables 1-1 to 4-2, the steel sheet of the present invention example, which is manufactured by the manufacturing method of the present invention, has a predetermined chemical composition specified in the present invention, and contains a microstructure at a 1 / 4 depth position, in area ratios, of 80% or more martensite, 0 to 15% total of ferrite, bainite, and pearlite, and 0 to 10% retained austenite, and the microstructure of the surface layer portion contains, in area ratios, a total of ferrite, bainite, and pearlite of 60% or more and a total of martensite and retained austenite of 0 to 40%, the average C content in the range from 10 μm from the surface along the plate thickness direction to 20 μm from the surface along the plate thickness direction is 0.085% or less, the maximum C content in the range from 30 μm from the surface along the plate thickness direction is 0.130% or less, the tensile strength is 1470 MPa or more, and the VDA bend angle is 75° or more, has high strength and excellent bendability, and LME cracking during spot welding is suppressed. In contrast, in the comparative examples, either the chemical composition or the manufacturing method was outside the scope of the present invention, and accordingly, at least one of the chemical composition, the microstructure at the 1 / 4 depth position, the microstructure of the surface layer, the average C content in the range from 10 μm from the surface along the thickness direction to 20 μm from the surface along the thickness direction, the maximum C content in the range from the surface along the thickness direction to 30 μm from the surface along the thickness direction, the tensile strength, and the VDA bend angle was outside the scope of the present invention. As a result, either the strength or the bendability was poor, or LME cracking during spot welding was not sufficiently suppressed. [Industrial Applicability]

[0090] According to the present invention, it is possible to provide a steel sheet having high strength and excellent bendability and capable of suppressing LME cracking during spot welding, and a manufacturing method thereof, which are therefore highly industrially applicable.

Claims

1. The chemical composition, in mass%, is C: 0.18-0.30%, Si: 0.50-2.00%, Mn: 2.4 to 3.5%, Al: 0.001-0.100%, B: 0.0001 to 0.0050%, Nb: 0.035-0.100%, P: 0.015% or less, S: 0.0030% or less, N: 0.0200% or less, O: 0.0030% or less, Ti: 0 to 0.050%, Cr: 0-1.000%, Mo: 0-0.500%, W: 0-0.500%, Co: 0 to 0.500%, V: 0 to 0.100%, Ta: 0-0.100%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, As: 0 to 0.050%, Ni: 0-1.000%, Cu: 0 to 1.000%, Ca: 0-0.050%, Zr: 0 to 0.050%, Mg: 0 to 0.050%, REM: 0 to 0.100%, and The balance is Fe and impurities. and When the position of 1 / 4 of the plate thickness from the surface along the plate thickness direction is defined as the 1 / 4 depth position, and the range from the surface to 30 μm is defined as the surface layer portion, The microstructure at the 1 / 4 depth position has an area ratio of Martensite: 80% or more, Sum of ferrite, bainite, and pearlite: 0 to 15% Retained austenite: 0 to 10% Contains The microstructure of the surface layer portion has an area ratio of Total of ferrite, bainite, and pearlite: 60% or more, Total of martensite and retained austenite: 0 to 40% Including, The average C content in the range from a position 10 μm from the surface along the plate thickness direction to a position 20 μm from the surface along the plate thickness direction is 0.085 mass% or less, The maximum C content in the range from the surface to a position of 30 μm along the plate thickness direction is 0.130 mass% or less, The tensile strength is 1470 MPa or more, The VDA bending angle is 75° or more. steel plate.

2. The chemical composition is, in mass %, Ti: 0.001 to 0.050%, Cr: 0.001-1.000%, Mo: 0.010-0.500%, W: 0.001-0.500%, Co: 0.010-0.500%, V: 0.001-0.100%, Ta: 0.001 to 0.100%, Sn: 0.001 to 0.050%, Sb: 0.001 to 0.050%, As: 0.001 to 0.050%, Ni: 0.010 to 1.000%, Cu: 0.001 to 1.000%, Ca: 0.001-0.050%, Zr: 0.001 to 0.050%, Mg: 0.0001 to 0.050%, and REM: 0.001~0.100% Contains one or more of the following: The steel sheet according to claim 1.

3. The surface has a hot-dip galvanized layer. The steel sheet according to claim 1 or 2.

4. The hot-dip galvanized layer is a galvannealed layer. The steel sheet according to claim 3.

5. A method for manufacturing the steel plate according to claim 1, A heating step of heating a slab having the chemical composition according to claim 1 to a heating temperature of 1180°C or higher; a hot rolling step of hot rolling the slab after the heating step to obtain a hot-rolled steel sheet; a coiling step of starting cooling of the hot-rolled steel sheet at 800°C or higher, cooling the hot-rolled steel sheet to a coiling temperature at an average cooling rate of 50°C / s or higher, and coiling the hot-rolled steel sheet at the coiling temperature of 400°C or lower; a cold rolling step of cold rolling the hot-rolled steel sheet after the coiling step under conditions in which a sheet thickness reduction rate is 5 to 50% to obtain a cold-rolled steel sheet; an annealing step of annealing the cold-rolled steel sheet; Including, The hot rolling process includes rough rolling to convert the slab into the hot-rolled steel sheet, and finish rolling to further hot-roll the hot-rolled steel sheet after the rough rolling by rolling down multiple passes, In the finish rolling, the surface temperature of the hot-rolled steel sheet before the start of the final three passes is set to 900 ° C. or less, and the rolling reduction in each of the final three passes is set to 30% or more, In the annealing step, As the annealing, the cold-rolled steel sheet is heated to 840°C at an average heating rate of 5°C / s or more, and held in a temperature range of 840 to 900°C for 60 seconds or more in an atmosphere having a dew point of -15 to 10°C. A method for producing a steel sheet.

6. The steel sheet according to claim 1 or 2, parts.

7. The steel sheet has a hot-dip galvanized layer on the surface.

7. The component of claim 6.

8. The hot-dip galvanized layer is a galvannealed layer.

8. The component of claim 7.

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