Steel sheet, method for producing same, and component

Abstract

This steel sheet has a specific chemical composition, and the microstructure thereof at the position of 1 / 4 depth contains, in terms of an area ratio, 80% or more of martensite, a total of 0-15% of ferrite, bainite, and pearlite, and 0-10% of retained austenite. The microstructure of a surface layer part contains, in terms of an area ratio, a total of 60% or more of ferrite, bainite, and pearlite, and a total of 0-40% martensite and retained austenite. The average C content in a range between the position of 10 µm from the surface in the sheet thickness direction to the position of 20 µm from the surface in the sheet thickness direction is 0.085 mass% or less, the maximum C content in a range from the surface to the position of 30 µm in the sheet thickness direction is 0.130 mass% or less, the tensile strength is 1,470 MPa or more, and the VDA bending angle is 75° or more.

Description

Steel plate, its manufacturing method, and parts

[0001] This application claims priority to Japanese Patent Application No. 2024-069146, filed on April 22, 2024, the contents of which are incorporated herein by reference.

[0002] Today, with the highly specialized nature of industrial technology, 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 vehicle weight and improve fuel efficiency, due to concerns about 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 galvannealed steel sheets, which have a zinc coating 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 and excellent in galvanizability; 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, high-strength hot-dip galvanized steel sheets and high-strength galvannealed steel sheets for automotive parts have issues. Specifically, resistance spot welding is primarily used in processes such as assembling automobile bodies and installing parts. Resistance spot welding is a resistance welding method in which overlapping base materials are clamped between the tips of properly shaped electrodes and current and pressure are concentrated in a relatively small area to locally heat the material. However, when galvanized steel sheets (hot-dip galvanized steel sheets, electrogalvanized steel sheets, or galvannealed hot-dip galvanized steel sheets) are resistance spot welded for assembling automobile bodies and / or parts, cracks known as liquid metal embrittlement (LME) cracks can occur in the spot welds. LME cracks occur when the heat generated during resistance spot welding melts the zinc in the galvanized layer, causing the molten zinc to penetrate into the grain boundaries of the steel sheet structure at the weld, resulting in tensile stress acting on the resulting structure. 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 location. The higher the strength of the steel sheet, the higher the susceptibility to LME cracking tends to be. Since the occurrence of 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] In response to the above-mentioned problems, techniques 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 graded from the surface layer to the central layer of the sheet thickness, even during spot welding, the softer layer (second layer) with a larger block diameter is able to bear strain when deformation is caused, making it possible to suppress excessive increase in strain in the outermost layer (first layer), thereby suppressing the occurrence of spot-weld LME cracking.

[0007] However, in Patent Document 2, no consideration is given to bendability, and it is believed that there is room for improvement in terms of bendability.

[0008] International Publication No. 2016 / 111275 International Publication No. 2021 / 251276

[0009] As described above, no steel sheet has been proposed to date 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 including the steel sheet.

[0010] The present inventors have investigated methods for improving strength and bendability while simultaneously suppressing LME cracking during spot welding. As a result, they have made the following discoveries: (A) To obtain high strength, it is effective to control the chemical composition and the microstructure at the 1 / 4 depth position. (B) Bendability is improved by providing a soft layer within a certain range from the surface. (C) The soft layer can be formed by decarburization. (D) By increasing the amount of decarburization in the soft layer more than usual and setting the average C concentration and maximum C concentration within a predetermined range, the occurrence of LME cracking can be suppressed. (E) To obtain a soft layer with 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 view of the above findings. The gist of the present invention is as follows. [1] A steel sheet according to one aspect of the present invention 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 to 0.500%, W: 0 to 0.500%, Co: 0 to 0.500%, V: 0 to 0.100%, Ta: 0 to 0.100%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, As: 0 to 0.050%, Ni: 0 to 1.000%, Cu: 0 to 1.000%, Ca: 0 to 0.050%, Zr: 0 to 0.050%, Mg: 0 to 0.050%, REM: 0 to 0.100%, and the balance: Fe and impurities, and When a position that is 1 / 4 of the plate 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, martensite: 80% or more, a total of ferrite, bainite, and pearlite: 0 to 15%, and retained austenite: 0 to 10%, and the microstructure of the surface layer portion contains, in area ratios, a total of ferrite, bainite, and pearlite: 60% or more, and a total of martensite and retained austenite: 0 to 40%, 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, and the maximum C content in the 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.[2] The steel sheet according to [1], wherein the chemical composition is, in mass%, 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 The steel sheet 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] The steel sheet according to [3] may have the hot-dip galvanized layer be an alloyed hot-dip galvanized layer. [5] A method for producing a steel sheet according to another aspect of the present invention includes a heating step of heating a slab having the chemical composition described in [1] to a heating temperature of 1180°C or more, 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 more, cooling the hot rolled steel sheet to a coiling temperature at an average cooling rate of 50°C / s or more, and coiling the hot rolled steel sheet at the coiling temperature of 400°C or less, a cold rolling step of cold rolling the hot rolled steel sheet after the coiling step under conditions such that a thickness reduction rate is 5 to 50% to obtain a cold rolled steel sheet, and a rolling step of annealing the cold rolled steel sheet. and an annealing step in which the hot-rolled steel sheet is subjected to rough rolling to convert the slab into the hot-rolled steel sheet, and finish rolling in which the hot-rolled steel sheet after the rough rolling is further hot-rolled by multiple passes, wherein the finish rolling sets the surface temperature of the hot-rolled steel sheet before the start of the final three passes to 900°C or less, and the rolling reduction in each of the final three passes is 30% or more, and the annealing step includes heating the cold-rolled steel sheet to 840°C at an average heating rate of 5°C / s or more and holding the temperature in the 840 to 900°C temperature range for 60 seconds or more in an atmosphere with a dew point of -15 to 10°C. [6] A part according to another aspect of the present invention includes the steel sheet according to [1] or [2]. [7] In the part according to [6], the steel sheet may have a hot-dip galvanized layer on its surface.[9] In the component according to [7], the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer.

[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.

[0013] A steel sheet according to one embodiment of the present invention (the steel sheet according to the present embodiment), a part including the steel sheet, and a manufacturing method thereof will be described. In the present embodiment, a position that is ¼ of the sheet thickness from the surface along the sheet thickness direction is defined as a ¼ depth position, and the range from the surface to 30 μm is defined as a surface layer portion. Here, the surface that serves as the reference for the ¼ 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> A steel plate according to this embodiment has a predetermined chemical composition, a microstructure at a quarter depth position containing, in area percentages, 80% or more martensite, 0 to 15% total of ferrite, bainite, and pearlite, and 0 to 10% retained austenite, a microstructure in a surface layer portion containing, in area percentages, 60% or more total of ferrite, bainite, and pearlite and 0 to 40% total of martensite and retained austenite, an 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, a 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, a tensile strength of 1470 MPa or more, and a VDA bend angle of 75° or more. Each of these will be described below.

[0015] [Chemical Composition] The chemical composition of the steel sheet according to this embodiment will be described. 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 plate. 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 decreases and the bendability deteriorates. 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 effective in increasing the strength of steel sheet. To achieve the above effect, the Si content is set to 0.50% or more. The Si content is preferably 0.65% or more. On the other hand, excessive Si content may lead to embrittlement of the steel sheet, resulting in reduced manufacturability and workability. Therefore, the Si content is set to 2.00% or less. The Si content is 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 fully obtain the above-mentioned effects. 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) is suppressed in the surface layer of the steel sheet, 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. The Al content is preferably 0.005% or more. On the other hand, if Al is added in excess, not only will the effect saturate and lead to increased costs, but the transformation temperature of the steel will rise, increasing the load during hot rolling and significantly impairing the flatness of the hot-rolled steel sheet, making it difficult to carry out 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 a beneficial element 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 become the starting points for voids during press forming may form in the steel, deteriorating 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, thereby forming a decarburized layer that is less susceptible to LME cracking. Therefore, to achieve the above effect, 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 void generation 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, and 0% is acceptable. However, taking into consideration the time and cost required for removing 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, and 0% is acceptable. However, taking into consideration the time and cost required for removing 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 the steel sheet and deteriorates the bendability and hole expandability of the steel sheet. If the N content exceeds 0.0200%, the above 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 will increase significantly. Therefore, the N content may be set to 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 also 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. Meanwhile, 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 need to be contained, the lower limit is 0%.

[0027] (Ti: 0 to 0.050%) Ti is an element effective in controlling the morphology of carbides. Therefore, Ti may be contained. To obtain the above effect, 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 effect, 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. 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 to 0.500%) W is a carbide-forming element and is effective in increasing the strength of steel sheet. 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 costs will also increase. For this reason, 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 sheet. Therefore, Co may be contained. To obtain the above effect, 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, and the hole expandability and bendability may decrease. 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 effective in controlling the morphology of carbides, and is also effective in improving the toughness of steel sheet. 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 deteriorates ductility and may reduce 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 effective in controlling the morphology of carbides and improving the strength of the steel sheet. Therefore, Ta may be contained. 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 sheet when scrap is used as the raw material for the steel sheet. Furthermore, Sn is an element that may cause a decrease in the hole expandability and bendability of the steel sheet due to the embrittlement of ferrite. For this reason, 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% leads to 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 sheet when scrap is used as a raw material for the steel sheet. 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. For this reason, the lower the Sb content, the better. The Sb content is 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 0.001% or more.

[0036] (As: 0 to 0.050%) Like Sn and Sb, As is an element that can be contained in steel sheet when scrap is used as the raw material for the steel sheet. As is an element that strongly segregates at grain boundaries and may cause a decrease in hole expandability and bendability. For this reason, the lower the As content, the better. The As content is 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 0.001% or more.

[0037] (Ni: 0 to 1.000%) Ni is an element effective in improving the strength of steel sheet. 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, and the hole expandability and bendability may decrease. 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 effect, 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 in hot rolling. In addition, there is a risk of reduced hole expandability and bendability due to the formation of coarse inclusions. Therefore, when Cu is contained, the Cu content is set to 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 in small amounts. Therefore, Ca may be contained. To obtain the above effect, 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 points for crack generation during cold forming, so if the Ca content is too high, hole expandability and bendability may deteriorate. For this reason, 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 in small amounts. Therefore, Zr may be contained. To obtain the above effect, 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 steel sheet. Therefore, Mg may be contained. 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, there is a risk that the hole expandability and bendability will decrease due to the formation of coarse inclusions. Therefore, if Mg is contained, the Mg content is set to 0.050% or less. The Mg content is preferably 0.040% or less.

[0042] (REM: 0 to 0.100%) REM is an element that effectively controls the morphology of sulfides, even 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 may be 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, and refers collectively to two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). The "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, and O, with the balance being Fe and impurities, or may include C, Si, Mn, Al, B, Nb, P, S, N, and O and one or more optional elements, with the balance being Fe and impurities. Here, impurities are elements that are mixed in during industrial steel production due to raw materials such as ore and scrap, and various factors in the production process, and whose presence is permitted to the extent that they do not impair the properties of the steel sheet according to this embodiment. They also include 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 common method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). However, elements that are difficult to measure using ICP-AES may be measured by other methods. For example, C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method. The measurement sample is collected 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 position] 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 to 15%) Ferrite is a soft phase formed during intercritical annealing or slow cooling after annealing. Ferrite improves the ductility of steel sheets when mixed with hard phases such as martensite, but to achieve a predetermined high strength, the area fraction of ferrite must be limited. Generally, bainite is formed by holding the steel at 400 to 550°C for a certain period of time during the cooling process after holding at the annealing temperature. Bainite is softer than martensite and therefore has the effect of improving ductility. However, to achieve a predetermined high strength, its area fraction must be limited, as with ferrite. Pearlite is a structure containing cementite within its structure and consumes C (carbon) in the steel, which contributes to improving strength. Therefore, an excessive area fraction of pearlite reduces the strength of the steel sheet. Therefore, the total area fraction of ferrite, bainite, and pearlite is set to 15% or less. These structures (phases) do not necessarily need to be included, so the lower limit of the area fraction is 0%.

[0048] (Residual austenite: 0 to 10%) 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 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, crack initiation points are more likely to occur, 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] The identification of each structure and calculation of its area fraction are performed by first determining the area to be retained austenite in a given observation area, and then identifying ferrite, bainite, martensite, or pearlite in the same observation area. It is desirable to collect samples within a range of 100 mm from the edge in the width direction. However, if this is difficult due to the small size of the original material, samples can be collected from the center of the material. Specifically, the area fraction of retained austenite is measured as follows. After mirror-finishing the observation surface, the observation surface is polished by colloidal silica polishing or electrolytic polishing. Using an EBSD attached to a scanning electron microscope, diffracted electrons are measured in a 100 μm × 100 μm square region, 100 μm in the thickness direction and 100 μm in the rolling direction, centered at a quarter-depth position in the steel sheet, at intervals of 0.2 μm (grid-like arrangement) in both the thickness direction and the rolling direction. The resulting pseudo-Kikuchi pattern is analyzed to identify the crystal orientation and crystal system. The accelerating voltage of the incident electrons is 15 kV. The sample preparation conditions are within the range recommended in the "Crystal Orientation Measurement Standard for Material Evaluation by Electron Backscatter Diffraction (EBSD) Method" standard published by the Japan Society for Materials Science. Measurement points detected as FCC phase from 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 to be the area fraction of retained austenite. The area fractions of ferrite, bainite, martensite, and pearlite can be measured using the following method. Specifically, the same square area as 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). The area fractions of each structure are calculated by point counting at 2 μm intervals (grid arrangement) from the resulting secondary electron micrograph. The observation area is 1200 μm to reduce 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 sheet (a plane parallel to both the longitudinal and transverse directions of the sheet) is polished to a quarter depth position and finished by mirror polishing, and then a Mn concentration map of a 500 μm × 500 μm area is obtained using an EPMA. When the solidification segregation of Mn is measured as streaks, the longitudinal direction of the streak pattern is determined to be the rolling direction. When the rolling direction of the steel sheet is known in advance, the rolling direction of the steel sheet may be determined without using the above determination method. Furthermore, even when the steel sheet is processed into a part, the rolling direction can be determined using the above method for a weakly processed portion of the part (for example, a flat portion that has received relatively little processing).

[0051] Each structure is determined as follows: Ferrite has a granular or acicular shape and does not contain iron-based carbides. Bainite has a lath-like shape (lath structure) and is a region in which iron-based carbides with a major axis of 20 nm or more are present within the lath structure and the carbides extend in the same direction. Fresh martensite has a lath-like shape (lath structure) and is a region in which iron-based carbides with a major axis of 20 nm or more are present within the lath structure and the carbides extend in multiple different directions. Furthermore, a region in which ferrite and cementite are in a lamellar shape is considered to be pearlite. When identifying ferrite, bainite, martensite, and pearlite, the above-described visual identification is not performed on regions determined to be retained austenite in the previous retained austenite identification. However, the area ratio is calculated as a ratio to the total number of measurement points in the observation range, including the retained austenite region.

[0052] When the total area ratio of each structure obtained by the above evaluation method is different from 100%, the area ratio of each structure is multiplied by 100 / (total area ratio of each structure), and the value obtained is used as the area ratio of each structure.

[0053] [Microstructure of Surface Layer Portion] The microstructure of the surface layer portion has a 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 to 40%) If the total area ratio of one or both of martensite and retained austenite exceeds 40%, bendability deteriorates. Therefore, the total area ratio of these is set to 40% or less.

[0056] The identification and calculation of the area ratio of each structure (phase) in the surface layer portion can be performed using the same method 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 portion, the measurement position is a square region from the surface of the steel sheet to a depth of 30 μm (a square region of 30 μm in the sheet thickness direction × 30 μm in the rolling direction, with one side at the surface).

[0057] [Average C Content in the Range from 10 μm from the Surface to 20 μm from the Surface in the Plate Thickness Direction] [Maximum C Content in the Range from 30 μm from the Surface to 30 μm from the Surface in the Plate Thickness Direction] A high C content makes LME cracking more likely. Therefore, in the steel plate 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 10 μm from the surface to 20 μm from the surface in the plate 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 is a position with a high C content (concentration) near the surface, LME cracking is more likely to occur during processes in which the steel plate is exposed to high temperatures, such as welding, due to the influence of C diffusing from C enrichment points near the surface to 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 deep 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. However, in steel sheets with a C content of 0.18 mass% or more at the 1 / 4 depth position, it is not possible to form a decarburized layer having the above-mentioned average C content and maximum C content by ordinary high dew-point annealing alone. Therefore, when manufacturing the steel sheet according to this embodiment, the content of Nb, which suppresses recrystallization during rolling, is increased, and the manufacturing conditions, particularly the conditions in the hot rolling process and the annealing process, are controlled as described below.

[0058] The average C content in the range from 10 μm from the surface to 20 μm from the surface in the thickness direction, and the maximum C content in the range from the surface to 30 μm from the surface in 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 predetermined position. During the measurement, samples are taken from the same positions as in the microstructure observation, and measurements are performed on these samples so that the depth of the measurement range in the 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 a steel sheet has a surface plating layer (such as a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer), the surface position of the steel sheet is defined as follows: That is, the Mn concentration profile is 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 at a depth of 200 μm), and the position where the Mn content is half the average value of the Mn content in the depth range of 190 to 200 μm is defined as the surface (most superficial layer position) of the steel sheet. The C profile is determined by measuring the C element concentration profile from the data used for the Mn measurement, with the most superficial layer position defined above as the origin, and determining 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, and the maximum C content in the range from the surface to a position 30 μm along the sheet thickness direction. However, the entire measurement value is corrected so that the average C content in the depth range of 190 to 200 μm matches 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 reducing the weight of an automobile body and improving impact resistance. There is no upper limit to the tensile strength. In order to ensure sufficient bendability, the tensile strength may be 1650 MPa or less.

[0060] [VDA bend 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] [Thickness] The thickness of the steel plate according to this embodiment is not limited, but if it is too thick, it becomes difficult to obtain the effect of reducing the vehicle body weight, so it is preferably 2.3 mm or less. It is more preferably 1.6 mm or less, and even more preferably 1.2 mm or less. Furthermore, if the thickness is too thin, it becomes difficult to control the thickness, so it may be 0.8 mm or more, or 1.0 mm or more.

[0062] [Plated Layer] The steel sheet according to this embodiment may have a zinc-plated layer on its surface. Providing a plated layer on its 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 locations 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] <Component> The component according to this embodiment is 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 as needed by welding or the like. Therefore, the component according to this embodiment at least partially includes 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 a position 10 μm from the surface along the thickness direction to a position 20 μm from the surface along the thickness direction, the maximum C content in the range from a position 30 μm from the surface along the thickness direction, 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 effects described above regardless of the manufacturing method, as long as it has the above-described characteristics. However, a manufacturing method including the following steps is preferable because 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 in which the slab after the heating step is hot-rolled to obtain a hot-rolled steel sheet; (III) a coiling step in which the hot-rolled steel sheet is cooled and coiled at a coiling temperature of 400°C or lower; (IV) a cold rolling step in which the hot-rolled steel sheet after the coiling step is cold-rolled under conditions such that the thickness reduction rate is 5 to 50% to obtain a cold-rolled steel sheet; and (V) an annealing step in which the cold-rolled steel sheet is annealed. Furthermore, the part according to this embodiment can be manufactured by further 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 as required, and (VII) a welding step of joining the steel plate according to the present embodiment to another steel material by welding as required. Each of these steps will be explained below. Known conditions can be applied to conditions and steps not explained.

[0066] [Heating Step] 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 lower than 1180°C, the NbC formed in the casting step cannot be solutionized, and the effect of suppressing recrystallization by Nb cannot be obtained in the subsequent hot rolling step. 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 lower in terms of fuel costs.

[0067] The chemical composition of the slab to be subjected to the heating process does not substantially change in the manufacturing process except for a certain range from the surface to become 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 of the target steel plate at a 1 / 4 depth position.

[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, 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 n-2th pass, the n-1th pass, and the nth pass, three passes) is set to 30% or higher. It is preferable that the finish rolling be performed by continuous rolling using a rolling mill having multiple stands. Dislocations are introduced into the austenite by the above 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 reduction rates 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 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 lower. Also, preferably, the reduction rates in each of the final three passes are 35% or higher. There is no upper limit to the reduction rate in the final three passes, but in terms of equipment load, each of the reduction rates in the final three passes may be 50% or lower.

[0069] [Coiling 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 equipment 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 number of lattice defects can be introduced into the austenite after reverse transformation. If the average cooling rate is slow or the coiling temperature is higher than 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 process, the hot-rolled steel sheet after the coiling process is cold-rolled under conditions where the 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 reduction in dislocation density due to recrystallization in the annealing process is promoted. 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 breakage during cold rolling, the steel sheet may be reheated to 200°C or less before cold rolling. If the reheating rate exceeds 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 in an atmosphere with a dew point of -15 to 10°C, at an annealing temperature in the temperature range of 840 to 900°C for 60 seconds or more. This annealing process results in a predetermined decarburized layer. By annealing a steel sheet that contains a certain amount of lattice defects through control in the previous process under the above conditions, decarburization is promoted more than usual, and the target decarburized layer is obtained. If the dew point is less than -15°C or the holding time is less than 60 seconds, sufficient decarburization is not achieved. Furthermore, if the annealing temperature is less than 840°C, strength may be insufficient. If the annealing temperature is higher than 900°C, the austenite grains become coarse, resulting in an insufficient amount of soft ferrite formed in the surface layer. If the average heating rate up to 840°C is less than 5°C / s, recrystallization progresses during heating, the amount of lattice defects introduced by hot rolling decreases, and decarburization progresses insufficiently. The average heating rate is not limited as long as it is 5°C / s or more, but is preferably 50°C / s or less in terms of temperature controllability when the maximum temperature is reached. If the dew point is above 10°C, oxides of elements other than C are generated in the surface layer, inhibiting the decarburization reaction, forming a C-enriched layer in the surface layer, 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 perspective of productivity. After the maximum temperature is reached, quenching and tempering to obtain a martensite structure, and an isothermal holding process to obtain ferrite, bainite, and pearlite structures are performed.

[0072] [Plating step] 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 a hot-dip galvanized layer is formed on the surface as a plating layer to produce 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 a temperature higher than 425°C and lower than 600°C to carry out hot-dip galvanizing.

[0073] [Alloying Step] 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 an 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 when it is performed during cooling in the annealing step, and is not included in the thermal history to be controlled in the annealing step when it is performed after cooling to room temperature in the annealing step.

[0074] [Processing step] When processing is performed, the steel sheet according to the present embodiment is processed to 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, bulging, etc.

[0075] [Welding process] When the welding process is performed, the steel plate according to the present embodiment (including that which has undergone a processing process) and another steel material are joined by welding such as spot welding. The welding conditions are not limited, and known conditions may be applied.

[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 having 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, and the average cooling rate from the start of cooling to the coiling temperature was set to 50°C / s or higher, and the hot-rolled steel sheet was coiled at the coiling temperature shown in Tables 2-1 and 2-2. The hot-rolled steel sheet after the coiling process was 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 sheet was 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 of the obtained steel sheets were evaluated in the same manner as described above. In addition, the tensile strength, bendability, and LME resistance during spot welding were evaluated in the following manner.

[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. At that time, the limit bending angle was evaluated when the direction parallel to the rolling direction was set as the bending ridge line. If the VDA bending angle was 75° or more, it was determined that the specimen had excellent bendability.

[0080] [LME Resistance] The obtained steel sheet and two commercially available GA steel sheets with a tensile strength of 980 MPa were stacked on top of each other, and spot welding was performed at a current value and current flow time resulting in a nugget diameter of 5 mm. The angle (strike angle) between the spot welding electrode and the sheet 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 length of a straight line connecting the crack initiation point on the sheet surface to the end point of the crack in 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 crack or a maximum crack length of less than 30 μm were judged to have excellent LME resistance (OK).

[0081]

[0082]

[0083]

[0084]

[0085]

[0086]

[0087]

[0088]

[0089] According to Tables 1-1 to 4-2, the steel plate 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, martensite: 80% or more, a total of ferrite, bainite, and pearlite: 0 to 15%, and retained austenite: 0 to 10%. The microstructure of the surface layer portion contains, in area ratios, a total of ferrite, bainite, and pearlite: 60% or more, and a total of martensite and retained austenite: 0 to 40%. 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% or less, and the maximum C content in the range from the surface to a position 30 μm 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. The steel plate 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 therefore 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, 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.

[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 is therefore highly industrially applicable.

Claims

1. Chemical composition, in mass%, is: 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 to 0.500%, W: 0 to 0.500%, Co: 0 to 0.500%, V: 0 to 0.100%, Ta: 0 to 0.100%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, As: 0 to 0.050%, Ni: 0 to 1.000%, Cu: 0 to 1.000%, Ca: 0 to 0.050%, Zr: 0 to 0.050%, Mg: 0 to 0.050%, REM: 0 to 0.100%, and the balance: Fe and impurities, and when a position from the surface to 30 μm in the sheet thickness direction 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, martensite: 80% or more, a total of ferrite, bainite, and pearlite: 0 to 15%, and retained austenite: 0 to 10%. the microstructure of the surface layer portion comprises, in area ratios, a total of ferrite, bainite, and pearlite: 60% or more, and a total of martensite and retained austenite: 0 to 40%; 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 steel sheet has a tensile strength of 1470 MPa or more; and a VDA bending angle of 75° or more.

2. The chemical composition is, in mass%, 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%, 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 to 0.050%, The steel sheet according to claim 1, containing one or more of: 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 claim 1 or 2, having a hot-dip galvanized layer on the surface.

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

5. A method for manufacturing a steel sheet comprising: a heating step of heating a slab having the chemical composition described in 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 to cool 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 such that the 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, wherein the hot rolling step comprises rough rolling to obtain the hot rolled steel sheet, and finish rolling to further hot roll the hot rolled steel sheet after the rough rolling by multiple passes of reduction, a method for manufacturing a steel sheet, wherein 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; and in the annealing step, the cold-rolled steel sheet is heated to 840°C at an average temperature increase 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 part comprising the steel sheet according to claim 1 or 2.

7. The part according to claim 6, wherein the steel sheet has a hot-dip galvanized layer on the surface.

8. The component according to claim 7, wherein the hot-dip galvanized layer is a galvannealed layer.

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