Steel sheet, manufacturing method therefor, and component

Abstract

This steel sheet has a predetermined chemical composition and has, when a position corresponding to 1 / 4 of the sheet thickness from the surface along the sheet thickness direction is defined as a 1 / 4 depth position, a microstructure at the 1 / 4 depth position which includes, in terms of area ratio, at least 80% of martensite, a total of 0-15% of ferrite, bainite, and pearlite, and 0-10% of retained austenite, wherein when a region in the martensite in which the average interval of grain boundaries having a crystal orientation angle difference of at least 50º is at most 2 μm is defined as fine martensite, the area ratio of the fine martensite in the martensite is at least 7%, and the tensile strength is at least 1,470 MPa.

Description

Steel plate, its manufacturing method, and parts

[0001] This application claims priority to Japanese Patent Application No. 2024-069143, filed on April 22, 2024, the contents of which are incorporated herein by reference.

[0002] In today's world, where industrial technology fields have become highly specialized, 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 vehicle bodies and improve fuel efficiency, in consideration of the global environment. Furthermore, among automotive components, collision members are required to have high strength, preferably high strength and excellent bendability, from the viewpoint of absorbing energy during a collision. In particular, in recent years, high-strength steel sheets having a tensile strength of 1470 MPa or more are in demand.

[0003] Resistance spot welding is primarily used in processes such as automobile body assembly and component installation. Resistance spot welding involves clamping overlapping base materials between the tips of properly shaped electrodes and concentrating current and pressure on a relatively small area to locally heat them. To increase the tensile strength of steel sheets, the carbon content is typically increased. However, increasing the carbon content reduces the strength of spot welds, as assessed by CTS and other standards. Reduced spot weld strength makes steel sheets more susceptible to fracture when used in crushable parts (such as front side frames, which are required to absorb energy by significantly deforming during a collision). In other words, even if the strength of the steel sheet is increased, the crushable strength achieved is only equivalent to that of low-strength steel sheets, thereby reducing the benefits of using high-strength steel sheets.

[0004] For example, in Patent Document 1, the structure at a position of 1 / 4 of the plate thickness from the surface includes, by volume ratio, 80.0% or more of tempered martensite, more than 2.5% and less than 10.0% of retained austenite, 0% or more and 15.0% or less of ferrite and bainite in total, 0% or more and 3.0% or less of martensite, and the remaining structure, and in the structure, the maximum random specific intensity Iq is 4.0 or less, the average diameter of regions Rq having an orientation within 10° from the crystal orientation where the random specific intensity Iq is maximum is 10.0 μm or less, and the areal density of the regions Rq is 1000 pieces / mm ２ The present invention discloses a high-strength steel plate having a tensile strength of 1,310 MPa or more, a uniform elongation of 5.0% or more, and a TS×λ of 35,000 MPa·% or more, which has excellent formability, a problem inherent in high-strength steel plates, and also has sufficient bendability and hydrogen embrittlement resistance, as well as a manufacturing method thereof.

[0005] However, Patent Document 1 does not take into consideration the prevention of fracture from the spot welds when applied to crushable parts.

[0006] International Publication No. 2021 / 070951

[0007] As described above, no steel sheet has been proposed to date that has high strength and that has high joint strength at spot welds when spot welded. Therefore, an object of the present invention is to provide a steel sheet that has high strength and that has high joint strength at spot welds when spot welded, and a method for manufacturing the same. Preferably, an object of the present invention is to provide a steel sheet that has high strength and high bendability and that has high joint strength at spot welds when spot welded, a method for manufacturing the same, and a component including the steel sheet.

[0008] The present inventors have investigated methods for improving the strength of spot welds while simultaneously improving their strength. They also investigated methods for further improving bendability. As a result, they have made the following findings: (A) To obtain high strength, it is effective to control the chemical composition and the microstructure at the quarter depth position. (B) To increase the strength of spot welds, it is effective to limit the C content to 0.190% or less and utilize fine martensite. (C) To obtain fine martensite, it is effective to introduce lattice defects such as dislocations into austenite by low-temperature rolling during the manufacturing process and utilize these lattice defects (grain boundaries, dislocations) in martensitic transformation during the annealing process. (D) Bendability is improved by providing a decarburized layer within a certain range from the surface.

[0009] 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 embodiment of the present invention has a chemical composition, in mass %, of C: 0.160 to 0.190%, 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%, Mo: 0.050 to 0.500%, 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%, 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 at 1 / 4 of the plate thickness from the surface along the plate thickness direction is defined as a 1 / 4 depth position, the 1 / 4 depth [2] The steel sheet according to [1], wherein, when a range from the surface to 30 μm is defined as a surface layer portion, the microstructure of the surface layer portion comprises, in terms of 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 when a region in the martensite where the crystal orientation angle difference is 50° or more and the average spacing of grain boundaries is 2 μm or less is defined as fine martensite, the area ratio of the fine martensite in the martensite is 7% or more, and the tensile strength is 1470 MPa or more. [2] The steel sheet according to [1], wherein, when a range from the surface to 30 μm is defined as a surface layer portion, the microstructure of the surface layer portion comprises, in terms of area ratios, 60% or more of ferrite, pearlite, and bainite, and 0 to 40% of a total of martensite and retained austenite, and the VDA bending angle is 75° or more.[3] The steel sheet according to [1] or [2], wherein the chemical composition is, in mass%, Ti: 0.001 to 0.050%, Cr: 0.001 to 1.00%, 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: 0.0 The steel sheet may contain one or more of: Cr: 0.01 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%. [4] The steel sheet according to [1] or [2] may have a hot-dip galvanized layer on the surface. [5] The steel sheet according to [3] may have a hot-dip galvanized layer on the surface. [6] The steel sheet according to [4] may have the hot-dip galvanized layer as a galvannealed layer. [7] The steel sheet according to [5] may have the hot-dip galvanized layer as a galvannealed layer. [8] 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 cooling the hot rolled steel sheet from 800°C or more to a coiling temperature of 400°C or less at an average cooling rate of 50°C / s or more and coiling the hot rolled steel sheet at the coiling temperature, a cold rolling step of cold rolling the hot rolled steel sheet after the coiling step under conditions such that a 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 method includes rough rolling to obtain a rolled steel sheet, and finish rolling in which the hot-rolled steel sheet after the rough rolling is further hot-rolled by rolling down in multiple passes. In the finish rolling, the surface temperature of the hot-rolled steel sheet before the start of the final three passes is 900°C or less, and the rolling reduction in each of the final three passes is 30% or more. In the annealing step, the cold-rolled steel sheet is heated to a maximum heating temperature of 800°C or more and less than 840°C so that the average heating rate from 400°C to the maximum heating temperature is 5°C / s or more, and is held at the maximum heating temperature for 30 to 90 seconds. After the holding, the steel sheet is cooled to 300°C or less at an average cooling rate of 10 to 50°C / s.[9] In the method for producing a steel sheet according to [8], in the annealing step, the annealing may be performed in an atmosphere having a dew point of -15 to 10°C, and between the holding and the cooling, the steel sheet may be slowly cooled so that the average cooling rate in the temperature range of 750 to 650°C is 1 to 5°C / s.

[10] A part according to another aspect of the present invention includes the steel sheet according to any one of [1] to [3].

[11] In the part according to

[10] , the steel sheet may have a hot-dip galvanized layer on a surface.

[12] In the part according to

[11] , the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer.

[0010] According to the above aspects of the present invention, it is possible to provide a steel plate that has high strength and that has high joint strength at spot welds when spot welded, a method for manufacturing the same, and a part that includes the steel plate.

[0011] FIG. 2 is a schematic diagram for explaining a method for measuring the average spacing of grain boundaries in martensite.

[0012] A steel sheet according to one embodiment of the present invention (steel sheet according to this embodiment) and a part, as well as methods for manufacturing the same, will be described. In this 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 steel sheet may have a plating layer on the surface, but 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), 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.

[0013] <Steel Sheet> The steel sheet according to this embodiment has a predetermined chemical composition, and the microstructure at a quarter depth contains, in area ratios, 80% or more martensite, 0 to 15% total of ferrite, bainite, and pearlite, and 0 to 10% retained austenite. In the martensite, the area ratio of regions (fine martensite) in which the average spacing of grain boundaries is 2 μm or less and the crystal orientation angle difference is 50° or more (area ratio of fine martensite in martensite) is 7% or more. The tensile strength is 1470 MPa or more. Furthermore, the steel sheet according to this embodiment preferably has a microstructure in the surface layer that contains, in area ratios, 60% or more total of ferrite, pearlite, and bainite, and 0 to 40% total of martensite and retained austenite. The VDA bend angle is 75° or more. Each of these is described below.

[0014] [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 %.

[0015] (C: 0.160 to 0.190%) C is an essential element for increasing the strength of steel plate. If the C content is less than 0.160%, sufficient tensile strength cannot be obtained. Therefore, the C content is set to 0.160% or more. The C content is preferably 0.165% or more. On the other hand, if the C content exceeds 0.190%, the weldability decreases. Therefore, the C content is set to 0.190% or less. From the viewpoint of suppressing deterioration of weldability, the C content is preferably 0.180% or less.

[0016] (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 0.60% or more, and more preferably 0.70% 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 1.80% or less or 1.50% or less, and more preferably 1.20% or less.

[0017] (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 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 hydrogen embrittlement resistance of the weld deteriorates, and the strength of the weld decreases due to hydrogen embrittlement. Therefore, the Mn content is set to 3.5% or less. The Mn content is preferably 3.3% or less.

[0018] (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.

[0019] (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 effect, 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 void generation during press forming may be formed in the steel, deteriorating the workability of the steel sheet. For this reason, the B content is set to 0.0050% or less. The B content is preferably 0.0045% or less.

[0020] (Nb: 0.035 to 0.100%) Nb is an element effective in suppressing recrystallization. In the steel sheet according to this embodiment, the dislocation density of the structure before reverse transformation can be increased by suppressing recrystallization during rolling and the annealing process. This makes it possible for lattice defects to exist even after γ (austenite) transformation. In order 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. The Nb content is preferably 0.080% or less.

[0021] (Mo: 0.050 to 0.500%) Mo is an element that is effective in increasing the strength of steel sheet and also suppresses recrystallization and grain growth. To obtain the above effects, the Mo content is set to 0.050% or more. The Mo content is preferably 0.070% or more, and more preferably 0.100% 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.

[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, 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. The Ti content is preferably 0.045% 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] (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.

[0030] (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.

[0031] (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 effect, 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.

[0032] (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.

[0033] (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.

[0034] (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.

[0035] (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 a 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 is more preferably 0.020% 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.

[0036] (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.

[0037] (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.

[0038] (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.

[0039] (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.

[0040] (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.

[0041] (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 may be generated, which may reduce workability, fracture resistance, hole expandability, and bendability. Therefore, when REM is contained, the REM content is 0.100% or less, and preferably 0.060% or less. Here, REM is a rare earth metal (rare earth element), and is a collective term for 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.

[0042] (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, Mo, P, S, N, and O, with the balance being Fe and impurities, or may include C, Si, Mn, Al, B, Nb, Mo, 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. Impurities also include elements that are not intentionally added to the steel sheet.

[0043] 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.

[0044] [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 %".

[0045] (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.

[0046] (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%.

[0047] (Residual austenite: 0 to 10%) Retained austenite is a structure that contributes to improving elongation through transformation induced plasticity (TRIP). Therefore, it may be contained. In the steel sheet according to this embodiment, 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, crack initiation points are likely to occur, and bendability and the like deteriorate. Therefore, the area fraction of retained austenite is set to 10% or less. The area fraction of retained austenite is preferably 5% or less.

[0048] 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 of the steel sheet, at intervals of 0.1 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 defined as retained austenite, and the ratio of the number of measurement points detected as FCC phase to the total number of measurement points is defined as 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 1 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.

[0049] 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).

[0050] 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.

[0051] 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.

[0052] (In martensite, the area ratio of regions where the average spacing of grain boundaries with a crystal orientation angle difference of 50° or more is 2 μm or less is 7% or more.) In the steel sheet according to this embodiment, the area ratio of each structure in the microstructure is set within the above range, and the martensite is made fine. By making the martensite fine, the toughness of the steel is improved. In this case, fracture is suppressed even in spot welds, and joint strength is increased. Specifically, when a boundary in martensite where the crystal orientation angle difference is 50° or more is defined as a grain boundary, a region where the average spacing of this grain boundary is 2 μm or less is defined as fine martensite, and the area ratio of fine martensite in martensite (occupying the martensite) is set to 7% or more. In martensite where the average spacing of grain boundaries with a crystal orientation angle difference of 50° or more exceeds 2 μm, the effect of improving tensile strength is small, and if the tensile strength is 1470 MPa or more, this may cause a decrease in joint strength due to a decrease in toughness. Therefore, the area ratio of the region (fine martensite) where the average spacing of grain boundaries having a crystal orientation angle difference of 50° or more is 2 μm or less is specified. If the area ratio of this fine martensite is less than 7%, the effect of improving tensile strength and joint strength cannot be sufficiently obtained. The area ratio of fine martensite in martensite may be 100%, 50% or less, or 30% or less. In the steel sheet according to this embodiment, such fine martensite is formed by lattice defects caused by ausforming under the manufacturing conditions described below.

[0053] The average spacing of martensite grain boundaries is obtained by analyzing the regions determined to be martensite by the point counting method using TSL OIM Analysis 7 for the measurement data obtained by the EBSD method. First, the martensite region is defined using the following method. The group of measurement points determined to be martensite by the point counting method is divided into a 5 μm × 5 μm square mesh consisting of the plate thickness direction and the direction perpendicular to the plate thickness direction, with the points determined to be martensite as vertices. Only measurement points where all four vertices of the square are determined to be martensite are used as part of the mesh. For example, if it is not possible to draw a square where all four points are determined to be martensite in the boundary region with other phases, the measurement points are not used as part of the mesh. Using FIG. 1 as an example, martensite (M) is divided into meshes (5) with 5 μm intervals, as indicated by dashed lines, and grain boundaries (50) with a crystal orientation angle difference of 50° or more (thick solid lines in martensite (M)) are displayed in the region where this mesh (5) exists using TSL OIM Analysis 7. Four measurement lines (1) (dashed lines) are drawn within the mesh (5) at equal intervals of 1 μm in the thickness direction and in the direction perpendicular to the thickness direction, and the number of intersections (points indicated by single arrows) between the measurement lines (1) within the mesh and the grain boundaries (50) with a crystal orientation angle difference of 50° or more is counted. The value obtained by dividing the total length of the measurement lines (1) within the mesh (40 μm) by the number of intersections with the grain boundaries within the mesh is defined as the "average spacing of martensite grain boundaries" within the mesh. This measurement is performed on all meshes within the martensite region within the observation area, and meshes with an average spacing of 2 μm or less are considered to be fine martensite grains. The ratio of the number of meshes determined to be fine martensite to the number of meshes of all martensite grains is taken as the area ratio of fine martensite grains.

[0054] [Microstructure of the surface layer portion] The microstructure of the surface layer portion has a particularly large effect on bendability. Therefore, in order to improve bendability, it is preferable that the microstructure of the surface layer portion contains each structure at the following area ratios.

[0055] (Total of ferrite, bainite, and pearlite: 60% or more) Ferrite, bainite, and pearlite contribute to improving the bendability of the steel sheet. Therefore, it is preferable that the total area ratio of these is 60% or more. The total area ratio of ferrite, bainite, and pearlite is more preferably 80% or more. The upper limit of these area ratios is not limited and may be 100%.

[0056] (Total of martensite and retained austenite: 0 to 40%) If the total area ratio of martensite and retained austenite exceeds 40%, bendability deteriorates. Therefore, it is preferable that the total area ratio of these is 40% or less.

[0057] 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).

[0058] [Tensile strength] The steel plate 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. The upper limit of the tensile strength is not limited, but may be 1600 MPa or less in order to ensure weldability.

[0059] [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.

[0060] [Thickness] The thickness of the steel plate according to the present embodiment is not limited, but if the thickness is too thick, it becomes difficult to obtain the effect of reducing the weight of the vehicle body, so the thickness is preferably 1.6 mm or less. More preferably, it is 1.2 mm or less. On the other hand, if the thickness is too thin, the strength of the welded portion decreases, so the thickness may be 1.0 mm or more.

[0061] [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.

[0062] <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 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. The component according to this embodiment is, for example, a crushable component.

[0063] A sample is taken from the steel material constituting the component 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, microstructure at a 1 / 4 depth position, area ratio of fine martensite in martensite, and tensile strength are equivalent to those of the steel plate according to this embodiment, then it can be said that the component according to this embodiment includes the steel plate according to this embodiment.

[0064] <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.

[0065] [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.

[0066] 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.

[0067] [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 to convert the slab into a hot-rolled steel sheet, and finish rolling to further hot-roll 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 900°C or less, and the reduction ratio 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) is set to 30% or more. There is no lower limit for the rolling start temperature, but 820°C or higher is preferred to suppress the formation of a soft phase in the surface layer. It is preferable that the finish rolling be performed continuously using a rolling mill with 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. 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 rates in each of the final three passes are 35% or more. There is no upper limit to the reduction rate in the final three passes, but in terms of equipment load, the reduction rates in each of the final three passes may be 50% or less.

[0068] [Coiling Process] In the coiling process, the hot-rolled steel sheet after the hot rolling process is cooled and coiled at a coiling temperature of 400°C or lower. Coiling at a low temperature allows dislocations introduced into austenite (γ) to remain even after transformation. If the coiling temperature exceeds 400°C, sufficient dislocations do not remain. Furthermore, to prevent the formation of a soft structure, water cooling is performed after the end of rolling. Specifically, cooling is started at 800°C or higher, and the average cooling rate from the start of cooling to the coiling temperature (cooling stop temperature) is 50°C / s or higher. The cooling start temperature may be 850°C or lower. 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 lower. By performing the above cooling and coiling at a low temperature, a large number of lattice defects can be introduced into γ 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) do 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.

[0069] [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 dislocation density is reduced by recrystallization in the annealing process. In this case, the desired decarburized layer cannot be obtained. Therefore, the thickness reduction rate is set to 50% or less. On the other hand, if the thickness reduction rate is too low, thickness variation in hot rolling becomes a problem, so the thickness reduction rate is set to 5% or more. From the viewpoint of suppressing fracture during cold rolling, the steel sheet may be reheated to 200°C or less before cold rolling. If the thickness reduction rate exceeds 200°C, the dislocation density decreases, and the desired fine martensite structure cannot be obtained.

[0070] [Annealing Process] In the annealing process, the cold-rolled steel sheet is heated from 400°C to the maximum heating temperature (annealing temperature) at an average heating rate of 5°C / s or more, and the maximum heating temperature is set to 800°C or higher but less than 840°C. The maximum heating temperature is then held for 30 to 90 seconds. If the average heating rate from 400°C to the maximum heating temperature (800°C or higher but less than 840°C) is low, the amount of γ lattice defects after the reverse transformation from α (ferrite) to γ ​​(austenite) is significantly reduced due to diffusion transformation and coarsening of recrystallized grains, and the martensite obtained by the subsequent transformation is not refined. Therefore, the average heating rate in this temperature range is set to 5°C / s or more. The average heating rate may be 50°C / s or less. Furthermore, if the annealing temperature is less than 800°C or the holding time is less than 30 seconds, the reverse transformation from α to γ ​​may be insufficient, resulting in an insufficient amount of martensite produced by subsequent cooling. On the other hand, if the annealing temperature is 840°C or higher or the holding time exceeds 90 seconds, the amount of lattice defects necessary for martensite refinement decreases due to recovery and grain growth, making it impossible to obtain fine martensite. After holding, quenching is performed to obtain martensite. The average cooling rate for quenching is 10 to 50°C / s, and the cooling stop temperature is 300°C or lower. If the average cooling rate is less than 10°C / s or the cooling stop temperature exceeds 300°C, sufficient quenching may not be achieved and the desired martensite fraction may not be obtained. On the other hand, if the average cooling rate exceeds 50°C / s, temperature deviations within the plate may occur, making it difficult to obtain a homogeneous material. Since special equipment is required to set the cooling stop temperature below room temperature, the cooling stop temperature should be set to room temperature or higher. The cooling start temperature for the above quenching may be immediately after the maximum heating temperature, or it may be the temperature after isothermal holding to obtain bainite, as described below, or the end temperature of slow cooling to obtain a soft surface layer. To obtain a partial bainite structure, the steel sheet may be held in the temperature range of 550 to 400°C for 100 seconds or less before or after quenching. By annealing a steel sheet that contains a certain amount of lattice defects through control in the previous process under the above conditions, a metal structure mainly composed of fine martensite can be obtained.

[0071] When controlling the structure of the surface layer to improve bendability, annealing is preferably performed in an atmosphere with a dew point of -15 to 10°C. Furthermore, in order to obtain a soft structure in the surface layer, slow cooling is preferably performed at an average cooling rate of 1 to 5°C / s in the temperature range of 750 to 650°C before quenching after reaching the maximum heating temperature. By controlling the dew point during annealing and the cooling rate in the temperature range of 750 to 650°C, the surface layer is appropriately decarburized, and the metal structure of the surface layer can have a total area ratio of ferrite, bainite, and pearlite of 60% or more.

[0072] When the martensite is to be tempered martensite, tempering at 150 to 300° C. may be carried out before or after the plating step.

[0073] [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.

[0074] [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.

[0075] [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.

[0076] [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.

[0077] 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 galvanizing was performed after the maximum heating temperature in the annealing step was reached 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.

[0078] The microstructure of the surface layer of the obtained steel sheets was observed at a 1 / 4 depth position in the same manner as described above. The results are shown in Tables 3-1 and 3-2.

[0079] The resulting steel sheets were evaluated for tensile strength, bendability, and strength of spot welds by the following methods.

[0080] [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 results are shown in Tables 4-1 and 4-2.

[0081] [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. The results are shown in Tables 4-1 and 4-2.

[0082] [Strength of Spot Welds] The strength of spot welds was evaluated by joint strength. Specifically, cross tensile test specimens were prepared by spot welding. The shape of the cross tensile test specimens conformed to JIS Z3138:1989, and the specimens were taken so that the longitudinal direction (150 mm) of the specimens was perpendicular to the rolling direction. Spot welding was performed at a current value and current flow time that resulted in a nugget diameter of 5 mm. The CTS (cross tensile strength) of the prepared test specimens was measured according to JIS Z3137:1989. If the CTS was 6 kN or more, the spot welds were judged to have excellent strength (OK in the front joint strength column). On the other hand, if the CTS was less than 6 kN, the spot welds were judged to have poor strength (NG in the front joint strength column). The results are shown in Tables 4-1 and 4-2.

[0083]

[0084]

[0085]

[0086]

[0087]

[0088]

[0089]

[0090]

[0091] As can be seen from Tables 1-1 to 4-2, all of the inventive examples had a tensile strength of 1470 MPa or more, and the strength of the spot welds (joint strength) was high when spot welded. In contrast, the comparative examples had low tensile strength or low strength of the spot welds.

[0092] According to the present invention, it is possible to provide a steel plate having high strength and achieving high joint strength at spot welds when spot welded, a method for manufacturing the same, and a part including the steel plate, which are therefore highly industrially applicable.

[0093] M Martensite 1 Measurement line at 1 μm intervals 5 Mesh at 5 μm intervals 50 Grain boundary with a crystal orientation angle difference of 50° or more

Claims

1. Chemical composition, in mass%, is: C: 0.160-0.190%, Si: 0.50-2.00%, Mn: 2.4-3.5%, Al: 0.001-0.100%, B: 0.0001-0.0050%, Nb: 0.035-0.100%, Mo: 0.050-0.500%, P: 0.015% or less, S: 0.0030% or less, N: 0.0200% or less, O: 0.0030% or less, Ti: 0-0.050%, Cr: 0-1.000%, W: 0-0.500%, Co: 0-0.500%, V: 0-0.100%, 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 1 / 4 of the sheet thickness along the sheet thickness direction is defined as a 1 / 4 depth position, the microstructure at the 1 / 4 depth position comprises, in area ratios, martensite: 80% or more, total of ferrite, bainite, and pearlite: 0 to 15%, and retained austenite: 0 to 10%. When a region in the martensite where the crystal orientation angle difference is 50° or more and the average spacing of grain boundaries is 2 μm or less is defined as fine martensite, the area ratio of the fine martensite in the martensite is 7% or more, and the steel sheet has a tensile strength of 1470 MPa or more.

2. The steel sheet according to claim 1, wherein, when the range from the surface to 30 μm is defined as the surface layer portion, the microstructure of the surface layer portion contains, in area ratios, a total of ferrite, pearlite, and bainite: 60% or more, and a total of martensite and retained austenite: 0 to 40%, and the VDA bending angle is 75° or more.

3. The chemical composition is, in mass%, Ti: 0.001 to 0.050%, Cr: 0.001 to 1.00%, 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%, Zr: 0.001 to 0.050%, The steel sheet according to claim 1 or 2, containing one or more of: Mg: 0.0001 to 0.050%; and REM: 0.001 to 0.100%.

4. The steel sheet according to claim 1 or 2, having a hot-dip galvanized layer on the surface.

5. The steel sheet according to claim 3, having a hot-dip galvanized layer on the surface.

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

7. The steel sheet according to claim 5, wherein the hot-dip galvanized layer is a galvannealed layer.

8. 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 cooling the hot rolled steel sheet from 800°C or higher at an average cooling rate of 50°C / s or higher to a coiling temperature of 400°C or lower and coiling it at the coiling temperature; 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 of the slab to obtain the hot rolled steel sheet, and finish rolling of further hot rolling the hot rolled steel sheet after the rough rolling by multiple passes of reduction, a method for producing 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 a maximum heating temperature of 800°C or more but less than 840°C so that an average heating rate from 400°C to the maximum heating temperature is 5°C / s or more, the maximum heating temperature is held for 30 to 90 seconds, and after the holding, the steel sheet is cooled to 300°C or less at an average cooling rate of 10 to 50°C / s.

9. The method for producing a steel sheet according to claim 8, wherein in the annealing step, the annealing is carried out in an atmosphere with a dew point of -15 to 10°C, and between the holding and the cooling, the steel sheet is slowly cooled so that the average cooling rate in the temperature range of 750 to 650°C is 1 to 5°C / s.

10. A part comprising the steel sheet according to claim 1 or 2.

11. The part according to claim 10, wherein the steel sheet has a hot-dip galvanized layer on the surface.

12. The component according to claim 11, wherein the hot-dip galvanized layer is a galvannealed layer.

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