Steel plate, its manufacturing method, and parts

Abstract

This steel plate has a predetermined chemical composition, and when the position from the surface to 1 / 4 of the plate thickness along the plate thickness direction is defined as the 1 / 4 depth position, the microstructure at the 1 / 4 depth position contains, in area ratios, 80% or more of martensite, 0 to 15% of the total of ferrite, bainite, and pearlite, and 0 to 10% of retained austenite, and when the martensite in the region 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.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet, a manufacturing method thereof, and a part. This application claims priority based on Japanese Patent Application No. 2024-069143, filed on April 22, 2024, the contents of which are incorporated herein by reference. [Background technology]

[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 automobile bodies and improve fuel efficiency, in consideration of the global environment. Furthermore, among automotive components, collision components are required to have high strength, preferably both high strength and excellent bendability, in order to absorb energy during a collision. In particular, in recent years, there has been a demand for high-strength steel sheets having a tensile strength of 1470 MPa or more.

[0003] Resistance spot welding is primarily used in the assembly of automobile bodies and the installation of 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. When attempting to increase the tensile strength of steel plate, the carbon content is usually increased. However, as the carbon content increases, the strength of spot welds, as evaluated by CTS and other standards, decreases. If the strength of spot welds decreases, fractures are more likely to occur at the spot welds when the steel plate is used in crushable parts (parts such as front side frames that are required to absorb energy by undergoing large deformation during a collision). In other words, even if the strength of the steel plate is increased, only the same crushing strength as that of low-strength steel plate can be obtained, which reduces the benefits of using high-strength steel plate.

[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 remainder, and in the structure, the maximum random specific strength 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 strength 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 sheet having a tensile strength of 1310 MPa or more, a uniform elongation of 5.0% or more, and a TS × λ of 35000 MPa·% or more, which has excellent formability, a problem inherent in high-strength steel sheets, 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 spot welds when applied to crushable parts. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2021 / 070951 Summary of the Invention [Problem to be solved by the invention]

[0007] As described above, conventionally, there have been no proposals for a steel plate 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 having high strength and 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 having high strength and high bendability and high joint strength at spot welds when spot welded, a method for manufacturing the same, and a component including the steel sheet. [Means for solving the problem]

[0008] The present inventors have investigated methods for improving the strength of spot welds while also improving the strength of the steel sheet. They have also investigated methods for further improving the bendability of the steel sheet. As a result, they have obtained the following findings. (A) In order to obtain high strength, it is effective to control the chemical composition and the microstructure at the 1 / 4 depth position. (B) In order to increase the strength of spot welds, it is effective to keep the C content to 0.190% or less and utilize fine martensite. (C) In order to obtain fine martensite, it is effective to introduce lattice defects such as dislocations into the austenite by low-temperature rolling during the manufacturing process, and to utilize these lattice defects (grain boundaries, dislocations) in the martensite transformation during the annealing process. (D) By providing a decarburized layer within a certain range from the surface, bendability is improved.

[0009] The present invention has been made in light of the above findings. [1] A steel sheet according to an 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 : 0-0.100%, Ta: 0-0.100%, Sn: 0-0.050%, Sb: 0-0.050%, As: 0-0.050%, Ni: 0-1.000%, Cu: 0-1.000%, Ca: 0-0.050%, Zr: 0-0.050%, Mg: 0-0.050%, REM: 0-0.100%, and the balance: Fe and impurities, and when a position at 1 / 4 of the plate thickness from the surface along the plate thickness direction is defined as the 1 / 4 depth position, The microstructure at the lowermost position contains, in area ratios, 80% or more of martensite, 0 to 15% of the total of ferrite, bainite, and pearlite, and 0 to 10% of retained austenite, and when fine martensite is defined as a region in the martensite where the average spacing of grain boundaries is 2 μm or less and the crystal orientation angle difference is 50° or more, the area ratio of the fine martensite in the martensite is 7% or more and the tensile strength is 1470 MPa or more. [2] In the steel sheet described in [1], when the range from the surface to 30 μm is defined as a surface layer portion, the microstructure of the surface layer portion may contain, 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 may be 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 alloy may contain one or more of the following: 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] In the steel sheet according to [4], the hot-dip galvanized layer may be a galvannealed layer. [7] In the steel sheet according to [5], the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer. [8] A method for producing a steel sheet according to another aspect of the present invention comprises: [1] A method for producing a steel sheet according to the present invention,[1] A heating step of heating a slab having the chemical composition described above 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 it at the coiling temperature; a cold rolling step of cold-rolling the hot-rolled steel sheet after the coiling step under conditions where the sheet thickness reduction rate is 5 to 50% to obtain a cold-rolled steel sheet; and an annealing step of annealing the cold-rolled steel sheet, wherein the hot rolling step includes rough rolling the slab to obtain the hot-rolled steel sheet; and finish rolling, in which the hot-rolled steel sheet after the rough rolling is further hot-rolled by rolling reduction in multiple passes, wherein 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, and 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, the maximum heating temperature is held for 30 to 90 seconds, and after the holding, the temperature is cooled to 300°C or less at an average cooling rate of 10 to 50°C / s. [9] [8] In the method for producing a steel sheet according to the present invention, 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 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 component according to another aspect of the present invention includes the steel sheet according to any one of [1] to [3].

[11]

[10] The part according to the present invention may be such that the steel sheet has a hot-dip galvanized layer on the surface thereof.

[12] In the component according to

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

[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. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is a schematic diagram for explaining a method for measuring the average spacing of grain boundaries in martensite. DETAILED DESCRIPTION OF THE INVENTION

[0012] A steel plate according to one embodiment of the present invention (a steel plate according to the present embodiment), a part, and methods for manufacturing these will be described. In this embodiment, the position of 1 / 4 of the plate thickness from the surface along the plate thickness direction is defined as the 1 / 4 depth position, and the range from the surface to 30 μm is defined as the surface layer. Here, the steel sheet may have a plating layer on its surface, but the surface serving as the reference for the 1 / 4 depth position and the surface layer portion is the surface of the base steel sheet excluding the plating layer when the steel sheet has a plating layer (when the steel sheet is a plated steel sheet having a base steel sheet and a plating layer).

[0013] <Steel plate> The steel plate according to this embodiment has a predetermined chemical composition, and the microstructure at the 1 / 4 depth position contains, in area ratios, 80% or more of martensite, 0 to 15% of the total of ferrite, bainite, and pearlite, and 0 to 10% of retained austenite, and in the martensite, the area ratio of regions (fine martensite) in which the average spacing of grain boundaries having a crystal orientation angle difference of 50° or more is 2 μm or less (area ratio of fine martensite in martensite) is 7% or more, and 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 includes, in area ratio, a total of ferrite, pearlite, and bainite: 60% or more, and a total of martensite and retained austenite: 0 to 40%, and has a VDA bending angle of 75° or more. Each of these will be explained below.

[0014] [Chemical composition] The chemical composition of the steel sheet according to this embodiment will be described below. Unless otherwise specified, "%" indicating the content of each element in the chemical composition always means mass %.

[0015] (C: 0.160 to 0.190%) C is an essential element for increasing the strength of steel sheets. 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 will decrease. Therefore, the C content is set to 0.190% or less. From the viewpoint of suppressing the deterioration of the weldability, the C content is preferably set to 0.180% or less.

[0016] (Si: 0.50 to 2.00%) Si is a solid solution strengthening element and is an element that is effective in increasing the strength of steel sheets. To achieve this 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 cause 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 obtain the above effect sufficiently. Therefore, the Mn content is set to 2.4% or more. The Mn content is preferably 2.6% or more. On the other hand, if the Mn content exceeds 3.5%, the 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, and preferably 0.005% or more. On the other hand, if Al is added excessively, the effect saturates, leading to an increase in costs, and the transformation temperature of the steel rises, increasing the load during hot rolling and significantly impairing the flatness of the hot-rolled steel sheet, making it difficult to perform the subsequent cold rolling process. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.090% or less.

[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 an element that is beneficial for increasing the strength of steel sheets. To achieve the above effects, the B content is set to 0.0001% or more. On the other hand, if the B content exceeds 0.0050%, coarse B oxides and borides that 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. 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 to maintain lattice defects even after γ (austenite) transformation. To achieve this 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, which become the starting points for voids during press forming, may precipitate, deteriorating the workability of the steel sheet. For this reason, the Nb content is set to 0.100% or less. 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 even 0% is acceptable. However, taking into consideration the time and cost required to remove P, the P content is set to 0.015% or less. The P content is preferably 0.013% or less, and more preferably 0.010% or less.

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

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

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

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

[0027] (Ti: 0 to 0.050%) Ti is an element that is effective in controlling the morphology of carbides. Therefore, Ti may be contained. To obtain the above effects, the Ti content is preferably 0.001% or more. On the other hand, if the Ti content exceeds 0.050%, coarse Ti oxides or Ti carbonitrides may be present in the steel, which may reduce the workability of the steel sheet. Therefore, if Ti is contained, the Ti content is set to 0.050% or less. 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 effects, the Cr content is preferably 0.001% or more. On the other hand, if the Cr content exceeds 1.000%, Cr may segregate in the center of the steel sheet, forming coarse Cr carbides, which may reduce cold formability. Therefore, if Cr is contained, the Cr content is set to 1.000% or less. The Cr content is preferably 0.800% or less or 0.600% or less.

[0029] (W:0~0.500%) W is a carbide-forming element and is an element effective in increasing the strength of steel sheets. Therefore, W may be contained. To obtain the above effects, the W content is preferably 0.001% or more. The W content is more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if the W content is too high, not only will the effect saturate but the cost will also increase. Therefore, if W is contained, the W content is set to 0.500% or less. The W content is preferably 0.400% or less, and more preferably 0.300% or less.

[0030] (Co: 0 to 0.500%) Co is an element that is effective in increasing the strength of steel sheets, and therefore Co may be contained. To obtain the above effects, the Co content is preferably 0.010% or more, and more preferably 0.050% or more. On the other hand, if the Co content is too high, the ductility of the steel sheet may decrease, resulting in decreased hole expandability and bendability. Therefore, if Co is contained, the Co content is set to 0.500% or less. The Co content is preferably 0.400% or less or 0.300% or less.

[0031] (V: 0 to 0.100%) V is an element that is effective in controlling the morphology of carbides and is also effective in improving the toughness of steel sheets. Therefore, V may be contained. To obtain the above effects, the V content is preferably 0.001% or more. On the other hand, if the V content exceeds 0.100%, a large number of fine V carbides precipitate, which increases the strength of the steel sheet but significantly reduces ductility and workability. Therefore, if V is contained, the V content is set to 0.100% or less. The V content is preferably 0.080% or less.

[0032] (Ta: 0 to 0.100%) Ta is an element that is effective in controlling the morphology of carbides and improving the strength of the steel sheet. Therefore, Ta may be added. To obtain the above effects, the Ta content is preferably 0.001% or more. On the other hand, if the Ta content is too high, a large number of fine Ta carbides will precipitate, which may reduce the ductility, hole expandability, and bendability of the steel sheet. Therefore, if Ta is contained, the Ta content is set to 0.100% or less. The Ta content is preferably 0.020% or less, and more preferably 0.011% or less.

[0033] (Sn: 0 to 0.050%) Sn is an element that can be contained in steel sheets when scrap is used as the raw material for the steel sheets. Furthermore, Sn is an element that may cause a decrease in the hole expandability and bendability of the steel sheets due to the embrittlement of ferrite. Therefore, the lower the Sn content, the better. The Sn content is set to 0.050% or less, and preferably 0.040% or less. The Sn content may be 0%, but reducing the Sn content to less than 0.001% would result in an excessive increase in refining costs, so the Sn content may be set to 0.001% or more.

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

[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 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. Therefore, the lower the As content, the better. The As content is set to 0.050% or less, and preferably 0.040% or less. The As content 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 set to 0.001% or more.

[0036] (Ni: 0 to 1.000%) Ni is an element that is effective in improving the strength of steel sheets. Therefore, Ni may be contained. To obtain the above effect, the Ni content is preferably 0.001% or more, and more preferably 0.010% or more. On the other hand, if the Ni content is too high, the ductility of the steel sheet may decrease, resulting in decreased hole expandability and bendability. Therefore, when Ni is contained, the Ni content is set to 1.000% or less. The Ni content is preferably 0.600% or less, and more preferably 0.300% or less.

[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 effects, the Cu content is preferably 0.001% or more. On the other hand, if the Cu content is too high, red shortness may occur, which may reduce productivity during hot rolling. Furthermore, the formation of coarse inclusions may also reduce hole expandability and bendability. Therefore, if Cu is contained, the Cu content should be 1.000% or less. The Cu content is preferably 0.700% or less or 0.600% or less, and more preferably 0.300% or less.

[0038] (Ca: 0 to 0.050%) Ca is an element that is effective in controlling the morphology of sulfides even in small amounts. Therefore, Ca may be contained. To obtain the above effects, the Ca content is preferably 0.001% or more. On the other hand, if the Ca content is too high, coarse Ca oxides may be generated. These Ca oxides become the starting point for cracks during cold forming, so if the Ca content is too high, hole expandability and bendability may deteriorate. Therefore, if Ca is contained, the Ca content is set to 0.050% or less. The Ca content is preferably 0.030% or less.

[0039] (Zr: 0 to 0.050%) Zr is an element that is effective in controlling the morphology of sulfides even in small amounts. Therefore, Zr may be contained. To obtain the above effects, the Zr content is preferably 0.001% or more. On the other hand, if the Zr content is too high, coarse Zr oxides may be generated, which may reduce the hole expandability and bendability. Therefore, if Zr is contained, the Zr content is set to 0.050% or less. The Zr content is preferably 0.040% or less.

[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 the steel sheet. Therefore, Mg may be added. To obtain the above effects, the Mg content is preferably 0.0001% or more. On the other hand, if the Mg content is too high, coarse inclusions may form, which may reduce the hole expandability and bendability. Therefore, if Mg is contained, the Mg content is set to 0.050% or less, and preferably 0.040% or less.

[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 are generated, which may reduce workability, fracture resistance, hole expandability, and bendability. Therefore, if REM is contained, the REM content is set to 0.100% or less, and preferably 0.060% or less. Here, REM stands for Rare Earth Metal, a collective term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). "REM content" refers to the total content of these rare earth elements.

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

[0043] The chemical composition of the steel sheet according to this embodiment may be measured by a general method. For example, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) can be used for measurement. However, elements that are difficult to measure with ICP-AES can be measured by other methods. For example, C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-non-dispersive infrared absorption method. The sample for measurement is taken from around the 1 / 4 depth position. When the chemical composition of the slab is known, the chemical composition of the slab may be used as the chemical composition of the steel sheet according to this embodiment.

[0044] [Microstructure at 1 / 4 depth] Next, the microstructure (metal structure) at the 1 / 4 depth position of the steel plate according to this embodiment will be described. In the description of the microstructure of the steel plate according to this embodiment, the structure fraction is expressed as an area fraction. Therefore, unless otherwise specified, "%" represents "area %".

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

[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, the area fraction of retained austenite is preferably 5% or more. On the other hand, if the area fraction of retained austenite is excessive, the grain size of the retained austenite becomes large. Such retained austenite with a large grain size becomes coarse and hard martensite after deformation. In this case, cracks are likely to start, and bendability and other properties 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] To identify each structure and calculate its area ratio, first determine the area that should be retained austenite in a specified observation area, and then identify ferrite, bainite, martensite, or pearlite in the same observation area. It is desirable to take samples from within 100 mm from the edge in the width direction, but if the original material is small and this is difficult, take samples from the center of the material. Specifically, the area fraction of retained austenite is measured as follows. After mirror-finishing the observation surface, colloidal silica polishing or electrolytic polishing are used to polish the observation surface. Using an EBSD (Electron Backscatter Diffraction) (EBSD) attached to a scanning electron microscope, diffracted electrons are measured in a 100 μm × 100 μm square area, centered at 1 / 4 depth of the steel sheet, measuring 100 μm in the thickness direction and 100 μm in the rolling direction, at intervals of 0.1 μm (grid arrangement) in both the thickness direction and the rolling direction. The crystal orientation and crystal system are identified by analyzing the resulting pseudo-Kikuchi pattern. The incident electron accelerating voltage is 15 kV. The sample preparation conditions are within the range recommended by the Japan Society for Materials Science's standard, "Crystal Orientation Measurement Standard for Materials Evaluation by Electron Backscatter Diffraction (EBSD) Method." The measurement points detected as FCC phase in the measurement data are considered retained austenite, and the ratio of the number of measurement points detected as FCC phase to the total number of measurement points is considered the area fraction of retained austenite. The area ratios of ferrite, bainite, martensite, and pearlite can be measured by the following method. Specifically, the same square area as that used for the observation of retained austenite above is etched with nital solution and photographed (magnification: 5000x) with a field emission scanning electron microscope (FE-SEM). From the obtained secondary electron image of the structure, a point counting method is used at intervals of 1 μm (grid arrangement), and the ratio of each structure to the total number of measurement points is taken as the area ratio. The observation area is set to 1200 μm to minimize variations depending on the observation location. 2 If the area of ​​one field of view is insufficient, the area should be 1200 μm or more. 2 It is sufficient to measure multiple fields of view so that the observation area is equal to or larger than 2500 μm. 2 The following may also be used.

[0049] In this embodiment, the rolling direction is determined by the following method. The Z-plane of the plate (the plane parallel to both the longitudinal and transverse directions of the plate) is polished to a depth of 1 / 4 and then mirror-polished. An Mn concentration map of a 500 μm × 500 μm area is then obtained using an EPMA. When the solidification segregation of Mn is measured as streaks, the longitudinal direction of the streaks is determined to be the rolling direction. When the rolling direction of a steel sheet is known in advance, the rolling direction of the steel sheet may be determined without using the above-mentioned determination method. Furthermore, even when a steel sheet is processed into a part, the rolling direction can be determined using the above-mentioned method for a lightly processed part of the part (for example, a flat part that has received relatively little processing).

[0050] Each structure is judged as follows: Ferrite has a granular or needle-like shape and does not contain iron-based carbides inside. Bainite has a lath-like shape (lath structure) and is a region where iron-based carbides with a major axis of 20 nm or more exist inside the lath structure and the carbides elongate in the same direction. Fresh martensite has a lath-like shape (lath structure) and is a region where iron-based carbides with a major axis of 20 nm or more exist inside the lath structure and the carbides elongate in multiple different directions. Furthermore, the region where ferrite and cementite are in a lamellar shape is called pearlite. When identifying ferrite, bainite, martensite, and pearlite, the above-mentioned visual identification is not performed on areas that have been determined to be retained austenite in the preceding identification of retained austenite. However, the area ratio is calculated as a ratio to the total number of measurement points in the observation range, including the retained austenite area.

[0051] If the total area ratio of each texture obtained by the above evaluation method is different from 100%, the area ratio of each texture is multiplied by 100 / (total area ratio of each texture).

[0052] (In martensite, the area ratio of regions with an average spacing of 2 μm or less and a crystal orientation angle difference of 50° or more 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 refined. By refining the martensite, the toughness of the steel is improved. In this case, fracture is suppressed even in spot welds, and joint strength is increased. Specifically, in martensite, if the boundaries where the crystal orientation angle difference is 50° or more are considered grain boundaries, the regions where the average spacing of these grain boundaries is 2 μm or less are defined as fine martensite, and the area fraction of fine martensite in martensite (occupying the martensite) is set to 7% or more. Martensite where the average spacing of grain boundaries where the crystal orientation angle difference is 50° or more exceeds 2 μm may not improve tensile strength significantly, and in cases where the tensile strength is 1470 MPa or more, it may cause a decrease in joint strength due to a decrease in toughness. Therefore, the area fraction of the regions (fine martensite) where the average spacing of grain boundaries where the crystal orientation angle difference is 50° or more and is 2 μm or less is specified. If the area fraction of this fine martensite is less than 7%, the effect of improving tensile strength and joint strength cannot be sufficiently obtained. The area fraction 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 manufacturing conditions as 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 the measurement data obtained by the EBSD method using TSL OIM Analysis7. First, the martensite region is defined using the following method. The group of measurement points identified as martensite using the point counting method is divided into a 5 μm x 5 μm square mesh consisting of points in the thickness direction and perpendicular to the thickness direction, with the points identified as martensite as vertices. Only measurement points where all four vertices of the square are identified as martensite are used as part of the mesh. For example, if it is not possible to draw a square where all four points are identified as martensite in the boundary region with other phases, the measurement points are not used as part of the mesh. Using Figure 1 as an example, the martensite (M) is divided into meshes (5) with 5 μm intervals, as indicated by the dashed-dotted lines. In the region where the meshes (5) exist, grain boundaries (50) with a crystal orientation angle difference of 50° or more (thick solid lines in the martensite (M)) are displayed 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. The number of intersections (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 total length of the measurement lines (1) within the mesh (40 μm) divided by the number of intersections with the grain boundaries within the mesh is defined as the "average spacing of martensite grain boundaries" within that 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 in all martensite is taken as the area ratio of fine martensite grains.

[0054] [Microstructure of the surface layer] 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, the total area ratio of these is preferably 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: 0-40%) If the total area ratio of martensite and retained austenite exceeds 40%, bendability deteriorates, so 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 can be done in the same way as the calculation of the area ratio of each structure at the 1 / 4 depth position, except for the measurement position. In the case of the surface layer, the measurement position is a square area from the surface of the steel plate to a depth of 30 μm (a square area with one side at the surface, 30 μm in the plate thickness direction × 30 μm in the rolling direction).

[0058] [Tensile strength] The steel sheet according to this embodiment has a tensile strength (TS) of 1470 MPa or more, which is a strength that contributes to weight reduction and impact resistance of automobile bodies. There is no upper limit to the tensile strength, but in order to ensure weldability, it may be 1600 MPa or less.

[0059] [VDA bending angle] In the steel plate according to this embodiment, the VDA bend angle is set to 75° or more to suppress fracture during a collision. The VDA bend angle can be determined by a bending test in accordance with VDA (German Association of the Automotive Industry) standard 238-100. Due to the nature of the test, the VDA bend angle is set to 180° or less.

[0060] Plate Thickness Although the thickness of the steel plate according to this embodiment is not limited, 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, and more preferably 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] [Plating layer] The steel sheet according to this embodiment may have a zinc-plated layer on the surface. Providing a plated layer on the surface improves corrosion resistance. Even if automotive steel sheets are strengthened, they may not be able to be thinned below a certain thickness due to concerns about holes due to corrosion. One of the purposes of strengthening steel sheets is to reduce weight by thinning them, so even if a steel sheet is developed, its application areas will be limited if its corrosion resistance is low. One possible method for solving these problems is to apply a highly corrosion-resistant coating, such as hot-dip galvanizing, to the steel sheet. The steel sheet according to this embodiment is capable of hot-dip galvanizing because the steel sheet composition is controlled as described above. The hot-dip galvanized layer may be an alloyed hot-dip galvanized layer.

[0062] <Parts> The part according to this embodiment can be obtained by cutting the steel plate according to this embodiment to a predetermined size as needed, processing it into a predetermined shape by pressing or the like as needed, and joining it to other members by welding or the like as needed. Therefore, the component according to this embodiment includes, at least in part, the steel plate according to this embodiment having the above-described characteristics. The part according to this embodiment is, for example, a crushable part.

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

[0065] [Heating process] In the heating step, the slab having a predetermined chemical composition obtained through the casting step is heated to a heating temperature of 1180°C or higher. If the heating temperature is less than 1180°C, the NbC formed in the casting process cannot be brought into solution, and the effect of Nb in suppressing recrystallization cannot be obtained in the subsequent hot rolling process. The heating temperature is preferably 1220°C or higher. There is no upper limit to the heating temperature, but the heating temperature may be 1350° C. or less in terms of fuel costs.

[0066] The chemical composition of the slab to be subjected to the heating process does not change substantially during the manufacturing process except for a certain range from the surface that becomes the decarburized layer, for example, at a 1 / 4 depth position. Therefore, the chemical composition of the slab may be made equivalent to the chemical composition at a 1 / 4 depth position of the target steel plate.

[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, which converts the slab into a hot-rolled steel sheet, and finish rolling, which further hot-rolls the hot-rolled steel sheet after rough rolling by multiple passes. In the hot rolling process, in finish rolling, the surface temperature of the hot-rolled steel sheet before the start of the final three passes is set to 900°C or less, and the 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) is set to 30% or more. There is no lower limit for the rolling start temperature, but it is preferably 820°C or higher 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 having multiple stands. Dislocations are introduced into the austenite by the rolling. In the steel sheet according to this embodiment, recrystallization is suppressed by Nb, so the introduced dislocations are carried over to subsequent processes. If the surface temperature of the steel sheet before the start of the final three passes of finish rolling (for example, before the start of the fifth pass in the case of seven passes) exceeds 900°C, or if any of the rolling reductions in the final three passes is less than 30%, sufficient dislocations cannot be introduced. Preferably, the surface temperature of the hot-rolled steel sheet before the start of the final three passes is 860°C or less. Also, preferably, the reduction ratio in each of the final three passes is 35% or more. There is no upper limit to the reduction ratio in the final three passes, but in terms of equipment load, the reduction ratio in each of the final three passes may be 50% or less.

[0068] [Winding 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 less. By coiling at a low temperature, dislocations introduced into austenite (γ) can be retained even after transformation. If the coiling temperature is higher than 400°C, sufficient dislocations will not remain. Furthermore, in order 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 facility 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 the γ after reverse transformation. If the average cooling rate is slow or the coiling temperature exceeds 400°C, sufficient lattice defects (dislocations and grain boundaries) will not remain. From the viewpoint of lattice defects, a lower coiling temperature is preferable. However, if the coiling temperature is less than 250°C, the steel sheet will become excessively hardened, and there is a risk of the steel sheet breaking during cold rolling. Therefore, the coiling temperature is preferably 250°C or higher.

[0069] [Cold rolling process] In the cold rolling step, the hot rolled steel sheet after the coiling step is cold rolled under conditions where the sheet thickness reduction rate (rolling reduction rate) is 5 to 50% to obtain a cold rolled steel sheet. If the sheet thickness reduction rate exceeds 50%, the reduction in dislocation density due to recrystallization during the annealing process is promoted. In this case, it is not possible to obtain the desired decarburized layer. Therefore, the sheet thickness reduction rate is set to 50% or less. 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. From the perspective of suppressing fracture during cold rolling, the steel may be reheated to 200°C or less before cold rolling. Reheating to above 200°C reduces the dislocation density, making it impossible to obtain the desired fine martensite structure.

[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, the maximum heating temperature is set to 800°C or more but less than 840°C, and the maximum heating temperature is 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) will be significantly reduced due to diffusion transformation and coarsening of recrystallized grains, and the martensite obtained by the subsequent transformation will not be refined. Therefore, the average heating rate in this temperature range is set to 5°C / s or higher. The average heating rate may be 50°C / s or lower. 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, and the amount of martensite formed during subsequent cooling may be insufficient. 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 refining martensite decreases due to recovery and grain growth, and fine martensite cannot be obtained. 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 is higher than 300°C, the plate may not be sufficiently hardened 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, and a homogeneous material may not be obtained. Since special equipment is required to stop the cooling at a temperature lower than room temperature, the cooling stop temperature should be set to room temperature or higher. The starting temperature for cooling for the above quenching may be immediately after the maximum heating temperature, or may be the temperature after isothermal holding for obtaining bainite, which will be described later, or the ending temperature of slow cooling for obtaining a soft surface layer. To obtain a partial bainite structure, the steel may be held in a 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 up to 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 at an average cooling rate of 1 to 5°C / s in the temperature range of 750 to 650°C is preferably performed 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 process] When a plating layer is formed on the surface of the steel sheet, a plating step may be further carried out. The plating step may be carried out during the cooling after the above-mentioned holding in the annealing step, or may be carried out after once cooling to room temperature. When forming a hot-dip galvanized layer on the surface as a plating layer and manufacturing a cold-rolled steel sheet (hot-dip galvanized steel sheet) having a hot-dip galvanized layer, the steel sheet may be immersed in a plating bath at an equivalent temperature while the steel sheet temperature is above 425°C and below 600°C to perform hot-dip galvanization.

[0074] [Alloying process] When producing 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 steel sheet may be subjected to an alloying heat treatment in which the steel sheet is heated to, for example, more than 450°C and less than 600°C, to convert the galvannealed layer into an galvannealed layer. The thermal history associated with the plating process and the alloying process is included in the thermal history to be controlled in the annealing process if it is performed during the cooling of the annealing process, but is not included in the thermal history to be controlled in the annealing process if it is performed after cooling to room temperature in the annealing process.

[0075] [Processing process] When processing is performed, the steel plate according to this embodiment is formed into a predetermined size and / or shape in the processing step. The processing method may be a known method, for example, shearing, punching, drilling, bending, pressing, stretching, or the like to process into a predetermined size and / or shape.

[0076] [Welding process] When the welding step is performed, the steel plate according to the present embodiment (including that which has undergone a processing step) and another steel material are joined by welding such as spot welding. There are no limitations on the welding conditions, and known conditions may be applied. [Example]

[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 with multiple stands and the reduction ratios in each of the final three passes were as shown in Tables 2-1 and 2-2. After hot rolling, the hot-rolled steel sheet was water-cooled from a temperature of 800°C or higher at an average cooling rate of 50°C / s or higher from the start of cooling to the coiling temperature, and was coiled at the coiling temperatures shown in Tables 2-1 and 2-2. The hot-rolled steel sheets after the coiling process were cold-rolled at the thickness reduction rates shown in Tables 2-1 and 2-2 to obtain cold-rolled steel sheets with thicknesses of 0.8 to 1.6 mm. The cold-rolled steel sheets after cold rolling were annealed under the conditions shown in Tables 2-1 and 2-2. In some examples, hot-dip 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 sheet at the 1 / 4 depth position was observed 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 JIS No. 5 tensile test pieces from the steel sheets in a direction perpendicular to the rolling direction and conducting tensile tests on these test pieces 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. The limiting bending angle was evaluated when the bending ridgeline was parallel to the rolling direction. A VDA bending angle of 75° or more was determined to have excellent bendability. The results are shown in Tables 4-1 and 4-2.

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

[0083] [Table 1-1]

[0084] [Table 1-2]

[0085] [Table 2-1]

[0086] [Table 2-2]

[0087] [Table 3-1]

[0088] [Table 3-2]

[0089] [Table 4-1]

[0090] [Table 4-2]

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

[0092] According to the present invention, it is possible to provide a steel sheet 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 sheet, which are therefore highly industrially applicable. [Explanation of symbols]

[0093] M martensite 1 Measurement lines spaced 1 μm apart 5 5μm mesh 50 Grain boundary with a crystal orientation angle difference of 50° or more

Claims

1. The chemical composition, in mass%, is C: 0.160-0.190%, Si: 0.50-2.00%, Mn: 2.4 to 3.5%, Al: 0.001-0.100%, B: 0.0001 to 0.0050%, Nb: 0.035-0.100%, 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 to 0.050%, Cr: 0-1.000%, W: 0-0.500%, Co: 0 to 0.500%, V: 0 to 0.100%, Ta: 0-0.100%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, As: 0 to 0.050%, Ni: 0-1.000%, Cu: 0 to 1.000%, Ca: 0-0.050%, Zr: 0 to 0.050%, Mg: 0 to 0.050%, REM: 0 to 0.100%, and The balance is Fe and impurities. and When a position of 1 / 4 of the plate thickness from the surface along the plate thickness direction is defined as a 1 / 4 depth position, the microstructure at the 1 / 4 depth position has an area ratio of Martensite: 80% or more, Sum of ferrite, bainite, and pearlite: 0 to 15% Retained austenite: 0 to 10% Including, 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, The tensile strength is 1470 MPa or more. steel plate.

2. When the range from the surface to 30 μm is defined as a surface layer portion, the microstructure of the surface layer portion has an area ratio of Total of ferrite, pearlite, and bainite: 60% or more, Total of martensite and retained austenite: 0 to 40% Including, The VDA bending angle is 75° or more. The steel sheet according to claim 1.

3. The chemical composition is, in mass %, Ti: 0.001 to 0.050%, Cr: 0.001-1.00%, W: 0.001-0.500%, Co: 0.010-0.500%, V: 0.001-0.100%, Ta: 0.001 to 0.100%, Sn: 0.001 to 0.050%, Sb: 0.001 to 0.050%, As: 0.001 to 0.050%, Ni: 0.010 to 1.000%, Cu: 0.001 to 1.000%, Ca: 0.001-0.050%, Zr: 0.001 to 0.050%, Mg: 0.0001 to 0.050%, and REM: 0.001~0.100% Contains one or more of the following: The steel sheet according to claim 1 or 2.

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

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

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

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

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

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

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

11. The steel sheet has a hot-dip galvanized layer on the surface. The component of claim 10.

12. The hot-dip galvanized layer is a galvannealed layer. The component of claim 11.