Cold-rolled steel sheets and steel members

Abstract

This cold-rolled steel sheet has a predetermined chemical composition, and when the sheet thickness in units of millimeters is defined as t and a position range of t / 8 to 3t / 8 from the surface in the sheet thickness direction is defined as a t / 4 position, a microstructure at the t / 4 position contains, in terms of volume ratio, at least 95% of martensite. In this microstructure, the volume ratio of tempered martensite is 95% or less, the prior austenite grain size is 7.0 μm or less, and the grain boundary segregation degree of Mn is 2.0 or less.

Description

[Technical Field]

[0001] The present invention relates to a cold-rolled steel sheet and a steel member. This application claims priority based on Japanese Patent Application No. 2023-061317, filed on April 5, 2023, the contents of which are incorporated herein by reference. [Background technology]

[0002] In today's highly specialized industrial technology fields, materials used in each technology 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 cold-rolled steel sheets to reduce the weight of vehicles and improve fuel efficiency, due to considerations of the global environment. Among automotive steel sheets, cold-rolled steel sheets used in body frame components in particular are now required to have high strength, and furthermore, high formability is required to expand their applications. Examples of the properties required for automotive steel sheets include a tensile strength (TS) of 1310 MPa or more and excellent bendability. Furthermore, in recent years, high-strength hot-dip galvanized steel sheets and high-strength alloyed hot-dip galvanized steel sheets, which have a zinc plating layer on the surface of the steel sheet, have also been used to ensure sufficient corrosion resistance of vehicle bodies and parts.

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

[0004] However, there are challenges with the use of high-strength galvanized and galvannealed steel sheets for automotive parts. Resistance spot welding is primarily used for processes such as assembling automobile bodies and installing parts. Resistance spot welding involves clamping overlapping base materials between the tips of properly shaped electrodes, concentrating current and pressure on a relatively small area to apply localized heat. However, when galvanized steel sheets (hot-dip galvanized, electrogalvanized, or galvannealed) are resistance-welded for the assembly of car bodies and / or parts, a type of cracking called liquid metal embrittlement (LME) can occur in the spot welds. LME cracking occurs when the heat generated during resistance spot welding melts the zinc in the galvanized layer, penetrating the grain boundaries of the steel sheet structure at the weld, and applying tensile stress to the resulting cracks. The conditions for cracking to occur are that the molten zinc comes into contact with the solid steel plate during welding, and that tensile stress (strain) acts at that point. The higher the strength of the steel plate, the higher the susceptibility to LME cracking tends to be. Patent Document 1 does not disclose a steel sheet having a tensile strength of 1310 MPa or more and excellent bendability, nor does it consider any measures against LME cracking.

[0005] To address the above issues, technologies have been proposed to improve the LME resistance of galvanized steel sheets during spot welding. For example, Patent Document 2 discloses a steel sheet in which, in a cross-sectional structure cut in the width direction perpendicular to the rolling direction, the block diameter is specified in a first depth region of 1 to 10 μm from the surface, a second depth region of 10 to 60 μm from the surface, and a third depth region of 60 μm to ¼ of the sheet thickness from the surface. Patent Document 2 shows that by using a three-layer structure in which the block diameter is controlled to be inclined from the surface layer of the plate thickness toward the central layer of the plate thickness, even during spot welding, the block diameter is large when deformation occurs, and the softer layer (second layer) bears the strain, making it possible to suppress excessive increases in strain in the outermost layer (first layer), and thereby suppressing the occurrence of spot weld LME cracks.

[0006] However, Patent Document 2 does not take into consideration bendability, and it is believed that there is room for improvement in terms of bendability. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2016 / 111275 [Patent Document 2] International Publication No. 2021 / 251276 Summary of the Invention [Problem to be solved by the invention]

[0008] As described above, conventionally, no steel sheet having high tensile strength, excellent bendability, and excellent LME resistance has been disclosed. Therefore, an object of the present invention is to provide a cold-rolled steel sheet having high tensile strength, excellent bendability, and excellent LME resistance, and a steel member obtained by processing this cold-rolled steel sheet. Here, the cold-rolled steel sheet includes a plated steel sheet having a plating layer formed on its surface, and the steel member includes a steel member such as a surface-treated steel material having a plating layer formed on its surface. [Means for solving the problem]

[0009] The present inventors have investigated methods for improving the tensile strength, bendability, and LME resistance of cold-rolled steel sheets, and as a result have obtained the following findings. In the microstructure, the tensile strength can be increased by increasing the volume fraction of martensite. Increasing the Al content in the chemical composition of cold-rolled steel improves LME resistance. However, increasing the Al content also increases the Ac3 point, which leads to larger prior γ grains and reduced bendability when heat treatment, as is commonly done in the manufacture of cold-rolled steel, is applied. To reduce the prior γ grain size, it is effective to repeatedly transform between the γ single phase and shear-type transformed structures (bainite and martensite). Suppressing Mn segregation to prior γ grain boundaries further improves bendability.

[0010] The present invention has been made in light of the above findings. [1] A cold-rolled steel sheet according to one embodiment of the present invention comprises, in mass%, C: 0.180% or more and 0.400% or less, Si: 0.050% or more and 1.000% or less, Mn: 2.00% or more and 4.00% or less, Al: 0.10% or more and 2.00% or less, Ti: 0.010% or more and 0.200% or less, B: 0.0010% or more and 0.0100% or less, N: 0.0010% or more and 0.0100% or less, P: 0% or more and 0.0100% or less, .0400% or less, S: 0% or more, 0.0100% or less, O: 0% or more, 0.0060% or less, Cr: 0% or more, 0.50% or less, Ni: 0% or more, 1.00% or less, Cu: 0% or more, 1.00% Below, Mo: 0% or more, 0.500% or less, Nb: 0% or more, 0.200% or less, V: 0% or more, 0.500% or less, W: 0% or more, 0.100% or less, Ta: 0% or more, 0.100% or less, Sn : 0% or more, 0.050% or less, Co: 0% or more, 0.500% or less, Sb: 0% or more, 0.050% or less, As: 0% or more, 0.050% or less, Mg: 0% or more, 0.050% or less, Ca: 0% or more, 0.040% or less, REM: 0% or more, 0.050% or less, Bi: 0% or more, 0.050% or less, Zr: 0% or more, 0.050% or less, Sr: 0% or more, 0.050% or less, the balance being F The plate has a chemical composition consisting of e and impurities, and when the plate thickness in mm is t and the range from the surface in the plate thickness direction to the t / 8 position to the 3t / 8 position is defined as the t / 4 position, the microstructure at the t / 4 position contains, in volume fraction, 95% or more of martensite, and the volume fraction of tempered martensite is 95% or less, the prior austenite grain size is 7.0 μm or less, and the grain boundary segregation degree of Mn is 2.0 or less. the law of nature , The grain boundary segregation degree of Mn is a value obtained by dividing the Mn concentration of the prior austenite grain boundary by the average Mn content of the steel sheet measured by ICP-AES, where the grain boundary appearing on the cleavage plane at the t / 4 position is regarded as a prior austenite grain boundary, and the average value of the Mn concentrations obtained by measuring five different grain boundaries among the prior austenite grain boundaries by Auger electron spectroscopy is defined as the Mn concentration of the prior austenite grain boundary. . [2] The cold-rolled steel sheet according to [1] may have a hot-dip galvanized layer on the surface. [3] In the cold-rolled steel sheet according to [2], the hot-dip galvanized layer may be a galvannealed layer. [4] A steel member according to another embodiment of the present invention is a member including a processed portion and a non-processed portion, wherein at least the non-processed portion contains, in mass%, C: 0.180% or more and 0.400% or less, Si: 0.050% or more and 1.000% or less, Mn: 2.00% or more and 4.00% or less, Al: 0.10% or more and 2.00% or less, Ti: 0.010% or more and 0.200% or less, B: 0.0010% or more and 0.0100% or less, N: 0.00 10% or more, 0.0100% or less, P: 0% or more, 0.0400% or less, S: 0% or more, 0.0100% or less, O: 0% or more, 0.0060% or less, Cr: 0% or more, 0.50% or less, Ni: 0% or more, 1 .00% or less, Cu: 0% or more, 1.00% or less, Mo: 0% or more, 0.500% or less, Nb: 0% or more, 0.200% or less, V: 0% or more, 0.500% or less, W: 0% or more, 0.100% or less, Ta: 0% or more, 0.100% or less, Sn: 0% or more, 0.050% or less, Co: 0% or more, 0.500% or less, Sb: 0% or more, 0.050% or less, As: 0% or more, 0.050% or less, Mg: 0% or more, 0. 050% or less, Ca: 0% or more, 0.040% or less, REM: 0% or more, 0.050% or less, Bi: 0% or more, 0.050% or less, Zr: 0% or more, 0.050% or less, Sr: 0% or more, 0.050% or less and the balance being Fe and impurities, and when the thickness in mm is t and the range from the surface in the thickness direction from the t / 8 position to the 3t / 8 position is defined as the t / 4 position, the microstructure at the t / 4 position contains, by volume fraction, 95% or more of martensite, and the volume fraction of tempered martensite is 95% or less, the prior austenite grain size is 7.0 μm or less, and the grain boundary segregation degree of Mn is 2.0 or less. the law of nature , The grain boundary segregation degree of Mn is a value obtained by dividing the Mn concentration of the prior austenite grain boundary by the average Mn content of the steel sheet measured by ICP-AES, where the grain boundary appearing on the cleavage plane at the t / 4 position is regarded as a prior austenite grain boundary, and the average value of the Mn concentrations obtained by measuring five different grain boundaries among the prior austenite grain boundaries by Auger electron spectroscopy is defined as the Mn concentration of the prior austenite grain boundary. . [5] The steel member according to [4] may have a hot-dip galvanized layer on the surface. [6] In the steel member according to [5], the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer. [Effects of the Invention]

[0011] According to the above-described aspects of the present invention, it is possible to provide a cold-rolled steel sheet having high tensile strength, excellent bendability, and excellent LME resistance, and a steel member obtained by processing this cold-rolled steel sheet. [Brief explanation of the drawings]

[0012] [Figure 1A] FIG. 1 is a diagram showing a manner in which two steel plates are spot-welded together. [Figure 1B] FIG. 1 is a diagram showing a mode of current control when spot welding two steel plates. [Figure 2] FIG. 10 is a diagram showing how cracks occur after spot welding. [Figure 3] FIG. 2 is a diagram showing a cross section of a steel member produced in an example. DETAILED DESCRIPTION OF THE INVENTION

[0013] A cold-rolled steel sheet according to one embodiment of the present invention (steel sheet according to this embodiment) and a steel member according to one embodiment of the present invention (steel member according to this embodiment) will be described. In this embodiment, the thickness of the cold-rolled steel sheet in mm is t, and the range from the surface in the thickness direction to the t / 8 position to the 3t / 8 position is described as the t / 4 position. Similarly, the thickness in mm of the unprocessed part of the steel member (if it is made of steel plate, this is the thickness of the steel plate) is defined as t, and the range from the surface in the thickness direction from the t / 8 position to the 3t / 8 position is defined as the t / 4 position.

[0014] <Cold rolled steel plate> The steel sheet according to this embodiment has a predetermined chemical composition, and at the t / 4 position, the microstructure contains, in volume fraction, 95% or more of martensite, the volume fraction of tempered martensite is 95% or less, the prior austenite grain size is 7 μm or less, and the grain boundary segregation degree of Mn is 2.0 or less. Each is explained in more detail below.

[0015] [Chemical composition] The reasons for limiting the chemical composition of the steel sheet according to this embodiment will be explained below. Unless otherwise specified, "%" relating to the content of each element constituting the chemical composition means "mass %."

[0016] C: 0.180% or more, 0.400% or less C (carbon) is an essential element for ensuring the strength of steel sheet. By setting the C content to 0.180% or more, the desired high strength can be obtained. The C content is preferably 0.200% or more, and more preferably 0.220% or more. On the other hand, in order to ensure workability and weldability, the C content is set to 0.400% or less, preferably 0.380% or less, and more preferably 0.360% or less.

[0017] Si: 0.050% or more, 1.000% or less Silicon (Si) is an element that contributes to improving the strength of steel sheet by suppressing temper softening of martensite in addition to solid solution strengthening. To obtain this effect, the Si content is set to 0.050% or more. On the other hand, in order to ensure the weldability of the steel sheet, the Si content is set to 1.000% or less, preferably 0.700% or less, and more preferably 0.500% or less.

[0018] Mn: 2.00% or more, 4.00% or less Mn (manganese) is a strong austenite stabilizing element and is an element that is effective in increasing the strength of steel sheets. To obtain these effects, the Mn content is set to 2.00% or more. The Mn content is preferably 2.20% or more, and more preferably 2.40% or more. On the other hand, if the Mn content is high, the weldability and low-temperature toughness decrease. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.60% or less, and more preferably 3.20% or less.

[0019] Al: 0.10% or more, 2.00% or less Al (aluminum) is an element used for deoxidizing steel, and like Si, it suppresses the formation of iron carbides and the temper softening of martensite, thereby effectively improving the strength of the steel sheet. Furthermore, Al is an element that contributes to improving LME resistance. To achieve the above effects, the Al content is set to 0.10% or more. The Al content is preferably more than 0.10%, more preferably 0.15% or more, and even more preferably 0.20% or more. On the other hand, if Al is added in excess, the effect will saturate, resulting in unnecessary cost increases, and coarse carbonitrides may precipitate during casting, embrittling the slab and causing cracks. Therefore, the Al content is set to 2.00% or less. The Al content is preferably 1.20% or less, and more preferably 0.80% or less. The Al content referred to here is the acid-soluble Al content (sol. Al content).

[0020] Ti: 0.010% or more, 0.200% or less Ti (titanium) is an element that is effective in securing solute B, which contributes to improving hardenability, by becoming TiN and fixing N. To obtain this effect, the Ti content is set to 0.010% or more. On the other hand, if the Ti content exceeds 0.200%, coarse carbonitrides may precipitate, which may reduce formability. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.180% or less, and more preferably 0.160% or less.

[0021] B: 0.0010% or more, 0.0100% or less Boron (B) is an element that segregates at austenite grain boundaries during welding, strengthening the grain boundaries and contributing to improved LME resistance. B also improves the hardenability of steel and contributes to increasing the strength of steel sheets. In order to obtain the above effects, the B content is set to 0.0010% or more, preferably 0.0015% or more, and more preferably 0.0020% or more. On the other hand, if the B content exceeds 0.0100%, carbides and nitrides are formed, the above effects become saturated, and hot workability deteriorates. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less, more preferably 0.0050% or less, and even more preferably 0.0030% or less.

[0022] N: 0.0010% or more, 0.0100% or less N (nitrogen) is an element that combines with Al to precipitate as AlN, contributing to the refinement of the structure. To obtain this effect, the N content is set to 0.0010% or more, and preferably 0.0020% or more. On the other hand, if the N content exceeds 0.0100%, coarse nitrides are formed in the steel, which deteriorates bendability and hole expandability. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, and more preferably 0.0060% or less.

[0023] P: 0% or more, 0.0400% or less P (phosphorus) is a solid solution strengthening element and is effective in increasing the strength of steel sheets, but excessive content deteriorates weldability and toughness. Therefore, the P content is set to 0.0400% or less. The P content is preferably 0.0350% or less, 0.0300% or less, or 0.0200% or less. The P content may be 0%, but reducing the P content too much increases the cost of dephosphorization. Therefore, from an economical standpoint, the P content may be set to 0.0010% or more.

[0024] S: 0% or more, 0.0100% or less S (sulfur) is an element contained as an impurity and forms MnS in steel, which deteriorates toughness and hole expandability. Therefore, the S content is set to 0.0100% or less, which is the range in which deterioration of toughness and hole expandability is not significant. The S content is preferably 0.0050% or less, 0.0040% or less, or 0.0030% or less. Although the S content may be 0%, extremely reducing the S content increases the cost of desulfurization. Therefore, from an economical standpoint, the S content may be set to 0.0001% or more or 0.0010% or more.

[0025] O: 0% or more, 0.0060% or less O (oxygen) is an element contained as an impurity, and if its content exceeds 0.0060%, it forms coarse oxides in the steel, degrading bendability and hole expandability. Therefore, the O content is set to 0.0060% or less. The O content is preferably 0.0050% or less, and more preferably 0.0040% or less. The O content may be 0%, but from the viewpoint of manufacturing costs, the O content may be set to 0.0001% or more.

[0026] The basic chemical composition of the steel sheet according to this embodiment includes the above elements (basic elements), with the balance being Fe and impurities. Here, "impurities" refer to components that are mixed in during industrial production of steel sheet due to various factors in raw materials such as ore and scrap, and in the manufacturing process, and whose inclusion is permitted to the extent that they do not have a clear adverse effect on the effects of the steel sheet according to this embodiment. The steel sheet according to this embodiment may contain the following elements (optional elements) in place of a portion of Fe, as needed. These elements do not necessarily have to be contained, so the lower limit is 0%. The following elements may be mixed in from raw material scrap or the like, but may be intentionally or unintentionally contained in the steel sheet as long as the content is equal to or less than the upper limit value described below. For example, the following elements may be contained in the steel sheet as a result of being contained in raw material scrap or the like for the steel sheet.

[0027] Cr: 0% or more, 0.50% or less Ni: 0% or more, 1.00% or less Cu: 0% or more, 1.00% or less Cr (chromium), Ni (nickel), and Cu (copper) are all elements that contribute to improving strength. Therefore, one or more selected from these elements may be contained as needed. To achieve the above effects, the content of one or more selected from Cr, Ni, and Cu is preferably 0.01% or more, and more preferably 0.10% or more. On the other hand, a Cr content exceeding 0.50%, a Ni content exceeding 1.00%, or a Cu content exceeding 1.00% may degrade pickling property, weldability, and hot workability. Therefore, the Cr content is set to 0.50% or less, the Ni content is set to 1.00% or less, and the Cu content is set to 1.00% or less. The Cr content may be set to 0.40% or less, 0.30% or less, or 0.10% or less. The Ni content may be set to 0.80% or less, 0.60% or less, or 0.20% or less. The Cu content may be set to 0.80% or less, 0.60% or less, or 0.20% or less.

[0028] Mo: 0% or more, 0.500% or less Mo (molybdenum), like Mn, is an element that improves the hardenability of steel and contributes to improving its strength. Therefore, Mo may be added as needed. To obtain the above effects, the Mo content is preferably 0.010% or more, more preferably 0.100% or more. On the other hand, if the Mo content exceeds 0.500%, hot workability may deteriorate, which may result in a decrease in productivity. Therefore, the Mo content is set to 0.500% or less. The Mo content is preferably 0.400% or less, more preferably 0.300% or less, and even more preferably 0.100% or less.

[0029] Nb: 0% or more, 0.200% or less V: 0% or more, 0.500% or less Both Nb (niobium) and V (vanadium) are elements that contribute to improving the strength of steel sheets through precipitation strengthening, grain refinement through grain growth inhibition, and dislocation strengthening through recrystallization inhibition. Therefore, one or more elements selected from these elements may be added as needed. To achieve the above effects, it is preferable to add one or both of 0.001% or more of Nb and 0.001% or more of V to the steel sheet. On the other hand, a Nb content exceeding 0.200% or a V content exceeding 0.500% may precipitate coarse carbonitrides, reducing formability. Therefore, the Nb content is set to 0.200% or less, and the V content is set to 0.500% or less. The Nb content is preferably 0.180% or less, more preferably 0.150% or less, and even more preferably 0.100% or less. The V content is preferably 0.400% or less, more preferably 0.300% or less, and even more preferably 0.100% or less.

[0030] W: 0% or more, 0.100% or less Ta: 0% or more, 0.100% or less Sn: 0% or more, 0.050% or less Co: 0% or more, 0.500% or less A: 0% or more, 0.050% or less W (tungsten), Ta (tantalum), Sn (tin), Co (cobalt), and As (arsenic) are elements that contribute to improving the strength of steel sheets by precipitation strengthening and suppressing grain coarsening. Therefore, these elements may be contained. To obtain this effect, it is preferable to contain one or more of these elements, with the W content being 0.001% or more, the Ta content being 0.001% or more, the Sn content being 0.001% or more, the Co content being 0.001% or more, and the As content being 0.001% or more. On the other hand, if these elements are present in large amounts, various properties of the steel sheet may be impaired. Therefore, the W content is set to 0.100% or less, the Ta content to 0.100% or less, the Sn content to 0.050% or less, the Co content to 0.500% or less, and the As content to 0.050% or less. The W content is preferably 0.080% or less, more preferably 0.050% or less, and even more preferably 0.030% or less. The Ta content is preferably 0.080% or less, more preferably 0.050% or less, and even more preferably 0.030% or less. The Sn content is preferably 0.040% or less, more preferably 0.030% or less, and even more preferably 0.010% or less. The Co content is preferably 0.400% or less, more preferably 0.300% or less, and even more preferably 0.100% or less. The As content is preferably 0.040% or less, more preferably 0.030% or less, and further preferably 0.010% or less.

[0031] Sb: 0% or more, 0.050% or less Mg: 0% or more, 0.050% or less Ca: 0% or more, 0.040% or less REM: 0% or more, 0.050% or less Zr: 0% or more, 0.050% or less Bi: 0% or more, 0.050% or less Sb (antimony), Mg (magnesium), Ca (calcium), REM (rare earth metal), Zr (zirconium), Bi (bismuth), and Sr (strontium) are all elements that contribute to improving formability. Therefore, one or more elements selected from these elements may be contained as needed. To achieve the above effects, it is preferable to contain one or more elements selected from Sb, Mg, Ca, REM, Zr, Bi, and Sr, with each content being 0.001% or more. The content of each element is more preferably 0.002% or more. On the other hand, a content of Sb, Mg, REM, Zr, Bi, or Sr exceeding 0.050% or a content of Ca exceeding 0.040% may degrade pickling properties, weldability, and hot workability. Therefore, the contents of Sb, Mg, REM, Zr, Bi, and Sr are all set to 0.050% or less, and the Ca content is set to 0.040% or less. The contents of Sb, Mg, Ca, REM, Zr, Bi, and Sr are preferably set to 0.035% or less, 0.030% or less, or 0.010% or less. In this embodiment, REM refers to rare earth elements, a collective term for Sc, Y, and lanthanoids, a total of 17 elements, and the REM content is the total content of these elements.

[0032] As described above, the chemical composition of the base steel plate of the steel plate according to this embodiment includes basic elements with the balance consisting of Fe and impurities, or includes basic elements and further includes one or more optional elements (elements other than impurities whose lower limit of content is 0% in the above description) with the balance consisting of Fe and impurities.

[0033] The chemical composition of the base steel sheet 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) of chips in accordance with JIS G 1201:2014. In this case, the chemical composition is the average content across the entire sheet thickness. C and S, which are difficult to measure using ICP-AES, may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method. The analytical sample is taken so as to obtain the average chemical composition throughout the thickness of the base steel plate, as described in JIS G 0417: 1999. Specifically, the analytical sample is taken from the t / 4 position, avoiding the ends in the width direction of the base steel plate.

[0034] [Microstructure (metal structure) at t / 4 position] (Martensite: 95% by volume or more) (Tempered martensite: 95% by volume or less) In the steel sheet according to this embodiment, the volume fraction of martensite is set to 95% or more in order to ensure a tensile strength of 1310 MPa or more. If the volume fraction of martensite is less than 95%, sufficient tensile strength cannot be ensured. The volume fraction of martensite may be 100%. In this embodiment, martensite includes fresh martensite and tempered martensite (including self-tempered martensite). However, if the volume fraction of tempered martensite exceeds 95%, sufficient tensile strength cannot be obtained, so the volume fraction of tempered martensite is set to 95% or less. That is, the volume fraction of tempered martensite is 95% or less, and the total volume fraction of tempered martensite and fresh martensite is 95% or more. The total volume fraction of tempered martensite and fresh martensite may be 100%. Fresh martensite is effective in contributing to higher strength. Therefore, when increasing tensile strength, it is preferable to increase the area fraction of fresh martensite. For example, fresh martensite is 85% or more, 90% or more, or 95% or more. On the other hand, fresh martensite is a brittle structure and has poor formability, so tempered martensite is preferable in terms of formability. Specifically, when formability is important, the volume fraction of tempered martensite is preferably 80% or more.

[0035] The remainder of the microstructure may be one or more selected from ferrite, pearlite, bainite, and retained austenite.

[0036] The volume fraction of each structure (each phase) at the t / 4 position is determined by the following procedure. Specifically, the volume fractions of ferrite, pearlite, bainite, fresh martensite, and tempered martensite were determined by taking a test specimen from a position at any point in the rolling direction of the steel sheet and at the center in the width direction. A longitudinal cross section parallel to the rolling direction (i.e., a cross section parallel to both the rolling direction and the thickness direction) was polished, and the metal structure revealed by nital etching at the quarter-tip position was observed using an SEM. SEM observations were performed at a magnification of 3000x, with five fields of view at the quarter-tip position, each 30 μm in the thickness direction and 50 μm in the rolling direction. The area fractions of each structure were measured from the observed images, and their average values ​​were calculated. Since there was no change in the structure in the direction perpendicular to the rolling direction (the steel sheet width direction), the area fractions of the longitudinal cross section parallel to the rolling direction were considered to be equal to the volume fractions, and the area fractions obtained by the structure observation were used as the respective volume fractions.

[0037] When measuring the area ratio of each structure, the region where the substructure is not revealed and has low brightness is defined as ferrite. The region with a layered structure of ferrite and cementite is defined as pearlite. The region where the substructure is not revealed and has high brightness is defined as fresh martensite or retained austenite. The region where the substructure is revealed is defined as tempered martensite or bainite.

[0038] Bainite and tempered martensite are further distinguished by careful observation of intragranular carbides. Specifically, tempered martensite is composed of martensite laths and cementite formed within the laths. Since there are two or more types of crystal orientation relationships between martensite laths and cementite, the cementite that constitutes tempered martensite has multiple variants. On the other hand, bainite is classified into upper bainite and lower bainite. Upper bainite is easily distinguished from tempered martensite because it is composed of lath-shaped bainitic ferrite and cementite formed at the lath interfaces. Lower bainite is composed of lath-shaped bainitic ferrite and cementite formed within the laths. Unlike tempered martensite, the crystal orientation relationship between bainitic ferrite and cementite is one type, and the cementite that constitutes lower bainite has the same variant. Therefore, lower bainite and tempered martensite are distinguished based on the cementite variant. On the other hand, fresh martensite and retained austenite cannot be clearly distinguished by SEM observation, so the volume fraction of martensite is calculated by subtracting the volume fraction of retained austenite, calculated by the method described below, from the volume fraction of the structure determined to be martensite or retained austenite.

[0039] The volume fraction of retained austenite is determined by taking a test piece from an arbitrary position in the rolling direction of the steel plate and at the center position in the width direction, chemically polishing the rolled surface from the surface of the steel plate to a position 1 / 4 of the plate thickness, and quantifying it from the (200) and (210) surface integral intensities of ferrite and the (200), (220), and (311) surface integral intensities of austenite using MoKα radiation.

[0040] (Prior austenite grain size (prior gamma grain size): 7.0 μm or less) In the steel sheet according to this embodiment, the prior austenite grain size is 7.0 μm or less in the microstructure at the t / 4 position. By refining the prior austenite grain size, bendability and LME resistance are improved. If the prior austenite grain size exceeds 7.0 μm, bendability and LME resistance become insufficient. Although there is no lower limit for the prior austenite grain size, unnecessarily refining the prior austenite grain size is economically disadvantageous, for example, requiring special equipment, and therefore the prior austenite grain size may be 4.0 μm or more. Here, the prior austenite grain size is the equivalent diameter of a circle surrounded by the prior austenite grain boundary when the grain boundary appearing on the cleavage plane is considered to be the prior austenite grain boundary.

[0041] (Mn grain boundary segregation: 2.0 or less) Furthermore, in the steel sheet according to this embodiment, in the microstructure at the t / 4 position, the degree of grain boundary segregation of Mn, which is the ratio of the Mn content at the grain boundaries to the average Mn content in the base material (the Mn content described in the above-mentioned chemical composition), is 2.0 or less. Mn is an element that easily segregates at grain boundaries. In the steel sheet according to this embodiment, the degree of Mn segregation at grain boundaries is reduced (the Mn content at grain boundaries is 2.0 times or less the average Mn content), thereby improving bendability. If the degree of Mn segregation at grain boundaries exceeds 2.0, bendability is poor. Although there is no lower limit for the degree of Mn segregation at grain boundaries, if the degree of Mn segregation at grain boundaries is less than 1.3, productivity decreases. Therefore, from the viewpoint of economic rationality, the degree of Mn segregation may be 1.3 or more.

[0042] The prior austenite grain size at the t / 4 position and the degree of grain boundary segregation of Mn are determined by the following method. A test piece is taken from any position in the rolling direction of the steel plate and at the center in the width direction (if the center position is unknown, it is at a position 50 mm or more away from the edge), and a notch is made at the center in the rolling direction of the test piece. The notched test piece is cooled to near liquid nitrogen temperature and subjected to impact fracture in a vacuum chamber to obtain a cleavage plane. In a cleavage plane at the t / 4 position, the grain boundary that appears on the cleavage plane observed by SEM observation is considered to be the prior austenite grain boundary, and the circle-equivalent diameter of the area surrounded by the prior austenite grain boundary is taken as the prior austenite grain size. In addition, the Mn concentration is measured for five different grain boundaries on the cleavage plane at the t / 4 position by Auger electron spectroscopy. The acceleration voltage and current value for the measurement are 10 kV and 10 nA, respectively. The average of the Mn concentrations at the five grain boundaries obtained as a result of the measurement is defined as the Mn concentration at the prior austenite grain boundary. The value obtained by dividing this by the average Mn content of the steel sheet measured by ICP-AES described above is defined as the degree of grain boundary segregation. If the prior austenite grain boundary does not appear during the measurement, the prior austenite grain boundary can be made to appear by previously carrying out the following hydrogen charging. That is, for a notched test piece, hydrogen is charged at a current density of 0.1 mA / cm in a solution with an ammonium thiocyanate concentration of 20 g / L or more. 2 Cathodic hydrogen charging is then performed for at least 24 hours. After cathodic hydrogen charging, electrogalvanization may be performed to prevent hydrogen desorption. Then, impact fracture is performed in a vacuum chamber. Increasing the concentration of ammonium thiocyanate or increasing the current density makes it easier to reveal prior austenite grain boundaries.

[0043] [Characteristics] (tensile strength) The steel sheet according to this embodiment is targeted to have a tensile strength (TS) of 1310 MPa or more, which is a strength that contributes to reducing the weight of automobile bodies. Although there is no upper limit to the tensile strength, if the tensile strength is too high, there is a risk of reduced formability, so the tensile strength may be set to 1600 MPa or less. The tensile strength (TS) is determined by taking a JIS No. 5 tensile test piece from the steel plate in the direction perpendicular to the rolling direction and conducting a tensile test in accordance with JIS Z 2241:2011. If it is not possible to obtain a JIS No. 5 tensile test piece, the tensile strength can be estimated by Vickers hardness. Specifically, a hardness test is conducted at the t / 4 position of the cross section, with a load of 1 kgf, in accordance with JIS Z 2244-1:2020. The obtained hardness (Hv) is multiplied by 3.3 to obtain the tensile strength TS. The target Vickers hardness is 400 Hv or more.

[0044] (bending) In the steel sheet according to this embodiment, the chemical composition and microstructure are controlled as described above, and therefore the steel sheet has excellent bendability and LME resistance. As for bendability, for example, the product of the maximum bending angle evaluated in a bending test according to the VDA standard and the tensile strength TS is preferably 130,000 (MPa·degree) or more. Regarding LME resistance, it is preferable that LME cracking does not occur when spot welding is performed on a material having a zinc-plated layer or when the mating material is a zinc-plated steel sheet.

[0045] Plate Thickness The thickness of the steel plate according to this embodiment is not limited, but is preferably 1.0 to 3.0 mm from the viewpoint of achieving both weight reduction of the automobile body and improvement of collision safety.

[0046] [Plating layer] The steel sheet according to this embodiment may have a zinc-plated layer. The presence of a zinc-plated layer improves corrosion resistance. In the case of automotive steel sheets, there are cases where they cannot be thinned below a certain thickness even if they are strengthened due to concerns about holes due to corrosion. Since one of the purposes of increasing the strength of steel sheets is to reduce weight by thinning them, even if a high-strength steel sheet is developed, its application will be limited if its corrosion resistance is low. To solve these problems, a highly corrosion-resistant zinc-plated layer may be formed on the surface of the steel sheet. The galvanized layer may be a hot-dip galvanized layer or an alloyed hot-dip galvanized layer. A hot-dip galvanized layer is preferable from the viewpoint of cost, and an alloyed hot-dip galvanized layer is preferable because it has excellent weldability and paintability due to the incorporation of Fe into the hot-dip galvanized layer by the alloying treatment. Furthermore, an upper layer of plating may be applied on the galvanized layer for the purpose of improving paintability and weldability. Furthermore, in the steel sheet according to this embodiment, various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment, etc. may be applied on the hot-dip galvanized layer.

[0047] In the case of a plated steel sheet having a plating layer (a plated steel sheet having a base material and a plating layer formed on the surface of the base material), the surface used as the reference for the t / 4 position is the surface of the base material excluding the plating layer. The chemical composition of the steel sheet is also the chemical composition of the base material excluding the plating layer.

[0048] <Steel parts> The steel member according to this embodiment is obtained by processing the steel sheet according to this embodiment (including the cold-rolled steel sheet and the plated steel sheet). That is, it has a processed portion and a non-processed portion. Even when the cold-rolled steel sheet becomes a steel member, the chemical composition and microstructure do not change at least in the non-processed portion, so the steel member according to this embodiment has the same characteristics as the steel sheet according to this embodiment in the non-processed portion. That is, at least the non-machined portion has, in mass%, C: 0.180% or more and 0.400% or less, Si: 0.050% or more and 1.000% or less, Mn: 2.00% or more and 4.00% or less, Al: 0.10% or more and 2.00% or less, Ti: 0.010% or more and 0.200% or less, B: 0.0010% or more and 0.0100% or less, N: 0.0010% or more and 0.0100% or less, P: 0% or more and 0.0400% or less, S: 0% or more and 0.0100% or less, O: 0% or more and 0.0060% or less, Cr: 0% or more and 0.50% or less, Ni: 0% or more and 1.00% or less, Cu: 0% or more and 1.00% or less, Mo: 0% or more, 0.500% or less, Nb: 0% or more, 0.200% or less, V: 0% or more, 0.500% or less, W: 0% or more, 0.100% or less, Ta: 0% or more, 0.100% or less, Sn: 0% or more, 0.050% or less, Co: 0% or more, 0.500% or less, Sb: 0% or more, 0.050% or less, As: 0% or more, 0.050% or less, Mg: 0% or more, 0.050% or less, Ca: 0% or more, 0.040% or less, REM: 0% or more, 0.050% or less, Bi: 0% or more, 0.050% or less, Zr: 0% or more, 0.050% or less, Sr The alloy has a chemical composition containing 0% or more and 0.050% or less of manganese, with the balance being Fe and impurities, and when the thickness in mm is t and the range from the surface in the thickness direction to the t / 8 position to the 3t / 8 position is defined as the t / 4 position, the microstructure at the t / 4 position contains, by volume fraction, 95% or more of martensite, and the volume fraction of tempered martensite is 95% or less, the prior austenite grain size is 7.0 μm or less, and the grain boundary segregation degree of Mn is 2.0 or less. In the steel member according to this embodiment, the non-processed portion is a flat portion whose thickness does not change from that of the steel plate used as a base material, while the processed portion is a portion whose thickness changes from that of the steel plate used as a base material or a portion having a constant curvature. The steel member according to this embodiment is obtained from a cold-rolled steel sheet that has high tensile strength, excellent bendability, and excellent LME resistance, and has properties equivalent to those of a cold-rolled steel sheet at least in the processed portion, and therefore has excellent collision resistance properties. The steel members are applied to, for example, bumper reinforcements, side sills, floor cloths, center pillars, and the like. The steel member may be joined to another member. When the steel member is joined by welding, the non-machined portion excludes the welded portion (nugget portion and heat-affected zone).

[0049] <Manufacturing method> Next, a preferred example of a method for manufacturing a steel sheet according to this embodiment will be described. According to this manufacturing method, the steel sheet according to this embodiment can be obtained. However, the manufacturing method described below does not limit the scope of the steel sheet according to this embodiment. A steel sheet that satisfies the above requirements is considered to be a steel sheet according to this embodiment, regardless of its manufacturing method.

[0050] Specifically, the steel sheet according to this embodiment is obtained by a manufacturing method including the following steps. (I) a hot rolling process in which a slab having a predetermined chemical composition is heated and hot-rolled to form a hot-rolled steel sheet; (II) a coiling step of cooling the hot-rolled steel sheet after the hot rolling step to a coiling temperature and coiling it at the coiling temperature; (III) a cold rolling step of cold rolling the hot-rolled steel sheet after the coiling step at a rolling reduction of 20% or more to obtain a cold-rolled steel sheet; (IV) a first annealing step in which the cold-rolled steel sheet is heated to a temperature of not less than the Ac3 point and not more than 900°C, held for 10 to 600 seconds, and then cooled to a temperature not more than the Bs point at an average cooling rate of not less than 20°C / second; (V) a temper rolling step of, as necessary, temper rolling the cold-rolled steel sheet after the first annealing step at a rolling reduction of 1.00% or less; (VI) A second annealing step in which the cold-rolled steel sheet after the first annealing step or the temper rolling step is heated to a temperature of not less than the Ac3 point and not more than 900°C, held for 10 to 600 seconds, then cooled at an average cooling rate of not less than 20°C / second to a temperature range of not less than the Ms point - 100°C and not more than the Bs point, held in that temperature range for 60 to 600 seconds, and then cooled to not more than 200°C at an average cooling rate of not less than 20°C / second.

[0051] The steel member according to this embodiment can be obtained by subjecting the cold-rolled steel sheet (the steel sheet according to this embodiment) obtained above to the following steps. (VII) A processing step in which the steel plate according to this embodiment is processed into a predetermined shape.

[0052] The Ac3 point can be calculated using the following formula: Ac3(℃)=910-203×[C] 1 / 2 +44.7×[Si]-30×[Mn]+700×[P]-20×[Cu]-15.2×[Ni]-11×[Cr]+31.5×[Mo]+400×[Ti]+104×[V]+120×[Al] The Ms point is the temperature at which martensite begins to form during cooling after quenching. In the manufacturing method according to this embodiment, the value calculated by the following formula is considered to be the Ms point. Ms(℃)=541-474×[C] / (1-Sα / 100)-15×[Si]-35×[Mn]-17×[Cr]-17×[Ni]+19×[Al] The Bs point is the temperature at which bainite transformation begins during cooling after quenching. In the manufacturing method according to this embodiment, the value calculated by the following formula is considered to be the Bs point. Bs(℃)=820-290×[C] / (1-Sα)-37×[Si]-90×[Mn]-65×[Cr]-50×[Ni]+70×[Al] Here, the [element symbols] included in the formulas for calculating the Ac3, Ms, and Bs points indicate the amount of each element (unit: mass%) contained in the steel sheet. Also, the symbol Sα included in the formulas is the ferrite fraction (unit: volume %) of the steel sheet at the time when heating for quenching is completed. However, it is difficult to determine the ferrite area fraction of a steel plate during manufacturing. Therefore, a steel plate that has undergone a temperature history similar to that of the actual steel plate manufacturing process is prepared in advance, and the ferrite area fraction at the center of the steel plate is determined. This ferrite area fraction is then used to calculate the Ms and Bs points. The ferrite fraction of a steel plate largely depends on the heating temperature used for quenching. Therefore, when considering cooling conditions, the manufacturing conditions of the process prior to cooling must first be determined, and a steel plate is manufactured under those manufacturing conditions. Then, the ferrite fraction of this steel plate is measured, thereby enabling Sα to be determined. Each step will be described below.

[0053] [Hot rolling process] In the hot rolling step, the slab is heated and hot rolled to obtain a hot-rolled steel sheet. The heating temperature is not limited, but is, for example, 1000 to 1350°C, taking into consideration the rolling load, scale loss, etc. The finish rolling end temperature is not limited either, but is preferably 850°C or higher. If the metal structure after the hot rolling process and the coiling process is an acicular structure, it is advantageous for further refining the prior γ grain size, but if the finish rolling end temperature is less than 850°C, ferrite and / or pearlite with a small aspect ratio is formed, making it difficult to increase the proportion of acicular structures (bainite or martensite) in the steel sheet. The method for producing the slab is not limited. It is sufficient to produce a steel billet having the above-mentioned chemical composition (the same chemical composition as that of the steel sheet according to the present embodiment) by melting, refining, and casting. For example, the slab can be produced by continuous casting or a thin slab caster.

[0054] [Winding process] In the coiling process, the hot-rolled steel sheet after the hot rolling process is cooled to a coiling temperature and then coiled. The cooling conditions and coiling temperature are not limited, but as described above, by setting the finish rolling temperature to 850°C or higher, cooling to 500°C or lower at an average cooling rate of 20°C / second or higher, and coiling in that temperature range, the structure of the steel sheet after the coiling step can be made into an acicular structure. This is more preferable in terms of refining the prior austenite grain size.

[0055] [Cold rolling process] In the cold rolling process, the hot-rolled steel sheet after the coiling process is cold-rolled at a rolling reduction (cumulative rolling reduction) of 20.0% or more, which allows dislocations to be introduced as diffusion paths for Mn, and reduces the degree of grain boundary segregation of Mn in the subsequent two annealing processes. If the cold rolling reduction is less than 20.0%, the degree of grain boundary segregation of Mn will be high in the finally obtained cold-rolled steel sheet. Although there is no upper limit to the rolling reduction, it may be set to 60.0% or less in terms of the cold rolling load. Before cold rolling, pickling may be carried out under known conditions, if necessary.

[0056] [First annealing process] In the first annealing step, the cold-rolled steel sheet is heated to a temperature of not less than the Ac3 point and not more than 900°C, held for 10 to 600 seconds, and then cooled to a temperature not more than the Bs point at an average cooling rate of not less than 20°C / second. This results in an acicular microstructure (martensite or bainite). This acicular structure allows for finer prior austenite grain size in the final cold-rolled steel sheet. Furthermore, the degree of grain boundary segregation of Mn is reduced by diffusion through dislocations introduced during the cold rolling process. If the heating temperature is lower than the Ac3 point or the holding time at the annealing temperature is shorter than 10 seconds, the gamma transformation is insufficient, and a desirable microstructure cannot be finally obtained. On the other hand, if the annealing temperature exceeds 900°C, the austenite grains become coarse. Furthermore, if the holding time at the annealing temperature exceeds 600 seconds, the austenite grains become coarse and productivity decreases. Furthermore, if the cooling stop temperature is higher than the Bs point, ferrite, pearlite, etc. are generated, and the microstructure does not become an acicular structure.

[0057] [Temper rolling process] The temper rolling step is not essential, but if it is performed, the cold-rolled steel sheet after the first annealing step is subjected to temper rolling at a reduction rate of 1.00% or less. Temper rolling is preferable because it is possible to introduce dislocations as diffusion paths for Mn, and in the subsequent annealing, it is possible to reduce the degree of grain boundary segregation of Mn. However, from the viewpoint of manufacturability, the rolling reduction may be set to 1.00% or less.

[0058] [Second annealing process] In the second annealing process, the cold-rolled steel sheet after the first annealing process or after the temper rolling process is heated to a temperature (annealing temperature) of not less than the Ac3 point and not more than 900°C, held for 10 to 600 seconds, then cooled at an average cooling rate of not less than 20°C / second to a temperature range of not less than the Ms point - 100°C and not more than the Bs point (°C), held in that temperature range for 60 to 600 seconds, and after holding, cooled to not more than 200°C at an average cooling rate of not less than 20°C / second. This process reverse-transforms the acicular structure into austenite single phase, and then transforms it into a martensite-based structure, thereby refining the prior austenite grain size. At the same time, the diffusion of Mn reduces the degree of Mn segregation at grain boundaries. If the annealing temperature is below the Ac3 point or the holding time in the temperature range of Ac3 to 900°C is less than 10 seconds, the transformation to austenite is insufficient and the desired microstructure cannot be obtained. On the other hand, if the annealing temperature exceeds 900° C., the austenite grains become coarse. If the holding time at the annealing temperature exceeds 600 seconds, the austenite grains become coarse and productivity decreases. Furthermore, if the average cooling rate after heating is less than 20°C / sec, or if the cooling stop temperature (subsequent holding temperature) is above the Bs point, ferrite, pearlite, etc. will be generated, and a microstructure mainly composed of martensite will not be obtained. If the cooling stop temperature is less than the Ms point - 100°C, the martensite that is finally obtained becomes brittle, resulting in poor bendability. Furthermore, if the subsequent holding time in the temperature range of Ms point -100°C to Bs point is less than 60 seconds, the martensite finally obtained will be brittle and have poor bendability.If the holding time exceeds 600 seconds, the volume fraction of martensite will be low. In this embodiment, the term "maintained" means that the steel sheet temperature is between Ms point -100°C and Bs point, and temperature changes are allowed as long as it is within this temperature range. When galvanizing is performed, the steel sheet may be immersed in a hot-dip galvanizing bath during this holding period. Alternatively, the hot-dip galvanized steel sheet may be subjected to an alloying treatment to produce a galvannealed hot-dip galvanized steel sheet. In this case, the temperature of the steel sheet can be held as described above by utilizing the heat applied to the steel sheet during hot-dip galvanizing and alloying. Known conditions can be applied to both methods. After holding, the untransformed austenite is transformed into martensite by cooling to 200°C or below at an average cooling rate of 20°C / second or more.

[0059] [Processing process] In the processing step, the cold-rolled steel sheet after the second annealing step is processed into a predetermined shape to obtain a steel member. The shape is not limited and may be determined depending on the part to which it is applied. Examples of shapes include a hat shape, a U-shape, and a rectangular cylindrical shape. [Example]

[0060] Example 1 Slabs having the chemical compositions shown in Tables 1A to 1C were produced by continuous casting. The slabs were heated to the heating temperatures shown in Tables 2A and 2B and hot-rolled to obtain hot-rolled steel sheets, with the finish rolling temperatures being the temperatures shown in Tables 2A and 2B. However, since cracks occurred in the slabs of Nos. 38 and 42, further tests were not carried out. The hot-rolled steel sheets after hot rolling were cooled to the coiling temperature at the average cooling rates shown in Tables 2A and 2B, coiled at the coiling temperature, and cooled to room temperature. Thereafter, the coiled hot-rolled steel sheets were uncoiled, pickled, and then cold-rolled at the cumulative reductions shown in Tables 2A and 2B to obtain cold-rolled steel sheets of 1.4 mm. Thereafter, the steel sheets were subjected to first annealing under the conditions in Tables 2A and 2B, in which the holding time at the heating temperature (annealing temperature) was set to 10 seconds or more and 600 seconds or less. Thereafter, temper rolling was carried out under the conditions shown in Tables 2A and 2B (however, temper rolling was not carried out in some cases (marked "-" in the table)). Thereafter, second annealing was performed under the conditions shown in Tables 3A and 3B. In addition, in some examples, the steel sheet was immersed in a hot-dip galvanizing bath during the second annealing to form a hot-dip galvanized layer. Furthermore, in some of the examples, an alloying treatment was performed to convert the hot-dip galvanized layer into a galvannealed layer. In the tables, CR indicates a cold-rolled steel sheet, GI indicates a galvannealed steel sheet having a hot-dip galvanized layer, and GA indicates a galvannealed steel sheet having a galvannealed layer.

[0061] The volume fraction of each phase, the prior austenite grain size, and the degree of grain boundary segregation of Mn were measured for the structure at the t / 4 position of the base steel sheet of the obtained steel sheets (cold-rolled steel sheet, hot-dip galvanized steel sheet, and alloyed hot-dip galvanized steel sheet) by the methods described above. The results are shown in Tables 4A and 4B.

[0062] The tensile strength (TS), bendability, and LME resistance of the obtained steel sheets were evaluated as follows, and the results are shown in Tables 4A and 4B.

[0063] Tensile Strength (TS) JIS No. 5 tensile test pieces were taken from the steel sheets in the direction perpendicular to the rolling direction, and the strength was determined by conducting a tensile test in accordance with JIS Z 2241:2011. If the tensile strength was 1310 MPa or more, it was determined that the desired strength was achieved.

[0064] [Bendability] A bending test was carried out in accordance with VDA standard VDA238-100 to determine the maximum bending angle. A specimen was judged to have excellent bendability when the product of the maximum bending angle and TS was 130,000 MPa·degree or more.

[0065] [LME resistance] The LME resistance of the steel sheets was evaluated by the following method. A commercially available galvannealed steel sheet was prepared as the mating material, and two of these sheets were spot welded together with the steel sheet obtained above, and the occurrence of LME in the steel sheet was evaluated by cross-sectional observation. 1A shows a mode of spot welding two steel sheets. In welding, the counterpart material (galvannealed steel sheet) was always used as steel sheet 1d, and the steel sheet to be evaluated was always used as steel sheet 1e, and the two sheets were overlapped and spot welded. Fig. 1B shows the current control when spot welding two steel sheets. The vertical axis I of the graph in Fig. 1B represents the current value, and the horizontal axis t represents time. Steel sheets 1d and 1e were overlapped and spot welded with a pair of electrodes 4a and 4b. The welding conditions were as follows: Electrodes 4a, 4b: DR type electrode made of Cr-Cu, tip outer diameter: 8mm, R: 40mm Pressure P: 450kgf Electrode inclination angle (angle between electrode center line 5 and vertical line 6) θ: 3° Upslope: None First energization time t1: 0.2 seconds No-power TC: 0.04 seconds Second energization time t2: 0.4 seconds Current ratio I1 / I2:0.7 Holding time after power off: 0.1 seconds

[0066] The LME was evaluated by polishing the cross section of the steel sheet, including the center of the nugget, and observing it with an SEM using the same method as for observing the microstructure. The presence or absence of cracks was evaluated at three locations: an internal crack 3a between the steel sheets, an external crack 3b at the contact point between the steel sheet and the spot welding electrode, and an external crack 3c in the steel sheet portion not in direct contact with the electrode, as shown in Figure 2. Figure 2 shows the observed locations in a joint 1 made by spot welding three overlapping steel sheets, but the observed locations are similar for a joint made of two sheets. If a crack longer than 0.2 mm was found in even one of the three locations, it was determined that LME cracks were present. If no LME cracks were present, the specimen was deemed to have excellent LME resistance.

[0067] Some of the steel sheets shown in the table are not plated. However, even if the steel sheet is not plated, the surface of the steel sheet on the 1e side is in contact with the zinc-plated surface of the steel sheet 1d. Therefore, even if the surface of the steel sheet on the 1e side is a cold-rolled steel sheet that is not zinc-plated, it is possible to evaluate the LME resistance.

[0068] [Table 1A]

[0069] [Table 1B]

[0070] [Table 1C]

[0071] [Table 2A]

[0072] [Table 2B]

[0073] [Table 3A]

[0074] [Table 3B]

[0075] [Table 4A]

[0076] [Table 4B]

[0077] As shown in Tables 1A to 4B, the cold-rolled steel sheets of the present invention have chemical compositions, microstructures, and grain boundary segregation degrees of Mn within the ranges of the present invention, and have high tensile strength, excellent bendability, and excellent LME resistance. In contrast, in the comparative examples, one or more of the chemical composition, microstructure, and degree of grain boundary segregation of Mn are outside the range of the invention, and one or more of the tensile strength, bendability, and LME resistance are inferior.

[0078] Example 2 A portion of the cold-rolled steel sheet obtained in Example 1 was processed into a hat-shaped member. A commercially available galvannealed steel sheet was used as a backing plate, and the backing plate was joined by spot welding at the flange portion to produce a steel member having a closed cross-section structure as shown in Fig. 3. The welding conditions were the same as those in Example 1.

[0079] The volume fraction of each phase, the prior austenite grain size, and the degree of grain boundary segregation of Mn were measured by the above-mentioned methods for the structure of the top plate, which was the unprocessed portion of the obtained steel member, at the t / 4 position in the center. Although not shown in the table, the results were all equivalent to those of the cold-rolled steel sheet.

[0080] Further, test pieces were taken from the top plate portion, which was the non-machined portion of the steel member, and the tensile strength and bendability were evaluated in the same manner as in Example 1. In addition, a cross section of the steel plate including the center of the nugget was polished from the welded portion, and the presence or absence of LME cracking was evaluated in the same manner as in Example 1. At this time, similarly to Example 1, the presence or absence of cracks was evaluated at three locations: inner crack 3a, outer crack 3b, and outer crack 3c in the steel plate portion not in direct contact with the electrode. If a crack exceeding 0.2 mm in length was found in any of the three locations, it was determined that LME cracks were present. The results are shown in Table 5.

[0081] [Table 5]

[0082] As shown in Table 5, the unprocessed area had properties equivalent to those of the cold-rolled steel sheet. [Industrial Applicability]

[0083] According to the present invention, it is possible to provide a cold-rolled steel sheet having high tensile strength, excellent bendability, and excellent LME resistance. Even after being made into a steel member, this cold-rolled steel sheet maintains high tensile strength, excellent bendability, and excellent LME resistance, at least in the unprocessed portion. Therefore, the present invention will greatly contribute to industrial development, such as by helping to solve global environmental problems through weight reduction of automobile bodies. [Explanation of symbols]

[0084] 1. Joint 1a steel plate 1b steel plate 1c steel plate 1d steel plate 1e steel plate 2 Nuggets 3a Internal crack between steel plates 3b External crack at the contact point between the steel plate and the spot welding electrode 3c External crack in steel plate part 4a electrode 4b electrode 5 Electrode center line 6 Vertical Lines P pressure θ Electrode tilt angle t1 First energization time t2 Second energization time tc non-energized period

Claims

1. In mass%, C: 0.180% or more, 0.400% or less, Si: 0.050% or more, 1.000% or less, Mn: 2.00% or more, 4.00% or less, Al: 0.10% or more, 2.00% or less, Ti: 0.010% or more, 0.200% or less, B: 0.0010% or more, 0.0100% or less, N: 0.0010% or more, 0.0100% or less, P: 0% or more, 0.0400% or less, S: 0% or more, 0.0100% or less, O: 0% or more, 0.0060% or less, Cr: 0% or more, 0.50% or less, Ni: 0% or more, 1.00% or less, Cu: 0% or more, 1.00% or less, Mo: 0% or more, 0.500% or less, Nb: 0% or more, 0.200% or less, V: 0% or more, 0.500% or less, W: 0% or more, 0.100% or less, Ta: 0% or more, 0.100% or less, Sn: 0% or more, 0.050% or less, Co: 0% or more, 0.500% or less, Sb: 0% or more, 0.050% or less, As: 0% or more, 0.050% or less, Mg: 0% or more, 0.050% or less, Ca: 0% or more, 0.040% or less, REM: 0% or more, 0.050% or less, Bi: 0% or more, 0.050% or less, Zr: 0% or more, 0.050% or less, Sr: 0% or more, 0.050% or less, Including, The balance has a chemical composition consisting of Fe and impurities, When the plate thickness in mm is t and the range from the surface to the plate thickness direction from the t / 8 position to the 3t / 8 position is t / 4 position, At the t / 4 position, The microstructure contains, in volume fraction, martensite: 95% or more, and the volume fraction of tempered martensite is 95% or less; The prior austenite grain size is 7.0 μm or less, The grain boundary segregation degree of Mn is 2.0 or less, The degree of grain boundary segregation of Mn is a value obtained by dividing the Mn concentration of the prior austenite grain boundary by the average Mn content of the steel sheet measured by ICP-AES, where the grain boundary appearing on the cleavage plane at the t / 4 position is regarded as a prior austenite grain boundary, and the average value of Mn concentrations obtained by measuring five different grain boundaries among the prior austenite grain boundaries by Auger electron spectroscopy is defined as the Mn concentration of the prior austenite grain boundary. Cold rolled steel plate.

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

3. The hot-dip galvanized layer is a galvannealed layer. The cold rolled steel sheet according to claim 2.

4. A member including a processed portion and a non-processed portion, At least the non-processed portion is In mass%, C: 0.180% or more, 0.400% or less, Si: 0.050% or more, 1.000% or less, Mn: 2.00% or more, 4.00% or less, Al: 0.10% or more, 2.00% or less, Ti: 0.010% or more, 0.200% or less, B: 0.0010% or more, 0.0100% or less, N: 0.0010% or more, 0.0100% or less, P: 0% or more, 0.0400% or less, S: 0% or more, 0.0100% or less, O: 0% or more, 0.0060% or less, Cr: 0% or more, 0.50% or less, Ni: 0% or more, 1.00% or less, Cu: 0% or more, 1.00% or less, Mo: 0% or more, 0.500% or less, Nb: 0% or more, 0.200% or less, V: 0% or more, 0.500% or less, W: 0% or more, 0.100% or less, Ta: 0% or more, 0.100% or less, Sn: 0% or more, 0.050% or less, Co: 0% or more, 0.500% or less, Sb: 0% or more, 0.050% or less, As: 0% or more, 0.050% or less, Mg: 0% or more, 0.050% or less, Ca: 0% or more, 0.040% or less, REM: 0% or more, 0.050% or less, Bi: 0% or more, 0.050% or less, Zr: 0% or more, 0.050% or less, Sr: 0% or more, 0.050% or less, Including, The balance has a chemical composition consisting of Fe and impurities, When the thickness in mm is t and the range from the surface in the thickness direction from the t / 8 position to the 3t / 8 position is defined as the t / 4 position, At the t / 4 position, The microstructure contains, in volume fraction, martensite: 95% or more, and the volume fraction of tempered martensite is 95% or less; The prior austenite grain size is 7.0 μm or less, The grain boundary segregation degree of Mn is 2.0 or less, The degree of grain boundary segregation of Mn is a value obtained by dividing the Mn concentration of the prior austenite grain boundary by the average Mn content of the steel sheet measured by ICP-AES, where the grain boundary appearing on the cleavage plane at the t / 4 position is regarded as a prior austenite grain boundary, and the average value of Mn concentrations obtained by measuring five different grain boundaries among the prior austenite grain boundaries by Auger electron spectroscopy is defined as the Mn concentration of the prior austenite grain boundary. steel parts.

5. The surface has a hot-dip galvanized layer. The steel member according to claim 4.

6. The hot-dip galvanized layer is a galvannealed layer. The steel member according to claim 5.

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