Steel sheets, plated steel sheets and automotive parts

Abstract

The present invention addresses the problem of providing a steel sheet and a plated steel sheet having high LME resistance. The steel sheet according to the present invention has a tensile strength of 780 MPa or more, includes a predetermined chemical component, and has a surface roughness of 3.0 μm or less in Ra, wherein the depth having a C concentration of 0.02% or less as measured by GDS in a thickness direction from the surface of the steel sheet is 3 μm or more, and the thickness of a layer having an area ratio of ferrite of 90% or more in the sheet thickness direction from the surface of the steel sheet is 3 μm or more, the steel sheet satisfying 0.45 ≤ I(110) / (I(110) + I(200) + I(211)) ≤ 0.90, where I(110) is the diffraction intensity corresponding to the (110) plane, I(200) is the diffraction intensity corresponding to the (200) plane, and I(211) is the diffraction intensity corresponding to the (211) plane, in oblique incident X-ray diffraction at an incident angle of 1° with respect to the surface of the steel sheet.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet and a plated steel sheet. More specifically, the present invention relates to a steel sheet and a plated steel sheet having high LME resistance. [Background technology]

[0002] In recent years, efforts have been made to increase the strength of steel sheets used in various fields such as automobiles, home appliances, building materials, etc. For example, in the automobile field, the use of high-strength steel sheets has been increasing with the aim of reducing the weight of vehicle bodies to improve fuel efficiency.

[0003] When welding zinc-based plated steel sheets, particularly high-strength steel sheets, as described in Patent Document 1, for example, weldability may be reduced due to liquid metal embrittlement (LME) cracking.

[0004] Patent Document 2 describes a steel sheet that suppresses LME cracking and improves weldability, and describes a steel sheet that has Si oxide particles with a particle size of 20 nm or more in the surface layer at a density of 3000 to 6000 particles / mm 2 The patent discloses a steel sheet having a suitable grain size distribution at a number density of 1000.degree. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2019 / 116531 [Patent Document 2] International Publication No. 2020 / 218575 Summary of the Invention [Problem to be solved by the invention]

[0006] In order to suppress the occurrence of LME cracking, for example, it is effective to prevent Zn and other elements contained in the coating layer from penetrating into the steel sheet whose metal structure has been transformed into austenite during welding. In this respect, there is room for improvement.

[0007] In view of the above circumstances, an object of the present invention is to provide a steel sheet and a plated steel sheet having high LME resistance. [Means for solving the problem]

[0008] The present inventors have intensively investigated means for solving the above problems, and as a result, have found that by imparting strain to a steel sheet before annealing using a shot material under appropriate conditions to create an appropriate surface condition, and then performing high dew-point annealing, the surface layer of the steel sheet is decarburized and a layer with a high ferrite fraction in which ferrite is randomly oriented is formed, thereby making it possible to suppress LME.

[0009] The present invention was made based on the above findings and through further investigation, and the gist of the present invention is as follows.

[0010] (1) A steel plate having a tensile strength of 780 MPa or more, the chemical composition of which is, in mass%, C: 0.05 to 0.40%, Si: 0.5 to 3.0%, Mn: 0.1 to 5.0%, sol. Al: 0 to 3.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0 to 0.0100%, Ti: 0 to 0.150%. 0%, Nb: 0~0.1500%, V: 0~0.150%, Cr: 0~2.00%, Ni: 0~2.00%, Cu: 0~2.0000%, Mo: 0~1.00%, W:0~1.000%, Ca:0~0.1000%, Mg:0~0.100%, Zr:0~0.100%, Hf:0~0.100%, REM:0~0.1000% a surface roughness Ra of the steel sheet of 3.0 μm or less, a depth of 3 μm or more from the steel sheet surface in the thickness direction at which the C concentration measured by GDS is 0.02% or less, and a thickness of a layer of 90% or more from the steel sheet surface in the thickness direction, the thickness of which is 3 μm or more, and a layer of 90% or more ferrite area ratio in the thickness direction from the steel sheet surface in oblique incidence X-ray diffraction with an incident angle of 1° to the steel sheet surface satisfies 0.45≦I(110) / (I(110)+I(200)+I(211))≦0.90, where I(110) is the diffraction intensity corresponding to the (110) plane, I(200) is the diffraction intensity corresponding to the (200) plane, and I(211) is the diffraction intensity corresponding to the (211) plane.

[0011] (2) The steel sheet according to (1) above, characterized in that the depth at which the C concentration is 0.02% or less is 10 μm or more in the thickness direction from the surface of the steel sheet.

[0012] (3) The steel plate according to (1) or (2) above, characterized in that 0.45≦I(110) / (I(110)+I(200)+I(211))≦0.75 is satisfied.

[0013] (4) The steel sheet according to any one of (1) to (3), characterized in that the thickness of a layer having an area ratio of ferrite of 90% or more in the thickness direction from the surface of the steel sheet is 8 μm or more.

[0014] (5) The steel sheet according to any one of (1) to (4), characterized in that the surface roughness Ra of the steel sheet is 2.0 μm or less.

[0015] (6) A plated steel sheet comprising any one of the steel sheets (1) to (5) above, and a Zn-containing plating layer on at least a portion of the surface thereof.

[0016] (7) An automobile member comprising the steel sheet according to any one of (1) to (5) above or the plated steel sheet according to (6) above. [Effects of the Invention]

[0017] According to the present invention, a steel sheet and a plated steel sheet having high LME resistance can be obtained. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 shows an example of the results of grazing incidence X-ray diffraction measurements when the ferrite phase is randomly oriented and when it is not. [Figure 2] FIG. 1 is a diagram illustrating the positions of cracks targeted in the LME resistance evaluation in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention will be described below. The present invention is not limited to the following embodiments. First, an outline of the improvement of LME resistance in the present invention will be described.

[0020] LME cracking occurs when, for example, the metallographic structure of a steel sheet is heated and transformed into austenite during spot welding, and molten zinc produced by melting the coating penetrates the austenite grain boundaries in the surface layer of the steel sheet. The molten zinc that penetrates the austenite grain boundaries embrittles the steel sheet, and the tensile stress applied to the steel sheet during welding is thought to cause LME cracking. The inventors came up with the idea of ​​utilizing the metallographic structure of the surface layer of a steel sheet as a method for improving LME resistance. Specifically, the surface layer of a steel sheet is designed to have a metallographic structure mainly composed of a ferrite phase with a low carbon concentration and low LME susceptibility, and the ferrite phase is randomly oriented to suppress the occurrence of LME. Herein, "LME resistance" refers to the property of a steel sheet that suppresses LME cracking, and "LME susceptibility" refers to the property of a steel sheet that is susceptible to LME cracking.

[0021] The random orientation of the ferrite phase means that the characteristics of the ferrite grain boundaries are averaged as a whole. In other words, it means that the crystal orientation of each ferrite particle in the ferrite phase is randomly oriented. The random orientation of the ferrite particles prevents grain boundaries oriented in a specific direction from being unevenly distributed and from being connected continuously or intermittently. LME cracking is thought to occur when Zn from the plating invades grain boundaries with locally low grain boundary energy. In other words, if there are continuous grain boundaries with locally low grain boundary energy, Zn from the plating will concentrate there, making LME cracking more likely to occur. By averaging the characteristics of the grain boundaries as a whole, grain boundaries with locally low grain boundary energy will no longer be connected continuously, preventing localized concentration of Zn from the plating, and as a result, it is thought that LME resistance will be improved.

[0022] In the present invention, the random orientation of the ferrite phase is expressed by the following conditional formula: That is, a steel sheet satisfying the following conditional formula means that the ferrite phase is randomly oriented.

[0023] (conditional expression) In the case of oblique incidence X-ray diffraction with an incident angle of 1° on the steel sheet surface, the diffraction intensity corresponding to the (110) plane is defined as I(110), the diffraction intensity corresponding to the (200) plane is defined as I(200), and the diffraction intensity corresponding to the (211) plane is defined as I(211). 0.45≦I(110) / (I(110)+I(200)+I(211))≦0.90

[0024] In order to obtain the steel sheet having the above-described configuration, in the present invention, when manufacturing the steel sheet, strain is imparted to the steel sheet after cold rolling, and then the steel sheet is annealed at a high dew point. This promotes decarburization and facilitates the formation of a ferrite phase on the steel sheet surface. Furthermore, the present invention was made based on the finding that the orientation of the ferrite phase can be randomized by controlling the temperature at which humidification is started. The present invention will be described in detail below.

[0025] [Tensile strength] The steel plate according to the present invention has a tensile strength of 780 MPa or more, i.e., it is a high-strength steel plate. The present invention suppresses LME that occurs in high-strength steel plates. Specifically, the steel plate according to the present invention has a tensile strength of 780 MPa or more. The upper limit of the tensile strength is not particularly limited, but may be, for example, 2000 MPa or less from the viewpoint of ensuring toughness. The tensile strength is measured in accordance with JIS Z 2241:2011 using a JIS No. 5 tensile test piece with the longitudinal direction perpendicular to the rolling direction and the plate thickness direction. The tensile strength may be 980 MPa or more, 1180 MPa or more. When the rolling direction of the steel plate cannot be identified, a JIS No. 5 test piece may be taken with the longitudinal direction aligned in any direction on the surface of the steel plate.

[0026] [Chemical composition] The chemical composition of the steel sheet will be described below. The "%" in relation to the chemical composition means "% by mass." Furthermore, in the numerical range of the chemical composition, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0027] (C: 0.05 to 0.40%) C (carbon) is an element that ensures the strength of steel. To obtain a tensile strength of 780 MPa or more, which is the target of the present invention, the C content is set to 0.05% or more. To prevent the C concentration in the surface layer (described later) from becoming too high, and taking weldability into consideration, the C content is set to 0.40% or less. The C content may be 0.08% or more, 0.10% or more, or 0.15% or more. The C content may be 0.37% or less, 0.35% or less, or 0.30% or less.

[0028] (Si: 0.5 to 3.0%) Silicon (Si) is an element that promotes ferrite stabilization and decarburization. The inclusion of Si promotes decarburization in the surface layer through the pretreatment and heat treatment described below, and stabilizes the ferrite in the surface layer, thereby improving LME resistance. To achieve this effect, the Si content is set to 0.5% or more. If the Si content is too high, even with high-dew-point annealing, external oxidation progresses, forming oxides (scale) in the surface layer of the steel sheet. Conversely, decarburization at the outermost surface is suppressed, reducing the effect of improving LME resistance. Taking this into consideration, the Si content is set to 3.0% or less. The Si content may be 0.6% or more, 0.7% or more, or 0.8% or more. The Si content may be 2.5% or less, 2.0% or less, or 1.5% or less.

[0029] (Mn: 0.1 to 5.0%) Manganese (Mn) is an element that is effective in improving the strength of steel by obtaining a hard structure. In consideration of the strength of the steel, the Mn content is set to 0.1% or more. In addition, in consideration of the deterioration of workability due to Mn segregation, the Mn content is set to 5.0% or less. The Mn content may be 0.5% or more, 1.0% or more, or 1.5% or more. The Mn content may be 4.5% or less, 4.0% or less, or 3.5% or less.

[0030] (sol.Al:0-3.0%) Aluminum (Al) is an element that, like Si, promotes ferrite stabilization and decarburization by dissolving in steel. Sol-Al refers to acid-soluble Al that is soluble in acid and does not form oxides such as Al2O3. The amount of Al measured is determined by subtracting the insoluble residue on the filter paper generated during the Al analysis process. In the steel sheet of the present invention, the role of sol-Al can also be achieved by adding Si, so sol-Al is not essential, and the lower limit of the sol-Al content is 0%. If the sol-Al content is too high, even with high-dew-point annealing, external oxidation progresses, forming oxides (scale) on the surface of the steel sheet. This suppresses decarburization on the outermost surface, reducing the effectiveness of improving LME resistance. Taking this into consideration, the sol-Al content is set to 3.0% or less. The sol-Al content may be 0.1% or more, 0.3% or more, or 0.5% or more. The sol-Al content may be 2.0% or less, 1.5% or less, or 1.0% or less.

[0031] As described above, since excessive addition of Si and sol.Al reduces LME resistance, the total content of Si and sol.Al is preferably 1.8% or less, and may be 1.7%, 1.6%, or 1.5% or less.

[0032] (P:0.0300% or less) P (phosphorus) is an impurity generally contained in steel. If the P content exceeds 0.0300%, weldability may be reduced. Therefore, the P content is set to 0.0300% or less. The P content may be 0.0200% or less, 0.0100% or less, or 0.0050% or less. It is preferable that no P is contained, and the lower limit of the P content is 0%. From the viewpoint of dephosphorization costs, the P content may be more than 0%, 0.0001% or more, or 0.0005% or more.

[0033] (S:0.0300% or less) S (sulfur) is an impurity generally contained in steel. If the S content exceeds 0.0300%, weldability will decrease, and further, the amount of MnS precipitation will increase, which may decrease workability such as bendability. Therefore, the S content is set to 0.0300% or less. The S content may be 0.0100% or less, 0.0050% or less, or 0.0020% or less. It is preferable that no S is contained, and the lower limit of the S content is 0%. From the viewpoint of desulfurization costs, the S content may be more than 0%, 0.0001% or more, or 0.0005% or more.

[0034] (N:0.0100% or less) N (nitrogen) is an impurity generally contained in steel. If the N content exceeds 0.0100%, weldability may be reduced. Therefore, the N content is set to 0.0100% or less. The N content may be 0.0080% or less, 0.0050% or less, or 0.0030% or less. It is preferable that N is not contained, and the lower limit of the N content is 0%. From the viewpoint of manufacturing costs, the N content may be more than 0%, 0.0005% or more, or 0.0010% or more.

[0035] (O:0.0030% or less) O (oxygen) is an element that forms oxides and reduces the workability of steel sheets. If the O content is too high, excessive oxides are generated, and the workability of steel sheets is likely to decrease. Therefore, the O content is set to 0.0030% or less. The O content may be 0.0026% or less, 0.0024% or less, 0.0020% or less, or 0.0018% or less. It is preferable that O is not contained, and the lower limit of the O content is 0%. From the viewpoint of production costs, the O content may be more than 0%, 0.0005% or more, or 0.0010% or more.

[0036] (B: 0 to 0.0100%) Boron (B) is an element that improves hardenability, contributes to improving strength, and segregates at grain boundaries to strengthen the grain boundaries and improve toughness, so it may be contained as needed. Since it is not an essential element, the lower limit of the B content is 0%. This effect can be obtained even with a small amount of B, but when B is contained, the B content is preferably 0.0001% or more. Furthermore, from the viewpoint of ensuring sufficient toughness, the B content is set to 0.0100% or less. The B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more. The B content may be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less.

[0037] (Ti: 0 to 0.1500%) Titanium (Ti) precipitates as TiC during cooling of the steel, contributing to improved strength, and may be included as needed. Since it is not an essential element, the lower limit of the Ti content is 0%. While this effect can be achieved even with trace amounts, the Ti content, if included, is preferably 0.0001% or more. The Ti content may be 0.0003% or more, or 0.0005% or more. However, excessive Ti content may result in the formation of coarse TiN, which may impair toughness, so the Ti content is set to 0.1500% or less. The Ti content may be 0.1000% or less, 0.0500% or less, 0.0050% or less, or 0.0020% or less.

[0038] (Nb: 0 to 0.1500%) Niobium (Nb) is an element that contributes to improving strength by improving hardenability, and may be contained as needed. Since it is not an essential element, the lower limit of the Nb content is 0%. This effect can be obtained even with a small amount of Nb contained, but when Nb is contained, the Nb content is preferably 0.0001% or more. The Nb content may be 0.0005% or more, or 0.0010% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the Nb content is set to 0.1500% or less. The Nb content may be 0.1000% or less, 0.0600% or less, or 0.0200% or less.

[0039] (V:0~0.150%) V (vanadium) is an element that contributes to improving strength by improving hardenability, so it may be contained as needed. Since it is not an essential element, the lower limit of the V content is 0%. This effect can be obtained even with a small amount of V, but when V is contained, the V content is preferably 0.001% or more. The V content may be 0.003% or more, 0.005% or more, or 0.008% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the V content is set to 0.150% or less. The V content may be 0.100% or less, 0.060% or less, or 0.020% or less.

[0040] (Cr: 0 to 2.00%) Cr (chromium) is effective in improving the hardenability and strength of steel, so it may be contained as needed. Since Cr is not an essential element, the lower limit of the Cr content is 0%. This effect can be obtained even with a small amount of Cr, but if Cr is contained, the Cr content is preferably 0.001% or more. The Cr content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive Cr content may form a large amount of Cr carbide, which may actually impair hardenability, so the Cr content is set to 2.00% or less. The Cr content may be 1.80% or less, 1.50% or less, 0.50% or less, or 0.20% or less.

[0041] (Ni: 0 to 2.00%) Ni (nickel) is effective in improving the hardenability of steel and increasing its strength, so it may be contained as needed. Since it is not an essential element, the lower limit of the Ni content is 0%. This effect can be obtained even with a small amount of Ni, but if Ni is contained, the Ni content is preferably 0.001% or more. The Ni content may be 0.01% or more, 0.02% or more, or 0.05% or more. However, because excessive addition of Ni increases costs, the Ni content is set to 2.00% or less. The Ni content may be 1.80% or less, 1.50% or less, 0.50% or less, or 0.20% or less.

[0042] (Cu: 0 to 2.0000%) Cu (copper) is effective in improving the hardenability of steel and increasing its strength, so it may be contained as needed. Since it is not an essential element, the lower limit of the Cu content is 0%. This effect can be obtained even with a small amount of Cu, but when Cu is contained, the Cu content is preferably 0.0001% or more. The Cu content may be 0.0002% or more, or 0.0005% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness and cracking of the slab after casting, the Cu content is set to 2.0000% or less. The Cu content may be 1.8000% or less, 1.5000% or less, 0.0050% or less, or 0.0020% or less.

[0043] (Mo: 0-1.00%) Mo (molybdenum) is effective in improving the hardenability of steel and increasing its strength, so it may be contained as needed. Since it is not an essential element, the lower limit of the Mo content is 0%. This effect can be obtained even with a small amount of Mo, but when Mo is contained, the Mo content is preferably 0.001% or more. The Mo content may be 0.01% or more, 0.02% or more, or 0.03% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the Mo content is set to 1.00% or less. The Mo content may be 0.80% or less, 0.60% or less, or 0.20% or less.

[0044] (W:0~1.000%) W (tungsten) is effective in improving the hardenability of steel and increasing its strength, so it may be contained as needed. Since it is not an essential element, the lower limit of the W content is 0%. This effect can be obtained even with a small amount of W, but when W is contained, the W content is preferably 0.001% or more. The W content may be 0.002% or more, or 0.003% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the W content is set to 1.000% or less. The W content may be 0.800% or less, 0.600% or less, 0.300% or less, 0.100% or less, or 0.020% or less.

[0045] (Ca: 0 to 0.1000%) Ca (calcium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, so it may be contained as needed. Since it is not an essential element, the lower limit of the Ca content is 0%. This effect can be obtained even with a small amount of Ca content, but if Ca is contained, the Ca content is preferably 0.0001% or more. The Ca content may be 0.0002% or more, or 0.0003% or more. On the other hand, since excessive Ca content may cause noticeable deterioration in surface properties, the Ca content is set to 0.1000% or less. The Ca content may be 0.0800% or less, 0.0500% or less, 0.0300% or less, 0.0100% or less, or 0.0010% or less.

[0046] (Mg: 0 to 0.100%) Magnesium (Mg) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, so it may be contained as needed. Since it is not an essential element, the lower limit of the Mg content is 0%. This effect can be obtained even with a small amount of Mg, but when Mg is contained, the Mg content is preferably 0.0001% or more. The Mg content may be 0.0005% or more, or 0.0008% or more. On the other hand, excessive Mg content may cause noticeable deterioration in surface properties, so the Mg content is set to 0.100% or less. The Mg content may be 0.090% or less, 0.080% or less, 0.030% or less, 0.010% or less, or 0.002% or less.

[0047] (Zr: 0 to 0.100%) Zr (zirconium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, so it may be contained as needed. Since it is not an essential element, the lower limit of the Zr content is 0%. This effect can be obtained even with a small amount of Zr, but if Zr is contained, the Zr content is preferably 0.001% or more. The Zr content may be 0.005% or more, or 0.010% or more. On the other hand, if Zr is contained in excess, deterioration of surface properties may become apparent, so the Zr content is set to 0.100% or less. The Zr content may be 0.050% or less, or 0.030% or less.

[0048] (Hf: 0 to 0.100%) Hf (hafnium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, so it may be contained as needed. Since it is not an essential element, the lower limit of the Hf content is 0%. This effect can be obtained even with a small amount of Hf, but if contained, the Hf content is preferably 0.0001% or more. The Hf content may be 0.0003% or more, or 0.0005% or more. On the other hand, excessive Hf content may cause noticeable deterioration of surface properties, so the Hf content is set to 0.100% or less. The Hf content may be 0.050% or less, 0.030% or less, 0.010% or less, 0.005% or less, or 0.002% or less.

[0049] (REM: 0 to 0.1000%) REM (rare earth elements) contribute to inclusion control, particularly to the fine dispersion of inclusions, and thus enhance toughness. Therefore, they may be added as needed. Since they are not essential elements, the lower limit of REM content is 0%. While this effect can be achieved even with trace amounts, the REM content, if present, is preferably 0.0001% or more. The REM content may be 0.0003% or more, or 0.0005% or more. However, excessive REM content can significantly deteriorate surface properties, so the REM content is set to 0.1000% or less. The REM content may be 0.0500% or less, 0.0300% or less, 0.0100% or less, 0.0050% or less, or 0.0020% or less. REM stands for rare earth metal and refers to elements belonging to the lanthanide series. REM is typically added as misch metal.

[0050] In the steel sheet according to the present invention, the balance other than the above chemical components consists of Fe and impurities. In the steel sheet according to the present invention, the balance may be Fe and impurities, i.e., the balance may consist only of Fe and impurities. Here, impurities refer to components that are mixed in during industrial production of steel sheet due to various factors in the manufacturing process, including raw materials such as ore and scrap, and that do not adversely affect the LME resistance of the steel sheet according to the present invention, i.e., are contained in a range that allows the steel sheet according to the present invention to obtain the LME resistance required.

[0051] The chemical composition of the steel sheet may be analyzed by an elemental analysis method known to those skilled in the art, for example, inductively coupled plasma mass spectrometry (ICP-MS). However, C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method. O may be measured using an inert gas fusion-infrared absorption method. These analyses may be performed on samples collected from the steel sheet according to JIS G0417:1999.

[0052] Next, the surface layer portion of the steel sheet will be described. Here, the surface layer portion of the steel sheet refers to a layered region having a depth of a predetermined distance from the surface of the steel sheet (sheet surface of the steel sheet) in the sheet thickness direction of the steel sheet. In the case of a plated steel sheet, the surface of the steel sheet (sheet surface of the steel sheet) refers to the surface of the steel sheet (sheet surface) excluding the plating. Note that the predetermined distance in the sheet thickness direction can be the longer (deeper) length (depth) of the "depth at which the C concentration of the surface layer is 0.02% or less" or the "depth at which the area ratio of the ferrite phase is 90% or more," which will be described later.

[0053] [Surface roughness Ra] The steel sheet of the present invention has a surface roughness Ra of 3.0 μm or less. The surface roughness Ra is the arithmetic mean roughness Ra defined in JIS B0601:2013. In the case of a plated steel sheet, the surface roughness Ra is the surface roughness of the interface between the steel sheet and the plated layer, excluding the plating.

[0054] In the present invention, when measuring the surface roughness Ra, 10 measurement points are randomly selected on the surface of the steel sheet in accordance with JIS B 0601:2013 so that the distance between each measurement point is 1 mm or more. At each measurement point, the surface profile is measured using a laser microscope (for example, Keyence Corporation's "VK-X3000"). Specifically, an image is taken at a magnification of 20 times using the laser microscope, and the arithmetic mean roughness (Ra) at each measurement point is determined using a reference length of 2000 μm in the captured image. The arithmetic mean value of the arithmetic mean roughness (Ra) at each measurement point is defined as the "surface roughness Ra."

[0055] [Depth where the surface carbon concentration is 0.02% or less] In the steel sheet of the present invention, the depth from the surface of the steel sheet in the sheet thickness direction at which the C concentration measured by GDS (glow discharge spectroscopy) is 0.02% or less is 3 μm or more.

[0056] Since LME susceptibility decreases as the C concentration decreases, LME resistance can be improved by lowering the C concentration in the surface layer. Also, since C is an austenite stabilizing element, having a low C content stabilizes the layer with low LME susceptibility.

[0057] Such a surface layer structure (metal structure in the surface layer portion of the steel sheet) can be obtained as a decarburized layer produced by setting the chemical composition of the steel sheet as described above and performing the pretreatment process and annealing process described below.

[0058] Since a depth of 3 μm or more where the C concentration is 0.02% or less contributes to improving LME resistance, there is no particular upper limit to the depth where the C concentration is 0.02% or less. The depth where the C concentration is 0.02% or less may be, for example, 50 μm or less, 40 μm or less, or 30 μm or less. The depth where the C concentration is 0.02% or less is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 10 μm or more, 15 μm or more, or 20 μm or more.

[0059] The GDS measurement is performed at five measurement points in the sheet thickness direction, and the arithmetic mean value of the depth of the region where the C concentration is 0.02% or less at each measurement point is defined as the depth where the C concentration in the surface layer is 0.02% or less. The five measurement points are randomly selected so that there is an interval of 5 mm or more between each measurement point on the steel sheet surface. The measurement conditions are as follows. Naturally, measurement results can be obtained even if the following conditions, for example the measurement device, are not followed, but if differences arise in the measurement results, the steel sheet according to the present invention is identified by measurement results according to the following conditions.

[0060] Equipment: High-frequency glow discharge optical emission spectrometer (LECO Japan, model number GDS850A) Ar gas pressure: 0.3MPa Anode diameter: 4mmφ RF output: 30W Measurement time: 200 to 1500 seconds

[0061] [Thickness of high ferrite layer (layer with ferrite area ratio of 90% or more)] In the steel sheet of the present invention, the thickness of a layer in which the area ratio of the ferrite phase is 90% or more (hereinafter referred to as "high ferrite layer") in the thickness direction from the surface of the steel sheet is 3 μm or more.

[0062] A thickness of 3 μm or more of the high ferrite layer contributes to improving LME resistance, so there is no particular upper limit to the thickness. The thickness of the high ferrite layer may be, for example, 100 μm or less, 80 μm or less, 60 μm or less, or 40 μm or less. The thickness of the high ferrite layer is preferably 5 μm or more, more preferably 8 μm or more, and even more preferably 10 μm or more, or 20 μm or more.

[0063] The structure other than ferrite in the high ferrite layer is not limited, and may be, for example, one or more of martensite, bainite, and cementite.

[0064] The thickness of the high ferrite layer is measured by analyzing secondary electron images of the observation cross section, which is mechanically polished to a mirror finish and then etched with nital, using SEM observation. A field-emission scanning electron microscope (e.g., JEOL "JSM 7000F," accelerating voltage: 15 kV) is used for SEM observation. The observation field of view of the observation cross section is a depth range of 500 μm from the surface of the steel sheet (sheet surface) in the sheet thickness direction (longitudinal direction of the observation cross section) and a width range of 600 μm in the direction perpendicular to the sheet thickness direction (lateral direction of the observation cross section). Five observation fields are observed in the direction perpendicular to the sheet thickness direction (lateral direction of the observation cross section) with an interval of 1000 μm or more between each observation field of view, and secondary electron images are obtained. The observation resolution is 1280 × 960 pixels. In the case of plated steel sheets, the surface of the steel sheet (sheet surface) refers to the surface of the steel sheet excluding the plating.

[0065] The ferrite fraction is calculated for the five obtained secondary electron images using the point counting method. More specifically, a grid with equal spacing is first drawn on the secondary electron image. Next, the number of grid points at each grid point where the structure is ferrite is determined and divided by the total number of grid points to measure the ferrite fraction. The greater the total number of grid points, the more accurately the area fraction can be determined. In this embodiment, the grid spacing is 2 μm × 2 μm, and the total number of grid points is 1,500.

[0066] In a secondary electron image, a region with relatively low brightness and no visible substructure can be determined to be ferrite. Here, substructure refers to a transformed structure formed inside a prior austenite phase, such as lath or block. In a secondary electron image, ferrite is observed as a region with relatively low brightness and a relatively monotonous spread of brightness and color tone. In the present invention, there is no particular need to distinguish metal structures other than ferrite. However, the following criteria are used to distinguish tempered martensite, pearlite, ferrite, fresh martensite, retained austenite, or bainite in secondary electron images. A region with intragranular substructure (lath boundary, block boundary) and where carbides are precipitated with multiple variants is determined to be tempered martensite. Furthermore, a region where cementite is precipitated in a lamellar form is determined to be pearlite. A region with high brightness and where the substructure is not revealed by etching is determined to be fresh martensite or retained austenite. A region that does not fall into any of the above categories is determined to be bainite. Simply put, the area ratio of the ferrite phase can be determined by distinguishing between ferrite and other structures.

[0067] Ferrite has low LME susceptibility. From the viewpoint of improving LME resistance, it is preferable that the surface layer structure of the steel sheet is mainly composed of ferrite. Such a surface layer structure can be obtained by setting the chemical composition of the steel sheet as described above and performing the pretreatment process and annealing process described below.

[0068] [Diffraction intensity ratio of ferrite phase by grazing incidence X-ray diffraction (XRD)] In the steel sheet of the present invention, when an oblique incidence X-ray diffraction analysis is performed at an incident angle of 1° with respect to the steel sheet surface, the diffraction intensity corresponding to the (110) plane is defined as I(110), the diffraction intensity corresponding to the (200) plane is defined as I(200), and the diffraction intensity corresponding to the (211) plane is defined as I(211). 0.45≦I(110) / (I(110)+I(200)+I(211))≦0.90 The condition expression is satisfied.

[0069] The value of the middle part of the conditional expression, "I(110) / (I(110)+I(200)+I(211))", is preferably 0.85 or less, more preferably 0.80 or less, and even more preferably 0.75 or less. This conditional expression means that the ferrite phase is randomly oriented. When the ferrite phase is completely randomly oriented, the value of the middle part is 0.67.

[0070] Grazing incidence X-ray diffraction (also called grazing incidence XRD, low angle incidence XRD, or oblique XRD) is a measurement technique in which the incident angle of the incident X-rays is set small and the detector is scanned (the detection angle is changed) while maintaining the incident angle. This allows efficient detection of information on the orientation of crystal grains up to a depth of about several micrometers in the surface layer of the sample. In the present invention, the incident angle of the X-rays is fixed at 1°, and the orientation of ferrite in the surface layer of the steel sheet is detected. The incident angle is the angle between the surface of the sample (steel sheet) and the incident direction of the incident X-rays.

[0071] FIG. 1 shows examples of the results of oblique incidence XRD analysis when the ferrite phase is randomized (b) and not (a). (a) shows the results of oblique incidence XRD analysis of a normal (conventional) steel sheet, which shows that the ferrite phase is oriented in the (110) direction. Therefore, the value of the middle part of the conditional equation, "I(110) / (I(110)+I(200)+I(211))," is relatively large at 0.91. (b) shows the results of oblique incidence XRD analysis of the steel sheet of the present invention, which shows that the orientation in the (110) direction is smaller than in (a). Therefore, the value of the middle part of the conditional equation, "I(110) / (I(110)+I(200)+I(211))," is relatively small at 0.58.

[0072] [Plating layer] The steel sheet of the present invention may have a coating layer, as described below. When a coating layer is present, the depth at which the C concentration is 0.02% or less in GDS measurement and the starting point of the layer thickness at which the area ratio of the ferrite phase is 90% or more is the interface between the steel sheet and the coating layer. In the present invention, the interface between the steel sheet and the coating layer is defined as follows. First, the Fe content in the thickness direction of the coated steel sheet is measured by GDS measurement. The value with the highest Fe content is defined as the Fe content of the steel sheet. The point at which the Fe content is 93% of the Fe content of this steel sheet is defined as the "interface between the steel sheet and the coating layer."

[0073] <Plated steel sheet> The plated steel sheet according to the present invention has a Zn-containing plating layer on the above-described steel sheet according to the present invention. This plating layer may be formed on one or both sides of the steel sheet. Alternatively, it may be formed only on a part of the surface. The plating layer may be subjected to an alloying treatment.

[0074] [Chemical composition of plating layer] The chemical composition of the plating layer is not limited as long as it contains Zn. Examples of plating layers containing Zn include Zn-0.2%Al (GI), Zn-(0.3-1.5)%Al, Zn-4.5%Al, Zn-0.09%Al-10%Fe (GA), Zn-1.5%Al-1.5%Mg, Zn-11%Al-3%Mg-0.2%Si, Zn-11%Ni, and Zn-15%Mg.

[0075] The chemical composition of the plating layer can be determined by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the steel sheet, and measuring the resulting solution by inductively coupled plasma (ICP) emission spectroscopy. For example, a 10% by mass hydrochloric acid solution containing 0.06% by mass of an inhibitor (Ivit 710K, manufactured by Asahi Chemical Industry Co., Ltd.) may be used as the acid solution containing the inhibitor for dissolving the plating layer.

[0076] The thickness of the plating layer may be, for example, 3 to 50 μm. The coating weight of the plating layer is not particularly limited, but may be, for example, 10 to 170 g / m per side.2 In the present invention, the coating weight of the plating layer is determined by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the steel sheet has been added, and measuring the change in weight before and after the plating layer is peeled off by pickling.

[0077] The roughness of the interface between the steel sheet and the plating layer (surface roughness Ra) corresponds to the roughness of the surface of the steel sheet described above, and the surface roughness Ra is 3.0 μm or less. Considering the adhesion of the plating, the surface roughness Ra is preferably 2.0 μm or less. The roughness of the interface (surface roughness Ra) may be the surface roughness of the steel sheet measured after removing the plating layer. The plating layer is removed by pickling by dissolving the plating layer in an acid solution to which an inhibitor has been added.

[0078] It should be noted that the steel sheet of the present invention can exhibit the effect of improving LME resistance even if it is not provided with a zinc coating. Generally, when non-galvanized steel sheets are spot welded together, LME cracking does not occur unless the spot weld is exposed to molten zinc. However, when one steel sheet is galvanized and the other is not galvanized, molten zinc is generated at the overlapping surface of the steel sheets during welding. Therefore, the molten zinc may come into contact with the surface of the non-galvanized steel sheet, causing LME cracking.

[0079] The thickness of the steel sheet and plated steel sheet of the present invention is not particularly limited. For example, it can be 0.1 to 3.2 mm. The thickness may be 0.2 mm or more, 0.4 mm or more, or 0.6 mm or more. The thickness may be 3.0 mm or less, 2.5 mm or less, 2.0 mm or less, or 1.8 mm or less.

[0080] 《Manufacturing method》 Next, a method for manufacturing a steel sheet according to the present invention will be described. The steel sheet according to the present invention can be obtained, for example, by a manufacturing method including a casting step in which molten steel with adjusted chemical composition is cast to form a steel billet, a hot rolling step in which the steel billet is hot-rolled to obtain a hot-rolled steel sheet, a coiling step in which the hot-rolled steel sheet is coiled, a cold rolling step in which the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet, a pretreatment step in which the cold-rolled steel sheet is pretreated (shot blasted), and an annealing step in which the pretreated cold-rolled steel sheet is annealed. Alternatively, the hot-rolled steel sheet may be pickled after the hot rolling step and then cold-rolled directly without being coiled.

[0081] <Casting process> The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a conventional method such as continuous casting or ingot casting.

[0082] <Hot rolling process> A hot-rolled steel plate can be obtained by hot-rolling a steel slab obtained by casting. The hot-rolling step is carried out by reheating the cast steel slab directly or after cooling it once, and then hot-rolling it. When reheating is carried out, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot-rolling step, rough rolling and finish rolling are usually carried out. The temperature and reduction rate of each rolling step may be appropriately changed depending on the desired metal structure and plate thickness. For example, the finishing temperature of finish rolling may be 900 to 1050°C, and the reduction rate of finish rolling may be 10 to 50%.

[0083] <Winding process> The hot-rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature may be changed as appropriate depending on the desired metal structure, etc., and may be, for example, 500 to 800°C. The hot-rolled steel sheet may be recoiled before or after coiling and subjected to a predetermined heat treatment. Alternatively, the hot-rolled steel sheet may be pickled after the hot-rolling step and then subjected to the cold-rolling step described below without performing the coiling step.

[0084] <Cold rolling process> After subjecting the hot-rolled steel sheet to pickling or the like, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio of the cold rolling may be appropriately changed depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold-rolling step, the sheet may be cooled to room temperature, for example, by air cooling.

[0085] <Pretreatment process> In order to obtain the above-mentioned surface layer structure of the steel sheet, it is necessary to carry out a predetermined pretreatment and then anneal the steel sheet.

[0086] The pretreatment involves shot blasting, in which spherical abrasives are projected onto the surface of the cold-rolled steel sheet. There are no particular limitations on the abrasives that can be used, but for example, steel balls (shot) with a median particle size of 40 to 450 μm can be used. Examples of such steel balls (shot) include TSH30 manufactured by WINOA IKK JAPAN. The amount of shot projected is 5 to 400 kg / m. 2 This makes it possible to introduce strain into the surface layer of the steel sheet without increasing the surface roughness Ra of the steel sheet. By carrying out such shot blasting, decarburization is promoted in the annealing process described below, and a stable ferrite structure can be efficiently formed in the surface layer of the steel sheet. The greater the amount of shot blasting, the greater the effect of improving LME resistance. However, at a fixed amount of shot blasting, for example 400 kg / m 2 Above this, the effect saturates. 2 The level per unit time and unit area is 4.0×10 -4 kg / (mm 2 ·min).

[0087] <Annealing process> The cold-rolled steel sheet that has undergone the pretreatment process is then annealed.

[0088] In the present invention, an annealing process is performed on a steel sheet that has been strained by shot blasting, including holding the steel sheet at a predetermined holding temperature and a high dew point. The heating rate to the predetermined holding temperature is not particularly limited, and may be 1 to 10°C / second. The dew point is controlled by humidification control from 300°C or higher, preferably from 450 to 550°C. That is, the dew point (humidification) control start temperature is 300°C or higher and lower than 600°C, preferably within the range of 450 to 550°C. The dew point without dew point (humidification) control is usually lower than -30°C. The dew point (high dew point) during annealing is set to -30 to 20°C to promote decarburization. The dew point (high dew point) during annealing is preferably -10°C or higher. The dew point during annealing is preferably 5°C or lower. The predetermined holding temperature (maximum heating temperature) in the annealing step is 750 to 900°C, preferably 770 to 870°C, to promote decarburization. The holding time at the holding temperature (maximum heating temperature) in the annealing step is 20 to 300 seconds, preferably 50 to 200 seconds. The atmosphere is preferably a non-oxidizing atmosphere, such as N2-1 to 10 vol% H2 or N2-2 to 4 vol% H2.

[0089] By setting the dew point, holding temperature, and holding time within the above ranges, decarburization can be promoted, the C concentration in the surface layer can be reduced, and the ferrite phase fraction can be appropriately controlled. Furthermore, by setting the dew point (humidification) control start temperature within the above range, decarburization in the surface layer of the steel sheet can be promoted. At the same time, internal oxidation of Si and Mn progresses rapidly, and internal oxides are rapidly formed. The formed internal oxides function as nucleation sites, resulting in randomization of the ferrite phase orientation. If the dew point (humidification) control start temperature is too low, external oxidation progresses and internal oxidation of Si and Mn does not progress, making it difficult to randomize the ferrite phase orientation.

[0090] Annealing is performed under tension of 1 to 20 MPa. Applying tension during annealing makes it possible to introduce strain into the steel sheet more effectively, accelerating decarburization of the surface layer.

[0091] By carrying out the above-mentioned steps, decarburization is promoted in the surface layer portion of the steel sheet, and a steel sheet can be obtained in which the surface layer portion of the steel sheet has a structure mainly composed of randomly oriented ferrite phase.

[0092] <<Method for manufacturing plated steel sheets>> The plated steel sheet according to the present invention can be obtained by carrying out a plating treatment step of forming a plating layer on the surface of the steel sheet produced as described above.

[0093] <Plating process> The plating process may be carried out according to a method known to those skilled in the art. The plating process may be carried out, for example, by hot-dip plating or electroplating. Preferably, the plating process is carried out by hot-dip plating. The plating conditions may be appropriately set taking into consideration the chemical composition, thickness, and coating amount of the desired plating layer. After the plating process, a known alloying process may be carried out to form alloyed plating.

[0094] The steel sheet and plated steel sheet according to the present invention have high strength and high LME resistance, and therefore can be suitably used in a wide range of fields, such as automobiles, home appliances, and building materials. They can be particularly suitably used in the automotive field. Steel sheets and plated steel sheets used for automobiles are often spot welded, which makes LME cracking more likely to occur. Therefore, when the steel sheet and plated steel sheet according to the present invention are used as automotive steel sheets for automotive components, the effect of the present invention, that is, high LME resistance, is suitably exhibited. [Example]

[0095] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0096] <Test No. 1> Molten steel adjusted to the chemical composition shown in Test No. 1 in Table 1 was melted in a blast furnace and cast by continuous casting to obtain a slab. The obtained slab was heated to 1200°C and hot-rolled with a finish rolling ending temperature of 950°C and a finish rolling reduction of 30% to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was coiled at a coiling temperature of 650°C, pickled, and then cold-rolled with a reduction of 50% to obtain a cold-rolled steel sheet. The cold-rolled steel sheet had a thickness of 1.6 mm.

[0097] Next, the surface of the obtained cold-rolled steel sheet was blasted with TSH30 manufactured by WINOA IKK JAPAN at a blast rate of 5 kg / m 2 The cold-rolled steel sheet after the shot blasting treatment had a surface roughness Ra of 2.9 μm.

[0098] Next, the shot-blasted cold-rolled steel sheet was annealed in a furnace with an oxygen concentration of 20 ppm or less in an N2-4%H2 gas atmosphere by increasing the temperature to 500°C at a rate of 6.0°C / s, and then increasing the temperature to 800°C at a rate of 2.0°C / s and holding for 40 seconds. At this time, dew point control was started so that the dew point would be 0°C from 300°C. The annealing was performed with the steel sheet under tension of 5.0 MPa.

[0099] Furthermore, the annealed steel sheet was immersed in a 450°C hot-dip galvanizing bath (Zn-0.14%Al) for 3 seconds, then pulled out at 100 mm / s, and the coating weight was reduced to 50 g / m using N2 wiping gas. 2 Thereafter, alloying treatment was carried out at 520°C for 30 seconds to obtain a galvannealed steel sheet.

[0100] <Test No. 2~55> Steel sheets or plated steel sheets were produced under the same conditions as in Example 1, except that the chemical compositions of the steel sheets were as shown in Table 1 or Table 2, the pretreatment process conditions and annealing process conditions were as shown in Table 3, and the coating type was as shown in Table 4. Note that in Test No. 32, shot blasting was omitted, and in Test No. 35, surface treatment was performed by grinding using a brush instead of shot blasting. Regarding the coating types in Table 4, "a" denotes alloyed hot-dip galvanizing, "b" denotes hot-dip galvanizing in which the alloying treatment in Test No. 1 was omitted, "c" denotes a coating bath of Zn-1.5%Al-1.5%Mg in which the alloying treatment was omitted, and "non-plated" denotes a cold-rolled steel sheet that was not subjected to a coating treatment.

[0101] [Table 1]

[0102] [Table 2]

[0103] [Table 3]

[0104] (surface roughness after pretreatment) The "surface roughness after pretreatment" shown in Table 3 was the surface roughness Ra of the steel sheet measured after the pretreatment process and before the annealing process in accordance with JIS B 0601:2013.

[0105] Rating AA: 2.0 μm or less Grade A: Over 2.0 μm, 3.0 μm or less Rating B: Over 3.0 μm

[0106] The annealed steel sheets and plated steel sheets were evaluated as follows.

[0107] (surface roughness Ra) After the annealing process, or the annealing process and the plating process, the surface roughness Ra of the unplated steel sheet was measured, and the surface of the plated steel sheet was measured after removing the plating. The plating layer was removed by dissolving the plating layer in a 10 mass% hydrochloric acid solution containing 0.06 mass% inhibitor (Ivit 710K, manufactured by Asahi Chemical Industry Co., Ltd.) that inhibits corrosion of the base steel sheet.

[0108] (Steel surface structure) A 30mm x 30mm sample was cut from the steel plate and subjected to GDS measurement, measurement of the high ferrite layer thickness, and grazing incidence XRD analysis using the methods described above. The results are shown in Table 4 in the columns "C≦0.02% depth," "High ferrite layer thickness," and "Grazing incidence XRD (middle value of the conditional equation)," respectively.

[0109] (tensile strength) For each steel plate, a JIS No. 5 tensile test piece was taken, and a tensile test was carried out in accordance with JIS Z 2241:2011 to determine the tensile strength. The obtained tensile strength value was evaluated as follows.

[0110] Rating AAA: 1180MPa or more Rating AA: 980 MPa or more, less than 1180 MPa Rating A: 780 MPa or more, less than 980 MPa

[0111] (LME resistance) A 50mm x 100mm specimen was cut from each steel plate, and a mating steel plate of the same size was also prepared. These two specimens were spot welded to produce welded joints using a dome-radius welding electrode with a tip diameter of 8mm at a 5° impact angle, 4.0kN pressure, 1.6 seconds of welding time, and 13kA of welding current. The steel plates used were those listed in the "Mating Material" column in Table 4. "Same type" for the mating material indicates that the same type of steel plate as the steel plate with the corresponding test number (Test No.) was used as the mating steel plate. "Unplated, same type" indicates that the same type of steel plate as the steel plate with the corresponding test number (Test No.) was used as the mating steel plate, but without plating. "GA, same type" indicates that the same type of steel plate as the steel plate with the corresponding test number (Test No.) was used as the mating steel plate, but with galvanneal plating. In addition, "GI270IF" indicates that a commercially available hot-dip galvanized steel sheet with a tensile strength of 270 MPa was used as the mating steel sheet, and "GA590" indicates that a commercially available alloyed hot-dip galvanized steel sheet with a tensile strength of 590 MPa was used as the mating steel sheet.

[0112] The evaluation method for LME resistance will be described with reference to FIG. 2 . LME resistance was evaluated by the length of an LME crack (crack 11 immediately outside the pressure welded portion) that occurred immediately outside the pressure welded portion of the welded portion 2 formed by spot welding two overlapping steel sheets 1. The two steel sheets 1 refer to the steel sheet with each test number (Test No.) and its mating steel sheet. The "immediately outside the pressure welded portion" of the welded portion refers to the portion outside the pressure welded portion 3, which is the portion of the overlapping surface of the two steel sheets that was pressure welded by spot welding, and refers to a position near the pressure welded portion 3 (within a range of approximately 1 mm outward from the end of the pressure welded portion 3). The crack length of the crack 11 immediately outside the pressure welded portion was evaluated. The spot welding test was performed three times, and the crack 11 immediately outside the pressure welded portion with the longest length was evaluated. The evaluation criteria were as follows: In this example, a rating of A or higher (i.e., ratings A, AA, and AAA) was determined to be excellent in LME resistance.

[0113] Rating AAA: 0μm Rating AA: Over 0 μm, less than 60 μm Rating A: 60 μm or more, less than 120 μm Rating B: 120 μm or more

[0114] The results of each evaluation are shown in Table 4.

[0115] [Table 4]

[0116] <Test No. 5-1~5-7> Tests Nos. 5-1 to 5-7 were performed under the same conditions as Test No. 5 shown in Tables 1, 3, and 4, except that the dew point in the annealing process was set to the value shown in Table 5, the holding temperature was set to 860°C, and the mating material was set to the "same type." The results are shown in Table 5.

[0117] [Table 5]

[0118] <Test No. 3-1~3-7> Tests Nos. 3-1 to 3-7 were performed under the same conditions as Test No. 3 shown in Tables 1, 3, and 4, except that the dew point control start temperature and holding temperature in the annealing step shown in Table 6 were set to 860° C. The results are shown in Table 6.

[0119] [Table 6]

[0120] The steel sheets of Test Nos. 1 to 25, 37 to 55, 3-2 to 3-6, and 5-1 to 5-7 are examples of the present invention and had high LME resistance. On the other hand, the steel sheets of Test Nos. 26 to 36, 3-1, and 3-7, which are comparative examples whose chemical compositions or manufacturing conditions were outside the specified conditions of the present invention, did not meet the pass criteria for LME resistance.

[0121] In Test No. 32, shot blasting was not performed, so strain was not introduced into the surface layer of the steel sheet. This is thought to be due to the lack of decarburization, and the depth at which the C concentration was 0.02% or less in the GDS measurement was shallow. Furthermore, internal oxidation of Si and Mn did not progress, and the orientation of the ferrite phase was not randomized. As a result, the LME resistance was poor.

[0122] In test No. 35, pretreatment was performed by grinding with a brush instead of shot blasting, which prevented sufficient strain from being introduced into the surface layer. This is thought to have prevented internal oxidation of Si and Mn during the annealing process, preventing randomization of the ferrite phase orientation. As a result, the specimen had poor LME resistance.

[0123] In Test No. 36, the dew point control start temperature during the annealing process was low, which is thought to have caused external oxidation to progress, preventing decarburization and resulting in a shallow depth at which the C concentration was 0.02% or less in the GDS measurement. Furthermore, it is thought that internal oxidation of Si and Mn did not progress, preventing randomization of the ferrite phase orientation. As a result, the LME resistance was poor. [Industrial Applicability]

[0124] According to the present invention, it is possible to provide a high-strength steel sheet and a plated steel sheet having high LME resistance, and the steel sheet and the plated steel sheet can be suitably used for applications such as automobiles, home appliances, and building materials, particularly for automobiles. Therefore, the present invention has extremely high industrial applicability. [Explanation of symbols]

[0125] 1 steel plate 2 Welded parts 3 Pressure welding part 11 Cracks just outside the pressure weld

Claims

1. A steel plate having a tensile strength of 780 MPa or more, Chemical composition, in mass%, C: 0.05-0.40%, Si: 0.5-3.0%, Mn: 0.1 to 5.0%, sol. Al: 0 to 3.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0 to 0.0100%, Ti: 0 to 0.1500%, Nb: 0 to 0.1500%, V: 0 to 0.150%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.0000%, Mo: 0-1.00%, W: 0-1.000%, Ca: 0-0.1000%, Mg: 0-0.100%, Zr: 0 to 0.100%, Hf: 0-0.100%, REM: 0~0.1000% and the balance being Fe and impurities, The surface roughness Ra of the steel plate is 3.0 μm or less, In the thickness direction from the steel sheet surface, the depth at which the C concentration is 0.02% or less is 3 μm or more, The thickness of a layer having an area ratio of ferrite of 90% or more in the thickness direction from the steel sheet surface is 3 μm or more, In the case of oblique incidence X-ray diffraction with an incident angle of 1° to the steel sheet surface, the diffraction intensity corresponding to the (110) plane is defined as I(110), the diffraction intensity corresponding to the (200) plane is defined as I(200), and the diffraction intensity corresponding to the (211) plane is defined as I(211). 0.45≦I(110) / (I(110)+I(200)+I(211))≦0.90 fulfill A steel plate characterized by:

2. The steel sheet according to claim 1, wherein the depth at which the C concentration is 0.02% or less is 10 μm or more in the thickness direction from the surface of the steel sheet.

3. 2. The steel sheet according to claim 1, wherein 0.45≦I(110) / (I(110)+I(200)+I(211))≦0.75 is satisfied.

4. 2. The steel sheet according to claim 1, wherein a thickness of a layer having an area ratio of ferrite of 90% or more in a thickness direction from the surface of the steel sheet is 8 μm or more.

5. 2. The steel sheet according to claim 1, wherein the surface roughness Ra of the steel sheet is 2.0 μm or less.

6. A plated steel sheet comprising the steel sheet according to any one of claims 1 to 5, and a Zn-containing plating layer on at least a portion of the surface thereof.

7. An automotive member comprising the steel sheet according to any one of claims 1 to 5.

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