Steel sheet, plated steel sheet, and automobile member

Abstract

The present invention addresses the problem of providing a steel sheet and a plated steel sheet that have high LME resistance. This steel sheet has a prescribed chemical composition, a tensile strength of at least 780 MPa, and a surface roughness Ra of greater than 3.0 μm. The depth in the thickness direction from the surface of the steel sheet to which the C concentration is no more than 0.02% is at least 8 μm, the thickness in the sheet thickness direction from the surface of the steel sheet of a layer that has a ferrite area percentage of at least 90% is greater than 3.0 μm, and when I(110), I(200), and I(211) are the diffraction intensities that respectively correspond to the (110) plane, the (200) plane, and the (211) plane from grazing incidence x-ray diffraction at an incidence angle of 1° to the surface of the steel sheet, 0.45≤I(110) / (I(110)+I(200)+I(211))≤0.90.

Description

Steel sheets, plated steel sheets and automotive parts

[0001] The present invention relates to a steel sheet and a plated steel sheet, and more particularly to a steel sheet and a plated steel sheet having high LME resistance.

[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, weldability may be reduced due to liquid metal embrittlement (LME) cracking, as described in Patent Document 1, for example.

[0004] In addition, Patent Document 2 discloses a steel sheet that suppresses LME cracking and improves weldability, and describes a steel sheet having a surface layer containing silicon oxide particles with a particle size of 20 nm or more at a rate of 3,000 to 6,000 particles / mm 2 The patent discloses a steel sheet having a suitable grain size distribution at a number density of 1000.degree.

[0005] International Publication No. WO 2019 / 116531 International Publication No. WO 2020 / 218575

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

[0008] The present inventors have intensively investigated means for solving the above-mentioned problems, and as a result, have found that by applying strain to a steel sheet before annealing using a shot material under appropriate conditions to create an appropriate surface state, and then performing high dew-point annealing, the surface layer of the steel sheet is decarburized, and further a layer with a high ferrite fraction in which ferrite is randomly oriented is formed, and as a result, it is 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. the steel sheet having a surface roughness Ra of 3.0 μm, and a balance consisting of Fe and impurities; a depth at which the C concentration is 0.02% or less from the surface of the steel sheet in the thickness direction from the surface of the steel sheet is 8 μm or more, a layer in which the area ratio of ferrite is 90% or more has a thickness of 3 μm or more from the surface of the steel sheet in the thickness direction from the surface of the steel sheet, and in oblique incidence X-ray diffraction at an incident angle of 1° to the steel sheet surface, the following relationship is satisfied: 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 15 μm or more in the thickness direction from the surface of the steel sheet.

[0012] (3) The steel sheet according to (1) or (2) above, characterized in that 0.50≦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 in which the area ratio of ferrite is 90% or more is 20 μm or more in the thickness direction from the surface of the steel sheet.

[0014] (5) The steel sheet according to any one of (1) to (4), characterized in that the surface roughness of the steel sheet is 3.5 μm or more in terms of Ra.

[0015] (6) A plated steel sheet comprising the steel sheet according to any one of (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.

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

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

[0019] The present invention will be described below. However, 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, during spot welding, the metallographic structure of a steel sheet is heated and transformed into austenite, and molten zinc produced by melting the coating penetrates into 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 of the present invention came up with the idea of ​​utilizing the metallographic structure of the surface layer of the steel sheet as a method for improving LME resistance. Specifically, the surface layer of the steel sheet has 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.

[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 crystal orientation of the ferrite particles prevents grain boundaries oriented in a specific direction from being unevenly distributed and connected continuously or intermittently. LME cracking is thought to occur when Zn from the plating invades grain boundaries where the grain boundary energy is locally low. In other words, if there are continuous grain boundaries where the grain boundary energy is locally low, 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 where the grain boundary energy is locally low will no longer be connected continuously, preventing local 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 formula) In the case of oblique incidence X-ray diffraction at an incident angle of 1° to the steel sheet surface, when the diffraction intensity corresponding to the (110) plane is I(110), the diffraction intensity corresponding to the (200) plane is I(200), and the diffraction intensity corresponding to the (211) plane is I(211), the following condition is satisfied: 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., 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. "%" regarding the chemical composition means "mass %." Furthermore, in the numerical range of the chemical composition, a numerical range expressed using "to" means a range that includes the numerical values ​​written 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. In order 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. In order to prevent the C concentration in the surface layer, which will be 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%) Si (silicon) is an element that promotes ferrite stabilization and decarburization. By including Si, decarburization progresses in the surface layer through the pretreatment and heat treatment described below, and the ferrite in the surface layer is stabilized, 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 if high dew-point annealing is performed, external oxidation progresses and oxides (scale) are formed in the surface layer of the steel sheet. Conversely, decarburization at the outermost surface is suppressed, and the effect of improving LME resistance is reduced. In consideration of this point, 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%) Mn (manganese) is an element 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 to 3.0%) Aluminum (aluminum) 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 that is 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 if high dew-point annealing is performed, external oxidation progresses, forming oxides (scale) on the surface layer of the steel sheet. Conversely, decarburization on the outermost surface is suppressed, and the effect of improving LME resistance is reduced. In consideration of this point, 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 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. The total content of Si and sol. Al may be 1.7% or less, 1.6% or less, 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 no N is 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 sheet. If the O content is too high, oxides are generated in excess, and the workability of the steel sheet is likely to be reduced. 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%) B (boron) is an element that improves hardenability and contributes to improving strength, and also segregates at grain boundaries to strengthen the grain boundaries and improve toughness, so it may be contained as needed. Since B 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%) Ti (titanium) is an element that precipitates as TiC during cooling of the steel and contributes to improving strength, so it may be contained as needed. Since it is not an essential element, the lower limit of the Ti content is 0%. This effect can be obtained even with a small amount of Ti, but when Ti is contained, the Ti content is preferably 0.0001% or more. The Ti content may be 0.0003% or more, or 0.0005% or more. On the other hand, if it is contained in excess, coarse TiN may be formed, 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%) Nb (niobium) 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 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 to 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 content, 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 of steel and increasing its strength, 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 content, but when 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, if contained in excess, a large amount of Cr carbide is formed, which may adversely 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, and 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 when 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. On the other hand, since 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 the strength of steel, 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 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 to 1.00%) Mo (molybdenum) is effective in improving the hardenability of steel and increasing the strength of steel, and 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 to 1.000%) W (tungsten) is effective in improving the hardenability of steel and increasing its strength, and 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 content, 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 when 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, if it is contained in excess, deterioration of surface properties may become apparent, so 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%) Mg (magnesium) 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, if Mg is contained in excess, deterioration of surface properties may become apparent, 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, and therefore 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 contained, but when contained, the Zr content is preferably 0.001% or more. The Zr content may be 0.005% or more, 0.010% or more. On the other hand, if 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, 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, and therefore 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 contained, but when 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, if contained excessively, deterioration of surface properties may become apparent, 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 element) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be contained as needed. Since it is not an essential element, the lower limit of the REM content is 0%. This effect can be obtained even with a small amount of REM, but when REM is contained, the REM content is preferably 0.0001% or more. The REM content may be 0.0003% or more, or 0.0005% or more. On the other hand, excessive REM content may cause deterioration of surface properties to become apparent, 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 is an abbreviation for Rare Earth Metal and refers to an element belonging to the lanthanide series. REM is usually 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, that is, 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 production process, including raw materials such as ore and scrap, and that do not adversely affect the LME resistance and hydrogen desorption property of the steel sheet according to the present invention, that is, impurities that are contained in a range that allows the LME resistance and hydrogen desorption property required for the steel sheet of the present invention to be obtained.

[0051] The chemical components of the steel sheet may be analyzed using 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 more than 3.0 μm. 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 roughness of the interface between the steel sheet and the plating 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, "VK-X3000" manufactured by Keyence Corporation). Specifically, an image is taken at a magnification of 20 times using a 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] As the roughness increases, hydrogen is more easily desorbed, improving hydrogen embrittlement resistance. By making the surface roughness Ra greater than 3.0 μm, hydrogen embrittlement resistance is improved. From the viewpoint of hydrogen embrittlement resistance, the surface roughness Ra is preferably 3.2 μm or more, more preferably 3.4 μm or more, 3.5 μm or more, 3.6 μm or more, 3.8 μm or more, or 4.0 μm or more.

[0056] [Depth at which the C concentration in the surface layer is 0.02% or less] In the steel sheet of the present invention, the depth at which the C concentration measured by GDS (glow discharge spectroscopy) is 0.02% or less in the sheet thickness direction from the surface of the steel sheet is 8 μm or more.

[0057] Since the susceptibility to LME decreases as the carbon concentration decreases, reducing the carbon concentration in the surface layer improves LME resistance. Furthermore, since carbon is an austenite-stabilizing element, reducing its content stabilizes the layer with low LME susceptibility. Furthermore, when the carbon content in the surface layer is low, hydrogen that has penetrated into the steel is more easily released, improving hydrogen desorption. This is presumably because hydrogen passes more easily due to the low presence of carbon, an interstitial element, in the ferrite phase.

[0058] 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 a pretreatment process and an annealing process, which will be described later.

[0059] Since a depth of 8 μm or more at which the C concentration is 0.02% or less contributes to improving LME resistance, there is no particular upper limit to the depth at which the C concentration is 0.02% or less. The depth at which 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 at which the C concentration is 0.02% or less is preferably 10 μm or more, more preferably 12 μm or more, and even more preferably 15 μm or more, or 20 μm or more.

[0060] 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 taken 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 conditions below, for example the measurement device, etc., are not followed. However, if differences arise in the measurement results, the steel sheet according to the present invention is identified by the measurement results according to the conditions below.

[0061] Apparatus: High-frequency glow discharge optical emission analyzer (manufactured by LECO Japan LLC, model number "GDS850A" Ar gas pressure: 0.3 MPa Anode diameter: 4 mmφ RF output: 30 W Measurement time: 200 to 1500 seconds

[0062] [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 the layer with a ferrite phase area ratio of 90% or more (hereinafter referred to as "high ferrite layer") in the thickness direction from the steel sheet surface is 3 μm or more.

[0063] Since a thickness of 3 μm or more of the high ferrite layer contributes to improving LME resistance, 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.

[0064] 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.

[0065] 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. For SEM observation, a field-emission scanning electron microscope (e.g., JEOL Ltd.'s "JSM 7000F," accelerating voltage: 15 kV) is used. The observation field of view in the observation cross section is a depth range of 500 μm from the surface of the steel sheet (sheet surface) in the thickness direction (longitudinal direction of the observation cross section) and a width range of 600 μm in the direction perpendicular to the thickness direction (lateral direction of the observation cross section). In the observation cross section, five observation fields are observed with an SEM in the direction perpendicular to the thickness direction (lateral direction of the observation cross section) with an interval of 1000 μm or more between each observation field, 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.

[0066] The ferrite fraction is calculated for the five obtained secondary electron images by the point counting method. More specifically, a grid with equal intervals 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 the present invention, the grid spacing is 2 μm × 2 μm, and the total number of grid points is 1,500.

[0067] 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 the prior austenite phase, such as laths or blocks. 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 boundaries, block boundaries) and carbide precipitation with multiple variants is determined to be tempered martensite. Furthermore, a region with lamellar precipitation of cementite is determined to be pearlite. A region with high brightness and no substructure 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.

[0068] 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 a structure 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.

[0069] [Diffraction intensity ratio of ferrite phase by grazing incidence X-ray diffraction (XRD)] In grazing incidence X-ray diffraction at an incident angle of 1° to the steel sheet surface, the steel sheet of the present invention satisfies the condition: 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.

[0070] 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. Furthermore, I(110) / (I(110)+I(200)+I(211)) is preferably 0.50 or more. The 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.

[0071] Grazing incidence X-ray diffraction (also referred to as grazing incidence XRD, low angle incidence XRD, or tilted XRD) is a measurement technique in which the incident angle of the incident X-rays is set small and only 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 down 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° to detect the orientation of ferrite in the surface layer of the steel sheet. The incident angle is the angle between the surface of the sample (steel sheet) and the incident direction of the incident X-rays.

[0072] FIG. 1 shows examples of the results of grazing incidence XRD analysis when the ferrite phase is randomized (b) and not (a). (a) shows the results of grazing 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 expression, "I(110) / (I(110)+I(200)+I(211))," is relatively large at 0.91. (b) shows the results of grazing 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 expression, "I(110) / (I(110)+I(200)+I(211))," is relatively small at 0.58.

[0073] [Plated Layer] The steel sheet of the present invention may have a plated layer as described below. When a plated layer is present, the depth at which the C concentration is 0.02% or less in GDS measurement and the starting point of the thickness of the layer at which the area ratio of the ferrite phase is 90% or more are the interface between the steel sheet and the plated layer excluding the plating. In the present invention, the interface between the steel sheet and the plated layer is defined as follows. First, the Fe content in the thickness direction of the plated 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 the steel sheet is defined as the "interface between the steel sheet and the plated layer."

[0074] <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.

[0075] [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 that contain Zn include Zn-0.2%Al (GI), Zn-(0.3 to 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.

[0076] 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 mass % hydrochloric acid solution containing 0.06 mass % 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.

[0077] 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.

[0078] The roughness (surface roughness Ra) of the interface between the steel sheet and the plating layer corresponds to the roughness of the surface of the steel sheet described above, and the surface roughness Ra is greater than 3.0 μm. From the viewpoint of hydrogen desorption, the surface roughness Ra may be 3.2 μm or more, 3.4 μm or more, 3.6 μm or more, 3.8 μm or more, or 4.0 μm or more. 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.

[0079] 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 zinc plating. Generally, when non-galvanized steel sheets are spot welded together, LME cracking does not occur unless the spot weld is in contact with 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.

[0080] 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.

[0081] <<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 of casting molten steel with adjusted chemical composition to form a steel billet, a hot rolling step of hot-rolling the steel billet to obtain a hot-rolled steel sheet, a coiling step of coiling the hot-rolled steel sheet, a cold rolling step of cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet, a pretreatment step of pretreating the cold-rolled steel sheet (grit blasting), and an annealing step of annealing the pretreated cold-rolled steel sheet. Alternatively, the hot-rolled steel sheet may be pickled after the hot rolling step and then cold-rolled directly without being coiled.

[0082] <Casting Step> The conditions for the casting step are not particularly limited. For example, after melting in a blast furnace or an electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a method such as ordinary continuous casting or casting by an ingot method.

[0083] <Hot Rolling Process> A hot-rolled steel plate can be obtained by hot-rolling the steel slab obtained by casting. The hot rolling process is carried out by reheating the cast steel slab directly or after cooling it once, followed by hot rolling. When reheating is carried out, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot rolling process, rough rolling and finish rolling are usually carried out. The temperature and reduction rate of each rolling step may be changed as appropriate 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%.

[0084] <Coiling step> 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 without performing the coiling step, and then subjected to the cold-rolling step described below.

[0085] <Cold Rolling Step> 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.

[0086] <Pretreatment Step> In order to obtain the above-described structure of the surface layer of the steel sheet, it is necessary to carry out a predetermined pretreatment and then anneal the steel sheet.

[0087] The pretreatment involves grit blasting, in which angular shot material is projected onto the surface of the cold-rolled steel sheet. There are no particular limitations on the type of shot material that can be used, but for example, polygonal steel grit with an average particle size of 100 to 500 μm can be used. An example of such grit is TGD-30 manufactured by WINOA IKK JAPAN. This makes it possible to increase the surface roughness Ra of the steel sheet while introducing strain into the surface layer of the steel sheet. The grit projection dose is 5 to 400 kg / m 2 By carrying out such grit blasting, decarburization is promoted in the annealing step described below, and a stable ferrite structure can be efficiently formed in the surface layer of the steel sheet. The greater the blasting amount, the greater the effect of improving LME resistance. However, it is preferable to use a constant blasting amount, for example, 400 kg / m 2 Above this, the effect is saturated. 2 The level per unit time and unit area is 4.0 x 10 -4 kg / (mm 2 ・min).

[0088] <Annealing Step> The cold-rolled steel sheet that has been subjected to the pretreatment step is annealed.

[0089] In the present invention, the steel sheet to which strain has been imparted by grit blasting is subjected to an annealing process, which includes 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 and lower than 600°C, preferably from 400 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, in order 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.

[0090] 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 of 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 orientation of the ferrite phase. 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 orientation of the ferrite phase.

[0091] Annealing is performed under a tension of, for example, 1 to 20 MPa. Applying tension during annealing makes it possible to introduce strain into the steel sheet more effectively, and promotes decarburization of the surface layer.

[0092] 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.

[0093] <<Method for Producing Plated Steel Sheet>> 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.

[0094] <Plating Treatment Step> The plating treatment may be carried out according to a method known to those skilled in the art. The plating treatment may be carried out, for example, by hot-dip plating or by electroplating. Preferably, the plating treatment is carried out by hot-dip plating. The plating treatment conditions may be appropriately set taking into consideration the chemical composition, thickness, and coating amount of the desired plating layer. After the plating treatment step, a known alloying treatment step may be carried out to form alloyed plating.

[0095] The steel sheet and plated steel sheet according to the present invention have high strength and high LME resistance and hydrogen desorption properties, 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, namely high LME resistance, is suitably exhibited.

[0096] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0097] <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 steel slab. The obtained steel 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.

[0098] Next, the surface of the obtained cold-rolled steel sheet was blasted with TGD-30 manufactured by WINOA IKK JAPAN Co., Ltd. at a blast rate of 5 kg / m. 2 The cold rolled steel sheet after the grit blasting treatment had a surface roughness Ra of 3.3 μm.

[0099] Subsequently, the cold-rolled steel sheet that had been subjected to the grit blasting treatment was subjected to an annealing treatment in which the temperature was raised to 500°C at a rate of 6.0°C / sec in an N-4% H gas atmosphere in a furnace with an oxygen concentration of 20 ppm or less, and then further raised to 800°C at a rate of 2.0°C / sec and held at that temperature for 40 seconds. At this time, dew point control was started so that the dew point would become 0°C from 300°C. The annealing treatment was performed with the steel sheet subjected to a tension of 5.0 MPa.

[0100] Furthermore, the annealed steel sheet was immersed in a hot dip galvanizing bath (Zn-0.14% Al) at 450°C for 3 seconds, then pulled out at 100 mm / sec, 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.

[0101] <Test Nos. 2 to 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, the grit blasting treatment was omitted, and in Test No. 35, surface treatment by grinding with a brush was performed instead of the grit blasting treatment. Regarding the coating types in Table 4, "a" denotes alloyed hot-dip galvanizing, "b" denotes hot-dip galvanizing where the alloying treatment in Example 1 was omitted, "c" denotes a coating bath containing Zn-1.5%Al-1.5%Mg where the alloying treatment was omitted, and "non-plated" denotes a cold-rolled steel sheet that was not subjected to a coating treatment.

[0102]

[0103]

[0104]

[0105] (Surface roughness after pretreatment) The "surface roughness after pretreatment" shown in Table 3 was measured in accordance with JIS B 0601:2013 for the surface roughness of the steel sheet after the pretreatment step and before the annealing step.

[0106] Grade AA: 3.5 μm or more Grade A: More than 3.0 μm, less than 3.5 μm Grade B: 3.0 μm or less

[0107] The annealed steel sheets and plated steel sheets were evaluated as follows.

[0108] (Surface Roughness Ra) After the annealing step, or the annealing step and the plating step, the surface roughness Ra of the steel sheet surface was measured for unplated steel sheets, and for plated steel sheets, the surface roughness Ra of the steel sheet surface exposed by removing the plating was measured. The plating 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 suppresses corrosion of the base steel sheet.

[0109] (Steel surface layer structure) A 30 mm x 30 mm 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 (value of the middle side of the conditional equation)," respectively.

[0110] (Tensile strength) A JIS No. 5 tensile test piece was taken from each steel plate, and a tensile test was performed in accordance with JIS Z 2241: 2011 to determine the tensile strength. The obtained tensile strength value was evaluated as follows.

[0111] Grade AAA: 1180 MPa or more Grade AA: 980 MPa or more, less than 1180 MPa Grade A: 780 MPa or more, less than 980 MPa

[0112] (LME Resistance) A sample cut to a size of 50 mm x 100 mm was taken from each steel plate, and a mating steel plate of the same size was prepared. These two samples were spot welded to produce welded joints using a dome radius welding electrode with a tip diameter of 8 mm at an impact angle of 2°, a pressure of 4.0 kN, a welding time of 0.8 seconds, and a current of 12 kA. The mating steel plates used were steel plates of the types 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 was not plated. "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 was not plated. In addition, "GI270IF" indicates that a commercially available hot-dip galvanized steel sheet having 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 having a tensile strength of 590 MPa was used as the mating steel sheet.

[0113] 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 weld) that occurred immediately outside the pressure welded portion of the weld 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 weld" 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.

[0114] Rating AAA: 0 μm Rating AA: More than 0 μm, less than 60 μm Rating A: 60 μm or more, less than 120 μm Rating B: 120 μm or more

[0115] (Evaluation of Hydrogen Embrittlement Resistance) Two specimens measuring 550 mm × 100 mm were cut from each steel sheet. One specimen was electrochemically charged with hydrogen (the specimen immediately after hydrogen charging), while the other specimen was electrochemically charged with hydrogen and then left to stand at room temperature (25°C, relative humidity 30%) for 12 hours (the specimen left to stand for 12 hours after hydrogen charging). The amount of diffusible hydrogen in these specimens was measured using a thermal desorption method. Specifically, the specimens were heated to 400°C in a heating furnace equipped with gas chromatography, and the total amount of hydrogen released until the temperature cooled to 250°C was taken as the amount of diffusible hydrogen.

[0116] The hydrogen embrittlement resistance (amount of accumulated hydrogen in the sample) was evaluated by calculating the diffusible hydrogen amount after standing for 12 hours / the diffusible hydrogen amount (%) immediately after hydrogen charging, and was evaluated according to the following criteria.

[0117] Grade AAA: 5% or less Grade AA: 10% to less than 5% Grade A: 30% to less than 10% Grade B: Over 30%

[0118] The results of each evaluation are shown in Table 4.

[0119]

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

[0121] Test No. 32 was not subjected to grit blasting, so no strain was introduced into the surface layer. Therefore, decarburization was not promoted, 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. Furthermore, the roughness of the interface between the steel sheet and the coating was reduced. As a result, the LME resistance and hydrogen embrittlement resistance were inferior.

[0122] In Test No. 35, pretreatment by brush grinding was performed instead of grit blasting, which prevented 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. Furthermore, the roughness of the interface between the steel sheet and the coating was reduced. As a result, the LME resistance and hydrogen embrittlement resistance were poor.

[0123] In Test No. 36, the dew point control start temperature in 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. It is also 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.

[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.

[0125] 1 Steel plate 2 Welded portion 3 Pressure welded portion 11 Crack directly outside the pressure welded portion

Claims

1. A steel plate having a tensile strength of 780 MPa or more, and having chemical components, in mass %, of C: 0.05 to 0.40%, Si: 0.5 to 3.0%, Mn: 0.1 to 5.0%, sol. Al: 0-3.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0-0.0100%, Ti: 0-0.1500%, 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%, and the balance being Fe and impurities, A steel sheet characterized in that the surface roughness Ra of the steel sheet exceeds 3.0 μm, the depth at which the C concentration is 0.02% or less is 8 μm or more in the thickness direction from the steel sheet surface, the thickness of a layer in which the area ratio of ferrite is 90% or more is 3 μm or more in the sheet thickness direction from the steel sheet surface, and in oblique incidence X-ray diffraction with an incidence angle of 1° on the steel sheet surface, when the diffraction intensity corresponding to the (110) plane is I(110), the diffraction intensity corresponding to the (200) plane is I(200), and the diffraction intensity corresponding to the (211) plane is I(211), the relationship satisfies 0.45≦I(110) / (I(110)+I(200)+I(211))≦0.

90.

2. The steel plate according to claim 1, characterized in that the depth at which the C concentration is 0.02% or less is 15 μm or more in the thickness direction from the surface of the steel plate.

3. The steel plate according to claim 1, characterized in that 0.50≦I(110) / (I(110)+I(200)+I(211))≦0.75 is satisfied.

4. The steel plate according to claim 1, characterized in that the thickness of the layer having an area ratio of ferrite of 90% or more in the thickness direction from the surface of the steel plate is 20 μm or more.

5. The steel plate according to claim 1, characterized in that the surface roughness of the steel plate is 3.5 μm or more in terms of Ra.

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 of the steel sheet.

7. An automobile component comprising the steel sheet according to any one of claims 1 to 5.