Title of invention: Steel sheet, hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet
Technical field
[0001]
　The present invention relates to a steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet.
Background technology
[0002]
　High-strength steel sheets are used as steel sheets for automobiles in order to reduce the weight of automobiles, improve fuel efficiency, reduce carbon dioxide emissions, and ensure passenger safety. In recent years, in order to sufficiently secure the corrosion resistance of the vehicle body and parts, in addition to the high-strength hot-dip galvanized steel sheet, high-strength galvannealed steel sheet is also used (for example, see Patent Documents 1 to 4).
[0003]
　However, if spot welding is performed on a high-strength galvanized steel sheet and an alloyed hot-dip galvanized steel sheet or a high-strength cold-rolled steel sheet and a galvanized steel sheet are spot-welded for assembly of a vehicle body and/or parts, A crack called a liquid metal embrittlement crack (Liquid Metal Embrittlement: LME) may occur in the spot weld. The LME is a crack generated by melting zinc in the galvanized layer by heat generated during spot welding, invading the crystal grain boundaries of the steel sheet structure of the welded portion, and applying tensile stress to the state.
[0004]
　LME remarkably occurs when a high-strength TRIP steel sheet (transformation-induced plastic steel sheet) is spot-welded. The high-strength TRIP steel sheet is a steel sheet that has higher C, Si, and Mn concentrations than ordinary high-strength steel sheets and contains retained austenite, and thus has excellent energy absorption capability and press formability.
[0005]
　Further, LME is generally generated during spot welding of galvanized high strength steel plate. However, even if the high-strength cold-rolled steel sheet is not galvanized, LME is generated when the zinc melted in the galvanized steel sheet comes into contact with the high-strength cold-rolled steel sheet during spot welding with the galvanized steel sheet. There is.
[0006]
　As a technique for suppressing molten metal embrittlement cracking, Patent Document 5 discloses a plated steel sheet having an alloyed hot-dip galvanized surface, the base steel of which is C: 0.04 to 0.25% by mass. , Si: 0.01 to 2.0% by mass, Mn: 0.5 to 3.0% by mass, P: 0.1% by mass or less, S: 0.03% by mass or less, and further Ti: 0. 001 to 0.1 mass%, Nb: 0.001 to 0.1 mass%, V: 0.01 to 0.3 mass%, Mo: 0.01 to 0.5 mass%, Zr: 0.01 to 0.5% by mass of one or more, and the balance being Fe and inevitable impurities, and having an area ratio of 40 to 95% ferrite and bainite, pearlite, martensite, or two or more of them. Further, an alloyed hot-dip galvanized high-strength steel sheet having a metal structure composed of retained austenite with a volume ratio of 1 to 10% and excellent in workability and hot-dip metal embrittlement cracking resistance has been proposed.
[0007]
　Further, in Patent Document 6, C: 0.05 to 0.20%, Si: 0.5 to 2.0%, Mn: 1.0 to 2.5%, and the balance Fe and unavoidable in mass%. Of the intergranular oxidation of the hot-rolled steel sheet by hot-rolling the base steel containing specific impurities, cooling it at a cooling rate of 30°C/sec or more after hot-rolling, and winding it at 450-580°C. The thickness is 5 μm or less, the hot-rolled steel sheet is cold-rolled , Fe-based electroplating treatment is performed on the cold-rolled steel sheet so as to have an adhesion amount of 3 g/m 2 or more, and the cold-rolled steel sheet is alloyed and melted. There has been proposed a method for producing an alloyed hot dip galvanized steel sheet for spot welding, which makes the grain boundary oxidation depth of the alloyed hot dip galvanized steel sheet 5 μm or less by performing a galvanizing treatment.
Prior art documents
Patent literature
[0008]
Patent Document 1: JP 2005-060742 A
Patent Document 2: JP 2004-323970 A
Patent Document 3: JP 2006-233333 A
Patent Document 4: JP 2004-315960 A
Patent Document 5: JP 2006-265671
Patent Document 6: JP-A-2008-231493
Summary of the invention
Problems to be Solved by the Invention
[0009]
　The steel sheet described in Patent Document 5 refines austenite generated during spot welding by the pinning action of precipitates of additive elements and/or composite precipitates to complicate the diffusion and penetration route of molten zinc, It is to prevent invasion. However, merely complicating the diffusion and penetration route of molten zinc is not always sufficient to improve the resistance to brittle cracking of molten metal.
[0010]
　Further, when the amount of the additional element forming the complex precipitate that acts as pinning is increased, the strength and the resistance to molten metal embrittlement cracking are improved. However, on the other hand, since the ductility and toughness decrease, it is difficult to apply the steel sheet of Patent Document 5 as a steel sheet for automobiles that requires complicated and severe processing.
[0011]
　The steel sheet manufactured by the method described in Patent Document 6 melts even when spot welding is performed under the conditions of large current and large heat input such that scattering occurs by setting the grain boundary oxidation depth to 5 μm or less. It is possible to suppress the occurrence of metal embrittlement cracking. However, if spot welding is performed on a portion having a large residual stress after processing, molten zinc easily penetrates into the crystal grain boundaries of the welded portion, and molten metal embrittlement cracking easily occurs.
[0012]
　An object of the present invention is to provide a steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet which are excellent in resistance to brittle cracking of hot metal.
Means for solving the problem
[0013]
　The present invention has been made to solve the above problems, and has as its gist the following steel sheet, hot-dip galvanized steel sheet, and alloyed hot-dip galvanized steel sheet.
[0014]
　(1) The chemical composition of the base material is% by mass,
　C: 0.17 to 0.40%,
　Si: 0.10 to 2.50%,
　Mn: 1.00 to 10.00%,
　P:0 0.001 to 0.03%,
　S: 0.0001 to 0.02%,
　Al: 0.001 to 2.50%,
　N: 0.0001 to 0.010%,
　O: 0.0001 to 0.010 %,
　Ti:0 to 0.10%,
　Nb:0 to 0.10%,
　V:0 to 0.10%,
　B:0 to 0.010%,
　Cr:0 to 2.00%,
　Ni:0 ~2.00%,
　Cu:0~2.00%,
　Mo:0~2.00%,
　Ca:0~0.50%,
　Mg:0~0.50%,
　REM:0~0.50% The
　balance: Fe and impurities,
　having an internal oxide layer in which at least a part of the grain boundaries is covered with an oxide, to a depth of 5.0 μm or more from the surface of the base material, and
　A
　steel sheet having a grain boundary coverage of the oxide of 60% or more in a region from the surface of the base material to a depth of 5.0 μm .
[0015]
　(2)
　The steel sheet according to (1) above , which has a decarburized layer from the surface of the base material to a depth of 50 μm or more .
[0016]
　(3)
　The steel sheet according to (1) or (2) above , which has a nickel electroplating layer on the surface of the base material .
[0017]
　(4)
　The steel sheet according to any one of (1) to (3) , which has a tensile strength of 980 MPa or more .
[0018]
　(5) A
　hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the steel sheet according to any one of (1) to (4) above .
[0019]
　(6) 　The hot-dip galvanized steel sheet according to (5), wherein the amount of the hot-dip galvanized layer deposited is 70 g/m 2 or less
.
[0020]
　(7) A galvannealed steel sheet having a galvannealed layer on the surface of the steel sheet according to any one of (1) to (4) above
　.
[0021]
　(8) 　The alloyed hot-dip galvanized steel sheet according to (7), wherein the amount of the alloyed hot-dip galvanized layer deposited is 70 g/m 2 or less
.
[0022]
　(9)
　The alloyed hot-dip galvanized steel sheet according to (7) or (8), wherein the alloyed hot-dip galvanized layer contains, in mass%, Fe: 7.0 to 15.0% .
Effect of the invention
[0023]
　According to the present invention, it is possible to obtain a steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet having excellent resistance to hot-dip metal embrittlement cracking.
Brief description of the drawings
[0024]
FIG. 1 is a diagram schematically showing an aspect of LME generated in a welded portion.
FIG. 2 is a diagram schematically showing that the solid solution state of oxygen changes in the steel sheet due to a change in tensile stress during heat treatment. (A) shows a solid solution state of oxygen when a strong tensile stress is applied, and (b) shows a solid solution state of oxygen when a weak tensile stress is applied.
FIG. 3 is a schematic diagram for explaining the process of calculating the grain boundary coverage. (A) shows the grain boundary oxide of the steel surface layer photographed by SEM-backscattered electron image, and (b) shows the grain boundary MAP with a crystal orientation difference of 15° or more at the same position. Further, (c) shows a portion covered with the oxide in the crystal grain boundary, and (d) shows a portion not covered with the oxide.
FIG. 4 is a diagram showing an aspect of a test for evaluating resistance to brittle cracking of molten metal. (A) shows the aspect which spot-welds two steel plates, (b) shows the aspect of electric current control at the time of spot-welding two steel plates.
MODE FOR CARRYING OUT THE INVENTION
[0025]
　FIG. 1 schematically shows an aspect of LME generated in the welded portion. By stacking the steel plates 1a, 1b, and 1c and spot-welding them to form the nugget 2, the three steel plates can be joined. At this time, as shown in FIG. 1, inner cracks 3a occur between the steel plates, outer cracks 3b occur at the contact portion between the steel plate and the spot welding electrode, and outer cracks 3c occur at the steel plate portion not in direct contact with the electrodes. There is something to do.
[0026]
　As described above, in LME, zinc of the plating layer melted by heat during welding penetrates into the crystal grain boundaries of the welded structure, and the grain boundaries are embrittled. .. LME can occur not only when three steel plates are overlapped and welded as illustrated in FIG. 1, but also when two or four steel plates are overlapped and spot welded.
[0027]
　The present inventors have paid attention to the state of the surface layer of the steel sheet, and have earnestly conducted research on a method for suppressing the generation of LME due to molten metal (in particular, molten zinc), and have obtained the following findings.
[0028]
　When a heat treatment is performed on a steel sheet whose base material contains easily oxidizable elements such as Si and Mn under predetermined conditions, the easily oxidizable elements are not formed on the surface of the steel sheet but on the crystal grain boundaries inside the steel sheet. Oxides containing are sometimes formed.
[0029]
　When spot welding was performed on various steel plates, it was found that the LME generation tended to be suppressed in the steel plates in which the above-mentioned internal oxide was produced. It is considered that the invasion of molten zinc during welding was suppressed by previously covering the crystal grain boundaries in the base metal surface layer with the internal oxide.
[0030]
　Therefore, in order to perform further verification and suppress the generation of LME, the layer in which the internal oxidation occurs (hereinafter, referred to as “internal oxide layer”) is allowed to exist up to a predetermined depth, and the crystal by the oxide is formed. It has been found that it is important to increase the coverage of grain boundaries (hereinafter referred to as "grain boundary coverage").
[0031]
　In addition, when the present inventors conducted a study on a method for producing a steel sheet satisfying the above conditions, it was found that control of heat treatment conditions when forming an internal oxide layer is important.
[0032]
　The oxide generated on the surface layer of the steel sheet during annealing is classified into external oxidation and internal oxidation depending on the oxygen potential in the annealing atmosphere. This change in morphology is determined by the competition between the flux due to the diffusion of the easily oxidizable element from the thickness center of the steel sheet to the surface and the flux due to the diffusion of oxygen from the surface of the steel sheet to the thickness center.
[0033]
　When the oxygen potential in the atmosphere is low, or when the dew point is low, the flow rate of diffusion of oxygen into the steel sheet is low, and the flow rate of diffusion of easily oxidizable elements to the surface of the steel sheet is relatively high. Will be generated.
[0034]
　Therefore, in order to cover the crystal grain boundaries with an oxide, it is necessary to generate an internal oxide, and it is essential to increase the oxygen potential or the dew point in the atmosphere during annealing.
[0035]
　It was also found that the grain boundary cannot be sufficiently covered with the internal oxide only by controlling the atmosphere during the heat treatment. Therefore, we investigated a method of efficiently covering the grain boundaries with internal oxides.
[0036]
　As a result, while setting the heat treatment temperature to a high value, the tensile stress is applied to the steel sheet, and the heat treatment is performed in a state where the crystal lattice is expanded, so that oxygen is efficiently distributed in the lattice inside the crystal grains of the steel sheet surface layer. It has been found that it becomes possible to form a solid solution and the coverage rate of the internal oxide on the grain boundary is also improved.
[0037]
　In addition, in order to increase the coverage of the grain boundary with the internal oxide, it is necessary to alternately apply a strong stress and a weak stress instead of making the tensile stress constant.
[0038]
　As shown in FIG. 2A, in the state where a strong tensile stress is applied, oxygen forms a solid solution in the crystal grain boundaries and in the grains. Then, as shown in FIG. 2B, when the tensile stress becomes weaker, the crystal lattice becomes narrower, so that the oxygen dissolved in the crystal grains moves to the grain boundaries, is stabilized there, and exists as a precipitate. Like After that, when a strong stress is applied again, new oxygen is dissolved from the outside into the crystal grains. By repeating this, the amount of oxides precipitated at the crystal grain boundaries increases, and the grain boundary coverage increases.
[0039]
　The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.
[0040]
　(A) Chemical composition of base material The
　reasons for limiting each element are as follows. In addition, in the following description, "%" regarding the content means "mass %".
[0041]
　C: 0.17 to 0.40%
　Carbon (C) is an element necessary for improving the strength of steel sheet. If the C content is less than 0.17%, sufficient retained austenite cannot be obtained, and it becomes difficult to achieve both high strength and high ductility. On the other hand, if the C content exceeds 0.40%, the weldability is significantly reduced. Therefore, the C content is set to 0.17 to 0.40%. The C content is preferably 0.20% or more, and preferably 0.35% or less.
[0042]
　Si: 0.10 to 2.50%
　Silicon (Si) is an element that contributes to improvement of steel plate strength by suppressing temper softening of martensite in addition to solid solution strengthening. Further, Si is important for suppressing precipitation of iron-based carbides in austenite in a steel sheet whose workability is improved by transformation-induced plasticity (TRIP effect) of retained austenite, and for securing a retained austenite volume ratio of the steel sheet structure. It is an element.
[0043]
　When the Si content is less than 0.10%, the hardness of the tempered martensite is significantly reduced, and retained austenite cannot be sufficiently obtained, resulting in insufficient workability. On the other hand, when the Si content exceeds 2.50%, the steel sheet becomes brittle, the ductility decreases, the plating property decreases, and non-plating easily occurs. Therefore, the Si content is set to 0.10 to 2.50%. The Si content is preferably 0.50% or more, and preferably 2.00% or less.
[0044]
　Mn: 1.00 to 10.00%
　Manganese (Mn) is an element that enhances the hardenability and contributes to the improvement of the steel plate strength. If the Mn content is less than 1.00%, a soft structure is generated during cooling after annealing, and it becomes difficult to secure the strength. On the other hand, if the Mn content exceeds 10.00%, the oxidizability during the reduction/annealing lowers the plating property and also reduces the workability and weldability. Therefore, the Mn content is 1.00 to 10.00%. The Mn content is preferably 1.30% or more, and from the viewpoint of weldability, it is preferably 5.00% or less.
[0045]
　P: 0.001 to 0.03%
　Phosphorus (P) is an element that has the effect of increasing the strength of the steel sheet and suppressing the penetration of molten zinc into the steel sheet structure. If the P content is less than 0.001%, the above effects cannot be sufficiently obtained. On the other hand, if the P content exceeds 0.03%, the steel sheet becomes brittle due to the segregation of P at the grain boundaries. Therefore, the P content is set to 0.001 to 0.03%. The P content is preferably 0.005% or more, and preferably 0.02% or less.
[0046]
　S: 0.0001 to 0.02%
　Sulfur (S) is an element that causes hot embrittlement and impairs weldability and corrosion resistance. If the S content is less than 0.0001%, the manufacturing cost is significantly increased, so the S content is substantially 0.0001% or more. On the other hand, if the S content exceeds 0.02%, the hot workability, weldability and corrosion resistance are significantly reduced. Therefore, the S content is set to 0.0001 to 0.02%. The S content is preferably 0.0010% or more, and preferably 0.01% or less.
[0047]
　Al: 0.001 to 2.50%
　Aluminum (Al) is a deoxidizing element and is an element that suppresses the formation of iron-based carbides and contributes to the improvement of strength. If the Al content is less than 0.001%, the deoxidizing effect cannot be sufficiently obtained. On the other hand, when the Al content exceeds 2.50%, the ferrite fraction increases and the strength decreases. Therefore, the Al content is 0.001 to 2.50%. The Al content is preferably 0.005% or more, and more preferably 2.00% or less.
[0048]
　N: 0.0001 to 0.010%
　Nitrogen (N) is an element that forms a nitride, impairs stretch flangeability, and causes blowholes during welding. In order to reduce the N content to less than 0.0001%, the manufacturing cost is significantly increased, so that the N content is substantially 0.0001% or more. On the other hand, when N exceeds 0.010%, stretch flangeability is significantly deteriorated, and blow holes are generated during welding. Therefore, the N content is set to 0.0001 to 0.010%. The smaller the N content, the more preferable, but from the viewpoint of manufacturing cost, it is preferably 0.0010% or more. Further, the N content is preferably 0.008% or less.
[0049]
　O: 0.0001 to 0.010%
　Oxygen (O) is an element that forms an oxide and inhibits stretch flangeability. If the O content is less than 0.0001%, the manufacturing cost is significantly increased, so that the O content is substantially 0.0001% or more. On the other hand, if the O content exceeds 0.010%, the stretch flangeability is significantly reduced. Therefore, the O content is set to 0.0001 to 0.010%. The O content is preferably as small as possible, but from the viewpoint of manufacturing cost, it is preferably 0.0010% or more. Further, the O content is preferably 0.007% or less.
[0050]
　Ti: 0 to 0.10%
　Nb: 0 to 0.10%
　V: 0 to 0.10%
　Titanium (Ti), niobium (Nb) and vanadium (V) are all precipitation strengthening and crystal grain growth. It is an element that contributes to the improvement of steel sheet strength by strengthening the fine grains by suppressing and dislocation strengthening by suppressing recrystallization. Therefore, one or more selected from these elements may be contained if necessary.
[0051]
　However, if the content of any of the elements exceeds 0.10%, coarse carbonitrides precipitate and the formability deteriorates. Therefore, the contents of Ti, Nb and V are all set to 0.10% or less. In order to obtain the above effect, the content of one or more selected from Ti, Nb and V is preferably 0.005% or more, more preferably 0.010% or more. ..
[0052]
　B: 0 to 0.010%
　Boron (B) is an element that segregates to austenite grain boundaries during welding, strengthens the crystal grain boundaries, and contributes to the improvement of molten metal embrittlement cracking resistance. Therefore, B may be contained if necessary. However, if the B content exceeds 0.010%, carbides and nitrides are generated, the above effects are saturated, and hot workability is deteriorated. Therefore, the B content is 0.010% or less. The B content is preferably 0.005% or less. In order to obtain the above effect, the B content is preferably 0.0005% or more, more preferably 0.0008% or more.
[0053]
　Cr: 0 to 2.00%
　Ni: 0 to 2.00%
　Cu: 0 to 2.00%
　Chromium (Cr), nickel (Ni) and copper (Cu) are all elements that contribute to the improvement of strength. Is. Therefore, one or more selected from these elements may be contained if necessary.
[0054]
　However, if the content of any element exceeds 2.00%, the pickling property, the weldability and the hot workability deteriorate. Therefore, the contents of Cr, Ni and Cu are all set to 2.00% or less. The content of these elements is preferably 1.50% or less. In order to obtain the above effect, the content of one or more selected from Cr, Ni and Cu is preferably 0.01% or more, more preferably 0.10% or more. ..
[0055]
　Mo: 0 to 2.00%
　Molybdenum (Mo) is an element that, like Mn and Ni, enhances the hardenability of steel and contributes to the improvement of strength. Therefore, Mo may be contained if necessary. However, if the Mo content exceeds 2.00%, the hot workability is reduced and the productivity is reduced. Therefore, the Mo content is 2.00% or less. The Mo content is preferably 1.50% or less. When it is desired to obtain the above effects, the Mo content is preferably 0.01% or more, more preferably 0.10% or more.
[0056]
　Ca: 0 to 0.50%
　Mg: 0 to 0.50%
　REM: 0 to 0.50%
　Calcium (Ca), magnesium (Mg) and rare earth element (REM) all contribute to the improvement of formability. It is an element that does. Therefore, one or more selected from these elements may be contained if necessary.
[0057]
　However, if the content of any element exceeds 0.50%, the pickling property, the weldability and the hot workability deteriorate. Therefore, the contents of Ca, Mg and REM are all set to 0.50% or less. The content of these elements is preferably 0.35% or less. In order to obtain the above effect, the content of one or more selected from Ca, Mg and REM is preferably 0.0001% or more, and more preferably 0.0010% or more. ..
[0058]
　Further, when Ca, Mg and REM are contained in a complex manner, the total content thereof is preferably 0.50% or less, more preferably 0.35% or less.
[0059]
　Here, in the present invention, REM indicates a total of 17 elements of Sc, Y and lanthanoid, and the content of REM means the total content of these elements. Incidentally, the lanthanoid is industrially added in the form of misch metal.
[0060]
　In the chemical composition of the steel sheet of the present invention, the balance is Fe and impurities.
[0061]
　Here, the "impurities" are components that are mixed by ores, raw materials such as scrap, and various factors of the manufacturing process when the steel sheet is industrially manufactured, and are allowed within a range that does not adversely affect the present invention. Means something.
[0062]
　(B) Internal Oxidation Layer
　The steel sheet according to the present invention has an internal oxidation layer from the surface of the base material to a depth of 5.0 μm or more. The internal oxide layer is a layer in which at least a part of the crystal grain boundaries of the base material is covered with an oxide of an easily oxidizable element such as Si or Mn. By covering the crystal grain boundaries with the oxide, it becomes possible to suppress the intrusion of the molten metal into the crystal grain boundaries during welding and also suppress the LME cracking during welding.
[0063]
　Further, when an easily oxidizable element such as Si or Mn exists as an oxide at the crystal grain boundary, the concentration of the oxide on the surface of the base material is suppressed. The oxide formed on the surface of the base material lowers the wettability of the hot-dip plated metal and causes non-plating. Therefore, by forming the internal oxide layer, the occurrence of non-plating can be prevented and the plating property can be improved.
[0064]
　Further, the grain boundary coverage of the oxide needs to be 60% or more in the region from the surface of the base material to a depth of 5.0 μm. The grain boundary coverage is a ratio (%) of the length of the crystal grain boundary coated with the oxide to the total length of the crystal grain boundary in the above region. If the depth in which the internal oxide layer exists is less than 5.0 μm or the grain boundary coverage is less than 60%, the effect of improving the molten metal embrittlement cracking resistance of the steel sheet cannot be obtained.
[0065]
　The depth at which the internal oxide layer exists is preferably 5.5 μm or more, and more preferably 6.0 μm or more. The grain boundary coverage is preferably 70% or more, more preferably 80% or more. Although the grain boundary coverage is most preferably 100%, a great deal of restrictions on manufacturing conditions are required for realization, resulting in a large increase in manufacturing cost. Therefore, the upper limit is set to less than 100%.
[0066]
　In the present invention, the depth at which the internal oxide layer exists and the grain boundary coverage as shown in FIG. 3 are determined by the following method. A scanning electron microscope (SEM) and crystal orientation analysis by backscattered electrons (SEM-EBSD) are used for observing the structure. First, a sample for microstructure observation is taken from a steel plate so that the structure of the plate thickness cross section can be observed.
[0067]
　In the sample after being collected, the surface parallel to the rolling direction and perpendicular to the plate thickness was wet-polished with emery paper, and further buffed with diamond abrasive grains having an average diameter of 1 μm. To make the observation surface a mirror surface. Subsequently, in order to remove the strain introduced into the polishing surface by the mechanical polishing, colloidal silica polishing is performed using a suspension containing alcohol as a solvent.
[0068]
　In the colloidal silica polishing, if the load applied during polishing increases, strain may be further introduced. Therefore, it is important to suppress the load during polishing. Therefore, in colloidal silica polishing, Vibromet 2 manufactured by BUEHLER may be used to perform automatic polishing for 1 hour at an output of 40%.
[0069]
　However, when electrolytic polishing or chemical etching or the like is applied in the process of removing the strain introduced by mechanical polishing, the oxide melts, and the actual condition of the oxide existing on the grain boundaries cannot be grasped by observation. The same caution is required when polishing is performed with water as a solvent, and the water-soluble oxide is dissolved during polishing with water as a solvent, and the internal oxide on the grain boundary cannot be observed. Therefore, it is necessary to adopt a process that does not include the above procedure in the polishing finishing process.
[0070]
　The surface layer of the sample prepared by the above procedure is observed by SEM and SEM-EBSD. Of the observation magnifications of 1000 to 9000, a magnification in which the number of ferrite crystal grains in the microstructure is 10 or more is selected, and is 3000 times, for example.
[0071]
　First, as shown in FIG. 3A, the oxides present at the grain boundaries are confirmed by the backscattered electron image in the SEM. In the backscattered electron image, since the color tone changes depending on the atomic number (or mass), the oxide and the steel structure can be easily distinguished.
[0072]
　Then, in the structure observation of the backscattered electron image, for example, when the state in which the atomic number (or mass) is small is set to be displayed in “black color tone”, the oxide having a small mass with respect to iron is observed in a black color image. It will be displayed inside (see FIG. 3(a)). Under this observation condition, the structure of the surface layer of the steel sheet in 5 fields of view is photographed to confirm the existence state of the internal oxide.
[0073]
　Then, at the same position as the visual field observed by the above SEM-backscattered electron image, B.M. C. C. -Obtain iron crystal orientation data. The measurement magnification may be selected from 1000 to 9000, and may be the same magnification as the observation of the SEM-backscattered electron image described above, for example. In addition, the measurement interval (STEP) may be a magnification of 0.01 to 0.1 μm, and 0.05 μm may be selected.
[0074]
　B. obtained under these measurement conditions C. C. -In the crystal orientation MAP data of iron, a boundary having a crystal orientation difference of 15° or more is defined as a crystal grain boundary, except for a region where the confidence value (CI value) is less than 0.1. The CI value is a numerical value that is an index of the reliability of the crystal orientation determination shown in the analysis software, and generally, if the value is less than 0.1, the reliability is low.
[0075]
　When an oxide is present at the crystal grain boundary of ferrite, B. C. C. -Since iron crystal orientation data cannot be obtained, there are many regions having CI values ​​of less than 0.1 between adjacent crystal grains. In this case, although the crystal grain boundary cannot be clearly confirmed, at the boundary where the misorientation of the crystal grains of the adjacent ferrite is 15° or more, the crystal grain boundary passes through the center of the region where the CI value is less than 0.1. Is drawn on the MAP.
[0076]
　In the ferrite grain boundary MAP obtained by the above procedure (see FIG. 3( b )), as shown in FIG. Oxide coating length"). Subsequently, as shown in FIG. 3D, the length of the crystal grain boundary not covered with the oxide (hereinafter referred to as “oxide non-covering length”) is measured. Then, the grain boundary coverage (%) is calculated by dividing the obtained oxide coating length by the lengths of all the crystal grain boundaries.
[0077]
　(C) Decarburized Layer
　The steel sheet according to the present invention preferably has a decarburized layer from the surface of the base material to a depth of 50 μm or more. The decarburized layer is a carbon-deficient layer existing near the surface of the base material. In the decarburized layer, the hardness decreases as the carbon content decreases. In the present invention, in the surface layer of the base material, the surface layer region having a hardness of 80% or less with respect to the average hardness of the region having a plate thickness of 2/5 to 3/5 is a decarburized layer.
[0078]
　As described above, the tensile stress acts in a state where the molten metal has entered the crystal grain boundaries of the welded portion, so that LME is likely to occur. When the soft decarburized layer is present on the surface layer of the base material, stress is reduced and cracking is less likely to occur. Therefore, it is preferable that the decarburized layer exists from the surface of the base material to a depth of 50 μm or more.
[0079]
　The depth at which the decarburized layer exists is preferably more than 80 μm, and more preferably 100 μm or more. Although the upper limit is not particularly defined, even if it exceeds 150 μm, the effect of suppressing the generation of LME is saturated, and on the contrary, the tensile strength is lowered and the withstand load during bending deformation is lowered. Therefore, the depth at which the decarburized layer exists is preferably 150 μm or less.
[0080]
　(D) Tensile Strength
　As described above, when the steel sheet according to the present invention is used as a steel sheet for automobiles, it is desired to have high strength. The mechanical properties are not particularly limited, but the tensile strength is preferably 980 MPa or more, more preferably 1050 MPa or more, and further preferably 1100 MPa or more. If the tensile strength exceeds 2000 MPa, the residual stress during welding increases, so that the internal oxide layer on the grain boundaries becomes cracked, and the effect of suppressing LME cracking remarkably decreases. Therefore, the upper limit of the tensile strength is preferably 2000 MPa.
[0081]
　(E) Plating Layer
　The steel sheet according to the present invention may have a hot-dip galvanized layer on the surface. By providing the hot-dip galvanized layer on the surface of the steel sheet, the corrosion resistance is improved.
[0082]
　The hot dip galvanized layer may be alloyed. In the alloyed hot-dip galvanized layer, since Fe is incorporated in the hot-dip galvanized layer by the alloying treatment, excellent weldability and paintability are obtained.
[0083]
　The amount of the hot-dip galvanized layer or the alloyed hot-dip galvanized layer is not particularly limited. However, if the amount of adhesion is too large, the amount of molten zinc during welding will increase. Therefore, from the viewpoint of more effectively suppressing the generation of LME, it is preferable that the amount of adhesion is 70 g/m 2 or less, and more preferably 60 g/m 2 or less.
[0084]
　Further, when the surface has an alloyed hot-dip galvanized layer, the higher the Fe concentration in the plated layer, the easier the alloying reaction during spot welding proceeds, and the amount of hot-dip zinc present during welding can be reduced. Therefore, the Fe concentration in the galvannealed layer is preferably 7.0% by mass or more, and more preferably 9.0% by mass or more.
[0085]
　On the other hand, when the Fe concentration of the alloyed hot-dip galvanized layer exceeds 15.0% by mass, the ratio of the Γ phase, which is an intermetallic compound having poor workability, becomes high in the alloyed hot-dip galvanized layer, resulting in press forming. In the middle, cracking of the plating layer may occur, and so-called powdering phenomenon may cause a peeling phenomenon of the plating due to plastic deformation during press molding. Therefore, the Fe concentration of the galvannealed layer is preferably 15.0% by mass or less, and more preferably 13.0% by mass or less.
[0086]
　(F) Nickel electroplating layer
　The steel sheet according to the present invention may have a nickel electroplating layer on the surface of the base material. The presence of the nickel electroplated layer causes the zinc and nickel to fuse during spot welding, increasing the solidification temperature of the molten zinc. As a result, the molten zinc solidifies before entering the crystal grain boundaries, so that the generation of LME is effectively suppressed.
[0087]
　(G) Manufacturing Method
　The steel sheet according to the present invention can be manufactured , for example, by annealing a hot rolled steel sheet or a cold rolled steel sheet under predetermined conditions.
[0088]
　There are no particular restrictions on the manufacturing conditions for the hot-rolled steel sheet or the cold-rolled steel sheet. For example, a hot-rolled steel sheet can be manufactured by casting molten steel having the above-described chemical composition under normal conditions to form a slab and then hot rolling under normal conditions.
[0089]
　The cast steel billet may be once cooled to a temperature of 500° C. or lower, reheated, and then hot-rolled. However, if it stays in the temperature range of 500 to 800° C. for a long time, an oxide film of an easily oxidizable element will grow on the surface of the steel slab. As a result, the content of the easily oxidizable element is reduced in the surface layer of the base material, and then the internal oxide layer is less likely to be formed. Therefore, after casting, it is preferable to perform reheating to a predetermined temperature and hot rolling before the surface temperature of the steel slab is lowered to 800° C. or lower.
[0090]
　Further, a cold rolled steel sheet can be manufactured by cold rolling the above hot rolled steel sheet under normal conditions.
[0091]
　Next, the annealing conditions for forming the internal oxide layer will be described in detail below. The annealing can be performed by, for example, a continuous annealing line.
[0092]
　 In
　order to prevent diffusion of easily oxidizable elements to the surface of the steel sheet and promote internal oxidation, it is important to control the oxygen potential in the heating zone during annealing. Specifically, the annealing is preferably performed in an atmosphere containing 0.1 to 30% by volume of hydrogen and H 2 O having a dew point of −40 to 20° C. , with the balance being nitrogen and impurities. More preferably, an atmosphere containing 0.5 to 20% by volume of hydrogen and H 2 O having a dew point of −30 to 15° C. , further preferably 1 to 10% by volume of hydrogen and H 2 O having a dew point of −20 to 10° C. It is an atmosphere that includes.
[0093]
　The annealing furnace is roughly divided into three regions: pre-tropical zone, heating zone, and soaking zone. Then, in the steel sheet according to the present invention, the atmosphere in the heating zone is set to the above condition. Atmosphere control is possible even in the pre-tropics and soaking zones. However, in the pre-tropical zone, the atmospheric temperature is low, and the flux of diffusion of oxygen and oxidizable elements is significantly reduced. Further, in the soaking zone, the retention temperature is high, and austenite is generated in the tissue, so that the flux of diffusion of oxygen and easily oxidizable elements is significantly reduced. That is, the influence of the atmosphere control in the pre-tropics and the soaking zones on the grain boundary coverage of the internal oxide layer is small.
[0094]
　 In
　order to efficiently dissolve oxygen into the steel sheet during annealing, the annealing temperature needs to exceed 750°C and 900°C or less. This is because if the annealing temperature is 750° C. or lower, the internal oxide layer may not be sufficiently formed. On the other hand, if the annealing temperature exceeds 900° C., it causes plate breakage, excessive decarburization and surface flaws in the threading step. The annealing temperature is preferably 780°C or higher, and preferably 840°C or lower.
[0095]
　 In
　order to efficiently dissolve oxygen into the steel sheet, a tensile stress of 3 to 150 MPa is applied to the steel sheet in the region of 300° C. or higher in the heating zone during annealing. If the minimum tensile stress to be applied is less than 3 MPa, the steel sheet will be scraped and the manufacturability will be reduced. Further, if the maximum tensile stress applied is less than 3 MPa, the effect of expanding the crystal lattice and making oxygen easy to form a solid solution cannot be sufficiently obtained. From the viewpoint of increasing the grain boundary coverage of the internal oxide layer, the maximum tensile stress is preferably 15 MPa or more. On the other hand, if the maximum tensile stress exceeds 150 MPa, drawing and breaking of the plate in the threading step are caused.
[0096]
　Further, as described above, in order to increase the coverage rate of the oxide on the grain boundary, the tensile stress is not made constant, but strong stress and weak stress are alternately applied. This is because when a strong stress is applied, oxygen is dissolved in the lattice inside the grain, and when the applied stress is weakened, the oxygen dissolved in the lattice diffuses toward the grain boundary (Fig. This is because precipitates (oxides) are generated on the grain boundaries.
[0097]
　In order to satisfy the grain boundary coverage of 60% or more specified by the steel sheet in the present invention, the difference between the maximum tensile stress and the minimum tensile stress (hereinafter, referred to as “maximum-minimum stress difference”) is 2 MPa or more. It is preferable that it is 4 MPa or more. Further, in order to satisfy the grain boundary coverage of 80% or more, the maximum-minimum stress difference is preferably 20 MPa or more.
[0098]
　Therefore, when repetitive stress is applied, it is preferable to increase the difference in strength. The tensile stress applied to the steel sheet can be changed, for example, by appropriately adjusting the feed rate and the frictional force of each roller during the continuous annealing line passing, and the tensile stress can be changed from the tension measured by the pinch roller to the tensile stress. Can be asked.
[0099]
　When the surface of the steel sheet is subjected to hot dip galvanizing, for example, it may be passed through the continuous annealing galvanizing line and then through the continuous hot dip galvanizing line.
[0100]
　There are no particular restrictions on the composition and temperature of the plating bath in which the steel sheet is dipped when performing hot dip galvanizing. For example, the composition of the plating bath is mainly Zn, and the effective Al amount (the value obtained by subtracting the total Fe amount from the total Al amount in the plating bath) is preferably 0.050 to 0.250 mass %.
[0101]
　If the amount of effective Al in the plating bath is less than 0.050% by mass, the penetration of Fe into the plating layer may proceed excessively and the plating adhesion may deteriorate. On the other hand, when the effective Al amount in the plating bath exceeds 0.250% by mass, an Al-based oxide that inhibits the movement of Fe atoms and Zn atoms is generated at the boundary between the steel sheet and the plating layer, and the plating adhesion is improved. It may decrease. The effective Al amount in the plating bath is more preferably 0.065 mass% or more, and even more preferably 0.180 mass% or less.
[0102]
　The plating bath is Ag, B, Be, Bi, Ca, Cd, Co, Cr, Cs, Cu, Ge, Hf, I, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, It may contain one or more selected from Pb, Rb, S, Si, Sn, Sr, Ta, Ti, V, W, Zr and REM.
[0103]
　The plating bath temperature is preferably 450 to 490°C. When the plating bath temperature is lower than 450° C., the viscosity of the plating bath excessively increases, it becomes difficult to control the thickness of the plating layer, and the appearance of the hot-dip galvanized steel sheet may be impaired. On the other hand, when the plating bath temperature exceeds 490° C., a large amount of fumes are generated, which may make safe plating operation difficult. The plating bath temperature is more preferably 455°C or higher, and more preferably 480°C or lower.
[0104]
　The steel plate temperature when the steel plate is immersed in the plating bath is preferably 440 to 500°C. If the steel plate temperature is lower than 440° C., the plating temperature is maintained at 450 to 490° C., so that it is necessary to apply a large amount of heat to the plating bath, which increases the manufacturing cost. On the other hand, when the steel plate temperature when the steel plate is immersed in the plating bath exceeds 500° C., in order to maintain the plating bath temperature at 490° C. or lower, a facility for removing a large amount of heat from the plating bath is required, and the manufacturing cost Rises. The steel plate temperature is more preferably 450°C or higher, and more preferably 490°C or lower.
[0105]
　After pulling up the steel sheet from the plating bath, it is preferable to blow a high-pressure gas containing nitrogen as a main component on the surface of the steel sheet to remove excess zinc so that the amount of deposited coating becomes a proper amount.
[0106]
　When the hot dip galvanized layer is subjected to an alloying treatment, the steel sheet having the hot dip galvanized layer is heated in a temperature range of 450 to 600°C. If the alloying temperature is less than 450°C, alloying may not proceed sufficiently. On the other hand, when the alloying temperature exceeds 600° C., the alloying proceeds too much, and the Γ phase is generated, so that the Fe concentration in the plating layer may exceed 15%. The alloying temperature is more preferably 470° C. or higher, and more preferably 580° C. or lower.
[0107]
　Since the alloying temperature needs to be changed depending on the composition of the steel sheet and the degree of formation of the internal oxide layer, it may be set while confirming the Fe concentration in the plating layer.
[0108]
　The steel sheet according to the present invention is a steel sheet that can be applied to all welding in which LME can occur during welding, such as spot, MIG, TIG, and laser. In particular, when spot welding is applied, the molten metal embrittlement cracking resistance in the spot welded portion is remarkably excellent.
[0109]
　Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to these examples.
Example 1
[0110]
　Steel having the chemical composition shown in Table 1 was melted and cast into steel pieces. Then, each steel slab cooled to the temperature shown in Table 2 was reheated to 1220° C. and then hot-rolled to produce a hot-rolled steel sheet having a thickness of 2.8 mm. Then, after performing pickling, cold rolling with a reduction rate shown in Table 2 was performed to obtain a cold rolled steel sheet. The obtained cold rolled steel sheet was annealed under the conditions shown in Table 2. During the annealing, the maximum tensile stress and the minimum tensile stress were controlled by adjusting the friction coefficient of roll rotation and the average value of the tensile stress applied to the steel sheet. The maximum-minimum stress difference was measured by using the variation value of the value every 30 seconds.
[0111]
　Next, some of the steel sheets were subjected to plating treatment under the conditions shown in Table 2 to obtain hot dip galvanized steel sheets (GI steel sheets) or galvannealed steel sheets (GA steel sheets). The effective Al amount in the plating bath was 0.1% by mass.
[0112]
　Furthermore, a nickel electroplating layer was provided on the surface of the base material in some of the steel plates, GI steel plates and GA steel plates. By the above, each test material was obtained.
[0113]
[table 1]

[0114]
[Table 2]

[0115]
　Then, a test piece for microstructure observation was taken from each test material so that the structure of the plate thickness section could be observed. Subsequently, in a test piece after being collected, a surface parallel to the rolling direction and perpendicular to the plate thickness was wet-polished with emery paper, and further, a buff using diamond abrasive grains having an average diameter of 1 μm. Polishing was performed to make the observation surface a mirror surface.
[0116]
　Further, in order to remove the strain introduced into the polishing surface by the mechanical polishing, colloidal silica polishing was performed using a suspension containing alcohol as a solvent. In the polishing with colloidal silica, Vibromet 2 manufactured by BUEHLER was used, and automatic polishing was performed for 1 hour at an output of 40%.
[0117]
　The surface layer of the test piece prepared by the above procedure was observed by SEM and SEM-EBSD. The SEM used for the measurement is JSM-7001F manufactured by JEOL. Of the observation magnifications of 1000 to 9000, a magnification in which the number of ferrite crystal grains in the microstructure was 10 or more was selected. Then, the oxides present at the grain boundaries were confirmed by the backscattered electron image in the SEM. Then, the structure of the steel sheet surface layer in 5 fields of view was photographed to confirm the existence state of the internal oxide.
[0118]
　Then, at the same position as the visual field observed by the above SEM-backscattered electron image, B.M. C. C. -Obtained crystal orientation data of iron. The measurement by EBSD was performed using the EBSD detector attached to the SEM, and the magnification of the measurement was the same magnification as the observation of the SEM-backscattered electron image. Further, the measurement interval (STEP) of the test piece was set to 0.05 μm. At this time, in the present invention, software “OIM Data Collection ™　(ver. 7)” manufactured by TSL Solutions Co., Ltd. was used as the crystal orientation data acquisition software　 .
[0119]
　B. obtained under these measurement conditions C. C. -In the crystal orientation MAP data of iron, a boundary having a crystal orientation difference of 15° or more was defined as a crystal grain boundary, except for a region where the confidence value (CI value) was less than 0.1. At this time, in the present invention, software "OIM Analysis ™　(ver. 7)" manufactured by TSL Solutions Co., Ltd. was used as the data analysis software of the crystal orientation　 .
[0120]
　In addition, when an oxide exists in the crystal grain boundary of ferrite, B. C. C. -Since iron crystal orientation data cannot be obtained, there are many regions having CI values ​​of less than 0.1 between adjacent crystal grains. In this case, although the crystal grain boundaries cannot be clearly confirmed, the crystal grain boundaries pass through the center of the region where the CI value is less than 0.1 at the boundary where the orientation difference between the crystal grains of the adjacent ferrite is 15° or more. Was drawn on the MAP.
[0121]
　In the ferrite grain boundary MAP obtained by the above procedure, the grain boundary coverage (%) was calculated by dividing the oxide coating length by all the grain boundary lengths.
[0122]
　Subsequently, the depth at which the decarburized layer exists was measured using the above test piece. Specifically, the Vickers hardness is measured in the depth direction from the base metal surface of each test piece in steps of 20 μm to a depth of 300 μm and in the region of the plate thickness 2/5 to 3/5 of the test material. It was At this time, the test force was 10 gf. Then, the decarburized layer was a surface layer region in which the hardness was reduced to 80% or less of the average hardness in the region of the plate thickness 2/5 to 3/5.
[0123]
　Next, with respect to the test material having the plating layer on the surface of the base material, the adhesion amount (g/m 2 ) of the plating layer and the Fe concentration (mass %) were measured. Further, with respect to the test material having the nickel electroplating layer on the surface of the base material, the adhesion amount (g/m 2 ) of the nickel electroplating layer was measured. The Fe concentration (mass %) of the plated layer was measured using an electron beam microanalyzer analyzer (EPMA). The instrument used for the measurement was JXA-8500F manufactured by JEOL.
[0124]
　Furthermore, JIS No. 5 tensile test pieces were taken from the direction (width direction) perpendicular to the rolling direction and the thickness direction of each test material, and a tensile test was performed according to JIS Z 2241 to measure the tensile strength (TS). ..
[0125]
　Then, each test material was used to evaluate the molten metal embrittlement cracking resistance according to the following procedure.
[0126]
　FIG. 4 shows a state of a test for evaluating the resistance to brittle cracking of molten metal. FIG. 4(a) shows a mode of spot welding two steel plates, and FIG. 4(b) shows a mode of current control when spot welding two steel plates. The steel plate 1d and the steel plate 1e were overlapped and spot-welded with the pair of electrodes 4a and 4b. The welding conditions are as follows.
[0127]
　Electrodes 4a, 4b: DR-type electrode made of Cr-Cu, tip outer diameter: 8 mm, R: 40 mm
　　　　　　　　　Applied pressure P: 450 kg
　Inclination angle of electrode (angle formed by electrode center line 5 and vertical line 6) θ: 3°
　　up Slope: None
　　First energization time t1: 0.2 seconds
　　Non-energization tc: 0.04 seconds
　　Second energization time t2: 0.4 seconds
　　Current ratio I1/I2: 0.7
　　Hold time after completion of energization: 0. 1 second
[0128]
　In addition, the test No. The alloyed hot-dip galvanized steel sheet shown in 24 is always used as the steel sheet 1d of FIG. 4, two steel sheets are overlapped and spot welded with the steel sheet to be evaluated as 1e, and the cross-sectional observation of the LME occurrence state of the steel sheet on the 1e side is performed. evaluated.
[0129]
　Here, as shown in Table 2 and Table 3, the test No. For the samples 1 to 3, 6, 7, 10 to 22, 25, 28, 31, 34, 37 to 46, a part of the steel plate on the 1e side was tested using a cold-rolled steel plate that was not plated. .. Even in this case, since the surface of the steel plate on the 1e side is in contact with the galvanized surface of the steel plate 1d, even if the surface of the steel plate on the 1e side is a cold-rolled steel plate, the molten metal embrittlement crack resistance Sex can be evaluated.
[0130]
　The mode of LME is such that the steel plate cross section including the center of the nugget is polished and SEM observation is performed by the same method as described above, the inner crack 3a between the steel plates, the outer crack 3b at the contact portion between the steel plate and the spot welding electrode, and the electrode. The cracks at the three locations of the outer crack 3c of the steel plate portion not in direct contact with were evaluated by the following crack ratings.
[0131]
　1: There is no crack in any place.
　2: A crack is present at any one of the locations, and its length is 60 μm or less.
　3: Cracks were observed at 2 or more and 3 or less points, and the length of each crack was 60 μm or less.
　4: The crack length exceeds 60 μm at any one or more places.
[0132]
　The results are shown in Table 3.
[0133]
[Table 3]

[0134]
　As can be seen from the results in Table 3, the test No. Since Nos. 1 to 5 and 11 to 24 all satisfy the requirements of the present invention, the crack score is 1 to 3, and the molten metal embrittlement cracking resistance is excellent. The test No. In Nos. 1, 14 and 17, after casting, the slab was once cooled to a temperature of 500° C. or lower and reheated, so that the cracking score was 3, which is more than the other examples of the present invention, and is resistant to molten metal embrittlement cracking. The result was poor.
[0135]
　On the other hand, the test No. Nos. 6 to 10 and 25 to 30 were due to inappropriate annealing conditions, at least one of the depth of presence of the internal oxide layer and the grain boundary coverage ratio was out of the range, and the cracking score was 4 and the resistance was high. The molten metal embrittlement cracking property deteriorated. The test No. In No. 9, the maximum-minimum stress difference applied was 2 MPa or more, but the dew point during annealing was extremely low, and the grain boundary coverage was lowered. As described above, when the composition, the annealing conditions, etc. are out of the range defined by the present invention, the grain boundary coverage may be reduced even when the maximum-minimum stress difference is large. In addition, the test No. In No. 10, the tensile stress was not applied during annealing, so the grain boundary coverage was reduced.
[0136]
　Test No. Since the chemical compositions of 31 to 36 and 38 to 46 were out of the regulation, the crack cracking rating was 4 regardless of the manufacturing conditions, and the molten metal embrittlement cracking resistance was deteriorated. In addition, the test No. In No. 37, the C content was less than the lower limit value, so that although the molten metal embrittlement cracking resistance was good, the strength decreased.
[0137]
　The test No. 33 and No. 33. In No. 44, the maximum-minimum stress difference is small, but the grain boundary coverage is high. This is a test No. 33 and No. 33. In No. 44, since the Si or Al content is higher than the range specified by the present invention, it is considered that a large amount of oxide was formed. However, since the above composition is out of the specified range of the present invention, the resistance to brittle cracking of molten metal deteriorates.
Example 2
[0138]
　Then, in order to investigate the influence of the rating on the characteristics, the test No. Welding was performed in the same manner as in Example 1 using three test materials 2, 4, and 24. The welding conditions are as follows.
[0139]
　Electrodes 4a, 4b: DR-type electrode made of Cr-Cu, tip outer diameter: 8 mm, R: 40 mm
　　　　　　　　　Applied pressure P: 450 kg
　Inclination angle of electrode (angle formed by electrode center line 5 and vertical line 6) θ: 1 to 10 °
　　Upslope: None
　　First energization time t1: 0.2 seconds
　　Non-energization tc: 0.04 seconds
　　Second energization time t2: 0.4 seconds
　　Current ratio I1/I2: 0.7
　　Hold time after completion of energization: 0.1 seconds
[0140]
　As in Example 1, the test No. The alloyed hot-dip galvanized steel sheet shown in 24 is always used as the steel sheet 1d of FIG. 4, two steel sheets are overlapped and spot welded with the steel sheet to be evaluated as 1e, and the cross-sectional observation of the LME occurrence state of the steel sheet on the 1e side is performed. evaluated. The length of cracks after welding was adjusted by changing the tilt angle of the electrode to 3 to 10°. The larger the inclination angle, the more the residual stress generated in the surface layer of the steel sheet during welding increases, so that LME cracking is more likely to occur.
[0141]
　And the cross tension strength (CTS) was evaluated using the steel plate after welding. The results are shown in Table 4.
[0142]
[Table 4]

[0143]
　As can be seen from the results of Table 4, in the range of 1 to 3, the relative CTS value is 0.9 or more with respect to the CTS value when the tilt angle θ of the electrode is 1°. On the other hand, when the score is 4, the relative CTS value is less than 0.6, and it can be seen that the characteristics are significantly deteriorated.
Industrial availability
[0144]
　According to the present invention, it is possible to obtain a steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet having excellent resistance to hot-dip metal embrittlement cracking.
The scope of the claims
[Claim 1]
　The chemical composition of the base material is% by mass,
　C: 0.17 to 0.40%,
　Si: 0.10 to 2.50%,
　Mn: 1.00 to 10.00%,
　P: 0.001 to 0.03%,
　S: 0.0001 to 0.02%,
　Al: 0.001 to 2.50%,
　N: 0.0001 to 0.010%,
　O: 0.0001 to 0.010%,
　Ti : 0 to 0.10%,
　Nb: 0 to 0.10%,
　V: 0 to 0.10%,
　B: 0 to 0.010%,
　Cr: 0 to 2.00%,
　Ni: 0 to 2. 00%,
　Cu: 0 to 2.00%,
　Mo: 0 to 2.00%,
　Ca: 0 to 0.50%,
　Mg: 0 to 0.50%,
　REM: 0 to 0.50%,
　balance: Fe and impurities, which
　have an internal oxide layer in which at least a part of the grain boundaries are covered with an oxide from the surface of the base material to a depth of 5.0 μm or more, and
　A
　steel sheet having a grain boundary coverage of the oxide of 60% or more in a region from the surface of the base material to a depth of 5.0 μm .
[Claim 2]

　The steel sheet according to claim 1, 　which has a decarburized layer from the surface of the base material to a depth of 50 μm or more .
[Claim 3]

　The steel plate according to claim 1 or 2, 　which has a nickel electroplating layer on the surface of the base material .
[Claim 4]

　The steel sheet according to any one of claims 1 to 3 　, which has a tensile strength of 980 MPa or more .
[Claim 5]
　A
　hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the steel sheet according to any one of claims 1 to 4 .
[Claim 6]
　The hot-dip galvanized steel sheet according to claim 5, 　wherein an adhesion amount of the hot-dip galvanized layer is 70 g/m 2 or less
.
[Claim 7]
　An
　alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer on the surface of the steel sheet according to any one of claims 1 to 4 .
[Claim 8]
　The 　galvannealed steel sheet according to claim 7, wherein an amount of the alloyed hot-dip galvanized layer deposited is 70 g/m 2 or less
.
[Claim 9]
　The
　galvannealed steel sheet according to claim 7 or claim 8, wherein the galvannealed layer contains Fe: 7.0 to 15.0% by mass .