Welded joint

Abstract

Invention details;

Description

Welded joints

[0001] The present invention relates to a welded joint, and more particularly to a welded joint that suppresses LME cracking during manufacturing.

[0002] In recent years, efforts have been made to increase the strength of steel sheets used in various fields, such as automobiles, home appliances, and building materials. For example, in the automobile field, the use of spot-welded welded joints 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, deterioration of weldability due to liquid metal embrittlement (LME) cracking can become a problem, as described in Patent Document 1, for example. LME cracking is thought to occur when the surface layer of the steel sheet transforms to austenite during welding, molten zinc penetrates into the grain boundaries, embrittling the steel sheet, and further when tensile stress is applied to the steel sheet during welding.

[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 prevent LME cracking during the production of welded joints, it is effective to suppress the penetration of Zn and other elements contained in the coating layer into the austenitic transformed steel sheet. 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 welded joint in which LME cracking during manufacturing is suppressed.

[0008] The present inventors have intensively investigated means for solving the above-mentioned problems, and as a result, have found that by blasting an abrasive at a steel sheet before annealing under appropriate conditions to impart strain to create an appropriate surface condition, and then performing high dew-point annealing, the surface layer of the steel sheet is decarburized and a layer with a low cementite fraction is formed, thereby making it possible to suppress LME.

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

[0010] (1) A welded joint comprising a plurality of overlapping steel plates and a spot weld that joins the plurality of steel plates, wherein the spot weld has a nugget, an indentation formed by pressing down with an electrode, and a weld shoulder that is a peripheral portion of the indentation, and at least one of the plurality of steel plates is a plated steel plate having a plating layer containing Zn formed on at least a surface corresponding to the overlapping surface of the plurality of steel plates, and at least one of the plurality of steel plates that constitutes the overlapping surface is a high-strength steel plate having a tensile strength of 780 MPa or more, and the chemical composition of the high-strength steel plate is, in mass %, C: 0.08 to 0.40%, Si: 0.4 to 2.0%, Mn: 0.1 to 5.0%, sol. Al: 0 to 2.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0 to 0.0100%, Ti: 0 to 0.1500%, Nb: 0 to 0.150%, V: 0 to 0.150%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 to 2.0000%, Mo: 0 to 1.00%, W: 0 to 1.000%, Ca: 0 to 0.1000%, Mg: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, REM: 0 to 0.1000%, a welded joint characterized in that the balance is Fe and impurities, and in a region 5 mm or more away from the outer edge of the nugget, the depth from the surface of the high-strength steel plate in the depth direction is 3 μm or more where the C concentration as measured by GDS is 0.01% or less, and the surface roughness of the high-strength steel plate is more than 3 μm in arithmetic mean roughness Ra, and the thickness of a layer in which the area ratio of cementite is 10% or less is 8 μm or more in a range of 0 to 100 μm outward from the outer edge of the weld shoulder.

[0011] (2) The welded joint according to (1), characterized in that, in at least one of the overlapping surfaces formed by a pair of adjacent steel plates among the plurality of steel plates, at least one of the pair of steel plates has a plating layer containing Zn, and at least one of the pair of steel plates is a high-strength steel plate having a tensile strength of 780 MPa or more.

[0012] (3) The welded joint according to (1) or (2), characterized in that the total content of Si and sol. Al is less than 1.8%.

[0013] (4) The welded joint according to any one of (1) to (3), characterized in that the depth at which the C concentration measured by GDS is 0.01% or less in the depth direction of the base steel sheet starting from the interface between the plating layer and the base steel sheet is 5 μm or more.

[0014] (5) The welded joint according to any one of (1) to (3), characterized in that the depth at which the C concentration measured by GDS is 0.01% or less in the depth direction of the base steel sheet starting from the interface between the plating layer and the base steel sheet is 7 μm or more.

[0015] According to the present invention, it is possible to obtain a welded joint in which LME cracking during manufacturing is suppressed.

[0016] It is a figure explaining the weld joint of this invention. It is a figure showing the layer formed around the weld part of the weld joint of this invention. It is a figure explaining the LME resistance evaluation in Example.

[0017] The present invention will be described below. The present invention is not limited to the following embodiments. First, an outline of a configuration for improving LME resistance during production of a welded joint according to the present invention will be described.

[0018] When a plated steel sheet is spot-welded during the production of a welded joint, the coating melts and the surface layer of the steel sheet is heated, transforming the steel sheet structure to austenite. During this process, the hot-dip coating penetrates into the steel sheet structure along the austenite grain boundaries, embrittling the grain boundaries. Therefore, when stress is applied to the steel sheet, LME cracking is likely to occur at the grain boundaries. It is believed that LME cracking is particularly likely to occur during welding, as tensile stress is applied to the steel sheet. The welded joint of the present invention improves LME resistance during the production of the welded joint by virtue of the structure formed in the surface layer of the steel sheet that constitutes the welded joint. In this specification, the surface layer of the steel sheet refers to the range from the outermost surface of the steel sheet to a depth of 100 μm.

[0019] Since LME cracking is likely to occur when carbon is present in the surface layer of a steel sheet, keeping the carbon concentration in the surface layer of the steel sheet low is effective in preventing LME cracking. Normally, when a steel sheet is heated, such as during annealing, external oxidation occurs, forming oxides (scale) on the steel sheet surface, making it difficult for decarburization to proceed. Therefore, the carbon concentration in the surface layer of the steel sheet is unlikely to decrease. Meanwhile, in the steel sheet constituting the welded joint of the present invention, a region where the carbon concentration measured by GDS is 0.01% or less exists 3 μm or more from the steel sheet surface in the depth direction from the steel sheet surface. This means that the carbon concentration in the surface layer of the steel sheet is low.

[0020] However, even if the C concentration in the surface layer of the steel sheet is low, if C is present as carbide (cementite), the hot-dip coating may penetrate into the steel sheet along the grain boundaries of the cementite and become the initiation point of LME cracking. Therefore, it is believed that the presence of C in the surface layer of the steel sheet as cementite may lead to a decrease in LME resistance. Therefore, in the steel sheet constituting the welded joint of the present invention, the layer having an area ratio of cementite of 10% or less has a thickness of 8 μm or more in the depth direction from the steel sheet surface. In other words, the steel sheet constituting the welded joint of the present invention has improved LME resistance during the production of the welded joint by controlling the form of C in the surface layer.

[0021] Thus, the present inventors have found that in order to reduce the area ratio of cementite in the surface layer, it is important to impart strong strain to the surface layer of a steel sheet and then anneal it, as well as to control the dew point during annealing. In the present invention, by imparting a predetermined surface roughness to the surface of a steel sheet and imparting strong strain to the surface layer, it is possible to promote oxygen diffusion into the steel sheet and reduce the C concentration in the surface layer of the steel sheet. The present inventors have also found that by setting the C concentration and cementite area ratio in the surface layer of a steel sheet as described above, a structure with a low cementite area ratio can be maintained even during the production of a welded joint, and LME resistance during the production of a welded joint can be improved, and have thus completed the present invention.

[0022] The present invention will be described in detail below.

[0023] <<Welded Joint>> First, with reference to FIG. 1 , a welded joint of the present invention will be described. As shown in FIG. 1( a), the welded joint of the present invention is produced by spot welding a plurality of (two in FIG. 1 ) steel sheets 1 using a welding electrode A. In this embodiment, a plurality of (two in FIG. 1 ) steel sheets 1 are spot-welded in an overlapping state. Spot welding is performed in the region where the steel sheets overlap when viewed from a direction perpendicular to their surfaces. In this embodiment, the surfaces where the steel sheets overlap and come into contact with each other are referred to as overlapping surfaces 8. In this case, if only one of the overlapping steel sheets is a plated steel sheet, the surface where the plated surface of one plated steel sheet comes into contact with the surface of the other unplated steel sheet is referred to as the "lapped surface." Furthermore, if both overlapping steel sheets are plated steel sheets, the surface where the plated surfaces of the respective steel sheets come into contact is referred to as the "lapped surface." In the case where none of the overlapping steel sheets is plated, the surface where the steel sheet surfaces overlap is referred to as the "overlapping surface."

[0024] FIG. 1( b ) shows a welded joint formed by spot welding two steel plates. The welded joint of the present invention includes multiple overlapping steel plates 1 and a spot weld 2 joining the multiple steel plates. The spot weld 2 includes a nugget 3, an indentation 4 formed by the welding electrode, and a weld shoulder 5. The weld shoulder 5 refers to the sloped portion of the edge (periphery) of the indentation 4. A heat-affected zone 6 is formed around the spot weld 2, which is a portion of the unmelted steel plate where the structure, metallurgical properties, mechanical properties, etc. have been changed by the welding heat. The portion other than the heat-affected zone 6 is the "non-heat-affected zone." The portion of the spot weld 2 that is 5 mm or more away from the outer edge of the nugget can be determined to be the non-heat-affected zone.

[0025] As described below, the multiple steel sheets that constitute the welded joint of the present invention can include both plated steel sheets and unplated steel sheets. Furthermore, one or more of the multiple steel sheets is a high-strength steel sheet having a tensile strength of 780 MPa or more. If at least one of the multiple steel sheets is a high-strength steel sheet having a tensile strength of 780 MPa or more, the other steel sheets may be steel sheets having a tensile strength of less than 780 MPa or steel sheets having a tensile strength of 780 MPa or more.

[0026] In the following description, the term "high strength steel plate" refers to a steel plate that constitutes the welded joint of the present invention and has a tensile strength of 780 MPa or more.

[0027] As described above, LME cracking occurs when hot-dip galvanization is present on the surface of a high-strength steel sheet during welding. For example, considering a welded joint consisting of two steel sheets, if at least one of the two steel sheets is high-strength and has hot-dip galvanization on the overlapping surface, LME cracking may occur at the position corresponding to the overlapping surface. Alternatively, even if one steel sheet is a relatively low-strength galvanized steel sheet that has hot-dip galvanization at least on the overlapping surface and the other steel sheet is an unplated high-strength steel sheet, hot-dip galvanization may be present on the overlapping surface of the steel sheets during welding, and the hot-dip galvanization will come into contact with the unplated high-strength steel sheet, which may result in LME cracking in the high-strength steel sheet. Since the welded joint of the present invention suppresses LME cracking during manufacturing even in such cases, it may also include a welded joint in which one steel sheet is a relatively low-strength galvanized steel sheet (but has hot-dip galvanization at least on the overlapping surface) and the other steel sheet is an unplated high-strength steel sheet. In addition, the welded joint of the present invention may also include a welded joint in which one side is made of a relatively low-strength unplated steel plate and the other side is made of a high-strength steel plate that has been zinc-plated, or a welded joint in which one side is made of a relatively low-strength zinc-plated steel plate and the other side is made of a high-strength steel plate that has been zinc-plated, or a welded joint in which both sides are made of high-strength steel plates that have been zinc-plated.

[0028] For example, in the welded joint of the present invention, at least one of the pair of adjacent steel plates among the plurality of steel plates may have a plating layer containing Zn, and at least one of the pair of steel plates may be a high-strength steel plate having a tensile strength of 780 MPa or more.

[0029] When at least one of the steel sheets constituting a welded joint is a high-strength steel sheet and the high-strength steel sheet is located at the outermost position (at a position where the welding electrode can come into direct contact with the high-strength steel sheet during spot welding), LME cracking may occur during spot welding, even if all of the steel sheets constituting the welded joint are unplated steel sheets. Specifically, if another steel sheet having a zinc-plated outermost layer is spot-welded immediately before the spot welding, the molten zinc may adhere to the welding electrode. When such a welding electrode is used, the plating adhered to the welding electrode may melt during spot welding and adhere to the unplated high-strength steel sheet. This may cause the molten zinc to penetrate the surface of the high-strength steel sheet, resulting in LME cracking. The welded joint of the present invention can also suppress such LME cracking during the production of the welded joint, even if all of the steel sheets constituting the welded joint are unplated steel sheets.

[0030] [Tensile Strength of High-Strength Steel Plate] The present invention suppresses LME that occurs in high-strength steel plates during the production of welded joints. At least one of the steel plates constituting the welded joint according to the present invention is a high-strength steel plate. Specifically, the high-strength steel plate is a steel plate having a tensile strength of 780 MPa or more. The upper limit of the tensile strength is not particularly limited, but from the viewpoint of ensuring toughness, it may be, for example, 2000 MPa or less. The tensile strength may be measured in accordance with JIS Z 2241:2011 using a JIS No. 5 tensile test specimen whose longitudinal direction is perpendicular to the rolling direction and the plate thickness direction. The tensile strength may be 880 MPa or more, 980 MPa or more, 1080 MPa or more, or 1180 MPa or more. The tensile strength may be 1900 MPa or less, or 1800 MPa or less.

[0031] When the rolling direction of a steel sheet is unclear, the following method, for example, is employed to identify the rolling direction of the steel sheet. After mirror-polishing a thickness cross section of the steel sheet, the S concentration is measured using an electron probe microanalyzer (EPMA). Measurement conditions include an acceleration voltage of 15 kV and a measurement pitch of 1 μm, and a distribution image is measured over a 500 μm square area in the center of the sheet thickness. At this time, an elongated region with a high S concentration is determined to be an inclusion such as MnS. Observation may be performed from multiple fields of view. Next, using the thickness cross section initially observed by the above method as a reference, a plane parallel to a plane rotated in 5° increments in the range of 0° to 180° around the thickness direction is observed by the above method. The average value of the major axis lengths of the multiple inclusions in each obtained cross section is calculated for each cross section, and the cross section with the largest average value of the major axis length of the inclusions is identified. The direction parallel to the longitudinal axis of the inclusions in the cross section is determined to be the rolling direction.

[0032] If it is not possible to obtain test specimens for measuring tensile strength from the steel plates constituting the welded joint, the hardness (Vickers hardness) of the steel plate can be measured in a non-heat-affected zone located at a distance of 5 mm or more from the outer edge of the spot weld, and the tensile strength value can be estimated using the following correlation equation (Correlation Between Static Strength Parameters, Hasegawa, Norihiko, Arai, Junichi, and Tanaka, "Materials," Vol. 39, No. 442, pp. 859-863). Here, the "heat-affected zone" refers to the unmelted portion of the steel plate where changes in structure, metallurgical properties, mechanical properties, etc. have occurred due to welding heat, and the "non-heat-affected zone" refers to the portion other than the heat-affected zone. The portion 5 mm or more from the outer edge of the spot weld can be determined to be the non-heat-affected zone.

[0033] Hv = 0.301 x TS + 5.701 (where Hv is Vickers hardness and TS is tensile strength (unit: MPa)). In other words, if the hardness is about 240 Hv or more, it can be considered that the tensile strength is 780 MPa or more.

[0034] The hardness of the steel plate is measured at a half-depth position in the non-heat-affected zone of the steel plate constituting the welded joint. The hardness measurement is performed in accordance with JIS Z 2244:2009. The measurement load is 200 gf. The hardness of the high-strength steel plate in the non-heat-affected zone located 5 mm or more away from the outer edge of the spot weld may be 245 Hv or more, 250 Hv or more, 260 Hv or more, 270 Hv or more, 300 Hv or more, or 340 Hv or more.

[0035] [Chemical Composition of High-Strength Steel Plate] Next, the chemical composition of the high-strength steel plate constituting the welded joint will be described. Hereinafter, "%" 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.

[0036] (C: 0.08 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 0.08 to 0.40%, taking into consideration the balance with weldability. The C content may be 0.10% or more, 0.12% or more, or 0.15% or more. The C content may be 0.40% or less, 0.35% or less, or 0.30% or less.

[0037] (Si: 0.4 to 2.0%) 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 0.4 to 2.0%. The Si content may be 0.5% or more, 0.6% or more, 0.7% or more, or 0.8% or more. The Si content may be 1.8% or less, 1.7% or less, 1.6% or less, or 1.5% or less.

[0038] (Sol. Al: 0 to 2.0%) Al (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 high-strength steel plate that constitutes the welded joint of the present invention, the role of Al can also be fulfilled by the inclusion of Si. Therefore, Al is not essential, and the lower limit of the sol. Al content is 0%. From the viewpoint of LME resistance, the sol. Al content is set to 2.0% or less. The sol. Al content may be 0.1% or more, 0.2% or more, or 0.3% or more. The sol. Al content may be 1.5% or less, 1.2% or less, or 1.0% or less.

[0039] Since Si and Al are elements that reduce LME resistance, the total content of Si and sol. Al is preferably less than 1.8%, and may be less than 1.7%, or less than 1.6%.

[0040] (Mn: 0.1 to 5.0%) Mn (manganese) is an element effective in improving the strength of steel by obtaining a hard structure. Taking into consideration the balance between the strength of the steel and the reduction in workability due to Mn segregation, the Mn content is set to 0.1 to 5.0%. 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.

[0041] (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.

[0042] (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.

[0043] (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 production costs, the N content may be more than 0%, 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0044] (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. The B content may be 0.0002% or more, or 0.0003% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the B content is set to 0.0100% or less. The B content may be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less.

[0045] (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 Ti 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.

[0046] (Nb: 0 to 0.150%) 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 0.002% or more. The Nb content may be 0.005% or more, or 0.010% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the Nb content is set to 0.150% or less. The Nb content may be 0.100% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0047] (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, but when V is contained, the V content is preferably 0.003% or more. The V content may be 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.

[0048] (Cr: 0 to 2.00%) Cr (chromium) is effective in improving the hardenability of steel and increasing its strength, and 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 0.01% or more. The Cr content may be 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.

[0049] (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 contained, but when Ni is contained, the Ni content is preferably 0.001% or more, more preferably 0.02% or more, and even more preferably 0.05% or more. On the other hand, since excessive addition of Ni increases costs, the Ni content is 2.00% or less, preferably 1.80% or less, more preferably 1.50% or less, and even more preferably 0.50% or less, and 0.20% or less.

[0050] (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, cracking of the slab after casting, and a decrease in weldability, 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.

[0051] (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 0.01% or more. The Mo content may be 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.

[0052] (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 is 0.800% or less, 0.600% or less, 0.300% or less, 0.100% or less, or 0.020% or less.

[0053] (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, and therefore 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.

[0054] (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, and therefore 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.

[0055] (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, or 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, or 0.030% or less.

[0056] (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.

[0057] (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, but when 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, since excessive content may cause deterioration of surface properties to become apparent, 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.

[0058] The balance of the steel plate constituting the welded joint according to the present invention, other than the above chemical components, consists of Fe and impurities. Here, the impurities refer to components that are mixed in during industrial production of steel plate due to various factors in the manufacturing process, including raw materials such as ore and scrap, and that do not adversely affect LME cracking during production of the welded joint according to the present invention.

[0059] The chemical components of the steel sheet may be analyzed by an elemental analysis method known to those skilled in the art, for example, inductively coupled plasma mass spectrometry (ICP-MS). However, C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method. These analyses may be performed on samples collected from the steel sheet by a method in accordance with JIS G0417:1999.

[0060] Next, the structure of the vicinity of the welded portion of the welded joint will be described. First, the structure of the high-strength steel plate in the non-heat-affected zone will be described.

[0061] [C concentration] In the welded joint of the present invention, in a non-heat-affected zone that is 5 mm or more away from the outer end of the spot weld, the C concentration measured by GDS (glow discharge spectroscopy) in the depth direction from the surface of the high-strength steel plate is 0.01% or less to a depth of 3 μm or more.

[0062] Since the susceptibility to LME decreases as the carbon concentration decreases, a low carbon concentration in the surface layer improves LME resistance. Furthermore, since carbon is an austenite stabilizing element, a low carbon content stabilizes the layer with low LME susceptibility, as described below.

[0063] Such a surface layer structure can be obtained by setting the chemical composition of the steel sheet as described above and subjecting it to the pretreatment and heat treatment described below.

[0064] Since the effect of improving LME resistance can be obtained as long as the depth at which the C concentration is 0.01% or less is 3 μm or more, there is no particular upper limit to the depth. The depth at which the C concentration is 0.01% or less may be 50 μm or less, 40 μm or less, or 30 μm or less. The depth at which the C concentration is 0.01% or less may be 5 μm or more, 7 μm or more, 10 μm or more, 15 μm or more, or 20 μm or more.

[0065] The C concentration is measured by GDS five times in the thickness direction of the high-strength steel plate at different positions on the surface, and the average value of these measurements is taken as the C concentration. The measurement conditions are as follows:

[0066] 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

[0067] The starting point of the "depth" for GDS measurement is the surface of the high-strength steel sheet, or, if the high-strength steel sheet has a coating layer (described later), the interface between the steel sheet and the coating layer. The interface between the steel sheet and the coating layer is defined as the position where the Fe concentration measured by GDS measurement is 93% of the Fe concentration at a depth of 150 μm.

[0068] [Surface Roughness] In the welded joint of the present invention, in the non-heat-affected zone located 5 mm or more away from the outer edge of the spot weld, the surface of the high-strength steel sheet, or, if the high-strength steel sheet has a plating layer described below, the roughness of the interface between the steel sheet and the plating layer is greater than 3.0 μm in terms of arithmetic mean roughness Ra defined by JIS B0601:2013. In the pretreatment process described below, strain is introduced while increasing the surface roughness of the steel sheet. This improves LME resistance. The upper limit of the surface roughness is not limited, but may be 20.0 μm or less, 15.0 μm or less, or 10.0 μm or less. The roughness of the interface between the steel sheet and the plating layer may be the surface roughness of the steel sheet measured after removing the plating. The plating is removed by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the steel sheet has been added.

[0069] Next, the structure of the heat-affected zone of the high-strength steel sheet will be described.

[0070] [Low-cementite layer] In the welded joint of the present invention, in the heat-affected zone located at a distance of 0 to 100 μm outward from the outer end of the weld shoulder, the thickness of a layer in which the area ratio of cementite is 10% or less (hereinafter referred to as "low-cementite layer") is 8 μm or more in the depth direction from the surface of the high-strength steel plate.

[0071] FIG. 2 shows an example of a microstructure photograph of a plated high-strength steel sheet as an example of a steel sheet constituting a welded joint of the present invention. FIG. 2 is an SEM microstructure photograph taken at 1000x magnification of the surface layer of a plated high-strength steel sheet. FIG. 2 shows a cross section parallel to the thickness direction of the steel sheet, with the upper side of the drawing representing the steel sheet surface, and shows the plated layer 13 and its surface. The surface layer of the steel sheet contains a low-cementite layer 11, which has a low carbon concentration, is composed mainly of ferrite 14, and has an area ratio of cementite 15 of 10% or less. A hard structure 12 containing a relatively large amount of cementite 15 is present closer to the interior of the steel sheet (lower side of the drawing) than the low-cementite layer 11. The relatively bright (bright) layer on the surface side of the low-cementite layer 11 (upper side of the drawing) is the plated layer 13. Ferrite and cementite can be distinguished by the difference in brightness in the SEM image. If the steel plate portion in Figure 1 is classified into areas with relatively high brightness (bright) and areas with relatively low brightness (dark), the areas with relatively high brightness can be determined to be cementite, and the areas with relatively low brightness can be determined to be ferrite.

[0072] Since cementite tends to segregate at grain boundaries, if the cementite content is high, the molten coating penetrates the grain boundaries, making LME more likely to occur. Therefore, by providing a thick low-cementite layer, LME is less likely to occur even when the coating is molten, and LME resistance can be improved. Such a surface layer structure can be obtained by setting the chemical composition of the high-strength steel sheet as described above and performing the pretreatment and heat treatment described below.

[0073] Since the effect of improving LME resistance can be obtained as long as the low cementite layer has a thickness of 8 μm or more, there is no particular upper limit to the thickness. The thickness of the low cementite layer may be 50 μm or less, 40 μm or less, or 30 μm or less. The thickness of the low cementite layer may be 10 μm or more, or 20 μm or more.

[0074] The structure of the low-cementite layer other than cementite is not limited. For example, it may be one or more of martensite, bainite, and ferrite. Since ferrite has low LME susceptibility, a structure mainly composed of ferrite is preferable from the viewpoint of improving LME resistance.

[0075] The thickness of the low cementite layer is determined by nital etching the C-section of the steel sheet (a thickness-wise cross section parallel to the rolling direction (L-direction)) and observing a 50 μm × 50 μm field of view including the surface layer of the steel sheet at 1000x magnification using an SEM. From the structural morphology on the SEM image obtained by SEM observation, it is possible to distinguish between hard structures such as martensite and bainite that contain relatively large amounts of cementite and ferrite. The thickness of the low cementite layer is measured within a measurement range of 500 μm in the L-direction, and the measurement range is divided into five ranges spaced 1000 μm apart in the L-direction. The average value of the thickness of the low cementite layer in the thickness direction over the five measurement ranges is taken as the average value. Here, the area ratio of cementite refers to the area ratio determined by observation of the C-section. When an L cross section (a plate thickness cross section perpendicular to the rolling direction) is observed locally in the thickness direction, even if there is a location where the area ratio of cementite exceeds 10%, there is no problem as long as the area ratio of cementite is 10% or less in a C cross section at a depth of up to 5 μm.

[0076] When the high-strength steel sheet has a plating layer described later, the starting point of the thickness of the low-cementite layer having a cementite area ratio of 10% or less is the interface between the high-strength steel sheet and the plating layer.

[0077] [Plated Layer] A Zn-containing plated layer may be provided on part or all of the surface of one or more of the steel sheets constituting the welded joint of the present invention. For example, a plated layer may be provided on the outermost surface of the overlapping steel sheets, or on the surface that forms the overlapping surface. A plated layer may also be provided on both surfaces. The plated layer is not particularly limited as long as it contains Zn. Examples 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, Zn-15%Mg, Zn-20%Al-7%Mg, and Zn-30%Al-10%Mg.

[0078] 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. The acid solution containing the inhibitor may be, for example, a 10 mass % hydrochloric acid solution containing 0.06 mass % inhibitor (Ibit, manufactured by Asahi Chemical Industry Co., Ltd.).

[0079] 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 weight change before and after the plating layer is peeled off by pickling. The thickness of the plating layer may be 5 μm or more, 10 μm or more, 15 μm or more, or 20 μm or more. The thickness of the plating layer may be 40 μm or less, or 30 μm or less. The coating weight of the plating layer is 20 g / m per side. 2 Above, 30g / m 2 Above, 40g / m 2 or more, or 50 g / m 2 The coating weight of the plating layer may be 150 g / m or more per side. 2 Below, 130g / m 2 Below, 120g / m 2 or less than 100 g / m 2 It may be the following:

[0080] Of the multiple steel sheets constituting the welded joint of the present invention, as long as one or more steel sheets constituting the overlapping surfaces are high-strength steel sheets and one or more steel sheets constituting the overlapping surfaces are plated steel sheets, the other steel sheets may be high-strength steel sheets or steel sheets other than high-strength steel sheets. That is, with respect to the two steel sheets constituting the overlapping surfaces, it is possible that both are high-strength steel sheets or only one is a high-strength steel sheet. The properties of the steel sheets other than the high-strength steel sheets are not limited. The tensile strength of the steel sheets other than the high-strength steel sheets may be, for example, 390 MPa or more, 490 MPa or more, or 590 MPa or more.

[0081] In the welded joint of the present invention, when a high-strength steel plate is used as the steel plate of the outermost layer, the occurrence of LME cracking in the outermost layer of the welded joint can also be suppressed. LME cracking in the outermost layer of a welded joint can occur when the steel plate of the outermost layer is a high-strength steel plate with a high C concentration and has zinc plating on the surface layer side, or when hot-dip zinc plating is attached to the welding electrode. Examples of LME cracking in the outermost layer of a welded joint include cracks in an indentation by the welding electrode (cracks directly below the welding electrode) and cracks in an inclined portion (shoulder) formed on the periphery of the indentation (cracks in the weld shoulder).

[0082] Furthermore, with regard to the steel sheets constituting the overlapping surfaces, both steel sheets constituting the overlapping surfaces may be plated, or only one of the steel sheets may be plated. Examples of combinations of steel sheets constituting the overlapping surfaces include: (1) both high-strength steel sheets that are plated; (2) one high-strength steel sheet that is plated and the other high-strength steel sheet that is not plated; (3) one high-strength steel sheet that is plated and the other high-strength steel sheet that is not plated; (4) one high-strength steel sheet that is plated and the other high-strength steel sheet that is not plated; and (5) one high-strength steel sheet that is not plated and the other high-strength steel sheet that is not plated. According to the welded joint of the present invention, LME cracking during manufacturing can be suppressed regardless of any combination.

[0083] The thickness of the high-strength steel plate and the steel plate other than the high-strength steel plate that constitute the weld joint of the present invention is not particularly limited. For example, it can be 0.6 to 3.2 mm. The thickness may be 0.8 mm or more, or 1.0 mm or more. The thickness may be 3.0 mm or less, 2.6 mm or less, 2.4 mm or less, 2.2 mm or less, 2.0 mm or less, or 1.8 mm or less.

[0084] <<Method for manufacturing welded joints>>

[0085] Next, a method for manufacturing a welded joint according to the present invention will be described. First, a method for manufacturing a high-strength steel plate that constitutes a welded joint will be described.

[0086] <Method for manufacturing high-strength steel sheet> The high-strength steel sheet constituting the welded joint according to the present invention can be obtained by a manufacturing method including, for example, 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 grit-blasting the cold-rolled steel sheet, 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 as is without being coiled.

[0087] [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 conventional method such as continuous casting or ingot casting.

[0088] [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 ratio 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 ratio of finish rolling may be 10 to 50%.

[0089] [Coiling process] The hot-rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature may be changed as appropriate depending on the desired metal structure, etc., and may be, for example, 500 to 800°C. The hot-rolled steel sheet may be subjected to a predetermined heat treatment by recoiling before or after coiling. Alternatively, the hot-rolled steel sheet may be pickled after the hot rolling process without being coiled, and then cold-rolled as described below.

[0090] [Cold Rolling Process] After subjecting the hot-rolled steel sheet to pickling or the like, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio of the cold rolling may be appropriately changed depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold rolling process, the sheet may be cooled to room temperature, for example, by air cooling.

[0091] [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.

[0092] The pretreatment involves grit blasting the surface of the cold-rolled steel sheet using an angular abrasive. While there are no particular limitations on the abrasive that can be used, for example, polygonal steel grit with an average particle size of 100 to 500 μm, preferably 120 to 420 μm, and more preferably 180 to 350 μm, can be used. An example of such grit is TGD-30 manufactured by WINOA IKK JAPAN. This allows for the introduction of strain while increasing the surface roughness. The grit projection amount 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. 2 The projection amount per unit time and unit area at this level is 4.0 x 10 -4 kg / (mm 2 The surface roughness of the steel sheet after pretreatment is maintained in the steel sheet and plated steel sheet according to the present invention after the annealing step and plating step (including the alloying step) described below.

[0093] [Annealing step] The pretreated cold-rolled steel sheet is annealed under a high dew point (high dew point annealing). The annealing is performed under a tension of 1 to 20 MPa. Applying tension during annealing makes it possible to introduce strain into the steel sheet more effectively, accelerating decarburization of the surface layer.

[0094] The holding temperature in the annealing step is set to 750 to 900°C. The holding temperature may be set to 770 to 870°C. By setting the holding temperature in this range, it is possible to promote decarburization, reduce the C concentration in the surface layer, and reduce cementite. The rate of temperature increase up to the holding temperature is not particularly limited, but may be set to 1 to 10°C / second.

[0095] The holding time at the holding temperature in the annealing step is set to 40 to 300 seconds. The holding time may be set to 50 to 250 seconds. By setting the holding time in this range, it is possible to promote decarburization, reduce the C concentration in the surface layer, and reduce cementite.

[0096] The atmosphere in the annealing step has a dew point of −30 to 20° C. The dew point may be −10 to 5° C. The atmosphere may be, for example, N ２ -1~10vol%H ２ , or N ２ -2 to 4 vol% H ２ If the dew point is too high or too low, a phase containing oxides of Si, Mn, Al, etc. is formed on the outside of the steel sheet, preventing decarburization. Furthermore, the interdiffusion of the plating components and the steel components is inhibited, resulting in insufficient plating performance.

[0097] By using the manufacturing method including the above-described steps, it is possible to obtain a high-strength steel sheet in which decarburization is promoted in the surface layer of the high-strength steel sheet and cementite is reduced.

[0098] <Method for manufacturing plated steel sheet> The plated steel sheet can be obtained by performing a plating treatment to form a plating layer containing Zn on the high-strength steel sheet manufactured as described above, or on any steel sheet that can constitute the weld joint according to the present invention.

[0099] The plating process may be performed according to a method known to those skilled in the art. The plating process may be performed, for example, by hot-dip galvanization or electroplating. Preferably, the plating process is performed by hot-dip galvanization. The plating conditions may be appropriately set taking into consideration the chemical composition, thickness, and coating weight of the desired plating layer. For example, the steel sheet may be immersed in a hot-dip galvanizing bath of adjusted chemical composition at 420 to 480°C for 1 to 10 seconds, and then withdrawn at 20 to 200 mm / sec, with the coating weight controlled by N2 wiping gas. After the plating process, an alloying process may be performed to obtain alloyed plating. The alloying process may be performed, for example, at 500 to 550°C for 10 to 60 seconds.

[0100] <Spot welding process> The above-described steel plate and high-strength steel plate are overlapped and spot-welded to obtain a welded joint. A plurality of steel plates may be overlapped as long as the high-strength steel plates are overlapped. Among the overlapping steel plates, the high-strength steel plate may be overlapped at any position (in the lamination direction or in the plate thickness direction) as long as it can exhibit LME resistance. The spot welding conditions are not particularly limited. For example, spot welding can be performed using a dome radius welding electrode with a tip diameter of 8 mm, with a pressure of 4.0 kN, a current flow time of 0.5 seconds, and a current flow of 12 kA.

[0101] The welded joint according to the present invention is capable of suppressing LME cracking during manufacturing, and therefore can be suitably used in a wide range of fields such as automobiles, home appliances, building materials, etc. It is particularly preferable that it be used in the automobile field.

[0102] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0103] Example No. 1 Molten steel adjusted to the chemical composition shown in No. 1 of 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 end 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.

[0104] Next, TGD-30 manufactured by WINOA IKK JAPAN was used as a shot material and shot at a dose of 5 kg / m onto the cold-rolled steel sheet. ２ Grit blasting was performed.

[0105] Before the annealing step, the surface roughness of the steel sheet was measured in accordance with JIS B 0601: 2013. That is, 10 locations were randomly selected on the surface of the steel sheet, the surface profile at each location was measured using a contact surface roughness meter, and the arithmetic mean roughness Ra obtained by arithmetically averaging the surface roughness at these locations was evaluated as follows.

[0106] Grade AA: Over 4.0 μm Grade A: Over 3.0 μm, 4.0 μm or less Grade B: 3.0 μm or less

[0107] Then, the dew point was set to 0°C, the holding temperature was set to 800°C, and the holding time was set to 40 seconds. The oxygen concentration was set to 20 ppm or less in the furnace. ２ -4% H ２ Annealing was performed in a gas atmosphere. The temperature increase rate during annealing was 6.0°C / sec up to 500°C, and 2.0°C / sec from 500°C to the holding temperature. The annealing was performed under a tension of 5.0 MPa.

[0108] Following the annealing treatment, the steel sheet was subjected to a plating treatment and then to an alloying treatment to obtain a galvannealed steel sheet (a high-strength steel sheet with an alloyed hot-dip galvannealed coating). The plating treatment involved immersion in a 450°C hot-dip galvanizing bath (Zn-0.14% Al) for 3 seconds. After immersion, the steel sheet was withdrawn at 100 mm / s, and the coating weight was reduced to 50 g / m using N2 wiping gas. 2 The alloying treatment was carried out at 520°C for 30 seconds.

[0109] Two of the manufactured alloyed hot-dip galvanized steel sheets were overlapped with each other, and spot welding was performed using a dome radius type welding electrode with a tip diameter of 8 mm at an impact angle of 2°, a pressure of 4.0 kN, a current flow time of 0.5 seconds, and a current flow current of 12 kA to manufacture a welded joint, and the LME resistance during manufacturing was evaluated.

[0110] Examples 2 to 28, Comparative Examples 29 to 40 Welded joints were produced in the same manner as in Example 1, except that the chemical compositions were as shown in Table 1, the conditions for the pretreatment step and annealing step were as shown in Table 2, and the plating type was as shown in Table 3, and the LME resistance during production was evaluated. Note that No. 36 was not subjected to grit blasting, and No. 40 was subjected to surface treatment by grinding with a brush instead of grit blasting.

[0111] In Table 3, "a" indicates Zn-0.14%Al galvannealed hot-dip galvanization, "b" indicates Zn-0.14%Al galvannealed hot-dip galvanization without alloying treatment, and "c" indicates Zn-1.5%Al-1.5%Mg. "Unplated" indicates that no plating was applied. Note that No. 2, an example of an "unplated" steel sheet that was not plated, was overlapped with No. 1, a galvannealed steel sheet, to produce a welded joint.

[0112]

[0113]

[0114] <Evaluation of Microstructure Around Weld> (Depth Where C Concentration is 0.01% or Less) A sample was taken by cutting the welded joint, and five GDS measurements were performed in the sheet thickness direction at a position that would be the non-heat-affected zone, 5 mm or more away from the outer edge of the nugget of the spot weld, starting from the interface between the plating layer and the steel sheet. The depth where the C concentration was 0.01% or less was determined, and this is shown in the column "C≦0.01% depth" in Table 3.

[0115] (Roughness of steel sheet surface or steel sheet / plating interface) Furthermore, the surface roughness of the high-strength steel sheet, or the surface roughness of the exposed high-strength steel sheet after removing the plating in the case of a plating-equipped high-strength steel sheet, was measured at a position that would become a non-heat-affected zone 5 mm or more away from the outer edge of the nugget of the spot weld in the same manner as before annealing, and the results are shown in the column "Roughness of steel sheet surface or steel sheet / plating interface" in Table 3.

[0116] (Thickness of Layer with Area Ratio of Cementite of 10% or Less) The thickness of the low-cementite layer was measured from the surface of the high-strength steel plate in the thickness direction of the steel plate at a position 0 to 100 μm outward from the outer end of the weld shoulder, and the results are shown in the column of "Cementite Thickness of 10% or Less" in Table 3.

[0117] (Tensile strength evaluation) For each steel plate used in producing a welded joint, a JIS No. 5 tensile test piece was taken, with the longitudinal direction being perpendicular to the rolling direction and the plate thickness direction, and a tensile test was carried out in accordance with JIS Z 2241:2011 to determine the tensile strength, which was evaluated as follows: Steel plates that received a rating of A or higher (i.e., ratings A, AA, and AAA) correspond to high-strength steel plates that constitute the welded joint of the present invention.

[0118] Rating AAA: 1180 MPa or more Rating AA: 980 MPa or more, less than 1180 MPa Rating A: 780 MPa or more, less than 980 MPa Rating B: Less than 780 MPa

[0119] (LME Resistance) Evaluation of LME resistance will be described with reference to FIG. 3 . LME resistance was evaluated by the length of an LME crack (crack 24 immediately outside the pressure welded portion) that occurred immediately outside the pressure welded portion 23 of the spot welded portion 22 formed by spot welding two overlapping steel sheets 21 (hereinafter referred to as "immediately outside the pressure welded portion"). The "immediately outside the pressure welded portion" refers to a portion of the overlapping surface of the two steel sheets outside the portion (pressed portion) that was press-welded by spot welding, and refers to a position in the vicinity of the pressure welded portion (within a range of approximately 1 mm outward from the edge of the pressure welded portion). The length of the crack 24 immediately outside the pressure welded portion was evaluated. The evaluation criteria were as follows. In this example, a rating of A or higher (i.e., rating A, AA, AAA) was determined to have excellent LME resistance. Note that the spot welding test was performed multiple times until cracks 24 immediately outside the pressure welded portion occurred three or more times, and the crack 24 immediately outside the pressure welded portion with the longest length was evaluated.

[0120] Grade AAA: 0 μm (no cracks) Grade AA: More than 0 μm, less than 60 μm Grade A: 60 μm or more, less than 120 μm Grade B: 120 μm or more

[0121] The results of each evaluation are shown in Table 3.

[0122]

[0123] No. 29 is a comparative example in which the steel sheet has a high C content. It is believed that the high C content of the steel sheet prevented the C concentration in the surface layer of the steel sheet from decreasing even after high-dew-point annealing. Therefore, the depth at which the C concentration in the non-heat-affected zone was 0.01% or less and the thickness of the layer in the heat-affected zone where the cementite area ratio was 10% or less were reduced. Furthermore, the roughness of the steel sheet / coating interface was reduced. As a result, the LME resistance during the production of welded joints was poor.

[0124] No. 30 is a comparative example in which the Si content of the steel sheet is low. It is believed that because the Si content of the steel sheet was low, decarburization did not progress in the surface layer even when high-dew-point annealing was performed. Therefore, the depth where the C concentration in the non-heat-affected zone was 0.01% or less and the thickness of the layer where the cementite area ratio in the heat-affected zone was 10% or less were small. Furthermore, the roughness of the steel sheet / coating interface was small. As a result, the LME resistance during weld joint production was poor.

[0125] No. 31 is a comparative example in which the steel sheet had a high Si content. Because the steel sheet had a high Si content, external oxidation progressed, even when high-dew-point annealing was performed, resulting in the formation of oxides (scale) on the surface layer of the steel sheet, which is thought to have suppressed decarburization at the outermost surface. As a result, the depth at which the C concentration in the non-heat-affected zone was 0.01% or less and the thickness of the layer in the heat-affected zone where the cementite area ratio was 10% or less were reduced. Furthermore, the roughness of the steel sheet / coating interface was reduced. As a result, the LME resistance during weld joint production was poor.

[0126] No. 32 is a comparative example in which the steel sheet had a high sol. Al content. Because the steel sheet had a high sol. Al content, external oxidation progressed, forming oxides (scale) on the surface layer of the steel sheet even after high-dew-point annealing, which is thought to have suppressed decarburization at the outermost surface. As a result, the thickness of the layer in the heat-affected zone with a cementite area ratio of 10% or less was reduced. Furthermore, the roughness of the steel sheet / coating interface was reduced. As a result, the LME resistance during weld joint production was poor.

[0127] In No. 33, the holding temperature during annealing was low, which is thought to have prevented sufficient decarburization during annealing. As a result, the thickness of the layer in the heat-affected zone where the cementite area ratio was 10% or less was small. In addition, the roughness of the steel sheet / coating interface was small. As a result, the LME resistance during weld joint production was poor.

[0128] In No. 34, the holding temperature during annealing was high, which is thought to have prevented sufficient decarburization during annealing. As a result, the depth of the non-heat-affected zone where the C concentration was 0.01% or less was small. In addition, the roughness of the steel sheet / coating interface was small. As a result, the LME resistance during weld joint production was poor.

[0129] In No. 35, the holding time during annealing was short, which is thought to be why decarburization was not sufficiently promoted during annealing. As a result, the depth of the non-heat-affected zone where the C concentration was 0.01% or less and the thickness of the layer in the heat-affected zone where the cementite area ratio was 10% or less were small. In addition, the roughness of the steel sheet / coating interface was small. As a result, the LME resistance during weld joint production was poor.

[0130] No. 36 is a comparative example in which grit blasting was not performed in the pretreatment step. It is believed that the absence of grit blasting led to no strain being introduced into the surface of the steel sheet, preventing decarburization during annealing. As a result, the depth of the non-heat-affected zone where the C concentration was 0.01% or less and the thickness of the layer in the heat-affected zone where the cementite area ratio was 10% or less were reduced. Furthermore, the roughness of the steel sheet / coating interface was reduced. As a result, the LME resistance during weld joint production was poor.

[0131] No. 37 is a comparative example in which the amount of grit blasting used in the grit blasting treatment was large. As a result, it is believed that the roughness of the steel sheet / coating interface was reduced, decarburization progressed too much, and the tensile strength of the steel sheet was reduced. In addition, the LME resistance during the production of the welded joint was also reduced.

[0132] In No. 38, the dew point during annealing was low, which is thought to have led to the formation of a phase containing oxides of Si, Mn, Al, etc. on the exterior of the steel sheet, preventing decarburization. As a result, the depth of the non-heat-affected zone where the C concentration was 0.01% or less and the thickness of the layer in the heat-affected zone where the cementite area ratio was 10% or less were reduced. Furthermore, the roughness of the steel sheet / coating interface was reduced. As a result, the LME resistance during weld joint production was poor.

[0133] In No. 39, the dew point during annealing was high, which is thought to have led to the formation of a phase containing oxides of Si, Mn, Al, etc. on the exterior of the steel sheet, preventing decarburization. As a result, the depth of the non-heat-affected zone where the C concentration was 0.01% or less and the thickness of the layer in the heat-affected zone where the cementite area ratio was 10% or less were reduced. Furthermore, the roughness of the steel sheet / coating interface was reduced. As a result, the LME resistance during weld joint production was poor.

[0134] No. 40 is a comparative example in which surface treatment was performed by brush grinding instead of grit blasting. It is believed that brush grinding did not introduce sufficient strain into the steel sheet surface, preventing decarburization during annealing. As a result, the depth of the non-heat-affected zone where the C concentration was 0.01% or less and the thickness of the layer in the heat-affected zone where the cementite area ratio was 10% or less were reduced. Furthermore, the roughness of the steel sheet / coating interface was reduced. As a result, the LME resistance during weld joint production was poor.

[0135] Nos. 1 to 28 are examples of the present invention, and had high LME resistance. It was confirmed that examples in which the depth of the non-heat-affected zone where the C concentration was 0.01% or less and the thickness of the layer in the heat-affected zone where the cementite area ratio was 10% or less were large had particularly excellent LME resistance during the production of welded joints.

[0136] 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.

[0137] REFERENCE SIGNS LIST 1 steel plate 2 spot weld 3 nugget 4 depression (indentation) 5 weld shoulder 6 heat-affected zone 8 lapped surface 11 low-cementite layer 12 hard structure 13 plating layer 14 ferrite 15 cementite 21 steel plate 22 spot weld 23 pressure weld 24 crack directly outside pressure weld A welding electrode

Claims

DEPCT6907 / 10 / 25681. A weld joint consisting of more than one stacked thin steel plate and a spot welded section connecting the thin steel plates, where the spot welded section has a fused weld area, a dimpled section pressed by the electrode, and a shoulder of the weld which is the surrounding edge of the dimpled section, at least one thin steel plate is an annealed thin steel plate provided with an annealed layer consisting of Zn which is formed on at least one surface that coincides with the stacked thin steel plates, among the thin steel plates. A minimum of one type of thin steel sheet that forms an overlapping skin is high-strength thin steel sheet with a tensile strength of 780 megapascals or more. The chemical composition of high-strength thin steel sheet includes (in percentage by mass): C: 0.08 to 0.40 percent, Si: 0.4 to 2.0 percent, Mn: 0.1 to 5.0 percent, Sol.Al: 0 to 2.0 percent, P: 0.0300 percent or less, S: 0.0300 percent or less, N: 0.0100 percent or less, B: 0 to 0.0100 percent, Ti: 0 to 0.1500 percent, Nb: 0 to 0.150 percent, V: 0 to 0.150 percent, Cr: 0 to 2.00 percent, Ni: 0 to 2.00 percent, Cu: 0 to 2.0000 percent, Mo: 0 to 1.00 percent, W: 0 to 1.000 percent, Ca: 0 to 0.1000 percent, Mg: 0 to 0.100 percent, Zr: 0 to 0.100 percent, Hf: 0 to 0.100 percent, REM: 0 to 0.1000 percent, and the remainder is Fe and impurities, in the area 5 mm or more from the outer edge of the weld zone, in the depth direction from the surface of the spread steel. High-strength thin steel sheets where the concentration of C, measured by GDS, is 0.01 percent or less, i.e., 3 micrometers or more, and the surface roughness of the high-strength thin steel sheet in the arithmetic mean roughness Ra is greater than 3 micrometers, and in the range of 0 to 100 micrometers from the outer edge of the weld shoulder to the outer thickness in the depth direction from the surface of the high-strength thin steel sheet of the layer with a cementite area ratio of 10 percent or less, i.e., 8 micrometers or more.2.

1. A weld joint under Reputation 1 where at least one of the overlapping surfaces formed by a pair of adjacent thin steel plates, at least one of the adjacent thin steel plates is provided with a quenching layer composed of Zn, and at least one of the adjacent thin steel plates is a high-strength thin steel plate with a tensile strength of 780 MPa or more; 2. A weld joint under Reputation 1 or 2 where the total amount of Si and sol.Al is less than 1.8 percent; 3. A weld joint under Reputation 1 or 2 where the depth in the depth direction of the base thin steel plate, starting from the common surface of the quenching layer and the base thin steel plate, where the concentration of C measured by GDS is 0.01 percent or less, is 5 micrometers or more; 4. A weld joint under Reputation 1 or 2 where the depth in the depth direction of the base thin steel plate, starting from the common surface of the quenching layer and the base thin steel plate, where the concentration of C measured by GDS is 0.01 percent or less, is 7 micrometers or more.

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