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 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 problems, and as a result, have found that by imparting strain to a steel sheet before annealing using a shot material under appropriate conditions to create an appropriate surface state, 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 portion pressed down by an electrode, and a weld shoulder that is the periphery of the indentation portion, and at least one steel plate arranged outermost among the plurality of steel plates 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 measured by GDS is 0.01% or less, and the surface roughness of the high-strength steel plate is 3.0 μm or less in arithmetic mean roughness Ra, and the thickness of a layer having an area ratio of cementite of 10% or less is 5 μm or more in the depth direction from the surface of the high-strength steel plate in a range of 0 to 100 μm from the outer edge of the weld shoulder.

[0011] [2] The welded joint of [1], characterized in that one or more of the plurality of steel plates has a plating layer containing Zn on part or all of the surface thereof.

[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 the GDS is 0.01% or less 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 the GDS is 0.01% or less is 7 μm or more.

[0015] The present invention also includes the following aspects.

[0016] (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 an indentation formed by pressing down with an electrode and a weld shoulder, and at least one of the plurality of steel plates is a plated steel plate comprising a base steel plate and a plating layer containing Zn, and the plating layer is formed on at least a surface corresponding to the overlapping surface of the plurality of steel plates, the plated steel plate has a tensile strength of 780 MPa or more, and the chemical composition of the base 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 2.0%, Ni: 0 to 2.00%, Cu: 0 to 2.0000%, Mo: 0 to 1.00%, W: 0 to 1.00%, 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%, and the balance being Fe and impurities. a non-heat-affected zone located 5 mm or more from the weld shoulder, a depth of 3 μm or more in a depth direction of the base steel sheet starting from the interface between the plating layer and the base steel sheet where the C concentration measured by GDS is 0.01% or less, and a roughness Ra of 3.0 μm or less at the interface between the plating layer and the base steel sheet; and a heat-affected zone located 0 to 100 μm from the weld shoulder, a thickness of 5 μm or more in a depth direction of the base steel sheet starting from the interface between the plating layer and the base steel sheet.

[0017] (2) The welded joint according to (1), characterized in that the total content of Si and sol. Al is less than 1.8%.

[0018] (3) The welded joint according to (1) or (2), 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.

[0019] (4) The welded joint according to (1) or (2), characterized in that the depth at which the C concentration measured by GDS is 0.01% or less is 7 μm or more in the depth direction of the base steel sheet starting from the interface between the plating layer and the base steel sheet.

[0020] According to the present invention, it is possible to obtain a welded joint in which LME cracking during manufacturing is suppressed.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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. For this reason, it is thought 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 thickness in the depth direction from the steel sheet surface of the layer in which the area ratio of cementite is 10% or less is 5 μm or more. 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 morphology of C in the surface layer.

[0026] As described above, 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 perform annealing, as well as to control the dew point during annealing. In the present invention, by imparting strong strain to the surface layer without increasing the surface roughness of the steel sheet, 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 the 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.

[0027] The present invention will be described in detail below.

[0028] <<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."

[0029] 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 a welding electrode, and a weld shoulder 5. The weld shoulder 5 refers to the inclined portion of the edge (periphery) of the indentation 4. An outer weld shoulder 6 is the outer portion of the weld shoulder 5. A heat-affected zone 7 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 7 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.

[0030] 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.

[0031] 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.

[0032] 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 composed of two steel sheets, if at least one of the two steel sheets is high-strength and has hot-dip galvanization at the overlapping surface, LME cracking may occur near the surface of the steel sheet at the position corresponding to the overlapping surface. Furthermore, even if one steel sheet is a relatively low-strength galvanized steel sheet and the other is a high-strength ungalvanized steel sheet, hot-dip galvanization may occur on the overlapping surface of the steel sheets during welding. Therefore, the hot-dip galvanization may come into contact with the surface of the high-strength ungalvanized steel sheet, potentially causing LME cracking. The welded joint of the present invention includes both galvanized and ungalvanized steel sheets that constitute the welded joint. For example, a welded joint composed of one steel sheet with a relatively low strength galvanized steel sheet and the other with a high-strength ungalvanized steel sheet may also be included in the welded joint of this embodiment. 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 sheet and the other side is made of a high-strength galvanized steel sheet, a welded joint in which one side is made of a relatively low-strength galvanized steel sheet and the other side is made of a high-strength galvanized steel sheet, or a welded joint in which both sides are made of high-strength galvanized steel sheets.

[0033] 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 using such a welding electrode, 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 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.

[0034] [Tensile Strength of High-Strength Steel Plate] Since 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 has 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 with the longitudinal direction perpendicular to the rolling 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.

[0035] 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.

[0036] Hv = 0.301 x TS + 5.701 (where Hv is Vickers hardness and TS is tensile strength (unit: MPa))

[0037] 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.

[0038] The hardness of the steel plate is measured at a position in the non-heat-affected zone of the steel plate constituting the welded joint, at a 1 / 2 depth. The hardness measurement is performed in accordance with JIS Z 2244:2009. The measurement load is 200 gf. The hardness of the steel plate in the non-heat-affected zone, which is 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.

[0039] [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.

[0040] (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.

[0041] (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.

[0042] (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.

[0043] 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%.

[0044] (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.

[0045] (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.

[0046] (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.

[0047] (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.

[0048] (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 0.0090% or less, 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0030% or less, or 0.0020% or less.

[0049] (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 0.0002% or more, or 0.0003% or more. On the other hand, if it is contained in excess, coarse TiN may be formed, which may impair toughness, so the Ti content is set to 0.1500% or less. The Ti content may be 0.1350% or less, 0.1200% or less, 0.0900% or less, 0.0600% or less, 0.0450% or less, 0.0300% or less, 0.0150% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0050] (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 0.001% or more. The Nb content may be 0.002% or more, 0.004% or more, 0.006% or more, or 0.007% 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 0.135% or less, 0.120% or less, 0.095% or less, 0.065% or less, 0.050% or less, 0.030% or less, or 0.020% or less.

[0051] (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 0.001% or more. The V content may be 0.003% or more, 0.005% or more, or 0.006% 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 0.135% or less, 0.120% or less, 0.095% or less, 0.065% or less, 0.050% or less, 0.045% or less, 0.025% or less, or 0.020% or less.

[0052] (Cr: 0 to 2.00%) Cr (chromium) is effective in improving the hardenability of steel and increasing its strength, so it may be contained as needed. Since Cr is not an essential element, the lower limit of the Cr content is 0%. This effect can be obtained even with a small amount of Cr content, but when Cr is contained, the Cr content is preferably 0.001% or more. The Cr content may be 0.01% or more, 0.02% or more, 0.04% or more, 0.06% or more, or 0.07% or more. On the other hand, if Cr is 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, 1.60% or less, 1.25% or less, 0.85% or less, 0.65% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0053] (Ni: 0 to 2.00%) Ni (nickel) is effective in improving the hardenability of steel and increasing its strength, and may be contained as needed. Since it is not an essential element, the lower limit of the Ni content is 0%. This effect can be obtained even with a small amount of Ni, but when Ni is contained, the Ni content is preferably 0.001% or more. The Ni content may be 0.01% or more, 0.03% or more, 0.04% or more, or 0.05% or more. On the other hand, since excessive addition of Ni increases costs, the Ni content is set to 2.00% or less. The Ni content may be 1.80% or less, 1.60% or less, 1.25% or less, 0.85% or less, 0.65% or less, 0.40% or less, 0.25% or less, or 0.15% or less.

[0054] (Cu: 0 to 2.0000%) Cu (copper) 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 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 0.0004% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness and 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, 1.6000% or less, 1.2000% or less, 0.8000% or less, 0.6000% or less, 0.4000% or less, 0.2000% or less, 0.1000% or less, 0.0070% or less, 0.0050% or less, 0.0035% or less, 0.0020% or less, or 0.0015% or less.

[0055] (Mo: 0 to 1.00%) Mo (molybdenum) is effective in improving the hardenability of steel and increasing its strength, and may therefore be contained as needed. Since it is not an essential element, the lower limit of the Mo content is 0%. This effect can be obtained even with a small amount of Mo, but when Mo is contained, the Mo content is preferably 0.001% or more. The Mo content may be 0.01% or more, 0.03% or more, 0.05% or more, or 0.06% 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 0.90% or less, 0.80% or less, 0.65% or less, 0.45% or less, 0.35% or less, 0.30% or less, or 0.20% or less.

[0056] (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, but when W is contained, the W content is preferably 0.001% or more. The W content may be 0.002% or more, or 0.003% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the W content is set to 1.000% or less. The W content may be 0.900% or less, 0.800% or less, 0.600% or less, 0.400% or less, 0.300% or less, 0.200% or less, 0.100% or less, 0.050% or less, 0.025% or less, 0.015% or less, or 0.010% or less.

[0057] (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 0.0900% or less, 0.0800% or less, 0.0600% or less, 0.0400% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0050% or less, 0.0025% or less, 0.0015% or less, or 0.0010% or less.

[0058] (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 contained, but when Mg is contained, the Mg content is preferably 0.0001% or more. The Mg content may be 0.0002% or more, 0.0003% or more, or 0.0005% 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, 0.060% or less, 0.040% or less, 0.030% or less, 0.020% or less, 0.010% or less, 0.005% or less, 0.003% or less, or 0.002% or less.

[0059] (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 content, but when Zr is contained, the Zr content is preferably 0.001% or more. The Zr content may be 0.003% or more, 0.006% or more, 0.009% or more, or 0.010% or more. On the other hand, if Zr is contained in excess, deterioration of surface properties may become apparent, so the Zr content is set to 0.100% or less. The Zr content may be 0.090% or less, 0.085% or less, 0.065% or less, 0.050% or less, 0.040% or less, or 0.030% or less.

[0060] (Hf: 0 to 0.100%) Hf (hafnium) is an element that contributes to inclusion control, particularly to finely dispersing 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 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if contained in excess, deterioration of surface properties may become apparent, so the Hf content is set to 0.100% or less. The Hf content may be 0.090% or less, 0.080% or less, 0.060% or less, 0.040% or less, 0.030% or less, 0.020% or less, 0.010% or less, 0.005% or less, 0.003% or less, or 0.002% or less.

[0061] (REM: 0 to 0.1000%) REM (rare earth element) is an element that contributes to inclusion control, particularly to finely dispersing inclusions, and has the effect of increasing toughness, and therefore may be contained as needed. Since it is not an essential element, the lower limit of the REM content is 0%. This effect can be obtained even with a small amount of REM contained, but when REM is contained, the REM content is preferably 0.0001% or more. The REM content may be 0.0003% or more, 0.0004% or more, or 0.0005% or more. On the other hand, if it is contained in excess, deterioration of surface properties may become apparent, so the REM content is set to 0.1000% or less. The REM content may be 0.0900% or less, 0.0800% or less, 0.0600% or less, 0.0400% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0040% or less, 0.0025% or less, or 0.0015% or less. REM is an abbreviation for Rare Earth Metal and refers to elements belonging to the lanthanide series. REM is usually added as misch metal.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] [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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] The GDS measurement is performed five times in the sheet thickness direction, and the average value is taken as the C concentration. The measurement conditions are as follows:

[0070] 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

[0071] 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.

[0072] [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 3.0 μm or less in terms of arithmetic mean roughness Ra defined by JIS B0601:2013. Higher roughness increases the likelihood of cracking due to stress concentration. The interface roughness may be 2.5 μm or less, or 2.0 μm or less in terms of Ra. 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.

[0073] Next, the structure of the heat-affected zone of the high-strength steel sheet will be described.

[0074] [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 from 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 5 μm or more in the depth direction from the surface of the high-strength steel plate.

[0075] Figure 2 shows an example of a microstructure photograph near the surface layer of a high-strength steel plate constituting a welded joint of the present invention. Figure 2 is a SEM microstructure photograph taken at 1000x magnification of the surface layer of a high-strength steel plate constituting a welded joint. Figure 2 shows a cross section parallel to the thickness direction of the steel plate, with the steel plate surface at the top. Figure 2 also shows the vicinity of a weld shoulder 5, which is the inclined portion of a depression created by a welding electrode, and a weld shoulder outer portion 6, which is the outer portion of the weld shoulder 5. The steel plate surface contains a low-cementite layer 11, which has a low C concentration, is mainly composed of ferrite, and has an area ratio of cementite of 10% or less. A hard structure 12 containing a relatively large amount of cementite is present closer to the interior of the steel plate than the low-cementite layer 11 (lower in the figure). Ferrite and cementite can be distinguished by the difference in brightness in the SEM image. If the steel plate portion in Figure 2 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.

[0076] Since cementite tends to segregate at grain boundaries, if the cementite content is high, the molten coating penetrates into the grains, 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 steel sheet as described above and performing the pretreatment and heat treatment described below.

[0077] Since the effect of improving LME resistance can be obtained as long as the low cementite layer has a thickness of 5 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.

[0078] 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.

[0079] 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.

[0080] When the steel sheet has a plating layer described below, 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 steel sheet and the plating layer.

[0081] [Plated Layer] A Zn-containing plated layer may be provided on part or all of the surface of one or more of the multiple steel sheets constituting the welded joint of the present invention. Note that, if the plated layer is provided on a high-strength steel sheet, it does not need to be located at the overlapping surface. For example, the plated layer may be provided on the outermost surface of a high-strength steel sheet among multiple overlapping steel sheets. Specifically, when a high-strength steel sheet provided with a plated layer (hereinafter referred to as the first steel sheet) is overlapped with a steel sheet not provided with a plated layer (hereinafter referred to as the second steel sheet), the first steel sheet may be overlapped on the second steel sheet with the plated layer facing up. In this case, no plated layer is present at the overlapping surface between the first steel sheet and the second steel sheet. Note that, if the first steel sheet is a high-strength steel sheet, the second steel sheet may be either a high-strength steel sheet or a non-high-strength steel sheet. Furthermore, the number of steel sheets is not limited to two, i.e., the first steel sheet and the second steel sheet. Three or more steel sheets may be used, resulting in two or more overlapping surfaces between the high-strength steel sheet and another steel sheet.

[0082] Furthermore, in the welded joint of the present invention, a plating layer may be provided on the surface that constitutes the overlapping surface between the first steel sheet and the second steel sheet. As long as a plating layer is present on the overlapping surface, the plating layer may be provided on both the first steel sheet and the second steel sheet, or on one of the first steel sheet and the second steel sheet. The plating layer is not particularly limited as long as it contains Zn. Examples include Zn-0.2%Al, Zn-0.5%Al, Zn-1.5%Al-1.5%Mg, Zn-20%Al-7%Mg, and Zn-30%Al-10%Mg.

[0083] Note that the case where none of the multiple steel sheets constituting the weld joint of the present invention is provided with a plating layer is also included. For example, when welding a first steel sheet and a second steel sheet, neither of which is provided with a plating layer, if a welding electrode with a plating attached thereto comes into contact with the high-strength steel sheet, the molten zinc from the plating may penetrate into the surface layer of the high-strength steel sheet.

[0084] 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.).

[0085] 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 2The 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:

[0086] 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.

[0087] 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).

[0088] 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.

[0089] <<Method for Manufacturing Welded Joint>> 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.

[0090] <Method for manufacturing high-strength steel sheet> The 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 having 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 shot-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.

[0091] [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.

[0092] [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%.

[0093] [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.

[0094] [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.

[0095] [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.

[0096] The pretreatment involves shot blasting the surface of the cold-rolled steel sheet using a spherical abrasive. While there are no particular limitations on the abrasive that can be used, for example, steel balls with a median particle size of 40 to 450 μm, preferably 120 to 420 μm, and more preferably 180 to 350 μm can be used. For example, TSH30 manufactured by WINOA IKK JAPAN can be used. The shot blasting rate is 5 to 400 kg / m. 2 This makes it possible to introduce strain without increasing the surface roughness. By performing such shot 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. Note that the shot dose is 400 kg / m 2 The level per unit time and unit area 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.

[0097] [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, thereby promoting decarburization of the surface layer.

[0098] 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.

[0099] 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.

[0100] The atmosphere used 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, N2-1 to 10 vol% H2, or N2-2 to 4 vol% H2. By setting the atmosphere in these ranges, decarburization can be promoted, the C concentration in the surface layer can be reduced, and cementite can be reduced. Furthermore, if the dew point is too high or too low, a phase containing oxides of Si, Mn, Al, etc. can be formed on the outside of the steel sheet, inhibiting interdiffusion between the plating components and the steel components and resulting in insufficient plating performance.

[0101] By using a manufacturing method including the above-described steps, decarburization is promoted in the surface layer of the steel sheet, and a steel sheet with reduced cementite can be obtained.

[0102] <Method for producing plated steel sheet> A plated steel sheet can be obtained by performing a plating treatment to form a plating layer containing Zn on the steel sheet produced as described above.

[0103] 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. The alloying process may be performed, for example, at 500 to 550°C for 10 to 60 seconds.

[0104] <Spot welding step> A plurality of the above-described steel sheets are stacked and spot-welded to obtain a welded joint. The conditions for spot welding 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.

[0105] 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.

[0106] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0107] 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.

[0108] Next, the cold-rolled steel sheet was blasted with TSH30 manufactured by WINOA IKK JAPAN Co., Ltd. at a rate of 5 kg / m 2 Shot blasting was carried out.

[0109] 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.

[0110] Grade AA: 2.0 μm or less Grade A: Over 2.0 μm, 3.0 μm or less Grade B: Over 3.0 μm

[0111] Thereafter, annealing was performed in a furnace with an oxygen concentration of 20 ppm or less in an N-4% H gas atmosphere at a dew point of 0°C, a holding temperature of 800°C, and a holding time of 40 seconds. The heating 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.

[0112] 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. 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.

[0113] Two of the manufactured steel plates were overlapped, 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 welding time of 0.5 seconds, and a current of 12 kA to manufacture a welded joint, and the LME resistance during manufacturing was evaluated.

[0114] Examples 2 to 24, Comparative Examples 25 to 37 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 process and annealing process 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 in No. 33, a steel plate with increased surface roughness achieved by skin pass was used. In No. 37, surface treatment was performed by grinding with a brush instead of shot blasting.

[0115] In Table 3, "a" indicates Zn-0.14%Al alloyed hot-dip galvanized steel, "b" indicates Zn-0.14%Al hot-dip galvanized steel without alloying treatment, and "c" indicates Zn-1.5%Al-1.5%Mg. "Unplated" indicates that no plating was performed. Note that for No. 2, an example of an "unplated" steel sheet that was not plated, a welded joint was produced using a welding electrode that had been previously spot-welded 10 or more times with a zinc-containing plated steel sheet.

[0116]

[0117]

[0118] <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 become 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 Table 3 as "C≦0.01% depth."

[0119] (Roughness of steel sheet surface or steel sheet / plating interface) Furthermore, 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, the surface roughness of the high-strength steel sheet, and for plated steel sheets, the surface roughness of the exposed steel sheet after removing the plating, were measured in the same manner as before annealing, and the results are shown in Table 3 under "Roughness of steel sheet surface or steel sheet / plating interface".

[0120] (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 positions 0 to 100 μm outward from the outer end of the weld shoulder, and is shown in "Thickness of Cementite of 10% or Less" in Table 3. The thickness was measured at five equally spaced points within a range of 500 μm, and the average value was calculated.

[0121] (Tensile strength evaluation) For each steel plate used in manufacturing the welded joint, a JIS No. 5 tensile test piece was taken with the longitudinal direction perpendicular to the rolling direction, and a tensile test was performed in accordance with JIS Z 2241:2011 to determine the tensile strength, which was evaluated as follows: A steel plate with an evaluation of A or higher corresponds to a high-strength steel plate constituting the welded joint of the present invention.

[0122] 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

[0123] (LME Resistance) Evaluation of LME resistance will be described with reference to Fig. 3. LME resistance was evaluated based on the length of LME cracks (cracks 21 in the shoulder and outside the shoulder) that occurred in the weld shoulder 5 and outside the weld shoulder 6 of a spot weld 2 formed by overlapping two steel plates 1 and spot welding them together. The evaluation was made as follows based on the length of the cracks 21 in the shoulder and outside the shoulder. In this example, if the evaluation was A or higher, it was determined that the LME resistance was excellent and that the problem of the present invention was solved.

[0124] Rating AAA: 0 μm Rating AA: More than 0 μm, less than 60 μm Rating A: 60 μm or more, less than 120 μm Rating B: 120 μm or more

[0125] The results of each evaluation are shown in Table 3.

[0126]

[0127] No. 25 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 when high-dew-point annealing was performed. 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. As a result, the LME resistance during the production of welded joints was poor.

[0128] No. 26 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. As a result, the LME resistance during weld joint production was poor.

[0129] No. 27 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, forming oxides (scale) on the surface layer of the steel sheet, even when high-dew-point annealing was performed, which is thought to have suppressed decarburization at the outermost surface. 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. As a result, the LME resistance during weld joint production was poor.

[0130] No. 28 is a comparative example in which the sol. Al content of the steel sheet is high. Because the sol. Al content of the steel sheet was high, external oxidation progressed, forming oxides (scale) on the surface layer of the steel sheet, even when high-dew-point annealing was performed, which is thought to have suppressed decarburization on the outermost surface. Therefore, the thickness of the layer in the heat-affected zone with a cementite area ratio of 10% or less was reduced. As a result, the LME resistance during weld joint production was poor.

[0131] In No. 29, the holding temperature during annealing was low, which is thought to be why decarburization was not sufficiently promoted 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. As a result, the LME resistance during weld joint production was poor.

[0132] In No. 30, the holding temperature during annealing was high, 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 was small. As a result, the LME resistance during weld joint production was poor.

[0133] In No. 31, 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 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 was 10% or less in the heat-affected zone were small. As a result, the LME resistance during weld joint production was poor.

[0134] No. 32 did not undergo shot blasting in the pretreatment process, which is thought to be why strain was not introduced into the surface of the steel sheet and decarburization did not progress during annealing. 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 was 10% or less in the heat-affected zone were small. As a result, the LME resistance during weld joint production was poor.

[0135] In No. 33, a steel sheet with a large surface roughness was used, which is thought to have resulted in a large roughness at the steel sheet / coating interface after annealing, making stress concentration more likely to occur. As a result, the LME resistance during weld joint production was poor.

[0136] In No. 34, the amount of shot used in the shot blasting treatment was large, which is thought to have resulted in excessive decarburization, resulting in a decrease in the tensile strength of the steel plate.

[0137] In No. 35, 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. Therefore, 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. As a result, the LME resistance during weld joint production was poor.

[0138] In No. 36, 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. Therefore, 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. As a result, the LME resistance during weld joint production was poor.

[0139] In No. 37, surface treatment was performed by grinding with a brush instead of shot blasting, which is thought to have prevented sufficient strain from being introduced into the surface of the steel sheet, preventing decarburization during annealing. This resulted in a decrease in the depth of the non-heat-affected zone where the C concentration was 0.01% or less, and a decrease in the thickness of the layer in the heat-affected zone where the cementite area ratio was 10% or less. This resulted in poor LME resistance during the production of welded joints.

[0140] On the other hand, Nos. 1 to 24 are examples of the present invention, in which LME was suppressed during production. It was confirmed that the examples in which 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 large had particularly excellent LME resistance during weld joint production.

[0141] According to the present invention, it is possible to provide a welded joint having high LME resistance, and the steel sheet and 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.

[0142] REFERENCE SIGNS LIST 1 steel plate 2 spot weld 3 nugget 4 depression (indentation) 5 weld shoulder 6 outside weld shoulder 7 heat-affected zone 8 lapped surface 11 low-cementite layer 12 hard structure 21 cracks in shoulder and outside shoulder A welding electrode

Claims

DEPCT6906 / 10 / 25681. A weld joint consisting of more than one stacked thin steel plates and a spot welded section connecting more than one thin steel plate, where the spot welded section consists of the weld area, the electrode indentation, and the weld shoulder of the edge of the indentation; at least one thin steel plate arranged on the outermost of the more than one thin steel plate is a high-strength thin steel plate with a tensile strength of 780 MPascals or more. The chemical composition of high-strength thin steel sheets includes (in percentage by 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 1.00%.The concentrations are: 0.00%, 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%, and the remainder is Fe and impurities; in the area separated from the outer end of the weld area by a distance of 5 millimeters or more, in the depth direction from the surface of the high-strength thin steel sheet, the depth at which the concentration of C is measured by GDS is 0.01% or less, i.e., 3 micrometers or more, and the surface roughness of the high-strength thin steel sheet, the arithmetic mean roughness Ra, is 3.0 micrometers.

1. A weld joint under claim 1 where the surface of one or more thin steel plates is partially or entirely coated with a layer containing 10 percent or less cementite; and in the range of 0 to 100 micrometers from the outer end of the weld shoulder, in the depth direction from the surface of the thin high-strength steel plate, the thickness of the layer with a cementite area ratio of 10 percent or less is 5 micrometers or more.

2. A weld joint under claim 1 where the surface of one or more thin steel plates, among more than one, is partially or entirely coated with a layer containing Zn.

3. A weld joint under claim 1 or 2 where the total amount of Si and Sol.

4. Weld joints under claim 1 or 2 where the depth of concentration of C measured by GDS is 0.01 percent or less, i.e., 5 micrometers or more; 5. Weld joints under claim 1 or 2 where the depth of concentration of C measured by GDS is 0.01 percent or less, i.e., 7 micrometers or more.

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