Welded joint

Abstract

The present invention addresses the problem of providing a welded joint in which LME cracks during production are suppressed. This welded joint is characterized by being provided with a high-strength steel plate having a tensile strength of 780 MPa or more, the high-strength steel plate having a prescribed chemical composition, and in a non-heat-affected part located at a distance of 5 mm or more from the outer end of a spot-welded part, the depth at which the C concentration measured by GDS is 0.01% or less in the depth direction of the high-strength steel plate is 3 [mu] m or more, and the thickness of the high-strength steel plate is 10 [mu] m or more. The roughness of the surface of the high-strength steel sheet exceeds 3.0 [mu] m, and the thickness of a layer having an area ratio of cementite of 10% or less in the depth direction of the high-strength steel sheet is 8 [mu] m or more in a heat-affected part located 0-100 [mu] m from the welding shoulder.

Description

Technical Field

[0001] The present invention relates to a welded joint, and more particularly to a welded joint in which LME cracking during manufacturing is suppressed. Background Art

[0002] In recent years, the strength of steel sheets used in various fields, including automobiles, home appliances, and building materials, has been increasing. For example, in the automotive field, the use of welded joints made by spot welding high-strength steel sheets is increasing to improve fuel efficiency and reduce vehicle body weight.

[0003] When welding zinc-plated steel sheets, particularly high-strength steel sheets, deterioration in weldability due to liquid metal embrittlement (LME) cracking can sometimes be a problem, as described in Patent Document 1. LME cracking is believed to occur when the surface layer of the steel sheet transforms into austenite during welding, molten zinc intrudes into the grain boundaries, embrittles the steel sheet, and further tensile stress is applied to the steel sheet during welding.

[0004] It should be noted that Patent Document 2 discloses a steel plate having improved weldability by suppressing LME cracks, wherein the surface layer of the steel plate has 3000 to 6000 cracks / mm. 2 A steel plate in which Si oxide particles having a particle size of 20 nm or more are present in an appropriate particle size distribution at a number density of 100 nm.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: International Publication No. 2019 / 116531

[0008] Patent Document 2: International Publication No. 2020 / 218575 Summary of the Invention

[0009] Problems to be solved by the invention

[0010] In order to prevent LME cracking during the production of welded joints, it is effective to suppress the intrusion of Zn and the like contained in the plating layer into the steel sheet after austenite transformation. In this regard, there is room for improvement.

[0011] The present invention has been made in view of such actual circumstances, and a technical object thereof is to provide a welded joint in which LME cracks are suppressed during manufacturing.

[0012] Means used to solve problems

[0013] The present inventors conducted intensive research into means for solving the above-mentioned technical problems and found that LME can be suppressed by applying strain to a pre-annealed steel sheet under appropriate conditions, thereby achieving an appropriate surface condition, and then performing high dew point annealing to decarburize the steel sheet surface, thereby forming a layer with a low cementite fraction.

[0014] The present invention has been completed as a result of further studies based on the above findings, and the gist of the present invention is as follows.

[0015] (1) A weld joint, characterized in that it comprises a plurality of overlapping steel plates and a spot weld portion joining the plurality of steel plates, the spot weld portion having a nugget, an indentation portion pressed by an electrode, and a weld shoulder portion as a peripheral portion of the indentation portion, at least one of the plurality of steel plates being a plated steel plate having a Zn-containing plating layer 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 constituting the overlapping surface being a tensile strength steel plate. A high-strength steel plate having a strength of 780 MPa or more, wherein the chemical composition of the high-strength steel plate comprises, by mass%, C: 0.08-0.40%, Si: 0.4-2.0%, Mn: 0.1-5.0%, sol.Al: 0-2.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0-0.0100%, Ti: 0-0.1500%, Nb: 0-0.1 50%, V: 0-0.150%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.0000%, Mo: 0-1.00%, W: 0-1.000%, Ca: 0-0.1000%, Mg: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100%, REM: 0-0.1000%, the balance being Fe and impurities, at a distance of 5 m from the outer end of the nugget. m or more, the depth at which the C concentration measured by GDS is 0.01% or less is 3 μm or more from the surface of the high-strength steel plate in the depth direction, and the surface roughness of the high-strength steel plate exceeds 3 μm in terms of arithmetic mean roughness Ra, and within a range of 0 to 100 μm from the outer end of the weld shoulder toward the outside, the thickness of a layer having an area ratio of cementite of 10% or less is 8 μm or more from the surface of the high-strength steel plate in the depth direction.

[0016] (2) The weld joint according to (1) above is 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 coating 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.

[0017] (3) The weld joint according to (1) or (2) above, characterized in that the total content of Si and sol.Al is less than 1.8%.

[0018] (4) A weld joint according to any one of (1) to (3) above, characterized in that, in the depth direction of the base steel plate starting from the interface between the coating layer and the base steel plate, the depth at which the C concentration measured by GDS is 0.01% or less is 5 μm or more.

[0019] (5) A weld joint according to any one of (1) to (3) above, characterized in that, in the depth direction of the base steel plate starting from the interface between the coating layer and the base steel plate, the depth at which the C concentration measured by GDS is 0.01% or less is 7 μm or more.

[0020] Effects of the Invention

[0021] According to the present invention, it is possible to obtain a welded joint in which LME cracking during production is suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a diagram illustrating the welded joint of the present invention.

[0023] Figure 2 This is a diagram showing layers formed around the weld portion of the weld joint of the present invention.

[0024] Figure 3 This is a diagram illustrating the LME resistance evaluation in Examples. DETAILED DESCRIPTION

[0025] The present invention will be described below. The present invention is not limited to the following embodiments. First, a summary of the structure of the welded joint of the present invention that improves LME resistance during manufacture will be described.

[0026] During the manufacture of welded joints, the plating melts when spot welding is performed on the plated steel sheets, and the surface portion of the steel sheets is heated, causing the steel sheet structure to transform into austenite. At this time, the molten plating penetrates into the steel sheet structure along the grain boundaries of the austenite, thereby embrittlement of the grain boundaries. Therefore, when stress is applied to the steel sheet, LME cracks are likely to occur at the grain boundaries. In particular, tensile stress is applied to the steel sheet during welding, so it is believed that LME cracks are likely to occur. The welded joint of the present invention improves the LME resistance of the welded joint during manufacture by forming a structure on the surface layer of the steel sheet constituting the welded joint. It should be noted that, 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.

[0027] When the element C is present in the surface layer of a steel plate, LME cracks are more likely to form. Therefore, keeping the C concentration in the surface layer of the steel plate low is effective in preventing LME cracks. Generally, when a steel plate is heated, as in annealing, external oxidation occurs, forming oxides (scale) on the surface of the steel plate, making decarburization difficult. Consequently, it is difficult to reduce the C concentration in the surface layer of the steel plate. Meanwhile, in the steel plates constituting the welded joint of the present invention, a region having a C concentration of 0.01% or less as measured by GDS exists at a depth of 3 μm or more from the surface of the steel plate. This indicates that the C concentration in the surface layer of the steel plate is low.

[0028] However, even when the carbon concentration in the steel plate surface layer is low, if carbon exists in the form of carbides (cementite), there is a risk that the molten plating will penetrate into the steel plate along the grain boundaries of the cementite, becoming the starting point of LME cracks. Therefore, it is believed that when carbon exists as cementite in the steel plate surface layer, it may sometimes lead to a decrease in LME resistance. Therefore, in the steel plate 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 plate surface. In other words, the steel plate constituting the welded joint of the present invention is a steel plate in which the LME resistance during welded joint manufacturing is improved by controlling the morphology of carbon in the surface layer.

[0029] Thus, the present inventors discovered that in order to reduce the area ratio of cementite in the surface layer, in addition to applying strong strain to the surface layer of the steel plate and annealing it, it is important to control the dew point during annealing. In the present invention, by applying a predetermined surface roughness to the surface of the steel plate and applying strong strain to the surface layer, it is possible to promote the diffusion of oxygen into the interior of the steel plate and reduce the carbon concentration in the surface layer of the steel plate. The present inventors then discovered that by setting the carbon concentration and area ratio of cementite in the surface layer of the steel plate as described above, a structure with a low area ratio of cementite can be maintained during the manufacture of the welded joint, thereby improving the LME resistance of the welded joint during manufacture, thereby completing the present invention.

[0030] Hereinafter, the present invention will be described in detail.

[0031] Welding Joints

[0032] First, refer to Figure 1 The welded joint of the present invention is described. Figure 1 As shown in (a), the welded joint of the present invention is formed by using a welding electrode A to weld multiple ( Figure 1 In this embodiment, a plurality of (in Figure 12 pieces) steel plates 1 are spot welded in an overlapping state. For these multiple steel plates, spot welding is performed on the areas where the steel plates overlap when viewed from a direction perpendicular to their surfaces. In the present embodiment, the surfaces where the steel plates overlap and contact each other are set as overlapping surfaces 8. In this case, when only one of the steel plates to be overlapped is a plated steel plate, the surface where the plated surface of the plated steel plate on one side contacts the surface of the non-plated steel plate on the other side is set as the "overlapping surface". In addition, when all the steel plates to be overlapped are plated steel plates, the surfaces where the plated surfaces of the respective steel plates contact each other are set as the "overlapping surface". When none of the steel plates to be overlapped form plated non-plated steel plates, the surfaces where the steel plate surfaces overlap each other are set as the "overlapping surface".

[0033] exist Figure 1 (b) shows a weld joint in which two steel plates are welded by spot welding. The weld joint of the present invention comprises a plurality of overlapping steel plates 1 and a spot weld portion 2 for joining the plurality of steel plates. The spot weld portion 2 comprises a weld nugget 3, a depression (indentation) 4 pressed down by the welding electrode, and a weld shoulder 5. The weld shoulder 5 refers to the inclined portion of the edge (peripheral portion) of the depression 4. A heat-affected portion 6 is formed around the spot weld portion 2, which is a portion of the steel plate that has not melted and whose structure, metallurgical properties, mechanical properties, etc. have changed due to welding heat. The portion other than the heat-affected portion 6 is a "non-heat-affected portion". The portion that is more than 5 mm away from the outer end of the weld nugget of the spot weld portion 2 can be judged as a non-heat-affected portion.

[0034] As described below, the multiple steel plates constituting the welded joint of the present invention may include both plated steel plates and unplated, non-plated steel plates. Furthermore, at least one of the multiple steel plates may be a high-strength steel plate having a tensile strength of 780 MPa or greater. If at least one of the multiple steel plates is a high-strength steel plate having a tensile strength of 780 MPa or greater, the remaining steel plates may have a tensile strength of less than 780 MPa or a tensile strength of 780 MPa or greater.

[0035] In the following description, the "high-strength steel plate" refers to a steel plate constituting the weld joint of the present invention, and is a steel plate having a tensile strength of 780 MPa or more.

[0036] As mentioned above, LME cracks occur when molten zinc coating is present on the surface of a high-strength steel sheet during welding. For example, consider a welded joint consisting of two steel sheets, where at least one of the two sheets is high-strength and zinc coating is present on the overlapping surface. LME cracks may occur at a location corresponding to the overlapping surface. Furthermore, even if one sheet is a zinc-plated steel sheet with a lower strength, at least on the overlapping surface, and the other sheet is an unplated high-strength steel sheet, LME cracks may occur in the high-strength steel sheet due to the presence of molten zinc coating on the overlapping surface during welding. The molten zinc coating may also come into contact with the unplated high-strength steel sheet, causing LME cracks to form in the high-strength steel sheet. The welded joint of the present invention also suppresses LME cracks during manufacturing in such situations, and therefore may also include welded joints formed by a zinc-plated steel sheet with a lower strength (plated at least on the overlapping surface) and an unplated high-strength steel sheet. In addition, a welded joint formed by a non-plated steel plate with relatively low strength on one side and a high-strength steel plate with zinc plating on the other side, a welded joint formed by a zinc-plated steel plate with relatively low strength on one side and a high-strength steel plate with zinc plating on the other side, or a welded joint formed by high-strength steel plates with zinc plating on both sides, etc., may also be included in the welded joint of the present invention.

[0037] For example, in the weld joint of the present invention, in at least one of the overlapping surfaces formed by a pair of adjacent steel plates among a plurality of steel plates, at least one of the pair 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.

[0038] If at least one of the multiple steel plates comprising a weld joint is a high-strength steel plate, and this high-strength steel plate is positioned on the outermost side of the multiple steel plates comprising the weld joint (where the welding electrode can directly contact the plate during spot welding), LME cracking may occur during spot welding, even if all the steel plates comprising the weld joint are unplated. Specifically, if spot welding is performed on another steel plate with a zinc-plated outermost layer immediately prior, the molten zinc plating may adhere to the welding electrode. When such a welding electrode is used, the plating adhered to the welding electrode melts during spot welding, and this plating may adhere to the unplated high-strength steel plate. As a result, the molten zinc may intrude into the surface layer of the high-strength steel plate, causing LME cracking. The weld joint of the present invention can suppress such LME cracking during manufacture, even when all the steel plates comprising the weld joint are unplated.

[0039] [Tensile strength of high-strength steel sheets]

[0040] The present invention is an invention for suppressing LME that occurs during the manufacture of welded joints of high-strength steel plates. One or more of the steel plates constituting the welded joint of 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; from the perspective of ensuring toughness, it may be, for example, 2000 MPa or less. The tensile strength can be measured using a JIS No. 5 tensile test piece whose longitudinal direction is perpendicular to the rolling direction and the plate thickness direction, in accordance with JIS Z 2241:2011. 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.

[0041] When the rolling direction of the steel plate is unclear, the following method is used as a method for determining the rolling direction of the steel plate. After the plate thickness section of the steel plate is finished by mirror polishing, the S concentration is measured using an electron probe micro analyzer (EPMA). The measurement conditions are to set the acceleration voltage to 15kV, set the measurement interval to 1μm, and measure the distribution image of a 500μm square range in the center of the plate thickness. At this time, the extended area with a high S concentration is judged to be an inclusion such as MnS. Observation can also be performed in multiple fields of view. Then, based on the plate thickness section initially observed by the above method, the above method is used to observe the surface parallel to the surface rotated in the range of 0° to 180° with the plate thickness direction as the axis in 5° units. The average value of the length of the long axis of multiple inclusions in each section calculated according to each section is determined to be the section with the largest average value of the length of the long axis of the inclusion. The direction in the section parallel to the long axis direction of the inclusion is judged as the rolling direction.

[0042] It should be noted that when it is impossible to take a test piece for tensile strength measurement from the steel plate constituting the welded joint, as an alternative, the hardness (Vickers hardness) of the steel plate in the non-heat-affected zone located at a distance of more than 5 mm from the outer end of the spot weld can be measured. The tensile strength value can be estimated based on the following correlation formula (Correlation between Static Strength Parameters, Norihiko Hasegawa, Junichi Arai, Michishichi Tanaka, "Materials", Vol. 39, No. 442, P859-863). Here, "heat-affected zone" refers to the portion of the steel plate that has not melted due to changes in structure, metallurgical properties, mechanical properties, etc. caused by welding heat, and "non-heat-affected zone" refers to the portion other than the heat-affected zone. The portion that is more than 5 mm away from the outer end of the spot weld can be determined to be the non-heat-affected zone.

[0043] Hv=0.301×TS+5.701

[0044] (Here, Hv is Vickers hardness, and TS is tensile strength (unit: MPa)) That is, if the hardness is approximately 240 Hv or higher, it can be considered that the tensile strength is 780 MPa or higher.

[0045] The hardness of the steel plate is measured at a depth of 1 / 2 of the non-heat-affected zone (NAZ) of the steel plate constituting the welded joint. 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 NZ at a distance of 5 mm or more from the outer end of the spot weld can be 245 Hv or higher, 250 Hv or higher, 260 Hv or higher, 270 Hv or higher, 300 Hv or higher, or 340 Hv or higher.

[0046] [Chemical composition of high-strength steel sheets]

[0047] Next, the chemical composition of the high-strength steel plate constituting the welded joint will be described. Hereinafter, "%" in chemical composition refers to "mass %." Furthermore, within the numerical ranges of chemical composition, numerical ranges indicated by "to" refer to ranges that include the numerical values ​​before and after "to" as the lower and upper limits.

[0048] (C: 0.08-0.40%)

[0049] Carbon (C) is an element that ensures the strength of steel. To achieve a tensile strength of 780 MPa or higher, as targeted by the present invention, and considering the balance with weldability, the C content is set to 0.08% to 0.40%. The C content can be 0.10% or higher, 0.12% or higher, or 0.15% or higher. The C content can be 0.40% or lower, 0.35% or lower, or 0.30% or lower.

[0050] (Si: 0.4-2.0%)

[0051] Silicon (Si) is an element that promotes ferrite stabilization and decarburization. The inclusion of Si promotes surface decarburization during the pretreatment and heat treatment described below, stabilizing the ferrite in the surface layer and improving LME resistance. To achieve this effect, the Si content is set to 0.4-2.0%. The Si content can be 0.5% or more, 0.6% or more, 0.7% or more, or 0.8% or more. The Si content can be 1.8% or less, 1.7% or less, 1.6% or less, or 1.5% or less.

[0052] (sol.Al: 0-2.0%)

[0053] Al (aluminum) is an element that promotes ferrite stabilization and decarburization in the same way as Si by being dissolved in steel. Sol.Al refers to acid-soluble Al that is soluble in acid without becoming oxides such as Al2O3. It is determined by subtracting the insoluble residue on the filter paper produced during the analysis of Al. In the high-strength steel plate constituting the welded joint of the present invention, the effect of Al is also obtained by containing 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 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.

[0054] Note that Si and Al are also elements that reduce LME resistance, so the total content of Si and sol.Al is preferably less than 1.8%. The total content of Si and sol.Al can be less than 1.7% or less than 1.6%.

[0055] (Mn: 0.1-5.0%)

[0056] Mn (manganese) is an element effective in increasing the strength of steel sheets by creating a hard structure. Considering the balance between steel strength and reduced workability due to Mn segregation, the Mn content is set to 0.1-5.0%. The Mn content can be 0.5% or higher, 1.0% or higher, or 1.5% or higher. The Mn content can be 4.5% or lower, 4.0% or lower, or 3.5% or lower.

[0057] (P: 0.0300% or less)

[0058] Phosphorus (P) is an impurity commonly found in steel. A P content exceeding 0.0300% may reduce weldability. Therefore, the P content is set to 0.0300% or less. The P content can be 0.0200% or less, 0.0100% or less, or 0.0050% or less. P is preferably absent, and the lower limit of the P content is 0%. From the perspective of dephosphorization costs, the P content may exceed 0%, be 0.0001% or more, or be 0.0005% or more.

[0059] (S: 0.0300% or less)

[0060] S (sulfur) is an impurity commonly found in steel. When the S content exceeds 0.0300%, there is a risk of reduced weldability and decreased workability, such as bendability, due to increased precipitation of MnS. Therefore, the S content is set to 0.0300% or less. The S content can be 0.0100% or less, 0.0050% or less, or 0.0020% or less. S is preferably absent, and the lower limit of the S content is 0%. From the perspective of desulfurization costs, the S content can exceed 0%, be 0.0001% or more, or be 0.0005% or more.

[0061] (N: 0.0100% or less)

[0062] Nitrogen (N) is an impurity commonly found in steel. A N content exceeding 0.0100% may reduce weldability. Therefore, the N content is set to 0.0100% or less. The N content can be 0.0080% or less, 0.0050% or less, or 0.0030% or less. N is preferably absent, and the lower limit of the N content is 0%. From the perspective of manufacturing costs, the N content can be greater than 0%, 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0063] (B: 0 to 0.0100%)

[0064] B (boron) is an element that improves hardenability and contributes to the improvement of strength, and segregates at grain boundaries to strengthen grain boundaries and thus improve toughness, so it can be contained as needed. Since it is not an essential element, the lower limit of the B content is 0%. This effect can be obtained even if a trace amount is contained, but the B content when contained is preferably 0.0001% or more. The B content can 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 can be 0.0080% or less, 0.0060% or less, 0.0040% or less or 0.0020% or less.

[0065] (Ti: 0 to 0.1500%)

[0066] Ti (titanium) is an element that precipitates in the form of TiC during cooling of steel and contributes to the improvement of strength, so it can be contained as needed. Since it is not an essential element, the lower limit of the Ti content is 0%. This effect can be achieved even if a trace amount is contained, but when contained, the Ti content is preferably 0.0001% or more. The Ti content can be 0.0003% or more or 0.0005% or more. On the other hand, when contained excessively, there is a risk of coarse TiN being generated and toughness being impaired, so the Ti content is set to 0.1500% or less. The Ti content can be 0.1000% or less, 0.0500% or less, 0.0050% or less, or 0.0020% or less.

[0067] (Nb: 0-0.150%)

[0068] Nb (niobium) is an element that contributes to increased strength by improving hardenability, so it can be contained as needed. Since it is not an essential element, the lower limit of the Nb content is 0%. This effect can be achieved even with a trace amount, but when included, the Nb content is preferably 0.002% or more. The Nb content can be 0.005% or more or 0.010% or more. On the other hand, from the perspective of ensuring sufficient toughness, the Nb content is set to 0.150% or less. The Nb content can be 0.100% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0069] (V: 0~0.150%)

[0070] V (vanadium) is an element that contributes to the improvement of strength by improving hardenability, so it can be contained as needed. Since it is not an essential element, the lower limit of the V content is 0%. This effect can be achieved even if a trace amount is contained, but the V content when contained is preferably 0.003% or more. The V content can 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 can be 0.100% or less, 0.060% or less, or 0.020% or less.

[0071] (Cr: 0-2.00%)

[0072] Cr (chromium) is effective in improving the hardenability of steel and the strength of steel, so it can be contained as needed. Since it is not an essential element, the lower limit of the Cr content is 0%. This effect can be achieved even if a trace amount is contained, but the Cr content when contained is preferably 0.01% or more. The Cr content can be 0.05% or more or 0.10% or more. On the other hand, when it is contained excessively, there is a risk that a large amount of Cr carbides will be formed, which may impair the hardenability, so the Cr content is set to 2.00% or less. The Cr content can be 1.80% or less, 1.50% or less, 0.50% or less, or 0.20% or less.

[0073] (Ni: 0-2.00%)

[0074] Ni (nickel) is effective for improving the hardenability of steel and improving the strength of steel, so it can also 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 if it contains a trace amount, but the Ni content when contained is preferably more than 0.001%, more preferably more than 0.02%, and further preferably more than 0.05%. On the other hand, excessive addition of Ni can lead to rising costs, so the Ni content is less than 2.00%, preferably less than 1.80%, more preferably less than 1.50%, and further preferably less than 0.50% or less than 0.20%.

[0075] (Cu: 0-2.0000%)

[0076] Cu (copper) is effective for improving the hardenability of steel and the strength of steel, so it can also 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 if it contains a trace amount, but the Cu content when contained is preferably 0.0001% or more. The Cu content can be 0.0002% or more or 0.0005% or more. On the other hand, from the viewpoint of suppressing the reduction of toughness, the cracks of the slab after casting, and the reduction of weldability, the Cu content is set to 2.0000% or less. The Cu content can be 1.8000% or less, 1.5000% or less, 0.0050% or less or 0.0020% or less.

[0077] (Mo: 0-1.00%)

[0078] Mo (molybdenum) is effective in improving the hardenability and strength of steel, so it can be contained as needed. Since it is not an essential element, the lower limit of the Mo content is 0%. This effect can be achieved even if a trace amount is contained, but the Mo content when contained is preferably 0.01% or more. The Mo content can be 0.02% or more or 0.03% or more. On the other hand, from the perspective of suppressing the reduction of toughness, the Mo content is set to 1.00% or less. The Mo content can be 0.80% or less, 0.60% or less, or 0.20% or less.

[0079] (W: 0~1.000%)

[0080] W (tungsten) is effective for improving the hardenability of steel and the strength of steel, so it can also be contained as needed. Since it is not an essential element, the lower limit of the W content is 0%. This effect can be achieved even if a trace amount is contained, but the W content when contained is preferably 0.001% or more. The W content can be 0.002% or more or 0.003% or more. On the other hand, from the viewpoint of suppressing the reduction of 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.

[0081] (Ca: 0 to 0.1000%)

[0082] Ca (calcium) is an element that helps control inclusions, especially fine dispersion of inclusions, and has the effect of improving toughness, so it can 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 if it is contained in a trace amount, but the Ca content when contained is preferably 0.0001% or more. The Ca content can be 0.0002% or more or 0.0003% or more. On the other hand, when it is contained excessively, the deterioration of the surface properties sometimes becomes obvious, so the Ca content is set to 0.1000% or less. The Ca content can be 0.0800% or less, 0.0500% or less, 0.0300% or less, 0.0100% or less, or 0.0010% or less.

[0083] (Mg: 0-0.100%)

[0084] Mg (magnesium) is an element that helps control inclusions, especially fine dispersion of inclusions, and has the effect of improving toughness, so it can 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 if it is contained in a trace amount, but the Mg content when contained is preferably 0.0001% or more. The Mg content can be 0.0005% or more or 0.0008% or more. On the other hand, when it is contained excessively, the deterioration of the surface properties sometimes becomes obvious, so the Mg content is set to 0.100% or less. The Mg content can be 0.090% or less, 0.080% or less, 0.030% or less, 0.010% or less, or 0.002% or less.

[0085] (Zr: 0 to 0.100%)

[0086] Zr (zirconium) is an element that helps control inclusions, especially fine dispersion of inclusions, and has the effect of improving toughness, so it can be contained as needed. Since it is not an essential element, the lower limit of the Zr content is 0%. This effect can be achieved even with a trace amount, but when it is contained, the Zr content is preferably 0.001% or more. The Zr content can be 0.005% or more or 0.010% or more. On the other hand, when contained excessively, the deterioration of surface properties sometimes becomes obvious, so the Zr content is set to 0.100% or less. The Zr content can be 0.050% or less or 0.030% or less.

[0087] (Hf: 0~0.100%)

[0088] Hf (Hafnium) is an element that helps control inclusions, especially fine dispersion of inclusions, and has the effect of improving toughness, so it can be contained as needed. Since it is not an essential element, the lower limit of the Hf content is 0%. This effect can be achieved even if it is contained in a trace amount, but when it is contained, the Hf content is preferably 0.0001% or more. The Hf content can be 0.0003% or more or 0.0005% or more. On the other hand, when it is contained excessively, the deterioration of the surface properties sometimes becomes obvious, so the Hf content is set to 0.100% or less. The Hf content can be 0.050% or less, 0.030% or less, 0.010% or less, 0.005% or less, or 0.002% or less.

[0089] (REM: 0~0.1000%)

[0090] REM (rare earth elements) are elements that contribute to inclusion control, especially fine dispersion of inclusions, and have the effect of improving toughness, so they can be contained as needed. Since they are not essential elements, the lower limit of the REM content is 0%. This effect can be obtained even if a trace amount is contained, but the REM content when contained is preferably 0.0001% or more. The REM content can be 0.0003% or more or 0.0005% or more. On the other hand, when contained excessively, the deterioration of surface properties sometimes becomes obvious, so the REM content is set to 0.1000% or less. The REM content can be 0.0500% or less, 0.0300% or less, 0.0100% or less, 0.0050% or less, or 0.0020% or less. It should be noted that REM is the abbreviation of Rare Earth Metal, which refers to elements belonging to the lanthanide series. REM is usually added in the form of mixed rare earths.

[0091] In the steel plate constituting the welded joint of the present invention, the balance other than the above-mentioned chemical components consists of Fe and impurities. Impurities herein refer to components that are introduced into raw materials such as ores and scrap during industrial steel plate production due to various factors during the manufacturing process, and do not adversely affect LME cracking during the production of the welded joint of the present invention.

[0092] The chemical composition of the steel plate can be analyzed using elemental analysis methods known to those skilled in the art, such as inductively coupled plasma mass spectrometry (ICP-MS). C and S can be measured using the combustion-infrared absorption method, while N can be measured using the inert gas fusion-thermal conductivity method. These analyses can be performed using samples collected from the steel plate in accordance with JIS G0417:1999.

[0093] Next, the microstructure of the welded portion of the welded joint and its surroundings will be described. First, the microstructure of the high-strength steel plate in the non-heat-affected portion will be described.

[0094] [C concentration]

[0095] In the welded joint of the present invention, in the non-heat-affected zone located at a distance of 5 mm or more from the outer end of the spot weld, the depth in the depth direction from the surface of the high-strength steel plate at which the C concentration is 0.01% or less as measured by GDS (glow discharge spectrometry) is 3 μm or more.

[0096] Since LME sensitivity decreases as the C concentration decreases, LME resistance is improved by reducing the C concentration in the surface layer. In addition, C is an austenite stabilizing element, so a small amount of C stabilizes the layer with low LME sensitivity described later.

[0097] Such a surface structure can be obtained by adjusting the chemical composition of the steel plate as described above and performing the pretreatment and heat treatment described later.

[0098] If the depth at which the C concentration is 0.01% or less is 3 μm or greater, LME resistance can be improved, so the upper limit of this depth is not particularly limited. The depth at which the C concentration is 0.01% or less can 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 can be 5 μm or greater, 7 μm or greater, 10 μm or greater, 15 μm or greater, or 20 μm or greater.

[0099] The C concentration was measured by GDS five times in the plate thickness direction while changing the position on the surface of the high-strength steel plate, and the average value thereof was defined as the C concentration. The measurement conditions were as follows.

[0100] Apparatus: High-frequency glow discharge luminescence analyzer (manufactured by LECO Japan Co., Ltd., model "GDS850A")

[0101] Ar gas pressure: 0.3MPa

[0102] Cathode diameter: 4mmφ

[0103] RF power: 30W

[0104] Measuring time: 200 to 1500 seconds

[0105] The starting point of the "depth" for GDS measurement is the surface of the high-strength steel sheet. If the high-strength steel sheet has a coating (described below), it is the interface between the steel sheet and the coating. The interface between the steel sheet and the coating is defined as the point where the Fe concentration measured by GDS reaches 93% at a depth of 150 μm.

[0106] [Surface roughness]

[0107] In the welded joint of the present invention, the surface roughness of the high-strength steel plate in the non-heat-affected zone located at a distance of 5 mm or more from the outer end of the spot weld, and when the high-strength steel plate has a coating (described below), the roughness at the interface between the steel plate and the coating, measured as the arithmetic mean roughness Ra defined in JIS B0601:2013, exceeds 3.0 μm. In the pretreatment step described below, the surface roughness of the steel plate is increased and strain is introduced. This improves LME resistance. The upper limit of the surface roughness is not limited and can be 20.0 μm or less, 15.0 μm or less, or 10.0 μm or less. The roughness at the interface between the steel plate and the coating can be the surface roughness of the steel plate measured after removing the coating. Removal of the coating is performed by dissolving the coating in an acid solution to which an inhibitor for suppressing corrosion of the steel plate has been added.

[0108] Next, the structure of the high-strength steel plate in the heat-affected zone will be described.

[0109] [Low cementite layer]

[0110] In the weld 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 the layer having an area ratio of cementite of less than 10% (hereinafter referred to as the "low cementite layer") from the surface of the high-strength steel plate in the depth direction is 8 μm or more.

[0111] Figure 2 , an example of a microstructure photograph of a plated high-strength steel plate is shown as an example of a steel plate constituting the welded joint of the present invention. Figure 2 This is a SEM microstructure photograph of the vicinity of the surface layer of a plated high-strength steel sheet, taken at a magnification of 1000 times. Figure 2It is a cross section of the steel plate parallel to the plate thickness direction. The upper side in the figure is the surface of the steel plate, indicating the coating 13 and its surface. There is a layer on the surface of the steel plate with a low C concentration and a cementite 15 mainly composed of ferrite 14 with an area ratio of less than 10%, namely the low cementite layer 11. On the inner side of the steel plate (the lower side in the figure) closer to the low cementite layer 11 (the lower side in the figure), there is a hard structure 12 containing a relatively large amount of cementite 15. The layer with a relatively high brightness (bright) on the surface side of the low cementite layer 11 (the upper side in the figure) is the coating 13. Ferrite and cementite can be distinguished by the difference in brightness in the SEM image. When Figure 1 When the steel plate portion is classified into a portion with relatively high brightness (bright) and a portion with relatively low brightness (dark), the portion with relatively high brightness can be determined as cementite, and the portion with relatively low brightness can be determined as ferrite.

[0112] Cementite tends to segregate at grain boundaries. Therefore, when cementite increases, molten plating invades the grain boundaries, making LME more likely to form. Therefore, by providing a thicker low-cementite layer, LME is less likely to form even when the plating melts, improving LME resistance. This surface structure can be achieved by using the chemical composition of the high-strength steel sheet as described above, through the pretreatment and heat treatment described below.

[0113] If the thickness of the low cementite layer is 8 μm or greater, the LME resistance can be improved, so the upper limit of the thickness is not particularly limited. The thickness of the low cementite layer can be 50 μm or less, 40 μm or less, or 30 μm or less. The thickness of the low cementite layer can be 10 μm or more, or 20 μm or more.

[0114] The structure other than cementite of the low cementite layer is not limited. For example, it can be any one or more of martensite, bainite, and ferrite. Since ferrite has low LME sensitivity, a structure mainly composed of ferrite is preferred from the perspective of improving LME resistance.

[0115] The thickness of the low cementite layer is obtained by subjecting the C section of the steel plate (the plate thickness section parallel to the rolling direction (L direction)) to nitric acid corrosion and observing the 50 μm × 50 μm field of view containing the surface layer of the steel plate at a magnification of 1000 times by SEM. According to the organizational morphology on the SEM image obtained by SEM observation, hard structures such as martensite and bainite containing relatively more cementite can be distinguished from 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 set to 5 ranges at intervals of 1000 μm in the L direction. In the 5 measurement ranges, the average value of the thickness of the low cementite layer in the plate thickness direction is set. Here, the area ratio of cementite refers to the area ratio obtained by observation in the C section. When an L section (a section of the plate thickness perpendicular to the rolling direction) is observed locally in the middle of the thickness direction, even if there is a location where the area ratio of cementite exceeds 10%, there will be no problem as long as the area ratio of cementite in the C section at a depth of 5 μm is less than 10%.

[0116] 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.

[0117] [Plating]

[0118] The surfaces of one or more of the multiple steel plates constituting the welded joint of the present invention may be partially or entirely coated with a Zn-containing coating. For example, the coating may be provided on the outermost surfaces of the multiple overlapping steel plates, or on the surfaces constituting the overlapping surfaces. Coatings may also be provided on both surfaces. There are no particular limitations on the coating as long as it contains Zn. Examples include Zn-0.2% Al(GI), Zn-(0.3-1.5)% Al, Zn-4.5% Al, Zn-0.09% Al-10% Fe(GA), Zn-1.5% Al-1.5% Mg, Zn-11% Al-3% Mg-0.2% Si, Zn-11% Ni, Zn-15% Mg, Zn-20% Al-7% Mg, and Zn-30% Al-10% Mg.

[0119] The chemical composition of the coating can be determined by dissolving the coating in an acid solution to which an inhibitor for inhibiting corrosion of the steel sheet has been added, and measuring the resulting solution by ICP (inductively coupled plasma) emission spectroscopy. The acid solution to which the inhibitor has been added can be, for example, a 10% by mass hydrochloric acid solution to which 0.06% by mass of an inhibitor (manufactured by Asahi Chemical Industries, Ltd., Ibit) has been added.

[0120] The thickness of the plating layer can be, for example, 3 to 50 μm. The amount of the plating layer applied is not particularly limited, but can be, for example, 10 to 170 g / m per side. 2In the present invention, the coating adhesion amount is determined by dissolving the coating in an acid solution to which an inhibitor for inhibiting corrosion of the steel plate is added, and by the weight change of the coating before and after pickling and stripping. The thickness of the coating may be greater than 5 μm, greater than 10 μm, greater than 15 μm or greater than 20 μm. The thickness of the coating may be less than 40 μm or less than 30 μm. The coating adhesion amount may be 20 g / m per single side. 2 Above, 30g / m 2 Above, 40g / m 2 Above or 50g / m 2 The coating weight can be 150g / m per side. 2 Below, 130g / m 2 Below, 120g / m 2 Below or 100g / m 2 the following.

[0121] Of the multiple steel plates comprising the welded joint of the present invention, one or more of the steel plates forming the overlapping surface is a high-strength steel plate. If one or more of the steel plates forming the overlapping surface is a plated steel plate, the other steel plates may be high-strength steel plates or may be steel plates other than high-strength steel plates. That is, both of the two steel plates forming the overlapping surface may be high-strength steel plates, or only one may be a high-strength steel plate. The properties of the steel plates other than the high-strength steel plates are not limited. The tensile strength of the steel plates other than the high-strength steel plates may, for example, be 390 MPa or greater, 490 MPa or greater, or 590 MPa or greater.

[0122] In the welded joint of the present invention, even when a high-strength steel plate is used as the outermost steel plate, the occurrence of LME cracks in the outermost layer of the welded joint can be suppressed. LME cracks in the outermost layer of the welded joint can occur when the outermost steel plate is a high-strength steel plate with a high carbon concentration, is zinc-plated on the surface side, or when molten zinc plating adheres to the welding electrode. Examples of LME cracks in the outermost layer of the welded joint include cracks in the indentation caused by the welding electrode (cracks directly below the welding electrode) and cracks formed in the inclined portion (shoulder) around the edge of the indentation (weld shoulder cracks).

[0123] In addition, regarding the steel plates constituting the overlapping surface, plating may be applied to both steel plates constituting the overlapping surface, or may be applied to only one of the steel plates. Examples of combinations of steel plates constituting the overlapping surface include (1) both being plated high-strength steel plates, (2) one being a plated high-strength steel plate and the other being an unplated high-strength steel plate, (3) one being a plated high-strength steel plate and the other being a steel plate other than a plated high-strength steel plate, (4) one being a plated high-strength steel plate and the other being a steel plate other than a plated high-strength steel plate, and (5) one being an unplated high-strength steel plate and the other being a plated steel plate other than a plated high-strength steel plate. According to the welded joint of the present invention, LME cracking during manufacturing can be suppressed regardless of any combination.

[0124] The thickness of the high-strength steel plate and the steel plates other than the high-strength steel plate that constitute the welded joint of the present invention is not particularly limited. For example, it can be 0.6 to 3.2 mm. The thickness can be 0.8 mm or greater, or 1.0 mm or greater. The thickness can 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.

[0125] Manufacturing methods of welded joints

[0126] Next, the method for producing the welded joint of the present invention will be described. First, the method for producing the high-strength steel plate constituting the welded joint will be described.

[0127] <Method for Manufacturing High-Strength Steel Sheet>

[0128] The high-strength steel plate constituting the welded joint of the present invention can be obtained, for example, by the following manufacturing method: a casting step in which molten steel with adjusted chemical composition is cast to form a steel sheet; a hot rolling step in which the steel sheet is hot-rolled to obtain a hot-rolled steel sheet; a coiling step in which the hot-rolled steel sheet is coiled; a cold rolling step in which the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; a pretreatment step in which the cold-rolled steel sheet is sandblasted; and an annealing step in which the pretreated cold-rolled steel sheet is annealed. Alternatively, after the hot rolling step, the hot-rolled steel sheet may be pickled and then directly cold-rolled without being coiled.

[0129] [Casting process]

[0130] The conditions of the casting process are not particularly limited. For example, after smelting in a blast furnace, electric furnace, or the like, various secondary smelting processes may be performed, followed by casting by conventional continuous casting, ingot casting, or thin slab casting.

[0131] [Hot rolling process]

[0132] Hot-rolled steel sheets are obtained by hot rolling the steel sheets obtained by casting. The hot rolling process is carried out by reheating the cast steel sheets directly or temporarily cooling them and then hot rolling them. In the case of reheating, the heating temperature of the steel sheets can 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 can be appropriately changed according to the desired metal structure and plate thickness. For example, the end temperature of the finish rolling can be 900 to 1050°C, and the reduction ratio of the finish rolling can be 10 to 50%.

[0133] [Coiling process]

[0134] Hot-rolled steel sheets can be coiled at a specified temperature. The coiling temperature can be adjusted appropriately based on the desired metallurgical structure, for example, 500-800°C. The hot-rolled steel sheets can also be uncoiled before or after coiling and subjected to a specified heat treatment. Alternatively, the hot-rolled steel sheets can be pickled after the hot rolling process without coiling and then cold-rolled as described below.

[0135] [Cold rolling process]

[0136] After pickling, etc., the hot-rolled steel sheet is cold-rolled to produce a cold-rolled steel sheet. The cold-rolling reduction ratio can be appropriately adjusted depending on the desired metallographic structure and sheet thickness, and can be, for example, 20 to 80%. After the cold-rolling step, the sheet is cooled to room temperature, for example, by air cooling.

[0137] [Pre-treatment process]

[0138] In order to obtain the above-described structure of the surface layer of the steel plate, it is necessary to perform a predetermined pretreatment and then perform annealing.

[0139] Pre-treatment includes using angular abrasives to implement sandblasting on the surface of the cold-rolled steel sheet. The abrasives that can be used are not particularly limited. For example, a polygonal steel abrasive (Grid) 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. As such an abrasive, for example, TGD-30 manufactured by WINOA IKK JAPAN Co., Ltd. can be cited. Thus, strain can be introduced while increasing the roughness of the surface. The projection amount of the abrasive can be 5 to 400 kg / m 2 By performing such a sandblasting treatment, decarburization can be promoted in the annealing process described later, and a ferrite-stable structure can be efficiently formed on the surface of the steel plate. It should be noted that the projection amount is 400 kg / m 2 The amount of projection per unit time and per unit area is 4.0×10 -4 kg / (mm 2·min). It should be noted that the surface roughness of the steel sheet after the pretreatment can be maintained also in the steel sheet and the plated steel sheet of the present invention after the annealing step and the plating step (including the alloying step) described later.

[0140] [Annealing process]

[0141] Annealing the pre-treated cold-rolled steel sheets at a high dew point (high dew point annealing). Annealing is performed under a tension of 1 to 20 MPa. Applying tension during annealing effectively introduces strain into the steel sheets, promoting decarburization of the surface layer.

[0142] The holding temperature in the annealing step is set at 750-900°C. Alternatively, the holding temperature can be 770-870°C. This range promotes decarburization, reduces the carbon concentration in the surface layer, and reduces cementite. The heating rate to the holding temperature is not particularly limited; it can be 1-10°C / second.

[0143] The holding time at the holding temperature in the annealing step is set to 40 to 300 seconds. The holding time can be 50 to 250 seconds. By setting the holding time in this range, decarburization can be promoted, the carbon concentration in the surface layer can be reduced, and cementite can be reduced.

[0144] The atmosphere during the annealing step should have a dew point of -30 to 20°C. Alternatively, the dew point can be -10 to 5°C. For example, the atmosphere can be N₂-1 to 10 vol% H₂, or N₂-2 to 4 vol% H₂. If the dew point is too high or too low, a phase containing oxides of Si, Mn, Al, and the like forms on the exterior of the steel sheet, hindering decarburization. Furthermore, interdiffusion between the plating components and the steel components may be hindered, resulting in insufficient plating properties.

[0145] According to the production method including the above-mentioned steps, a high-strength steel sheet in which decarburization is promoted in the surface layer and cementite is reduced can be obtained.

[0146] <Method for Manufacturing Plated Steel Sheet>

[0147] 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 produced as described above, or on any steel sheet that can constitute the weld joint of the present invention.

[0148] The plating treatment can be carried out according to methods well known to those skilled in the art. The plating treatment can be carried out, for example, by melt plating or by electroplating. Preferably, the plating treatment is carried out by melt plating. The conditions for the plating treatment can be appropriately set in consideration of the chemical composition, thickness and adhesion amount of the desired coating. For example, it can be immersed in a molten zinc plating bath of 420 to 480°C with adjusted chemical composition for 1 to 10 seconds, and after dipping, it can be pulled out at 20 to 200 mm / second, and the coating adhesion amount can be controlled by N2 wiping gas. After the plating treatment, alloying treatment can be carried out to form alloyed plating. The alloying treatment can be carried out, for example, at 500 to 550°C for 10 to 60 seconds.

[0149] <Spot welding process>

[0150] The above-mentioned steel plate and the high-strength steel plate are overlapped and spot welded to obtain a welded joint. As long as the high-strength steel plates are overlapped, multiple steel plates can be overlapped. Among the multiple overlapping steel plates, the high-strength steel plates can be overlapped at any position (position in the stacking direction or the thickness direction) as long as they can show LME resistance. The conditions for spot welding are not particularly limited. For example, a dome radius welding electrode with a front diameter of 8mm can be used, with a pressure of 4.0kN, a power-on time of 0.5 seconds, and a power-on current of 12kA for spot welding.

[0151] The welded joint of the present invention can suppress LME cracking during production and can therefore be suitably used in a wide range of fields, including automobiles, home appliances, and building materials, and is particularly preferably used in the automotive field.

[0152] Example

[0153] The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to these Examples.

[0154] <Example No. 1>

[0155] Molten steel adjusted to the chemical composition listed in Table 1, No. 1, was melted in a blast furnace and cast by continuous casting to produce steel slabs. The resulting steel slabs were heated to 1200°C, finished at a temperature of 950°C, and hot rolled at a reduction ratio of 30% to produce hot-rolled steel sheets. The resulting hot-rolled steel sheets were coiled at a coiling temperature of 650°C, pickled, and then cold rolled at a reduction ratio of 50% to produce cold-rolled steel sheets. The thickness of the cold-rolled steel sheets was 1.6 mm.

[0156] Next, the cold-rolled steel sheet was projected using TGD-30 manufactured by WINOA IKK JAPAN Co., Ltd. at a projection rate of 5 kg / m 2 Sandblasted.

[0157] Before the annealing step, the surface roughness of the steel sheet was measured in accordance with JIS B 0601: 2013. Specifically, 10 locations were randomly selected on the surface of the steel sheet, and the surface profile was measured at each location using a contact-type surface roughness meter. The surface roughness values ​​at these locations were then arithmetic averaged to obtain the arithmetic average roughness Ra, which was evaluated as follows.

[0158] Evaluation AA: more than 4.0μm

[0159] Evaluation A: more than 3.0 μm and 4.0 μm or less

[0160] Evaluation B: 3.0 μm or less

[0161] Annealing was then performed in a furnace with an oxygen concentration of 20 ppm or less in an N₂-4% H₂ 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 / second to 500°C and 2.0°C / second from 500°C to the holding temperature. Annealing was performed under a tension of 5.0 MPa.

[0162] After annealing, plating, and alloying, an alloyed hot-dip zinc-plated steel sheet (a high-strength steel sheet with alloyed hot-dip zinc plating) was obtained. The plating process was performed by immersing the steel sheet in a hot-dip zinc plating bath (Zn-0.14% Al) at 450°C for 3 seconds. After immersion, the steel sheet was withdrawn at a speed of 100 mm / s and the coating weight was controlled to 50 g / m² using N2 wiping gas. 2 , alloying treatment was performed at 520°C for 30 seconds.

[0163] Two overlapping hot-dip zinc-coated alloyed steel sheets were spot welded using a dome-radius welding electrode with an 8mm tip diameter at a 2° tapping angle, a 4.0kN force, a 0.5-second current, and a 12kA current. The welded joints were then evaluated for LME resistance during fabrication.

[0164] <Examples 2 to 28, Comparative Examples 29 to 40>

[0165] Welded joints were produced in the same manner as in Example 1, except that the chemical composition was set as shown in Table 1, the pretreatment and annealing conditions were set as shown in Table 2, and the plating type was set as shown in Table 3. The LME resistance during production was evaluated. Note that in No. 36, sandblasting was not performed, while in No. 40, surface treatment using brush grinding was performed instead of sandblasting.

[0166] In Table 3, "a" indicates Zn-0.14% Al alloyed hot-dip zinc plating, "b" indicates unalloyed Zn-0.14% Al alloyed hot-dip zinc plating, and "c" indicates Zn-1.5% Al-1.5% Mg alloyed hot-dip zinc plating. Furthermore, "unplated" indicates no plating. Note that the unplated "unplated" steel sheet, Example No. 2, was superimposed on the alloyed hot-dip zinc plated steel sheet, Example No. 1, to produce a welded joint.

[0167]

[0168] Table 2

[0169]

[0170] ※The underlined part indicates that the production conditions of the present invention are not met.

[0171] <Microstructure Evaluation around Welding Area>

[0172] (Depth where C concentration is 0.01% or less)

[0173] The weld joint was cut and a sample was taken. GDS measurements were performed five times in the plate thickness direction at a position that was a non-heat-affected zone at a distance of more than 5 mm from the outer end of the weld nugget of the spot weld, starting from the interface between the coating and the steel plate. The depth at which the C concentration was 0.01% or less was determined and is shown in the "C ≤ 0.01% Depth" column of Table 3.

[0174] (Roughness of steel sheet surface or steel sheet / plating interface)

[0175] Furthermore, the surface roughness of the high-strength steel plate at a position that serves as a non-heat-affected zone that is 5 mm or more away from the outer end of the nugget of the spot weld, and in the case of a plated high-strength steel plate, the surface roughness of the high-strength steel plate exposed after the plating is removed, was measured by the same method as before annealing, and the results are shown in the "Steel Plate Surface or Steel Plate / Plating Interface Roughness" column of Table 3.

[0176] (Thickness of the layer having an area ratio of cementite of 10% or less)

[0177] In addition, the thickness of the low cementite layer was measured from the surface of the high-strength steel plate in the plate thickness direction at a position 0 to 100 μm outward from the outer end of the weld shoulder, and is shown in the column "Thickness of cementite 10% or less" in Table 3.

[0178] (Tensile Strength Evaluation)

[0179] For each steel plate used to produce the welded joint, a JIS No. 5 tensile test piece was taken, with the longitudinal direction perpendicular to the rolling direction and the plate thickness direction. Tensile testing was performed in accordance with JIS Z 2241:2011 to determine the tensile strength and evaluate it as follows. A rating of A or higher (i.e., ratings A, AA, or AAA) indicates that the steel plate constitutes the high-strength steel plate of the welded joint of the present invention.

[0180] Rating AAA: 1180MPa or above

[0181] Rating AA: 980 MPa or more and less than 1180 MPa

[0182] Evaluation A: 780 MPa or more and less than 980 MPa

[0183] Evaluation B: less than 780MPa

[0184] (LME resistance)

[0185] Reference Figure 3 The evaluation of LME resistance is described below. LME resistance is evaluated by spot welding two overlapping steel sheets 21. The length of LME cracks (cracks 24 near the outside of the crimping portion) generated near the outside of the crimping portion 23 of the resulting spot weld 22 (hereinafter referred to as "near the outside of the crimping portion"). The "near the outside of the crimping portion" refers to the area outside the portion of the overlapping surface of the two steel sheets crimped together by spot welding (the crimping portion), near the crimped portion (approximately 1 mm outward from the end of the crimped portion). The length of cracks 24 near the outside of the crimping portion is evaluated. The evaluation criteria are as follows. In this example, a rating of A or higher (i.e., ratings A, AA, or AAA) is considered excellent LME resistance. It should be noted that the spot welding test is repeated until cracks 24 near the outside of the crimping portion occur three or more times, and the longest crack 24 near the outside of the crimping portion is evaluated.

[0186] Rating AAA: 0μm (no cracks)

[0187] Evaluation AA: more than 0 μm and less than 60 μm

[0188] Evaluation A: 60 μm or more and less than 120 μm

[0189] Evaluation B: 120 μm or more

[0190] Table 3 shows the evaluation results.

[0191] Table 3

[0192]

[0193] ※Underlined elements indicate that the elements are outside the scope of the present invention or that the expected characteristics are not obtained.

[0194] Steel No. 29 is a comparative example with a high carbon content in the steel plate. It is believed that due to the high carbon content in the steel plate, the carbon concentration in the surface layer of the steel plate did not decrease even after high dew-point annealing. Consequently, the depth at which the carbon concentration in the non-heat-affected zone (NAZ) is 0.01% or less and the thickness of the layer at which the cementite area fraction in the HZ is 10% or less are reduced. Furthermore, the roughness of the steel plate / plating interface is reduced. Consequently, LME resistance during weld joint fabrication is poor.

[0195] No. 30 is a comparative example in which the Si content of the steel plate is low. It is believed that due to the low Si content in the steel plate, decarburization in the surface layer did not progress even after high dew-point annealing. Consequently, the depth at which the carbon concentration in the non-heat-affected zone is 0.01% or less and the thickness of the layer at which the cementite area fraction in the heat-affected zone is 10% or less are reduced. Furthermore, the roughness of the steel plate / plating interface is reduced. Consequently, LME resistance during weld joint fabrication is poor.

[0196] No. 31 is a comparative example in which the steel sheet contains a high Si content. It is believed that due to the high Si content in the steel sheet, external oxidation progressed even after high dew-point annealing, forming oxides (scale) on the surface of the steel sheet, which suppressed decarburization at the outermost surface. Consequently, the depth at which the carbon concentration in the non-heat-affected zone (NAZ) is less than 0.01% and the thickness of the layer at which the cementite area ratio in the HZ is less than 10% are reduced. Furthermore, the roughness of the steel sheet / plating interface is reduced. As a result, LME resistance during weld joint production is poor.

[0197] Steel No. 32 is a comparative example in which the sol. Al content of the steel plate is high. This high sol. Al content is believed to have caused external oxidation to proceed even after high dew-point annealing, forming oxides (scale) on the surface of the steel plate and suppressing decarburization at the outermost surface. Consequently, the thickness of the layer with a cementite area fraction of 10% or less in the heat-affected zone was reduced. Furthermore, the roughness of the steel plate / plating interface was reduced. Consequently, LME resistance during weld joint fabrication was poor.

[0198] It is believed that the holding temperature during annealing for No. 33 was low, so decarburization was not sufficiently promoted during annealing. Consequently, the thickness of the layer with a cementite area fraction of 10% or less in the heat-affected zone was reduced. Furthermore, the roughness of the steel plate / plating interface was reduced. Consequently, LME resistance during weld joint production was poor.

[0199] It is believed that the holding temperature during annealing for No. 34 was high, so decarburization was not sufficiently promoted during annealing. Consequently, the depth of the non-heat-affected zone where the carbon concentration was 0.01% or less was reduced. Furthermore, the roughness of the steel plate / plating interface was reduced. Consequently, LME resistance during weld joint production was poor.

[0200] It is believed that the short holding time during annealing in No. 35 prevented sufficient decarburization during annealing. Consequently, the depth of the non-heat-affected zone (NAZ) with a carbon concentration of 0.01% or less and the thickness of the layer in the HZ with a cementite area ratio of 10% or less were reduced. Furthermore, the roughness of the steel plate / plating interface was reduced. Consequently, LME resistance during weld joint production was poor.

[0201] No. 36 is a comparative example that did not undergo sandblasting during the pretreatment process. It is believed that the lack of sandblasting did not introduce strain into the steel plate surface, and decarburization did not occur during annealing. Consequently, the depth of the non-heat-affected zone (NAZ) at which the carbon concentration is 0.01% or less and the thickness of the layer in the HZ at which the cementite area fraction is 10% or less were reduced. Furthermore, the roughness of the steel plate / plating interface was reduced. Consequently, LME resistance during weld joint production was poor.

[0202] No. 37 is a comparative example with a large shot blasting volume. As a result, it is believed that the roughness of the steel plate / plating interface decreased, decarburization progressed excessively, and the tensile strength of the steel plate decreased. Furthermore, LME resistance during weld joint production was also reduced.

[0203] In steel No. 38, due to the low dew point during annealing, a phase containing oxides of Si, Mn, Al, and the like formed on the exterior of the steel plate, failing to promote decarburization. Consequently, the depth at which the carbon concentration in the non-heat-affected zone reached 0.01% or less, and the thickness of the layer at which the cementite area fraction in the heat-affected zone reached 10% or less, were reduced. Furthermore, the roughness of the steel plate / plating interface decreased. Consequently, LME resistance during weld joint fabrication was poor.

[0204] In No. 39, due to the high dew point during annealing, a phase containing oxides of Si, Mn, Al, and the like formed on the outer surface of the steel plate, failing to promote decarburization. Consequently, the depth at which the carbon concentration in the non-heat-affected zone reached 0.01% or less and the thickness of the layer at which the cementite area fraction in the heat-affected zone reached 10% or less were reduced. Furthermore, the roughness of the steel plate / plating interface decreased. Consequently, LME resistance during weld joint fabrication was poor.

[0205] No. 40 is a comparative example in which brush grinding was used instead of sandblasting. It is believed that brush grinding did not introduce sufficient strain into the steel plate surface, and decarburization was not achieved during annealing. As a result, the depth of the non-heat-affected zone (NAZ) with a carbon concentration of 0.01% or less and the thickness of the layer with a cementite area fraction of 10% or less in the HZ were reduced. Furthermore, the roughness of the steel plate / plating interface was reduced. Consequently, LME resistance during weld joint fabrication was poor.

[0206] Examples No. 1 to 28 represent the present invention and demonstrate high LME resistance. Examples with a carbon concentration of 0.01% or less in the non-heat-affected zone and a thicker layer with a cementite area fraction of 10% or less in the heat-affected zone demonstrate particularly excellent LME resistance during weld joint fabrication.

[0207] Industrial applicability

[0208] According to the present invention, high-strength steel sheets and plated steel sheets having high LME resistance can be provided, which are suitable for use in applications such as automobiles, home appliances, and building materials, particularly automobiles. Therefore, the present invention has extremely high industrial applicability.

[0209] Description of Reference Numerals

[0210] 1 steel plate

[0211] 2 Spot welding parts

[0212] 3 nugget

[0213] 4 Depression (indentation)

[0214] 5 Welding shoulder

[0215] 6 Heat-affected zone

[0216] 8 Overlapping Faces

[0217] 11 Low carburized layer

[0218] 12 Hard tissue

[0219] 13 Plating

[0220] 14 Ferrite

[0221] 15 Cementite

[0222] 21 Steel Plate

[0223] 22 spot welding part

[0224] 23 Crimping part

[0225] 24 Cracks near the outside of the crimped part

[0226] A. Welding Electrode

Claims

1. A welded joint, characterized in that: It includes a plurality of overlapping steel plates and spot welding portions for joining the plurality of steel plates. The spot weld portion includes a nugget, an indentation portion pressed by an electrode, and a weld shoulder portion as a peripheral portion of the indentation portion. At least one of the plurality of steel sheets is a plated steel sheet having a Zn-containing plating layer formed on at least a surface corresponding to a surface on which the plurality of steel sheets overlap. One or more of the plurality of steel plates constituting the overlapping surface is a high-strength steel plate having a tensile strength of 780 MPa or more. The chemical composition of the high-strength steel plate contains C：0.08～0.40％、 Si: 0.4-2.0%, Mn: 0.1-5.0%, sol.Al: 0~2.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B：0～0.0100％、 Ti: 0~0.1500%, Nb: 0-0.150%, V：0～0.150％、 Cr：0～2.00％、 Ni: 0-2.00%, Cu: 0-2.0000%, Mo: 0-1.00%, W：0～1.000％、 Ca: 0~0.1000%, Mg: 0~0.100%, Zr：0～0.100％、 Hf: 0~0.100%, REM: 0~0.1000%, The balance is Fe and impurities. In an area 5 mm or more away from the outer end of the nugget, The depth from the surface of the high-strength steel sheet to the depth direction at which the carbon concentration measured by GDS is 0.01% or less is 3 μm or more, and the surface roughness of the high-strength steel sheet exceeds 3 μm in terms of arithmetic mean roughness Ra, In the range of 0 to 100 μm from the outer end of the welding shoulder to the outside, The thickness of a layer having an area ratio of cementite of 10% or less is 8 μm or more in the depth direction from the surface of the high-strength steel sheet.

2. The weld joint according to claim 1, wherein: In at least one of 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 includes 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.

3. The weld joint according to claim 1 or 2, characterized in that: The total content of Si and sol.Al is less than 1.8%.

4. The weld joint according to claim 1 or 2, characterized in that: In the depth direction of the base steel sheet starting from the interface between the coating layer and the base steel sheet, the depth at which the C concentration measured by GDS is 0.01% or less is 5 μm or more.

5. The welded joint according to claim 1 or 2, characterized in that: In the depth direction of the base steel sheet starting from the interface between the coating layer and the base steel sheet, the depth at which the C concentration measured by GDS is 0.01% or less is 7 μm or more.

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