Welded joint, and joining structure for automobile member

Abstract

The present invention addresses the problem of providing a welded joint in which LME during manufacture is inhibited.　In this welded joint, which comprises a plurality of steel plates and a spot-welded part, one or more of the plurality of steel plates is a plated steel plate, and at least one of the plurality of steel plates is a high-strength steel plate having a prescribed chemical component. In a first region (non-heat-affected zone) of the high-strength steel plate, the surface roughness Ra is 3.0 μm or below, the depth from the surface at which the C concentration is 0.02% or lower is 3 μm or above, and the relationship 0.45 ≤ I(110) / (I(110) + I(200) + I(211)) ≤ 0.90 maintains in oblique incidence X-ray diffraction with an incidence angle of 1° on the surface of the high-strength steel plate, where I(110) is the diffraction intensity corresponding to the (110) plane, I(200) is the diffraction intensity corresponding to the (200) plane, and I(211) is the diffraction intensity corresponding to the (211) plane. In a second region (heat-affected zone) of the high-strength steel plate, the thickness of the surface layer having a ferrite area ratio of 90% or above is 3 μm or greater.

Description

Welded joints and joining structures for automobile components

[0001] The present invention relates to a welded joint and a joining structure of an automobile component. More specifically, the present invention relates to a welded joint and a joining structure of an automobile component 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 (high tensile strength and hardness), weldability may be reduced due to liquid metal embrittlement (LME) cracking, as described in Patent Document 1, for example.

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

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

[0006] In order to suppress the occurrence of LME cracking 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 steel sheet whose metal structure has been transformed into austenite during welding, for example. In this respect, there is room for improvement.

[0007] In view of the above circumstances, an object of the present invention is to provide a welded joint in which LME cracking during manufacturing is suppressed.

[0008] The present inventors have intensively investigated means for solving the above-mentioned problems, and as a result, have found that by applying strain to a steel sheet before annealing using a shot material under appropriate conditions to create an appropriate surface state, and then performing high dew-point annealing, the surface layer of the steel sheet is decarburized, and further a layer with a high ferrite fraction in which ferrite is randomly oriented is formed, and as a result, it is possible to suppress LME.

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

[0010] (1) A welded joint comprising a plurality of overlapping steel plates and a spot weld that joins the plurality of steel plates, wherein the spot weld has a nugget, an indentation formed by pressing down with an electrode, and a weld shoulder that is a peripheral portion of the indentation, and at least one of the plurality of steel plates is a plated steel plate having a 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 that constitutes the overlapping surface is a high-strength steel plate having a hardness of 200 Hv or more in a first region that is a surface region that is 5 mm or more away from the outer end of the nugget toward the outside of the nugget in the direction in which the plate surfaces of the steel plates extend, and the high-strength steel plate has a chemical composition, in mass %, of C: 0.05 to 0.40%, Si: 0.5 to 3.0%, Mn: 0.1 to 5.0%, sol. Al: 0-3.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0-0.0100%, Ti: 0-0.1500%, Nb: 0- 0.150%, V: 0-0.150%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.0000%, Mo: 0-1.00%, W: 0-1.000%, C a: 0 to 0.1000%, Mg: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, REM: 0 to 0.1000%, with the balance being Fe and impurities, and in the first region, the high-strength steel plate has a surface roughness Ra of 3.0 μm or less, and a C concentration of 0.02% or less from the surface of the high-strength steel plate in a plate thickness direction. The depth is 3 μm or more, and in oblique incidence X-ray diffraction with an incident angle of 1° to the surface of the high-strength steel plate, when the diffraction intensity corresponding to the (110) plane is I(110), the diffraction intensity corresponding to the (200) plane is I(200), and the diffraction intensity corresponding to the (211) plane is I(211), the relationship of 0.45≦I(110) / (I(110)+I(200)+I(211))≦0.90 is satisfied, and in the second region of the high-strength steel plate, which is a surface layer region located at a distance of 0 to 100 μm from the weld shoulder toward the outside of the spot weld in the direction in which the plate surface of the high-strength steel plate expands, the thickness of a layer in which the area ratio of ferrite is 90% or more in the plate thickness direction from the surface of the high-strength steel plate is 3 μm or more,When the steel sheet is a plated steel sheet, the surface of the steel sheet is an interface between the steel sheet and a plating layer of the plated steel sheet.

[0011] (2) The welded joint according to (1) above, characterized in that in the first region, 0.45≦I(110) / (I(110)+I(200)+I(211))≦0.75 is satisfied.

[0012] (3) The welded joint according to (1) or (2), characterized in that in the first region, the depth at which the C concentration is 0.02% or less is 10 μm or more.

[0013] (4) The welded joint according to any one of (1) to (3), wherein the surface roughness Ra in the first region is 2.0 μm or less.

[0014] (5) The welded joint according to any one of (1) to (4), wherein in the second region, the thickness of the layer in which the area ratio of the ferrite is 90% or more is 8 μm or more.

[0015] (6) A joining structure for automobile components, in which the welded joint of any one of (1) to (5) is used to join the automobile components when the plurality of steel plates are used as automobile components.

[0016] According to the present invention, it is possible to obtain a welded joint in which LME cracking during manufacturing is suppressed.

[0017] Fig. 1 is a diagram illustrating a weld shoulder of a welded joint of the present invention. Fig. 2 is a diagram illustrating an example of the results of grazing incidence X-ray diffraction measurement when the ferrite phase is randomly oriented and when it is not. Fig. 3 is an example of a structural photograph taken by SEM of the vicinity of the surface layer at a distance of 0 to 100 μm from the weld shoulder of a welded joint of the present invention. Fig. 4 is a diagram illustrating the positions of cracks targeted in the LME resistance evaluation in the examples.

[0018] The present invention will be described below. However, the present invention is not limited to the following embodiments. First, an outline of the improvement of LME resistance in the present invention will be described.

[0019] LME cracking occurs when, for example, during spot welding, the metallographic structure of a steel sheet is heated and transformed into austenite, and molten zinc produced by melting the coating penetrates into the austenite grain boundaries in the surface layer of the steel sheet. The molten zinc that penetrates the austenite grain boundaries embrittles the steel sheet, and the tensile stress applied to the steel sheet during welding is thought to cause LME cracking. The inventors have come up with the idea of ​​utilizing the metallographic structure of the surface layer of the steel sheet that constitutes a welded joint as a method for improving LME resistance. Specifically, the surface layer of the steel sheet has a metallographic structure mainly composed of a ferrite phase with a low carbon concentration and low LME susceptibility, and the ferrite phase is randomly oriented to suppress the occurrence of LME. Herein, "LME resistance" refers to the property of a steel sheet that suppresses LME cracking, and "LME susceptibility" refers to the property of a steel sheet that is susceptible to LME cracking.

[0020] The random orientation of the ferrite phase means that the characteristics of the ferrite grain boundaries are averaged as a whole. In other words, it means that the crystal orientation of each ferrite particle in the ferrite phase is randomly oriented. The random orientation of the crystal orientation of the ferrite particles prevents grain boundaries oriented in a specific direction from being unevenly distributed and connected continuously or intermittently. LME cracking is thought to occur when Zn from the plating invades grain boundaries where the grain boundary energy is locally low. In other words, when grain boundaries where the grain boundary energy is locally low exist in succession, Zn from the plating concentrates there, making LME cracking more likely to occur. By averaging the characteristics of the grain boundaries as a whole, grain boundaries where the grain boundary energy is locally low are no longer connected continuously, preventing local concentration of Zn from the plating. As a result, it is thought that LME can be suppressed during the production of welded joints.

[0021] In the present invention, the random orientation of the ferrite phase is expressed by the following conditional formula: That is, a steel sheet satisfying the following conditional formula means that the ferrite phase is randomly oriented.

[0022] (Conditional formula) In the case of oblique incidence X-ray diffraction at an incident angle of 1° to the steel sheet surface, when the diffraction intensity corresponding to the (110) plane is I(110), the diffraction intensity corresponding to the (200) plane is I(200), and the diffraction intensity corresponding to the (211) plane is I(211), the following condition is satisfied: 0.45≦I(110) / (I(110)+I(200)+I(211))≦0.90

[0023] In order to configure the steel sheet constituting the welded joint as described above, in the present invention, when manufacturing the steel sheet constituting the welded joint, strain is imparted to the cold-rolled steel sheet, and then annealing is performed at a high dew point. This promotes decarburization and makes it easier to form a ferrite phase on the steel sheet surface. Furthermore, the present invention was made based on the finding that the orientation of the ferrite phase can be randomized by controlling the temperature at which humidification begins.

[0024] The present invention will be described in detail below. First, a welded joint of the present invention will be described with reference to FIG. 1 . The welded joint of the present invention includes a plurality of overlapping steel plates 1 and a spot weld 2 joining the plurality of steel plates. The spot weld 2 has an indentation 3, a weld shoulder 4, and a nugget 5 formed on the surface of the steel plate 1 pressed down by an electrode. The weld shoulder 4 is the peripheral edge of the indentation 3 and refers to the sloped portion from the edge of the indentation 3 to the outer edge 4a of the weld shoulder. A pressure weld 6 is formed around the nugget 5, where the two steel plates 1 are pressure welded together. The first region 21 is a surface region (non-heat-affected zone) that is 5 mm or more away from the outer edge of the nugget in the direction in which the steel plate surfaces expand. The second region 22 is a surface region (heat-affected zone) that is 0 to 100 μm away from the weld shoulder in the direction in which the high-strength steel plate surfaces expand. The plurality of steel sheets may include both plated steel sheets and unplated steel sheets, and may also include both high-strength steel sheets and relatively low-strength steel sheets.

[0025] [Plated Steel Sheet] One or more of the plurality of steel sheets is a plated steel sheet. The plated steel sheet comprises a base steel sheet and a plating layer. The plating layer contains Zn and is formed on at least the surface corresponding to the overlapping surfaces of the plurality of steel sheets. 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. The plating layer may be formed on a surface other than the surface corresponding to the overlapping surfaces of the steel sheets.

[0026] The chemical composition of the plating layer can be determined by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the steel sheet, and measuring the resulting solution by inductively coupled plasma (ICP) emission spectroscopy. For example, a 10 mass % hydrochloric acid solution containing 0.06 mass % inhibitor (Ivit 710K, manufactured by Asahi Chemical Industry Co., Ltd.) may be used as the acid solution containing the inhibitor for dissolving the plating layer.

[0027] 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 from the change in weight before and after removal of the plating layer. The plating layer is dissolved in an acid solution containing an inhibitor that suppresses corrosion of the base steel sheet, and the coating weight is determined from the change in weight before and after pickling and stripping of the plating layer. After removal of the plating layer, the base steel sheet is washed with water and dried.

[0028] [High-strength steel plate] At least one of the plurality of steel plates is a high-strength steel plate. Here, "high strength" means that the hardness in the first region is 200 Hv or more. The hardness of the steel plate is measured at a position of the first region of the steel plate constituting the weld joint, at a position of 1 / 2 depth. The hardness measurement is performed in accordance with JIS Z 2244:2009. The measurement load is 200 gf. The hardness in the first region may be 240 Hv or more, 270 Hv or more, or 340 Hv or more.

[0029] Here, the one or more plated steel sheets and the one or more high-strength steel sheets may be different steel sheets, or may be the same high-strength galvanized steel sheet that is high in strength and has been subjected to galvanization.

[0030] LME cracking occurs when hot-dip galvanization is present on the surface of a high-strength steel sheet during welding. For example, considering a welded joint consisting of two steel sheets, if at least one of the two steel sheets is high-strength and has hot-dip galvanization on the overlapping surface, LME cracking may occur. 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 be present on the overlapping surface of the steel sheets during welding, and the hot-dip galvanization may come into contact with the high-strength ungalvanized steel sheet, which may result in LME cracking. Since the welded joint of the present invention suppresses LME cracking during manufacturing even in such cases, it may also include a welded joint consisting of one steel sheet being a relatively low-strength galvanized steel sheet and the other being a high-strength ungalvanized steel sheet. In addition, the welded joint of the present invention may also include a welded joint in which one side is made of a relatively low-strength unplated steel 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.

[0031] The thickness of the steel plate constituting the welded joint of the present invention is not particularly limited. For example, it can be 0.1 to 3.2 mm. The thickness may be 0.2 mm or more, 0.4 mm or more, or 0.6 mm or more. The thickness may be 3.0 mm or less, 2.5 mm or less, 2.0 mm or less, or 1.8 mm or less.

[0032] [Chemical Composition] Of the steel plates constituting the welded joint of the present invention, the high-strength steel plate described above has the chemical composition described below. 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.

[0033] (C: 0.05 to 0.40%) C (carbon) is an element that ensures the strength of steel, and the C content is set to 0.05% or more. In order to prevent the C concentration in the surface layer, which will be described later, from becoming too high, and taking weldability into consideration, the C content is set to 0.40% or less. The C content may be 0.08% or more, 0.10% or more, or 0.15% or more. The C content may be 0.37% or less, 0.35% or less, or 0.30% or less.

[0034] (Si: 0.5 to 3.0%) Si (silicon) is an element that promotes ferrite stabilization and decarburization. By including Si, decarburization progresses in the surface layer through the pretreatment and heat treatment described below, and the ferrite in the surface layer is stabilized, thereby improving LME resistance. To achieve this effect, the Si content is set to 0.5% or more. If the Si content is too high, even if high dew-point annealing is performed, external oxidation progresses and oxides (scale) are formed in the surface layer of the steel sheet, which in turn suppresses decarburization at the outermost surface, thereby reducing the effect of improving LME resistance. In consideration of this point, the Si content is set to 3.0% or less. The Si content may be 0.6% or more, 0.7% or more, or 0.8% or more. The Si content may be 2.5% or less, 2.0% or less, or 1.5% or less.

[0035] (Mn: 0.1 to 5.0%) Mn (manganese) is an element that is effective in improving the strength of steel by obtaining a hard structure. In consideration of the strength of the steel, the lower limit of the Mn content is set to 0.1%. In addition, in consideration of the deterioration of workability due to Mn segregation, the Mn content is set to 5.0% or less. The Mn content may be 0.5% or more, 1.0% or more, or 1.5% or more. The Mn content may be 4.5% or less, 4.0% or less, or 3.5% or less.

[0036] (Sol. Al: 0 to 3.0%) Aluminum (aluminum) is an element that, like Si, promotes ferrite stabilization and decarburization by dissolving in steel. Sol. Al refers to acid-soluble Al that is soluble in acid and does not form oxides such as Al2O3. The amount of Al measured is determined by subtracting the insoluble residue on the filter paper that is generated during the Al analysis process. In the steel sheet constituting the welded joint of the present invention, the role of sol. Al can also be achieved by adding Si, so sol. Al is not essential, and the lower limit of the sol. Al content is 0%. If the sol. Al content is too high, even if high dew-point annealing is performed, external oxidation progresses, forming oxides (scale) on the surface layer of the steel sheet. Conversely, decarburization on the outermost surface is suppressed, reducing the effect of improving LME resistance. Taking this into consideration, the sol. Al content is set to 3.0% or less. The sol. Al content may be 0.1% or more, 0.3% or more, or 0.5% or more. The Al content may be 2.0% or less, 1.5% or less, or 1.0% or less.

[0037] As described above, since excessive addition of Si and sol. Al reduces LME resistance, the total content of Si and sol. Al is preferably 1.8% or less. The total content of Si and sol. Al may be 1.7% or less, 1.6% or less, or 1.5% or less.

[0038] (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.

[0039] (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.

[0040] (N: 0.0100% or less) N (nitrogen) is an impurity generally contained in steel. If the N content exceeds 0.0100%, weldability may be reduced. Therefore, the N content is set to 0.0100% or less. The N content may be 0.0080% or less, 0.0050% or less, or 0.0030% or less. It is preferable that no N is contained, and the lower limit of the N content is 0%. From the viewpoint of manufacturing costs, the N content may be more than 0%, 0.0005% or more, or 0.0010% or more.

[0041] (O: 0.0030% or less) O ​​(oxygen) is an element that forms oxides and reduces the workability of steel sheet. If the O content is too high, oxides are generated in excess, and the workability of the steel sheet is likely to be reduced. Therefore, the O content is set to 0.0030% or less. The O content may be 0.0026% or less, 0.0024% or less, 0.0020% or less, or 0.0018% or less. It is preferable that O is not contained, and the lower limit of the O content is 0%. From the viewpoint of production costs, the O content may be more than 0%, 0.0005% or more, or 0.0010% or more.

[0042] (B: 0 to 0.0100%) B (boron) is an element that improves hardenability and contributes to improving strength, and also segregates at grain boundaries to strengthen the grain boundaries and improve toughness, so it may be contained as needed. Since B is not an essential element, the lower limit of the B content is 0%. This effect can be obtained even with a small amount of B, but when B is contained, the B content is preferably 0.0001% or more. Furthermore, from the viewpoint of ensuring sufficient toughness, the B content is set to 0.0100% or less. The B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more. The B content may be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less.

[0043] (Ti: 0 to 0.1500%) Ti (titanium) is an element that precipitates as TiC during cooling of the steel and contributes to improving strength, so it may be contained as needed. Since it is not an essential element, the lower limit of the Ti content is 0%. This effect can be obtained even with a small amount of Ti, but when Ti is contained, the Ti content is preferably 0.0001% or more. The Ti content may be 0.0003% or more, or 0.0005% or more. On the other hand, if it is contained in excess, coarse TiN may be formed, which may impair toughness, so the Ti content is set to 0.1500% or less. The Ti content may be 0.1000% or less, 0.0500% or less, 0.0050% or less, or 0.0020% or less.

[0044] (Nb: 0 to 0.1500%) Nb (niobium) is an element that contributes to improving strength by improving hardenability, so it may be contained as needed. Since it is not an essential element, the lower limit of the Nb content is 0%. This effect can be obtained even with a small amount of Nb contained, but when Nb is contained, the Nb content is preferably 0.0001% or more. The Nb content may be 0.0005% or more, or 0.0010% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the Nb content is set to 0.1500% or less. The Nb content may be 0.1000% or less, 0.0600% or less, or 0.0200% or less.

[0045] (V: 0 to 0.150%) V (vanadium) is an element that contributes to improving strength by improving hardenability, so it may be contained as needed. Since it is not an essential element, the lower limit of the V content is 0%. This effect can be obtained even with a small amount of V content, but when V is contained, the V content is preferably 0.001% or more. The V content may be 0.003% or more, 0.005% or more, or 0.008% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the V content is set to 0.150% or less. The V content may be 0.100% or less, 0.060% or less, or 0.020% or less.

[0046] (Cr: 0 to 2.00%) Cr (chromium) is effective in improving the hardenability of steel and increasing its strength, and may therefore be contained as needed. Since Cr is not an essential element, the lower limit of the Cr content is 0%. This effect can be obtained even with a small amount of Cr content, but when Cr is contained, the Cr content is preferably 0.001% or more. The Cr content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, if 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, 1.50% or less, 0.50% or less, or 0.20% or less.

[0047] (Ni: 0 to 2.00%) Ni (nickel) is effective in improving the hardenability of steel and increasing its strength, and may be contained as needed. Since it is not an essential element, the lower limit of the Ni content is 0%. This effect can be obtained even with a small amount of Ni, but when Ni is contained, the Ni content is preferably 0.001% or more. The Ni content may be 0.01% or more, 0.02% or more, or 0.05% or more. On the other hand, since excessive addition of Ni increases costs, the Ni content is set to 2.00% or less. The Ni content may be 1.80% or less, 1.50% or less, 0.50% or less, or 0.20% or less.

[0048] (Cu: 0 to 2.0000%) Cu (copper) is effective in improving the hardenability of steel and increasing the strength of steel, so it may be contained as needed. Since it is not an essential element, the lower limit of the Cu content is 0%. This effect can be obtained even with a small amount of Cu, but when contained, the Cu content is preferably 0.0001% or more. The Cu content may be 0.0002% or more, or 0.0005% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness and cracking of the slab after casting, the Cu content is set to 2.0000% or less. The Cu content may be 1.8000% or less, 1.5000% or less, 0.0050% or less, or 0.0020% or less.

[0049] (Mo: 0 to 1.00%) Mo (molybdenum) is effective in improving the hardenability of steel and increasing the strength of steel, and may be contained as needed. Since it is not an essential element, the lower limit of the Mo content is 0%. This effect can be obtained even with a small amount of Mo, but when Mo is contained, the Mo content is preferably 0.001% or more. The Mo content may be 0.01% or more, 0.02% or more, or 0.03% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the Mo content is set to 1.00% or less. The Mo content may be 0.80% or less, 0.60% or less, or 0.20% or less.

[0050] (W: 0 to 1.000%) W (tungsten) is effective in improving the hardenability of steel and increasing its strength, and may be contained as needed. Since it is not an essential element, the lower limit of the W content is 0%. This effect can be obtained even with a small amount of W content, but when W is contained, the W content is preferably 0.001% or more. The W content may be 0.002% or more, or 0.003% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the W content is set to 1.000% or less. The W content may be 0.800% or less, 0.600% or less, 0.300% or less, 0.100% or less, or 0.020% or less.

[0051] (Ca: 0 to 0.1000%) Ca (calcium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, so it may be contained as needed. Since it is not an essential element, the lower limit of the Ca content is 0%. This effect can be obtained even with a small amount of Ca content, but when Ca is contained, the Ca content is preferably 0.0001% or more. The Ca content may be 0.0002% or more, or 0.0003% or more. On the other hand, if it is contained in excess, deterioration of surface properties may become apparent, so the Ca content is set to 0.1000% or less. The Ca content may be 0.0800% or less, 0.0500% or less, 0.0300% or less, 0.0100% or less, or 0.0010% or less.

[0052] (Mg: 0 to 0.100%) Mg (magnesium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, so it may be contained as needed. Since it is not an essential element, the lower limit of the Mg content is 0%. This effect can be obtained even with a small amount of Mg, but when Mg is contained, the Mg content is preferably 0.0001% or more. The Mg content may be 0.0005% or more, or 0.0008% or more. On the other hand, if Mg is contained in excess, deterioration of surface properties may become apparent, so the Mg content is set to 0.100% or less. The Mg content may be 0.090% or less, 0.080% or less, 0.030% or less, 0.010% or less, or 0.002% or less.

[0053] (Zr: 0 to 0.100%) Zr (zirconium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be contained as needed. Since it is not an essential element, the lower limit of the Zr content is 0%. This effect can be obtained even with a small amount of Zr contained, but when contained, the Zr content is preferably 0.001% or more. The Zr content may be 0.005% or more, 0.010% or more. On the other hand, if contained in excess, deterioration of surface properties may become apparent, so the Zr content is set to 0.100% or less. The Zr content may be 0.050% or less, 0.030% or less.

[0054] (Hf: 0 to 0.100%) Hf (hafnium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be contained as needed. Since it is not an essential element, the lower limit of the Hf content is 0%. This effect can be obtained even with a small amount of Hf contained, but when contained, the Hf content is preferably 0.0001% or more. The Hf content may be 0.0003% or more, or 0.0005% or more. On the other hand, if contained excessively, deterioration of surface properties may become apparent, so the Hf content is set to 0.100% or less. The Hf content may be 0.050% or less, 0.030% or less, 0.010% or less, 0.005% or less, or 0.002% or less.

[0055] (REM: 0 to 0.1000%) REM (rare earth element) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be contained as needed. Since it is not an essential element, the lower limit of the REM content is 0%. This effect can be obtained even with a small amount of REM, but when REM is contained, the REM content is preferably 0.0001% or more. The REM content may be 0.0003% or more, or 0.0005% or more. On the other hand, excessive REM content may cause deterioration of surface properties to become apparent, so the REM content is set to 0.1000% or less. The REM content may be 0.0500% or less, 0.0300% or less, 0.0100% or less, 0.0050% or less, or 0.0020% or less. REM is an abbreviation for Rare Earth Metal and refers to an element belonging to the lanthanide series. REM is usually added as misch metal.

[0056] In the high-strength steel plate constituting the welded joint according to the present invention, the balance other than the above chemical components consists of Fe and impurities. In the steel plate according to the present invention, the balance may be Fe and impurities, that is, the balance may consist only of Fe and impurities. Here, impurities refer to components that are mixed in during industrial production of steel 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.

[0057] The chemical components of the steel sheet may be analyzed using an elemental analysis method known to those skilled in the art, for example, inductively coupled plasma mass spectrometry (ICP-MS). However, C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method. O may be measured using an inert gas fusion-infrared absorption method. These analyses may be performed on samples collected from the steel sheet according to JIS G0417:1999.

[0058] [Tensile Strength] The high-strength steel plate constituting the welded joint according to the present invention may have a tensile strength of 780 MPa or more. The present invention aims to suppress LME, which occurs when welding high-strength steel plates, i.e., steel plates with high tensile strength and hardness, to produce a welded joint. The welded joint according to the present invention can suppress LME even when using steel plates with a tensile strength of 780 MPa or more. The upper limit of the tensile strength is not particularly limited, but may be, for example, 2000 MPa or less from the viewpoint of ensuring toughness. The tensile strength is measured in accordance with JIS Z 2241:2011 using a JIS No. 5 tensile test specimen with the longitudinal direction perpendicular to the rolling direction and the plate thickness direction. The tensile strength may be 980 MPa or more, or 1180 MPa or more.

[0059] Next, the structure of the welded joint will be described. Here, the surface layer of the steel sheet refers to a layered region having a depth of a predetermined distance from the surface of the steel sheet (sheet surface of the steel sheet) in the sheet thickness direction of the steel sheet. In the case of a plated steel sheet, the surface of the steel sheet (sheet surface of the steel sheet) refers to the surface of the steel sheet (sheet surface) excluding the plating. The predetermined distance in the sheet thickness direction can be the longer (deeper) length (depth) of the "depth at which the C concentration of the surface layer is 0.02% or less" or the "depth at which the area ratio of the ferrite phase is 90% or more," which will be described later.

[0060] <Characteristics of the first area>

[0061] [Interface between coating layer and base steel sheet, or surface roughness Ra of steel sheet] In the high-strength steel sheet constituting the welded joint of the present invention, in the first region, the interface between the coating layer and base steel sheet when the high-strength steel sheet has a coating layer, or the surface roughness Ra of the high-strength steel sheet when the high-strength steel sheet does not have a coating layer is 3.0 μm or less in terms of arithmetic mean height Ra defined by JIS B0601:2013. Higher roughness makes cracks more likely to occur due to stress concentration. The surface roughness Ra may be 2.0 μm or less. When the high-strength steel sheet has a coating layer, the interface roughness Ra is the surface roughness Ra of the steel sheet measured after removing the coating. The coating layer is removed by dissolving the coating layer in an acid solution to which an inhibitor that suppresses corrosion of the base steel sheet has been added, as in the measurement of the coating weight described above.

[0062] In the present invention, when measuring the surface roughness Ra, 10 measurement points are randomly selected on the surface of the steel sheet in accordance with JIS B 0601:2013 so that the distance between each measurement point is 1 mm or more. The surface profile at each measurement point is measured using a laser microscope (for example, "VK-X3000" manufactured by Keyence Corporation). Specifically, an image is taken at a magnification of 20 times using a laser microscope, and the arithmetic mean roughness (Ra) at each measurement point is determined using a reference length of 2000 μm in the captured image. The arithmetic mean value of the arithmetic mean roughness (Ra) at each measurement point is defined as the "surface roughness Ra."

[0063] [Depth where C concentration in surface layer is 0.02% or less] In the high-strength steel sheet constituting the welded joint of the present invention, the depth where the C concentration is 0.02% or less as measured by GDS (glow discharge spectroscopy) is 3 μm or more from the steel sheet surface in the sheet thickness direction. The starting point in the depth direction is the interface between the coating layer and the base steel sheet when the high-strength steel sheet has a coating layer, or the surface of the high-strength steel sheet when the high-strength steel sheet does not have a coating layer.

[0064] Since the susceptibility to LME decreases as the carbon concentration decreases, the resistance to LME is improved by lowering the carbon concentration in the surface layer. Furthermore, since carbon is an austenite stabilizing element, a low carbon content stabilizes the layer with low LME susceptibility.

[0065] Such a surface layer structure (metal structure in the surface layer of the steel sheet) can be obtained as a decarburized layer produced by setting the chemical composition of the steel sheet as described above and performing the pretreatment and annealing described below.

[0066] Since a depth of 3 μm or more where the C concentration is 0.02% or less contributes to improving LME resistance, there is no particular limitation on the upper limit of the depth where the C concentration is 0.02% or less. The depth where the C concentration is 0.02% or less may be, for example, 50 μm or less, 40 μm or less, or 30 μm or less. The depth where the C concentration is 0.02% or less is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 10 μm or more, 15 μm or more, or 20 μm or more.

[0067] The GDS measurement is performed at five measurement points in the sheet thickness direction, and the arithmetic mean value of the depth of the region where the C concentration is 0.02% or less at each measurement point is taken as the depth where the C concentration in the surface layer is 0.02% or less. The five measurement points are randomly selected so that there is an interval of 5 mm or more between each measurement point on the steel sheet surface. The measurement conditions are as follows. Naturally, measurement results can be obtained even if the conditions below, for example the measurement device, etc., are not followed. However, if differences arise in the measurement results, the steel sheet according to the present invention is identified by the measurement results according to the conditions below.

[0068] 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

[0069] [Diffraction intensity ratio of ferrite phase by grazing incidence X-ray diffraction (XRD)] In the high-strength steel plate constituting the welded joint of the present invention, in grazing incidence X-ray diffraction at an incident angle of 1° to the steel plate surface, the conditional expression of 0.45≦I(110) / (I(110)+I(200)+I(211))≦0.90 is satisfied, where the diffraction intensity corresponding to the (110) plane is I(110), the diffraction intensity corresponding to the (200) plane is I(200), and the diffraction intensity corresponding to the (211) plane is I(211).

[0070] The value of the middle part of the conditional expression, "I(110) / (I(110)+I(200)+I(211))", is preferably 0.85 or less, more preferably 0.80 or less, and even more preferably 0.75 or less. This conditional expression means that the ferrite phase is randomly oriented. When the ferrite phase is completely randomly oriented, the value of the middle part is 0.67.

[0071] Grazing incidence X-ray diffraction (also referred to as grazing incidence XRD, low-angle incidence XRD, or tilted XRD) is a measurement technique in which the incident angle of incident X-rays is set small and the detector is scanned (the detection angle is changed) while maintaining the incident angle. This allows for efficient detection of information on the orientation of crystal grains up to a depth of approximately several micrometers in the surface layer of the sample. In the present invention, the X-ray incident angle is fixed at 1° to detect the orientation of ferrite in the surface layer of the steel sheet. The incident angle is the angle between the surface of the sample (steel sheet) and the direction of incidence of the incident X-rays. If the high-strength steel sheet has a coating layer, the coating layer is removed before measurement. The coating layer is removed by dissolving the coating layer in an acid solution containing an inhibitor that suppresses corrosion of the base steel sheet, as in the above-mentioned coating weight measurement.

[0072] Figure 2 shows examples of the results of oblique incidence XRD analysis when the ferrite phase is randomized (b) and not (a). (a) shows the results of oblique incidence XRD analysis of a normal (conventional) steel plate, which shows that the ferrite phase is oriented in the (110) direction. Therefore, the value of the middle part of the conditional expression, "I(110) / (I(110)+I(200)+I(211))", is relatively large at 0.91. (b) shows the results of oblique incidence XRD analysis of a steel plate constituting a welded joint of the present invention, which shows that the orientation in the (110) direction is smaller than in (a). Therefore, the value of the middle part of the conditional expression, "I(110) / (I(110)+I(200)+I(211))", is relatively small at 0.58.

[0073] <Characteristics of the second area>

[0074] [Thickness of high ferrite layer (layer with ferrite area ratio of 90% or more)] In the high-strength steel plate constituting the welded joint of the present invention, in the second region, the thickness of a layer with a ferrite area ratio of 90% or more (hereinafter referred to as "high ferrite layer") in the thickness direction of the base steel plate starting from the interface between the plating layer and the base steel plate is 3 μm or more.

[0075] A high ferrite layer having a thickness of 3 μm or more contributes to suppressing LME during the production of a welded joint, so there is no particular upper limit to the thickness. The thickness of the high ferrite layer may be, for example, 100 μm or less, 80 μm or less, 60 μm or less, or 40 μm or less. The thickness of the high ferrite layer is preferably 5 μm or more, more preferably 8 μm or more, and even more preferably 10 μm or more, or 20 μm or more.

[0076] The structure of the high ferrite layer other than ferrite is not limited, and may be, for example, one or more of martensite, bainite, and cementite.

[0077] The thickness of the high ferrite layer is measured by analyzing secondary electron images of the observation cross section, which is mechanically polished to a mirror finish and then etched with nital, using SEM observation. For SEM observation, a field-emission scanning electron microscope (e.g., JEOL Ltd.'s "JSM 7000F," accelerating voltage: 15 kV) is used. The observation field of view in the observation cross section is a depth range of 500 μm from the surface of the steel sheet (sheet surface) in the thickness direction (longitudinal direction of the observation cross section) and a width range of 600 μm in the direction perpendicular to the thickness direction (lateral direction of the observation cross section). In the observation cross section, five observation fields are observed with an SEM in the direction perpendicular to the thickness direction (lateral direction of the observation cross section) with an interval of 1000 μm or more between each observation field, and secondary electron images are obtained. The observation resolution is 1280 × 960 pixels. In the case of plated steel sheets, the surface of the steel sheet (sheet surface) refers to the surface of the steel sheet excluding the plating.

[0078] The ferrite fraction is calculated for the five obtained secondary electron images by the point counting method. More specifically, a grid with equal intervals is first drawn on the secondary electron image. Next, the number of grid points at each grid point where the structure is ferrite is determined and divided by the total number of grid points to measure the ferrite fraction. The greater the total number of grid points, the more accurately the area fraction can be determined. In the present invention, the grid spacing is 2 μm × 2 μm, and the total number of grid points is 1,500.

[0079] In a secondary electron image, a region with relatively low brightness and no visible substructure can be determined to be ferrite. Here, substructure refers to a transformed structure formed inside the prior austenite phase, such as laths or blocks. In a secondary electron image, ferrite is observed as a region with relatively low brightness and a relatively monotonous spread of brightness and color. In the present invention, there is no particular need to distinguish metal structures other than ferrite. However, the following criteria are used to distinguish tempered martensite, pearlite, ferrite, fresh martensite, retained austenite, or bainite in secondary electron images. A region with intragranular substructure (lath boundaries, block boundaries) and where carbides are precipitated with multiple variants is determined to be tempered martensite. Furthermore, a region where cementite is precipitated in a lamellar form is determined to be pearlite. A region with low brightness and no visible substructure is determined to be ferrite. A region with high brightness and where the substructure is not revealed by etching is determined to be fresh martensite or retained austenite. The area that does not fall into any of the above categories is judged to be bainite. Simply put, the area ratio of the ferrite phase can be calculated by distinguishing between ferrite and other structures.

[0080] Figure 3 shows an example of an SEM micrograph of the surface layer of a steel plate constituting a welded joint of the present invention, located 0 to 100 μm away from the weld shoulder. In the micrograph of Figure 3, the white area is ferrite. It can be seen that the surface layer is mainly composed of ferrite.

[0081] Ferrite has low LME susceptibility. From the viewpoint of suppressing LME during the production of welded joints, it is preferable for the surface layer structure of the steel sheet to be mainly composed of ferrite. Such a surface layer structure can be obtained by setting the chemical composition of the steel sheet as described above and performing the pretreatment process and annealing process described below.

[0082] When the steel sheet has a plating layer, the depth at which the C concentration is 0.02% or less and the thickness at which the ferrite area ratio is 90% or more as measured by GDS start from the interface between the plating layer and the base steel sheet.

[0083] <<Manufacturing Method>> 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.

[0084] The high-strength steel sheet constituting the welded joint according to the present invention can be obtained by a manufacturing method including, for example, a casting step of casting molten steel with adjusted chemical composition to form a steel billet, a hot rolling step of hot-rolling the steel billet to obtain a hot-rolled steel sheet, a coiling step of coiling the hot-rolled steel sheet, a cold rolling step of cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet, a pretreatment step of pretreating the cold-rolled steel sheet (shot blasting), and an annealing step of annealing the pretreated cold-rolled steel sheet. Alternatively, the hot-rolled steel sheet may be pickled and cold-rolled as is without being coiled after the hot rolling step.

[0085] <Casting Step> The conditions for the casting step are not particularly limited. For example, after melting in a blast furnace or an electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a method such as ordinary continuous casting or casting by an ingot method.

[0086] <Hot Rolling Process> A hot-rolled steel plate can be obtained by hot-rolling the steel slab obtained by casting. The hot rolling process is carried out by reheating the cast steel slab directly or after cooling it once, followed by hot rolling. When reheating is carried out, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot rolling process, rough rolling and finish rolling are usually carried out. The temperature and reduction rate of each rolling step may be changed as appropriate depending on the desired metal structure and plate thickness. For example, the finishing temperature of finish rolling may be 900 to 1050°C, and the reduction rate of finish rolling may be 10 to 50%.

[0087] <Coiling step> The hot-rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature may be changed as appropriate depending on the desired metal structure, etc., and may be, for example, 500 to 800°C. The hot-rolled steel sheet may be recoiled before or after coiling, and subjected to a predetermined heat treatment. Alternatively, the hot-rolled steel sheet may be pickled after the hot-rolling step without performing the coiling step, and then subjected to the cold-rolling step described below.

[0088] <Cold Rolling Step> After subjecting the hot-rolled steel sheet to pickling or the like, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio of the cold rolling may be appropriately changed depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold rolling step, the sheet may be cooled to room temperature, for example, by air cooling.

[0089] <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.

[0090] The pretreatment involves shot blasting, in which spherical abrasives are projected onto the surface of the cold-rolled steel sheet. There are no particular limitations on the abrasives that can be used, but for example, steel balls (shot) with a median particle size of 40 to 450 μm can be used. Examples of such steel balls (shot) include TSH30 manufactured by WINOA IKK JAPAN. The amount of shot projected is 5 to 400 kg / m. 2 This makes it possible to introduce strain into the surface layer of the steel sheet without increasing the surface roughness Ra of the steel sheet. By carrying out 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. The greater the amount of shot blasting, the greater the effect of improving LME resistance. However, at a fixed amount of shot blasting, for example 400 kg / m, 2 Above this, the effect is saturated. 2 The level per unit time and unit area is 4.0 x 10 -4 kg / (mm 2 ・min).

[0091] <Annealing Step> The cold-rolled steel sheet that has been subjected to the pretreatment step is annealed.

[0092] In the present invention, a steel sheet that has been strained by shot blasting is subjected to an annealing process, which includes holding the steel sheet at a predetermined temperature and a high dew point. The heating rate to the predetermined holding temperature is not particularly limited, and may be 1 to 10°C / second. The dew point is controlled by humidification control from 300°C or higher but lower than 600°C, preferably from 450 to 550°C. That is, the dew point (humidification) control start temperature is 300°C or higher but lower than 600°C, preferably within the range of 450 to 550°C. The dew point without dew point (humidification) control is usually lower than -30°C. The dew point (high dew point) during annealing is set to -30 to 20°C to promote decarburization. The dew point (high dew point) during annealing is preferably -10°C or higher. The dew point during annealing is preferably 5°C or lower. The predetermined holding temperature (maximum heating temperature) in the annealing step is 750 to 900°C, preferably 770 to 870°C, in order to promote decarburization. The holding time at the holding temperature (maximum heating temperature) in the annealing step is 20 to 300 seconds, preferably 50 to 200 seconds. The atmosphere is preferably a non-oxidizing atmosphere, such as N2-1 to 10 vol% H2 or N2-2 to 4 vol% H2.

[0093] By setting the dew point, holding temperature, and holding time within the above ranges, decarburization can be promoted, the C concentration in the surface layer can be reduced, and the ferrite phase fraction can be appropriately controlled. Furthermore, by setting the dew point (humidification) control start temperature within the above range, decarburization of the surface layer of the steel sheet can be promoted. At the same time, internal oxidation of Si and Mn progresses rapidly, and internal oxides are rapidly formed. The formed internal oxides function as nucleation sites, resulting in randomization of the orientation of the ferrite phase. If the dew point (humidification) control start temperature is too low, external oxidation progresses and internal oxidation of Si and Mn does not progress, making it difficult to randomize the orientation of the ferrite phase.

[0094] Annealing is performed under a tension of, for example, 1 to 20 MPa. Applying tension during annealing makes it possible to introduce strain into the steel sheet more effectively, and promotes decarburization of the surface layer.

[0095] By carrying out the above-mentioned steps, decarburization is promoted in the surface layer portion of the steel sheet, and a high-strength steel sheet can be obtained in which the surface layer portion of the steel sheet has a structure mainly composed of randomly oriented ferrite phase.

[0096] <Method for manufacturing plated steel sheet> The plated steel sheet constituting the welded joint according to the present invention can be obtained by performing a plating treatment to form a Zn-containing plating layer on the surface of the steel sheet. The plating layer may be formed on the high-strength steel sheet manufactured as described above.

[0097] <Plating Treatment Step> The plating treatment may be carried out according to a method known to those skilled in the art. The plating treatment may be carried out, for example, by hot-dip plating or by electroplating. Preferably, the plating treatment is carried out by hot-dip plating. The plating treatment conditions may be appropriately set taking into consideration the chemical composition, thickness, and coating amount of the desired plating layer. After the plating treatment step, a known alloying treatment step may be carried out to form alloyed plating.

[0098] <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 5 to 10 mm, with a pressure of 1.0 to 5.0 kN, a current flow time of 0.2 to 1.2 seconds, and a current flow of 6 to 14 kA.

[0099] The welded joint according to the present invention can be suitably used in a wide range of fields, such as automobiles, home appliances, building materials, etc., because LME cracking during manufacturing is suppressed. In particular, it can be suitably used in the automobile field as a joining structure for joining automobile components.

[0100] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0101] <Test No. 1> Molten steel adjusted to the chemical composition shown in Test No. 1 in Table 1 was melted in a blast furnace and cast by continuous casting to obtain a steel slab. The obtained steel slab was heated to 1200°C and hot-rolled with a finish rolling ending temperature of 950°C and a finish rolling reduction of 30% to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was coiled at a coiling temperature of 650°C, pickled, and then cold-rolled with a reduction of 50% to obtain a cold-rolled steel sheet. The cold-rolled steel sheet had a thickness of 1.6 mm.

[0102] Next, the surface of the obtained cold-rolled steel sheet was blasted with TSH30 manufactured by WINOA IKK JAPAN Co., Ltd. at a blast rate of 5 kg / m. 2 The cold rolled steel sheet after the shot blasting treatment had a surface roughness Ra of 2.9 μm.

[0103] Subsequently, the shot-blasted cold-rolled steel sheet was subjected to an annealing treatment in which the temperature was raised to 500°C at a rate of 6.0°C / sec in an N-4% H gas atmosphere in a furnace with an oxygen concentration of 20 ppm or less, and then further raised to 800°C at a rate of 2.0°C / sec and held at that temperature for 40 seconds. At this time, dew point control was started so that the dew point would become 0°C from 300°C. The annealing treatment was performed with a tension of 5.0 MPa applied to the steel sheet.

[0104] Furthermore, the annealed steel sheet was immersed in a hot dip galvanizing bath (Zn-0.14% Al) at 450°C for 3 seconds, then pulled out at 100 mm / sec, and the coating weight was reduced to 50 g / m using N2 wiping gas. 2 Thereafter, alloying treatment was carried out at 520°C for 30 seconds to obtain a galvannealed steel sheet.

[0105] The obtained galvannealed steel sheet and one steel sheet of the same type as a counterpart were stacked one on top of the other, and spot welding was performed using a dome radius welding electrode with a tip diameter of 8 mm at an impact angle of 5°, a pressure of 4.0 kN, a welding time of 1.6 seconds, and a current of 13 kA to produce a welded joint, and the LME resistance during production was evaluated. Note that, hereinafter, the steel sheets produced in the examples will also be referred to as "example steel sheets," and the steel sheets combined when producing the welded joint will also be referred to as "counterpart steel sheets."

[0106] <Test Nos. 2 to 55> Steel sheets or plated steel sheets were prepared under the same conditions as in Example 1, except that the chemical composition of the steel sheets was as shown in Table 1 or Table 2, the pretreatment process conditions, the annealing process conditions were as shown in Table 3, and the plating conditions were as shown in Table 4. Test No. 32 omitted shot blasting, and Test No. 35 performed surface treatment by brush grinding instead of shot blasting. Regarding the plating type in Table 4, "a" indicates alloyed hot-dip galvanizing, "b" indicates hot-dip galvanizing without the alloying treatment in Example 1, "c" indicates a Zn-1.5%Al-1.5%Mg plating bath without the alloying treatment, and "non-plated" indicates a cold-rolled steel sheet that was not plated. Welded joints were produced under the same welding conditions as in Test No. 1, using the steel sheets listed in Table 4 as the mating materials. The "same type" for the mating material indicates that the same type of steel sheet as in the examples was used as the mating steel sheet. "Non-plated same type" indicates that the counterpart steel sheet used was a steel sheet of the same type as that of the example but not plated, "GA same type" indicates that the counterpart steel sheet used was a steel sheet of the same type as that of the steel sheet of the corresponding test number (Test No.) but plated with alloyed zinc. "GI270IF" indicates that the counterpart steel sheet used was a commercially available hot-dip galvanized steel sheet with a tensile strength of 270 MPa, and "GA590" indicates that the counterpart steel sheet used was a commercially available hot-dip galvanized steel sheet with a tensile strength of 590 MPa.

[0107]

[0108]

[0109]

[0110] (Surface roughness after pretreatment) The "surface roughness after pretreatment" shown in Table 3 is the surface roughness Ra of the steel sheet measured in accordance with JIS B 0601:2013 after the pretreatment step and before the annealing step.

[0111] Grade AA: 2.0 μm or less Grade A: Over 2.0 μm, 3.0 μm or less Grade B: Over 3.0 μm

[0112] The welded joints manufactured by spot welding were subjected to the following evaluations.

[0113] (First Region) The hardness of the example steel plate was measured in the first region of the welded joint using the method described above. The hardness of the steel plate was measured at a position half the depth of the example steel plate in accordance with JIS Z 2244:2009. The measurement load was 200 gf. The hardness was evaluated as follows:

[0114] Rating AAA: 340Hv or more Rating AA: 270Hv or more, less than 340Hv Rating A: 200Hv or more, less than 270Hv

[0115] The surface roughness Ra of the unplated steel sheets was measured in the same manner as before annealing, and the plating was removed from the exposed base steel sheets. The plating was removed by dissolving the plating layer in a 10 mass % hydrochloric acid solution containing 0.06 mass % inhibitor (Ivit 710K, manufactured by Asahi Chemical Industry Co., Ltd.) that inhibits corrosion of the base steel sheet.

[0116] Furthermore, a 30 mm × 30 mm cut sample was taken from the first region of the welded joint, and five GDS measurements were performed in the thickness direction using the method described above to determine the depth at which the C concentration was 0.02% or less. The "C≦0.02% depth" in Table 4 is the average value of the depths at which the C concentration was 0.02% or less, determined by five GDS measurements.

[0117] In addition, samples cut to 20 mm x 20 mm were collected, and the grazing incidence X-ray diffraction intensity I(110) at an incidence angle of 1° from the (110) plane of the steel sheet surface, the grazing incidence X-ray diffraction intensity I(200) at an incidence angle of 1° from the (200) plane of the steel sheet surface, and the grazing incidence X-ray diffraction intensity I(211) at an incidence angle of 1° from the (211) plane of the steel sheet surface were measured. In Table 4, "grazing incidence XRD" refers to the value of I(110) / (I(110) + I(200) + I(211)). For plated steel sheets, the plating layer was dissolved in a 10 mass% hydrochloric acid solution containing 0.06 mass% inhibitor (Ibit 710K, manufactured by Asahi Chemical Industry Co., Ltd.) to suppress corrosion of the base steel sheet. After removing the plating layer, the grazing incidence X-ray diffraction intensity was measured.

[0118] (Second Region) Samples cut to 25 mm x 15 mm were collected from the second region of the welded joint and subjected to nital etching. The T-section of each sample was observed with an SEM to measure the thickness of the layer with an area ratio of ferrite phase of 90% or more (high ferrite layer). The thickness was measured at five equally spaced points within a range of 500 μm in the T direction, and the average value was calculated. Here, the starting point of the "thickness" is the surface of the steel sheet for unplated steel sheets, and the starting point is the interface between the plating layer and the base steel sheet for plated steel sheets.

[0119] (LME Resistance) A method for evaluating LME resistance will be described with reference to FIG. 4 . LME resistance was evaluated by the length of an LME crack (crack 11 immediately outside the pressure weld) that occurred immediately outside the pressure welded portion 2 formed by spot welding two overlapping steel sheets 1. The two steel sheets 1 refer to the steel sheet with each test number (Test No.) and its mating steel sheet. The "immediately outside the pressure welded portion" of the weld refers to the outer portion of the pressure welded portion 6, which is the portion of the overlapping surface of the two steel sheets that is pressure welded by spot welding, and refers to a position near the pressure welded portion 6 (within a range of approximately 1 mm outward from the end of the pressure welded portion 6). The crack length of the crack 11 immediately outside the pressure welded portion was evaluated. The spot welding test was performed three times, and the crack 11 immediately outside the pressure welded portion with the longest length was evaluated. The evaluation criteria were as follows. In this example, a rating of A or higher (i.e., ratings A, AA, and AAA) was determined to be excellent in LME resistance.

[0120] 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

[0121] The results of each evaluation are shown in Table 4.

[0122]

[0123] Test Nos. 1 to 25 and 37 to 55 are examples of the present invention and had high LME resistance.

[0124] In Test No. 32, shot blasting was not performed, so strain was not introduced into the surface layer of the steel plate. Therefore, decarburization was not promoted, and the depth at which the C concentration was 0.02% or less in the GDS measurement was shallow. Furthermore, internal oxidation of Si and Mn did not progress, and the orientation of the ferrite phase was not randomized. As a result, the LME suppression during weld joint production was poor.

[0125] In Test No. 35, pretreatment by brush grinding was performed instead of shot blasting, so strain was not sufficiently introduced into the surface layer. Therefore, it is thought that internal oxidation of Si and Mn did not progress during the annealing process, and the orientation of the ferrite phase was not randomized. As a result, the LME suppression during weld joint production was inferior.

[0126] In Test No. 36, the dew point control start temperature in the annealing process was low, which is thought to have caused external oxidation to progress, preventing decarburization and resulting in a shallow depth at which the C concentration was 0.02% or less in the GDS measurement. It is also thought that internal oxidation of Si and Mn did not progress, preventing randomization of the ferrite phase orientation. As a result, the welded joint had poor LME resistance during production.

[0127] According to the present invention, it is possible to provide a welded joint in which LME is suppressed during manufacturing, and the welded joint can be suitably used in applications such as automobiles, home appliances, and building materials, particularly automobiles. Therefore, the present invention has extremely high industrial applicability.

[0128] REFERENCE SIGNS LIST 1 steel plate 2 spot weld 3 indentation 4 weld shoulder 4a outer end of weld shoulder 5 nugget 6 pressure weld 11 crack directly outside pressure weld 21 first region 22 second region

Claims

1. A welded joint comprising: a plurality of overlapping steel plates; and a spot weld portion joining the plurality of steel plates, wherein the spot weld portion has a nugget, an indentation portion pressed down by an electrode, and a weld shoulder portion which is a peripheral portion of the indentation portion, and at least one of the plurality of steel plates is a plated steel plate having a plating layer containing Zn formed on at least a surface corresponding to the overlapping surface of the plurality of steel plates, and at least one of the plurality of steel plates constituting the overlapping surface is a high-strength steel plate having a hardness of 200 Hv or more in a first region which is a surface layer region located 5 mm or more away from an outer end of the nugget toward the outside of the nugget in a direction in which the sheet surfaces of the steel plates extend, and the high-strength steel plate has chemical compositions, in mass %, of C: 0.05-0.40%, Si: 0.5-3.0%, Mn: 0.1-5.0%, sol. Al: 0-3.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0-0.0100%, Ti: 0-0.1500%, Nb: 0-0.150%, 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%, and the balance being Fe and impurities, In the first region, the high strength steel plate has a surface roughness Ra of 3.0 μm or less, the depth at which the C concentration is 0.02% or less in the plate thickness direction from the surface of the high strength steel plate is 3 μm or more, and in an oblique incidence X-ray diffraction with an incidence angle of 1° on the surface of the high strength steel plate, when the diffraction intensity corresponding to the (110) plane is I(110), the diffraction intensity corresponding to the (200) plane is I(200), and the diffraction intensity corresponding to the (211) plane is I(211), the relationship of 0.45≦I(110) / (I(110)+I(200)+I(211))≦0.90 is satisfied, anda second region of the high-strength steel plate which is a surface region located at a distance of 0 to 100 μm from the weld shoulder toward the outside of the spot weld in a direction in which the plate surface of the high-strength steel plate extends, the second region having a ferrite area ratio of 90% or more has a thickness of 3 μm or more in a plate thickness direction from the surface of the high-strength steel plate; provided that, when the steel plate is the plated steel plate, the surface of the steel plate is an interface between the steel plate and a plating layer of the plated steel plate.

2. The welded joint according to claim 1, characterized in that in the first region, 0.45≦I(110) / (I(110)+I(200)+I(211))≦0.75 is satisfied.

3. The welded joint according to claim 1, characterized in that in the first region, the depth at which the C concentration is 0.02% or less is 10 μm or more.

4. The welded joint according to claim 1, characterized in that in the first region, the surface roughness Ra is 2.0 μm or less.

5. The welded joint according to claim 1, characterized in that in the second region, the thickness of the layer in which the area ratio of ferrite is 90% or more is 8 μm or more.

6. A joining structure for automobile components, in which the welded joint according to any one of claims 1 to 5 joins the automobile components when the plurality of steel plates are used as automobile components.

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