Spot welded joint and method for manufacturing spot welded joint

Abstract

The spot welded joint (1) according to one aspect of the present invention satisfies the conditions θ≦20.0 and 0.7≦D NUG / D IND ≦1.0, 0.5≦T IND / T PAR , and D NUG ≥ 6.0, θ is the angle between the edge of the indentation made in the zinc-based coating layer of the high-strength steel sheet and the surface of the high-strength steel sheet, and D NUG is the diameter of the nugget, and D IND is the diameter of the indentation (121) of the zinc-based coating layer on the high-strength steel plate, and T IND is the minimum thickness of the spot welded joint (1) inside the indentation (121), and T PAR is the total thickness of the steel plates outside the indentation (121).

Description

[Technical Field]

[0001] The present invention relates to a spot welded joint and a method for manufacturing a spot welded joint. This application claims priority based on Japanese Patent Application No. 2024-008710, filed on January 24, 2024, the contents of which are incorporated herein by reference. [Background technology]

[0002] Zinc-based plating layers dramatically improve the corrosion resistance of steel materials due to their sacrificial corrosion protection effect, and therefore are used as surface treatment layers for a variety of steel materials.

[0003] However, the zinc contained in the zinc-based plating layer can cause LME (Liquid Metal Embrittlement) cracking. When spot welding a zinc-based plated steel sheet with a zinc-based plating layer, the zinc contained in the zinc-based plating layer melts. The molten zinc penetrates the grain boundaries of the steel sheet and embrittles them. When stress is applied to the embrittled grain boundaries, cracks easily occur at the grain boundaries.

[0004] The conditions for LME cracking during spot welding are: Molten zinc comes into contact with solid steel. Tensile stress is applied to the contact area between the molten zinc and the solid steel plate. Furthermore, the higher the tensile strength of a steel sheet, the higher its susceptibility to LME cracking. Various studies have been conducted to prevent LME cracking.

[0005] Patent Document 1 discloses a resistance spot welding method including a step of welding a workpiece made of overlapping steel plates using a resistance spot welding device, at least one of which is a zinc-plated steel plate. In the welding step, the method discloses a resistance spot welding method in which the cooling rate of a high-tensile steel plate having a higher tensile strength than the other steel plates is increased.

[0006] Patent Document 2 discloses a resistance spot welding method for resistance spot welding a sheet set in which at least one steel sheet out of a plurality of overlapping steel sheets has a zinc-based plating layer, the method comprising the steps of: placing the sheet set between a pair of electrodes arranged opposite each other, each having a main electrode, at least one of which has an auxiliary electrode arranged near the main electrode and movable independently of the main electrode; passing current between the main electrode and the auxiliary electrode arranged near the main electrode to partially remove the zinc-based plating layer; and passing current between the pair of main electrodes while applying pressure to the sheet set, thereby joining the sheet set by resistance spot welding. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2022-15124 [Patent Document 2] Japanese Patent Publication No. 2020-142251 Summary of the Invention [Problem to be solved by the invention]

[0008] When spot welding two or more overlapping steel sheets to create a welded joint, disturbances can cause LME cracking. Disturbances in spot welding include the impact angle and the gap. The impact angle is the angle between the central axis of the spot welding electrode and the normal direction of the steel sheets. The gap is the size of the gap between the overlapping steel sheets. If spot welding is performed with a large impact angle and gap, LME cracking may occur on the surface that comes into contact with the electrode.

[0009] The frequency of LME cracking (sometimes called external cracking) on the surface of spot-welded joints correlates with the frequency of expulsion. Explosion is the phenomenon in which the base metal is locally overheated and molten and splashes during lap resistance welding, or the metal itself. The number and length of external cracks tend to increase as the expulsion increases and the weld becomes thinner. Explosion is particularly likely to occur when the welding current is increased to increase the nugget diameter and ensure joint strength, or when there is a gap. It is difficult to predict and prevent expulsion. Therefore, preventing external cracking is similarly difficult.

[0010] In spot welding, the depression on the surface of the base material caused by the electrode tip as a result of welding is called an indentation. Cracks in the center of the indentation have little effect on joint strength and are often not considered a problem. However, cracks at the edge of the indentation (sometimes called the shoulder) and in the area slightly outside the indentation can reduce joint strength. Therefore, it is desirable to suppress LME cracks at the edge of the indentation and in its vicinity.

[0011] The technology of Patent Document 1 suppresses LME cracking by making the contact area between the high-tensile steel sheet and the first electrode larger than the contact area between the other steel sheets and the second electrode, thereby making the cooling rate of the high-tensile steel sheet higher than that of the other steel sheets. However, if the heat input is increased to enlarge the nugget diameter, sufficient cooling cannot be achieved, and it is thought that LME cracking cannot be suppressed.

[0012] The technology of Patent Document 2 suppresses LME cracking by using an auxiliary electrode to partially remove the zinc-based plating layer before spot welding. However, it may be difficult to provide an auxiliary electrode to a spot welding device.

[0013] In view of the above circumstances, an object of the present disclosure is to provide a spot-welded joint in which a zinc-based plating layer formed on the surface of a high-strength steel plate faces the outside of the spot-welded joint, a sufficiently large nugget diameter is ensured, and LME cracking at the edge (shoulder) of an indentation formed in the zinc-based plating layer can be suppressed, and a method for manufacturing a spot-welded joint. [Means for solving the problem]

[0014] The gist of the present disclosure is as follows.

[0015] (1) A spot welded joint according to one aspect of the present invention is a spot welded joint comprising a plurality of stacked steel plates, a nugget joining the plurality of steel plates, and a zinc-based plating layer provided on one or both sides of one or more of the steel plates, wherein one or both of the steel plates arranged on a surface of the spot welded joint is a high-strength steel plate having a tensile strength of 980 MPa or more, the zinc-based plating layer is arranged on a surface of the spot welded joint that is also a surface of a high-strength steel plate, the zinc-based plating layer has an indentation, and satisfies the following formulas 1 to 4: θ≦20.0……(Formula 1) 0.7≦D NUG / D IND ≦1.0……(Formula 2) 0.5≦T IND / T PAR ......(Formula 3) D NUG ≧6.0……(Formula 4) θ is the angle (unit: degrees) formed between the edge of the indentation provided in the zinc-based plating layer of the high-strength steel sheet and the surface of the high-strength steel sheet, measured on a cross section passing through the center of the indentation and perpendicular to the surface of the spot-welded joint, and D NUG is the diameter (unit: mm) of the nugget measured at the cross section along the joint interface between the high-strength steel plate provided with the zinc-based plating layer disposed on the surface of the spot-welded joint and the adjacent steel plate, and D INDis the diameter (unit: mm) of the indentation in the zinc-based coating layer provided on the high-strength steel sheet, measured on the cross section, and T IND is the minimum thickness (unit: mm) of the spot weld joint inside the indentation measured at the cross section, and T PAR is the total thickness (unit: mm) of the plurality of steel plates outside the indentation. (2) In the spot welded joint described in (1) above, T IND / T PAR is less than 0.8.

[0016] (3) A manufacturing method of a spot welded joint according to another aspect of the present invention is a manufacturing method of a spot welded joint comprising a step of spot welding, using a pair of electrodes, to a sheet assembly formed by stacking a plurality of steel sheets, wherein one or both of the steel sheets arranged on the surface of the sheet assembly are high-strength steel sheets having a tensile strength of 980 MPa or more, a zinc-based plating layer is arranged on a surface of the sheet assembly that is also a surface of the high-strength steel sheet, a tip of the electrode in contact with the zinc-based plating layer provided on the high-strength steel sheet has a central part and a peripheral part surrounding the central part, the electrode in contact with the zinc-based plating layer has a nominal diameter D of 16 mm or less, a diameter of the central part is d, and a radius of curvature R in a range of d / 2 or more and d / 2+1.5 mm or less from a central axis of the electrode is more than 9.5 mm, and the spot welding is performed so as to satisfy the following formula: 6.0≦S≦20.0……(Formula 5) 2.5×(T PAR / 2)≦P……(Formula 6) S is the value (unit: kA sec) obtained by integrating the current (unit: kA) flowing through the pair of electrodes over the current application time (unit: sec), and T PAR is the total thickness (unit: mm) of the steel plates included in the plate set, and P is the pressure (unit: kN) of the pair of electrodes. (4) Preferably, in the manufacturing method of a spot-welded joint described in (3) above, the electrode in contact with the zinc-based plating layer provided on the high-strength steel plate is a dome radius electrode, the diameter d of the center is 5 to 10 mm, the radius of curvature R of the center is 30 to 50 mm, and the radius of curvature r of the peripheral portion is more than 9.5 mm but less than R. [Effects of the Invention]

[0017] According to the present disclosure, it is possible to provide a spot-welded joint in which a zinc-based plating layer formed on the surface of a high-strength steel plate faces the outside of the spot-welded joint, a sufficiently large nugget diameter is ensured, and LME cracking at the edge (shoulder) of the indentation formed in the zinc-based plating layer can be suppressed, and a method for manufacturing a spot-welded joint. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a cross-sectional schematic view of an example of a spot-welded joint. [Figure 2] FIG. 1 is a cross-sectional schematic view of an example of a spot-welded joint. [Figure 3] FIG. 1 is a cross-sectional schematic view of an example of a spot-welded joint. [Figure 4] FIG. 10 is an enlarged perspective view of the edge of an indentation in a spot welded joint with a cut surface. [Figure 5] FIG. 2 is a schematic diagram showing the positional relationship between an electrode and an indentation during spot welding. [Figure 6] FIG. 1 is a cross-sectional schematic diagram of a DR (dome radius) type electrode. DETAILED DESCRIPTION OF THE INVENTION

[0019] (1. Spot welded joints) The present inventors focused on the shape of the edge 211 of the tip 21 of the spot welding electrode 2, which is the location that comes into contact with the edge 1212 of the indentation. The present inventors discovered that by increasing the radius of curvature of the edge 211 of the electrode and decreasing the angle θ of the edge 1212 of the indentation, it is possible to suppress LME cracking in spot welding with a large heat input. The present inventors were then able to achieve both an increase in the nugget diameter and suppression of LME cracking at the edge 1212 of the indentation. A spot-welded joint 1 according to one embodiment of the present disclosure, obtained based on the above findings, will be described in detail below.

[0020] (Steel plate 11) The spot-welded joint 1 is obtained by spot welding a plurality of stacked steel plates 11. The number of steel plates 11 can be any value equal to or greater than two. As exemplified in FIGS. 2 and 3, the number of steel plates 11 may be two. As exemplified in FIG. 1, the number of steel plates 11 may be three or more. The steel plates 11 of the spot-welded joint 1 may be formed into a member by bending. For example, the spot-welded joint 1 may be a hat-shaped member, and one or more steel plates 11 may be hat-shaped steel. In this case, a plurality of nuggets 12 are provided on the flange of the hat-shaped member along the extension direction of the flange.

[0021] One or more of the steel plates 11 are high-strength steel plates 111. The high-strength steel plates 111 are steel plates 11 having a tensile strength of 980 MPa or more. As illustrated in FIG. 2, all of the steel plates 11 in the spot-welded joint 1 may be high-strength steel plates 111. On the other hand, as illustrated in FIGS. 1 and 3, one or more of the steel plates 11 in the spot-welded joint 1 may be low-strength steel plates 112. The low-strength steel plates 112 are steel plates 11 having a tensile strength of less than 980 MPa.

[0022] One or more of the high-strength steel plates 111 are disposed on the surface of the spot-welded joint 1. In other words, one or both of the steel plates 11 disposed on the surface of the spot-welded joint 1 are high-strength steel plates 111. For example, in the spot-welded joint 1 shown in FIGS. 1 and 2, both of the steel plates 11 disposed on the surface of the spot-welded joint 1 are high-strength steel plates 111. On the other hand, as in the spot-welded joint 1 shown in FIG. 3, a high-strength steel plate 111 may be disposed on only one surface.

[0023] (Zinc-based plating layer 13) The spot welded joint 1 has a zinc-based plating layer 13. The zinc-based plating layer 13 is a plating in which the proportion of zinc in the chemical composition is 50 mass % or more. Examples of the zinc-based plating layer 13 include pure zinc plating, hot-dip galvanizing, electrogalvanizing, and alloyed hot-dip galvanizing. The zinc-based plating layer 13 has a sacrificial anticorrosion effect, which serves to enhance the corrosion resistance of the spot welded joint 1.

[0024] The zinc contained in the zinc-based plating layer 13 melts during spot welding and penetrates into the grain boundaries of the heat-affected zone. The molten zinc that penetrates into the grain boundaries embrittles the grain boundaries. When stress is applied to the embrittled grain boundaries, cracks easily occur at the grain boundaries. The brittle cracks that occur in the zinc-based plating layer 13 are called LME (Liquid Metal Embrittlement) cracks.

[0025] The zinc-based plating layer 13 is provided on one or both surfaces of one or more steel sheets 11. The zinc-based plating layer 13 is disposed at least on a surface that is the surface of the spot-welded joint 1 and also the surface of the high-strength steel sheet 111. In other words, the high-strength steel sheet 111 disposed on the surface of the spot-welded joint 1 is a zinc-based plating steel sheet 11 having a zinc-based plating layer 13, and this zinc-based plating layer 13 faces outward from the spot-welded joint 1. For example, the zinc-based plating layers 13 of the spot-welded joint 1 in FIG. 1 are disposed on both surfaces of the spot-welded joint 1. Both of these zinc-based plating layers 13 are disposed on the surfaces of the high-strength steel sheets 111. The zinc-based plating layer 13 of the spot-welded joint 1 in FIG. 2 is disposed on only one surface of the spot-welded joint 1. This zinc-based plating layer 13 is disposed on the surface of the high-strength steel sheet 111. 3 is disposed on both surfaces of the spot-welded joint 1. One of these zinc-based plating layers 13 is disposed on the surface of the high-strength steel plate 111.

[0026] Naturally, a zinc-based plating layer 13 may be further provided on the surface of the low-strength steel sheet 112. Furthermore, a zinc-based plating layer 13 may be further provided inside the spot-welded joint 1. For example, the spot-welded joint 1 illustrated in FIG. 3 is formed by stacking a high-strength steel sheet 111 having zinc-based plating layers 13 on both sides and a low-strength steel sheet 112 having zinc-based plating layers 13 on both sides. The spot-welded joint 1 illustrated in FIG. 3 has four zinc-based plating layers 13. The uppermost zinc-based plating layer 13 is disposed on the surface of the spot-welded joint 1, which is also the surface of the high-strength steel sheet 111. Meanwhile, the second and third zinc-based plating layers 13 from the top are disposed at the joint interface 14 of the spot-welded joint 1. The fourth zinc-based plating layer 13 from the top, which is disposed on the surface of the spot-welded joint 1, is disposed on the surface of the low-strength steel sheet 112. The configuration illustrated in FIG. 3 is also naturally acceptable for the spot-welded joint 1 according to this embodiment. The term "joint interface" is defined in JIS Z 3001-6:2013 "Welding terminology - Part 6: Resistance welding" as "the surface where parts come into contact and face each other for joining."

[0027] (Indentation 121) An indentation 121 is provided on the zinc-based plating layer 13 disposed on the surface of the spot-welded joint 1, which is also the surface of the high-strength steel plate 111. The indentation 121 is defined in JIS Z 3001-6:2013 "Welding Terminology - Part 6: Resistance Welding" as "a depression on the surface of the base material caused by the electrode tip and disk electrode as a result of welding in lap resistance welding." In spot welding, the tip 21 of the electrode 2 is pressed into the surface of the steel plate 11, thereby forming the indentation 121.

[0028] LME cracking is highly likely to occur in the edge 1212 of the indentation 121 formed in the zinc-based coating layer 13 of the high-strength steel sheet 111. This is because all the conditions for LME cracking are met in this region. The conditions for LME cracking are as follows: The molten zinc comes into contact with the solid steel plate 11. Tensile stress is applied to the contact area between the molten zinc and the solid steel plate 11. Furthermore, the higher the tensile strength of the steel sheet, i.e., in the case of a high-strength steel sheet 111, the higher the susceptibility to LME cracking tends to be. All of these conditions are met at the edge 1212 of the indentation 121 formed in the zinc-based coating layer 13 of the high-strength steel sheet 111. For example, in the spot-welded joint 1 shown in FIG. 1, LME cracking is likely to occur in both the upper indentation 121 and the lower indentation 121. In the spot-welded joint 1 shown in FIGS. 2 and 3, LME cracking is likely to occur in the upper indentation 121.

[0029] For ease of explanation, (1) the surface of the spot-welded joint 1 on which (2) the high-strength steel plate 111 is disposed and (3) the zinc-based plating is disposed will be referred to as the "easily embrittled surface E." Furthermore, (A) the high-strength steel plate 111 disposed on the surface of the spot-welded joint 1 and (B) the zinc-based plating layer 13 disposed on the surface of the spot-welded joint 1 will be referred to as the "easily embrittled steel sheet 111E." In FIG. 1 , both surfaces of the spot-welded joint 1 are the easily embrittled surfaces E. In FIGS. 2 and 3 , only the upper surface of the spot-welded joint 1 is the easily embrittled surface E. The lower surface of the spot-welded joint 1 in FIG. 2 is not the easily embrittled surface E because it is not provided with a zinc-based plating. The lower surface of the spot-welded joint 1 in FIG. 3 is not the easily embrittled surface E because it is not provided with a high-strength steel plate 111. 3. Furthermore, although the high-strength steel plate 111 and the zinc-based plating are disposed at the joint interface 14 in FIG.

[0030] In order to prevent LME cracking at the edge 1212 of the indentation on the embrittlement-prone surface E and to increase the nugget diameter, the spot-welded joint 1 according to this embodiment satisfies the following formula. θ≦20.0……(Formula 1) 0.7≦D NUG / D IND ≦1.0……(Formula 2) 0.5≦T IND / T PAR ......(Formula 3) D NUG ≧6.0……(Formula 4) The symbols in the above formula are defined as follows: θ: Angle (unit: degrees) formed between the edge 1212 of the indentation 121 provided in the zinc-based plating layer 13 of the high-strength steel sheet 111 and the surface of the high-strength steel sheet 111, measured on a cross section passing through the center of the indentation 121 and perpendicular to the surface of the spot-welded joint 1 D NUG: Diameter (unit: mm) of the nugget 12 along the joint interface 14 between the high-strength steel plate 111 provided with the zinc-based plating layer 13 disposed on the surface of the spot-welded joint 1 and the adjacent steel plate 11, measured at the cross section. D IND : diameter of the indentation 121 in the zinc-based plating layer 13 provided on the high-strength steel plate 111 measured at the cross section (unit: mm) T IND : The minimum thickness of the spot welded joint 1 inside the indentation 121 measured at the cross section (unit: mm) T PAR : Total thickness of the plurality of steel plates 11 outside the indentation 121 (unit: mm) These formulas will be explained in detail below. In the following explanation, "a cross section passing through the center of the indentation 121 and perpendicular to the surface of the spot-welded joint 1" will be simply referred to as "cross section." All of Figs. 1 to 3 show a cross section passing through the center of the indentation 121 and perpendicular to the surface of the spot-welded joint 1. The center of the indentation 121 can be identified by observing the surface of the spot-welded joint 1 with the naked eye. By cutting the spot-welded joint 1 along a line passing through the center of the indentation 121 and adjusting the cross section appropriately, it is possible to determine θ, D NUG , D IND , T IND A cross section can be formed to measure the above.

[0031] (For θ and Eq. 1) θ is a value for evaluating the curvature of the edge 1212 of the indentation 121 provided on the embrittlement surface E. Fig. 4 shows an enlarged perspective view of the outer edge 1211 and edge 1212 of the indentation 121 of the spot-welded joint 1 on which a cut surface has been formed. Below, the definitions of the outer edge 1211 and edge 1212 and the method for measuring θ will be explained with reference to Fig. 4.

[0032] The dashed-dotted line marked with the symbol X in Figure 4 is an imaginary line that follows the surface of the high-strength steel sheet 111 outside the indentation 121. In Figure 4, the dashed-dotted line X is drawn not along the surface of the high-strength steel sheet 111 but along the surface of the zinc-based plating layer 13 provided on the surface of the high-strength steel sheet 111, but for ease of explanation. In Figure 4, the zinc-plated layer is drawn as if it were extremely thick for ease of explanation. However, when measuring θ in an actual spot-welded joint 1, the zinc-plated layer is very thin, so the presence of the zinc-plated layer can be ignored.

[0033] When measuring θ, the outer edge 1211 of the indentation 121 in the cross section is defined as the intersection of the dashed-dotted line X (an imaginary line along the surface of the high-strength steel plate 111 outside the indentation 121) and the inner surface of the indentation 121. The edge 1212 of the indentation 121 is defined as the region from the outer edge 1211 of the indentation 121 to a position 200 μm away along the dashed-dotted line X. The edge 1212 is sometimes referred to as the shoulder of the indentation 121.

[0034] 4 is an imaginary line connecting the inner edge of the edge 1212 and the outer edge 1211 (i.e., the outer edge of the edge 1212). The dot-dash line Y is considered to be an imaginary line along the edge 1212. θ substituted into Equation 1 is the angle between the dot-dash line X and the dot-dash line Y.

[0035] Note that θ is measured at both ends of the indentation 121 provided on the embrittlement surface E in the cross section of the nugget. If both surfaces of the spot-welded joint 1 are embrittlement surfaces E, the angle is measured at both ends of the indentation 121 on both surfaces. For example, in the spot-welded joint 1 of FIG. 1, θ is measured at both ends of the upper indentation 121 and at both ends of the lower indentation 121. At each of the four locations (both ends of the upper indentation 121 and both ends of the lower indentation 121), θ must satisfy Equation 1.

[0036] On the other hand, in the indentation 121 formed on a surface that is not the embrittlement-prone surface E, θ does not need to satisfy Equation 1. For example, the lower indentation 121 in FIG. 2 does not have a zinc-based plating layer 13. Therefore, θ at both ends of the lower indentation 121 in FIG. 2 is not particularly limited. Furthermore, the lower indentation 121 in FIG. 3 has a zinc-based plating layer 13, but this is not formed on the surface of the high-strength steel plate 111. Therefore, θ at both ends of the lower indentation 121 in FIG. 3 is not particularly limited. In the spot-welded joint 1 in FIG. 2 or FIG. 3, θ is measured at both ends of the upper indentation 121. It is sufficient that θ at each of the two locations satisfies Equation 1.

[0037] As shown in the above formula 1, θ is set to 20.0 degrees or less. θ may be 19.0 degrees or less, 18.0 degrees or less, or 16.0 degrees or less.

[0038] (D IND , D NUG , and for Eq. 2) D IND is the diameter of the indentation 121 on the embrittlement-prone surface E. NUG is the nugget diameter for the embrittlement steel plate 111E. Hereinafter, D IND and D NUG 1 to 3, for the sake of convenience, the zinc-based plating layer 13 is depicted as being very thick. However, in reality, the zinc-based plating layer 13 is usually very thin compared to the thickness of the steel sheet 11 and the diameter of the nugget 12. By observing the cross section of an actual spot-welded joint 1, it is possible to measure D. NUG When measuring the above and other values, the surface of the zinc-based plating layer 13 and the surface of the high-strength steel sheet 111 can be regarded as the same.

[0039] D IND is the diameter of the indentation 121 formed on the embrittlement-prone surface E. The diameter of the indentation 121 is the distance between the two outer edges 1211 of the indentation 121 as determined in the cross section. The method for determining the outer edges 1211 of the indentation is as described above with reference to FIG. 4. NUGis the nugget diameter measured along the joining interface 14 between the embrittlement steel plate 111E and the steel plate 11 adjacent thereto.

[0040] When both surfaces of the spot welded joint 1 are embrittlement surfaces E, that is, when the number of embrittlement steel plates 111E is two, it is necessary for each of the two embrittlement steel plates 111E to satisfy Equation 2. For example, in the spot welded joint 1 shown in FIG. 1, the number of embrittlement steel plates 111E is two. D of the indentation 121 of the upper embrittlement steel plate 111E IND and D of the indentation 121 of the lower embrittlement steel plate 111E. IND Also, D measured at the joint interface 14 of the upper embrittlement steel plate 111E is different from NUG and D measured at the joint interface 14 of the lower embrittlement steel plate 111E. NUG In the spot welded joint 1 shown in FIG. 1, D IND and D NUG Also, the other easily embrittled steel plate 111E D IND and D NUG It is also necessary to satisfy Formula 2. Although not shown, the same applies to the spot welded joint 1 in which two embrittlement-prone steel plates 111E are stacked together.

[0041] As shown in the above formula 2, in the embrittlement steel plate 111E, D NUG / D IND is between 0.7 and 1.0. NUG / D IND may be 0.75 or more, 0.8 or more, or 0.85 or more. NUG / D IND may be 0.95 or less, 0.9 or less, or 0.85 or less.

[0042] (T IND , T PAR , and for Eq. 3) T IND is the minimum thickness of the spot welded joint 1 inside the indentation 121, measured in cross section. PARis the total thickness of the steel plates 11 outside the indentation 121. Hereinafter, with reference to FIGS. 1 to 3, T IND and T PAR The measurement method will be explained.

[0043] As illustrated in FIGS. 1 to 3, indentations 121 are formed on two surfaces of the spot welded joint 1. The minimum value of the thickness of the spot welded joint 1 inside the indentation 121, i.e., the minimum value of the distance between the bottoms of the two indentations 121 along the direction perpendicular to the surface of the spot welded joint 1 outside the indentation 121, is T IND Typically, the thickness of the spot welded joint 1 is smallest at the center of the indentation 121. The thickness of the zinc-based plating layer 13 is included in the thickness of the spot welded joint 1.

[0044] T IND is measured at the cross section, while T PAR does not have to be measured at the cross section. PAR is the total thickness of the steel plates 11 outside the indentation 121. Therefore, by measuring the thickness of each of the steel plates 11 at a location away from the spot weld using a known tool such as a vernier caliper and adding them up, T PAR The gap between the steel plates 11 (sheet separation) is obtained by PAR The thickness of the spot weld joint 1 shall be measured at the location where there is no sheet separation. PAR The thickness of the zinc-based plating layer 13 is included in the thickness of the steel sheet 11. Alternatively, if the thicknesses of all the steel sheets are given as information in advance, the values can be simply summed up to obtain T without measuring. PAR It is also possible to do so.

[0045] As shown in Equation 3 above, T IND / T PAR is 0.5 or more. T IND / T PAR may be 0.55 or more, 0.6 or more, 0.7 or more, or 0.8 or more.

[0046] (Regarding Equation 4) As shown in Equation 4, D NUG In the spot welded joint 1 having two bonding interfaces 14 for the embrittlement steel plate 111E as illustrated in FIG. 1, the D NUG must satisfy Equation 4. Note that D shown on the right side of Equation 4 NUG The lower limit may be 6.2 mm, 6.5 mm, or 6.8 mm.

[0047] (Action and effect) The surface of the spot welded joint 1 according to this embodiment is provided with a zinc-based plating layer 13. The zinc-based plating layer 13 has the effect of increasing the corrosion resistance of the spot welded joint 1.

[0048] However, the zinc-based plating layer 13 may cause LME cracking in the high-strength steel sheet 111. LME cracking at the edge 1212 of the indentation (sometimes referred to as the shoulder of the indentation 121) reduces the joining strength of the spot-welded joint 1. It is necessary to prevent LME cracking at least at the edge 1212 of the indentation. Therefore, the present inventors focused on the shape of the electrode 2 for spot welding. The present inventors attempted to suppress edge cracking by improving the shape of the region of the tip 21 of the electrode 2 that contacts the edge 1212 of the indentation (see FIG. 5 ). Hereinafter, a portion of the tip 21 of the electrode 2 that includes the region that contacts the edge 1212 of the indentation will be referred to as the “edge 211 of the electrode.” The definition of the edge 211 of the electrode will be described later, but the edge 1212 of the indentation contacts a part of the edge 211 of the electrode.

[0049] The present inventors have found that making the radius of curvature at the edge 211 of the electrode larger than usual is effective in preventing LME cracking.

[0050] The inventors predict that if the radius of curvature at the edge 211 of the electrode is made larger than usual, the following effects will be produced. (1) Cooling effect of the electrode edge 211 If the radius of curvature at the edge 211 of the electrode is made larger than usual, the contact area between the edge 211 of the electrode and the high-strength steel plate 111 will be larger than usual. As a result, the current density will decrease and the amount of heat dissipated from the electrode 2 to the high-strength steel plate 111 will increase. As a result, it is expected that the maximum temperature at the edge 211 of the electrode will be reduced. (2) Reduced scattering effect When the contact area between the electrode edge 211 and the high-strength steel plate 111 is larger than usual, the compressive stress applied to the weld is reduced compared to the normal electrode 2. As a result, it is believed that the amount of spatter is reduced. When the amount of spatter is reduced, it is believed that the tensile stress generated at the edge 1212 of the indentation and on the outside of the indentation 121 is also reduced. Furthermore, when the electrode 2 suppresses the occurrence of spatter, it becomes easier to increase the heat input and enlarge the nugget diameter. (3) Stress reduction effect When the contact area between the edge 211 of the electrode and the high-strength steel plate 111 is larger than usual, the amount by which the edge 211 of the electrode presses into the edge 1212 of the indentation and the resulting local bending deformation are reduced compared to the normal electrode 2. As a result, it is presumed that the tensile stress generated at the edge 1212 of the indentation is also reduced. It is believed that optimizing the radius of curvature at edge 211 of the electrode brings about the effects of reducing both temperature and stress as described above, and exerts the effect of suppressing LME cracking at edge 1212 of the indentation.

[0051] In the spot-welded joint 1 according to this embodiment produced by the above-described means, θ is 20.0 degrees or less. If an electrode 2 and other welding conditions are selected that make θ exceed 20.0 degrees, the stress when the edge 211 of the electrode is pressed into the high-strength steel plate 111 will increase, and LME cracking will likely occur.

[0052] In addition, in the spot welded joint 1 according to this embodiment, D NUG / D IND When this condition is satisfied, the contact area between the edge 211 of the electrode and the high-strength steel plate 111 increases, and D NUG / D INDAs the value approaches 0.7, the distance from the nugget to the shoulder increases, which makes it difficult for the temperature at the shoulder to rise, further enhancing the crack suppression effect. This effect can be expected especially when large expulsions occur.

[0053] Furthermore, in the spot welded joint 1 according to this embodiment, T IND / T PAR In other words, it is permissible for the electrode 2 to be pressed deep into the steel sheet 11, and for the thickness of the spot welded joint 1 inside the indentation 121 to be reduced to about 0.5 of the total thickness of the steel sheet 11. IND / T PAR In a spot welded joint 1 where T is approximately 0.5, large expulsion occurs during manufacturing, and the electrode 2 may be pushed deep into the weld. In the spot welded joint 1 according to this embodiment, the occurrence of such large expulsion is permitted. In normal spot welding, T IND / T PAR If large expulsion occurs, such that T is approximately 0.5, LME cracking will occur. However, in the spot welded joint 1 according to this embodiment, the shape of the electrode 2 is improved to make θ 20.0 degrees or less, so LME cracking will not occur even if large expulsion occurs. However, T IND / T PAR If T is small, the joint strength may decrease. IND / T PAR is preferably 0.6 or more, or 0.7 or more.

[0054] When large expulsion is permitted, it is easy to manufacture the spot welded joint 1. Furthermore, when large expulsion is permitted, it is easy to increase the heat input and enlarge the nugget diameter. As a result, in the spot welded joint 1 according to this embodiment, D NUG ≧6.0 When the nugget diameter is 6.0 mm or more, the joining strength of the spot welded joint 1 is significantly increased.

[0055] The most basic aspect of the spot welded joint 1 according to this embodiment has been described above. A more preferred aspect will now be described.

[0056] (T IND / T PAR upper limit of In the spot welded joint 1 according to this embodiment, T IND / T PAR may be 0.8 or less. That is, the total depth of the indentations 121 formed on the top and bottom of the nugget 12 may be 20% or more of the total thickness of the steel plate 11. Furthermore, T IND / T PAR It may be 0.75 or less, or 0.7 or less. Although scattering should be avoided, T IND / T PAR If a high heat input condition is selected that generates expulsion such that the expulsion factor is 0.8 or less, it becomes easier to ensure a sufficiently large nugget diameter of 6.0 mm or more.

[0057] (2. Manufacturing method of spot welded joint 1) Next, a method for manufacturing a spot welded joint 1 according to another aspect of the present disclosure will be described. The definitions of terms used in the description of the spot welded joint 1 according to this embodiment and various preferred aspects of the spot welded joint 1 according to this embodiment also apply to the method for manufacturing the spot welded joint 1 according to this embodiment. The spot welded joint 1 described above can be manufactured using the method for manufacturing the spot welded joint 1 according to this embodiment. However, the manufacturing method described below does not limit the scope of the spot welded joint 1 according to this embodiment described above.

[0058] (board set) The method for manufacturing the spot-welded joint 1 includes a step of spot welding to a sheet assembly. The sheet assembly is a welding base material formed by stacking a plurality of steel sheets 11. One or both of the steel sheets 11 arranged on the surface of the sheet assembly are steel sheets 11 having a tensile strength of 980 MPa or more, i.e., high-strength steel sheets 111. Furthermore, a zinc-based plating layer 13 is arranged on at least the surface of the sheet assembly that is also the surface of the high-strength steel sheet 111. In other words, the high-strength steel sheet 111 is used as the steel sheet 11, and the high-strength steel sheet 111 is stacked on another steel sheet 11 with the zinc-based plating layer 13 facing outward from the sheet assembly.

[0059] Various aspects of the steel sheet 11 in the spot-welded joint 1 according to this embodiment can be applied to a sheet assembly. For example, a zinc-based plating layer 13 may be provided on only one side of a high-strength steel sheet 111, or on both sides. The sheet assembly may include a low-strength steel sheet 112 having a tensile strength of less than 980 MPa. The low-strength steel sheet 112 may have a zinc-based plating layer 13 provided on one or both sides. That is, the low-strength steel sheet 112 may serve as the steel sheet 11.

[0060] (Shape of tip 21 of electrode 2) Spot welding is performed using a pair of spot welding electrodes 2. The spot welding electrodes 2 are rod-shaped electrodes that directly contact the base material during spot welding to pass the welding current and transmit the welding pressure (see JIS Z 3001-6:2013). The electrodes 2 are provided with a flow path through which a refrigerant can flow. When spot welding is performed, the refrigerant cools the tip 21 of the electrodes 2. This transfers heat from the welded portion to the tip 21 of the electrodes 2, lowering the temperature of the welded portion. In other words, a typical spot welding electrode 2 has the effect of cooling the welded portion.

[0061] In the manufacturing method of the spot-welded joint 1 according to this embodiment, it is necessary to appropriately select the shape of the tip 21 of the electrode 2 that contacts the zinc-based plating layer 13 provided on the high-strength steel sheet 111 (i.e., the electrode 2 that contacts the embrittlement surface E). This prevents LME cracking of the high-strength steel sheet 111. The shape of the tip 21 of the electrode 2 that contacts a location where the zinc-based plating layer 13 is not provided is not particularly limited. Furthermore, the shape of the tip 21 of the electrode 2 that contacts the zinc-based plating layer 13 provided on the low-strength steel sheet 112 is also not particularly limited. For example, in the example shown in FIG. 5 , the upper electrode 2 contacts the zinc-based plating layer 13 provided on the high-strength steel sheet 111. The shape of the tip 21 of the upper electrode 2 needs to be appropriately selected. On the other hand, the lower electrode 2 does not contact the zinc-based plating layer 13 provided on the high-strength steel sheet 111. Therefore, the shape of the tip 21 of the lower electrode 2 is not particularly limited.

[0062] Generally, spot welding electrodes are broadly classified into electrodes whose tip 21 has a central portion and a peripheral portion, with the central portion and the peripheral portion having different radii of curvature, as illustrated in FIG. 6 , and electrodes whose tip has a uniform radius of curvature. Examples of electrodes whose tip has a central portion and a peripheral portion, with the central portion and the peripheral portion having different radii of curvature, include a dome-radius electrode (DR-type electrode), a truncated cone electrode (CF-type electrode), and a truncated cone-radius electrode (CR-type electrode) defined in JIS C 9304:1999. Examples of electrodes whose tip has a uniform radius of curvature include a flat electrode (F-type electrode), a radius electrode (R-type electrode), and a dome-shaped electrode (D-type electrode) defined in JIS C 9304:1999. In the manufacturing method of a spot-welded joint 1 according to this embodiment, the tip of the electrode 2 in contact with the embrittlement-prone surface E may have any shape as illustrated in FIG. 6 . However, it is necessary to set the radius of curvature of at least the region in contact with the edge 1212 of the indentation 121 within a predetermined range.

[0063] An electrode having a central portion 21C and a peripheral portion 21P, as illustrated in FIG. 6 , will be described. In the electrode used in the manufacturing method of the spot welded joint 1 according to this embodiment, the diameter of the central portion 21C is defined as d. Furthermore, the range from d / 2 to d / 2+1.5 mm from the central axis of the electrode 2 is defined as the edge portion 211 of the electrode 2. The edge portion 211 of the electrode 2 is part of the peripheral portion 21P of the electrode 2. That is, as described above, part of the edge portion 211 of the electrode 2 contacts the edge portion 1212 of the indentation 121. 6 indicates the nominal diameter (outer diameter) of the electrode 2, and the symbol R1 in FIG. 6 indicates the radius of curvature of the center portion 21C of the electrode 2. The nominal diameter D of the electrode 2 in contact with the embrittlement surface E is 16 mm or less. Preferably, the nominal diameter D of the electrode 2 in contact with the embrittlement surface E is 15 mm or less, or 14 mm or less.

[0064] For the tip of an electrode that is normally distributed as a standard product, the radius of curvature r of the peripheral part including the edge 211 of the electrode 2 relative to the nominal diameter D is specified in JIS C 9304:1999. For example, when the nominal diameter D is 16 mm, the radius of curvature r is specified as 8 mm, and when the nominal diameter D is 20 mm, the radius of curvature r is specified as 10 mm.

[0065] On the other hand, in this embodiment, the radius of curvature r of the edge 211 of the electrode 2 that contacts the embrittlement surface E during spot welding is set to 9.5 mm or more. The edge 211 of the electrode 2 may be set to 10.0 mm or more. There is no particular upper limit to the radius of curvature r of the edge 211 of the electrode 2, but it may be, for example, 15.0 mm or less, 13.0 mm or less, 11.0 mm or less, or 10.0 mm or less. In such an electrode 2, the radius of curvature r in the region that contacts the edge 1212 of the indentation 121 is larger than the nominal diameter D of electrodes that are normally distributed as standard products.

[0066] (Spot welding conditions) Spot welding is performed so as to satisfy the following formula. 6.0≦S≦20.0……(Formula 5) 2.5×(T PAR / 2)≦P……(Formula 6) The symbols in the above formula are defined as follows: S: The current (unit: kA) flowing through the pair of electrodes 2 integrated over the current flow time (unit: sec) (unit: kA sec) T PAR : Total thickness of steel plates 11 included in the plate assembly (unit: mm) P: pressure applied by the pair of electrodes 2 (unit: kN)

[0067] (For S and Equation 5) S is a value used to evaluate the amount of heat input, and its unit is kA·sec. When spot welding is performed with a constant current, S is the current (kA) multiplied by the welding time (sec). When spot welding is performed under welding conditions where the current value is not constant, such as upslope welding or downslope welding, S is the integral of the welding waveform over the welding time. Note that post-heat treatment (heat treatment performed on the nugget 12 after it has been completed) is not included in spot welding. This is because post-heat treatment does not change the nugget diameter or other shapes of the weld. Therefore, the heat input during post-heat treatment is not included in S.

[0068] S is set to 6.0 kA·sec or more and 20.0 kA·sec or less. The higher the heat input, the larger the nugget diameter can be. In the manufacturing method of the spot-welded joint 1 according to this embodiment, the shape of the electrode 2 is optimized as described above, making LME cracking less likely to occur. Therefore, it is easy to increase the heat input. The lower limit of S is set to a value higher than that of normal spot welding. However, from the viewpoint of maintaining the soundness of the weld, the heat input is set to 20.0 kA·sec or less. S may be 6.5 kA·sec or more, 8.0 kA·sec or more, or 10.0 kA·sec or more. S may be 19.0 kA·sec or less, 18.0 kA·sec or less, or 15.0 kA·sec or less.

[0069] (T PAR , P, and Eq. 6) T PARis the total thickness (mm) of the steel plates 11 included in the plate assembly. The total thickness of the steel plates 11 included in the plate assembly is equal to the total thickness outside the indentation 121 of the steel plates 11 included in the spot-welded joint 1. Therefore, the same symbol is assigned to both. P is the pressure (kN) of the pair of electrodes 2.

[0070] The pressure P is determined according to the total thickness of the plate assembly. That is, the pressure P is 2.5 × (T PAR The applied pressure P is 2.8 × (T PAR / 2) or more, 3.0×(T PAR / 2) or more, or 3.2 × (T PAR / 2) or more.

[0071] The pressure P may be constant during spot welding, or may be varied within a range that satisfies Equation 6. In other words, as long as Equation 6 is always satisfied until the end of spot welding, the pressure can be varied as appropriate. The end of spot welding refers to the end of the welding current (main current) that is used to form the weld. Post-current application is not included in spot welding. The pressure during post-current application does not have to satisfy Equation 6.

[0072] (Action and effect) In the method for manufacturing a spot-welded joint 1 according to this embodiment, spot welding is performed using an electrode 2 having a larger-than-normal radius of curvature in the region that contacts the edge 1212 of the indentation 121. This makes it possible to prevent LME cracking at the edge 1212 of the indentation, as described above.

[0073] In addition, in the manufacturing method of the spot welded joint 1 according to this embodiment, the heat input S is set to 6.0 kA·sec or more, and the pressure P is set to 2.5×(T PAR The heat input S is set to a value greater than usual. This allows the diameter of the nugget 12 to be significantly enlarged.

[0074] In normal spot welding, increasing the heat input increases expulsion, deepening the indentation 121 and increasing the LME cracking at the edge 1212 of the indentation. However, in the manufacturing method for spot-welded joint 1 according to this embodiment, the occurrence of expulsion is suppressed by optimizing the shape of tip 21 of electrode 2. Furthermore, even if some expulsion occurs, LME cracking is suppressed.

[0075] The above has described the most basic aspect of the method for manufacturing the spot welded joint 1 according to this embodiment. A more preferred aspect will now be described.

[0076] (Dome radius electrode) A suitable example of the electrode 2 in contact with the embrittlement surface E is a dome radius electrode (DR electrode). The tip 21 of the dome radius electrode has a central portion 21C and a peripheral portion 21P surrounding the central portion 21C, and the radius of curvature R1 of the central portion 21C is larger than the radius of curvature r of the peripheral portion. An example of a side view of a dome radius electrode is described in JIS C 9304:1999 "Spot Welding Electrodes." The central portion 21C is sometimes referred to as the dome portion, and the peripheral portion 21P is sometimes referred to as the radius portion. When a dome radius electrode is used, it is preferable that the diameter d of the central portion is 5 to 10 mm, the radius of curvature R1 of the central portion 21C is 30 to 50 mm, and the radius of curvature r of the peripheral portion 21P is greater than 8.0 mm but less than R1.

[0077] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited thereto and can be modified as appropriate without departing from the technical spirit thereof. Below, more preferred examples of the spot-welded joint 1 according to the present embodiment and the manufacturing method thereof will be described. Unless otherwise specified, the embodiments described below are applicable to both the spot-welded joint 1 and the manufacturing method thereof.

[0078] (Chemical composition and metal structure of steel plate 11) There are no particular limitations on the chemical compositions and metal structures of the high-strength steel sheet 111 and the low-strength steel sheet 112. For example, the base steel sheet 11 of the hot-dip galvanized steel sheet 11 disclosed in International Publication No. 2020 / 162561 can be suitably used as the high-strength steel sheet 111 of the spot-welded joint 1 according to this embodiment.

[0079] (Thickness of steel plate 11) The thickness of the steel plates 11 is not particularly limited. The plate thickness ratio of the plate assembly is also not particularly limited. The plate thickness ratio of the plate assembly is the value obtained by dividing the total plate thickness of the plate assembly by the thickness of the thinner of the steel plates 11 arranged on the surface of the plate assembly. For example, when the spot welded joint 1 is an automobile part, the steel plates 11 are preferably two thick high-strength steel plates 111 and one thin low-strength steel plate 112. The high-strength steel plate 111 is a frame member of the automobile. The low-strength steel plate 112 is an exterior member of the automobile. The plate thickness of the high-strength steel plate 111 is preferably 1.0 to 2.5 mm, for example. The plate thickness of the low-strength steel plate 112 is preferably 0.4 to 1.2 mm, for example.

[0080] (Hardness of steel plate 11) There is no particular limitation on the hardness of the steel plate 11. There is a good correlation between the hardness and tensile strength of steel, and the Vickers hardness of a high-strength steel plate 111 having a tensile strength of 980 MPa or more is usually about 330 HV or more. The hardness of the high-strength steel plate 111 may be 360 HV or more, 390 HV or more, or 420 HV or more.

[0081] The Vickers hardness of the steel plate 11 is measured outside the heat-affected zone in accordance with JIS Z 2244:2009 "Vickers hardness test - Test method." The test force is 4.9N.

[0082] (Tensile strength of steel plate 11) The tensile strength of the high-strength steel plate 111 can be any value equal to or greater than 980 MPa. For example, the tensile strength of the high-strength steel plate 111 may be equal to or greater than 1200 MPa, equal to or greater than 1300 MPa, or equal to or greater than 1400 MPa. The tensile strength of the high-strength steel plate 111 may be equal to or less than 1270 MPa, equal to or less than 1200 MPa, or equal to or less than 1130 MPa. The tensile strength of the low-strength steel plate 112 may be equal to or greater than 330 MPa, equal to or greater than 390 MPa, or equal to or greater than 480 MPa. The tensile strength of the low-strength steel plate 112 may be equal to or less than 480 MPa, equal to or less than 420 MPa, or equal to or less than 360 MPa.

[0083] The tensile strength of the steel plate 11 is measured in accordance with JIS Z 2241:2011 "Methods for tensile testing of metallic materials." Test specimens are taken from outside the heat-affected zone. If it is difficult to take test specimens from the steel plate 11 of the spot-welded member, the tensile strength of the steel plate 11 may be estimated by measuring the Vickers hardness of the steel plate 11 using the above-mentioned method and converting the Vickers hardness to tensile strength using the hardness conversion table of SAE J417. [Example]

[0084] The effects of one embodiment of the present disclosure will be explained in more detail using examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present disclosure. The present disclosure is not limited to this example of conditions. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the disclosure and achieve the purpose.

[0085] Spot welding was performed under various conditions on various sheet assemblies formed by stacking multiple steel sheets. The presence or absence of expulsion during spot welding was confirmed. The shape of the spot welded joints obtained by spot welding and the presence or absence of cracks were also confirmed.

[0086] The steel sheets that make up the sheet assemblies are shown in Table 1. All sheet assemblies were manufactured by stacking two steel sheets of the same construction. All steel sheets were 1.6 mm thick zinc-based plated steel sheets with zinc-based plating layers on both surfaces.

[0087] [Table 1]

[0088] The spot welding conditions are shown in Table 2. In all spot welding, the configuration of the pair of electrodes was the same. All spot welding was performed using dome radius electrodes (DR electrodes), with a nominal diameter D of 16 mm and a curvature radius R1 of 40 mm at the center.

[0089] The welding was performed in two stages, including a period during which a small welding current was applied and a period during which a large welding current was applied. The welding current and welding time for the first stage and the welding current and welding time for the second stage are listed in the "Welding Conditions" column. The welding pressure P was 4 kN in all examples. The holding time (the time from the end of the welding current application to the start of opening the electrodes) is also listed in Table 2.

[0090] [Table 2]

[0091] Table 3 shows the shape of the spot-welded joints. Ten spot-welded joints were created for each condition. One of the ten was selected and its shape was measured using the method described above. Note that in all spot-welded joints, the steel plates on both surfaces were high-strength steel plates, and both surfaces were coated with zinc-based plating. Therefore, the parameters indicating the shape of the spot-welded joints were measured as follows:

[0092] θ (the angle between the edge of the indentation made in the zinc-based plating layer of the high-strength steel sheet and the surface of the high-strength steel sheet, measured on a cross section passing through the center of the indentation and perpendicular to the surface of the spot-welded joint) was measured at both ends of the upper indentation and both ends of the lower indentation on the cross section of the spot-welded joint.

[0093] D NUG(The diameter of the nugget measured in a cross section along the joint interface between a high-strength steel plate provided with a zinc-based plating layer disposed on the surface of a spot-welded joint and an adjacent steel plate) was measured along the joint interface between two high-strength steel plates.

[0094] D IND The diameter of the indentation in the zinc-based plating layer provided on the high-strength steel sheet, measured on the cross section, was measured for both the upper indentation and the lower indentation.

[0095] [Table 3]

[0096] Table 4 shows the occurrence of expulsion during spot welding and the occurrence of cracks in spot-welded joints. Ten spot-welded joints were created for each condition. The presence or absence of expulsion was visually confirmed after ten spot welds. Conditions in which expulsion occurred in one or more spot welds were marked "Yes" in the "Expulsion" column. Table 4 also lists the number of spot-welded joints in which cracks occurred among the ten spot-welded joints. The presence or absence of cracks was also visually confirmed. Cases in which the number of cracks was three or less were determined to be examples in which cracking was sufficiently suppressed.

[0097] [Table 4]

[0098] In Test No. 1, Test No. 5, and Test No. 10, the radius of curvature r of the peripheral part of the electrode was insufficient. As a result, in the spot-welded joints of Test No. 1, Test No. 5, and Test No. 10, θ at all measurement points did not satisfy Equation 1. As a result, cracks occurred frequently in the spot-welded joints of Test No. 1, Test No. 5, and Test No. 10.

[0099] In Test No. 9, the radius of curvature r of the peripheral part of the electrode was also insufficient. In Test No. 9, cracking could be suppressed by reducing the heat input. However, in Test No. 9, the nugget diameter D NUG did not satisfy Equation 4.

[0100] On the other hand, in the spot welding of Test Nos. 2 to 4 and Test Nos. 6 to 8, the electrode shape was appropriate. Also, in the spot welding of Test Nos. 2 to 4 and Test Nos. 6 to 8, Equation 5 regarding the heat input and Equation 6 regarding the welding force were satisfied. In the spot-welded joints of Test Nos. 2 to 4 and Test Nos. 6 to 8, θ satisfied Equation 1 at all measurement points, and D NUG and D IND In addition, in the spot-welded joints of Test Nos. 2 to 4 and Test Nos. 6 to 8, T IND / T PAR satisfies Equation 3, and D NUG satisfied Equation 4. In the spot-welded joints of Test Nos. 2 to 4 and Test Nos. 6 to 8, the occurrence of cracks was sufficiently suppressed. [Explanation of symbols]

[0101] 1 Spot welded joints 11 Steel plate 111 High strength steel plate E Easily embrittlement surface 111E Brittle steel plate 112 Low strength steel plate 12 Nuggets 121 Indentation 1211 Outer edge of indentation 1212 Edge of indentation 13 Zinc-based plating layer 14 Bonding interface 2 electrodes 21 Tip 21C Center of electrode 21P Periphery of electrode 211 Edge of electrode R1 Radius of curvature at center d center diameter r Radius of curvature of the periphery D nominal diameter

Claims

1. Multiple stacked steel plates; a nugget joining a plurality of the steel plates; a zinc-based plating layer provided on one or both sides of one or more of the steel sheets; A spot welded joint comprising: One or both of the steel plates arranged on the surface of the spot welded joint is a high-strength steel plate having a tensile strength of 980 MPa or more, the zinc-based plating layer is disposed on a surface of the spot-welded joint that is also a surface of the high-strength steel plate, The zinc-based plating layer has an indentation formed thereon, The following formulas 1 to 4 are satisfied: θ≦20.0……(Formula 1) 0.7≦D ＮＵＧ / D ＩＮＤ ≦1.0………(Formula 2) 0.5≦T ＩＮＤ / T ＰＡＲ ………(Formula 3) D ＮＵＧ ≧6.0………(Formula 4) θ is the angle (unit: degrees) formed between the edge of the indentation provided in the zinc-based plating layer of the high-strength steel sheet and the surface of the high-strength steel sheet, measured on a cross section passing through the center of the indentation and perpendicular to the surface of the spot-welded joint, D ＮＵＧ is the diameter (unit: mm) of the nugget measured at the cross section along the joint interface between the high-strength steel plate provided with the zinc-based plating layer disposed on the surface of the spot-welded joint and the steel plate adjacent thereto, D ＩＮＤ is the diameter (unit: mm) of the indentation in the zinc-based coating layer provided on the high-strength steel sheet, measured on the cross section, T ＩＮＤ is the minimum thickness (unit: mm) of the spot welded joint inside the indentation measured at the cross section, T ＰＡＲ is the total thickness (unit: mm) of the plurality of steel plates outside the indentation.

2. T ＩＮＤ / T ＰＡＲ The spot welded joint according to claim 1, wherein the σ is 0.8 or less.

3. A method for manufacturing a spot welded joint, comprising a step of spot welding a plate assembly formed by stacking a plurality of steel plates using a pair of electrodes, One or both of the steel plates arranged on the surface of the plate assembly is a high-strength steel plate having a tensile strength of 980 MPa or more, a zinc-based plating layer is disposed on the surface of the sheet assembly, which is also the surface of the high-strength steel sheet; a tip of the electrode in contact with the zinc-based plating layer provided on the high-strength steel plate has a central portion and a peripheral portion surrounding the central portion, The electrode in contact with the zinc-based plating layer has a nominal diameter D of 16 mm or less, a diameter of the center portion is d, and a radius of curvature R in a range of d / 2 or more and d / 2+1.5 mm or less from a central axis of the electrode is more than 9.5 mm, The spot welding is performed so as to satisfy the following formula: 6.0≦S≦20.0……(Formula 5) 2.5×(T ＰＡＲ / 2)≦P………(Formula 6) S is a value (unit: kA sec) obtained by integrating the current (unit: kA) flowing through the pair of electrodes over the current application time (unit: sec), T ＰＡＲ is the total thickness (unit: mm) of the steel plates included in the plate assembly, P is the pressure applied by the pair of electrodes (unit: kN) A method for manufacturing spot welded joints.

4. the electrode in contact with the zinc-based plating layer provided on the high-strength steel plate is a dome radius electrode, The diameter d of the center portion is 5 to 10 mm, The radius of curvature R of the central portion is 30 to 50 mm, The radius of curvature r of the peripheral portion is greater than 9.5 mm and less than R The method for manufacturing a spot welded joint according to claim 3.

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