Spot-welded joint and method for producing spot-welded joint

Abstract

A spot-welded joint (1) according to an aspect of the present invention satisfies θ ≤ 20.0, 0.7 ≤ DNUG / DIND ≤ 1.0, 0.5 ≤ TIND / TPAR, and DNUG ≥ 6.0, where θ is the angle between the edge of an indentation provided in a galvanized layer of a high-strength steel plate and the surface of the high-strength steel plate, DNUG is the diameter of the nugget, DIND is the diameter of the indentation (121) in the galvanized layer provided on the high-strength steel plate, TIND is the minimum thickness of the spot-welded joint (1) inside the indentation (121), and TPAR is the total thickness of a plurality of steel plates outside the indentation (121).

Description

Spot welded joint and method for manufacturing spot welded joint

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

[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, zinc contained in a zinc-based coating layer may cause liquid metal embrittlement (LME) cracking. When a zinc-based coated steel sheet having a zinc-based coating layer is spot welded, the zinc contained in the zinc-based coating layer melts. The molten zinc penetrates into the grain boundaries of the steel sheet and embrittles the grain boundaries. When stress is applied to the embrittled grain boundaries, cracks easily occur at the grain boundaries.

[0004] The conditions for LME cracking to occur during spot welding are: - Molten zinc comes into contact with the steel sheet in a solid state - Tensile stress is applied to the contact area between the molten zinc and the solid steel sheet. In addition, the higher the tensile strength of the steel sheet, the higher the sensitivity to LME cracking tends to be. Various studies have been conducted to prevent LME cracking.

[0005] Patent Document 1 discloses a resistance spot welding method that includes 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 cooling rate of a high-tensile steel plate, which has a higher tensile strength than the other steel plates, is set to be higher than the cooling rate of the other steel plates.

[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.

[0007] JP 2022-15124 A JP 2020-142251 A

[0008] When two or more overlapping steel sheets are spot-welded to produce a welded joint, disturbances can cause LME cracking. Disturbances in spot welding include, for example, 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. When spot welding is performed with a large impact angle and gap, LME cracking may occur on the surface in 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 refers to 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 the occurrence of 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 therefore 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, it is thought that sufficient cooling cannot be achieved and LME cracking cannot be suppressed.

[0012] The technique 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.

[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 (Equation 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 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, 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 0.8 or less.

[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 including 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 (Equation 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), and T ＰＡＲ is the total thickness (unit: mm) of the steel plates included in the plate assembly, 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 central portion is 5 to 10 mm, the radius of curvature R of the central portion is 30 to 50 mm, and the radius of curvature r of the peripheral portion is more than 9.5 mm and less than R.

[0017] According to the present disclosure, it is possible to provide a spot-welded joint in which a zinc-based plating layer provided 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 can be suppressed at the edge (shoulder) of the indentation formed in the zinc-based plating layer, and a method for manufacturing a spot-welded joint.

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

[0019] (1. Spot-welded joint) 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 aspect of the present disclosure, obtained based on the above findings, will be described in detail below.

[0020] (Steel plates 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 refer to 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 refer to 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 Figures 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 Figure 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 corrosion protection effect, thereby enhancing 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 plated 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. The zinc-based plating layers 13 of the spot-welded joint 1 in Fig. 3 are 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 to be joined."

[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 at 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 of the conditions for LME cracking are met in this region. The conditions for LME cracking are: molten zinc comes into contact with the steel sheet 11 in a solid state; and tensile stress is applied to the contact area between the molten zinc and the steel sheet 11 in a solid state. Furthermore, the higher the tensile strength of the steel sheet, i.e., in the case of the 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] Hereinafter, for ease of explanation, (1) the surface of the spot-welded joint 1, (2) the high-strength steel plate 111 disposed thereon, and (3) the surface on which the zinc-based plating is disposed, will be referred to as the "easily embedded surface E." Also, (A) the high-strength steel plate 111 disposed on the surface of the spot-welded joint 1, and (B) the high-strength steel plate 111 provided with the zinc-based plating layer 13 disposed on the surface of the spot-welded joint 1 will be referred to as the "easily embedded steel sheet 111E." In FIG. 1 , both surfaces of the spot-welded joint 1 are the easily embrittlement surface E. In FIGS. 2 and 3 , only the upper surface of the spot-welded joint 1 is the easily embrittlement surface E. The lower surface of the spot-welded joint 1 in FIG. 2 is not provided with the zinc-based plating, and therefore is not the easily embrittlement surface E. The lower surface of the spot welded joint 1 in Fig. 3 does not have the high-strength steel plate 111 arranged thereon, and is therefore not an easily embrittled surface E. In addition, the joint interface 14 in Fig. 3 has the high-strength steel plate 111 arranged thereon and is also coated with zinc-based plating, but is not a surface of the spot welded joint 1, and is therefore not an easily embrittled surface E.

[0030] In order to prevent LME cracking at the edge 1212 of the indentation on the embrittlement 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 (Equation 4) The symbols included in the above equation 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 D: 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 described above ＩＮＤT: diameter of the indentation 121 in the zinc-based plating layer 13 provided on the high-strength steel plate 111 measured on the cross section (unit: mm) ＩＮＤ T: Minimum thickness of the spot welded joint 1 inside the indentation 121 measured at the cross section (unit: mm) ＰＡＲ : total thickness of the plurality of steel plates 11 outside the indentation 121 (unit: mm) Details of these formulas will be explained 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 Figures 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] (Regarding θ and Equation 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 a method for measuring θ will be described with reference to Fig. 4.

[0032] The dashed-dotted line marked with the symbol X in Fig. 4 is a virtual line that follows the surface of the high-strength steel sheet 111 outside the indentation 121. In Fig. 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, but this is for ease of explanation. In Fig. 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 (a virtual 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 that runs 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. When 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 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 disposed thereon. 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 disposed thereon, but this is not disposed 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 also 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 formed on the embrittlement surface E. ＮＵＧ is the nugget diameter for the embrittlement steel plate 111E. 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 values ​​of the thickness, thickness, and other values, the surface of the zinc-based plating layer 13 and the surface of the high-strength steel plate 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. ＮＵＧ is the nugget diameter measured along the joining interface 14 between the embrittlement steel plate 111E and the adjacent steel plate 11.

[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. ＩＮＤ and D of the indentation 121 of the lower embrittlement steel plate 111E. ＩＮＤ In addition, 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 embrittlement steel plate 111E ＩＮＤ and DＮＵＧ It is also necessary to satisfy Formula 2. Although not shown, the same applies to the spot welded joint 1 in which two embrittlement steel plates 111E are stacked.

[0041] As shown in the above formula 2, in the embrittlement steel plate 111E, D ＮＵＧ / D ＩＮＤ is set to be 0.7 or more and 1.0 or less. 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. ＰＡＲ is the total thickness of the steel plates 11 outside the indentation 121. Hereinafter, with reference to FIGS. ＩＮＤ and T ＰＡＲ The measurement method will be explained.

[0043] 1 to 3, an indentation 121 is formed on two surfaces of the spot welded joint 1. The minimum value of the thickness of the spot welded joint 1 inside this 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 need 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 ＰＡＲThe gap between the steel plates 11 (sheet separation) is obtained by ＰＡＲ By measuring the thickness of the spot weld joint 1 in the place where there is no sheet separation, T 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 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 the above formula 3, T ＩＮＤ / T ＰＡＲ is 0.5 or more. 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] (Operation 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 exerts 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 joint 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, the portion of the tip 21 of the electrode 2 that includes the region that contacts the edge 1212 of the indentation is 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 portion of the edge 211 of the electrode.

[0049] The 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 increasing the radius of curvature of the electrode edge 211 will produce the following effects: (1) Cooling effect of the electrode edge 211: Increasing the radius of curvature of the electrode edge 211 will increase the contact area between the electrode edge 211 and the high-strength steel plate 111. 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, the maximum temperature at the electrode edge 211 is expected to decrease. (2) Splash reduction effect: Increasing the contact area between the electrode edge 211 and the high-strength steel plate 111 will reduce the compressive stress applied to the weld compared to a normal electrode 2. As a result, the amount of spatter is expected to decrease. A reduction in the amount of spatter is expected to reduce the tensile stress generated at the edge 1212 of the indentation and outside the indentation 121. Furthermore, if the electrode 2 suppresses the occurrence of expulsion, it becomes easier to increase the heat input and enlarge the nugget diameter. (3) Stress Reduction Effect When the contact area between the electrode edge 211 and the high-strength steel plate 111 is larger than usual, the amount by which the electrode edge 211 presses into the indentation edge 1212 and the resulting local bending deformation are reduced compared to a normal electrode 2. As a result, it is presumed that the tensile stress generated at the indentation edge 1212 is also reduced. It is believed that optimizing the curvature radius of the electrode edge 211 reduces both the temperature and stress as described above, and exhibits the effect of suppressing LME cracking at the indentation edge 1212.

[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 it is thought that LME cracking will be more likely to 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 portion 211 of the electrode and the high-strength steel plate 111 increases, and D ＮＵＧ / D ＩＮＤAs the value of ρ approaches 0.7, the distance from the nugget to the shoulder increases, which makes it difficult for the temperature of the shoulder to rise, further enhancing the crack suppression effect. In particular, when large expulsions occur, the crack suppression effect can be expected.

[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 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 In the spot welded joint 1 according to this embodiment, T ＩＮＤ / T ＰＡＲ In other words, 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 Although scattering should be avoided, T ＩＮＤ / T ＰＡＲ If a large heat input condition is selected that generates expulsion to the extent 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 manufacturing method of 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 manufacturing method of the spot welded joint 1 according to this embodiment. The manufacturing method of the spot welded joint 1 according to this embodiment makes it possible to manufacture the spot welded joint 1 described above. However, the manufacturing method described below does not limit the scope of the spot welded joint 1 according to this embodiment described above.

[0058] (Plate assembly) The manufacturing method for the spot-welded joint 1 includes a step of spot welding to a plate assembly. A plate assembly is a welding base material formed by stacking a plurality of steel plates 11. One or both of the steel plates 11 arranged on the surface of the plate assembly are steel plates 11 with a tensile strength of 980 MPa or more, i.e., high-strength steel plates 111. Furthermore, a zinc-based plating layer 13 is arranged on at least the surface of the plate assembly that is also the surface of the high-strength steel plate 111. In other words, the high-strength steel plate 111 is used as the steel plate 11, and the high-strength steel plate 111 is stacked on another steel plate 11 with the zinc-based plating layer 13 facing outward from the plate 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, the high-strength steel sheet 111 may have a zinc-based plating layer 13 on only one side thereof, or on both sides thereof. 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 on one or both sides thereof. 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 electrode 2 also has a flow path inside it through which a refrigerant can flow. When spot welding is performed, the refrigerant cools the tip 21 of the electrode 2. This transfers heat from the welded portion to the tip 21 of the electrode 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. Note that 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 dome radius electrodes (DR-type electrodes), truncated cone electrodes (CF-type electrodes), and truncated cone radius electrodes (CR-type electrodes) defined in JIS C 9304:1999. Examples of electrodes whose tip has a uniform radius of curvature include flat electrodes (F-type electrodes), radius electrodes (R-type electrodes), and dome electrodes (D-type electrodes) 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 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. The area 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, a part of the edge portion 211 of the electrode 2 contacts the edge portion 1212 of the indentation 121. Note that the symbol D in FIG. 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 central portion 21C of the electrode 2. The nominal diameter D of the electrode 2 contacting the embrittlement surface E is 16 mm or less. Preferably, the nominal diameter D of the electrode 2 contacting 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 portion including the edge portion 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. The upper limit of the radius of curvature r of the edge 211 of the electrode 2 is not particularly limited, but 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 (Equation 6) The symbols included in the above equation are defined as follows: S: Value (unit: kA·sec) obtained by integrating the current (unit: kA) flowing through the pair of electrodes 2 over the current application time (unit: sec) T ＰＡＲ : total thickness of steel sheets 11 included in the sheet assembly (unit: mm) P: pressure force of a pair of electrodes 2 (unit: kN)

[0067] (Regarding S and Equation 5) S is a value for evaluating the amount of heat input, and its unit is kA·sec. When spot welding is performed with a constant current, S is the value obtained by multiplying the current (kA) by the current application time (sec). When spot welding is performed under current application conditions in which the current value is not constant, such as upslope current application or downslope current application, S is the value obtained by integrating the current waveform with the current application time. Note that post-current application (current application performed for the purpose of heat treating the nugget 12 after the nugget 12 is completed) is not included in spot welding. This is because post-current application does not change the nugget diameter or other shapes of the weld. Therefore, the heat input during post-current application 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 Equation 6) T ＰＡＲ is the total thickness (mm) of the steel sheets 11 included in the sheet assembly. The total thickness of the steel sheets 11 included in the sheet assembly is equal to the total thickness outside the indentation 121 of the steel sheets 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 determined as follows: 2.5×(T ＰＡＲ The pressure P is set to 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 flow 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] (Operation and Effect) In the method for manufacturing a spot welded joint 1 according to this embodiment, spot welding is performed using an electrode 2 with 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 normal. 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 likelihood of LME cracking at the edge 1212 of the indentation. However, in the manufacturing method for spot-welded joint 1 according to this embodiment, expulsion is suppressed by optimizing the shape of the tip 21 of the electrode 2. Furthermore, even if some expulsion occurs, LME cracking is suppressed.

[0075] The most basic aspect of the method for manufacturing the spot welded joint 1 according to this embodiment has been described above. 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-type 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 sheet 11) There are no particular limitations on the chemical composition and metal structure 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 WO 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 Plates 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) The hardness of the steel plate 11 is not particularly limited. 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 4.9 N.

[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.

[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 constituting the sheet assemblies are shown in Table 1. All sheet assemblies were manufactured by stacking two steel sheets of the same configuration. All steel sheets were 1.6 mm thick zinc-based plated steel sheets with zinc-based plating layers on both surfaces.

[0087]

[0088] The spot welding conditions are shown in Table 2. In all spot welding, the configuration of the pair of electrodes was the same. In all spot welding, dome radius electrodes (DR electrodes) were used, 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]

[0091] Table 3 shows the shapes 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 disposed on both surfaces were high-strength steel plates, and both surfaces were coated with zinc-based plating layers. Therefore, parameters indicating the shape of the spot-welded joints were measured as follows:

[0092] θ (the angle formed by 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) 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 (diameter of the nugget measured in a cross section along the joining 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 joining 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 each of the upper and lower indentations.

[0095]

[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 weldings. Conditions in which expulsion occurred in one or more spot weldings were marked "present" in the "Expulsion Presence / Absence" 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]

[0098] In Test No. 1, Test No. 5, and Test No. 10, the radius of curvature r of the peripheral portion of the electrode was insufficient. Therefore, 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, the electrode shape was appropriate in the spot welding of Test No. 2 to Test No. 4 and Test No. 6 to Test No. 8. Furthermore, in the spot welding of Test No. 2 to Test No. 4 and Test No. 6 to Test No. 8, Equation 5 regarding the heat input and Equation 6 regarding the welding pressure were satisfied. In the spot-welded joints of Test No. 2 to Test No. 4 and Test No. 6 to Test No. 8, θ satisfied Equation 1 at all measurement points, and D ＮＵＧ and D ＩＮＤ satisfied Formula 2. 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 ＮＵＧsatisfied Formula 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.

[0101] DESCRIPTION OF SYMBOLS 1 Spot welded joint 11 Steel plate 111 High strength steel plate E Brittle surface 111E Brittle steel plate 112 Low strength steel plate 12 Nugget 121 Indentation 1211 Outer edge of indentation 1212 Edge of indentation 13 Zinc-based plating layer 14 Joint interface 2 Electrode 21 Tip 21C Center of electrode 21P Peripheral part of electrode 211 Edge of electrode R1 Radius of curvature of center d Diameter of center r Radius of curvature of peripheral part D Nominal diameter

Claims

1. A spot welding 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 the surface of the spot welding joint are high-strength steel plates having a tensile strength of 980 MPa or more, the zinc-based plating layer is arranged on the surface of the spot welding joint and on the surface of the high-strength steel plate, the zinc-based plating layer is provided with indentations, and 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: degree) formed by the edge of the indentation provided in the zinc-based plating layer of the high-strength steel plate and the surface of the high-strength steel plate, measured in a cross-section passing through the center of the indentation and perpendicular to the surface of the spot welding joint, D ＮＵＧ is the diameter (unit: mm) of the nugget along the joint interface between the high-strength steel plate provided with the zinc-based plating layer arranged on the surface of the spot welding joint and the adjacent steel plate, measured in the cross-section, D ＩＮＤ is the diameter (unit: mm) of the indentation of the zinc-based plating layer provided on the high-strength steel plate, measured in the cross-section, T ＩＮＤ is the minimum value of the thickness of the spot welding joint inside the indentation (unit: mm), measured in the cross-section, T ＰＡＲ is the total thickness (unit: mm) of the plurality of steel plates outside the indentation, a spot welding joint.

2. T ＩＮＤ / T ＰＡＲ The spot welding joint according to claim 1, wherein T 3. A method for manufacturing a spot weld joint, comprising a step of spot welding a stack of a plurality of steel plates using a pair of electrodes, wherein one or both of the steel plates arranged on the surface of the stack of plates is a high-strength steel plate having a tensile strength of 980 MPa or more, a zinc-based plating layer is arranged on the surface of the stack of plates and on the surface of the high-strength steel plate, the 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, in the electrode in contact with the zinc-based plating layer, the nominal diameter D is 16 mm or less, the diameter of the central portion is d, and the radius of curvature R in the range of d / 2 or more and d / 2 + 1.5 mm or less from the central axis of the electrode is more than 9.5 mm, the spot welding is performed so as to satisfy the following formulae: 6.0 ≦ S ≦ 20.0 ……… (Formula 5) 2.5×(T ＰＡＲ / 2) ≦ P ……… (Formula 6) wherein S is a value (unit: kA·sec) obtained by integrating the current (unit: kA) flowing through the pair of electrodes over the energization time (unit: sec), T ＰＡＲ is the total thickness (unit: mm) of the steel plates included in the stack of plates, and P is the pressing force (unit: kN) of the pair of electrodes. A method for manufacturing a spot weld joint.

4. The electrode in contact with the zinc-based plating layer provided on the high-strength steel sheet is a dome radius electrode, the diameter d of the central portion is 5 to 10 mm, the radius of curvature R of the central portion is 30 to 50 mm, and 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 weld joint according to claim 3.

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