Spot-welded joint

Abstract

A spot-welded joint according to one aspect of the present disclosure is provided with a plurality of stacked steel sheets and a spot-welded part having a nugget for joining the plurality of steel sheets. At least one of the steel sheets is a plated steel sheet having a base steel sheet and a plating layer disposed on one or both surfaces of the base steel sheet. The base steel sheet has a prescribed chemical component. The Vickers hardness of the base steel sheet is not less than 85 HV to less than 320 HV. The plating layer is a ternary plating layer containing Mg, Zn, and Al. The plating layer is disposed on the surface of the spot-welded joint. The minimum thickness of the spot-welded part is 50% or more of the total thickness of the plurality of steel sheets. On the surface of the spot-welded joint on which the plating layer is disposed, the size of the angle formed by a shoulder part of an indentation of the spot-welded part and the surface of the plated steel sheet on which the indentation is provided is not more than 20 degrees.

Description

Spot welded joints

[0001] This disclosure relates to a spot welded joint. This application claims priority under Japanese Patent Application No. 2025-067330, filed in Japan on April 16, 2025, the contents of which are incorporated herein by reference.

[0002] Zinc-based plating layers dramatically improve the corrosion resistance of steel materials through sacrificial corrosion protection. Therefore, zinc-based plating layers are used as surface treatment layers for various types of steel.

[0003] However, the zinc contained in the zinc-based plating layer can cause LME (Liquid Metal Embrittlement) cracking. When spot welding a zinc-plated steel sheet having 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, making the grain boundaries brittle. When stress is applied to the brittle grain boundaries, cracks easily occur at the grain boundaries.

[0004] The conditions under which LME cracking occurs during spot welding are: - Contact between molten zinc and a solid steel sheet, and - Application of tensile stress to the contact area between the molten zinc and the solid steel sheet. Various studies have been conducted to prevent LME cracking. For example, Patent Document 1 discloses techniques for preventing LME cracking.

[0005] Patent No. 7502713

[0006] In the technology described in Patent Document 1, a steel plate having a tensile strength of 980 MPa or more is subjected to a greater pressure than usual. By increasing the pressure, the current density is reduced, promoting heat dissipation from the weld to the electrode. This suppresses the temperature rise of the plate assembly surface in contact with the electrode, thereby suppressing grain coarsening.

[0007] However, applying this technology to steel plates with a tensile strength of less than 980 MPa or a Vickers hardness of less than 320 HV may increase the indentation depth and reduce the thickness of the weld. A reduction in weld thickness can decrease the joint strength of the spot weld. To suppress LME cracking in such steel plates, alternative methods are needed.

[0008] In view of the above circumstances, the object of this disclosure is to provide a spot-welded joint that has high corrosion resistance, ensures sufficient thickness of the spot weld, and prevents LME cracking.

[0009] The gist of this disclosure is as follows:

[0010] (1) A spot welded joint according to one aspect of the present disclosure is a spot welded joint comprising a plurality of stacked steel plates and a spot weld having a nugget for joining the plurality of steel plates, wherein one or more of the steel plates are plated steel plates having a base steel plate and a plating layer disposed on one or both sides of the base steel plate, and the composition of the base steel plate is, in mass%, C: 0.005 to 0.200%, Si: 0 to 1.00%, Mn: 0.08 to 3.00%, Al: 0.005 to 0.5%, Ti: 0 to 0.1000%, B: 0 to 0.0080%, P: 0.020% or less, S: 0 (1) Preferably, in the spot welded joint described in (1) above, the base steel sheet contains 0.010% or less of B and 0.010% or less of N, the Vickers hardness of the base steel sheet is 85 HV or more and less than 320 HV, the plating layer is a ternary plating layer containing Mg, Zn, and Al, the plating layer is disposed on the surface of the spot welded joint, the minimum thickness of the spot weld is 50% or more of the total thickness of the plurality of steel sheets, and on the surface of the spot welded joint on which the plating layer is disposed, the angle between the shoulder of the indentation of the spot weld and the surface of the plated steel sheet on which the indentation is provided is 20 degrees or less. －６ Formula A (3) Preferably, in the spot welded joint described in (1) above, the mass% B content, mass% N content, and mass% Ti content of the base steel plate satisfy the following formulas B and C: (mass% B) × (mass% N) ≥ 1.5 × 10 －６Formula B (mass%Ti) / (mass%N)≧0.3 Formula C (4) Preferably, in the spot welded joint described in any one of (1) to (3) above, the base steel sheet contains, by mass%, Si: 0 to 0.80%, Mn: 0.08 to 1.80%, and B: 0.0010 to 0.0080%. (5) Preferably, in the spot welded joint described in any one of (1) to (4) above, the average B content measured in the surface layer of the base steel sheet, which is a region with a depth of 9 μm or more and 20 μm or less from the surface facing the outside of the spot welded joint, is 7.0 ppm or more. (6) Preferably, in the spot welded joint described in any one of (1) to (5) above, the diameter of the nugget is less than 6.0 mm. (7) Preferably, in the spot welded joint described in any one of the above items (1) to (6), in the shoulder portion where the plating layer is provided, the size of the angle formed between the shoulder portion and the surface of the plated steel sheet on which the indentation is provided is the largest, the plating layer contains 10.0 area % or more of MgZn ２ (8) Preferably, in the spot welded joint described in any one of (1) to (7) above, the chemical composition of the plating layer contains, by mass%, Al: 5.0 to 15.0%, Mg: 1.0 to 8.0%, with the remainder being Zn and impurities. (9) Preferably, in the spot welded joint described in any one of (1) to (8) above, the chemical composition of the base steel sheet is Al: 0.005 to 0.3%.

[0011] According to this disclosure, it is possible to provide a spot-welded joint that has sufficient thickness in the spot weld, high corrosion resistance, and can prevent LME cracking.

[0012] This is a schematic cross-sectional view of a spot welded joint. This is a magnified schematic view of the shoulder of the indentation. This is a schematic view of the surface region.

[0013] A spot welded joint 1 according to one aspect of the present disclosure comprises a plurality of stacked steel plates and a spot weld 12 having a nugget 121 for joining the plurality of steel plates, wherein one or more steel plates are plated steel plates 11C having a base steel plate 111 and a plating layer 112 disposed on one or both sides of the base steel plate 111, and the base steel plate 111 has a composition in mass%, of C: 0.005 to 0.200%, Si: 0 to 1.00%, Mn: 0.08 to 3.00%, Al: 0.005 to 0.5%, Ti: 0 to 0.1000%, B: 0 to 0.0080%, P: 0.020% or less, S The plating layer contains 0.010% or less of 0.010% or less of N, the Vickers hardness of the base steel sheet 111 is 85 HV or more and less than 320 HV, the plating layer 112 is a ternary plating layer containing Mg, Zn, and Al, the plating layer 112 is placed on the surface of the spot welded joint 1, the minimum thickness tw of the spot weld is 50% or more of the total thickness of the multiple steel sheets, and on the surface of the spot welded joint 1 on which the plating layer 112 is placed, the angle θ between the shoulder portion 1222 of the indentation 122 of the spot weld 12 and the surface of the plated steel sheet on which the indentation 122 is provided is 20 degrees or less.

[0014] (Steel Plate) The spot-welded joint 1 is obtained by spot-welding multiple overlapping steel plates. The number of steel plates can be any value of 2 or more. As illustrated in Figure 1, the number of steel plates may be 2. On the other hand, the number of steel plates may be 3 or more.

[0015] The steel plate of the spot-welded joint 1 may be bent. For example, the spot-welded joint 1 may be a hat-shaped member, and one or more steel plates may be hat-shaped steel. In this case, multiple nuggets 121 are provided on the flange of the hat-shaped member, along the direction of extension of the flange.

[0016] (Spot Weld 12) The spot welded joint 1 has a spot weld 12. The term "weld" is defined in JIS Z 3001-1:2018 as "a general term for the portion including the weld metal and the heat-affected zone." Following this standard, this disclosure defines the term "spot weld 12" as "a general term for the portion including the nugget 121 and the heat-affected zone 123." However, as will be described later, the spot weld 12 may not include the heat-affected zone 123. If the heat-affected zone 123 does not exist, the spot weld 12 means the nugget 121 and its surrounding area.

[0017] A nugget 121 is a molten and solidified portion that forms in the weld area during overlap resistance welding. As shown in the schematic cross-sectional view in Figure 1, the spot weld area 12 has a nugget 121. The nugget 121 joins multiple overlapping steel plates.

[0018] The heat-affected zone 123 is the unmelted portion of the base material where the structure, metallurgical properties, and mechanical properties have changed due to welding heat. The heat-affected zone 123 is sometimes referred to as the HAZ. The heat-affected zone 123 is formed around the nugget 121.

[0019] However, the spot weld portion 12 of the spot welded joint 1 according to this embodiment does not necessarily have a heat-affected zone 123. This is because the heat-affected zone 123 that exists immediately after the completion of spot welding can be eliminated by heat treatment of the spot weld portion 12. For example, in a tailored blank comprising a plurality of steel plates and a spot weld portion 12 joining the plurality of steel plates, a heat-affected zone 123 exists in the spot weld portion 12. However, in a press-formed product obtained by hot press forming the tailored blank, there may be cases where no heat-affected zone 123 remains in the spot weld portion 12.

[0020] An indentation 122 is further formed in the spot weld 12. An indentation 122 is a depression in the base material surface caused by the electrode tip as a result of welding in overlap resistance welding. When the spot welded joint 1 is viewed in plan along a direction perpendicular to its surface, the nugget 121 and the indentation 122 overlap.

[0021] A sheet separation 13 may be formed around the nugget 121 of the spot welded portion 12. The sheet separation 13 refers to a gap between plates generated around the nugget 121 as a result of welding in lap resistance welding.

[0022] (Plated Steel Sheet 11C) At least one of the steel sheets is a plated steel sheet 11C. The plated steel sheet 11C includes a base steel sheet 111 and a plated layer 112 disposed on one or both surfaces of the base steel sheet 111. In the spot welded joint 1 illustrated in FIG. 1, the upper steel sheet is the plated steel sheet 11C. The lower steel sheet is a steel sheet having no plated layer 112, that is, a non-plated steel sheet 11N. Although the plated layer 112 is provided only on the upper surface of the plated steel sheet 11C in FIG. 1, it may be provided on both surfaces.

[0023] The plated steel sheet 11C satisfies all of the requirements regarding (i) the chemical composition of the base steel sheet 111, (ii) the hardness of the base steel sheet 111, (iii) the chemical composition of the plated layer 112, and (iv) the arrangement of the plated layer 112. In the present disclosure, unless otherwise specified, "plated steel sheet 11C" means a plated steel sheet that satisfies all of these requirements (i) to (iv). Hereinafter, these requirements will be described.

[0024] (i. Chemical Composition of Base Steel Sheet 111 of Plated Steel Sheet 11C) The base steel sheet 111 has a predetermined chemical composition. Hereinafter, the chemical composition of the base steel sheet 111 will be specifically described. In the present disclosure, the unit "%" for the content of an element means mass%. The chemical composition of the non-plated steel sheet 11N is not particularly limited, and may be the same as the chemical composition of the base steel sheet 111 of the plated steel sheet 11C.

[0025] (C: 0.005 to 0.200%) C improves the strength of the base steel sheet 111. Therefore, the C content of the base steel sheet 111 is set to 0.005% or more. Preferably, the C content of the base steel sheet 111 is 0.010% or more, 0.020% or more, 0.0400% or more, or 0.080% or more.

[0026] On the other hand, by setting the C content of the base steel sheet 111 to a predetermined value or less, the workability of the base steel sheet 111 can be ensured. Therefore, the C content of the base steel sheet 111 is set to 0.200% or less. Preferably, the C content of the base steel sheet 111 is 0.180% or less, 0.150% or less, or 0.120% or less.

[0027] (Si: 0 to 1.00%) Si may not be contained in the base steel sheet 111. Therefore, the lower limit of the Si content of the base steel sheet 111 is 0%. On the other hand, Si suppresses the formation of carbides and improves the toughness of the steel sheet. Therefore, preferably, the Si content of the base steel sheet 111 is 0.01% or more, 0.05% or more, or 0.10% or more.

[0028] On the other hand, by setting the Si content of the base steel sheet 111 to a predetermined value or less, cracking of the slab during the manufacturing stage of the base steel sheet 111 can be suppressed. Further, by setting the Si content of the base steel sheet 111 to a predetermined value or less, the LME cracking resistance can be further improved. Therefore, the Si content of the base steel sheet 111 is set to 1.00% or less. Preferably, the Si content of the base steel sheet 111 is 0.90% or less, 0.80% or less, or 0.60% or less.

[0029] (Mn: 0.08 to 3.00%) Mn improves the strength of the base steel sheet 111. Therefore, the Mn content of the base steel sheet 111 is set to 0.08% or more. Preferably, the Mn content of the base steel sheet 111 is 0.10% or more, 0.50% or more, or 1.00% or more.

[0030] On the other hand, by setting the Mn content of the base steel sheet 111 to a predetermined value or less, the formation of MnS is suppressed, and the ductility of the base steel sheet 111 can be enhanced. Therefore, the Mn content of the base steel sheet 111 is set to 3.00% or less. Preferably, the Mn content of the base steel sheet 111 is 2.50% or less, 2.00% or less, or 1.80% or less.

[0031] (Al: 0.005 to 0.5%) Al deoxidizes steel. Therefore, the Al content of the base steel sheet 111 is 0.005% or more. Preferably, the Al content is 0.030% or more, 0.050% or more, or 0.070% or more.

[0032] On the other hand, by keeping the Al content below a predetermined value, cracking of the slab during the manufacturing stage of the base steel sheet 111 can be suppressed. Therefore, the Al content of the base steel sheet 111 is set to 0.5% or less. Preferably, the Al content of the base steel sheet 111 is 0.4% or less, 0.3% or less, or 0.2% or less.

[0033] (Ti: 0 to 0.1000%) Ti does not have to be included in the base steel sheet 111. Therefore, the lower limit of the Ti content of the base steel sheet 111 is 0%. On the other hand, Ti precipitates in the steel as precipitates such as carbides or nitrides. These precipitates refine the metal structure of the base steel sheet 111 through a pinning effect. Furthermore, these precipitates increase the strength and yield ratio of the base steel sheet 111 through precipitation strengthening. Therefore, preferably, the Ti content of the base steel sheet 111 is 0.0010% or more, 0.0030% or more, or 0.0100% or more.

[0034] On the other hand, by setting the Ti content of the base steel sheet 111 to a predetermined value or less, the excessive formation of precipitates can be suppressed and the hole-expanding properties of the steel sheet can be ensured. Therefore, the Ti content of the base steel sheet 111 is set to 0.1000% or less. Preferably, the Ti content of the base steel sheet 111 is 0.0900% or less, 0.0600% or less, or 0.0500% or less.

[0035] (B: 0 to 0.0080%) B does not have to be included in the base steel sheet 111. Therefore, the lower limit of the B content in the base steel sheet 111 is 0%. On the other hand, B improves the strength of the base steel sheet 111. Furthermore, B suppresses LME cracking. This is because LME cracking occurs when molten zinc penetrates into the prior austenite grain boundaries, while B prevents molten zinc from penetrating the grain boundaries by reducing the grain boundary energy of the prior austenite grain boundaries. Therefore, preferably, the B content of the base steel sheet 111 is 0.0010% or more, 0.0020% or more, or 0.0030% or more.

[0036] On the other hand, by keeping the B content of the base steel sheet 111 below a predetermined value, the excessive formation of B precipitates can be suppressed, and the strength of the steel sheet can be ensured. Therefore, the B content of the base steel sheet 111 is 0.0080% or less. Preferably, the B content of the base steel sheet 111 is 0.0070% or less, 0.0060% or less, or 0.0050% or less.

[0037] (P: 0.020% or less) P does not have to be included in the base steel sheet 111. Therefore, the lower limit of the P content of the base steel sheet 111 is 0%. On the other hand, by allowing a small amount of P in the base steel sheet 111, the refining cost of the base steel sheet 111 can be reduced. Therefore, preferably, the P content of the base steel sheet 111 is 0.001% or more, 0.002% or more, or 0.005% or more.

[0038] Furthermore, by setting the P content of the base steel sheet 111 to below a predetermined value, the weldability of the base steel sheet 111 can be improved. Also, by setting the P content of the base steel sheet 111 to below a predetermined value, cracking of the slab during the manufacturing stage of the base steel sheet 111 can be suppressed. Therefore, the P content of the base steel sheet 111 is 0.020% or less. Preferably, the P content of the base steel sheet 111 is 0.018% or less, 0.016% or less, or 0.010% or less.

[0039] (S: 0.010% or less) S does not have to be included in the base steel sheet 111. Therefore, the lower limit of the S content of the base steel sheet 111 is 0%. On the other hand, by allowing a small amount of S in the base steel sheet 111, the refining cost of the base steel sheet 111 can be reduced. Therefore, preferably, the S content of the base steel sheet 111 is 0.001% or more, 0.002% or more, or 0.003% or more.

[0040] Furthermore, by keeping the S content of the base steel sheet 111 below a predetermined value, the formation of MnS can be suppressed, and the ductility of the base steel sheet 111 can be improved. Therefore, the S content of the base steel sheet 111 is 0.010% or less. Preferably, the S content of the base steel sheet 111 is 0.009% or less, 0.008% or less, or 0.006% or less.

[0041] (N: 0.010% or less) N does not have to be included in the base steel sheet 111. Therefore, the lower limit of the N content of the base steel sheet 111 is 0%. On the other hand, by allowing a small amount of N in the base steel sheet 111, the refining cost of the base steel sheet 111 can be reduced. Therefore, preferably, the N content of the base steel sheet 111 is 0.001% or more, 0.002% or more, or 0.003% or more.

[0042] Furthermore, by keeping the N content of the base steel sheet 111 below a predetermined value, cracking of the slab during the manufacturing stage of the base steel sheet 111 can be suppressed. Therefore, the N content of the base steel sheet 111 is 0.010% or less. Preferably, the N content of the base steel sheet 111 is 0.009% or less, 0.008% or less, or 0.006% or less.

[0043] (Other elements) The remainder of the composition of the base steel sheet 111 is mainly iron. The remainder of the composition of the base steel sheet 111 may contain impurities and other elements. Examples of elements that the base steel sheet 111 may contain are O, Nb, V, Cu, Cr, Mo, Ni, B, Ca, Mg, REM, Bi, Ta, Zr, Co, Zn, W, Sb, As, and Sn.

[0044] (ii. Vickers hardness of base steel sheet 111 of plated steel sheet 11C) The Vickers hardness of the base steel sheet 111 is 85 HV or more and less than 320 HV. By setting the Vickers hardness of the base steel sheet 111 to 85 HV or more, the strength of the spot welded joint 1 can be increased. Furthermore, by setting the Vickers hardness of the base steel sheet 111 to less than 320 HV, the ductility and formability of the base steel sheet 111 can be increased.

[0045] Preferably, the Vickers hardness of the base steel sheet 111 is 90 HV or higher, 100 HV or higher, or 120 HV or higher. Preferably, the Vickers hardness of the base steel sheet 111 is 300 HV or lower, 290 HV or lower, or 280 HV or lower. The tensile strength of the base steel sheet 111 having such Vickers hardness is generally 270 MPa or higher and less than 980 MPa.

[0046] The hardness of the unplated steel sheet 11N is not particularly limited. For example, the hardness of the base steel sheet 111 described above can be applied to the unplated steel sheet 11N.

[0047] (iii. Chemical composition of the plating layer 112 of the plated steel sheet 11C) The plating layer 112 is a ternary plating layer mainly composed of Mg, Zn, and Al. A ternary plating layer containing Mg, Zn, and Al has excellent corrosion resistance. Therefore, the ternary plating layer is applied to the surface of various components where corrosion resistance is required.

[0048] A preferred example of a ternary plating layer is a plating layer 112 in which the proportion of Zn in the chemical composition is 50% by mass or more, i.e., a zinc-based plating layer. Examples of zinc-based plating layers include hot-dip galvanized layers, electro-galvanized layers, and alloyed hot-dip galvanized layers. The zinc-based plating layer enhances the corrosion resistance of the spot-welded joint 1 through a sacrificial corrosion protection effect. The hot-dip zinc-aluminum-magnesium alloy plating layer specified in JIS G 3323:2019 is also a preferred example of a ternary plating layer. Paragraph 5.1 of the said standard discloses a plating bath composition in which Al: 5.0% to 13.0%, Mg: 2.0% to 4.0%, elements other than Al, Mg, and Zn: 1.0% or less, and Zn: remainder (including impurities). The chemical composition of the plating layer 112 according to this embodiment may be within the above range. More specific examples of ternary plating layers will be described later.

[0049] However, the Zn contained in the ternary plating layer melts during spot welding and penetrates the grain boundaries of the heat-affected zone 123. The molten zinc that penetrates the grain boundaries embrittles them. When stress is applied to the embrittlement of the grain boundaries, cracks easily occur at the grain boundaries. Brittle cracks caused by molten zinc in the weld are called LME (Liquid Metal Embrittlement) cracks.

[0050] (iv. Arrangement of the plating layer 112 on the plated steel sheet 11C) The plating layer 112 is provided on one or both sides of one or more steel sheets. In addition, the plating layer 112 is provided on at least the surface of the spot welded joint 1. In other words, the steel sheet provided on the surface of the spot welded joint 1 is the plated steel sheet 11C, and this plating layer 112 is oriented outwards from the spot welded joint 1.

[0051] Naturally, the plating layer 112 may also be provided at the joining interface inside the spot-welded joint 1. The term "joint interface" is defined in JIS Z 3001-6:2013 as "the surfaces that come into contact with each other and face each other in order to join them." In the spot-welded joint 1 according to this embodiment, the joining surfaces of the multiple steel plates are the joining interfaces.

[0052] As described above, the spot welded joint 1 according to this embodiment must have one or two plated steel sheets 11C that satisfy all of the above requirements regarding (i) the chemical composition of the base steel sheet 111, (ii) the hardness of the base steel sheet 111, (iii) the chemical composition of the plating layer 112, and (iv) the arrangement of the plating layer 112. On the other hand, if there is a plated steel sheet 11C that satisfies all of the above requirements (i) to (iv), the composition of the remaining steel sheets is not particularly limited. As shown in Figure 1, an unplated steel sheet 11N may be included in the spot welded joint 1. Also, a plated steel sheet that does not satisfy one or more of the above requirements may be included in the spot welded joint 1. For example, a high-strength plated steel sheet in which the hardness of the base steel sheet 111 exceeds 320 HV may be included in the spot welded joint 1.

[0053] (Minimum thickness tw of spot weld) The minimum thickness tw of the spot weld is 50% or more of the total thickness of the multiple steel plates. The minimum thickness tw of the spot weld is the minimum thickness of the spot weld 12 inside the indentation 122, measured in the cross-section of the spot weld 12 that passes through approximately the center of the indentation 122 and is approximately perpendicular to the surface of the spot weld joint 1. The total thickness of the multiple steel plates is the sum of the thicknesses of each of the multiple steel plates outside the indentation 122.

[0054] As illustrated in Figure 1, indentations 122 are formed on the two surfaces of the spot welded joint 1. The minimum thickness of the spot welded joint 1 inside these indentations 122, that is, the minimum distance between the bottoms of the two indentations 122 along a direction perpendicular to the surface of the spot welded joint 1 outside the indentations 122, is the minimum thickness tw of the spot weld. Typically, the thickness of the spot welded joint 1 is smallest at the center of the indentations 122. Note that the thickness of the plating layer 112 is included in the thickness of the spot welded joint 1.

[0055] The total thickness of the multiple steel plates is the sum of the thicknesses of the steel plates included in the spot weld 12, measured outside the indentation 122. In the spot welded joint 1 illustrated in Figure 1, the total thickness of the multiple steel plates is t1 + t2. The minimum thickness tw of the spot weld is measured in cross-section, but the total thickness of the multiple steel plates does not need to be measured in cross-section. The total thickness of the multiple steel plates can be obtained by measuring the thickness of each of the multiple steel plates at a location away from the spot weld 12 using a known tool such as a caliper and summing them up.

[0056] The thickness of the plating layer 112 is included in the total thickness of the multiple steel plates. On the other hand, the thickness of the sheet separation 13 is not included in the total thickness of the multiple steel plates. In areas where there is no sheet separation 13, the total thickness of the multiple steel plates may be determined by measuring the thickness of the spot welded joint 1. If the thickness of all the steel plates is given as information in advance, these values ​​may simply be added together and considered as the total thickness of the multiple steel plates without measurement.

[0057] In the spot-welded joint 1 according to this embodiment, the spot-welded portion 12 is made sufficiently thick. Specifically, the minimum thickness tw of the spot-welded portion is 50% or more of the total thickness of the multiple steel plates. In the spot-welded joint 1 illustrated in Figure 1, tw / (t1+t2) is 50% or more. Preferably, the minimum thickness tw of the spot-welded portion is 55% or more, 60% or more, or 70% or more of the total thickness of the multiple steel plates.

[0058] (Angle θ of the shoulder portion 1222) In the spot welded joint 1 according to this embodiment, the angle of the shoulder portion 1222 on the surface of the spot welded joint 1 on which the plating layer 112 is provided is 20 degrees or less. The angle θ of the shoulder portion 1222 is the size of the angle formed between the shoulder portion 1222 of the indentation 122 of the spot welded portion 12 and the surface of the plated steel sheet 11C on which the indentation 122 is provided. The shoulder portion 1222 and the angle θ of the shoulder portion 1222 will be described below.

[0059] Figure 2 shows an enlarged view of the shoulder portion 1222. The dashed line labeled X in Figure 2 is a virtual line along the surface of the plated steel sheet 11C outside the indentation 122. When measuring θ, the outer edge 1221 of the indentation 122 in cross-section is defined as the intersection of the dashed line X (a virtual line along the surface of the plated steel sheet 11C outside the indentation 122) and the inner surface of the indentation 122. The shoulder portion 1222 of the indentation 122 is defined as the region on the surface of the indentation 122 from the outer edge 1221 of the indentation 122 to a position 200 μm away along the dashed line X.

[0060] The dashed line denoted Y in Figure 2 is a hypothetical line connecting the two ends of the shoulder portion 1222 in the cross-section. One end of the shoulder portion 1222 is the outer edge 1221 of the indentation 122. The other end of the shoulder portion 1222 is a point on the surface of the indentation 122, 200 μm away from the outer edge 1221 of the indentation 122 along the dashed line X. The dashed line Y is considered to be a hypothetical line along the shoulder portion 1222. The size of the angle between the shoulder portion 1222 of the indentation 122 and the surface of the plated steel sheet 11C, i.e., the angle θ of the shoulder portion 1222, is the inferior angle between the dashed line X and the dashed line Y.

[0061] In the spot-welded joint 1 according to this embodiment, the angle θ of the shoulder portion 1222 is 20 degrees or less. Preferably, the angle θ of the shoulder portion 1222 is 19 degrees or less, 18 degrees or less, or 15 degrees or less.

[0062] In addition, in the cross-section of the spot weld 12, there are four shoulder portions 1222 of the indentation 122. Of these, the above-mentioned angle requirement must be met for all of the shoulder portions 1222 of the indentation 122 of the plated steel sheet 11C that satisfy the above requirements: (i) the chemical composition of the base steel sheet 111, (ii) the hardness of the base steel sheet 111, (iii) the chemical composition of the plating layer 112, and (iv) the arrangement of the plating layer 112.

[0063] On the other hand, the angle of the shoulder portion 1222 in steel plates that do not meet the above requirements is not limited. That is, the angle of the shoulder portion 1222 in the indentation 122 provided on the surface of the unplated steel plate 11N and the plated steel plate that does not meet one or more of the above requirements is not limited. In the spot welded joint 1 of Figure 1, the angles of the two shoulder portions 1222 provided on the upper surface must be within the above range. On the other hand, the angles of the two shoulder portions 1222 provided on the lower surface are not limited.

[0064] (Effects) The spot welded joint 1 according to this embodiment has a plating layer 112 on at least one surface. The plating layer 112 is a ternary plating layer with high corrosion resistance. Therefore, the spot welded joint 1 according to this embodiment also has high corrosion resistance.

[0065] However, in conventional spot-welded joints 1, LME cracking is extremely likely to occur in the shoulder portion 1222 and surrounding area of ​​the indentation 122 formed in the plating layer 112. This is because all the conditions for LME cracking to occur are met in that region. The conditions for LME cracking to occur are: - Molten zinc comes into contact with the solid steel plate - Tensile stress is applied to the contact area between the molten zinc and the solid steel plate. All of these conditions are met in the shoulder portion 1222 and surrounding area of ​​the indentation 122 formed in the plating layer 112. For example, in the spot-welded joint 1 shown in Figure 1, LME cracking is likely to occur in the shoulder portion 1222 of the upper indentation 122.

[0066] Therefore, in the spot-welded joint 1 according to this embodiment, the angle of the shoulder portion 1222 in the indentation 122 formed in the plating layer 112 is set to 20 degrees or less. By using welding conditions that set the angle of the shoulder portion 1222 to 20 degrees or less, the stress introduced into the shoulder portion 1222 and its surrounding area can be reduced. This makes it possible to suppress the occurrence of LME cracks on the surface of the spot-welded joint 1.

[0067] Furthermore, in the spot-welded joint 1 according to this embodiment, the minimum thickness tw of the spot weld is set to 50% or more of the total thickness of the steel plate. As a result, in the spot-welded joint 1 according to this embodiment, the depth of the indentation 122 is reduced, ensuring the thickness of the spot weld 12 and increasing the joint strength of the spot weld 12.

[0068] The means for setting the minimum thickness tw of the spot weld and the angle of the shoulder portion 1222 within the above-mentioned range are not particularly limited. The inventors were able to obtain the spot welded joint 1 according to this embodiment by suitably selecting welding conditions and electrode shape. Suitable examples of welding conditions and electrode shape will be described later.

[0069] The most basic embodiment of the spot-welded joint 1 according to this embodiment has been described above. A more preferred embodiment will be described below.

[0070] (Preferred chemical composition of base steel sheet 111) Preferably, the mass% B content and mass% N content of the base steel sheet 111 satisfy the following formula A: (mass% B) × (mass% N) ≤ 1.5 × 10 －６ Formula A: When the chemical composition of the base steel sheet 111 satisfies Formula A, the precipitation of BN is suppressed. As a result, there is a possibility that LME resistance due to solid solution B will improve, and the risk of slab cracking due to BN precipitation will be reduced.

[0071] Alternatively, preferably, the mass% B content, mass% N content, and mass% Ti content of the base steel sheet 111 satisfy the following formulas B and C: (mass% B) × (mass% N) ≥ 1.5 × 10 －６ Formula B (mass%Ti) / (mass%N) ≥ 0.3 Formula C When the chemical composition of the base steel sheet 111 satisfies formulas B and C, even if there is a possibility of BN precipitation, solid solution B can be secured and LME resistance can be ensured by adding a predetermined amount of Ti.

[0072] In addition to specifying the ratios of N content, B content, and Ti content, the Si content, Mn content, and B content may be further limited. For example, as described above, it is preferable that the base steel sheet 111 contains, by mass%, Si: 0 to 0.80%, Mn: 0.08 to 1.80%, and B: 0.0010 to 0.0080%.

[0073] (B content in the surface region 1111 of the base steel sheet 111) In this disclosure, the surface region 1111 of the base steel sheet 111 is defined as the region of the base steel sheet 111 with a depth of 9 μm or more and 20 μm or less from the surface facing the outside of the spot welded joint 1. Figure 3 shows an enlarged cross-sectional view of the surface region 1111. Preferably, the average B content measured in the surface region 1111 is 7.0 ppm or more. In this disclosure, the unit "ppm" means ppmw, i.e., parts per million weight.

[0074] B atoms have the property of becoming concentrated at the prior austenite grain boundaries in the heat-affected zone 123. Furthermore, B prevents molten zinc from penetrating the grain boundaries by reducing the grain boundary energy of the prior austenite grain boundaries. LME cracking occurs when molten zinc penetrates the prior austenite grain boundaries. Therefore, B contained in the surface region 1111 of the base steel sheet 111 suppresses delamination of the prior austenite grain boundaries and suppresses LME cracking. To obtain the above effects, it is preferable that the average B content in the surface region 1111 be 7.0 ppm or more. More preferably, the average B content in the surface region 1111 is 8.0 ppm or more, 9.0 ppm or more, or 10.0 ppm or more.

[0075] Furthermore, B contained in parts of the base steel sheet 111 other than the surface region 1111 is less likely to exhibit an LME crack suppression effect. Therefore, the B content in parts other than the surface region 1111 is not particularly limited. B may be concentrated only in the surface region 1111 of the base steel sheet 111.

[0076] (Diameter D of Nugget 121) Preferably, the diameter D of the nugget 121 is less than 6.0 mm. The diameter D of the nugget 121 refers to the diameter of the nugget 121 measured at the joint interface by a cross-sectional test of the spot welded portion 12. The method of the cross-sectional test is specified in JIS Z 3139:2009 "Methods for Cross-Section Testing of Spot, Projection and Seam Welded Joints". FIG. 1 shows the measurement position of the diameter D of the nugget 121.

[0077] Incidentally, when the spot welded joint 1 has three or more steel sheets, two or more joint interfaces exist. When a plurality of joint interfaces exist, it is preferable that the diameter of the nugget 121 measured at the joint interface between the plated steel sheet 11C disposed on the surface of the spot welded joint 1 and the steel sheet adjacent thereto is less than 6.0 mm. When the spot welded joint 1 has three or more steel sheets and the plated steel sheets 11C are disposed on both surfaces of the spot welded joint 1, it is preferable that the diameter of the nugget 121 measured at the joint interface between at least one of the plated steel sheets 11C and the steel sheet in contact therewith is less than 6.0 mm.

[0078] From the viewpoint of increasing the joint strength, the larger the diameter D of the nugget 121, the more preferable. On the other hand, the smaller the diameter D of the nugget 121, the more difficult it is for LME cracking to occur. Therefore, from the viewpoint of further suppressing LME cracking, it is preferable that the diameter D of the nugget 121 is less than 6.0 mm. More preferably, the diameter D of the nugget 121 is 5.8 mm or less, 5.5 mm or less, or 5.2 mm or less. The lower limit value of the diameter D of the nugget 121 is not particularly limited, and is, for example, 4.5 mm or more, 4.8 mm or more, or 5.0 mm or more.

[0079] (Structure of Plating Layer 112 at Shoulder Portion 1222) Preferably, at least among the shoulder portions provided with the plating layer, at the shoulder portion where the angle formed by the shoulder portion and the surface of the plated steel sheet provided with the indentation is the largest, the plating layer 112 contains 10.0 area% or more of MgZn ２ phase. More preferably, in all the shoulder portions, the plating layer 112 contains 10.0 area% or more of MgZn ２ phase. MgZn ２ phase suppresses LME cracking. Therefore, MgZn ２By increasing the amount of phase at least in the shoulder portion 1222 where LME cracking is likely to occur, LME cracking can be further suppressed. More preferably, the plating layer 112 contains 12.0 area % or more, 15.0 area % or more, or 20.0 area % or more of MgZn in the shoulder portion 1222. ２ Includes phase.

[0080] (Chemical composition of the plating layer 112) Preferably, the chemical composition of the plating layer 112 contains, by mass%, Al: 5.0 to 15.0%, Mg: 1.0 to 8.0%, with the remainder being Zn and impurities. A ternary plating layer having such a chemical composition has extremely excellent corrosion resistance. The chemical composition of the plating layer 112 may also contain elements other than Zn, Al, and Mg. Examples of elements contained in the plating layer 112 include Si and Ca.

[0081] (Method for Manufacturing Spot Welded Joint 1) Next, preferred examples of the method for manufacturing the spot welded joint 1 according to this embodiment will be described below. However, the manufacturing method described below is not limited to the spot welded joint 1. Any spot welded joint 1 that satisfies the above requirements is considered to be the spot welded joint 1 according to this embodiment, regardless of its manufacturing method. Furthermore, the preferred embodiments of the spot welded joint 1 described above can also be applied to the manufacturing method of the spot welded joint 1.

[0082] In the method for manufacturing the spot-welded joint 1 according to this embodiment, first, multiple steel plates are stacked on top of each other. In this disclosure, the stacked steel plates to be welded are referred to as a plate assembly.

[0083] One or more of the multiple steel sheets are plated steel sheets 11C. The plated steel sheet 11C has a base steel sheet 111 and a plating layer 112 disposed on one or both sides of the base steel sheet 111. In the plated steel sheet 11C, the chemical composition and Vickers hardness of the base steel sheet 111 are within the above range, and the plating layer 112 is a ternary plating layer containing Mg, Zn, and Al. When stacking multiple steel sheets, the plating layer 112 of the plated steel sheet 11C is placed on the surface of the sheet assembly.

[0084] Next, the plate assembly is spot-welded. Specifically, the plate assembly is sandwiched between a pair of spot-welding electrodes, and while applying pressure to the plate assembly, a welding current is passed through it. The spot-welding conditions are determined to satisfy the following equation. In other words, the spot welding conditions are determined such that the following equation is satisfied: 2 ≤ (I ２ ×t ｔｏｔａｌ ×time) ×10 ６ / (F × η × T) ２ ) ≤ 10

[0085] In the above formula, I: welding current (kA) t ｔｏｔａｌ : Total thickness of steel plates included in the plate assembly (mm) F: Pressing force (kN) η: Amount of plating layer 112 adhering to the surface in contact with the electrode (g / m ２ ) T: Eutectic temperature of the plating layer 112 (°C) time: Welding time (seconds)

[0086] The welding current (I) is the current that flows to form the spot weld 12. The post-heat current, which flows after the welding current for heat treatment of the spot weld 12, is not included in the symbol I of the above formula. However, it is permissible to flow a post-heat current through the spot weld 12.

[0087] The applied pressure (F) is the force applied between a pair of electrodes during spot welding. If the applied pressure is not constant, the welding conditions should be set so that both the maximum and minimum values ​​of the applied pressure during the welding time satisfy the above formula.

[0088] The amount of plating layer 112 (η) on the surface in contact with the electrode is the mass of the plating layer 112 per unit area on the surface in contact with the electrode. When plating layers are provided on both sides of a plated steel sheet, the weight of the plating layer on the mating surface of the steel sheets of the spot-welded joint is not included in the symbol η in the above formula.

[0089] The eutectic temperature (T) of the plating layer 112 is the temperature at which a liquid phase appears when the temperature of the plating layer 112 is increased. When the temperature of the plating layer 112 exceeds the eutectic temperature, the plating layer 112 enters a solid-liquid coexistence state. An estimated value of the eutectic temperature obtained by simulation using the chemical composition of the plating layer 112 as a variable may be used as the eutectic temperature of the plating layer 112. The eutectic temperature can be estimated, for example, using integrated thermodynamic calculation software Thermo-Calc.

[0090] Welding time (I) is the time during which the welding current is passed, and is also called weld time. Note that if the welding current (I) is not controlled by constant current control, the "I" in the above formula will be... ２ Replace the "×time" component with the value obtained by integrating the square of the welding current with respect to the welding time.

[0091] Furthermore, when plated steel sheets 11C satisfying the requirements (1) to (4) above are placed on both surfaces of the plate assembly, and the plating layer 112 is placed on both surfaces of the plate assembly, η and T on one surface of the plate assembly may differ from those on the other surface. In such cases, the above formula must be satisfied on both surfaces of the plate assembly.

[0092] Furthermore, during spot welding, the spot welding electrode in contact with the plating layer 112 is a dome radius electrode (DR type electrode). A DR type electrode is an electrode in which the radius of curvature R at the tip is greater than the radius of curvature r at the peripheral part of the tip. Examples and schematic diagrams of DR type electrodes are shown in JIS C 9304:1999 "Spot welding electrodes". The radius of curvature r at the peripheral part of the spot welding electrode is greater than 8 mm.

[0093] The reason why the eutectic temperature and deposition amount of the plating layer 112 are included in the welding condition formula is that the liquid phase of the plating layer 112 affects spot welding. The eutectic temperature of the ternary plating layer containing Mg, Zn, and Al is lower than the melting point of the steel sheet. During spot welding, the liquid phase of the plating layer 112 inevitably forms around the electrode. The resistance value of the liquid phase is higher than that of the solid phase. Therefore, the liquid phase of the plating layer 112 inhibits spot welding. The amount of liquid phase increases as the heat input increases. Also, the amount of liquid phase increases as the deposition amount increases. By determining the welding conditions according to the above formula, the influence of the liquid phase can be mitigated.

[0094] Furthermore, the reason why the welding condition formula includes applied pressure is to keep the minimum thickness tw of the spot weld within the aforementioned range and to prevent LME cracking. If the applied pressure is too low, the spot weld becomes unstable. On the other hand, if the applied pressure is too high, the electrode will penetrate too deeply into the plate assembly, making it impossible to secure the minimum thickness tw of the spot weld. In addition, the stress applied to the shoulder portion 1222 will increase, making LME cracking more likely to occur in the shoulder portion 1222.

[0095] The reason why welding current and welding time are included in the welding condition formula is to keep the minimum thickness tw of the spot weld within the above range and to prevent LME cracking. If the heat input is too large, the amount of molten metal becomes excessive. As a result, similar to when the pressure is too high, the electrode penetrates deeply into the plate assembly, and the minimum thickness tw of the spot weld cannot be secured. Furthermore, the stress applied to the shoulder portion 1222 becomes large, making LME cracking more likely to occur at the shoulder portion 1222. On the other hand, if the heat input is too small, joining the steel plates becomes difficult.

[0096] The reason for limiting the electrode shape is to prevent LME cracking. The peripheral part of the electrode tip is the part that contacts the shoulder 1222 of the indentation 122. By setting the radius of curvature r of the peripheral part to a predetermined value or greater, the stress applied to the shoulder 1222 of the indentation 122 can be reduced, thereby reducing LME cracking at the shoulder 1222.

[0097] Furthermore, by setting the welding conditions to satisfy the above formula and selecting the electrode shape as described above, the stress applied to the shoulder portion 1222 during spot welding is reduced, and the angle θ of the shoulder portion 1222 falls within the above range.

[0098] While embodiments of the present disclosure have been described above, the disclosure is not limited thereto and can be modified as appropriate without departing from the technical idea. A more preferred example of the spot welded joint 1 according to this embodiment will be described below.

[0099] (Thickness of steel plates) The thickness of the steel plates is not particularly limited. The plate thickness ratio is also not particularly limited. The plate thickness ratio is the value obtained by dividing the sum of the thicknesses of the multiple steel plates included in the spot weld 12 by the thickness of the thinner of the steel plates placed on the surface of the spot welded joint 1. For example, if the spot welded joint 1 is an automobile part, it is preferable to use two thick high-strength steel plates and one thin low-strength steel plate. The high-strength steel plates are structural members of the automobile. The low-strength steel plates are exterior members of the automobile. The thickness of the high-strength steel plates is preferably, for example, 1.0 to 2.5 mm. The thickness of the low-strength steel plates is preferably, for example, 0.4 to 1.2 mm.

[0100] (Thickness and amount of plating layer 112) The thickness and amount of plating layer 112 are not particularly limited. The thickness of the plating layer 112 is preferably, for example, 0.01 mm to 0.07 mm. The amount of plating layer 112 is, for example, 60 to 275 g / m². ２ It is preferable to do so.

[0101] (Measurement Method) The measurement methods for the various parameters specified in this disclosure are as follows:

[0102] The chemical composition of the steel sheet is determined by inductively coupled plasma mass spectrometry (ICP-MS). However, the C and S content is determined by combustion-infrared absorption spectrometry. The N content is determined by inert gas fusion-thermal conductivity spectrometry. These component analyses are performed on samples taken in accordance with JIS G 0417:1999 "Iron and steel - Sampling and preparation of samples for chemical composition determination". The standard states that "the sample for analysis shall be free of surface coatings, moisture, dirt, and other contaminants." This "surface coating" includes the plating layer 112 in this disclosure.

[0103] The hardness of the steel plate shall be measured according to the Vickers hardness test method specified in JIS Z 2244-1:2020. The measurement position shall be within ±1 / 4 t from the center of the plate thickness, where t is the thickness of the steel plate. The test force shall be 200 gf.

[0104] The minimum thickness tw of the spot weld is measured in a cross-section of the spot weld 12 that passes approximately through the center of the indentation 122 and is approximately perpendicular to the surface of the spot weld joint 1. In this cross-section, the thickness of the spot weld 12 inside the indentation 122 is measured along a direction perpendicular to the surface of the spot weld joint 1. The minimum thickness obtained in this way is considered to be the minimum thickness tw of the spot weld.

[0105] The total thickness of the multiple steel plates is obtained by measuring the thickness of each steel plate included in the spot weld 12 outside the indentation 122 and calculating the sum of these values. The minimum thickness tw of the spot weld is measured in cross-section, but the total thickness of the multiple steel plates does not need to be measured in cross-section. The thickness of each steel plate can be measured at a location away from the spot weld 12 using a known tool such as a caliper. If the thicknesses of all the steel plates are given as information beforehand, these values ​​can simply be added together and considered as the total thickness of the multiple steel plates without measurement.

[0106] The average mass percentage content of B in the surface region 1111 is determined by GDS (glow discharge spectroscopy). The GDS analysis conditions are in accordance with those described in International Publication No. 2024 / 053667. The GDS analysis conditions for measuring C concentration described in International Publication No. 2024 / 053667 can be adapted for measuring B concentration in the surface region 1111 of the spot welded joint according to this embodiment. The depth of the analysis region is 9 μm to 20 μm from the surface facing the outside of the spot welded joint 1. The following is a quote from International Publication No. 2024 / 053667: "Apparatus: High-frequency glow discharge emission spectrometer (manufactured by LECO Japan LLC, model number "GDS850A") Ar gas pressure: 0.3 MPa Anode diameter: 4 mmφ RF output: 30 W Measurement time: 200 to 1500 seconds"

[0107] The diameter of the nugget 121 shall be measured in accordance with JIS Z 3139:2009 "Test Method for Sectional Measurement of Spot, Projection, and Seam Welds". According to this standard, the cross-sectional test specimen shall be cut along a vertical plane passing through the center of the weld point. However, the weld point may be unknown. In this case, the center of the indentation 122 shall be considered the weld point.

[0108] The MgZn contained in the plating layer 112 in the shoulder portion 1222 ２ The area ratio of the phase is, in principle, determined in accordance with the method described in International Publication No. 2022 / 085287. ２ Area of ​​the "phase" and "(Al-Zn) phase and MgZn ２ Lamellar tissue in which phases are arranged in layers contains layered MgZn ２ Phase area and "Zn / Al / MgZn ２ Layered MgZn contained in the ternary eutectic structure ２ The sum of the phase area and the MgZn area in this disclosure ２This is the area ratio of the phases. The measurement field of view is positioned inside the plating layer 112 provided on the shoulder portion 1222. The number of measurement fields of view is five, as described in International Publication No. 2022 / 085287. The sample size described in International Publication No. 2022 / 085287 does not necessarily have to be applied to the spot welded joint according to this embodiment. The following is a quote from International Publication No. 2022 / 085287: "The structure of the plating layer (area ratio of each phase, lamellar spacing) is measured by the following method. A sample of 25 mm in the direction perpendicular to the rolling direction and 15 mm in the rolling direction is taken from the plated steel sheet according to this embodiment, and the surface of the plating layer of this sample is embedded in resin so that it becomes the observation surface, and after polishing, an SEM image and an elemental distribution image by EDS are obtained. Lamellar structure of the plating layer, massive MgZn ２ Phase, Zn / Al / MgZn ２ The area ratios of ternary eutectic structures, (Al-Zn) dendrites, and other intermetallic compounds are calculated from cross-sectional EDS mapping images of the plating layer, obtained from five different samples, one field of view for each sample (magnification 1500x: 60 μm × 50 μm / field of view), for a total of five fields of view. At this time, the (Al-Zn) phase and MgZn are considered. ２ If the phases are arranged in layers and the lamellar spacing is 4 μm or less, it is judged to be a lamellar structure, and the (Al-Zn) phase and MgZn ２ Even if the phases are adjacent, the (Al-Zn) phase or MgZn ２ If the minor axis of the phase is greater than 4 μm, then they are called (Al-Zn) dendrites and massive MgZn. ２ Based on the determination of the phases, the Zn phase, α phase, and MgZn phase have lamellar spacing of 4 μm or less. ２ If it is a lamellar tissue, then Zn / Al / MgZn ２ If a ternary eutectic structure is determined and more than 10% of a metal other than (Zn, Al, Mg, Si) is present in the phase, it is determined to be another intermetallic compound. Furthermore, the lamellar spacing of the lamellar structure is determined by measuring the distance to the adjacent phase for the phase with the lowest area ratio among the phases forming the lamellar structure, based on SEM observation, and calculating the average of 10 such measurements. Also, lamellar structure, Zn / Al / MgZn ２ Zn phase and MgZn, which constitute the ternary eutectic structure. ２The area ratio of the Al phase can be determined by using image processing software to draw lines around the region containing the tissue on the cross-sectional SEM image, and then calculating the area of ​​the enclosed region.

[0109] The chemical composition of the plating layer 112 can be determined by dissolving the plating layer 112 in an acid solution to which an inhibitor that suppresses corrosion of the steel sheet has been added, and measuring the resulting solution by ICP (inductively coupled plasma) emission spectroscopy. The acid solution to which the inhibitor has been added is, for example, a 10% hydrochloric acid solution to which 0.06% by mass of inhibitor (manufactured by Asahi Chemical Industries, Ltd., ibit) has been added.

[0110] The eutectic temperature of the plating layer 112 can be easily determined by evaluating the plating layer 112 using a known thermal analysis method. Alternatively, as described above, an estimated value of the eutectic temperature obtained by a simulation using the chemical composition of the plating layer 112 as a variable may be used as the eutectic temperature of the plating layer 112.

[0111] The amount of plating layer 112 is determined by the adhesion amount test specified in paragraph 6 of JIS H 0401:2021. The thickness of plating layer 112 is determined by cross-sectional observation of the plating layer 112.

[0112] The effects of one aspect of this disclosure will be further illustrated by the examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effectiveness of this disclosure. This disclosure is not limited to these examples of conditions. This disclosure may adopt various conditions as long as they do not depart from its gist and achieve its objectives.

[0113] Spot-welded joints were manufactured using the plated steel sheets shown in the table below. The "Thickness" column in Table 1 indicates the thickness of the plated steel sheet. The "Strength" column indicates the tensile strength of the plated steel sheet in MPa. The "Hardness" column indicates the Vickers hardness of the base steel sheet, estimated from its tensile strength. The Vickers hardness was estimated based on the following formula: σ Ｂ = 0.32 × HV The above formula is formula 25 disclosed in "Work Hardening in Ferritic Steel" by Setsuo Takagi, Iron and Steel, Vol. 105, 2019, No. 10, p. 953. HV is the Vickers hardness in units of GPa, and σ ＢThis is the tensile strength in units of GPa. The units of tensile strength listed in the "Strength" column of Table 1 are MPa, and since Vickers hardness needs to be unitless in this disclosure, the inventors have converted the above formula as follows: [Vickers hardness (unitless)] = 102 × [Tensile strength (MPa)] / (0.32 × 1000) In the above formula, "102" is a coefficient for converting Vickers hardness in units of GPa to unitless Vickers hardness. In the above formula, "1000" is a coefficient for converting tensile strength in units of GPa to tensile strength in units of MPa.

[0114] The "Plating Eutectic Temperature" column lists the eutectic temperature of the plating layer on the plated steel sheet. The eutectic temperature is a value estimated using the integrated thermodynamic calculation software Thermo-Calc. The "Plating Type" column lists the composition of the plating layer. "Zn-6Al-3Mg-0.2Si" is a ternary plating layer whose main component is Zn, and which contains 6 mass% Al, 3 mass% Mg, and 0.2 mass% Si. "Zn-11Al-3Mg-0.2Si" is a ternary plating layer whose main component is Zn, and which contains 11 mass% Al, 3 mass% Mg, and 0.2 mass% Si.

[0115]

[0116] Table 2 lists the chemical composition, in mass percent, of the base steel sheet used for plating. The remainder of the base steel sheet's chemical composition consisted of iron and impurities. Table 2 also includes the B content in the surface layer of the base steel sheet. A "-" symbol indicates that the corresponding element was not added during the manufacturing process of the base steel sheet.

[0117]

[0118] Two of these plated steel sheets were stacked together to prepare a plate assembly. Spot welding was then performed on the plate assembly. Table 3 lists the types of plated steel sheets included in the plate assembly, and the radius of curvature r of the peripheral part of the tip of the pair of electrodes. Also, the welding parameters obtained by the following formula, namely "(I ２ ×t ｔｏｔａｌ ×time) ×10 ６ / (F × η × T) ２) is also listed in Table 3. I: Welding current (kA) t ｔｏｔａｌ : Total thickness of steel plates included in the plate assembly (mm) F: Pressing force (kN) η: Amount of plating layer (g / m ２ ) T: Eutectic temperature of the plating layer (°C) time: Welding time (seconds)

[0119]

[0120] The spot welded joints obtained by the procedure described above were evaluated. Specifically, the ratio of the minimum thickness of the spot weld to the total thickness of the multiple steel plates, and the size of the angle between the shoulder of the indentation and the surface of the plated steel plate (shoulder angle θ) were measured. Furthermore, the diameter of the nugget and the MgZn of the plating layer at the shoulder were measured. ２ The phase area ratio was also measured. These values ​​are listed in Table 4.

[0121] The plating layer was applied to both surfaces of the spot-welded joint. Therefore, there were four shoulders to be evaluated in the cross-section. In this experiment, the angles of the four shoulders were measured. The maximum value of the shoulder angle was recorded in the "Shoulder Angle" column of the table. Spot-welded joints with a value of 20 degrees or less in the "Shoulder Angle" column have an angle of 20 degrees or less at all four shoulders, where the angle between the shoulder and the surface of the plated steel sheet with the indentation is 20 degrees or less.

[0122] Next, the LME crack resistance of the spot-welded joints was evaluated. The evaluation criteria were as follows: A: No LME cracks, or LME crack length is less than 50 μm B: LME crack length is 50 μm or more and less than 100 μm C: LME crack length is 100 μm or more and less than 200 μm D: LME crack length is 200 μm or more The method for measuring the length of the LME crack was as follows: The cross-section of the spot weld, passing through approximately the center of the indentation and approximately perpendicular to the surface of the spot-welded joint, was observed with an optical microscope. If an LME crack was visible at the shoulder, a magnified photograph of the LME crack was taken, and the length of the LME crack along the thickness direction was measured by image analysis of the photograph. "Length along the thickness direction" refers to the distance between the start and end points of the LME crack measured along the thickness direction. The component of the LME crack perpendicular to the thickness direction is ignored in the measurement of the LME length. If there were two or more LME cracks, the above evaluation was performed based on the longest LME crack length.

[0123] Spot welded joints that received an LME cracking resistance rating of A, B, or C were judged to have excellent LME cracking resistance. The evaluation results are shown in Table 4.

[0124]

[0125] In Example 2, the welding parameter values ​​were above the appropriate range. Furthermore, the electrode shape was inappropriate in Example 2. As a result, the stress on the shoulder of the indentation was excessive, and the shoulder angle exceeded the appropriate range. Consequently, the LME cracking resistance of the spot-welded joint in Example 2 was poor. The main reason for the above-appropriate welding parameter values ​​in Example 2 was that the welding current was too high.

[0126] In Example 5, the Si content of the base steel sheet was excessive. Therefore, the LME cracking resistance of the spot-welded joint in Example 5 was inferior.

[0127] In Example 7, the welding parameter values ​​were above the appropriate range. Furthermore, the electrode shape was inappropriate. As a result, the stress on the shoulder of the indentation was excessive, and the shoulder angle exceeded the appropriate range. Consequently, the LME cracking resistance of the spot-welded joint in Example 7 was poor. Moreover, the minimum thickness of the spot weld in Example 7 was below the appropriate range. The main reason for the above-appropriate welding parameter values ​​in Example 7 was that the welding current was too high. A large amount of spatter occurred during the spot welding in Example 7. This is considered to be the cause of the thin spot weld.

[0128] In Example 10, the welding parameter values ​​exceeded the appropriate range. As a result, the minimum thickness of the spot weld in Example 10 fell below the appropriate range. Furthermore, the LME resistance of the spot welded joint in Example 10 was inferior. The main reasons why the welding parameter values ​​in Example 10 exceeded the appropriate range were that the pressing force was too high and the energizing time was too long. As a result, it is thought that the electrodes dug significantly into the plate assembly, causing the spot weld to become thin.

[0129] In Example 14, the Si content of the base steel sheet was excessive. Therefore, the LME cracking properties of the spot-welded joint in Example 14 were inferior.

[0130] On the other hand, in cases where the manufacturing conditions were appropriate, spot-welded joints with good LME resistance were obtained. Furthermore, the thickness of the spot weld was ensured in these spot-welded joints.

[0131] 1 Spot welded joint 11C Plated steel sheet 111 Bare steel sheet 1111 Surface layer 112 Plating layer 11N Unplated steel sheet 12 Spot welded area 121 Nugget 122 Indentation 1221 Outer edge 1222 Shoulder 123 Heat-affected zone 13 Sheet separation tw Minimum thickness of spot welded area t1, t2 Thickness of steel sheet θ Shoulder angle, D Nugget diameter

Claims

1. A spot-welded joint comprising: a plurality of stacked steel plates; and a spot-welded section having a nugget for joining the plurality of steel plates, wherein one or more of the steel plates are plated steel plates having a base steel plate and a plating layer disposed on one or both sides of the base steel plate, the base steel plate contains, by mass%, C: 0.005 to 0.200%, Si: 0 to 1.00%, Mn: 0.08 to 3.00%, Al: 0.005 to 0.5%, Ti: 0 to 0.1000%, B: 0 to 0.0080%, P: 0.020% or less, S: 0.010% or less, and N: 0.010% or less, and the Vickers hardness of the base steel plate is 85 HV or more and less than 320 HV. A spot welded joint in which the plating layer is a ternary plating layer containing Mg, Zn, and Al, the plating layer is disposed on the surface of the spot welded joint, the minimum thickness of the spot weld is 50% or more of the total thickness of the plurality of steel plates, and the angle between the shoulder of the indentation of the spot weld and the surface of the plated steel plate on which the indentation is provided is 20 degrees or less on the surface of the spot welded joint on which the plating layer is disposed.

2. The mass% B content and mass% N content of the base steel sheet satisfy the following formula A: (mass% B) × (mass% N) ≤ 1.5 × 10 －６ A spot welded joint according to feature A of 1.

3. The mass% B content, mass% N content, and mass% Ti content of the base steel sheet satisfy the following formulas B and C: (mass% B) × (mass% N) ≥ 1.5 × 10 －６ Formula B (mass%Ti) / (mass%N)≧0.3 Formula C The spot welded joint according to claim 1.

4. The spot welded joint according to claim 1, characterized in that the base steel sheet contains, by mass%, Si: 0 to 0.80%, Mn: 0.08 to 1.80%, and B: 0.0010 to 0.0080%.

5. The spot welded joint according to claim 1, characterized in that the average content of B measured in the surface layer region of the base steel plate, which is a region with a depth of 9 μm or more and 20 μm or less from the surface facing the outside of the spot welded joint, is 7.0 ppm or more.

6. The spot-welded joint according to any one of claims 1 to 5, characterized in that the diameter of the nugget is less than 6.0 mm.

7. In the shoulder portion on which the plating layer is provided, the shoulder portion where the angle between the shoulder portion and the surface of the plated steel sheet on which the indentation is provided is the largest, the plating layer contains 10.0 area % or more of MgZn. ２ A spot welded joint according to any one of claims 1 to 5, characterized by including a phase.

8. The spot welded joint according to any one of claims 1 to 5, characterized in that the chemical composition of the plating layer contains, by mass%, Al: 5.0 to 15.0%, Mg: 1.0 to 8.0%, with the remainder being Zn and impurities.

9. A spot-welded joint according to any one of claims 1 to 5, characterized in that the chemical composition of the base steel sheet is Al: 0.005 to 0.3%.

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