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

Abstract

The invention relates to a spot-welded joint and a method for manufacturing the spot-welded joint. A spot-welded joint according to one embodiment of the present invention is provided with a first steel plate, a second steel plate, and a spot-welded part that joins the first steel plate and the second steel plate, the first steel plate and the second steel plate being directly overlapped, the tensile strength of the first steel plate being 1500 MPa or more, the tensile strength of the second steel plate being not more than the tensile strength of the first steel plate, and the tensile strength of the second steel plate being not more than the tensile strength of the first steel plate. The Vickers hardness from the surface of the first steel sheet to a depth of 20 [mu] m is 95% or less of the Vickers hardness at a position 1 / 4 of the sheet thickness of the base material portion of the first steel sheet, the carbon equivalent Ceq of the base material portion of the first steel sheet is 0.22% or more, HC = Ceq * TS2 * (1-(TS2 * t23) / (TS1 * t13)) / [square of t2] is 170 or more, and the spot welding portion is provided with a nugget and a crimped portion located around the nugget and joining the first steel sheet and the second steel sheet. The Vickers hardness at a position 20 [mu] m on the first steel plate side of the outer peripheral end of the crimped portion is lower than the Vickers hardness at a position 1 / 4 of the plate thickness of the base material portion of the first steel plate by Hv50 or more.

Description

Technical Field

[0001] The invention relates to a spot welding joint and a method for manufacturing the spot welding joint.

[0002] This application claims priority based on Tokugan Application No. 2022-089551 filed in Japan on June 1, 2022, the contents of which are incorporated herein by reference. Background Art

[0003] Resistance spot welding can join steel plates quickly and at low cost, and is therefore used in various applications, such as joining high-strength steel plates, which are materials for automobile body parts.

[0004] On the other hand, in the automotive field, the steel sheets are becoming stronger and thinner, which can reduce the weight of automobile body parts while maintaining their strength, thereby improving fuel efficiency and other effects.

[0005] Here, in high-strength steel plates, such as steel plates with tensile strengths of 1300MPa, 1500MPa, and 1800MPa, the reduction in joint strength of the joint of the spot welding joint and hydrogen embrittlement become issues. One of the reasons is that as the C content of the nugget (molten portion) increases, the toughness of the nugget decreases. In high-strength steel plates, heat treatments under various conditions are applied to optimize the metal structure. Thus, the toughness of the high-strength steel plate is ensured. However, if the high-strength steel plate is spot welded, the metal structure changes in the nugget and the HAZ (Heat Affected Zone) around it. As a result, the embrittlement of the nugget occurs. From the viewpoint of ensuring the joint strength of the spot welding joint, a joining method that suppresses the embrittlement of the nugget is required.

[0006] In the prior art, methods have been proposed to achieve both joint strength and hydrogen embrittlement prevention by increasing the diameter of the nugget and modifying the nugget by post-energization. However, these methods have the problem of increasing production cycles and narrowing the range of appropriate welding conditions.

[0007] In addition, research is also underway to improve joint strength and prevent hydrogen embrittlement by providing a decarburized layer on the surface of the steel plate before spot welding.

[0008] For example, Patent Document 1 discloses a joining structure, characterized in that it comprises: a first component, which is made of high-strength steel; a second component, which overlaps with the above-mentioned first component and is made of high-strength steel; a surface soft layer, which is formed on at least one of the overlapping surfaces of the above-mentioned first component and the above-mentioned second component or the overlapping surfaces of the above-mentioned second component and the above-mentioned first component; a molten solidification portion, which is formed by melting and solidifying the above-mentioned first component and the above-mentioned second component; and a heat-affected portion, which is formed around the above-mentioned molten solidification portion, the total thickness of the above-mentioned surface soft layer is not less than 5μm and not more than 200μm, the carbon content of the above-mentioned molten solidification portion is not less than 0.35 mass%, and the maximum Vickers hardness of the above-mentioned surface soft layer in the above-mentioned heat-affected portion is not less than 100Hv and not more than 500Hv.

[0009] Patent Document 2 discloses a steel plate having excellent spot weld fracture resistance, wherein the ratio of the hardness Hs of the steel plate surface layer to the hardness Hm of the steel plate center portion (Hs / Hm) is 0.4 or more and 0.8 or less.

[0010] Prior art literature

[0011] Patent Literature

[0012] Patent Document 1: Japanese Patent Application Publication No. 2020-82103

[0013] Patent Document 2: Japanese Patent Application Publication No. 2017-2384 Summary of the invention

[0014] Problems to be solved by the invention

[0015] The general spot welding part is composed of a nugget as a molten solidification part, a crimping part formed around the nugget, and a HAZ (Heat Affected Zone). The crimping part refers to the area where two steel plates are solid-phase joined even though molten solidification has not occurred. The crimping part is also called a crimping surface or a plastic metal ring. The HAZ refers to the area where the metal structure, metallurgical properties, and mechanical properties have changed due to the influence of welding heat even though molten solidification has not occurred.

[0016] The inventors have found that when spot welding is performed on a plate group consisting of a plurality of steel plates including one or more high-strength steel plates and having different tensile strengths and / or thicknesses, cracks of a unique morphology may occur. Specifically, Figure 5 As shown, cracks developing in the thickness direction of the steel plate were confirmed in the HAZ near the outer peripheral end of the crimped portion. In addition, the outer peripheral end of the crimped portion was visually recognized as the crimped portion end on the opposite side of the nugget in the cross section of the nugget.

[0017] For example, even if the carbon content of the nugget is diluted by forming a plate set of high carbon steel and low carbon steel, such HAZ cracks near the outer peripheral end of the rolled surface cannot be eliminated. In addition, as described in Patent Documents 1 or 2, the risk of HAZ cracks near the outer peripheral end of the crimped portion is low in plate sets of the same strength and thickness, and in plate sets of high carbon steel and mild steel. Regarding the suppression of such cracks, Patent Documents 1 or 2 do not conduct sufficient research.

[0018] In view of the above situation, the subject of the present invention is to provide a spot welding joint and a method for manufacturing a spot welding joint, wherein the spot welding joint is composed of multiple steel plates including one or more high-strength steel plates, and these steel plates have different tensile strengths and / or thicknesses, which can suppress HAZ cracks near the outer peripheral side end of the crimping part.

[0019] Means for solving problems

[0020] The gist of the present invention is as follows.

[0021] (1) A spot welding joint according to one embodiment of the present invention comprises: a first steel plate; a second steel plate; and a spot welding portion, wherein the first steel plate and the second steel plate are joined, the first steel plate and the second steel plate are directly overlapped, the tensile strength of the first steel plate is 1500 MPa or more, the tensile strength of the second steel plate is less than the tensile strength of the first steel plate, and the Vickers hardness from the surface of the first steel plate to a depth of 20 μm is the Vickers hardness of the base material portion of the first steel plate at a position 1 / 4 of the plate thickness. 95% or less, the carbon equivalent Ceq of the above-mentioned base material portion of the above-mentioned first steel plate defined by formula 1 is more than 0.22%, the index HC defined by formula 2 is more than 170, the above-mentioned spot weld portion comprises a molten core, and a crimping portion located around the above-mentioned molten core and joining the above-mentioned first steel plate to the above-mentioned second steel plate, and the Vickers hardness of 20μm on the side of the above-mentioned first steel plate at the outer peripheral side end portion of the above-mentioned crimping portion is lower than the Vickers hardness at the position of 1 / 4 of the plate thickness of the above-mentioned base material portion of the above-mentioned first steel plate by more than Hv50.

[0022] Ceq=C+Si / 90+(Mn+Cr) / 100 (Formula 1)

[0023] HC=Ceq×TS2×(1-(TS2×t2 3 ) / (TS1×t1 3 )) / √t2 (Formula 2)

[0024] Here, the element symbol recorded in the above formula 1 is the content of the element corresponding to the element symbol at the 1 / 4 part of the plate thickness of the above base material part of the above first steel plate in unit mass %, the symbol TS1 recorded in the above formula 2 is the tensile strength of the above first steel plate in unit MPa, t1 is the plate thickness of the above first steel plate in unit mm, TS2 is the tensile strength of the above second steel plate in unit MPa, and t2 is the plate thickness of the above second steel plate in unit mm.

[0025] (2) In the spot welded joint described in (1) above, preferably, the base material portion of the first steel plate includes a martensitic structure.

[0026] (3) Preferably, in the spot welding joint described in (1) or (2), the Vickers hardness from the surface of the first steel plate to a depth of 50 μm is less than 95% of the Vickers hardness of the parent material portion of the first steel plate at a position of 1 / 4 of the plate thickness, and the Vickers hardness of 50 μm on the first steel plate side of the outer peripheral end portion of the crimped portion is lower than the Vickers hardness of the parent material portion of the first steel plate at a position of 1 / 4 of the plate thickness by more than Hv50.

[0027] (4) Preferably, in the spot weld joint described in any one of the above (1) to (3), the Vickers hardness from the above-mentioned surface of the above-mentioned first steel plate to a depth of 100 μm is less than 95% of the Vickers hardness of the above-mentioned base material portion of the above-mentioned first steel plate at a position of 1 / 4 of the plate thickness, and the Vickers hardness of 100 μm on the above-mentioned first steel plate side of the above-mentioned outer peripheral side end portion of the above-mentioned crimping portion is lower than the above-mentioned Vickers hardness at a position of 1 / 4 of the plate thickness of the above-mentioned base material portion of the above-mentioned first steel plate by more than Hv50.

[0028] (5) Preferably, in the spot welding joint described in any one of the above (1) to (4), the Vickers hardness from the above-mentioned surface of the above-mentioned first steel plate to a depth of 200 μm is less than 95% of the Vickers hardness of the above-mentioned base material portion of the above-mentioned first steel plate at a position of 1 / 4 of the plate thickness, and the Vickers hardness of 200 μm on the above-mentioned first steel plate side of the above-mentioned outer peripheral side end portion of the above-mentioned crimping portion is lower than the above-mentioned Vickers hardness at a position of 1 / 4 of the plate thickness of the above-mentioned base material portion of the above-mentioned first steel plate by more than Hv50.

[0029] (6) Another aspect of the present invention is a method for manufacturing a spot welding joint, comprising: overlapping a first steel plate and a second steel plate; and spot welding the first steel plate and the second steel plate, wherein the tensile strength of the first steel plate is greater than 1500 MPa, the tensile strength of the second steel plate is less than the tensile strength of the first steel plate, the Vickers hardness from the surface of the first steel plate to a depth of 20 μm is less than 95% of the Vickers hardness of a base material portion of the first steel plate at a position of 1 / 4 of the plate thickness, the carbon equivalent Ceq of the base material portion of the first steel plate defined by formula 1 is greater than 0.22%, and the index HC defined by formula 2 is greater than 170.

[0030] Ceq=C+Si / 90+(Mn+Cr) / 100 (Formula 1)

[0031] HC=Ceq×TS2×(1-(TS2×t2 3 ) / (TS1×t1 3 )) / √t2 (Formula 2)

[0032] Here, the element symbol recorded in the above formula 1 is the content of the element corresponding to the element symbol at the 1 / 4 part of the plate thickness of the above base material part of the above first steel plate in unit mass %, the symbol TS1 recorded in the above formula 2 is the tensile strength of the above first steel plate in unit MPa, t1 is the plate thickness of the above first steel plate in unit mm, TS2 is the tensile strength of the above second steel plate in unit MPa, and t2 is the plate thickness of the above second steel plate in unit mm.

[0033] (7) Preferably, in the method for manufacturing a spot welded joint described in (6) above, the Vickers hardness from the surface of the first steel plate to a depth of 50 μm is less than 95% of the Vickers hardness at the position of 1 / 4 of the plate thickness of the base material portion of the first steel plate.

[0034] (8) Preferably, in the method for manufacturing a spot welded joint described in (6) or (7) above, the Vickers hardness from the surface of the first steel plate to a depth of 100 μm is less than 95% of the Vickers hardness at the position of 1 / 4 of the plate thickness of the base material portion of the first steel plate.

[0035] (9) Preferably, in the method for manufacturing a spot welded joint described in any one of (6) to (8) above, the Vickers hardness from the above-mentioned surface of the above-mentioned first steel plate to a depth of 200 μm is less than 95% of the Vickers hardness at the above-mentioned 1 / 4 plate thickness position of the above-mentioned base material portion of the above-mentioned first steel plate.

[0036] <Additional Notes>

[0037] (1) A spot welding joint according to another embodiment of the present invention comprises: two steel plates, including at least one high-strength steel plate having a tensile strength of 1300 MPa or more; and a spot welding portion having a nugget and a crimping surface around the nugget, wherein the two steel plates are joined, and when the tensile strengths of the two steel plates are different, the steel plate having the higher tensile strength is set as the first steel plate, and the steel plate having the lower tensile strength is set as the second steel plate; and when the tensile strengths of the two steel plates are the same, the thicker one is set as the second steel plate. When the steel plate is set as the first steel plate and the thinner steel plate is set as the second steel plate, in the base material portion of the first steel plate, the carbon equivalent Ceq within the plate thickness represented by the following formula 1 is greater than 0.22%, the index HC represented by the following formula 2 is greater than 170, and the Vickers hardness of the surface portion of the first steel plate at the outer peripheral end portion of the crimping surface measured in a cross section passing through the center of the weld core and perpendicular to the surface of the steel plate is lower than the Vickers hardness of a 1 / 4 portion of the plate thickness of the first steel plate by more than HV50.

[0038] Ceq=C+Si / 90+(Mn+Cr) / 100 (Formula 1)

[0039] HC=Ceq×TS2×(1-(TS2×t2 3 ) / (TS1×t1 3 )) / √t2 (Formula 2)

[0040] Here, the element symbol recorded in the above formula 1 is the content of the element corresponding to the element symbol in the above plate thickness of the above base material part of the above first steel plate in unit mass %, the symbol TS1 recorded in the above formula 2 is the tensile strength of the above first steel plate in unit MPa, t1 is the plate thickness of the above first steel plate in unit mm, TS2 is the tensile strength of the above second steel plate in unit MPa, and t2 is the plate thickness of the above second steel plate in unit mm.

[0041] (2) A spot welding joint according to another embodiment of the present invention comprises: three or more steel plates, including one or more high-strength steel plates having a tensile strength of 1300 MPa or more; and a spot welding portion having a nugget and a pressure-welding surface around the nugget, wherein the three or more steel plates are joined, and in at least one of the combinations of the high-strength steel plate and the steel plates connected thereto, when the tensile strengths of the two steel plates are different, the steel plate having the higher tensile strength is set as the first steel plate, and the steel plate having the lower tensile strength is set as the second steel plate, and in the two steel plates, the pressure-welding portion comprises a spot welding portion, wherein the spot welding portion comprises a spot welding portion, and a pressure-welding ... When the tensile strength of the plates is the same, the thicker steel plate is set as the first steel plate and the thinner steel plate is set as the second steel plate. In the parent material portion of the first steel plate, the carbon equivalent Ceq inside the plate thickness represented by the following formula 1 is greater than 0.22%, the index HC represented by the following formula 2 is greater than 170, and the Vickers hardness of the surface portion of the first steel plate at the outer peripheral end of the crimping surface measured in a cross section passing through the center of the weld core and perpendicular to the surface of the steel plate is lower than the Vickers hardness of a 1 / 4 portion of the plate thickness of the first steel plate by more than HV50.

[0042] Ceq=C+Si / 90+(Mn+Cr) / 100 (Formula 1)

[0043] HC=Ceq×TS2×(1-(TS2×t2 3 ) / (TS1×t1 3 )) / √t2 (Formula 2)

[0044] Here, the element symbol recorded in the above formula 1 is the content of the element corresponding to the element symbol in the above plate thickness of the above base material part of the above first steel plate in unit mass %, the symbol TS1 recorded in the above formula 2 is the tensile strength of the above first steel plate in unit MPa, t1 is the plate thickness of the above first steel plate in unit mm, TS2 is the tensile strength of the above second steel plate in unit MPa, and t2 is the plate thickness of the above second steel plate in unit mm.

[0045] (3) In the spot welding joint described in (2) above, in at least the combination with the largest index HC among the combinations of the high-strength steel plate and the steel plate connected thereto, the carbon equivalent Ceq inside the plate thickness is greater than 0.22%, the index HC is greater than 170, and the Vickers hardness of the surface portion of the first steel plate at the outer peripheral end portion of the crimping surface is lower than the Vickers hardness of the 1 / 4 portion of the plate thickness of the first steel plate by more than HV50.

[0046] (4) In the spot welding joint described in (2) above, in all combinations of the high-strength steel plate and the steel plate connected thereto, the carbon equivalent Ceq inside the plate thickness is greater than 0.22%, the index HC is greater than 170, and the Vickers hardness of the surface portion of the first steel plate at the outer peripheral end portion of the crimping surface is lower than the Vickers hardness of the 1 / 4 portion of the plate thickness of the first steel plate by more than HV50.

[0047] (5) In the spot welded joint described in any one of (1) to (4) above, the first steel plate may have a decarburized layer having a depth of 5 to 200 μm.

[0048] (6) In the spot welded joint described in any one of (1) to (5) above, the tensile strength of the first steel plate may be 1500 MPa or more.

[0049] (7) In the spot welded joint described in any one of (1) to (6) above, the first steel plate may be a hot-pressed steel plate.

[0050] (8) An automobile component according to another embodiment of the present invention includes the spot welded joint described in any one of (1) to (7).

[0051] (9) A method for manufacturing a spot welded joint according to another embodiment of the present invention comprises the following steps: spot welding two steel plates including one or more high-strength steel plates having a tensile strength of 1300 MPa or more to form a spot weld portion having a nugget and a crimping surface around the nugget and joining the two steel plates, wherein, when the tensile strengths of the two steel plates are different, the steel plate having the higher tensile strength is set as the first steel plate and the steel plate having the lower tensile strength is set as the second steel plate, and when the tensile strengths of the two steel plates are the same, the spot weld portion is formed by spot welding the two steel plates. When the thicker steel plate is set as the first steel plate and the thinner steel plate is set as the second steel plate, in the base material portion of the first steel plate, the carbon equivalent Ceq inside the plate thickness represented by the following formula 1 is greater than 0.22%, and the index HC represented by the following formula 2 is greater than 170, so that the Vickers hardness of the surface portion of the first steel plate at the outer peripheral side end of the crimping surface measured in a cross section passing through the center of the weld core and perpendicular to the surface of the steel plate is lower than the Vickers hardness of a 1 / 4 portion of the plate thickness of the first steel plate by more than HV50.

[0052] Ceq=C+Si / 90+(Mn+Cr) / 100 (Formula 1)

[0053] HC=Ceq×TS2×(1-(TS2×t2 3 ) / (TS1×t1 3 )) / √t2 (Formula 2)

[0054] Here, the element symbol recorded in the above formula 1 is the content of the element corresponding to the element symbol in the above plate thickness of the above base material part of the above first steel plate in unit mass %, the symbol TS1 recorded in the above formula 2 is the tensile strength of the above first steel plate in unit MPa, t1 is the plate thickness of the above first steel plate in unit mm, TS2 is the tensile strength of the above second steel plate in unit MPa, and t2 is the plate thickness of the above second steel plate in unit mm.

[0055] (10) A method for manufacturing a spot welded joint according to another embodiment of the present invention comprises the following steps: spot welding three or more steel plates including one or more high-strength steel plates having a tensile strength of 1300 MPa or more to form a spot weld portion having a nugget and a crimping surface around the nugget and joining the three or more steel plates, wherein in at least one of the combinations of the high-strength steel plate and the steel plate connected thereto, when the tensile strengths of the two steel plates are different, the steel plate having the higher tensile strength is set as the first steel plate, and the steel plate having the lower tensile strength is set as the second steel plate When the tensile strength of the two steel plates is the same, the thicker steel plate is set as the first steel plate and the thinner steel plate is set as the second steel plate, in the base material portion of the first steel plate, the carbon equivalent Ceq inside the plate thickness represented by the following formula 1 is greater than 0.22%, and the index HC represented by the following formula 2 is greater than 170, so that the Vickers hardness of the surface portion of the first steel plate at the outer peripheral end of the crimping surface measured in a cross section passing through the center of the weld core and perpendicular to the surface of the steel plate is lower than the Vickers hardness of the 1 / 4 portion of the plate thickness of the first steel plate by more than HV50.

[0056] Ceq=C+Si / 90+(Mn+Cr) / 100 (Formula 1)

[0057] HC=Ceq×TS2×(1-(TS2×t2 3 ) / (TS1×t1 3 )) / √t2 (Formula 2)

[0058] Here, the element symbol recorded in the above formula 1 is the content of the element corresponding to the element symbol in the above plate thickness of the above base material part of the above first steel plate in unit mass %, the symbol TS1 recorded in the above formula 2 is the tensile strength of the above first steel plate in unit MPa, t1 is the plate thickness of the above first steel plate in unit mm, TS2 is the tensile strength of the above second steel plate in unit MPa, and t2 is the plate thickness of the above second steel plate in unit mm.

[0059] (11) In the method for manufacturing a spot welding joint described in (10) above, in at least the combination with the largest index HC among the combinations of the high-strength steel plate and the steel plate connected thereto, the carbon equivalent Ceq inside the plate thickness is greater than 0.22%, the index HC is greater than 170, and the Vickers hardness of the surface portion of the first steel plate at the outer peripheral end portion of the crimping surface is lower than the Vickers hardness of the 1 / 4 portion of the plate thickness of the first steel plate by more than HV50.

[0060] (12) In the method for manufacturing a spot welding joint described in (10) above, in all combinations of the high-strength steel plate and the steel plate connected thereto, the carbon equivalent Ceq inside the plate thickness is greater than 0.22%, the index HC is greater than 170, and the Vickers hardness of the surface portion of the first steel plate at the outer peripheral end portion of the crimping surface is lower than the Vickers hardness of the 1 / 4 portion of the plate thickness of the first steel plate by more than HV50.

[0061] (13) In the method for producing a spot welded joint according to any one of (9) to (12), the first steel plate may have a decarburized layer having a depth of 5 to 200 μm.

[0062] Effects of the Invention

[0063] According to the present invention, a spot welding joint and a method for manufacturing a spot welding joint can be provided, wherein the spot welding joint is composed of multiple steel plates including one or more high-strength steel plates, and these steel plates have different tensile strengths and / or thicknesses, which can suppress HAZ cracks near the outer peripheral end of the crimping portion. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Figure 1 It is a schematic diagram of a cross section passing through the center of the nugget and perpendicular to the surface of the steel sheet of a spot welded joint having two steel sheets according to the first embodiment.

[0065] Figure 2 It is an enlarged cross-sectional view of a crimping portion of a spot welding joint according to the first embodiment.

[0066] Figure 3 This is a schematic diagram of a cross section of a spot welded joint including three steel plates and one high-strength steel plate, passing through the center of the nugget and perpendicular to the surface of the steel plates.

[0067] Figure 4 This is a schematic diagram of a cross section of a spot welded joint including three steel plates and two high-strength steel plates, passing through the center of the nugget and perpendicular to the surfaces of the steel plates.

[0068] Figure 5 This is an enlarged cross-sectional view of a crimped portion of a conventional spot welded joint. DETAILED DESCRIPTION

[0069] (First embodiment: spot welding joint)

[0070] like Figure 1 As shown, the spot welding joint 1 of the first embodiment of the present invention comprises a first steel plate 111, a second steel plate 112, and a spot welding portion 12 for joining the first steel plate 111 and the second steel plate 112, the first steel plate 111 and the second steel plate 112 are directly overlapped, the tensile strength of the first steel plate 111 is 1500 MPa or more, the tensile strength of the second steel plate 112 is less than the tensile strength of the first steel plate 111, and the Vickers hardness from the surface of the first steel plate 111 to a depth of 20 μm is 1 / the plate thickness of the parent material portion of the first steel plate 111 The spot weld 12 has a nugget 121 and a crimping portion 122 around the nugget for joining the first steel plate 111 and the second steel plate 112, and the Vickers hardness of 20 μm on the side of the first steel plate 111 at an outer peripheral end 122E of the crimping portion 122 is lower than the Vickers hardness of the first steel plate 111 at a position 1 / 4 of the plate thickness of the parent material portion of the first steel plate 111 by more than Hv50.

[0071] Ceq=C+Si / 90+(Mn+Cr) / 100 (Formula 1)

[0072] HC=Ceq×TS2×(1-(TS2×t2 3 ) / (TS1×t1 3 )) / √t2 (Formula 2)

[0073] Here, the element symbol recorded in Formula 1 is the content of the element corresponding to the element symbol at the 1 / 4 part of the plate thickness of the base material part of the first steel plate 111, expressed in unit mass %, the symbol TS1 recorded in Formula 2 is the tensile strength of the first steel plate 111 expressed in unit MPa, t1 is the plate thickness of the first steel plate 111 expressed in unit mm, TS2 is the tensile strength of the second steel plate 112 expressed in unit MPa, and t2 is the plate thickness of the second steel plate 112 expressed in unit mm.

[0074] (Steel Plate 11)

[0075] The welding base materials of the spot welding joint 1 of the first embodiment are two or more steel plates 11. Hereinafter, the case where the number of steel plates 11 is two will be described first, and then the case where the number of steel plates 11 is three or more will be described.

[0076] In the spot welded joint 1 of the first embodiment, one or both of the two steel plates 11 are high-strength steel plates. In this specification, a high-strength steel plate is defined as a steel plate having a tensile strength of 1300 MPa or more or 1500 MPa or more.

[0077] In addition, for convenience, the steel plate 11 with higher tensile strength among the two steel plates 11 is defined as a first steel plate 111, and the other steel plate 11 is defined as a second steel plate 112. Figure 1 In the described spot welding joint 1, only the first steel plate 11 with hatching from the bottom is a high-strength steel plate, and the other steel plates 11 are soft steel plates. Therefore, the steel plate 11 with hatching is regarded as the first steel plate 111, and the steel plate 11 without hatching is regarded as the second steel plate 112. If the two steel plates 11 have the same tensile strength, the thicker steel plate 11 is defined as the first steel plate 111, and the thinner steel plate 11 is defined as the second steel plate 112. In addition, when the plate thickness and tensile strength of the two steel plates are the same, the index HC described later becomes 0, so it is not considered in the spot welding joint 1 of the first embodiment.

[0078] like Figure 1 As shown in the example, in the spot welding joint 1, the first steel plate 111 and the second steel plate 112 are directly overlapped. In addition, during spot welding, the deformation amount of the first steel plate 111 is smaller than the deformation amount of the second steel plate 112. Therefore, after the spot welding is completed, the first steel plate 111 is applied with tensile stress from the second steel plate 112. This will be described later.

[0079] In addition, in the spot welding joint 1 of the first embodiment, it is preferred that the Vickers hardness from the surface of the first steel plate 111 to a depth of 20 μm is less than 95% of the Vickers hardness at the 1 / 4 position of the plate thickness of the parent material portion of the first steel plate 111. This means that the decarburized layer depth of the first steel plate 111 is at least 20 μm. The decarburized layer depth and its effect will be described later. In addition, the 1 / 4 position of the plate thickness refers to the position of the depth of 1 / 4t1 from the surface of the first steel plate 111 when the plate thickness of the first steel plate 111 is expressed as t1. The 1 / 4 position of the plate thickness is a different concept from the 1 / 4 portion of the plate thickness described later.

[0080] In the base material portion of the first steel plate 111 , the carbon equivalent Ceq is 0.22% or more. The carbon equivalent is a value calculated by the following formula 1.

[0081] Ceq=C+Si / 90+(Mn+Cr) / 100 (Formula 1)

[0082] Here, the element symbol recorded in Formula 1 is the content of the element corresponding to the element symbol in the 1 / 4 portion of the plate thickness of the parent material part of the first steel plate 111 in unit mass %. The parent material part of the first steel plate 111 refers to an area of ​​the first steel plate 111 that is not affected by heat during welding. The 1 / 4 portion of the plate thickness refers to a region with a depth of more than 1 / 8t1 and less than 1 / 4t1 from the surface of the first steel plate 111 when the plate thickness of the first steel plate 111 is expressed as t1. In addition, the inside of the plate thickness refers to a range of 1 / 4t1 to 3 / 4t1 excluding the depth t1 / 4 from the surface of the first steel plate 111 when the plate thickness of the first steel plate 111 is expressed as t1. By measuring the chemical composition of the 1 / 4 portion of the plate thickness or the inside of the plate thickness of the base material part of the first steel plate 111 by a combustion method (infrared absorption method), a spectroscopic analysis method, etc., it is possible to obtain the carbon equivalent Ceq of the 1 / 4 portion of the plate thickness or the inside of the plate thickness of the base material part of the first steel plate 111. The Ceq of the 1 / 4 portion of the plate thickness is substantially consistent with the Ceq of the inside of the plate thickness.

[0083] The carbon equivalent Ceq is an indicator of hardenability. The higher the carbon equivalent Ceq, the higher the tensile strength, so it is more preferred. However, the higher the carbon equivalent Ceq, the easier it is for the spot weld 12 to become brittle. Therefore, in the calculation formula of the indicator HC described later, the carbon equivalent Ceq is used as an indicator of the brittleness of the first steel plate 111. The carbon equivalent Ceq of the parent material part of the first steel plate 111 may also be greater than 0.25%, greater than 0.30%, or greater than 0.35%. The upper limit value of the carbon equivalent Ceq of the parent material part of the first steel plate 111 is not particularly limited, but for example, it may be less than 0.55% or less than 0.50%.

[0084] The first steel plate 111 may also be a high-strength steel plate exceeding 1300MPa level. For example, the tensile strength of the first steel plate may be above 1500MPa level, or may be above 1800MPa level. The tensile strength of the first steel plate 111 is, for example, above 1500MPa or above 1800MPa. Here, with respect to the hot-pressed steel plate, "1500MPa level (1800MPa level)" means that if appropriate quenching is performed, it becomes a tensile strength of 1500MPa or more (1800MPa or more), and it may also become a tensile strength of less than 1500MPa (less than 1800MPa) depending on the quenching conditions.

[0085] In addition, as long as at least one of the two steel plates 11 is a high-strength steel plate, other various forms of the steel plate 11 are not particularly limited. For example, the composition and metal structure of the steel plate 11 are not particularly limited, and a preferred form for ensuring a tensile strength of 1300 MPa or higher can be selected as needed.

[0086] The steel plate 11 is preferably a hot-pressed steel plate, that is, a steel plate (steel component) manufactured by hot pressing. The hot-pressed steel plate contains a martensitic structure produced by quenching. Therefore, the base material portion of the steel plate 11, especially the base material portion of the first steel plate 111, may also contain a martensitic structure. The martensitic structure may be any of primary martensite and tempered martensite. The amount of martensitic structure is not particularly limited, but for example, it is preferred to set the amount of martensite at 1 / 4 of the plate thickness of the base material portion of the first steel plate 111 to within a range of 80% to 100%, and more preferably to 90 to 100%. In addition, the steel plate before hot pressing (steel plate for hot pressing) may also be used as the steel plate 11.

[0087] In the spot welded joint 1 of the first embodiment, the “steel plate” is a concept including not only steel having a plate shape as a whole but also steel having only a portion thereof such as a member having a flange portion.

[0088] The steel plate 11 may have a coating on its surface. The coating may be, for example, hot-dip galvanizing, alloyed hot-dip galvanizing, or aluminum plating. This can improve the corrosion resistance of the spot welded joint 1.

[0089] The thickness of the steel plate 11 is not particularly limited. For example, when the spot welded joint 1 is used as an automotive component, the thickness t1 of the first steel plate 111 can be set to 1.0 to 2.6 mm, and the thickness t2 of the second steel plate 112 can be set to 0.5 to 2.3 mm. In addition, the thickness t1 of the first steel plate 111 and the thickness t2 of the first steel plate 111 refer to the thickness of the spot welded portion 12. When a steel plate with an irregular thickness such as a spliced ​​blank is used as the first steel plate, the thickness measured at the welded portion is regarded as t1 or t2.

[0090] (Spot welding part 12)

[0091] In the spot welding joint 1 of the first embodiment, two steel plates 11 composed of high-strength steel plates are joined by a spot welding portion 12. Figure 1 as well as Figure 2 As shown, the spot welding portion 12 has a nugget 121 as a molten solidification portion, and a crimping portion 122. The crimping portion 122 is formed around the nugget 121. The crimping portion 122 joins the first steel plate 111 and the second steel plate 112 around the nugget 121. The crimping portion 122 is a region where the two steel plates 11 are solid-phase joined although molten solidification has not occurred, and is also referred to as a crimping surface or a plastic metal ring. In addition, a HAZ 123 is formed around the nugget 121. The HAZ 123 is a region where the metal structure, metallurgical properties, mechanical properties, etc. change due to the influence of welding heat although molten solidification has not occurred.

[0092] In addition, the spot welding joint 1 may also have a plurality of spot welding parts 12. A joint in which at least one of the plurality of spot welding parts 12 satisfies the requirements of the spot welding joint 1 of the first embodiment is regarded as the spot welding joint 1 of the first embodiment. In addition to the spot welding parts 12, the spot welding joint 1 may also have a joining unit such as a laser welding part, an adhesive part, and a rivet joint part.

[0093] (Indicator HC)

[0094] In the spot welding joint 1 of the first embodiment, the index HC defined by the following formula 2 is 170 or more.

[0095] HC=Ceq×TS2×(1-(TS2×t2 3 ) / (TS1×t1 3 )) / √t2 (Formula 2)

[0096] Here, the meanings of the symbols described in Formula 2 are as follows.

[0097] TS1: tensile strength of the first steel plate 111 in MPa

[0098] t1: Plate thickness of the first steel plate 111 in mm

[0099] TS2: tensile strength of the second steel plate 112 in MPa

[0100] t2: Plate thickness of the second steel plate 112 in mm

[0101] Ceq: Carbon equivalent of the first steel plate 111 calculated by the above formula 1

[0102] HC is an index of the susceptibility of HAZ cracks to occur in the first steel plate near the outer peripheral end 122E of the crimped portion. The index HC will be described below.

[0103] The present inventors have found that when spot welding is performed on a plate group consisting of a plurality of steel plates 11 including one or more high-strength steel plates and having different tensile strengths and / or thicknesses, cracks C of a unique shape are generated. Specifically, Figure 5 As shown, a crack C that develops in the thickness direction of the steel plate 11 is confirmed in the HAZ 123 near the outer peripheral end 122E of the crimping portion 122. In addition, when the spot weld 12 is viewed from above, the crimping portion 122 is formed in a ring shape around the nugget 121. Therefore, in the cross section of the nugget 121, the outer peripheral end 122E of the crimping portion 122 can be visually recognized as the end on the opposite side of the nugget 121.

[0104] In view of the above circumstances, the present inventors conducted various FEM analyses and found the following.

[0105] (1) After spot welding two steel plates 11 having the same strength and thickness, stress is generated at the butting surfaces of the two steel plates 11 to cause the nugget 121 to peel off.

[0106] (2) After spot welding two steel plates 11 having different strengths and thicknesses, high stress is also generated at the outer peripheral end 122E of the crimping portion 122 of the steel plate 11 that is less likely to deform among the two steel plates 11 .

[0107] Regarding (2), the following is a detailed description. Conventional spot welding is often performed in a state where there are some gaps between the steel plates 11. This gap is called a plate gap. When spot welding is performed in a state where there is a plate gap, the steel plate 11 is deformed due to the pressing force of the electrode. Here, in the case where the strength and thickness of the two steel plates 11 are different, the steel plate 11 with lower strength and / or smaller plate thickness, that is, the second steel plate 112, is deformed more due to the pressing force of the electrode. Then, when the spot welding is completed and the electrode is released, the deformed second steel plate 112 intends to return to its original shape, and tensile stress is applied to the first steel plate 111. As a result, the tensile stress is concentrated on the outer peripheral end 122E of the crimping portion 122 of the first steel plate 111. Then, a crack C is generated in the HAZ123 near the outer peripheral end 122E of the crimping portion 122 of the first steel plate 111.

[0108] The inventors first determined the conditions under which stress is easily generated at the outer peripheral end 122E of the crimping portion 122, and then prevented cracks near the outer peripheral end 122E of the crimping portion 122. As a result, the inventors found that the conditions under which stress is easily generated at the outer peripheral end 122E of the crimping portion 122 are as follows.

[0109] The greater the difference in strength and thickness between the first steel plate 111 and the second steel plate 112, the greater the deformation of the second steel plate 112.

[0110] The greater the tensile strength of the second steel plate 112, the greater the stress applied by the second steel plate 112 to the first steel plate 111.

[0111] The smaller the nugget diameter, the greater the stress per unit area

[0112] Therefore, the present inventors defined an index HC of the crack susceptibility of the HAZ 123 of the first steel plate 111 in the vicinity of the outer peripheral end 122E of the crimping portion 122 in consideration of these conditions.

[0113] The formula (2) that defines the index HC contains “(1-(TS2×t2 3 ) / (TS1×t1 3))" is an index value of the strength-thickness balance of the first steel plate 111 and the second steel plate 112. The larger the index value, the greater the deformation of the second steel plate 112 during spot welding. The smaller the tensile strength TS2 of the second steel plate 112 is relative to the tensile strength TS1 of the first steel plate 111, the larger the index value. In addition, the smaller the thickness t2 of the second steel plate 112 is relative to the thickness t1 of the first steel plate 111, the larger the index value.

[0114] The "TS2" and "1-(TS2×t2 3 ) / (TS1×t1 3 The product of "TS2" and "(1-(TS2×t2 3 ) / (TS1×t1 3 The product of ))" is an index value of the tensile strength applied by the second steel plate 112 to the first steel plate 111 after spot welding. "Ceq" included in Formula 2 is an index value of the brittleness of the first steel plate 111. In addition, "√t2" included in Formula 2, that is, t2 0.5 It is an indicator of the diameter of the weld nugget.

[0115] When the index HC becomes 170 or more, the HAZ 123 of the first steel plate 111 becomes easy to crack near the outer peripheral end 122E of the crimping portion 122. On the other hand, a spot welding joint 1 having an index HC of 170 or more is required in various mechanical structural components. For example, at the manufacturing site of automotive components, high-strength and thicker steel plates are used as skeleton components, and low-strength and thinner steel plates are used as exterior components, and they are spot welded very frequently. Therefore, by setting the index HC of the spot welding joint 1 to 170 or more, the application range of the spot welding joint 1 can be expanded. The index HC may also be 180 or more, 190 or more, or 200 or more.

[0116] (Difference ΔHv between the Vickers hardness of the outer peripheral end 122E of the crimping portion 122 at 20 μm on the first steel plate 111 side and the Vickers hardness of the base material portion of the first steel plate 111 at a position of 1 / 4 of the plate thickness)

[0117] When the HC index is 170 or above, Figure 5 As shown, the HAZ 123 of the first steel plate 111 near the outer peripheral end 122E of the crimping portion 122 is the area most susceptible to cracking. Therefore, in the spot welding joint 1 of the first embodiment, the Vickers hardness of the outer peripheral end 122E of the crimping portion 122 on the first steel plate 111 side 20 μm is made lower than the Vickers hardness at the position of 1 / 4 of the plate thickness of the first steel plate 111 by more than Hv50. Figure 2As shown, the Vickers hardness of 20 μm on the first steel plate 111 side of the outer peripheral end 122E of the crimping portion 122 (i.e., the Vickers hardness of the surface portion of the first steel plate 111 at the outer peripheral end 122E of the crimping portion 122) is the Vickers hardness measured at a portion D of the first steel plate 111 at a depth of 20 μm from the outer peripheral end 122E of the crimping portion 122.

[0118] By setting ΔHv to Hv50 or more, HAZ cracks of the first steel plate 111 can be extremely effectively prevented. The difference ΔHv between the Vickers hardness of the outer peripheral end 122E of the press-bonded surface 122 at 20 μm on the first steel plate 111 side and the Vickers hardness at a position of 1 / 4 of the plate thickness of the first steel plate 111 can also be set to Hv60 or more, Hv70 or more, or Hv80 or more.

[0119] The Vickers hardness at 20 μm on the first steel plate 111 side of the outer peripheral end 122E of the crimping portion 122 (i.e., the Vickers hardness of the surface layer of the first steel plate 111 at the outer peripheral end 122E of the crimping portion 122) is measured in accordance with JIS Z 2244: 2009. The measurement surface is a cross section that passes through the center of the nugget 121 and is perpendicular to the surface of the steel plate 11. Figure 2 As shown, the measurement position is a position 20 μm deep from the crimping portion 122 inside the first steel plate 111 on a line passing through the outer peripheral end 122E of the crimping portion 122 and being substantially perpendicular to the crimping portion 122. In addition, the measurement surface is observed under a microscope to determine the position of the outer peripheral end 122E of the crimping portion 122. In a conventional spot welding joint 1, a few gaps are generated between the steel plates 11 due to the separation of the sheets. The crimping end can also be visually identified on the measurement surface.

[0120] The Vickers hardness at the 1 / 4 position of the plate thickness of the first steel plate 111 is also measured according to JIS Z 2244: 2009. The measuring surface is a cross section passing through the center of the weld nugget 121 and perpendicular to the surface of the steel plate 11. The measuring position is a position at a depth of 1 / 4 of the thickness t1 of the first steel plate 111 from the surface of the first steel plate 111. In addition, the measuring position is outside the weld nugget 121 and the HAZ 123. In addition, the HAZ softening portion outside the HAZ hardened portion is sometimes not clearly visually recognizable even after etching. However, the width of the HAZ softening portion is approximately 2 to 3 mm. Therefore, as long as the Vickers hardness at the 1 / 4 position of the plate thickness is measured at a position that is sufficiently away from the outer edge of the HAZ 123 visually recognizable by etching (for example, more than 5 mm), the hardness of the base material portion can be reliably determined.

[0121] When measuring the Vickers hardness of the surface layer and the position of 1 / 4 of the plate thickness, the test force was set to 10 gf. This enables comparison of the two measured values.

[0122] The method for making ΔHv greater than or equal to Hv50 is not particularly limited. For example, by spot welding after performing surface decarburization treatment on the first steel plate 111, ΔHv can be made greater than or equal to Hv50. Therefore, in the spot welding joint 1 of the first embodiment, the first steel plate 111 may also have a decarburized layer with a depth of 5 to 200 μm. The depth of the decarburized layer of the first steel plate 111 may also be made greater than or equal to 10 μm, greater than or equal to 20 μm, or greater than or equal to 50 μm. In addition, the depth of the decarburized layer of the first steel plate 111 may also be made less than or equal to 190 μm, less than or equal to 180 μm, or less than or equal to 150 μm.

[0123] In the cross section of the spot weld 12 formed perpendicularly to the surface of the steel plate 11 , the carbon concentrations in the surface layer and inside of the first steel plate 111 are locally analyzed and compared, thereby easily determining whether the first steel plate 111 has a decarburized layer.

[0124] For the convenience of measurement, the depth of the decarburized layer of the first steel plate 111 is determined based on the Vickers hardness distribution in the depth direction of the first steel plate 111. When measuring the Vickers hardness, the test force is set to 10 gf. The Vickers hardness measurement according to JIS Z 2244:2009 is continuously performed from the surface of the first steel plate 111 toward the inside. In the case where the first steel plate 111 has a decarburized layer, the deeper the measurement point, the greater the Vickers hardness. In the surface layer of the first steel plate 111, the area where the Vickers hardness becomes 95% or less of the plate thickness 1 / 4 position of the first steel plate 111 is regarded as the decarburized layer, and the thickness of the area is regarded as the depth of the decarburized layer. For example, when the Vickers hardness from the surface of the first steel plate 111 to a depth of 20 μm is less than 95% of the Vickers hardness at the plate thickness 1 / 4 position of the parent material part of the first steel plate 111, the depth of the decarburized layer of the first steel plate 111 is at least 20 μm. In this case, ΔHv can be made equal to or greater than Hv50.

[0125] In addition, the decarburized layer may be provided on both surfaces of the first steel plate 111 , or may be provided only on the side of the surface of the first steel plate 111 that is in contact with the second steel plate 112 .

[0126] The decarburized layer is composed of a structure containing at least one of ferrite, bainite and martensite. The closer to the surface of the steel plate, the lower the carbon content, and thus the amount of ferrite as a low-carbon structure increases. However, if the layer composed of the ferrite-based structure becomes excessively thick, the strength of the base material decreases. Therefore, the thickness of the layer composed of the ferrite-based structure contained in the decarburized layer on the surface of the first steel plate 111 is preferably less than 5μm. More preferably, the thickness of the layer composed of the ferrite-based structure is less than 4.5μm, less than 4.0μm or less than 3.5μm. In addition, the ferrite-based structure is defined as a structure composed of ferrite with an area ratio of 70% or more when observing the cross section. In this embodiment, the thickness of the decarburized layer is a different concept from the thickness of the layer composed of the ferrite-based structure. As described above, the thickness of the decarburized layer is determined by hardness measurement, but the thickness of the layer composed of the ferrite-based structure is determined by the structure observation described below.

[0127] The tissue observation was performed using a scanning electron microscope. Prior to the observation, the sample for tissue observation was ground by wet grinding based on sandpaper and diamond abrasives with an average particle size of 1 μm. After the observation surface was finished into a mirror surface, the tissue was etched with a 3% nitrate alcohol solution. The observation magnification was set to 3000 times, and 10 images of a 50 μm×50 μm field of view were randomly taken. The observation field of view was made to include an area from the surface of the steel plate to a depth of 5 μm. The proportion of the tissue was determined by the following steps. For the obtained tissue image, lattice-shaped imaginary lines arranged at intervals of 0.5 μm in the vertical direction and 0.5 μm in the horizontal direction were arranged within the range from the surface of the steel plate to a depth of 5 μm. Then, the area ratio of ferrite inside each lattice surrounded by the imaginary lines was measured. Ferrite is a block-shaped grain that does not contain iron carbides with a long diameter of more than 100 nm inside. The area of ​​the ferrite was divided by the area of ​​the lattice (0.25 μm 2 ) is the area ratio of ferrite in one lattice. Thus, lattices with a ferrite area ratio of 70% or more and lattices with a ferrite area ratio of less than 70% are determined. In the decarburized layer, the closer to the surface of the steel plate, the greater the ferrite area ratio. According to the above steps, a layer composed of lattices with a ferrite area ratio of 70% or more can be determined. The thickness of this layer is regarded as the thickness of a layer composed of a ferrite main body structure.

[0128] On the other hand, in order to make ΔHv Hv50 or more, a method other than surface decarburization treatment can also be used. For example, when the number density of precipitates is controlled and only the surface layer of the first steel plate 111 is softened, ΔHv can be made Hv50 or more. In addition, by making the first steel plate 111 a composite steel plate composed of a combination of mild steel and high-strength steel, ΔHv can also be made Hv50 or more.

[0129] (Spot welding joint 1 having three or more steel plates 11)

[0130] Next, the spot welding joint 1 in which the number of steel plates 11 is three or more will be described.

[0131] So far, the spot welding joint 1 in which the number of steel plates 11 is two has been described. Figure 3 or Figure 4 As shown, the spot welding joint 1 can have more than three steel plates 11 including one or more high-strength steel plates having a tensile strength of 1300 MPa or more or 1500 MPa or more, and a spot welding portion 12 joining the three or more steel plates 11. In this spot welding joint 1, the high-strength steel plate is most likely to generate cracks. Therefore, crack countermeasures need to be taken in the combination of the high-strength steel plate and the steel plate 11 connected thereto. On the other hand, crack countermeasures are not required in the combination of the steel plate 11 that is not a high-strength steel plate.

[0132] For example, in Figure 3 In the spot welded joint 1 shown, only the first hatched steel plate 11 from the bottom is a high-strength steel plate, and the other steel plates 11 are mild steel plates. Figure 3 In the spot welding joint 1, the steel plate 11 arranged at the bottom is thicker. In this case, the combination of the first steel plate 11 from the bottom and the second steel plate 11 from the bottom is equivalent to "a combination of a high-strength steel plate and a steel plate connected thereto". For this combination, it is necessary to apply the same structure as the spot welding joint 1 in which the number of steel plates 11 is two. At this time, the first steel plate 11 from the bottom is regarded as the first steel plate 111. On the other hand, the third steel plate 11 from the bottom and the second steel plate 11 from the bottom are both soft steel plates. For the combination of the third steel plate 11 from the bottom and the second steel plate 11 from the bottom, it is not necessary to apply the same structure as the spot welding joint 1 in which the number of steel plates 11 is two.

[0133] In addition, Figure 3 In the spot welding joint 1 shown, there is only one "combination of a high-strength steel plate and a steel plate connected thereto". On the other hand, there are sometimes more than two "combinations of a high-strength steel plate and a steel plate connected thereto". For example, in the case where the spot welding joint 1 is composed of two high-strength steel plates and one soft steel plate, or in the case where it is composed of three high-strength steel plates, there are two "combinations of a high-strength steel plate and a steel plate connected thereto". In this case, it is sufficient to apply the same structure as the spot welding joint 1 having two steel plates 11 to at least one of the multiple "combinations of high-strength steel plates and steel plates connected thereto". Thereby, the effect of suppressing HAZ cracks near the outer peripheral end of the crimping portion can be obtained.

[0134] It is more preferred that the same structure as the spot welding joint 1 with two steel plates 11 is applied to at least the combination with the largest index HC among the multiple "combinations of high-strength steel plates and steel plates connected thereto". The combination with the largest index HC is the combination with the highest risk of cracks. Therefore, it is more preferred to take crack countermeasures for the combination with the largest index HC. It is more preferred that the same structure as the spot welding joint 1 with two steel plates 11 is applied to all combinations of the multiple "combinations of high-strength steel plates and steel plates connected thereto".

[0135] For example, in Figure 4 In the spot welded joint 1 shown, the first steel plate 11 and the second and hatched steel plate 11 from the bottom are high-strength steel plates of the same strength, and the third steel plate 11 from the bottom is a soft steel plate. Figure 4 In the spot welded joint 1, the steel plate 11 arranged at the bottom has a greater thickness. In this case, the combination A of the first steel plate 11 from the bottom and the second steel plate 11 from the bottom and the combination B of the second steel plate 11 from the bottom and the third steel plate 11 from the bottom are respectively equivalent to "a combination of a high-strength steel plate and a steel plate connected thereto".

[0136] It is sufficient to apply the same structure as the spot welding joint 1 with two sheets of steel plates 11 to at least one of the combination A and the combination B. On the other hand, the index HC of the combination A among the combination A and the combination B is larger, and HAZ cracks are more likely to occur. Therefore, it is preferred that the same structure as the spot welding joint 1 with two sheets of steel plates 11 is applied to at least the combination A among the combination A and the combination B. More preferably, the same structure as the spot welding joint 1 with two sheets of steel plates 11 is applied to both the combination A and the combination B. The second steel plate 11 from the bottom is regarded as the second steel plate 112 in the combination with the first steel plate 11 from the bottom, and is regarded as the first steel plate 111 in the combination with the third steel plate 11 from the bottom.

[0137] Except for the above aspects, the spot welding joint 1 having three or more steel plates 11 has the same structure as the spot welding joint 1 having two steel plates 11. The preferred embodiment of the spot welding joint 1 having two steel plates 11 can also be applied to the spot welding joint 1 having three or more steel plates 11.

[0138] (Preferred depth of decarburized layer)

[0139] The Vickers hardness from the surface of the first steel plate 111 to a depth of 50 μm is preferably less than 95% of the Vickers hardness at a position of 1 / 4 of the plate thickness of the parent material portion of the first steel plate 111. In other words, the decarburized layer depth of the first steel plate 111 is preferably greater than 50 μm. In this case, the Vickers hardness of 50 μm on the first steel plate side of the outer peripheral end of the crimping portion can be made lower than the Vickers hardness at a position of 1 / 4 of the plate thickness of the parent material portion of the first steel plate by more than Hv50. In addition, the Vickers hardness from the surface of the first steel plate 111 to a depth of 100 μm is preferably less than 95% of the Vickers hardness at a position of 1 / 4 of the plate thickness of the parent material portion of the first steel plate 111. In other words, the decarburized layer depth of the first steel plate 111 is preferably greater than 100 μm. In this case, the Vickers hardness of 100 μm on the first steel plate side of the outer peripheral end of the crimping portion can be made lower than the Vickers hardness of the 1 / 4 position of the plate thickness of the parent material portion of the first steel plate by more than Hv50. In addition, the Vickers hardness from the surface of the first steel plate 111 to a depth of 200 μm is preferably less than 95% of the Vickers hardness of the 1 / 4 position of the plate thickness of the parent material portion of the first steel plate 111. In other words, the decarburized layer depth of the first steel plate 111 is preferably more than 200 μm. In this case, the Vickers hardness of 200 μm on the first steel plate side of the outer peripheral end of the crimping portion can be made lower than the Vickers hardness of the 1 / 4 position of the plate thickness of the parent material portion of the first steel plate by more than Hv50. As a result, HAZ cracks of the first steel plate 111 can be more effectively prevented.

[0140] The Vickers hardness of X μm (X is an arbitrary number) on the first steel plate side of the outer peripheral end of the crimping portion is the Vickers hardness measured at a depth of X μm from the outer peripheral end 122E of the crimping portion 122 in the first steel plate 111 .

[0141] The Vickers hardness of the outer peripheral end 122E of the crimping portion 122 at the first steel plate 111 side at X μm (X is an arbitrary number) or the Vickers hardness of 100 μm on the first steel plate 111 side is measured according to JIS Z 2244: 2009. The measurement surface is a cross section passing through the center of the nugget 121 and perpendicular to the surface of the steel plate 11. The measurement position is a position at a depth of X μm inside the first steel plate 111 from the crimping portion 122 on a line passing through the outer peripheral end 122E of the crimping portion 122 and substantially perpendicular to the crimping portion 122. The method for determining the test force when measuring the Vickers hardness is based on the method for measuring the Vickers hardness of 20 μm on the first steel plate 111 side of the outer peripheral end 122E of the crimping portion 122.

[0142] (Second embodiment: Automobile member)

[0143] The automotive component of the second embodiment of the present invention has the spot welding joint 1 of the first embodiment. The automotive component is often manufactured by joining a thick high-strength steel plate to a thin mild steel plate, but according to the automotive component of the second embodiment, HAZ cracks near the outer peripheral end 122E of the crimping portion 122 can be eliminated.

[0144] (Third Embodiment: Method for Manufacturing Spot Welded Joint 1)

[0145] The manufacturing method of the spot welding joint 1 of the third embodiment of the present invention is a manufacturing method of the spot welding joint 1 having two or more steel plates 11. Specifically, the manufacturing method of the spot welding joint 1 of the third embodiment includes: overlapping the first steel plate 111 and the second steel plate 112; and spot welding the first steel plate 111 and the second steel plate 112, the tensile strength of the first steel plate 111 is 1500MPa or more, the tensile strength of the second steel plate 112 is less than the tensile strength of the first steel plate 111, the Vickers hardness from the surface of the first steel plate 111 to a depth of 20μm is less than 95% of the Vickers hardness at a position of 1 / 4 of the plate thickness of the parent material part of the first steel plate 111, the carbon equivalent Ceq of the parent material part of the first steel plate 111 defined by formula 1 is 0.22% or more, and the index HC defined by formula 2 is 170 or more.

[0146] Ceq=C+Si / 90+(Mn+Cr) / 100 (Formula 1)

[0147] HC=Ceq×TS2×(1-(TS2×t2 3 ) / (TS1×t1 3 )) / √t2 (Formula 2)

[0148] Here, the element symbol recorded in Formula 1 is the content of the element corresponding to the element symbol in the 1 / 4 portion of the plate thickness of the base material part of the first steel plate 111 in unit mass %, the symbol TS1 recorded in Formula 2 is the tensile strength of the first steel plate 111 in unit MPa, t1 is the plate thickness of the first steel plate 111 in unit mm, TS2 is the tensile strength of the second steel plate 112 in unit MPa, and t2 is the plate thickness of the second steel plate 112 in unit mm. As a result, the spot welding joint 1 in which the HAZ cracks near the outer peripheral side end 122E of the crimping portion 122 are eliminated can be easily obtained. The preferred upper and lower limits of the numerical values ​​such as the carbon equivalent are based on the spot welding joint 1 of the first embodiment.

[0149] There is no particular limitation on the method for making the difference ΔHv between the Vickers hardness of 20 μm on the first steel plate side of the outer peripheral end 122E of the crimping portion 122 and the Vickers hardness at a position of 1 / 4 of the plate thickness of the first steel plate 111 be Hv50 or more. For example, the first steel plate 111 used for spot welding may have a decarburized layer with a depth of 5 to 200 μm. Alternatively, the Vickers hardness from the surface of the first steel plate 111 used for spot welding to a depth of 20 μm may be less than 95% of the Vickers hardness at a position of 1 / 4 of the plate thickness of the parent material portion of the first steel plate 111. Thus, ΔHv can be made greater than Hv50. The method for measuring the depth of the decarburized layer and the preferred depth of the decarburized layer are based on the spot welding joint 1 of the first embodiment.

[0150] As with the spot welding joint 1 of the first embodiment, in the method for manufacturing the spot welding joint 1 of the third embodiment, the number of the steel plates 11 may be two or more than three. As described above with respect to the spot welding joint 1 of the first embodiment, when the number of the steel plates 11 is more than three, crack countermeasures need to be taken in the combination of the high-strength steel plate and the steel plate 11 connected thereto. On the other hand, crack countermeasures do not necessarily need to be taken in the combination of the steel plates 11 that are not high-strength steel plates.

[0151] As with the spot welded joint of the first embodiment, in the method for manufacturing the spot welded joint of the third embodiment, it is preferred that the Vickers hardness from the surface of the first steel plate 111 to a depth of 50 μm is 95% or less of the Vickers hardness at a position of 1 / 4 of the plate thickness of the parent material portion of the first steel plate 111. More preferably, the Vickers hardness from the surface of the first steel plate 111 to a depth of 100 μm is 95% or less of the Vickers hardness at a position of 1 / 4 of the plate thickness of the parent material portion of the first steel plate 111. It is further preferred that the Vickers hardness from the surface of the first steel plate 111 to a depth of 200 μm is 95% or less of the Vickers hardness at a position of 1 / 4 of the plate thickness of the parent material portion of the first steel plate 111. Thus, HAZ cracks of the first steel plate 111 can be more effectively prevented.

[0152] Example

[0153] The effect of one embodiment of the present invention is described in more detail by way of example. However, the conditions in the example are only a conditional example adopted to confirm the feasibility and effect of the present invention. The present invention is not limited to this conditional example. As long as the present invention does not depart from the gist of the present invention and achieves the purpose of the present invention, various conditions can be adopted.

[0154] After applying oil to various plate groups consisting of the first steel plate and the second steel plate shown in Table 1, spot welding was performed. During spot welding, a plate gap of 1.5 mm (backing plate spacing of 40 mm) was set between the steel plates. The electrode was a chromium copper electrode with a front end diameter of Φ6 mm and a dome radius of R40. The welding conditions were a pressure of 600 kgf, a power-on time of 0.2 seconds, and a holding time of 1.0 seconds. The welding voltage was changed by 0.3 kA at a time within the range of 4.5 to 9.0 kA, thereby producing various sizes of nuggets under various conditions.

[0155] After welding, the spot welded joint is stored at room temperature for more than 48 hours. Afterwards, the cross section of the spot weld is observed to investigate whether there is a HAZ crack near the outer peripheral end of the crimping portion. In any case, the larger the nugget diameter, the more suppressed the crack. In other words, it is found that the smaller the nugget diameter, the easier it is to crack. Therefore, the nugget diameter 4√t is adopted as the judgment criterion. T is the thickness of the thinner one of the first steel plate and the second steel plate. For examples in which no cracks are generated when the nugget diameter is about 4√t, it is recorded as "qualified" in the "crack judgment" column. For other examples, it is recorded as "unqualified" in the "crack judgment" column. The Ceq, ΔHv and decarburization layer depth of the first steel plate are determined by the above method. In addition, when measuring the Vickers hardness, the test force is set to 10gf. In the example where the first steel plate is not provided with a decarburization layer, the decarburization layer depth is recorded as "-". In these examples, ΔHv is all below Hv30, so it is recorded as "-" in the table. Furthermore, in all the examples having the decarburized layer, the thickness of the ferrite-based layer was less than 5.0 μm.

[0156]

Table 1

[0157]

[0158] Comparative Examples 1 to 4 are spot welded joints with an index HC of less than 170. In these comparative examples, HAZ cracks near the outer peripheral end of the crimped portion did not occur. Therefore, in spot welded joints with an index HC of less than 170, HAZ cracks near the outer peripheral end of the crimped portion do not become a problem.

[0159] Comparative Examples 5 to 19 are spot welded joints having an index HC of 170 or more but ΔHv of less than 50. In these comparative examples, HAZ cracks occurred near the outer peripheral end of the crimped portion.

[0160] Examples 20 to 37 are spot welded joints having an index HC of 170 or more and a ΔHv of 50 or more. In these examples, HAZ cracks in the vicinity of the outer peripheral end of the crimped portion were suppressed.

[0161] Explanation of symbols

[0162] 1: spot welding joint; 11: steel plate; 111: first steel plate; 112: second steel plate; 12: spot welding part; 121: nugget; 122: crimping part; 123: HAZ; C: crack; D: pit.

Claims

1. A spot welding joint, comprising: 1st steel plate; The second steel plate; and The spot welding portion joins the first steel plate and the second steel plate. The first steel plate is directly overlapped with the second steel plate. The tensile strength of the first steel plate is 1500 MPa or more. The tensile strength of the second steel plate is less than the tensile strength of the first steel plate. The Vickers hardness from the surface of the first steel plate to a depth of 20 μm is 95% or less of the Vickers hardness at a position of 1 / 4 of the plate thickness of the base material portion of the first steel plate. The carbon equivalent Ceq of the base material portion of the first steel plate defined by Formula 1 is 0.22% or more, The index HC defined by formula 2 is 170 or more. The spot welding portion includes a nugget and a pressure welding portion located around the nugget and joining the first steel plate and the second steel plate. The Vickers hardness of the outer peripheral end of the crimping portion at 20 μm on the first steel plate side is lower than the Vickers hardness of the base material portion of the first steel plate at a position of 1 / 4 of the plate thickness by Hv50 or more. Ceq=C+Si / 90+(Mn+Cr) / 100 (Formula 1) HC=Ceq×TS2×(1-(TS2×t2 3 ) / (TS1×t1 3 )) / √t2 (Formula 2) Here, the element symbol described in the above formula 1 is the content of the element corresponding to the element symbol at the 1 / 4 thickness portion of the above base material portion of the above first steel plate in unit mass %, Symbol TS1 recorded in the above formula 2 is the tensile strength of the above-mentioned first steel plate in unit MPa, t1 is the plate thickness of the above-mentioned first steel plate in unit mm, TS2 is the tensile strength of the above-mentioned second steel plate in unit MPa, and t2 is the plate thickness of the above-mentioned second steel plate in unit mm.

2. The spot welding joint according to claim 1, wherein: The base material portion of the first steel plate includes a martensitic structure.

3. The spot welding joint according to claim 1 or 2, wherein: The Vickers hardness of the first steel plate from the surface to a depth of 50 μm is 95% or less of the Vickers hardness of the base material portion of the first steel plate at a position of 1 / 4 of the plate thickness, The Vickers hardness of the outer peripheral end portion of the crimped portion at 50 μm on the first steel plate side is lower than the Vickers hardness of the base material portion of the first steel plate at a position of ¼ of the plate thickness by at least Hv50.

4. The spot welding joint according to claim 1 or 2, wherein: The Vickers hardness of the first steel plate from the surface to a depth of 100 μm is 95% or less of the Vickers hardness of the base material portion of the first steel plate at a position of 1 / 4 of the plate thickness, The Vickers hardness of the outer peripheral end portion of the crimped portion at 100 μm on the first steel plate side is lower than the Vickers hardness of the base material portion of the first steel plate at a position of 1 / 4 of the plate thickness by at least Hv50.

5. The spot welding joint according to claim 1 or 2, wherein: The Vickers hardness of the first steel plate from the surface to a depth of 200 μm is 95% or less of the Vickers hardness of the base material portion of the first steel plate at a position of 1 / 4 of the plate thickness, The Vickers hardness of 200 μm on the first steel plate side of the outer peripheral end portion of the crimped portion is lower by Hv50 or more than the Vickers hardness at the ¼ plate thickness position of the base material portion of the first steel plate.

6. A method for manufacturing a spot welding joint, comprising: Overlapping the first steel plate with the second steel plate; as well as spot welding the first steel plate and the second steel plate, The tensile strength of the first steel plate is 1500 MPa or more. The tensile strength of the second steel plate is less than the tensile strength of the first steel plate. The Vickers hardness from the surface of the first steel plate to a depth of 20 μm is 95% or less of the Vickers hardness at a position of 1 / 4 of the plate thickness of the base material portion of the first steel plate. The carbon equivalent Ceq of the base material portion of the first steel plate defined by Formula 1 is 0.22% or more, The index HC defined by formula 2 is 170 or more. Ceq=C+Si / 90+(Mn+Cr) / 100 (Formula 1) HC=Ceq×TS2×(1-(TS2×t2 3 ) / (TS1×t1 3 )) / √t2 (Formula 2) Here, the element symbol described in the above formula 1 is the content of the element corresponding to the element symbol at the 1 / 4 thickness portion of the above base material portion of the above first steel plate in unit mass %, Symbol TS1 recorded in the above formula 2 is the tensile strength of the above-mentioned first steel plate in unit MPa, t1 is the plate thickness of the above-mentioned first steel plate in unit mm, TS2 is the tensile strength of the above-mentioned second steel plate in unit MPa, and t2 is the plate thickness of the above-mentioned second steel plate in unit mm.

7. The method for manufacturing a spot welding joint according to claim 6, wherein: The Vickers hardness of the first steel plate from the surface to a depth of 50 μm is 95% or less of the Vickers hardness of the base material portion of the first steel plate at a position having a 1 / 4 thickness.

8. The method for manufacturing a spot welding joint according to claim 6, wherein: The Vickers hardness of the first steel plate from the surface to a depth of 100 μm is 95% or less of the Vickers hardness of the base material portion of the first steel plate at a position having a 1 / 4 thickness.

9. The method for manufacturing a spot welding joint according to claim 6, wherein: The Vickers hardness of the first steel plate from the surface to a depth of 200 μm is 95% or less of the Vickers hardness of the base material portion of the first steel plate at a position having a 1 / 4 thickness.

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