Spot welded joint, manufacturing method for spot welded joint, high strength steel member for spot welded joint, and manufacturing method for high strength steel member for spot welded joint

Abstract

A spot-welded joint according to one aspect of the present invention comprises: a plurality of steel plate portions including a high-strength steel plate portion having a Vickers hardness of 530 HV or more; a nugget where the high-strength steel plate portion and other steel plate portions among the plurality of steel plate portions are joined together; and a corona bond which is formed in a ring shape around the nugget in a plan view from the direction perpendicular to the surface of the high-strength steel plate portion and in which the high-strength steel plate portion and other steel plate portions are solid-phase joined together, wherein in the region where the corona bond is present, there is no plating layer, and the corona bond has an oxygen concentration of 15.0 mass% or less at a position 50 μm outside the outer periphery of the nugget in a plan view.

Description

[Technical field]

[0001] The present invention relates to a spot welded joint, a method for manufacturing a spot welded joint, a high-strength steel member for a spot welded joint, and a method for manufacturing a high-strength steel member for a spot welded joint. This application claims priority based on Japanese Patent Application No. 2022-040299, filed on March 15, 2022, the contents of which are incorporated herein by reference. [Background technology]

[0002] In spot welded joints manufactured by spot welding high-strength steel members, there is a problem that the joint strength of the welded part is low. In particular, when the tensile strength of the high-strength steel member is 1800 MPa or more (or the Vickers hardness of the high-strength steel member is 530 HV or more), the tensile shear strength (TSS) of the spot welded part is significantly reduced. This is thought to be because the large amount of C contained in the high-strength steel member causes embrittlement of the welded part.

[0003] Various studies have been conducted to improve the joining strength of spot welded joints made of high strength steel plates.

[0004] Patent Document 1 discloses a spot-welded component in which a plurality of steel plates are spot-welded, at least one of the plurality of steel plates being a high-strength cold-rolled steel plate having a tensile strength of 780 MPa or more and having no plating layer on its surface, and at least one of the plurality of steel plates being a zinc-based plated steel plate having a zinc-based plating layer on its surface, and the surface layer Zn concentration inside the corona bond of the spot weld is 1 mass % or more and less than 25 mass %.

[0005] Patent Document 2 discloses a steel plate for spot welding having a tensile strength of 400 to 700 MPa, a C content in the composition of the base material in the range of 0.05 to 0.12 mass%, a carbon equivalent Ceqt expressed by the following formula {Ceqt=C+Si / 30+Mn / 20+2P+4S} in the range of 0.18 mass% or more and 0.22 mass% or less, a carbon equivalent Ceqh expressed by the following formula {Ceqh=C+Si / 40+Cr / 20} in the range of 0.08 mass% or more, and further an average oxygen concentration OC (%) measured by GDS analysis at a depth of 3 μm from the surface of the steel plate in the range expressed by the following formula {OC≦0.5}.

[0006] Patent Document 3 describes a method for manufacturing a steel sheet having an average composition in a nugget that contains, by mass%, P: 0.03% or less, S: 0.01% or less, and O: 0.02% or less, and further contains one or more of Mg, La, and Ce in a total amount of 0.005% or less, with the balance being Fe and unavoidable impurities, the average hardness in the nugget being 320 to 500 HV, and the distribution density of oxide-based inclusions with a particle diameter of 1 μm or more present in the nugget being 20 pieces / mm 2 and a high-strength resistance welded joint in which the maximum grain size of all inclusions is 50 μm or less and the metal structure in the nugget is composed of tempered martensite and lower bainite. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2020-179413 [Patent Document 2] Japanese Patent Application Publication No. 2012-102370 [Patent Document 3] Japanese Patent Application Publication No. 2014-180698 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the maximum tensile strength of the steel plate that is the subject of discussion in Patent Document 1 is 1570 MPa. Patent Document 1 does not discuss at all high-strength steel members with a tensile strength of 1800 MPa or more (or a Vickers hardness of 530 HV or more), in which embrittlement of spot welds is even more severe.

[0009] The tensile strength of the steel plate that is the subject of the study in Patent Document 2 is 700 MPa at most. Patent Document 2 makes no study whatsoever on high-strength steel members with a tensile strength of 1800 MPa or more (or a Vickers hardness of 530 HV or more), in which embrittlement of spot welds is even more severe. Furthermore, Patent Document 2 attempts to improve the weld strength of spot welded joints through the oxygen concentration of the base steel plate, but does not particularly study the configuration of spot welded joints.

[0010] The Vickers hardness of the nugget disclosed in Patent Document 3 is within the range of 320HV to 500HV. Considering the nugget hardness, it is estimated that the tensile strength of the steel plate considered in Patent Document 3 is less than 1800MPa (or a Vickers hardness of less than 530HV). It is estimated that Patent Document 3 does not consider at all high-strength steel members with a tensile strength of 1800MPa or more (or a Vickers hardness of 530HV or more), in which embrittlement of spot welds is more significant.

[0011] In view of the above circumstances, an object of the present invention is to provide a spot welded joint that includes one or more high-strength steel plate portions having a Vickers hardness of 530 HV or more and has excellent TSS, and a manufacturing method thereof. Another object of the present invention is to provide a high-strength steel member for a spot welded joint that allows the manufacture of a spot welded joint that has a Vickers hardness of 530 HV or more and has excellent TSS, and a manufacturing method thereof. [Means for solving the problem]

[0012] The gist of the present invention is as follows.

[0013] (1) A spot welded joint according to a first embodiment of the present invention comprises a plurality of steel plate portions including a high-strength steel plate portion having a Vickers hardness of 530 HV or more, a nugget joining the high-strength steel plate portion to another of the plurality of steel plate portions, and a corona bond formed in a ring shape around the nugget in a planar view from a direction perpendicular to the surface of the high-strength steel plate portion and solid-state joining the high-strength steel plate portion to the other of the steel plate portions, wherein no plating layer is present in the region where the corona bond is present, and the oxygen concentration of the corona bond at a position 50 μm outside the outer periphery of the nugget in the planar view is 15.0 mass% or less. (2) In the spot welded joint described in (1) above, the high strength steel plate portion preferably has a carbon content of more than 0.30 mass%. (3) Preferably, in the spot welded joint described in (1) or (2) above, the carbon content of the nugget is 0.29 mass% or more. (4) Preferably, in the spot welded joint according to any one of (1) to (3) above, the diameter of the nugget measured along the boundary line between adjacent steel plate portions in a cross section passing through the center of the nugget and perpendicular to the surface of the steel plate portion is 4.5√t or more, where t is the smaller of the plate thicknesses of the two adjacent steel plate portions forming the boundary line.

[0014] (5) A method for manufacturing a spot welded joint according to a second embodiment of the present invention includes the steps of: providing a high-strength steel member having a surface treatment portion and a Vickers hardness of 530 HV or more; overlapping a plurality of the steel members so that the surface treatment portion is located on a mating surface between the high-strength steel member and another steel member; and spot welding the plurality of steel members so that a nugget and a corona bond are contained inside the surface treatment portion in a plan view perpendicular to the surface treatment portion, wherein in the overlapping step, no plating layer is provided on the surface treatment portion and an area in contact with the surface treatment portion, the ten-point average roughness of the surface treatment portion is 3.5 μm or less, and the oxygen concentration of the surface treatment portion is 18.0 mass% or less. (6) Preferably, the method for manufacturing a spot welded joint described in (5) above further includes, before the step of providing a high-strength steel member, a step of forming the surface treatment portion on a surface of the high-strength steel member. (7) Preferably, the method for manufacturing a spot welded joint described in (6) above further comprises a step of hot stamping a high-strength steel plate before forming the surface treatment portion, and in the step of forming the surface treatment portion, the surface treatment portion is formed on a surface of the hot stamped high-strength steel member. (8) Preferably, the method for manufacturing a spot welded joint described in (6) or (7) above includes a step of removing a portion of the plating layer on a surface of the high-strength steel member before forming the surface-treated region, and in the step of forming the surface-treated region, the surface-treated region is formed in the area from which the plating layer has been removed. (9) In the method for producing a spot welded joint according to any one of (5) to (8) above, the high-strength steel members preferably have a carbon content of more than 0.30 mass%. (10) In the method for manufacturing a spot welded joint according to any one of (5) to (9) above, preferably, the nugget formed by the spot welding has a carbon content of 0.29 mass % or more. (11) Preferably, in the method for manufacturing a spot welded joint according to any one of (5) to (10) above, the diameter of the nugget formed by the spot welding is 4.5√t or more, where the diameter of the nugget refers to the size of the nugget measured along a boundary line between adjacent steel members in a plane that passes through a center of the nugget and is perpendicular to a surface of the steel members, and t refers to the smaller of the sheet thicknesses of the two adjacent steel members that form the boundary line. (12) Preferably, in the method for producing a spot welded joint according to any one of (6) to (11) above, the surface treatment portion is formed by shot blasting or wet blasting.

[0015] (13) A high-strength steel member for a spot-welded joint according to a third embodiment of the present invention has a Vickers hardness of 530 HV or more, has a surface treatment area not provided with a plating layer, the ten-point average roughness of the surface treatment area is 3.5 μm or less, and the oxygen concentration of the surface treatment area is 18.0 mass% or less. (14) Preferably, in the high-strength steel member for a spot welded joint according to (13) above, the carbon content is more than 0.30 mass%. (15) Preferably, in the high-strength steel component for a spot welded joint described in (13) or (14) above, the surface treatment portion is provided over the entire surface of the high-strength steel component.

[0016] (16) A fourth embodiment of the present invention relates to a manufacturing method of a high-strength steel component for a spot-welded joint, the method comprising the steps of forming a surface treatment on a surface of a high-strength steel component having a Vickers hardness of 530 HV or more, wherein no plating layer is provided on the surface treatment, the ten-point average roughness of the surface treatment is 3.5 μm or less, and the oxygen concentration of the surface treatment is 18.0 mass% or less. (17) Preferably, the method for manufacturing a high-strength steel member for a spot welded joint described in (16) above further comprises a step of hot stamping a high-strength steel plate before forming the surface treatment portion, and in the step of forming the surface treatment portion, the surface treatment portion is formed on a surface of the hot stamped high-strength steel member. (18) Preferably, the method for manufacturing a high-strength steel component for a spot-welded joint described in (16) or (17) above includes a step of removing a part of a plating layer on a surface of the high-strength steel component before forming the surface-treated portion, and in the step of forming the surface-treated portion, the surface-treated portion is formed in the area from which the plating layer has been removed. (19) Preferably, in the method for producing a high-strength steel member for a spot-welded joint according to any one of (16) to (18) above, the high-strength steel member has a carbon content of more than 0.30 mass%. (20) Preferably, in the method for producing a high-strength steel member for a spot-welded joint according to any one of (16) to (19) above, the surface treatment portion is provided over an entire surface of the high-strength steel member. (21) Preferably, in the method for producing a high-strength steel member for a spot-welded joint according to any one of (16) to (20) above, the surface treatment portion is formed by shot blasting or wet blasting. Effect of the Invention

[0017] According to the present invention, it is possible to provide a spot-welded joint that includes one or more high-strength steel plate portions having a Vickers hardness of 530 HV or more and has excellent TSS, and a manufacturing method thereof. Furthermore, according to the present invention, it is possible to provide a high-strength steel member for a spot-welded joint that can produce a spot-welded joint having a Vickers hardness of 530 HV or more and has excellent TSS, and a manufacturing method thereof. [Brief description of the drawings]

[0018] [Figure 1] 1 is a cross-sectional view of a spot welded joint according to a first embodiment. FIG. [Figure 2A] FIG. 1 is a diagram showing the relationship between nugget diameter and surface oxygen concentration of a corona bond. [Figure 2B] FIG. 1 is a diagram showing the relationship between nugget diameter and surface oxygen concentration of a corona bond. [Figure 3A] FIG. 2 is a schematic diagram of a surface treatment portion forming step S1. [Figure 3B] FIG. 13 is a schematic diagram of step S2 of overlapping steel members. [Figure 3C] FIG. 13 is a schematic diagram of step S3 of spot welding steel members. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] <Spot welded joint according to the first embodiment> First, a spot welded joint 1 according to a first embodiment of the present invention will be described. As shown in the cross-sectional view of Fig. 1, the spot welded joint 1 according to the first embodiment includes a plurality of steel plate parts 11 including a high-strength steel plate part 11H having a Vickers hardness of 530HV or more, a nugget 121 that joins the high-strength steel plate part 11H and the other steel plate parts 11 among the plurality of steel plate parts 11, and a corona bond 122 that is formed in a ring shape around the nugget 121 in a plan view from a direction perpendicular to the surface of the high-strength steel plate part (hereinafter referred to as a plan view) and solid-state joins the high-strength steel plate part 11H and the other steel plate parts 11, and no plating layer 13 is present in the region where the corona bond 122 is formed, and the oxygen concentration of the corona bond 122 at a position 50 μm outside from the outer periphery of the nugget 121 in the plan view is 15.0 mass% or less. Hereinafter, the surface oxygen concentration of the corona bond 122 formed on the surface of the high-strength steel plate portion 11H at a position 50 μm outside from the outer periphery of the nugget may be simply referred to as the “surface oxygen concentration of the corona bond.” Also, the region between the high-strength steel plate portion 11H and the other steel plate portion 11 where the corona bond 122 exists may be referred to as the “corona bond portion 123.”

[0020] In the spot welded joint 1 according to the first embodiment, the TSS is improved by modifying the corona bond 122. The present inventors have found that by reducing the oxygen concentration in the corona bond 122 formed on the high-strength steel plate portion 11H, the TSS of the spot welded joint 1 including the high-strength steel plate portion 11H having a Vickers hardness of 530 HV or more is dramatically improved. Hereinafter, the spot welded joint 1 according to the first embodiment will be described in detail.

[0021] (Steel plate part 11, high strength steel plate part 11H) Although the manufacturing method will be described in detail later, the spot welded joint 1 is formed by spot joining steel members formed from a steel plate that serves as a base material (joint material). The steel plate portion 11 refers to a flat portion of the spot welded joint 1 that is on the same surface as the spot welded portion among the portions that correspond to the steel members or steel plates before spot welding. For example, if the steel member before spot welding is a hat-shaped part, the steel plate portion 11 is a spot-welded flange portion. Alternatively, if the spot welded joint 1 is formed by spot welding to a flat plate part, the steel plate portion 11 is the entire flat plate part. The number of steel plate parts 11 may be two as exemplified in FIG. 1, or may be three or more. In addition, one or more of the multiple steel plate parts 11 are high-strength steel plate parts 11H having a Vickers hardness of 530HV or more. This dramatically increases the base material strength of the spot welded joint 1. There is a good correlation between the tensile strength of steel and the Vickers hardness, and a Vickers hardness of 530HV corresponds to a tensile strength of about 1800MPa. A Vickers hardness of 580HV corresponds to a tensile strength of about 2000MPa. According to the conventional technology, when the Vickers hardness of the steel plate portion 11 is 530 HV or higher, the TSS of the spot welded joint 1 decreases. However, in the spot welded joint 1 according to the first embodiment, the TSS is ensured by reducing the oxygen concentration of the corona bond 122 described later. The number of high-strength steel plate portions 11H is not limited. In the spot welded joint 1 illustrated in Fig. 1, the high-strength steel plate portion 11H is combined with a steel plate portion 11 having a Vickers hardness of less than 530HV, but two high-strength steel plate portions 11H may be combined. Hereinafter, for convenience, the steel plate portion 11 having a Vickers hardness of less than 530HV will be referred to as the "normal steel plate portion 11".

[0022] (Nugget 121 and Coronabond 122) The nugget 121 is a molten and solidified portion that occurs in a welded portion during spot welding. The nugget 121 joins at least the high-strength steel plate portion 11H among the plurality of steel plate portions 11 to the other steel plate portions 11. Preferably, the nugget 121 joins all of the plurality of steel plate portions 11. The corona bond 122 is a ring-shaped solid-state bond between two steel plate portions 11 around the nugget 121 during spot welding. The corona bond 122 solid-state bonds the high-strength steel plate portion 11H and another steel plate portion 11. However, in the corona bond 122, the steel plate portion 11 is not melted and solidified, and the two steel plate portions 11 are only solid-state bonded. In the spot welded joint 1 illustrated in Fig. 1, the steel plate portion 11 has only one overlapping surface, and therefore only one corona bond 122 is formed for each spot weld. If the steel plate portion 11 has multiple overlapping surfaces, a corona bond 122 is formed on each of the multiple overlapping surfaces. Hereinafter, the portion that joins the multiple steel plate portions 11 and includes the nugget 121 and the corona bond 122 will be collectively referred to as the "spot welded portion 12."

[0023] (Oxygen concentration of Coronabond 122 and plating availability) As a result of extensive investigations by the present inventors into the TSS of the spot welded joint 1 including the high-strength steel plate portion 11H, the following points have become clear.

[0024] First, in a spot welded joint 1 including a high-strength steel plate portion 11H having a Vickers hardness of 530 HV or more, it was revealed that the corona bond 122 has a large effect on the joint strength of the spot welded joint 1. Since a large amount of C contained in the high-strength steel plate portion 11H causes embrittlement of the welded joint and reduces the joint strength of the molten and solidified portion, it is estimated that the joint strength of the corona bond 122 has a large effect on the joint strength of the spot welded joint 1 in relative terms.

[0025] Next, a good correlation was found between the oxygen concentration of the corona bond 122 formed on the surface of the high-strength steel plate portion 11H and the TSS of the spot-welded joint 1. The lower the oxygen concentration, the higher the TSS. This is presumably because the higher the oxygen concentration of the corona bond 122, the more oxides there are in the corona bond 122, which causes the corona bond 122 to peel off.

[0026] On the other hand, no correlation was found between the oxygen concentration of the corona bond between two normal steel plate parts 11 and the TSS. For example, in a spot welded joint consisting of normal steel plate parts 11 and not including a high strength steel plate part 11H, even if the oxygen concentration of the corona bond was reduced, no TSS improvement effect was obtained. In addition, in the prior art of a spot welded joint not including a high strength steel plate part 11H having a Vickers hardness of 530HV or more, the relationship between the oxygen concentration of the corona bond, oxides, and the TSS has not been reported. It is presumed that this is because in a spot welded joint consisting of normal steel plate parts 11 and not including a high strength steel plate part 11H, the corona bond has almost no effect on the joining strength of the spot welded joint 1.

[0027] Furthermore, in a spot welded joint 1 consisting of two ordinary steel plate portions 11 and one high-strength steel plate portion 11H, even if the oxygen concentration of the corona bond between the two ordinary steel plate portions 11 was reduced, no improvement in TSS was obtained.

[0028] In addition, when a coating such as hot-dip galvanizing is contained in the corona bond portion 123 between the high-strength steel plate portion 11H and another adjacent steel plate portion 11, the TSS improvement effect due to the reduction in oxygen concentration cannot be obtained. When spot welding is performed in a state where a coating layer is present on the contact surface between the steel members, in addition to oxides, inclusions of the coating are formed in the corona bond portion 123. It is presumed that these inclusions derived from the coating components prevent the realization of the TSS improvement effect. Here, the corona bond portion 123 refers to the region where the corona bond 122 is present between the high-strength steel plate portion 11H and another steel plate portion 11.

[0029] For the above reasons, in the spot welded joint 1 according to the first embodiment, the plating layer 13 is not present in the corona bond portion that joins the high strength steel plate portion 11H and the other steel plate portion 11. The plating layer 13 does not have to be present at least in the corona bond portion between the high strength steel plate portion 11H and the other steel plate portion 11 (i.e., the plating layer 13 is excluded from the corona bond portion). The plating layer 13 may be formed on the surface of the high strength steel plate portion 11H that is not overlapped with the other steel plate portion 11, since the corona bond 122 is not formed thereon. The plating layer 13 may also be formed in the area other than the corona bond portion on the surface of the high strength steel plate portion 11H that is overlapped with the other steel plate portion 11. The plating layer 13 outside the corona bond portion of the high strength steel plate portion 11H does not adversely affect the TSS of the spot welded joint 1. The plating layer may be present in the corona bond portion between the normal steel plate portions 11.

[0030] Furthermore, the oxygen concentration of the corona bond 122 that solid-phase bonds the high-strength steel plate portion 11H and the other steel plate portion 11 at a position 50 μm outside from the outer periphery of the nugget 121 in a plan view is set to 15.0 mass% or less. Preferably, the oxygen concentration of the corona bond 122 at a position 50 μm outside from the outer periphery of the nugget 121 is 14.0 mass% or less, 13.0 mass% or less, 12.0 mass% or less, or 10.0 mass% or less. The lower limit of the oxygen concentration of the corona bond 122 between the high-strength steel plate portion 11H and the other steel plate portion 11 is not particularly limited, and may be 0 mass%. However, it is difficult to completely remove oxygen in consideration of the manufacturing cost of the spot welded joint 1, etc. Therefore, the oxygen concentration of the corona bond 122 between the high-strength steel plate portion 11H and the other steel plate portion 11 may be 1.0 mass% or more, 2.0 mass% or more, or 5.0 mass% or more. The oxygen concentration of the corona bond 122 that is not in contact with the high-strength steel plate portion 11H is not particularly limited. As described above, there is no correlation between the oxygen concentration and the TSS at the mating surface of the normal steel plate portion 11 having a tensile strength of less than 1800 MPa (i.e., a Vickers hardness of less than 530 HV).

[0031] As long as the spot welded joint 1 according to the first embodiment satisfies the above-mentioned requirements, it is possible to obtain the effect of improving the TSS. On the other hand, by applying the preferred embodiments described below, it is possible to further improve the TSS and other mechanical properties of the spot welded joint 1 according to the first embodiment.

[0032] (Carbon content of high strength steel plate portion 11H: preferably more than 0.30 mass%) The carbon content of the high-strength steel plate portion 11H is not particularly limited, but is preferably, for example, more than 0.30 mass%, 0.31 mass% or more, 0.33 mass% or more, 0.36 mass% or more, or 0.40 mass% or more. This provides the strength improvement effect of the technology disclosed herein. On the other hand, by setting the carbon content of the high-strength steel plate portion 11H to 0.50 mass% or less, 0.48 mass% or less, 0.45 mass% or less, 0.42 mass% or less, or 0.40 mass% or less, the hardness of the nugget 121 can be reduced, and the joining strength of the spot-welded joint 1 can be further increased. The carbon content of the normal steel plate portion 11 that may be combined with the high-strength steel plate portion 11H is also not particularly limited, but may be, for example, 0.2 mass% or less. Note that C contained in the high-strength steel plate portion 11H may cause embrittlement of the welded portion. However, in the spot-welded joint 1 according to the first embodiment, the TSS is increased by using a corona bond. Therefore, even if the high-strength steel plate portion 11H has a high carbon content, the joining strength of the welded portion is increased in the spot-welded joint 1 according to the first embodiment.

[0033] (Carbon content of nugget 121: preferably 0.29 mass% or more) The carbon content of the nugget 121 is not particularly limited, and may be, for example, 0.29 mass% or more, 0.31 mass% or more, 0.35 mass% or more, or 0.38 mass% or more. This provides the strength improvement effect of the technology of the present disclosure. On the other hand, the carbon content of the nugget 121 may be 0.45 mass% or less, 0.42 mass% or less, or 0.38 mass% or less. This prevents excessive hardening of the nugget 121 and further increases the joint strength of the spot welded joint 1.

[0034] (Diameter of nugget 121: preferably 4.5√t or more) The diameter of the nugget 121 is not particularly limited, and various values ​​can be selected within a range in which the high-strength steel plate portion and the other steel plate portion can be joined. On the other hand, according to the results of experiments conducted by the present inventors, it has become clear that the larger the diameter of the nugget 121, the lower the oxygen concentration of the corona bond 122 and the more the TSS is improved.

[0035] This is presumably due to the amount of thermal expansion during the formation of the nugget 121. As shown in Figures 2A and 2B, the nugget 121 thermally expands during spot welding. This thermal expansion applies pressure to the corona bond 122 in the outward direction of the nugget 121. It is believed that this pressure causes plastic flow in the corona bond portion, expelling the oxides Ox to the outside of the corona bond portion. The greater the heat input during spot welding and the larger the nugget diameter, the greater the pressure applied to the corona bond 122. Therefore, it is believed that the larger the nugget diameter, the greater the amount of oxides Ox expelled, further improving the TSS.

[0036] According to the results of experiments by the present inventors, when the diameter D of the nugget 121 is 4.5√t or more, a further improvement in TSS was observed. Here, the diameter D of the nugget 121 is the size of the nugget 121 measured along the boundary line between the adjacent steel plate parts 11 in a cross section passing through the center of the nugget 121 and perpendicular to the surface of the steel plate part 11. "t" refers to the smaller of the plate thicknesses of the two adjacent steel plate parts 11 that form the boundary line that serves as the reference for measuring the nugget 121. When the number of steel plate parts 11 included in the spot welded joint 1 is three or more and there are two or more boundary lines, two or more Ds and two or more ts corresponding to the two or more boundary lines are specified. For example, when the number of steel plate portions 11 included in the spot welded joint 1 is two, the boundary line between the steel plate portions 11 is one. The diameter D of the nugget 121 is the width of the nugget 121 measured along the boundary line, and t is the thickness of the thinner of the two steel plate portions 11. It is preferable that D≧4.5√t is satisfied at the boundary line. In this case, a further improvement in TSS is observed. When the number of steel plate parts 11 included in the spot welded joint is three or more, the steel plate parts 11 have two or more boundary lines. In this case, it is preferable that the diameter D of the nugget 121 and the thinner plate thickness t of the steel plate parts 11 that form the boundary line satisfy D≧4.5√t at at least one boundary line, and it is more preferable that D≧4.5√t is satisfied at all boundary lines. It is more preferable that the diameter of the nugget 121 is 4.8√t or more, or 5.0√t or more. In addition, it is preferable that the diameter D of the nugget 121 and the thinner plate thickness t of the steel plate parts 11 that form the boundary line satisfy D≧4.5√t at the boundary line formed by the high-strength steel plate parts 11H.

[0037] (Thickness, hardness, tensile strength, and surface treatment of steel plate portion 11) As described above, the Vickers hardness of the high-strength steel plate portion 11H is 530HV or more. From the viewpoint of improving the strength of the spot-welded joint 1, the higher the Vickers hardness of the high-strength steel plate portion 11H, the more preferable it is. Therefore, the Vickers hardness of the high-strength steel plate portion 11H may be 550HV or more, 580HV or more, 600HV or more, or 620HV or more. The upper limit of the Vickers hardness of the high-strength steel plate portion 11H is not particularly limited, but from the viewpoint of improving the joining strength of the spot-welded joint 1, the Vickers hardness of the high-strength steel plate portion 11H may be 700HV or less, 680HV or less, or 650HV or less. The tensile strength of the high-strength steel plate portion 11H is not particularly limited. In general, there is a good correlation between the tensile strength of steel and the Vickers hardness, and a Vickers hardness of 530HV corresponds to a tensile strength of about 1800MPa. A Vickers hardness of 580HV corresponds to a tensile strength of about 2000MPa. Therefore, if the hardness of the high-strength steel plate portion 11H is specified, there is no need to specify the tensile strength of the high-strength steel plate portion 11H separately. On the other hand, the tensile strength of the high-strength steel plate portion 11H may be specified to be 1900MPa or more, 2000MPa or more, or 2100MPa or more. The upper limit of the tensile strength of the high-strength steel plate portion 11H is not particularly limited. For example, the tensile strength of the high-strength steel plate portion 11H may be specified to be 2800MPa or less, 2500MPa or less, or 2300MPa or less. The tensile strength of the ordinary steel plate portion 11 having a Vickers hardness of less than 530 HV to be combined with the high-strength steel plate portion 11H is also not particularly limited. For example, the tensile strength of the ordinary steel plate portion 11 may be specified to be 1700 MPa or less, 1500 MPa or less, or 1200 MPa or less.

[0038] The metal structure of the steel plate portion 11 and the high-strength steel plate portion 11H is not particularly limited. For example, 90 area % or more, 92 area % or more, or 95 area % or more of the metal structure of the high-strength steel plate portion 11H may be martensite. Note that martensite is a concept including both fresh martensite, which is martensite that has not been tempered, and tempered martensite, which is martensite that has been tempered. The upper limit of the proportion of martensite in the metal structure of the high-strength steel plate portion 11H is not particularly limited, but the proportion may be, for example, 100 area % or less, 99 area % or less, or 98 area % or less.

[0039] In the spot welded joint 1 according to the first embodiment, it is required that no plating layer is present in the region (corona bond portion) where the corona bond 122 that solid-phase bonds the high-strength steel plate portion 11H and the other steel plate portion 11 exists. On the other hand, as long as this requirement is met, the steel plate portion 11 may have a plating layer 13 on a part or all of its surface. For example, the plating layer 13 arranged on the surface of the plate assembly does not adversely affect the corona bond 122. In addition, the plating layer 13 present between two adjacent steel plate portions 11 also does not adversely affect the corona bond 122 as long as it is arranged outside the corona bond portion. Examples of the plating layer 13 include, for example, hot-dip galvanizing, hot-dip alloy galvanizing, alloy hot-dip galvanizing, electrolytic galvanizing, electrolytic Al plating, and the like. In addition, a surface treatment film such as painting and a chemical conversion coating may be arranged on a part or all of the surface of the steel plate portion 11.

[0040] (Measurement method) A method for measuring parameters defining the spot welded joint 1 according to the first embodiment will be described below. It is not necessary to perform all of the measurements described below on a single sample of spot weld. In general, two or more spot welds are provided on the same steel plate part in a spot welded joint, and the parameters of these spot welds are approximately the same. Therefore, the results of measurements performed on samples of two or more spot welds can be considered to be the measurement results of the parameters of one spot weld. Even when determining whether or not a manufacturing method for a spot welded joint corresponds to a manufacturing method for a spot welded joint according to the second embodiment described later, the results of measurements performed on samples of two or more spot welds can be considered to be the measurement results of the parameters of one spot weld. The method for measuring the Vickers hardness of the steel plate portion 11 is as follows. In a cross-sectional view, a cut surface perpendicular to the surface of the steel plate portion is prepared at a position 10 mm or more away from the outer periphery of the corona bond. Of the cut surfaces for measuring the carbon content of the nugget 121 described later, a position 10 mm or more away from the outer periphery of the corona bond can also be used as a sample for measuring the hardness of the steel plate portion. In this cut surface, the Vickers hardness is measured at a position 1 / 4t from the surface of the steel plate portion. Three measurement points are used. The load is 0.5 kgf. The measurement points are arranged approximately parallel to the surface of the steel plate portion, and the measurement points are spaced 0.5 mm apart. The average value of the measurement values ​​at the three points is regarded as the Vickers hardness of the steel plate portion. The cut surface for measuring the hardness of the steel member can be formed at any flat location, such as a flange portion. The tensile strength of the steel plate portion 11 is measured by a tensile test in accordance with JIS Z 2241:2011 "Method of tensile test for metallic materials". The test piece is a No. 13B test piece. The test piece is taken from the flat portion. The test piece is taken from the outside of the heat-affected zone of the spot welded portion. The location from which the test piece is taken may be far away from the nugget. For example, when the high-strength steel plate portion is a flange portion of a hat-shaped member, the tensile test piece may be taken from the flange portion where the nugget is provided, or from the vertical wall portion or the horizontal wall portion. This measurement method is also applied to the measurement of the tensile strength of the steel member before spot welding, which will be described later. The method for measuring the oxygen concentration of the corona bond 122 at a position 50 μm outside from the outer periphery of the nugget 121 is as follows. First, a peel test is performed on the spot welded joint 1 in accordance with the provisions of JIS Z 3144:2013 "Field test method for spot and projection welds" to fracture the spot weld 12. This causes the surface (also called fracture surface) of the corona bond 122 that solid-phase bonds the high-strength steel plate portion and the other steel plate portion to be revealed. Next, the oxygen concentration of the corona bond 122 is measured by surface EDX analysis. The acceleration voltage is 15 kV. The position of the measurement point is the corona bond 122 at a position 50 μm outside from the outer periphery of the nugget 121 in a plan view. The number of measurement points is 20 for one corona bond 122. The measurement points are distributed at approximately equal intervals along the outer periphery of the nugget 121. When the nugget 121 is circular, the angle between adjacent measurement points and the center of the nugget 121 is approximately 18°. This measurement is performed on the surface (fracture surface) of the high-strength steel plate portion 11H that was solid-state joined by the corona bond 122. However, when high-strength steel plate portions 11H are solid-state joined to each other, measurements are performed at 20 points on the surface (fracture surface) of each of the two high-strength steel plate portions 11H, resulting in a total of 40 measurement results. The average value of these measurement results is regarded as the oxygen concentration of the corona bond 122 that solid-state joins the high-strength steel plate portion to another steel plate portion, at a position 50 μm outside from the outer periphery of the nugget 121.

[0041] The method for determining whether or not the corona bond 122 has a plating layer is as follows. The surface (fracture surface) of the corona bond 122 is exposed using the same procedure as for measuring the oxygen concentration of the corona bond 122. The chemical composition of the plating layer differs from the chemical composition of the base steel sheet. Therefore, the presence or absence of the plating layer can be easily visually confirmed by EDX analysis of the corona bond surface (fracture surface). The presence or absence of the plating layer may also be determined for the sample used to measure the oxygen concentration of the corona bond 122.

[0042] The carbon content of the steel plate portion 11 is measured by JIS G 1211-3:2018 "Iron and steel - Carbon determination method - Part 3: Combustion - Infrared absorption method". The sample for measuring the carbon content is extracted from a location 10 to 30 mm away from the center of the spot weld. When extracting the sample, the surface of the steel plate portion is scraped off by 0.1 mm or more, thereby removing plating and contamination. This measurement method is also applied to the measurement of the carbon content of steel members before spot welding, which will be described later. The method for measuring the carbon amount of the nugget 121 is as follows. First, the nugget 121 is cut on a plane that passes through the center of the nugget 121 when the nugget 121 is viewed in a plan view in a direction perpendicular to the surface of the steel plate portion 11 and is perpendicular to the surface of the steel plate portion 11. Next, the cut surface is etched to expose the nugget 121. The area of ​​each steel plate portion of the nugget 121 exposed on the cut surface is measured, and the carbon amount of the nugget 121 is calculated by dividing the sum of the product of the carbon amount of each steel plate and the area of ​​the nugget 121 on each steel plate by the area of ​​the nugget 121. The "area of ​​the nugget 121" refers to the area of ​​the nugget measured on the cut surface. "The area of ​​each steel plate portion of the nugget 121" refers to the area of ​​each nugget region that exists on the side of each steel plate portion among the divided nugget regions, which are obtained by dividing the nugget 121 on the cut surface using a straight line connecting the corona bonds appearing at both ends of the nugget 121 as a boundary line (hereinafter referred to as the boundary line). Specifically, the carbon content of the nugget is calculated by the following formula. Carbon content of nugget (%) = (S1×C1+S2×C2+...+Sn×Cn) / (S1+S2+...+Sn) S1: Nugget area of ​​the first steel plate (mm2 ) C1: Carbon content of the first steel plate part (mass%) S2: nugget area of ​​the second steel plate (mm 2 ) C2: Carbon content of the second steel plate (mass%) Sn: nugget area of ​​the nth steel plate (mm 2 ) Cn: Carbon content of the nth steel plate part (mass%) For example, in the cross-sectional view of the spot welded joint shown in Figure 1, if the steel plate portion 11 arranged on the upper side is regarded as the "first steel plate portion" and the high-strength steel plate portion 11H arranged on the lower side is regarded as the "second steel plate portion," the dashed line shown inside the nugget 121 is the boundary line, the area of ​​the region above the dashed line in the nugget 121 is S1, and the area of ​​the region below the dashed line in the nugget 121 is S2. If they are on the same steel plate part 11, the oxygen concentration of the corona bond around a nugget other than the nugget that was the subject of the carbon concentration measurement is considered to have been measured as the oxygen concentration of the corona bond around the same nugget. Specifically, a sample is taken from a spot-welded joint as the same plate part having multiple spot welds, two adjacent spot welds among them are measured, and the measurement results for each spot weld are considered to be the measurement results for the same spot weld.

[0043] The diameter of the nugget 121 is measured as follows: First, the nugget 121 is cut along a plane that passes through the center of the nugget 121 when the nugget 121 is viewed in a plan view in a direction perpendicular to the surface of the steel plate portion 11 and is perpendicular to the surface of the steel plate portion 11. Next, the cut surface is etched to reveal the nugget 121. Then, the diameter of the nugget for the thinnest steel plate portion is measured. In JIS Z 3001-6:2013, the diameter of the nugget portion measured at the joint interface by a cross-sectional test of the spot weld is defined as the nugget diameter. In this embodiment, the nugget diameter refers to the nugget width measured on the boundary line between two adjacent steel plate portions. When there are two or more boundary lines, the nugget diameter is measured at each boundary line. The diameter of the nugget may be measured on the cross section of the nugget after the carbon concentration measurement. On the other hand, the diameter of a nugget other than the nugget that was the subject of the oxygen concentration measurement and the carbon concentration measurement may be measured as the diameter of the same nugget, so long as it is on the same steel plate part 11. Specifically, a sample is taken from the spot-welded joint as the same plate-like part having a plurality of spot welds, two adjacent spot welds among them are taken as the measurement subjects, and the average value of the measurement results for each spot weld is regarded as the measurement result for the same spot weld.

[0044] <Method for manufacturing spot welded joint 1 according to the second embodiment> Next, a method for manufacturing a spot welded joint 1 according to a second embodiment of the present invention will be described. As illustrated in Fig. 3A to Fig. 3C, the method for manufacturing a spot welded joint 1 according to the second embodiment includes a step of forming a surface treatment portion 24 on a surface of a high-strength steel member 2H having a Vickers hardness of 530HV or more, a step of overlapping a plurality of steel members 2 so that the surface treatment portion 24 is disposed on a mating surface between the high-strength steel member 2H and another steel member 2, and a step of spot welding the plurality of steel members 2 so that the nugget 121 and the corona bond 122 are located inside the surface treatment portion 24 in a plan view perpendicular to the surface of the surface treatment portion 24. In the overlapping step, no plating layer is provided on the surface treatment portion 24 and the area in contact with the surface treatment portion 24, the ten-point average roughness of the surface treatment portion 24 is 3.5 μm or less, and the oxygen concentration of the surface treatment portion 24 is 18.0 mass% or less. The term "steel member 2" refers to the material (i.e., base material) of the "steel plate portion 11" of the spot welded joint 1 according to the first embodiment. Of the spot-welded steel member 2, the flat portion on the same surface as the spot weld is the steel plate portion 11. Similarly, the term "high-strength steel member 2H" refers to the material of the "high-strength steel plate portion 11H" of the spot welded joint 1 according to the first embodiment. Of the spot-welded high-strength steel member 2H, the flat portion on the same surface as the spot weld is the high-strength steel plate portion 11H.

[0045] (Step S1 of forming surface treatment part 24) First, as shown in FIG. 3A, the surface treatment portion 24 is formed on the surface of a high-strength steel member 2H having a Vickers hardness of 530HV or more. However, when a spot-welded joint is manufactured by providing a high-strength steel plate on which the surface treatment portion 24 is provided in advance, the step S1 of forming the surface treatment portion 24 can be omitted from the manufacturing method of the spot-welded joint. The "surface" of the high-strength steel member 2H on which the surface treatment portion 24 is formed means a surface on which an electrode for spot welding can be placed. Therefore, the "surface" is a concept that does not include an "end surface" (i.e., a cut surface formed by cutting a steel member from a steel plate, which is a raw material). As will be described later, a nugget 121 and a corona bond 122 are formed on the surface treatment portion 24. By controlling the surface state of the surface treatment portion 24 in advance, the oxygen concentration of the corona bond 122 can be set to 15.0 mass% or less.

[0046] In the surface treatment section 24, the ten-point mean roughness of the surface treatment section 24 is set to 3.5 μm or less, and the oxygen concentration is set to 18.0 mass% or less. By setting the ten-point mean roughness of the surface treatment section 24 of the high-strength steel member 2H to 3.5 μm or less, the surface area of ​​the surface treatment section 24 can be reduced, and the amount of oxygen in the surface treatment section 24 can be reduced. In addition, according to the results of experiments conducted by the present inventors, oxides are discharged to the outside of the corona bond 122 due to thermal expansion of the nugget 121 during spot welding, and the oxygen concentration decreases. Therefore, if the ten-point mean roughness of the surface treatment section 24 is set to 3.5 μm or less and the oxygen concentration of the surface treatment section 24 is set to 18.0 mass% or less before spot welding, the oxygen concentration of the corona bond 122 formed on the surface treatment section 24 can be set to 15.0 mass% or less. Note that when the high-strength steel member 2H and the normal steel member 2 are combined, it is not necessary to control the ten-point mean roughness and the oxygen concentration on the surface of the normal steel member 2 that contacts the high-strength steel member 2H. The oxygen concentration of the surface treatment portion 24 may be 16.5 mass% or less, 16.0 mass% or less, or 15.0 mass% or less. The surface treatment portion having the above-mentioned configuration may be formed on the surface of a high-strength steel member 2H having at least a Vickers hardness of 530 HV or more. More preferably, the surface treatment portion having the above-mentioned configuration is also formed on the surface of an ordinary steel member 2 having a Vickers hardness of less than 530 HV.

[0047] The method for measuring the Vickers hardness of the high-strength steel member 2H is the same as the method for measuring the Vickers hardness of the steel plate portion of the spot-welded joint 1 according to the first embodiment, except for the measurement position. The measurement position is a position at 1 / 4t from the surface of the high-strength steel member 2H on the cross section of the high-strength steel member 2H. The method for measuring the ten-point average roughness of the surface treatment part 24 of the high-strength steel member 2H is as follows. Based on JIS B 0601:2013, Appendix JA "Ten-point average roughness", the ten-point average roughness of any measurement point on the surface treatment part 24 is measured. The definition of ten-point average roughness is based on the 2013 standard. The measurement length is 4 mm, and the cutoff is 0.8 mm. The method for measuring the oxygen concentration in the surface treatment portion 24 of the high-strength steel member 2H is the same as the method for measuring the oxygen concentration in the corona bond 122 of the spot welded joint 1 according to the first embodiment, except for the peel test and the measurement position. That is, in the surface treatment portion 24 of the high-strength steel plate portion 11H, a surface EDX analysis may be performed at an acceleration voltage of 15 KV. However, since the nugget 121 is not formed in the surface treatment portion 24, the interval between measurement points is set to 10 μm, the number of measurement points is set to 5, and the positions of these measurement points are set to any positions inside the surface treatment portion 24.

[0048] The steel member 2 and the high-strength steel member 2H may have a plating layer. However, the surface treatment portion 24 is not provided with a plating layer. When the high-strength steel member 2H is a non-plated steel member 2, removal of the plating layer is not necessary, and the ten-point average roughness and oxygen concentration of the surface treatment portion 24 are set within the above-mentioned ranges. When the high-strength steel member 2H is a plated steel member 2, it is necessary to first remove the plating layer from the surface-treated portion 24, and then adjust the ten-point average roughness and oxygen concentration of the surface-treated portion 24 to fall within the above-mentioned ranges. If the plating layer is removed and the surface-treated portion is formed in a single treatment, the temperature of the surface-treated portion will rise, making it difficult to adjust the oxygen concentration to fall within the above-mentioned ranges.

[0049] The surface treatment portion 24 may be formed on the surface of the high-strength steel member 2H. When two high-strength steel members 2H are overlapped, both high-strength steel members 2H are provided with the surface treatment portion 24. On the other hand, when the high-strength steel member 2H is overlapped with an ordinary steel member 2 having a Vickers hardness of less than 530 HV, it is not necessary to provide the surface treatment portion 24 on the ordinary steel member 2. Furthermore, when the ordinary steel member 2 to be overlapped with the high-strength steel member 2H is a plated steel member 2, it is necessary to remove at least the plating layer from at least a part of this plated steel member 2.

[0050] When the high-strength steel member 2H is disposed on the surface of the plate assembly, the surface treatment portion 24 may be provided on one surface of the high-strength steel member 2H. When the high-strength steel member 2H is disposed between two other steel members 2, the surface treatment portion 24 is provided on both surfaces of the high-strength steel member 2H. In either case, the surface treatment portion 24 may be provided only on a part of the surface of the high-strength steel member 2H, or may be provided over the entire surface. For example, when the high-strength steel member is a hat-shaped member having a flange portion, a vertical wall portion, and a horizontal wall portion (top plate portion), spot welding is performed at the flange portion. The surface treatment portion may be formed only on the surface of the flange portion, or may be formed on the surface of any part including the flange portion, the vertical wall portion, and the horizontal wall portion. The surface treatment portion may be provided on one or both of the inner surface and the outer surface of the hat-shaped member. However, as described above, it is not necessary to provide the surface treatment portion on the end surface.

[0051] (Step S2 of stacking multiple steel members 2) Next, as shown in FIG. 3B, a plurality of steel members 2 including the high-strength steel member 2H described above are overlapped. At this time, the surface treatment portion 24 formed on the high-strength steel member 2H is arranged on the mating surface of the steel member 2. Also, the surface treatment portion 24 formed on the high-strength steel member 2H is not in contact with the plating layer. That is, not only the surface treatment portion 24 but also the area in contact with the surface treatment portion 24 is not provided with a plating layer. When the high-strength steel members 2H are overlapped with each other, the surface treatment portions 24 are overlapped with each other, so no special measures are required. On the other hand, when the steel member 2 to be overlapped with the high-strength steel member 2H is a normal steel plate and has a plating layer on the overlapping surface, it is necessary to remove the plating layer in advance as described above, and align the steel member 2 so that the removed portion of the plating layer and the surface treatment portion 24 overlap.

[0052] (Step S3 of spot welding multiple steel members 2) 3C, a plurality of steel members 2 are spot-welded. At this time, the steel members 2 are spot-welded by overlapping the surface treatment portion 24 with the impact point, which is the position where the electrode E and the steel members 2 come into contact, so that the nugget and corona bond are contained inside the surface treatment portion 24 in a plan view from a direction perpendicular to the surface of the surface treatment portion 24. By manufacturing the spot welded joint 1 according to the above-mentioned procedure, it is possible to form the nugget 121 and the corona bond 122 in the surface treatment portion 24, which has a low ten-point mean roughness and oxygen concentration and is not provided with a plating layer. That is, the manufacturing method according to the second embodiment makes it possible to prevent the presence of a plating layer in the corona bond 122 formed on the surface of the high-strength steel plate portion 11H of the spot welded joint 1, and to set the oxygen concentration of the corona bond 122 formed on the surface of the high-strength steel plate portion 11H to 15.0 mass% or less.

[0053] The manufacturing method of the spot welded joint 1 according to the second embodiment can improve the TSS as long as the above-mentioned requirements are satisfied. On the other hand, the preferred aspects of the spot welded joint 1 according to the first embodiment can also be applied to the manufacturing method according to the second embodiment.

[0054] (Hot stamping and removal of plating layer) The manufacturing method of the spot welded joint 1 may further include a step of hot stamping the high strength steel plate. In this case, it is preferable that the high strength steel plate is hot stamped before the surface treatment portion 24 is formed on the high strength steel plate. Then, in the step of forming the surface treatment portion 24, it is preferable that the surface treatment portion 24 is formed on the surface of the hot stamped high strength steel plate. This makes it possible to avoid an increase in the oxygen concentration of the surface treatment portion 24 due to heating for hot stamping. The manufacturing method of the spot welded joint 1 may include a step of removing a part of the plating layer on the surface of the high-strength steel member before forming the surface treatment portion. In this case, in the step of forming the surface treatment portion, it is preferable to form the surface treatment portion in the area where the plating layer has been removed. This makes it possible to manufacture a corona bond without a plating layer.

[0055] (Carbon content of high-strength steel member 2H: preferably more than 0.30 mass%) (Carbon content of nugget 121: preferably 0.29 mass% or more) For example, similarly to the spot welded joint 1 according to the first embodiment, in the manufacturing method according to the second embodiment, the carbon content of the high-strength steel member 2H may be more than 0.30 mass%. Also, the carbon content of the nugget 121 may be 0.29 mass% or more. This makes it possible to further improve various mechanical properties of the spot welded joint 1. The preferable upper and lower limit values ​​of the carbon content of the high-strength steel plate portion 11H and the nugget 121 exemplified in the first embodiment can also be applied to the high-strength steel member 2H and the nugget 121 in the manufacturing method according to the second embodiment. The carbon content of the nugget 121 can be controlled via the carbon content and plate thickness of the steel member 2. The higher the carbon content of the steel member 2, the higher the carbon content of the nugget 121. In addition, when manufacturing the spot welded joint 1 by combining a high carbon steel member and a low carbon steel member, the carbon content of the nugget 121 can be increased by increasing the ratio of the plate thickness of the high carbon steel member to the total plate thickness t. The method for measuring the carbon content of the high-strength steel member 2H and the oxygen content of the nugget 121 is the same as the method for measuring the carbon content of the high-strength steel plate portion 11H of the spot welded joint 1 according to the first embodiment and the oxygen content of the nugget 121.

[0056] (Hardness of high-strength steel 2H: 530HV or higher) (Metal structure of high strength steel member 2H: preferably 90% or more of the area is martensite) As described above, the Vickers hardness of the high-strength steel member 2H is 530HV or more. From the viewpoint of improving the strength of the spot-welded joint 1, the higher the Vickers hardness of the high-strength steel member 2H, the more preferable it is. Therefore, the Vickers hardness of the high-strength steel member 2H may be 550HV or more, 580HV or more, 600HV or more, or 620HV or more. The upper limit of the Vickers hardness of the high-strength steel member 2H is not particularly limited, but from the viewpoint of improving the joining strength of the spot-welded joint 1, the Vickers hardness of the high-strength steel member 2H may be 700HV or less, 680HV or less, or 650HV or less. The metal structure of the steel member 2 and the high-strength steel member 2H is not particularly limited. For example, 90 area % or more, 92 area % or more, or 95 area % or more of the metal structure of the high-strength steel member 2H may be martensite. Note that martensite is a concept that includes both fresh martensite, which is martensite that has not been tempered, and tempered martensite, which is martensite that has been tempered. The upper limit of the proportion of martensite in the metal structure of the high-strength steel member 2H is not particularly limited, but the proportion may be, for example, 100 area % or less, 99 area % or less, or 98 area % or less.

[0057] (Diameter of nugget 121: preferably 4.5√t or more) The diameter D of the nugget 121 may be 4.5√t or more. Here, the diameter D of the nugget 121 is the size of the nugget 121 measured along the boundary line between adjacent steel members 2 in a plane that passes through the center of the nugget 121 and is perpendicular to the surface of the steel member 2. "t" refers to the smaller of the sheet thicknesses of the two adjacent steel members 2 that form the boundary line that serves as the reference for measuring the nugget 121. When the number of steel members 2 to be spot welded is three or more and there are two or more boundary lines, Ds of 2 or more and ts of 2 or more corresponding to each of the two or more boundary lines are specified. By satisfying D≧4.5√t, the amount of oxides discharged from the corona bond 122 can be increased, and the oxygen concentration of the corona bond 122 can be further reduced. The diameter of the nugget 121 can be controlled by the amount of heat input in the spot welding. The larger the diameter of the nugget 121, the larger the diameter of the corona bond 122. Therefore, it is preferable to determine the size of the surface treatment portion 24 so that the corona bond 122 is formed inside the surface treatment portion 24. When the number of steel members 2 is three or more and there are two or more boundary lines, it is preferable that D≧4.5√t is satisfied at at least one boundary, and it is even more preferable that D≧4.5√t is satisfied at all boundaries. In addition, it is preferable that the diameter D of the nugget 121 and the thinner plate thickness t of the steel members 2 that form the boundary line satisfy D≧4.5√t at the boundary line formed by the high-strength steel members 2H. The method for measuring the diameter of the nugget 121 is the same as the method for measuring the diameter of the nugget 121 of the spot welded joint 1 according to the first embodiment.

[0058] (Means for forming the surface treatment portion 24: preferably shot blasting or wet blasting) The means for forming the surface treatment portion 24 is not particularly limited, but is preferably shot blasting or wet blasting. Shot blasting is a surface treatment in which a projecting material, which is an abrasive powder, is sprayed. Wet blasting is a surface treatment in which a slurry obtained by mixing an abrasive and a liquid is sprayed.

[0059] The high-strength steel member 2H having a Vickers hardness of 530HV or more is preferably manufactured through heat treatment such as quenching and tempering, and hot stamping. Therefore, a large amount of oxide scale adheres to the surface of the high-strength steel member 2H. The oxide scale is usually removed by means of pickling, shot blasting, and the like. However, the inventors confirmed that the oxygen concentration of the high-strength steel member 2H after removing the oxide scale by these ordinary means exceeded 18.0 mass%, and the ten-point average roughness also exceeded 3.5 μm.

[0060] Therefore, the inventors of the present invention conducted various investigations and found that the oxygen concentration of the high-strength steel member 2H could be reduced to 18.0 mass% by wet blasting. It was not known that the effect of wet blasting in reducing the oxygen concentration of the high-strength steel member 2H was greater than that of shot blasting. In addition, wet blasting has a disadvantage that it takes longer to remove oxide scale than shot blasting. This is because wet blasting has a smaller processing force than shot blasting because water resistance is added when the abrasive collides. In addition, wet blasting has a disadvantage that it requires additional steps such as the installation of drainage equipment and drying because water is used. Therefore, no examples of using wet blasting as a pretreatment for spot welding have been reported so far. However, in the manufacturing method according to the second embodiment, based on the above-mentioned knowledge of the inventors of the present invention, wet blasting is adopted as a preferred surface treatment means. The material of the abrasive used in wet blasting is preferably, for example, steel, zirconia, stainless steel, or alumina. The shape of the abrasive is preferably, for example, spherical or grid-shaped. The average diameter of the abrasive is preferably, for example, 0.03 to 0.50 mm. During wet blasting, the air pressure for spraying the blasting material (i.e., slurry obtained by mixing the abrasive and liquid) is preferably, for example, 0.2 to 0.8 MPa. The projection distance (i.e., the distance between the nozzle for spraying the blasting material and the high-strength steel member 2H) is preferably, for example, 1 to 30 cm.

[0061] It is considered possible to reduce the oxygen concentration and ten-point average roughness of the high-strength steel member 2H by reducing the diameter of the particles injected in the shot blasting. However, in the shot blasting, heat such as frictional heat is generated during the surface processing, and there is a risk that the oxygen concentration of the high-strength steel member 2H will increase. Therefore, when performing the surface processing of the high-strength steel member 2H using the shot blasting, it is preferable to appropriately adjust the conditions such as the particle ejection speed and to use a cooling means in addition to the particle diameter. In addition, reducing the particle diameter of the shot blasting has disadvantages such as the effect of air resistance becoming greater, making it difficult for the particles to reach the high-strength steel member 2H, the particles being easily humidified, not being ejected evenly, or the nozzle being clogged, and dust being easily stirred up, which may lead to a deterioration of the working environment. The material of the particles (projection material) used in shot blasting is preferably, for example, steel, zirconia, stainless steel, or alumina. The shape of the projection material is preferably, for example, spherical or grid-shaped. The average diameter of the projection material is preferably, for example, 0.03 to 0.70 mm. During shot blasting, the air pressure for spraying the projection material is preferably, for example, 0.2 MPa to 1.0 MPa. The projection distance (i.e., the distance between the nozzle for spraying the projection material and the high-strength steel member 2H) is preferably, for example, 5 to 40 cm. The blasting pressure in wet blasting or shot blasting, i.e., the air pressure used to spray the blasting material, is determined by measuring the pressure of the air pipe that feeds the blasting material. The particle size of the blasting material in wet blasting or shot blasting is determined by measuring 20 random particles under a microscope and calculating the average value. The blasting distance in wet blasting or shot blasting is determined by measuring the distance from the tip of the blasting material spray nozzle to the workpiece.

[0062] In addition, the oxygen concentration and the ten-point average roughness can also be reduced by grinding or cutting. By grinding or cutting, it is also possible to remove the plating layer. However, as with shot blasting, grinding and cutting also generate heat such as frictional heat during surface processing, which may increase the oxygen concentration of the high-strength steel member 2H. Therefore, when performing surface processing of the high-strength steel member 2H using grinding or cutting, it is preferable to appropriately set conditions such as the processing speed or use a cooling means. In addition, grinding or cutting has the disadvantages of high processing costs and long processing times. For the above reasons, wet blasting is considered to be the most preferable means for forming the surface treatment portion 24. When the surface treatment portion is formed by grinding, it is preferable to suppress frictional heat by controlling the peripheral speed, which is the rotation speed of the grinding wheel, the feed rate, the amount of cut, and the type and flow rate of the grinding fluid. When the surface treatment portion is formed by cutting, it is preferable to suppress frictional heat by controlling the rotation speed, feed rate, and amount of cut of the tool.

[0063] <High-strength steel member for spot welded joint according to the third embodiment> Next, a high-strength steel member for a spot-welded joint according to a third embodiment of the present invention will be described. The high-strength steel member for a spot-welded joint according to the third embodiment has a Vickers hardness of 530 HV or more, a surface treatment portion having no plating layer, a ten-point average roughness of the surface treatment portion of 3.5 μm or less, and an oxygen concentration of 18.0 mass% or less.

[0064] The high-strength steel member according to the third embodiment has a Vickers hardness of 530 HV or more. Therefore, according to the high-strength steel member according to the third embodiment, a spot welded joint 1 with very high strength can be manufactured. However, the spot welded portion 12 obtained by spot welding a high-strength steel member having a Vickers hardness of 530 HV or more is inferior in TSS. Therefore, the high-strength steel member according to the third embodiment has a surface treatment portion 24 without a plating layer, the ten-point average roughness of the surface treatment portion 24 is 3.5 μm or less, and the oxygen concentration of the surface treatment portion 24 is 18.0 mass% or less. In the corona bond 122 obtained by spot welding this surface treatment portion 24, there is no plating layer, and further, the oxygen concentration is 15.0 mass% or less. This allows the TSS of the spot welded joint 1 to be dramatically increased.

[0065] As long as the high-strength steel member for a spot-welded joint according to the third embodiment satisfies the above-mentioned requirements, it can obtain the effect of improving the TSS of the spot-welded joint 1. Meanwhile, preferred aspects of the spot-welded joint 1 according to the first embodiment and the manufacturing method for the spot-welded joint 1 according to the second embodiment can also be applied to the high-strength steel member according to the third embodiment. The method for measuring the Vickers hardness of the high-strength steel member in the third embodiment is the same as the method for measuring the Vickers hardness of the high-strength steel member described in the manufacturing method of the spot welded joint 1 in the second embodiment. The method for measuring the ten-point mean roughness of the surface treatment portion 24 of the high-strength steel component in the third embodiment is the same as the method for measuring the ten-point mean roughness of the surface treatment portion 24 described in the manufacturing method of the spot welded joint in the second embodiment. The method for measuring the oxygen concentration in the surface treatment part 24 of the high-strength steel component in the third embodiment is the same as the method for measuring the oxygen concentration in the surface treatment part described in the manufacturing method of the spot welded joint in the second embodiment.

[0066] (Carbon content: preferably more than 0.30 mass%) (Location of surface treatment 24: preferably the entire surface of one or both surfaces) For example, like the spot welded joint 1 according to the first embodiment, the carbon content of the high-strength steel member for the spot welded joint according to the third embodiment may be more than 0.30 mass%. This can further improve various mechanical properties of the spot welded joint 1. Note that C contained in the high-strength steel member may cause embrittlement of the welded joint. However, according to the high-strength steel member for the spot welded joint according to the third embodiment, the corona bond is modified using a surface treatment device to increase the joint strength of the joint of the spot welded joint. Therefore, even if the carbon content is high, the high-strength steel member for the spot welded joint according to the third embodiment can produce a welded joint with high joint strength. The carbon content of the high-strength steel member for the spot welded joint is more preferably 0.31 mass% or more, 0.33 mass% or more, 0.36 mass% or more, or 0.40 mass% or more. The carbon content of the high-strength steel member for the spot welded joint is more preferably 0.50 mass% or less, 0.48 mass% or less, 0.45 mass% or less, 0.42 mass% or less, or 0.40 mass% or less. Similarly to the manufacturing method of the spot welded joint 1 according to the second embodiment, the surface treatment portion 24 may be provided over the entire surface of one or both of the high-strength steel members. The method for measuring the carbon content of the high-strength steel member is the same as the method for measuring the carbon content of the high-strength steel plate portion 11H of the spot-welded joint 1 according to the first embodiment. However, the sample for measuring the carbon content is extracted from a location 10 to 30 mm away from the end face of the high-strength steel member. When extracting the sample, the surface of the steel plate portion is scraped off by 0.1 mm or more to remove plating and contamination.

[0067] <Manufacturing method of high strength steel member according to the fourth embodiment> Next, a manufacturing method of a high-strength steel member 2H for spot-welded joints according to a fourth embodiment of the present invention will be described. The manufacturing method of a high-strength steel member 2H for spot-welded joints according to the fourth embodiment of the present invention includes a step of forming a surface treatment portion 24 on the surface of a high-strength steel member having a Vickers hardness of 530 HV or more, in which no plating layer is provided on the surface treatment portion 24, the ten-point average roughness of the surface treatment portion 24 is 3.5 μm or less, and the oxygen concentration of the surface treatment portion 24 is 18.0 mass% or less.

[0068] The high-strength steel member 2H obtained by the manufacturing method according to the fourth embodiment has a Vickers hardness of 530 HV or more. Therefore, according to the manufacturing method of the high-strength steel member according to the fourth embodiment, it is possible to provide a steel plate portion 11 from which a spot-welded joint 1 with extremely high strength can be manufactured. However, the spot welded portion 12 obtained by spot welding a high-strength steel member having a Vickers hardness of 530 HV or more is inferior in TSS. Therefore, in the manufacturing method of the high-strength steel member according to the fourth embodiment, a surface treatment portion 24 is formed that has a ten-point average roughness of 3.5 μm or less, an oxygen concentration of 18.0 mass% or less, and is not plated. In the corona bond 122 obtained by spot welding this surface treatment portion 24, there is no plated layer 13, and furthermore, the oxygen concentration is 15.0 mass% or less. Therefore, according to the manufacturing method of the high-strength steel member according to the fourth embodiment, it is possible to provide a high-strength steel member that can dramatically increase the TSS of the spot-welded joint 1. As long as the above-mentioned requirements are satisfied, the manufacturing method for the high-strength steel member according to the fourth embodiment can achieve the effect of improving the TSS of the spot-welded joint 1. Meanwhile, preferred aspects of the spot-welded joint 1 according to the first embodiment, the manufacturing method for the spot-welded joint 1 according to the second embodiment, and the high-strength steel member for the spot-welded joint according to the third embodiment can also be applied to the manufacturing method for the high-strength steel member according to the fourth embodiment. The method for measuring the Vickers hardness of the high-strength steel member is the same as the method for measuring the Vickers hardness of the high-strength steel plate portion in the manufacturing method of the spot welded joint 1 according to the second embodiment. The method for measuring the ten-point mean roughness and oxygen concentration of the surface treatment portion 24 of the high-strength steel member is the same as the method for measuring the ten-point mean roughness and oxygen concentration of the surface treatment portion 24 in the manufacturing method of the spot welded joint according to the second embodiment.

[0069] (Hot stamping and removal of plating layer) The manufacturing method of the high-strength steel member may further include a step of hot stamping the high-strength steel plate. In this case, it is preferable that the high-strength steel plate is hot stamped before the surface treatment portion 24 is formed on the high-strength steel plate. Then, in the step of forming the surface treatment portion 24, it is preferable that the surface treatment portion 24 is formed on the surface of the hot stamped high-strength steel member. This makes it possible to avoid an increase in the oxygen concentration of the surface treatment portion 24 due to heating for hot stamping. The manufacturing method of the spot welded joint 1 may include a step of removing a part of the plating layer on the surface of the high-strength steel member before forming the surface treatment portion 24. In this case, in the step of forming the surface treatment portion, it is preferable to form the surface treatment portion in the area where the plating layer has been removed. This makes it possible to provide a high-strength steel member that can produce a corona bond without a plating layer.

[0070] (Carbon content: preferably more than 0.30 mass%) (Location of surface treatment 24: preferably the entire surface of one or both surfaces) (Means for forming the surface treatment portion 24: preferably wet blasting) For example, like the high-strength steel plate portion 11H of the spot welded joint 1 according to the first embodiment, the carbon content of the high-strength steel member 2H used in the manufacturing method according to the fourth embodiment may be more than 0.30 mass%. This can further improve various mechanical properties of the spot welded joint 1. Also, like the manufacturing method of the spot welded joint 1 according to the second embodiment, the surface treatment portion 24 may be provided over the entire surface of one or both of the high-strength steel members 2H. Also, like the manufacturing method of the spot welded joint according to the second embodiment, the means for forming the surface treatment portion 24 is not particularly limited, but preferably, this means is shot blasting or wet blasting. Also, the surface treatment portion 24 may be formed by grinding. Various processing conditions for forming the surface treatment portion 24 exemplified in the second embodiment may be applied to the manufacturing method according to the fourth embodiment. The method for measuring the carbon content of the high strength steel member 2H is the same as the method for measuring the carbon content of the high strength steel member according to the third embodiment. EXAMPLES

[0071] The effects of one embodiment of the present invention will be explained more specifically with reference to an example. However, the conditions in the example are merely an example of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0072] Example 1 Two high-strength steel plates with a thickness of 1.6 mm and a Vickers hardness of 580 HV (tensile strength of 2000 MPa) were spot-welded to produce spot-welded joints. However, in the example of No. 20, a spot-welded joint was obtained by spot-welding two ordinary steel plates with a thickness of 1.6 mm and a Vickers hardness of 310 HV (tensile strength of 980 MPa) instead of the high-strength steel plates described above, and in the example of No. 22, a spot-welded joint was obtained by spot-welding two ordinary steel plates with a thickness of 1.6 mm and a Vickers hardness of 460 HV (tensile strength of 1500 MPa) instead of the high-strength steel plates described above. The high-strength steel plate with a tensile strength of 2000 MPa was heat-treated to simulate a steel member after hot stamping, and the steel plate with a tensile strength of 980 MPa and the steel plate with a tensile strength of 1500 MPa were ordinary cold-rolled steel plates that were not heat-treated.

[0073] The surfaces of the two steel sheets were each formed with a surface treatment prior to spot welding. The ten-point average roughness and oxygen concentration of the surface treatment are shown in Table 1.

[0074] The oxygen concentration of the corona bond and the nugget diameter of various spot welded joints obtained by the above procedure were measured and listed in Table 1. Furthermore, the TSS increase rate of various spot welded joints was evaluated and listed in Table 1.

[0075] The TSS increase rate is the rate of increase in TSS relative to the TSS standard value shown in Table 2. The TSS standard value is the TSS of a spot welded joint obtained by subjecting two high-strength steel plates, each with a thickness of 1.6 mm and a tensile strength, to normal shot blasting treatment and then spot welding. Since the TSS differs depending on the nugget diameter, Table 2 lists standard values ​​according to the nugget diameter. The meanings of the symbols for the TSS increase rate shown in Table 1 are as follows: Spot welded joints with TSS below the standard value: D - Spot welded joints with a TSS increase rate of more than 0% and 5% or less compared to the standard value: C - Spot welded joints with a TSS increase rate of more than 5% and less than 20% compared to the standard value: B - Spot welded joints with a TSS increase rate of 20% or more compared to the standard value: A

[0076] [Table 1]

[0077] [Table 2]

[0078] In Comparative Examples 2, 5, and 7, the oxygen concentration of the corona bond was outside the range of the invention. In these Comparative Examples, no increase in TSS was observed relative to the TSS standard value. It is presumed that the reason for the high oxygen concentration of the corona bond in these Comparative Examples is that the oxygen concentration in the surface treatment area before spot welding was excessive.

[0079] In Comparative Example 20, the oxygen concentration of the corona bond was within the invention range, but no increase in TSS was observed relative to the TSS standard value. This is presumably because the tensile strength of the base metal of the spot-welded joint in Comparative Example 20 was 980 MPa in all cases. Similarly, in Comparative Example 22, the oxygen concentration of the corona bond was within the invention range, but no increase in TSS was observed relative to the TSS standard value. This is presumably because the tensile strength of the base metal of the spot-welded joint in Comparative Example 22 was 1500 MPa in all cases. The correlation between the oxygen concentration of the corona bond and TSS was confirmed only in high-strength steel plates with a Vickers hardness of 530 HV or more.

[0080] In Comparative Example 21, the oxygen concentration of the corona bond was outside the range of the invention. In these Comparative Examples, no increase in TSS was observed relative to the TSS standard value. The reason why the oxygen concentration of the corona bond was high in this Comparative Example is presumably because the ten-point average roughness of the surface treatment area before spot welding was excessive, and the oxygen concentration of the corona bond increased after spot welding.

[0081] On the other hand, in examples within the scope of the present invention, it was possible to provide spot welded joints that were excellent in TSS while including one or more high-strength steel plates having a Vickers hardness of 530 HV or more.

[0082] Example 2 Various high-strength steel plates shown in Table 3 were subjected to hot stamping to manufacture high-strength steel members. In the hot stamping, the high-strength steel plate was heated to 890°C, held for 5 minutes, and then cooled using a water-cooled mold. Then, a test piece was taken out from the flat surface of the high-strength steel member by laser cutting. These test pieces of the high-strength steel member were subjected to wet blasting according to the conditions shown in Table 4, shot blasting according to the conditions shown in Table 5, or cutting according to the conditions shown in Table 6, thereby forming a surface treatment portion. The ten-point average roughness and oxygen concentration of the surface treatment portion were measured by the above-mentioned means and are shown in Table 7.

[0083] [Table 3]

[0084] [Table 4]

[0085] [Table 5]

[0086] [Table 6]

[0087] [Table 7]

[0088] Wet blasting was performed under appropriate conditions for sample No. 1. The ten-point average roughness and oxygen concentration of the surface treatment formed by wet blasting for sample No. 1 were appropriate. For sample No. 2, the projection distance during wet blasting was too long. For sample No. 2, the oxygen concentration in the surface treatment area formed by wet blasting was excessive. This is presumably because the projection distance was too long, and therefore the removal of oxides present on the surface of the steel member was insufficient. In sample No. 3, the diameter of the abrasive used in wet blasting was too large. In sample No. 3, both the ten-point average roughness and the oxygen concentration of the surface treatment formed by wet blasting were excessive. This is presumably because the diameter of the abrasive was too large, which roughened the surface of the steel component and also insufficient removal of oxides present on the surface.

[0089] In sample No. 4, shot blasting was performed under appropriate conditions. In sample No. 4, the ten-point average roughness and oxygen concentration of the surface treatment formed by shot blasting were appropriate. The shot distance during shot blasting was too long for sample No. 5. The oxygen concentration in the surface treatment area formed by shot blasting was excessive for sample No. 5. This is presumably because the shot distance was too long, and therefore the removal of oxides present on the surface of the steel member was insufficient. In sample No. 6, the diameter of the shot material used in shot blasting was too large. In sample No. 6, both the ten-point average roughness and the oxygen concentration of the surface treatment area formed by shot blasting were excessive. This is presumably because the diameter of the shot material was too large, which roughened the surface of the steel member and also insufficient removal of oxides present on the surface.

[0090] Cutting was performed under appropriate conditions for sample No. 7. The ten-point average roughness and oxygen concentration of the surface treatment formed by cutting were appropriate for sample No. 7. The cutting depth during cutting was too large for sample No. 8. The ten-point average roughness of the surface treatment formed by cutting for sample No. 8 was excessive. This is presumably because the cutting depth was too large, which caused chattering and roughened the surface.

[0091] In sample No. 9, cutting was performed under appropriate conditions. In sample No. 9, the ten-point average roughness and oxygen concentration of the surface treatment portion formed by cutting were appropriate. For reference, the TSS of sample 9 was measured and evaluated based on the TSS evaluation criteria used in Example 1, and the TSS increase rate of sample 9 was 20% or more. The standard values ​​of the TSS increase rate in the welded joint combining steel member No. 1 and steel member No. 2 listed in Table 3 are 23 kN when the nugget diameter is 5.0 mm, 25 kN when the nugget diameter is 5.5 mm, and 27 kN when the nugget diameter is 6.0 mm.

[0092] For sample 10, a steel member having a zinc-based plating layer was used as the starting material. For sample 10, the zinc-based plating layer of the steel member was first removed to form a plating-removed portion. Next, after waiting for the temperature of the plating-removed portion to drop to room temperature, cutting was performed to form the surface treatment portion. For sample 10, the zinc-based plating layer was removed before forming the surface treatment portion, and then cutting was performed under appropriate conditions. For sample No. 10, the ten-point average roughness and oxygen concentration of the surface treatment portion formed by cutting were appropriate. For sample 11, a steel member having a zinc-based plating layer was used as the starting material, as in sample 10. However, for sample 11, the zinc-based plating layer was not removed before cutting to form the surface treatment portion. Specifically, by cutting the area where the zinc-based plating layer was present under condition G1, the removal of the zinc-based plating layer and the formation of the surface treatment portion were carried out in a single process. For sample No. 11, the oxygen concentration in the surface treatment portion formed by cutting became excessive. [Explanation of symbols]

[0093] 1 Spot welded joints 11 Steel plate section 11H High strength steel plate part 12 Spot welds 121 Nugget 122 Coronabond 123 Coronabond Department 13 Plating layer 2 Steel parts 2H high strength steel parts 24 Surface treatment section E-electrode Oxide

Claims

1. A plurality of steel plate portions including a high-strength steel plate portion having a Vickers hardness of 530 HV or more; A nugget that joins the high strength steel plate portion and another steel plate portion among the plurality of steel plate portions; a corona bond that is formed in a ring shape around the nugget in a plan view from a direction perpendicular to a surface of the high-strength steel plate portion and that solid-state bonds the high-strength steel plate portion and the other steel plate portion; Equipped with No plating layer is present in the area where the corona bond is present, The oxygen concentration of the corona bond at a position 50 μm outside from the outer periphery of the nugget in the plan view is 15.0 mass% or less. Spot welded joints.

2. The spot welded joint according to claim 1, characterized in that the high strength steel plate portion has a carbon content of more than 0.30 mass%.

3. 3. The spot welded joint according to claim 1, wherein the carbon content of the nugget is 0.29 mass% or more.

4. A spot welded joint as described in claim 1 or 2, characterized in that the diameter of the nugget measured along the boundary line between adjacent steel plate portions in a cross section passing through the center of the nugget and perpendicular to the surface of the steel plate portion is 4.5√t or more. Here, t refers to the smaller of the thicknesses of the two adjacent steel plate portions that form the boundary line.

5. Providing a high strength steel member having a Vickers hardness of 530 HV or more, the high strength steel member having a surface treatment; A step of overlapping a plurality of the steel members so that the surface treatment portion is disposed on a mating surface between the high strength steel member and another steel member; spot welding the steel members together so that a nugget and a corona bond are located inside the surface treatment portion when viewed from a plane perpendicular to the surface treatment portion; Equipped with In the overlapping step, no plating layer is provided on the surface treatment portion and the area in contact with the surface treatment portion, The ten-point average roughness of the surface treatment is 3.5 μm or less, and the oxygen concentration of the surface treatment is 18.0 mass% or less. A method for manufacturing spot welded joints.

6. The method for manufacturing a spot welded joint according to claim 5, further comprising the step of forming the surface treatment on a surface of the high strength steel member before the step of providing the high strength steel member.

7. The method further comprises the step of hot stamping the high strength steel sheet before forming the surface treatment portion; In the step of forming the surface treatment, the surface treatment is formed on the surface of the hot stamped high strength steel member. The method for manufacturing a spot welded joint according to claim 6 .

8. a step of removing a part of a plating layer on a surface of a high-strength steel member before forming the surface treatment part, In the step of forming the surface treatment portion, the surface treatment portion is formed in the area from which the plating layer has been removed. The method for manufacturing a spot welded joint according to claim 6 or 7.

9. The method for manufacturing a spot welded joint according to claim 5 or 6, characterized in that the carbon content of the high strength steel member is more than 0.30 mass%.

10. 7. The method for manufacturing a spot welded joint according to claim 5, wherein the carbon content of the nugget formed by the spot welding is 0.29 mass% or more.

11. 7. The method for manufacturing a spot welded joint according to claim 5, wherein a diameter of the nugget formed by the spot welding is 4.5√t or more. Here, the diameter of the nugget is the size of the nugget measured along the boundary line between adjacent steel members in a plane passing through the center of the nugget and perpendicular to the surface of the steel member, and t is the smaller of the plate thicknesses of the two adjacent steel members that form the boundary line.

12. The method for manufacturing a spot welded joint according to claim 6 or 7, characterized in that the surface treatment portion is formed by shot blasting or wet blasting.

13. Vickers hardness is 530 HV or more, A surface treatment portion is provided with no plating layer, The ten-point average roughness of the surface treatment area is 3.5 μm or less, The oxygen concentration of the surface treatment portion is 18.0 mass% or less, as measured by performing a surface EDX analysis at an accelerating voltage of 15 KV. High strength steel components for spot welded joints.

14. The high strength steel member for spot welded joint according to claim 13, characterized in that the carbon content is more than 0.30 mass%.

15. 15. The high strength steel member for a spot welded joint according to claim 13 or 14, wherein the surface treatment portion is provided over the entire surface of the high strength steel member.

16. The method includes the step of forming a surface treatment on a surface of a high-strength steel member having a Vickers hardness of 530 HV or more, No plating layer is provided on the surface treatment portion, The ten-point average roughness of the surface treatment area is 3.5 μm or less, The oxygen concentration of the surface treatment portion is 18.0 mass% or less, as measured by performing a surface EDX analysis at an accelerating voltage of 15 KV. A manufacturing method for high strength steel components for spot welded joints.

17. The method further comprises the step of hot stamping the high strength steel sheet before forming the surface treatment portion; In the step of forming the surface treatment, the surface treatment is formed on the surface of the hot stamped high strength steel member. The method for manufacturing a high strength steel component for a spot welded joint according to claim 16.

18. a step of removing a part of a plating layer on a surface of a high-strength steel member before forming the surface treatment part, In the step of forming the surface treatment portion, the surface treatment portion is formed in the area from which the plating layer has been removed.

18. A method for manufacturing a high strength steel member for a spot welded joint according to claim 16 or 17.

19. The method for manufacturing a high strength steel member for a spot welded joint according to claim 16 or 17, characterized in that the carbon content of the high strength steel member is more than 0.30 mass%.

20. The method for manufacturing a high strength steel member for a spot welded joint according to claim 16 or 17, characterized in that the surface treatment portion is provided over the entire surface of the high strength steel member.

21. The method for manufacturing a high strength steel member for a spot welded joint according to claim 16 or 17, characterized in that the surface treatment portion is formed by shot blasting or wet blasting.

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