Overlap welded joint, automotive skeletal member, and method for manufacturing overlap welded joint

Abstract

This lap welded joint comprises a plurality of steel plates that are partially or entirely overlapped, a spot welded portion joining the overlapped parts of two or more of the steel plates, and an arc welded portion disposed so as to temper a nugget of the spot welded portion, wherein: at least one of the spot welded steel plates is a high-strength steel plate having a tensile strength of at least 780 MPa; a part in which a measured value of a hardness of the nugget of the spot welded portion is smallest is located between a center of the nugget and the arc welded portion; and a difference between a minimum value of the measured value of the hardness of the nugget and a maximum value of the measured value of the hardness of the nugget is at least equal to 25 HV.

Description

Technical Field

[0001] The present invention relates to a lap welding joint, a skeletal member for an automobile, and a method for manufacturing a lap welding joint. This application claims priority based on Japanese Patent Application No. 2022-023284 filed in Japan on February 17, 2022, and incorporates the content herein by reference.

Background Art

[0002] For the purpose of weight reduction of automobiles and improvement of collision safety, the application of high-strength steel sheets to automobile parts has been promoted. However, spot welding joints composed of high-strength steel sheets have a problem that the joint strength is likely to decrease. Specifically, when the tensile strength of the base metal steel sheet is 780 MPa or more, the joint strength decreases.

[0003] Various techniques for improving the joint strength of welding joints composed of high-strength steel sheets have been provided so far.

[0004] Patent Document 1 discloses a method for manufacturing a laser-welded structural member, which includes overlapping one steel sheet having a bent portion and a flange following the bent portion with one or more other steel sheets at the flange, performing first laser welding on the overlapping portion to form a first laser-welded portion, and after the temperature of the first laser-welded portion drops below the Mf point, performing second laser welding on a region near the first laser-welded portion on the opposite side of the bent portion with respect to the formed first laser-welded portion to form a second laser-welded portion, and annealing the heat-affected zone of the first laser-welded portion by the second laser welding so that the hardness of the heat-affected zone is 90% or less of the hardness of the heat-affected zone of the second laser-welded portion.

[0005] Patent Document 2 discloses a method for manufacturing a welded joint including a first steel plate, a second steel plate stacked on the first steel plate, and a quenched nugget joining the first steel plate and the second steel plate. The method includes applying a first electrode to the first steel plate at a portion A which is an outer portion in the plate surface direction of the nugget in a plane parallel to the first steel plate of the welded joint, applying a second electrode to the second steel plate at a portion B which is an outer portion in the plate surface direction of the nugget in a plane parallel to the first steel plate of the welded joint and is located on the opposite side of the portion A with the nugget interposed therebetween, and passing an electric current through the welded joint between the first electrode and the second electrode.

[0006] Patent Document 3 discloses a welding method for a superposed portion where a plurality of steel plate members are joined at a superposed portion, and at least one of the plurality of steel plate members contains a martensite structure. The method includes a spot welding step of forming a spot welded portion having a nugget at the superposed portion, and a molten solidified portion forming step of irradiating a laser beam to form a molten solidified portion that crosses the end of the nugget between the nugget and a position 3 mm or more away from the end of the nugget outward, and forming the depth of the molten solidified portion to be 50% or more of the respective plate thicknesses of the steel plate members containing the martensite structure at a position 1 mm away from the end of the nugget outward.

[0007] Patent Document 4 discloses a method for manufacturing a spot welded joint using a plurality of steel plates including a steel plate having a tensile strength of 440 MPa or more. The method includes a first step of sandwiching the superposed plurality of steel plates with a pair of first electrodes and passing an electric current with a predetermined energization current I1 and energization time t1 to form a nugget, a second step of releasing the pair of first electrodes and cooling until the temperature of the nugget reaches below the Mf point, and a third step of sandwiching the portions of the plurality of steel plates corresponding to the nugget with a pair of second electrodes and passing an electric current with a predetermined energization current I3 and energization time t3 to temper the nugget. The energization conditions in the first step and the third step satisfy I3 2 ×t3 < I1 2The manufacturing method is disclosed, which is characterized by satisfying ×t1.

[0008] Patent Document 5 discloses a welding method in which a welded portion having a nugget portion is formed by spot-welding heat-treated steel plates stacked together, and a high-frequency current is passed through the welded portion to temper it so that the hardness of the nugget portion is equal to or lower than that of the heat-treated steel plate.

[0009] Patent Document 6 discloses a spot welding method for high-strength thin steel plates, in which when spot-welding two or more stacked high-strength thin steel plates while sandwiching them with a pair of electrodes and applying a clamping pressure, after welding the first point, the position of the electrode is moved, and after the welded portion of the first point is cooled to a temperature below the Mf point, the second point is welded so as to partially overlap the welded portion of the first point.

[0010] Patent Document 7 discloses a welding method for a metal plate, in which after temporarily fixing a metal plate stacked on a base material to the base material by spot-welding the ends of the metal plate along the edges of the ends at substantially equal intervals, when fillet-welding the edge of the metal plate, the spot-welding is performed with a nugget diameter of 2 mm or more, the center of the nugget being 5 to 20 mm from the edge and the interval being 20 mm or less.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Summary of the Invention

Problems to be Solved by the Invention

[0012] However, in recent years, the requirements for the joint strength of welded joints have been increasing more and more. An even further improvement in the joint strength of welded joints by means different from Patent Documents 1 to 7 is earnestly desired.

[0013] An object of the present invention is to provide a lap welded joint composed of a plurality of steel plates including high-strength steel plates and having high joint strength, a skeletal member for an automobile, and a method for manufacturing the lap welded joint.

Means for Solving the Problems

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

[0015] (1) The lap welding joint according to one aspect of the present invention includes a plurality of steel plates that are partially or entirely overlapped, a spot welding portion that joins two or more of the steel plates, and an arc welding bead formed on one or more of the steel plates. The lap welding joint is such that the spot welding portion is disposed at an overlapping portion where the plurality of steel plates are overlapped in the lap welding joint, the arc welding bead is formed on one or more of the steel plates on which the spot welding portion is formed, one or more of the steel plates joined by the spot welding portion are high-strength steel plates with a tensile strength of 780 MPa or more, and when viewed in plan from the thickness direction of the lap welding joint, a straight line connecting the center of the indentation of the spot welding portion and the point on the edge of the arc welding bead closest to the center of the indentation is included, and in a cross-section perpendicular to the overlapping portion of the steel plates, when the Vickers hardness at a depth of 1 / 4 of the plate thickness of the high-strength steel plate from the spot welding surface of the high-strength steel plate is continuously measured along the spot welding surface, the portion where the hardness measurement value of the nugget of the spot welding portion is minimized is between the center of the nugget and the arc welding bead, and the difference between the minimum value of the hardness measurement value of the nugget and the maximum value of the hardness measurement value of the nugget is 25 HV or more. (2) In the lap welding joint described in (1) above, in the cross-section, the difference between the minimum value of the hardness measurement value of the nugget and the maximum value of the hardness measurement value of the nugget may be 40 HV or more. (3) In the lap welding joint described in (1) or (2) above, two or more of the steel plates may be joined by both the spot welding portion and the arc welding bead. (4) In the lap welding joint described in any one of (1) to (3) above, when viewed in plan from the thickness direction of the lap welding joint, the shortest distance between the center of the indentation of the spot welding portion and the edge of the arc welding bead may be 17 mm or less. (5) In the lap welding joint described in any one of (1) to (4) above, when viewed in plan from the thickness direction of the lap welding joint, the interval between the edge of the indentation of the spot welding portion and the edge of the arc welding bead may be more than 0 mm. (6) In the lap welding joint according to any one of (1) to (5) above, the width of the arc welding bead may be 3 mm or more. (7) In the lap welding joint according to any one of (1) to (6) above, the number of steel plates is 3 or more, one or more of the steel plates are outside the spot welding portion, and the steel plate outside the spot welding portion and the steel plate joined by the spot welding portion may be joined by the arc welding bead. (8) In the lap welding joint according to any one of (1) to (7) above, the tensile strength of the high-strength steel plate may be 1700 MPa or more.

[0016] (9) The vehicle skeletal member according to another aspect of the present invention has the lap welding joint according to any one of (1) to (8) above.

[0017] (10) The method for manufacturing a lap welding joint according to another aspect of the present invention is a method for manufacturing a lap welding joint for manufacturing the lap welding joint according to any one of (1) to (8) above, the method comprising: a step of overlapping a part or all of a plurality of steel plates; a step of spot welding the overlapping portion of the steel plates to form a spot welding portion; and a step of arc welding one or more of the steel plates to form an arc welding bead. In the method for manufacturing a lap welding joint, one or more of the steel plates to be spot welded are high-strength steel plates having a tensile strength of 780 MPa or more, and the nugget of the spot welding portion is tempered by the welding heat of the arc welding. (11) In the method for manufacturing a lap welding joint according to (10) above, when viewed in plan from the thickness direction of the lap welding joint, the distance between the edge of the indentation of the spot welding portion and the edge of the arc welding bead may be more than 0 mm. (12) In the method for manufacturing a lap welding joint according to (10) or (11) above, the heat input of the arc welding may be 1000 J / cm or more, and when viewed in plan from the thickness direction of the lap welding joint, the shortest distance between the center of the indentation of the spot welding portion and the edge of the arc welding bead may be 17 mm or less. (13) In the method for manufacturing a lap-welded joint according to any one of (10) to (12) above, the method for manufacturing the lap-welded joint may further include a step of adding one or more steel plates to the two or more steel plates spot-welded before the arc welding, and joining the two or more steel plates spot-welded and the added steel plates by the arc welding. (14) In the method for manufacturing a lap-welded joint according to any one of (10) to (13) above, the tensile strength of the high-strength steel plate may be 1700 MPa or more.

Effects of the Invention

[0018] According to the present invention, it is possible to provide a lap-welded joint composed of a plurality of steel plates including a high-strength steel plate and having a high joint strength, a vehicle body frame member, and a method for manufacturing a lap-welded joint.

Brief Description of the Drawings

[0019]

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MODE FOR CARRYING OUT THE INVENTION

[0020] As shown in the plan view of FIG. 1A and the cross-sectional view of FIG. 1B, the overlap welding joint 1 according to the first embodiment of the present invention includes a plurality of steel plates 11 that are partially or entirely overlapped, a spot welding portion 12 that joins two or more steel plates 11, and an arc welding portion 13 formed on one or more of the steel plates. Here, the overlapping portion 111 is a portion where two or more steel plates 11 are overlapped in the overlap welding joint 1. That is, the overlapping portion 111 has a laminated structure composed of two or more steel plates 11, and the direction in which the surface of the overlapping portion 111 extends is generally parallel to the direction in which the steel plate 11 extends at the portion where the steel plates 11 are overlapped, and is generally perpendicular to the thickness direction of the steel plate 11 at the portion where the steel plates 11 are overlapped. The two or more steel plates 11 do not need to be overlapped at portions other than the overlapping portion 111, for example, as shown in FIGS. 3 and 4. In the overlapping portion 111, the two or more overlapped steel plates 11 are joined by spot welding, and the spot welding portion 12 is formed. The arc welding portion 13 is formed on at least one of the steel plates 11 on which the spot welding portion 12 is formed. One or more of the spot-welded steel plates 11 are high-strength steel plates 11H having a tensile strength of 780 MPa or more. Further, when viewed in plan from the thickness direction of the overlap welding joint 1, a straight line connecting the center C of the indentation 121 of the spot welding portion 12 and the point P closest to the center C of the indentation 121 at the edge of the arc welding portion 13 (for example, IB-IB in FIG. 1A and IIB-IIB in FIG. 2A) is included, and in a cross-section perpendicular to the overlapping portion 111 of the steel plate 11, when the Vickers hardness at the position L at a depth of 1 / 4 of the plate thickness t of the high-strength steel plate 11H is continuously measured along the spot welding surface 11HS of the high-strength steel plate 11H, the portion where the hardness measurement value of the nugget 122 of the spot welding portion 12 is minimized is between the center of the nugget 122 and the arc welding portion 13. Further, the difference between the minimum value of the hardness measurement value of the nugget 122 and the maximum value of the hardness measurement value of the nugget 122 is 25 HV or more. Hereinafter, the overlap welding joint 1 according to the present embodiment will be described in detail.

[0021] The lap welding joint 1 according to this embodiment has a plurality of steel plates 11. Two or more of the plurality of steel plates 11 are partially or entirely overlapped, and the overlapping portion 111 of the lap welding joint 1 is joined by the spot welding portion 12. The overlapping portion 111 is the portion where the steel plates 11 are overlapped. In the configuration illustrated in FIG. 1B, the upper plate is a press-formed member having a flange portion, and a flange portion, which is a part of the upper plate, is overlapped with the lower plate, and the overlapping portion 111 is joined by the spot welding portion 12. On the other hand, both the upper plate and the lower plate have a flat plate shape, and the whole of them may be lap-welded.

[0022] One or more of the steel plates 11 joined by the spot welding portion 12 are high-strength steel plates 11H having a tensile strength of 780 MPa or more. Thereby, the strength of the mechanical parts to which the lap welding joint 1 according to this embodiment is applied can be improved. In such a high-strength steel plate 11H, there is a problem that the joint strength of the spot welding joint decreases. However, in the lap welding joint 1 according to this embodiment, this problem is addressed by tempering the nugget 122 using the heat input when forming the arc welding portion 13 described later.

[0023] In the lap welding joint 1 according to this embodiment, the spot welding portion 12 means the nugget 122 and its peripheral portion. Specifically, the spot welding portion 12 is a concept including the nugget 122, which is the welding metal, and the indentations 121 formed above and below the nugget 122. Strictly speaking, it is the nugget 122 of the spot welding portion 12 that joins the steel plates 11.

[0024] The configuration of the spot welding portion 12 is not particularly limited and can be appropriately changed according to the type of the steel plate 11. Generally, the nugget diameter of the joint portion of the automotive skeletal member is within the range of 3√t to 7√t. "t" is the thickness of the thinner one of the two steel plates 11 arranged on the surface of the overlapping portion 111 joined by the spot welding portion 12. This nugget diameter may be applied to the spot welding portion 12 of the fillet weld joint 1 according to the present embodiment. From the viewpoints of ensuring joint strength and preventing hydrogen embrittlement, more preferably, the nugget diameter is within the range of 4√t to 7√t.

[0025] The tensile strength of the high-strength steel plate 11H is preferably 980 MPa or more, 1300 MPa or more, 1500 MPa or more, 1700 MPa or more, or 1900 MPa or more. The upper limit value of the tensile strength of the high-strength steel plate 11H is not particularly defined, but for example, the tensile strength may be 2700 MPa or less, 2600 MPa or less, or 2500 MPa or less. In the fillet weld joint 1 illustrated in FIGS. 1A and 1B, the high-strength steel plate 11H and the low-strength steel plate 11 having a tensile strength of less than 780 MPa are joined by the spot welding portion 12. On the other hand, two or more high-strength steel plates 11H may be joined by the spot welding portion 12.

[0026] The type of the high-strength steel plate 11H is not particularly limited. Examples of the high-strength steel plate 11H include DP steel plates, TRIP steel plates, duplex structure steel plates, martensite steel plates, and hot-stamped steel plates. Further, the high-strength steel plate 11H may be a cold-rolled steel plate or a hot-rolled steel plate.

[0027] The high-strength steel plate 11H with a tensile strength of 780 MPa or more and the low-strength steel plate with a tensile strength of less than 780 MPa that may be included in the lap welding joint 1 according to this embodiment may be a plated steel plate or a non-plated steel plate. Examples of the plated steel plate include GI plated steel plate, GA plated steel plate, EG plated steel plate, Zn-Ni plated steel plate, Zn-Al plated steel plate, Zn-Mg plated steel plate, and Zn-Mg-Al plated steel plate. When a zinc-based hot-stamped steel plate is included in the lap welding joint 1, zinc oxide may be included in the surface layer of the Fe-Zn or Fe-Zn-Ni solid solution phase. When an aluminum-based hot-stamped steel plate is included in the lap welding joint 1, a plurality of intermetallic compound layers of the Al-Fe-Si system may be formed, and further, ZnO or a black film may be formed on the intermetallic compound layer. When a non-plated hot-stamped steel plate is included in the lap welding joint 1, in order to remove the scale generated in the hot stamping process, shot-blasted ones may be used.

[0028] There is no particular limitation on the plate thickness of the high-strength steel plate 11H. Generally, the plate thickness of the steel plate used for automobile parts or the vehicle body is 0.6 to 3.2 mm. This plate thickness may be applied to the high-strength steel plate 11H of the lap welding joint 1 according to this embodiment. Also, the number of stacked steel plates 11 included in the overlapping portion 111 joined by the spot weld portion 12 is preferably in the range of, for example, 2 to 4 sheets. The steel plate 11 spot-welded to the high-strength steel plate 11H may be a high-strength steel plate 11H or a low-strength steel plate.

[0029] There is a problem that the joint strength is low in the spot weld portion obtained by spot-welding high-strength steel plates. This is considered to be because the nugget of the spot weld portion is embrittled by a large amount of C contained in the high-strength steel plate. Therefore, the inventors arranged the arc weld portion 13 in the lap welding joint 1 so as to temper the nugget 122 of the spot weld portion 12 by the heat input when forming the arc weld portion 13.

[0030] The arc welding portion 13 is formed by arc welding. Arc welding has a larger heat input compared to spot welding. Therefore, deformation of the steel plate 11 may occur around the arc welding portion 13. In addition, a heat-affected zone is formed over a wide range around the arc welding portion 13. This heat-affected zone may also reduce the strength of the steel plate 11. In addition, the working efficiency of arc welding is lower than that of spot welding. For the above reasons, in the manufacture of automobile bodies, spot welding is exclusively used for joining the high-strength steel plate 11H. Arc welding is usually limited to use in locations where spot welding cannot be performed due to reasons such as the inability to arrange electrodes for spot welding.

[0031] However, the inventors have found that by arranging the arc welding portion 13 so that the heat input during the formation of the arc welding portion 13 temper the nugget 122 of the spot welding portion 12, the joint strength of the spot welding portion 12 is dramatically improved. When the inventors examined in detail the spot welding portion 12 with improved joint strength, they found that the heat input during arc welding tempered the nugget 122, and the nugget 122 was softened. The inventors considered that the softening of the nugget 122 improved the brittleness of the nugget 122 and thus improved the joint strength. For the above reasons, the overlapping weld joint 1 according to the present embodiment has an arc welding portion 13 arranged to temper the nugget 122 of the spot welding portion 12. In the present embodiment, in order to surely transfer the heat input during the formation of the arc welding portion 13 to the nugget 122, the arc welding portion 13 is formed on at least one of the steel plates 11 on which the spot welding portion 12 is formed (i.e., on which the nugget 122 is formed).

[0032] In general, the "welded part" means a part that joins two or more materials. However, as shown in FIGS. 1A and 1B, in the lap-welded joint 1 according to the present embodiment, the arc-welded part 13 does not necessarily need to join two or more steel plates 11. That is, the arc-welded part 13 may be an arc-welded bead that does not join two or more workpieces to be welded. The main purpose of forming the arc-welded part 13 is to normalize and soften the nugget 122. The arc-welded part 13 is provided to improve the joint strength of the spot-welded part 12 without joining two or more steel plates 11.

[0033] On the other hand, as shown in FIGS. 2A and 2B, the arc-welded bead may be formed so as to connect two or more steel plates 11, and the arc-welded part 13 may join two or more steel plates 11. That is, two or more of the plurality of steel plates 11 included in the lap-welded joint 1 may be joined by both the spot-welded part 12 and the arc-welded part 13. In this case, the arc-welded part 13 improves the joint strength of the spot-welded part 12 through improving the toughness of the nugget 122, and also improves the joint strength of the lap-welded joint 1 itself.

[0034] As described above, in the lap-welded joint 1 according to the present embodiment, the nugget 122 is normalized by the heat input when forming the arc-welded part 13, and the hardness of the nugget 122 is reduced. The hardness of the nugget 122 is evaluated by the following procedure. (1) Identify the center C of the indentation 121 of the spot weld 12. The indentation 121 is a depression formed by the electrode being pressed into the steel plate 11 during spot welding. When viewed in plan from the thickness direction of the lap weld joint 1, the indentation 121 is usually substantially circular. If the shape of the indentation 121 is not circular, then among the indentations 121, the most recessed part is identified as the center C of the indentation 121. The shape of the indentation 121 and its center can be easily determined by visually observing the overlapping portion 111 of the lap weld joint 1 from the thickness direction of the lap weld joint 1. As illustrated in FIGS. 20 and 21 described later, when another steel plate 11 or the like is provided so as to cover the indentation 121, it may be removed and then the indentation 121 may be observed. Also, in the examples of FIGS. 20 and 21, the indentation 121 can be visually recognized on the lower surface of the lap weld joint 1. In the spot weld 12, the indentations 121 are formed above and below the nugget 122. The centers C of these indentations 121 usually coincide when the lap weld joint 1 is viewed in plan from the thickness direction. Therefore, the indentation 121 may be observed on the lower surface of the lap weld joint 1 and its center C may be identified. (2) Identify the point P closest to the center C of the indentation 121 on the edge of the arc weld 13. The edge of the arc weld 13 refers to the outer periphery of the so-called arc weld bead. The edge of the arc weld 13 can be easily determined by visually observing the arc weld 13 from the thickness direction of the lap weld joint 1. When another steel plate 11 or the like is provided so as to cover the arc weld 13, it may be removed and then the arc weld 13 may be observed. (3) Draw a straight line connecting the center C of the indentation 121 and the point P closest to the center C of the indentation 121 on the edge of the arc weld 13. For example, the dashed line IB-IB in FIG. 1A and the dashed line IIB-IIB in FIG. 2A are the said straight lines. The said straight line is drawn on the surface of the overlapping portion 111. If the said straight line can be identified, it is not necessarily required to draw the said straight line. (4) Cut the overlapping portion 111 of the steel plate 11 along the straight line. As a result, a cross-section that includes the straight line and is perpendicular to the overlapping portion 111 of the steel plate 11 (in other words, a cross-section parallel to the thickness direction of the fillet weld joint 1) is formed. For example, the cross-sectional views IB-IB in FIG. 1B and IIB-IIB in FIG. 2B are such cross-sections. If necessary, the cross-section may be polished to an extent that hardness measurement is possible. (5) Measure the hardness of the portion of the nugget 122 included in the high-strength steel plate 11H in the cross-section. Specifically, measure the Vickers hardness of the nugget 122 at a depth of 1 / 4 of the plate thickness t of the high-strength steel plate 11H continuously along the spot weld surface 11HS of the high-strength steel plate. Here, "measuring continuously" means, for example, measuring at a measurement pitch of 1 mm or less. The broken line L shown in FIGS. 1B and 2B is the hardness measurement position L. It is also preferable to measure the hardness of the peripheral portion of the nugget 122 at this time. Note that the spot weld surface 11HS is the surface of the two surfaces of the high-strength steel plate 11H that is joined to the other steel plate 11 by the nugget 122 of the spot weld portion 12. The direction in which the spot weld surface 11HS extends in the cross-section coincides with the direction in which the overlapping portion 111 extends in the cross-section. The hardness measurement conditions may be a measurement load of 500 gf or 1000 gf. Since substantially the same value can be obtained under any condition, the measurement conditions according to the shape of the measurement portion, etc. can be appropriately adopted.

[0035] Note that depending on the combination of the steel plates 11 in the overlapping portion 111, there may be two or more spot welding surfaces 11HS of the high-strength steel plate 11H. For example, when three steel plates 11 are spot welded in the overlapping portion 111 and the high-strength steel plate 11H is located at the center of the plate stack, in the lap-welded joint 1, the number of spot welding surfaces 11HS of the high-strength steel plate 11H is two. In such a case, two or more hardness measurement positions L may be set based on each of the two or more spot welding surfaces 11HS, and hardness measurement may be performed in accordance with the above-described procedure at each position. If the requirements described below are satisfied at at least one of the plurality of hardness measurement positions L, the lap-welded joint 1 according to the present embodiment is regarded as such. Further, it is preferable that the requirements described below are satisfied at all of the plurality of hardness measurement positions L.

[0036] When measuring the hardness of the nugget 122 and its peripheral portion in accordance with the above procedures (1) to (5), a hardness curve as shown in FIGS. 32 and 33, for example, is obtained. The hardness curve of the normal spot welding portion 12 where the arc welding portion 13 is not provided usually becomes like the graph of "without arc welding" in the upper left stage of FIG. 32. In the hardness curve in the upper left stage of FIG. 32, significant softening portions are observed in the heat-affected zones at both ends of the nugget 122, while the hardness inside the nugget 122 is substantially uniform. On the other hand, the hardness curve of the spot welding portion 12 according to the present embodiment in which the arc welding portion 13 is provided becomes, for example, like "4 mm" in the upper right stage of FIG. 32. The hardness inside the nugget 122 becomes lower as it is closer to the arc welding portion 13. This is because the highest temperature reached during arc welding is higher as it is closer to the arc welding portion 13, and thus the tempering temperature is higher, and the degree of softening due to tempering becomes larger.

[0037] When annealing the armature 122 using the arc welding portion 13, the portion where the hardness measurement value of the armature 122 is minimized, that is, the armature softening portion, is formed between the center of the armature 122 and the arc welding portion 13 (i.e., on the arc welding portion 13 side from the center of the armature 122) inside the armature 122. The center of the armature 122 usually coincides with the center C of the indentation 121 of the spot welding portion 12 when viewed in a plan view from the thickness direction of the fillet weld joint 1. As a method of annealing the armature 122, high-frequency heating or the like is known. Usually, when annealing the armature 122, the armature 122 is annealed evenly and the armature softening portion is not formed. On the other hand, when annealing the armature 122 using the arc welding portion 13, the annealing temperature becomes higher in the place closer to the arc welding portion 13. Therefore, in this case, the armature 122 is annealed unevenly. The fact that the armature softening portion is formed between the center of the armature 122 and the arc welding portion 13 means that the armature 122 is annealed by the welding heat when forming the arc welding portion 13. When the armature 122 is annealed by the heat input when forming the arc welding portion 13, the armature softening portion is often formed at the end of the armature 122. On the other hand, when the arc welding portion 13 is formed so that the distance between the armature 122 and the arc welding portion 13 becomes very small, for example, when the arc welding portion 13 is formed so that the spot welding portion 12 and the arc welding portion 13 overlap, the end portion of the armature 122 is re-quenched during arc welding, and the armature softening portion may be formed at a location away from the end of the armature 122.

[0038] By creating the above hardness curve, the minimum value (hardness measurement value at the softest part of the nugget 122) and the maximum value of the hardness measurement values of the nugget 122 can be specified. In the lap joint 1 according to the present embodiment, the difference ΔHV between the minimum value and the maximum value of the hardness measurement values of the nugget 122 is 25 HV or more. When ΔHV is less than 25 HV, the brittleness of the nugget 122 cannot be sufficiently improved, and the joint strength of the lap joint 1 does not sufficiently increase. The larger ΔHV is, the more preferable it is. For example, it may be 30 HV or more, 35 HV or more, 40 HV or more, or 50 HV or more. Although the upper limit value of ΔHV is not particularly defined, for example, it may be 270 HV or less, 220 HV or less, or 170 HV or less. When a part of the nugget 122 overlaps with the arc welding metal in the cross section, the shape of the nugget 122 before arc welding may be estimated based on the shape of another part of the nugget 122.

[0039] As long as ΔHV is 25 HV or more, the distance between the spot weld portion 12 and the arc weld portion 13 is not particularly limited. According to the results of the experiments by the present inventors, for example, when viewed in plan from the thickness direction of the lap joint 1, if the shortest distance between the center C of the indentation 121 of the spot weld portion 12 and the edge of the arc weld portion 13 is 17 mm or less, 15 mm or less, or 13 mm or less, it is possible to sufficiently anneal the nugget 122 and further expand ΔHV. However, if the heat input during arc welding is increased, even if the shortest distance between the center C of the indentation 121 of the spot weld portion 12 and the edge of the arc weld portion 13 exceeds 17 mm, it was possible to make ΔHV 25 HV or more.

[0040] Even if the distance between the spot welding part 12 and the arc welding part 13 is close, it is possible to make ΔHV 25 HV or more. For example, on the right side of the overlap welding joint 1 shown in the photograph of FIG. 30, a state where the indentation 121 of the spot welding part 12 and the arc welding part 13 overlap is shown. However, even in this spot welding part 12, ΔHV could be made 25 HV or more. When this overlap welding joint 1 was cut and observed in cross section, the nugget 122 and the arc welding part 13 did not overlap. On the other hand, when a slight gap was provided between the spot welding part 12 and the arc welding part 13, there was a tendency for ΔHV to increase more easily. Therefore, for example, the distance between the indentation 121 of the spot welding part 12 and the edge of the arc welding part 13 may be more than 0 mm, 1 mm or more, 2 mm or more, or 4 mm or more.

[0041] There is no particular limitation on the size of the arc welding part 13 either. On the other hand, according to the experimental results of the present inventors, the width of the arc welding part 13 is preferably 3 mm or more. The larger the width of the arc welding part 13, the larger the heat input amount during arc welding, and the nugget 122 of the spot welding part 12 can be sufficiently annealed. The width of the arc welding part 13 may be 4 mm or more, 5 mm or more, or 6 mm or more. There is no need to define the upper limit value of the width of the arc welding part 13, but for example, the width of the arc welding part 13 may be 15 mm or less. Note that the "width of the arc welding part 13" can be the length in a direction perpendicular to the one direction (longitudinal direction) when the arc welding bead is long in one direction, for example, when it extends linearly, and perpendicular to the thickness direction of the overlap welding joint 1.

[0042] As described above, the number of steel plates 11 of the overlap welding joint 1 according to the present embodiment is not limited. Also, the shape of the steel plate 11 is not particularly limited. Therefore, the overlap welding joint 1 according to the present embodiment can have various shapes. Hereinafter, suitable examples will be described. In the examples listed below, any of the plurality of steel plates 11 may be the high-strength steel plate 11H. Therefore, in the drawings corresponding to the examples listed below, for convenience, the reference numerals of all the steel plates 11 are "11".

[0043] The arc welding portion 13 shown in FIG. 2B is an overlapping fillet welding portion that joins the surface of one steel plate 11 and the end face of the other steel plate 11. On the other hand, as shown in FIG. 3, one of the plurality of steel plates 11 may have a bent portion, and the arc welding portion 13 may be a fillet welding portion that joins the surface of one steel plate 11 and the surface of the bent portion of the other steel plate 11. As shown in FIG. 4, the end faces of two steel plates 11 arranged on substantially the same plane may be joined by the arc welding portion 13.

[0044] Holes for arc welding may be provided in the steel plate 11. For example, as shown in the plan view of FIG. 5A and the cross-sectional view of FIG. 5B, holes are provided in the steel plate 11, and a welded portion obtained by overlapping fillet welding the surface of the steel plate 11 in contact with this steel plate 11 and the inner end face of the hole in this steel plate 11 may be used as the arc welding portion 13 of the overlapping weld joint 1 according to the present embodiment.

[0045] The structure of a joggled joint may be applied to the arc welding portion 13. A joggled lap joint is a welded joint in which one member of an overlapping joint is stepped so that the base metal surfaces are substantially flush, as described in JIS Z 3001-1:2018. FIGS. 6 and 7 show examples of cross-sections of the arc welding portion 13 to which the joggled structure is applied. In the joggled structure shown in FIG. 6, there is a gap between the arc welding portion 13 and the step provided in the steel plate 11. In the joggled structure shown in FIG. 7, there is no gap between the arc welding portion 13 and the step provided in the steel plate 11. That is, in the joggled structure shown in FIG. 7, the step of the steel plate 11 and the end face of the steel plate 11 overlapped on this steel plate 11 are joined. In any case of adopting the joggled structure of FIGS. 6 and 7, the effect of making the stress flow straight can be obtained. As a result, the merit of improving the energy transfer efficiency at the time of collision and improving the static strength and fatigue strength of the joint can be obtained.

[0046] The arc welding portion 13 may be an arc spot welding portion 13. The arc spot welding portion 13 is a welded portion obtained by spot welding using arc welding. An example of the arc spot welding portion 13 is shown in the plan view of FIG. 8A and the cross-sectional view of FIG. 8B. Note that the arc spot welding portion 13 illustrated in FIGS. 8A and 8B does not join two steel plates 11, but can exhibit the effect of improving the joint strength by tempering the spot welding portion 12. On the other hand, an arc spot welding portion 13 that joins two steel plates 11 may be formed by previously drilling a hole in one steel plate 11 and then performing arc welding so as to transfer a filler metal to the hole.

[0047] In the example of the lap welding joint 1 described above, the direction in which the spot welding portion 12 and the arc welding portion 13 are arranged is perpendicular to the extending direction of the end portion of the steel plate 11. However, of course, the angle formed by the direction in which the spot welding portion 12 and the arc welding portion 13 are arranged and the extending direction of the end portion of the steel plate 11 is not limited. Further, the nugget 122 of one spot welding portion 12 may be tempered by the heat input when forming two or more arc welding portions 13. An example of such a combination of configurations is the lap welding joint 1 in which the arc welding portion 13 is arranged between a plurality of spot welding portions 12, as shown in FIG. 9. Even in the arrangement as shown in FIG. 9, the spot welding portion 12 can be tempered during arc welding, and ΔHV can be 25 HV or more.

[0048] In addition, when the nugget 122 of one spot weld 12 is tempered by the heat input during the formation of a plurality of arc welds 13, the cross section for measuring the hardness of the nugget 122 of the spot weld 12 may be formed along a straight line connecting the center C of the indentation 121 of the spot weld 12 and the point P closest to the center C of the indentation 121 at the edge of the plurality of arc welds 13. Regarding this matter, the lower one of the two spot welds 12 in FIG. 9 will be described as an example. Two arc welds 13A and 13B are arranged adjacent to the upper left and lower left of this spot weld 12, respectively. In the upper arc weld 13A, the point P1 closest to the center C of the nugget 122 of the spot weld 12 is at the lower right of the arc weld 13A. In the lower arc weld 13B, the point P2 closest to the center C of the nugget 122 of the spot weld 12 is at the upper right of the arc weld 13B. The distance between C and P1 is smaller than the distance between C and P2. In this case, the overlapping portion 111 may be cut along the straight line connecting C and P1, and the hardness of the nugget 122 may be measured.

[0049] In the example of the lap weld joint 1 described above, the end of the steel plate 11 extended linearly. On the other hand, the shape of the end of the steel plate 11 can be changed variously. An example of the shape of the end is a wave shape. FIGS. 10 and 11 show examples of a plan view of the lap weld joint 1 in which the end of the steel plate 11 is wave-shaped. In any of the lap weld joints 1 in FIGS. 10 and 11, the end of the steel plate 11 is wave-shaped composed of convex portions and concave portions, and the spot welds 12 are arranged inside the convex portions. In the lap weld joint 1 in FIG. 10, the arc welds 13 are provided along the ends of the convex portions, and in the lap weld joint 1 in FIG. 11, the arc welds 13 are provided along the ends of the concave portions. In any configuration, the weight of the steel plate 11 can be reduced. Also, in any configuration, an effect of improving the joint strength can be obtained.

[0050] In the example of the lap weld joint 1 described above, the number of steel plates 11 was two. On the other hand, the number of steel plates 11 may be three or more. Hereinafter, an example of the lap weld joint 1 having three or more steel plates 11 will be described.

[0051] Figure 12 shows an example in which the overlapping portion 111 of three steel plates 11 is joined by one spot welding portion 12, and further, two steel plates 11 are joined by one fillet arc welding portion 13. In this example, the steel plates 11 not joined by the arc welding portion 13 are firmly joined to the adjacent steel plates 11 by the spot welding portion 12 tempered by the arc welding portion 13.

[0052] Figure 13 shows an example in which the overlapping portion 111 of three steel plates 11 is joined by one spot welding portion 12, and further, three steel plates 11 are joined by one fillet arc welding portion 13. In this example, all the steel plates 11 are firmly joined by two types of welding portions.

[0053] Figure 14 shows an example in which the overlapping portion 111 of three steel plates 11 is joined by one spot welding portion 12, and further, three steel plates 11 are joined using two fillet arc welding portions 13. Specifically, in this example, the steel plate 11 arranged on one surface of the lap joint 1 and the central steel plate 11 are joined using one of the two arc welding portions 13, and the steel plate 11 arranged on the other surface of the lap joint 1 and the central steel plate 11 are joined using the other of the two arc welding portions 13. In this example, all the steel plates 11 are firmly joined using three welding portions. Also in this example, the cross section for hardness evaluation of the nugget 122 is formed based on the arc welding portion 13 closer to the nugget 122.

[0054] Figure 15 shows an example in which the holes illustrated in Fig. 5B are applied to one steel plate 11 in the example shown in Fig. 14. In this example, arc welding holes are provided in the central one of the three overlapping steel plates 11. And fillet arc welding portions 13 are arranged on the inner end surfaces of the holes.

[0055] Figures 16A and 16B also show an example in which the overlapping portion 111 of three steel plates 11 is joined by one spot welding portion 12 and holes are used for arc welding of the three steel plates 11. FIG. 16A is a plan view of the lap welding joint 1, and FIG. 16B is a cross-sectional view taken along the dashed-dotted line shown in FIG. 16A. In the example shown in FIGS. 16A and 16B, among the three steel plates 11, elongated holes are provided in those facing one surface of the lap welding joint 1 and those arranged in the center, and these elongated holes are overlapped. Among the three steel plates 11, those facing the other surface of the lap welding joint 1 are not provided with holes. And an arc welding portion 13 is arranged so as to transfer the filler metal to the entire inside of the two overlapped elongated holes. The spot welding portion 12 is arranged side by side with the arc welding portion 13 along the extending direction of the elongated hole.

[0056] Figures 17A, 17B, and 17C also show an example in which the overlapping portion 111 of three steel plates 11 is joined by one spot welding portion 12 and holes are used for arc welding of the three steel plates 11. FIG. 17A is a plan view of the lap welding joint 1, FIG. 17B is a cross-sectional view taken along the dashed-dotted line XVIIB-XVIIB shown in FIG. 17A, and FIG. 17C is a cross-sectional view taken along the dashed-dotted line XVIIC-XVIIC. In this example, the spot welding portion 12 is arranged side by side with the arc welding portion 13 along the direction perpendicular to the extending direction of the elongated hole. Also, in this example, the arc welding portion 13 is provided only in a part of the elongated hole. Regarding other configurations, this example is the same as FIGS. 16A and 16B.

[0057] In the examples of FIGS. 12 to 17C, all of the plurality of steel plates 11 included in the lap welding joint 1 were overlapped in part and joined by the spot welding portion 12. However, only a part of the steel plates 11 included in the lap welding joint 1 may be joined by the spot welding portion 12. In this case, the steel plates 11 that are not joined by the spot welding portion 12 and are outside thereof may be joined using the arc welding portion 13.

[0058] FIG. 18 shows an example in which only two of the three steel plates 11 are spot welded. The steel plate 11 that is not spot welded and the steel plate 11 that is spot welded are joined by the fillet arc weld portion 13.

[0059] FIG. 19 also shows an example in which only two of the three steel plates 11 are spot welded. The steel plate 11 that is not spot welded and the steel plate 11 that is spot welded are joined by the T-fillet arc weld portion 13. Here, the end face of the steel plate 11 that is not spot welded abuts against the surface of the steel plate 11 that is spot welded.

[0060] FIG. 20 also shows an example in which only two of the three steel plates 11 are spot welded. A hole is provided in the steel plate 11 that is not spot welded, and the inner end face of the hole in the steel plate 11 that is not spot welded and the surface of the steel plate 11 that is spot welded are joined by the fillet arc weld portion 13.

[0061] FIG. 21 also shows an example in which only two of the three steel plates 11 are spot welded. A hole is provided in the steel plate 11 that is not spot welded, and the steel plate 11 that is not spot welded and the steel plate 11 that is spot welded are joined by the arc spot weld portion 13 shown in FIGS. 8A and 8B.

[0062] In the examples of FIGS. 20 and 21, the indentation 121 of the spot weld portion 12 is covered by the arc welded steel plate 11. However, when evaluating the hardness of the nugget 122, the center C of the indentation 121 can be specified by removing the arc welded steel plate 11. Also, in the examples of FIGS. 20 and 21, the indentation 121 can be visually recognized on the lower surface of the lap weld joint 1. In the spot weld portion 12, indentations 121 are formed above and below the nugget 122, and the centers C of these indentations 121 usually coincide when the lap weld joint 1 is viewed in plan from the thickness direction. Therefore, the indentation 121 may be observed on the lower surface of the lap weld joint 1 and its center may be specified.

[0063] FIG. 22 also shows an example in which only two of the three steel plates 11 are spot welded. The steel plate 11 that is not spot welded and the steel plate 11 that is spot welded are joined by a T-joint fillet arc weld 13. However, unlike the example of FIG. 19, in the example of FIG. 22, the end face of the steel plate 11 that is spot welded abuts against the surface of the steel plate 11 that is not spot welded.

[0064] Note that the example of FIG. 22 includes three spot welds 12. Of these, the right spot weld 12 and the center spot weld 12 are located near the arc weld 13, but are spaced far from the arc weld 13. In this case, the right spot weld 12 and the center spot weld 12 may not be fully annealed during arc welding and may not meet the above-mentioned requirements for nugget hardness. However, even in such a case, if the left spot weld 12 meets the above-mentioned requirements for nugget hardness, the example shown in FIG. 22 is regarded as the overlap weld joint 1 according to this embodiment. When a plurality of spot welds 12 are provided in the overlap joint 1, it is not necessary for all of them to meet the above-mentioned requirements for nugget hardness. If the above-mentioned requirements for nugget hardness are met only at the locations where joint strength is particularly required in the overlap weld joint 1, sufficient joint strength can be imparted to the overlap weld joint 1.

[0065] As described above, various exemplary forms of the overlap weld joint 1 according to this embodiment have been described, but the present invention is not limited to these examples. It is also possible to appropriately combine the above-described examples, and it is also possible to apply a well-known joint structure not described above to the overlap weld joint 1 according to this embodiment.

[0066] Next, an automotive skeletal member according to the second embodiment of the present invention will be described. The automotive skeletal member according to the present embodiment has the lap welding joint 1 according to the first embodiment. Note that the lap welding joint 1 according to the first embodiment may be applied only to a part of the joint portion of the automotive skeletal member, or may be applied to all of it. The portion where the lap welding joint 1 according to the first embodiment is applied has high joint strength. That is, the automotive skeletal member according to the present embodiment has high joint strength even though it includes the high-strength steel plate 11H whose joint strength at the joint portion is likely to decrease.

[0067] Examples of automotive skeletal members include bumper reinforcements, A-pillars, B-pillars, side sills, roof rails, floor members connected from front side members, front side members, front side member kick portions, rear side members, front suspension towers, tunnel reinforcements, dash panels, torque boxes, seat skeletons, seat rails, and frames of battery cases. By applying the lap welding joint 1 according to the present embodiment to some or all of these automotive skeletal members, excellent joint strength can be exhibited.

[0068] Those in which the lap welding joint 1 according to the present embodiment is applied to the joint portion between these automotive skeletal members and a pillar are also regarded as the automotive skeletal members according to the present embodiment. The joint portion between the automotive skeletal member and the pillar is, for example, the joint portion between the B-pillar reinforcement and the side sill, the joint portion between the front side member of an electric vehicle and the side sill, the joint portion between the B-pillar and the roof rail, the joint portion between the roof cross member and the roof rail, the joint portion between the side sill and the A-pillar, the joint portion between the dash panel and the tunnel, and the attachment root portion of the front side member, etc.

[0069] FIG. 23A shows a perspective view of the bumper reinforcement 21. FIG. 23B shows a cross-sectional view taken along line XXIIIB-XXIIIB of the bumper reinforcement of FIG. 23A, and FIG. 23C shows a cross-sectional view taken along line XXIIIC-XXIIIC of the bumper reinforcement of FIG. 23A. The cross-sectional structure shown in FIG. 28B, which is composed of three steel plates and has high strength, may be applied to the central portion of the bumper reinforcement 21, which is the portion that collides with an obstacle. On the other hand, in portions other than the central portion, the cross-sectional structure shown in FIG. 23C, which is composed of two steel plates and is lightweight, may be applied. In any of the cross-sectional structures, one or more flange portions can be bent, and arc welding portions can be provided at the bent portions. Thereby, the joining strength of the arc welding portions can be further improved. By arc welding, the nugget can be softened to improve the joining strength. Thereby, it is possible to prevent a decrease in energy transmission due to breakage of the joint portion of the bumper reinforcement 21 during a frontal collision of the automobile.

[0070] FIG. 24A shows a plan view of the floor member 22 joined to the floor. FIG. 24B shows a cross-sectional view taken along line XVIXB-XVIXB of the floor member of FIG. 24A. In FIG. 24B, the floor 24 is joined so as to be sandwiched between the front side member 23, which is the lower member, and the floor member 22, which is the upper member. The floor member 22 receives a load from the front side member 23 during a frontal collision. By arc welding, the nugget is softened to improve the joining strength of the joint portion, thereby preventing breakage of the joint portion even when a load is transmitted from the front side member 23.

[0071] FIG. 25A shows a perspective view of the front side member 23. FIG. 25B shows an enlarged view of the left side of the portion surrounded by the two dashed lines in FIG. 25A, and FIG. 25C shows an enlarged view of the right side of the portion surrounded by the two dashed lines in FIG. 25A. At the joint shown in FIG. 25B, the arc welding portion is formed in the recess of the flange of the upper plate as shown in FIG. 11. As a result, the joint shown in FIG. 25B is lap fillet welded and has high joint strength. Also, at the joint shown in FIG. 25B, since the arc welding portion is housed in the recess, interference with other members can be prevented and obstacles in subsequent processes after welding can be prevented.

[0072] At the joint shown in FIG. 25C, the arc welding portion is formed on the convex portion of the flange of the lower plate. As a result, the joint shown in FIG. 25C is lap fillet welded and has high joint strength. Note that the convex portion of the flange of the lower plate may have holes for bolt and nut fastening for joining with other members.

[0073] FIG. 26A shows a perspective view of the joint between the B-pillar reinforcement 25 and the side sill reinforcement 26. The horizontal member described below FIG. 26A is the side sill reinforcement 26, and the vertical member described above FIG. 26A is the B-pillar reinforcement 25. At the joint between the two, spot welding and arc welding are used in combination.

[0074] FIG. 26B shows an enlarged view of the portion marked with an arrow in FIG. 26A. The joint between the B-pillar reinforcement 25 and the side sill reinforcement 26 is a location prone to breakage during a side collision of the vehicle. By providing an arc welding portion at this location, breakage can be further prevented. Also, at the joint shown in FIG. 26B, the arc welding portion is formed in the recess of the flange of the upper plate as shown in FIG. 11. As a result, the joint shown in FIG. 26B is lap fillet welded and has high joint strength.

[0075] FIG. 27 shows a perspective view of the joint between the front side member 23 and the side sill 27 of an electric vehicle. The member on the left side is the front side member 23, and the member on the right side is the side sill 27. The front side member 23 and the side sill 27 are joined by a central joint member 28. In an electric vehicle, a large space for arranging a battery is provided on the floor. Therefore, when a frontal collision occurs in the electric vehicle, it is necessary to transmit the load applied from the front side member 23 to the side sill 27. For this reason, in this member, it is necessary to increase the shape offset. As the shape offset increases, the moment increases, so the spot weld portion of the joint is likely to break. Therefore, also at the joint between the front side member 23 and the side sill 27, it is preferable to use spot welding and arc welding in combination.

[0076] Next, a method for manufacturing the overlap welding joint 1 according to the third embodiment of the present invention will be described. As shown in FIG. 28A, the method for manufacturing the overlap welding joint 1 according to the present embodiment includes a step S1 of overlapping a part or all of a plurality of steel plates 11, a step S2 of spot-welding the overlapping portion 111 of the steel plates 11 to form a spot weld portion 12, and a step S3 of arc-welding one or more steel plates 11 to form an arc weld portion 13 so as to temper the nugget 122 of the spot weld portion 12. Here, one or more of the steel plates 11 to be spot-welded are made of a high-strength steel plate 11H having a tensile strength of 780 MPa or more. Further, the nugget of the spot weld portion 12 is tempered by the welding heat of the arc welding.

[0077] Hereinafter, a method for manufacturing the overlap welding joint 1 according to the third embodiment will be described. Needless to say, the various preferred examples cited in the description of the overlap welding joint 1 according to the first embodiment can be applied to the method for manufacturing the overlap welding joint 1 according to the third embodiment.

[0078] In the step S1 of overlapping the steel plates 11, a plurality of steel plates 11 are overlapped. In S1, all regions of the steel plates 11 may be overlapped, or only some of them may be overlapped. Also, in S1, it is not necessary to overlap all the steel plates 11 constituting the overlap welding joint 1, and only the steel plates 11 to be spot-welded need to be overlapped. At this time, a gap may occur on the overlapping surface, but from the viewpoint of ensuring spot welding quality, the gap is preferably 2.5 mm or less, and more preferably the gap is 1.5 mm or less.

[0079] Here, one or more of the steel plates 11 to be spot-welded are made into high-strength steel plates 11H with a tensile strength of 780 MPa or more. The preferred embodiment of the high-strength steel plate 11H conforms to the embodiment exemplified in the description of the first embodiment. Also, regarding the number, shape, positional relationship, etc. of the steel plates 11, the embodiments exemplified in the description of the first embodiment can be appropriately applied. For example, the tensile strength of the high-strength steel plate 11H may be 1700 MPa or more.

[0080] In the subsequent spot welding step S2, the overlapping portion 111 of the steel plates 11 is spot-welded. Thereby, a spot welding portion 12 for joining the overlapping portion 111 of the steel plates 11 is formed. The spot welding conditions and the spot welding apparatus are not particularly limited, and known conditions and apparatuses can be appropriately adopted. Hereinafter, a preferred example of spot welding is shown.

[0081] The resistance spot welding machine may be an inverter DC type resistance spot welding machine or a single-phase AC spot welding machine. The pressurizing mechanism of the resistance spot welding machine may be pressurization by a servo motor or pressurization by air. Also, the shape of the gun may be any of a stationary type, a C type, and an X type.

[0082] There are no particular restrictions on the electrodes for resistance spot welding. Examples of suitable electrodes include DR type electrodes with a tip diameter of 5 to 9 mm. The upper and lower electrodes may be the same or different. The material of the electrodes may be any of chromium copper, zirconium copper, or alumina-dispersed copper electrodes. From the viewpoint of suppressing welding between the electrodes and the steel sheet, generation of surface scabbing, and cracking due to liquid metal embrittlement (LME) during welding of zinc-based plated steel sheets, it is preferable that the material of the electrodes be alumina-dispersed copper.

[0083] There are no particular restrictions on the pressing force during resistance spot welding. The pressing force may be controlled to a constant value from the start to the end of resistance spot welding, or the pressing force may be changed according to the welding stage. The pressing force is desirably, for example, 200 to 800 kgf.

[0084] There is no particular limitation on the energization time of resistance spot welding, but it may be, for example, from 0.15 seconds to 2.0 seconds. From the perspective of reducing the risk of cracking in the welded joint due to hydrogen embrittlement of the spot welded part in the case of a gap, a longer energization time is desirable. There is also no particular limitation on the current value of resistance spot welding, but it may be, for example, from 5 kA to 13 kA. It is desirable to set the current value to a value at which no spatter occurs from the perspective of ensuring joint strength and preventing LME, but it may also be within 2.2 kA from the spatter generation current. The current value may be controlled to be constant from the start to the end of resistance spot welding, or the current value may be changed according to the welding stage. In the range within 1.0 second, upslope energization or downslope energization may be performed. The upslope has an effect of suppressing spatter generation when there is a gap between steel plates and suppressing hydrogen embrittlement cracking of the welded joint due to hydrogen derived from rust preventive oil, and the downslope is desirable because it has an effect of suppressing cracking of the welded joint due to LME and cracking of the welded joint due to hydrogen embrittlement. Also, before the main energization for growing the nugget, preliminary energization may be performed one or more times at a current value lower than the main energization. This makes it possible to suppress the generation of spatter and suppress hydrogen embrittlement cracking derived from rust preventive oil. Also, after the main energization for growing the nugget, multiple post-energizations may be performed for modifying the metal structure of the nugget and relaxing solidification segregation. This can obtain a further improvement in joint strength and an effect of suppressing hydrogen embrittlement. There is no particular limitation on the electrode holding time either, but it may be, for example, from 0 seconds to 1.0 seconds. From the perspective of preventing cracking of the welded joint due to LME when using a galvanized steel sheet, 0.15 seconds or more is desirable, and from the perspective of suppressing cracking due to hydrogen embrittlement in ultra-high strength steel sheets, it is desirable to be 0.7 seconds or less so that the temperature at the time of electrode release does not drop too much, and optimally it is desirable to be 0.55 seconds or less.

[0085] And in the subsequent arc welding process S3, arc welding is performed to anneal the lug 122 of the spot weld 12 by utilizing the welding heat of the arc welding. This arc welding does not necessarily have to join two or more workpieces to be welded. This is because the arc welding is carried out to anneal the spot weld 12 by the heat input of the arc. Therefore, the arc welding may be performed on only one steel plate 11, thereby forming an arc weld 13 (arc welding bead) as shown in FIG. 1B. On the other hand, the arc welding may be performed on two or more steel plates 11, thereby forming an arc weld 13 as shown in FIG. 2B.

[0086] As long as the spot weld 12 is annealed to obtain the overlap weld joint 1 according to the first embodiment, the location where the arc welding is performed and the arc welding conditions are not particularly limited. Generally, the closer the distance between the spot weld 12 and the arc weld 13 is, the higher the maximum heating temperature of the spot weld 12 during the arc welding rises. Also, the larger the heat input amount during the arc welding is, the higher the maximum heating temperature of the spot weld 12 during the arc welding rises. The higher the maximum heating temperature is, the larger the amount of annealing softening of the spot weld 12 becomes. However, if the maximum heating temperature is too high, tempering of the spot weld 12 occurs and the spot weld 12 hardens. Considering these matters, the location where the arc welding is performed and the heat input amount during the arc welding may be appropriately selected. For example, the location where the arc welding is performed and the heat input amount during the arc welding are appropriately selected so that the difference ΔHV between the minimum value and the maximum value of the hardness measurement value of the lug 122 measured by the above-described measurement method is 25 HV or more.

[0087] A preferred example of arc welding is to set the heat input of arc welding to 1000 J / cm or more, and when viewed in plan from the thickness direction of the overlap weld joint 1, the shortest distance between the center C of the indentation 121 of the spot weld portion 12 and the edge of the arc weld portion 13 is 17 mm or less. In addition to these, the maximum temperature reached by the softest part of the nugget 122 may be set to 300°C to 720°C. Thereby, a softest part of the nugget 122 is provided between the center of the nugget 122 of the spot weld portion 12 and the arc weld portion 13, and the difference ΔHV between the minimum value and the maximum value of the hardness measurement value of the nugget 122 can be easily made 25 HV or more. Also, the arc welding position may be determined such that the indentation 121 of the spot weld portion 12 and the arc weld portion 13 overlap, but from the viewpoint of sufficiently avoiding re-hardening of the spot weld portion 12, the distance between the edge of the indentation 121 of the spot weld portion 12 and the edge of the arc weld portion 13 is preferably determined to be more than 0 mm. The heat input may be set to 1500 J / cm or more. Note that the heat input is the energy per unit length of the arc weld bead. The heat input H can be calculated by the following formula using the arc voltage E (V), the arc current I (A), and the welding speed ν (cm / min). H = 60EI / ν

[0088] Examples of suitable conditions for other arc welding are described below. Arc welding includes, for example, consumable electrode type gas shielded arc welding using an iron welding wire, MIG brazing using a Cu alloy wire, but other types of arc welding may also be used. When the arc welding is MAG welding, for example, Ar+CO2 gas, Ar+CO2+O2 gas, Ar+O2 gas, etc. may be used as the shielding gas. When the arc welding is CO2 gas welding, CO2 gas may be used as the shielding gas. As the wire for arc welding, a welding wire in the range of YGW12 to YGW17 may be used. When high joint strength is required, a high-strength wire with a weld metal hardness of about 280 to 480 may be used. Also, when blowholes in the GA-plated steel sheet are a problem, a wire corresponding to the galvanized steel sheet may be used. In addition, when cracks due to hydrogen embrittlement occur around the welded part, a wire made of austenitic stainless steel such as SUS309 or a duplex stainless steel wire may be used. Cracks due to hydrogen embrittlement are suppressed by forming an austenite structure with a high diffusible hydrogen absorption capacity in the welded part. Also, the arc welding may be any of pulse welding, short arc welding, and CMT welding. CMT welding is a desirable welding method because there is little spatter in the welded part. Note that synchronous feed welding and super active wire welding may also be used. Although these welding methods have different names depending on the manufacturer, they are essentially the same welding method as CMT.

[0089] When the arc welding is MIG brazing, as the shielding gas, for example, Ar gas or a gas containing a trace amount of oxidizing gas in Ar can be used. The wire used in MIG brazing can be, for example, a Cu-Al based wire, a Cu-Si based wire, etc. When it is necessary to further improve the joint strength by the arc welded part 13, it is preferable to perform arc welding using a Cu-Al based wire.

[0090] Examples of the positional relationship between arc welding and spot welding conform to the positional relationship between the arc welding portion 13 and the spot welding portion 12 in the lap welding joint 1 described above. That is, the positional relationship between arc welding and spot welding may be appropriately set so that various forms exemplified in FIG. 1A and the like can be realized.

[0091] In addition, in the manufacturing method of the lap welding joint 1 according to the present embodiment, all of the plurality of steel plates 11 included in the lap welding joint 1 may be overlapped in a part thereof and spot welded. On the other hand, only a part of the steel plates 11 included in the lap welding joint 1 may be spot welded. In this case, the steel plate 11 that was not the target of spot welding may be arc welded to the spot welded steel plate 11.

[0092] Therefore, as shown in FIG. 28B, the manufacturing method of the lap welding joint 1 according to the present embodiment may further include a step S4 of adding one or more steel plates 11 to two or more spot welded steel plates 11 before the arc welding step S3. Then, by the arc welding step S3, two or more spot welded steel plates 11 and the added (i.e., not spot welded) steel plates 11 may be joined. The arc welding may be a fillet arc welding for obtaining a lap welding joint 1 as exemplified in FIG. 18, or a butt arc welding for obtaining a composite structure of a lap joint and a T joint as exemplified in FIG. 19. Therefore, in the step S4 of adding the steel plate 11, the steel plate 11 to be arc welded may be overlapped or butted against the spot welded steel plate 11. Depending on the shape of the member and the structure of the manufacturing line, the step S4 of adding the steel plate 11 may be provided before the spot welding step S2. However, even in this case, the arc welding step S3 needs to be performed after the spot welding step S2.

[0093] In the case of a plate assembly including a galvanized steel sheet (alloyed hot-dip galvanized steel sheet, hot-dip galvanized steel sheet), there are cases where zinc on the overlapping surface evaporates due to the heat of arc welding, and defects such as pits occur in the arc-welded part. In this process, since a minute gap is formed between the superposed steel sheets due to thermal deformation during spot welding, zinc vapor escapes from the gap. Therefore, it is difficult for defects such as pits to occur during arc welding. However, in the manufacture of parts having a structure in which it is difficult to form a gap, cases where defects such as pits are likely to occur are also assumed. In such a case, a press forming process may be performed in advance so that a small gap (0.1 mm to 1.5 mm) is formed on the overlapping surface of the steel sheets, and minute protrusions may be provided on at least one of the steel sheets in the vicinity of the arc-welded part.

Example

[0094] The effects of one aspect of the present invention will be further specifically described by way of examples. However, the conditions in the examples are merely one set of conditions adopted for confirming the feasibility and effects of the present invention. The present invention is not limited to this one set of conditions. The present invention can adopt various conditions without departing from the gist of the present invention and as long as the object of the present invention is achieved.

[0095] (Example 1) Two identical high-strength steel sheets were superposed and spot welded. Then, these two high-strength steel sheets were arc welded. Details of the steel sheets and welding conditions are as follows. · Thickness of high-strength steel sheet: 1.6 mm · Tensile strength of high-strength steel sheet: As shown in Table 1 · Pressing force for spot welding: 400 kgf · Current for spot welding: 5.6 kA · Welding time for spot welding: 0.33 seconds · Holding time for spot welding: 0.17 seconds · Current for arc welding: 80 A · Voltage for arc welding: 15.6 V · Welding speed for arc welding: 30 cm / min · Wire for arc welding: YM-24T · Shielding gas for arc welding: Ar + 20% CO2 ·Position relationship between spot weld and arc weld: As described in Table 1. A schematic cross-sectional view of the lap weld joint described as "arc weld on the upper plate" is shown in Fig. 29A, and a schematic cross-sectional view of the lap weld joint described as "arc weld on the lap surface" is shown in Fig. 29B.

[0096] The hardness of the spot weld part of the lap weld joint obtained by the above procedure was measured by the method described above, and the difference ΔHV between the minimum value and the maximum value of the hardness measurement value of the nugget was calculated. ΔHV is described in Table 1.

[0097] Furthermore, a chisel test was performed on the spot weld parts of various lap weld joints thus obtained. The chisel test was carried out in accordance with JIS Z 3144:2013 "Field test method for spot and projection welds", and the fracture mode was classified into either plug fracture or interfacial fracture. The lap weld joint with a fracture mode of plug fracture was judged to have excellent joint strength. The chisel test results are described in Table 1.

[0098]

Table 1

[0099] For the lap weld joint where the difference ΔHV between the minimum value and the maximum value of the hardness measurement value of the nugget was 25 HV or more, the fracture mode caused by the chisel test was plug fracture. These lap weld joints are presumed to have excellent joint strength. On the other hand, in the lap weld joint where the difference ΔHV between the minimum value and the maximum value of the hardness measurement value of the nugget was less than 25, the joint strength of the nugget was low and interfacial fracture occurred.

[0100] (Example 2) Two identical hot stamping steel plates were overlapped and spot welded. Then, these two hot stamping steel plates were arc welded. The details of the steel plates and welding conditions are as follows. ·Thickness of hot stamping steel plate: 1.6 mm ·Tensile strength of hot stamping steel plate: 2350 MPa · Chemical composition of the hot stamp steel plate: 0.45C - 0.2Si - 0.6Mn - 0.008P - 0.002S - Cr, Nb, Ti, B · Pressure of spot welding: 400 kgf · Current of spot welding: 7 kA · Welding time of spot welding: 0.33 seconds · Holding time of spot welding: 0.17 seconds · Current of arc welding: 80 A · Voltage of arc welding: 15.6 V · Welding speed of arc welding: 30 cm / min · Wire of arc welding: YM - 24T · Shielding gas of arc welding: Ar + 20% CO2 · Positional relationship between the spot welded part and the arc welded part: As described in Table 2 For reference, an external photograph of the test piece is shown in Fig. 30. The bead extending horizontally at the lower part of the test piece shown in Fig. 30 is the arc welding bead. Further, the dent formed adjacent to the bead is the indentation of the spot welded part.

[0101] The hardness of the spot welded part of the lap joint obtained by the above procedure was measured by the method described above. Further, the chisel test was performed on the spot welded part. The evaluation results are described in Table 2. Further, a photograph of the chisel test results is shown in Fig. 31.

[0102]

Table 2

[0103] For the lap joint where the difference ΔHV between the minimum value and the maximum value of the hardness measurement value of the nugget was 25 HV or more, the fracture form caused by the chisel test was plug fracture. These lap joints are presumed to have excellent joint strength. On the other hand, in the lap joint where the difference ΔHV between the minimum value and the maximum value of the hardness measurement value of the nugget was less than 25, the joint strength of the nugget was low and interfacial fracture occurred.

[0104] For reference, the hardness measurement results of the lap joint without an arc welding part and the lap joints with the distances between the edge of the arc welding part and the center of the indentation being 4 mm, 6 mm, 14 mm, and 18 mm are shown in FIG. 32.

[0105] The nugget of the lap joint without an arc welding part is the nugget of a normal spot welding part. The hardness inside the nugget is uniform, and a HAZ softening part is formed outside the nugget.

[0106] On the other hand, in the nuggets of the lap joints where an arc welding part is provided to temper the nugget of the spot welding part and the distance between the edge of the arc welding part and the center of the indentation is 4 mm, 6 mm, or 14 mm, the hardness decreases as it approaches the arc welding part. In these lap joints, the hardness of the closer end of both ends of the nugget to the arc welding part is the minimum value of the hardness measurement value of the nugget, and the hardness of the farther end from the arc welding part is the maximum value of the hardness measurement value of the nugget. The lap joint in which the arc welding part is formed and ΔHV is appropriate had excellent joint strength.

[0107] In all lap joints, the nugget hardness was measured at the 3 / 4t part of the upper plate. The upper plate refers to the upper of the two steel plates included in the cross-sectional photograph. In this embodiment, the arc welding torch faced the upper plate. The 3 / 4t part of the upper plate refers to the position at a depth of 3 / 4 of the thickness of the upper plate from the surface of the upper plate corresponding to the surface of the lap joint. On the other hand, in the lap joint where the distance between the edge of the arc welding part and the center of the indentation is 14 mm, for reference, the nugget hardness was also measured at the 3 / 4t part of the lower plate. The hardness measurement results were the same in any measurement part.

[0108] Even if an arc welding portion is provided near the spot welding portion, if ΔHV is not appropriate, the joint strength cannot be increased. In the nugget of the overlap weld joint in which an arc welding portion was provided near the spot welding portion and the distance between the edge of the arc welding portion and the center of the indentation was 18 mm, ΔHV was small and the joint strength was low. This is presumably because the tempering of the spot welding portion by arc welding was insufficient. However, if the heat input during arc welding is increased, it is predicted that the joint strength can be improved even if the distance between the edge of the arc welding portion and the center of the indentation is 18 mm.

[0109] (Example 3) Two identical hot-stamped steel sheets were overlapped and spot welded. Next, these two hot-stamped steel sheets were arc welded. The details of the steel sheet and the welding conditions are as follows. · Thickness of hot-stamped steel sheet: 1.6 mm · Tensile strength of hot-stamped steel sheet: 1780 MPa · Chemical composition of hot-stamped steel sheet: 0.29C - 0.2Si - 1.8Mn - 0.012P - 0.003S - Cr, Nb, Cu, Ni, Ti, B · Pressure for spot welding: 400 kgf · Current for spot welding: 6.7 kA · Welding time for spot welding: 0.33 seconds · Holding time for spot welding: 0.17 seconds · Current for arc welding: 70 A · Voltage for arc welding: 14.2 V · Welding speed for arc welding: 40 cm / min · Wire for arc welding: YM - 24T, φ1.2 mm · Shielding gas for arc welding: Ar + 20% CO2 · Positional relationship between the spot welding portion and the arc welding portion: As described in Table 3

[0110] The hardness of the spot welding portion of the overlap weld joint obtained by the above procedure was measured by the method described above. Further, the above-mentioned chisel test was performed on the spot welding portion. The evaluation results are described in Table 3.

[0111]

Table 3

[0112] For the overlap weld joints where the difference ΔHV between the minimum and maximum hardness measurement values of the nugget was 25 HV or more, the fracture mode caused by the chisel test was plug fracture. These overlap weld joints were presumed to have excellent joint strength. On the other hand, in the overlap weld joints where the difference ΔHV between the minimum and maximum hardness measurement values of the nugget was less than 25, the joint strength of the nugget was low and interfacial fracture occurred.

[0113] For reference, Fig. 33 shows the hardness measurement results of the overlap weld joints without an arc weld, and the overlap weld joints where the distance between the edge of the arc weld and the center of the indentation was 6 mm, 9 mm, and 18 mm.

[0114] The nugget of the overlap weld joint without an arc weld is the nugget of a normal spot weld. The hardness inside the nugget was uniform, and a HAZ softening zone was formed outside the nugget.

[0115] On the other hand, in the nugget of the overlap weld joint where an arc weld was provided to temper the nugget of the spot weld and the distance between the edge of the arc weld and the center of the indentation was 6 mm or 9 mm, the hardness decreased as it approached the arc weld. In these overlap weld joints, the hardness of the closer of the two ends of the nugget to the arc weld was the minimum value of the hardness measurement value of the nugget, and the hardness of the farther end from the arc weld was the maximum value of the hardness measurement value of the nugget. The overlap weld joints in which the arc weld was formed and ΔHV was appropriate had excellent joint strength.

[0116] However, even when an arc weld is provided near the spot weld, if ΔHV is not appropriate, the joint strength cannot be increased. In the nugget of the fillet weld where an arc weld was provided near the spot weld and the distance between the edge of the arc weld and the center of the indentation was 18 mm, ΔHV was small and the joint strength was low. This is presumably because the tempering of the spot weld by the arc weld was insufficient. However, it is predicted that if the heat input during arc welding is increased, the joint strength can be improved even when the distance between the edge of the arc weld and the center of the indentation is 18 mm.

[0117] (Example 4) Using two identical hot-stamped steel sheets, the hat member shown in Fig. 34 was manufactured. The flange portion of the hat member was spot welded. Then, in the inventive example, an arc weld was formed to temper the nugget of the spot weld. The details of the steel sheet and welding conditions are as follows. · Thickness of hot-stamped steel sheet: 1.6 mm · Tensile strength of hot-stamped steel sheet: 2000 MPa · Chemical composition of hot-stamped steel sheet: 0.34C - 0.2Si - 1.3Mn - 0.008P - 0.001S - Cr, Nb, Ti, B · Pressing force for spot welding: 400 kgf · Current for spot welding: 7.3 kA · Welding time for spot welding: 0.33 s · Holding time for spot welding: 0.17 s · Spot welding interval: 50 mm · Current for arc welding: 80 A · Voltage for arc welding: 15.6 V · Welding speed for arc welding: 30 cm / min · Wire for arc welding: YM - 24T · Shielding gas for arc welding: Ar + 20% CO2

[0118] In the hat member of the invention example, arc welding was performed such that the distance between the center of the indentation of the spot weld and the edge of the arc weld was 5 mm. The length of the arc weld metal was 30 mm, and arc welding was carried out at 8 locations. On the other hand, in the hat member of the comparative example, only spot welding was performed. Further, these hat members were heated to 170 °C and held for 20 minutes. This corresponds to the heat history during electrocoating baking performed on the automotive skeletal member. Then, a three-point bending test was conducted on these hat members. The location indicated by the downward arrow shown in Fig. 34 is the position where the bending load is applied. Also, when applying the bending load, both ends of the hat member were supported using a support member.

[0119] The displacement-load curve, which is the result of the bending test, is shown in Fig. 35. The lower curve is the test result of the comparative example where only spot welding was performed. In this comparative example, as shown in the photograph of Fig. 36A, breakage occurred at the spot weld during the bending test. In the curve, breakage of the spot weld occurred at the location where the load dropped rapidly. On the other hand, the upper curve is the test result of the invention example where an arc weld was formed to anneal the spot weld. In this invention example, as shown in the photograph of Fig. 36B, breakage did not occur at the spot weld, and high member performance was obtained.

Explanation of Reference Numerals

[0120] 1 Overlap Weld Joint 11 Steel Plate 11H High-Strength Steel Plate 11HS Spot Weld Surface of High-Strength Steel Plate 111 Overlap Portion 12 Spot Weld 121 Indentation 122 Nugget 13 Arc Weld 21 Bumper Reinforcement 22 Floor Member 23 Front Side Member 24 Floor 25 B-Pillar Reinforcement 26 Side Sill Reinforcement 27 Side Sill 28 Coupling member Center of C indentation Point on the edge of the P arc weld closest to the center of the indentation L Hardness measurement position t Thickness of high-strength steel sheet

Claims

1. A plurality of steel plates, one or more of which are overlapped, Spot welds for joining two or more of the steel plates, Arc weld beads formed on one or more of the steel plates, A lap weld joint comprising: The spot welds are arranged at the overlapping portion where the plurality of steel plates are overlapped in the lap weld joint, The arc weld beads are formed on one or more of the steel plates on which the spot welds are formed, One or more of the steel plates joined by the spot welds are high-strength steel plates with a tensile strength of 780 MPa or more, When viewed in plan from the thickness direction of the lap weld joint, a straight line connecting the center of the indentation of the spot weld and the point on the edge of the arc weld bead closest to the center of the indentation, and in a cross-section perpendicular to the overlapping portion of the steel plates, when the Vickers hardness at a depth of 1 / 4 of the plate thickness of the high-strength steel plate from the spot weld surface of the high-strength steel plate is continuously measured along the spot weld surface, The portion where the hardness measurement value of the nugget of the spot weld is minimum is between the center of the nugget and the arc weld bead, A lap weld joint in which the difference between the minimum value and the maximum value of the hardness measurement value of the nugget is 25 HV or more.

2. The lap weld joint according to claim 1, wherein in the cross-section, the difference between the minimum value and the maximum value of the hardness measurement value of the nugget is 40 HV or more.

3. The lap weld joint according to claim 1, wherein two or more of the steel plates are joined by both the spot welds and the arc weld beads.

4. The lap weld joint according to claim 1, wherein when viewed in plan from the thickness direction of the lap weld joint, the shortest distance between the center of the indentation of the spot weld and the edge of the arc weld bead is 17 mm or less.

5. The lap weld joint according to claim 1, wherein when viewed in plan from the thickness direction of the lap weld joint, the distance between the edge of the indentation of the spot weld and the edge of the arc weld bead is more than 0 mm.

6. The lap weld joint according to claim 1, wherein the width of the arc weld bead is 3 mm or more.

7. The number of the steel plates is 3 or more, One or more of the steel plates are outside the spot weld portion. The overlapping welded joint according to claim 1, wherein the steel plate outside the spot weld portion and the steel plate joined by the spot weld portion are joined by the arc weld bead.

8. The overlapping welded joint according to claim 1, wherein the tensile strength of the high-strength steel plate is 1700 MPa or more.

9. An automotive skeletal member having the overlapping welded joint according to any one of claims 1 to 8.

10. A method for manufacturing an overlapping welded joint for manufacturing the overlapping welded joint according to any one of claims 1 to 8, a step of overlapping a part or all of a plurality of steel plates; a step of spot-welding the overlapping portion of the steel plates to form a spot weld portion; a method for manufacturing an overlapping welded joint, comprising a step of arc-welding one or more of the steel plates to form an arc weld bead, wherein one or more of the steel plates to be spot-welded are high-strength steel plates having a tensile strength of 780 MPa or more, and a method for manufacturing an overlapping welded joint, wherein the nugget of the spot weld portion is tempered by the welding heat of the arc welding.

11. The method for manufacturing an overlapping welded joint according to claim 10, wherein when viewed in plan from the thickness direction of the overlapping welded joint, the distance between the edge of the indentation of the spot weld portion and the edge of the arc weld bead is more than 0 mm.

12. wherein the heat input of the arc welding is 1000 J / cm or more, and the method for manufacturing an overlapping welded joint according to claim 10, wherein when viewed in plan from the thickness direction of the overlapping welded joint, the shortest distance between the center of the indentation of the spot weld portion and the edge of the arc weld bead is 17 mm or less.

13. Before the arc welding, the method for manufacturing the overlapping welded joint further comprises a step of adding one or more steel plates to two or more of the steel plates spot-welded, and the method for manufacturing an overlapping welded joint according to claim 10, wherein the two or more steel plates spot-welded and the added steel plates are joined by the arc welding.

14. The method for manufacturing an overlapping welded joint according to claim 10, wherein the tensile strength of the high-strength steel plate is 1700 MPa or more.

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