Weld joint

Abstract

To provide a weld joint capable of suppressing breaking in an HAZ softened part and having excellent joint strength.SOLUTION: A weld joint 10 includes: a first steel plate member 12 having a tensile strength of equal to or greater than 2.0 GPa; a second steel plate member 14; and a resistance spot welding part 16b. The maximum value of a Vickers hardness of a first influence part 19a and a first specific range 26a at a first depth position P1 of 3t / 4 and at a second depth position P2 of 3t / 8 to the second steel plate member 14 side from a surface of the first steel plate member 12 is equal to or less than 75% of an average Vickers hardness of a base material of the first steel plate member 12. The minimum value of the Vickers harness of the first influence part 19a at the first position P1 and the second position P2 is equal to or greater than 75% of an average Vickers hardness of the first steel plate member 12 in the first specification range 26a.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a welded joint produced by resistance spot welding. [Background technology]

[0002] Welded joints (hereinafter also referred to as resistance spot-welded joints) formed by welding multiple steel plate members by resistance spot welding are used as automotive parts. Resistance spot-welded joints used as automotive parts are required to have excellent joint strength capable of suppressing fracture during a vehicle collision. Specifically, resistance spot-welded joints used as automotive parts are required to have excellent tensile shear strength (TSS) and cross tensile strength (CTS). The shear tensile strength refers to the tensile strength when a load is applied to a weld in the shear direction, and the cross tensile strength refers to the tensile strength when a load is applied to a weld in the peel direction. The shear load refers to a load applied in a direction parallel to the surface of each steel plate member, separating the steel plate members from each other. The peel load refers to a load applied in a direction separating the steel plate members from each other in the thickness direction of each steel plate member.

[0003] Therefore, techniques for improving the CTS and TSS of welded joints have been proposed. For example, Patent Document 1 discloses a resistance spot welding method in which a nugget formed by main current application is transformed into martensitic structure by a subsequent cooling process, and then the martensitic structure is tempered by re-current application. Patent Document 1 describes that the above method can improve the CTS and TSS. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6958765 Summary of the Invention [Problem to be solved by the invention]

[0005] When high-strength steel plates are welded by resistance spot welding, HAZ softening generally occurs at the spot weld. Welded joints used as automotive parts may be subjected to in-plane tensile stress during a vehicle collision. It is known that when a welded joint has the above-mentioned HAZ softening, its crashworthiness is reduced. Specifically, when in-plane tensile stress is applied to a welded joint, strain may concentrate in the HAZ softening, resulting in fracture. The method disclosed in Patent Document 1, as described above, considers improving the CTS and TSS of the welded joint, but does not consider measures to prevent fractures that occur in the HAZ softening when in-plane tensile stress is applied. Note that in-plane tensile stress refers to tensile stress that occurs in each steel plate member constituting the welded joint in a direction parallel to the surface of each steel plate member.

[0006] Therefore, an object of the present invention is to provide a welded joint that can suppress fracture in the HAZ-softened portion and has excellent joint strength. [Means for solving the problem]

[0007] As mentioned above, when high-strength steel plates are welded by resistance spot welding, HAZ softening occurs. In particular, when high-strength steel plates with a tensile strength of 2.0 GPa or more are resistance spot welded, the weld (part of the HAZ (A during welding) C3 This causes significant hardening in the weld (areas heated above the weld point) and the weld metal. This significant hardening makes the weld more susceptible to premature fracture when subjected to shear or peel loads.

[0008] Therefore, the inventors conducted various studies on a configuration for simultaneously avoiding fracture due to the HAZ softening and suppressing premature fracture in the weld metal and the HAZ hardening in resistance spot welded joints including high-strength steel plates with a tensile strength of more than 2.0 GPa. As a result, they found that by appropriately adjusting the hardness of each part of the weld by performing heat treatment on the weld using an external heat source (e.g., tempering by laser light irradiation) after resistance spot welding, it is possible to simultaneously avoid fracture in the HAZ softening and suppress premature fracture in the weld metal and the HAZ hardening.

[0009] The present invention was made based on the above findings.

[0010] (1) A welded joint according to one embodiment of the present invention is A first steel plate member having a tensile strength of 2.0 GPa or more, a second steel plate member, and a plurality of resistance spot welds joining the first steel plate member and the second steel plate member, the resistance spot weld includes a weld metal; When any one of the plurality of resistance spot welds is a target weld, a cross section passing through the center of the target weld and parallel to the thickness direction of the first steel plate member is a first cross section, a cross section passing through the center of the target weld, perpendicular to the first cross section, and parallel to the thickness direction is a second cross section, the thickness of the first steel plate member is t1, the surface of the first steel plate member opposite to the second steel plate member is a first front surface, the surface of the first steel plate member facing the second steel plate member is a first back surface, a portion of the first steel plate member and the weld metal facing the first steel plate member that is more inward than 2.5 mm outward from the outer edge of the weld metal on the first back surface as viewed in the thickness direction is a first affected portion, and a range of 2.5 mm to 5.0 mm outward from the outer edge of the weld metal on the first back surface as viewed in the thickness direction of the first steel plate member is a first specific range, In the first cross section and the second cross section, the maximum values ​​of Vickers hardness of the first affected portion at a first depth position of 3t1 / 4 and a second depth position of 3t1 / 8 from the first surface of the first steel plate member toward the second steel plate member are each 75% or less of the average Vickers hardness of the base material of the first steel plate member, In the first cross section and the second cross section, the maximum values ​​of Vickers hardness at the first depth position and the second depth position of the first steel plate member in the first specific range are each 75% or less of the average Vickers hardness of a base material of the first steel plate member, At the first depth position and the second depth position of the first cross section and the second cross section, respectively, the minimum value of the Vickers hardness of the first affected portion is 75% or more of the average Vickers hardness of the first steel plate member in the first specific range.

[0011] (2) In the welded joint of (1) above, the second steel plate member may have a tensile strength of less than 2.0 GPa.

[0012] (3) In the welded joint of (1) above, the Vickers hardness at the first depth position and the second depth position intermediate between the target weld and the resistance spot weld adjacent to the target weld may each be 75% or less of the average Vickers hardness of the base material of the first steel plate member.

[0013] (4) In the welded joint of (1) above, the Vickers hardness at the first depth position and the second depth position intermediate between the target weld and the resistance spot weld adjacent to the target weld may each be more than 75% of the average Vickers hardness of the base material of the first steel plate member.

[0014] (5) In the welded joint of (1) above, When a thickness of the second steel plate member is t2, a surface of the second steel plate member opposite to the first steel plate member is a second front surface, a surface of the second steel plate member on the first steel plate member side is a second back surface, a portion of the second steel plate member and the weld metal on the second steel plate member side that is more inward than 2.5 mm outward from the outer edge of the weld metal on the second back surface as viewed from the thickness direction is a second affected portion, and a range of 2.5 mm to 5.0 mm outward from the outer edge of the weld metal on the second back surface as viewed from the thickness direction of the second steel plate member is a second specific range, In the first cross section and the second cross section, the maximum value of Vickers hardness of the second affected portion at a third depth position that is inside the outer edge of the weld metal on the second back surface as viewed from the thickness direction and is 3t² / 4 from the second surface of the second steel plate member toward the first steel plate member is 90 to 110% of the maximum value of Vickers hardness of the first affected portion at the first depth position as viewed from the thickness direction and inside the outer edge of the weld metal on the first back surface, In the first cross section and the second cross section, the average Vickers hardness of the second steel plate member within the second specific range at the third depth position and at a fourth depth position 3t2 / 8 from the second surface toward the first steel plate member may each be 90 to 110% of the average Vickers hardness of the base material of the second steel plate member.

[0015] (6) In the welded joint of (1) above, In the first cross section and the second cross section, the maximum value of Vickers hardness of the first affected zone at a position inside an outer edge of the weld metal on the first back surface and at the first depth position as viewed in the thickness direction is greater than the maximum value of Vickers hardness of the first affected zone at a position inside an outer edge of the weld metal on the first back surface and at the second depth position as viewed in the thickness direction, In the first cross section and the second cross section, the average Vickers hardness of the first steel plate member within the first specific range and at the first depth position may be greater than the average Vickers hardness of the first steel plate member within the first specific range and at the second depth position.

[0016] (7) In the welded joint of (1) above, The first steel plate member has a flange portion extending in a first direction and a wall portion rising from an edge portion of the flange portion in a second direction perpendicular to the first direction when viewed from a thickness direction of the flange portion, The flange portion and the second steel plate member may be welded together by the plurality of resistance spot welds.

[0017] (8) In the welded joint of (7) above, In the first affected portion of a cross section passing through the center of the target weld and parallel to the second direction and the thickness direction, the maximum Vickers hardness of the first steel plate member at the first depth position in a portion closer to the wall portion than the outer edge of the weld metal on the first reverse surface as viewed from the thickness direction may be greater than the maximum Vickers hardness of the first steel plate member at the first depth position in a portion closer to the tip of the flange portion than the outer edge of the weld metal on the first reverse surface as viewed from the thickness direction.

[0018] (9) In the welded joint of (8) above, The Vickers hardness of the base material of the first steel plate member may be the Vickers hardness of the first steel plate member in the wall portion. [Effects of the Invention]

[0019] According to the present invention, it is possible to obtain a welded joint that can suppress fracture in the HAZ-softened portion and has excellent joint strength. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a diagram showing a welded joint according to one embodiment of the present invention. [Figure 2] FIG. 2 is a diagram for explaining the hardness of the resistance spot weld and each of the surrounding areas. [Figure 3] FIG. 3 is a diagram for explaining the hardness of the resistance spot weld and each of the surrounding areas. [Figure 4]FIG. 4 is a diagram for explaining another method of irradiating laser light. [Figure 5] FIG. 5 is a diagram for explaining the hardness of the resistance spot weld and each of the surrounding areas. [Figure 6] FIG. 6 is a diagram for explaining a method for measuring Vickers hardness. [Figure 7] FIG. 7 is a diagram showing another example of the arrangement of a plurality of resistance spot welds. [Figure 8] FIG. 8 is a diagram showing a sub-size plate-shaped test piece. [Figure 9] FIG. 9 shows the test piece used in the in-plane tensile test. [Figure 10] FIG. 10 shows the test piece used in the shear tensile test. [Figure 11] FIG. 11 shows the test piece used in the cross tension test. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, a welded joint according to an embodiment of the present invention will be described with reference to the drawings.

[0022] (Schematic structure of welded joint) Figure 1 shows a welded joint according to one embodiment of the present invention, specifically, Figure 1(a) is a schematic perspective view showing a part of the welded joint, and Figure 1(b) is a schematic cross-sectional view showing part AA of Figure 1(a).

[0023] As shown in Fig. 1, a welded joint 10 according to this embodiment has a first steel plate member 12 and a second steel plate member 14. The first steel plate member 12 has a tensile strength of 2.0 GPa or more, and the second steel plate member 14 has a tensile strength of less than 2.0 GPa. In this embodiment, the tensile strength of the first steel plate member 12 is, for example, less than 4.0 GPa. In this specification, 2.0 GPa means 2000 MPa, and 4.0 GPa means 4000 MPa.

[0024] In this embodiment, the first steel plate member 12 may be made of a steel plate A described below, and the second steel plate member 14 may be made of a steel plate B described below.

[0025] (Steel plate A) Chemical composition, in mass%, is: C: 0.33 to 0.70%, Si: 0.01 to 2.00%, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.100% or less, Al: 0.010 to 0.100%, Cr: 0.01 to 1.00%, Nb: 0 to 0.200%, Ti: 0 to 0.200%, Mo: 0 to 1.00%, B: A steel plate containing Zn: 0-0.0100%, Co: 0-4.00%, Ni: 0-2.00%, Cu: 0-1.00%, V: 0-1.00%, W: 0-1.00%, Ca: 0-0.0100%, Mg: 0-0.0100%, REM: 0-0.0100%, Sb: 0-0.100%, Zr: 0-0.100%, Sn: 0-1.00%, As: 0-0.100%, and the remainder being Fe and impurities.

[0026] (Steel plate B) Chemical composition, in mass%, is: C: 0.07 to 0.70%, Si: 0.01 to 2.00%, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.100% or less, Al: 0.010 to 0.100%, Cr: 0.01 to 1.00%, Nb: 0 to 0.200%, Ti: 0 to 0.200%, Mo: 0 to 1.00%, B: A steel plate containing Zn: 0-0.0100%, Co: 0-4.00%, Ni: 0-2.00%, Cu: 0-1.00%, V: 0-1.00%, W: 0-1.00%, Ca: 0-0.0100%, Mg: 0-0.0100%, REM: 0-0.0100%, Sb: 0-0.100%, Zr: 0-0.100%, Sn: 0-1.00%, As: 0-0.100%, and the remainder being Fe and impurities.

[0027] The impurities refer to components that are mixed in due to raw materials such as ore and scrap or other factors when industrially manufacturing steel sheets, and are not components that are intentionally added to the steel sheets according to this embodiment.

[0028] In this embodiment, a steel plate containing, for example, 0.07 to 0.33% by mass of C is preferably used as the material for the second steel plate member 14. The thickness of the first steel plate member 12 is, for example, 1.0 to 2.6 mm, and the thickness of the second steel plate member 14 is, for example, 0.6 to 2.0 mm. Note that, if the first steel plate member 12 is plated, the thickness of the first steel plate member 12 means the thickness including the plated layer. The same applies to the thickness of the second steel plate member 14. Note that there are no particular restrictions on the plated layers formed on the first steel plate member 12 and the second steel plate member 14, but for example, an Al-based plated layer or a Zn-based plated layer is formed.

[0029] In this embodiment, the first steel plate member 12 is a hat-shaped member, and the second steel plate member 14 is a flat closure. The first steel plate member 12 has a pair of flange portions 12a (only one of the flange portions 12a is shown in FIG. 1(a)), a pair of wall portions 12b rising from the pair of flange portions 12a (only one of the wall portions 12b is shown in FIG. 1(a)), and a top plate portion 12c connecting the pair of wall portions 12b. The flange portions 12a are provided to extend in a first direction D1. The wall portions 12b are provided to rise from edges of the flange portions 12a in a second direction D2 perpendicular to the first direction D1, as viewed from the thickness direction of the flange portions 12a. The first direction D1 is a direction along the boundary (ridge) between the flange portions 12a and the wall portions 12b, as viewed from the thickness direction of the flange portions 12a. The method for manufacturing the first steel plate member 12 is not particularly limited, but it may be manufactured by cold press forming or hot stamping.

[0030] A pair of flange portions 12a of the first steel plate member 12 are joined to the second steel plate member 14 by a plurality of resistance spot welds. FIG. 1 shows a plurality of resistance spot welds 16a-16d that join one of the flange portions 12a to the second steel plate member 14. In this embodiment, the resistance spot welds 16a-16d are arranged at intervals so as to be aligned in a first direction D1. As shown in FIG. 1(b), each of the resistance spot welds 16a-16d includes a weld metal 18, a first heat-affected zone 20a formed in the first steel plate member 12, and a second heat-affected zone 20b formed in the second steel plate member 14. The first heat-affected zone 20a and the second heat-affected zone 20b are heat-affected zones formed when the first steel plate member 12 and the second steel plate member 14 are resistance spot welded together.

[0031] The tensile strength of the first steel plate member 12 and the second steel plate member 14 was measured using test specimens taken from areas not affected by the resistance spot welds or the laser beam irradiation described below. Specifically, the test specimens were cut from flat sections at a position sufficiently distant (at least 30 mm away) from the resistance spot welds joining the first steel plate member 12 and the second steel plate member 14. The test specimens were as specified in ASTM A370-22 (width: 10 mm, total length: 100 mm, parallel section width: 6.25 mm, parallel section length: 32 mm, gauge length: 25 mm, thickness: the original thickness of the first steel plate member 12 and the second steel plate member 14), as shown in FIG. 8 . The test was performed at a crosshead displacement rate of 5.0 mm / min. Other conditions were in accordance with JIS Z 2241:2022. The flat portion refers to a portion in which the radius of curvature of both surfaces in the thickness direction (the radius of curvature as viewed from a direction perpendicular to the cut surface when the test specimen is cut out) is 500 mm or more. When the first steel plate member 12 and the second steel plate member 14 are plated, the tensile strength is measured using the test specimen in its plated state. The original cross-sectional area of ​​the test specimen used to calculate the tensile strength is the original cross-sectional area of ​​the test specimen including the plating layer. In this specification, a position 30 mm or more away from multiple resistance spot welds refers to a position 30 mm or more away from any of the resistance spot welds and a position 30 mm or more away from a line connecting the resistance spot welds.

[0032] The welded joint 10 according to this embodiment is manufactured by joining a first steel plate member 12 and a second steel plate member 14 using a known resistance spot welding method, followed by heat treatment of the resistance spot welds using an external heat source. In this embodiment, a laser beam is irradiated (scanned) onto a region 100 enclosed by a two-dot chain line in FIG. 1( a) on one flange portion 12a of the first steel plate member 12 (more specifically, the surface of the flange portion 12a opposite the second steel plate member 14 in the thickness direction). This tempers the resistance spot welds 16a-16d and their surrounding regions on the first steel plate member 12 and the second steel plate member 14. As a result, the hardness of the resistance spot welds 16a-16d and their surrounding regions can be appropriately adjusted. Although not described further, the other flange portion 12a (not shown) of the first steel plate member 12 is also irradiated with a laser beam, thereby tempering the region including the resistance spot welds.

[0033] The conditions for irradiating the laser beam are not particularly limited, as long as they are set so that the hardness of the resistance spot weld and its surrounding areas meets the conditions described below. In this embodiment, the laser beam is irradiated onto the region 100 by scanning the laser beam in a predetermined beam shape (e.g., a rectangular shape with a width greater than the diameter of the resistance spot weld) in a predetermined direction. A known semiconductor laser can be used as the oscillator. The processing point power is set to, for example, 250 to 1500 W, and the scanning speed is set to 10 to 150 cm / min. The beam shape is set, for example, to a rectangular shape with a width of 10 to 30 mm and a length in the scanning direction of 5 to 20 mm on the surface irradiated with the laser beam.

[0034] Below, we will explain the hardness of resistance spot weld 16b and its surrounding areas, assuming that resistance spot weld 16b is the target weld. However, in this embodiment, other resistance spot welds 16a, 16c, 16d and their surrounding areas also have the same hardness as resistance spot weld 16b and its surrounding areas (for example, within ±10% of the hardness).

[0035] 2 and 3 are diagrams illustrating the hardness of the resistance spot weld 16b and its surrounding areas. Specifically, FIG. 2(a) is a schematic cross-sectional view of the welded joint 10 shown in FIG. 1(b), enlarging the area surrounding the resistance spot welds 16b and 16c. Note that hatching indicating cross sections has been omitted in FIG. 2(a) and FIG. 3(a), which will be described later, to avoid cluttering the drawings. The same applies to FIG. 5(a), which will be described later. The cross sections shown in FIGS. 1(b) and 2(a) are cross sections (first cross sections) that pass through the center of the resistance spot weld 16b and are parallel to the thickness direction of the first steel plate member 12 (the flange portion 12a in this embodiment). In this embodiment, the cross sections shown in FIGS. 1(b) and 2(a) are cross sections parallel to the first direction D1 (the direction parallel to the extension direction of the boundary (ridge portion) between the flange portion 12a and the wall portion 12b). Fig. 2(b) is a schematic diagram showing the hardness of each portion of the first steel plate member 12 in the cross section (first cross section) shown in Fig. 2(a). Fig. 2(c) is a schematic diagram showing the hardness of each portion of the second steel plate member 14 in the cross section shown in Fig. 2(a).

[0036] In this specification, the "center of the resistance spot weld" in a cross section passing through the center of the resistance spot weld and parallel to the thickness direction of the first steel plate member refers to the center of a circle inscribed in the outer edge of the weld metal on the surface of the first steel plate member facing the second steel plate member (corresponding to the first back surface 13b shown in Figure 2(a) and described below) when viewed from the thickness direction. The center of the circle inscribed in the outer edge includes not only the exact center of the inscribed circle but also an area enclosed by a circle with a radius of 1.0 mm centered on the center of the inscribed circle. In this specification, the "thickness direction of the first steel plate member" in a cross section passing through the center of the resistance spot weld and parallel to the thickness direction of the first steel plate member refers to the thickness direction of the portion of the first steel plate member joined to the second steel plate member by the resistance spot weld. In other words, the "thickness direction of the first steel plate member" in a cross section passing through the center of the resistance spot weld and parallel to the thickness direction of the first steel plate member refers to the overlapping direction of the first steel plate member and the second steel plate member at the resistance spot weld (hereinafter simply referred to as the overlapping direction). Furthermore, in this specification, the "depth position" in a cross section passing through the center of the resistance spot weld and parallel to the thickness direction of the first steel plate member refers to the depth position in a direction parallel to the "thickness direction of the first steel plate member." In other words, in this specification, the "depth position" in a cross section passing through the center of the resistance spot weld and parallel to the thickness direction of the first steel plate member refers to the depth position in a direction parallel to the overlapping direction.

[0037] Fig. 3(a) is a cross-sectional view showing the BB portion of Fig. 2(a). The cross-section shown in Fig. 3(a) is a cross-section (second cross-section) that passes through the center of the resistance spot welded portion 16b, is perpendicular to the first cross-section, and is parallel to the thickness direction of the first steel plate member 12 (in this embodiment, the flange portion 12a). In this embodiment, the cross-section shown in Fig. 3(a) is a cross-section parallel to the second direction D2. Fig. 3(b) is a schematic diagram showing the hardness of each portion of the first steel plate member 12 in the cross-section (second cross-section) shown in Fig. 3(a). Fig. 3(c) is a schematic diagram showing the hardness of each portion of the second steel plate member 14 in the cross-section shown in Fig. 3(a).

[0038] In the following description, of the two surfaces 13a, 13b in the thickness direction of the first steel plate member 12, the surface 13a opposite to the second steel plate member 14 will be referred to as the first front surface 13a, and the surface 13b on the second steel plate member 14 side will be referred to as the first back surface 13b. Furthermore, of the two surfaces 15a, 15b in the thickness direction of the second steel plate member 14, the surface 15a opposite to the first steel plate member 12 will be referred to as the second front surface 15a, and the surface 15b on the first steel plate member 12 side will be referred to as the second back surface 15b. In Figures 2(a) and 3(a), the first depth position P1 and the second depth position P2 of the first steel plate member 12, and the third depth position P3 and the fourth depth position P4 of the second steel plate member 14 are respectively indicated by dotted lines. The first depth position P1 indicates a position at a depth of 3t1 / 4 from the first surface 13a toward the second steel plate member 14 (t1 is the thickness of the first steel plate member 12), the second depth position P2 indicates a position at a depth of 3t1 / 8 from the first surface 13a toward the second steel plate member 14. The third depth position P3 indicates a position at a depth of 3t2 / 4 from the second surface 15a toward the first steel plate member 12 (t2 is the thickness of the second steel plate member 14), and the fourth depth position P4 indicates a position at a depth of 3t2 / 8 from the second surface 15a toward the first steel plate member 12.

[0039] 2(a) and 3(a), the first specific area 26a and the second specific area 26b around the resistance spot weld 16b are hatched to clearly show their positions. The first specific area 26a refers to a range of the first steel plate member 12 extending 2.5 mm to 5.0 mm outward from the outer edge of the weld metal 18 on the first back surface 13b, as viewed in the thickness direction of the first steel plate member 12 (the flange portion 12a in this embodiment). The second specific area 26b refers to a range of the second steel plate member 14 extending 2.5 mm to 5.0 mm outward from the outer edge of the weld metal 18 on the second back surface 15b, as viewed in the thickness direction. In this embodiment, the first specific area 26a and the second specific area 26b are regions tempered by laser light irradiation. In this embodiment, the first specific area 26a and the second specific area 26b are regions outside the first heat-affected zone 20a and the second heat-affected zone 20b. In FIG. 1(a), the first specific area 26a surrounding the resistance spot weld 16b is indicated by a dashed line. As shown in FIG. 1(a), the first specific area 26a is annularly formed so as to surround the resistance spot weld 16b. The same applies to the second specific area 26b.

[0040] In the following description, the first affected zone 19a refers to a portion of the first steel plate member 12 and the weld metal 18 on the first steel plate member 12 side that is more than 2.5 mm inward from the outer edge of the weld metal 18 on the first back surface 13b when viewed in the thickness direction. The second affected zone 19b refers to a portion of the second steel plate member 14 and the weld metal 18 on the second steel plate member 14 side that is more than 2.5 mm inward from the outer edge of the weld metal 18 on the second back surface 15b when viewed in the thickness direction. In this embodiment, the first affected zone 19a includes the portion of the weld metal 18 on the first steel plate member 12 side and the first heat-affected zone 20a, and the second affected zone 19b includes the portion of the weld metal 18 on the second steel plate member 14 side and the second heat-affected zone 20b.

[0041] As shown by the dashed-dotted line in FIG. 2( b ), in the state where the first steel plate member 12 and the second steel plate member 14 are resistance spot welded together (before laser beam irradiation), the first heat-affected zone 20 a of the first steel plate member 12 contains a softened portion (HAZ softened portion) whose hardness is significantly reduced compared to the base material, and a hardened portion (HAZ hardened portion) whose hardness is increased compared to the base material. In this state, as described above, when in-plane tensile stress is applied to the weld joint 10, strain may concentrate in the HAZ softened portion, causing fracture. Furthermore, when a load in the shear or peel direction is applied to the resistance spot weld 16 b, early fracture is likely to occur in the weld metal 18 or the HAZ hardened portion. Before laser beam irradiation, the hardness of the weld metal 18 on the first steel plate member 12 side is equivalent to the hardness of the weld metal 18 on the second steel plate member 14 side. For example, before the laser beam is irradiated, the hardness at an arbitrary position of the weld metal 18 on the second steel plate member 14 side is 90 to 110% of the hardness at the first depth position P1 of the weld metal 18 on the first steel plate member 12 side.

[0042] In this embodiment, to prevent the above-described fracture, the hardness of the resistance spot weld 16b and the surrounding areas is adjusted to an appropriate hardness by tempering with laser light irradiation. The method for measuring the hardness of the resistance spot weld 16b and the surrounding areas, and the hardness requirements, are described below.

[0043] First, a method for measuring the hardness of each portion will be described. In this embodiment, hardness is measured in the following manner in a cross section (first cross section: in this embodiment, the cross section shown in FIG. 2(a)) that passes through the center of the resistance spot welded portion 16b and is parallel to the thickness direction of the first steel plate member 12, and in a cross section (second cross section: in this embodiment, the cross section shown in FIG. 3(a)) that passes through the center of the resistance spot welded portion 16b, is perpendicular to the first cross section, and is parallel to the thickness direction.

[0044] FIG. 6 is a diagram illustrating a method for measuring Vickers hardness. FIGS. 6(a) and 6(b) show end surfaces (measurement surfaces: first and second cross sections) obtained by cutting a welded joint 10 to measure Vickers hardness. The first cross section shown in FIG. 6(a) is a cross section that passes through the center of the resistance spot weld and is parallel to the thickness direction of the first steel plate member 12 and the first direction D1 (see FIG. 1). The second cross section shown in FIG. 6(b) is a cross section that passes through the center of the resistance spot weld and is parallel to the thickness direction of the first steel plate member 12 and the second direction D2 (see FIG. 1). The following describes a case in which the welded joint 10 is cut to obtain the second cross sections, the Vickers hardness is measured at depth positions P1 to P4 in the second cross section, and then the welded joint 10 is further cut to obtain the first cross sections, and the Vickers hardness is measured at depth positions P1 to P4 in the first cross section. When cutting the welded joint 10 in this manner, as shown in Fig. 6(a), a cut plane L1 exists in the first cross section so as to divide the resistance spot weld into left and right halves. Fig. 6(b) also shows a line L2 (hereinafter referred to as the center line L2) in the second cross section, which passes through the midpoint of a line connecting one outer edge and the other outer edge of the weld metal 18 on the first back surface 13b and extends in the thickness direction (overlapping direction) of the first steel plate member 12. Figs. 6(a) and 6(b) also show a pair of lines L3 that pass through the outer edges of the weld metal 18 on the first back surface 13b and extend in the thickness direction of the first steel plate member 12.

[0045] As shown in FIGS. 6(a) and 6(b), recesses 11a and 11b are formed on the first surface 13a of the first steel plate member 12 and the second surface 15a of the second steel plate member 14 by resistance spot welding. The thicknesses of the portions of the first steel plate member 12 and the second steel plate member 14 where the recesses 11a and 11b are formed are smaller than the thicknesses of the remaining portions. Therefore, it is necessary to define a depth position common to the portions where the recesses 11a and 11b are formed and the remaining portions. Therefore, in this embodiment, a first depth position P1 and a second depth position P2 are defined based on a virtual plane P10 passing through the inner edge 27a of the first specific range 26a. Furthermore, a third depth position P3 and a fourth depth position P4 are defined based on a virtual plane P11 passing through the inner edge 27b of the second specific range 26b.

[0046] In this embodiment, the Vickers hardness of the entire range of the first specific area 26a, the second specific area 26b, the first affected area 19a, and the second affected area 19b along the first depth position P1, the second depth position P2, the third depth position P3, and the fourth depth position P4 is measured at 0.50 mm intervals with a test force (measurement load) of 1.0 kgf (9.807 N). Furthermore, from the Vickers hardness measurements thus obtained, the average Vickers hardness (average value of the measurements) of the first steel plate member 12 and the second steel plate member 14 in the first specific area 26a and the second specific area 26b is calculated. In this embodiment, as described above, the first cross section includes a cut surface L1. In this embodiment, the Vickers hardness is measured at each of the depth positions P1 to P4 for each of the two portions divided by the cut surface L1. At this time, for each of depth positions P1 to P4 of the two divided portions, a position 0.3 mm away from the cut surface L1 in a direction perpendicular to the thickness direction of the first steel plate member 12 (position indicated by a dashed circle in FIG. 6(a)) is set as the measurement start position for Vickers hardness, and Vickers hardness is measured in order in the direction away from the cut surface L1. For each of depth positions P1 to P4 of the second cut surface, a position on the center line L2 of the weld metal 18 is set as the measurement start position for Vickers hardness. If an indentation (indentation) formed when measuring Vickers hardness overlaps with a blowhole in the weld metal 18, the measurement value measured at that indentation is excluded.

[0047] In addition, for a portion of first affected zone 19a that is outside the outer edge of weld metal 18 on first back surface 13b (portion outside line L3) when viewed in the thickness direction (overlapping direction) of first steel plate member 12, Vickers hardness was measured at a test force of 1.0 kgf at 0.50 mm intervals, and the Vickers hardness was measured on both sides (positions 0.25 mm from the center of the indentation) where the maximum value was measured, with a test force of 500 gf (4.903 N). The maximum of the three measured values, namely the maximum Vickers hardness measured with a test force of 1.0 kgf and the Vickers hardness (two locations) measured with a test force of 500 gf, is defined as the maximum Vickers hardness of first affected zone 19a in the portion outside the outer edge of weld metal 18. However, if the measurement position for Vickers hardness at the test force of 500 gf is located inside the first specific range 26a or the outer edge of the weld metal 18 (inside the line L3), the measurement value at that measurement position is excluded from the determination of the maximum value. Note that the center of the indentation is the part corresponding to the apex of the indenter when measuring the Vickers hardness.

[0048] Furthermore, for the portion of first affected zone 19a outside the outer edge of weld metal 18 on first back surface 13b (portion outside line L3), the Vickers hardness was measured at a test force of 1.0 kgf at 0.50 mm intervals, and the Vickers hardness was also measured on both sides (positions 0.25 mm from the center of the indentation) of the indentation where the minimum value was measured, using a test force of 500 gf (4.903 N).The minimum of the three measured values, namely the minimum Vickers hardness measured at a test force of 1.0 kgf and the Vickers hardness (at two locations) measured at a test force of 500 gf, was determined to be the minimum Vickers hardness of the portion of first affected zone 19a outside the outer edge of weld metal 18. However, if the measurement position of the Vickers hardness at the test force of 500 gf is located inside the first specific range 26a or the outer edge of the weld metal 18 (inside the straight line L3), the measurement value at that measurement position is excluded from determining the minimum value.

[0049] 6(c) and 6(d), when an indentation (indentation) 29 formed when measuring the Vickers hardness at the first depth position P1 and the second depth position P2 is located on line L3 (a position corresponding to the outer edge of the weld metal 18 on the first back surface 13b when viewed from the thickness direction of the first steel plate member 12), it is confirmed whether the center of the indentation 29 is located on the inner side (the weld metal 18 side) or the outer side (the opposite side from the weld metal 18) of line L3. Then, when the center of the indentation 29 is located on the inner side (the weld metal 18 side) of line L3 as shown in FIG. 6(c), the Vickers hardness measured based on the indentation 29 is the Vickers hardness of a portion on the first back surface 13b that is inner than the outer edge of the weld metal 18 when viewed from the thickness direction of the first steel plate member 12. 6(d), when the center of the dent 29 is located outside the line L3 (the side opposite the weld metal 18), the Vickers hardness measured based on the dent 29 is the Vickers hardness of a portion on the first back surface 13b outside the outer edge of the weld metal 18 when viewed in the thickness direction of the first steel plate member 12. When the center of the dent 29 is located on the line L3, the Vickers hardness measured based on the dent 29 is the Vickers hardness of a portion on the first back surface 13b inside the outer edge of the weld metal 18 and a portion on the first back surface 13b outside the outer edge when viewed in the thickness direction of the first steel plate member 12. Similarly, when measuring the Vickers hardness at the third depth position P3 and the fourth depth position P4, the Vickers hardness of the portion on the second back surface 15b inside the outer edge of the weld metal 18 when viewed in the thickness direction of the first steel plate member 12 or the Vickers hardness of the portion outside the outer edge is determined based on the position of the center of the dent 29. Although detailed explanation will be omitted, even when the depression 29 is formed in other boundary portions (the boundary portion between the first affected portion 19a and the first specific area 26a, the boundary portion between the second affected portion 19b and the second specific area 26b, the boundary portion between the first specific area 26a and the portion outside the first specific area 26a, and the boundary portion between the second specific area 26b and the portion outside the second specific area 26b), the Vickers hardness of which portion is determined based on the position of the center of the depression 29. Regarding Vickers hardness, measurement conditions other than those specified in this specification shall be in accordance with JIS Z 2244-1:2020.

[0050] Next, the hardness requirements for each part will be explained.

[0051] (Requirement 1) The welded joint 10 of this embodiment satisfies the following (Requirement 1a) and (Requirement 1b) in a cross section (first cross section: in this embodiment, the cross section shown in Figure 2(a)) that passes through the center of the resistance spot weld 16b and is parallel to the thickness direction of the first steel plate member 12 (flange portion 12a), and in a cross section (second cross section: in this embodiment, the cross section shown in Figure 3(a)) that passes through the center of the resistance spot weld 16b, is perpendicular to the first cross section, and is parallel to the thickness direction. (Requirement 1a) "The maximum value of the Vickers hardness of the first affected zone 19a at each of the first depth position P1 and the second depth position P2 is 75% or less of the average Vickers hardness of the base material of the first steel plate member 12." (Requirement 1b) "In the first specific range 26a, the maximum value of the Vickers hardness at each of the first depth position P1 and the second depth position P2 of the first steel plate member 12 is 75% or less of the average Vickers hardness of the base material of the first steel plate member 12."

[0052] In this embodiment, the average Vickers hardness of the base material of the first steel plate member 12 is defined as the average Vickers hardness of the base material of the first steel plate member 12, which is defined as the average Vickers hardness of the base material of the first steel plate member 12. Specifically, the Vickers hardness is measured at 10 locations (flat portions) located 30 mm or more away from the resistance spot welds at a depth of t1 / 4 and a depth of 3t1 / 4 from one surface in the thickness direction of the first steel plate member 12, and the average value is defined as the average Vickers hardness of the base material of the first steel plate member 12. The Vickers hardness is measured with a test force (measuring load) of 1.0 kgf (9.807 N). In this embodiment, for example, the Vickers hardness measured by the above method on the wall portion 12b can be defined as the Vickers hardness of the base material of the first steel plate member 12. It should be noted that the "thickness direction" of the first steel plate member 12 when measuring the Vickers hardness of the base material of the first steel plate member 12 means the thickness direction of the first steel plate member 12 at the measurement location.

[0053] As shown by the dashed-dotted lines in FIGS. 2(b) and 3(b), the Vickers hardness of the weld metal 18 and a portion of the first heat-affected zone 20a is high after resistance spot welding (as-welded state). However, in this embodiment, the first affected zone 19a and the first specific region 26a, including the weld metal 18 and the first heat-affected zone 20a, are tempered by laser light irradiation so that the Vickers hardness of the first affected zone 19a and the first specific region 26a satisfies the above-mentioned (Requirement 1). This prevents significant hardening from remaining in the resistance spot weld 16b. As a result, early fracture of the resistance spot weld 16b due to loads in the shear and peel directions can be suppressed.

[0054] The maximum Vickers hardness of the first affected zone 19a at the first depth position P1 is preferably 73% or less of the average Vickers hardness of the base material of the first steel plate member 12. The maximum Vickers hardness of the first affected zone 19a at the second depth position P2 is preferably 70% or less of the average Vickers hardness of the base material of the first steel plate member 12. In the first specific range 26a, the maximum Vickers hardness of the first steel plate member 12 at the first depth position P1 is preferably 71% or less of the average Vickers hardness of the base material of the first steel plate member 12. In the first specific range 26a, the maximum Vickers hardness of the first steel plate member 12 at the second depth position P2 is preferably 67% or less of the average Vickers hardness of the base material of the first steel plate member 12.

[0055] (Requirement 2) Furthermore, the welded joint 10 according to this embodiment satisfies the following requirements at the first cross section and the second cross section. "At each of the first depth position P1 and the second depth position P2, the minimum value of the Vickers hardness of the first affected portion 19a is 75% or more of the average Vickers hardness of the first steel plate member 12 in the first specific range 26a."

[0056] As shown by the dashed-dotted lines in FIGS. 2(b) and 3(b), in the state after resistance spot welding (as-welded state), the hardness of the softened portion (HAZ-softened portion) of the first heat-affected zone 20a is significantly lower than the hardness of the surrounding base material of the first steel plate member 12. However, in this embodiment, the first specific range 26a of the first steel plate member 12 is tempered by laser light irradiation so that the Vickers hardness of the first affected zone 19a, including the first heat-affected zone 20a, satisfies the above-mentioned (Requirement 2). This reduces the difference in hardness between the softened portion (HAZ-softened portion) of the first heat-affected zone 20a and the surrounding base material of the first steel plate member 12. As a result, when in-plane tensile stress is applied to the welded joint 10, strain concentration in the HAZ-softened portion can be suppressed, and fracture in the HAZ-softened portion can be suppressed.

[0057] At the first depth position P1, the minimum Vickers hardness of the first affected portion 19a is preferably 86% or more of the average Vickers hardness of the first steel plate member 12 in the first specific range 26a. At the second depth position P2, the minimum Vickers hardness of the first affected portion 19a is preferably 88% or more of the average Vickers hardness of the first steel plate member 12 in the first specific range 26a.

[0058] (Requirement 3) The welded joint 10 according to this embodiment further satisfies, for example, the following (Requirement 3a) and (Requirement 3b) at the first cross section and the second cross section. (Requirement 3a) "The maximum Vickers hardness of the second affected zone 19b at the third depth position P3, which is located inside the outer edge of the weld metal 18 on the second back surface 15b when viewed in the thickness direction of the first steel plate member 12 (in this embodiment, the flange portion 12a), is 90 to 110% of the maximum Vickers hardness of the first affected zone 19a at the first depth position P1, which is located inside the outer edge of the weld metal 18 on the first back surface 13b when viewed in the thickness direction of the first steel plate member 12 (in this embodiment, the flange portion 12a)." (Requirement 3b) "The average Vickers hardness of the second steel plate member 14 within the second specific range 26b at the third depth position P3 and the fourth depth position P4 is 90 to 110% of the average Vickers hardness of the base material of the second steel plate member 14."

[0059] In this embodiment, the average Vickers hardness of the base material of the second steel plate member 14 is defined as the average Vickers hardness of the base material of the second steel plate member 14, which is the average Vickers hardness of the base material of the second steel plate member 14. Specifically, the Vickers hardness is measured at 10 locations (flat portions) located 30 mm or more away from the resistance spot welds at a depth of t1 / 4 and a depth of 3t1 / 4 from one surface in the thickness direction of the second steel plate member 14, and the average value is defined as the average Vickers hardness of the base material of the second steel plate member 14. The Vickers hardness is measured with a test force (measuring load) of 1.0 kgf (9.807 N). Note that when measuring the Vickers hardness of the base material of the second steel plate member 14, the "thickness direction" of the second steel plate member 14 refers to the thickness direction of the second steel plate member 14 at the measurement locations.

[0060] During resistance spot welding, the materials of the first steel plate member 12 and the second steel plate member 14 are mixed together, so that in the state after resistance spot welding (as-welded state), the portion of the weld metal 18 on the first steel plate member 12 side and the portion on the second steel plate member 14 side have the same hardness. Therefore, as shown by the dashed-dotted lines in FIGS. 2(c) and 3(c), in the state after resistance spot welding (as-welded state), the hardness of the portion of the weld metal 18 on the second steel plate member 14 side is greater than the hardness of the base material of the second steel plate member 14. However, in this embodiment, when a laser beam is irradiated onto the first surface 13a of the first steel plate member 12, heat is conducted from the first steel plate member 12 side of the weld metal 18 to the second steel plate member 14 side. This heat tempers not only the weld metal 18 on the first steel plate member 12 side, but also a portion of the weld metal 18 on the second steel plate member 14 side. Specifically, weld metal 18 around mating surfaces 24 (first back surface 13b and second back surface 15b) is tempered so that the Vickers hardness of weld metal 18 satisfies the above-mentioned (Requirement 3a). This sufficiently prevents resistance spot weld 16b from prematurely fracture due to loads in the shear direction and peel direction.

[0061] As described above, when the first surface 13a of the first steel plate member 12 is irradiated with laser light, heat is conducted from the first steel plate member 12 to the second steel plate member 14 via the weld metal 18. However, the temperature rise of the second steel plate member 14 is kept low compared to the temperature rise of the first steel plate member 12. In particular, the amount of heat conducted to the second steel plate member 14 within the second specific range 26b is small. Therefore, the second steel plate member 14 within the second specific range 26b is hardly tempered so as to satisfy the above-mentioned (Requirement 3b). Therefore, a decrease in the strength of the second steel plate member 14 can be sufficiently suppressed. Note that the amount of heat conducted to the second heat-affected zone 20b of the second steel plate member 14 is greater than the amount of heat conducted to the second specific range 26b, but the degree of tempering of the second heat-affected zone 20b is small. In this embodiment, the Vickers hardness of the second heat-affected zone 20b after the laser irradiation is about 90% of the Vickers hardness of the second heat-affected zone 20b before the laser irradiation.

[0062] (Requirement 4) The welded joint 10 according to this embodiment further satisfies, for example, the following (Requirement 4a) and (Requirement 4b) at the first cross section and the second cross section. (Requirement 4a) "The maximum Vickers hardness of the first affected zone 19a at a first depth position P1 and on the inside of the outer edge of the weld metal 18 on the first back surface 13b when viewed in the thickness direction of the first steel plate member 12 (in this embodiment, the flange portion 12a) is greater than the maximum Vickers hardness of the first affected zone 19a at a second depth position P2 and on the inside of the outer edge of the weld metal 18 on the first back surface 13b when viewed in the thickness direction of the first steel plate member 12 (in this embodiment, the flange portion 12a)." (Requirement 4b) "The average Vickers hardness of the first steel plate member 12 within the first specific range 26a and at the first depth position P1 is greater than the average Vickers hardness of the first steel plate member 12 within the first specific range 26a and at the second depth position P2."

[0063] In this embodiment, a laser beam is irradiated onto the first surface 13a of the first steel plate member 12. Therefore, the portion of the first steel plate member 12 on the first surface 13a side is more easily tempered than the portion on the first back surface 13b side. Specifically, the resistance spot welds 16b and the first steel plate member 12 are tempered so that the Vickers hardness of the weld metal 18 satisfies the above-mentioned (requirement 4a) and the Vickers hardness of the first steel plate member 12 in the first specific range 26a satisfies the above-mentioned (requirement 4b).

[0064] (Requirement 5) The welded joint 10 according to this embodiment further satisfies the following requirements, for example. "In the first affected zone 19a of a cross section (the second cross section in this embodiment) that passes through the center of the resistance spot weld 16b and is parallel to the second direction D2 and the thickness direction of the first steel plate member 12 (flange portion 12a), the maximum Vickers hardness of the first steel plate member 12 at a first depth position P1 in a portion of the first back surface 13b that is closer to the wall portion 12b than the outer edge of the weld metal 18 when viewed from the thickness direction is greater than the maximum Vickers hardness of the first steel plate member 12 at the first depth position P1 in a portion of the weld metal 18 that is closer to the tip end of the flange portion 12a than the outer edge of the weld metal 18."

[0065] The volume of the portion of the first steel plate member 12 closer to the wall portion 12b than the weld metal 18 in the second direction D2 is larger than the volume of the portion of the weld metal 18 closer to the tip of the flange portion 12a (the opposite side to the wall portion 12b). As a result, when laser light is irradiated onto the flange portion 12a of the first steel plate member 12, heat removal is promoted in the portion of the periphery of the resistance spot weld 16b closer to the wall portion 12b, but heat removal is suppressed in the portion of the flange portion 12a closer to the tip. In this case, tempering is more advanced in the portion of the flange portion 12a closer to the wall portion 12b than in the portion of the weld metal 18. As a result, the Vickers hardness of the portion of the weld metal 18 closer to the wall portion 12b in the second direction D2 is greater than the Vickers hardness of the portion of the weld metal 18 closer to the tip of the flange portion 12a (the opposite side to the wall portion 12b).

[0066] (Requirement 6) The welded joint 10 according to this embodiment further satisfies the following requirements, for example. "The Vickers hardness at a first depth position P1 and a second depth position P2 that are intermediate positions between resistance spot weld 16b and resistance spot weld 16c adjacent to resistance spot weld 16b is each 75% or less of the average Vickers hardness of the base material of first steel plate member 12."

[0067] The Vickers hardness at the intermediate position between resistance spot welds 16b and 16c can be measured in the cross section shown in Figure 2(a) (a cross section parallel to the arrangement direction (first direction D1) of resistance spot welds 16b, 16c and the thickness direction of first steel plate member 12 (flange portion 12a)). Note that the intermediate position between resistance spot welds 16b and 16c refers to a position equidistant from weld metal 18 of resistance spot welds 16b and weld metal 18 of resistance spot welds 16c in the arrangement direction of resistance spot welds 16b, 16c. At each of the first depth position P1 and the second depth position P2, measurements are taken at five locations at 0.5 mm intervals (the distance between the centers of the depressions) within a range of ±3 mm centered on the above-mentioned intermediate position, with a test force (measurement load) of 1.0 kgf (9.807 N), and the maximum of the measured values ​​is taken as the maximum Vickers hardness at each of the first depth position P1 and the second depth position P2.

[0068] In this embodiment, as shown in FIG. 1(a), a laser beam is irradiated onto region 100. Therefore, as shown in FIG. 2(b), the region between resistance spot weld 16b and resistance spot weld 16c adjacent to resistance spot weld 16b is also tempered, similar to the periphery of resistance spot weld 16b. As a result, the maximum Vickers hardness values ​​at first depth position P1 and second depth position P2, which are intermediate positions between resistance spot weld 16b and resistance spot weld 16c, are 75% or less of the average Vickers hardness of the base material of the first steel plate member 12. In this embodiment, for example, the entire region between resistance spot weld 16b and resistance spot weld 16c adjacent to resistance spot weld 16b is tempered by laser beam irradiation. In this embodiment, the intermediate positions between resistance spot weld 16a and resistance spot weld 16b and the intermediate positions between resistance spot weld 16c and resistance spot weld 16d are also tempered in the same manner.

[0069] (Requirement 7) The welded joint 10 may satisfy the following requirement instead of the above (Requirement 6). "The Vickers hardness at a first depth position P1 and a second depth position P2 that are intermediate positions between resistance spot weld 16b and resistance spot weld 16c adjacent to resistance spot weld 16b is each more than 75% of the average Vickers hardness of the base material of first steel plate member 12."

[0070] The conditions for measuring the Vickers hardness at the intermediate position between resistance spot welds 16b and 16c are the same as the conditions for measuring the Vickers hardness at the intermediate portion in (Requirement 6) above.

[0071] In the above-described embodiment, as shown in FIG. 1(a), a case has been described in which the laser beam is scanned across region 100 to continuously irradiate multiple resistance spot welds 16a-16d with the laser beam. However, the method of laser beam irradiation is not limited to the above example. FIG. 4 is a diagram illustrating another method of laser beam irradiation. FIG. 5 is a diagram illustrating the hardness of resistance spot weld 16b and its surrounding areas. Specifically, FIG. 5(a) is a schematic cross-sectional view showing an enlarged view of the surrounding areas of resistance spot welds 16b and 16c, and FIG. 5(b) is a schematic view showing the hardness of each portion of the first steel plate member 12 in the cross section shown in FIG. 5(a).

[0072] As shown in FIG. 4, in this embodiment, the resistance spot welds 16a-16d and their surrounding areas are tempered by irradiating the spaced-apart regions 100a-100d, which are provided for each of the resistance spot welds 16a-16d, with laser light. In this case, as shown in FIG. 5(b), the intermediate position between the resistance spot welds 16a and 16b and their surrounding areas are not tempered by the laser light. Therefore, the Vickers hardness at the first depth P1 and the second depth P2, which are intermediate positions between the resistance spot welds 16b and 16c, is greater than 75% of the average Vickers hardness of the base material of the first steel plate member 12. In this embodiment, the Vickers hardness at the first depth P1 and the second depth P2, which are intermediate positions between the resistance spot welds 16b and 16c, is 90-110% of the average Vickers hardness of the base material of the first steel plate member 12.

[0073] (Requirement 8) The welded joint 10 according to this embodiment further satisfies, for example, the following (Requirement 8a) and (Requirement 8b) at the first cross section and the second cross section. (Requirement 8a) "The maximum Vickers hardness of the first affected zone 19a at the first depth position P1 and inside the outer edge of the weld metal 18 on the first back surface 13b when viewed in the thickness direction of the first steel plate member 12 (in this embodiment, the flange portion 12a) is 300 or more." (Requirement 8b) "The maximum Vickers hardness of the second affected zone 19b at the third depth position P3, which is located inside the outer edge of the weld metal 18 on the second back surface 15b when viewed in the thickness direction of the first steel plate member 12 (in this embodiment, the flange portion 12a), is 300 or more."

[0074] By making the welded joint 10 satisfy the above (Requirement 8a) and (Requirement 8b), it is possible to sufficiently prevent fractures from occurring in the weld metal (particularly at the joint interface between the first steel plate member 12 and the second steel plate member 14) when a load in the shear direction is applied to the resistance spot weld. In other words, it is possible to ensure sufficiently excellent shear tensile strength.

[0075] The maximum Vickers hardness of the first affected zone 19a at the first depth position P1, which is located inside the outer edge of the weld metal 18 on the first back surface 13b when viewed in the thickness direction of the first steel plate member 12 (in this embodiment, the flange portion 12a), is preferably 350 or more, and more preferably 402 or more. The maximum Vickers hardness of the second affected zone 19b at the third depth position P3, which is located inside the outer edge of the weld metal 18 on the second back surface 15b when viewed in the thickness direction of the first steel plate member 12 (in this embodiment, the flange portion 12a), is preferably 350 or more, and more preferably 399 or more.

[0076] (Variation) In the above-described embodiment, the first cross section is parallel to the thickness direction of the first steel plate member 12 and the first direction D1, but the first cross section is not limited to a direction parallel to the first direction D1. That is, the hardness of each portion may satisfy the above-described requirements in an arbitrary first cross section that passes through the center of the resistance spot weld and is parallel to the thickness direction of the first steel plate member, and in a second cross section that passes through the center of the resistance spot weld, is perpendicular to the first cross section, and is parallel to the thickness direction.

[0077] In the above embodiment, the case where the plurality of resistance spot welds 16a-16d and their surrounding areas meet the hardness requirements has been described, but it is not necessary for all resistance spot welds and their surrounding areas to meet the hardness requirements. For example, only resistance spot welds in areas where it is particularly important to ensure joint strength may be tempered by laser light irradiation.

[0078] In the above embodiment, a case has been described in which a plurality of resistance spot welds are formed in a single row, but a plurality of resistance spot welds 16 may also be arranged in a plurality of rows (two rows in this embodiment) as shown in Fig. 7. In this case, the laser light may be irradiated to each row, or to each resistance spot weld 16 individually. Alternatively, the laser light may be irradiated to only specific resistance spot welds 16 among the plurality of resistance spot welds 16.

[0079] In the above-described embodiment, the flange portion 12a and the wall portion 12b extend linearly along the first direction D1 when viewed in the thickness direction of the flange portion 12a. However, the shapes of the flange portion and the wall portion are not limited to the above-described example. For example, the boundary (ridge portion) between the flange portion and the wall portion may be curved when viewed in the thickness direction of the flange portion. In this case, the direction along the boundary when viewed in the thickness direction of the flange portion corresponds to the first direction. Furthermore, the cross section passing through the center of the weld and parallel to the second direction and the thickness direction of the flange portion refers to a cross section passing through the center of the weld portion and parallel to the thickness direction of the flange portion, which is perpendicular to the boundary at any position on the boundary when viewed in the thickness direction of the flange portion.

[0080] In the above-described embodiment, the case where the tensile strength of the second steel plate member 14 is less than 2.0 GPa has been described. However, the tensile strength of the second steel plate member 14 may be 2.0 GPa or more. In this case, for example, the laser beam may be irradiated not only to the first surface 13a of the first steel plate member 12 but also to the second surface 15a of the second steel plate member 14 so that each portion satisfies the above-described hardness requirements. Note that when irradiating only one of the first steel plate member 12 and the second steel plate member 14 with the laser beam, it is preferable to irradiate the first surface 13a of the flange portion 12a of the first steel plate member 12 (hat-shaped member) with the laser beam. When the tensile strength of the second steel plate member 14 is 2.0 GPa or more, for example, a steel plate having the same chemical composition as the steel plate used as the material of the first steel plate member 12 can be used as the material of the second steel plate member 14.

[0081] In the above-described embodiment, the welded joint 10 includes a first steel plate member 12 and a second steel plate member 14. However, the welded joint 10 may include other steel plate members. For example, the first steel plate member 12, the second steel plate member 14, and the third steel plate member may be joined in this order by resistance spot welds. In this case, the resistance spot welds and their surrounding areas only need to satisfy the hardness requirements described above. If the tensile strength of the third steel plate member is 2.0 GPa or greater, a steel plate member with a tensile strength of less than 2.0 GPa is used as the second steel plate member 14. In this case, for example, laser light is irradiated from both the first steel plate member 12 side and the third steel plate member side. If the tensile strength of the third steel plate member is 2.0 GPa or greater, for example, a steel plate with the same chemical composition as the steel plate used as the first steel plate member 12 can be used as the material for the third steel plate member.

[0082] In the above embodiment, the first steel plate member 12 is a hat-shaped member and the second steel plate member 14 is a closure. However, the shapes of the first steel plate member and the second steel plate member are not limited to the above example and can be changed as appropriate depending on the intended use of the welded joint. For example, both the first steel plate member and the second steel plate member may be hat-shaped members. Note that, if both the first steel plate member and the second steel plate member are hat-shaped members and the tensile strengths of both the first steel plate member and the second steel plate member are 2.0 GPa or more, it is preferable to irradiate both the first steel plate member and the second steel plate member with laser light.

[0083] (Usage example) The welded joints according to the embodiments of the present invention can be used as various automobile parts (assembly parts) such as front pillars, center pillars, side sills, front side members, and rear side members that constitute an automobile body.

[0084] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples. [Example]

[0085] Steel plate members having the chemical compositions shown in Table 1 below were used as the first steel plate member or the second steel plate member, and welded joints of Test Nos. 1 to 15 having the same shape as the welded joint 10 shown in Figure 1 were produced by resistance spot welding.

[0086] [Table 1]

[0087] Of the welded joints produced, for the welded joints of Test Nos. 2 to 6, 9, and 15, laser light was irradiated to each resistance spot weld as shown in Figure 4. For Test Nos. 7 and 8, laser light was irradiated continuously to multiple resistance spot welds as shown in Figure 1. For Test Nos. 10 to 14, tempering was performed in a heating furnace. The welded joint of Test No. 1 was left as welded. The laser light irradiation conditions for the welded joints of Test Nos. 2 to 9 and 15 were as follows: Processing point output: 360~800W Scanning speed: 30~60cm / min Beam shape: Width 10-14mm, length in scanning direction 10mm

[0088] The tempering temperatures in the heating furnace were 250°C, 350°C, 400°C, 450°C, and 550°C in the order of Test Nos. 10 to 14. The tempering time was 1200 seconds.

[0089] The tensile strength of the first steel plate member, the tensile strength of the second steel plate member, and the hardness of each portion of the welded joints of Test Nos. 1 to 15 were measured according to the methods described above. Furthermore, for each of Test Nos. 1 to 15, an in-plane tensile test, a shear tensile test, and a cross tensile test were performed using test specimens (described below) prepared under the same conditions as the welded joints (resistance spot welding conditions, tempering conditions). It was confirmed that the hardness of the resistance spot weld and its surrounding area for each test specimen was equivalent to the hardness of the resistance spot weld and its surrounding area for the corresponding welded joint (welded joint prepared under the same conditions).

[0090] FIG. 9 shows the test specimen used in the in-plane tensile test, where (a) is a front view of the test specimen and (b) is a plan view of the test specimen. As shown in FIG. 9, a JIS No. 5 tensile test specimen was prepared using a first steel plate member, and a second steel plate member (length: 40 mm, width: 25 mm) was joined to the center of the JIS No. 5 tensile test specimen by resistance spot welding. Furthermore, for Test Nos. 2 to 9 and 15, a laser beam was irradiated onto the center of the first steel plate member (JIS No. 5 tensile test specimen) to prepare a welded joint that would serve as the test specimen for the in-plane tensile test. The laser beam was scanned over a distance of 20 mm in the longitudinal direction of the first steel plate member. For Test Nos. 10 to 14, tempering was performed in a heating furnace to prepare a welded joint that would serve as the test specimen for the in-plane tensile test. The test specimen for Test No. 1 was left as welded. A tensile load was applied to the first steel plate member (JIS No. 5 tensile test piece) of the welded joint (test piece) thus prepared to fracture the welded joint, and the fracture location was examined.

[0091] The shear tensile test was performed according to the method specified in JIS Z 3136 1999 to measure the shear tensile strength (TSS). The tensile speed was set to 10 mm / min, and the maximum tensile load until the test specimen broke was measured to determine the shear tensile strength. Figure 10 shows the test specimen used in the shear tensile test, with (a) being a front view of the test specimen and (b) being a plan view of the test specimen. As shown in Figure 10, the first steel plate member and the second steel plate member were overlapped with each other with an overlap of 40 mm, and the first steel plate member and the second steel plate member were joined by resistance spot welding so that a weld was formed in the center of the overlap. Furthermore, for Test Nos. 2 to 9 and 15, a laser beam was irradiated from the first steel plate member side to prepare a welded joint that would serve as the test specimen for the shear tensile test. The laser beam was scanned over a distance of 20 mm in the width direction of the first steel plate member. For test Nos. 10 to 14, tempering was carried out in a heating furnace to prepare welded joints that would serve as test pieces for shear tensile tests. The test piece for test No. 1 was left as welded.

[0092] Cross tension tests were performed according to the method specified in JIS Z 3137 1999 to measure cross tension strength (CTS). The tension speed was set to 10 mm / min, and the maximum tensile load until the test specimen broke was measured to determine the cross tension strength. Figure 11 shows the test specimen used in the cross tension test, with (a) being a front view of the test specimen and (b) being a plan view of the test specimen. As shown in Figure 11, a first steel plate member and a second steel plate member, each having the same length (150 mm) and width (50 mm), were overlapped and joined by resistance spot welding so that a weld was formed in the center of the overlap. Furthermore, for Test Nos. 2 to 9 and 15, a laser beam was irradiated from the first steel plate member side to prepare a welded joint that would serve as a test specimen for the cross tension test. The laser beam was scanned 20 mm along the length of the first steel plate member. For test Nos. 10 to 14, tempering was carried out in a heating furnace to prepare welded joints that would serve as test pieces for cross tension tests. The test piece for test No. 1 was left as welded.

[0093] The measurement results of tensile strength and hardness are shown in Tables 2 to 4 below, and the results of the in-plane tensile test, shear tensile test, and cross tensile test are shown in Table 5 below. In Tables 2 and 3 below, the outer edge of the weld metal of the first steel plate member refers to the outer edge of the weld metal on the first back surface of the first steel plate member when viewed in the thickness direction of the first steel plate member (flange portion). In Table 4 below, the outer edge of the weld metal of the second steel plate member refers to the outer edge of the weld metal on the second back surface of the second steel plate member when viewed in the thickness direction of the first steel plate member (flange portion). The minimum Vickers hardness values ​​of the first affected zone at the first depth position and the second depth position in the first cross section were consistent with the minimum Vickers hardness values ​​of the portions outside the outer edge of the weld metal (corresponding to the portions outside line L3 in FIG. 6). Similarly, the minimum Vickers hardness values ​​of the first affected zone at the first depth position and the second depth position in the second cross section were consistent with the minimum Vickers hardness values ​​of the portions outside the outer edge of the weld metal. In the in-plane tensile test, test pieces that did not fracture in the weld metal or HAZ were judged to have excellent strength against in-plane tensile stress (indicated by "O" in Table 5). In the shear tensile test, test pieces with a TSS of 16.3 kN or more were judged to have excellent shear tensile strength. In the cross tensile test, test pieces with a CTS of 4.6 kN or more were judged to have excellent cross tensile strength.

[0094] [Table 2]

[0095] [Table 3]

[0096] [Table 4]

[0097] [Table 5]

[0098] As shown in Table 2, all of the test pieces of Test Nos. 2 to 7, in which the hardness of each portion satisfied the requirements of the present invention, had excellent strength against in-plane tensile stress, as well as excellent shear tensile strength and cross tensile strength. On the other hand, the test pieces of Test Nos. 1, 8 to 11, and 13 to 15, which did not satisfy the above-mentioned (Requirement 1), did not satisfy the above-mentioned conditions (TSS: 16.3 kN or more, CTS: 4.6 kN or more) for at least one of the shear tensile strength (TSS) and cross tensile strength (CTS). In Test Nos. 8 and 9, the temperature rise at the laser beam irradiated portion was insufficient, so the hardness of the first affected portion and the first specific region could not be adequately reduced. In Test No. 15, the temperature on the laser beam irradiated surface side (first surface side) reached the hardening temperature and hardened, so the hardness of the first affected portion and the second depth position of the first specific region could not be adequately reduced. In Test No. 12, which was tempered in a heating furnace, both the TSS and CTS satisfied the above conditions, but the tensile strength of the first steel plate member was less than 2.0 GPa, which did not satisfy the requirements of the present invention. In addition, in Test Nos. 13 and 14, which were tempered in a heating furnace like Test No. 12, the tensile strength of the first steel plate member was also less than 2.0 GPa.

[0099] Furthermore, in the in-plane tensile tests of the test pieces of Test Nos. 1 and 8 to 12, which did not satisfy the above-mentioned (Requirement 2), fracture occurred in the weld metal or the softened HAZ. In other words, excellent strength against in-plane tensile stress could not be achieved. From the above results, it was found that by satisfying the requirements of the present invention, excellent strength against in-plane tensile stress, as well as excellent shear tensile strength and cross tensile strength can be achieved. [Industrial Applicability]

[0100] According to the present invention, it is possible to obtain a welded joint that can suppress fracture in the HAZ-softened portion and has excellent joint strength. [Explanation of symbols]

[0101] 10 Welded joints 12 First steel plate member 12a Flange 12b Wall section 12c Top plate 14 Second steel plate member 16, 16a to 16d Resistance spot welds 18 Weld Metal 19a 1st affected part 19b 2nd affected part 20a First heat affected zone 20b 2nd heat affected zone 24 Mating surface 26a First specific range 26b Second specific range

Claims

1. The steel sheet includes a first steel plate member having a tensile strength of 2.0 GPa or more, a second steel plate member, and a plurality of resistance spot welds joining the first steel plate member and the second steel plate member, the resistance spot weld includes a weld metal; Any one of the plurality of resistance spot welds is designated as a target weld, a cross section passing through the center of the target weld and parallel to the thickness direction of the first steel plate member is designated as a first cross section, a cross section passing through the center of the target weld, perpendicular to the first cross section, and parallel to the thickness direction is designated as a second cross section, and the thickness of the first steel plate member is designated as t １ a first surface is a surface of the first steel plate member opposite to the second steel plate member, a first back surface is a surface of the first steel plate member facing the second steel plate member, a first affected portion is a portion of the first steel plate member and the weld metal on the first steel plate member side that is more inward than 2.5 mm outward from the outer edge of the weld metal on the first back surface as viewed in the thickness direction, and a first specific range is a range of 2.5 mm to 5.0 mm outward from the outer edge of the weld metal on the first back surface as viewed in the thickness direction of the first steel plate member, In the first cross section and the second cross section, a distance of 3t from the first surface of the first steel plate member to the second steel plate member is １ / 4 first depth position and 3t １ the maximum value of the Vickers hardness of the first affected portion at a second depth position of 1 / 8 is 75% or less of the average Vickers hardness of the base material of the first steel plate member, In the first cross section and the second cross section, the maximum values ​​of Vickers hardness at the first depth position and the second depth position of the first steel plate member in the first specific range are each 75% or less of the average Vickers hardness of a base material of the first steel plate member, A welded joint, wherein the minimum value of the Vickers hardness of the first affected portion at the first depth position and the second depth position of the first cross section and the second cross section, respectively, is 75% or more of the average Vickers hardness of the first steel plate member in the first specific range.

2. The welded joint according to claim 1 , wherein the second steel plate member has a tensile strength of less than 2.0 GPa.

3. 2. The welded joint according to claim 1, wherein the Vickers hardness at the first depth position and the second depth position intermediate positions between the target weld and the resistance spot weld adjacent to the target weld is 75% or less of the average Vickers hardness of the base material of the first steel plate member.

4. 2. The welded joint according to claim 1, wherein the Vickers hardness at the first depth position and the second depth position intermediate positions between the target weld and the resistance spot weld adjacent to the target weld is greater than 75% of the average Vickers hardness of the base material of the first steel plate member.

5. The thickness of the second steel plate member is t ２ a second surface is a surface of the second steel plate member opposite to the first steel plate member, a second back surface is a surface of the second steel plate member facing the first steel plate member, a second affected portion is a portion of the second steel plate member and the weld metal facing the second steel plate member that is more inward than 2.5 mm outward from the outer edge of the weld metal on the second back surface as viewed in the thickness direction, and a second specific range is a range of 2.5 mm to 5.0 mm outward from the outer edge of the weld metal on the second back surface as viewed in the thickness direction of the second steel plate member, In the first cross section and the second cross section, a portion of the second steel plate member that is located inside an outer edge of the weld metal on the second back surface as viewed in the thickness direction and that is 3t from the second surface of the second steel plate member toward the first steel plate member is ２ the maximum value of the Vickers hardness of the second affected zone at a third depth position of 1 / 4 is 90 to 110% of the maximum value of the Vickers hardness of the first affected zone at a position inside an outer edge of the weld metal on the first back surface as viewed in the thickness direction and at the first depth position, In the first cross section and the second cross section, within the second specific range and from the third depth position and the second surface to the first steel plate member side, ２ The welded joint according to claim 1, wherein the average Vickers hardness of the second steel plate member at a fourth depth position of 1 / 8 is 90 to 110% of the average Vickers hardness of the base material of the second steel plate member.

6. In the first cross section and the second cross section, the maximum value of Vickers hardness of the first affected zone at a position inside an outer edge of the weld metal on the first back surface and at the first depth position as viewed in the thickness direction is greater than the maximum value of Vickers hardness of the first affected zone at a position inside an outer edge of the weld metal on the first back surface and at the second depth position as viewed in the thickness direction, 2. The welded joint according to claim 1, wherein, in the first cross section and the second cross section, an average Vickers hardness of the first steel plate member within the first specific range and at the first depth position is greater than an average Vickers hardness of the first steel plate member within the first specific range and at the second depth position.

7. The first steel plate member has a flange portion extending in a first direction and a wall portion rising from an edge portion of the flange portion in a second direction perpendicular to the first direction when viewed from a thickness direction of the flange portion, The welded joint according to claim 1 , wherein the flange portion and the second steel plate member are welded together by the plurality of resistance spot welds.

8. 8. The welded joint according to claim 7, wherein, in the first affected zone in a cross section passing through a center of the target weld and parallel to the second direction and the thickness direction, the maximum Vickers hardness of the first steel plate member at the first depth position in a portion closer to the wall portion than an outer edge of the weld metal on the first back surface as viewed in the thickness direction is greater than the maximum Vickers hardness of the first steel plate member at the first depth position in a portion closer to a tip end of the flange portion than the outer edge of the weld metal on the first back surface as viewed in the thickness direction.

9. The welded joint according to claim 8 , wherein the Vickers hardness of the base material of the first steel plate member is the Vickers hardness of the first steel plate member in the wall portion.

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