Lap-welded joint and method for manufacturing lap-welded joint

Abstract

To provide a lap-welded joint which can be suppressed in cracking of a weld zone and a method for manufacturing the lap-welded joint.SOLUTION: A lap-welded joint 1 is provided, which has: a plurality of metal plates 10 and 20; a laser-welded zone 30 jointing the plates 10 and 20; resistance spot-welded zone 40 and satisfies at least either of condition (1) and condition (2). The condition (1): a dent 41 of the resistance welded zone 40 laps on a part or the whole of the end edge 31 of the laser welded zone 30. The condition (2): the resistance spot welded zone 40 is present in a vicinity of the end edge 31 of the laser welded zone 30, a HAS softened part 42 with a Vickers hardness of 90% or less than the maximum Vickers hardness of the laser welded zone 30 is present in a part or the whole of the end edge 31 of the laser welded zone 30, and in the HAS softened part 42, the Vickers hardness decreases toward the center of the resistance spot welded zone 40.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a lap weld joint and a method for manufacturing a lap weld joint. [Background technology]

[0002] In the manufacture of automotive components, the use of high-strength steel plates is being promoted in order to reduce the weight of vehicle bodies and improve safety in the event of a collision. Meanwhile, laser welding is sometimes used as a means for joining metal plates such as steel plates together. Welded joints joined by welding are required to have sufficient joint strength. Therefore, various efforts have been made to increase joint strength, such as combining welding methods. For example, Patent Document 1 attempts to increase joint strength by combining laser welding and spot welding. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-033706 Summary of the Invention [Problem to be solved by the invention]

[0004] Here, for example, a steel plate having a carbon content of 0.1 mass % or more is a high-strength steel plate having a tensile strength of approximately 980 MPa or more. Although the joint strength is improved as described above, when a plate assembly including such a high-strength steel plate is welded by laser welding, cracks may occur in the welded portion.

[0005] Furthermore, when assembling parts (e.g., bumper reinforcements, door beams, floor members, side sill reinforcements, battery frames, A-pillar reinforcements, B-pillar reinforcements, seat frames, etc.) by stacking high-strength steel plates and laser welding them in a linear stitch pattern, there is a problem in that such parts are prone to cracking due to tensile stress concentrating at the deepest part of the crater at the end of each linear stitch weld.

[0006] The present invention has been made in view of the above, and an object of the present invention is to provide a lap welded joint in which cracking of the welded portion is suppressed, and a method for manufacturing the lap welded joint. [Means for solving the problem]

[0007] (1) A lap weld joint according to one aspect of the present invention is A lap welded joint having a plurality of metal plates, a laser welded portion joining the plurality of metal plates, and a resistance spot welded portion, At least one of the following conditions (1) and (2) is satisfied: A lap welded joint characterized by: Condition (1): The end of the laser weld is partially or entirely overlapped with the dent of the resistance spot weld. Condition (2): The resistance spot weld is present near the end of the laser weld, and a HAZ-softened zone having a Vickers hardness of 90% or less of the maximum Vickers hardness of the laser weld is present in part or all of the end of the laser weld, and the Vickers hardness decreases in the HAZ-softened zone toward the center of the resistance spot weld. (2) In the lap welded joint described in (1) above, The length of the laser welded portion may be 5 to 300 mm. (3) In the lap welded joint described in (1) or (2) above, The laser weld may be formed on only one surface of the lap weld joint. (4) In the lap welded joint described in any one of (1) to (3) above, The plurality of metal plates may be a plurality of steel plates. (5) In the lap welded joint described in any one of (1) to (4) above, The chemical composition of at least one steel plate among the plurality of steel plates is C: 0.10~0.60% by mass, Si:0.01~3.50% by mass, Mn: 0.1 to 5.5 mass%, and The total of P and S may be 0.030% by mass or less. (6) A method for manufacturing a lap welded joint according to one aspect of the present invention includes: A method for manufacturing a lap welded joint having a plurality of metal plates, a laser welded portion joining the plurality of metal plates, and a resistance spot welded portion, a stacking step of stacking the plurality of metal plates; a laser welding step of welding the overlapping metal plates by laser welding; a resistance spot welding step of performing resistance spot welding in the vicinity of an end portion of the laser weld formed by the laser welding, The resistance spot welding step is performed within 10 minutes after the end of the laser welding step, The resistance spot welding process satisfies at least one of the following conditions (3) and (4): A method for manufacturing a lap welded joint. Condition (3): The resistance spot welding is performed so that the dent of the resistance spot weld overlaps part or all of the end of the laser weld. Condition (4): The resistance spot welding is performed so that a part or all of the end portion of the laser weld is tempered by the resistance spot welding. (7) In the method for manufacturing a lap welded joint described in (6) above, The length of the laser welded portion may be 5 to 300 mm. (8) In the method for manufacturing a lap welded joint described in (6) or (7), The laser weld may be formed on only one surface of the lap weld joint. (9) In the method for manufacturing a lap welded joint according to any one of (6) to (8), The plurality of metal plates may be a plurality of steel plates. (10) In the method for manufacturing a lap welded joint according to any one of (6) to (9), The chemical composition of at least one steel plate among the plurality of steel plates is C: 0.10~0.60% by mass, Si:0.01~3.50% by mass, Mn: 0.1 to 5.5 mass%, and The total of P and S may be 0.030% by mass or less. [Effects of the Invention]

[0008] According to the lap welded joint and the method for manufacturing the lap welded joint of the present invention, cracking of the welded portion can be suppressed. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic plan view illustrating a lap welded joint according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram for explaining a lap welded joint according to one embodiment of the present invention, and is a schematic cross-sectional view taken along the line AA in FIG. 1. [Figure 3] 3 is a diagram for explaining a lap welded joint according to one embodiment of the present invention, and is a schematic cross-sectional view taken along the line BB in FIG. 2. FIG. [Figure 4] FIG. 10 is a schematic plan view illustrating a lap welded joint in which the impact mark of the resistance spot weld does not overlap part or all of the end of the laser weld. [Figure 5] FIG. 5 is a diagram for explaining a lap welded joint in which the impact mark of the resistance spot weld does not overlap part or all of the end of the laser weld, and is a schematic cross-sectional view taken along the AA cross-sectional line in FIG. 4. [Figure 6]FIG. 10 is a schematic plan view illustrating another example of a lap welded joint in which the impact mark of the resistance spot weld does not overlap part or all of the end of the laser weld. [Figure 7] FIG. 1 is a schematic plan view illustrating a lap welded joint according to an embodiment of the present invention. [Figure 8] FIG. 2 is a schematic cross-sectional view illustrating a state in which metal plates are overlapped, illustrating a method for manufacturing a lap welded joint according to one embodiment of the present invention. [Figure 9] FIG. 2 is a schematic cross-sectional view illustrating a state after laser welding has been performed, illustrating a method for manufacturing a lap welded joint according to one embodiment of the present invention. [Figure 10] FIG. 10 is a diagram for explaining a method for manufacturing a lap welded joint according to one embodiment of the present invention, and is a schematic plan view showing the state after laser welding. [Figure 11] FIG. 10 is a diagram for explaining a manufacturing method of a lap welded joint according to one embodiment of the present invention, and is a schematic cross-sectional view showing a state in which resistance spot welding has been performed after laser welding has been performed. [Figure 12] FIG. 10 is a diagram for explaining a method for manufacturing a lap welded joint according to one embodiment of the present invention, and is a schematic plan view showing a state in which resistance spot welding has been performed after laser welding has been performed. [Figure 13] FIG. 10 is a schematic plan view illustrating a modified example of a lap welded joint according to an embodiment of the present invention. [Figure 14] FIG. 1 is a schematic cross-sectional view illustrating a lap welded joint according to one embodiment of the present invention, showing an example in which a laser weld is formed partially in the thickness direction of the metal plate. DETAILED DESCRIPTION OF THE INVENTION

[0010] The inventors investigated the cause of the cracks that occur in the welds described above and inferred that hydrogen embrittlement cracking occurs due to the concentration of welding residual stress (tensile stress) in a depression at the end of the weld. Generally, in laser welding, molten steel is forced to flow in the direction opposite to the welding direction, resulting in a lack of molten steel at the end where the weld ends, resulting in a depression (crater). This depression occurs when laser welding ordinary steel sheets. However, in steel sheets with a low carbon content, the welds are not hard and have low susceptibility to hydrogen embrittlement cracking, so hydrogen embrittlement cracking tends to occur rarely. However, it was thought that hydrogen embrittlement cracking is particularly pronounced in high-strength steel sheets (tensile strength of approximately 980 MPa or more) with a carbon content of 0.1% by mass or more. From the above investigations, the present inventors have concluded that hydrogen embrittlement cracking can be suppressed by applying a treatment to the deepest recessed portion of the crater in the laser weld.

[0011] Hereinafter, embodiments of the present invention will be described using examples, but it is obvious that the present invention is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present invention are obtained. Furthermore, the components of the following embodiments can be combined with each other. Furthermore, in this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. In this specification, the term "process" includes not only an independent process, but also a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved.

[0012] [Lap weld joint] A lap weld joint according to this embodiment is shown in Figure 1. Figure 1 is a schematic plan view of the lap weld joint 1 as seen from a direction perpendicular to the plate surfaces of the metal plates. A lap weld joint is a weld joint that includes a welded portion and in which at least portions of the metal plates to be joined overlap each other. As shown in FIG. 1, the lap welded joint 1 according to this embodiment is a lap welded joint 1 having a plurality of metal plates (metal plate 10 and metal plate 20), a laser welded portion 30 that joins the plurality of metal plates (metal plate 10 and metal plate 20), and a resistance spot welded portion 40.

[0013] The lap welded joint according to this embodiment satisfies at least one of the following conditions (1) and (2). Condition (1): The dent 41 of the resistance spot weld 40 overlaps part or all of the end 31 of the laser weld 30 . Condition (2): A resistance spot weld 40 is present near the end 31 of the laser weld 30, and a HAZ-softened portion 42 having a Vickers hardness of 90% or less of the maximum Vickers hardness of the laser weld 30 is present in part or all of the end 31 of the laser weld 30, and the Vickers hardness of the HAZ-softened portion 42 decreases toward the center of the resistance spot weld 40.

[0014] (Condition (1)) One of the conditions for the lap welded joint 1 according to this embodiment is that the dent 41 of the resistance spot weld 40 overlaps part or all of the end 31 of the laser weld 30. Figure 1 shows an example of a lap welded joint 1 that satisfies condition (1).

[0015] The laser welded portion 30 is a portion where steel plates or the like are melted and solidified by irradiation with a laser beam during laser welding, and contains the weld metal that melted and solidified during welding. When no filler (filler metal) is used during laser welding, the source of material for the laser welded portion 30 is the multiple metal plates being joined. When a filler metal is used during laser welding, the source of material for the laser welded portion 30 is the multiple metal plates being joined and the filler. When the metal plates are plated, the components of the plating also melt and become material that constitutes the weld metal of the laser welded portion 30. In addition to elements from these sources, the weld metal of the laser welded portion 30 may also incorporate oxygen and nitrogen from the air, as well as unavoidable impurities. Generally, in laser welding, molten metal flows in the direction opposite to the traveling direction of the laser beam (the direction indicated by arrow P in FIG. 1). As a result, a crater 32 is formed at the end 31 of the weld metal (also called a bead) of a laser welded portion 30 formed by laser welding. The end 31 of the laser welded portion 30 is the final solidification portion of the weld metal, and the crater 32 is formed because there is a shortage of molten metal in this area.

[0016] The end portion 31 of the laser welded portion 30 is in the range of 5.0 mm along the extending direction of the laser welded portion 30 from the end E of the laser welded portion 30 where the crater 32 exists. 1, a dent 41 of a resistance spot weld 40 overlaps a part of an end portion 31 of a laser weld 30. In FIG. 1, the length L is 5.0 mm, and this range is defined as end portion 31.

[0017] FIG. 2 shows a cross-sectional view of the lap weld joint 1 of FIG. The cut surface of the lap welded joint 1 in FIG. 2 is a plane parallel to the extension direction of the laser weld 30 in FIG. 1 (the direction along the weld line of the laser weld 30) and the thickness direction of the metal plates 10 and 20, passing through the center of the laser weld 30, and is parallel to the X and Z coordinate axes. The cross-sectional view of FIG. 2 is a cross-sectional view taken along the arrow AA in FIG. 1. The X, Y, and Z coordinate axes in FIG. 1 and other figures are perpendicular to each other. The metal plate 10 has one plate surface 10a and the other plate surface 10b, and the metal plate 20 has one plate surface 20a and the other plate surface 20b. In this example, the plate surface 10b of the metal plate 10 faces the plate surface 20a of the metal plate 20.

[0018] 1 and 2 show an example in which a laser weld 30 is formed by irradiating a laser beam from the plate surface 10a side (positive side of the Z coordinate axis) of the metal plate 10. In this example, the laser weld 30 is formed over the entire thickness direction of the metal plate 10 and the metal plate 20. In this case, a crater can be formed at the end of the weld metal on both the side irradiated with the laser beam and the opposite side. On the other hand, the laser weld 30 may be formed throughout the entire thickness direction of the metal plate 10, and may be formed from one plate surface 20a of the metal plate 20 to the plate surface 20b, up to halfway in the thickness direction.

[0019] As will be described later, in the present invention, after laser weld 30 is formed in a laser welding process, resistance spot weld 40 is formed in a resistance spot welding process. Therefore, dent 41 in resistance spot weld 40 causes part or all of end 31 of laser weld 30 to be crushed in the thickness direction of the metal plate (Z coordinate axis direction). In this way, the dent 41 made during resistance spot welding overlaps with the end 31 of the laser weld 30, flattening the most recessed part of the crater 32 present at the end 31 of the laser weld 30 and the area nearby, suppressing stress concentration at the most recessed part, thereby suppressing hydrogen embrittlement cracking. This is thought to be because the tiny solidification cracks that occur at the most recessed part of the crater 32 when the laser weld 30 is formed are filled by the flow of material when the dent 41 is formed, and do not become the starting point for stress concentration.

[0020] The end E of the laser weld 30 is identified by observing the appearance of a sample of the lap welded joint 1 as shown in FIG. 1. The laser weld 30 (also called the weld metal) can be visually distinguished from the metal plate 10 or the metal plate 20. The surface of the metal plate 10 or the metal plate 20 is generally formed with few irregularities. On the other hand, because the laser weld 30 has been melted once, a wave pattern called a ripple is often formed on its surface, and it is from these surface differences that the laser weld 30 can be distinguished from the metal plate 10 or the metal plate 20.

[0021] The dent 41 in the resistance spot weld 40 is identified by observing a sample of the lap welded joint 1 as shown in Figure 1. The sample is measured with a laser displacement meter, and the area recessed by 0.01 mm or more from the surface of the lap welded joint 1 is identified as the dent 41. The indentation 41 of the resistance spot weld 40 is not particularly limited, but is generally circular.

[0022] Fig. 3 is a cross-sectional view taken along the arrow BB in Fig. 2. The cross section in Fig. 3 is parallel to the plate surface of the metal plate 10 or the metal plate 20. As shown in Fig. 3, a portion of the end portion 31 of the laser welded portion 30 has disappeared due to a nugget 43 formed by the resistance spot welded portion.

[0023] If the resistance spot weld 40 is formed in the resistance spot welding process and then the laser weld 30 is formed in the laser welding process, the dent 41 in the resistance spot weld 40 will not overlap part or all of the end 31 of the laser weld 30. In this case, as shown in FIG. 4 , the laser weld 30, including the end 31, is present inside the dent 41. From this state of the laser weld 30, it can be confirmed that the dent 41 in the resistance spot weld 40 does not overlap part or all of the end 31 of the laser weld 30. In this case, also in the cross section of the lap welded joint, the laser weld 30 extends to the nugget 43 formed by resistance spot welding, as shown in the cross section of Fig. 5. The cut surface of the lap welded joint 1 in Fig. 5 is a plane that is parallel to the extension direction of the laser weld 30 in Fig. 4 (the direction along the weld line of the laser weld 30) and the thickness direction of the metal plates 10 and 20, passes through the center of the laser weld 30, and is parallel to the X coordinate axis and the Z coordinate axis.

[0024] Furthermore, the positional relationship between the terminal end 31 of the laser welded portion 30 and the dent 41 of the resistance spot welded portion 40 as shown in Figure 6 does not satisfy condition (1) that the dent 41 of the resistance spot welded portion 40 overlaps part or all of the terminal end 31 of the laser welded portion 30.

[0025] (Condition (2)) In the lap welded joint 1 of this embodiment, one of the conditions is that a resistance spot weld 40 is present near the terminal end 31 of the laser weld 30, a HAZ softened portion having a Vickers hardness of 90% or less of the maximum Vickers hardness of the laser weld 30 is present in part or all of the terminal end 31 of the laser weld 30, and the Vickers hardness of the HAZ softened portion 42 decreases toward the center of the resistance spot weld 40.

[0026] The HAZ-softened portion 42 is a portion that occurs in the resistance spot weld 40 due to tempering during resistance spot welding. Generally, the HAZ has a HAZ-hardened portion 44 that has a higher Vickers hardness than the metal plate being welded, and the HAZ-softened portion 42 that has a lower Vickers hardness.

[0027] The HAZ-softened zone 42 is formed approximately concentrically with the resistance spot weld 40. The HAZ-hardened zone 44 and the HAZ-softened zone 42 are formed in this order from the center of the resistance spot weld 40 outward. The HAZ-softened zone 42 is affected by heat during welding, but its temperature does not rise above the transformation point. Therefore, tempering occurs in the HAZ-softened zone 42, resulting in a lower Vickers hardness than the metal plate 10 or the metal plate 20.

[0028] The presence of a HAZ softened portion 42 in part or all of the terminal end 31 of the laser welded portion 30 means that when the state shown in Figure 2 is observed, the range of the HAZ softened portion 42 includes part or all of the terminal end 31 of the laser welded portion 30.

[0029] (Method for measuring Vickers hardness and method for identifying HAZ softened areas) The Vickers hardness of the laser welded portion 30 and the HAZ-softened portion 42 is determined as follows using a Vickers hardness tester. From the lap welded joint 1 to be measured, a plane parallel to the extension direction of the laser weld 30 (the direction along the weld line of the laser weld 30) and passing through the center of the laser weld 30 is cut out over an area of ​​approximately 40 mm perpendicular to the surface of the metal plate 10 or 20, so as to include the end portion 31. The cut-out portion is then embedded in resin, polished to a mirror finish, and then etched with metal flow to prepare a sample. By etching with metal flow, the melted and solidified portions, i.e., the nugget 43 of the resistance spot weld 40 and the laser weld 30, appear to have a different contrast from the unmelted and solidified portions, making them easy to distinguish. This sample has a cross section, for example, as shown in FIG. 2. For this sample, the Vickers hardness of the laser welded portion 30 is measured at 0.2 mm intervals in a direction parallel to the plate surfaces of the metal plates 10 and 20 at positions 0.2 mm away in the thickness direction of the metal plates 10 and 20 from the overlapping surface of the metal plates 10 and 20. The Vickers hardness is measured under a load of 300 gf. Of the Vickers hardness values ​​of the laser welded portion 30 obtained by the above measurement, five points in descending order of magnitude are extracted, and the average value of the Vickers hardness values ​​of these five points is defined as the maximum Vickers hardness of the laser welded portion 30 .

[0030] Next, the region where the Vickers hardness is 90% or less of the maximum Vickers hardness of the laser weld 30 is identified as the HAZ-softened region 42. Note that even if the laser weld 30 is short, the Vickers hardness of the HAZ-softened region 42 decreases toward the center of the resistance spot weld 40, so the region where the Vickers hardness is 90% or less of the maximum Vickers hardness, which is the average of the (maximum) five points, may be defined as the HAZ-softened region 42. If a nugget 43 is present at the measurement location, the nugget 43 is not included in the Vickers hardness measurement. The nugget 43 is corroded by metal flow and etched more deeply, appearing to have a different contrast, making it possible to distinguish it from the laser weld 30 and the HAZ softened portion 42.

[0031] The above measurements are performed on both of the multiple metal plates, and it is sufficient that any one of the multiple metal plates meets the requirement that a HAZ softened portion having a Vickers hardness of 90% or less of the maximum Vickers hardness of the laser welded portion 30 exists in part or all of the end portion 31 of the laser welded portion 30.

[0032] If the lap weld joint 1 has been processed by a press or the like, work hardening occurs at the bent portion, making it impossible to measure the Vickers hardness accurately. Therefore, when measuring a processed lap weld joint 1, it is best to use a flat sample that has not been bent as much as possible as the sample area.

[0033] In the HAZ-softened portion 42 , the Vickers hardness decreases toward the center of the resistance spot weld 40 . In the cross section of the sample to be measured for Vickers hardness as described above, it is sufficient that the Vickers hardness decreases from the end of the HAZ-softened zone 42 toward a point on the line passing through the center of the laser weld 30 and its extension C, where the distance between the line passing through the center of the laser weld 30 and its extension C (C in FIG. 7) is shortest and the center of the resistance spot weld 40 is the shortest. Using this point as an index, if the hardness decreases from the end of the HAZ-softened zone 42 toward this point, it can be considered that the Vickers hardness decreases toward the center of the resistance spot weld 40.

[0034] For example, as shown in FIG. 7, even if the center of the resistance spot weld 40 is offset from the extension direction of the laser weld 30, the requirements of this embodiment are met as long as the terminal end 31 of the laser weld 30 and the resistance spot weld 40 are in a positional relationship that satisfies at least one of the above conditions (1) and (2).

[0035] By providing the resistance spot weld 40 in the vicinity of the end portion 31, the end portion 31 is tempered and its hardness is reduced, thereby making it possible to suppress hydrogen embrittlement cracking.

[0036] The end of the laser weld is a range of 5.0 mm from the end (end) of the laser weld along the length of the laser weld (the direction in which the laser beam traveled during laser welding).

[0037] Note that, although laser weld 30 joins metal plate 10 and metal plate 20, which are multiple overlapping metal plates, metal plate 10 and metal plate 20 do not necessarily have to be joined at resistance spot weld 40. Resistance spot weld 40 may be formed for the purposes of suppressing stress concentration by forming dents 41 and tempering by HAZ softened portion 42.

[0038] [Manufacturing method for lap welded joints] The method for manufacturing a lap welded joint according to this embodiment is as follows: A method for manufacturing a lap welded joint (1) having a plurality of metal plates (10 and 20), a laser welded portion (30) joining the plurality of metal plates (10 and 20), and a resistance spot welded portion (40), a stacking step of stacking a plurality of metal plates 10 and 20; a laser welding step of welding the overlapping metal plates 10 and 20 together by laser welding; and a resistance spot welding step of performing resistance spot welding in the vicinity of the end of the laser welded portion 30 formed by laser welding.

[0039] The lap welded joint 1 according to this embodiment is basically the same as the lap welded joint 1 described above, and therefore a description thereof will be omitted here.

[0040] (Laminating process) In the overlapping step, the metal plate 10 and the metal plate 20 are overlapped as shown in Fig. 8. Fig. 8 is a cross-sectional view of the metal plate 10 and the metal plate 20 taken along a plane perpendicular to the plate surfaces of the metal plate 10 and the metal plate 20. In the overlapping step, at least a portion of the metal plates 10 and 20, which are a plurality of metal plates, is overlapped, and the overlapped portion is subsequently subjected to laser welding and resistance spot welding.

[0041] (Laser welding process) In the laser welding step, the metal plates 10 and 20 that have been overlapped in the overlapping step are welded by laser welding. In the laser welding process, as shown in FIG. 9, laser welding is performed to weld and join overlapping metal plates 10 and 20. In the example of FIG. 9, a laser beam is irradiated from the plate surface 10a side of metal plate 10. The direction indicated by arrow P is the direction of movement during laser beam irradiation. At this stage, a crater 32 is formed at the end 31 of laser weld 30 formed by laser welding.

[0042] Fig. 10 is a diagram showing a state in which the laser welded portion 30 has been formed, and is a plan view seen from the plate surface 10a side of the metal plate 10 in Fig. 9. The cross-sectional view of Fig. 9 is a cross-sectional view taken along the arrow AA in Fig. 10.

[0043] (Resistance spot welding process) In the resistance spot welding step, resistance spot welding is performed near the end 31 of the laser welded portion 30 formed by laser welding. In the resistance spot welding process, resistance spot welding is performed on the vicinity of and including end portion 31 of laser welded portion 30 formed by laser welding, as shown in Fig. 11. In the example of Fig. 11, end portion 31 of laser welded portion 30 is overlapped with dent 41 and softened HAZ 42 of resistance spot welded portion 40, but the position at which resistance spot welding is performed is not limited to the example of Fig. 11, as long as it satisfies at least one of conditions (3) and (4) described below.

[0044] Fig. 12 is a diagram showing a state in which resistance spot welding is performed to form resistance spot welds 40 after forming laser welds 30, and is a plan view seen from the plate surface 10a side of metal plate 10 in Fig. 11. The cross-sectional view of Fig. 11 is a cross-sectional view taken along the arrow AA in Fig. 12.

[0045] In the method for manufacturing a lap welded joint according to this embodiment, the resistance spot welding step is performed within 10 minutes after the end of the laser welding step. By performing the resistance spot welding in the resistance spot welding step within 10 minutes after performing the laser welding in the laser welding step, the effect of suppressing weld cracking can be obtained.

[0046] In the method for manufacturing a lap welded joint according to this embodiment, the resistance spot welding step satisfies at least one of the following conditions (3) and (4).

[0047] Condition (3): Resistance spot welding is performed so that the dent 41 of the resistance spot weld overlaps part or all of the end 31 of the laser welded portion 30 . After the laser welding process, resistance spot welding is performed so as to satisfy condition (3), whereby end portion 31 of laser welded portion 30 is crushed and the most depressed portion of crater 32 that existed in end portion 31 and the area nearby are flattened. By satisfying condition (3), the dent 41 of resistance spot welded portion 40 overlaps part or all of end portion 31 of laser welded portion 30, as described above.

[0048] Condition (4): Resistance spot welding is performed so that a part or the whole of the end portion 31 of the laser welded portion 30 is tempered by the resistance spot welding. After the laser welding process, resistance spot welding is performed so as to satisfy condition (4), thereby forming the HAZ-softened zone 42 as described above. Within the range of this HAZ-softened zone 42, the terminal end 31 is tempered, reducing its hardness and thereby suppressing hydrogen embrittlement cracking.

[0049] The welding conditions for resistance spot welding are preferably a pressure and current sufficient to produce a nugget wider than the laser weld. If the resistance spot welding current is too low, the end 31 of the laser weld 30 will not plastically deform, and the HAZ softened zone 42 will not be formed, which is undesirable. If the resistance spot weld 40 is too far from the end 31, the HAZ softened zone 42 will not be able to surround part or all of the end 31, preventing proper tempering, which is undesirable.

[0050] Note that, although metal plate 10 and metal plate 20, which are multiple overlapping metal plates, are joined by laser welded portion 30, metal plate 10 and metal plate 20 do not necessarily have to be joined at resistance spot welded portion 40. Resistance spot welding may be performed for the purposes of suppressing stress concentration by forming dents 41 or tempering by HAZ softened portion 42.

[0051] The relationship between the size of the dent 41 in the resistance spot weld 40 and the size of the HAZ-softened portion 42 is not limited to the form shown in Fig. 1 etc. For example, as shown in Fig. 13, when viewed in a direction perpendicular to the plate surface of the metal plate 10 or metal plate 20, the HAZ-softened portion 42 and the HAZ-hardened portion 44 formed inside the metal plate 10 or metal plate 20 may be smaller than the dent 41 formed on the plate surface of the metal plate 10 or metal plate 20. When the heat input of the spot welding is relatively small, there are cases where only the HAZ-softened zone 42 is formed without the formation of the HAZ-hardened zone 44. On the other hand, when the heat input of the spot welding is relatively large, there are cases where the HAZ-softened zone 42, the HAZ-hardened zone 44, and the nugget 43, each of which has a diameter smaller than the dent 41, are formed.

[0052] In the lap welded joint 1 according to the above embodiment, the length of the laser welded portion 30 may be 5 to 300 mm. From the viewpoint of further suppressing cracking in the welded portion, the length of the laser welded portion 30 is preferably 5 mm or more, more preferably 10 mm or more, and even more preferably 15 mm or more. On the other hand, if the length of the laser welded portion 30 exceeds 300 mm, the time required for laser welding may increase, resulting in a loss of productivity. From this viewpoint, the length of the laser welded portion 30 is preferably 300 mm or less, more preferably 200 mm or less, and even more preferably 100 mm or less. The length of the laser welded portion 30 is the length passing through the center of the laser welded portion 30 in the width direction at each point in the extension direction of the laser welded portion 30.

[0053] The laser welded portion 30 may be formed on only one surface of the lap welded joint 1. This has the effect of further suppressing weld cracks. As shown in Figure 14, the laser weld 30 may be formed throughout the entire thickness direction of the metal plate 10, and may be formed from one plate surface 20a of the metal plate 20 to one plate surface 20b, up to halfway in the thickness direction.

[0054] In the lap welded joint 1 according to the above embodiment, the plurality of metal plates is preferably a plurality of steel plates. In the lap welded joint according to this embodiment, it is more preferable that the plurality of steel plates are high-strength steel plates having a tensile strength of 980 MPa or more. When the above-described weld cracks occur, fractures caused by the weld cracks at the end portion 31 may propagate along the extension direction of the laser welded portion 30, and weld cracks may form throughout the entire laser welded portion 30. The greater the tensile strength of the steel plates, the greater the tensile stress applied to the end portion 31 of the laser welded portion 30 after laser welding, increasing the risk of weld cracks occurring at the end portion 31 of the laser welded portion 30. From the viewpoint of increasing the strength of the lap welded joint, the higher the strength of the steel plates, the more preferable. For example, the tensile strength of the multiple steel plates may be 980 MPa or more, 1000 MPa or more, or 1100 MPa or more. The higher the tensile strength of the steel plates, the greater the tensile stress applied to the end portion 31 of the laser welded portion 30 after welding. However, in the lap welded joint according to this embodiment, cracking is suppressed by satisfying the above-mentioned conditions. The tensile strength of the steel plates is measured in accordance with JIS Z 2241 by cutting test pieces from the steel plates according to the size of the parts. The test pieces are cut from flat portions at least 5 mm away from the welded portion.

[0055] In the lap welded joint 1 according to the above embodiment, the chemical composition of at least one steel plate among the plurality of steel plates is C: 0.10~0.60% by mass, Si:0.01~3.50% by mass, Mn: 0.1 to 5.5 mass%, and The total of P and S is preferably 0.030 mass % or less.

[0056] The chemical composition of the above-mentioned steel sheet is measured by sample preparation and quantitative analysis in accordance with the standards listed in Table 1 of JIS G0321 (2010). The sample used to measure these chemical compositions is taken from one location (preferably a location) at least 5 mm away from the laser weld 30 of the steel sheet, and the measured values ​​of this sample are used. If the steel sheet has a plating layer on its surface, the plating layer can be removed by mechanical grinding before analyzing the chemical composition.

[0057] The shape of laser weld 30 when viewed from the plate surface side of the metal plate is not particularly limited, and may be linear, C-shaped, wavy, bent, curved, bent, etc. In the above example, the width of laser weld 30 formed by laser welding is not particularly limited, and may be designed to suit the desired joint strength and the shape of the steel plates to be joined. [Example]

[0058] The present invention will be specifically described below by way of examples, but the present invention is not limited thereto.

[0059] In this example, two steel plates were prepared as the plurality of metal plates, and these were overlapped and welded to form a lap welded joint. The chemical composition, tensile strength and thickness of each of the steel plates A to C used were as shown in Table 1.

[0060] The chemical compositions of the steel plates described above were prepared and quantitatively analyzed in accordance with the standards listed in Table 1 of JIS G0321 (2010). The sample used to measure these chemical compositions was taken from one location (preferable location) at least 5 mm away from the weld of the steel plate, and the measured values ​​of this sample were used. The tensile strength of the steel plate was measured in accordance with JIS Z 2241 by cutting out test pieces according to the size of the part from the steel plate. The test pieces were cut out from a flat part at least 5 mm away from the weld. The thickness of the steel plate was measured at five points using a caliper, and the arithmetic mean value of these measurements was taken as the thickness of the steel plate.

[0061] [Table 1]

[0062] Two steel plates were overlapped in the combinations (plate combinations) shown in Table 2, and laser welding and resistance spot welding were performed. In addition, the above steel sheet is coated with 3g / m2 The specimens were subjected to the experiment after being coated with anti-rust oil. Regarding the welding order, in Table 2, "L" indicates an experimental example in which only laser welding was performed, "L → S" indicates an experimental example in which laser welding was performed followed by resistance spot welding, and "S → L" indicates an experimental example in which resistance spot welding was performed followed by laser welding.

[0063] Regarding the positional relationship between the end of the laser weld and the dent or HAZ-softened portion of the resistance spot weld, experimental examples in which the dent of the resistance spot weld overlapped part or all of the end of the laser weld (those satisfying condition (1) of the above embodiment) were marked with a "○" in the "Condition (1)" category, while experimental examples in which this condition was not satisfied were marked with an "×" in the "Condition (2)" category. Furthermore, experimental examples in which the end of the laser weld had a HAZ-softened portion with a Vickers hardness of 90% or less of the maximum Vickers hardness of the laser weld or less, and in which the Vickers hardness decreased toward the center of the HAZ (those satisfying condition (2) of the above embodiment), were marked with a "○" in the "Condition (2)" category, while experimental examples in which this condition was not satisfied were marked with an "×" in the "Condition (2)" category.

[0064] The "welding time" item was the time from when laser welding was performed to when resistance spot welding was performed. In the "Laser weld length" section, the length along the extension direction of the laser weld was measured from the appearance as shown in Figure 1.

[0065] "Spot weld diameter" refers to the nugget diameter (see JIS Z 3001-6 2021 Figure 5). The nugget diameter was measured in accordance with JIS Z 3139. The spot weld diameter was calculated using the thickness of the thinner steel plate in the sheet assembly as t.

[0066] In the "Laser Welding Penetration" category, experimental examples in which a laser weld was formed across the entire thickness of two steel plates were classified as "full penetration," while experimental examples in which a laser weld was formed partway through the thickness of one of the two steel plates were classified as "partial penetration."

[0067] The laser welding was carried out under the following conditions: Speed: 3~5m / min Laser output at processing point: 3 to 5 kW Pressurized air was blown at a height of 100 mm above the molten pool to remove the laser-induced plume.

[0068] Resistance spot welding was carried out under the following conditions: A stationary servo pressure single-phase AC (50Hz) welding machine was used. The electrode used was a dome radius chromium copper electrode with a tip R40. The welding conditions were a pressure of 300-600 kg, a squeeze time of 50 cycles, a welding time of 16-20 cycles, a welding current of 2.0 kA-8.0 kA, and a holding time of 8 cycles.

[0069] (Evaluation of weld cracks) Three lap welded joints were fabricated under the same conditions, and each lap welded joint was evaluated for weld cracking. The evaluation was carried out 15 minutes after the last laser welding or resistance spot welding. Cases in which cracks occurred in all test pieces were rated x (bad), cases in which no cracks occurred in some test pieces were rated ○ (good), and cases in which no cracks occurred in any test pieces were rated ◎ (very good). The presence or absence of cracks was confirmed by visually inspecting the test pieces after welding.

[0070] [Table 2] As can be seen from the results in Table 2, each example that met the requirements of the present application achieved good results in the evaluation of weld cracking. [Industrial Applicability]

[0071] According to the lap welded joint and the method for manufacturing the lap welded joint of the present invention, cracking of the welded portion can be suppressed. Therefore, the present invention is extremely useful industrially. [Explanation of symbols]

[0072] 1. Lap weld joint 10, 20 metal plate 30 Laser welded section 40 Resistance spot welds

Claims

1. A lap welded joint having a plurality of metal plates, a laser welded portion joining the plurality of metal plates, and a resistance spot welded portion, At least one of the following conditions (1) and (2) is satisfied: A lap welded joint characterized by: Condition (1): The dent of the resistance spot weld overlaps part or all of the end of the laser weld. Condition (2): The resistance spot weld is present near the end of the laser weld, and a HAZ-softened zone having a Vickers hardness of 90% or less of the maximum Vickers hardness of the laser weld is present in part or all of the end of the laser weld, and the Vickers hardness decreases in the HAZ-softened zone toward the center of the resistance spot weld.

2. The length of the laser weld is 5 to 300 mm.

2. The lap weld joint according to claim 1.

3. The laser weld is formed on only one surface of the lap weld joint.

3. The lap welded joint according to claim 1 or 2.

4. The plurality of metal plates are a plurality of steel plates.

3. The lap welded joint according to claim 1 or 2.

5. The chemical composition of at least one steel plate among the plurality of steel plates is C: 0.10 to 0.60% by mass, Si: 0.01 to 3.50% by mass, Mn: 0.1 to 5.5 mass%, and The total of P and S: 0.030% by mass or less, 5. The lap weld joint according to claim 4.

6. A method for manufacturing a lap welded joint having a plurality of metal plates, a laser welded portion joining the plurality of metal plates, and a resistance spot welded portion, a stacking step of stacking the plurality of metal plates; a laser welding step of welding the overlapping metal plates by laser welding; a resistance spot welding step of performing resistance spot welding in the vicinity of an end portion of the laser weld formed by the laser welding, The resistance spot welding step is performed within 10 minutes after the end of the laser welding step, The resistance spot welding process satisfies at least one of the following conditions (3) and (4): A method for manufacturing a lap welded joint. Condition (3): The resistance spot welding is performed so that the dent of the resistance spot weld overlaps part or all of the end of the laser weld. Condition (4): The resistance spot welding is performed so that a part or all of the end portion of the laser weld is tempered by the resistance spot welding.

7. The length of the laser weld is 5 to 300 mm. The method for manufacturing a lap welded joint according to claim 6 .

8. The laser weld is formed on only one surface of the lap weld joint.

8. The method for manufacturing a lap welded joint according to claim 6 or 7.

9. The plurality of metal plates are a plurality of steel plates.

8. The method for manufacturing a lap welded joint according to claim 6 or 7.

10. The chemical composition of at least one steel plate among the plurality of steel plates is C: 0.10 to 0.60% by mass, Si: 0.01 to 3.50% by mass, Mn: 0.1 to 5.5 mass%, and The total of P and S: 0.030% by mass or less, The method for manufacturing a lap welded joint according to claim 9 .

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