Overlap laser-welded joint, method for manufacturing an overlap laser-welded joint, and structural member for automobile body

Abstract

To provide a laser lap welding joint capable of suppressing welding cracks at a bead tail end, a method for manufacturing the same, and a structure member for an automobile body.SOLUTION: The laser lap welding joint comprises; a plurality of metal plates; and a first bead that is a linearly extending laser welding part. The ratio G / T of a total value G of sizes of gaps among the metal plates to a total value T of thicknesses of the metal plates is 0-18%. The length L1 of the first bead is 10-100 mm. The laser lap welding joint further comprises a second bead. The length L2 of the second bead is shorter than the length L1. In a tail end surrounding region, the length of a region of the second bead having a distance of 2 mm or shorter from the width end of the first bead is 2 mm or shorter along the center axis of the first bead. The second bead is isolated from the center axis of the first bead. The second bead is not included in a start end surrounding region.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a laser lap welding joint, a method for manufacturing a laser lap welding joint, and a structural member for an automobile body.

Background Art

[0002] A lap joint is a welded joint obtained by welding a plurality of overlapped metal plates. One of the welding means for manufacturing a lap joint is laser welding. Laser lap welding is a welding method in which a laser beam is irradiated onto one surface of a plurality of overlapped metal plates, and the metal plates are melted and solidified to join these metal plates.

[0003] Laser lap welding can join a narrow area such as a flange portion of a hat-shaped member at high speed. However, laser lap welding has a problem that cracks are likely to occur at the final solidification portion at the end of the weld bead.

[0004] In laser welding, a flow of molten metal occurs in the direction opposite to the traveling direction of the laser. Therefore, a depression called a crater occurs at the end portion of the bead formed by laser welding. Further, after the laser welding is completed, a tensile stress is applied to the end portion of the bead. This is because after the laser welding is completed, the welded portion is rapidly cooled and shrinks due to heat extraction from the welded portion to its periphery. When a tensile stress is applied to the end portion of the bead where the crater is formed, the end portion of the bead may break so as to be torn perpendicularly to the extending direction of the bead. In this case, the breakage at the end portion may progress along the bead, and cracks may be formed over the entire bead.

[0005] In recent years, there has been an increasing trend towards using high-strength materials for mechanical structural components. For example, in automotive body components, particularly structural components that form the car's frame, high-strength steel sheets with a tensile strength of 980 MPa or higher are increasingly being used to improve the strength and rigidity of the vehicle body. However, the higher the tensile strength of the metal sheet, the greater the tensile stress applied to the bead end after laser welding, increasing the risk of cracking at the bead end. If a crack occurs along the entire length of the bead, the static strength of the joint, such as shear strength and peel strength, decreases, and the fatigue strength also decreases significantly. For these reasons, there is a strong demand for technology to prevent cracking at the bead end in high-strength metal sheets.

[0006] Patent Document 1 discloses a lap laser welded joint, a method for manufacturing the same, and a structural member for an automobile body having the same welded joint, in which, when intermittently irradiating a laser beam onto one side surface of a steel plate formed by overlapping multiple steel plates, a welded joint is formed in which a linear first joint and a linear subsequent joint following the first joint are arranged in a row, and at least the total gap G between the steel plates constituting the welded joint is set to within 0 to 15% of the total thickness T of the steel plates constituting the welded joint, and the direction of movement of the welding head that irradiates the laser beam and the scanning direction of the laser beam are in opposite directions, so that the welding start end of the first joint and the welding end of the subsequent joint adjacent to the first joint face each other, and the welding start end and welding end of the subsequent joints face each other, and the various dimensions of the joint are controlled to an appropriate range, the welded joint is formed so that cracks do not occur at the welding end of the joint and the peel strength is excellent.

[0007] Patent Document 2 describes a laser-welded joint in which multiple steel plates, each having a thickness of 0.5 to 3.0 mm and coated with oil on its surface, are overlapped and laser-welded, including at least one high-tensile steel plate having a tensile strength of 780 MPa or higher, wherein the overlapping portion of the multiple steel plates has a weld bead that penetrates from the surface of the uppermost steel plate to the back surface of the lowermost steel plate, and the minimum Vickers hardness of the weld bead is 350 Hv or higher, and the front A laser-welded joint is disclosed, characterized in that the weld metal in the weld bead contains, by mass%, C: 0.05~0.35%, Si: 0.01~2.5%, Mn: 0.5~5.0%, P: 0.05% or less, S: 0.01% or less, with the remainder being Fe and unavoidable impurities, satisfying C+Si / 5≦0.4, and the front bead width WS and back bead width WB in the weld bead satisfy WS+WB≦3.5.

[0008] Patent Document 3 describes a method in which multiple steel plates are overlapped or butted together, and a laser is irradiated onto the surface of the uppermost steel plate at the overlapping or butted portion, melting the steel plate down to the back surface of the lowermost steel plate to form a weld. In this method, the absolute humidity is 2 g / m² within 10 minutes after the weld is formed. 3 A steel structure is disclosed characterized by supplying the following shielding gas to the weld on the surface of the uppermost steel plate, while re-irradiating the weld metal with a laser one or more times so as not to penetrate to the back surface of the lowermost steel plate which is in contact with the atmosphere, thereby re-melting the weld metal for the number of irradiations.

[0009] Patent Document 4 discloses a lap laser welding method in which welding is performed by scanning a laser beam along the edges of multiple stacked workpieces, characterized in that the laser beam is irradiated obliquely to the workpiece surface from a direction tilted 45 to 90 degrees with respect to the beam scanning direction, and the bead is tilted in a cross section perpendicular to the beam scanning direction.

[0010] Patent Document 5 discloses a laser welding method for welding multiple metal members together by irradiating a portion where multiple metal members are overlapped with laser light, wherein at least one of the multiple metal members is formed from a plated steel sheet in which the base material is covered with a coating material having a lower melting point than the base material, and the method comprises a pre-treatment step in which, with the in-plane position of one of the metal members determined, processing is performed from one side of the one metal member to generate a protrusion that rises from the other opposite side, and a welding step in which the one metal member with the protrusion generated is overlapped with other metal members while maintaining the in-plane position with the protrusion interposed, and laser light is irradiated to the portion where the multiple metal members are overlapped to weld the multiple metal members together. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] International Publication No. 2020 / 194669 [Patent Document 2] Japanese Patent Publication No. 2019-188407 [Patent Document 3] Japanese Patent Publication No. 2012-240083 [Patent Document 4] Japanese Patent Publication No. 2008-296236 [Patent Document 5] International Publication No. 2014 / 126172 [Overview of the project] [Problems that the invention aims to solve]

[0012] The technology described in Patent Document 1 suppresses welding cracks by making the bead end J-shaped. However, the technology in Patent Document 1 requires the bead end to be J-shaped and cannot be applied to members with a small width at the weld, for example, the width of the flange. Also, the length of the bead must be above a predetermined value and cannot be applied to short beads. The technologies in Patent Documents 2 to 5 do not consider the relaxation of tensile stress at the bead end. Furthermore, the technology in Patent Document 3 has the problem of requiring a long welding time.

[0013] In view of the above circumstances, the object of the present invention is to provide a lap laser welded joint capable of suppressing welding cracks at the bead end, a method for manufacturing a lap laser welded joint, and a structural member for an automobile body. [Means for solving the problem]

[0014] The gist of this invention is as follows:

[0015] (1) An overlapping laser-welded joint according to one aspect of the present invention comprises a plurality of overlapping metal plates and a first bead which is a linearly extending laser-welded portion that joins the plurality of metal plates, wherein the ratio G / T of the sum of the gaps between the plurality of metal plates G to the sum of the thicknesses T of the plurality of metal plates is 0 to 18%, the length L1 of the first bead measured along the central axis along the extending direction of the first bead is 10 to 100 mm, the overlapping laser-welded joint further comprises a second bead, the length L2 of the second bead measured along the central axis along the extending direction of the second bead is smaller than L1, and in a plan view of the overlapping laser-welded joint as seen from the thickness direction, a first imaginary line passing through the end of the first bead and perpendicular to the central axis of the first bead at the end, and the first bead extending from the end of the first bead In the end peripheral region, which is the area between a first point 5 mm away from the central axis of the first bead toward the starting end of the bead and a second imaginary line perpendicular to the central axis of the first bead at the first point, the length of the area in the second bead that is 2 mm or less away from the widthwise end of the first bead is 2 mm or more toward the central axis of the first bead, and the second bead is separated from the central axis of the first bead and does not include the start peripheral region, which is the area between a third imaginary line passing through the starting end of the first bead and perpendicular to the central axis of the first bead at the starting end and a fourth imaginary line passing through a second point 1 / 3 × L1 away from the starting end of the first bead toward the end of the first bead toward the starting end of the first bead and perpendicular to the central axis of the first bead at the second point. (2) In the overlapping laser welded joint described in (1) above, in a plan view of the overlapping laser welded joint as seen from the thickness direction, the second bead may be formed on both sides of the first bead with reference to the central axis of the first bead. (3) In the overlapping laser welded joint described in (1) or (2) above, the second bead may be separated from a fifth imaginary line that passes through a third point 2 mm away from the end of the first bead, in the direction opposite to the start of the first bead, along the central axis of the first bead, and that is perpendicular to the central axis of the first bead at the third point. (4) In the overlapping laser welded joint described in any one of the above items (1) to (3), the first bead may be formed only on one surface of the overlapping laser welded joint. (5) In the overlapping laser-welded joint described in any one of the above items (1) to (4), the second bead may be formed only on one of the metal plates arranged on the outermost surface of the overlapping laser-welded joint, and the penetration depth of the second bead may be 1 / 2 or more of the thickness of the metal plate on which the second bead is formed. (6) In the lap laser welded joint described in any one of the above items (1) to (5), the plurality of metal plates are a plurality of steel plates, and the component composition of one or more of the plurality of steel plates may include C: 0.05 to 0.5 mass%, Si: 0.1 to 3.5 mass%, Mn: 0.1 to 5.5 mass%, and P and S: total 0.03 mass% or less. (7) In the lap laser welded joint described in any one of the above items (1) to (6), the plurality of metal plates may be a plurality of steel plates, and one or more of the plurality of steel plates may have a tensile strength of 980 MPa or more. (8) In the lap laser welded joint described in any one of the above paragraphs (7), the tensile strength of the steel plate on which the second bead is formed among the plurality of steel plates may be 980 MPa or more.

[0016] (9) A method for manufacturing a lap laser welded joint according to another aspect of the present invention is a method for manufacturing a lap laser welded joint comprising the steps of: overlapping a plurality of metal plates; and irradiating with a first laser to form a first bead, which is a linearly extending laser welded portion that joins the plurality of metal plates, wherein in the overlapping, the ratio G / T of the total size of the gaps between the metal plates G to the total thickness T of the metal plates is 0 to 18%, and the length L1 of the first bead, measured along the central axis along the extending direction of the first bead, is 10 to 100 mm, and the method for manufacturing a lap laser welded joint further comprises the step of irradiating with a second laser to form a second bead, wherein the length L2 of the second bead, measured along the central axis along the extending direction of the second bead, is smaller than L1, and in a plan view of the lap laser welded joint as seen from the thickness direction, a first imaginary line passing through the end of the first bead and perpendicular to the central axis of the first bead at the end, and the first bead In the peripheral end region, which is the area between the first point, which is 5 mm away from the end of the first bead towards the starting end of the first bead, and a second imaginary line perpendicular to the central axis of the first bead at the first point, the length of the region in the second bead where the distance from the widthwise end of the first bead is 2 mm or less is set to 2 mm or more along the central axis of the first bead, thereby separating the second bead from the central axis of the first bead, and the first bead The second bead is not included in the region around the starting end, which is the area between a third imaginary line passing through the starting end of the bead and perpendicular to the central axis of the first bead at the starting end, and a fourth imaginary line passing through a second point located (1 / 3) × L1 away from the starting end of the first bead along the central axis of the first bead toward the end of the first bead and perpendicular to the central axis of the first bead at the second point, and the second bead is formed before 10 seconds have elapsed from the end of the first laser irradiation. (10) In the method for manufacturing a lap laser welded joint described in (9) above, the second laser irradiation may be performed after the first laser irradiation. (11) In the method for manufacturing the overlapping laser welding joint according to (9) above, the first laser irradiation may be performed after the second laser irradiation. (12) In the method for manufacturing the overlapping laser welding joint according to any one of (9) to (11) above, in a plan view seen from the thickness direction of the overlapping laser welding joint, the second bead may be formed on both sides of the first bead with respect to the central axis of the first bead. (13) In the method for manufacturing the overlapping laser welding joint according to any one of (9) to (12) above, from the terminal end of the first bead, in a direction opposite to the starting end of the first bead, passing through a third point 2 mm away along the central axis of the first bead, and from a fifth virtual line perpendicular to the central axis of the first bead at the third point, the second bead may be spaced apart. (14) In the method for manufacturing the overlapping laser welding joint according to any one of (9) to (13) above, the first bead may be formed only on one side of the overlapping laser welding joint. (15) In the method for manufacturing the overlapping laser welding joint according to any one of (9) to (14) above, the penetration depth of the second bead may be set to 1 / 2 or more of the thickness of the metal plate on which the second bead is formed.

[0017] (16) The structural member for an automobile body according to another aspect of the present invention includes the overlapping laser welding joint according to any one of (1) to (8) above.

[0018] (17) A lap laser welded joint according to one aspect of the present invention comprises a plurality of overlapping metal plates, a first bead which is a linearly extending laser weld that joins the end face of a first metal plate which is the outermost metal plate among the plurality of metal plates and the surface of a second metal plate which is overlapped with the first metal plate, and joins the plurality of metal plates, wherein the ratio G / T of the sum of the gap sizes G of the plurality of metal plates to the sum of the thicknesses T of the plurality of metal plates is 0 to 18%, the length L1 of the first bead measured along the central axis along the extending direction of the first bead is 10 to 100 mm, and the lap laser welded joint further comprises at least the surface of the second metal plate A second bead is present, and the length L2 of the second bead, measured along the central axis in the extending direction of the second bead, is smaller than L1, and in the mating surface of the second metal plate superimposed with the first metal plate, in the peripheral end region which is the area between a first imaginary line passing through the end of the first bead and perpendicular to the central axis of the first bead at the end, and a second imaginary line passing through a first point 5 mm away from the end of the first bead towards the beginning of the first bead along the central axis of the first bead and perpendicular to the central axis of the first bead at the first point, in the second bead Furthermore, the length of the region where the distance from the widthwise end of the first bead is 2 mm or less is 2 mm or more along the central axis of the first bead, and the second bead is not included in the starting end peripheral region which is the region between a third imaginary line that is separated from the central axis of the first bead and passes through the starting end of the first bead and perpendicular to the central axis of the first bead at the starting end, and a fourth imaginary line that passes through a second point located (1 / 3) × L1 away from the starting end of the first bead towards the end of the first bead along the central axis of the first bead and perpendicular to the central axis of the first bead at the second point. (18) In the overlapping laser welding joint described in (17) above, on the mating surface of the second metal plate that is overlapped with the first metal plate, the second bead may be formed on both sides of the first bead with respect to the central axis of the first bead. (19) In the overlapping laser welding joint described in (17) or (18) above, from the terminal end of the first bead, in a direction opposite to the starting end of the first bead, passing through a third point 2 mm away along the central axis of the first bead, and from a fifth imaginary line perpendicular to the central axis of the first bead at the third point, the second bead may be separated. (20) In the overlapping laser welding joint described in any one of (17) to (19) above, the first bead may be formed only on one surface of the overlapping laser welding joint. (21) In the overlapping laser welding joint described in any one of (17) to (20) above, the penetration depth of the second bead in the second metal plate may be 1 / 2 or more of the thickness of the second metal plate. (22) In the overlapping laser welding joint described in any one of (17) to (21) above, the plurality of metal plates are a plurality of steel plates, and the component composition of one or more of the plurality of steel plates may include C: 0.05 to 0.5 mass%, Si: 0.1 to 3.5 mass%, Mn: 0.1 to 5.5 mass%, and P and S: a total of 0.03 mass% or less. (23) In the overlapping laser welding joint described in any one of (17) to (22) above, the plurality of metal plates are a plurality of steel plates, and the tensile strength of one or more of the plurality of steel plates may be 980 MPa or more. (24) In the overlapping laser welding joint described in (23) above, the tensile strength of the steel plate on which the second bead is formed among the plurality of steel plates may be 980 MPa or more.

[0019] (25) A method for manufacturing a lap laser welded joint according to another aspect of the present invention is a method for manufacturing a lap laser welded joint comprising the steps of overlapping a plurality of metal plates and irradiating a first laser to form a first bead which is a linearly extending laser weld that joins the plurality of metal plates, wherein in the step of irradiating the first laser, at least the end face of the first metal plate disposed on the outermost surface of the plurality of metal plates and the surface of the second metal plate overlapped with the first metal plate are joined, and in the overlapping, the size of the gap between the metal plates The ratio G / T of the measured value G to the total thickness T of the metal plates is set to 0 to 18%, the length L1 of the first bead, measured along the central axis in the direction of extension of the first bead, is set to 10 to 100 mm, and the method for manufacturing the overlapping laser-welded joint further comprises the step of irradiating a second laser so as to form a second bead on at least the second metal plate, the length L2 of the second bead, measured along the central axis in the direction of extension of the second bead, is set to be smaller than L1, and at the mating surface of the second metal plate overlapping with the first metal plate, In the peripheral end region, which is the area between a first imaginary line passing through the end of the first bead and perpendicular to the central axis of the first bead at the end, and a second imaginary line passing through a first point 5 mm away from the end of the first bead towards the beginning of the first bead along the central axis of the first bead and perpendicular to the central axis of the first bead at the first point, the length of the region in the second bead that is 2 mm or less from the widthwise end of the first bead is set to be 2 mm or more along the central axis of the first bead, and the second bead The second bead is formed before 10 seconds have elapsed since the end of the first laser irradiation, and is not included in the starting end peripheral region, which is the region between a third imaginary line passing through the starting end of the first bead and perpendicular to the central axis of the first bead at the starting end, and a fourth imaginary line passing through a second point located (1 / 3) × L1 away from the starting end of the first bead along the central axis of the first bead toward the end of the first bead and perpendicular to the central axis of the first bead at the second point. (26) In the method for manufacturing a lap laser welded joint described in (25) above, the second laser irradiation may be performed after the first laser irradiation. (27) In the method for manufacturing a lap laser welded joint described in (25) above, the first laser irradiation may be performed after the second laser irradiation. (28) In the method for manufacturing an overlapping laser-welded joint described in any one of the above paragraphs (25) to (27), the second bead may be formed on both sides of the first bead with respect to the central axis of the first bead at the mating surface of the second metal plate that is overlapped with the first metal plate. (29) In the method for manufacturing an overlapping laser-welded joint described in any one of the above paragraphs (25) to (28), the second bead may be separated from a fifth imaginary line that passes through a third point 2 mm away from the end of the first bead and in the direction opposite to the start of the first bead, along the central axis of the first bead, and that is perpendicular to the central axis of the first bead at the third point. (30) In the method for manufacturing an overlapping laser-welded joint described in any one of the above items (25) to (29), the first bead may be formed on only one side of the overlapping laser-welded joint. (31) In the method for manufacturing an overlapping laser welded joint described in any one of the above items (25) to (30), the penetration depth of the second bead in the second metal plate may be 1 / 2 or more of the thickness of the second metal plate.

[0020] (32) A structural member for an automobile body according to another aspect of the present invention comprises an overlapping laser-welded joint as described in any one of (17) to (24) above. [Effects of the Invention]

[0021] According to the present invention, it is possible to provide a lap laser welded joint capable of suppressing welding cracks at the bead end, a method for manufacturing a lap laser welded joint, and a structural member for an automobile body. [Brief explanation of the drawing]

[0022] [Figure 1] This is a schematic diagram of an overlapping laser-welded joint viewed from the thickness direction in a plan view. [Figure 2] This is an enlarged view of the stress relaxation region of the second bead. [Figure 3A] This is an enlarged cross-sectional view of an overlapping laser welded joint including the first bead and the second bead in the first embodiment. [Figure 3B] This is an enlarged cross-sectional view of an overlapping laser welded joint including the first bead and the second bead in the same embodiment. [Figure 4] This is an enlarged cross-sectional view of the overlapping laser welded joint including the first bead in the same embodiment. [Figure 5A] This is a schematic diagram of tensile stress in a typical lap laser welded joint without a second bead. [Figure 5B] This is a schematic diagram of tensile stress in an overlapping laser welded joint with a second bead. [Figure 6] This is a schematic diagram of an overlapping laser-welded joint viewed from the thickness direction in a plan view. [Figure 7A] This is an enlarged cross-sectional view of an overlapping laser welded joint including the first bead and the second bead in the second embodiment. [Figure 7B] This is an enlarged cross-sectional view of an overlapping laser welded joint including the first bead and the second bead in the same embodiment. [Figure 8] This is an enlarged cross-sectional view of the overlapping laser welded joint including the first bead in the same embodiment. [Figure 9] This is an enlarged cross-sectional view showing a modified example of an overlapping laser-welded joint according to one embodiment of the present invention. [Figure 10] This is a photograph of a lap laser-welded joint under condition No. 5. [Figure 11] This is a photograph of a lap laser-welded joint under condition No. 8. [Figure 12] This is a photograph of a lap laser-welded joint under condition No. 12. [Figure 13] This is a photograph of a lap laser-welded joint under condition No. 20. [Figure 14]This is a photograph of a lap laser-welded joint under condition No. 29. [Figure 15] This is a photograph of a lap laser-welded joint under condition No. 33. [Figure 16] This is a photograph of a lap laser-welded joint under condition No. 36. [Modes for carrying out the invention]

[0023] [First Embodiment] <Overlapping laser-welded joint 1> Hereinafter, a lap laser welded joint according to the first embodiment of the present invention will be described with reference to the drawings as appropriate. The lap laser welded joint 1 according to this embodiment has a plurality of overlapping metal plates 10 and a first bead 11 which is a linearly extending laser weld that joins the plurality of metal plates 10. The ratio G / T of the sum of the gap sizes G between the multiple metal plates 10 to the sum of the thicknesses T of the multiple metal plates 10 is 0 to 18%. The length L1 of the first bead 11, measured along the central axis 11X in the direction of extension of the first bead 11, is 10 to 100 mm. Furthermore, the overlapping laser-welded joint 1 according to this embodiment further has a second bead 12. The second bead 12 is spaced away from the central axis 11X of the first bead 11, and at least a portion of it is located in a terminal peripheral region EA defined with respect to the end of the first bead 11. Specifically, in the terminal peripheral region EA of the first bead 11, the length of the region in the second bead 12 that is 2 mm or less from the widthwise end of the first bead 11 is 2 mm or more along the central axis 11X of the first bead 11. In addition, the starting peripheral region SA of the first bead 11 does not include the second bead 12. Note that the widthwise end of the first bead 11 means the edges on both sides with respect to the extending direction of the first bead 11. The end of the first bead 11 is the intersection of the outer circumference of the first bead 11 and the central axis 11X, where the crater 111 is located. The end region EA and the start region SA will be described later.

[0024] (Metal plate 10) Multiple metal plates 10 are the base material for the overlapping laser-welded joint 1. The type, thickness, and presence or absence of surface treatment of the metal plates are not particularly limited, as long as they are suitable for laser welding. The number of metal plates 10 is also not particularly limited and can be any number of two or more.

[0025] A suitable example of the multiple metal plates 10 is a multiple steel plate, a multiple Al plate, etc. When the metal plates 10 are steel plates, the component composition of the steel plate is not particularly limited, and a suitable component composition can be applied depending on the application of the overlapping laser-welded joint. For example, one or more of the multiple steel plates may have a component composition containing C: 0.05~0.5 mass%, Si: 0.1~3.5 mass%, Mn: 0.1~5.5 mass%, and P and S: total 0.03 mass% or less, with the remainder being Fe and impurities. Since steel plates having such a component composition have high strength, excellent strength can be imparted to the overlapping laser-welded joint 1. However, when conventional overlapping laser welding is performed on a steel plate having such a component composition, welding cracks tend to occur at the end of the bead. However, in the overlapping laser-welded joint 1 according to this embodiment, welding cracks are suppressed by the second bead 12, which will be described later.

[0026] Furthermore, from the viewpoint of increasing the strength of the overlapping laser-welded joint 1, it is preferable that the strength of the metal plates 10 is higher. For example, if the multiple metal plates 10 are multiple steel plates, the tensile strength of one or more of these steel plates may be 980 MPa or higher, 1000 MPa or higher, or 1100 MPa or higher. The higher the tensile strength of the steel plate, the greater the tensile stress applied to the end of the bead after welding. However, in the overlapping laser-welded joint 1 according to this embodiment, the tensile stress is reduced by the second bead 12, which will be described later. Also, a steel plate with a tensile strength of 980 MPa or higher, i.e., a high-strength steel plate, may be placed on the outermost surface of the overlapping laser-welded joint or on the inside. Generally, high-strength steel plates have a high carbon content, and if a high-strength steel plate is included somewhere in a plate assembly made up of overlapping steel plates to be welded, the carbon content of the welded area will be high, making welding cracks more likely to occur. However, in the overlapping laser-welded joint 1 according to this embodiment, by utilizing the second bead 12, which will be described later, the effect of preventing welding cracks can be effectively achieved even in a plate assembly that includes a high-strength steel plate. Furthermore, if the multiple metal plates 10 are multiple steel plates, and one or more of them are high-strength steel plates, it is preferable that the second bead 12, described later, is fused into the high-strength steel plate. It is even more preferable that the positional relationship between the first bead 11 and the second bead 12, described later, is satisfied in the high-strength steel plate. On the other hand, even if the second bead 12 is formed only on steel plates with a tensile strength of less than 980 MPa that are combined with the high-strength steel plate, as long as the positional relationship between the first bead 11 and the second bead 12, described later, is satisfied, the effect of easing the tensile stress at the end of the bead and suppressing welding cracks in the high-strength steel plate can be sufficiently obtained.

[0027] (Bead) In the overlapping laser-welded joint 1 according to this embodiment, a first bead 11 and a second bead 12 shorter than the first bead 11 are formed on a plurality of metal plates 10. Generally, "bead" refers to the raised portion due to welding, but in this embodiment, a bead mark formed by removing the raised portion of the bead by polishing or the like is also considered a "bead". Even if the bead is flattened, the effect of the overlapping laser-welded joint according to this embodiment is not impaired. By optimizing the spacing between the plurality of metal plates 10, as well as the shape and positional relationship of the beads, welding cracks at the end of the beads can be prevented. The shape and positional relationship of the beads will be described below with reference to Figure 1, etc. Note that the shape of the beads does not necessarily coincide on the front and back surfaces of the overlapping laser-welded joint 1. If the shape and positional relationship of the beads described below are satisfied on at least one surface of the overlapping laser-welded joint 1, the effect of suppressing welding cracks can be obtained. Therefore, a joint in which the bead shape and positional relationship described below are within the range described later on at least one side is considered to be an overlap laser welded joint 1 according to this embodiment. Preferably, the bead shape and positional relationship described below are satisfied on both sides of the overlap laser welded joint 1. Unless otherwise specified, the bead shape and positional relationship described below are as viewed from above when the overlap laser welded joint 1 is viewed from the thickness direction of the overlap laser welded joint 1.

[0028] (First bead 11) The first bead 11 is a linearly extending laser weld that joins multiple metal plates 10. The shape of the first bead 11 is not particularly limited as long as it is linear; it may be a straight line, a curve, or even bent. For example, the first bead 11 may be C-shaped or L-shaped. Since the first bead 11 joins multiple metal plates 10, when viewed in cross-section, as illustrated in Figures 3A and 3B, the first bead 11 extends in the thickness direction so as to span all the metal plates 10. However, it is not necessary for the first bead 11 to penetrate all the metal plates 10. As illustrated in Figure 3B, the first bead 11 may be formed on only one side of the overlapping laser weld joint. Alternatively, multiple first beads 11 may join several metal plates 10 each, thereby joining all the metal plates 10. When the first bead 11 is formed on only one side of the overlapping laser-welded joint, the crater 111 of the first bead 11 becomes smaller, further suppressing weld cracking.

[0029] (G gap between multiple metal plates 10) In laser welding, a flow of weld metal occurs in the direction opposite to the direction of laser propagation. As a result, a depression called a crater 111 is formed at the end of the first bead 11. Craters 111 can cause welding cracks, so it is preferable for them to be as small as possible. Here, the crater 111 can be reduced by reducing the gaps between the multiple metal plates 10. For the reasons above, the ratio G / T of the sum of the gaps between the multiple metal plates 10 G to the sum of the thicknesses of the multiple metal plates 10 T is set to be within the range of 0 to 18%. G is the gap between the two metal plates 10 when there are two metal plates 10, and the sum of the gaps between the metal plates 10 when there are three or more metal plates 10. For example, in the overlapping laser-welded joint 1 illustrated in Figure 3A, G is the sum of gap g1 and gap g2, and T is the sum of the plate thicknesses t1, t2, and t3. A smaller G / T ratio is preferable, and may be 15% or less, 12% or less, or 10% or less.

[0030] The size of the gap between the metal plates 10 is measured at the cross-section of the end of the first bead 11. The cross-section is taken from the end 11E of the first bead 11, along the central axis 11X of the first bead 11, at a point 2.5 mm away from the starting end 11S. The cross-section is also perpendicular to the central axis 11X of the first bead at the aforementioned position. At this cross-section, the sum of the gap sizes G between the multiple metal plates 10 and the sum of the thicknesses T of the multiple metal plates 10 are measured. As shown in Figure 4, the gap size is also measured at both ends of the first bead 11 in the cross-section. The average value of the gap size gl on the left side of the first bead 11 and the gap size gr on the right side is considered to be the size of the gap between the metal plates 10.

[0031] (Length L1 of the first bead 11) The length L1 of the first bead 11, measured along the central axis 11X in the direction of extension of the first bead 11, is set to 10 to 100 mm. If the length L1 of the first bead 11 is less than 10 mm, it becomes difficult to suppress welding cracks. From the viewpoint of further suppressing welding cracks, the length L1 of the first bead 11 may be set to 12 mm or more, 15 mm or more, or 20 mm or more. On the other hand, if the length L1 of the first bead 11 exceeds 100 mm, the time required for laser welding increases, which may impair productivity. The length L1 of the first bead 11 may be set to 90 mm or less, 80 mm or less, or 70 mm or less.

[0032] Furthermore, if the workpiece is large, multiple first beads 11 may be provided in the overlapping laser-welded joint 1. In this case, a second bead 12, described later, should be placed near the end of each of the multiple first beads 11. However, it is not necessary to place a second bead 12 at the end of all first beads 11 included in the overlapping laser-welded joint 1. The second bead 12 may be formed only in areas where welding cracks are a particular concern.

[0033] (Second bead 12) The overlapping laser-welded joint 1 has a second bead 12 in addition to the first bead 11. The second bead 12 is positioned near the end 11E of the first bead 11. The length L2 of the second bead 12 is shorter than the length L1 of the first bead 11. The length L2 of the second bead 12 is the size of the second bead 12, measured along the central axis in the direction of extension of the second bead 12. If the second bead 12 is short and the direction of extension of the second bead cannot be determined, the major axis of the second bead 12 is considered to be the length L2 of the second bead 12.

[0034] One of the causes of weld cracks at the end of a weld bead formed by laser welding is the tensile stress applied to the end of the bead after laser welding. Therefore, reducing the tensile stress applied to the end of the bead can suppress the occurrence of weld cracks in that area. This tensile stress is caused by the contraction of the weld and its surroundings as the temperature of the weld decreases after the completion of laser welding. The inventors diligently investigated means to reduce this tensile stress. The inventors found that by forming another bead near the end of the bead before the temperature of the end of the bead decreases, the tensile stress applied to the end can be dispersed, thereby suppressing weld cracks.

[0035] Figure 5A is a conceptual diagram of the tensile stress applied to the first bead 11 when the second bead 12 is absent. A crater 111 is formed at the end of the first bead 11. Additionally, tensile stress is applied to the end of the first bead 11 due to the contraction of the base material heated by laser welding. The arrows in Figure 5A illustrate the tensile stress applied to the end. When the end of the bead is torn by this tensile stress, a weld crack C occurs.

[0036] On the other hand, Figure 5B is a conceptual diagram of tensile stress when a second bead 12 is formed near the end of the first bead 11. Most of the tensile stress caused by the shrinkage of the base material is applied to the second bead 12. In addition, the tensile stresses cancel each other out between the first bead 11 and the second bead 12. Therefore, the second bead 12 formed near the end of the first bead 11 can significantly suppress the tensile stress applied to the end of the first bead 11.

[0037] For the reasons stated above, in the overlapping laser-welded joint 1 according to this embodiment, a second bead 12 is used as a secondary bead to reduce the tensile stress applied to the end of the first bead 11. However, if the size of the second bead 12 and the positional relationship between the first bead 11 and the second bead 12 are inappropriate, welding cracks cannot be sufficiently suppressed even if the second bead 12 is provided. The size of the second bead 12 and other related aspects will be described in detail below.

[0038] Hereinafter, in order to define the size and position of the second bead 12, the terms "end peripheral region EA" and "start peripheral region SA" of the first bead 11 will be used. The region EA surrounding the end of the first bead 11 is the region between the first virtual line VL1 and the second virtual line VL2, as shown in Figure 1. The first virtual line VL1 is a virtual line that passes through the end 11E of the first bead 11 and is perpendicular to the central axis 11X of the first bead 11 at the end 11E. The second virtual line VL2 is a virtual line that passes through a first point P1 located 5 mm away from the end 11E of the first bead 11 along the central axis 11X of the first bead 11 toward the start end 11S of the first bead 11, and is perpendicular to the central axis 11X of the first bead 11 at the first point P1. The region SA surrounding the starting end of the first bead 11 is the region between the third virtual line VL3 and the fourth virtual line VL4, as shown in Figure 1. The third virtual line VL3 is a virtual line that passes through the starting end 11S of the first bead 11 and is perpendicular to the central axis 11X of the first bead 11 at the starting end 11S. The fourth virtual line VL4 is a virtual line that passes through a second point P2 located (1 / 3) × L1 away from the central axis 11X of the first bead 11, along the central axis 11X of the first bead 11, toward the end 11E of the first bead 11, and is perpendicular to the central axis 11X of the first bead 11 at the second point P2.

[0039] (Positional relationship between the second bead 12 and the terminal peripheral region EA) First, the second bead 12 must be at least partially included in the terminal peripheral region EA. A bead formed outside the terminal peripheral region EA is not considered to have the effect of relieving the tensile stress at the terminal portion of the first bead 11.

[0040] (Length of the stress relaxation region 121 of the second bead 12) Furthermore, if the distance between the second bead 12 and the first bead 11 is too large, the second bead 12 will not be able to exert its tensile stress relaxation effect. For this reason, in the terminal peripheral region EA, the length of the region in the second bead 12 where the distance from the widthwise end of the first bead 11 is 2 mm or less is set to be 2 mm or more along the central axis 11X of the first bead 11. Here, the distance in the second bead 12 from the widthwise end of the first bead 11 is the distance between the widthwise end of the first bead and the widthwise end of the second bead on a virtual line perpendicular to the central axis of the first bead on the surface of the overlapping laser-welded joint when viewed from the thickness direction of the overlapping laser-welded joint in plan view. However, if the widthwise end of the first bead and the widthwise end of the second bead are touching or overlapping in a virtual line perpendicular to the central axis of the first bead, the distance of the widthwise end of the second bead 12 from the widthwise end of the first bead 11 shall be zero. For the sake of explanation, the area in the terminal peripheral region EA where the distance of the second bead 12 from the widthwise end of the first bead 11 is 2 mm or less shall be referred to as the stress relaxation region 121. In other words, in the overlapping laser welded joint 1 according to this embodiment, the length of the stress relaxation region 121 must be 2 mm or more along the central axis 11X of the first bead 11. For reference, an enlarged view of the second bead 12 shown in Figure 1 is shown in Figure 2. In Figure 2, the shaded area in the second bead 12 is the stress relaxation region 121. Here, the method for measuring the length of the stress relaxation region 121 is to measure the maximum length of the second bead 12 along the central axis 11X of the first bead 11 in the region between imaginary lines that are 2 mm from the end in the width direction of the first bead 11 on imaginary lines perpendicular to the central axis 11X of the first bead 11 within the terminal peripheral region EA. However, the stress relaxation region 121 may be continuous or discontinuous, as long as the length of the stress relaxation region 121 is 2 mm or more along the central axis 11X of the first bead 11.

[0041] Beads positioned far from the first bead 11 are not considered to have any effect in relieving tensile stress. Therefore, only the area of ​​the second bead 12 that is within 2 mm of the first bead 11 is considered to be the area that exhibits a stress-relieving effect, i.e., the stress-relieving area 121. Furthermore, if the stress-relieving area 121 is too small, it is not considered that the effect of relieving tensile stress will be obtained. For this reason, the length of the stress-relieving area 121 is set to 2 mm or more. The length of the stress-relieving area 121 is the value obtained by measuring the length of the stress-relieving area 121 along the central axis 11X of the first bead 11. The length of the stress-relieving area 121 may be 2.5 mm or more, 3 mm or more, or 4 mm or more. Since the width of the terminal peripheral area EA is 5 mm, the maximum length of the stress-relieving area 121 is 5 mm.

[0042] (Positional relationship between the second bead 12 and the central axis 11X of the first bead 11) The second bead 12 may be in contact with the first bead 11. Even when the second bead 12 and the first bead 11 are in contact, the tensile stress applied to the end of the first bead 11 is relieved. However, if the area where the second bead 12 and the first bead 11 overlap is too large, the tensile stress may not be sufficiently relieved. Therefore, the second bead 12 is separated from the central axis 11X of the first bead 11. That is, the second bead 12 does not overlap with the central axis 11X of the first bead 11.

[0043] (Positional relationship between the second bead 12 and the starting edge region SA) The second bead 12 is provided for the purpose of relieving the tensile stress applied to the end of the first bead 11. However, even if the second bead 12 is extended far from the end of the first bead 11, the effect of relieving the tensile stress will not be obtained. For example, if the second bead 12 is extended toward the starting end 11S of the first bead 11 so that the second bead 12 overlaps with the starting end peripheral region SA, manufacturing efficiency will be impaired, while the effect of relieving the tensile stress will not improve. Therefore, considering manufacturing efficiency, the second bead 12 is formed so as to be excluded from the starting end peripheral region SA. Note that if the first bead 11 has a significantly bent shape such as a U-shape, the starting end peripheral region SA may extend to the area around the end 11E of the first bead 11. As described above, the area around the end 11E is the region where the second bead 12 should be placed. In this case, the starting end region SA may be considered as the region that satisfies the above requirements and is 7 mm away from the end of the first bead 11 in the width direction.

[0044] Furthermore, extending the second bead 12 toward the opposite side of the starting end 11S of the first bead 11 is also undesirable from a manufacturing efficiency perspective. Therefore, for example, when a fifth imaginary line VL5 is defined as a virtual line that passes through a third point P3 2 mm away from the end 11E of the first bead 11, in the direction opposite to the starting end 11S of the first bead 11, along the central axis 11X of the first bead 11, and perpendicular to the central axis 11X of the first bead 11 at the third point P3, it is preferable that the second bead 12 is separated from the fifth imaginary line VL. This further improves the manufacturing efficiency of the overlapping laser welded joint 1.

[0045] Furthermore, the length L2 of the second bead 12 described above may be 7 mm or less, 6 mm or less, or 5 mm or less. As long as the length of the stress relaxation region 121 is 2 mm or more, shortening the length L2 of the second bead 12 is preferable because it can improve the manufacturing efficiency of the overlapping laser welded joint 1.

[0046] The shape of the second bead 12 in plan view has been described above, but the cross-sectional shape of the second bead 12 is not particularly limited. The first bead 11 needs to penetrate the metal plates 10 in order to join multiple metal plates 10, but the second bead 12 does not need to penetrate the metal plates 10. As shown in Figure 3A, the second bead 12 may penetrate the metal plates 10 from one surface to the other of the overlapping laser-welded joint 1. On the other hand, as shown in Figure 3B, the second bead 12 may be formed only on the metal plate 10 on the outermost surface of the overlapping laser-welded joint 1 and may not extend to the other metal plates 10. Even in this case, the second bead 12 can exert the effect of relieving tensile stress. Weld cracks in the first bead 11 usually occur near the surface of the first bead 11. Therefore, if the tensile stress is relieved on the surface of the first bead 11, weld cracks can be suppressed.

[0047] On the other hand, in order to further improve the tensile stress suppression effect of the second bead 12, a lower limit value for the penetration depth of the second bead 12 may be set. For example, if the penetration depth of the second bead 12 is shallow, and as shown in Figure 3B, the second bead 12 is provided only on one of the metal plates 10a of the metal plates 10 arranged on the outermost layer of the overlapping laser-welded joint 1, the penetration depth of the second bead 12 may be 1 / 2 or more of the thickness t of the metal plate 10a arranged on the outermost layer of the overlapping laser-welded joint 1 and on which the second bead 12 is formed. In this case, tensile stress can be sufficiently suppressed while shortening the manufacturing time of the second bead 12.

[0048] In the configuration shown in Figure 1, one second bead 12 is formed at the end of one first bead 11. On the other hand, as shown in Figure 6, two second beads 12 may be formed at the end of one first bead 11. Specifically, the second beads 12 may be provided on both sides of the first bead 11 with respect to its central axis 11X. This further relieves the tensile stress at the end of the first bead 11, thereby further suppressing weld cracking. Furthermore, two or more second beads 12 may be provided on one side of the central axis 11X, or three or more second beads 12 may be formed at the end of one first bead 11. In addition, as long as one or more second beads 12 are provided, further beads that do not satisfy the above-described requirements for second beads 12 may be formed near the end of the first bead 11.

[0049] <Method for manufacturing overlapping laser-welded joints> Next, a method for manufacturing the overlapping laser-welded joint according to this embodiment will be described. This manufacturing method allows for the suitability of manufacturing the overlapping laser-welded joint 1 according to this embodiment described above. However, even if an overlapping laser-welded joint is obtained by a method other than the manufacturing method described below, it will be considered as the overlapping laser-welded joint 1 according to this embodiment if it satisfies the above requirements.

[0050] (Superposition, and first laser irradiation) The manufacturing method for the overlapping laser-welded joint according to this embodiment includes the steps of overlapping a plurality of metal plates 10 and irradiating with a first laser to form a first bead 11, which is a linearly extending laser-welded portion that joins the plurality of metal plates 10. Here, when overlapping the metal plates 10, the ratio G / T of the total size of the gaps between the metal plates 10 to the total thickness T of the metal plates 10 is set to 0 to 18%. This makes it possible to reduce the depth of the crater 111 formed at the end of the first bead 11. In addition, the length L1 of the first bead 11, measured along the central axis 11X along the extending direction of the first bead 11, is set to 10 to 100 mm. By setting the upper limit of L1 to 100 mm, the manufacturing efficiency of the overlapping laser-welded joint 1 can be improved, and by setting the lower limit of L1 to 10 mm, welding cracks can be suppressed. The preferred numerical ranges exemplified in the overlapping laser-welded joint 1 according to this embodiment described above can be appropriately applied to G / T and L1 in the manufacturing method according to this embodiment. Similarly, with respect to other configurations, preferred embodiments of the overlapping laser-welded joint according to this embodiment can be applied to the manufacturing method according to this embodiment.

[0051] (Second laser irradiation) The method for manufacturing an overlapping laser-welded joint according to this embodiment further includes the step of irradiating a second laser so as to form a second bead 12 that is shorter than the first bead 11 near the end of the first bead 11.

[0052] In the second laser irradiation, in the terminal peripheral region EA, which is the region between a first imaginary line VL1 passing through the end 11E of the first bead 11 and perpendicular to the central axis 11X of the first bead 11 at the end 11E, and a second imaginary line VL2 passing through a first point P1 5 mm away from the end 11E of the first bead 11 along the central axis 11X of the first bead 11 toward the starting end 11S of the first bead 11 and perpendicular to the central axis 11X of the first bead 11 at the first point P1, the length of the region in the second bead 12 that is 2 mm or less from the end of the first bead 11 in the width direction must be 2 mm or more along the central axis 11X of the first bead 11. This ensures the tensile stress relaxation effect of the second bead 12.

[0053] Furthermore, in the second laser irradiation, the starting end region SA, which is the region between a third imaginary line VL3 that passes through the starting end 11S of the first bead 11 and is perpendicular to the central axis 11X of the first bead 11 at the starting end 11S, and a fourth imaginary line VL4 that passes through a second point P2 located (1 / 3) × L1 away from the starting end 11S of the first bead 11 towards the end 11E of the first bead 11 along the central axis 11X of the first bead 11 and is perpendicular to the central axis 11X of the first bead 11 at the second point P2, must not include the second bead 12. This improves the manufacturing efficiency of overlap laser welded joints.

[0054] Furthermore, the second bead 12 needs to be spaced away from the central axis 11X of the first bead 11. This ensures that the tensile stress relaxation effect of the second bead 12 is maintained.

[0055] In addition, it is necessary to form at least one second bead 12 before 10 seconds have elapsed from the end of the first laser irradiation. Weld cracks at the end of the first bead 11 occur several tens of seconds after the end of the first laser irradiation. Naturally, even if the second bead 12 is formed after the weld crack has occurred, the weld crack suppression effect of the second bead 12 will not be exhibited. According to the inventors' experimental results, no weld cracks occur for at least 10 seconds after the end of the first bead 11 is formed. Based on this finding, the timing of the second laser irradiation was defined as described above.

[0056] Furthermore, "forming the second bead 12" means forming a second bead 12 that meets the requirements described above. For example, even if the second laser irradiation is started before 10 seconds have elapsed since the end of the first laser irradiation, if the starting position of the laser irradiation is far from the end of the first bead 11, and the formation of the second bead 12 that meets the requirements described above is not completed 10 seconds after the end of the first laser irradiation, then the requirements for the second laser irradiation according to this embodiment cannot be said to be met.

[0057] On the other hand, if the second bead 12 can be formed before 10 seconds have elapsed since the end of the first laser irradiation, the timing of the start of the second laser irradiation is not particularly limited. The second laser irradiation may be performed before the first laser irradiation, so that the second bead 12 is formed in advance before the first bead 11 is formed. In this case, by ending the first laser irradiation near the pre-formed second bead 12, the second bead 12 can be made to satisfy the above requirements. In particular, when using metal plates that have undergone surface treatment such as plating, it is preferable to form the second bead 12 in advance before the first laser irradiation process from the viewpoint of suppressing blowholes. By forming the second bead 12 in advance before the first laser irradiation process, an appropriate gap can be formed between the metal plates 10 in the step of stacking the multiple metal plates 10 before the first laser irradiation process, and the surface treatment agent components evaporated by the laser irradiation can be discharged from that gap. Alternatively, a second laser irradiation may be performed after the first laser irradiation. In particular, from the viewpoint of further suppressing welding cracks at the bead end, it is preferable to perform the second laser irradiation after the first laser irradiation. By performing the second laser irradiation after the first laser irradiation, the second bead 12 can override the tensile stress applied to the end of the first bead 11, thereby further suppressing cracking. If the second laser irradiation is performed two or more times to form two or more second beads 12, the second laser irradiation can be performed both before and after the first laser irradiation. If the second laser irradiation is performed twice, at least one of the second laser irradiations must be performed in such a way that it forms the second bead 12 before 10 seconds have elapsed since the end of the first laser irradiation. It is most preferable that all second laser irradiations are performed in such a way that this requirement is met.

[0058] In addition, in the manufacturing method according to this embodiment, preferred configurations of the overlap laser-welded joint 1 according to this embodiment can be appropriately adopted. For example, the second bead 12 may be provided on both sides of the first bead 11 with respect to the central axis 11X of the first bead 11. The second bead 12 may be separated from a fifth imaginary line VL5 that is perpendicular to the central axis 11X of the first bead 11 at the third point P3, passing 2 mm away from the end 11E of the first bead 11 in the direction opposite to the start end 11S of the first bead 11. The first bead 11 may be formed on only one side of the overlap laser-welded joint 1. The penetration depth of the second bead 12 may be 1 / 2 or more of the thickness of the metal plate 10 on which the second bead 12 is formed.

[0059] [Second Embodiment] <Layer laser welded joint 1A> Hereinafter, a lap laser welding joint according to a second embodiment of the present invention will be described with reference to the drawings as appropriate. In the laser welding joint 1 according to the first embodiment, a plurality of metal plates 10 were joined together by a first bead 11 on the surfaces of each metal plate. However, in the lap laser welding joint 1A according to this embodiment, as shown in Figure 7A for example, the end face of the first metal plate 10b, which is the outermost metal plate 10 among the plurality of metal plates 10, and the surface of the second metal plate 10c, which is superimposed on the first metal plate 10b, are joined by a first bead 11A. That is, the laser welding joint 1A according to this embodiment differs from the laser welding joint 1 according to the first embodiment in that it is a lap fillet joint. In this embodiment, the configuration of the joint surface superimposed on the first metal plate 10b in the second metal plate 10c is the same as in Figure 1. Therefore, Figure 1 can also be described as a schematic diagram of the overlapping laser-welded joint 1A, viewed from the thickness direction of the overlapping laser-welded joint 1A, showing the mating surface of the second metal plate 10c and the first metal plate 10b. Therefore, the laser-welded joint 1A according to this embodiment comprises a plurality of overlapping metal plates 10, and a first bead 11A which is a linearly extending laser-welded portion that joins the end face of the first metal plate 10b and the surface of the second metal plate 10c, and joins the plurality of metal plates. In the laser-welded joint 1A, the ratio G / T of the sum of the gap sizes G between the multiple metal plates 10 to the sum of the thicknesses T of the multiple metal plates 10 is 0 to 18%. The length L1 of the first bead 11A, measured along the central axis in the direction of extension of the first bead 11, is 10 to 100 mm. Furthermore, the overlapping laser-welded joint 1A also has a second bead 12 on the surface of at least the second metal plate 10c, and the length L2 of the second bead 12, measured along the central axis in the direction of extension of the second bead 12, is smaller than L1. In the mating surface of the second metal plate 10c superimposed with the first metal plate 10b, in the terminal peripheral region EA, which is the region between a first imaginary line VL1 passing through the end 11E of the first bead 11A and perpendicular to the central axis 11X of the first bead 11A at the end 11E, and a second imaginary line VL2 passing through a first point P1 5 mm away from the end 11E of the first bead 11A toward the starting end 11S of the first bead 11 and perpendicular to the central axis 11X of the first bead 11A at the first point P1, the length of the region 121 in the second bead 12 where the distance from the widthwise end of the first bead 11A is 2 mm or less is 2 mm or more along the central axis 11X of the first bead 11A. The second bead 12 is separated from the central axis 11X of the first bead 11A. Furthermore, the second bead 12 is not included in the starting end peripheral region SA, which is the region between a third imaginary line VL3 that passes through the starting end 11S of the first bead 11A and is perpendicular to the central axis 11X of the first bead 11A at the starting end 11S, and a fourth imaginary line VL4 that passes through a second point P2 located (1 / 3) × L1 away from the starting end 11S of the first bead 11A towards the end 11E of the first bead 11A and is perpendicular to the central axis 11X of the first bead 11A at the second point P2. The various components of the laser welded joint 1A according to this embodiment will be described in more detail below. The positional relationship between the second bead 12 and the terminal peripheral region EA, the length of the stress relaxation region 121 of the second bead 12, and the positional relationship between the second bead 12 and the central axis 11X of the first bead 11A are the same as in the first embodiment, so a detailed explanation is omitted here.

[0060] (First bead 11A) The first bead 11A is a linearly extending fillet laser weld that joins the end face of the first metal plate 10b and the surface of the second metal plate 10c among the multiple metal plates 10, and also joins the multiple metal plates. The shape of the first bead 11A is not particularly limited as long as it is linear. For example, the first bead 11A is shaped along the end face of the first metal plate 10b. The first bead 11A joins at least the end face of the first metal plate 10b and the surface of the second metal plate 10c among the multiple metal plates 10, but in order to join the multiple metal plates 10, when the first bead 11 is viewed in cross-section, as illustrated in Figure 7A, the first bead 11A may extend in the thickness direction so as to span all the metal plates 10. The first bead 11A does not need to penetrate all of the metal plates 10; as illustrated in Figure 7B, the first bead 11A may be formed on only one side of the overlapping laser welded joint.

[0061] (Second bead 12) As described above, the second bead 12 in the overlapping laser-welded joint 1A according to this embodiment is the same as the second bead 12 in the laser-welded joint 1 according to the first embodiment. In this embodiment, the second bead 12 may be formed by irradiating the first metal plate 10b with a laser, penetrating the first metal plate 10b and reaching the second metal plate 10c. In this case, the weld formed on the second metal plate 10c in the second bead 12 satisfies the above requirements, thereby relieving the tensile stress at the end of the first bead 11A. Alternatively, the second bead 12 may be formed by directly irradiating the second metal plate 10c with a laser.

[0062] Furthermore, at the joint surface where the second metal plate 10c and the first metal plate 10b are superimposed, the second bead 12 may be provided on both sides of the first bead 11A, with reference to the central axis 11X of the first bead 11A. This further relieves the tensile stress at the end of the first bead 11A, thereby further suppressing weld cracking.

[0063] Furthermore, the penetration depth of the second bead 12 in the second metal plate 10c may be 1 / 2 or more of the thickness t of the second metal plate 10c. In this case, tensile stress can be sufficiently suppressed while shortening the manufacturing time of the second bead 12.

[0064] (G gap between multiple metal plates 10) The size of the gap between the metal plates 10 is basically measured in the same way as in the first embodiment. However, the size of the gap between the first metal plate 10b and the second metal plate 10c is measured at the end of the first bead 11A on the side where the first metal plate 10b and the second metal plate 10c overlap in the cross-section, as shown in Figure 8.

[0065] <Method for manufacturing overlapping laser-welded joints> Next, a method for manufacturing the overlapping laser-welded joint according to this embodiment will be described. This manufacturing method allows for the suitability of manufacturing the overlapping laser-welded joint 1A according to this embodiment described above. However, even if an overlapping laser-welded joint is obtained by a method other than the manufacturing method described below, it will be considered as the overlapping laser-welded joint 1 according to this embodiment if it satisfies the above requirements.

[0066] The method for manufacturing a lap laser-welded joint according to this embodiment differs from the method for manufacturing a lap laser-welded joint according to the first embodiment in that, in the step of performing a first laser irradiation, at least the end face of the first metal plate, which is placed on the outermost surface of the plurality of metal plates, and the surface of the second metal plate, which is superimposed on the first metal plate, are joined together. Therefore, the method for manufacturing a lap laser welded joint according to this embodiment is a method for manufacturing a lap laser welded joint comprising the steps of overlapping a plurality of metal plates 10 and irradiating with a first laser to form a first bead 11A, which is a linearly extending laser welded portion that joins the plurality of metal plates 10. In the first laser irradiation step, at least the end face of the first metal plate 10b, which is positioned on the outermost surface of the plurality of metal plates 10, and the surface of the second metal plate 10c, which is superimposed on the first metal plate 10b, are joined together. In this superposition, the ratio G / T of the total size G of the gaps between the metal plates 10 to the total thickness T of the metal plates 10 is set to 0-18%, and the length L1 of the first bead, measured along the central axis 11X in the direction of extension of the first bead 11A, is set to 10-100 mm. The manufacturing method for the overlapping laser-welded joint according to this embodiment further comprises the step of irradiating with a second laser so as to form a second bead 12 on at least the second metal plate 10c. The length L2 of the second bead 12, measured along the central axis along the extending direction of the second bead 12, is made smaller than L1, and at the mating surface where the second metal plate 10c and the first metal plate 10b are overlapped, a first imaginary line VL1 passes through the end 11E of the first bead 11A and is perpendicular to the central axis 11X of the first bead 11A at the end 11E, and the distance from the end 11E of the first bead 11A to the start end 11 In the terminal peripheral region EA, which is the region between a first point P1 that is 5 mm away from the central axis 11X of the first bead 11A toward S and a second imaginary line VL2 that is perpendicular to the central axis 11X of the first bead 11A at the first point P1, the length of the region 121 in the second bead 12, where the distance from the end of the first bead 11A in the width direction is 2 mm or less, is set to be 2 mm or more along the central axis 11X of the first bead 11A. Furthermore, the second bead 12 is separated from the central axis 11X of the first bead 11A, and the second bead 12 is not included in the starting end peripheral region SA, which is the region between a third imaginary line VL3 that passes through the starting end 11S of the first bead 11A and is perpendicular to the central axis 11X of the first bead 11A at the starting end 11S, and a fourth imaginary line VL4 that passes through a second point P2 located (1 / 3) × L1 away from the starting end 11S of the first bead 11A towards the end 11E of the first bead 11A along the central axis 11X of the first bead 11A and is perpendicular to the central axis 11X of the first bead 11A at the second point P2. Furthermore, the second bead 12 is formed before 10 seconds have elapsed since the end of the first laser irradiation.

[0067] Furthermore, in the manufacturing method according to this embodiment, preferred conditions for the manufacturing method of the overlapping laser-welded joint according to the first embodiment can be appropriately adopted.

[0068] <Structural components for automobile bodies> Another aspect of the present invention relates to a structural member for an automobile body, which includes a lap laser welded joint according to this embodiment. Examples of structural members for an automobile body include A-pillars, B-pillars, roof rails, side sills, floor cross members, bumpers, crash boxes, instrument panel reinforcements, seat frames, and battery cases. By applying the lap laser welded joint according to this embodiment to the flange portion of these members, it is possible to obtain a structural member for an automobile body that offers excellent productivity and suppresses the occurrence of weld cracks.

[0069] Up to this point, the lap laser welded joint, the method for manufacturing the lap laser welded joint, and the structural member for an automobile body of the present invention have been described with reference to embodiments. However, the technical scope of the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention.

[0070] For example, in the second embodiment, laser fillet welding may be performed on either of the two surfaces in the thickness direction of the overlapping laser-welded joint 1A. That is, on both surfaces in the thickness direction of the overlapping laser-welded joint 1A, the edge of the outermost metal plate and the surface of the metal plate adjacent to that metal plate in the thickness direction may be fillet-welded by laser.

[0071] Furthermore, the first bead 11 according to the first embodiment and the first bead 11A according to the second embodiment may be combined. For example, as shown in Figure 9, if the laser lap weld joint 1B comprises three metal plates 10 and the first bead 11A, which is a fillet weld, is formed on the first metal plate 10b and the second metal plate 10c, the first bead 11 may join the second metal plate 10c and the metal plate 10 positioned on the opposite side of the first metal plate 10b with respect to the second metal plate 10c. In this case, a second bead 12 is provided to relieve tensile stress at the end of the first bead 11, and a second bead 12 is provided to relieve tensile stress at the end of the first bead 11A. [Examples]

[0072] The effects of one aspect of the present invention will be further explained in detail by the examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as it does not depart from the spirit of the invention and achieves the objectives of the present invention.

[0073] [Example 1] Two steel plates having the tensile strength, thickness, and chemical composition listed in Table 1 were overlapped and laser-welded under various conditions to form a first bead. Chemical components other than those listed in Table 1 are Fe and impurities. In some cases, a second laser irradiation and a third laser irradiation were performed to form a second bead and a third bead near the end of the first bead. More specifically, the second and third beads were formed so as to be located within the peripheral region EA of the end. However, under condition No. 25, the second bead was formed extending from a position 2.6 mm from the end of the first bead towards the start end, and under condition No. 26, the second bead was formed extending from a position 7.0 mm from the end of the first bead towards the start end, and the presence or absence of weld cracks was checked in the various overlapping laser-welded joints obtained as a result. The welding process sequence is shown in Table 2, and other welding conditions and evaluation results of overlapping laser-welded joints are shown in Tables 3 and 4. Representative photographs of laser-welded joints are shown in Figures 10 to 13.

[0074] Note that both the second bead and the third bead correspond to the second bead of the overlapping laser-welded joint according to this embodiment described above. However, for convenience, of the two second beads provided, the one formed later will be referred to as the third bead. Similarly, if the second laser irradiation is performed twice, the one performed later will be referred to as the third laser irradiation.

[0075] The meanings of the symbols listed in the table are as follows: • "L1", "L2", "L3": The length of the first, second, or third bead, measured along the central axis in the direction of extension of the first, second, or third bead. If the bead is short and the direction of extension of the second or third bead cannot be determined, the major axis of the bead is used. • "W1", "W2", "W3": The width of the first, second, or third bead, measured along the direction perpendicular to the extension direction of the first, second, or third bead. If the bead is short and the extension direction of the second or third bead cannot be determined, use the shorter diameter of the bead. • "H12S" and "H13S": The distance between the starting point of the second or third bead and the central axis of the first bead. • "H12E" and "H13E": The distance between the end of the second or third bead and the central axis of the first bead. • "Output": The laser output during the first to third laser irradiations. "Velocity" refers to the speed at which the laser irradiation position moves during the first to third laser irradiations.

[0076] Whether the central axis of the second bead and the central axis of the first bead overlap can be determined from H12S, H12E, and W2. For example, in condition No. 5, the distance between the start and end points of the second bead and the central axis of the first bead is sufficiently greater than half the width of the second bead, so it can be seen that the central axis of the second bead and the central axis of the first bead do not overlap. On the other hand, in condition No. 23, the distance between the start point of the second bead and the central axis of the first bead is less than half the width of the second bead, so it can be seen that the central axes of the second bead and the first bead overlap near the start point.

[0077] "Stress relaxation region length" refers to the length of the area in the terminal peripheral region EA where the distance from the widthwise end of the first bead to the second or third bead is 2 mm or less. If neither the second nor third bead is present, "0" is written in this column. If only the second bead is present, the stress relaxation region length of the second bead is written in this column. If both the second and third beads are present, the larger of the stress relaxation region lengths of each bead is written in the "Stress Relaxation Region Length" column. The stress relaxation region length was measured on the joint surface on the laser irradiation side.

[0078] The size of the gap between the steel plates was controlled by inserting a spacer between them. The "plate gap G" listed in the table is the value measured at the cross-section of the end of the first bead 11. The measurement method was the one described above. In laser irradiation under conditions No. 12 to 14, the plate gap G was controlled by using a spacer corresponding to the plate gap G. On the other hand, in laser irradiation under condition No. 24, laser irradiation was performed without using a spacer. For reference, the table also includes the ratio of the size of the gap between the two steel plates to the sum of the thicknesses of the two steel plates.

[0079] "Penetration degree" refers to the state of the first bead on the side opposite to the laser irradiation side. If the steel plates are joined by the first bead, but the first bead is not formed on the joint surface opposite to the laser irradiation side, this column is marked "Partial." If the steel plates are joined by the first bead, and the first bead is also formed on the joint surface opposite to the laser irradiation side, this column is marked "Complete."

[0080] Weld crack resistance was evaluated by preparing three test specimens under identical conditions and determining the number of weld cracks that occurred in these specimens. Cases where cracks occurred in all specimens were rated as "×", cases where no cracks occurred in any specimen were rated as "〇", and cases where cracks occurred in some specimens were rated as "△". Cases rated as "〇" or "△" were considered to have excellent weld crack resistance.

[0081] [Table 1]

[0082] [Table 2]

[0083] [Table 3]

[0084] [Table 4]

[0085] Under conditions No. 1 to 4, the second bead did not form near the end of the first bead. Under these conditions No. 1 to 4, weld cracks occurred in all test specimens. Under condition No. 7, the second bead formed near the end of the first bead, but the gap between the first and second beads was 3 mm or more. In other words, the second bead did not include a stress relaxation region. Weld cracks occurred in all test specimens under condition No. 7 as well. Under condition No. 11, the second and third weld beads were formed near the end of the first weld bead, but the distance between the first weld bead and the second and third weld beads was 3 mm or more. In other words, neither the second nor the third weld bead contained a stress relaxation region. Weld cracks occurred in all test specimens under condition No. 11 as well. Under condition No. 14, the G / T ratio was over 18%. Weld cracks occurred in all test specimens under condition No. 14 as well. In conditions No. 19 and No. 20, the second and third beads formed near the end of the first bead, and some of them were sufficiently close to the first bead; however, the stress relaxation regions of these beads were less than 2 mm. In conditions No. 19 and No. 20, weld cracks occurred in all test specimens. Under condition No. 22, the laser irradiation sequence was second laser irradiation, third laser irradiation, and then first laser irradiation. Under condition No. 22, the distance between the first bead and the second and third beads was 3 mm or more. That is, neither the second nor the third bead contained a stress relaxation region. Under condition No. 22, weld cracks occurred in all test specimens. Under condition No. 23, the second and third weld beads were not separated from the central axis of the first bead, and therefore did not exhibit a stress-relaxing effect. As a result, weld cracks occurred in all test specimens under condition No. 23 as well. In addition, in conditions No. 7, 11, 14, 19, 20, 22, and 23, evaluation was also performed on the joint surface opposite to the laser irradiation side, but similar to the laser irradiation side, the requirements of the present invention were not met.

[0086] On the other hand, under conditions No. 5, 6, 8-10, 12, 13, 15-18, 21, and 24-26, the requirements of the present invention were met at least on the joint surface on the laser irradiation side. In the overlapping laser-welded joints obtained under these conditions, welding cracks at the bead end were suppressed. Furthermore, under condition No. 24, the order of laser irradiation was second laser irradiation, third laser irradiation, and then first laser irradiation. In addition, under condition No. 24, the second and third beads were formed before overlapping the steel plates. As a result, under condition No. 24, an appropriate gap between plates was created without the use of spacers.

[0087] [Example 2] Two steel plates having the tensile strength, thickness, and chemical composition listed in Table 5 were stacked together, and fillet welds were performed by laser irradiation under various conditions to form a first bead. Specifically, the end face of one of the two steel plates (the first metal plate) was joined to the surface of the other metal plate (the second metal plate) by laser welding. The chemical composition of each steel plate, other than the components shown in Table 5, consisted of Fe and impurities. In some examples, a second and a third laser irradiation were performed to form a second and a third bead near the end of the first bead. More specifically, the second and third beads were formed on the second metal plate so that they were located within the peripheral region EA. The presence or absence of weld cracks in the various lap laser welded joints obtained was checked. The welding process sequence is shown in Table 6, and other welding conditions and evaluation results of the lap laser welded joints are shown in Tables 7 and 8. Photographs of representative laser welded joints are shown in Figures 14 to 16.

[0088] As in Example 1, both the second and third beads correspond to the second bead of the overlapping laser-welded joint according to the present embodiment described above. For convenience, of the two second beads, the one formed later will be referred to as the third bead. Similarly, if the second laser irradiation is performed twice, the later one will be referred to as the third laser irradiation.

[0089] The length of the stress relaxation region was measured on the surface of the second metal plate on the laser irradiation side. When the second laser irradiation was applied to the first metal plate, the length of the stress relaxation region of the second bead was measured at the mating surface between the second metal plate and the first metal plate formed by the second laser irradiation. Specifically, the first metal plate was peeled away from the second metal plate, and the length of the stress relaxation region of the second bead was measured at the mating surface between the second metal plate and the first metal plate.

[0090] The size of the gap between the steel plates was controlled by inserting a spacer between them. The "plate gap G" listed in Table 8 is the value measured at the cross-section of the end of the first bead 11. The measurement method was the same as described above. For laser irradiation under conditions No. 39 and 40, the plate gap G was controlled by using a spacer corresponding to the plate gap G. For reference, Table 8 also includes the ratio of the size of the gap between the two steel plates to the sum of the thicknesses of the two steel plates.

[0091] The "penetration degree" in Table 8 was described according to the same criteria as in Example 1. Weld crack resistance was evaluated according to the same criteria as in Example 1.

[0092] [Table 5]

[0093] [Table 6]

[0094] [Table 7]

[0095] [Table 8]

[0096] Under conditions No. 27 and 28, the second bead was not formed near the end of the first bead on the second metal plate. Under these conditions No. 27 and 28, weld cracks occurred in all test specimens. In conditions No. 31, 44, 46, and 48, the second and third beads were formed near the end of the first bead in the second metal plate, but the spacing between the first and second beads, and between the first and third beads, was 3 mm or more. That is, neither the second nor the third bead contained a stress relaxation region. In conditions No. 31, 44, 46, and 48, weld cracks occurred in all test specimens. Under condition No. 34, the second bead was formed near the end of the first bead on the second metal plate, but the gap between the first and second beads was 3 mm or more. In other words, the second bead did not include a stress relaxation region. Weld cracks occurred in all test specimens under condition No. 34 as well. Under condition No. 35, the first metal plate was subjected to a second laser irradiation, but the laser output was low, and a second bead was not formed on the second metal plate. Weld cracks occurred in all test specimens under condition No. 35 as well. Under condition No. 38, a second laser irradiation was performed on the second metal plate, and a second bead was formed near the end of the first bead on the second metal plate. However, the gap between the first and second beads was 3 mm or more. In other words, the second bead did not include a stress relaxation region. Weld cracks occurred in all test specimens under condition No. 38 as well. Under condition No. 40, the G / T ratio was over 18%. Weld cracks occurred in all test specimens under condition No. 40 as well.

[0097] On the other hand, under conditions No. 29, 30, 32, 33, 36, 37, 39, 41, 42, 43, 45, 47, and 49, the requirements of the present invention were met at least on the joint surface on the laser irradiation side. In the overlapping laser-welded joints obtained under these conditions, welding cracks at the bead end were suppressed. [Explanation of symbols]

[0098] 1. 1A Overlap Laser Welded Joint 10, 10a metal plate 10b First metal plate 10c Second metal plate 11, 11A First bead 11E End of the first bead 11S First bead start 11X Central axis of the first bead 111 Craters 12. The second bead 121 Stress relaxation region of the second bead VL1 First virtual line VL2 Second virtual line EA Terminus Region VL3 Third virtual line VL4, the fourth virtual line SA starting point surrounding area VL5 Fifth virtual line P1 First point P2 Second point P3 Third point C. Weld crack

Claims

1. Multiple metal plates stacked on top of each other, A first bead is a linearly extending laser weld that joins multiple metal plates, A lap laser welded joint comprising, The ratio G / T of the sum of the gaps between the multiple metal plates G to the sum of the thicknesses of the multiple metal plates T is 0 to 18%. The length L1 of the first bead, measured along the central axis in the direction of extension of the first bead, is 10 to 100 mm. The aforementioned overlapping laser-welded joint further has a second bead, The length L2 of the second bead, measured along the central axis in the direction of extension of the second bead, is smaller than L1. In a plan view of the overlapping laser-welded joint as seen from the thickness direction, in the terminal peripheral region which is the area between a first imaginary line passing through the end of the first bead and perpendicular to the central axis of the first bead at the end, and a second imaginary line passing through a first point 5 mm away from the end of the first bead towards the starting end of the first bead along the central axis of the first bead and perpendicular to the central axis of the first bead at the first point, the length of the region in the second bead that is 2 mm or less from the end of the first bead in the width direction is 2 mm or more along the central axis of the first bead. The second bead is spaced apart from the central axis of the first bead. The second bead is not included in the starting end peripheral region, which is the region between a third imaginary line passing through the starting end of the first bead and perpendicular to the central axis of the first bead at the starting end, and a fourth imaginary line passing through a second point located (1 / 3) × L1 away from the starting end of the first bead along the central axis of the first bead toward the end of the first bead and perpendicular to the central axis of the first bead at the second point. Overlap laser welded joint.

2. The overlapping laser-welded joint according to claim 1, characterized in that, in a plan view of the overlapping laser-welded joint as seen from the thickness direction, the second bead is formed on both sides of the first bead with reference to the central axis of the first bead.

3. The overlapping laser welded joint according to claim 1 or 2, characterized in that the second bead is separated from a fifth imaginary line perpendicular to the central axis of the first bead at the third point, passing through a third point 2 mm away from the end of the first bead in the direction opposite to the starting end of the first bead.

4. The overlapping laser welded joint according to claim 1 or 2, characterized in that the first bead is formed only on one surface of the overlapping laser welded joint.

5. The second bead is formed on only one of the metal plates arranged on the outermost layer of the overlapping laser-welded joint. The overlapping laser welded joint according to claim 1 or 2, characterized in that the penetration depth of the second bead is 1 / 2 or more of the thickness of the metal plate on which the second bead is formed.

6. The aforementioned plurality of metal plates are a plurality of steel plates, The component composition of one or more of the aforementioned plurality of steel plates is C: 0.05 to 0.5 mass%, Si: 0.1 to 3.5 mass%, Mn: 0.1 to 5.5 mass%, and P and S: Total 0.03 mass% or less including The overlapping laser-welded joint according to claim 1 or 2.

7. The aforementioned plurality of metal plates are a plurality of steel plates, The lap laser welded joint according to claim 1 or 2, characterized in that one or more of the plurality of steel plates have a tensile strength of 980 MPa or more.

8. The overlapping laser welded joint according to claim 7, characterized in that the tensile strength of the steel plate on which the second bead is formed among the plurality of steel plates is 980 MPa or more.

9. The process of stacking multiple metal plates, A step of irradiating with a first laser to form a first bead, which is a linearly extending laser welded portion that joins a plurality of the aforementioned metal plates, A method for manufacturing an overlapping laser-welded joint comprising: In the aforementioned overlapping configuration, the ratio G / T of the total size of the gaps between the metal plates G to the total thickness of the metal plates T is set to 0 to 18%. The length L1 of the first bead, measured along the central axis in the direction of extension of the first bead, is set to 10 to 100 mm. The method for manufacturing the overlapping laser-welded joint further comprises a step of irradiating with a second laser to form a second bead, The length L2 of the second bead, measured along the central axis in the direction of extension of the second bead, is made smaller than L1. In a plan view of the overlapping laser-welded joint as seen from the thickness direction, in the terminal peripheral region which is the area between a first imaginary line passing through the end of the first bead and perpendicular to the central axis of the first bead at the end, and a second imaginary line passing through a first point 5 mm away from the end of the first bead towards the starting end of the first bead along the central axis of the first bead and perpendicular to the central axis of the first bead at the first point, the length of the region in the second bead that is 2 mm or less from the end of the first bead in the width direction is 2 mm or more along the central axis of the first bead. The second bead is separated from the central axis of the first bead. The region around the starting end, which is the area between a third imaginary line passing through the starting end of the first bead and perpendicular to the central axis of the first bead at the starting end, and a fourth imaginary line passing through a second point located (1 / 3) × L1 away from the starting end of the first bead along the central axis of the first bead toward the end of the first bead and perpendicular to the central axis of the first bead at the second point, excludes the second bead. The second bead is formed before 10 seconds have elapsed since the end of the first laser irradiation. A method for manufacturing overlapping laser-welded joints.

10. The method for manufacturing an overlapping laser-welded joint according to claim 9, characterized in that the second laser irradiation is performed after the first laser irradiation.

11. The method for manufacturing an overlapping laser-welded joint according to claim 9, characterized in that the first laser irradiation is performed after the second laser irradiation.

12. A method for manufacturing an overlapping laser-welded joint according to any one of claims 9 to 11, characterized in that, in a plan view of the overlapping laser-welded joint as seen from the thickness direction, the second bead is formed on both sides of the first bead with reference to the central axis of the first bead.

13. A method for manufacturing an overlapping laser welded joint according to any one of claims 9 to 11, characterized in that the second bead is separated from a fifth imaginary line that is perpendicular to the central axis of the first bead at the third point, passing through a third point 2 mm away from the end of the first bead in the direction opposite to the starting end of the first bead.

14. A method for manufacturing an overlapping laser-welded joint according to any one of claims 9 to 11, characterized in that the first bead is formed only on one side of the overlapping laser-welded joint.

15. A method for manufacturing an overlapping laser welded joint according to any one of claims 9 to 11, characterized in that the penetration depth of the second bead is 1 / 2 or more of the thickness of the metal plate on which the second bead is formed.

16. A structural member for an automobile body comprising the overlapping laser-welded joint according to claim 1 or 2.

17. Multiple metal plates stacked on top of each other, A first bead is a linearly extending laser weld that joins the end face of a first metal plate, which is the outermost metal plate among the multiple metal plates, and the surface of a second metal plate superimposed on the first metal plate, and also joins the multiple metal plates. A lap laser welded joint comprising, The ratio G / T of the sum of the gaps between the multiple metal plates G to the sum of the thicknesses of the multiple metal plates T is 0 to 18%. The length L1 of the first bead, measured along the central axis in the direction of extension of the first bead, is 10 to 100 mm. The overlapping laser-welded joint further has a second bead on at least the surface of the second metal plate, The length L2 of the second bead, measured along the central axis in the direction of extension of the second bead, is smaller than L1. In the mating surface of the second metal plate superimposed with the first metal plate, in the end peripheral region which is the area between a first imaginary line passing through the end of the first bead and perpendicular to the central axis of the first bead at the end, and a second imaginary line passing through a first point 5 mm away from the end of the first bead towards the starting end of the first bead along the central axis of the first bead and perpendicular to the central axis of the first bead at the first point, the length of the region in the second bead that is 2 mm or less from the end of the first bead in the width direction is 2 mm or more along the central axis of the first bead. The second bead is spaced apart from the central axis of the first bead. The second bead is not included in the starting end peripheral region, which is the region between a third imaginary line passing through the starting end of the first bead and perpendicular to the central axis of the first bead at the starting end, and a fourth imaginary line passing through a second point located (1 / 3) × L1 away from the starting end of the first bead along the central axis of the first bead toward the end of the first bead and perpendicular to the central axis of the first bead at the second point. Overlap laser welded joint.

18. The overlapping laser welding joint according to claim 17, characterized in that, at the mating surface of the second metal plate overlapping the first metal plate, the second bead is formed on both sides of the first bead with reference to the central axis of the first bead.

19. The overlapping laser welded joint according to claim 17 or 18, characterized in that the second bead is separated from a fifth imaginary line perpendicular to the central axis of the first bead at the third point, passing through a third point 2 mm away from the end of the first bead in the direction opposite to the starting end of the first bead.

20. The lap laser welded joint according to claim 17 or 18, characterized in that the first bead is formed only on one surface of the lap laser welded joint.

21. The overlapping laser welded joint according to claim 17 or 18, characterized in that the penetration depth of the second bead in the second metal plate is 1 / 2 or more of the thickness of the second metal plate.

22. The aforementioned plurality of metal plates are a plurality of steel plates, The component composition of one or more of the aforementioned plurality of steel plates is C: 0.05 to 0.5 mass%, Si: 0.1 to 3.5 mass%, Mn: 0.1 to 5.5 mass%, and P and S: Total 0.03 mass% or less including The overlapping laser-welded joint according to feature 17 or 18.

23. The aforementioned plurality of metal plates are a plurality of steel plates, The overlapping laser-welded joint according to claim 17 or 18, characterized in that one or more of the plurality of steel plates have a tensile strength of 980 MPa or more.

24. The lap laser welded joint according to claim 23, characterized in that the tensile strength of the steel plate on which the second bead is formed is 980 MPa or more among the plurality of steel plates.

25. The process of stacking multiple metal plates, A step of irradiating with a first laser to form a first bead, which is a linearly extending laser welded portion that joins a plurality of the aforementioned metal plates, A method for manufacturing an overlapping laser-welded joint comprising: In the first laser irradiation step, at least the end face of the first metal plate, which is placed on the outermost surface of the plurality of metal plates, and the surface of the second metal plate, which is superimposed on the first metal plate, are joined together. In the aforementioned overlapping configuration, the ratio G / T of the total size of the gaps between the metal plates G to the total thickness of the metal plates T is set to 0 to 18%. The length L1 of the first bead, measured along the central axis in the direction of extension of the first bead, is set to 10 to 100 mm. The method for manufacturing the overlapping laser-welded joint further comprises the step of irradiating with a second laser so as to form a second bead on at least the second metal plate, The length L2 of the second bead, measured along the central axis in the direction of extension of the second bead, is made smaller than L1. In the mating surface of the second metal plate superimposed with the first metal plate, in the end peripheral region which is the area between a first imaginary line passing through the end of the first bead and perpendicular to the central axis of the first bead at the end, and a second imaginary line passing through a first point 5 mm away from the end of the first bead towards the starting end of the first bead along the central axis of the first bead and perpendicular to the central axis of the first bead at the first point, the length of the region in the second bead that is 2 mm or less from the end of the first bead in the width direction is 2 mm or more along the central axis of the first bead. The second bead is separated from the central axis of the first bead. The region around the starting end, which is the area between a third imaginary line passing through the starting end of the first bead and perpendicular to the central axis of the first bead at the starting end, and a fourth imaginary line passing through a second point located (1 / 3) × L1 away from the starting end of the first bead along the central axis of the first bead toward the end of the first bead and perpendicular to the central axis of the first bead at the second point, excludes the second bead. The second bead is formed before 10 seconds have elapsed since the end of the first laser irradiation. A method for manufacturing overlapping laser-welded joints.

26. The method for manufacturing an overlapping laser-welded joint according to claim 25, characterized in that the second laser irradiation is performed after the first laser irradiation.

27. The method for manufacturing an overlapping laser-welded joint according to claim 25, characterized in that the first laser irradiation is performed after the second laser irradiation.

28. A method for manufacturing an overlapping laser welding joint according to any one of claims 25 to 27, characterized in that, at the mating surface of the second metal plate overlapping with the first metal plate, the second bead is formed on both sides of the first bead with reference to the central axis of the first bead.

29. A method for manufacturing an overlapping laser welded joint according to any one of claims 25 to 27, characterized in that the second bead is separated from a fifth imaginary line perpendicular to the central axis of the first bead at the third point, passing through a third point 2 mm away from the end of the first bead in the direction opposite to the starting end of the first bead.

30. A method for manufacturing an overlapping laser-welded joint according to any one of claims 25 to 27, characterized in that the first bead is formed only on one side of the overlapping laser-welded joint.

31. A method for manufacturing an overlapping laser welded joint according to any one of claims 25 to 27, characterized in that the penetration depth of the second bead in the second metal plate is 1 / 2 or more of the thickness of the second metal plate.

32. A structural member for an automobile body comprising the overlapping laser-welded joint according to claim 17 or 18.

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