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

Abstract

To provide a lap laser welding joint capable of suppressing welding cracks, a structure member for an automobile body, and a method for manufacturing the lap laser welding joint.SOLUTION: This lap laser welding joint is configured such that a ratio G / T of a total thickness value G of gaps among a plurality of metal plates to a total thickness value T is 0-15%, a laser welding part has a first bead and a second bead provided at the terminal of the fist bead, the second bead extends to both sides of the first bead, the effective angle of the second bead is 40 degrees or lower on each of both sides of the first bead, a ratio L2 / W2 of a length L2 along the center axis of the second bead to a width W2 with respect to the center axis of the second bead is 2.0 or higher, there is only one crater in the laser welding part, the crater is present in the second bead, and a cater deepest part is present between extension lines of both edges of the first bead.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] An overlapping joint is a welded joint obtained by welding a plurality of overlapped metal plates. One of the welding means for manufacturing an overlapping joint is laser welding. Overlapping laser welding is a welding method in which a laser beam is irradiated onto one surface of a plurality of overlapped metal plates to melt and solidify the metal plates, thereby joining these metal plates.

[0003] Overlapping laser welding can join a narrow area such as a flange portion of a hat-shaped member at high speed. Conventionally, in manufacturing an overlapping laser welded joint, a laser beam is intermittently irradiated onto the surface of a plurality of overlapped steel plates, and the steel plates at the sites irradiated with the laser beam are melted and solidified, thereby forming a welded portion in which short linear joints are continuously arranged in a row.

[0004] However, overlapping laser welding has a problem that cracking easily occurs at the final solidification portion at the end of the weld bead. In laser welding, a flow of molten metal occurs in the direction opposite to the traveling direction of the laser. Therefore, a dent 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 a 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 1180 MPa or higher are increasingly being used to improve the strength and rigidity of the vehicle body. However, as the tensile strength of the metal sheet increases, the tensile stress applied to the bead end after laser welding increases, raising 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 made 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 within the range of 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 there is no cracking at the welding end of the joint and the peel strength is excellent.

[0007] Patent Document 2 discloses an impact-absorbing member that has excellent impact absorption characteristics during automobile collisions and can protect occupants by reliably and effectively absorbing collision energy. This impact-absorbing member consists of a cylindrical body having a laser-welded bead on the flange for joining a first member having a flange and a second member. The laser-welded bead is composed of a plurality of first regions and a plurality of second regions that are alternately formed spaced apart in the longitudinal direction of the flange, and the projected length in the width direction of the flange of the first regions is greater than the projected length in the width direction of the flange of the second regions.

[0008] Patent Document 3 discloses a welding structure with high joint strength that does not experience stress concentration in the weld bead, that is, does not cause continuous fracture of the heat-affected zone due to insufficient strength at the welding start and end points or welding defects. It also discloses a welding method and welding apparatus that can reduce the effects of welding defects that occur when welding with a high-energy beam. In this invention, in a welding structure in which two members are joined by overlap welding, at least one of the weld beads formed in a single pass, specifically the start end and the end end, is formed by bending laterally relative to the weld bead in the middle section. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] International Publication No. 2020 / 194669 [Patent Document 2] Japanese Patent Publication No. 2008-161911 [Patent Document 3] Japanese Patent Publication No. 2003-290951 [Overview of the project] [Problems that the invention aims to solve]

[0010] However, even with these technologies, it is difficult to sufficiently suppress cracking, especially in laser welding of high-strength steel plates. In the technology of Patent Document 1, there is a risk of crack propagation along the curved joint having a J-shape. In the technology of Patent Document 2, the laser welded area is composed of a pair of first regions and a second region formed between them, and if the first region is formed as a point, or in the case of a U-shaped weld or a modified U-shaped weld, there is a risk of crack propagation to the second region. On the other hand, in the case of an I-shaped weld, it is thought that this suppresses cracking in the second region. However, in the first region, there is a risk of crack propagation along its extension direction. In the technology of Patent Document 3, the laser welded area is composed of a starting point, an intermediate part, and an ending part, and if these are U-shaped or roughly U-shaped, or if the additional weld bead is formed at a position offset from the welding end point of the weld bead, there is a risk of crack propagation or occurrence in the weld bead (intermediate part). On the other hand, in the case of I-shaped welding, it is thought that cracking in the middle section is suppressed. However, there is a risk that cracks may propagate along the direction of extension at the starting and ending points.

[0011] 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, a structural member for an automobile body, and a method for manufacturing a lap laser welded joint. [Means for solving the problem]

[0012] The gist of this invention is as follows:

[0013] (1) An overlapping laser-welded joint according to one aspect of the present invention comprises a plurality of overlapping metal plates and a laser-welded portion for joining the plurality of metal plates, wherein the ratio G / T of the total thickness of the gaps between the plurality of metal plates to the total thickness of the plurality of metal plates to 0 to 15%, and on at least one side of the overlapping laser-welded joint, the laser-welded portion has a first bead for joining the plurality of metal plates and a second bead provided at the end of the first bead, the second bead extends on both sides with respect to a central axis perpendicular to the width direction of the first bead, and the first bead The effective angle of the second bead, which is the smaller of the angles between a virtual line passing through the intersection of the central axis and the outer edge of the second bead and perpendicular to the central axis of the first bead, and the central axis perpendicular to the width direction of the second bead, is 40 degrees or less on each of the two sides of the first bead; the ratio L2 / W2 of the length L2 of the second bead along the central axis to the width W2 of the second bead with respect to the central axis is 2.0 or more; only one crater exists in the laser welded area, the crater is located in the second bead, and the deepest part of the crater is located between the extensions of both edges of the first bead. (2) In the overlapping laser welded joint described in (1) above, the length L2 of the second bead along the central axis may be less than the length L1 of the first bead along the central axis on at least one side of the overlapping laser welded joint. (3) In the overlapping laser welded joint described in (1) or (2) above, the length L2 of the second bead along the central axis may be 12.0 mm or less on at least one side of the overlapping laser welded joint. (4) In the lap laser welded joint described in any one of the above items (1) to (3), the penetration depth D2 of the second bead may be less than the penetration depth D1 of the first bead. (5) In the overlapping laser welded joint described in any one of the above items (1) to (4), an additional bead does not need to be provided at the starting end of the first bead. (6) In the overlapping laser-welded joint described in any one of the above items (1) to (5), the laser-welded portion may be present only on one side of the overlapping laser-welded joint. (7) In the lap laser welded joint described in any one of the above items (1) to (6), the plurality of metal plates are a plurality of steel plates, and the chemical composition of one or more of the plurality of steel plates may contain 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. (8) In the lap laser welded joint described in any one of the above items (1) to (7), 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. (9) The lap laser welded joint described in any one of the above items (1) to (8) may be a lap fillet joint.

[0014] (10) 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 (1) to (9) above.

[0015] (11) A method for manufacturing a lap laser welded joint according to another aspect of the present invention comprises the steps of: performing a first laser welding on a plurality of overlapping metal plates to form a first bead that joins the plurality of metal plates; and performing a second laser welding on the metal plates on at least one surface of the lap laser welded joint to form a second bead, wherein the lap laser welded joint has a laser welded portion having the first bead and the second bead, the ratio G / T of the total thickness of the gap between the plurality of metal plates G to the total thickness of the plurality of metal plates T is 0 to 15%, and on at least one side of the lap laser welded joint, the second bead is provided at the end of the first bead, and the second bead is provided at the end of the first The second bead extends on both sides with respect to a central axis perpendicular to the width direction of the bead, and the effective angle of the second bead, which is the smaller of the angle between a virtual line passing through the intersection of the central axis of the first bead and the outer edge of the second bead and perpendicular to the central axis of the first bead and the central axis perpendicular to the width direction of the second bead, is set to 40 degrees or less on each side of the first bead, the ratio L2 / W2 of the length L2 of the second bead along the central axis to the width W2 of the second bead with respect to the central axis is set to 2.0 or more, only one crater exists in the laser welded area, the crater is located in the second bead, and the deepest part of the crater is located between the extensions of both edges of the first bead. (12) In the method for manufacturing a lap laser-welded joint described in (11) above, laser irradiation may be performed continuously from the start of the first laser welding to the end of the second laser welding. (13) In the method for manufacturing an overlapping laser-welded joint described in (11) or (12) above, the second laser welding may be terminated at the point where the end of the first bead and the second bead overlap. (14) In the method for manufacturing a lap laser welded joint described in (11) to (13) above, the lap laser welded joint may be a lap fillet joint. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a lap laser welded joint capable of suppressing welding cracks, a structural member for an automobile body, and a method for manufacturing a lap laser welded joint.

Brief Description of the Drawings

[0017] [Figure 1] It is a plan view of an overlapping laser welding joint according to an embodiment of the present invention, in which the laser welding part has a T shape. [Figure 2] It is a plan view of an overlapping laser welding joint according to the same embodiment, in which the laser welding part has an I shape. [Figure 3] It is a plan view of an overlapping laser welding joint according to the same embodiment, in which the laser welding part has a Y shape. [Figure 4] It is a plan view of an overlapping laser welding joint in which the laser welding part is linear and the effective angle is 90 degrees. [Figure 5] It is a plan view of an overlapping laser welding joint according to the same embodiment, in which the first bead and the second bead are curved. [Figure 6A] It is a cross-sectional view perpendicular to the central axis 121X of the first bead 121 in the overlapping laser welding joint according to the same embodiment. [Figure 6B] It is an enlarged cross-sectional view perpendicular to the central axis 121X of the first bead 121 in the overlapping laser welding joint according to the same embodiment. [Figure 7A] It is a cross-sectional view along the central axis 121X of the first bead 121 in the overlapping laser welding joint according to the same embodiment. [Figure 7B] It is a cross-sectional view along the central axis 121X of the first bead 121 in the overlapping laser welding joint according to the same embodiment. [Figure 8A] It is a schematic diagram of the tensile stress during solidification of the final solidified part in a conventional overlapping laser welding joint. [Figure 8B] It is a schematic diagram of the tensile stress during solidification of the final solidified part in the overlapping laser welding joint according to the same embodiment. [Figure 9A] It is a schematic diagram of a manufacturing method of an overlapping laser welding joint in which the first laser welding and the second laser welding are performed continuously. [Figure 9B] It is a schematic diagram of a manufacturing method of an overlapping laser welding joint in which the first laser welding and the second laser welding are performed intermittently. [Figure 10] This is a plan view of a modified example of the same embodiment, which is a fillet joint with a T-shaped laser welded section. [Figure 11] This is a cross-sectional view of a cross-section taken in the thickness direction at the position of the central axis of the second bead of an overlapping fillet joint, which is a modified example of the same embodiment. [Figure 12] This is a photograph of the overlapping laser-welded joint in Comparative Example No. 5. [Figure 13] This is a photograph of the overlapping laser-welded joint in Example No. 17. [Figure 14] This is a photograph of the overlapping laser-welded joint in Example No. 26. [Figure 15] This is a photograph of the overlapping fillet joint in example No. 44. [Modes for carrying out the invention]

[0018] <Overlapping laser-welded joint 1> An overlapping laser-welded joint 1 according to one aspect of the present invention, as shown in Figures 1 to 3, comprises a plurality of overlapping metal plates 11 and a laser-welded portion 12 that joins the plurality of metal plates 11. Here, the ratio G / T of the total thickness G of the gaps between the plurality of metal plates 11 to the total thickness T of the plurality of metal plates 11 is 0 to 15%, and on at least one side of the overlapping laser-welded joint 1, the laser-welded portion 12 has a first bead 121 that joins the plurality of metal plates 11 and a second bead 122 provided at the end of the first bead 121, the second bead 122 extends on both sides with respect to a central axis 121X perpendicular to the width direction of the first bead 121, passes through the intersection P of the central axis 121X of the first bead 121 and the outer edge of the second bead 122 and is perpendicular to the central axis 121X of the first bead 121 The effective angles θ1 and θ2 of the second bead 122, which are the smaller angles between the imaginary line VL and the central axis 122X perpendicular to the width direction of the second bead 122, are 40 degrees or less on each side of the first bead 121, the ratio L2 / W2 of the length L2 of the second bead 122 along the central axis 122X to the width W2 of the second bead 122 with respect to the central axis 122X is 2.0 or more, there is only one crater 120 in the laser welded joint 12, the crater 120 is located in the second bead 122, and the deepest part of the crater 120 is located between the extensions of both edges of the first bead 121. The overlapping laser welded joint 1 according to this embodiment will now be described in detail.

[0019] (Metal plate 11) Multiple metal plates 11 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 11 are not particularly limited, as long as they are suitable for laser welding. The number of metal plates 11 is also not particularly limited and can be any number of two or more.

[0020] A suitable example of the multiple metal plates 11 is a plurality of steel plates, a plurality of Al plates, etc. Alternatively, a combination of steel plates and Al plates may be used to form the multiple metal plates 11. When the metal plates 11 are steel plates, the chemical composition of these steel plates is not particularly limited, and a suitable chemical composition can be applied depending on the application of the overlapping laser-welded joint 1. For example, one or more of the multiple steel plates may contain a chemical composition of 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. In this case, the remainder of the chemical composition of the steel plates may include Fe and impurities. Since steel plates with such a chemical 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 steel plates with such a chemical 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 122, which will be described later.

[0021] 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 11 is high. For example, if the multiple metal plates 11 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 at 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 122, which will be described later.

[0022] Furthermore, steel plates with a tensile strength of 980 MPa or higher, i.e., high-strength steel plates, may be placed on the outermost surface of the overlapping laser-welded joint 1, 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 122 described later, the effect of preventing welding cracks can be effectively exhibited even in plate assemblies that include high-strength steel plates. When the multiple metal plates 11 are multiple steel plates, and one or more of them are high-strength steel plates, it is preferable that the second bead 122 described later penetrates to the high-strength steel plate. Furthermore, it is even more preferable that the positional relationship between the first bead 121 and the second bead 122 described later is satisfied in the high-strength steel plate. On the other hand, even if the second bead 122 is formed only on steel plates with a tensile strength of less than 980 MPa that are combined with high-strength steel plates, as long as the positional relationship between the first bead 121 and the second bead 122 described later is satisfied, the effect of relieving tensile stress at the end of the bead and suppressing welding cracks in the high-strength steel plate can be sufficiently obtained.

[0023] The metal sheet 11 may be plated or unplated. Examples of plating include GI plating, GA plating, EG plating, Zn-Ni plating, Zn-Al plating, Zn-Mg plating, and Zn-Mg-Al plating. If the metal sheet 11 is a zinc-based hot-stamped steel sheet, zinc oxide may be included in the surface layer of the Fe-Zn or Fe-Zn-Ni solid solution phase. If the metal sheet 11 is an aluminum-based hot-stamped steel sheet, multiple Al-Fe-Si intermetallic compound layers may be formed, and furthermore, ZnO or a black coating may be formed on the intermetallic compound layers. If the metal sheet 11 is an unplated hot-stamped steel sheet, it may be shot-blasted to remove the scale generated during the hot-stamping process.

[0024] (G gap between multiple metal plates 11) In laser welding, a flow of weld metal occurs in the direction opposite to the direction of laser propagation. Therefore, a depression called a crater 120 is generally formed at the end of the bead formed by laser welding. Craters 120 can cause weld cracks, so it is preferable for them to be as small as possible. Here, the crater 120 can be reduced by reducing the gap between multiple metal plates 11. For the reasons above, the ratio G / T of the total thickness of the gaps between multiple metal plates 11 to the total thickness of the multiple metal plates 11 is set to be within the range of 0 to 15%. G is the size of the gap between the two metal plates 11 when there are two metal plates 11, and the total thickness of the gaps between the metal plates 11 when there are three or more metal plates 11. For example, in the cross-sectional view of the overlapping laser-welded joint 1 illustrated in Figure 6A, G is the sum of gap g1 and gap g2, and T is the sum of plate thicknesses t1, t2, and t3. A smaller G / T ratio is preferable, and may be 12% or less, 10% or less, or 8% or less.

[0025] The size of the gap between the metal plates 11 is measured in the cross-section of the first bead 121. The cross-section is defined as a cross-section cut along a virtual line VL that passes through the intersection point P of the central axis 121X of the first bead 121 and the outer edge of the second bead 122, and is perpendicular to the central axis 121X of the first bead. In this cross-section, the sum of the thicknesses of the gaps between the multiple metal plates 11, G, and the sum of the thicknesses of the multiple metal plates 11, T, are measured. Also, as shown in Figure 6B, the size of the gap is measured at both ends of the first bead 121 in the cross-section. The average value of the gap size gl on the left side of the first bead 121 and the gap size gr on the right side is considered to be the size of the gap between the metal plates 11.

[0026] (Laser welded section 12) In the overlapping laser-welded joint 1 according to this embodiment, a laser-welded section 12 is formed on a plurality of metal plates 11, having a first bead 121 that joins the plurality of metal plates 11, and a second bead 122 provided at the end of the first bead 121. Generally, "bead" refers to the raised portion created by 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 1 according to this embodiment is not impaired. By optimizing the spacing between the plurality of metal plates 11, as well as the shape and positional relationship of the bead, welding cracks at the end of the bead can be prevented. The shape and positional relationship of the bead will be described below with reference to Figure 1, etc.

[0027] Note that the shape of the bead does not necessarily coincide between the front and back surfaces of the overlapping laser-welded joint 1. If the bead shape and positional relationship 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 surface is considered to be the overlapping laser-welded joint 1 according to this embodiment. The bead shape and positional relationship described below may also be satisfied on both surfaces of the overlapping laser-welded joint 1. Also, unless otherwise specified, the bead shape and positional relationship described below are as viewed from above when the overlapping laser-welded joint 1 is viewed from the thickness direction of the overlapping laser-welded joint 1.

[0028] (First bead 121) The first bead 121 is a linearly extending laser weld 12 that joins multiple metal plates 11. The shape of the first bead 121 is not particularly limited as long as it is linear. The first bead 121 may be straight, curved, or even bent. For example, the first bead 121 may be C-shaped or L-shaped.

[0029] Since the first bead 121 joins multiple metal plates 11, when viewed in cross-section, the first bead 121 usually extends in the thickness direction, spanning all the metal plates 11, as illustrated in Figures 7A and 7B. However, the first bead 121 does not need to penetrate all the metal plates 11. As illustrated in Figure 7B, the first bead 121 may be formed on only one side of the overlapping laser welded joint 1. Alternatively, if there are two or more first beads 121 in the overlapping welded joint, each first bead 121 does not need to span all the metal plates 11. For example, by forming a first bead 121 that joins a portion of the multiple metal plates 11 on one side of the plate assembly, and forming a first bead 121 that joins the rest of the multiple metal plates 11 on the other side of the plate assembly, all of the multiple metal plates 11 can be joined by the first bead 121.

[0030] (Length L1 of the first bead 121) The length L1 of the first bead 121 is the length measured along the central axis 121X perpendicular to the width direction of the first bead 121. In other words, the length L1 of the first bead 121 is the distance from the start end of the first bead 121 to the end end of the first bead 121, or the distance from the start end of the first bead 121 to the intersection point of the two outer edges of the central axis 121X perpendicular to the width direction of the first bead 121 and the second bead 122 that is further away from the start end 121S of the first bead 121. As shown in Figure 1, etc., the overlapping portion of the first bead 121 and the second bead 122 is also included in L1. Hereafter, for convenience, the "central axis 121X perpendicular to the width direction of the first bead 121" may be simply referred to as the "central axis 121X of the first bead 121". Also, if the first bead 121 is a curve or a folded line, the central axis 121X of the first bead 121 will also be a curve or a folded line.

[0031] The length L1 of the first bead 121 is not particularly limited, but is preferably 10 to 100 mm. By setting the length L1 of the first bead 121 to 10 mm or more, welding cracks can be suppressed more effectively. From the viewpoint of further suppressing welding cracks, the length L1 of the first bead 121 may be 12 mm or more, 15 mm or more, or 20 mm or more. On the other hand, by setting the length L1 of the first bead 121 to 100 mm or less, the time required for laser welding can be shortened and productivity can be improved. The length L1 of the first bead 121 may be 90 mm or less, 80 mm or less, or 70 mm or less.

[0032] Furthermore, if the material to be welded is large, multiple first beads 121 may be provided in the overlapping laser-welded joint 1. In this case, a second bead 122, described later, should be placed at the end of each of the multiple first beads 121. However, it is not necessary to place a second bead 122 at the end of all first beads 121 included in the overlapping laser-welded joint 1. The second bead 122 may be formed only in areas where welding cracks are a particular concern.

[0033] (Second bead 122) The overlapping laser-welded joint 1 has a second bead 122 provided at the end of the first bead 121. Whether the second bead 122 is provided at the end of the first bead 121 can be determined from the position of the crater 120. If the second bead 122 is provided at the beginning of the first bead 121, the crater 120 will be formed at the end of the first bead 121 and at the second bead 122 provided at the beginning of the first bead 121. The fact that the crater 120 is formed only on the second bead is evidence that the second bead 122 is provided at the end of the first bead 121.

[0034] As shown in Figures 1 to 3, the second bead 122 has a shape that extends on both sides with respect to the central axis 121X which is perpendicular to the width direction of the first bead 121. The second bead 122 has the effect of preventing welding cracks C in the crater 120 by changing the extension direction of the final solidified portion of the first bead 121.

[0035] Furthermore, in order to obtain the effect of preventing welding cracks by the second bead 122, at least on one side of the overlapping laser-welded joint 1, (A) Effective angle of the second bead 122, (B) Aspect ratio of the second bead 122, and (C) Number and location of craters 120 formed in the second bead 122 These need to be controlled. These matters are explained in detail below.

[0036] (A Effective angles θ1, θ2 of the second bead 122) The effective angles θ1 and θ2 of the second bead 122 are defined with respect to a virtual line VL that passes through the intersection point P of the central axis 121X of the first bead 121 and the outer edge of the second bead 122, and is perpendicular to the central axis 121X of the first bead. Specifically, the effective angles θ1 and θ2 of the second bead 122 are defined as the smaller of the angles made between the virtual line VL and the central axis 122X perpendicular to the width direction of the second bead 122. However, if the virtual line VL and the central axis 122X perpendicular to the width direction of the second bead 122 are parallel, as shown in Figure 1, for example, then the effective angles θ1 and θ2 of the second bead are set to 0°. Hereinafter, "the central axis 122X perpendicular to the width direction of the second bead 122" may be simply referred to as "the central axis 122X of the second bead 122".

[0037] As mentioned above, the second bead 122 extends on both sides of the central axis 121X of the first bead 121. The effective angles θ1 and θ2 of the second bead 122 may not coincide on both sides of the central axis 121X. In this embodiment, for convenience, the effective angles of the second bead 122 extending on both sides of the first bead 121 are distinguished by being described as θ1 and θ2. In Figures 1 to 3, for convenience, the angle between the second bead 122 and the first bead 121 that protrudes upward from the plane of the paper is described as θ1, and the angle between the second bead 122 and the first bead 121 that protrudes downward from the plane of the paper is described as θ2. However, since θ1 and θ2 are equivalent, the angle on either side may be considered as θ1.

[0038] In the overlapping laser-welded joint 1 illustrated in Figure 4, the effective angles θ1 and θ2 of the second bead 122 are 90°, and the first bead 121 and the second bead 122 are formed parallel to each other. In this case, welding cracks C occur in the laser-welded joint 12. According to the inventors' experiments, welding cracks C could be sufficiently prevented by setting the effective angles θ1 and θ2 of the second bead 122 to 40° or less. Therefore, in the overlapping laser-welded joint 1 according to this embodiment, the effective angles θ1 and θ2 of the second bead 122 are both 40° or less. Preferably, θ1 and θ2 are 35° or less, 30° or less, or 15° or less. θ1 and θ2 may also be 0°.

[0039] On the other hand, in the overlapping laser-welded joint 1 illustrated in Figure 1, the second bead 122 is formed in a straight line and intersects the first bead 121 at a right angle. In this case, when the laser-welded section 12 has a T-shape, both θ1 and θ2 are 0°, and therefore, the figures for θ1 and θ2 are omitted in Figure 1.

[0040] In the overlapping laser-welded joint 1 illustrated in Figure 2, the second bead 122 is formed in a straight line and intersects the first bead 121 at an angle. In this way, even when the laser-welded section 12 has an "I" shape or an inverted "I" shape, if both θ1 and θ2 are between 0° and 40°, the second bead 122 has the effect of preventing welding cracks C.

[0041] In the overlapping laser-welded joint 1 illustrated in Figure 3, the second bead 122 is formed in a bent shape and intersects the first bead 121 symmetrically in the plane of the paper. Thus, even when the laser-welded section 12 has a Y-shape, if both θ1 and θ2 are between 0° and 40°, the second bead 122 has the effect of preventing welding cracks C.

[0042] In all of the overlapping laser-welded joints 1 illustrated in Figures 1 to 3, θ1 and θ2 have equal values. However, naturally, θ1 and θ2 may be different as long as they are within the range of 40° or less. Also, in all of the overlapping laser-welded joints 1 illustrated in Figures 1 to 3, the second bead 122 extends linearly from the first bead 121. However, naturally, the second bead 122 may be curved. In this case, as in the overlapping laser-welded joint 1 illustrated in Figure 5, the effective angles θ1 and θ2 of the second bead are defined as the angle between the line connecting the intersection point Q of the central axis 121X of the first bead and the central axis 122X of the second bead, and a point R 1.0 mm away from the intersection point along the central axis 122X of the second bead, and the imaginary line VL. However, if the first bead 121 is curved, when extending the central axis 121X of the first bead from the first bead 121 towards the second bead 122, it is extended as a straight line perpendicular to the imaginary line VL.

[0043] (B Aspect ratio of the second bead 122) The aspect ratio of the second bead 122 is defined as the ratio L2 / W2 of the length L2 of the second bead 122 along the central axis 122X to the width W2 of the second bead 122 relative to the central axis 122X. Even if the second bead 122 is not straight, as shown in Figure 5, the length L2 is defined as the length of the second bead 122 along the central axis 122X. L2 is considered to roughly coincide with the length of the weld line of the second bead 122. Furthermore, if the width W2 of the second bead 122 is not uniform, the maximum value of the width of the second bead 122 is considered to be the width W2 of the second bead 122. Note that the boundary between the second bead 122 and the metal plate 11, and the boundary between the second bead 122 and the first bead 121 can be easily identified by visual inspection. Therefore, L2 and W2 can be measured by visually identifying the outer edge of the second bead 122.

[0044] If the aspect ratio of the second bead 122 is too small, the second bead 122 cannot effectively prevent welding cracks C. For example, if the second bead 122 is approximately circular and has an aspect ratio of 1, it cannot change the long axis direction of the crater 120. According to the inventors' experimental results, if the aspect ratio of the second bead 122 is 2.0 or higher, the long axis direction of the crater 120 can be sufficiently changed. In this case, the second bead 122 can prevent welding cracks C. The aspect ratio of the second bead 122 may be 2.5 or higher, 3.0 or higher, or 5.0 or higher.

[0045] (C Number and location of craters 120 formed in the second bead 122) A crater 120 is a depression formed at the end of the laser weld 12. In laser welding, the molten metal flows in the direction opposite to the direction of laser LZ. Therefore, at the end of the laser weld 12, i.e., the final solidification area, there is insufficient molten metal, and a depression is formed. As shown in Figure 8A, tensile stress is applied in the width direction of the first bead 121, that is, in the width direction of the crater 120, causing a weld crack C to occur.

[0046] In the overlapping laser-welded joint 1 according to this embodiment, it is necessary to have only one crater 120. For example, if both the crater 120 formed at the end of the first bead 121 and the crater 120 formed in the second bead 122 are included in the laser-welded joint 12, there will be two potential points of risk that could be the starting point of a crack, increasing the likelihood of a weld crack C occurring.

[0047] Furthermore, the crater 120 must be located in the second bead 122, and the deepest part of the crater 120 must be located between the extensions 121E of both edges of the first bead 121. If the deepest part of the crater 120 is located outside the extensions 121E of both edges of the first bead and forms at the end of the second bead 122, a weld crack will occur along the second bead 122. By controlling the position of the crater 120 as described above, the second bead 122 can be prevented from developing a weld crack C.

[0048] The overlapping laser-welded joint 1 according to this embodiment can suppress welding cracks by possessing the above-described features. The inventors hypothesize the mechanism as follows. Figure 8A is a conceptual diagram of the tensile stress applied to the first bead 121 and the welding crack C when the second bead 122 is absent. A crater 120 is formed at the end of the first bead 121. In addition, tensile stress is applied to the end of the first bead 121 as the base material heated by laser welding shrinks. This tensile stress is applied largely in the width direction of the first bead. The arrows shown in Figure 8A illustrate the tensile stress applied to the end. When the end of the bead is torn by this tensile stress, a welding crack C occurs. In particular, welding crack C is likely to occur when tensile stress is applied perpendicular to the longitudinal direction of the crater 120. Furthermore, the welding crack C occurs along the extension direction of the first bead 121 and propagates and grows along the first bead 121.

[0049] On the other hand, Figure 8B is a conceptual diagram of tensile stress when a second bead 122 is formed at the end of the first bead 121. When the second bead 122 is formed with the above-mentioned characteristic points, welding cracks C can be suppressed. This is presumed to be because the longitudinal direction of the crater 120 is changed by the second bead 122. Tensile stress is largely applied in the width direction of the first bead. By changing the longitudinal direction of the crater 120, the tensile stress applied in the direction perpendicular to the longitudinal direction of the crater can be reduced, and welding cracks C can be suppressed. However, if the second bead 122 extends only to one side with respect to the central axis 121X of the first bead 121, the above effect cannot be fully obtained.

[0050] The overlapping laser-welded joint 1 according to this embodiment can suppress welding cracks by possessing the above-described features. On the other hand, welding cracks can be suppressed even more effectively by optimizing the shape of the first bead 121 and the second bead 122 in plan view, as well as the penetration depth. Preferred embodiments of the overlapping laser-welded joint 1 according to this embodiment will be described below.

[0051] (Length L2 of the second bead 122) It is preferable that, on at least one side of the overlapping laser-welded joint 1, the length L2 of the second bead 122 along the central axis 122X is less than the length L1 of the first bead 121 along the central axis 121X. Furthermore, it is even more preferable that, on at least one side of the overlapping laser-welded joint 1, the length L2 of the second bead 122 along the central axis 122X is 12.0 mm or less, 8.0 mm or less, or 5.0 mm or less. This further suppresses the formation of a crater 120 at the end of the second bead 122 and more effectively suppresses welding cracks C of the second bead 122.

[0052] (Penetration depth D1 of the first bead 121, and penetration depth D2 of the second bead 122) The second bead 122 is provided to suppress welding cracks and is not intended for joining multiple metal plates 11. Therefore, the penetration depth D2 of the second bead 122 may be small. For example, as shown in Figures 7A and 7B, which are cross-sectional views of the overlapping portion of the first bead 121 and the second bead 122, the penetration depth D2 of the second bead 122 may be less than the penetration depth D1 of the first bead 121. In general, the penetration depth of a laser welding bead is proportional to the amount of heat input during laser welding. Also, the depth of the crater 120 of the laser-welded area 12 is proportional to the amount of heat input to the laser-welded area 12. The smaller the penetration depth of the bead and the smaller the heat input, the smaller the depth of the crater 120. Therefore, by reducing the laser power ratio, which is the ratio of the output during the second laser welding that forms the second bead 122 to the output during the first laser welding that forms the first bead 121, the penetration depth D2 of the second bead 122 can be made smaller than the penetration depth D1 of the first bead 121, and the depth of the crater 120 formed together with the second bead 122 can be reduced, thereby more effectively preventing weld cracks.

[0053] As shown in Figures 7A and 7B, the second bead 122 of the laser weld 12 may be present on only one side of the overlapping laser welded joint 1. As mentioned above, the smaller the penetration depth D2 of the second bead 122, the smaller the depth of the crater 120 formed in the final solidification portion of the second bead 122 can be. Furthermore, as shown in Figure 7B, the penetration depth D2 of the second bead 122 may be made even smaller so that it is less than the thickness of the metal plate 11 on which the second bead 122 is provided. In other words, there may be zero metal plates 11 penetrated by the second bead 122. As mentioned above, the second bead 122 does not need to join multiple metal plates 11.

[0054] Since the crater 120 can be made smaller, it is preferable that the penetration depth D1 of the first bead 121 be small. The effect of the crater 120 can also be further mitigated by suppressing the heat input when forming the first bead 121. Therefore, as shown in Figure 7B, the entire laser weld 12, including the first bead 121 and the second bead 122, may exist on only one side of the overlapping laser welded joint 1.

[0055] The penetration depth D1 can be measured, similar to the gap G, by etching a cross-section cut along a virtual line VL that passes through the intersection point P of the central axis 121X of the first bead 121 and the outer edge of the second bead 122, and is perpendicular to the central axis 121X of the first bead, and observing it with an optical microscope. The penetration depth D2 can be measured similarly by etching a cross-section cut along the central axis 121X of the first bead 121 and observing it with an optical microscope.

[0056] A second bead 122 is provided at the end of the first bead 121, but the shape of the starting end 121S of the first bead 121 is not particularly limited. Generally, no crater 120 is formed at the starting end of the laser welding bead. Therefore, the starting end of the laser welding bead does not become the starting point of a weld crack. To further improve manufacturing efficiency, it is preferable to leave the starting end 121S of the first bead 121 in the as-welded state and not provide an additional bead.

[0057] Next, a structural member for an automobile body according to another aspect of the present invention will be described. The structural member for an automobile body according to another aspect of the present invention includes a lap laser welding 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 welding 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 is highly productive and suppresses the occurrence of weld cracks.

[0058] Next, a method for manufacturing an overlapping laser-welded joint according to another aspect of the present invention will be described. This manufacturing method allows for the suitability of manufacturing the overlapping laser-welded joint 1 according to the present 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 an overlapping laser-welded joint 1 according to the present embodiment if it satisfies the above requirements.

[0059] The method for manufacturing a lap laser-welded joint according to this embodiment is as follows: (S1) A step of performing first laser welding on multiple overlapping metal plates 11 to form a first bead 121 that joins the multiple metal plates 11, (S2) A step of performing a second laser welding on the metal plate 11 on at least one surface of the overlapping laser-welded joint to form a second bead 122. The lap laser welded joint 1 has a laser welded section 12 having a first bead 121 and a second bead 122. Here, the ratio G / T of the total thickness G of the gaps between the multiple metal plates 11 to the total thickness T of the multiple metal plates 11 is set to 0 to 15%. Furthermore, on at least one side of the lap laser welded joint 1, the second bead 122 is provided at the end of the first bead 121, and the second bead 122 extends on both sides with respect to the central axis 121X perpendicular to the width direction of the first bead 121, and the smaller angle between the imaginary line VL passing through the intersection P of the central axis 121X of the first bead 121 and the central axis 122X perpendicular to the width direction of the second bead 122 is the second The effective angles θ1 and θ2 of the second bead 122 are set to 40 degrees or less on each side of the first bead 121, the ratio L2 / W2 of the length L2 along the central axis 122X of the second bead 122 to the width W2 of the second bead 122 with respect to the central axis 122X is set to 2.0 or more, only one crater 120 is present in the laser welded area 12, the crater 120 is located in the second bead 122, and the deepest part of the crater 120 is located between the extensions 121E of both edges of the first bead 121.

[0060] First, a first laser welding is performed on multiple metal plates 11 to form a first bead 121 that joins the multiple metal plates 11. Then, a second laser welding is performed on the end of the first bead 121 to form a second bead. At this time, the ratio G / T of the total thickness G of the gap between the multiple metal plates 11 to the total thickness T of the multiple metal plates 11, the effective angle of the second bead 122, the aspect ratio of the second bead 122, and the number and position of the craters 120 formed in the second bead 122 are kept within the predetermined range described above. The reason for this is as explained with respect to the overlapping laser-welded joint 1 according to this embodiment.

[0061] The first and second laser welding may be performed continuously as shown in Figure 9A, or intermittently as shown in Figure 9B. Specifically, in the method shown in Figure 9A, laser LZ irradiation is performed continuously from the start of the first laser welding to the end of the second laser welding, without any stop time. On the other hand, in the method shown in Figure 9B, laser LZ irradiation is stopped when the formation of the first bead 121 is completed. Then, the tip of the laser irradiation axis LX is moved to a target position corresponding to the upper or lower end of the second bead 122, and then the second laser welding is started. The overlapping laser-welded joint 1 according to this embodiment can be manufactured by either method. When manufacturing the joint using the method shown in Figure 9B, a stop time may be provided between the end of the first laser welding and the start of the second laser welding, but from the viewpoint of work efficiency, the stop time may be 60 seconds or less, 50 seconds or less, 30 seconds or less, or 10 seconds or less.

[0062] The method for shaping the laser-welded area 12 within the predetermined range described above is not particularly limited. For example, the effective angle of the second bead 122 can be easily changed by controlling the laser LZ irradiation position. Furthermore, the aspect ratio of the second bead 122 can be controlled, for example, through the amount of heat input and welding speed in laser welding.

[0063] The means for having only one crater 120 in the laser-welded area 12, having the crater 120 in the second bead 122, and having the deepest part of the crater 120 located between the extensions 121E of both edges of the first bead 121 are not particularly limited. For example, the number and position of the craters 120 can be controlled by controlling the termination position of the second laser welding for forming the second bead 122. Specifically, it is preferable to terminate the second laser welding at the point where the first bead 121 and the second bead 122 overlap. For example, in the example of second laser welding shown in Figures 9A and 9B, after forming a second bead 122 of a predetermined shape, the laser LZ is returned to the point where the first bead 121 and the second bead 122 overlap. This terminates the second laser welding at the point where the first bead 121 and the second bead 122 overlap.

[0064] By ending the second laser welding at the point where the first bead 121 and the second bead 122 overlap, the final solidified portion of the second bead 122 becomes the point where the first bead 121 and the second bead 122 overlap. As a result, a crater 120 is formed at the point where the end of the first bead 121 and the second bead 122 overlap, and the deepest part of the crater 120 lies between the extensions 121E of both edges of the first bead 121. Furthermore, even if a crater 120 is formed on the first bead 121, by ending the second laser welding at the point where the first bead 121 and the second bead 122 overlap, the crater 120 of the first bead 121 can be overwritten by the crater 120 of the second bead 122, resulting in a single crater 120.

[0065] On the other hand, the second laser welding may be terminated at a location other than where the first bead 121 and the second bead 122 overlap. Since a large amount of heat is stored in the region where the first bead 121 and the second bead 122 overlap, cooling and solidification are delayed. Therefore, if the length L2 of the second bead 122 is small, specifically if the length L2 of the second bead 122 is 5 mm or less, the region where the first bead 121 and the second bead 122 overlap becomes the final solidification area, and a crater 120 is formed there. However, this is not the case if the length L2 of the second bead 122 is large, specifically if the length L2 of the second bead 122 is greater than 5 mm, in which case the crater 120 may be formed at the end of the second bead 122, or in both that region and the region where the first bead 121 and the second bead 122 overlap.

[0066] Furthermore, the preferred embodiment described with respect to the overlapping laser-welded joint 1 can also be applied to the manufacturing method of the overlapping laser-welded joint 1.

[0067] Up to this point, we have described overlapping laser-welded joints, structural members for automobile bodies, and methods for manufacturing overlapping laser-welded joints 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.

[0068] <Layer laser welded joint (layer fillet joint) 1A> For example, the lap laser welded joint according to this embodiment may be a lap fillet joint. In the lap fillet joint 1A, for example as shown in Figure 10, the end face of a first metal plate 11A, which is the outermost metal plate 11 among a plurality of metal plates 11, and the surface of a second metal plate 11B which is superimposed on the first metal plate 11A are joined by a first bead 121A.

[0069] The lap fillet joint 1A is basically the same as the lap laser welded joint 1 described above, except that the first bead 121A joins the end face of the first metal plate 11A and the surface of the second metal plate 11B. Therefore, the lap fillet joint 1A is a lap fillet joint comprising a plurality of overlapping metal plates 11 and a laser welded portion that joins the plurality of metal plates 11, wherein the ratio G / T of the total thickness G of the gap between the plurality of metal plates 11 to the total thickness T of the plurality of metal plates 11 is 0 to 15%, and on at least one side of the lap fillet joint 1A, the laser welded portion has a first bead 121A that joins the plurality of metal plates 11 and a second bead 122A provided at the end of the first bead 121A, the second bead 122A extends on both sides with respect to a central axis 121AX perpendicular to the width direction of the first bead 121A, and the central axis 121AX of the first bead 121A and the outer edge of the second bead 122A The effective angle of the second bead, which is the smaller of the angles between the imaginary line VL passing through the intersection point P and perpendicular to the central axis 121AX of the first bead 121A and the central axis 122AX perpendicular to the width direction of the second bead 122A, is 40 degrees or less on each side of the first bead 121A, the ratio L2 / W2 of the length L2 of the second bead 122A along the central axis 122AX to the width W2 of the second bead 122A with respect to the central axis 122AX is 2.0 or more, there is only one crater 120A in the laser welded area, the crater 120A is located in the second bead 122A, and the deepest part of the crater 120A is located between the extensions 121AE of both edges of the first bead 121A.

[0070] (First bead 121A) The first bead 121A is a fillet laser weld that extends linearly, joining the end face of the first metal plate 11A and the surface of the second metal plate 11B among the multiple metal plates 11. The shape of the first bead 121A is not particularly limited as long as it is linear. For example, the first bead 121A is shaped along the end face of the first metal plate 11A. The first bead 121A joins at least the end face of the first metal plate 11A and the surface of the second metal plate 11B among the multiple metal plates 11, but it may extend in the thickness direction so as to span all the metal plates 11 in order to join the multiple metal plates 11. Furthermore, the first bead 121A does not need to penetrate all the metal plates 11, and the first bead 121A may be formed only on one side of the overlapping laser welded joint.

[0071] Even in an overlapping fillet joint 1A, the effect of preventing welding cracks by the second bead 122A can be obtained by controlling (A) the effective angle of the second bead 122A, (B) the aspect ratio of the second bead 122, and (C) the number and position of the craters 120 formed in the second bead 122, at least on one side of the overlapping fillet joint 1A.

[0072] (G gap between multiple metal plates 11) The size of the gap between the metal plates 11 in the overlapping fillet joint 1A is basically measured in the same way as described above. However, the size of the gap between the first metal plate 11A and the second metal plate 11B is measured at the end of the first bead 121A on the side where the first metal plate 11A and the second metal plate 11B overlap in the cross-section, as shown in Figure 11.

[0073] The method for manufacturing the lap fillet joint 1A differs from the method for manufacturing the laser-welded joint 1 in that, in the step of forming the first bead, at least the end face of the first metal plate 11A, which is placed on the outermost surface of the plurality of metal plates 11, and the surface of the second metal plate 11B, which is superimposed on the first metal plate 11A, are joined together. However, the other steps are the same as those for manufacturing the laser-welded joint 1. Therefore, the method for manufacturing the lap fillet joint 1A comprises the steps of: first laser welding the end face of the first metal plate 11A, which is placed on the outermost surface of the plurality of superimposed metal plates 11, and the surface of the second metal plate 11B, which is superimposed on the first metal plate 11A, to form a first bead that joins the plurality of metal plates 10; and second laser welding the first metal plate 11A and the second metal plate 11B to form a second bead 122A. The lap fillet joint 1A has a laser welded section having a first bead 121A and a second bead 122A. The ratio G / T of the total thickness G of the gaps between the multiple metal plates 11 to the total thickness T of the multiple metal plates 11 is set to 0 to 15%. On at least one side of the lap fillet joint 1A, the second bead 122A is provided at the end of the first bead 121A, and the second bead 122A extends on both sides with respect to the central axis 121AX perpendicular to the width direction of the first bead 121A. The smaller of the angles between the imaginary line VL passing through the intersection point P of the central axis 121AX of the first bead 121A and the central axis 122AX perpendicular to the central axis 121AX of the first bead 121A and the central axis 122AX perpendicular to the width direction of the second bead 122A is the second The effective angle of the second bead 122A is set to 40 degrees or less on each side of the first bead 121A, the ratio L2 / W2 of the length L2 along the central axis 122AX of the second bead 122A to the width W2 of the second bead 122A with respect to the central axis 122AX is set to 2.0 or more, only one crater 120A exists in the laser welded area, the crater 120A is located on the second bead 122A, and the deepest part of the crater 120A is located between the extensions 121AE of both edges of the first bead 121A. [Examples]

[0074] 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.

[0075] [Example 1] Two steel plates having the tensile strength, thickness, and 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 welding was performed to form a second bead at the end of the first bead. If the length L2 along the central axis of the second bead exceeded 5.0 mm, after forming the second bead, the laser was returned to the point where the first and second beads overlapped, and the second laser welding was terminated at the overlapping point. The presence or absence of weld cracks in the various overlapping laser-welded joints obtained in this way was checked. The shape of the laser-welded joint and the evaluation results are shown in Table 2. In Table 2, values ​​outside the scope of the invention are underlined.

[0076] The size of the gap between the steel plates, as shown in Table 2, was controlled by inserting spacers between the steel plates. The "plate gap G" shown in Table 2 is the value measured in a cross-section cut along a virtual line VL that passes through the intersection point P of the central axis 121X of the first bead 121 and the outer edge of the second bead 122, and is perpendicular to the central axis 121X of the first bead. The measurement method was the one described above.

[0077] The "weld shape" described in Table 2 refers to the shape of the weld when the overlapping laser welded joint is viewed from above. "Straight line" refers to a weld shape in which the first bead and the second bead extend in the same direction, as shown in Figure 4, for example. "T-shape" refers to a T-shape as shown in Figures 1, 12, and 13, for example. Figure 12 is a photograph of overlapping laser welded joint No. 5, and Figure 13 is a photograph of overlapping laser welded joint No. 17. "Y-shape" refers to a shape in which the second bead 122 is inclined in the opposite direction to the starting end of the first bead 121, as shown in Figures 3 and 14, for example. Figure 14 is a photograph of overlapping laser welded joint No. 26. "I-shape" refers to an I-shape as shown in Figure 2, for example. "Arrow" refers to a shape in which the second bead 122 is inclined toward the starting end of the first bead 121.

[0078] The "laser power ratio" listed in Table 2 refers to the ratio of the laser power output during the second laser welding process to the laser power output during the first laser welding process.

[0079] The "degree of penetration" in Table 2 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] Cracking was evaluated by preparing three test specimens under identical conditions and determining the number of weld cracks that occurred in these specimens.

[0081] Table 2 shows the number of test specimens that did not crack as the numerator and the number of test specimens "3" as the denominator. For cases where welding cracks occurred in all test specimens, the welding crack resistance was evaluated as "×", for cases where no welding cracks occurred in any test specimens, the welding crack resistance was evaluated as "〇", and for cases where welding cracks occurred in some test specimens, the welding crack resistance was evaluated as "△". Cases evaluated as "〇" or "△" were evaluated as having excellent welding crack resistance.

[0082] [Table 1] [Table 2]

[0083] Although a second bead was formed in comparative examples No. 1 and No. 2, the shape of the weld was linear, and the effective angle of the second bead was inappropriate. Therefore, welding cracks could not be suppressed in comparative examples No. 1 and No. 2.

[0084] In comparative examples No. 3 to 6, although a second bead was formed, the aspect ratio L2 / W2 of the second bead was insufficient. Therefore, welding cracks could not be suppressed in comparative examples No. 3 to 6.

[0085] Comparative examples No. 11, 12, 15, 16, 21, and 22 had a plate gap G of 0.6 mm, and the G / T ratio for these comparative examples was 18.8%. Therefore, welding cracks could not be suppressed in comparative examples No. 11, 12, 15, 16, 21, and 22.

[0086] In comparative examples No. 23, 24, 27, 28, 31, and 32, the effective angle of the second bead was inappropriate. As a result, the crack suppression effect of the second bead was not obtained in these comparative examples, and welding cracks could not be suppressed.

[0087] On the other hand, the invention example in which L2 / W2, angle θ1, angle θ2, and G / T were all appropriate was evaluated as "○" or "△", indicating excellent resistance to weld cracking.

[0088] [Example 2] Two steel plates A to C, having the tensile strength, thickness, and composition described in Table 3, were stacked together. The end face of the first metal plate, placed on the outermost layer, and the surface of the second metal plate stacked on top of the first metal plate were laser-welded under various conditions to form a first bead. Chemical components other than those shown in Table 3 are Fe and impurities. In some cases, a second laser welding was performed to form a second bead at the end of the first bead. If the length L2 along the central axis of the second bead exceeded 5.0 mm, after forming the second bead, the laser was returned to the point where the first and second beads overlapped, and the second laser welding was terminated at the overlapping point. The presence or absence of welding cracks in the various overlapping laser-welded joints obtained in this way was checked. The shape of the laser-welded joint and the evaluation results are shown in Table 4. In Table 4, values ​​outside the scope of the invention are underlined.

[0089] The size of the gap between the steel plates, as shown in Table 4, was controlled by inserting spacers between the steel plates. The "plate gap G" shown in Table 4 is the value measured in a cross-section cut along a virtual line passing through the intersection point P of the central axis of the first bead and the outer edge of the second bead, and perpendicular to the central axis of the first bead. The measurement method was the one described above.

[0090] In the "Weld Shape" section of Table 4, "T-shape" refers to a T-shape like the one shown in Figure 15. Figure 15 is a photograph of the lap fillet weld joint No. 44, where the upper plate corresponds to the first metal plate and the lower plate corresponds to the second metal plate. In addition, the "straight line," "Y-shape," "I-shape," and "arrow" in Table 4 are the same shapes as in Example 1, formed across the first and second metal plates.

[0091] The "degree of penetration" in Table 4 was described according to the same criteria as in Example 1. Furthermore, crack evaluation was performed according to the same criteria as in Example 1.

[0092] [Table 3]

[0093] [Table 4]

[0094] In comparative example No. 35, although a second bead was formed, the shape of the weld was linear, and the effective angle of the second bead was inappropriate. Furthermore, the aspect ratio L2 / W2 of the second bead was insufficient. Therefore, welding cracking could not be suppressed in comparative example No. 35.

[0095] In comparative examples No. 36 and No. 37, although a second bead was formed, the aspect ratio L2 / W2 of the second bead was insufficient. Therefore, welding cracks could not be suppressed in comparative examples No. 36 and No. 37.

[0096] Comparative examples No. 43, 45, and 48 had a plate gap G of 0.6 mm, and the G / T ratio for these comparative examples was 18.8%. Therefore, welding cracks could not be suppressed in comparative examples No. 43, 45, and 48.

[0097] In comparative examples No. 49, 51, and 53, the effective angle of the second bead was inappropriate. As a result, the crack suppression effect of the second bead was not obtained in these comparative examples, and welding cracks could not be suppressed.

[0098] On the other hand, the invention example in which L2 / W2, angle θ1, angle θ2, and G / T were all appropriate was evaluated as "○" or "△" for any of steel grades A to C, and demonstrated excellent resistance to weld cracking. [Explanation of Symbols]

[0099] 1. Overlap laser-welded joint 1A Overlap laser welded joint (overlap fillet joint) 11, 11A, 11B metal plate 12, 12A laser welded section Craters 120 and 120A 121, 121A First bead 121X, 121AX: Central axis of the first bead 121S, 121AS: Starting point of the first bead 121E, 121AE: Extensions of both edges of the first bead. 122, 122A Second bead 122X, 122AX: Central axis of the second bead P: The intersection of the central axis of the first bead and the outer edge of the second bead. A virtual line passing through intersection point P of the VL and perpendicular to the central axis of the first bead. θ1, θ2 Effective angle of the second bead L1 Length along the central axis of the first bead L2 Length along the central axis of the second bead W1: Width relative to the central axis of the first bead W2: Width relative to the central axis of the second bead LZ Laser LX laser irradiation axis M Laser irradiation means C. Weld crack

Claims

1. Multiple overlapping metal plates, A laser welding section for joining multiple metal plates, A lap laser welded joint comprising, The ratio G / T of the total thickness G of the gaps between the multiple metal plates to the total thickness T of the multiple metal plates is between 0 and 15%. On at least one side of the aforementioned overlapping laser-welded joint, The laser-welded portion has a first bead that joins a plurality of metal plates, and a second bead provided at the end of the first bead. The second bead extends on both sides with respect to a central axis perpendicular to the width direction of the first bead, and the effective angle of the second bead, which is the smaller of the angle between a virtual line passing through the intersection of the central axis of the first bead and the outer edge of the second bead and perpendicular to the central axis of the first bead, and the central axis perpendicular to the width direction of the second bead, is 40 degrees or less on each of the sides of the first bead. The ratio L2 / W2 of the length L2 of the second bead along the central axis to the width W2 of the second bead with respect to the central axis is 2.0 or greater. The laser-welded area contains only one crater. The crater is located in the second bead. The deepest part of the crater is located between the extensions of the two edges of the first bead. Overlap laser welded joint.

2. On at least one side of the overlapping laser-welded joint, The length L2 of the second bead along the central axis is less than the length L1 of the first bead along the central axis. The overlapping laser-welded joint according to feature 1.

3. On at least one side of the overlapping laser-welded joint, The length L2 of the second bead along the central axis is 12.0 mm or less. The overlapping laser-welded joint according to claim 1 or 2.

4. The overlapping laser welded joint according to claim 1 or 2, characterized in that the penetration depth D2 of the second bead is less than the penetration depth D1 of the first bead.

5. The overlapping laser welded joint according to claim 1 or 2, characterized in that no additional bead is provided at the starting end of the first bead.

6. The overlapping laser-welded joint according to claim 1 or 2, characterized in that the laser-welded portion exists only on one side of the overlapping laser-welded joint.

7. The multiple metal plates are multiple steel plates, The chemical composition of one or more of the aforementioned 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 The overlapping laser welding joint according to claim 1 or 2, characterized by containing the following:

8. The multiple metal plates are multiple steel plates, The overlapping laser-welded joint according to claim 1 or 2, characterized in that one or more of the multiple steel plates have a tensile strength of 980 MPa or more.

9. The lap laser welded joint according to claim 1 or 2, wherein the lap laser welded joint is a lap fillet joint.

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

11. A step of performing a first laser welding on multiple overlapping metal plates to form a first bead that joins the multiple metal plates, The process involves performing a second laser welding on the metal plate on at least one surface of the overlapping laser-welded joint to form a second bead. A method for manufacturing an overlapping laser-welded joint comprising: The aforementioned overlapping laser-welded joint has a laser-welded section having the first bead and the second bead, The ratio G / T of the total thickness G of the gaps between the multiple metal plates to the total thickness T of the multiple metal plates is set to 0 to 15%. On at least one side of the aforementioned overlapping laser-welded joint, The second bead is provided at the end of the first bead. The second bead extends on both sides of the central axis perpendicular to the width direction of the first bead, and the effective angle of the second bead, which is the smaller of the angle between a virtual line passing through the intersection of the central axis of the first bead and the outer edge of the second bead and perpendicular to the central axis of the first bead, and the central axis perpendicular to the width direction of the second bead, is set to 40 degrees or less on each of the sides of the first bead. The ratio L2 / W2 of the length L2 of the second bead along the central axis to the width W2 of the second bead with respect to the central axis is set to 2.0 or more. The laser-welded area contains only one crater. The crater is located in the second bead. The deepest part of the crater is located between the extensions of the two edges of the first bead. A method for manufacturing overlapping laser-welded joints.

12. The method for manufacturing an overlapping laser-welded joint according to claim 11, characterized in that laser irradiation is performed continuously from the start of the first laser welding to the end of the second laser welding.

13. The method for manufacturing an overlapping laser-welded joint according to claim 11 or 12, characterized in that the second laser welding is terminated at the point where the end of the first bead and the second bead overlap.

14. The method for manufacturing an overlapping laser-welded joint according to claim 11 or 12, wherein the overlapping laser-welded joint is an overlapping fillet joint.

Images