Lap laser spot welded joint and method for manufacturing the same, and structural member for automobile body

Abstract

Provided are a lap laser spot welded joint and a method for manufacturing the same, and a structural member for an automobile body having the welded joint, wherein, when a plurality of steel sheets are overlapped, and a laser beam is intermittently irradiated to one side surface of the overlapped steel sheets to form a welded portion including joining portions arranged in a columnar shape to manufacture a lap laser spot welded joint, the shape of the joining portions is made into large oblong joining portions by at least causing the laser beam to scan in a spiral shape from the outside to the inside of an oblong shape while self-rotating in an oblong shape drawn by combining a semicircle and a straight line, and by controlling the welding conditions so that the minor axis width D1, the major axis width D2, the ratio (D2 / D1) of the major axis width D2 to the minor axis width D1, and the minimum thickness u of the final solidified portion of the oblong joining portions satisfy prescribed conditions, cracks are not generated at the welded terminal portions of the joining portions, and the peel strength is also excellent.

Description

Technical Field

[0001] This invention relates to an lap laser spot welding head, a method for manufacturing the same, and a structural component for an automobile body having the aforementioned lap laser spot welding head. Background Technology

[0002] In the welding of structural components (strength components) of automobile bodies with flanges, resistance spot welding is commonly used. However, resistance spot welding has several problems, including long welding time, reduced heat generation due to current shunting preventing a reduction in the welding spacing, and the need for a certain amount of space to set up the welding torch. To address these issues, in recent years, lap laser spot welding technology has been researched and promoted as a replacement for traditional resistance spot welding. Here, lap laser spot welding refers to a welding method that involves irradiating a laser beam onto one side of multiple overlapping steel plates to melt and join the plates.

[0003] Traditionally, for lap laser spot welding, a laser beam is intermittently irradiated onto the surfaces of multiple overlapping steel plates, causing the irradiated areas to melt and solidify. This creates a continuous series of weld joints, either straight or circular, thus joining the plates. However, in the case of straight joints, lap laser welding presents a problem where cracks easily form at the final solidification point near the weld termination. Similarly, in the case of circular joints, cracks easily form at the final solidification point in the center of the joint. Once cracks form, they propagate along the entire length of the joint, leading to a decrease in not only static strengths such as shear strength and peel strength, but also a significant reduction in fatigue strength. In recent years, high-tensile steel plates have been widely used in automotive body components, particularly structural components (strength components) serving as the skeleton, to improve the strength and rigidity of the body. Therefore, the reduction in static and fatigue strength of weld joints caused by cracks at the joints has become a serious problem.

[0004] Therefore, various methods have been studied to prevent cracks at the weld termination of the joint during laser beam welding of overlapping steel plates. For example, Patent Document 1 discloses preventing weld cracks by making the lower side of the lap weld protrude and setting the welding start position to be separate from the flange end. Patent Document 2 discloses a technique for preventing weld cracks by irradiating the lap surface with a laser from an oblique direction towards the end of the lap surface. Patent Documents 3 and 4 disclose techniques for preventing weld cracks by reheating or welding the area around the already welded portion or the post-weld portion. Patent Document 5 discloses a technique for preventing weld cracks by welding the lap surface into an elliptical shape. Furthermore, Patent Document 6 discloses a technique for preventing weld cracks by optimizing the steel plate composition and optimizing the ratio of weld width to weld thickness.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2007-229740

[0008] Patent Document 2: Japanese Patent Application Publication No. 2008-296236

[0009] Patent Document 3: Japanese Patent Application Publication No. 2012-240083

[0010] Patent Document 4: Japanese Patent Application Publication No. 2012-240086

[0011] Patent Document 5: Japanese Patent Application Publication No. 2017-113781

[0012] Patent Document 6: Japanese Patent Application Publication No. 2018-001197 Summary of the Invention

[0013] The problem that the invention aims to solve

[0014] However, in the method described in Patent Document 1, the protruding steel plate on the lower side of the lap weld becomes redundant, limiting the design of the components. Furthermore, in the method described in Patent Document 2, the laser is irradiated from an oblique direction, making it difficult to form a molten portion on the lap surface when the overlapping plates are separated, resulting in insufficient weld depth and difficulty in ensuring adequate strength. Additionally, in the methods described in Patent Documents 3 and 4, reheating or welding is required around the already welded portion and the post-weld portion, increasing the welding time. Furthermore, the method described in Patent Document 5 limits the welded portion to a circular or near-circular elliptical shape, making it difficult to ensure sufficient weld strength. Moreover, in the method described in Patent Document 6, stress tends to concentrate at the weld termination, making it difficult to prevent cracks from forming at the weld termination of short, straight joints.

[0015] The present invention is made in view of the above-mentioned problems existing in the prior art, and its object is to provide an lap weld joint having a weld portion formed in a row by intermittently irradiating a laser beam to form a joint (weld point), an lap laser spot weld joint that does not produce cracks in the final solidification of the joint and has excellent peel strength of the weld portion, and a method for manufacturing the same, and to provide a structural component for automobile body having the lap laser spot weld joint.

[0016] Methods for solving problems

[0017] In order to solve the above-mentioned problems, the inventors of this application conducted repeated and in-depth research focusing on the shape and size of the joints (weld points) constituting the welded part formed by laser welding. As a result, it was found that in order to prevent cracking in the final solidification part of the joint, it is effective to make the joint a larger oblong shape than the conventional linear, circular, or elliptical shapes, and to control the various dimensions of the oblong joint within an appropriate range, and thus the present invention was developed.

[0018] Based on the above insights, the present invention is an lap laser spot welding head having a welded portion formed by lapping multiple steel plates, characterized in that the total gap G between the steel plates constituting the welded portion is in the range of 0 to 15% of the total thickness T of the steel plates constituting the welded portion, the welded portion includes discontinuously arranged elongated oval joints, and the elongated oval joints satisfy all of the following formulas (1) to (5):

[0019] 1.0≤T≤6.0 (1),

[0020] 2.0≤D1≤8.0 (2),

[0021] 6.0≤D2≤15.0 (3),

[0022] 1.1≤D2 / D1≤5.0 (4),

[0023] 0.6≤u / T≤1.0 (5),

[0024] Where T is the total thickness (mm) of the steel plate constituting the welded part.

[0025] D1 is the minor axis width (mm) of the oblong joint.

[0026] D2 is the width of the major axis of the oblong joint (mm).

[0027] u represents the minimum thickness (mm) of the final solidified portion of the oblong joint.

[0028] The lap laser spot welding head of the present invention is characterized in that at least one of the steel plates contains C: 0.07-0.4% by mass, Si: 0.2-3.5% by mass, Mn: 1.8-5.5% by mass, P+S: less than 0.03% by mass, Al: less than 0.08% by mass and N: less than 0.010% by mass, with the balance being Fe and unavoidable impurities.

[0029] Furthermore, the lap laser spot welding joint of the present invention is characterized in that, in addition to the above-mentioned composition, the steel plate also contains at least one component from group A and group B:

[0030] Group A consists of one or two ingredients selected from Ti: 0.0005–0.01 wt% and Nb: 0.005–0.050 wt%.

[0031] Group B consists of one or more ingredients selected from Cr: less than 1.0% by mass, Mo: less than 0.50% by mass, and B: less than 0.10% by mass.

[0032] Furthermore, the lap laser spot welding head of the present invention is characterized in that at least one of the steel plates is a high-tensile steel plate with a tensile strength of 980 MPa or more.

[0033] In addition, the present invention provides a method for manufacturing an lap laser spot welded joint, wherein multiple steel plates are overlapped vertically, and a laser beam is intermittently irradiated onto one side surface of the overlapped steel plates to form a welded joint comprising an elongated oval joint arranged in a row to manufacture an lap laser spot welded joint. The manufacturing method is characterized in that the total gap G between the steel plates constituting the welded joint is in the range of 0 to 15% of the total thickness T of the steel plates constituting the welded joint. With regard to the elongated oval joint, the laser beam is made to rotate in a spiral manner from the outside to the inside of the elongated oval while drawing an elongated oval composed of a combination of a semicircle and a straight line. Furthermore, at least one of the following factors—laser power, focal position, welding speed, spin radius, amount of movement per spin, and beam diameter—is controlled so that the elongated oval joint satisfies all of the following equations (1) to (5):

[0034] 1.0≤T≤6.0 (1),

[0035] 2.0≤D1≤8.0 (2),

[0036] 6.0≤D2≤15.0 (3),

[0037] 1.1≤D2 / D1≤5.0 (4),

[0038] 0.6≤u / T≤1.0 (5),

[0039] Where T is the total thickness (mm) of the steel plate constituting the welded part.

[0040] D1 is the minor axis width (mm) of the oblong joint.

[0041] D2 is the width of the major axis of the oblong joint (mm).

[0042] u represents the minimum thickness (mm) of the final solidified portion of the oblong joint.

[0043] In addition, the present invention is a structural component for an automobile body having any of the aforementioned lap laser spot welding heads.

[0044] The effects of the invention

[0045] According to the present invention, by making the joint of the weld portion of the lap laser spot weld joint, which constitutes a laser beam weld of multiple overlapping steel plates, into a larger elongated oval joint than before, it is possible to manufacture lap laser spot weld joints that not only reliably suppress cracks at the final solidification point but also exhibit excellent peel strength of the weld portion. Furthermore, the lap laser spot weld joint of the present invention can form elongated oval joints with a wide range of major-to-minor-axis ratios, thus increasing the freedom of component design and enabling the development of lighter, more rigid, and stronger components. Therefore, the lap laser spot weld joint of the present invention can be applied to structural components (strength components) that form the skeleton of an automobile body. Attached Figure Description

[0046] Figure 1 This is a perspective view showing an example of a conventional lap laser spot welded joint.

[0047] Figure 2 These are schematic diagrams illustrating the welded portion of a conventional lap laser spot welded joint. (a) is a top view, and (b) is a cross-sectional view (AA) of (a).

[0048] Figure 3 These are schematic diagrams illustrating the welding portion of the lap laser spot welding head of the present invention. (a) is a top view, and (b) is a BB cross-sectional view of (a).

[0049] Figure 4 This is a diagram illustrating the welding method used in the manufacture of the welded joint of the present invention.

[0050] Figure 5 This is a diagram illustrating an example of the scanning trajectory of a laser beam used to obtain the elongated joint of the present invention.

[0051] Figure 6 The figures illustrate the welding position of the lap laser spot welding head of the present invention. (a) is a top view and (b) is a CC cross-sectional view of (a).

[0052] Figure 7 This is a perspective view illustrating a peel test piece having an lapped laser spot welded joint used in an embodiment of the present invention. Detailed Implementation

[0053] The following describes the lap laser spot welding head of the present invention, its manufacturing method, and a structural component for an automobile body having the lap laser spot welding head.

[0054] <Layered Laser Spot Welded Joint>

[0055] Figure 1This is a perspective view showing an example of a conventional lap laser spot weld joint. The lap laser spot weld joint 1 is a structure formed by overlapping at least two steel plates. Figure 1 In the example shown, two steel plates, a steel plate 2 with a roughly cap-shaped cross-section and a flange portion 2b extending outward from the front end of the longitudinal wall portion 2a and a flat, panel-shaped steel plate 3, are overlapped with the flange portion 2b facing the steel plate 3 to form a joint surface. A laser beam is irradiated from above the flange portion 2b towards its surface, forming a molten portion (molten metal portion) that at least penetrates the steel plate 2 and solidifying it to form a joint (weld point), thereby performing welding. It should be noted that a heat-affected zone (HAZ) exists around the aforementioned molten portion, but the joint of the present invention refers only to the molten portion excluding the heat-affected zone.

[0056] The welding portion of the aforementioned lap laser spot welding head 1 is such that one side of the welding head, which serves as the laser beam source, extends along the length direction of the longitudinal wall portion 2a. Figure 1 The laser beam is intermittently irradiated onto the surface of flange 2b while moving in the direction of the arrow in the image. The result is as follows: Figure 1 As shown, elliptical joints (weld points) are continuously formed in rows on the joint surface of steel plate 2.

[0057] Figure 2 It is shown in Figure 1 The schematic diagram of the conventional welded portion formed on the flange portion 2b of the lap laser spot welding head shown is (a) a top view of the joint portion constituting the above-mentioned welded portion as viewed from above the flange portion 2b, and (b) a cross-sectional view showing the AA section shown in (a).

[0058] In conventional laser beam welding, if a slender elliptical joint 14 with a large minor-to-major axis ratio is formed in the molten portion, welding cracks 5 begin to appear from the center 14a, which becomes the final solidified portion. Therefore, it is necessary to reduce the minor-to-major axis ratio of the molten portion, which presents a challenge in improving weld strength. The reason for this is that, if... Figure 2 As shown in (b), a large amount of sputtering occurs in conventional laser beam welding, so the central part 14a, which becomes the final solidified part, becomes too thin, resulting in tensile stress from the outer periphery of the molten part outwards. Figure 2 The force (in the direction of arrow σa shown in (a)) is concentrated. On the other hand, if the melting diameter of the joint is increased in order to improve the weld strength, other problems such as burn-through may occur.

[0059] It should be noted that the crack at the weld end is generated from the surface to the back side of the final solidified part of the joint. Whether it is generated can be confirmed by visual inspection, but for a more reliable determination, it is preferable to cut the final solidified part of the welded joint along the width direction and observe and determine the cut surface using, for example, an optical microscope magnified to about 10 times.

[0060] Therefore, in order to reduce stress concentration in the final solidification portion of the joint during laser beam welding, the inventors of this application have repeatedly studied countermeasures to prevent the thickness of the central portion of the final solidification portion of the joint from decreasing.

[0061] As a result, as described later, by causing the laser beam to rotate in a spiral motion from the outside to the inside of the oblong shape, drawing a shape composed of a semicircle and a straight line, a... Figure 3 The elongated joint shown in (a) can suppress sputtering, and thus, as Figure 3 As shown in (b), the minimum thickness u of the central portion 4a of the joint, which becomes the final solidified portion, can be increased. Therefore, by forming the above-described elongated oval joint, the tensile stress on the final solidified portion 4a of the joint, from the outer periphery of the molten portion toward the outside, can be significantly reduced compared to conventional welding methods. Figure 3 As shown by arrow σb in (a), cracks in the final solidified portion can be prevented. Here, "oblong" in this invention refers to a shape obtained by connecting two circles of equal radius with a common external tangent.

[0062] By employing the aforementioned larger elongated oval joint, cracks in the final solidification portion of the joint can be significantly reduced. However, according to further research by the inventors of this application, in order to more reliably prevent cracks in the final solidification portion of the joint and to ensure that the peel strength of the weld is sufficient, in addition to employing the aforementioned elongated oval joint, the aforementioned elongated oval joint must also satisfy 0≤G / T≤0.15 (described later) and satisfy all of the following equations (1) to (5):

[0063] 1.0≤T≤6.0 (1)

[0064] 2.0≤D1≤8.0 (2)

[0065] 6.0≤D2≤15.0 (3)

[0066] 1.1≤D2 / D1≤5.0 (4)

[0067] 0.6≤u / T≤1.0 (5)

[0068] Where T: Total thickness of the steel plates constituting the welded section (mm)

[0069] D1: Minor axis width of the oblong joint (mm)

[0070] D2: Major axis width of the oblong joint (mm)

[0071] u: Minimum thickness (mm) of the final solidified portion of the oblong joint.

[0072] The following is a detailed explanation.

[0073] 0≤G / T≤0.15

[0074] For the lap laser spot welding head of the present invention, the ratio (G / T) of the total gap G between the steel plates constituting the welding part to the total thickness T of the steel plates constituting the welding part should be 0 to 0.15, that is, the ratio of the total gap G between the steel plates constituting the welding part to the total thickness T of the steel plates constituting the welding part should be in the range of 0 to 15%. The reason for this is that if the ratio of G to T exceeds 15%, the depth of the arc crater at the welding end becomes deeper, and stress becomes more easily concentrated. Preferably, it is in the range of 0 to 10%.

[0075] 1.0≤T≤6.0

[0076] Furthermore, the lap laser spot welding joint of the present invention requires that the total thickness T of the multiple steel plates be in the range of 1.0 to 6.0 mm. When the total thickness is thinner than 1.0 mm or thicker than 6.0 mm, burn-through is likely to occur after irradiation with the laser beam, making lap welding difficult. Therefore, the welding joint of the present invention has a total thickness T of 1.0 to 6.0 mm for the steel plates constituting the joint. Preferably, it is in the range of 2.0 to 5.0 mm.

[0077] 2.0≤D1≤8.0

[0078] Furthermore, the lap laser spot welding head of the present invention requires that the minor axis width D1 of the elongated oval joint constituting the weld portion be in the range of 2.0 to 8.0 mm. If D1 is greater than 8.0 mm, burn-through will occur. Preferably, it is 6.0 mm or less. On the other hand, from the viewpoint of ensuring sufficient joint strength, D1 is 2.0 mm or more. Preferably, it is 4.0 mm or more.

[0079] 6.0 ≤ D² ≤ 15.0

[0080] Furthermore, the lap laser spot welding head of the present invention requires that the major axis width D2 of the elongated oval joint constituting the welded portion be in the range of 6.0 to 15.0 mm. If D2 is greater than 15.0 mm, welding cracks will occur. Preferably, it is 13.0 mm or less. On the other hand, the lower limit of D2 is not specified, but from the viewpoint of ensuring sufficient joint strength, it is set to 6.0 mm. Preferably, it is 8.0 mm or more.

[0081] 1.1≤D2 / D1≤5.0

[0082] Furthermore, the lap laser spot welding head of the present invention requires that the ratio (D2 / D1) of the major axis width D2 to the minor axis width D1 of the elongated oval joint constituting the weld portion is in the range of 1.1 to 5.0. If the ratio (D2 / D1) is greater than 5.0, welding cracks will occur. Preferably, it is 4.0 or less. On the other hand, welding cracks are also prone to occur when the ratio (D2 / D1) is less than 1.1. Preferably, it is 1.5 or more.

[0083] 0.6 ≤ u / T ≤ 1.0

[0084] Furthermore, the lap laser spot welding joint of the present invention requires that the ratio (u / T) of the minimum thickness u of the molten portion generated at the center portion 4a of the final solidified portion of the joint to the total plate thickness T of the steel plates constituting the joint is in the range of 0.6 to 1.1. If the above ratio (u / T) is less than 0.6, the tensile stress on the center portion 4a of the final solidified portion from the outer periphery of the molten portion toward the outside increases, making it impossible to prevent cracking. Preferably, it is 0.7 or more. On the other hand, the thickness u of the molten portion at the center portion of the final solidified portion is usually less than the total plate thickness T of the plurality of steel plates due to sputtering, so the ratio (u / T) is 1.0 or less.

[0085] Next, the steel plate constituting the lap laser spot welding head of the present invention will be described.

[0086] It should be noted that, Figures 1-4 The image shows an example of overlapping two steel plates to form an lapped laser spot weld joint. Of course, three or more steel plates can also be overlapped to form a welded joint.

[0087] Steel plate thickness

[0088] Regarding the total thickness of the steel plates constituting the lap laser spot weld joint of the present invention, it has been explained above, but the thickness of each steel plate is not particularly limited, and can be within the range of 0.5 to 3.2 mm, which is commonly used for outer panels and structural components (strength components) of automobile bodies. Furthermore, the multiple steel plates can all be of the same thickness or can be of different thicknesses. For example, in... Figure 1 In the case of the lap laser spot welding head 1 with the shape shown, the thickness t2 of the upper steel plate 2 can be 0.6 to 1.8 mm and the thickness t3 of the lower steel plate 3 can be 1.0 to 2.5 mm. Alternatively, the thickness t2 of the upper steel plate 2 and the thickness t3 of the lower steel plate 3 can be the same, 0.5 to 3.2 mm.

[0089] Composition of steel plates

[0090] Furthermore, there are no particular limitations on the composition of the plurality of steel plates constituting the lap laser spot welding joint of the present invention. Preferably, at least one steel plate has the following composition as described below, containing C: 0.07 to 0.4% by mass, Si: 0.2 to 3.5% by mass, Mn: 1.8 to 5.5% by mass, P+S: less than 0.03% by mass, Al: less than 0.08% by mass, and N: less than 0.010% by mass, with the balance being Fe and unavoidable impurities.

[0091] C: 0.07–0.4% by mass

[0092] Carbon (C) is an element that helps improve the strength of steel. By containing 0.07% by mass or more, precipitation strengthening and transformation strengthening effects can be achieved. Furthermore, by keeping the C content at 0.4% by mass or less, the precipitation of coarse carbides can be avoided, ensuring the desired strength and workability. Therefore, a C content in the range of 0.07 to 0.4% by mass is preferred. More preferably, it is in the range of 0.15 to 0.3% by mass.

[0093] Si: 0.2–3.5% by mass

[0094] Si is an element with excellent solid solution strengthening properties, and its content of 0.2% by mass or more can improve the strength of steel. Furthermore, by keeping the Si content at 3.5% by mass or less, excessive solidification of the weld heat-affected zone can be suppressed, preventing deterioration of the weld heat-affected zone's toughness and resistance to low-temperature cracking. Therefore, a Si content in the range of 0.2% to 3.5% by mass is preferred. More preferably, it is in the range of 1.0% to 2.5% by mass.

[0095] Mn: 1.8–5.5% by mass

[0096] Mn is an effective element for improving hardenability and suppressing the precipitation of coarse carbide, and is preferably present at 1.8% by mass or more. Furthermore, by keeping the Mn content at 5.5% by mass or less, it is possible to suppress the increase in grain boundary embrittlement sensitivity and prevent deterioration of toughness and resistance to low-temperature cracking. Therefore, an Mn content in the range of 1.8 to 5.5% by mass is preferred. More preferably, it is in the range of 2.0 to 3.5% by mass.

[0097] P+S: less than 0.03% by mass

[0098] P and S are harmful elements that negatively affect the ductility and toughness of steel. By keeping the total content of P and S below 0.03% by mass, it is possible to prevent a decrease in ductility and toughness, and ensure the desired strength and workability. Therefore, it is preferable that the total content of P and S is below 0.03% by mass. More preferably, it is below 0.02% by mass.

[0099] Al: less than 0.08% by mass

[0100] Al is an element added as a deoxidizer during the steelmaking process, typically at a concentration of 0.01% by mass or more. However, if the Al content exceeds 0.08% by mass, inclusions such as alumina increase, and the adverse effects on fatigue resistance become more pronounced. Therefore, the Al content is 0.08% by mass or less, preferably in the range of 0.02% to 0.07% by mass.

[0101] N: less than 0.010% by mass

[0102] Nitrogen (N) is an element that significantly deteriorates the aging resistance of steel, and its content should be minimized. In particular, if N exceeds 0.010% by mass, the deterioration of aging resistance becomes significant; therefore, the N content should be 0.010% by mass or less. It should be noted that there is no particular lower limit for the N content, but from the viewpoint of preventing increased manufacturing costs, it is preferably around 0.001% by mass.

[0103] In addition, in order to further improve the strength of the steel plate and the peel strength of the welded part, it is preferable that at least one steel plate constituting the welded joint of the present invention contains, in addition to the above-mentioned components, at least one of the components of group A and group B.

[0104] Group A: One or two of the following: Ti: 0.0005–0.01 wt% and Nb: 0.005–0.050 wt%.

[0105] Both Ti and Nb have the effect of forming and precipitating carbides and nitrides, and suppressing austenite coarsening during annealing in steel sheet manufacturing. To obtain the above effects, it is preferable to contain 0.0005% by mass or more of Ti and 0.005% by mass or more of Nb, one or both selected from Ti and Nb. However, even if Ti and Nb are contained in excess, the above effects will saturate, leading only to an increase in raw material costs. In addition, since the recrystallization temperature is increased, there is a possibility that the metal structure after annealing during steel sheet manufacturing may become uneven and the tensile flangeability may be impaired. Furthermore, there is a possibility that the yield ratio may increase due to an increase in the amount of carbide or nitride precipitation, and the shape freeze-thawability may deteriorate. Therefore, when Ti and / or Nb are contained, Ti is 0.01% by mass or less, and Nb is 0.050% by mass or less. More preferably, the content is in the range of Ti: 0.0006 to 0.0080% by mass, and Nb: 0.010 to 0.040% by mass.

[0106] Group B: One or more of the following: Cr: less than 1.0% by mass, Mo: less than 0.50% by mass, and B: less than 0.10% by mass.

[0107] Cr, Mo, and B are effective elements for improving the hardenability of steel. To achieve the above effects, it is preferable to contain one or more of the following: Cr: 0.01% by mass or more, Mo: 0.004% by mass or more, and B: 0.0001% by mass or more. However, even if these elements are contained in excess, the above effects will saturate, leading only to an increase in raw material costs. Therefore, when Cr, Mo, and B are present, it is preferable to add Cr: 1.0% by mass or less, Mo: 0.50% by mass or less, and B: 0.10% by mass or less. More preferably, the range is Cr: 0.02 to 0.50% by mass, Mo: 0.010 to 0.10% by mass, and B: 0.001 to 0.03% by mass.

[0108] The balance of at least one steel plate constituting the welded joint of the present invention, other than the above-mentioned components, is Fe and unavoidable impurities.

[0109] Tensile strength of steel plate

[0110] Furthermore, it is preferable that at least one of the steel plates constituting the lap laser spot weld joint of the present invention is a high-tensile steel plate with a tensile strength TS of 980 MPa or higher. If at least one steel plate is such a high-tensile steel plate, the lap laser spot weld joint can achieve high bonding strength, and even in the case of welding defects occurring in conventional elliptical joints, if it is the oblong joint of the present invention, the stress concentration towards the final solidification portion is small, thus preventing welding cracks. Therefore, for example, it is preferable that at least one of the steel plates has the above-described composition and a tensile strength TS of 980 MPa or higher. It should be noted that the multiple steel plates constituting the lap laser spot weld joint of the present invention can be steel plates with the same composition and strength, or steel plates with different compositions and strengths.

[0111] <Manufacturing Method of Lap Laser Spot Welded Joint>

[0112] Next, use Figures 4-6 This invention describes a method for manufacturing an lap laser spot welding head.

[0113] In the manufacturing method of the lap laser spot welded joint of the present invention, a laser beam is intermittently irradiated onto the surface of the uppermost steel plate among the overlapping steel plates to sequentially form a joint 4, thereby forming a welded part and manufacturing a welded joint. Figure 4 In the example shown, for the lap laser spot welding head 1 of the present invention, multiple steel plates 2 and 3 are overlapped and intermittently irradiated with a laser beam 6 onto the surface of the uppermost steel plate 2, and lap laser beam welding is performed by forming the joint portion 4 of the steel plates 2 and 3 in a row and continuously.

[0114] As described above, in this invention, multiple overlapping steel plates are welded on one side. By employing one-sided welding, the working space required for welding can be reduced.

[0115] It should be noted that, in single-sided welding, from the viewpoint of preventing burn-through during welding, it is preferable to irradiate the side of the thicker steel plate among the multiple overlapping steel plates. On the other hand, from the viewpoint of preventing non-joining caused by incomplete penetration, it is preferable to irradiate the side of the thinner steel plate. It should also be noted that, when the steel plates are of the same thickness, the laser beam can be irradiated from either side of the steel plate.

[0116] Importantly, in this invention, the lap laser spot welding head is configured such that the ratio (G / T) of the total gap G between the steel plates constituting the weld portion to the total thickness T of the steel plates constituting the weld portion is 0 to 0.15; that is, the ratio of the total gap G between the steel plates constituting the weld portion to the total thickness T of the steel plates constituting the weld portion is in the range of 0 to 15%. This is because if the ratio of G to T exceeds 15%, the depth of the crater at the weld termination becomes deeper, and stress becomes more easily concentrated. A range of 0 to 10% is preferred.

[0117] Furthermore, the most important aspect of this invention is, as Figure 5 As shown in (a), by making the laser beam 6 spin while scanning in a spiral motion from the outside to the inside of the elongated circle in a manner that depicts a combination of semicircles and straight lines, a larger-than-usual elongated oval joint is formed. As described above, by making the laser beam spin while welding, sputtering can be suppressed; therefore, as... Figure 3 As shown in (b), the thickness u of the central portion 4a, which becomes the final solidified portion of the joint, can be increased, thus preventing excessive stress concentration at that location. Therefore, cracking can be prevented.

[0118] Here, as Figure 5 As shown in (a) and (b), the size of the aforementioned oblong joint can be changed by adjusting the spin radius r when the laser beam spins, the distance traveled by each spin (i.e., the advance amount c), the length L of the straight section of the oblong shape traced by the laser beam, and the radius R of the arc section. It should be noted that the aforementioned spin radius r, advance amount c, length L of the straight section of the oblong shape, and radius R of the arc section need to be adjusted so that the minor axis width D1, major axis width D2, and minimum thickness u of the final solidified portion of the aforementioned oblong joint satisfy the following equations (2) to (5):

[0119] 2.0≤D1≤8.0 (2)

[0120] 6.0≤D2≤15.0 (3)

[0121] 1.1≤D2 / D1≤5.0 (4)

[0122] 0.6≤u / T≤1.0 (5).

[0123] Here, as the type of laser beam used in the laser beam welding described above, a fiber laser, a disk laser, etc., can be used, for example. Furthermore, in order to satisfy the above (2) to (5), it is preferable that the laser beam irradiation is performed within the range of power: 1.0 to 6.0 kW, focal position: -5 mm to +5 mm from the surface of the steel plate irradiating the laser beam, beam diameter: 0.2 to 0.6 mm, and laser beam scanning speed: 5.0 to 10.0 m / min. More preferably, the laser power is within the range of 3.0 to 5.0 kW, focal position: from the surface of the steel plate irradiating the laser beam to +5 mm from the surface of the steel plate, beam diameter: 0.3 to 0.5 mm, and laser beam scanning speed: 6.0 to 9.0 m / min.

[0124] It should be noted that in the above description, the shape of the joint of the welded part constituting the present invention is described as oblong, but it can also be elliptical as long as it satisfies the above (2) to (5).

[0125] <Structural Components for Automobile Body>

[0126] Next, the structural component for automobile body of the present invention will be described.

[0127] As an example where the lap laser spot welding head of the present invention can be preferably used, there is a structural component (strength component) that becomes part of the skeleton of a car body. The aforementioned Figure 1 The component shown is composed of a steel plate 2, which serves as a frame component and has a roughly cap-shaped cross-section, and a steel plate 3, which serves as a panel component. The flange 2b of the steel plate 2 and the steel plate 3, which is positioned opposite the flange 2b, are joined by a weld to form a closed cross-section. This weld includes a series of elongated oval joints 4 formed by the aforementioned laser beam welding. For components with this shape to be used in strong components of automobile bodies, excellent weld strength is important from the viewpoint of ensuring crash safety. The lap laser spot weld of the present invention, because it exhibits no cracks in the final solidification of the joint and has sufficient peel strength, can therefore be applied, for example, to structural components such as center pillars and roof rails of automobile bodies.

[0128] Here, regarding the preferred location for forming the welded portion when using the lap laser spot welding head of the present invention to manufacture structural components for automobile bodies, etc., such as... Figure 6 As shown, the example is the case where two steel plates 2 and 3 with L-shaped cross-sections having flange portions 2b and 3b are overlapped with their flange portions facing each other and laser beam welding is performed from one side. Figure 6(a) is a top view of the overlapping flange portions, showing a weld portion formed on the flange portions, comprising a series of continuous elongated oval joint portions. Figure 6 (b) is a cross-sectional view of the CC section shown in (a) above.

[0129] exist Figure 6 In terms of the preferred location for forming the welded part, when the distance from the center line of the thickness of the steel plates 2 and 3 to the center of the width of the elongated joint formed on the flange is defined as the weld position X, the weld position X preferably satisfies the following formula (5):

[0130] 5t≤X≤8t (5)

[0131] Where t: the thickness (mm) of the thickest steel plate in the steel plate constituting the welded part.

[0132] For example, when the thickness t of the thickest steel plate is 2 mm, the preferred welding position X is in the range of 10 to 16 mm.

[0133] This is because if the weld position X is less than 5t, the weld metal is prone to breakage during the peel test, resulting in reduced peel strength. On the other hand, if the weld position X is greater than 8t, the torque on the first joint 4 and subsequent joint 5 becomes too large during the peel test, and the peel strength still decreases. A more preferred range for X is 6t ≤ X ≤ 7t. By forming the weld at the above-mentioned position, the peel strength of the welded joint of two overlapping steel plates with a total thickness of 2 to 5 mm can be 12.0 kN or more.

[0134] It should be noted that, regarding the above-mentioned welding position X, equation (5) is not limited to the one described above. Figure 6 The T-shaped lap laser spot weld joint shown is formed by overlapping two steel plates with L-shaped cross-sections. For example, it can also be applied to... Figure 1 The lap laser spot weld joint obtained by laser beam welding of the frame component (steel plate 2) and panel component (steel plate 3) with a cross-sectional shape roughly hat-shaped is shown. In this case, the base point (0 point) of the welding position X can be set as the center of the plate thickness of the longitudinal wall 2a of the frame component with a cross-sectional shape roughly hat-shaped.

[0135] Example

[0136] A sample with a width of 100 mm and a length of 150 mm was collected from a high-tensile steel plate with any of the following thicknesses (1.2 mm, 1.6 mm, and 2.0 mm) and a tensile strength (TS) of 590–1180 MPa, as shown in Table 1 (A–J). This sample was then bent into an L-shape with a long side of 120 mm and a short side of 30 mm to form an L-shaped steel plate. Here, the long side of the L-shaped steel plate corresponds to… Figure 1 ( Figure 4 The longitudinal wall 2a of ) has a short side corresponding to Figure 1 ( Figure 6 The flange portion 2b of the flange. Next, as... Figure 7 As shown, after the two L-shaped steel plates 7 are overlapped with their short sides facing each other, a laser beam is irradiated onto the overlapped portion in the atmosphere to form an intermittently arranged elongated joint, thus preparing a T-shaped peel test piece 8.

[0137] It should be noted that when performing the above-mentioned overlapping laser beam welding, a fiber laser with a beam diameter of 0.4mmφ at the focal position is used, and the focal position is set to the upper surface of the overlapping steel plates. Figure 7 Based on the surface of the upper steel plate 7 shown in Table 2, various changes are made to the gap G between the two steel plates, the power P of the irradiated laser beam, the scanning speed v, the radius r of the laser beam during spin, the amount of progress c per spin, and the length L of the straight section and the radius R of the arc section of the oblong shape depicted by the laser beam. This results in various variations in the minor axis width D1, major axis width D2, and minimum thickness u of the final solidified portion of the oblong joint, as shown in Table 2. At this time, the welding position X forming the welded portion is set to 6.5 times the thickest plate thickness t (fixed).

[0138] For the T-shaped peel test piece prepared in the above manner, visual inspection and penetrant testing are used to determine whether cracks or burn-through have occurred in the final solidified part of the weld, especially the joint.

[0139] Next, for the aforementioned T-shaped peel test piece, a tensile test was conducted at a speed of 10 mm / min, with the length direction of the long side of the two L-shaped steel plates as the tensile direction, to determine the peel strength (maximum load). It should be noted that in this embodiment, a peel strength of 12.0 kN or higher is considered "qualified".

[0140] [Table 1]

[0141]

[0142] [Table 2-1]

[0143]

[0144] [Table 2-2]

[0145]

[0146] [Table 2-3]

[0147]

[0148] The results of the determination of the presence or absence of welding cracks and the results of the peel strength test are shown in Table 2.

[0149] According to the results, for the test pieces (No. 1, 9, 17, 25, 33, 41, 49, 57, 65 and 73) obtained by lap laser beam welding under the conditions of the present invention, no cracks were generated in the final solidification part of the joint, and there was no burn-through. The peel strength was also above 12.0kN.

[0150] In contrast, for test pieces No.2, 10, 18, 26, 34, 42, 50, 58, 66 and 74, since the minimum thickness u of the final solidified part was less than 60% of the total plate thickness T, although there was no burn-through, cracks were generated in the final solidified part of the joint, and the peel strength was less than 12.0 kN.

[0151] In addition, for test pieces No.3, 11, 19, 27, 35, 43, 51, 59, 67 and 75, since the gap G of the welded part was greater than 15% of the total thickness T of the steel plate, although there was no burn-through, cracks were generated in the final solidification part of the joint, and the peel strength was less than 12.0kN.

[0152] In addition, for test pieces No.4, 12, 20, 28, 36, 44, 52, 60, 68 and 76, since the short axis width D1 of the joint is less than 2 mm, although there is no burn-through, cracks are generated in the final solidification part of the joint, and the peel strength is less than 12.0 kN.

[0153] In addition, for test pieces No.5, 13, 21, 29, 37, 45, 53, 61, 69 and 77, since the short axis width D1 of the joint was greater than 8 mm, although there were no cracks in the final solidified part of the joint, burn-through occurred.

[0154] In addition, for test pieces No.6, 14, 22, 30, 38, 46, 54, 62, 70 and 78, since the short axis width D1 of the joint was greater than 15 mm, although there was no burn-through, cracks occurred in the final solidification part of the joint.

[0155] In addition, for test pieces No.7, 15, 23, 31, 39, 47, 55, 63, 71 and 79, since the ratio of the major axis width D2 to the minor axis width D1 of the joint (D2 / D1) is greater than 5.0, cracks occurred in the final solidified part of the joint, even though there was no burn-through.

[0156] In addition, for test pieces No. 8, 16, 24, 32, 40, 48, 56, 64, 72 and 80, since the ratio of the major axis width D2 to the minor axis width D1 of the joint (D2 / D1) was less than 1.1, cracks occurred in the final solidified part of the joint, even though it did not burn through.

[0157] Furthermore, for No. 81 and 82, test results are shown for test pieces obtained by lap laser beam welding of two steel plates with different strength grades under the conditions of the present invention. The results show that, although it is a combination of 590MPa and 980MPa grades, No. 81, whose steel composition is within the preferred range of the present invention, not only did not have weld cracks but also obtained excellent peel strength. However, for No. 82, whose steel composition is outside the preferred range of the present invention, weld cracks occurred and the peel strength was less than 12.0kN.

[0158] As described above, in the examples of lap laser beam welding according to the present invention, good lap laser spot welded joints with the characteristics desired by the present invention were obtained. In contrast, in the comparative examples that did not meet the conditions of the present invention, good lap laser spot welded joints were not obtained.

[0159] Industrial availability

[0160] The technology of this invention enables high-speed and low-strain welding, and is therefore applicable to automotive structural components with flanges.

[0161] Explanation of reference numerals in the attached figures

[0162] 1: Overlapping laser spot welding joint

[0163] 2, 3: Steel plates

[0164] 2a: Longitudinal wall portion of steel plate 2

[0165] 2b: Flange portion of steel plate 2

[0166] 4.14: Joint (solder point)

[0167] 4a, 14a: The final solidified part (center part) of the joint.

[0168] 5: Cracks in the final solidified part of the joint

[0169] 6: Laser beam

[0170] 7: L-shaped steel plate

[0171] 7a: Long side of L-shaped steel plate

[0172] 7b: Width of L-shaped steel plate

[0173] 8: Peeling test piece

[0174] S: The starting point of laser beam irradiation during the formation of the joint.

[0175] E: The end of laser beam irradiation during the formation of the joint.

[0176] σa, σb: Stress acting on the final solidified portion of the joint.

[0177] T: Total thickness of the steel plates forming the joint

[0178] u: Minimum thickness of the final solidified portion of the joint

[0179] D1: Short axis width of the oblong joint

[0180] D2: The width of the major axis of the oblong joint

[0181] G: Total gap between the steel plates forming the joint

[0182] X: Welding position

[0183] 0: Base point of welding position

Claims

1. An lap laser spot welding head, comprising a welding section formed by overlapping multiple steel plates, characterized in that, The total gap G between the steel plates constituting the welded part is within the range of 0% to 15% of the total thickness T of the steel plates constituting the welded part. The welded portion comprises discontinuously arranged elongated oval joints, and The oblong joint satisfies all of the following equations (1) to (5): 1.0≤T≤6.0 （1）， 2.0≤D1≤8.0 （2）， 6.0≤D2≤15.0 （3）， 2.2≤D2 / D1≤5.0 (4) 0.6≤u / T≤1.0 (5) in, T represents the total thickness (mm) of the steel plate that makes up the welded section. D1 is the width of the minor axis (mm) of the oblong joint. D2 is the width of the major axis of the oblong joint (mm). u represents the minimum thickness (mm) of the final solidified portion of the oblong joint. Furthermore, the location of the welded part satisfies: 5t ≤ X ≤ 8t. The oblong joint is formed by rotating a laser beam while moving along the contour of an oblong shape that combines a semicircle and a straight line, and simultaneously scanning in a spiral motion from the outside to the inside of the oblong shape. Here, with the center line of the steel plate thickness as the base point (0 point), the distance from that point to the center of the width of the oblong joint formed on the flange is set as the welding position X, and t is the thickness (mm) of the thickest steel plate constituting the welded part.

2. The lap laser spot welding head according to claim 1, characterized in that, At least one of the components in the steel plate contains C: 0.07-0.4% by mass, Si: 0.2-3.5% by mass, Mn: 1.8-5.5% by mass, P+S: less than 0.03% by mass, Al: less than 0.08% by mass, and N: less than 0.010% by mass, with the balance being Fe and unavoidable impurities.

3. The lap laser spot welding head according to claim 2, characterized in that, In addition to the aforementioned composition, the steel plate also contains at least one component from group A and group B: Group A consists of one or two ingredients selected from Ti: 0.0005–0.01 wt% and Nb: 0.005–0.050 wt%. Group B consists of one or more ingredients selected from Cr: less than 1.0% by mass, Mo: less than 0.50% by mass, and B: less than 0.10% by mass.

4. The lap laser spot welding head according to any one of claims 1 to 3, characterized in that, At least one of the steel plates is a high-tensile steel plate with a tensile strength of 980 MPa or higher.

5. A method for manufacturing lap laser spot welded joints, wherein, Multiple steel plates are overlapped vertically, and a laser beam is intermittently irradiated onto one side of the overlapping steel plates to form a weld joint comprising elongated oval joints arranged in a continuous row, thus creating an lap laser spot weld joint. The manufacturing method is characterized in that... The total gap G between the steel plates constituting the welded part is within the range of 0% to 15% of the total thickness T of the steel plates constituting the welded part. Regarding the oblong joint, the laser beam spins as it moves along the contour of the oblong shape, which is composed of a semicircle and a straight line, while simultaneously scanning in a spiral motion from the outside to the inside of the oblong shape. At least one of the following parameters is controlled: laser power, focal position, welding speed, spin radius, amount of movement per spin, and beam diameter, so that the oblong joint satisfies all of the following equations (1) to (5), and the formation position of the welded part satisfies: 5t ≤ X ≤ 8t: 1.0≤T≤6.0 （1）， 2.0≤D1≤8.0 （2）， 6.0≤D2≤15.0 （3）， 1.1≤D2 / D1≤5.0 (4) 0.6≤u / T≤1.0 (5) Where T is the total thickness (mm) of the steel plate constituting the welded part. D1 is the width of the minor axis (mm) of the oblong joint. D2 is the width of the major axis of the oblong joint (mm). u represents the minimum thickness (mm) of the final solidified portion of the oblong joint. With the center line of the steel plate thickness as the base point (0 point), the distance from that point to the center of the width of the oblong joint formed on the flange is set as the welding position X, and t is the thickness (mm) of the thickest steel plate constituting the welded part.

6. A structural component for automobile body, comprising an lap laser spot welded joint as described in any one of claims 1 to 4.

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