Laser-welded overlap joints: A method for producing laser-welded overlap joints and automotive frame components.

Abstract

------20 / 05 / 2562------(OCR) The objective is to provide an overlapping joint from laser welding as a method for manufacturing joints. Lapping seams from laser welding and automotive frame components, including laser-welded lap joints. This invention is a laser-welded overlap joint that includes the weld area formed by welding. Laser overlapping is performed on the overlapping areas, which include a number of overlapping steel plates. Welding areas include... The main weld penetrates the steel plate at the overlapping areas and the final weld area forms at the ends. One of the main welding areas and has a basin and welding area conforming to formulas (1) to (4). L(greater than or equal to) 15.0(1) 10.0 (greater than or equal to) L2 (greater than or equal to) 21c (2) t1(greater than or equal to)2dc(3) wc(greater than or equal to)dc(4) ------------ The objective is to provide an overlapping joint from laser welding as a method for manufacturing joints. Lapping seams from laser welding and automotive frame components, including laser-welded lap joints. This invention is a laser-welded overlap joint that includes the weld area formed by welding. Laser overlapping is performed on the overlapping areas, including a number of overlapping steel plates in the welding area, as well as other areas. The main weld penetrates the steel plate at the overlapping areas and the final weld area forms at the ends. One of the main welding areas and has a basin and welding area conforming to formulas (1) to (4). L is greater than or equal to 15.0(1) 10.0 greater than or equal to L2 greater than or equal to 21c(2) tl greater than or equal to 2dc(3) wc is greater than dc(4)

Description

Lap laser welded joint, manufacturing method for lap laser welded joint, and automotive frame part

[0001] The present invention relates to a laser lap welded joint, a method for manufacturing a laser lap welded joint, and an automobile frame component having the laser lap welded joint.

[0002] Resistance spot welding has traditionally been used to weld automotive structural components with flanges. However, resistance spot welding has several problems: it takes a long time to complete the welding process; the welding pitch cannot be narrowed because heat generation is reduced due to current shunting; and the welding machine is subject to spatial limitations due to the gun. To solve these problems, the use of lap laser welding has been considered in addition to conventional resistance spot welding. Here, lap laser welding refers to a welding method in which a laser beam is irradiated onto the surfaces of multiple overlapping steel sheets to join the steel sheets.

[0003] In lap laser welding, a laser beam is irradiated linearly onto the surfaces of multiple overlapping steel sheets, melting and solidifying the irradiated portions of the steel sheets, thereby forming a molten zone (weld). This process joins the overlapping steel sheets to form a lap laser welded joint. However, lap laser welding presents a problem: cracks are prone to occur at the end of the linear molten zone, and once they occur, they can propagate along the entire length of the molten zone. When cracks occur and propagate in the weld metal, they not only reduce the static strength of the lap laser welded joint, such as its shear and peel strength, but also significantly reduce its fatigue strength due to crack propagation from the cracks. In recent years, higher-strength, high-tensile steel sheets have been used in automotive body parts, particularly frame parts, to improve the strength and rigidity of the vehicle body. Therefore, a reduction in the static and fatigue strength of the joint due to cracks in the weld zone is a serious problem.

[0004] Therefore, various techniques have been disclosed as methods for suppressing the occurrence and propagation of lap laser weld cracks that occur when overlapping steel sheets are laser welded.

[0005] For example, Patent Document 1 discloses a technique for preventing weld cracks by protruding the lower steel plate of an lap weld and positioning the welding start position away from the flange end. Patent Document 2 discloses a technique for preventing weld cracks by irradiating the end of the overlapping surface with a laser from an oblique angle. Patent Documents 3 and 4 disclose techniques for preventing weld cracks by reheating or welding a portion that has already been welded or the periphery of the welded portion. Patent Document 5 discloses a technique for preventing weld cracks by welding overlapping surfaces in an elliptical shape.

[0006] JP 2007-229740 A JP 2008-296236 A JP 2012-240083 A JP 2012-240086 A JP 2017-113781 A

[0007] However, in the method described in Patent Document 1, the lower steel plate of the lap weld is protruded, resulting in an excess protruding portion, which restricts the part design.

[0008] In the method described in Patent Document 2, the laser is irradiated from an oblique angle, so if there is a gap between the overlapping plates, the molten part is not properly formed at the overlapping surface, resulting in insufficient penetration, making it difficult to ensure strength.

[0009] The methods described in Patent Documents 3 and 4 have the problem that a portion that has already been welded or the periphery of that welded portion is reheated or welded, and therefore reheating or welding requires additional welding time.

[0010] The method described in Patent Document 5 involves welding overlapping surfaces into an elliptical shape, and is not applicable to weld cracks in linear welds.

[0011] In view of the above problems, the present invention aims to provide a lap laser welded joint that can suppress the occurrence and propagation of cracks at the end of the fusion zone and has good peel strength of the welded joint, a method for manufacturing this lap laser welded joint, and an automotive frame part that has this lap laser welded joint.

[0012] As a result of investigations to solve the above problems, the present inventors have made the following findings.

[0013] In the present invention, attention was paid to (a) the total length of the fusion zone, (b) the length of the final weld zone, (c) the length, depth, and width of the crater at the end of the lap laser weld, and (d) the thickness of the uppermost steel plate among the multiple overlapping steel plates. It was discovered that by controlling all of the relationships (a) to (d) above, that is, by forming a weld by lap laser welding that satisfies the following formulas (1) to (4), it is possible to suppress the occurrence and propagation of cracks at the end of the weld. In the present invention, the fusion zone and the weld heat-affected zone are collectively referred to as the weld zone. L≧15.0 ... (1) 10.0≧L2≧2l ｃ ... (2) t1 ≥ 2d ｃ ... (3) w ｃ >d ｃ ... (4) where L is the total length of the weld (unit: mm), L2 is the length of the final weld (unit: mm), l ｃ is the length of the crater in the final weld (unit: mm), t1 is the thickness of the top steel plate in the overlapping part (unit: mm), d ｃ is the crater depth of the final weld (unit: mm), w ｃ is the width of the crater at the final weld (unit: mm).

[0014] Furthermore, it was found that by controlling the total thickness of the stacked steel plates and the total size of the gaps between the stacked steel plates, stress concentration in the molten part at the overlapping surface can be suppressed, thereby further improving the peel strength.

[0015] The present invention was completed based on the above findings and has the following gist: [1] A lap laser welded joint having a weld joined by lap laser welding at an overlapping portion of a plurality of overlapping steel plates, the weld comprising a main weld penetrating the steel plates of the overlapping portion and a final weld having a crater formed at one end of the main weld, the weld being a lap laser welded joint that satisfies the following formulas (1) to (4): L≧15.0 ... (1) 10.0≧L2≧2l ｃ ... (2) t1 ≥ 2d ｃ... (3) w ｃ >d ｃ ... (4) where L is the total length of the weld (unit: mm), L2 is the length of the final weld (unit: mm), l ｃ is the length of the crater in the final weld (unit: mm), t1 is the thickness of the top steel plate in the overlapping part (unit: mm), d ｃ is the crater depth of the final weld (unit: mm), w ｃis the width (unit: mm) of the crater in the final weld. [2] The lap laser welded joint according to [1] above, wherein the total size of the gap between the steel plates in the overlapping portion is 0% to 15% of the total thickness of the plurality of steel plates. [3] The lap laser welded joint according to [1] or [2] above, wherein at least one steel plate among the plurality of steel plates has a chemical composition containing, by mass%, C: more than 0.07% to 0.25%, P+S: less than 0.03%, Mn: 1.8% to 3.0%, Si: more than 1.2% to 2.5%, with the balance being Fe and unavoidable impurities. [4] The lap laser welded joint according to any one of [1] to [3] above, further containing, in addition to the chemical composition, one or two selected from the following groups A and B:

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[0093] [7] The method for manufacturing a lap laser welded joint according to [6], wherein the laser welding includes a main welding process for forming a main weld and a final welding process for forming a final weld having a crater, and at least one of the laser output, welding speed, focal position, and beam diameter in the final welding process is controlled under conditions that satisfy formulas (5) to (7) so as to form the weld that satisfies formulas (1) to (4). L≧15.0 ... (1) 10.0≧L2≧2l ｃ ... (2) t1 ≥ 2d ｃ ... (3) w ｃ >d ｃ ... (4) Pｉ ≧P ｆ ≧(1 / 4)P ｉ ... (5) v ｉ ≧v ｆ ≧(1 / 4)v ｉ ... (6) f ｉ ≦f ｆ ≦20.0 (7) where L is the total length of the weld (unit: mm), L2 is the length of the final weld (unit: mm), l ｃ is the length of the crater in the final weld (unit: mm), t1 is the thickness of the top steel plate in the overlapping part (unit: mm), d ｃ is the crater depth of the final weld (unit: mm), w ｃ is the width of the crater at the final weld (unit: mm), P ｉ is the laser output of this welding process (unit: kW), P ｆ is the laser output in the final welding process (unit: kW), v ｉ is the welding speed of this welding process (unit: m / min), v ｆ is the welding speed in the final welding process (unit: m / min), f ｉ is the focal position of the main welding process (unit: mm), f ｆ is the focal position (unit: mm) in the final welding step. [8] An automobile frame part having the lap laser welded joint according to any one of [1] to [5] above.

[0016] According to the present invention, the occurrence and propagation of cracks at the end of the weld can be suppressed in lap laser welding of multiple overlapping steel sheets, and therefore a lap laser welded joint with good peel strength can be manufactured. In addition, the lap laser welded joint of the present invention has an excellent appearance, making it suitable for use as an automotive structural member, and automotive frame parts having the lap laser welded joint of the present invention can be manufactured.

[0017] FIG. 1 is a perspective view showing an example of a lap laser welded joint of the present invention. FIG. 2(A) is a schematic diagram illustrating the weld end of a conventional lap laser welded joint, and FIG. 2(B) is a schematic diagram illustrating the weld end of a lap laser welded joint of the present invention. FIG. 3(A) is a cross-sectional view of the lap laser welded joint of FIG. 2(A) along line A-A, and FIG. 3(B) is a cross-sectional view of the lap laser welded joint of FIG. 2(B) along line B-B. FIG. 4(A) is a top view and cross-sectional view illustrating the weld of a conventional lap laser welded joint, and FIG. 4(B) is a top view and cross-sectional view illustrating the weld of a lap laser welded joint of the present invention. FIG. 5 is a perspective view illustrating a welding method for a lap laser welded joint of the present invention. FIG. 6(A) is a top view illustrating the position of the weld (melt zone) of a lap laser welded joint of the present invention, and FIG. 6(B) is a cross-sectional view of line C-C in FIG. 6(A). FIG. 7 is a diagram showing an example of a lap laser welded joint in an embodiment of the present invention. 8A and 8B are graphs showing an example of lap laser welding conditions in the manufacturing method of a lap laser welded joint of the present invention, in which FIG. 8A shows the relationship between laser output and welding time, FIG. 8B shows the relationship between welding speed and welding time, and FIG. 8C shows the relationship between focal position and welding time.

[0018] The laser-welded lap joint, the method for manufacturing the laser-welded lap joint, and the automotive frame part of the present invention will be described below with reference to the drawings. Note that the present invention is not limited to this embodiment.

[0019] <Laser-welded lap joint> The laser-welded lap joint of the present invention has a welded portion joined by lap laser welding at an overlapping portion of a plurality of overlapping steel plates. The welded portion comprises a main welded portion that penetrates the steel plates of the overlapping portion and a final welded portion having a crater formed at one end of the main welded portion, and the welded portion satisfies the following formulas (1) to (4): L≧15.0 (1) 10.0≧L2≧2l ｃ ... (2) t1 ≥ 2d ｃ ... (3) w ｃ >d ｃ ... (4) where L is the total length of the weld (unit: mm), L2 is the length of the final weld (unit: mm), l ｃis the length of the crater in the final weld (unit: mm), t1 is the thickness of the top steel plate in the overlapping part (unit: mm), d ｃ is the crater depth of the final weld (unit: mm), w ｃ is the width of the crater at the final weld (unit: mm).

[0020] An embodiment of such a lap laser welded joint 1 of the present invention will be described with reference to Figures 1 to 4. Figure 1 is a perspective view showing an example of the lap laser welded joint 1 of the present invention. Figure 2(A) is a schematic diagram showing the weld end portion of a conventional lap laser welded joint, and Figure 2(B) is a schematic diagram showing the weld end portion of a lap laser welded joint of the present invention. Figure 3(A) is a cross-sectional view taken along line A-A in Figure 2(A), and Figure 3(B) is a cross-sectional view taken along line B-B in Figure 2(B). Figure 4(A) is a top view and a cross-sectional view showing the weld portion of a conventional lap laser welded joint, and Figure 4(B) is a top view and a cross-sectional view showing the weld portion of a lap laser welded joint of the present invention.

[0021] First, with reference to FIG. 1, a laser-welded lap joint 1 of the present invention will be described.

[0022] In the laser lap welded joint 1 of the present invention, at least two steel plates are overlapped. In the example shown in FIG. 1 , two steel plates are used: a steel plate 2 having a generally hat-shaped cross section with a vertical wall portion 2a and a flange portion 2b extending outward from the tip of the vertical wall portion 2a, and a steel plate 3 having a flat panel shape. The steel plates 2 and 3 are overlapped so as to face each other, and the region (flat region) of the flange portion 2b of the steel plate 2 forms the joining surface. The two overlapping steel plates 2 and 3 are joined by lap laser welding at the flange portion 2b. In lap laser welding, a fusion zone is formed that penetrates at least one of the steel plates 2 and 3 and joins the steel plates 2 and 3. This fusion zone and the weld heat-affected zone of this fusion zone form the weld zone 4.

[0023] The laser welding is performed by intermittently irradiating the laser beam 7 while moving the flange portion 2b in the longitudinal direction along the vertical wall portion 2a. As a result, a plurality of welds 4 having a substantially linear surface shape are formed on the joining surfaces of the steel plates 2, 3, as shown in Figure 1. Here, an example will be described in which two steel plates 2, 3 are overlapped as the lap laser welded joint 1 of the present invention, but three or more steel plates may be overlapped.

[0024] Next, the technical concept of the present invention and the configuration of weld 4 will be described with reference to Figures 2 to 4. Figures 2(A), 3(A), and 4(A) show weld 14 and its periphery in a conventional lap laser welded joint, while Figures 2(B), 3(B), and 4(B) show weld 4 and its periphery in a lap laser welded joint of the present invention.

[0025] In conventional lap laser welding methods, a weld is formed by welding at a constant power and speed. Therefore, the size of the keyhole, which is the controlling factor for the fusion width, remains constant until the latter half of the welding, and a weld 14 as shown in FIG. 4(A) is generally formed. Specifically, the width W of the weld 14 is substantially the same from the start S to the end E of the weld 14. The steel sheets 2 and 3 at the overlapping portion are fused by welding, penetrating all of the steel sheets 2 and 3. At the end E of the weld 14, a crater 15 with a large cross-sectional area is formed in the upper steel sheet 2.

[0026] In a lap laser welded joint welded by the conventional lap laser welding method described above, the depth of the crater 15 at the end of the weld 14 is deeper than the thickness of the steel plate of the uppermost layer. Specifically, as shown in Figures 2(A) and 3(A), the depth d ｃ penetrates the steel plate 2 and is equal to or greater than the thickness of the steel plate 2. In the case of such a shape, tensile stress (force in the direction of arrow Fa shown in FIG. 2(A)) directed outward from the outer periphery of the molten part is concentrated in the central part, which becomes the final solidification part at the end E of the weld 14. It has been found that this can cause solidification cracks 16 to occur and propagate at the end part of the weld 14.

[0027] In contrast, in the lap laser welding method of the present invention for forming the shape of the weld 4, which will be described later, as shown in Figure 4(B), the width W of the weld 4 is formed to be approximately the same from the start S of the weld 4 to a predetermined position, but is formed to gradually narrow from the predetermined position to the end E. The steel sheets 2, 3 at the overlapping portion are melted through all of the steel sheets 2, 3 in the first half of welding up to the predetermined position, but the amount of melting of the lower steel sheet 3 decreases in the second half of welding after the predetermined position. At the end E of the weld 4, a small crater 5 is formed in the upper steel sheet 2.

[0028] As shown in Figures 4(A) and 4(B), the size of the crater 5 formed at the end E of the weld 4 is smaller (shallower) than the thickness of the uppermost steel plate, compared to the conventional crater 15. Specifically, as shown in Figures 2(B) and 3(B), the depth d ｃ The depth of the weld is shallow and does not penetrate the steel plate 2. In the case of such a shape, the concentration of the tensile stress (the force in the direction of the arrow Fb shown in FIG. 2B ) directed from the outer periphery of the fusion zone to the center (the center of the final solidification part of the end E) is small at the end E of the weld 4. This makes it possible to prevent solidification cracks from occurring at the end part of the weld 4.

[0029] Based on the above-described technical concept, in the laser-welded lap joint 1 of the present invention, the surface dimensions of the weld 4 are adjusted to a predetermined range. In particular, it is important to reduce the size of the crater 5 at the end of the weld.

[0030] 4(B), the weld 4 is composed of a main weld 4a that penetrates the steel plates at the overlapping portion, and a final weld 4b that has a crater 5 formed adjacent to one end of the main weld 4a. The weld 4 satisfies the above-mentioned formulas (1) to (4).

[0031] (Total length L (mm) of welded portion 4: L≧15.0 mm) If the total length L of welded portion 4 is shorter than 15.0 mm, the length L2 (mm) of the final welded portion 4b, which will be described later, cannot be sufficiently ensured, resulting in weld cracks. Therefore, the total length L of welded portion 4 is set to 15.0 mm or more (formula (1) above). Preferably, it is set to 20.0 mm or more. Note that there is no particular upper limit specified for the total length L of welded portion 4, but from the viewpoint of preventing the welding time of the parts from becoming too long, it is preferably set to 40.0 mm or less. More preferably, it is set to 30.0 mm or less.

[0032] (Length L2 (mm) of final weld 4b: 10.0 ≧ L2 ≧ 21 ｃ ) Length l of the crater 5 of the final weld 4b ｃ is greater than half of the length L2 of the final weld 4b, the ratio of the crater 5 to the end portion (final weld 4b) of the weld 4 is large, and the occurrence of weld cracks cannot be suppressed. ｃ The length L2 (mm) of the final weld 4b is set to be equal to or less than half of the length L2 (mm) of the final weld 4b. ｃ (the above formula (2)), and preferably 7.0 mm or more. From the viewpoint of suppressing the occurrence of weld cracks, the upper limit of the length L2 of the final weld 4b is set to 10.0 mm or less, and preferably 9.5 mm or less.

[0033] (Thickness t1 (mm) of the uppermost steel plate in the overlapping portion: t1≧2d ｃ ) Depth d of the crater 5 of the final weld 4b ｃ However, if the thickness t1 of the steel sheet at the top layer (the uppermost part of the overlapped portion of multiple steel sheets) of the overlapped portion is greater than half, underfilling on the steel sheet surface becomes significant. As a result, tensile stress is more concentrated at the end E of the welded portion 4, making it more likely for weld cracks to occur. Furthermore, if weld cracks occur, as will be shown in the examples of the present invention described later, fracture is likely to occur from the weld cracked portion in a test to evaluate peel strength, and the peel strength targeted by the present invention may not be achieved. Therefore, the depth d of the crater 5 at the final welded portion 4b is ｃis set to be 1 / 2 or less of the thickness t1 (mm) of the uppermost steel plate in the overlapping portion. In other words, the thickness t1 of the uppermost steel plate in the overlapping portion is set to be 1 / 2 or less of the depth d of the crater 5 in the final weld 4b. ｃ (the above formula (3)). In the following description, "the plate thickness t1 of the uppermost steel plate in the overlapping portion" refers to "t2" in the example shown in FIG. 4(B) which corresponds to the above t1.

[0034] In the present invention, the depth d of the crater 5 of the final welded portion 4b is ｃ Although the lower limit is not specified, from the viewpoint of practical construction, the depth d of the crater 5 of the final welded portion 4b ｃ is preferably set to 1 / 3 or less of the thickness t1 (mm) of the uppermost steel plate in the overlapping portion. In other words, the thickness t1 of the uppermost steel plate in the overlapping portion is set to 1 / 3 or less of the depth d of the crater 5 in the final weld 4b. ｃ More than three times (3d ｃ It is preferable that the thickness t1 of the uppermost steel plate in the overlapping portion is 0.7 mm or more. From the viewpoint of efficiently performing welding without reducing the laser output as much as possible, the depth d of the crater 5 in the final weld portion 4b is 0.7 mm or more. ｃ is preferably set to be 1 / 10 or more of the thickness t1 (mm) of the steel plate of the uppermost layer in the overlapping portion. In other words, the thickness t1 of the steel plate of the uppermost layer in the overlapping portion is set to be 1 / 10 or more of the depth d of the crater 5 of the final weld 4b. ｃ Less than 10 times (10 x d ｃ ≧t1).

[0035] (The width w of the crater 5 of the final welded portion 4b ｃ (mm): w ｃ >d ｃ ) Depth d of the crater 5 of the final weld 4b ｃ Even if the thickness t1 of the uppermost steel plate in the overlapping portion is less than 1 / 2, the depth d of the crater 5 in the final welded portion 4b ｃ is the width w of the crater 5 of the final weld 4b ｃIn the above cases, the underfill on the steel sheet surface becomes significant, which causes tensile stress to concentrate more at the end E of the weld 4, making it more likely for weld cracks to occur. Furthermore, if weld cracks occur, as will be shown in the examples described later, fractures are likely to occur from the weld cracked area in tests to evaluate peel strength, and the peel strength targeted by the present invention may not be achieved. Therefore, the depth d of the crater 5 in the final weld 4b ｃ is the width w of the crater 5 of the final weld 4b ｃ That is, the width w of the crater 5 of the final weld 4b is less than ｃ is the depth d of the crater 5 of the final weld 4b ｃ Preferably, the width w of the crater 5 of the final weld 4b is larger than ｃ The depth d of the crater 5 of the final welded portion 4b is 0.15 mm or more. ｃ Although the lower limit of is not specified, it is preferably 0.30 mm or more from the viewpoint of practical construction. In addition, if the size of the crater 5 of the final weld 4b becomes large, the underfill becomes large. Therefore, the width w of the crater 5 of the final weld 4b ｃ is preferably 2 mm or less.

[0036] As described above, the weld 4 of the present invention maximizes the above-mentioned effect of reducing the size of the crater 5 because the main weld 4a and final weld 4b are formed within the above-mentioned specified range. As a result, excessive stress concentration at the end E of the weld 4 can be prevented, and weld cracks can be prevented. This makes it possible to prevent weld defects at the end of the weld even when the minimum total length L of the weld 4 is as short as 15.0 mm.

[0037] The laser-welded lap joint 1 of the present invention can obtain the characteristics aimed at by the present invention through the above-described configuration, but in addition to the above-described configuration, the following configuration can be added as necessary.

[0038] (Total Size of Gaps (mm) Between Steel Plates with Respect to Total Thickness T (mm) of Multiple Steel Plates: 0% to 15%) In the present invention, the total size of gaps G between steel plates at the overlapping portion of multiple steel plates (hereinafter sometimes referred to as the total gap) can be 0% to 15% of the total thickness T (mm) of the multiple steel plates. In the example shown in FIG. 4(B), the gap (gap) between the steel plates is only between steel plates 2 and 3. Therefore, when the total gap is G mm and the total thickness T of the multiple steel plates is (t2 + t3) mm, the relationship is expressed as 0%≦G / (t2 + t3)≦15%. By adjusting the total gap G (mm) with respect to the total thickness T (mm) of the multiple steel plates within the above range, the amount of underfill is reduced, stress concentration at the weld formed at the overlapping surface is suppressed, and weld cracking is suppressed, thereby further improving peel strength. More preferably, the total gap G (mm) with respect to the total thickness T (mm) of the multiple steel plates is 5% or more. More preferably, it is set to 10% or less.

[0039] (Composition of Steel Plate) The composition of the steel plate used in the lap laser welded joint 1 of the present invention is not particularly limited, but the composition of at least one of the multiple steel plates to be overlapped can be, for example, a composition containing, in mass%, C: more than 0.07% and not more than 0.25%, P + S: less than 0.03%, Mn: 1.8% or more and 3.0% or less, Si: more than 1.2% and 2.5% or less, with the balance being Fe and inevitable impurities. Hereinafter, % in each composition refers to mass%.

[0040] (C: more than 0.07% and not more than 0.25%) When the C content exceeds 0.07%, it is possible to obtain the effect of precipitation strengthening. On the other hand, when the C content is 0.25% or less, it is possible to ensure the desired high strength and workability without inviting the precipitation of coarse carbides. Therefore, it is preferable that the C content be more than 0.07% and not more than 0.25%. More preferably, the C content is 0.10% or more and 0.21% or less.

[0041] (P + S: Less than 0.03%) When the total amount of P and S (P + S) is less than 0.03%, the ductility and toughness are not reduced, and the desired high strength and workability can be ensured. Therefore, the total amount of P and S (P + S) is preferably less than 0.03%.

[0042] (Mn: 1.8% or more and 3.0% or less) When the Mn content is 1.8% or more, sufficient hardenability can be ensured, making it difficult for coarse carbides to precipitate. On the other hand, when the Mn content is 3.0% or less, the grain boundary embrittlement susceptibility increases, making it difficult for the toughness and cold cracking resistance to deteriorate. Therefore, the Mn content is preferably 1.8% or more and 3.0% or less. More preferably, the Mn content is 1.9% or more. The Mn content is more preferably 2.7% or less, and even more preferably 2.5% or less.

[0043] (Si: more than 1.2% and not more than 2.5%) When the Si content exceeds 1.2%, it is possible to obtain a sufficient effect of increasing the strength of the steel through solid solution. On the other hand, when the Si content is not more than 2.5%, the hardening of the weld heat affected zone is unlikely to increase, and the toughness and cold cracking resistance of the weld heat affected zone are unlikely to deteriorate. Therefore, the Si content is preferably more than 1.2% and not more than 2.5%. More preferably, the Si content is not less than 1.3% and not more than 1.5%.

[0044] (Balance: Fe and Inevitable Impurities) The balance other than the above-mentioned component composition is Fe and unavoidable impurities. Examples of the unavoidable impurities include Al: 0.015 to 0.050%, N: 0.002 to 0.005%, etc.

[0045] In addition, in order to further improve the steel sheet strength and peel strength, in addition to the above-mentioned component composition, one or two selected from the following Group A and Group B may be further contained as necessary.

[0046] (Group A: one or two elements selected from, by mass%, Ti: 0.005% or more and 0.01% or less, and Nb: 0.005% or more and less than 0.050%) Ti and Nb precipitate as carbides or nitrides and have the effect of suppressing coarsening of austenite during annealing. Therefore, when Ti and Nb are contained, it is preferable to contain at least one of them. When Ti and Nb are contained to obtain this effect, it is preferable to contain 0.005% or more of Ti and 0.005% or more of Nb, respectively. However, even if these elements are contained in excess, the effects of the above-mentioned actions may saturate and become uneconomical. In addition, the recrystallization temperature during annealing may increase, the metal structure after annealing may become nonuniform, and stretch flangeability may be impaired. Furthermore, the amount of precipitation of carbides or nitrides may increase, the yield ratio may increase, and shape fixability may also deteriorate. Therefore, when Ti and Nb are contained, the Ti content is preferably 0.01% or less, and the Nb content is preferably less than 0.050%, more preferably less than 0.0080%, and more preferably less than 0.040%.

[0047] (Group B: one or more elements selected from, by mass%, Cr: 1.0% or less, Mo: 0.50% or less, and B: 0.10% or less) Cr, Mo, and B are elements that improve the hardenability of steel. Therefore, one or more of these elements may be contained. However, excessive inclusion of these elements may saturate the above-mentioned effects and become uneconomical. Therefore, when Cr, Mo, and B are contained, the Cr content is set to 1.0% or less, the Mo content is set to 0.50% or less, and the B content is set to 0.10% or less, respectively. More preferably, the Cr content is set to 0.50% or less. More preferably, the Mo content is set to 0.10% or less. More preferably, the B content is set to 0.03% or less, and even more preferably, the B content is set to 0.0030% or less. Furthermore, preferably, the Cr content is set to 0.01% or more. Preferably, the Mo content is set to 0.004% or more. Preferably, the B content is 0.0001% or more.

[0048] (Tensile strength of steel plate) Of the multiple steel plates used in the lap laser welded joint 1 of the present invention, at least one steel plate can be a high-tensile steel plate having a tensile strength TS of 980 MPa or more. Even if at least one steel plate is the above-mentioned high-tensile steel plate, the lap laser welded joint 1 can obtain high joining strength and prevent the occurrence of welding defects. For example, it is preferable that at least one steel plate of the multiple steel plates has the above-mentioned chemical composition and has a tensile strength TS of 980 MPa or more. Note that the multiple steel plates may be steel plates of the same type and shape, or steel plates of different types and shapes.

[0049] (Thickness of Steel Plates) In the present invention, the thickness t of the multiple steel plates to be lap-welded is not particularly limited, but is preferably within the range of 0.5 mm≦t≦3.2 mm, for example. Steel plates having a thickness within this range can be suitably used as automotive exterior panels and automotive frame members. The thicknesses of the multiple steel plates may all be the same or different.

[0050] Specifically, in the case of the lap laser welded joint 1 shown in Fig. 1 etc., it is preferable that the thickness t2 of the upper steel plate 2 satisfies 0.6 mm ≤ t2 ≤ 1.2 mm and the thickness t3 of the lower steel plate 3 is 1.0 mm ≤ t3 ≤ 2.5 mm. Alternatively, it is preferable that the thickness t2 of the upper steel plate 2 and the thickness t3 of the lower steel plate 3 both satisfy the ranges 0.5 mm ≤ t2 ≤ 3.2 mm and 0.5 mm ≤ t3 ≤ 3.2 mm.

[0051] In the present invention, "weld cracking" refers to cold cracking that occurs at the weld end portion of the weld 4 and propagates from the weld end E to the weld start S. The presence or absence of weld cracking can be determined by cutting the weld 4 after welding and checking for the presence or absence of cracks. The presence or absence of cracks can also be confirmed visually. For a clearer determination, for example, the cut surface of the weld can be checked by magnifying it by about 10 times using an optical microscope. When taking a cross-sectional photograph of the weld, a precision cutting machine is used to cut out a portion 5 mm away from the weld end perpendicular to the welding direction. The weld cracks are observed penetrating from the front surface to the back surface of the weld 4.

[0052] <Method of manufacturing lap laser welded joint> First, a method of manufacturing the above-described lap laser welded joint 1 of the present invention will be described with reference to Fig. 5. Fig. 5 is a diagram for explaining an example of a method of welding the lap laser welded joint 1 of the present invention.

[0053] The manufacturing method of the lap laser welded joint 1 of the present invention is a manufacturing method of the lap laser welded joint 1 described above, in which a plurality of steel plates are stacked in the vertical direction, and then lap laser welding is performed in which a laser is irradiated onto the surface of the topmost steel plate among the overlapping portions of the stacked steel plates, forming a weld 4 in the overlapping portion and joining them.

[0054] In the present invention, one-side welding is performed on a plurality of stacked steel plates. By performing one-side welding, space saving can be achieved in an automobile body part assembly line. For one-side welding, it is preferable to perform lap laser welding from the side of the steel plate with the larger plate thickness among the plurality of stacked steel plates. This makes it possible to prevent burn-through. When the steel plates have the same plate thickness, lap laser welding can be performed in order from either side.

[0055] In the example shown in Figure 5, the lap laser welded joint 1 of the present invention can be obtained by overlapping multiple steel plates 2, 3 and performing lap laser welding in which a laser beam 7 is irradiated linearly onto the surface of the outermost steel plate 2 so as to form a weld 4 in the steel plates 2, 3.

[0056] In the above-described lap laser welding, the laser beam 7 is continuously irradiated while being scanned linearly. For example, as shown in Fig. 5, the main weld 4a and the final weld 4b are formed consecutively to form the weld 4. In this case, by continuously performing the main welding process for forming the main weld 4a and the final welding process for forming the final weld 4b, the effect of reducing the size of the crater 5 can be maximized. This is preferable because it prevents excessive stress concentration at the end E of the weld 4 (see Figs. 2(B) and 3(B)) and the occurrence of cracks.

[0057] The lap laser welding of the present invention includes a main welding process for forming a main weld 4a and a final welding process for forming a final weld 4b having a crater 5. The main welding process is a main welding process that penetrates all of the steel plates in the overlapping portion in the first half of the welding. The final welding process is a final welding process that follows the main welding process and aims to reduce the size of the crater 5 formed at the end E of the weld 4 while adjusting the welding conditions described below up to the end E of the weld 4. It is preferable to control at least one of the laser output, welding speed, focal position, and beam diameter of the final welding process so that the main weld 4a and the final weld 4b having the crater 5 form a weld 4 that satisfies the above formulas (1) to (4). Note that the main welding process is performed under constant conditions.

[0058] For example, the laser beam may be a fiber laser, a disk laser, etc. Preferably, the beam diameter is 0.3 to 0.8 mm, the laser output is 2.0 to 5.0 kW, the focal position is in the range from on the surface of the outermost layer of the steel sheet to 0 to 20 mm above the surface of the outermost layer of the steel sheet, and the welding speed is 2.0 to 5.0 m / min.

[0059] More preferably, when forming the main weld 4a, the beam diameter is controlled to 0.5 to 0.8 mm, the laser output is controlled to 2.5 to 4.5 kW, the focal position is controlled to a range from on the surface of the outermost layer of the steel sheet to 20 mm above the surface of the outermost layer of the steel sheet, and the welding speed is controlled to a range from 2.5 to 4.5 m / min. Furthermore, when forming the final weld 4b, the beam diameter is controlled to 0.2 to 0.6 mm, the laser output is controlled to 0.5 to 3.0 kW, the focal position is controlled to a range from on the surface of the outermost layer of the steel sheet to 30 mm above the surface of the outermost layer of the steel sheet, and the welding speed is controlled to a range from 2.0 to 4.0 m / min.

[0060] More preferably, the laser output, welding speed, and focal position are controlled so that the relationship between the laser output Pi, welding speed vi, and focal position fi in the main welding step and the laser output Pf, welding speed vf, and focal position ff in the final welding step is as shown in the graphs of Figures 8(A) to 8(C). Specifically, at least one of the laser output, welding speed, focal position, and beam diameter in the final welding step can be controlled under conditions that satisfy equations (5) to (7). P ｉ ≧P ｆ ≧(1 / 4)P ｉ ... (5) v ｉ ≧v ｆ ≧(1 / 4)v ｉ ... (6) f ｉ ≦f ｆ ≦20.0 (7) where P ｉ is the laser output of this welding process (unit: kW), P ｆ is the laser output in the final welding process (unit: kW), v ｉ is the welding speed of this welding process (unit: m / min), v ｆ is the welding speed in the final welding process (unit: m / min), f ｉ is the focal position of the main welding process (unit: mm), f ｆ is the focal position in the final welding process (unit: mm).

[0061] P ｉ ≧P ｆ ≧(1 / 4)P ｉ ... (5) Laser output P in the final welding process ｆ (kW) is the laser output P of this welding process ｉ If the laser output power exceeds 100 W, the amount of underfill increases and burn-through may occur. ｆ is set to be equal to or less than (Pi x 0.5).

[0062] On the other hand, the laser output P ｆ is the laser output P ｉ If the laser output power is less than 1 / 4 of the laser power P in the final welding process, the laser power is insufficient to melt the steel plate. As a result, a sufficient welding line length cannot be secured, and the peel strength may be insufficient.ｆ is set to be equal to or greater than (Pi x 1 / 3).

[0063] v ｉ ≧v ｆ ≧(1 / 4)v ｉ ... (6) Welding speed v in the final welding process ｆ (m / min) is the welding speed v of this welding process ｉ (m / min), the depth d of the crater 5 of the final weld 4b ｃ More preferably, the welding speed v ｆ (v ｉ × 0.8) or less.

[0064] On the other hand, the welding speed of the final welding process v ｆ is the welding speed v of this welding process ｉ If the welding speed is less than ¼ of v, the size of the crater 5 in the final weld 4b will become large, and burn-through may occur. ｆ (v ｉ ×1 / 2) or more.

[0065] f ｉ ≦f ｆ ≦20.0 (7) Focal position f of final welding process ｆ (mm) is the focal position f of the main welding process ｉ If it is less than (mm), the depth d of the crater 5 of the final weld 4b ｃ More preferably, the focal position f ｆ is (f ｉ ×1.2) or more.

[0066] On the other hand, the focal position f ｆ However, if the focal position f exceeds 20.0 mm, the laser power density is insufficient to melt the steel sheet. As a result, a sufficient welding line length cannot be secured, and the peel strength may be insufficient. ｆ shall be 15.0 mm or less.

[0067] In the welding method of the present invention, for example, steel plates having the above-mentioned chemical composition and a tensile strength TS of 980 MPa or more can be used as the steel plates 2 and 3. The thicknesses t2 and t3 of the multiple steel plates 2 and 3 can be 0.5 mm≦t2≦3.2 mm and 0.5 mm≦t3≦3.2 mm, respectively, and the gap between the steel plates 2 and 3 can be 0% to 15% of the total thickness of the steel plates 2 and 3. The reasons for using these steel plates are the same as those described above.

[0068] Next, an example of a suitable welding position in the laser-welded lap joint 1 of the present invention will be described with reference to FIG.

[0069] Figure 6 is a diagram illustrating an example of a suitable position of the weld (melt zone) 4 in the lap laser welded joint 1 of the present invention. Figure 6(A) is a top view showing the periphery of the weld 4 of two steel plates 2 and 3, and Figure 6(B) is a cross-sectional view taken along line CC in Figure 6(A). In the explanation of Figure 6, the steel plate 2 will also be referred to as the flange portion 2b, and the steel plate 3 will also be referred to as the frame component.

[0070] In the example shown in Figures 6(A) and 6(B), the position coordinate of the end of the contact position between the flange portion 2b of the upper steel plate 2 and the frame component of the lower steel plate 3 is set to 0. The coordinate system is expressed as (-) for the outer end of the flange portion 2b and (+) for the vertical wall portion 2a of the approximately hat-shaped flange portion 2b (only a portion of the approximately hat-shaped shape is shown in Figure 6). The steel plate thickness at the thickest part of the approximately hat-shaped flange portion 2b is set to t (mm). In this case, it is preferable to perform welding using a one-sided welding method at a welding position X (mm) expressed by the following equation (8). This allows the peel strength of an L-shaped tensile test specimen, as shown in Figure 7, having a total plate thickness T of 2 to 5 mm and a flange portion 2b length of 50 mm, to be 1.2 kN or more. -2t ≧ X ≧ -4t (8) Here, we explain the reason for setting X as in equation (8).

[0071] If the welding position X is closer to the contact end of the flange portion 2b than -2t, the weld metal portion may fracture more easily during a tensile test and the peel strength may be lower. On the other hand, if the welding position X is farther from the contact end of the flange portion 2b than -4t, the moment acting on the weld portion 4 may be larger and the peel strength may be lower. Therefore, it is preferable to set the welding position X as shown in the above formula (8).

[0072] <Automotive frame part> An example of a part for which the laser-welded lap joint 1 of the present invention can be suitably used is an automotive frame part. In the case of the automotive frame part shown in Fig. 1 above, a steel plate 2 that is a frame part having a generally hat-shaped cross section and a steel plate 3 that is a panel part are used. The automotive frame part has a closed cross section formed by welding a flange portion 2b of the frame part (steel plate 2 shown in Fig. 1) and a panel part (steel plate 3 shown in Fig. 1) that is arranged opposite this flange portion 2b by the welding method described above to form the weld 4 described above.

[0073] The automotive frame part of the present invention is preferably applied to, for example, center pillars, roof rails, etc. For these parts, it is important to ensure peel strength from the viewpoint of collision safety. As described above, the center pillar to which the automotive frame part of the present invention is applied has sufficient peel strength.

[0074] As described above, according to the present invention, by overlapping a plurality of steel plates including at least one high-tensile steel plate, forming a weld 4, and welding and joining them, it is possible to obtain a lap laser welded joint 1 in which no welding defects occur on the front and back surfaces of the steel plates.

[0075] The weld 4 of the present invention can reduce the size of the crater at the end of the fusion zone, thereby reducing the concentration of tensile stress in the center of the final solidification zone. This suppresses the occurrence and propagation of cracks at the end of the fusion zone, making it possible to produce a lap laser welded joint 1 with high peel strength and excellent durability.

[0076] Furthermore, compared to conventional lap laser welding, weld cracking can be suppressed even with a shorter fusion length (15 mm), which is expected to increase the degree of freedom in component design and improve strength by welding multiple areas that require greater peel strength.

[0077] Furthermore, the lap laser welded joint 1 of the present invention has an excellent appearance and can therefore be suitably used as a structural member of an automobile. For example, by using high-strength steel plates as the steel plates to be joined, it can be used as an automobile frame part. By using such a lap laser welded joint 1, it is possible to obtain an automobile frame part or the like having high peel strength.

[0078] The functions and effects of the present invention will be described below using examples, but the present invention is not limited to the following examples.

[0079] In this example, steel sheets having the chemical compositions shown in Table 1 were used as test materials.

[0080] The steel plates had thicknesses of 1.2 mm, 1.6 mm, or 2.0 mm, and widths of 50 mm. These steel plates were bent into an L-shaped cross-sectional shape as shown in Figure 7. The L-shaped steel plate 8 has a long side 8a and a short side 8b. The long side 8a corresponds to the vertical wall portion 2a of the steel plate 2 of the lap laser welded joint 1 shown in Figure 1 above, and the short side 8b corresponds to the flange portion 2b.

[0081] Two L-shaped steel plates 8 of the same steel type and thickness were used, and their short sides 8b were overlapped. The overlapped portions (flat portions shown in FIG. 7 ) were then intermittently lap-laser welded at multiple locations in the longitudinal direction to form weld beads (welds 4), producing L-shaped test pieces (hereinafter referred to as test pieces). Here, the test piece size was 120 mm for the long side 8a (vertical wall length), 50 mm for the short side 8b (test piece width), and 30 mm for the overlapped portion (flange width). The gap (plate gap) G between the upper and lower L-shaped steel plates 8 was appropriately adjusted within a range of 0 to 20% according to the plate thickness.

[0082] The conditions for the weld 4 formed by lap laser welding are shown in Tables 2-1 and 2-2. As shown in Figure 7, the coordinates indicating the welding position are expressed in a coordinate system in which the contact position closest to the vertical wall in the overlapping part of the two L-shaped steel plates 8 is set to 0, the outside of the overlapping part of the test piece is (-), and the vertical wall side of the test piece is (+). The welding position at this time is X, the total length of the fusion part (weld) 4 is L, the length of the end (final weld 4b) of the weld 4 is L2, and the length of the crater 5 of the final weld 4b is L3. ｃ The depth of the crater 5 in the final weld 4b is d ｃ , the width of the crater 5 of the final weld 4b is w ｃ The test was carried out by changing the values ​​of each.

[0083] A fiber laser with a beam diameter of 0.6 mm at the focal position was used for the laser welding. The size of the crater 5 in the final weld 4b was adjusted in the main welding process and the final welding process so that the laser output, welding speed, and focal position were in the relationship shown in each graph of Figures 8(A) to 8(C). The processing point distance was on the surface of the outermost layer of the steel plate. Note that the focal position of the lap laser welding was set to 0 on the surface of the topmost steel plate of the overlapping steel plates (in the example shown in Figure 7, the surface of the upper L-shaped steel plate 8), and the positive direction was the vertical upward direction relative to the L-shaped steel plate 8. Welding was performed in the atmosphere.

[0084] The peel strength was measured by an L-shaped tensile test in which L-bent steel plates 8 were overlapped as shown in Figure 7, lap-welded with a laser, and a tensile load was applied from both sides of the welded steel plates. The tensile test was performed at a speed of 10 mm / min in accordance with JIS Z3136. A peel strength of 1.2 kN or more was evaluated as "pass" because it was deemed to have high bonding strength.

[0085] Furthermore, the occurrence of cracks can be determined by visual inspection and penetrant testing. In this example, the occurrence of cracks was determined visually as described above. Specifically, the obtained test specimen was cut in a direction perpendicular to the welding direction at a position 5 mm away from the center of the crater 5 of the final weld 4b toward the welding start side. The cut surface was confirmed under an optical microscope at 10x magnification. One specimen was observed under each condition. If the crack penetrated from the front surface to the back surface of the weld 4, it was determined that a weld crack was present.

[0086] The results of the evaluation of weld cracking and peel strength are shown in Tables 2-1 and 2-2.

[0087]

[0088]

[0089]

[0090] As shown in Tables 2-1 and 2-2, the test pieces of the invention examples had a peel strength of 1.2 kN or more, and no welding defects occurred.

[0091] On the other hand, among the test pieces of the comparative examples, No. 2, No. 10, No. 18, No. 26, No. 34, and No. 42, the total plate gap G was larger than 15% of the total plate thickness T, and the depth d of the crater 5 at the end E of the welded portion 4 was ｃ Because the depth was deep (the value was large), weld cracks occurred.

[0092] In addition, in Nos. 4, 12, 20, 28, 36, and 44, the total length L of the welded portion 4 was shorter than 15.0 mm, and therefore weld cracks occurred.

[0093] In addition, in Nos. 5, 13, 21, 29, 37, and 45, the length L2 of the final weld 4b was small, so weld cracks occurred.

[0094] In addition, No. 6, No. 14, No. 22, No. 30, No. 38, and No. 46, the depth d of the crater 5 at the end E of the weld 4 ｃ Because the depth was deep (the value was large), weld cracks occurred.

[0095] In addition, No. 7, No. 15, No. 23, No. 31, No. 39, and No. 47, the length l of the crater 5 at the end E of the weld 4 ｃ As the welding temperature was too high, weld cracks occurred.

[0096] In addition, No. 8, No. 16, No. 24, No. 32, No. 40, and No. 48, the width w of the crater 5 at the end E of the weld 4 ｃ Because the welding temperature was small, weld cracks occurred.

[0097] As described above, in the examples of the present invention in which lap laser welding was performed according to the present invention, good lap laser welded joints having the properties targeted by the present invention were obtained. In contrast, in the comparative examples in which the welding conditions of the present invention were not met, good lap laser welded joints were not obtained.

[0098] REFERENCE SIGNS LIST 1 lap laser welded joint 2 steel plate 3 steel plate 4 welded portion 4a main welded portion 4b final welded portion 5 crater 7 laser beam 8 steel plate 14 welded portion 15 crater 16 crack L total length of welded portion L2 length of final welded portion l ｃ Crater length d ｃ Crater depth w ｃ Crater width W Weld width G Plate gap

Claims

------20 / 05 / 2019------(OCR)1. Laser welding overlap joint consisting of a welding area formed by laser welding in the overlap area, which includes a number of steel plates overlapping another plate, where the welding area includes the main welding area which penetrates the steel plates in the overlap area and the final welding area which forms at one end of the main welding area and has a depression and where the welding area conforms to formulas (1) to (4):L(greater than or equal to) greater than or equal to A equals 15.0(1)10.0(greater than or equal to)L2(greater than or equal to)21c(2)t1(greater than or equal to)2dc(3)Wc>dc(4)where L is the total length (in mm) of the welding area, L2 is the length (in mm) of the final welding area, lc is the length (in mm) of the basin in the final welding area, t1 is the thickness (in mm) of the top steel plate in the overlapping section, dc is the depth (in mm) of the basin in the final welding area, and wc is the width (in mm) of the basin in the final welding area. 2.Laser lap joints under Reputation 1 where the overall size of the gap between steel plates in the lap section is 0% or greater and 15% or less than the overall thickness of a certain number of steel plates; 3. Laser lap joints under Reputation 1 or 2 where at least one of the steel plates of a certain number of steel plates contains the following chemical compositions in % by mass: C: greater than 0.07% and 0.25% or less; P+S: less than 0.03%; Mn: 1.8% or greater and 3.0% or less; and Si: greater than 1.2% and 2.5% or less, with the balance being Fe and unavoidable impurities.

4. Overlap joints from laser welding as per claims 1 through 3, any one of which is further combined with, in addition to that chemical composition, one or two elements selected from the following groups A and B: Group A: in % by mass, one or two elements selected from Ti: 0.005% or more and 0.01% or less and Nb: 0.005% or more and less than 0.050% and Group B: in % by mass, one or two or more elements selected from Cr: 1.0% or less, Mo: 0.50% or less and B: 0.10% or less.

5. Laser welding lap joints under any of the claims 1 to 4, in which at least one of the steel plates is a high tensile steel plate with a tensile strength of 980 MPa or more.

6. Methods for the production of laser welding lap joints under any of the claims 1 to 5, in which the assembly method involves vertically overlapping several steel plates onto another and performing a laser welding operation in which the surface of the top steel plate in the overlapping area, which includes several overlapping steel plates, is irradiated with a laser beam to form the welding area in the overlapping area, thus connecting several overlapping steel plates together. 7.The method for producing overlapping welds under laser welding under claim 6, where laser welding includes the primary welding step of the primary weld formation area and the final welding step of the final weld formation area with a basin, and where at least one of the laser power, welding speed, focal position and beam diameter in the final welding step are controlled under conditions which satisfy Formulas (5) to (7) in such a way that the formed weld area satisfies Formulas (1) to (4):L(greater than or equal to)15.0(1)10.0(greater than or equal to)L2(greater than or equal to)21C(2)t1(greater than or equal to)2dc(3)Wc>dc(4)Pi(greater than or equal to)Pf>(1 / 4)Pi(5)vi(greater than or equal to)vf(greater than or equal to)(1 / 4)vi(6)fi(less than or equal to)ff(less than or equal to)20.0(7)where L is the total length (in mm) of the welding area, L2 is the length (in mm) of the final welding area, lc is the length (in mm) of the basin in the final welding area, t1 is the thickness (in mm) of the top steel plate in the overlapping section, and dc is the depth (in mm).) of the basin in the final welding area wc is the width (in mm) of the basin in the final welding area Pi is the laser power (in kW) in the main welding step pf is the laser power (in kW) in the final welding step Vi is the welding speed (in m / min) in the main welding step vf is the welding speed (in m / min) in the final welding step f is the focus position (in mm) in the main welding step and ff is the focus position (in mm) in the final welding step 8. Automotive frame components are comprised of laser-welded joints pursuant to any one of Claims 1 through 5.------------1. Laser-welded overlapping joints comprised of a welding area formed by laser welding in the overlapping area, which includes a number of steel plates overlapping another plate, where the welding area includes the main welding area which penetrates the steel plates in the overlapping area and the final welding area which is formed at one end of the main welding area and has a basin and where the welding area conforms to formulas (1) through (4):L greater than or equal to 15.0(1)10.0 greater than or equal to L2 greater than or equal to 21c(2)tl greater than or equal to 2dc(3)Wc greater than dc(4)where L is the total length (in mm) of the welding area, L2 is the length (in mm) of the final welding area, lc is the length (in mm) of the basin in the final welding area, tl is the thickness (in mm) of the top steel plate in the overlap area, dc is the depth (in mm) of the basin in the final welding area, and Wc is the width (in mm) of the basin in the final welding area.

2. Laser welding overlap joints according to claim 1 where the overall dimensions of The gap between steel plates in the overlapping section is 0% or greater and 15% or less than the overall thickness of a certain number of steel plates.

3. Laser welding overlapping joints under claim 1 or 2 where at least one of the steel plates of a certain number of steel plates contains the following chemical elements by mass: C: greater than 0.07% and 0.25% or less; P+S: less than 0.03%; Mn: 1.8% or greater and 3.0% or less; and Si: greater than 1.2% and 2.5% or less, where the balance is Fe and unavoidable impurities. 4.

5. Any laser lap joint under Reservations 1 through 3 shall, in addition to the chemical composition, include one or two elements selected from the following Groups A and B: Group A: in % by mass one or two elements selected from Ti: 0.005% or more and 0.01% or less and Nb: 0.005% or more and less than 0.050% and Group B: in % by mass one or two or more elements selected from Cr: 1.0% or less, Mo: 0.50% or less and B: 0.10% or less.

6. Any laser lap joint under Reservations 1 through 4 in which at least one of the sheets of a certain number of steel plates is a high tensile steel plate with a tensile strength of 980 MPa or more.Any method for the production of overlapping joints by laser welding under claims 1 to 5, in which the method involves the vertical overlapping of several steel plates on top of another and the laser welding operation in which the surface of the topmost steel plate in the overlapping area, which includes several overlapping steel plates, is irradiated with a laser beam to form the weld area in the overlapping area, thus connecting the several overlapping steel plates together.

7. Any method for the production of overlapping joints by laser welding under claim 6, in which the laser welding includes the primary welding step of the primary weld area formation and the final welding step of the final weld area formation with a basin, and in which at least one of the laser power, welding speed, focal position and beam diameter in the final welding step is controlled under conditions which satisfy formulas (5) to (7) in such that the formed weld area satisfies formulas (1) to (4):L greater than or equal to 15.0(1)10.0 greater than or equal to L2 greater than or equal to 21c(2)tl greater than or equal to 2dc(3)wc greater than or equal to dc(4)Pi greater than or equal to Pf greater than or equal to Pf(1 / 4)Pi(5)Vi greater than or equal to vf greater than or equal to (1 / 4)vi(6)fi less than or equal to ff less than or equal to 20.0 (7)L is the total length (mm) of the welding area, L2 is the length (mm) of the final welding area, lc is the length (mm) of the basin in the final welding area, tl is the thickness (mm) of the top steel plate in the overlapping section, dc is the depth (mm) of the basin in the final welding area, wc is the width (mm) of the basin in the final welding area, P; is the power of the laser (kW) in the main welding step, Pf is the power of the laser (kW) in the final welding step, vi is the welding speed (m / min) in the main welding step, vf is the welding speed (m / min) in the final welding step, f is the focus position (mm) in the main welding step, and ff is the focus position (mm) in the final welding step. 8.The vehicle frame components are assembled with joints created by laser welding as per any one of claims 1 through 5;