Method for manufacturing welded joint

Abstract

Provided is a method for manufacturing a welded joint, whereby excellent peel strength can be obtained even when a high strength material is used as a material to be processed. The method for manufacturing a welded joint comprises: a preparation step for preparing a material to be processed, the material having two overlapped metal plates and a weld metal part for joining the two metal plates; and an irradiation step for irradiating, with a laser, the material to be processed, under a condition satisfying a prescribed relationship.

Description

Welded joint manufacturing method

[0001] The present invention relates to a method for manufacturing a welded joint.

[0002] Laser welding uses laser light as a heat source. Therefore, laser welding allows for more reliable and easier control of heat input than arc welding such as TIG welding and MIG welding. Therefore, in laser welding, it is possible to reduce thermal distortion by appropriately setting welding conditions such as laser output, scanning speed, and shielding gas flow rate. Furthermore, because laser welding can be performed from one side, it is particularly suitable for assembling and welding complex components such as automobile bodies.

[0003] As an example of such laser welding technology, Patent Document 1 discloses a method for manufacturing a laser-welded structural member, comprising: overlapping one steel plate having a bent portion and a flange continuing from the bent portion with one or more other steel plates at the flange; performing a first laser weld at the overlapping portion to form a first laser weld; after the temperature of the first laser weld falls below the Mf point, performing a second laser weld in a region near the first laser weld on the opposite side of the bent portion with respect to the first laser weld to form a second laser weld; and tempering a heat-affected zone of the first laser weld by the second laser welding to set the hardness of the heat-affected zone to 90% or less of the hardness of the heat-affected zone of the second laser weld, thereby manufacturing a laser-welded structural member. Furthermore, Patent Documents 2 to 4 also disclose technologies related to laser welding.

[0004] JP 2010-012504 A, JP 2012-240086 A, JP 2023-087674 A, JP 2023-143843 A

[0005] In recent years, in order to meet demands for improved fuel economy and safety of automobiles, the application of high-strength metal plates, for example, metal plates having a tensile strength of 980 MPa or more (hereinafter also referred to as high-strength materials), to automobile bodies has been promoted. In automobile bodies, for example, multiple steel plates may be overlapped and welded together. In such welded joints of automobile bodies, it is required to stably obtain high joint strength.

[0006] However, in conventional laser welding including the techniques of Patent Documents 1 to 4, when a high-strength material is used as the workpiece, the joint strength of the welded joint, particularly the peel strength (hereinafter simply referred to as peel strength) of a welded joint obtained by lap welding (hereinafter also referred to as a lap welded joint) may not be sufficient, and improvements in this regard are desired.

[0007] The present invention has been developed to meet the above-mentioned demands, and provides a method for manufacturing a welded joint that achieves excellent peel strength even when a high-strength material is used as the workpiece. Note that in this disclosure, all numerical ranges expressed using "to" mean ranges that include the numerical values ​​written before and after "to" as the lower and upper limits, respectively.

[0008] The present inventors conducted extensive research to achieve the above-mentioned objective and discovered the following: When high-strength materials such as steel plates are laser welded, heat input from the laser irradiation generates an embrittled structure near the weld metal. This embrittled structure reduces peel strength. To improve peel strength, tempering the vicinity of the inter-sheet edge of the (existing) weld metal while suppressing the heat input from the laser irradiation without forming a new weld metal to join the metal plates is effective. Specifically, when a peel load is applied to a lap welded joint, stress concentrates at the inter-sheet edge of the weld metal, and this area becomes the initiation point for cracks. Furthermore, when a new weld metal to join the metal plates is formed or when the heat input from the laser irradiation becomes excessive, a new embrittled structure is formed near the laser-irradiated area. As a result, for example, the inter-sheet edge of the new weld metal becomes the initiation point for cracks, which then propagate to the (existing) weld metal, making it more likely to fracture. Therefore, to improve peel strength, it is effective to temper the vicinity of the end of the weld metal part while suppressing the heat input due to laser irradiation without forming a new weld metal part that joins the metal sheets (hereinafter also referred to as tempering the end of the weld metal part). - To effectively temper the end of the weld metal part, it is important to irradiate the vicinity of the weld metal part with a laser and ensure that the heat input caused by the laser irradiation is propagated to the end of the weld metal part by an appropriate amount. In particular, it is important to irradiate the vicinity of the weld metal part with a laser under conditions that satisfy the relationship of the following formula (1): T x 0.7 + (L + 0.9) ２ / (10×S)+W×0.05 ≦ K ≦ 2+W+((0.2×S)×(L+1) ２ ) / 10-1 / T (1) In the formula, K: laser output (kW) in the irradiation process, W: width (mm) of the laser irradiated area on the surface of the workpiece in the irradiation process, L: center-to-center distance (mm) in the width direction between the laser irradiated area on the surface of the workpiece and the weld metal part in the irradiation process, S: operating scanning speed of the laser in the irradiation process (m / min), and T: plate thickness (mm) of the metal plate of the workpiece that is irradiated with the laser.

[0009] The present invention has been completed based on the above findings and further investigations. That is, the gist and configuration of the present invention are as follows.

[0010] 1. A method for manufacturing a welded joint, comprising: a preparation step of preparing a workpiece having two overlapping metal plates and a weld metal portion joining the two metal plates; and an irradiation step of irradiating the workpiece with a laser, wherein the irradiation step satisfies the relationship of the following formula (1): T x 0.7 + (L + 0.9). ２ / (10×S)+W×0.05 ≦ K ≦ 2+W+((0.2×S)×(L+1) ２ ) / 10-1 / T (1) In the formula, K: laser output (kW) in the irradiation process, W: width (mm) of the laser irradiated area on the surface of the workpiece in the irradiation process, L: center-to-center distance (mm) in the width direction between the laser irradiated area on the surface of the workpiece and the weld metal part in the irradiation process, S: operating scanning speed of the laser in the irradiation process (m / min), and T: plate thickness (mm) of the metal plate of the workpiece that is irradiated with the laser.

[0011] 2. The method for manufacturing a welded joint according to 1 above, wherein the irradiation step satisfies the relationship of the following formula (2): 0 ≦ F ≦ 18 + L + (0.8 × K ３ ) / S-L / T (2) In the formula, F: defocus amount of the laser in the irradiation process (mm), K: laser output power (kW) in the irradiation process, W: width (mm) of the laser irradiated area on the surface of the workpiece in the irradiation process, L: center-to-center distance in the width direction between the laser irradiated area on the surface of the workpiece in the irradiation process and the weld metal part (mm), S: laser scanning speed (m / min) in the irradiation process, and T: thickness (mm) of the metal plate of the workpiece that is irradiated with the laser.

[0012] 3. The method for manufacturing a welded joint according to 1 or 2 above, wherein the welded metal part is a laser-welded metal part.

[0013] 4. The method for manufacturing a welded joint according to any one of 1 to 3, wherein the Vickers hardness of the inter-plate end portion of the welded metal part after irradiating with the laser in the irradiation step is 95% or less of the Vickers hardness of the base material, and the Vickers hardness of the base material is the larger of the Vickers hardnesses of the base materials of the two metal plates.

[0014] 5. The method for manufacturing a welded joint according to any one of 1 to 4 above, wherein the penetration depth of the workpiece by the laser irradiation in the irradiation step is less than T, and T is the plate thickness (mm) of a metal plate of the workpiece that is irradiated with the laser.

[0015] 6. The method for manufacturing a welded joint according to any one of 1 to 4 above, wherein the penetration depth of the workpiece by the laser irradiation in the irradiation step is less than 0.50 × T, where T is the plate thickness (mm) of the metal plate of the workpiece that is irradiated with the laser.

[0016] 7. The method for manufacturing a welded joint according to any one of 1 to 6 above, wherein at least one of the metal plates has a tensile strength of 980 MPa or more.

[0017] According to the present invention, excellent joint strength can be obtained even when high-strength materials are used as the workpieces, thereby further promoting the application of high-strength steel sheets to automobile bodies. This further improves automobile fuel efficiency and safety, which is extremely advantageous industrially. Furthermore, since melting of the metal sheets during the irradiation process can be suppressed, the generation of spatter can also be suppressed. Therefore, this method is extremely advantageous in terms of appearance quality and productivity, particularly when manufacturing welded joints with shapes that are prone to spatter generation and difficult to remove, such as welded joints using L-shaped metal sheets as shown in Figure 3, where the gap between the metal sheets becomes large at the laser irradiation position (in the Y direction, as described below) during the irradiation process.

[0018] 1 is a schematic diagram (top view) showing an example of an implementation outline of an irradiation step of a method for manufacturing a welded joint according to an embodiment of the present invention; FIG. 2 is a schematic diagram (YZ plan view) showing an example of an implementation outline of an irradiation step of a method for manufacturing a welded joint according to an embodiment of the present invention; and FIG. 3 is a schematic diagram of a welded joint using an L-shaped metal plate.

[0019] A method for manufacturing a welded joint according to one embodiment of the present invention will be described below. The X, Y, and Z directions in Figures 1 to 3 are as follows: In the figures, reference numerals 1 and 2 denote a metal plate (first metal plate), 2 a metal plate (second metal plate), 3 a welded metal portion (first welded metal portion), 4 a laser irradiation area (second welded metal portion) in the irradiation step, 5 a laser, 6 an inter-plate end portion of the welded metal portion (first welded metal portion), 7 a flange portion of the first metal plate, 8 a wall portion of the first metal plate, 9 a flange portion of the second metal plate, and 10 a wall portion of the second metal plate. X direction: laser scanning direction and length direction of the weld metal portion of the weld joint (workpiece) (hereinafter simply referred to as the X direction); Y direction: direction perpendicular to the laser scanning and width direction of the weld metal portion of the weld joint (workpiece) (direction perpendicular to the X direction and perpendicular to the Z direction described below; hereinafter simply referred to as the Y direction); Z direction: thickness direction of the weld joint (workpiece) (hereinafter simply referred to as the Z direction);

[0020] [1] Preparation Step In the preparation step, a workpiece is prepared that has two or more overlapping metal plates and a weld metal portion that joins the two or more metal plates.

[0021] For example, the workpiece can be prepared by overlapping two or more metal plates and welding the overlapping portions of the metal plates. The welding method is not particularly limited, and examples thereof include laser welding and arc welding. Laser welding is particularly preferred. That is, the welded metal part is preferably a laser-welded metal part, i.e., a welded metal part formed by laser welding. The welding conditions are not particularly limited, and may be conventional.

[0022] The type of metal plate is not particularly limited, and examples include high-melting-point alloys such as steel plates. Specific steel types include general structural steel and carbon steel, such as rolled steel for welded structures according to JIS G 3106 (2020) and carbon steel for mechanical structures according to JIS G 4051 (2016). In particular, in steel plates with a tensile strength of 980 MPa or more, embrittlement near the weld metal zone becomes significant due to the heat input associated with laser irradiation. That is, in welded joints using metal plates (steel plates) with a tensile strength of 980 MPa or more as the base material, improvement in joint strength is particularly required. Therefore, the tensile strength of at least one of the metal plates is preferably 980 MPa or more, more preferably 1180 MPa or more. The upper limit of the tensile strength of the metal plate is not particularly limited. The tensile strength of the metal plate is preferably, for example, 2000 MPa or less. The tensile strength of all the metal plates constituting the material to be treated may preferably be 980 MPa or more, more preferably 1180 MPa or more.

[0023] The thickness of the metal plate is not particularly limited. For example, the thickness of the metal plate is preferably 0.8 mm or more, more preferably 1.0 mm or more. The thickness of the metal plate is preferably 3.0 mm or less, more preferably 2.0 mm or less. In this case, the metal plate can be advantageously applied in the automobile assembly process.

[0024] The thickness of the workpiece (the total thickness of the metal plates that make up the workpiece) is not particularly limited. For example, the thickness of the workpiece is preferably 0.5 mm or more, more preferably 1.0 mm or more. The thickness of the workpiece is preferably 3.0 mm or less, more preferably 2.0 mm or less.

[0025] The weld metal zone is a region where molten metal solidifies. The weld metal zone can be defined, for example, as follows. First, a cross-sectional sample in the Z direction (a sample with a cross section perpendicular to the X direction (YZ plane)) as shown in FIG. 2 is cut from the workpiece (welded joint) and mirror-polished. The cross-sectional sample is then observed using an optical microscope or SEM at a magnification of 100x. The weld metal zone, the heat-affected zone formed adjacent to the weld metal zone, and the interface between the base material of the metal plate are determined based on the difference in color tone (contrast) of each structure observed in the obtained image and the contrast of the interface, thereby defining the weld metal zone and the heat-affected zone. The weld zone is composed of the weld metal zone and the heat-affected zone. The weld metal zone and the heat-affected zone can also be defined in a welded joint obtained through the irradiation step [2] described below using the same method. As described above, the weld metal zone is preferably a laser-welded metal zone.

[0026] As shown in Fig. 2, the inter-plate end of the weld metal part is the location where the interface between the metal plates and the interface of the weld metal part intersect in a Z-direction cross section (a plane perpendicular to the X direction (YZ plane)) of the workpiece (welded joint). In other words, the inter-plate end of the weld metal part can also be said to be the Y-direction end of the weld metal part at the same Z-direction level as the interface between the metal plates in the cross section.

[0027] The shape of the weld metal portion is not particularly limited. For example, when the weld metal portion extends linearly as shown in FIG. 1 , the weld metal portion may or may not span the longitudinal ends of the workpiece in the X direction. The width (length in the Y direction) of the weld metal portion is preferably 0.2 to 5.0 mm, more preferably 0.2 to 2.0 mm. Furthermore, as long as the metal plates are joined together, the weld metal portion may or may not penetrate the workpiece. Note that the length and width of the weld metal portion refer to the length and width of the weld metal portion on the surface of the workpiece that is irradiated with a laser in the irradiation step described below (i.e., the surface of the first metal plate described below).

[0028] In the following description, the metal plate irradiated with the laser in the irradiation step [2] described later is also referred to as the first metal plate, and the other metal plate is also referred to as the second metal plate. Also, the welded metal part of the workpiece is also referred to as the first welded metal part, and the area where the metal is melted and solidified by the irradiation with the laser in the irradiation step [2] described later is also referred to as the second welded metal part.

[0029] [2] Irradiation step: Next, the workpiece prepared in the preparation step is irradiated with a laser. For example, when using a workpiece in which overlapping metal plates are joined by a linearly extending first weld metal portion as shown in Figure 1, the irradiation step involves linearly irradiating the workpiece with a laser along the first weld metal portion. In this case, it is important to irradiate the vicinity of the first weld metal portion with the laser under conditions that satisfy the relationship of the above-mentioned formula (1) and temper the inter-sheet end portion of the first weld metal portion.

[0030] T×0.7+(L+0.9) ２ / (10×S)+W×0.05 ≦ K ≦ 2+W+((0.2×S)×(L+1) ２ ) / 10-1 / T (1) The laser is irradiated onto the workpiece under conditions that satisfy the relationship of the above formula (1). This allows the end of the first welded metal portion to be effectively tempered. As a result, the strength of the welded joint is improved. Here, K (laser output (kW) in the irradiation process) is T×0.7+(L+0.9) ２ If K is less than / (10×S)+W×0.05, the heat input will be insufficient and the end portion between the plates of the first welded metal part will not be able to be tempered effectively. ２ If K exceeds 0.2 + T × 0.7 + (L + 0.9), the heat input becomes excessive, which leads to a decrease in the strength of the metal plate that is the base material. As a result, the peel strength decreases. K is preferably 0.2 + T × 0.7 + (L + 0.9) ２ K is preferably 0.8 × {2 + W + ((0.2 × S) × (L + 1)} / (10 × S) + W × 0.05 or more. ２ ) / 10-1 / T} or less.

[0031] 0≦F≦18+L+(0.8×K ３) / S-L / T (2) The laser is irradiated onto the workpiece under conditions that satisfy the relationship of the above formula (2) in addition to the relationship of the above formula (1). This makes it possible to more effectively temper the inter-plate end of the first welded metal part. As a result, the joint strength of the welded joint is further improved. F (the defocus amount (mm) of the laser in the irradiation step) is more preferably 0≦F≦15+L+(0.8×K ３ ) / S−L / T. The defocus amount is the amount of deviation (absolute value) of the laser focal position in the Z direction (thickness direction) from the surface of the workpiece (first metal plate) closer to the laser light source, as shown in FIG.

[0032] Furthermore, as long as the above formula (1), preferably the above formula (2), is satisfied, the laser irradiation conditions are not particularly limited.

[0033] For example, W (the width of the laser irradiation area on the surface of the workpiece in the irradiation process) is preferably 0.6 to 2.0 mm from the viewpoint of interference with the first weld metal portion. L (the widthwise center-to-center distance between the laser irradiation area on the surface of the workpiece in the irradiation process and the first weld metal portion) is preferably 0.1 to 3.0 mm from the viewpoint of effectively propagating the heat input accompanying laser irradiation to the inter-plate end of the first weld metal portion. L is more preferably 2.0 mm or less. S (the laser scanning speed in the irradiation process) is preferably 3.0 to 10.0 m / min from the viewpoint of takt time in the automobile assembly process. S is more preferably 5.0 m / min or more. S is more preferably 8.0 m / min or less. T (the plate thickness of the metal plate irradiated with the laser, i.e., the first metal plate, among the workpieces) is preferably 0.8 to 3.0 mm. T is more preferably 1.0 mm or more. T is more preferably 2.0 mm or less. In particular, it is desirable that W, L, S, and T satisfy the above formula (1), preferably the above formula (2), within the above ranges.

[0034] Furthermore, from the viewpoint of more effectively tempering the sheet-to-sheet end portion of the first welded metal part, the maximum temperature reached at the sheet-to-sheet end portion of the first welded metal part (hereinafter also referred to as the maximum temperature reached at the sheet-to-sheet end portion) is preferably 300°C or higher and 700°C or lower. The maximum temperature reached at the sheet-to-sheet end portion is more preferably 650°C or lower. Note that the sheet-to-sheet end portion of the first welded metal part here refers to the sheet-to-sheet end portion of the first welded metal part that is closer to the laser irradiation position in the Y direction. The same applies to the following.

[0035] Here, the maximum temperature at the end between the plates may be measured using, for example, a thermocouple. In one example, a minute hole is drilled in the first welded metal part so as to reach the vicinity of the end between the plates, and a thermocouple is inserted into the hole to measure the temperature at the end between the plates.

[0036] Vickers hardness of the sheet-to-sheet end of the first welded metal portion after laser irradiation in the irradiation step (hereinafter also referred to as post-irradiation Vickers hardness): 95% or less of the base material Vickers hardness. By setting the post-irradiation Vickers hardness to 95% or less of the base material Vickers hardness, it is possible to ensure the toughness of the sheet-to-sheet end of the first welded metal portion, where stress concentrates when a peel load is applied, and to improve the peel strength. Therefore, the post-irradiation Vickers hardness is preferably 95% or less, more preferably 90% or less, of the base material Vickers hardness. Note that there is no particular limitation on the lower limit of the post-irradiation Vickers hardness. For example, the post-irradiation Vickers hardness is preferably 70% or more of the base material Vickers hardness. Here, the base material Vickers hardness is the larger of the Vickers hardnesses of the base materials of the two metal plates used as the treated material.

[0037] The Vickers hardness is measured by a Vickers hardness test in accordance with JIS Z 2244-1:2020. The test force is 300 N, and the test force is held for 15 seconds. The post-irradiation Vickers hardness is measured, for example, as follows. That is, a Z-direction cross-sectional sample (a sample having a cross section on a plane perpendicular to the X direction (YZ plane)) as shown in FIG. 2 is cut out from the welded joint after laser irradiation in the irradiation process. Then, starting from the inter-plate end of the first weld metal part on the side where the laser was irradiated, the Vickers hardness is measured at three points at 100 μm intervals in the Y direction toward the widthwise center of the first weld metal part, and the average value of these three points is taken as the post-irradiation Vickers hardness. The base material Vickers hardness may be measured in the base material of the metal plate (areas other than the first weld metal part and its heat-affected zone) before the irradiation process, or may be measured in the base material of the metal plate (areas other than the first weld metal part and its heat-affected zone, and the second weld metal part and its heat-affected zone) after the irradiation process.

[0038] Penetration depth of the workpiece due to laser irradiation (penetration depth of the second weld metal portion, hereinafter also referred to as D): Less than T As described above, to improve peel strength, it is effective to temper the vicinity of the sheet-to-sheet end of the first weld metal portion while suppressing the heat input due to laser irradiation without forming a new weld metal portion that joins the metal sheets. Therefore, D is preferably less than T, more preferably less than 0.50 × T, even more preferably 0.45 × T or less, and even more preferably 0.40 × T or less. The lower limit of D is not particularly limited as long as it can effectively heat the sheet-to-sheet end of the first weld metal portion of the workpiece, and may be 0. In other words, it is not necessary to form a second weld metal portion. D is more preferably 0.10 × T or more, and even more preferably 0.25 × T or more. Note that D can be controlled, for example, by adjusting K, F, W, L, and S within the above-mentioned ranges.

[0039] Here, the penetration depth of the second weld metal zone may be measured, for example, as follows. Specifically, a cross-sectional sample in the Z direction (a sample with a cross section in a plane perpendicular to the X direction (YZ plane)) as shown in FIG. 2 is cut from the weld joint obtained through the irradiation process and mirror-polished. The cross-sectional sample is then observed using an optical microscope or SEM at a magnification of 10x. Based on the difference in color tone (contrast) of each structure and the contrast of the interface observed in the obtained image, the second weld metal zone, the heat-affected zone formed adjacent to the second weld metal zone, and the interface between the base metal of the metal plate (and, if necessary, the interface between the first weld metal zone and its heat-affected zone and the second weld metal zone) are determined, thereby defining the second weld metal zone. The distance from the surface of the first metal plate, which is the laser irradiated surface, to the deepest point in the thickness direction of the second weld metal zone is measured as the penetration depth of the second weld metal zone. It is also possible to visually inspect the cross-sectional sample and determine that the second weld metal portion is less than T if the second weld metal portion does not reach the second metal plate.

[0040] Furthermore, the laser may be irradiated on one side of the first welded metal part, or on both sides of the first welded metal part. In the former case, the laser is irradiated on the side where stress concentrates when a peeling load is applied (for example, in the case of FIG. 3 , the right side of the first welded metal part (between the first welded metal part and the wall part)). In this case, the laser may be irradiated over the entire length of the first welded metal part in the X direction, or may be irradiated on a portion of the first welded metal part in the X direction (for example, so that the length of the laser irradiated area in the X direction includes 80% or more of the length of the first welded metal part).

[0041] The conditions other than those mentioned above are not particularly limited and may be in accordance with conventional methods. For example, the laser used may be a fiber laser or a CO ２ Examples of the laser include a laser, a YAG laser, and a semiconductor laser.

[0042] The functions and effects of the present invention will be described below with reference to examples, but the present invention is not limited to the following examples.

[0043] For the sheet assembly described in Table 1, an L-shaped first metal plate and a second metal plate having a wall portion and a flange portion were overlapped at the flange portion as shown in FIG. 3, and the overlapping portion between the first metal plate and the second metal plate was linearly laser welded to prepare the treated material described in Table 2. Here, both the first metal plate and the second metal plate had a length a (X direction): 120 mm, a flange width b (Y direction): 50 mm, a wall height c (Z direction): 70 mm, an overlap width d (Y direction): 30 mm, and a distance e (Y direction) between the widthwise end of the flange portion and the width center position of the first weld metal portion: 20 mm. The first weld metal portion was spaced 40 mm from each of the longitudinal ends of the flange portion in the X direction. A fiber laser was used. The laser output was 3.5 kW, the beam diameter at the focal position was a constant 0.6 mmφ, and the scanning speed and defocus amount of the laser were adjusted so that the first weld metal portion penetrated the workpiece (the overlapping portion of the first metal plate and the second metal plate). Laser welding was performed in the atmosphere.

[0044] Next, a laser was irradiated parallel to the first weld metal portion between the first weld metal portion and the wall portion of the prepared workpiece under the conditions shown in Table 2 to obtain a welded joint. A fiber laser was used. The laser irradiation was performed in the atmosphere. The length of the laser irradiation area in the X direction was 40 mm, the same as the length of the first weld metal portion. Furthermore, as with the first weld metal portion, the laser irradiation area was spaced 40 mm from each of the longitudinal ends of the flange portion in the X direction. The maximum temperature reached at the end between the plates, the Vickers hardness after irradiation, and the penetration depth of the second weld metal portion were measured as described above.

[0045] The peel strength of each welded joint thus obtained was evaluated in the following manner. That is, a tensile load (peel load) was applied to each welded joint obtained in the direction of the arrow shown in Figure 3, and the tensile load at which the welded joint peeled was defined as the peel strength of the welded joint. The pulling speed was 10 mm / min. For comparison, a material that was the same as each of the above-mentioned treated materials but was not irradiated with laser (hereinafter also referred to as unirradiated material) was separately prepared, and the peel strength of this unirradiated material was measured in the above manner. The peel strength of each welded joint was then evaluated according to the following criteria using the peel strength increase ratio calculated according to the following formula. The evaluation results are also shown in Table 2. [Peel strength increase ratio] = [peel strength of welded joint (kN)] ÷ [peel strength of unirradiated material (kN)] Evaluation criteria A (pass, very excellent): Peel strength increase ratio is 1.2 or more B (pass, excellent): Peel strength increase ratio is 1.0 or more and less than 1.2 C (fail, poor): Peel strength increase ratio is less than 1.0

[0046]

[0047]

[0048] As shown in Table 2, all of the inventive examples achieved excellent peel strength even when high-strength materials were used as the treated materials. On the other hand, none of the comparative examples achieved excellent peel strength.

[0049] REFERENCE SIGNS LIST 1 Metal plate (first metal plate) 2 Metal plate (second metal plate) 3 Welded metal part (first welded metal part) 4 Laser irradiation area in irradiation step (second welded metal part) 5 Laser 6 End between sheets of welded metal part (first welded metal part) 7 Flange part of first metal plate 8 Wall part of first metal plate 9 Flange part of second metal plate 10 Wall part of second metal plate

Claims

1. A preparation step involves preparing a workpiece having two overlapping metal plates and a welded metal portion that joins the two metal plates. The irradiation step involves irradiating the material to be treated with a laser, It has, A method for manufacturing a welded joint, wherein the irradiation step satisfies the following relationship (1). T×0.7+(L+0.9) ２ / (10×S)+W×0.05 ≦ K ≦ 2+W+((0.2×S)×(L+1) ２ ) / 10-1 / T ・・・(1) During the ceremony, K: Laser output (kW) during the irradiation process. W: Width of the laser irradiation area on the surface of the material to be treated during the irradiation process (mm) L: Distance (mm) between the center of the laser irradiation area on the surface of the workpiece and the weld metal in the width direction during the irradiation process. S: Laser operation scanning speed (m / min) during the irradiation process, and T: The thickness (mm) of the metal plate that will be irradiated with the laser among the materials to be treated. That is the case.

2. A method for manufacturing a welded joint according to claim 1, wherein the irradiation step satisfies the following relationship (2). 0 ≦ F ≦ 18+L+(0.8×K ３ ) / S-L / T ・・・(2) During the ceremony, F: Laser defocus amount (mm) during the irradiation process, K: Laser output (kW) during the irradiation process. W: Width of the laser irradiation area on the surface of the material to be treated during the irradiation process (mm) L: Distance (mm) between the center of the laser irradiation area on the surface of the workpiece and the weld metal in the width direction during the irradiation process. S: Laser scanning speed (m / min) during the irradiation process, and T: The thickness (mm) of the metal plate that will be irradiated with the laser among the materials to be treated. That is the case.

3. The method for manufacturing a welded joint according to claim 1 or 2, wherein the welded metal portion is a laser-welded metal portion.

4. The method for manufacturing a welded joint according to claim 1 or 2, wherein the Vickers hardness of the inter-plate end of the welded metal portion after irradiation with the laser in the irradiation step is 95% or less of the Vickers hardness of the base material, and the Vickers hardness of the base material is the larger of the Vickers hardnesses of the base materials of the two metal plates.

5. The method for manufacturing a welded joint according to claim 1 or 2, wherein the penetration depth of the workpiece due to laser irradiation in the irradiation step is less than T, and T is the thickness (mm) of the metal plate irradiated with the laser among the workpiece.

6. The method for manufacturing a welded joint according to claim 1 or 2, wherein the penetration depth of the workpiece due to laser irradiation in the irradiation step is less than 0.50 × T, and T is the thickness (mm) of the metal plate that is irradiated with the laser among the workpiece.

7. The method for manufacturing a welded joint according to claim 1 or 2, wherein at least one of the metal plates has a tensile strength of 980 MPa or more.