Overlap laser-welded joint and method for manufacturing the same

Abstract

The objective is to provide a lap laser welded joint and a method for manufacturing the same. The lap laser welded joint of the present invention comprises two or more overlapping steel plates and a welded joint, wherein when the laser-irradiated side of the overlapping steel plates is the welded joint surface and the non-irradiated side is the welded joint back surface, the penetration width W of the welded joint surface F and the penetration width W on the back of the welded area B The total thickness t of the superimposed steel plates satisfies equation (1). 2.10 ≤ (W F / W B ) + (0.25 × t) ≤ 3.80 …(1)

Description

[Technical Field]

[0001] This invention relates to a lap laser welding joint and a method for manufacturing the same. [Background technology]

[0002] Laser welding uses a high-energy-density laser heat source, resulting in a narrow heat-affected zone in the weld area and reduced thermal distortion. Therefore, laser welding is used in the manufacturing process of thin steel sheets and the assembly process of automotive parts.

[0003] In the automotive parts sector in particular, tailored blanks, which involve butt welding two steel plates of different thicknesses and strengths together to form a single plate, and then press-forming the resulting integrated plate, are attracting attention. Furthermore, the application of laser welding to lap-welded joints of multiple steel plates with large total thicknesses, where resistance spot welding results in unstable heat generation and makes joining difficult, is also gaining interest.

[0004] In lap laser welding joints, where multiple steel plates are joined together using a laser, if plated steel plates are used for joining, the heat from the laser during welding causes the plating components between the steel plates to dissolve in the molten pool. During cooling, these dissolve as bubbles and are expelled from the surface of the molten pool. However, during the solidification process of the molten metal, bubbles present inside the molten pool may not be expelled to the outside and may remain as pits on the surface of the weld and blowholes within the weld, which may be judged as porosity, a welding defect.

[0005] The above-mentioned challenges exist in overlapping laser welding of plated steel sheets, and one example of a technology that solves these problems is Patent Document 1. Patent Document 1 discloses a laser welding method that involves multiple steps, a pretreatment step and a welding step, for laser irradiation. In the pretreatment step, a temperature difference is generated on the upper and lower surfaces of the lower steel sheet by laser heat input, causing it to deform into a V shape. As a result, a gap is created between the steel sheets, so that the plating components between the steel sheets do not dissolve into the molten pool during the welding step, but remain in the gap between the steel sheets, and as a result, it is possible to suppress the occurrence of porosity. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2011-156572 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, in the method described in Patent Document 1 above, the steel plate is deformed by laser irradiation in the pretreatment step, and then the same position is irradiated with a laser in the welding step to form the welded part. As a result, laser irradiation under different conditions is performed in multiple steps, which presents challenges such as the need to optimize the laser irradiation conditions for each plate assembly and the length of the construction time.

[0008] The present invention has been made in view of the above-mentioned problems, and aims to provide a lap laser welded joint and a method for manufacturing the same. The present invention makes it possible to manufacture a welded joint in only one welding step without requiring pretreatment steps such as deformation of steel plates, and makes it possible to reduce the occurrence of porosity such as pits and blowholes in the welded portion of the resulting lap laser welded joint. [Means for solving the problem]

[0009] To solve the above problems, the inventors of this invention conducted extensive research on the influence of laser irradiation conditions on bubbles generated during laser welding. As a result, they found that porosity in the weld can be suppressed by appropriately controlling the penetration shape of the weld.

[0010] This invention is based on the above findings, and its gist is as follows. [1] comprising two or more overlapping steel plates and a welded joint, When the superimposed steel plates are treated as follows, with the laser-irradiated side being the surface of the weld and the non-irradiated side being the back surface of the weld, When the penetration width W on the surface of the welded part F (mm) and the penetration width W on the back surface of the welded part B (mm) and the total plate thickness t (mm) of the superposed steel plates satisfy the formula (1), a lap laser welded joint. 2.10 ≦ (W F / W B )+(0.25×t) ≦ 3.80 …(1) 〔2〕 The lap laser welded joint according to the above 〔1〕, wherein at least one of the steel plates is a plated steel plate. 〔3〕 A method for manufacturing the lap laser welded joint according to the above 〔1〕 or 〔2〕, having a welding process of joining two or more superposed steel plates by laser welding, in the welding process, the welding conditions of the laser welding are such that laser light is irradiated onto the steel plates from a main laser irradiation part and at least one sub-laser irradiation part, and a method for manufacturing a lap laser welded joint, wherein the sub-laser output P2 (W) and the center-to-center distance Dce (μm) between the laser irradiation positions from the main laser irradiation part and the sub-laser irradiation part satisfy the formula (3). 0.40 ≦ P2 / Dce ≦ 1.40 …(3) 〔4〕 The welding conditions further include that the ratio of the main laser output P1 (W) to the sub-laser output P2 (W) satisfies the formula (4), and the method for manufacturing the lap laser welded joint according to the above 〔3〕. 1.8 ≦ P1 / P2≦ 16.0 …(4) 〔5〕 The welding conditions further include that the welding speed V (mm / s), the total plate thickness t (mm) of the steel plates, and the main laser output P1 (W) satisfy the formula (5), and the method for manufacturing the lap laser welded joint according to the above 〔3〕 or 〔4〕. 9.5 ≦ P1 / (V×t) ≦ 28.5 …(5) 〔6〕 There is at least one laser beam from the sub-laser irradiation part, A method for manufacturing an overlapping laser welded joint according to any one of [3] to [5] above, wherein the laser light from the sub-laser irradiation unit is irradiated around the laser light from the main laser irradiation unit. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a lap laser-welded joint capable of suppressing porosity, which can cause a decrease in joint strength, and a method for manufacturing the same. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 illustrates the welding state in a conventional lap laser welding apparatus. [Figure 2] Figure 2(A) is a schematic diagram illustrating laser irradiation during welding using the conventional welding apparatus shown in Figure 1, and Figure 2(B) is a schematic diagram illustrating the cross-section of the welded joint obtained by that welding. [Figure 3] Figure 3 illustrates the welding state in a lap laser welding apparatus according to one embodiment of the present invention. [Figure 4] Figures 4(A) to 4(C) are schematic diagrams illustrating the laser irradiation shape of the present invention. [Figure 5] Figure 5 is a schematic diagram illustrating an lap welded joint, which is one embodiment of the present invention. [Figure 6] Figure 6 is a schematic diagram illustrating a cross-section of a welded joint in an lap welded joint, which is one embodiment of the present invention. [Figure 7] Figures 7(A) and 7(B) are schematic diagrams illustrating the laser irradiation positions of the main and sub-lasers of the present invention. [Figure 8] Figure 8 is a schematic diagram illustrating laser irradiation during welding using the welding apparatus used in the present invention shown in Figure 3. [Figure 9] Figure 9 is a schematic diagram illustrating the bead start and end portions of a weld bead in the present invention. [Modes for carrying out the invention]

[0013] One embodiment of the present invention will be described in detail below. However, the present invention is not limited to this embodiment.

[0014] First, I will explain the technical concept behind this invention.

[0015] Figure 1 shows an example of a conventional laser welding apparatus. Figure 2(A) shows a diagram illustrating laser irradiation during lap laser welding using the laser welding apparatus of Figure 1, and Figure 2(B) shows a cross-sectional view in the plate thickness direction of the welded joint in the lap laser welded joint (sometimes referred to as a "welded joint" in this specification) obtained by that welding.

[0016] Figure 1 shows two steel plates being laser-welded together using a laser welding device. While at least two steel plates are typically used, three or more plates may be used. Furthermore, at least one plated steel plate may be included among the plates being overlapped. When a plated steel plate is included, the plates should be overlapped so that the plated side faces the other plates. Here, we will describe an example where the plated steel plate is on top and the other steel plates are on the bottom.

[0017] As shown in Figure 1, a conventional laser welding apparatus 20 comprises at least a laser oscillator 21, a light guide 22, and a laser processing head 23. The laser beam emitted from the laser oscillator 21 is focused by a focusing unit 23a in the laser processing head 23 via the light guide 22, and then irradiated onto the stacked steel plates 1. The laser light 24 is irradiated from a laser irradiation unit 23b provided on the laser processing head 23 of the laser welding apparatus 20. The laser light 24 heats the steel plates 1, and the molten pool 5 formed by localized melting solidifies to form a weld bead 6, thereby joining the steel plates together. As shown in Figure 1, there is only one laser beam 24.

[0018] In conventional lap laser welding, the heat input from the laser beam 24 during welding causes the plating components between the steel plates (i.e., the overlapping portion 2 of the steel plates 1) to dissolve in the molten pool 5, and during cooling, they attempt to be discharged from the surface of the molten pool as bubbles. However, because the cooling rate is relatively high and the melting width on the surface of the molten pool is narrow, as shown in Figure 2(B), some of the bubbles in the molten pool are not discharged to the outside and remain in the welded joint 7 as porosity 4. The inventors believe that this causes stress concentration in the welded joint, which in turn reduces the joint strength. In Figure 2(B), the region labeled 8 is the heat-affected zone, and the region labeled 9 is the weld metal.

[0019] Therefore, the inventors diligently investigated lap laser-welded joints and their manufacturing methods that suppress porosity, which can cause a decrease in joint strength, and have good joint strength. As a result, they obtained the following findings.

[0020] In the case of a conventional laser welding apparatus shown in Figure 1, the heat input from the laser beam 24 is localized as shown in Figure 2(A). As a result, the penetration shape of the weld is narrow on both the front and back surfaces of the overlapping steel plates 1, 1, and consequently, porosity occurs in the weld.

[0021] The inventors focused on the ease with which bubbles are discharged during welding and considered that porosity could be suppressed if the area from which bubbles are discharged was increased. Specifically, when the laser irradiation side is the top surface of the stacked steel plates (i.e., the surface of the steel plate) and the non-laser irradiation side is the bottom surface (i.e., the back surface of the steel plate), the melting width on the surface of the steel plate could be made larger than the melting width on the back surface of the steel plate.

[0022] To increase the melting width on the surface of the steel plate, it is necessary to change the heat input distribution by the laser beam. Conventional techniques to change this heat input distribution involve defocusing the laser beam to increase the beam diameter. However, with this method, as the melting width on the surface of the steel plate increases, the melting width in the center and back of the weld also increases, which increases the amount of plating components dissolved in the molten pool, making it difficult to effectively remove porosity.

[0023] Based on this, the inventors considered that applying heat input by laser irradiation at multiple points rather than a single point would be effective in eliminating the porosity described above, and they discovered that splitting the laser beam into a main and a secondary beam, and irradiating with the main and secondary laser beams simultaneously, is effective.

[0024] The present invention was completed based on the technical concept described above. An example of a laser welding apparatus capable of realizing laser irradiation in this overlapping laser welding (hereinafter sometimes simply referred to as "laser welding") is the laser welding apparatus 10 shown in Figure 3, which is described below.

[0025] Next, with reference to Figure 3, an example of a laser welding apparatus suitably used in the present invention will be described. Figure 3 shows one embodiment of the overlapping laser welding of the present invention.

[0026] The laser welding apparatus 10 shown in Figure 3 comprises at least a laser oscillator 11, a light guide path 12, and a laser processing head 13. The laser processing head 13 has at least a focusing unit 13a and a laser irradiation unit 13b. Although not shown in the figure, the laser welding apparatus 10 may also include, in addition to the above configurations, a shielding gas injection device for injecting shielding gas, a processing unit such as a CPU for executing a processing program, and a control unit for controlling the components of the laser irradiation unit 13b and the shielding gas injection device.

[0027] In the example shown in Figure 3, the laser welding apparatus 10 guides the laser beam emitted from the laser oscillator 11 through the light guide path 12 to the focusing unit 13a (e.g., a focusing lens) in the laser processing head 13, where it is focused by the focusing lens 13a. Then, the main and secondary laser beams 14a and 14b are simultaneously irradiated onto the overlapping steel plates 1 from the laser irradiation unit 13b. At this time, to prevent oxidation, shielding gas is injected from a shielding gas injection device to shield the welding area. The main and secondary laser beams 14a and 14b heat the steel plates 1, and the molten pool 5 formed by locally melting the steel plates solidifies to form a weld bead 6, thereby joining the overlapping steel plates 1.

[0028] The light guide path 12 is a transmission path that transmits the emitted laser beam to the laser processing head 13, and examples include a transmission cable or a mirror.

[0029] The focusing unit 13a is used to focus the transmitted laser beam, and examples include a collimating lens and a focusing lens.

[0030] The laser irradiation unit 13b is a means for irradiating the overlapping steel plates 1 with main and secondary laser beams 14a and 14b. Laser irradiation from this laser irradiation unit 13b makes it possible to realize the shape of the molten area described in the above technical concept. This "shape of the molten area" is a shape that focuses on the ease of discharging air bubbles during welding while reducing the amount of plating components dissolved in the molten pool. As shown in Figure 6 described later, it is a shape in which the expansion of the molten width at the overlapping portion 2 of the steel plates is suppressed, while the molten width on the surface of the steel plates is larger than the molten width on the back surface of the steel plates.

[0031] In the present invention, the laser irradiation unit 13b comprises two or more laser irradiation units capable of irradiating main and sub-laser beams. For example, the laser irradiation unit 13b consists of one main laser irradiation unit 13b1 and at least one (i.e., one or more) sub-laser irradiation units 13b2.

[0032] The purpose of the main laser irradiation is to melt the stacked steel plates with the large thermal energy of the main laser.

[0033] The secondary laser irradiation unit 13b2 is positioned to irradiate at least one secondary laser beam 14b around the irradiation area of ​​the main laser irradiation unit 13b1. The purpose of secondary laser irradiation is to increase the melting width on the surface side of the steel plate. Therefore, the secondary laser irradiation unit 13b2 can be appropriately positioned around the main laser irradiation unit 13b1, with the main laser irradiation unit 13b1 at its center, to achieve the above purpose. For example, as shown in the example laser irradiation configuration in Figure 4(B) described later, the secondary laser irradiation unit 13b2 can be positioned in front of the main laser irradiation unit 13b1.

[0034] The method for achieving these beam mode controls is not particularly limited. For example, one method involves installing a beam splitter or distribution mirror inside the laser processing head 13 to split and shape the laser beam, or attaching a diffractive optical element to branch and shape the laser beam.

[0035] Next, with reference to Figure 4, the positional relationship between the main and secondary laser beams emitted from the laser irradiation unit 13b will be explained in detail.

[0036] In the present invention, there is preferably at least one laser beam from the sub-laser irradiation unit 13b2 (hereinafter sometimes referred to as "sub-laser beam (14b)"), and the sub-laser beam 14b is irradiated around the laser beam from the main laser irradiation unit 13b1 (hereinafter sometimes referred to as "main laser beam (14a)").

[0037] Examples of laser beam irradiation patterns from the main laser irradiation unit 13b1 and the sub-laser irradiation unit 13b2 include the shapes shown in Figures 4(A) to 4(C). As shown in these figures, it is preferable that one or more sub-laser beams 14b are irradiated with respect to the main laser beam 14a at one or more positions on the front, rear, right, and left sides in the welding direction, in line symmetric with respect to the welding line.

[0038] For example, as shown in Figure 4(A), when the irradiation point of the main laser beam 14a is taken as the center point, the sub-laser beam 14b can be arranged so that it forms a circle composed of a point cloud around the center point.

[0039] Alternatively, as shown in Figure 4(B), for example, when the irradiation point of the main laser beam 14a is considered the center point, the sub-laser beam 14b can be positioned so that it forms a straight line composed of a point cloud at a position forward or backward in the welding direction relative to the center point. Note that the example of positioning at the backward position is omitted.

[0040] Alternatively, as shown in Figure 4(C), the secondary laser beam 14b can be arranged so that, when the irradiation point of the main laser beam 14a is considered the center point, the secondary laser beam 14b forms a hexagon composed of a point cloud around the center point. Although not shown in the illustration, the secondary laser beam 14b may also be arranged in a polygonal shape such as a square or an octagon.

[0041] As described above, the laser irradiation pattern is shaped by a diffractive optical element inserted into the laser processing head 13 to form the main laser beam and the sub-laser beam. The point cloud constituting the sub-laser beam can be adjusted as needed in accordance with the temperature distribution control of the molten pool 5, by adjusting the number of points, beam diameter, and laser output.

[0042] As described above, in the present invention, it is important to use the above-mentioned laser welding apparatus to irradiate the overlapping steel plates with a primary laser, and simultaneously irradiate at least one secondary laser around the primary laser irradiation. This makes it possible to effectively suppress porosity, which can cause a decrease in joint strength, and to obtain an overlapping laser-welded joint with good joint strength.

[0043] Next, the overlapping laser welding joint of the present invention will be described.

[0044] Figure 5 shows an example of the overlapping laser-welded joint of the present invention, and Figure 6 is an enlarged cross-sectional view in the thickness direction of the plate showing the welded portion and its surrounding area of ​​the welded joint. Figure 6 shows the cross-sectional view along line AA in Figure 5.

[0045] As shown in Figure 5, the welded joint of the present invention comprises two or more overlapping steel plates 1 and a welded joint. At least one of these two or more overlapping steel plates may be a plated steel plate. The steel plates will be described later, so their explanation is omitted here.

[0046] Furthermore, as shown in Figure 6, when the laser-irradiated side of the overlapping steel plates is the weld surface and the non-irradiated side is the weld surface, the penetration width W of the weld surface is F (Unit: mm) and the penetration width W on the back surface of the weld. B(Unit: mm) The total plate thickness t (unit: mm) of the steel plates overlapped satisfies the following formula (1).

[0047] [The penetration width W F at the front surface of the welded part and the penetration width W B at the back surface of the welded part and the relationship with the total plate thickness t] 2.10 ≤ (W F / W B ) + (0.25 × t) ≤ 3.80 …(1) Here, W F and W B in formula (1) will be described. The above-mentioned "penetration width W F at the front surface of the welded part" refers to the distance of the line segment connecting two points that are the ends of the weld metal (i.e., the melting boundary) in the direction perpendicular to the weld line on the steel plate surface. Also, the above-mentioned "penetration width W B at the back surface of the welded part" refers to the distance of the line segment connecting two points that are the ends of the weld metal (i.e., the melting boundary) in the direction perpendicular to the weld line on the steel plate back surface. These penetration widths W F , W B can be measured by the following method. As described in the examples below, for example, in the region excluding the start and end portions of the weld bead of the lap laser welded joint (see Fig. 9), it can be measured by observing the cross-section in the plate thickness direction perpendicular to the weld line.

[0048] This formula (1) is a newly defined relational expression by the inventor of the present invention, and it is a relational expression that stipulates that the penetration shape of the welded part is appropriately controlled.

[0049] When the value of "(W F / W B ) + (0.25 × t)" shown in formula (1) is less than 2.10, the expansion of the penetration width W F on the steel plate surface is insufficient, and the effect of porosity suppression cannot be obtained. On the other hand, when the value of "(W F / W B ) + (0.25 × t)" shown in formula (1) is greater than 3.80, the penetration width W FIf this value becomes excessive, the heat-affected zone expands excessively, causing a decrease in joint strength. Therefore, this value should be between 2.10 and 3.80. Preferably, this value is 2.50 or higher, and more preferably 3.30 or lower. More preferably, this value is 2.70 or higher, and more preferably 3.20 or lower.

[0050] Furthermore, the welded joint in the welded joint of the present invention may have the following constituent elements in addition to the above constituent elements.

[0051] [Depth Wc of overlapping steel plates] (preferred conditions) 1.10 ≤ (W C / W B ) + (0.04 × d) ≤ 1.60 …(2) Here, W in equation (2) C Let's explain d. As shown in Figure 6, the above-mentioned "penetration width W of the overlapping portion of the steel plate" C "The penetration width W" refers to the distance of the line segment connecting two points that form the weld metal edge (i.e., the molten boundary) perpendicular to the weld line on the overlapping surface of steel plates. C As shown in Figure 6, this corresponds to the joint diameter at the overlapping portion 2 of the two steel plates. Also, "d" above refers to the distance from the back surface of the steel plate to the overlapping surface of the steel plate (i.e., the overlapping portion 2).

[0052] Although not shown in the diagram, when there are three or more overlapping steel plates, the distance between the line segments connecting two points that form the weld metal ends (i.e., molten boundaries) perpendicular to the weld lines in the multiple overlapping sections 2 is Wc. It is preferable that all of these Wc values ​​satisfy equation (2).

[0053] (2) The equation "(W C / W B If the value of (W) + (0.04 × d) is less than 1.10, sufficient flow will not be obtained in the molten pool of the lower plate, and bubbles generated in the molten pool will not be able to be discharged, resulting in increased porosity. On the other hand, the value of (W) shown in equation (2) C / W BIf the value of ") + (0.04 × d)" is greater than 1.60, the amount of plating components between steel sheets that melt into the molten pool increases, making porosity more likely to occur. Therefore, this value should be between 1.10 and 1.60. This value is preferably 1.22 or higher, and preferably 1.57 or lower.

[0054] Next, the method for manufacturing the overlapping laser-welded joint of the present invention will be described.

[0055] The present invention relates to a method for manufacturing a lap laser-welded joint formed by welding two or more overlapping steel plates. This manufacturing method comprises at least a welding step of joining two or more overlapping steel plates by laser welding. In this welding step, multiple laser beams are simultaneously irradiated onto the overlapping steel plates from the main laser irradiation unit and the sub-laser irradiation unit of a laser welding apparatus, and the butted steel plates are joined by locally melting and solidifying them.

[0056] As described above, in the present invention, it is important to use the laser welding apparatus 10 shown in Figure 3 above to irradiate the overlapping steel plates with a primary laser, and to simultaneously irradiate the area around the primary laser with one or more secondary lasers. This allows for control of the temperature distribution of the molten pool, and the melting width of the molten pool on the surface and back of the steel plates can be set to the range defined by equation (1). Furthermore, the molten pool in the overlapping portion of the steel plates can also be set to the range defined by equation (2). The welding conditions will be described in detail below.

[0057] <Welding Process> In this welding process, the positional relationship between the main laser and the sub-laser, as well as the output relationship between each laser, satisfy the following welding conditions.

[0058] [Steel plate] In this invention, at least one plated steel sheet may be included among the two or more steel sheets to be superimposed. This plated steel sheet refers to alloyed hot-dip galvanized steel sheet (GA), non-alloyed hot-dip galvanized steel sheet (GI), electro-galvanized steel sheet (EG), etc.

[0059] Furthermore, as mentioned above, from the viewpoint of applying the present invention to the manufacture of automobile parts, the steel plates used for laser welding are those with a plate thickness (t) in the range of 0.5 to 4.0 mm. If the plate thickness (t) is less than 0.5 mm, the amount of steel plate to be molten is small, which may cause the weld area to melt through and make welding impossible. On the other hand, if the plate thickness (t) exceeds 4.0 mm, it becomes difficult to melt the back surface of the steel plate to form a keyhole, and welding defects such as undercuts may occur on the unmelted back surface.

[0060] Furthermore, it is preferable that the steel sheet be a cold-rolled steel sheet or a plated steel sheet with a tensile strength of 270 MPa to 2.0 GPa.

[0061] The overlapping steel plates described above may be of the same thickness or different thicknesses. Furthermore, the same type of steel plates may be overlapped, or different types of steel plates may be overlapped. When overlapping plated steel plates, the plates should be overlapped so that the sides with the plating layer face each other.

[0062] Furthermore, no specific provisions are made regarding the composition of the steel plate mentioned above.

[0063] [Welding conditions] In the present invention, laser light is simultaneously irradiated onto the steel plate from a main laser irradiation unit and at least one sub-laser irradiation unit, and the laser light is irradiated under welding conditions such that the sub-laser output P2 (unit: W) and the distance Dce (unit: μm) between the centers of the laser irradiation positions from the main laser irradiation unit and the sub-laser irradiation unit satisfy the following equation (3).

[0064] Here, we will explain the welding conditions using laser welding with the laser beam irradiation configuration shown in Figure 4(A) above as an example. In this example, the number of sub-laser beam points is 16, and the beam diameter of the main and sub-lasers is 300 μm.

[0065] Figure 7(A) shows a magnified view of the laser irradiation unit 13b and its surroundings, and Figure 7(B) shows a magnified view of the area X enclosed by the rectangular frame in Figure 7(A). For ease of understanding, Figure 7(A) shows a cross-section in the thickness direction of the plate on the weld line, and some of the secondary laser beams are omitted from the illustration. Figure 8 shows a diagram illustrating the laser irradiation during welding according to the present invention.

[0066] [Relationship between sub-laser output P2 and the distance Dce between the centers of the main laser irradiation unit and the sub-laser irradiation unit] 0.40 ≦ P2 / Dce ≦ 1.40 …(3)

[0067] In the present invention, the "distance between the centers of the main laser irradiation position and the sub-laser irradiation position Dce" refers to the distance between the irradiation positions of the main laser beam 14a and the sub-laser beam 14b on the steel plate surface, as shown in Figure 7(A). Specifically, as shown in Figure 7(B), the distance between the center of the main laser beam of the main laser beam 14a on the steel plate surface and the center of the sub-laser beam of the sub-laser beam 14b on the steel plate surface is the distance between the centers of the main laser beam 14a on the steel plate surface and the distance between the centers of the sub-laser beam 14b on the steel plate surface.

[0068] For example, in the irradiation configuration shown in Figure 4(A), the sub-lasers are irradiated along the circumference, so Dce is always the radius of the circle. The same applies to the regular polygonal irradiation configuration shown in Figure 4(C); the sub-laser irradiation positions are points on the same circumference, so Dce is always the same value. In the linear irradiation configuration shown in Figure 4(B), where the sub-laser irradiation positions do not lie on the same circumference, Dce is the distance between the center of the main laser irradiation position and the center of each sub-laser irradiation position, and the above welding conditions must be satisfied at all Dce.

[0069] (3) If the value of "P2 / Dce" shown in equation (3) is less than 0.40, the influence of the sub-laser on the temperature distribution of the molten pool near the main laser irradiation position will be insufficient, and the penetration width W on the surface of the steel plate will be small. F The expansion effect cannot be obtained. On the other hand, if the value of "P2 / Dce" shown in equation (3) is greater than 1.40, the penetration width W on the steel plate surface FIf this value becomes excessive, an effective penetration shape for suppressing porosity cannot be obtained. In other words, the penetration shape does not satisfy the relationship shown in equation (1). Also, the heat-affected zone expands excessively, which may cause a decrease in joint strength. Therefore, this value should be between 0.40 and 1.40. This value is preferably 0.50 or higher, and preferably 1.30 or lower. This value is preferably 0.65 or higher, and preferably 1.20 or lower.

[0070] In the present invention, by satisfying these welding conditions, a welded joint having the above-described characteristics can be obtained. According to the present invention, as shown in Figure 8, heat input by the secondary laser beam 14b can be applied to the outer portion of the keyhole formed by the main laser beam 14a. As a result, the penetration shape of the weld becomes such that the penetration width on the surface of the steel plate is wider than the penetration width on the back surface of the steel plate, and as a result, the discharge of air bubbles during welding is promoted and porosity can be suppressed.

[0071] Furthermore, in this invention, the effectiveness can be further improved by having the following welding conditions in addition to the welding conditions described above.

[0072] [Dce: Distance between the centers of the main laser irradiation position and the sub-laser irradiation position] (Preferred condition) In the present invention, the distance Dce between the centers of the main laser irradiation section and the sub-laser irradiation section is preferably 400 μm or more and 1500 μm or less.

[0073] If Dce is less than 400 μm, the distance Dce between the centers of the main and sub-lasers is small relative to the beam diameters, resulting in insufficient heat input dispersion. This makes it difficult to obtain the effect of expanding the surface penetration width through multiple laser irradiations. Therefore, it is preferable that the distance Dce between the centers of the main laser irradiation position and the sub-laser irradiation position be 400 μm or more. It is even more preferable that the distance Dce between the centers be 500 μm or more.

[0074] On the other hand, when the distance Dce between the centers of the main laser irradiation position and the sub-laser irradiation position is greater than 1500 μm, the penetration by the main laser irradiation and the penetration by the sub-laser irradiation do not become one, and an effective penetration shape for suppressing porosity cannot be obtained. In other words, the relationship shown in equation (1) is not satisfied. For this reason, it is preferable that the distance Dce between the centers of the main laser irradiation position and the sub-laser irradiation position be 1500 μm or less. It is more preferable that the distance Dce between the centers be 1300 μm or less.

[0075] [Ratio of main laser output P1 to sub-laser output P2] (Preferred conditions) In the present invention, it is preferable that the welding conditions described above further satisfy equation (4) in terms of the ratio of the main laser output P1 to the sub-laser output P2. 1.8 ≤ P1 / P2 ≤ 16.0 …(4)

[0076] If the ratio of the main laser output P1 (unit: W) to the sub-laser output P2 (unit: W) (i.e., P1 / P2) is less than 1.8, the laser heat input is dispersed, making it difficult to obtain the effect of stabilizing the molten pool. Therefore, it is preferable that the ratio of the main laser output P1 to the sub-laser output P2 be 1.8 or higher. This ratio is more preferably 2.0 or higher, and even more preferably 4.0 or higher.

[0077] On the other hand, if this ratio is greater than 16.0, the effect of heat input from the sub-laser becomes insufficient, making it difficult to obtain a stable molten pool. Therefore, it is preferable that the ratio of the main laser output P1 to the sub-laser output P2 (i.e., P1 / P2) be 16.0 or less. It is more preferable that this ratio be 15.0 or less, and even more preferable that it be 12.0 or less.

[0078] [Relationship between welding speed V, plate thickness t, and main laser output P1] (Optimal conditions) In the present invention, it is preferable that the welding conditions described above further satisfy equation (5) with respect to the welding speed V, the total thickness t of the steel plate, and the main laser output P1. 9.5 ≦ P1 / (V×t) ≦ 28.5 …(5)

[0079] If the value of "P1 / (V×t)", calculated from the welding speed V (unit: mm / s), plate thickness t (unit: mm), and main laser output P1 (unit: W), is less than 9.5, the area (i.e., volume) melted by the laser heat input becomes too small, making the molten pool prone to turbulence. This can destabilize the weld shape and cause porosity. Therefore, it is preferable that this value be 9.5 or higher. It is even more preferable that this value be 9.7 or higher, and even more preferable that it be 11.5 or higher.

[0080] On the other hand, if the value of "P1 / (V×t)" is greater than 28.5, excessive heating of the molten pool by the laser may increase spatter scattering from the molten pool, potentially resulting in an unsatisfactory weld shape. Therefore, it is preferable to keep this value 28.5 or less. It is more preferable to keep this value 23.5 or less, and even more preferable to keep it 22.0 or less.

[0081] [Beam diameters of the main laser and sub-laser] (Preferred conditions) In the present invention, the above welding conditions are further preferably such that the beam diameters of the main and secondary lasers are 200 μm or more and 900 μm or less. If the beam diameters of the main and secondary lasers are less than 200 μm, the area of ​​the molten pool becomes too small, making it difficult to control the penetration width as defined in equation (1). On the other hand, if the beam diameters of the main and secondary lasers are greater than 900 μm, the heat input from the lasers is dispersed, making it impossible to obtain the desired penetration. Therefore, it is preferable that the beam diameters of the main and secondary lasers are 200 μm or more and 900 μm or less. It is more preferable that the beam diameters of each laser are 300 μm or more, and even more preferable that they are 800 μm or less. [Examples]

[0082] The following describes some embodiments of the present invention. These are just examples and are not limited to them.

[0083] First, two or more steel plates were prepared as shown in Table 1, and the steel plates were stacked vertically as shown in Figure 3. Here, the stacked steel plates were numbered from the bottom up, starting with the first plate, the second plate, and so on. Unplated steel plates (cold-rolled steel plates) and plated steel plates were used as the steel plates.

[0084] Next, laser welding was performed on two or more overlapping steel plates under the welding conditions shown in Tables 2 and 3 to fabricate overlapping laser-welded joints. The laser welding apparatus shown in Figure 3, described above, was used for laser welding. A fiber laser oscillator with a maximum output of 12 kW was used as the laser oscillator. The laser irradiation shape from the main and sub-laser irradiation units was shaped by a diffractive optical element inserted into the laser processing head. In the "Sub-laser beam irradiation shape" column of Table 2, it is indicated which of the irradiation shapes shown in Figures 4(A) to 4(C) was used, in this case, one of the circular, linear, or hexagonal irradiation shapes.

[0085] The tensile strength of the steel plate used in this embodiment was in the range of 270 MPa to 2.0 GPa.

[0086] The resulting lap laser-welded joints were used to evaluate the occurrence of porosity according to the following test method. In addition, the penetration width of each weld was measured using the method described above.

[0087] <Measurement of weld penetration depth> In the region of the weld bead 6 of the overlapping laser-welded joint, excluding the bead start and end portions 6a, the thickness-direction cross-section perpendicular to the weld line was observed at five arbitrary locations on the weld bead. Figure 9 shows a diagram illustrating the start and end portions of the weld bead 6. As shown in Figure 9, the "bead start and end portions 6a" refers to the range from the bead start to a length of 10 mm toward the center of the weld bead 6, and the range from the bead end to a length of 10 mm toward the center of the weld bead 6. The "thickness-direction cross-section perpendicular to the weld line" refers to the cross-sectional view along line AA shown in Figure 9.

[0088] Each cross-section was subjected to Nital etching, and then cross-sectional images were taken using an optical microscope to determine the penetration width W of the weld surface.F , the penetration width W on the back of the welded joint B The penetration width W between the plates in the welded joint. C We measured it.

[0089] <Polo City's Evaluation> In the region of the weld bead 6 of the overlapping laser-welded joint, excluding the bead start and end portions 6a, the cross-sections in the thickness direction perpendicular to the weld line were observed at five arbitrary locations on the weld bead.

[0090] Nital etching was performed on each cross-section, and then cross-sectional images were taken using an optical microscope to check for the presence or absence of porosity in the weld (see Figure 2(B)). The occurrence of porosity in the weld at five arbitrary locations was checked and evaluated according to the following criteria. [standard] • If the ratio of the porosity area to the total weld area is 1.0% or less: Pass (Grade A) • If the ratio of porosity area to the total weld area is greater than 1.0% but 3.0% or less: Pass (Grade B) • If the ratio of porosity area to the total weld area is greater than 3.0% but less than or equal to 5.0%: Pass (Evaluation C) • If the ratio of porosity area to weld area is greater than 5.0%: Fail (Grade F)

[0091] [Table 1]

[0092] Overlap laser welding was performed on the plate assemblies shown in Table 1, and the penetration depth and porosity area ratio were evaluated. The evaluation results are shown in Tables 2 and 3.

[0093] [Table 2]

[0094] [Table 3]

[0095] As is clear from Tables 2 and 3, in welds No. 1-12, 15, and 16 (examples of the present invention), overlapping laser-welded joints with a porosity area ratio of 5.0% or less were obtained. Among these that were evaluated as "acceptable," welds No. 1-6 and 16 had a porosity area ratio of 1.0% or less, and welds No. 7-9 and 15 had a porosity area ratio of 3.0% or less, resulting in laser-welded joints with even less porosity.

[0096] In other words, it has been found that the present invention can effectively suppress porosity, which can cause a decrease in joint strength.

[0097] In contrast, welds No. 13-14 (comparative examples) were deemed "failure (evaluation F)" because the porosity area ratio was greater than 5.0%. [Explanation of Symbols]

[0098] 1 steel plate 2 overlapping section 5. Melting pool 4 Polo City 6. Weld bead 6a Bead start and end section 7. Welded section 8. Heat-affected zone 9. Weld metal 10 Laser welding equipment 11. Laser Oscillator 12 Light guide 13 Laser processing head 13a Light-gathering section 13b Laser irradiation area 14a Main laser beam 14b Sub-laser light 20 Laser welding equipment 21 Laser Oscillator 22 Light guide 23 Laser processing head 23a Light-gathering section 23b Laser irradiation area 24 Laser light

Claims

1. It comprises two or more overlapping steel plates and a welded joint, When the superimposed steel plates are treated as follows, with the laser-irradiated side being the surface of the weld and the non-irradiated side being the back surface of the weld, The penetration width W on the surface of the welded joint. Ｆ (mm) and the penetration width W on the back surface of the welded area. Ｂ A lap laser-welded joint in which the total plate thickness t (mm) of the overlapping steel plates satisfies equation (1). 2.10 ≦ (W Ｆ / W Ｂ )+(0.25×t) ≦ 3.80 …(1)

2. The overlapping laser-welded joint according to claim 1, wherein at least one of the steel plates is a plated steel plate.

3. A method for manufacturing an overlapping laser-welded joint according to claim 1 or 2, The welding process involves joining two or more overlapping steel plates using laser welding. In the aforementioned welding process, The welding conditions for the aforementioned laser welding are: Laser light is irradiated onto the steel plate from the main laser irradiation unit and at least one sub-laser irradiation unit. Furthermore, the secondary laser output P ２ A method for manufacturing an overlapping laser-welded joint, wherein (W) and the distance Dce (μm) between the centers of the laser irradiation positions from the main laser irradiation unit and the sub-laser irradiation unit satisfy equation (3). 0.40 ≦ P ２ / Dce ≦ 1.40 …(3)

4. The welding conditions described above are further, Main laser output P １ (W) and the aforementioned sub-laser output P ２ A method for manufacturing an overlapping laser-welded joint according to claim 3, wherein the ratio of (W) satisfies equation (4). 1.8 ≦ P １ / P ２ ≦ 16.0 …(4)

5. The welding conditions described above are further, The welding speed V (mm / s), the total thickness t (mm) of the steel plate, and the main laser output P. １ A method for manufacturing an overlapping laser-welded joint according to claim 3, wherein (W) and (5) satisfy formula (5). 9.5 ≦ P １ / (V×t) ≦ 28.5 …(5)

6. The laser beam from the aforementioned sub-laser irradiation unit is at least one, The method for manufacturing an overlapping laser welded joint according to claim 3, wherein the laser light from the sub-laser irradiation unit is irradiated around the laser light from the main laser irradiation unit.

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