Method for producing laser butt welded joint

Abstract

The purpose of the present invention is to provide a method for producing a laser butt welded joint. The present invention is a method for producing a laser butt welded joint that is formed by welding butting portions of two steel sheets, said method comprising a welding step for joining the steel sheets by simultaneously irradiating the butting portions with laser beams from a main laser emission unit and a sub-laser emission unit, wherein the respective thicknesses t of the steel sheets are 0.5-6.0 mm, and in the welding step, the laser beams are emitted under welding conditions in which the center-to-center distance Dce between the respective laser irradiation positions of the main laser emission unit and the sub-laser emission unit is 300-3000 μm and the ratio between the main laser output P1 and the sub-laser output P2 satisfies formula (1). (1): 1.6≤P1 / P2≤16.0

Description

Method for manufacturing butt laser welded joints

[0001] The present invention relates to a method for manufacturing a butt-welded joint by butt-laser welding multiple thin steel plates together.

[0002] Laser welding uses a high-energy-density laser heat source, resulting in a narrow heat-affected zone and reduced thermal distortion. Therefore, laser welding is used in the manufacturing process of thin steel sheets and the assembly process of automotive parts. In particular, in the automotive parts sector, tailored blanks—where two steel sheets of different thicknesses and strengths are laser-welded together to form a single sheet, which is then press-formed—are attracting attention.

[0003] In these butt laser welding processes, increasing the welding speed is required to improve productivity. However, high-speed butt laser welding can sometimes result in welding defects such as undercuts.

[0004] Butt laser welding has the above-mentioned problems, and patent documents 1 and 2 are examples of technologies that solve these problems. Patent document 1 discloses a plasma arc combined laser welding method. In this plasma arc combined laser welding method, a preceding plasma gas flow is irradiated onto the steel plates at the butt joint without melting them, and welding is performed using a YAG laser or semiconductor laser with a shorter wavelength than a carbon dioxide laser. This makes it possible to prevent humping and to achieve a high welding speed.

[0005] Furthermore, Patent Document 2 discloses a galvanoscanner-type laser welding method in which a galvanoscanner is used to trace a spiral or zigzag trajectory with a laser at the butt joint of two plate materials. With this laser welding method, stable and excellent welding strength can be obtained by controlling the butt joint distance of the plate materials and the amplitude of the laser within an appropriate range.

[0006] Japanese Patent Publication No. 2002-103071, International Publication No. 2020 / 179029

[0007] However, the welding method described in Patent Document 1 controls both laser welding and plasma arc welding, resulting in a large and complex apparatus. Furthermore, it is necessary to determine the welding conditions while considering the interaction between the laser and the plasma arc. Consequently, equipment costs increase, and when the material or plate thickness changes, it takes a considerable amount of time to derive appropriate welding conditions for both laser welding and plasma arc welding.

[0008] Furthermore, in the welding method described in Patent Document 2, the laser irradiation direction is controlled using a galvanometer scanner, which causes the length of the laser path to change. Depending on the target position, the beam shape may become elliptical, reducing the heat input density. As a result, there is a problem in that a consistent welding quality cannot be obtained.

[0009] The present invention has been made in view of the above problems, and aims to provide a method for manufacturing a butt laser-welded joint that uses only a laser heat source for welding, eliminates the need for special operations such as controlling the laser irradiation position during welding, and suppresses undercuts that can cause a decrease in joint strength.

[0010] To solve the above problems, the inventors focused on the laser irradiation conditions that affect the stability of laser welding in welding methods using only a laser heat source, and conducted extensive research. As a result, they found that by simultaneously irradiating the base material with multiple lasers and appropriately controlling the distance between each irradiated laser and the power ratio of each laser, it is possible to suppress the occurrence of undercuts that can cause a decrease in joint strength.

[0011] The present invention is based on the above findings, and its gist is as follows: [1] A method for manufacturing a butt laser welded joint formed by welding the butt joints of two steel plates, comprising a welding step of simultaneously irradiating the butt joint with laser light from a main laser irradiation unit and a sub-laser irradiation unit to join the steel plates, wherein the thickness t of the steel plates is 0.5 to 6.0 mm, and in the welding step, the distance Dce between the centers of the laser irradiation positions from the main laser irradiation unit and the sub-laser irradiation unit is 300 to 3000 μm, and the laser light is irradiated under welding conditions such that the ratio of the main laser output P1 to the sub-laser output P2 satisfies equation (1). 1.6 ≤ P1 / P2 ≤ 16.0 …(1) [2] The welding conditions further include a welding speed V of 30 to 340 mm / s, and the welding speed V, the plate thickness t, and the main laser output P1 satisfying equation (2), as described in [1] above. 24 ≤ P1 / (V × t) ≤ 144 …(2) [3] The welding conditions further include a main laser beam diameter D1 and a sub-laser beam diameter D2 satisfying equation (3), as described in [1] or [2] above. 0.40 ≤ (D1 + D2) / Dce ≤ 3.00 …(3) [4] The welding conditions further include at least one laser beam from the sub-laser irradiation unit, and the laser beam from the sub-laser irradiation unit is irradiated around the laser beam from the main laser irradiation unit, as described in any one of [1] to [3] above.

[0012] According to the present invention, it is possible to provide a method for manufacturing a butt laser-welded joint that can suppress undercuts that may cause a decrease in joint strength.

[0013] Figure 1 is a diagram illustrating the welding state in a conventional butt laser welding apparatus. Figures 2(A) and 2(B) are schematic diagrams illustrating laser irradiation and molten pool during welding in the conventional welding apparatus shown in Figure 1, and Figure 2(C) is a schematic diagram illustrating the cross-section of the welded joint obtained by that welding. Figure 3 is a diagram illustrating the welding state in a butt laser welding apparatus according to one embodiment of the present invention. Figures 4(A) to 4(C) are schematic diagrams illustrating the laser irradiation shape of the present invention. Figures 5(A) and 5(B) are schematic diagrams illustrating the laser irradiation positions of the main and sub-lasers of the present invention. Figures 6(A) and 6(B) are schematic diagrams illustrating laser irradiation and molten pool during welding in the welding apparatus used in the present invention shown in Figure 3, and Figure 6(C) is a schematic diagram illustrating the cross-section of the welded joint obtained by that welding. Figure 7 is a schematic diagram illustrating the bead start and end portions of the weld bead in the present invention. Figure 8 is a schematic diagram illustrating the undercut in the present invention.

[0014] One embodiment of the present invention will be described in detail below. However, the present invention is not limited to this embodiment.

[0015] First, I will explain the technical concept behind this invention.

[0016] Figure 1 shows an example of a conventional laser welding apparatus. Figures 2(A) and 2(B) illustrate the laser irradiation and molten pool shape during butt laser welding using the laser welding apparatus of Figure 1, and Figure 2(C) shows a cross-sectional view in the plate thickness direction of the welded joint obtained by that welding.

[0017] Figure 1 shows a laser welding apparatus being used to butt-weld two steel plates. As shown in Figure 1, a conventional laser welding apparatus 20 includes 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 light-gathering unit 23a in the laser processing head 23 via the light guide 22, and then irradiated onto the butt joint 2 where the end faces of two steel plates 1 are joined together. The laser light 24 is irradiated from a laser irradiation unit 23b provided in the laser processing head 23 to heat the steel plates 1, and the molten pool 3 formed by locally melting the steel plates solidifies to form a weld bead 5, thereby joining the steel plates together. As shown in Figure 1, there is only one laser beam 24.

[0018] In conventional butt welding, as shown in Figures 2(A) and 2(B), a keyhole is formed by irradiation with laser light 24, penetrating to the back surface of the steel plate, and the molten metal cools and solidifies behind the laser irradiation area. In such cases, the temperature difference between the center and the outer edges of the molten pool 3 is relatively large. Also, the width of the molten pool 3 is smaller on the back surface of the steel plate compared to the surface of the steel plate (i.e., the laser irradiation surface). As a result, the solidification timing of the molten pool 3 is earlier on the outer edges than in the center of the molten pool 3, and earlier on the back surface of the steel plate than on the surface of the steel plate. Because of this, the molten metal cannot flow sufficiently, and welding defects such as undercuts occur at the ends of the welded joint. As shown in Figures 2(C) and 8, undercuts 4 occur on the back side and the front side of the steel plate. Therefore, the inventors believe that stress concentration occurs in the welded joint, and this causes a decrease in joint strength. Note that the area indicated by reference numeral 8 in Figure 2(C) is the heat-affected zone, and the area indicated by reference numeral 9 is the weld metal.

[0019] The inventors of this invention have diligently studied methods to effectively suppress undercuts that can cause a decrease in joint strength, and have obtained the following findings.

[0020] In the case of the conventional laser welding apparatus shown in Figure 1, differences in cooling rates occur between the center and outer parts of the molten pool 3, as well as between the surface and back of the steel plate. As a result, the penetration shape becomes an inverted triangle, as shown in Figures 2(B) and 2(C), and an undercut 4 occurs at the end of the weld 7. Therefore, the inventors focused on the cooling rate of the molten pool and considered that if the cooling rate could be made uniform throughout the entire molten pool 3, that is, if the penetration shape could be made into a rectangle with equal melting width on the front and back surfaces of the steel plate (see Figures 6(B) and 6(C) described later), the undercut 4 could be suppressed.

[0021] In order to achieve a uniform cooling rate in the molten pool 3, it is necessary to make the temperature of the molten pool 3 uniform. In conventional technology, it is possible to suppress sputter generation by increasing the beam diameter through defocusing, which shifts the focus of the laser beam, thereby reducing the molten metal flow velocity. However, since the local heat input pattern does not change, it is difficult to achieve a uniform cooling rate in the molten pool 3.

[0022] Therefore, the inventors considered that applying heat input by laser irradiation at multiple points rather than a single point would be effective in achieving a uniform cooling rate. They found that the laser could be divided into a main and a sub-laser, and that the main and sub-laser beams could be irradiated simultaneously (see Figure 3, etc., described later). It should be noted that while many studies have been conducted in recent years on the use of multiple laser beams, their purpose is to suppress sputtering, and the suppression of undercuts by utilizing the simultaneous irradiation of main and sub-laser beams, as in the present invention, has not been studied.

[0023] This invention was completed based on the above technical concept. Details regarding the configuration of the laser welding apparatus and the laser irradiation mode that enable laser irradiation in the butt laser welding (hereinafter sometimes simply referred to as "laser welding") of this invention will be described later.

[0024] 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 an example of butt laser welding according to the present invention.

[0025] 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 figures, 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.

[0026] 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 (for example, a focusing lens) in the laser processing head 13, and focuses the beam with the focusing lens 13a. Then, the main and sub laser beams 14a and 14b are simultaneously irradiated from the laser irradiation unit 13b onto the butt joint 2 of the two steel plates 1. At this time, to prevent oxidation, shielding gas is injected from a shielding gas injection device to shield the butt joint 2. The main and sub laser beams 14a and 14b heat the steel plates 1, and the molten pool 3 formed by locally melting the steel plates solidifies to form a weld bead 5, thereby joining the steel plates 1.

[0027] 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 and a mirror.

[0028] The focusing unit 13a is used to focus the transmitted laser beam, and examples include a collimating lens and a focusing lens.

[0029] The laser irradiation unit 13b is a means for irradiating the butt joint 2 of the steel plate with laser beams 14a and 14b. By irradiating with laser from this laser irradiation unit 13b, it is possible to achieve the shape of the molten portion described in the above technical concept (i.e., to make the melted shape a rectangle in which the melted width is equal on both the front and back surfaces of the steel plate). In this invention, the laser irradiation unit 13b comprises two or more irradiation units. Specifically, the laser irradiation unit 13b comprises one main laser irradiation unit 13b １ and at least one sub-laser irradiation unit 13b ２ It is composed of and .

[0030] Main laser irradiation unit 13b １ is arranged so as to be able to irradiate the main laser beam 14a on the line of the butted portion 2 of the two steel plates (that is, on the welding line). The purpose of the main laser irradiation is to melt the two butted steel plates 1 with the large thermal energy of the main laser and form a keyhole (see Fig. 6(A)) to achieve stable penetration welding.

[0031] Sub - laser irradiation unit 13b ２ is arranged so as to be able to irradiate at least one or more sub - laser beams 14b around the irradiation position of the main laser irradiation unit 13b １ . The purpose of the sub - laser irradiation is to apply heat input that can uniformize the temperature distribution of the molten metal, and thus the cooling rate. Therefore, if the sub - laser irradiation unit 13b ２ is centered on the main laser irradiation unit 13b １ and appropriately arranged around this main laser irradiation unit 13b １ , the above - mentioned purpose can be achieved. For example, as shown in the laser irradiation mode example of Fig. 4(B) described later, the sub - laser irradiation unit 13b ２ may be arranged side by side in front of the main laser irradiation unit 13b １ in the welding direction.

[0032] Note that the method for realizing these beam mode controls is not particularly limited. For example, a method of attaching a beam splitter, a distribution mirror, etc. in the laser processing head 13 to split and shape the laser beam, or a method of attaching a diffractive optical element to branch and shape the laser beam can be mentioned.

[0033] Next, referring to Figs. 4(A) - 4(C), the positional relationship between the main and sub - laser beams irradiated from the laser irradiation unit 13b will be specifically described.

[0034] In the present invention, the laser beam from the sub - laser irradiation unit 13b ２ (hereinafter may be referred to as "sub - laser beam 14b") is at least one, and it is preferable that the sub - laser beam 14b is irradiated around the laser beam from the main laser irradiation unit 13b １ (hereinafter may be referred to as "main laser beam 14a").

[0035] Main laser irradiation unit 13b １ and sub-laser irradiation unit 13b ２ As the irradiation form of the laser light from, for example, the shapes shown in FIGS. 4(A) to 4(C) can be mentioned. As shown in these figures and FIG. 3, one or two or more sub-laser lights 14b are preferably irradiated to any one or more positions on the front side, rear side, right side, and left side in the welding direction with respect to the main laser light 14a.

[0036] For example, as shown in FIG. 4(A), when the irradiation location of the main laser light 14a is the center point, the sub-laser light 14b may be arranged so as to form a circle composed of a point group around the center point. Also, for example, as shown in FIG. 4(B), when the irradiation location of the main laser light 14a is the center point, the sub-laser light 14b may be arranged so as to form a straight line composed of a point group at a position in front of the center point in the welding direction. Also, for example, as shown in FIG. 4(C), when the irradiation location of the main laser light 14a is the center point, the sub-laser light 14b may be arranged so as to form a hexagon composed of a point group around the center point. Although illustration is omitted, the arrangement of the sub-laser light 14b may be a polygonal shape such as a quadrilateral or an octagon.

[0037] As described above, the laser irradiation form shapes the main laser light and the sub-laser light by means of a diffractive optical element inserted into the laser processing head 13 or the like. The point group constituting the sub-laser light may be appropriately adjusted in the number of points, beam diameter, and laser output according to the temperature distribution control of the molten pool 3.

[0038] Next, a method for manufacturing the butt joint laser welded joint of the present invention will be described.

[0039] The present invention is a method for manufacturing a butt joint laser welded joint formed by welding the butt joint portion of two steel plates. This manufacturing method includes at least a welding process of butt-joining two steel plates into one steel plate. In this welding process, the end faces of the two steel plates are abutted against each other, and a plurality of laser lights are simultaneously irradiated from the main laser irradiation unit and the sub-laser irradiation unit of the laser welding apparatus to the butt joint portion, and the abutted steel plates are locally melted and solidified to be joined. The above two steel plates may be steel plates with the same plate thickness or steel plates with different plate thicknesses.

[0040] In the present invention, as described above, it is important to perform the main laser irradiation on the butting portion of the two steel plates and simultaneously perform one or more sub-laser irradiations around the main laser irradiation. This is because it becomes possible to control the temperature distribution of the molten pool and make the cooling rate uniform throughout the molten pool. The welding conditions will be specifically described below.

[0041] <Welding Process> In this welding process, the positional relationship between the main laser and the sub-laser and the output relationship of each laser satisfy the following welding conditions.

[0042] <Steel Plate> From the perspective of applying the present invention to the manufacture of automotive parts as described above, the steel plate used for laser welding is targeted at those with a plate thickness (t) in the range of 0.5 to 6.0 mm. If the plate thickness (t) is less than 0.5 mm, the amount of the molten steel plate is small, so there is a risk that the welded portion will melt and fall off, making it impossible to weld. On the other hand, when the plate thickness (t) exceeds 6.0 mm, it becomes difficult to melt the back surface of the steel plate to form a keyhole, and welding defects such as undercut may occur on the unmelted back surface.

[0043] Also, the steel plate is preferably a cold-rolled steel plate with a tensile strength of 270 MPa to 2.0 GPa.

[0044] Note that the steel plate composition of the above steel plate is not particularly specified.

[0045] <Welding Conditions> In the present invention, laser light is irradiated under welding conditions in which the center-to-center distance Dce between the main laser irradiation position and the sub-laser irradiation position is 300 to 3000 μm, and the ratio of the main laser output P1 to the sub-laser output P2 satisfies the following formula (1).

[0046] Here, taking the laser welding in the laser light irradiation form shown in FIG. 4(A) above as an example, the welding conditions will be described. In this example, the number of sub-laser lights is 16, and the beam diameters of the main and sub-lasers are 300 μm.

[0047] Figure 5(A) shows an enlarged view of the laser irradiation unit 13b and its surroundings, and Figure 5(B) shows an enlarged view of the area X enclosed by the rectangular frame in Figure 5(A). For ease of understanding, Figure 5(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. Figures 6(A) and 6(B) show diagrams illustrating the laser irradiation and molten pool during welding according to the present invention, and Figure 6(C) shows a diagram illustrating the cross-section of the welded joint obtained by the present invention.

[0048] [Dce: Distance between the centers of the main laser irradiation position and the sub-laser irradiation position] In the present invention, the above-mentioned "Dce: Distance between the centers of the main laser irradiation position and the sub-laser irradiation position" refers to the distance (in μm) between the irradiation positions of the main laser beam 14a and the sub-laser beam 14b on the surface of the steel plate, as shown in Figure 5(A). Specifically, as shown in Figure 5(B), the distance between the center of the main laser beam of the main laser beam 14a on the surface of the steel plate and the center of the sub-laser beam of the sub-laser beam 14b on the surface of the steel plate is the above-mentioned distance between the centers Dce.

[0049] 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), where 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.

[0050] By controlling Dce to the specified range described above, as shown in Figures 6(A) and 6(B), heat input from the sub-laser beam 14b can be applied to the outer portion of the keyhole formed by the main laser beam 14a, reducing the temperature difference between the center and outer portion of the molten pool 3 that occurred in conventional butt welding. Furthermore, as the molten metal heated in the outer portion flows sufficiently while maintaining a high temperature, the width of the molten pool 3 on the back surface of the steel plate can also be increased. As a result, as shown in Figure 6(C), a rectangular penetration shape with equal melting width on the front and back surfaces of the steel plate can be obtained, suppressing undercuts 4. Note that the area labeled 8 in Figure 6(C) is the heat-affected zone, and the area labeled 9 is the weld metal.

[0051] When the distance Dce between the centers of the main laser irradiation position and the sub-laser irradiation position, as shown in Figure 5(A), is less than 300 μm, the distance Dce is small relative to the beam diameters of the main and sub-lasers, resulting in insufficient heat dispersion. This makes it difficult to obtain the effect of uniformizing the temperature of the molten pool through multiple laser irradiations. Therefore, the distance Dce between the centers of the main laser irradiation position and the sub-laser irradiation position should be 300 μm or more. Preferably, the distance Dce should be 360 ​​μm or more.

[0052] On the other hand, if the distance Dce between the centers of the main laser irradiation position and the sub-laser irradiation position is greater than 3000 μm, the influence of the sub-laser on the temperature distribution of the molten pool near the main laser irradiation position becomes insufficient, and the effect of uniformizing the temperature of the molten pool may not be obtained. For this reason, the distance Dce between the centers of the main laser irradiation position and the sub-laser irradiation position should be 3000 μm or less. Preferably, this distance Dce should be 2300 μm or less.

[0053] [Ratio of main laser output P1 to sub-laser output P2] 1.6 ≤ P1 / P2 ≤ 16.0 ... (1) By controlling the ratio of the main laser output P1 to the sub-laser output P2 within the specified range described above, as shown in Figures 6(A) and 6(B), the heat input from the sub-laser beam 14b can be appropriately applied to the outer part of the keyhole formed by the main laser beam 14a. This reduces the temperature difference between the center and the outer part of the molten pool 3 that occurred in conventional butt welding. In addition, the molten metal heated in the outer part flows sufficiently while maintaining a high temperature, which also expands the width of the molten pool 3 on the back surface of the steel plate. Furthermore, as shown in Figure 6(B), the solid-liquid boundary is located further back. As a result, as shown in Figure 6(C), a rectangular penetration shape can be obtained in which the melting width on the surface and back surface of the steel plate are equal, and undercuts 4 are suppressed.

[0054] 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 1.6, the heat input from the sub-laser becomes relatively large, resulting in an excessively large melting area, making it difficult to obtain a uniform temperature distribution. Therefore, the ratio of the main laser output P1 to the sub-laser output P2 should be 1.6 or higher. This laser output ratio (P1 / P2) is preferably 2.4 or higher, and more preferably 6.5 or higher.

[0055] On the other hand, if the ratio of the main laser output P1 to the sub-laser output P2 is greater than 16.0, the effect of heat input from the sub-laser becomes insufficient, and the temperature uniformity effect of the molten pool cannot be obtained. For this reason, the ratio of the main laser output P1 to the sub-laser output P2 should be 16.0 or less. Preferably, this laser output ratio should be 15.0 or less.

[0056] The numerical values ​​described regarding the relationship between laser outputs P1 and P2 define the relationship between the main laser and a single sub-laser point.

[0057] By controlling the welding conditions and steel plate thickness as described above, it is possible to suppress spatter scattering, which leads to a decrease in the volume of molten metal, as shown in Figure 6(B), while ensuring sufficient flow of the high-temperature molten metal. As a result, a butt laser-welded joint with a weld that does not have undercuts (see Figure 6(C)) can be obtained.

[0058] Furthermore, in order to more effectively improve the stability of the molten pool, controlling the following factors in addition to the welding conditions mentioned above is also effective.

[0059] In the present invention, it is preferable that the welding speed V is 30 to 340 mm / s, and that the welding speed V, plate thickness t, and main laser output P1 satisfy the following equation (2).

[0060] [Welding speed V: 30 to 340 mm / s] (Preferred conditions) If the welding speed V is less than 30 mm / s, the amount of heat input into the molten pool per unit time becomes excessive, which may result in the weld melting through and making welding impossible. For this reason, it is preferable that the welding speed V be 30 mm / s or more. More preferably, the welding speed V is 40 mm / s or more, and even more preferably 60 mm / s or more.

[0061] On the other hand, when the welding speed V is greater than 340 mm / s, it becomes difficult to stably form keyholes, and undercuts may occur. For this reason, it is preferable to set the welding speed V to 340 mm / s or less. More preferably, the welding speed V should be 320 mm / s or less, and even more preferably 250 mm / s or less.

[0062] [Welding speed V, plate thickness t, and main laser output P1] (Preferred conditions) 24 ≤ P1 / (V × t) ≤ 144 ... (2) 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 24, the area (i.e., volume) that melts due to the heat input from the main laser becomes too small, and the molten pool becomes easily disturbed. This destabilizes the shape of the weld and can cause undercuts. For this reason, it is preferable that the value of "P1 / (V × t)" be 24 or more. More preferably, this value is 26 or more, and even more preferably 30 or more.

[0063] On the other hand, if the value of "P1 / (V×t)" is greater than 144, the heating of the molten pool due to the heat input from the main laser becomes excessive, which increases spatter scattering from the molten pool and may prevent the acquisition of a good weld shape. For this reason, it is preferable to set the value of "P1 / (V×t)" to 144 or less. More preferably, this value should be 70 or less.

[0064] Furthermore, in the present invention, it is preferable that the main laser beam diameter D1 and the sub-laser beam diameter D2 satisfy the following equation (3).

[0065] [Main laser beam diameter D1 and sub-laser beam diameter D2] (Preferred conditions) 0.40 ≤ (D1 + D2) / Dce ≤ 3.00 ... (3) If the value calculated from the main laser beam diameter D1 (unit: μm) and sub-laser beam diameter D2 (unit: μm) shown in Figure 5(B), and the distance Dce between the centers of the main laser irradiation area and the sub-laser irradiation area, i.e., the value of "(D1 + D2) / Dce", is less than 0.40, the influence of the sub-laser on the temperature distribution of the molten pool near the main laser irradiation area will be insufficient. As a result, it will be difficult to obtain a temperature uniformity effect on the molten pool. For this reason, it is preferable that the value of "(D1 + D2) / Dce" be 0.40 or more. More preferably, this value should be 0.48 or more.

[0066] On the other hand, if the value of "(D1 + D2) / Dce" is greater than 3.00, the intercenter distance Dce is small relative to the beam diameters of the main and sub-lasers, resulting in insufficient heat dispersion. This makes it difficult to obtain the effect of uniformizing the temperature of the molten pool through irradiation by multiple lasers. Therefore, it is preferable that the value of "(D1 + D2) / Dce" be 3.00 or less. More preferably, this value should be 2.85 or less, and even more preferably 1.60 or less.

[0067] The following describes some embodiments of the present invention. These are just examples and are not limited to them.

[0068] Two steel plates with the thickness (t) shown in Table 1 were prepared, and laser welding was performed on the butt joint between the ends of the two steel plates using the welding conditions shown in Table 1 to create a butt laser-welded joint. 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.

[0069] Furthermore, the laser irradiation patterns from the main and sub-laser irradiation units were configured using a diffractive optical element inserted into the laser processing head to shape the irradiation pattern shown in Figure 4(A), i.e., the main laser beam at the center point and 16 sub-laser beams on its circumference; the irradiation pattern shown in Figure 4(B), i.e., the main laser beam at the center point and 5 sub-laser beams in a straight line in front of it; and the irradiation pattern shown in Figure 4(C), i.e., the sub-laser beams at the 6 vertices of a hexagon centered on the main laser beam. In this embodiment, No. 8 in Table 1 corresponds to the irradiation pattern in Figure 4(B), and No. 9 corresponds to the irradiation pattern in Figure 4(C). In the column Dce for the center-to-center distance in No. 8 of Table 1, the distances between the center point and the five points are indicated as a range, and the calculated values ​​are also indicated as a range in the (3) equation column of No. 8.

[0070] The tensile strength of the steel plate used in this embodiment was in the range of 270 MPa to 2.0 GPa.

[0071] The resulting butt laser-welded joints were used to evaluate the occurrence of undercuts according to the following test method.

[0072] <Occurrence of Undercuts> In the region of the weld bead 5 of a butt laser-welded joint, excluding the bead start and end portions 6, the cross-sections perpendicular to the weld line in the thickness direction were observed at five arbitrary locations on the weld bead. Figure 7 shows a diagram illustrating the start and end portions of the weld bead 5. As shown in Figure 7, the "bead start and end portions 6" refer to the range from the bead start to a length of 10 mm toward the center of the weld bead 5, and the range from the bead end to a length of 10 mm toward the center of the weld bead 5. The "cross-section perpendicular to the weld line in the thickness direction" refers to the cross-sectional view along line A-A shown in Figure 7.

[0073] Nital etching was applied to each cross-section, and then cross-sectional images were taken using an optical microscope to check for the presence or absence of undercuts 4 on the front and back surfaces of the weld, as shown in Figure 8. The occurrence of undercuts was checked on the front and back surfaces of the weld at five arbitrary locations, i.e., a total of 10 locations, and evaluated according to the following criteria: [Criteria] - If undercuts occur in 2 locations or less: Pass (Evaluation A) - If undercuts occur in more than 2 locations but 5 locations or less: Pass (Evaluation B) - If undercuts occur in more than 5 locations: Fail (Evaluation F)

[0074]

[0075] As is clear from Table 1, in welds No. 1 to 13 (examples of the present invention), butt laser welded joints with five or fewer undercuts were obtained. Of these, welds No. 1 to 9, which were evaluated as "acceptable," had two or fewer undercuts, resulting in butt laser welded joints with even fewer undercuts. In other words, it was found that the present invention can effectively suppress undercuts that can cause a decrease in joint strength.

[0076] In contrast, welds No. 14-17 (comparative examples) were deemed "failure (evaluation F)" because there were more than five instances of undercutting.

[0077] 1 Steel plate 2 Butt joint 3 Weld pool 4 Undercut 5 Weld bead 6 Bead start / end 7 Welded area 8 Heat-affected zone 9 Weld metal 10 Laser welding apparatus 11 Laser oscillator 12 Light guide 13 Laser processing head 13a Focusing unit 13b Laser irradiation unit 14a Main laser beam 14b Sub-laser beam 20 Laser welding apparatus 21 Laser oscillator 22 Light guide 23 Laser processing head 23a Focusing unit 23b Laser irradiation unit 24 Laser beam

Claims

1. A method for manufacturing a butt laser-welded joint formed by welding the butt joints of two steel plates, comprising a welding step of simultaneously irradiating the butt joint with laser light from a main laser irradiation unit and a sub-laser irradiation unit to join the steel plates, wherein the thickness t of the steel plates is 0.5 to 6.0 mm, and in the welding step, the distance Dce between the centers of the laser irradiation positions from the main laser irradiation unit and the sub-laser irradiation unit is 300 to 3000 μm, and the laser light is irradiated under welding conditions such that the ratio of the main laser output P1 to the sub-laser output P2 satisfies equation (1). 1.6 ≤ P1 / P2 ≤ 16.0 …(1) 2. The method for manufacturing a butt laser-welded joint according to claim 1, wherein the welding conditions further include a welding speed V of 30 to 340 mm / s, and the welding speed V, the plate thickness t, and the main laser output P1 satisfy equation (2). 24 ≤ P1 / (V × t) ≤ 144 … (2) 3. The welding conditions further include satisfying equation (3) of the main laser beam diameter D1 and the sub-laser beam diameter D2, as described in claim 1 or 2, for the method of manufacturing a butt laser welded joint. 0.40 ≤ (D1 + D2) / Dce ≤ 3.00 …(3) 4. The method for manufacturing a butt laser welded joint according to claim 1 or 2, wherein there is at least one laser beam from the sub-laser irradiation unit, and the laser beam from the sub-laser irradiation unit is irradiated around the laser beam from the main laser irradiation unit.

5. The method for manufacturing a butt laser welded joint according to claim 3, wherein there is at least one laser beam from the sub-laser irradiation unit, and the laser beam from the sub-laser irradiation unit is irradiated around the laser beam from the main laser irradiation unit.

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