Method for manufacturing welded joint of steel plates

Abstract

In a preparation step of the method for manufacturing a welded joint of steel plates according to the present disclosure, two steel plates are prepared, at least one of which has a carbon content of 0.30 mass% or greater. In a welding step, laser arc hybrid welding is performed on the two steel plates, and F1 defined by expression (1) is 2.10 or more.　(1): F1 = \{(WL + WA) / V\} / \{t × ([C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo / 4 + [V] / 14)\}, wherein: t is a plate thickness (mm) at a welding point; [element symbol] is the content in mass% of a corresponding element of the steel plate having a higher carbon content; WL is an output (kW) of laser welding; WA is an output (kW) of arc welding; and V is a welding speed (m / min).

Description

Manufacturing method for steel plate welded joints

[0001] The present disclosure relates to a method for manufacturing a welded joint, and more particularly to a method for manufacturing a steel plate welded joint.

[0002] BACKGROUND ART Arc welding, laser welding, and laser-arc hybrid welding are known as methods for producing a steel plate welded joint by butt welding or lap welding two steel plates.

[0003] Arc welding is a method in which an arc is generated between a welding wire (electrode) and a steel plate, and the heat of the arc melts and joins the steel. Examples of arc welding include MAG welding, carbon dioxide gas arc welding, MIG welding, and TIG welding. Arc welding is a heat conduction type welding method, and has low penetration. Therefore, it is difficult to increase the welding speed in order to input heat deep into the steel plate. Therefore, it is difficult to improve productivity.

[0004] On the other hand, laser welding is a method of melting and joining steel using a laser beam. Compared to arc welding, laser welding allows heat to be input deeper into steel plates, enabling faster welding speeds. However, laser welding involves a small amount of heat input. Therefore, the cooling rate of the weld metal and the heat-affected zone (HAZ) is rapid. As a result, the hardness of the weld metal and the HAZ may become excessively high.

[0005] Laser-arc hybrid welding is a welding method that combines laser welding and arc welding. Laser-arc hybrid welding can join steel plates at a faster welding speed than arc welding. Furthermore, because welding is performed using a welding wire, it is possible to prevent the hardness of the weld metal from becoming excessively high, as occurs with laser welding. Laser-arc hybrid welding is disclosed, for example, in International Publication No. 2021 / 131560 (Patent Document 1).

[0006] International Publication No. 2021 / 131560

[0007] In the manufacturing process of wear-resistant parts for automotive applications, such as gears, which require wear resistance, a steel plate is cold-worked (pressed) into a predetermined shape to produce an intermediate product, which is then quenched and tempered. The quenched and tempered intermediate product is then welded. The steel plate used as the raw material for such wear-resistant parts may have its hardness increased by quenching. Therefore, the steel plate has excellent hardenability. Therefore, when the quenched and tempered intermediate product is welded, the hardness of the weld metal and the weld heat-affected zone (HAZ) may become excessively high. Therefore, in order to suppress such an increase in hardness of the weld metal and the HAZ, conventionally, the hardness of the weld metal and the HAZ has been suppressed by preheating the intermediate product before welding or by post-welding heating (PWHT) of the weld metal and the HAZ after welding.

[0008] Recently, there has been a demand for higher strength and thinner steel sheets for the purpose of weight reduction. Therefore, in order to achieve higher strength and thinner steel sheets, there is a demand for the application of steel sheets with a higher C content of 0.30% or more than conventional steel sheets. However, the higher the C content of a steel sheet, the more likely it is that the hardness of the weld metal and HAZ after welding will increase. Therefore, there is an increasing need to suppress the increase in hardness of the weld metal and HAZ by performing preheating and / or postheating.

[0009] However, if preheating or postheating is performed, the manufacturing cost increases and the production efficiency decreases. Therefore, there is a need for a manufacturing method that can suppress an excessive increase in hardness of the weld metal and HAZ even when preheating or postheating is omitted.

[0010] An object of the present disclosure is to provide a method for manufacturing a steel plate welded joint that can sufficiently suppress the hardness of the weld metal and HAZ after welding, even when preheating and post-heating are not performed, in the manufacture of a steel plate welded joint including a steel plate having a C content of 0.30% or more by mass.

[0011] The method for manufacturing a steel plate welded joint according to the present disclosure includes a preparation step and a laser-arc hybrid welding step. In the preparation step, two steel plates are prepared, with at least one of the steel plates having a C content of 0.30% or more by mass. In the laser-arc hybrid welding step, laser-arc hybrid welding is performed on the two steel plates to manufacture a steel plate welded joint. The laser-arc hybrid welding step includes a laser welding step and an arc welding step. In the laser welding step, laser welding is performed by scanning a laser beam on the steel plates to weld them. In the arc welding step, arc welding is performed following the scanning of the laser beam. In the laser welding step and the arc welding step, F1 defined by formula (1) is set to 2.10 or more. F1={(WL+WA) / V} / {t×([C]+[Si] / 24+[Mn] / 6+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14)} (1) Here, t in formula (1) is the plate thickness (mm) at the welded portion, and when the welded portion corresponds to a butt weld of two steel plates, t is the plate thickness if the two steel plates have the same plate thickness, or t is the plate thickness (mm) of the thinner of the two steel plates if the two steel plates have different plate thicknesses, and when the welded portion corresponds to a lap weld of two steel plates, t is the sum of the plate thicknesses (mm) of the two steel plates. In formula (1), the [element symbol] is the content by mass% of the corresponding element in the steel plate having the higher C content of the two steel plates, WL is the output (kW) of laser welding, WA is the output (kW) of arc welding, and V is the welding speed (m / min).

[0012] In the manufacturing method of a steel plate welded joint disclosed herein, in the manufacturing of a steel plate welded joint including a steel plate having a C content of 0.30% or more by mass, even when preheating and post-heating are not performed, the hardness of the weld metal and HAZ after welding can be sufficiently suppressed.

[0013] FIG. 1A is a schematic diagram of a steel plate welded joint for explaining t in F1. FIG. 1B is a schematic diagram of a steel plate welded joint different from FIG. 1A for explaining t in F1. FIG. 1C is a schematic diagram of a steel plate welded joint different from FIGS. 1A and 1B for explaining t in F1. FIG. 1D is a schematic diagram of a steel plate welded joint different from FIGS. 1A to 1C for explaining t in F1. FIG. 1E is a schematic diagram of a steel plate welded joint different from FIGS. 1A to 1D for explaining t in F1. FIG. 2 is a schematic diagram of a laser-arc hybrid welding apparatus used in the method for manufacturing a steel plate welded joint of the first embodiment. FIG. 3 is a cross-sectional photographic image perpendicular to the extending direction of the weld metal of a steel plate welded joint manufactured by laser-arc hybrid welding. Fig. 4 shows the Vickers hardness distribution in the width direction of the weld metal from the center position in the width direction of the weld metal in a cross section perpendicular to the extending direction of the weld metal when bead-on welding simulating butt welding was performed on a steel plate having a C content of 0.36% by mass using laser welding, arc welding, and laser-arc hybrid welding. Fig. 5 is a schematic diagram of a test piece used in the Vickers hardness test. Fig. 6 shows the relationship between F1 and the maximum Vickers hardness (HV) of the weld metal in laser-arc hybrid welding when the C content of at least one steel plate is 0.30% or more. Fig. 7 shows the relationship between F1 and the maximum Vickers hardness (HV) of the HAZ in laser-arc hybrid welding when the C content of at least one steel plate is 0.30% or more. Fig. 8 shows the same Vickers hardness distribution as Fig. 4 and is used to explain a method for measuring the softened width. Fig. 9 is a diagram showing the relationship between F1 and the softened width in laser-arc hybrid welding when at least one of the steel plates has a C content of 0.30% or more. Fig. 10 is a schematic diagram for explaining a method for manufacturing a steel plate welded joint according to a second embodiment.

[0014] The present inventors have investigated a method for manufacturing a steel plate welded joint that can sufficiently suppress the hardness of the weld metal and HAZ after welding, even when preheating and post-heating are not performed in the manufacture of a steel plate welded joint including a steel plate having a C content of 0.30% or more by mass.

[0015] Laser-arc hybrid welding, which combines laser welding and arc welding, can achieve a faster welding speed than arc welding alone. Therefore, laser-arc hybrid welding is a welding method suitable for manufacturing methods that increase productivity. In laser-arc hybrid welding, there are two cases: laser welding is performed first and then arc welding (Case 1), and arc welding is performed first and then laser welding (Case 2).

[0016] In laser-arc hybrid welding, the laser welding head is generally positioned so that the laser irradiates perpendicularly to the surface of the steel plate, minimizing the melt depth (i.e., melt depth = plate thickness) in order to maximize the penetration of the laser weld. Furthermore, the arc welding head (arc torch) is positioned at an angle relative to the steel plate. Note that with laser welding, the laser head may be positioned at an angle of 0 to 10 degrees from the perpendicular to the steel plate surface to protect the laser welding optical system from reflected light and spatter generated during welding.

[0017] In arc welding with the above arrangement, Case 1 is welding using the forward welding method, and Case 2 is welding using the backward welding method. In arc welding, the forward welding method (Case 1) produces a wider weld bead width than the backward welding method (Case 2). In the forward welding method, the arc discharges in the direction of travel, so the arc preheats the steel plate. This results in a gentler temperature gradient when the weld metal solidifies, and the cooling rate of the weld metal decreases. As a result, the weld bead width increases. Note that as the cooling rate of the weld metal decreases, the cooling rate of the HAZ also decreases.

[0018] Based on the above considerations, the inventors have concluded that, in the production of steel plate welded joints including steel plates having a C content of 0.30% or more by mass, it is effective to adopt Case 1, in which arc welding is a forward welding method, among the laser-arc hybrid welding processes.

[0019] Therefore, the present inventors investigated and examined factors that cause the hardness of the weld metal and HAZ to increase excessively in the manufacturing method of a steel plate welded joint that is premised on laser-arc hybrid welding in Case 1. As a result, the present inventors obtained the following findings.

[0020] The hardenability of a steel plate correlates with the increase in hardness of the weld metal and HAZ. On the other hand, the increase in hardness of the weld metal and HAZ can be suppressed as the heat input during welding increases. If the heat input is large, the cooling rate after welding decreases. Therefore, the increase in hardness can be suppressed.

[0021] Based on the above findings, the inventors of the present invention have considered that the hardness of the weld metal and HAZ after welding can be suppressed even without preheating or postheating by adjusting the heat input of laser welding and the heat input of arc welding according to the content of elements that contribute to the hardenability of the steel sheet to be welded. Therefore, the inventors of the present invention have investigated the relationship between the content of elements that contribute to the hardenability of the steel sheet, the laser welding output WL (kW), the arc welding output WA (kW), the welding speed V (m / min), and the hardness distribution of the welded steel sheet weld joint after welding. As a result, they have found that the hardness of the weld metal and HAZ after welding can be sufficiently suppressed even without preheating or postheating when F1, defined by the following formula (1), is 2.10 or more. F1={(WL+WA) / V} / {t×([C]+[Si] / 24+[Mn] / 6+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14)} (1) Here, t in formula (1) is the plate thickness (mm) at the welded portion, and when the welded portion corresponds to a butt weld of two steel plates, t is the plate thickness if the two steel plates have the same plate thickness, or t is the plate thickness (mm) of the thinner of the two steel plates if the two steel plates have different plate thicknesses, and when the welded portion corresponds to a lap weld of two steel plates, t is the sum of the plate thicknesses (mm) of the two steel plates. In formula (1), the [element symbol] is the content by mass% of the corresponding element in the steel plate having the higher C content of the two steel plates, WL is the output (kW) of laser welding, WA is the output (kW) of arc welding, and V is the welding speed (m / min).

[0022] The method for manufacturing a steel plate welded joint of this embodiment, which has been completed based on the above technical concept, includes the following steps.

[0023] The method for manufacturing a steel plate welded joint of this embodiment includes a preparation step and a laser-arc hybrid welding step. In the preparation step, two steel plates are prepared, at least one of which has a C content of 0.30% or more by mass. In the laser-arc hybrid welding step, laser-arc hybrid welding is performed on the two steel plates to manufacture a steel plate welded joint. The laser-arc hybrid welding step includes a laser welding step and an arc welding step. In the laser welding step, laser welding is performed by scanning a laser beam on the steel plates to weld them. In the arc welding step, arc welding is performed following the scanning of the laser beam. In the laser welding step and the arc welding step, F1 defined by formula (1) is set to 2.10 or more. F1={(WL+WA) / V} / {t×([C]+[Si] / 24+[Mn] / 6+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14)} (1) Here, t in formula (1) is the plate thickness (mm) at the welded portion, and when the welded portion corresponds to a butt weld of two steel plates, t is the plate thickness if the two steel plates have the same plate thickness, or t is the plate thickness (mm) of the thinner of the two steel plates if the two steel plates have different plate thicknesses, and when the welded portion corresponds to a lap weld of two steel plates, t is the sum of the plate thicknesses (mm) of the two steel plates. In formula (1), the [element symbol] is the content by mass% of the corresponding element in the steel plate having the higher C content of the two steel plates, WL is the output (kW) of laser welding, WA is the output (kW) of arc welding, and V is the welding speed (m / min).

[0024] 1A to 1E are schematic diagrams of a steel plate welded joint for explaining t in F1. Referring to Fig. 1A and Fig. 1B, when the welded portion corresponds to a butt weld 51 (also referred to as weld metal 51) of two steel plates 50A and 50B, t in F1 is the thickness TH of the steel plate 50A. Ａ (mm) and the thickness TH of the steel plate 50B Ｂ In the case of FIG. 1A, the thickness TH of the steel plate 50A is Ａ (mm) is the thickness TH of the steel plate 50B Ｂ (mm). In this case, t is the plate thickness TH Ａ (or plate thickness TH ＢIn the case of FIG. 1B, the thickness TH of the steel plate 50B is Ｂ (mm) is the thickness TH of the steel plate 50A Ａ (mm) thinner than the plate thickness TH Ｂ (mm).

[0025] As shown in FIGS. 1C to 1E, when the welding location corresponds to the lap weld 51 of two steel plates 50A and 50B, t is the thickness TH of the steel plate 50A. Ａ (mm) and the thickness TH of the steel plate 50B Ｂ (mm).

[0026] In the method for manufacturing a steel plate welded joint of this embodiment, the laser welding power WL (kW), the arc welding power WA (kW), and the welding speed V (m / min) are adjusted based on the carbon equivalent, which is an index of the hardenability of two steel plates, to set F1 to 2.10 or more. In this case, when two steel plates, at least one of which has a C content of 0.30% by mass or more, are welded together to form a steel plate welded joint, the maximum Vickers hardness of the weld metal and HAZ can be reduced to or below the hardness of the base metal region, which is the region other than the HAZ of the quenched steel plate. Preferably, the Vickers hardness of the weld metal and HAZ can be reduced to or below 500 HV.

[0027] In the laser welding process and the arc welding process, F1 may be set to 2.80 or less.

[0028] In this embodiment, in the steel plate constituting the steel plate welded joint, the region other than the weld and HAZ that is not affected by the heat of welding is referred to as the "base metal region." Furthermore, when welding is performed, the region of the weld metal and HAZ that is lower in hardness than the base metal region in a cross section perpendicular to the extending direction of the weld metal is referred to as the "softened region." Softened regions always occur during welding. In the hardness distribution of a steel plate welded joint, it is preferable that the number of regions with locally low hardness be as small as possible. In other words, it is preferable that the softened region be as narrow as possible.

[0029] If F1 is set to 2.80 or less, the width of the softened region in a cross section perpendicular to the extending direction of the weld metal can be narrowed. For example, the width of the softened region can be kept to less than 10.00 mm.

[0030] The method for manufacturing a steel plate welded joint of this embodiment will be described below.

[0031] [First embodiment] Fig. 2 is a schematic diagram of a laser-arc hybrid welding apparatus used in a manufacturing method of a steel plate welded joint according to the first embodiment. The lower part of Fig. 2 is a plan view of two steel plates to be welded during butt welding.

[0032] Referring to FIG. 2 , laser-arc hybrid welding apparatus 1 includes a laser welding section 10 , an arc welding section 20 , a welding position adjusting section 30 , and a control device 40 .

[0033] The laser welding portion 10 includes a laser oscillator 11, a laser head 12, and a cable 13. The laser oscillator 11 generates laser light 14. The laser light 14 may be a fiber laser, a solid-state laser such as a YAG laser, or a gas laser such as a carbon dioxide laser. The laser head 12 is connected to the laser oscillator 11 via the cable 13. The cable 13 is, for example, an optical fiber.

[0034] The laser head 12 is disposed above the butt joint of two steel plates 50A and 50B to be butt-welded. In the following description, the steel plates 50A and 50B are collectively referred to as steel plates 50. The laser head 12 irradiates the butt joint of the steel plates 50 with laser light 14 transmitted from the laser oscillator 11 via a cable 13 to form a weld metal 51. In Fig. 2, the laser head 12 may receive the laser light 14 from the laser oscillator 11 via a mirror (not shown) instead of the cable 13.

[0035] The arc welding section 20 includes a power supply 21, an arc torch 22, and cables 23 and 24. The power supply 21 is connected to the arc torch 22 via the cable 23. The power supply 21 is further electrically connected to the steel plate 50 via the cable 24. A steel welding wire 25 is supplied to the arc torch 22 as an electrode. The arc torch 22 is an arc torch used for one or more of MAG welding, carbon dioxide gas arc welding, MIG welding, and TIG welding.

[0036] The power supply device 21 applies a voltage between the arc torch 22 and the steel plate 50 to generate an arc between the steel welding wire 25 and the steel plate 50 .

[0037] The welding position adjustment unit 30 adjusts the relative positions of the laser head 12 and the arc torch 22 with respect to the steel plate 50, and moves the laser head 12 and the arc torch 22 relative to the steel plate 50 during laser-arc hybrid welding. The welding position adjustment unit 30 may move the laser head 12 and the arc torch 22 integrally relative to the steel plate 50, or may move the laser head 12 and the arc torch 22 separately relative to the steel plate 50. The welding position adjustment unit 30 may move the laser head 12 and the arc torch 22 relative to the steel plate 50 while the steel plate 50 is fixed, or may move the steel plate 50 relative to the laser head 12 and the arc torch 22 while the laser head 12 and the arc torch 22 are fixed.

[0038] The control device 40 controls the laser-welded portion 10, the arc-welded portion 20, and the welding position adjustment unit 30. Specifically, the control device 40 adjusts the irradiation timing and output power WL (kW) of the laser beam 14 for laser welding. The control device 40 also adjusts the output timing of the arc for arc welding. The control device 40 also controls the welding position adjustment unit 30 to adjust the positions of the laser head 12 and the arc torch 22 and the welding speed V (m / min). The control device 40 also adjusts the output power WA (kW) for arc welding.

[0039] The control device 40 is a computer including a CPU, memory, a storage device, and an interface (not shown). The code of a control program stored in the storage device is read into the memory, and the CPU retrieves and executes the code from the memory, thereby causing the control device 40 to control the laser welding section 10, the arc welding section 20, and the welding position adjustment section 30. Note that the control device 40 may have a configuration other than a computer including a CPU, memory, a storage device, and an interface.

[0040] [Method for manufacturing a steel plate welded joint according to this embodiment] The above-described method for manufacturing a steel plate welded joint using the laser-arc hybrid welding apparatus 1 includes a preparation step and a laser-arc hybrid welding step. Each step will be described below.

[0041] [Preparation Step] In the preparation step, two steel plates 50A and 50B are prepared. At least one of the steel plates 50 (steel plate 50A and steel plate 50B) has a C content of 0.30% or more by mass. The steel plate 50 may be, for example, a high-tensile steel plate or a high-carbon steel plate. The steel plate 50A may be of the same steel type as the steel plate 50B, or may be of a different steel type. As the steel plate having a C content of 0.30% or more by mass, a quenched steel plate or an unquenched steel plate may be used. The hardness of the steel plate is not particularly limited. Preferably, a quenched steel plate is used.

[0042] The chemical composition of a steel sheet having a C content of 0.30% or more is not particularly limited, but may contain, for example, by mass%, C: 0.30 to 0.60%, Si: more than 0% but not more than 0.50%, Mn: 0.20 to 1.80%, P: more than 0% but not more than 0.030%, S: more than 0% but not more than 0.030%, Cr: 0 to 0.80%, Mo: 0 to 0.80%, Ni: 0 to 0.80%, Ti: 0 to 0.500%, V: 0 to 0.80%, and B: 0 to 0.0060%, with the balance being Fe and impurities. The steel sheet may also contain, instead of Fe in the above chemical composition, other optional elements in a total amount of 1.0% or less.

[0043] The two prepared steel plates 50A and 50B are butted together. Specifically, the steel plates 50A and 50B are fixed to a stage 60 so that the end of the steel plate 50A and the end of the steel plate 50B are butted together. The cable 24 is connected to the stage 60.

[0044] [Laser-arc hybrid welding process] In the laser-arc hybrid welding process, the steel plates 50A and 50B are butted together in a preparation process, and then laser-arc hybrid welding is performed. At this time, under the control of the control device 40, the welding position adjustment unit 30 moves the laser head 12 and the arc torch 22 relative to the steel plates 50 in the X direction at a welding speed V (m / min). Here, the X direction is the extension direction of the weld metal 51, which corresponds to the extension direction of the butt joint in this embodiment. The laser-arc hybrid welding process includes a laser welding process and an arc welding process. That is, in the laser-arc hybrid welding process of this embodiment, laser welding is first performed at the butt joint of the steel plates 50, and then arc welding is performed.

[0045] [Laser Welding Process] In the laser welding process, the laser head 12 and the arc torch 22 move in the X direction relative to the steel plate 50 at a welding speed V (m / min), while the laser head 12 scans the butt joint between the steel plates 50A and 50B with the laser beam 14 under the control of the control device 40 to perform laser welding. When irradiated with the laser beam 14, the laser beam 14 inputs a sufficient amount of heat from the surface of the steel plate 50 to a deep position. Note that the laser welding is non-filler welding. In other words, no filler metal (filler) is used in laser welding.

[0046] [Arc Welding Process] In the arc welding process, the arc torch 22 applies an arc perpendicularly to the butt joint between the steel plates 50A and 50B under the control of the control device 40 to perform arc welding. A steel welding wire 25 is used in the arc welding. In this embodiment, a general-purpose steel welding wire having a C content of 0.30% or less by mass is used as the steel welding wire 25. The general-purpose steel welding wire is, for example, one or more types selected from the group consisting of solid wires for MAG welding and MIG welding specified in JIS Z 3312:2009 and flux-cored wires for arc welding specified in JIS Z 3313:2009. The chemical composition of the steel welding wire 25 is not particularly limited, but may contain, for example, C: more than 0% and not more than 0.30%, Si: 0% to 1.10%, Mn: 0 to 2.60%, P: more than 0% and not more than 0.030%, S: more than 0% and not more than 0.040%, Cu: 0 to 0.50%, Cr: 0 to 0.70%, Mo: 0 to 0.65%, Al: 0 to 0.50%, one or more of Ti and Zr: 0 to 0.30% in total, and the balance consisting of Fe and impurities.

[0047] Fig. 3 is a cross-sectional photographic image perpendicular to the extending direction of the weld metal of a steel plate welded joint produced by the above-described laser arc hybrid welding. Referring to Fig. 3, the steel plate welded joint produced by laser arc hybrid welding includes a weld metal 51, a weld heat-affected zone (HAZ) 52, and a base metal region 53. The HAZ 52 is a region of the steel plate 50 (50A and 50B) that has been affected by the welding heat. The base metal region 53 is a region of the steel plate 50 (50A and 50B) that has not been affected by the welding heat. A steel plate welded joint having the configuration shown in Fig. 3 is produced by laser arc hybrid welding.

[0048] As described above, in laser-arc hybrid welding, the laser welding process performed before the arc welding process inputs heat from the surface to the deep regions near the butt joint of the steel plates 50. By performing forward arc welding on steel plates in this state, the weld bead width can be increased. This prevents the cooling rate after welding from becoming excessively fast, and prevents an excessive increase in hardness in the weld metal and HAZ.

[0049] [Regarding F1] In the present embodiment, F1 defined by formula (1) is set to 2.10 or more in the laser welding process and the arc welding process. This point will be described below.

[0050] F1 is defined as in formula (1): F1 = {(WL + WA) / V} / {t × ([C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14)} (1) Here, t in formula (1) is the plate thickness (mm) at the welded portion, and when the welded portion corresponds to the butt welded portion of two steel plates 50A and 50B, t is the plate thickness TH of the two steel plates 50A and 50B. Ａ and T.H. Ｂ If they are equal, the plate thickness is set to the plate thickness TH of the two steel plates 50A and 50B. Ａ and T.H. Ｂ If the thicknesses are different, the thickness (mm) of the thinner of the two steel plates is used. The [element symbol] represents the mass % content of the corresponding element of the steel plate with the higher C content between the two steel plates 50A and 50B. Specifically, [C] is substituted with the C content (mass %) of the steel plate with the higher C content between the steel plates 50A and 50B. [Si] is substituted with the Si content (mass %) of the steel plate with the higher C content between the steel plates 50A and 50B. [Mn], [Ni], [Cr], [Mo], and [V] are substituted with the Mn content (mass %), Ni content (mass %), Cr content (mass %), Mo content (mass %), and V content (mass %) of the steel plate with the higher C content between the steel plates 50A and 50B, respectively. For elements not contained, "0" is substituted for the corresponding [element symbol]. WL is the laser welding power (kW). WA is the arc welding output (kW). V is the welding speed (m / min). The arc welding output WA (kW) is calculated by multiplying the current (A) set during arc welding by the voltage (V) / 1000.

[0051] F1 is an index related to the hardness of the weld metal and HAZ. The hardness of the weld metal and HAZ is mainly affected by the chemical composition of the steel plates 50A and 50B to be welded, the laser welding output WL, the arc welding output WA, and the welding speed V. As described above, the C content of the general-purpose steel welding wire 25 used for arc welding is 0.30% or less. Therefore, the C content of the general-purpose steel welding wire 25 is equal to or less than that of the steel plate having a C content of 0.30% or more, which is the target material of this embodiment. Therefore, F1 uses the element contents in the chemical composition of the steel plate.

[0052] FIG. 4 is a diagram showing an example of the Vickers hardness distribution in the width direction of the weld metal from the center position in the width direction of the weld metal in a cross section perpendicular to the extending direction of the weld metal when bead-on welding simulating butt welding is performed on a steel plate having a C content of 0.36% by mass by laser welding, arc welding, and laser-arc hybrid welding.

[0053] 4, in the case of laser welding, the maximum Vickers hardness values ​​at the weld and HAZ exceed the Vickers hardness (500 HV in FIG. 4) of the base metal region of the steel plate, and the hardness increases excessively due to welding. In the case of arc welding, the excessive increase in hardness at the weld and HAZ is suppressed, but the welding speed is 0.3 m / min, which is slower than the welding speed (3.0 m / min) of laser welding and laser-arc hybrid welding, and therefore production efficiency decreases.

[0054] In contrast, in the case of laser-arc hybrid welding, if the F1 value is less than 2.10 (F1 = 1.85 in FIG. 4 ), the Vickers hardness of the weld metal and HAZ exceeds the Vickers hardness of the base metal region of the steel plate (500 HV in FIG. 4 ). However, if the F1 value is 2.10 or more (F1 = 2.24 and 2.95 in FIG. 4 ), the maximum Vickers hardness of both the weld metal and the HAZ is equal to or less than the Vickers hardness of the base metal region (500 HV in FIG. 4 ), and the increase in hardness of the weld metal and the HAZ is sufficiently suppressed to be equal to or less than the Vickers hardness of the base metal region of the steel plate.

[0055] [Method for Measuring Vickers Hardness of Weld Metal and HAZ] Here, the maximum Vickers hardness of the weld metal and the maximum Vickers hardness of the HAZ are measured by the following method. A test specimen including a cross section perpendicular to the extending direction of the weld metal of a steel plate is taken. FIG. 5 is a schematic diagram of the test specimen. Referring to FIG. 5, the cross section 100S of the test specimen 100 is 50 mm in the direction (width direction) perpendicular to the extending direction of the weld metal 51. The test specimen is prepared so that the width center position of the weld metal 51 is located at the width center position of the cross section 100S of the test specimen 100. The cross section 100S is the measurement surface 100S.

[0056] The reference measurement point Pc is the center position of the weld metal on the measurement surface 100S (i.e., the center position of the width of the measurement surface) and the center position of the plate thickness. The reference measurement point Pc is set to the origin "0". Measurement points m are selected from the reference measurement point Pc in one width direction of the measurement surface 100S (hereinafter referred to as the + direction) at 0.25 mm intervals up to +23 mm. A Vickers hardness test in accordance with JIS Z 2244-1:2020 is performed on the selected measurement points m to obtain the Vickers hardness (HV). At this time, the test force is 1.0 kN.

[0057] On the measurement surface 100S, the weld metal 51, the HAZ 52, and the base metal region 53, which is the region of the steel plate 50 (50A and 50B) other than the HAZ 52, are identified by the contrast of the metal structure revealed by etching the measurement surface 100S after the Vickers hardness test. Based on the identified weld metal 51, the HAZ 52, and the base metal region 53 and the obtained Vickers hardness, the maximum Vickers hardness (HV) of the weld metal 51 and the maximum Vickers hardness (HV) of the HAZ 52 are determined by the following method.

[0058] The maximum Vickers hardness value in the region of the weld metal 51 identified by contrast is defined as the maximum Vickers hardness value (HV) of the weld metal 51. The measurement point of the minimum Vickers hardness value (HV) in the region of the HAZ 52 identified by contrast is defined as m0. The maximum value of the Vickers hardness values ​​at a measurement point m in the region of the HAZ 52 that is closer to the reference measurement point Pc than the measurement point m0 ​​is defined as the maximum Vickers hardness value (HV) of the HAZ 52.

[0059] Fig. 6 is a diagram showing the relationship between F1 and the maximum Vickers hardness (HV) of the weld metal in laser-arc hybrid welding when at least one steel plate in the steel plate 50 has a C content of 0.30% or more. Fig. 7 is a diagram showing the relationship between F1 and the maximum Vickers hardness (HV) of the HAZ in laser-arc hybrid welding when at least one steel plate in the steel plate 50 has a C content of 0.30% or more. Figs. 6 and 7 were created from the results obtained in the examples described below, and in this example, a steel plate 50 with a Vickers hardness (Vickers hardness of the base metal region) of 500 HV was used.

[0060] 6 and 7 , as F1 increases, the maximum Vickers hardness values ​​of the weld metal and HAZ decrease. In a welded joint made of steel plate with a C content of 0.30% or more, when F1 is 2.10 or more, the maximum Vickers hardness value of the weld metal becomes equal to or less than the Vickers hardness (500 HV) of the base metal region of the steel plate 50 (see FIG. 6 ), and the maximum Vickers hardness value of the HAZ also becomes equal to or less than the Vickers hardness (500 HV) of the base metal region (see FIG. 7 ). Therefore, in the method for manufacturing a steel plate welded joint of this embodiment, F1 is adjusted to 2.10 or more.

[0061] The lower limit of F1 is preferably 2.50, more preferably 2.70, and even more preferably 3.00.

[0062] As described above, in the manufacturing method of a steel plate welded joint of this embodiment, in the laser-arc hybrid welding process, the arc welding process is performed after the laser welding process, and F1 is adjusted to be 2.10 or more in the laser welding process and the arc welding process. This allows for a faster welding speed than with arc welding alone, while sufficiently suppressing the hardness of the weld metal and HAZ after welding without performing preheating or postheating.

[0063] [Preferable Upper Limit of F1] Preferably, in the above-described method for producing a steel plate welded joint, F1 is further adjusted to 4.50 or less in the laser welding step and the arc welding step of the laser-arc hybrid welding step.

[0064] As described above, in steel plate welded joints, it is required to sufficiently suppress an excessive increase in hardness in the weld metal and HAZ in consideration of workability. In steel plate welded joints, it is also preferable that the width of the softened region (softened width), which is a region whose hardness is lower than that of the base metal region of the steel plate, is as small as possible. The softened region is formed by the weld metal and HAZ. In other words, the softened region is formed in the region affected by the heat during welding.

[0065] The softened width, which is the width of the softened region, is determined by the following method. Based on the method described above in [Method for measuring Vickers hardness of weld metal and HAZ], test specimens are taken from the steel plate weld joint, and the Vickers hardness distribution in the width direction of the steel plate weld joint is obtained. Based on the obtained Vickers hardness, the softened width (mm) is determined by the following method.

[0066] Figure 8 shows the same Vickers hardness distribution as Figure 4 and is a diagram for explaining the method for measuring the softening width. The arithmetic mean value of the Vickers hardness in the range of 20 to 23 mm in the positive direction from the reference measurement point Pc is defined as the average hardness HB (HV) of the base material region (i.e., the steel sheet). Referring to Figure 8, when the Vickers hardnesses of five consecutive measurement points mi, mi+1, mi+2, mi+3, and mi+4 in the positive direction from the measurement point m0 ​​with the minimum Vickers hardness value are all within ±1% of the average hardness HB for the first time, the distance between the measurement point mi closest to the measurement point m0 ​​and the measurement point m0 ​​is defined as the softening width (mm).

[0067] 8, when only arc welding is performed, the softened width Wa is excessively wide. On the other hand, when laser-arc hybrid welding is performed, the softened width Wh is significantly narrower than when only arc welding is performed.

[0068] Fig. 9 is a diagram showing an example of the relationship between F1 and softened width in laser-arc hybrid welding when at least one of the steel sheets has a C content of 0.30% or more, and was created from the results obtained by the test method shown in the examples described later.

[0069] Referring to FIG. 9, if F1 is 4.50 or less, the softened width can be kept below 10.00 mm.

[0070] If a further reduction in the softening width is desired, the preferred upper limit of F1 is 2.80. Referring to Fig. 9, when F1 exceeds 2.80, the softening width remains almost constant even when F1 increases. On the other hand, when F1 is 2.80 or less, the softening width decreases as F1 decreases. In other words, in Fig. 9, an inflection point exists when F1 is near 2.80. Therefore, if a further reduction in the softening width is desired, F1 is preferably set to 2.80 or less.

[0071] [Second embodiment] In the first embodiment, a method for manufacturing a steel plate welded joint using laser arc hybrid welding when two steel plates are butt-welded has been described. However, the method for manufacturing a steel plate welded joint of this embodiment is not limited to butt welding. The method for manufacturing a steel plate welded joint of this embodiment can also be applied to lap welding. Below, a method for manufacturing a steel plate welded joint of this embodiment in the case of lap welding will be described.

[0072] Fig. 10 is a schematic diagram for explaining a method for manufacturing a steel plate welded joint according to a second embodiment. Referring to Fig. 10, the configuration of the laser-arc hybrid welding apparatus 1 is the same as that shown in Fig. 2. A plan view and a side view of steel plates 50 (steel plates 50A and 50B) are shown at the bottom of Fig. 10. In this embodiment, lap welding is performed by vertically overlapping a portion of steel plate 50A with a portion of steel plate 50B.

[0073] When lap welding is performed, laser-arc hybrid welding can be performed in the same manner as the method for manufacturing a steel plate welded joint in Embodiment 1. Hereinafter, a method for manufacturing a steel plate welded joint in the case of lap welding will be described.

[0074] The manufacturing method of this embodiment includes a preparation step and a laser-arc hybrid welding step, similar to the first embodiment.

[0075] In the preparation step, two steel plates to be stacked are prepared. At least one of the steel plates 50 (steel plate 50A and steel plate 50B) has a C content of 0.30% or more by mass. The steel plate 50 may be, for example, a high-tensile steel plate or a high-carbon steel plate. The steel plate 50A may be of the same steel type as the steel plate 50B, or may be of a different steel type.

[0076] In the laser-arc hybrid welding process, after laser welding, arc welding is performed in accordance with the scanning of the laser light. That is, in the laser-arc hybrid welding of the second embodiment, as in the first embodiment, laser welding is first performed at the joint of the steel sheets 50 (the portion where the steel sheets 50A and 50B are overlapped), and then arc welding is performed. In the arc welding, as in the first embodiment, a general-purpose steel welding wire 25 (having a C content of 0.30% or less) is used.

[0077] In the laser-arc hybrid welding process, as in the first embodiment, F1 defined by formula (1) is adjusted to be 2.10 or more. F1 = {(WL + WA) / V} / {t × ([C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14)} (1) Here, t in formula (1) is the plate thickness (mm) of the steel plate at the welding location, and when the welding location corresponds to the lap weld 51 (weld metal 51) of two steel plates 50A and 50B, t is the plate thickness TH of the two steel plates 50A and 50B. Ａ and T.H. Ｂ The element symbol, power WA, power WL, and welding speed V in F1 are defined the same as those in F1 in the first embodiment.

[0078] In the case of lap welding, the plate thickness of the joint may be thicker than in butt welding. Even when the plate thickness of the joint is thick, the laser-arc hybrid welding of this embodiment is effective. Specifically, in the laser-arc hybrid welding process of this embodiment, as described above, laser welding is performed before arc welding. Compared to arc welding, laser welding can input heat from the surface of the steel plate to a deeper position. Therefore, heat can be input throughout the plate thickness of the joint (lapped portion) during laser welding. Furthermore, because forward arc welding is performed, the weld bead width can be widened even when the plate thickness of the joint is thick. As a result, the cooling rate after welding can be prevented from becoming excessively fast, and an excessive increase in hardness in the weld metal and HAZ can be suppressed.

[0079] In this embodiment, F1 is further adjusted to be 2.10 or more. Therefore, as shown in Figures 6 and 7, the hardness of the weld metal and HAZ after welding can be sufficiently suppressed. Note that, in the second embodiment as well, the preferred upper limit of F1 is 4.50. In this case, the softened width after welding can be suppressed to less than 10.00 mm. The more preferred upper limit of F1 is 2.80.

[0080] Steel plates were prepared having the chemical compositions shown in Table 1. The plate thickness TH of steel plate A and steel plate B was 2.6 mm. The V content of both steel plate A and steel plate B was 0%.

[0081]

[0082] The Vickers hardness of steel plate A was determined by the following method. The Vickers hardness (HV) was determined in accordance with JIS Z 2244-1:2020 at any five points at the center position of the plate thickness of steel plate A. The test force was 1.0 kN. The arithmetic mean value of the obtained Vickers hardnesses was taken as the Vickers hardness (HV) of steel plate A. The Vickers hardness of steel plate B was also determined by the same method as steel plate A. The Vickers hardness of both steel plate A and steel plate B was 500 HV.

[0083] Bead-on welding simulating butt welding was performed using the steel plates in Table 1, and weld metal and HAZ were formed on the steel plates with each test number. The length of the weld metal was approximately 100 mm. The welding method used was either laser welding only, arc welding only, or laser-arc hybrid welding. In the "Welding Type" column in Table 2, "Hybrid" indicates that laser-arc hybrid welding was performed. "Laser" indicates that laser welding only was performed. "Arc" indicates that arc welding only was performed. For test numbers in which arc welding or laser-arc hybrid welding was performed, YGW12 (C content: 0.02 to 0.15% by mass) specified in JIS Z 3312:2009 was used as the steel welding wire. For laser welding, non-filler welding was performed.

[0084]

[0085] The output WL (kW) of the laser welding process, the current (A), voltage (V), output WA (kW), welding speed V (m / min) and F1 value of the arc welding process for each test number are shown in Table 2. In this example, bead-on welding simulating butt welding was performed, and therefore the plate thickness TH (mm) of the corresponding steel plate shown in Table 1 was substituted for t in F1.

[0086] [Evaluation Test] A Vickers hardness test was performed on the welded steel plate based on the method described above in [Method for measuring Vickers hardness of weld metal and HAZ]. Specifically, a test specimen 100 including a cross section perpendicular to the extending direction of the weld metal of the steel plate of each test number was taken. The cross section 100S of the test specimen 100 had a dimension of 50 mm in the direction (width direction) perpendicular to the extending direction of the weld metal 51, and its thickness was defined as the plate thickness t (mm). The test specimen 100 was prepared so that the width center position of the weld metal 51 was located at the width center position of the cross section 100S of the test specimen 100. The cross section 100S was defined as the measurement surface 100S.

[0087] The central position of the weld metal on the measurement surface 100S (i.e., the central position of the width of the measurement surface) and the central position of the plate thickness was set as the reference measurement point Pc, and the reference measurement point Pc was set as the origin "0." Measurement points m were selected from the reference measurement point Pc in one width direction of the measurement surface 100S (hereinafter referred to as the + direction) at 0.25 mm intervals up to +23 mm. A Vickers hardness test in accordance with JIS Z 2244-1:2020 was performed on the selected measurement points m to obtain the Vickers hardness (HV). The test force was 1.0 kN.

[0088] In the measurement surface 100S, the weld metal 51, the HAZ 52, and the base metal region 53, which is the region of the steel plate 50 other than the HAZ 52, were identified by the contrast of the metal structure revealed by etching the measurement surface 100S after the Vickers hardness test. Based on the obtained Vickers hardness, the maximum Vickers hardness value in the identified region of the weld metal 51 was defined as the maximum Vickers hardness (HV) of the weld metal 51. In addition, in the region of the HAZ 52 identified by the contrast, a measurement point m0 ​​of the minimum Vickers hardness value (HV) was identified. In the region of the HAZ 52, the maximum value of the Vickers hardness at a measurement point m closer to the reference measurement point Pc than the measurement point m0 ​​where the Vickers hardness was minimum was defined as the maximum Vickers hardness (HV) of the HAZ 52.

[0089] Furthermore, the arithmetic mean value of the Vickers hardness in the range of 20 to 23 mm in the positive direction from the reference measurement point Pc was defined as the average hardness HB (HV) of the base material region 53. When the Vickers hardnesses of all five consecutive measurement points mi, mi+1, mi+2, mi+3, and mi+4 in the positive direction from the measurement point m0 ​​with the minimum Vickers hardness value were within the average hardness HB ±1% for the first time, the distance between the measurement point mi closest to the measurement point m0 ​​and the measurement point m0 ​​was defined as the softening width (mm).

[0090] The determined maximum Vickers hardness (HV) of the weld metal is shown in the "Weld metal hardness MAX (HV)" column in Table 2. The determined maximum Vickers hardness (HV) of the HAZ is shown in the "HAZ hardness MAX (HV)" column in Table 2. The determined softened width (mm) is shown in the "Softened width (mm)" column in Table 2.

[0091] [Evaluation Results] Referring to Table 2, in Test Nos. 1, 2, 4, 5, and 11, laser-arc hybrid welding was used as the welding method, and F1 was 2.10 or more. Therefore, even when the C content of the steel plate was 0.30% or more, the maximum Vickers hardness of the weld metal and HAZ after welding was equal to or less than the Vickers hardness (500 HV) of the base metal region of the steel plate, and the hardness of the weld metal and HAZ was sufficiently suppressed. Furthermore, F1 in Test Nos. 1, 2, 4, 5, and 11 was 4.50 or less. Therefore, the softened width was less than 10.00 mm, and the softened width was sufficiently suppressed. Among Test Nos. 1, 2, 4, 5, and 11, F1 in Test Nos. 1 and 4 was 2.80 or less. Therefore, the softened width was further suppressed compared to Test Nos. 2, 5, and 11.

[0092] On the other hand, in test numbers 3, 10, 12, and 13, although laser-arc hybrid welding was performed, F1 was less than 2.10. Therefore, the maximum Vickers hardness values ​​of the weld metal and HAZ all exceeded 500 HV.

[0093] Only laser welding was performed in test numbers 6 to 8 and 14 to 16. As a result, the maximum Vickers hardness values ​​of the weld metal and HAZ all exceeded the Vickers hardness (500 HV) of the base metal region of the steel plate.

[0094] In test numbers 9 and 17, only arc welding was performed, resulting in a slower welding speed than in laser-arc hybrid welding.

[0095] Steel plates having the chemical compositions shown in Table 1 were prepared. The plate thickness TH of steel plate A and the plate thickness TH of steel plate B were both 2.6 mm. Furthermore, steel plate C having the chemical composition shown in Table 3 was prepared. The plate thickness TH of steel plate C was 1.6 mm. The C content of steel plate C was less than 0.30%.

[0096]

[0097] Steel plate welded joints were manufactured by overlapping steel plate A in Table 1 with steel plate C in Table 3, or by overlapping steel plate B in Table 1 with steel plate C in Table 3, and performing lap welding. Specifically, when steel plate A, which has a higher C content than steel plate C, was used as the target material, lap welding was performed in which the target material was overlapped under steel plate C, and lap welding was performed in which the target material was overlapped on steel plate C. Furthermore, when steel plate B, which has a higher C content than steel plate C, was used as the target material, lap welding was performed in which the target material was overlapped under steel plate C, and lap welding was performed in which the target material was overlapped on steel plate C.

[0098] In all cases, the length of the weld metal was approximately 100 mm. The welding method used was either laser welding only, arc welding only, or laser-arc hybrid welding. In Table 4, "Hybrid" in the "Welding Type" column indicates that laser-arc hybrid welding was performed. "Laser" indicates that laser welding only was performed. "Arc" means that arc welding only was performed. For test numbers in which arc welding or laser-arc hybrid welding was performed, YGW12 (C content: 0.02 to 0.15% by mass) specified in JIS Z 3312:2009 was used as the steel welding wire. For laser welding, non-filler welding was performed.

[0099]

[0100] The output WL (kW) of the laser welding process, the current (A), voltage (V), output WA (kW), welding speed V (m / min), and F1 value of the arc welding process for each test number are shown in Table 4. Note that, since lap welding was performed in Example 2, the sum of the plate thickness TH of the subject material (Steel Plate A or Steel Plate B) and the plate thickness TH of Steel Plate C (i.e., 2.6 (mm) + 1.6 (mm) = 4.2 (mm)) was substituted for t of F1.

[0101] [Evaluation Test] Using the same test method as in Example 1, for each test number, the maximum Vickers hardness of the weld metal of the target material, the maximum Vickers hardness of the HAZ, and the softened width (mm) were determined when lap welding was performed with the target material (steel plate A or steel plate B) overlapped below steel plate C. In Table 4, the respective values ​​are shown in the "Weld metal hardness MAX (HV)" column under "Target material", the "HAZ hardness MAX (HV)" column under "Target material", and the "Softened width (mm)" column under "Target material".

[0102] Furthermore, when lap welding was performed in which the target material (steel plate A or steel plate B) was overlapped on steel plate C, the maximum Vickers hardness of the weld metal of the target material, the maximum Vickers hardness of the HAZ, and the softened width (mm) were determined. In Table 4, the respective values ​​are shown in the "Weld metal hardness MAX (HV)" column under "On target material", the "HAZ hardness MAX (HV)" column under "On target material", and the "Softened width (mm)" column under "On target material".

[0103] [Evaluation Results] Referring to Table 4, in Test Nos. 3, 9, and 10, laser-arc hybrid welding was used as the welding method, and F1 was 2.10 or more. Therefore, even when the C content of the steel plate was 0.30% or more, the maximum Vickers hardness of the weld metal and HAZ after welding was 500 HV or less, and the hardness of the weld metal and HAZ was sufficiently suppressed. Furthermore, F1 of Test Nos. 3, 9, and 10 was 4.50 or less. Therefore, the softened width was less than 10.00 mm, and the softened width was sufficiently suppressed. Note that, among Test Nos. 3, 9, and 10, F1 of Test Nos. 3 and 9 was 2.80 or less. Therefore, the softened width was further suppressed compared to Test No. 10.

[0104] On the other hand, in Test Nos. 1, 2, and 8, although laser-arc hybrid welding was performed, F1 was less than 2.10. Therefore, the maximum Vickers hardness value of the HAZ exceeded the Vickers hardness (500 HV) of the base metal region of the steel plate.

[0105] Only laser welding was performed in Test Nos. 4, 5, 11, and 12. As a result, the maximum Vickers hardness of the weld metal and / or HAZ exceeded the Vickers hardness (500 HV) of the base metal region of the steel plate.

[0106] In test numbers 6, 7, 13 and 14, only arc welding was performed, resulting in a slower welding speed than in laser-arc hybrid welding.

[0107] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

[0108] REFERENCE SIGNS LIST 1 Laser arc hybrid welding device 10 Laser welded portion 20 Arc welded portion 14 Laser light 50, 50A, 50B Steel plate 51 Weld metal

Claims

1. A method for manufacturing a steel plate welded joint, comprising: a preparation step of preparing two steel plates, at least one of which has a C content of 0.30% by mass or more; and a laser-arc hybrid welding step of performing laser-arc hybrid welding on the two steel plates to manufacture the steel plate welded joint, wherein the laser-arc hybrid welding step includes: a laser welding step of performing laser welding by scanning a laser beam on the steel plates to weld them; and an arc welding step of performing arc welding in accordance with the scanning of the laser beam, and wherein in the laser welding step and the arc welding step, F1, defined by formula (1), is set to 2.10 or more. F1={(WL+WA) / V} / {t×([C]+[Si] / 24+[Mn] / 6+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14)} (1) Here, t in formula (1) is the plate thickness (mm) at the welded portion, and when the welded portion corresponds to a butt weld of two steel plates, if the plate thicknesses of the two steel plates are equal, t is the plate thickness, and if the plate thicknesses of the two steel plates are different, t is the plate thickness (mm) of the thinner of the two steel plates, and when the welded portion corresponds to a lap weld of two steel plates, t is the sum (mm) of the plate thicknesses of the two steel plates. In formula (1), the [element symbol] is the content by mass % of the corresponding element in the steel plate having the higher C content of the two steel plates, WL is the output (kW) of the laser welding, WA is the output (kW) of the arc welding, and V is the welding speed (m / min).

2. A method for manufacturing a steel plate welded joint according to claim 1, wherein F1 is set to 2.80 or less in the laser welding process and the arc welding process.

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