Laser welded joint and manufacturing method thereof

Abstract

A laser-welded joint having excellent press formability of the welded portion, even when using steel plates with a tensile strength of 980 MPa or more, is provided. The laser-welded joint has a first steel plate, a second steel plate, and a butt weld between the first and second steel plates, in which either or both of the first and second steel plates are high-strength steel plates with a tensile strength of 980 MPa or more, and the weld heat-affected zone of the high-strength steel plates satisfies the relationship of the following formula (1): H s-max -H s-min ≦200 (1)

Description

[Technical Field]

[0001] The present invention relates to a laser welded joint and a manufacturing method thereof. In particular, the present invention relates to a laser welded joint used in a tailored blank (hereinafter also referred to as a laser welded joint for a tailored blank) and a manufacturing method thereof. [Background technology]

[0002] A tailored welded blank (TWB) is a single blank made by welding multiple steel plates of different thicknesses and materials together using laser welding or other methods before press forming. For example, in the automotive field, the application of tailored blanks enables the optimal placement of the steel plates used as raw materials. This allows for both lighter car bodies and collision safety. Specifically, tailored blanks are used for automotive parts such as door inners, side panel inners, and side members.

[0003] As a technology related to tailored blanks, for example, Patent Document 1 describes: "A laser-butt-welded thin steel plate, characterized in that the width of the heat-affected softened zone near the weld of the welded steel plate is 25% or less of the plate thickness, and the tensile strength of at least one of the base materials is 780 MPa or more." has been disclosed.

[0004] Furthermore, Patent Document 2 states: "A method for strengthening a butt-welded workpiece, characterized by plastically deforming a heat-affected softened portion near the weld of the workpiece, which is made by butt-welding plate materials together." has been disclosed. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-218500 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-082685 Summary of the Invention [Problem to be solved by the invention]

[0006] One of the required properties of laser welded joints for tailored blanks is crack resistance in the weld and weld heat-affected zone during press forming (hereinafter also referred to as press formability of the weld). For example, if excessive strain is concentrated in the weld or weld heat-affected zone during press forming of a tailored blank, cracks will occur originating from the strain-concentrated area.

[0007] In recent years, steel sheets for automobiles have been made even stronger, and when high-strength steel sheets are used for tailored blanks, excellent press formability of welded portions is also required.

[0008] However, with the techniques disclosed in Patent Documents 1 and 2, when high-strength steel sheets are used for tailored blanks, excellent press formability of the welded joints may not necessarily be obtained, and improvements in this area are currently desired.

[0009] The present invention was developed in light of the above-mentioned current situation, and aims to provide a laser-welded joint that exhibits excellent press formability of the weld, even when using high-strength steel plate, particularly steel plate with a tensile strength of 980 MPa or more. Another aim of the present invention is to provide a method for manufacturing the above-mentioned laser-welded joint. Note that in this disclosure, any numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits, respectively. [Means for solving the problem]

[0010] The inventors have conducted extensive research to achieve the above object and have come to the following findings. (1) To improve the press formability of welds, it is important to prevent fractures due to cracks originating from the softened HAZ by suppressing strain concentration in the softened area (hereinafter referred to as the HAZ) of the weld heat-affected zone (hereinafter referred to as the HAZ). (2) In particular, during press forming of a tailored blank, the tailored blank undergoes bending deformation, which tends to cause strain to concentrate in the surface layer of the HAZ-softened portion. Here, the surface layer refers to the surface layer on the laser incident side, more specifically, the region up to a depth of 300 μm from the surface on the laser incident side. Furthermore, when a high-strength steel plate, particularly a steel plate with a tensile strength of 980 MPa or more, is used for the tailored blank, strain concentration in the surface layer of the HAZ-softened portion becomes significant. (3) In order to prevent such strain concentration in the softened HAZ, it is effective to minimize the difference in hardness in the HAZ. More specifically, by keeping the difference between the maximum and minimum Vickers hardness in the surface layer of the HAZ of high-strength steel sheet to 200 or less, it becomes possible to obtain excellent press formability of the weld, even when using steel sheet with a tensile strength of 980 MPa or more. (4) In addition, in order to make the difference between the maximum Vickers hardness and the minimum Vickers hardness in the surface layer of the HAZ of a high-strength steel plate 200 or less, it is important to properly control the laser welding conditions, in particular, the relationship between the laser power, the welding speed, and the average Vickers hardness in the surface layer and at the center of the plate thickness of the high-strength steel plate to be welded. The present invention was completed based on the above findings and further investigations.

[0011] That is, the gist and configuration of the present invention are as follows. 1. A laser welded joint having a first steel plate, a second steel plate, and a butt weld between the first steel plate and the second steel plate, Either one or both of the first steel plate and the second steel plate is a high-strength steel plate having a tensile strength of 980 MPa or more, A laser welded joint in which the weld heat affected zone of the high strength steel plate satisfies the relationship of the following formula (1): H s-max -Hs-min ≦200 (1) where: H s-max : Maximum Vickers hardness of the surface layer on the laser incident side of the weld heat affected zone of high strength steel plate, H s-min : Minimum Vickers hardness of the surface layer on the laser incident side of the weld heat affected zone of high strength steel plate, is.

[0012] 2. The laser welded joint according to 1 above, wherein the base metal portion of the high-strength steel plate satisfies the relationship of the following formula (2). 0.20≦H bs / H bm ≦0.90 (2) where: H bm : Average Vickers hardness at the center of the thickness of the base material of the high-strength steel plate, H bs : Average Vickers hardness of the surface layer on the laser incident side of the base material of the high-strength steel plate, is.

[0013] 3. A laser welded joint as described in 1 or 2 above, wherein the weld heat affected zone of the high strength steel plate satisfies the relationship of the following formula (3): 50≦H m-min -H s-min ≦300 (3) where: H m-min : Minimum Vickers hardness at the center of the plate thickness in the heat-affected zone of a high-strength steel plate, H s-min : Minimum Vickers hardness of the surface layer on the laser incident side of the weld heat affected zone of high strength steel plate, is.

[0014] 4. A method for manufacturing a laser welded joint, comprising: The method includes a step of performing laser welding on a workpiece formed by butting a first steel plate and a second steel plate together to obtain a laser welded joint, Either one or both of the first steel plate and the second steel plate is a high-strength steel plate having a tensile strength of 980 MPa or more, A method for manufacturing a laser welded joint, wherein the laser welding satisfies the relationship of the following formula (4). (V / 3)+2≦P≦[(V / 3)+9]×(H bm / H bs ) ···(4) where: P: Laser power (kW), V: welding speed (m / min), H bm : Average Vickers hardness at the center of the thickness of high-strength steel plate, H bs : Average Vickers hardness of the surface layer on the laser incident side of the high-strength steel plate, is.

[0015] 5. The high-strength steel plate satisfies the following formula (2): 5. The method for producing a laser welded joint according to 4 above, wherein the laser welding satisfies the relationship of the following formula (5). 0.20≦H bs / H bm ≦0.90 (2) (V / 3)+(D / 10)+2≦P≦[(V / 3)+8]×(H bm / H bs )+(D / 10) ···(5) where: P: Laser power (kW), V: welding speed (m / min), D: Defocus amount (mm) of the focal position relative to the surface of the workpiece during laser irradiation, H bm : Average Vickers hardness at the center of the thickness of high-strength steel plate, H bs : Average Vickers hardness of the surface layer on the laser incident side of the high-strength steel plate, is. [Effects of the Invention]

[0016] According to the present invention, even when using steel plate with a tensile strength of 980 MPa or more, it is possible to obtain a laser-welded joint that exhibits excellent press formability at the weld, and more preferably, a laser-welded joint that exhibits excellent static tensile properties in addition to the press formability at the weld. By applying the laser-welded joint of the present invention to automobile parts, such as tailored blanks for vehicle body frame parts, it is possible to reduce the vehicle weight (and thus reduce CO2 emissions during vehicle operation) while further improving the safety performance of the automobile. Furthermore, the laser-welded joint of the present invention has a high degree of freedom in shape and can be manufactured with high production efficiency and consistent quality, making it extremely advantageous in terms of manufacturability. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 2 is a schematic diagram showing an example of a cross section of a laser welded joint. [Figure 2] FIG. 10 is a schematic diagram illustrating an example of a defocus amount. [Figure 3] FIG. 10 is a schematic diagram illustrating an example of a defocus amount. [Figure 4] FIG. 2 is a schematic diagram showing an example of a crack form in an Erichsen test. [Figure 5] FIG. 2 is a schematic diagram showing an example of a crack form in an Erichsen test. [Figure 6] FIG. 2 is a schematic diagram showing an example of a crack form in an Erichsen test. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will be described based on the following embodiments: First, a laser welded joint according to one embodiment of the present invention will be described.

[0019] [1] Laser welded joints A laser welded joint according to one embodiment of the present invention comprises: A laser welded joint having a first steel plate, a second steel plate, and a butt weld between the first steel plate and the second steel plate, Either one or both of the first steel plate and the second steel plate is a high-strength steel plate having a tensile strength of 980 MPa or more, The weld heat affected zone of the high strength steel plate satisfies the relationship of the following formula (1). H s-max -H s-min ≦200 (1) where: H s-max : Maximum Vickers hardness in the surface layer of the weld heat affected zone of high strength steel plate, H s-min : Minimum Vickers hardness in the surface layer of the weld heat affected zone of high strength steel plate, is.

[0020] [1-1] First steel plate and second steel plate In a laser welded joint according to one embodiment of the present invention, a high-strength steel plate having a tensile strength of 980 MPa or more is used for either or both of the first steel plate and the second steel plate that constitute the laser welded joint.

[0021] Tensile strength: 980 MPa or more When the strength of the steel plate constituting a laser welded joint increases, particularly when the tensile strength reaches 980 MPa or more, the Vickers hardness of the entire laser welded joint, including the butt weld and the weld heat-affected zone, which are made of weld metal, increases. This leads to strain concentration in the surface layer of the softened HAZ, which in turn significantly reduces the press formability of the weld. In other words, laser welded joints using steel plates with a tensile strength of 980 MPa or more require improved press formability of the weld. Therefore, the tensile strength of the high-strength steel plate is set to 980 MPa or more, preferably 1180 MPa or more. There is no particular upper limit to the tensile strength of the high-strength steel plate. For example, the tensile strength of the high-strength steel plate is preferably 2500 MPa or less.

[0022] The chemical composition of the high-strength steel plate is not particularly limited. For example, the chemical composition of the high-strength steel plate may be In mass%, C: 0.04 to 0.40%, Si: 0.01 to 2.50% Mn: 1.00~5.00%, P: 0.050% or less, S: 0.010% or less, Ti: 0 to 0.20% Al: 0.01 to 0.30% and B: 0~0.0100%, and optionally containing one or more optional added elements selected from Cr, Ni, Mo, W, V, Nb, Cu, N, and O in a total amount of 10% or less (if these optional added elements are contained, more preferably in a total amount of 0.1% or more), with the balance being Fe and unavoidable impurities.

[0023] The above-mentioned high-strength steel plate may be used for either the first steel plate or the second steel plate, and a steel plate having a tensile strength of less than 980 MPa (hereinafter also referred to as a general-purpose steel plate) may be used for the other.Furthermore, the above-mentioned high-strength steel plate may be used for both the first steel plate and the second steel plate.

[0024] In addition, both the first steel sheet and the second steel sheet (in other words, the above-mentioned high-strength steel sheet and / or general-purpose steel sheet used for the first steel sheet and the second steel sheet) may be surface-treated steel sheets (hereinafter also referred to as plated steel sheets) having a metal plating layer on the surface of the base steel sheet. The type and composition of the metal plating layer are not particularly limited. Examples of the metal plating layer include a Zn-based plating layer (a plating layer with a Zn content of more than 50% by mass) and an Al-based plating layer (a plating layer with an Al content of more than 50% by mass). When corrosion resistance is required, a Zn-based plating layer is preferable to an Al-based plating layer. This is because a Zn-based plating layer reduces the corrosion rate of the base steel sheet due to the sacrificial corrosion protection effect of Zn. Examples of Zn-based plating layers include hot-dip galvanized layers (GI), galvannealed layers (GA), electrogalvanized layers (EG), Zn-Ni-based plating layers (e.g., plating layers containing 10 to 25 mass% of Ni in addition to Zn), Zn-Al-based plating layers, Zn-Mg-based plating layers, and Zn-Al-Mg-based plating layers. Examples of Al-based plating layers include Al-Si-based plating layers (e.g., plating layers containing 10 to 20 mass% of Si in addition to Al). The coating weight of the metal plating layer on the plated steel sheet is not particularly limited. For example, the coating weight of the metal plating layer is set to 120 g / m per side from the viewpoint of weldability. 2 In addition, the coating weight of the metal plating layer is preferably 20 g / m per side from the viewpoint of ensuring rust prevention. 2 It is preferable that the above is set.

[0025] The thickness of the first steel plate and the second steel plate is preferably 0.5 mm or more and 5.0 mm or less. The thickness of the first steel plate and the second steel plate is more preferably 0.7 mm or more. The thickness of the first steel plate and the second steel plate is more preferably 2.0 mm or less. The thickness of the first steel plate and the second steel plate may be the same or different.

[0026] Furthermore, the high-strength steel plates used for the first steel plate and the second steel plate each have a HAZ adjacent to the butt weld and a base metal adjacent to the HAZ. As will be described later, it is important to reduce the difference between the maximum Vickers hardness and the minimum Vickers hardness in the surface layer of the HAZ of the high-strength steel plate. The butt weld, the HAZ, and the base metal can be defined, for example, as described later.

[0027] [1-2] Butt weld The butt weld that constitutes the laser welded joint according to one embodiment of the present invention is the portion where the first steel plate and the second steel plate are butt-joined, and is made of weld metal. The chemical composition of the weld metal is not particularly limited, and examples include a chemical composition similar to that of the high-strength steel plate, and a chemical composition that is a mixture of the chemical compositions of the first steel plate and the second steel plate.

[0028] [1-3] Vickers hardness In a laser welded joint according to one embodiment of the present invention, it is extremely important that the relationship in equation (1) above is satisfied.

[0029] H s-max -H s-min ≦200 (1) As mentioned above, in order to improve the press formability of the weld, it is important to prevent fracture due to cracks originating from the HAZ softened portion by suppressing strain concentration in the HAZ softened portion. In particular, when a tailored blank is press-formed, the tailored blank is bent and deformed, so strain tends to concentrate in the surface layer of the HAZ softened portion. Furthermore, when a high-strength steel plate, particularly a steel plate with a tensile strength of 980 MPa or more, is used for the tailored blank, strain concentration in the surface layer of the HAZ softened portion becomes significant. In order to suppress such strain concentration in the HAZ softened portion, it is effective to reduce the hardness difference in the HAZ as much as possible. Here, H s-max -H s-minIf H exceeds 200, strain concentration in the softened HAZ is promoted, and sufficient press formability of the weld cannot be obtained, especially when using steel plates with a tensile strength of 980 MPa or more. Therefore, the relationship of the above formula (1) must be satisfied. s-max -H s-min is preferably 150 or less, more preferably 120 or less. s-max -H s-min The lower limit of is not particularly limited and may be 0. However, since it is difficult to completely eliminate the hardness difference in the HAZ when welding high-strength steel plates, H s-max -H s-min is preferably 30 or more. When high strength steel plates are used for both the first steel plate and the second steel plate, the relationship of the above formula (1) is satisfied in the heat-affected zones of the respective high strength steel plates. where: H s-max : Maximum Vickers hardness in the surface layer of the weld heat affected zone of high strength steel plate, H s-min : Minimum Vickers hardness in the surface layer of the weld heat affected zone of high strength steel plate, is.

[0030] 0.20≦H bs / H bm ≦0.90 (2) In a laser welded joint according to one embodiment of the present invention, it is preferable to further satisfy the relationship of the above formula (2). In addition to the press formability of the welded portion, a tailored blank may be required to have excellent static tensile properties. Here, H is the ratio of the average Vickers hardness at the surface layer of the base material of the high-strength steel plate to the average Vickers hardness at the center of the plate thickness of the base material of the high-strength steel plate. bs / H bm By controlling H to the range of 0.20 to 0.90, it is possible to improve the press formability of the welded portion as well as the static tensile properties. Therefore, it is preferable to satisfy the relationship of the above formula (2). bs / H bm is preferably 0.30 or more. bs / H bmis preferably 0.80 or less. When high-strength steel plates are used for both the first steel plate and the second steel plate, it is preferable that the base metal portions of the respective high-strength steel plates satisfy the relationship of the above formula (2). where: H bm : Average Vickers hardness at the center of the thickness of the base material of the high-strength steel plate, H bs : Average Vickers hardness in the surface layer of the base material of high-strength steel plate, is.

[0031] The mechanical properties of the base material of the high-strength steel plate that constitutes the laser welded joint usually remain the same as the mechanical properties of the high-strength steel plate that is the welded material. Therefore, as will be described later, the average Vickers hardness at the center of the plate thickness and the surface layer of the high-strength steel plate that is the welded material is also H bm and H bs It is expressed as follows.

[0032] 50≦H m-min -H s-min ≦300 (3) In a laser welded joint according to one embodiment of the present invention, it is preferable to further satisfy the relationship of the above formula (3). As mentioned above, a tailored blank may be required to have excellent static tensile properties in addition to press formability of the weld. When a tailored blank is press-formed, the tailored blank undergoes bending deformation, which tends to cause strain to concentrate in the surface layer of the HAZ softened portion. On the other hand, when a static tensile load is applied to a tailored blank, strain also tends to concentrate in the HAZ softened portion. However, in this case, the strain is dispersed in the thickness direction of the laser welded joint. Therefore, to improve the static tensile properties, it is necessary to increase the minimum Vickers hardness at the center of the thickness of the HAZ of a high-strength steel sheet, which can be said to be the average hardness in the thickness direction, and thereby to increase the H m-min -H s-min It is important to increase Hm-min -H s-min If H is less than 50, the effect of improving the static tensile properties may be small. m-min -H s-min In order to make H exceed 300, the carbon equivalent of the high-strength steel sheet may be excessively large, which may result in production limitations. Therefore, it is preferable to satisfy the relationship of the above formula (3). m-min -H s-min is preferably 60 or more. m-min -H s-min is preferably not more than 170. When high-strength steel plates are used for both the first steel plate and the second steel plate, it is preferable that the relationship of the above formula (3) be satisfied in the base metal portion of each high-strength steel plate. where: H m-min : Minimum Vickers hardness at the center of the plate thickness in the heat-affected zone of a high-strength steel plate, H s-min : Minimum Vickers hardness in the surface layer of the weld heat affected zone of high strength steel plate, is.

[0033] Also, H s-max , H s-min and H m-min is measured in accordance with JIS Z 2244-1:2020. s-max , H s-min and H m-min is measured, for example, as follows.

[0034] As shown in Figure 1, the laser-welded joint is cut so that the cross section in the plate thickness direction perpendicular to the weld line (welding direction) is the cut surface. Next, the cut surface is polished, and the boundary between the first steel plate and the butt weld (also referred to as the first bond portion) and the boundary between the second steel plate and the butt weld (also referred to as the second bond portion) are determined on the cut surface to define the butt weld. The first bond portion and the second bond portion may be determined by a conventional method. Then, the region of the first steel plate from the first bond portion to a position 5 mm (toward the first steel plate in the direction perpendicular to the weld) (hereinafter also referred to as the HAZ-base metal boundary of the first steel plate) is defined as the HAZ, and the remaining region is defined as the base metal. Similarly, in the second steel plate, the area from the second bond (towards the second steel plate in the direction perpendicular to the weld) to a position 5 mm (hereinafter also referred to as the HAZ-base metal boundary of the second steel plate) is defined as the HAZ, and the other area is defined as the base metal.

[0035] In Figure 1, reference numeral 1 denotes the butt weld, 2 denotes the first steel plate, 2-1 denotes the HAZ, 2-2 denotes the base metal, 3 denotes the second steel plate, 3-1 denotes the HAZ, 3-2 denotes the base metal, 4 denotes the first bond, 5 denotes the second bond, 6 denotes the HAZ-base metal boundary of the first steel plate, and 7 denotes the HAZ-base metal boundary of the second steel plate. Also, x denotes the direction perpendicular to the weld, and y denotes the plate thickness direction. The direction perpendicular to the weld is the direction perpendicular to the welding direction and the plate thickness direction.

[0036] When a high-strength steel plate is used for the first steel plate, a Vickers hardness test conforming to JIS Z 2244-1:2020 is performed at a representative position in the surface layer of the HAZ of the high-strength steel plate (a position 20 μm deep from the surface on the laser incident side of the high-strength steel plate) and at the center of the plate thickness, from the first bond portion to the boundary between the HAZ and the base metal of the first steel plate, at intervals of 200 μm in the direction perpendicular to the weld (x direction), to measure the Vickers hardness. The indentation load is 20 g force, and the indentation time is 15 s. The maximum value of the Vickers hardness measured at the surface layer of the HAZ of the high-strength steel plate is defined as H s-max , the minimum value is H s-min In addition, the minimum value of the Vickers hardness measured at the center of the plate thickness of the HAZ of the high-strength steel plate is defined as H m-minFurthermore, when a high-strength steel plate is used for the second steel plate, a Vickers hardness test is carried out in the same manner as above, and the H value of the high-strength steel plate used for the second steel plate is s-max , H s-min and H m-min Measure.

[0037] Also, H bm and H bs is measured in accordance with JIS Z 2244-1:2020. bm and H bs is measured, for example, as follows.

[0038] The laser welded joint is cut in the same manner as above, and the HAZ and base material of the first steel plate, the butt weld, and the HAZ and base material of the second steel plate are defined on the cut surface as shown in Figure 1. If a high-strength steel plate is used as the first steel plate, a Vickers hardness test is performed in accordance with JIS Z 2244-1:2020 at three arbitrary points, at a representative position in the surface layer of the base material of the high-strength steel plate (a position 20 μm deep from the surface on the laser incident side of the high-strength steel plate) and at the center position of the plate thickness, to measure the Vickers hardness. The indentation load is 20 g force, and the indentation time is 15 s. The average Vickers hardness at the center position of the plate thickness and the surface layer of the base material of the high-strength steel plate is then calculated as H bm and H bs In addition, when a high-strength steel plate is used for the second steel plate, a Vickers hardness test is carried out in the same manner as above, and the H value of the high-strength steel plate used for the second steel plate is bm and H bs Measure.

[0039] In addition, the Vickers hardness (H s-max , H s-min and H bs ) deviation of the measurement position is permissible as long as it is at a depth of 20 μm ± 10 μm from the surface of the high-strength steel plate. However, if the Vickers hardness varies greatly depending on the depth position, the average Vickers hardness at 10 μm, 20 μm, and 30 μm from the surface of the HAZ of the high-strength steel plate is used to calculate H s-max , Hs-min and H bs In addition, the Vickers hardness (H m-min and H bm ) deviation of the measurement position is acceptable as long as it is at a depth position of ±100 μm from the center of plate thickness of high strength steel plate. However, if the Vickers hardness varies greatly depending on the depth position, the average value of the Vickers hardness at the center of plate thickness -100 μm, the center of plate thickness, and the center of plate thickness +100 μm is used to calculate H m-min and H bm It is sufficient to determine the following.

[0040] The configuration other than that described above is not particularly limited, and the same configuration as that of a conventional laser welded joint can be appropriately adopted.

[0041] [2] Laser welded joint manufacturing method Next, a method for manufacturing a laser welded joint according to one embodiment of the present invention will be described.

[0042] A method for manufacturing a laser welded joint according to one embodiment of the present invention includes: The method includes a step of performing laser welding on a workpiece formed by butting a first steel plate and a second steel plate together to obtain a laser welded joint, Either one or both of the first steel plate and the second steel plate is a high-strength steel plate having a tensile strength of 980 MPa or more, In the laser welding, the relationship of the following formula (4) is satisfied. (V / 3)+2≦P≦[(V / 3)+9]×(H bm / H bs ) ···(4) where: P: Laser power (kW), V: welding speed (m / min), H bm : Average Vickers hardness at the center of the thickness of high-strength steel plate, H bs : Average Vickers hardness in the surface layer of high-strength steel plate, is.

[0043] A method for manufacturing a laser welded joint according to one embodiment of the present invention will be described below. Note that the tensile strength, chemical composition, plate thickness, Vickers hardness, etc. of the first and second steel plates to be welded are the same as those described in [1] Laser welded joint above, so will not be described here.

[0044] (V / 3)+2≦P≦[(V / 3)+9]×(H bm / H bs ) ···(4) The inventors have conducted various studies and have found that in order to manufacture a laser welded joint that satisfies the above formula (1), it is necessary to determine the ratio of the average Vickers hardness at the surface layer at the center of the thickness of the high-strength steel plate to the average Vickers hardness at the center of the thickness of the high-strength steel plate to be welded in laser welding. bm / H bs It was found that it is important to properly control the relationship between the laser power P and the welding speed V, which greatly affect the amount of welding heat input, depending on the welding conditions. Here, P is [(V / 3) + 9] × (H bm / H bs ), the welding heat input becomes excessive, and the hardenability increases in a part of the HAZ (the area near the heat input part). In other words, the peak temperature during welding in a part of the HAZ reaches the Ac3 point or higher, and that part becomes a very hard martensite structure. s-max On the other hand, in other areas of the HAZ, there are areas where the peak temperature during welding is below the Ac1 point, and in these areas, tempering of martensite is promoted. s-min decreases. In other words, it becomes extremely difficult to satisfy the above formula (1). On the other hand, if P is less than (V / 3)+2, the welding heat input becomes too small, and welding defects such as undercuts are likely to occur in the weld metal. As a result, geometric stress concentration during press forming becomes significant, and press formability deteriorates. In addition, geometric stress concentration when a static tensile load is applied also becomes significant, and static tensile properties also deteriorate. Therefore, the above formula (4) is satisfied. P is preferably (V / 3)+3 or more. Furthermore, P is preferably [(V / 3)+7]×(H bm / H bs ) is as follows. where: P: Laser power (kW), V: welding speed (m / min), H bm : Average Vickers hardness at the center of the thickness of high-strength steel plate, H bs : Average Vickers hardness in the surface layer of high-strength steel plate, is.

[0045] The inventors have determined that the upper limit of P on the right side of the above formula (4) is H bm / H bs We believe that the reason why it is proportional to H is as follows. bm / H bs When H exceeds 1, it means that the hardness of the surface layer of the high-strength steel plate to be welded is smaller than the hardness at the center of the plate thickness. As the hardness of the surface layer of the high-strength steel plate to be welded decreases, tempering becomes more difficult due to, for example, a decrease in the proportion of martensite in the microstructure and a decrease in carbon equivalent. This makes it possible to s-min Therefore, the upper limit of P is relatively suppressed. bm / H bs changes in proportion to

[0046] (V / 3)+(D / 10)+2≦P≦[(V / 3)+8]×(H bm / H bs )+(D / 10) ···(5) To produce a laser-welded joint that satisfies the relationship of Equation (3), it is important to use high-strength steel sheets that satisfy the relationship of Equation (2) for the first and second steel sheets to be welded, and then perform laser welding under conditions that satisfy the relationship of Equation (5). Here, the defocus amount D of the focal position relative to the surface of the workpiece during laser irradiation (hereinafter also referred to as the defocus amount) is the absolute value of the deviation of the laser focal position in the thickness direction from the surface of the workpiece closer to the laser light source, as shown in Figures 2 and 3. Figure 2 is a schematic diagram showing the defocus amount when the focal position is shifted toward the laser light source. Figure 3 is a schematic diagram showing the defocus amount when the focal position is shifted away from the laser light source. In Figures 2 and 3, reference numerals 8, 9, 10, and 11 denote the workpiece, laser light source, laser beam, and laser focal position, respectively. where: P: Laser power (kW), V: welding speed (m / min), D: Defocus amount (mm) of the focal position relative to the surface of the workpiece during laser irradiation, H bm : Average Vickers hardness at the center of the thickness of high-strength steel plate, H bs : Average Vickers hardness in the surface layer of high-strength steel plate, is.

[0047] The energy density of laser light is maximum at the laser focal position, and decreases with increasing distance from the laser focal position. In other words, the energy density decreases as the defocus amount increases, and as a result, the welding heat input also tends to decrease. Therefore, by optimizing the welding conditions taking the defocus amount into consideration, specifically by shifting the range of P to the high power side as the defocus amount increases, the above-mentioned effects, particularly the effect of improving static tensile properties, can be more advantageously obtained. In addition, it is possible to manufacture laser welded joints that satisfy the relationship of the above formula (3). P is preferably (V / 3) + (D / 10) + 2.5 or more. In addition, P is preferably [(V / 3) + 6] × (H bm / H bs )+(D / 10) or less.

[0048] The welding conditions other than those mentioned above are not particularly limited, and may be in accordance with conventional methods.

[0049] For example, V is preferably 1.0 m / min or more and 10.0 m / min or less. If V is less than 1.0 m / min, the construction time may increase. On the other hand, if V exceeds 10.0 m / min, welding stability may decrease, resulting in increased spatter. V is more preferably 2.0 m / min or more. Furthermore, V is more preferably 7.0 m / min or less.

[0050] Laser welding may or may not require the use of a shielding gas. The type of shielding gas used is not particularly limited, and examples include Ar, He, CO2, O2, N2, and mixtures thereof.

[0051] In addition, a filler wire may or may not be used in laser welding. As the type of filler wire, for example, a solid wire, a flux-cored wire, or a metal-cored wire can be used. Among them, from the viewpoint of the cost of the wire, it is preferable to use a solid wire. The component composition of the filler wire is, for example, in mass %, as follows: C: 0.03~0.2%, Si: 0.005 to 2.00%, Mn: 0.05 to 5.00%, P: 0.050% or less, S: 0.010% or less, Ti: 0 to 0.20% Al: 0 to 0.30% and O: 0 to 0.01%, and An example of a composition is one that optionally contains one or more optional elements selected from Cr, Ni, Mo, W, V, B, Nb, Cu, and N in a total amount of 10% or less (if these optional elements are contained, more preferably a total amount of 0.01% or more), with the balance being Fe and unavoidable impurities. When a filler wire is used, it is desirable to adjust the wire diameter and feed rate to prevent the weld from becoming too thick.

[0052] The distance (gap) between the first and second steel plates to be butted together is preferably 1.0 mm or less, more preferably 0.5 mm or less, and from the viewpoint of welding stability, it is desirable that it be close to 0 (zero) mm. [Example]

[0053] The first and second steel plates shown in Table 1 were butt-joined as the welded materials, and laser welding was performed under the conditions shown in Table 2 to obtain laser-welded joints. In all of the above laser welding, a fiber laser with a maximum output of 13 kW and a beam diameter of 0.5 mm was used. The entire width of the first and second steel plates was welded. In some cases, welding was performed using a filler wire equivalent to YGW11 specified in JIS Z 3312:2009.

[0054] The obtained laser welded joint was subjected to H s-max , H s-min , H m-min , H bm and H bs The measurement results are shown in Table 3.

[0055] The press formability and static tensile properties of the welded joints were evaluated as follows. The evaluation results are shown in Table 3.

[0056] -Evaluation of press formability of welded joints Test pieces with the shapes shown in Figures 4 to 6 were taken from the center of the weld direction of the laser-welded joint, and an Erichsen test (stretch test) was performed in accordance with JIS Z 2247: 2006. The positions of cracks in the test pieces were then confirmed, and the press formability of the weld was evaluated according to the following criteria. A (pass, excellent): Cracks occurred in the direction perpendicular to the weld (x direction) ±10° as shown in Figure 4. F (Fail): Cracks occur in directions other than ±10° perpendicular to the weld (x direction) (especially cracks in the weld direction (z direction) at butt welds and HAZs as shown in Figures 5 and 6). 4 to 6, reference numeral 1 denotes the butt weld, 2 the first steel plate, 3 the second steel plate, 12 the crack, and 13 the outer line of the stretched portion. Also, x denotes the direction perpendicular to the weld, and z the welding direction.

[0057] -Static tensile property evaluation A No. 5 tensile test piece, as specified in JIS Z 2241:2011, was taken from the laser-welded joint so that the direction perpendicular to the weld was the longitudinal direction, and a tensile test was carried out. The test speed was 10 mm / s. Next, the fracture position of the test piece was confirmed and the joint efficiency was calculated, and the static tensile properties were evaluated according to the following criteria. A (pass, particularly excellent): The fracture location is in the base material. B (Pass, Excellent): The fracture location is the HAZ or butt weld and the joint efficiency is 90% or more. F (Fail): The fracture location is HAZ or butt weld and the joint efficiency is less than 90% Here, the joint efficiency was calculated as follows. If the fracture location is HAZ Joint efficiency (%) = Maximum load during tensile test (N) x 100 / (Thickness of steel plate with HAZ on fractured side (mm) x Width of parallel part of test piece (mm) x Tensile strength of steel plate on fractured side (MPa)) If the fracture occurs at a butt weld The smaller of the joint efficiency for the first steel plate and the joint efficiency for the second steel plate Joint efficiency (%) for the first steel plate = Maximum load during tensile test (N) × 100 / (Thickness of the first steel plate (mm) × Width of the parallel part of the test piece (mm) × Tensile strength of the first steel plate (MPa)) Joint efficiency (%) for the second steel plate = Maximum load during tensile test (N) x 100 / (Thickness of second steel plate (mm) x Width of parallel part of test piece (mm) x Tensile strength of second steel plate (MPa))

[0058] [Table 1]

[0059] [Table 2]

[0060] [Table 3]

[0061] As shown in Table 3, all of the inventive examples had excellent press formability of the welded joint. The inventive examples also had excellent static tensile properties. On the other hand, the comparative examples did not have sufficient press formability of the welded joint. [Explanation of symbols]

[0062] 1: Butt weld 2: First steel plate 2-1:HAZ 2-2: Base metal part 3: Second steel plate 3-1:HAZ 3-2: Base metal part 4: First bond section 5: Second bond section 6: HAZ-base metal boundary of the first steel plate 7: HAZ-base metal boundary of the second steel plate 8: Material to be joined 9: Laser light source 10: Laser light 11: Laser focal position 12: Crack 13: Outer wire of the bulging molding part

Claims

1. A laser welded joint having a first steel plate, a second steel plate, and a butt weld between the first steel plate and the second steel plate, Either one or both of the first steel plate and the second steel plate is a high-strength steel plate having a tensile strength of 980 MPa or more, A laser welded joint, wherein the weld heat affected zone of the high strength steel plate satisfies the relationship of the following formula (1): H ｓ－ｍａｘ -H ｓ－ｍｉｎ ≦200 ・・・(1) where: H ｓ－ｍａｘ : Maximum Vickers hardness of the surface layer on the laser incident side of the weld heat affected zone of the high strength steel plate, H ｓ－ｍｉｎ : Minimum Vickers hardness of the surface layer on the laser incident side of the weld heat affected zone of the high strength steel plate, is.

2. 2. The laser welded joint according to claim 1, wherein the base metal portion of the high-strength steel plate satisfies the relationship of the following formula (2). 0.20≦H ｂｓ / H ｂｍ ≦0.90 ・・・(2) where: H ｂｍ : Average Vickers hardness at the center of the thickness of the base material of the high-strength steel plate, H ｂｓ : Average Vickers hardness of the surface layer on the laser incident side of the base material of the high-strength steel plate, is.

3. 3. The laser welded joint according to claim 1, wherein the weld heat affected zone of the high strength steel plate satisfies the following formula (3): 50≦H ｍ－ｍｉｎ -H ｓ－ｍｉｎ ≦300 ・・・(3) where: H ｍ－ｍｉｎ : Minimum Vickers hardness at the center of the plate thickness in the heat-affected zone of the high-strength steel plate, H ｓ－ｍｉｎ : Minimum Vickers hardness of the surface layer on the laser incident side of the weld heat affected zone of the high strength steel plate, is.

4. A method for manufacturing a laser welded joint, comprising: The method includes a step of performing laser welding on a workpiece formed by butting a first steel plate and a second steel plate together to obtain a laser welded joint, Either one or both of the first steel plate and the second steel plate is a high-strength steel plate having a tensile strength of 980 MPa or more, A method for manufacturing a laser welded joint, wherein the laser welding satisfies the relationship of the following formula (4). (V / 3)+2≦P≦[(V / 3)+9]×(H ｂｍ / H ｂｓ ) ・・・(4) where: P: laser output (kW), V: welding speed (m / min), H ｂｍ : Average Vickers hardness at the center of the plate thickness of the high-strength steel plate, H ｂｓ : Average Vickers hardness of the surface layer on the laser incident side of the high-strength steel plate, is.

5. The high-strength steel plate satisfies the relationship of the following formula (2), The method for manufacturing a laser welded joint according to claim 4, wherein the laser welding satisfies the following formula (5): 0.20≦H ｂｓ / H ｂｍ ≦0.90 ・・・(2) (V / 3)+(D / 10)+2≦P≦[(V / 3)+8]×(H ｂｍ / H ｂｓ )+(D / 10) ・・・(5) where: P: laser output (kW), V: welding speed (m / min), D: Defocus amount (mm) of the focal position relative to the surface of the workpiece during laser irradiation, H ｂｍ : Average Vickers hardness at the center of the plate thickness of the high-strength steel plate, H ｂｓ : Average Vickers hardness of the surface layer on the laser incident side of the high-strength steel plate, is.

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