Laser welded joint and method for manufacturing same

Abstract

Provided is a laser welded joint excellent in press moldability of a welded part even when a steel plate having a tensile strength of 980 MPa or more is used. This laser welded joint has a first steel plate, a second steel plate, and a butt-welded part of the first steel plate and the second steel plate, wherein: either or both of the first steel plate and the second steel plate are high-strength steel plates having a tensile strength of 980 MPa or more; and the relationship of the following formula (1) is satisfied in a weld heat-affected zone of the high-strength steel plate.　(1) Hs-max-Hs-min≤200

Description

Laser welded joint and manufacturing method thereof

[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.

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

[0003] As a technology relating to tailored blanks, for example, Patent Document 1 discloses "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."

[0004] Furthermore, Patent Document 2 discloses a "method for strengthening a butt-welded workpiece, characterized in that the heat-affected softened portion near the weld of the workpiece, which is made by butt-welding plate materials together, is strengthened by plastically deforming it."

[0005] JP 2006-218500 A JP 2010-082685 A

[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 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 portion may not necessarily be obtained, and improvements in this regard are currently desired.

[0009] The present invention was developed in view of the above-described 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-described 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.

[0010] The inventors conducted extensive research to achieve the above-mentioned objectives and discovered the following. (1) To improve the press formability of a weld, it is important to prevent fracture due to cracking originating from the HAZ-softened zone by suppressing strain concentration in the softened region (hereinafter also referred to as the HAZ-softened zone) of the weld heat-affected zone (hereinafter also referred to as the HAZ-softened zone). (2) In particular, during press forming of a tailored blank, the tailored blank undergoes bending deformation, which tends to cause strain concentration in the surface layer of the HAZ-softened zone. 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 zone becomes significant. (3) To suppress such strain concentration in the HAZ-softened zone, it is effective to minimize the hardness difference in the HAZ. More specifically, by making the difference between the maximum Vickers hardness and the minimum Vickers hardness in the surface layer portion of the HAZ of a high-strength steel plate 200 or less, it is possible to obtain excellent press formability of the weld, even when using a steel plate with a tensile strength of 980 MPa or more. (4) Furthermore, in order to make the difference between the maximum Vickers hardness and the minimum Vickers hardness in the surface layer portion of the HAZ of a high-strength steel plate 200 or less, it is important to properly control the laser welding conditions, particularly the laser output, welding speed, and the relationship between the average Vickers hardness in the surface layer portion 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 of the present invention is 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, wherein either or both of the first steel plate and the second steel plate 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 plate satisfies the relationship of the following formula (1). 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.

[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 plate 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,

[0013] 3. The laser welded joint according to 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 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.

[0014] 4. A method for manufacturing a laser welded joint, comprising a step of obtaining a laser welded joint by laser welding a first steel plate and a second steel plate that are butt-joined together, wherein one or both of the first steel plate and the second steel plate are high-strength steel plates with a tensile strength of 980 MPa or more, and 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 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,

[0015] 5. The method for manufacturing a laser welded joint according to 4 above, wherein the high-strength steel plate satisfies the relationship of the following formula (2), and 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 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, and 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,

[0016] According to the present invention, even when a steel plate having a tensile strength of 980 MPa or more is used, it is possible to obtain a laser welded joint having excellent press formability at the welded portion, and more preferably a laser welded joint having excellent static tensile properties in addition to the press formability at the welded portion. By applying the laser welded joint of the present invention to automobile parts, for example, tailored blanks for frame parts of an automobile body, it is possible to reduce the weight of the automobile (and thus reduce CO2 emissions during automobile driving). ２ This makes it possible to further improve the safety performance of automobiles while achieving a reduction in CO2 emissions. 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 stable quality, making it extremely advantageous in terms of manufacturability.

[0017] Fig. 1 is a schematic diagram showing an example of a cross section of a laser welded joint. Fig. 2 is a schematic diagram showing an example of a defocus amount. Fig. 3 is a schematic diagram showing an example of a defocus amount. Fig. 4 is a schematic diagram showing an example of a crack morphology in an Erichsen test. Fig. 5 is a schematic diagram showing an example of a crack morphology in an Erichsen test. Fig. 6 is a schematic diagram showing an example of a crack morphology in an Erichsen test.

[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 joint A laser-welded joint according to one embodiment of the present invention is 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, wherein 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, and 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 of the weld heat affected zone of the high strength steel plate, H ｓ－ｍｉｎ : minimum Vickers hardness at the surface layer of the weld heat affected zone of the high strength steel plate,

[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 constituting the laser welded joint.

[0021] Tensile strength: 980 MPa or more When the steel plate constituting a laser welded joint is strengthened, particularly when the tensile strength is 980 MPa or more, the Vickers hardness of the entire laser welded joint, including the butt weld and the weld heat-affected zone formed by the weld metal, increases. This leads to strain concentration in the surface layer of the HAZ-softened zone, and ultimately to a significant decrease in the press formability of the weld. In other words, in laser welded joints using steel plates with a tensile strength of 980 MPa or more, improvement in the press formability of the weld is particularly required. Therefore, the tensile strength of the high-strength steel plate is set to 980 MPa or more, preferably 1180 MPa or more. The upper limit of the tensile strength of the high-strength steel plate is not particularly limited. 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 sheet is not particularly limited. For example, the chemical composition of the high-strength steel sheet can be, in mass %, C: 0.04 to 0.40%, Si: 0.01 to 2.50%, Mn: 1.00 to 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 to 0.0100%, optionally containing one or more optional additional elements selected from Cr, Ni, Mo, W, V, Nb, Cu, N, and O in a total amount of 10% or less (when these optional additional elements are contained, more preferably in a total amount of 0.1% or more), with the balance being Fe and unavoidable impurities.

[0023] The 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 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 a 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 mass%) and an Al-based plating layer (a plating layer with an Al content of more than 50 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 the Zn-based plating layer include a hot-dip galvanized layer (GI), a galvannealed layer (GA), an electrogalvanized layer (EG), a Zn-Ni-based plating layer (e.g., a plating layer containing 10 to 25 mass% of Ni in addition to Zn), a Zn-Al-based plating layer, a Zn-Mg-based plating layer, and a Zn-Al-Mg-based plating layer. Examples of the Al-based plating layer include an Al-Si-based plating layer (e.g., a plating layer 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. From the viewpoint of weldability, the coating weight of the metal plating layer is, for example, 120 g / m per side. 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 portion 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 portion 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 thereof 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 of the above formula (1) 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－ｍｉｎ If H exceeds 200, strain concentration in the HAZ softened portion is promoted, and sufficient press formability of the welded portion cannot be obtained, particularly when using a steel plate with a tensile strength of 980 MPa or more. Therefore, the relationship of the above formula (1) is 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, 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 zone of each high-strength steel plate. ｓ－ｍａｘ : Maximum Vickers hardness of the surface layer of the weld heat affected zone of the high strength steel plate, H ｓ－ｍｉｎ : minimum Vickers hardness at the surface layer of the weld heat affected zone of the high strength steel plate,

[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 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 ｂｍ is 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 portion of each high-strength steel plate satisfies the relationship of the above formula (2). ｂｍ : Average Vickers hardness at the center of the plate thickness of the base material of the high-strength steel plate, H ｂｓ : average Vickers hardness of the surface layer of the base material of the high-strength steel plate,

[0031] The mechanical properties of the base material of the high-strength steel plate constituting the laser welded joint usually remain the same as the mechanical properties of the high-strength steel plate used as the welded material. Therefore, as will be described later, the average Vickers hardness at the center of thickness and the surface layer of the high-strength steel plate used as 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 described 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, so strain tends 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 HAZ thickness of a high-strength steel plate, 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 H ｍ－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 plate may be excessively increased, 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－ｍｉｎis preferably 170 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 portion of each high-strength steel plate satisfies the relationship of the above formula (3). ｍ－ｍｉｎ : Minimum Vickers hardness at the center of the plate thickness of the weld heat affected zone of the high strength steel plate, H ｓ－ｍｉｎ : minimum Vickers hardness at the surface layer of the weld heat affected zone of the high strength steel plate,

[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 (toward the first 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 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 region 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 region is defined as the base metal.

[0035] In Figure 1, reference numeral 1 denotes a butt weld, 2 denotes a first steel plate, 2-1 denotes a HAZ, 2-2 denotes a base metal, 3 denotes a second steel plate, 3-1 denotes a HAZ, 3-2 denotes a base metal, 4 denotes a first bond, 5 denotes a second bond, 6 denotes a HAZ-base metal boundary of the first steel plate, and 7 denotes a HAZ-base metal boundary of the second steel plate. Also, x denotes a direction perpendicular to the weld, and y denotes a plate thickness direction. The weld perpendicular direction is a 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 is performed in accordance with JIS Z 2244-1:2020 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 position of the plate thickness, from the first bond portion to the HAZ-base metal boundary 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 designated 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－ｍｉｎ Furthermore, 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 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 portion of the first steel plate, the butt weld portion, and the HAZ and base material portion of the second steel plate are defined on the cut surface as shown in Figure 1. When 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 portion of the base material portion 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 values ​​of the Vickers hardness at the center position of the plate thickness and the surface layer portion of the base material portion of the high-strength steel plate are respectively calculated as H ｂｍ and H ｂｓIn 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 of the high-strength steel plate used for the second steel plate is ｂｍ and H ｂｓ Measure.

[0039] The Vickers hardness (H ｓ－ｍａｘ , H ｓ－ｍｉｎ and H ｂｓ ) is acceptable as long as the measurement position 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 the H ｓ－ｍａｘ , H ｓ－ｍｉｎ and H ｂｓ In addition, the Vickers hardness (H ｍ－ｍｉｎ and H ｂｍ Regarding the Vickers hardness, deviation of the measurement position is permissible as long as it is at a depth position of ±100 μm from the center of the plate thickness of the 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 the plate thickness -100 μm, the center of the plate thickness, and the center of the plate thickness +100 μm is used to calculate the 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] Method for Manufacturing Laser Welded Joint 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 a step of obtaining a laser welded joint by laser welding a first steel plate and a second steel plate that are butt-joined together, wherein one or both of the first steel plate and the second steel plate are high-strength steel plates having a tensile strength of 980 MPa or more, and 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 plate thickness of the high-strength steel plate, H ｂｓ : average Vickers hardness in the surface layer of the high-strength steel plate,

[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, and therefore 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 set the ratio H ｂｍ / H ｂｓ It has been found that it is important to appropriately 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 speed. 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 part near the heat input part). That is, when the peak temperature during welding in a part of the HAZ exceeds Ac ３ The temperature rises above this point, and the region becomes a very hard martensite structure. ｓ－ｍａｘ On the other hand, in other areas of the HAZ, the peak temperature during welding is １ There are areas where the temperature is below the temperature point, and in these areas, tempering of martensite is promoted. ｓ－ｍｉｎdecreases. 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 ) or less, where P: laser power (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 in the surface layer of the high-strength steel plate,

[0045] The inventors have determined that the right side of the above formula (4), i.e., the upper limit of P, is H ｂｍ / H ｂｓ 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 is smaller, tempering becomes more difficult due to, for example, a decrease in the proportion of martensite in the microstructure and a decrease in carbon equivalent. ｓ－ｍｉｎ Therefore, the upper limit of P is set to H ｂｍ / H ｂｓ changes in proportion to

[0046] (V / 3)+(D / 10)+2≦P≦[(V / 3)+8]×(H ｂｍ / H ｂｓ) + (D / 10) ... (5) Furthermore, to produce a laser welded joint that satisfies the relationship of the above formula (3), it is important to use high-strength steel sheets that satisfy the relationship of the above formula (2) for the first and second steel sheets of the workpieces, and then perform laser welding under conditions that satisfy the relationship of the above formula (5). Here, the defocus amount D of the focal position relative to the workpiece surface during laser irradiation (hereinafter also referred to as the defocus amount) is the amount (absolute value) of deviation of the laser focal position in the plate thickness direction from the workpiece surface 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 workpieces, the laser light source, the laser beam, and the laser focal position. 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, and H ｂｍ : Average Vickers hardness at the center of the plate thickness of the high-strength steel plate, H ｂｓ : average Vickers hardness in the surface layer of the high-strength steel plate,

[0047] The energy density of a laser beam 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 in accordance with the increase in the defocus amount, the above-mentioned effects, particularly the effect of improving static tensile properties, can be more advantageously obtained. Furthermore, 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. Furthermore, 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, the welding stability may decrease and the amount of spatter may increase. 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 thereof include Ar, He, CO ２ , O ２ , N ２ and mixed gases thereof.

[0051] In addition, a filler wire may or may not be used in laser welding. Examples of the filler wire that can be used include a solid wire, a flux-cored wire, and a metal-cored wire. Among these, a solid wire is preferred from the viewpoint of wire cost. Examples of the composition of the filler wire include, in mass %, C: 0.03 to 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 optionally containing one or more optional additional elements selected from Cr, Ni, Mo, W, V, B, Nb, Cu, and N in a total amount of 10% or less (more preferably 0.01% or more in total when these optional additional elements are contained), with the balance being Fe and unavoidable impurities. When using filler wire, 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. From the viewpoint of welding stability, it is desirable that the distance be close to 0 (zero) mm.

[0053] The first and second steel plates shown in Table 1 were butted together 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 laser welding, a fiber laser with a maximum output of 13 kW and a beam diameter of 0.5 mm was used. The entire widths of the first and second steel plates were welded. In some examples, 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 in the following manner. The evaluation results are shown in Table 3.

[0056] Evaluation of press formability of welded joints Test specimens with the shapes shown in Figures 4 to 6 were taken from the center of the welded joint in the welding direction, and an Erichsen test (stretch test) was performed in accordance with JIS Z 2247:2006. The locations of cracks in the test specimens were confirmed, and the press formability of the welded joints was evaluated according to the following criteria: A (pass, excellent): Cracks occurred in an orientation of ±10° from the direction perpendicular to the weld (x direction) as shown in Figure 4; F (fail): Cracks occurred in an orientation other than ±10° from the direction perpendicular to the weld (x direction) (particularly cracks in the welding direction (z direction) in the butt weld and HAZ as shown in Figures 5 and 6). In Figures 4 to 6, reference numerals 1, 2, and 3 denote the butt weld, the first steel plate, the second steel plate, 12, and the outer line of the stretched portion, respectively. Furthermore, x denotes the direction perpendicular to the weld, and z denotes the welding direction.

[0057] Evaluation of static tensile properties No. 5 tensile test specimens, as specified in JIS Z 2241:2011, were taken from the laser-welded joints so that the direction perpendicular to the weld was the longitudinal direction, and tensile tests were conducted. The test speed was 10 mm / s. Next, the fracture location on the test specimens 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 was in the base material; B (pass, excellent): The fracture location was in the HAZ or butt weld, and the joint efficiency was 90% or more; F (fail): The fracture location was in the HAZ or butt weld, and the joint efficiency was less than 90%. Here, the joint efficiency was calculated as follows. ) ・When the fracture position is in the HAZ: Joint efficiency (%) = Maximum load during tensile test (N) × 100 / (Thickness (mm) of steel plate having HAZ on the fractured side × Width of parallel portion of test piece (mm) × Tensile strength of steel plate on the fractured side (MPa)) ・When the fracture position is in 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 (mm) of the first steel plate × Width of parallel portion of test piece (mm) × Tensile strength of the first steel plate (MPa)) Joint efficiency (%) for the second steel plate = Maximum load during tensile test (N) × 100 / (Thickness (mm) of the second steel plate × Width of parallel portion of test piece (mm) × Tensile strength of the second steel plate (MPa))

[0058]

[0059]

[0060]

[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.

[0062] 1: Butt welded portion 2: First steel plate 2-1: HAZ 2-2: Base material portion 3: Second steel plate 3-1: HAZ 3-2: Base material portion 4: First bond portion 5: Second bond portion 6: HAZ-base material portion boundary of first steel plate 7: HAZ-base material portion boundary of second steel plate 8: Welded material 9: Laser light source 10: Laser beam 11: Laser focal position 12: Crack 13: Outer line of stretch-formed portion

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, wherein either or both of the first steel plate and the second steel plate 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 plate satisfies the relationship of the following formula (1). 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.

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 plate 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, 3. The laser welded joint according to claim 1 or 2, 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 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.

4. A method for manufacturing a laser welded joint, comprising a step of obtaining a laser welded joint by laser welding a first steel plate and a second steel plate that are butt-joined together, wherein one or both of the first steel plate and the second steel plate are high-strength steel plates with a tensile strength of 980 MPa or more, and 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 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, 5. The method for manufacturing a laser welded joint according to claim 4, wherein the high-strength steel plate satisfies the relationship of the following formula (2), and 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 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, and 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,

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