Laser welded joint and method for manufacturing a laser welded joint

Abstract

A laser welding joint (1) comprises a plurality of steel plates (11) and (12) and a welding metal (20) for joining the plurality of steel plates (11) and (12), wherein at least one of the plurality of steel plates (11) and (12) is a high-strength steel plate with a Vickers hardness of 320 HV or higher, the amount of Al penetrated into the base material of the plurality of steel plates (11) and (12) is greater than 0.10 mass% and less than or equal to 2.10 mass%, the average amount of Al in the welding metal (20) is greater than or equal to 0.30 mass% and less than or equal to 2.00 mass%, and when the average amount of Al, average amount of Si, and average amount of Mn in the welding metal (20) are denoted as [Al], [Si], and [Mn] respectively, Equation 1 ([Al] / ([Al]+[Si]+[Mn])≥0.150) is satisfied, and a slag containing Al is on the surface of the welding metal (20). A laser welded joint characterized by covering 30.0% or more, and a method for manufacturing the same.

Description

Technology Field

[0001] The present invention relates to a laser welded joint and a method for manufacturing a laser welded joint.

[0002] The present application claims priority based on Japanese Patent Application No. 2023-007149 filed in Japan on January 20, 2023, and incorporates the contents thereof herein by reference. Background Technology

[0003] A laser welded joint is manufactured by arranging multiple steel plates by overlapping or butting the ends of the steel plates together, irradiating a laser beam onto the joint, melting the steel plates or fillers in the irradiated area, and solidifying the molten metal to form weld metal, and has a structure in which multiple steel plates are joined by weld metal. Prior art literature

[0004] Japanese Patent Publication No. 2011-31249 The problem to be solved

[0005] Laser welds of high-strength steel plates with a strength of 980 MPa or higher, which have been used in recent years for the purpose of improving the strength and rigidity of vehicle bodies, have a problem in that the weld metal is prone to hardening due to the high carbon content, and low-temperature cracking (cracking due to hydrogen embrittlement) is likely to occur at the ends of the weld metal. When such cracking occurs, the crack propagates along the entire length of the weld metal, which is mainly formed in a straight line, so not only is static strength, such as shear strength and peel strength of the weld joint, reduced, but fatigue strength is also significantly reduced. Therefore, a method to prevent low-temperature cracking in laser welds of high-strength steel plates is required.

[0006] Furthermore, in conventional laser welded joints, the weld metal becomes embrittled due to diffusible hydrogen introduced during welding, leading to cases where cracks occur during the forming process following the fabrication of the laser welded joint or after welding during the assembly of parts. Therefore, methods are required to reduce diffusible hydrogen in the laser weld area or to improve the hydrogen embrittlement resistance of the laser weld area.

[0007] Meanwhile, as described in Patent Document 1, it has been reported that adding a filler during welding lowers the hardness of the weld, thereby making it difficult to cause low-temperature cracking in the weld, but there was a problem that the strength (hardness) of the weld was lowered.

[0008] The present invention has been made in consideration of the above, and aims to provide a laser welded joint in which low-temperature cracking is suppressed and strength as a joint is secured, and a method for manufacturing the same. means of solving the problem

[0009] (1) A laser welding joint according to one embodiment of the present invention is a laser welding joint comprising a plurality of steel plates and a welding metal for joining the plurality of steel plates, and

[0010] At least one of the plurality of steel plates is a high-strength steel plate with a Vickers hardness of 320 HV or higher, and

[0011] The amount of Al penetrated into the base material of the plurality of steel plates is greater than 0.10 mass% and less than or equal to 2.10 mass%, and

[0012] The average Al content of the weld metal is 0.30 mass% or more and 2.00 mass% or less, and

[0013] When the average Al, average Si, and average Mn amounts of the above weld metal are denoted as [Al], [Si], and [Mn], respectively, the following Equation 1 is satisfied, and

[0014] Slag containing Al covers 30.0% or more of the surface of the weld metal.

[0015] It is characterized by the fact that.

[0016] [Al] / ([Al]+[Si]+[Mn])≥0.150 … Equation 1

[0017] (2) The laser welding joint described in (1) above may have an average amount of Al in the weld metal relative to the amount of Al in the base metal of the plurality of steel plates, with a ratio of 0.80 to 1.20.

[0018] (3) The laser welded joint described in (1) or (2) above has a chemical composition of the weld metal in mass%,

[0019] C: 0.1 to 0.6%,

[0020] Si: 0.005 to 3.0%,

[0021] Mn: 0.5 to 3.0%,

[0022] Al: 0.30 to 2.00%,

[0023] P: 0.04% or less,

[0024] S: 0.01% or less,

[0025] N: 0.1% or less,

[0026] O: 0.1% or less,

[0027] Cu: 0 to 1.0%,

[0028] Nb+Ti+V: 0 to 0.3%,

[0029] Ca+REM: 0 to 0.01%,

[0030] B: 0 to 0.005%,

[0031] Cr: 0 to 2.0%,

[0032] Ni: 0 to 1.0%,

[0033] Mo: 0 to 1.0%,

[0034] Sn: 0.1% or less,

[0035] Mg: 0 to 0.01%,

[0036] Sb: 0 to 0.1%,

[0037] As: 0 to 0.1%,

[0038] Remainder: May contain Fe and impurities.

[0039] (4) The laser welding joint described in any one of claims (1) to (3) above may have at least one of the plurality of steel plates being a non-plated steel plate or a zinc-plated steel plate.

[0040] (5) The laser welded joint described in any one of (1) to (4) above may have a Vickers hardness of 450 HV or higher of the weld metal.

[0041] (6) The laser welding joint described in any one of (1) to (5) above may be an overlapping laser welding joint, a butt laser welding joint, or a fillet laser welding joint.

[0042] (7) In any one of the above (1) to (6) laser welded joints, at least one of the plurality of steel plates may be a high-strength steel plate with a tensile strength of 980 MPa or more.

[0043] (8) The laser welded joint described in any one of claims (1) to (7) above has a chemical composition of at least one of the plurality of steel plates in mass%,

[0044] C: Exceeding 0.15% and up to 0.5%,

[0045] Si: 0.1 to 3.5%,

[0046] Mn: 0.2 to 5.5%,

[0047] Al: Exceeding 0.10% and up to 2.10%,

[0048] N: 0.1% or less,

[0049] O: 0.1% or less,

[0050] P+S: 0.050% or less,

[0051] Cu: 0 to 1.0%,

[0052] Nb+Ti+V: 0 to 0.3%,

[0053] Ca+REM: 0 to 0.01%,

[0054] B: 0 to 0.005%,

[0055] Cr: 0 to 2.0%,

[0056] Ni: 0 to 1.0%,

[0057] Mo: 0 to 1.0%,

[0058] Sn: 0.1% or less,

[0059] Mg: 0 to 0.01%,

[0060] Sb: 0 to 0.1%,

[0061] As: 0 to 0.1%

[0062] It contains, and the remainder may contain Fe and impurities.

[0063] (9) A method for manufacturing a laser welded joint according to one embodiment of the present invention is a method for manufacturing a laser welded joint described in any one of claims (1) to (8), and

[0064] The process of preparing the above plurality of steel plates, and

[0065] The process of arranging the above plurality of steel plates, and,

[0066] A laser welding process for forming the welding metal for joining the plurality of steel plates.

[0067] including

[0068] In the above laser welding process, laser welding is performed in an environment with an oxygen concentration of 3.0% or higher.

[0069] It is characterized by the fact that.

[0070] (10) In the method for manufacturing a laser welded joint described in (9) above, the laser beam source in the laser welding process may be a solid-state laser device or a semiconductor laser device. Effects of the invention

[0071] According to the laser welding joint and the method of manufacturing the same according to the present invention, low-temperature cracking can be suppressed and strength as a joint can also be secured. Brief explanation of the drawing

[0072] Figure 1 is a schematic cross-sectional view of a cut section to illustrate an overlapping laser welded joint. Figure 2 is a schematic plan view of the laser welding joint (1) seen from the forward side of the Z coordinate axis of Figure 1. Figure 3 is a schematic cross-sectional view of a cut section to illustrate a butt laser welded joint. FIG. 4 is a schematic plan view of the laser welding joint (1) seen from the forward side of the Z coordinate axis of FIG. 3. Figure 5 is a schematic cross-sectional view of a cut section to illustrate a T-shaped fillet laser welded joint. FIG. 6 is a schematic plan view of the laser welding joint (1) seen from the forward side of the Z coordinate axis of FIG. 5. Figure 7 is a schematic cross-sectional view of a cut section to illustrate an overlapping fillet laser welded joint. FIG. 8 is a schematic plan view of the laser welding joint (1) seen from the forward side of the Z coordinate axis of FIG. 7. FIG. 9 is a schematic cross-sectional view of a cut section to explain the penetration cross-sectional area of ​​a steel plate in an overlapping laser welded joint. FIG. 10 is a schematic cross-sectional view of a cut section to explain the penetration cross-sectional area of ​​a steel plate in a butt laser welded joint. FIG. 11 is a schematic cross-sectional view of a cut section to explain the penetration cross-sectional area of ​​a steel plate in a T-shaped fillet laser welded joint. FIG. 12 is a schematic cross-sectional view of a cut section to explain the penetration cross-sectional area of ​​a steel plate in an overlapping fillet laser welded joint. Figure 13 (A) is an example of an SEM image of a measurement location, and (B) is an example of a binarized image of the mapping of Al at the measurement location of (A). Figure 14 (A) is an example of an SEM image of a measurement location, and (B) is another example of a binarized image of the mapping of Al at the measurement location of (A). Specific details for implementing the invention

[0073] Generally, it is believed that low-temperature cracking occurs when three elements—diffusible hydrogen entering the weld zone, tensile stress applied to the weld zone, and the hardened structure of the weld zone—overlap and exceed a limit. The inventors conducted research with the aim of reducing the diffusible hydrogen among these factors. The inventors discovered that by adding Al (aluminum) to the steel plates being joined, the surface of the molten pool during laser welding is covered with slag containing Al, making it difficult for gases containing diffusible hydrogen to come into contact with the molten pool, thereby preventing hydrogen from entering the molten pool. From this result, the inventors arrived at the idea that low-temperature cracking in the laser weld zone can be prevented. Furthermore, the inventors discovered that by using steel plates as the source of Al for the weld metal, rather than fillers, the effect of preventing low-temperature cracking can be further enhanced without lowering the strength of the weld zone.

[0074] Embodiments of the present invention are described below by way of example, but it is obvious that the present invention is not limited to the examples described below. In the following description, specific numerical values ​​or materials may be exemplified, but other numerical values ​​or materials may be applied as long as the effects of the present invention are obtained. Furthermore, each component of the following embodiments may be combined with one another.

[0075] Furthermore, in this specification, a numerical range expressed using "to" means a range that includes the values ​​described before and after "to" as lower and upper limits. In this specification, the term "process" is included not only in independent processes but also in cases where the intended purpose of the process is achieved, even if it cannot be clearly distinguished from other processes.

[0076] [Laser Welded Joint]

[0077] The laser welding joint according to the present embodiment is a laser welding joint comprising a plurality of steel plates and a welding metal for joining the plurality of steel plates.

[0078] (Steel plate)

[0079] Multiple steel plates are the base material of the laser welded joint. The thickness of the steel plates (also referred to as plate thickness) is not particularly limited as long as it is suitable for laser welding. The number of steel plates is also not particularly limited and can be any number of two or more. In addition, in the following examples, a laser welded joint in which two steel plates are joined through weld metal is described as an example.

[0080] In the laser welded joint according to the present embodiment, the chemical composition of one or more of the steel plates among the plurality of steel plates is, in mass%, C: greater than 0.15% and less than or equal to 0.5%, Si: 0.1 to 3.5%, Mn: 0.2 to 5.5%, Al: greater than 0.10% and less than or equal to 2.10%, N: 0.1% or less, O: 0.1% or less, P+S: 0.050% or less, Cu: 0 to 1.0%, Nb+Ti+V: 0 to 0.3%, Ca+REM: 0 to 0.01%, B: 0 to 0.005%, Cr: 0 to 2.0%, Ni: 0 to 1.0%, Mo: 0 to 1.0%, Sn: 0.1% or less, Mg: 0 to 0.01%, Sb: 0 to 0.1%, As: 0 to It contains 0.1%, and the remainder may contain Fe and impurities. Since steel sheets with this chemical composition have high strength, they can provide excellent strength to laser welded joints.

[0081] The chemical composition of the steel plate described above is determined by sample preparation and quantitative analysis in accordance with the standards listed in Table 1 of JIS G0321 (2010). As a sample to be used for measuring the chemical composition, one sample is taken from a location (preferably a location) spaced at least 5 mm away from the weld of the steel plate, and the measurement value of this sample is used. If the steel plate has a plating layer on its surface, the plating layer can be removed by mechanical grinding before the chemical composition is analyzed.

[0082] (Welded metal)

[0083] Weld metal is the area where steel plates, etc., are melted and solidified by irradiation with a laser beam during laser welding; it is the metal that melts and solidifies during welding within a part of the weld zone. When fillers are not used during laser welding, the material source for the weld metal is the multiple steel plates being joined. When fillers are used during laser welding, the material source for the weld metal is the multiple steel plates being joined and the filler. If the steel plates are plated, the components of the plating also melt and become the material constituting the weld metal. In addition to elements from these sources, oxygen and nitrogen from the air are introduced into the weld metal, and unavoidable impurities may also be introduced.

[0084] FIGS. 1 to 8 illustrate various forms of a laser welding joint according to the present embodiment. However, the laser welding joint to which the present invention is applied is not limited to these examples, and any shape of laser welding joint and its manufacturing method can be applied as long as it satisfies the requirements of the present invention. In addition, the X, Y, and Z coordinate axes of FIGS. 1 to 8 are each orthogonal to each other.

[0085] (Overlapping laser welded joint)

[0086] FIG. 1 is a schematic cross-sectional view of a cross section for illustrating an overlapping laser welding joint in which at least a portion of the steel plates being joined overlap each other. The cross-sectional plane of the laser welding joint (1) in FIG. 1 is a plane perpendicular to the extension direction of the weld metal (direction along the weld line) and parallel to the X and Z coordinate axes. The laser welding joint (1) shown in FIG. 1 comprises a steel plate (11), a steel plate (12) overlapping the steel plate (11), and a weld metal (20) formed across the plate thickness direction of the steel plate (11) and the steel plate (12). The steel plate (11) has one plate surface (11a) and the other plate surface (11b), and the steel plate (12) has one plate surface (12a) and the other plate surface (12b). In this example, the plate surface (11b) of the steel plate (11) and the plate surface (12a) of the steel plate (12) are opposite each other.

[0087] The example of FIG. 1 is an example in which a laser beam is irradiated from the plate surface (11a) side (the side in the forward direction of the Z coordinate axis) of the steel plate (11) to form a weld metal (20). In the example of FIG. 1, the weld metal (20) is formed over the entire plate thickness direction of the steel plate (11) and the steel plate (12). However, the overlapping laser weld joint is not limited to the example of FIG. 1, and the weld metal (20) may be formed over the entire plate thickness direction of the steel plate (11), and the weld metal (20) may be formed from one plate surface (12a) of the steel plate (12) toward the plate surface (12b) up to the middle of the plate thickness direction.

[0088] FIG. 2 is a schematic plan view of a laser welding joint (1) viewed from the forward side of the Z coordinate axis of FIG. 1. FIG. 1 is a cross-sectional view of the laser welding joint (1) shown in FIG. 2, cut at the cross-sectional line AA. In the example of FIG. 2, the weld metal (20) is formed to extend along the Y coordinate axis. The direction of propagation of the laser beam when forming the weld metal (20) is the forward direction of the Y coordinate axis. A crater (201) exists at the end portion (21) of the weld metal (20).

[0089] Generally, in laser welding, molten metal flows in a direction opposite to the direction of travel of the laser beam. Because of this, a depression called a crater is formed at the end of the weld metal (also called a bead) formed by laser welding. The end of the weld metal formed by laser welding is the final solidified part of the weld metal, and since there is a lack of molten metal in this area, a crater is formed.

[0090] (Butt laser welded joint)

[0091] FIG. 3 is a schematic cross-sectional view of a butt laser welding joint in which the end faces of the steel plates being joined face each other. The cross-sectional view of the laser welding joint (1) in FIG. 3 is a plane perpendicular to the extension direction of the welding metal (the direction of propagation of the laser beam) and is parallel to the X and Z coordinate axes. The laser welding joint (1) shown in FIG. 3 includes steel plates (11) and steel plates (12) arranged so that their end faces are butted against each other, and welding metal (20) that joins the steel plates (11) and steel plates (12) in a direction parallel to their respective plate faces. The steel plate (11) has one plate face (11a) and the other plate face (11b), and the steel plate (12) has one plate face (12a) and the other plate face (12b). In this example, the plate surface (11a) of the steel plate (11) and the plate surface (12a) of the steel plate (12) are facing in the same direction.

[0092] The example of FIG. 3 is an example in which a laser beam is irradiated from the plate surface (11a) side (the side in the forward direction of the Z coordinate axis) of the steel plate (11) to form a weld metal (20). In the example of FIG. 3, the weld metal (20) is formed over the entire plate thickness direction of the steel plate (11) and the steel plate (12). However, the butt laser weld joint is not limited to the example of FIG. 3, and the weld metal (20) may be formed up to the middle of the plate thickness direction of the steel plate (11) and the steel plate (12).

[0093] FIG. 4 is a schematic plan view of the laser welding joint (1) viewed from the forward side of the Z coordinate axis of FIG. 3. FIG. 3 is a cross-sectional view of the cut section of the laser welding joint (1) shown in FIG. 4, cut at the cross-sectional line AA. In the example of FIG. 4, the weld metal (20) is formed to extend along the Y coordinate axis. In addition, the end surface (11A) of the steel plate (11) and the end surface (12A) of the steel plate (12) are in contact. The direction of propagation of the laser beam when forming the weld metal (20) is the forward direction of the Y coordinate axis. A crater (201) is present at the end portion (21) of the weld metal (20).

[0094] In the example of FIG. 4, the end surfaces of the steel plates (11) and (12) are straight, and weld metal (20) is formed along the end surfaces. The weld metal (20) may be formed over the entire end surface of the steel plates to be butted, or may be formed intermittently or partially on the end surface of the steel plates to be butted. In addition, the end surfaces of the steel plates to be joined are not limited to straight lines and may have a predetermined shape required for the parts being manufactured.

[0095] In addition, the end surfaces of the butted steel plates may be in contact with each other, or have a gap between them.

[0096] (Fillet laser welded joint)

[0097] FIG. 5 is a schematic cross-sectional view of a fillet laser welding joint (also referred to as a T-shaped fillet welding joint) for explaining a fillet laser welding joint in which the end surface of one steel plate is joined to the plate surface of another steel plate, and weld metal is formed at the corner formed by the plate surface of the steel plate. The steel plate (11) of FIG. 5 has one plate surface (11a) and the other plate surface (11b), and the steel plate (12) has one plate surface (12a) and the other plate surface (12b). In the laser welding joint (1) shown in FIG. 5, the plate surface of the steel plate (11) and the plate surface of the steel plate (12) are arranged perpendicularly to each other, and the end surface (11A) of the steel plate (11) comes into contact with the plate surface (12a) of the steel plate (12), so that weld metal (20) is formed at the corner formed by the steel plate (11) and the steel plate (12). The cross-sectional plane of the laser weld joint (1) in Fig. 5 is a plane perpendicular to the extension direction of the weld metal (the direction of travel of the laser beam) and is parallel to the X coordinate axis and the Z coordinate axis.

[0098] In addition, the positional relationship between the plate surface of the steel plate (11) and the plate surface of the steel plate (12) is not limited to being vertical, and the plates may intersect. Furthermore, the end surface of one steel plate may not come into contact with the plate surface of the other steel plate, and there may be a gap between the steel plates. The weld metal (20) may be formed along the entire length of the corner section, or it may be formed intermittently or partially along the corner section. In addition, the end surface of the steel plate to be joined is not limited to being straight, and may have a predetermined shape required for the manufactured part. In addition, the plate surface of the steel plate to be joined is not limited to being flat, and may have irregularities or cuts in parts.

[0099] In the example of FIG. 5, weld metal (20) is formed from the plate surface (11a) of the steel plate (11) and the plate surface (12a) of the steel plate (12) to the middle of the plate thickness direction.

[0100] FIG. 6 is a schematic plan view of the laser welding joint (1) viewed from the forward side of the Z coordinate axis of FIG. 5. FIG. 5 is a cross-sectional view of the cut section of the laser welding joint (1) shown in FIG. 6, cut at the cross-sectional line AA. In the example of FIG. 6, the weld metal (20) is formed to extend along the Y coordinate axis. The direction of propagation of the laser beam when forming the weld metal (20) is the forward direction of the Y coordinate axis. A crater (201) is present at the end portion (21) of the weld metal (20).

[0101] In addition, as another form of the laser welding joint (1) exemplified in FIGS. 5 and 6, welding metal (20) may be formed on both plate surfaces (11a and 11b) of the steel plate (11). Such welding metal (20) may be formed, for example, by irradiating sequential laser beams from the forward and negative sides of the X coordinate axis of FIG. 5. In addition, in such a laser welding joint (1), welding metal (20) may be formed along the plate thickness direction from plate surface (12a) to plate surface (12b) of the steel plate (12).

[0102] FIG. 7 is a schematic cross-sectional view of a fillet laser welding joint (also referred to as an overlapping fillet laser welding joint) in which at least a portion of the steel plates being joined overlap each other and the end surface of one steel plate and the plate surface of the other steel plate are joined. The cross-sectional view of the laser welding joint (1) in FIG. 7 is a plane perpendicular to the direction of extension of the weld metal (the direction of propagation of the laser beam) and is parallel to the X and Z coordinate axes. The laser welding joint (1) shown in FIG. 7 comprises a steel plate (11), a steel plate (12) superimposed on the steel plate (11), and a weld metal (20) formed across the end surface of the steel plate (11) and the plate surface of the steel plate (12). The steel plate (11) has one plate surface (11a) and the other plate surface (11b), and the steel plate (12) has one plate surface (12a) and the other plate surface (12b). In this example, the plate surface (11b) of the steel plate (11) and the plate surface (12a) of the steel plate (12) are facing each other.

[0103] In the example of FIG. 7, at the end surface (11A) of the steel plate (11), weld metal (20) is formed over the entire thickness direction of the steel plate (11), and weld metal (20) is formed from the plate surface (12a) of the steel plate (12) toward the plate surface (12b) along the middle of the thickness direction. Additionally, weld metal (20) may be formed over the entire thickness direction of the steel plate (12) from the plate surface (12a) toward the plate surface (12b).

[0104] FIG. 8 is a schematic plan view of the laser welding joint (1) seen from the forward side of the Z coordinate axis of FIG. 7. FIG. 7 is a cross-sectional view of the cut section of the laser welding joint (1) shown in FIG. 8, cut at the cross-sectional line AA. In the example of FIG. 8, the weld metal (20) is formed to extend along the Y coordinate axis. The direction of propagation of the laser beam when forming the weld metal (20) is the forward direction of the Y coordinate axis. A crater (201) is present at the end portion (21) of the weld metal (20).

[0105] In the example described above, the width or length of the weld metal (20) formed by laser welding is not particularly limited and can be designed to suit the desired joint strength or the shape of the steel plate being joined. In addition, the weld metal (20) is not limited to a straight shape and may, for example, be curved or bent on the surface of the steel plate.

[0106] In addition, the end portion (21) of the weld metal (20) in this embodiment is not necessarily located at the end of the weld metal (20), and if the laser is moved slightly back along the weld line and the laser beam irradiation is terminated at a location other than the end of the weld metal, the end portion (21) having a crater (201) may be formed at a location other than the end of the weld metal (20).

[0107] In addition, in the example described above, the thickness of the steel plate (11) and the steel plate (12) being joined is made the same, but the thickness of the steel plate being joined is not particularly limited. The thickness of the steel plate (plate thickness) is determined by measuring the thickness at five points using a measuring instrument such as a caliper or a micrometer, and the arithmetic mean value of these measurements is used as the thickness of the steel plate.

[0108] (Crater at the end)

[0109] A crater may be formed at the end of the weld metal on either the side where the laser beam is irradiated or the opposite side. After the laser welding is completed, the weld metal rapidly cools and shrinks due to heat dissipation from the weld to its surroundings, thereby applying tensile stress to the end of the weld metal in a direction perpendicular to the extension direction of the weld line. As tensile stress is applied to the end of the weld metal where the crater is formed, the end of the weld metal may fracture in a direction perpendicular to the tensile stress (the extension direction of the weld line). In the laser weld joint according to the present embodiment, it is possible to prevent such fracture by satisfying the following requirements.

[0110] (Vickers hardness of steel plates)

[0111] In the laser welded joint according to the present embodiment, at least one of the plurality of steel plates is a high-strength steel plate with a Vickers hardness of 320 HV or higher. When fracture occurs as described above, the fracture of the end portion may propagate along the extension direction of the weld metal, and there is a risk that a crack may form throughout the entire weld metal. As the Vickers hardness of the steel plate increases, the susceptibility to hydrogen embrittlement cracking increases, and the risk of cracking occurring at the end portion of the weld metal increases. If a crack occurs in the weld metal along its entire length, the static strength of the joint, such as shear strength and peel strength, decreases, and fatigue strength also decreases significantly. Therefore, low-temperature cracking is a particular problem for high-strength steel plates.

[0112] The Vickers hardness of the steel plate shall be measured in accordance with JIS Z 2204. The measurement location for Vickers hardness shall be a cross-section including the plate thickness of the steel plate, and the measurement shall be taken at a position 1 / 4 of the plate thickness in the plate thickness direction. The indentation load during measurement shall be 300gf or more. The Vickers hardness of the steel plate shall be measured at 10 points with a pitch of 0.2mm and the arithmetic mean value thereof shall be used. The location where the test specimen is cut shall be a flat section spaced at least 5mm away from the weldment.

[0113] From the perspective of increasing the strength of the laser welded joint, it is desirable for the strength of the steel plate to be higher. For example, the Vickers hardness of one or more of the steel plates among the plurality of steel plates may be 320 HV or higher, 328 HV or higher, or 363 HV or higher. Additionally, the Vickers hardness of all of the steel plates may be 320 HV or higher, 328 HV or higher, or 363 HV or higher. Although the higher the Vickers hardness of the steel plate, the higher the susceptibility of the weld metal to hydrogen embrittlement cracking, in the laser welded joint according to the present embodiment, diffusible hydrogen is reduced by meeting the requirements described below, thereby suppressing low-temperature cracking.

[0114] (Tensile strength of steel plates)

[0115] In the laser welded joint according to the present embodiment, it is preferable that at least one of the plurality of steel plates is a high-strength steel plate with a tensile strength of 980 MPa or more. When fracture occurs as described above, there is a risk that the fracture of the end portion will progress along the extension direction of the weld metal and that cracks will form throughout the entire weld metal. As the tensile strength of the steel plate increases, the tensile stress applied to the end portion of the weld metal after the laser welding is completed increases, and the risk of cracks occurring at the end portion of the weld metal increases. If cracks occur in the weld metal along its entire length, static strengths such as shear strength and peel strength of the joint are reduced, and fatigue strength is also significantly reduced. For this reason, low-temperature cracking is a particular problem in high-strength steel plates.

[0116] From the perspective of increasing the strength of a laser welded joint, it is desirable for the strength of the steel plate to be higher. For example, the tensile strength of one or more of the steel plates among the plurality of steel plates may be 980 MPa or higher, 1000 MPa or higher, or 1100 MPa or higher. In addition, the tensile strength of all of the plurality of steel plates may be 980 MPa or higher, 1000 MPa or higher, or 1100 MPa or higher. As the tensile strength of the steel plate increases, the tensile stress applied to the end portion of the weld metal after welding increases; however, in the laser welded joint according to the present embodiment, diffusible hydrogen is reduced and low-temperature cracking is suppressed by meeting the requirements described below.

[0117] The tensile strength of the steel plate is measured by cutting a test specimen according to the size of the part in accordance with JIS Z 2241. The location where the test specimen is cut is a flat section spaced at least 5 mm away from the weld.

[0118] Vickers hardness and tensile strength of a steel plate are both parameters representing the strength of the steel plate, and can be converted to each other by the following Equation A. In Equation A, TS represents the tensile strength of the steel plate, and HV represents the Vickers hardness of the steel plate.

[0119] TS=2.87×HV+59 … Equation A

[0120] (Al penetration amount in base material)

[0121] In the laser welded joint according to the present embodiment, the amount of Al penetrated into the base material of a plurality of steel plates is greater than 0.10 mass% and less than or equal to 2.10 mass%. The reason for defining the amount of Al penetrated into the base material in this way is to supply the Al necessary to stably form the Al slag described later to the weld metal from each steel plate being joined.

[0122] The amount of Al penetrated into the base material is defined as the sum of the values ​​obtained by multiplying the amount of Al in each joined steel plate by the ratio of the penetration cross-sectional area of ​​that steel plate. That is, the amount of Al penetrated into the base material is defined by the following Equation B.

[0123] Amount of Al penetrated into base material = ([Ratio of the penetration cross-sectional area of ​​steel plate (1)] × [Amount of Al in steel plate (1)]) + ([Ratio of the penetration cross-sectional area of ​​steel plate (2)] × [Amount of Al in steel plate (2)]) ··· + ([Ratio of the penetration cross-sectional area of ​​steel plate n] × [Amount of Al in steel plate n]) … Equation B

[0124] In the case of an overlapping laser welding joint or a butt laser welding joint as described above, the penetration cross-sectional area of ​​the steel plate shall be the area where weld metal is formed in a cutting plane perpendicular to the plate surface of each steel plate being joined. Specifically, the penetration cross-sectional area of ​​each steel plate shall be the area enclosed by the plate surface and / or end surface of each steel plate and the boundary between the steel plate and the weld metal. However, if the location of the plate surface or end surface of the steel plate cannot be determined from the cutting plane, the location of the plate surface or end surface may be determined by extending the plate surface or end surface of the portion of the laser welding joint where weld metal is not formed.

[0125] FIG. 9 shows a schematic cross-sectional view of a cut section to explain an example of the penetration cross-sectional area of ​​a steel plate in an overlapping laser welding joint. In the overlapping laser welding joint (1) shown in FIG. 9, it is assumed that the amount of Al in the steel plate (11) is 0.15 mass%, the amount of Al in the steel plate (12) is 0.20 mass%, and the thickness t1 of the steel plate (11) and the thickness t2 of the steel plate (12) are both 1.6 mm. In the steel plate (11), a weld metal (20) is formed throughout the entire thickness direction of the plate, and the penetration cross-sectional area m1 of region M1 in the weld metal is 2.4 mm 2 And, in the steel plate (12), weld metal (20) is formed up to about half the thickness direction of the plate, and the penetration cross-sectional area m2 of region M2 in the weld metal is 1.2 mm 2 am.

[0126] Additionally, in the example of FIG. 9, the respective plate surfaces (11a, 11b, and 12a) of the steel plate (11) and steel plate (12) are extended along the X coordinate axis to specify the cross-sectional areas of regions M1 and M2. In FIG. 9, regions M1 and M2 are each shown as areas enclosed by a dashed line within the weld metal (20).

[0127] The ratio of the penetration cross-sectional area of ​​the steel plate (11) is m1 / (m1+m2)=2.4 / (2.4+1.2), and the ratio of the penetration cross-sectional area of ​​the steel plate (12) is m2 / (m1+m2)=1.2 / (2.4+1.2). Therefore, the above-described formula B is expressed as {2.4 / (2.4+1.2)×0.15}+{1.2 / (2.4+1.2)×0.20}, and the amount of Al penetrating the base material is approximately 0.17 mass%.

[0128] FIG. 10 shows a schematic cross-sectional view of a cut section to explain an example of the penetration cross-sectional area of ​​a steel plate in a butt laser welded joint. In the example of FIG. 10, the respective end surfaces (not shown) of the butt-joined steel plates (11) and steel plates (12), where weld metal is not formed, are extended along the Y coordinate axis, and the plate surface (11a) of the steel plate (11) and the plate surface (12a, 11b, 12a, and 12b) of the steel plate (12), where weld metal is not formed, are extended along the X coordinate axis to specify the cross-sectional areas of regions M1 and M2, respectively. The dotted line in FIG. 10 represents a line extending the plate surface and end surface of the steel plates. In FIG. 10, regions M1 and M2 are each shown as areas enclosed by the dotted line within the weld metal (20). FIG. 10 also corresponds to the cross-sectional view of a cut section in FIG. 3.

[0129] FIG. 11 shows a schematic cross-sectional view of a cut section to explain an example of the penetration cross-sectional area of ​​a steel plate in a T-shaped fillet laser welded joint. In the example of FIG. 11, the end surface (11A) of the steel plate (11) where weld metal is not formed and the plate surface (12a) of the steel plate (12) are extended in the direction along the X coordinate axis, and the plate surface (11a) of the steel plate (11) where weld metal is not formed is extended in the direction along the Z coordinate axis to specify the cross-sectional areas of regions M1 and M2, respectively. The dotted line in FIG. 11 represents a line extending the plate surface and the end surface of the steel plate. In FIG. 11, regions M1 and M2 are each shown as areas enclosed by the dotted line within the weld metal (20). FIG. 11 also corresponds to the cross-sectional view of a cut section in FIG. 5.

[0130] FIG. 12 shows a schematic cross-sectional view of a cut section to explain an example of the penetration cross-sectional area of ​​a steel plate in an overlapping fillet laser welded joint. In the example of FIG. 12, the end surface (11A) of the steel plate (11) where weld metal is not formed is extended in the direction along the Y coordinate axis, and the plate surface (11b) of the steel plate (11) and the plate surface (12a) of the steel plate (12) where weld metal is not formed are extended in the direction along the X coordinate axis to specify the cross-sectional areas of regions M1 and M2, respectively. The dotted line in FIG. 12 represents a line extending the plate surface and the end surface of the steel plate. In FIG. 12, regions M1 and M2 are each shown as areas enclosed by the dotted line within the weld metal (20). FIG. 12 also corresponds to the cross-sectional view of a cut section in FIG. 7.

[0131] The ratio of the penetration cross-sectional area of ​​the steel plates is the ratio of the penetration cross-sectional area to the total sum of the penetration cross-sectional areas of all joined steel plates.

[0132] Since there may be an abnormal region of about 5 mm at the beginning and end of the weld metal, when measuring the penetration cross-sectional area of ​​the steel plate, a cross-section is taken from the portion excluding this abnormal region. After grinding the taken cross-section and etching it with a picrol, the penetration depth is measured using an optical microscope at 20 to 80 magnification. In laser welding, since there are cases where the penetration shape changes slightly due to pulsation, it is preferable to take and measure about three cross-sections. When multiple cross-sections are measured, the penetration cross-sectional area of ​​the steel plate is calculated by arithmetically averaging the penetration cross-sectional areas.

[0133] (Average Al content of weld metal)

[0134] In the laser welded joint according to the present embodiment, the average Al content of the weld metal is 0.30 mass% or more and 2.00 mass% or less. As described below, in order for slag containing Al to cover 30.0% or more of the surface of the weld metal, the average Al content of the weld metal needs to be 0.30% or more. Furthermore, from the perspective of ensuring the formation of slag, it is more preferable that the average Al content of the weld metal be 0.50% or more. In addition, by making the average Al content of the weld metal 2.00 mass% or less, the hardness of the weld metal is not reduced, and the joint strength of the weld can be secured.

[0135] The average amount of Al in the weld metal is measured by emission spectroscopic analysis. For the sample used in emission spectroscopic analysis, the entire portion of the weld metal is taken, excluding the area within 3 mm from the beginning and end. However, if the weld line is short and a sufficient amount of sample cannot be obtained, the beginning and end may be included in the sample. Samples are taken from three locations within this range, and the arithmetic mean of the measured amounts of Al in these samples is taken as the average amount of Al.

[0136] In addition, when measuring the average Al content of the weld metal, the measurement is performed including the slag. Therefore, a cross-section perpendicular to the elongation direction of the weld metal is cut and used as a sample for emission spectroscopic analysis.

[0137] Furthermore, weld metal can be distinguished from steel plates by visual inspection. The surface of a steel plate basically forms a surface with minimal irregularities. On the other hand, because weld metal has been molten once, wavy patterns known as ripples often form on its surface, and these differences in surface texture allow for differentiation between weld metal and steel plates.

[0138] (Average Al content, average Si content, average Mn content of weld metal)

[0139] In the laser welded joint according to the present embodiment, when the average amount of Al, average amount of Si, and average amount of Mn of the weld metal are denoted as [Al], [Si], and [Mn], respectively, the following Equation 1 is satisfied.

[0140] [Al] / ([Al]+[Si]+[Mn])≥0.150 … Equation 1

[0141] The average Si and Mn amounts of the weld metal, like the average Al amount, are measured by emission spectroscopy.

[0142] The denominator of Equation 1 represents the total amount of oxide-forming elements in the weld metal. The slag described below contains Al. Compared to slag containing other elements such as Si or Mn, this Al-containing slag is more likely to have a wider surface area of ​​the weld metal and is effective in suppressing hydrogen intrusion. By satisfying Equation 1, the surface area of ​​the weld metal covered by the Al-containing slag becomes 30.0%.

[0143] It is more preferable that the right-hand side of Equation 1 above be 0.211, 0.250, 0.300, or 0.500. By doing so, the area ratio of the slag containing Al covering the surface of the weld metal can be more reliably secured.

[0144] (Area ratio of slag containing Al)

[0145] In the laser welded joint according to the present embodiment, slag containing Al covers 30% or more of the surface of the weld metal. Hydrogen intrusion can be suppressed by the slag containing Al covering 30.0% or more of the surface of the weld metal, that is, by the area ratio of the slag containing Al on the surface of the weld metal being 30.0% or more. From the perspective of enhancing the effect of suppressing hydrogen intrusion, it is more preferable that the area ratio of the slag containing Al be 32.0% or more, 50.0% or more, or 70.0% or more.

[0146] In order to make the area ratio of slag containing Al on the surface of the weld metal 30% or more, the above-mentioned amount of Al penetrating the base metal needs to be greater than 0.10 mass% and less than or equal to 2.10 mass%. That is, at least 0.10 mass% of Al needs to be supplied from the base steel plate to the weld metal. For example, if there is no supply of Al from the base steel plate to the weld metal and Al is supplied only from the filler material or plating, the Al concentration in the weld metal becomes non-uniform, and regions with insufficient Al concentration occur, making it impossible to make the area ratio of slag containing Al 30.0% or more.

[0147] The area percentage of Al-containing slag on the surface of the weld metal is determined by analyzing the central portion of the weld metal using EDX (Energy Dispersive X-ray Analysis). Specifically, the area percentage of Al-containing slag is determined by the following procedure.

[0148] (1) Among the welded laser welded joints, the part that does not include the beginning and end portions of the weld metal, for example, the central portion of the weld metal (the surface where the weld metal is formed), is observed using a scanning electron microscope (SEM).

[0149] The field of view for observation by SEM is set as follows: vertical (in the width direction of the weld metal): greater than the width of the weld metal (a range that includes all of the weld metal within the field of view in the width direction of the weld metal), and horizontal (in the direction of the weld line): 3 to 5 times the width of the weld metal in the direction of the weld line.

[0150] (2) To investigate the distribution of Al-containing slag on the surface of the weld metal, mapping analysis is performed using EDX over the entire field of view of observation by SEM. The EDX mapping image is captured at 1280×1024 pixels and is displayed in 256 steps (levels) in the range from the minimum value to the maximum value of Al concentration. As a device used, the IT300 (acceleration voltage: 15kV, irradiation current: 7.5nA) manufactured by Nihon Denshi Co., Ltd. can be used.

[0151] (3) For any point, quantitative analysis is performed to find a point where the Al concentration is 3.0% (e.g., 3.0 to 3.1%), and the level (algae) of the pixel on the mapping image corresponding to the location of the point is specified. Point analysis is performed under the same conditions as the mapping analysis (acceleration voltage: 15 kV, irradiation current: 7.5 nA).

[0152] (4) In the mapping image by EDX, pixels that are lighter than the specific step (seaweed) in (3) are determined to be slag with an Al concentration of 3.0% or more and classified as white, while other pixels are classified as black and binarized. Within the range of the weld metal (e.g., between the two dashed lines shown in Fig. 14), the slag area ratio is calculated by (area ratio of slag containing Al) = (area of ​​white part within the range of the weld metal within the observation field) / (area of ​​the weld metal within the observation field). In addition, depending on the settings of the mapping image, pixels that are darker than a specific stage (seaweed) in (3) above may be determined as slag with an Al concentration of 3.0% or more and classified as black, and other pixels classified as white and binarized, and the slag area ratio may be calculated by (area ratio of slag containing Al) = (area of ​​black part in the range of weld metal within the observation field) / (area of ​​weld metal within the observation field). In addition, the processing of (1) to (4) above may be performed multiple times (5 times), and the average value of the multiple (5) slag area ratios calculated is used as the final slag area ratio.

[0153] The central part of the weld metal is a range that is spaced apart from the boundary between the weld metal and the base metal plate by at least the shorter of 0.4 mm or 0.3 times the width of the weld metal in a direction perpendicular to the extension direction of the weld metal, with respect to the part excluding the abnormal region as described above. With respect to the part excluding the abnormal region, the width of the weld metal is measured at three places in a direction perpendicular to the extension direction of the weld metal, and the arithmetic mean of the values ​​is taken as the width of the weld metal.

[0154] In addition, for overlapping laser weld joints or butt laser weld joints, observation is performed using a scanning electron microscope from a direction perpendicular to the surface of the base steel plate. For T-fillet laser weld joints or overlapping fillet laser weld joints, observation is performed using a scanning electron microscope while rotating at an arbitrary angle around the extension direction of the weld metal from the direction in which the surface of the weld metal is visible, and the value at the angle in which the highest Al concentration is detected is adopted as the Al concentration.

[0155] Then, as shown in Equation C below, the area of ​​the weld metal in each field of view mapped with the Al element is calculated. The ratio of the area of ​​the Al-containing slag to the area of ​​the weld metal is calculated, and the arithmetic mean value of the area ratios of the Al-containing slag at all measurement points is calculated as the area ratio of the Al-containing slag.

[0156] Area ratio of slag containing Al = (Area of ​​slag containing Al) / (Surface area of ​​weld metal) … Equation C

[0157] FIGS. 13 and 14 show (A) an example of an SEM image of a measurement location and (B) an example of a binarized image of the mapping image of Al at the measurement location of (A). FIG. 13 is an example where the average amount of Al in the weld metal is 0.03 mass%, and FIG. 14 is an example where the average amount of Al in the weld metal is 0.70 mass%. As shown in these examples, by performing a binarization transformation on the image mapping the Al element and calculating the ratio of the area to the area of ​​the weld metal, the area ratio of the range in which Al-containing slag covers the weld metal (area ratio of Al-containing slag) can be calculated. In FIG. 14, the white areas represent Al-containing slag. Also, in FIGS. 13 and 14, the dotted lines represent the boundary between the surface of the weld metal and the surface of the steel plate.

[0158] In the laser welded joint according to the present embodiment, it is more preferable that the ratio of the average amount of Al in the weld metal to the amount of Al penetrated into the base material of a plurality of steel plates is 0.80 to 1.20. As described above, in order for the slag containing Al to cover the surface of the weld metal by 30.0% or more, it is preferable that the Al in the weld metal originates from the steel plate which is the base material. By supplying Al from the base material to the weld metal, Al is uniformly supplied within the weld metal, thereby sufficiently satisfying the requirement that the area ratio of the slag containing Al on the surface of the weld metal is 30.0% or more.

[0159] For example, when a filler with a higher Al concentration than the Al concentration of the base material is used, Al is supplied to the weld metal not only from the steel plate but also from the filler used during laser welding. In addition, Al may be supplied to the weld metal from the plating layer provided on the steel plate. In such cases, the ratio of the average Al amount of the weld metal to the Al amount penetrated into the base material of multiple steel plates exceeds 1.00.

[0160] On the other hand, if the plating or filler does not contain Al, or if the Al concentration of the plating or filler is lower than the Al concentration of the base material, a small amount of Al contained in the molten steel may be lost as slag by combining with oxygen. In addition to the Al lost as slag, the average Al concentration of the weld metal may be lower than the Al concentration of the base material because weld metal is formed by mixing the plating or filler with the base material. In such cases, the ratio of the average Al amount of the weld metal to the Al amount penetrating the base material of multiple steel plates becomes 1.00 or less.

[0161] In the laser welded joint according to the present embodiment, it is more preferable that the chemical composition of the weld metal is as follows in mass%.

[0162] C: 0.1 to 0.6%,

[0163] Si: 0.005 to 3.0%,

[0164] Mn: 0.5 to 3.0%,

[0165] Al: 0.30 to 2.00%,

[0166] P: 0.04% or less,

[0167] S: 0.01% or less,

[0168] N: 0.1% or less,

[0169] O: 0.1% or less,

[0170] Cu: 0 to 1.0%,

[0171] Nb+Ti+V: 0 to 0.3%,

[0172] Ca+REM: 0 to 0.01%,

[0173] B: 0 to 0.005%,

[0174] Cr: 0 to 2.0%,

[0175] Ni: 0 to 1.0%,

[0176] Mo: 0 to 1.0%,

[0177] Sn: 0.1% or less,

[0178] Mg: 0 to 0.01%,

[0179] Sb: 0 to 0.1%,

[0180] As: 0 to 0.1%,

[0181] Remainder: Contains Fe and impurities.

[0182] By having such a chemical composition, the laser welded joint according to the present embodiment has the advantage of being able to further reduce the concentration of diffusible hydrogen and further suppress the occurrence of low-temperature cracking. In addition, the elements of the weld metal are measured by emission spectroscopic analysis, and C and S are measured by infrared absorption.

[0183] In the laser welded joint according to the present embodiment, it is more preferable that the plurality of steel plates are non-plated steel plates or zinc-plated steel plates. It is preferable that the Al in the weld metal originates from the base steel plate. Therefore, in order to effectively utilize the Al of the base material, it is preferable to use non-plated steel plates or zinc-plated steel plates. Examples of plating include GI plating, GA plating, EG plating, Zn-Ni plating, Zn-Mg plating, etc.

[0184] All of the multiple steel plates may be non-plated steel plates, or any of the multiple steel plates may be non-plated steel plates. Likewise, all of the multiple steel plates may be zinc-plated steel plates, or any of the multiple steel plates may be zinc-plated steel plates.

[0185] In the laser welded joint according to the present embodiment, it is preferable that the Vickers hardness of the weld metal be 450 HV or higher. By having a Vickers hardness of 450 HV or higher, strength as a joint can be secured. In the laser welded joint according to the present embodiment, it is more preferable that the Vickers hardness of the weld metal be 500 HV or higher. By having a Vickers hardness of 500 HV or higher, strength as a joint can be further secured.

[0186] The Vickers hardness of the weld metal is measured by the following method. A sample is taken by cutting a laser welded joint containing the weld metal in a plane that intersects the extension direction of the weld metal and is perpendicular to the plane of the joined steel plate. This sample is polished (for the weld metal to be easily identified, it may be etched after polishing, for example, using a picrol), and the hardness of the weld metal is measured at three or more points using a Vickers hardness tester in accordance with JIS Z2244 (Vickers hardness test), and the arithmetic mean is calculated as the Vickers hardness of the weld metal. As for the measurement conditions for Vickers hardness, an example can be given with a load of 300 gf, a pitch of 0.2 mm, and a holding time of 10 seconds.

[0187] [Method for manufacturing laser welded joints]

[0188] Hereinafter, a method for manufacturing a laser welded joint according to the present embodiment is described. According to this manufacturing method, the laser welded joint (1) according to the present embodiment can be suitably manufactured. However, it is obvious that a laser welded joint obtained by a method other than the manufacturing method described below is considered to be the laser welded joint (1) according to the present embodiment, provided that it satisfies the requirements of the above.

[0189] The method for manufacturing a laser welded joint according to the present embodiment is,

[0190] A process for preparing multiple steel plates (S1: preparation process), and

[0191] A process for arranging multiple steel plates (S2: arrangement process), and

[0192] It includes a laser welding process (S3: laser welding process) for forming the welding metal for joining a plurality of steel plates.

[0193] In the preparation process, steel plates to be joined are prepared. The thickness and chemical composition of the steel plates to be joined may be adopted from the embodiments described above.

[0194] In the placement process, the steel plates prepared in the above preparation process are placed at predetermined positions. In the placement process, the steel plates are appropriately arranged to form the aforementioned overlapping laser welding joints, butt laser welding joints, and fillet laser welding joints, in accordance with the shape of the product being manufactured.

[0195] In the laser welding process, laser welding is performed on a plurality of steel plates arranged in the above-mentioned batching process. In the laser welding process, a laser beam is irradiated onto the steel plates to melt them, and the molten metal solidifies between the steel plates to form weld metal, thereby joining the steel plates together.

[0196] In the laser welding process according to the present embodiment, laser welding is performed in an environment with an oxygen concentration of 3.0% or higher. In the area where the metal is melted by irradiating the laser beam (also referred to as the molten zone), elements from the external environment are introduced in addition to the constituent metals of the steel plate, filler, and plating. However, by performing laser welding in an environment with an oxygen concentration of 3.0% or higher and satisfying the chemical composition of the weld metal as described in Equation 1 above, an appropriate amount of oxygen can be supplied to the weld metal, and the area ratio of the Al-containing slag formed on the surface of the weld metal can be 30.0% or higher.

[0197] In conventional laser welding of high-strength steel plates, it was common practice to use a shielding gas such as argon to prevent oxidation. However, in the method for manufacturing a laser welded joint according to the present embodiment, by performing laser welding in an environment with an oxygen concentration of 3.0% or higher, a slag containing a desired amount of Al can be formed at the end portion of the weld metal.

[0198] The conditions of the laser beam are not particularly limited, but from the perspective of productivity, it is desirable to use a solid-state laser device or a semiconductor laser device.

[0199] In the laser welding process, a filler may be used. The chemical composition of the filler is not particularly limited, but from the perspective of increasing the source of Al, a filler having a chemical composition of, for example, Si: 0.5 to 0.8 mass%, Mn: 1.0 to 2.0 mass%, Al: 0.50 to 1.00 mass%, or Si: 0.7 mass%, Mn: 1.5 mass%, Al: 0.70 mass% may be used. When Al is included in the chemical composition of the filler, Al derived from the filler is also included in the weld metal.

[0200] The laser welded joint according to the above embodiment can be preferably used in automotive parts. Examples of automotive parts to which the laser welded joint according to the above embodiment is applied include A-pillars, B-pillars, roof rails, side sills, floor cross members, bumpers, crash boxes, instrument panel reinforcements, seat frames, battery cases, etc. By applying the laser welded joint according to the present embodiment as a joint for these parts, low-temperature cracking can be suppressed, strength as a joint can be secured, and a product with excellent productivity can be manufactured.

[0201] Examples

[0202] The present invention will be described in detail below with reference to examples, but the invention is not limited thereto.

[0203] For steel plates A to O of each chemical composition (the remainder being Fe and impurities) shown in Table 1, laser welded joints were fabricated using various combinations of steel plates shown in Table 2. Specifically, two steel plates were overlapped using the combinations of steel plates shown in Table 2, and overlapping laser welding was performed.

[0204] In addition, the chemical composition of the steel plate was measured by emission spectroscopic analysis as described in the above embodiment. The tensile strength and plate thickness of the steel plate were measured by the method described in the above embodiment. The Vickers hardness of the steel plate was calculated from Equation A based on the tensile strength.

[0205] In the example where a filler was used, the chemical composition of the filler was set to Si: 0.7 mass%, Mn: 1.5 mass%, and Al: 0.70 mass%.

[0206] The oxygen concentration in the laser welding process was as described in Table 2. In addition, conditions that do not satisfy the requirements of the present invention were underlined.

[0207]

[0208]

[0209] (Al penetration amount in base material of steel plate)

[0210] The amount of Al penetrated into the base material of the steel plate was calculated based on the ratio of the chemical composition of the steel plate and the penetration cross-sectional area measured by emission spectroscopic analysis, as described in the above embodiment.

[0211] (Average Al amount, average Si amount, and average Mn amount of weld metal)

[0212] The average Al, average Si, and average Mn amounts of the weld metal were measured by emission spectroscopic analysis as described in the above embodiment. In addition, Equation 1 was calculated based on these measured values.

[0213] The ratio of the average amount of Al in the weld metal to the amount of Al penetrated into the base metal of multiple steel plates (Al ratio) was calculated based on the above measurements.

[0214] In addition, based on the measurements, the sum of the average Al, average Si, and average Mn amounts of the weld metal (Al+Si+Mn) was calculated.

[0215] (Vickers hardness of weld metal)

[0216] For each laser welded joint No. 1 to 29, the Vickers hardness of the weld metal was measured.

[0217] Specifically, a laser welded joint containing weld metal was cut in a plane that intersects the extension direction of the weld metal and is perpendicular to the surface of the joined steel plate to take a sample, and this sample was polished and etched with a picrol, and measured using a Vickers hardness tester in accordance with the method of JIS Z2244 (Vickers hardness test). The load was set to 300 gf, the measurement position was set to a pitch of 0.2 mm, and the holding time was set to 10 seconds. Multiple measurements were taken at a position 1 / 4 of the plate thickness from the surface of the opposing steel plate along the butt interface of the steel plate, and the arithmetic mean of these measurements was calculated as the Vickers hardness of the weld metal.

[0218] In addition, having a Vickers hardness of 450 HV or higher was accepted. This is because if the Vickers hardness of the weld metal is low, the joint strength will be reduced.

[0219] (Area ratio of the weld metal covered by Al-containing slag)

[0220] For each laser welded joint of No. 1 to 29, the area ratio of Al-containing slag covering the weld metal in the central part of the weld metal (Al slag area ratio) was measured. Specifically, it was measured as follows.

[0221] The area ratio of Al-containing slag on the surface of the weld metal was determined by excluding the region (beginning and end portions of the weld metal) that constitutes an abnormal part of the laser weld joint, and by analyzing the central portion of the weld metal described in the above embodiment using a scanning electron microscope (IT300 manufactured by Nihon Denshi Co., Ltd.) by EDX (Energy Dispersive X-ray Analysis). Specifically, quantitative analysis was performed on the central portion of the weld metal within the range described in the above embodiment. Then, as described in the above embodiment, locations where an Al concentration of 3.0% or more was recognized were determined to be Al-containing slag, and a binarization threshold was determined to obtain a binarized image.

[0222] And, as described in the above embodiment, the area of ​​the weld metal in the mapping image was calculated, and the area ratio of the slag containing Al to the area of ​​the weld metal was calculated based on the binarized image.

[0223] (Assessment of cracks)

[0224] For each laser welded joint from No. 1 to 29, a crack evaluation test was performed.

[0225] Specifically, within 72 hours after welding, a laser welded joint containing the weld metal was cut along a plane that intersects the extension direction of the weld metal and is perpendicular to the plane of the joined steel plate to obtain a sample. This sample was then polished, etched with a picrol, and observed under an optical microscope to determine the presence or absence of cracks. The crack evaluation was performed three times, and the result was marked as “○ (Good)” if no cracks were found in any of the tests, and “× (Bad)” if even one crack was found. Since cracks grow from craters, the crack evaluation was performed at locations near the crater (within 5 mm) at the end. For each laser welded joint, the crack evaluation test was performed with a repetition count of 3 (n3).

[0226] In Comparative Examples 1 to 4, 6, 7, 9 to 11, none of the requirements of the present invention were satisfied, and cracks occurred in the weld metal and the HAZ (heat-affected zone).

[0227] In Comparative Examples 5 and 8, no cracks were observed, but the Vickers hardness did not meet the criteria.

[0228] Meanwhile, as can be understood from the results of Table 2, the laser welded joint according to the present invention shows good results in the crack evaluation test, and low-temperature cracking is suppressed. In addition, from the results of Table 2, it can be seen that by appropriately controlling the oxygen concentration in the laser welding process, the area of ​​the end portion of the weld metal covered by Al-containing slag can be made to a desired range. Industrial applicability

[0229] According to the laser welding joint and the method for manufacturing the same of the present invention, low-temperature cracking can be suppressed and strength as a joint can be secured, making it very useful in industry. Explanation of the symbols

[0230] 1: Laser welded joint 11, 12: Steel plate 20: Welding metal 21: The end 201: Crater

Claims

Claim 1 A laser welding joint comprising a plurality of steel plates and a welding metal for joining the plurality of steel plates, wherein at least one of the plurality of steel plates is a high-strength steel plate with a Vickers hardness of 320 HV or higher, the amount of Al penetrated into the base material of the plurality of steel plates is greater than 0.10 mass% and less than or equal to 2.10 mass%, the average amount of Al in the welding metal is greater than 0.30 mass% and less than or equal to 2.00 mass%, and when the average amount of Al, average amount of Si, and average amount of Mn in the welding metal are denoted as [Al], [Si], and [Mn], respectively, the following Equation 1 is satisfied, and a slag containing Al covers 30.0% or more of the surface of the welding metal. [Al] / ([Al]+[Si]+[Mn])≥0.150 … Equation 1 Claim 2 A laser welded joint according to claim 1, characterized in that the ratio of the average amount of Al in the weld metal to the amount of Al penetrated into the base material of the plurality of steel plates is 0.80 to 1.

20. Claim 3 In claim 1, the chemical composition of the weld metal is, in mass%, C: 0.1 to 0.6%, Si: 0.005 to 3.0%, Mn: 0.5 to 3.0%, Al: 0.30 to 2.00%, P: 0.04% or less, S: 0.01% or less, N: 0.1% or less, O: 0.1% or less, Cu: greater than 0%, 1.0% or less, Nb+Ti+V: greater than 0%, 0.3% or less, Ca+REM: greater than 0%, 0.01% or less, B: greater than 0%, 0.005% or less, Cr: greater than 0%, 2.0% or less, Ni: greater than 0%, 1.0% or less, Mo: greater than 0%, 1.0% or less, Sn: greater than 0%, 0.1% or less, Mg: 0% A laser welded joint characterized by containing: excess, 0.01% or less, Sb: excess, 0.1% or less, As: excess, 0.1% or less, remainder: Fe and impurities. Claim 4 A laser welding joint according to claim 1, characterized in that at least one of the plurality of steel plates is a non-plated steel plate or a zinc-plated steel plate. Claim 5 A laser welded joint according to claim 1, characterized in that the Vickers hardness of the weld metal is 450 HV or higher. Claim 6 A laser welding joint characterized in that, in claim 1, it is an overlapping laser welding joint, a butt laser welding joint, or a fillet laser welding joint. Claim 7 A laser welded joint according to claim 1, characterized in that at least one of the plurality of steel plates is a high-strength steel plate having a tensile strength of 980 MPa or more. Claim 8 In claim 1, the chemical composition of one or more of the plurality of steel plates is, in mass%, C: greater than 0.15% and less than or equal to 0.5%, Si: 0.1 to 3.5%, Mn: 0.2 to 5.5%, Al: greater than 0.10% and less than or equal to 2.10%, N: 0.1% and less, O: 0.1% and less, P+S: 0.050% and less, Cu: greater than 0% and less than or equal to 1.0%, Nb+Ti+V: greater than 0% and less than or equal to 0.3%, Ca+REM: greater than 0% and less than or equal to 0.01%, B: greater than 0% and less than or equal to 0.005%, Cr: greater than 0% and less than or equal to 2.0%, Ni: greater than 0% and less than or equal to 1.0%, Mo: greater than 0% and less than or equal to 1.0%, Sn: greater than 0% and less than or equal to 0.1% A laser welded joint characterized by containing, less than or equal to, Mg: greater than 0%, less than or equal to 0.01%, Sb: greater than 0%, less than or equal to 0.1%, and As: greater than 0%, less than or equal to 0.1%, with the remainder being Fe and impurities. Claim 9 A method for manufacturing a laser welded joint as described in any one of claims 1 to 8, comprising a process of preparing a plurality of steel plates, a process of arranging the plurality of steel plates, and a laser welding process for forming a weld metal for joining the plurality of steel plates, wherein in the laser welding process, laser welding is performed in an environment with an oxygen concentration of 3.0% or higher. Claim 10 A method for manufacturing a laser welded joint according to claim 9, characterized in that, in the laser welding process, the source of the laser beam is a solid-state laser device or a semiconductor laser device.

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