Butt weld joint, tailored blank, and method for manufacturing butt weld joint

Abstract

A butt weld joint having two high-strength steel plates and a weld metal that joins the high-strength steel plates to each other, said butt weld joint being characterized in that the high-strength steel plates include a normal section and a HAZ section that is adjacent to the weld metal, the normal section has a Vickers hardness of 372 HV or higher, the HAZ section has a HAZ hardened section that has a higher Vickers hardness than the normal section and a HAZ softened section that has a lower Vickers hardness than the normal section, the average width of the weld metal is 1.00-1.65 mm, and among the 500 gf load Vickers hardnesses when locations at 1 / 4 in the thickness direction of the high-strength steel plates from the surfaces of the high-strength steel plates are measured at 0.15 mm intervals, the average Vickers hardness of the weld metal is 60.00-83.00% of the maximum hardness of the HAZ hardened section.

Description

Butt welded joint, tailored blank and method for manufacturing butt welded joint

[0001] This application claims priority to Japanese Patent Application No. 2024-026405, filed February 26, 2024, the contents of which are incorporated herein by reference.

[0002] To reduce the weight of automobiles, the use of tailored blanks made of high-strength steel plates is expected. A tailored blank is formed by joining multiple types of steel plates by welding. The tailored blank is subjected to press working, bending, cutting, and the like to form a desired product shape. For example, Patent Document 1 discloses a technology relating to a tailored blank in which the ends of steel plates are joined by welding. Furthermore, Patent Document 2 discloses a technology in which a portion of a welded portion cut after the manufacture of a tailored blank is partially softened by heat treatment in order to improve the workability of the welded portion.

[0003] Japanese Patent Publication No. 2021-122914 Japanese Patent No. 7172107

[0004] The present inventors have found that when cold-pressing the above-described tailored blank, particularly a tailored blank made of a high-strength steel plate having a Vickers hardness of 372 HV or more, hydrogen embrittlement occurs due to hardening of the weld metal at the weld, and cracks may occur if press forming is performed shortly after welding.

[0005] Factors that cause hydrogen embrittlement include the hardness of the weld metal, hydrogen absorbed into the weld metal during welding, and stress introduced into the weld due to processing. The inventors have focused on the hardness of the weld metal among these factors to study ways to suppress cracking due to hydrogen embrittlement. Note that cracking can be suppressed by performing press forming after sufficient time has passed since welding until hydrogen has diffused out of the weld metal (e.g., after 24 hours have passed), but this increases the time from welding to press forming, resulting in lower productivity.

[0006] The invention of the present disclosure has been made in consideration of the above, and has an object to provide a butt-welded joint, a tailored blank, and a method for manufacturing a butt-welded joint that can suppress cracking in the welded portion even when pressed shortly after welding.

[0007] (1) A butt-welded joint according to one aspect of the present disclosure is a butt-welded joint having two high-strength steel plates and a weld metal joining the high-strength steel plates, wherein the high-strength steel plates include a normal portion and a HAZ portion adjacent to the weld metal, wherein the normal portion has a Vickers hardness of 372 HV or more, the HAZ portion has a HAZ-hardened portion having a higher Vickers hardness than the normal portion, and a HAZ-softened portion having a lower Vickers hardness than the normal portion, the weld metal has an average width of 1.00 to 1.65 mm, and the average Vickers hardness of the weld metal is 60.00 to 83.00% of the maximum hardness of the HAZ-hardened portion, measured at 0.15 mm intervals from the surface of the high-strength steel plate at a load of 500 gf. (2) In the butt-welded joint described in (1) above, the average Vickers hardness of the weld metal may be lower than the Vickers hardness of the normal portion of the high-strength steel plate. (3) In the butt-welded joint described in (1) or (2) above, the area ratio of a structure having a BCC structure in the structure of the weld metal may be 50% or more. (4) In the butt-welded joint described in any one of (1) to (3) above, the plate thickness of the high-strength steel plate may be 0.8 to 2.0 mm. (5) In the butt-welded joint described in any one of (1) to (4) above, the aspect ratio of the weld metal, defined by the average width of the weld metal to the average value of the plate thicknesses of the two high-strength steel plates, may be less than 2. (6) A tailored blank according to one aspect of the present disclosure is characterized by including the butt-welded joint described in any one of (1) to (5) above.(7) A manufacturing method of a butt-welded joint according to an aspect of the present disclosure is a manufacturing method of a butt-welded joint having two high-strength steel plates and a weld metal joining the high-strength steel plates, the manufacturing method comprising: a step of arranging the high-strength steel plates so that the welding surfaces of the high-strength steel plates face each other with a gap therebetween; and a step of butt-welding the high-strength steel plates using a filler wire, wherein the high-strength steel plates have a Vickers hardness of 372 HV or more, a size of the gap is 0.4 to 1.0 mm, a C content of the filler wire is more than 0 mass % and 0.10 mass % or less, a value obtained by dividing the size of the gap by the C content of the filler wire is 25.0 or less, and a thickness of the high-strength steel plates is 0.8 to 2.0 mm.

[0008] According to the butt welded joint, tailored blank, and method for manufacturing a butt welded joint disclosed herein, cracking can be suppressed even when pressing is performed shortly after welding.

[0009] The present disclosure relates to a butt-welded joint, a weld metal, and a HAZ portion of the butt-welded joint, and a method for measuring the Vickers hardness of the weld metal and the HAZ portion of the butt-welded joint.

[0010] Hereinafter, embodiments of the invention according to the present disclosure will be described using examples, but it is clear that the invention according to the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be used as examples, but other numerical values ​​and materials may be applied as long as the effects of the invention according to the present disclosure are obtained. Furthermore, the components of the following embodiments can be combined with each other. Furthermore, in this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits. In this specification, the term "process" includes not only an independent process, but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

[0011] The butt-welded joint according to this embodiment is a butt-welded joint having two high-strength steel plates and a weld metal that joins the high-strength steel plates. As shown in Fig. 1 , in the butt-welded joint 1, a high-strength steel plate 10 and a high-strength steel plate 20 are connected by a weld metal 30. Fig. 1 is a schematic cross-sectional view for illustrating a butt-welded joint according to one embodiment of the present disclosure, showing a cross section perpendicular to a weld line L along which the weld metal 30 is formed. Fig. 1 shows a case in which the thickness of the high-strength steel plate 10 is thinner than the thickness of the high-strength steel plate 20.

[0012] (High-strength steel plate) The high-strength steel plate 10 includes a normal portion 11 and a HAZ portion 12 adjacent to the weld metal 30. The high-strength steel plate 20 includes a normal portion 21 and a HAZ portion 22 adjacent to the weld metal 30.

[0013] (Normal portion) The normal portion 11 and the normal portion 21 are portions that are not affected by heat due to welding when providing the weld metal 30. For example, in laser welding, the normal portion 11 or the normal portion 21 is generally a portion that is 10 mm or more away from the toe of the weld metal 30 along a direction parallel to the plate surface of the high-strength steel plate 10 or the high-strength steel plate 20 and perpendicular to the weld line L of the butt-welded joint 1. The normal portion 11 or the normal portion 21 can be distinguished from the HAZ portion 12 or the HAZ portion 22 by Vickers hardness. The normal portion 11 and the normal portion 21 have a Vickers hardness of 372 HV or more.

[0014] FIG. 2 shows a distribution diagram of Vickers hardness in the butt-welded joint 1 according to this embodiment. In FIG. 2, the vertical axis represents Vickers hardness (HV), and the horizontal axis represents the relative position in the direction of extension of the plate surfaces in a plane perpendicular to the weld line L (corresponding to the horizontal direction in FIG. 1). The distribution diagram in FIG. 2 plots the Vickers hardness values ​​measured at a load of 500 gf at 0.15 mm intervals in a direction parallel to the surfaces of the high-strength steel plates at a position ¼ of the thickness from the surface of the thinner high-strength steel plate on the side with the smaller step. The area A in FIG. 2 represents the weld metal 30, the area B represents the HAZ-hardened zones 12h and 22h, and the area C represents the HAZ-softened zones 12s and 22s. The dotted lines in FIG. 2 represent the Vickers hardness of the normal portion of the high-strength steel plate. In the case of butt welded joint 1 in the distribution diagram shown in Figure 2, the Vickers hardness of the normal portion is 500 HV. Note that in the case of butt welded joint 1 in the distribution diagram shown in Figure 2, the Vickers hardness of the normal portions of the two high-strength steel plates is the same. From Figure 2, it can be seen that the Vickers hardness is higher in the HAZ-hardened portion (range B) than in the normal portion, and that the Vickers hardness is lower in the HAZ-softened portion (range C) than in the normal portion.

[0015] The tensile strength of the normal portion 11 or the normal portion 21 is measured, for example, in accordance with JIS Z 2241 (2011), by preparing a JIS No. 5B-shaped test piece and conducting a tensile test using a tensile tester. By this method, the tensile strength is measured at, for example, three locations, and the arithmetic mean value of these values ​​is defined as the tensile strength of the normal portion 11 or the normal portion 21. From the viewpoint of reducing the weight of the member, it is more preferable that the tensile strength of the normal portion 11 or the normal portion 21 is 1180 MPa or more, 1300 MPa or more, or 1470 MPa or more.

[0016] The Vickers hardness of the normal portion 11 or 21 is measured using a Vickers hardness tester according to the following procedure. A plane perpendicular to the weld line L of the butt-welded joint 1 to be measured, including the weld metal 30, is cut out over a range of approximately 30 mm in a direction parallel to the surfaces of the high-strength steel plates 10 and 20, embedded in resin, polished to a mirror finish, and then etched in a picric acid alcohol solution to prepare a sample. The cutout position excludes a range of 30 mm from the welding start point and welding end point. If the welded joint has been processed by a press or the like, work hardening occurs in the bent portion, making it impossible to accurately measure the Vickers hardness. Therefore, when measuring a processed welded joint, it is best to collect a sample from a flat surface that has not been bent as much as possible. Note that the surface used to measure the Vickers hardness of the normal portion 11 or 21 may be the same as the surface used to measure the Vickers hardness of the HAZ portion 12, which will be described later. For this sample, the Vickers hardness is measured at five or more points at positions 10 mm or more away from the center of the weld metal 30 in a direction parallel to the surfaces of the high-strength steel plates 10 and 20 and 100 μm or more away from the front and back surfaces of the butt-welded joint 1, and the arithmetic mean value of these measurements is defined as the Vickers hardness of the ordinary portion 11 or the ordinary portion 21. The Vickers hardness is measured under a load of 500 gf. From the viewpoint of reducing the weight of the member, it is more preferable that the Vickers hardness of the ordinary portion 11 or the ordinary portion 21 be 402 HV or more, or 440 HV or more.

[0017] The tensile strength and Vickers hardness can be converted from psi using the JIS Z 8413 and Z 8438 conversion tables.

[0018] (HAZ portion) The HAZ portion 12 has a HAZ-hardened portion 12h having a higher Vickers hardness than the normal portion 11, and a HAZ-softened portion 12s having a lower Vickers hardness than the normal portion 11. The HAZ-hardened portion 12h is located closer to the weld metal 30 than the HAZ-softened portion 12s. The HAZ-hardened portion 12h is a portion that is rapidly cooled after reaching a high temperature of approximately 900 to 1500°C due to heat input during welding. Therefore, the HAZ-hardened portion 12h is heated to a transformation point of approximately 900°C or higher and then quenched, resulting in a structure having a BCC structure including martensite as the main component. The HAZ-softened portion 12s is affected by heat during welding, but its temperature does not rise above the transformation point. Therefore, tempering occurs in the HAZ-softened portion 12s, resulting in a lower Vickers hardness than the normal portion 11.

[0019] The Vickers hardness of the HAZ 12 is determined using a Vickers hardness tester as follows. A plane perpendicular to the weld line L of the butt-welded joint 1 to be measured is cut out over a range of approximately 30 mm in a direction parallel to the surfaces of the high-strength steel plate 10 and the high-strength steel plate 20, including the weld metal 30. The cutout is embedded in resin, polished to a mirror finish, and then etched in a picric acid alcohol solution to prepare a sample. The cutout position excludes a range of 30 mm from the welding start point and welding end point. If the welded joint has been processed by a press or the like, work hardening occurs at the bent portion, making it impossible to measure the Vickers hardness accurately. Therefore, when measuring a processed welded joint, it is best to collect a flat sample that is as unbent as possible.

[0020] For this sample, the Vickers hardness is measured at 0.15 mm intervals in a direction parallel to the surface of the high-strength steel plate 10 at a position that is 1 / 4 of the thickness of the high-strength steel plate 10 or 20 from the surface. As shown in FIG. 3 , when the high-strength steel plate 10 and the high-strength steel plate 20 have different thicknesses, first, as shown in FIG. 3 , the high-strength steel plate 10 having the smaller thickness is identified. Then, the Vickers hardness is measured at 0.15 mm intervals in a direction parallel to the surface of the high-strength steel plate 10 at a position that is 1 / 4 of the thickness of the high-strength steel plate 10 from the surface of the high-strength steel plate 10. Note that the position that is 1 / 4 of the thickness from either the front or back surface of the high-strength steel plate 10 may be used. However, it is assumed that the high-strength steel plate 20 is present at a height that is 1 / 4 of the thickness from the surface of the high-strength steel plate 10.

[0021] The Vickers hardness was measured under a load of 500 gf. When the two high-strength steel plates have different thicknesses, the thinner steel plate was designated as the high-strength steel plate 10. When the two high-strength steel plates have the same thickness, either high-strength steel plate may be designated as the high-strength steel plate 10. The Vickers hardness measurement position in the thickness direction may be a position inside the high-strength steel plate 10, which is ¼ of the thickness of the high-strength steel plate 10 from the surface of the high-strength steel plate 10. FIG. 3 illustrates a case where the Vickers hardness was measured at a position ¼ of the thickness of the high-strength steel plate 10 from the surface of the high-strength steel plate 10 on the lower side of the figure. The dots in FIG. 3 indicate Vickers hardness measurement positions, which are arranged at 0.15 mm intervals in a direction parallel to the surface of the high-strength steel plate 10, at positions ¼ of the thickness of the high-strength steel plate 10 from the surface of the high-strength steel plate 10. Alternatively, the Vickers hardness may be measured at a position that is ¼ of the thickness of the high-strength steel plate 10 from the surface of the high-strength steel plate 10 on the upper side in the figure.

[0022] The Vickers hardness is measured as described above, and the area relatively harder than the normal portion 11 is defined as the HAZ-hardened portion 12h. The area relatively lower in hardness than the normal portion 11 is defined as the HAZ-softened portion 12s.

[0023] The HAZ portion 22 has a HAZ-hardened portion 22h having a higher Vickers hardness than the normal portion 21, and a HAZ-softened portion 22s having a lower Vickers hardness than the normal portion 21. The HAZ-hardened portion 22h is located closer to the weld metal 30 than the HAZ-softened portion 22s.

[0024] The HAZ-hardened portion 22h and the HAZ-softened portion 22s are subjected to the same thermal influence as the HAZ-hardened portion 12h and the HAZ-softened portion 12s. The Vickers hardness of the HAZ portion 22 is determined by the same method as the Vickers hardness of the HAZ portion 12.

[0025] (Weld Metal) The weld metal 30 is a portion of the steel plate or the like that is melted and solidified by the heat input during welding. It is a metal that is included in part of the weld and melts and solidifies during welding. The source of the material for the weld metal 30 is the multiple steel plates to be joined and the filler wire. If the steel plates are plated, the components of the plating also melt and become materials that make up the weld metal. In addition to the elements from these sources, the weld metal may also incorporate oxygen and nitrogen from the air, as well as unavoidable impurities.

[0026] The average width of the weld metal 30 is 1.00 to 1.65 mm. The average width of the weld metal 30 is determined as follows: Two flat surfaces perpendicular to the weld line L of the weld metal 30 of the butt-welded joint 1 are cut out, embedded in resin, mirror-polished, and etched in a picric acid alcohol solution to prepare cross-sectional samples. The cutting position is any flat surface excluding a range of 30 mm from the welding start point and end point. The measurement surface for the average width of the weld metal 30 may be the same as the measurement surface used to measure the Vickers hardness of the HAZ 12.

[0027] The weld metal 30 is an area in which solidification structures such as a cellular structure, a cellular dendritic structure, and a dendritic dendritic structure can be observed, and can generally be identified by microscopic observation. For the weld metal 30 of the cross-sectional samples taken on each plane as described above, the width W of the narrowest part of the weld metal 30 is measured in a direction parallel to the surfaces of the high-strength steel plate 10 and the high-strength steel plate 20, as shown in Figure 1. The arithmetic mean value of the widths of the weld metal 30 at two locations is then taken as the average width of the weld metal 30.

[0028] The weld metal 30 can be distinguished from the high-strength steel plate 10 and the high-strength steel plate 20 by visual observation of a microscope image at 10x magnification. The surfaces of the high-strength steel plate 10 and the high-strength steel plate 20 are basically formed with few irregularities. On the other hand, since the weld metal 30 has been melted once, a wave pattern called ripples is often formed on its surface, and it is from these surface differences that the weld metal 30 can be distinguished from the high-strength steel plate 10 and the high-strength steel plate 20.

[0029] The average Vickers hardness of the weld metal 30 is 60.00 to 83.00% of the maximum hardness of the HAZ-hardened zone 12h or the HAZ-hardened zone 22h.

[0030] The average Vickers hardness of the weld metal 30 is determined using a Vickers hardness tester as follows. A plane perpendicular to the weld line L of the butt-welded joint 1 to be measured, including the weld metal 30, is cut out over a range of approximately 30 mm in a direction parallel to the surfaces of the high-strength steel plates 10 and 20, embedded in resin, mirror-polished, and etched in a picric acid alcohol solution to prepare a sample. The cutout position excludes a range of 30 mm from the welding start point and welding end point. If the weld joint has been processed by a press or other method, work hardening occurs at the bent portion, making it difficult to measure the Vickers hardness accurately. Therefore, when measuring a processed weld joint, it is best to collect a sample from a flat surface that is as unbent as possible. The measurement surface for the average Vickers hardness of the weld metal 30 may be the same as the measurement surface used to measure the Vickers hardness of the HAZ 12. For this sample, the Vickers hardness is measured in the thickness direction of the high-strength steel plate 10 at positions that are ¼ of the thickness of the high-strength steel plate 10 from the surface on the side where the step between the high-strength steel plate 10 and the high-strength steel plate 20 is smaller, at intervals of 0.15 mm in a direction parallel to the surface of the high-strength steel plate 10. The Vickers hardness is measured under a load of 500 gf. When the high-strength steel plate 10 and the high-strength steel plate 20 have different thicknesses, the thinner steel plate is taken as the high-strength steel plate 10. The arithmetic mean value of the Vickers hardness of the weld metal 30 is taken as the average Vickers hardness of the weld metal 30.

[0031] The maximum hardness of the HAZ hardened zone 12h or the HAZ hardened zone 22h is the highest Vickers hardness measured as described above in the HAZ hardened zone 12h of the high-strength steel plate 10 and the HAZ hardened zone 22h of the high-strength steel plate 20.

[0032] The chemical composition of the high-strength steel plate 10 and the high-strength steel plate 20 preferably contains: C (carbon): 0.09 to 0.35 mass%, Si (silicon): 0.01 to 0.98 mass%, Mn (manganese): 1.2 to 3.8 mass%, P (phosphorus): 0.001 to 0.050 mass%, S (sulfur): 0.001 to 0.050 mass%, Ti (titanium): 0.00 to 0.50 mass%, Al (aluminum): 0.001 to 1.000 mass%, Nb (niobium): 0.000 to 0.500 mass%, and B (boron): 0.0000 to 0.0100 mass%, with the balance being Fe (iron) and impurities. Impurities refer to components contained in raw materials or components mixed in during the manufacturing process, but not intentionally added. For example, trace amounts of elements other than Fe, such as W, Mg, and V, may be mixed in as impurities. The chemical compositions of the high-strength steel plate 10 and the high-strength steel plate 20 may contain other elements in addition to the impurities as long as they do not impair the effects of the butt welded joint 1. The chemical composition of the high-strength steel plate 10 and the high-strength steel plate 20 may be the same as or different from each other.

[0033] Although the high-strength steel plates 10 and 20 having such chemical compositions are strong, they have the aspect of being prone to cracking due to hydrogen embrittlement as described above in the weld metal 30. In the butt-welded joint 1 according to this embodiment, even when the high-strength steel plates 10 and 20 having the above-described chemical compositions are used, cracking in the weld can be suppressed even if the weld is pressed shortly after welding.

[0034] The amount of C contained in a steel plate is measured by collecting chips from the inside of the steel plate to a depth of 0.3 mm or more from the surface and measuring the collected chips using the well-known high-frequency combustion method (combustion-infrared absorption method). Chips are collected from three locations, and the arithmetic mean of the measured values ​​is taken as the amount of C contained in the steel plate. The amount of S is measured by the high-frequency combustion method (combustion-infrared absorption method) in the same way as the amount of C. The amounts of other elements are measured by the well-known spark discharge optical emission spectroscopy.

[0035] In the butt-welded joint 1 according to this embodiment, it is more preferable that the average Vickers hardness of the weld metal 30 is lower than the Vickers hardness of the HAZ hardened portions of the high-strength steel plate 10 and the high-strength steel plate 20. This has the advantage of better preventing cracking of the weld when pressed shortly after welding. As described above, the average Vickers hardness of the weld metal 30 is calculated based on the measured value measured by a Vickers hardness tester. The Vickers hardness of the high-strength steel plate 10 and the high-strength steel plate 20, i.e., the Vickers hardness of each of the ordinary portions 11 and 21, is determined by the method described above.

[0036] In the butt welded joint 1 according to this embodiment, it is more preferable that the area ratio of the structure having a BCC structure, such as ferrite or martensite, in the structure of the weld metal 30 is 50% or more. This has the advantage of stabilizing the hardness of the weld metal. The crystal structure of the structure of the weld metal 30 is identified by measuring and analyzing the diffraction pattern using EBSD. Using the results, it is possible to distinguish between a BCC structure and an FCC structure, and to calculate the area ratio of BCC. If the structure is a BCC structure, it is ferrite, bainite, or martensite. If the structure is an FCC structure, it is austenite.

[0037] In the butt welded joint 1 according to this embodiment, the thickness of the high-strength steel plate 10 and the high-strength steel plate 20 is more preferably 0.8 to 2.0 mm. This has the advantage of enabling the weight of automotive components to be reduced. The thickness of the steel plate is measured using a microscope after cutting the steel plate perpendicular to its surface, embedding it in resin, and mirror-polishing it. The thickness of the steel plate may also be measured using a vernier caliper or the like. The thickness of the high-strength steel plate 10 or the high-strength steel plate 20 is measured at three locations, and the arithmetic mean value of these measurements is taken as the thickness of the high-strength steel plate 10 or the high-strength steel plate 20.

[0038] In the butt welded joint 1 according to this embodiment, it is more preferable that the aspect ratio of the weld metal 30, which is defined by the average width of the weld metal 30 relative to the average value of the plate thicknesses of the high-strength steel plates 10 and 20, is less than 2. This has the advantage that formability is less likely to decrease. Average width of the weld metal / average value of the plate thicknesses of the high-strength steel plates = aspect ratio of the weld metal 30. The average width of the weld metal 30 is as described above. The average value of the plate thicknesses of the high-strength steel plates 10 and 20 is the average value of the plate thicknesses of the high-strength steel plates 10 and 20 calculated using the method described above.

[0039] [Method for manufacturing a butt welded joint] A method for manufacturing a butt welded joint according to this embodiment will be described below. This manufacturing method makes it possible to manufacture a butt welded joint that can suppress cracking in the welded portion even when pressed shortly after welding.

[0040] The method for manufacturing a butt-welded joint according to this embodiment is a method for manufacturing a butt-welded joint having two high-strength steel plates and a weld metal that joins the high-strength steel plates together, and includes the steps of: arranging the high-strength steel plates so that the welding surfaces of the high-strength steel plates face each other with a gap between them (arranging step); and butt-welding the high-strength steel plates together using a filler wire (welding step).

[0041] (Arrangement process) In the arrangement process, the high-strength steel plates are arranged with their welding surfaces facing each other with a gap between them. The welding surfaces of the high-strength steel plates refer to the end surfaces of the high-strength steel plates that are used for butt welding. The welding surfaces of the opposing high-strength steel plates have shapes that correspond to each other. The shape of the welding surfaces of the high-strength steel plates when viewed from the front surface side of the high-strength steel plates may be straight, curved, or a combination of these.

[0042] The two high-strength steel plates have a Vickers hardness of 372 HV or more. The tensile strength of the high-strength steel plates is measured by processing the steel plates into a JIS No. 5B shape and conducting a tensile test in accordance with JIS Z 2241 (2011) using a tensile testing machine. Using this method, the tensile strengths are measured at two locations, and the arithmetic mean value of these values ​​is taken as the tensile strength of each high-strength steel plate. From the perspective of reducing the weight of the member, it is more preferable that the tensile strength of the high-strength steel plate be 1180 MPa or more, 1300 MPa or more, or 1470 MPa or more.

[0043] The Vickers hardness of a high-strength steel plate is measured by cutting out planes perpendicular to the surface of the steel plate at any two locations, embedding them in resin, and mirror-polishing them. The interior of the steel plate is measured at a distance of 100 μm or more from the front and back surfaces of the steel plate using a Vickers hardness tester. The Vickers hardness measurement condition is a load of 500 gf. Using this method, the Vickers hardness is measured at five points on each plane, for a total of 10 points, and the arithmetic mean value of these is taken as the Vickers hardness of the high-strength steel plate. From the perspective of reducing the weight of the member, it is more preferable that the Vickers hardness of the high-strength steel plate be 402 HV or more, 440 HV or more. The two high-strength steel plates can be the high-strength steel plate 10 and the high-strength steel plate 20 of the above embodiment.

[0044] In the placement process, the size of the gap between the welded surfaces of the high-strength steel plates, i.e., the width of the gap, is 0.4 to 1.0 mm. The size of the gap is the shortest distance from the welded surface of one high-strength steel plate to the welded surface of the other high-strength steel plate when viewed in a plan view from a direction perpendicular to the surface of the opposing high-strength steel plate at each point on the welded surface of the high-strength steel plate. In terms of design, it is desirable that the size of the gap does not change over the entire length of the welded surfaces of the high-strength steel plates. By setting the size of the gap between the welded surfaces of the high-strength steel plates within this range, it is possible to obtain the desired Vickers hardness of the weld metal and the HAZ portion as described above.

[0045] (Welding process) In the welding process, high-strength steel plates are butt-welded to each other by laser welding using a filler wire. By irradiating the high-strength steel plates with a laser beam while supplying the filler wire, the two high-strength steel plates and the filler wire melt, and the molten metal solidifies between the two high-strength steel plates to form a weld metal, thereby joining the two high-strength steel plates.

[0046] In the welding process, weld metal is formed by supplying a filler wire and performing laser welding. The filler wire is, for example, a solid wire or a flux-cored wire. In this embodiment, the weld metal refers to a part of the weld that is melted and solidified during welding. Here, the term "molten and solidified metal" refers to both the metal derived from the molten high-strength steel sheet and the metal derived from the molten filler wire. Therefore, the weld metal refers to the metal obtained by melting and mixing a part of the high-strength steel sheet with the filler wire.

[0047] The C content of the filler wire is more than 0 mass% and 0.10 mass% or less. By setting the C content of the filler wire within this range, the average Vickers hardness of the weld metal can be made lower than the Vickers hardness of the high-strength steel plate. The C content of the filler wire is measured by collecting chips from the filler wire and using the well-known high-frequency combustion method (combustion-infrared absorption method) on the collected chips. Chips are collected at three locations, and the arithmetic mean value of the respective measured values ​​is taken as the C content of the steel plate.

[0048] Other preferred chemical compositions of the filler wire are as follows: Si: 0.25 to 1.50%, Mn: 0.5 to 2.8%, Al: 0.001 to 0.300%, Ti: 0.01 to 0.30%, P: more than 0% but not more than 0.05%, S: more than 0% but not more than 0.05%, Cu: 0 to 0.50%, with the balance consisting of Fe, B, Cr, Ni, Mo, V, and impurities. Regarding the chemical composition of the filler wire, chips are collected from the filler wire, and the amount of each element is measured using the well-known high-frequency combustion method (combustion-infrared absorption method). Chips are collected from three locations, and the arithmetic mean of the respective measured values ​​is taken as the amount of the element contained in the steel sheet.

[0049] The value obtained by dividing the gap size by the C content of the filler wire is 25.0 or less. When the gap size varies over the entire length of the weld surface of the high-strength steel plate, the maximum gap size is taken as the gap size value. As described below, the butt-welded joint according to this embodiment is characterized by reducing the hardness of the weld metal, but if the hardness of the weld metal is reduced excessively, there is a risk of the strength of the butt-welded joint being reduced. By setting the value obtained by dividing the gap size by the C content of the filler wire within this range, excessive reduction in the hardness of the weld metal can be suppressed.

[0050] The conditions for the laser beam are not particularly limited, but a commonly used CO ２ A solid-state laser such as a laser, fiber laser, or disk laser may be used. A semiconductor laser may also be used. The laser output is preferably 2 kW to 6 kW from the viewpoint of welding speed and cost. The laser scanning speed is preferably 3 m / min to 10 m / min from the viewpoint of productivity. The filler wire feed speed is preferably 3 m / min to 10 m / min from the viewpoint of cost. The shielding gas may be CO ２ When a laser is used, it is preferable to use helium gas or argon gas. When a solid-state laser or semiconductor laser is used, it is preferable to use argon gas, nitrogen gas, compressed air, or the like as a shielding gas, but it may be performed in the atmosphere without using a shielding gas.

[0051] The butt weld joint of the above embodiment can be preferably used as a welded portion of a tailored blank.

[0052] Conventionally, filler metals have been added when welding common steel types to harden the weld metal and prevent fractures at the weld metal. Furthermore, in butt welding, even a gap of only 0.1 to 0.2 mm when steel plates are butted together can cause a dent in the weld, reducing joint strength. To prevent a reduction in joint strength, steel plates are typically butted together without a gap to prevent dents, and a filler metal is added to harden the weld and stabilize joint strength. On the other hand, the butt-welded joint according to the above embodiment aims to prevent hydrogen embrittlement by softening the weld metal through the addition of a filler wire when butt-welding high-strength steel plates with a Vickers hardness of 372 HV or higher. Furthermore, when manufacturing the butt-welded joint according to this embodiment, the welding surfaces of the high-strength steel plates are opposed to each other with a gap therebetween, and the size of the gap between the plates during butt welding is ensured to be 0.4 mm to 1.0 mm, and this gap is filled with filler to increase the dilution rate by the filler wire. In this case, a filler wire with a lower C content (carbon content) than the high-strength steel plate is used, which is characterized by reducing the hardness of the weld metal.

[0053] The invention according to the present disclosure will be specifically described below using examples, but the invention according to the present disclosure is not limited thereto.

[0054] In this example, two steel plates were prepared, and their ends were butted together to form a butt-welded joint by laser welding. The tensile strength and Vickers hardness of each of the steel plates A to D used were as shown in Table 1. The tensile strength of the steel plate was measured by processing the steel plate into a JIS No. 5B shape and conducting a tensile test in accordance with JIS Z 2241 (2011) using a tensile testing machine. Using this method, the tensile strength was measured at two locations, and the arithmetic average value was used as the tensile strength of each steel plate. The Vickers hardness of the high-strength steel plate was measured by cutting out planes perpendicular to the surface of the steel plate at any two locations on the steel plate, embedding them in resin, and mirror-polishing them. The interior of the steel plate, at least 100 μm away from the front and back surfaces of the steel plate, was measured using a Vickers hardness tester. The Vickers hardness measurement condition was a load of 500 gf. Using this method, the Vickers hardness was measured at five points on each plane, for a total of 10 points, and the arithmetic mean value of these measurements was taken as the Vickers hardness of the high-strength steel plate. Chips were collected from the interior of the steel plate at a depth of 0.3 mm or more from the surface, and the collected chips were subjected to a high-frequency combustion method (combustion-infrared absorption method) to measure the carbon content of the steel plate. Chips were collected from three locations, and the arithmetic mean value of the respective measured values ​​was taken as the carbon content of the steel plate.

[0055] The carbon content of each of the filler wires A to C used in welding was as shown in Table 2. The carbon content of the filler wire was measured by collecting chips from the filler wire and subjecting the collected chips to a high-frequency combustion method (combustion-infrared absorption method). Chips were collected from three locations, and the arithmetic mean value of the respective measured values ​​was taken as the carbon content of the filler wire.

[0056]

[0057]

[0058] Material 1, material 2, and filler wire were selected in the combinations shown in Tables 3 and 4, and welded joints were produced for each experimental example. The thickness of the steel plate selected as material 1 or material 2 was measured using a microscope and is shown in Table 3. The size of the gap between the butted steel plates was varied from 0.0 mm to 1.2 mm, as shown in Tables 3 and 4. Filler wire diameters of 0.6 mm, 0.9 mm, and 1.2 mm were used as appropriate, depending on the size of the gap between the steel plates.

[0059] Laser welding was performed under the following conditions: Laser power and wire speed were set to ensure stable welding for each gap. Speed: 5 m / min Shielding gas: Air

[0060] After preparing butt-welded joints, two planes perpendicular to the weld line were cut out from each weld joint, embedded in resin, mirror-polished, and then etched in a picric acid alcohol solution to prepare cross-sectional samples for observing the shape of the weld. The welds in the two cross-sectional samples were designated as weld 1 and weld 2, respectively. The cut-out positions excluded ranges of 30 mm from the welding start point and welding end point. The width of the narrowest part of the weld metal in each cross-sectional sample was measured in a direction parallel to the plate surface of the steel plate, and the arithmetic mean value of the widths of weld 1 and weld 2 was taken as the average width of the weld metal.

[0061] The Vickers hardness of the weld metal and the HAZ was measured using a Vickers hardness tester. For the above-mentioned weld 1 or weld 2, as shown in Figure 3, the Vickers hardness was measured at 0.15 mm intervals in a direction parallel to the surface of the thinner high-strength steel plate at a position that was 1 / 4 of the thickness of the high-strength steel plate from the surface of the thinner high-strength steel plate. The arithmetic mean value of these values ​​was then adopted as the average Vickers hardness of the weld metal. The Vickers hardness was measured under a load of 500 gf.

[0062] The maximum hardness of the HAZ hardened zone in each of Material 1 and Material 2 was measured using a Vickers hardness tester, similar to the measurement of the Vickers hardness of the weld metal, and the largest value among these was taken as the maximum hardness of the HAZ hardened zone. The numerical value (%) was calculated by dividing the average Vickers hardness of the weld metal by the maximum hardness of the HAZ hardened zone. The maximum hardness of the HAZ hardened zone was taken as the higher of the maximum hardnesses of Material 1 and Material 2.

[0063] In each experimental example, the aspect ratio of the weld metal was calculated by dividing the average width of the weld metal by the average thickness of materials 1 and 2. The crystal structure of the metallographic structure of the weld metal was identified by measuring and analyzing the diffraction pattern using EBSD. Using the results, the BCC structure and the FCC structure were distinguished, and the area ratio of the structure having the BCC structure was calculated.

[0064] The following evaluations were carried out. The results are shown in Tables 3 and 4. (Evaluation of press formability) A bending test was carried out on each butt-welded joint 8 hours or 24 hours after welding. In the bending test, the welded joint was bent at 90 degrees with an R8 so that the weld line was bent in a V-shape. Each butt-welded joint that had undergone the bending test was observed using a microscope at a magnification of 40x to check for the presence or absence of cracks on the weld metal surface. Those in which no cracks were observed using the microscope were rated "good (◯)". Those in which cracks were observed using the microscope were rated "bad (X)".

[0065] (Measurement of tensile strength) Test pieces were prepared in a JIS No. 5B shape including a welded portion, and a tensile test was performed using a tensile tester. The weld line was arranged perpendicular to the tensile direction. After the tensile test, the test pieces were visually observed, and those that showed even partial fracture within the weld metal were rated as "× (bad)," and those that did not were rated as "◯ (good)."

[0066]

[0067]

[0068] As can be seen from the results in Tables 3 and 4, each of the Examples that met the requirements of the present application achieved good results in all of the above evaluations.

[0069] However, when the width of the weld was narrow, i.e., when the gap between the butt joints of materials 1 and 2 was narrow or 0.0 mm, a large amount of filler could not be fed, and the value obtained by dividing the average Vickers hardness of the weld metal by the maximum hardness of the HAZ-hardened zone increased. As a result, the press formability results were poor. Furthermore, when the width of the weld was wide, i.e., when the gap between the butt joints of materials 1 and 2 was wide, burn-through occurred, and stable welding could not be achieved along the entire length of the weld. Furthermore, when the value obtained by dividing the gap size by the C content of the filler wire exceeded 25.0, the average Vickers hardness of the weld metal relative to the maximum hardness of the HAZ-hardened zone was less than 60%, resulting in fracture at the weld metal in a tensile test and reduced joint strength as a butt-welded joint. Furthermore, when the C content of the filler wire exceeded 0.10 mass%, the value obtained by dividing the average Vickers hardness of the weld metal by the maximum hardness of the HAZ-hardened zone increased, and the press formability results were poor.

[0070] The butt-welded joint, tailored blank, and method for manufacturing a butt-welded joint according to the present disclosure can suppress cracking in the welded portion even when pressing is performed shortly after welding. Therefore, the present disclosure is extremely useful industrially.

[0071] 1 butt weld joint 10, 20 high strength steel plate 30 weld metal

Claims

1. A butt weld joint having two high-strength steel plates and weld metal joining the high-strength steel plates, wherein the high-strength steel plates include a normal portion and a HAZ portion adjacent to the weld metal, wherein the normal portion has a Vickers hardness of 372 HV or more, and the HAZ portion has a HAZ-hardened portion having a Vickers hardness higher than that of the normal portion, and a HAZ-softened portion having a Vickers hardness lower than that of the normal portion, wherein the average width of the weld metal is 1.00 to 1.65 mm, and wherein the average Vickers hardness of the weld metal is 60.00 to 83.00% of the maximum hardness of the HAZ-hardened portion, measured at 0.15 mm intervals at a load of 500 gf from the surface of the high-strength steel plate at a position 1 / 4 of the way in the thickness direction of the high-strength steel plate.

2. The butt-welded joint according to claim 1, characterized in that the average Vickers hardness of the weld metal is lower than the Vickers hardness of the normal portion of the high-strength steel plate.

3. A butt welded joint according to claim 1 or 2, characterized in that the area ratio of the structure of the weld metal having a BCC structure is 50% or more.

4. A butt-welded joint according to claim 1 or 2, characterized in that the thickness of the high-strength steel plate is 0.8 to 2.0 mm.

5. A butt-welded joint according to claim 1 or 2, characterized in that the aspect ratio of the weld metal, defined by the average width of the weld metal relative to the average thickness of the two high-strength steel plates, is less than 2.

6. A tailored blank comprising the butt weld joint according to claim 1 or 2.

7. A method for manufacturing a butt-welded joint having two high-strength steel plates and a weld metal joining the high-strength steel plates, comprising the steps of: arranging the high-strength steel plates so that the welding surfaces of the high-strength steel plates face each other with a gap between them; and butt-welding the high-strength steel plates using a filler wire; wherein the high-strength steel plates have a Vickers hardness of 372 HV or more, the size of the gap is 0.4 to 1.0 mm, the carbon content of the filler wire is more than 0 mass% and 0.10 mass% or less, the value obtained by dividing the size of the gap by the carbon content of the filler wire is 25.0 or less, and the thickness of the high-strength steel plates is 0.8 to 2.0 mm.

Images