Butt welded joints, tailored blanks, and methods for manufacturing butt welded joints

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

Technical Field

[0001] The present disclosure relates to a butt weld joint, a tailored blank, and a method for manufacturing a butt weld joint. This application claims priority based on Japanese Patent Application No. 2024-026405 filed in Japan on February 26, 2024, and incorporates its content herein by reference.

Background Art

[0002] For the purpose of weight reduction of automobiles, the utilization of tailored blanks using high-strength steel sheets is expected. A tailored blank is a joined product of multiple types of steel sheets by welding. Pressing, bending, cutting, etc. are performed on the tailored blank so as to obtain a desired product shape. For example, Patent Document 1 discloses a technique related to a tailored blank in which the ends of steel sheets are joined by welding. Further, Patent Document 2 discloses a technique for partially softening a part of a weld portion that is cut after manufacturing a tailored blank by heat treatment for the purpose of improving the workability of the weld portion.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the above-described tailored blank, particularly when cold pressing a tailored blank using a high-strength steel sheet having a Vickers hardness of 372 HV or more, the inventors have found that hydrogen embrittlement occurs due to hardening of the weld metal in the weld portion, and cracks may occur when press forming is performed within a short time after welding.

[0005] Factors contributing to hydrogen embrittlement include the hardness of the weld metal, the hydrogen incorporated into the weld metal during welding, and the stress introduced into the weld during processing. The inventors focused on the hardness of the weld metal and investigated how to suppress cracking caused by hydrogen embrittlement. While cracking can be suppressed by performing press forming after sufficient time has elapsed for hydrogen to diffuse out of the weld metal (for example, after 24 hours), this increases the time between welding and press forming, resulting in lower productivity.

[0006] The invention disclosed herein has been made in view of the above, and aims 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 area even when pressed a short time after welding. [Means for solving the problem]

[0007] (1) A butt welded joint according to one aspect of the present disclosure is: A butt welded joint comprising two high-strength steel plates and a weld metal that joins the two high-strength steel plates, The high-strength steel plate includes a normal portion and a HAZ portion adjacent to the weld metal, The aforementioned normal part has a Vickers hardness of 372 HV or higher. The HAZ portion comprises 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 average width of the weld metal is 1.00 to 1.65 mm. The Vickers hardness of the weld metal, measured at 0.15 mm intervals from the surface of the high-strength steel plate at a position 1 / 4 of the way through the thickness direction of the high-strength steel plate, under a load of 500 gf, is 60.00 to 83.00% of the maximum hardness of the HAZ hardened portion. It is characterized by the following: (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, In the microstructure of the weld metal, the area ratio of the structure having a BCC structure may be 50% or more. (4) In the butt welded joint described in any one of the above items (1) to (3), The 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 the above items (1) to (4), The aspect ratio of the weld metal, defined by the average width of the weld metal with respect to the average thickness of the two high-strength steel plates, may be less than 2. (6) A tailored blank relating to one aspect of this disclosure is: The invention is characterized by comprising a butt welded joint as described in any one of the above items (1) to (5). (7) A method for manufacturing a butt welded joint according to one aspect of the present disclosure 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, The process involves arranging the high-strength steel plates so that their welded surfaces face each other with a gap between them, A process of butt welding the high-strength steel plates together using filler wire, It has, The aforementioned high-strength steel plate has a Vickers hardness of 372 HV or higher. The size of the aforementioned gap is 0.4 to 1.0 mm. The amount of carbon in the filler wire is greater than 0% by mass and less than or equal to 0.10% by mass. The value obtained by dividing the size of the gap by the amount of carbon in the filler wire is 25.0 or less. The thickness of the aforementioned high-strength steel plate is 0.8 to 2.0 mm. It is characterized by the following: [Effects of the Invention]

[0008] According to the butt welded joint, tailored blank, and manufacturing method of the butt welded joint described herein, cracking can be suppressed even if the joint is pressed a short time after welding. [Brief explanation of the drawing]

[0009] [Figure 1] It is a schematic cross-sectional view for explaining a butt-welded joint according to an embodiment of the present disclosure. [Figure 2] It is a graph for explaining the distribution of Vickers hardness with respect to the butt-welded joint according to an embodiment of the present disclosure. [Figure 3] It is a view for explaining a method for measuring Vickers hardness for a weld metal and a HAZ portion in a butt-welded joint.

Mode for Carrying Out the Invention

[0010] Hereinafter, embodiments of the invention according to the present disclosure will be described with examples, but it is self-evident 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 exemplified, but other numerical values and materials may be applied as long as the effects of the invention according to the present disclosure can be obtained. Further, each component of the following embodiments can be combined with each other. Also, in this specification, a numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value. In this specification, the term "step" includes not only an independent step but also this term as long as the intended purpose of the step is achieved even when it cannot be clearly distinguished from other steps.

[0011] The butt-welded joint according to the present embodiment is a butt-welded joint having two high-strength steel plates and a weld metal for joining the high-strength steel plates to each other. As shown in FIG. 1, in the butt-welded joint 1, the high-strength steel plate 10 and the high-strength steel plate 20 are connected by the weld metal 30. FIG. 1 is a schematic cross-sectional view for explaining a butt-welded joint according to an embodiment of the present disclosure, and represents a cross-section perpendicular to the weld line L where the weld metal 30 is formed. In FIG. 1, a case where the thickness of the high-strength steel plate 10 is thinner than the thickness of the high-strength steel plate 20 is shown.

[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 part) The normal areas 11 and 21 are parts that are not affected by the heat of welding when the weld metal 30 is applied. For example, in laser welding, the normal areas 11 or 21 are generally the parts of the high-strength steel plate 10 or high-strength steel plate 20 that are parallel to the plate surface and perpendicular to the weld line L of the butt weld joint 1, and are located at least 10 mm away from the toe of the weld metal 30. The normal areas 11 or 21 can be distinguished from the HAZ areas 12 or 22 by their Vickers hardness. The normal parts 11 and 21 have a Vickers hardness of 372 HV or higher.

[0014] Figure 2 shows the distribution of Vickers hardness in the butt welded joint 1 according to this embodiment. In Figure 2, the vertical axis represents Vickers hardness (HV), and the horizontal axis represents the relative position in the direction in which the plate surface extends on a plane perpendicular to the weld line L (corresponding to the horizontal direction in Figure 1). The distribution in Figure 2 shows the Vickers hardness measured at 0.15 mm intervals in a direction parallel to the surface of the high-strength steel plate, at a position that is 1 / 4 of the thickness from the surface of the thinnest high-strength steel plate of the two high-strength steel plates, and the values ​​were plotted. In Figure 2, area A represents the weld metal 30, area B represents the HAZ hardened areas 12h and 22h, and area C represents the HAZ softened areas 12s and 22s. The dotted line in Figure 2 represents the Vickers hardness of the normal portion of the high-strength steel plate. In the case of butt welded joint 1 shown in the distribution diagram in Figure 2, the Vickers hardness of the normal portion is 500 HV. Note that in the case of butt welded joint 1 shown in the distribution diagram in Figure 2, the Vickers hardness of the normal portion 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 area (area B) than in the normal portion, and lower in the HAZ softened area (area C) than in the normal portion.

[0015] The tensile strength of the normal section 11 or normal section 21 is measured by, for example, preparing a test specimen of JIS No. 5B shape in accordance with JIS Z 2241 (2011) and conducting a tensile test using a tensile testing machine. Using this method, the tensile strength is measured at, for example, three locations, and the arithmetic mean of these measurements is taken as the tensile strength of the normal section 11 or normal section 21. From the viewpoint of reducing the weight of the components, it is more preferable that the tensile strength of the normal section 11 or normal section 21 be 1180 MPa or more, 1300 MPa or more, or 1470 MPa or more.

[0016] The Vickers hardness of the normal section 11 or normal section 21 is measured using a Vickers hardness tester in the following procedure. A sample is prepared by cutting out a plane perpendicular to the weld line L of the butt welded joint 1 to be measured, including the weld metal 30, in a direction parallel to the surfaces of the high-strength steel plates 10 and 20, within a range of approximately 30 mm, embedding it in resin, polishing it to a mirror finish, and then etching it with a picric acid alcohol solution. The cutting position should exclude a range of 30 mm from the welding start point and welding end point. If the welded joint has been processed by pressing or other means, work hardening occurs in the bent portion, making it impossible to measure the Vickers hardness accurately. Therefore, when measuring a processed welded joint, it is best to take a sample of the flat surface that has not been bent as much as possible. Note that the measurement surface for the Vickers hardness of the normal portion 11 or normal portion 21 may be the same as the measurement surface used for measuring the Vickers hardness of the HAZ portion 12 described later. For this sample, the Vickers hardness of five or more points is measured at positions at least 10 mm away from the center of the weld metal 30 and at least 100 μm away from the front and back surfaces of the butt weld joint 1, in a direction parallel to the surfaces of the high-strength steel plates 10 and 20. The arithmetic mean of these measurements is taken as the Vickers hardness of the normal section 11 or normal section 21. The Vickers hardness is measured under a load of 500 gf. From the viewpoint of reducing the weight of the components, it is more preferable that the Vickers hardness of the normal section 11 or normal section 21 be 402 HV or higher and 440 HV or higher, respectively.

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

[0018] (HAZ Department) The HAZ portion 12 has a HAZ hardened portion 12h with a higher Vickers hardness than the normal portion 11, and a HAZ softened portion 12s with a lower Vickers hardness than the normal portion 11. The HAZ hardened portion 12h is located on the weld metal 30 side of the HAZ softened portion 12s. The HAZ hardened area 12h is a region that was rapidly cooled after reaching a high temperature of approximately 900-1500°C due to the heat input during welding. As a result, in the HAZ hardened area 12h, the structure mainly consists of a BCC structure containing martensite, due to being heated above the transformation point of approximately 900°C and then quenched. The HAZ softened area 12s is affected by heat during welding, but its temperature does not rise above the transformation point. As a result, tempering occurs in the HAZ softened area 12s, and its Vickers hardness becomes lower than that of the normal area 11.

[0019] The Vickers hardness of the HAZ section 12 is determined using a Vickers hardness tester as follows: A sample is prepared by cutting out a plane perpendicular to the weld line L of the butt welded joint 1 to be measured, within a range of approximately 30 mm in a direction parallel to the surfaces of the high-strength steel plates 10 and 20, including the weld metal 30, embedding it in resin, polishing it to a mirror finish, and then etching it with a picric acid alcohol solution. The cutting position should exclude the range of 30 mm from the welding start point and welding end point. If the welded joint has been processed by pressing or other means, work hardening occurs in the bent portion, making it impossible to accurately measure the Vickers hardness. Therefore, when measuring processed welded joints, it is best to take a sample of the flat surface that has not been bent as much as possible.

[0020] For this sample, the Vickers hardness is measured at a position that is 1 / 4 of the thickness of the high-strength steel plate 10 or high-strength steel plate 20 from the surface of the high-strength steel plate 10 or high-strength steel plate 20, at intervals of 0.15 mm in a direction parallel to the surface of the high-strength steel plate 10. As shown in Figure 3, if the high-strength steel plate 10 and the high-strength steel plate 20 have different thicknesses, first, as shown in Figure 3, identify the high-strength steel plate 10, which has the smaller thickness. Then, measure the Vickers hardness at a position that is 1 / 4 of the thickness of the high-strength steel plate 10 from its surface, at 0.15 mm intervals in a direction parallel to the surface of the high-strength steel plate 10. Note that this position can be taken from either the front or back surface of the high-strength steel plate 10. However, it is assumed that the high-strength steel plate 20 is located at a height position that is 1 / 4 of the thickness from the surface of the high-strength steel plate 10.

[0021] The Vickers hardness measurement conditions shall be a load of 500 gf. If the two high-strength steel plates have different thicknesses, the thinner plate shall be designated as high-strength steel plate 10. If the two high-strength steel plates have the same thickness, either plate may be designated as high-strength steel plate 10, and the measurement position for Vickers hardness in the thickness direction may be located one-quarter of the thickness of high-strength steel plate 10 from the surface of high-strength steel plate 10. Figure 3 illustrates the case where the Vickers hardness is measured at a position that is 1 / 4 of the thickness of the high-strength steel plate 10 from the surface of the lower high-strength steel plate 10 in the figure. The points in Figure 3 indicate the Vickers hardness measurement positions that are placed 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 in the figure. Alternatively, the Vickers hardness may be measured at a position that is 1 / 4 of the thickness of the high-strength steel plate 10 from the surface of the upper high-strength steel plate 10 in the figure.

[0022] The Vickers hardness is measured as described above, and the area where the hardness is relatively higher than that of the normal area 11 is defined as the HAZ hardened area 12h. Conversely, the area where the hardness is relatively lower than that of the normal area 11 is defined as the HAZ softened area 12s.

[0023] The HAZ portion 22 has a HAZ hardened portion 22h with a higher Vickers hardness than the normal portion 21, and a HAZ softened portion 22s with a lower Vickers hardness than the normal portion 21. The HAZ hardened portion 22h is located on the weld metal 30 side of the HAZ softened portion 22s.

[0024] The HAZ hardened section 22h and the HAZ softened section 22s are subjected to the same thermal effects as the HAZ hardened section 12h and the HAZ softened section 12s. The Vickers hardness of the HAZ section 22 is determined using the same method as the Vickers hardness of the HAZ section 12.

[0025] (Weld metal) Weld metal 30 is the portion of the steel plate, etc., that has melted and solidified due to the heat input during welding. It is included in the weld area and is the metal that melted and solidified during welding. The source of the weld metal 30 is the multiple steel plates to be joined and the filler wire. If the steel plate is plated, the plating components also melt and become part of the weld metal. In addition to elements from these sources, the weld metal may also contain 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 planes perpendicular to the weld line L are cut from the weld metal 30 of the butt welded joint 1, embedded in resin, mirror-polished, and then etched with picric acid alcohol solution to prepare cross-sectional samples. The cutting positions are any flat area excluding the 30 mm range from the weld start and end points. The surface used to measure the average width of the weld metal 30 may be the same surface used to measure the Vickers hardness of the HAZ section 12.

[0027] The weld metal 30 is within the range where solidification structures such as cellular structures, cellular dendrite structures, and dendritic dendrite structures can be observed, and are generally identifiable by microscopic observation. For the weld metal 30 of the cross-sectional samples taken from 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 surface of the high-strength steel plate 10 and the high-strength steel plate 20, as shown in Figure 1. The arithmetic mean of the widths of the weld metal 30 at the two locations is taken as the average width of the weld metal 30.

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

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

[0030] The average Vickers hardness of the weld metal 30 is determined using a Vickers hardness tester as follows: A sample is prepared by cutting out a plane perpendicular to the weld line L of the butt welded joint 1 to be measured, including the weld metal 30, in a direction parallel to the surfaces of the high-strength steel plates 10 and 20, within a range of approximately 30 mm, embedding it in resin, mirror polishing it, and etching it with a picric acid alcohol solution. The cutting position should exclude the range of 30 mm from the welding start point and welding end point. If the welded joint has been processed by pressing or other means, work hardening occurs in the bent portion, making it impossible to measure the Vickers hardness accurately. Therefore, when measuring processed welded joints, it is best to take a sample of the flat surface that has not been bent as much as possible. Note that the measurement surface for the average Vickers hardness of the weld metal 30 may be the same as the measurement surface used for measuring the Vickers hardness of the HAZ portion 12. For this sample, the Vickers hardness is measured at 0.15 mm intervals 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 on the side with the smaller step difference between the high-strength steel plate 10 and the high-strength steel plate 20. The Vickers hardness measurement conditions are set to a load of 500 gf. If the thicknesses of the high-strength steel plate 10 and the high-strength steel plate 20 are different, the thinner steel plate is designated as the high-strength steel plate 10. Then, the arithmetic mean 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 portion 12h or the HAZ-hardened portion 22h shall be the highest Vickers hardness among the HAZ-hardened portion 12h of the high-strength steel plate 10 and the HAZ-hardened portion 22h of the high-strength steel plate 20, as measured as described above.

[0032] The chemical compositions of high-strength steel plate 10 and high-strength steel plate 20 are as follows: C (carbon): 0.09~0.35% by mass, Si (silicon): 0.01~0.98% by mass, Mn (manganese): 1.2-3.8 mass%, P (phosphorus): 0.001~0.050% by mass, S (sulfur): 0.001~0.050% by mass, Ti (Titanium): 0.00~0.50 mass%, Al (aluminum): 0.001 to 1.000 mass%, Nb (niobium): 0.000~0.500 mass%, B (Boron): 0.0000~0.0100 mass%, It contains, It is preferable that the remainder contains Fe (iron) and impurities. Impurities refer to components contained in the raw materials or components mixed in during the manufacturing process, and not components that were intentionally included. For example, trace amounts of components other than Fe, such as W, Mg, and V, may be included as impurities. In addition to impurities, other elements may be included in the chemical composition of the high-strength steel plate 10 and the high-strength steel plate 20, as long as they do not impair the effect of the butt welded joint 1. The chemical composition of the high-strength steel plate 10 and the chemical composition of the high-strength steel plate 20 may be the same or different.

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

[0034] The carbon (C) content in steel sheets is measured by taking chips from a depth of 0.3 mm or more from the surface of the steel sheet and measuring the carbon content of the collected chips using a well-known high-frequency combustion method (combustion-infrared absorption method). Chips are collected from three locations, and the arithmetic mean of the measured values ​​from each location is taken as the carbon content of the steel sheet. The sulfur (S) content is measured in the same way as the carbon content using the high-frequency combustion method (combustion-infrared absorption method). The amounts of other elements are measured using a known spark discharge emission spectrometry method.

[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 portion 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 a short time after welding. The average Vickers hardness of the weld metal 30 is calculated based on the Vickers hardness measured using a Vickers hardness tester, as described above. The Vickers hardness of the high-strength steel plate 10 and the high-strength steel plate 20, i.e., the Vickers hardness of the normal portion 11 and the normal portion 21, respectively, are 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 structures having a BCC structure, such as ferrite or martensite, in the microstructure of the weld metal 30 is 50% or more. This has the advantage of stabilizing the hardness of the weld metal. The microstructure of the weld metal 30 is identified by measuring and analyzing the diffraction pattern using EBSD. Using these results, BCC and FCC structures can be distinguished, and the area fraction of BCC can be calculated. If it is a BCC structure, it consists of ferrite, bainite, and martensite. If it is an FCC structure, it consists of austenite.

[0037] In the butt-welded joint 1 according to this embodiment, it is more preferable that the plate thickness of the high-strength steel plate 10 and the high-strength steel plate 20 be 0.8 to 2.0 mm. This has the advantage of enabling weight reduction of automotive components. The thickness of the steel plate is measured using a microscope after cutting a section of the steel plate perpendicular to its surface, embedding it in resin, and performing mirror polishing. Alternatively, the thickness of the steel plate may be measured using calipers or similar tools. The thickness of the high-strength steel plate 10 or high-strength steel plate 20 is measured at three locations, and the arithmetic mean of these measurements is taken as the thickness of the high-strength steel plate 10 or 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 thickness of the high-strength steel plates 10 and 20, is less than 2. This has the advantage of preventing a decrease in formability. The ratio of the weld metal 30 is given by: average width of weld metal / average thickness of high-strength steel plate = aspect ratio of weld metal 30. The average width of weld metal 30 is as described above. The average thickness of high-strength steel plate 10 and high-strength steel plate 20 is the average of the thickness of high-strength steel plate 10 and high-strength steel plate 20 calculated using the method described above.

[0039] [Manufacturing method for butt welded joints] The method for manufacturing a butt-welded joint according to this embodiment is described below. This manufacturing method makes it possible to produce a butt-welded joint that can suppress cracking in the welded area even when pressed a short time 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, The process of arranging high-strength steel plates by placing the welded surfaces of the high-strength steel plates facing each other with a gap in between (arrangement process), The process of butt welding high-strength materials together using filler wire (welding process), It holds.

[0041] (Placement process) In the placement process, the high-strength steel plates are positioned with their welded surfaces facing each other, leaving a gap between them. The welded surface of the high-strength steel plate refers to the end surface of the high-strength steel plate that is used for butt welding. The welded surfaces of the opposing high-strength steel plates have corresponding shapes. The shape of the welded surface of the high-strength steel plate, when viewed from the surface side of the high-strength steel plate, may be a straight line, a curve, or a combination of these.

[0042] The two high-strength steel plates have a Vickers hardness of 372 HV or higher. The tensile strength of high-strength steel plates is 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 is measured at two locations, and the arithmetic mean of these values ​​is taken as the tensile strength of each high-strength steel plate. From the viewpoint of reducing the weight of the components, it is more preferable for the tensile strength of the high-strength steel plates to be 1180 MPa or higher, 1300 MPa or higher, and 1470 MPa or higher.

[0043] The Vickers hardness of high-strength steel sheets is determined by cutting out planes perpendicular to the surface of the steel sheet at two arbitrary locations, embedding them in resin, and performing mirror polishing. The Vickers hardness is then measured using a Vickers hardness tester at a distance of 100 μm or more from the front and back surfaces of the steel sheet. The measurement conditions for Vickers hardness are a load of 500 gf. Using this method, the Vickers hardness is measured at 5 points from each plane, for a total of 10 points, and the arithmetic mean of these values ​​is taken as the Vickers hardness of the high-strength steel sheet. From the viewpoint of reducing the weight of the components, it is more preferable for the Vickers hardness of the high-strength steel plate to be 402HV or higher, or 440HV or higher. The two high-strength steel plates can be the high-strength steel plate 10 and the high-strength steel plate 20 described in the above embodiment.

[0044] In the arrangement process, the size of the gap between the welded surfaces of the high-strength steel plates, i.e., the gap width, 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 from 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. Ideally, the size of the gap should not change over the entire length of the welded surface of the high-strength steel plate in terms of design. By setting the gap size between the welded surfaces of high-strength steel plates within this range, the desired Vickers hardness of the weld metal and HAZ (Heat-Absorbing Zone) can be obtained as described above.

[0045] (Welding process) In the welding process, high-strength steel plates are butt-welded together using laser welding and filler wire. By supplying filler wire and irradiating the high-strength steel plates with a laser beam, 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 weld metal, thereby joining the two high-strength steel plates together.

[0046] In the welding process, filler wire is supplied and laser welding is performed to form the weld metal. Filler wires include, for example, solid wires and flux-cored wires. In this embodiment, weld metal refers to a part of the weld that has melted and solidified during welding. Here, "melted and solidified metal" refers to both the metal derived from the molten high-strength steel plate and the metal derived from the molten filler wire. Therefore, weld metal refers to the metal formed when a part of the high-strength steel plate and the filler wire melt and mix together.

[0047] The carbon content of the filler wire is greater than 0% by mass and less than or equal to 0.10% by mass. By setting the carbon content of the filler wire within this range, the average Vickers hardness of the weld metal can be reduced to that of the high-strength steel plate. The carbon content of the filler wire is determined by taking chips from the filler wire and measuring them using the well-known high-frequency combustion method (combustion-infrared absorption method). Chips are collected from three locations, and the arithmetic mean of the measurements from each location is taken as the carbon content in the steel sheet.

[0048] Other preferred chemical compositions for the filler wire include the following: Si: 0.25~1.50% Mn: 0.5~2.8% Al: 0.001~0.300% Ti: 0.01~0.30% P: More than 0% and less than 0.05% S: More than 0% and less than 0.05% Cu: 0~0.50% The remainder consists of Fe, as well as B, Cr, Ni, Mo, V, and impurities. Regarding the chemical composition of the filler wire, the elemental amounts of each component are measured by taking chips from the filler wire and measuring the collected chips using the well-known high-frequency combustion method (combustion-infrared absorption method). Chips are collected at three locations, and the arithmetic mean of the measured values ​​from each location is taken as the elemental amount contained in the steel sheet.

[0049] The value obtained by dividing the gap size by the filler wire's carbon content is 25.0 or less. If the gap size changes along the entire length of the weld surface of the high-strength steel plate, the maximum value of the gap size shall be used as the gap size value. As described later, the butt welded joint according to this embodiment is characterized by a reduction in the hardness of the weld metal. However, if the hardness of the weld metal is reduced excessively, there is a risk that the strength of the butt welded joint will decrease. By setting the value obtained by dividing the gap size by the C content of the filler wire to this range, it is possible to suppress an excessive reduction in the hardness of the weld metal.

[0050] The laser beam conditions are not particularly limited, but solid-state lasers such as CO2 lasers, fiber lasers, and disk lasers, which are commonly used, may be used. Semiconductor lasers may also be used. The laser output is preferably 2kW to 6kW from the viewpoint of welding speed and cost. The laser scanning speed is preferably 3m / min to 10m / min from the viewpoint of productivity. The filler wire feeding speed is preferably 3m / min to 10m / min from the viewpoint of cost. When using a CO2 laser, it is preferable to use helium or argon gas as the shielding gas. When using a solid-state laser or semiconductor laser, it is preferable to use argon gas, nitrogen gas, or compressed air as the shielding gas, but the procedure may also be carried out in the atmosphere without using a shielding gas.

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

[0052] Traditionally, when welding common types of steel, fillers were added to harden the weld metal and suppress fracture in the weld metal. Furthermore, in butt welding, even a gap of only 0.1-0.2 mm when steel plates are joined could cause dents in the weld, reducing the joint strength. To suppress this reduction in joint strength, the steel plates were typically joined without gaps to prevent dents, and fillers were added to harden the weld and stabilize the joint strength. On the other hand, in the butt-welded joint according to the above embodiment, hydrogen embrittlement is suppressed by softening the weld metal through the addition of filler using filler wire in butt welding of 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 placed facing each other with a gap between them, and a gap of 0.4 mm to 1.0 mm is secured between the plates during butt welding. This gap is then 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 that of the high-strength steel plate is used to reduce the hardness of the weld metal. [Examples]

[0053] The invention described herein will be explained in detail below with reference to examples, but the invention is not limited thereto.

[0054] In this example, two steel plates were prepared, and their ends were joined together to create a butt welded joint by laser welding. For each of the steel plates A to D used, the tensile strength and Vickers hardness were as shown in Table 1. The tensile strength of the steel plates was 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 strength was measured at two points, and the arithmetic mean of these values ​​was taken as the tensile strength of each steel plate. The Vickers hardness of high-strength steel sheets was measured by cutting out planes perpendicular to the surface of the steel sheet at two arbitrary locations, embedding them in resin, and performing mirror polishing. The Vickers hardness was measured using a Vickers hardness tester at a distance of 100 μm or more from the front and back surfaces of the steel sheet. The measurement conditions for Vickers hardness were a load of 500 gf. Using this method, the Vickers hardness was measured at 5 points from each plane, for a total of 10 points, and the arithmetic mean of these values ​​was taken as the Vickers hardness of the high-strength steel sheet. Steel chips were collected from a depth of 0.3 mm or more from the surface of the steel plate, and the carbon content of the steel plate was measured by high-frequency combustion (combustion-infrared absorption method) on the collected chips. Chips were collected from three locations, and the arithmetic mean of the measured values ​​from each location was taken as the carbon content of the steel plate.

[0055] The amount of C in each of the filler wires A to C used for welding was as shown in Table 2. The carbon content of the filler wire was determined by taking chips from the filler wire and measuring the carbon content of the chips using high-frequency combustion (combustion-infrared absorption method). Chips were taken from three locations, and the arithmetic mean of the measurements from each location was used as the carbon content of the filler wire.

[0056] [Table 1]

[0057] [Table 2]

[0058] Material 1, Material 2, and filler wire were selected in the combinations listed in Tables 3 and 4, and welded joints were fabricated for each experimental example. The plate 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. The diameter of the filler wire was appropriately used depending on the size of the gap between the steel plates, with diameters of 0.6 mm, 0.9 mm, and 1.2 mm being used.

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

[0060] After fabricating butt weld joints, two sections perpendicular to the weld line were cut out from each joint, embedded in resin, and mirror-polished. These sections were then etched with a picric acid alcohol solution to create 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 cutting positions excluded areas within 30 mm of the weld start and end points. For each cross-sectional sample, the width of the narrowest portion of the weld metal was measured in a direction parallel to the surface of the steel plate, and the arithmetic mean 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 area was measured using a Vickers hardness tester. For either weld 1 or weld 2 described above, as illustrated in Figure 3, the Vickers hardness was measured at a position that is 1 / 4 of the thickness of the thinnest high-strength steel plate from the surface of the thinnest high-strength steel plate, at 0.15 mm intervals in a direction parallel to the surface of the thinnest high-strength steel plate. The arithmetic mean of these values ​​was adopted as the average Vickers hardness of the weld metal. The Vickers hardness was measured under a load of 500 gf.

[0062] For each of Material 1 and Material 2, the maximum hardness of the HAZ-hardened area was determined by measuring the hardness using a Vickers hardness tester, similar to the Vickers hardness measurement of the weld metal. The highest value among these measurements was taken as the maximum hardness of the HAZ-hardened area. The average Vickers hardness of the weld metal was divided by the maximum hardness of the HAZ-hardened area to calculate a percentage (%). The maximum hardness of the HAZ-hardened area was the higher of the maximum hardness of the HAZ-hardened areas 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 material 1 and material 2. The microstructure of the weld metal was identified by measuring and analyzing the diffraction pattern using EBSD. Using these results, BCC and FCC structures were distinguished, and the area percentage of the microstructure containing the BCC structure was calculated.

[0064] The following evaluations were conducted. The results are shown in Tables 3 and 4. (Evaluation of press formability) A bending test was performed on each butt welded joint 8 or 24 hours after welding. In the bending test, the welded joint was bent to a 90-degree angle with a radius of R8 so that the weld line bent into a V shape. The surface of the weld metal of each butt welded joint that underwent the bending test was observed using a microscope at a magnification of 40x to check for cracks. Items that showed no cracks during microscopic observation were marked "○ (good)". Items that showed cracks during microscopic observation were marked "× (bad)".

[0065] (Measurement of tensile strength) Test specimens were prepared in the JIS No. 5B shape, including the weld, and tensile tests were performed using a tensile testing machine. The weld line was positioned perpendicular to the tensile direction. After the tensile test, the test specimens were visually inspected, and any specimens that fractured within the weld metal were marked "× (bad)," while all others were marked "〇 (good)."

[0066] [Table 3]

[0067] [Table 4]

[0068] As can be seen from the results in Tables 3 and 4, each embodiment that satisfies the requirements of the present application yielded favorable results in all of the above evaluations.

[0069] However, when the width of the weld was narrow, that is, when the gap between material 1 and material 2 was narrow or 0.0 mm, a large amount of filler could not be supplied, and the value obtained by dividing the average Vickers hardness of the weld metal by the highest hardness of the HAZ hardened area became high. As a result, the press formability was poor. Also, when the width of the weld was wide, that is, when the gap between material 1 and material 2 was wide, burn-through occurred, and stable welding could not be achieved over the entire length of the weld. Furthermore, if the value obtained by dividing the gap size by the carbon content of the filler wire exceeds 25.0, the average Vickers hardness of the weld metal becomes less than 60% of the maximum hardness of the HAZ hardened area, resulting in fracture in the weld metal during tensile testing and a decrease in the joint strength of the butt welded joint. Furthermore, when the carbon content of the filler wire exceeded 0.10 mass%, the value obtained by dividing the average Vickers hardness of the weld metal by the highest hardness of the HAZ hardened area increased, resulting in poor press formability. [Industrial applicability]

[0070] The butt welded joint, tailored blank, and method for manufacturing the butt welded joint described herein can suppress cracking in the welded joint even when pressed shortly after welding. Therefore, the invention described herein is extremely useful in industry. [Explanation of Symbols]

[0071] 1. Butt welded joint 10, 20 High strength steel plate 30 Weld metal

Claims

1. A butt welded joint having two high-strength steel plates and a weld metal that joins the two high-strength steel plates together, The high-strength steel plate includes a normal portion and a HAZ portion adjacent to the weld metal. The aforementioned normal part has a Vickers hardness of 372 HV or higher. The HAZ portion comprises 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 average width of the weld metal is 1.00 to 1.65 mm. At a load of 500 gf, the Vickers hardness of the weld metal, measured at 0.15 mm intervals at a position 1 / 4 of the way from the surface of the high-strength steel plate in the thickness direction, is 60.00 to 83.00% of the maximum hardness of the HAZ hardened portion. A butt welded joint characterized by the following.

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

3. In the microstructure of the weld metal, the area ratio of the microstructure having a BCC structure is 50% or more. The butt welded joint according to claim 1 or 2, characterized by the features described above.

4. The thickness of the aforementioned high-strength steel plate is 0.8 to 2.0 mm. The butt welded joint according to claim 1 or 2, characterized by the features described above.

5. 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. The butt welded joint according to claim 1 or 2, characterized by the features described above.

6. A butt welded joint according to claim 1 or 2, A tailored blank characterized by the following features.

7. 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, The process involves arranging the high-strength steel plates so that their welded surfaces face each other with a gap between them, A process of butt welding the high-strength steel plates together using filler wire, It has, The aforementioned high-strength steel plate has a Vickers hardness of 372 HV or higher. The size of the gap is 0.4 to 1.0 mm. The amount of carbon in the filler wire is greater than 0% by mass and less than or equal to 0.10% by mass. The value obtained by dividing the size of the gap by the amount of carbon in the filler wire is 25.0 or less. The thickness of the aforementioned high-strength steel plate is 0.8 to 2.0 mm. A method for manufacturing a butt welded joint, characterized by the above.

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