Butt joint, tailor-welded blank and method for manufacturing a butt joint

Abstract

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

Description

Technical Field

[0001] This disclosure relates to a butt weld joint, a butt weld blank, and a method for manufacturing the butt weld joint.

[0002] This application claims priority based on Japanese Patent Application No. 2024-026405, filed in Japan on February 26, 2024, the contents of which are incorporated herein by reference. Background Technology

[0003] To achieve lightweighting of automobiles, it is desirable to effectively utilize welded blanks made from high-strength steel sheets. These welded blanks are formed by joining multiple steel sheets together through welding. The welded blanks are then subjected to stamping, bending, and cutting processes to achieve the desired product shape.

[0004] For example, Patent Document 1 discloses a technique related to a welded blank formed by joining the ends of steel plates together by welding. Furthermore, Patent Document 2 discloses a technique in which a portion of the welded part, which is cut off after the manufacture of the welded blank, is locally softened by heat treatment for the purpose of improving the workability of the welded part.

[0005] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2021-122914 Patent Document 2: Japanese Patent No. 7172107 Summary of the Invention

[0006] The problem that the invention aims to solve

[0007] The inventors have discovered that in the above-mentioned welded blanks, especially when cold pressing is performed on welded blanks made of high-strength steel plates with a Vickers hardness of 372 HV or higher, hydrogen embrittlement occurs in the welded part due to the hardening of the weld metal. If stamping is performed shortly after welding, cracking may sometimes occur.

[0008] The main causes of hydrogen embrittlement include the hardness of the weld metal, the introduction of hydrogen into the weld metal during welding, and the stress introduced into the weld joint through processing. The inventors focused on the hardness of the weld metal and investigated the suppression of cracking caused by hydrogen embrittlement. It should be noted that when stamping is performed after welding, allowing sufficient time until hydrogen diffuses out of the weld metal (e.g., after 24 hours), cracking can be suppressed, but the time from welding to stamping becomes longer, resulting in lower productivity.

[0009] This disclosure was made in view of the above circumstances, and its object is to provide a method for manufacturing a butt weld joint, a butt weld blank, and a butt weld joint that can suppress cracking at the weld even if stamping is performed shortly after welding.

[0010] means for solving problems

[0011] (1) One aspect of the present disclosure relates to a butt-welded joint characterized in that it is a butt-welded joint having two high-strength steel plates and a weld metal that joins the high-strength steel plates together. The high-strength steel plate includes a general section and a HAZ section adjacent to the weld metal. The Vickers hardness of the normal part is above 372 HV. The HAZ portion has a HAZ hardened portion with a higher Vickers hardness than the normal portion and a HAZ softened portion with a lower Vickers hardness than the normal portion. The average width of the weld metal is 1.00~1.65mm. In the Vickers hardness test at a load of 500 gf obtained by measuring the position at 1 / 4 of the thickness direction of the high-strength steel plate at 0.15 mm intervals from the surface of the high-strength steel plate, the average Vickers hardness of the weld metal is 60.00 to 83.00% of the highest hardness of the HAZ hardened portion.

[0012] (2) In the butt weld 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.

[0013] (3) In the butt weld joint described in (1) or (2) above, the area ratio of the structure having a BCC structure in the structure of the weld metal may be 50% or more.

[0014] (4) In any of the above (1) to (3) welded joints, the thickness of the high-strength steel plate may be 0.8 to 2.0 mm.

[0015] (5) In any of the above (1) to (4) weld joints, the aspect ratio of the weld metal, as defined by the average width of the weld metal relative to the average thickness of the two high-strength steel plates, may be less than 2.

[0016] (6) The welding blank of one embodiment of the present disclosure is characterized in that it includes the butt weld joint of any one of (1) to (5) above.

[0017] (7) A method for manufacturing a butt-welded joint according to one aspect of the present disclosure is characterized in that it is a method for manufacturing a butt-welded joint having two high-strength steel plates and a weld metal for joining the high-strength steel plates together, comprising: The process of arranging the high-strength steel plates by placing their weld surfaces facing each other with a gap between them; and The process of butt-welding the high-strength steel plates together using filler wire. The high-strength steel plate has a Vickers hardness of 372 HV or higher. The size of the gap is 0.4~1.0 mm. The carbon content of the filler wire exceeds 0% by mass and is less than 0.10% by mass. The value obtained by dividing the size of the gap by the value of the C amount of the filler wire is 25.0 or less. The thickness of the high-strength steel plate is 0.8~2.0mm.

[0018] Invention Effects

[0019] According to the manufacturing method of the butt weld joint, the butt weld blank and the butt weld joint disclosed herein, cracking can be suppressed even if stamping is performed shortly after welding. Attached Figure Description

[0020] Figure 1 This is a schematic cross-sectional view of a welded joint used to illustrate one embodiment of the present disclosure.

[0021] Figure 2 This is a graph illustrating the distribution of Vickers hardness of a welded joint according to an embodiment of this disclosure.

[0022] Figure 3 This is a diagram illustrating a method for measuring Vickers hardness in weld metal and the HAZ region of a weld joint. Detailed Implementation

[0023] The following description illustrates embodiments of the invention disclosed herein, but the invention disclosed herein is obviously not limited to the examples described below. In the following description, specific values ​​and materials are sometimes illustrated, but other values ​​and materials can be applied as long as the effects of the invention disclosed herein can be achieved. Furthermore, the constituent elements of the following embodiments can be combined with each other.

[0024] Furthermore, the numerical range indicated by "~" refers to the range of values ​​before and after "~" as the lower and upper limits. In this specification, the term "process" includes not only independent processes, but also processes that cannot be clearly distinguished from other processes, as long as the desired purpose of the process can be achieved.

[0025] The butt weld joint in this embodiment is a butt weld joint having two high-strength steel plates and a weld metal that joins the high-strength steel plates together. For example... Figure 1 As shown, in the butt weld joint 1, the high-strength steel plate 10 and the high-strength steel plate 20 are connected by the weld metal 30.

[0026] Figure 1This is a schematic cross-sectional view of a welded joint used to illustrate an embodiment of the present disclosure, showing a section perpendicular to the weld line L on which the weld metal 30 is formed. Figure 1 The diagram shows a case where the thickness of the high-strength steel plate 10 is thinner than that of the high-strength steel plate 20.

[0027] (High-strength steel plate)

[0028] 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.

[0029] (Usually)

[0030] Normal portions 11 and 21 are portions not affected by the heat generated during welding when the weld metal 30 is placed. For example, in laser welding, normally, the portion extending more than 10 mm from the weld toe of the weld metal 30 along a direction parallel to the surface of the high-strength steel plate 10 or 20 and perpendicular to the weld line L of the weld joint 1 is normal portion 11 or normal portion 21. Normal portion 11 or normal portion 21 can be distinguished from HAZ portion 12 or HAZ portion 22 by Vickers hardness.

[0031] The Vickers hardness of ordinary part 11 and ordinary part 21 is above 372HV.

[0032] Figure 2 This is a Vickers hardness distribution diagram showing the weld joint 1 in this embodiment. Figure 2 In the diagram, the vertical axis represents Vickers hardness (HV), and the horizontal axis represents the direction of extension of the plate surface in a plane perpendicular to the weld line L (equivalent to...). Figure 1 The relative position on the horizontal axis (in the middle). Figure 2 The distribution diagram is obtained by measuring the Vickers hardness at 500gf at 0.15mm intervals in a direction parallel to the surface of the high-strength steel plate, starting from the surface of the side with the smaller height difference between the two high-strength steel plates (the surface of the thinner high-strength steel plate), at a position of 1 / 4 of its thickness, and plotting the value.

[0033] Figure 2 The range of A represents the weld metal 30, the range of B represents the HAZ hardened portion 12h and 22h, and the range of C represents the HAZ softened portion 12s and 22s. Figure 2 The dashed line represents the Vickers hardness of a typical portion of a high-strength steel plate. Figure 2 In the case of weld joint 1 shown in the distribution diagram, the Vickers hardness of the typical portion is 500 HV. It should be noted that, in... Figure 2 In the case of weld joint 1 shown in the distribution diagram, the Vickers hardness of the normal portions of the two high-strength steel plates is the same. Figure 2It can be seen that the Vickers hardness of the hardened HAZ region (range B) is higher than that of the normal region, and the Vickers hardness of the softened HAZ region (range C) is lower than that of the normal region.

[0034] The tensile strength of the ordinary part 11 or the ordinary part 21 is determined, for example, according to JIS Z 2241 (2011), by preparing a JIS 5 B-shaped test piece and performing a tensile test using a tensile testing machine. By this method, for example, the tensile strength at three points is measured, and their arithmetic mean is taken as the tensile strength of the ordinary part 11 or the ordinary part 21.

[0035] From the viewpoint of lightweight components, the tensile strength of the general part 11 or the general part 21 is more preferably 1180 MPa or more, 1300 MPa or more, or 1470 MPa or more.

[0036] The Vickers hardness of the normal part 11 or the normal part 21 is measured using a Vickers hardness tester in the following order.

[0037] The weld metal 30, contained in a plane perpendicular to the weld line L, is cut out in a direction parallel to the surfaces of the high-strength steel plates 10 and 20, within a range of approximately 30 mm. The cut is then embedded in resin, ground to a mirror finish, and etched with a picric acid alcohol solution to create a sample. The cut location does not include the 30 mm range from the weld start and end points. In the case of welded joints processed by stamping or similar methods, work hardening occurs at the bend, making accurate Vickers hardness measurement impossible. Therefore, when measuring processed welded joints, it is best to collect a sample from a plane that is as flat as possible without bending. It should be noted that the Vickers hardness measurement surface for the general section 11 or general section 21 can also be the same as the measurement surface used in the Vickers hardness measurement of the HAZ section 12 described later.

[0038] For this sample, Vickers hardness was measured at five or more points at locations at least 10 mm away from the center of the weld metal 30 and at least 100 μm away from the back surface of the 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 was taken as the Vickers hardness of the normal portion 11 or normal portion 21. The Vickers hardness was measured under a load of 500 gf.

[0039] From the viewpoint of lightweight components, the Vickers hardness of the general part 11 or the general part 21 is more preferably 402HV or above and 440HV or above.

[0040] Tensile strength and Vickers hardness can be converted from psi according to the conversion tables of JIS Z 8413 and Z 8438.

[0041] (HAZ Department)

[0042] HAZ section 12 has a HAZ hardened section 12h with a Vickers hardness higher than that of the normal section 11 and a HAZ softened section 12s with a Vickers hardness lower than that of the normal section 11. The HAZ hardened section 12h is located on the side closer to the weld metal 30 than the HAZ softened section 12s.

[0043] The HAZ hardened section 12h is a region that is rapidly cooled after being heated to a high temperature of approximately 900~1500°C during welding. Therefore, in the HAZ hardened section 12h, the microstructure is mainly composed of martensite and has a BCC structure, which is heated to approximately 900°C or higher, which is the phase transformation point, and then quenched.

[0044] The HAZ softened section 12s is the part that is affected by the heat during welding but whose temperature does not rise above the phase transformation point. Therefore, tempering occurs in the HAZ softened section 12s, and the Vickers hardness is lower than that of the normal section 11.

[0045] HAZ uses a Vickers hardness tester to determine the Vickers hardness of section 12 as follows.

[0046] The weld metal 30, contained in a plane perpendicular to the weld line L, is cut out in an area of ​​approximately 30 mm in a direction parallel to the surfaces of the high-strength steel plates 10 and 20. The cut is then embedded in resin, ground to a mirror finish, and etched with a picric acid alcohol solution to prepare the specimen. The cut location does not include the 30 mm range from the weld start and end points. In cases where the weld joint is processed by stamping or similar methods, work hardening occurs at the bend, making accurate Vickers hardness measurement impossible. Therefore, when measuring processed weld joints, it is best to collect specimens from the flat surface that is as unbent as possible.

[0047] For this sample, Vickers hardness was measured at 0.15 mm intervals in a direction parallel to the surface of the high-strength steel plate 10 or 20, at a position from the surface of the high-strength steel plate 10 or 20 to 1 / 4 of the thickness of the high-strength steel plate 10 or 20.

[0048] like Figure 3 As shown, when the thicknesses of high-strength steel plate 10 and high-strength steel plate 20 are different, firstly, as... Figure 3 As shown, the high-strength steel plate 10, with the smaller thickness, is selected from the specific high-strength steel plate 10 and high-strength steel plate 20. Then, Vickers hardness is measured at intervals of 0.15 mm in a direction parallel to the surface of the high-strength steel plate 10, at a position from the surface of the high-strength steel plate 10 to 1 / 4 of its thickness. It should be noted that the position can also be set to 1 / 4 of the thickness from either the front or back surface of the high-strength steel plate 10. The high-strength steel plate 20 is located at a height position of 1 / 4 of its thickness from the surface of the high-strength steel plate 10.

[0049] The Vickers hardness test was performed under a load of 500 gf. It should be noted that when two high-strength steel plates have different thicknesses, the thinner plate is used as high-strength steel plate 10. When two high-strength steel plates have the same thickness, any high-strength steel plate can be used as high-strength steel plate 10, and the Vickers hardness test can be performed from the inner side of the high-strength steel plate 10, at a position equal to 1 / 4 of its thickness.

[0050] exist Figure 3 The figure illustrates the Vickers hardness measured at a position where the thickness of the high-strength steel plate 10 is 1 / 4 of the surface of the high-strength steel plate 10. Figure 3 The dots represent the Vickers hardness measurement positions located at 0.15 mm intervals in a direction parallel to the surface of the high-strength steel plate 10, from the surface of the high-strength steel plate 10 to a position at 1 / 4 of the thickness of the high-strength steel plate 10. It should be noted that the Vickers hardness at a position at 1 / 4 of the thickness of the high-strength steel plate 10 can also be measured from the surface of the high-strength steel plate 10 on the upper side of the figure.

[0051] The Vickers hardness test as described above is performed, and the range where the hardness is relatively higher than the normal range 11 is defined as the HAZ hardened range 12h. Conversely, the range where the hardness is relatively lower than the normal range 11 is defined as the HAZ softened range 12s.

[0052] HAZ section 22 has a HAZ hardened section 22h with a Vickers hardness higher than that of the normal section 21 and a HAZ softened section 22s with a Vickers hardness lower than that of the normal section 21. The HAZ hardened section 22h is located on the weld metal 30 side, while the HAZ softened section 22s is located on the weld metal 30 side.

[0053] The HAZ hardened portion 22h and the HAZ softened portion 22s are subjected to the same thermal effects as the HAZ hardened portion 12h and the HAZ softened portion 12s. The Vickers hardness of the HAZ portion 22 is determined using the same method as that used for the Vickers hardness of the HAZ portion 12.

[0054] (Welding metal)

[0055] Weld metal 30 is the part of the steel plate or the like that melts and solidifies due to the heat input during welding. It is a part of the weld that melts and solidifies during welding. The material supply source for weld metal 30 is the multiple steel plates being joined and filler wire.

[0056] When steel plates are coated, the coating components also melt and become part of the weld metal. It should be noted that, in addition to the elements from these supply sources, oxygen and nitrogen from the air are introduced into the weld metal, and sometimes unavoidable impurities are also introduced.

[0057] The average width of the weld metal 30 is 1.00~1.65mm. The average width of the weld metal 30 is determined as follows.

[0058] Two sections are cut into a plane perpendicular to the weld line L of the weld metal 30 of the weld joint 1, resin is embedded in the cut sections, mirror polishing is performed, and cross-sectional specimens are prepared by etching with a picric acid alcohol solution. The cut sections can be any plane other than 30 mm from the start and end points of the weld. It should be noted that the surface used to measure the average width of the weld metal 30 can also be the same as the surface used in the Vickers hardness test of the HAZ section 12.

[0059] The weld metal 30 is the range in which solidified structures such as honeycomb structure, honeycomb dendritic structure, and dendritic structure are observed, which can generally be identified by microscopic observation.

[0060] For the weld metal 30 of the cross-sectional specimens collected in each of the planes described above, such as Figure 1 As shown, the width W of the narrowest portion of the weld metal 30 in a direction parallel to the surfaces of the high-strength steel plate 10 and the high-strength steel plate 20 is measured. Furthermore, 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.

[0061] It should be noted that, through visual observation using a microscope at 10x magnification, weld metal 30 can be identified as high-strength steel plate 10 and high-strength steel plate 20. The surfaces of high-strength steel plate 10 and high-strength steel plate 20 generally have minimal unevenness. On the other hand, since weld metal 30 is melted once, it often forms a wavy pattern on its surface, and this surface difference can be used to distinguish weld metal 30 from high-strength steel plate 10 and high-strength steel plate 20.

[0062] The average Vickers hardness of weld metal 30 is 60.00 to 83.00% of the highest hardness of the HAZ hardened portion 12h or HAZ hardened portion 22h.

[0063] Using a Vickers hardness tester, determine the average Vickers hardness of weld metal 30 as follows.

[0064] The weld metal 30, which will be the object of the test, is contained in a plane perpendicular to the weld line L of the weld joint 1. This plane is cut out in an area of ​​approximately 30 mm in a direction parallel to the surfaces of the high-strength steel plates 10 and 20. The cut is then embedded in resin, mirror-polished, and etched with a picric acid alcohol solution to prepare the sample. The cut location does not include the area within 30 mm from the weld start and end points. In the case of welded joints processed by stamping or other methods, work hardening occurs at the bend, making accurate Vickers hardness measurement impossible. Therefore, when measuring processed welded joints, it is best to collect a sample from a plane that is as flat as possible without bending. It should be noted that the surface used for measuring the average Vickers hardness of the weld metal 30 can also be the same as the surface used for measuring the Vickers hardness of the HAZ section 12.

[0065] For this sample, Vickers hardness was measured at 0.15 mm intervals in the thickness direction of the high-strength steel plate 10, from the surface of the side with the smaller height difference between the high-strength steel plate 10 and the high-strength steel plate 20 to a position one-quarter of the thickness of the high-strength steel plate 10, in a direction parallel to the surface of the high-strength steel plate 10. The Vickers hardness was measured under a load of 500 gf. It should be noted that when the thicknesses of the high-strength steel plate 10 and the high-strength steel plate 20 are different, the thinner steel plate is used as the high-strength steel plate 10.

[0066] Then, the arithmetic mean of the Vickers hardness of weld metal 30 is taken as the average Vickers hardness of weld metal 30.

[0067] The highest hardness of the HAZ hardened portion 12h or HAZ hardened portion 22h is the highest Vickers hardness among the HAZ hardened portions 12h of the high-strength steel plate 10 and the HAZ hardened portions 22h of the high-strength steel plate 20 as measured above.

[0068] The preferred chemical composition for high-strength steel plate 10 and high-strength steel plate 20 is as follows: contain C (carbon): 0.09~0.35% by mass Si (silicon): 0.01~0.98% by mass Mn (manganese): 1.2~3.8% by mass P (phosphorus): 0.001~0.050% by mass S (sulfur): 0.001~0.050% by mass Ti (titanium): 0.00~0.50% by mass Al (aluminum): 0.001~1.000% by mass Nb (Niobium): 0.000~0.500% by mass B (boron): 0.0000~0.0100% by mass The balance includes Fe (iron) and impurities. Impurities refer to components contained in the raw materials or mixed in during the manufacturing process that are not intentionally present. For example, trace amounts of components other than Fe, such as W, Mg, and V, may sometimes be mixed in. As part of the chemical composition of high-strength steel plate 10 and high-strength steel plate 20, other elements may be included besides impurities, provided they do not impair the effect on the weld joint 1. The chemical composition of high-strength steel plate 10 may be the same as or different from that of high-strength steel plate 20.

[0069] High-strength steel plates 10 and 20, having such chemical compositions, are high-strength, but the weld metal 30 is prone to cracking due to hydrogen embrittlement, as described above. In the weld joint 1 of this embodiment, even when using high-strength steel plates 10 and 20 with the chemical compositions described above, cracking of the weld can be suppressed even when stamping is performed shortly after welding.

[0070] The carbon (C) content in the steel plate was determined by collecting shavings from within the plate at a depth of 0.3 mm or more above the surface. The shavings were measured using a known high-frequency combustion method (combustion-infrared absorption method). Shavings were collected at three locations, and the arithmetic mean of the measured values ​​was taken as the C content in the steel plate. The sulfur (S) content was determined similarly using the high-frequency combustion method (combustion-infrared absorption method). Other elemental contents were determined using known spark discharge emission spectrometry.

[0071] In the weld joint 1 of 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 provides the advantage of further preventing cracking of the weld joint when pressed shortly after welding.

[0072] As described above, the average Vickers hardness of the weld metal 30 is determined by a Vickers hardness tester, and the calculation is based on the measured value. The Vickers hardness of the high-strength steel plate 10 and the high-strength steel plate 20, i.e., the Vickers hardness of the ordinary portion 11 and the ordinary portion 21 respectively, is determined by the above method.

[0073] In the weld joint 1 of this embodiment, the area fraction of ferrite, martensite, and other structures with a BCC structure in the microstructure of the weld metal 30 is more preferably 50% or more. This provides the advantage of stable hardness in the weld metal.

[0074] The microstructure of weld metal 30 was identified by measuring and resolving the diffraction pattern in EBSD. Using the results, BCC and FCC structures could be distinguished, and the area ratio of BCC could be calculated. If it is a BCC structure, the crystal structure is ferrite, bainite, or martensite. If it is an FCC structure, the crystal structure is austenite.

[0075] In the butt weld joint 1 of 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 provides the advantage of enabling lightweighting of automotive components.

[0076] Regarding the thickness of the steel plate, the steel plate is cut along a direction perpendicular to the surface, embedded in resin, mirror-polished, and measured using a microscope. It should be noted that the thickness of the steel plate can also be measured using vernier calipers. 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.

[0077] In the weld joint 1 of this embodiment, it is more preferable that the aspect ratio of the weld metal 30, defined by the average width of the weld metal 30 relative to the average thickness of the high-strength steel plate 10 and the high-strength steel plate 20, is less than 2. This provides the advantage that the formability is less likely to be compromised.

[0078] The average width of the weld metal / the average thickness of the high-strength steel plate = the aspect ratio of the weld metal 30. The average width of the weld metal 30 is as described above. The average thickness of the high-strength steel plate 10 and the high-strength steel plate 20 is calculated using the method described above.

[0079] [Method for manufacturing welded joints]

[0080] The manufacturing method of the butt weld head according to this embodiment will be described below. According to this manufacturing method, it is possible to manufacture a butt weld head that can suppress cracking at the weld even if it is stamped shortly after welding.

[0081] The method for manufacturing a butt weld joint in this embodiment is a method for manufacturing a butt weld joint having two high-strength steel plates and a weld metal that joins the high-strength steel plates together, comprising: The process of arranging high-strength steel plates by placing their weld surfaces facing each other with a gap between them (the arrangement process); and The process of butt welding high-strength steel plates together using filler wire (welding process).

[0082] (Configuration process)

[0083] In the assembly process, high-strength steel plates are arranged with their welding surfaces facing each other, spaced apart. The welding surface of a high-strength steel plate refers to the end face used for butt welding. The welding surfaces of the opposing high-strength steel plates have corresponding shapes. The shape of the welding surface of the high-strength steel plate, when viewed from the surface side of the high-strength steel plate, can be a straight line, a curve, or a combination of these.

[0084] The Vickers hardness of the two high-strength steel plates is above 372HV.

[0085] The steel plate was processed into a JIS 5B shape, and a tensile test was performed using a tensile testing machine according to JIS Z 2241 (2011) to determine the tensile strength of the high-strength steel plate. The tensile strength was measured at two points, and their arithmetic mean was taken as the tensile strength of each high-strength steel plate. From the viewpoint of lightweight components, the tensile strength of the high-strength steel plate is more preferably 1180 MPa or higher, 1300 MPa or higher, or 1470 MPa or higher.

[0086] To determine the Vickers hardness of high-strength steel plates, two planes perpendicular to the steel plate surface are cut at any two points, resin is embedded in the planes, and mirror polishing is performed. The Vickers hardness is then measured using a Vickers hardness tester at a depth of at least 100 μm from the surface to the back of the steel plate. The measurement conditions are set to a load of 500 gf. Using this method, the Vickers hardness is measured at 5 points on each plane, for a total of 10 points. The arithmetic mean of these measurements is taken as the Vickers hardness of the high-strength steel plate.

[0087] From the perspective of lightweight components, the Vickers hardness of high-strength steel plates is more preferably above 402HV or above 440HV.

[0088] Two high-strength steel plates can be high-strength steel plate 10 and high-strength steel plate 20 as described above.

[0089] In the assembly process, the gap between the welded surfaces of the high-strength steel plates, i.e., the gap width, is 0.4~1.0 mm. The gap size is defined as the shortest distance from one welded surface of one high-strength steel plate to another when viewed from a direction perpendicular to the surface of the opposing high-strength steel plate at various points on the welded surfaces. Preferably, the gap size is designed to remain constant along the entire length of the welded surfaces of the high-strength steel plates.

[0090] By setting the gap between the weld surfaces of the high-strength steel plates to this range, the desired weld metal and Vickers hardness of the HAZ region as described above can be obtained.

[0091] (Welding process)

[0092] In the welding process, high-strength steel plates are butt-welded together using laser welding and filler wire. By supplying filler wire while irradiating the high-strength steel plates with a laser beam, the two high-strength steel plates and the filler wire melt together. The molten metal solidifies between the two high-strength steel plates to form weld metal, thereby joining the two high-strength steel plates.

[0093] In the welding process, laser welding is performed by supplying filler wire to form weld metal. Filler wires include, for example, solid wire and flux-cored wire.

[0094] In this embodiment, weld metal refers to a portion of the weld, that is, the metal that melts and solidifies during welding. Here, "molten and solidified metal" refers to both the metal derived from molten high-strength steel plate and the metal derived from molten filler wire. Therefore, weld metal refers to the metal formed by the fusion of a portion of high-strength steel plate and filler wire.

[0095] The carbon content of the filler wire is greater than 0% by mass and less than 0.10% by mass.

[0096] By setting the C value of the filler wire to this range, the average Vickers hardness of the weld metal can be lower than that of the high-strength steel plate.

[0097] The carbon content of the filler wire was determined by collecting powder from the filler wire and measuring the collected powder using a known high-frequency combustion method (combustion-infrared absorption method). The powder was collected at three locations, and the arithmetic mean of the measured values ​​was taken as the carbon content in the steel plate.

[0098] The following are preferred as other chemical compositions for filler wire. 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 balance includes Fe, as well as B, Cr, Ni, Mo, V, and impurities.

[0099] Regarding the chemical composition of the filler wire, the elemental amounts of each component were determined by collecting shavings from the filler wire and measuring the collected shavings using a known high-frequency combustion method (combustion-infrared absorption method). Shavings were collected at three locations, and the arithmetic mean of the measured values ​​was taken as the elemental amount contained in the steel plate. This was then used for the determination.

[0100] The value obtained by dividing the gap size by the C-value of the filler wire is 25.0 or less. When the gap size varies along the entire length of the weld surface of the high-strength steel plate, the maximum value of the gap size is set as the gap size value.

[0101] As described below, in the butt weld joint of this embodiment, the hardness of the weld metal is reduced. However, if the hardness of the weld metal is reduced excessively, the strength of the butt weld joint may decrease. By setting the value obtained by dividing the gap size by the amount of filler wire (C) within this range, excessive reduction in the hardness of the weld metal can be suppressed.

[0102] There are no particular restrictions on the laser beam conditions; commonly used solid-state lasers such as CO2 lasers, fiber lasers, and disk lasers can be used. Alternatively, semiconductor lasers can also be used.

[0103] From the perspective of welding speed and cost, the laser output is preferably 2kW to 6kW. From the perspective of productivity, the laser scanning speed is preferably 3m / min to 10m / min. From the perspective of cost, the filler wire supply speed is preferably 3m / min to 10m / min.

[0104] When using a CO2 laser, helium or argon is preferred as the protective gas. When using a solid-state laser or a semiconductor laser, argon, nitrogen, or compressed air is preferred as the protective gas, but it is also possible to operate in the atmosphere without a protective gas.

[0105] The butt weld joint of the above embodiment can preferably be used as the welding part of the welded blank.

[0106] Traditionally, filler was added when welding common steel grades to harden the weld metal and prevent fracture at the weld joint. Furthermore, in butt welding, even with a gap of approximately 0.1 to 0.2 mm between the steel plates, indentation can occur at the weld joint, reducing joint strength. To prevent this reduction in joint strength, the steel plates are typically butt-jointed without gaps to suppress indentation, and filler is added to harden the weld joint and stabilize the joint strength.

[0107] On the other hand, in the butt weld joint of the above embodiment, in the butt welding of high-strength steel plates with a Vickers hardness of 372 HV or higher, the weld metal is softened by adding filler wire, thereby suppressing hydrogen embrittlement. Furthermore, when manufacturing the butt weld joint of this embodiment, the welding surfaces of the high-strength steel plates are placed opposite each other with a gap between them, and the gap between the plates is ensured to be 0.4 mm to 1.0 mm during butt welding. This gap is filled with filler to increase the dilution rate of the filler wire. At this time, a filler wire with a lower C content (carbon content) than that of the high-strength steel plate is used as the filler wire, thereby reducing the hardness of the weld metal.

[0108] Example

[0109] The following examples illustrate the invention disclosed herein, but the invention disclosed herein is not limited thereto.

[0110] In this embodiment, two steel plates are prepared, their ends are joined together, and a welded joint is made by laser welding.

[0111] The tensile strength and Vickers hardness of the steel plates A through D used are listed in Table 1. The tensile strength of the steel plates was determined by machining the steel plates into a JIS 5 B shape and performing a tensile test according to JIS Z 2241 (2011) using a tensile testing machine. The tensile strength was measured at two points using this method, and their arithmetic mean was taken as the tensile strength of each steel plate.

[0112] To determine the Vickers hardness of high-strength steel plates, two planes perpendicular to the steel plate surface are cut at any two points, resin is embedded in the planes, and mirror polishing is performed. The Vickers hardness is then measured using a Vickers hardness tester at a depth of at least 100 μm from the surface to the back of the steel plate. The measurement conditions are set to a load of 500 gf. Using this method, the Vickers hardness is measured at 5 points on each plane, for a total of 10 points. The arithmetic mean of these measurements is taken as the Vickers hardness of the high-strength steel plate.

[0113] Cutting powder was collected from within the steel plate at a depth of 0.3 mm or more above the surface. The collected powder was subjected to a high-frequency combustion method (combustion-infrared absorption method) to determine the carbon content of the steel plate. The powder was collected at three locations, and the arithmetic mean of the measured values ​​was taken as the carbon content in the steel plate.

[0114] The C content of filler wires A to C used in welding is recorded in Table 2.

[0115] The carbon content of the filler wire was determined by collecting powder from the filler wire and performing a high-frequency combustion method (combustion-infrared absorption method) on the collected powder. The powder was collected at three locations, and the arithmetic mean of the measured values ​​was taken as the carbon content in the filler wire.

[0116]

[0117]

[0118] Welded joints for each experimental example were fabricated using materials 1 and 2 and filler wire selected from the combinations described in Tables 3 and 4. The thickness of the steel plates selected as material 1 or material 2 was measured using a microscope, as shown in Table 3.

[0119] As shown in Tables 3 and 4, the gap between the butt-jointed steel plates is varied from 0.0 mm to 1.2 mm. The diameter of the filler wire is appropriately selected based on the gap between the steel plates, using diameters of 0.6 mm, 0.9 mm, and 1.2 mm.

[0120] Laser welding is performed under the following conditions: The laser output and wire speed are set to ensure stable welding relative to each gap. Speed: 5m / min Protective gas: Atmosphere

[0121] After fabricating the weld joints, for each weld joint, two sections are cut out on a plane perpendicular to the weld line. Resin is embedded in these sections, and after mirror polishing, they are etched with a picric acid alcohol solution to create cross-sectional specimens for observing the shape of the weld. The welded portions in the two cross-sectional specimens are designated as weld 1 and weld 2, respectively. A 30mm radius from the weld start and end points is removed as the cutout locations.

[0122] For the weld metal of each cross-section specimen, the width of the narrowest part in the direction parallel to the surface of the steel plate is measured, and the arithmetic mean of the widths of weld part 1 and weld part 2 is taken as the average width of the weld metal.

[0123] The Vickers hardness of the weld metal and the HAZ region was measured using a Vickers hardness tester. For weld 1 or weld 2 as described above, such as... Figure 3 As illustrated, Vickers hardness was measured at 0.15 mm intervals in a direction parallel to the surface of the thin high-strength steel sheet, from the surface to a position one-quarter of the sheet's thickness. The arithmetic mean of these values ​​was then used as the average Vickers hardness of the weld metal. The Vickers hardness was measured under a load of 500 gf.

[0124] Regarding the maximum hardness of the HAZ hardened portion of each of materials 1 and 2, similar to the determination of the Vickers hardness of weld metal, a hardness test was performed using a Vickers hardness tester, and the maximum value was taken as the maximum hardness of the HAZ hardened portion.

[0125] The value (%) is obtained by dividing the average Vickers hardness of the weld metal by the highest hardness of the HAZ hardened portion. The highest hardness of the HAZ hardened portion is the higher of the highest hardness of the HAZ hardened portions of material 1 and material 2.

[0126] 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 the plates of material 1 and material 2.

[0127] The microstructure of weld metal is identified by measuring and resolving diffraction patterns in EBSD to determine its crystal structure. Using the results, BCC and FCC structures are distinguished, and the area fraction of the microstructure with the BCC structure is calculated.

[0128] The following evaluation was conducted. The results are shown in Tables 3 and 4.

[0129] (Evaluation of stamping formability)

[0130] Bending tests were performed on each weld joint 8 or 24 hours after welding. In the bending test, the weld joint was bent at an R8 angle to 90 degrees by bending the weld line into a V-shape. For each weld joint that underwent the bending test, the weld metal surface was observed for cracks using a microscope at 40x magnification.

[0131] When observing under a microscope, cases where no cracking is detected are marked as "〇 (Good)". When observing under a microscope, cases where cracking is detected are marked as "× (Poor)".

[0132] (Determination of tensile strength)

[0133] A test piece of JIS 5B shape containing the welded part was fabricated and subjected to a tensile test using a tensile testing machine. The weld line was positioned perpendicular to the tensile direction. After the tensile test, the test piece was visually inspected, and any part that broke within the weld metal was marked as "× (poor)," while the rest was marked as "〇 (good)."

[0134]

[0135] As can be seen from the results in Tables 3 and 4, the above evaluations are all good results in each embodiment that meets the requirements of this application.

[0136] However, when the width of the weld joint is narrow, i.e., the gap between material 1 and material 2 is narrow or 0.0 mm, the filler wire cannot be fed in large quantities, and the value obtained by dividing the average Vickers hardness of the weld metal by the highest hardness of the HAZ hardened portion becomes higher. Therefore, the stamping formability is poor. Furthermore, when the width of the weld joint is wide, i.e., the gap between material 1 and material 2 is large, burn-through occurs, making stable welding impossible along the entire length of the weld joint.

[0137] Furthermore, when the value obtained by dividing the gap size by the C amount of filler wire exceeds 25.0, the average Vickers hardness of the weld metal is less than 60% of the highest hardness of the HAZ hardened portion, and the weld metal fractures in the tensile test, resulting in a decrease in the joint strength of the weld joint.

[0138] In addition, when the carbon content of the filler wire exceeds 0.10% by mass, the average Vickers hardness of the weld metal divided by the highest hardness of the HAZ hardened part becomes higher, resulting in poor stamping formability.

[0139] Industrial availability According to the welding joint, welding blank, and manufacturing method of the welding joint disclosed herein, even if stamping is performed shortly after welding, cracking at the weld can be suppressed. Therefore, the invention disclosed herein is extremely useful in industry.

[0140] Explanation of reference numerals in the attached figures 1. Welding joint 10 and 20 high-strength steel plates 30 Welding metal

Claims

1. A butt-welding joint, characterized in that, It is a butt weld joint consisting of two high-strength steel plates and weld metal that joins the high-strength steel plates together. The high-strength steel plate includes a general section and a HAZ section adjacent to the weld metal. The Vickers hardness of the normal part is above 372 HV. The HAZ portion has a HAZ hardened portion with a higher Vickers hardness than the normal portion and a HAZ softened portion with a lower Vickers hardness than the normal portion. The average width of the weld metal is 1.00~1.65mm. In the Vickers hardness test at a load of 500 gf obtained by measuring the position at 1 / 4 of the thickness direction of the high-strength steel plate at 0.15 mm intervals from the surface of the high-strength steel plate, the average Vickers hardness of the weld metal is 60.00 to 83.00% of the highest hardness of the HAZ hardened portion.

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

3. The butt weld joint according to claim 1 or 2, characterized in that, In the microstructure of the weld metal, the area fraction of the microstructure having a BCC structure is 50% or more.

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

5. The butt weld 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 weldable blank, characterized in that, It has the butt weld joint as described in claim 1 or 2.

7. A method for manufacturing a welded joint, characterized in that, It is a method for manufacturing a butt-welded joint having two high-strength steel plates and a weld metal for joining the high-strength steel plates together, comprising: The process of arranging the high-strength steel plates by placing their weld surfaces facing each other with a gap between them; and The process of butt-welding the high-strength steel plates together using filler wire. The high-strength steel plate has a Vickers hardness of 372 HV or higher. The size of the gap is 0.4~1.0 mm. The carbon content of the filler wire exceeds 0% by mass and is less than 0.10% by mass. The value obtained by dividing the size of the gap by the value of the C amount of the filler wire is 25.0 or less. The thickness of the high-strength steel plate is 0.8~2.0mm.

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