Brazed lap joint, method for manufacturing brazed lap joint, and automotive component

Abstract

A brazed lap joint according to one embodiment of the present invention has a first steel sheet and a second steel sheet that overlap each other, a zinc-based plating layer positioned on the overlapping surface of the first steel sheet and the second steel sheet, and a brazed section that includes a brazed metal that joins the end surface of the first steel sheet and the surface of the second steel sheet, and a heat-affected section around the brazed metal. The concentration of B atoms at the prior austenite grain boundary of the second steel sheet other than the brazed section is 2.0 atm% or more, the average effective crystal grain size of the second steel sheet other than the brazed section is 7.0 [mu] m or less, and the average effective crystal grain size of the second steel sheet is 15.0 [mu] m or less at the weld root section, the hem section, and the intermediate section between the weld root section and the hem section. The Vickers hardness of the brazing metal is 250 or less, and the weld waist height on the second steel plate side of the brazed lap joint is 2.0 mm or more.

Description

Technical Field

[0001] The present invention relates to a brazed lap joint (also referred to as a brazed lap joint), a method for manufacturing a brazed lap joint, and an automotive component.

[0002] This application claims priority based on Japanese Patent Application No. 2022-206635 filed in Japan on December 23, 2022, and the contents are incorporated herein by reference. Background Art

[0003] Lap joints require sufficient joint strength and corrosion resistance. One of the means to improve the corrosion resistance of lap joints is zinc plating. Zinc plating has the effect of dramatically improving the corrosion resistance of steel materials by replacing the chemical corrosion protection effect. Therefore, zinc plating is used as a surface treatment layer for various steel materials.

[0004] For example, Patent Document 1 discloses a Zn-Al-Mg based plated steel sheet, wherein the base steel sheet has the following composition: in mass %, it contains C: 0.05-0.25%, Si: 1.5% or less, Mn: 1-2%, N: 0.005% or less, Ti: 3.43×N-0.05%, B: 0.0003-0.01%, Cr: 0.5-2%, and further contains one or more of Nb: 0.3% or less, V: 1% or less, Mo: 1% or less, and Zr: 1% or less as required, and the remainder is Fe and unavoidable impurities, Mn+1.29Cr≥2.05, the base steel sheet contains a ferrite phase+5 volume % or more of a martensite phase, and the Mn segregation of the base steel sheet satisfies the range of Mn maximum concentration (mass %) / Mn minimum concentration (mass %)≤2, and the tensile strength of the plated steel sheet is 590 MPa or more, and the yield ratio is less than 0.7.

[0005] Patent document 2 discloses a hot-dip galvanized steel sheet, wherein a hot-dip galvanized layer is provided on at least one surface of a base steel sheet, the base steel sheet having a specified composition, containing by volume fraction ferrite: 0% to 50%, retained austenite: 0% to 30%, tempered martensite: 5% or more, primary martensite: 0% to 10% and the total of pearlite and cementite: 0% to 5%, wherein, in the presence of residual structure, the residual structure comprises bainite, the concentration of B atoms at the original austenite grain boundaries is 2.0 atm% or more, and the average effective grain size is 7.0 μm or less.

[0006] Patent document 3 discloses a high-strength steel plate having the following composition: containing C, Si, Mn, P, S, Al, and N, and [%Si], [%Mn], [%P], [%Mo], and [%Cr] satisfying a prescribed relationship, and the remainder containing Fe and unavoidable impurities, and having the following steel structure: having ferrite, tempered martensite and bainite, quenched martensite, and retained austenite, the amount of diffusible hydrogen in the steel plate is less than 0.60 mass ppm, the softened thickness of the surface layer is greater than 5 μm and less than 150 μm, and the corresponding grain boundary frequency of the surface layer of the steel plate after a high-temperature tensile test is less than 0.45, and the tensile strength of the high-strength steel plate is greater than 1180 MPa.

[0007] Patent document 4 discloses a spot welding component, which is a spot welding component formed by spot welding a plurality of steel plates, at least one of the plurality of steel plates is a high-strength cold-rolled steel plate having a tensile strength of 780 MPa or more without a coating on the surface, at least one of the plurality of steel plates is a zinc-plated steel plate having a zinc-plated layer on the surface, and the surface Zn concentration in the interior of the plastic metal ring zone (corona bond) of the spot welding portion is 1 mass % or more and less than 25 mass %.

[0008] Prior art literature

[0009] Patent Literature

[0010] Patent Document 1: Japanese Patent Application Publication No. 2009-228079

[0011] Patent Document 2: International Publication No. 2020 / 162561

[0012] Patent Document 3: International Publication No. 2021 / 019947

[0013] Patent Document 4: Japanese Patent Application Publication No. 2020-179413 Summary of the invention

[0014] Problems to be solved by the invention

[0015] When welding zinc-plated steel sheets, LME (Liquid Metal Embrittlement) cracking becomes a problem. Zn, the main component of the zinc-plated layer, melts due to welding heat, diffuses and invades the grain boundaries of the steel sheet, embrittles the grain boundaries. In addition, tensile stress is introduced into the weld portion heated by welding heat due to thermal contraction of the base material and weld metal caused by the temperature drop. LME cracking is a phenomenon in which the grain boundaries embrittled by molten zinc crack due to the tensile stress (or tensile strain) introduced into the weld portion.

[0016] In addition, even in bare steel sheets, that is, steel sheets without coating, LME cracks may sometimes occur when they are welded in a superimposed manner with zinc-plated steel sheets. This is because when welding heat is applied to the bare steel sheet while the zinc-plated layer is in contact, molten zinc diffuses and penetrates into the grain boundaries of the bare steel sheet. In Patent Document 4, LME cracks generated in steel sheets without zinc-plated layers are referred to as "acquired LME cracks".

[0017] According to the Zn-Al-Mg steel plate disclosed in Patent Document 1, even under severe welding conditions, molten metal embrittlement cracking can be stably suppressed. In addition, Patent Document 1 states that molten metal embrittlement cracking can be suppressed by increasing the interatomic bonding force through the segregation of B at the grain boundaries. However, Patent Document 1 does not make any research on the grain size and welding conditions for suppressing LME cracking.

[0018] According to the hot-dip galvanized steel sheet disclosed in Patent Document 2, press formability and hydrogen embrittlement resistance after plastic working are improved. However, Patent Document 2 does not make any study on LME cracking.

[0019] The high-strength steel plate disclosed in Patent Document 3 is considered to have excellent LME resistance. In addition, Patent Document 3 discloses the following: by controlling the corresponding grain boundary frequency of the surface layer of the steel plate after the high-temperature tensile test to 0.45 or less and controlling the surface softening thickness to 5 μm or more and 150 μm or less, the LME resistance is improved. However, Patent Document 3 does not make any research on the grain boundary characteristics, the crystal grain size of the welded portion, the welding conditions, etc. for suppressing LME cracking.

[0020] The spot welded component disclosed in Patent Document 4 is considered to be able to suppress LME cracking. In addition, Patent Document 4 discloses the following idea: by controlling the surface Zn concentration in the plastic metal ring region of the spot weld, the occurrence of LME cracking can be suppressed. However, Patent Document 4 does not make any research on LME cracking of joints other than spot welded joints, such as lap fillet welded joints.

[0021] In view of the above circumstances, an object of the present invention is to provide a brazed lap joint having high joint strength and suppressed LME cracking, a method for producing the same, and an automobile part having suppressed LME cracking.

[0022] Means for solving problems

[0023] The gist of the present invention is as follows.

[0024] (1) A brazed lap joint according to one embodiment of the present invention comprises: a first steel plate and a second steel plate which are overlapped; a zinc-based plating layer located on the overlapping surfaces of the first steel plate and the second steel plate; and a brazed portion, the brazed portion including a brazing metal which joins the end surface of the first steel plate and the surface of the second steel plate, and a heat-affected zone around the brazing metal, wherein the concentration of B atoms at the prior austenite grain boundaries of the second steel plate excluding the brazed portion is 2.0 atm% or more, the average effective grain size of the second steel plate excluding the brazed portion is 7.0 μm or less, the average effective grain size of the second steel plate at a weld root (also referred to as a “weld root”), a seam edge, and an intermediate portion between the weld root and the seam edge is 15.0 μm or less, the Vickers hardness of the brazing metal is 250 or less, and the weld waist height on the second steel plate side of the brazed lap joint is 2.0 mm or more.

[0025] (2) Preferably, in the brazed lap joint described in (1) above, the second steel plate has a tensile strength of 980 MPa or more.

[0026] (3) A method for manufacturing a brazed lap joint according to another embodiment of the present invention is a method for manufacturing a lap joint having a zinc-based coating on the overlapping surfaces and wherein the end face of a first steel plate and the surface of a second steel plate are brazed, wherein the concentration of B atoms at the prior austenite grain boundaries of the second steel plate is 2.0 atm% or more, and the average effective grain size of the second steel plate is 7.0 μm or less, the manufacturing method comprising: bringing a metal platform whose interior is cooled by cooling water into close contact with the second steel plate, using a brazing wire having a main component of Cu and a diameter of 0.8 to 1.4 mm as a filler metal, and performing brazing in such a manner that a brazing heat input Q calculated by the following formula is 190 to 270 J / mm.

[0027] Q=V×I / v

[0028] Here, V is the voltage in V, I is the current in A, and v is the brazing speed in mm / s.

[0029] (4) Preferably, in the method for manufacturing a brazed lap joint described in (3) above, the tensile strength of the second steel plate is 980 MPa or more.

[0030] (5) In the method for producing a brazed lap joint described in (3) or (4) above, it is preferred that the brazing be MIG brazing.

[0031] (6) An automobile component according to another aspect of the present invention includes the brazed lap joint described in (1) or (2).

[0032] Effects of the Invention

[0033] According to the present invention, it is possible to provide a brazed lap joint having high joint strength and suppressed LME cracking, a method for producing the same, and an automobile part having suppressed LME cracking. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a schematic diagram of a cross section perpendicular to the end surface of the first steel plate and perpendicular to the surface of the second steel plate (ie, a cross section perpendicular to the brazing direction) of a brazed lap joint according to one embodiment of the present invention.

[0035] Figure 2 It is an enlarged schematic diagram of a cross section of the weld root portion which is perpendicular to the end surface of the first steel plate and perpendicular to the surface of the second steel plate.

[0036] Figure 3 It is an enlarged schematic diagram of a cross section of the seam allowance that is perpendicular to the end surface of the first steel plate and perpendicular to the surface of the second steel plate.

[0037] Figure 4 It is an enlarged schematic diagram of a cross section of a middle portion between a weld root portion and a seam edge portion, which is perpendicular to the end surface of the first steel plate and perpendicular to the surface of the second steel plate.

[0038] Figure 5 This is a cross-sectional photograph illustrating the weld waist height.

[0039] Figure 6 This is a photograph of B atoms concentrated at the prior austenite grain boundaries in the heat-affected zone obtained by SIMS analysis. DETAILED DESCRIPTION

[0040] (1. Brazed lap joint 1)

[0041] The brazed lap joint 1 according to the first embodiment of the present invention is as follows Figure 1 As shown in the example, the first steel sheet 11 and the second steel sheet 12 are overlapped, the zinc-based coating 14 is located on the overlapping surface of the first steel sheet 11 and the second steel sheet 12, and the brazing portion 13 includes a brazing metal 131 that joins the end surface 111 of the first steel sheet 11 and the surface 121 of the second steel sheet 12, and a heat-affected zone 132 around the brazing metal 131, and the B at the prior austenite grain boundary of the second steel sheet 12 other than the brazing portion 13. The atomic concentration is greater than 2.0atm%, the average effective grain size of the second steel plate 12 excluding the brazing portion 13 is less than 7.0μm, the average effective grain size of the second steel plate 12 at the weld root R, the seam edge T, and the middle portion M between the weld root R and the seam edge T is less than 15.0μm, the Vickers hardness of the brazing metal 131 is less than 250, and the weld waist height L on the second steel plate 12 side of the brazed lap joint 1 is greater than 2.0mm.

[0042] (First Steel Plate 11, Second Steel Plate 12, and Brazing Portion 13)

[0043] In the brazed lap joint 1 of the first embodiment, as Figure 1 As shown in the example, the first steel plate 11 and the second steel plate 12 overlap. Furthermore, the end surface 111 of the first steel plate and the surface 121 of the second steel plate are joined by the brazing metal 131. For example, a lap joint brazed by fillet welds and a lap joint brazed by plungers have such a structure.

[0044] The so-called lap joint is a joint in which the parts are placed in parallel at an angle of 0°≤α≤5° and overlap each other as specified in JIS Z 3000-1:2018 "Welding Terminology - Part 1: General". The so-called brazing is a joining method that uses a brazing material with a melting point of 450°C or above to make the base material as non-melting as possible as specified in JIS Z 3000-3:2018 "Welding Terminology - Part 3: Brazing", also known as brazing. The so-called fillet brazing is a fillet weld performed by brazing. The so-called fillet weld is a weld with a triangular cross-section between components without providing a groove in a lap joint, T-joint, fillet joint, etc. as specified in JIS Z 3000-1:2018 "Welding Terminology - Part 1: General". The so-called plug brazing is a plug weld performed by brazing. Plug welding is welding performed on an opening of one of the superimposed base materials as specified in JIS Z 3000-1:2018 “Welding Terms - Part 1: General”.

[0045] In general lap fillet welding, in most cases, the steel plate with the weld metal provided on the end face is arranged on the upper side, and the steel plate with the weld metal provided on the surface is arranged on the lower side. Therefore, the steel plate with the weld metal provided on the end face is sometimes referred to as the "upper plate", and the steel plate with the weld metal provided on the surface is sometimes referred to as the "lower plate". In the brazed lap joint 1 of the first embodiment, the first steel plate 11 with the brazing metal 131 provided on the end face 111 corresponds to the upper plate, and the second steel plate 12 with the brazing metal 131 provided on the surface 121 corresponds to the lower plate.

[0046] The brazing metal 131 is formed by solidifying a filler metal such as a filler wire for a brazing material after melting. The brazing metal 131 is formed by brazing such as MIG brazing and MAG brazing. In the brazed lap joint 1 of the first embodiment, the base materials are the first steel plate 11 and the second steel plate 12. Figure 1 As shown in FIG. 1 and FIG. 2 , the first steel sheet 11 and the second steel sheet 12 are preferably not melted by brazing as much as possible. Figure 2As shown in , the first steel plate 11 and the second steel plate 12 may be slightly melted, and the brazing metal 131 may be melted into the first steel plate 11 and the second steel plate 12 .

[0047] A heat-affected portion 132 is formed around the brazing metal 131. In other words, in the first steel plate 11 and the second steel plate 12, the area around the brazing metal 131 is set as the heat-affected portion 132. The so-called heat-affected portion 132 is a portion of the base material that has not been melted and whose structure, metallurgical properties, mechanical properties, etc. are changed by the heat during brazing. The heat-affected portion 132 can be easily identified by etching the cross section of the brazing portion 13. The so-called brazing portion 13 is a general term for a portion including the brazing metal 131 and the heat-affected portion 132.

[0048] (Zinc coating 14)

[0049] A zinc coating 14 is provided on the surface of one or both of the first steel plate 11 and the second steel plate 12. The zinc coating 14 is a coating with zinc as the main component. For example, a coating with a zinc content of 50% by mass or more is equivalent to a coating with zinc as the main component. Examples of the zinc coating 14 include pure zinc coating, hot-dip galvanizing, electroplating zinc, alloyed hot-dip galvanizing, etc. The zinc coating 14 has the function of improving the corrosion resistance of the brazed lap joint 1 by replacing the anti-corrosion effect.

[0050] The zinc coating 14 is disposed at least on the overlapping surface of the first steel sheet 11 and the second steel sheet 12. That is, at least one of the first steel sheet 11 and the second steel sheet 12 is a base steel sheet portion of the zinc-coated steel sheet, and the first steel sheet 11 and the second steel sheet 12 are overlapped in such a manner that the zinc coating 14 is located on the overlapping surface. Figure 1 In the brazed lap joint 1 illustrated in the figure, the zinc-based coating 14 is provided on one side of the first steel plate 11, but the zinc-based coating 14 may be provided on both sides of the first steel plate 11. In addition, the zinc-based coating 14 may be provided on one side or both sides of the second steel plate 12, and the first steel plate 11 and the second steel plate 12 may be overlapped in such a manner that the zinc-based coating 14 is located on the overlapping surface.

[0051] The zinc-based plating layer 14 disposed on the overlapped surface of the lap joint may cause LME cracking in the steel sheet. LME cracking is particularly likely to occur in the second steel sheet 12, which is a steel sheet whose surface is joined to another steel sheet.

[0052] (Concentration of B atoms at the prior austenite grain boundary of the second steel sheet 12 except the brazed portion 13)

[0053] As described above, in the second steel plate 12, the area around the brazing metal 131 is set as the heat-affected portion 132. Therefore, a part of the second steel plate 12 is included in the brazing portion 13, and its structure, metallurgical properties, mechanical properties, etc. are different from those before brazing. On the other hand, the structure, metallurgical properties, mechanical properties, etc. in the area outside the brazing portion 13 in the second steel plate 12 (that is, the second steel plate 12 excluding the heat-affected portion 132) are substantially the same as before brazing. In this embodiment, the area outside the brazing portion 13 is sometimes referred to as the base material portion.

[0054] In the base material portion of the second steel plate 12, the concentration of B atoms at the original austenite grain boundaries is set to 2.0 atm% or more. That is, the concentration of B atoms at the original austenite grain boundaries of the second steel plate 12 except the brazing portion 13 is 2.0 atm% or more. The concentration of B atoms at the original austenite grain boundaries of the second steel plate 12 except the brazing portion 13 is further preferably 2.2 atm% or more, 2.5 atm% or more, or 3.0 atm% or more. In the second steel plate 12 except the brazing portion 13, the upper limit of the concentration of B atoms at the original austenite grain boundaries is not particularly limited. From the viewpoint of preventing excessive hardening of the second steel plate 12, it is further preferred to set the B atom concentration to 7.0 atm% or less, 6.0 atm% or less, or 5.0 atm% or less. The concentration of B atoms at the original austenite grain boundaries of the second steel plate 12 except the brazing portion 13 can be determined by the STEM-EELS method described later.

[0055] It should be noted that the so-called original austenite grain boundary is a trace of the grain boundary of austenite as a high-temperature structure. The second steel plate 12 is temporarily heated to the austenite temperature region during its manufacturing process, and the metal structure is mainly made into austenite and then cooled to room temperature. Through cooling, part or all of the austenite is transformed into a structure such as martensite and disappears, but the grain boundary of the austenite grain remains in the second steel plate 12 after the structural phase transformation. The remaining grain boundary is called the original austenite grain boundary. Molten zinc easily diffuses and invades the original austenite grain boundary. Therefore, the original austenite grain boundary is an area where LME cracking is easily generated.

[0056] (Average effective grain size of the second steel sheet 12 excluding the brazed portion 13)

[0057] Furthermore, in the base material portion of the second steel plate 12, the average effective grain size is set to be less than 7.0 μm. That is, the average effective grain size of the second steel plate 12 excluding the brazing portion 13 is less than 7.0 μm. As a result, the hydrogen embrittlement resistance of the second steel plate 12 is improved, and the delayed fracture resistance of the brazed lap joint 1 is improved. The so-called average effective grain size is the average grain size of the grain when the area surrounded by the grain boundary with an orientation difference of more than 15 degrees is defined as the grain. The average effective grain size of the second steel plate 12 excluding the brazing portion 13 can be determined using EBSD described later.

[0058] The average effective grain size of the second steel plate 12 excluding the brazing portion 13 is further preferably 6.5 μm or less, 6.0 μm or less, 5.5 μm or less, or 5.0 μm or less. The lower limit of the average effective grain size of the second steel plate 12 excluding the brazing portion 13 is not particularly limited. From the viewpoint of improving the manufacturing efficiency of the second steel plate 12, it is preferred that the average effective grain size be set to 2.0 μm or more, 2.5 μm or more, or 3.0 μm or more.

[0059] It should be noted that the type of crystal grains is not particularly limited. As described below, it is not the type of crystal grains that affects LME cracking resistance, but the amount of crystal boundaries (that is, the total length of the crystal boundaries). Crystal grains are, for example, ferrite, martensite, and retained austenite.

[0060] (Average effective grain size of the second steel plate 12 at the weld root portion R, the weld edge portion T, and the intermediate portion M between the weld root portion and the weld edge portion)

[0061] In the brazed lap joint 1 of the first embodiment, the average effective grain size of the second steel plate 12 at the weld root R, the seam allowance T, and the middle portion M between the weld root and the seam allowance is also specified. The average effective grain size of the second steel plate 12 at the weld root R, the seam allowance T, and the middle portion M can also be determined using EBSD described later.

[0062] The weld root R is the area near the weld root r of the brazed lap joint 1. The weld root r is the intersection of the end face 111 of the first steel plate and the surface 121 of the second steel plate joined to each other by the brazing metal 131. In the case where there is a gap between the first steel plate 11 and the second steel plate 12 near the lower end of the end face 111 of the first steel plate, the intersection of the straight line perpendicular to the surface 121 of the second steel plate and in contact with the end face 111 of the first steel plate and the surface 121 of the second steel plate is regarded as the weld root r. For example, Figure 2 In the enlarged cross-sectional view of the weld root R shown in FIG, the intersection of the dotted line Z and the surface 121 of the second steel plate is the weld root r, and the periphery thereof is the weld root R. The so-called seam allowance T is the vicinity of the seam allowance t of the brazed lap joint 1. The so-called seam allowance t is as follows: Figure 3As shown in FIG. 1 , the surface 121 of the second steel plate intersects the surface of the brazing metal 131. The middle portion M between the weld root and the seam edge (hereinafter referred to as the "middle portion M") is as shown in FIG. Figure 4 As shown in FIG. 1 , the vicinity of the midpoint m of the weld root and the seam edge of the brazed lap joint 1 is shown.

[0063] Any of the weld root R, the seam allowance T, and the middle portion M is a region where stress is easily concentrated in the process of cooling the brazed lap joint 1 after the completion of brazing. Therefore, the weld root R, the seam allowance T, and the middle portion M are portions where LME cracking is easily generated. In the brazed lap joint 1 of the first embodiment, the average effective grain size of the second steel plate 12 at the weld root R, the seam allowance T, and the middle portion M is set to 15.0 μm or less. It is further preferred that the average effective grain size of the second steel plate 12 at the weld root R, the seam allowance T, and the middle portion M is set to 13.0 μm or less, 11.0 μm or less, 10.0 μm or less, or 9.0 μm or less. The lower limit of the average effective grain size of the second steel plate 12 at the weld root R, the seam edge T and the middle portion M is not particularly limited, but from the viewpoint of improving the manufacturing efficiency of the brazed lap joint 1, the average effective grain size is preferably set to 6.0 μm or more, 7.0 μm or more, or 8.0 μm or more.

[0064] (Vickers hardness of brazing metal 131)

[0065] In the brazed lap joint 1 of the first embodiment, the Vickers hardness of the brazing metal 131 is set to 250 or less. The Vickers hardness of the brazing metal 131 may be set to 240 or less, 220 or less, or 200 or less. The lower limit of the Vickers hardness of the brazing metal 131 is not particularly limited, but from the viewpoint of improving the bonding strength of the brazed lap joint 1, the Vickers hardness of the brazing metal 131 may be set to 100 or more, 120 or more, or 140 or more. The Vickers hardness of the brazing metal 131 can be determined by a Vickers hardness test described later.

[0066] (Weld seam height L on one side of the second steel plate 12)

[0067] The weld waist height L of the brazed lap joint 1 on the second steel plate 12 side is 2.0 mm or more. The weld waist height L on the second steel plate 12 side may also be 2.5 mm or more, 3.0 mm or more, or 3.5 mm or more.

[0068] Here, the so-called "weld waist height" is defined as the distance from the weld root of the joint to the seam edge of the fillet weld in JIS Z 3001-1:2018. Figure 5In this way, the so-called weld waist height L on the second steel plate 12 side is the distance between the weld root r and the seam edge t measured in the cross section of the brazed portion along the direction perpendicular to the end surface 111 of the first steel plate. It should be noted that the cross section for measuring the weld waist height is perpendicular to the end surface 111 of the first steel plate 11 and perpendicular to the surface 121 of the second steel plate 12.

[0069] (Effect)

[0070] LME cracking of the steel plate is caused by applying tensile stress to the steel plate in the state where molten zinc exists on the surface of the steel plate. During the cooling process of the brazed lap joint 1 after brazing, tensile stress is easily applied to the weld root R, the seam edge T, and the middle portion M of the second steel plate 12. In addition, in the brazed lap joint 1 of the first embodiment, since the zinc-based plating layer 14 is provided on the superimposed surface of the second steel plate 12, molten zinc exists on the surface 121 of the second steel plate during the cooling process of the brazed lap joint 1 after brazing. Therefore, the brazed lap joint 1 of the first embodiment contains a factor that causes LME cracking.

[0071] However, in the brazed lap joint 1 of the first embodiment, the Vickers hardness of the brazing metal 131 is suppressed to less than 250. Therefore, when the brazing metal 131 is cooled after brazing, the brazing metal 131 reduces the stress introduced into the weld root portion R, the seam edge portion T, and the middle portion M of the second steel plate 12.

[0072] In addition, in the brazed lap joint 1 of the first embodiment, the average effective grain size of the second steel plate 12 excluding the brazed portion is set to 7.0 μm or less. That is, the average effective grain size of the second steel plate 12 before brazing is set to 7.0 μm or less. Thus, the average effective grain size at the weld root R, the seam edge T and the middle portion M of the second steel plate 12 can be set to 15.0 μm or less. By receiving the brazing heat input, the grains of the heat-affected portion of the second steel plate 12 grow. The average effective grain size of the heat-affected portion after brazing becomes a value corresponding to the brazing heat input and the average effective grain size before brazing. By reducing the average effective grain size before brazing, the coarsening of the grain size in the heat-affected portion (especially the weld root R, the seam edge T and the middle portion M) after brazing can be suppressed.

[0073] By reducing the average effective grain size of the weld root R, the weld edge T, and the middle portion M, the amount of grain boundaries at the weld root R, the weld edge T, and the middle portion M can be increased, and the LME sensitivity of the weld root R, the weld edge T, and the middle portion M can be reduced. By increasing the amount of grain boundaries, the concentration of zinc intruding into the grain boundaries is diluted, and the embrittlement of the grain boundaries can be suppressed.

[0074] In addition, in the brazed lap joint 1 of the first embodiment, the concentration of B atoms at the prior austenite grain boundaries of the second steel plate 12 excluding the brazed portion is set to 2.0 atm% or more. The B atoms segregated at the prior austenite grain boundaries increase the peel strength of the prior austenite grain boundaries. Figure 6 As shown in the example, B atoms have the property of concentrating at the prior austenite grain boundaries in the heat-affected zone 132. Therefore, the concentration of B atoms at the prior austenite grain boundaries of the seam edge portion T, the weld root portion R, and the middle portion M of the second steel plate 12 is further increased compared to the base material portion. Moreover, at the seam edge portion T, the weld root portion R, and the middle portion M of the second steel plate 12, the LME sensitivity is further reduced.

[0075] In addition, in the brazed lap joint 1 of the first embodiment, the weld waist height L on the second steel plate 12 side is 2.0 mm or more. As a result, the tensile shear strength of the brazed lap joint 1 can be improved. For example, for the brazed lap joint 1 of the first embodiment, when a shear tensile test is performed on a test piece with a parallel portion width set to 45 mm, the shear tensile strength TSS (Tensile Shear Strength) can be set to 11.25 kN or more (i.e., 0.25 kN / mm or more).

[0076] For the above reasons, in the brazed lap joint 1 of the first embodiment, although there is a factor causing LME cracking, LME cracking is strongly suppressed. Although the zinc-based plating layer becomes a factor causing LME cracking, on the other hand, it has various effects such as dramatically improving the corrosion resistance of the brazed lap joint 1. Furthermore, the brazed lap joint 1 of the first embodiment has high joint strength. According to the brazed lap joint 1 of the first embodiment, the reliability of the joint having the zinc-based plating layer can be improved, and the scope of application can be expanded.

[0077] The most basic aspect of the brazed lap joint 1 according to the first embodiment has been described above. Next, a more preferred aspect of the brazed lap joint 1 according to the first embodiment will be described.

[0078] (Upper limit of the weld waist height L on the second steel plate 12 side)

[0079] By reducing the heat input during brazing and reducing the weld waist height L on the second steel plate 12 side, the coarsening of the grain size of the heat-affected portion can be further suppressed. Therefore, the weld waist height L on the second steel plate 12 side may also be 6.0 mm or less, 5.8 mm or less, or 5.0 mm or less.

[0080] (Weld seam height on one side of the first steel plate 11)

[0081] The weld waist height on the first steel plate 11 side is preferably 1.0 mm or more, and may be 1.3 mm or more, 1.5 mm or more, or 2.0 mm or more. Here, the weld waist height on the first steel plate 11 side is the distance between the weld root r and the seam edge at the end face 111 of the first steel plate 11 when the end face 111 of the first steel plate 11 has a seam edge. When the brazing metal 131 completely covers the end face 111 of the first steel plate 11 and there is no seam edge on the end face 111 of the first steel plate 11, for convenience, the weld waist height on the first steel plate 11 side is set to the distance between the surface of the first steel plate 11 (the surface on the side away from the surface 121 of the second steel plate 12) and the surface 121 of the second steel plate 12. That is, the maximum value of the weld waist height on the first steel plate 11 side is the distance between the surface of the first steel plate 11 (the surface on the side away from the surface 121 of the second steel plate 12) and the surface 121 of the second steel plate 12. This interval is equal to the thickness of the first steel plate 11 when there is no gap between the first steel plate 11 and the second steel plate 12 near the lower end of the end surface 111 of the first steel plate. In order to improve the bonding strength (joint strength), it is preferred that the brazing metal 131 completely covers the end surface 111 of the first steel plate 11, that is, the weld waist height on the first steel plate 11 side is the maximum.

[0082] (Tensile strength of steel plate)

[0083] One or both of the first steel plate 11 and the second steel plate 12 are preferably high-strength steel plates with a tensile strength of 980 MPa or more, 1000 MPa or more, 1100 MPa or more, or 1300 MPa or more. Thus, the strength of the brazed lap joint 1 is improved. It should be noted that the higher the tensile strength of the steel plate, the higher the LME sensitivity of the steel plate. However, in the brazed lap joint 1 of the first embodiment, the hardness of the brazing metal 131, the average effective grain size of the seam edge T, the weld root R and the middle part M of the second steel plate 12, and the B atom concentration of the original austenite grain boundary of the second steel plate 12 are set within a suitable range, thereby suppressing LME cracking. Therefore, even if the tensile strength of the first steel plate 11 and / or the second steel plate 12 is increased, LME cracking will not be a problem. It should be noted that the second steel plate 12 as the lower plate has a tendency to easily produce LME cracking compared with the first steel plate 11. When the second steel plate 12 is a high-strength steel plate, the brazed lap joint 1 of the first embodiment has further advantages over the conventional brazed lap joint 1 or welded joint.

[0084] It should be noted that it is not necessary to limit the plate thickness of the first steel plate 11 and the second steel plate 12, but it can be set to 0.8 to 3.2 mm, for example. As needed, the lower limit of the first steel plate 11 can be set to 1.0 mm, 1.2 mm, or 1.6 mm, and the upper limit can be set to 3.2 mm, 3.0 mm, 2.8 mm, 2.6 mm, 2.3 mm, or 2.0 mm. Similarly, the lower limit of the second steel plate 12 can be set to 1.0 mm, 1.2 mm, or 1.6 mm, and the upper limit can be set to 3.2 mm, 3.0 mm, 2.8 mm, 2.6 mm, 2.3 mm, or 2.0 mm.

[0085] (2. Method for manufacturing brazed lap joint 1)

[0086] A method for manufacturing a brazed lap joint 1 according to a second embodiment of the present invention is a method for manufacturing a lap joint 1 having a zinc-based coating 14 on the overlapping surfaces, and wherein an end face 111 of a first steel plate 11 and a surface 121 of a second steel plate 12 are brazed, wherein the concentration of B atoms at the prior austenite grain boundaries of the second steel plate 12 is greater than 2.0 atm%, and the average effective grain size of the second steel plate 12 is less than 7.0 μm, and the manufacturing method comprises: bringing a metal platform whose interior is cooled by cooling water into close contact with the second steel plate 12, using a brazing wire having Cu as a main component and a diameter of 0.8 to 1.4 mm as a filler metal, and brazing in a manner such that a brazing heat input Q calculated by the following formula is 190 to 270 J / mm.

[0087] Q=V×I / v

[0088] Here, V is the voltage in V, I is the current in A, and v is the brazing speed in mm / s.

[0089] (First steel sheet 11, second steel sheet 12, zinc-based coating 14)

[0090] During brazing, a zinc-based coating 14 is disposed at least on the overlapping surfaces of the first steel plate 11 and the second steel plate 12. Therefore, one or both of the first steel plate 11 and the second steel plate 12 have a zinc-based coating 14 on the surface. In addition, in the second steel plate 12, the concentration of B atoms at the original austenite grain boundary is greater than 2.0 atm%, and the average effective crystal grain size is less than 7.0 μm. The structure and positional relationship of the first steel plate 11, the second steel plate 12, and the zinc-based coating 14 are based on the brazed lap joint 1 of the first embodiment. The preferred scheme exemplified in the description of the brazed lap joint 1 of the first embodiment may also be adopted for the first steel plate 11, the second steel plate 12, and the zinc-based coating 14 in the manufacturing method of the brazed lap joint 1 of the second embodiment.

[0091] (Brazing)

[0092] The method for manufacturing the brazed lap joint 1 of the second embodiment includes the step of brazing the end surface 111 of the first steel plate and the surface 121 of the second steel plate that overlap each other. Thus, brazing metal 131 is formed by joining the end surface 111 of the first steel plate and the surface 121 of the second steel plate.

[0093] Brazing is a joining method that uses a brazing material (filler metal) having a melting point of 450°C or more to make the base material as non-melting as possible as described above. Preferably, brazing is performed while shielding the wire electrode and the brazing portion 13 with an inert gas. A preferred example of brazing is MIG brazing. MIG brazing is a welding method in which the protection mechanism of metal extremely inert gas shielded welding specified in JIS Z 3001-7:2018 "Welding Terms-Part 7: Arc Welding" is transferred to brazing. Specifically, MIG brazing is brazing that is performed while using an inert gas (shielding gas) such as Ar to protect the brazing portion. According to MIG brazing, a brazed lap joint with good mechanical properties can be obtained. On the other hand, from the perspective of welding costs, brazing can also be set to MAG brazing. MAG brazing is a welding method that applies the protection mechanism of metal active gas shielded arc welding specified in JIS Z 3001-7:2018 "Welding Terminology - Part 7: Arc Welding" to brazing. 2 Gas and CO 2 The brazing may be performed while the brazing portion is protected by an active gas obtained by using a gas. In addition, the brazing may be, for example, lap fillet brazing or plunger brazing.

[0094] (Brazing conditions)

[0095] In brazing, the brazing heat input Q is set to 190 to 270 J / mm. The brazing heat input Q is a value obtained by the following formula, and the unit is J / mm.

[0096] Q=V×I / v

[0097] The symbols included in the above formula have the following meanings.

[0098] V: Brazing voltage (unit: V)

[0099] I: Brazing current (unit: A)

[0100] v: Brazing speed (unit: mm / s)

[0101] It should be noted that the so-called brazing speed is the speed of moving the welding gun used for brazing. The brazing heat input Q can also be set to 200 J / mm or more, 210 J / mm or more, or 230 J / mm or more. The brazing heat input Q can also be set to 260 J / mm or less, 250 J / mm or less, or 240 J / mm or less.

[0102] If the brazing heat input Q exceeds the above range, the second steel plate 12, which is the lower steel plate in normal brazing, melts. In the case where the back of the lower steel plate is a galvanized steel plate, if the penetration of the lower steel plate increases, LME cracks may occur not only at the weld root, seam edge and its middle part, but also at the back side of the lower steel plate. On the other hand, if the brazing heat input Q is less than 190 kJ / mm, the weld waist height is small and the tensile shear strength is reduced.

[0103] In addition, brazing is performed in a state where the second steel plate 12 is in close contact with the metal platform. For example, by setting the surface shape of the platform to be flat, removing foreign matter such as dust from the surface of the platform, and placing a flat second steel plate on the platform, the second steel plate 12 can be in close contact with the metal platform.

[0104] A flow path through which cooling water can flow is provided inside the platform. During brazing, cooling water is circulated in the flow path to cool the inside of the platform. Using the platform, during brazing, the temperature rise of the second steel plate 12 is suppressed, and the coarsening of the crystal grain size at the heat-affected portion is prevented. For example, a copper plate after internal water cooling can also be set as the platform.

[0105] (Filler Metal Composition)

[0106] In addition, in brazing, a brazing wire having Cu as the main component and a diameter of 0.8 to 1.4 mm is used. The welding wire having Cu as the main component is, for example, a welding wire having a Cu content of 50% by mass or more. The Cu content of the brazing wire may also be 70% by mass or more, 80% by mass or more, or 90% by mass or more. As a specific example of a welding wire having Cu as the main component, a Cu-7% Al welding wire can be cited. According to such a welding wire, the Vickers hardness of the brazing metal 131 can be set to 250 or less.

[0107] (Effect)

[0108] In the manufacturing method of the brazed lap joint 1 of the present embodiment, the concentration of B atoms at the original austenite grain boundary in the second steel plate 12 provided for brazing is set to 2.0 atm% or more. In addition, the heat input during brazing promotes the concentration of B at the original austenite grain boundary at the heat-affected zone 132 of the brazed portion 13. As described in relation to the welded joint of the first embodiment, the B atoms segregated at the original austenite grain boundary increase the peeling strength of the original austenite grain boundary. As a result, the LME sensitivity is reduced at the seam edge portion T, the weld root portion R, and the middle portion M of the second steel plate 12.

[0109] In the method for manufacturing the brazed lap joint 1 of the present embodiment, the brazing heat input Q is set to 190 J / mm or more. This ensures the weld waist height L on the second steel plate 12 side and improves the bonding strength of the brazed lap joint 1.

[0110] Furthermore, in the manufacturing method of the brazed lap joint 1 of the present embodiment, the average effective grain size of the second steel plate 12 before brazing is set to 7.0 μm or less, and the brazing heat input Q is suppressed to 270 J / mm or less. In addition, in the manufacturing method of the brazed lap joint 1, the second steel plate is made to fit closely with the metal platform after the internal cooling. The average effective grain size of the heat-affected zone after brazing becomes a value corresponding to the brazing heat input and the average effective grain size before brazing. By reducing the average effective grain size before brazing and reducing the brazing heat input, the hypertrophy of the grains in the heat-affected zone 132 can be suppressed. By reducing the average effective grain size of the steel plate, the amount of grain boundaries of the steel plate can be increased, and the LME sensitivity of the steel plate can be further reduced. By increasing the amount of grain boundaries, the concentration of zinc intruding into the grain boundaries is diluted, and the embrittlement of the grain boundaries can be suppressed.

[0111] The most basic aspect of the method for producing the brazed lap joint 1 according to the second embodiment has been described above. Next, a more preferred aspect of the method for producing the brazed lap joint 1 according to the second embodiment will be described.

[0112] (Filler Metal Composition)

[0113] The chemical composition of the brazing wire with Cu as the main component contains, for example, Al: 0-30%, Si: 0-20%, Mn: 0-10% in unit mass %, and the remainder includes Cu and impurities. The so-called impurities are, for example, components mixed in due to raw materials such as ore or scrap or various factors in the manufacturing process when the brazing wire is manufactured industrially, and are components that are allowed within the range that does not adversely affect the brazing metal 131. As a specific method for selecting the filler metal, for example, a preliminary test is performed to select a wire with a Cu content of 50% or more that can set the Vickers hardness of the brazing metal 131 to 250 or less.

[0114] It should be noted that in MIG brazing, the first steel plate 11 and the second steel plate 12 as the base materials may be slightly melted and melted into the brazing metal 131. The components derived from these steel plates become impurities of the brazing metal 131. Therefore, the chemical composition of the brazing wire may not be consistent with the chemical composition of the brazing metal 131. However, according to the above-mentioned brazing heat input, the melting amount of the first steel plate 11 and the second steel plate 12 is small, and these components do not excessively increase the hardness of the brazing metal 131.

[0115] (Tensile strength of steel plate)

[0116] One or both of the first steel plate 11 and the second steel plate 12 are preferably high-strength steel plates having a tensile strength of 980 MPa or more, 1000 MPa or more, 1100 MPa or more, or 1300 MPa or more. Thus, the strength of the brazed lap joint 1 is improved. When the second steel plate 12 is set to a high-strength steel plate, the method for manufacturing the brazed lap joint 1 of the second embodiment has further advantages over the conventional method for manufacturing a joint.

[0117] (3.Auto parts)

[0118] The automobile component of the third embodiment of the present invention has the brazed lap joint 1 of the first embodiment. Therefore, the automobile component of the third embodiment has high corrosion resistance due to the zinc-based plating layer 14, while LME cracking is suppressed. The automobile component is, for example, a center pillar.

[0119] (Other Embodiments)

[0120] The above describes the embodiments of the present invention, but the present invention is not limited thereto and can be appropriately changed without departing from the technical concept of the invention. The following describes the modified examples of the present invention. It should be noted that, unless otherwise specified, the scheme described below can be applied to all of the first embodiment, the second embodiment, and the third embodiment.

[0121] (Feeding speed of filler metal)

[0122] The feed speed of the filler metal is not particularly limited, but from the viewpoint of further improving the stability of brazing, it is preferred to set the feed speed of the filler metal to within the range of 3 to 6 m / min. By setting the feed speed of the filler metal to less than 6 m / min relative to the brazing heat input, the melting of the filler metal can be promoted, and brazing defects can be further suppressed. On the other hand, by setting the feed speed of the filler metal to more than 3 m / min relative to the brazing heat input, the amount of brazing metal 131 can be increased, and poor joining can be further suppressed. It should be noted that in conventional brazing, the voltage and current are determined in a manner to achieve the set feed speed.

[0123] (Chemical composition of brazing metal 131)

[0124] As long as the hardness of the brazing metal 131 can be set within the above-mentioned range, the chemical composition of the brazing metal 131 is not particularly limited. The brazing metal 131 preferably contains Cu as a main component, for example. An example of the chemical composition of the brazing metal 131 containing Cu as a main component contains Al: 0-30%, Si: 0-20% and Mn: 0-10% in unit mass %, and the remainder contains Cu and impurities. The so-called impurities are substances mixed into the brazing material during the manufacturing process of the filler metal as the material of the brazing metal 131. In addition, the first steel plate 11 and the second steel plate 12 are sometimes melted and mixed into the brazing metal 131. The components derived from the steel plates are also regarded as impurities in the chemical composition of the brazing metal 131. For example, by using Cu-7%Al welding wire as a filler metal for MIG brazing, a brazing metal 131 having the above-mentioned chemical composition can be obtained.

[0125] (Chemical Composition and Metal Structure of the First Steel Plate 11 and the Second Steel Plate 12)

[0126] In the brazed lap joint 1 of the present embodiment, the average effective grain size of the lower plate, i.e., the second steel plate 12, which is prone to LME cracking, and the B concentration of the prior austenite grain boundary are specified. As long as these elements are set within the above range, the chemical composition and metal structure of the second steel plate 12 are not particularly limited. The chemical composition and metal structure of the first steel plate 11 are also not particularly limited. For example, the base steel plate of the hot-dip galvanized steel plate disclosed in Patent Document 2 (International Publication No. 2020 / 162561) can be suitably used as the first steel plate 11 and the second steel plate 12 of the brazed lap joint 1 of the present embodiment.

[0127] (Measurement method)

[0128] The concentration of B atoms at the original austenite grain boundaries of the second steel plate 12 at the outside of the brazing portion 13 is determined by the STEM-EELS method. Specifically, it is determined, for example, by the method disclosed in METALLURGICAL AND MATERIALS TR ANSACTIONS A: vol.45A, p.1877-1888. First, a sample is collected with the cross section of the second steel plate 12 at the outside of the brazing portion 13 as the observation surface, and the observation surface is mechanically polished and finished into a mirror surface, and then electrolytically polished. The cross section is perpendicular to the end face of the first steel plate and perpendicular to the surface of the second steel plate. Next, in one or more observation fields in the range of 1 / 8 thickness to 3 / 8 thickness with 1 / 4 thickness from the surface as the center on the observation surface, a total of 2.0×10 -9 m 2 The above area is subjected to crystal structure and orientation analysis by SEM-EBSD method to identify the original austenite grain boundary. Then, the area containing the original austenite grain boundary is extracted by FIB processing in SEM. Afterwards, Ar ion milling is used to thin the film to about 70nm. For the test piece after thin filming, the line analysis of the concentration of B atoms is performed by electron energy loss spectroscopy (EELS) along the line that crosses the original austenite grain boundary by aberration correction STEM. The scanning step during line analysis is set to 0.1nm. Since B atoms are segregated at the original austenite grain boundary, the maximum value of the concentration of B atoms obtained is regarded as the concentration of B atoms at the original austenite grain boundary. The number of measurements is set to 3 times, and the average value of the measurement results is used as the concentration of B atoms.

[0129] The average effective grain size of the second steel plate 12 outside the brazing portion 13 is evaluated by the SEM-EBSD (Electron Back Scatter Diffraction) method. First, a sample is collected with the cross section of the second steel plate 12 outside the brazing portion 13 as the observation surface, and the observation surface is mechanically polished and finished into a mirror surface, and then electrolytic polishing is performed. The cross section is perpendicular to the end face of the first steel plate and perpendicular to the surface of the second steel plate. Next, in one or more observation fields in the observation surface with a range of 1 / 8 thickness to 3 / 8 thickness with a distance of 1 / 4 thickness as the center, the crystal structure and orientation analysis are performed by the SEM-EBSD method. The analysis of the data obtained by the EBSD method uses "OIM Analysys 6.0" manufactured by TSL. In addition, the score interval (step) is set to 0.03 to 0.20 μm. The area judged to be FCC iron by the observation results is set as retained austenite. Furthermore, a grain boundary map is obtained by taking the boundary where the crystal orientation difference is more than 15 degrees as the grain boundary. And the value calculated by the following formula was made into the average effective crystal grain size.

[0130] [Mathematical formula 1]

[0131]

[0132] In the formula, N represents the number of grains contained in the evaluation area of ​​the average effective crystal grain size, Ai represents the area of ​​the i-th (i=1, 2, ··, N) grain, and di represents the equivalent circle diameter of the i-th grain. These data can be easily obtained by EBSD analysis.

[0133] The method for measuring the average effective grain size of the second steel plate 12 at the weld root R, the seam edge T, and the middle portion M is the same as the method for measuring the average effective grain size of the second steel plate 12 outside the brazed portion 13, except for the measurement location. The measurement is performed in the cross section of the brazed portion 13. The cross section is perpendicular to the end surface of the first steel plate and perpendicular to the surface of the second steel plate. The measurement location A1 of the average effective grain size of the second steel plate at the weld root is as follows: Figure 2 As shown in FIG, the weld root r is a rectangular area within a range of ±250 μm in the extension direction of the surface 121 of the second steel plate and within a range of 200 μm in the depth direction of the second steel plate 12 with the weld root r as the starting point. The measurement position A2 of the average effective crystal grain size of the second steel plate at the seam edge is as follows: Figure 3As shown in FIG, a rectangular area is formed within a range of ±250 μm in the extension direction of the surface 121 of the second steel plate with the seam edge t as the starting point and within a range of 200 μm in the depth direction of the second steel plate 12 with the seam edge t as the starting point. The measurement position A3 of the average effective grain size of the second steel plate at the middle portion M of the weld root portion and the seam edge portion is as follows Figure 4 As shown in , a rectangular area is defined within a range of ±250 μm in the extension direction of the surface 121 of the second steel plate with the midpoint m of the weld edge t and the weld root r as the starting point, and within a range of 200 μm in the depth direction of the second steel plate 12 with the midpoint m as the starting point. The average effective grain sizes of these measurement locations A1, A2, and A3 (i.e., the average effective grain sizes of the three locations of the weld root, the weld edge, and the intermediate portion between the weld root and the weld edge) must all be 15.0 μm or less.

[0134] The hardness of the brazing metal 131 is determined by Vickers hardness measurement. The measurement is performed in the cross section of the brazing portion. The cross section is perpendicular to the end face of the first steel plate and perpendicular to the surface of the second steel plate. The Vickers hardness test is in accordance with JIS Z 2244-1:2020. The test force is set to 9.807N (i.e., HV1). The number of measurements is set to 5 times. The interval between the 5 measurement points is set to 0.5mm. The 5 measurement points are arranged along a straight line that is parallel to the surface 121 of the second steel plate and passes through the center of the plate thickness of the first steel plate 11. For example Figure 1 The dotted line Y described in is a straight line parallel to the surface 121 of the second steel plate and passing through the center of the thickness of the first steel plate 11. In addition, the measurement point located in the middle of the five measurement points is arranged at the midpoint of the intersection of the above-mentioned straight line and the surface of the brazing metal 131 and the intersection of the above-mentioned straight line and the end surface 111 of the first steel plate (for example Figure 1 The average value of the Vickers hardness at the five measurement points was regarded as the hardness of the brazing metal 131.

[0135] The weld waist height L on the second steel plate side is determined by cross-sectional observation. The measurement is performed in the cross section of the brazed portion. The cross section is perpendicular to the end face of the first steel plate and perpendicular to the surface of the second steel plate. As long as the measurement is made by known means Figure 5 The distance between the weld root r and the seam edge t shown in the figure is sufficient. The distance between the weld root r and the seam edge t is regarded as the weld waist height L on the second steel plate side.

[0136] The chemical composition of the brazing metal 131 was evaluated using EPMA. The measurement point was set to be a straight line ( Figure 1 The dotted line Y) is on the straight line and is the midpoint of the intersection of the straight line and the surface of the brazing metal 131 and the intersection of the straight line and the end surface 111 of the first steel plate ( Figure 1As a method of chemical analysis, after preparing a test piece, other methods such as emission spectroscopy may be used.

[0137] Example

[0138] The effect of one scheme of the present invention is described more specifically by way of an embodiment. However, the conditions in the embodiment are only a conditional example adopted to confirm the feasibility and effect of the present invention. The present invention is not limited to this conditional example. As long as the purpose of the present invention is achieved without departing from the gist of the present invention, the present invention may adopt various conditions.

[0139] (Experiment 1)

[0140] Plate groups 1 to 15 described in Table 1 were MIG brazed using the filler metals described in Table 2 to produce brazed lap joints 1 to 15. The voltage value, current value, brazing speed, and brazing heat input during MIG brazing were set as described in Table 3. The average effective grain size of the second steel plate at the weld root, seam edge, and middle of the brazed lap joints 1 to 15, and the hardness of the brazed metal were measured and recorded in Table 4. In addition, the LME cracking frequency and hydrogen embrittlement frequency in the brazed lap joints 1 to 15 were investigated, and the results are recorded in Table 4.

[0141] It should be noted that in any of the steel plates described in Table 1, the average effective grain size is 7.0 μm or less, and the concentration of B atoms at the prior austenite grain boundary is 2.0 atm% or more. The remainder of the chemical composition of the filler metal described in Table 2 is impurities. In addition, in all MIG brazings, the feed speed of the filler metal is set within the range of 3.5 to 7.0 m / min. Values ​​outside the appropriate range are underlined.

[0142] The method for measuring the average effective grain size of the second steel plate at the weld root, the seam edge and the middle portion, the hardness of the brazing metal and the weld waist height on one side of the second steel plate is set as described above. It should be noted that the maximum measurement result (i.e., the maximum value of the average effective grain size) among the measurement results of the average effective grain size of the second steel plate at the weld root, the seam edge and the middle portion is recorded in Table 4.

[0143] The frequency of LME cracking in the brazed lap joints was evaluated by the following method. Ten brazed lap joints were made for each condition by the above-mentioned means, and the cross sections of the brazed parts were observed to confirm the presence or absence of LME cracking. The number of brazed lap joints in which LME cracking occurred was the frequency of LME cracking. When LME cracking did not occur in all the ten test pieces, it was determined that LME cracking was sufficiently suppressed.

[0144] The shear tensile strength of the brazed lap joint is evaluated by the following method. From the brazed lap joint 1, a shear tensile test piece is collected in which the extension direction of the end face 111 of the first steel plate 11 is the test piece width (parallel portion width) direction and the test piece width (parallel portion width) is 45 mm. A tensile load (shear tensile load) is applied in the direction in which the first steel plate 11 and the second steel plate 12 are separated from each other, and the shear tensile strength of the brazed lap joint is calculated from the load at the moment the test piece breaks. A brazed lap joint with a shear tensile strength of 11.25 kN or more (i.e., 0.25 kN / mm or more) is judged to have excellent joint strength and is recorded as "GOOD" in the table.

[0145] Table 2

[0146] No Welding materials Fe Cu Al 1 Cu-7%AIφ1mm 0.4 93 6.3 2 Cu-7%Alφ1mm 0.4 93 6.3 3 Cu-7%AIφ1mm 0.5 93 6.2 4 Cu-7%Alφ1mm 0.4 93 6.3 5 YM24T welding wire for mild steel φ1mm 99 <![CDATA[ 0 ]]> 0.06 6 YM24T welding wire for mild steel φ1mm 99 <![CDATA[ 0 ]]> 0.06 7 Cu-7%Alφ1mm 0.4 93 6.3 8 Cu-7%Alφ1mm 0.4 93 6.3 9 Cu-7%Alφ1mm 0.4 93 6.3 10 Cu-7%Alφ1mm 0.4 93 6.3 11 Cu-7%Alφ1mm 0.5 93 6.2 12 Cu-7%Alφ1mm 0.4 93 6.3 13 Cu-7%Alφ1mm 0.4 93 6.3 14 Cu-7%Alφ1mm 0.4 93 6.3 15 Cu-7%Alφ1mm 0.4 93 6.3

[0147] Table 3

[0148]

[0149] Table 4

[0150]

[0151] In the brazed lap joints of test numbers 3, 4, 6, 8, and 11, the average effective grain size of the second steel plate at the weld root, seam edge, or middle portion was too large. As a result, LME cracking occurred in these brazed lap joints. For test numbers 3, 4, 6, 8, and 11, it is estimated that the grains were coarsened due to excessive brazing heat input Q.

[0152] In addition, the hardness of the brazed metal was too high in the brazed lap joints of test numbers 5 and 6. As a result, LME cracking occurred in these brazed lap joints. In test numbers 5 and 6, it is estimated that the hardness of the brazed metal could not be suppressed because the composition of the filler metal was mainly Fe.

[0153] The weld waist height was insufficient in the brazed lap joint of test number 10. As a result, the joint strength was insufficient in the brazed lap joint of test number 10. In test number 10, it is estimated that the sufficient weld waist height could not be ensured because the heat input Q was insufficient.

[0154] In the brazed lap joint of Test No. 12, the average effective grain size of the second steel plate at the weld root, seam edge, or middle portion was too large. As a result, LME cracking occurred in the brazed lap joint of Test No. 12. In Test No. 12, it is estimated that the second steel plate was not cooled during brazing, so the grains were coarsened.

[0155] On the other hand, in the brazed lap joint where the brazing heat input Q and the cooling conditions of the second steel plate were appropriate, the average effective grain size of the second steel plate at the weld root, seam edge and middle portion, and the hardness of the brazing metal were within appropriate ranges, no LME cracking occurred at all.

[0156] (Supplementary Items)

[0157] (A) A brazed joint according to another embodiment of the present invention comprises: a first steel plate and a second steel plate overlapped with each other, and a brazed portion, the brazed portion including a brazing metal joining an end surface of the first steel plate and a surface of the second steel plate, and a heat-affected zone around the brazing metal, wherein one or both of the first steel plate and the second steel plate have a zinc-based plating layer on a surface, the zinc-based plating layer is arranged on a joining surface of the first steel plate and the second steel plate, outside the brazed portion, a concentration of B atoms at a prior austenite grain boundary of the second steel plate is 2.0 atm% or more, outside the brazed portion, an average effective grain size of the second steel plate is 7.0 μm or less, at a weld root portion, a seam edge portion, and a middle portion between the weld root portion and the seam edge portion, the average effective grain size of the second steel plate is 15.0 μm or less, and the hardness of the brazing metal is HV250 or less.

[0158] (B) Preferably, in the brazed joint described in (A), the second steel sheet has a tensile strength of 980 MPa or more.

[0159] (C) A method for manufacturing a brazed joint according to another embodiment of the present invention comprises a step of brazing an end face of a first steel plate and a surface of a second steel plate that are overlapped to form a brazing metal, wherein one or both of the first steel plate and the second steel plate have a zinc-based coating on the surface, the zinc-based coating is arranged on the joint surface of the first steel plate and the second steel plate, in the second steel plate, the concentration of B atoms at the prior austenite grain boundary is 2.0 atm% or more, and the average effective crystal grain size is 7.0 μm or less, and in the brazing, a brazing heat input Q calculated by substituting a voltage value V in V, a current value I in A, and a brazing speed v in m / min into the following formula is set within a range of 1500 to 3000 J / 10 mm, and in the brazing, the hardness of the brazing metal is set to HV250 or less.

[0160] Q=0.6×V×I / v

[0161] (D) In ​​the method for manufacturing a brazed joint described in (C), preferably, the hardness of the brazing metal is set to HV250 or less by setting the filler metal to a brazing wire having a main component of Cu and a diameter of 0.8 to 1.4 mm.

[0162] (E) Preferably, in the method for manufacturing a brazed joint described in (C) or (D), the second steel sheet has a tensile strength of 980 MPa or more.

[0163] (F) In the method for manufacturing a soldered joint according to any one of (C), (D) and (E) above, it is preferred that the brazing be MIG brazing.

[0164] (G) An automobile component according to another embodiment of the present invention includes the brazing joint described in (A) or (B).

[0165] Description of Reference Numerals

[0166] 1 Brazed lap joints

[0167] 11 First Steel Plate

[0168] 111 End surface of the first steel plate

[0169] 12 Second Steel Plate

[0170] 121 Surface of the second steel plate

[0171] 13 Brazing Department

[0172] 131 Brazing Metal

[0173] 132 Heat affected area

[0174] 14 Zinc coating

[0175] R Weld root

[0176] r Weld root

[0177] T seam edge

[0178] T seam edge

[0179] M The middle part between the weld root and the weld edge

[0180] m The middle of the weld root and the seam edge

[0181] L Weld waist height on the second steel plate side

[0182] A1 Measurement location of the average effective grain size of the second steel plate at the weld root

[0183] A2 Measurement location of the average effective grain size of the second steel plate at the seam edge

[0184] A3 Measurement location of the average effective grain size of the second steel plate at the middle of the weld root and the seam edge

Claims

1. A brazed lap joint having: Overlapping a first steel plate and a second steel plate; a zinc coating located on the overlapping surface of the first steel plate and the second steel plate; and a brazed portion including a brazing metal joining the end surface of the first steel plate and the surface of the second steel plate, and a heat-affected zone around the brazing metal, The concentration of B atoms at the prior austenite grain boundaries of the second steel sheet excluding the brazed portion is 2.0 atm% or more, The average effective grain size of the second steel sheet excluding the brazed portion is 7.0 μm or less, The average effective grain size of the second steel plate at the weld root portion, the seam edge portion, and the intermediate portion between the weld root portion and the seam edge portion is 15.0 μm or less, The Vickers hardness of the brazing metal is 250 or less, The weld waist height of the brazed lap joint on the second steel plate side is greater than 2.0 mm.

2. The brazed lap joint according to claim 1, characterized in that The tensile strength of the second steel plate is greater than 980 MPa.

3. A method for manufacturing a brazed lap joint, characterized in that: The invention relates to a method for manufacturing a lap joint in which the overlapped surfaces have a zinc coating and the end surface of the first steel plate and the surface of the second steel plate are brazed. The concentration of B atoms at the prior austenite grain boundaries of the second steel sheet is 2.0 atm% or more, and the average effective grain size of the second steel sheet is 7.0 μm or less, The manufacturing method comprises: A metal platform whose interior is cooled by cooling water is brought into close contact with the second steel plate. A brazing wire with a diameter of 0.8 to 1.4 mm and containing Cu as the main component is used as the filler metal. Brazing is performed so that the brazing heat input Q calculated by the following formula is 190 to 270 J / mm. Q=V×I / v Here, V is the voltage in V, I is the current in A, and v is the brazing speed in mm / s.

4. The method for manufacturing a brazed lap joint according to claim 3, characterized in that: The tensile strength of the second steel plate is greater than 980 MPa.

5. The method for manufacturing a brazed lap joint according to claim 3 or 4, characterized in that: The brazing was set as MIG brazing.

6. An automobile component comprising the brazed lap joint according to claim 1 or 2.

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