Brazed lap joint, method for manufacturing a brazed lap joint, and automotive parts

Abstract

The brazed lap joint according to an embodiment of the present invention has: an overlapped first steel sheet and second steel sheet; a zinc-system plating layer residing at the overlapping surface between the first steel sheet and the second steel sheet; and a braze region comprising a brazing metal that joins the end face of the first steel sheet and the surface of the second steel sheet and a heat affected zone at the perimeter of the brazing metal. The concentration of B atoms at the prior austenite grain boundary in the second steel sheet, excluding the braze region, is at least 2.0 atm%, and the average effective grain size of the second steel sheet, excluding the braze region, is 7.0 µm or less. The average effective grain size of the second steel sheet is 15.0 µm or less in a root region, terminal region, and middle region between the root region and terminal region, the Vickers hardness of the brazing metal is 250 or less, and the leg length on the side of the second steel sheet of the brazed lap joint is at least 2.0 mm.

Description

Technical Field

[0001] The present invention relates to a brazed overlapping joint, a method for manufacturing a brazed overlapping joint, and automotive parts. This application claims priority based on Japanese Patent Application No. 2022-206635 filed in Japan on December 23, 2022, and incorporates its content herein.

Background Art

[0002] Sufficient joining strength and corrosion resistance are required for overlapping joints. One means for improving the corrosion resistance of overlapping joints is a zinc-based plating layer. The zinc-based plating layer has the function of dramatically enhancing the corrosion resistance of steel materials due to the sacrificial anticorrosion effect. Therefore, the zinc-based plating layer is used as a surface treatment layer for various steel materials.

[0003] For example, Patent Document 1 discloses a Zn-Al-Mg-based plated steel sheet having a composition in which the base steel sheet contains, in mass%, C: 0.05 to 0.25%, Si: 1.5% or less, Mn: 1 to 2%, N: 0.005% or less, Ti: 3.43×N to 0.05%, B: 0.0003 to 0.01%, Cr: 0.5 to 2%, and optionally one or more of Nb: 0.3% or less, V: 1% or less, Mo: 1% or less, Zr: 1% or less, with the balance being Fe and unavoidable impurities, and Mn + 1.29Cr ≧ 2.05, the base steel sheet being composed of a ferrite phase + 5% by volume or more of a martensite phase, and the Mn segregation of the base steel sheet being in a range satisfying Mn maximum concentration (mass%) / Mn minimum concentration (mass%) ≦ 2, and having a tensile strength of 590 MPa or more and a yield ratio of less than 0.7.

[0004] Patent Document 2 discloses a hot-dip galvanized steel sheet having a hot-dip galvanized layer on at least one surface of a base steel sheet, wherein the base steel sheet has a predetermined composition and contains, by volume fraction, ferrite: 0% to 50%, retained austenite: 0% to 30%, tempered martensite: 5% or more, fresh martensite: 0% to 10%, and the sum of pearlite and cementite: 0% to 5%, and if a remaining structure exists, the remaining structure consists of bainite, the concentration of B atoms at the prior austenite grain boundaries is 2.0 atm% or more, and the average effective grain size is 7.0 μm or less.

[0005] Patent Document 3 discloses a high-strength steel sheet having a composition in which C, Si, Mn, P, S, Al, and N are contained, and [%Si], [%Mn], [%P], [%Mo], and [%Cr] satisfy predetermined relationships, with the remainder being Fe and unavoidable impurities; having ferrite, tempered martensite and bainite, quenched martensite and retained austenite; having a diffusible hydrogen content in the steel sheet of 0.60 ppm by mass or less; a surface softening thickness of 5 μm to 150 μm; and a corresponding grain boundary frequency of the steel sheet surface after a high-temperature tensile test of 0.45 or less; and having a tensile strength of 1180 MPa or more.

[0006] Patent Document 4 discloses a spot-welded member in which a plurality of steel plates are spot-welded, wherein at least one of the plurality of steel plates is a high-strength cold-rolled steel plate with a tensile strength of 780 MPa or more and has no plating layer on its surface, and at least one of the plurality of steel plates is a zinc-plated steel plate having a zinc-plated layer on its surface, and the surface Zn concentration inside the corona bond of the spot-welded portion is 1% by mass or more and less than 25% by mass. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2009-228079 [Patent Document 2] International Publication No. 2020 / 162561 [Patent Document 3] International Publication No. 2021 / 019947 [Patent Document 4] Japanese Patent Publication No. 2020-179413 [Overview of the project] [Problems that the invention aims to solve]

[0008] When welding zinc-plated steel sheets, LME (Liquid Metal Embrittlement) cracking is a problem. Zinc, the main component of the zinc-plated layer, melts due to the heat of welding and diffuses into the grain boundaries of the steel sheet, causing them to become brittle. In addition, tensile stress is introduced into the weld area, which has been heated by the welding heat, due to the thermal contraction of the base metal and weld metal as the temperature decreases. LME cracking is a phenomenon in which grain boundaries, which have been brittle by molten zinc, crack due to the tensile stress (or tensile strain) introduced into the weld area.

[0009] Furthermore, even in bare steel sheets, i.e., steel sheets without a plating layer, LME cracking can occur when they are overlapped and welded with zinc-plated steel sheets. This is because when welding heat is applied while the zinc-plated layer is in contact with the bare steel sheet, the molten zinc diffuses into the grain boundaries of the bare steel sheet. In Patent Document 4, LME cracking that occurs in steel sheets without a zinc-plated layer is referred to as "transferred LME cracking."

[0010] According to the Zn-Al-Mg steel sheet disclosed in Patent Document 1, molten metal embrittlement cracking can be reliably suppressed even under severe welding conditions. Furthermore, Patent Document 1 states that molten metal embrittlement cracking is suppressed by B segregating at the grain boundaries and increasing interatomic bonding forces. However, Patent Document 1 does not consider grain size, welding conditions, or other factors necessary to suppress LME cracking.

[0011] According to the hot-dip galvanized steel sheet disclosed in Patent Document 2, press formability and hydrogen embrittlement resistance after plastic deformation are improved. However, Patent Document 2 does not address LME cracking at all.

[0012] The high-strength steel sheet disclosed in Patent Document 3 is said to have excellent LME resistance. Furthermore, Patent Document 3 discloses that LME resistance can be improved by controlling the corresponding grain boundary frequency of the steel sheet surface after a high-temperature tensile test to 0.45 or less, and the surface softening thickness to 5 μm or more and 150 μm or less. However, Patent Document 3 does not consider any grain boundary characteristics, crystal grain size of the welded area, or welding conditions for suppressing LME cracking.

[0013] The spot-welded member disclosed in Patent Document 4 is said to be able to suppress induced LME cracking. Furthermore, Patent Document 4 discloses that the occurrence of induced LME cracking can be suppressed by controlling the surface Zn concentration inside the corona bond of the spot weld. However, Patent Document 4 does not consider LME cracking of joints other than spot-welded joints, such as lap fillet joints.

[0014] In view of the above circumstances, the object of the present invention is to provide a brazed lap joint with high joint strength and suppressed LME cracking, a method for manufacturing the same, and an automobile part with suppressed LME cracking. [Means for solving the problem]

[0015] The gist of this invention is as follows:

[0016] (1) A brazed lap joint according to one aspect of the present invention comprises a brazed portion consisting of a first steel plate and a second steel plate stacked on top of each other, a zinc-based plating layer on the overlapping surfaces of the first steel plate and the second steel plate, a brazing metal that joins the end face 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 in the root portion, the toe portion, and the intermediate portion between the root portion and the toe portion is 15.0 μm or less, the Vickers hardness of the brazing metal is 250 or less, and the leg length on the second steel plate side of the brazed lap joint is 2.0 mm or more. (2) Preferably, in the brazed lap joint described in (1) above, the tensile strength of the second steel plate is 980 MPa or more.

[0017] (3) Another embodiment of the present invention relates to a method for manufacturing a brazed lap joint, wherein the overlapping surfaces have a zinc-based plating layer, and the end face of a first steel plate and the surface of a second steel plate are brazed together, wherein the concentration of B atoms in 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, and the second steel plate is brought into close contact with a metal surface plate whose interior is cooled by cooling water, and brazing is performed using a brazing wire whose main component is Cu and whose diameter is 0.8 to 1.4 mm as a filler material, 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 units of V, I is the current in units of A, and v is the brazing rate in units of mm / s. (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. (5) Preferably, in the method for manufacturing a brazed lap joint described in (3) or (4) above, the brazing is MIG brazing.

[0018] (6) An automotive part according to another aspect of the present invention includes the brazed overlapping joint described in the above (1) or (2).

Advantages of the Invention

[0019] According to the present invention, it is possible to provide a brazed overlapping joint with high joint strength and suppressed LME cracking, a method for manufacturing the same, and an automotive part with suppressed LME cracking.

Brief Description of the Drawings

[0020] [Figure 1] It is a schematic view of a cross section (i.e., a cross section perpendicular to the brazing direction) of the brazed overlapping joint according to one aspect of the present invention, which is perpendicular to the end face of the first steel plate and perpendicular to the surface of the second steel plate. [Figure 2] It is an enlarged schematic view of a cross section of the root part, which is perpendicular to the end face of the first steel plate and perpendicular to the surface of the second steel plate. [Figure 3] It is an enlarged schematic view of a cross section of the stop part, which is perpendicular to the end face of the first steel plate and perpendicular to the surface of the second steel plate. [Figure 4] It is an enlarged schematic view of a cross section of the middle part between the root part and the stop part, which is perpendicular to the end face of the first steel plate and perpendicular to the surface of the second steel plate. [Figure 5] It is a cross-sectional photograph for explaining the leg length. [Figure 6] It is a photograph of B atoms concentrated at the prior austenite grain boundaries in the heat-affected zone obtained by SIMS analysis.

Embodiments for Carrying Out the Invention

[0021] (1. Brazed Overlapping Joint 1) As illustrated in Figure 1, the brazed lap joint 1 according to the first embodiment of the present invention comprises a first steel plate 11 and a second steel plate 12 that are stacked, a zinc-based plating layer 14 on the overlapping surface of the first steel plate 11 and the second steel plate 12, a brazing metal 131 that joins the end face 111 of the first steel plate 11 and the surface 121 of the second steel plate 12, and a brazing portion 132 that surrounds the brazing metal 131, and the prior austerity of the second steel plate 12 excluding the brazing portion 13. The concentration of B atoms at the tenite grain boundaries is 2.0 atm% or more, the average effective grain size of the second steel sheet 12 excluding the brazed portion 13 is 7.0 μm or less, the average effective grain size of the second steel sheet 12 in the root portion R, the toe portion T, and the intermediate portion M between the root portion R and the toe portion T is 15.0 μm or less, the Vickers hardness of the brazing metal 131 is 250 or less, and the leg length L on the second steel sheet 12 side of the brazed lap joint 1 is 2.0 mm or more.

[0022] (First steel plate 11, second steel plate 12, and brazed portion 13) In the brazed lap joint 1 according to the first embodiment, a first steel plate 11 and a second steel plate 12 are overlapped, as illustrated in Figure 1. The end face 111 of the first steel plate and the surface 121 of the second steel plate are joined by brazing metal 131. For example, fillet brazed lap joints and plug brazed lap joints have such a configuration.

[0023] A lap joint is a joint in which parts are placed parallel to each other at an angle of 0°≦α≦5° and overlap each other, as defined in JIS Z 3000-1:2018 "Welding Terminology - Part 1: General". Brazing is a joining method in which the base material is joined using a solder with a melting point of 450°C or higher, while minimizing the melting of the base material, and is also called brazing. Fillet brazing is a fillet weld performed by brazing. Fillet welding is a weld in which a triangular cross-section is formed between members without creating a groove, as defined in JIS Z 3000-1:2018 "Welding Terminology - Part 1: General". Plug welding, as defined in JIS Z 3000-1:2018 "Welding Terminology - Part 1: General", is a welding method performed on a hole drilled in one of two overlapping base materials.

[0024] In typical lap fillet welding, the steel plate with weld metal on its end face is often placed on top, and the steel plate with weld metal on its surface is often placed on the bottom. Therefore, the steel plate with weld metal on its end face is sometimes called the "top plate," and the steel plate with weld metal on its surface is sometimes called the "bottom plate." In the brazed lap joint 1 according to the first embodiment, the first steel plate 11 with brazing metal 131 on its end face 111 is the top plate, and the second steel plate 12 with brazing metal 131 on its surface 121 is the bottom plate.

[0025] The brazing metal 131 is the portion formed when a filler wire or other welding material melts and then solidifies. The brazing metal 131 is formed by brazing, for example, MIG brazing and MAG brazing. In the brazed lap joint 1 according to the first embodiment, the base materials are the first steel plate 11 and the second steel plate 12. As shown in Figure 1, it is preferable that the first steel plate 11 and the second steel plate 12 are not melted by brazing as much as possible. However, as shown in Figure 2, the first steel plate 11 and the second steel plate 12 may be slightly melted, and the brazing metal 131 may be fused into the first steel plate 11 and the second steel plate 12.

[0026] A heat-affected zone 132 is formed around the brazing metal 131. In other words, in the first steel plate 11 and the second steel plate 12, the region around the brazing metal 131 is the heat-affected zone 132. The heat-affected zone 132 is the unmelted base material where the structure, metallurgical properties, and mechanical properties have changed due to the heat during brazing. The heat-affected zone 132 can be easily identified by etching the cross-section of the brazed portion 13. The brazed portion 13 is a general term for the portion including the brazing metal 131 and the heat-affected zone 132.

[0027] (Zinc-based plating layer 14) A zinc-based plating layer 14 is provided on one or both surfaces of the first steel plate 11 and the second steel plate 12. The zinc-based plating layer 14 is a plating layer mainly composed of zinc. For example, a plating layer with a zinc content of 50% by mass or more is considered a zinc-based plating. Examples of zinc-based plating layers 14 include pure zinc plating, hot-dip galvanizing, electro-galvanizing, alloyed hot-dip galvanizing, etc. The zinc-based plating layer 14 enhances the corrosion resistance of the brazed lap joint 1 through a sacrificial corrosion protection effect.

[0028] The zinc-based plating layer 14 is provided on at least 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 the base steel sheet portion of the zinc-based plated steel sheet, and the first steel sheet 11 and the second steel sheet 12 are stacked such that the zinc-based plating layer 14 is located on the overlapping surface. In the brazed lap joint 1 illustrated in Figure 1, the zinc-based plating layer 14 is provided on one side of the first steel sheet 11, but the zinc-based plating layer 14 may be provided on both sides of the first steel sheet 11. Alternatively, the zinc-based plating layer 14 may be provided on one or both sides of the second steel sheet 12, and the first steel sheet 11 and the second steel sheet 12 may be stacked such that this zinc-based plating layer 14 is located on the overlapping surface.

[0029] The zinc-based plating layer 14 applied to the overlapping surface of the lap joint can cause LME cracking in the steel sheet. LME cracking is particularly likely to occur in the steel sheet whose surface is joined to another steel sheet, i.e., the second steel sheet 12.

[0030] (Concentration of B atoms at the prior austenite grain boundaries of the second steel sheet 12, excluding the brazed portion 13) As described above, in the second steel sheet 12, the region surrounding the brazing metal 131 is designated as the heat-affected zone 132. Therefore, a portion of the second steel sheet 12 is included in the brazed portion 13, and its structure, metallurgical properties, and mechanical properties are different from those before brazing. On the other hand, the structure, metallurgical properties, and mechanical properties of the region of the second steel sheet 12 outside the brazed portion 13 (i.e., the second steel sheet 12 excluding the heat-affected zone 132) are substantially the same as those before brazing. In this embodiment, the region outside the brazed portion 13 may be referred to as the base material.

[0031] In the base material portion of the second steel sheet 12, the concentration of B atoms at the prior austenite grain boundaries is set to 2.0 atm% or higher. That is, the concentration of B atoms at the prior austenite grain boundaries of the second steel sheet 12 excluding the brazed portion 13 is 2.0 atm% or higher. It is even more preferable that the concentration of B atoms at the prior austenite grain boundaries of the second steel sheet 12 excluding the brazed portion 13 be 2.2 atm% or higher, 2.5 atm% or higher, or 3.0 atm% or higher. In the second steel sheet 12 excluding the brazed portion 13, there is no particular upper limit to the concentration of B atoms at the prior austenite grain boundaries. From the viewpoint of preventing excessive hardening of the second steel sheet 12, it is even more preferable, for example, to set the B atom concentration to 7.0 atm% or lower, 6.0 atm% or lower, or 5.0 atm% or lower. The concentration of B atoms at the prior austenite grain boundaries of the second steel sheet 12 excluding the brazed portion 13 can be determined by the STEM-EELS method described later.

[0032] Prior austenite grain boundaries are the remnants of grain boundaries in austenite, a high-temperature structure. The second steel sheet 12 is heated to the austenite temperature range during its manufacturing process, and after its metallic structure is mainly austenite, it is cooled to room temperature. During cooling, some or all of the austenite transforms into structures such as martensite and disappears, but the grain boundaries of the austenite grains remain in the second steel sheet 12 after the structural transformation. These remaining grain boundaries are called prior austenite grain boundaries. Molten zinc easily diffuses into prior austenite grain boundaries. Therefore, prior austenite grain boundaries are regions where LME cracking is likely to occur.

[0033] (Average effective grain size of the second steel plate 12 excluding the brazed portion 13) Furthermore, in the base material portion of the second steel sheet 12, the average effective grain size is set to 7.0 μm or less. That is, the average effective grain size of the second steel sheet 12 excluding the brazed portion 13 is 7.0 μm or less. This improves the hydrogen embrittlement resistance of the second steel sheet 12 and improves the delayed fracture resistance of the brazed lap joint 1. The average effective grain size is the average grain size of a crystal grain, defined as a region surrounded by a grain boundary with an orientation difference of 15 degrees or more. The average effective grain size of the second steel sheet 12 excluding the brazed portion 13 can be determined using EBSD, which will be described later.

[0034] It is even more preferable that the average effective grain size of the second steel sheet 12 excluding the brazed portion 13 is 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 sheet 12 excluding the brazed portion 13 is not particularly limited. From the viewpoint of improving the manufacturing efficiency of the second steel sheet 12, it is preferable that the average effective grain size be 2.0 μm or more, 2.5 μm or more, or 3.0 μm or more.

[0035] The type of crystal grain is not particularly limited. As will be discussed later, what affects LME crack resistance is not the type of crystal grain, but the amount of grain boundaries (i.e., the total length of the grain boundaries). Examples of crystal grains include ferrite, martensite, and retained austenite.

[0036] (Average effective grain size of the second steel sheet 12 in the root portion R, the toe portion T, and the intermediate portion M between the root portion and the toe portion) In the brazed lap joint 1 according to the first embodiment, the average effective grain size of the second steel plate 12 in the root portion R, the toe portion T, and the intermediate portion M between the root portion and the toe portion is also defined. The average effective grain size of the second steel plate 12 in the root portion R, the toe portion T, and the intermediate portion M can also be determined using EBSD, which will be described later.

[0037] The root section R refers to the region near the root r of the brazed lap joint 1. The root r is the intersection of the end face 111 of the first steel plate and the surface 121 of the second steel plate, which are joined together by the brazing metal 131. If 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 a straight line perpendicular to the surface 121 of the second steel plate and tangent to the end face 111 of the first steel plate and the surface 121 of the second steel plate is considered to be the root r. For example, in the enlarged cross-sectional view of the root section R shown in Figure 2, the intersection of the dashed line Z and the surface 121 of the second steel plate is the root r, and the area around it is the root section R. The toe section T refers to the region near the toe section t of the brazed lap joint 1. The toe section t is the point where the surface 121 of the second steel plate and the surface of the brazing metal 131 intersect, as shown in Figure 3. The intermediate portion M between the root and toe (hereinafter simply referred to as "intermediate portion M") is the area near the midpoint m of the root and toe of the brazed lap joint 1, as shown in Figure 4.

[0038] The root portion R, the toe portion T, and the intermediate portion M are all areas where stress tends to concentrate during the cooling process of the brazed lap joint 1 after the brazing is completed. Therefore, the root portion R, the toe portion T, and the intermediate portion M are areas where LME cracking is likely to occur. In the brazed lap joint 1 according to the first embodiment, the average effective grain size of the second steel plate 12 in the root portion R, the toe portion T, and the intermediate portion M is set to 15.0 μm or less. It is even more preferable that the average effective grain size of the second steel plate 12 in the root portion R, the toe portion T, and the intermediate portion M be 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 in the root portion R, the toe portion T, and the intermediate portion M is not particularly limited, but from the viewpoint of improving the manufacturing efficiency of the brazed lap joint 1, it is preferable that the average effective grain size be 6.0 μm or more, 7.0 μm or more, or 8.0 μm or more.

[0039] (Vickers hardness of brazing metal 131) In the brazed lap joint 1 according to 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 increasing the joint 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.

[0040] (Length L of the side of the second steel plate 12) The leg length L on the side of the second steel plate 12 of the brazed lap joint 1 is 2.0 mm or more. The leg length L on the side of the second steel plate 12 may be 2.5 mm or more, 3.0 mm or more, or 3.5 mm or more.

[0041] Here, "leg length" is defined in JIS Z 3001-1:2018 as the distance from the root of the joint to the toe of the fillet weld. Specifically, as shown in Figure 5, the leg length L on the side of the second steel plate 12 is the distance between the root r and the toe t, measured in the cross-section of the brazed joint along a direction perpendicular to the end face 111 of the first steel plate. The cross-section for measuring the leg length is perpendicular to the end face 111 of the first steel plate 11 and perpendicular to the surface 121 of the second steel plate 12.

[0042] (Effects and Benefits) LME cracking in steel plates occurs when tensile stress is applied to the steel plate while molten zinc is present 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 root portion R, toe portion T, and intermediate portion M of the second steel plate 12. Furthermore, in the brazed lap joint 1 according to the first embodiment, a zinc-based plating layer 14 is provided on the overlapping surface of the second steel plate 12, so during the cooling process of the brazed lap joint 1 after brazing, molten zinc is present on the surface 121 of the second steel plate. Therefore, the brazed lap joint 1 according to the first embodiment contains factors that cause LME cracking.

[0043] However, in the brazed lap joint 1 according to the first embodiment, the Vickers hardness of the brazing metal 131 is suppressed to 250 or less. Therefore, when the brazing metal 131 is cooled after brazing, the stress introduced by the brazing metal 131 into the root portion R, toe portion T, and intermediate portion M of the second steel plate 12 is reduced.

[0044] Furthermore, in the brazed lap joint 1 according to 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. This makes it possible to set the average effective grain size of the root portion R, toe portion T, and intermediate portion M of the second steel plate 12 to 15.0 μm or less. The grains in the heat-affected zone of the second steel plate 12 grow due to the heat input during brazing. The average effective grain size of the heat-affected zone after brazing is a value corresponding to the heat input during brazing and the average effective grain size before brazing. By reducing the average effective grain size before brazing, it is possible to suppress the coarsening of the grain size in the heat-affected zone after brazing (especially the root portion R, toe portion T, and intermediate portion M).

[0045] By reducing the average effective grain size of the root portion R, toe portion T, and intermediate portion M, the amount of grain boundaries in the root portion R, toe portion T, and intermediate portion M can be increased, thereby reducing the LME susceptibility of the root portion R, toe portion T, and intermediate portion M. By increasing the amount of grain boundaries, the concentration of zinc penetrating the grain boundaries is diluted, and grain boundary embrittlement is suppressed.

[0046] In addition, in the brazed lap joint 1 according to the first embodiment, the concentration of B atoms at the prior austenite grain boundaries of the second steel sheet 12 excluding the brazed portion is set to 2.0 atm% or higher. B atoms segregated at the prior austenite grain boundaries increase the delamination strength of the prior austenite grain boundaries. Furthermore, as illustrated in Figure 6, B atoms have the property of becoming concentrated at the prior austenite grain boundaries in the heat-affected zone 132. Therefore, the concentration of B atoms at the prior austenite grain boundaries in the toe portion T, root portion R, and intermediate portion M of the second steel sheet 12 is further increased than in the base material portion. As a result, LME susceptibility is further reduced in the toe portion T, root portion R, and intermediate portion M of the second steel sheet 12.

[0047] Furthermore, in the brazed lap joint 1 according to the first embodiment, the leg length L on the side of the second steel plate 12 is 2.0 mm or more. This makes it possible to increase the tensile shear strength of the brazed lap joint 1. For example, when a shear tensile test is performed on a test piece with a parallel section width of 45 mm using the brazed lap joint 1 according to the first embodiment, the shear tensile strength (TSS) can be set to 11.25 kN or more (i.e., 0.25 kN / mm or more).

[0048] For the reasons stated above, in the brazed lap joint 1 according to the first embodiment, LME cracking is strongly suppressed despite the presence of factors that cause LME cracking. The zinc-based plating layer, while being a factor in the occurrence of LME cracking, has various effects such as dramatically improving the corrosion resistance of the brazed lap joint 1. Furthermore, the brazed lap joint 1 according to the first embodiment has high joint strength. According to the brazed lap joint 1 according to the first embodiment, the reliability of joints having a zinc-based plating layer can be improved and the range of application can be expanded.

[0049] The most basic embodiment of the brazed lap joint 1 according to the first embodiment has been described. Next, a more preferred embodiment of the brazed lap joint 1 according to the first embodiment will be described.

[0050] (Upper limit of leg length L on the side of the second steel plate 12) By reducing the amount of heat input during brazing and thereby reducing the leg length L on the second steel plate 12 side, the coarsening of the grain size in the heat-affected zone can be further suppressed. Therefore, the leg length L on the second steel plate 12 side may be 6.0 mm or less, 5.8 mm or less, or 5.0 mm or less.

[0051] (Leg length on the side of the first steel plate 11) The leg length 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 leg length on the first steel plate 11 side is the distance between the root r and the toe at the end face 111 of the first steel plate 11, if there is a toe at the end face 111 of the first steel plate 11. If the brazing metal 131 completely covers the end face 111 of the first steel plate 11 and there is no toe at the end face 111 of the first steel plate 11, the leg length on the first steel plate 11 side is, for convenience, the distance between the surface of the first steel plate 11 (the surface on the side furthest from the surface 121 of the second steel plate 12) and the surface 121 of the second steel plate 12. In other words, the maximum value of the leg length on the first steel plate 11 side is the distance between the surface of the first steel plate 11 (the surface on the side furthest from the surface 121 of the second steel plate 12) and the surface 121 of the second steel plate 12. This gap is the same as 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 face 111 of the first steel plate. To improve the joint strength, it is preferable that the brazing metal 131 completely covers the end face 111 of the first steel plate 11, that is, that the leg length on the side of the first steel plate 11 is at its maximum value.

[0052] (Tensile strength of steel plate) Preferably, one or both of the first steel plate 11 and the second steel plate 12 are 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. This increases the strength of the brazed lap joint 1. Note that the higher the tensile strength of the steel plate, the higher the LME susceptibility of the steel plate. However, in the brazed lap joint 1 according to the first embodiment, LME cracking is suppressed by keeping the hardness of the brazing metal 131, the average effective grain size of the toe T, root R, and intermediate M of the second steel plate 12, and the B atom concentration of the prior austenite grain boundaries of the second steel plate 12 within an appropriate range. 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. Note that the second steel plate 12, which is the lower plate, tends to be more susceptible to LME cracking than the first steel plate 11. When the second steel plate 12 is a high-strength steel plate, the brazed lap joint 1 according to the first embodiment has an even greater advantage over conventional brazed lap joints 1 or welded joints. While there is no need to limit the thickness of the first steel plate 11 and the second steel plate 12, they may be, for example, 0.8 to 3.2 mm. If necessary, the lower limit of the first steel plate 11 may be 1.0 mm, 1.2 mm, or 1.6 mm, and its upper limit may be 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 may be 1.0 mm, 1.2 mm, or 1.6 mm, and its upper limit may be 3.2 mm, 3.0 mm, 2.8 mm, 2.6 mm, 2.3 mm, or 2.0 mm.

[0053] (2. Method for manufacturing brazed lap joint 1) 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 in which a zinc-based plating layer 14 is present on the overlapping surface, and the end face 111 of a first steel plate 11 and the surface 121 of a second steel plate 12 are brazed together, wherein the concentration of B atoms in the prior austenite grain boundaries of the second steel plate 12 is 2.0 atm% or more, and the average effective grain size of the second steel plate 12 is 7.0 μm or less, and the second steel plate 12 is brought into close contact with a metal surface plate whose interior is cooled by cooling water, and brazing is performed using a brazing wire whose main component is Cu and whose diameter is 0.8 to 1.4 mm as a filler material, 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 units of V, I is the current in units of A, and v is the brazing rate in units of mm / s.

[0054] (First steel plate 11, second steel plate 12, zinc-based plating layer 14) During brazing, a zinc-based plating layer 14 is applied to at least the overlapping surfaces of the first steel sheet 11 and the second steel sheet 12. Therefore, one or both of the first steel sheet 11 and the second steel sheet 12 have a zinc-based plating layer 14 on their surfaces. Furthermore, in the second steel sheet 12, the concentration of B atoms at the prior austenite grain boundaries is 2.0 atm% or more, and the average effective grain size is 7.0 μm or less. The configuration and positional relationship of the first steel sheet 11, the second steel sheet 12, and the zinc-based plating layer 14 are the same as those of the brazed lap joint 1 according to the first embodiment. The preferred embodiments exemplified in the description of the brazed lap joint 1 according to the first embodiment can also be adopted for the first steel sheet 11, the second steel sheet 12, and the zinc-based plating layer 14 in the manufacturing method of the brazed lap joint 1 according to the second embodiment.

[0055] (Brazing) The manufacturing method for the brazed lap joint 1 according to the second embodiment includes a step of brazing the end face 111 of the overlapping first steel plate and the surface 121 of the second steel plate. This forms a brazing metal 131 that joins the end face 111 of the first steel plate and the surface 121 of the second steel plate.

[0056] Brazing, as described above, is a joining method that uses a filler material (brazing agent) with a melting point of 450°C or higher to join materials while minimizing the melting of the base material. Preferably, brazing is performed while shielding the wire electrode and the brazed joint 13 with an inert gas. A suitable example of brazing is MIG brazing. MIG brazing is an application of the shielding mechanism used in MIG welding, as defined in JIS Z 3001-7:2018 "Welding Terminology - Part 7: Arc Welding," to brazing. Specifically, MIG brazing is brazing performed while shielding the brazed joint with an inert gas (shielding gas) such as Ar. MIG brazing yields brazed lap joints with good mechanical properties. On the other hand, from the viewpoint of welding costs, MAG brazing may be used instead. MAG brazing is an application of the shielding mechanism used in MAG welding, as defined in JIS Z 3001-7:2018 "Welding Terminology - Part 7: Arc Welding," to brazing. Specifically, MAG brazing is a brazing method that uses an activated gas, which is a mixture of an inert gas, O2 gas, and CO2 gas, to shield the brazed joint. The brazing can also be, for example, lap fillet brazing or plug brazing.

[0057] (Brazing conditions) In brazing, the brazing heat input Q is set to 190-270 J / mm. The brazing heat input Q is a value obtained by the following formula, and its unit is J / mm. Q = V × I / v The meanings of the symbols in the above formula are as follows: V: Brazing voltage (unit: V) I: Brazing current (unit: A) v: Brazing speed (unit: mm / s) The brazing speed refers to the speed at which the brazing torch is moved. The brazing heat input Q may be 200 J / mm or more, 210 J / mm or more, or 230 J / mm or more. The brazing heat input Q may be 260 J / mm or less, 250 J / mm or less, or 240 J / mm or less.

[0058] If the brazing heat input Q exceeds the above range, the second steel plate 12, which is the lower steel plate in normal brazing, will melt. If the back surface of the lower steel plate is a galvanized steel plate, if the penetration of the lower steel plate is large, LME cracking may occur not only in the root, toe and intermediate parts, but also on the back surface of the lower steel plate. On the other hand, if the brazing heat input Q is less than 190 kJ / mm, the leg length will be small and the tensile shear strength will decrease.

[0059] Furthermore, the brazing is performed with the second steel plate 12 in close contact with the metal surface plate. For example, by making the surface of the surface plate flat and removing foreign matter such as dust from the surface of the surface plate, and then placing the flat second steel plate on top of the surface plate, the second steel plate 12 can be made to adhere closely to the metal surface plate.

[0060] The surface plate is provided with a channel through which cooling water can circulate. During brazing, cooling water is circulated through the channel to cool the inside of the surface plate. The surface plate is used to suppress the temperature rise of the second steel plate 12 during brazing and to prevent the coarsening of the grain size in the heat-affected zone. For example, a copper plate with a water-cooled interior may be used as the surface plate.

[0061] (Composition of filler material) In addition, for brazing, a brazing wire whose main component is Cu and has a diameter of 0.8 to 1.4 mm is used. A wire whose main component is Cu is, for example, a wire with a Cu content of 50% by mass or more. The Cu content of the brazing wire may be 70% by mass or more, 80% by mass or more, or 90% by mass or more. A specific example of a wire whose main component is Cu is a Cu-7%Al wire. With such a wire, the Vickers hardness of the brazing metal 131 can be made 250 or less.

[0062] (Effects and Benefits) In the manufacturing method of the brazed lap joint 1 according to this embodiment, the concentration of B atoms at the prior austenite grain boundaries in the second steel sheet 12 used for brazing is set to 2.0 atm% or more. Furthermore, the heat input during brazing promotes the enrichment of B at the prior austenite grain boundaries in the heat-affected zone 132 of the brazed portion 13. As described with respect to the welded joint according to the first embodiment, B atoms segregated at the prior austenite grain boundaries increase the peel strength of the prior austenite grain boundaries. As a result, LME susceptibility is reduced at the toe T, root R, and intermediate M of the second steel sheet 12.

[0063] Furthermore, in the manufacturing method of the brazed lap joint 1 according to this embodiment, the brazing heat input Q is set to 190 J / mm or more. This ensures the leg length L on the second steel plate 12 side and increases the joint strength of the brazed lap joint 1.

[0064] Furthermore, in the manufacturing method of the brazed lap joint 1 according to this 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 in close contact with a metal surface plate whose interior is cooled. The average effective grain size of the heat-affected zone after brazing is 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, grain enlargement in the heat-affected zone 132 is suppressed. By reducing the average effective grain size of the steel plate, the amount of grain boundaries in the steel plate can be increased, and the LME susceptibility of the steel plate can be further reduced. By increasing the amount of grain boundaries, the concentration of zinc penetrating the grain boundaries is diluted, and embrittlement of the grain boundaries is suppressed.

[0065] The most basic embodiment of the method for manufacturing the brazed lap joint 1 according to the second embodiment has been described. Next, a more preferred embodiment of the method for manufacturing the brazed lap joint 1 according to the second embodiment will be described.

[0066] (Composition of filler material) The chemical composition of a brazing wire mainly composed of Cu is, for example, that it contains Al: 0-30%, Si: 0-20%, and Mn: 0-10% by mass, with the remainder being Cu and impurities. Impurities refer to components that are mixed in during the industrial manufacture of brazing wire, such as raw materials like ore or scrap, or due to various factors in the manufacturing process, and are acceptable as long as they do not adversely affect the brazing metal 131. As a specific method for selecting a filler material, for example, a preliminary test can be conducted to select a wire with a Cu content of 50% or more that can achieve a Vickers hardness of 250 or less for the brazing metal 131.

[0067] In MIG brazing, the base materials, the first steel plate 11 and the second steel plate 12, may slightly melt and dissolve into the brazing metal 131. Components derived from these steel plates become impurities in the brazing metal 131. Therefore, the chemical composition of the brazing wire and the chemical composition of the brazing metal 131 may not match. However, given the brazing heat input described above, the amount of melting 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.

[0068] (Tensile strength of steel plate) Preferably, one or both of the first steel plate 11 and the second steel plate 12 are 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. This increases the strength of the brazed lap joint 1. When the second steel plate 12 is a high-strength steel plate, the manufacturing method of the brazed lap joint 1 according to the second embodiment has an even greater advantage over conventional joint manufacturing methods.

[0069] (3. Automotive parts) An automotive part according to the third embodiment of the present invention has a brazed lap joint 1 according to the first embodiment. Therefore, the automotive part according to the third embodiment has high corrosion resistance due to the zinc-based plating layer 14, while LME cracking is suppressed. An automotive part is, for example, a center pillar.

[0070] (Other embodiments) Although embodiments of the present invention have been described above, the present invention is not limited thereto and can be modified as appropriate without departing from the technical spirit of the invention. Modifications of the present invention will be described below. Unless otherwise specified, the embodiments described below are applicable to all of the first, second, and third embodiments.

[0071] (Feeding speed of filler material) The feed rate of the filler material is not particularly limited, but from the viewpoint of further improving the stability of the brazing, it is preferable to set the feed rate of the filler material in the range of 3 to 6 m / min. By setting the feed rate of the filler material to 6 m / min or less relative to the brazing heat input, the melting of the filler material can be promoted and brazing defects can be further suppressed. On the other hand, by setting the feed rate of the filler material to 3 m / min or more relative to the brazing heat input, the amount of brazing metal 131 can be increased and joining defects can be further suppressed. In normal brazing, the voltage and current are determined in order to achieve the set feed rate.

[0072] (Chemical composition of brazing metal 131) The chemical composition of the brazing metal 131 is not particularly limited, as long as the hardness of the brazing metal 131 can be kept within the above range. Preferably, the brazing metal 131 is mainly composed of Cu. An example of the chemical composition of brazing metal 131 mainly composed of Cu is that, by unit mass%, it contains Al: 0-30%, Si: 0-20%, and Mn: 0-10%, with the remainder being Cu and impurities. Impurities are those that are mixed into the brazing metal during the manufacturing process of the filler material that becomes the material for the brazing metal 131. In addition, the first steel plate 11 and the second steel plate 12 may melt and be mixed into the brazing metal 131. Components derived from the steel plates are also considered impurities in the chemical composition of the brazing metal 131. For example, brazing metal 131 having the above chemical composition can be obtained by performing MIG brazing using Cu-7%Al wire as the filler material.

[0073] (Chemical composition and microstructure of the first steel plate 11 and the second steel plate 12) In the brazed lap joint 1 according to this 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 boundaries are defined. As long as these elements are within the above range, the chemical composition and microstructure of the second steel plate 12 are not particularly limited. The chemical composition and microstructure of the first steel plate 11 are also not particularly limited. For example, the base steel plate of a hot-dip galvanized steel sheet 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 according to this embodiment.

[0074] (Measurement method) The concentration of B atoms at the prior austenite grain boundaries of the second steel sheet 12 outside the brazed joint 13 is determined by the STEM-EELS method. Specifically, it is determined by the method disclosed, for example, in METALLUGICAL AND MATERIALS TRANSACTIONS A: vol. 45A, pp. 1877-1888. First, a sample is taken from the cross section of the second steel sheet 12 outside the brazed joint 13, which is used as the observation surface. The observation surface is mechanically polished to a mirror finish, and then electropolished. The cross section is perpendicular to the end face of the first steel sheet and perpendicular to the surface of the second steel sheet. Next, in one or more observation fields in the range of 1 / 8 to 3 / 8 thickness centered on 1 / 4 thickness from the surface on the observation surface, a total of 2.0 × 10⁻⁶ -9 m 2The above area is subjected to crystal structure and orientation analysis using SEM-EBSD to identify the prior austenite grain boundaries. Next, the region containing the prior austenite grain boundaries is extracted using FIB processing within the SEM. Subsequently, it is thinned to approximately 70 nm using Ar ion milling or the like. For the thinned specimen, line analysis of the B atom concentration is performed along a line crossing the prior austenite grain boundary using electron energy-loss spectroscopy (EELS) with aberration-corrected STEM. The scanning step for line analysis is set to 0.1 nm. Since B atoms segregate at the prior austenite grain boundaries, the maximum value of the obtained B atom concentration is considered to be the B atom concentration at the prior austenite grain boundary. Three measurements are performed, and the average value of the measurement results is taken as the B atom concentration.

[0075] The average effective grain size of the second steel sheet 12 outside the brazed joint 13 is evaluated by SEM-EBSD (Electron Backscatter Diffraction). First, a sample is taken from the cross section of the second steel sheet 12 outside the brazed joint 13, which is used as the observation surface. The observation surface is mechanically polished to a mirror finish, and then electropolished. The cross section is perpendicular to the end face of the first steel sheet and perpendicular to the surface of the second steel sheet. Next, crystal structure and orientation analysis is performed by SEM-EBSD in one or more observation fields in the range of 1 / 8 to 3 / 8 thickness, centered on 1 / 4 thickness from the surface of the observation surface. TSL's "OIM Analysys 6.0" is used to analyze the data obtained by the EBSD method. The step distance between evaluation points is set to 0.03 to 0.20 μm. Regions that are judged to be FCC iron from the observation results are considered retained austenite. Furthermore, a grain boundary map is obtained by defining the boundaries where the crystal orientation difference is 15 degrees or more as grain boundaries. The value calculated using the following formula is defined as the average effective grain size.

number

[0076] The method for measuring the average effective grain size of the second steel sheet 12 at the root R, toe T, and intermediate M is the same as the method for measuring the average effective grain size of the second steel sheet 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 face of the first steel sheet and perpendicular to the surface of the second steel sheet. The measurement location A1 for the average effective grain size of the second steel sheet at the root is a rectangular area, as shown in Figure 2, within a range of ±250 μm in the extending direction of the surface 121 of the second steel sheet, starting from the root r, and within a range of 200 μm in the depth direction of the second steel sheet 12, starting from the root r. The measurement location A2 for the average effective grain size of the second steel sheet at the toe is a rectangular area, as shown in Figure 3, within a range of ±250 μm in the extending direction of the surface 121 of the second steel sheet, starting from the toe t, and within a range of 200 μm in the depth direction of the second steel sheet 12, starting from the toe t. The measurement location A3 for the average effective grain size of the second steel sheet in the intermediate part M between the root and toe is a rectangular area, as shown in Figure 4, within a range of ±250 μm in the extending direction of the surface 121 of the second steel sheet, starting from the midpoint m between the toe t and the root r, and within a range of 200 μm in the depth direction of the second steel sheet 12, starting from the midpoint m. The average effective grain size of these measurement locations A1, A2, and A3 (i.e., the average effective grain size of the three locations: the root, the toe, and the intermediate part between the root and the toe) must all be 15.0 μm or less.

[0077] The hardness of the brazing metal 131 is determined by Vickers hardness testing. The measurement is performed on the cross-section of the brazed joint. 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 conforms to JIS Z 2244-1:2020. The test force is 9.807 N (i.e., HV1). Five measurements are taken. The spacing between the five measurement points is 0.5 mm. The five measurement points are arranged parallel to the surface 121 of the second steel plate and along a straight line passing through the center of the thickness of the first steel plate 11. For example, the dashed line Y shown in Figure 1 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. Furthermore, the middle of the five measurement points is positioned at the midpoint (for example, the × mark in Figure 1) between the intersection of the aforementioned straight line and the surface of the brazing metal 131 and the intersection of the aforementioned straight line and the end face 111 of the first steel plate. The average value of the Vickers hardness at the five measurement points is considered to be the hardness of the brazing metal 131.

[0078] The leg length L on the second steel plate side is determined by cross-sectional observation. The measurement is performed on the cross-section of the brazed joint. 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 distance between the root r and the toe t, as shown in Figure 5, can be measured by known means. The distance between the root r and the toe t is considered to be the leg length L on the second steel plate side.

[0079] The chemical composition of the brazing metal 131 is evaluated using EPMA. The measurement point is on a straight line (dashed line Y in Figure 1) that is parallel to the surface 121 of the second steel sheet and passes through the center of the thickness of the first steel sheet 11, and is the midpoint (marked with an "x" in Figure 1) between the intersection of the straight line and the surface of the brazing metal 131 and the intersection of the straight line and the end face 111 of the first steel sheet. As a method of chemical analysis, a test specimen may be prepared and other methods such as emission spectrometry may be used. [Examples]

[0080] The effects of one aspect of the present invention will be further explained in detail by the examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as it does not depart from the spirit of the invention and achieves the objectives of the present invention.

[0081] (Experiment 1) Lap joints 1-15 were manufactured by MIG brazing using the filler material described in Table 2 on plate assemblies 1-15 as described in Table 1. The voltage, current, brazing speed, and heat input during MIG brazing were as described in Table 3. The average effective grain size of the second steel plate and the hardness of the brazing metal were measured at the root, toe, and intermediate sections of the brazed lap joints 1-15 and are described in Table 4. In addition, the frequency of LME cracking and hydrogen embrittlement in the brazed lap joints 1-15 were investigated and the results are described in Table 4.

[0082] Furthermore, in all of the steel plates listed in Table 1, the average effective grain size was 7.0 μm or less, and the concentration of B atoms at the prior austenite grain boundaries was 2.0 atm% or more. The remainder of the chemical composition of the filler material listed in Table 2 consisted of impurities. In addition, the filler material feeding rate was within the range of 3.5 to 7.0 m / min in all MIG brazing. Values ​​outside the appropriate range are underlined.

[0083] The measurement methods for the average effective grain size of the second steel sheet at the root, toe, and intermediate sections, the hardness of the brazing metal, and the leg length on the side of the second steel sheet were as described above. The largest measurement result (i.e., the maximum value of the average effective grain size) among the measurement results for the average effective grain size of the second steel sheet at the root, toe, and intermediate sections is shown in Table 4.

[0084] The frequency of LME cracking in brazed lap joints was evaluated using the following method. Ten brazed lap joints were prepared for each condition using the method described above, and the cross-sections of the brazed joints were observed to check for the presence or absence of LME cracking. The number of brazed lap joints in which LME cracking occurred was defined as the LME cracking frequency. If no LME cracking occurred in any of the ten test specimens, it was determined that LME cracking was sufficiently suppressed.

[0085] The shear tensile strength of brazed lap joints was evaluated using the following method. From the brazed lap joint 1, a shear tensile test specimen was taken with a specimen width (parallel section width) of 45 mm, where the extension direction of the end face 111 of the first steel plate 11 was in the direction of the specimen width (parallel section width). A tensile load (shear tensile load) was applied in the direction that separates the first steel plate 11 and the second steel plate 12 from each other, and the shear tensile strength of the brazed lap joint was calculated from the load at the time the specimen fractured. Brazed lap joints with a shear tensile strength of 11.25 kN or more (i.e., 0.25 kN / mm or more) were judged to have excellent joint strength and were marked "GOOD" in the table.

[0086] [Table 1]

[0087] [Table 2]

[0088] [Table 3]

[0089] [Table 4]

[0090] In the brazed lap joints of test numbers 3, 4, 6, 8, and 11, the average effective grain size of the second steel plate in the root, toe, or intermediate sections was too large. As a result, LME cracking occurred in these brazed lap joints. In test numbers 3, 4, 6, 8, and 11, it is presumed that grain coarsening occurred because the brazing heat input Q was excessive.

[0091] Furthermore, in the brazed lap joints of test numbers 5 and 6, the hardness of the brazing metal was excessive. As a result, LME cracking occurred in these brazed lap joints. It is presumed that in test numbers 5 and 6, the filler material was mainly Fe, which was insufficient to suppress the hardness of the brazing metal.

[0092] In the brazed lap joint of test number 10, the leg length was insufficient. As a result, the joint strength of the brazed lap joint of test number 10 was insufficient. In test number 10, it is presumed that the heat input Q was insufficient, which prevented sufficient leg length from being secured.

[0093] In the brazed lap joint of test number 12, the average effective grain size of the second steel plate was too large at the root, toe, or intermediate sections. As a result, LME cracking occurred in the brazed lap joint of test number 12. It is presumed that grain coarsening occurred in test number 12 because the second steel plate was not cooled during brazing.

[0094] On the other hand, in brazed lap joints where the brazing heat input Q and the cooling conditions of the second steel plate were appropriate, and the average effective grain size of the second steel plate in the root, toe, and intermediate sections, as well as the hardness of the brazing metal, were within the appropriate range, no LME cracking occurred at all.

[0095] (Additional notes) (A) A brazed joint according to another embodiment of the present invention comprises a first steel plate and a second steel plate stacked on top of each other, a brazing metal that joins the end face of the first steel plate and the surface of the second steel plate, and a brazed portion including 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 their surface, the zinc-based plating layer is arranged on the mating surface of the first steel plate and the second steel plate, the concentration of B atoms at the prior austenite grain boundaries of the second steel plate outside the brazed portion is 2.0 atm% or more, the average effective grain size of the second steel plate outside the brazed portion is 7.0 μm or less, the average effective grain size of the second steel plate is 15.0 μm or less at the root portion, the toe portion, and the intermediate portion between the root portion and the toe portion, and the hardness of the brazing metal is HV250 or less. (B) Preferably, in the brazed joint described in (A) above, the tensile strength of the second steel plate is 980 MPa or more. (C) A method for manufacturing a brazed joint according to another embodiment of the present invention comprises the step of forming a brazed metal by brazing the end faces of a first steel plate and the surface of a second steel plate that are stacked, wherein one or both of the first steel plate and the second steel plate have a zinc-based plating layer on their surface, the zinc-based plating layer is placed on the joint surface of the first steel plate and the second steel plate, the concentration of B atoms in the prior austenite grain boundaries of the second steel plate is 2.0 atm% or more, and the average effective grain size is 7.0 μm or less, the amount of heat Q applied to the braze, calculated by substituting the voltage value V in unit V, the current value I in unit A, and the brazing rate v in unit m / min into the following formula, is in the range of 1500 to 3000 J / 10 mm, and the hardness of the brazed metal is HV250 or less. Q = 0.6 × V × I / v (D) Preferably, in the method for manufacturing a brazed joint described in (C) above, the filler material is a brazing wire whose main component is Cu and whose diameter is 0.8 to 1.4 mm, thereby making the hardness of the brazed metal HV250 or less. (E) Preferably, in the method for manufacturing a brazed joint described in (C) or (D) above, the tensile strength of the second steel plate is 980 MPa or more. (F) Preferably, in the method for manufacturing a brazed joint described in any one of (C), (D), and (E) above, the brazing is MIG brazing. (G) An automotive part according to another embodiment of the present invention comprises the brazed joint described in (A) or (B) above. [Explanation of symbols]

[0096] 1. Brazed lap joint 11 Daiichi Steel Plate 111 End face of the first steel plate 12 Second steel plate 121 Surface of the second steel plate 13 Brazed section 131 Brazed metal 132 Heat-affected zone 14. Zinc-based plating layer R Root Section r root T Toe t toe M Root section and intermediate section of the toe end m Route and intermediate end L Leg length on the side of the second steel plate A1 Measurement location of the average effective grain size of the second steel plate in the root section A2 Measurement location of the average effective grain size of the second steel plate at the toe end. A3 Measurement location of the average effective grain size of the second steel plate in the intermediate part of the root and toe sections.

Claims

1. The first steel plate and the second steel plate are stacked on top of each other, A zinc-based plating layer on the overlapping surface of the first steel sheet and the second steel sheet, A brazed portion comprising a brazing metal that joins the end face of the first steel plate and the surface of the second steel plate, and a heat-affected zone surrounding the brazing metal, A brazed lap joint having, The concentration of B atoms in 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. In the root portion, the toe portion, and the intermediate portion between the root portion and the toe portion, the average effective grain size of the second steel sheet is 15.0 μm or less. The Vickers hardness of the aforementioned brazing metal is 250 or less. The leg length on the side of the second steel plate of the brazed lap joint is 2.0 mm or more. Brazed lap joint.

2. The tensile strength of the second steel plate is 980 MPa or more. The brazed lap joint according to feature 1.

3. A method for manufacturing an overlapping joint in which a zinc-based plating layer is present on the overlapping surface, and the end face of the first steel plate and the surface of the second steel plate are brazed together, 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 metal surface plate, whose interior is cooled by cooling water, is brought into close contact with the second steel plate. Using a brazing wire whose main component is Cu and has a diameter of 0.8 to 1.4 mm as a filler material, Brazing is performed so that the brazing heat input Q, calculated by the following formula, is between 190 and 270 J / mm. A method for manufacturing brazed lap joints, characterized by the following features. Q = V × I / v Here, V is the voltage in units of V, I is the current in units of A, and v is the brazing speed in units of mm / s.

4. The tensile strength of the second steel plate is 980 MPa or more. The method for manufacturing a brazed lap joint according to feature 3.

5. The brazing described above is MIG brazing. A method for manufacturing a brazed lap joint according to claim 3 or 4.

6. An automotive part comprising a brazed lap joint according to claim 1 or 2.

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