Projection welded joint and method for manufacturing the same

Abstract

A projection-welded joint according to one embodiment of the present disclosure comprises a steel sheet, a steel member, and a projection welding unit for joining the steel sheet and the steel member. The projection welding unit comprises: a melt solidification part; and a corona bond part located around the melt solidification part. The amount of carbon at the sheet thickness center of the steel sheet is 0.25 mass% or more, and the hardness at an outer edge of the corona bond part is lower than that at a position that is 1 / 4 of the sheet thickness of the steel sheet on the outer side of the corona bond part. Preferably, the steel sheet has a softened layer within a range of 20 μm depth from the surface on the side to which the steel member is joined. The hardness of the the softened layer is 95% or less of the hardness at the position that is 1 / 4 of the sheet thickness of the steel sheet.

Description

[Technical Field]

[0001] The present disclosure relates to a projection weld joint and a method for manufacturing a projection weld joint. This application claims priority based on Japanese Patent Application No. 2022-089552, filed on June 1, 2022, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, in the automotive field, there has been an increasing need to use high-strength steel sheets in vehicle bodies and parts, etc., in order to reduce the weight of vehicle bodies and improve collision safety in order to improve fuel efficiency and reduce carbon dioxide emissions. For example, in automotive structural members such as front side members, center pillars, and hinge reinforcements, a configuration is adopted in which steel members such as nuts are welded to parts made of high-strength steel sheets.

[0003] In the manufacture of automotive structural components in which steel members are joined, a method of joining nuts to the surface of high-strength steel sheets using the projection welding method is commonly used. Projection welding is a resistance welding method in which protrusions formed at the welding point of one base material are brought into contact with the welding point of the other base material and an electric current is passed through, limiting the generation of resistance heat to a relatively small, specific area. When a nut is projection welded to a steel sheet, a nut with multiple protrusions is pressed against the steel sheet and current is passed through the protrusions, locally heating the contact interface between each protrusion and the steel sheet, resulting in melting and solidification or solid-state welding.

[0004] However, in projection welded joints made of such high-strength steel plates, hydrogen embrittlement of the welds has become a problem. The higher the strength of the steel, the more pronounced the adverse effects of hydrogen embrittlement become.

[0005] Regarding projection welding, for example, Patent Document 1 discloses a method for manufacturing a sintered part for welding, which is made of an iron-carbon alloy and is to be subjected to welding, characterized in that after powder molding and before sintering, a portion of the green compact that will become the weld is locally heated to decarbonize.

[0006] Patent Document 2 describes a power transmission drive shaft in which a balance weight is joined to a part of a hollow shaft constituting the drive shaft by projection welding, and the hollow shaft has a tensile strength of 70 kgf / mm 2 A high-strength drive shaft for power transmission is disclosed, which is the above steel pipe, and in which the ratio (d / D) of the diameter d of the joint to the projection diameter D at the joint between the steel pipe and the balance weight is 0.6 to 1.0.

[0007] Patent Document 3 discloses a projection welding method in which, when welding one member and another member, at least one of which is aluminum-plated, the welding process is divided into three stages: a first stage using a small current, a second stage using a large current, and a third stage using a medium current. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 6-49504 [Patent Document 2] Japanese Patent Application Publication No. 7-317844 [Patent Document 3] Japanese Patent Application Laid-Open No. 2004-42089 Summary of the Invention [Problem to be solved by the invention]

[0009] Patent Documents 1 to 3 propose methods for preventing hydrogen embrittlement cracking by modifying the shape or composition of the protrusions of the steel members or by modifying the welding conditions, but these methods have problems such as an increase in takt time and restrictions on the nut shape and welding conditions. In addition, steel members to be projection-welded generally have multiple protrusions. During projection welding, current is passed through the steel member while pressing it toward the steel plate with the protrusions in contact with the surface of the steel plate. By passing current at a high current density through the multiple protrusions and the areas where they contact, the multiple protrusions and their surroundings are heated simultaneously, forming a molten solidified zone. This makes it difficult to prevent hydrogen embrittlement cracking through optimization of welding conditions. This is because it is difficult to strictly match the pressure and heat input of the multiple protrusions. Even if optimal welding conditions are met for some protrusions, hydrogen embrittlement cracking cannot be suppressed in the projection-welded joint if the welding conditions for other protrusions are inappropriate. Furthermore, considering dimensional errors, it is difficult to perfectly match the shapes of the multiple protrusions on the steel member. This also makes it difficult to suppress hydrogen embrittlement cracking.

[0010] Various methods for suppressing hydrogen embrittlement have been proposed in the technical field of spot welding. Like projection welding, spot welding is a type of resistance welding. However, it is difficult to adapt methods for suppressing hydrogen embrittlement in spot welding to projection welding. This is because the heat generation phenomenon caused by electrical current in spot welding is completely different from that in projection welding. In spot welding, two or more stacked metal sheets are heated by electrical current while being clamped between rod-shaped electrodes. During this process, the base metal sheets are in surface contact. Therefore, the shape of the current path in spot welding is completely different from that in projection welding. Furthermore, in spot welding, the pressure and heat input at multiple welds do not need to be strictly consistent. Therefore, the above-mentioned issues associated with projection welding do not exist in spot welding. This also makes it difficult to adapt methods for suppressing hydrogen embrittlement in spot welding to projection welding.

[0011] In view of the above circumstances, an object of the present disclosure is to provide a projection welded joint that is composed of a steel member and a steel plate having an internal carbon content of 0.25 mass% or more, and that is capable of suppressing hydrogen embrittlement of multiple welds, and a method for manufacturing a projection welded joint. [Means for solving the problem]

[0012] The gist of the present disclosure is as follows.

[0013] (1) A projection welded joint according to one embodiment of the present disclosure comprises a steel plate, a steel member, and a projection weld that joins the steel plate and the steel member, the projection weld comprising a molten solidified portion and a corona bond portion around the molten solidified portion, the carbon content at the center of the thickness of the steel plate being 0.25 mass% or more, and the hardness of the outer end of the corona bond portion being lower than the hardness at a position 1 / 4 of the thickness of the steel plate outside the corona bond portion. (2) Preferably, in the projection welded joint of (1) above, the steel plate has a softened layer in a range of 20 μm deep from the surface on the side where the steel member is joined, and the hardness of the softened layer is 95% or less of the hardness at the 1 / 4 position of the plate thickness of the steel plate. (3) Preferably, in the projection welded joint of (2) above, a location within a depth range of 20 μm to 200 μm from the surface of the steel plate on the side where the steel member is joined has a location where the hardness is 95% of the hardness at the 1 / 4 plate thickness position of the steel plate. (4) Preferably, in the projection welded joint of any one of (1) to (3) above, the Vickers hardness of the outer end of the corona bond portion is Hv700 or less. (5) Preferably, in the projection welded joint of any one of (1) to (4) above, there are a plurality of the molten solidified portions, and the cross-sectional area of ​​the first molten solidified portion in a cross section along the thickness direction of the steel plate is 1.5 times or more the cross-sectional area of ​​the second molten solidified portion in a cross section along the thickness direction of the steel plate.

[0014] (6) A method for manufacturing a projection welded joint according to another aspect of the present disclosure is a method for manufacturing a projection welded joint according to any one of (1) to (5) above, comprising pressing a protrusion of a steel member having a protrusion against a steel plate and passing current through the protrusion to perform projection welding, wherein the carbon content at the center of the thickness of the steel plate is 0.25 mass% or more, the steel plate has a softened layer within a range of 20 μm deep from the surface on the side where the protrusion is pressed, and the hardness of the softened layer is 95% or less of the hardness at a position 1 / 4 of the plate thickness of the steel plate. (7) Preferably, in the method for manufacturing a projection welded joint described in (6) above, a location having 95% of the hardness at a position 1 / 4 of the plate thickness of the steel plate is located within a depth range of 20 μm to 200 μm from the surface of the side of the steel plate against which the protrusion is pressed.

[0015] (Additional notes) (1) A projection welded joint according to one embodiment of the present disclosure comprises a steel plate, a steel member joined to a surface of the steel plate, and a plurality of welds that are arranged at rotationally symmetric positions in a plan view along a direction perpendicular to the surface of the steel plate and join the steel plate and the steel member, the welds having a molten solidified portion, and a pressure-welded surface and a heat-affected zone formed around the molten solidified portion, the carbon content inside a base material portion of the steel plate that is outside the heat-affected zone is 0.25 mass% or more, and in all of a plurality of cross sections that pass through a line connecting the center of each of the plurality of welds and the rotation center of the plurality of welds and are perpendicular to the surface of the steel plate, the Vickers hardness of the surface layer portion of the steel plate at the outer end of the pressure-welded surface is lower than the Vickers hardness of a 1 / 4 plate thickness portion of the base material portion of the steel plate. (2) In the projection welded joint described in (1) above, the base material portion of the steel plate may have a decarburized layer on the surface on the side where the steel member is joined, and the depth of a region in the decarburized layer having a hardness that is 5% or more lower than the Vickers hardness of the 1 / 4 plate thickness portion of the base material portion may be 5 to 200 μm. (3) In the projection welded joint described in (1) or (2) above, the Vickers hardness of the surface layer portion of the steel plate at the outer end of the pressure welding surface may be HV700 or less in all of the multiple cross sections. (4) In the projection welded joint described in any one of (1) to (3) above, the ratio Amax / Amin of the maximum value Amax of the cross-sectional area of ​​the plurality of molten solidified portions measured at each of the plurality of cross sections to the minimum value Amin of the cross-sectional area of ​​the plurality of molten solidified portions may be 1.5 or more. (5) In the projection welded joint according to any one of (1) to (4) above, the steel plate may be a hot stamped steel plate. (6) In the projection welded joint according to any one of (1) to (5) above, the steel member may be a nut. (7) An automobile part according to another embodiment of the present disclosure has the projection weld joint described in any one of (1) to (6) above. (8) A method for manufacturing a projection-welded joint according to another aspect of the present disclosure includes a step of projection-welding a plurality of projection welding protrusions of a steel member onto a surface of a steel plate to form a plurality of welds, wherein during the projection welding, the protrusions are arranged in rotationally symmetric positions in a plan view along a direction perpendicular to the surface of the steel plate, the welds have a molten solidified portion, and a pressure-welded surface and a heat-affected zone around the molten solidified portion, the carbon content inside the steel plate is 0.25 mass% or more, and in all of a plurality of cross sections passing through a line connecting the center of each of the plurality of welds and the rotation center of the plurality of welds and perpendicular to the surface of the steel plate, the Vickers hardness of the surface layer portion of the steel plate at the outer end of the pressure-welded surface is lower than the Vickers hardness of a 1 / 4 plate thickness portion of the base material portion outside the heat-affected zone of the steel plate. (9) In the method for manufacturing a projection welded joint described in (8) above, the steel plate may have a decarburized layer on the surface, and the depth of a region in the decarburized layer having a hardness that is 5% or more lower than the Vickers hardness at the 1 / 4 plate thickness portion of the steel plate may be 5 to 200 μm. [Effects of the Invention]

[0016] According to the present disclosure, it is possible to provide a projection welded joint that is composed of a steel member and a steel plate having an internal carbon content of 0.25 mass% or more and that can suppress hydrogen embrittlement of multiple welds, and a method for manufacturing a projection welded joint. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a cross-sectional view of a projection welded joint according to the present embodiment. [Figure 2] FIG. 2 is an enlarged cross-sectional view of a welded portion of a projection welded joint according to the present embodiment. [Figure 3] FIG. 1 is a plan view of a projection welded joint according to an embodiment of the present invention. [Figure 4] FIG. 1 is a schematic diagram of a crack occurring in a conventional projection welded joint. DETAILED DESCRIPTION OF THE INVENTION

[0018] As shown in Figures 1 to 3, a projection welded joint 1 according to one embodiment of the present disclosure comprises a steel plate 11, a steel member 12, and a projection weld 13 joining the steel plate 11 and the steel member 12, wherein the projection weld 13 comprises a molten solidified portion 131 and a corona bond portion 133 around the molten solidified portion 131, the carbon content at the center of the thickness of the steel plate 11 is 0.25 mass% or more, and the hardness of the outer end 133E of the corona bond portion 133 is lower than the hardness at a 1 / 4 position of the thickness of the steel plate 11 outside the corona bond portion 133.

[0019] First, the technical concept of the projection welded joint 1 according to this embodiment will be described below.

[0020] The present inventors conducted a detailed investigation into the fracture morphology of projection welded joints in which hydrogen embrittlement cracking occurred. As a result, they found that crack C in the weld very often originates from the edge of the corona bond portion 133 (pressure weld surface) formed around the molten solidified portion 131, or in its vicinity, as shown in Fig. 4. They also found that when the projection welded joint is viewed in a plan view along a direction perpendicular to the surface of the steel plate, cracks are likely to occur at the outer weld edge of the steel member.

[0021] Generally, it is believed that the main factors that affect the hydrogen embrittlement of steel are the strength of the steel, the stress applied to the steel, and the hydrogen that has penetrated into the steel. Therefore, the inventors attempted to reduce the surface hardness of the steel sheet 11 at the outer end 133E of the corona bond portion 133.

[0022] Here, the outer end 133E of the corona bond portion 133 refers to the end of the corona bond portion 133 that is located on the outer side of the steel member 12, as shown in the cross-sectional view of the weld in FIG. 2 . The cross section at which the outer end 133E is identified is a cross section that passes through a line connecting the centers of the projection welds 13 and the centers of the projection welds 13 and is perpendicular to the surface of the steel plate 11. When the projection welds 13 are arranged in rotationally symmetrical positions, the center of the projection welds 13 is the center of rotation. When the projection welds 13 are arranged in non-rotationally symmetrical positions, the center of the projection welds 13 is the center of the fastening components of the steel member 12. For example, when the steel member 12 has a nut shape, the center of the through hole of the nut is considered to be the center of the projection welds 13. When the steel member 12 has a bolt shape, the central axis of the bolt is considered to be the center of the projection welds 13. The line connecting the center of the projection weld 13 itself and the centers of the multiple projection welds 13 is, for example, the dashed dotted line shown in the plan view of Figure 3. Furthermore, the centers of each of the multiple projection welds 13 and the centers of the multiple projection welds 13 are identified in a plan view along a direction perpendicular to the surface of the steel plate 11. Hereinafter, unless otherwise specified, "a cross section passing through a line connecting the center of each of the multiple projection welds 13 and the center of the multiple projection welds and perpendicular to the surface of the steel plate" will be simply referred to as "cross section." The outer end 133E defined above is the point farthest from the center of the multiple projection welds 13.

[0023] By attempting to reduce the surface hardness of the steel sheet 11 at the outer end 133E of the corona bond portion 133, the inventors were able to significantly reduce the frequency of hydrogen embrittlement cracking. This method does not increase the takt time or impose restrictions on the nut shape or welding conditions. Furthermore, since this method does not require softening the interior of the steel sheet 11, the tensile strength of the steel sheet 11 can be maintained high. Additionally, this method does not require uniform welding conditions for multiple projection welds 13. Even if the conditions of the multiple projection welds 13 are significantly different, the projection welded joint 1 according to this embodiment can suppress hydrogen embrittlement cracking in all of the multiple projection welds 13.

[0024] Next, we will explain the specific configuration of the projection-welded joint 1 according to this embodiment. The projection-welded joint 1 has a steel plate 11, a steel member 12 joined to the surface of the steel plate 11, and a projection weld 13 joining the steel plate 11 and the steel member 12 together.

[0025] The projection weld 13 has a molten solidified portion 131, a corona bond portion 133 formed around the molten solidified portion 131, and a heat-affected portion 132. The molten solidified portion 131 is a portion formed when the steel plate 11 and the steel member 12 are first melted and then solidified by projection welding. The heat-affected zone 132 (HAZ) refers to a portion of the base material in the steel plate 11 and the steel member 12 where the structure, metallurgical properties, and mechanical properties have changed due to welding heat but where the base material remains unmelted. The corona bond portion 133 is a solid-state bond between the steel member 12 and the steel plate 11 formed around the molten solidified portion 131. The corona bond portion 133 is a solid-state bond similar to the corona bond formed in a ring shape around the nugget in a spot weld. The corona bond portion 133 is also sometimes referred to as a pressure weld portion or a pressure weld surface.

[0026] In addition, in this embodiment, the portion of the steel plate 11 that is outside the heat-affected zone 132 is referred to as the base material portion 111 of the steel plate 11. Furthermore, the base material portion 111 of the steel plate 11 may also be simply referred to as the "base material portion 111."

[0027] (Steel plate 11) The steel plate 11 is a steel plate having a carbon content of 0.25% by mass or more at the plate thickness center, or a steel plate having a carbon content of 0.25% by mass or more inside the base metal portion 111. Such a steel plate can improve the crash resistance of the projection welded joint 1 and the automotive parts to which it is applied. The carbon content of the steel plate 11 at the plate thickness center or the carbon content of the base metal portion 111 of the steel plate 11 may be 0.27% by mass or more, 0.30% by mass or more, more than 0.30% by mass, 0.31% by mass or more, 0.32% by mass or more, 0.33% by mass or more, 0.34% by mass or more, or 0.35% by mass or more.

[0028] It is recognized by those skilled in the art that steel plates with a carbon content of more than 0.30% by mass are very susceptible to cracking during projection welding. However, in the projection-welded joint 1 according to this embodiment, hydrogen embrittlement of the weld is suppressed. Therefore, in the projection-welded joint 1 according to this embodiment, the carbon content of the steel plate 11 can be increased to more than 0.30% by mass.

[0029] On the other hand, there is no particular limitation on the carbon content in the surface layer of the steel sheet 11. As will be described later, a softened layer may be provided on the surface of the steel sheet 11. Other configurations of the steel sheet 11 are not particularly limited, but preferred examples will be given below.

[0030] The chemical components of the steel plate 11 other than the carbon content at the plate thickness center (or inside the base material portion 111) are not particularly limited. For example, the chemical components other than carbon at the plate thickness center (or inside the base material portion 111) of the steel plate 11 may contain, in mass %, Si: 2.00% or less, Mn: 0.05 to 5.00%, P: 0.100% or less, and S: 0.0100% or less. In this case, the balance of the chemical components includes Fe and impurities.

[0031] The tensile strength of the steel plate 11 is not particularly limited, and may be, for example, 980 MPa or more, 1200 MPa or more, or 1500 MPa or more. The steel plate 11 may also be a hot-stamped steel plate, i.e., a steel plate (steel member) manufactured by hot stamping. Alternatively, the steel plate 11 may be a steel plate for hot stamping before hot stamping. In the projection-welded joint 1 according to this embodiment, the term "steel plate" is a concept that encompasses not only steel that is entirely plate-shaped, but also steel that is only partially plate-shaped, such as a member having a flange portion.

[0032] The steel sheet 11 may have a plating layer on its surface. Examples of the plating layer include hot-dip galvanizing, alloyed hot-dip galvanizing, and aluminum plating. This can improve the corrosion resistance of the projection welded joint 1.

[0033] The thickness of the steel plate 11 is not particularly limited. For example, when the projection-welded joint 1 is used as an automotive part, the thickness of the steel plate 11 may be within a range of 0.5 to 3.0 mm. Generally, the thicker the steel plate 11, the greater the stress applied to the projection weld 13 during the cooling process after projection welding, making it more susceptible to hydrogen embrittlement cracking. However, in the projection-welded joint according to this embodiment, hydrogen embrittlement cracking can be suppressed by softening the outer end 133E of the corona bond portion 133, as described below. Furthermore, for example, two or more overlapping steel plates may be projection-welded. In this case, hydrogen embrittlement cracking can be suppressed by softening the outer end 133E of the corona bond portion 133 between the steel plate and the steel member.

[0034] As illustrated in Fig. 1, a through hole may be provided in the steel plate 11. When the steel member 12 is a nut, the bolt can be attached to the projection welded joint 1 so as to pass through the projection welded joint 1 by aligning the hole in the nut with the through hole. On the other hand, the steel plate 11 may be a flat plate.

[0035] (Steel member 12) The steel member 12 is a steel mechanical component joined to the surface of the steel plate 11. The steel member 12 has multiple protrusions before projection welding. These protrusions are pressed against the steel plate 11 during projection welding, forming a current path. The protrusions then concentrate current at the contact surfaces between the multiple protrusions and the steel plate 11, forming projection welds 13 that join the steel member 12 and the steel plate 11. To ensure a uniform joining state of the projection welds 13, the multiple protrusions on the steel member 12 before projection welding are preferably arranged in rotationally symmetric positions. Therefore, the projection welds 13 formed on the steel member 12 after projection welding are preferably arranged in rotationally symmetric positions in a plan view along a direction perpendicular to the surface of the steel plate 11, as shown in FIG. 3 . The detailed configuration of the projection welds 13 is as described above.

[0036] To reliably join the steel member 12 to the steel plate 11, the number of protrusions is set to two or more. For example, the number of protrusions may be three, four, five, or six. As will be described later, from the viewpoint of ensuring uniform contact between each of the multiple protrusions and the steel plate 11, it is most preferable to set the number of protrusions to three. This allows the number of projection welds 13 to be two, three, four, five, or six.

[0037] The shape of the steel member 12 is not particularly limited. For example, when the projection welded joint 1 is used as an automobile part, the steel member 12 is preferably a bolt or a nut. However, various shapes can be applied to the steel member 12 depending on the application of the projection welded joint 1. The strength, chemical composition, etc. of the steel member 12 are also not particularly limited. For example, the material of the steel member 12 may be S10C, S25C, SWCH steel, etc.

[0038] (Hardness of outer end 133E of corona bond portion 133) A molten and solidified zone 131 and a heat-affected zone 132 are formed at the projection weld 13 between the steel member 12 and the steel plate 11. According to various experiments conducted by the present inventors, it has been found that hydrogen embrittlement cracking is likely to occur in the surface layer of the steel plate 11 at the outer end 133E of the corona bond 133, as shown in Fig. 4. Here, the outer end 133E of the corona bond 133 refers to the end located outside the steel member 12, of both ends of the corona bond 133 in the cross section of the projection weld 13, as shown in Figs. 2 and 3.

[0039] Therefore, in the projection welded joint 1 according to this embodiment, the Vickers hardness of the surface layer of the steel plate 11 at the outer end 133E of the corona bond portion 133 between the steel member 12 and the steel plate 11, measured at the cross section of the projection weld 13, is defined as the "hardness of the outer end 133E of the corona bond portion 133." The hardness of the outer end 133E of the corona bond portion 133 is set lower than the hardness at a quarter-thickness position of the steel plate outside the corona bond portion 133. Preferably, the hardness of the outer end 133E of the corona bond portion 133 is set lower than the Vickers hardness at a quarter-thickness position of the base material portion 111 of the steel plate 11. Hydrogen embrittlement cracking is more likely to occur as the strength of the steel increases. Therefore, hydrogen embrittlement cracking can be prevented by softening the steel at the locations where hydrogen embrittlement cracking initiates.

[0040] Furthermore, this requirement is met for all of the multiple projection welds 13. That is, in all cross sections of each of the multiple projection welds 13, the hardness of the outer end 133E of the corona bond portion 133 is made lower than the hardness at a 1 / 4 plate thickness position of the steel plate 11 outside the corona bond portion 133. Preferably, in all cross sections of each of the multiple projection welds 13, the hardness of the outer end 133E of the corona bond portion 133 is made lower than the Vickers hardness at a 1 / 4 plate thickness position of the base material portion 111 of the steel plate 11. This suppresses hydrogen embrittlement cracking in all of the projection welds 13, and the joining strength of the steel members 12 can be maintained at an extremely high level.

[0041] The absolute value ΔHv of the difference between the hardness of the outer end 133E of the corona bond portion 133 and the hardness at the 1 / 4 position of the plate thickness of the steel plate outside the corona bond portion 133 may be greater than Hv0 for all projection welds 13. Preferably, ΔHv is Hv10 or greater, Hv30 or greater, Hv50 or greater, or Hv100 or greater for all projection welds 13. Since a larger ΔHv is preferable, there is no need to specify an upper limit. However, to maintain the strength of the steel plate, ΔHv may be, for example, Hv250 or less, Hv200 or less, or Hv150 or less for all projection welds 13. Note that the above-mentioned ΔHv may also be defined as the absolute value of the difference between the hardness of the outer end 133E of the corona bond portion 133 and the hardness at the 1 / 4 position of the plate thickness of the base material portion 111 of the steel plate 11.

[0042] Furthermore, the absolute value of the hardness of outer end 133E of corona bond portion 133 may be specified. Specifically, in all welds, the Vickers hardness of outer end 133E of corona bond portion 133 may be Hv700 or less, Hv650 or less, or Hv600 or less. This makes it possible to more effectively suppress hydrogen embrittlement cracking in the welds.

[0043] There are no particular limitations on the method for softening the outer end 133E of the corona bond portion 133 more than the 1 / 4 thickness position of the steel plate outside the corona bond portion 133 (or the 1 / 4 thickness portion of the base material portion 111 of the steel plate 11). For example, by performing projection welding on a steel plate 11 that has been subjected to a surface softening treatment such as surface decarburization, the difference ΔHv between the Vickers hardness of the outer end 133E of the corona bond portion 133 and the Vickers hardness of the 1 / 4 thickness position of the steel plate outside the corona bond portion 133 (or the 1 / 4 thickness portion of the base material portion 111 of the steel plate 11) can be made to exceed Hv0. Therefore, in the projection-welded joint 1 according to this embodiment, the steel plate 11 may have a softened layer. The softened layer is, for example, a decarburized layer. Hereinafter, the depth of the softened layer is defined as the depth of a region in the softened layer that has a hardness that is 5% or more lower than the hardness at the 1 / 4 position of the plate thickness of the steel plate 11 (particularly preferably, the base material portion 111 of the steel plate 11). In other words, the hardness of the softened layer is 95% or less of the hardness at the 1 / 4 position of the plate thickness of the steel plate 11 (particularly preferably, the base material portion 111 of the steel plate 11).

[0044] The softening layer may be provided on both sides of the steel plate 11 or may be provided only on the surface of the steel plate 11 that comes into contact with the steel member 12 .

[0045] It is preferable that the steel plate 11 has a softened layer in a range of at least 20 μm deep from the surface on the side where the steel member 12 is joined. For example, the softened layer depth of the steel plate 11 may be 25 μm or more, 30 μm or more, 40 μm or more, or 50 μm or more. The softened layer depth of the steel plate 11 may also be 200 μm or less, 190 μm or less, 80 μm or less, or 150 μm or less.

[0046] Alternatively, a location within a depth range of 20 μm to 200 μm from the surface of the steel plate 11 on the side where the steel member 12 is joined may have a hardness that is 95% of the hardness at the 1 / 4 position in the plate thickness of the steel plate 11. In this case, the softened layer depth is within a range of 20 μm to 200 μm, further enhancing the effect of suppressing hydrogen embrittlement cracking.

[0047] Furthermore, the smaller the hardness of the softened layer, the greater the effect of suppressing hydrogen embrittlement cracking. Therefore, the softened layer depth may be defined as the depth of a region having a hardness that is 8% or more, 10% or more, or 12% or more lower than the hardness at the 1 / 4 position of the plate thickness of the base material portion 111, and the above-mentioned numerical value of the softened layer depth may be applied.

[0048] On the other hand, in order to make ΔHv exceed Hv0, it is also possible to use means other than surface layer softening treatment (for example, surface layer decarburization treatment). For example, even when only the surface layer of the steel sheet 11 is softened by controlling the number density of precipitates, ΔHv can be made to exceed Hv0. Furthermore, ΔHv can also be made to exceed Hv0 by making the steel sheet 11 a multi-layer steel sheet combining mild steel and high-strength steel, or by tempering only the surface layer of the steel sheet.

[0049] In the projection-welded joint 1 according to this embodiment, there may be a plurality of projection welds 13, and therefore a plurality of molten solidified portions 131. The welding state of the plurality of projection welds 13 may be uneven. For example, the cross-sectional area of ​​the first molten solidified portion in a cross section along the thickness direction of the steel plate 11 may be 1.5 times or more the cross-sectional area of ​​the second molten solidified portion in a cross section along the thickness direction of the steel plate 11. The first molten solidified portion is any molten solidified portion in the projection-welded joint 1 including a plurality of molten solidified portions 131. The second molten solidified portion is any molten solidified portion in the projection-welded joint 1 including a plurality of molten solidified portions 131, and has a smaller cross-sectional area than the first molten solidified portion.

[0050] Alternatively, the ratio Amax / Amin of the maximum value Amax of the cross-sectional area of ​​the molten solidified portion 131 included in the projection weld 13 to the minimum value Amin of the cross-sectional area of ​​the molten solidified portion 131 may be 1.5 or more.

[0051] Here, the cross-sectional area of ​​the molten solidified portion 131 refers to the cross-sectional area of ​​the molten solidified portion 131 measured in multiple cross sections that pass through a line connecting the center of each of the multiple welds and the center of the multiple welds and are perpendicular to the surface of the steel plate. By measuring the cross-sectional area of ​​the molten solidified portion 131 in each of the multiple cross sections, Amax and Amin can be identified. Note that when the projection welds 13 are provided in rotationally symmetric positions in a plan view along a direction perpendicular to the surface of the steel plate 11, the cross-sectional area of ​​the molten solidified portion 131 is measured in multiple cross sections that pass through a line connecting the center of each of the multiple projection welds 13 and the rotation center of the multiple projection welds 13 and are perpendicular to the surface of the steel plate 11.

[0052] The cross-sectional area of ​​the molten solidified portion 131 is an index of the heat input at the projection weld 13. When the cross-sectional area of ​​the first molten solidified portion and the cross-sectional area of ​​the second molten solidified portion are the same, or when Amax / Amin is 1, there is a high probability that the multiple projection welds 13 were manufactured under exactly the same welding conditions. However, it is difficult to manufacture multiple projection welds 13 under the same conditions. The surface condition of the steel plate 11 during projection welding, distortion of the steel plate 11, deviations in the pressure applied to the steel member 12, and dimensional errors of the protrusions of the steel member 12 are disturbance factors that cause the conditions of the multiple projection welds 13 to vary. In this case, residual stress is generated between the multiple projection welds 13, and this residual stress may cause hydrogen embrittlement cracking.

[0053] The larger the value obtained by dividing the cross-sectional area of ​​the first molten solidified portion by the cross-sectional area of ​​the second molten solidified portion, the more disadvantageous it is from the viewpoint of suppressing hydrogen embrittlement cracking of the welded portion 13. On the other hand, if the cross-sectional area of ​​the first molten solidified portion is allowed to be 1.5 times or more the cross-sectional area of ​​the second molten solidified portion, projection welding becomes easy to perform, and the projection-welded joint 1 according to this embodiment can be applied to various components. Furthermore, in the projection-welded joint 1 according to this embodiment, even if the cross-sectional area of ​​the first molten solidified portion is 1.5 times or more the cross-sectional area of ​​the second molten solidified portion, hydrogen embrittlement cracking can be suppressed by setting ΔHv to exceed Hv0. Therefore, in the projection-welded joint 1 according to this embodiment, the cross-sectional area of ​​the first molten solidified portion is preferably 1.5 times or more the cross-sectional area of ​​the second molten solidified portion. The cross-sectional area of ​​the first molten solidified portion may be 1.6 times or more, 1.8 times or more, or 2.0 times or more the cross-sectional area of ​​the second molten solidified portion.

[0054] Similarly, a larger Amax / Amin is more disadvantageous in terms of suppressing hydrogen embrittlement cracking of the welded portion 13. On the other hand, if an Amax / Amin ratio of 1.5 or greater is acceptable, projection welding can be easily performed, and the projection-welded joint 1 according to this embodiment can be applied to a variety of components. Furthermore, in the projection-welded joint 1 according to this embodiment, even if Amax / Amin is 1.5 or greater, hydrogen embrittlement cracking can be suppressed by setting ΔHv to greater than Hv0. Therefore, in the projection-welded joint 1 according to this embodiment, Amax / Amin is preferably 1.5 or greater. Amax / Amin may also be 1.6 or greater, 1.8 or greater, or 2.0 or greater.

[0055] (Methods for measuring carbon content, Vickers hardness, softened layer depth, and cross-sectional area of ​​molten solidified portion) The carbon content inside the base material portion 111 of the steel plate 11 can be determined by grinding at least 0.3 mm of the surface layer of the base material portion 111 of the steel plate 11 and measuring the composition of the resulting ground surface by combustion (infrared absorption), spectroscopic analysis, or X-ray fluorescence analysis. The carbon content at the thickness center of the steel plate 11 can be determined by grinding approximately t / 2 mm (t is the thickness of the steel plate 11) of the surface layer of the steel plate 11 and measuring the composition of the resulting ground surface by combustion (infrared absorption), spectroscopic analysis, or X-ray fluorescence analysis. Note that the carbon content inside the base material portion 111 of the steel plate 11 and the carbon content at the thickness center of the steel plate 11 often coincide with each other.

[0056] The hardness of the outer end 133E of the corona bond portion 133 (i.e., the Vickers hardness of the surface layer of the steel plate 11 at the outer end 133E of the corona bond portion 133) can be determined by the following method. First, the projection weld 13 is cut along a plane that passes through a line connecting the center of the projection weld 13 itself and the centers of the multiple projection welds 13 and is perpendicular to the surface of the steel plate 11. If the projection welds 13 are provided at rotationally symmetric positions in a plan view along the direction perpendicular to the surface of the steel plate 11, the projection weld 13 is cut along a plane that passes through a line connecting the center of the projection weld 13 itself and the rotation centers of the multiple projection welds 13 and is perpendicular to the surface of the steel plate 11. For example, the cross section passing through the dashed dotted line shown in Figure 3 is a plane that passes through the center of the projection weld 13 itself and the center of the projection welds 13.

[0057] Next, the cut surface is appropriately adjusted to identify the outer end 133E of the corona bond portion 133. At the corona bond portion 133, the steel plate 11 and the steel member 12 are solid-state welded together, but there is a small gap between the steel plate 11 and the steel member 12 outside the corona bond portion 133. The end of this gap is the end of the corona bond portion 133. There are two ends of the corona bond portion 133 on the cut surface, and of these, the end farthest from the center of the steel member 12 is the outer end 133E of the corona bond portion 133.

[0058] Hardness is measured by a Vickers test. Therefore, hardness refers to Vickers hardness. The Vickers test is performed in accordance with JIS Z 2244:2009. The measurement depth is 20 μm. In other words, the Vickers test is performed so that the center of the indentation D is formed at a depth of 20 μm from the surface.

[0059] The hardness of the steel plate 11 outside the corona bond portion 133 at the 1 / 4 plate thickness position is also measured in accordance with JIS Z 2244:2009. The measurement surface is a cross section that passes through a line connecting the centers of the multiple projection welds 13 and the centers of the multiple projection welds 13 and is perpendicular to the surface of the steel plate 11. The 1 / 4 plate thickness position is a position that is 1 / 4 of the thickness of the steel plate from the surface of the steel plate. Therefore, the measurement position is a position that is approximately 1 / 4 of the thickness t of the steel plate 11 from the surface of the steel plate 11. The measurement position is also outside the projection welds 13.

[0060] The softened HAZ outside the hardened HAZ may not be clearly visible even after etching. However, since the width of the softened HAZ is generally 2 to 3 mm, the hardness of the base metal can be reliably measured by measuring the Vickers hardness at a position 1 / 4 of the plate thickness at a sufficient distance (for example, 5 mm or more) from the outer edge of the HAZ 132 that can be seen by etching.

[0061] When measuring the Vickers hardness at the surface layer and at the 1 / 4 position of the plate thickness, the test force is set to 10 gf, which makes it possible to compare the measured values ​​of both.

[0062] The presence or absence of a softened layer can be easily determined by analyzing the carbon concentration in the surface layer and the interior of the steel sheet 11 and comparing them. The depth of the softened layer in the steel sheet 11 is determined based on the Vickers hardness distribution in the depth direction of the steel sheet 11. Vickers hardness measurements are performed continuously from the surface of the steel sheet 11 toward the interior. If the steel sheet 11 has a softened layer, the deeper the measurement point, the greater the Vickers hardness. In the surface layer of the steel sheet 11, a region where the Vickers hardness is 5% or more lower than at the 1 / 4 position of the steel sheet 11 thickness is considered to be a softened layer, and the thickness of this region is considered to be the softened layer depth. In other words, in the surface layer of the steel sheet 11, a region where the Vickers hardness is 95% or less of the 1 / 4 position of the steel sheet thickness is considered to be a softened layer, and the thickness of this region is considered to be the softened layer depth.

[0063] The cross-sectional area of ​​the molten solidified portion 131 is determined by cross-sectional observation. The measurement plane for the cross-sectional area is a cross section that passes through a line connecting the center of each of the multiple projection welds 13 and the center of rotation of the multiple projection welds 13 and is perpendicular to the surface of the steel plate 11. The cross section is subjected to metal flow corrosion to reveal the molten solidified portion 131. The region where dendrites are observed is considered to be the molten solidified portion 131. A microphotograph of this molten solidified portion 131 is taken, and the cross-sectional area of ​​the molten solidified portion 131 is determined by analyzing the photograph.

[0064] (Automotive parts) Next, an automotive part according to this embodiment will be described below. The automotive part according to this embodiment includes the projection welded joint 1 according to the embodiment described above. As a result, the automotive part according to this embodiment is made of steel members and a steel plate with an internal (thickness center) carbon content of 0.25% by mass or more, and has excellent resistance to hydrogen embrittlement cracking. Examples of automotive parts include side sills, kick reinforcements, B-pillars, B-pillar hinge reinforcements (including patches), B-pillar lowers, A-pillar lowers, and bumper reinforcements.

[0065] (Manufacturing method for projection welded joints) Next, a method for manufacturing a projection-welded joint 1 according to this embodiment will be described. The method for manufacturing a projection-welded joint 1 according to this embodiment includes pressing the protruding portion of a steel member 12 having a protrusion against a steel plate 11 and passing current through the protruding portion to perform projection welding, wherein the carbon content at the center of the thickness of the steel plate 11 is 0.25 mass % or more, the steel plate 11 has a softened layer in a range of 20 μm deep from the surface on the side against which the protruding portion is pressed, and the hardness of the softened layer is 95% or less of the hardness at a position 1 / 4 of the plate thickness of the steel plate 11. As a result, the manufacturing method according to this embodiment can suppress hydrogen embrittlement cracking in the projection weld 13.

[0066] The method for making the hardness of the outer end 133E of the corona bond portion 133 (i.e., the Vickers hardness of the surface layer portion of the steel sheet 11 at the outer end 133E of the corona bond portion 133) lower than the hardness at the 1 / 4 position of the sheet thickness of the steel sheet 11 outside the corona bond portion 133 (or the Vickers hardness at the 1 / 4 position of the sheet thickness of the base material portion 111 of the steel sheet 11) is not particularly limited. For example, it is preferable to provide a softened layer on the surface of the steel sheet 11. In this softened layer, it is preferable that the depth of a region having a hardness that is 5% or more lower than the Vickers hardness at the 1 / 4 position of the sheet thickness of the steel sheet 11 is 5 to 200 μm. Furthermore, it is preferable that the steel sheet 11 has a softened layer within a range of 20 μm deep from the surface on the side where the protrusion is pressed, and the hardness of the softened layer is 95% or less of the hardness at the 1 / 4 position of the sheet thickness of the steel sheet 11. More preferably, the hardness of the steel plate 11 is 95% of that at the 1 / 4 plate thickness position at a location within a depth range of 20 μm to 200 μm from the surface on the side of the steel plate 11 against which the protrusion is pressed. The more preferred embodiments of the softened layer exemplified for projection welded joints can also be applied to the manufacturing method of projection welded joints.

[0067] The projection welding conditions are as follows: The pressure is preferably 200 to 600 kgf, for example. More preferably, the lower limit of the pressure is 300 kgf. Even more preferably, the upper limit of the pressure is 500 kgf. The welding current is preferably 8 to 15 kA, for example. Even more preferably, the lower limit of the welding current is 10 kA. The current application time is preferably 100 to 300 msec, for example. Even more preferably, the lower limit of the current application time is 120 msec (6 cycles when the AC power supply frequency is 50 Hz). Even more preferably, the upper limit of the current application time is 300 msec (15 cycles when the AC power supply frequency is 50 Hz).

[0068] There are no particular limitations on the configurations of the steel members 12 and the steel plates 11. For example, the configuration of the projection welded joint 1 according to the present embodiment described above can be applied to the manufacturing method according to the present embodiment. [Example]

[0069] The effects of one embodiment of the present disclosure will be explained in more detail using examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present disclosure. The present disclosure is not limited to this example of conditions. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the present disclosure and the object of the present disclosure is achieved.

[0070] Various projection welded joints were produced by projection welding steel members to various steel plates.

[0071] Before projection welding, the steel sheets were heated to 900°C and held there for 4 minutes, followed by hot stamping with a bottom dead center holding time of 1 minute. For unplated steel sheets, scale was removed by shot blasting. The thickness, carbon content at the center of the steel sheet thickness, surface treatment method, and softened layer depth of the steel sheets were as shown in Table 1. The softened layer depth listed in Table 1 is the depth of the region with a Vickers hardness 5% or more lower than the Vickers hardness at 1 / 4 of the base material thickness. The steel member was an M8 square nut with four projections for projection welding. The carbon content at the center of the thickness of each steel sheet was the same as the carbon content inside the base material of each steel sheet.

[0072] The welding conditions were as follows: One cycle was 1 / 60 seconds. Pressure: 300kgf Current duration: 8 cycles Welding current: 13kA Holding time: 30 cycles

[0073] After the projection welding was completed, the hardness of the outer edge of the corona bond (i.e., the Vickers hardness of the surface layer of the steel sheet at the outer edge of the corona bond) and the hardness at a position 1 / 4 of the plate thickness of the steel sheet outside the corona bond (i.e., the Vickers hardness at a position 1 / 4 of the plate thickness of the base material of the steel sheet) were measured at a measurement load of 10 gf in the cross section of the four welds, and the absolute value of the difference between these hardnesses, ΔHv, was calculated. The hardness measurement results for the welds where the Vickers hardness of the surface layer of the steel sheet at the outer edge of the corona bond was greatest are listed in Table 1. Note that if the Vickers hardness of the surface layer of the steel sheet at the outer edge of the corona bond was greater than the Vickers hardness at a position 1 / 4 of the plate thickness of the base material of the steel sheet, ΔHv was omitted.

[0074] Furthermore, the cross-sectional area of ​​the molten solidified portion was measured in the cross section of the four welds, and the ratio Amax / Amin of the maximum cross-sectional area Amax to the minimum cross-sectional area Amin was calculated and shown in Table 1. Amax can be regarded as the cross-sectional area of ​​the first molten solidified portion in the cross section along the plate thickness direction of the steel plate, and Amin can be regarded as the cross-sectional area of ​​the second molten solidified portion in the cross section along the plate thickness direction of the steel plate.

[0075] Furthermore, the cross sections of the projection welds were checked for the presence or absence of hydrogen cracking. The number of projection welds in which cracks occurred is recorded in the "Number of cracks" column of Table 1. Projection welded joints with one or more cracks were recorded as "Fail" in the "Hydrogen embrittlement cracking" column of Table 1, and projection welded joints with zero cracks were recorded as "Pass" in the "Hydrogen embrittlement cracking" column of Table 1.

[0076] [Table 1]

[0077] Examples 1 to 5 are comparative examples in which the Vickers hardness of the surface layer of the steel plate at the outer end of the corona bond was higher than the Vickers hardness of the base material of the steel plate at 1 / 4 of the plate thickness. In these examples, hydrogen embrittlement cracking occurred in one or more welds.

[0078] On the other hand, in Examples 6 to 19, the Vickers hardness of the surface layer of the steel sheet at the outer end of the corona bond was lower than the Vickers hardness of the base material of the steel sheet at 1 / 4 of the sheet thickness. In these examples, hydrogen embrittlement was suppressed in all welds. [Explanation of symbols]

[0079] 1 Projection welded joint 11 Steel plate 111 Base material part 12 Steel parts 13 Projection welding section (welding section) 131 Melted and solidified part 132 Heat-affected zone 133 Corona bond part (pressure contact surface) 133E Outer end of corona bond part (pressure contact surface) D recess

Claims

1. a steel plate, a steel member, and a projection weld that joins the steel plate and the steel member; The projection weld portion includes a molten solidification portion and a corona bond portion around the molten solidification portion, The carbon content at the center of the thickness of the steel plate is 0.25% by mass or more, the hardness of the outer end of the corona bond portion is lower than the hardness of the steel plate at a ¼ position of the plate thickness outside the corona bond portion; Projection welded joints.

2. The steel plate has a softened layer in a range of 20 μm deep from the surface on the side where the steel member is joined, The hardness of the softened layer is 95% or less of the hardness at the 1 / 4 plate thickness position of the steel plate. The projection weld joint of claim 1.

3. A location within a range of 20 μm to 200 μm deep from the surface of the steel plate on the side where the steel member is joined has a location that is 95% of the hardness at the 1 / 4 position of the plate thickness of the steel plate. The projection weld joint of claim 2.

4. The Vickers hardness of the outer end of the corona bond portion is Hv700 or less. The projection weld joint according to any one of claims 1 to 3.

5. There are multiple molten solidified portions, and the cross-sectional area of ​​the first molten solidified portion in the cross section along the thickness direction of the steel plate is 1.5 times or more the cross-sectional area of ​​the second molten solidified portion in the cross section along the thickness direction of the steel plate. The projection weld joint according to any one of claims 1 to 3.

6. There are multiple molten solidified portions, and the cross-sectional area of ​​the first molten solidified portion in the cross section along the thickness direction of the steel plate is 1.5 times or more the cross-sectional area of ​​the second molten solidified portion in the cross section along the thickness direction of the steel plate. The projection weld joint of claim 4.

7. Pressing the protrusion of the steel member having the protrusion against the steel plate; and applying current to the protrusions to perform projection welding; Equipped with The carbon content at the center of the thickness of the steel plate is 0.25% by mass or more, The steel plate has a softened layer in a range of 20 μm deep from the surface on the side where the protrusion is pressed, The hardness of the softened layer is 95% or less of the hardness at a 1 / 4 position of the plate thickness of the steel plate. A method for manufacturing the projection weld joint of claim 1.

8. 8. The method for manufacturing a projection welded joint according to claim 7, wherein a location within a depth range of 20 μm to 200 μm from the surface of the steel plate on the side against which the protrusion is pressed has a location where the hardness is 95% of that at a position 1 / 4 of the plate thickness of the steel plate.

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