Projection welded joint and method for manufacturing projection welded joint

Abstract

The invention relates to a projection welded joint and a method for manufacturing the projection welded joint. A projection-welded joint according to one embodiment of the present invention is provided with a steel plate, a steel member, and a projection-welded section for joining the steel plate and the steel member, the projection-welded section being provided with a melt-solidified section and a plastic metal ring section positioned around the melt-solidified section, and the carbon content at the plate thickness center of the steel plate being 0.25 mass% or more. The hardness of the outer end portion of the plastic metal ring portion is lower than the hardness of the steel plate at a position of 1 / 4 plate thickness outside the plastic metal ring portion. It is preferable that the steel sheet has a softening layer on the surface on the side to which the steel member is joined to a depth of 20 [mu] m, and that the hardness of the softening layer is 95% or less of the hardness of the steel sheet at a position 1 / 4 of the sheet thickness.

Description

Technical Field

[0001] The present invention relates to a projection welding joint and a method for manufacturing the projection welding joint.

[0002] This application claims priority based on Tokugan Application No. 2022-089552 filed in Japan on June 1, 2022, the contents of which are incorporated herein by reference. Background Art

[0003] In recent years, in the automotive field, in order to improve the weight reduction and collision safety of the vehicle body for the purpose of reducing fuel consumption and carbon dioxide emissions, the demand for the use of high-strength steel plates in vehicle bodies and components is increasing. For example, in automotive structural parts such as front longitudinal beams, center pillars, and hinge reinforcements, a structure in which steel parts such as nuts are welded to components formed of high-strength steel plates is adopted.

[0004] When manufacturing automotive structural parts that are joined to steel parts, a method of joining nuts to the surface of high-strength steel plates using projection welding is generally adopted. Projection welding refers to resistance welding that is performed by bringing a protrusion formed at a welding portion of a base material on one side into contact with a welding portion of a base material on the other side and passing an electric current, thereby limiting the generation of resistance heat to a relatively small specific portion. When projection welding a nut to a steel plate, a nut provided with a plurality of protrusions is pressed against the steel plate, and electricity is applied to these protrusions, thereby locally heating the contact interface between each protrusion and the steel plate, causing them to melt and solidify or solid-phase join.

[0005] However, in such projection welds made of high-strength steel sheets, hydrogen embrittlement at the weld becomes a problem. The higher the strength of the steel, the more pronounced the adverse effects of hydrogen embrittlement become.

[0006] Regarding projection welding, for example, Patent Document 1 discloses a method for producing a sintered component for welding, which is composed of an iron-carbon alloy and is used for welding. The method is characterized in that after powder molding and before sintering, a portion of the powder compact to be a welded portion is locally heated to perform decarburization.

[0007] Patent Document 2 discloses a high-strength drive shaft for power transmission, in which a counterweight is joined to a portion of a hollow shaft constituting the drive shaft by projection welding, and the hollow shaft has a tensile strength of 70 kgf / mm 2 In the above steel pipe, the ratio (d / D) of the diameter d of the joint portion between the steel pipe and the counterweight to the diameter D of the protruding portion is 0.6 to 1.0.

[0008] Patent document 3 discloses a projection welding method, characterized in that when welding at least one aluminum-plated component to another component, the welding process is divided into three stages: a first stage with a small current, a second stage with a large current, and a third stage with a medium current.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent Application Laid-Open No. 6-49504

[0012] Patent Document 2: Japanese Patent Application Laid-Open No. 7-317844

[0013] Patent Document 3: Japanese Patent Application Publication No. 2004-42089 Summary of the invention

[0014] Problems to be solved by the invention

[0015] Patent Documents 1 to 3 propose methods of preventing hydrogen embrittlement cracking by studying the shape or composition of the protrusion of a steel part or studying welding conditions. However, these methods have problems such as increased tact time and restrictions on nut shape and welding conditions.

[0016] In addition, the steel parts to be projection welded generally have multiple protrusions. During projection welding, the steel parts are pressed toward the steel plate while electricity is applied while the protrusions are in contact with the surface of the steel plate. By applying electricity to multiple protrusions and the areas where they are in contact at a higher current density, multiple protrusions and their surroundings are heated simultaneously to form a molten solidified portion. Due to this situation, it is difficult to prevent hydrogen embrittlement cracks by optimizing the welding conditions. This is because it is difficult to make the pressure and input heat of multiple protrusions strictly consistent. Even if the optimal welding conditions are met in some protrusions, hydrogen embrittlement cracks cannot be suppressed in the projection weld joint when the welding conditions are inappropriate in other protrusions. In addition, when considering dimensional errors, it is also difficult to make the shapes of multiple protrusions of the steel parts completely consistent. Due to this situation, it is also difficult to suppress hydrogen embrittlement cracks.

[0017] In addition, various methods for suppressing hydrogen embrittlement have been proposed in the technical field of spot welding. Spot welding, like projection welding, is a type of resistance welding. However, it is difficult to transfer the method for suppressing hydrogen embrittlement in spot welding to projection welding. This is because the phenomenon of heating by electricity in spot welding is completely different from that in projection welding. In spot welding, two or more overlapping metal plates are clamped by rod-shaped electrodes while being heated by electricity. At this time, the metal plates serving as the base material are in surface contact. Therefore, the shape of the current path in spot welding is completely different from that in projection welding. In addition, in spot welding, it is not necessary to strictly make the applied pressure and input heat of multiple welding parts consistent. Therefore, the above-mentioned issues in projection welding do not exist in spot welding. Due to this situation, it is also difficult to transfer the method for suppressing hydrogen embrittlement in spot welding to projection welding.

[0018] In view of the above, an object of the present invention is to provide a projection welded joint comprising a steel component and a steel plate having an internal carbon content of 0.25 mass % or more, and capable of suppressing hydrogen embrittlement of a plurality of welded portions, and a method for producing the projection welded joint.

[0019] Means for solving problems

[0020] The gist of the present invention is as follows.

[0021] (1) A projection weld joint according to one embodiment of the present invention comprises a steel plate, a steel component, and a projection weld portion for joining the steel plate and the steel component, wherein the projection weld portion comprises a molten solidification portion and a plastic metal ring portion located around the molten solidification portion, the carbon content at the center of the plate thickness of the steel plate is greater than 0.25 mass %, and the hardness of the outer end portion of the plastic metal ring portion is lower than the hardness at a position of 1 / 4 of the plate thickness of the steel plate outside the plastic metal ring portion.

[0022] (2) Preferably, in the projection weld joint of (1) above, a softened layer is provided on the steel plate within a range of 20 μm from the surface of the side to which the steel component is joined, and the hardness of the softened layer is less than 95% of the hardness of the steel plate at a position of 1 / 4 of the plate thickness.

[0023] (3) Preferably, in the projection weld of (2) above, there is a portion of the steel plate having a hardness of 95% at a position of 1 / 4 of the plate thickness within a range of 20 μm to 200 μm from the surface of the side to which the steel component is joined.

[0024] (4) Preferably, in the projection weld joint according to any one of (1) to (3), the Vickers hardness of the outer end portion of the plastic metal ring portion is Hv700 or less.

[0025] (5) Preferably, in any one of the projection welds (1) to (4) above, the molten solidified portions have a plurality of 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 at least 1.5 times the cross-sectional area of ​​the second molten solidified portion in the cross section along the thickness direction of the steel plate.

[0026] (6) Another embodiment of the present invention is a method for manufacturing a projection weld joint, which is a method for manufacturing a projection weld joint according to any one of the above (1) to (5), comprising: pressing the projection of a steel component having a projection against a steel plate; and performing projection welding by applying electricity to the projection, wherein the carbon content at the center of the thickness of the steel plate is greater than 0.25% by mass, and a softening layer is provided on the steel plate within a range from the surface of the side where the projection is pressed to a depth of 20 μm, and the hardness of the softening layer is less than 95% of the hardness at a position 1 / 4 of the thickness of the steel plate.

[0027] (7) Preferably, in the method for manufacturing a projection weld joint of the above-mentioned (6), a portion of the steel plate having a hardness of 95% of the above-mentioned hardness at a position of 1 / 4 of the plate thickness is located within a range of 20 μm to 200 μm from the surface of the side where the above-mentioned protrusion is pressed.

[0028] (Supplementary Items)

[0029] (1) A projection weld joint according to one embodiment of the present invention comprises: a steel plate; a steel component joined to the surface of the steel plate; and a plurality of welds arranged at rotationally symmetrical positions in a plan view along a direction perpendicular to the surface of the steel plate, for joining the steel plate to the steel component, the welds comprising a molten solidified portion, and a crimping surface and a heat-affected portion formed around the molten solidified portion, the carbon content of the outside of the heat-affected portion of the steel plate, i.e., the inside of a parent material portion, is 0.25% by mass or more, and in all of a plurality of cross sections passing through a line connecting the centers of each of the plurality of welds and the rotation centers of the plurality of welds and perpendicular to the surface of the steel plate, the Vickers hardness of the surface portion of the steel plate at the outer end of the crimping surface is lower than the Vickers hardness of a portion having a thickness of 1 / 4 of the parent material portion of the steel plate.

[0030] (2) In the projection weld joint described in (1) above, the base material portion of the steel plate may have a decarburized layer on the surface of the side to which the steel component is joined, and in the decarburized layer, a region 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 has a depth of 5 to 200 μm.

[0031] (3) In the projection weld joint described in (1) or (2) above, in all of the plurality of cross sections, the Vickers hardness of the surface portion of the steel plate at the outer end portion of the press-welded surface may be HV700 or less.

[0032] (4) In the projection weld joint described in any one of the above (1) to (3), a ratio Amax / Amin of a maximum value Amax of the cross-sectional areas of the plurality of molten solidified portions measured in each of the plurality of cross sections to a minimum value Amin of the cross-sectional areas of the plurality of molten solidified portions may be greater than 1.5.

[0033] (5) In the projection weld joint described in any one of (1) to (4) above, the steel plate may be a hot-pressed steel plate.

[0034] (6) In the projection weld joint described in any one of (1) to (5) above, the steel member may be a nut.

[0035] (7) An automobile component according to another aspect of the present invention includes the projection weld joint described in any one of (1) to (6).

[0036] (8) Another embodiment of the present invention is a method for manufacturing a projection welded joint, comprising the step of forming a plurality of welds by projection welding a plurality of projection welding protrusions of a steel component on the surface of a steel plate, wherein during the projection welding, the protrusions are arranged at rotationally symmetrical positions in a plan view along a direction perpendicular to the surface of the steel plate, the welds have a molten solidification portion, and a crimping surface and a heat-affected portion around the molten solidification portion, the carbon content inside the steel plate is greater than 0.25% by mass, and in all of the plurality of cross-sections perpendicular to the surface of the steel plate and connecting the centers of each of the plurality of welds with the rotation centers of the plurality of welds, the Vickers hardness of the surface portion of the steel plate at the outer end of the crimping surface is lower than the Vickers hardness of the outside of the heat-affected portion of the steel plate, i.e., the 1 / 4 portion of the base material portion of the plate thickness.

[0037] (9) In the method for manufacturing a projection weld joint described in (8) above, the steel plate may be a steel plate having a decarburized layer on the surface, and in the decarburized layer, a region having a hardness that is 5% or more lower than the Vickers hardness of 1 / 4 of the thickness of the steel plate has a depth of 5 to 200 μm.

[0038] Effects of the Invention

[0039] According to the present invention, a projection welded joint including a steel member and a steel plate having an internal carbon content of 0.25 mass % or more and capable of suppressing hydrogen embrittlement of a plurality of welded portions, and a method for manufacturing the projection welded joint can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a cross-sectional view of a projection welding joint according to the present embodiment.

[0041] Figure 2 It is an enlarged cross-sectional view of a weld portion of a projection welding joint according to the present embodiment.

[0042] Figure 3 It is a top view of the projection welding joint according to the present embodiment.

[0043] Figure 4 is a schematic diagram of cracks generated in a conventional projection weld joint. DETAILED DESCRIPTION

[0044] like Figure 1 to Figure 3As shown, a projection weld joint 1 of one embodiment of the present invention comprises a steel plate 11, a steel component 12, and a projection weld portion 13 for joining the steel plate 11 and the steel component 12, the projection weld portion 13 comprises a molten solidification portion 131, and a plastic metal ring portion 133 located around the molten solidification portion 131, the carbon content at the center of the plate thickness of the steel plate 11 is greater than 0.25 mass%, and the hardness of the outer end portion 133E of the plastic metal ring portion 133 is lower than the hardness of the 1 / 4 position of the plate thickness of the steel plate 11 outside the plastic metal ring portion 133.

[0045] First, the technical concept of the projection welding joint 1 according to the present embodiment will be described below.

[0046] The present inventors have conducted a detailed investigation into the failure mode of projection welds that have hydrogen embrittlement cracks. Figure 4 As shown, the crack C of the weld portion is often generated starting from the end of the plastic metal ring portion 133 (pressing surface) formed around the molten solidified portion 131 or its vicinity. In addition, it was also found that when the projection weld joint is viewed from above in a direction perpendicular to the surface of the steel plate, cracks are easily generated at the weld end on the outside of the steel part.

[0047] It is generally believed that the main factors affecting hydrogen embrittlement of steel are the strength of steel, stress applied to steel, and hydrogen intruding into steel. Therefore, the present inventors tried to reduce the surface hardness of the steel plate 11 at the outer end 133E of the plastic metal ring portion 133.

[0048] Here, if Figure 2 As shown in the cross-sectional view of the weld portion, the outer end 133E of the plastic metal ring portion 133 is the end of the plastic metal ring portion 133, which is located on the outside of the steel component 12, of the two ends. In addition, the cross section of the outer end 133E is determined by a line connecting the center of each of the plurality of projection welds 13 and the center of the plurality of projection welds 13 and is perpendicular to the surface of the steel plate 11. In the case where these projection welds 13 are arranged at rotationally symmetrical positions, the center of the plurality of projection welds 13 refers to the center of rotation. In the case where these projection welds 13 are arranged at non-rotationally symmetrical positions, the center of the plurality of projection welds 13 refers to the center of the fastening component possessed by the steel component 12. For example, in the case where the steel component 12 has a nut shape, the center of the through hole of the nut is regarded as the center of the plurality of projection welds 13. In the case where the steel component 12 has a bolt shape, the central axis of the bolt is regarded as the center of the plurality of projection welds 13. The line connecting the center of the projection weld 13 itself and the center of the plurality of projection welds 13, for example, is Figure 3The dot-dash line recorded in the top view of the steel plate 11. In addition, the center of each of the plurality of projection welds 13 and the center of the plurality of projection welds 13 are determined in the top view along the direction perpendicular to the surface of the steel plate 11. Hereinafter, unless otherwise specified, "a cross section perpendicular to the surface of the steel plate and passing through a line connecting the center of each of the plurality of projection welds and the center of the plurality of projection welds" is simply recorded as "a cross section". The outer end 133E defined as above is the portion farthest from the center of the plurality of projection welds 13.

[0049] As a result of attempting to reduce the surface hardness of the steel plate 11 at the outer end 133E of the plastic metal ring portion 133, the inventors were able to significantly suppress the frequency of hydrogen embrittlement cracks. According to this method, there is no increase in production cycle time, no restrictions on the nut shape and welding conditions, etc. In addition, in this method, it is not necessary to soften the inside of the steel plate 11, so the tensile strength of the steel plate 11 can be ensured at a high level. Moreover, according to this method, it is not necessary to make the welding conditions of the plurality of projection welds 13 uniform. For example, even if the states of the plurality of projection welds 13 differ greatly, in the projection weld joint 1 of the present embodiment, hydrogen embrittlement cracks can be suppressed in all of the plurality of projection welds 13.

[0050] Next, a specific configuration of the projection welding joint 1 of the present embodiment will be described. The projection welding joint 1 includes 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 .

[0051] The projection weld 13 has a molten solidification portion 131, a plastic metal ring portion 133 formed around the molten solidification portion 131, and a heat-affected zone 132. The molten solidification portion 131 is a portion where the steel plate 11 and the steel component 12 are temporarily melted and then solidified due to projection welding. The heat-affected zone 132 (Heat Affected Zone, HAZ) refers to the portion of the base material that is not melted in the steel plate 11 and the steel component 12, although the structure, metallurgical properties and mechanical properties change due to welding heat. In addition, the plastic metal ring portion 133 refers to the solid-phase bonding portion between the steel component 12 and the steel plate 11 formed around the molten solidification portion 131. The plastic metal ring portion 133 is the same solid-phase bonding portion as the plastic metal ring formed in an annular shape around the nugget in the spot weld. The plastic metal ring portion 133 is sometimes also referred to as a crimping portion or a crimping surface.

[0052] In the present embodiment, the portion of the steel plate 11 located outside the heat-affected zone 132 is referred to as the base material portion 111 of the steel plate 11. In addition, the base material portion 111 of the steel plate 11 may be simply referred to as “base material portion 111”.

[0053] (Steel Plate 11)

[0054] The steel plate 11 is a steel plate having a carbon content of 0.25 mass % or more at the center of the plate thickness or a steel plate having a carbon content of 0.25 mass % or more inside the base material portion 111. Such a steel plate can improve the collision resistance of the projection weld joint 1 and the automotive component to which the projection weld joint 1 is applied. The carbon content at the center of the plate thickness of the steel plate 11 or the carbon content inside the base material portion 111 of the steel plate 11 may be 0.27 mass % or more, 0.30 mass % or more, more than 0.30 mass %, 0.31 mass % or more, 0.32 mass % or more, 0.33 mass % or more, 0.34 mass % or more, or 0.35 mass % or more.

[0055] Those skilled in the art will recognize that a steel plate having a carbon content exceeding 0.30 mass % is very prone to cracking during projection welding. However, in the projection welding joint 1 of the present embodiment, hydrogen embrittlement of the weld is suppressed. Therefore, in the projection welding joint 1 of the present embodiment, the carbon content of the steel plate 11 can be increased to exceed 0.30 mass %.

[0056] On the other hand, the carbon content in the surface layer of the steel plate 11 is not particularly limited. As described later, a softened layer may be provided on the surface of the steel plate 11. The other structures of the steel plate 11 are not particularly limited, but preferred examples are given below.

[0057] The chemical composition of the steel plate 11 other than the carbon content at the center of the plate thickness (or inside the parent material portion 111) is not particularly limited. For example, as chemical components other than carbon at the center of the plate thickness (or inside the parent material portion 111) of the steel plate 11, Si: 2.00% or less, Mn: 0.05-5.00%, P: 0.100% or less, S: 0.0100% or less may be contained in terms of unit mass%. In this case, the remainder of the chemical composition contains Fe and impurities.

[0058] 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. In addition, the steel plate 11 may be a hot-pressed steel plate, that is, a steel plate (steel component) manufactured by hot pressing. Alternatively, the steel plate 11 may be a steel plate for hot pressing before hot pressing. In the projection weld joint 1 of the present embodiment, the "steel plate" is a concept that includes not only steel that is in the form of a plate as a whole, but also steel that is only partially in the form of a plate, such as a component having a flange portion.

[0059] The steel plate 11 may have a coating on its surface. The coating may be, for example, hot-dip galvanizing, alloyed hot-dip galvanizing, or aluminum plating. This can improve the corrosion resistance of the projection welded joint 1.

[0060] The thickness of the steel plate 11 is not particularly limited. For example, when the projection weld joint 1 is used as an automotive component, the thickness of the steel plate 11 can be within the range of 0.5 to 3.0 mm. In general, the thicker the steel plate 11, the greater the stress applied to the projection weld portion 13 during the cooling process after projection welding, and the easier it is to generate hydrogen embrittlement cracks. However, in the projection weld joint of the present embodiment, as described later, hydrogen embrittlement cracks can be suppressed by softening the outer end 133E of the plastic metal ring portion 133. In addition, for example, projection welding can be performed on two or more overlapping steel plates. In this case, hydrogen embrittlement cracks can be suppressed by softening the outer end 133E of the plastic metal ring portion 133 of the steel plate in contact with the steel component.

[0061] like Figure 1 As shown in the example, a through hole may be provided in the steel plate 11. When the steel member 12 is a nut, the bolt is attached to the projection weld joint 1 so as to penetrate the projection weld joint 1 by aligning the hole of the nut with the axis of the through hole. On the other hand, the steel plate 11 may be a flat plate.

[0062] (Steel component 12)

[0063] The steel component 12 is a steel mechanical component that is joined to the surface of the steel plate 11. The steel component 12 has a plurality of protrusions before projection welding. These protrusions are pressed against the steel plate 11 during projection welding and serve as current paths. Furthermore, the protrusions concentrate the current on the contact surfaces between the plurality of protrusions and the steel plate 11, thereby forming a projection weld 13 that joins the steel component 12 to the steel plate 11. In order to make the joining state of the projection weld 13 uniform, it is preferred that the plurality of protrusions of the steel component 12 before projection welding be arranged in rotationally symmetrical positions. Therefore, it is preferred that, as Figure 3 As shown, the projection welds 13 formed on the steel member 12 after projection welding are provided at rotationally symmetrical positions in a plan view along a direction perpendicular to the surface of the steel plate 11. The detailed configuration of the projection welds 13 is as described above.

[0064] In order to reliably join the steel component 12 to the steel plate 11, the number of protrusions is 2 or more. For example, the number of protrusions may be 3, 4, 5, or 6. As described later, from the viewpoint of making the contact state between each of the plurality of protrusions and the steel plate 11 uniform, it is most preferable to set the number of protrusions to 3. Therefore, the number of projection welds 13 may be 2, 3, 4, 5, or 6.

[0065] The shape of the steel component 12 is not particularly limited. For example, when the projection weld joint 1 is used as an automobile component, the steel component 12 is preferably a bolt or a nut. However, various shapes can be applied to the steel component 12 depending on the purpose of the projection weld joint 1. The strength and chemical composition of the steel component 12 are also not particularly limited. For example, the raw material of the steel component 12 can also be S10C or S25C, or SWCH steel, etc.

[0066] (Hardness of the Outer End 133E of the Plastic Metal Ring 133)

[0067] The projection weld 13 between the steel member 12 and the steel plate 11 forms a melt-solidified portion 131 and a heat-affected portion 132. According to various experiments conducted by the inventors, it has been found that Figure 4 As shown in FIG. 1 , hydrogen embrittlement cracks are easily generated on the surface of the steel plate 11 at the outer end 133E of the plastic metal ring portion 133. Figure 2 as well as Figure 3 As shown, the outer end portion 133E of the plastic metal ring portion 133 is an end portion located on the outer side of the steel member 12 of both ends of the plastic metal ring portion 133 in the cross section of the projection weld portion 13 .

[0068] Therefore, in the projection weld joint 1 of the present embodiment, the Vickers hardness of the surface portion of the steel plate 11 at the outer end 133E of the plastic metal ring portion 133 of the steel component 12 and the steel plate 11 measured in the cross section of the projection weld portion 13 is defined as "the hardness of the outer end 133E of the plastic metal ring portion 133". In addition, the hardness of the outer end 133E of the plastic metal ring portion 133 is lower than the hardness of the steel plate at the 1 / 4 position of the plate thickness outside the plastic metal ring portion 133. Preferably, the hardness of the outer end 133E of the plastic metal ring portion 133 is lower than the Vickers hardness of the base material portion 111 of the steel plate 11 at the 1 / 4 position of the plate thickness. The higher the strength of the steel, the easier it is to generate hydrogen embrittlement cracks. Therefore, by softening the steel at the position that becomes the starting point of the hydrogen embrittlement cracks, hydrogen embrittlement cracks can be prevented.

[0069] Furthermore, this requirement is satisfied in all of the plurality of projection welds 13. That is, in all of the cross sections of the plurality of projection welds 13, the hardness of the outer end 133E of the plastic metal ring portion 133 is lower than the hardness of the 1 / 4 position of the plate thickness of the steel plate 11 outside the plastic metal ring portion 133. Preferably, in all of the cross sections of the plurality of projection welds 13, the hardness of the outer end 133E of the plastic metal ring portion 133 is lower than the Vickers hardness of the 1 / 4 position of the plate thickness of the base material portion 111 of the steel plate 11. Thus, hydrogen embrittlement cracks are suppressed in all of the projection welds 13, and the bonding strength of the steel component 12 can be maintained extremely high.

[0070] The absolute value ΔHv of the difference between the hardness of the outer end 133E of the plastic metal ring portion 133 and the hardness of the steel plate at the 1 / 4 position of the plate thickness outside the plastic metal ring portion 133 is sufficient as long as it exceeds Hv0 in all the projection welds 13. Preferably, ΔHv is greater than Hv10, greater than Hv30, greater than Hv50, or greater than Hv100 in all the projection welds 13. The larger ΔHv is, the better, so there is no need to specify its upper limit, but in order to maintain the strength of the steel plate, for example, ΔHv can also be less than Hv250, less than Hv200, or less than Hv150 in all the projection welds 13. In addition, the above-mentioned ΔHv can also be defined as the absolute value of the difference between the hardness of the outer end 133E of the plastic metal ring portion 133 and the hardness of the steel plate 111 at the 1 / 4 position of the plate thickness.

[0071] In addition, the absolute value of the hardness of the outer end 133E of the plastic metal ring portion 133 may also be specified. Specifically, in all welded portions, the Vickers hardness of the outer end 133E of the plastic metal ring portion 133 may be Hv700 or less, Hv650 or less, or Hv600 or less. Thus, hydrogen embrittlement cracking of the welded portion can be more effectively suppressed.

[0072] The method for softening the outer end 133E of the plastic metal ring portion 133 more than the 1 / 4 position of the plate thickness of the steel plate outside the plastic metal ring portion 133 (or the 1 / 4 portion of the plate thickness of the parent material portion 111 of the steel plate 11) is not particularly limited. For example, by projection welding the steel plate 11 that has been subjected to a surface softening treatment such as a surface decarburization treatment, the difference ΔHv between the Vickers hardness of the outer end 133E of the plastic metal ring portion 133 and the Vickers hardness of the 1 / 4 position of the plate thickness of the steel plate outside the plastic metal ring portion 133 (or the 1 / 4 portion of the plate thickness of the parent material portion 111 of the steel plate 11) can be made to exceed Hv0. Therefore, in the projection welding joint 1 of the present embodiment, the steel plate 11 may also have a softened layer. The softened layer is, for example, a decarburized layer. Hereinafter, in the softened layer, the depth of a region having a hardness lower by 5% or more than the hardness at a position of 1 / 4 of the plate thickness of the steel plate 11 (particularly preferably the base material portion 111 of the steel plate 11) is defined as the softened layer depth. In other words, the hardness of the softened layer is 95% or less of the hardness at a position of 1 / 4 of the plate thickness of the steel plate 11 (particularly preferably the base material portion 111 of the steel plate 11).

[0073] In addition, the softened layer may be provided on both surfaces of the steel plate 11 , or may be provided only on the side of the surface of the steel plate 11 that contacts the steel member 12 .

[0074] In the steel plate 11, it is preferred that a softened layer be present within a range of 20 μm from the surface of the side to which the steel member 12 is joined. For example, the softened layer depth of the steel plate 11 may be, for example, 25 μm or more, 30 μm or more, 40 μm or more, or 50 μm or more. In addition, the softened layer depth of the steel plate 11 may be set to 200 μm or less, 190 μm or less, 80 μm or less, or 150 μm or less.

[0075] Alternatively, a portion having a hardness of 95% of the hardness at a position of 1 / 4 of the plate thickness of the steel plate 11 may exist in a portion within a range of 20 μm to 200 μm from the surface of the side to which the steel member 12 is joined. In this case, the softening layer depth is within a range of 20 μm to 200 μm, and the effect of suppressing hydrogen embrittlement cracking can be further improved.

[0076] In addition, the smaller the hardness of the softening layer, the higher the effect of suppressing hydrogen embrittlement cracking. Therefore, the above-mentioned softening layer depth value can also be applied on the basis of defining the softening layer depth as the depth of the region having a hardness lower than the hardness at the 1 / 4 position of the plate thickness of the parent material part 111 by 8%, 10% or 12%.

[0077] On the other hand, in order to make ΔHv exceed Hv0, a method other than surface softening treatment (such as surface decarburization treatment) may be used. For example, even when only the surface of the steel plate 11 is softened by controlling the number density of precipitates, ΔHv can be made to exceed Hv0. In addition, ΔHv can be made to exceed Hv0 by making the steel plate 11 a multilayer steel plate composed of a combination of mild steel and high-strength steel, or by tempering only the surface of the steel plate.

[0078] In the projection welding joint 1 of the present embodiment, there may be a plurality of projection welding portions 13, and thus there may be a plurality of molten solidified portions 131. Furthermore, the welding state of the plurality of projection welding portions 13 may also be non-uniform. For example, the cross-sectional area of ​​the first molten solidified portion of the cross section along the plate thickness direction of the steel plate 11 may also be 1.5 times or more of the cross-sectional area of ​​the second molten solidified portion of the cross section along the plate thickness direction of the steel plate 11. The first molten solidified portion is any molten solidified portion in the projection welding joint 1 including the plurality of molten solidified portions 131. The second molten solidified portion is any molten solidified portion in the projection welding joint 1 including the plurality of molten solidified portions 131, and is a molten solidified portion having a smaller cross-sectional area than the first molten solidified portion.

[0079] 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 portion 13 to the minimum value Amin of the cross-sectional area of ​​the molten solidified portion 131 may be 1.5 or more.

[0080] Here, the cross-sectional area of ​​the molten solidified portion 131 is the cross-sectional area of ​​the molten solidified portion 131 measured in a plurality of cross sections perpendicular to the surface of the steel plate by a line connecting the center of each of the plurality of welds and the center of the plurality of welds. By measuring the cross-sectional area of ​​the molten solidified portion 131 in a plurality of cross sections, Amax and Amin can be determined. In addition, in the case where the projection weld 13 is arranged at a rotationally symmetrical position 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 a plurality of cross sections perpendicular to the surface of the steel plate 11 by a line connecting the center of each of the plurality of projection welds 13 and the rotation center of the plurality of projection welds 13.

[0081] The cross-sectional area of ​​the molten solidified portion 131 becomes an index of the input heat of the projection weld portion 13. When the cross-sectional area of ​​the first molten solidified portion is the same as that of the second molten solidified portion or when Amax / Amin is 1, it is highly likely that a plurality of projection welds 13 are manufactured under exactly the same welding conditions. However, it is difficult to manufacture a plurality of projection welds 13 under the same conditions. The surface state of the steel plate 11 during projection welding, the strain of the steel plate 11, the deviation of the pressure applied to the steel component 12, the dimensional error of the protrusion of the steel component 12, etc. become interference factors, causing deviations in the states of the plurality of projection welds 13. In this case, residual stress is generated between the plurality of projection welds 13, and this residual stress sometimes causes hydrogen embrittlement cracks.

[0082] From the viewpoint of suppressing hydrogen embrittlement cracks in the weld 13, 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. On the other hand, when the cross-sectional area of ​​the first molten solidified portion is allowed to be more than 1.5 times the cross-sectional area of ​​the second molten solidified portion, projection welding can be easily implemented, and the projection weld joint 1 of the present embodiment can be applied to various components. In addition, in the projection weld joint 1 of the present embodiment, even if the cross-sectional area of ​​the first molten solidified portion is more than 1.5 times the cross-sectional area of ​​the second molten solidified portion, hydrogen embrittlement cracks can be suppressed by making ΔHv exceed Hv0. Therefore, in the projection weld joint 1 of the present embodiment, it is preferred that the cross-sectional area of ​​the first molten solidified portion is more than 1.5 times the cross-sectional area of ​​the second molten solidified portion. The cross-sectional area of ​​the first molten solidified portion may be more than 1.6 times, more than 1.8 times, or more than 2.0 times the cross-sectional area of ​​the second molten solidified portion.

[0083] Similarly, from the viewpoint of suppressing hydrogen embrittlement cracking of the weld 13, the larger the Amax / Amin is, the more disadvantageous it is. On the other hand, when Amax / Amin is allowed to be 1.5 or more, projection welding can be easily performed, and the projection welding joint 1 of the present embodiment can be applied to various components. In addition, in the projection welding joint 1 of the present embodiment, even if Amax / Amin is 1.5 or more, hydrogen embrittlement cracking can be suppressed by making ΔHv exceed Hv0. Therefore, in the projection welding joint 1 of the present embodiment, it is preferred that Amax / Amin is 1.5 or more. Amax / Amin can also be 1.6 or more, 1.8 or more, or 2.0 or more.

[0084] (Methods for measuring carbon content, Vickers hardness, softening layer depth, and cross-sectional area of ​​molten solidified portion)

[0085] By grinding the surface layer of the base material portion 111 of the steel plate 11 by more than 0.3 mm, the composition of the ground surface obtained by the combustion method (infrared absorption method), spectroscopic analysis method or fluorescent X-ray analysis is measured, thereby the carbon content inside the base material portion 111 of the steel plate 11 can be determined. By grinding the surface layer of the steel plate 11 by about t / 2 mm (t is the plate thickness of the steel plate 11), the composition of the ground surface obtained by the combustion method (infrared absorption method), spectroscopic analysis method or fluorescent X-ray analysis is measured, thereby the carbon content at the plate thickness center of the steel plate 11 can be determined. In addition, the carbon content inside the base material portion 111 of the steel plate 11 is often consistent with the carbon content at the plate thickness center of the steel plate 11.

[0086] The hardness of the outer end 133E of the plastic metal ring portion 133 (i.e., the Vickers hardness of the surface portion of the steel plate 11 at the outer end 133E of the plastic metal ring portion 133) can be determined by the following method. First, the projection weld 13 is cut along a plane passing through a line connecting the center of the projection weld 13 itself and the centers of a plurality of projection welds 13 and perpendicular to the surface of the steel plate 11. In a case where the projection weld 13 is arranged at a rotationally symmetrical position in a top view along a direction perpendicular to the surface of the steel plate 11, the projection weld 13 is cut along a plane passing through a line connecting the center of the projection weld 13 itself and the rotation centers of a plurality of projection welds 13 and perpendicular to the surface of the steel plate 11. For example, by Figure 3 The cross section indicated by the dashed line in FIG. 2 is a plane passing through the center of the projection weld 13 itself and the center of the projection weld 13 .

[0087] Next, the cut surface is appropriately adjusted to determine the outer end 133E of the plastic metal ring portion 133. The steel plate 11 and the steel component 12 are solid-phase bonded in the plastic metal ring portion 133, but a small gap exists between the steel plate 11 and the steel component 12 outside the plastic metal ring portion 133. The end of the gap is the end of the plastic metal ring portion 133. There are two ends of the plastic metal ring portion 133 in the cut surface, and the end of one of them that is far from the center of the steel component 12 is the outer end 133E of the plastic metal ring portion 133.

[0088] The hardness is measured by the Vickers test. Therefore, the hardness is the Vickers hardness. The Vickers test is carried out according to JIS Z 2244: 2009. The measurement depth is 20 μm. That is, the Vickers test is carried out in such a way that the center of the pit D is formed at a position of 20 μm in depth from the surface.

[0089] The hardness of the 1 / 4 position of the plate thickness of the steel plate 11 outside the plastic metal ring portion 133 is also measured according to JIS Z2244: 2009. The measurement surface is a cross section that passes through a line connecting the center of each of the plurality of projection welds 13 and the center of the plurality of projection welds 13 and is perpendicular to the surface of the steel plate 11. In addition, the 1 / 4 position of the plate thickness is a depth position of 1 / 4 of the thickness of the steel plate from the surface of the steel plate. Therefore, the measurement position is a depth position of approximately 1 / 4 of the thickness t of the steel plate 11 from the surface of the steel plate 11. In addition, the measurement position is the outside of the projection weld 13.

[0090] In addition, the HAZ softened portion outside the HAZ hardened portion may not be clearly visually identified even after etching. On the other hand, the width of the HAZ softened portion is approximately 2 to 3 mm, so as long as the Vickers hardness at a position of 1 / 4 of the plate thickness is measured at a position sufficiently away (for example, 5 mm or more) from the outer edge of the HAZ132 that can be visually identified by etching, the hardness of the base material portion can be reliably measured.

[0091] When measuring the Vickers hardness of the surface layer and the position of 1 / 4 of the plate thickness, the test force was set to 10 gf. This enables comparison of the two measured values.

[0092] By analyzing the carbon concentration of the surface layer and the inside of the steel plate 11 and comparing the two, the presence or absence of a softening layer can be easily determined. The depth of the softening layer of the steel plate 11 is determined based on the Vickers hardness distribution in the depth direction of the steel plate 11. The Vickers hardness measurement is continuously performed from the surface of the steel plate 11 toward the inside. In the case where the steel plate 11 has a softening layer, the deeper the measuring point, the greater the Vickers hardness. In the surface layer of the steel plate 11, the area where the Vickers hardness is lower than the 1 / 4 position of the plate thickness of the steel plate 11 by more than 5% is regarded as a softening layer, and the thickness of the area is regarded as the depth of the softening layer. That is, in the surface layer of the steel plate 11, the area where the Vickers hardness is less than 95% of the 1 / 4 position of the plate thickness of the steel plate is regarded as a softening layer, and the thickness of the area is regarded as the depth of the softening layer.

[0093] The cross-sectional area of ​​the molten solidified portion 131 is determined by cross-sectional observation. The measurement surface of the cross-sectional area is a cross-sectional area perpendicular to the surface of the steel plate 11 and passing through a line connecting the center of each of the plurality of projection welds 13 and the rotation center of the plurality of projection welds 13. The molten solidified portion 131 is visualized by subjecting the cross section to metal flow corrosion. The region where the dendrite is observed is regarded as the molten solidified portion 131. The cross-sectional area of ​​the molten solidified portion 131 is determined by taking a microscope photograph of the molten solidified portion 131 and analyzing the photograph.

[0094] (Automotive components)

[0095] Next, the automotive component of the present embodiment is described below. The automotive component of the present embodiment has the projection weld joint 1 of the embodiment described above. Thus, the automotive component of the present embodiment is composed of a steel component and a steel plate having a carbon content of 0.25% by mass or more in the interior (center of the plate thickness), and has excellent resistance to hydrogen embrittlement cracking. The automotive component is, for example, a side beam, a recoil reinforcement, a B-pillar, a hinge reinforcement of the B-pillar (including a connector), a B-pillar lower component, an A-pillar lower component, a bumper reinforcement, etc.

[0096] (Production method of projection welding joint)

[0097] Next, the manufacturing method of the projection welding joint 1 of the present embodiment is described. The manufacturing method of the projection welding joint 1 of the present embodiment includes: a step of pressing the projection of the steel component 12 having the projection to the steel plate 11; and a step of applying electricity to the projection to perform projection welding, wherein the carbon content of the center of the plate thickness of the steel plate 11 is 0.25 mass% or more, and there is a softened layer on the steel plate 11 from the surface of the side pressed by the projection to a depth of 20 μm, and the hardness of the softened layer is 95% or less of the hardness at the position of 1 / 4 of the plate thickness of the steel plate 11. As a result, the manufacturing method of the present embodiment can suppress hydrogen embrittlement cracking of the projection welding portion 13.

[0098] The method of making the hardness of the outer end 133E of the plastic metal ring portion 133 (i.e., the Vickers hardness of the surface layer of the steel plate 11 at the outer end 133E of the plastic metal ring portion 133) lower than the hardness of the 1 / 4 position of the plate thickness of the steel plate 11 outside the plastic metal ring portion 133 (or, the Vickers hardness of the 1 / 4 position of the plate thickness of the base material portion 111 of the steel plate 11) is not particularly limited, but for example, it is preferred to provide a softening layer on the surface of the steel plate 11. In the softening layer, the depth of the region having a hardness lower than the Vickers hardness of the 1 / 4 position of the plate thickness of the steel plate 11 by 5% or more is preferably 5 to 200 μm. In addition, it is preferred that a softening layer is provided on the steel plate 11 in the range of 20 μm from the surface of the side pressed by the protrusion, and the hardness of the softening layer is less than 95% of the hardness of the 1 / 4 position of the plate thickness of the steel plate 11. More preferably, the portion of the steel plate 11 within a depth range of 20 μm to 200 μm from the surface of the side pressed by the protrusion has a hardness of 95% of the hardness at a position 1 / 4 of the thickness of the steel plate 11. The more preferred embodiment of the softened layer exemplified for the projection weld can also be applied to the method for manufacturing the projection weld joint.

[0099] The conditions for projection welding are as follows. The pressure is preferably 200 to 600 kgf, for example. More preferably, the lower limit of the pressure is 300 kgf. Still more preferably, the upper limit of the pressure is 500 kgf. The welding current is preferably 8 to 15 kA, for example. More preferably, the lower limit of the welding current is 10 kA. The power-on time is preferably 100 to 300 msec, for example. More preferably, the lower limit of the power-on time is 120 msec (6 cyc when the frequency of the AC power supply is 50 Hz). Still more preferably, the upper limit of the power-on time is 300 msec (15 cyc when the frequency of the AC power supply is 50 Hz).

[0100] The configurations of the steel member 12 and the steel plate 11 are not particularly limited. For example, the configuration of the projection welding joint 1 of the present embodiment described above can be applied to the manufacturing method of the present embodiment.

[0101] Example

[0102] The effect of one embodiment of the present invention is described in more detail by way of example. However, the conditions in the example are only a conditional example adopted to confirm the feasibility and effect of the present invention. The present invention is not limited to this conditional example. The present invention may adopt various conditions as long as it does not depart from the gist of the present invention and achieves the purpose of the present invention.

[0103] Various projection welds are produced by projection welding steel parts onto various steel plates.

[0104] For the steel plate, before projection welding, after heating to 900°C and holding for 4 minutes, hot pressing with a bottom dead center holding time of 1 minute was performed. In the non-plated steel plate, the scale was removed by shot peening. The plate thickness of the steel plate, the carbon content at the center of the plate thickness of the steel plate, the surface treatment method of the steel plate, and the softening layer depth of the steel plate are shown in Table 1. In addition, the softening layer depth recorded in Table 1 is the depth of the area having a hardness that is 5% or more lower than the Vickers hardness at the 1 / 4 position of the plate thickness of the parent material. The steel part is an M8 square nut with 4 protrusions for projection welding. The carbon content at the center of the plate thickness of each steel plate is the same as the carbon content inside the parent material part of each steel plate.

[0105] The welding conditions are as follows: 1 cycle is 1 / 60 seconds.

[0106] Pressure: 300kgf

[0107] Power-on time: 8 cycles

[0108] Welding current: 13kA

[0109] Holding time: 30 cycles

[0110] After the projection welding is completed, the hardness of the outer end of the plastic metal ring (i.e., the Vickers hardness of the surface of the steel plate at the outer end of the plastic metal ring) and the hardness of the steel plate at the 1 / 4 position of the plate thickness outside the plastic metal ring (i.e., the Vickers hardness of the base material of the steel plate at the 1 / 4 position of the plate thickness) are measured in the cross sections of the four welds by measuring a load of 10 gf, and the absolute value ΔHv of the difference between these hardnesses is calculated. Then, the hardness measurement results of the weld where the Vickers hardness of the surface of the steel plate at the outer end of the plastic metal ring becomes the largest are recorded in Table 1. In addition, when the Vickers hardness of the surface of the steel plate at the outer end of the plastic metal ring is greater than the Vickers hardness of the base material of the steel plate at the 1 / 4 position of the plate thickness, the record of ΔHv is omitted.

[0111] Furthermore, the cross-sectional area of ​​the molten solidified portion was measured in the cross sections of the four welds, and the ratio Amax / Amin of the maximum value Amax and the minimum value Amin of the cross-sectional area was calculated and recorded in Table 1. In addition, Amax can be regarded as the cross-sectional area of ​​the first molten solidified portion of 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 of the cross section along the plate thickness direction of the steel plate.

[0112] Furthermore, the presence or absence of hydrogen cracks in the cross section of the projection weld was confirmed. The number of projection welds with cracks is recorded in the "Number of Cracks" column of Table 1. For projection welds with one or more cracks, it is recorded as "unqualified" in the "Hydrogen Embrittlement Cracks" column of Table 1, and for projection welds with zero cracks, it is recorded as "qualified" in the "Hydrogen Embrittlement Cracks" column of Table 1.

[0113] [Table 1]

[0114]

[0115] Examples No. 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 plastic metal ring portion is higher than the Vickers hardness of the base material portion of the steel plate at a position of 1 / 4 of the plate thickness. In these examples, hydrogen embrittlement cracks occurred in one or more welded portions.

[0116] On the other hand, Examples No. 6 to 19 are embodiments of the present invention in which the Vickers hardness of the surface layer of the steel plate at the outer end of the plastic metal ring portion is lower than the Vickers hardness of the base material portion of the steel plate at a position of 1 / 4 of the plate thickness. In these examples, hydrogen embrittlement is suppressed in all welds.

[0117] Explanation of symbols

[0118] 1: projection weld joint; 11: steel plate; 111: base material part; 12: steel component; 13: projection weld part (welding part); 131: molten solidification part; 132: heat-affected part; 133: plastic metal ring part (crimping surface); 133E: outer end part of the plastic metal ring part (crimping surface); D: pit.

Claims

1. A projection welding joint, wherein: The projection weld joint includes a steel plate, a steel component, and a projection weld portion that joins the steel plate and the steel component. The projection welding portion comprises a molten solidification portion and a plastic metal ring portion located around the molten solidification portion. The carbon content of the steel plate at the center of the plate thickness is 0.25 mass % or more. The hardness of the outer end portion of the plastic metal ring portion is lower than the hardness of a position at 1 / 4 of the plate thickness of the steel plate outside the plastic metal ring portion.

2. The projection welding joint according to claim 1, wherein: The steel plate has a softened layer in the range of 20 μm from the surface on the side to which 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.

3. The projection welding joint according to claim 2, wherein: In a portion of the steel plate within a depth range of 20 μm to 200 μm from the surface of the side to which the steel member is joined, there is a portion having 95% of the hardness at a position of 1 / 4 of the plate thickness of the steel plate.

4. The projection welding joint according to any one of claims 1 to 3, wherein: The Vickers hardness of the outer end portion of the plastic metal ring portion is Hv700 or less.

5. The projection welding joint according to any one of claims 1 to 4, wherein: There are a plurality of molten solidified portions, and a cross-sectional area of ​​a first molten solidified portion in a cross section along the thickness direction of the steel plate is 1.5 times or more of a cross-sectional area of ​​a second molten solidified portion in a cross section along the thickness direction of the steel plate.

6. A method for manufacturing a projection welding joint, which is the method for manufacturing a projection welding joint according to claim 1, comprising: pressing the protrusion of the steel member having the protrusion against the steel plate; as well as The protrusion is energized to perform projection welding. The carbon content of the steel plate at the center of the plate thickness is 0.25 mass % or more. The steel plate has a softened layer in the range of 20 μm from the surface of the side pressed by the protrusion. The hardness of the softened layer is 95% or less of the hardness at a position of 1 / 4 of the plate thickness of the steel plate.

7. The method for manufacturing a projection welding joint according to claim 6, wherein: The steel plate has a portion having a hardness of 95% of that at a position of 1 / 4 of the plate thickness within a range of 20 μm to 200 μm in depth from the surface of the side pressed by the protrusion.

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