Projection welding joint and method for manufacturing the same

Abstract

Provided are a projection welded joint having improved peel strength, and a manufacturing method therefor. A projection welded joint according to the present invention has a steel sheet and a nut. The steel sheet has a predetermined component composition. The minimum hardness Hvb (HV) of a region b and the base material hardness Hvm (HV) of the steel sheet satisfy Hvb / Hvm≤0.90. The region b includes the center line of the nut of the projection welded joint, and is defined as follows in a cross section passing through the center of the joint between the steel sheet and the nut. An end point on the inner circumferential surface side of the nut in contact with the steel sheet is defined as A; an end point on the outer circumferential surface side is defined as B; a point, inside the steel sheet, that is moved 0.50 mm in the plate thickness direction of the steel sheet from said point B is defined as C; a point, inside the steel sheet, that is moved 0.50 mm in the plate thickness direction of the steel sheet from said point A is defined as D; and the inside of a rectangle formed by connecting said point A, point B, point C, and point D is defined as the region b.

Description

[Technical Field]

[0001] The present invention relates to a projection welded joint comprising a steel plate and a nut having a projection, which are projection welded together via the projection, and a method for manufacturing the same. [Background technology]

[0002] In recent years, various types of high-strength steel sheets (high-tensile steel) have been increasingly used in automobile bodies to improve fuel efficiency through weight reduction and to ensure collision safety. Furthermore, when resistance spot welding cannot be used in the automobile assembly process, arc welding or bolt fastening is employed. Bolt fastening is also frequently used for joining dissimilar materials such as steel sheets and aluminum. In bolt fastening, a nut with a projection (protrusion) is project-welded (resist-welded) to the steel sheet, and then other plates are fastened with bolts to complete the assembly.

[0003] One of the quality assurance criteria for projection welding is the indentation peel strength (also simply called peel strength). However, with the increasing strength of steel plates in recent years, the joint between the steel plate and the nut becomes more brittle, making it difficult to ensure sufficient peel strength.

[0004] Patent Document 1 describes a projection welded joint in which the area ratio between the joint and the nominal diameter portion of the nut or bolt satisfies a predetermined relationship, and the maximum hardness of the joint and heat-affected zone is 550 HV or less. Furthermore, Patent Document 1 reports that high joint strength can be obtained by increasing the applied pressure during projection welding. In addition, Patent Document 2 reports that high joint strength can be obtained by tempering the joint after projection welding by applying a post-welding current value lower than the current value used during welding. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2012-157900 [Patent Document 2] Japanese Patent Publication No. 2013-78784 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, conventional methods have a drawback: when projection welding high-strength steel plates, a decrease in peel strength occurs, leaving room for improvement.

[0007] In view of the above issues, the present invention aims to provide a projection welding joint and a method for manufacturing the same that have improved peel strength. [Means for solving the problem]

[0008] The inventors of this invention have diligently studied to solve the above problems and have obtained the following findings. High-strength steel plates have a higher carbon content than conventional steel plates, making the joints more prone to embrittlement, which reduces the peel strength of projection welded joints. To improve the peel strength of projection welded joints using high-strength steel plates, it is effective to alleviate stress concentration by softening the joint surface between the nut and the steel plate to be welded. In particular, by defining region b as region b and reducing the hardness of region b, including the area near the boundary between the steel plate and the nut and a part of the joint, the peel strength can be improved. This is because softening the hardness of region b improves the toughness of region b and further reduces residual stress.

[0009] In other words, the gist of the present invention is as follows:

[0010] [1] A projection welded joint comprising a steel plate and a nut having a projection, wherein the two are projection welded together via the projection, The aforementioned steel plate is, by mass%, C: 0.05~0.50%, Si: 0.1~2.0%, Mn: 1.5~4.0%, P: 0.10% or less, S: 0.005% or less, N: 0.010% or less, and O: 0.030% or less including, and optionally Al: 2.00% or less, B: 0.0050% or less, Ca: 0.005% or less, Cr: 1.00% or less, Cu: 0.80% or less, Ni: 1.00% or less, Mo: 1.00% or less, Ti: 0.200% or less, V: 0.500% or less, Nb: 0.080% or less, Sb: 0.200% or less, and Sn: 0.200% or less comprising at least one element selected from the group consisting of, with the balance being Fe and inevitable impurities, having a component composition A projection welded joint in which the minimum hardness Hvb (HV) of region b in the steel plate according to the following definition and the base material hardness Hvm (HV) of the steel plate satisfy the following formula (1). Hvb / Hvm ≦ 0.90 ···(1) The region b is specified as follows in a cross section including the center line of the nut of the projection welded joint and passing through the center of the joint between the steel plate and the nut. With the inner peripheral surface side end point of the nut in contact with the steel plate being A and the outer peripheral surface side end point being B, the point inside the steel plate that has moved 0.50 mm in the plate thickness direction of the steel plate from point B is C, and the point inside the steel plate that has moved 0.50 mm in the plate thickness direction of the steel plate from point A is D. The interior of the rectangle formed by connecting points A, B, C, and D is defined as the region b.

[0011] [[ID=4D]] [2] A method for manufacturing a projection welded joint by projection welding a steel plate and a nut having a protrusion through the protrusion, where the steel plate contains, in mass %, C: 0.05 - 0.50%, Si: 0.1 - 2.0%, Mn: 1.5 - 4.0%, P: 0.10% or less, S: 0.005% or less, N: 0.010% or less, and O: 0.030% or less containing, and optionally Al: 2.00% or less, B: 0.0050% or less, Ca: 0.005% or less, Cr: 1.00% or less, Cu: 0.80% or less, Ni: 1.00% or less, Mo: 1.00% or less, Ti: 0.200% or less, V: 0.500% or less, <00> Nb: 0.080% or less, Sb: 0.200% or less, and Sn: 0.200% or less comprising at least one element selected from the group consisting of, with the balance being Fe and inevitable impurities, having a component composition wherein the projection welding is a main energization step of energizing for a time t1 (ms) at a current value I1 (kA) to form a joint between the steel plate and the nut, a non - energization step of providing a non - energization period of a non - energization time t , , , , , c1 , , , , , c1 , , , (ms) satisfying the following formula (2), a first post - energization step of energizing for a time t2 (ms) satisfying the following formula (4) at a current value I2 (kA) satisfying the following formula (3), a second post - energization step of energizing for a time t3 (ms) satisfying the following formula (6) at a current value I3 (kA) satisfying the following formula (5), a method for manufacturing a projection welded joint having 400 ≤ t c1 ≤ 800 ···(2) I1 ≤ I2 ≤ 3.00I1 ···(3) 80 ≤ t2 ≤ 250 ···(4) I3 < I2 ···(5) 180 ≤ t3 ≤ 1000 ···(6) [Effects of the Invention]

[0012] This invention can provide a projection welded joint with improved peel strength. [Brief explanation of the drawing]

[0013] [Figure 1] This is a cross-sectional view including the center line of a nut used in one embodiment of the present invention. [Figure 2] This is a partial cross-sectional view of a projection welded joint according to one embodiment of the present invention, including the center line of the nut and passing through the center of the joint between the steel plate and the nut. [Figure 3] (a) A nut with three protrusions, and (b) A top view of a nut with four protrusions, used in one embodiment of the present invention. [Figure 4] This figure shows region b and hardness measurement points in region b of a cross-section of a projection welded joint according to one embodiment of the present invention. [Figure 5] This figure shows a measurement region 30 for the base material hardness Hvm and hardness measurement points within the cross-section of a projection welded joint according to one embodiment of the present invention. [Modes for carrying out the invention]

[0014] The following describes embodiments of the projection welding joint and its manufacturing method according to the present invention. Note that the embodiments described below are examples of the present invention and do not limit the configuration of the present invention.

[0015] [Steel plate] First, the component composition of the steel plate in the projection welded joint according to the present invention will be described. In the following description, the "%" notation for component composition refers to "mass%" unless otherwise specified.

[0016] C: 0.05~0.50% Carbon (C) is an element that contributes to strengthening steel sheets. If the carbon content of a steel sheet is less than 0.05%, the strength of the steel sheet will be low, and it will be extremely difficult to manufacture a steel sheet with a tensile strength of 780 MPa or more. Therefore, the carbon content of the steel sheet should be 0.05% or more. Preferably, the carbon content of the steel sheet should be 0.10% or more. On the other hand, if the carbon content of the steel sheet exceeds 0.50%, although the strength of the steel sheet will be high, the amount of hard martensite will be excessive, and the number of microvoids will increase. Furthermore, the joints will harden excessively and embrittlement will progress, making it difficult to improve the peel strength. Therefore, the carbon content of the steel sheet should be 0.50% or less. Preferably, the carbon content of the steel sheet should be 0.45% or less.

[0017] Si: 0.1~2.0% A Si content of 0.1% or more in steel sheets effectively strengthens them. Furthermore, since Si is a ferrite-forming element, it favorably promotes ferrite formation at the edges of joints. Therefore, the Si content of steel sheets should be 0.1% or more. A Si content of 0.2% or more is preferable. On the other hand, if the Si content of steel sheets exceeds 2.0%, while the steel sheet is strengthened, it may negatively affect its toughness. Therefore, the Si content of steel sheets should be 2.0% or less. A Si content of 1.8% or less is preferable.

[0018] Mn: 1.5~4.0% If the Mn content is 1.5% or more, high peel strength can be obtained without prolonged cooling. Therefore, the Mn content of the steel plate should be 1.5% or more. Preferably, the Mn content of the steel plate should be 2.0% or more. On the other hand, if the Mn content of the steel plate exceeds 4.0%, embrittlement of the weld or cracking associated with embrittlement becomes significant, making it difficult to improve the peel strength. Therefore, the Mn content of the steel plate should be 4.0% or less. Preferably, the Mn content of the steel plate should be 3.5% or less.

[0019] P:0.10% or less Although phosphorus (P) is an unavoidable impurity, if the P content of the steel sheet exceeds 0.10%, strong segregation appears at the edges of the welded joint, making it difficult to improve the peel strength. Therefore, the P content of the steel sheet should be 0.10% or less. Preferably, the P content of the steel sheet is 0.05% or less, and more preferably 0.02% or less. There is no particular lower limit to the P content of the steel sheet. However, excessive reduction leads to increased costs, so it is preferable that the P content of the steel sheet be 0.005% or more.

[0020] S: 0.005% or less S is an element that segregates at grain boundaries and embrittles steel sheets, and is therefore an unavoidable element. Furthermore, if the S content of steel sheets exceeds 0.005%, it reduces the local deformability of the steel sheet along with sulfides. For this reason, the S content of steel sheets should be 0.005% or less. Preferably, the S content of steel sheets should be 0.004% or less, and more preferably 0.003% or less. There is no particular lower limit to the S content of steel sheets. However, excessive reduction leads to increased costs, so it is preferable that the S content of steel sheets be 0.001% or more.

[0021] N: 0.010% or less N is an unavoidable element, and if the N content of steel sheets exceeds 0.010%, it degrades the aging resistance of the steel sheets. Therefore, the N content of steel sheets should be 0.010% or less. Preferably, the N content of steel sheets should be 0.008% or less. There is no particular lower limit to the N content of steel sheets. However, excessive reduction leads to increased costs, so it is preferable that the N content of steel sheets be 0.001% or more.

[0022] O: 0.030% or less If the oxygen content of steel sheets exceeds 0.030%, non-metallic inclusions are formed, degrading the cleanliness and toughness of the steel sheets. Therefore, the oxygen content of steel sheets should be 0.030% or less. Preferably, the oxygen content of steel sheets should be 0.020% or less. There is no particular lower limit to the oxygen content of steel sheets. However, excessive reduction leads to increased costs, so it is preferable that the oxygen content of steel sheets be 0.005% or more.

[0023] In addition to the above component composition, the steel sheet used in this invention may optionally contain one or more elements selected from Al, B, Ca, Cr, Cu, Ni, Mo, Ti, V, Nb, Sb, and Sn. The remainder of the component composition is Fe and unavoidable impurities.

[0024] Al: 2.00% or less Al is an element that can control the structure for austenite fineness, but adding large amounts deteriorates toughness. Therefore, when adding Al, the Al content should be 2.00% or less. Preferably, the Al content should be 1.50% or less. Furthermore, there is no particular lower limit to the Al content, but from the viewpoint of obtaining sufficient effects from the addition of Al, the Al content is preferably 0.01% or more, and more preferably 1.20% or more.

[0025] B: 0.0050% or less B is an element that can improve hardenability and strengthen steel sheets, but its effect saturates when added in large quantities. Therefore, when adding B, the B content should be 0.0050% or less. Preferably, the B content should be 0.0010% or less. Furthermore, there is no particular lower limit to the B content, but from the viewpoint of obtaining sufficient effects from the addition of B, it is preferable that the B content be 0.0003% or more.

[0026] Ca: 0.005% or less Ca is an element that can contribute to improving the workability of steel sheets, but adding large amounts will degrade toughness. Therefore, when adding Ca, the Ca content should be 0.005% or less. Preferably, the Ca content should be 0.004% or less. Furthermore, there is no particular lower limit to the Ca content, but from the viewpoint of obtaining sufficient effects from the addition of Ca, it is preferable that the Ca content be 0.001% or more.

[0027] Cr:1.00% or less Cr is an element that can improve the strength of steel sheets by improving hardenability, but adding large amounts may degrade the toughness of the joints. Therefore, when adding Cr, the Cr content should be 1.00% or less. Preferably, the Cr content should be 0.80% or less. Furthermore, there is no particular lower limit to the Cr content, but from the viewpoint of obtaining sufficient effects from the addition of Cr, it is preferable that the Cr content be 0.01% or more.

[0028] Cu:0.80% or less Cu is an element that can contribute to improving the strength of steel sheets, but adding large amounts will degrade toughness. Therefore, when adding Cu, the Cu content should be 0.80% or less. Preferably, the Cu content should be 0.60% or less. Furthermore, there is no particular lower limit to the Cu content, but from the viewpoint of obtaining sufficient effects from the addition of Cu, it is preferable that the Cu content be 0.006% or more.

[0029] Ni: 1.00% or less Ni is an element that can contribute to improving the strength of steel plates, but adding large amounts will degrade their toughness. Therefore, when adding Ni, the Ni content should be 1.00% or less. Preferably, the Ni content should be 0.80% or less. Furthermore, there is no particular lower limit to the Ni content, but from the viewpoint of obtaining sufficient effects from the addition of Ni, it is preferable that the Ni content be 0.005% or more.

[0030] Mo: 1.00% or less Mo is an element that can contribute to improving the strength of steel plates, but adding it in large quantities degrades its toughness. Therefore, when adding Mo, the Mo content should be 1.00% or less. Preferably, the Mo content should be 0.80% or less. Furthermore, there is no particular lower limit to the Mo content, but from the viewpoint of obtaining sufficient effects from the addition of Mo, it is preferable that the Mo content be 0.006% or more.

[0031] Ti: 0.200% or less Ti is an element that can strengthen steel sheets by improving hardenability, but if added in large quantities, it forms carbides, and the toughness deteriorates significantly due to precipitation hardening. Therefore, when adding Ti, the Ti content should be 0.200% or less. Preferably, the Ti content should be 0.150% or less. Furthermore, there is no particular lower limit to the Ti content, but from the viewpoint of obtaining sufficient effects from the addition of Ti, it is preferable that the Ti content be 0.003% or more.

[0032] V:0.500% or less V is an element that can strengthen steel by controlling its structure through precipitation hardening, but adding it in large quantities can lead to a deterioration of the toughness of the joint. Therefore, when adding V, the V content should be 0.500% or less. Preferably, the V content should be 0.300% or less. Furthermore, there is no particular lower limit to the V content, but from the viewpoint of obtaining sufficient effects from the addition of V, it is preferable that the V content be 0.005% or more.

[0033] Nb: 0.080% or less Nb is an element that can improve cross-tensile strength and delayed fracture resistance after resistance welding by forming fine carbonitrides. However, adding large amounts of Nb not only significantly reduces elongation but also severely impairs toughness. Therefore, when adding Nb, the Nb content should be 0.080% or less. Preferably, the Nb content should be 0.070% or less, and more preferably 0.060% or less. Furthermore, there is no particular lower limit to the Nb content, but from the viewpoint of fully obtaining the effect of Nb addition, it is preferable that the Nb content be 0.005% or more.

[0034] Sb: 0.200% or less Sb is an element that can suppress nitriding and oxidation of the steel sheet surface, but adding large amounts reduces toughness. Therefore, when adding Sb, the Sb content should be 0.200% or less. Furthermore, there is no particular lower limit to the Sb content, but from the viewpoint of obtaining sufficient effects from the addition of Sb, it is preferable that the Sb content be 0.002% or more.

[0035] Sn: 0.200% or less Sn is an element that can suppress nitriding and oxidation of the steel sheet surface, and adding Sn stabilizes the material, but adding large amounts reduces toughness. Therefore, when adding Sn, the Sn content should be 0.200% or less. Furthermore, there is no particular lower limit to the Sn content, but from the viewpoint of obtaining sufficient effects from the addition of Sn, it is preferable that the Sn content be 0.002% or more.

[0036] The steel sheet having the above-described component composition preferably has a tensile strength of 980 MPa or higher, and more preferably 1180 MPa or higher. However, when the tensile strength of the steel sheet is 980 MPa or higher, the carbon content is high, which presents a problem in that conventional projection welding tends to cause the joint to become brittle and the indentation peel strength to decrease. According to the present invention, even with a high-strength steel sheet having a tensile strength of 980 MPa or higher, the decrease in peel strength can be suppressed by softening the hardness of region b, thereby easing stress concentration. Furthermore, by reducing the residual stress in region b, the peel strength of the projection weld can be improved. Note that the above effects can also be obtained when using a steel sheet with a tensile strength of less than 980 MPa. On the other hand, there is no particular upper limit to the tensile strength of the steel sheet, but the tensile strength is generally 2000 MPa or less.

[0037] From the perspective of targeting general automotive steel sheets, the thickness of the steel sheet is preferably 0.8 mm to 2.3 mm.

[0038] The steel sheet used in the present invention may have a plating layer on its surface, for example, a steel sheet having a zinc plating layer (zinc-plated steel sheet). The zinc plating layer includes known zinc plating layers, and includes hot-dip galvanized layers, electro-galvanized layers, Zn-Al plating layers, and Zn-Ni layers. Furthermore, the steel sheet used in the present invention may be a steel sheet having an alloyed zinc plating layer after being subjected to an alloying treatment after zinc plating. The zinc plating layer may be applied to only one side of the steel sheet or to both sides.

[0039] [nut] The nut used in this invention is a nut having a projection. Figure 1 shows a cross-section of a nut 10 used in one embodiment of the present invention, including the center line 12. The nut 10 has a projection 16 on a surface 14 perpendicular to the center line 12. In Figure 1, two projections 16 are provided symmetrically on the surface 14 perpendicular to the center line 12, at 180° intervals with the center line 12 as the axis of rotation. Note that three or four projections 16 may be provided symmetrically with respect to the center line 12 as the axis of rotation. The shape of the nut 10 and the projections 16 may be, for example, the square welded nut (type 1C) and various projection shapes described in JIS B 1196:2010, respectively.

[0040] The composition of the nut is not particularly limited, but for example, S25C (JIS G4051) may be used. In particular, when the carbon content in the nut's composition is 0.05% by mass or more, a suitable hardness in region b can be obtained. Therefore, the carbon content of the nut is preferably 0.05% by mass or more, and more preferably 0.07% by mass or more. On the other hand, when the carbon content in the nut's composition is 0.40% by mass or less, a suitable hardness in region b can be obtained. Therefore, the carbon content of the nut is preferably 0.40% by mass or less, and more preferably 0.38% by mass or less.

[0041] When three or more protrusions are provided on the surface perpendicular to the centerline of the nut used in this invention, during projection welding described later, there will be three or more contact points between the protrusions and the steel plate, allowing for a suitably stable contact state. Therefore, it is preferable that the number of protrusions provided on the surface perpendicular to the centerline of the nut used in this invention be three or more. On the other hand, when four or fewer protrusions are provided on the surface perpendicular to the centerline of the nut used in this invention, it is preferable that during projection welding, weak contact points on the protrusions occur, preventing an imbalance in heat input and effectively suppressing a decrease in the strength of the joint. Therefore, it is preferable that the number of protrusions provided on the surface perpendicular to the centerline of the nut used in this invention be four or fewer.

[0042] [Projection welding joints] The projection welded joint according to the present invention is a projection welded joint in which the steel plate and the nut described above are projection welded via the projection of the nut, thereby forming a joint between the steel plate and the nut. Here, the "joint between the steel plate and the nut" refers to the portion where the steel plate and the nut are joined by fusion welding or solid-state welding, and is a portion derived from the projection of the nut.

[0043] The projection welded joint according to the present invention is characterized in that the minimum hardness Hvb(HV) of region b in the steel plate according to the following definition and the base material hardness Hvm(HV) of the steel plate satisfy the following formula (1). Hvb / Hvm ≤ 0.90 ···(1)

[0044] Figure 2 shows a cross-section of a projection welded joint 100 according to one embodiment of the present invention, including the center line 12 of the nut 10 and passing through the center of the joint between the steel plate 20 and the nut 10. Let A be the endpoint on the inner circumferential surface side of the nut 10 that is in contact with the steel plate 20, and B be the endpoint on the outer circumferential surface side. Let C be a point inside the steel plate 20 moved 0.50 mm in the thickness direction of the steel plate 20 from point B, and let D be a point inside the steel plate 20 moved 0.50 mm in the thickness direction of the steel plate 20 from point A. The area inside the rectangle formed by connecting points A, B, C, and D is defined as region b. Figure 2 also shows the measurement area 30 for the base material hardness Hvm.

[0045] Since the joint originates from the projection of the nut, the center of the joint corresponds to the position of the center of the projection of the nut. Figure 3 shows top views of (a) a nut 10A with three projections 16 and (b) a nut 10B with four projections 16 used in one embodiment of the present invention. The cutting line shown in Figure 3 is a line that passes through the center line of the nut and the center of the projection 16. After projection welding the steel plate and the nut, the projection welded joint is cut along the cutting line shown in Figure 3 to obtain the cross section shown in Figure 2.

[0046] When the ratio Hvb / Hvm between the minimum hardness Hvb of region b and the base material hardness Hvm of the steel plate is 0.90 or less, region b is sufficiently softened, and the indentation peel strength is improved. Therefore, the Hvb / Hvm of the projection welded joint according to the present invention is set to 0.90 or less. Hvb / Hvm is preferably 0.87 or less, and more preferably 0.85 or less. On the other hand, when Hvb / Hvm is 0.60 or more, the softening effect in region b is appropriately obtained. Therefore, Hvb / Hvm is preferably 0.60 or more.

[0047] The minimum hardness Hvb in region b is determined as follows. Figure 4 shows an enlarged view of region b. In region b, the hardness measurement points 32 are set at 0.20 mm intervals in a direction perpendicular to the thickness direction of the steel plate, starting from a point moved 0.20 mm or 0.40 mm from point B in the direction of point C. At each of the hardness measurement points 32 in region b, a measurement load of 300 gf is applied with an indenter for 15 seconds in accordance with JIS Z 2244 (2020), and the Vickers hardness is measured. The lowest value among the obtained measurement results is taken as the minimum hardness Hvb of region b.

[0048] If the minimum hardness Hvb in region b is 300 HV or higher, a suitable softening effect can be obtained in region b. Therefore, it is preferable that the minimum hardness Hvb in region b be 300 HV or higher, and more preferably 320 HV or higher. On the other hand, if the minimum hardness Hvb in region b is 650 HV or lower, a suitable softening effect can be obtained in region b. Therefore, it is preferable that the minimum hardness Hvb in region b be 650 HV or lower, and more preferably 600 HV or lower.

[0049] The base material hardness Hvm of the steel plate is determined as follows. As shown in Figure 2, in the cross-section of the projection welded joint 100, point E1 is set to a point 7.00 mm away from point B on the line formed by the surface of the steel plate 20. Point E2 is set to a point 0.60 mm further away from point E1 on the line formed by the surface of the steel plate 20. Point E3 is set to a point inside the steel plate 20, 0.60 mm away from point E2 in the thickness direction of the steel plate 20. Point E4 is set to a point inside the steel plate 20, 0.60 mm away from point E1 in the thickness direction of the steel plate 20. The area inside the square formed by connecting points E1, E2, E3, and E4 is defined as the measurement area 30 for the base material hardness Hvm. Point E1 is set to a point 7.00 mm away from point B in order to avoid the influence of heat during welding. Figure 5 shows an enlarged view of the measurement area 30 for the base material hardness Hvm. The hardness measurement points 32 are set at 0.20 mm intervals in the thickness direction of the steel plate 20, starting from a point 0.20 mm away from point E1 in the thickness direction of the steel plate 20. At the total of 9 hardness measurement points 32 shown in Figure 5, a measurement load of 300 gf was applied with an indenter for 15 seconds in accordance with JIS Z 2244 (2020), and the Vickers hardness was measured at each point. The average value obtained was defined as the base material hardness Hvm. As described above, the base material hardness Hvm of the steel plate is measured at its cross-section, so the presence or absence of a plating layer does not affect it.

[0050] When the base material hardness Hvm of the steel sheet is 400HV or higher, a suitable softening effect can be obtained in region b. Therefore, it is preferable that the base material hardness Hvm of the steel sheet is 400HV or higher. On the other hand, when the base material hardness Hvm of the steel sheet is 700HV or lower, a suitable softening effect can be obtained in region b. Therefore, it is preferable that the base material hardness Hvm of the steel sheet is 700HV or lower, and more preferably 650HV or lower.

[0051] [Manufacturing method for projection welded joints] The present invention relates to a method for manufacturing a projection welded joint, which involves projection welding a steel plate and a nut having a projection via the projection. The steel plate has the aforementioned component composition, and the projection welding is performed in a main energizing step where current is applied at a current value I1 (kA) for an energizing time t1 (ms) to form a joint between the steel plate and the nut, and a non-energing time t satisfying the following equation (2) c1 The manufacturing method comprises a non-energizing step that provides a non-energizing period of (ms), a first post-energizing step in which current is supplied at a current value I2 (kA) satisfying the following equation (3) for an energizing time t2 (ms) satisfying the following equation (4), and a second post-energizing step in which current is supplied at a current value I3 (kA) satisfying the following equation (5) for an energizing time t3 (ms) satisfying the following equation (6). 400≦t c1 ≤800 ···(2) I1 ≤ I2 ≤ 3.00 I1 ···(3) 80 ≤ t² ≤ 250 ···(4) I3 <I2···(5) 180 ≤ t3 ≤ 1000 ···(6)

[0052] [Main energization process] The main energizing process is the process of forming the joint between the steel plate and the nut. In the main energizing process, the joint is formed by applying current I1 for a duration t1. However, as long as the joint can be formed, the conditions for the main energizing process described below are not particularly limited.

[0053] When the current value I1 in the main energizing process is 7.0kA or higher, the steel plate and the nut can be joined effectively. When the current value I1 is less than 7.0kA, after joining the steel plate and the nut, if the center of the joint is cut and the cross-section is examined under a microscope, a gap is often observed between the steel plate and the nut. Therefore, the current value I1 in the main energizing process is preferably 7.0kA or higher, and more preferably 8.0kA or higher. On the other hand, when the current value I1 is 30.0kA or lower, the joint expands too much, causing molten metal to splatter outside the nut, and the projection portion of the nut to be completely crushed, rendering the joint unusable. This can be effectively prevented. Therefore, the current value I1 is preferably 30.0kA or lower, and more preferably 28.0kA or lower.

[0054] If the energizing time t1 of the main energizing process is 70 ms or more, a suitable and stable joint can be obtained. Therefore, the energizing time t1 of the main energizing process is preferably 70 ms or more, more preferably 100 ms or more, and even more preferably 160 ms or more. On the other hand, if the energizing time t1 is 500 ms or less, the joint expands too much, causing molten metal to splatter outside the nut, and the projection portion of the nut to be completely crushed, rendering the joint unusable. This can be suitably prevented. Therefore, the energizing time t1 of the main energizing process is preferably 500 ms or less, and more preferably 400 ms or less.

[0055] When the pressing force in the main energizing process is 3.0 kN or more, the energizing diameter is of a suitable size, and scattering can be effectively prevented. Therefore, the pressing force in the main energizing process is preferably 3.0 kN or more, and more preferably 3.5 kN or more. On the other hand, when the pressing force in the main energizing process is 6.0 kN or less, the energizing diameter does not expand and the joint can be effectively secured. Therefore, the pressing force in the main energizing process is preferably 6.0 kN or less, and more preferably 5.5 kN or less. Note that the pressing force may be limited by the capacity of the equipment used, so it may be adjusted as appropriate to obtain the required joint diameter.

[0056] [First non - energization process and first post - energization process] After the main energization process, in the projection welded joint, particularly for region b, softening treatment is performed. Specifically, after the main energization process, a non - energization process, a first post - energization process, and a second post - energization process are carried out. By softening region b, relieving stress concentration during the peel test, and reducing residual stress, the peel strength can be improved.

[0057] In the first non - energization process, in order to obtain the effect of softening, the projection welded joint is cooled to sufficiently lower the temperature of region b.

[0058] In the first non - energization process, the welded part is cooled by maintaining a non - energized state for the cooling time t c1 (ms) shown in Equation (2). 400 ≦ t c1 ≦ 800 ···(2) The cooling time t of the first non - energization process c1 If it is less than 400 ms, the effect of the first post - energization process cannot be obtained and the joint may have a brittle structure. Therefore, the cooling time t c1 shall be 400 ms or more. The cooling time t c1 is preferably 450 ms or more, and more preferably 500 ms or more. On the other hand, if the cooling time t c1 exceeds 800 ms, the welding time will be prolonged. Therefore, the cooling time t of the first non - energization process c1 shall be 800 ms or less.

[0059] Following the first non - energization process, the first post - energization process is carried out. In the first post - energization process, energization is performed to raise the temperature of region b to an appropriate temperature range. The above - mentioned "appropriate temperature range" refers to the temperature range for softening the hardness of region b. In the first post - energization process, energization is carried out for the energization time t2(ms) that satisfies the following Equation (4) with the current value I2(kA) that satisfies the following Equation (3). I1 ≦ I2 ≦ 3.00I1···(3) 80 ≦ t2 ≦ 250 ···(4)

[0060] The inventors have found that region b can be softened by performing a first post-energization step with a current value equivalent to or greater than that of the main energization step. By reducing the hardness of region b through the post-energization step, it becomes possible to alleviate the stress concentration applied to region b during the indentation peel test.

[0061] If the current value I2 in the first post-energization step is less than I1, region b cannot be effectively softened. Therefore, the current value I2 in the first post-energization step should be I1 or greater, preferably 1.10I1 or greater. On the other hand, if the current value I2 in the first post-energization step exceeds 3.00I1, the temperature of region b will be A c3 There is a high probability that the temperature will exceed the point (the temperature at which the transformation from ferrite to austenite is completed), ultimately leading to embrittlement. As a result, toughness at the joint cannot be obtained. Therefore, the current value I2 in the first post-energization process should be 3.00I1 or less, and preferably 2.80I1 or less.

[0062] As described above, the first post-energization step rapidly raises the temperature in a short time, so the energization time t2 in the heating process should be 80 ms or more. Preferably, the energization time t2 should be 100 ms or more. On the other hand, if the energization time t2 exceeds 250 ms, the temperature of region b will be A c3 There is a possibility that the value will be exceeded. Therefore, the energizing time t2 should be 250ms or less, and preferably 220ms or less.

[0063] The projection welding according to the present invention involves a second post-energizing step, in which, after the first post-energizing step, current is applied at a current value I3 (kA) satisfying the following equation (5) for an energizing time t3 (ms) satisfying the following equation (6). Performing the second post-energizing step allows for more effective softening of region b, reducing residual stress and easing stress concentration. I3 <I2···(5) 180 ≤ t3 ≤ 1000 ···(6)

[0064] If the current value I3 in the second post-energization step is I2 or greater, appropriate temperature control cannot be performed to more effectively soften region b. Therefore, the current value I3 in the second post-energization step should be less than I2. The current value I3 is preferably 0.90I2 or less, and more preferably 0.80I2 or less. On the other hand, if the current value I3 in the second post-energization step is 0.20I2 or greater, the softening effect on region b can be suitably obtained. Therefore, the current value I3 in the second post-energization step is preferably 0.20I2 or greater, and more preferably 0.30I2 or greater.

[0065] The second post-energization step is a step for softening region b, and in order to effectively soften region b, the energizing time is the same as that of the first post-energization step. Therefore, the energizing time t3 of the second post-energization step is 180 ms or more, and preferably 200 ms or more. On the other hand, for the same reason, the energizing time t3 of the second post-energization step is 1000 ms or less. The energizing time t3 of the second post-energization step is preferably 900 ms or less.

[0066] Following the second post-energization step, the de-energization step and post-energization step may be repeated. Repeating the de-energization step and post-energization step is preferable because it increases the effect of softening region b. On the other hand, if the number of repetitions of the de-energization step and post-energization step is too large, the effect will saturate, so the number of repetitions of this step is preferably less than 10 times, and more preferably less than 4 times.

[0067] As described above, the projection welding method of the present invention can alleviate stress concentration and reduce residual stress by softening region b through appropriate control of the welding conditions in the post-heat treatment process. In other words, the projection welded joint obtained by this welding method can have improved indentation peel strength. Therefore, even when welding steel plates that contain a relatively large amount of carbon in their composition, specifically high-strength steel plates with a tensile strength of 980 MPa or more and a carbon content of 0.05 to 0.50 mass%, as described above, the peel strength can be further improved.

[0068] For processes and conditions not described in this specification, conventional methods may be used. [Examples]

[0069] As an embodiment of the present invention, projection welded joints were fabricated by projection welding steel plates and nuts. Steel plates (Steel Plate A to Steel Plate M) with tensile strengths of 780 MPa to 1800 MPa and plate thicknesses of 1.0 to 1.8 mm were used as test specimens. Table 1 shows the tensile strength (TS), plate thickness, and component composition of the steel plates A to Steel Plate M used. The remainder of the component composition listed in Table 1 consists of Fe and unavoidable impurities, and "-" indicates that the content of that element is below the detection limit.

[0070] Cold-rolled steel sheets or GA steel sheets (alloyed hot-dip galvanized steel sheets) were prepared as test specimens. The size of the test specimens was 50 mm on each side, and a hole with a diameter of 11 mm was drilled in the center of each specimen. In addition, an M8 welding nut was prepared, which had three protrusions symmetrically spaced at 120° intervals around the center line of the nut. The specimens were set in an AC welding machine so that the center of the hole in the specimen and the center of the threaded hole in the nut coincided, and welding was performed under the welding conditions shown in Table 2 to obtain a welded body. The steel type of the M8 welding nut was S25C (JIS G4051). The resistance welding conditions were to use a single-phase AC (50 Hz) resistance welding machine with a servo motor pressure attached to the welding gun, and a projection welded joint was fabricated. The pair of electrode tips used were flat electrodes with a diameter of 30 mm. A bolt was fixed to the nut hole of the obtained projection welded joint to fabricate a bolt fastening test specimen.

[0071] [Table 1]

[0072] [Table 2]

[0073] The hardness and peel strength characteristics of the base material and region b were measured for the obtained projection welded joints and bolted test specimens using the method described below.

[0074] [Hardness evaluation] The hardness of the base material and region b was measured as follows: A projection welded joint was cut to include the center line of the nut and pass through the center of the joint between the steel plate and the nut to obtain a test specimen. After ultrasonic cleaning of the test specimen, it was embedded in resin to obtain a sample. The thickness cross section of the obtained sample was polished and etched using a picric acid solution. Hardness measurements were performed on the thickness cross section of the etched sample as described above, and the results are shown in Table 2. In the "Hardness Judgment" column of Table 2, if the hardness measurement result satisfies formula (1) (i.e., if Hvb / Hvm ≤ 0.90), it is marked as a pass and marked as "○", and if it does not satisfy formula (1), it is marked as a fail and marked as "×".

[0075] [Peel strength evaluation] Using the obtained bolt fastening test specimens, the maximum load at which the nut peeled off the steel plate was measured by an indentation peel test in accordance with JIS B1196:2010, and this measured value was defined as the peel strength (kN). In this invention, the lower limit of JIS B1196:2010 (3.73kN) was used as the standard for peel strength. If the peel strength was less than 3.73kN, it was evaluated as "×" as having poor peel strength; if it was between 3.73kN and less than 6.00kN, it was evaluated as "△" as having peel strength equivalent to conventional methods; and if it was 6.00kN or more, it was evaluated as "○" as having superior peel strength. Table 2 shows the measured peel strength and its evaluation results.

[0076] As shown in Table 2, the projection welded joint of the inventive example was a good welded joint with excellent peel strength. In contrast, a good projection welded joint could not be obtained in the comparative example. [Industrial applicability]

[0077] This invention can provide a projection welded joint with improved peel strength. [Explanation of symbols]

[0078] 100 Projection Welded Joints 10 nuts 10A Nut with three protrusions 10B A nut with four protrusions. 12. Center line of the nut 14. A surface perpendicular to the center line of the nut. 16 Protrusion 20 steel plate 30 Measurement range of base material hardness Hvm 32 hardness measurement points

Claims

1. A projection welded joint comprising a steel plate and a nut having a projection, wherein the two are projection welded together via the projection, The aforementioned steel plate is, by mass%, C: 0.05-0.50%, Si: 0.1-2.0%, Mn: 1.5-4.0%, P: 0.10% or less, S: 0.005% or less, N: 0.010% or less, and O: 0.030% or less This includes, and optionally, Al: 2.00% or less, B: 0.0050% or less, Ca: 0.005% or less, Cr: 1.00% or less, Cu: 0.80% or less, Ni: 1.00% or less, Mo: 1.00% or less Ti: 0.200% or less, V: 0.500% or less, Nb: 0.080% or less, Sb: 0.200% or less, Sn: 0.200% or less The component composition comprises at least one element selected from the group consisting of the following, with the remainder being Fe and unavoidable impurities. A projection welded joint in which the minimum hardness Hvb(HV) of region b in the steel plate according to the following definition and the base material hardness Hvm(HV) of the steel plate satisfy the following equation (1). Hvb / Hvm≦0.90...(1) Region b is defined as follows in a cross-section of the projection welded joint that includes the center line of the nut and passes through the center of the joint between the steel plate and the nut: Let A be the inner circumferential end point of the nut that contacts the steel plate, and B be the outer circumferential end point. Let C be a point inside the steel plate moved 0.50 mm from point B in the thickness direction of the steel plate, and let D be a point inside the steel plate moved 0.50 mm from point A in the thickness direction of the steel plate. Region b is defined as the inside of the rectangle formed by connecting points A, B, C, and D.

2. A method for manufacturing a projection welded joint by projection welding a steel plate and a nut having a projection via the projection, The aforementioned steel plate is, by mass%, C: 0.05-0.50%, Si: 0.1-2.0%, Mn: 1.5-4.0%, P: 0.10% or less, S: 0.005% or less, N: 0.010% or less, and O: 0.030% or less This includes, and optionally, Al: 2.00% or less, B: 0.0050% or less, Ca: 0.005% or less, Cr: 1.00% or less, Cu: 0.80% or less, Ni: 1.00% or less, Mo: 1.00% or less Ti: 0.200% or less, V: 0.500% or less, Nb: 0.080% or less, Sb: 0.200% or less, Sn: 0.200% or less The component composition comprises at least one element selected from the group consisting of the following, with the remainder being Fe and unavoidable impurities. The aforementioned projection welding, Current value I １ (kA) for energizing time t １ The main energizing step involves energizing for (ms) to form a joint between the steel plate and the nut, Non-energized time t that satisfies the following equation (2) ｃ１ A non-energizing process that includes a non-energizing period of (ms), Current value I that satisfies the following equation (3) ２ (kA) and the energizing time t that satisfies the following equation (4) ２ A first post-energization step in which power is applied for (ms), Current value I that satisfies the following equation (5) ３ (kA) and the energizing time t that satisfies the following equation (6) ３ A second post-energization process in which power is applied for (ms), A method for manufacturing a projection welded joint having [a specific feature]. 400≦t ｃ１ ≦800 ・・・(2) I １ ≦I ２ ≦3.00I １ ・・・(3) 80≦t ２ ≦250 ・・・(4) I ３ <I ２ ・・・(5) 180≦t ３ ≦1000 ・・・(6)

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