Projection welding joint and method for manufacturing the same

Abstract

Provided are: a projection weld joint with improved peel strength and delayed fracture resistance; and a method for manufacturing same. A projection weld joint according to the present invention has a steel plate and a nut, the steel plate has a predetermined component composition, and a region a minimum hardness HVa (HV) and a steel plate base material hardness HVm (HV) satisfy the relationship Hva / Hvm≤0.90. The region a is specified as follows in a cross-section including the center line of the nut and passing through the center of the joint between the steel plate and the nut of the projection weld joint. An end point of the nut on an inner peripheral surface side in contact with the steel plate is designated as A, and an end point on an outer peripheral surface side is designated as B, a length of a line segment connecting the end points A, B is designated as 2L, a point moved by L / 2 from B to A is designated as C1, a point moved by L on a line formed by the surface of the steel plate from B is designated as C2, points inside the steel plate moved by 0.80 mm in the plate thickness direction of the steel plate from C2 and C1 are respectively designated as points C3 and C4, and the inside of a rectangle formed by connecting C1 to C4 is designated as the region a.

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). 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. Furthermore, the increased strength of steel plates also leads to increased susceptibility to delayed fracture, i.e., hydrogen embrittlement, which is another challenge. Additionally, the influence of rust-preventive oils and plating layers on the steel plate surface can lead to hydrogen being incorporated into the weld, increasing the likelihood of delayed fracture.

[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 problems such as reduced peel strength and delayed fracture when projection welding high-strength steel plates, and there was room for improvement.

[0007] In view of the above issues, the present invention aims to provide a projection welded joint and a method for manufacturing the same that have improved peel strength and delayed fracture resistance. [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. Furthermore, delayed fracture is partly caused by the hardness of the joint at the fracture initiation point and the influence of segregation and inclusions present in the joint. Therefore, to suppress delayed fracture, it is effective to prevent hardening of the joint and reduce residual stress. In particular, by designating the region including the boundary between the steel plate and the nut and a part of the joint as region a, and reducing the hardness of region a, the resistance to delayed fracture can be improved. This is because softening the hardness of region a improves the toughness of region a, and further reducing residual stress makes delayed fracture less likely to occur.

[0009] That is, the gist configuration of the present invention is as follows.

[0010] [1] A projection welding joint in which a steel plate and a nut having a protrusion are projection welded through the protrusion, where the steel plate contains, in mass %, C: 0.05 to 0.50%, Si: 0.1 to 2.0%, Mn: 1.5 to 4.0%, P: 0.10% or less, S: 0.005% or less, N: 0.010% or less, and O: 0.030% or less and further optionally contains [[ID=​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​The area a is specified as follows in a cross-section that includes the center line of the nut of the projection welded joint and passes 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 length of the line segment connecting the end points A and B is 2L. Let the point obtained by moving L / 2 from point B towards point A on the said line segment be C1, the point obtained by moving L from point B along the line formed by the surface of the steel plate continuous with the said line segment be C2, the point inside the steel plate obtained by moving 0.80 mm in the thickness direction of the steel plate from point C2 be C3, and the point inside the steel plate obtained by moving 0.80 mm in the thickness direction of the steel plate from point C1 be C4. The interior of the rectangle formed by connecting points C1, C2, C3, and C4 is defined as the area a.

[0011] [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, 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 and further 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, [[ID=3B]] 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 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, A main energizing step involves energizing with a current value I1 (kA) for an energizing time t1 (ms) to form a joint between the steel plate and the nut, Non-energized time t that satisfies the following equation (2) c1 A first non-energizing process that includes a non-energizing period of (ms), A first post-energization step is performed by energizing with a current value I2 (kA) that satisfies the following equation (3) for an energizing time t2 (ms) that satisfies the following equation (4), It has, and optionally, Non-energized time t that satisfies the following equation (5) c2 A second de-energizing step is to provide a de-energizing period of (ms), followed by a second post-energizing step in which current is supplied at a current value I3 (kA) that satisfies the following equation (6) for an energizing time t3 (ms) that satisfies the following equation (7), A method for manufacturing a projection welded joint having [a specific feature]. 10≦t c1 ≤300 ···(2) I1 ≤ I2 ≤ 3.00 I1 ···(3) 30 ≤ t² ≤ 300 ···(4) 0 <t c2 ≤400 ···(5) 0.70I2 ≤ I3 ≤ 1.30I2 ···(6) 0 <t3≦300 ···(7) [Effects of the Invention]

[0012] This invention can provide a projection welded joint with improved peel strength and delayed fracture resistance. [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 a and hardness measurement points in region a of a cross-section of a projection welded joint according to one embodiment of the present invention. [Figure 5] This figure shows the measurement region 30 for the base material hardness Hvm and the 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 a, thereby easing stress concentration. Furthermore, by reducing the residual stress in region a, the peel strength of the projection weld can be improved, and the delayed fracture resistance 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 a 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 a 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 Hva(HV) of region a 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). Hva / 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, and let the length of the line segment 24 connecting endpoints A and B be 2L. Let C1 be the point obtained by moving L / 2 from point B toward point A on the line segment 24, let C2 be the point obtained by moving L from point B toward point C4 on the line formed by the surface of the steel plate 20 that is continuous with the line segment 24, let C3 be the point inside the steel plate 20 obtained by moving 0.80 mm in the thickness direction of the steel plate 20 from point C2, and let C4 be the point inside the steel plate 20 obtained by moving 0.80 mm from point C1 in the thickness direction of the steel plate 20, and let the inside of the rectangle formed by connecting points C1, C2, C3, and C4 be region a. 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 Hva / Hvm between the minimum hardness Hva of region a and the base material hardness Hvm of the steel plate is 0.90 or less, region a is sufficiently softened, improving the indentation peel strength and delayed fracture resistance. Therefore, the Hva / Hvm of the projection welded joint according to the present invention is set to 0.90 or less. Hva / Hvm is preferably 0.87 or less, and more preferably 0.85 or less. On the other hand, when Hva / Hvm is 0.60 or more, even if region a has become entirely tempered martensite, it still retains a certain degree of hardness, thus providing a suitable indentation peel strength. Therefore, Hva / Hvm is preferably 0.60 or more, and more preferably 0.70 or more.

[0047] The minimum hardness Hva in region a is determined as follows. Figure 4 shows an enlarged view of region a. In region a, the hardness measurement points 32 are set at 0.20 mm intervals from point C3 in the direction of the steel plate thickness and in the direction perpendicular to the thickness direction. Note that measurements are not taken on the line formed by the surface of the steel plate (the line connecting points C1 and C2), as this is the edge of the steel plate. At each of the hardness measurement points 32 in region a, 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 Hva of region a.

[0048] If the minimum hardness Hva of region a is 300 HV or higher, region a is appropriately softened, and a suitable indentation peel strength is obtained by relieving stress concentration. Therefore, the minimum hardness Hva of region a is preferably 300 HV or higher, and more preferably 320 HV or higher. On the other hand, if the minimum hardness Hva of region a is 700 HV or lower, it becomes possible to soften region a, and a suitable peel strength is obtained. Therefore, the minimum hardness Hva of region a is preferably 700 HV or lower, and more preferably 650 HV or lower.

[0049] Furthermore, from the viewpoint of suitably obtaining the effects of the present invention, it is more preferable that the minimum hardness Hva' of region a' satisfies the following formula (1)'. Here, as shown in Figure 2, let E be a point inside the steel plate 20 moved 0.80 mm in the thickness direction from point B, and region a' is defined as the area inside the rectangle formed by connecting points B, C2, C3, and E. The minimum hardness Hva' of region a' is preferably 300 HV or more, and more preferably 320 HV or more, similar to Hva. Also, the minimum hardness Hva' of region a' is preferably 700 HV or less, and more preferably 650 HV or less, similar to Hva. The minimum hardness Hva' of region a' can be determined in the same way as the measurement method for Hva. Hva' / Hvm≦0.90 ···(1)'

[0050] 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 D1 is set to a point 7.00 mm away from point B on the line formed by the surface of the steel plate 20 continuous with line segment 24. Point D2 is set to a point 0.60 mm further away from point D1 on the line formed by the surface of the steel plate 20. Point D3 is set to a point inside the steel plate 20, 0.60 mm away from point D2 in the thickness direction of the steel plate 20. Point D4 is set to a point inside the steel plate 20, 0.60 mm away from point D1 in the thickness direction of the steel plate 20. The area inside the square formed by connecting points D1, D2, D3, and D4 is defined as the measurement area 30 for the base material hardness Hvm. Point D1 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 direction of the thickness of the steel plate 20, starting from a point 0.20 mm away from point D1 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.

[0051] When the base material hardness Hvm of the steel sheet is 400HV or higher, a suitable softening effect can be obtained in region a. 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 a. Therefore, it is preferable that the base material hardness Hvm of the steel sheet is 700HV or lower, and more preferably 650HV or lower.

[0052] [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 process includes a first de-energizing step which provides a de-energizing period of (ms), and a first post-energizing step which provides current I2 (kA) that satisfies the following equation (3) for an energizing time t2 (ms) that satisfies the following equation (4), and optionally, a de-energizing period t that satisfies the following equation (5) c2 The manufacturing method comprises a second non-energizing step which includes a non-energizing period of (ms), and a second post-energizing step which includes energizing with a current value I3 (kA) that satisfies the following equation (6) for an energizing time t3 (ms) that satisfies the following equation (7). 10≦t c1 ≤300 ···(2) I1 ≤ I2 ≤ 3.00 I1 ···(3) 30 ≤ t² ≤ 300 ···(4) 0 <t c2 ≤400 ···(5) 0.70I2 ≤ I3 ≤ 1.30I2 ···(6) 0 <t3≦300 ···(7)

[0053] [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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] [First non - energization process and first post - energization process] After the main energization process, in the projection welded joint, particularly for region a, a softening treatment is performed. Specifically, after the main energization process, a first non - energization process and a first post - energization process are carried out. By softening region a, relieving stress concentration during the peel test, and reducing residual stress, delayed fracture can be avoided.

[0058] 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 a.

[0059] In the first non - energization process, during the cooling time t c1 (ms) shown in Equation (2), the welded part is cooled by maintaining a non - energized state. 10 ≦ t c1 ≦ 300 ···(2) The cooling time t of the first non - energization process c1 If it is less than 10 ms, since the cooling time is short, the effect of the first post - energization process cannot be obtained and the joint part may become a brittle structure. Furthermore, due to the temperature history resulting from the combination of the main energization process and the first post - energization process, the effect of post - energization after joining cannot be obtained. Therefore, the cooling time t c1 shall be 10 ms or more. The cooling time t c1 is preferably 20 ms or more, and more preferably 40 ms or more. On the other hand, if the cooling time t c1 exceeds 300 ms, the welding time becomes long. Therefore, the cooling time t of the first non - energization process c1 shall be 300 ms or less.

[0060] Following the first non - energization process, a first post - energization process is carried out. In the first post - energization process, energization is performed to raise the temperature of region a to an appropriate temperature range. The above - mentioned "appropriate temperature range" refers to the temperature range for softening the hardness of region a. In the first post - energization process, energization is carried out for a conduction time t2 (ms) that satisfies the following Equation (4) at a current value I2 (kA) that satisfies the following Equation (3). I1 ≦ I2 ≦ 3.00I1···(3) 30 ≤ t² ≤ 300 ···(4)

[0061] The inventors have found that by performing a first post-energization step with a current value equivalent to or greater than that of the main energization step, it is possible to soften a region a located away from the joint. Here, the appropriate temperature for softening region a, i.e., the temperature at the center of the joint, is A c1 By setting the temperature above the point (the temperature at which the transformation from ferrite and cementite to austenite begins), the temperature of region a is increased to A c1 It is particularly important to rapidly raise the temperature to just below the point in a short amount of time. This allows for effective softening of region a.

[0062] If the current value I2 in the first post-energization step is less than I1, region a 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 a 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.

[0063] 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 30 ms or more. Preferably, the energization time t2 should be 40 ms or more. On the other hand, if the energization time t2 exceeds 300 ms, the temperature of region a may become too high, potentially causing it to become brittle again. Therefore, the energization time t2 should be 300 ms or less, preferably 100 ms or less, and more preferably 80 ms or less.

[0064] The projection welding according to the present invention further optionally involves a non-energizing time t after the first post-energizing step that satisfies the following formula (5). c2A second de-energizing step may be performed, which includes a de-energizing period of (ms), followed by a second post-energizing step in which current is supplied at a current value I3 (kA) satisfying the following equation (6) for an energizing time t3 (ms) satisfying the following equation (7). Performing the second de-energizing step and the second post-energizing step allows for more effective softening of region a, reducing residual stress and easing stress concentration. Therefore, it is preferable to perform the second de-energizing step and the second post-energizing step after the first post-energizing step. 0 <t c2 ≤400 ···(5) 0.70I2 ≤ I3 ≤ 1.30I2 ···(6) 0 <t3≦300 ···(7)

[0065] Non-energizing time t of the second non-energizing process c2 If is 0ms, the temperature of region a may rise too high during the second post-energization process. Furthermore, if I2 and I3 are equal, it is assumed that the first post-energization process and the second post-energization process cannot be distinguished. Therefore, the non-energization time t of the second non-energization process c2 The duration of the second non-energizing process is greater than 0ms, and preferably 20ms or more. c2 If this exceeds 400ms, the effect of subsequent energization becomes difficult to obtain. Therefore, the non-energizing time t of the second non-energizing process is c2 The response time should be 400ms or less, and preferably 350ms or less.

[0066] If the current value I3 in the second post-energization process is less than 0.70I2, it is not possible to perform appropriate temperature control to more effectively soften region a. Therefore, when performing the second post-energization process, it is preferable that the current value I3 be 0.70I2 or higher, and preferably 0.75I2 or higher. On the other hand, if the current value I3 in the second post-energization process exceeds 1.30I2, the temperature at the joint end may reach a temperature at which it melts again. Therefore, when performing the second post-energization process, it is preferable that the current value I3 be 1.30I2 or lower, and preferably 1.20I2 or lower.

[0067] In the present invention, the second non-energizing step and the second post-energizing step are optional. Therefore, the second post-energizing step does not need to be performed; that is, the energizing time t3 may be set to 0 ms. In this case, the second non-energizing step is also omitted. Generally, projection welded joints cool naturally after projection welding, so if the energizing step is not performed afterward, there is no need to provide a non-energizing step for cooling. Therefore, when the second non-energizing step and the second post-energizing step are performed, the energizing time t3 is preferably greater than 0 ms and 40 ms or more. On the other hand, the second post-energizing step is a step to soften region a, and it is preferable that the energizing time is the same as that of the first post-energizing step. Therefore, the energizing time t3 of the second post-energizing step is 300 ms or less, preferably 100 ms or less, and more preferably 80 ms or less. The non-energizing step and the post-energizing step may be repeated after the second post-energizing step. Repeating the non-energizing process and the post-energizing process is preferable because it increases the effect of softening region a. On the other hand, if the number of repetitions of the non-energizing process and the post-energizing process is too large, the effect will saturate. Therefore, the number of repetitions of this process is preferably less than 10 times, and more preferably less than 4 times.

[0068] As described above, the projection welding method of the present invention can alleviate stress concentration and reduce residual stress by softening region a 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.

[0069] For processes and conditions not described in this specification, conventional methods may be used. [Examples]

[0070] 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.

[0071] 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.

[0072] [Table 1]

[0073] [Table 2]

[0074] The hardness, peel strength, and delayed fracture resistance of the base material and region a of the obtained projection welded joints and bolted test specimens were measured using the method described below.

[0075] [Hardness evaluation] The hardness of the base material and region a 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. The test specimen was ultrasonically cleaned and then 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 equation (1) (i.e., if Hva / Hvm ≤ 0.90), it is marked as a pass and marked as a fail and marked as a fail.

[0076] [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.

[0077] [Evaluation of delayed fracture tolerance] After allowing the projection welded joints to stand for 24 hours, the joints were cut, resin was applied, and the cross-section of the welds was observed. The cross-sections were visually inspected to evaluate whether delayed fracture had occurred. Joints where delayed fracture occurred were marked with "×", and those where delayed fracture did not occur were marked with "○".

[0078] As shown in Table 2, the projection welded joint of the inventive example was a good welded joint with excellent peel strength and delayed fracture resistance. In contrast, a good projection welded joint could not be obtained in the comparative example. [Industrial applicability]

[0079] This invention can provide a projection welded joint with improved peel strength and delayed fracture resistance. [Explanation of Symbols]

[0080] 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 24 Line segment formed by the joint surface of the steel plate with the nut 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 Hva (HV) of region a 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). Hva / Hvm≦0.90...(1) Region a 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 endpoint on the inner circumferential surface side of the nut that is in contact with the steel plate, and B be the endpoint on the outer circumferential surface side. Let the length of the line segment connecting endpoints A and B be 2L. Let C1 be the point obtained by moving L / 2 from point B toward point A on the line segment. Let C2 be the point obtained by moving L from point B toward point B toward point C3 on the line formed by the surface of the steel plate that is continuous with the line segment. Let C3 be the point obtained by moving 0.80 mm from point C2 toward point C3 in the thickness direction of the steel plate and moving 0.80 mm from point C1 toward point C4 in the thickness direction of the steel plate and moving 0.80 mm inside the steel plate. Region a is defined as the interior of the rectangle formed by connecting points C1, C2, C3, and C4.

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 first 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), It has, and optionally, Non-energized time t that satisfies the following equation (5) ｃ２ A second de-energizing step involves providing a de-energizing period of (ms), followed by a current value I that satisfies the following equation (6). ３ (kA) and the energizing time t that satisfies the following equation (7) 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]. 10≦t ｃ１ ≦300 ・・・(2) I １ ≦I ２ ≦3.00I １ ・・・(3) 30≦t ２ ≦300 ・・・(4) 0<t ｃ２ ≦400 ・・・(5) 0.70I ２ ≦I ３ ≦1.300 ２ ・・・(6) 0<t ３ ≦300 ・・・(7)

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