Projection welding joint and manufacturing method thereof

Abstract

Provided are a projection welded joint having improved peel strength and delayed fracture resistance, and a method for manufacturing the same. This projection welded joint has a steel plate and a nut, the steel plate having a prescribed component composition, and the minimum hardness Hvc (HV) of region c and the hardness Hvm (HV) of the base material of the steel plate satisfy Hvc / Hvm < = 0.85. The region c is determined as follows in a cross-section of the projection weld including the center line of the nut and passing through the center of the joint. A is the end point on the inner peripheral surface side of the nut in contact with the steel plate, B is the end point on the outer peripheral surface side of the nut, t (mm) is the plate thickness of the steel plate, A1 and A2 are the points moved by 0.20 mm and 2 / 3 t from the point A in the plate thickness direction of the steel plate, B1 and B2 are the points moved by 0.20 mm and 2 / 3 t from the point B in the plate thickness direction of the steel plate, and t (mm) is the plate thickness of the steel plate. The interior of a rectangle formed by connecting the point A1, the point B1, the point B2, and the point A2 is defined as a region c.

Description

TECHNICAL FIELD

[0001] The present application relates to a projection welding joint of a steel sheet and a nut having a projection portion by projection welding of the projection portion, and a manufacturing method thereof. BACKGROUND

[0002] In recent years, from the viewpoint of lightening for improving fuel efficiency and ensuring collision safety, various high-strength steel sheets (high-tension steel) are applied to automobile bodies. In addition, in automobile assembly processes, in cases where resistance spot welding cannot be used, arc welding, bolt fastening is used. Furthermore, bolt fastening is also widely used for joining of dissimilar materials such as steel sheets and aluminum. In bolt fastening, a nut having a projection portion is projection welded (resistance welded) with a steel sheet, and then other sheets are assembled with bolt fastening.

[0003] As one of the quality assurance of projection welding, press-in peel strength (also simply referred to as peel strength) can be cited. With the high-strength of steel sheets in recent years, the joint of a steel sheet and a nut is easily embrittled, and thus there is a problem in that it is difficult to ensure the peel strength. Furthermore, with the high-strength of steel sheets, the delayed fracture, that is, the hydrogen embrittlement sensitivity becomes high, which is also a problem. In addition, there is also a problem that hydrogen penetrates into the welded portion due to the influence of rust preventive oil, plating layer, or the like on the surface of the steel sheet, and the delayed fracture easily occurs.

[0004] Patent Document 1 describes a projection welding joint in which the area ratio of the joint portion to the nominal diameter portion of the nut or bolt satisfies a prescribed relationship, and the maximum value of the hardness of the joint portion and the heat affected portion is 550 HV or less. Furthermore, Patent Document 1 reports that a high joint strength is obtained by increasing the pressing force at the time of projection welding. In addition, Patent Document 2 reports that a high joint strength is obtained by tempering the joint portion by post-energization at a current value lower than the current value at the time of welding after projection welding.

[0005] PRIOR ART DOCUMENTS

[0006] PATENT DOCUMENTS

[0007] Patent Document 1: Japanese Patent Application Publication No. 2012-157900

[0008] Patent Document 2: Japanese Patent Application Publication No. 2013-78784 SUMMARY

[0009] However, the conventional method has a problem in that, when a high-strength steel sheet is projection welded, the peel strength decreases and delayed fracture occurs, and there is room for improvement.

[0010] In view of the above problems, an object of the present application is to provide a projection welding joint and a manufacturing method thereof in which the peel strength and the delayed fracture resistance are improved.

[0011] The present inventors have conducted intensive studies in order to solve the above problems, and as a result, have arrived at the following findings. The carbon content of a high-strength steel sheet is higher than that of conventional steel sheets, and the joining portion is easily embrittled, so the peel strength of a projection-welded joint is reduced. In order to increase the peel strength of a projection-welded joint using a high-strength steel sheet, it is effective to relieve stress concentration by softening the joining surface of the welded nut and the steel sheet. In addition, one cause of delayed fracture is the influence of the hardness of the joining portion that becomes a fracture initiation point, and segregation and inclusions present in the joining portion. Therefore, in order to suppress delayed fracture, it is effective to prevent hardening of the joining portion and to reduce residual stress. In particular, by setting a region c that is a region including the vicinity of the boundary between the steel sheet and the nut and a portion of the joining portion, and reducing the hardness of the region c, it is possible to improve the delayed fracture resistance. This is because by softening the hardness of the region c, the toughness of the region c is improved, and further, the residual stress is reduced, so delayed fracture is less likely to occur.

[0012] That is, the gist of the present application is as follows.

[0013] [1] A projection-welded joint that is a projection-welded joint of a steel sheet and a nut having a projection portion, in which the steel sheet and the nut are projection-welded by the projection portion,

[0014] The steel sheet has a composition containing, 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 at least one element selected from 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, with the remainder consisting of Fe and unavoidable impurities

[0015] The minimum hardness Hvc (HV) of the region c according to the following definition in the steel sheet satisfies the following equation (1) with respect to the base material hardness Hvm (HV) of the steel sheet.

[0016] Hvc / Hvm ≤ 0.85 ··· (1)

[0017] The region c is determined in a cross section of a center line of the projection welded joint including the nut and passing through a center of a joint of the steel sheet and the nut as follows. Endpoints of an inner peripheral surface side of the nut in contact with the steel sheet are set as A and B, a sheet thickness of the steel sheet in the cross section is set as t (mm), points inside the steel sheet moved by 0.20 mm and 2 / 3t in a sheet thickness direction of the steel sheet from the point A are set as Al and A2, respectively, points inside the steel sheet moved by 0.20 mm and 2 / 3t in the sheet thickness direction of the steel sheet from the point B are set as Bl and B2, respectively, and an inside of a rectangle formed by connecting the points Al, Bl, B2, and A2 is set as the region c.

[0018] [2] A method of manufacturing a projection welded joint, which is a method of manufacturing a projection welded joint by projection welding a steel sheet and a nut having a projection portion through the projection portion,

[0019] The steel sheet has a composition containing, 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, further optionally containing at least one element selected from 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, with the remainder consisting of Fe and unavoidable impurities,

[0020] The projection welding has the following procedures:

[0021] A main current application procedure in which a current value I1 (kA) is applied for a current application time t1 (ms) to form a joint of the steel sheet and the nut;

[0022] A non-current application procedure in which a non-current application time t c1 (ms) satisfying the following equation (2) is set for a non-current application period;

[0023] A first post-current application procedure in which a current value I2 (kA) satisfying the following equation (3) is applied for a current application time t2 (ms) satisfying the following equation (4); and

[0024] A second post-current application procedure in which a current value I3 (kA) satisfying the following equation (5) is applied for a current application time t3 (ms) satisfying the following equation (6).

[0025] 800 < t c1... (2)

[0026] 1.10 < I2 < 1.80 I1... (3)

[0027] 50 < t2 < 200... (4)

[0028] I3 < 0.70 I2... (5)

[0029] 300 < t3 < 1000... (6)

[0030] The present application can provide a projection welded joint with improved peel strength and resistance to delayed fracture. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 is a sectional view of a nut including a center line used in an embodiment of the present application.

[0032] Figure 2 is a partial sectional view of a projection welded joint of an embodiment of the present application including a center line of a nut and passing through a center of a joint of a steel sheet and the nut.

[0033] Figure 3 is a plan view of (a) a nut provided with three protruding portions and (b) a nut provided with four protruding portions used in an embodiment of the present application.

[0034] Figure 4 is a chart showing a region c of a section of a projection welded joint of an embodiment of the present application and hardness measurement points in the region c.

[0035] Figure 5 is a chart showing a measurement region 30 of a base material hardness Hvm of a section of a projection welded joint of an embodiment of the present application and hardness measurement points in the region 30. DETAILED DESCRIPTION

[0036] Hereinafter, an embodiment of a projection welded joint of the present application and a method of manufacturing the same will be described. Note that the embodiment described below is one example of embodying the present application, and the present application is not limited to this specific example.

[0037] [Steel Sheet]

[0038] First, the composition of a steel sheet in a projection welded joint of the present application will be described. Note that in the following description, the "% " expression of the composition means "mass %" unless otherwise specified.

[0039] C: 0.05 to 0.50%

[0040] C is an element that contributes to the strengthening of the steel sheet. When the C content of the steel sheet is less than 0.05%, the strength of the steel sheet becomes low, and it is extremely difficult to produce a steel sheet having a tensile strength of 780 MPa or more. Therefore, the C content of the steel sheet is 0.05% or more. The C content of the steel sheet is preferably 0.10% or more. On the other hand, when the C content of the steel sheet exceeds 0.50%, the strength of the steel sheet becomes high, but the amount of hard martensite becomes excessive, and the microvoids increase. In addition, the joint portion is excessively hardened, and embrittlement also proceeds, so it is difficult to improve the peel strength. Therefore, the C content of the steel sheet is 0.50% or less. The C content of the steel sheet is preferably 0.45% or less.

[0041] Si: 0.1 to 2.0%

[0042] When the Si content of the steel sheet is 0.1% or more, it effectively functions to strengthen the steel sheet. In addition, since Si is a ferrite-forming element, it is advantageous for the generation of ferrite at the end portion of the joint portion. Therefore, the Si content of the steel sheet is 0.1% or more. The Si content of the steel sheet is preferably 0.2% or more. On the other hand, when the Si content of the steel sheet exceeds 2.0%, the steel sheet is strengthened, but sometimes adversely affects the toughness. Therefore, the Si content of the steel sheet is 2.0% or less. The Si content is preferably 1.8% or less.

[0043] Mn: 1.5 to 4.0%

[0044] When the Mn content is 1.5% or more, high peel strength can be obtained even without performing long-time cooling. Therefore, the Mn content of the steel sheet is 1.5% or more. The Mn content of the steel sheet is preferably 2.0% or more. On the other hand, when the Mn content of the steel sheet exceeds 4.0%, embrittlement of the welded portion or cracks accompanying embrittlement significantly occur, so it is difficult to improve the peel strength. Therefore, the Mn content of the steel sheet is 4.0% or less. The Mn content of the steel sheet is preferably 3.5% or less.

[0045] P: 0.10% or less

[0046] P is an unavoidable impurity, but when the P content of the steel sheet exceeds 0.10%, strong segregation occurs at the end portion of the joint portion in the welded portion, so it is difficult to improve the peel strength. Therefore, the P content of the steel sheet is 0.10% or less. The P content of the steel sheet is preferably 0.05% or less, and more preferably 0.02% or less. It should be noted that the lower limit of the P content of the steel sheet is not particularly limited. However, excessive reduction leads to an increase in cost, so the P content of the steel sheet is preferably 0.005% or more.

[0047] S: 0.005% or less

[0048] S is an element that segregates at the grain boundaries and embrittles the steel sheet, and is an element that is inevitably contained. In addition, when the S content of the steel sheet exceeds 0.005%, the sulfides and the local deformability of the steel sheet are reduced. Therefore, the S content of the steel sheet is 0.005% or less. The S content of the steel sheet is preferably 0.004% or less, and more preferably 0.003% or less. It should be noted that the lower limit of the S content of the steel sheet is not particularly limited. However, excessive reduction leads to an increase in cost, and therefore the S content of the steel sheet is preferably 0.001% or more.

[0049] N: 0.010% or less

[0050] N is an element that is inevitably contained, and when the N content of the steel sheet exceeds 0.010%, the aging resistance of the steel sheet is deteriorated. Therefore, the N content of the steel sheet is 0.010% or less. The N content of the steel sheet is preferably 0.008% or less. It should be noted that the lower limit of the N content of the steel sheet is not particularly limited. However, excessive reduction leads to an increase in cost, and therefore the N content of the steel sheet is preferably 0.001% or more.

[0051] O: 0.030% or less

[0052] When the O content of the steel sheet exceeds 0.030%, non-metallic inclusions are generated, and the cleanliness and the toughness of the steel sheet are deteriorated. Therefore, the O content of the steel sheet is 0.030% or less. The O content of the steel sheet is preferably 0.020% or less. It should be noted that the lower limit of the O content of the steel sheet is not particularly limited. However, excessive reduction leads to an increase in cost, and therefore the O content of the steel sheet is preferably 0.005% or more.

[0053] The steel sheet used in the present application can be added with one or two or more elements selected from Al, B, Ca, Cr, Cu, Ni, Mo, Ti, V, Nb, Sb, and Sn, in addition to the above-described component composition, as necessary. In addition, the remaining portion of the component composition is Fe and inevitable impurities.

[0054] Al: 2.00% or less

[0055] Al is an element that can control the structure by austenite fine-grainization, but the toughness is deteriorated when added in a large amount. Therefore, when Al is added, the Al content is 2.00% or less. The Al content is preferably 1.50% or less. In addition, the lower limit of the Al content is not particularly limited, but from the viewpoint of sufficiently obtaining the addition effect of Al, the Al content is preferably 0.01% or more, and more preferably 1.20% or more.

[0056] B: 0.0050% or less

[0057] B is an element capable of improving the hardenability and strengthening the steel sheet, but its effect is saturated when added in a large amount. Therefore, when B is added, the B content is 0.0050% or less. The B content is preferably 0.0010% or less. In addition, the lower limit of the B content is not particularly limited, but from the viewpoint of sufficiently obtaining the addition effect of B, the B content is preferably 0.0003% or more.

[0058] Ca: 0.005% or less

[0059] Ca is an element capable of contributing to improvement in the workability of the steel sheet, but the toughness deteriorates when added in a large amount. Therefore, when Ca is added, the Ca content is 0.005% or less. The Ca content is preferably 0.004% or less. In addition, the lower limit of the Ca content is not particularly limited, but from the viewpoint of sufficiently obtaining the addition effect of Ca, the Ca content is preferably 0.001% or more.

[0060] Cr: 1.00% or less

[0061] Cr is an element capable of improving the strength of the steel sheet by improving the hardenability, but there is a risk that the toughness of the joint deteriorates when added in a large amount. Therefore, when Cr is added, the Cr content is 1.00% or less. The Cr content is preferably 0.80% or less. In addition, the lower limit of the Cr content is not particularly limited, but from the viewpoint of sufficiently obtaining the addition effect of Cr, the Cr content is preferably 0.01% or more.

[0062] Cu: 0.80% or less

[0063] Cu is an element capable of contributing to improvement in the strength of the steel sheet, but the toughness deteriorates when added in a large amount. Therefore, when Cu is added, the Cu content is 0.80% or less. The Cu content is preferably 0.60% or less. In addition, the lower limit of the Cu content is not particularly limited, but from the viewpoint of sufficiently obtaining the addition effect of Cu, the Cu content is preferably 0.006% or more.

[0064] Ni: 1.00% or less

[0065] Ni is an element capable of contributing to improvement in the strength of the steel sheet, but the toughness deteriorates when added in a large amount. Therefore, when Ni is added, the Ni content is 1.00% or less. The Ni content is preferably 0.80% or less. In addition, the lower limit of the Ni content is not particularly limited, but from the viewpoint of sufficiently obtaining the addition effect of Ni, the Ni content is preferably 0.005% or more.

[0066] Mo: 1.00% or less

[0067] Mo is an element that can contribute to an increase in the strength of the steel sheet, but when added in a large amount, the toughness deteriorates. Therefore, when Mo is added, the Mo content is 1.00% or less. The Mo content is preferably 0.80% or less. In addition, the lower limit of the Mo content is not particularly limited, but from the viewpoint of sufficiently obtaining the addition effect of Mo, the Mo content is preferably 0.006% or more.

[0068] Ti: 0.200% or less

[0069] Ti is an element that can improve hardenability and strengthen the steel sheet, but when added in a large amount, carbides are formed, and the toughness significantly deteriorates due to the precipitation hardening thereof. Therefore, when Ti is added, the Ti content is 0.200% or less. The Ti content is preferably 0.150% or less. In addition, the lower limit of the Ti content is not particularly limited, but from the viewpoint of sufficiently obtaining the addition effect of Ti, the Ti content is preferably 0.003% or more.

[0070] V: 0.500% or less

[0071] V is an element that can strengthen the steel by controlling the structure through precipitation hardening, but when added in a large amount, the toughness of the joint deteriorates. Therefore, when V is added, the V content is 0.500% or less. The V content is preferably 0.300% or less. In addition, the lower limit of the V content is not particularly limited, but from the viewpoint of sufficiently obtaining the addition effect of V, the V content is preferably 0.005% or more.

[0072] Nb: 0.080% or less

[0073] Nb is an element that can improve the cross tensile strength and the delayed fracture resistance after resistance welding by forming fine carbonitrides, but when added in a large amount, not only the elongation significantly decreases, but also the toughness is significantly impaired. Therefore, when Nb is added, the Nb content is 0.080% or less. The Nb content is preferably 0.070% or less, and more preferably 0.060% or less. In addition, the lower limit of the Nb content is not particularly limited, but from the viewpoint of sufficiently obtaining the addition effect of Nb, the Nb content is preferably 0.005% or more.

[0074] Sb: 0.200% or less

[0075] Sb is an element that can suppress nitriding and oxidizing on the surface of the steel sheet, but when added in a large amount, the toughness decreases. Therefore, when Sb is added, the Sb content is 0.200% or less. In addition, the lower limit of the Sb content is not particularly limited, but from the viewpoint of sufficiently obtaining the addition effect of Sb, the Sb content is preferably 0.002% or more.

[0076] Sn: 0.200% or less

[0077] Sn is an element capable of suppressing nitriding and oxidizing on the surface of the steel sheet, and by adding Sn, the material is stabilized, but when added in a large amount, the toughness is reduced. Therefore, when Sn is added, the Sn content is 0.200% or less. In addition, the lower limit of the Sn content is not particularly limited, but from the viewpoint of sufficiently obtaining the effect of the addition of Sn, the Sn content is preferably 0.002% or more.

[0078] The tensile strength of the steel sheet having the above composition is preferably 980 MPa or more, and more preferably 1180 MPa or more. However, when the tensile strength of the steel sheet is 980 MPa or more, the carbon content becomes high, and thus there is a problem in that when the conventional projection welding is performed, the joint portion is easily embrittled, and the press-in peel strength is reduced. According to the present application, even in the case of a high-strength steel sheet having a tensile strength of 980 MPa or more, by softening the hardness of the region c, stress concentration can be alleviated, and thus the reduction in the peel strength can be suppressed. In addition, by reducing the residual stress of the region c, the peel strength of the projection welded portion can be improved, and the delayed fracture resistance can be improved. It should be noted that even in the case where a steel sheet having a tensile strength of less than 980 MPa is used, the above effects can be obtained. On the other hand, the upper limit of the tensile strength of the steel sheet is not particularly limited, but the tensile strength is usually 2000 MPa or less.

[0079] From the viewpoint of targeting a general automotive steel sheet, the thickness of the steel sheet is preferably 0.8 mm to 2.3 mm.

[0080] The steel sheet used in the present application can have a plated layer on the surface of the steel sheet, and for example, can be a steel sheet having a zinc plated layer (a zinc plated steel sheet). The zinc plated layer includes well-known zinc plated layers, including a hot-dip zinc plated layer, an electroplated zinc plated layer, a Zn-Al plated layer, and a Zn-Ni layer, and the like. In addition, the steel sheet used in the present application can be subjected to alloying treatment after the zinc plating treatment, and can be made into a steel sheet having an alloyed zinc plated layer. It should be noted that the zinc plated layer can be applied to only one side of the steel sheet, or can be applied to both sides.

[0081] [Nut]

[0082] The nut used in the present application is a nut having a protrusion portion. Figure 1 A cross section including a center line 12 of a nut 10 used in one embodiment of the present application is shown. The nut 10 has a protrusion portion 16 in a plane 14 perpendicular to the center line 12. In the present embodiment, the protrusion portion 16 is provided in the plane 14 at an interval of 180° from the center line 12. Figure 1 In the present embodiment, two protrusion portions 16 are symmetrically provided at an interval of 180° from the center line 12 in the plane 14 perpendicular to the center line 12 with the center line 12 as the rotation axis. It should be noted that the protrusion portion 16 can be symmetrically provided at three points or four points with the center line 12 as the rotation axis. The shapes of the nut 10 and the protrusion portion 16 can be, for example, a JIS B 1196:2010 described quadrangular welded nut (1C shape) and various protrusion shapes, respectively.

[0083] The composition of the nut is not particularly limited, and for example, S25C (JIS G4051) can be used. In particular, when the C content in the composition of the nut is 0.05 mass% or more, the hardness of region c can be appropriately obtained. Therefore, the C content of the nut is preferably 0.05 mass% or more, and more preferably 0.07 mass% or more. On the other hand, when the C content in the composition of the nut is 0.40 mass% or less, the hardness of region c can be appropriately obtained. Therefore, the C content of the nut is preferably 0.40 mass% or less, and more preferably 0.38 mass% or less.

[0084] In the case where the surface of the nut perpendicular to the center line used in the present application is provided with three or more protruding portions, the contact positions of the protruding portions with the steel sheet are three or more at the time of projection welding described later, and the contact state can be appropriately stabilized. Therefore, the number of the protruding portions provided on the surface of the nut perpendicular to the center line used in the present application is preferably three or more. On the other hand, in the case where the surface of the nut perpendicular to the center line used in the present application is provided with four or less protruding portions, the generation of a protruding portion having weak contact at the time of projection welding, which causes imbalance in input heat, can be appropriately prevented, and the reduction in the strength of the joint portion can be appropriately prevented. Therefore, the number of the protruding portions provided on the surface of the nut perpendicular to the center line used in the present application is preferably four or less.

[0085] [Projection welded joint]

[0086] The projection welded joint of the present application is a projection welded joint in which the above-described steel sheet and the nut are projection welded by the protruding portion of the nut, and a joint portion of the steel sheet and the nut is formed. Here, the "joint portion of the steel sheet and the nut" is a portion in which the steel sheet and the nut are joined by fusion joining or solid phase joining, and is a portion derived from the protruding portion of the nut.

[0087] The projection welded joint of the present application is characterized in that the minimum hardness Hvc (HV) of region c defined below in the steel sheet and the base material hardness Hvm (HV) of the steel sheet satisfy the following formula (1).

[0088] Hvc / Hvm ≤ 0.85 ··· (1)

[0089] Figure 2A cross section of a projection-welded joint 100 according to one embodiment of the present application including a center line 12 of a nut 10 and passing through the center of the joint of the steel sheet 20 and the nut 10. The end point of the inner peripheral surface side of the nut 10 in contact with the steel sheet 20 is set as A, and the end point of the outer peripheral surface side is set as B. The thickness of the steel sheet in this cross section is set as t (mm). Points inside the steel sheet 20, which are 0.20 mm and 2 / 3 t in the thickness direction of the steel sheet 20 from the point A, are set as A1 and A2, respectively. Points inside the steel sheet 20, which are 0.20 mm and 2 / 3 t in the thickness direction of the steel sheet 20 from the point B, are set as B1 and B2, respectively. The inside of a rectangle formed by connecting the points A1, B1, B2, and A2 is set as a region c. Note that, Figure 2 A measurement region 30 of the base material hardness Hvm is also illustrated.

[0090] 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 (a) a nut 10A provided with three projections 16 and (b) a nut 10B provided with four projections 16 used in one embodiment of the present application are shown in plan view. Figure 3 The cutting line shown is a line passing through the center line of the nut and the center of the projection 16. After projection welding of the steel sheet and the nut, the projection-welded joint is cut along the cutting line Figure 3 The projection-welded joint is cut along the cutting line shown, and thus a cross section as shown Figure 2 is obtained.

[0091] When the ratio Hvc / Hvm of the minimum hardness Hvc of the region c to the base material hardness Hvm of the steel sheet is 0.85 or less, the region c is sufficiently softened, and thus the press-in peel strength and the delay fracture resistance are improved. Therefore, the Hvc / Hvm of the projection-welded joint of the present application is 0.85 or less. The Hvc / Hvm is preferably 0.83 or less, and more preferably 0.80 or less. On the other hand, when the Hvc / Hvm is 0.55 or more, the region c can be appropriately tempered. Therefore, the Hvc / Hvm is preferably 0.55 or more.

[0092] The minimum hardness Hvc of the region c is calculated as follows. Figure 4 An enlarged view of the region c is shown. In the region c, hardness measurement points 32 are set at intervals of 0.20 mm in a direction perpendicular to the thickness direction of the steel sheet from points 0.10 mm or 0.30 mm in the direction of the point B1 from the point B2. At each hardness measurement point 32 of the region c, the Vickers hardness is measured by a load of 300 gf for 15 seconds in accordance with JIS Z 2244 (2020) using a pressure head. The minimum value among the obtained measurement results is set as the minimum hardness Hvc of the region c.

[0093] When the minimum hardness Hvc of the region c is 250 HV or more, stress concentration can be appropriately alleviated by softening the region c. Therefore, the minimum hardness Hvc of the region c is preferably 250 HV or more. On the other hand, when the minimum hardness Hvc of the region c is 650 HV or less, stress concentration can be appropriately alleviated by softening the region c. Therefore, the minimum hardness Hvc of the region c is preferably 650 HV or less.

[0094] The base material hardness Hvm of the steel sheet is calculated as follows. As shown in FIG. 6, in the cross section of the projection weld joint 100, a point 7.00 mm away from a point B on the surface of the steel sheet 20 is set as Cl. A point 0.60 mm away from the point Cl on the surface of the steel sheet 20 is set as C2, a point 0.60 mm inside the steel sheet 20 in the thickness direction of the steel sheet 20 from the point C2 is set as C3, and a point 0.60 mm inside the steel sheet 20 in the thickness direction of the steel sheet 20 from the point Cl is set as C4. The inside of a square formed by connecting the points Cl, C2, C3, and C4 is set as a measurement region 30 of the base material hardness Hvm. The point Cl is set as a point 7.00 mm away from the point B in order not to be affected by heat at the time of welding. Figure 2 An enlarged view of the measurement region 30 of the base material hardness Hvm is shown in FIG. 7. Hardness measurement points 32 are set at intervals of 0.20 mm in the thickness direction of the steel sheet and in a direction perpendicular to the thickness direction of the steel sheet from a point 0.20 mm away from the point Cl in the thickness direction of the steel sheet 20. In the measurement region 30, a total of 9 hardness measurement points 32 are set. Figure 5 An enlarged view of the measurement region 30 of the base material hardness Hvm is shown in FIG. 7. Hardness measurement points 32 are set at intervals of 0.20 mm in the thickness direction of the steel sheet and in a direction perpendicular to the thickness direction of the steel sheet from a point 0.20 mm away from the point Cl in the thickness direction of the steel sheet 20. In the measurement region 30, a total of 9 hardness measurement points 32 are set. Figure 5 As shown in FIG. 7, a total of 9 hardness measurement points 32 are set, and the Vickers hardness of each point is measured using a pressure head load of 300 gf for 15 seconds in accordance with JIS Z 2244 (2020), and the average value obtained is set as the base material hardness Hvm. Note that, as described above, the base material hardness Hvm of the steel sheet is measured in the cross section thereof, and therefore the presence or absence of a plated layer does not affect the measurement.

[0095] When the base material hardness Hvm of the steel sheet is 400 HV or more, softening effects can be appropriately obtained in the region c. Therefore, the base material hardness Hvm of the steel sheet is preferably 400 HV or more. On the other hand, when the base material hardness Hvm of the steel sheet is 700 HV or less, softening effects can be appropriately obtained in the region c. Therefore, the base material hardness Hvm of the steel sheet is preferably 700 HV or less, more preferably 650 HV or less.

[0096] [Method for manufacturing projection weld joint]

[0097] The method for manufacturing the projection-welded joint of the present application is a method for manufacturing a projection-welded joint by projection welding a steel sheet and a nut having a projection portion through the projection portion. Also, in the manufacturing method, the steel sheet has the above-described composition, and the projection welding has the following requirements: a main current application process of applying current with a current value I1 (kA) during a current application time t1 (ms) to form a joint portion of the steel sheet and the nut; a non-current application process of setting a non-current application time t c1 (ms) that satisfies the following equation (2) during a non-current application period; a first post-current application process of applying current with a current value I2 (kA) that satisfies the following equation (3) during a current application time t2 (ms) that satisfies the following equation (4); and a second post-current application process of applying current with a current value I3 (kA) that satisfies the following equation (5) during a current application time t3 (ms) that satisfies the following equation (6).

[0098] 800 < t c1 ··· (2)

[0099] 1.10 I1 ≤ I2 ≤ 1.80 I1 ··· (3)

[0100] 50 ≤ t2 ≤ 200 ··· (4)

[0101] I3 ≤ 0.70 I2 ··· (5)

[0102] 300 ≤ t3 ≤ 1000 ··· (6)

[0103] [Main current application process]

[0104] The main current application process is a process of forming a joint portion of the steel sheet and the nut. In the main current application process, the joint portion is formed by applying current with a current value I1 during a current application time t1. Note that the conditions of the main current application process described below are not particularly limited as long as the joint portion can be formed.

[0105] When the current value I1 of the main current application process is 7.0 kA or more, the steel sheet and the nut can be appropriately joined. When the current value I1 is less than 7.0 kA, a gap between the steel sheet and the nut is often observed when the center of the joint portion is cut and the cross section is observed using a microscope after the steel sheet and the nut are joined. Therefore, the current value I1 of the main current application process is preferably 7.0 kA or more, and more preferably 8.0 kA or more. On the other hand, when the current value I1 is 30.0 kA or less, the joint portion can be appropriately prevented from excessively expanding, resulting in molten metal flying out to the outside of the nut in the form of scattering, and the projection portion of the nut becoming a completely flattened shape and failing to function as a joint portion. Therefore, the current value I1 is preferably 30.0 kA or less, and more preferably 28.0 kA or less.

[0106] When the power-on time t1 of the main power-on process is 70 ms or more, the joint portion can be appropriately and stably obtained. Therefore, the power-on time t1 of the main power-on process is preferably 70 ms or more, more preferably 100 ms or more, and further preferably 160 ms or more. On the other hand, when the power-on time t1 is 500 ms or less, the joint portion can be appropriately prevented from excessively expanding, resulting in molten metal flying out to the outside of the nut in a scattered form, and the protruding portion of the nut becoming a completely flattened shape and failing to function as the joint portion. Therefore, the power-on time t1 of the main power-on process is preferably 500 ms or less, and more preferably 400 ms or less.

[0107] When the pressing force of the main power-on process is 3.0 kN or more, the power-on diameter is an appropriate size, and scattering can be appropriately prevented. Therefore, the pressing force of the main power-on process is preferably 3.0 kN or more, and more preferably 3.5 kN or more. On the other hand, when the pressing force of the main power-on process is 6.0 kN or less, the power-on diameter can be appropriately prevented from expanding, and the joint portion can be appropriately ensured. Therefore, the pressing force of the main power-on process is preferably 6.0 kN or less, and more preferably 5.5 kN or less. Note that the pressing force is sometimes limited by the capacity of the device used, and thus can be appropriately adjusted to obtain a desired joint diameter.

[0108] [First non-power-on process and first post-power-on process]

[0109] After the main power-on process, the projection-welded joint, particularly the region c, is subjected to a softening treatment. Specifically, after the main power-on process, a non-power-on process, a first post-power-on process, and a second post-power-on process are performed. By softening the region c, stress concentration at the time of a peel test is alleviated, and residual stress is reduced, and thus delayed fracture can be avoided.

[0110] In the first non-power-on process, the projection-welded joint is cooled to sufficiently lower the temperature of the region c in order to obtain a softening effect.

[0111] In the first non-power-on process, the welded portion is cooled by maintaining a non-power-on state during a cooling time t c1 (ms) shown in Expression (2).

[0112] 800 < t c1 ... (2)

[0113] When the cooling time t c1 of the first non-power-on process is 800 ms or less, the region c cannot be sufficiently softened. Thus, even if the subsequent processes are performed, the region c becomes a brittle structure, and thus stress concentration is easily caused at the time of a press-in peel test, and a crack easily expands. Therefore, the cooling time t c1 exceeds 800 ms. The cooling time t c1Preferably, it is 850 ms or more, and more preferably, it is 900 ms or more. On the other hand, the cooling time t c1 long, resulting in an increase in the processing time. Therefore, the cooling time t c1 Preferably, it is 2500 ms or less.

[0114] After the first non-energization process, a first post-energization process is performed. Energization is performed to raise the temperature of the region c to an appropriate temperature region for softening the hardness of the region c. The above "appropriate temperature region" refers to a temperature region for softening the hardness of the region c. In the first post-energization process, energization is performed with a current value I2 (kA) satisfying the following equation (3) during an energization time t2 (ms) satisfying the following equation (4).

[0115] 1.10 I1≤ I2≤ 1.80 I1 ··· (3)

[0116] 50≤ t2≤ 200 ··· (4)

[0117] The present inventors have found that by performing the first post-energization process with a current value equal to or higher than that of the main energization process, the region c can be softened. By the post-energization process, the region c is softened and the hardness is reduced, whereby stress concentration applied to the region c at the time of the press-in peeling test can be mitigated.

[0118] When the current value I2 of the first post-energization process is less than 1.10 I1, the region c cannot be effectively softened. Therefore, the current value I2 of the first post-energization process is 1.10 I1 or more, and preferably, it is 1.20 I1 or more. On the other hand, when the current value I2 of the first post-energization process exceeds 1.80 I1, there is a high possibility that the temperature of the region c exceeds the A c3 point (temperature at which the phase transition from ferrite to austenite is completed), resulting in embrittlement. As a result, the toughness of the joint cannot be obtained. Therefore, the current value I2 of the first post-energization process is 1.80 I1 or less, and preferably, it is 1.70 I1 or less.

[0119] As described above, the first post-energization process rapidly raises the temperature in a short time, and therefore, the energization time t2 of the temperature raising process is 50 ms or more. The energization time t2 is preferably 60 ms or more. On the other hand, when the energization time t2 exceeds 200 ms, there is a possibility that the temperature of the region c exceeds the A c3 point. Therefore, the energization time t2 is 200 ms or less, and preferably, it is 180 ms or less.

[0120] The projection welding of the present application can be performed with a second post-energization process after the first post-energization process, in which energization is performed with a current value I3 (kA) satisfying the following equation (5) during an energization time t3 (ms) satisfying the following equation (6). By performing the second post-energization process, softening of the region c can be more effectively performed, and residual stress can be reduced, whereby stress concentration can be easily mitigated.

[0121] I3≤ 0.70 I2...(5)

[0122] 300 ≤ t3 ≤ 1000...(6)

[0123] When the current value I3 of the second post-energization process exceeds 0.70 I2, proper temperature control cannot be performed to more effectively perform softening of the region c. Therefore, the current value I3 of the second post-energization process is 0.70 I2 or less, preferably 0.65 I2 or less, and more preferably 0.60 I2 or less. On the other hand, when the current value I3 of the second post-energization process is 0.15 I2 or more, stress concentration due to softening of the region c can be properly relaxed. Therefore, the current value I3 of the second post-energization process is preferably 0.15 I2 or more, and more preferably 0.20 I2 or more.

[0124] The second post-energization process is a process for performing softening of the region c, and when the energization time t3 is 300 ms or more, softening can be effectively performed. Therefore, the energization time t3 of the second post-energization process is 300 ms or more, preferably 350 ms or more, and more preferably 400 ms or more. On the other hand, when the energization time t3 exceeds 1000 ms, the temperature rises, and it can be possible that the temperature again rises to the vicinity of the melting point. Therefore, the energization time t3 of the second post-energization process is 1000 ms or less, and preferably 900 ms or less.

[0125] After the second post-energization process, the non-energization process and the post-energization process can be further repeated. By repeating the non-energization process and the post-energization process, the effect of softening the region b becomes greater, and therefore is preferred. On the other hand, when the number of repetitions of the non-energization process and the post-energization process is too great, the effect saturates, and therefore the number of repetitions of the process is preferably less than 10 times, and more preferably less than 4 times.

[0126] As described above, the projection welding method of the present application can soften the region c by properly controlling the welding conditions of the post-heat treatment process, thereby relaxing stress concentration and reducing residual stress. That is, the projection welded joint obtained by the welding method can improve the press-in peel strength. Therefore, even in the case where a steel sheet whose component composition as a steel sheet contains a large amount of C, specifically, a high-strength steel sheet whose tensile strength is 980 MPa or more and whose C content is 0.05 to 0.50 mass% as described above, the peel strength can be further improved.

[0127] Note that, for the processes and conditions not described in the present specification, conventional methods can be used.

[0128] Example

[0129] As an embodiment of the present application, a projection welded joint was produced by projection welding of a steel sheet and a nut. Test pieces were produced using steel sheets having a tensile strength of 780 MPa to 1800 MPa and a sheet thickness of 1.0 to 1.8 mm (Steel Sheet A to Steel Sheet M). Table 1 shows the tensile strength (TS), sheet thickness, and composition of Steel Sheet A to Steel Sheet M used. Note that the remaining portion of the composition shown in Table 1 is Fe and inevitable impurities, and "-" indicates that the content of the element is below the detection lower limit.

[0130] As test pieces, cold-rolled steel sheets or GA steel sheets (galvannealed steel sheets) of each steel sheet were prepared. The size of the test pieces was 50 mm in the long side and 50 mm in the short side, and a hole having a diameter of 11 mm was formed in the center of the test piece. In addition, an M8 welding nut having three protruding portions symmetrically arranged at intervals of 120° along the center line of the nut was prepared. The test piece was set in an alternating current welding machine so that the center of the hole of the test piece coincided with the center of the threaded hole of the nut, and welding was performed under the welding conditions shown in Table 2 to obtain a welded body. The steel type of the above-mentioned M8 welding nut was S25C (JIS G4051). The resistance welding was performed using a resistance welding machine of a single-phase alternating current (50 Hz) using a servo motor pressurization type electrode mounted on a welding gun to produce a projection welded joint. Note that the pair of electrode chips used was a flat type electrode having a diameter of 30 mm. A bolt was fixed to the nut hole of the obtained projection welded joint to produce a bolt fastening test piece.

[0131]

[0132]

[0133] For the obtained projection welded joint and the bolt fastening test piece, the hardness, peel strength, and delayed fracture resistance of the base material and region c were measured by the following methods.

[0134] [Evaluation of Hardness]

[0135] The hardness of the base material and region c was measured as follows. The projection welded joint was cut at a position including the center line of the nut and passing through the center of the joint between the steel sheet and the nut to produce a test piece, and the test piece was ultrasonically cleaned and then resin-embedded to obtain a sample. The sheet thickness section of the obtained sample was polished and etched using a picric acid solution. The hardness was measured in the sheet thickness section of the etched sample as described above, and the results are shown in Table 2. In the "hardness evaluation" column in Table 2, the hardness measurement results were evaluated as "O" when they satisfied Equation (1) (i.e., when Hvc / Hvm≤0.85 was satisfied), and as "X" when they did not satisfy Equation (1).

[0136] [Evaluation of Peel Strength]

[0137] The maximum load at the time of peeling of the nut from the steel sheet was measured by the press-in peeling test according to JIS B 1196:2010 using the obtained bolt fastening test piece, and the measured value was taken as the peeling strength (kN). In the present application, the lower limit value (3.73 kN) of JIS B 1196:2010 was taken as a reference of the peeling strength, and less than 3.73 kN was evaluated as "X" as a poor peeling strength, 3.73 kN or more and less than 6.00 kN was evaluated as "Δ" as a peeling strength equivalent to the conventional one, and 6.00 kN or more was evaluated as "O" as an excellent peeling strength. Table 2 shows the measured peeling strengths and the evaluation results thereof.

[0138] [Anti-delayed fracture property evaluation]

[0139] After the projection-welded joint was left to stand for 24 hours, the joint portion was cut, resin-embedded, and then the cross section of the welded portion was observed. The cross section was confirmed by visual observation, and evaluated for the presence or absence of delayed fracture. The presence of delayed fracture was evaluated as "X", and the absence of delayed fracture was evaluated as "O".

[0140] As shown in Table 2, the projection-welded joints of the inventive examples were good welded joints having excellent peeling strength and anti-delayed fracture properties. On the other hand, good projection-welded joints were not obtained in the comparative examples.

[0141] Industrial applicability

[0142] The present application can provide a projection-welded joint having improved peeling strength and anti-delayed fracture properties.

[0143] Explanation of symbols

[0144] 100 projection-welded joint

[0145] 10 nut

[0146] 10A nut provided with three protruding portions

[0147] 10B nut provided with four protruding portions

[0148] 12 center line of the nut

[0149] 14 surface of the nut perpendicular to the center line

[0150] 16 protruding portion

[0151] 20 steel sheet

[0152] 30 measurement region of the base material hardness Hvm

[0153] 32 hardness measurement point

Claims

1. A projection weld joint, wherein a steel plate and a nut having a protrusion are formed by projection welding through said protrusion. The steel plate has the following composition by mass: C: 0.05-0.50%, Si: 0.1-2.0%, Mn: 1.5-4.0%, P: less than 0.10%, S: less than 0.005%, N: less than 0.010%, and O: less than 0.030%, further optionally containing at least one element selected from Al: less than 2.00%, B: less than 0.0050%, Ca: less than 0.005%, Cr: less than 1.00%, Cu: less than 0.80%, Ni: less than 1.00%, Mo: less than 1.00%, Ti: less than 0.200%, V: less than 0.500%, Nb: less than 0.080%, Sb: less than 0.200%, and Sn: less than 0.200%, with the remainder consisting of Fe and unavoidable impurities. The minimum hardness Hvc of region c in the steel plate, as defined below, satisfies the following equation (1) with the hardness Hvm of the base material of the steel plate, where... The units for Hvc and Hvm are HV. Hvc / Hvm≤0.85···(1) The region c is defined in the cross section of the welded joint that includes the centerline of the nut and passes through the center of the joint between the steel plate and the nut as follows: the endpoint of the inner circumferential surface of the nut in contact with the steel plate is designated as A, the endpoint of the outer circumferential surface is designated as B, the thickness of the steel plate in the cross section is designated as t, the points inside the steel plate that are moved 0.20 mm and 2 / 3 t from point A in the thickness direction of the steel plate are designated as A1 and A2 respectively, the points inside the steel plate that are moved 0.20 mm and 2 / 3 t from point B in the thickness direction of the steel plate are designated as B1 and B2 respectively, and the interior of the rectangle formed by connecting points A1, B1, B2 and A2 is designated as the region c, where the unit of the thickness t is mm.

2. A method for manufacturing a projection welded joint, comprising projection welding a steel plate to a nut having a protrusion through the protrusion to manufacture the projection welded joint. The steel plate has the following composition by mass: C: 0.05-0.50%, Si: 0.1-2.0%, Mn: 1.5-4.0%, P: less than 0.10%, S: less than 0.005%, N: less than 0.010%, and O: less than 0.030%, further optionally containing at least one element selected from Al: less than 2.00%, B: less than 0.0050%, Ca: less than 0.005%, Cr: less than 1.00%, Cu: less than 0.80%, Ni: less than 1.00%, Mo: less than 1.00%, Ti: less than 0.200%, V: less than 0.500%, Nb: less than 0.080%, Sb: less than 0.200%, and Sn: less than 0.200%, with the remainder consisting of Fe and unavoidable impurities. The projection welding process includes the following steps: In the main energizing process, current value I1 is applied during the energizing time t1 to form the joint between the steel plate and the nut; For non-energized processes, set a non-energized time t that satisfies the following formula (2). c1 During periods when no power is supplied; The first post-energization process is to energize the current value I2 that satisfies the following equation (3) during the energization time t2 that satisfies the following equation (4); and The second energizing process is to energize the current value I3 that satisfies the following equation (5) during the energizing time t3 that satisfies the following equation (6). in, The units of I1, I2, and I3 are kA, and t1 and t2 are also kA. c1 The units for t1, t2, and t3 are milliseconds (ms). 800＜t c1 ···（2） 1.10I1≤I2≤1.80I1···(3) 50≤t2≤200 ···（4） I3≤0.70I2 ···(5) 300≤t3≤1000 ···（6）。

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