Joint member manufacturing method
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- JFE STEEL CORP
- Filing Date
- 2025-10-09
- Publication Date
- 2026-08-06
Smart Images
Figure JP2025035883_06082026_PF_FP_ABST
Abstract
Description
Manufacturing method for joining members
[0001] This invention relates to a method for manufacturing a joining member.
[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 or bolt with a projection portion (protrusion) is project-welded (resistance-welded) to the steel sheet, and then other plates are fastened to complete the assembly.
[0003] Examples of quality assurance for projection-welded joints include delayed fracture resistance and peel strength. In particular, with the increasing strength of steel plates, delayed fracture resistance, i.e., susceptibility to hydrogen embrittlement, has become a challenge. There is also the issue that hydrogen can be incorporated into the weld due to the influence of rust-preventive oils and plating layers on the surface of the steel plate, making delayed fracture more likely.
[0004] The following methods are used to address delayed fracture of projection-welded joints. Patent documents 1 and 2 disclose a method for manufacturing projection-welded joints, which includes a first energizing step to form the joint, followed by a cooling step for quenching and a second energizing step for tempering.
[0005] International Publication No. 2023 / 181564, International Publication No. 2023 / 182266
[0006] Patent documents 1 and 2 describe a first energizing process to form the joint, followed by a cooling process for quenching and a second energizing process for tempering, which suppresses delayed fracture. However, a problem with this approach is that the cooling process and the second energizing process lengthen the welding process.
[0007] In view of the above issues, the present invention aims to provide a method for manufacturing a jointed member that shortens the welding time compared to conventional methods and exhibits superior delayed fracture resistance and peel strength.
[0008] The inventors of the present invention have diligently studied to solve the above problems and have obtained the following findings. When projection welding a nut or bolt to a steel plate having a predetermined component composition, a post-energizing process is performed after the main energizing process. By setting the current value and energizing time of the post-energizing process within a predetermined range, it is possible to manufacture a jointed member with excellent delayed fracture resistance and peel strength in a short welding time.
[0009] In other words, the gist of the present invention is as follows:
[0010] [1] A method for manufacturing a joint member by projection welding a steel plate and a nut or bolt having a projection via the projection, wherein the steel plate contains, by 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 optionally Al: 1.000% 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 The material has a composition comprising at least one element selected from the group consisting of Nb: 0.080% or less, Sb: 0.200% or less, and Sn: 0.200% or less, with the remainder being Fe and unavoidable impurities, and the projection welding is performed with a current value of I １ (kA) for energizing time t １ A main energizing step involves energizing for (ms) to form a joint between the steel plate and the nut or bolt, followed by a current value I that satisfies the following formula (1). ２ (kA) and the energizing time t that satisfies the following equation (2) ２ A method for manufacturing a joining member comprising a first post-energizing step of energizing for a duration of (ms), and 0.45 × I １ ≤I ２ ≤ 0.95 × I １ ... (1) 50 ≤ t ２ ≤ 200 ... (2)
[0011] [2] Next, following the first post - energization step, a current value I satisfying the following formula (3) ３ (kA) and an energization time t satisfying the following formula (4) ３ (ms) for the second post - energization step. The manufacturing method of the joining member according to [1] above. 0.45×I ２ ≦ I ３ ≦ 0.95×I ２ ・・・(3) 40 ≦ t ３ ≦ 180 ・・・(4)
[0012] According to the present invention, a joining member excellent in stress - relaxation fracture resistance characteristics and peel strength can be manufactured with a welding time shorter than before.
[0013] It is a cross - sectional view including the center line of the nut used in one embodiment of the present invention. It is a graph showing the relationship between the current value and time when performing the main energization step, the first post - energization step, and the second post - energization step in the manufacturing method according to one embodiment of the present invention.
[0014] Hereinafter, the manufacturing method of the joining member according to the present invention will be described. The embodiments described below are an example embodying the present invention, and do not limit the configuration of the present invention with its specific examples.
[0015] [Steel plate] First, the component composition of the steel plate, which is one of the weld - able materials, in the manufacturing method of the joining member according to one embodiment of the present invention will be described. In the following description, the “%” indication of the component composition refers to “mass%” unless otherwise specified.
[0016] C: 0.05 - 0.50% C is an element that contributes to the strengthening of the steel plate. If the C content of the steel plate is less than 0.05%, the strength of the steel plate will be low, and it will be extremely difficult to produce a steel plate with a tensile strength of 780 MPa or more. Therefore, the C content of the steel plate should be 0.05% or more. Preferably, the C content of the steel plate should be 0.10% or more. On the other hand, when the C content of the steel plate exceeds 0.50%, although the strength of the steel plate increases, the amount of hard martensite becomes excessive and microvoids increase. Furthermore, the joint part is excessively hardened and embrittlement progresses, making it difficult to suppress delayed fracture. Therefore, the C content of the steel plate should be 0.50% or less. Preferably, the C content of the steel plate should be 0.45% or less.
[0017] Si: 0.1 - 2.0% When the Si content of the steel plate is 0.1% or more, it effectively acts on the strengthening of the steel plate. Also, since Si is a ferrite-forming element, it predominantly works on the formation of ferrite at the end of the joint part. Therefore, the Si content of the steel plate should be 0.1% or more. Preferably, the Si content of the steel plate should be 0.2% or more. On the other hand, when the Si content of the steel plate exceeds 2.0%, although the steel plate is strengthened, it may have an adverse effect on toughness. Therefore, the Si content of the steel plate should be 2.0% or less. Preferably, the Si content of the steel plate should be 1.8% or less.
[0018] Mn: 1.5 - 4.0% When the Mn content of the steel plate is 1.5% or more, excellent delayed fracture resistance characteristics can be obtained. 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, when the Mn content of the steel plate exceeds 4.0%, embrittlement of the welded part or cracks accompanying embrittlement become显著, making it difficult to suppress delayed fracture. 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 suppress delayed fracture. Therefore, the P content of the steel sheet should be 0.10% or less. Preferably, the P content of the steel sheet should be 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, but excessive reduction will lead 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 an element that is inevitably included. Furthermore, if the S content of steel sheets exceeds 0.005%, it reduces the local deformability of the steel sheet along with sulfides. Therefore, 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, but excessive reduction will lead 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, but excessive reduction will lead 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, but excessive reduction will lead to increased costs, so it is preferable that the oxygen content of steel sheets be 0.005% or more.
[0023] The steel sheet used in this invention may, in addition to the above component composition, optionally contain at least one element selected from the group consisting of Al, B, Ca, Cr, Cu, Ni, Mo, Ti, V, Nb, Sb, and Sn. The remainder of the component composition consists of Fe and unavoidable impurities.
[0024] Al: 1.000% or less. Al is an element that enables microstructure control by refining the austenite grain. Therefore, it is preferable that the Al content of the steel sheet be 0.015% or more. On the other hand, excessive Al content leads to a deterioration of toughness. Therefore, when Al is included, the Al content of the steel sheet should be 1.000% or less, preferably 0.500% or less, and more preferably 0.100% or less.
[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 B is included, the B content of the steel sheet should be 0.0050% or less. Preferably, the B content of the steel sheet 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 of the steel sheet 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 a large amount will degrade the toughness. Therefore, when Ca is included, the Ca content of the steel sheet should be 0.005% or less. Preferably, the Ca content of the steel sheet 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 of the steel sheet 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 a large amount may deteriorate the toughness of the joints. Therefore, when Cr is included, the Cr content of the steel sheet should be 1.00% or less. Preferably, the Cr content of the steel sheet 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 of the steel sheet 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 a large amount will degrade the toughness. Therefore, when Cu is included, the Cu content of the steel sheet should be 0.80% or less. Preferably, the Cu content of the steel sheet 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 of the steel sheet be 0.006% or more.
[0029] Ni: 1.00% or less. Ni is an element that can contribute to improving the strength of steel sheets, but adding a large amount will degrade the toughness. Therefore, when Ni is included, the Ni content of the steel sheet should be 1.00% or less. Preferably, the Ni content of the steel sheet 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 of the steel sheet 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 a large amount will degrade the toughness. Therefore, when Mo is included, the Mo content of the steel plate should be 1.00% or less. Preferably, the Mo content of the steel plate 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 of the steel plate be 0.006% or more.
[0031] Ti: 0.200% or less. Ti is an element that can improve hardenability and strengthen steel sheets, but if added in large quantities, it forms carbides, and the toughness deteriorates significantly due to precipitation hardening. Therefore, when Ti is included, the Ti content of the steel sheet should be 0.200% or less. Preferably, the Ti content of the steel sheet 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 of the steel sheet be 0.003% or more.
[0032] V: 0.500% or less. V is an element that can strengthen steel by controlling the structure through precipitation hardening, but adding it in large quantities can lead to deterioration of the toughness of the joint. Therefore, when V is included, the V content of the steel sheet should be 0.500% or less. Preferably, the V content of the steel sheet should be 0.300% or less, and more preferably 0.100% 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 of the steel sheet 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, if added in large quantities, it not only significantly reduces elongation but also significantly impairs toughness. Therefore, when Nb is included, the Nb content of the steel sheet should be 0.080% or less. Preferably, the Nb content of the steel sheet 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 of the steel sheet 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 a large amount reduces toughness. Therefore, when Sb is included, the Sb content of the steel sheet should be 0.200% or less. Preferably, the Sb content of the steel sheet should be 0.100% or less, and more preferably 0.050% 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 of the steel sheet 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. Adding Sn stabilizes the material, but adding a large amount reduces toughness. Therefore, when Sn is included, the Sn content of the steel sheet should be 0.200% or less. Preferably, the Sn content of the steel sheet should be 0.100% or less, and more preferably 0.050% 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 of the steel sheet 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. When the tensile strength of the steel sheet is 980 MPa or higher, the carbon equivalent tends to be high, which presents a problem in that delayed fracture is more likely to occur when conventional projection welding is performed. According to the present invention, even with a high-strength steel sheet having a tensile strength of 980 MPa or higher, residual stress is reduced by performing the post-current treatment process under predetermined conditions, making delayed fracture less likely to occur. The above effect 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 or more and 2.3 mm or less.
[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-based plating layer (zinc-plated steel sheet). The zinc-based plating layer includes, for example, a hot-dip galvanized layer, an electro-galvanized layer, a Zn-Al plating layer, a Zn-Al-Mg plating layer, and a Zn-Ni layer. Furthermore, the steel sheet used in the present invention may be a steel sheet having an alloyed zinc-plated layer after being subjected to an alloying treatment following zinc plating. The zinc plating layer may be applied to only one side of the steel sheet or to both sides.
[0039] [Nut or Bolt] The nut or bolt used in the present invention is a nut or bolt 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 in the circumferential direction with respect to the center line 12. The projections 16 may be provided at three or four points at equal or unequal intervals with respect to the center line 12. 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. The shape of the bolt may be the bolt described in JIS B 1195:2015. To stabilize the contact between the projection and the steel plate, the number of projections on the nut or bolt is preferably three or more, and more preferably four or more.
[0040] The composition of the nut or bolt is not particularly limited, but for example, S25C (JIS G4051) may be used. In particular, when the carbon content in the composition of the nut or bolt is 0.05% by mass or more, peel strength can be preferably obtained. Therefore, the carbon content of the nut or bolt 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 composition of the nut or bolt is 0.40% by mass or less, peel strength can be preferably obtained. Therefore, the carbon content of the nut or bolt is preferably 0.40% by mass or less, and more preferably 0.38% by mass or less.
[0041] [Method for Manufacturing Joining Members] A method for manufacturing joining members according to one embodiment of the present invention is a method for manufacturing joining members by projection welding a steel plate and a nut or bolt having a projection via the projection. The steel plate has the above-mentioned component composition, and the projection welding is performed with a current value of 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 or bolt, followed by a current value I that satisfies the following equation (1). ２ (kA) and the energizing time t that satisfies the following equation (2) ２ The system comprises a first post-energizing step in which current is supplied for a duration of (ms). Furthermore, optionally following the first post-energizing step, a current value I satisfying the following equation (3) is supplied. ３ (kA) and the energizing time t that satisfies the following equation (4) ３ A second post-energizing step may be included, in which energy is supplied for a duration of (ms). Figure 2 shows the relationship between current value and time when the main energizing step, the first post-energizing step, and the second post-energizing step are performed. 0.45 × I １ ≤I ２ ≤ 0.95 × I １ ... (1) 50 ≤ t ２ ≤200 ... (2) 0.45 × I ２ ≤I ３ ≤ 0.95 × I ２ ... (3) 40 ≤ t ３ ≤ 180 ... (4)
[0042] (Main energizing process) The main energizing process is the process of forming the joint between the steel plate and the nut or bolt. In the main energizing process, the current value I １ Power supply time t １ The joint is formed by applying current during this process. The main energizing process is a process of applying a constant current, that is, a process of applying current by constant current control. However, as long as the joint can be formed, the conditions for the main energizing process described below are not particularly limited.
[0043] Current value I in the main energization process １ If the current is less than 7.0 kA, after joining the steel plate and the nut or bolt, 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 or bolt. Therefore, the current value I of the main energizing process １ It is preferably 7.0 kA or more, and more preferably 8.0 kA or more. On the other hand, the current value I １ If the current exceeds 30.0 kA, the joint expands too much, causing molten metal to splatter and fly out onto the outside of the nut or bolt, completely crushing the projection portion of the nut or bolt and rendering the joint unusable. Therefore, the current value I in the main energizing process is １ It is preferably 30.0 kA or less, and more preferably 28.0 kA or less.
[0044] Main energizing process energizing time t １ If the energizing time is 70 ms or more, the joint can be obtained in a suitable and stable manner. Therefore, the energizing time t of the main energizing process １ The energizing time t is preferably 70 ms or more, more preferably 100 ms or more, and even more preferably 120 ms or more. １ If the energizing time exceeds 250 ms, the joint expands too much, causing molten metal to splatter and fly out onto the outside of the nut or bolt, completely crushing the projection portion of the nut or bolt and rendering the joint unusable. Therefore, the energizing time t of the main energizing process １ The time is preferably 250 ms or less, and more preferably 220 ms or less.
[0045] 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 adequately 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.
[0046] (First Post-Energy-Enhancing Process) Following the main energy-enhancing process, the first post-energy-enhancing process is performed. The first post-energy-enhancing process is a process in the manufacturing method of the joined member that slowly cools the temperature of the joint and plays a role in reducing residual stress in the joint. The first post-energy-enhancing process is a process of applying a constant current value, that is, a process of applying energy by constant current control. It is not necessary to provide a non-energy-enhancing period such as a cooling process between the main energy-enhancing process and the first post-energy-enhancing process, and it is preferable to perform the main energy-enhancing process and the first post-energy-enhancing process continuously.
[0047] Current value I of the first post-energization process ２ 0.45 × I １ If the current value is less than I, the cooling rate will be too fast, making it difficult to reduce residual stress and thus preventing improvement in delayed fracture resistance. Therefore, the current value I of the first post-energization step ２ is 0.45 × I １ That concludes the explanation. On the other hand, the current value I ２ 0.95 × I １ If the current value exceeds this, the temperature will rise too high, causing the joint to expand excessively and potentially resulting in molten metal splattering outside the nut or bolt, thus preventing improvement in delayed fracture resistance. Therefore, the current value I of the first post-energization process ２ is 0.95 × I １ The following applies: 0.92 × I １ The following are preferable.
[0048] The energizing time t of the first post-energizing process ２If the energizing time t is less than 50 ms, the joint will not cool slowly and the residual stress will hardly change, making it impossible to improve the delayed fracture resistance. Therefore, the energizing time t of the first post-energizing process ２ The energizing time t of the first post-energizing step is 50 ms or more, and preferably 70 ms or more. ２ If the energizing time exceeds 200 ms, there is a concern that the joint will expand too much, causing molten metal to splatter outside the nut or bolt, making it impossible to improve the delayed fracture resistance and peel strength. Therefore, the energizing time t of the first post-energizing process ２ The duration should be 200 ms or less, and 180 ms is preferred.
[0049] (Second Post-Energy Energization Process) Following the first post-energization process, an optional second post-energization process may be performed. The second post-energization process is a process in the manufacturing method of the joined member in which the temperature of the joint is cooled more gradually, further reducing the residual stress of the joint and obtaining excellent peel strength. The second post-energization process is a process in which a constant current value is applied, that is, a process in which energy is applied by constant current control. It is not necessary to provide a non-energizing period such as a cooling process between the first post-energization process and the second post-energization process, and it is preferable to perform the first and second post-energization processes continuously.
[0050] Current value I of the second post-energization process ３ 0.45 × I ２ The above conditions are met to obtain a suitable effect of reducing residual stress. Therefore, the current value I of the second post-energization step ３ is 0.45 × I ２ The above is preferable. On the other hand, the current value I of the second post-energization step ３ 0.95 × I ２ The following conditions effectively prevent the temperature of the joint from rising too high. Therefore, the current value I of the second post-energization step is... ３ is 0.95 × I ２ Preferably, the following is true: 0.92 × I ２ The following is more preferable:
[0051] The energizing time t of the second post-energizing process ３When the energizing time is 40 ms or more, the effect of reducing residual stress is preferably obtained. Therefore, the energizing time t of the second post-energizing process is ３ The energizing time t of the second post-energizing process is preferably 40 ms or more, and more preferably 50 ms or more. ３ If the energizing time is 180 ms or less, it is possible to effectively prevent the temperature of the joint from rising too high. Therefore, the energizing time t of the second post-energizing step ３ The time is preferably 180 ms or less, and more preferably 160 ms or less.
[0052] As described above, the method for manufacturing a jointed member according to one embodiment of the present invention can appropriately control the welding conditions in the post-energization process, thereby appropriately slowing down the temperature of the joint and reducing residual stress. In other words, the jointed member obtained by this manufacturing method can suppress delayed fracture. 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, delayed fracture can be suppressed.
[0053] For processes and conditions not described in this specification, conventional methods may be used.
[0054] As an embodiment of the present invention, a joint member was fabricated by projection welding a steel plate and a nut. Steel plates (steel types A to M) with a tensile strength of 780 MPa to 1800 MPa and a plate thickness 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 each steel type 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.
[0055]
[0056] 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 the long side and 50 mm on the short side, and a hole with a diameter of 11 mm was drilled in the center of the test specimen. In addition, an M8 welding nut was prepared, which has three protrusions provided at 120° intervals in the circumferential direction with the center line of the nut as the center. The test specimen was set in an AC welding machine so that the center of the hole and the center of the threaded hole of the nut coincided, and welding was performed under the welding conditions shown in Table 2 to obtain a joined member. The steel type of the M8 welding nut was S25C (JIS G4051). The resistance welding conditions were to be met using a single-phase AC (50 Hz) resistance welding machine with a servo motor pressure attached to the welding gun, and the joined member was fabricated. The pair of electrode tips used were flat electrodes with a diameter of 30 mm. Furthermore, for the test specimens used in the peel strength evaluation described below, bolts were fixed into the nut holes of the joining members to create bolt-fastened test specimens.
[0057] [Evaluation of Delayed Fracture Resistance] After the obtained jointed members were left to stand for 24 hours, the joint was cut, resin was embedded, and the cross-section of the weld was observed. The cross-section was visually inspected to evaluate whether delayed fracture occurred. Those in which delayed fracture occurred were evaluated as "×", and those in which delayed fracture did not occur were evaluated as "○". The evaluation results are shown in Table 2.
[0058] [Peel Strength Evaluation] Using the obtained bolt fastening test pieces, 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 (3.73 kN) was used as the standard for peel strength, and if the peel strength was less than 3.73 kN, it was evaluated as "×" as having inferior peel strength. If the peel strength was 3.73 kN or more and less than 6.00 kN, it was evaluated as "△" as having peel strength equivalent to conventional methods, if it was 6.00 kN or more and less than 9.00 kN, it was evaluated as "○" as having superior peel strength, and if it was 9.00 kN or more, it was evaluated as "◎" as having even superior peel strength. Table 2 shows the measured peel strength and its evaluation results.
[0059]
[0060] As shown in Table 2, the example of the present invention exhibited excellent resistance to delayed fracture, while the comparative example, manufactured under conditions outside the scope of the present invention, showed inferior resistance to delayed fracture. Furthermore, the peel strength of the example of the present invention was excellent, equivalent to or better than that of conventional products, and even better peel strength was obtained in the example in which a second post-energization process was performed.
[0061] According to the present invention, it is possible to manufacture a jointed member with superior delayed fracture resistance and peel strength in a shorter welding time than conventional methods.
[0062] 10 Nut 12 Center line 14 Surface perpendicular to the center line 16 Projection
Claims
1. A method for manufacturing a joint member by projection welding a steel plate and a nut or bolt having a projection via the projection, wherein the steel plate contains, by 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 optionally, Al: 1.000% 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. The material has a composition comprising at least one element selected from the group consisting of Nb: 0.080% or less, Sb: 0.200% or less, and Sn: 0.200% or less, with the remainder being Fe and unavoidable impurities, and the projection welding is performed with a current value of I １ (kA) for energizing time t １ A main energizing step involves energizing for (ms) to form a joint between the steel plate and the nut or bolt, followed by a current value I that satisfies the following formula (1). ２ (kA) and the energizing time t that satisfies the following equation (2) ２ A method for manufacturing a joining member comprising a first post-energizing step of energizing for a duration of (ms), and 0.45 × I １ ≤I ２ ≤ 0.95 × I １ ... (1) 50 ≤ t ２ ≤ 200 ... (2) 2. After the first post-energization step, a current value I satisfying the following formula (3) ３ (kA) and a second post-energization step of energizing for a conduction time t satisfying the following formula (4) ３ (ms). The method for manufacturing a joint member according to claim 1. 0.45 × I ２ ≦ I ３ ≦ 0.95 × I ２ ・・・(3) 40 ≦ t ３ ≦ 180 ・・・(4)