Resistance spot welding method and method for manufacturing welded joint

Abstract

Provided is a resistance spot welding method suited for manufacturing a welded joint that exhibits superior delayed fracture resistance. In the resistance spot welding method, two or more superimposed steel plates are sandwiched between a pair of welding electrodes and energized while being pressurized to form a nugget on the superimposed surfaces of the steel plates, and the steel plates are thereby bonded, the resistance spot welding method being characterized in that, after the bonding, a steady magnetic field is applied to a weld mark generated on the surface of the bonded steel plates through the bonding such that the magnetic flux density becomes 0.1-15 T at an angle θ, formed by a surface normal direction of the bonded steel plates and the application direction of the steady magnetic field, exceeding 0°.

Description

[Technical field]

[0001] The present invention relates to a resistance spot welding method and a method for manufacturing a welded joint using the resistance spot welding method. [Background technology]

[0002] Resistance spot welding is widely used for processing the exterior of vehicles such as automobiles because of the good appearance of the finished product. Resistance spot welding is a technique for joining metals by applying pressure to the metals. Specifically, resistance spot welding is a technique for joining metals by placing electrodes on both sides of two or more metals (e.g., steel plates) to be joined, gradually melting the metals by applying a moderate amount of pressure while passing an electric current through them, and then cooling the metals to solidify the molten parts, thereby joining the metals together. The heat-affected zone that has undergone melting due to joining and is formed at and around the part where the metals are joined is called a nugget. The part joined via the nugget is called a welded joint.

[0003] In resistance spot welding, high tensile stress remains in the nugget during the metal melting and solidification process. In addition, during the above melting and solidification process during welding, rust-preventive oil, moisture, plating film, surface treatment agent, etc. that were present on the steel sheet surface are absorbed into the metal, generating or penetrating hydrogen. This hydrogen tends to accumulate in the tensile stress area, and as a result, delayed fracture occurs in the welded joint due to the residual stress and hydrogen in the nugget in the metal after welding and cooling, which is a problem. Delayed fracture is a phenomenon in which a metal suddenly breaks after a certain time has passed since the completion of processing such as welding, even though the stress applied to the metal is below its yield strength.

[0004] On the other hand, high-strength steel sheets are sometimes used as steel sheets for vehicles such as automobiles in order to improve the crashworthiness of the vehicle body by increasing its strength. Generally, high-strength steel sheets are steel sheets whose strength is increased by adding not only a large amount of C but also various alloy elements, but they have a high susceptibility to hydrogen embrittlement. Therefore, the delayed fracture described above is a particularly serious problem in the resistance spot welding of high-strength steel sheets.

[0005] In response to this problem of delayed fracture, Patent Document 1 discloses that after welding current is applied at a certain pressure, post-current is applied at a higher pressure than this pressure, and the electrode is held, thereby reducing the tensile residual stress in the weld and improving the delayed fracture resistance. Patent Document 1 also discloses that after the electrode holding, further "post-weld heat treatment" is performed at 120 to 220°C for 100 to 6000 seconds, which reduces the amount of hydrogen that has penetrated into the weld and is advantageous in preventing delayed fracture. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6194765 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the technology of Patent Document 1 focuses on reducing the tensile residual stress by optimizing the pressure and current pattern to prevent delayed fracture, and there is room for further improvement in hydrogen embrittlement of steel sheets. Moreover, in the technology of Patent Document 1, the weld is rapidly cooled by the cooling time without current flow provided between the welding current flow and the post-current flow, so that a lot of hydrogen remains without diffusing to the outside of the nugget, increasing the amount of residual hydrogen in the nugget, and there is a concern that it is difficult to suppress delayed fracture caused by residual hydrogen. In addition, even if the "post-welding heat treatment" disclosed in Patent Document 1 is performed to prevent residual hydrogen, there is a further concern that the cost of the heat treatment equipment will increase, and that the material properties will change due to the change in the structure of the steel sheet caused by the heat treatment. Therefore, in order to obtain a welded joint that exhibits better delayed fracture resistance, it was necessary to investigate a method that can better control the hydrogen remaining in the nugget during resistance spot welding.

[0008] In view of the above problems, an object of the present invention is to provide a resistance spot welding method and a method for manufacturing a welded joint that are capable of obtaining a welded joint that exhibits excellent delayed fracture resistance by improving hydrogen embrittlement. [Means for solving the problem]

[0009] In order to solve the above problems, the present inventors have intensively studied ways to improve the delayed fracture resistance of the welded joint by releasing hydrogen generated or entering into the nugget during resistance spot welding to the outside of the steel sheet. As a result, the present inventors have obtained a new finding that applying a steady magnetic field to the steel sheet (joint) after joining is effective for improving the delayed fracture resistance of the welded joint without causing any change in the material due to the microstructural change caused by heat treatment. The inventors have discovered that in resistance spot welding, if a steady magnetic field is applied under specified conditions to a steel sheet after joining in which a nugget is formed, a welded joint exhibiting excellent resistance to delayed fracture can be easily obtained.

[0010] The present invention has been made based on the above findings, and the gist of the present invention is as follows. [1] A resistance spot welding method in which two or more overlapping steel sheets are clamped between a pair of welding electrodes, a current is passed through the steel sheets while applying pressure to the steel sheets, and a nugget is formed on the overlapping surfaces of the steel sheets to join the steel sheets together, a resistance spot welding method including, after the joining, applying a steady magnetic field to a weld mark formed on the surface of the steel sheet after the joining by the joining, such that an angle θ between a surface normal direction of the joined steel sheet and a direction of application of the steady magnetic field exceeds 0° and a magnetic flux density is 0.1 to 15 T.

[0011] In this specification, a "nugget" is usually formed on the overlapping surface between steel sheets (see reference numbers 12 and 22 in Figures 1 and 3) and cannot be directly seen from the surface of the steel sheets after resistance spot welding (see reference numbers 11 and 21 in Figures 1 and 3). However, it is possible to confirm that a "nugget" has been formed by the weld marks (see reference number 6 in Figure 2 and reference numbers 13 and 23 in Figure 3), which are resistance spot weld points that appear on the surfaces of the overlapping steel sheets during this welding. In this specification, the "magnetic flux density" can be measured, for example, according to the method described below.

[0012] [2] The resistance spot welding method according to claim 1, wherein the constant magnetic field is applied for 1 second or more. [3] The resistance spot welding method according to claim 1 or 2, wherein at least one of the steel plates has a tensile strength of 780 MPa or more. [4] The resistance spot welding method according to any one of 1 to 3 above, wherein at least one of the steel plates has a plating coating on at least one surface. [5] The resistance spot welding method according to 4 above, wherein the plated coating is a hot-dip galvanized coating or a galvannealed hot-dip galvanized coating. [6] A method for manufacturing a welded joint, comprising joining steel plates together using any one of the resistance spot welding methods according to [1] to [5] to obtain a welded joint. Effect of the Invention

[0013] According to the resistance spot welding method of the present invention, even when steel sheets are joined together, the problem of delayed fracture can be effectively avoided without causing changes in the properties of the steel sheets due to microstructural changes caused by heat treatment. Also, according to the manufacturing method of the welded joint of the present invention, a welded joint exhibiting excellent delayed fracture resistance can be easily obtained. [Brief description of the drawings]

[0014] [Figure 1] FIG. 2 is a schematic diagram showing the formation of a nugget and joining of steel sheets together in accordance with an embodiment of the present invention; [Diagram 2]FIG. 2 is a plan view of one surface side of the steel sheets after joining according to one embodiment of the present invention. [Diagram 3] FIG. 2 is a schematic diagram showing how a steady magnetic field is irradiated onto a weld mark on a surface of a steel plate after the steel plates are joined together according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0016] In the resistance spot welding method of the present invention, two or more overlapping sheets, for example steel sheets 1, 2, are clamped between a pair of welding electrodes 4, 5 and current is passed through them while applying pressure, forming a nugget 3 on the overlapping surfaces (overlapped portions) 12, 22 of the steel sheets, and after the steel sheets are joined together, a steady magnetic field is applied at a predetermined angle θ and with a predetermined magnetic flux density to at least one of the weld marks 13, 23 produced on the surfaces of the overlapping steel sheets during joining.

[0017] Furthermore, according to the resistance spot welding method of the present invention, hydrogen that mainly accumulates in the nugget can be efficiently released to the outside of the steel sheet, thereby making it possible to effectively and simply avoid the problem of delayed fracture of spot welds without causing changes in material quality due to structural changes caused by heat treatment.

[0018] The method for producing a welded joint of the present invention has the same characteristics as the resistance spot welding method of the present invention described above. According to the method for producing a welded joint of the present invention, a welded joint having excellent delayed fracture resistance can be easily obtained.

[0019] Here, the reason why the delayed fracture resistance of steel sheets can be improved by applying a steady magnetic field to the steel sheets after joining is not clear, but the present inventors speculate as follows. That is, when a steady magnetic field is applied under a predetermined condition to the weld mark generated on the surface of the overlapped steel sheets during joining, the shape of the steel sheets changes due to the magnetostrictive effect. The change in shape is caused by the expansion of the lattice spacing, and the potential energy for diffusion decreases, so that the diffusion rate of hydrogen in the steel sheets increases and the hydrogen is desorbed from the surface. In this way, it is presumed that the magnetic field applied to the steel sheets under the predetermined conditions sufficiently and efficiently reduces the diffusible hydrogen in the steel, and furthermore sufficiently and efficiently reduces the hydrogen that accumulates in the steel, particularly in the nugget, which is a tensile residual stress portion, and therefore the delayed fracture resistance of the welded joint can be improved.

[0020] The resistance spot welding method of the present invention will be described in detail below according to several embodiments, but the resistance spot welding method of the present invention is not limited thereto. In addition, the method for manufacturing a welded joint of the present invention has the same features as those described in detail for the resistance spot welding method of the present invention, and the method for manufacturing a welded joint of the present invention is also not limited to the embodiments described below.

[0021] <Joining steel plates> In the resistance spot welding method of the present invention, hydrogen can be efficiently released from the nugget after joining the steel sheets. Therefore, the process of joining a plurality of steel sheets together to obtain a welded body is not particularly limited, and may be in accordance with general resistance spot welding conditions. General resistance spot welding energization conditions may be, for example, a current of 1 kA to 15 kA, an energization time of 100 ms to 2000 ms, and a pressure of 0.5 kN to 10 kN.

[0022] In one embodiment of the present invention shown in Figs. 1 and 2, a pair of welding electrodes 4, 5 are pressed against the surfaces 11, 21 of two overlapping steel sheets 1, 2 and electricity is applied. At this time, the current-applied portions of the overlapping surfaces 12, 22 of the steel sheets melt once due to resistance heating, and then solidify to form a nugget 3. In this manner, in the resistance spot welding method, the steel sheets 1, 2 are joined via the solid nugget 3. This nugget 3 does not usually appear directly on the surfaces 11, 21 of the joined steel sheets. However, on the surfaces 11, 21 of the joined steel sheets, a burnt mark and / or a dented weld mark 6 is generated at the location where the welding electrodes 4, 5 were pressed. Therefore, it can be confirmed that the nugget 3 exists inside the weld mark 6 of the resistance spot in the sheet thickness direction, and this weld mark 6 is also called the "nugget-equivalent surface" in the process of applying a steady magnetic field described later.

[0023] <<Steel plate characteristics>> The steel plate used in the resistance spot welding method of the present invention is not particularly limited, but is preferably a high-strength steel plate. Specifically, the tensile strength of at least one of the multiple steel plates to be joined is preferably 780 MPa or more, more preferably 1000 MPa or more, and even more preferably 1300 MPa or more. It is even more preferable that all of the multiple steel plates to be joined have the above tensile strength. When the tensile strength of the steel plate to be joined is less than 780 MPa, the degree of tensile residual stress generated in the nugget by resistance spot welding is small, so that delayed fracture is difficult to occur in the obtained welded joint. On the other hand, the higher the strength of the steel plate to be joined as described above, the more easily hydrogen is generated or penetrated into the nugget by resistance spot welding, and delayed fracture is likely to occur in the welded joint, so that the effect of improving the delayed fracture resistance of the welded joint by applying a steady magnetic field is enhanced. The tensile strength of the steel plate is not particularly limited, but can be 3000 MPa or less.

[0024] The composition of the steel sheet is not particularly limited, but is preferably the composition that can be used to form the high-strength steel sheet described above. As the composition of the high-strength steel sheet, for example, a steel sheet having a C content of 0.05 mass% or more and 0.50 mass% or less can be suitably used.

[0025] <<Surface treatment for steel sheets>> Since the resistance spot welding method of the present invention is a welding method in which a steady magnetic field is applied in a non-contact manner and is not affected by the surface state of the steel sheets, any surface treatment such as plating can be applied to the steel sheets to be joined in order to impart desired properties to the steel sheets to be joined. It is preferable that at least one of the multiple steel sheets to be joined has been surface-treated on at least one surface. The at least one surface may be a surface corresponding to the surface of the steel sheets after joining, or a surface corresponding to the overlapping surface. The plating film by surface treatment may be any of organic plating, inorganic plating, and metal plating, and may be plated according to a known method. Among them, from the viewpoint of preventing rust and corrosion, it is preferable that the plating film is a hot-dip galvanized (GI) film or a hot-dip galvannealed (GA) film.

[0026] <Application of a steady magnetic field> Next, in the resistance spot welding method of the present invention, after the above-mentioned steel sheets are joined together, a steady magnetic field is intentionally applied to at least one of the weld marks (surfaces equivalent to the nugget). Here, when applying the steady magnetic field, it is essential that the steady magnetic field has a magnetic flux density of 0.1 T or more and 15 T or less, and the angle θ between the surface normal direction of the joined steel sheets and the application direction of the steady magnetic field is greater than 0°. By controlling the angle θ as described above, hydrogen can be efficiently released from the nugget, and delayed fracture of the welded joint due to hydrogen embrittlement can be effectively and simply reduced without causing changes in the material due to structural changes caused by heat treatment. In the present invention, the steady magnetic field is applied without contacting the steel sheet.

[0027] <<Magnetic flux density>> From the viewpoint of promoting hydrogen diffusion and sufficiently desorbing hydrogen contained in the steel sheet, the magnetic flux density at the weld mark on the surface of the steel sheet after joining is preferably 0.1 T or more, more preferably 0.2 T or more, and even more preferably 0.5 T or more. On the other hand, taking into consideration the performance of a general magnetic field application device, the magnetic flux density at the weld mark on the surface of the steel sheet after joining is preferably 15 T or less, and more preferably 14 T or less. The "magnetic flux density" can be measured near the weld mark using a Teslameter, for example, by placing the probe of the Teslameter in a non-contact position directly above the weld mark. The magnetic flux density received by the weld mark (surface equivalent to the nugget) can be adjusted, for example, by adjusting the number of turns of the coil of the magnetic field application device or the current value.

[0028] <<Angle θ between the surface normal direction of the steel sheet after joining and the normal direction of the magnetic pole surface>> It is also an important condition of the present invention that the angle θ between the surface normal direction of the steel sheet after joining and the direction of application of the steady magnetic field is greater than 0°. Even if a steady magnetic field is applied with an angle θ of 0°, the direction in which the lattice spacing spreads is parallel to the sheet surface, so that the diffusion of hydrogen outside the steel sheet is not promoted and the amount of hydrogen in the nugget is not sufficiently reduced. The larger the angle θ, the closer the direction in which the lattice spacing spreads to perpendicular to the sheet surface, so that the residual hydrogen in the steel, especially in the nugget, is released more, thereby more suppressing delayed fracture. The angle θ is preferably 15° or more, more preferably 30° or more. The upper limit of the angle θ is 90°, and in this case, the steady magnetic field is applied parallel to the steel sheet surface.

[0029] <<Magnetic field application time>> If the time for which the magnetic field is applied to the weld mark (surface equivalent to the nugget) is short, it may not be sufficient to desorb the hydrogen remaining in the nugget to the outside of the steel sheet, and the amount of hydrogen in the steel may not be reduced satisfactorily. Therefore, the time for which the steady magnetic field is applied is preferably 1 second or more, more preferably 5 seconds or more, and even more preferably 10 seconds or more. On the other hand, applying a steady magnetic field to the weld mark for 3600 seconds or more reduces productivity, so the time for applying the steady magnetic field is preferably less than 3600 seconds, more preferably 1800 seconds or less, and even more preferably 1500 seconds or less.

[0030] <<Time from start of current flow to start of application of steady magnetic field>> Delayed fracture caused by resistance spot welding using a welding electrode may occur between 180 and 720 minutes, with the start of current flow being 0 seconds. It is preferable to apply a steady magnetic field before such delayed fracture occurs, and suppress or eliminate hydrogen accumulation in the nugget, which is a tensile stress portion of the steel sheet after joining. From this viewpoint, it is preferable to apply a steady magnetic field to the weld mark (surface equivalent to the nugget) within 360 minutes from the start of current flow to the steel sheet, more preferably within 180 minutes, and even more preferably within 60 minutes. From the viewpoint of avoiding the risk of delayed fracture as much as possible, the shorter the time from the start of current flow to the start of application of the steady magnetic field, the more advantageous it is. Therefore, the lower limit of the time from the start of current flow to the start of application of the steady magnetic field is not particularly limited, but considering the time required for current flow itself, the lower limit of the above time is usually 10 seconds.

[0031] <<Remaining hydrogen content in the nugget>> In the nugget after the resistance spot welding according to the present invention, the amount of remaining hydrogen is preferably 0.5 ppm or less in mass fraction, more preferably 0.3 ppm or less, and may be 0 ppm. Hydrogen remaining in the nugget causes hydrogen embrittlement in the welded joint, so the smaller the amount of remaining hydrogen, the better. In general, the higher the strength of the steel plate, the more likely delayed fracture is to occur when the resistance spot welding is performed. However, in the present invention, a steady magnetic field is applied under predetermined conditions, so that the amount of remaining hydrogen can be effectively reduced even in the case of high strength steel plate.

[0032] <<Magnetic field application device>> A typical device (magnetic field application device) that generates a magnetic field and applies it to an object can be used to apply the steady magnetic field. Examples of the magnetic field application device include electromagnets. In one example, the magnetic field application device can be a pair of electromagnets located on the upper steel plate side and the lower steel plate side, respectively. Each electromagnet can have an iron core, a coil that winds around the iron core, and a driving power source for passing a current through the coil. By turning on the driving power source and passing a continuous direct current through the coil, each electromagnet can be magnetized and a steady magnetic field can be generated. Each electromagnet has a magnetic pole face that is located toward the weld mark (nugget-equivalent surface) at a predetermined interval, and by controlling the direction of the current flowing through the coil, the magnetic pole face of one electromagnet can be made an N pole and the magnetic pole face of the other electromagnet can be made an S pole. This pair of magnetic pole faces can be made to face each other across the weld marks (nugget-equivalent surfaces) on the upper and lower surfaces of the overlapped steel plates. In this way, the stationary magnetic field generated by the pair of electromagnets can be made such that its main magnetic flux is directed from the magnetic pole face (N pole) of one electromagnet to the magnetic pole face (S pole) of the other electromagnet, and this direction can be set as the direction of application of the stationary magnetic field. In this case, the direction of the stationary magnetic field coincides with the normal direction of the magnetic pole faces. This allows the stationary magnetic field to be applied uniformly at a predetermined angle θ with respect to the normal direction of the surface of the steel sheet after joining. In this case, the angle θ can be said to be the angle between the normal direction of the magnetic pole faces and the normal direction of the magnetic pole faces, and the normal direction of the magnetic pole faces can be said to be a straight line connecting the magnetic pole faces and the weld marks. In this specification, the term "continuous direct current" means a direct current whose current value is maintained continuously (preferably constant) rather than in a pulsed manner. In addition, in this specification, the term "stationary magnetic field" means a magnetic field that is maintained continuously rather than in a pulsed manner, and includes a magnetic field formed by a stationary magnet and a magnetic field formed by an electromagnet to which a continuous direct current is supplied.

[0033] As long as the above-mentioned predetermined steady magnetic field is applied to the weld mark (surface equivalent to the nugget), the method of installing the magnetic field application device is not particularly limited. For example, the magnetic field application device 30 may be installed so that the steady magnetic field is applied at an angle θ to at least one side of the weld mark (reference numerals 13 and 23 in FIG. 3) in the shortest straight line distance (L in FIG. 3). When there are multiple weld marks on one surface of the steel sheet after joining, one magnetic field application device may be provided for each weld mark 13 or 23, or one or more magnetic field application devices capable of applying a steady magnetic field across multiple nugget-equivalent surfaces on one surface may be provided. Moreover, the magnetic field application devices may be provided facing both surfaces 11, 21 of the steel sheet after joining.

[0034] Furthermore, when there are multiple weld marks on one surface, the steady magnetic field may be applied to only one of the weld marks, or to any multiple weld marks, or to all of the weld marks, or to the entire one surface of the steel sheet after joining. In particular, considering that hydrogen is likely to remain in the nugget, when there are multiple weld marks on one surface, it is preferable to apply a steady magnetic field to the multiple weld marks, and it is more preferable to apply a steady magnetic field to all of the weld marks.

[0035] From the viewpoint of enhancing the effect of the above-mentioned predetermined frequency on delayed fracture resistance, the shortest linear distance between the surface of the steel sheet and the magnetic field application device is preferably within 15 m, and more preferably within 5 m.

[0036] In the present invention, the amount of hydrogen remaining in the nugget can be reduced without performing heat treatment. Therefore, according to the present invention, compared to conventional techniques in which heat treatment is performed after welding, it is possible to obtain a welded joint that exhibits excellent delayed fracture resistance while avoiding the risk that the component composition and / or microstructure of the steel plate will be changed from the desired state due to heat. In addition, the present invention does not require a heating device to deal with hydrogen embrittlement, and is also advantageous in terms of work time and work costs.

[0037] Furthermore, the present invention, which employs a simple technique of applying a steady magnetic field without contacting the steel sheet, can be particularly advantageously used in resistance spot welding in automobile manufacturing, which requires a large number of fine welds. EXAMPLES

[0038] The present invention will be described in more detail with reference to the following examples. The following examples are merely preferred examples of the present invention, and the present invention is not limited to these examples.

[0039] Two steel plates, measuring 150 mm in the longitudinal direction, 50 mm in the lateral direction, and 1.4 mm in thickness, were used as the lower steel plate 1, which was placed vertically below the lower steel plate 1, and the upper steel plate 2, which was placed vertically above the lower steel plate 1. The tensile strengths of the lower steel plate 1 and the upper steel plate 2, and the presence or absence of a plating film on the surface and overlapping surface of the steel plates, are shown in Table 1. The results are for cases where no plating treatment was applied (CR) and cases where plating treatment was applied (hot-dip galvanized (GI), galvannealed hot-dip galvanized (GA), with a coating weight of 50 g / m per side). 2 )

[0040] The tensile strength was determined by preparing JIS No. 5 tensile test pieces from each steel plate in a direction perpendicular to the rolling direction and conducting tensile tests in accordance with the provisions of JIS Z 2241 (2011).

[0041] As shown in Figures 1 and 2, a welded joint was obtained by clamping a plate set consisting of two overlapping steel plates (lower steel plate 1 and upper steel plate 2) between a pair of welding electrodes (lower electrode 4 and upper electrode 5) and joining them under the joining (current application) conditions shown in Table 1. This joining resulted in weld marks 6 (schematically represented by ellipses) on the surfaces of the steel plates (welded joint) after joining. The above-mentioned steps were performed while the lower electrode 4 and the upper electrode 5 were constantly water-cooled and the steel sheet was kept at room temperature (20° C.). Chromium copper DR type electrodes with a tip diameter (tip diameter) of 6 mm and a curvature radius of 40 mm were used as the lower electrode 4 and the upper electrode 5. The pressure during joining was controlled by driving the lower electrode 4 and the upper electrode 5 with a servo motor, and a single-phase AC with a frequency of 50 Hz was supplied during current application.

[0042] In this way, weld marks 6 were observed on the surfaces 11, 21 of the lower steel sheet 1 and the upper steel sheet 2 after joining, as shown in Fig. 2. A nugget 3, as shown in Fig. 1, was formed on the overlapping surfaces 12, 22 of the lower steel sheet 1 and the upper steel sheet 2 along the plate thickness direction from the weld marks 6. Two welded joints were produced under each current flow condition in order to measure the amount of hydrogen remaining in the nugget before and after the application of a steady magnetic field.

[0043] After the steel sheets were joined by applying current as described above, a steady magnetic field was applied from one side of the surface of the joined steel sheet (welded joint) toward the weld mark 6 (surface equivalent to the nugget) under the conditions described in Table 1 after the "time from the start of current application to the start of application of the steady magnetic field" described in Table 1 had elapsed. The application of the steady magnetic field was performed using an electromagnet installed at a position where the length of the straight line connecting the weld mark 6 and the magnetic pole surface (the shortest straight line distance, L in Figure 3) was 0.5 m, and the magnetic flux density was controlled by the current value. The magnetic flux density was measured by placing the probe tip of a Tesla meter (Lake Shore F41 type) parallel to the steel sheet surface at a position directly above the weld mark and 5 cm above the surface. The angle θ is the angle between the normal direction of the surface of the joined steel sheet (welded joint) and the straight line connecting the weld mark 6 and the magnetic pole surface.

[0044] The obtained welded joint was left at room temperature (20°C) in the air for 24 hours, and whether or not delayed fracture occurred after leaving it was judged visually. Furthermore, when no nugget peeling or cracks were found visually from the surface, the cross section in the plate thickness direction including the center of the nugget was observed with an optical microscope (×50 magnification) to confirm the presence or absence of cracks in the cross section. When nugget peeling (the phenomenon in which the nugget peels into two at the joint interface) was observed, it was rated as D, when a crack was visually observed from the surface, it was rated as C, when a crack that did not reach the surface was observed in the cross section including the center of the nugget, it was rated as B, and when no crack was confirmed from the cross section. The results are shown in Table 1. When no cracks were confirmed from the cross section (A) and when a crack that did not reach the surface was observed in the cross section (B), the welded joint was judged to have excellent delayed fracture resistance.

[0045] The amount of hydrogen remaining in the nugget was measured by thermal desorption analysis. Regarding the amount of hydrogen remaining before the application of a steady magnetic field, a sample was obtained by cutting a welded joint obtained under each current flow condition that had not been subjected to a steady magnetic field to a size of 1 cm x 1 cm x plate thickness so that the resistance spot weld point was included in the center, and the sample was degreased with ethanol and then subjected to thermal desorption analysis. Regarding the amount of hydrogen remaining after the application of a steady magnetic field, a sample was obtained by cutting a welded joint obtained under each current flow condition that had been subjected to a steady magnetic field to a size of 1 cm x 1 cm x plate thickness so that the resistance spot weld point was included in the center, and the sample was degreased with ethanol and then subjected to thermal desorption analysis. The sample was heated at 200°C / hour, and the amount of hydrogen released from the sample was quantified every 5 minutes using a gas chromatograph, and the hydrogen release rate (wt / min) at each temperature was obtained. The amount of hydrogen released was calculated by integrating the hydrogen release rate obtained. The cumulative amount of hydrogen released up to 210°C was divided by the mass of the sample to obtain the part per million (ppm) remaining in the nugget as a mass fraction, which is shown in Table 1.

[0046] [Table 1] TIFF0007626253000002.tif223110

[0047] From Table 1, it can be seen that in all of the welded joints obtained through the application of a steady magnetic field according to specific conditions, the amount of residual hydrogen in the nugget was sufficiently reduced, and as a result, no delayed fracture was observed, and good delayed fracture resistance was demonstrated. In particular, good delayed fracture resistance was achieved even in high-strength steel plates, which were previously prone to delayed fracture. On the other hand, in the welded joints No. 1, 2, 10, 16, and 17, which were not subjected to the application of a steady magnetic field or where the application conditions of the steady magnetic field were outside the specific range, the amount of residual hydrogen in the nugget was high and delayed fracture occurred, meaning that delayed fracture due to residual hydrogen in the nugget could not be suppressed. [Industrial Applicability]

[0048] According to the resistance spot welding method of the present invention, it is possible to effectively avoid the problem of delayed fracture after joining steel sheets together. Furthermore, according to the manufacturing method of the welded joint of the present invention, it is possible to easily obtain a welded joint that exhibits excellent delayed fracture resistance. Therefore, the present invention is suitable for resistance spot welding of high-strength steel sheets, and can be suitably used in the manufacturing process of vehicle parts such as automobiles and the assembly process of vehicle bodies. [Explanation of symbols]

[0049] 1 Steel plate (lower steel plate) 11 Surface 12. Surface of steel plates overlapping each other 13 Weld marks 2 Steel plate (upper steel plate) 21 Surface 22 Steel plate overlapping surface 23 Weld marks 3. Nugget 4 Welding electrode (lower electrode) 5 Welding electrode (upper electrode) 6 Weld marks 30 Magnetic field application device

Claims

1. A resistance spot welding method comprising: clamping two or more overlapping steel sheets between a pair of welding electrodes; applying a current while applying pressure to the steel sheets; and forming a nugget on the overlapping surfaces of the steel sheets to join the steel sheets together, the method comprising the steps of: After the joining, a steady magnetic field is applied to the weld mark generated on the surface of the steel sheet after the joining by the joining so that the angle θ between the surface normal direction of the steel sheet after the joining and the application direction of the steady magnetic field is an angle exceeding 0° and the magnetic flux density is 0.1 to 15 T, the time from the start of the current flow to the start of the application of the steady magnetic field is 10 seconds or more.

2. The resistance spot welding method according to claim 1 , wherein the constant magnetic field is applied for a time period of 1 second or more.

3. 3. The resistance spot welding method according to claim 1, wherein at least one of the steel plates has a tensile strength of 780 MPa or more.

4. The resistance spot welding method according to claim 1 or 2, wherein at least one of the steel plates has a plating coating on at least one surface.

5. The resistance spot welding method according to claim 3 , wherein at least one of the steel sheets has a plated coating on at least one surface.

6. The resistance spot welding method according to claim 4, wherein the plated coating is a hot-dip galvanized coating or a hot-dip galvannealed coating.

7. The resistance spot welding method according to claim 5 , wherein the plated coating is a hot-dip galvanized coating or a hot-dip galvannealed coating.

8. A method for manufacturing a welded joint, comprising joining steel plates together using the resistance spot welding method according to claim 1 or 2 to obtain a welded joint.

9. A method for manufacturing a welded joint, comprising joining steel plates together using the resistance spot welding method according to claim 3 to obtain a welded joint.

10. A method for manufacturing a welded joint, comprising joining steel plates together using the resistance spot welding method according to claim 4 to obtain a welded joint.

11. A method for manufacturing a welded joint, comprising joining steel plates together using the resistance spot welding method according to claim 5 to obtain a welded joint.

12. A method for manufacturing a welded joint, comprising joining steel plates together using the resistance spot welding method according to claim 6 to obtain a welded joint.

13. A method for manufacturing a welded joint, comprising joining steel plates together using the resistance spot welding method according to claim 7 to obtain a welded joint.

Images