Resistance spot welding method and method for manufacturing welded joint

Abstract

Provided is a resistance spot welding method suitable for producing a welded joint exhibiting excellent delayed fracture resistance characteristics. Provided is a resistance spot welding method in which two or more overlapped steel plates are sandwiched by a pair of welding electrodes, the steel plates are energized while being pressurized, nuggets are formed on the overlapping surfaces of the steel plates, and the steel plates are joined to each other, and applying a constant magnetic field to a weld mark generated on the surface of the joined steel sheet by the joining such that the magnetic flux density is 0.1-15 T at an angle such that the angle (theta) formed between the normal direction of the surface of the joined steel sheet and the direction in which the constant magnetic field is applied is greater than 0 DEG.

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 Art

[0002] Resistance spot welding is widely used for processing the appearance of vehicles such as automobiles because of the good appearance of the finished product. Resistance spot welding is one of the technologies that applies pressure to metals to join metals to each other. Specifically, resistance spot welding refers to the following technology: electrodes are contacted from both sides of two or more metals to be joined (such as steel plates), and the metals are gradually melted by applying appropriate pressure while passing electricity, and then the molten part is solidified by cooling the metal, thereby joining the metals to each other. The heat-affected zone formed at the part where the metals are joined and the surrounding area that undergoes melting due to the joining is called a nugget. In addition, the part joined via the nugget is called a weld joint.

[0003] Here, in the resistance spot welding, high tensile stress remains in the nugget during the melting and solidification process of the metal. In addition, during the above-mentioned melting and solidification process during welding, rust-proof oil, moisture, plating film, surface treatment agent, etc. present on the surface of the steel plate are brought into the metal to generate hydrogen or invade hydrogen. Since this hydrogen easily accumulates in the tensile stress part, as a result, in the metal after welding and cooling, the residual stress and hydrogen in the nugget cause delayed fracture in the weld joint, which becomes a problem.

[0004] Delayed fracture refers to a phenomenon in which the metal suddenly breaks after a certain period of time has passed since the completion of a process such as welding, even if the stress applied to the metal is below the yield strength.

[0005] On the other hand, in order to improve the collision resistance by increasing the strength of the vehicle body, high-strength steel sheets are sometimes used as steel sheets for vehicles such as automobiles. Generally speaking, high-strength steel sheets are steel sheets that have not only a large amount of C but also various alloy elements added to improve strength, but are highly sensitive to hydrogen embrittlement. Therefore, the above-mentioned delayed fracture becomes a particularly serious problem in resistance spot welding of high-strength steel sheets.

[0006] For such delayed fracture problem, Patent Document 1 discloses that after welding is energized at a predetermined pressure, post-energization is performed at a pressure higher than the pressure, and the electrode is further held, thereby reducing the tensile residual stress in the weld and improving the delayed fracture resistance. In addition, Patent Document 1 discloses that after the electrode is held, a "post-welding heat treatment" is further performed at 120 to 220°C for 100 to 6000 seconds, which reduces the amount of hydrogen intruding into the weld and helps prevent delayed fracture.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent No. 6194765 Summary of the invention

[0010] However, Patent Document 1 is a technology that specifically focuses on the application force and the rationalization of the power-on mode to reduce the tensile residual stress for delayed fracture, and there is still room for further improvement in the hydrogen embrittlement of the steel plate. In addition, in the technology of Patent Document 1, regarding the hydrogen embrittlement, since the non-power-on cooling time set between the welding power-on and the post-power-on is used to quickly cool the weld, a large amount of hydrogen will not diffuse to the outside of the molten core and remain, and the amount of residual hydrogen in the molten core increases, so there is a concern that it is difficult to suppress the delayed fracture caused by residual hydrogen. In addition, with respect to the residual hydrogen, if the "heat treatment after welding" disclosed in Patent Document 1 is performed, there is a concern that it is impossible to avoid the increase in the cost of the heat treatment equipment and the change in the structure of the steel plate caused by the heat treatment, which will change the material properties.

[0011] Therefore, in order to obtain a weld joint that exhibits more excellent delayed fracture resistance, it is necessary to study a method for better controlling the hydrogen remaining in the nugget during resistance spot welding.

[0012] The present invention has been made in view of the above problems, and an object of the present invention is to provide a resistance spot welding method and a method for manufacturing a welded joint, which can obtain a welded joint exhibiting excellent delayed fracture resistance by improving hydrogen embrittlement.

[0013] In order to solve the above problems, the present inventors have conducted intensive research on a method for improving the delayed fracture resistance of the obtained welded joint by allowing hydrogen generated or intruded in the nugget to escape to the outside of the steel plate during resistance spot welding. As a result, the present inventors have made the following new discovery: applying a constant magnetic field to the steel plates (joint) after joining is effective in improving the delayed fracture resistance of the welded joint, and is not accompanied by changes in the material due to changes in the structure caused by heat treatment.

[0014] Furthermore, the present inventors have found that, in resistance spot welding, if a constant magnetic field is applied under predetermined conditions to steel plates after joining in which a nugget has been formed, a weld joint exhibiting excellent delayed fracture resistance can be easily obtained.

[0015] The present invention has been accomplished based on the above findings, and the gist of the present invention is as follows.

[0016] [1] A resistance spot welding method, comprising: clamping two or more overlapping steel plates with a pair of welding electrodes, applying pressure to the steel plates while applying electricity, forming a molten core on the overlapping surfaces of the steel plates and joining the steel plates to each other, wherein the method includes: after the above-mentioned joining, applying a constant magnetic field to the weld mark generated on the surface of the above-mentioned steel plates after the above-mentioned joining in a manner such that the angle θ formed by the surface normal direction of the above-mentioned steel plates after the above-mentioned joining and the direction of application of the constant magnetic field is greater than 0°, and the magnetic flux density is 0.1 to 15T.

[0017] Here, in this specification, the "nugget" is generally located on the surface where the steel plates overlap each other (see Figure 1 , 3 12, 22) side, and cannot be welded from the surface of the steel plate after resistance spot welding (refer to Figure 1 , 3 11, 21) can be directly visually confirmed, but it can be determined by the resistance spot welding points, i.e., welding marks, generated on the surfaces of the overlapping steel plates (see Figure 2 Symbol 6, Figure 3 Symbols 13, 23) are used to confirm that a "nugget" is formed during welding.

[0018] In this specification, the "magnetic flux density" can be measured by, for example, the method described later.

[0019] [2] The resistance spot welding method according to item 1 above, wherein the time for applying the constant magnetic field is 1 second or longer.

[0020] [3] The resistance spot welding method according to item 1 or 2 above, wherein at least one of the steel plates has a tensile strength of 780 MPa or more.

[0021] [4] The resistance spot welding method according to any one of 1 to 3 above, wherein at least one of the steel sheets has a plated film on at least one surface.

[0022] [5] The resistance spot welding method according to item 4 above, wherein the plated film is a hot-dip galvanized film or an alloyed hot-dip galvanized film.

[0023] [6] A method for producing a welded joint, comprising joining steel plates together using the resistance spot welding method described in any one of [1] to [5] to obtain a welded joint.

[0024] According to the resistance spot welding method of the present invention, even if the steel plates are joined together, the material of the steel plates will not change due to the change in structure caused by heat treatment, and the problem of delayed fracture can be well avoided. In addition, according to the manufacturing method of the welded joint of the present invention, a welded joint with excellent delayed fracture resistance can be easily obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a schematic diagram showing a state where a nugget is formed and steel plates are joined together according to one embodiment of the present invention.

[0026] Figure 2 This is a plan view showing steel plates after joining, viewed from one surface side, according to one embodiment of the present invention.

[0027] Figure 3 This is a schematic diagram showing a state in which a constant magnetic field is irradiated onto a weld mark on the surface of the joined steel plates after joining the steel plates according to one embodiment of the present invention. DETAILED DESCRIPTION

[0028] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0029] In the resistance spot welding method of the present invention, two or more overlapping steel plates 1 and 2 are clamped by a pair of welding electrodes 4 and 5, and current is applied while being pressed to form a weld nugget 3 on the overlapping surfaces (overlapping portions) 12 and 22 of the steel plates. After the steel plates are joined together, a constant magnetic field is applied at a specified angle θ and a specified magnetic flux density to at least one of the weld marks 13 and 23 generated on the surfaces of the overlapping steel plates during the joining.

[0030] Furthermore, according to the resistance spot welding method of the present invention, the problem of delayed fracture of the spot weld can be avoided well and simply by allowing hydrogen mainly accumulated in the nugget to escape to the outside of the steel plate efficiently, and there will be no change in the material due to the change in the structure caused by the heat treatment.

[0031] The method for producing a welded joint of the present invention has the same features as the resistance spot welding method of the present invention described above, and can easily produce a welded joint having excellent delayed fracture resistance.

[0032] Here, the reason why the delayed fracture resistance of the steel sheets can be improved by applying a constant magnetic field to the joined steel sheets is not clear, but the present inventors speculate as follows.

[0033] That is, by applying a constant magnetic field under specified conditions to the weld mark generated on the surface of the overlapping steel plates during joining, the shape of the steel plates changes due to the magnetostrictive effect. This shape change is caused by the expansion of the lattice spacing, which reduces the potential energy for diffusion, increases the diffusion rate of hydrogen in the steel plates, and separates from the surface. As a result, the magnetic field applied to the steel plates under specified conditions can fully and efficiently reduce the diffusible hydrogen in the steel, and further fully and efficiently reduce the hydrogen accumulated in the steel, especially in the tensile residual stress portion, i.e., the nugget, so that the delayed fracture resistance of the welded joint can be improved.

[0034] The resistance spot welding method of the present invention is described in detail below based on some 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 about the resistance spot welding method of the present invention, and the method for manufacturing a welded joint of the present invention is not limited to the embodiments described below.

[0035] <Joining of Steel Plates>

[0036] In the resistance spot welding method of the present invention, hydrogen can be efficiently released from the nugget after the steel plates are joined. Therefore, the process until a plurality of steel plates are joined to obtain a welded body is not particularly limited, and general resistance spot welding conditions may be used. General resistance spot welding power-on conditions may be, for example, a current of 1 kA to 15 kA, a power-on time of 100 ms to 2000 ms, and a pressure of 0.5 kN to 10 kN.

[0037] exist Figure 1 , 2 In one embodiment of the present invention shown, a pair of welding electrodes 4 and 5 are pressed on the surfaces 11 and 21 of the two overlapping steel plates 1 and 2 and energized. At this time, the energized parts of the overlapping surfaces 12 and 22 of the steel plates are temporarily melted due to resistance heating, and then solidified to form a nugget 3. In this way, in the resistance spot welding method, the steel plates 1 and 2 are joined via the nugget 3 in a solid state. The nugget 3 does not usually appear directly on the surfaces 11 and 21 of the joined steel plates. However, a burn mark and / or a concave weld mark 6 is generated at the position where the welding electrodes 4 and 5 are pressed on the surfaces 11 and 21 of the joined steel plates. Therefore, the presence of the nugget 3 can be confirmed inside the weld mark 6 of the resistance spot welding in the plate thickness direction, and in the process of applying a constant magnetic field described later, the weld mark 6 is referred to as "the surface equivalent to the nugget".

[0038] <<Characteristics of Steel Plate>>

[0039] 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, it is preferred that the tensile strength of at least one of the multiple steel plates to be joined is 780MPa or more, more preferably 1000MPa or more, and further preferably 1300MPa or more. In addition, it is further preferred that the multiple steel plates to be joined all have the above-mentioned tensile strength. When the tensile strength of the steel plates to be joined is less than 780MPa, since the degree of tensile residual stress generated in the nugget by resistance spot welding is low, it is difficult to produce delayed fracture in the resulting welded joint. On the other hand, the higher the strength of the steel plates to be joined, the easier it is to produce or invade hydrogen in the nugget due to resistance spot welding, and the easier it is to produce delayed fracture in the welded joint, so the improvement effect of the delayed fracture resistance of the welded joint brought about by applying a constant magnetic field is improved. It should be noted that the tensile strength of the steel plate is not particularly limited and can be 3000MPa or less.

[0040] The composition of the steel sheet is not particularly limited, but is preferably a composition that can form the above-mentioned high-strength steel sheet. As the composition of the high-strength steel sheet, for example, a steel sheet having a C content of 0.05 mass % to 0.50 mass % can be suitably used.

[0041] <<Surface treatment of steel plates>>

[0042] The resistance spot welding method of the present invention is a welding method that applies a constant magnetic field in a non-contact manner and has no effect on the surface state of the steel plate. Therefore, any surface treatment such as plating can be applied to give the joined steel plates desired characteristics. Preferably, at least one of the plurality of joined steel plates is subjected to surface treatment on at least one surface. The at least one surface may be a surface corresponding to the surface of the joined steel plate or a surface corresponding to the overlapping surface.

[0043] The plated film based on the surface treatment can be a plated film based on any one of organic plating, inorganic plating, and metal plating, and the plating can be performed according to a known method. Among them, from the viewpoint of being able to prevent rust and corrosion, the plated film is preferably a hot-dip galvanized (GI) film or an alloyed hot-dip galvanized (GA) film.

[0044] <Application of a constant magnetic field>

[0045] Next, in the resistance spot welding method of the present invention, after the steel plates are joined to each other, a constant magnetic field is intentionally applied to at least one welding mark (equivalent to the surface of the nugget). Here, when applying the constant magnetic field, it is important to make the constant magnetic field with a magnetic flux density of 0.1T to 15T satisfy that the angle θ formed by the surface normal direction of the joined steel plates and the application direction of the constant magnetic field is greater than 0°. By controlling the angle θ as described above, hydrogen can be efficiently released from the nugget, and the delayed fracture of the weld joint caused by hydrogen embrittlement can be well and simply reduced, and there is no change in the material caused by the change in the structure caused by the heat treatment.

[0046] It should be noted that the application of the constant magnetic field in the present invention is performed in a non-contact manner on the steel plate.

[0047] <<Magnetic flux density>>

[0048] From the viewpoint of promoting the diffusion of hydrogen and fully separating the hydrogen contained in the steel sheet, the magnetic flux density of 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 further preferably 0.5 T or more. On the other hand, considering the performance of a conventional magnetic field application device, the magnetic flux density of the weld mark on the surface of the steel sheet after joining is preferably 15 T or less, more preferably 14 T or less. The "magnetic flux density" can be measured near the weld mark using a Tesla meter, for example, by placing a Tesla meter probe at a non-contact position directly above the weld mark.

[0049] The magnetic flux density received by the weld mark (corresponding to the surface of the nugget) can be adjusted by adjusting the number of turns and current value of the coil of the magnetic field applying device, for example.

[0050] <<Angle θ between the surface normal direction of the steel plates after joining and the normal direction of the magnetic pole surface>>

[0051] It is also an important constituent condition of the present invention that the angle θ formed by the surface normal direction of the steel plate after joining and the direction of application of the constant magnetic field is greater than 0°. Even if the constant magnetic field is applied at an angle θ of 0°, since the expansion direction of the lattice spacing is parallel to the plate surface, it will not promote the diffusion of hydrogen out of the steel plate, and the amount of hydrogen in the molten core will not be sufficiently reduced. The larger the angle θ, the closer the expansion direction of the lattice spacing is to the plate surface, so the residual hydrogen is further released from the steel, especially from the molten core, thereby further suppressing delayed fracture. The angle θ is preferably greater than 15°, and more preferably greater than 30°. The upper limit of the angle θ is 90°, at which time the constant magnetic field is applied parallel to the surface of the steel plate.

[0052] <<Magnetic field application time>>

[0053] If the time for applying the magnetic field to the weld mark (equivalent to the surface of the nugget) is short, it is not enough to make the hydrogen remaining in the nugget escape to the outside of the steel plate, and sometimes the amount of hydrogen in the steel cannot be reduced well. Therefore, the time for applying the constant magnetic field is preferably 1 second or more, more preferably 5 seconds or more, and even more preferably 10 seconds or more.

[0054] On the other hand, applying a constant magnetic field to the weld mark for more than 3600 seconds will reduce productivity. Therefore, the time for applying the constant magnetic field is preferably less than 3600 seconds, more preferably 1800 seconds or less, and further preferably 1500 seconds or less.

[0055] <<Time from the start of power supply to the start of application of a steady magnetic field>>

[0056] When the start of power-on is set to 0 seconds, delayed fracture caused by resistance spot welding using welding electrodes may sometimes occur between 180 minutes and 720 minutes. It is preferred to apply a constant magnetic field before such delayed fracture occurs, so as to suppress and eliminate hydrogen accumulation in the tensile stress portion of the steel plate after joining, i.e., the nugget. From this point of view, it is preferred to apply a constant magnetic field to the weld mark (equivalent to the surface of the nugget) within 360 minutes from the start of power-on of the steel plate, more preferably less than 180 minutes, and further preferably within 60 minutes. From the point of view of minimizing the risk of delayed fracture, the shorter the time from the start of power-on to the start of application of the constant magnetic field, the more advantageous it is. Therefore, the lower limit of the time from the start of power-on to the start of application of the constant magnetic field is not particularly limited. Considering the time required for power-on itself, the lower limit of the above time is usually 10 seconds.

[0057] <<Residual Hydrogen Amount in the Nut>>

[0058] Furthermore, in the nugget after resistance spot welding according to the present invention, the amount of residual hydrogen is preferably less than 0.5 ppm, more preferably less than 0.3 ppm, and of course may be 0 ppm in terms of mass fraction. Since the hydrogen remaining in the nugget is the cause of hydrogen embrittlement of the weld joint, the less the amount of residual hydrogen, the better. Generally speaking, the more resistance spot welding of high-strength steel plates is, the more likely delayed fracture will occur. In the present application, since a constant magnetic field is applied under specified conditions, the amount of residual hydrogen can be well reduced even in the case of high-strength steel plates.

[0059] <<Magnetic field application device>>

[0060] The application of a constant magnetic field can use a common device (magnetic field applying device) that generates a magnetic field and applies it to an object. As a magnetic field applying device, for example, an electromagnet can be cited. In one example, the magnetic field applying device can be a pair of electromagnets located on the upper steel plate side and the lower steel plate side, respectively. Each electromagnet has an iron core, a coil wound around the iron core, and a driving power supply for flowing current in the coil. By turning on the driving power supply to allow a continuous DC current to flow in the coil, each electromagnet can be magnetized to generate a constant magnetic field. Each electromagnet has a magnetic pole face that is located at a predetermined interval toward the weld mark (equivalent to the surface of the nugget), and by controlling the direction of the current flowing through the coil, the magnetic pole face of the electromagnet on one side can be an N pole, and the magnetic pole face of the electromagnet on the other side can be an S pole. The pair of magnetic pole faces can be opposed to each other across the weld mark (equivalent to the surface of the nugget) on the upper and lower surfaces of the overlapping steel plates. Thus, the constant magnetic field generated by a pair of electromagnets can be made such that the main magnetic flux is directed from the magnetic pole face (N pole) of the electromagnet on one side toward the magnetic pole face (S pole) of the other electromagnet, and its direction can be the direction of application of the constant magnetic field. In this case, the direction of the constant magnetic field is consistent with the normal direction of the magnetic pole face. Thus, the constant magnetic field can be uniformly applied at a predetermined angle θ relative to the surface normal direction of the steel plate after joining. In this case, the angle θ can be referred to as the angle formed by the normal direction of the magnetic pole face and the normal direction of the magnetic pole face, and the normal direction of the magnetic pole face can be referred to as the straight line connecting the magnetic pole face and the welding mark. It should be noted that in this specification, "continuous direct current" refers to a direct current whose current value is not pulsed but maintained continuously (preferably constantly). In addition, in this specification, "constant magnetic field" refers to a magnetic field that is not pulsed but maintained continuously, including a magnetic field formed by a stationary magnet and a magnetic field formed by an electromagnet that supplies a continuous direct current.

[0061] As long as a predetermined constant magnetic field is applied to the weld mark (equivalent to the surface of the nugget), the method of setting the magnetic field application device is not particularly limited. For example, the direction of applying the constant magnetic field may be such that the shortest straight line distance ( Figure 3 L) at an angle θ to the welding mark ( Figure 3 The magnetic field applying device 30 is set in a manner to apply a constant magnetic field to at least one side of the symbols 13 and 23).

[0062] When there are multiple welding marks on one surface of the joined steel plate, a magnetic field applying device may be provided for each welding mark 13 or 23, or one or more magnetic field applying devices may be provided, and the magnetic field applying device may apply a constant magnetic field to multiple surfaces corresponding to the nuggets on one surface. In addition, the magnetic field applying device may be provided opposite to the two surfaces 11 and 21 of the joined steel plate.

[0063] In addition, when there are multiple weld marks on one surface, a constant magnetic field may be applied to only one of the weld marks, or to any of the multiple weld marks, or to all of the weld marks, or to the entire surface of the steel plate 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 preferred to apply a constant magnetic field to the multiple weld marks, and it is more preferred to apply a constant magnetic field to all of the weld marks.

[0064] In addition, from the viewpoint of the effect of improving delayed fracture resistance by the above-mentioned predetermined frequency, the shortest straight-line distance between the surface of the steel plate and the magnetic field application device is preferably within 15 m, more preferably within 5 m.

[0065] In the present invention, the residual hydrogen in the nugget can be reduced without performing a heat treatment. Therefore, according to the present invention, compared with the conventional technology of performing a heat treatment after welding, the risk of the component composition and / or microstructure of the steel plate changing from a desired state due to heat can be avoided, and a welded joint exhibiting excellent delayed fracture resistance can be obtained. In addition, the present invention does not require a heating device for dealing with hydrogen embrittlement, which is also advantageous in terms of operation time and operation cost.

[0066] Furthermore, the present invention, which adopts a simple method of applying a constant magnetic field without contacting the steel plate, can be particularly advantageously used in resistance spot welding in automobile manufacturing, for example, which requires many fine welding operations.

[0067] Example

[0068] The present invention is further specifically described according to the following examples. The following examples illustrate a preferred example of the present invention, and the present invention is not limited to the examples.

[0069] Two steel plates with a length of 150 mm in the long side direction and a length of 50 mm in the short side direction and a thickness of 1.4 mm were used as a lower steel plate 1 arranged on the lower side in the vertical direction and an upper steel plate 2 arranged on the upper side in the vertical direction of the lower steel plate 1. The tensile strength 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, which is the case where no plating treatment is applied (CR) or the case where plating treatment is applied (hot-dip galvanizing (GI), alloyed hot-dip galvanizing (GA), and the adhesion amount is 50 g / m on one side). 2 )

[0070] The tensile strength is the tensile strength obtained by preparing a JIS No. 5 tensile test piece from each steel plate in a direction perpendicular to the rolling direction and performing a tensile test in accordance with the provisions of JIS Z 2241 (2011).

[0071] like Figure 1 and Figure 2 As shown in FIG. 1 , a plate group consisting of two overlapping steel plates (lower steel plate 1 and upper steel plate 2) is clamped by a pair of welding electrodes (lower electrode 4 and upper electrode 5) and joined under the joining (energization) conditions described in Table 1, thereby obtaining a welded joint. This joining produces a weld mark 6 (schematically represented by an ellipse) on the surface of the joined steel plates (welded joint).

[0072] The above steps are performed while the lower electrode 4 and the upper electrode 5 are always water-cooled and the steel plate is at room temperature (20° C.).

[0073] As the lower electrode 4 and the upper electrode 5, DR-shaped electrodes made of chromium copper with a front end diameter (front end diameter) of 6 mm and a curvature radius of 40 mm are used. In addition, the pressure during joining is 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 is supplied when power is turned on.

[0074] Thus, in the surfaces 11 and 21 of the lower steel plate 1 and the upper steel plate 2 after joining, as shown in FIG. Figure 2 As shown in FIG. 1 , a weld mark 6 is observed. Then, a weld mark 6 is formed on the overlapping surfaces 12 and 22 of the lower steel plate 1 and the steel plate 2 along the plate thickness direction. Figure 1 The nugget 3 is schematically shown. In order to measure the amount of residual hydrogen in the nugget before and after the application of the constant magnetic field, two weld joints were produced under each current supply condition.

[0075] As described above, after the steel plates are joined together by energizing, after the "time from the start of energizing to the start of application of the constant magnetic field" shown in Table 1 has passed, a constant magnetic field is applied from one side of the surface of the steel plates (welded joint) after joining toward the weld mark 6 (corresponding to the surface of the nugget) under the conditions shown in Table 1. The application of the constant magnetic field uses the length of the straight line connecting the weld mark 6 and the magnetic pole surface (the shortest straight line distance, Figure 3 The magnetic flux density is controlled by the current value. The magnetic flux density is measured by placing the front end of the probe of the Tesla meter (F41 model manufactured by LakeShore) parallel to the steel plate surface at a height of 5 cm above the weld mark, just above the weld mark. The angle θ is the angle between the surface normal direction of the steel plate (welded joint) after joining and the straight line connecting the weld mark 6 and the magnetic pole surface.

[0076] The obtained welded joint was left to stand in the atmosphere at room temperature (20°C) for 24 hours, and visual observation was used to determine whether delayed fracture occurred after standing. Furthermore, when no peeling or cracking of the nugget was observed by visual observation from the surface, the cross section in the plate thickness direction including the center of the nugget was observed with an optical microscope (×50 times) to confirm the presence or absence of cracks in the cross section. The case where peeling of the nugget (the phenomenon that the nugget peels off in two at the joint interface) was observed was evaluated as D, the case where cracks were visually observed from the surface was evaluated as C, the case where the cross section in the plate thickness direction including the center of the nugget was observed, and the case where cracks that did not reach the surface were observed on the cross section was evaluated as B, and the case where cracks were not confirmed from the cross section was evaluated as A. The results are shown in Table 1. The case where no cracks were confirmed from the cross section (A) and the case where cracks that did not reach the surface were observed in the cross section (B) were judged to be excellent in delayed fracture resistance of the welded joint.

[0077] The amount of residual hydrogen in the nugget was determined by thermal desorption spectrum analysis. Regarding the amount of residual hydrogen before the application of a constant magnetic field, a sample was obtained by cutting a weld joint obtained under various power-on conditions without applying a constant magnetic field into 1cm×1cm×plate thickness in a manner that includes a resistance spot welding point in the center, and degreasing with ethanol, and then performing thermal desorption spectrum analysis. In addition, regarding the amount of residual hydrogen after the application of a constant magnetic field, a sample was obtained by cutting a weld joint applied with a constant magnetic field in the above-mentioned weld joint into 1cm×1cm×plate thickness in a manner that includes a resistance spot welding point in the center, and degreasing with ethanol, and then performing thermal desorption spectrum analysis. The sample was heated at 200°C / hour, and the amount of hydrogen released from the sample was quantified every 5 minutes by gas chromatography to obtain the hydrogen release rate (wt / min) at each temperature. The hydrogen release rate was accumulated and the amount of hydrogen released was calculated. In addition, the parts per million of the value obtained by dividing the cumulative value of the amount of hydrogen released until 210° C. by the mass of the sample is recorded in Table 1 as the amount of residual hydrogen in the nugget in terms of mass fraction (wt. ppm).

[0078]

[0079]

[0080] As can be seen from Table 1, in the welded joints obtained by applying a constant magnetic field under specific conditions, the amount of residual hydrogen in the nugget can be sufficiently reduced, and as a result, no delayed fracture is confirmed, and good delayed fracture resistance is exhibited. In particular, even high-strength steel plates that are prone to delayed fracture in the past can achieve good delayed fracture resistance. On the other hand, in the welded joints No. 1, 2, 10, 16, and 17 where a constant magnetic field is not applied or the conditions for applying a constant magnetic field are outside a specific range, the amount of residual hydrogen in the nugget is high and delayed fracture occurs, and delayed fracture caused by residual hydrogen in the nugget cannot be suppressed.

[0081] Industrial Applicability

[0082] According to the resistance spot welding method of the present invention, the problem of delayed fracture after joining steel plates can be well avoided. In addition, according to the method for manufacturing a welded joint of the present invention, a welded joint that exhibits excellent delayed fracture resistance can be easily obtained. Therefore, the present invention is suitable for resistance spot welding of high-strength steel plates, and can be applied to the manufacturing process of vehicle parts such as automobiles and the assembly process of vehicle bodies.

[0083] Explanation of symbols

[0084] 1 steel plate (lower steel plate)

[0085] 11 Surface

[0086] 12 Overlapping surfaces of steel plates

[0087] 13 Welding marks

[0088] 2 steel plates (upper steel plate)

[0089] 21 Surface

[0090] 22 Overlapping surfaces of steel plates

[0091] 23 welding marks

[0092] 3. Melt core

[0093] 4 welding electrode (lower electrode)

[0094] 5. Welding electrode (upper electrode)

[0095] 6. Welding marks

[0096] 30 Magnetic field application device

Claims

1. A resistance spot welding method, wherein two or more overlapping steel plates are clamped by a pair of welding electrodes, and the steel plates are pressed and energized to form a nugget on the overlapping surfaces of the steel plates to join the steel plates. in, include: After the joining, a constant magnetic field is applied to the welding mark generated on the surface of the joined steel plate by the joining in a manner that the angle θ formed by the surface normal direction of the joined steel plate and the application direction of the constant magnetic field is greater than 0°, and the magnetic flux density is 0.1 to 15T.

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

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 claims 1 to 3, wherein: At least one of the steel sheets has a plated film on at least one surface.

5. The resistance spot welding method according to claim 4, wherein: The plating film is a hot-dip galvanized film or an alloyed hot-dip galvanized film. 6 . A method for producing a welded joint, comprising joining steel plates together using the resistance spot welding method according to claim 1 to obtain a welded joint.

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