Resistance spot welding method, manufacturing method for welded member, and resistance spot welding device

Abstract

To provide a new method for suppressing hydrogen penetration into a nugget in resistance spot-welding.SOLUTION: A resistance spot-welding method comprises a melting process in which overlapping surfaces of steel plates are melt by electrical conduction between electrodes. This method further comprises a first removal process in which before the melting process, electrical conduction is performed between the electrodes in the state that joint surfaces, which are planned melting locations in the melting process, are not brought into contact with each other, and in the state that an electrically conducting path, which does not pass through the joint surfaces, is formed between the electrodes.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a resistance spot welding method, a method for manufacturing a welded member, and a resistance spot welding apparatus. [Background technology]

[0002] When steel sheets are resistance spot welded, there is a problem of hydrogen embrittlement cracking due to hydrogen penetration into the nugget. Patent Document 1 confirms that oil present on the overlapping surfaces of steel sheets causes hydrogen penetration, and that the amount of hydrogen that penetrates increases as the contact area between the steel sheets increases when pressure is applied during resistance spot welding, and discloses a resistance spot welding method that can suppress hydrogen penetration into the nugget, which is a cause of hydrogen embrittlement cracking. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2020-49541 A Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present disclosure is to provide a new method for suppressing hydrogen penetration into a nugget in resistance spot welding. [Means for solving the problem]

[0005] The inventors have considered reducing hydrogen-causing substances such as oil present on the overlapping surfaces of the steel sheets before melting the overlapping surfaces of the steel sheets as a method for suppressing hydrogen penetration into the nugget. The inventors have come up with the idea of ​​passing a current between the electrodes in a state where the overlapping surfaces (joint surfaces) at the planned welding points are not in contact with each other, and passing a current through a current path formed by an existing weld point. The inventors have also come up with the idea that even if there is no existing weld point, it is sufficient to form a current path in some other manner.

[0006] The present disclosure is based on this idea, and the gist of the disclosure is as follows.

[0007] [1] A resistance spot welding method including a melting step of melting overlapping surfaces of steel sheets by passing current between a pair of opposing electrodes, Prior to the melting step, a first removal step of removing hydrogen-causing substances by passing current between the electrodes in a state where the joining surfaces, which are to be melted in the melting step, are not in contact with each other and in a state where a current path that does not pass through the joining surfaces is formed between the electrodes; Further provided is a resistance spot welding method. (explanation) According to the above method, hydrogen-causing substances (oil, water, etc.) are removed from the joining surface (the area to be melted in the melting process) by the first removal process prior to the melting process, so that the amount of hydrogen-causing substances trapped at the joining surface during the melting process is reduced, and hydrogen penetration into the nugget is suppressed. The first removal step is performed by passing a current between the electrodes while the joining surfaces are not in contact with each other and while a current path that does not pass through the joining surfaces is formed between the electrodes. Therefore, the current mainly flows through the current path that does not pass through the joining surfaces. Since the joining surfaces are not in contact with each other, the hydrogen-causing substances present at the joining surfaces are not trapped between the steel sheets, but are evaporated by Joule heat generated by the current and discharged to the outside.

[0008] [2] In the first removal step, the state in which the bonding surfaces are not in contact with each other is This is achieved by measuring a resistance value between the electrodes and controlling the pressure applied by the electrodes or the position of the electrodes based on the measured resistance value. 2. The resistance spot welding method according to claim 1. (explanation) When the bonding surfaces change from a non-contact state to a contact state, the resistance between the electrodes changes. Specifically, when the bonding surfaces come into contact, the resistance between the electrodes decreases. Therefore, by controlling the pressure applied by the electrodes or the positions of the electrodes based on the measured resistance, it is possible to realize a state in which the bonding surfaces do not come into contact with each other.

[0009] [3] In the first removal step, the state in which the bonding surfaces are not in contact with each other is The position of the electrode or the gap of the bonding surface is monitored by a distance measuring device or an imaging device, and the pressure applied by the electrode or the position of the electrode is controlled based on the monitoring result. 2. The resistance spot welding method according to claim 1.

[0010] [4] After the first removing step and before the melting step, a second removal step of contacting the bonding surfaces with each other by a pressure force smaller than the pressure force applied by the electrodes in the melting step, and removing hydrogen-causing substances by passing a current between the electrodes in a state in which a current path that does not pass through the bonding surfaces is formed between the electrodes; Further preparations: The method for producing a welded member according to any one of [1] to [3]. (explanation) In the second removal process, the joining surfaces are in contact with each other, so that a current flows through the joining surfaces. Therefore, in the second removal process, Joule heat is generated more intensively near the joining surfaces than in the first removal process. As a result, it is possible to shorten the time required to remove the hydrogen-causing substances. In addition, the joining surfaces are brought into contact with each other using a pressure force smaller than that in the melting process, so that the contact area between the steel sheets is relatively small. Therefore, even if the hydrogen-causing substances remaining after the first removal process are trapped between the steel sheets, the amount is small.

[0011] [5] In the second removal step, the state in which the bonding surfaces are brought into contact with each other is While measuring the resistance between the electrodes, the pressure or the current, or both the pressure and the current, are gradually increased; This is realized by stopping the increase in the applied pressure or the current value, or both the applied pressure and the current value, based on the measured resistance value. [4] A resistance spot welding method according to the present invention.

[0012] [6] In the second removal step, the state in which the bonding surfaces are brought into contact with each other is The position of the electrode or the gap of the bonding surface is monitored by a distance measuring device or an imaging device, and the position of the electrode or the pressure applied by the electrode is controlled based on the monitoring result. [4] A resistance spot welding method according to the present invention.

[0013] [7] The current path that does not pass through the joining surface is a nugget that has already been formed by resistance spot welding. The resistance spot welding method according to any one of [1] to [6].

[0014] [8] The steel plate includes at least one steel plate having a tensile strength of 1400 MPa or more; The resistance spot welding method according to any one of [1] to [7].

[0015] [9] A method for manufacturing a welded component having a plurality of nuggets, comprising: One or more of the plurality of nuggets are formed by the resistance spot welding method according to any one of [1] to [8]. A method for manufacturing welded components. (explanation) According to this aspect, a welded component having a nugget in which hydrogen penetration is suppressed can be obtained. When the resistance spot welding method disclosed herein is used, hydrogen penetration into the nugget can be suppressed, but time is required for the first removal step. Therefore, by setting the ratio of nuggets formed by the resistance spot welding method disclosed herein among the multiple nuggets included in the welded component to an appropriate ratio, it is possible to form nuggets that are less likely to cause hydrogen embrittlement cracking in appropriate places while reducing the time required to manufacture the welded component. The appropriate ratio is preferably 10 to 40%, and more preferably 20 to 30%.

[0016]

[10] The nugget formed by the resistance spot welding method is a martensite structure or a tempered martensite structure, or both. A method for manufacturing a welded component according to [9].

[0017]

[11] A resistance spot welding apparatus comprising: a welding machine including an electrode; and a control unit that performs resistance spot welding by controlling welding conditions of the welding machine, The control unit is A melting step is performed by passing current between the electrodes to melt the overlapping surfaces of the steel sheets; Prior to the melting step, A means for performing a first removal step of passing a current between the electrodes in a state where the joining surfaces, which are to be melted in the melting step, are not in contact with each other; Equipped with Resistance spot welding equipment.

[0018]

[12] The control unit is The measuring device further includes a means for measuring a resistance value between the electrodes, In the first removal step, the state in which the bonding surfaces are not in contact with each other is This is achieved by measuring a resistance value between the electrodes and controlling the pressure applied by the electrodes or the position of the electrodes based on the measured resistance value.

[11] A resistance spot welding apparatus according to the present invention. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is a diagram showing a resistance spot welding method according to a first embodiment, in which FIG. 1(A) shows a first removing step, and FIG. 1(B) shows a melting step. [Diagram 2] 2A to 2C are diagrams showing a resistance spot welding method according to a second embodiment, in which FIG. 2A shows a first removing step, FIG. 2B shows a second removing step, and FIG. 2C shows a melting step. [Diagram 3] FIG. 3 shows the samples prepared in Examples 1 and 2. [Figure 4] FIG. 4 is a graph showing the amount of absorbed hydrogen as a result of Example 2. [Diagram 5] FIG. 5 shows an example of a resistance spot welding device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] First, a first embodiment of the method according to the present disclosure will be described with reference to FIG. 1(A) and FIG. 1(B).

[0021] Resistance spot welding is a welding method in which an electric current is passed through materials to be welded, causing a portion of the materials to melt due to Joule heat, thereby joining the materials to be welded. In this embodiment, a welded component is manufactured by welding two steel plates (a first plate 11 and a second plate 12) together by resistance spot welding. The resistance spot welding method of this embodiment includes a first removal step for removing hydrogen-causing substances such as oil present on the joining surfaces of the materials to be welded (meaning the locations to be melted in the melting step) prior to a step of melting the materials to be welded (melting step).

[0022] (First removal process) FIG. 1(A) shows the first removal step.

[0023] The first removal step is performed by sandwiching the portion to be welded between a pair of electrodes 21, 22 and passing a current between the pair of electrodes 21, 22. The positions of the steel sheets with which the electrodes 21, 22 come into contact in the first removal step are the same as the positions of the steel sheets with which the electrodes 21, 22 come into contact in the subsequent melting step. In other words, the contact positions of the electrodes 21, 22 with respect to the steel sheets 11, 12 are not moved between the first removal step and the melting step.

[0024] The first removal step is performed in a state where a current path that does not pass through the bonding surface is formed between the electrodes 21 and 22. In the present embodiment shown in Fig. 1, a welded point 31 (nugget) is formed at a position different from the melted portion in the melting step (see Fig. 1(B)). This forms a current path that passes through the first plate 11 from the portion where the first electrode 21 is in contact, through the first plate 11, via the welded point 31, through the second plate 12, and to the portion where the second electrode 22 is in contact on the second plate 12.

[0025] The first removal step is performed in a state where the overlapping surfaces (joint surfaces) of the steel plates 11, 12 at the intended welding locations are not in contact with each other. In this specification, the term "joint surface" refers to the overlapping surface of steel plates at a location to be welded, and is the location to be melted in the melting step.

[0026] As shown by the arrows in Figure 1(A), the current in the first removal process mainly flows through a current path that does not pass through the joining surfaces. Since the current is passed without the joining surfaces being in contact with each other, the hydrogen-causing substances present on the joining surfaces are not trapped between the steel sheets, but are evaporated by Joule heat generated by the current and discharged to the outside.

[0027] (Melting process) FIG. 1(B) shows the melting process. The melting process is a process in which a large amount of Joule heat is generated at the joining surface by appropriately controlling the pressure and current value, thereby melting the joining surface. The melted portion is cooled by a holding process following the melting process to form a nugget. The melting process to form the nugget may be performed by passing current multiple times. After the nugget is formed, current may be passed to perform a heat treatment of the nugget.

[0028] In the melting process, the joining surfaces are brought into contact with each other by the pressure of the electrodes 21, 22, so that a current flows through the joining surfaces. Therefore, as shown by the arrows in Fig. 1(B), a part of the current flows through a current path that does not pass through the joining surfaces, but most of the current flows through the joining surfaces. The welding conditions in the melting process, such as the current value, pressure, and current flow time, are not particularly limited as long as they are conditions that allow the formation of a desired nugget. The pressure applied in the melting step can be determined according to the total thickness T (mm) of the overlapped steel sheets. The pressure applied in the melting step can be, for example, 50 to 120% of a reference pressure F (kN) calculated by the following formula (1). F = 0.53 × T + 2.4 (1)

[0029] Since the hydrogen-causing substances present on the joining surface are reduced by the first removal process, the amount of hydrogen that penetrates into the nugget during the melting process is reduced.

[0030] (Conditions for the first removal step) The current value in the first removal step is preferably smaller than the current value in the melting step, but from the viewpoint of shortening the welding time, it is preferably 1 / 5 or more of the current value in the melting step. The current value in the first removal step can be, for example, 2.0 kA to 4.0 kA.

[0031] The energization time in the first removal step is not particularly limited, but can be set to 0.05 to 1.00 s. The energization time in the first removal step is, for example, 0.20 s or more, and preferably 0.60 s or more.

[0032] It is preferable that there is no current-free period between the current-flow period in the first removal step and the current-flow period in the melting step, but there may be a current-free period between the current-flow period in the removal step and the current-flow period in the melting step. The pressure in the first removal step can be determined according to the total sheet thickness T (mm) of the overlapped steel sheets. The pressure in the first removal step can be, for example, 10 to 45% of the reference pressure F (kN) calculated by the following formula (1), and can be preferably 10 to 29%. F = 0.53 × T + 2.4 (1)

[0033] (Gap between steel plates) In the first removal step, the overlapping surfaces of the steel plates at the planned welding location may have a gap of, for example, 0.5 mm or more. (Distance from existing weld point) The distance from the welding point (point to be welded) to the existing weld point 31 (current carrying portion) is not particularly limited, but is, for example, 15 to 60 mm.

[0034] (Nugget organization) The structure of the nugget in the finished welded component is not particularly limited, but may be, for example, a martensite structure or a tempered martensite structure. The martensite structure can be produced by rapidly cooling the nugget in a holding step after the melting step. Since the martensite structure is highly susceptible to hydrogen embrittlement, the method of the present disclosure is particularly useful when the nugget is a martensite structure. In addition, the structure of a nugget made of a steel plate with a high carbon content may contain retained austenite.

[0035] (Steel plate as the material to be welded) The thickness of the steel plate as the material to be welded is not particularly limited, but is, for example, 0.5 to 3.0 mm, and is, for example, 0.8 to 3.0 mm. The total plate thickness at the welded portion is not particularly limited, but is, for example, 5.2 mm or less.

[0036] The steel plate as the material to be welded is not particularly limited, but is, for example, a high-strength steel plate having a tensile strength of 900 MPa or more (or 1400 MPa or more). When high-strength steel plates are resistance spot welded, the problem of hydrogen embrittlement cracking tends to become prominent, so the method of the present disclosure is particularly useful when the material to be welded is a high-strength steel plate. In this disclosure, "steel plate" includes plated steel plate.

[0037] (Welded components as finished products) The use of the welded member as a finished product is not particularly limited, but for example, it is for automobiles. High-strength steel plates used as steel plates for automobiles have a higher carbon content than low-strength steel plates, and the welded parts become hard martensite. Furthermore, steel plates for automobiles are generally welded while being coated with oil such as rust-preventive oil. Therefore, the method of the present disclosure is particularly useful when manufacturing welded members for automobiles.

[0038] Next, a method for realizing a state in which the bonding surfaces are not in contact with each other in the first removal step will be described.

[0039] When the bonding surfaces change from a non-contact state to a contact state, the resistance between the electrodes changes. Therefore, by controlling the pressure applied by the electrodes based on the measured resistance, the bonding surfaces can be brought into a state where they are not in contact with each other. For example, when it is detected from a decrease in the resistance that the bonding surfaces have come into contact with each other, the pressure can be reduced to bring the bonding surfaces back into a state where they are not in contact with each other.

[0040] Next, a method according to a second embodiment of the present disclosure will be described.

[0041] FIG. 2 shows the spot welding method according to the second embodiment. As shown in FIG. 2, the spot welding method according to the second embodiment differs from the method according to the first embodiment in that it includes a second removal step (see FIG. 2(B)) after the first removal step (see FIG. 2(A)) and before the melting step (see FIG. 2(C)). The second removal process is a process in which the joining surfaces are brought into contact with each other using a pressure force that is smaller than the pressure force of the electrodes in the melting process, and a current is passed between the electrodes in a state in which a current path is formed between the electrodes that does not pass through the joining surfaces.

[0042] The second removal step also removes hydrogen-causing substances from the bonding surfaces, similarly to the first removal step. Specifically, as shown by the arrows in FIG. 2(B), the second removal process is performed with the joining surfaces in contact with each other, so that a current flows through the joining surfaces. Therefore, in the second removal process, Joule heat is generated more effectively near the joining surfaces than in the first removal process. As a result, it is possible to shorten the time required to remove the hydrogen-causing substances. In addition, the joining surfaces are brought into contact with each other using a pressure force smaller than that in the melting process, so the contact area between the steel sheets is small. Therefore, even if the hydrogen-causing substances remaining after the first removal process are trapped between the steel sheets, the amount will be small.

[0043] (Conditions for the second removal step) The current value in the second removal step is preferably smaller than the current value in the melting step, but from the viewpoint of shortening the welding time, it is preferably at least ¼ or at least ½ of the current value in the melting step. The current value in the second removal step can be, for example, 2.0 kA to 4.0 kA. The energization time in the second removal step is not particularly limited, but can be set to 0.05 to 1.00 s. The energization time in the second removal step is, for example, 0.20 s or more, and preferably 0.60 s or more.

[0044] The pressure applied in the second removal step may be the same as the pressure applied in the first removal step. The steel plate is softened by Joule heat generated by the current passed in the first and second removal steps, so that it is possible to bring the mating surfaces into contact at the welding points even with the same pressing force as in the first removal step. According to this method, there is no need to change the pressing force between the first and second removal steps, making it easy to control.

[0045] Of course, the pressure in the second removal step may be greater than the pressure in the first removal step. For example, in the second removal process, the pressure is gradually increased, and the increase in pressure is stopped when contact of the joining surfaces is detected from a change (decrease) in the resistance value between the electrodes, thereby achieving a contact state with a small contact area. Alternatively, in the second removal step, the current value may be gradually increased to soften the steel plate by Joule heat, thereby bringing the joining surfaces into contact. In this case, too, a contact state with a small contact area can be achieved by controlling the current value while measuring the resistance between the electrodes. When the current value is gradually increased to bring the joining surfaces into contact, an excessively high current value can cause excessive scattering when the surfaces come into contact, so appropriate settings are required. In addition, in order to bring the bonding surfaces into contact in the second removal step, both the pressure and the current value may be gradually increased. In addition, the pressure force, current value, and current flow time that can achieve a contact state with a small contact area can be confirmed in advance through experiments, and the second removal step can be carried out based on these conditions, thereby achieving a contact state with a small contact area and allowing current to flow. Here, as a method for suppressing hydrogen penetration into the nugget, it is possible to simply reduce the pressure applied in the welding process to reduce the amount of hydrogen-causing substances trapped. However, if the pressure applied in the welding process is too small (the contact area is too small), the current flow is unstable, causing splashing, or an appropriate nugget diameter cannot be obtained. According to this embodiment, since the current can be applied with a small contact area while utilizing the existing welded points in the second removal process, the occurrence of excessive splashing and explosion can be suppressed. In addition, since the hydrogen-causing substances can be removed in the first removal process and the second removal process, the current can be applied with a higher pressure applied in the welding process than in the first removal process and the second removal process. This solves the problems of splashing, explosion, and inability to obtain an appropriate nugget diameter, which are concerns when the welding process is performed with a low pressure applied.

[0046] From the standpoint of reducing the welding time, it is preferable that the current flow period in the first removal step, the current flow period in the second removal step, and the current flow period in the melting step are all continuous and that there is no period of no current flow between each of the periods.

[0047] 1 and 2, for the sake of explanation, an example is given in which two steel plates are overlapped, but three or more steel plates may be used. When there are three or more steel plates, there will be two or more joint surfaces, and in order to fully obtain the effect of the first removal step, all of the two or more joint surfaces must be in a state where they are not in contact with each other. Therefore, when there are three or more steel plates as the material to be welded, in the first removal step, all of the two or more joint surfaces must be in a state where they are not in contact with each other.

[0048] (Resistance spot welding equipment) Resistance spot welding according to the method of the present disclosure may be performed under automatic control by a computer. Figure 7 shows a resistance spot welding apparatus 50 which is an example of the resistance spot welding apparatus of the present disclosure.

[0049] Resistance spot welding apparatus 50 includes a welding machine 51 including electrodes 21, 22, and a control unit 52 that controls welding machine 51. Welding machine 51 includes a power source 56 for applying a voltage to electrodes 21, 22, an actuator 57 for moving electrodes 21, 22, and the like, and control unit 52 controls welding machine 51 to control welding conditions such as current value, pressure (or electrode position), and current application time, and performs resistance spot welding by the various methods described above.

[0050] The welding machine 51 may be configured to output information on the pressure applied by the electrodes 21, 22 to the control unit 52, so that the control unit 52 may have a function of measuring the resistance value between the electrodes 21, 22. In this case, the control unit 52 can realize a state in which the joining surfaces are not in contact with each other (the state of the first removal step) by controlling the pressure applied by the electrodes 21, 22 (or the position of the electrodes) based on the measured resistance value. Also, the control unit 52 can realize a state in which the contact surfaces are in contact with each other over a small area (the state of the second removal step) by gradually increasing the pressure applied by the electrodes 21, 22 while measuring the resistance value between the electrodes 21, 22 and stopping the increase in the pressure applied based on the measured resistance value. Furthermore, the control unit 52 may be connected to a distance measuring device or an imaging device and have a function of monitoring the positions of the electrodes 21, 22 or the gap between the bonding surfaces. In this case, the control unit 52 controls the pressure applied by the electrodes 21, 22 or the positions of the electrodes based on the monitoring results regarding the gap between the bonding surfaces, thereby realizing a state in which the bonding surfaces are not in contact with each other or a state in which the bonding surfaces are not in contact with each other.

[0051] The control unit 52 is realized by, for example, a computer having a processor 53 and a memory 54. The processor 53 is, for example, configured by a CPU (Central Processing Unit), and the memory 54 is, for example, configured by a ROM (Read Only Memory), a RAM (Random Access Memory), and a storage. A program 55 is stored in the memory 54. The processor 53 reads and executes the program 55 to control the welding machine 51, thereby performing resistance spot welding by the various methods described above. EXAMPLES

[0052] As Example 1, an experiment was carried out to check the presence or absence of fractures and cracks after resistance spot welding.

[0053] Two unplated steel sheets (thickness 1.6 mm) with a tensile strength of 1500 MPa were stacked and welded together as shown in Figure 3, and the presence or absence of fractures or cracks at the welded part in the center of the test piece was confirmed. The experimental conditions and results are shown in Table 1.

[0054] [Table 1]

[0055] In Table 1, "presence or absence of fracture / crack" refers to the presence or absence of fracture or crack about 24 hours after welding by this current (melting process). Cracks were confirmed by cutting the welded part to be evaluated in the longitudinal direction of the test piece and observing the cross section.

[0056] The experimental conditions for each experimental example are as follows. In No. 1, pre-energization was not performed, that is, neither the first removal step nor the second removal step was performed. In No. 2, prior to the main current application (melting step), a preliminary current application was performed with a pressure of 4.0 kN. However, as described below, this preliminary current application does not correspond to the first removal step of the present disclosure. In No. 3, a preliminary current application (first removal step) was performed with a pressure of 1.0 kN before the main current application (melting step). In addition, a contact step (second removal step) was performed between the preliminary current application (first removal step) and the main current application (melting step).

[0057] The observations from the results of each experimental example are as follows. In No. 1, since the first and second removal steps were not performed, it is believed that hydrogen from the rust-preventive oil applied to the mating surfaces of the steel sheets penetrated into the nugget. In No. 2, the pre-current application pressure was large at 4.0 kN, so the steel sheets came into contact with each other during the pre-current application, preventing the rust-preventive oil from escaping to the outside, and it is believed that hydrogen penetrated into the nugget during the subsequent main current application. Note that the pre-current application in No. 2 was performed with the steel sheets in contact with each other, so it does not correspond to the first removal step of the present disclosure. In No. 3, since the pressure applied during pre-current application was small at 1.0 kN, it is considered that the steel sheets did not come into contact with each other during pre-current application, the rust-preventive oil evaporated by Joule heat was discharged to the outside, and hydrogen did not penetrate into the nugget. Note that No. 3 corresponds to the method according to the second embodiment. EXAMPLES

[0058] As Example 2, an experiment was carried out to measure the amount of absorbed hydrogen after resistance spot welding.

[0059] In Example 2, similarly to Example 1, a sample was prepared by stacking two non-plated steel sheets (sheet thickness 1.6 mm) having a tensile strength of 1500 MPa and welding them as shown in FIG. The experimental conditions are shown in Table 2. Regarding the current flow time, 1 cyc is 0.02 seconds. The sample was immersed in liquid nitrogen immediately after welding to cool it down and prevent the release of absorbed hydrogen.

[0060] [Table 2]

[0061] The experimental conditions and intent of each experimental example are as follows. Normal 1: This is set as a general welding condition to serve as a reference. Normal 2: In order to confirm whether lowering the pressure would have an effect, the pressure during the main current application (melting process) was lowered compared to normal 1. (Note that if the pressure is lowered to less than 3.0 kN, splashing will occur.) Long-term power supply: In order to confirm whether there is an effect simply by increasing the heat input, the power supply time was increased compared to normal 1. · Invention Examples 1 to 3: In order to confirm the effect of the method of the present disclosure, a preliminary current application (first removal step) was carried out before the main current application (melting step).

[0062] FIG. 4 shows a graph showing the amount of absorbed hydrogen as the result of each experimental example. The amount of absorbed hydrogen was measured by cutting a 10x10mm piece surrounding the nugget in the weld to be evaluated while appropriately cooling it with liquid nitrogen, immersing it in acetone to remove the rust-preventive oil, and then measuring the amount of hydrogen using the thermal desorption method. The thermal desorption method was performed at a heating rate of 100℃ / h until the temperature reached 200℃, and the amount of hydrogen detected was the amount of absorbed hydrogen.

[0063] The results of each experimental example are as follows: ·Normal 1: Standard Normal 2: By lowering the pressure during main current application, the amount of hydrogen that penetrated was slightly reduced. Long-term current application: The amount of absorbed hydrogen increased by increasing the heat input. · Invention Examples 1 to 3: By carrying out the preliminary energization (first removal step), the amount of absorbed hydrogen was extremely small or was zero.

[0064] It was confirmed from Example 2 that the first removal step prior to the melting step has a significant effect on reducing the amount of absorbed hydrogen.

[0065] 〔supplementary explanation〕 The current-carrying part that forms a current path that does not go through the joint surface may be other than an existing weld point, and may be, for example, a part where steel plates are fastened together with a bolt, a part where steel plates are in contact with each other by being clamped and pressurized, or a part welded by welding other than resistance welding (laser welding, arc welding, etc.). In the above, an example has been described in which the pressure applied by the electrodes is controlled to realize a state in which the joining surfaces are not in contact with each other, or a state in which the joining surfaces are in contact with each other. However, the present disclosure is not limited to this. For example, the distance between the joining surfaces may be controlled by directly controlling the positions of the electrodes. For example, when overlapping steel plates are sandwiched between a pair of electrodes, the distance between the joining surfaces can be controlled by controlling the distance between the pair of electrodes. In the above, an example has been described in which the resistance value between the electrodes is measured to detect whether the bonding surfaces are in contact with each other, and control (control of the pressure force or the position of the electrodes) is performed based on this. However, the present disclosure is not limited to this. For example, the position of the electrodes or the gap between the bonding surfaces may be monitored by a distance measuring device or an imaging device (camera), and control may be performed based on the monitoring results.

[0066] In addition, in the above, an example has been described in which no adhesive is applied between the steel sheets, and therefore no adhesive is applied to the joining surfaces, but the present disclosure is not limited to this. An adhesive may be applied to the joining surfaces as long as the adhesive does not completely inhibit the action mechanism in the first removal step (i.e., the action mechanism by which hydrogen-causing substances are removed). [Explanation of symbols]

[0067] 11,12 Steel plate 21,22 electrode 31 Already welded 32 Nugget

Claims

1. A resistance spot welding method including a melting step of melting overlapping surfaces of steel sheets by passing current between a pair of opposing electrodes, Prior to the melting step, a first removal step of removing hydrogen-causing substances by passing current between the electrodes in a state where the joining surfaces, which are to be melted in the melting step, are not in contact with each other and in a state where a current path that does not pass through the joining surfaces is formed between the electrodes; Further preparation, In the first removal step, the state in which the bonding surfaces are not in contact with each other is This is achieved by measuring a resistance value between the electrodes and controlling the pressure applied by the electrodes or the position of the electrodes based on the measured resistance value. Resistance spot welding method.

2. A resistance spot welding method including a melting step of melting overlapping surfaces of steel sheets by passing current between a pair of opposing electrodes, Prior to the melting step, a first removal step of removing hydrogen-causing substances by passing current between the electrodes in a state where the joining surfaces, which are to be melted in the melting step, are not in contact with each other and in a state where a current path that does not pass through the joining surfaces is formed between the electrodes; Further preparation, After the first removing step and before the melting step, a second removal step of contacting the bonding surfaces with each other by a pressure force smaller than the pressure force applied by the electrodes in the melting step, and removing hydrogen-causing substances by passing a current between the electrodes in a state in which a current path that does not pass through the bonding surfaces is formed between the electrodes; Further preparations: Resistance spot welding method.

3. In the first removal step, the state in which the bonding surfaces are not in contact with each other is The position of the electrode or the gap of the bonding surface is monitored by a distance measuring device or an imaging device, and the pressure applied by the electrode or the position of the electrode is controlled based on the monitoring result.

3. The resistance spot welding method of claim 2.

4. In the second removal step, the state in which the bonding surfaces are in contact with each other is While measuring the resistance between the electrodes, the pressure or the current, or both the pressure and the current, are gradually increased; This is realized by stopping the increase in the applied pressure or the current value, or both the applied pressure and the current value, based on the measured resistance value. The resistance spot welding method according to claim 2 or 3.

5. In the second removal step, the state in which the bonding surfaces are in contact with each other is The position of the electrode or the gap of the bonding surface is monitored by a distance measuring device or an imaging device, and the position of the electrode or the pressure applied by the electrode is controlled based on the monitoring result. The resistance spot welding method according to claim 2 or 3.

6. The current path that does not pass through the joining surface is a nugget that has already been formed by resistance spot welding. The resistance spot welding method according to any one of claims 1 to 5.

7. The steel plate includes at least one steel plate having a tensile strength of 1400 MPa or more. The resistance spot welding method according to any one of claims 1 to 6.

8. A method for manufacturing a welded component having a plurality of nuggets, comprising the steps of: One or more of the plurality of nuggets are formed by the resistance spot welding method according to any one of claims 1 to 7. A method for manufacturing welded components.

9. The nugget formed by the resistance spot welding method has a martensite structure or a tempered martensite structure, or both. The method for manufacturing a welded member according to claim 8.

10. A resistance spot welding apparatus comprising a welding machine including an electrode and a control unit for performing resistance spot welding by controlling welding conditions of the welding machine, The control unit is A melting step is performed by passing current between the electrodes to melt the overlapping surfaces of the steel sheets; Prior to the melting step, a means for performing a first removal step of removing hydrogen-causing substances by passing current between the electrodes in a state where the joining surfaces, which are to be melted in the melting step, are not in contact with each other and in a state where a current path that does not pass through the joining surfaces is formed between the electrodes; Equipped with The control unit is The measuring device further includes a means for measuring a resistance value between the electrodes, In the first removal step, the state in which the bonding surfaces are not in contact with each other is This is achieved by measuring a resistance value between the electrodes and controlling the pressure applied by the electrodes or the position of the electrodes based on the measured resistance value. Resistance spot welding equipment.

11. A resistance spot welding apparatus comprising a welding machine including an electrode and a control unit for performing resistance spot welding by controlling welding conditions of the welding machine, The control unit is A melting step is performed by passing current between the electrodes to melt the overlapping surfaces of the steel sheets; Prior to the melting step, a means for performing a first removal step of removing hydrogen-causing substances by passing current between the electrodes in a state where the joining surfaces, which are to be melted in the melting step, are not in contact with each other and in a state where a current path that does not pass through the joining surfaces is formed between the electrodes; After the first removing step and before the melting step, a means for performing a second removal step of removing hydrogen-causing substances by applying a current between the electrodes in a state in which the bonding surfaces are brought into contact with each other using a pressure force smaller than the pressure force applied by the electrodes in the melting step and a current path that does not pass through the bonding surfaces is formed between the electrodes; Equipped with Resistance spot welding equipment.

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