JP2018144098A - Spot-welding device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a resistance spot-welding device which can form a weld joint having a delayed fracture-resistant property.SOLUTION: A spot-welding device for welding plural overlapped steel plate members comprises: an electrode drive mechanism which drives at least one of weld electrodes which are so located as to face each other, thereby applying a pressurizing force to the steel plate member; a weld electric power source which conducts electric current between the weld electrodes; and a control unit which synchronously controls the electrode drive mechanism and the weld electrode, and changes a weld current and the pressurizing force of the electrode based on a predetermined sequence. The control unit conducts electric current to a welded portion of the steel plate with a conduction current I1 while applying a pressurizing force P1 thereto by the weld electrode, and subsequently, cools the welded portion by conducting a conduction current Ic thereto while applying the pressurizing force P1 thereto. Subsequently, the control unit applies a high pressurizing force P2 to the welded portion during a pressurizing time tf while conducting electric current thereto with a conduction current I2, and thereafter, immediately applies a low pressurizing force P3 to the welded portion during a pressurizing time ti, and repeatedly performs increasing/decreasing of a pressurizing force two times or more.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a spot welding apparatus for resistance spot welding of steel plate members, and more particularly to a spot welding apparatus used for assembling automobile parts and vehicle bodies using high strength steel plates.

  In the field of automobiles, in order to preserve the environment, there is a demand for improved collision safety as well as improved fuel efficiency by reducing the weight of the vehicle body. Therefore, various efforts have been made so far in order to reduce the thickness by using a high-strength steel plate and to optimize the vehicle body structure to reduce the weight of the vehicle body and improve the collision safety.

Spot welding by resistance welding is mainly used in the manufacture of parts such as automobiles and welding in the assembly of vehicle bodies.
In order to achieve the strength of high-strength steel sheets, the carbon equivalent of the base metal is large, and in spot welding, the welds are quenched immediately after heating, so the spot welds of high-strength steel sheets are martensite. It becomes a structure, and the hardness increases and the toughness decreases at the welded part and the heat-affected zone. Further, in the welded portion, the tensile residual stress of the welded joint is increased due to transformation expansion and contraction that occur locally.

  In spot welding of high-strength steel sheets, there is a method using two-stage energization in which post-energization (temper energization) is further performed after main energization for forming the nugget as a method for improving the toughness of the spot welded portion and ensuring the joint strength. By this method, the spot welded portion (nugget portion and heat affected zone) can be annealed by post-energization to reduce the hardness of the welded portion.

On the other hand, there is a problem of delayed fracture (hydrogen embrittlement) when spot-welding a high-strength steel plate. This delayed fracture is mainly governed by three factors: the hardness of the steel sheet, the residual stress, and the amount of hydrogen in the steel sheet.
As described above, the welded portion is high in hardness and has a large tensile residual stress. Therefore, if hydrogen intrusion occurs, the welded portion is likely to cause delayed fracture. When such delayed fracture occurs, sufficient strength cannot be obtained in terms of tensile strength and fatigue strength, which are the quality indicators of welded joints, and corrosion will occur if moisture enters the part (crack). This causes a problem that the strength further decreases. These problems are one factor that hinders the weight reduction (thinning) of the vehicle body due to the application of high-strength steel sheets.

For such a problem, as shown in FIG. 4, in Patent Document 1, the post-energization after the main energization for forming the nugget is performed by increasing the applied pressure from the applied pressure P1 during the main energization, A spot welding technique is disclosed in which a compressive residual stress is introduced around the welded portion by applying high pressure P2 to improve delayed fracture resistance.
Further, in Patent Document 2, after performing spot welding between a PC steel bar and an auxiliary reinforcing bar, the spot welded portion is subjected to a striking process using an ultrasonic vibrator, and a compressive residual stress is applied to the welded portion. A technique for improving delayed fracture resistance of a high strength PC steel rod is described.

Japanese Patent Laying-Open No. 2015-93282 JP 2006-104550 A

The technique disclosed in Patent Document 1 is an effective technique for suppressing delayed fracture because it can reduce the tensile residual stress of the welded portion, but it is further desired to improve the suppression of delayed fracture.
Moreover, although patent document 2 can expect an effect in the point of suppression of delayed fracture, there exists a problem which requires processes and apparatus other than welding.
This invention makes it a subject to provide the resistance spot welding apparatus which can form stably the welded joint which has high delayed fracture resistance, without using a special apparatus in view of such a situation.

  The present inventors diligently studied a means for solving the above problems. As a result, when applying pressure to the weld using a welding electrode while energizing after formation of the nugget, the welding electrode is repeatedly raised and lowered alternately after applying a certain pressure or more. By controlling in this way, when a high pressure and a low pressure are alternately applied to the welded portion, the peening effect as in Patent Document 2 can be obtained, and the delayed fracture resistance of the welded joint can be remarkably improved. I found.

The present invention has been made based on such knowledge, and the gist thereof is as follows.
(1) In a spot welding apparatus for welding a plurality of steel plate members on which at least welds are superimposed,
A pair of welding electrodes arranged opposite to each other and approaching or separating; an electrode driving mechanism for applying pressure to the steel plate member by driving at least one of the welding electrodes in a direction approaching or separating; and the pair of pairs A welding power source for energizing a current between welding electrodes, and a control device that controls the electrode driving mechanism and the welding power source in synchronization, and changes the welding current and the pressure applied to the electrodes based on a predetermined sequence. ,
The control device forms a molten metal by applying an electric current I1 while applying a pressing force P1 to the welded portion of the steel plate member with the welding electrode, and then cooling while applying the pressing force P1. During the time tc, the welding current of the steel sheet member is cooled by setting the energization current to be an energization current Ic lower than the energization current I1, and then energizing with an energization current I2 lower than the energization current I1 and higher than the energization current Ic. The pressurizing force rises and falls by applying a pressurizing force P2 higher than the pressurizing force P1 during the pressurizing time tf and immediately thereafter applying a pressurizing force P3 lower than the pressurizing force P2 during the pressurizing time ti. A spot welding apparatus that controls the electrode driving mechanism and the welding power source based on a sequence that is repeated more than once.

(2) The control device includes welding current I1 (kA), welding current Ic (kA), welding current I2 (kA), pressure P1 (kN), pressure P2 (kN), pressure P3 (kN), The resistance spot welding apparatus according to claim 1, wherein the pressurization time tf (s) and the pressurization time ti (s) are controlled within a range satisfying the relations of the following formulas (1) to (6). .
0 ≦ Ic <I1 (1)
0.3 ≦ I2 / I1 <1.0 (2)
P2 / P1 ≧ 1.2 (3)
tf ≦ 0.2 (4)
P3 <P2 (5)
ti ≦ 0.2 (6)

  By performing welding using the spot welding apparatus of the present invention, compressive residual stress can be effectively applied around the nugget, so that a welded joint with improved hydrogen embrittlement resistance can be obtained.

It is a figure which shows an example of the spot welding apparatus to which this invention is applied. It is a figure which shows an example of the electrode drive mechanism of a spot welding apparatus. It is a figure which shows an example of the control pattern of the welding electrode pressurization force and energization current which the control apparatus in the spot welding apparatus to which this invention is applied. It is a figure which shows an example of the pattern of the pressurizing force and energization current in the conventional welding electrode.

  In the present invention, in order to perform the same effect as the impact treatment as shown in Patent Document 2 with the welding electrode, the increase and decrease of the welding pressure by the welding electrode are repeated during the post-heating energization period after the nugget is formed. This is a spot welding apparatus incorporating a control means capable of performing a compressive residual stress application process.

Before describing the spot welding apparatus, first, a compressive residual stress application process (hereinafter referred to as a peening process) using a welding electrode will be described.
In spot welding, two steel plates are overlapped, and a welding electrode made of a copper alloy or the like is sandwiched between two steel plates from both sides, while pressing the pressure as P1, as shown in FIG. Then, the energization current I is energized as I1, and a molten metal is formed. Thereafter, with the applied pressure P1 applied, an elliptical cross-section is formed between the two steel plates 1 by the heat removal by the water-cooled welding electrode 2 or the heat conduction to the steel plate itself as the conduction current Ic during the cooling time tc. A weld metal (nugget) is formed.

  When the plate set of the spot welded portion includes high-strength steel plates, the welded joints (nuggets and surrounding heat-affected zones) of the welded joints obtained by spot welding are baked during the cooling process, resulting in a martensitic structure. . In addition, heat shrinkage occurs during the cooling process, and in particular, the end portion of the nugget is in a state where tensile stress remains.

  Therefore, after forming the nugget by spot welding, when applying a compressive residual stress to the welded part and its surroundings by applying a pressurizing force higher than P1 while energizing the energizing current as I2 lower than I1, FIG. As shown in Fig. 2, peening is performed by applying pressure for P2 for a pressurization time tf, and then immediately increasing and lowering the pressurization applied for pressurization time ti by setting the pressurization pressure to P3 lower than P2 twice or more. Apply processing.

  Since the energization current I2 is allowed to flow during the peening process, the welded portion becomes a high temperature, yield strength is reduced, and plastic deformation during peening is facilitated. For this reason, it is considered that the reduction of the tensile residual stress is promoted. Furthermore, since the applied pressure is increased or decreased during peening, the contact area between the electrode and the welded portion is increased or decreased. Since the load per contact area, that is, the stress locally increases or decreases, plastic deformation seems to further progress. In addition, it is presumed that the microstructure of the welded portion, the solidification segregation partial breakage of the embrittlement element, etc. occur and the delayed fracture resistance is improved.

  In the peening process of FIG. 3, the change in the applied pressure is indicated by a rectangular wave (pulse waveform). However, as long as the applied pressure is increased or decreased, a sine wave or a similar waveform can be used. . In the case of such a waveform, the pressurizing force and pressurizing time are, for example, values at the 90% position of the amplitude.

Next, a spot welding apparatus capable of performing the above peening process will be described.
The present invention can be basically realized by incorporating a control device capable of performing the peening process described above into a conventional spot welder. Hereinafter, a stationary spot welder will be described as an example.

An example of a stationary spot welder is shown in FIG. The spot welder body 1 is provided with an upper arm 2 and a lower arm 3, and an upper electrode holder 5 driven by an electrode driving mechanism 4 is attached to the upper arm 2. One movable welding electrode 6 is attached. A lower electrode holder 7 is fixed to the lower arm 3, and the other fixed welding electrode 8 is attached to the tip of the lower arm 3 so as to face the movable welding electrode 6.
A welding power source 20 is connected to the upper electrode holder 5 made of a conductive member via an electrical connection portion 21, and a welding power source 20 is connected to a similar lower electrode holder 7 via an electrical connection portion 22, A welding current is supplied to the welding electrodes 6 and 8.

  An example of the electrode drive mechanism 4 is shown in FIG. In this example, a ball screw shaft 12 is supported by a support member 11 fixed to the upper arm 2 so that the ball screw shaft 12 can rotate. A nut block 13 is incorporated in the ball screw shaft 12, and the ball screw shaft 12 is driven by a servo motor 15. The nut block 13 is moved up and down along the guide rails 14 and 14 by the rotation. As shown in FIG. 1, the upper electrode holder is attached to the nut block 13 via the mounting member 19. By attaching 5, the upper electrode holder 5 performs an approaching / separating operation (pressing / releasing operation of the welding electrode) with respect to the lower electrode holder 7.

  In FIG. 2, as a power transmission mechanism, a belt gear 16 is attached coaxially to the ball screw shaft 12, a belt gear 17 is attached coaxially to the motor shaft, and both gears 16, 17 are connected by a timing belt 18. An example in which the rotational force of the servo motor 15 is transmitted to the rotation of the ball screw shaft 12 is shown.

The welding power source 20 and the servo motor 15 are connected to a control device 30 that controls their operations based on a predetermined sequence.
The control device 30 includes an electrode drive control unit 32 that controls the servo motor 15, a welding current control unit 33 that controls the welding power source, a main control unit 31 that outputs an operation command to these control units, and the like. The electrode drive control means 32 is connected to a pressure detection means (for example, a strain gauge) 34 provided on the movable electrode holder 5, and the welding current control means 33 is connected between the electrical connection portion 22 and the welding power source 20. An inserted current detection means (for example, an ammeter) 35 is connected.

  The main control means 31 has conditions relating to the sequence of one welding cycle as shown in FIG. 3, that is, a welding current I1 and its energization time t1, a welding current Ic and a cooling time tc, a welding current I2 and its energization time t2, Pressure P1 and pressurization time t1, pressurization P2 and pressurization time tf, pressurization P3 and pressurization time ti, other squeeze time and slope time, and time to change the pressurization force and current It is set in advance, and a command of a required pressure and current value is output to the electrode drive control means 32 and the welding current control means 33 every unit time.

  The electrode drive control means 32 controls the servo motor 15 so that the detected value from the pressure detection means 34 becomes the commanded pressure from the main control means 31, so that the tip position of the movable welding electrode 6, that is, the pressure applied. Control the pressure. Further, the welding current control means 32 controls the welding power source 20 so that the detection value from the current detection means 35 becomes the current value commanded from the main control means 31.

  In the present invention, in order to implement a preset control sequence based on the welding electrode pressure and energization current pattern shown in FIG. 3, the main control means 31 uses the welding electrodes 6 and 8 at the welding points of the steel plate member. While applying the pressing force P1, the energizing current I1 is energized to form a molten metal. Subsequently, the energizing current is set to an energizing current Ic lower than the energizing current I1 during the cooling time tc while the pressing force P1 is applied. The welded portion of the steel plate member is cooled, and subsequently, the energization current is lower than the energization current I1 and the energization current I2 is higher than the energization current Ic, and the pressurization time tf is higher than the pressurization pressure P1. Electrode driving control means based on a sequence in which the pressing force P3 lower than the pressing force P2 is applied for the pressurizing time ti and the rising and lowering of the pressing force is repeated twice or more. And it controls the second and welding current control means 33 comprises means for controlling the electrode driving mechanism 4 and the welding power source 20.

  In the above, the spot welding apparatus of the present invention has been described by taking a stationary spot welding machine as an example, but a spot welding gun attached to a welding robot may be used. The driving mechanism for driving the welding electrode is a mechanical mechanism using a ball screw shaft in FIGS. 1 and 2 and has been described with an example in which the driving control is performed by a servo motor. However, the driving mechanism is not limited to this. The present invention can be similarly applied to a mechanism in which a fluid actuator such as an air cylinder is used as a mechanism and the drive is controlled by a servo valve.

Spot welding using the welding apparatus described above will be described.
In spot welding, welding electrodes 6 and 8 are pressed from both sides so as to sandwich the welded portions of a plurality of stacked steel plates, and energized while pressing between the steel plates to form molten metal (main energization period). The molten metal is rapidly cooled and solidified by heat removal from the water-cooled electrode and heat conduction to the steel plate itself after completion of the main energization, forming an elliptical cross-section nugget between the steel plates (cooling period) After the nugget is formed, a process of repeatedly raising and lowering the applied pressure is performed while post-energization is performed (peening process period).

<Main energization period>

In this energization, for example, a dome radius type welding electrode having a tip diameter of 6 to 8 mm, pressurization pressure P1: 1.5 to 6.0 kN, energization time t1: 0.1 to 1.0 s (5 to 50 cycles, power supply) Welding is performed at a frequency of 50 Hz) and an energization current I1: 4 to 15 kA. The nugget diameter can be a general range of 3.0√t to 5.0√t, where t (mm) is the thickness of the thinnest steel plate. Further, if the welding conditions and the electrode shape are appropriately selected, a nugget having a diameter less than or exceeding this range can be formed.
The basic pressurization and energization pattern at the time of main energization are not particularly limited, and within the range of the above-mentioned applied pressure, energization time, and energization current, it depends on the material and thickness of the steel plate member at the welding location. Accordingly, the optimum conditions may be appropriately adjusted. The current value at the start of energization may not be immediately applied current, but may be gradually increased (up-slope) from 0 (zero) or a low current exceeding 0 until the current value becomes the energized current.

<Cooling period>
After the energization time in the main energization has elapsed, the energization current value is lowered to a value that is low enough to cause solidification of the molten metal while the welding electrode pressure is maintained, or the energization is stopped and the weld is cooled. The cooling time tc is not particularly limited, and may be 0.04 to 0.4 s although it is sufficient that a weld metal (nugget) is formed and depends on the thickness of the steel plate. The energization current Ic during the cooling time must satisfy the following condition (1). When Ic is greater than I1, solidification of the molten metal does not proceed. Ic is preferably 0 (kA).
0 ≦ Ic <I1 (1)

<Peening period>
In the peening process performed after spot welding, in the previous example, the welding power source 20 and the servo motor 15 are controlled by the command of the control device 30 so that the welding electrode 6 and 8 is energized with the energization current I2, and a high pressure is applied. A process of alternately and repeatedly applying P2 and a lower pressure P3 is performed.
At this time, the energization current I2, the pressures P2 and P3, and the pressurization times tf and ti satisfy the following conditions (2) to (3).
0.3 ≦ I2 / I1 <1.0 (2)
P2 / P1 ≧ 1.2 (3)
tf ≦ 0.2 (4)
P3 <P2 (5)
ti ≦ 0.2 (6)

(Energizing current and energizing time during peening process)
It is assumed that the energization current I2 during the peening process satisfies the above (2). The temperature of the welded part in the peening process becomes appropriate, plastic deformation of the welded part becomes easy, and the tensile residual stress of the welded part is reduced. In addition, by setting the energization current I2 to be less than the energization current I1 at the time of spot welding, expansion of the nugget is suppressed in the peening process. If the peening process is performed while enlarging the nugget, solidification of the nugget may become unstable, resulting in scattering, a welded portion being depressed, and energy being wasted. The ratio (I2 / I1) of the energization current I2 to the energization current I1 is set to be 0.3 or more and less than 1.0. If it is less than 0.3, the effect of reducing the tensile residual stress by the peening treatment may be insufficient.

  The energization time t2 during the peening process is from the start of the application of the pressure P2 to the release. Note that the current value may be gradually increased from Ic (upslope) until the current value becomes the energization current I2, or the current value may be immediately changed to the energization current I2.

(Pressurizing pressure during peening process, pressurization time)
In order to reduce the tensile residual stress, the high pressure P2 during the peening process is 1.2 times or more the pressure P1 during spot welding. Preferably, it is 1.3 times or more. Although an upper limit is not specifically limited, In order to avoid the excessive pressurization to a welding part, 2.5 times or less are preferable.
Further, the upper limit of the pressurizing time tf at the high pressurizing force P2 is set to 0.4 s in order to reduce the tensile residual stress of the welded portion. Preferably it is 0.2 s. The lower limit is preferably 0.02 s.

  The low pressure P3 during the peening process needs to be P3 <P2. As a result, the applied pressure can be increased and decreased. Preferably, P2 / P3 ≧ 1.2. It is preferable that the applied pressure P3 is approximately the same as the applied pressure P1 during spot welding. Although the value can be lower than P1, the lower limit of P3 is determined by the capability of the apparatus. The upper limit of the pressurization time ti is preferably set to 0.4 s because it is preferable to make it as short as possible from the viewpoint of productivity. The lower limit is set to 0.02 s in consideration of the apparatus capability, that is, the stability of the pressure control.

(Number of repetitions of increasing and decreasing pressure)
The number of repetitions of raising and lowering the pressing force (one time with one pressurization P2 and the next one pressurization P3) is two or more. After raising and lowering the pressing force two or more times, the tensile residual stress of the welded portion can be reduced by finally applying the pressing force P2 for tf. For this reason, the number of repetitions of increasing and decreasing the pressing force is two or more. The upper limit of the number of repetitions is not particularly limited, but is preferably 10 times in order to shorten the work time.

  Further, the repeated pressing force P2 may be the same pressing force or different pressing force as long as it is within the range of the above formula (3), and within the range of the above formula (4). , All may have the same pressurization time or different pressurization times. However, it is preferable in terms of work efficiency that the pressure and pressurization time in the repetition process are all the same.

  Further, the repeated pressurization to the pressurizing force P3 may be the same pressurization force or different pressurization pressures as long as the relationship of the above formula (5) is satisfied, and is within the range of the above formula (6). If so, all may have the same pressurization time or different pressurization times. However, it is preferable in terms of work efficiency that the pressure and pressurization time in the repetition process are all the same.

  Needless to say, the spot welding apparatus of the present invention can be used for spot welding of a general steel plate member, and particularly when applied to welding of a steel plate member produced using a high-strength steel plate of 980 MPa or more, delay resistance due to peening treatment. The effect of improving destructibility appears remarkably. In that case, not only the case where all the steel plates have a tensile strength of 980 MPa or more, but also the case where at least one of them has the above-described tensile strength.

  The plate thickness of the steel plate is preferably in the range of 0.5 to 3.2 mm. Even if the plate thickness is less than 0.5 mm, the effect of improving the delayed fracture characteristics of the welded portion can be obtained, but the stress load on the welded portion during the tensile test is low, and the tensile residual stress generated in the welded portion Since the value of is low, delayed fracture is unlikely to occur. Further, even if the plate thickness is over 3.2 mm, the effect of improving the delayed fracture characteristics of the welded portion can be obtained, but it may be difficult to reduce the weight of the member.

Next, an example of welding using the welding apparatus of the present invention will be described.
The alloyed hot dip galvanized steel sheet shown in Table 1 was prepared. Table 2 shows the spot welding conditions set in the main control means of the welding apparatus, and Table 3 shows the same peening conditions set in the same manner.
In each test number, two steel plates having the same steel plate number were welded to prepare test pieces. In spot welding, a dome radius type electrode having a diameter of 16 mm and a tip of 6 mm was used. The number of repetitions n shown in Table 3 is the number of cycles when applying the pressurizing force P2 during the pressurizing time tf and applying the pressurizing force P3 during the pressurizing time ti as one cycle. Further, after increasing and decreasing the pressurizing force a predetermined number of times, finally, the pressurizing force P2 was applied during the pressurizing time tf.

  The test piece after welding was subjected to a hydrochloric acid immersion test. This test was conducted by examining the presence or absence of cracks after the test piece was immersed in 0.15 N hydrochloric acid for 100 hours. Presence or absence of cracks, the spot welded joint formed by spot welding is cut in a cross section perpendicular to the plate surface and passing through the center of the nugget, cut out the test piece containing the nugget from this cut piece, the cut surface is polished, The polished cut surface was observed with an optical microscope. This test was carried out on 10 test pieces, and the number of cracks at that time was confirmed. Table 4 shows the results of the hydrochloric acid immersion test.

  As shown in Table 4, in the peening process, a test number in which a pressurizing force P2 higher than the pressurizing force P1 and a pressurizing force lower than P2 are alternately repeated twice or more, and the energizing current I2 is set to an energizing current lower than I1. In 1, 2, 6 and 7, no spot was generated in the hydrochloric acid immersion test, and spot welded joints having excellent delayed fracture characteristics were obtained.

  On the other hand, test numbers 4 and 8 are such that the applied pressure P2 is constant as shown in FIG. 4 and do not repeat the increase and decrease of the applied pressure. Test number 3 is the relationship of the energized current I2 to the energized current Ic. In Test No. 5, the applied pressure P2 was not greater than the applied pressure P1, and cracking occurred in the hydrochloric acid immersion test, and sufficient delayed fracture characteristics were not obtained.

  In the apparatus of the present invention, since the peening treatment of the welded portion is performed after spot welding, a welded joint with improved hydrogen embrittlement resistance can be obtained. Therefore, the present invention has high industrial applicability.

DESCRIPTION OF SYMBOLS 1 Welding machine main body 2 Upper arm 3 Lower arm 4 Electrode drive mechanism 5 Upper electrode holder 6 Movable welding electrode 7 Lower electrode holder 8 Fixed welding electrode 11 Support member 12 Ball screw shaft 13 Nut block 14 Guide rail 15 Servo motor 16, Reference Signs List 17 Gear for belt 18 Timing belt 19 Mounting member 20 Welding power supply 21, 22 Electrical connection part 30 Control device 31 Main control means 32 Electrode drive control means 33 Welding current control means 34 Pressure detection means 35 Current detection means I Current carrying current I1 Spot Current applied during welding I2 Current applied during peening treatment Ic Current applied during cooling time P Pressure P1 Pressure applied during spot welding P2 Pressure applied when pressure increases P3 Pressure applied when pressure decreases t1 Spot welding Energizing time t2 When energizing during the peening process Interval tc Cooling time tf Pressurization time when pressurization rises ti Pressurization time when pressurization falls

Claims (2)

In a spot welding apparatus that welds at least a plurality of steel plate members on which welds are superimposed,
A pair of welding electrodes arranged opposite to each other and approaching or separating; an electrode driving mechanism for applying pressure to the steel plate member by driving at least one of the welding electrodes in a direction approaching or separating; and the pair of pairs A welding power source for energizing a current between welding electrodes, and a control device that controls the electrode driving mechanism and the welding power source in synchronization, and changes the welding current and the pressure applied to the electrodes based on a predetermined sequence. ,
The control device forms a molten metal by applying an electric current I1 while applying a pressing force P1 to the welded portion of the steel plate member with the welding electrode, and then cooling while applying the pressing force P1. During the time tc, the welding point of the steel plate member is cooled with an energization current Ic lower than the energization current I1, and then the energization current is lower than the energization current I1 and higher than the energization current Ic. While applying current, an applied pressure P2 higher than the applied pressure P1 is applied during the pressurizing time tf, and immediately thereafter an applied pressure P3 lower than the applied pressure P2 is applied during the applied pressure time ti. A spot welding apparatus, wherein the electrode driving mechanism and the welding power source are controlled based on a sequence in which the descent is repeated twice or more. The control device includes welding current I1 (kA), welding current Ic (kA), welding current I2 (kA), pressure P1 (kN), pressure P2 (kN), pressure P3 (kN), pressurization time. 2. The resistance spot welding apparatus according to claim 1, wherein tf (s) and pressurizing time ti (s) are controlled within a range satisfying relationships of the following formulas (1) to (6).
0 ≦ Ic <I1 (1)
0.3 ≦ I2 / I1 <1.0 (2)
P2 / P1 ≧ 1.2 (3)
tf ≦ 0.2 (4)
P3 <P2 (5)
ti ≦ 0.2 (6)

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