CN113766990B - Resistance spot welding method and manufacturing method of resistance spot welding joint - Google Patents
Resistance spot welding method and manufacturing method of resistance spot welding joint Download PDFInfo
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- CN113766990B CN113766990B CN201980095708.0A CN201980095708A CN113766990B CN 113766990 B CN113766990 B CN 113766990B CN 201980095708 A CN201980095708 A CN 201980095708A CN 113766990 B CN113766990 B CN 113766990B
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K11/00—Resistance welding; Severing by resistance heating
- B23K11/10—Spot welding; Stitch welding
- B23K11/11—Spot welding
- B23K11/115—Spot welding by means of two electrodes placed opposite one another on both sides of the welded parts
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K11/00—Resistance welding; Severing by resistance heating
- B23K11/10—Spot welding; Stitch welding
- B23K11/11—Spot welding
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K11/00—Resistance welding; Severing by resistance heating
- B23K11/24—Electric supply or control circuits therefor
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K37/00—Auxiliary devices or processes, not specially adapted to a procedure covered by only one of the preceding main groups
- B23K37/003—Cooling means
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Abstract
The object is to provide a resistance spot welding method and a method for manufacturing a resistance spot welding joint. The present invention provides a resistance spot welding method for joining welding electrodes by overlapping 2 or more steel plates and clamping them by 1, wherein the welding method includes an initial energization step and a main energization step for forming nuggets having a predetermined nugget diameter as energization, and spatter is generated in the initial energization step.
Description
Technical Field
The present invention relates to a resistance spot welding method and a method for manufacturing a resistance spot welding joint.
Background
Resistance spot welding is widely used in assembly of a vehicle body such as an automobile, and resistance spot welding is performed at up to several thousand points in a 1-part vehicle body. Resistance spot welding is to apply pressure to the welding electrodes while overlapping 2 or more steel plates with each other and applying electricity while sandwiching them between upper and lower 1 pairs of welding electrodes. Thus, nuggets of a predetermined size are formed at the joint portions of the steel plates, and the steel plates are joined to obtain a welded joint.
In recent years, from the viewpoint of environmental protection, CO of automobiles has been demanded 2 The reduction of the emission amount is achieved by reducing the thickness of the vehicle body by using a high-strength steel sheet, thereby achieving the reduction of the weight of the vehicle body, that is, the improvement of fuel economy. However, in general, high-strength steel sheets contain not only a large amount of C but also various alloying elements to improve strength and increase hydrogen embrittlement sensitivity. In addition, in the resistance spot welding, rust preventive oil, moisture, a plating layer, and the like on the surface of a steel sheet are involved in a weld metal (molten portion) during melting and solidification at the time of welding, and therefore remain as a hydrogen source that is a main cause of delayed fracture after cooling.
Therefore, when welding high-strength steel sheets by resistance spot welding, the occurrence of delayed fracture due to hydrogen entering the weld metal having high hydrogen embrittlement sensitivity at the time of welding becomes a problem in the welded portion of the obtained welded joint.
As a method for preventing delayed fracture of a welded portion, for example, patent document 1 discloses a technique for controlling residual stress of the welded portion and preventing delayed fracture by increasing a pressurizing force and reducing a current immediately after welding energization (main energization). Further, for example, patent document 2 discloses a technique of controlling the structure and hardness of a welded portion and preventing delayed fracture by increasing the pressurizing force immediately after welding energization (main energization) and performing energization after a cooling time without energization has elapsed.
Prior art literature
Patent literature
Patent document 1: japanese patent application laid-open No. 2015-93282
Patent document 2: international publication No. 2014/171495
Disclosure of Invention
Problems to be solved by the invention
As described above, in resistance spot welding of a high-strength steel sheet, hydrogen is involved in the weld metal. Therefore, in resistance spot welding of high-strength steel sheets, it is important to increase the strength of the welded joint and to reduce the amount of hydrogen remaining in the welded portion in order to prevent delayed fracture.
However, the techniques of patent document 1 and patent document 2 are not techniques for reducing the hydrogen amount at the welded portion in order to prevent delayed fracture. In addition, in these techniques, if the pressurizing force is excessively increased in a molten state of the nugget immediately after the welding is energized, the plate thickness of the welded portion tends to be reduced, and there is a problem that the strength of the obtained welded joint is lowered or the appearance of the welded portion is impaired.
Therefore, the problem of occurrence of delayed fracture due to hydrogen entering the weld metal having high hydrogen embrittlement sensitivity during welding is also present in the resistance spot welding of other steel sheets as well as in the case of resistance spot welding of high-strength steel sheets for automobiles.
The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a resistance spot welding method and a method for manufacturing a resistance spot welding joint, which can suppress delayed fracture of a welded portion.
Means for solving the problems
The inventors of the present invention studied the behavior of hydrogen entering the weld metal during welding, which is a factor of delayed fracture, in order to suppress delayed fracture of a welded joint obtained by resistance spot welding of a high-strength steel sheet having a high tensile strength, and have found the following knowledge.
As described above, first, hydrogen enters the welded portion during welding. The diffusion of hydrogen is slower as the temperature is lowered, so that a large amount of hydrogen does not diffuse out of the nugget and remains due to rapid cooling after welding. Thereafter, as time passes, hydrogen is accumulated in a portion where large tensile stress represented by the notch shape of the nugget end portion is concentrated, thereby generating delayed fracture.
Therefore, more hydrogen is discharged from the nugget during welding, and the residual hydrogen is reduced, which is effective for suppressing the delayed fracture.
Accordingly, the present inventors have conducted intensive studies on suitable resistance spot welding conditions capable of reducing the residual hydrogen amount in the welded portion. The results are described below.
In the power-on step, first, splash is generated from the joined surfaces of the steel plates, and the hydrogen source present on the joined surfaces of the steel plates is discharged as splash. As a result, the mixing of hydrogen into the nugget in the subsequent power-on step can be reduced, and the delayed fracture resistance of the welded joint can be improved. In the case where the spatter is generated at the later stage of the energization process, it is difficult to reduce the hydrogen that has been mixed into the nugget before the spatter is generated. As a result, the delayed fracture may not be suppressed, and the growth of the nugget may be affected, so that a large nugget diameter may not be ensured.
Therefore, by dividing the energization process into 2 stages, specifically, a 1 st energization process (initial energization process described later) for the purpose of generating spatters and a 2 nd energization process (main energization process described later) for the purpose of forming nuggets thereafter, spatters can be generated at the initial stage of the energization process and can be suppressed at the later stage of the energization process.
Further, by providing the 1 st energization step (initial energization step) as described above, the adhering matter such as moisture, oil, dirt, or the like existing on the joined surfaces of the steel plates can be discharged together with splashes, and the joined surfaces of the steel plates can be kept clean, and the steel plates can be moderately softened before nugget formation by energization heating. This can maintain the contact state between the steel plates well, and can provide an effect of improving the delayed fracture resistance. Further, the effect of forming the nugget having a large nugget diameter more stably in the 2 nd power-on step (main power-on step) can be obtained at the same time.
The present invention has been made based on the above-described knowledge, and the gist of the present invention is as follows.
[1] A resistance spot welding method for joining 2 or more steel plates by sandwiching them with 1 pair of electrodes and applying electricity while pressurizing, wherein,
the energizing includes:
an initial electrifying procedure; and
a main power-on step of forming a nugget having a predetermined nugget diameter,
splash is generated in the initial energization process.
[2] The resistance spot welding method according to item [1], wherein the welding voltage Vs (V) at the time point of the spatter generation satisfies the following formula (1),
Vs≥0.7×Va···(1)
wherein Va: a welding voltage (V) of 5ms or less before spatter generation,
Vs: the welding voltage (V) at the spatter generating time point.
[3] The resistance spot welding method according to item [1] or [2], wherein the current value I1 (kA) in the initial energization step satisfies the following formula (2),
1.1×I2≤I1≤5×I2···(2)
wherein, I1: a current value (kA) in the initial energization step,
I2: current value (kA) in the main energization step.
[4] The resistance spot welding method according to any one of [1] to [3], further comprising a cooling step of cooling the nugget by applying an electric current with a current value Ic (kA) satisfying the following formula (3) between the initial energizing step and the main energizing step,
0≤Ic≤I1···(3)
wherein Ic: current value (kA) in cooling step,
I1: current value (kA) in the initial energization step.
[5] A method for manufacturing a resistance spot welding joint, wherein the resistance spot welding method according to any one of [1] to [4] is used.
ADVANTAGEOUS EFFECTS OF INVENTION
According to the present invention, delayed fracture of the welded portion can be suppressed, and thus, an industrially exceptional effect is achieved.
Drawings
Fig. 1 is a cross-sectional view schematically illustrating resistance spot welding according to an embodiment of the present invention.
Fig. 2 is a diagram illustrating an example of a welded joint used in an embodiment of the present invention, in which fig. 2 (a) is a plan view and fig. 2 (b) is a side view.
Fig. 3 is a graph showing an example of the energization pattern in the resistance spot welding method of the present invention.
Detailed Description
The resistance spot welding method and the method for manufacturing a resistance spot welding joint according to the present invention will be described below with reference to the drawings. The present invention is not limited to this embodiment.
First, a resistance spot welding method according to the present invention will be described with reference to fig. 1.
The invention joins more than 2 steel plates by resistance spot welding. Fig. 1 schematically illustrates an example of a resistance spot welding method. An example of resistance spot welding 2 sheets is shown in fig. 1.
More than 2 steel plates are overlapped. In the example shown in fig. 1, 2 steel sheets, that is, a steel sheet disposed on the lower side (hereinafter referred to as a lower steel sheet 1) and a steel sheet disposed on the upper side (hereinafter referred to as an upper steel sheet 2), are superimposed.
The stacked steel plates (lower steel plate 1 and upper steel plate 2) are sandwiched between welding electrodes (electrodes) 4 and 5 by 1 arranged in the vertical direction, and energized in an energizing mode described later while being pressurized. In the example shown in fig. 1, the electrode disposed on the lower side of the steel plate is referred to as a lower electrode 4, and the electrode disposed on the upper side of the steel plate is referred to as an upper electrode 5.
The overlapped steel plates are energized while being pressed in a state of being sandwiched by the welding electrodes 1, and the nugget 3 having a desired size is formed by resistance heat generation, so that the overlapped steel plates are joined to obtain a welded joint. In the present invention, 3 or more steel sheets may be superimposed and subjected to resistance spot welding, and in this case, a welded joint may be obtained in the same manner as in the welding method described above, but this is not shown.
The apparatus for carrying out the resistance spot welding method of the present invention is not particularly limited as long as it can be pressurized by the lower electrode 4 and the upper electrode 5 and can control the pressurizing force at will. Conventionally known devices such as a cylinder, a servo motor, and the like can be used. The configuration for supplying current and controlling the current value at the time of energization is not particularly limited, and conventionally known devices can be used. In addition, both direct current and alternating current are also applicable to the present invention. In the case of ac, "current" means "effective current".
The form of the tips of the lower electrode 4 and the upper electrode 5 is not particularly limited. Examples thereof include JIS C9304: DR (dome radius) (eccentric), R (radius) and D (dome) in 1999. The diameters of the tips of the lower electrode 4 and the upper electrode 5 are, for example, 4mm to 16mm. The resistance spot welding was performed in a state where the electrode was always water-cooled.
In the present invention, the steel grade of the steel sheet subjected to resistance spot welding is not particularly limited. Preferably, at least 1 sheet of the steel sheets to be stacked is a high-strength steel sheet having a carbon equivalent Ceq (%) of 0.17% or more and a tensile strength of 780MPa or more represented by the following formula (7). In a high-strength steel sheet having a Ceq (%) of 0.17% or more and a tensile strength of 780MPa or more, delayed fracture of the resistance spot welded portion is particularly likely to be a problem, and therefore the effect of the present invention can be obtained more effectively. If Ceq (%) exceeds 0.60 (%), the resistance spot welding portion has excessively high sensitivity to delay damage, and suppression of delay damage is difficult even when the method of the present invention is used. Therefore, ceq (%) is preferably 0.60% or less.
Of course, the resistance spot welding method of the present invention can be applied to a steel sheet having Ceq (%) of less than 0.17% and/or tensile strength of less than 780 MPa. In the example shown in fig. 1, the lower steel sheet 1 and the upper steel sheet 2 are high-strength steel sheets having a carbon equivalent Ceq (%) of 0.17% or more and a tensile strength of 780MPa or more, which are represented by the following formula (7).
Ceq(%)=C+Si/30+Mn/20+2P+4S···(7)
The symbol of the element in formula (7) represents the content (mass%) of each element, and the element not contained is set to 0.
The thickness of the steel sheet subjected to resistance spot welding is not particularly limited. For example, the diameter is preferably in the range of 0.5mm to 3.0 mm. Since the steel sheet having a sheet thickness within this range can be suitably used as an automobile member.
The steel sheet subjected to resistance spot welding may be a steel sheet having a plating layer on the surface thereof by a plating treatment. In the present invention, examples of plating include Zn plating and Al plating. Examples of the Zn plating include hot dip zinc (GI), zn-Ni plating, and Zn-Al plating. Examples of the Al plating include Al-Si plating (e.g., al-Si plating containing 10 to 20 mass% of Si). The molten plating layer may be an alloyed molten plating layer after alloying. Examples of the alloyed hot-dip plating layer include an alloyed hot-dip zinc (GA) layer.
Further, 2 or more steel sheets subjected to resistance spot welding may be the same or different. That is, the steel plates may be of the same kind and shape, or of different kinds and/or shapes. The surface-treated steel sheet having a coating layer may be superposed on the steel sheet having no coating layer.
Next, the energization pattern in the resistance spot welding method according to the present invention will be described.
The present invention is a resistance spot welding method in which 2 or more steel plates are overlapped and sandwiched by 1 pair of electrodes, and the overlapped steel plates are joined while pressurizing and energizing to form nuggets. In the example shown in fig. 1, the steel plates 1, 2 sandwiched between the electrodes 4, 5 are energized in a specific pattern while being pressurized. The current application of the present invention includes an initial current application step and a main current application step for forming a nugget having a predetermined nugget diameter.
First, in the initial power-on step, power is supplied at a higher current value than in the main power-on step, whereby spatter is generated in the step. That is, in the initial power-on step, the hydrogen source and the splashes existing on the joining surfaces of the steel plates are discharged together, and a good contact state between the steel plates is ensured.
In the present invention, it is important to generate spatter in the initial power-on step. When the generation of the spatter is a step subsequent to the initial energization step (for example, a cooling step, a main energization step, which will be described later), a lot of hydrogen is mixed into the nugget before the spatter is generated. Therefore, it is difficult to obtain a hydrogen reducing effect by the splashing, and a delayed fracture suppressing effect is not obtained. In addition, when the effect of reducing hydrogen is to be more remarkably exhibited, it is effective to shorten the energization time before the splash generation and to suppress the mixing of hydrogen to the minimum.
In the present invention, it is preferable that the splash is generated within 200ms from the start of the energization in the initial energization step. More preferably, the splash is generated within 100ms from the start of the energization of the initial energization step.
In addition, the spatter generated in the initial power-on step is preferably small-sized spatter (hereinafter, also referred to as small spatter) in order to stably form a nugget having a large diameter in the main power-on step described below. When the voltage between the electrodes is measured in resistance spot welding, the resistance between the electrodes decreases when spatter is generated, and therefore the decrease in voltage is expressed as a measured value. In the present invention, the size of the splash is controlled by the voltage drop amount at the time of the splash generation. Specifically, it is preferable to set the current value and the pressurizing force in the initial energization step so that the inter-electrode voltage Vs (V) at the time point when the spatter is generated satisfies the following expression (1). The splash generated by energizing in such a manner as to satisfy the formula (1) means the small splash in the present invention.
Vs≥0.7×Va···(1)
Wherein Va: an inter-electrode voltage (V) of 5ms or less before the generation of spatter,
Vs: the inter-electrode voltage (V) at the time point of splash generation.
When the inter-electrode voltage Vs (V) at the time point of the spatter generation is lower than (0.7×va), the scale of the spatter is large, and a good current-carrying state cannot be ensured in the main current-carrying process, so that a nugget having a large nugget diameter (hereinafter, also referred to as a diameter) cannot be stably formed. Thus, the inter-electrode voltage Vs (V) at the time point of the spatter generation is set to (0.7×va) or more. In order to more significantly exert the effect of maintaining the contact state between the steel plates well and stably forming nuggets having a large diameter in the main energization step, it is effective to suppress the scale of the spatter to a minimum, and therefore, it is preferable to set the inter-electrode voltage Vs (V) at the time point of the spatter generation to (0.8×va) or more. In general, as described above, when spatter is generated during spot welding, the inter-electrode voltage decreases. That is, it is considered that the inter-electrode voltage is not increased due to the occurrence of spatter, and therefore, it is not assumed that (1.0×va) or more is present in the above formula (1).
After the initial energization step, a main energization step for forming a nugget of a predetermined diameter is performed. In the main power-on step, power-on conditions such as a current value and a power-on time for forming the nugget and pressurizing conditions are not particularly limited, and conventionally known welding conditions may be employed.
For example, from the viewpoint of forming a nugget of an appropriate diameter, the current value in the main current-carrying step is preferably 1.0kA to 15.0kA, and the pressurizing force in the main current-carrying step is preferably 1.0kN to 9.0 kN. The energization time in the main energization step is preferably 100ms or more and 1000ms or less. The main energization step may be a multistage energization or multistage pressurization step in which the current value or the pressurization force is changed in the main energization step.
In the present invention, the nugget having the predetermined nugget diameter is preferably 3 to 6 v t (t: plate thickness) (mm).
In the present invention, a cooling step described later may be further provided between the initial energization step and the main energization step.
Next, specific energization conditions for the initial energization step for realizing the resistance spot welding method of the present invention will be described.
In the initial energization step, the current value I1 (kA) is preferably set so as to satisfy the following expression (2).
1.1×I2≤I1≤5×I2···(2)
Wherein, I1: a current value (kA) in the initial energization step,
I2: current value (kA) in the main energization step.
When the current value I1 (kA) in the initial energization step is lower than (1.1×i2), it is difficult to generate spatter in the initial energization step. As a result, the hydrogen in the nugget cannot be reduced, and the effect of suppressing the delayed fracture cannot be obtained. When the current value I1 (kA) exceeds (5×i2), the scale of the generated spatter increases, and it is difficult to stably form a nugget having a large diameter in the subsequent main energization step. In the case where the effect of suppressing the delayed destruction of the small spatter generated in the initial energization step and the effect of stably forming the nugget having a large diameter in the main energization step are to be more remarkably obtained, the current value I1 in the initial energization step is more preferably set to 1.3×i2.ltoreq.i1, and even more preferably set to i1.ltoreq.3×i2.
The energization time in the initial energization step is preferably 300ms or less. If the current is applied for a period of time exceeding 300ms, the possibility of occurrence of large-scale splashing increases, and it may be difficult to stably form a nugget having a large diameter in a subsequent main current application step. More preferably, the time is 140ms or less.
Next, a cooling process under appropriate conditions for the resistance spot welding method according to the present invention will be described with reference to fig. 3. Fig. 3 shows an example of the energization pattern having a cooling step.
As described above, in the present invention, a cooling step of cooling the nugget by applying electric current to the nugget at a current value Ic (kA) satisfying the following expression (3) may be provided between the initial electric current application step and the main electric current application step.
0≤Ic≤I1···(3)
Wherein Ic: current value (kA) in cooling step,
I1: current value (kA) in the initial energization step.
By providing the cooling step, the contact state between the steel plates which are once disturbed by the occurrence of spatter can be stabilized again, and the effect of forming the nugget more stably in the subsequent main energization step can be obtained. If the current value Ic (kA) in the cooling step exceeds the current value I1 (kA) in the initial energization step, the possibility of occurrence of spatter in the cooling step increases, and the effect of ensuring the contact state between the steel plates may not be obtained. In the cooling step, the current flow pattern in the cooling step is not particularly limited as long as the current value Ic in the cooling step is in the range satisfying the formula (3) for the purpose of stabilizing the contact state between the steel plates without generating spatter, and may be a non-current flow step, a multi-stage current flow step, or a down (down) current flow step in which no current flow is performed.
The time of the cooling step is preferably 500ms or less. On the other hand, if the energization is performed for a time exceeding 500ms in the cooling step, the total time of the welding step itself may become long, and productivity may be lowered.
Fig. 3 shows an example of the energization pattern having a cooling step between the initial energization step and the main energization step. In the example shown in fig. 3, after the initial energization step of the current value I1 (kA) and the energization time t1 (ms), the cooling step of the current value Ic (kA) and the energization time tc (ms) is performed, and then the main energization step of the current value I2 (kA) and the energization time t2 (ms) is performed. Here, the case where the current is supplied for a certain period of time by the current satisfying the formula (3) is shown as the cooling step, but the non-supply step, the multi-stage supply step, or the downstream supply step may be performed as described above.
Next, a method of manufacturing the resistance spot welding joint will be described.
The present invention is a method for manufacturing a resistance spot welding joint using the resistance spot welding method described above. In the method for manufacturing a resistance spot welding joint according to the present invention, for example, 2 or more steel plates are stacked and sandwiched between a pair of welding electrodes, and resistance spot welding is performed while applying pressure and applying current under the welding conditions of the respective steps described above, thereby forming nuggets of a desired size and obtaining a resistance spot welding joint. The steel plate, the welding conditions, and the like are the same as those described above, and therefore, the description thereof is omitted.
As described above, according to the present invention, delay damage of the welded portion can be suppressed. Further, since the spatter having a small size satisfying the above condition of the inter-electrode voltage is generated in the initial energization step, the nugget having a large diameter can be stably formed in the subsequent main energization step.
Further, according to the present invention, the entry of hydrogen into the weld metal having high hydrogen embrittlement sensitivity can be effectively suppressed, and therefore, the above-described effects can be obtained similarly in the resistance spot welding of other steel sheets, not only in the case of resistance spot welding of high-strength steel sheets for automobiles.
Examples
The operation and effects of the present invention will be described below with reference to examples. The present invention is not limited to the following examples.
In the embodiment of the present invention, as shown in fig. 1, the lower steel sheet 1 and the upper steel sheet 2 are overlapped and resistance spot welded. The resistance spot welding is performed at normal temperature, and is performed in a state where the lower electrode 4 and the upper electrode 5 are always water-cooled. The lower electrode 4 and the upper electrode 5 were each formed as a DR-shaped electrode made of chromium copper having a diameter of the tip (tip diameter) of 6mm and a radius of curvature of 40 mm. The pressurizing force is controlled by driving the lower electrode 4 and the upper electrode 5 with a servo motor, and a single-phase alternating current having a frequency of 50Hz is supplied at the time of energization. The following 2 steel grades were used for the welded steel sheets.
(Steel type I) A steel sheet having a tensile strength of 1320MPa, ceq (%) represented by the formula (7) of 0.37%, a long side of 150mm, a short side of 50mm, a sheet thickness of 1.4mm, and no plating treatment
(Steel type II) had a tensile strength of 1470MPa, ceq (%) represented by the formula (7) of 0.40%, a long side of 150mm, a short side of 50mm, a plate thickness of 1.4mm, and a plating treatment (melting)Zinc melt plating (GI) with an adhesion of 50g/m on average on one side 2 ) Steel sheet of (2)
In the case of the plate group at the time of welding, the same 2 pieces of combination of steel grade I was set as plate group a, the same 2 pieces of combination of steel grade II was set as plate group B, and the different 2 pieces of combination of steel grade I and steel grade II was set as plate group C, and experiments were conducted with the objective of evaluating the delayed fracture characteristics and the nugget stability.
Here, a welded joint used in the test will be described with reference to fig. 2. Fig. 2 (a) is a plan view of the welded joint, and fig. 2 (b) is a side view thereof. As shown in fig. 2 (a) and 2 (b), between 2 sheets of steel sheets 1 and 2 (length in the longitudinal direction is 150mm and length in the width direction is 50 mm) of the steel type, spacers 6 having a thickness of 2.0mm and a side length of 50mm were sandwiched from both sides and temporarily welded, and then the centers of the sheet groups obtained by overlapping 2 sheets were welded under the conditions shown in table 1. As shown in fig. 2 (b), temporary welding portions at both ends of the plate group are designated as temporary welding spots 8, and a welding portion at the center of the plate group is designated as a welding spot 7.
During welding, the current value was adjusted so that the nugget diameter became about 3.5 ∈t (t: plate thickness) (mm) under all conditions. In the case of a steel sheet having a sheet thickness of 1.4mm, 3.5 v/t=4.14 mm.
The delay fracture characteristics were evaluated as follows. The obtained welded joint was left to stand in the atmosphere at normal temperature (20 ℃) for 24 hours, and then the presence or absence of delayed fracture of the welded portion was examined. The welding was performed under all conditions with n=3, and the mark "o" where no delayed fracture occurred after standing for 24 hours and the mark "x" where the mark occurred were shown in table 2.
Regarding the judgment of the delayed fracture, it was judged that the delayed fracture occurred when the detachment of the nugget (the phenomenon that the nugget is detached into two at the joint interface) was visually observed after the welding. As a final determination of the delay fracture characteristics, a condition mark ". Very good" in which no delay fracture occurs for 3 times in n=3, a condition mark ". O" in which delay fracture occurs only 1 time in n=3, and a condition mark "×" in which delay fracture occurs 2 times or more in n=3 are shown in table 2, respectively.
The same test body was used to evaluate the stability of the nugget. The evaluation of the nugget stability was performed as follows. Regarding the nugget stability, the sign "very" for nugget diameters of 3.5 v t or more was obtained for all of n=3, the sign "good" for nugget diameters of 3.5 v t or more was obtained for n=2 of n=3, and the sign "delta" for nugget diameters of 3.5 v t or more, which was n=1 or less of n=3, was obtained, and is shown in table 2. The "(∈)" shown in table 2 indicates a nugget diameter of less than 3.5 Δt.
In this example, the nugget diameter was calculated by cutting the nugget diameter at the center of the welded portion after welding, etching the obtained cross section with a bitter aqueous solution (picric acid aqueous solution), and then measuring the length of the corroded nugget structure.
As is clear from table 2, in the inventive example, the occurrence of delayed fracture in the welded joint was suppressed. In the invention example, the generated spatter is small, and the effect of stably forming the nugget can be obtained in addition to the delayed fracture suppression effect. In particular, in the embodiment provided with the cooling step, the nugget diameter is 3.5 v t or more under all conditions where n=3, and the effect of forming the nugget more stably can be obtained.
In contrast, in the comparative example, delay damage cannot be suppressed.
Description of the reference numerals
1. Lower steel plate
2. Upper steel plate
3. Nugget
4. Lower electrode
5. Upper electrode
6. Spacing piece
7. Welding spot
8. Temporary welding spot
Claims (3)
1. A resistance spot welding method for joining 2 or more steel plates by sandwiching them with 1 pair of electrodes and applying electricity while pressurizing, wherein,
the energizing includes:
an initial electrifying procedure; and
a main power-on step of forming a nugget having a predetermined nugget diameter,
in the initial energization step, spatter is generated from the joint surfaces of the 2 or more steel plates,
the current value I1 in the initial energization step satisfies the following expression (2),
1.1×I2≤I1≤5×I2 · · · (2)
wherein, I1: the current value in the initial electrifying process is in the unit of kA,
I2: the current value in the main energizing step is kA,
the welding voltage Vs at the time point of the spatter generation satisfies the following expression (1),
Vs≥0.7×Va · · · (1)
wherein Va: the welding voltage before 5ms of splash generation is expressed as V,
Vs: the welding voltage at the spatter generating time point is given in V.
2. The resistance spot welding method according to claim 1, further comprising a cooling step of stabilizing a contact state between the steel plates by applying an electric current at an electric current value Ic satisfying the following formula (3) between the initial energizing step and the final energizing step,
0≤Ic≤I1 · · · (3)
wherein Ic: the current value in the cooling step is kA,
I1: the current value in the initial energization step is in kA.
3. A method of manufacturing a resistance spot welding joint using the resistance spot welding method according to claim 1 or 2.
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