阅读说明：本技术 电阻点焊方法、电阻点焊接头的制造方法 (Resistance spot welding method and method for manufacturing resistance spot welded joint ) 是由 川边直雄 松田广志 村上善明 于 2019-04-24 设计创作，主要内容包括：以提供电阻点焊方法、电阻点焊接头的制造方法为目的。本发明是一种电阻点焊方法,是将2张以上的钢板重合并用1对焊接电极进行夹持,边加压边通电进行接合的电阻点焊方法,作为通电,具有初始通电工序、和形成具有规定的熔核直径的熔核的正式通电工序,在初始通电工序内产生飞溅。(An object is to provide a resistance spot welding method and a method for manufacturing a resistance spot welded joint. The present invention provides a resistance spot welding method in which 2 or more steel plates are stacked and clamped by 1 pair of welding electrodes, and welding is performed while applying pressure, and the resistance spot welding method includes an initial energization step and a main energization step for forming a nugget having a predetermined nugget diameter as energization, and spatters are generated in the initial energization step.)

1. A resistance spot welding method for joining steel sheets of 2 or more sheets by superposing the sheets and clamping the sheets by 1 pair of electrodes and applying a voltage thereto,

the energization includes:

an initial electrifying procedure; and

a main energization step of forming a nugget having a predetermined nugget diameter,

spattering is generated in the initial energization step.

2. The resistance spot welding method according to claim 1, wherein a welding voltage Vs (V) at a time point of the spatter generation satisfies the following formula (1),

Vs≥0.7×Va ···(1)

wherein, Va: welding voltage (V) 5ms before generated by spatter,

Vs: welding voltage (V) at the spatter generating time point.

3. The resistance spot welding method according to claim 1 or 2, wherein a 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: the current value (kA) in the main energization step.

4. The resistance spot welding method according to any one of claims 1 to 3, further comprising a cooling step of cooling the nugget by conducting current at a current value ic (kA) satisfying the following formula (3) between the initial energization step and the main energization step,

0≤Ic≤I1 ···(3)

wherein, Ic: a current value (kA) in the cooling step,

I1: current value (kA) in the initial energization step.

5. A method for manufacturing a resistance spot welding joint by using the resistance spot welding method according to any one of claims 1 to 4.

Technical Field

The present invention relates to a resistance spot welding method and a method for manufacturing a resistance spot welded joint.

Background

Resistance spot welding is widely used in the assembly of vehicle bodies of automobiles and the like, and resistance spot welding is performed at up to several thousand points in 1 part of the vehicle body. Resistance spot welding is performed by stacking 2 or more steel plates and applying pressure while sandwiching upper and lower 1 pairs of welding electrodes. Thereby, a nugget of a predetermined size is formed at the joint portion of the steel sheets and the steel sheets are joined to obtain a welded joint.

In recent years, from the viewpoint of environmental protection, CO of automobiles has been demanded₂Reduction in emissions leads to weight reduction of the vehicle body, that is, improvement in fuel economy, by making the vehicle body thinner using a high-strength steel sheet. However, high-strength steel sheets generally contain a large amount of C and various alloying elements added to improve strength, and the hydrogen embrittlement sensitivity is increased. In resistance spot welding, rust preventive oil, moisture, a plating layer, and the like on the surface of a steel sheet are involved in the weld metal (molten portion) during melting and solidification at the time of welding, and therefore remain as a hydrogen source which causes delayed fracture after cooling.

Therefore, when high-strength steel sheets are welded by resistance spot welding, the occurrence of delayed fracture due to hydrogen entering into the weld metal having high hydrogen embrittlement sensitivity at the time of welding becomes a problem in the welded portion of the resulting welded joint.

As a method for preventing delayed fracture of a welded portion, for example, patent document 1 discloses a technique for preventing delayed fracture by controlling residual stress of a welded portion by increasing a pressing force and reducing a current immediately after welding energization (main energization). For example, patent document 2 discloses a technique of preventing delayed fracture by increasing a pressurizing force immediately after welding energization (main energization) and performing energization after a cooling time without energization has elapsed, thereby controlling the structure and/or hardness of a welded portion.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2015-93282

Patent document 2: international publication No. 2014/171495

Disclosure of Invention

Problems to be solved by the invention

As described above, resistance spot welding of high-strength steel sheets has a problem that hydrogen enters weld metal. Therefore, in resistance spot welding of high-strength steel sheets, it is important to increase the strength of a welded joint and to reduce the amount of hydrogen remaining in a welded portion in order to prevent delayed fracture.

However, the techniques of patent documents 1 and 2 are not techniques for reducing the amount of hydrogen in the welded portion in order to prevent delayed fracture. Further, in these techniques, when the pressurizing force is excessively increased in a state where the nugget immediately after welding energization is melted, the plate thickness of the welded portion is likely to be reduced, and there are problems such as a decrease in the strength of the resulting welded joint or a deterioration in the appearance of the welded portion.

Therefore, such a problem that hydrogen enters a weld metal having high hydrogen embrittlement sensitivity during welding and causes delayed fracture occurs is similarly present not only in resistance spot welding of a high-strength steel sheet for automobiles but also in resistance spot welding of other steel sheets.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a resistance spot welding method and a method for manufacturing a resistance spot welded joint, which can suppress delayed fracture of a welded portion.

Means for solving the problems

The present inventors have investigated behavior of hydrogen entering into a weld metal at the time of welding, which is a main cause 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 obtained the following findings.

As described above, first, hydrogen enters the welded portion at the time of welding. Since hydrogen diffuses more slowly at a lower temperature, a large amount of hydrogen remains without diffusing from the nugget due to rapid cooling after welding. Then, as time passes, hydrogen collects at a portion where a large tensile stress is concentrated, which is represented by the notch shape of the nugget end portion, thereby generating delayed fracture.

Therefore, it is effective to discharge more hydrogen from the nugget during welding and reduce the remaining hydrogen, which is effective in suppressing delayed fracture.

Therefore, the present inventors have conducted extensive studies on suitable resistance spot welding conditions that can reduce the amount of residual hydrogen in the welded portion. The results are explained below.

In the energization step, first, the hydrogen source present on the mating surface of the steel sheet can be discharged as splashes by generating the splashes from the mating surface of the steel sheet. As a result, the contamination of hydrogen into the nugget in the subsequent energization step can be reduced, and the delayed fracture resistance of the welded joint can be improved. When the spatter is generated in the latter stage of the energization step, it is difficult to reduce the hydrogen mixed into the nugget before the generation of the spatter. As a result, delayed fracture may not be suppressed, growth of nuggets may be affected, and a large nugget diameter may not be ensured.

Therefore, by dividing the energization step into 2 stages, specifically, into the 1 st energization step (the initial energization step described later) for the purpose of generating the spatters and the 2 nd energization step (the final energization step described later) for the purpose of forming the nuggets thereafter, the spatters can be generated in the initial stage of the energization step and the spatters can be suppressed in the latter stage of the energization step.

Further, by providing the first energization step (initial energization step) as described above, the adhered matter such as moisture, oil, or dirt present on the steel sheet faying surface can be discharged together with the splashes to keep the steel sheet faying surface clean, and the steel sheet can be softened appropriately before nugget formation by energization heating. This can maintain the contact state between the steel sheets well, and can provide an effect of improving the delayed fracture resistance. Further, the effect of more stably forming nuggets having a large nugget diameter in the 2 nd energization step (main energization step) can also be obtained.

The present invention has been made based on the above knowledge, and the gist thereof is as follows.

[1] A resistance spot welding method for joining steel sheets of 2 or more sheets by superposing the sheets and clamping the sheets by 1 pair of electrodes and applying a voltage thereto,

the energization includes:

an initial electrifying procedure; and

a main energization step of forming a nugget having a predetermined nugget diameter,

spattering is generated in the initial energization step.

[2] According to the resistance spot welding method described in [1], a welding voltage Vs (V) at a time point of the spatter generation satisfies the following formula (1),

Vs≥0.7×Va···(1)

wherein, Va: welding voltage (V) 5ms before generated by spatter,

Vs: welding voltage (V) at the spatter generating time point.

[3] According to the resistance spot welding method as set forth in [1] or [2], 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: the 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 current at a current value ic (kA) satisfying the following formula (3) between the initial energization step and the main energization step,

0≤Ic≤I1···(3)

wherein, Ic: a current value (kA) in the cooling step,

I1: current value (kA) in the initial energization step.

[5] A method for manufacturing a resistance spot welded joint, using the resistance spot welding method according to any one of [1] to [4 ].

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, delayed fracture of the welded portion can be suppressed, and therefore, an extra industrial effect is obtained.

Drawings

Fig. 1 is a cross-sectional view schematically showing 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, fig. 2(a) is a plan view thereof, and fig. 2(b) is a side view thereof.

Fig. 3 is a graph showing an example of the energization pattern in the resistance spot welding method of the present invention.

Detailed Description

Hereinafter, a resistance spot welding method and a method of manufacturing a resistance spot welded joint according to the present invention will be described with reference to the drawings. The present invention is not limited to the embodiment.

First, a resistance spot welding method according to the present invention will be described with reference to fig. 1.

The present invention joins 2 or more steel plates by resistance spot welding. Fig. 1 schematically illustrates an example of the resistance spot welding method. Fig. 1 shows an example of resistance spot welding 2 steel plates.

Firstly, overlapping more than 2 steel plates. In the example shown in fig. 1, 2 steel plates, that is, a steel plate disposed on the lower side (hereinafter, referred to as a lower steel plate 1) and a steel plate disposed on the upper side (hereinafter, referred to as an upper steel plate 2), are stacked.

The overlapped steel plates (lower steel plate 1 and upper steel plate 2) are sandwiched by 1 pair of welding electrodes (electrodes) 4 and 5 arranged in the vertical direction, and current is applied in a current application mode described later while applying pressure. 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 thus-stacked steel plates are subjected to edge pressing and energization while being sandwiched between 1 pair of welding electrodes, a nugget 3 having a desired size is formed by resistance heating, and the stacked steel plates are joined to each other, thereby obtaining a welded joint. In the present invention, 3 or more steel sheets may be stacked and resistance spot-welded, and in this case, a welded joint can be obtained by performing the same welding method as described above, but the welded joint 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 pressurize by the lower electrode 4 and the upper electrode 5 and can control the pressurizing force arbitrarily. Conventionally known devices such as an air cylinder, a servo motor, and the like can be used. The configuration for supplying a 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 alternating current, "current" means "effective current".

The form of the tip of the lower electrode 4 and the upper electrode 5 is not particularly limited. Examples thereof include JIS C9304: DR (dome radius) shape (eccentric shape), R (radius) shape (spherical head shape), D (dome) shape (round flat head shape) and the like described in 1999. The diameters of the tips of the lower electrode 4 and the upper electrode 5 are, for example, 4mm to 16 mm. The resistance spot welding is performed in a state where the electrode is always water-cooled.

In the present invention, the steel type of the steel sheet to be resistance spot welded 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, which is represented by the following formula (7). In the 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 effects of the present invention can be more effectively obtained. When Ceq (%) exceeds 0.60 (%), the resistance spot welded portion has too high sensitivity to delayed fracture, and it is difficult to suppress delayed fracture 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 a Ceq (%) of less than 0.17% and/or a 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)

In the formula (7), the element symbol indicates the content (mass%) of each element, and the element not contained is 0.

The thickness of the steel sheet subjected to resistance spot welding is not particularly limited. For example, it is preferably in the range of 0.5mm to 3.0 mm. Since a steel sheet having a sheet thickness within this range can be suitably used as a member for automobiles.

The steel sheet to be resistance spot welded may be a steel sheet having a plating layer on the surface thereof by plating. In the present invention, examples of the plating include Zn plating and Al plating. Examples of the Zn-based plating include hot-dip Galvanizing (GI), Zn-Ni-based plating, and Zn-Al-based plating. Examples of the Al-based plating include Al-Si-based plating (e.g., Al-Si-based 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 coating layer include an alloyed hot-dip Galvanized (GA) layer.

The 2 or more sheets of steel subjected to resistance spot welding may be the same or different. That is, the steel sheets may be of the same kind and shape, or of different kinds and/or shapes. The surface-treated steel sheet having a plating layer may be superposed on a steel sheet having no plating layer.

Next, the energization pattern in the resistance spot welding method of the present invention will be described.

The present invention relates to a resistance spot welding method for joining two or more steel sheets by superposing the steel sheets, clamping the steel sheets by 1 pair of electrodes, applying a voltage, and applying a current to the steel sheets to form nuggets. In the example shown in fig. 1, the steel sheets 1 and 2 held between the electrodes 4 and 5 are energized in a specific pattern while being pressed. The energization of the present invention includes an initial energization step and a main energization step for forming a nugget having a predetermined nugget diameter.

First, in the initial energization step, the energization is performed at a higher current value than in the main energization step, and thus the spatters are generated in the step. That is, in the initial energization step, the hydrogen source and the splashes existing on the steel sheet faying surface are discharged together, and a good contact state between the steel sheets is ensured.

In the present invention, it is important to generate spatters in the initial energization step. When the generation of the spatters is a step after the initial energization step (for example, a cooling step and a main energization step described later), a large amount of hydrogen is mixed into the nuggets before the generation of the spatters. Therefore, it is difficult to obtain the hydrogen reduction effect by the spattering, and the delayed destruction suppression effect is not obtained. In addition, when it is desired to exhibit the hydrogen reduction effect more remarkably, it is effective to shorten the energization time before the splash is generated and to suppress the mixing of hydrogen to the minimum.

In the present invention, it is preferable that the spatter is generated within 200ms from the start of energization in the initial energization step. More preferably, the spatter is generated within 100ms from the start of the energization in the initial energization step.

The spatter generated in the initial energization step is preferably small in scale (hereinafter, also referred to as small spatter) in order to stably form nuggets having a large diameter in the later-described main energization step. When the voltage between the electrodes is measured in resistance spot welding, if spatter is generated, the resistance between the electrodes is reduced, and therefore the reduction in voltage is expressed in the measured value. In the present invention, the magnitude of the splash is controlled by the voltage drop amount at the time of the splash generation. Specifically, it is preferable that the current value and the pressurizing force in the initial energization step are set so that the voltage vs (v) between the electrodes at the time point when the spatters are generated satisfies the following expression (1). The spatter generated by the energization satisfying the formula (1) means a small spatter as referred to in the present invention.

Vs≥0.7×Va···(1)

Wherein, Va: voltage (V) between electrodes 5ms before spatter generation,

Vs: voltage (V) between electrodes at the time of spattering generation.

When the voltage vs (v) between the electrodes at the time of spatter generation is lower than (0.7 × Va), the spatter is large in scale, and a good current supply state cannot be ensured in the main current supply step, and therefore, a nugget having a large nugget diameter (hereinafter, also referred to as a diameter) cannot be stably formed. Therefore, the voltage vs (v) between the electrodes at the time of the spatter generation is set to (0.7 × Va) or more. In order to more significantly exhibit the effect of maintaining a good contact state between the steel sheets and stably forming a nugget having a large diameter in the main energization step, it is effective to minimize the scale of the spatters, and therefore, it is preferable to set the voltage vs (v) between the electrodes at the time of spatter generation to (0.8 × Va) or more. As described above, generally, when spatter is generated during spot welding, the voltage between electrodes is reduced. That is, it is considered that the voltage between the electrodes does not increase due to the generation of the spatters, and therefore (1.0 × Va) or more is not assumed in the above formula (1).

After the initial energization step, a main energization step is performed to form a nugget having a predetermined diameter. In the main energization step, energization conditions such as a current value and energization time for forming the nugget, and pressurization conditions are not particularly limited, and conventionally known welding conditions can be employed.

For example, from the viewpoint of forming a nugget having an appropriate diameter, the current value in the main energization step is preferably 1.0kA or more and 15.0kA or less, and the pressing force in the main energization step is preferably 1.0kN or more and 9.0kN or less. The energization time in the main energization step is preferably 100ms to 1000 ms. The main energization step may be a multi-stage energization or multi-stage pressurization step in which the current value or the pressurization force changes in the main energization step.

In the present invention, the nugget having the predetermined nugget diameter is preferably 3 v t to 6 v t (t: sheet thickness) (mm) in nugget diameter.

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 realizing the initial energization step of 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 formula (2).

1.1×I2≤I1≤5×I2···(2)

Wherein, I1: a current value (kA) in the initial energization step,

I2: the current value (kA) in the main energization step.

If the current value I1(kA) in the initial energization step is lower than (1.1 × I2), spatters may be less likely to be generated in the initial energization step. As a result, hydrogen in the melting nuclei cannot be reduced, and the effect of suppressing delayed destruction cannot be obtained. When the current value I1(kA) exceeds (5 × I2), the scale of the spatters generated increases, and it may be difficult to stably form a nugget having a large diameter in the subsequent main energization step. In the case where it is desired to more significantly obtain the effect of suppressing the delayed fracture by the small spatters generated in the initial energization step and the effect of stably forming the nuggets having a large diameter in the main energization step, the current value I1 in the initial energization step is more preferably set to 1.3 × I2 ≦ I1, and still more preferably set to I1 ≦ 3 × I2.

The energization time in the initial energization step is preferably 300ms or less. When the energization is performed for a time longer than 300ms, the possibility of large-scale spatters is increased, and it may be difficult to stably form nuggets having a large diameter in a subsequent main energization step. More preferably, it is set to 140ms or less.

Next, a cooling step, which is an appropriate condition for the resistance spot welding method of the present invention, will be described with reference to fig. 3. Fig. 3 shows an example of the energization pattern having the cooling step.

As described above, in the present invention, a cooling step of cooling the nuggets by energization at the current value ic (ka) satisfying the following expression (3) may be provided between the initial energization step and the main energization step.

0≤Ic≤I1···(3)

Wherein, Ic: a current value (kA) in the cooling step,

I1: current value (kA) in the initial energization step.

By providing the cooling step, the contact state between the steel sheets once disturbed by the generation of the spatters can be stabilized again, and the nugget can be formed more stably in the subsequent main energization step. If the current value ic (kA) in the cooling step exceeds the current value I1(kA) in the initial energization step, the possibility of spattering in the cooling step increases, and the effect of ensuring the contact state between the steel sheets may not be obtained. In the cooling step, since the purpose is to stabilize the contact state between the steel sheets without generating spatters, the energization pattern of the cooling step is not particularly limited as long as the current value Ic in the cooling step is in a range satisfying the formula (3), and may be a non-energization step, a multi-stage energization step, or a downstream (downslope) energization step in which energization is not performed.

The time for the cooling step is preferably 500ms or less. On the other hand, if the energization is performed for a time longer than 500ms in the cooling step, the total time of the welding step itself may be increased, and the productivity may be reduced.

Fig. 3 shows an example of an energization pattern having a cooling step between an initial energization step and a main energization step. In the example shown in fig. 3, after the initial energization step at current value I1(kA) and energization time t1(ms), the cooling step at current value ic (kA) and energization time tc (ms) is performed, and then the main energization step at current value I2(kA) and energization time t2(ms) is performed. Here, as the cooling step, the case where energization is performed for a certain time with a current satisfying the formula (3) is shown, but as described above, the non-energization step, the multi-stage energization step, or the downstream energization step may be performed.

Next, a method of manufacturing the resistance spot welded joint will be described.

The present invention is a method for manufacturing a resistance spot welded joint using the above resistance spot welding method. In the method of manufacturing a resistance spot welded joint according to the present invention, for example, 2 or more steel sheets are stacked and sandwiched by a pair of welding electrodes, and resistance spot welding is performed by applying current under the welding conditions of the above-described respective steps while pressing the sheets, thereby forming nuggets having a desired size, and obtaining a resistance spot welded joint. The steel sheet, 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, delayed fracture of the welded portion can be suppressed. Further, since the spatter of a small scale satisfying the condition of the voltage between the electrodes 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, since hydrogen can be effectively prevented from entering the weld metal having high hydrogen embrittlement sensitivity, the above-described effects can be obtained not only in the case of resistance spot welding of a high-strength steel sheet for automobiles but also in resistance spot welding of other steel sheets.

Examples

The operation and effect of the present invention will be described below with reference to examples. The present invention is not limited to the following examples.

In the example of the present invention, as shown in fig. 1, resistance spot welding was performed by overlapping the lower steel plate 1 and the upper steel plate 2. 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. Both the lower electrode 4 and the upper electrode 5 were made of chrome-copper DR electrodes having a tip diameter (tip diameter) of 6mm and a radius of curvature of 40 mm. Further, the lower electrode 4 and the upper electrode 5 are driven by a servo motor to control the pressurizing force, 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 non-plated steel sheet having a tensile strength of 1320MPa, a Ceq (%) of 0.37%, a long side of 150mm, a short side of 50mm, and a sheet thickness of 1.4mm, and represented by formula (7)

(Steel type II) tensile Strength of 1470MPa, Ceq (%) represented by formula (7) of 0.40%, long side of 150mm, short side of 50mm, plate thickness of 1.4mm, plating treatment (hot-dip Galvanizing (GI), adhesion amount of 50g/m on average on one side²) Steel plate of

The plate groups at the time of welding were subjected to tests for the purpose of evaluating delayed fracture characteristics and nugget stability, with 2 combinations of the same type of steel type I as the plate group a, 2 combinations of the same type of steel type II as the plate group B, and 2 combinations of different types of steel type I and steel type II as the plate group C.

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, in the resistance spot welding, 2 sheets of the steel sheets 1 and 2 (length in the longitudinal direction is 150mm, length in the width direction is 50mm) were temporarily welded by sandwiching spacers 6 having a thickness of 2.0mm and a side length of 50mm from both sides, and the centers of the plate groups obtained by stacking 2 steel sheets were welded under the conditions shown in table 1. As shown in fig. 2(b), the provisional welding points at both ends of the plate group are provisional welding points 8, and the welding point at the center of the plate group is welding point 7.

In the welding, the current value was adjusted so that the nugget diameter became about 3.5 v t (t: sheet thickness) (mm) under all conditions. In the case of a steel sheet having a thickness of 1.4mm, 3.5 √ t is 4.14 mm.

The delayed fracture characteristics were evaluated as follows. The obtained welded joint was left to stand in the atmosphere at normal temperature (20 ℃) and after 24 hours, 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 marking symbol "o" indicating that delayed fracture did not occur after the welding was left standing for 24 hours and the marking symbol "x" indicating that delayed fracture occurred are shown in table 2.

Regarding the determination of delayed fracture, it was determined that delayed fracture occurred when separation of nuggets was visually observed after welding (two nugget separations occurred at the joint interface). As final judgment of the delayed fracture characteristic, a condition mark "excellent" indicating that no delayed fracture occurred for 3 times of n-3, a condition mark "good" indicating that delayed fracture occurred only for 1 time of n-3, and a condition mark "×" indicating that delayed fracture occurred for 2 or more times of n-3 are shown in table 2.

In addition, the same test piece was used to evaluate the nugget stability. The nugget stability was evaluated as follows. Regarding the nugget stability, the mark "excellent" in which the nugget diameter of 3.5 v t or more was obtained for all of n-3, the mark "ao" in which the nugget diameter of 3.5 v t or more was obtained for n-2 in n-3, and the mark "Δ" in which the mark n-1 in n-3 or less was obtained for 3.5 v t or more were each shown in table 2. In addition, "(↓)" shown in table 2 indicates a nugget diameter lower than 3.5 √ t.

In the present example, the nugget diameter was calculated by cutting at the center of the welded portion after welding, etching the obtained cross section with a bitter acid aqueous solution (bitter aqueous solution), and measuring the length of the eroded nugget structure.

As can be seen from table 2, in the invention examples, the occurrence of delayed fracture in the welded joint was suppressed. In the invention example in which the generated spatters are small spatters, the effect of stably forming nuggets can be obtained in addition to the delayed fracture suppression effect. In particular, in the example provided with the cooling step, the nugget diameter is 3.5 √ t or more under all conditions where n is 3, and an effect of forming the nugget more stably can be obtained.

In contrast, in the comparative example, delayed destruction cannot be suppressed.

Description of the reference numerals

1 lower steel plate

2 go up the steel sheet

3 nugget

4 lower electrode

5 Upper electrode

6 spacer

7 welding spot

8 temporary welding points.