KR20190089192A - Resistance spot welding method - Google Patents

Abstract

Provided is a resistance spot welding method capable of suppressing the occurrence of runoff during welding and reduction in the thickness of a welded plate and suppressing delayed fracture of the welded joint. A resistance spot welding method in which two or more steel plates are superimposed on each other and sandwiched between a pair of welding electrodes and are energized while applying pressure to form a nugget and joining the steel sheet together, characterized in that a steel plate is pressed by a first pressing force F ₁ (kN) The main energizing step of forming the nugget part by energizing with the current I ₁ (㎄) and the energization time t _a (2) shown by the formula (2) by the second current I ₂ (㎄) ㎳) has a power application process after the power application to the cooling unit nugget during, and after the power application process a, the pressure delay time represented by (3) from the start of the power application process equation after t _b (㎳) of claim 1 for holding a first pressing force F ₁ for a pressing step, a first second resistance spot welding method has a pressing step of pressing a pressing process a second pressing force F ₂ (kN) is then represented by formula (4) to.

Description

Resistance spot welding method

The present invention relates to a resistance spot welding method.

BACKGROUND ART Resistance spot welding is widely used for assembling a vehicle body such as an automobile, and resistance spot welding of several thousand points is carried out from one vehicle body. In the resistance spot welding, two or more steel plates are superimposed on each other, clamped by a pair of upper and lower welding electrodes, and energized while being pressurized, thereby forming a nugget of a predetermined size at the joining portion of the steel plate, Thereby obtaining a welded joint.

In recent years, reduction of CO ₂ emission of automobiles has been demanded from the viewpoint of environmental protection, and reduction in weight of the vehicle body, that is, improvement in fuel economy, has been achieved by adopting a high strength steel sheet for the vehicle body. However, high-strength steel sheets generally have a high hydrogen embrittlement sensitivity because they are strengthened by adding not only a large amount of C but also various alloying elements. In the resistance spot welding, anti-rust oil, water or a plating layer on the surface of the steel sheet is caused to melt in the weld metal (melting portion) during the welding and solidification process at the time of welding, It remains as a wish.

As described above, when the high-strength steel sheet is welded with the resistance spot, the occurrence of delayed fracture due to the intrusion of hydrogen into the weld metal with high hydrogen embrittlement susceptibility at the time of welding at the welded portion of the obtained weld joint becomes a problem. Therefore, in the resistance spot welding of the high-strength steel sheet, it is important to reduce the amount of hydrogen remaining in the welded portion in order to prevent the delayed fracture while increasing the strength of the welded joint.

As a method for preventing the delayed fracture of the welded portion, Patent Document 1 discloses a technique of controlling the residual stress of the welded portion by increasing the pressing force immediately after welding energization (main energizing) and decreasing the current, And the like. Patent Document 2 discloses a technique for preventing delayed fracture by controlling the structure and hardness of the welded portion by increasing the pressing force immediately after welding current (main current application) and energizing after a lapse of the cooling time before power-off. However, these techniques do not reduce the amount of hydrogen in the welded portion, and also cause a problem that expulsion occurs easily because the pressing force is increased in a state where the nugget immediately after welding current is melted, There is a problem that the strength of the resulting welded joint is lowered or the appearance of the welded portion is deteriorated. Also, in Patent Document 1, a technique of raising the pressing force after elapse of the cooling time before power-off after welding is also disclosed. However, in this technique, since the welded portion is rapidly cooled by the cooling time before neutralization, most of the hydrogen is left without diffusing from the nugget and the remaining amount of hydrogen in the nugget is increased, so that it is difficult to suppress the delayed breakdown.

Patent Document 3 discloses a technique for improving the strength of a welded joint by providing a pressing force holding time for holding a pressing force at the time of welding energization before powering up the welding and then increasing the pressing force before the welding . However, in this technique, since the welded portion is rapidly cooled by the pressurizing force holding time before neutralization, the residual amount of hydrogen in the nugget increases, and delayed fracture tends to occur.

The problem that delayed fracture occurs due to the intrusion of hydrogen into the weld metal having high hydrogen embrittlement susceptibility at the time of welding, the problem that the weld is generated, and the problem that the plate thickness of the welded portion is reduced is due to the fact that the high strength steel plate for automobiles The present invention is not limited to the case of resistance spot welding but also exists in resistance spot welding of other steel sheets.

Japanese Laid-Open Patent Publication No. 2015-93282 International Publication No. WO2014 / 171495 A1 Japanese Laid-Open Patent Publication No. 2010-110816

SUMMARY OF THE INVENTION It is an object of the present invention to provide a resistance spot welding method which is capable of suppressing the occurrence of sparks during welding and the reduction of the thickness of a welded portion and suppressing delayed fracture of the welded joint.

The present inventors investigated the behavior of hydrogen entering the weld metal at the time of 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 large tensile strength, I got recognition.

First, hydrogen enters the welded portion at the time of welding as described above. Since hydrogen diffuses more slowly at a low temperature, most of the hydrogen is left without diffusing from the nugget by quenching after welding. Thereafter, with the elapse of time, hydrogen accumulates at a portion where a large tensile stress represented by a notch type of the nugget end is concentrated, and delayed fracture occurs.

Therefore, it is effective to suppress delayed fracture by discharging more hydrogen from the nugget during welding and reducing the amount of remaining hydrogen.

The inventors of the present invention have studied the suitable resistance spot welding conditions capable of reducing the amount of hydrogen remaining in the welded portion. The results are described below.

In the welding process, after the energization of the welding (main energization) and the energization after the energization, the diffusion of hydrogen from the welded portion is promoted by maintaining the high temperature condition where hydrogen easily diffuses, and the resistance to delayed fracture of the welded joint is improved Could know. In addition, it has been found that by increasing the pressing force in the post-energizing process, it is possible to maintain the welded portion in a high-temperature state and in a large compressive stress state in which hydrogen is more likely to be discharged.

However, if the pressing force is increased immediately after the welding current is applied, a large force is applied to the nugget in a molten state which has little rigidity, which may cause problems such as runoff during welding and reduction of the thickness of the welded portion. It has been found that when the solidification of the molten portion is started and the molten portion is cooled to a temperature at which the rigidity is restored, the pressing force is increased to suppress the occurrence of the above-mentioned problems and to give hydrogen with a large compressive stress .

The present invention has been made on the basis of the above-described recognition, and its point is as follows.

[1] A resistance spot welding method in which two or more steel plates are superimposed on each other and sandwiched between a pair of welding electrodes and energized while being pressed to form a nugget,

A main energizing step of energizing the steel sheet with a first current I ₁ (㎄) while pressurizing the steel sheet with a first pressing force F ₁ (kN)

There is provided a post-energizing step for energizing the energization time t _a (ms) and cooling the neck portion by the second current I ₂ (㎄) represented by the following formula (1), following the main energization step,

Wherein the energization step after the energizing step includes a first pressurizing step of maintaining the first pressurizing force F ₁ (kN) during the pressurization delay time t _b (ms) shown by the following expression (3) And a second pressing step of pressing the first pressing step with a second pressing force F ₂ (kN) shown by the following equation (4).

0 < I ₂ < I ₁ (1)

20? T _a? 400 (2)

10? T _b? T _a (3)

F ₁ < F _{2 < /} = 3F ₁ (4)

[2] The resistance spot welding method according to [1], wherein at least one of the steel sheets is a high strength steel sheet having a carbon equivalent Ceq (%) of 0.2% or more and a tensile strength of 780 MPa or more,

Ceq = C + Si / 30 + Mn / 20 + 2P + 4S (5)

(The symbol of the element in the formula (5) represents the content (mass%) of each element)

[3] The post-energizing step is a step of gradually decreasing the current value from the first current I ₁ (㎄) during the down-slope energizing time t _c (ms) represented by the following equation (6) Energization step,

(1) or (2), which is a subsequent energization step of maintaining the current value at the end of the down slope energization process for the latter energization time t _d (ms) represented by the following formula (7) Spot welding method.

20? T _c? T _a (6)

t _d = t _{a -} t _c (7)

[4] The second pressurizing process is an upslope pressurizing process in which the pressurizing force is gradually increased from the first pressurizing force F ₁ (kN) during the up-slope pressurizing time t _e represented by the following expression (8) ,

In any one of [1] to [3] composed of a late pressing step of maintaining the pressing force at the time of up-slope pressing process ends for reviews pressing time t _f (㎳) represented by then formula (9) in the art up slope pressing process ≪ / RTI >

0 < t _e & lt _; 200 (8)

0? _Tf (9)

According to the present invention, even in the case of performing resistance spot welding of a high-strength steel sheet, it is possible to suppress problems such as occurrence of warping at the time of welding and reduction in thickness of a welded plate thickness, It is possible to obtain a welded joint having a high joint strength, a good appearance of the welded portion and an excellent delayed fracture resistance without causing a warping at the time of welding.

1 is a cross-sectional view schematically showing an example of a resistance spot welding method.
Fig. 2 is a graph showing an example of the energizing pattern and the pressing pattern of the resistance spot welding method according to the present invention.
3 is a graph showing an example of the energizing pattern and the pressing pattern of the resistance spot welding method according to the present invention.
4 is a graph showing an example of the energizing pattern and the pressing pattern of the resistance spot welding method according to the present invention.
5 is a graph showing an example of the energizing pattern and the pressing pattern of the resistance spot welding method according to the present invention.
6 is a plan view and a side view showing a test piece for resistance spot welding.

(Mode for carrying out the invention)

The resistance spot welding method of the present invention is a resistance spot welding method in which two or more steel plates are superimposed one upon another and sandwiched between a pair of welding electrodes and energized while being pressed to form a nugget and to join the steel plate, the pressing force F ₁ (kN), while the pressure in the first current I ₁ (㎄) a second current that appears energization by the following subsequent to main power application process and, art the main energizing step of forming a nugget equation (1) by a I ₂ (㎄ ), And a post-energizing step of energizing the energizing element for a period of energization time t _a (ms) represented by the following formula (2) to cool the nugget element, A first pressing step of holding the first pressing force F ₁ (kN) during the time t _b (ms) and a second pressing step of pressing the first pressing step F ₁ (kN) by the second pressing force F ₂ (kN) 2 pressure step The.

0 < I ₂ < I ₁ (1)

20? T _a? 400 (2)

10? T _b? T _a (3)

F ₁ < F _{2 < /} = 3F ₁ (4)

In the present invention, two or more steel sheets are bonded by resistance spot welding. Fig. 1 is a cross-sectional view schematically showing an example of a resistance spot welding method, and shows an example of performing resistance spot welding of two steel plates. Hereinafter, the resistance spot welding method of the present invention will be described with reference to Fig.

First, two or more steel plates are overlapped with each other. In Fig. 1, a steel plate (hereinafter referred to as a lower steel plate 1) arranged on the lower side and a steel plate (hereinafter referred to as an upper steel plate 2) arranged on the upper side are overlapped with each other.

The steel of the steel plate subjected to resistance spot welding in the present invention is not particularly limited, but it is preferable that at least one steel sheet is a high strength steel sheet having a carbon equivalent Ceq (%) of 0.2% or more and a tensile strength of 780 MPa or more . In FIG. 1, the lower steel plate 1 and / or the upper steel plate 2 is preferably a high strength steel plate having a carbon equivalent of 0.2% or more and a tensile strength of 780 MPa or more as represented by the following formula (5). This is because, in a steel sheet having a Ceq (%) of 0.2% or more and a tensile strength of 780 MPa or more, delayed fracture of the resistance spot welded part tends to be a problem. Of course, the present invention can also be applied to the resistance spot welding of the present invention to a steel sheet having a Ceq (%) of less than 0.2% but a tensile strength of less than 780 MPa.

Ceq = C + Si / 30 + Mn / 20 + 2P + 4S (5)

(The symbol of the element in the formula (5) represents the content (mass%) of each element)

In the present invention, the thickness of the steel plate subjected to resistance spot welding is not particularly limited, but is preferably within a range of 1.0 mm or more and 2.0 mm or less, for example. A steel sheet having a plate thickness within this range can be suitably used as an automotive member.

The steel plate subjected to resistance spot welding may be plated to have a plated layer on its surface. Examples of the plating include Zn-based plating and Al-based plating. Examples of the Zn-based plating include hot-dip galvanizing (GI), Zn-Ni-based plating, and Zn-Al-based plating. As the Al-based plating, Al-Si based plating (for example, Al-Si based plating containing 10 to 20% by mass of Si) and the like can be mentioned. The molten plated layer may be an alloyed molten plated layer. As the alloying hot-dip coating layer, for example, a galvannealed hot dip galvanizing (GA) layer can be mentioned.

Further, two or more steel sheets subjected to resistance spot welding may be the same or different, and may be steel sheets of the same type or of the same shape, or may be steel sheets of different types or different shapes.

Subsequently, a pair of welding electrodes, that is, a lower layer (hereinafter referred to as lower electrode 4) and an upper layer (hereinafter referred to as upper electrode 5) (1) and the upper steel plate (2)) and pressurizes the steel plate. The configuration for pressing by the lower electrode 4 and the upper electrode 5 and for controlling the pressing force is not particularly limited and conventionally known devices such as an air cylinder and a servo motor can be used. There is also no particular limitation on a configuration for supplying current during energization and for controlling the current value, and a device known in the art can be used. The present invention can be applied to both DC and AC. In the case of AC, "current" means "effective current". The shape of the tips of the lower electrode 4 and the upper electrode 5 is not particularly limited. For example, DR type (dome-radius shape) described in JIS C 9304: 1999, R type (Radial type), and D type (domed type). The tip diameter of the electrode is, for example, 4 mm to 16 mm. Resistance spot welding is performed in a state in which the electrode is always water-cooled.

Thus, the welded joints are obtained by energizing the steel plates overlapping each other while pressurizing the steel plates sandwiched between the pair of welding electrodes to form a nugget by resistance heating and joining the steel plates overlapped with each other. In the present invention, this pressing and energizing are performed in a specific pattern. Specifically, in the present invention, for example, as shown in Figure 2, the parts of the nugget while pressing the overlapping steel plate with a first pressing force F ₁ (kN) by feeding the electric current to the first current I ₁ (㎄) formed After the main energizing step and the main energizing step, the nugget part is cooled by energizing for the energization time t _a (ms) represented by the above formula (2) with the second current I ₂ (나타나) And an energizing process. After completion of the post-energization process, the energization is stopped. Figs. 2 to 5 are graphs showing an example of the energizing pattern and the pressing pattern of the resistance spot welding method according to the present invention. Fig. In the energizing pattern and the pressing pattern shown in Figs. 2 to 5, the ordinate is a current value or a pressing force, and both the current value and the pressing force become large as it goes upward in the axial direction on the graph.

The main energizing step is a step of forming a nugget portion to become the nugget 3 when solidifying. The energizing condition and the pressurizing condition for forming the nugget portion are not particularly limited, and conventionally used welding conditions can be employed. For example, the first current I ₁ is 1.0 to 15 kV, and the first pressing force F ₁ is 2.0 kN to 7.0 kN. The time of the main energization step is not particularly limited, and is, for example, 100 ms or more and 1000 ms or less. The term "nugget" refers to a molten and solidified portion generated in welding at the time of superimposed resistance welding, and "nugget" means a molten portion that becomes nugget when solidified (that is, a molten portion before solidification).

In the post-electrification step, the nugget portion is cooled by energizing for the energization time t _a (ms) represented by the above-mentioned formula (2) with the second current I ₂ (나타나) shown by the formula (1) That is, the current value is reduced to the second current I ₂ (㎄) less than the first current I ₁ (㎄) and the energization is performed for the energization time t _a (ms) represented by the above-mentioned formula (2) . That is, by performing the post-electrification step, the solidification of the nugget portion is started, and the temperature region capable of promoting the hydrogen diffusion is maintained for a long time. The first current I ₁ in the equation (1) is a current value at the end of the main energization process.

The second since the current I ₂ has become less than in the molten state does not disclose an even nugget portion solidified to, after the power application process not less than I ₁ the first current, a pressing force than that of the first pressing process at the second pressing step to be described later There arises a problem such as a reduction in the thickness of the welded portion (weld metal and heat affected portion) and the strength of the resulting welded joint is lowered or the appearance of the welded portion is deteriorated, so that a good welded joint can not be obtained.

Further, when the energization time t _a of the post-energization process is less than 20 ms, it is impossible to maintain the high-temperature state, which is the temperature range for promoting the hydrogen diffusion, for a long time. In addition, when the energization time t _a is 400 ms or more, the total time of the welding process itself becomes long, and the productivity decreases.

The post-energizing step includes a first pressurizing step for holding the first pressurizing force F ₁ for the pressurizing delay time t _b (ms) as shown in the above-mentioned equation (3) from the start of the post-electrification step, And a second pressurizing step of pressurizing by the second pressing force F ₂ represented by the above-mentioned formula (4). That is, the pressing force in the post-electrification step is set to be the first pressing force F ₁ which is the pressing force in the main energization step during the press delay time t _b from the start of the post-electrification step to the equation (3) 1 current I ₁ to the second current I ₂ and is increased to the second pressing force F ₂ (kN) expressed by the equation (4) after the lapse of the pressing delay time t _b . The (4) first pressing force F ₁ and the first pressing force of the first pressing step in the expression ₁ F is a pressing force at the end state the power application process. It is preferable that the second pressing force F ₂ satisfies the expression (4), but it is preferable that the second pressing force F ₁ satisfies 1.20F _1? F ₂ when the pressing force is constant in the second pressing step. Specifically, in the case of having an upslash pressing step When the back pressure is not constant, it is preferable that the time to satisfy 1.20F _1? F ₂ in the _second pressurizing process is 20% or more of the up-slope pressurizing process.

When the pressurization delay time t _b is less than 10 ms, the nugget portion gives a high pressing force in a molten state without almost starting to solidify, and problems such as occurrence of runoff and reduction of the thickness of the welded portion occur, Is not obtained. If the pressurization delay time t _b is larger than the energization time t _a , the temperature of the nugget portion is excessively lowered, so that the compressive stress state is not introduced and the effect of efficiently discharging the hydrogen from the welded portion can not be obtained.

When the second pressing force F ₂ is equal to or less than the first pressing force F ₁ , a large compressive stress can not be imparted to the flange portion, and hydrogen in the welding portion can not be efficiently discharged. If the second pressing force F ₂ is larger than 3F ₁ , an excessive dent is formed in the welded portion, which causes a problem of deteriorating the joint strength or deteriorating the appearance.

As in Patent Document 1, if the pressing force is increased after the lapse of the cooling time before the power is turned on after the energization of the welding, the welding portion is rapidly cooled by the cooling time before the power-off. Therefore, most of the hydrogen is caught in the solidification of the weld metal and does not diffuse from the inside of the ladle, so that the amount of the remaining hydrogen in the nugget is increased, and delayed breakage is likely to occur.

In the present invention, by performing the above-mentioned specific main energization process and the post-energization process, it is possible to suppress the occurrence of runoff at the time of welding and the reduction of the thickness of the welded plate, and to suppress delayed fracture of the welded joint. The resistance spot welding method of the present invention can suppress the occurrence of spatter at the time of welding and the reduction of the thickness of the welded part plate and suppress the delayed fracture of the welded joint. , And is suitable for welding high-strength steel sheets for automobiles.

The second current I ₂ , the second pressing force F ₂ , the energizing time t _a, and the pressing delay time t _b are not particularly limited as long as the above conditions are satisfied.

As shown in Fig. 3, the post-energizing step gradually decreases the current value during the down-slope energization time t _c (ms) represented by the following equation (6) from the first current I ₁ (7), which is followed by a down-slope energizing step, to maintain the current value at the end of the down-slope energizing step for the latter energizing time t _d (ms) . That is, after the down slope energization in which the current value is gradually decreased from the first current I ₁ during the down slope energization time t _c (ms) shown by the following expression (6) in the post-energization step, The current value at the end of the down slope energization may be maintained for the latter energization time t _d (ms) represented by the equation (7). Further, in the case of t _a = t _c , the latter energization step is not performed and the current in the latter energization step is made only by the down-slope energization step.

20? T _c? T _a (6)

t _d = t _{a -} t _c (7)

As described above, by performing the down slope energizing step capable of suppressing the abrupt change in the current value at the start of the post-energizing step, the hydrogen gas can be efficiently discharged by maintaining the high temperature state, which is the temperature range in which the hydrogen diffusion can be promoted, . In this case, the second current I ₂ in the later energization process satisfies 0.3 I ₁ ≤I ₂ <0.95 I ₁ (where the first current I ₁ is the current value at the end of the main energization process), and the pressurization delay time t _b It is preferable that t _{b &} gt _{; =} 20. The second current I ₂ in the down-slope energizing process may satisfy the above-described formula (1) and gradually decrease from the first current I ₁ .

The second pressing step is, to 8 in the up-slope pressing time t _e (㎳) pressing force to the first pressing force gradually increases from the F ₁ while appearing formula (increasing of the power application process and then, as shown in Fig. 4, or step-by-step , And an up-slope pressurizing process for maintaining the press-down force at the end of the up-slope pressurizing process for the latter pressurizing time t _f (ms) shown by the following formula (9) . That is, after the second pressurizing step performs the up-slope pressurization in which the pressurizing force is gradually increased from the first pressurizing force F ₁ during the up-slap pressurizing time t _e represented by the following expression (8) Latter pressing time represented by the equation _f t while the pressure may be kept at the time of up-slope pressure end. In the case of t _f = 0, the latter pressurizing step is not performed, and the pressing force of the second pressurizing step is made only by the up-slope pressing step. The upper limit value of t _f is not particularly limited, but is preferably t _f? 400.

0 &lt; t _e & lt _; 200 (8)

0? _Tf (9)

By performing the upslope pressing process capable of suppressing the abrupt change of the pressing force at the beginning of the second pressing process, the pressing process is performed at the high temperature condition, which is the temperature range where the hydrogen diffusion can be promoted without exerting an excessive force on the welding portion So that the effect of hydrogen emission can be exerted more remarkably. In this case, the second pressing force F ₂ is the pressing step, reviews 1.20F ₁ ≤F ₂ (Expression of the first force F _1, the pressing force being at the end of the main energizing step) preferably satisfying. The second pressing force F ₂ in the up-slope pressurizing step may be satisfied by satisfying the above-mentioned expression (4) and gradually increasing from the first pressing force F ₁ .

As shown in Figure 5, with regard to the current value, after the power application process is a down slope power application to the above (6) formula as shown downslope energization time t _c (㎳) gradually decreases the value of current from the first current I ₁ during And a post-electrification step of maintaining the current value at the end of the down-slope energization process for the latter energization time t _d (ms) shown by the above-mentioned expression (7) following the downslope energization process. , And the second pressurizing step of the post-energizing step includes an upslope pressing step in which the pressing force is gradually increased from the first pressing force F ₁ during the upslope pressing time t _e represented by the above-mentioned equation (8) 9 may be made such that the pressing time t _f late late pressing step of maintaining the pressing force at the time of up-slope end during the pressing step shown by the formula.

The pressing force may be applied at the same time as the start of the main energization process or may be given before the main energization process as shown in Figs. The pressing force may be stopped at the same time as the end of the post-energizing process (the electrode may be separated from the steel plate), or the pressing force may be maintained in the non-conductive state after the end of the post-energizing process, as shown in Figs.

The current value of the main energizing step (the first current I ₁₎ is, also may be 2 to constant as shown in Figure 5, also, as to gradually increase in the main power application process (growing, or multi-stage power application step by step increase) .

The pressing force (first pressing force F ₁ ) in the main energizing process may be constant as shown in Figs. 2 to 5, and may be gradually increased (gradually increased or stepwise increased) during the main energizing process Maybe.

In the above, mainly, the case of welding two steel plates is described, but the same is applicable to a case of welding three or more steel plates.

Example

EXAMPLES Hereinafter, the present invention will be described with reference to examples for the purpose of further understanding of the present invention, but the present invention is not limited to these examples.

(Inventive and comparative examples)

As shown in Fig. 1, the lower steel plate 1 and the upper steel plate 2 were overlapped with each other to perform resistance spot welding. The resistance spot welding was performed at normal temperature, and the electrode was always kept in a water-cooled state. The lower electrode 4 and the upper electrode 5 were each made of a CR type electrode having a diameter (tip diameter) of 6 mm at the tip and a radius of curvature of 40 mm. The pressing force was controlled by driving the lower electrode 4 and the upper electrode 5 with a servo motor, and a single-phase alternating current with a frequency of 50 Hz was supplied at the time of energization. (Tensile strength 1470 MPa, Ceq (%) expressed by the formula (5) is 0.4%, long side is 100 mm, short side is 100 mm) for the lower steel plate 1 and the upper steel plate 2 in the test numbers 1 to 28, (Tensile strength: 1470 MPa, Ceq (%) in the formula (5) was 0.4%, long side was 100 mm, short side was 30 mm, plate thickness was 1.6 mm, no plating treatment) , A plate thickness of 1.6 mm and a plating treatment (hot dip galvanizing (GI), the amount of adhesion being 50 g / m 2 per side)) was used. The tensile strength is a tensile strength obtained by preparing a tensile test specimen of JIS No. 5 in parallel with the rolling direction from each steel sheet and performing a tensile test according to JIS (Japanese Industrial Standard) Z 2241: 2011.

6, a spacer 6 having a thickness of 1.6 mm and a size of 30 mm square was sandwiched between two steel plates (100 mm in the long direction and 30 mm in the short direction) And the center of the combined structure of steel sheets in which the two steel plates were in contact with each other was welded under the conditions described above and in Table 1-1 and Table 1-2. Fig. 6 is a plan view (Fig. 6 (a)) and a side view (Fig. 6 (b)) showing a test piece for resistance spot welding. Reference numeral 7 in Fig. 6 denotes a welding point and 8 denotes an available contact point. Further, in all the times (inventive and comparative examples), the first current I ₁ in the main energization step was set to a constant value. In the post-energization process, the down-slope energization is not performed in the sequence numbers 2 to 9, 16 to 23, 30 to 37, and 44 to 51, the second current I ₂ is set to a constant value, and the sequence numbers 10 to 12, 24 to 26 , 38 to 40, and 52 to 54, down slope energization is performed to linearly reduce the current value, and the second current I ₂ after the down slope energization is set to a constant value. In the post-energization process, the current values of the energization after making the sequence numbers 13 to 14, 27 to 28, 41 to 42, 55 to 56, The first pressing force F ₁ of the first pressing process in the main energization process and the post-energization process was set to a constant value in all the times (inventive and comparative example). The second pressing force is not applied in the up-slope pressures in the cases 2 to 8, 10, 11, 13, 16 to 22, 24, 25, 27, 30 to 36, 38 to 39, 41, 44 to 50, 52 to 53, F ₂ is set to a constant value and up-slope pressurization is performed to increase the pressing force linearly in the second pressurizing process in the times 9, 12, 14, 23, 26, 28, 37, 40, 42, 51, 54, , And the second pressing force F ₂ after the up-slope pressing is set to a constant value. In the case where down slope energization or non-energization is performed, in Table 1-1 and Table 1-2, the "down slope energization" or "non-energization" and the subsequent current values are listed in the second current column In the column of the energization time, the time for energizing or de-energizing the down slope and the energizing time thereafter are described in this order. Similarly, in the case where the upslope pressurization is carried out, the "upslope pressurization" and the pressurizing force thereafter are described in the order of the second pressurizing force in Table 1-1 and Table 1-2, , The time of the up-slope pressurization and the pressurization time thereafter are described in this order. In Table 1-1 and Table 1-2, the value described in the cycle of the single-phase alternating current (frequency 50 Hz) as described above and the value converted in ms (one cycle is 20 ms) are described.

Of the comparative examples of Table 1-1 and Table 1-2, the sequence numbers 1, 15, 29, and 43 are examples in which the post-energization process is not performed. Sequences 3, 17, 31, and 45 are examples of increasing current in the post-energization process. Examples 7, 21, 35, and 49 are examples in which the pressing force is lowered in the second pressurizing process. The sequence numbers 8, 11, 25, 36, 39, and 53 are examples in which the pressing force is increased immediately after the end of the main energization process. The sequence numbers 13 to 14, 27 to 28, 41 to 42, and 55 to 56 are examples in which electricity is not supplied immediately after the main energization process.

The obtained welded joint was allowed to stand at room temperature (20 캜) in the atmosphere, and after 24 hours passed, the depth of penetration and the presence of delayed fracture were investigated. The evaluation of the welded joint was evaluated by three items of the depth of penetration of the welded portion, the occurrence of the creep at the weld, and the delayed fracture after the welding, and the results are shown in Tables 1-1 and 1-2. For the depth of penetration, the plate thickness after welding was 70% or more before welding, and the case where the plate thickness was less than 70% was evaluated as X. Regarding the skidding, it was rated that the skidding did not occur at the time of welding, and the skidding occurred when the skidding occurred. With respect to the delayed breakage, the case where delayed breakage did not occur after standing for 24 hours was rated as &quot;? &Quot; With regard to the determination of delayed fracture, it was determined that delamination occurred when the nugget was peeled off (the phenomenon that the nugget was peeled at two from the bonding interface) after the welding was observed with the naked eye. Among the above evaluation items, those in which all the results were &quot;? &Quot; The nugget diameters of the obtained weld joints are also shown in Tables 1-1 and 1-2. The nugget diameter is the maximum diameter at the abutment surface of the two steel plates, and t is the thickness (mm) of the thinnest steel plate among the overlapped steel plates.

As is evident from Tables 1-1 and 1-2, all of the inventive examples obtained good welded joints, whereas the comparative examples did not provide good welded joints.

(Table 1-1)

(Table 1-2)

1: Lower steel plate
2:
3: Nugget
4: Lower electrode
5: Upper electrode
6: Spacer
7: welding spot
8: Available contacts

Claims (4)

A resistance spot welding method in which two or more steel plates are superimposed one upon another and sandwiched between a pair of welding electrodes and energized while being pressed to form a nugget,
A main energizing step of energizing the steel sheet with a first current I ₁ (㎄) while pressurizing the steel sheet with a first pressing force F ₁ (kN)
There is provided a post-energizing step for energizing the energization time t _a (ms) and cooling the neck portion by the second current I ₂ (㎄) represented by the following formula (1), following the main energization step,
Wherein the energization step after the energizing step includes a first pressurizing step of maintaining the first pressurizing force F ₁ (kN) during the pressurization delay time t _b (ms) shown by the following expression (3) And a second pressing step of pressing the first pressing step with a second pressing force F ₂ (kN) shown by the following equation (4).
0 &lt; I ₂ &lt; I ₁ (1)
20? T _a? 400 (2)
10? T _b? T _a (3)
F ₁ &lt; F _{2 &lt; /} = 3F ₁ (4) The method according to claim 1,
Wherein at least one of said steel sheets is a high strength steel sheet having a carbon equivalent Ceq (%) of 0.2% or more and a tensile strength of 780 MPa or more as represented by the following formula (5).
Ceq = C + Si / 30 + Mn / 20 + 2P + 4S (5)
(The symbol of the element in the formula (5) represents the content (mass%) of each element) 3. The method according to claim 1 or 2,
The down-slope energization step in which the post-energizing step gradually reduces the current value from the first current I ₁ (㎄) for the down slope energization time t _c (ms) represented by the following expression (6)
And a subsequent energization step of maintaining the current value at the end of the down slope energization process for the latter energization time t _d (ms) represented by the following equation (7) following the downslope energization step.
20? T _c? T _a (6)
t _d = t _{a -} t _c (7) 4. The method according to any one of claims 1 to 3,
Wherein the second pressurizing step includes an upslope pressing step for gradually increasing the pressing force from the first pressing force F ₁ (kN) during the upslope pressing time t _e represented by the following expression (8)
And a subsequent pressing step of maintaining the pressing force at the end of the upslope pressing step during the latter pressing time t _f (ms) shown by the following equation (9) following the upslamp pressing step.
0 &lt; t _e & lt _; 200 (8)
0? _Tf (9)

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