JP2003236674A - Method and equipment of spot welding of high tensile steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of spot welding of high tensile steel plate and equipment for the method by which the generation of expulsion caused by a rapid increase in electric resistance due to a large hardness of a tensile steel plate and the poor fitness in the earlier stage of welding is suppressed, a sufficient nugget diameter is assured, the generation limit of the expulsion is expanded, an electric current path is surely secured, and the generation of crack is prevented by relaxing the decrease in the gradient of hardness of a heat-affected zone (HAZ) and a stress concentration due to the expansion of the HAZ. <P>SOLUTION: The spot welding is performed in a process which is provided with a first step S1 in which a nugget is generated by gradually increasing a supplied current to the high tensile steel plate, a second step S2 in which the supplied current is decreased after the first step S1, and a third step S3 in which a regular welding is performed by increasing the electric current after the second step S2 and the supplied current is gradually decreased. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for spot welding a high strength steel plate such as a roof reinforcement as a rigid member for a vehicle body or other high strength steel plate.

[0002]

2. Description of the Related Art Generally, spot welding is performed by superposing steel plates 71 and 72 as shown in FIG.
By sandwiching the electrodes 1, 72 with a pair of electrodes facing each other, and energizing in a pressurized state, Joule heat causes resistance heating to melt the steel plates 71, 72 between the sandwiched portions, thereby nugget (nugget) fusion. Part) 73 is formed and joined.

The above-mentioned spot welding conditions are steel plates 71, 7
The optimum current range and pressurizing force suitable for production are variously set depending on the material, plate thickness, number of superposed sheets, etc. The welding strength becomes higher as the energizing current increases so that the nugget 73 grows.
(Small droplets of droplets) easily occur.

The optimum current range is indicated by the range of the minimum current for ensuring the diameter of the nugget 73 (nugget diameter) for ensuring the bonding strength at each pressing force and the maximum current at which dust does not occur, and is generally stable. It is desirable that the proper current range is 1 KA or more in order to ensure the above welding quality.

This proper current range varies depending on the shape of the electrode, and normally, the CF having a flat tip as shown in FIG.
Although a type electrode 74 and an R type electrode 75 having a spherical tip as shown in FIG. 9 are used, it is necessary to secure a stable electrode contact area in order to suppress the resistance in the initial stage of energization. The R-type electrode 75 (see FIG. 9), which is less affected by disturbances such as the gap between the steel plates 71 and 72 and the deviation of the electrode striking angle, is more advantageous.

On the other hand, the energization method includes a one-step energization method in which energization is performed only once (see FIG. 10) and a two-step energization method in which preliminary energization is performed before main energization as shown in FIG.
-104849), that is, when the gap between the steel plates 71 and 72 as the work is large, there is a two-step energization method in which the work contact surfaces are made familiar to each other by pre-energization and then the main contact is performed.

By the way, a high-tensile steel sheet having a tensile strength of 500 MPa or more contains a large amount of alloying elements as compared with a mild steel sheet and has a high hardness (the hardness of a general mild steel sheet is Hv9
The hardness of the high-strength steel plate is Hv150, while it is about 0.
It is difficult to get used to, so it is a normal one-step energization method.
In (see FIG. 10), a sufficient proper current range cannot be obtained, and the HAZ portion 76 (abbreviation of heat affected zone) is a weld metal portion whose heat treatment is affected by heat and changes in the metal structure and characteristics. There is a problem in that the hardness of the part) becomes high and the fatigue strength decreases.

In order to verify the problem of this one-step energization method, spot welding was performed using a nitrided steel plate (test piece) as a high-strength steel plate. As a result, according to the CF type electrode 74 shown in FIG. Although the proper current range described above could be secured, the proper current range was 1 KA or less in the R-type electrode 75 shown in FIG.

Here, the CF type electrode 74 (see FIG. 8) is used for 1 KA.
The above proper current range was obtained because the test piece of the nitrided steel plate was used for welding with no gap between the plates,
It is more familiar than the R-type electrode 75 (see FIG. 9) and it is possible to suppress dust generation early, but in actual production, plate gaps occur due to the shape of equipment and parts, and It is considered that the appropriate current range is deteriorated as compared with the case where the R-type electrode 75 is used, because the hitting angle of the electrode changes.

That is, in the actual production, as described above, the R-type electrode 75 which is not easily affected by the disturbance (see FIG. 9).
It is desirable to use the R type electrode, and it is necessary to secure an appropriate current range with the R type electrode 75.

A HAZ portion 76 (see FIG. 7) was obtained as a result of performing a tensile shearing test on a nitrided steel plate spot-welded with an R-shaped electrode 75 in the above-described one-step energization method.
A crack occurred at the boundary between the base metal and the base metal, and the strength decreased.

The reason why the above-mentioned appropriate current range is difficult to obtain is that the hardness of the steel sheet is increased to about Hv300 by nitriding and that Hv80 is generated in the surface layer of about 15 μm.
With a layer of nitrogen compound having a high hardness of about 0, the familiarity in the initial stage of welding was poor and the current-carrying resistance rapidly increased, and the fact that nitriding caused a large amount of solid solution of nitrogen and carbon in the steel sheet to lower the melting point. This is because both of them cause the dust to easily occur.

The reason why cracking occurs at the boundary between the HAZ part 76 and the base metal is that nitriding causes a large amount of nitrogen and carbon to form a solid solution in the steel sheet, lowering the transformation point and increasing hardenability. , The heat of welding increases the hardness of the HAZ part 76,
This is because the altitude of the part is steeply sloped and the strain is concentrated.

Therefore, in order to suppress the above-mentioned generation of dust, it is effective to increase the pressing force. However, in a high-tensile steel plate such as a nitrided steel plate, its hardness is high, so that an excessive pressing force is required. It will be difficult to perform spot welding using existing equipment.

In order to solve such a problem, FIG.
The two-step energization method shown in Fig. 12 and the two-step energization method shown in Fig. 12 have been invented.

As described above, in the two-step energization method shown in FIG. 11, the work contact surfaces are made to conform to each other by the first-step energization (preliminary energization), and welding is performed by the second-step energization (main energization). This is a method for obtaining an appropriate current range (Japanese Patent Laid-Open No. 11-104849).
(See Japanese Patent Publication).

According to this conventional method, although there is an advantage that an appropriate current range can be obtained, the hardness gradient of the HAZ portion 76 cannot be improved, and a crack occurs at the boundary between the HAZ portion 76 and the base material. There was a point.

On the other hand, in the two-step energization method shown in FIG. 12, a spot welding current is applied to the work piece for welding in the first energization,
Tempering current is temporarily applied to the work by the second-stage energization to perform the tempering process, and then about 35K.
The work is cold-pressurized by applying an excessive N pressure (see Japanese Patent Laid-Open No. 2001-170776).

According to the conventional method shown in FIG. 12, HA
Although the hardness gradient of the Z portion 76 is improved, there is a problem that it is difficult to obtain an appropriate current range. In addition, it is possible to reduce tensile residual stress and apply compressive residual stress by applying additional pressure (see cold pressure treatment) using the electrode after welding, but the welding time is approximately 300 cycles (approximately 5 seconds) and long,
In particular, the processing time until the end of additional pressurization is as long as 530 to 540 cycles (however, 1 cycle is 1/60 seconds), and
Since it is necessary to add pressure equipment separately, there is a problem that spot welding cannot be performed using existing equipment.

[0020]

SUMMARY OF THE INVENTION According to the present invention, a nugget is produced by gradually increasing the current passed through a high-strength steel sheet, and then the current is once dropped and then the current is raised. By performing main welding and gradually lowering the energizing current, the hardness of the high-strength steel plate is high and the conformability at the beginning of welding is poor, so the energizing resistance rises sharply and dust occurs. It is possible to secure a sufficient nugget diameter, expand the limit of dust generation, and ensure a current-carrying path, and to prevent the heat-affected zone (H
The purpose of the present invention is to provide a spot welding method for a high-strength steel plate and a device therefor capable of preventing the occurrence of cracks by relaxing the hardness gradient of the AZ part) and relaxing the stress concentration due to the expansion of the HAZ part range. And

[0021]

A spot welding method for a high-strength steel sheet according to the present invention is a spot-welding method for a high-strength steel sheet for spot-welding a high-strength steel sheet, wherein a current applied to the high-strength steel sheet is gradually changed. First step of generating nugget by increasing the temperature, a second step of decreasing the current after the first step, and a step of increasing the current after the second step to perform the main welding and to gradually change the energizing current. Spot welding is performed by a process including a third step of lowering.

The high-strength steel sheet having the above-mentioned structure may be set to a gas soft-nitrided or salt-bath-nitrided (tufftrided) nitrided steel sheet (tensile strength of about 1000 MPa) or a hardened steel sheet. According to the above configuration, in the first step, the energizing current to the high-tensile steel plate is gradually increased to generate the nugget, and then the second step is performed.
In the step, the energizing current is once decreased, then in the third step, the energizing current is increased to perform the main welding, and the energizing current is gradually decreased after the main welding.

By the treatment of the above-mentioned first step, the hardness of the high-tensile steel plate is high (when the nitrided steel plate is used as the high-tensile steel plate, the Vickers hardness of the surface layer thereof is as high as Hv800), and the welding is performed. Due to the poor initial compatibility, the energization resistance is prevented from rapidly increasing and dust is suppressed, ensuring a sufficient nugget diameter and expanding the dust generation limit, and ensuring the energization path. It is possible to secure and obtain an appropriate current range.

Further, by the process of the above-mentioned second step, by lowering the current, the temperature of the material (high-tensile steel plate) can be once suppressed, and the generation of dust can be further suppressed. Further, due to the effect of the above-mentioned third step treatment, in particular, tempering for gradually decreasing the energizing current, the concentration of stress is relieved from the gentle slope of the hardness gradient of the HAZ portion and the expansion of the HAZ portion range. It is possible to suppress the occurrence of cracks.

In one embodiment of the present invention, the high-strength steel sheet has a tensile strength set to a value higher than 500 MPa. According to the above configuration, even under the disadvantageous conditions that the tensile strength ≧ 500 MPa, the hardness of the material to be welded, so-called work, is high, and the hardenability is good,
In particular, the process of the first step makes it possible to secure the energization path while securing the conformability at the initial stage of welding.

In one embodiment of the present invention, a predetermined time lag is provided between the end of the second step and the start of the third step. According to the above configuration,
The current is once decreased after the nugget (or nugget nucleus) is generated by the first-stage energization, and a predetermined time lag is provided between the time of the current drop and the start of the main energization. If current is continuously applied when the nugget temperature due to the first-stage current application is high, the material tends to melt due to the low melting point of the material and dust occurs, but the temperature is suppressed by the above-mentioned predetermined time lag. Therefore, it is possible to prevent the occurrence of dust even better.

In one embodiment of the present invention, the object to be welded is set to a surface-hardened steel plate in which at least the surface to be welded is hardened. According to the above configuration, since the surface-hardened steel plate is used as the object to be welded, the conformability of the surface to be welded deteriorates, but even in such a case,
In particular, the process of the first step makes it possible to reliably obtain the energization path while ensuring the conformability at the initial stage of welding.

In one embodiment of the present invention, the surface-hardened steel plate is a nitrided steel plate and / or a hardened steel plate. According to the above configuration, since the nitrided steel plate or / and the hardened steel plate whose surface layer has a hardness of about Hv800 is used as the object to be welded, its surface hardness is particularly high, and the familiarity of the surface to be welded is extremely poor. Even in such a case, the energization path can be surely ensured while ensuring the conformability at the initial stage of welding by the process of the first step.

In one embodiment of the present invention, the surface-hardened steel sheet is formed into a predetermined shape before being hardened, subjected to a hardening treatment after the molding, and then spot-welded at the hardening portion. .

According to the above construction, since the surface hardened steel sheet is formed into a predetermined shape before the hardening treatment, the formability can be ensured, and thereafter, the hardening treatment and the spot welding by the above method are performed. The required physical properties can be obtained.

In one embodiment of the present invention, the energizing current during the main welding in the third step is set higher than the energizing current during the nugget formation in the first step. According to the above configuration, the energization current during the nugget generation in the first step (the energization current during preliminary energization) can be set to a relatively low current at which dust does not occur.
The occurrence of dust can be reliably suppressed, and since the main welding is performed with a high-value energizing current (main energizing) with respect to the current in the first step, this main welding can be ensured.

In one embodiment of the present invention, the period from the start of the current increase in the first step to the end of the current decrease in the third step is executed within 125 cycles. According to the above configuration, the time required for welding is 1
Since it is within 25 cycles (however, 1 cycle is 1/60 seconds), preferably within 50 cycles, it is possible to secure a welding time with high productivity.

The spot welding apparatus according to the present invention is a spot welding apparatus for spot welding a high-strength steel plate,
A pair of electrodes arranged to face each other, a pressurizing means for pressurizing the electrode, and a control means for controlling the energization current to the electrode, the control means gradually increasing the energization current, high tension steel plate After generating the nugget in the
After that, the current is increased to perform the main welding, and the energizing current is controlled to be gradually decreased.

According to the above construction, first, the energizing current is gradually increased to generate the nugget on the high-tensile steel plate as the object to be welded, so that the high-tensile steel plate has high hardness and poor conformability at the initial stage of welding. Due to this, the energization resistance suddenly increased,
The generation of dust can be suppressed, a sufficient nugget diameter can be secured, the limit of dust generation can be expanded, and an energization path can be reliably secured to obtain a sufficient proper current range.

Further, by temporarily lowering the current after the nugget is formed, the temperature of the material (workpiece as the member to be welded) is once suppressed (temperature rise is suppressed), and the generation of dust can be further suppressed.

Further, the current is increased to perform the main welding, and the energizing current is gradually decreased to obtain the effect of tempering. Therefore, the hardness of the HAZ part is made gentle and the HAZ part range is expanded. Therefore, it is possible to reduce the concentration of stress, that is, strain, to suppress the occurrence of cracks, and to perform spot welding with existing equipment.

In one embodiment of the present invention, the energizing current during main welding (current during preliminary energization) is set higher than the energizing current during nugget formation (current during main energization). According to the above configuration, the energization current during nugget generation can be set to a relatively low current at which dust does not occur, so it is possible to reliably suppress the occurrence of dust, and the energization current during nugget generation. Since the main welding is performed with a high value of energizing current, the main welding can be ensured.

[0038]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. The drawing shows a spot welding method and apparatus for high-tensile steel plates having a tensile strength of ≧ 500 MPa. First, the structure of a high-tensile-rigidity spot welding apparatus will be described with reference to FIG.

As shown in FIG. 1, this spot welding apparatus includes a welding gun 1 and a control unit 2. The welding gun 1 described above is moved by operating a cylinder 3 as a pressurizing device.
(Up and down) and pressurized upper electrode 4 (one movable electrode)
And a lower electrode 6 (the other fixed electrode) fixed to the arm 5 so as to face the upper electrode 4 in the vertical direction, a welding transformer 7, and a robot connecting portion 8.

Here, it is desirable to use R-type electrodes (see FIG. 9) as the pair of upper and lower electrodes 4 and 6 described above. On the other hand, the control unit 2 includes an input operation unit 9 and a C as a control unit.
It has a PU 10, a ROM 11, and a RAM 12.

The CPU 10 drives and controls the cylinder 3 and the welding transformer 7 in accordance with a program stored in the ROM 11 based on a signal from the input operation section 9, and the RAM 12 (storage means) has an energization pattern (see FIG. 3). And necessary data such as the value of the energizing current and the number of energizing cycles are stored.

The above-mentioned CPU 10 is a control means for controlling the energizing current to the electrodes 4 and 6 via the welding transformer 7, and this CPU 10 gradually increases the energizing current to the high tensile steel plate during welding (see FIG. 3). (See line a), and then a relatively low current (see line b in FIG. 3) that does not cause dust is generated to generate a nugget (see FIG. 7) in the high-strength steel plate, and then the energizing current is temporarily changed. The current is lowered to zero (see line c in FIG. 3), and after a predetermined time lag d from this current downstream time, the energization current is increased again (see line e in FIG. 3) to generate the nugget current (FIG. 3). The main welding current (see line f in FIG. 3) is higher than that in the main welding, and the current gradually decreases to zero after the main welding (line g in FIG. 3). Control) to control tempering. It is structured to control.

FIG. 2 is a process diagram showing a spot welding method in the case of spot welding a high-strength steel plate having a tensile strength of ≧ 500 MPa using the above spot welding apparatus, which is a pair in the first process (upslope process U1). Between two electrodes 4 and 6, for example, a so-called work piece to be welded is sandwiched between a mild steel sheet containing a small amount of alloying elements and a nitrided steel sheet containing a large amount of alloying elements (an example of a surface-hardened steel sheet having at least the surface to be welded hardened). ,
Under the condition that a predetermined pressurizing force (several KN level) that can be applied by the existing equipment is urged by the cylinder 3, the energizing current to the work is gradually increased in 5 to 30 cycles (line a in FIG. 3). reference).

Next, in the second step (energization step) U2, a relatively low current is applied to the work for 5 to 30 cycles to generate a nugget (see FIG. 7) (see line b in FIG. 3). ).

After the first step S1 consisting of the above-mentioned first step U1 and second step U2, the energizing current is once decreased to zero in the third step (cool down step) U3.
(See line C in Figure 3).

Second step S in the third step U3
From the end of 2 to the next third step S3, a predetermined cycle in the fourth step U4 (time lag step)
It waits for the elapse of (specifically, several cycles) (see the predetermined time lag d in FIG. 3).

After the elapse of the predetermined time lag d described above, FIG.
In the fifth step U5, the CPU 10 increases the energizing current through the welding transformer 7 and performs the main welding for 5 to 30 cycles at an energizing current higher than the current at the time of producing the nugget (FIG. 3). Line f)).

After the completion of the main welding in the fifth step U5, in the sixth step (downslope step) of FIG.
10 gradually changes the energizing current to zero over 10 to 30 cycles to give the work a tempering effect (see line g in FIG. 3). In addition, the fifth step U and the sixth step described above.
The third step S3 is formed by the process U6. In this case, the period from the current rise start time t ₀ in the first step S1 to the current fall end time t _n in the third step S3 can be set within 125 cycles, preferably within 50 cycles, and the spot can be set within this short time. Welding can be performed. Further, time points t _{0 to} t _n shown in FIG.
Until then, a predetermined pressing force is applied to the work.

Further, as the above-mentioned work, before the hardening treatment by nitriding or quenching, the work is molded into a predetermined shape by a press machine or the like in advance, and after this molding, the hardening treatment by nitriding or quenching is carried out, and then the hardening treated portion Spot welding is performed by the method shown in the process diagram of FIG.

The result of the welding test performed on the product of this example spot-welded by the above method is shown by a solid line h in FIG. Here, the characteristic diagram of FIG. 4 is a characteristic diagram showing the change in hardness, with the horizontal axis representing the distance from the center of the nugget and the vertical axis representing the hardness (Vickers hardness). In this case, the nitride steel sheet having a thickness of 0.8mm was gas soft or salt bath nitriding as nitriding treatment (Tufftride), plate thickness and the mild steel plate of 0.7mm and two-ply, diameter 5.0 mm ^phi R type electrodes 4, 6
Was used for spot welding.

In the product of this example, the width of the hardness inclined portion from the HAZ portion to the base material is about 1.0 mm, the hardness gradient can be made gentle and the range of the HAZ portion can be expanded. As a result, the concentration of strain can be suppressed and the occurrence of cracks can be prevented.

The suitable current range can be expanded to 1.0 KA for salt bath nitriding (tuftride) as the nitriding treatment, and the appropriate current range can be 2.0 for gas soft nitriding.
I was able to expand to KA.

In order to make a comparison with the above-mentioned embodiment,
Under the same combination condition of steel sheets, the results of the welding test of spot welding by the conventional one-step energization method (see FIG. 10) are shown by solid lines i and j in FIGS. 5 and 6, respectively.

The comparative example shown by the solid line i is a CF type electrode (see FIG. 8), which is a current of 8.8 KA applied for 15 cycles under a pressure of 3,43 KN. The comparative example shown by the solid line j and by the dotted line j in FIG. 4 uses an R-type electrode (see FIG. 9), and a current of 6.2 KA was applied for 15 cycles under a pressurizing condition of a pressing force of 2.94 KN. It is a thing. The characteristic indicated by the dotted line k in FIG. 5 is the test result when spot welding was performed using both mild steel plates without using any high-tensile steel plates.

The conventional method shown by solid lines i and j in FIGS. 5 and 6 has a low proper current range of 0.4 KA when a gas nitrocarburized steel sheet is used as the nitriding steel sheet, and a salt bath nitriding sheet is used as the nitriding steel sheet. The proper current range is 0.2KA when using the steel plate
And the result was even lower. Further, the hardness distribution from the HAZ portion to the base material has a steep gradient, and the width thereof is about 0.
It was as small as 5 mm.

In the characteristics of the solid lines i and j obtained by the conventional method described above, the reason why the proper current range is narrow is that the nitrided steel sheet is hard and a very hard compound layer is formed on the surface thereof, so that the familiarity at the initial stage of welding is good. It deteriorates, the energization path becomes small, and this causes rapid heat generation, and since the amount of solid solution of nitrogen and carbon is large, the melting point of the nitrided steel sheet lowers, it melts early and dust occurs. This is because the current at the dust generation limit becomes low.

Further, as a result of measuring the shear tensile strength of the comparative example products shown by solid lines i and j in FIGS. 5 and 6, cracks occurred at the boundary between the HAZ part and the base metal with a high probability, and the strength was lowered. did. This is because a large amount of nitrogen and carbon are in solid solution, the transformation point is lowered, the hardenability is enhanced, the hardness of the HAZ part is increased, and the width of the hardness inclined part from the HAZ part to the base metal is increased. Is as narrow as about 0.5 mm, the hardness becomes steep, strain tends to concentrate, and cracking occurs.

In this example product (see solid line h in FIG. 4) spot-welded to each of the comparative example products (see solid lines i and j) by the method of the above-described embodiment, the HAZ portion is used as the base material. The width of the hardness sloped part to about 1.0 mm can be expanded, the hardness gradient can be made gentle, and the HAZ part range can be expanded, thus suppressing the concentration of strain, It was possible to prevent the occurrence of cracks, and the appropriate current range could be expanded to 1.0 KA or 2.0 KA.

As described above, the spot-welding method for a high-tensile steel sheet of the above-mentioned embodiment is a spot-welding method for a high-tensile steel sheet for spot-welding a high-tensile steel sheet, and the energizing current to the high-tensile steel sheet is gradually changed. The first step S1 of generating a nugget by raising (see line a)
Second step of decreasing current (see line c) after step S1
Current increase after step S2 and second step S2
(Refer to line e) to carry out main welding (see line f) and gradually decrease the energizing current (see line g) Third
Spot welding is performed by the process including step S3.

According to this configuration, in the first step S1,
The nugget is generated by gradually increasing the energizing current of the high-tensile steel plate, the energizing current is once decreased in the next second step S2, and then the energizing current is increased in the third step S3 to perform the main welding. In addition, the energizing current is gradually decreased after the main welding.

By the above-described first step S1, it is possible to prevent the energization resistance from rapidly increasing and the occurrence of dust due to the high hardness of the high-tensile steel plate and the poor compatibility in the initial stage of welding. It is possible to secure a sufficient nugget diameter and increase the limit of dust generation, and to reliably secure a current-carrying path to obtain an appropriate current range.

Further, by the above-mentioned processing of the second step S2, by lowering the current, the temperature of the material (high-tensile steel plate) can be once suppressed, and the generation of dust can be further suppressed. Furthermore, due to the effect of the above-described third step S3, that is, tempering, which gradually decreases the energizing current (see line g), the HAZ part has a gentle slope, and
It is possible to suppress the concentration of stress due to the expansion of the Z portion range and suppress the occurrence of cracks.

In addition, the tensile strength of the high-tensile steel plate is set to a value higher than 500 MPa, including 500 MPa. According to this structure, even under the disadvantageous conditions that the tensile strength ≧ 500 MPa and the material to be welded, that is, the hardness of the work is high and the hardenability is good, the familiarity at the initial stage of welding is ensured by the process of the first step S1. At the same time, it is possible to secure an energization path.

From the end of the second step S2 to the third step.
A predetermined time lag d (see FIG. 3) is provided before the start of step S3. According to this configuration, the current is once decreased (see line c) after the nugget is generated by the first-stage energization (see line b), and the predetermined time lag d is provided from the current drop time to the start of the main energization. is there. If current is continuously applied when the nugget temperature due to the first-stage current is high, the material is easily melted due to the low melting point of the material and dust occurs, but the temperature is suppressed by the predetermined time lag d. As a result, the generation of dust can be prevented even better.

Further, the object to be welded is set to a surface-hardened steel plate in which at least the surface to be welded is hardened. According to this configuration, since the surface-hardened steel plate described above is used as the object to be welded, the conformability of the surface to be welded deteriorates. Even in such a case, the first step S1
By this process, it is possible to reliably obtain the energization path while ensuring the conformability at the initial stage of welding.

In addition, the surface hardened steel sheet is set to a nitrided steel sheet and / or a hardened steel sheet. According to this configuration, since the nitrided steel plate and / or the hardened steel plate is used as the object to be welded, its surface hardness is particularly high and the conformability of the surface to be welded is poor. By the process of step S1, it is possible to reliably secure the energization path while ensuring the conformability at the initial stage of welding.

The surface-hardened steel sheet is formed into a predetermined shape before being hardened, subjected to a hardening treatment after the molding, and then spot-welded at the hardening portion.

According to this structure, since the surface-hardened steel sheet is formed into a predetermined shape before the hardening treatment, the formability can be ensured, and thereafter the hardening treatment and the spot welding by the above method are performed. The required physical properties can be obtained.

Further, the energizing current during the main welding in the third step S3 is set to be higher than the energizing current during the nugget formation in the first step S1. According to this configuration, the energizing current when the nugget is generated in the first step S1 is
Since the current can be set to a relatively low level at which dust does not occur, it is possible to ensure that the generation of dust is suppressed, and the main welding is performed with a high value of the energizing current with respect to the current in the first step S1. Since this is performed, this main welding can be ensured.

[0070] Moreover, the current starts rising in the first step S1 to the current fall ended in the third step S3, that is until the time _t 0 ~t _n shown in FIG. 3 125
It is executed within a cycle. According to this configuration, the time required for welding is 125 cycles (however, 1 cycle is 1/60 seconds), preferably within 50 cycles, so that it is possible to secure a welding time with high productivity.

On the other hand, the spot welding apparatus of the above embodiment is
A spot welding apparatus for spot-welding a high-strength steel plate, comprising a pair of electrodes 4, 6 facing each other, the electrode 4,
Pressurizing means (see cylinder 3) for pressurizing 6 and control means (CPU 1 for controlling energizing current to the electrodes 4, 6)
0)), the control means (see CPU 10) gradually increases the energizing current (see line a) to generate a nugget on the high-strength steel sheet (see line b), and then lowers the current (see line c). ), And then the current is increased (see line e) to perform the main welding (see line f), and the energizing current is gradually decreased (see line g).

According to this structure, first, the energizing current is gradually increased to generate the nugget in the high-tensile steel plate as the object to be welded. Therefore, the high-tensile steel plate has high hardness and poor conformability in the initial stage of welding. Due to this, the energization resistance suddenly increased,
The generation of dust can be suppressed, a sufficient nugget diameter can be secured, the limit of dust generation can be expanded, and an energization path can be reliably secured to obtain a sufficient proper current range.

Further, by temporarily lowering the current after the nugget is formed, the temperature of the material can be temporarily suppressed and the generation of dust can be further suppressed.

Further, the current is increased to perform the main welding, and the energizing current is gradually decreased to obtain the effect of tempering. Therefore, the hardness gradient of the HAZ part is made gentle and the range of the HAZ part is expanded. Therefore, it is possible to reduce the concentration of stress, that is, strain, to suppress the occurrence of cracks, and to perform spot welding with existing equipment.

Further, the energizing current during the main welding is set higher than the energizing current during the nugget formation. According to this configuration, the energizing current during nugget generation can be set to a relatively low current at which dust does not occur, so that the occurrence of dust can be reliably suppressed, and the energizing current during nugget generation can be suppressed. Since the main welding is performed with a high value of energizing current, the main welding can be ensured.

In the correspondence between the constitution of the present invention and the above-mentioned embodiment, the high-tensile steel plate of the present invention corresponds to the nitrided steel plate or the hardened steel plate of the tensile strength ≧ 500 MPa of the embodiment, and the electrode is added in the same manner. The pressurizing means for pressing corresponds to the cylinder 3 and the control means corresponds to the CPU 10. However, the present invention is not limited to the configuration of the above-described embodiment.

Although the two-step energization pattern is used in the above embodiment, it may be a multi-step energization pattern.

[0078]

According to the present invention, the nugget is formed by gradually increasing the current passing through the high-strength steel sheet, then decreasing the current once and then increasing the current flowing through the main welding. In addition, the energization current is gradually decreased, so that the hardness of the high-strength steel plate is high and the conformability at the initial stage of welding is poor, so that the energization resistance rises sharply and suppresses the occurrence of dust. And secure a sufficient nugget diameter,
In addition to increasing the dust occurrence limit, it is possible to reliably secure the current-carrying path, and also to make the hardness slope of the heat-affected zone (HAZ part) gentle and to concentrate stress or strain by expanding the HAZ part range. There is an effect that it can be relaxed and cracking can be prevented.

[Brief description of drawings]

FIG. 1 is a system diagram showing a spot welding apparatus for a high-strength steel sheet according to the present invention.

FIG. 2 is a process diagram showing a spot welding method for a high-strength steel sheet according to the present invention.

FIG. 3 is an explanatory view showing an energization pattern by the method of FIG.

FIG. 4 is a characteristic diagram showing a change in hardness with respect to the distance from the center of the nugget.

FIG. 5 is a characteristic diagram showing a change in hardness according to a conventional one-step energization method.

FIG. 6 is a characteristic diagram showing a change in hardness according to a conventional one-step energization method.

FIG. 7 is an explanatory diagram of a nugget and a HAZ part.

FIG. 8 is an explanatory diagram of a CF type electrode.

FIG. 9 is an explanatory diagram of an R-type electrode.

FIG. 10 is an explanatory diagram showing an energization pattern of a conventional one-step energization method.

FIG. 11 is an explanatory diagram showing an energization pattern of a conventional two-step energization method.

FIG. 12 is an explanatory view showing a conventional energization and pressurization pattern of an additional pressurization method.

[Explanation of symbols]

S1 ... the first step S2 ... the second step S3 ... Third step d ... predetermined time lag 3 ... Cylinder (pressurizing means) 4, 6 ... Electrodes 10 ... CPU (control means)

Claims (10)

[Claims] 1. A spot welding method for a high-strength steel sheet for spot-welding a high-strength steel sheet, comprising: a first step of gradually increasing an energizing current to the high-strength steel sheet to produce a nugget; The spot welding is performed by a process including a second step of decreasing the current after the first step and a third step of increasing the current and performing the main welding after the second step and gradually decreasing the energizing current. Spot welding method for tensile steel sheets. 2. The high-strength steel plate has a tensile strength of 500M.
The spot welding method for a high-strength steel sheet according to claim 1, wherein the spot welding method is set to a value higher than that including Pa. 3. The spot-welding method for high-tensile steel sheet according to claim 1, wherein a predetermined time lag is provided between the end of the second step and the start of the third step. 4. The spot welding method for a high-strength steel plate according to claim 1, wherein the object to be welded is set to a surface-hardened steel plate in which at least the surface to be welded is hardened. 5. The spot welding method for a high-strength steel sheet according to claim 4, wherein the surface-hardened steel sheet is a nitrided steel sheet and / or a hardened steel sheet. 6. The surface-hardened steel sheet is formed into a predetermined shape before being hardened, and is then subjected to a hardening treatment, and thereafter,
The spot welding method for a high-strength steel sheet according to claim 5, wherein the hardening treatment site is spot-welded. 7. The spot welding method for a high-strength steel sheet according to claim 1, wherein the energizing current during the main welding in the third step is set higher than the energizing current during the nugget formation in the first step. 8. The spot-welding method for a high-strength steel sheet according to claim 1, wherein the process from the start of the current increase in the first step to the end of the current decrease in the third step is executed within 125 cycles. 9. A spot welding apparatus for spot-welding a high-strength steel sheet, comprising: a pair of electrodes arranged to face each other; a pressurizing means for pressurizing the electrodes; and a control means for controlling a current supplied to the electrodes. The control means gradually increases the energizing current to generate a nugget in the high-tensile steel plate, and then lowers the current, and then increases the current to perform main welding, and controls to gradually lower the energizing current. Spot welding equipment. 10. The spot welding apparatus according to claim 9, wherein the energizing current during the main welding is set higher than the energizing current during the nugget formation.