JP7305396B2 - Spot welding method for galvanized steel sheets - Google Patents

Description

The present invention relates to a spot welding method for joining a plurality of steel sheets including at least galvanized steel sheets.

In recent years, steel sheets used for automobile structural members are required to have high strength and workability comparable to low-strength steel sheets in order to achieve both weight reduction of the vehicle body and improvement of collision safety. In addition, zinc-based steel sheets, which have a zinc-based coating layer formed on the steel sheet surface, are widely used as structural members for automobiles because of their excellent corrosion resistance, and various high-strength zinc-based steel sheets have been developed. .

Automobile structural members have complex shapes and must satisfy various required properties. are applied.

Spot welding is mainly used to join steel plates together when assembling structural members of automobiles. In this spot welding, a plate assembly including a plurality of steel plates is sandwiched between a pair of electrodes and energized while being pressurized to melt and solidify the joint interface of the steel plates to form a nugget and join the steel plates together. The energization at this time is performed within a welding current range in which no flash occurs, but it is known that cracks occur in the weld even in such a welding current range.

This kind of weld cracking occurs because the zinc-based coating layer on the surface of the steel sheet is melted by the Joule heat during welding to become molten zinc, and tensile stress due to thermal contraction of the steel sheet is applied to the welded area. is attributed to so-called liquid metal embrittlement (LME), in which the grain boundary is invaded to reduce the grain boundary strength.

Such cracks caused by liquid metal embrittlement (hereinafter sometimes referred to as "LME cracks") occur when at least one of a plurality of steel sheets to be welded is a galvanized steel sheet.

As a technique for preventing the occurrence of LME cracking, for example, Patent Documents 1 and 2 include:
"The spot welding conditions are adjusted so that the nugget formed by spot welding has a nugget diameter d0 or more defined by the following formula (1) and a molten residual thickness of 0.05 mm or more. A spot welding method for tensile galvanized steel sheets.

d0=k√t (1)
where d0: desired nugget diameter (mm)
k: coefficient; a coefficient selected between 3 and 6 according to the construction conditions t: steel plate thickness (mm)
is proposed.

On the other hand, in Patent Document 3, as a countermeasure against contamination and abrasion of the electrode tip when aluminized steel sheets or an aluminized steel sheet and another metal plate are superimposed and spot-welded, "a constant As a post-process of the main welding process in which electricity is supplied in an alternating current cycle, a downslope process is added to gradually reduce the amount of electricity to gradually reduce the heat conduction from the welded part to the electrode tip, and the cooling speed of the welded part is increased. Gentle, Spot Welding Method for Aluminized Steel Sheets." has been proposed.

Japanese Patent No. 3849539 Japanese Patent No. 3849525 JP 2006-212649 A

In Patent Documents 1 and 2, if the penetration of the nugget in the plate thickness direction is relatively small and the molten residual thickness defined by the shortest distance from the steel plate surface to the molten metal is increased, that is, if a flat nugget is formed , is a technique developed under the idea that the occurrence of LME cracks in welds can be suppressed.

However, it is difficult to sufficiently relax the tensile stress generated in the cooling process after the end of the main energization only by controlling the residual thickness of the melt. Therefore, when a plurality of high-strength galvanized steel sheets are superimposed and spot-welded, there is a problem that the occurrence of LME cracks in the welded portion cannot be sufficiently suppressed.

On the other hand, in Patent Document 3, when spot welding is performed with at least an aluminum plated steel sheet as a welding target, contamination and wear of the electrode tip are more likely to occur than when spot welding is performed on a zinc-based plated steel sheet. By adding it, even if the electrode tip is contaminated and worn, it is a technology that gives a sufficient current density to the welding spot and forms a nugget of a necessary and sufficient size.

However, in the technique of Patent Document 3, no study has been made as to what kind of effect the downslope process exerts on LME cracks that are likely to occur when zinc-based plated steel sheets are spot-welded.

The present invention has been made in view of the circumstances as described above, and its object is to provide a spot welding method capable of suppressing the occurrence of LME cracks in welds when joining a plurality of steel sheets including zinc-based plated steel sheets. to do.

One aspect of the present invention is
In joining a plate assembly including two or more steel plates, at least one of which is a zinc-based plated steel plate, by spot welding,
including a downslope step of stepwise decreasing the current value while maintaining the pressurization after main energization while pressurizing the plate assembly, and a time t (ms) from the start to the end of the downslope step. is in the range of 16≦t≦4000.

In the spot welding method of this embodiment, at least one of the two or more steel plates is preferably a steel plate having a tensile strength of 980 MPa or more.

As a preferred embodiment of the present invention, the step of stopping the energization and holding the pressurization after the down slope step may be performed for 16 ms or more.

ADVANTAGE OF THE INVENTION According to this invention, when spot-welding two or more steel sheets including a galvanized steel sheet, generation|occurrence|production of LME crack in a weld zone can be suppressed. Such a spot welding method is useful as a joining method for structural members that require high strength, such as automobile parts, structural materials and reinforcing materials for automobiles.

The present inventors have investigated from various angles the conditions for suppressing the occurrence of LME cracks in the weld zone when spot welding two or more steel sheets including a galvanized steel sheet. Then, spot welding is performed including a downslope process in which a plate assembly including two or more steel plates to be joined is energized while being pressurized, and then the current value is lowered step by step while the pressurization is maintained. For example, it has been found that a weld zone in which occurrence of LME cracking is suppressed can be obtained.

LME cracking in the weld zone occurs in the cooling process after the end of the main energization, in the presence of molten zinc in which the zinc-based coating layer on the steel sheet surface has melted, tensile stress due to thermal contraction of the steel sheet is applied to the weld zone, and the melting occurs. It is considered that zinc penetrates grain boundaries and lowers the grain boundary strength. The tensile stress is generated when the pressurization disappears after the main energization is completed. In addition, the tensile stress is not uniformly applied inside the welded portion, and it is considered that the position and degree of occurrence of LME cracks change due to the locally high tensile stress and the presence of molten zinc.

The inventors of the present invention expected that the tensile stress caused by thermal contraction of the steel sheet can be alleviated to some extent by continuing to apply compressive stress to the weld zone even after the end of the main energization. However, it has been found that the occurrence of LME cracks cannot be sufficiently suppressed only by maintaining the pressure.

Therefore, if spot welding is performed including a down-slope process in which the current value is gradually lowered while maintaining the pressure after main energization is performed while pressurizing the sheet assembly, the welded portion suppresses the occurrence of LME cracks. was obtained, and the conditions were further investigated.

As a result, the present inventors have found that a weld zone in which the occurrence of LME cracking is suppressed can be obtained by setting the time from the start to the end of the downslope process within a predetermined range, and completed the present invention.

That is, the spot welding method of the present embodiment, which has been able to achieve the above object, is a method for joining a set of plates including two or more steel plates, at least one of which is a zinc-plated steel plate, by spot welding. After main energization while pressurizing, a downslope step in which the current value is gradually lowered while maintaining the pressurization, and the time t (ms) from the start to the end of the downslope step is set to 16 ≤ The range of t≦4000 is characterized.

First, each requirement defined in the spot welding method of this embodiment will be described.

[A plate assembly containing two or more steel sheets, at least one of which is a galvanized steel sheet]
In the spot welding method of the present embodiment, at least one plate set to be welded is a zinc-based plated steel plate. When the plate assembly does not include a zinc-based plated steel sheet, no molten metal is generated on the surface of the steel sheet during spot welding, so LME cracking does not occur. Therefore, in order to exhibit the effect of suppressing the occurrence of LME cracks by the spot welding method of the present embodiment, at least one of the steel sheets included in the set of sheets is a galvanized steel sheet.

The type of the zinc-based plated steel sheet is not limited in any way. Also includes As for the type of zinc-based plating, various types of zinc-based plating such as Zn--Mg--Al-based plating can be used. In the spot welding method of the present embodiment, the number of plates is usually about 2-3. However, the number of board sets is not limited to this, and four or more board sets may be used as necessary.

[Perform spot welding including the down slope process]
In the spot welding method of the present embodiment, it is important to carry out spot welding including a down slope step after applying the main current while pressurizing the plate assembly. By adding a down-slope process in which the current is stepped down after the main energization, the cooling rate of the welded part is reduced and the deformation resistance of the welded part can be reduced. Tensile stress relaxation upon application is obtained. It should be noted that although the pressurization at the time of main energization is usually performed by electrodes, other pressurizing means may be employed.

[Time t (ms; milliseconds) from the start to the end of the downslope process: 16 ≤ t ≤ 4000]
In order to suppress the occurrence of LME cracking, it is important to relax the tensile stress generated in the cooling process after the end of the main energization. time t”) must be set within an appropriate range.
If the downslope time t is less than 16 ms, the cooling rate of the weld zone cannot be reduced sufficiently, and the effect of suppressing the occurrence of LME cracks cannot be exhibited.

Therefore, the downslope time t must be 16 ms or longer. The lower limit of the downslope time t is preferably 166 ms or longer, more preferably 250 ms or longer, and even more preferably 500 ms or longer.

However, if the downslope time t becomes too long, the amount of heat input increases and the temperature of the surface of the steel sheet becomes extremely high, so LME cracking is more likely to occur. Therefore, the downslope time t must be 4000 ms or less. The upper limit of the downslope time t is preferably 2000 ms or less, more preferably 1000 ms or less.

A preheating energization before the main energization and a post-heating energization after the main energization may be performed if necessary, or a tempering energization may be performed after the downslope process to temper the welded portion. The conditions for the preheating energization, the main energization, and the post-heating energization are not limited as long as the conditions for the downslope process are satisfied, and normal energization conditions may be used.

By performing spot welding while satisfying each of the above requirements, the effect of suppressing the occurrence of LME cracks in the weld zone is exhibited.

The reason why the spot welding method of the present embodiment achieves the above effects has not been clarified completely, but it can be considered as follows. That is, by adding a down-slope process in which the current is stepped down after the main energization, the cooling rate of the welded portion is reduced, and the deformation resistance of the welded portion can be reduced. It is presumed that the effect of suppressing the occurrence of LME cracks is maximized by the relaxation of tensile stress due to the application of stress. In addition, since the weld zone is held at a high temperature for a longer period of time, the reduction in the absolute value of the tensile stress due to recovery or the like is also presumed to act favorably on the effect of suppressing the occurrence of LME cracks.

In the spot welding method of this embodiment, it is preferable to satisfy the following requirements as necessary.

[At least one sheet is a steel plate having a tensile strength of 980 MPa or more]
The tensile strength of the steel plates included in the set of plates is not particularly limited, but it is preferable that at least one steel plate has a tensile strength of 980 MPa or more. As described above, structural members of automobiles are required to have various properties. By including at least one steel plate with a tensile strength of 980 MPa or more, it greatly contributes to weight reduction of the vehicle body and improvement of crash safety. can.

[Electricity is stopped after the downslope process, and the process of maintaining pressurization is performed]
In the spot welding method of the present embodiment, a step of maintaining pressure may be added after stopping the energization in the main energization and the down slope process. By including such a pressurization and holding step, the tensile stress relaxation effect is further exhibited, and the welded structure has a more excellent property that suppresses the occurrence of LME cracking (this property is called “LME cracking resistance”). can get things.

In the case of adding the pressurization and holding step as described above, the pressurization holding time (hereinafter sometimes referred to as "hold time") is 16 ms or more in order to exert the effect of relaxing the tensile stress. is preferably 80 ms or more. However, if the hold time is too long, the time required for spot-welding a large number of welding points will increase, and the productivity of the member will decrease. More preferably, it is 500 ms or less.

[Other requirements]
The chemical composition of the steel plates included in the set of plates is not limited at all. For example, it may be a steel plate that satisfies the chemical composition of ordinary high-strength steel or mild steel. The base steel sheet of the steel sheet or galvanized steel sheet may be either a hot-rolled steel sheet or a cold-rolled steel sheet. Also, the steel sheet production conditions (heat treatment conditions, annealing pattern, plating layer formation conditions, etc.) are not limited at all, and the usual conditions may be followed.

Although pressurization is also performed during main energization, there is no need to supply welding current at the same time as pressurization. good.

In the spot welding method of the present embodiment, the pressure method (servo method, air method), current method (direct current, alternating current), etc. of the spot welder are not limited at all. As for the frequency of the current, either 50 Hz or 60 Hz may be applied, or a high frequency of 100 Hz may be used. Each of the above conditions does not affect the effects of the present invention.

Any spot welder that can control the current value and the applied pressure can be applied to the spot welding method of the present embodiment.

It is preferable that the current value for main energization is a current value to the extent that dust does not occur. The generation of dust is also affected by the applied pressure, and the higher the applied pressure, the less likely dust is generated. . Also, the energization pattern during the main energization is not limited at all, and either single-stage energization or multi-stage energization of two or more stages can be adopted.

The current value at the end of the downslope process is not limited at all as long as it is within a range in which LME cracking resistance due to the downslope process can be obtained. Also, the slope of the current value (current reduction rate) during the downslope process is not limited as long as it is within a range in which LME cracking resistance due to the downslope process can be obtained.

The applied pressure is preferably at least 250 kgf (2450 N) or more from the viewpoint of ensuring a current range in which a sound nugget diameter can be obtained. More preferably, it is 300 kgf (2940 N) or more. However, if the pressure is too high, the wear of the electrodes will increase and the productivity will decrease.

The striking angle of the electrode (the angle formed by the perpendicular to the surface of the steel sheet and the center line of the electrode) is preferably 5° or less. The striking angle of the electrode should be as small as possible in order to prevent LME cracking. Also, in order to ensure the strength of the welded portion, it is preferable that the striking angle of the electrode is small in order to form an appropriate nugget diameter. From this point of view, the striking angle of the electrode is preferably 5° or less.

As described above, in one aspect of the present invention, when spot welding a plate assembly including two or more steel plates, at least one of which is a zinc-based plated steel plate, the main current is applied while pressing the plate assembly. Later, a downslope step is included in which the current value is lowered stepwise while the pressure is maintained, and the time t (ms) from the start to the end of the downslope step is set in the range of 16 ≤ t ≤ 4000. It is a spot welding method.

By adopting such a configuration, it is possible to realize a spot welding method capable of suppressing the occurrence of LME cracks in welded portions when welding a plurality of steel sheets including zinc-based plated steel sheets.

In the spot welding method of the present embodiment, at least one of the two or more steel plates to be welded is preferably a steel plate having a tensile strength of 980 MPa or more.

By including at least one steel plate having a tensile strength of 980 MPa or more in the object to be welded, it is possible to greatly contribute to weight reduction of the vehicle body and improvement of collision safety.

Further, in the spot welding method of the present embodiment, the step of holding the pressurization may be performed for 16 ms or more after stopping the energization in the main energization and the down slope step.

By including such a pressurizing and holding step, the effect of relaxing the tensile stress is further exhibited, and a welded structure having more excellent resistance to LME cracking can be obtained.

Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to the following examples, and it is also possible to implement it by adding changes within the scope that can conform to the gist of the above and below. All of them are included in the technical scope of the present invention.

Various steel plate symbols a to d for the strength class (steel type), plate thickness, and plating type shown in Table 1 were used. These steel plates were spot-welded under the welding conditions shown in Table 2 below to produce welded structures. The "current value" shown in Table 2 indicates an effective value in the case of alternating current. The steel plates 1 to 3 were stacked in this order, the portions to be joined of the steel plates were stacked, and pressure was applied with an electrode to conduct electricity.

At this time, a servo-type stationary AC spot welding machine was used, and the electrode was a cap tip type (DR type) electrode made of a Cu—Cr alloy with a tip diameter of 8 mm. Two-step energization was adopted for the energization pattern during the main energization, and the current value of the latter stage was set to be higher than the current value of the former stage. The pressure applied from the start to the end of spot welding was kept constant at 350 kgf (3430 N), and the striking angle of the electrode was kept constant at 5°. Spot welding was carried out at room temperature (25° C.), and the electrodes were cooled by constantly flowing water into the electrodes during the spot welding.

After cutting the welded part of the obtained welded structure in the direction parallel to the striking angle and in the thickness direction of the steel plate, the cross section is mirror-polished and etched, and the presence or absence of LME cracks and nuggets are checked by a projector (magnification: 10 times). observed the diameter. LME cracking was evaluated according to the following criteria.

(Evaluation criteria)
○: No LME cracks ×: LME cracks present

These results are shown in Table 2 below together with the applicable steel types of Steel Plates 1 to 3 (steel plate symbols a to d in Table 1) and welding conditions (down slope time t, hold time). Note that Table 2 shows both values expressed in units of [cyc/60 Hz] (cyc: cycle) and values converted into [ms] for the down slope time t and the hold time.

From these results, it can be considered as follows. Test no. 1 to 4, 7, 9 to 12, 15, and 16 are invention examples that satisfy the requirements specified in this embodiment, no LME cracking occurs, and excellent LME cracking resistance. . Regarding the nugget diameter, when t _min is the plate pressure of the thinnest steel plate among the plates above and below the nugget measurement position, both were 4√t _min (see Patent Documents 1 and 2).

On the other hand, in Comparative Examples that do not satisfy the requirements defined in this embodiment, the LME cracking resistance is deteriorated. Specifically, Test No. In Nos. 5, 6, 8, 13, 14 and 17, no downslope process was added and the cooling rate of the welded portion was high, so the deformation resistance of the welded portion increased and compressive stress was applied to the welded portion due to pressurization. The effect of suppressing the occurrence of LME cracking cannot be maximized by the provision of. In addition, since the time during which the welded portion is held at a high temperature is shortened, the tensile stress mitigation effect due to recovery or the like is not exerted, and the LME cracking resistance is deteriorated. Also, the nugget diameter was 4√t _min in all cases.

Claims (3)

In joining a plate assembly including two or more steel plates, at least one of which is a zinc-based plated steel plate, by spot welding,
including a downslope step of stepwise decreasing the current value while maintaining the pressurization after main energization while pressurizing the plate assembly, and a time t (ms) from the start to the end of the downslope step. is in the range of 250 ≤ t ≤ 4000. The spot welding method according to claim 1, wherein at least one of the two or more steel plates is a steel plate having a tensile strength of 980 MPa or more. The spot welding method according to claim 1 or 2, wherein the step of stopping the energization and holding the pressurization after the down slope step is performed for 16 ms or longer.