JPH04371371A - Resistance spot welding method - Google Patents

Abstract

PURPOSE:To extend the service life of electrodes in mixed spot welding. CONSTITUTION:When zinc plated steel sheets or cold rolled steel sheets which are not plated are subjected to resistance spot welding alternately, collision energy when the above-mentioned steel sheets are pressurized by the upper and lower electrodes is regulated to <=0.4 joule.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resistance spot welding method with excellent electrode life in mixed spot welding of zinc-plated steel sheets and cold-rolled steel sheets used as rust-proof steel sheets for automobiles.

0002

BACKGROUND OF THE INVENTION In recent years, galvanized steel sheets such as electrogalvanized and alloyed galvanized steel sheets, which have excellent rust prevention properties, have been widely used as automobile materials. These steel plates, along with conventional unplated cold-rolled steel plates, are used according to the required performance of each part, and are assembled into a car body mainly by resistance spot welding. Resistance spot welding involves sandwiching the welded joint of two overlapping steel plates between copper alloy rod-shaped electrodes and applying a large current for a short period of time while applying strong pressure, leaving a go stone-shaped nugget (a trace of solidified molten steel) on the contact surface. It is a welding method to form. Therefore, the nugget portion is not visible from the outside of the stacked steel plates, and only a slight depression pressed by the electrode is visible on the surface.

0003

[Problem to be Solved by the Invention] By the way, electrode life is not much of a problem when spot welding zinc-coated steel sheets is performed continuously, but in actual production lines, electrode life is not a problem when welding zinc-coated steel sheets together. Mixed dot welding is performed in which welding of unplated cold-rolled steel sheets is inserted in the middle and alternately performed, and in the case of this mixed dot welding, the life of the electrode is extremely short. That is, the nugget formed during welding of zinc-based plated steel sheets gradually becomes ring-shaped with no adhesion in the center, and eventually no nugget is formed at all, making welding impossible. For this reason, electrodes must be replaced or cleaned (dressed) frequently.
The current situation is that this is causing a significant decline in production efficiency. Furthermore, the influence of the impact energy of the electrode on the mixed spot weldability when pressing the steel plate with the electrode is not generally considered.

The present invention focuses on this collision energy and aims to extend the life of the electrode in mixed spot welding by controlling it.

[0005]

[Means for Solving the Problems] The present invention has a plate thickness of 0.6
When carrying out resistance spot welding alternately between zinc-plated steel plates of 0.8mm or 0.8mm or between unplated cold-rolled steel plates, the collision energy when the steel plates are pressurized by upper and lower electrodes is set to 0.4. It is characterized by being less than joules.

[0006]

[Function] As a result of intensive study on the phenomena that occur in the electrode and weld zone during mixed continuous dot welding, the inventors found that during welding between zinc-based plated steel sheets, zinc in the plating layer diffuses into the electrode. This is caused by the fact that the temperature during welding reaches a high temperature of 600 to 800 °C, which causes the tip of the electrode to soften, which promotes wear due to flattening and deformation of the tip of the electrode when welding cold-rolled steel plates together. The cause-and-effect relationship became clear: this caused the diameter of the electrode tip to expand, and eventually no nuggets were formed when welding galvanized steel sheets.

Furthermore, one of the causes of the deterioration of electrode life is that when two steel plates are stacked and pressurized, the electrode collides with the steel plate at a considerable speed, causing deformation of the tip of the electrode. elucidated. That is, when mixed spot welding is performed by changing only the collision speed of the electrode against the steel plate while keeping other conditions constant, flattening and wear of the electrode tip are accelerated as the collision speed increases. Also, as a matter of course, the weight of the moving part of the electrode has a large effect on the collision between the electrode and the steel plate, so considering both the collision speed and the weight of the moving part of the electrode, it depends on the collision energy of the electrode. According to experiments, the life of the electrode can be significantly improved if the collision energy is set to 0.4 joules or less.

Means for reducing the collision energy to 0.4 joules or less include reducing the weight of the electrode movable part and lowering the collision speed between the electrode and the steel plate, and methods for controlling the collision speed include cylinder sliding. There are various factors such as dynamics (friction), cylinder capacity, flow resistance of pressurized air into the cylinder, and spacing between upper and lower electrodes before welding, and by adjusting any of these factors, collision energy can be reduced to 0. It is possible to set it to 4 joules or less.

[0009] The thickness of the steel plate in the present invention is limited to 0.6 to 0.8 mm since it is intended for ordinary galvanized steel plates and cold rolled steel plates for automobiles, and the corresponding collision energy regulation is applied. Although the value was set to 0.4 Joule or less, it goes without saying that the basic idea of regulating the collision energy with respect to the electrode life is not limited to the above plate thickness.

0010

[Examples] Table 1 shows the experimental results of Examples of the present invention and Comparative Examples. The experimental conditions are as follows. Electrode shape: Cone-shaped (CF) type, tip diameter:
5.0mm, material: chrome copper welding conditions pressure: 190 kgf, energization method: 5
10 cycles per second with 0 Hz power supply Continuous dot speed: 1 dot/2 seconds Welding current: Continuous dot welding The nugget diameter at the 1st dot is
A current value of 5 mm is set for each of the zinc-based plated steel sheet and the cold-rolled steel sheet.

[0011] Mixing cycle: 25 points of zinc-based plated steel plate →
25 repetitions of cold rolled steel plate. Electrode life The number of dots when the nugget type falls below 4.5 √t (t is the plate thickness). Test materials: Zinc-based galvanized steel sheet: 0.7 mm thick, alloyed galvanized steel sheet with a basis weight of 45/45 g/m2 Cold rolled steel sheet: 0.7 mm thick steel sheet

0012

[Table 1]

[0013] The zinc-based plating referred to in the present invention refers to a single layer or a stack of two or more layers of zinc plating, zinc-nickel alloy plating, zinc-iron alloy plating, or other zinc alloy plating. The plating method is not limited, and may be electroplating, hot-dip plating, alloyed hot-dip plating, vapor deposition plating, or the like.

[0014]

According to the present invention, when resistance spot welding is performed alternately between galvanized steel sheets and cold-rolled steel sheets, the impact energy when pressurizing the steel sheets with the upper and lower electrodes can be reduced to 0.4.
By setting it to joule or less, the life of the electrode during mixed continuous spot welding can be significantly extended. As a result, the frequency of electrode replacement or maintenance at the manufacturing site is significantly reduced compared to the conventional method, improving productivity.

Claims (1)

[Claims] Claim 1: When carrying out resistance spot welding alternately between galvanized steel plates with a thickness of 0.6 to 0.8 mm or between unplated cold-rolled steel plates, the steel plates are welded with upper and lower electrodes. A resistance spot welding method characterized in that the collision energy during pressurization is 0.4 Joule or less.