JPH04118177A - Resistance spot welding method for composite coated steel plates - Google Patents

Abstract

PURPOSE:To improve the service life of an electrode by using the electrode which is worked in a tapered shape on an electrode base part between the projecting type curved-surface-shaped electrode tip and an electrode main body. CONSTITUTION:The composite coated steel plates having, by weight, 0.2-10.0g/m<2> coating layer with organic resin as a main body on galvanized steel sheets are subjected to resistance spot welding. The tip 2 of the electrode used for resistance spot welding is then made in the projecting type curved surface shape. The electrode base part 4 between this electrode tip 2 and the electrode main body 1a is worked in the tapered shape. Consequently, the enlargement rate of the electrode tip diameter in accordance with increase of the number of spots in resistance spot welding of the various composite coated steel plates can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for resistance spot welding of composite coated steel sheets with excellent corrosion resistance used for automobiles, home appliances, building materials, etc.

<Prior Art> As surface-treated steel sheets with excellent corrosion resistance, various composite coated steel sheets having a coating layer mainly composed of an organic resin on a zinc or zinc-based alloy plated steel sheet have been developed. However, in resistance spot welding of these steel plates, the diffusion of zinc into the tip of the electrode used for -M and the reduction in electrical conductivity due to the presence of the resin coating layer, which is originally an insulator, make it difficult to perform continuous welding. The disadvantage is that the electrode life is shortened.

In order to suppress such deterioration of weldability, examples in which the alloy components of the plating layer and the components of the coating layer are limited (Japanese Patent Application Laid-Open No. 1996-2991)
0798 Publication) and examples in which the surface roughness is limited (see JP-A-2-145774), but in both cases, deterioration of weldability due to a decrease in electrical conductivity due to the presence of a resin coating layer can be avoided. The deterioration is particularly noticeable when a dome-shaped electrode with a rounded spherical electrode tip as shown in FIG. 4 is used. However, when this dome-shaped electrode is used for resistance spot welding of Zn-Ni plated steel sheets that are not coated with resin, the above-mentioned disadvantages do not occur.

In addition, there are examples of improving electrode life by combining the materials of the electrode itself (such as Japanese Patent Application Laid-Open No. 1-258875), but with composite electrodes made by combining special materials, the cost of the electrode itself is high. There is an economic problem as it becomes too expensive.

<Problems to be Solved by the Invention> The present invention has been made in order to provide a method for improving the electrode life in resistance spot welding of zinc-based composite coated steel sheets having such a resin coating without using any special materials. It is something that

In other words, the electrode life during continuous dot welding of composite coated steel sheets by resistance spot welding deteriorates particularly when a dome-shaped electrode with a rounded spherical electrode tip is used. The problem was that it was difficult to obtain a sufficient electrode life just by improving the composite coating. Therefore, Fig. 4 (al
When a CF type electrode as shown in Fig. 1 cannot be used, it is necessary to ensure weldability at the expense of properties such as corrosion resistance. The present invention provides a method for improving electrode life in welding composite coated steel sheets in such cases.

<Means for Solving the Problems> The present invention provides: (i) a coating layer mainly composed of an organic resin on a zinc or zinc-based alloy plated steel sheet having a weight of 0.2 to 10.0 g;
In resistance spot welding of a composite coated JFil plate, an electrode is used in which the tip of the electrode has a convex curved surface and the base of the electrode between this electrode tip and the electrode body is processed into a tapered shape. A resistance spot welding method for composite coated steel plates, in which the tip of a cylindrical electrode material 1 is processed into a hemispherical shape with a radius of 1/2 of its diameter, and the electrode is determined by the thickness of the material to be welded. Resistance spot welding of the composite coated plate as described in the previous item (2) using an electrode processed into a tapered shape so that the ridgeline 5 between the tip 2 and the electrode base 4 is located inside the ridgeline 6 between the hemispherical tip 1b It's a method.

For Kusaku Next, the present invention will be explained in detail.

The life of the electrode in continuous dot welding for resistance spot welding of zinc or zinc-based alloy coated steel sheets is limited by the diffusion of zinc into the electrode tip, which accelerates the wear of the electrode tip, which promotes the expansion of the tip diameter and increases the current density. It is said that the number of hits decreases rapidly until no nuggets are formed.

In the case of a composite coated steel sheet, since it has a coating layer mainly composed of resin, the electrical conductivity between the electrode and the steel sheet decreases and damage to the electrode is accelerated, so the life of the electrode is even shorter.

The present inventors conducted a detailed study on the electrode life in continuous resistance spot welding of such composite coated steel sheets, and found that the electrode life greatly depends on the shape of the electrode base between the curved surface of the electrode tip and the electrode body. The present invention is based on this finding that the influence of the freely adjustable shape of the tip is rather small.

In other words, when using a welding machine such as an X-type welding gun of a spot welding machine, in which the movement of the electrode does not coincide with the center line of the electrode, a dome-shaped electrode with a rounded spherical electrode tip is used to prevent the electrode from touching unevenly. However, in the case of such an electrode, the expansion rate of the tip diameter per unit wear depth of the electrode tip is large, resulting in a short electrode life. Therefore, if the shape of the electrode base is made conical, for example, it is possible to reduce the rate of expansion of the tip diameter and improve the life of the electrode. In this case, since the shape of the electrode tip itself does not affect the expansion rate of the tip diameter, the convex curved surface portion ensures good contact with the electrode when using the above-mentioned X-shaped welding gun or the like. An example of the shape of the electrode for achieving such an effect is shown in FIG.

Next, the method of processing into the shape of the electrode according to the present invention will be explained based on FIG. 1.

First, the tip of the cylindrical electrode material 1 is processed into a hemispherical tip 1b with a radius R that is 1/2 of its diameter 2R, as shown in FIG. 1(a). Next, as shown in FIG. 2(B), the ridgeline of the electrode base 4 between the spherical electrode tip 2 and the cylindrical electrode main body 1a, the size of which is empirically determined depending on the thickness of the material to be welded, etc. 5 is processed into a tapered shape so that it falls within the ridgeline 6 of the spherical tip as shown in FIG. 1(C).

As a shape other than the tapered shape, for example, the shapes shown in Fig. 2 (a) and 3) can be considered, but the tapered shape shown in Fig. 1 (C) or Fig. 3 is most suitable for ease of processing. I think that the.

<Example> Examples of the present invention will be described below.

Various composite steel plates are produced using the resins shown in Table 1, and two of these composite M plates are stacked to form the electrode shown in FIG. 1 (C) according to the present invention and the conventionally used dome shown in FIG. An electrode life test was conducted using continuous point welding using a type electrode.
Table 2 summarizes the details and test results of the manufactured composite steel plate.

The welding conditions are Pressure crab 200kgf, Current: 79kA.

Current application time: 10 cycles (50 Hz), dot speed: 2 seconds/5 pots, and the electrode life was evaluated as the number of dots that could ensure a nugget diameter of 4T.

When the method according to the present invention is applied, the life of any electrode having a convex curved surface portion can be significantly improved.

<Effects of the Invention> As is clear from the results in Table 2, by applying the method of the present invention, the expansion rate of the electrode tip diameter due to the increase in the number of welding points in resistance spot welding of various composite coated steel sheets can be reduced. By reducing this, electrode life is improved.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an electrode shape according to the present invention and an example of its manufacturing process, FIG. 2 is a sectional view of another example of the electrode shape according to the present invention, and FIG. 3 is a diagram showing a conventional electrode shape. Cross section, 4th
The figure is a cross-sectional view of a conventional electrode shape. 1... Cylindrical electrode material, la... Electrode body,
1b...Semispherical tip, 2...Electrode tip,
4...1 is the scratching base, 5...the ridgeline of the electrode base, 6...the ridgeline of the spherical tip.

Claims (1)

[Claims] 1. In resistance spot welding of a composite coated steel plate in which a coating layer mainly composed of an organic resin is coated with a weight of 0.2 to 10.0 g/m^2 on a zinc or zinc-based alloy coated steel plate, A method for resistance spot welding of composite coated steel sheets, characterized by using an electrode having a convex curved electrode tip and a tapered electrode base between the electrode tip and the electrode body. 2. Cut the tip of the cylindrical electrode material (1) into 1/2 of its diameter.
The ridge line (5) between the electrode tip (2) and the electrode base (4) is determined by the thickness of the material to be welded.
2. The method of resistance spot welding a composite coated plate according to claim 1, wherein an electrode is used which is tapered so that the electrode is located inside the ridge line (6) with the hemispherical tip (1b).