JPH06336664A - Production of zinc-aluminum plated steel sheet excellent in spot weldability - Google Patents

Abstract

PURPOSE:To improve the spot weldability of a steel sheet hot dip plated with a Zn-Al alloy having a specified compsn. by spraying silicon-contg. water on the surface of the plated steel sheet held at a certain temp. and forming a silica coating film. CONSTITUTION:A steel sheet is dipped in a Zn-Al hot dip plating bath consisting of 3-7wt.% Al, <=0.5wt.% other metals and the balance Zn, and silicon-contg. water is sprayed on the surface of the plated steel sheet at 250 deg.C. The amt. of Si in the silicon-contg. water is regulated to 0.05-10% and the silicon-contg. water is prepd. by dissolving or dispersing silica powder, colloidal silica, a silane coupling agent or various silicates in water. Since the sprayed silicon- contg. water is freed of the water by drying and forms a tough silica coating film on the surface of the plating, the number of weldable spots by spot welding is increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a zinc-aluminum plated steel sheet having excellent spot weldability.

[0002]

2. Description of the Related Art Recently, the amount of surface-treated steel sheets used has greatly increased in order to cope with the prolongation of the service life of automobiles, home appliances, building materials and the like. Among these surface-treated steel sheets, especially Zn-5 wt% Al-based hot-dip galvanized steel sheet is
Compared with the conventional hot-dip galvanized steel sheet, it has been excellent in corrosion resistance and workability of the plated layer, and therefore, it has been mainly used for long-term rust-guaranteed building materials.

However, a problem with using this Zn-5 wt% Al hot-dip galvanized steel sheet is that the weldability is poor, and the melting point of the Zn-5 wt% Al hot-dip galvanized layer is 390 ° C. Since the melting point is lower than the melting point of the galvanized layer of 419 ° C., the galvanized layer is easily melted during spot welding, the contact area of the plate surface is increased, the current density is reduced, and the nugget diameter is decreased. Will lead to a decrease in

Due to such an adverse effect, the number of weldable spots which can obtain a more appropriate nugget diameter, that is, the so-called electrode life is extremely shortened, the frequency of welding tip replacement is increased in the welding process, and the productivity is increased. There is the disadvantage of decreasing
There is a strong demand for improvement.

As a method for improving the weldability of a galvannealed steel sheet, Japanese Patent Laid-Open No. 02-004
Japanese Patent Publication No. 983 describes that an oxide film containing ZnO as a main component is formed. Although this method is effective in improving weldability, an industrially stable ZnO film can be easily formed. It is very difficult to form and has not yet been put to practical use.

[0006]

DISCLOSURE OF THE INVENTION In order to solve the various problems of the prior art described above, the present invention has been earnestly studied by the present inventor, and as a result of repeated studies, the plating layer at the time of spot welding was found. Zinc with excellent spot weldability, which does not melt and does not reduce the current density and has good productivity.
We have developed a method for manufacturing aluminum-based plated steel sheets.

[0007]

The feature of the method for producing a zinc-aluminum-based plated steel sheet having excellent spot weldability according to the present invention is that the steel sheet is immersed in a zinc-aluminum-based hot dip bath and then the steel sheet is It is to spray a solution containing silicon at a temperature of 250 ° C. or higher to form a silica film on the plating surface.

The method for producing the zinc-aluminum plated steel sheet having excellent spot weldability according to the present invention will be described in detail.

In the method for producing a zinc-aluminum-based plated steel sheet excellent in spot weldability according to the present invention, the zinc-aluminum-based plated steel sheet contains Al in an amount of 3 wt% or more and 7% or more.
less than wt%, other metals less than 0.5 wt%, balance Zn
Steel plate manufactured by immersing the steel plate in a plating bath consisting of That is, after the steel sheet is immersed in the zinc-aluminum hot-dip galvanizing bath, the temperature of the plated steel sheet is 2
Spraying the silicon-containing aqueous solution at a temperature of 50 ° C. or higher means that the water of the silicon-containing aqueous solution is evaporated and removed at a temperature of 250 ° C. or higher, and a silica film is firmly formed on the plating layer surface. Because it is done.

As a method of spraying this silicon-containing aqueous solution,
A method using a general mist spray header is effective, and it is desirable that the apparatus can uniformly spray in the plate width direction as the spray direction.

The Si content of the silicon-containing aqueous solution is 0.05
10% is preferable, and if it is less than 0.05%, a dense silica film is not formed on the plating surface, so the number of weldable spots cannot be improved, and if it exceeds 10%, it gives a weldability. The effect is not improved, and on the contrary, there is a problem that the chemical conversion treatment is not performed smoothly.

The silicon used can be made into an aqueous solution by dissolving or dispersing silica powder, colloidal silica, a silane coupling agent and various silicates in water.

As described above, by forming a silica film on the surface of the plating layer, the number of weldable spots increases. Although the theoretical basis for this is not clear, Zn-Cu is formed on the plating surface and the tip of the welding tip. It seems that this is because the formation of the alloy layer is suppressed.

[0014]

[Example] Using an aluminum-killed steel plate having a thickness of 0.8 mm obtained by casting and processing after melting by a normal melting method, Zn-Al containing 4.5 wt% of Al
Plating was performed by immersing in an alloy plating bath for 2 seconds. Then, the coating adhesion amount at this time is 100 g / m ² . Then, an aqueous solution containing silica gel at room temperature was sprayed on the surface of the plating layer for 0.5 seconds to form a film.

Welding conditions Electrode shape: 16φ dome type Pressing force: 220 kgf Welding time: 12 cycles (60 Hz) Welding current: 11 kA

Under these welding conditions, two plated steel sheets were superposed on each other, and a continuous welding point test of spot welding was performed to measure the nugget diameter after shear tension. The value was 4.5.
The electrode life was defined as the number of dots when the diameter became mmφ.

The results are shown in Table 2. Judgment Criteria ◯: 2000 points or more Δ: 1000 to 2000 points ×: 1000 points or less

[0018]

[Table 1]

[0019]

As described above, since the method for producing a zinc-aluminum-based plated steel sheet excellent in spot weldability according to the present invention has the above-mentioned structure, the electrode maintenance or the electrode in the spot welding operation is performed. The replacement frequency is extremely low, and it has an excellent effect of improving productivity.

Claims (1)

[Claims] 1. A spot weldability characterized by forming a silica film on a plating surface by immersing a steel sheet in a zinc-aluminum hot-dip galvanizing bath and then spraying an aqueous solution containing silicon at a steel sheet temperature of 250 ° C. or higher. Of excellent zinc-aluminum plated steel sheet.