JPH02270546A - Surface treated steel sheet excellent in corrosion resistance, weldability and coating brightness of reflectivity - Google Patents

Abstract

PURPOSE:To obtain a steel sheet which is excellent in corrosion resistance and weldability and capable of securing coating brightness of reflectivity on the plated surface by specifying the surface roughness of the steel sheet in an organic film side, the surface roughness thereof in a plated surface side and waviness of the surface. CONSTITUTION:The surface roughness Ra of a steel sheet in an organic film side is regulated to roughness of 1.0 - 5.0 times of thickness of an organic film. The surface roughness of the steel sheet in a plated surface side is regulated to 0.4 - 1.0mum. The waviness of the surface is regulated to <=0.7mum in Wca indication. Thereby the surface-treated steel sheet is obtained which is excellent in corrosion resistance, weldability and coating brightness of reflectivity. The improving effect of weldability due to control of roughness of the steel sheet is shown from >=1.0 ratio of Ra of the steel sheet to film thickness of the organic film. This improved effect is remarkably shown as the ratio is made larger. When Ra is made large, the uniformly coating property of the organic film is deteriorated and therefore the upper limit of the ratio of Ra to the film thickness of the organic film is 5.0. Since Ra in the organic film surface side is made larger than thickness of the organic film, the holding property of press oil is enhanced and the lower limit value of Ra of the steel sheet in the plated surface side can be lowered to 0.4 from 0.6mum. Therefore the brightness of reflectivity after coating is enhanced because Wca can be made smaller.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention improves the weldability and paint clarity of thin organic film type surface-treated steel sheets that are used in the automobile and home appliance fields and have excellent press workability and corrosion resistance. It is something to improve.

(Prior art and its problems) In recent years, in the automobile and home appliance fields, there has been a strong demand for zinc or zinc-based alloy coated steel sheets to have high rust resistance and high image clarity. Among these, many steel sheets coated with a thin organic coating have been proposed for high rust prevention. Highly rust-proofed steel plate made by applying water-dispersed emulsion resin to zinc-plated or zinc-alloy plated steel sheet G
This patent publication No. 55-51032, Japanese Patent Application Publication No. 61-584
There is one mentioned in the publication.

In addition, in 1983, a product made by blending a water-dispersible emulsion resin with an organic composite silicate and coating it with a thin film was published.
Examples include those described in Japanese Patent Application Laid-Open No. 60-149786. Furthermore, the present inventors have also developed an organic coated steel sheet that does not elute harmful substances such as chromium during the surface treatment process at customers, can be baked at low temperatures, has excellent corrosion resistance, high workability, and has excellent electrodeposition coating properties. We proposed the steel plate described in the publication.

However, since these steel sheets do not contain conductive materials such as metal powder in the coating film, they have the disadvantage that the thicker the coating, the higher the current carrying resistance (which leads to a decrease in weldability.Especially on the organic coating surface. In the case of a steel sheet whose opposite surface is also plated with zinc or zinc-based alloy to improve corrosion resistance, weldability is significantly reduced.

On the other hand, it is known that the surface roughness of the steel plate has an effect on high image clarity.
In the publication, the surface waviness of the steel plate is expressed as Wca, which is 0.7a.
It is proposed that it be less than m. W for sharpness
Although it is desirable that ca be smaller, since Ra and Wca are in a proportional relationship, if Wca is made smaller, Ra will also be smaller, resulting in a problem of reduced pressability.

Therefore, there was a problem that Wca could not be made sufficiently small.

(Problems to be solved by the invention) The present invention solves the above problems, has excellent corrosion resistance and weldability,
Another object of the present invention is to provide a steel sheet that also ensures the sharpness of the coating on the plated surface opposite to the organic coating surface.

(Means for Solving the Problems) The gist of the present invention is that one side of a steel sheet has a chromate film on the upper layer of zinc plating, zinc alloy plating, or zinc-based composite alloy plating, and an organic film is applied thereon. In steel sheets that are coated with organic coating and the side opposite to the organic coating is galvanized, zinc-based alloy plating, or zinc-based composite alloy plating, the surface roughness Ra of the steel plate on the organic coating side is
The roughness of the steel plate on the plated side should be 0.4 to 1.0 ptn, and the surface waviness should be 0.7 μl or less in Wca. This is a surface-treated steel sheet with excellent corrosion resistance, weldability, and paint clarity.

In order to ensure corrosion resistance, the present inventors investigated various ways to improve weldability without reducing the organic coating thickness, adjusted the roughness of the steel plate on the organic coating side, and set the Ra to the organic coating thickness. It was discovered that the weldability was improved by making the steel sheet thicker (thicker), and that the pressability did not deteriorate even if the roughness of the steel plate on the opposite side to the organic coating surface was reduced, and this led to the proposal of the present invention. It is.

In addition, as the zinc plating, zinc-based alloy plating, and zinc-based composite alloy plating applied to the present invention, in the electroplating system, galvanized steel sheet, zinc-nickel alloy plated steel sheet,
Zinc-iron alloy plated steel sheet, or 5iOz, Tie, based on zinc-nickel or zinc-iron, A7,
0. , ZrO, BaCr0n, and other metal oxides are dispersed therein, such as zinc-based composite alloy plated steel sheets, etc.
Hot-dip coatings include galvanized steel sheets, alloyed galvanized steel sheets, zinc-aluminum alloy plated steel sheets, etc., and those obtained by known methods may be used.

(effect) The effect on the constituent factors of the present invention will be described below.

By adjusting the roughness of the steel plate on the organic coating side and making Ra larger than the thickness of the organic coating, when the electrode is pressed against the steel plate during welding, the organic coating deforms and flows into the recesses on the steel plate surface, causing the electrode and organic coating to deform. Contact occurs with the underlying plating. Therefore, the contact resistance value between the steel plate and the electrode is reduced, and weldability is improved.

The effect of improving weldability by controlling the roughness of the steel sheet becomes apparent when the ratio of Ra of the steel sheet to the thickness of the organic film is 1.0 or more, and the larger the ratio, the more remarkable the improvement effect becomes.

However, as Ra increases, the uniform coverage of the organic film decreases, so the upper limit of the ratio of Ra to the thickness of the organic film is 5.
It is 0. Preferably 1.5 to 3.0.

In the present invention, the composition and thickness of the organic film are not limited, but preferably the film thickness is 0.2 to 3I.
rm range is desirable, also! ! ! A steel plate having a chromate film to ensure adhesion between the film and the plating also falls within the scope of the present invention.

The image clarity after painting on the plated surface side can be improved by reducing the surface waviness of the steel plate surface to 0.7- or less in terms of filtered center line waviness Wca. However, if Wca is made smaller, Ra will also be made smaller, and as shown in JP-A-63-25
As shown in Japanese Patent No. 5320, mold galling occurs during pressing.In the present invention, since Ra on the organic coating side is made larger than the organic coating thickness, the retention of press oil is improved, and the plated surface is The lower limit of Ra of the side steel plate can be lowered from 0.6n to 0.4prrr, and therefore Wca
can be made smaller, improving image clarity after painting.

(Example) Predetermined zinc plating, zinc-based alloy plating, or zinc-based composite alloy plating shown in Table 1 was applied to aluminum killed steel sheets with a thickness of 0.8 m with various surface Ra and Wca changes using a known plating method. After that, one side was subjected to chromate treatment, and the upper layer was painted and baked with an organic film.

The weldability, sharpness after painting, and die galling resistance during pressing of the single-sided organic coated and single-sided plated steel sheets produced in this manner were investigated.

The results are summarized in Table 1.

All of the steel plates 1 to 15 of the present invention examples were spot welded to 3
The number of consecutive welding points was 000 or more.

Furthermore, there was no plate breakage during pressing, and the coating clarity showed a high N5IC value of 80 or higher.

(Effects of the Invention) The present invention provides a method for improving weldability by controlling the surface roughness of a steel plate, which can be easily performed industrially, and also improving pressability and paint clarity on the plated surface side. This is an invention with great industrial benefits.

Claims (1)

[Scope of Claims] One side of a steel sheet has a chromate film on the top layer of zinc plating, zinc alloy plating, or zinc composite alloy plating, and an organic film is coated thereon, and the opposite side of the organic film is For steel plates whose surface side is galvanized, zinc-based alloy plated, or zinc-based composite alloy plated, the surface roughness Ra of the organic coating side steel plate is 1.0 of the organic coating thickness.
~5.0 times the roughness, and the surface roughness of the steel plate on the plated side is 0.4 to 1.0 μm, and the surface waviness is 0.0 in Wca.
A surface-treated steel sheet with excellent corrosion resistance, weldability, and paint clarity, characterized by having a thickness of 7 μm or less.