Method of producing organic composite-plated steel sheet

The present invention provides a method for producing a highly corrosion resistant organic composite-plated steel sheet which is inhibited from liberation of chromium, which comprises forming a chromate film comprising a composition of the following (A) as a first layer at a coating amount of 5-150 mg/m.sup.2 as a solid film on the surface of a zinc or zinc alloy plated or zinc composite alloy plated steel sheet and then, coating on the first layer an organic solvent coating composition of the following (B) as a second layer at a thickness of 0.3-5 .mu.m as a solid film. PA0 (A) coating type chromate bath composition PA1 1 Cr.sup.6+ : 1-30 g/l, PA1 2 Cr.sup.3+ : 1-30 g/l, PA1 3 Cr.sup.6+ /Cr.sup.3+ weight ratio: 0.2-2.0, PA1 4 HF+H.sub.3 PO.sub.4 : 1-60 g/l, PA1 5 HF+H.sub.3 PO.sub.4 /Cr.sup.3+ weight ratio: 0.5-3.5, and PA1 6 silane coupling agent/Cr.sup.6+ molar ratio: 0.05-0.3 PA0 (B) organic solvent coating composition PA1 (a) a bisphenol epoxy resin having a number-average molecular weight of 300-100,000 in an amount of 30% by weight or more based on the solid content of the coating composition; PA1 (b) at least one curing agent selected from the group consisting of polyisocyanate compounds and blocked polyisocyanate compounds at 1/10-20/10 in a weight ratio to solid content of the epoxy resin; PA1 (c) fumed silica of 0.1-100 m.mu. in an average particle size in an amount of 5-50% by weight based on the solid content of the coating composition; PA1 (d) a ketone type organic solvent in an amount of 40% by weight or more based on the weight of the coating composition; and the solid content of the coating composition being in the range of 10-50% by weight.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an organic composite-plated steel sheet 
provided with a thin film coating thereon, more particularly to a method 
for producing the organic composite-plated steel sheet prevented from 
liberation of chromium and high in corrosion resistance. 
2. Description of Related Art 
Recently, reduction of cost and enhancement of the rust resistance of zinc- 
or zinc alloy-plated steel sheets (hereinafter referred to as "zinc 
alloy-plated steel sheets") has been highly desired by the fields of 
automobiles and appliances, and researches for the development of new 
products have been increasingly conducted. 
In the production of the organic composite-plated steel sheets by applying 
a thin film coating on the above-mentioned zinc plated steel sheet which 
has been chromated, the chromating has been required to be carried out by 
coating. 
In order to meet this requirement, it is necessary to develop a hardly 
soluble chromate coating type treatment technique which inhibits chromium 
from liberation from the coated chromate film. 
A proposal to make the chromate film hardly soluble has been reported in 
JP-A-50-158535. 
This patent application discloses a chromating solution of a chromic 
anhydride--phosphoric acid--water-soluble or water-dispersible polymer 
compound type, in which at least 70% of hexavalent chromium ions have been 
reduced with a reducing agent such as ethylene glycol. However, the 
chromate film produced by this technique contains polymers and so, is 
inferior in weldability though superior in hard-solubility, corrosion 
resistance and coatability. 
The chromate solution disclosed in JP-B-61-58552 is of a chromic 
acid--chromic acid reduction product--silica sol type. However, there is 
the problem that when the surface-treated steel sheet having the chromate 
film formed thereon is worked and coated, mainly hexavalent chromium ions 
in the chromate film are readily liberated by washing with water before 
the coating, resulting in reduction of the corrosion resistance of the 
film. 
Furthermore, use of silane coupling agents for reduction of hexavalent 
chromium ions in the chromating solution is disclosed in JP-A-58-22383 and 
62-83478. Films formed by the processes mentioned therein are all 
excellent in adhesion, but the chromate films formed by the process 
mentioned in the former patent application are inferior in the alkali 
resistance, because they are formed with chromating solution containing no 
phosphoric acid and the chromate films formed by the process mentioned in 
the latter patent application are also inferior in the alkali resistance. 
Properties of respective components in the above-mentioned conventional 
chromating processes were examined to find that organic polymers and 
silica have tendencies to enhance the corrosion resistance but deteriorate 
the alkali resistance, and silane coupling agents have a tendency to 
reduce hexavalent chromium ions in the chromating solution to cause 
transient decrease in the hexavalent chromium ion concentration of the 
chromating solution and to make the corrosion resistance of the chromate 
film unstable. 
As mentioned above, various processes have been disclosed, but in all of 
them, the properties of the chromate films depend greatly upon drying 
conditions in the chromating process 
Furthermore, it has been proposed in JP-B-55-51032 and JP-A-59-162278 and 
61-584 to increase the rust resistance by coating an aqueous solution of a 
mixture of a water-dispersible emulsion resin with chromic acid or a 
chromate compound of a high rust resistance on the surface of zinc-plated 
or zinc alloy-plated steel sheets. However, all of the resulting films are 
high in rust resistance, but sweating or liberation of chromium with an 
aqueous treating solution are seen. Thus, there are some practical 
problems. Moreover, JP-A-60-50181 and 60-149786 propose to carry out the 
thin film coating of a blend of a water-dispersible emulsion resin and an 
organic composite silicate (silica sol, a silane coupling agent or the 
like). These processes do not offer the problems of the liberation of 
chromium, but they have problems in the properties of the coated film, 
especially water resistance, because the film contains silica in the form 
of stable colloid and so alkali ions or ammonium ions. Furthermore, 
solvent type coating compositions containing fine silica is very high in 
the viscosity owing to their structural viscosity, and coating of a 
uniform thin film is impossible. It can be considered to rupture the 
structural viscosity by the hydrogen bond of alcohols to reduce the 
viscosity, but this cannot be applied to systems using a polyisocyanate 
compound as a curing agent. 
As mentioned above, the thin film coated rustprevented steel sheets require 
1 higher corrosion resistance, 2 further improvement in adhesion to a 
coating (especially, cationically electrodeposited film) formed on the 
steel sheets, and 3 reduction of baking temperatures 
(140.degree.-170.degree. C.) for improvement of the working strength of 
the steel sheets. Various coating compositions have been proposed in order 
to meet such requirements. The above requirements are especially important 
to the coating systems using polyisocyanate compounds as curing agents. 
Since the proposed coating compositions have the problem of thixotropy, 
there have not yet been obtained any practically usable, organic 
solvent-soluble epoxy resin coating compositions as the solvent-type 
compositions or coating compositions containing polyisocyanate compounds 
or silica powders for the thin film type rust-prevented steel sheets. 
SUMMARY OF THE INVENTION 
The primary object of the present invention is to provide organic 
composite-plated steel sheets which are inhibited from liberation of 
harmful substances such as chromium from the sheets during the surface 
treatment step conducted by users, and excellent in the corrosion 
resistance by low-temperature baking of the thin film coating.