Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Patent
1993-03-09
1995-07-04
Zimmerman, John
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
428626, 428632, 428659, 4273882, 427410, B32B 1508
Patent
active
054298806
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a coated steel sheet with a composite layer containing an organic substance, and having an excellent cosmetic corrosion resistance and excellent image clarity which is also excellent in anti-powdering property, weldability and paint adhesion, and which is beneficial for use in making automobile bodies, electric appliances, etc., and also to a process for manufacturing the same.
BACKGROUND ART
The corrosion of automobile bodies by the salts which are sprinkled on road surfaces to prevent their freezing during the wintertime has become a big social problem recently in North America, northern Europe, and other countries or regions having a cold winter. The use of a coated steel sheet having an improved corrosion resistance instead of a conventional cold rolled steel strip has, therefore, been increasing as one of the measures for improving the corrosion resistance of automobile bodies.
Examples of the coated steel sheets are organic composite coated steel sheet, as disclosed in Japanese Patent Applications laid open under Nos. Sho 64-8033 and Hei 2-15177. These steel sheets comprise a steel sheet plated with zinc or a zinc alloy, and coated with a first layer formed by a chromate coating, and a second layer formed thereon by an organic coating composed of an organic resin which comprises a base resin obtained by adding one or more basic nitrogen atoms and two or more primary hydroxyl groups to the ends of molecules or an epoxy resin, a polyisocyanate compound and a blocked isocyanate compound, the organic resin film further containing specific proportions of silica and a sparingly soluble chromate. These strips are excellent in corrosion resistance, weldability, anti-powdering property and paint adhesion.
The recent inclination toward high-class and original automobiles has come to require a greater importance to be placed on the quality of a paint-finished body surface, particularly its image clarity. The organic composite coated steel sheet as hereinabove described has, however, the drawback of being somewhat inferior to the conventional zinc or a zinc alloy plated steel sheet in image clarity as desired of the outer surface of the outer panel of the automobile body, though it may have a good perforation corrosion resistance as required of the inner surfaces of the outer and inner panels of the automobile body. Moreover, the known organic composite coated steel sheet cannot necessarily be said to be capable of forming a satisfactorily cosmetic corrosion resistance outer surface on the outer panel of the automobile body.
Under these circumstances, it is an object of this invention to provide an organic composite coated steel sheet which can form an excellent cosmetic corrosion resistance and image clarity on, for example, the outer surface of the outer panel of an automobile body, and a process for manufacturing the same.
DISCLOSURE OF THE INVENTION
The steel sheet of this invention having a composite coating containing an organic resin has a first layer a chromate layer formed on the surface of a zinc or a zinc alloy plate steel sheet, and having a coating weight of 10 to 200 mg/m.sup.2 in terms of metallic chromium, and as a second layer formed on the first layer an organic resin film containing a blocked urethane-modified epoxy resin and a rust-preventing additive in a ratio of 90/10 to 40/60 (by weight), and having a thickness of 0.2 to 2 microns. The basic composition of the blocked urethane-modified epoxy resin is as shown at i) below: epoxy resin (A) which comprises an epoxy resin and a polyfunctional amine, and a blocked urethane (B) which comprises a polyol, a polyisocyanate and a blocked agent, in an A/B ratio of 95/5 to 50/50 (by weight).
The use of a modified epoxy resin obtained by adding a monoisocyanate, as well as a polyfunctional amine, to an epoxy resin enables the formation of a film having a still higher level of corrosion resistance. In this case, the composition of the blocked urethane-modified epoxy resin is as shown at ii) below: epoxy
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Kikuta Yoshio
Kobori Kimio
Kubota Takahiro
Ogata Hideaki
Watanabe Toyofumi
NKK Corporation
Zimmerman John
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