HOT-DIP Zn-Al-Mg COATED STEEL SHEET EXCELLENT IN CORROSION RESISTANCE AND SURFACE APPEARANCE AND PROCESS FOR THE PRODUCTION THEREOF

摘要

A steel sheet having on the surface thereof a coating layer which is formed by hot dipping and which consists of 4.0 to 10 wt.% of Al, 1.0 to 4.0 wt.% of Mg and the balance consisting of Zn and unavoidable impurities, the coating layer having a metallic structure in which a proeutectic Al phase or both of a proeutectic Al phase and a single Zn phase are dispersed in a matrix of an Al/Zn/Zn2Mg ternary eutectic structure. The coating layer having such a metallic structure can be formed by properly controlling the rate of cooling the coating layer adhering to the steel strip pulled up from a plating bath in continuous plating equipment and the temperature of the plating bath and/or by adding proper amounts of Ti and B into the plating bath. The streaking inherent in such a coated steel sheets is inhibited by controlling the configuration of a magnesium-containing film generated until the solidification of the coating layer or by the addition of a proper amount of Be into the plating bath.