| 摘要 |
PROBLEM TO BE SOLVED: To produce a galvannealed steel sheet excellent in corrosion resistance, formability and continuous spottability and moreover reduced in the weldable electric current limit in spot weldability and to provide a method for inexpensively producing it. SOLUTION: This zinc composite plated steel sheet is the one that at least one side of a base material is provided with a zinc composite plating layer contg. 0.5 to 5% Co and 0.5 to 5% high polymer organic matter composed of a starch decomposed product, and the surface is provided with a film contg. an amorphous reactant product of phosphoric acid and zinc by 10 to 300 mg/m2 as phosphorus. The steel sheet can easily be produced by applying electroplating to at least one side of a base material by using an electrogalvanizing bath contg. 0.5 to 5 wt.% Co and 0.5 to 5 wt.% high polymer organic matter composed of starch decomposed product, next coating the surface with an aq. soln. of phosphoric acid in which the concn. of phosphoric acid is controlled to 1 to 20 wt.% by the amt. by which the coating weight after the coating of the reaction product of phosphoric acid and zinc in the surface of the plating layer is controlled to 10 to 300 mg/m2 as phosphorus and executing drying without washing by water.
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