| 摘要 |
PROBLEM TO BE SOLVED: To improve fuel corrosion resistance after working by allowing cracks, having respectively specified density and size, to occur in a Zn alloy plating film of a surface treated steel sheet having the Zn alloy electroplating film and a chromate film. SOLUTION: A Zn-X alloy electroplating film, containing at least one kind selected from 3-18%, by weight, Ni, 0.02-3% Co, 25-45% Mn, and 8-20% Cr, is formed on a steel sheet by (5 to 50)g/m<2> coating weight per side. Further, a chromate film is formed on the above film by (10 to 200)mmg/m<2> coating weight expressed in terms of metallic Cr. At this time, cracking is allowed to occur in the Zn-X alloy plating film by means of chemical etching, etc. The density of cracks is regulated to a value in the range of 1000 to 150000 pieces in the region of a visual field of 1×1mm at the surface of the plating. Further, the cracks of <=0.5μm maximum width comprise >=90% and the cracks having a depth not smaller than 80% of the thickness of the plating film comprise >=80%.
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