| 摘要 |
PURPOSE:To improve the weldability of a steel sheet and the corrosion resistance after painting by immersing the steel sheet having a formed metallic tin layer with many recesses and projections in a chromating soln. before electrolysis so as to limit the distribution of a chromate film according to the recesses and projections. CONSTITUTION:Before a chromate film is formed by electrolysis on a metallic tin layer with many recesses and projections formed on the surface of a steel sheet, the steel sheet is immersed in a chromating soln. to form a chromate film consisting of metallic chromium and chromium oxide hydrate on the metallic tin layer. The thickness of the chromate film is regulated according to the recesses and projections so as to satisfy formulae XM>=2, XM+XO=4-18, YM>=4 and YM+YO>=8 [where XM is the amount (mg/m<2>) of metallic chromium on the projections, XO is the amount (mg/m<2>) of chromium oxide hydrate on the projections, YM is the amount (mg/m<2>) of metallic chromium on the recesses and YO is the amount (mg/m<2>) of chromium oxide hydrate on the recesses].
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