| 摘要 |
PURPOSE:To manufacture a tin-free steel sheet superior in the secondary stickability to paint by performing the inverse electrolysis by AC current in manufacturing said sheet by the inverse electrolysis, electrolytic chromic acid treatment after chromium plating of the steel sheet. CONSTITUTION:A cold rolled steel sheet is electrolyzed in NaOH aqueous solution and the oil is removed, then rinsed, pickled by dil. sulfuric acid aqueous soln. and rinsed. This is plated electrically as cathodes in Cr plating bath contg. CrO3 and H2SO4, to form films of Cr and Cr2O3 on the steel plate surface. Next, the inverse electrolytic treatment by AC voltage in which positive and negative voltages are impressed alternately is performed for the steel sheet in the same plating bath, and the anodic and cathodic electrolysises are performed repeatedly. Said plate is rinsed, then electrolytically Cr acid treated in electrolyte contg. CrO3 and F ions, rinsed and dried. Thereby, the tin free steel sheet having double-coated film composed of Cr plated layer and Cr hydrated oxide film as the lower and upper layers respectively, and superior in the secondary stickability to paint is manufactured.
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