PRODUCTION OF TIN-FREE STEEL PLATE

PURPOSE:To improve the adhesive strength between a metallic Cr film and a Cr hydrous oxide film as well as between the Cr hydrous oxide film and paint by plating the surface of a tin-free steel plate for retort treatment in a Cr plating bath then electrolyzing the same reverse and further subjecting the steel plate to an electrolytic Cr acid treatment in a Cr acid treatment soln. CONSTITUTION:In the production of a tin-free steel plate for retort treatment, first it is plated in a Cr plating bath, whereby Cr plating layers of 20-200mg/ m metal per one surface is formed. Next, this is subjected to reverse electrolysis in this Cr plating bath, whereby the Cr hydrous oxide formed during Cr plating and the Cr plating assistant anions therein are eluted. After the steel plate is thoroughly washed, it is subjected to an electrolytic C acid treatment in an aq. Cr acid soln., whereby a Cr hydrous oxide film of 5-50mg/m is formed. When painting is coated on this surface, the adhesive strength between the metallic Cr plating layer and the Cr hydrous oxide film and between the hydrous oxide film and paint is improved and retort treatment resistance is improved.