TIN-FREE STEEL HAVING EXCELLENT ADHESIVE PROPERTIES

PURPOSE:To obtain economically a tin-free steel having excellent resistance to retort treatment by specifying respectively the S/O value and the value of an alkali soluble component in a chromium hydroxide film in the stage of subjecting a steel sheet as an anode to a reverse electrolysis after Cr plating then further to an electrolytic chromic acid treatment. CONSTITUTION:A thin steel sheet is cathodically treated in an aq. soln. congt. Cr, for example, an aq. sulfuric acid soln. contg. CrO3, Na2SiF6, by which electroplating consisting essentially of metallic Cr is accomplished. The electroplated steel sheet as an anode is in succession thereto subjected to a reverse electrolytic treatment in an aq. soln. contg. Cr. The thin steel sheet after the reverse electrolytic treatment is subjected to an electrolytic chromic acid treatment in an aq. soln. contg. >=1 kind of CrO3, K2Cr2O7 and dichromate. The S/O in the hydrated chromic oxide film on the surface of the tin-free steel (TFS) obtd. in this case is regulated to >=1.8 and the alkali soluble component to >=30%, respectively, by which the TFS having extremely high adhesive properties is obtd.