METHOD FOR PRETREATMENT IN THE PRODUCTION OF TIN-FREE STEEL

摘要

A tin-free steel sheet having a surface layer of hydrated chromium oxide and an intermediate layer of metallic chromium between the surface layer and the steel base, is produced by pre-treating a degreased steel sheet by subjecting it to an anodic treatment for activating the surface of the steel sheet without forming thereon hydrated chromium oxide, and immediately thereafter to a cathodic treatment for forming a film of hydrated chromium oxide on the steel sheet, each of the anodic and cathodic treatments being carried out at a temperature below 60.degree.C and in an acid electrolyte consisting of at least one chromium compound selected from the group consisting of chromic acid, alkali metal chromates and dichromates, ammonium chromate and dichromate. The quantity of electricity used is at least 0.25 coulombs/dm2 in the anodic treatment and ranges from 0.25 to 20 coulombs/dm2 in the cathodic treatment such that the hydrated chromium oxide is formed in an amount of 3 to 20 mg/m2 as chromium. The steel sheet thus pre-treated is thereafter subjected either to an electrolytic chromic acid treatment or to a chromium plating followed by an electrolytic chromic acid treatment so as to farm the aforesaid surface layer of hydrated chromium oxide and intermediate layer of metallic chromium, the hydrated chromium oxide film forming part of the surface layer. The tin-free steel sheet produced in accordance with the invention has improved lacquer adhesion after aging in hot water and under retort conditions.