Process for producing surface-treated steel sheet superior in weldability and paint-adhesion

Abstract

A surface-treated steel sheet is provided, at a low cost, which has high weldability and is useful for a material of containers of foods and beverages. The surface treatment is conducted by applying flattened granular tin coating deposits having a specified diameter and high adhesion on a steel sheet surface at a specified plated area ratio, and applying thereon a metallic chromium coating and a chromium hydrate oxide coating. In the surface treatment, firstly, the surface of a steel sheet is subjected to tin-plating in an acidic tin plating bath containing a conventionally used brightener in an amount of from 0,001 to 0.05 g/l so as to form flattened granular tin coating deposits having a diameter of 0.4-2.4 .mu. at a plated area ratio of 5 to 30% and to improve the adhesion to the steel sheet and decrease falling-off of the granular tin coating deposits until completion of chrome-plating, thereby the variation of the amount of tin coating being decreased. Subsequently, the sheet is plated with metallic chromium in an amount of 50-150 mg/m.sup.2, and further chromium hydrate oxide layer is formed in an amount of 2-40 mg/m.sup.2 as chromium. Thus a surface-treated steel sheet, which has satisfactory surface appearance, paint adhesiveness, under-paint-coating corrosion resistance of conventional tin-free steel sheets, and stable weldability, is producible at a low cost.

Dependencies are inferred from the claim text.