Manufacturing of one-side galvanized and galvannealed steel sheet by masking coat.

Abstract

A process for producing one-side galvannealed steel sheet by masking coat has been developed. The functions of masking coat consisting of sodium silicate, sodium borate, magnesia, titania and alumina are in masking and repelling from molten zinc and easily exfoliating by light bending after galvannealing heat treatment. The ungalvannealed side surface has an excellent cosmetic corrosion resistance due to fine phosphate coating originating from the very small quantity of zinc deposition of several mg/m2. The galvannealed layer can be free from powdering by reducing the coating weight to as low as 20 g/m2 on one side with the aid of flame wiping and air knife. Deep drawable steel sheet can be made in a continuous galvanizing line without pre-or post-annealing by adopting an extra low carbon steel stabilized with niobium. Thus produced galvannealed steel sheet is considered to be an excellent material against both perforation and cosmetic corrosion for car body exposed panels.