SURFACE SELECTIVE OXIDE REDUCTION DURING CONTINUOUS ANNEALING OF ADVANCED HIGH STRENGTH AND ULTRA-HIGH STRENGTH STEEL GRADES

摘要

Enabling the production of defect-free Zn and Zn-alloy coated Advanced high strength steels (AHSS) and ultra-high strength steels (UHSS) grades in conventional hot dip galvanizing (HDG) lines has become a central aspect of advanced steel products and processing research. The surface selective oxidation of alloying additions such as Mn, Si, and Al during the continuous annealing of AHSS and UHSS grades results in the deterioration of the coating quality on galvanized sheet steel. The present contribution reviews the methods to avoid the surface defects on hot dip galvanized sheet steel. One method is to anneal the strip in a high dew point atmosphere, which results in internal, rather than external, oxidation. Another method is strip surface pre-oxidation followed by annealing reduction, thereby creating a pure Fe layer. The present contribution will introduce a new approach to improve the galvanizability by the surface selective oxide reduction using the reduction potential of N_2 + H_2 gas atmospheres.