Influence of Si Oxide on Fe-Zn Galvannealing Reaction of Si-added Steel Sheet

Abstract

The influences of external and internal Si oxides on the galvannealing reaction of 0-1.8%Si-added steel were investigated. Internal and external Si oxides formed by recrystallization annealing at the dew points of 223-263 K. The distribution of Si oxides in the subsurface region of the substrate steel was analyzed using glow discharge optical emission spectroscopy, reflection electron microscopy, and transmission electron microscopy. External Si oxides suppressed the galvannealing reaction. Then the ζ phase formed locally at the interface between the substrate steel and the zinc coating after galvanizing, which suggests that the external Si oxides at the steel surface acted as a barrier to the Fe-Zn reaction. When Si oxidized internally, a depleted zone of solute Si was observed in the subsurface region of the substrate steel. In this case, the galvannealing reaction was accelerated in comparison with that when Si did not oxidize. Furthermore, decrease of the solute Si content led to acceleration of the galvannealing reaction, nucleation of the ζ phase, and growth of the Γ phase. From these results, it was concluded that depletion of solute Si attributable to internal oxidation of Si accelerated the galvannealing reaction.