Effect of Coating Structure on Powdering Resistance of Galvannealed Steel Sheet

Galvannealed steel sheets containing 10wt%Fe in coated layer were prepared in various galvannealing temperature and cooling rate. Powdering resistances were evaluated by a draw bead test. X-ray diffraction examination revealed an attribution of the coating structure to powdering resistance.

Higher galvannealing temperature(500∼550°C) deteriorates powdering resistance, but slower cooling from higher temperature improves it. At low galvannealing temperature(440°C), powdering resistance is rather good independent of cooling rate. Г₁-phase(Fe₅Zn₂₁) is formed along the interface between coating and steel when samples are galvannealed at lower temperature(≦500°C) or cooled at slower cooling rate from higher galvannealing temperature. The results suggest that the increase of Г₁ ratio [Г₁/(Г+Г₁)] in the coating is inclined to improve powdering resistance.