Effects of Galvanizing and Galvannealing Conditions on Powdering Characteristics of Galvannealed Steel Sheet

Abstract

The effects of various manufacturing conditions on powdering characteristics were investigated. Powdering is suppressed by an increase in Al content of the bath, a decrease in the bath temperature and a decrease in the galvannealing temperature. All of these conditions tend to delay the alloying progress. In the case of low Al content (0.12%), ζ-phase (FeZn₁₃) abruptly, appears and forms homogeneous layer at the initial stage of alloying, and then completely changes into δ₁-phase (FeZn₇) in a short period. When the Al content is high (0.16%), the alloying progress is slow and ζ-phase appears heterogeneously at localized spots with time lags. The ζ-phase which appeared late remains unchanged up to the δ₁-phase. As the hardness of ζ-phase is lower than that of δ₁-phase, the ζ-phase deforms preferentially and relaxes a part of stress induced by forming, therefore powdering is suppressed.