Reactive wetting during hot-dip galvanizing of high manganese alloyed steel

The present study discusses reactive wetting during hot-dip galvanizing of high Mn alloyed steel (X-IP1000, 23 wt.% Mn) and is focused on investigating the influence of the metallic Mn concentration in the steel bulk composition on phase formation at the interface steel/coating. Samples were in-line bright annealed (1100 °C/ 60 s in N2-5%H2 at DP −50 °C) prior hot-dipping to avoid external MnO on the steel surface. This approach was applied to avoid influencing the wetting reaction by an aluminothermic MnO reduction, because this is considered to lead to an unwanted zeta-phase (FeZn13) formation in the coating by hot-dipping of Mn alloyed steels (< 5.0 wt.% Mn). The influence of hot-dipping parameters, which are contributing to the kinetics of the wetting reaction, was examined in terms of varying bath-Al content (0.17 and 0.22 wt.%), bath temperature (440–500 °C) and strip entry temperature (420–520 °C). The structure and chemical composition of both galvanized coating and interface steel/coating were characterized. While external MnO was verifiably avoided, brittle zeta-phase distinctively appeared at the interface steel coating together with the typical Fe2Al5 phase. This shows that the model of aluminothermic MnO reduction failed in the present case. This study suggests an alternative model explaining the appearance of zeta-phase with the removal of bath-Al by metallic Mn, which is dissolved out of the steel bulk into the Zn bath. The present investigation shows that alloying elements in the steel bulk may influence coating quality not only “indirectly” by external formation of nonwettable oxides, but also “directly” by influencing phase equilibria and kinetics of the wetting reaction. Understanding these phenomena will improve processing of (high) alloyed steel concepts as well as industrial Zn bath management.