On the reactive wetting of a medium-Mn advanced high-strength steel during continuous galvanizing

The effects of process atmosphere oxygen partial pressure (pO2) and Sn-addition on the reactive wetting of a prototype medium manganese third generation advanced high strength steel by a 0.20 wt% Al (dissolved) zinc bath was determined through galvanizing simulations. External MnO layers with thicknesses ranging from 19 nm to 45 nm were formed after annealing for 120 s at 690 °C under a variety of experimental process atmospheres. Despite the presence of surface oxides prior to immersion, an integral zinc coating and good coating adherence was obtained on all steel substrates, with the exception of the reference steel annealed under the lowest pO2−50 °C dew point process atmosphere, which exhibited poor reactive wetting. Excellent reactive wetting was demonstrated by the Sn-containing steels annealed under the −30 °C and +5 °C atmospheres. It was determined that the Al uptake as well as the population of the Fe-Al intermetallics at the coating/steel interface increased as a result of annealing under the higher pO2 atmospheres and adding Sn to the steel. The significant improvement in reactive wetting of the Sn-added steel was attributed to the decreased surface enrichment of Mn and the discrete distribution of the fine external oxide particles formed on the surface of these steel prior to dipping; which allowed for progression of reactive wetting through mechanisms such as the aluminothermic reduction of MnO, oxide bridging and oxide lift-off by the molten galvanizing bath.