Influence of Minor Alloying Elements on Selective Oxidation and Reactive Wetting of CMnSi TRIP Steel during Hot Dip Galvanizing

The influence of the addition of minor alloying elements on the selective oxidation and the reactive wetting of CMnSi transformation-induced plasticity (TRIP) steels was studied by means of galvanizing simulator tests. Five TRIP steels containing small alloying additions of Cr, Ni, Ti, Cu, and Sn were investigated. After intercritical annealing (IA) at 1093 K (820 °C) in a N₂ + 5 pct H₂ gas atmosphere with a dew point of 213 K (−60 °C), two types of oxides were formed on the strip surface: Mn-rich xMnO·SiO₂ (x > 1.5) and Si-rich xMnO·SiO₂ (x < 0.3) oxides. The addition of the minor alloying elements changed the morphology of the Si-rich oxides from a continuous film to discrete islands and this improved the wettability by molten Zn. The improved wetting effect of the minor alloying elements was attributed to an increased area fraction of the surface where the oxides were thinner, enabling a direct unhindered reaction between Fe and the Al in the liquid Zn and the formation of the inhibition layer during the hot dip galvanizing. The addition of a small amount of Sn is shown to significantly decrease the density of Zn-coating defects on CMnSi TRIP steels.