Selective Oxidation and Reactive Wetting During Hot-Dip Galvanizing of a 1.0 pct Al-0.5 pct Si TRIP-Assisted Steel

Selective oxidation and reactive wetting during continuous galvanizing were studied for a low-alloy transformation induced plasticity (TRIP)-assisted steel with 0.2 pct C, 1.5 pct Mn, 1.0 pct Al and 0.5 pct Si. Three process atmospheres were tested during annealing prior to galvanizing: 220 K (-53 A degrees C) dew point (dp) N-2-20 pct H-2, 243 K (-30 A degrees C) dp N-2-5 pct H-2 and 278 K (+5 A degrees C) dp N-2-5 pct H-2. The process atmosphere oxygen partial pressure affected the oxide chemistry, morphology and thickness. For the 220 K (-53 A degrees C) dp and 243 K (-30 A degrees C) dp process atmospheres, film and nodule-type manganese, silicon and aluminum containing oxides were observed at the surface. For the 278 K (+5 A degrees C) dp atmosphere, MnO was observed at the grain boundaries and as thicker localized surface films. Oxide morphology, thickness and chemistry affected reactive wetting, with complete wetting being observed for the 220 K (-53 A degrees C) dp and 243 K (-30 A degrees C) dp process atmospheres and incomplete reactive wetting being observed for the 278 K (+5 A degrees C) dp atmosphere. Complete reactive wetting for the 220 K (-53 A degrees C) dp and 243 K (-30 A degrees C) dp process atmospheres was attributed to a combination of zinc bridging of oxides, aluminothermic reduction of surface oxides and wetting of the oxides. Incomplete wetting for the 278 K (+5 A degrees C) dp atmosphere was attributed to localized thick MnO films.