Effects of Oxygen Partial Pressure on Oxidation Behavior of CMnSi TRIP Steel in an Oxidation-Reduction Scheme

Abstract

An oxidation-reduction scheme is an alternative approach for improving the galvanizability of advanced high-strength steel in the continuous hot-dip galvanizing process. Here, we investigated the effect of oxygen partial pressure (PO2) on the oxidation behavior of a transformation-induced plasticity steel containing 1.5 wt% Si and 1.6 wt% Mn during heating to and holding for 60 s at 700 degrees C under atmospheres with various PO2 values. Irrespective of PO2, a thin amorphous Si-rich layer of Si-Mn-O was formed underneath the Fe oxide scale (a Fe2O3/Fe3O4 bilayer) in the heating stage. In contrast to Si, Mn tended to segregate at the scale surface as (Fe, Mn)(2)O-3. The multilayered structure of (Fe, Mn)(2)O-3/Fe2O3/Fe3O4/amorphous Si-Mn-O remained even after extended oxidizing at 700 degrees C for 60 s. Fe2O3 was the dominantly growing oxide phase in the scale. The enhanced growth rate of Fe2O3 with increasing PO2 resulted in the formation of more Kirkendall voids in the amorphous Si-rich layer and a less Mn segregation at the scale surface. The mechanisms underlying the absence of FeO and the formation of Kirkendall voids are discussed.