Selective Oxidation of a CMnSi Steel during Heating to 1000 °C: Reversible SiO2 Oxidation

Many multiphase cold-rolled high-strength steels require complex thermal processing, including a (partial) austenitization to reach the desired microstructure. However, of importance for their galvanizability, the selective oxidation of these steels in the austenitic region has not been studied extensively. The present work aims at characterizing the selective oxidation of a CMnSi steel during heating up to 1030 degrees C by X-ray photoelectron spectroscopy (XPS) surface analysis. A phenomenological description of the evolution of the surface composition during heating and soaking in a N-2-5 pct H-2 atmosphere with a dew point of -50 degrees C is presented. Four distinct stages of selective oxidation were identified during heating; manganese oxidation, SiO2 film growth, a transition stage, and a steady state with boron nitride (BN) passivation. During the transition stage, the previously formed SiO2 film disappears, resulting in increased iron concentration at the surface. Simultaneously, a BN film forms on the uncovered metallic iron, inhibiting any further oxidation of the surface. It is demonstrated that the SiO2 oxidation is reversible and temperature controlled.