Selective Oxidation of a Medium-Mn Third Generation Advanced High Strength Steel during Austenitizing and Intercritical Annealing

The effect of the simulated continuous galvanizing line N2−5 vol% H2 process atmosphere oxygen partial pressure (pO2) on the external and internal selective oxidation of a prototype medium-Mn third generation (3G) advanced high strength steel was determined during a two-stage heat treatment cycle (i.e., austenitizing and intercritical annealing) which had previously yielded 3G properties. Thick external oxides (∼200 nm) were observed after the austenitizing heat treatment, regardless of the process atmosphere pO2 employed. An intermediate flash pickling step was successful in reducing the external oxide thickness significantly (to ∼30 nm) along with revealing some extruded metallic Fe nodules on the surface. The austenitizing heat treatment also resulted in a solute-depleted surface layer with a minimum thickness of 2 μm. This solute-depleted layer inhibited the formation of external oxides during intercritical annealing, resulting in a surface similar to that observed after flash pickling comprising a near-pure Fe surface with isolated, nodular external oxides. These surfaces are promising in terms of successful reactive wetting of this prototype medium-Mn steel during subsequent continuous hot-dip galvanizing.