Surface Selective Oxide Reduction During the Intercritical Annealing of Medium Mn Steel

Third generation advanced high-strength steels achieve an excellent strength–ductility balance using a cost-effective alloy composition. During the continuous annealing of medium Mn steel, the formation of an external selective oxide layer of MnO has a negative impact on the coating quality after galvanizing. A procedure to reduce the selective oxide was therefore developed. It involves annealing in the temperature range of 1073 K to 1323 K (800 °C to 1050 °C) in a HN_x gas atmosphere. Annealing at higher temperatures and the use of larger H₂ volume fractions are shown to make the gas atmosphere reducing with respect to MnO. The reduction of the surface MnO layer was observed by SEM, GDOES, and cross-sectional TEM analysis.