Selective Oxidation and Sub-Surface Phase Transformation during Austenitic Annealing of TWIP Steels

Abstract

The selective oxidation of Al-free and Al-added Twinning Induced Plasticity (TWIP) steels during full austenitic annealing at 800°C in N2+10%H2 atmosphere at a dew point of -17°C was investigated by means of HR-TEM of FIB cross-sectional samples. For Al-free TWIP steel, a dense MnO layer was formed on the surface. Crystalline c-xMnO.SiO2(x2) particles and amorphous a-xMnO.SiO2(x<0.9) particles were found at the MnO layer/steel matrix interface. In the subsurface, Mn depletion resulted in the transformation of the austenite to the ferrite phase in a narrow zone. For Al-added TWIP steel, a continuous outer MnO layer and a transition layer consisting of amorphous a-xMnO.SiO2(x<0.9) and crystalline c-MnO.Al2O3(0.8<x<1.2) were formed. The interface between the a-xMnO.SiO2(x<0.9) and c-MnO.Al2O3(0.8<x<1.2) layers had rough structure and 20~50nm diameter voids were formed at the interface. Meanwhile, a narrow Mn-depleted ferrite layer was also formed in the subsurface. The void formation is very likely related to Kirkendall effect occurring during the oxides formation. The thick MnO layer and the voids constitute major challenges to the successful hot dip galvanization of TWIP steels in industrial HDG lines.