Selective Oxidation and Sub-Surface Phase Transformation of TWIP Steel during Continuous Annealing

The selective oxidation of Twinning Induced Plasticity (TWIP) steel during annealing at 800 degrees C in a N-2+10%H-2 gas atmosphere with a dew point of -17 degrees C and -3 degrees C was investigated by means of high resolution transmission electron microscopy of cross-sectional samples. The annealing resulted in the selective oxidation of Mn and Si and the austenite-to-ferrite phase transformation of the sub-surface region. In the low dew point atmosphere, the annealing resulted in the formation of a MnO layer at the surface. Crystalline c-xMnO center dot SiO2 (x >= 2) particles and amorphous a-xMnO center dot SiO2 (x<0.9) particles were found at the interface between the MnO layer and the steel matrix. In a narrow zone of the sub-surface, the Mn depletion resulted in the transformation of the initial austenite. In the high dew point atmosphere, a thicker MnO layer was formed on the surface and no mixed manganese-silicon oxides particles were observed at the MnO/steel matrix interface. In the sub-surface, Mn was significantly depleted in the range of 2-3 mu m below the surface and the initial austenite in this zone was transformed to ferrite. MnO particles were found at the grain boundaries and in the interior of grains.