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

Abstract

AbstractThe selective oxidation of Twinning Induced Plasticity (TWIP) steel during annealing at 800 °C in a N2 + 10%H2 gas atmosphere with a dew point of −17 °C and −3 °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 · SiO2 (x ≥ 2) particles and amorphous a–xMnO · 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 µm 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.