Effect of dew point on the formation of surface oxides of twinning-induced plasticity steel

The surface oxides of twinning-induced plasticity (TWIP) steel annealed at 800 °C for 43 s were investigated using transmission electron microscopy. During the annealing process, the oxygen potential was controlled by adjusting the dew point in a 15%H2-N2 gas atmosphere. It was found that the type of surface oxides formed and the thickness of the oxide layer were determined by the dew point. In a gas mixture with a dew point of − 20 °C, a MnO layer with a thickness of ~ 100 nm was formed uniformly on the steel surface. Under the MnO layer, a MnAl2O4 layer with a thickness of ~ 15 nm was formed with small Mn2SiO4 particles that measured ~ 70 nm in diameter. Approximately 500 nm below the MnAl2O4 layer, Al2O3 was formed at the grain boundaries. On the other hand, in a gas mixture with a dew point of − 40 °C, a MnAl2O4 layer with a thickness of ~ 5 nm was formed on most parts of the surface. On some parts of the surface, Mn2SiO4 particles were formed irregularly up to a thickness of ~ 50 nm. Approximately 200 nm below the MnAl2O4 layer, Al2O3 was found at the grain boundaries. Thermodynamic calculations were performed to explain the experimental results. The calculations showed that when aO2 > ~ 1.26 × 10− 28, MnO, MnAl2O4, and Mn2SiO4 can be formed together, and the major oxide is MnO. When aO2 is in the range of 1.26 × 10− 28-2.51 × 10− 31, MnO is not stable but MnAl2O4 is the major oxide. When aO2 < ~ 2.51 × 10− 31, only Al2O3 is stable. Consequently, the effective activity of oxygen is considered the dominant factor in determining the type and shape of surface oxides of TWIP steel.