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 43s 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 ~100nm was formed uniformly on the steel surface. Under the MnO layer, a MnAl2O4 layer with a thickness of ~15nm was formed with small Mn2SiO4 particles that measured ~70nm in diameter. Approximately 500nm 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 ~5nm was formed on most parts of the surface. On some parts of the surface, Mn2SiO4 particles were formed irregularly up to a thickness of ~50nm. Approximately 200nm 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.