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 MnAl ₂O₄ 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, Mn ₂SiO₄ 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 a_O2 > ~ 1.26 × 10 ^{- 28}, MnO, MnAl₂O₄, and Mn₂SiO ₄ can be formed together, and the major oxide is MnO. When a _O2 is in the range of 1.26 × 10^{- 28}-2.51 × 10^{- 31}, MnO is not stable but MnAl₂O₄ is the major oxide. When a_O2 < ~ 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.