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 a_O2 > ∼ 1.26 × 10⁻²⁸, MnO, MnAl₂O₄, and Mn₂SiO₄ 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. - Highlights: • The surface oxides of TWIP steel annealed at 800 °C were investigated using TEM. • The surface oxides were determined by the dew point during the annealing process. • The activity of oxygen is the major factor determining the oxides of TWIP steel