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

Abstract

The surface oxides of twinning-induced plasticity (TWIP) steel annealed at 800 degrees 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%H-2-N-2 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 degrees C, a MnO layer with a thickness of similar to 100 nm was formed uniformly on the steel surface. Under the MnO layer, a MnAl2O4 layer with a thickness of similar to 15 nm was formed with small Mn2SiO4 particles that measured similar to 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 degrees C, a MnAl2O4 layer with a thickness of similar to 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 similar to 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) > similar to 1.26 x 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 x 10(-28)-2.51 x 10(-31) MnO is not stable but MnAl2O4 is the major oxide. Whena(O2) < similar to 2.51 x 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. (C) 2014 Elsevier Inc. All rights reserved.