On the Transition of Internal to External Selective Oxidation on CMnSi TRIP Steel

The selective oxidation of a CMnSi transformation-induced plasticity (TRIP) steel during intercritical annealing (IA) in a N₂ + 10 pct H₂ gas atmosphere with a dew point (DP) in the range from 213 K to 278 K (?60 °C to +5 °C) was investigated by transmission electron microscopy. The decarburization during IA resulted in a fully ferritic matrix at the TRIP steel surface. Annealing in high DP gas atmospheres resulted in a reduction of the oxide layer thickness at the surface and an increase of the depth of the subsurface internal oxidation. The experimental results were compared to the calculations of the DP for the transition from internal to external oxidation based on the Wagner model. The evolution of the surface oxide composition during annealing was analyzed thermodynamically by means of the chemical potential diagram for the surface oxides. In the high DP atmosphere conditions, mainly, Mn-rich xMnO·SiO₂ (1 < x < 2) oxides were formed at the surface, while Si-rich xMnO·SiO₂ (x < 1) oxides were formed by internal oxidation. The use of a high DP gas atmosphere is therefore advantageous to induce internal selective oxidation and reduce the amount of surface oxides. It also leads to the formation of Mn-rich xMnO·SiO₂ (1 < x < 2) oxides.