Grain size control by oxide dispersion in austenitic stainless steel

The difference in the deoxidation condition between Al-Ca system and Si-Mn system was discussed in terms of grain growth behavior of an austenitic stainless steel (Fe-17%Cr-9%Ni-3%Cu-low C, N alloy). In the steel deoxidized by Si-Mn, oxide inclusion exists as MnO-SiO2 particles in the as-cast ingot. However, once this steel is annealed at 1523K for 3.6ks, a part of MnO-SiO2 particles decomposes and MnO-Cr2O3 particles are newly formed on the annealing. The particle size of MnO-Cr2O3 is about 0.2 mum span and this size is much smaller than that of MnO-SiO2 particles (about 1 mum). This oxide transition from MnO-SiO2 to MnO-Cr2O3 is very useful for suppressing the grain growth of recrystallized austenite grains on annealing at 1373K after 65% cold working because the reprecipitated fine oxide particles pin the austenite grain boundary effectively. The relation between austenite grain size and oxide particles dispersion is not explained by the well-known Zener's relationship but done by the Doherty's theory in which a half of particles are thought on grain boundary and play a role to pin the grain boundary. On the other hand, in the steel deoxidized by Al-Ca, stable Al2O3-CaO particles are formed in the as-cast ingot. This oxide is so stable that it never causes the oxide transition on annealing like that in the steel deoxidized by Si-Mn. Therefore, the grain refining through recrystallization process is never expected in the steel deoxidized by Al-Ca.