Grain Size Control by Oxide Dispersion in Austenitic Stainless Steel

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Abstract
The difference in the deoxidation condition between Al-Ca system and Si-Mn system was discussed in terms of grain growth behavior of anaustenitic 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 3.6 ks, 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 0.2μm span and this size is much smaller than that of MnO-SiO2 particles (about 1μm). This oxide transition from MnO-SiO2to 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 wellknown 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.