Effect of multi-phase oxide particles on TiN crystallization and solidification structure in Ti-added ferritic stainless steel

The effect of TiN crystallization on the solidification structure of Ti-added ferritic stainless steel were studied in Ti/Mg, Ti/Ca, Ti/Mg/Al, Ti/Ca/Al, Al/Ti/Ca and Ti/Mg(Ca)/Ca(Mg) deoxidations carried out at 1 600°C using an Fe-17.5(11)%Cr-0.25%Mn-0.20%Si-0.2 to 0.3%Ti-0.01%C-0.003 to 0.04% N alloy on a mass percent basis. Compositional analysis of the oxide particles and TiN layers generated during the deoxidation using scanning electron microscopy in conjunction with an electron probe microanalyzer, coupled with observation of the morphology and homogeneity of complex TiN+oxide particles, revealed that the TiN layers were formed through the postulated mechanism of the co-crystallization of TiN and oxide. Ti/Mg and Ti/Mg/Al deoxidations performed in an Al₂O₃ crucible yielded a very fine solidification structure at high N contents (>200 ppm) because of the presence of complex particles consisting of the TiN layer with a small amount of oxide phases. In the Ti/Ca, Ti/Ca/Al and Al/Ti/Ca deoxidations using an Al₂O ₃ crucible, a relatively fine solidification structure was observed when a low oxide content was present in the TiN layer. Ti/Ca/A and Al/Ti/Ca deoxidations carried out using an MgO crucible in the presence or absence of MgO-CaO-Al₂O₃ slag gave rise to solidification structures that were relatively finer in comparison with those obtained using an Al ₂O₃ crucible. Very fine structures were observed at low N contents (36 to 80 ppm) in the Ti/Mg/Ca and Ti/Ca/Mg deoxidations using an Al₂O₃ crucible because of the effective surface composition of the oxide particles for δ-phase solidification.