Mechanism for TiN refinement and resultant heterogeneous nucleation of δ-Fe in magnesium-rare earth-treated ultrapure ferritic stainless steel

The effect of magnesium rare-earth (Mg-RE) treatments on TiN epitaxial nucleation to refine the as-cast structure of ultrapure ferritic stainless steel was investigated. Four samples with different melt treatments were prepared, namely an initial sample, a low-RE-treated sample, a high-RE-treated sample, and a Mg-RE-treated sample. The results reveal that TiN can be refined after the RE/Mg-RE treatment because of the co-crystallization of TiN and RE-Mg-Al-O. Moreover, the size of the inclusions increased with the RE content owing to the decreasing number of oxides. Compared to the low-RE treated sample, the average inclusion size of the Mg-RE-treated sample was further reduced because of the presence of more oxides. These results suggest that the number of oxides is a key factor in TiN refinement. However, few changes were observed in the 3D morphology of the composite TiN under such circumstances. In addition, compared with the Initial sample, the RE/Mg-RE treatment yielded finer solidification structures, indicating that the addition of RE/Mg-RE modifiers decreased the nucleation barrier. The solidification structure coarsened with increasing RE content because of the decrease in the inclusion number density, particularly for TiN. The Mg-RE treatment yielded the finest solidification structure with completely equiaxed grains. In addition, high temperature laser confocal microscopy was successfully applied to observe the in-situ nucleation-growth of δ-Fe, showing that the average grain size after Mg-RE treatment decreased by 38.09%. To reveal these phenomena, a new interfacial wetting-lattice mismatch heterogeneous nucleation model is established, accounting for the high-temperature wetting angle (θ1). Using differential scanning calorimetry, the upper limit of the heterogeneous nucleation rate (Iheter.) of δ-Fe was determined from the number density of all the TiN inclusions. Crystallization of primary δ-Fe and Iheter. depend on the critical degree of undercooling (∆Tc∗).