Grain Refinement Mechanism of the δ-Ferrite in Steels Through Cerium Addition

To investigate and clarify the roles and mechanism of Ce addition in grain refinement of primary δ-ferrite during solidification of steels, a Fe-4wt pct Si alloy, which can retain the primary δ-ferrite to room temperature after solidification, was used to evaluate the roles of Ce in refining the δ-ferrite. Experimental results showed that addition of small amount of Ce led to significant reduction in average grain size of the as-cast Fe-4wt pct Si alloy from 1906 ± 160 µm without Ce addition down to 998 ± 70 µm at 0.0260 wt pct Ce addition. For the purpose of investigating the grain refinement mechanism of Ce, the Equilib and Phase Diagram modules in the FactSage Software System were used to simulate the formation sequence and calculate fractions of the Ce-containing inclusions above the liquidus in various Fe-4wt pct Si alloys with different Ce, S and O contents. The predicted formation sequence was Ce₂O₃ > Ce₂O₂S > Ce₂S₃/Ce₃S₄. However, due to the low fraction of the oxide and sulfides formed, only the oxysulfide, Ce₂O₂S, was observed and verified in the experimental alloys at room temperature in a transmission electron microscopy and a scanning electron microscope that was equipped with the electron backscatter diffraction and energy-dispersive X-ray spectroscopy. Based on the crystallographic calculations using the edge-to-edge matching model together with previously determined orientation relationships between the δ-ferrite and Ce₂O₂S, it is considered that the heterogeneous nucleation of δ-ferrite on the in situ-formed Ce₂O₂S particles in the Fe-4wt pct Si alloy melt played a key role in grain refinement of the δ-ferrite. An inductively coupled plasma mass spectrometry was used to measure the amount of Ce solute in the ferrite after conducting low-temperature electrolysis to separate and filter out the Ce compounds. The calculation of the growth restriction parameter (β) using the Phase Diagram module in FactSage Software System indicated an almost negligible role of the Ce solute in the grain refinement of δ-ferrite. The present work clarifies previous uncleanness in understanding the grain refinement mechanism of δ-ferrite through Ce addition.