Although the melting point of rare earth oxide is very high, it is unstable at steelmaking temperature and has strong activity. It is easy to react with other oxides to form various complex oxides.This characteristic has both positive and very important negative effects.
For austenitic steels, CeO2 does not match the austenitic lattice to a high degree and cannot be used as the heterogeneous nucleus on which austenitic grains precipitate. However, when CeAlO3 is formed by the action of Al2O3 in the liquid steel, the degree of mismatch with the austenitic lattice is greatly reduced, which is very beneficial to the grain refinement of austenitic steels, which is a positive effect.
For carbon steel and low alloy steel, CeO2 does not match the delta-ferrite crystal lattice to a great extent, and it should be the heterostructure nucleus that can be precipitated as the delta-ferrite crystal.However, there are various fine oxide inclusions in the liquid steel. As long as CeO2 formed after adding rare earth elements acts with other oxides (especially SiO2) to form complex oxides, it will lose its function as heterostructure nucleus, thus the grain refinement of carbon steel and low alloy steel is still difficult to be controlled stably.In addition, rare earth oxides react with Al2O3 and SiO2 in liquid steel and refractories for ladles to form viscous oxides and silicates, which not only easily lead to macroscopic slag inclusion defects in castings, but also to blockage of the pouring holes in the bottom pouring ladle.This was once a major problem affecting the application of rare earth elements in cast steel and two important negative effects.
Therefore, a better understanding of the activity of rare earth oxides at high temperatures is of great importance for the application of rare earth elements in cast steel.
To understand the interaction between rare earth oxides and refractories used in steelmaking at elevated temperatures, Tuttle’s research team recently conducted two batches of research and tests in this regard, which they also call “phase equilibrium tests”.