Rare earth elements used in foundry industry are mainly light rare earth elements such as cerium and lanthanum, which have strong activity. Under the temperature of molten steel, they can easily interact with oxygen and sulfur in molten steel to form oxide and sulfide with high melting point.For the most commonly used cerium, the melting point of CeO2 is 2347 C, Ce2O3 is 2142 C and CeS is 2450 C.

However, rare earth oxides are highly active and unstable at high temperatures. They are easy to combine with oxides such as Al2O3, Si O2 and Fe O in molten steel to form complex oxides or with sulfides to form complex sulfur-oxides.

After adding rare earth elements into high temperature steel liquid, it is easy to interact with oxygen and sulphur with high dispersion in steel liquid, forming many fine non-metallic inclusions, which of course may act as heterostructure nucleus during solidification and crystallization of steel liquid.

More than 30 years ago, Gao Ruizhen of Beijing Iron and Steel Institute of China and J.J.Moore of the United Kingdom have noticed that the addition of rare-earth elements in cast steel can refine grain size. However, as the effect is not obvious and unstable in carbon steel and low-alloy steel, they mainly study the desulfurization and purification effect of rare-earth elements in steel and have not made further research on grain refinement.