Rare earth has a strong chemical affinity with impurities such as oxygen and sulfur in molten steel. At steelmaking temperature (about 1873K), rare earth can react with impurities such as oxygen and sulfur in molten steel to form rare earth oxides, sulfides or oxysulfides. The deoxidation equilibrium constant or desulfurization equilibrium constant of these reactions is lower than or close to that of commonly used deoxidizers and desulfurizers. Therefore, rare earth steel additives can be used as strong deoxidizers and desulfurizers for molten steel to purify it. Commonly used rare earth steel additives are mainly mixed rare earth metals or high-grade (RE>27%) rare earth ferrosilicon alloys, and sometimes rare earth oxides with mixed reducing agents are also used.
Oxides, sulfides or oxysulfides generated by the reaction of rare earth with oxygen and sulfur in molten steel can partially remain in the molten steel and become inclusions in the steel. Due to the high melting point of these inclusions, they can serve as inhomogeneous nucleation centers when the molten steel solidifies, and play a role in refining the solidification structure of the steel; and because these inclusions are not easily deformed at the rolling temperature, they still maintain a small spherical or spindle shape, so that the morphology of inclusions in the steel can be controlled, thereby avoiding or overcoming the anisotropy of steel properties caused by the extension and deformation of other types of inclusions (such as MnS) during hot pressure processing, and making the longitudinal, transverse and thickness directions of the steel properties tend to be consistent. This kind of modification of rare earth treated steel is currently the most important application content of rare earth steel additives.
