Valence electron structure analysis of the purification effect of rare earth elements on steel

Based on the Empirical Electron Theory of Solids and Molecules (EET), the unit cell models of compounds formed by rare earth (RE) elements in steel with O, S and low-melting-point harmful elements such as As, Sb, Bi and Pb, as well as Fe–Ce intermetallic phases, are established using the bond length difference (BLD) method, and their valence electron structures are calculated. The characteristics of RE oxides and sulphides formed in steel are explained using phase valence electron structure parameters, and their formation sequence and conditions are analysed. The results show that RE deoxidise first and then desulphurise in steel, RE oxysulphides act as intermediate products during the transition from deoxidation to desulphurisation, and deoxidation remains dominant during their formation. Moreover, the type and distribution of RE compounds with O and S in steel are related to the contents of oxygen and sulphur. After the reactions with oxygen and sulphur are completed, residual RE, if any, will react with low-melting-point impurity elements. Attention should be paid to controlling the addition amount of RE in steel to avoid precipitation of Fe–Ce intermetallic phases caused by excessive REs, which would contaminate the molten steel.