Rare earth additions to steel

Abstract

The theory and practice of rare earth additions to steel are reviewed. On the basis of thermodynamic data it is predicted that rare earth elements have a very strong affinity for oxygen and sulphur and are capable of reducing the oxygen and sulphur contents of steel as well as modifying the inclusions remaining after solidification. The formation of rare earth-lead compounds which should offset the detrimental effects of lead is also predicted. Laboratory and plant experience of rare earth treatment of steel shows the thermodynamic predictions to be true. The different methods of adding rare earth metals to steel are summarized and it is shown that recoveries can be improved by using plunging and encapsulation techniques instead of simply adding the alloy to the metal stream during tapping or teeming. The efficiency of rare earth treatment is also enhanced by the use of stable refractor iessuch as magnesia. The composition, constitution, and methods of identification of rare earth oxides, sulphides, and oxysulphides are discussed. The beneficial effects of rare earth additions on transverse impact and through-thickness ductility properties are discussed in terms of their elimination of alumina stringers and elongated manganese sulphide phases and the substitution by small, non-deformable rare earth inclusions. Finally, the author mentions briefly the possible future areas of study in the field of rare earth additions to steel.