Effect of rare earth elements    

    
When rare earths (elements) like cerium, lanthanides are added to steel they can react to form an oxide, sulphide or oxysulphides:

 

Correct addition of rare earth elements like cerium forms oxysulphide inclusion.

It must be noted that rare earths have high atomic weights, and their oxides, sulphides and oxysulphides have high density 5 to 6 g/  as compared with other oxides. These inclusions do not float easily. Rare earths are strong deoxidizers and desulphrizers as well.

As an example of inclusion engineering, oxide inclusions have been designed to make them useful to increase the machine tool life. Subramanian et al has designed tool steel to be self lubricating by modifying the rheology of oxide inclusions. Glassy inclusions are designed to soften at the tool/chip interface temperature so that a viscous layer of glass can lubricate the tool-chip interface. The interested reader may go through the references.

References:

• R. J. Fruehan: ladle metallurgy
• R. Kiessling and N. lange: Nonmetallic inclusions in steel.
• A. Ghosh: secondary steelmaking
• Subramanian et.al: iron making and steelmaking 2004, vol. 31 p249
• C. Gaslick et.al: iron making and steelmaking 2002, vol.29 No.2.