Sulphide shape control

has a low melting point and elongate in the direction of rolling and becomes stringer type inclusions. These inclusions greatly reduce the transverse mechanical properties of steel. also causes steel to be more susceptible to hydrogen induced cracking. Stringer type inclusions should be modified if not eliminated.

Formation of requires thermodynamically that the solubility product   

 

at the solidification temperature. During solidification, Mn and S are rejected from the solid steel causing an increase in their concentration in the remaining liquid. The Mn and S enriches in the interdendritic region and MnS forms during the end of solidification.

For sulphide shape control, it is necessary to desulphurize steel to very low value say . Addition of calcium to aluminum killed steel will first form calcium aluminate inclusion. If calcium aluminate inclusion has excess  content, it has high sulphide capacity as well. This will result in precipitation of  which forms a duplex inclusion in which a  ring surrounds calcium aluminate core. This type of inclusion is spherical and does not elongate.