Abstract
The solidification structure, solute segregation, constitution and distribution of inclusions in rare earth (RE) treated large ingots have been investigated. A concentration region, called “The Rare Earth Zone” which is bound by [%RE][%S] =2.7-8.0 and [%RE][%S]<=15×10-5 on RE vs. S coordinates, has been found to give a satisfactory sulphide shape control without causing any harmful accumulation of RE-sulphides and RE-oxysulphides in the sedimental zone of the ingots. A theoretical model has been given to successfully account for the sedimental zone accumulation, anomalous A-segregation, and the reversion, named “S-Reversal”, of the segregation pattern in the RE-treated ingots. The constitution and origin of inclusions typical in the RE-treated ingots have also been interpreted in terms of the model supplemented by thermodynamic considerations.
