Thermodynamic Evaluation of Inclusion Formation during Cooling and Solidification of Low Carbon Si-Mn Killed Steel

Abstract

The primary endogenous inclusions are inherent and specific to the deoxidation process and may contribute significantly to the steel properties besides affecting its castability during continuous casting. Additional (secondary) inclusions form during cooling and subsequent freezing of liquid steel during continuous casting operation. Therefore, in order to produce clean steel it is extremely important to understand clearly their formation mechanisms. Rejection of solutes by the solidifying dendrites causes segregation of solutes in the interdendritic liquid with consequent build-up of their thermodynamic super saturation. Rejected solute elements subsequently react to give rise to secondary inclusions in the solidifying liquid steel. The present investigation aimed at development of an appropriate thermodynamic calculation procedure for the prediction of inclusion compositions formed during cooling and solidification of liquid steel. The model has been applied to an inclusion sensitive grade of steel. Segregation of various solutes with progress of freezing was calculated using the Clyne–Kurz microsegregation equation. A sequential computation procedure involving segregation equation and thermodynamic equilibrium calculations by the Factsage-06 thermodynamic software was developed. Compositions of inclusions at various stages of solidification were calculated. Model predictions were finally compared with literature as well as with observed inclusion compositions determined in several continuously cast billet samples using SEM-EDS. Reasonably good correspondence between model predictions and observed inclusions have been obtained. The model has provided deeper insight into the formation mechanisms of various nonmetallic inclusions in steel and provided the required guidance for controlling them.

Keywords:

• Billets
• Casting
• Inclusions
• Segregation
• Steelmaking
• Thermodynamics

ACKNOWLEDGMENTS

The author would like to thank the Management of Tata Steel, India, for providing permission to publish the work. Valuable guidance provided by Prof. A. Ghosh, IIT Kanpur is also gratefully acknowledged.

Notes

Note: *Estimated.