Thermodynamics of Secondary Steelmaking Operations: Desulfurization, Dephosphorization, and Inclusion Modifications Processes.

A sulfur content less than 10 ppm is desirable to prevent cracks formation during solidification and to improve material properties. This low sulfur content can be obtained by desulfurizing steels with either oxides or calcium or rare earth elements. Dephosphorization processes can be performed in both oxidizing and reducing conditions. Thermodynamic calculations show that phosphorous oxidation is hindered by FeO formation. When P//2O//5 activity is less than 1. 3 multiplied by 10** minus **1**0 it is possible to oxidize phosphorus instead of iron. Such a low value of P//2O//5 activity is possible only when another oxide reacts to form a stable phosphate. Calcium and sodium oxides are suitable for this purpose. To avoid MnS precipitation it is necessary to lower sulfur concentration to less than 50 ppm, when Mn content ranges between 1. 0% and 1. 5%. A calcium treatment is therefore useful and the inclusions formed (CaS) are not detrimental for steel quality.