(V) Thermoslag software – An attempt to simplify slag properties

Slags play a very important role in steelmaking processes. Apart from protecting the molten metal from atmospheric oxidation, the slag serves, during steel refining, as a medium to gather unwanted elements in steel like phosphorus and sulphur and also as an intermediate phase to transfer useful alloying elements like manganese or vanadium to the steel. An understanding of the steel-slag reactions and modelling them requires a systematization of the thermochemical and thermophysical properties of multi-component slags. An attempt was made at the Royal Institute of Technology to create a common base for modelling both the thermochemical and thermophysical properties of slags like viscosity and density. The initial model was based on Temkin-type of slag description. In order circumvent complexity regarding the nature and population of the various silicate species in the slag melt, a Lumsden approach was followed wherein all the silicate entities are considered to “break up” into Si⁴⁺ and O^2-. The entropy of mixing was considered as the sum of the entropies for the two sub-groupings. Redlich-Kister polynomial was used to describe the excess terms. The model was able to describe the thermodynamic properties including sulphide capacities of six component slags. Using the same base, the viscosities were successfully described using Eyring equation. It was found difficult to link densities/molar volumes based on the above description. A CALPHAD approach was adopted, and the densities could be described reasonably well, depending on the availability of experimental data. Further work is in progress regarding remodelling slag viscosities and other physical properties by CALPHAD approach.