Different thermodynamic models are currently used to describe the chemical reactions in the steelmaking process. Beyond the knowledge of phase diagrams, a very good description of the liquid phase is needed to describe with accuracy the steelmaking reactions such as slag/metal equilibrium, inclusions precipitation and solidification of steels. Interaction parameter formalism, first proposed by Wagner [1], is generally used for dilute solutions and hot metal whereas the sub-lattice model, developed in the Thermo-Calc software, is used for more concentrated solutions. However, these models are not able to describe with accuracy the thermodynamic behaviour of some interstitial elements such as sulphur or oxygen in concentrated solutions. Thus, the “central atoms” model, introduced by Lupis and Elliott [2, 3], and simultaneously developed by Mathieu and co-workers [4, 5] under the denomination of “surrounded atom” model has been applied to the description of the liquid steels. Its basic concept consists in using, as elementary energetic support of the solution, the atom in the force field of its nearest neighbours. The partition function is described in terms of probabilities associated with different compositions of the nearest neighbour shell and in terms of the influence of these configurations on the potential and vibrational energies of the central atom. In the formalism developed by Foo and Lupis [6] to describe Fe-based solutions (liquid or austenite), few parameters are required to describe complex metallic solutions. Thus, a good prediction of the model is expected. A quaternary system containing substitutional and interstitial elements is described with only twelve parameters (six binary and six ternary ones). In the present study, the thermodynamic assessment of the liquid Fe-Al-Cr-Mn-Ni-Si-C-N-O-P-S system was performed. The parameters were assessed from literature data on limiting binary and ternary systems. To describe the solidification of steels, the “central atoms” model was coupled with the sub-lattice model used to describe the solid phases. The calculated liquidus and solidus temperatures of several high-alloyed steels are compared with experimental data.
[1] C. Wagner, Thermodynamics of alloys, Addison-Wesley, Reading, MA, 1962.
[2] C. H P. Lupis, Doct. Thesis, MIT (Jan. 1965)
[3] C. H. P. Lupis, J. F. Elliott, Acta Met., Vol. 15, October 1967, 265.
[4] J. C. Mathieu, Doct. Thesis, Univ. of Grenoble (March 1967)
[5] P. Hicter, J. C. Mathieu, F. Durand, E. Bonnier Adv. in Phys., 16, 1967, 523.
[6] E-Hsin Foo, C. H. P. Lupis, Acta Met., Vol. 21, October 1973, 1409.