The stability of pseudobrookite type solid solutions in the binary section FeTi,05-Ti30, is investigated in the temperature range 1060" to 1360°C by the quench method combined with chemical analysis and X-ray diffractometry. These data are then combined with all other available phase equilibrium and thermodynamic data in order to obtain a set of optimized thermodynamic equations for the phases in the FeTi03-Ti0,-Ti20,-Fe system. Models for the ilmenite and pseudobrookite solutions are developed. The good agreement between the calculated phase equilibria and the measurements indicates the essential correctness of the thermodynamic data and the phase diagrams. ~ *) Present address: Thyssen Stahl AG, Forschung, Zentrales Qualitatsund Prufwesen, D-47161 Duisburg, Germany developed for the ilmenite and pseudobrookite solutions. The parameters of these models are found by optimization of phase equilibrium data from the literature and from the experimental part of the present study.

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The use of thermodynamic, statistical, and light and electron-optical microscopic methods has made it possible to analyse the course of reactions during steel-plant-internal hot metal desulphurisation. The theoretical dependence between oxygen and sulphur activity has been used to develop an EMF-measurement-based technique for determining the sulphur content as the hot metal is desulphurised. The absence of magnesium sulphide from the collection of phases in the final slag because of the greater stability and secondary formation of calcium sulphide has been explained with the aid of thermo-chemical calculations.The increasing demands placed on steel properties as a result of technological innovations have so far involved a steady decrease of the sulphur content to as low as 10 ppm. As steel properties have improved, the philosophy for hot metal desulphurisation has consequently had to be modi-