The non-metallic inclusions in steel of controlled size, distribution and composition play an important role in obtaining required structures. The MnO-SiO2-TiO2 inclusions, which are the result of steel deoxidization with Mn, Si and Ti, act as the nucleation centers of acicular ferrite from austenite. This structure offers good mechanical properties and may be obtained even in the case of large austenite grains, eg. in heat affected zone on welding. However, this function of inclusions requires proper chemical composition, which in turn results from the parameters of preceding operations. The aim of the present work is an attempt of designing the steel composition prior to casting, which would allow for obtaining proper MnO-SiO2-TiO2 inclusions during steel crystallization. This was accomplished on the basis of thermodynamic calculations of equilibrium distribution of components between liquid oxide phase MnO-SiO2-(Al2O3)-TiO2(Ti2O3) and liquid steel for decreasing temperature, i.e. the conditions corresponding to the behavior in casting tundish and casting mould. The presence of Al2O3 in inclusions must be also considered, as aluminum is common deoxidizing agent and it strongly affects the inclusions composition. The calculations were carried out by means of both FACTSage and authors' own programs. The analysis of phase diagrams MnO-SiO2-Al2O3 and MnO-SiO2-TiO2 allowed to decide, if oxide solution was liquid in the solidus temperature of steel, which is important for the action of inclusions as the nucleation centers.
This work was financed by the Polish Ministry of Education and Science - contract nr 220.127.116.119 (AGH-UST Kraków).