Consideration of phase equilibria in Fe-Al-Ti-O system and its importance in steel cleanliness during casting process

Abstract

Phase diagram of oxide systems composed of Fe t O-Al 2 O 3 -TiO x was reviewed based on available experimental phase diagram data and CALPHAD thermodynamic database. Most recent updated thermodynamic database was used in order to predict phase equilibria of this oxide system under controlled oxygen partial pressure. In particular, phase equilibria of the oxide system in contact with liquid steel containing Al and Ti, with air, and with refractory for casting process were calculated. A series of experiments were carried out in order to validate the thermodynamic calculations. It was found that, apart from solid alumina, a liquid oxide composed of Fe t O-Al 2 O 3 -TiO x could be formed at the interface between the refractory and the steel, when the liquid steel contains very low C (ultra low C steel grade). The formation of liquid oxide seems to be responsible for clog materials during continuous casting of ultra low C steel. Moreover, this steel can be reoxidized in a tundish by entrapped air. This also generates a liquid oxide mixed with a solid oxide, which significantly affect cleanliness of the steel. Oxidation behavior of the Al-Ti containing steel in the tundish and in the nozzle refractory is comparably discussed. © 2019 Associazione Italiana di Metallurgia. All rights reserved.