Enhancing Steel Cleanliness in Tundishes by applying Physical and Mathematical Modeling

The tundish, being the last vessel which stores steel in liquid state, provides the final opportunity to enhance its quality. Hence, it must be ensured that the tundish acts as a refiner rather than a contaminator. It has been noticed that reoxidation of liquid steel from ambient air aspiration through the ladle slide gate, can seriously degrade the steel quality. As a preventive measure, the reoxidation-prone region is flooded with an inert gas such as argon to prevent aspiration of air. However, some argon bubbles themselves can get aspirated into the liquid steel stream flowing from the ladle into the tundish. Thus, an upwelling buoyant gas-liquid plume is formed, which sweeps off the overlying slag layer and subsequently gives rise to an exposed region of molten steel, known as the Tundish Open Eye (TOE). In the present work, TOE formation in two different tundish systems viz., a 12-tonne four-strand billet caster tundish, and a 63-tonne two-strand slab caster tundish, has been studied. A holistic approach, including plant trials, physical modeling, mathematical modeling, dimensional analysis and mechanistic modeling, has been taken up to investigate the TOE phenomenon. Additionally, the effects of eccentric open eye formation and non-isothermal flows on steel quality were also analyzed. Plant trials were carried out to study the variation of TOE size by using high definition (HD) videography. The total oxygen and inclusion analyses were carried out using the LECO and ASPEX techniques respectively. The mathematical modeling for both the water/oil/air system and liquid steel/slag/argon gas system was performed using the Volume of Fluid (VOF) method, and discrete phase method (DPM), coupled with the standard k-ε turbulence model. The mathematical model was validated with the experimental results from physical modeling and plant trials. A mechanism of formation of the TOEs and emulsification of the slag layer was proposed based on the results. Critical argon gas flow rates were also determined, for which there is negligible or no TOE formation. Operating under these flow rates ensure better steel quality due to prevention of reoxidation through TOE.Ph.D