Forced flotation of inclusions in tundish

Abstract

With increased usage of steel for critical applications, the demand for cleaner steel has increased. Existing technologies have been improved and new technologies are being introduced all along the process route to minimise the size and quantity of inclusions. In continuous casting, the function of tundish has changed from its early role as a liquid steel buffer to a multifunctional vessel, dominating quality adjustments, specifically inclusion flotation. To meet stringent cleanliness requirements, a methodology of forced flotation of inclusions by controlled bubbling of inert gas in tundish was developed and investigated. Location of the bubbling arrangement was optimised using the flow profile generated by water modelling experiments. Optimum gas flowrates were calculated mathematically and further adjusted with a series of experiments. Extensive plant scale trials were used to optimise the process parameters and bring the technology to practice. There was a 33-70% reduction in inclusions of size greater than 50 mm and a 13-36% reduction in inclusions 25-50 mm. Inclusion removal reduced at higher casting speeds. Under steady state conditions, efficiency of inclusion flotation was found to be a function of casting speed. Additionally, a significant reduction in oxide build-up and nozzle clogging was observed thus lengthening refractory life.

Novel concept of fine inclusion removal using carbonate powder injection through the ladle shroud

“…Hence, the efficiency of inclusion removal has been one of the most important criteria in evaluating the metallurgical performance of any given tundish design. Numerous studies have been conducted to optimize the designs of tundish using various flow control devices [3][4][5] such as weirs, dams, impact pads, turbulence inhibitors, etc. In order to promote the floatation of inclusions, these devices can improve the removal of larger inclusions, greater than 50 μm, but do not necessarily promote the removal of smaller sized inclusions, owing to their extremely low Stokes velocities.…”