Assessment of CaO-Al2O3 Based Mold Flux System for High Aluminum TRIP Casting

Today, the demands for Advanced High Strength Steels (AHSS) have gradually increased due to their ability to reduce vehicle weight as a means to save energy, reduce the environmental impact while simultaneously improving passenger safety. However, AHSS often require the addition of large amounts of alloying elements such as aluminum and this can make it difficult to cast sound slabs without surface defects. When casting high aluminum AHSS, due to the reaction between aluminum in steel and silica in mold flux, the viscosity and crystallization characteristics of the mold slag changes drastically, and deteriorates mold lubrication. Therefore, it is critical to limit the reaction between Al in steel and mold slag and at the same time to provide consistent and adequate mold slag in-use properties. This paper describes the development of non-traditional lime-alumina based mold fluxes which have the potential to reduce slag-steel interaction during casting of high aluminum TRIP steel. Several trial casts of 1.45% Al TRIP steel have been conducted on a pilot caster to examine the performance of mold fluxes developed. When the lime-alumina based mold fluxes were applied successfully, alumina pickup was reduced to less than 5% as compared to 15% alumina pickup for corollary trial casts using conventional lime-silica mold fluxes. The developed lime-alumna mold fluxes showed improved in-mold performance as indicated by enhanced lubrication and stable mold heat transfer, again compared to lime-silica fluxes. Cast slabs from the trials using these lime-alumina fluxes have periodic and sound oscillation marks and minimized defects.