Conventional mold powders for the continuous casting process contain network formers (SiO2), network modifiers (CaO, MgO, Na2O, K2O), intermediate oxides (Al2O3) and carbon. For reducing the viscosity of the slag system, mold powder normally contains fluoride. However, high amount of fluorine causes excessive erosion of the submerged entry nozzle and fluoride emissions are detrimental to the environment and pose a health hazard. Thus, for developing new F-free mold powders FactSage®- calculations and laboratory tests were performed. The selection of the most suitable composition took into account the melting point, the viscosity and the crystallization behavior. The new designed F-free mold powder and a standard mold powder were tested and compared at a pilot plant, which is equipped with a 5t induction furnace, an electrical plasma heating system, a degassing plant, and a vertical casting machine.
For the trials at the pilot plant, the process parameters, like casting speed or oscillation, were fixed. The casting procedures showed an ordinary melting behavior of the mold powders. Furthermore, casting using a F-free mold powder has a lower mold powder consumption. The metallurgical investigations of the bloom (Baumann and macro structure etching) reveal no abnormalities, i.e. the distribution of sulfur in steel is equally and there are no blowholes visible. The mineralogical investigations of the pilot caster slag films show, that the F-free mold powder has a higher crystallization ability and a more complex crystal structure. Furthermore, the wear of the SEN (submerged entry nozzle) was also examined in detail; the wear of the alumina SEN material is lower for the new developed F-free mold slag.
To sum up, the results show a satisfactory performance of the F-free mold powder, with a uniform melting behavior, a sufficient lubrication of the strand and a high-quality product. Furthermore, the pilot plant is well suited for testing new designed mold powders.