Estudo da cinética de cristalização de pós fluxantes vítreos à base de CaO-Al2O3 (livres de flúor e SiO2) para uso em lingotamento contínuo de placas de aços TRIP/TWIP

Abstract

High-aluminum steels (TRIP / TWIP) are modern steels highly demanded by the automotive industry and present some difficulties during the continuous casting process, such as high viscosity slag, poor plate quality and breakouts. The present work studies the crystallization of unconventional flux powders for use in the casting of TRIP / TWIP steels. Crystallization kinetics and structure of glassy CaO-Al 2 O 3 -B 2 O 3 -Na 2 O-Li 2 O fluoride-free mold fluxes for samples with different CaO/Al 2 O 3 ratios were investigated by Differential scanning calorimeter (DSC), X-ray diffraction (XRD) and Field-emission environmental scanning electron microscopy (FE-SEM/EDS) techniques. The first crystal, which precipitates during heating from glass, is Ca 12 Al 14 O 33 , followed by CaO. For the first crystal, which precipitates as plate- like (two-dimensional), it was found, when using the Matusita-Sakka model, coherence between the calculated Avrami parameters and the images got from electron microscope. In the same way, coherence was found for the second event – CaO precipitation –, which growths two-dimensional or three-dimensional, depending on the CaO/Al 2 O 3 ratio. The most important event (in terms of energy liberated and amount of crystals) is Ca 12 Al 14 O 33 precipitation. For this event the effective activation energy for crystallization, E G , decreases with the increase of CaO/Al 2 O 3 ratio. The activation energy for crystallization reflects the energy barrier for crystallization. Thus, it can be concluded that mold fluxes crystallization is enhanced when increasing the CaO/Al 2 O 3 ratio.