Abstract

The horizontal heat extraction and lubrication are considered key factors in the continuous casting of the steel in order to avoid product defects and operation problems. Synthetic slags named mould fluxes are used to achieve good lubrication and controlled heat transfer. Commercial mould fluxes are constituted by several oxides, fluorine compounds and carbonaceous materials. The fluorine compounds control physical properties and have relevant influence on the crystallization of the slag during casting operation. Nevertheless, fluorine emissions during operation could cause health problems and equipment corrosion. For these reasons, nowadays fluorine free mould fluxes are developed.
In order to compare the crystallization process of mould fluxes (with and without fluorine compounds) samples of both types of fluxes were melted, quenched and heat treated at 600ºC, 700ºC and 850 ºC. Samples were identified as: A (with 10.4 % of F), B (with 6% B2O3) and BL (with 6% B2O3, 4% Li2O). It is relevant to mention that the proportion and type of crystalline phases affect the thermal conductivity of the mould fluxes during steel solidification. Also, the viscosity behaviour is affected by crystallization process. In this paper, a microscopy study was carried out applying light and scanning electron microscopy (SEM) on all the samples. The information was correlated with DTA and X ray diffraction (XRD) results obtained at different temperatures. The chemical composition of phases was determined by EDS (SEM). The information obtained allows us to identify the crystallization mechanism in all the samples and increase the knowledge on fluorine free mould flux behaviour in the continuous casting mould. In the specimens BL it was detected a liquid-liquid phase separation process prior to crystallization.