Formation and morphology of non-metallic inclusions in aluminium killed steels

(eng) Non-metallic inclusions are a major issue during the production of high performance steels, as they influence the microstructure and structural properties to a large extent. They are often considered as harmful to the final product quality and to the steel processing productivity, so that many industrial efforts are directed towards improving inclusion removal. Another, more positive approach is to use non-metallic inclusions to produce steels with enhanced or tailored properties. In both cases, the key issue is to control the characteristics of the inclusion population in the liquid steel, such as number, composition, morphology, size and spatial distribution. In this doctoral research, the influence of the main inclusion-related process parameters, like the dissolved aluminium (Al) and oxygen (O) content in liquid iron (Fe), on the formation and morphology of Al2O3 inclusions was studied. Deoxidation and reoxidation phenomena in liquid Fe were investigated during lab-scale experiments under rigorously controlled conditions. First of all, a new technique was developed to enable the observation of inclusions in the initial stage of the deoxidation process, by bringing a piece of Al in contact at 1600°C with liquid Fe containing different dissolved O levels. A significant increase in inclusion number and a change in inclusion morphology from angular to spherical was observed with increasing dissolved O content. The shape transition was explained by the evolution of the supersaturation and the availability of oxygen at the reaction front. Secondly, the inclusion formation and characteristics during reoxidation were investigated by exposing Fe-Al melts to different oxidizing atmospheres with controlled oxygen partial pressure (pO2). Dendrites, clusters, aggregates and bars were observed near the melt surface at high pO2, while compact and faceted shapes were found at low pO2, i.e. under lower supersaturation conditions. Finally, to assist in the interpretation of the experimental results on the inclusion characteristics, a coupled diffusion/reaction model was formulated to estimate the spatial and time dependent Al and O concentration profiles as a function of selected process parameters.