Growth and removal of inclusions during ladle refining

Abstract: The overall purpose of this thesis work has been to further our understanding of the growth and removal of inclusions in gas- and induction-stirred ladles. The primary focus has been on alumina inclusions.Growth mechanisms were studied using data from fundamental mathematical models of gas- and induction-stirred ladles. The results showed the turbulence mechanism to be the most dominant in alumina inclusion growth. The dynamic growth and removal of inclusions in a gas-stirred ladle was studied using mathematical modelling. The model results showed concentration gradients of inclusions. The effect was most obvious in the steel flow past the removal sites: top slag, ladle refractory, and gas plume (bubble flotation). A new removal model was developed for large spherical caps bubbles.In order to verify the predicted concentration gradients for the size population of inclusions, three experiments were carried out in production. The sampling equipment enabled sampling at five different positions and different locations at the same time. The results showed that concentration gradients of inclusions do exist both in induction-stirred and gas-stirred ladles. A theoretical analysis showed that the drag force on the inclusions to be the dominating force and that therefore inclusions follow the fluid flow.The cluster behaviour of alumina inclusions were examined on steel samples taken in an industrial-scale deoxidation experiment in a ladle. The samples were examined by microscope and the results used to study cluster growth. It was found that there was rapid cluster growth due to collision during stirring and that at the end of the deoxidation experiment a majority of the small inclusions were bound in clusters. The cluster growth data determined using the microscopic results were compared with predicted cluster-growth data. A method was developed for converting the experimental data observed per unit area into data given per unit volume and vice versa. An expression for the collision diameter of the cluster was also developed. The results showed that the predicted cluster growth agreed well with the microscopic observations for the assumptions made in the growth model.