A study on molten steel/slag/refractory reactions during ladle steel refining

by Jansson, Sune

During the production of steel the degree of oxide inclusions partly depends on the reaction of the melt with the furnace lining, the ladle lining and the pouring system. The refractory material may be eroded by the molten steel and slag as well as corroded through chemical reactions with the slag and molten steel and the deoxidation products. In this report the effects of revolution speed, temperature and steel composition on the rate of dissolution of MgO-C refractory samples into Al-deoxidised molten steel and CaO-Al2O3-SiO2-MgO slag were examined by the rotating cylinder method. The steel was deoxidised by adding metallic aluminium. Cylinders of MgO-C refractory material were immersed in Al-deoxidised molten steel in the range of 1600 to 1700°C and were rotated at 100 to 800 rpm during different times. The experimental results show that the rate of dissolution of MgO-C refractory materials increased with the temperature, rotating speed of the cylinder and immersion time. This supports the assumption that the diffusion of magnesium through the slag boundary layer formed around the refractory samples would be the rate-determining step. Mass transfer coefficients calculated on the basis of experimental results are in good agreement with earlier published results. The formation of a thin oxide layer at the interface is due the reaction between magnesium vapour and the CO generated by the reaction MgO and C in the refractory walls. The oxide inclusions formed in the steel have been shown mainly to consist of MgO, Al2O3 and a mixture of them. The rate of dissolution of solid MgO-C into liquid CaO-Al2O3-SiO2-MgO slag at different temperatures was studied under conditions of forced convection by rotating cylindrical refractory specimens in a stationary crucible containing the molten slag. The corrosion rate was calculated from the change in diameter of the cylindrical specimens. The specimens were rotated for 15 to 120 minutes at speed of 100 to 400 rpm in the molten slag. The rate of corrosion increased with temperature and with rotating speed of the rod and decreased when the slag was nearly saturated with MgO. The experimental results confirm the assumption that the diffusion of magnesium oxide through the slag phase boundary layer controls the corrosion process. The corrosion mechanism seems to be the dissolution of elements in the refractory materials into the slag, followed by penetration into the pores and grain boundaries. Finally, grains are loosened from the refractory into the slag. Key words: reoxidation, aluminium killed steel, inclusions, refractory, molten slag, corrosion rate, corrosion mechanism. 3 4