A Study on the Influence of Steel, Slag or Gas on Refractory Reactions

University dissertation from Stockholm : KTH

Abstract: During the production of steel the oxide inclusion content 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 commercial MgO-C refractory samples into Al-deoxidised molten steel and CaOAl2O3- SiO2-MgO slag were examined by the rotating cylinder method. The study also includes tests with slag were doloma refractory samples are examined by the same method.Cylinders of MgO-C refractory material were immersed in to steel that was deoxidised by adding metallic aluminium. This was carried out in the temperature range of 1873 to 1973C° and at rotational speeds of 100 to 800 rpm for different holding times. The experimental results show that the rate of dissolution of MgO-C refractory materials increased with the temperature, rotational speed 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 for pure ceramics.A formation of a thin oxide layer at the interface was found. It 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 to mainly 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 similar to the MgO-C refractory/steel experiments. The corrosion rate was calculated from the change in diameter of the cylindrical specimens. The specimens were rotated for 15 to 120 minutes at speeds 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.The investigation of doloma and doloma-carbon showed that the dissolution of magnesia into the slag was determining the corrosion rate. As for the other experiments, steel/MgO-C refractory and slag/MgO-C refractory, the corrosion rate was calculated from the change in diameter of the cylindrical specimens. The specimens were rotated for 15 to 120 minutes at speeds of 100 to 400 rpm in the molten slag. The results from the study showed that refractory materials that were impregnated with carbon had a much better slag resistance than the refractory that contained no carbon. This is due to the higher wetting angle between carbon and slag.Corrosion of MgO-C refractories in different gas atmospheres consisting of air, Ar, CO or Ar/CO was also studied. Experiments were carried out in the temperature range 1173 K to 1773 K and for holding times between 2 to 120 min. The reaction rate of the MgO-C material was determined from measurements of the weight loss of the samples. The results showed that the refractory weight loss increased with an increased temperature or an increased holding time. The thermodynamic conditions and the experimental results show that magnesium gas and carbon monoxide gas should form during ladle refining of steel when the refractory material consists of MgO-C.