A kinetic model to predict chemical composition changes in molten steel, slag, and inclusions in ladle refining was developed and used to elucidate the mechanism underlying the change in the chemical composition of the inclusions. The coupled reaction model was applied to estimate the reaction between molten steel/slag and molten steel/inclusion originating from the slag. The thermodynamic calculation software, FactSage6.3, was employed to obtain the activity of each component in the slag phase. Empirical equations were applied to the reaction between the slag and the refractory. The resulting model can calculate changes in (1) the composition of each element in the molten steel, slag, and the inclusion originating from the slag, (2) the amount of inclusion originating from the slag and the deoxidation products, and (3) the ratio of the inclusion originating from the slag and the deoxidation products to the total inclusion. The calculated results were found to agree with the operational results of a 165 t ladle refining process reported in the literature. The deoxidation products altered from alumina to a MgO·Al2O3 spinel-type inclusion due to an increase in the Mg content of steel. In the average composition changes of each element in the total inclusions, calculated results for the MgO and Al2O3 contents were also found to agree with the operational results.
The formation of MgO·Al2O3 spinel-type inclusions has often been reported even when Mg is not added during treatment. Many researchers have investigated the dissolution behavior of Mg from slag; however, studies on the reaction between molten steel and MgO-type refractory are limited. In this paper, the inclusion composition of Al-deoxidized steel melted in a MgO crucible, and mass transfer rates of Mg from a MgO rod and MgO in slag to Al-deoxidized molten steel were investigated. These studies clarified that Mg dissolved in the molten steel and the spinel formed not only with the reaction of molten steel and slag but also with the reaction of molten steel and MgO crucible. The dissolution rate of Mg from MgO rod increased as the rotation rate and Al content in steel increased. The MgO·Al2O3 spinel layer formed at the interface between the metal and MgO rod. The Mg content was higher for the reaction between molten steel and MgO in slag compared to the reaction between molten steel and MgO rods, as it equilibrated with MgO activity in slag.
A kinetic model to simulate the reactions in a ladle furnace was developed in the previous paper. The following parameters were considered in this model; (1) ratio of the entrapment of slag in the molten steel, (2) ratio of the floatation of the deoxidation products and inclusions originating from the slag, (3) ratio of the agglomeration of deoxidation products with inclusions originating from the slag and (4) ratio of the volume of the bulk zone to the total volume of molten steel and that of slag phase. These parameters were optimized using sensitivity calculation by comparison with operational results as the parameters affected the amount and composition of inclusions.Then, the method to suppress the formation of MgO·Al2O3 spinel-type inclusion was discussed using the optimized parameters. The calculated results showed that the formation of MgO·Al2O3 spinel-type inclusion could be suppressed by optimizing the additional amount of Al, initial content of MgO in the slag, and slag basicity in addition to the Ca treatment. The changes in the inclusions calculated using the kinetic model were in good agreement with those predicted by the phase stability diagram. The developed model was useful for optimizing the operation of a ladle furnace.
It is well known that the composition of inclusions is determined by alloying elements and by reaction with slag. For example, MgO·Al2O3 spinel-type inclusions form, even though Mg is not added, due to the supply of Mg through the reaction between slag and metal. To clarify the mechanism of compositional changes in inclusions, the authors have developed a kinetic model to simulate the reactions during the ladle refining process. In this study, experiments were conducted using an induction furnace, and the compositional changes in molten steel, slag, and inclusions were investigated. The inclusions were analyzed by P-SEM, which incorporates an automatic analysis system. By the application of the developed simulation model to these experiments, the validity of the model was evaluated. The inclusion composition gradually changed from Al2O3 to MgO·Al2O3 after the addition of Al, and the inclusions originating from slag were also observed at all times. The compositional change of the deoxidation product by the model calculation corresponded well to the observed variation in the composition of inclusions, and the calculated composition of inclusions originating from slag also agreed with the experimental results. The rate of compositional change increased with increasing Ar gas flow rate, and this tendency was captured well by the model. Therefore, the validity of the developed model is considered to be confirmed.
The effect of Al addition in the middle of the metal-slag reaction on the formation of MgO•Al 2 O 3 spinel-type inclusions was investigated by laboratory-scale experiments and a kinetic model calculation in order to reduce the spinel-type inclusions in high-carbon steel. As results of the experiments, the total Mg content in the steel and average content of MgO in the inclusions were relatively low before Al addition, and spinel-type inclusions were hardly formed. After Al addition, spinel-type inclusions formed when CaO/SiO 2 and CaO/Al 2 O 3 in the slag were high, and the total Mg content in the steel and average MgO content in the inclusions were also higher. On the other hand, formation of spinel-type inclusions was suppressed at lower CaO/SiO 2 and CaO/Al 2 O 3 in the slag. Therefore, the experimental results indicated that addition of Al at the midpoint in the reaction and control of the slag composition were effective for suppression of spinel-type inclusions. However, spinel-type inclusions formed soon after Al addition in slag with higher CaO/SiO 2 and higher CaO/Al 2 O 3. To evaluate the effect of midpoint addition of Al on the actual process, a kinetic model calculation under virtual conditions was carried out. According to the calculation, the increase in the content of Mg in the steel under actual-scale conditions was slower than that in the laboratory, and formation of spinel-type inclusions could be avoided.
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