Reoxidation of Al-Killed Ultra-Low C Steel by FetO in RH Slag: Experiment, Reaction Rate Model Development, and Mechanism Analysis

To elucidate the reoxidation mechanism of Al-killed ultra-low C steel by FetO-containing slag, the kinetics of a reaction 2Al + 3(FetO) = (Al2O3) + 3Fe between the steel and CaO-Al2O3-FetO-MgOsat, slag was investigated mostly at 1823 K. Al contents (total and soluble), and total 0 content in steel samples were measured during the reactions under various initial compositions of slag ((pct CaO)(0)/(pct Al2O3)(0), (FetO)(0)), and the reaction temperature. The experimental results were analyzed using the reaction rate model developed in the present study, which is based on probable rate controlling step and employing CALPHAD thermodynamics using FactSage thermochemical software and databases. When the (pct FetO)(0) was higher than 10, the rate model could explain the measured data with an assumption that the rate was solely controlled by mass transport of Al in the steel. However, mixed transport control theory should be used to interpret the reaction rate when the (pct FetO)(0) was lower than 10. Decreasing (pct FetO) during the reoxidation reaction changes the reaction mechanism in terms of the mode of rate-controlling step. The mass transport coefficient of Al in the steel (k(Al)(M) was 5 x 10(-4) m s(-1) at 1823 K (1550 degrees C), which is in favorable agreement with those in the literature. The mass transport coefficients of Al2O3 was formulated to depend on the viscosity of the slag. In the mixed transport control regime, the apparent mass transport coefficient gradually decreased due to the slow mass transport of Al2O3 as a resistance to the overall mass transport. This was also additionally supported by evaluating the activation energy of the apparent mass transport coefficient, which turned out to increase as the reoxidation reaction proceeds. Therefore, it can be concluded that the reaction mechanism gradually changes during the reoxidation reaction. (pct CaO)(0)/(pct Al2O3)(0) ratio affects the reoxidation rate only when (FetO)(0) was low.