In the present work, a mathematical model was developed to understand the multiphase flow behavior in a Rheinsahl-Heraeus (RH) reactor by using the Euler-Euler approach, and the effects of initial bubble diameter, nonequilibrium expansion of bubble caused by sudden thermal effect and sharp pressure drop, and various interphase forces were considered and clarified. The simulation results of mixing time, liquid circulation rate, and local liquid velocity in RH agree well with the measured results. The result indicates that the initial bubble diameter has a weak impact on the multiphase flow but that the bubble expansion has a tremendous impact on it for an actual RH. Meanwhile, the drag force and turbulent dispersion force strongly influence the multiphase flow, whereas the lift force and virtual mass force only have negligible influence on it. Furthermore, the turbulent dispersion force should be responsible for reasonable prediction of multiphase flow behavior in the RH reactor.

Nozzle blockage is a major problem during continuous casting of Al-containing steel. Herein, we analyzed the thermodynamic equilibrium behavior between aluminum and oxygen in steel at 1873 K (1600°C) and demonstrated that, the dissolved [O] initially decreases with increasing the dissolved [Al] until approximately 0.1 wt pct [Al], and after that, the dissolved [O] increases with dissolved [Al]. Thus, for high-aluminum steel with 1.0 wt pct dissolved [Al], the precipitation of Al 2 O 3 inclusion can be avoided during cooling from deoxidation temperature to the liquidus temperature, if the actual dissolved [O] can be kept from increasing when the dissolved [Al] further increases from 0.1 to 1.0 wt pct. Hence, a method of inclusion control for highaluminum steel without traditional Ca treatment technology was proposed based on the thermodynamic analysis. Industrial tests confirmed that low-melting point Ca-aluminate inclusions were observed typically through a slag washing with SiO 2-minimized high-basicity slag during tapping, accompanied by two-step Al-adding process for production of high-aluminum steel. Moreover, there was no nozzle clogging occurred for five heats of continuous casting.

Controlling the morphology of the sulfide inclusion is of vital importance in enhancing the properties of structural steel. Long strip-shaped sulfides in hot-rolled steel can spherize when, instead of the inclusion of pure single-phase MnS, the guest is a complex sulfide, such as an oxide-sulfide duplex and a solid-solution sulfide particle. In this study, the inclusions in a commercial rolled structural steel were investigated. Spherical and elongated oxide-sulfide duplex as well as single-phase (Mn,Ca)S solid solution inclusions were observed in the steel. A thermodynamic equilibrium between the oxide and sulfide inclusions was proposed to understand the oxide-sulfide duplex inclusion formation. Based on the equilibrium solidification principle, thermodynamic discussions on inclusion precipitation during the solidification process were performed for both general and resulfurized structural steel. The predicted results of the present study agreed well with the experimental ones.

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To study the effect of refining slag on the compositions of molten steel and inclusions, the reaction between ship plate steel and different slag systems (slag A-CaO/Al 2 O 3 : 1.0, SiO 2 : 5 mass-% and slag B-CaO/Al 2 O 3 : 1.5, SiO 2 : 9 mass-%) was investigated by laboratory-scale high-temperature equilibrium experiments. Results showed that the desulphurisation capacity of the two slags was very similar, and the average sulphur content for both was 0.002 mass-% in steel, but the deoxidation capacity of slag B was slightly higher than slag A. The amount of inclusions <5 μm was more in steel balanced with slag B than in that balanced with slag A. To generate inclusions smaller than 5 μm, and spherical liquid CaO-MgO-Al 2 O 3 -SiO 2 inclusions, and to decrease the T[O] (total oxygen) content in steel, refining slag composition should be: CaO/Al 2 O 3 ∼ 1.5, and SiO 2 ∼ 9 mass-%. Slag system optimisation can reduce or even eliminate calcium treatment during production. T[O] content in the slab could be controlled to 15-21 ppm, with typical microinclusions of ≤10 μm, and CaO-Al 2 O 3 -MgO and CaO-Al 2 O 3 -CaS systems in molten steel.