Numerical Simulation on Inclusion and Bubble Entrapment in Solidified Shell in Model Experiment and in Mold of Continuous Caster with DC Magnetic Field

Abstract

A model experiment investigating entrapment of inclusions and bubbles on the solidified shell was performed using molten steel, and the conditions for inclusion and bubble entrapment and mechanism of entrapment were studied. The results were applied to the flow behavior in the casting mold of a continuous caster.1. At the solid–liquid interface, entrapment of inclusions is greatly reduced by the existence of a low velocity flow, e.g., 0.05 m/s.2. The above-mentioned interfacial flow velocity dependency of inclusion entrapment is considered to be largely influenced by changes in the thickness of the concentration boundary layer, which depend on the interfacial flow velocity. Specifically, bubbles and inclusions which enter the concentration boundary layer are drawn to the solid–liquid interface by a suction force which is several orders larger than the Saffman's force.3. In addition to the above-mentioned suction force, the so-called cleaning effect is determined by fluid-dynamic forces such as drag force, etc. which act on particles, and furthermore, by resident time of particles at the solid–liquid interface, which depends on the solidification rate.4. In a FC mold with a 2-stage electromagnetic brake, flotation of bubbles entrained in the jet flow from the nozzle is accelerated with the large DC magnetic field. This is attributed to the braking effect of the DC field on the nozzle jet and the upward flow by the buoyancy of the bubbles. As a result, the interfacial flow velocity can be normalized by increasing the strength of the magnetic field, and entrapment of large bubbles and inclusions can be reduced.