Numerical Analysis of Surface Tension Gradient Effect on the Behavior of Gas Bubbles at the Solid/Liquid Interface of Steel

The mechanism which governs the behavior of bubbles in the vicinity of the solidification front where both the concentration and temperature gradients exist was investigated and the effect of the solutal and thermal marangoni forces to the entrapment of bubble were compared and discussed. When a bubble approaches the solid/liquid interface, it first encounters the thermal boundary layer, which is much thicker than the concentration boundary layer, and experiences the thermal marangoni force. This force, which occurs due to the temperature gradient in the thermal boundary layer, pushes the bubble away from the solidification front if the sulfur concentration is lower than the critical value which was found to be around 47–60 ppm, but pulls it toward the solidification front if the sulfur concentration is higher than the critical value. Only if the bubble passes through the thermal boundary layer successfully, it then arrives at the concentration boundary layer where a strong solutal marangoni force comes into action on the bubble to pull it to the solid/liquid interface. The above analysis predicts that the bubble-induced surface defect of a cast be strongly affected by the sulfur concentration and there exists a critical sulfur concentration above which the surface defect becomes increasing. The above prediction was validated by the CFD (Computational Fluid Dynamics) simulation and the trial at the commercial casting plant, and they were in good agreement with each other.