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

Abstract

Ar injection into the nozzle is widely employed to reduce the clogging of the submerged entry nozzle (SEN) in the continuous casting of steel. However, some of these bubbles are attracted to the solidification front in the mold. Finally, they are entrapped at the solidifying shell. These entrapped bubbles become the major cause of severe defects of the final products.The objective of this study is to clarify the mechanism which governs the behavior of bubbles at the solid/liquid interface of steel where both the concentration and temperature gradients exist. The effect of the solutal and thermal marangoni forces to the entrapment of bubble were compared and discussed by the numerical analysis, the water model experiment and the industrial plant trials in the present study.First, when a bubble approaches to the solid/liquid interface of steel, it initially encounters the thermal boundary layer, which is about 10~100 times 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 or pulls it toward the solidification front depending on the sulfur concentration. Below a critical concentration of the sulfur(47~60 ppm) in the liquid steel, the bubble is pushed away from the solidification front, and vice versa. 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.Second, the marangoni force can be thought as a product of the effective area A of a bubble and the surface tension gradient K. K is the sum of thermal and solutal terms, KT and KC. Although KT is about 104 times smaller than KC, the thermal effective area AT is about 3000~4000 times larger than the solutal effective area AC and it reaches even more than 104 times when bubble diameter is 2 mm. It means that the larger size of bubbles is, the more effect of the thermal marangoni becomes. Therefore, the order of magnitude analysis in the present study shows that the thermal marangoni force not only exerts to the bubble before it enters into the solutal boundary layer, but also has a significant contribution to the total force acting on the bubble compared with the solutal marangoni force for a large size of bubble.The CFD simulation is conducted by using commercial software FLUENT® to validate the effect of the thermal marangoni force on the behavior of bubbles in the liquid Fe-S binary system. When a bubble is exposed to the temperature field with different sulfur concentrations, the marangoni flow is developed toward opposite direction on the surface of a bubble depending on the sulfur concentration. Results of the CFD simulation for the single bubble correspond to results obtained from analytical equations developed in the present study.Numerical 2-D simulation of the gas bubble behavior during the steel solidification is carried out by using the developed analytical equations of the thermal marangoni force. As discussed above about the entrapment sequences, 2-D simulations including solidification of steel show that the entrapment behavior of the bubble is mostly determined by the thermal boundary layer and so-generated thermal marangoni force.The water model experiment was also conducted in order to validate the effect of the thermal marangoni force to the behavior of bubbles in the continuous casting mold. Bubbles were introduced into the horizontal channel, and the movement was observed for different temperature gradients. The movement of bubbles in the thermal boundary layer was affected by the thermal marangoni force, which changes the tangential angle of bubble pathline in the thermal boundary layer. The 3-dimensional CFD simulation of the continuous casting in the actual plant is conducted by using FLUENT to predict bubble related defects in the mold. Comparing with the solutal marangoni model, the thermal marangoni model gives number and distribution changes as sulfur concentration increases from 30 ppm to 150 ppm. In addition, the more superheat gives the more number of the entrapment as the thermal marangoni model predicted when the sulfur concentration is more than a critical point. The results show that the thermal marangoni force contributes significantly on the entrapment behavior of bubbles, contrary to the conclusions made in previous investigations. The sampling of plant trials were conducted in POSCO Gwangyang works in order to validate the above 3-D analysis. All of the above analyses and predictions were in good agreement with data obtained from plant trials. Therefore, the thermal marangoni model shows reasonable pattern and tendency of the bubble entrapment.The S-value is suggested from the thermal marangoni model as: The suggested S-value is compared with Mukai-value which is based on the solutal marangoni force. S-value shows great agreement with plant trials and POCAST data as well while Mukai-value does not. Therefore S-value can be a good indicator of bubble–related surface defect for practical usage of the steel industry.