INVESTIGATIONS ON INCLUSION TRANSPORT AND SOLIDIFICATION BEHAIVOUR IN CONTINUOUS CASTING PROCESS USING MULTIPHASE ANALYSIS

Abstract

In continuous casting process, the quality of casting depends on interrelated complex phenomena in tundish and mold. The presence of inclusions in large numbers can lead to excessive casting repair or rejection of casting. Tundish, also serving as a metallurgical reactor, removes inclusion by altering the liquid steel flow. Combinations of different mechanisms such as buoyancy rising, flow modifiers, turbulent inhibitor box(TIB), gas bubbling etc., are employed to enhance the inclusion removal in tundish. Inclusions that are not removed in tundish, are likely to come in mold. Slag entrainment and inclusions generation due to SEN erosion are other potential sources of impurities that may harm the casting quality. Extensive research has been carried out to understand the interrelated multiphase phenomena in the continuous casting process with an aim to enhance inclusion removal. In most of the tundish inclusion studies, ladle originated inclusions are considered. However, erosion of refractory lining due to flow induced wall shear stress is one of the severe problems that shop floor personnel face in tundish operations. It decreases the lining life and increases overall operational cost. Natural convection due to thermal buoyancy directly influences flow characteristics and inclusion transport. With gas bubbling, the removal rate of small size inclusion can be enhanced. Use of in-mold electromagnetic stirrer (M-EMS) in continuous casting mold has been an effective means to get clean steel. However, sometimes their application results in slag entrainment and inclusion particles from the meniscus due to its position. More importantly, the presence of slag layer at the top of the meniscus and the inclusion particles coming through the tundish outlet pose a great challenge as they are likely to get trapped in the swirl flow developed due to the stirrer. Study of inclusion particles in steel coming through the slag layer necessitates the analysis to be carried out in a multiphase environment. With the shortcomings mentioned above, this study has attempted to fill some of the voids in numerical multiphase modeling of the continuous casting process. Three-dimensional fluid flow study has been carried out to investigate the flow induced wall shear stress. High shear stress zones(HSSZ) are taken as potential inclusion generation sites. Different sizes of inclusions are injected from those sites and their paths are tracked. It is found that the shape of TIB significantly affects the flow induced wall shear stress and inclusion removal rate. Results indicate that tundish with TIB 3 arrangement (inward draft with closing) exhibited minimum wall shear stress at all walls. TIB 2 (Outward draft) coupled tundish gives the highest removal rate in the case of bottom wall originated small size inclusions. Further, gas bubbling curtain modeling using the Euler-Euler approach is performed for the different locations on the bottom wall of the tundish and quantitative analysis of tundish performance is presented using residence time distribution (RTD) curves. The results show that big circulation loop generated due to thermal buoyancy assists in inclusion removal and mixing at each outlet. Gas bubbling increases the molten metal flow velocity in the central region of tundish, leading to a decrease in the dimensionless number Gr/Re2 near the outlets. Hence, the dominance of natural convection decreases. A significant increase in the inclusion removal rate is observed. However, the reported inclusion removal rate in gas bubbling cases is found to be independent of particle size and curtain location. A multiphase model involving solidification, slag-metal interface and inclusion transport is developed for billet caster mold in the continuous casting process. A novel approach based on the volume fraction criterion is used with the help of user defined function to study the inclusion removal during the continuous casting steelmaking operation in the mold. The effect of shifting EMS down the mold on inclusion entrapment and interface fluctuation has been studied. The numerical results reveal that both intensity and size of the recirculation loop decrease on shifting the EMS away from the meniscus. While a tangential velocity increases significantly in the presence of EMS. Stirring generated due to EMS decreases the temperature and increases the uniformity in liquid fraction in the region above the stirrer. Interface fluctuation decrease as the distance between meniscus and EMS center increases. The effect of EMS on incoming inclusion from SEN is more pronounced as EMS shifts downward. Stirring pulls down most of the interface originated inclusion particles of size 2 μm and a few 100 μm in the mold, which is otherwise trapped back in slag layer when EMS is off. This effect diminishes with the downward movement of the stirrer. Moreover, two SEN designs that differ in port angles are considered for the study of flow pattern, solidification thickness, interface fluctuation and inclusion transport & removal during the process. The results show that the upper circulation loop formed in the 15° port angle case is responsible for higher inclusion removal, high interface level, and low solidification thickness in the meniscus region as compared to 30° case. Strong rotational flow coupled with oppositely directed swirl flow provides high tangential velocity near the solidification front and high axial velocity in the core region, resulting in increasing shell thickness fluctuation and decreasing the superheated steam penetration. Inclusion entrapment in solid shell increases for both 15° and 30° cases, however, inclusion removal decrease for 15° case and increase for 30° on applying the M-EMS.