INVESTIGATION ON GRADE INTERMIXING IN TUNDISH DURING LADLE CHANGE OVER

Abstract

Continuous casting set up consists of three components namely ladle, tundish and mold. The ladle receives the molten material from furnace and delivers it into the tundish from where it is fed into the water cooled copper mold. The fluid flow pattern inside the tundish directly affects the performance of the tundish in terms of metallurgical phenomena like mixing, grade change and inclusion removal processes inside the tundish because it is the last reservoir before the metal entering into the mold and solidified in the form of slabs/billets. Tundishes, in terms of their shape and use of flow modifiers (dams, turbo stop etc.), are designed to provide optimum flow characteristics. One of the few important objectives with which a tundish is effectively utilized is minimizing the formation of intermixed grade steels during ladle change-over in continuous casting process. When the old ladle (representing old grade of steel) is emptied, a new ladle (representing new grade of steel) replaces the old one resulting into continuous supply of molten steel to the mould. The intermixed grade steel is produced due to the mixing of new grade steel with the old grade steel. The chemical composition of intermixed grade steel lies between the composition of old and new grade steel. Thus, the intermixed grade steel have less demand in market and usually is considered as scrap material. Since the ladle change-over method results into formation of large intermixed amount which is very difficult to be avoided completely, efforts can be made to minimize it. Grade intermixing phenomenon in tundish is largely affected by operating variables namely, inflow rate in to tundish, outflow rate and residual volume of steel in the tundish. A comprehensive analysis of mixing behavior of two grades will be useful study for steelmakers to optimize or minimize the intermixed grade steel. Most of the previous numerical investigations have assumed steady state operating bath level of molten metal in tundish during physical and numerical investigations. However, bath height of molten steel varies in tundish during the ladle change-over operation. The change in bath height of molten steel in tundish has a significant impact on intermixing phenomenon inside the tundish. In addition to this, one of the major challenges that remain in continuous casting process is the accurate prediction of surface separating immiscible fluids, like steel-slag or slag-air. These must be dealt with available computational technologies. Numerical simulations can predict the interface accurately by studying the process and the parameters affecting it, since actual experiment is extremely difficult to be carried out. In the present study, experimental investigation to predict intermixing was conducted on a 1/4th scale model billet caster tundish fabricated with Perspex sheet using water as the working fluid. The results from numerical investigations were validated against the v experimental results obtained from aqueous model. In validation study, the concentration of fluid was measured at the outlets of tundish. The concentration of exiting fluid varied due to the mixing of new grade into the old grade steel. It was found that the F-curves obtained from numerical investigation reasonably matched with the experimental results. Transient, 3-dimensional, multiphase, isothermal analysis using numerical investigation has been carried out in conjunction with Modified-HRIC interface interpolation scheme. The model predicts the effect of inflow rate, outflow rate and residual volume on the formation of intermixed grade steel during ladle change-over process. The investigations were carried out on bare tundish as well as on tundish with advance pouring box (APB). It was observed that increase in inflow rate caused decrement in intermixing time for all cases in bare tundish and tundish with APB. Intermixing time for all outlets of bare tundish has been observed to be nearly equal, but has been found to be different in case of tundish with APB at different inflow rates. Residual volume of old grade was found to have significant impact on the grade mixing in tundish. The intermixing time was found less for low residual volume of tundish. The effect of outflow rate has insignificant effect on mixing. Further, a detailed assessment of five different Reynolds-averaged Navier–Stokes (RANS) two equation turbulence models have been carried out. The turbulence models have been compared for approximation of intermixed amount as well as accurate prediction of bath height in the tundish during sequential castings. The phase volume fractions (air and steel) continuously change as the bath height of tundish changes during teeming operation from ladle to tundish. Coupled level-set volume of fluid (CLSVOF), a combination of volume of fluid (VOF) method with Level-set (LS) method has been employed to predict the free surface level during the ladle change over process. Out of the five different turbulence models, RNG k-ε turbulence model has shown better approximation of F-curve and interface profile. SST k-ω model has predicted poorly among all the turbulence models. With an aim to minimize the intermixed amount, tundish shapes have been changed, which alters the fluid flow phenomenon inside the tundish and is expected to even affect the shear stress caused on the refractory lining of the tundish walls. In this perspective, numerical investigations have been carried out on three different shapes of tundish from fluid dynamic point of view to investigate the grade mixing and generation of tundish wall shear stress. It was observed that V-shape tundish performed better than boat & T-shape tundish. The formation of intermixed grade was highest in T-shape tundish followed by boat shape tundish. The wall vi shear stress values at the bottom as well as longitudinal walls of three different designs have been calculated, since it is reasonable to assume that erosion of the refractory lining, which occurs due to thermal, chemical and mechanical attack, can be well predicted by the wall shear stress values at the tundish walls. It has been seen that front wall of T-shape tundish develops more wall shear stress as compared to boat shape and V-shape tundishes. The front walls of boat shape and V-shape tundish are greatly affected up to mid length of tundish. Average wall shear stress values are found to be maximum for V-shape tundish as compared to other two configuration tundishes. Statistical modelling has also been carried out to optimize the placement of Flow Control Devices (FCDs) in order to minimize the grade mixing in tundish. The FCDs includes depth of ladle-shroud, APB wall inclination angle, height of the dam and position of dam in the tundish. Response surface methodology (RSM) was applied to analyse the grade mixing in tundish. The results have been predicted for each outlet and average output of results have been studied. It was seen that APB wall inclination angle has most significant impact on intermixed amount. The average intermix amount can be significantly minimized by using proper values of APB wall inclination angle, dam height and position. It was seen that shroud depth had no major impact on mixing. The outcome relations of various parameters were verified in order to test the accuracy of model.