INTERMIXING MODEL OF CONTINUOUS-CASTING DURING A GRADE TRANSITION

To investigate the composition distribution that develops in continuou sly cast steel during a grade change, an efficient, accurate, and user -friendly computational model has been developed. The model is fully t ransient and consists of three submodels, which account for mixing in the tundish, mixing in the liquid core of the strand, and solidificati on. The first submodel of mixing in the tundish consists of two plug f low zones, two back-mixing boxes, and two dead volumes. The second sub model solves a one-dimensional (1-D) diffusion equation in series with two back-mixing boxes to calculate concentration histories in the str and, and the third submodel transforms these histories into slab compo sitions. The model was calibrated using both concentration histories m easured on tundish water models and calculations from a three-dimensio nal (3-D) model. It was then verified with several sets of composition measurements along the surface and centerlines of slabs. The model is capable of tracking mixing phenomena for arbitrary tundish filling an d casting speed histories. It has been used to compare the effects of different grade change procedures on the amount of intermixed steel, i ncluding standard sequence casting, flying tundish change, and inserti on of grade separators. Mixing in the strand was found to be very impo rtant. Without a grade separator, a flying tundish change had very lit tle benefit on reduced intermixing, for the typical conditions conside red.