Modeling Study of Intermixing in Tundish and Strand during a Continuous-casting Grade Transition

A 1-D, transient mathematical model (MIX1D) has been developed to simulate the final composition distributions produced within continuous-cast slabs and blooms during an arbitrary grade transition. This work applies the model to investigate the effect of process variables on the extent of intermixing during a typical ladle change operation, where mixing occurs in both the tundish and the liquid core of the strand, prior to final solidification into slabs or blooms. The model was developed, calibrated, and verified with composition measurements on tundish outflow and at the surface and centerlines of slabs and blooms cast during grade transitions. Several different strategies were investigated to minimize intermixing. Results show that the amount of intermixed steel generally decreases with lower tundish weight at ladle open, increased holding time before refilling, increased plug flow through the tundish, decreased mold thickness, decreased mold width, casting the most stringent grade first, and using lower average casting speed during the grade change. However, the optimal philosophy depends greatly on the size of the tundish and the extent of the mix (which includes combinations ranging from lenient to stringent for either the old or new grades cast). Using the results of this work, grade changes can be customized for a given casting operation to minimize the amount of downgraded steel.