Numerical Simulation of Metal Flow and Heat Transfer during Twin Roll Strip Casting

摘要

How to start up the casting operation is one of the serious problems encounted in twin-roll strip casting process. During the initial pouring stage, the metal pool should be progressively developed to reach the optimal pool level and the solidified metal layers formed on the surfaces of two opposite-rotating rolls should be welded together before the dummy sheet is withdrawn from the minimum roll gap. Improper operation with inaccurate withdrawal time often results in unstable process and products as well as the breakup of cast strip, which will be detrimental to the formability and subsequent coiling of cast strip. Therefore, the proper withdrawal time of dummy sheet plays a very important role in the initial stage of a vertical twin-roll casting process. The accurate withdrawal time is, however, very sensitive to the variations of thermal distribution, flow conditions and metal-roll interactions. In order to elucidate the fundamental transport phenomena in twin-roll casting, a commerical software called ProCAST was employed to simulate the transient fluid flow, heat transfer and solidification behaviors during the early stage of the process in this study. The coupled set of governing differential equations for mass, momentum and energy blance were solved with the finite element method and the transient free surface problem was treated with a Volume of Fluids (VOF) approach. An enthalpy method was employed to handle the phase change during solidification. The advantage of this model is the great capability to treat the problems of moving boundary and free surface of fluid. With this mathematical model, the filling sequences, flow patterns and corresponding temperature profiles in the metal pool under an actual casting condition of a vertical twin-roll strip casting process were simulated in this study. From the calculated results, how the metal pool progressively develops between the rolls and how the solidified layer grows in thickness with time on the rotating rolls are possibly revealed. Moreover, the metal pool level, the solidification front as well as the position of solidification end could be obtained. With these available informations, the optimal withdrawal time of the strip casting process can be thoughtfully determined.