Experimental Simulating For Energy Shift And Transfer In Mushy Zone Of Fast Solidification Roll-casting

Continuous roll-casting has become a major technology to produce metal planks and strips for its characters such as short process, high productivity and good product qualities. The fast-solidification roll-casting for thin-gauge strip (thickness gauge is thinner than 2mm, roll-casting speed is beyond 10m/min) markedly advanced over the conventional roll-casting. This paper is funded by one sub-project of the State Key Project of Foundmental Research(Grant No. 1999064906): The Multi-coupling Mechanisms of Energy Shift and Microstructure Formation in Transient Solidification and Continuously Deep Deformation Process.Generally the roll-casting zone was considered as an integral zone when the metal's behaviors in the roll-casting zone were studied. The EHTC (Interfacial Heat Transfer Coefficient) was regarded as a constant or a linear function of the temperature or the press when the boundary condition in the mushy zone were dealed with, but the influences of the surface figuration of the rollers, the thermophysical properties of the metal of the rollers and the medium between the rollers on heat transfer at the interface and final solidification microstructure were neglected. Researchers put emphasis on classification of the roll-casting microstructure and process analysis of defect formation about the research on microstructure and properties of roll-casting product.In this paper, the mushy zone is separated from the roll-casting zone. The process of metal's solidification in the mushy zone is abstracted into a unidirectional solidification with high temperature gradient, and the heat transfer and microstructure formation are studied. The influences of the external fields (material and surface roughness of the chill, the superheat and medium at the interface) on heat transfer during metal (aluminum)'s solidification and microstructure are studied by the instruments designed by myself. The adaptive inverse heat conduction method with automatic control is introduced to solve IHCP (Inverse Heat Conduction Problem) When the temperature at the interface is calculated, which the basic algorithm and program are made. From the experiments and calculation, it is obvious that under the condition of unidirectional solidification with high temperature gradient, the interfacial heat flux and IHTC do not linearly change and the SDAS(secondary dendrite arm spacing) increases with distance from the interface and decreases withIHTC. Heat extraction increases with increasing surface smoothness of the chill, resulting hi a decrease hi SDAS. It is observed that with a smoother chill surface (roughness <3.25um ), the solidified shell is rougher than the chill. On the other hand, as the chill surface roughened (>3.25um), the surface of the solidified shell is smoother than that of the chill. The IHTC is greater with higher superheat or greater thermal diffusivity of the chill. The interfacial heat flux and IHTC decreases when graphite is added at the interface, and the solidification microstructure becomes rougher. Under the same or similar process condition, comparisons among microstructure and macro-properties of general hot-rolling, conventional twin-roll casting and thin-gauge continuous roll-casting are carried out, the difference is tried to explain from the view of temperature distribution.