Heat-transfer conditions in hot-top mould and cold-shut formation mechanism on DC continuously cast non-ferrous alloy rods

Abstract

The existence and severity ql swlace defects such as cold shuts on direct-chill continuously cast products depend to a large extent on the heat-transfer conditions in the mould. An examination of the heat-transfer conditions at the mould/insert junction of a hot-top continuous-casting mould was performed as part of a study of the cold-shut formation mechanism on horizontal continuously cast 20 mm diameter lead, tin, and zinc alloy rods. It was found that there are two stages in the formation of a cold shut: intellace-controlled solidification, followed by diffusion-controlled solidification. The effective heat-transfer coefficient at the mould/insert junction lras found to be comparable to the very large values found during splat cooling. Using a steady-state numerical model of the solidification process, the concept of a critical casting speed for cold-shut formation was developed. If the actual casting speed is greater than this critical speed, cold shuts will not be produced. Alternatively, the critical casting speed can be lowered, and cold shuts eliminated, if the mould temperature is increased, the eflective heat-transfer coefficient in the mould is reduced, or the liquid metal superheat is substantially increased.