MOLD HEAT-TRANSFER AND SOLIDIFICATION DURING HORIZONTAL CONTINUOUS-CASTING

The mould of the pilot horizontal caster at Chaparral Steel (Midlothian, TX, USA) has been instrumented with thermocouples in order to determine axial profiles of copper temperature and, via a mathematical model, steel-to-mould heat flux. In addition, billet samples were cut during the trial for metallographic examination; and the water velocity in the mould cooling channel was measured. Thus the peak flux close to the breakring was determined to be 17 MW m-2, well above values of 3-5 MW m-2 that have been measured on conventional, oil lubricated billet moulds. The high heat flux can be attributed to the lack of lubricant intervening between the steel and mould wall and not to the ferrostatic head of molten steel in the tundish as originally supposed. The axial heat flux profile has been used as input to a mathematical model of solidification adjacent to the breakring to predict witness mark formation quantitatively. The model, which confirms the mechanism of witness mark formation proposed earlier in the literature, was validated by comparing predictions of primary witness mark depth to measurements made on the billet samples cast with varying superheat, stroke frequency, and casting speed. It has been shown that witness mark depth decreases significantly with increasing stroke frequency particularly in the range 0.8-3.3 Hz. Increasing superheat of the liquid steel also decreases witness mark depth although this influence is smaller than that of stroke frequency. Finally, the model has revealed that design changes to the mould that effectively reduce the peak heat flux have the potential of significantly decreasing primary witness mark depth.