UBC Theses and Dissertations
Solidification in the mold of a continuous billet caster Prasad, Bommaraju V. S. S. Rama
Billet samples obtained from a large number of heats were examined to study different aspects of solidification in the mould; columnar-equiaxed transition, segregation bands, off-corner cracks, rhomboidity, sub-surface structure and oscillation marks. With the aid of two mathematical models the influence of heat extraction on early solidification and billet quality has been elucidated. In the carbon range 0.13-0.36%, increasing carbon and phosphorous (0.020-0.060%) were seen to cause an early columnar-equiaxed transition . Higher heat transfer rates as carbon is increased from 0.13 to 0.36% were seen to promote the growth of equiaxed zone. A steep rise in the length of columnar zone in steels with carbon content more than 0.38% was observed for the first time in billet casting inspite of high heat transfer rates. Examination of macroetches revealed that there exist two distinct bands within 10-11. mm off the edge of the billets. These were found to delineate the shell profile at an instant of time. With the help of the one-dimensional model it was shown that the white band which is closer to the surface forms 450-550 mm below the meniscus where the solidifying shell is subjected to a constant cooling rate and the solidification front proceeds at a minimal speed. The shell thickness corresponding to the second band which was dark or deeply etched was found to have developed at the midface very close to the bottom of the mould. Based on the pool profile exhibited by these bands in the transverse sections it has been concluded that while solidification proceeds slowly at the midface after the formation of the white band 450-550 mm below the meniscus it appears to have virtually stopped at the off-corner/corner area. Thin white bands at the obtuse angle corners of the billets which are a result of improper heat extraction could lead to rhomboid billets in the sub-mould regions because of differential contraction of adjacent faces. Study of these bands has shown that in the majority of the billets heat transfer on the four faces is not identical indicating that the mould billet gap is different on the four sides. When one of the mould walls was constrained, rhomoidity was found to be minimum showing that wall movement gives rise to different cooling capacities of curved and straight walls. Off-corner cracking was seen to be aggravated by mould water velocities of 6.5-7.2 m/s and Mn/S ratios lower than 22-25. Absence of secondary cooling water was found to accentuate cracking at the off-corners. It was suggested that these cracks would form when the midface bulges in the lower parts of the mould or immediately after the billet's exit from the mould wherein according to the one-dimensional unsteady state model the surface was found to undergo extensive amount of reheating. A two-dimensional unsteady-state mathematical model was developed to study the possibility of meniscus solidification observed in the the structure around the oscillation marks on the billet surfaces. It was found that the heat flux necessary to allow growth of solid over the meniscus has to be much larger than those obtained from experiments conducted in a separate study. A new mechanism has been formulated to explain meniscus solidification and the formation of oscillation marks in billet casting. The negative taper in billet moulds constrained only at the top was thought to be the potential cause of these periodic depressions.
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