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Meniscus thermal analysis for detecting defects in continuous cast slabs Lai, Winky Wan Kei

Abstract

Transverse corner cracking was found to be a predominant problem in continuously cast slabs with peritectic carbon contents (0.09 - 0.16 wt % C). This thesis is based on an industrial plant trial carried out at Dofasco's No.l straight mould continuous caster for low carbon (0.04 - 0.06 wt % C) and peritectic grade (0.09 - 0.10 wt % C) steels. Embedded thermocouples around the mould periphery located 12, 100, and 250 mm below the meniscus were used to measure the thermal response of the strand during casting. The objective of this study is to establish a cross-correlation between the mould thermal response and the quality of the cast product. In the analysis, it was found that the A-series thermocouples which were especially instrumented for this plant trial, located just 12 mm below the meniscus, were the most sensitive to thermal events at the meniscus. It was found that the A-series thermocouples around the mould perimeter were capable of monitoring the metal level activity across the mould, but the magnitude of the signals were too attenuated for use as a tool in metal level control. Transverse corner cracks were only found on the peritectic grade slabs and these cracks tended to form more frequently around transient events such as width or SEN changes, although they also formed during periods of steady-state. It was found that the effect of thermal deviation at the edges of a broad face has a significant impact on the quality of peritectic grade slabs. Transverse corner cracks were found on peritectic grade slabs that were cast with a temperature deviation at the edges of the broad face exceeding 30°C. Corner cracking is related to differences in mould friction at the corners resulting from differences in heat transfer and mould flux viscosity. From the inspected samples, the slab corner with the lowest heat transfer was always found to be free of cracks. It was postulated that the reduced heat transfer at the colder mould corner has increased friction and the longitudinal stress from strand withdrawal was re-directed to the three corners with a lower friction.

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