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Investigation of transverse depression formation during the continuous casting of steel using real-time mould thermal data Adjei-Sarpong, Hanson Mark


With the need to achieve defect-free steel increasing, traditional methods of billet quality evaluation in the cold state are becoming inadequate as corrective actions can only be taken after the fact. Real-time monitoring of the billet casting process using signals from thermocouples embedded in the wall and other mechanical sensors has become attractive in predicting the occurrence of defects and controlling the appropriate operating parameters to minimize the generation of quality problems on-line. This study examined the formation of transverse depressions in the continuous casting of steel billets using real-time mould thermal data. A plant trial involving an instrumented mould was carried out at a mini mill designated as Company G. Signals from thermocouples installed in the mould wall, as well as from casting speed and metal level controllers, were sampled at 100 Hz during the casting of steel grades ranging from 0.12 to 0.83 %C with oil lubrication. Three billet samples corresponding to the beginning, middle and the end of each heat were collected and later subjected to metallographic analysis for cracks and solidification structures. Two different shapes of the depressions; "nose"- and smooth-type were identified during the billet samples analysis. Analysis of the mould thermal data revealed that a transverse depression is manifested as a drop in mould temperature as the depression passes by a given thermocouple position. The depressions were seen to initiate close to the meniscus, and travel down the mould length at the speed at which the strand is withdrawn. A rise in metal level was found to precede the formation of a depression. A mechanism has been proposed to explain these observations. Metal level fluctuations, oil flow rate, and casting speed (relative to section size), are critical parameters to the generation of the depressions. The mechanism of depression formation and other findings from this study conform to other related observations made in various steel plants. Other factors that were seen to influence the formation of the depressions include steel composition and mould taper.

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