Mould response and its impact on billet quality

In the past three decades, continuous casting has emerged as a dominant steel production technology. Global competition and customer expectations are driving the mini-mills to improve billet quality and increase productivity. At the core of billet casting technology is the water-cooled, oscillating copper mould. Mould interaction with the billet, both thermal and mechanical, governs billet quality and productivity. The heat extraction capability of the billet mould has been well addressed in the literature, but the mechanical response of the mould, also fundamental to the process, remains less studied. Quality issues relating to the casting operation include cracks, shape defects and breakouts. Excessive mould-billet friction can certainly contribute to these defects, in addition to restricting caster productivity. Further, wobbly mould oscillation is believed to contribute to cracks and off-squareness. It is remarkable to note that even though controlling friction is a necessity to the continuous casting process, few attempts have been made to monitor it. The main objectives of this study were: to quantify the mechanical response of the mould with force and kinematic sensors; to evaluate mould-billet binding using mathematical models; and to provide practical recommendations for on-line monitoring. A series of five industrial plant trials were conducted using instrumented moulds at two Canadian mini-mills. In addition to logging mould temperature, casting speed and metal level, new sensors were installed and tested to measure mould oscillation and mould-strand friction. Billet samples were obtained and process variables and upsets were recorded for correlation with the logged data. Near the end of this work, a prototype on-line system was tested to record mould oscillation parameters and machine forces. This study has lead to a quantitative understanding of mould response through measurements of mould oscillation and friction on industrial casting machines. Oscillation monitoring is imperative for billet producers, since the machines were found to deviate from their design specifications. A highlight of this research was the quantification of mould-billet friction forces. Fundamental lubrication behaviour was elucidated with a force sensor, which is an excellent tool for evaluating lubrication and mould oscillation. Further, the force response varied as a function of process variables and upsets. Mathematical modelling of mould-billet binding has shown that the force signal responds mainly to lubrication effectiveness, and not the degree of binding. In the presence of a lubrication upset, however, high friction forces can be measured. When casting with oil lubrication, the friction response appeared to increase with increasing heat extraction, indicating that lubrication, heat transfer and friction are intimately linked. Modelling of binding has also lead to some recommendations for improvement in mould taper design.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat