Mould taper for high speed continuous casting of stainless steel billets

Over 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 through increases in casting speed. Although high speed casting trials are being carried out at many mini-mills, there is still a lack of fundamental understanding about the influence of casting speed on mould heat transfer. At the core of the technology is the water-cooled, oscillating copper mould. Mould interaction with the billet, both thermal and mechanical, governs billet quality and productivity. Heat transfer under high speed casting conditions needs to be determined in order to design the taper of the mould. The main objectives of this study were: to quantify the thermal-mechanical response of the mould in the casting of austenitic stainless steel and martensitic stainless steel with mathematical models; to evaluate the mould-billet interaction using mathematical models; and to provide practical recommendations for optimum mould taper design in high speed billet casting. Measurements were conducted on an operating billet casting machine at Atlas Steel to determine mould-wall temperature profiles for different steel grades, different casting speeds, mould flux types, oscillation frequency, off-center nozzle operation. The trial also involved data acquisition on mould displacement, casting speed, metal level. Samples of the billets cast at the trial were collected and process variables were recorded. An inverse heat conduction model was utilized to determine mould heat flux from measured mould wall temperatures and existing mathematical models were utilized to investigate mould/billet interaction and mould taper using the heat flux as input. Results from plant measurements, mathematical models and billet sample evaluation were used to correlate mould thermal response with transverse and longitudinal depressions, oscillation mark depths for different steel grades. It was found from this work that mould heat flux is strongly influenced by casting speed with mould flux lubrication. Increasing casting speed will consistently increase the heat flux while the shell thickness will be reduced owing to the reduction of the residence time of the strand in the mould. This study has also shown the effect of casting conditions such as steel grade, superheat and casting speed on mould taper requirements. Optimum mould tapers based on the evaluation of the interaction between the mould and the billets are recommended for austenitic and martensitic stainless steel billet casting.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat