Monitoring of Meniscus Thermal Phenomena with Thermocouples in Continuous Casting of Steel

Abstract

Many quality problems in continuous-cast steel are related to mold level fluctuations, stickers, deep oscillation marks, and other events at the meniscus. These phenomena may be detected by monitoring temperature signals in the wall of the copper mold. This work applies computational models of transient heat conduction to investigate the potential capabilities of mold thermocouples to detect such phenomena by computing the sensitivity of the detected signal to heat flux variations at the meniscus. The three-dimensional model is first validated with temperature data recorded in a commercial slab casting mold, and in a previous laboratory measurement. The method is capable of monitoring meniscus level, and to detect large surface level fluctuations. However, its ability to detect temperature fluctuations decreases with decreasing magnitude and duration of the level fluctuations and the distance of the thermocouple from the hot-face surface. Sensitivity calculations with the model are presented to quantify these detection limits. Finally, a new inverse heat-conduction model is applied to extract new insights into heat transfer at the meniscus from thermocouple measurements. Introduction During continuous slab casting, molten steel flows through a “Submerged Entry Nozzle” (SEN) into a water-cooled copper mold. The steel solidifies a thin shell, which contains the liquid and is withdrawn at a casting speed that matches the flow rate. Fluctuations of the position of the molten meniscus (metal level) disrupts solidification at the meniscus, entrains slag inclusions, and leads to many quality problems. These include deep oscillation marks, stickers, and even catastrophic “breakouts. Liquid level is usually measured with an expensive commercial system to maintain liquid level within +/a few millimeters, using a suspended eddy-current level sensor (which measures a single spot somewhere between SEN and the narrow face), or a radiation detector (which averages over a volume that is blocked by metal) [1]. Another potential method to quantify the metal level during continuous casting is to utilize the temperatures measured continuously by thermocouples (TC’s) embedded in the copper mold. This inexpensive method has been applied commercially by “LevelTherm” to control level within +/-20-30mm in billet/bloom casting [2]. If sufficiently accurate, this method would have the potential advantage of providing information around the entire perimeter of the meniscus, 119 Sensors, Sampling, and Simulation for Process Control Edited by: Brian G. Thomas, James A. Yurko, and Lifeng Zhang TMS (The Minerals, Metals & Materials Society), 2011