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Experiments and mathematical modelling of controlled runout table cooling in a hot rolling mill Liu, Zhengdong

Abstract

Controlled hot strip rolling is one of most important processes in the steel industry since approximately 200 million metric tons of steel are hot-rolled to flat products with gauges from less than 2 mm up to 25 mm annually. As has been widely recognized, runout table cooling is essential to the control of final mechanical properties and flatness of a steel strip in a hot strip mill. Therefore, a very significant effort has been mounted worldwide to quantify the thermal and metallurgical phenomena during runout table operation over the past decades. However, very limited cooling data has been obtained for industrial conditions even today, and the information available is not well understood. As a result, even though many of the runout table operations are considered to be relatively under control, little is known about what is really happening during water jet impingement and how far these operations are from the optimum This is the motivation behind this pioneer investigation. The ultimate goal of the research is to perform some industrial scale water jet impingement experiments and to employ the experimental data obtained to simulate runout table operation and further to gain a better understanding of the relationship among thermal profiles of a steel strip and adjustable processing parameters during runout table operation. A comprehensive literature survey has been created in which the knowledge accumulation on water jet impingement boiling heat transfer and modelling of controlled runout table heat transfer has been highlighted. A full-scale pilot runout table facility has been designed and constructed, which is the very first full-scale runout table test rig ever built in a laboratory worldwide. With the aid of the test rig, more than thirty water jet impingement tests have been conducted to attempt to quantify the relationship between the thermal evolution of a heated steel plate which is undergoing water jet impingement and the adjustable processing parameters during the cooling process. A 2-D finite element (FEM) numerical model and a transient inverse heat conduction (T-IHCP) model have been developed and implemented so as to calculate heat fluxes and heat transfer coefficients along the impinging surface of a heated steel plate based on thermal measurements at several discrete positions inside the steel plate. Experimental data obtained was used to modify the established AISI ROT model to improve its predictive capability on the prediction of coiling temperatures at Company S. Then, the modified AISI ROT model was validated by comparing its prediction to plant measurement for 24 mill coils from the company. Very good agreement was found. Downcoiler thermal model was also validated by comparing its prediction with industrial measurement. Very good agreement was also met in this regard. The applications of both the AISI ROT model/modified AISI ROT model and downcoiler thermal model to the steel industry have also been explored. Obtained experimental data was also well discussed and analyzed with the purpose of improvement of a runout table operation by altering adjustable processing parameters, such as cooling water temperature, jet impingement velocity, water flux, water jet diameter and water jet arrangement. With the help of experimental measurement and recorded visual observation, heat transfer mechanisms involved during water jet impingement cooling have also been discussed. This work has contributed new knowledge to water jet impingement boiling heat transfer and its application to runout table operation not only in the process modelling but in the process control. As a consequence, the progress has already led to a better understanding on the controlled runout table cooling practice in a hot strip mill.

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