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Modelling of the thermal evolution of steel strips cooled in the hot rolling runout table Hernandez-Avila, Victor Hugo

Abstract

The controlled cooling of steel on the runout table has become a standard practice in the production of flat hot-rolled products, and since its introduction by BISRA in 1957, has been adopted because it is a vital tool in meeting the increasing demand of steels with better quality and higher mechanical properties. Although approximately one third of the total steel produced in the world is cooled using laminar cooling in the runout table with some degree of success, little is known about what actually happens during cooling. Even though many of the runout table operations are relatively under control, it is still not known how far these operations are from the optimum. Whereas the influence of steel chemistry has been traditionally exploited to accomplish certain mechanical properties, the recently recognized effect of cooling on mechanical properties has been exploited poorly. This increases the importance of the study of the runout table cooling. The runout table process has been studied in detail during this work, and important observations leading to a better understanding of this operation were obtained. A mathematical model for the runout table cooling was developed, which solves the transient 1-D heat transfer in a steel strip moving through the cooling units in the runout table. The model takes into account the individual fluid flow characteristics of each jet such as its velocity, diameter, temperature, and the geometry of the nozzle arrays, and relates them to other process variables, such as strip speed, chemistry, and thickness, to predict the thermal evolution of the steel. The model was validated by comparing the predictions of the model for the processing of two plain carbon steels, A36 and DQSK, with measurements in ten different runout table operations. Very good agreement with measurements was obtained. The boundary conditions were obtained by the mathematical modelling of the boiling curves during water jet cooling based on the application of the macrolayer evaporation mechanism, which lead to accurate predictions of the boiling curves, which compare very well with most of the reported measurements in the literature.

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