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Heat transfer during run-out table bottom jet cooling of steel Kashyap, Debanga


Accelerated cooling on a run-out table in a hot mill governs the final microstructure and mechanical properties of thermo-mechanically controlled processed (TMCP) steels. Thus, it is crucial to delineate the heat transfer mechanisms and develop models adept to predict the temperature history of a steel strip or plate on a run-out table. In this work, controlled accelerated cooling experiments under transient conditions were performed on stationary steel specimens using an impinging bottom planar jet of water. Spatial sub-surface temperature history was recorded from 700 °C to below the saturation point of water. A two-dimensional inverse heat conduction algorithm was employed to quantify the heat fluxes and surface temperatures in order to obtain representative boiling curves. A range of water flow rates (160-300 l/min), water temperatures (10-40 °C) and jet inclinations (0-20 degrees) were investigated to examine the influence of process parameters on heat extraction rates. The obtained boiling curves show the presence of different heat transfer regimes with varying surface temperatures, i.e. nucleate boiling and transition boiling. Heat extraction rates increase with increasing water flow rates and decreasing water temperatures, particularly in the transition boiling region. Heat flux values show a strong dependence with respect to distance from the stagnation line, demarcating an impingement zone and a parallel flow zone. Characteristic boiling curves represent the two zones. A shoulder in the transition boiling region is observed to be an important feature in the boiling curves of the impingement zone. Based on the experimental data, empirical correlations have been proposed for the heat fluxes, considering the amalgamated effect of process parameters and distance from the stagnation line. Boiling curves for transient cooling have been constructed in the impingement zone and the parallel flow zone for a range of cooling temperatures relevant to the production of TMCP steels.

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