Numerical investigation of heat transfer and coil cooling on a runout table in hot strip rolling

The runout table (ROT) cooling acts as an effective metallurgical tool to provide the desired cooling rate (CR) so as to obtain the required transformation product in the hot rolling process. Prediction and control of temperature in ROT is important to ensure microstructure and mechanical properties of thermo-mechanically processed steel strip. In the present study, a mathematical model is developed to simulate the thermal and metallurgical behaviour of the steel strip on a ROT. The model takes into account the effect of phase transformation by incorporating fundamental equation explaining the kinetics of austenite decomposition of steel under water jet impingement cooling. Particular attention is paid to account for the variation in heat transfer coefficient due to temperature change and operating conditions. Results from the proposed model are compared with available results and validated with industrial operational data. Various parametric studies are undertaken to examine the effects of finish roll temperature (FRT), strip thickness, strip velocity and cooling pattern on the strip temperature and coiling temperature (CT). It is seen that an increase in strip motion by 50% raises the CT by 10%. A change in strip thickness and FRT also alters the CT significantly. With the decrease in strip thickness and FRT, the CT also decreases. It is observed that early cooling with active headers at the beginning of ROT cools the strip faster than with late cooling. The average CR is influenced by the position and number of active headers. The overall cooling increases by up to 13% for continuous cooling when compared with late distributed cooling, while the later consumes 88% less water than continuous cooling scenario. The proposed approach can determine phase transformations over a wide range of operating conditions with high degree of accuracy and can contribute to efficient energy estimations.