Simulation of Runout Table Cooling

Accelerated cooling on the runout table of hot mills has become a key technology to produce thermo-mechanically controlled processed (TMCP) steel plates and strips. During runout table cooling austenite decomposition takes place and determines the final microstructure and, hence, the properties of the hot-rolled steel. There is an increased tendency to produce higher strength TMCP steels with complex microstructures including bainite and martensite. To tailor these microstructures, it is required to carefully design runout table cooling paths and lower the cooling stop and coiling temperature, respectively, for producing flat products with homogeneous mechanical properties. Thus, simulation of runout table cooling is a crucial aspect of process modeling. In the present paper, the status of runout table simulation approaches is reviewed. In particular, the three boiling mechanisms of water cooling, that is, nucleate, transition, and film boiling are discussed. The development of appropriate heat transfer coefficients is rather mature for nucleate and film boiling, respectively. Modeling and controlling the transition boiling regime below the Leidenfrost temperature remains a challenge as heat extraction rates increase with decreasing steel temperature. The status of heat transfer simulations for transition boiling is thus discussed in detail. Currently, the proposed heat transfer correlations, while increasingly based on the underlying physics, still contain a number of empirical parameters that require tuning with experimental and/or mill data. The review is limited to information in the open literature while recognizing that a number of proprietary in-house runout table cooling models exist that are developed either by equipment makers or steel companies to control accelerated cooling to lower cooling stop or coiling temperatures.