Modeling the Accelerated Cooling Process in Heavy Steel Plate Production
Research output: Thesis › Doctoral Thesis
In the production of thermo-mechanically rolled heavy steel plates accelerated cooling is a decisive process step which enables the adjustment of specific mechanical-technological properties. Immediately after the last rolling pass the plates are conveyed through the accelerated cooling line, where they are cooled by water spraying applied to the top and the bottom surface. The heat transfer conditions at the top surface and the bottom surface are not identical. Therefore, a temperature field which is non-symmetrical with respect to the midplane and hence a certain degree of warpage are prevalent during cooling. The specific choice of the cooling parameters allows minimizing the warpage after completed cooling. Optimal cooling parameters enable the production of plates with a high degree of flatness even before the leveling process. Numerical algorithms become increasingly important in the assessment of the cooling parameters. The models are required to permit reliable predictions of the thermo-mechanical behavior of the plates within reasonable computation times. The focus of this thesis is therefore the development of efficient numerical models for the accelerated cooling process. On the level of structural modeling two types of models are in the center of interest. (i) is a comprehensive 3D-model and (ii) is a 1D-model especially developed for the accelerated cooling process, with only three kinematic degrees of freedom. Due the fact that the 1D-model has such a low number of degrees of freedom it leads to an enormous reduction of the computation time. At the same time it provides a good resolution across the thickness. On the level of material modeling the phase transformation of austenite which is accompanied by transformation induced plasticity is to be taken into account. In order to achieve an efficient integration of the nonlinear material phenomena fully implicit integration algorithms are formulated which account for the plane stress state prevailing in the plates as well as transformation induced plasticity. Cooling experiments at the accelerated cooling line serve as a benchmark for the simulation results. In addition, the simulation results obtained by the 3D-model are compared with those obtained by the 1D-model. The influence of the plate length and the conveying velocity on a representative measure characterizing the curvature of the plate is investigated by means of the 3D-model. Moreover, the influence of material properties is examined. To this end, a systematic variation of the transformation kinetics is performed. Furthermore, the effect of transformation induced plasticity is analyzed.