UBC Theses and Dissertations
The prediction of the evolution of microstructure during hot rolling of steel strips Devadas, Christopher
A mathematical model to predict the through-thickness temperature distribution in a steel strip during hot rolling in the finishing stands has been developed. The model is based on one-dimensional heat conduction within the strip and the work rolls and takes into account the cooling due to descaler sprays, roll chilling, interstand cooling due to radiation and the heat generated due to friction and deformation in the roll gap. A submodel to predict the cyclic steady state temperature distribution of the work rolls has also been formulated. The contact region of the strip and roll is characterized by an interface heat-transfer coefficient which was computed from the results obtained from physical simulation of the industrial hot rolling mill on CANMET's pilot mill. To verify the heat-transfer model, industrial trials were conducted in which the surface temperature of the strip was monitored with pyrometers at several locations. To obtain an accurate prediction of the roll forces in the roll bite, Orowans' model, as formulated by Alexander, was modified to incorporate the temperature variation within the strip. This involved a force balance on each nodal volume for a series of vertical slices throughout the roll gap. To formulate the constitutive equations to characterize the high temperature mechanical behaviour of three steels (0.34% C, 0.05% C and 0.074% C-0.024% Nb), hot compression tests were conducted on the Cam-plastometer and Gleeble. It has been found that the hyperbolic sine relationship, coupled with the Ludwik characterization of the stress-strain curve as proposed by Baragar, gave excellent prediction of flow stresses during the hot rolling process. It has been shown that the roll force prediction obtained by the inclusion of the through-thickness temperature was approximately 12% larger than that obtained using a uniform temperature. The effect of lubrication on the roll force has been examined from the stand point of a change in the coefficient of friction and the associated reduction of the heat-transfer coefficient. A computer model has been formulated using existing empirical relationships to predict the evolution of the microstructure during the hot rolling of steel strip. Isothermal restoration and recrystallization results were obtained from tests conducted on the Cam-plastometer and the Gleeble. These results were used to test and validate the re-crystallization relationships chosen from the literature. The principle of additivity has been employed so as to enhance the applicability of isothermal recrystallization kinetics to non-isothermal applications, such as the hot rolling process. IRSID's model has been utilized to characterize the static recrystallization that occurs after the deformation process, whereas Sellars' model was employed to describe metadynamic recrystallization. The grain growth kinetics for plain-carbon steels have been found to be adequately described by a power law relationship with the exponent having a value of 7.5. The microstructure obtained from the CANMET pilot mill tests showed excellent agreement with the model-predicted degree of recrystallization and resulting grain size. The model was used to predict the changes in microstructure associated with changes in the rolling schedules. It was found that the resulting austenite grain size produced for a given gauge was less dependent on the initial grain size, provided that conventional rolling temperatures were employed and that recrystallization occurred during or after the roll passes. The model also confirmed that low rolling temperatures are required to produce a fine austenite grain size.
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