| The need to manufacture high quality steel products that meet specific requirements dictates the control of steel processing. For flat products, controlled rolling in the finishing mill followed by accelerated cooling on the run-out table of a hot strip mill are the final processing steps before coiling the hot band. These three processing steps significantly influence the final microstructure and thus, the mechanical properties of the hot rolled steel. This work examines the austenite-to-ferrite phase transformation under run-out table conditions for two plain-carbon and two single microalloyed commercial low carbon steels. The austenite decomposition was quantified using diametral dilation measurements on a Gleeble 1500 thermomechanical simulator.; A novel method was developed to characterize the austenite-to-ferrite phase transformation kinetics simulating the industrial, non-isothermal operating conditions. This technique adopts the additivity rule, utilizing the grain size-modified Avrami equation, back-calculating the effective isothermal Avrami equation solely from continuous cooling test data. In this way, it permits the modelling of the austenite decomposition kinetics in low carbon steels where isothermal tests are difficult to perform. A more fundamental approach based on a carbon diffusion model incorporating a solute drag-like effect (SDLE) was also employed to describe the transformation kinetics of austenite to ferrite. The accuracy of the diffusion model could be improved by including the austenite grain size distribution rather than a mean grain size as an initial condition.; For high strength, low alloy, Nb microalloyed (HSLA-Nb) steel, the presence of Nb retards austenite recrystallization. creating a temperature, T nr, below which the rolling strain will be accumulated as the steel progresses from stand to stand. The retained strain enhances the nucleation of the ferrite during austenite decomposition and results in enhanced strength and toughness properties. Rolling under no-recrystallization conditions with the accumulation of strain, i.e. controlled rolling, is a commonly employed rolling practice in the last stands of finish rolling. Thus, for the HSLA-Nb grade, the effect of retained strain on the austenite decomposition has been evaluated by performing continuous cooling transformation tests after deformation below Tnr.; Acceleration of the transformation and additional ferrite grain refinement was obtained as a result of the prior deformation which increased the ferrite nucleation rate by introducing additional nucleation sites both on the austenite grain boundary and within the deformed grains at crystallographic defects. (Abstract shortened by UMI.)... |
