UBC Theses and Dissertations
Modelling of austenite-to-ferrite transformation behaviour in low carbon steels during run-out table cooling Pandi, Rassoul
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. The conducted tests were designed to simulate the microstructural conditions present at the exit of the finishing mill, i.e., the retained strain, the austenite grain size at the exit of finishing mill and the accelerated cooling conditions of the run-out table; the austenite decomposition kinetics were measured during the accelerated cooling simulation. Hot rolling which involves progressive stages of deformation and recrystallization of austenite during the concomitant reduction of temperature results in refined austenite grains. Refining the austenite grain raises the transformation temperature, increases the ferrite nucleation density and thereby refines the resulting ferrite grains. Accelerated cooling lowers the transformation temperature with associated ferrite grain refinement. 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, Tnr, 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. However, the degree of grain refinement was strongly affected by the initial austenite grain size and the cooling conditions; the effect was small for fine austenite grains and accelerated cooling, whereas for larger austenite grains and lower cooling rates, significant additional grain refinement was observed which increased with increasing retained strain. It is important to note that the retained strain increases the austenite decomposition temperature for a given cooling rate and therefore augments the production of a more equilibrium microstructure, i.e., more polygonal ferrite and pearlite.