De Moor, EmmanuelSchweitzer, Jessie A.2024-07-032024-07-032023https://hdl.handle.net/11124/179106Includes bibliographical references.2023 Fall.There has been continued demand for hot-rolled, microalloyed steels with high strength, high toughness, and good weldability for applications such as oil and gas pipelines. Thermomechanical controlled processing (TMCP) coupled with accelerated cooling has been shown to produce high quality as-rolled plate and reduce costs. Judicious alloying with Nb and Mo can provide strengthening through grain refinement and transformation strengthening, while maintaining toughness. It is therefore pertinent to understand the relationships between deformation, alloying, and accelerated cooling to optimize TMCP schedules, alloy levels, and resultant plate microstructures. Four 0.05C microalloyed steel compositions were studied with Nb and Mo content ranging from 0.03-0.045 and 0.03-0.15 wt pct, respectively. Dilatometry was performed in situ during simulated TMCP processing with a Gleeble® 3500 system to study transformation behavior with different levels Nb and Mo content as well as different levels of deformation. Continuous cooling transformation (CCT) diagrams were constructed for each alloy utilizing cooling rates between 2 °C/s and a target 100 °C/s and 2 deformation levels, -0.4 and -0.6 total true strain in the austenite regime, to develop a range of microstructures. Isothermal testing to evaluate transformation after 2 levels of deformation was also performed to closer simulate plate coiling and examine isothermal transformation kinetics. Additionally, dilatometry testing without prior deformation was performed to decouple deformation and alloying effects on the kinetics of phase transformations. Increased deformation in the austenite non-recrystallization region increased formation of polygonal ferrite and acicular ferrite transformation through the creation of additional nucleation sites and led to finer resultant microstructures across all cooling rates. With increased cooling rate, transformation start temperatures were reduced and non-polygonal transformation products, such as acicular ferrite, bainite, and martensite, were favored. Increased Nb and Mo content increased the hardenability of the steel. Both were found to retard the polygonal ferrite transformation. However, Mo was found to have a complex effect. In the high Mo alloy, the bainite transformation kinetics were increased and bainite transformation was favored compared to acicular ferrite transformation.born digitalmasters thesesengCopyright of the original work is retained by the author.microalloyed steelphase transformationthermomechanical simulationText2024-06-25