Microstructural Evolution in Medium Manganese Steels during Quenching and High-Temperature Partitioning Process: A combined experimental and modelling approach

An effective way for the automotive industry to tackle the growing concern of CO2 emissions from automobiles is to reduce the overall weight of the vehicle, without compromising its performance and passenger safety. With the increasing demand of steels with enhanced properties in the last decade, the development of advanced high strength steels (AHSSs) has been focused on the design of complex microstructures leading to exceptional combinations of strength and ductility. One such steel is Quenching and Partitioning (Q&P) steel, which is typically composed of a high strength phase, martensite, and a softer phase, austenite, which contributes to the ductility of the material. The main strategy in developing Q&P steels involves partitioning of carbon, an interstitial alloying element, from supersaturated martensite (α׀, formed in an initial quenching step from the austenitisation temperature) into austenite (γ) during an isothermal holding (partitioning stage) to enhance the thermal and mechanical stability of austenite. If the partitioning step is subjected at higher temperatures, substitutional austenite-stabilising alloying elements, such as manganese, may partition to the austenite and significantly enhance the stability of austenite in the final microstructure. Keeping this in mind, experimental and modelling approaches are employed in this Ph.D. thesis to investigate the microstructural evolution and the mechanisms involved during the quenching and high-temperature partitioning process in five different medium manganese steels.