Understanding and improving thermodynamic stability of austenite in low carbon carbide free bainitic steels via ausforming process

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen, Diagramme (2023). = Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023
Carbide-free bainitic (CFB) steel has become a new forefront of advanced high-strength steels owing to their outstanding balance in mechanical properties. Due to a thermodynamic instability of austenite in low carbon CFB steels, formations of only primary phase bainitic ferrite and secondary carbon enriched retained austenite phase are impracticable. The untransformed austenite at high temperatures could partially transform into fresh martensite during cooling operation, depending on the local carbon concentration in the austenite. A general consequence is that an excessive formation of fresh martensite may deteriorate ductility, despite the enhanced strength of the steel. Thus, controlling the thermodynamic stability of austenite has been a challenging issue in developing low-carbon carbide-free bainitic (CFB) steels, besides increasing mean carbon content and chemical compositions. Ausforming as a thermomechanical heat treatment process is applied to compromise the formation of fresh martensite and to balance the phase constituent of the steels. This process combines plastic deformation of the untransformed austenite with the conventional process of isothermal heat treatment. Parameters of ausforming, such as deformation temperature, strain, and strain rate, are of significant importance in defining appropriate conditions for desirable microstructures and mechanical properties. The correlation between the ausforming conditions throughout the kinetics behavior of isothermal bainitic transformation, factors inherent in the martensite transformation, hardness, and tensile properties have been established. A unified physics-based model has been developed based on nucleation rate theory to provide a better understanding of how ausforming influences the variations of activation energy, corresponding driving energy, and the evolution of carbon enrichment in austenite. In addition, the impact of the chemical compositions has been conducted to reveal a limitation of ausforming with respect to the deformation strain on improving the thermodynamic stability of austenite against the formation of fresh martensite. Throughout the dissertation, a systematic investigation in heterogeneous microstructure and mechanical properties subjected to ausforming conditions allows for establishing advanced high-strength steels with reasonable hardness and improved strength and ductility.
Published by RWTH Aachen University, Aachen