Press hardening is a thermomechanical process that enables the production of ultra-high-strength steel components with complex geometries and superior crash performance, making it essential for lightweight automotive structures. The final properties of press-hardened steels are governed by a sequence of coupled phenomena, including austenitization, hot deformation of austenite, phase transformation during cooling, and subsequent low-temperature thermal treatments. While these relationships are relatively well established for conventional 1.5 GPa grades such as 22MnB5, the underlying mechanisms in next-generation 2.0 GPa steels remain insufficiently explored. This lack of knowledge limits the ability to accurately predict microstructural evolution and optimise industrial press hardening processes for these advanced materials.

The aim of this thesis was to investigate how microstructural evolution governs the performance of ultra-high-strength steels for press hardening applications, with a particular focus on commercially available 2.0 GPa grades benchmarked against conventional 1.5 GPa steels. The influence of post-quench thermal treatments, including tempering, auto-tempering, and paint baking, on martensitic microstructures and mechanical performance was evaluated. A low-temperature tempering regime (180-200 °C) effectively enhanced yield strength while preserving the high tensile strength and good ductility of the martensitic microstructure. In contrast, higher tempering temperatures (250-300 °C) promoted martensite recovery and carbide coarsening, leading to a pronounced reduction in strength and the onset of tempered martensite embrittlement. The implications of these microstructural changes for fracture resistance were further examined by considering the effects of baking treatment and prior austenite grain size. Fracture toughness was found to be strongly governed by carbon content, with the 2.0 GPa grade exhibiting significantly lower resistance to crack propagation compared to the 1.5 GPa steel. The paint baking treatment enhanced fracture toughness in both steels, with a more pronounced improvement in the higher-strength grade. In addition, prior austenite grain size exerted a secondary but systematic influence, where finer grain structures provided consistently higher resistance to crack propagation. The effect of thermomechanical processing was investigated through simulations of press hardening conditions using Gleeble testing. The results demonstrated that deformation of austenite significantly altered phase transformation kinetics, where increasing strain and decreasing deformation temperature accelerated diffusive transformations, resulting in a reduced martensite fraction in the final microstructure. Austenite grain growth was modelled, and the effects of austenitization temperature and holding time on bending performance were also investigated. The developed grain growth model predicted the evolution of grain size under conditions of homogeneous growth. The results indicated that austenitization temperature has no significant effect on bending performance for short holding times, while only a minor reduction in bending performance was observed at longer austenitization times. Finally, transformation mechanisms beyond conventional martensitic pathways were explored through the study of quenching and partitioning treatments in a high-silicon steel using in-situ high-energy synchrotron X-ray diffraction. The findings demonstrated that pre-existing martensite accelerates carbide-free bainitic transformation by promoting nucleation and affecting carbon partitioning, providing insight into alternative microstructure design strategies with potential relevance for press hardening applications.

In summary, this thesis establishes comprehensive process-structure-property relationships across the press hardening value chain, providing a foundation for improved process design, predictive modelling, and the development of ultra-high-strength steels with enhanced performance.