With the continuous development of the automobile industry, the use of advanced high-strength steel in vehicles has become increasingly prevalent. Press hardening steels, known for their ultra-high strength, are gaining significant traction in this domain. Components manufactured using press hardening exhibit high dimensional accuracy and minimal spring back. The press hardening components are produced by heating the steel blank at the austenitization temperature, followed by hot forming and rapid cooling using pressing dies. However, despite the advancement in press hardening technologies and the widespread adoption of ultra-high-strength steels, certain challenges remain in further improving the performance of these materials, particularly in understanding the effect of processing treatments. 

This work studies the behavior of two ultra-high strength press hardening steels (PHS2000 and PHS1500) after undergoing heat treatment methods, specifically focusing on low-temperature tempering and press hardening. The objective is to increase the understanding of microstructure evolution and its influence on mechanical performance. The first method involved quenching the steels in oil followed by tempering at four temperatures in the range of 180-300 °C, while the second utilized die quenching to targeted temperatures, followed by air cooling to induce auto-tempering. Tensile properties, hardness, and microstructure changes were tested to understand how these treatments affect the steels' mechanical properties. The tensile properties of the steels investigated are influenced by the auto-tempering of martensite that occurs during the processes of oil quenching and die cooling. This phenomenon allows the steel to attain an optimal balance, demonstrating ultra-high strength exceeding 1900 MPa and good elongation at fracture, reaching around 8% or slightly higher. The tensile tests and microstructure analysis implied that low-temperature tempering (180-200 °C) could improve the yield strength of the steel as well as the elongation with a small reduction in the strength of the steel, in which the tempering effect caused precipitation strengthening and reducing of residual stresses in the microstructure. It was found that tempering at 300 °C promotes the formation and coarsening of cementite carbide, which led to a deterioration in the tensile elongation of the steels.

The fracture toughness and bending properties of the steels were evaluated following oil quenching, press hardening, and a combination of press hardening with subsequent bake hardening. PHS2000 displayed brittle fracture characteristics and lower fracture toughness, while PHS1500 exhibited ductile fracture behavior and higher fracture toughness, with similar trends observed in three-point bending results. STEM and EDS analyses identified precipitates of varying compositions and morphologies, with coarse precipitates acting as key factors that limit grain refinement and contribute to stress concentration, thereby influencing fracture toughness and bending properties.