Tailoring Microstructure and Mechanical Performance in Advanced High-Strength Steels Via Optimized Thermal Treatment Regimes.

This thesis addresses the critical challenge of tailoring the microstructure and mechanical performance of Advanced High-Strength Steels (AHSS) through the design and optimization of thermal treatment regimes. A systematic investigation was conducted on Dual-Phase (DP980) and Quenched & Partitioned (Q&P) steels, focusing on the precise influence of intercritical annealing temperatures, cooling rates, and tempering treatments. Utilizing advanced microstructural characterization (SEM, EBSD, XRD, TEM) and comprehensive mechanical testing (tensile, impact, fatigue), this research established explicit correlations between specific optimized thermal parameters, the resulting microstructures (e.g., controlled ferrite-martensite ratios, stable retained austenite content, tailored carbide precipitation), and their enhanced mechanical properties. Key findings demonstrate how optimized intercritical annealing in DP980 precisely controls the ferrite-martensite balance, leading to superior strength-ductility synergy, and how optimized partitioning parameters in Q&P steels maximize stable retained austenite to significantly enhance impact toughness and fatigue resistance. Ultimately, this work provides an optimized and predictive framework for designing bespoke thermal treatment routes, enabling the precise tailoring of properties in HSS for demanding engineering applications.