Effect of prior austenite grain size on strength-ductility trade-off, phase evolution, and fracture behavior in a fully austenitizated QP steel: Insights into TRIP behavior

This study investigates the effects of primary austenite grain size (PAGS) on the strength-ductility balance, phase development, and fracture properties of hot-forged quench and partitioning (QP) steels (Fe-0.23 %C-1.25 %Si-1.77 %Mn), which were fully austenitized at five different temperature-time combinations. Microstructural analysis using optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) revealed that as PAGS decreased, the volume of blocky retained austenite (RA) increased. Fresh martensite/retained austenite (FM/RA) islands were predominantly located at triple junctions of primary austenite grain boundaries (PAGBs) and RA grains. X-ray diffraction (XRD) patterns showed that RA stability and carbon content increased with decreasing PAGS, while tempered martensite (TM) and FM fractions decreased. Macro-hardness increased with larger PAGS. Tensile testing revealed that the fine-grained sample exhibited superior strength and nearly identical toughness compared to the coarse-grained specimens. However, the fine-grained sample failed to follow the Hall-Petch relation due to an increased fraction of soft austenite and reduced hard martensite phases. Total elongation (TE) decreased as PAGS reduced. Additionally, the sample austenitized at higher temperatures exhibited better work hardening behavior. Fracture surface analysis of the deformed QP samples (D-QP) showed numerous micro-dimples, indicating a primarily ductile fracture mode, but cleavage facets were also present. The combination of granular and intergranular fracture types contributed to the weak TE observed in all samples.