Divergent mechanical responses driven by local microstructural evolution during low-temperature aging in advanced high-strength steels

Advanced high-strength steels (AHSSs) utilize hard phases, such as martensite and bainite, which are typically tempered to improve their strength–ductility balance. Low-temperature aging (LTA) below 200 °C enables the redistribution of interstitial carbon atoms, thereby improving the mechanical properties without altering the alloy composition. Although LTA has been extensively studied in single-phase ferritic steels, its effects on complex-phase (CP) AHSSs remain underexplored. This study investigated the origin of the divergent mechanical responses between global and local properties in CP AHSSs during LTA. The mechanical properties were evaluated at both global (yield strength and Vickers hardness) and local (nanohardness) scales, and the microstructural evolution was analyzed using high-resolution transmission electron microscopy. Although the macroscale microstructures—primarily tempered martensite and lower bainite—remained largely unchanged, nanoscale transformations were evident. Supersaturated carbon atoms diffused toward the boundaries and pre-existing carbides, promoting θ-carbide growth in the bainitic regions and the progressive ε*-carbide to ε-carbide to θ-carbide transition in the martensitic regions. Interestingly, the yield strength and Vickers hardness increased with aging, whereas the nanohardness steadily declined. This contrasting behavior is attributed to two competing mechanisms: matrix softening, caused by a reduction in the solid solution and precipitation hardening, and interfacial strengthening, driven by carbon segregation, which enhances interfacial cohesion and impedes dislocation motion. These findings highlight the critical role of nanoscale carbon redistribution in controlling the mechanical behavior, suggesting a practical and cost-effective approach for optimizing the performance of third-generation AHSSs through a simple low-temperature treatment.