Microstructure–property correlation and strain partitioning behavior in medium-carbon carbide-free bainitic steel

The correlation between the microstructure, properties, and strain partitioning behavior in a medium-carbon carbide-free bainitic steel was investigated through a combination of experiments and representative volume element simulations. The results reveal that as the austempering temperature increases from low to intermediate, the optimal balance of properties shifts from strength–toughness to plasticity–toughness. The formation of fine bainitic ferrite plates and bainite sheaves under low austempering temperature (270 °C) enhances both strength and toughness. Conversely, the wide size and shape distribution of the retained austenite (RA) obtained through austempering at intermediate temperature (350 °C) contribute to increased work-hardening capacity, resulting in enhanced plasticity. The volume fraction of the ductile film-like RA plays a crucial role in enhancing impact toughness under relatively higher austempering temperatures. In the simulations of tensile deformation, the concentration of equivalent plastic strain predominantly manifests in the bainitic ferrite neighboring the martensite, whereas the equivalent plastic strain evenly spreads between the thin film-like retained austenite and bainitic ferrite. It is predicted that the cracks will occur at the interface between martensite and bainitic ferrite where the strain is concentrated, and eventually propagate along the strain failure zone.