Abstract The present study makes an effort to improve strength-toughness combination of a high carbon carbide free nanostructured bainite by investigating the effect of content, size and morphology of constituent phases when austempered at 250-350°C. Tension tests showed that the tensile strength increases sharply with refinement of bainitic microstructure with decrease in austempering temperature while retaining significant ductility. However, degradation of toughness was observed with an increase in strength. Processing-microstructure-properties correlations have been formulated in addition to detailed electron microscopy to explain the experimental trends.