The potential of mechanical twinning in ultrafine retained austenite to enhance high cycle fatigue property of advanced bainitic steels

•The HCF property was greatly enhanced by tailoring retained austenite in bainitic steel.•Mechanism of sub-surface non-inclusion induced fatigue crack initiation was discussed.•Blocky retained austenite experienced deformation-induced martensitic transformation.•Mechanical twinning occurred in film-like retained austenite at fatigue crack initiation area.