Evolution of tensile properties with transformation temperature in medium-carbon carbide-free bainitic steel

Abstract Different morphologies of carbide-free bainite were obtained through a series of isothermal heat treatments of a new medium-carbon bainitic steel, and the evolution of microstructures during tensile deformation was then observed. The results showed that the strength-ductility balance could reach its highest value near 350 °C. The retained austenite sustaining martensitic transformation, long bainite ferrite sheaf, and phase transformation dynamics were the main factors that caused high plasticity of the steel at 350 °C isothermal transformation. It is noteworthy that 350 °C is also a phase change sensitive point for most bainitic steels. Maintaining high work hardening rate at high strain is beneficial to increase elongation, which is attributed to the continuous martensitic transformation of the retained austenite with high volume fraction. The (200) austenite peak was separated using the Gaussian multi-peaks fitting method. It was found that the (200) austenite peak moves to the left with the increase of strain. The proportion of low angle peaks also increased with strain. This indicates that the transformation of the retained austenite always occurs in the low carbon region.