Strain-rate effects on mechanical behavior and austenite transformation kinetics in carbide-free bainitic steel

The mechanical response of a carbide-free bainitic (CFB) steel was investigated over a wide range of strain rates to elucidate the interplay between deformation and austenite-to-martensite transformation. Uniaxial tensile tests were conducted at nominal strain rates from 10 -4 s -1 to 8 × 10 2 s -1 . To characterise the transformation kinetics, we introduce a novel test methodology based on a linear-strain-path sample. The results reveal a positive strain-rate sensitivity of strength. Ductility exhibits a non-monotonic dependence on strain rate, with the highest values occurring under dynamic loading. Plastic anisotropy increases steadily with strain rate. Damage analysis and fractography confirm a ductile fracture mechanism, showing larger dimples at higher strain rates. A more complete and accelerated austenite-to-martensite transformation is observed with increasing strain rate. To represent the observed kinetics, a modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) model is proposed incorporating a strain-rate-dependent term. This allows the transformation behavior to be described across a broad range of strain rates, which to our knowledge has not been reported previously. The findings demonstrate that the strain-rate dependent behaviour of CFB steel arises from multiple interacting mechanisms beyond transformation-induced plasticity and provide valuable data for the development and constitutive modelling of CFB steels under dynamic loading.