Impact of hot and warm deformation on the bainite morphology and kinetics during continuous cooling of low- and medium-C carbide-free bainitic steels

• Thermomechanical processing slows-down the kinetics of the bainite transformation. • Mechanical stabilization was connected to the dislocation density of austenite. • Bainite inherits the low-angle misorientation substructure from the deformed austenite. • The pairing-behaviour of continuous cooled bainite deviates from predictive models of isothermal conditions. This work aims to describe the microstructural evolution of bainite during continuous cooling from austenite with varying grain size, morphology and defect density in two carbide-free bainitic steels with low and medium C contents. To achieve these conditions, hot and warm deformations were applied, and compared to an undeformed condition. Thermomechanical treatments were performed with a dilatometer and monitored in-situ via high-energy X-Ray diffraction. EBSD was employed for identifying the variant pairing behaviour and misorientation distributions. In comparison to the undeformed condition, a delay in the transformation kinetics was observed for the deformed conditions of both steels. The delay was correlated to the dislocation density of austenite before the onset of the bainitic transformation. Due to the delay, the bainite transformation temperature was lower in the deformed conditions. In spite of this, there was an increase in the frequency of low-angle misorientation boundaries as given by the misorientation distributions. This effect is unexpected in view of established thermodynamic models for prediction of the bainite morphology. This discrepancy was attributed to the substructure introduced by the deformation, which was inherited by bainite. A microstructure evolution history was proposed based on the concurring metallurgical phenomena during the deformation and subsequent continuous cooling.