A novel thermomechanical approach to produce a fine ferrite and low-temperature bainitic composite microstructure

A novel thermomechanical processing was developed in the present study to produce a unique microstructure consisting of fine ferrite grains (i.e. ∼4 μm on average) and low-temperature bainite in a relatively low-carbon steel with a modest hardenability. The thermomechanical route consisted of warm deformation of supercooled austenite followed by reheating in the ferrite region and then cooling to the bainitic transformation regime (i.e. 400-200 °C). The low-temperature bainite consisted of high dislocation density bainitic laths and very fine retained austenite films. This microstructure offered a high work hardening rate leading to a unique combination of ultimate tensile strength and elongation. This was due to the presence of ductile fine ferrite grains and hard low-temperature bainitic ferrite laths with retained austenite films. The microstructural characteristics of bainite were studied using optical microscopy in conjunction with scanning and transmission electron microscopy, electron backscatter diffraction and atom probe tomography techniques.