Quantitative characterisation of pearlite morphology in hot-rolled carbon steel

During the hot rolling of carbon steel, austenite phase transforms into a pearlitic morphology, which essentially is a matrix of ferrite lamellae (α-Fe) and cementite (Fe₃C). This transformation occurs at the cooling bed after an equalisation temperature of around 600 °C Pearlitic steels find their use in ropes for bridges and elevators, rails, and tyre cords among others. Characterisation of microstructure has not been broadly applied to pearlitic steels because of their complex microstructures. Therefore, the characterisation of this morphology becomes inevitable, in order to identify potential weaknesses in the matrix. In this study, hot-rolled reinforcement bars (rebars) produced from recycled steel and direct reduced iron (DRI), were used for microstructural examination using standard metallurgical procedures. Although the optical microscope (OM) and scanning electron microscope (SEM) were used to obtain qualitative microstructure, they could not characterise the pearlite morphology quantitatively because of their three-dimensional (3D) limitation. Hence, the image analyser - Gwyddion Software, was used to quantify the pearlite morphology of these Y16 rebars. The results indicate that the pearlite colony is characterised by 3D single interpenetrating crystals of ferrite and cementite running parallel to each other due to their common growth during the transformation process of austenite. It was further observed that, the dimensional properties of the phases in the morphology in terms of their width and Interlamellar spacing (S), including the roughness of the pearlite colony can vary significantly. These results could be used to enhance the processing methodology of the industrial production processes.