THE INFLUENCE OF PLASTIC-DEFORMATION AND COOLING RATES ON THE MICROSTRUCTURAL CONSTITUENTS OF AN ULTRA-LOW CARBON BAINITIC STEEL

An experimental ultra-low carbon bainitic steel was prepared to investigate the effect of a prior compressive deformation on the morphology of the transformation product during continuous cooling. It is found that at the higher cooling rate the deformed austenite tends to form the non-parallel plates of acicular ferrite, and that at the lower cooling rate the deformed austenite tends to form the parallel plates of bainitic ferrite. The orientation relationships between adjacent grains of acicular ferrite have been studied using the analysis of axis-angle pair. The result shows that the adjacent variants have nearly the same orientation in space, which is analogous to the case in alloy-steel weld metals. Furthermore, based on a thermodynamic analysis, it is indicated that acicular ferrite transformation (from deformed austenite) also exhibits the phenomenon of incomplete reaction, where the reaction ceases well before the residual austenite achieves its equilibrium carbon concentration.