Effect of hot deformation parameters on flow stress and microstructure in a low carbon microalloyed steel

Hot deformation behavior of low carbon microalloyed steel was investigated by single-pass compression tests using a thermomechanical simulator. After the specimens were subjected to different hot deformation processes, their microstructures were obtained by constant cooling rate and then examined using optical microscopy and electron backscattering diffraction (EBSD) technique. The results showed that the absence of stress peak in the flow stress curves does not necessarily indicate the absence of dynamic recrystallization (DRX) because in some cases the partial DRX phenomenon does occur when the stress peaks are absent in the flow curves. The metallurgical events that occur during hot deformation directly influence the nature of phase transformation products. High dislocation density in the deformed austenite can promote the intragranular nucleation of acicular ferrite, while the DRX austenite grains prefer to form lath bainite/martensite. The EBSD results confirmed that the microstructures that were transformed from prior pancaked austenite grains always contain higher fraction of middle misorientation angle boundaries and larger average local strain as compared to those with prior DRX grain shape.