Effects of Plastic Stress on Transformation Plasticity and Microstructure of a Carbide-Free Bainite Steel

Thermal simulation experiments were conducted, the microstructures were observed by scanning electron microscope (SEM) and transmission electron microscopy (TEM), and the bainite orientation was examined by electron backscattering diffraction (EBSD). The effects of plastic stress on transformation plasticity (TP) strain were investigated. The results show that the TP strain increases nonlinearly with the applied stress. It increases slowly versus the stress in the elastic region, and there is a sharp increase in TP strain if the stress exceeds the parent austenite yield strength because of the combined effect of prior deformation and stress. As the stress continues to increase, the bainite transformation amount decreases, leading to a slower increasing rate of TP strain. In addition, it is confirmed by EBSD that the TP strain is accompanied by the variant selection in microstructures, so that the occurrence of TP is consistent with the Magee mechanism. Moreover, the TP strain is more obvious at lower transformation temperatures due to the increased bainite transformation amount and more organized bainite structure.