Characterizations of Dynamic Strain-induced Transformation in Low Carbon Steel

Dynamic strain-induced transformation of the low carbon steel Q235 at 770 °C and 850 °C leads to fine ferrite grains. The microstructure characterization and mechanism of the fine ferrite grain were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) technique. The results show that strain-induced microstructure is the mixed microstructure of ferrite and pearlite, with cementite randomly distributed on ferrite grain boundaries and the grains interiors. EBSD images of grain boundaries demonstrate that high angle grain boundaries (HAGBs) are dominant in both of the deformation induced microstructures occurring below and above Ae3, with only a few low angle grain boundaries (LAGBs) existing in the grain interiors. It implies that the dynamic strain-induced transformation (DSIT) happens above and below Ae3 temperature and has the same phase transition mechanisms. The refinement of ferrite is the cooperative effect of DSIT and continuous dynamic recrystallization (CDRX) of ferrite. Besides, DSIT is deemed as an incomplete carbon diffusion phase transition through the analysis of microstructure and the previous simulated results. The strengths of the Q235 steel with refined ferrite and pearlite structure get doubled than the initial state without treated by DSIT and the residual stress in the refined structure is partly responsible for the ductility loss.