Characterizations of Dynamic Strain-induced Transformation in Low Carbon Steel

Abstract

Dynamic strain-induced transformation of the low carbon steel Q235 at 770 ‐ C and 850 ‐ C leads to flne ferrite grains. The microstructure characterization and mechanism of the flne ferrite grain were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscattered difiraction (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 reflnement of ferrite is the cooperative efiect of DSIT and continuous dynamic recrystallization (CDRX) of ferrite. Besides, DSIT is deemed as an incomplete carbon difiusion phase transition through the analysis of microstructure and the previous simulated results. The strengths of the Q235 steel with reflned ferrite and pearlite structure get doubled than the initial state without treated by DSIT and the residual stress in the reflned structure is partly responsible for the ductility loss.