Development of ferrite/bainite bands and study of bainite transformation retardation in HSLA steel during continuous cooling

The development of banded structure in HSLA steel during continuous cooling has been systematically studied using dilatometry and microstructural observations. At low cooling rates (< 10 A degrees C/min), the microstructure contains alternating bands of ferrite and a mixture of pearlite and bainite. At higher cooling rate (> 10 A degrees C/min), the formation of pearlite is suppressed, and the corresponding microstructure consists of banded ferrite and bainite. Dilatometric analysis indicates that as ferrite and pearlite transformation at low rates (or ferrite transformation at higher rates) completes, Manganese (Mn) and Carbon (C) concentration in austenite can retard the bainitic transformation and result in the transformation stagnancy phenomenon. The magnitude of retardation and the bainite morphology are affected by Mn and C concentration. The increased cooling rate decreases Mn and C content in bainite, and then the length of the stagnant stage decreases and bainite morphology changes from lower bainite to acicular ferrite.