Effect of Deformation Temperature on Microstructure Evolution in ARB Processed Ultralow Carbon IF Steel

The effect of deformation temperature on the microstructure evolution was investigated in the range from room temperature to 600°C for an ultralow carbon interstitial free steel deformed to high strain by accumulative roll-bonding (ARB). In the whole temperature range, the microstructure was being subdivided by deformation-induced boundaries, and the spacing of such boundaries decreased with increasing the applied strain. The mechanism of this microstructure evolution is known as grain subdivision. In the warm-temperature ARB at 400°C and above, a quite uniform lamellar boundary structure elongated to the rolling direction was obtained after high strain, where the boundary spacing tended to be saturated above strain of approximately 4 and the saturated boundary spacing became smaller when the deformation temperature was lower. It is suggested that the microstructure after high strain deformation is determined by a balance of deformation-induced grain subdivision and restoration processes such as recovery and short-range boundary migration, and that the saturated boundary spacing is explained as a function of the Zener–Hollomon parameter. However, localized shear banding occurred at high strain in the room-temperature ARB, leading to inhomogeneities in the microstructure. It is therefore considered that a moderate deformation temperature as well strain rate has to be chosen to avoid shear localization and obtain homogeneous nanostructures.