Microstructures and Mechanical Properties of Ultra Low Carbon Interstitial Free Steel Severely Deformed by a Multi-Stack Accumulative Roll Bonding Process

Abstract

An ultra low carbon interstitial free (IF) steel was severely deformed by the six-layer stack accumulative roll-bonding (ARB) process for improvement of the mechanical properties. As-received material with 1 mm in thickness showed a recrystallization structure with average grain diameter of 27 μm. The ARB was conducted at ambient temperature after deforming the as-received material to 0.5 mm thick by cold rolling. The ARB was performed for six-layer stacked, i.e. 3 mm thick sheet, up to 3 cycles (an equivalent strain of ∼7.1). In each ARB cycle, the stacked sheets were, first, deformed to 1.5 mm thick by the first pass, and then reduced to 0.5 mm thick, equals to the starting thickness, by multipass rolling without lubrication. The specimen after 3 cycles of ARB was annealed for 1.8 ks at various temperatures ranging from 673 K to 1073 K. The tensile strength of the ARB processed materials increased largely with the number of ARB cycles, after 3 cycles it reached a maximum of 1.12 GPa, which is about 4 times larger than that of the initial material. The elongation dropped largely after the cold rolling prior to the ARB, however it remains almost constant during the subsequent ARB process. Transmission Electron Microscopy revealed that the ARB processed materials exhibited a dislocation cell and/or subgrain structure with relatively high dislocation density. The selected area diffraction (SAD) patterns suggested that the orientation difference between neighboring cells was very small. The annealing up to 873 K resulted in gradual decrease in the strength due to the static recovery. The annealing above 873 K resulted in recrystallization and normal grain growth, and thereby a significant drop in the strength and recovery in ductility.