Microstructures and Mechanical Properties of Ultra Low Carbon Interstitial Free Steel Severely Deformed by a Multi-Stack Accumulative Roll Bonding ProcessThis Paper was Presented at the Fall Meeting of the Japan Institute of Metals, held in Fukuoka, on September 24, 2001

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 1mm in thickness showed a recrystallization structure with average grain diameter of 27mm. The ARB was conducted at ambient temperature after deforming the as-received material to 0.5mm thick by cold rolling. The ARB was performed for six-layer stacked, i.e. 3mm thick sheet, up to 3 cycles (an equivalent strain of 7:1). In each ARB cycle, the stacked sheets were, first, deformed to 1.5mm thick by the first pass, and then reduced to 0.5mm 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 673K to 1073K. The tensile strength of the ARB processed materials increased largely with the number of ARB cycles, after 3 cycles it reached a maximum of 1.12GPa, 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 873K resulted in gradual decrease in the strength due to the static recovery. The annealing above 873K resulted in recrystallization and normal grain growth, and thereby a significant drop in the strength and recovery in ductility.