Microstructures and Mechanical Properties of Ultra Low Carbon IF Steel Processed by Accumulative Roll Bonding Process

Abstract

An ultra low carbon IF steel was severely strained by accumulative roll-bonding (ARB) process and subsequently annealed for 0.5 h at various temperatures ranging from 673 to 1073 K for strengthening by grain refinement. The ARB process was carried out up to 5 cycles (an apparent equivalent strain of 4.0) at ambient temperature without lubrication. The as-ARBed specimen exhibited a dislocation cell structure with relatively high dislocation density, rather than a well-defined ultrafine grained structure. However, the subsequent annealing at 673 or 773 K resulted in the formation of ultrafine grains in the structure. This indicates that the heating (recovery) is necessary for the formation of ultrafine grains in the present IF steel severely deformed by ARB process. The tensile strength of the as-ARB processed IF steel increased with strain, reached a maximum of 813 MPa, which is about 3 times higher than the initial value. The elongation dropped largely from 60 to below 10% at the 1st cycle, but it hardly changed from the 2nd cycle even if the ARB cycle increased. The strength decreased gradually and the elongation increased with annealing temperature. Changes in the mechanical properties with the annealing temperatures corresponded well with the changes in the microstructures.