Effect of Cold Rolling and Annealing on Texture Evolution and Mechanical Properties of IF Steel Sheet

The dependence of textural evolution, deformation microstructure and flow stress on cold-rolling reductions of an interstitial-free (IF) steel sheet has been investigated using the X-ray diffraction and the transmission electron microscopy techniques as well as tensile tests. The development of both alpha-and gamma-fiber orientations is observed during cold-rolling to reductions of 13% to 75%. It is also noted that deformation microstructures consist of geometrically necessary boundaries and incidental dislocation boundaries. Dislocation and grain boundary hardening leads to increase in flow stress with the rolling reduction. A fall in ductility seen between rolling reductions of 13% to 46% is attributed to the onset of localized shearing. The re-gaining ductility is concerned with the more uniform size of cell blocks in two dimensions with an increase of rolling reduction to 57%. Further annealing tests were carried out for the sample with 75% of rolling reduction to obtain an improved ductility. However, the sample exhibited a very small increment in ductility (similar to 3%) and an obvious decrease in strength under an annealed temperature of 500 degrees C. With an increase in annealing temperature to 750 degrees C, the ductility impressively increased to 29% at the expense of strength, decreasing from 710 MPa to 210 MPa due to the process of recovery and recrystallization. These results clearly demonstrate that, without geometrically necessary boundaries and incidental dislocation boundaries, the lack of strain hardening at room temperature is related to dynamic recovery.