The Formation of Microstructures in Cold Rolled Interstitial-Free Steel

Abstract

The development of the cold rolling microstructure in an Interstitial-Free (IF) steel has been systematically investigated using transmission electron microscopy (TEM). The microstructure originally develops 3D dislocation meshes which relax into cells, which refine until microbands form. It is postulated that from then on, deformation concentrates in the microbands, rendering further cell refinement unnecessary. The cells remained ∼1 mm in diameter with a cell misorientation of ∼1° over the strain range 4-50%. The crystallographic measurement and deformation geometry calculation reveal that the habit plane of a microband has the largest Schmid factor; and when one 〈111〉 slip direction is intensively activated in this plane, one set of microbands is formed on this plane. Two sets of microbands form if two 〈111〉 slip directions are strongly, and nearly equally activated. In the case of microband-free crystals, up to 7 slip systems have similar Schmid factors and thus are activated concurrently. This leads to homogeneous deformation and as a result, no microbands form. The process of forming microbands is considered to arise from a dense dislocation sheet containing positive and negative Burgers vectors so that net rotation is almost zero across the wall of dislocations. An instability and unbalancing of this situation leads to the formation of a second wall of one sign, leaving the original wall with the opposite sign net Burgers vector. This is consistent with the fact that the interior of the microband is rotated away from identical neighboring matrices. At rolling reduction of ∼50% shear bands form on the same slip systems as one set of microbands. Shear banding involves two deformation mechanisms; dislocation gliding and rigid-body rotation. Dislocation gliding mechanism makes shear bands parallel to one set of microbands.