Structure and properties of heavily cold rolled iron and high strength low alloy and low carbon steels

The development of microstructure in α iron and steels during cold rolling has been investigated in the strain range ε = 0.5–4.5. In the low strain region (ε > 1.5), primary and secondary microbands were identified. The substructure development with strain has been explained in terms of a model in which the volume fraction of micro bands increases with increasing strain at the expense of a uniform cell structure matrix. At large strains, no ‘crystallographic’ microbands were detected, and it is suggested that in this region, slip is controlled by more short range events. The effect of initial grain size on strain hardening has been considered by comparing the behaviour of a coarse grained iron, a medium grain size low carbon steel, and an ultrafine grained high strength low alloy steel. The observations suggest that during cold rolling the flow mechanism will change from being substructure controlled (σ = σ₀ + kd⁻¹, where d is the subboundary separation) to grain boundary controlled (σ = σ₀ + kd^−½, where d is the high angle boundary separation) when the separation of high angle grain boundaries owing to the imposed strain decreases to about 1 μm.