Effects of Foil Thickness to Grain Size ( t/d ) Ratio and Prestraining on Tensile Response, Microformability and Crystallographic Texture of Ultra-Low Carbon Steel Thin Foils

Ultra-low carbon (ULC) steels, containing a carbon content ~ 0.055 wt%, have been used in several applications in the form of thin foils. However, there are limited studies on the effects of foil thickness ( t ) to grain size ( d ) ratio and foil condition on the tensile response and formability of thin ULC steel foils. In the present work, the tensile and forming behaviours of ULC steel foils of thickness about 400 µm were evaluated in both annealed and prestrained (by cold reduction to 2–7%) conditions as a function of t/d ratio and followed by detailed texture evolution analysis. Vacuum annealing was used to achieve varying t/d ratios in the specimens. Additionally, thin ULC steel foils of 100 μm thickness in annealed condition were also used for examining the thickness effect. Microstructural analysis was performed using the electron backscattered diffraction technique. Microformability was assessed by a miniaturised Nakazima test setup with specimen geometries designed to produce three different strain paths. The annealed foils displayed a typical yield-point phenomenon, but the total yield-point elongation decreased with decreasing thickness and grain size. The foils exhibited typical Hall–Petch strengthening, cold work hardening, and forming limit curves; however, there were substantial reductions in both tensile strength and ductility, and consequently, the forming strains, with decreasing the t/d ratio. The tensile response and formability of the foils were adversely affected by both thinning and prestraining. The texture studies revealed the formation of a γ-fibre i.e., < 111 >||normal direction, and its intensity varied significantly with the t/d ratio and mode of strain path. Graphical Abstract