Reduction in Planar Anisotropy of Pure Niobium Sheet by Asymmetric Rolling

Abstract

Cold rolled niobium normally shows high planar anisotropy in mechanical properties which causes earing in deep-drawing. Niobium is a bcc refractory metal, but its deformation and recrystallization textures have not been understood sufficiently. In this study, pure niobium sheets have been reduced from 2.0 to 0.5 mm in thickness by four-pass cold rolling. Either symmetric rolling or asymmetric rolling was performed as the cold rolling. The asymmetric rolling was realized by differential-speed rolling with speed ratio of 1.4. Elongated grains with α-fiber and γ-fiber texture components developed by the symmetric rolling. On the other hand, elongated grains are less developed by the asymmetric rolling. Changes in texture and microstructures in symmetric rolling and subsequent annealing are similar to those of α-Fe. During recrystallization after the asymmetric rolling, α-fiber does not develop well and equiaxed microstructure is formed. It is formed that planar anisotropy of r-value, i.e. Δr-value, is improved by the asymmetric rolling subsequently followed by annealing.