A rolling-annealing cycle for enhanced deep drawing properties in interstitial free steels

Abstract

To give good drawability, steel needs high volume fractions of the annealing texture component {111} 〈hkl〉 and a low fraction of ε {100} 〈0k1〉. This is achieved in conventional Nb + Ti stabilized interstitial free (IF) steels by a cold rolling (CR) reduction of 85 per cent followed by annealing at 750–850°C for a few minutes. In this research, a detailed investigation of two-stage deformation processes was undertaken in which the total reduction in thickness was kept constant at 80 per cent, with a first and second rolling interrupted by recrystallization (RX) before the final recrystallization anneal was made. The texture produced is a rather flat γ recrystallization fibre of relatively high intensity at a reasonable final grain size. A second experiment, involving rolling ferrite at 700°C, produced a strong rolling texture and a well-developed {111} 〈hkl〉 texture after annealing at 710°C, and so this material was also subjected to further rolling and annealing. The intermediate annealing between warm rolling (WR) and subsequent cold rolling significantly improved the intensity and uniformity of the final {111} texture compared with metal that was cold rolled without intermediate annealing. An investigation into the mechanisms involved in recrystallization revealed that the {111} 〈hkl〉 oriented grains were subject to orientation splitting involving rotations around 〈111〉 ND, and this process of deformation banding produced the necessary lattice curvature for nucleation of the {111} recrystallization texture components essential for good deep drawability.