Microstructure and mechanical behaviour of Nb- containing high Al ferritic low density steels

Abstract

Two high Al ferritic low density steels, Fe-11Al-5Mn-1Nb-1C (Alloy 1) and Fe-11Al-10Mn-1Nb-1C (Alloy 2) were prepared by vacuum induction melting. Their recrystallization behaviour is investigated. Hot rolling of the steels was carried out at 1100 °C. The temperature was chosen to ensure that the alloys were hot rolled in the intercritical region. Hot rolling led to a ferritic microstructure which was only partially recrystallized even after 92% rolling reduction. Annealing of hot rolled alloys at 1000 °C for 1 h results in a significant Static Recrystallization of the ferrite matrix. This suggests that dynamic recrystallization is difficult in these high Al ferritic steels. A further reduction of 33% was given to the hot rolled and annealed alloys by cold rolling. Annealing of cold rolled alloys at 1000 °C for 1 h leads to almost complete recrystallization. The hot rolled alloys exhibit better Yield and Ultimate Tensile Strength whereas the annealed alloys show higher total elongation. The best combination of properties is obtained in cold rolled and annealed samples. Alloy 2 exhibit superior tensile properties in comparison to Alloy 1. NbC precipitates pin the grain boundaries and may delay recrystallization. This necessitates further cold working and annealing to achieve complete recrystallization. NbC precipitates result in a significant refinement in grain size in annealed alloys. Nb addition also permits cold rolling without edge cracking. Thus, Nb may be an important alloying addition for high Al ferritic low density steels.