Effects of Temperatures of Rolling and Annealing on Microstructures and Tensile Properties of Low Carbon Ferritic Low Density Steels

Abstract

The microstructural modification and the effect of cold/warm rolling and annealing on tensile properties of ferritic low density steels containing different amounts of carbon (0.0035 mass%, Steel 1 and 0.04 mass%, Steel 2) and aluminium (6.8 mass%, Steel 1 and 9.7 mass%, Steel 2) were investigated with respect to amount of ferrite, grain size and formation of Fe₃Al intermetallic. The Steel 1 was deformed by cold rolling at room temperature and Steel 2 was deformed through warm rolling at 250°C to avoid prior cracks formation. After rolling, samples of Steels 1 and 2 were annealed at 900°C for 5 minutes. The yield strength, ultimate tensile strength and ductility increase while yield ratio decreases with annealing. This is due to formation of Fe₃Al intermetallic and reduction of ferrite grain size. Due to high interstitial Carbon in Steel 2, yield point phenomenon was observed which may reduce the formability of the steel. The cracks started during tensile test at the ferrite grains and ferrite grain boundaries and the fracture took place in the quasi cleavage mode in Steels 1 and 2. The grain refinement and the microstructural features due to rolling and annealing are accountable for the good combination of strength and ductility of the both steels.