Submicrocrystalline structures and tensile behaviour of stainless steels subjected to large strain deformation and subsequent annealing

Tensile behaviour of two steels with submicrocrystalline structures, i.e. a 304-type austenitic steel and an Fe-27%Cr-9%Ni austenitic-ferritic steel, was studied. The starting materials were subjected to large strain rolling and swaging to a total strain of ∼4 at ambient temperature. The severe deformation resulted in a partial martensitic transformation and the development of highly elongated austenite/ferrite (sub)grains aligned along the deformation axis. In the cold worked state, the transverse grain/subgrain size was about 100 nm in the 304-type steel and about 150 nm in the Fe-27%Cr-9%Ni steel. The grain refinement by severe plastic deformation resulted in increase of ultimate tensile strength to 2000 MPa and 1800 MPa in 304-type and Fe-27%Cr-9%Ni steels, respectively. The phase transformation and recrystallization took place concurrently upon annealing, leading to the development of submicrocrystalline structure consisting of austenite and ferrite grains. No significant softening took place under annealing at temperatures below 600°C. The tensile strength was 1920 MPa in 304-type steel and 1710 MPa in Fe-27%Cr-9%Ni steel after annealing at 500°C for 2 hours.