Effect of large strain cold rolling and subsequent annealing on microstructure and mechanical properties of an austenitic stainless steel

The microstructural evolution of an S304H steel during bar rolling to a strain of 4 and subsequent annealing as well as its effect on the mechanical properties were investigated. The cold working was accompanied by a strain-induced martensitic transformation, leading to the development of lamellar-type microstructure consisting of highly elongated austenite/ferrite subgrains with a mean transverse size of approximately 50 nm; the austenite volume fraction was approximately 0.35. This material exhibited a yield strength above 2000 MPa. The subsequent annealing resulted in grain coarsening following the ferrite → austenite reversion, which led to almost full austenitization at temperatures above 700 °C. The formation of the austenite/ferrite lamellar structure that mixed with separate equiaxed grains occurred after annealing at temperatures of T ≤ 700 °C. The grain coarsening was accompanied by a degradation in strength, although the yield strength of above 1000 MPa remained after 2 h of annealing at 700 °C. The discontinuous recrystallization of austenite resulted in the development of a relatively coarse-grained microstructure at T ≥ 800 °C.