Excellent combination of strength and ductility in 15Cr-2Ni duplex stainless steel based on ultrafine-grained austenite phase

The room-temperature duplex structure of duplex stainless steel is always metastable, which suggests that non-equilibrium phase transformation can be further exploited for producing duplex stainless steel having the same chemical compositions and phase constitution but different microstructures. This work uses rapid solidification obtained duplex stainless steel to expand heat-treatment temperature range for achieving 50/50 duplex structure. Research shows an equilibrium state for the phase constitution of duplex stainless steel after sufficiently long time annealing, and establishes the non-equilibrium kinetics diagram of ferrite-to-austenite transition in cold-rolled duplex stainless steel. It is then shown that the duplex stainless steel with about 50% austenite phase can be prepared using different non-equilibrium thermal process, of which the yield strength and elongation vary in the ranges of 306–499 MPa and 20–33%, respectively. The sample, which exhibits the best combination of yield strength (371 MPa) and elongation (33%), is attributed to the bimodal distribution of austenite grain size.