Martensite reversion and strain hardening of a 2304 lean duplex stainless steel subjected to cold rolling and isochronous annealing at low temperatures

Lean duplex stainless steels (LDSS) have a microstructure composed of ferrite and austenite phases. This microstructure provides good mechanical and corrosion resistances. The effects of cold rolling, leading to α′-martensite formation by strain-induced transformation and subsequent reversion annealing treatment were analyzed on the microstructural evolution and mechanical properties of an LDSS 2304. The steel was cold-rolled to a 60% reduction and annealed for 1800 s at 600, 700, 800, and 900 °C. The analysis of mechanical properties performed by tensile tests, along with microstructural analysis, allowed the characterization of each stage of strain hardening. The work hardening rate curves revealed well-defined stages involved in the steel deformation process, linked to the different mechanisms of plastic deformation of the microstructure. These results indicated that a transformation-induced plasticity, TRIP effect, occurred concurrently in the austenite after reversion at low annealing temperatures. The synergy and mutual competition from the coexistence of slipping and TRIP caused multiple-stage strain hardening. The shape of the strain-hardening curve for the selected annealing temperature range could be divided into two cases: a multiple-stage strain-hardening for the as-received and fully annealed sample (900 °C) to a characteristic three-stage strain-hardening from 600 to 800 °C annealing conditions. Microstructural observations revealed that the typical three-stage hardening was mainly related to strain-induced martensite formation and its reversion.