Martensite reversion and strain hardening of a 2304 lean duplex stainless steel subjected to cold rolling and isochronous annealing at low temperatures
Renata Mangini Santos, Daniella Gomes Rodrigues, Maria Luiza Dias Santos, Dagoberto Brandão Santos
Abstract
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.