Microstructural and magnetic characterization of a lean duplex steel: Strain-induced martensite formation and austenite reversion

Abstract

Strain-induced α′-martensite and austenite reversion in a cold rolled UNS S32304 lean duplex steel were tracked by means of magnetic measurements, with emphasis on both Ms (saturation magnetization) and Hc (coercive field) parameters. Grain-averaged quality metrics derived from EBSD (electron backscatter diffraction) analysis were also used to distinguish the phases during austenite reversion. The material was cold rolled to a true strain (ε) of 1.61 and subjected to isothermal and continuous annealing, the latter conducted in the presence of an external magnetic field. The evolution of the α′-martensite fraction upon straining and after isothermal annealing was monitored by coupling the Ms values and thermodynamic simulations, as well as from EBSD analysis. For the isothermally annealed material (ε = 1.61), the overall behavior of Ms and hardness displayed similar trends with a strong decrease for temperatures higher than 500 °C, suggesting austenite reversion. Results confirmed the occurrence of austenite reversion for the temperature interval investigated here. At 800 °C, austenite reversion is complete, and the steel is fully recrystallized. Besides, from the EBSD analysis, evidence of ferrite transformation into austenite was rather noticeable, in accordance with thermodynamic simulations and magnetic probing. Complementary electron channeling contrast imaging (ECCI) revealed that precipitation reactions mainly occur in the recrystallized austenite at 700 and 800 °C. The Hc behavior of both, the strained and annealed conditions was inferred to be mostly driven by microstructural changes in ferrite.