Tailoring the metastable reversed austenite from metastable Mn-rich carbides

Abstract

A two-steps tempering-partitioning process is designed for redistributing carbon and austenite stabilizer such as manganese in a cold rolled medium-manganese steel. After tempering at 450℃ for 0.5h and 1h, a large number of metastable Mn-rich M12C carbides and high density of NiAl-type nanoparticles were found, and the segregation of carbon at the dislocations/interfaces around NiAl precipitates was revealed by 3D atom probe tomography (APT). The competition between the M12C carbides and the dislocations/interfaces for carbon inhibits transformation of the metastable M12C carbides to more stable carbides. During the following partitioning at 630℃ for 0.5h, the metastable Mn-rich M12C dissolved and the fine reversed austenite nucleating on the metastable M12C carbides exhibits a sharp gradient Mn distribution. The tailored metastable austenite inherited from metastable M12C carbide shows strong enrichment of Mn and C (Mn: 18.1 at.%, C: 1.56 at.%), dispersed distribution (9.8 μm−2) and fine size (50-200 nm). With the nature of both hard particle and transitionable phase, the austenite couples Orowan strengthening and transformation induced plasticity (TRIP) assisted work hardening. Outstanding dynamic mechanical properties (yield strength: 1350 MPa, total elongation: 30%) were achieved due to this microstructure. Here we report a strategy for the design of ultrahigh-strength steels by making metastable austenite serve as a strengthening second phase in addition to the TRIP effect.