Manganese controlled transformation and twinning of the nanoscale austenite in low-carbon-medium-Mn steel

A Fe-10.5Mn-0.06C steel consisting of austenite and ferrite dual phases with a volume fraction ratio of 7:3 was fabricated via the quenching and partitioning (Q&P) processing. Tensile tests at various temperatures and strain rates were performed to reveal the mechanical properties of the steel, and the deformation microstructures were characterized by using TEM, t-EBSD and APT techniques. At a constant strain rate of 0.1 s-1, yield strength, tensile strength increase as the temperature decreases from 100 degrees C to -50 degrees C, and reach the highest values of 560 MPa and 1390 MPa at -50 degrees C, respectively. The specimens deformed at low temperatures (below 200 degrees C) exhibit a characteristic of three-stage work-hardening behavior, while those deformed at higher temperatures (200 and 300 degrees C) only show two-stage work-hardening curves. The extra hardening stage found at low temperatures is associated with the concurrent twinning and transformation in austenite with varying Mn contents as well as the interaction between twins and dislocations. With increasing temperature, the diffusion of Mn from austenite to ferrite is depressed, inhibiting the martensitic transformation. Consequently, the TRIP effect is weakened and the steel shows the reduced work-hardening rates at higher temperatures. In addition, a few nanoscale austenite residues (<200 nm) are remained even after -25 degrees C tensile deformtion, due to the highly localized Mn concentration of ~15 wt%. These findings provide a guidance for designing advanced materials with combinations of ultrahigh strength and good ductility.

Automated summary

A Fe-10.5Mn-0.06C steel, fabricated via quenching and partitioning processing, exhibited different mechanical properties and deformation characteristics at various temperatures. The steel showed three-stage work-hardening behavior at low temperatures and two-stage work-hardening curves at higher temperatures, which were associated with the interactions between twinning, transformation, and dislocations.