Nanoscale precipitation and ultrafine retained austenite induced high strength-ductility combination in a newly designed low carbon Cu-bearing medium-Mn steel

A novel low carbon Cu-bearing medium-Mn steel was designed and subjected to intercritical annealing (IA) and tempering (IAT). The yield strength and uniform elongation significantly increased from 659 MPa and 10% to 911 MPa and 20% by tempering. The microstructure of sample IAT was comprised of ferrite, ultrafine retained austenite, tempered martensite, and hierarchical Cu particles. Large size precipitates (9.7 ± 3.1 nm) were formed during intercritical annealing, while fine particles (1.85 ± 0.36 nm) were formed during tempering. Hierarchical Cu particles increased yield strength of ferrite by ~267 MPa, which compensated the strength loss induced by intercritical annealing and tempering. The carbon and alloying elements were further partitioned to austenite from martensite during tempering, which increased austenite stability. As a consequence, TRIP effect occurred over a wide strain regime, which contributed to a superior ductility. Before tempering, yielding of retained austenite was caused by martensite transformation at a low stress because of poor stability, which was avoided by the enhanced stability of retained austenite after tempering. As a result, the yield strength of austenite, and thereby the yield strength of the steel was increased.