Improved strength-ductility-toughness balance of a precipitation-strengthened low-carbon medium-Mn steel by adopting intercritical annealing-tempering process

The present study on low-carbon medium-Mn steel demonstrates that the newly proposed "intercritical annealing-tempering" process led to good combination of strength, ductility and impact toughness, by simultaneously optimizing austenite reverted transformation and Cu precipitation. Intercritical annealing at high temperature accelerated the dissolution of cementite particles, promoted the formation of reverted austenite, and then increased the total elongation and impact energy. The additional tempering at 500 degrees C facilitated intensive precipitation of Cu-rich precipitates in secondary martensite. The precipitates were fine, uniform in size and high in density, which provided a significant strengthening effect. Strengthening by nano-scale Cu-rich precipitates compensated softening associated with high temperature annealing, and increased the yield strength of tempered medium-Mn steel by 75 MPa. In addition to facilitating Cu precipitation, the introduction of secondary martensite also promoted nucleation of reverted austenite during tempering with sufficient C and Mn concentration that stabilized austenite and further improved ductility. Excellent mechanical properties, 960 MPa yield strength, 1010 MPa tensile strength, 29.2% total elongation and 150 J impact energy at room temperature were obtained in the precipitation-strengthened low-carbon medium-Mn steel, on intercritical annealing at 645 degrees C, followed by tempering at 500 degrees C for 2 h.