Microstructure and tensile properties of a Nb-Mo microalloyed 6.5Mn alloy processed by intercritical annealing and quenching and partitioning

The transformation behavior, microstructural evolution and mechanical properties were compared in a coldrolled Nb-Mo microalloyed 6.5Mn alloy after intercritical annealing (IA) and quenching and partitioning (Q & P), respectively. The thermodynamic calculation and theoretical analysis were used to determine the optimal heat treatment parameters. The Q & P samples exhibited relatively higher strength with relatively low ductility, mainly due to the hard martensite matrix, which resulted in continuous yielding behavior upon loading, whereas the IA samples showed the significantly improved ductility, which benefited from the more sufficient transformation-induced plasticity (TRIP) effects and the softer ultrafine ferrite matrix. The dependence of yield point elongation (YPE) of IA samples on grain size demonstrated that the YPE value was in the reverse proportional relationship to the average grain size, which agreed well with theoretical analysis.