Fig. 1 Schematic illustration of heat treatment procedures of the cold-rolled Nb-Mo microalloyed 6.5Mn steel: a IA process; b Q & P process

Fig. 2 Predicted variation in different phase fractions as a function of QT for the cold-rolled Nb-Mo microalloyed 6.5Mn steel treated by Q & P: \(\gamma_{\text{R}}\), \(\gamma_{{1{\text{st}}}}\), \(\alpha_{{1\text{st}}}^{{\prime }}\) and \(\alpha_{{2\text{nd}}}^{{\prime }}\), which represent the final retained austenite, austenite after initial quenching, primary martensite and secondary martensite, respectively

Fig. 3 SEM microstructures of the Nb-Mo microalloyed 6.5Mn steel: the as-cold rolled a, the Q & P processed after austenitized at b 800 °C; c 850 °C

Fig. 4 TEM morphologies of the cold-rolled Nb-Mo microalloyed 6.5Mn steel treated by Q & P with different austenization temperatures: a-c 800 °C; d-f 850 °C; b, f correspond to the dark field images of a, e, respectively. \(\gamma_{\text{R}}\) retained austenite; \(\alpha_{\text{B}}\) carbide-free bainite; \(\alpha_{\text{L}}^{{\prime }}\) lath martensite; \(\alpha_{\text{T}}^{{\prime }}\) twinning martensite

Fig. 5 X-ray diffraction spectra of the cold-rolled Nb-Mo microalloyed 6.5Mn steel treated by Q & P with different austenization temperatures

Fig. 6 SEM micrographs of the cold-rolled Nb-Mo microalloyed 6.5Mn steel treated by IA at 650 °C for different time: a 10 min; b 30 min; c 60 min; d 180 min; e 270 min; f 360 min. α ferrite; \(\gamma_{\text{R}}\) retained austenite; SF stacking fault

Fig. 7 a X-ray diffraction patterns; b the volume fraction of retained austenite (\({\text{V}}_{R}\)) of the cold-rolled Nb-Mo microalloyed 6.5Mn steel treated by IA at 650 °C for different holding time

Fig. 8 Schematic illustrations of microstructural evolution of the cold-rolled Nb-Mo microalloyed 6.5Mn steel treated by IA. \(\alpha^{{\prime }}\) deformed martensite; \(\alpha\) ferrite; \(\gamma_{\text{R}}\) retained austenite

Fig. 9 Schematic illustrations of microstructural evolution of the cold-rolled Nb-Mo microalloyed 6.5Mn steel treated by Q & P. \(\gamma\) austenite; \(\alpha_{\text{A}}^{{\prime }}\) quenched martensite; \(\gamma_{\text{A}}\) quenched austenite; \(\gamma_{\text{R}}\) retained austenite; \(\alpha_{\text{P}}^{{\prime }}\) partitioned martensite

Fig. 10 True stress-strain curves of the cold-rolled Nb-Mo microalloyed 6.5Mn steel after IA a, Q & P b processes

Fig. 11 a Grain size versus annealing time; b yield point elongation versus grain size of the cold-rolled Nb-Mo microalloyed 6.5Mn steel treated by IA