The microstructural evolutions of the cold rolled Fe-0.1C-5Mn steel during intercritical annealing were examined using combined advanced techniques.It was demonstrated that intercritical annealing results in an ultrafine granular ferrite and austenite duplex structure in cold rolled 0.1C-5Mn steel.The strong partitioning of manganese and carbon elements from ferrite to austenite was found during intercritical annealing by scanning transmission electron microscopy(STEM)and X-ray diffraction(XRD).Strong effects of boundary characters on the austenite formation were indicated by austenite fast nucleation and growth in the high angle boundaries but sluggish nucleation and growth in the low angle boundaries.The ultrafine grained duplex structure in 0.1C-5Mn was resulted from the the sluggish Mn-diffusion and the extra high Gibbs free energy of ferrite phase.Based on the analysis of the microstructure evolution,it was pointed out that the intercritical annealing of the medium-Mn steels could be applied to fabricate an ultrafine duplex grained microstructure,which would be a promising approach to develop the 3rd generation austomobile steels with excellent combination of strength and ductility. The microstructural evolutions of the cold rolled Fe-0.1C-5Mn steel during intercritical annealing were examined using combined advanced techniques. It was said that intercritical annealing results in an ultrafine granular ferrite and austenite duplex structure in cold rolled 0.1C-5Mn steel.The. strong partitioning of manganese and carbon elements from ferrite to austenite was found during intercritical annealing by scanning transmission electron microscopy (STEM) and X-ray diffraction (XRD). Strong effects of boundary characters on the austenite formation were indicated by austenite fast nucleation and growth in the high angle boundaries but sluggish nucleation and growth in the low angle boundaries. The ultrafine grained duplex structure in 0.1C-5Mn was resulting from the sluggish Mn-diffusion and the extra high Gibbs free energy of ferrite phase. Based on the analysis of the microstructure evolution, it was pointed out that the intercritical annealing of the medium-Mn steels could be applied to fabricate an ultrafine duplex grained microstructure, which would have a promising approach to develop the 3rd generation austomobile steels with excellent combination of strength and ductility.