The development and mechanical performances of new type air-cooled Mn series bainitic steels including granular bainitic steels,FGBA/BG duplex steels,CFB/M duplex steels,medium carbon bainite/martensite steels,cast bainitic steels invented by the authors are summarized.The novel series of bainitic steels are alloyed with Mn,and several series bainitic duplex microstructures can be easily obtained under the condition of air cooling through unique composition design.The invented idea,the principle of alloying design,the strengthening mechanism,and the evolution of the microstructure of new type air-cooled Mn series bainitic steels are presented.Furthermore,the applications in different fields of these Mn series air-cooled bainitic steels with different strength level are also introduced.It is suggested that the significance of the development of the air-cooled Mn series bainitic steel can be summarized as follows:reducing costs of both raw materials and production;good combination of strength and toughness;self-hardening with high bainitic hardenability by air cooling from hot working without additional quenching-tempering treatment or quenching procedure;large savings in energy resources;and reduced environmental pollution. The development and mechanical performances of new type air-cooled Mn series bainitic steels including granular bainitic steels, FGBA / BG duplex steels, CFB / M duplex steels, medium carbon bainite / martensite steels, cast bainitic steels invented by the authors are summarized. novel series of bainitic steels are alloyed with Mn, and several series of bainitic duplex microstructures can be easily obtained under the condition of air cooling through unique composition design. The invented idea, the principle of alloying design, the strengthening mechanism, and the evolution of the microstructure of new type air-cooled Mn series bainitic steels are presented. Still more, the applications in different fields of these Mn series air-cooled bainitic steels with different strength levels are also introduced. It is suggested that the significance of the development of the air -cooled Mn series bainitic steel can be rounded as follows: reducing costs of both raw materials and production; good combination of str ength and toughness; self-hardening with high bainitic hardenability by air cooling from hot working without additional quenching-tempering treatment or quenching procedure; large savings in energy resources; and reduced environmental pollution.