CCT curves of Mn-Si steels with different manganese contents or carbon contents were determined. The results show that the transformation range of bainite can be separated from that of ferrite when the manganese content approaches a certain content,and the incubation period of ferrite increases more significantly than that of bainite transformation with the increase of carbon content in Mn-Si steels. Furthermore,water-cooled bainitic steels without adding expensive alloying element were developed. Granular bainite was obtained when a bar with diameter of 300mm was cooled by water,and a mixed microstructure of granular bainite and martensite was obtained in water-cooled plate with thickness of 40mm. The developed water-cooled bainitic steels containing no expensive alloying element showed a good combination of strength and toughness. The tensile strength,yield strength,and toughness (AKU at -20℃) of bar with diameter of 300mm after water cooling were higher than 850MPa,620MPa,and 65J,respectively,and those of plate with thickness of 40mm after water cooling were higher than 1000MPa,800MPa,and 50J,respectively. CCT curves of Mn-Si steels with different manganese contents or carbon contents were determined. The results show that the transformation range of bainite can be separated from that of ferrite when the manganese content of certain content, and the incubation period of ferrite increases more significantly than that of bainite transformation with the increase of carbon content in Mn-Si steels. furthermore, water-cooled bainitic steels without adding expensive alloying element were developed. a mixed microstructure of granular bainite and martensite was obtained in water-cooled plate with thickness of 40mm. The developed water-cooled bainitic steels containing no expensive alloying element showed a good combination of strength and toughness. The tensile strength, yield strength, and toughness (AKU at -20 ° C) of bar with diameter of 300mm after water cooling were higher than 850MPa, 620MPa, and 65J, respectively, and those of plate with thickness of 40mm after water cooling were higher than 1000MPa, 800MPa, and 50J, respectively.