Abstract: Effect of tempering temperature on the combination of strength and toughness of air cooled carbide free bainite-martensite (CFB/M) microstructure and water quenched martensite (M) microstructure for a low carbon Mn-Si-Cr steel is experimentally investigated. The results show that CFB/M microstructure compared with M microstructure has a better combination of strength and toughness after intermediate temperature tempering. The fracture toughness of the steel with CFB/M microstructure should be characterized by JIC while that of the steel with M could be characterized by KIC after tempering. After tempering at 360 ℃,the combination of strength and toughness of the steel with CFB/M microstructure is s 0.2＝ 1 355 Mpa，sb＝1 600 Mpa，δ5＝13.5 %，j＝56.2 %， AK＝81 J, while that of the same steel with M microstructure is s0.2＝1 350 Mpa，sb＝1 617 Mpa，d 5＝14.1%，j＝59.5， AK＝67.5 J. The cause might be that CFB has a higher temper resistance than M. And carbon enriched retained austenite film in CFB with a good thermal stability might improve the toughness of the steel after tempering at intermediate temperature.

Key words: Carbide free bainite-martensite steel, Fracture toughness, Strength and toughness