Abstract: The effect of different austempering processes on microstructure evolution and mechanical property of low carbon ultra-high strength bainitic steels was studied. The results show that compared with the one-stage austempering process， the amount of bainite increases and the content of martensite decreases in the two-stage process， which result that the yield strength of the experimental steel increases 58% and is up to 1178MPa， the elongation increases from 7.7% to 14.4% and the impact toughness increases 16% and reaches 66J/cm². Comparing the strength and plasticity matching of the corresponding experimental steels under different heat treatments， it is found that the product of strength and elongation of the steel can reach 21888MPa·% under the A₃(240℃for 2h and then 270℃for 1h) two-stage process condition. Under the two-stage process conditions， the martensite band due to Mn segregation in the steel can be eliminated and a relatively uniform bainite structure can be obtained， so that the strength and toughness of the experimental steel can be improved simultaneously.

Key words: ultra-high strength bainitic steel, austempering, strength and plasticity matching, the product of strength and elongation, bainite