采用一步(300 ℃等温6 h)和两步(300 ℃等温2 h+250 ℃等温24 h)低温贝氏体转变工艺，研究了残留奥氏体对中碳微纳结构钢冲击韧性的影响，对不同热处理试样的显微组织、各相体积分数、大小角度晶界、有效晶粒尺寸与冲击性能进行表征和分析。结果表明，与一步贝氏体转变相比，两步贝氏体转变试样的冲击性能明显提高，-40 ℃冲击功从31 J提高到42 J，主要原因是第二步贝氏体转变时新形成的贝氏体铁素体分割细化块状未转变奥氏体，减少贝氏体等温后淬火过程中块状马氏体形成，在冲击过程中能够更好地使裂纹分叉甚至阻止裂纹的扩展，显著提高样品的韧性。

Micro/nano-structured bainitic steel provides a unique combination of ultra-high strength and high ductility due to their structure consisting of micro/nano-scale bainitic-ferrite and retained austenite, but the toughness is a little bit low. The retained austenite plays a leading role for the toughness, and it can significantly increase the toughness of micro/nano-structured bainitic steel by refining the size of blocky retained austenite and improving the content of film retained austenite. Simultaneously, the structure of retained austenite affects the stability of retained austenite, and even can change the micro-deformation and determine the toughness. This work has been refined retained austenite of medium-carbon bainitic steel by using two-step bainitic transformation to study phase transformation of retained austenite through heat treatment. The effect of retained austenite on the impact toughness in medium-carbon micro/nano-structured steels was analyzed by one (300 ℃ for 6 h) and two-step (300 ℃ for 2 h, then 250 ℃ for 24 h) bainitic transformation processes. The microstructure, phase fraction, misorientation, crystallographic grain size and impact energy of different heat treatment steels were observed, detected and analyzed. The results showed that the impact property of two-step bainitic transformation was significantly higher than that of one-step bainitic transformation in medium-carbon steel, which the impact energy in -40 ℃ increased from 31 J to 42 J. The main reason is the new bainitic ferrite was formed in two-step bainitic transformation, the untransformed retained austenite was divided and refined by new bainitic-ferrite, reducing the formation of massive martensite during water quenching after isothermal bainite process. It significantly improve the toughness of the steel because the fracture energy was increased, owing to making crack bifurcation and even preventing the propagation of cracks in the impact process. Through the above-mentioned studies, this research not only precisely refines the retained austenite structure, reveals the effect of retained austenite stability on deformation mechanism and resolves toughness mechanism, but also provides the theoretical guidance for the production of micro/nano-structured bainitic steels in combination with good toughness.