基于过冷奥氏体变形的纳米贝氏体强韧化机理研究

High-C high-Si nanostructured bainite steels can be obtained by low-temperature isothermal transformation, which has a combination of strength and fracture toughness comparable to maraging steels. However, the poor impact toughness of the nanostructured bainite steels is an issue that remains to be solved. The thermomechanical simulation results of the present research group show that the deformation strengthening of supercooled austenite can produce nanostructured bainite in medium-C steel by low-temperature isothermal transformation. In addition, the deformation-induced fibrous structure can not only result in the increase in resistance to crack growth due to the grain refinement along crack but also result in delamination fracture due to the preferred orientation of product phase bainite formed in displacive mode, which can improve the strength and toughness. Therefore, in this study we propose an idea for strengthening and toughening based on supercooled austenite deformation that can combine the two toughening routes, i.e. fibrous structure/delamination and lowering carbon content. We plan to study on the effect of rolling deformation of supercooled austenite on microstructure, tensile and impact properties, and delamination feature of nanostructured bainite steels with various carbon contents, and to examine fracture morphology in order to explore the fracture mode and the strengthening and toughening mechanism. By studying, we can put forward a route and theoretic basis for strengthening and toughening of nanostructured bainite steels and provide technical reference for the preparation of the steels with high strength and toughness.

高碳高硅钢低温等温转变得到纳米结构贝氏体组织，其强度和断裂韧性接近于马氏体时效钢的水平，但冲击韧性低的问题尚待解决。我们前期热机械模拟研究表明，利用过冷奥氏体变形强化实现中碳钢低温等温转变，可获得纳米结构贝氏体。此外，过冷奥氏体变形（特别是大变形）产生纤维组织既可以减小沿裂纹方向晶粒尺寸，增大裂纹扩展阻力，又可以导致切变转变产物贝氏体组织具有取向而使断口产生分层，从而提高强度和韧性。故本项目提出基于过冷奥氏体变形的强韧化思路，把降低纳米结构贝氏体的碳含量和纤维组织/分层两个韧化结合起来。研究过冷奥氏体轧制变形对不同碳含量纳米结构贝氏体组织特点、拉伸与冲击性能及分层断裂的影响，结合断口形貌分析断裂机制，揭示过冷奥氏体变形的纳米结构贝氏体强韧化机理。通过本项目研究，可为纳米结构贝氏体钢的强韧化提供有效途径和理论依据，并为高强韧纳米贝氏体钢的制备提供技术参考。

高碳高硅钢低温等温转变得到纳米结构贝氏体组织，其强度和断裂韧性接近于马氏体时效钢的水平。但对于低中碳钢，由于碳含量降低使Ms升高，不利于低温等温转变，而添加Ni等合金元素使Ms和Bs降低，且加入量增大使得Ms与Bs差很小，造成低温贝氏体转变“窗口”消失。我们前期热机械模拟研究表明，利用过冷奥氏体变形强化降低其Ms，可实现中碳钢低温等温转变，获得纳米结构贝氏体。此外，过冷奥氏体变形（特别是大变形）产生纤维组织，可以导致切变转变产物贝氏体组织具有取向，对强韧性必将产生影响。为此，本项目研究了过冷奥氏体轧制变形对低温等温转变组织和力学性能的影响，揭示了过冷奥氏体变形的纳米结构贝氏体强韧化机理。研究结果表明，温轧奥氏体在Ms以上10−40°C等温转变可得到贝氏体铁素体板条厚度为70±9−124±40nm的纳米贝氏体组织，残余奥氏体含量比未变形明显增大。等温转变温度降低，板条厚度减小，硬度提高到606−682HV1.0，与高碳纳米贝氏体钢相当。与未变形奥氏体等温转变相比，温轧奥氏体等温转变试样冲击功稍低，热轧奥氏体等温转变试样冲击功稍高。温轧奥氏体低温等温转变试样抗拉强度提高显著，如500°C温轧奥氏体200°C等温转变试样达到超高强度水平（2581MPa），高于高碳纳米贝氏体钢的水平；延伸率达到6.5−13.0%。强度2200MPa以上所对应的延伸率高于高碳纳米贝氏体钢的水平。温轧奥氏体低温等温转变纳米贝氏体超高强度主要源于组织细化到纳米尺度和温轧引入的位错遗传给贝氏体使位错密度升高。较高塑性主要源于温轧奥氏体等温转变组织含有较多残余奥氏体。本研究结果为纳米贝氏体钢的强韧化提供了新思路和理论依据，为超高强度纳米贝氏体钢的制造提供了技术参考。