残余奥氏体中的纳米Cu析出演变规律及其对奥氏体稳定性影响的研究

The transformation induced plasticity effect of retained austenite can effectively enhance ductility, meanwhile, stable retained austenite can lower the ductile-brittle transition temperature. So far, the concept of retained austenite was applied to develop advanced automobile steel, the third generation high strength low alloy steel and low-temperature steel. In general, retained austenite can be obtained through optimizing chemical composition and subsequent heat treatment. Our previous study showed that copper precipitation occurred in retained austenite, which not only decreases the stability of retained austenite, but also improve the mechanical properties. Apparently, it is curious that retained austenite remain stable after the precipitation of solution alloying element. The objective of this project is to study the precipitation behavior of copper in retained austenite through transmission electron microscopy and 3-D atom probe technology. By the thermodynamic, dynamic and linear elastic calculation, it is expected to make clear the formation condition of copper precipitates and the influence of hydrostatic stress field of copper precipitates on the stability of retained austenite for the purpose to reveal the stabilization mechanism of copper precipitates on retained austenite. We attempt to demonstrate the impact of copper precipitates in retained austenite on strength, ductility and toughness, and develop the theory of obtaining retained austenite using the effect of hydrostatic stress which is introduced by copper precipitates in retained austentie.

残余奥氏体的相变诱导塑性能同时提高强度和韧塑性，被广泛应用于汽车制造、先进钢开发及低温钢等领域。常用的获得残余奥氏体的途径为改变合金成分设计，及通过合适的热处理工艺。前期的工作发现，残余奥氏体中的铜析出有助于提高其稳定性，改善材料的力学性能。但目前为止，这一稳定化机制还不明确。本项目通过TEM技术对残余奥氏体中Cu析出的详细表征，Thermal-Calc动力学计算，结合3DAP技术对铜的富集行为进行定量化表征，同时建立铜析出相的应力场模型，研究应力场对奥氏体稳定性的影响。本项目旨在揭示铜析出相在残余奥氏体中的析出过程、析出规律及其稳定残余奥氏体的原理；阐明残余奥氏体中的铜析出对残余奥氏体塑性变形行为的影响，对材料力学性能的影响规律；建立利用析出相稳定奥氏体的理论体系，发展新的获取残余奥氏体的技术路线。

提高钢材的综合力学性能是本文研究的主要目的。通过添加少量的碳和微合金元素并利用多步临界热处理方法，获得了屈服强度在700MPa级别、800MPa级别等一系列钢材。由于在钢中组织调制出逆转变奥氏体和纳米析出相使得这类钢材兼顾高强度和高塑性。对钢材中的组织和纳米析出相的演化规律进行了系统研究，弄清了逆转变奥氏体中Cu析出相的产生机理及其对奥氏体稳定性的影响规律。通过聚焦离子束技术和场发射透射电子显微镜，观察到了逆转变奥氏体中存在9R Cu纳米析出相。根据纳米析出相的结构特征和奥氏体的生长规律证明了低合金钢中逆转变奥氏体中纳米Cu析出相来自于铁素体中形成的Cu析出相。随着临界回火保温时间延长，逆转变奥氏体的体积不断扩大，导致铁素体中形成的纳米Cu析出相被奥氏体所占有。同时，对Cu元素在铁素体中的析出行为进行了深入研究，得到了纳米Cu析出相形态、尺寸、密度、晶体结构随保温时间的演化规律。这些广泛存在的纳米析出相大幅度提高了钢材强度，也提高了奥氏体的屈服强度。这是本研究中钢兼顾高强度高塑性的主要原因。