基于“干扰贝氏体转变”工艺的纳米结构贝氏体钢及其组织调控机理

The third generation advanced high-strength steels (AHSSs) have been investigated intensively with the aim of saving resources and protecting environment by decreasing the weight of engineering components. Recently, the nanostructured bainitic steels consisting of nanoscaled bainitic ferrite and carbon-enriched filmy retained austenite have attracted many attentions, due to the excellent combination of strength, ductility and toughness. However, the formation of nanostructured bainite by isothermal holding at a low transformation temperature shows a very slow transformation rate, which leads to a very long heat treatment time. The “disturbed bainitic austempering” (DBAT) process that combines the bainitic isothermal transformation and quenching-partitioning (Q&P) processes could effectively reduce the heat treatment time. The DBAT process has a distinct microstructure evolution characteristic, compared with the bainitic isothermal transformation and Q&P processes. Therefore, in this project, based on the Mn-Si-Cr series bainitic steels, we attempt to study the carbon enrichment behaviors in the austenite during the DBAT process, through DIL805L quenching device, Thermo-Calc and DICTRA simulation and 3-D atom probe tomography (APT) techniques. The chemical composition and process parameters that affecting the carbon enrichment behaviors in the austenite would be explored. Then we would develop a prediction model to predict the carbon enrichment limit in austenite during the DBAT process. The microstructure evolution during the DBAT process would be investigated. Based on these studies, the mechanism of microstructure design of the nanostructured bainitic steels treated by the DBAT process would be revealed. The results of this project would provide a novel insight on the microstructure design of the nanostructured bainitic steels. Then the ductile super nanostructured bainitic steels with independent intellectual property rights would be developed.

纳米结构贝氏体钢具有纳米尺度的贝氏体铁素体和富碳的残余奥氏体，显示良好的强度、塑性和韧性匹配，是近年来研究的热点。“干扰贝氏体转变”工艺巧妙地将贝氏体等温转变和淬火-配分工艺相结合，有效地解决了纳米贝氏体钢热处理时间过长的技术难题。“干扰贝氏体转变”工艺具有不同于贝氏体等温转变和淬火-配分工艺的组织转变特征，本项目计划在Mn-Si-Cr系贝氏体钢的基础上，利用DIL805L淬火膨胀仪、Thermo-Calc和DICTRA模拟和3D原子探针等技术研究“干扰贝氏体转变”工艺过程中奥氏体碳富集行为，探讨影响奥氏体碳富集行为的成分和工艺因素，建立奥氏体碳富集极限的预测模型，研究“干扰贝氏体转变”工艺过程中组织转变的规律，揭示经“干扰贝氏体转变”处理的纳米结构贝氏体钢的微观组织调控机理，研究成果有助于丰富纳米贝氏体钢的组织调控手段和思路，进而发展具有自主知识产权的高强塑性纳米结构贝氏体钢。

纳米结构贝氏体钢具有纳米尺度的贝氏体铁素体和富碳的残余奥氏体，显示良好的强度、塑性和韧性匹配，具有良好的应用前景；如何加快贝氏体的转变速率、稳定钢中的残余奥氏体是纳米贝氏体钢应用过程中关键问题。本项目设计了3种低成本Mn-Si-Cr系贝氏体钢，研究了其相变行为；基于贝氏体等温转变与淬火-配分工艺结合的“干扰贝氏体转变”工艺，结合Thermal-Calc模拟、淬火膨胀仪、XRD分析和多尺度显微组织表征等技术研究了工艺参数对残余奥氏体碳富集行为的影响规律，揭示了影响残余奥氏体碳富集极限的关键成分和工艺因素；结果显示，“干扰贝氏体转变”工艺中残余奥氏体中的碳富集极限分布在T0线和PLE/NPLE线附近，当配分温度较高时，残余奥氏体中的碳含量接近PLE/NPLE线，当配分温度较低时，向T0线靠近。在此基础上，研究了“干扰贝氏体转变”工艺的组织演变规律，分析了影响残余奥氏体成分、形态、尺寸及亚结构的关键工艺参数；在低成本60Mn2Si贝氏体钢中获得强度级别为1500~1800MPa，延伸率20~35%，强塑积接近50GPa%的优异强塑性匹配。分析了“干扰贝氏体转变工艺”处理纳米结构贝氏体钢强塑性改善的机理，揭示了不同变形阶段不同形貌的残余奥氏体的塑性变形机制，发现残余奥氏体的亚结构（位错、层错、孪晶）和相界面是影响奥氏体塑性变形机制是关键组织因素。通过本项目，揭示的奥氏体碳富集极限的关键工艺因素，为进一步揭示贝氏体的相变机制、制定合理的热处理工艺具有一定的科学和工程意义；基于“干扰贝氏体转变”工艺，加快了贝氏体转变的速率，制备了具有优异强塑性的纳米贝氏体钢，拓宽了纳米贝氏体钢的工艺体系。