基于同步辐射的高强塑积含Cu/Ni中锰钢的复合强韧化机理研究

Because of the good combination of tensile strength and ductility and relatively low production cost of medium-Mn steels, researchers have aroused broad interest in studies of medium-Mn steels. However, the engineering application of medium-Mn steels is restricted due to the propagation of Lüders band during tensile deformation. Preliminary studies reveal that the low strength of ferrite and the austenite to martensite phase transformation during deformation are closely related to the propagation of Lüders band in medium-Mn steels. From the point view of strengthening and toughening mechanisms, we propose an approach for the development of medium-Mn steels to suppress the propagation of Lüders band, i.e. by adding Cu/Ni into medium-Mn steels, the strength of ferrite could be improved by the nanoprecipitation, the stacking fault energy of austenite increases, facilitating the deformation-induced twinning in austenite. Thus the strengthening and toughening mechanisms and mechanical behavior of medium-Mn steels could be tailored. This proposal will focus on the study of the key scientific issue on the correlations between the collaboration of nanoprecipitation strengthening and TRIP/TWIP toughening mechanisms and mechanical behavior of Cu/Ni-containing medium-Mn steels. The Cu/Ni-containing medium-Mn steels with the high product of tensile strength and ductility will be developed. The strengthening and toughening mechanisms of Cu/Ni-containing medium-Mn steels will be elucidated with in-situ synchrotron X-ray diffraction technique. The relationship among nanoprecipitation, deformation mechanisms, and Lüders band will be established. The successful execution of the proposal will contribute greatly to the development and engineering application of the third generation of advanced high strength steels of our nation.

中锰钢因其良好的强度塑性匹配和较低的成本受到了国内外学者的广泛关注，但拉伸过程中吕德斯带的扩展限制了其工程应用。前期研究发现，中锰钢中铁素体较低的强度以及变形过程中奥氏体向马氏体的转变与吕德斯带扩展有着紧密的联系。本项目提出从强韧化机理方面设计和开发中锰钢以抑制吕德斯带扩展的研究思路：通过Cu/Ni元素的添加，促进纳米相在铁素体中析出提高其强度，提高奥氏体层错能促进形变孪晶的产生，进而调控含Cu/Ni中锰钢的复合强韧化机理与力学行为。本项目以“含Cu/Ni中锰钢的纳米相析出强化和相变/孪晶诱导塑性的协同作用与力学行为的关联”科学问题为主线，研制综合力学性能优异的含Cu/Ni中锰钢，结合同步辐射原位表征技术阐明含Cu/Ni中锰钢的强韧化机理，建立含Cu/Ni中锰钢中纳米相析出强化和变形机制与吕德斯带扩展之间的关联。项目的成功实施对我国第三代先进汽车用钢的设计开发和工程应用具有重要的指导意义。

中锰钢因其具有良好的强度塑性匹配和较低的成本受到了国内外学者的广泛关注，但拉伸过程中吕德斯带的扩展限制了其工程应用。前期研究发现，中锰钢中铁素体较低的强度以及变形过程中奥氏体向马氏体的转变与吕德斯带扩展有着紧密的联系。本项目通过研究Cu/Ni元素对中锰钢组织与力学行为的影响发现，Ni元素的添加可以减小吕德斯应变，而Cu元素的添加不仅可以减小吕德斯应变还可以提高中锰钢的屈服强度。在此基础上，设计和制备了力学性能优异且强塑积大于60GPa%含铜中锰钢。原位同步辐射高能X射线衍射（HE-XRD）实验表明，由于中锰钢塑性变形过程中局部变形带的扩展，奥氏体步进地向马氏体转变。在塑性变形初期，奥氏体和铁素体均承受外加载荷。随着相变诱导塑性效应（TRIP）的发生，马氏体承担较大的外加载荷。随着退火温度的升高，奥氏体的力学稳定性降低。实验钢优异的加工硬化能力归因于奥氏体的适度稳定性以及TRIP效应和孪晶诱发塑性（TWIP）效应的协同作用。通过原位HE-XRD实验发现了铁素体的屈服强度高于奥氏体的屈服强度。进一步的强化机制分析表明，铁素体和奥氏体中固溶强化、位错强化和细晶强化对屈服强度的贡献彼此较为接近。铁素体屈服强度高于奥氏体的主要原因是富Cu纳米颗粒在铁素体中的析出强化作用。本项目的开展，对于研发新型中锰钢，调控其微观组织结构和强韧化机制，进一步提高其力学性能，具有重要的工程意义和理论价值，可以为第三代先进汽车用钢的设计开发和工程应用提供理论指导和技术支持。