残余奥氏体对BQ&P高强钢超高周疲劳行为的影响及其机理研究

Very high cycle fatigue (VHCF, i.e., enduring cyclic stress in excess of 10E8 cycles) behaviors of the 1500MPa grade high-strength steels have become an important subject to ensure the long-term safety of the structural components such as railway wheels and rails. The failure mechanism for VHCF is basically different from that for the conventional low cycle fatigue (LCF) and high cycle fatigue (HCF). Recently, the third-generation advanced high strength steels (AHSSs) have been ongoing to increase strength and ductility to higher levels. The excellent combination of strength and ductility of the AHSSs is mainly attributed to the transformation induced plasticity (TRIP) effect of the retained austenite (RA), which depends on the amount and stability of RA. However, the effect of RA on the VHCF behaviors of the AHSSs is still unclear. Therefore, in this project, we attempt to design the multiphase microstructure of the Mn-Si-Cr series bainite/martensite high strength steels through bainitic transformation plus quenching partitioning (BQ&P) process. Various multiphase microstructure (i.e., bainite, martensite and RA) with different fraction, morphology, size and distribution would be obtained after BQ&P treatment. The wide distribution ranges of the volume fraction and stability degree of the RA would be designed in especial. Then we aim to investigate the effect of RA on the VHCF behaviors of the BQ&P high strength steels in term of the volume fraction and stability degree of the RA. The effect of RA on the VHCF properties and the mode of the fatigue crack initiation would be studied. Then the prediction model between the volume fraction and stability degree of RA, the parameter of ODA areas and the VHCF properties of the BQ&P high strength steels would be developed. The multi-dimensional/multi-scale microstructural observations in the ODA areas of the fatigue fracture would be carried out through the step-by-step polishing, focused ion beam (FIB) system, high resolution transmission electron microscopy (HRTEM), electron backscatter diffraction (EBSD) and 3-D atom probe tomography (APT) techniques. Then, the role of the RA on the formation of the ODA in the “inclusion-induced crack initiation” and “non-inclusion induced crack initiation” would be explored. The crack mechanisms of the two fatigue crack initiation modes would be developed from the perspective of the effect of RA. Finally, the influence and its mechanism of the RA on the VHCF crack initiation of the high strength steels would be revealed. The results of this project will provide the experimental and theoretical guidelines of the design of the microstructure and process to develop the AHSSs with long-life and high ductility.

高强钢的超高周疲劳行为具有与传统疲劳不同的特征；残余奥氏体是改善先进高强钢强塑性的关键组织因素，但其对超高周疲劳行为的影响规律还不完全清楚。鉴于此，本项目计划以Mn-Si-Cr系贝/马复相高强钢为基础，通过BQ&P工艺调控钢中的复相组织（贝氏体、马氏体和残奥），特别是调控残奥的含量和稳定性，系统研究残奥对高强钢超高周疲劳性能及疲劳裂纹萌生方式的影响，建立“残奥含量及稳定性-ODA区参量-超高周疲劳性能”三者之间的预测模型。采用逐层抛光、聚焦离子束、高分辨透射电镜、电子背散射衍射和3D原子探针等技术对ODA区进行多方位/多尺度的精细组织观察，研究残奥在“夹杂物起裂”和“非夹杂起裂”的ODA区形成过程中的作用及其异同，提出残奥影响“夹杂物起裂”和“非夹杂起裂”裂纹萌生的模型，揭示残奥对高强钢超高周疲劳裂纹萌生的影响及其机理。研究成果将为长寿命高强塑性先进高强钢的组织工艺调控提供理论指导。

超高周疲劳是轨道交通、航空航天领域面临的重要课题，金属材料的超高周疲劳行为具有与传统疲劳不同的特征。残余奥氏体是改善先进高强钢强塑性的关键组织因素，但其对高周/超高周疲劳行为，尤其是疲劳裂纹萌生的影响规律还不完全清楚。本项目通过调控冶炼工艺和BQ&P热处理工艺调控钢中的夹杂物和显微组织，系统研究了残余奥氏体含量和稳定性对超高周疲劳性能及裂纹萌生行为的影响规律。项目阐明了夹杂物和显微组织对超高周疲劳性能的交互控制规律，提出了协同控制夹杂物和显微组织改善钢疲劳性能的原理；建立了基于裂纹萌生特征区参量的超高周疲劳强度、寿命预测模型；掌握了不同类型的残余奥氏体在裂纹萌生和初期扩展中的演变规律，揭示了残余奥氏体对疲劳裂纹萌生的作用机制；提出了贝氏体钢的内部组织起裂的微观机理模型，明确了控制贝氏体钢超高周疲劳损伤的组织基因；建立了残余奥氏体宏观稳定性与疲劳强度的定量模型，提出了通过调控残余奥氏体稳定性和相变路径以提升钢疲劳性能的思路。制备了抗拉强度为1410MPa，10E9周次疲劳强度为770MPa的低碳BQ&P钢，以及抗拉强度为1688MPa，10E9周次疲劳强度为875MPa的中碳BQ&P钢。研究成果对于进一步理解金属材料的超高周疲劳裂纹萌生机制、以及开发先进高强钢的长寿命化设计策略具有重要的科学与工程意义。