目的 研究Nb微合金化后渗碳层和基体的显微组织变化规律，及Nb微合金化对接触疲劳性能的影响，以实现齿轮的接触疲劳长寿命。方法 利用真空渗碳炉将Nb微合金化及未Nb微合金化齿轮用钢18CrNiMo7-6进行渗碳热处理，采用滚动接触疲劳试验机进行接触疲劳试验，通过扫描电子显微镜(SEM)、X射线衍射仪(XRD)、透射电子显微镜(TEM)、电子背散射衍射显微镜(EBSD)和洛氏硬度计等设备，对试样的组织及硬度进行检测，探讨Nb微合金化对接触疲劳性能的影响。结果 渗碳热处理后，表面组织为针状马氏体、残余奥氏体和碳化物，心部组织为板条马氏体。Nb微合金化渗碳层组织发生了细化，位错密度由7.52×10¹⁵ m^?2增加到8.75×10¹⁵ m^?2，残余奥氏体含量由23.6%降低至15.4%，渗碳层硬度由58.6HRC提高至59.4HRC，心部奥氏体晶粒平均尺寸由20.5 μm降低至16.3 μm。剥落坑表面粗糙且呈分层结构，起裂位置位于次表面;剥落坑在滚动接触应力作用下发生加工硬化，Nb微合金化和未Nb微合金化的加工硬化硬度均提高了1HRC左右，抗变形能力相差不大。Nb微合金化齿轮钢的接触疲劳寿命L10=3.2×10⁷，L50=8.2×10⁷;未Nb微合金化齿轮钢的接触疲劳寿命L10=2.0×10⁷，L50=6.4×10⁷。结论 Nb微合金化后，渗碳层组织细化，位错密度增大，显著抑制了裂纹的萌生，并且渗碳层的硬度稍有增加，综合作用使得齿轮钢的接触疲劳寿命L10和L50分别提高37.5%和22%。

The work aims to study the change of microstructure of carburized layer and matrix and its effect on contact fatigue performance after Nb microalloyed, so as to achieve long life contact fatigue life of gear. Carburizing heat treatment were carried out on gear steel 18CrNiMo7-6 with and without Nb microalloyed by means of vacuum carburizing furnace, contact fatigue tests were carried out by means of the rolling contact fatigue tester. Scanning electron microscope (SEM), X-ray diffraction (XRD), transmission electron microscope (TEM), electron backscattering diffraction microscope (EBSD) and rockwell hardness tester were used to detect the microstructure and hardness of the samples, and the influence of contact fatigue property was discussed. The results show that the surface was acicular martensite, residual austenite and carbide, and the matrix was lath martensite after carburizing heat treatment. The microstructure of the carburized layer after Nb microalloyed was refined, the dislocation density increased from 7.52×1015m?2 to 8.75×1015m?2, the content of residual austenite decreased from 23.6% to 15.4%, the hardness of the carburized layer increased from 58.6HRC to 59.4HRC, and the grain size of the core austenite decreased from 20.5 μm to 16.3 μm. Typical failure mode was spalling pit on contact surface. The spalling pit was rough and layered; The work hardening of spalling pits occured under the action of rolling contact stress, and the work hardening hardness of both Nb microalloyed and non-Nb microalloyed pits increases by about 1HRC, and the deformation resistance of spalling pits had little difference. For Nb microalloyed gear steel, fatigue lives L10=3.2×107, and L50=8.2×107; while for the gear steel without Nb microalloyed L10=2.0×107, L50=6.4×107. So after Nb microalloyed, the microstructure of the carburized layer was refined, and the dislocation density increased, which significantly inhibited the initiation of cracks. The hardness of the carburized layer increased slightly, and the rolling contact fatigue life L10 and L50 increased by 37.5% and 22%, respectively.