随着高寒地区铁路建设的不断推进，钢轨在低温环境下的疲劳性能引起了广泛关注。目前，许多研究聚焦于高强度钢在常温下的疲劳性能，但对于贝氏体钢轨钢的低温疲劳裂纹扩展行为，尤其是微观组织(特别是残余奥氏体)的影响机制，仍缺乏清晰的认识。本工作以热轧空冷处理的U20Mn2SiCrNiMo (简称U20Mn)贝氏体钢轨为研究对象，采用SEM、TEM、EBSD、XRD等手段对钢轨的显微组织进行了多尺度表征，研究了在室温(25 ℃)和-40 ℃下钢轨的常规力学性能与疲劳裂纹扩展行为。结果表明，热轧空冷处理贝氏体钢轨的显微组织以粒状贝氏体/马氏体复相组织为主，含有约10% (体积分数)的残余奥氏体。随着温度降低，钢轨的强度升高，而冲击韧性下降。疲劳裂纹扩展曲线与拟合的Paris公式表明，在一定应力强度因子范围内(8~18 MPa×m^1/2)，-40 ℃下具有更低的疲劳裂纹扩展速率；但疲劳断口观察结果表明，在-40 ℃时疲劳裂纹扩展呈现由韧性断裂向脆性断裂转变的趋势。分析认为，低温条件下钢轨强度的升高、韧性的降低以及钢中残余奥氏体稳定性的变化3者协同影响是造成贝氏体钢轨低温疲劳裂纹扩展速率变化的内在原因。

Bainitic rail steels have been extensively studied as a solution to the considerable fatigue and wear issues experienced with conventional pearlitic rails. With railway constructions in high-cold regions, the effect of    low temperatures on the fatigue performance of railways has garnered considerable attention. Despite several studies on the room-temperature fatigue performance of high-strength steels, the effect of the microstructure, particularly retained austenite (RA), on the low-temperature fatigue crack growth behavior in bainitic rail steels remains unclear. This study examined the low-temperature fatigue crack propagation behavior of U20Mn2SiCrNiMo (U20Mn) bainitic rail steel subjected to hot rolling and air cooling. The microstructure of the bainitic rail steel was characterized employing SEM, TEM, EBSD, and XRD. The conventional mechanical properties and fatigue crack propagation rate (da/dN; where a is the crack length and N is the number of stress cycles) were determined at room temperature (25 ℃) and −40 ℃. The results demonstrated that the U20Mn bainitic rail steel was primarily composed of a granular bainite/martensite multiphase structure, with approximately 10% (volume fraction) RA. A decrease in the experimental temperature increased the tensile and yield strength of the U20Mn rail steel; however, its impact toughness decreased. The da/dN vs. stress intensity factor (ΔK) curves for U20Mn rail steel at 20 ℃ and −40 ℃ indicated that the fatigue crack propagation rate reduced at low temperatures within the ΔK range of 8–20 MPa·m^1/2. However, an examination of the fatigue fracture surface revealed a transition from ductile to brittle fractures at −40 ℃. This indicates that the combined effects of the increased strength, decreased toughness, and changes in the stability of RA at low temperatures are the underlying factors responsible for the variation in the low-temperature fatigue crack propagation rate of bainitic rail steel. These findings are expected to guide the design of low-temperature fatigue-resistant bainitic rail steel by optimizing the volume fraction and RA stability.