图为DZ2（0.05V0Nb）钢低温疲劳断口形貌，0.05V钢在室温至-40℃温度区间，断口裂纹扩展区占比相当，-60℃时裂纹扩展区占比略有下降。图 (e)-(g)给出了-20℃试样裂纹起裂区、裂纹扩展区和瞬断区微观形貌，可见，疲劳断口为单点起裂，扩展区呈解理状，瞬断区呈韧窝状。

The figure shows the low-temperature fatigue fracture morphology of DZ2 (0.05V0Nb) steel. In the temperature range of 0.05V steel from room temperature to -40℃, the proportion of fracture crack growth zone is similar, and the proportion of crack growth zone is slightly decreased at -60℃. Figure (e)-(g) shows the micromorphology of the crack initiation zone, crack propagation zone and transient fracture zone of the sample at -20℃. It can be seen that the fatigue fracture is a single point of initiation, the expansion zone is like cleavage, and the transient fracture zone is like dimple.