某热轧机用十字万向关节在非关键受力处发生断裂。通过宏观观察、化学成分分析、力学性能测试、金相检验、断口分析等方法，分析了该十字万向关节的断裂原因。结果表明：十字万向关节断裂的主要原因为，材料内部存在孔洞群及大颗粒非金属夹杂物，成为裂纹源，并在往复运行中产生疲劳断裂；断裂的次要原因为材料内部非金属夹杂物较多，材料的强度和韧性均较差。

The cross universal joint for a hot rolling machine fractured at the position without high stress. The fracture reasons of the cross universal joint were analyzed by means of macroscopic observation, chemical composition analysis, mechanical property testing, metallographic examination and fracture analysis. The results show that:the main cause of fracture of the cross universal joint was the existing of voids and bulky non-metallic inclusions in the material, which resulted in crack source and fatigue cracking during reciprocating motion; the minor cause of the fracture was that a large number of non-metallic inclusions existed in the material, which resulted in the poor strength and toughness.