在扫描电子显微镜下观察分析了B级船板钢试样在原位拉伸过程中的断裂行为以及低温脆性断口中二次裂纹的扩展。结果表明:在拉伸变形过程中,微裂纹首先在试样缺口处形成,然后沿铁素体+珠光体界面扩展。加载过程中,多边形铁素体发生塑性变形,裂纹在基体内以“Z”字型扩展。在低温脆性断裂区,二次裂纹以穿晶方式通过铁素体基体,在扩展到珠光体区域时有时沿铁素体和珠光体的界面扩展,有时穿过珠光体区域扩展。 The fracture behavior of B-class steel plate specimens during in-situ tensile test and the expansion of secondary cracks in low-temperature brittle fracture were observed under a scanning electron microscope. The results show that during the tensile deformation, the micro-cracks are firstly formed at the notch of the sample and then extend along the ferrite + pearlite interface. During loading, the polygonal ferrite undergoes plastic deformation and the crack propagates in the “Z” shape within the matrix. In the low-temperature brittle fracture zone, secondary cracks pass through the ferrite matrix in a transgranular fashion and sometimes extend along the interfaces of ferrite and pearlite when expanded into the pearlite region, sometimes expanding through the pearlite region.