针对拉伸断口分离问题, 对热轧高强度低碳贝氏体钢进行了纵向常规拉伸实验和三主轴方向短试样拉伸实验. 结果表明: 在纵向和横向取样的拉伸实验中均发生了断口分离现象, 分离面均垂直于厚度方向, 即平行于轧面. 通过对断裂试样分离裂纹的SEM观察发现, 分离面具有明显的低塑性解理断裂特征.利用三主轴方向短试样拉伸实验证明了原始钢板在纵向、横向和厚度方向上具有相似的强度和塑性性能.通过有限元模拟的方法, 对颈缩过程中侧向拉伸应力的水平进行了估算, 发现即使在颈缩程度非常严重时,侧向拉应力仍远小于主拉伸应力. 由此提出了拉伸过程中沿厚度方向由塑性到脆性的转变机制, 并进一步揭示了断口分离并非意味着钢板沿厚度方向存在性能差异, 而是由于贝氏体自身特有的力学性能导致的,是经严重的拉伸塑性形变后织构状态演变、晶界重分布以及三向应力状态出现综合影响的结果.

Split fracture phenomenon in tensile tests of a high strength low carbon bainitic steel plate was investigated. Regular tensile tests in the longitudinal direction and short bar tensile tests in three principal axis directions were performed. The results showed that split fracture always occurred when the tensile direction was longitudinal or transverse. The splitting surface was almost parallel to the direction of thickness. According to SEM observation of splitting crack, it was found that the ‘split’ had features of cleavage fracture. Meanwhile, the original steel plate was experimentally proved to have similar strength and ductility in the longitudinal direction, the transverse direction and the direction of thickness. Based on a finite element analysis simulation, the lateral tensile stress within the necked zone was calculated to be much lower than the principle tensile stress. Thus, a mechanism transition from ductile to brittle during the tension process was proposed. Moreover, it was revealed that the split fracture is not definitely the result of lower performance along the direction of thickness, but it is caused by the comprehensive influence of texture evolution, the redistribution of grain boundaries and the state of three dimensional stresses with large tensile plastic deformation, which can be considered as the characteristic of mechanical property of bainitic steel.