Mash seam welders are not applied to continuous cold rolling lines. The presumption is that rolling fracture occurs under conditions of high reduction because of residual steps at the edge of the lap seam, the defective solid phase bonding line and the defective melted area. The flattening mechanism of residual steps has mainly been investigated, which is a characteristic of mash seam welding. Furthermore, a mash seam welder for preventing double lapped deformation is developed. In the process of development, it is concluded that the potential for rolling fracture at a double lapped defect is increased when the angle of the steps formed increases. On the basis of FEM and experimental analysis, in this report it is argued that rolling fracture is caused by double lapped defects similar to cracks. The conclusions are as follows: (1) The configuration of a rolling fracture caused by a double lapped defect is a shear-type ductile fracture. (2) During rolling in a bite around a double lapped defect, the mean normal stress is in a state of tension and the stress triaxiality is very large. (3) Rolling fracture caused by a double lapped defect is occurred in a roll bite.
