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This study focused on ruptures due to bending deformation of steel pipe that is operated with a high design factor. The design factor refers to the ratio of the hoop stress from internal pressure to the yield strength of pipe material. Bending experiments were conducted on steel pipes with a high design factor, and the failure mode was investigated. Ruptures occurred on the tensile side of the pipes and were accompanied by local reduction of the wall thickness (i.e., necking), which indicated strain localization. To evaluate the deformability of the pipes, a method based on finite-element analysis (FEA) was developed to predict the displacement at the rupture. The yield surface shape and true stress-strain relationship after uniform elongation had to be defined based on the actual properties of the bend material. The yield condition under the biaxial stress condition was measured with the biaxial tensile test and approximated with Hill's yield function. The true stress-strain relationship, including after uniform elongation, was determined by the inverse calibration method. Rupture by bending was predicted under these conditions. In contrast, the von Mises yield criterion, which is commonly used in cases of elastic-plastic FEA, overestimated the deformability; the true stress-strain relationship assumed by linear extrapolation after uniform elongation could not simulate the rupture. The FEA showed that rupture is dominated not by the fracture criterion of the material but by the initiation of strain localization, which is a deformation characteristic of the material. These ruptures are due to the rapid increase in local strain after the initiation of strain localization, which causes the fracture criterion to suddenly be reached.