Effect of Delamination on Ductile Fracture during the Impact Test in Ultra-Heavy Steel

Two industrial ship plate steels, one having delamination and the other having no delamination during the impact tests at − 60 and − 80 °C, were studied to clarify the effect of delamination on ductile fracture. The instrumented impact tests recorded the load and absorbed energy. The microhardness maps at the selective section of the broken specimen were drawn to display the tendency of strain hardening and the stress state during fracture. The delamination morphology was observed to clarify the reason for the delamination in heavy-gauge steel. The texture transform characteristics were correlated with the initial and deformed microstructures. The results demonstrated that the delamination formed before the main fracture surface and divided the stress concentration into two parts during the impact test. Furthermore, by transmitting much plastic deformation into the specimen far away from the fracture edge and giving full play to the characteristics of crystallographic texture rotation, the severe stress concentration and grain breakage could be avoided. At the same time, the region that should have been formed a radial zone with a plane strain state at the lower part of the fracture could be eliminated and still maintain a ductile fracture contributing to the absorbed energy of 63 J. The segregation of Mn element, different plastic deformation ability between ferrite and bainite, and the crystallographic texture of <001>//ND orientation caused the delamination.