Fracture mechanism of inclusions in thick plate center under tensile loading

Segregation of solute atoms in the center of thick plates of the tempered steel can cause an inhomogeneous structural transformation and generate micron-sized inclusions, which leads to lamellar tearing of thick plate and decreases the plasticity and toughness. The formation and fragmentation mechanisms of micron-sized inclusions, like MnS and (Nb, Ti)C, in the center of thick plates were investigated by using thermodynamic calculations, finite element simulations, and electron backscatter diffraction characterization techniques. The results show that micron-sized inclusions nucleate and grow in the liquid phase, and under tensile loading, they exhibit three fragmentation mechanisms. The local stress during the fragmentation of inclusions is lower than the critical fracture stress of adjacent grains, and phase boundaries can effectively impede crack propagation into the matrix. The existence of a low proportion of high-angle grain boundaries (58.1%) and high Kernel average misorientation value (0.534°) in the segregation band promotes inclusions fragmentation and crack propagation. The difference in crack initiation and propagation direction caused by the morphology of inclusions and physical properties, as well as different matrix arrest abilities, is the main reasons for the diversity of inclusion fragmentation.