Effect of Central Multiphase Microstructure of Thick Plates on Work Hardening and Crack Propagation

The effect of multiphase microstructure in the center segregation zones (CSZ) of high-strength steel thick plates on work hardening characteristics and crack initiation/propagation were studied. Microstructure was correlated with deformation damage behavior by using Taylor factor (TF) gradients, kernel average misorientation (KAM), and geometric necessary dislocation (GND) density. The results show that the segregation leads to a mixed structure of ferrite (soft phase) and martensite/bainite (hard phase) in the center of the thick plate. Compared to the 1/4 thickness (1/4 T) region, grain refinement occurred in the CSZ, with KAM values increasing from 0.24 to 0.49 and a decrease in the proportion of high-angle grain boundaries (> 15°) from 77.4 to 51.8%. In the process of deformation, due to the influence of grain refinement and adjacent martensite/bainite, ferrite grains were divided into structural units with different TF values. The higher KAM value and GND density at the interface between soft/hard phases resulted in severe work hardening. In addition, the presence of micron-sized inclusions in the CSZ caused local strain concentration and non-continuous deformation of the matrix, which induced crack initiation. The fracture surface showed a mixed mode of brittle cleavage fracture and ductile fracture.