Crystallographic analysis of the influence of cross section effect in thick plate for high-rise building on the hardness of abnormal banded structure

Revealing the high hardness mechanism of abnormal banded structure from the crystallography perspective can effectively prevent the local failure behavior of materials. In this study, the surface, 1/4 thickness (1/4 T), and center segregation samples of 70 mm thick plates for high-rise building were taken and treated at different cooling speeds (water cooling, oil cooling, air cooling). The effects of cooling rate and solute segregation on hardness were studied. The results showed that the mixed structure of lath martensite and lath bainite is formed in the surface, 1/4 T and center segregation zone (Center-SZ), and the microstructure hardness is 310 ± 10, 287 ± 10, 366 ± 17 HV, respectively. The increase in hardness was attributed to the refinement of the substructural size and the increase in the density of the large angle grain boundaries (> 15°, D_LAGBS). The analysis conducted using the Hall–Petch formula for validation and fitting led to the conclusion that the block acts as an effective structural unit for controlling hardness. The solute element segregation in the center-SZ enhances the variant selection in the phase transformation process and increases the fractions of V2, V3, and V5 variants. These variants formed block boundaries with the V1 variants, ultimately leading to a high block boundary density (> 55°, D_BBS). In addition, elements segregation increases the driving force of bainite phase transformation and decreases the driving force of martensite phase transformation, forming a high fraction of V1/V2 variant pairs in both phases. It can adjust the phase transformation strain, increase the D_LAGBS, and finally make the abnormal banded structure obtain a higher hardness value (366 ± 17 HV).