Enhanced ductility and toughness by tailoring heterogenous microstructure in an ultra-heavy gauge high strength steel with severe centerline segregation

Abstract

• New approach was offered to greatly enhance ductility and toughness with severe centerline segregation. • Heterogeneous microstructure was homogenized to fine multi-phase microstructure. • Centerline segregation produced more stable retained austenite. • Partitioning during annealing made local fluctuation of Mn concentration. • Similar local fluctuations of Mn concentration led to chemical homogeneity on the whole. A novel two-step intercritical heat treatment was applied to tailor the heterogenous microstructure in an ultra-heavy E500 offshore engineering steel with severe centerline segregation. Compared with the mechanical properties in the center of the studied steel after thermo-mechanical controlled processing, the yield strength of the steel is maintained above 550 MPa, the ductility and low-temperature toughness were greatly improved, the elongation was increased from 18.7% to 29.1%, and the low-temperature impact energy at −40 °C was enhanced from 16 to 163 J. The fundamental basis for the enhancement should be attributed to the more homogenous chemical composition distribution from the overall view and the fine multiphase microstructure with stable retained austenite obtained in the segregation region.