Influence of band microstructure on carbide precipitation behavior and toughness of 1 GPa-grade ultra-heavy gauge low-alloy steel

This study investigated the influence of band microstructure induced by centerline segregation on carbide precipitation behavior and toughness in an 80 mm-thick 1 GPa low-carbon low-alloy steel plate. The quarter-thickness (1/4t) and half-thickness (1/2t) regions of the plate exhibited similar ductility and toughness after quenching. After tempering, the 1/4t region exhibited ∼50% and ∼25% enhancements in both the total elongation and low-temperature toughness at −40°C, respectively, without a decrease in yield strength, whereas the toughness of the 1/2t region decreased by ∼46°%. After quenching, both the 1/4t and 1/2t regions exhibited lower bainite and lath martensite concentrations, but only the 1/2t region exhibited microstructure bands. Moreover, the tempered 1/4t region featured uniformly dispersed short rod-like M₂₃C₆ carbides, and spherical MC precipitates with diameters of ∼20–100 nm and <20 nm, respectively. The uniformly dispersed nanosized M₂₃C₆ carbides and MC precipitates contributed to the balance of high strength and high toughness. The band microstructure of the tempered 1/2t region featured a high density of large needle-like M₃C carbides. The length and width of the large M₃C carbides were ∼200–500 nm and ∼20–50 nm, respectively. Fractography analysis revealed that the high density of large carbides led to delamination cleavage fracture, which significantly deteriorated toughness.