Homogenization of Through-Thickness Microstructure and Mechanical Properties in Direct-Quenched High-Nb Q690 Steel via Tempering

Manufacturing heavy-gauge high-strength steel plates with uniform through-thickness properties is challenging due to the limited hardenability and significant cooling rate variations inherent to heavy sections. However, the mechanism governing microstructural homogenization across such large cross-sections remains not fully understood. This study investigates the through-thickness microstructure and mechanical properties of a 60 mm thick high-Nb microalloyed Q690 steel plate processed by direct quenching (AQ) and subsequent tempering at 530 °C and 580 °C. Characterization was performed at the surface (0t), quarter-thickness (1/4t), and core (1/2t) locations. Results revealed a pronounced gradient in the as-quenched state: while the surface consisted of fine lath martensite/bainite, the core formed coarse granular bainite containing blocky martensite-austenite (M-A) constituents. This microstructural heterogeneity resulted in poor core toughness (~24 J). High-temperature tempering at 580 °C promoted the complete decomposition of these metastable M-A constituents into ferrite and fine carbides, significantly improving the core impact energy to ~49 J. However, a toughness gradient persisted compared to the quarter-thickness (>120 J), attributed to the inherited coarse matrix and the formation of grain boundary carbides. Notably, high yield strength was maintained across the thickness despite matrix recovery. This is primarily attributed to a potent anti-softening effect provided by thermally stable (Nb,Ti,Mo)C nanoprecipitates, which generate strong Orowan strengthening. These findings highlight the critical role of optimizing the trade-off between M-A decomposition and carbide evolution in promoting the microstructural and property homogenization of heavy-gauge steels.