Thermomechanical Rolling of Air-Cooled Thick Nb-Ti-V-Ni High Strength Structural Plate

The effect of thermomechanical controlled processing (TMCP) strain and finishing temperature on the microstructure of as-rolled and air-cooled 50mm thick Nb-Ti-V-Ni microalloyed steel plate was investigated through laboratory simulations, incorporating variations in reheating, roughing and finishing practices. Laboratory simulations produced microstructures similar to those observed in industrial rolling owing to comparable total strain in the TMCP region. Larger total strains in the TMCP region promoted sub-grain formation and increased nucleation site density, leading to grain refinement. Recrystallisation was completely suppressed at the commencement of finishing in all TMCP schedules due to sufficiently lower starting temperatures. The extent of recrystallisation during finishing depended on the finishing temperature: i) partial softening after finishing above 910°C and ii) complete recrystallisation below 910°C due to substantial accumulated strain. Finishing below 910°C produced finer polygonal ferrite and pearlite microstructures. Microstructure and mechanical properties were fairly consistent when the finishing temperature was between 925 and 950°C. However, the sub-zero impact toughness can be significantly improved by employing lower finish temperatures or applying larger total TMCP strains. Interrupted accelerated cooling at 5°C/s after finishing significantly refined the microstructure.