Microstructural and mechanical properties development during quenching-partitioning-tempering process of Nb-V-Ti microalloyed ultra-high strength steel

The applicability of quenching-partitioning-tempering (Q-P-T) treatment to the Nb-V-Ti microalloyed ultra-high strength steel has been investigated by SEM, TEM, XRD examination and mechanical tests. After Q-P process, lath martensite and retained austenite were obtained. Three kinds of carbides have been identified, namely Ti-rich MC, Nb-rich MC and M3C. In addition to tempered martensite and retained austenite, M2C, M7C3 and M23C6 particles were formed after Q-P-T treatment. Two types of MC carbides were found to be the stable phases existing during tempering process. In both of the specimens after Q-P and tempering process, the width of lath martensite and the volume fraction of retained austenite increased with increase in partitioning temperature. After tempering process, a variation of mechanical properties was found when the partitioning temperature varies from 320 °C to 440 °C. The strength first decreased and then increased, impact toughness increased gradually with increase in partitioning temperature. There were no significant changes in reduction of area and elongation. The reduction of area was about 60% and elongation was around 20%.