Effect of V, Nb, and Ti microalloying on low-temperature impact fracture behavior of non-quenched and tempered forged steel

In this study, three kinds of non-quenched and tempered forged steels with V, V-Nb and V-Nb-Ti microalloying elements were designed. The microstructure and low-temperature impact fracture behavior of the three steels were comprehensively compared and analyzed. The results showed that the addition of Nb and Ti elements can significantly refine the grains and form precipitated particles to improve the strength of the steel. The fracture behavior of the three steels types at a low temperature of-28 degrees C was dominated by brittle cleavage fractures. V-Nb-Ti steel exhibited the worst low-temperature impact toughness. The SEM analysis result showed that coarse (V, Nb, Ti) (C, N) particles present in the steel acted as local cleavage initiation sites. No cleavage fracture caused by (V, Nb) (C, N) particles occurred in the V-Nb steel. The results showed that the high impact energy (70.4 J) at room temperature and low ductile-brittle transition temperature (DBTT,-20 degrees C) of the V-Nb steel were mainly attributable to the grain refinement of Nb. The micron-sized coarse (V, Nb, Ti) (C, N) particles were the main factor responsible for the low impact energy (35.5 J) at room temperature and high DBTT (-7 degrees C) of V-Nb-Ti steel and can act as the local cleavage initiation sites to dominate the cleavage fracture at low tem-perature and decrease the impact toughness.