A remarkable role of niobium precipitation in refining microstructure and improving toughness of A QT-treated 20CrMo47NbV steel with ultrahigh strength

The microstructures and niobium-contained precipitates of 20CrMo47NbV martensitic steel quenched at the different temperatures of 900–1000 °C and tempered at 650 °C were characterized by optical microscopy (OM), scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). The impact properties at varied temperatures from 0 °C to 80 °C were measured from all the as-quenched and tempered specimens, and the 50% FATTs were determined. The results show that the particles of Nb-rich (Nb,V)C and Nb-rich (Nb,Ti,V)(C,N) precipitated in 20CrMo47NbV steel during quenching. In comparison to 25CrMo48V steel without niobium, the impact toughness of QT-treated 20CrMo47NbV steel was significantly improved as a result of the refinement of austenite grain and final martensitic structures. This could be attributed to the role of niobium precipitation in restraining the austenite grain growth during the heating process of quenching.