Effect of V-Nb Composite Microalloying on Microstructure and Properties of Non-Quenched and Tempered Forged Steel

Herein, non-quenched and tempered forging steels containing V and V-Nb are designed, and the mechanical properties and microstructure of two steels are compared and analyzed. The comprehensive mechanical properties of V-Nb containing steel are as follows: the yield strength is 525.1 MPa, the impact energy A(kV) is 62.1 J at ambient temperature, and the elongation is 26.1%. It is shown in the results that the addition of Nb element can refine the grain size (17.2 mu m), increase the ferrite content (54.1%), and refine the lamellar spacing of pearlite (274 nm). The formation of V (C, N) particles on MnS inclusions can promote fine ferrite nucleation and growth, and Nb element can further promote ferrite nucleation by forming coarser (V, Nb) (C, N) particles. The difference of yield strength and hardness between the two steels is mainly caused by the difference of precipitation strengthening, the precipitation-strengthening increment of V-Nb containing steel is 18.31 MPa higher than that of V containing steel, which is because the coarser-size (V, Nb) (C, N) particles produce stronger precipitation-strengthening effect. But the large-sized MnS inclusions are beneficial to the increase of crack driving force and reduce the plasticity and toughness.