Effects of V-N microalloying on microstructure and properties of high-strength shipbuilding steel

V-N microalloying is an important approach for enhancing the mechanical properties of non-quenched and tempered shipbuilding steel plates. This study systematically characterized the microstructures of different microalloyed tested steels using OM(optical microscope)、SEM(scanning electron microscope)、TEM(transmission electron microscope) and EBSD(electron backscatter diffraction), and evaluated their mechanical properties using a universal tensile testing machine and a metal Charpy V-notch pendulum impact tester. The results indicate that V-N microalloying effectively refines the grain size, reducing the average grain diameter to below 10 μm. Specifically, the grain size of the V-N tested steel is 9. 1 μm, while the V-N-Ti tested steel exhibited the most significant refinement at 8 μm. More importantly, V-N microalloying promotes the formation of intragranular acicular ferrite and increases the proportion of high-angle grain boundaries, with the V-N tested steel achieving the highest proportion at 77% while the V-N-Ti tested steel reaches 63. 4%. Additionally, finely dispersed V(C,N) and(Ti,V)(C,N) precipitates are formed. After V-N microalloying, the strength and plasticity of the tested steels are significantly improved. The strength of the V-N-Ti tested steel is further enhanced compared with the V-N steel but its plasticity decreases. Under low-temperature impact testing at-60 ℃, the impact absorbed energy and crack propagation energy of the V-N microalloyed steels increase markedly. The fracture mode shifts from the brittle fracture dominated by quasi-cleavage in the 0 V-0 Ti tested steel to microvoid coalescence ductile fracture with numerous equiaxed dimples in the V-N tested steel. However, due to the reduced proportion of high-angle grain boundaries and coarsening of precipitates, the low-temperature toughness of the V-N-Ti tested steel deteriorated to some extent compared with the V-N steel. V-N microalloying achieves a comprehensive improvement in the strength and toughness of shipbuilding steel through a synergistic mechanism of "grain refinement+acicular ferrite regulation+nano-precipitation". While the addition of Ti enhances precipitation strengthening but also imposes constraints on toughness. The V-N steel achieves the optimal balance of strength, plasticity and low-temperature toughness.