The Nb-V-Ti-N-C System Microelements Coupling Precipitation Behavior and its Effects on Properties in High Strength Naval Steel

The Nb-V-Ti-N-C system microelements coupling precipitation behavior in high strength naval steel was thermodynamically analyzed. The effects of micron/nano particles on the microstructure, mechanical properties, and corrosion resistance were also studied by an in situ scanning electron microscopy (SEM) tensile test, transmission electron microscopy (TEM) analysis, and electrochemical polarization measurements. The results show that the solid solution amount of Nb, V, Ti, N, or C decreases in the steels as the temperature decreases. Carbonitrides begin to precipitate at 1506.39 °C in N1 steel, and the carbonitrides are nano-scale. Meanwhile, carbonitrides begin to precipitate at 1628.74 °C in N2 steel, which is 116.69 °C higher than the corresponding liquidus temperature of 1512.05 °C; carbonitrides with micron scale are formed in the metal melt. The tensile test revealed that with the increase in titanium content from 0.05% to 0.1%, the strength increases while the elongation decreases. The in situ SEM test results indicated that lower plasticity are associated with the carbonitrides of micron-scale, which are the micro crack sources under stress. Polarization test results indicated that pitting corrosion may easily occur at the abnormally large-sized (Nb, V, Ti)(C, N) carbonitrides.