Niobium, Vanadium, Titanium and Erbium, Hafnium, Tantalum and Chromium, Molybdenum, Tungsten and other elements are difficult of molten metal, belongs to Ⅳ B of the periodic table, VB and Ⅵ B, have the ability to form nitrides and carbides. The radius difference between Nb, V, Ti and Fe atoms is very small, and the face-centered cubic structure of nitride and carbide as well as the face-centered cubic and body centered cubic matrix of steel are coherent, which can be both dissolved in and precipitated under certain conditions.
(1) Comparison of the possibility of forming nitrides and carbides. The nitride of Ti is formed in the solidification stage of molten steel and is actually insoluble in austenite. Therefore, the grain size can be controlled in the hot working and heating process of steel and the weld seam during welding. In addition, due to the formation of TiN, free nitrogen in steel can be eliminated, which is beneficial against aging. The nitride and carbide of V are almost completely dissolved in austenite and have no effect on controlling austenite grain. The compounds of V were precipitated only during or after the gamma-phase transition. The precipitates were very small and had a very significant precipitation enhancement effect. Ti carbides and Nb nitride and carbides can be dissolved in the high temperature austenite zone and precipitated in the low temperature austenite zone. The binding effect of Nb, V and Ti on austenite grain boundaries can refine ferrite grains after phase transition.
(2) Comparison of the influence of deformation recrystallization. Solid solute atoms drag on diffusion-controlled reactions or phase transitions, thus pushing the recrystallization process to a higher temperature, while carbonitride precipitation promotes nucleation of phase transition and retards the growth of secondary grains. From this point of view, Nb and Ti microalloyed steel has higher recrystallization temperature and finer austenite grain. The recrystallization temperature of V microalloyed steel is low, and the V-Ti composite can give full play to the retarding effect on new grain boundary of multiple recrystallization.
(3) Comparison of precipitation strengthening effect. The increment of precipitation intensification depends on the number of precipitates and particle size, as well as the difference in lattice constants between the iron atoms of the coherent particle. The yield strength increment produced by precipitation intensification is Nb>Ti>V in the range of ~ 0.14% carbon content. To achieve the same dispersion enhancement effect as V, use 1/2 Nb. The strengthening effect was also restricted by the tendency to form precipitates in austenite, which was promoted by deformation. Therefore, the strengthening effect of NbC was only in the steel with low carbon content.
