OBRABOTKAMETALLOV Vol. 26 No. 3 2024 technology Sulfur and phosphorus are detrimental impurities and its content is limited to values of no more than 0.005 % and no more than 0.012 %, respectively, to ensure high values of impact energy during impactbending testing. Vanadium, niobium, and molybdenum are limited by the required mechanical properties. Calcium is an unavoidable technological impurity. With an increase in the calcium content in the hotrolled strip, corrosion-active non-metallic inclusions of the first kind are formed, which negatively affects the mechanical properties of rolled products and the corrosion resistance of steel. The precipitations of TiN and Nb(C,N) effectively reduce the growth of austenitic grains [14, 46]. VN, NbCN and TiC particles are stable at a normalization temperature of about 900 °C, which creates a sufficient volume fraction of fine particles to control grain growth [56–59]. The Nb solid solution and the precipitation of its carbonitrides retard the growth of austenite grains (Fig. 6). а b c d Fig. 6. Solubility of carbonitrides depending on temperature: a – vanadium nitride; b – niobium carbide; c – niobium nitride; d – titanium carbide [59] On the other hand, small Ti additives cause fine separation of nanoscale carbides, which limits the growth of austenitic grains at higher temperatures (1,200 °C) [15, 16]. Ti is released during steel solidification and causes a local concentration that promotes the precipitation of large TiN particles [15, 16]. TiC particles can also cause strengthening. Vanadium is the most versatile precipitation strengthening element and is effective in a variety of micro-alloy steel compositions, as well as higher carbon steels. It has been noted in the literature that vanadium carbonitrides V(C,N) can potentially create sites for ferrite nucleation. A small lattice mismatch between vanadium nitride (VN) (lattice parameters = 0.4139 nm) and ferrite (lattice parameters = 0.2865 nm) promotes ferrite nucleation [14, 45, 46].
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