Complex influence of carbon, niobium and vanadium on the mechanical properties of the structural steel S355N

The role of microalloying additions of V, Nb, Ti manifested mainly as a result of their influence on the formation of substitution solid solution; dispersion, shape and distribution of the carbides (carbonitrides); the structure of boundaries and the fine structure of the grains; reducing the negative impact of harmful impurities. It is important to precise knowledge of the required number of microalloying elements in the steel. The objective of the present article is to review the role of Nb and V in micro-alloyed steel S355N and to know how does they affect on the mechanical properties. For this, the volume of the total population in 1340 heats of steel S355N has been divided into 3 sampling selection: the first - with a carbon content at the lower limit Cmin = 0,15%, the second - the upper limit Cmax = 0,20% and the third - the average Cmed = 0,17% . The paper presents the results of mathematical modeling of complex influence of carbon, niobium and vanadium on the strength properties and impact toughness of sheet steel S355N. It has been established that the simultaneous increase of the content of carbon and niobium, carbon and vanadium causes an increase in yield strength and tensile strength. Carbon has a negligible influence on the impact strength KV-20 °C at constant of niobium and vanadium. Niobium with different carbon content positively affects on the impact strength, while vanadium - clearly negative. Increasing the vanadium content up to 0.06% when carbon content Cmax = 0,2% leads to a drop in impact energy up to 23 J at which this value does not satisfy the requirements of EN 10025. The required by EN10025 level of strength YTS?355 MPa; UTS=470-630 MPa and low temperature-temperature toughness KV-20 °C?40 J will be achieved when the content: C ?0,15%, Nb? 0,04%, V? 0,060%