Composition-property relationships for low alloyed S355J2 steel

The effects of vanadium and niobium microalloying on the microstructure and mechanical properties of structural steel have been studied. In low carbon steel containing vanadium and niobium, the contribution to hardening of the following mechanisms increases: grain refinement, hardening by dispersed precipitates of carbides and nitrides, dislocation hardening. These chemical elements refine the grains, enhance microstructure uniformity and mechanical properties of low alloyed steel. In the present work, statistical analysis of the acceptance testing results of the normalized rolled S355J2 steel has been performed. The sampling amounts to 15619 meltings. The meltings were divided into 2 parts: the first included the melting alloyed with vanadium, the second included the melting alloyed with niobium. The minimum yield strength was 373 MPa for melts with vanadium and 400 MPa for melts with niobium, which is higher than the required by 20-40 MPa. The values of the tensile strength fit into the required interval 470-630 MPa. The average values are ~545 MPa. The minimum values of the KV-20 impact energy are 3-4 times higher than the required 27 J. The average values of KV-20 (with vanadium) = 159 J and KV-20 (with niobium) = 168 J. Thus, steel S355J2 is charac-terized by a stable and high complex of mechanical characteristics. Tensile test and Charpy test at negative temperatures show good combinations of strength and ductility due to microalloying additions.In this paper the composition-property relationships are also shown: σ0,2 = 392%V + 408 (MPa), R = 0,4; KV-20 = 583%V + 134 (J), R=0,35; σ0,2 = 692%Nb + 402 (MPa), R = 0,46; KV-20 = 1204%V + 130 (J), R = 0,52. Microalloying with just vanadium in amounts of 0,01-0,10% or only with niobium 0,01-0,05% increases the yield strength of S355J2 steel by ~30 Mpa, the impact energy is increased by ~40 J. From the obtained «composition-property» models it follows that niobium has a more significant influence on the mechanical characteristics of S355J2 steel