PRECIPITATION AND GRAIN REFINEMENT IN VANADIUM-CONTAINING STEEIS

The present work has concentrated on the roles of vanadium, nitrogen and carbon in controlling the precipitation of V(C,N) in austenite and ferrite and their effects on:(i) grain refinement by promoting the formation of intragranular ferrite, and (ii) precipitalion strengthening by interphase and random precipitation.
The degree of precipitation strengthening of ferrite at a given vanadium content depends on the available quantities of carbon and nitrogen. It is concluded that nitrogen is a very reliable alloying element, increasing the yield strength of V-microalloyed steels by some 6 MPa for every 0.001%N, essentially independent of processing conditions. The role of tarbon in precipitation strengthening is complicated. The present results have shown that the precipitation strengthening of V-microalloyed steels increases significantly with the total C-content, ≈5.5MPa/0.01%C. The explanation is that the metastable equilibrium between ferrite and undercooled austenite greatly increases the solubility of carbon in ferrite, in the times available during transformation, thereby contributing to profuse nucleation of V(C,N) particles. This effect of carbon is particularly significant for medium carbon steels typically used for hot rolled bars and sections.
The experimental results strongly indicate that vanadium can be effectively used not only for precipitation strengthening but also for ferrite grain refinement. It was shown that vanadium contributes to the formation of two types of intragranulary nucleated ferrite; polygonal(idiomorph) ferrite and acicular (sideplate)ferrite. Intragranular polygonal ferrite nucleates on VN particles that grow in austenite during isothermal holding or slow cooling throughout the austenite range. Acicular ferrite microstructure forms in V- microalloyed steels during isothermal transformation at lower temperatures (~450 C). The acicular ferrite microstructure was obtained in V-microalloyed steels containing high, medium or very low nitrogen levels. This suggests that vanadium on its own can promote the formation of the acicular ferrite microstructure.