Vanadium - Transforming Possibilities

The Use of Vanadium in High Strength Low Alloy Steels

Abstract:

The microalloying elements V, Nb and Ti are commonly used to improve the properties of HSLA steels. Of these three microalloying elements, the carbides and nitrides of V have the highest solubility, and this influences the steel types in which V is used, and how they are processed. This paper will review the use of V in HSLA steels and give recent examples of how the high solubility of V carbides and nitrides is utilised in a range of steel types. V additions are well suited to medium and high carbon steels, as the high solubility of V carbo-nitrides, V(C,N), enables the formation of the fine precipitates necessary for precipitation strengthening. In concrete reinforcing bar, V(C,N) precipitation strengthening allows high strengths to be obtained combined with weldability and high ductility for use in seismic zones. In direct forging steels, V additions enable costly quench and temper heat treatments to be eliminated, and in bainitic forgings, increased hardenability allows improved properties to be achieved over a wider section size. V additions to rail steels have been shown to significantly increase rail life, and in wire rod steels, V not only increases strength, but decreases the formation of coarse grain boundary cementite which is detrimental to ductility. In low carbon steels, increased V additions allow significant volume fractions of fine precipitates to be produced, giving exceptionally high precipitation strengthening. This allows high strength single phase ferritic steels to be produced with excellent edge ductility for the automotive sector. At low processing temperatures such as those used for hot forming of structural hollow sections or the annealing of cold rolled dual phase steels, V(C,N) still has sufficient solubility to produce useful precipitation strengthening, and also significant grain refinement. Finally the high solubility of V(C,N) allows reduced reheating and equalisation temperatures to be used prior to rolling, and limits precipitation in austenite. This results in reduced cracking during continuous casting, and gives a minimal increase in rolling loads which allows increased rolling rates and the production of wider material.

Keywords:

vanadium, precipitation, mechanical properties, processing, products