The Role of Vanadium in Microalloyed Steels

Abstract:

The present monograph concerns the role of vanadium as a microalloying element in structural steels. A brief historical outline is followed by three chapters that are of a didactic nature, introducing the physical and chemical properties of vanadium in steels and, in particular, its reactions together with the elements carbon and nitrogen. Basic thermodynamic concepts and their application presented in chapter 2 where vanadium is also compared with other microalloying elements, titanium and niobium. The different behaviours of these elements in steel processing are shown to depend on the varying solubilities of their carbonitrides in austenite. The relatively high solubility of V(C,N) in austenite means that vanadium plays only a minor role during hot deformation and mostly remains available for precipitation strengthening during subsequent cooling. Chapters 3 and 4 discuss the precipitation behaviour of carbonitrides in austenite and ferrite respectively. Co-precipitation of VN together with TiN is often an important factor since TiN is the more stable compound and frequently acts as a substrate for VN in austenite. A relatively recent finding is the ease with which ferrite nucleates on VN particles, thus providing scope for grain refinement and improved mechanical properties. Mechanisms of precipitation of V(C,N) during or after the phase transformation to ferrite are described and analysed in relation to their strengthening contributions, including inter-phase precipitation and the symbiotic effects of carbon and nitrogen contents on uniform precipitation in ferrite.

Effects of vanadium and also other elements are described in connection with the structures and ductility problems encountered during continuous casting of conventional and thin slabs in chapter 5. Principles of thermomechanical controlled processing (TMPC) are outlined in chapter 6 with special reference to vanadium and the benefits of recrystallization controlled rolling (RCR) for optimisation of final strength and toughness. The most important factors are the deformation schedule, the steel composition, especially the nitrogen content, and the cooling conditions through the austenite to ferrite transformation range. Following these general principles, chapter 7 takes up specific product types where vanadium enhances their properties and discusses how the TMPC is most suitably carried out in each case. The product types considered are (i) heavy plate, (ii) strip, (iii) long products, (iv) seamless pipes and (v) forgings. In all of these, the presence of nitrogen plays an important role for enhancing strength through precipitation of V(C,N) in ferrite while in several cases grain refinement by nucleation of ferrite on VN particles in austenite contributes both strength and toughness.

The weldability of vanadium microalloyed products is discussed in chapter 8 in relation to microstructures and toughness properties of the heat affected zones (HAZs). In most situations, vanadium and the higher nitrogen levels used in these steels pose no problems for HAZ toughness. With high heat input welding there is loss of toughness caused by coarse grained ferrite formation but for low heat input conditions the HAZ toughness is remarkably high for both single and multipass welds.