Speaker
Description
It is the nature of microalloying elements, primarily Ti, Nb, and V, to form stable carbide and nitride compounds with interstitial carbon and nitrogen present in the steel matrix. The resulting fine-sized compound particles have the potency to retard or prevent phenomena such as recovery, recrystallization and grain growth. The controlled microalloy precipitation during thermomechanical processing is the basis for generating dedicated microstructural features resulting in advanced properties of such steels. Microalloy solubility in steel is determined by the solubility product, representing the maximum solute content (carbides/nitrides) in austenite or ferrite. Solute concentration increases exponentially with temperature. Initially, it is the aim to redissolve microalloy compounds ahead of hot processing to activate the microalloying elements for the subsequent processing steps. Therefore, it is decisive to define a sufficiently high soaking temperature for redissolving the microalloy compounds as much as possible. In later processing stages, on the contrary, it is often desirable to prevent dissolution of existing microalloy particles. In either case, the solubility product is the fundamental criterion determining the stability of microalloy compounds. Over the more than five decades that microalloying technology is in use many solubility products have been developed that, however, do not result in a unique dissolution temperature for nominally the same microalloy compound. This can be the consequence of the measurement technique used to determine the amount of dissolve microalloy as well as the presence of other alloying elements in the steel. This contribution critically reviews available solubility products in austenite and ferrite addressing the potential origins of variation. Furthermore, novel aspects of microalloy solubility in the intercritical ferrite range including the possible consequences for microstructural control will be discussed.