Challenges and opportunities in thermodynamic and kinetic modeling microalloyed HSLA steels using computational thermodynamics

Abstract

Most High Strength Low-Alloy (HSLA) steels rely on the control of the carbonitrides of Ti, Nb and V to achieve their properties. To properly control the distribution and size of precipitates the knowledge of the thermodynamics of the Fe–C–N-(Ti,Nb,V) systems is essential. Furthermore, understanding the kinetics of dissolution and precipitation of the carbonitrides is fundamental to properly design these steels and their processing. In this work, we emphasize Ti and Nb behavior in HSLA steels. First, the thermodynamic information concerning the stability of their carbonitrides in steel is reviewed, highlighting the difficulties associated with the variable C/N ratio of the carbonitrides and with the fact that there is also miscibility between Ti, Nb and V in the carbonitrides of the “NaCl” structure. These difficulties make the use of the compound energy formalism (CEF) and of computational thermodynamics extremely recommended. To demonstrate this, some frequently used solubility products are compared to results obtained with current computational thermodynamics databases. Currently, many efforts have been made to model the kinetics of dissolution and precipitation of these particles in steel. We focus on the potential advantages of using computational thermodynamics methods both for diffusion and for nucleation, growth and coalescence, in special using software based on Kampmann-Wagner numerical (KWN) theory. The potential of these methods is demonstrated in some selected applications. We emphasize the importance of interfacial energy in these models. Frequently the interfacial energy is used as an adjustable parameter in these models. Even when this is done, the models can be very useful for steel and process design, as long as the user is aware of the limitations associated to the adjustment.