Formation of carbonitrides and precipitation of alloying elements during thermomechanical treatment of high-strength low-alloy steels. Atomistic modeling

Abstract

Understanding the processes that control the formation of precipitates of alloying element, their composition and morphology is important for the choosing of optimal regimes of thermomechanical treatment, providing the required structural state and properties of low-alloyed steels. The use of modern methods of atomistic modeling and ab initio calculations to study the mechanisms and conditions of precipitation depending on the steel composition and temperature is discussed. The enthalpy of formation of Ti, V, Nb carbides and nitrides in γ-Fe was calculated using the methods of the theory of the electron density functional and the temperatures of their dissolution were determined in dependence on the composition of the steel. The results obtained agree with the experimental data and obtained by the CALPHAD method. Calculations of the electronic structure, enthalpy of mixing, energy of effective interactions between atoms of alloying and impurity elements have been performed. It was shown that an essential factor determining the nature of the interaction between alloying elements was the magnetic state of iron. It has been established that the enthalpy of mixing, which expresses the tendency of the system to decomposition, increases with the transition of α-Fe from the ferromagnetic to the paramagnetic state. In ferromagnetic α-Fe energetically, the formation of precipitates of Cu, as well as particles of a mixed composition of Cu‒Al, Cu‒Ni and an ordered phase of NiAl are preferable. It has been shown that precipitates Cu-based can be formed during technological times. The transition of α-Fe to the paramagnetic state results in increase of interactions between alloying elements (the exception is the interaction of Cu‒Cu and Cu‒Ni). As a result, additional combinations of alloying elements appear, the interaction between which lead to the formation of precipitates. At the same time, in γ-Fe, precipitates can be formed only based on Mo and Nb. The role of magnetism in the formation of precipitates and their influence on the γ→α-transformation is discussed. It has been shown that the results obtained make a base for enhancement technologies of high-strength low-alloyed steel production.