Mathematical Modelling of Nucleating and Growing Precipitates

Abstract

Steels with higher strength and better formability are increasingly required by the automotive industry because they can provide higher safety, reduce energy consumption and thus lead to a better environmental protection. To meet these requirements, it is a sustainable effort for steel industry to develop high strength formable steels. One of the commonly used methods for such a development is micro-alloying, that is, the addition of micro-alloying elements such as niobium, vanadium and titanium at a level of only a few hundredths of a weight percent results in a very pronounced strength-enhancing effect on the steels, provided that an appropriate heat treatment is applied. It is understood that the strength-enhancing effect primarily arises from a strong reduction in the average grain size of the ferrite, originating from the grain-refining effect during the austenisation treatment. The reason for the grain-refining effect is that the micro-alloying elements have a very strong affinity for the interstitial elements such as carbon and nitrogen, leading to the precipitation of extremely fine and widely distributed precipitates. The existence of the precipitates prevents the growth of austenite grains by means of Zener pinning. Therefore it is an essential issue for steel industry to have an accurate control of the nucleation and growth of the precipitates during thermomechanical processing of the steels. In this thesis we focus on several models describing the nucleation and growth of precipitates. This PhD research focusses on this subject.