Influence of non-metallic inclusions on the austenite-to-ferrite phase transformation

Abstract

During continuous casting of low-alloy steels non-metallic inclusions (e.g. aluminium nitride (AlN)) may precipitate and grow inside the austenite phase at temperatures above 1200 K. At lower temperatures austenite (γ) will partly transform to the ferrite (α) phase, which significantly influences the properties of the cast product. The kinetics of the γ/α phase transformation depends on the mobility of the interface as well as on the diffusion coefficients of interstitial as well as substitutional components. The driving force for the γ/α phase transformation can be separated in a chemical and a mechanical term. The actual chemical compositions in the γ- and in the α-phase very near to the interface determine the chemical contribution to the actual driving force. The mechanical driving force of the γ/α phase transformation is directly related to the strain energy and plastic work generated by the transforming spherical shell. Therefore, AlN-inclusions, producing large strains and stresses in their surroundings, may affect the γ/α phase transformation. Finite element (FE) calculations show that AlN-inclusions are able to trigger the γ/α phase transformation. This leads to the formation of ferrite at a higher temperature compared to a steel grade without AlN-precipitates. Finally, the kinetics of the γ/α phase transformation is determined numerically by a finite difference method.