Anisotropic dilatation behaviour during transformation of hot rolled steels showing banded structure

Dilatation measurements were carried out on cylindrical specimens taken from the longitudinal and the normal directions of hot rolled C-Mn steel strip, giving rather different results for the heat treatments that led to a banded ferrite-pearlite microstructure. Under those conditions, the standard methods of deriving the fractions transformed from the uniaxial dilatation signal give rise to systematic errors. A model describing the anisotropic dilatation behaviour for such an evolving banded structure is presented. The model reproduces the experimentally observed differences semiquantitatively as a function of the band orientation in the specimen. Calculations indicate that the most adequate description of the anisotropic dilatation behaviour during formation of an evolving banded structure should be based on a time dependent model, which accounts for the phase transformation history. The consequences of the anisotropic behaviour for determination of the fractions transformed are discussed.