Influence of thermomechanical treatment on the austenite-pearlite transformation in a high carbon Nb-microalloyed steel

Multipass torsion tests were carried out with a Nb-microalloyed high carbon steel, using different deformation sequences ill order to modify the austenite state prior to transformation. Both recrystallized and deformed austenite microstructures were investigated. After deformation different cooling rates were applied. The Electron Back Scattering Diffraction (EBSD) technique was applied to study the pearlite microstructure. The ferritic "orientation unit" was defined using a tolerance criterion of a 15degrees change ill orientation. A good correlation was observed between the units identified in this way and the cleavage facets measured from the fracture profiles ill SEM. The results show that accumulating strain ill the austenite before transformation does not affect the interlamellar spacing when similar cooling conditions are applied, but significantly refines the orientation unit.