The application of stereological analysis in understanding differences in toughness of V- and Nb-microalloyed steels of similar yield strength

The present paper describes an attempt to relate small variations in toughness of V- and Nb-microalloyed steels, characterized by almost similar yield strength and ferrite–pearlite microstructures, to stereological parameters. Stereological parameters derived from two-dimensional microstructure include mean intercept length (L¯α) of ferrite, ferrite grain boundary surface area per unit volume [SV]α, contiguity ratio (Cα) of ferrite grains, pearlite colony size, and pearlite interlamellar spacing. Stereological analysis was carried out as a function of depth from the surface using serial sectioning method. The analysis predicts that the toughness of the microalloyed steels is related to uniformity in the stereological parameters and the contiguity ratio. If the processing conditions such as cooling rate are increased, a convincingly higher toughness is obtained for Nb-containing steel in relation to V-microalloyed steels, at similar yield strength.