Microalloyed V-Nb-Ti and V steels - Part 1 - Stereological study of ferrite-pearlite microstructure and its relationship to toughness

Stereological analysis was applied to quantify the ferrite-pearlite microstructure and relate it to the toughness of similar strength V-Nb-Ti and V microalloyed steels. The V-Nb-Ti and the V steel were characterised by average yield strengths of ∼ 375 and 363 MPa, respectively and an elongation of ∼ 20%. The relationship between the microstructure and toughness was investigated in terms of stereological parameters, namely: (a) mean intercept length of polygonal ferrite and its distribution, (b) mean intercept length of pearlite colonies and their distribution, (c) ferrite grain boundary surface area per unit volume, (d) total grain boundary surface area per unit volume, (e) contiguity ratio of ferrite grains and pearlite colonies, and (f) pearlite interlamellar spacing and its distribution. Stereological analysis of core and flange sections of beams of both steels indicated that the core and flange sections of V steel are characterised by a marginally finer ferrite grain size and narrower grain size distribution, higher contiguity ratio of ferrite grains, finer pearlite interlamellar spacing and its distribution, and more uniform distribution of pearlite colony size, in comparison with respective core and flange sections of the V-Nb-Ti steel. These characteristics influence the toughness of the investigated steels.