THE EXPLOITATION OF X-RAY DIFFRACTION IN CHARACTERISATION OF STRENGTH OF HOT-ROLLED AND COLD-DRAWN FERRITIC-PEARLITIC STEEL

D. Šimek¹, D. Rafaja¹, M. Motylenko¹, V. Klemm¹, C. Ullrich¹, A. Oswald², R. Schmidtchen² and G. Lehmann²

¹Institute of Materials Science, TU Bergakademie Freiberg, D-09599 Freiberg, Germany

²Materials Forming Institute, TU Bergakademie Freiberg, D-09599 Freiberg, Germany

simek@fzu.cz

A series of samples of the C45 (0.45 wt.% C) steel was prepared by hot rolling with different thermomechanical history in an industrial-type rolling stage (e.g. rolling temperature and speed, cooling rate). The microstructure of resulting material was ferritic-pearlitic with a pearlite volume fraction ranging from 57 to 90%; the mean interlamellar spacing in the pearlite varied between 180 and 270 nm. An equally-spaced arrangement of misfit dislocations was found at the ferrite/cementite interfaces. The microstrain they generate was observed in the X-ray diffraction by means of anisotropic line broadening; the density of the dislocations was proved to be proportional to the density of the lamellas. The dislocation density was found to correlate with the ultimate tensile strength (UTS) of the steel in the tensile test as well as the density of the lamellas. Upon gradual cold drawing through conical dies, the dislocation density observed in X-ray increased up to about 50% of elongation, the UTS was still well-following its dependence on the dislocation density, while the density of pearlitic lamellas remained intact. The X-ray diffraction can thus be utilised for an instant and non-destructive estimation of UTS of hot-rolled ferritic-pearlitic steels and cold-drawn steels with moderate grade of cold deformation.

(extended contribution submitted)