Strain rate sensitivity of ultra-low carbon steels

Ultra-low carbon, interstitial free (IF) steels were prepared by an intermediate rolling reduction of 20% followed by a 3 or 9 day anneal at 400°C prior to further reduction and anneal. Previously described determination of the interstitial content using the Haasen plot intercept indicates that levels in the ppb range were attained. The strain rate sensitivity in the macro-plastic region defined by the slope of the Haasen plot increased after reducing solute levels below 1 ppm, and the work hardening mechanism was attributed to that of jogs and/or recombination of dissociated dislocations in bcc structures. A distinct strain rate sensitivity was found in the micro-strain region below 0.2% where the activation work was an order of magnitude larger than in the macro-plastic region indicating an activation distance of 4.5b. This measurement suggests that during micro-straining, glissile dislocations intersect, but as the jog density increases with straining, the dislocation cores become sessile and some recombination mechanism activates.