The mechanical behaviour and microstructures of interstitial free steel strained in monotonic or cyclic torsion at 1223 K

Abstract

The work hardening behavior of metals under cold cyclic or multiaxial deformation involves lower hardening rates than those caused by monotonic strain to the same total magnitude. The hardening is lower as the strain per cycle decreases. Similar studies under hot working conditions, for a limited number of cycles, also lead to lower hardening, as well as to changes in the shape of the flow curves and to delays in the initiation of Dynamic Recrystallization (DRX) and in the post-processing static softening kinetics. These results point to a decrease in stored energy caused by cyclic straining. A higher number of hot deformation cycles leads to lower steady state stresses (σ SSr) than those corresponding to monotonic deformation (σss). Besides, DRX can be suppressed and replaced by Dynamic Recovery (DRV), and the successive cycles have been associated with the repeated build-up and disintegration of dislocation structures. The present research discusses the hot deformation of an IF steel in the austenitic temperature range, for monotonie torsion or a large number of reversed torsions with various strain amplitudes (Δε). It was confirmed that σ SSr depends on the relative amplitude of the strain cycles (Δε/εp,. where εpp is the strain at the peak stress in the monotonie torsion of the material), and that lower values of Δεlεp eliminate DRX, which is replaced by DRV. The ferritic grain size after deformation into the cyclic steady state stress depends on the value of σSSr, similarly to results in the literature for monotonie torsion of IF steels. © 2006 ISIJ.