Effect of thermal treatment on the bending properties of pre-strained carbon steel

Previous studies on the bending behaviour of aged carbon steel strip after pre-strain in tension have shown the presence of an unusually large softening effect. The purpose of this work was to understand the mechanisms that lead to this, including the effect of re-ageing at various temperatures and also stress relief. It was found that the rate at which the elastic limit in bending increased as a function of the temperature treatment differed significantly from that found in previous investigations for tension–compression and torsion–reverse torsion deformation. The complete elimination of the Bauschinger effect was observed at temperatures far below that of recrystallization in contrast to most previous observations. While the yield phenomenon reappeared at modest ageing temperatures, inhomogeneous deformation in bending due to the development of a distinct upper yield stress was only found after stress relieving at higher temperatures. The work has practical implications as carbon steel sheet and strip are often pre-strained by skin pass rolling, tension levelling, or roller levelling to eliminate yield point effects. The properties of such pre-treated material are usually determined by tensile testing; the present work suggests that material softening resulting from such pre-treatment and the effect of ageing may be strain path dependent and that tensile data alone may be inadequate to determine material properties for use in the prediction of bending behaviour.

► Softening in bending cannot be attributed to macroscopic residual stresses alone.
► Bauschinger effect and the effect of ageing treatment are strain path dependent.
► The Bauschinger effect is eliminated at temperatures below recrystallization.
► Immediate re-appearance of the yield phenomenon in tension but not in bending.
► Unstable bending was observed; this follows the model proposed by Duncan et al.