Investigating the fluting defect in v-bending due to the yield-point phenomenon and its reduction via roller-leveling process

Abstract

Defects such as stretcher strain in drawing and fluting in v-bending result from the yield-point phenomenon of annealed or aged metals. This phenomenon can be diminished by applying the roller-leveling process to these materials composed of multiple up-and-down bending operations with a series of offset rolls. It is important to know an adequate process condition because excessive leveling can induce superficies defects while insufficient leveling cannot prevent the emergence of defects related to the yield-point phenomenon, which unfortunately remains still elusive. In the present study, experimental and numerical investigations are performed for the fluting defect in the v-bending process and its reduction by the roller-leveling process. The rate dependency and the cyclic characteristics of the yield-point phenomenon are first examined experimentally for pre-coated low carbon steel under uniaxial tension and cyclic loading conditions. Systematic evaluation for the effect of roller-leveling condition on the fluting in v-bending is then carried out. For numerical simulations, an implicit stress-integration procedure is formulated and implemented for a constitutive material model that can describe both the yield-point phenomenon and the Bauschinger effect. Material parameters are determined for the model to reproduce the experimental data under uniaxial tension and cyclic loadings. Results of finite element analysis for v-bending process at various roller-leveling conditions demonstrate the capability of the model. The proposed analysis procedure is expected to be useful in estimating a proper leveling condition to prevent potential defects as well as investigating the effect of the yield-point phenomenon in various metal forming processes.