Residual formability analysis of bent and remanufactured thin steel sheets

This study examines the plastic deformation and remaining formability of DC04 mild steel, a material frequently used in automotive applications, after undergoing V-bending and subsequent cold or warm flattening processes. The research aims to determine if previously bent sheets maintain sufficient ductility for an additional forming cycle, a key factor in advancing circular economy practices. Samples were cold-formed into V-bends of various angles and then reshaped through flattening, with critical process parameters controlled. Tensile tests were performed on both the original sheets and reshaped samples to study the impact of the bending index, dwell force, dwell time, and flattening temperature on mechanical properties. Geometric characterization of the samples before and after flattening showed localized permanent deformation and an "M" profile near the bend line, leading to a spring-forward effect and a "U" shape of the samples. Analysis of tensile stress-strain curves and regression models indicated a decrease in elongation at maximum stress with an increasing bending index, while the ultimate tensile strength remained relatively constant. Yield stress showed a minor increase with a higher bending index and flattening temperature. These results underscore the potential for reusing endof-life vehicle sheets, providing a more sustainable alternative to conventional recycling methods.