Predicting Yield Strength and Tensile Strength After Forming for Automotive Integral Body Structural Rail Type Parts

Samples of cold rolled and hot dip galvanized mild steel, microalloyed high strength steel, and dual phase steel were prestrained by bending and straightening, bending and straightening with superimposed tensile strain in a die, and cold rolling (dual phase steels only). In all three cases, the strain state was approximately plane strain. Stress-strain behavior was evaluated by conventional tensile testing of as-received and prestrained samples. For the mild and the high strength microalloyed steels, it is shown that the use of effective prestrain calculated assuming isotropy coupled with simple parabolic work hardening provides reasonable engineering estimates of the yield and tensile strength after prestraining if κ and n are taken from as-received tensile tests oriented coaxially to the restrain direction. It was also found that in bending and straightening, only the absolute average value of the bending strain should be used in calculating the effective prestrain. For steels with dual phase microstructures, this approach to predicting cold work strengthening does not provide useful engineering estimates.