Effect of Pre-strain and Strain Rate on Deformation and Fracture Behavior of Automotive Grade Interstitial Free Steel Sheets

The influence of pre-strain and strain rate on the deformation behavior of two automotive grades of interstitial free (IF) steel sheets, known as EIF and HIF, with a thickness of 0.7 mm, was investigated. Tensile tests were conducted on as-received (0% pre-strain) and pre-strained (5, 10, and 15%) steels at five different strain rates ranging from 3.33 × 10⁻⁵ to 1.66 × 10⁻¹ s⁻¹. The stress–strain data from these tests were analyzed to determine the strain-hardening exponent (n) and strain-hardening rate (dσ/dε), and to gain insights into the deformation behavior. The steels exhibited two distinct strain-hardening regimes, with a transition point at a true strain range of 0.035-0.05. Both n and dσ/dε of the steels decreased with increasing pre-strain. While n and dσ/dε decreased with increasing strain rate in the initial stage of deformation for as-received steel, these remained largely unaffected for the entire plastic deformation regime of pre-strained steels, which is a novel finding. Fractographic analysis revealed ductile fractures characterized by dimples. The size of these dimples decreased with increasing pre-strain but increased with increasing strain rate. TEM study of dislocation substructures and estimation of dislocation density were supplementary to this investigation.