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

The effect of pre-strain and strain rate on deformation behavior of two grades of interstitial free (IF) steel sheets, industrially referred as EIF and HIF, of thickness 0.7 mm have been investigated. Tensile tests are conducted with as-received (0% pre-strain) and pre-strained (5, 10, and 15%) steels at five different strain rates of 3.33 ×10-5, 3.33 ×10-4, 3.33 ×10-3, 3.33 ×10-2, and 1.66 ×10-1 s-1. The stress-strain data of the tensile tests are analyzed, and strain hardening exponent (n) or strain hardening rate (dσ/dε) is determined to understand the deformation behavior of the steels. It is observed that the strength of the selected IF steels increases with increasing pre-strain and strain rate, but ductility decreases. As-received and pre-strained EIF and HIF steels exhibit two distinct strain hardening regimes with a transition point at a true strain range of 0.035 to 0.05. EIF steel exhibits higher strain hardening exponent compared to HIF steel, and hence possesses higher drawability. Strain hardening exponent and strain hardening rate of the selected IF steels decrease with increasing prestrain. With increasing strain rate, strain hardening exponent (n) or strain hardening rate (dσ/dε) decreases in the initial stage of deformation for as-received steel only. Strain hardening behavior remains almost unaffected with increasing strain rate for the entire plastic deformation regime of pre-strained IF steels, and in the later stage of deformation of as-received steel, which is a new observation. Fractography shows dimples suggesting ductile fracture for the selected prestrain and strain rate range; the size of dimples decreases with prestrain but increases with strain rate. TEM study of dislocation substructures and estimation of dislocation density have been presented as necessary supplement.