Mechanical Behavior of a Rephosphorized Steel for Car Body Applications: Effects of Temperature, Strain Rate, and Pretreatment

Temperature and strain rate effects on the mechanical behavior of commercial rephosphorized, interstitial free steel have been investigated by uniaxial tensile testing, covering temperatures ranging from $−60°C$ to $+100°C$ and strain rates from $1×10−4s−1$ to $1×102s−1$ encompassing most conditions experienced in automotive crash situations. The effect of prestraining to 3.5% with or without successive annealing at $180°C$ for 30 min has also been evaluated. These treatments were used to simulate pressing of the plates and the paint-bake cycle in the production of car bodies. Yield and ultimate tensile strengths, ductility including uniform and total elongation and area reduction, thermal softening effect at high strain rate, and strain rate sensitivity of stress were determined and discussed in all cases. It was found that the Voce equation $[σ=σs−(σs−σ0)exp(ε/ε0)]$ can be fitted to the experimental true stress-true plastic strain data with good precision. The parameter values in this equation were evaluated and discussed. Furthermore, temperature and strain rate effects were examined in terms of thermal and athermal components of the flow stresses. Finally, a thermal activation analysis was performed.