Influence of Pre‐Straining and Bake Hardening on the Strain Rate Sensitivity of Automotive Sheet Steels

Abstract

The present investigation deals with the influence of pre-straining with or without bake hardening on the strain rate sensitivity of automotive sheet steels in typical crash conditions. The strain rate sensitivity m has been determined by means of dynamic tensile tests in the strain rate range 0.005-1000 s À1 and in the temperature range 233-373K. A bake hardening heat treatment at 170 8C for 20 min without pre-straining does not influence the m-value in comparison to the base material condition. A small pre-straining near plane strain condition, as commonly found in outer door panels, or a 10% uniaxial, plane strain and biaxial pre-straining, as typically used in formed automotive crash components, without bake hardening does not affect the m-value of sheet steels in comparison to the base material condition. Uniaxial 2% to 10% pre-straining, longitudinal or transverse to rolling direction with subsequent bake hardening, does not clearly change the m-value in comparison to the base material condition either. Small differences in the strain rate sensitivity behaviour are rather attributed to experimental scattering without real physical background.

Strain Rate Sensitivity of Pre‐Strained AISI 301LN2B Metastable Austenitic Stainless Steel

“…The strain rate sensitivity, therefore, does not depend on the pre‐straining condition, as previously observed in Figure 6 in the low plastic strain region. Such results are also in accordance with previous investigations suggesting the athermal nature of pre‐straining 18, 29…”

“…Generally the influence of strain rate and temperature on the dynamic properties of sheet steel grades in the strain rate range 10 −4 to 1000 s −1 can be well described by the thermal activation theory for automotive sheet steels 1, 18, 28. A strong reciprocity between strain rate and temperature is characteristic for dynamic deformation according to the theory of thermal activated plastic deformation.…”

“…The yield strength R p0.2 can be then modeled as follows:18, 28 where Δ G 0 and σ 0 * are the free activation enthalpy and effective stress at zero thermal activation ( T = 0 K) and m′ is the strain rate exponent.…”

“…Different quasi‐static pre‐deformation modes have been considered: 10% engineering strain for the uniaxial condition, 10% major true strain for the plane strain condition, and 5% major and 5% minor true strain for the equi‐biaxial condition. The pre‐straining has been performed transverse to rolling direction with a tensile test machine and a Marciniak tool with appropriate sample geometries and forming depths 1, 17–19. A Mises equivalent strain around 10% has been targeted for all forming conditions (Table 3).…”

Influence of Strain Rate, Temperature, Plastic Strain, and Microstructure on the Strain Rate Sensitivity of Automotive Sheet Steels

“…A significant increase of the m ‐value can be usually observed with increasing strain rate 7–10. The strain rate sensitivity may reach values well above 0.02, a level which is otherwise commonly assumed for steels 11–15. This increase is more significant for mild steels than for high strength steels (HSSs) 16, 17.…”