Strain Rate Sensitivity of Automotive Sheet Steels: Influence of Plastic Strain, Strain Rate, Temperature, Microstructure, Bake Hardening and Pre-Strain

This experimental work shows the different parameters influencing the strain rate sensitivity behaviour of automotive sheet steel grades in crash conditions. Most investigations have been performed in the strain rate range [10-3-200s-1] and temperature range [233-373K] with servohydraulic tensile testing machines. Additional Split-Hopkinson bar testing results up to 103s-1 have also been included at room temperature. The focus has been laid on the "apparent" strain rate sensitivity, determined based on multiple dynamic flow curves at constant strain rate in a semi-logarithmic (b10 values) or logarithmic (m values) way.It has been shown that the dynamic behaviour in the investigated strain rate and temperature range is clearly thermally activated for a wide range of automotive sheet steels. This means that an increase in strain rate is nearly equivalent to a decrease in temperature. The strain rate sensitivity values are dependent on the strain rate range considered, as well as on the temperature and plastic deformation range chosen. The strain rate sensitivity decreases with increasing plastic strain level due to a gradual exhausting of work hardening potential combined with adiabatic softening effects. The strain rate sensitivity increases with decreasing temperature or increasing strain rate, which is often omitted when considering literature data.The strain rate sensitivity is also dependent on the microstructure investigated. The strain rate sensitivity decreases strongly with increasing strength level, especially below 400MPa yield strength or 500MPa tensile strength, and is stabilised at a low level for AHSS and UHSS steel grades. The strain rate sensitivity decreases for single phase ferritic mild, HSS and HSLA steel grades, mainly due to solid solution alloying with Mn, Si and P elements. Long range mechanisms such as precipitation hardening, grain refinement or cold work do not influence the strain rate sensitivity. With increasing second hard phase content, the strain rate sensitivity decreases due to the decrease of relative volume content of strain rate sensitive ferrite in multiphase steels.The TRIP effect decreases the strain rate sensitivity in low alloy TRIP steels in comparison to dualphase steels. High alloy TRIP steels show some negative strain rate sensitivity in the low strain rate and high strain range. A significant decrease in the TRIP effect intensity is seen at strain rates above 1s-1 at high strain level, which is related to an adiabatic temperature increase.Uniaxial, plane strain or biaxial pre-straining up to around 0,10 equivalent strain does not influence the strain rate sensitivity of sheet steels in comparison to the as-delivered material. A bake hardening heat treatment with or without pre-straining does not influence the strain rate sensitivity significantly neither. Forming and/or bake hardening does not affect particularly the subsequent strain rate sensitivity in crash conditions. Cold work or bake hardening introduces obstacles to dislocations, which are rather of athermal nature, so that the strain rate sensitivity is not influenced.For high alloy TRIP steels, the magnitude of subsequent TRIP effect is increased with increasing pre-straining level, which slows down adiabatic stress softening quite effectively.This experimental work allows some reliable comparisons between different alloying concepts and helps to identify the parameters influencing effectively the strain rate sensitivity, such as strain rate, temperature, plastic deformation, solid solution alloying, second phase hardening and TRIP effect. This work delivers additionally a wide database for strain rate sensitivity values of automotive sheet steel grades, which can be referred to for further experimental and modelling investigations