Effects of Strain Rate and Temperature on the Work Hardening Behavior of High Strength Sheet Steels

The influence of strain rate on work hardening behavior has been determined for a variety of high strength steels including high-strength low-alloy (HSLA), dual phase (DP), and transformation-induced plasticity (TRIP) steels. Tensile testing was performed at true strain rates of 10^-3 s^-1 and 1.0 s^-1 to represent laboratory testing conditions and dynamic press-forming operations, respectively. Work hardening behavior is described by the conventional strain hardening exponent (n-value), the work hardening rate (dσ/dε), and the Shape-Tilt-Strength (STS) equation as an alternative approach. The effects of deformation temperature and temperature rise during deformation (adiabatic heating) on work hardening are also evaluated. Increasing the strain rate generally increases the work hardening rate at smaller strains, which may contribute to a broader initial strain distribution in press forming. At larger strains, the work hardening rate decreases with increasing strain rate due to adiabatic heating, thereby lowering the overall forming limits. The high-strain-rate formability decrease due to adiabatic heating is anticipated to be greater as strength level increases for a given degree of formability.