Strain Rate Sensitivity of Automotive Steels

Strain rate sensitivity is an important material property in the formability of sheet metal. In this study, strain rate sensitivity is evaluated for several different grades of steel. Strain rate sensitivity varies from 0.01 to 0.022 for the steels tested. It was found that formable steels such as IF and AKDQ steels have both high n-value (strain hardening) and m-value (strain rate sensitivity). Positive strain rate sensitivity results in a significant increase in the yield strength and tensile strength at higher strain rates. The n-value decreases with strain rate for all of the steels. The total elongation decreases slightly with strain rate for the lower strength steels but is constant or even increases slightly with strain rate for high strength steels. For a typical AKDQ steel, the increase in yield strength can be as high as 43% for an increase in strain rate from 0.002 /s to 2.0 /s. The yield strength at high strain rates can be calculated based on the yield strength at low strain rates and the strain rate ratio of the high strain rate to the low strain rate.