The Influences of Strain Rate on Mechanical Properties and Deformation Mechanisms of High-Mn and Medium-Mn TWIP-TRIP Steels

Abstract

High strain rates are investigated to better understand the behavior of advanced high strength steels (AHSS) in environments that simulate forming operations of complex automotive parts. The effects of quasi-static and dynamic strain rates (10-4 – 103 /s) on mechanical properties and deformation mechanisms were studied in single-phase high-Mn and multi-phase medium-Mn twinning and transformation-induced plasticity (TWIP-TRIP) steels. During medium-Mn alloy design, the stacking fault energy (SFE) of the austenite, (a minority phase in the multi-phase steel), was tuned to match the SFE of the high-Mn austenitic steel to better understand how microstructural differences may affect high strain rate behavior. High frame rate thermal imaging was used to measure the increase in tensile specimen temperature in situ, which allowed for the prediction of increase in SFE, based on a thermodynamic model. High strain rate tests were also interrupted and advanced characterization techniques, mostly based in electron microscopy, were used to quantify minor differences among all observed deformation mechanisms. All steels designed and investigated in this work exhibited an increase in yield and ultimate tensile strengths with increasing strain rate and are stronger than first generation AHSS, which improves the likelihood for implementing next generation AHSS into automotive bodies.