Strain Rate Effects on Mechanical Stability of Retained Austenite in TRIP Sheet Steels

Dynamic mechanical properties of TRIP steels with the same volume fraction but different stabilities of retained austenite were evaluated over a wide range of strain rates using a high-velocity hydraulic tensile testing machine. Tensile tests were performed at strain rates ranging from 10^-2 to 6×10²s^-1 and ultimate tensile strength, strain hardening behavior, and absorbed energy were evaluated. Strain control during high speed tensile testing was accomplished using a “stopper” attachment designed to limit strain within the gage section to an amount preset before testing. Strain was controlled successfully up to the highest strain rate examined, 200 s^-1. The methodology allowed, for the first time, the extent of austenite transformation to be monitored at incremental strains during a high-rate test. At all strain rates, the extent of martensite transformation was considerable after only a few strain percent, and was essentially complete well before the onset of necking.