Strain Rate Effect on Martensitic Transformation in a TRIP Steel Containing Carbide-Free Bainite 2019-01-0521

Adiabatic heating during plastic straining can slow the diffusionless shear transformation of austenite to martensite in steels that exhibit transformation induced plasticity (TRIP). However, the extent to which the transformation is affected over a strain rate range of relevance to automotive stamping and vehicle impact events is unclear for most third-generation advanced high strength TRIP steels. In this study, an 1180MPa minimum tensile strength TRIP steel with carbide-free bainite is evaluated by measuring the variation of retained austenite volume fraction (RAVF) in fractured tensile specimens with position and strain. This requires a combination of servo-hydraulic load frame instrumented with high speed stereo digital image correlation for measurement of strains and ex-situ synchrotron x-ray diffraction for determination of RAVF in fractured tensile specimens. Specifically, the potentially competing effects of strain rate on austenite transformation to martensite were investigated to determine which predominate at nominal strain rates of 0.5 s^-1, 5 s^-1, 50 s^-1 and 500 s^-1. A corresponding decrease in austenite volume fraction at a fixed true strain with strain rate suggests that austenite transformation to martensite with strain is accelerated with increased strain rate despite potential energetic inhibition of the transformation due to adiabatic heating. Increased transformation rate to martensite results in increased work hardening rates, strengths, and elongations with strain rate increases to 500 s^-1. Observations are discussed in the context of contributing mechanisms both favoring and inhibiting the strain-assisted transformation of austenite to martensite.