Effect of microstructure on retained austenite stability and work hardening of TRIP steels

TLDR: In this article, two distinct TRIP steel microstructures, equiaxed versus lamellar, and how microstructure affects the retained austenite transformation during uniaxial tensile loading were investigated.

Abstract: Retained austenite is a metastable phase in transformation induced plasticity (TRIP) steels that transforms into martensite under local stress and strain. This transformation improves sheet formability, allowing this class of higher strength steels to be used for applications such as automotive structural components. The current work studies two distinct TRIP steel microstructures, i.e. equiaxed versus lamellar, and how microstructure affects the austenite transformation during uniaxial tensile loading. Different heat treatments were employed to obtain the two microstructures, and the bainite hold times of the treatments were varied to change the volume fraction of retained austenite. Based on uniaxial tensile response and magnetic saturation measurements, the bainite hold time of 100 s was determined to produce the best results in terms of largest strain at the ultimate tensile strength and highest volume fraction of retained austenite. The work hardening of the samples with a 100 s bainite hold was evaluated by calculating the instantaneous n value as a function of strain. It was found that the lamellar microstructure has a lower maximum instantaneous n value than the equiaxed microstructure, but has higher work hardening values for strain levels greater than 0.05 and up to the ultimate tensile strength. This difference in work hardening behaviour corresponds directly to the transformation rate of retained austenite in the two microstructures. The slower rate of transformation in the lamellar microstructure allows for work hardening to persist at high strains where the transformation effect has already been exhausted in the equiaxed microstructure. The different rates of transformation can be attributed to the location, carbon content, and size of the retained austenite grains in the respective TRIP microstructures.