Effect of microstructure on retained austenite stability and tensile behaviour in an aluminum-alloyed TRIP steel

Microstructures, retained austenite (RA) stability and tensile behaviour were investigated for an aluminum-alloyed TRIP steel. Two distinct microstructural variants (equiaxed and lamellar) were studied: (i) at constant volume percent RA and (ii) at constant UTS. The RA in the lamellar variant transforms more slowly with tensile strain than for the equiaxed variant and stabilizes at ~80% transformed. The higher RA stability for lamellar microstructures is primarily due to the stress shielding effect of RA grains by the surrounding bainite. A secondary contributor is the elongated shape of the RA grains in the lamellar microstructure. The tensile work hardening behaviour correlates with RA transformation behaviour for the two model microstructural variants. The lamellar variant exhibits a relatively constant work hardening rate sustained up to the point of necking. By contrast, the equiaxed variant exhibits a maximum work hardening rate at low strain, followed by a monotonic decrease to the necking point. Comparing the results reported here with those of a previous study on silicon-alloyed TRIP steel shows that for both microstructural variants RA transforms more slowly with tensile strain and the UTS×El values are higher for the aluminum-alloyed steel.