Effect of Nb on the stability of retained austenite in hot-rolled TRIP steels based on dynamic transformation

Abstract

Two C–Mn–Al–Si TRIP steels with and without Nb addition were fabricated by the thermo-mechanical process based on dynamic transformation of undercooled austenite (DTUA), the microstructures and the mechanical properties of the two TRIP steels were investigated and the transformation kinetics of retained austenite (RA) to martensite during tensile straining were also analyzed using interrupted tensile tests, in combination with SEM, XRD and ICP-OES. Although the yield strength and the tensile strength of Nb steel are higher than that of Nb-free steel, the uniform elongation and the total elongation of Nb steel are lower, as well as the lower strain-hardening capability of Nb steel. That should be attributed to the higher stability of RA in Nb steel, resulting in the sluggish transformation kinetics of RA to martensite and the relatively weaker TRIP effect during tensile straining. The main reason for the higher stability of RA in Nb steel is the higher stacking fault energy (SFE) of RA due to the dissolved Nb in the final multi-phase microstructure. Another reason is that the addition of Nb retards the transformation of austenite to ferrite, resulting in that most of RA in Nb steel are layers between the bainitic ferrite laths while a large part of RA is blocky islands between the polygonal ferrite grains in Nb-free steel. The stability of the former type is higher and most of them transform to martensite at relatively larger strain.