The Mechanical Properties of Low Alloy TRIP-Aided Steel: The Role of Retained Austenite

Thermo-mechanical treatments were performed to exploit the recrystallisation and deformation induced ferrite transformation (DIFT) during processing and study their effect on the microstructure and strength-ductility balance of a low-carbon, low alloy CMnSiAl TRIP-aided steel containing a small amount of Mo and Nb. A particular focus of the work was to study the influence of prior transformation of retained austenite to martensite (i.e., absence of the TRIP effect) on the strength-ductility balance of the steel by comparing the tensile properties of samples quenched in water and in liquid nitrogen. It was observed that the formation of high carbon martensite as a result of the prior transformation of retained austenite does not necessarily lead to brittle failure. The work also revealed that the TRIP effect per se may not play a major role and it is the deformation of a composite microstructure comprising hard and soft phases, which allows TRIP aided steels to offer a better combination of mechanical properties (strength, ductility and strain hardening) in comparison with conventional high strength steels (HSS) like high strength interstitial free (HS-IF) steel and high strength low alloy steel (HSLA) and other first generation advanced high strength steels (AHSS) like dual phase (DP) steels. The new insight into the TRIP effect may further be exploited in the design of alloy composition and subsequent thermo-mechanical treatments.