Role of Al element in tailoring the austenite mechanical stability and tensile properties of medium Mn steels

Abstract

Al is generally employed as a key alloying element to tailor the austenite mechanical stability and mechanical properties in medium Mn TRIP steels. However, the effect of Al on the austenite stability has been limitedly studied, leaving contradictory results without a self-consistent explanation. This study addressed this issue through a systematic study of medium Mn TRIP steels with compositions of 0.2C–8Mn-0/1.5/3Al (wt.%), while providing new insights into the factors influencing austenite stability and mechanical properties. It was found that the variation of the Mn distribution in austenite from non-equilibrium to near-equilibrium state and the weakened strain incompatibility between phases due to sufficient recovery of tempered martensite can be achieved with the addition of Al, which contributed to slowed strain-induced martensite transformation (SIMT) and improved austenite stability in the experimental steels annealed at low temperatures in the two-phases region, resulting in a decrease in ultimate tensile strength (UTS) and an increase in total elongation (TE). Furthermore, fully recovered tempered martensite with the hardness much lower than that of austenite was obtained in the 3Al steel annealed at a high temperature in the two-phase region, which led to a significant reduction in yield strength and thus low UTS. Meanwhile, heterogeneous strain distribution was induced by the remarkable gap in hardness between the two phases, which can be responsible for the relatively high austenite mechanical stability. The varying strain states and variation of Mn distribution in austenite can be achieved for the medium Mn TRIP steels with different Al contents, contributing to a wide range of UTS&TE combination with high strength and elongation product (≥40 GPa·%). And the elucidation of the effect of different strain states also opens a new window to adjust the austenite stability and mechanical properties of medium Mn TRIP steels.