Microstructure, Mechanical, and Magnetic Properties of Medium Manganese Steel Subjected to ART Heat Treatment Cycle

Abstract

A type of low-carbon (0.19 wt.% C), medium-manganese (6.67% Mn) steel containing 0.78% Al was cast and converted into 2 mm thick wrought steel. This type of steel was subjected to austenitization at a temperature range of 600 to 800 °C with 20 °C steps, followed by air cooling, to develop the third-generation Advanced High-Strength Steel (AHSS). Microstructural changes were analyzed through Electron Backscattered Diffraction (EBSD), mechanical properties were assessed through tensile testing, and magnetic properties were evaluated and correlated to enable potential Non-Destructive Evaluation (NDE) of the heat treatment conditions and property variations. With an increase in austenitization temperature, the strengths (Yield Strength (YS) and Ultimate Tensile Strength (UTS)) were decreased, while total elongation in the steel increased up to around 680-700 °C, and then a reversing trend was found. The strength-elongation product was found to be maximum (> 30 GPa.%) at such a temperature range due to the Transformation Induced Plasticity (TRIP) effect from the presence of the maximum amount of Retained Austenite (RA) in addition to bainitic phases. A decrease in remanence and maximum induction was found with the increase in coercivity as RA content increased in the steel up to 700 °C, and then a reversing trend was found, with the exception of coercivity, which peaked at 720 °C. Hence, by measuring the maximum induction in the steel, the retained austenite content can be estimated, which is responsible for the TRIP effect in the steel.