Abstract

In order to establish the processing conditions to achieve the target of tensile strength of ∼1500 MPa with minimum retained austenite (RA) fraction of 10% in hot-rolled 3–4%Mn medium Mn steels, three hot-deformed steels with the base composition of Fe-0.3C–1Si-0.5Al (concentrations in wt.%) and variable contents of Mn (3 or 4 wt%), Ni (1 or 2 wt%), Mo (0.2 or 0.4 wt%) and V (0 or 0.2 wt%) were processed using both the quenching and partitioning (QP) and austenite reverse transformation (ART) treatments. Physical simulation experiments were carried out using small cylinders in a thermomechanical simulator and the differences in the amounts, morphology and stability of RA were established by using X-ray diffraction and electron probe microanalysis, besides standard material characterization techniques. Both processes provided relatively high fractions of austenite; up to 26 vol% using the QP route and up to 49 vol% by the ART treatment under specific conditions. These amounts were comparable to those predicted for the conditions of thermodynamic equilibrium using the ThermoCalc software. The stability of RA for various conditions was estimated in terms of the average carbon contents of the RA. In QP process RA had mostly blocky-like appearance while in ART process the majority of RA was lath-like shaped. The hardness measurements revealed that the level of hardness in the QP structures was higher than that of the ART samples, possibly affected by RA fraction but more obviously as a result of martensite tempered at relatively low temperatures in the QP and at high temperatures in the ART treated structures. Preliminary tensile properties were measured on selected laboratory hot-rolled samples and results analyzed based on RA characteristics. The preliminary results indicated that the target is achievable in the laboratory scale through both the processing routes.