Effect of Temperature and Deformation on the Stability of Retained Austenite in Closed-Die Forgings from High-Strength Martensitic Manganese-Silicon Steels

In advanced steels, retained austenite is an important phase in the final microstructure, which provides a favourable combination of strength and ductility. Advanced methods capable of achieving this include the Q-P process (Quenching and Partitioning). By this means, strength of around 2000 MPa and elongation of 10-15 % can be obtained. The main factors that determine whether retained austenite remains stable in the martensitic matrix include its morphology, particle size and distribution. A closed-die forging was made from an experimental steel containing 0.42 % C, 2.45 % Mn, 2.09 % Si, 1.34 % Cr and 0.56 % Ni and Q-P processed. Based on previous data measured in a real-world process, two basic heat treatment sequences were created and tested on a thermomechanical simulator. The two sequences differed in the cooling rates. Upon heat treatment, a martensitic microstructure with a retained austenite content of 13-17 % was obtained. The ultimate strength was in the range of 2100-2200 MPa with elongations A(5mm) 8-15 %. Austenite stability was tested by bringing the material to various temperatures (200, 300, 400, -18, -196 degrees C) and by cold forming at various strain rates (10(-3), 10(-1), 10 s(-1)). The volume fraction of carbon in austenite was calculated from the lattice parameters determined by X-ray diffraction.