Today’s advanced steels are required to possess high strength and ductility. One way of achieving a favourable ductility level and a very high ultimate strength is to stabilize retained austenite in martensitic matrix. Among heat treatment processes capable of producing such microstructure, there is the Q&P process (Quenching and Partitioning). It can produce microstructures consisting of martensite and a certain amount of retained austenite, which exhibit strengths above 2000 MPa and elongation levels of 10-15 %. For some processes, it is appropriate to depress the Ms and Mf temperatures and modify the ferritic and pearlitic transformations to occur at lower cooling rates. By choosing an appropriate steel chemistry, the retained austenite can be stabilized in martensitic matrix at lower temperatures than in the ordinary Q&P process. Four steels were selected for the present experimental programme. Several heat treatment sequences were tried with these steels and multiple parameters were varied: austenitizing temperature, cooling rate, quenching temperature and the carbon partitioning temperature.Final strengths of the steels were in the range of 1750-2400 MPa and their A5 mm elongation level was up to 10 %. The morphology and distribution of retained austenite dictate the resulting mechanical properties. The microstructures were studied using transmission electron microscopy. The distribution and morphology of retained austenite were examined using diffraction analysis (SAED) and bright and dark-field illumination. X-ray diffraction was employed to measure the volume fraction of retained austenite. The values were between 9 and 15 %.Keywords: Q-P process, retained austenite, AHSS, UHSS, transmission electron microscopy, XRD.
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