EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

A novel processing route of thermomechanical rolling followed by direct quenching and partitioning (DQ&P) developed at the University of Oulu has shown good potential for the development of tough ductile ultra-high strength steels both for structural and abrasion-resistant purposes. In this study, fresh attempts have been made to conduct DQ&P processing on medium carbon steels with different amount of Si and/or Al contents in order to understand the precise role of these alloying elements on the microstructural mechanisms, particularly at low quenching and partitioning temperatures. Right from designing lean, appropriate compositions using 0.4 wt.% carbon steels to establishing the DQ&P parameters based on physical simulation experiments conducted on a Gleeble thermomechanical simulator, the emphasis was essentially made to develop a process that was amenable for industrial strip rolling. While quenching to temperatures such as 150 °C ensured ≈75 % martensite in the microstructure, low temperature partitioning at temperatures as low as 200 °C largely circumvented the decomposition of austenite to bainite. Detailed interpretation of dilatation curves combined with electron microscopy enabled optimization of processing schedule for subsequent laboratory rolling. Ausforming in Tnr regime resulted in extensive refining of the martensite packets/laths besides fine division of interlath austenite. A preliminary characterization showed an all-round improvement of mechanical properties following DQ&P processing in comparison to just direct quenched (DQ) samples. Evaluation of DQ&P processed laboratory rolled samples confirmed achieving the desired martensite-austenite microstructures. Nature and morphology of interlath austenite, tempered and twinned martensite and different carbides were comprehensively characterized through high-resolution transmission electron microscopy. Possible existence of metastable hexagonal omega phase was identified within the boundaries of nano-twinned martensite. The paper will highlight recent advances made in the direction of DQ&P processing of medium carbon steels and the associated challenges.