Transformation-induced plasticity (TRIP)-assisted medium-Mn steels with a ferritic matrix containing considerable amounts of retained austenite are a promising candidate to fulfill the requirements for the third-generation of advanced high-strength steels (AHSS), which is currently under development. The influence of the intercritical annealing temperature and cooling rate on the final microstructure of a 0.1C3.5Mn and 0.1C5Mn steel, respectively, was elaborately investigated. Dilatometric experiments were carried out and additionally supported by microstructural observations. During soaking in the two-phase ferriteaustenite region and subsequent slow cooling the C and Mn concentration in the austenite increased and resulted in its chemical stabilization. The variation of the annealing temperature and cooling rate altered the amounts of ferrite, retained austenite, bainite, pearlite, and martensite in the final microstructure. Furthermore, two thermodynamical models for the prediction of the maximum retained austenite content and the optimal annealing temperature have been thoroughly evaluated in this work. It can be stated that the experimental data revealed a shift of the maximum retained austenite to higher annealing temperatures compared to the model calculations. As diffusional transformations were not considered in the applied models, slow cooling rates led to pronounced deviations between calculation and experiment, thereby showing the need for model adaptions.
The heat-treatment (HT) schedule and selected annealing parameters have a substantial effect on the microstructure and mechanical properties of medium-Mn-steels. The structure morphology depends on the fact, whether the austenite-reverted transformation takes place from deformed (one-step HT) or non-deformed (two-step HT) microstructures. Depending on the intercritical annealing temperature, the stability of the retained austenite can be altered to a large extent. As a result, the mechanical properties can be adjusted from high strength with excellent ductility to very high strength with reasonable ductility. The present contribution, therefore, elucidates the dependence of the microstructural characteristics and material behaviour on the HT parameters for medium-Mn alloy compositions with different Mn-contents. This paper is part of a Thematic Issue on Medium Manganese Steels.
TRansformation Induced Plasticity (TRIP)-assisted Medium-Mn steels, with Mn-contents in the range of 4–10 ma.-%, have recently gained a lot of interest due to their promising mechanical properties. This steel group contains ≥ 30 % of retained austenite, which is stabilized by Carbon and mainly Manganese during intercritical annealing. The present work investigated the influence of annealing temperature and cooling rate on the microstructural evolution by means of dilatometry. Two thermodynamical models for the prediction of optimal annealing temperature and maximum retained austenite content have also been thoroughly evaluated. For further characterization, scanning electron microscopy, EBSD, micro-hardness testing and X-ray diffraction were carried out. The investigations manifested a pronounced influence of both annealing temperature and cooling rate, on the phase fractions of ferrite, austenite and martensite, which must be taken into account by design of batch annealing route for Medium-Mn TRIP steels in order to obtain superior combination of strength and ductility.
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