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.
The effects of the intercritical annealing temperature and initial microstructure on the stability of retained austenite were investigated for a 0.1C-6Mn (wt-%) steel. Medium-Mn transformation-induced plasticity (TRIP) steels exhibit a strong dependence of their mechanical properties on the variation of intercritical annealing temperature. This behavior is strongly linked to the amount and stability of the retained austenite. Thus, interrupted tensile tests were used to examine the effect of annealing temperature on the stabilization of the retained austenite. Detailed microstructural investigations were employed to elaborate the effects of its chemical and mechanical stabilization. Furthermore, the final microstructure was varied by applying the batch annealing step to an initial non-deformed and deformed microstructure respectively. Retained austenite stability along with resulting mechanical properties of the investigated medium-Mn TRIP steel was significantly influenced as the amount and morphology of the respective phases altered as a consequence of both initial microstructure and applied intercritical annealing temperature.
The present work concentrates on the investigation of the damage behavior in the case of a batch annealed 0.1C6Mn medium‐Mn steel as a function of intercritical annealing temperature. The variation of this important annealing parameter results in the formation of different microstructures, consisting of altering amounts of ferrite, martensite, and retained austenite. Martensite and retained austenite change not only in their volume fraction, but also in the grain size and composition, which implies a modification of their hardness and for the latter microstructural compound, varying chemical, and mechanical stability. Therefore, a detailed investigation of non‐deformed and pre‐strained microstructures is performed by means of scanning electron microscopy (SEM) and interrupted tensile testing is carried out to determine the retained austenite stability. To understand the macro‐ and micro‐scale damage response of the investigated steel, tested tensile samples are analyzed with respect to their fracture behavior, as well as void appearance and their location within the microstructure. It is found that the stability of retained austenite and the presence of hard athermal martensite in the microstructure play the most important role, governing the overall damage behavior of the present medium‐Mn steel.
In the present work, the influence of the cooling time on the mechanical performance, hardness, and microstructural features of a double pulse resistance spot welded medium-Mn steel are investigated. Curves of the electrical resistance throughout the welding revealed that the cooling time strongly influences the heat generation during the second pulse. A second pulse after a short cooling time re-melts the center, and heat treats the edge of the primary fusion zone. This desired in-process heat treatment leads to a modification of the cast-like martensitic structure by recrystallization illustrated by electron backscatter diffraction measurements and to a homogenization of manganese segregations, visualized by energy-dispersive X-ray spectroscopy, which results in an enhanced mechanical performance during the cross tension strength test. In contrast, during excessively long cooling times, the resistance drops to a level where the heat generation due to the second pulse is too low to sufficiently re-heat the edge of the primary FZ. As a consequence, the signs of recrystallization disappear, and the manganese segregations are still present at the edge of the fusion zone, which leads to a deterioration of the mechanical properties.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.