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Herein, the Fe–5Mn–2Al–0.1C (wt%) and micro‐alloyed Fe–5Mn–2Al–0.1C–0.1Nb–0.2V (wt%) medium Mn steels are processed by a novel flash‐austenite‐reverse‐transformation (ART) annealing. The core–shell Mn‐gradient in austenite is observed and compositionally characterized at near‐atomic scale by using atom probe tomography. Compared with the conventional ART annealing, flash‐ART process doubled the austenite fraction in both materials, based on electron backscattering diffraction measurement. The rapid austenite reversion in flash‐ART annealed MMnS is attributed to the negligible partitioning local equilibrium controlled austenite kinetics at the flash temperature. The formation of core–shell Mn‐gradient is due to the different austenite growth kinetics and interface mobilities. Microalloying elements are found to form nano‐carbides, which leads to decelerating austenite reversion because of depleted carbon content in the initial martensitic microstructure, and retarding austenite growth according to pinning effect. [ABSTRACT FROM AUTHOR]

Developing medium-Mn steels (MMnS) demands a better understanding of the microstructure evolution during thermo-mechanical treatments (TMTs). This study demonstrates the relationship among processing, microstructure, and mechanical properties of a warm-rolled medium-Mn steel (MMnS) containing 1.5 wt. % Cu and 1.5 wt. % Ni. After short-time warm rolling (WR) in an intercritical temperature range, a significant quantity (40.6 vol.%) of austenite was reverted and retained after air cooling. The microstructure and tensile properties of the WR specimens were compared with two typical process routes, namely hot rolling+ cold rolling+ annealing+ tempering (CRAT) and warm rolling+ annealing+ tempering (WRAT). The WR specimen exhibited comparable tensile properties with the CRAT specimens (967 MPa yield strength, 1155 MPa tensile strength, 23% total elongation), with a remarkably shortened process route, which was derived from the dislocation accumulation and austenite reversion during rolling. The WR route stands out among the traditional CRAT and the extended WRAT routes for its excellent tensile properties and compact processing route. [ABSTRACT FROM AUTHOR]

A Fe–Al–Mn–Ni–C steel is designed explicitly to precipitate intermetallic phases in specific phases of a duplex steel. CALPHAD‐assisted alloy design is carried out with respect to an innovative "solublility‐assisted precipitation" (SAP) mechanism. A four phase equilibrium comprising of ferrite, austenite, κ‐phase (Fe3AlC), and B2‐phase (NiAl) is stabilized in a suitable annealing window. The processing conditions are adjusted according to elemental partitioning and consequential stabilization of κ‐phase in austenite and B2‐phase in ferrite. The feasibility of the SAP mechanism is validated by investigation of microstructure and precipitation evolution using electron microscopy and site‐specific atom probe tomography. A concept to direct precipitation of intermetallic phases to specific phases of a high‐Al alloyed duplex steel is proposed. CALPHAD‐assisted alloy and processing design is carried out to control elemental partitioning and facilitate the formation of intermetallic precipitates. A validation of the proposed concept is conducted by a multi‐scale experimental characterization using electron microscopy and atom probe tomography. [ABSTRACT FROM AUTHOR]

Medium-Mn steels are characterized by ultrafine-grained (UFG) duplex microstructure consisting of ferrite and a large amount of retained austenite. Intercritical annealing is of great importance to achieving the UFG duplex microstructure and adjusting amount as well as stability of retained austenite. In the present work, the influence of intercritical annealing temperature on the microstructure and mechanical properties was investigated in a cold-rolled medium-Mn steel Fe-12Mn-3Al-0.05C. Particularly, the dependence of microstructural morphology on intercritical annealing temperature was emphasized to reveal the genesis of the microstructural morphology in medium-Mn steels. The ferrite-austenite duplex microstructure manifested an elongated morphology in the specimen annealed at 555 °C, which inherited the lath structure of the cold-rolled state. The medium-Mn steel exhibited a continuous yielding behavior and a relatively low strain-hardening rate. With an increase in intercritical annealing temperature up to 650 °C, the amount of retained austenite increased and microstructure was partially recrystallized, showing a mixture of elongated and equiaxed grain morphologies. When the intercritical annealing was applied at 700 °C, the medium-Mn steel mainly exhibited recrystallized microstructure with equiaxed morphology. The optimal balance between the amount and the stability of retained austenite led to an enhancement of strain hardening and ductility. With a further increase in the intercritical annealing temperature to 750 °C, the medium-Mn steel possessed pronounced strain-hardening behavior at the beginning of the tensile deformation with deteriorated ductility. [ABSTRACT FROM AUTHOR]

Since highly loaded components, such as roller bearings, tend to have relatively high lifetime scatter caused by microstructural defects, an increased defect tolerance of the material can improve the performance as well as the reliability of these highly stressed components. To increase the defect tolerance of the roller bearing steel 100Cr6, two different laboratory melts with either 1.5 wt.-% Al or 1.5 wt.-% Si in addition to the standard composition were developed. After an adapted heat treatment, both steels exhibited a bainitic microstructure with a relatively high content of retained austenite with more than 20 vol.-%. Considering the effects of the retained austenite, a defined mechanical stability is essential to avoid excessive deformation-induced transformation from the retained austenite into α′-martensite, which can lead to shape deviations in components. However, besides the increased deformability of the austenitic phase, local phase transformations in the vicinity of defects can increase the defect tolerance. The fatigue tests performed at ambient temperature (AT) and at T = 100 °C, show a higher fatigue strength of the Si-alloyed steel due to a refined microstructure and revealed for both steels a relatively high austenite stability, since no macroscopic phase transformation was observed. The high austenite stability can be explained by a relatively high carbon content of the austenitic phase. However, in contrast to the Al-alloyed steel, a decrease in the fatigue strength and in the defect tolerance due to the increased temperature were observed for the Si-alloyed steel, which indicates a more pronounced phase transformation at AT. The lower carbon content and the finer distribution of the retained austenite, when compared to the Al-alloyed steel, are supposed to be the reason. While a smaller carbon content decreases the phase stability, the refined austenite distribution results in a higher probability of retained austenite presence in the vicinity of a defect. [ABSTRACT FROM AUTHOR]

In this study, the austenite transformation in a cold-rolled medium-Mn steel (MMnS; 7 wt% Mn) was adjusted by inter-critical annealing (IA) at 680 °C (above the Ac 1 temperature) and by partition annealing (PA) at 650 °C (below the Ac 1 temperature). The deformation behavior associated with the microstructural evolution, and crystallographic changes were investigated using in situ synchrotron X-ray diffraction during tensile deformation. Electron backscatter diffraction was used to characterize the microstructure. A considerable amount of austenite (approximately 30 and 20 vol%) was promoted by reversion transformation during the IA and PA treatments, respectively. The difference in deformation behaviors between the IA and PA specimens was attributed to the different mechanical stabilities of the reverted austenite. The relatively low mechanical stability of retained austenite (RA), due to less Mn enrichment during IA, led to a pronounced activation of the transformation-induced plasticity effect, which improved the strain hardening capacity, the ultimate tensile strength, and total elongation. However, the low recovered/recrystallized fraction of the ferrite phase resulting from PA contributed to a significant increase in the yield strength. The current understanding of the characteristics and mechanical stability of RA induced by annealing at different temperatures below and above the Ac 1 temperature will help in further optimizing annealing parameters to achieve better mechanical properties for MMnS. [ABSTRACT FROM AUTHOR]

A novel Cr-alloyed medium-Mn steel was designed and manufactured for the hot forming application in automobile. Compared to the typical press hardening steel (PHS) grade of 22MnB5, it possesses more excellent mechanical combination and enormously reduced oxidization after press hardening. The synchrotron-based high-energy X-ray diffraction experiment indicated that the solute C atoms could be partitioned from martensite to austenite during the baking process and then segregated on geometry necessary dislocations that had been generated due to the austenite-to-martensite transformation during the previous cooling. This led to much hardened austenite grains, which could bear more load partitioned and accommodate the greater deformation until they transformed to martensite during deformation than the non-baked austenite grains, thus greatly improving both strength and ductility. Moreover, the thickness of oxidization layer on the invented steel is less than 3 μm after the hot forming at 750−810°C, much thinner than the 100 μm-thick oxidization layer on 22MnB5. Such a tremendously reduced oxidation results from both the lower soaking temperature employed for the hot forming and the formation of dense Cr/Al/Si oxide band at the bottom of the oxidation layer, the latter is because the designed steel contains much higher contents of Cr and Al than 22MnB5. Image, graphical abstract [ABSTRACT FROM AUTHOR]