Medium Mn Research Articles

Influence of Intercritical Annealing on Microstructure, Ductility, and Toughness of Medium Mn Steels

Influence of Intercritical Annealing on Microstructure, Ductility, and Toughness of Medium Mn Steels

Achieving superior strength-ductility balance via heterogeneous structure and successive TWIP+TRIP effects in medium Mn steel

In the present work, the medium Mn steel with a strength - ductility product of 49.5 GPa·% was fabricated by a hot rolling, quenching and intercritical annealing process. The excellent mechanical property is attributed to the heterogeneous austenite morphologies, i.e., lath, granular and equiaxed austenite, and the successive TWIP + TRIP effects. The present work especially focus on two aspects: i) the evolution of micro-strain between different morphologies of austenite and ferrite during tensile deformation and its impact on the strain hardening caused by the transformation induced plasticity (TRIP) effect; and ii) the strengthening mechanisms of successive twin induced plasticity (TWIP) and TRIP effects in heterogeneous structures. Under different austenite/ferrite micro-strain interactions, the sequence of TRIP effect is in granular, lath and equiaxed austenite grains. Prior to this, the TWIP effect mainly enhance the strength of austenite, particularly equiaxed grains, which act as a high-strength skeleton structure, postponing the initiation of necking.

Microstrain partitioning, transformation induced plasticity, and damage evolution of a third generation medium Mn advanced high strength steel

An experimental Medium Mn (med-Mn) steel (0.15C-5.8Mn-1.8Al-0.71Si) with a martensitic starting microstructure, intercritically annealed at 685 °C for 120s, was discovered to have a large true strain at fracture (ɛf = 0.61) while also meeting established (strength x elongation) targets (28,809 MPa%), sustained monotonic work hardening and prolonged Transformation Induced Plasticity (TRIP) kinetics. This was found by varying the intercritical annealing (IA) temperature within a narrow temperature interval in order to isolate its impact on TRIP kinetics and damage development on such med-Mn steel. A comprehensive understanding of the microstructural damage processes leading to fracture is presented using quasi in-situ Scanning Electron Microscope tensile testing as well as X-ray Computed Microtomography. In addition, we precisely evaluated the TRIP kinetics of this steel using a combined Digital Image Correlation (DIC) and synchrotron-sourced High Energy X-ray Diffraction technique. With these methods, we demonstrate that an abundance of voids nucleate during deformation, but their growth can be suppressed by prolonging TRIP over a large strain range. Moreover, novel post-processing techniques to assess DIC acquired data at the microscopic scale have been used to gauge the severity of strain partitioning amongst phases and strain gradient evolution across dissimilar phase interfaces. By comparing it to another 3G TRIP-assisted steel and an ultrafine grained Dual Phase steel. Overall, it has been found that, in addition to carefully moderating TRIP kinetics, the introduction of polygonal ferrite, as is conventional in med-Mn steels, enhances the local forming properties and damage tolerance in 3G TRIP-assisted microstructures.

Effect of peak temperature on microstructure and mechanical performances of ART-Flash processed medium Mn steel

The combination of ART and Flash annealing has been utilized to enhance the mechanical properties of medium Mn AHSS, demonstrating significant effectiveness. However, the industrial application was constrained by the absence of research on the impact of critical processing variables. In this work, the peak temperature of the flash heating was varied in the intercritical field to manipulate the microstructure and mechanical response of a 5 wt% Mn steel. The mechanisms of microstructural evolution during heat treatment and deformation were thoroughly discussed through interrupted tensile testing. A collection of ultrafine multiphase structures, consisting of retained austenite (RA), ferrite, and martensite with flexible phase constituents, were constructed, resulting in an ultrahigh YS increment (∼450 MPa) and exceptional strength-ductility combination (YS: 1125 MPa, UTS: 1544 MPa, TE: ∼21 %). The significant strengthening is primarily attributed to the increase in α′-martensite, while the sustained TRIP effect induced by stable retained austenite is responsible for the appropriate ductility. The interaction between austenite and adjacent phases and grain size exhibit a crucial influence on the mechanical stability of austenite. Benefiting from the “shielding” effect provided by the surrounding martensite, the Flash-810 sample achieved great ductility, despite possessing only a 5 % volume fraction of retained austenite. The YS of the experimental steels is sensitive to peak temperature, while the UTS and ductility remains relatively stable. Our work suggests the substantial potential of intercritical flash heating in enhancing the mechanical response of medium Mn steels and the profound significance of controlling the peak temperature.

Design of tri-phase lamellar architectures for enhanced ductility in ultra-strong steel

Achieving exceptional ductility in ultra-high strength steels has long been a formidable challenge, particularly when the yield strength reaches 2.0 GPa. In this study, we have designed an innovative tri-phase lamellar microstructure in medium Mn steel that successfully achieves both an ultrahigh yield strength of approximately 2.0 GPa and an impressive uniform elongation ranging from 22.5 % to 24.5 %. The ultra-high yield strength of this steel is primarily due to the ultrafine lamellar grain with high-density dislocations and HDI strengthening resulting from meticulously designed tri-phase lamellae. The remarkable ductility is attributed to the synergistic action of multiple plasticity mechanisms. The inherently excellent plasticity of the lamellar ferrite, in coordination with the lamellar martensite, generates significant HDI hardening, thereby suppressing the onset of necking in the early stages of deformation. During the mid-stage of deformation, high strain activates the transformation-induced plasticity effect in the highly stable austenite, which remains stable due to substantial HDI hardening and effectively mitigates strain localization. In the later stages of deformation, delamination cracking relieves stress accumulation at the interfaces, delaying material failure. These mechanisms elevate yield strength and uniform elongation product to 47.3 GPa·%, showcasing the tri-phase lamellar structure’s potential for ultrahigh-strength, high-ductility steels.

Novel Mn and Si alloyed complex phase steel for automotive applications

Automotive industries require materials with higher strength, plasticity and crashworthiness, aiming for 3rd generation of advanced high strength steels (AHSS) with medium Mn, Si and minor alloying elements. In this investigation, 3rd generation AHSS were synthesized using a vacuum arc melting furnace with manganese (Mn) and silicon (Si) as major alloying elements as well as minor additions such as Cr, Al, Ni, etc. The steels thus developed were characterized using FE-SEM, XRD, microhardness tester, universal testing machine (UTM) and 3-dimensional Atom Probe Tomography (APT). The alloy steels after casting and homogenization were subjected to hot rolling at 1100 °C, with rolling exit temperature 900 ± 50 °C. The FE-SEM micrographs in SE mode revealed a complex phase microstructure with martensite, ferrite, bainitic ferrite and retained austenite in the specimens obtained after rolling and air cooling. The microhardness of the developed alloys is found in the range of 395–502 VHN in the hot-rolled and air cooled condition of the specimen. The tensile strength of alloys was measured to be in the range of 1412–1614 MPa with elongation of 12.12 %–18.92 %. The analysis of fracture surfaces after tensile tests for developed alloys revealed that Alloy 1 (Fe-4Mn-1.5Si) had dimples indicating ductile fracture while Alloy 2 (Fe-6Mn-1.5Si) has a mixture of dimples and facets, and Alloy 3 (Fe-8Mn-1.5Si) has lower dimples but larger facets, confirming quasi-ductile fracture with a lower ductility limit of 12.12 %. Atom Probe Tomography was performed to study the complex phase structure at nanoscale through re-distribution of carbon and other alloying elements. 3D APT revealed the presence of very fine retained austenite film of thickness ∼4–5 nm and carbon content of 6–8 at.%. This complex phase microstructure obtained after hot rolling and normalizing is made of very fine bainitic ferrite with film type retained austenite providing TRIP effect, in addition martensite and ferrite resulting in high strength.

Enhancing the Strength of Medium Mn Steel by Flash Treatment

Enhancing the Strength of Medium Mn Steel by Flash Treatment

Influence of martensite and grain size on the mechanical behavior of austenitic Fe–C–Mn–Ni–Al medium Mn steels with TWIP mechanism under uniaxial tension/compression and micromechanical creep

Medium Mn steels with induced plasticity mechanisms have effectively addressed the high-strength and ductility requirements for structural applications. However, there is still uncertainty about the impact of residual martensite on medium Mn steels with an austenitic matrix under uniaxial tension and compression. Furthermore, research on the influence of grain size through incomplete recrystallization in medium Mn alloys is scarce. In this context, our study focused on design and manufacturing a medium Mn Fe–C–Mn–Ni–Al alloy through controlled atmosphere casting incorporating martensite, aiming to analyze its effects under uniaxial stress conditions. Thermomechanical and annealing heat treatments were applied to control grain growth and nucleation of new grains, allowing us to explore how grain size affects the twinning process and mechanical reinforcement in the alloy. Microstructural characterization techniques, including optical and scanning electron microscopy, were employed to assess the mechanical response and damage mechanisms. Mechanical tests included tension, compression, and nanoindentation. The results indicated that residual martensite generates an accelerated damage mechanism under tension due to co-deformation at the interface with austenite, resulting in crack nucleation and rapid propagation through mode I fracture. Consequently, ductility in tension significantly decreases, while no apparent failure is observed during the compression deformation process. Furthermore, incomplete recrystallization was observed to enhance the creep response in both tension and compression. The strain hardening rate results revealed the activation of primary twinning in tension and both primary and secondary twinning in compression for annealed samples. Nanoindentation tests confirmed the presence of twinning and revealed that diffusion creep and grain boundary sliding are the predominant creep mechanisms.

Current-manipulated martensite transformation to enhance strength-ductility synergy in a medium Mn steel

In order to quantify the thermal and athermal effects during pulse current assisted deformation, the deformation behavior of medium manganese steel was studied using forced air cooling. At room temperature, the ultimate tensile strength of medium manganese steel is 1350 MPa and the total elongation is 47.3 %. However, in the pulsed current assisted deformation under forced air cooling, its strength and ductility are synergistically improved, with the ultimate tensile strength increased to 1380 MPa and the total elongation increased to 57 %. Athermal effects can delay deformation-induced martensite transformation by reducing austenite dislocation density and reducing stress concentration at austenite-ferrite phase boundaries, resulting in better strength and ductility in the pulsed tensile sample with forced air cooling. While the thermal effect increases the strain energy required for deformation-induced martensite transformation, resulting in a decrease in martensitic content and a decrease in accumulated dislocation density. Therefore, compared with the sample with forced air cooling, the ultimate tensile strength of the pulse tensile sample without forced air cooling is reduced.

Tailoring hetero-deformation induced strengthening to improve strength-ductility balance in warm rolled medium Mn steel

The heterogenous microstructures are designed through controlling warm rolling (WR) reduction in two-stage rolling process to control geometrically necessary dislocations (GND) density, greatly improving the strength and ductility balance of medium Mn steel in this contribution. With an elevated WR reduction in two-stage rolling process, the austenite grain size is refined and the grains change from equiaxed to lath-like in morphology, which can be attributed to the accumulation of stored strain energy during the rolling process. In addition, the volume fraction of partially recrystallized austenite also increases, which results in a higher GND density of austenite, providing a stronger hetero-deformation induced (HDI) strengthening. The stronger HDI hardening at low strain as well as the transformation-induced plasticity (TRIP) effect and twinning-induced plasticity (TWIP) effect results in a slower decrease in work hardening rate. These results lead to an enhanced yield strength (YS) without damaging ductility. The best strength and ductility combination with YS of 1088 MPa, ultimate tensile strength (UTS) of 1221 MPa and total elongation (TE) of 36.4 % has been achieved in the annealed sample with 82 % WR reduction.

Effects of temperature on the microstructural evolution and mechanical properties of copper − bearing medium manganese steel during tempering

This work introduced the effects of higher tempering temperatures on the microstructural evolution of Cu − bearing medium Mn steel. After quenching steel at 860 °C for 1 h, it was tempered at 600, 640, and 670 °C for 2 h. The results indicated that the low − temperature impact toughness initially increased and then decreased, while the yield strength consistently decreased with increasing temperature. At a tempering temperature of 640 °C, the steel exhibited a relatively high content of reversed austenite and demonstrated the best low − temperature toughness and plasticity. The low − temperature toughness and plasticity of the steel significantly diminished, when the temperature exceeded 640 °C. This is because the degree of alloying element enrichment in the reversed austenite is reduced, accompanied by an increase in dislocation density and the formation of twins. These reversed austenites exhibit a diminished ability to hinder crack nucleation and propagation. The yield strength decreased with increasing temperature, primarily due to the coarsening of the Cu particles, which reduces their precipitation strengthening effect. This study advances the development of high − strength tough medium − manganese steels.

Effect of dislocation density on microstructure and mechanical properties of cold-rolled medium Mn steel

This study investigated the effect of dislocation density on microstructural transformation and mechanical properties of Fe-7.23Mn-4.42Al-0.09C-1.99Si (wt.%) steel. By the tempering treatment, the dislocation density was reduced. The effect of of dislocation density on the plasticity of the steel was analyzed. The dislocation density was reduced from 2.3 × 10−16 m−2 to 1.6 10−16m−2 by tempering. Reducing dislocation density in the metastable austenite resulted in a higher metastable austenite transformation rate. And the low dislocation density steels exhibited a longer strain length during tensile deformation. It was obvserved that the dislocation density of the test steel decreased by 30% after tempering, the austenite transformation rate increased to 53.3%, and tensile strength and elongation increased to 1010 MPa and 44% respectively.

Enhancing mechanical properties of medium Mn steel by warm rolling based on laminated elemental segregation

The hot-rolled microstructure of medium Mn steel has coarse grains and severe elemental segregation, resulting in low strength and plasticity. Constructing a multiphase structure, refining the microstructure, and regulating elemental segregation enhance the mechanical properties. In this study, liquid nitrogen treatment created a layered distribution of austenite and martensite. Warm rolling was then used to reduce layer thickness and refine grain structure. After liquid nitrogen and warm rolling treatments, the strength and plasticity of medium Mn steel increased to 1270 MPa and 23.3%, respectively, far exceeding the hot-rolled state (724 MPa, 12.8%). Warm rolling also triggers austenite reverted transformation (ART) and introduces high-density dislocations, further improving austenite stability. This strengthening effect is higher than that from intercritical annealing alone. Improved austenite stability delays the transformation induced plasticity (TRIP) effect, preventing brittle fracture and enhancing deformation coordination between layers, significantly increasing the plastic deformation capacity of medium Mn steel.

Fabrication of Medium Mn Advanced High-Strength Steel with Excellent Mechanical Properties by Friction Stir Processing.

Friction stir processing (FSP) manufacturing technology was used to fabricate medium Mn advanced high-strength steel in this study. The mechanical properties and microstructure of the steel fabricated using FSP were investigated. The steel obtained a total elongation of 35.1% and a tensile strength of 1034.6 MPa, which is about 59% higher than that of the steel without FSP. After FSP, a gradient structure occurs along the thickness direction. Specifically, across the thickness direction from the base material zone to the transition zone and finally to the stirring zone, both the grain size and austenite fraction decrease while the dislocation density increases, which results from the simultaneous effect of severe plastic deformation and recrystallization during FSP. Due to the gradient structure, an obvious difference in the strain across the thickness direction of the steel occurs during the deformation process, resulting in significant hetero-deformation-induced (HDI) strengthening. The deformation mechanism analysis reveals that HDI strengthening and dislocation strengthening are the main factors in the improvement in the strength-ductility balance. The obtained knowledge sheds light on the process of fabricating medium Mn steels with excellent properties using FSP manufacturing technology.

Impacts of near-Ms austempering treatment on microstructure evolution and bainitic transformation kinetics of a medium Mn steel

Impacts of near-Ms austempering treatment on microstructure evolution and bainitic transformation kinetics of a medium Mn steel

The nonlinear analysis of Portevin-Le Chatelier (PLC) effect: An application to medium Mn steel

The instability of plastic flow in medium manganese steel was investigated through uniaxial tensile tests conducted at room temperature across various strain rates. The Portvin-Le Chatelier (PLC) effect was analyzed using statistical methods and a multifractal approach. The findings indicate that the zigzag stress–strain curves associated with instability plastic flow exhibit typical multifractal characteristics. Furthermore, the multifractal analysis quantifies the influence of strain rate and initial phase composition on the instability characteristics of the stress–strain curves. Specifically, it was observed that lower strain rates and austenite content correspond to a chaotic state, whereas higher strain rates and austenite content result in self-organizing critical dynamics. Finally, the underlying physical mechanisms driving these differing modes of behavior are discussed, with a focus on their relationship to strain rate and the initial austenitic phase fraction.

Preparation of high-strength and high-ductility medium Mn steel with chemistry heterogeneous via a two-step intercritical annealing process

Recently, the retained austenite (RA) with chemically heterogeneous has attracted great attention in medium Mn steel. In this work, we designed a two-step intercritical annealing (TIA) heat treatment process to prepare high-strength medium Mn steel with the chemical composition of Fe–0.20C–8.0Mn–1.6Si–1.3Al (wt.%). Compared with the normal intercritical annealing (IA) heat treatment process, the TIA process can not only increase the volume fraction of RA but also obtain the RA with inhomogeneous Mn distribution. Meanwhile, it was found that the Mn content on the outer side of this structured RA was higher than that on the inner side, forming a nanoscale Mn-enriched layer on the outer side of RA, which played a significant role in improving the stability of RA. Finally, we used the TIA process to obtain medium Mn steel with an ultimate tensile strength (UTS) of 1611.83 MPa, a total elongation (TE) of 45.02%, and a product of strength and elongation (PSE) of 72.56 GPa·%, which achieves a superior combination of high strength and high ductility in medium Mn steel.

Austenite Tailoring for Strength and Ductility Enhancement in Medium Mn Steel: A Brief Review

Austenite Tailoring for Strength and Ductility Enhancement in Medium Mn Steel: A Brief Review

The Effect of Laser Power on Microstructure and Mechanical Properties between Medium Mn Steel and DP980 Joint by Laser Welding

The Effect of Laser Power on Microstructure and Mechanical Properties between Medium Mn Steel and DP980 Joint by Laser Welding

Synergistic improvement of strength and ductility by high magnetic field assisted intercritical annealing in lightweight medium Mn steel

The microstructures and mechanical properties of lightweight medium Mn steels were studied by high magnetic field (HMF) assisted intercritical annealing process (HMF-IA). The results show that the volume fraction of retained austenite increased and reached the maximum value of 56.2 % when the magnetic flux density (MFD) was 10 T at first and then decreased with the increase of MFD. The application of 10 T HMF promoted the diffusion ability of carbon atoms into austenite by magnetic stress-driven dislocation movement and increased the volume fraction and grain size of austenite. HMF also played a critical role in dislocation movement and element partitioning during the IA process. Due to the wide range of bimodal size distribution of austenite after the HMF-IA process, stronger transformation induced plasticity effect and twin induced plasticity effect happened in the process of plastic deformation. For the sample annealed under 10 T, comparing with the sample annealed without HMF, the product of strength and elongation value increased from 45.7 GPa·% to 55.1 GPa·%. Additionally, it also kept a persistent high strain hardening. The present study provides a new understanding of the role of HMF in adjusting the relationship between microstructure and mechanical properties during the annealing process and opens up a new path for designing industrial medium Mn steel with low cost, high performance and simple process.