Medium Mn Steel Research Articles

Manipulation of the Stacking Fault Energy of a Medium-Mn Steel Through Temperature and Hierarchical Compositional Variation

Manipulation of the Stacking Fault Energy of a Medium-Mn Steel Through Temperature and Hierarchical Compositional Variation

Heterogeneous microstructure design by pre-manipulating ferrite recrystallization in a cold rolled medium Mn steel

The medium Mn steel (MMS), which is regarded as the most representative candidate of the third generation advanced high strength steels, has received widely attention during the last several decades with respect to the exceptional advantage of low cost and excellent strength-ductility properties. In this study, a microstructural strategy of developing heterogeneous microstructures in a cold rolled MMS is presented. By pre-manipulating occurrence of the ferrite recrystallization, both the lamellar-shaped and granular-shaped ultra-fine retained austenite can be obtained after the two-step intercritical annealing process. Various amounts of recrystallized ferrite and difform martensite can be obtained by adjusting the pre-annealing temperature, which can effectively contribute to producing the two types of heterogeneous retained austenite, i.e., lamellar and granular in the following annealing process. The heterogeneous-structured retained austenite enables an excellent strength–ductility combination and reduced Lüders strain in the cold-rolled MMS.

Analysis of phase transformation thermodynamics and kinetics and its relationship to structure-mechanical properties in a medium-Mn high strength steel

The difference in transformation mechanisms between the normal and reverse phase transformation in medium Mn steel was revealed using thermodynamic and kinetic analysis. The results indicate that the austenite formation during the reverse phase transformation is a fast reaction, significant amount of 46.9 vol% retained austenite is obtained after subjected partial reversion at 650 °C × 1 h. At the nucleation stage, the lath-like structure, dislocations, martensitic packet boundaries, and grain boundaries provide high-density nucleation sites for austenite formation. The critical temperature of austenite formation is reduced by the enrichment of Mn and C. However, no ferrite transformation occurred during the normal phase transformation (650 °C × 6 h isothermal treatment and subsequent furnace-cooling process). At the nucleation stage, the undersaturated Mn and C in the prior austenite grain interior provide insufficient concentration fluctuation, leading to a low nucleation rate. At the phase growth stage, the atomic mobility of Mn and C in the parent austenite (control the ferrite growth) is 1/69 and 1/282 than that in ferrite (control the austenite growth) at 650 °C. The differences in nucleation site density, driving force, and element diffusion rate lead to significant variations in phase transformation behaviors. The excellent mechanical property with the product of strength and elongation (PSE) exceeding 20 GPa % is achieved via the sustained transformation-induced plasticity (TRIP) effect by metastable retained austenite over a large strain range.

Investigating the influence of Si on dynamic recrystallization during hot deformation behavior of ultra-high-strength lightweight steel

In this work, we systematically evaluated the effect of Si on the dynamic recrystallization (DRX) during hot deformation behavior of a novel high-strength rolled Medium Mn steel. To this end, the novel steel was subjected to compression test in the temperatures of range temperature of 950–1150 °C and the strain rate of range of 0.01–10 s−1. Numerical simulation, constitutive analysis, different models and processing maps were also developed to predict the volume fraction of DRX, which was further confirmed through electron backscatter diffraction (EBSD). The results show that some disparities are existed between the model predictions and the results. The EBSD micrographs of samples displayed prominent discontinuous DRX peak in the true stress-strain curves exhibit conventional equiaxed DRX particles. Most specifically, in initial stages the addition of Si increased the DRX and grain boundary migration, and the later stages of deformation the DRX was triggered after achieving the steady state. Therefore, the final microstructure appears as elongated austenite or localized DRX. Furthermore, the work hardening stage before the new round of DRX nucleation caused the stress to continue to increase after the steady state. These findings provide valuable insights into the complex behavior of materials under the influence of Si addition and enhancing our understanding of microstructural evolution during hot deformation.

Effect of C on the superplasticity of medium Mn steel

We investigated the effect of C content on the superplasticity of medium Mn steel through high-temperature tensile testing of Fe–7Mn-(0–0.35)C (wt%) steels at temperatures ranging from 490 °C to 720 °C with an initial strain rate of 1.0 × 10−3 s−1. Superplasticity became evident at 550 °C for C-added steels and at 634 °C for the C-free steel. As all the C-added steels exhibited the highest elongation at 650 °C, we conducted in-depth analyses using the specimens tested at 650 °C. The elongation significantly increased with increasing C content up to 0.20 wt%. The results of strain rate sensitivity, apparent activation energy, and microstructural observations indicated that the low elongation of the 0C steel was attributed to the occurrence of dislocation creep, whereas the high elongations of both 0.10C and 0.20C steels were due to the occurrence of grain boundary sliding. The 0.20C steel exhibited higher elongation than the 0.10C steel due to its smaller average grain size and a larger area of α/γ interphase boundaries compared to the 0.10C steel. Meanwhile, the elongation of the 0.35C steel was not significantly improved compared to the 0.20C steel, despite having a smaller average grain size. This was because the 0.35C steel had a lower area of α/γ interphase boundaries, which resulted from the higher γ fraction. We believe the optimal C content for enhancing the superplasticity of Fe–7Mn steel is approximately 0.20 wt%, considering the 0.35C_T steel requires an additional tempering process, and its elongation was comparable to the 0.20C steel.

Investigations on austenite stability and martensitic transformation kinetics of a medium Mn steel under different strain states

Martensitic transformation has received wide attention due to its significant role in improving mechanical properties of medium Mn steels. In this paper, three typical strain states in sheet metal forming, uniaxial tension, biaxial tension and plane strain states, were selected to investigate the effect of the strain state on martensitic transformation of a medium Mn steel at room temperature. The experimental results show that the austenite stability of the medium Mn steel is seriously strain state dependent. The sample deformed under the plane strain state exhibits the lowest austenite stability, while it progressively increases for biaxial tension and uniaxial tension states. The microstructural evolution and the strain partitioning behavior of the material are characterized. It is indicated that the strains in austenite under different deformation modes are quite different even though the overall deformation conditions are similar, which leads to different transformation rates of γ → α’ transformation. Besides, the ε-martensitic transformation that only generates under the plane strain state further decreases the austenite stability. A strain state dependent α’-martensitic transformation kinetics law is proposed that incorporates the effect of the stress triaxiality and Lode angle parameter, which provides a reasonable prediction of γ → α’ transformation in the medium Mn steel over a wide range of strain states.

Enhancing hydrogen embrittlement resistance of TRIP-rich medium Mn steel by morphology optimization

Enhancing hydrogen embrittlement resistance of TRIP-rich medium Mn steel by morphology optimization

Effects of Quenching Temperature on the Microstructure and Mechanical Properties of a Strip-Cast Medium-Mn Steel Processed by Quenching and Partitioning

Twin-roll strip casting (TRSC), which is a low-energy and short process to produce strip steel, is a potential approach to produce advanced high-strength steels. Herein, a medium-Mn steel containing 4 wt% Mn was processed using a novel route involving TRSC, hot rolling and quenching and partitioning (QP) to explore the possibility of medium-Mn steel produced by TRSC plus QP process. The effects of quenching temperature on the microstructure and mechanical properties were studied. It was found that primary martensite and retained austenite (RA) were obtained at the quenching temperature of 140–180 °C, while primary martensite, RA and secondary martensite were obtained when the quenching temperature was 220–300 °C. With an increase in quenching temperature from 140 to 260 and to 300 °C, the RA fraction first increased from 15.4% to 31.8% and then decreased to 16.6%. The sample at a quenching temperature of 220 °C yielded mechanical properties with a yield strength of 992 MPa, tensile strength of 1159 MPa and total elongation of 20.4%. The superior mechanical properties were achieved by an optimum combination of high RA fraction (26.5%), appropriate mechanical stability of RA and a small number of the islands of secondary martensite and RA. Hence, the present study provides a viable processing route for medium-Mn steel.

Unusual step-like stress flow behavior and mechanisms of a 1.3 GPa grade medium-Mn steel with 51.2 % ductility

A Fe-8.9Mn-1.59Si-1.0Al-0.25C medium-Mn steel have achieved a superior combination of strength and ductility, i.e., the product of strength and elongation (PSE) of 66.5 GPa%. Meanwhile, an unusual step-like stress–strain curve due to heterogeneous deformation bands including Lüders band and multiple Portevin–Le Chatelier (PLC) bands was investigated systematacially. The repeated heterogeneous deformation is due to a combination of extrinsic and intrinsic factors. The former refers a small stress concentration near the grip ends of the tensile sample, and the latter is associated with low density of initial mobile dislocations, limited capacity of mobile dislocation multiplication and the presence of meta-stable austenite. As for meta-stable austenite, stress-assisted martensite transformation (SAMT) was confirmed experimentally, which will facilitate strain concentration and the starting of heterogeneous deformation. Moreover, before strain-induced martensite transformation (SIMT), it is the plastic deformation of austenite grains through the slip of partial dislocation and stacking faults (SFs) that contributes to a larger Lüders strain. Finally, we propose that whether it is possible to make the heterogeneous deformation to be a positive side for certain working conditions, which may open new doors for the application of medium-Mn steels. We will continue to do some exploratory studies in this direction next.

Partitioning temperature-dependent microstructures and mechanical properties of precipitation-hardened medium-Mn steel

This study reports the partitioning temperature-dependent microstructures and mechanical properties of precipitation-hardened medium Mn steel processed by room-temperature quenching and partitioning. The hot-rolled Fe–7Mn-0.3C–3Ni-1.5Si–1Al–1Cu-0.3V (wt.%) steel was processed by austenitization, water quenching to room-temperature, and partitioning treatment at 200, 300, 400, and 500 °C for 600 s. The samples before and after the partitioning process at temperatures below 400 °C show a microstructure consisting of lath martensite and retained austenite, whereas the sample partitioned at 500 °C shows a notably high fraction of retained austenite, owing to the formation of austenite grains during the partitioning process. The C partitioning during the partitioning process led to higher C contents in the samples processed by austenitization, quenching, and partitioning than in the sample processed by austenitization and quenching. The co-precipitation of VC precipitates and dispersed fine particles could exhibit a precipitation-strengthening effect. The sample partitioned at 200 °C shows an excellent strength-ductility combination, i.e., a yield strength of 740.9 MPa, an ultimate tensile strength of 1623.6 MPa, and a total elongation of 23.8%. Film-like retained austenite stabilized by carbon partitioning could exhibit high mechanical stability, leading to a gradual transformation induced plasticity (TRIP) effect and superior mechanical properties. In contrast, the samples partitioned at temperatures above 400 °C show poor ductility with brittle fracture, which could be related to the formation of martensite/retained austenite (M/A) constituents.

Partially recrystallized triplex-phase medium Mn steel strengthened by B2 precipitates

We report the annealing time-dependent microstructure–mechanical properties relationship of cold-rolled and annealed Fe–6Mn-0.05C–3Ni-1.5Al (wt.%) steel, containing Ni and Al to form NiAl B2 precipitates. The microstructures of the investigated materials after cold-rolling and intercritical annealing show a mixture of lath and equiaxed zones consisting of a triplex matrix phase (ferrite, austenite, and tempered martensite) with B2 precipitates. Recovery of the deformed martensite in the cold-rolled microstructure during annealing resulted in the formation of sub-boundaries and a continuous transition from partially-recrystallized tempered martensite to recrystallized ferrite. The investigated materials showed a decrease in strength and an increase in ductility and strain-hardening rate with increasing annealing time. A decrease in the fraction of tempered martensite, an increase in the fraction of recrystallized ferrite/austenite grains, and enhanced transformation-induced plasticity (TRIP) kinetics with increasing annealing time led to enhanced ductility and strain hardening of the materials. The mixed presence of less stable equiaxed austenite and more stable lath austenite resulted in sustained TRIP effect during tensile deformation and superior strain hardening capacity of the specimen annealed for 24 h. This study provides a novel microstructure design solution for medium Mn steels, and further optimization of the composition and processing will lead to the development of medium Mn steels with superior mechanical performance.

Optimizing site-specific specimen preparation for atom probe tomography by using hydrogen for visualizing radiation-induced damage

Atom probe tomography (APT) is extensively used to measure the local chemistry of materials. Site-specific preparation via a focused ion beam (FIB) is routinely implemented to fabricate needle-shaped specimens with an end radius in the range of 50 nm. This preparation route is sometimes supplemented by transmission Kikuchi diffraction (TKD) to facilitate the positioning of a region of interest sufficiently close to the apex. Irradiating the specimen with energetic electrons and ions can lead to the generation of vacancies and even amorphization of the specimen. These extrinsically created vacancies become crucial for probing the hydrogen or deuterium distribution since they act as a strong trap. Here, we investigated the feasibility of site-specific preparation of a two-phase medium-Mn steel containing austenite (fcc) and ferrite (bcc). Following gaseous charging of APT specimens in deuterium (D2), clusters enriched by up to 35 at.% D, are imaged after Pt deposition, conventional Ga-FIB preparation, and TKD conducted separately. These D-rich clusters are assumed to arise from the agglomeration of vacancies acting as strong traps. By systematically eliminating these preparation-induced damages, we finally introduce a workflow allowing for studying intrinsic traps for H/D inherent to the material.

Accelerating bainite transformation by concurrent pearlite formation in a medium Mn steel: Experiments and modelling

Bainite transformation has yet to be utilized and even thoroughly studied in medium Mn steels. Here, we investigate the isothermal bainite transformation in a 10Mn steel at 450 °C experimentally and theoretically, focusing on the effect of dislocations introduced by warm deformation. We show that the bainite transformation in the studied medium Mn steel exhibits extremely sluggish kinetics (on a time scale of days), concurrent with the pearlite formation. The introduced dislocations can significantly accelerate bainite transformation kinetics while also facilitating the pearlite reaction. This is likely the first report on the simultaneous occurrence of these two solid-state reactions in medium Mn steels. With respect to the roles of dislocations in the acceleration of bainite transformation observed in this work, we propose a new ‘carbon depletion mechanism’, in which dislocations-stimulated pearlite formation makes a twofold contribution: facilitating the formation of bainitic ferrite sub-units to further enhance the autocatalytic effect and preventing the carbon enrichment in the remaining austenite. On this basis, a physical model is developed to quantitatively understand the bainite transformation kinetics considering the effect of concurrent pearlite formation, revealing good agreements between model descriptions and experiment results. Our findings, herein, offer fundamental insights into the bainite transformation in medium Mn steels and uncover a previously unidentified role played by introduced dislocations in influencing the kinetics of bainite formation, which may guide its future application in manipulating microstructure for the development of advanced high-strength steels.

MECHANICAL BEHAVIOR OF AN HIGH STRENGHT STEEL (AHSS) WITH MEDIUM MN CONTENT IN TWO ROLLING CONDITIONS: HOT AND WARM

The microstructure before intercritical annealing of an AHSS (Advanced High Strength Steel) with medium Mn content plays an important role in the final mechanical properties, since the transformations occurring during annealing modify phases, composition, and morphology. The microstructural changes that occur during intercritical annealing treatment of a medium Mn steel were examined. Two starting material comes from different conditions, hot rolling at 1200°C and warm rolling (initial rolling at 1200°C and subsequent at 680°C). The mechanical properties were related to the transformation phenomena that occur in these steels, mainly TRIP (Transformation Induced Plasticity) and TWIP (Twinning Induced Plasticity) effects. The transformations were verified by SEM (Scanning Electron Microscopy) and X-RD (X-Ray Diffraction). Tensile strength values of 1111 MPa and 17% elongation were obtained by hot rolling route. For the warm rolling route, 35% deformation and a tensile strength of 1357 MPa were obtained. The strain hardening curve was analyzed, showing the presence of the TWIP effect subsequently "saw" behavior related to the discontinuous TRIP effect. The mechanic properties values are related to the difference in morphology phases present. An acicular morphology of a/? (ferrite/austenite) provides a higher value of tensile strength, but low elongation percentage, and a mixture of lamellar and globular morphologies, provides an optimized combination of strength and ductility. Key words: AHSS, medium manganese steel, rolling, heat treatment, discontinuous TRIP effect, TWIP.

Thermal deformation behavior and microstructure evolution of Fe−8.5Mn−1.5Al light−weight medium manganese steel

In this paper, Fe−8.5Mn−1.5Al light−weight medium Mn steel is used as the research object, and the hot compression test is conducted by Gleeble−3500 thermal simulation tester. The dynamic reversion and dynamic recrystallization (DRX) processes are analyzed according to the true stress−strain curve; the θ−σ work−hardening rate curve is plotted by P−J double differential method to analyze the influence law between work−hardening and dynamic reversion and recrystallization softening effect of the material; the DRX critical strain model is established to analyze the influence law of deformation parameters on DRX; Based on the dynamic material model theory, the hot working diagram and three−dimensional power dissipation diagram are constructed, and the best hot working area is determined by combining the microstructure of the material. Finally, based on the above research content, a systematic overview of the dynamic softening effects of DRX and dynamic recovery (DRV) will be provided.

Effect of heterogeneous microstructure on martensitic transformation behavior and mechanical properties of a cold rolling medium Mn steel

A heterostructured medium Mn steel with duplex phases (ferrite and austenite) and multiple degrees of grain sizes is fabricated by a cold rolling process combined with repeated annealing. The obtained sample exhibits excellent mechanical properties with the ultimate tensile strength above 1110 MPa and the total elongation of 34.4%, the mechanical properties being higher than those of the sample produced by conventional cold rolling. The microstructural evolution and the strain partitioning behavior of the material during deformation are investigated. It is indicated that the great strain hardening ability rely on the combination of the transformation induced plasticity (TRIP) effect and the hetero-deformation induced (HDI) hardening. The sample with heterostructure induces more sufficient martensitic transformation. Meanwhile, the generation of large grains intensifies the mechanical incompatibility and makes it possible to accommodate larger amount of geometrically necessary dislocations (GNDs), thus inducing more effective HDI strengthening and HDI hardening, so that the comprehensive mechanical properties have been improved.

Effects of soaking temperature on the microstructure and mechanical properties of a medium Mn steel for hot/warm forming

The increasing demands for better crashworthiness of vehicles require structural components with excellent energy absorption performance and anti-intrusion resistance. Here, we explored the potential of using a medium Mn steel with a 10 wt.% Mn content for hot/warm forming with the aim to produce automotive components with improved strength–ductility combination. The soaking temperature was varied from 670 to 870 °C for tuning the microstructure and the resultant mechanical properties. It is found that a duplex microstructure consisting of ferrite with a high dislocation density and retained austenite with a volume fraction of 53.5% is formed at 720 °C, leading to an exceptional strength–ductility combination (yield strength: 1016 MPa; ultimate tensile strength: 1407 MPa; total elongation: 32.7%). The energy absorbed before fracture is 39.8 GPa·%, which is four times more than that of the widely-used 22MnB5 boron steel. On basis of the experimental results, the effects of soaking temperature on austenite reverse transformation, ferrite recrystallization and martensitic transformation are discussed in depth to provide guidance for optimizing the strength–ductility combination of medium Mn steels processed by hot/warm forming.

Improvement in low-temperature toughness of Fe-6.5Mn-0.08C Medium-Mn steel by multi-step heat treatment

In this study, a novel multi-step heat treatment method, including intercritical annealing and tempering, is proposed to improve the low-temperature toughness of Fe-6.5Mn-0.08C medium-manganese (Mn) steel. The effects of the subsequent tempering treatment on the microstructural evolution and low-temperature toughness of medium-Mn steel were investigated and compared with those of a single intercritical annealing treatment. Medium-Mn steel subjected to intercritical annealing exhibited a duplex microstructure composed of martensite and nanolaminate-retained austenite. The subsequent tempering treatment after intercritical annealing generated a more homogenous distribution of retained austenite, together with an increase in the volume fraction of the retained austenite. The uniformly distributed retained austenite prevented Mn segregation at prior austenite grain boundaries, and the high fraction of retained austenite effectively inhibited crack propagation, leading to an improvement in the low-temperature toughness. In addition, the subsequent tempered medium-Mn steel contained a considerable amount of retained austenite, which could transform into martensite during the impact test, resulting in an impact fracture morphology that changed from cleavage facets to dimples at low temperatures.

Alloy composition and process design based on thermodynamic and kinetic simulation: The case of medium Mn steel

This paper presents a design method for alloy materials that can be applied to medium Mn steel containing Al and/or Si. By using thermodynamic calculations based on CALPHAD, a wide range of alloy composition spaces were systematically studied, and the relationship between retained austenite fraction and its stability with intercritical annealing temperature was investigated. Alloys that meet the Process Window (PW) were screened and their various characteristics of alloy elements within the process window were determined. Additionally, the study performs three different kinetic simulations of austenite growth and solute partitioning during isothermal annealing on alloys that meet PW. The prediction of the model is verified by comparing with the experimental data in the literature. The results show that the remote model is more suitable for the actual production of low temperature isothermal intercritical annealing. Therefore, the method provided in this work makes a guiding contribution to alloy development, which can determine the alloy composition and heat treatment process satisfying PW and optimize the overall properties.

Retained austenite grain size as a strength-plasticity relationship indicator in Al-alloyed medium-Mn steel processed by intercritical annealing

Although most studies focus on the impact of the fraction of retained austenite (RA) on the relationship between strength and plasticity, this research explores the influence of the mechanical stability of RA, which is primarily affected by its grain size. To investigate this, samples of 5% Mn steel with varying fractions of RA, as well as different sizes and concentrations of carbon (C) and manganese (Mn), were produced by adjusting the soaking time during intercritical annealing (IA) between 1 and 300 min at a temperature of 680 °C. The mechanical properties of the samples were evaluated using static tensile tests. Detailed microstructural analyses were conducted before and after deformation through hardness tests, X-ray diffraction measurements (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) images, TEM-EDS microanalysis, and electron backscatter diffraction (EBSD) phase and orientation maps. Additionally, thermodynamic calculations were used to supplement the experimental findings. The results suggest that both chemical enrichment and grain size reduction have comparable effects on stabilizing the RA. The wide variation of RA grain sizes from 0.02 to 0.15 μm3 promotes a gradual and continuous martensitic transformation under strain throughout the entire strain range, leading to a uniform elongation exceeding 19% in the steel analyzed. However, the presence of an appropriate average size of RA (from ∼0.07 to ∼0.11 μm3) is crucial because grains smaller than ∼0.02 μm3 impede martensitic transformation, while coarse grains characterized by a volume greater than ∼0.13 μm3 result in significant transformation in an early stage of deformation.