Medium Mn Steel Research Articles

Correlation between pre-strain and hydrogen embrittlement behavior in medium-Mn steel

In this study, we investigated the correlation between the pre-strain and hydrogen embrittlement (HE) mechanisms in medium-Mn steel. Intercritically annealed Fe-7Mn-0.2C-3Al (wt.%) steel, which showed a two-phase microstructure comprising α ferrite and γR retained austenite, was used as a model alloy. As the pre-strain level increased from 0 % to 45 %, the volume fraction of γR gradually decreased owing to the strain-induced α′ martensite transformation accompanied by an increase in dislocation density. The HE resistance decreased with increasing the pre-strain level because the sample with a higher pre-strain level revealed a higher amount of dissolved hydrogen, combined with a more extensive brittle fracture region owing to the enhanced diffusion and permeation of hydrogen from the reduced γR fraction. Additionally, the H-assisted crack in the sample without pre-strain was initiated and propagated from the γR grains when the strain-induced α′ phase was formed, because most of the dissolved hydrogen was concentrated in the γR grains, and these grains were predominantly deformed compared to the other phases. However, the pre-strained sample showed more pronounced multiple H-assisted cracking at the constituent phases, such as α and α′, because it exhibited relatively well-dispersed hydrogen atoms and reduced microstrain localization at the γR grains, due to the reduced γR fraction.

Realization of strength-ductility balance of 10 Mn-steel by tailoring symbiosis microstructure

Medium Mn steel is considered as a third-generation advanced high-strength steel. However, the bottle-neck problems of excessive strength and insufficient plasticity seriously limited its development. In this study, a warm rolling process was employed to refine grains and acquire high-density dislocations. Subsequently, a quenching-tempering process was employed to prepare a symbiosis microstructure containing multi-phase structures and double precipitates (κ- and V-carbides). On the premise of providing a gigapascal-level yield strength, the introduction of multi-phase structures activated various deformation mechanisms during the straining and contributed to a tensile strength of 1777 MPa and total elongation of 14.1 % in sample W500. In sample W700, the high-density dislocations in martensite gradually passed through the austenite/martensite interfaces and entered the neighboring austenite, enhancing the coordinated deformation capacity of martensite. Sample W700 exhibited a total elongation of 21.8 % and tensile strength of 1468 MPa. Combined with a microstructure analysis during the interrupted tensile deformation, the influences of martensite transformation, dislocation motion, double precipitates, and deformation twins on the work hardening and ductility are discussed in detail.

Dependence of mechanical properties on the phase composition of intercritically annealed medium-Mn steel as the main competitor of high-strength DP steels

Depending on the alloy composition, intercritical annealing may provide different phases in the microstructure. For low-alloyed dual-phase (DP) steels it is usually ferrite and martensite, while for medium-Mn steels retained austenite is also formed. In a present study, a wide intercritical temperature range was applied to a 5% Mn steel to investigate possible microstructure combinations: ranging from fully ferritic, through ferritic-austenitic, multiphase, to fully martensitic, which were next investigated in terms of mechanical properties to clarify the behavior of this type of material. The obtained results together with technological issues and economic indicators were next compared to mechanical properties of typical DP steels in order to assess the possibility of replacing this material in car production. The mechanical properties were evaluated using static tensile and hardness tests. The phase composition was determined qualitatively and quantitatively using dilatometry, X-ray diffraction measurements, and electron backscatter diffraction analysis. The results suggest that both initial austenite and martensite fractions have a decisive influence on the yielding and elongation of steel; however, the tensile strength depends mainly on the sum of martensite initially present in the microstructure and the strain-induced martensite formed from the plastically deformed austenite regardless of the initial retained austenite—martensite ratio. The results indicate superior total elongation of medium-Mn steels reaching 30% compared to DP steels with a similar strength level in the range between 900 and 1400 MPa. However, medium-Mn steels could be a significant competitor to dual phase steels only if some technological problems like discontinuous yielding and serrations are significantly reduced.

Crystal Plasticity Finite Element Modeling of the Influences of Ultrafine-Grained Austenite on the Mechanical Response of a Medium-Mn Steel

Medium manganese (medium-Mn) steel, one of the third-generation advanced high-strength steels (AHSS), delivers impressive mechanical properties such as high yield strength, ultimate tensile strength, and uniform elongation. One notable feature of medium-Mn steels is the presence of ultrafine-grained (UFG) austenite, achieved through phase transformation from the parent martensite phase during intercritical annealing. While, in general, UFG is considered a strengthening mechanism, the impact of UFG austenites in medium-Mn steel has not been fully studied. In this manuscript, we advance our previous work on crystal plasticity simulation based on the Taylor model to consider fully resolved high-fidelity microstructures and systematically study the influence of the UFG austenites. The original microstructure with UFG is reconstructed from a set of serial electron backscatter diffraction (EBSD) scans, where the exact grain morphology, orientation, and phase composition are preserved. This microstructure was further analyzed to identify the UFG austenites and recover them to their parent martensite before the intercritical annealing. These two high-fidelity microstructures are used for a comparative study using dislocation density-based crystal plasticity finite modeling to understand the impact of UFG austenites on both the local and overall mechanical responses.

A new perspective on Lüders band formation in medium-Mn steels based on Lüders-strain-rate and dislocation evolution

Propagating Lüders bands with pronounced strain localization are often observed in medium-Mn transformation-induced plasticity (TRIP) steels, which adversely limits the application of such alloys in engineering structures. The present work aims to understand this phenomenon from a dislocation-based perspective, with a focus on the specific role of dislocation multiplication. The material studied is a typical dual-phase medium-Mn steel with a ferrite matrix and a second phase of retained austenite. A warm-rolling process was used as a pre-straining approach to adjust the microstructure of the material, such that materials with two different microstructural states can be obtained: as-received material and pre-strained material. They show distinct differences in initial dislocation density. Stress-controlled rather than conventional strain-controlled tensile testing was adopted, which allowed the Lüders band to develop in an unconstrained manner. In this way, the intrinsic Lüders-strain-rate can be obtained. The results show that the Lüders band-related mechanical parameters, including Lüders strain, Lüders-strain-rate and yield stress drop, all depend on the initial microstructure state and the dislocation evolution process during Lüders band formation. In particular, Lüders-strain-rate is a critical microstructure-sensitive factor. It is closely related to the dislocation multiplication in the formation of Lüders band, which can be characterized and quantified using scanning transmission electron microscope and synchrotron X-ray diffraction. Finally, a quantitative relationship between Lüders-strain-rate and dislocation multiplication rate is established, which allows providing a better understanding of the exceptional Lüders band phenomenon in medium-Mn steels.

Impact Toughness Dependent on Annealing Temperatures in 0.16C-6.5Mn Forged Steel for Flywheel Rotors

For the application of forged medium-Mn steels on flywheel rotors, the effect of annealing temperatures from 300 °C to 650 °C on the impact toughness of 0.16C-6.5Mn forged steel was investigated to demonstrate the microstructural characteristics and austenite reverse transformation determining the impact toughness. The results obtained through standard Charpy V-notch impact tests at ambient temperature show that the impact absorbed energy holds at lower than 10 J almost constantly at annealing temperatures of 300 °C to 500 °C, and a representative intergranular fracture is presented. At an annealing temperature of 600 °C, the impact absorbed energy increases to 147 J, with the ductile fracture characteristics showing plenty of fine dimples, and the high impact toughness is attributed to the high volume fraction above 30% and the moderate stability of reverted austenite. Subsequently, the annealing temperature rises higher than 600 °C, the impact absorbed energy decreases, and the fracture morphology shows brittleness characterized by more flat facets of intergranular fractures and small quasi-cleavage facets, presumably corresponding to the insufficient transformation and twinning-induced plasticity effect due to weakening the Mn partitioning from quenched martensite to reverted austenite, which results in lower austenitic stability. Furthermore, the ductile-to-brittle transition temperature (DBTT) of the 0.16C-6.5Mn forged steel annealed at 600 °C, which holds the highest impact absorbed energy, and is explored for the possibility of flywheel rotor application in a service environment. The DBTT reaches −21 °C, obtained through the Boltzmann function, and the impact absorbed energy is approximately 72 J.

Modulation of δ-ferrite formation and austenite stability in Al-added medium-Mn steels and its role in the structure–property relationships

The present work investigates the influence of Al content on δ-ferrite formation and its volume fraction, resulting in different austenite stability, microstructure evolution, corresponding mechanical behaviour and underlying deformation mechanisms for three Fe-8Mn-XAl-1Si-0.2C steels with Al contents of 1.23 wt.% (1.25Al steel), 3.5 wt.% (3.5Al steel) and 5.1 wt.% (5Al steel), respectively. Steel ingots are casted and processed through hot deformation, entailing hot forging, hot rolling and subsequent intercritical annealing at various temperatures. It is observed that Al addition promotes the formation of δ-ferrite that enhances the mechanical stability of austenite. Diffusion simulation using TC-DICTRA supports that preferential partitioning of Mn and C from δ-ferrite to austenite leads to enhanced mechanical stability of austenite. Both maximum ( k ∼ 0.49) and minimum ( k ∼ 7.30) austenite stability, where obtained in 5Al and 1.25Al steels, respectively, followed by inferior mechanical properties. This occurs due to the presence of a higher δ-ferrite (∼ 65%) and no δ-ferrite fraction in 5Al and 1.25Al steels, respectively. However, achieving an intermediate δ-ferrite content of approximately 34% results in optimal stability of austenite ( k ∼ 2.50) in 3.5Al steel that results in an optimal microstructure comprising both intercritical α-ferrite and δ-ferrite, as well as excellent tensile properties as represented by an ultimate tensile strength of 825 MPa with ∼18% elongation.

Effect of warm rolling and heat treatment on heterogeneous austenite morphology and tensile property of 3Mn steel

Facing the current lack of clarity in temperature selection mechanisms for warm rolling in medium Mn steel, three warm rolling methods were conducted in 3Mn steel according to the rolling temperatures in the dual-phases region or ferrite region. Subsequently, the effect of heat treatment on the austenite morphologies and tensile properties of warm-rolling samples was studied. Rolling in dual-phase regions (750 °C) led to heterogeneous austenite morphology composed of equiaxed and lamellar grains while rolling in the ferrite region (600 °C) resulted in significant cementite precipitation and the absence of retained austenite. Warm rolling promoted the formation of dislocation cells in the lath-like grains, and they did not evolve into grain boundaries completely because low deformation temperature didn’t trigger the complete dynamic recrystallization. Tempering after rolling provided a significant enhancement in plasticity through the enrichment of C and Mn in austenite and the formation of multiple heterogeneous austenite morphologies with equiaxed, coarse lath-like and fine lamellar grains. In addition, superlattice structures in the matrix are formed through the formation of stacking fault networks and the spinodal decomposition. Ultimately, the exceptional properties were achieved in 3Mn steel, with an ultimate tensile strength of 1409 MPa and a total elongation of 27.3 %.

Effect of caliber rolling temperature on the deformation behavior and mechanical properties of dual-phase medium Mn steel

Dual-phase medium manganese steel (MMS) was caliber rolled at 800, 900 and 1000 °C, respectively, in order to clarify the deformation behavior and mechanical properties of warm deformed MMS. Herein, the microstructure evolution, tensile behavior and fracture characteristics exhibit obvious dependence on the caliber rolling temperature. The warm rolled steel contains martensite and austenite phases with heterogeneous grains including coarse fiber grains (CFG) and ultrafine equiaxed grains (UFG), which shows different microstructural stability. With the increase of rolling temperature, dynamic recrystallization (DRX) is more obvious, corresponding with the increasing percent of UFG, as well as the decreasing percent of CFG and martensite volume fraction. Meanwhile, high deformation temperature also leads to high austenite stability and the decreasing Transformation Induced Plasticity (TRIP) effect. The strength, strain hardening behavior and total elongation are gradually increased with the decrease of rolling temperature, which is attributed to severe strain partitioning and unique laminated structure in the caliber rolled MMS with low rolling temperature. Moreover, the warm rolled MMS reveals obvious ductile-ductile transition behavior in the temperature range from 25 °C to −60 °C. In addition, the low caliber rolling temperature also results into high impact energy, which is attributed to the high volume fraction of CFG.

Effect of Heat Input on Austenite Reversion, Recrystallization, and Ensuing Impact Toughness in Simulated Intercritical Coarse‐Grained Heat Affected Zone of Medium Mn Steel

A systematic experimental study has been conducted on the simulated intercritical coarse‐grained heat affected zone of 690 MPa grade heavy‐plate medium Mn steel by tailoring heat input of the first welding pass to obtain the influence of initial microstructure on the austenite reversion mechanism, recrystallization, and resultant impact toughness. Results reveal that prior austenite grain size significantly increases from 30 ± 11, 41 ± 14 to 54 ± 15 μm with the increasing heat input from 15, 30 to 50 kJ cm−1, which affect the nucleation site of reversed austenite, recrystallization mechanism, and final microstructure. Accordingly, impact toughness is better at the heat input of 30 kJ cm−1 (75.2 ± 0.8 J) in comparison with that of 15 kJ cm−1 (63.3 ± 13.3 J) and 50 kJ cm−1 (26.7 ± 6.5 J) at −40 °C, which is mainly ascribed to the formation of lamellar heterogeneous structures, multiplication of slip system and morphology of high‐angle grain boundaries.

Damage evolution mechanism around AlN non-metallic inclusions in Fe-8.5Mn–3Al-0.2C lightweight medium-Mn steel during warm forming

Based on the tensile test in two-phase region, the damage evolution mechanism around AlN non-metallic inclusions in Fe-8.5Mn–3Al-0.2C lightweight medium-Mn steel during warm forming was elucidated. The findings indicate that voids primarily originate from the bonding interfaces between AlN non-metallic inclusions and ferrite. According to the numerical simulation results, strain eventually concentrates at the bonding interface where the AlN inclusion is connected to the high-strain ferrite in the direction parallel to the load, which is in good agreement with the experimental results. TEM analysis reveals the semi-coherent interface between AlN and austenite as well as the incoherent interface between AlN and ferrite. The incoherent interface between AlN and ferrite exhibits higher interfacial energy and worse mechanical stability, accelerating the separation of the bonding interface between AlN and ferrite.

Effect of Si on the deformation behavior of retained austenite and annealed martensite in medium Mn steels

The present study systematically investigated the effect of Si additions from 0 to 4 wt % on the deformation mechanism of a Fe-0.2C-6.8Mn (wt. %) medium Mn steel. The duplex microstructure consisting of film-like retained austenite (RA) and lath-shaped annealed martensite (AM) was produced for different Si alloyed steels by annealing treatments. They have similar volume fraction of RA. The effect of Si content on the dynamic strain aging (DSA) of RA and the serration type of tensile curves, as well as the fine structure evolution within AM during deformation was studied in detail. For the different Si alloyed steels, their tensile curves all exhibit flow serrations, accompanied with the propagation of the Portevine-Le Châtelier (PLC) band caused by DSA in RA. The serration type was sensitive to Si content. For the high Si steel, only type B serrations appear. This is attributed to the high Si content in RA, promoting aged dislocations by C atoms due to the repulsive interaction between C and Si atoms. For the low Si and Si free steels, type A serrations occur after type B serrations disappear at a certain strain. The mechanical stability and Mn/Si contents of RA play an important role in the PLC band propagation. The Si content has also an important effect on the evolution of dislocation configuration within AM during deformation. In the low and Si free steels, the dislocation multiplication and tangles, as well as the formation of dislocation walls, dividing up the laths appear in the AM. While in the high Si steel, it is found that the dislocation cells appear within the AM due to Si addition promoting the formation of screw dislocations by neutron diffraction technique, and the cross slip of screw dislocations as a necessary mechanism for dislocation cell formation. The formation of dislocation configuration with low energy is believed to be another essential factor improving the mechanical properties, besides the transformation-induced plasticity effect. This work paves a new pathway towards tailoring dislocation configuration for designing a new generation advanced high-strength steel.

Austenite formation in a medium-Mn steel during intercritical annealing via in situ high-energy X-ray diffraction

The microstructural evolution of a prototype Fe-0.15C-5.56Mn-1.1Si-1.89Al medium-Mn third-generation advanced high strength steel (3G AHSS) with a martensite-ferrite (MF) starting microstructure during intercritical annealing was determined in situ using high energy X-ray diffraction (HEXRD). Intercritical annealing was carried out at 665 °C and 710 °C for 120 s and 240 s, followed by cooling to room temperature. HEXRD data were analyzed to monitor the austenite fraction and lattice parameters throughout the thermal cycle. During isothermal holding, the austenite fraction increased, up to 31% and 45% for the samples annealed for 120 s, and up to 33% and 46% for the samples annealed for 240 s at 665 °C and 710 °C, respectively. Observed changes in lattice parameters during isothermal holding were attributed to composition changes due to the partitioning of C between austenite and ferrite. Diffusion simulations using DICTRA were used to calculate solute partitioning during intercritical annealing, providing further insights into both austenite growth and the distribution of alloying elements within the austenite.

Wear Conditions-Dependent Tribological Behavior in Q&P Medium-Mn Steels: The Role of TRIP Effect and Strain Hardening

Wear Conditions-Dependent Tribological Behavior in Q&P Medium-Mn Steels: The Role of TRIP Effect and Strain Hardening

Plasticity enhancement and sustainable strain hardening mechanism of a novel developed medium Mn steel

A novel medium manganese steel denoted as Fe-8Mn-0.3C-1.5Si-3Al-0.2Nb (wt.%) is developed via the combination of two-stage warm rolling (TWR) and intercritical annealing (IA), which exhibits excellent strength and elongation of 1203 MPa and 47.2 %, respectively. The sustainable strain hardening and plasticity enhancement in the scenarios of various IA treatments are elaborated via the analysis of TRIP-assisted deformation and fracture mechanisms. It is indicated that the combination of the continuous/discontinuous TRIP effect and the dynamic balance between dislocation pile-ups and annihilation contribute to the sustainable strain hardening. The excellent plasticity is attributed to TRIP effect, plenty of deep/fine dimples and few cracks.

Effect of laser spot offset on microstructure and properties of laser welded joint of medium Mn/DP980 dissimilar steel

During laser welding of medium Mn (M−Mn) steel, the existence of Mn element at the grain boundaries (GBs) lead to the embrittlement of the joint. In this paper, a new strategy of dissimilar high strength steel welding and laser spot offset is introduced in the laser welding of M−Mn steel. This paper focuses on the influence of laser spot offset on the brittleness of laser welded joints of M−Mn/dual-phase 980 (DP980) dissimilar steel, and compares the differences in microstructure, element distribution and properties of joints under different processes. Compared with the welded joints of M−Mn/M−Mn steel and M−Mn/DP980 steel, the content of Mn element and Mn23C6 carbide in the GBs of the new welded joint was reduced, which greatly slowed down the propagation of intergranular cracks in the fusion zone (FZ). Finally, the plasticity of the welded specimen is greatly improved.

Effect of deformation mode on martensitic transformation in medium Mn steel

In the present work, we have investigated the effect of deformation modes namely uniaxial loading and plane strain loading on martensite transformation in medium Mn TRIP steels, since it plays an important role in influencing formability. Interrupted deformation experiments showed that the martensitic transformation proceeds continuously with plastic strain and is influenced by the specific deformation mode applied. Uniaxially loaded samples underwent a complete austenite to martensite transformation. On the other hand, under plane strain loading, only 70 % of the austenite transformed into martensite, even after fracture had occurred. Through synchrotron and EBSD experiments, we observed that under uniaxial loading, the transformation sequence followed the γ-ε-α′ pathway, whereas, under plane strain, a direct γ-α′ transformation was observed. The intermediate ε-martensite phase is known to facilitate the transformation from austenite to martensite. Consequently, the presence of ε-martensite resulted in a higher degree of austenite transformation in the uniaxial samples. We rationalize the difference in transformation sequences based on resolved shear stress calculations for the two deformation modes.

Review on the plastic instability of medium-Mn steels for identifying the formation mechanisms of Lüders and Portevin–Le Chatelier bands

Review on the plastic instability of medium-Mn steels for identifying the formation mechanisms of Lüders and Portevin–Le Chatelier bands

High temperature oxidation behavior and microstructure evolution of medium Mn steel

The phase transition, microstructure evolution and oxidation behavior of Fe-0.2C-12Mn-3Al-2Cu-xNi steel during the oxidation process for 120 min at 1050 °C-1200 °C were systematically studied. The twin boundaries will provide rapid channels for the oxygen atoms diffusion, while the (110) and (111) grain orientations show a low oxidation resistance property, which also then promote the oxygen atoms diffuse along grain boundaries. Cu and Ni enrichment areas were formed on the subsurface in the 1100 °C-1200 °C oxidation stage, which limited the ion diffusion rate on the subsurface and prevented the further growth of the oxide layer. The oxidation temperature, Mn concentration gradient and [CuNi] rich region on the subsurface contribute to multi-oxide layer structure: Mn deficient region, Mn rich region, transition layer and inner oxide layer. The Mn deficient region with rough surface is mainly composed of Fe2O3, Fe3O4 and Mn3O4, while the Mn rich region is mainly composed of FeO and MnO. The honeycomb transition layer and inner oxide layer is composed of FeO, MnO, MnAl2O4, and Al2O3.

Influence of boron addition on microstructure and mechanical properties of medium-Mn advanced high-strength steel

This study investigates the effects of 0.12 wt% boron addition on the microstructure and mechanical properties of a Fe-0.08C-4.5Mn-2.5Cr–1Si medium-Mn steel in as-rolled conditions and after the intercritical annealing process. In the as-rolled conditions, boron addition increased the strength and reduced the alloy ductility. In as-hot rolled conditions, an ultimate tensile strength (UTS)/elongation of 1195 MPa/5.5% and 1008 MPa/7% was achieved in the boron-added and boron-free alloys, respectively. However, the effect of boron on the intercritically annealed specimens was diverse. The intercritical annealing had no tangible effect on the mechanical properties of the hot-rolled specimen. However, the intercritical annealing at 700 °C for 600 s led to a UTS/elongation of 1029 MPa/19.5% in the boron-added alloy, the highest UTS/elongation combination among all specimens. The formation of carbo-boride precipitates on lath boundaries of the boron-added alloy possibly locked the boundaries thereby a temper-resistant martensitic structure was attained. The lath structure of the annealed alloy caused an enhanced strength in the boron-added specimen. Nonetheless, in the boron-free alloy, martensite was normally tempered and a considerable reduction in the strength was observed.