Properties Of Medium Mn Steels Research Articles

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.

From the alloy design to the microstructural and mechanical properties of medium manganese steels of the third generation of advanced high strength steels

The automotive industry faces increasing pressure to reduce vehicle weight, enhance safety, and lower production costs while also complying with stricter environmental regulations. Various solutions have been developed to efficiently respond to these conflicting requirements. The promising solution involves the development of new steel types of the third-generation of advanced high-strength steels (3GAHSS), these steels aim to bridge the gap between TWIP and TRIP steels. The so-called (MM) Medium Manganese steels combining an exceptional strength, good malleability, with lower production costs attributed to the reduced use of alloying elements like manganese (below 12wt. %). The medium-Mn steel is drawing more attention to be designed for future vehicle bodies and structures. Therefore, this paper aims to explore the relationship between chemical composition, microstructure, manufacturing processes, and mechanical properties of Medium Mn steels.

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.

Revealing the interdependence of load partitioning, dislocation density evolution and mechanical behavior of medium Mn steels

The load partitioning between inter-phases and intragranular of austenite, their dislocation density evolution, and mechanical behavior were investigated in two medium Mn steels by in-situ high-energy synchrotron x-ray diffraction during tensile loading. The investigated steels were produced with different hot rolling parameters resulting in different initial microstructures. The initial microstructures were characterized using scanning electron microscope and electron backscatter diffraction analysis. Transmission electron microscope with high angle annular dark field technique was furthermore used to provide the fine microstructural features of deformed specimens in order to supplement the in-situ analysis. Despite the differences in the initial microstructure, the load partitioning from the softer ferrite to the stronger α’-martensite (α’) was observed; while different degrees of load partitioning for intragranular of γ occurred. The sample with a relatively low degree of load partitioning exhibits the intersection of two bundles of stacking faults (SF), one with high density SF, and the other with low density and limited SF, as well as the occurrence of α′ in the intersecting region. The secondary SFs and the relatively intermediate density of primary SFs bundle are characterized in γ for the sample with relatively high degree of load partitioning. The load partitioning between intragranular of γ is found to have a coordination relationship with the dislocation density evolution of γ; after the load partitioning, dislocation density of γ rapidly rises. The sample with intersection of two bundles SFs and the occurrence of α′, exhibits the higher transformation rate of γ into ε-martensite/α’, resulting in the larger work hardening rate. In addition, the early load on α’ is found to contribute to a higher yield strength. These results reveal the importance of assessing both phase-specifc strain/stress and the dislocation density evolution in order to predict the macroscopic mechanical properties of medium Mn steels.

Micro‐ to Nanoscale Microstructural Differences Induced by Intercritical Annealing in a Hot‐Rolled Medium Manganese Steel

The effect of austenite‐reverted transformation (ART) annealing process on the chemical and microstructural properties of medium Mn steel (7 wt% Mn) is evidenced by a combination of electron probe micro‐analysis, electron back‐scattering diffraction, and scanning transmission electron microscopy. Hot‐rolled medium Mn steels with distinct dual‐phase austenite/martensite microstructures and identical bulk composition can be obtained via direct austenite reverted transformation (D‐ART) as compared to a reaustenized quenched (RAQ) + ART route. The different annealing sequences applied to the steel have an effect on the fraction of austenite, its morphology, chemical composition, and distribution. The D‐ART route produces a core–shell structure of Mn‐rich shell and Mn‐lean core inside the austenite grains, while the RAQ + ART route did not. Such systematic characterizations using a combination of different microscale and nanoscale techniques are essential as chemistry and morphological differences at the individual grain level provide a key understanding to the microstructure–property relationship of such steels.

Strong and ductile medium manganese steel processed by low-temperature partitioning

Conventional medium Mn steels processed by intercritical annealing (IA) demonstrate low yield strength. The present work investigates the effect of low-temperature partitioning (LTP) on the microstructure and tensile properties of medium Mn steel. It is found that medium Mn steel processed by LTP at 300°C possesses ultra-high yield strength of 1683 MPa and proper uniform elongation of 11.3%. The LTP process does not reduce the yield strength but substantially enhances the tensile ductility. The increase of the tensile ductility after LTP treatment can be ascribed to the reduction of the residual stress, martensitic transformation, and the well-developed ductile fracture behaviour. The present LTP process provides an alternative pathway to develop strong and ductile medium Mn steel.

Improving the strength and ductility of medium Mn steel by depleting the stress-assisted martensite

The present work investigates the role of martensite induced by either stress or strain on the tensile properties of medium Mn steel treated by quenching and partitioning/tempering. Different from the strain-induced martensite with isolated fine martensite blocks, the stress-assisted martensite is featured with large and continuous martensite blocks, which facilitates the propagation of cracks and results in the premature fracture and thus limited tensile ductility (12.8%). A pre-deformation and tempering strategy is employed to deplete the stress-assisted martensite while promote the strain-induced martensite, turning an original soft and brittle medium Mn steel into a strong and ductile one, leading to an increment of ultimate tensile strength and total elongation of 533 MPa and 28.4%, respectively.

Mechanism and Application of Reverse Austenitic Transformation in Medium Mn Steels: A Systematic Review

To meet the requirement of fuel efficiency and CO2 emission, tremendous efforts have been made in the last decade to improve the mechanical properties of medium Mn steels. In article 2200257, Guo, Hu, and Luo focus on the mechanism of the reverse austenitic transformation kinetics and several processing strategies that utilize these mechanisms for improving mechanical performance.

Role of Al element in tailoring the austenite mechanical stability and tensile properties of medium Mn steels

Al is generally employed as a key alloying element to tailor the austenite mechanical stability and mechanical properties in medium Mn TRIP steels. However, the effect of Al on the austenite stability has been limitedly studied, leaving contradictory results without a self-consistent explanation. This study addressed this issue through a systematic study of medium Mn TRIP steels with compositions of 0.2C–8Mn-0/1.5/3Al (wt.%), while providing new insights into the factors influencing austenite stability and mechanical properties. It was found that the variation of the Mn distribution in austenite from non-equilibrium to near-equilibrium state and the weakened strain incompatibility between phases due to sufficient recovery of tempered martensite can be achieved with the addition of Al, which contributed to slowed strain-induced martensite transformation (SIMT) and improved austenite stability in the experimental steels annealed at low temperatures in the two-phases region, resulting in a decrease in ultimate tensile strength (UTS) and an increase in total elongation (TE). Furthermore, fully recovered tempered martensite with the hardness much lower than that of austenite was obtained in the 3Al steel annealed at a high temperature in the two-phase region, which led to a significant reduction in yield strength and thus low UTS. Meanwhile, heterogeneous strain distribution was induced by the remarkable gap in hardness between the two phases, which can be responsible for the relatively high austenite mechanical stability. The varying strain states and variation of Mn distribution in austenite can be achieved for the medium Mn TRIP steels with different Al contents, contributing to a wide range of UTS&TE combination with high strength and elongation product (≥40 GPa·%). And the elucidation of the effect of different strain states also opens a new window to adjust the austenite stability and mechanical properties of medium Mn TRIP steels.

Study on the organization and properties of medium-Mn steel with V

V is an important strengthening element in alloy steels, and its addition to the steel significantly improves the overall performance of the material. This paper describes the principle of V for optimizing the properties of medium-Mn steels. The excellent properties of V-containing Mn steels and their applications in practical industrial production are presented.

A new insight into annealing parameters in tailoring the mechanical properties of a medium Mn steel

Generally, the mechanical properties of medium Mn steels are mainly tailored by adjusting temperature and duration of intercritical annealing (IA). However, in the present study, it is found that the temperature-rise-period in IA also plays a remarkable role in the control of the mechanical properties of the investigated medium Mn steel. A longer temperature-rise-period is conducive to the formation of austenite grains at different temperatures, resulting in more heterogeneous distribution of Mn element within an individual austenite grain and among different austenite grains. This heterogeneous microstructure leads to the difference in the yield strength among austenite laths and a slow strain-induced-martensite-transformation kinetics. Eventually, the yield strength and total elongation of the investigated steel has been significantly increased while high ultimate tensile strength is maintained. This study provides a new insight into comprehensively understanding the role of annealing parameters so as to improve the mechanical properties of medium Mn steels in industrial production.

New observations of the twinning effect and austenite stability in intercritical quenched and tempered steel with high strength

To optimize the formability and strength of hot-rolled Fe-10Mn-0.4C-2Al-0.6 V medium Mn steel, intercritical quenching and tempering processes were carried out. The strength of the steel was enhanced, and the Ludders platform was eliminated. The higher strength of the steel was attributed to the occurrence of a complex twinning effect, martensitic transformation and V-carbide precipitation during tensile deformation. In particular, the twin martensite structure retained after the quenching-tempering process served as another previous twin to accelerate the generation of nanomechanical twins in recrystallized austenite grain. The occurrence of transformation-induced plasticity (TRIP) of austenite with poor stability in non-recrystallized regions stimulated the TRIP and twinning-induced plasticity (TWIP) effects in austenite with high stability in recrystallized regions. Therefore, two pathways to improve the formability and optimize the mechanical properties of medium Mn steel by adjusting the quenching and tempering processes were proposed in this paper: (1) Manufacturing more martensite twin structures and (2) regulating the balance of austenite stability in both recrystallized and non-recrystallized regions.

Influence of Isothermal Treatment Prior to Initial Quenching of Q&P Process on Microstructure and Mechanical Properties of Medium Mn Steel

Influence of Isothermal Treatment Prior to Initial Quenching of Q&P Process on Microstructure and Mechanical Properties of Medium Mn Steel

Influence of silicon addition on intercritical annealing process and tensile properties of medium Mn steel

This paper presents research on the equilibrium phase fractions, microstructure, and tensile properties of a 0.2C–5Mn–1.5Al (mass %) steel with 0 or 0.5 (mass %) Si addition after intercritical annealing (IA), using thermodynamic simulation, scanning electron microscopy, transmission electron microscopy, X-ray diffractometry, and uniaxial tensile testing. The research results show that the addition of 0.5 Si (mass %) has no significant effect on the equilibrium phase fraction but influences the optimal IA temperature. Different IA temperatures result in the retained austenite having different morphologies. The retained austenite in the 730 °C sample (0.5 Si) has two different morphologies—polygonal and lath—while the 760 °C sample (0 Si) has only polygonal retained austenite. The optimal tensile properties of the 0.5 Si steel are better than those of the 0 Si steel due to the solution strengthening effect of Si and the higher stability of retained austenite. The tensile properties of the investigated 0.5 Si steel are excellent, with a tensile strength of over 1 GPa, elongation over 42.00%, and strength × ductility over 42 GPa%.

On the Variants of Thermal Process in Developing Strong and Ductile Medium Mn Steel

In this contribution, we investigate the influence of three heat treatments, including intercritical annealing (IA), quenching and partitioning (Q&P) and combination of IA and Q&P (IA-Q&P), on microstructure and mechanical properties of the medium Mn steel. The steel treated by IA process has largest volume fraction of austenite, which is responsible for the longest elongation and largest energy absoprtion with operation of both transformation-induced plasticity (TRIP) effect and twinning-induced plasticity (TWIP) effects. The medium Mn steel produced by IA-Q&P treatment has a mixture microstructure with retained austenite, ferrite and lath martensite. The existence of martensite in the IA-Q&P steel makes the higher yield strength than that of the IA steel. The better uniform elongation of the IA-Q&P steel than that of the Q&P steel is ascribed to the larger volume fraction of retained austenite and higher mechanical stability of austenite. The Q&P steel has highest yield stress due to its martensite matrix. It is expected that the tensile properties of medium Mn steel can be tuned by different thermal processes (IA, Q&P and IA-Q&P) to facilitate its broad automotive applications.

A novel two-step intercritical annealing process to improve mechanical properties of medium Mn steel

In this paper, we present a novel two-step intercritical annealing process, i.e. a-few-minute interruption during heating followed by a short intercritical annealing, to improve the mechanical properties of medium Mn steel without prolonging the intercritical annealing (IA) period. Both experimental investigations and numerical simulations were performed to investigate the microstructural inheritance between the two steps and its influence on the tensile properties. The increase of interruption temperature led to a gradual change of tensile flow curve from the discontinuous yielding to the continuous yielding at lower yield strength due to more martensite and less austenite formed after the IA at 670 °C. It is concluded that the two-step IA process can improve the mechanical properties of medium Mn steel when it is optimized, compared to the non-interrupted IA process. The strategy for optimizing this novel process is to carefully tailor cementite precipitates before annealing as the nuclei of austenite for promoting relatively coarse austenite grains formed with Mn gradients inside. This kind of austenitization on the deliberately introduced nuclei finally led to a considerable fraction of austenite grains retained adjacent to martensite, causing strength and ductility improved simultaneously. Moreover, we demonstrate that such a deliberate design of two-step IA process can be guided by the quantitative modeling of austenitization kinetics on a cementite nucleus.

Relative effect of C and Mn on strength-toughness of medium Mn steels

ABSTRACTIn order to find an alternative choice for structural load-bearing components, an effort was made to improve the impact toughness of medium Mn steel through inter-critical annealing treatment without significant reduction in strength. Besides, the relative effect of C and Mn contents on microstructure and mechanical properties of medium Mn steels was also studied. Fibrous microstructures with fine, alternate films of ferrite and martensite with retained austenite were obtained. The low-C, high-Mn steel showed a superior combination of yield strength, ductility and impact toughness due to finer microstructure and higher retained austenite fraction, as compared to high-C, low-Mn steel. Thus, the beneficial effect of Mn enrichment in stabilising retained austenite and improving mechanical properties by transformation induced plasticity effect becomes evident.

Effect of Annealing Processes on Microstructures and Mechanical Properties of Medium Mn Steel

Two annealing processes have been designed to process a medium Mn steel, namely, the traditional direct two-phase annealing process and the pre-quenching treatment before two-phase annealing process, called two stage annealing process. The experimental results indicated that different annealing processes resulted in completely different microstructures, and a significant difference in mechanical properties. The microstructures of the steel after direct two-phase annealing process were coarse tempered martensite matrix and retained austenite. In this case, the optimum mechanical properties with total elongation of 29.87 %, tensile strength of 932 MPa and UTS*TE of 27.84 GPa•% were achieved after annealing at 625 °C for 18 h. The annealing process with pre-quenching treatment could realize two kinds of retained austenite, including acicular austenite and blocky austenite. The process with pre-quenching treatment could improve mechanical properties as well as shorten the optimal annealing time. The steel with two stage annealing process achieved optimal mechanical properties after pre-quenching and annealing at 625 °C for 4 h with tensile strength of 1177 MPa, total elongation of 30.92 % and UTS*TE of 36.39 GPa•%.

Microstructure Evolution and Change in Mechanical Properties of Medium Mn Steels during Thermomechanical Processing

Medium manganese steels are nowadays energetically investigated as the third generation advanced high strength steels (AHSS) because of their excellent balance between material cost and mechanical properties. However, the phase transformation and microstructure evolution in medium manganese steels during various heat treatments and thermomechanical processing are still unclear. The present study firstly examined kinetics of static phase transformation behavior and microstructural change in a 3Mn-0.1C medium manganese steel. Hot compression tests were also carried out to investigate the influences of high-temperature thermomechanical processing on the microstructure evolution. It was found that ferrite transformation was quite slow in static conditions but greatly accelerated by hot compression in (austenite and ferrite) two phase region. Dual phase microstructures composed of martensite and ferrite with ferrite grain sizes of 1~2 μm were obtained, which exhibited superior mechanical properties.

Enhancing the Mechanical Properties of Al‐Containing Medium Mn Steel Through Warm Rolling and Intercritical Annealing

This investigation attempts to further enhance the mechanical properties of an Al‐containing medium Mn steel, Fe‐0.25C‐8.5Mn‐0.5Si‐2.5Al (wt%), through warm rolling (WR) and subsequent intercritical annealing (IA) treatments. Two schedules of WR processes were applied with a total thickness reduction of ≈60%. The results show that an ultrafine‐grained (UFG) lamellar structure was well developed after WR, while subsequent IA led to a coarsening of the microstructure as well as the formation of bimodal‐morphological ferrite and retained austenite (RA) grains, i.e., lamellar and equiaxed. Compared with warm rolled samples with clear discontinuous yielding and large yield point elongation, the IA treatment after WR could eliminate this phenomenon mainly due to microstructure coarsening. A significantly enhanced combination of strength and ductility (the product of the ultimate tensile strength and total elongation) as high as ≈75 GPa% was obtained for the WR + IA sample. This enhancement is notably superior to that of previously investigated medium Mn steels with similar Mn contents and is similar or even better than that of twinning‐induced plasticity (TWIP) steels with high‐Mn content. It is thus suggested that WR plus subsequent IA is a promising and feasible approach to further enhance the mechanical properties of medium Mn steel.