5th International High Manganese Steel Conference - HMNS 2022Hybrid conference

After a short introduction of voestalpine, the first part of this contribution will give a brief overview of high manganese steels produced and developed by voestalpine group and of cold formable steels for the automotive industry with a special focus towards the 3-GEN advanced high strength steels (AHSS). Regarding cold rolled strip steels, the relationship between global and local formability will be elucidated, while the challenges related to their weldability will also be presented.
In the second part of this contribution, the laboratory development of high-Mn, conventional medium-Mn and lean medium-Mn Quenching and Partitioning (Q&P) will be accentuated. The application of one-step and two-step heat treatment and the benefit of the latter one with respect to the avoidance of yield point elongation and achievement of optimal mechanical properties in conventional medium-Mn steels will be illustrated. To assure an extraordinary local ductility of these steel grades, the development of a low C variant will also be presented. As an aftermath of a relatively low Mn content, lean medium-Mn Q&P steels ensure an excellent balance between local and global formability. In order to predict their microstructural evolution, a new carbon constrained equilibrium (CCE) based model, including a novel Ms-formula, an incomplete C partitioning from matrix to retained austenite and mechanical stabilization of retained austenite, was developed. The interplay between ductility and toughness of these steel grades and the forecast of their development for GA coatings will also be discussed.
The last part of this contribution focuses on the industrial development of medium-Mn steels. First, the concept with a strength level of 780 MPa, having the Mn content of about 6 wt-%, will be discussed in detail. In this context, applied two-step annealing, resulting microstructure, mechanical and service in use properties, such as weldability and formability, will be presented. In terms of their excellent deep drawability, a number of potential applications for the manufacturing of the most complex structural parts for the automotive industry will be shown. Finally, the options for medium-Mn concepts with a strength level higher than 980 MPa will be introduced.

Medium Manganese Steels (MMnS) belong to the third-generation advanced high strength steels (3.G. AHSS) that have been developed because of their ecological and economic potential with a focus on automotive applications. Reducing weight, decreasing CO2 emission, simplifying processing are important drivers for their development.
This presentation deals with the interaction of chemical composition, processing, microstructure development and mechanical properties with a special focus on forging applications. The chemical composition range of Mn is from 4 to 10 weight-%, of C from 0.1 to 0,4 %, and also considers additions of Al, Cr, Si, V, Nb, Mo and B. The process steps discussed in detail include continuous casting, annealing, controlled cooling and tempering treatments. The special behavior of these steels during production as well as the characteristic microstructure and their particular mechanical properties are worked out.
In detail, the processability of MMnS is discussed in light of their hot ductility behavior. The findings indicate the prime role of precipitation, phase transformation and extension or shift of solidification intervals induced by the alloying concepts in controlling the hot ductility. It turns out that the formation of complex AlN and MnS precipitates as well as δ-ferrite solidification deteriorates the high temperature ductility.
Furthermore, the concept of MMnS is used to develop air-hardening forging steels. The alloy design and the heat treatment parameters have been varied with a focus on the prevention of Mn embrittlement as well as the formation of fine austenite grains during intercritical annealing. It is shown that the addition of B and Mo increase the impact toughness, although the effectiveness of each element varies depending on the heat treatment conditions. The impact toughness can be significantly increased by the introduction of a globular metastable austenitic phase. Compared to the reference quench+tempered steels an ultimate tensile strength level of more than 1300 MPa and an improved cyclic strength can be achieved. Thus, via a combined material, process and geometry optimization, these ductile air-hardening steels offer the possibility to save energy and CO2 emissions both by shortening the heat treatment and by light weighting the components. The critical aspects in controlling microstructures in MMnS are explained with respect to industrial applications.

Multiphase medium Manganese TRIP steels offer good combinations of high strength and high ductility and can be designed to be used in automotive industry for light weighting purposes. However, the deformation-driven martensitic transformation associated with the TRIP effect strongly deteriorates the materials’ resistance against hydrogen embrittlement (HE). We studied the hydrogen related damage mechanisms and developed an approach to increase the HE resistance by tailoring of the microstructure. We show by slow strain rate tensile tests that the HE resistance can be significantly increased by adding stable austenite (gamma(stable)) into an ultrafine microstructure. This ductile phase stops cracks by blunting and serves as a dead end for microcracks. In order to increase ductility by a TRIP effect, additional metastable austenite (gamma(metastable)) is required. Such tailored alfa/gamma(stable)/gamma(metastable) microstructures can be produced by simple step annealing: Our atom probe tomography results reveal that during intercritical annealing reverted austenite is formed with the equilibrium partitioning of Mn. This is due to a local equilibrium at the moving alfa/gamma interface. We exploited this for a medium Mn steel (0.2C–10Mn–3Al–1Si in wt.%) to form stable austenite (high Mn partitioning) at 700°C for crack blunting and metastable austenite (lower Mn partitioning) at 750°C to enhance the ductility by a TRIP effect. This approach results in a significantly increased HE resistance with similar high strength and high ductility for the alfa/gamma(stable)/gamma(metastable) microstructure in comparison to the conventional alfa/gamma(metastable) microstructure.

Hydrogen embrittlement of high strength steel is one of the critical issue when attempting to prolong the material life for a practical use. Because of this reason, macro/microstructural design for a prevention of hydrogen invasion is important to enhance hydrogen embrittlement resistance of materials. In is study, ultrasonic nanocrystalline surface modification (UNSM) treatment at elevated temperature was conducted to the high-Mn steel. Both ultrafine-grained layer and compressive residual stress at the surface region of the UNSM treated high-Mn steel not only provide an additional strength, but also prevent hydrogen invasion on the surface that reduces hydrogen localization. These beneficial effects enhance the mechanical property in a hydrogen-containing environment of the UNSM-treated high-Mn steel. Therefore, designing heterogeneous microstructure in high-strength steel can be a good strategy to improve both strength-ductility combination and prolonged life time in a hydrogen-rich environment.

The global formability of sheet materials corresponds to the ability to undergo uniform plastic deformation without the formation of a localized neck, whereas the local formability addresses the ability to undergo plastic deformation in a local area without fracture. Both the local and the global formability are therefore important properties that can become limiting factors for the application of medium manganese steel in the automotive industry. While for the stretch formability and drawability, the global formability is of critical importance, in particular the edge crack resistance relates to the local formability. In our contribution, we aim to present a damage mechanics based approach to evaluate the local and the global formability of medium manganese steel. For the characterization of local formability, our approach considers strain-based, state-of-stress dependent criteria for damage initiation and ductile fracture. It belongs to the group of phenomenological, coupled damage mechanics models which take damage-induced softening effects into account. On the other hand, the modified maximum force criterion is applied to predict the localization resistance. After introducing the simulation framework, we will develop material parameter identification strategies, show validation examples, and compare the localization, damage and fracture properties of a medium manganese steel to a conventional dualphase steel of grade DP1000. These studies will reveal significant differences between the two materials and allow to give requirements for future materials design.

Driven by the increasing demand for passenger safety and weight reduction in the automotive industry, advanced Fe-Mn-Al-C alloys with low density and specific high strength have gained great attention in recent decades. Unfortunately, these materials are prone to hydrogen embrittlement (HE) which impedes their further application. Here we focus on a high-Mn and high-Al lightweight steel with an austenite and ferrite two-phase microstructure and unravel the effects of H-associated decohesion and localized plasticity on its H-induced catastrophic failure. The HE in this alloy is driven by both, H-induced intergranular cracking along austenite-ferrite phase boundaries and H-induced transgranular cracking inside the ferrite phase. The former phenomenon is attributed to the mechanism of H-enhanced decohesion. For the latter damage behavior, systematic scanning electron microscopy-based characterization reveals that only parts of the transgranular cracks inside ferrite are straight (~52%) and along the {100} cleavage plane. Other such type of cracks shows a distinct deviation from the {100} plane at certain stages of crack propagation, which is associated with a mechanism transition from the H-enhanced decohesion of the ferrite cleavage planes to the H-associated localized plasticity occurring near the propagating crack tip. These mechanisms are further discussed based on a detailed comparison to the damage behavior at cryogenic temperature and on the nanoindentation results performed with in-situ H-charging. Those findings provide some new insights into the boundary conditions of different HE mechanisms in high-strength alloys, their interplay and synergistic effects on damage evolution.

Influence of nitrogen and carbon on mechanical properties of TWIP steels depending on grain size has been controversially discussed. In addition, especially in high manganese steels, the temperature dependence of the stacking fault energy can lead to the activation or suppression of different deformation mechanisms during a pre-deformation. In this way, a complex interaction between dislocation structures, twins and even martensite can take place and a unique microstructure can be obtained. As a result, diverse combinations of strength and ductility can be achieved. For a selected TWIP steel, this study investigates the influence of grain size and nitrogen content on mechanical properties as a function of temperature. Temperature dependence of yield strength and influence of nitrogen on grain refinement are investigated respectively between alloys. The microstructural analysis contributes to a better understanding of the deformation mechanisms and their interaction in different states.

Hadfield steel shows high strain hardening which can be linked to its ability to form twins during mechanical loading. Twinning in Hadfield steel is well documented. However, detwinning in Hadfield steel is only barely investigated but might be an essential effect as during cyclic loading detwinning provides an additional contribution to plastic deformation.
In order to gain more information regarding the deformation behavior in Hadfield steel, the present work concentrates on the twinning and detwinning behavior during interrupted tensile tests. The tensile tests were conducted in-situ within a scanning electron microscope (SEM) and interrupted at different elongations. To determine the formation of twins after every interruption, the electropolished tensile test sample was analyzed by electron backscatter diffraction (EBSD). It is shown that twinning in a certain region takes place within one cycle of tensile straining, whereas partial detwinning of the twinned regions takes place over a few tensile loading/unloading cycles. Furthermore, the kernel average misorientation of the twinned and detwinned region is analyzed. High values of misorientation are found in the detwinned regions, which can be linked to the detwinning process.

In order to reduce the vehicles weight constructors are focusing on stronger or lighter materials.
High Mn lightweight steels could be a response to both demands due to high mechanical properties (Yield Strength up to almost 1000 MPa) and low density (15% lower than a standard Hadfield steel and 16% lower than a typical stainless austenitic steel like AISI 306L).
Lightweight steels undergo thermal treatments to exploit precipitation hardening given by κ-carbide formation which, depending on the precipitation site, can give very different mechanical strengthening effect and affect differently the ductility and toughness of the material.
In this work, the effect of different thermal treatments on the microstructure and mechanical properties of a cold rolled austenitic high Mn lightweight steel has been investigated.
Material has been cold rolled from the “as Hot Rolled” condition with a thickness reduction of 80%. Then, after thermal treatments the material characterization has been carried out focusing on the ongoing recrystallization, the κ-carbide precipitation or the coexistence of both transformations.
Material has been solubilized and aged or directly aged with two different temperatures and different holding time (550°C and 600°C for 30min, 1h, 1.5h and 8h). Even if the temperatures of the thermal treatments are very close the microstructure and the mechanical properties appear to be very different.
Peculiar microstructure and non-equilibrium phases have been observed and analysed and astonishing mechanical properties has been observed.

The aim of the presentation is the explanation and description of structural phenomena occurring in high manganese steels with TWIP effect during dynamic deformation, taking into account the heat generated during the deformation and structure changes depending on the stacking- fault energy (SFE) affected by chemical composition. It has been assumed that the ability to activate specific deformation mechanisms, such as mechanical twinning - TWIP effect - in high manganese steels is determined by the SFE value affected primarily by the chemical composition of steel and the strain rate due to heat generated during deformation. The uniaxial tension tests will be conducted at a rate of 0.001 s-1 ÷ 2500 s-1. Thermocouple and A 2D finite element thermo-mechanical model of uniaxial tensile tests will be developed to determine local strain and increase temperature. Obtained results allow presenting the main deformation mechanism regarding strain rate and chemical composition

Recently, the new designing concepts for medium-Mn steel have been promoted such as micro-alloyed steels to improve mechanical properties. Several authors reported that micro-alloying was effective to enhance the strength and toughness of medium-Mn steels. In this study, we elucidated the unknown role of micro-alloying element addition to a medium-Mn steel and explored why a micro-alloyed steel exhibited higher strength without sacrificing ductility as compared to a plain medium-Mn steel. Steels of Fe-8Mn-0.2C-3Al-(0, 0.2)V (wt.%) compositions, cold-rolled and intercritically annealed, were used as model alloys. The annealed steels revealed dual-phase microstructures consisting of α ferrite and γR retained austenite with globular morphology. V-added steel revealed a smaller grain sizes and lower volume fractions of γR than V-free steel owing to formation of VC precipitates. The precipitates were mostly formed in the α phase rather than in the γR phase. The VC precipitates led to different chemical composition of consisting phases of the steels; the V-added sample exhibited a lower C content in γR and a higher C content in α than the V-free steel. This difference in chemical composition of V-added steel, which was originated from VC formation, leads to an enhanced strain hardening rate (SHR) at the later stage of yielding by more active twinning-induced plasticity (TWIP) and dynamic strain aging (DSA).

With an increasing demand for high-strength steel with lean-alloyed composition, the medium-Mn steels containing ~3-12 wt.% Mn and ~0.05-0.4 wt.% C have garnered significant attention as next-generation advanced high-strength steels. The α′ martensite microstructure could be observed after hot and cold rolling of the corresponding steel, and it turns to the two-phase microstructure of α ferrite plus γR retained austenite or tempered α′ plus γR after reversion treatment at intercritical temperature region of the steels. The large fraction of metastable γR triggers the transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) during plastic loading, leading to a remarkable combination of strength (< ~1400 MPa) and ductility (< ~50%). Interestingly, two different microstructural morphology could be obtained in the medium-Mn steels; hot-rolled steel reveals a nanolaminate morphology and cold-rolled steel shows a nanoscale globular morphology after annealing. The various mechanical responses such as tensile property, impact absorbed energy, hydrogen embrittlement resistance, and fatigue resistance are greatly influenced by the morphology characteristics. Here, we summarized the relation between microstructural morphology and mechanical responses in medium-Mn steel to derive the optimum microstructure for improved mechanical properties.

The strength of ausformed martensite is higher than that of as-quenched martensite. This is believed to be because the dislocation introduced by ausforming is inherited after quenching, and the dislocation density of ausformed martensite is higher than that of as-quenched martensite. However, there is little studies about the detailed relationship between the mechanical properties and microstructure (dislocation density, grain size, carbides, etc.). In this study, the microstructure of ausformed martensite was investigated in medium Mn steel with and without C (Fe-5Mn-0.1C and Fe-5Mn-0.02C alloys, respectively), and then, the strengthening mechanism of ausformed martensite was discussed. In this study, a medium Mn steel was prepared because the diffusional transformation does not occur during ausforming due to its excellent hardenability. In Fe-5Mn-0.02C alloy, the block size decreases by ausforming, while the hardness and dislocation density hardly varied by ausforming, which means that the hardness of ausformed martensite does not depend on the block size. On the other hand, in Fe-5Mn-0.1C alloy, the hardness and dislocation density increase with increasing the thickness reduction during ausforming. The carbides did not be observed with transmission electron microscope. These results suggest that the hardening by ausforming is mainly due to the increasing of the dislocation density enhanced by C. The effect of C on the dislocation introduction in ausformed martensite was discussed in terms of the interaction between C atoms and dislocations.

The strength-ductility balance of steels is strongly dependent on their microstructure characteristic which dominates strain distribution, such as texture, grain morphology, and dispersed hard- and/or soft-phases. It is generally known that addition of Mn above 10% decreases stacking fault energy, which results in the occurrence of γ→ε→α’ martensitic transformation. The microstructure formed via γ→ε→α’ martensitic transformation exhibits ultra-fine microstructure containing retained ε martensite. This ultra-fine microstructure of matrix would inhibit non-uniform deformation, whereas the hard-phase of ε martensite probably would induce the strain localization. In this study, the combination of SEM observation and digital image correlation (DIC) method was carried out to investigate the effect of ultra-fine microstructure and ε martensite on the strain distribution in the duplex martensitic medium Mn steel. An as-quenched 10%Mn-0.1%C steel was used in this study, which was electrical discharge machined to the small tensile test piece. The tensile test was performed in a SEM (Sigma500, Zeiss) with maintaining the acceleration voltage of 2 kV. The strain distribution was analyzed by DIC software (VIC-2D, Correlated Solutions). The dominant microstructure of 10%Mn-0.1%C steel was found to be α’ martensite with the block size of 0.4±0.2 µm. The ε martensite and retained γ dispersed in the matrix exhibited granular-like morphology, whose grain size was below 2 μm. The DIC analysis revealed that strain localization tended to occur along the longitudinal of blocks rather than the ε martensite and/or retained γ. In the presentation, we will also discuss the difference of strain distribution between coarse martensite and ultra-fine martensite.

Previously, some of the problems of cracking as well as edge cracking during two phase hot rolling (e.g. duplex stainless, high Mn steels) have been attributed to the effect of hot ductility, difference of flow behaviour between the different phases. There is limited work and knowledge regarding the exact cracking mechanism as a function of rolling pass number, temperature, mean flow stress or reduction percentages. Using a high-speed camera, it is possible to address this and extract valuable information for optimizing the rolling process for such steels.
In a lab environment, a high-speed camera setup was used while rolling high manganese and aluminium steel concepts (Fe-Mn-Al-C alloys). Data such as crack initiation, crack propagation with each rolling pass were obtained for various rolling schemes, considering different reduction ratios, starting rolling temperatures etc. Information from such experimental schemes was analysed based on alloying composition and thermodynamic equilibrium diagrams in the rolling temperature regions. Process maps were created incorporating data from the high-speed camera. Insights from such studies were used to avoid edge cracking in some alloy compositions by adjusting the processing parameters during rolling. Such studies can be used to further optimize processing of such alloys or variation in alloying elements to avoid such dual phase regimes which are prone to hot ductility.

Austenitic high manganese lightweight steels emerge as a potential candidate for structural applications in the automotive industry. The precipitation of κ-carbides in these materials upon age hardening treatment offers a great combination of strength, ductility and toughness, while the high Al content (8-11 wt. %) reduces the specific weight. In general, the size of κ-carbides at grain boundaries can be carefully controlled to render them harmless for room-temperature ductility and toughness. However, the effects of nano-sized grain boundary κ-carbides on the material’s mechanical properties under harsh conditions like H environment and low temperatures are less studied. In this work, we address the formation and growth of grain boundary κ-carbides in an austenitic high manganese lightweight steel and their effects on intergranular cracking, using a combination of experimental techniques including atom probe tomography (APT), electron backscattered diffraction (EBSD) and electron channeling contrast imaging (ECCI). The mechanisms of grain boundary decohesion under H and low-temperature conditions, along with their association with early-precipitated grain boundary κ-carbides, are systematically studied. Further alloying and microstructure solutions for enhancing damage resistance in such materials, including boron doping and grain boundary engineering, are also discussed.

The transformation stasis (TS) phenomenon refers to the occurrence of incomplete transformation by exhaustion of a driving force while a phase transformation proceeds, which was observed during isothermal bainitic transformation. However, in the present study, the TS phenomenon was observed during athermal γ austenite → ε martensitic transformation in a high Mn alloy. Namely, γ → ε martensitic transformation stopped at a certain temperature during continuous cooling and the volume fraction of ε martensite did not increase with further cooling. The TS phenomenon occurred in all specimens regardless of prior γ grain size, and the TS temperature decreased with decreasing prior γ grain size. The TS phenomenon was highly related to dynamic γ stabilization during γ → ε martensitic transformation, which occurred most likely due to dynamic γ grain refinement that a prior γ grain is subdivided into several subgrains by ε platelets. As the size of γ subgrains decreased, the shear stress necessary for martensitic transformation increased, resulting in the exhaustion of the driving force for γ → ε martensitic transformation. In addition, the characteristics of boundaries surrounding subgrains also influenced dynamic γ stabilization. Compared to incoherent and curved prior γ grain boundaries, coherent and flat γ/ε interphase boundaries revealed the higher suppression effect of γ → ε martensitic transformation. Therefore, even if the subgrain size was similar, γ → ε martensitic transformation occurred more actively in the fine grain-sized specimen with a higher fraction of prior γ grain boundaries surrounding subgrains than the coarse grain-sized specimen. Considering dynamic grain refinement and boundary characteristic, the driving force for γ → ε martensitic transformation was newly calculated to explain the TS phenomenon.

Metal additive manufacturing (AM) received enhanced scientific and industrial importance during the last decades, mostly due to the geometrical flexibility offered by AM. The development of novel, process-adapted metallic alloys is a key for further industrial implementation of AM. We address the computational and experimental development of new high-manganese steels with properties superior to existing steel concepts for AM. Target applications are filigree (sub-mm) lattice structures with enhanced energy-absorption capacity.
On the one hand, computational tools were used to consider the thermodynamics (CALPHAD), process behavior (FEM), solidification behavior (phase-field modelling) and deformation behavior (crystal-plasticity modelling, FEM), enabling a holistic integrated computational materials engineering (ICME) approach. On the other hand, the microstructures and mechanical properties of bulk and lattice-structure specimens were investigated experimentally. The influence of chemical composition within the system Fe-Mn-Al-C and of the AM processing conditions on the process-microstructure-properties-relationships will be discussed.

High-manganese steels (HMnS), which are the member of advanced high-strength steel (AHSS) class, stand out due to their ability to exhibit both high strength and ductility. They are identified as promising alloys for additive manufacturing, which is a novel method enabling the production of metallic structures with complex geometries such as lattice (cellular) structures for lightweight applications with high energy absorption capacity.
In HMnS, different deformation mechanisms such as transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) are activated by tailoring stacking fault energy via alloy design approach. Therefore, in addition to dislocation slip, twinning and ε-martensite transformation can be accommodated in HMnS as crystallographic deformation mechanisms. The underlying phenomena of these deformation mechanisms can be incorporated into a physics-based constitutive model in order to simulate the crystal plasticity behaviour of HMnS.
In this presentation, the effect of micro- and/or meso-structural features (related to additive manufacturing process), e.g., crystallographic texture and grain morphology, on the mechanical properties, which was determined by means of crystal plasticity simulations, will be discussed. In addition to bulk structures, lattice structures will also be explored. Thereby, a robust linkage between structure and properties, which correlates microstructural heterogeneity of additively manufactured HMnS with anisotropic mechanical behaviour, will be established.

In situ tensile and reversed cyclic tests were run on a TWIP steel in a SEM, with High-Resolution Digital Image Correlation (HR-DIC) measurements of the plastic strain field in a few selected grains prone to twinning, with a spatial resolution between 150 and 250nm, or under an AFM, with measurements of surface steps height at emerging deformation twins. Evidences of detwinning upon load reversal, as well as quantitative data on twinning/detwinning/retwinning were obtained. Detwinning and retwinning, which often were only partial, in spite of a fully reversed loading, did not seem to start at the onset of stress reversal. It required a sufficient variation of the stress, close to the twinning stress (estimated as 400 to 475 MPa) in absolute value, so that a mechanical hysteresis of the local twinned fraction occurred. Primary and secondary twinning along the same plane, inducing axial plastic strains in opposite directions, also allowed some grains to accommodate reversed plastic strain. Under fixed stress amplitude (± 500 MPa), the twin fraction in all monitored grains saturated at values between 0.5 and 3.5%, from the 2nd cycle, while under fixed plastic strain amplitude (± 0.5%), it increased in a ratchetting way during the whole cyclic hardening stage, reaching 0.5 to 5%. In both cases, however, the plastic strain amplitude accommodated by twinning/detwinning, which reached 0.35 - 0.42% in some grains during the 1st cycle, decreased down to less than 0.05% after 100 to 1000 cycles.

9%Ni steels, which have excellent strength and toughness in the low temperature range, are used as the cryogenic steel mainly used for LNG storage tanks, Mn is similar to Ni as an alloying element in steels, such as improvement of hardenability and stabilization of γ, so that Mn is expected as an alternative element to Ni. However, because the addition of Mn to steel causes remarkable intergranular fracture, the microstructure control to suppress the intergranular fracture is important for applying the medium Mn steels to cryogenic materials. In previous study, we found that toughness of 10%Mn-0.1%C steel could be improved by thermomecanical control process (TMCP).　However, the effect of intercritical annealing on the toughness of medium-Mn steel subjected to TMCP has not been investigated yet. In this study, we attempted to further strengthen the TMCPed 10%Mn-0.1%C steel by performing intercritical annealing.
10%Mn-0.1%C steel was austenitized at 1473 K, and then hot-rolled with the reduction of 75% at 973 K, followed by water-quenching. The quenched sample was annealed at 873 K for 1 h. In addition, a sample intercritical annealed without TMCP was also prepared as a reference specimen.
The toughness of intercritical annealed sample without TMCP was hardly improved, whose fracture mode was found to be intergranular fracture with orange peel like surface. On the other hand, the intercritical annealed sample with TMCP showed remarkable improvement with occurrence of fracture separation and the main fracture mode was intragranular fracture. It is considered that this is because the softening of the matrix by intercritical annealing contributed only in the case of intragranular fracture. In the presentation, we will also discuss the contribution of retained γ stabilized by intercritical annealing to the toughness.

Medium manganese sheet steels show an excellent combination of strength and ductility. Typically, they have a duplex type microstructure consisting of ferrite and austenite layers after cold rolling and subsequent intercritical annealing. The thermomechanical rolling produces a mixture of bainite, martensite and retained austenite instead of typical duplex microstructures. Their forming behaviour is not fully understood especially for thermomechanically processed conditions. Therefore, the present work aims at comparing the dynamic mechanical behaviour and microstructure of two medium manganese sheet steels (3Mn-Al and 5Mn-Al) alloyed with aluminum addition. The static tensile tests and rotary hammer tests at strain rates of 250, 500 and 1000 s-1 were applied. Mechanical properties under dynamic tensile loads were determined, which are significantly affected by a manganese content in a range from 3 to 5%. In both steels the tensile strength increased with increasing strain rate but the applied strain rate range had a moderate effect on the mechanical behavior. The higher strength properties showed the 5Mn-Al steel with a tensile strength level higher than 1200 MPa. On the other hand, the 3Mn-Al steel was characterized by better ductility due to a larger retained austenite amount and more enhanced TRIP effect. The mechanical properties of low-Mn steel were more strain-rate sensitive compared to the static mechanical behavior. The microstructure features and phase composition were assessed in detail using XRD and SEM tests. The morphology of structural constituents and quantification of microstructural details were done using advanced EBSD studies.

The intercritical annealing approach is widely used as a heat treatment for the formation of a dual-phase structure, composed of ferrite and austenite in medium-Mn alloys. By controlling the temperature, it is possible to control a fraction of both phases. Then, during deformation, the austenite undergoes martensite transformation, enhancing the plasticity of the steel. Depending on the mechanical stability of it, the transformation can occur at different stages of the deformation.
This work presents the effect of the double intercritical annealing of 5% medium-Mn steel on the grain size and mechanical stability of retained austenite. During the second intercritical annealing, the material was heated at a rate of 150°C/s to 850°C. This fast heating leads to formation of new, small austenite grains. The temperature was then lowered to 750°C, resulting in the formation of ferrite at austenite grain boundaries. As the newly formed austenite was small, the ferrite was also refined. Finally, the microstructure should be composed of large retained austenite (formed during the first intercritical annealing) and some fraction of small austenite and ferrite grains (formed during the second step). This could result in the continuous transformation of retained austenite into martensite during cold straining. In this way, it is possible to further enhance the plasticity of the steel during deformation.

Medium-Mn steels (MMnS) with very low (≤ 0.05 wt.%) carbon content exhibit improved cold-formability but limited strength due to the deficient strain hardening. A combination of nano-precipitation and enhanced transformation-induced plasticity (TRIP) effect is a promising approach to improve the strength and ductility of such MMnS. In this study, a low-carbon (0.05 wt.%) medium-Mn steel (MMnS) containing Cu and Ni (1.5Cu1.5Ni-alloy) was developed to influence the austenite stability and to stimulate the nano-precipitation. A reference MMnS without Ni and Cu (0Cu0Ni-alloy) was used to elucidate the effect of Cu and Ni on the austenite reversion and nano-precipitation behavior. The alloying with Cu and Ni was beneficial for the austenite reversion during the proposed shortened annealing period, which is revealed by using synchrotron X-ray diffraction (SY-XRD). During the subsequent tempering, the RA fraction of 1.5Cu1.5Ni-alloy further increased to 38.5 vol.% and pronouncedly contributed to its strain hardening behavior through the TRIP effect. In contrast, the RA fraction of the reference alloy remained almost unchanged. The high number density of Cu-based nano-precipitates with a mean diameter of approximately 4 nm was observed in the ferrite phase of 1.5Cu1.5Ni-alloy using three-dimensional atom probe tomography (3D-APT) and significantly increased the yield strength. The current study demonstrates the beneficial influence of Cu and Ni on austenite reversion and nano-precipitation behavior, which subsequently enabled an enhanced strain hardening with increased ductility and a high level of yield and ultimate tensile strength, respectively.

N alloyed austenitic stainless steels are known as good candidates for replacing commercial austenitic stainless steels (STS) like AISI 304 and 316, etc due to their high strength, excellent pitting resistance with low Ni content. Austenitic steels containing (10-20)Cr, (10-20)Mn and (C+N)>0.3% manufactured by conventional vacuum induction melting have been studied intensively for the last 2-3 decades. In addition, as the importance of hydrogen economy is growing, understanding the role of N in STS in hydrogen environment is also drawing a great attention. Due to the low yield strength of commercial grade austenitic STS, researches for the development of high strength STS has been done worldwide.
In the first part of this presentation, the recent progress in the research on new alloy design, mechanical properties, deformation mechanism, and corrosion resistance in the (C+N) alloyed austenitic stainless steels will be reviewed. Some the combined addition of C and N helps easy manufacturing, the importance of C/N ratio on mechanical properties and corrosion resistance has been actively studied recently. In the second part, the effects of N on hydrogen embrittlement behavior will be presented. Among them the effects of N on hydrogen diffusivity, crack initiation and propagation, and change of strain hardening rate are included.

Abstract: Advanced austenitic stainless steels are a group of austenitic stainless steels with a nickel content near or higher 30 wt% and a pitting resistance equivalent number (PREN) is near or higher than 40. They are widely used in the oil-gas and chemical industries. Due to high Ni content, these alloys are costly. Mn and N are two low cost alloying element for austenite stability. In this study, influence of Mn and N on mechanical properties and corrosion resistance of advanced austenitic stainless steels are discussed. Unexpectedly, a combination Mn (about 6 wt%) and N (about 0,25 wt%) can greatly increase both strength and elongation of the advanced steels. When tensile-tested at a cryogenic temperature, the strengths and elongations of the steels are further increased. EBSD and ECCI studies show that addition of manganese and nitrogen has increased amount of nano deformation twins in the steels at RT. At the cryogenic temperature, much more nano deformation twins in the steels can be observed. This indicates that twin induced plasticity, TWIP, has led to high elongations in the steels. These phenomena have been explained by the stacking fault energy and the critical stress for deformation twinning evaluated by ab initio simulation at RT and cryogenic temperature. The influence of Mn and N on the corrosion properties of the steels has also been studied with ASTM G-150. A pitting resistance equivalent number (PREN) with Mn is used to predict the corrosion properties. This paper will increase our understanding for the development of high performance and low cost advanced austenitic stainless steels.

1. INTRODUCTION
   Herein, the structural and chemical features of passive films formed on a high Cr-added high-Mn 18 (wt%) twinning-induced plasticity (TWIP) steel and on two different austenitic stainless (304L and 316L) steels were revealed via Cs-corrected scanning transmission electron microscopy (STEM) and atom probe tomography (APT).

2. EXPERIMENTAL
   With a goal of increasing the corrosion resistance of typical high-Mn TWIP steel series, high amount of Cr (17wt%) was added to typical 18 (wt%) TWIP steel, i.e., HCr-HMnS. While comparing the passivation abilities of the HCr-HMnS and two 304L and 316L stainless steels by potentiodynamic polarization curves, the thickness and chemical compositions of the material passivation films were examined via Cr-corrected STEM and APT.

3. RESULTS AND DISCUSSION
   Although the passive films of all the samples with similar Cr concentrations had the same thickness, unprecedented hexagonal wurtzite MnO inside the passive film of HCr-HMnS specimen was susceptible to corrosion cracking; this was not observed in the passive films of typical stainless steels. This MnO caused crack formation during potentiodynamic polarization test, suggesting that reducing the harmful MnO by adding Mo and Ni facilitates the development of high-Mn base stainless steels. Furthermore, higher MoO2 composition of the passive films on 316L stainless steels than 304L type series might would result in primarily the improved pitting resistance.

4. CONCLUSION
   We explored the origin of less passivation ability of the high Cr-added TWIP steel sample than the typical stainless steels. Furthermore, based on the chemical difference of passive films between the 316L and the 304L stainless steels, we suggested the primary cause of the improved pitting resistance of 316L stainless steels than 304L type series. We expect that alloying the Mo and Ni, which are capable of reducing the MnO-density, may be beneficial for developing high-Mn base stainless steel materials.

In this work, it was shown that quenching and partitioning (Q&P) processing can be applied to interstitial alloyed CrMnNi austenitic steels capable of martensitic transformation during cryogenic cooling. With the aid of Q&P, ultra-high strength properties can be achieved with exceptional elongation even in the as-cast state. In the present study, the alloy Fe-15Cr-3Mn-3Ni-0.16C-0.12N (concentrations in wt.%) was used in the cast condition to demonstrate its excellent suitability for Q&P. The solution annealed steel was quenched to -120 °C to produce about 44 vol.% α´-martensite in the austenitic steel matrix. Subsequently, partitioning was carried out at 450 °C for various partitioning times (Pts), namely 3, 15 and 30 min. The austenite lattice parameter was found to increase slightly with increasing Pt from 3 min to 30 min., proving the enrichment of C and N in the austenite. After the Q&P processing, tensile tests with an initial strain rate of approximately 4 × 10-4 s-1 were performed on round type tensile specimens until fracture. The steel partitioned for 3 min achieved an ultimate tensile strength (UTS) of 1470 MPa accompanied by a total elongation (TE) of 32% at room temperature. As the partitioning time increases to 30 min, the tensile strength decreases only slightly, whereas the very good deformability can be maintained. The experimental results show that almost constant mechanical properties can be produced even if the partitioning time is varied over a wide range. In addition, the Q&P-treated cast steel met the US Department of Energy's 2017 proposed criteria for wrought 3rd generation Advanced High Strength steels (AHSSs) (UTS ≥ 1200 MPa, TE ≥ 30%).

The growing demands of weight saving, emission reduction and passenger safety in the automotive industry have promoted new concepts for medium-Mn steels design to achieve an excellent combination of high strength and superior ductility. Tailoring the microstructural evolution for mechanical enhancement requires deep understanding of austenite reversion mechanism. The previous research work on the transformation kinetics revealed many discrepancies between experimental measurements and the theoretic prediction by the classical diffusive transformation theory. The theory also predicts a sharp Mn concentration gradient in austenite, i.e. Mn spike, near the austenite/ferrite phase interface after a short period of reverse transformation, which have not been experimentally confirmed until now.
In this study, we applied cyclic partial-transformation annealing in medium-Mn steel to reserve the transit elemental partitioning features of migration interface and to produce multi Mn spike in front of the 𝛼/𝛾 phase boundary in Fe-5Mn-2Al-0.1C (wt. %) medium-Mn steel. By DICTRA simulation, the annealing parameters are optimized in the homogenized samples, in order to include the region of interest in atom probe tomography (APT) tip. The qualitative and quantitative analyses of microstructure are achieved by EBSD and synchrotron X-ray diffraction technique (SYXRD). SYXRD technique detected a significant amount of austenite formed during transit cyclic partial-transformation annealing, showing high transformation kinetics. The C concentrations in constitution phases are in good consistency between APT and SYXRD analysis. Mn concentration profile reveals multi-spike characteristics, similar as predicted by DICTRA simulation. Extremely high austenite reversion kinetics is found to be controlled under the negligible partitioning local equilibrium (NPLE) state and be correlated with the preserved carbon enrichment.

In the last decade, the academic community extensively researched medium-Mn steels. This steel class has proved their position as one of the possible successor to classic AHSS due to excellent mechanical properties paired with still reasonable alloying costs. Nevertheless, several technical issues have to be resolved in order to enable a successful application of medium-Mn steels in a vehicle, which is the main target of this contribution. In this context, all presented results were acquired by a large-scale produced material, fulfilling the requirements for a batch-annealed medium-Mn steel with the strength level of 780 MPa, manufactured at the voestalpine steel plant in Linz, Austria.
The first presented topic is that of the often-observed presence of extensive yield point elongation (YPE). This work clearly shows that YPE can be avoided by a two-step heat treatment, altering the microstructural features such as morphology, effective grain size and texture by a simultaneous improvement of mechanical properties.
Concerning the robustness of the manufacturing process, the issue of the sensitivity of mechanical properties to the intercritical annealing temperature was referred to as a concern towards industrial scale production. Based on the large-scaled material, this work can emphasize that by a precise temperature control during a batch annealing cycle stable mechanical properties throughout an entire coil can be achieved since the sensitivity to annealing time is low.
The third addressed topic is the necessary suitability of the material for resistance spot welding process. In our first investigations, the material revealed rather low cross tension strength (CTS) after welding with standard parameters. Therefore, the chemical composition was adjusted and a double-pulse regime was established to vastly increase CTS.
Considering all the above-mentioned aspects, this contribution represents a compilation of critical points and possible solutions towards the large-scale implementation and subsequent use of the present material by the automotive industry.

The effects of starting microstructure and intercritical annealing (IA) temperature on the phase transformation kinetics and microstructural evolution of a prototype Fe-0.15C-5.56Mn-1.1Si-1.89Al medium-Mn third-generation advanced high strength steel (3G AHSS) were determined. The starting microstructures comprised i) an as-received cold rolled (CR) microstructure containing a significant fraction of ferrite, tempered martensite and cementite and ii) an austenitized and quenched martensite and ferrite (approximately 10 vol%, MF) microstructure. Based on the microstructural observations, two different scenarios for austenite formation during intercritical annealing have been proposed based on the starting CR or MF microstructures. In the case of the CR starting microstructure, isolated cementite at the ferrite grain boundaries and ferrite/cementite interfaces were austenite nucleation sites. However, the MF starting microstructure, which contained thin films of inter-lath retained austenite with very little cementite, showed a different mechanism. In this case, austenite either formed on the martensite lath boundaries or grew directly from the existing inter-lath retained austenite this case. The studies of austenite growth and solute partitioning during intercritical isothermal holding were interpreted using the DICTRA module of Thermo-Calc. In the case of the CR starting microstructures, pre-existing cementite particles dis not dissolve during the 120 s intercritical annealing isothermal hold, whereas cementite particles formed during heating in the MF microstructure and then were largely re-dissolved at higher temperatures. The sluggish cementite dissolution observed during CR intercritical annealing was found to be the result of Mn partitioning to cementite during isothermal holding. Simulations indicate that the time for achieving the same volume fraction of austenite is longer in the presence of microstructural cementite, explaining the accelerated austenite reversion kinetics observed for the MF starting microstructure samples. This paper will present and discuss the experimental and modelling efforts used to come to this conclusion.

This paper presents the results of recent investigations on the influence of the hot press forming (HPF) temperature cycles for a novel medium manganese, high ductility 1000MPa zinc coated steel (HQ1000-GI), with specific emphasis on the risks of liquid metal embrittlement (LME) induced cracking during resistance spot welding (RSW).

In this study, a zinc coated 7Mn steel was hot formed using several temperature cycles. Subsequently, the LME sensitivity of the hot formed parts was investigated using both dedicated resistance spot welding experiments as well as high temperature tensile testing. Tested samples were subjected to visual and microstructural investigations to reveal the occurrence of LME cracks and their relation with the substrate and coating conditions after hot forming.

For low temperature HPF cycles, and subsequently welded under very high heat input weld conditions, it is observed that the Medium manganese HQ1000-GI can be moderately sensitive to LME induced cracking in the heat affected zone of the resistance spot welds. At the same time, it is shown that the sensitivity to LME during welding is significantly reduced after hot forming at higher temperatures. A similar trend is obtained using high temperature tensile tests for time-temperature loads relevant for welding.

Analyses of the substrate coating interface shows that the heat cycle applied in the hot forming process affects both the substrate and the coating structure, which both add to the reduced LME sensitivity.

Combined with other recent application studies on the HQ1000-GI steel, it is shown that a hot forming process window can be defined for the that provides hot formed parts with excellent strength-ductility properties, low springback, good corrosion properties and with good weldability.

Hot rolled medium Mn steel was arc welded by employing a E-71T-GS flux cored filler in a butt joint with 3 mm in thickness. The initial microstructure of the sheet was composed of martensite and very little amount of austenite, the ultimate tensile strength in this condition was close to 1600 MPa with a negligible elongation. A post-heat treatment of intercritical annealing was proposed in order to promote the austenite reversion in the heat affected zone (HAZ) and to increase the amount and stability of this phase. Thermodynamic simulations and previous experimental results were used to determine the temperature and time for the IA treatment. Before IA the microstructure in the fusion zone (FZ) was completely lath α´-martensite, the heat affected zone (HAZ) comprises austenite and α´-martensite, and the base metal (BM) comprises only α´-martensite; after IA the major change was observed in the HAZ on which the austenite amount was increased. After welding microhardness values were the highest at the fusion zone and then a slightly decrement of 50 HV was detected in the HAZ while a significant decrement of 125 HV was observed in the base metal. After IA these differences in the microhardness were attenuated in the three areas. Finally, the steel experimented a reduction of around 50% of the tensile strength after welding without IA in comparison to the hot rolled steel.

The microstructure and tensile properties of an arc welding joint of a Medium Mn steel, with nominal composition of Fe-0.12C-1.7Al-1.8Si-10.4Mn-0.14V (wt%), before and after intercritical annealing (IA) treatment were analyzed. An E-71T-GS flux cored filler was used to weld with a butt joint the medium Mn sheets of 3mm thickness. The initial microstructure of the sheet was composed of retained austenite, α´-martensite and some ferrite. Thermodynamic simulations and experimental results were used to determine the optimal parameters for the IA treatment, in a previous work. The results revealed that before IA the microstructure of fusion zone (FZ) was nearly all martensite, the heat affected zone (HAZ) comprised austenite and α´-martensite, and the base metal (BM) had the initial microstructure that was mentioned above; after IA the major change was exhibited in the HAZ and BM zones which is reflected as an increment of austenite fraction. Microhardness profile after welding showed a notable difference between HAZ and BM compared to FZ, which presented the highest value; while, after IA a significant decrement in the microhardness of the FZ was observed and the values of the other areas were more uniform. The tensile strength of welded steel before IA was higher than the one obtained after IA. The steel experimented a reduction of around 40% of the tensile strength after welding in comparison to the initial condition.

It is state of the art, that the corrosion resistance of stainless steels is mainly controlled by the alloying elements Cr, Mo and N. Derived from that, the widely applied pitting resistance equivalent number (PREN), is used to estimate and compare the pitting corrosion resistance of such steels. -
In this work, the focus was laid on the role of the Mn influence on the passive layer. Therefore, a CrMnN and a CrNi austenitic stainless steel were chosen and examined by high resolution imaging methods like GDOES and TEM, in combination with electrochemical methods like cyclic voltametry and Mott-Schottky analysis.
The results of the analysis indicated that additionally to the varying Mn content, in the CrMnN alloy the Mn-enrichment was found to be shifted towards the surface in relation to the predominant Cr-enrichment of the passive layer. Thus implies that Mn can react with the corrosive medium.
This findings are put in context to the electrochemical results and subsequently a possible explanation for the differing behaviour of both alloying concepts and therefore passive layers is formulated.

The plastic deformation of solution annealed Fe-17Cr-8.6Mn-4Ni-0.17N-steel (concentrations in wt.%) and the resulting strain-induced α´-martensite evolution were investigated at different tensile test temperatures to provide an estimate of martensite formation during tube manufacture. For this purpose, the onset and evolution of α´-martensite were determined by in situ magnetic measurement during tensile straining at -40 °C and 20 °C. The triggering stress for martensite formation at 20 °C was 754 MPa, whereas the triggering stress at -40 °C declined to 680 MPa. In addition, the triggering strain decreased with decreasing test temperature. The strain-induced α´-martensite volume fraction was obtained by ex situ volumetric magnetic measurements. The microstructure characterization was done by light optical microscope (LOM) and scanning electron microscope (SEM) using electron backscatter diffraction (EBSD). Using EBSD, strain-induced α´-martensite platelets were detected within straight deformation bands in the austenite. The strain hardening curve at -40 °C shows a curve progression typical of metastable austenitic steels with pronounced strain-induced martensite formation and can be divided into 4 hardening stages. At this temperature, the studied steel exhibited the highest strain hardening rate. The amount of α´-martensite increased with decreasing test temperature, and reached a maximum volume fraction of 76 vol.% at -40 °C. The highest ductility of 83% was achieved at 40 °C, accompanied by a tensile strength of 732 MPa.

Keywords: TRIP/TWIP effect; strain-induced martensite formation; mechanical properties; strain hardening

The development of new-generation AHSS steels requires a careful optimization of their processing parameters. In the case of intercritically annealed medium-Mn steels, parameters as the initial microstructure, annealing time and temperature and cooling and heating rates are paramount in controlling the final response of the microstructure.
The present research concerns with the effect that increasing intercritical annealing time (1 to 300 min) has on the morphology, size distribution, and stability of the final microstructure constituents (austenite and ferrite). The steel with composition 0.16C, 4.7Mn, 1.6Al, 0.2Si, 0.2Mo in hot rolled conditions with initial microstructure of martensite was used. The temperature for the intercritical treatment of 680°C, has been selected on the basis of previous studies seeking for the highest fraction of stable retained austenite at room temperature after soaking for 60 min.
This work aims to clarify how intercritical annealing time affects retained austenite fraction, grain size and distribution of Mn, which are paramount in defining austenite stability (thermal and mechanical). Results thus obtained, indicated a significant increase of retained austenite fraction, lath thickness and grains merging with increasing annealing time. The retained austenite fraction increase from 8% to 35% when intercritical time increase from 1 to 300 min, also accompanied by a considerable decrease in the fraction of austenite grains with an area below 1 μm2 , from almost 100% to less than 20% respectively.

Medium-Mn steels belonging to the third generation of Advanced High Strength Steels (AHSS) are the most perspective lightweight materials for body-in-white automotive parts. These steels containing between 3 and 12 wt.% of Mn show a beneficial combination of high strength at sufficient ductility. Their excellent mechanical properties are attributed to the substantial fraction of metastable austenite, which is able to transform into martensite during cold straining, resulting in a high work hardening rate and necking retardation. Therefore, the mechanical properties of medium Mn steels are directly related to the fraction and mechanical stability of retained austenite.
Typically, medium-Mn steels are manufactured through a single-step intercritical annealing process, resulting in a duplex microstructure consisting of austenite and ferrite. Despite many beneficial properties, such multiphase microstructure has some disadvantages related to the high difference in the hardness of neighboring phases resulting in their poor stretch-flangeability and limited hole expansion ratio (HER). Limiting these problems is possible due to the use of novel double-step intercritical annealing allowing the replacement of a significant part of soft ferrite by low-C martensite.
Temperature and time of intercritical annealing are the key parameters that affect the fraction and mechanical stability of retained austenite. However, heating and cooling rates have also an impact on the microstructure evolution. Therefore, the present study concerns the influence of heating and cooling rates during the second intercritical annealing step carried out at 800°C and 850°C on the microstructure of 0.16C-5Mn-1.6Al-0.2Si steel. Microstructure analysis was performed by means of scanning electron microscopy and electron backscatter diffraction techniques.

Application of the double soaking treatment to medium-Manganese (Mn) steels has been shown to result in impressive combinations of tensile strength and uniform elongation. The two-step double soaking heat treatment is intended to produce an austenite-martensite microstructure by intercritical annealing followed by secondary soaking. The intercritical annealing step serves to partition Mn from ferrite to newly formed austenite, while the secondary soaking step is intended to transform the remaining Mn-lean ferrite into austenite (and subsequently into martensite upon quenching). If Mn is not appreciably redistributed during the secondary soaking step, the Mn-rich austenite (primary austenite) may be retained, while the Mn-lean austenite (secondary austenite) will transform to martensite upon quenching.

The present work discusses dilatometry results from a Fe-7Mn steel with an ultra-low residual carbon (C) concentration. These results indicate that a heterogeneous Mn distribution may be maintained in a fully austenitic microstructure during secondary soaking, and provide the basis for calculations of the Mn concentrations associated with the primary and secondary austenite. MICRESS® simulations were employed to help understand the evolution of microstructure during the multi-step heat treatment. These simulations corroborate the dilatometry results and provide further insight on the effects of the secondary soaking temperature on the ferrite-to-austenite transformation and Mn redistribution during secondary soaking. Finally, energy-dispersive X-ray spectroscopy results collected from a Fe-0.2C-4.5Mn steel using scanning transmission electron microscopy are presented, and demonstrate that a heterogeneous Mn distribution can also be maintained in a similar carbon-bearing steel after double soaking.

Segregation engineering by controlling the solute decoration state of certain interfaces has recently been proposed as a new method to tailor mechanical properties in metallic materials. Despite the advancements in thermodynamic understanding of grain boundary segregation, the segregation behaviour at phase boundary in multiphase steels has largely remained untouched. Such information is deemed to be a key to further improve the damage tolerance of multiphase steels. In this study, we aim to investigate the dominant mechanisms of phase boundary segregation, based on Gibbs adsorption theory and elemental partitioning between adjacent phases. A detailed investigation using atom probe tomography (APT) analysis was carried out on a medium Mn steel with a ferrite-austenite two phase microstructure, which was subjected to low temperature tempering at 450℃ for varying durations (2, 50 and 200 hours). While the Mn concentration at the austenite-ferrite phase boundary was consistent throughout the course of tempering, the level of C segregation showed a decreasing trend with increasing tempering time. The underlying mechanisms of the observed phase boundary segregation and its change with tempering time will be discussed in this talk. Furthermore, a detailed analysis will be presented pertaining to the effects of orientation relationship between adjacent phases on the C and Mn segregation behaviour.

Here, we suggest a new thermomechanical process, which is referred to as resetting process, to recover the reduced ductility in cold-worked medium-Mn steel. The objective of the present study is motivated by the fact that one automotive component sometimes requires the two different parts for compensation of opposing attributes. For example, the upper part of center pillar needs a high strength for the intrusion resistance, while the bottom part of it needs a high ductility to absorb the impact energy. To make such automotive components, tailor-welded blank and hot stamping processes have been widely used in industry. However, these processes could deteriorate the productivity, and lead to mechanical degradation at the welded zone. In the present study, through a simple heat treatment after cold forming, we realize that the original microstructure before deformation is restored by re-reversion of Mn-enriched martensitic phase, resulting in a recovered ductility. Namely, the present technique facilitates the use of single sheet steel for an automotive component comprised of two different parts to compensate opposing attributes.

Product quality, throughput, as well as energy and resource efficiency are the most important requirements in steel production. In order to meet these requirements, typically more than 15 parameters such as temperature, energy consumption and composition of the steel are monitored and the process is adjusted accordingly. The slag analysis results however, are due to extensive homogenization times only available 10 to 30 minutes after sampling. Therefore the results are evaluated post-mortem for following heats.

This "slag pathology" is particularly dramatic, since the slag analysis allows comprehensive conclusions on the process condition and is correspondingly valuable for process control. Insufficient slag composition can lead to increased refractory wear, greater oxidation of alloying agents such as Mn and other unbeneficial effects. Especially the loss of valuable alloying elements is problematic in multiple ways. They have to be purchased at high cost, melted down energy intensively, and then be disposed in a costly and time-consuming manner.

Various workarounds have been established to estimate or guess the slag composition, but none of them can compete with chemical analysis in terms of precision.

Laser Optical Emission Spectrometry (Laser OES) is increasingly establishing itself as an alternative. Due to up to 1.000 measurements per second it can homogenize data instead of physical samples. this allows a significant acceleration of the overall slag analysis time from the hot sample to the result down to 1-2 minutes. Along with an increased number of samples analyzed per heat, this enables a close monitoring as well as an in-situ process adjustment based on real analysis results.

A precise furnace control in a narrow process window close to the targeted optimum reduces the refractory wear, it minimizes the oxidation of alloying agents such as Mn into the slag and it improves energy efficiency.

This paper will present the effects of substrate chemistry and process atmosphere pO2 on the selective oxidation and reactive wetting of a series of prototype 0.2C-6Mn-xSi-yAl-zCr third-generation advanced high strength steels (3G AHSS). The substrates were first austenitized and quenched to room temperature, flash pickled, intercritically annealed and galvanized, where all annealing treatments were conducted in a N2-5% H2 process atmosphere under a variety of controlled dew points. All substrates demonstrated 3G-compatible properties.

Austenitizing the panels at a dew point of -30°C or -10°C resulted in the selective oxidation of Mn, Si, Al, and Cr, resulting in a compact external oxide layer as well as an extensive internal oxide network in a near pure Fe matrix. The external oxides were highly soluble in the hydrochloric acid pickling solution, and the resulting intermediate surfaces were relatively oxide-free apart from some discrete, dispersed nano-oxides. The ensuing intercritical annealing at a dew point of -30°C, -10°C, or +5°C did not significantly alter the surface structures.

High-quality and adherent galvanized coatings were formed after a 4 s immersion in a conventional 0.2 wt% Al (dissolved) bath. The substrate/coating interfacial structures were investigated using transmission electron microscopy equipped with electron energy loss spectroscopy (TEM-EELS), and it was determined that the primary reactive wetting mechanism was direct wetting of the substrate Fe by the galvanizing bath. Secondary mechanisms included oxide wetting, oxide cracking and lift-off, oxide bridging by the liquid bath metal, and infiltration of the bath metal into the substrate through channels left by the selective dissolution of some internal oxides during flash pickling. A fully-developed Fe2Al5-xZnx interfacial layer was observed. These results show that the two-stage processing route is promising for the production of galvanized medium-Mn 3G AHSS.

To increase the yield strength of HMnS and afterwards regain the capability for residual deformation we developed a novel thermomechanical treatment to manufacture a high manganese TRIP steel with excellent mechanical performance by combining the deformation-mechanisms SLIP, TWIP and subsequently activating the TRIP-effect. Our process resulted in ultra-high tensile strength of the high manganese steel. The so-called mechanism-controlled rolling and a subsequent recovery annealing lead to 1.6 GPa, with uniform elongations up to 15%.

The TRIP-steel was warm rolled at 200 °C, to suppress TRIP and activate TWIP as deformation mechanism. Thus, a high density of deformation twins and dislocations was introduced to the microstructure, avoiding martensite formation. A subsequent recovery annealing at elevated temperatures, reduced the dislocation density and while a high density of deformation twins was preserved. This combination of warm rolling and annealing allows the production of a fully austenitic nano-structured microstructure. If then deformed the TRIP effect is predominant in and depending on the tensile direction. The plastic deformation at ambient conditions creates martensite in a highly twinned microstructure which increases work hardening. The anisotropic behavior was analyzed as well as the hydrogen resistivity in slow strain rate tests. The HMnS exhibits an ultra-high yield strength and sufficient ductility, favorable properties for lightweight construction in automotive or aerospace industry.

Maraging steels have been widely used for the industrial application needed high strength materials such as aerospace technology and military industry. The corresponding steel is generally composed of substitutional elements such as 17-19 % Ni (in weight percent), and 8-12 % Co, 3-5 % Mo and 0.2-1.8 % Ti, and it contains a small amount of C (< 0.03 %). After hot rolling and subsequent aging, the steels reveal the α' martensitic matrix with intermetallic nanoprecipitates such as Ni3Mo (D0a-type), Ni3Ti (D024-type), Ni3Al (L12-type), and Fe2Mo (C14-type), resulting in a high yield strength of over 1.4 GPa and a high tensile strength of over 1.6 GPa. However, despite its remarkable mechanical response, the relative high materials cost of the steels driven by expensive alloying (e.g., a high amount of Ni, Mo, and Co) hinders the mass production. Therefore, in the present study, we proposed the new alloy design concept of maraging steel with the merit of materials cost, and the microstructural evolutions and mechanical response of the steel are systematically investigated. Regarding the alloy composition, the high Ni content in conventional maraging steel was partly replaced to Mn content, and the ~4 wt.% of Al was added to form an intermetallic nanoprecipitate (e.g., NiAl) and to reduce the weight of alloy.

The objective of the present study is to develop the steels with the reasonable materials cost to replace the Fe-9Ni (wt.%) steel known as the materials for the cryogenic application. We designed the Fe-xMn-yNi-0.1C steel, and the quenching-tempering (QT) process and quenching-lamellarizing-tempering (QLT) process were applied to corresponding steel to optimize the microstructure and mechanical properties. The QLT-processed steel revealed a higher impact absorbed energy at –196 °C (~193 J) than the QT-processed steel (~168 J), showing a similar value with Fe-9Ni steel. The soft martensitic matrix from two-step annealing and pronounced transformation-induced plasticity from retained austenite led to the high damage tolerance and improved toughness of QLT sample.

A decarbonized energy system is underway worldwide. The Paris Agreement was adopted by 196 Parties at COP 21 in Paris. Its goal is to keep global warming “below 2 °C above preindustrial levels, and to pursue effort to limit the temperature increase even further to 1.5 °C.” To achieve this long-term temperature goal, big changes are needed in the ways energy is produced, distributed and storage, as well as the decrease of fossil fuels and the use of renewable energies. In the future, electric vehicles will be of fundamental importance in the transport sector in order to be able to achieve these climate targets. Electrical drives are characterized by their high efficiency and enable the use of electrical power from renewable energies. The present work is focused on the development of a novel high-Mn steel with high strength and non-magnetic which will be used for components in the e-cars. This new steel is based in the system Fe-15Mn-4Al-0.6C (wt.%) and Ni, Cu and V were added to study their effect in the mechanical and physical properties. Four heats were produced, hot rolled and heat treated. Microstructure, mechanical and physical properties were studied after quenching and annealing treatment. The four heats show a 100 % austenitic microstructure without any detrimental phases, like ferrite and martensite. The new steels Fe-15Mn-4Al-0.6C allowed with Cu and V present the best combination of tensile strength, higher than 700 MPa, ductility and high toughness. These two alloys have a potential in terms of material technology, but also represents an attractive solution from an economic point of view compared with other non-magnetic steels.

A novel medium Mn steel with 8 wt% Mn was developed and processed through hot rolling and intercritical annealing. The steel possessed tensile properties of 1 GPa yield strength, 1.13 GPa tensile strength and ductility of 41%. The TWIP+TRIP mechanism was found to be operative in this steel. By further cold rolling the steel after intercritical annealing, the strain hardening rate was found to increase with increasing cold reduction but without a significant loss in ductility. It was found that cold rolling activated additional twinning systems as compared to uniaxial tension and also increased the amount of nucleation sites for strain-induced martensite. The large number of nucleation sites then led to an enhanced TRIP effect in subsequent tensile tests of the cold rolled steel. A scale up study from a laboratory 400 g to 5 kg ingot was also conducted on this alloy to determine the effects of segregation and delta-ferrite dissolution on the resulting tensile properties. It was found that the Mn segregation range could be reduced from 6.2 to 1.5 wt% but delta ferrite could not be fully dissolved within a standard reheating cycle of 1250 ˚C for 2 h. Nevertheless, the tensile properties were not significantly affected, demonstrating the increasing industrial readiness of medium Mn steels.

Understanding the deformation behavior of metals over a wide range of strain rates is of critical importance in a variety of engineering applications, such as vehicle collision, projectile penetration, explosion shock, and so on. Decades ago, studies on the strain rate dependent deformation behavior of various metals, including bcc metals (e.g. pure iron), fcc metals (e.g. Cu, Ni), and hcp metals (e.g. Ti), were reported, and the results show that all of these metals exhibit positive strain rate sensitivity (SRS). In the present study, we investigated the strain rate dependent mechanical behavior of a medium Mn steel fabricated by a room-temperature quenching and partitioning (RT-Q&P) processing. We found that the RT-Q&P medium Mn steel exhibits abnormally negative SRS. The underlying mechanism has been comprehensively investigated by interrupted tensile tests, synchrotron XRD, TEM and APT. We found that the interstitial carbon is the reason for the negative SRS, but not the TRIP effect. Our finding implies that we have to be extremely cautious when using high-carbon high-strength steels that are subjected to high-strain-rate deformation, such as automobile anti-collision components (e.g. A, B pillars, crash-box etc).

While excellent compromise of strength and ductility of Medium Mn third generation steels are well known and deeply studied, the understanding of intergranular brittleness observed at different stages of the manufacturing process remains a major challenge for the industrialization of these new steels. Recent studies carried out at ArcelorMittal on a third generation Medium Mn 0.15C-4Mn-1.5Si steel have highlighted different conditions favoring such a brittleness:
1) After tempering: As already well reported in the literature, temper embrittlement of martensite is associated with the segregation of certain impurities to grain boundaries in the 300°C-650°C range. It is especially shown here that the phenomenon is reversible, and that formation of stable retained austenite during tempering might be very beneficial through its effect on crack blunting and on segregation.
2) After quenching from full austenitic region: Despite a lower segregation in prior austenite grain boundaries compared to that of ferrite, the higher martensite strength could lead to a large embrittling effect, especially for lower soaking temperature.
3) After overaging treatment: Among various overageing conditions tested, this embrittlement has only been observed when bainite is partially formed during overageing. Closer look at fracture surfaces highlighted a full interface decohesion at the bainite-fresh martensite interfaces. It is thought that it is a consequence of low interface strength and high strength mismatch between bainite and fresh martensite. It is supposed that the low interface strength is a consequence of segregation occurring during the phase transformation (so called solute-drag). Ductility is fully recovered after a tempering treatment.

Press hardening of manganese-boron steels is one of the most efficient production processes for high strength automotive components. However, the residual formability of these sheet components is greatly limited by the formation of fully martensitic microstructure during in-die quenching, which makes them only to a limited extent suitable for crash-relevant anti-intrusion parts of the vehicle body. In order to extend the application range of press-hardened components, the use of third-generation advanced high strength steels achieves increasing attention.
Aim of the presented research is to analyze the potential of press hardening of lean medium manganese steel in combination with a quenching & partitioning (Q&P) treatment. For this reason, dilatometer investigations on Fe-0.3%C-5%Mn-1.5%Si with varying Q&P parameter were performed. By adjusting the heat treatment parameters, the microstructure and hence the mechanical properties were modified to fit the application´s load requirements. The results demonstrated that the performed Q&P treatments lead to multi-phase microstructures, consisting of martensite (tempered and fresh) and retained austenite resulting in high tensile strength, comparable to martensitic 22MnB5, and significantly improved total elongation up to 18 %. Regarding the ductility, it was shown that besides the adjustment of a sufficient austenite content, the reduction of fresh martensite is indispensable to prevent brittle failure. Finally, selected heat treatments were successfully reproduced in a laboratory-scale press hardening system equipped with a heatable hat-shaped pressing tool.

The effect of QP heat treatment on the phase transformation of Fe-8Mn-2.5Al-0.5Si-0.2C-1.6Cu (in wt.%) medium manganese steel was studied by thermal dilatometer and EBSD characterization. It was found that the key factor determining the microstructure (phase content) of QP heat treatment of medium manganese steel is quenching temperature. When the quenching temperature is low, a small amount of austenite after quenching can be stabilized in the partition process, and martensite transformation is not easy to occur during the second cooling after partition. However, this method of stabilizing austenite is limited because the upper limit of the final austenite content is the austenite content obtained at the first cooling. When the quenching temperature is high, although there is more austenite after quenching, the partition process has little effect on its stability due to the limited element diffusion, and a sharp transformation will occur during the second cooling. Therefore, moderate quenching temperature combined with the blending process is the key to obtain sufficient austenite. In addition, the morphology and size of martensite are determined by the first cooling stage when the quenching temperature is low, showing a large lath morphology. While, the small and needle-like martensite is mainly determined by the second cooling stage when the quenching temperature is high.

Materials used in the oil and gas industry are often exposed to some of the most aggressive industrial environments as the industry has moved towards increasingly more challenging prospects including high pressure, high temperature, ultra-deep water, and severe sour environment. Developing these prospects at a competitive cost while maintaining robust structural integrity is one of the most complex engineering challenges. It does, however, offer the potential to transform existing materials technologies and the opportunity to innovate. High Manganese steels (HMS) have been extensively studied for the applications in automotive industry. This presentation is to highlight the recent development of novel HMS metallurgy for energy industry applications. The proposed paper will describe i) selected R&D and commercialization examples of metallurgical technology development in energy industry with emphasis on unique aspects of industry applications, and ii) emerging opportunities to provide timely and cost-effective materials solutions for future energy industry prospects

Steels used in several aerospace applications, specifically landing gears, require high strength and fracture toughness, good ductility and corrosion resistance. However, currently used steels such as 300M and SAE 4340 are not corrosion resistant and therefore a coating is needed. Recently, a new non-toxic zinc-nickel coating is being developed; however, neither zinc-nickel coating nor chromium and toxic cadmium coatings do not solve the corrosion problem if it cracks. Ultimately, the use of stainless steel delivers a more robust solution, reducing maintenance time and cost of repairs caused by corrosion. Ultra high-strength stainless steels have been developed to replace the coated 300M and SAE 4340 steels. They are alloyed with Co (8-13 wt.%) and Ni (17-19 wt.%) and can be considered as maraging stainless steels. The high content of alloying elements and their complex production result in low viability of these maraging stainless steels. This paper presents the results of a newly developed high-strength low allowed maraging steel to be used in aerospace applications. The newly developed maraging steel (Fe-7Cr-7Mn-5Ni–3Mo-1Ti, wt.%) was produced by conventional steelmaking processes. Different heat treatments were carried out to determine the optimal parameters such as temperature, time, cooling rate to obtain a high strength properties. After quenching and tempering, the new material shows a martensitic microstructure. It presents yield and ultimate tensile strength higher than 1300 MPa and 1500 MPa, and hardness higher than 45 HRC. The Pitting Resistance Equivalent Number (PREN) is higher than 17. The newly developed X4CrMnNiMoTi7-7-5-3-1 maraging steel can be a new option to replace coated steels in aerospace applications due to the high mechanical strength.

Floating production assets rely on mooring chain integrity to ensure safe operation. The offshore industry has identified a number of technical gaps in the industry’s knowledge on mooring chain integrity. This has limited the ability of designers and operators to reliably estimate the degradation of mooring chain and to mitigate the risk of mooring line failures. The current code of practice recommends designing mooring systems based on uniform corrosion and factoring in a concurrent allowance for wear. Steel mooring chain retrieved from service exhibited both uniform and localized corrosion as well as wear damage. Pre-emptive chain replacements may be required if observed degradation exceeds design expectations; such replacements can bear significant cost. Carbon steel is conventionally employed for mooring chains, however, there are significant incentives to explore new materials technologies with improved seawater corrosion and wear performance for mooring chain applications.

High Manganese Steel (HMS) can offer improved properties over conventional carbon steel and is a potential candidate for use as mooring chain. In this study, several HMS chemistries have been assessed on various parameters of mooring chain performance, such as manufacturing, weldability of small scale bars, mechanical properties, and coupon fatigue testing. This paper aims to present the results which indicate the potential of HMS as mooring chain material.

Quenching and partitioning (Q&P) process has shown a great potential to achieve enhanced tensile properties in advanced high strength steel (AHSS). It is primarily attributed to the retained austenite in martensitic matrix that exhibits TRIP effect. In order to obtain austenite in the final microstructure, the cold-rolled steel is austenitized and then quenched to the temperature between Ms and Mf, followed by a partitioning process which allows the diffusion of carbon supersaturated in martensite into adjacent austenite. Consequently, the carbon enriched austenite can be retained upon final cooling to room temperature. According to the earlier works, the first quenching temperature after the austenitization has a significant influence on the mechanical properties because it determines the fraction of constituent phases. However, it is often difficult to control the first quenching temperature precisely in the manufacturing process, leading to the deviation in the mechanical properties. To overcome this difficulties, a room temperature Q&P process has been paid attention, in which the chemical composition of the steel is adjusted that the interval between Ms and Mf temperature includes the room temperature. However, high Mn content that is necessary to control Ms and Mf temperature possibly causes degradation of galvanizability and weldability. In the present study, we investigated the room temperature Q&P process starting from the chemically heterogeneous microstructure. Without increasing Mn content, highly heterogeneous Mn distribution in the initial microstructure successfully produced the Q&P microstructure consisting of martensite and retained austenite even subjected the room temperature Q&P process. Detailed microstructure evolution and corresponding mechanical properties will be discussed with respect to the influence of chemically heterogeneous initial microstructure.

During the last years, Quenching and Partitioning (Q&P) steels have gained strong interest in the steel industry due to their promising lightweight potential and crashworthiness. With their microstructure consisting of tempered martensite (α’’) and C-enriched retained austenite (RA), they are characterized by balanced ductility characteristics, which close the gap between the conventional Dual Phase (DP) and Complex Phase (CP) steels.
The aim of this work was to investigate the influence of the microstructure on the ductility of several lean medium Mn Q&P steels. Therefore, three chemical compositions with varying C-contents between 0.10 and 0.20 wt-%, 4.0 wt-% Mn and 1.5 wt-%Si were thoroughly examined with regard to their structure-properties relationship. To assess the formability behavior of the investigated steels, parameters derived from tensile testing were utilized. In particular, the true uniform strain (εu,true) was used to describe the global ductility, whereas the true thickness strain (ε3,true) was determined as a characteristic value for the local one.
The results clearly demonstrated a strong effect of the RA fraction and its mechanical stability on the ductility behavior of lean medium Mn Q&P steels. Generally, the optimum exploitation of the Transformation Induced Plasticity (TRIP) ensured high strain-hardening rates and therefore favored high global ductility. However, the strain-induced martensitic transformation resulted in increasing heterogeneity in the microstructure, remarkably impairing the local ductility. By an increase in C larger RA fractions could be stabilized, which shifted the ductility characteristics from rather local towards global. For this reason, a medium C-content of 0.15 wt-% was found to result in the optimum microstructure leading to the desired balanced formability in combination with an excellent strength-ductility combination.

In a thermal cycle consisting of a quenching and partitioning (Q&P) process, followed by a subsequent galvannealing process, the effect of Si and Al especially on the retained austenite in the microstructure was investigated by two alloy concepts 0.2C-4Mn-1.5Si and 0.2C-4Mn-1.5Al. Thereby the influence of a replacement of the alloy element Si by Al and their carbide retarding effect was investigated in the Q&P-temperature region until 400°C and also at higher galvannealing temperatures up to 560°C. Thereby samples were annealed by dilatometry to investigate the microstructure evolution. Afterwards retained austenite was measured by saturation magnitude magnification and X-ray diffraction method and microstructure was investigated by light optical microscopy, scanning electron microscopy and electron backscattered diffraction. As Si and Al have a different effect on the tempering behavior, respectively carbide precipitation in martensite, derivation of relative length change was calculated from dilatometry martensite tempering tests and heat flow was measured by differential scanning calorimetry starting from fully martensitic microstructure.
During traditional Q&P, Si-concept was more efficient in stabilizing high amounts of lath like and blocky RA compared to Al. This correlates well with the tempering experiments of martensite, as more carbide precipitation takes place around the Q&P temperature, respectively lower carbon partitioning potential for the Al-concept. At higher temperatures, during the galvannealing process, Al showed considerable benefit compared to Si, as it retards effectively the retained austenite decomposition into pearlite. For the Al-concept annealing at galvannealing temperatures results also in a good work hardening behavior due to the still effective TRIP-effect, whereby for the Si-concept RA was decomposed. These investigations showed that Si-alloyed grade in particular possesses excellent properties after Q&P process. In contrast, the Al-concept proved to be more suitable to fulfill the desired requirements subsequent to galvannealing.

As medium manganese steels are often treated in the intercritical domain to obtain bimodal retained austenite and ferrite microstructures, the application of a quenching and partitioning treatment to a medium manganese steel also promotes the retention of austenite at room temperature via carbon partitioning from martensite to austenite and by the austenite stabilizing effect of manganese in solution. The resulting microstructures are complex and they exhibit interesting mechanical properties that contrast with that of traditional Q&P steels, reaching high strength levels (1600 MPa) and ductilities (20%). However, the medium manganese Q&P microstructures are sensitive to the mechanical contrast brought by the presence of fresh martensite, often leading to early fractures. This research provides an understanding on the contribution of each constituent phase on the global mechanical properties by applying post-tempering treatments on a 5 wt.% Q&P steel. Three microstructure-types with different phases proportions were systematically investigated to assess the role primary martensite, retained austenite and fresh martensite on the work-hardening and ductility. The constituent phases were characterized with XRD, EBSD, TEM, EDX and nanoindentation. The mechanical contrast between phases was found to be a major responsible for the rapid transformation of metastable retained austenite to martensite during straining leading to early fractures. The reduction of the mechanical contrast between phases by post-tempering treatment led to the deployment of different degrees of serrations on the tensile curve, allowing to highlight qualitative conditions for the occurrence of this phenomenon.

Medium Manganese steels represent an important family of steels that, if successfully implemented, can contribute toward enabling light weighting for increased fuel economy and extended electric vehicle range. Attractive tensile properties are obtained albeit highly sensitive to processing path. A review will be presented highlighting physical metallurgy fundamentals of medium manganese steels pertaining to microstructural evolution, processing and properties interrelationships. Austenite stabilization and its mechanical stability will be discussed. Annealing responses as influenced by time, temperature, prior microstructure and cementite dissolution will be discussed. Microstructural models assuming equilibrium phase fractions and solute enrichment of austenite, and diffusional calculations in one- and two-dimensions will be presented. In addition, processing routes alternate to intercritical annealing will be discussed in particular the recently proposed double soaking where intercritical annealing is followed by secondary soaking at higher temperature and quenching to partially replace the lean Mn ferrite present at the intercritical temperature into martensite following quenching to room temperature.

The changes in the martensite structure of 0.1-0.2%C-2%Si-5-7% Mn steels with a decrease in the prior austenite grain size were investigated. Particularly, the austenite grain size, at which the single variant martensite was formed, was investigated. When the prior austenite grain size was 2 micrometers, martensite with a single variant (single block) structure was formed. These results indicate that the critical prior austenite grain size to form martensite with a single variant structure is 2 micrometers. The tensile strength (TS >1500 MPa), uniform elongation (UEL, 8%), and total elongation (TEL >14%) of the single-variant martensite were superior to that of the multi-packet multi-block martensite (TS = 1350 MPa, UEL = 6% and TEL = 13%). The superior properties of the single-variant martensite are a result of enhancement of its strain hardening rate due to the structural change. Synchrotron radiation analysis clarified the change in dislocation density with decreasing prior austenite grain size.

Abstract:
Samples from a medium manganese thermomechanical processed steel sheet show strongly preferred crystallographic orientations far from the random distribution. For the quantification of the retained austenite phase, a novel X-Ray diffraction (XRD) based method is presented that calculates the phase fraction from the measured texture data. By a geometrical averaging procedure in a large area of the reciprocal space, it is possible to define an orientation dependent phase fraction that can be related to the mechanical and magnetical properties along different directions. The structural information from XRD of the phases ferrite, retained austenite and epsilon martensite is used to setup a combined Electron Backscatter Diffraction (EBSD) and Energy Dispersive X-Ray detection (EDX) measurement on the scanning electron microscope (SEM). The distribution, shape and chemical composition of the phases is obtained with a high lateral resolution and is used to characterize the impact of deformation induced by an uniaxial reckling test on the microstructure. The filtered chemical composition of the individual phases is used to correct the atomic form factors and absorption calculations of the XRD postprocessing. The local misorientation of the EBSD data is evaluated with a kernel averaging method for each phase and correlated to the experimental data of the residual stresses measurement by XRD along different directions. The results of this orientation dependent analysis help to improve the understanding of the physical phenomena and mechanical strengthening mechanisms of medium manganese steels under deformation.

Medium Mn steels is a family of new steels which present ground-breaking properties due to their ultra-fine ferritic matrix and a high amount of retained austenite when processed through intercritical annealing. The high fraction of austenite is enabled thanks to carbon and manganese partition between austenite and ferrite at high temperature. Controlling the enrichment and the stability of austenite is crucial for the steel to show an efficient TRIP (transformation induced plasticity) effect.
The kinetics of enrichment in Mn and C of austenite depends on annealing conditions (intercritical annealing temperature and heating rate), the initial microstructure (cold-rolled or as quenched martensitic as well as the fraction of cementite) as well as the nominal composition of the steel. Moreover, hydrostatic stresses generated in austenite during the final cooling (due to the final martensite transformation or to the difference in thermal expansion coefficient between austenite and ferrite) also affects its apparent stability.

The purpose of this work is to decouple the effect of each aforesaid parameters on the stability of austenite. To do so, a single alloy (0.2C-4Mn-0.8Al-1.5Si) have been annealed at four intercritical temperatures, ranging from 680°C to 750°C after a controlled heating stage. Those temperatures have been chosen after thermodynamic equilibrium calculations. Before annealing, the plates have been manufactured in two different ways to vary the initial microstructures. They have been either simply cold-rolled or cold-rolled, fully annealed and quenched.
These thermal treatments have been performed on synchrotron beamline to follow in operando the phase transformations mechanisms using High Energy X-Ray Diffraction (HEXRD). The high energy (100 keV) coupled with a high-performance detector enable high acquisition rates (10 Hz) and thus time-resolved investigations even during the heating stage. HEXRD correlated with post-mortem wavelength dispersive spectroscopy allows an simultaneous tracking of the phase transformation sequences, the chemical partitioning and the internal stresses at phase scale.

The development of new advanced high-strength steels is traditionally a very tedious process, involving extensive experimental investigations by which composition and processing conditions are varied in a more or less erratic manner until the desired properties are achieved. This situation has a tremendous negative impact on alloy development time and costs. The Alloyneering alloy design approach encompasses the application of computational alloy thermodynamics, kinetics, and constitutive models, which can describe the microstructure evolution and link strength with microstructure, for the computational design of metallic alloys with tailored microstructure and properties. The procedure is integrated with optimization techniques for the selection of optimal alloy composition or processing parameters. An example illustrating the alloyneering approach is the design of Medium-Mn steels (MMnS) with tuned retained austenite stability for optimum TRIP-TWIP interactions which lead to enhanced strength/formability combinations. CALPHAD based thermodynamic and kinetic calculations, coupled with evolutionary and gradient based robust optimization techniques, enabled the design of optimal compositions and processing parameters that satisfy certain design criteria under the uncertainty found upon material production. Constitutive and transformation kinetics modeling of stress-assisted and strain-induced ε-Martensite, α΄-Martensite as well as mechanical twinning in a physical framework, followed to determine the mechanical behavior and the evolution of TRIP & TWIP upon loading. This example illustrates that the alloyneering approach could be used for the development of new advanced high-strength steels.

Hydrogen embrittlement (HE) is a persistent mode of failure in high-strength steels
which hinders their applicability in industry. The role of retained austenite (RA) in the
HE susceptibility of these steels is still unclear. There are two possible scenarios, firstly
since the solubility of H in austenite is remarkably larger than that of martensite, the
RA phase can act as a trap for H atoms and hinder them from diffusion to the critical
regions such as grain boundaries and junctions and damage the material. On the other
hand, if during the lifetime of the steel RA undergoes martensitic transformation it could
release excess trapped hydrogen atoms which might damage the material in the vicinity
of the former RA.
To shed more light on the role of RA, we probed the role of H atoms on the relative
stability of the fcc/bcc/hcp phases in Iron using the ab initio thermodynamics approach.
The results indicate that at low hydrogen chemical potentials regime the stability of the
fcc phase is slightly enhanced while at high hydrogen chemical potentials the bcc phase
dominates. The different excess volumes of the hydrogen-rich phases in steels can lead
to phase transformation under the cyclic load. Furthermore, for the scrutiny of the
decisive role of the interface, the interplay of the hydrogen atoms and carbon atoms at
the bcc-fcc phase boundary is investigated using density functional theory. Results
indicate that the phase boundary is indeed a trap for both H and C atoms. Interestingly,
the interaction of the C and H atoms at the phase boundary is repulsive i.e. presence of
C at the phase boundary prohibits binding of H atoms to it

In the present work, a metastable Cr-Mn-Ni steel with deformation-induced plasticity was investigated. The influence of martensite formation on the mechanical properties of an Fe-16Cr-6Ni-6Mn (concentrations in wt-%) austenitic stainless steel was studied by tensile tests between -70°C and 300°C. As-quenched martensite formation was observed at temperatures below -30°C. Deformation-induced α' martensite formation, on the other hand, was triggered below 100°C. Subsequently, a microstructure characterization was carried out. The results obtained by a semi empirical thermodynamic-mechanical calculation model were linked to a few experimental results from the tensile tests. The presented model combines chemical energy amounts from thermodynamic databases with mechanical energy amounts obtained by tensile tests with the aid of the conversion rule proposed by Patel and Cohen. The magnitudes of shear strain (γ_0=0.23) and dilatational strain (ε_0=0.023), required for the calculations, were obtained based on the martensite crystallography theory of Wechsler-Lieberman-Read. Thermodynamic calculations were performed using the ThermoCalc software with the database TCFE10. As a result, the Stress-Temperature-Transformation (STT) diagram showing the temperature dependence of proof stress, tensile strength, triggering stress for martensite formation and the associated critical transformation temperatures (M_S, M_d & T_d) is obtained. With the aid of the semi empirical thermodynamic-mechanical model it can be shown, that the mechanical properties and critical temperatures of a metastable austenitic steel can be determined at very low experimental effort.

In recent years, most of the studies on medium-Mn steels have focused on the chemical compositions, microstructural evolution, mechanical properties, and heat treatment schedules with different C and Mn contents. The micro/nano scale grain size of medium Mn steel after austenite reverse transformation (ART) provides a challenging situation to characterize the microstructure and evaluate chemical composition on bulk samples.
Here we used a combination of Electron Probe Micro-Analysis (EPMA) and Electron Back-Scattering Diffraction (EBSD) to characterize different medium Mn steels obtained via direct ART route vs. a reaustenitized quenched (RAQ) + ART route with different temperatures and soaking time.
Different routes along with different ART soaking times and temperatures ultimately affect the growth of retained austenite (RA) phase with different alloying compositions. This would further change the SFE of the RA phase affecting mechanical properties. Such characterizations are essential as chemistry differences and morphological differences provide a key understanding to the microstructure-property relationship of such steels.
A dedicated method was carefully designed to perform high-resolution quantitative carbon and manganese mappings on medium-Mn steels after ART processing materials. To enable direct comparison between chemical and structural information of the analysed regions, EPMA and EBSD measurements were performed at the same location. Highly sensitive and spatially resolved experiments provided a direct understanding to the achieved mechanical property based on the composition and morphology of sub-micrometer sized globular (“blocky”) and acicular (“film-like”) RA domains.

3rd generation AHSS (advanced high-strength steel) family encompasses different innovating microstructure concepts, among which medium Mn steels. They have ground-breaking properties due to their particular microstructures containing a nanostructured ferritic matrix and a large amount of metastable retained austenite. Under mechanical solicitation, strain induced martensitic transformation of austenite will occur, leading to an efficient TRIP effect (transformation induced plasticity).

The purpose of this work is to evaluate the stability of austenite during mechanical loading, accounting for stress partitioning between phases, the strain induced martensitic transformation and the mechanical instabilities (Lüders banding and Portevin-Le Chatelier effect) that are commonly reported in those steels. In situ High Energy X-Ray Diffraction (HEXRD) on synchrotron beamline during quasi-static tensile tests have been performed. The presented experiments have been realized on Petra-III P07 line at DESY (Hamburg). The high energy (100 keV) coupled with a high-performance detector enable high acquisition rates (10 Hz) and thus in situ time-resolved investigations. Mechanically induced martensitic transformations kinetics are determined using Rietveld refinements. The in situ evolution of 3D stress partitioning between the ferritic matrix and retained austenite was characterized by the sin²ψ method. Stress in fresh martensite is obtained through a micromechanical calculation. Digital image correlation has been carried out with 3D stereographic cameras to obtain the local strain near the diffracting volume and follow the mechanical instabilities.

Four microstructures produced by intercritical annealing between 680°C to 750°C have been studied. The samples have the same nominal composition, but the composition and fraction of each phase differ depending on the thermal treatment. The current study gives new insights into the effects of chemistry, grain size, initial phase fractions and mechanical instability on austenite stability.

Third generation Advanced High Strength Steels (AHSS), among which medium Mn steels, have potential applications for the automotive industry, due to their high balance between strength and ductility and their excellent forming properties. These good mechanical properties are possible thanks to the presence of a high amount of metastable retained austenite, which will transform into martensite during deformation, leading to an efficient TRIP (transformation induced plasticity) effect. Medium Mn steels microstructure usually consists of refined ferrite, retained austenite, and sometimes fresh martensite, and are obtained through intercritical annealing. Controlling the stability and the enrichment of the austenite phase during this thermal process is of vital importance for the steel to show an efficient TRIP effect.
The interplays between the different deformation mechanisms (dislocation gliding and strain-induced martensitic transformation) are not well understood in the literature. Moreover, the local gradient of alloying element composition exists inside grains of the same phase, which induces a gradient of austenite stability, and cementite precipitates can remain in the microstructure. Finally, mechanical instabilities, such as Lüders banding and the Portevin-Le Chatelier effect, are often reported in medium Mn steels, adding to the complexity of the deformation mechanism in such steels. Therefore, a local analysis in micro and nanoscale is necessary to understand the interactions between all those phenomena.
The contribution of this work is to characterize, via Transmission Electron Microscopy (TEM) and its associate analytical techniques (EDS, EELS) the stability of retained austenite, measuring the alloying element partitioning (Mn and C content) between retained austenite and the ferritic matrix. The characteristics of strain-induced martensite and deformation-induced defects (dislocation structures, twins…) are also inspected. Cold-rolled ferritic samples with composition 0.2C-4Mn-0.8Al-1.5Si were processed through different thermal treatments (intercritical annealing at various temperatures, with or without pre-austenitizing annealing) to provide the different medium Mn microstructures studied in this work.

Excellent combinations of strength and ductility have been observed in complex phase medium Mn steels, due to the activation of TRIP and TWIP in retained austenite, stabilized at room temperature via C and Mn partitioning. Limited attempts have been made in the past to model the deformation behavior and transformation kinetics of medium Mn steels. Previous studies relied on fitting the Olson-Cohen equation to phase fraction measurements in order to determine the kinetics of strain induced martensite or mechanical twinning, and then to calculate the ensuing mechanical response. In the present study a constitutive model describing the evolution of the flow stress upon loading as well as the kinetics of stress assisted and strain induced ε-, α΄-Martensite and mechanical twinning in a physical framework is presented. The elastoplastic response of each constituent phase is calculated as a function of the dislocation density evolution according to a Kocks-Mecking-Estrin type model and the macroscopic flow stress is calculated considering a rule of mixtures and the iso-work principle. The nucleation kinetics of ε-, α΄-Martensite and twin embryos and the subsequent growth of shear bands, as a function of the plastic strain facilitated in austenite were modeled in terms of the stacking fault energy of austenite. The nucleation of α΄-Martensite from shear band intersections and the consumption of ε and twins as α΄ grows, is considered, effectively modeling the evolution of phase fractions and the 𝛾 → 𝜀 → 𝛼΄, 𝛾 → 𝑡 → α΄ and 𝛾 → 𝛼΄ transformations upon loading, in fully austenitic and complex phase steels. The model could be used to accelerate the design of medium Mn steels with desired microstructural and mechanical properties by providing insight into the transformation kinetics and deformation behavior of the material.

High manganese steels are considered to be promising alloys for light weight applications due to their high strength and good ductility based on their special microstructure. During plastic deformation the mechanisms of twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) lead to a strong microstructure refinement within the material. Previous studies have shown that the TWIP and TRIP effect both depend on the applied stress condition during plastic forming. For an applied strain under tensile stress a larger number of twins and martensite lamellae has been observed compared to a strain under compression stress. During the sheet forming process several different stress conditions are applied to the material. Hence the amount of local tensile and pressure stresses directly influences the evolution of the local microstructure and thus the local mechanical properties. In order to consider those local property evolutions in sheet forming simulations a constitutive hardening model is needed which takes the correlation of local stress condition and microstructure evolution into account. For this purpose, the constitutive hardening model introduced by Steinmetz was extended by an empirical model, which considers the dependence of microstructure evolution on the stress condition. Within this empirical model the stress condition, represented by the Lode parameter, is correlated to one of the fitting input parameters of the hardening model. This special fitting input parameter has been identified and estimated by comparing the results of a sensitivity study of the constitutive hardening model input parameters on the one hand and a parameter study with different stress conditions as input for the crystal-plasticity simulations using DAMASK on the other hand. Finally, the extended constitutive hardening model has been used to calculate the hardening of a X30MnAl23-1 steel for different stress/strain conditions (uniaxial tensile stress, plane strain, simple shear), which is in good alignment with experimental data.

To control the production processes and design product properties of promising medium and high Mn steels, reliable solidification and material models are essential. High precision and experimentally validated thermodynamic data are essential data sources for all these models. As manganese is a segregating element, it is crucial to describe high manganese concentrations well, e.g. for the final stage of solidification.

But especially with higher manganese contents, there is a significant lack of experimental data. All published thermodynamic Calphad descriptions from Huang(1990), Djurovic(2011) and Kim(2015) refer to the solid-liquid transformations exclusively on the measurement data from Schuermann (1977). No other data/publications are available, and even Schuermann(1977) measure the solidus temperatures of high Mn steels only rarely.

As the mentioned thermodynamic descriptions show significant differences at higher Mn contents, an own data set of reliable data is necessary for the evaluation, selection, and if required for the assessment of the Calphad models. For this purpose, an own experimental study has been performed, and model alloys with Fe-Mn (up to 30 w.t.-% Mn), Fe – 10%Mn – C (up to 2.5 w.t.-% C) and Fe – 20%Mn – C (up to 2.5 w.t.-% C) were produced by induction melting and subsequent centrifugal spin casting. Since manganese has a strong tendency to evaporate when it melts and can quickly destroy measuring devices, a new measuring method was developed. Using a micro-DTA-protected setup with closed crucible by tantalum lids (local-getter and Mn “catcher”), all high-temperature phase transformations (TLiquid, TPeritectic, TSolid, TEutectic, TGamma-Delta) can be measured in equilibrium conditions. Based on these new experimental results, the thermodynamic description of Djurovic(2011) is identified as the most accurate one. Nevertheless, there are still significant deviations with Mn contents above 10 w.t.-% and further research is necessary and ongoing.