TWIP Steel Research Articles

Analysis of Strain Hardening Behavior of a High-Mn TWIP Steel Using Electron Microscopy and Cyclic Stress Relaxation

Analysis of Strain Hardening Behavior of a High-Mn TWIP Steel Using Electron Microscopy and Cyclic Stress Relaxation

Achieving Strength-Ductility Synergy in Nanotwinned Femnsic Twip Steels Prepared by Cryogenic Deformation at the Optimum Temperature

Achieving Strength-Ductility Synergy in Nanotwinned Femnsic Twip Steels Prepared by Cryogenic Deformation at the Optimum Temperature

Analysis of Strain Hardening Behavior of a High-Mn TWIP Steel Using Electron Microscopy and Cyclic Stress Relaxation

Analysis of Strain Hardening Behavior of a High-Mn TWIP Steel Using Electron Microscopy and Cyclic Stress Relaxation

Study on the Effect of Rare Earth Cerium on the Low Temperature Impact Properties of Twip Steel

Study on the Effect of Rare Earth Cerium on the Low Temperature Impact Properties of Twip Steel

Artificial Inclusion Environments—Replicating Industry in the Laboratory

The authors present a series of complementary test methods which were developed and used to investigate reactions between high aluminium steel and silica rich inclusions. Non-metallic inclusions (NMIs) cause many defects in the final steel product, therefore the ability to track their size, morphology and composition and correlate this with fundamental reaction kinetics provides important knowledge to support the production of clean quality steel products. Novel steel grades such as TRIP, TWIP and low-density steels have high aluminium contents; aluminium is a readily oxidisable species presenting the potential for instability and excessive reaction with commonly used mould powders that contain silica. A novel combination of techniques including HT-CLSM (High-Temperature Confocal Laser Scanning Microscope), XCT (X-ray computed tomography) and SEM/EDS (scanning electron microscopy/electron dispersive spectroscopy) have been used to study the interaction of entrained mould powder inclusions with steel at high temperatures simulating industrial conditions. This report presents a discussion on the development of techniques and samples to achieve representative and repeatable results that can provide information on the complex chemical and physical interaction phenomena with confidence. Each experimental technique had its own learning points and consequent results. Outcomes presented include possible confirmation of the chemical reaction rate controlling step being aluminium mass transfer; heterogeneous local environmental conditions including fluidity and chemical composition; and occurrence of spontaneous emulsification where the mould powder inclusion breaks apart into a cloud of smaller fragments.

Influences of carbon concentration on microstructure and tensile properties of Fe–18Mn–9Cr–2Al–xC steels

As an effort to develop corrosion resistant and highly formable austenitic steels, Fe–18Mn–9Cr–2Al–(0–0.5)C steels in wt. % were designed and their microstructures and tensile properties were investigated with particular attention being paid to the effects of carbon concentration. The carbon-free steel consisted of the austenite, δ-ferrite and ε-martensite phases in as heat-treated condition while those of carbon-added steels had an austenite single phase with the (Fe,Cr)23C6 carbides at grain boundaries. When tensile strained at room temperature, all the steels exhibited extensive deformation twinning, resulting in large work-hardening and excellent ductility. Deformation twinning was suppressed at 300 °C, and dynamic recrystallization took place partially at grain boundaries when strained at 600 °C. The addition of Cr and hence formation of carbides in carbon-added steels have little influence on plasticity of the conventional twinning-induced plasticity steels, suggesting the possibility of developing Cr-bearing TWIP steels with both corrosion resistance and excellent formability.

Direct tuning of the microstructural and mechanical properties of high-alloy austenitic steel by electron beam melting

In the present work austenitic stainless steel X2CrMnNi16-7-9 was additively manufactured using electron beam melting (EBM) technique. In order to reach the highest work of deformation processing parameters such as scan speed, beam current and volume energy were varied. Different properties of manufactured samples, including chemical composition, density, roughness, yield and tensile strength, ductility, grain morphology, texture, and phase composition were thoroughly examined by means of SEM, EDX, EBSD, tensile testing, Archimedes and other methods. The correlation between the parameters was analysed and optimal processing settings allowed to reach work of deformation up to 39 GPa with remarkable elongation up to 110%. It was also found that tuning of the Mn evaporation during EBM by varying the beam energy resulted in the change of stacking fault energy and martensitic transformation temperatures of the steel. This led to the change of the austenite stability and plasticity mechanisms, such as the occurrence of deformation induced martensite in the initially TWIP steel. Comparison of phase diagrams of the studied X2CrMnNi16–7–9 steel and similar X5CrMnNi16-6-6 steel has proven that the reason for in situ EBM grain refinement in the latter one is the presence of primary bcc phase field. • Manipulation of Mn content via EBM can be used to change CrMnNi steel properties. • Grain refinement during EBM of CrMnNi steel is due to bcc to fcc transition. • CrMnNi steel with 39 GPa·% work of deformation and 110% strain is produced by EBM. • Optimal processing window for EBM production of X2CrMnNi16-7–9 steel is revealed.

Annealing treatment induced ε martensite formation and evolution in TWIP steel

Crystallographic analyses on the orientation relationships of ε martensite variants were carried out by electron backscatter diffraction and transmission electron microscope in TWIP steel. Results show that the ε phases are mainly distributed in single or multiple initial variants with 70.5° angle on {111}γ planes during short-time annealing. Importantly, secondary variants, which are generally considered difficult to form by thermal-induced and are also not derived directly from matrix by the well-known simple shear process for the FCC to HCP transformation, were observed at the intersection between two initial variants with 19.5° rotation angle after long-time annealing. These successfully interpret the microstructural features of ε variants and their relationships.

The influence of grain size and precipitation and a boron addition on the hot ductility of a high Al, V containing TWIP steels

The hot ductility of V containing high Al, TWIP steels was determined at a 1000°C, when no dynamic recrystallisation occurred, and precipitation was too coarse to influence ductility. A change in grain size from ∼1250 to 500 µm caused the reduction of the area to increase by ∼25%, intimating that the improvement in ductility on adding boron is due to its segregation to the boundaries causing grain refinement on solidification. Fine VC precipitation was mainly responsible for deteriorating ductility. In these steels, the ductility of un-recrystallised austenite decreases gradually with increasing temperature from 700°C to 1000°C and this in combination with fine precipitation can markedly change the shape of the hot ductility curve from ductility decreasing to increasing with temperature.

Unique transition of yielding mechanism and unexpected activation of deformation twinning in ultrafine grained Fe-31Mn-3Al-3Si alloy

Tensile mechanical properties of fully recrystallized TWIP steel specimens having various grain sizes (d) ranging from 0.79 μm to 85.6 μm were investigated. It was confirmed that the UFG specimens having the mean grain sizes of 1.5 μm or smaller abnormally showed discontinuous yielding characterized by a clear yield-drop while the specimens having grain sizes larger than 2.4 μm showed normal continuous yielding. In-situ synchrotron radiation XRD showed dislocation density around yield-drop in the UFG specimen quickly increased. ECCI observations revealed the nucleation of deformation twins and stacking faults from grain boundaries in the UFG specimen around yielding. Although it had been conventionally reported that the grain refinement suppresses deformation twinning in FCC metals and alloys, the number density of deformation twins in the 0.79 μm grain-sized specimen was much higher than that in the specimens with grain sizes of 4.5 μm and 15.4 μm. The unusual change of yielding behavior from continuous to discontinuous manner by grain refinement could be understood on the basis of limited number of free dislocations in each ultrafine grain. The results indicated that the scarcity of free dislocations in the recrystallized UFG specimens changed the deformation and twinning mechanisms in the TWIP steel.

Study of dislocation-twin boundary interaction mechanisms in plastic deformation of TWIP steel by discrete dislocation dynamics and dislocation density-based modeling

Deformation twins contribute to the unique deformation behaviors and characteristics in the plastic deformation of TWIP steels since twin boundary (TB) blocks the movement of dislocations and absorbs them in deformation process. On the other hand, dislocations can traverse TB. However, there is still no consensus on how TB influences the plastic deformation of TWIP steels among prior researches. Therefore, exploring the interaction between dislocation and TB is critical to understand the effects of twins on the macro deformation behaviors and exploit the strengthening potential of the alloys. In this study, a dislocation-TB interactions model for TWIP steel was proposed, developed and implemented in discrete dislocation dynamics (DDD) simulation, the complicated dislocation reactions at the TB were determined by the energy criterion, which serves as a feasible approach to represent the micro deformation characteristics under different tension directions with respect to the twin plane normal of TWIP steel micropillar. Furthermore, the effect of dislocation type and reaction characteristic at the TB in DDD are incorporated into the conventional dislocation density-based (DDB) model, and then the improved DDB model is used to quantitatively describe the macro plastic behavior of TWIP steel micropillar. The DDD simulation results show that the dislocation-TB interactions are related to the dislocation type and the angular relationship between loading direction and twin plane normal. The TB has a significant strengthening effect if the loading direction is perpendicular to the twin plane (0°) due to the increase of the back stress induced by the activated 60° dislocation pileups. For other orientations (45° and 90°), however, the strain hardening becomes negligible. Meanwhile, the stress and dislocation density-strain curves under different directions with respect to the twin plane normal are predicted by the improved DDB model and have a good agreement with the DDD simulation and experimental results. The research thus advances the understanding of dislocation-TB interaction mechanisms in plastic deformation of TWIP steels.

Twinning engineering of high-entropy alloys: An exercise in process optimization and modeling

In a bid to improve the mechanical properties of high-entropy alloys, particularly, their strain hardening capability, we adapted the time-proven concept of ‘twinning engineering’, developed in the context of TWIP steels, to twinning-assisted high-entropy materials. The strategy chosen involved a two-step thermomechanical processing that consisted of low-temperature pre-straining and subsequent annealing. This approach was trialled on CoCrFeMnNi as an exemplary high-entropy alloy. The annealing conditions selected ensured that the deformation twins generated under low-temperature deformation were retained, whilst the dislocation density was recovered. The viability of this strategy was convincingly confirmed for room temperature deformation of the alloy. A constitutive model accounting for the effect of the pre-straining induced deformation twins was proposed. It was shown to provide a reliable description of the low-temperature and room-temperature deformation behavior of CoCrFeMnNi when deformation twins are involved.

Direct monitoring of twinning/detwinning in a TWIP steel under reversed cyclic loading

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 250 nm, 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 400–475 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 0.5–5%. 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.

Mechanism of twinning induced plasticity in austenitic lightweight steel driven by compositional complexity

The current work investigated the mechanical behaviors of a novel austenitic lightweight Fe-Mn-Al-Si-C steel that combined the characteristics of lightweight steel and TWIP steel, due to compositional complexity. The critical design principle lay in the synergetic addition of Al and Si to obtain simultaneously a stacking fault energy of 49.5 mJ/m2, short-range ordering, and density reduction. The initial strain hardening in this steel proceeded by microband-induced plasticity slip featuring the substructure of Taylor lattices due to planar-slip dislocations. It contributed to a high Hall-Petch coefficient of 673 MPa·μm0.5. In the latter stage of deformation, even though the stacking fault energy was not low, stacking faults and deformation twins were triggered when the critical stress was achieved. This transition behavior was first observed in lightweight steel and the overall effects led to an outstanding work hardening rate of about 2.4 GPa. This lightweight TWIP steel with a density of 6.98 g/cm3 possessed ultimate tensile strength of 800-950 MPa and total elongation of 67-77%. This study focused on its mechanical behaviors and developed a confined dislocation model to clarify the transition from dislocation glide to mechanical twinning.

Effect of microalloying with V and Ti on the microstructure and properties of electron beam welded thick high-Mn TWIP steel plates

In the present work, thick high-Mn TWIP steel plates with/without the addition of V and Ti were butt welded by electron beam welding (EBW) to study the effect of V and Ti on the microstructure and properties of the welded joints. It was observed that the fusion zones (FZs) of the joints consisted of two types of solidification structures, i.e. columnar dendritic structure and cellular structure. Strong texture component characterized by ⟨100⟩∥X-axis was developed by epitaxial growth of these solidification structures along ⟨100⟩ direction. Microalloying with V and Ti significantly refined the microstructure and led to the precipitation of secondary phase particles, which dramatically enhanced yield strength (YS). γ-ε transformation occurred when the REF (reference) grade joint was subjected to impact deformation at cryogenic temperature, which led to the decrease in Charpy impact energy. The addition of V and Ti reduced the occurrence frequency of internal defects, particularly the microsegregations of C and Mn, which contributed to the improvement of corrosion resistance.

Hot Deformation Behavior of a High-Mn TWIP Steel and Establishment of Strain-Compensated Constitutive Equation

Hot Deformation Behavior of a High-Mn TWIP Steel and Establishment of Strain-Compensated Constitutive Equation

Grain size altering yielding mechanisms in ultrafine grained high-Mn austenitic steel: Advanced TEM investigations

The underlying mechanism of discontinuous yielding behavior in an ultrafine-grained (UFG) Fe-31Mn-3Al-3Si (wt.%) austenitic TWIP steel was investigated by the use of advanced TEM technique with taking the plastic deformation mechanisms and their correlation with grains size near the macroscopic yield point into account. Typical yield drop mechanisms such as the dislocation locking by the Cottrell atmosphere due to the presence of interstitial impurities cannot explain the origin of this phenomenon in the UFG high-Mn austenitic TWIP steel. Here, we experimentally revealed that the plastic deformation mechanisms in the early stage of deformation, around the macroscopic yield point, show an obvious association with grain size. More specifically, the main mechanism shifts from the conventional slip in grain interior to twinning nucleated from grain boundaries with decreasing the grain size down to less than 1 μm. Our observation indicates that the grain size dependent deformation mechanisms transition is also deeply associated with the discontinuous yielding behavior as it could govern the changes in the grain interior dislocation density of mobile dislocations around the macroscopic yield point.

A new processing route to develop nano-grained structure of a TRIP-aided austenitic stainless-steel using double reversion fast-heating annealing

Abstract A novel processing route comprising double reversion annealing (DRA) was designed for developing bulk nano-grained (NG) structure of an austenitic stainless steel (Type 301LN). The new processing concept of DRA comprised two subsequent intrinsic type processes i.e., two times cold reductions (~53% and 63%) followed by fast induction heating (~200 °C/s) and short duration annealing at different temperatures (first at 690 °C/60s and second at 750–900 °C/0.1–1s). The NG structure revealed a remarkable improvement of the mechanical properties compared to the counterparts processed by single reversion annealing. Furthermore, outstanding combination of strength and formability is achieved for the DRA structures, significantly higher than those of high-Mn TWIP steels, low-alloy TRIP steels and 304 stainless steel. For instance, a superior combination of yield strength (~950–1030 MPa) and formability index (11.8–12.5 mm) obtained after DRA at 750 °C/0.1s and 800 °C/1 s, respectively. However, the corresponding values are 300 MPa and 12 mm for TWIP steels, 500 MPa and 10 mm for TRIP steels, and 270 MPa and 12 mm for 304 stainless steel. In order to reveal the effect of DRA on the stretch formability, Erichsen cup testing was conducted of both the initial and DRA steel specimens. Moreover, Erichsen cup testing also simulated by the finite element method (FEM) to survey further details of their deformation.

수직압을 받는 트윕강 보강패널의 극한거동

수직압을 받는 트윕강 보강패널의 극한거동

Twinning Engineering of a CoCrFeMnNi High-Entropy Alloy

Deformation-induced twinning has been a notable example of overcoming the strength/ductility trade-off dilemma as a strengthening mechanism. By borrowing this concept from the area of TWIP steels, we designed a thermomechanical treatment for a CoCrFeMnNi high-entropy alloy to improve its mechanical characteristics. We used pre-straining at 77 K to introduce deformation-induced twins in the microstructure of the alloy, and then recovered it by annealing at 773 K, while avoiding recrystallization. The deformation-induced twins generated by pre-straining at 77 K were retained after this heat treatment, whilst partial recovery of dislocations occurred. As a result, the room-temperature mechanical properties of the alloy, including its strain hardening ability, were improved substantially.