high-Mn Twinning-induced Plasticity Research Articles

Improved yield strength-ductility synergy of a high Mn TWIP steel by grain refinement and partial recrystallization

The industrial application of high Mn twinning-induced plasticity (TWIP) steels is limited by their low yield strength (YS). In this work, a combination of grain refinement and partial recrystallization was applied to increase the YS of a high Mn TWIP steel to 1215 MPa, along with a total elongation (TE) of 16 %. The high YS of the partially recrystallized steel mainly originates from the combination effect of high dislocation density up to 8.3 × 1014 m−2, fine grain size of about 1 μm in recrystallized regions, and dense mechanical twins with a small spacing of about 20 nm in unrecrystallized regions. This work provides a potential way to improve the YS-ductility synergy and broaden the application of TWIP steels.

Effects of cooling process on the microstructure and mechanical properties of a novel high-V high-Mn TWIP steel for cryogenic storage tank applications

In the present work, a novel high-V high-Mn twinning induced plasticity (TWIP) steel for cryogenic storage tank applications was developed. Furthermore, innovative two-stage cooling processes encompassing ultra-fast cooling and the subsequent furnace cooling were proposed. Effects of cooling process on the microstructure and mechanical properties of the experimental steel were investigated. It was observed that with the change in cooling process (the initial temperature of furnace cooling rised gradually), the dwell time of hot-rolled plates above 600 °C was extended. The extension in dwell time facilitated the precipitation of secondary phase particles, leading to the change in average diameter and volume fraction of secondary phase particles. The yield strength increased with the increasing initial temperature of furnace cooling due to the combined effect of small-sized secondary phase particles inside the grains and large-sized ones located at the grain boundaries. The occurrence of intergranular cracking, primarily caused by the presence of large-sized secondary phase particles at the grain boundaries, was the main factor responsible for the significant reduction in total elongation. The small-sized secondary phase particles inside the grains inhibited deformation twinning, while the large-sized ones at the grain boundaries weaken the binding force between grain boundaries, which ultimately resulted in a decrease in cryogenic impact toughness. The implementation of two-stage cooling processes effectively mitigated the trade-off between yield strength and cryogenic impact toughness, offering a novel approach and valuable insights for optimizing the strength and toughness characteristics of cryogenic high-Mn steels.

Atomic-scale understanding of twin intersection rotation and ε-martensite transformation in a high Mn twinning-induced plasticity steel

Twin intersections have been reported to significantly impact on macroscopic properties by relaxing localized stress concentration and accommodating strain. Here the atomic-scale structural characterization of the twin intersection region in a deformed high Mn twinning-induced plasticity (TWIP) steel was conducted using high-resolution transmission electron microscopy. Detailed microstructural features were revealed at the twin intersection region, including twin intersection rotation, low angle grain boundary and ε-martensite. The twin intersections were observed to preserve the face-centered cubic structure, which exhibits deviation angles of ∼0–15o with respect to the barrier twin depending on the localized stress concentration. Interestingly, the ε-martensite was observed in the vicinity of the twin intersection, which shows a wedge shape, distinct from the usual plate-like deformation-induced martensite. Therefore, we identified here a novel stress relaxation mechanism at the twin intersection region. Based on the detailed microstructural characterization of the twin interaction region at the atomic scale, the fundamental dislocation mechanisms of twin transmission and interaction were discussed. The research thus advances the understanding of twin intersection rotation and ε-martensite transformation by twin-twin interactions.

Effect of grain size on dynamic strain aging behavior of C-bearing high Mn twinning-induced plasticity steel

In this work, the effects of grain size on the tensile properties, serrated flow, Portevin-Le Châtelier (PLC) band evolution, dislocation density, and microstructure evolution of Fe–22Mn-0.6C steel were investigated using digital image correlation (DIC), X-ray diffraction (XRD), transmission electron microscope (TEM) and monotonic tensile tests. Results show that the yield strength, tensile strength, and work hardening value of Fe–22Mn-0.6C steel increase whereas its elongation decreases with decreasing grain size. The critical strain of serration decreases whereas the amplitude of serration increases with decreasing grain size. Moreover, the plateau of serrations is larger and that the strain concentration within PLC bands is more severe in fine-grained steel than in coarse-grained steel. The dynamic strain aging (DSA) behavior of the steel is enhanced by grain refinement. The speed of PLC band movement decreases faster in fine-grained steel than in coarse-grained steel, which results in a decrease in plasticity. XRD and TEM analyses show that the high dislocation density and unique twin substructure may be the main reasons for the strong DSA phenomenon observed in fine-grained steel.

Precipitation behavior of carbides and cryogenic toughness in heat-affected zone of high-Mn twinning-induced plasticity steel welded joint

The newly designed cryogenic high-Mn steel plates (Fe-23Mn-1.5Al-0.45C) were joined by metal inert gas (MIG) multi-pass welding process. To evaluate the inhomogeneity of microstructure and cryogenic toughness in heat-affected zone (HAZ) of welded joint, the impact samples were prepared by zonal sampling method. The microstructure features and embrittlement mechanism in HAZ of welded joint were systematically investigated. The results reveal that grains obviously grew in the coarse-grain HAZ (CGHAZ). C element segregated and nano-sized lamellar and granular (Cr, Mn) 7 C 3 carbides precipitated at the grain boundary in HAZ, and the size of precipitate carbides in CGHAZ and fine-grain HAZ (FGHAZ) was about 100 nm and 40 nm, respectively. The interface between (Cr, Mn) 7 C 3 and γ-austenite matrix in CGHAZ was incoherent with the lattice misfit of 45.13%. Higher fraction of low angle grain boundaries (LAGBs) appeared in CGHAZ (25.89%) and FGHAZ (11.99%), compared with the base metal (4.33%). Such microstructural differences can degrade the impact toughness in CGHAZ. From cryogenic Charpy impact tests, the impact toughness in different zones of the joint at each test temperature follows this sequence: base metal > fine-grain HAZ > coarse-grain HAZ > fusion line. These results can provide a fundamental understanding and guidance on the brittle failure of cryogenic high-Mn steel welded joint during service.

Strength–ductility enhancement in multi-layered sheet with high-entropy alloy and high-Mn twinning-induced plasticity steel

In this study, a high-entropy-alloy-cored multi-layered sheet (MLS) clad with an austenitic high-Mn twinning-induced-plasticity steel (HEA/high-Mn MLS) is proposed and its room- and cryogenic-temperature tensile behaviors were investigated. The MLS was fabricated using a commercial roll-bonding procedure. Its interface was well bonded without critical defects such as pores or voids. During the MLS fabrication and subsequent annealing, the interdiffusion of alloying elements and interfacial friction resulted in the formation of ε-martensite and V-rich carbides in the high-Mn and HEA layers, respectively, as well as grain refinement. Nevertheless, the interface remained strongly bonded after the tensile deformation at room and cryogenic temperatures, thereby providing larger strength and elongation than the values calculated based on the rule of mixtures. The improvement in tensile properties beyond the calculated values was interpreted by the activation of deformation twins, transformation-induced plasticity, and generation of geometrically necessary dislocations in the HEA/high-Mn interfacial region. Deformation twins were populated at both high-Mn and HEA layers at room temperature. However, at cryogenic temperatures, ε-martensite and body-centered cubic martensite were additionally formed in the high-Mn and HEA layers. Thus, both strength and ductility were considerably improved. In addition, the consequent good strength–ductility balance was comparable to or better than those of other HEAs or medium-entropy alloys. This indicates that our MLS can be an attractive design strategy to tailor various properties by controlling the thickness fraction of each layer and develop strong alloys for cryogenic applications.

Effect of Discontinuous Yielding on the Strain Hardening Behavior of Fine-Grained Twinning-Induced Plasticity Steel

The yielding of a high Mn twinning-induced plasticity steel was examined in three fine-grained specimens recrystallized at 700°C for 5 min with different cooling conditions. While the stress-strain curves of furnace-cooled and air-cooled specimens exhibit a stress drop at yielding, the drop was not observed in the water-quenched specimen. A simple analysis of the displacement data indicates the occurrence of localized deformation at the beginning of the plastic deformation in the three tensile specimens with different cooling conditions. The localized deformation of all three specimens was confirmed as Lüders strain by digital image correlation (DIC) analysis. Based on this observation, the role of yielding behavior on the strain hardening rate evolution at an early stage of the tensile deformation was discussed.

Corrosion Mechanisms of High-Mn Twinning-Induced Plasticity (TWIP) Steels: A Critical Review

Twinning-induced plasticity (TWIP) steels have higher strength and ductility than conventional steels. Deformation mechanisms producing twins that prevent gliding and stacking of dislocations cause a higher ductility than that of steel grades with the same strength. TWIP steels are considered to be within the new generation of advanced high-strength steels (AHSS). However, some aspects, such as the corrosion resistance and performance in service of TWIP steel materials, need more research. Application of TWIP steels in the automotive industry requires a proper investigation of corrosion behavior and corrosion mechanisms, which would indicate the optimum degree of protection and the possible decrease in costs. In general, Fe−Mn-based TWIP steel alloys can passivate in oxidizing acid, neutral, and basic solutions, however they cannot passivate in reducing acid or active chloride solutions. TWIP steels have become as a potential material of interest for automotive applications due to their effectiveness, impact resistance, and negligible harm to the environment. The mechanical and corrosion performance of TWIP steels is subjected to the manufacturing and processing steps, like forging and casting, elemental composition, and thermo-mechanical treatment. Corrosion of TWIP steels caused by both intrinsic and extrinsic factors has posed a serious problem for their use. Passivity breakdown caused by pitting, and galvanic corrosion due to phase segregation are widely described and their critical mechanisms examined. Numerous studies have been performed to study corrosion behavior and passivation of TWIP steel. Despite the large number of articles on corrosion, few comprehensive reports have been published on this topic. The current trend for development of corrosion resistance TWIP steel is thoroughly studied and represented, showing the key mechanisms and factors influencing corrosion processes, and its consequences on TWIP steel. In addition, suggestions for future works and gaps in the literature are considered.

Corrosion behaviour of resistance-spot-welded high-Mn austenitic TWIP steel

ABSTRACT High-Mn twinning-induced plasticity (TWIP) steels have been proposed to meet higher passenger safety and crash-safe vehicle frame requirements demanded by the automotive industry. The corrosion behaviour of welded Fe–30Mn–5Al–0.5C–6Cr TWIP steel using the resistance-spot-welding (RSW) technique was studied. Welds showed a microstructure consisting of primary austenite dendrites and interdendritic δ-ferrite in both fusion zone (FZ) and base metal (BM), which is in agreement with an austenitic-ferritic (AF) solidification mode. The results reveal that the refinement of the microstructure in the weld region does not significantly affect the corrosion resistance of this region, having a low corrosion current density of the order of 1 μA cm–2. It was concluded that Cr-rich second phase particles along the austenite/δ-ferrite interface can lead to Cr-depleted zones, increasing pitting corrosion susceptibility. It was found that the minimum Cr content at the Cr-depleted zones is nearly independent of the δ-ferrite volume fraction.

Enhancement of grain structure and mechanical properties of a high-Mn twinning-induced plasticity steel bearing Al–Si by fast-heating annealing

In this study, a cold-rolled Fe-0.01C-21.3Mn–3Al–3Si (wt.%) TWIP steel was undergone a fast-heating (FH) annealing at high temperatures of 1000–1200 °C and 2 s soaking time for grain refinement and controlling the phase structure and thereby to enhance the mechanical properties. For comparison, recrystallization annealing was conducted at lower temperatures of 650 and 700 °C for 180 s. The microstructural evolution of the FH annealed steel was surveyed using electron backscatter diffraction. The strain hardening behavior of the FH structures was studied by tensile tests. The tensile flow curves were also predicted by a phenomenological model based on the evolution of dislocation density during deformation. Fine mainly austenitic structure was promoted by FH annealing at 1000 and 1100 °C. At the lower temperatures of 650 and 700 °C, bands of finer grains, indicative of some inhomogeneity, were evident in the mostly austenitic recrystallized microstructure. However, at 1200 °C, the structure consisted of coarse austenite and ferrite with almost equal fractions. The FH annealed structures exhibited a remarkable improvement in the mechanical properties (a better combination of yield and tensile strength and ductility) compared to conventional long annealing cycles.

Fusion Weldabilities of Advanced High Manganese Steels: A Review

A large amount of manganese has been added to next-generation advanced high strength steels for automotive applications. The increased manganese content changes the microstructural and mechanical characteristics by varying both the stacking fault energy and the austenite stability, but it is known to deteriorate weldability. The current review provides a general strategy to address several problems in fusion welds of advanced high manganese steels from the viewpoint of microstructural transitions, plasticity mechanism, and mechanical properties based on the welding metallurgy. The importance of research on the weldabilities of advanced Mn steels was highlighted. Among the representative types of advanced manganese steels, this review focuses on high Mn twinning-induced plasticity steel, high Mn austenitic lightweight steel, and medium Mn transformation-induced plasticity steel. Specifically, this review suggests fundamental concepts for designing each steel with improved weld characteristics by presenting many research results on the correlations between the microstructure and mechanical properties of welds and heat-affected zones for these three different steel groups.

Effect of CO2 and O2 Mixed Injection on the Decarburization and Manganese Retention in High-Mn Twinning-Induced Plasticity Steels

In order to achieve decarburization and manganese retention simultaneously, CO2 was introduced in the smelting process of high-Mn twinning-induced plasticity (TWIP) steel. The effect of CO2 and O2 mixed injection on the decarburization and manganese retention was carried out based on the thermodynamics calculation and experimental research. The calculation results showed that the evaporation loss of manganese decreased dramatically with increasing CO2 blowing proportion, from the value close to 38 kg/t at pure O2 blowing to the value close to 4.4 kg/t at 40 vol pct CO2 proportion. The oxidation loss of manganese decreased from 70.5 to 41.7 kg/t when the CO2 blowing proportion increased from 0 to 40 vol pct. Additionally, the obtained data of the present experiments indicated that the introduction of CO2 could significantly lower the manganese loss and improve decarburization efficiency. The decarburization and manganese retention could be achieved simultaneously by replacing the partial oxygen with CO2 in the injected gas. Moreover, increasing the flow rate of the CO2-O2 gas mixture and increasing the melt temperature would improve decarburization efficiency but deteriorate the retention of manganese in the meantime.

Orientation Dependence on Plastic Flow Behavior of Hydrogen-Precharged Micropillars of High-Mn Steel

This study investigated the dependence of grain orientation on the hydrogen embrittlement (HE) characteristics of high-Mn twinning-induced plasticity steels. Single-crystal micropillars were fabricated to represent the five major texture components of face-centered cubic structure: brass, Goss, copper, cube, and S components. These were classified into three groups for discussion based on the microstructural and mechanical characteristics. The copper and cube micropillars showed the lowest HE resistance because of an exclusive formation of twin boundaries that led to a localized hydrogen concentration. Brass and Goss micropillars revealed multiple slips with increasing strain, which was different from the case of S orientation that exhibited the activation of single slip system. The results of this work suggest that increasing frequency of S component grains would enhance HE resistance owing to the inhibited twinning and suppressed dislocation mobility. This is supported by the superiority of S/cube bicrystal micropillar to Goss/cube micropillar in terms of HE resistance.

A novel top-down approach to make bulk nanostructured metal with low stacking fault energy

We report a novel method, named as stepping refining (STR) processing, to prepare three-dimensional nanograined bulk of high-Mn twinning-induced plasticity (TWIP) steel for the first time. After STR processing, the average grain size reduces to 20 nm—which is smaller than any reported grain size of TWIP steel—and the hardness reaches to 5.8 GPa, approximately 0.9 GPa above that of its nanotwinned counterpart with average twin-boundary spacing of 14 nm. The coarsening temperature of both nanograined and nanotwinned TWIP steels exceeds 350 °C, due to the low energy stored at grain/twin boundaries.

Investigation of the Performance of Flow Models for TWIP Steel

Modeling of metal processing requires constitutive laws able to represent the experimental material behavior. Of the large number of available empirical constitutive equations, only a subset may be fitted accurately to given experimental data. The present work is aimed at identifying the equations that can be used to model the ambient temperature mechanical behavior of high Mn twinning-induced plasticity (TWIP) steels. These are fitted to experimental data for TWIP900 and further compared in terms of their ability to predict springback. The reference springback value is determined experimentally for the same material. The study provides guidelines for the selection of the constitutive model in forming simulations for this type of steel.

Design of Fully Austenitic Medium Manganese Steels

Due to their higher ferrite potential compared to high Mn twinning-induced plasticity (TWIP) steels, medium Mn steels usually exhibit austenitic-ferritic microstructures, which makes them suitable for third-generation advanced high-strength steel applications. Nevertheless, the strain hardening characteristics of medium Mn steels are inferior to those of fully austenitic high Mn steels. The present work introduces alloy design strategies to obtain fully austenitic medium Mn steels capable of the TWIP effect. To achieve a fully austenitic microstructure, the martensite start temperature is reduced by raising the C concentration to above 1 mass-%, which in turn facilitates the formation of cementite. The formation of cementite during cooling from austenitization temperature is counteracted by alloying with Al. Microstructural examination of slowly-cooled Fe−Mn−Al−C and Fe−Mn−C steels indicated that Al changes the morphology of intergranular cementite from plate-shaped to equiaxed.

Improvement of circumferential ductility by reducing discontinuities in a high-Mn TWIP steel weldment

Recently, high-frequency electrical resistance welded (HF-ERW) pipes have been introduced as an application of high-Mn twinning induced plasticity (TWIP) steels. However, the excellent ductility of the TWIP steels has been underutilized in expandable pipe applications due to discontinuities in the weldment. This work presents two key ways to improve the pipe expandability or circumferential ductility. In-depth microstructural analysis and flaring tests were conducted to identify bond line discontinuities and to evaluate the circumferential ductility. These results indicate that ductility formed to be strongly related to the welding atmosphere and post-weld heat treatment (PWHT) conditions. The welding atmosphere was related to oxide defects, while the PWHT conditions were related to intrinsic discontinuities. The distribution and amount of penetrator defects along the bond line are the main factors related to the deterioration of the circumferential ductility, which is affected by the sheet metal compositions and decreased remarkably at low PO2 content. We also suggest that PWHT is necessary for improving the ductility by inducing changes in the bond line texture and hardness distribution. The results were verified based on changes in the crack path propagation.

Effects of Strain Rate and Temperature on Tensile Properties of High Mn Twinning Induced Plasticity Steels

Four types of high Mn TWIP(Twinning Induced Plasticity) steels were fabricated by varying the Mn and Al content, and the tensile properties were measured at various strain rates and temperatures. An examination of the tensile properties at room temperature revealed an increase in strength with increasing strain rate because mobile dislocations interacted rapidly with the dislocations in localized regions, whereas elongation and the number of serrations decreased. The strength decreased with increasing temperature, whereas the elongation increased. A martensitic transformation occurred in the 18Mn, 22Mn and 18Mn1.6Al steels tested at −196 oC due to a decrease in the stacking fault energies with decreasing temperature. An examination of the tensile properties at −196 oC showed that the strength of the non-Al added high Mn TWIP steels was high, whereas the elongation was low because of the martensitic transformation and brittle fracture mode. Although a martensitic transformation did not occur in the 18Mn1.9Al steel, the strength increased with decreasing temperature because many twins formed in the early stages of the tensile test and interacted rapidly with the dislocations.

Weldability of High-Mn Austenitic Twinning-Induced Plasticity (TWIP) Steel Microalloyed with Nb

ABSTRACTThe objective of this research work is to study the weldability of a Nb microalloyed TWIP steel through welding nuggets generated by Gas Tungsten Arc Welding process. Weldability was examined by microstructural changes in the fusion zone (FZ) and heat affected zone (HAZ) using light optical metallography (LOM), segregation in the nuggets was evaluated using elemental mappings of chemical analysis by Scanning Electron Microscopy and Electron Dispersive Spectroscopy (SEM-EDS), phase transformations were evaluated using X-ray diffraction (XRD) and the hardness properties were examined using Vickers microhardness testing (HV25). Experimental results show that microstructure of welding nuggets consists of austenitic dendritic grains in the FZ and equiaxed grains in the HAZ. FZ width and HAZ grain growth tend to increase as the heat input increases. Additionally, the studied Nb-containing TWIP steel showed segregation in the FZ, where Mn and Si segregated in the interdendritic regions, while Al and C preferentially segregated in dendritic areas. In general, the data obtained by XRD indicated that GTAW process did not affect austenite stability. Finally, the welding nuggets of studied TWIP steel showed lower microhardness values than the as-solution condition (starting condition). However, the heat affected zone showed hardened areas, which are associated with NbC precipitation hardening.

Deformation behaviour of low carbon high Mn twinning-induced plasticity steel

The present study deals with the deformation behaviour of low carbon and high manganese twinning-induced plasticity (TWIP) steel (Fe–21Mn–3Si–3Al–0.06C, wt%) through microstructural investigation. Low carbon with high manganese along with the addition of aluminium in TWIP steel results in lowering of specific weight with higher strain hardening due to the formation of mechanical twins during deformation. The full austenite phase is obtained after solution treatment and deformation twins appear and austenite grains become flattened during application of 10% to 50% cold deformation. The annealing twins are relatively coarser compared to the newly formed deformation twins. With the increasing amount of cold deformation, deformation twins and dislocation density are increased. Deformation twinning can be considered to be the dominant deformation mechanism during the course of cold rolling applied in the present study. The cold deformation results in the evolution of dislocation substructure, stacking faults, deformation twins and twin–dislocation interaction, which may be correlated with the lower stacking fault energy (∼24 mJ/m2) of the investigated steel. Excellent combination of strength and ductility has been obtained in the present TWIP steel with a small rolling reduction of 10% and 30%. With the increasing amount of cold deformation, tensile strength notably increases and maximum tensile strength is obtained at 50% cold-deformed sample along with the diminutive sacrifice of the ductility.