high-Mn Twinning-induced Plasticity Steel Research Articles

Superior strength and wear resistance of mechanically deformed High-Mn TWIP steel

In the present study, the mechanical and wear behaviour of the surface-mechanically treated high-manganese (high-Mn) twinning-induced plasticity (TWIP) steel were investigated. The TWIP alloy was first designed and fabricated via surface-mechanical attrition treatment (SMAT) system and the mechanical properties including strength, wear behaviour as well as the microstructural evolution were thereafter determined. Transmission electron microscopy (TEM) characterization revealed a typical dislocation as a result of the surface treatment as well as the formation of twin layers with a reduced stacking fault energy (SFE). Due to the ultra-fine grain refinement caused by plastic deformation during surface treatment, a microhardness value of 489 HV can be obtained after treatment. Likewise, the yield strength of the high-Mn TWIP steel could be enhanced from 360 MPa to 813 MPa and a reduction in elongation to failure of about 20 % can be achieved. The wear test showed that the treated TWIP steel possessed a reduced friction coefficient and improved wear resistance at different testing loads, attributed to the nanoscale refinement of grains induced during treatment. The strength, hardness, and wear resistance of the fabricated TWIP alloy improves significantly, thanks to surface treatment by SMAT.

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.

Cross-slip of extended dislocations and secondary deformation twinning in a high-Mn TWIP steel

Twinning-induced plasticity (TWIP) steels have become important materials in industry owing to the good combination of strength and ductility and high strain hardening rate. The excellent mechanical properties are highly related to the glide of dislocation and deformation twinning. However, the cross-slip behavior of the extended dislocation and the mechanism of deformation twinning are still controversial. Here, the partial dislocation motion and austenite twinning of a high-Mn steel at the early stage of deformation were investigated using in-situ tensile transmission electron microscope (TEM) technique. Results show that a large number of plane glide and cross-slip of extended dislocations can occur at the early stage of deformation. Extended dislocation nodes can be formed as a result of the reaction between adjacent extended dislocations on the same glide plane. In-situ tensile TEM experiments confirm two cross-slip models of partial dislocation: (1) the Friedel-Escaig model, cross-slip based on constriction of extend dislocation and re-dissociation; (2) the Fleischer model, cross-slip involving Lomer-Cottrell dislocation. Based on experimental results and energy calculations, it can be confirmed that the formation mechanism of austenite primary deformation twin induced by partial dislocations is different from that of secondary deformation twin. Grain boundary emits partial dislocation into the grain to form stable stacking fault which induces austenite primary deformation twin. The formation of secondary deformation twin is related to cross-slip of extended dislocation. Only the cross-slip of an extended dislocation containing a 90° partial dislocation can induce the formation of secondary deformation twin by introducing the Frank partial dislocation.

Hot working processing and microstructure characterisation of as-cast high manganese TWIP steel

ABSTRACT The hot compression of the as-cast Fe-21Mn-0.7C–0.1Si twinning induced plasticity (TWIP) steel was investigated at the deformation temperature range of 950–1100 °C and strain rates of 0.01–5 s−1, using a GleebleTM thermo-mechanical simulator, to determine the hot deformation behaviour and analyse the recrystallisation mechanism. The results showed that the flow curves were characterised by yield-point-elongation (YPE) and dynamic recrystallisation (DRX) as the principal restoration mechanisms. The activation energy (Q) was calculated to be 394.975 KJ mol−1, which implies that the recrystallisation is sluggish in the whole range of the deformation conditions. Furthermore, the processing maps were generated according to the dynamic material model (DMM). The processing map was subdivided into different domains for the microstructural observations. The ideal hot deformation parameters of as-cast high Mn TWIP steel were obtained in the deformation condition range of 1087–1098 °C/10−1.2-10−0.17 s−1. Moreover, the microstructure analysis revealed that the DRX grains were twinned and nucleated through the bulging mechanism at the serrated grain boundaries, accompanied by twin boundaries migration created in the DRX grains by growth accidents, which contributed significantly to the growth of the DRX.

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.

Hydrogen Embrittlement of Medium Mn Steels

Recent research efforts to develop advanced–/ultrahigh–strength medium-Mn steels have led to the development of a variety of alloying concepts, thermo-mechanical processing routes, and microstructural variants for these steel grades. However, certain grades of advanced–/ultrahigh–strength steels (A/UHSS) are known to be highly susceptible to hydrogen embrittlement, due to their high strength levels. Hydrogen embrittlement characteristics of medium–Mn steels are less understood compared to other classes of A/UHSS, such as high Mn twinning–induced plasticity steel, because of the relatively short history of the development of this steel class and the complex nature of multiphase, fine-grained microstructures that are present in medium–Mn steels. The motivation of this paper is to review the current understanding of the hydrogen embrittlement characteristics of medium or intermediate Mn (4 to 15 wt pct) multiphase steels and to address various alloying and processing strategies that are available to enhance the hydrogen-resistance of these steel grades.

Corrosion Behavior of High-Mn Austenitic Fe-Mn-Al-Cr-C Steels in NaCl and NaOH Solutions.

The corrosion behavior of austenitic Fe–Mn–Al–Cr–C twinning-induced plasticity (TWIP) and microband-induced plasticity (MBIP) steels with different alloying elements ranging from 22.6–30 wt.% Mn, 5.2–8.5 wt.% Al, 3.1–5.1 wt.% Cr, to 0.68–1.0 wt.% C was studied in 3.5 wt.% NaCl (pH 7) and 10 wt.% NaOH (pH 14) solutions. The results obtained using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques, alongside optical microscopy analysis, revealed pitting as the dominant corrosion mechanism in high-Mn TWIP steels. An X-ray diffraction analysis of the surface revealed that the main corrosion products were hematite (Fe2O3), braunite (Mn2O3), and hausmannite (Mn3O4), and binary oxide spinels were also identified, such as galaxite (MnAl2O4) and jacobsite (MnFe2O4). This is due to the higher dissolution rate of Fe and Mn, which present a more active redox potential. In addition, a protective Al2O3 passive film was also revealed, showing enhanced corrosion protection. The highest corrosion susceptibility in both electrolytes was exhibited by the MBIP steel (30 wt.% Mn). Pitting corrosion was observed in both chloride and alkaline solutions.

Effect of Thermo-Mechanical Processing on the Corrosion Behavior of Fe−30Mn−5Al−0.5C TWIP Steel

Electrochemical corrosion of thermo-mechanically processed (TMP) and recrystallized Fe−30Mn−5Al−0.5C twinning-induced plasticity (TWIP) steels containing 30 wt.% Mn was studied in a 1.0 wt.% NaCl electrolyte solution. The alkaline nature of the corrosion products containing manganese oxide (MnO) increases the dissolution kinetics of the TWIP steel in acid media, obtaining Mn2+ cations in solution, and producing the hydrogen evolution reaction (HER). X-ray photoelectron spectroscopy (XPS) surface analysis revealed an increased Al2O3 content of 91% in the passive layer of the recrystallized TWIP steel specimen, while in contrast only a 43% Al2O3 was found on the TMP specimen. Additionally, the chemical composition of the surface oxide layer as well as the TWIP alloy microstructure was analyzed by optical microscopy (OM) and scanning electron microscopy (SEM). The results indicate an enhanced corrosion attack for the TMP high-Mn TWIP steel.

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.

Shear and laser cutting effects on hydrogen embrittlement of a high-Mn TWIP steel

The influence of shear and laser cutting on hydrogen embrittlement of a hot-rolled Fe-18Mn-0.4C-1.2Al-1.8Cr TWIP (twinning induced plasticity) steel sheet in 3 mm thickness was investigated by hydrogen pre-charging and subsequent slow strain rate tests. Due to the severe plastic deformation during shearing, the ductility of the shear-cut specimen was significantly reduced in tensile test compared to the laser-cut specimen. After hydrogen pre-charging, the laser-cut specimen exhibited less susceptibility to hydrogen embrittlement than the shear-cut specimen. The fractographic analysis revealed that the shear-affected zone acted as crack initiation site during tensile test, which exhibited a flat fracture zone with high stress triaxiality. In hydrogen-charged condition, the flat fracture zone transited from large primary dimples to the mixed features with dimples and quasi-cleavage features.

Optimizing pulsed Nd: YAG laser welding of high-Mn TWIP steel using response surface methodology technique

In the present study, the microstructural and mechanical properties of laser beam-welded thin sheet twinning-induced plasticity (TWIP) steel were investigated. The pulsed neodymium: yttrium-aluminum-garnet (Nd: YAG) laser beam welding process parameters were modeled and optimized based on experimental data and statistical analysis using response surface methodology (RSM) technique. Process parameters range, i.e. the power input (2000–3000 W), welding speed (0.2–1 mm/min), and spot size (0.3–0.7 mm) were selected properly in order to obtain the desired mechanical properties. Main effects of each factor along with interaction effect with other factors were determined quantitatively. The predicted and actual values of the mechanical properties compared using analysis of variance (ANOVA) in order to verify the adequacy of the developed model. Optimal laser beam welding parameters were identified as the power input, welding speed and spot size of 2586 W, 0.53 mm/min, and 0.48 mm, respectively. Using parameters in the optimal conditions, a welding joint with tensile load of 2001 N (% 94 strength of the base metal) was obtained. In addition, the welding zone with an average grain size coarser than the one for the base metal and a random texture was identified.

Strain Hardening, Damage and Fracture Behavior of Al-Added High Mn TWIP Steels

The strain hardening and damage behavior of Al-added twinning induced plasticity (TWIP) steels were investigated. The study was focused on comparing two different alloying concepts by varying C and Mn contents with stacking fault energy (SFE) values of 24 mJ/m 2 and 29 mJ/m 2 . The evolution of microstructure, deformation mechanisms and micro-cracks development with increasing deformation was analyzed. Al-addition has led to the decrease of C diffusivity and reduction in tendency for Mn-C short-range ordering resulting in the suppression of serrated flow caused due to dynamic strain aging (DSA) in an alloy with 0.3 wt.% C at room temperature and quasi-static testing, while DSA was delayed in an alloy with 0.6 wt.% C. However, an alloy with 0.6 wt.% C showing DSA effect exhibited enhanced strain hardening and ductility compared to an alloy with 0.3 wt.% C without DSA effect. Twinning was identified as the most predominant deformation mode in both the alloys, which occurred along with dislocation glide. Al-addition has increased SFE thereby delaying the nucleation of deformation twins and prolonged saturation of twinning, which resulted in micro-cracks initiation only just prior to necking or failure. The increased stress concentration caused by the interception of deformation twins or slip bands at grain boundaries (GB) has led to the development of micro-cracks mainly at GB and triple junctions. Deformation twins and slip bands played a vital role in assisting inter-granular crack initiation and propagation. Micro-cracks that developed at manganese sulfide and aluminum nitride inclusions showed no tendency for growth even after large deformation indicating the minimal detrimental effect on the tensile properties.