High-manganese Twinning-induced Plasticity Steel Research Articles

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.

The effect of loading direction and pre-deformation on the low-cycle fatigue behavior of high-manganese twinning-induced plasticity steels

The low-cycle fatigue performance of high-manganese twinning-induced plasticity (TWIP) steels with the specimen axial direction of 0°, 22.5°, 45°, 67.5°, and 90° to the rolling direction is investigated in the range of the total strain amplitudes (Δε/2) of 0.6 % ∼ 4.4 % and two types of pre-deformed conditions (i.e., tension and tension-unloading). Experimental results show that the dependence of the fatigue life on the specimen orientation is obvious for lower strain amplitudes, and it gradually reduces with the increase of the applied strain level. The 0° orientations show the highest sensitivity to the pre-deformation process, and the tension-unloading pre-deformation exhibits the most severe change of the texture at fracture.

Hydrogen-induced hardening of a high-manganese twinning induced plasticity steel

High-manganese twinning-induced plasticity (TWIP) steels exhibit high strain hardening, high tensile strength, and high ductility, which make them attractive for structural applications. At low tensile strain rates, TWIP steels are prone to hydrogen embrittlement (HE). Here though, we study the hardening and strengthening resulting from electrochemical hydrogen-charging of a surface layer of a Fe-26.9Mn-0.28C (wt.%) TWIP steel. We observed a 20% increase in yield strength following the electrochemical hydrogen-charging, accompanied by a reduction in ductility from 75% to 10% at a tensile strain rate of 10−3s−1. The microstructural evolution during tensile deformation was examined at strain levels of 3%, 5% and 7% by electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) to study the dislocation structure of the hardened region. As expected, the microstructure of the hydrogen-hardened and the uncharged regions of the material evolve differently. The uncharged areas show entangled dislocation structures, indicating slip from a limited number of potentially coplanar slip systems. In contrast, hydrogen segregated to the grain boundaries, revealed by the deuterium-labelled atom probe tomography, delays the dislocation nucleation by blocking dislocation sources at the grain boundaries. The charged areas hence first show the formation of cells, indicating dislocation entanglement from more non-coplanar slip systems. With increasing strain, these cells dissolve, and stacking faults and strain-induced ε-martensite are formed, promoted by the presence of hydrogen. The influence of hydrogen on dislocation structures and the overall deformation mechanism is discussed in details.

Transmission Kikuchi Diffraction Mapping Induces Structural Damage in Atom Probe Specimens.

Measuring local chemistry of specific crystallographic features by atom probe tomography (APT) is facilitated by using transmission Kikuchi diffraction (TKD) to help position them sufficiently close to the apex of the needle-shaped specimen. However, possible structural damage associated to the energetic electrons used to perform TKD is rarely considered and is hence not well-understood. Here, in two case studies, we evidence damage in APT specimens from TKD mapping. First, we analyze a solid solution, metastable β-Ti-12Mo alloy, in which the Mo is expected to be homogenously distributed. Following TKD, APT reveals a planar segregation of Mo among other elements. Second, specimens were prepared near Σ3 twin boundaries in a high manganese twinning-induced plasticity steel, and subsequently charged with deuterium gas. Beyond a similar planar segregation, voids containing a high concentration of deuterium, i.e., bubbles, are detected in the specimen on which TKD was performed. Both examples showcase damage from TKD mapping leading to artefacts in the distribution of solutes. We propose that the structural damage is created by surface species, including H and C, subjected to recoil from incoming energetic electrons during mapping, thereby getting implanted and causing cascades of structural damage in the sample.

Reveal Hydrogen Behavior at Grain Boundaries in Fe–22Mn–0.6C TWIP Steel via In Situ Micropillar Compression Test

In this study, the effect of hydrogen on dislocation and twinning behavior along various grain boundaries in a high-manganese twinning-induced plasticity steel was investigated using an in situ micropillar compression test. The compressive stress in both elastic and plastic regimes was increased with the presence of hydrogen. Further investigation by transmission electron backscatter diffraction and scanning transmission electron microscope demonstrated that hydrogen promoted both dislocation multiplication and twin formation, which resulted in higher stress concentration at twin–twin and twin–grain boundary intersections.

Consequences of Deep Rolling at Elevated Temperature on Near-Surface and Fatigue Properties of High-Manganese TWIP Steel X40MnCrAl19-2

Due to pronounced work-hardening induced by the complex interplay of deformation mechanisms such as dislocation slip, twinning and/or martensitic phase transformation, high-manganese steels represent a class of materials well-suited for mechanical surface treatment. In the present study, the fatigue behavior of a high-mangsanese steel showing twinning-induced plasticity (TWIP) effect at room temperature (RT) was investigated after deep rolling at 550 °C. Results are compared to a former study discussing the behavior after RT deep rolling. Evolution of the near-surface microstructure was analyzed by scanning electron microscopy (SEM), microhardness measurements and residual stress depth profiles obtained by X-ray diffraction (XRD). Both uniaxial tensile tests and uniaxial tension-compression fatigue tests have been conducted in order to rationalize the macroscopic material behavior. Following deep rolling at 550 °C, SEM measurements employing electron backscatter diffraction (EBSD) revealed a heavily deformed surface layer as well as localized deformation twinning. Specimens showed inferior hardness and residual stress depth profiles when compared to RT deep rolled counterparts. Tensile tests indicated no difference between the conditions considered. Fatigue properties however were improved. Such behavior is rationalized by a more stable residual stress state induced by dynamic strain aging.

Investigating the dislocation reactions on \u03a33{111} twin boundary during deformation twin nucleation process in an ultrafine-grained high-manganese steel

Some of ultrafine-grained (UFG) metals including UFG twinning induced plasticity (TWIP) steels have been found to overcome the paradox of strength and ductility in metals benefiting from their unique deformation modes. Here, this study provides insights into the atomistic process of deformation twin nucleation at Σ3{111} twin boundaries, the dominant type of grain boundary in this UFG high manganese TWIP steel. In response to the applied tensile stresses, grain boundary sliding takes place which changes the structure of coherent Σ3{111} twin boundary from atomistically smooth to partly defective. High resolution transmission electron microscopy demonstrates that the formation of disconnection on Σ3{111} twin boundaries is associated with the motion of Shockley partial dislocations on the boundaries. The twin boundary disconnections act as preferential nucleation sites for deformation twin that is a characteristic difference from the coarse-grained counterpart, and is likely correlated with the lethargy of grain interior dislocation activities, frequently seen in UFG metals. The deformation twin nucleation behavior will be discussed based on in-situ TEM deformation experiments and nanoscale strain distribution analyses results.

Thermodynamic activity of FeO in CaO–SiO2–MnO–FeO–MgO molten slags

ABSTRACT To understand the thermodynamic characteristics of the melting process for high manganese twinning-induced plasticity steel, the activity coefficient of FeO in molten slag of MnO–CaO–SiO2–FeO–MgO slag system with up to 52 mass% MnO was measured at 1450°C by the experiments of the iron equilibrium between liquid silver and molten slag under the mixed gas atmosphere of CO, CO2 and Ar. The effects of MnO, FeO and the basicity on the activity coefficient of FeO in molten slag were discussed. The relationships between the activity coefficients of FeO, the concentrations of FeO and MnO and the basicity ((mass% CaO + mass% MgO)/mass% SiO2) were investigated by the regression analysis method. The results show that the activity coefficient of FeO was proportional to the reciprocal of FeO concentration, and increased slightly with the increasing MnO, but slowly decreased by increasing the basicity under the research conditions.

Tailoring Extra-Strength of a TWIP Steel by Combination of Multi-Pass Equal-Channel Angular Pressing and Warm Rolling

Increasing the yield stress of twinning-induced plasticity (TWIP) steels is a demanding task for modern materials science. This aim can be achieved by microstructure refinement induced by heavy straining. We feature the microstructural evolution and mechanical performance of a high-manganese TWIP steel subjected to deformation treatment by different combinations of equal channel angular pressing (ECAP) and rolling at different temperatures. The effect of microstructure on the tensile properties of the steel subjected to the multi-pass ECAP process and to subsequent rolling is reported as well. We show that the combined deformation procedure allows us to further increase the strength of the processed workpieces due to a gradual transition from a banded structure to a heterogeneous hierarchical microstructure consisting of fragments, dislocation configurations and nano- and micro-twins colonies. Rolling of multi-pass ECAP specimens at 375 °C allowed us to achieve an extraordinary strength, the highest among all the investigated cases, while the best trade-off between yield strength and elongation to failure was reached using multi-pass ECAP followed by rolling at 500 °C. This study shows a great potential of using combined deformation techniques to enhance the mechanical performance of TWIP steels.

Effect of dew point on hot-dip galvanizing behavior of a high-manganese TWIP steel for automotive application

The effect of dew points (− 50, − 10 and + 10 °C) on the galvanizing properties of a high-manganese twinning-induced plasticity (TWIP) steel was studied. Scanning electron microscopy (SEM), glow discharge optical emission spectrometry (GDOES) and X-ray photoelectron spectroscopy (XPS) were used for microscopic observation and qualitative analysis of the interfacial layer between the steel surface and the zinc layer after hot-dip galvanizing. SEM analysis results show that three different morphologies of metallic oxides are formed on the interfacial layer under the different dew points. GDOES results show that Al is present in the molten zinc, reacting with Fe on the steel surface to form Fe2Al5, which can increase the galvanizing properties. XPS results show that the valence states of Mn in the interfacial alloy layer are Mn2+ and Mn4+, and the valence states of Fe are Fe0, Fe2+ and Fe3+. The experimental results show that the hot-dip galvanizing performance is the best at − 10 °C and the formation of Mn and Fe intermetallic oxides has a bad effect on hot-dip galvanizing behavior of the high-manganese TWIP steel. The types of the formed surface oxides (MnO, Mn3O4, Mn2O3, FeO, and Fe2MnO4) on the surface of the steel sheet are confirmed. It can obtain the best hot-dip galvanizing performance of the high-manganese TWIP steel by controlling the dew point from − 10 to − 5 °C.

Unravelling the effect of hydrogen on microstructure evolution under low-cycle fatigue in a high-manganese austenitic TWIP steel

We systematically investigated the effect of hydrogen on the low-cycle fatigue (LCF) behaviour of a Fe–28Mn-0.3C (wt.%) twinning-induced plasticity (TWIP) steel by comparing the fatigue microstructure of samples with and without hydrogen pre-charging using electron channelling contrast imaging (ECCI) and cross-correlation electron backscatter diffraction (CC-EBSD). The results reveal that a complex interplay of several hydrogen-microstructure interaction mechanisms is involved in the observed LCF behaviour of the studied TWIP steel with hydrogen pre-charging; first, hydrogen assists the nucleation of stacking faults (SFs) and deformation-induced ε-martensite in the studied TWIP steel by reduction of local stacking fault energy (Suzuki effect) and stabilization of the hexagonal close-packed (hcp) ε-phase. The evolution of fatigue dislocation pattern is strongly retarded in the presence of hydrogen. The rapid formation of ε-martensite leads to a stronger cyclic hardening. Meanwhile, the impingement of ε-martensite plates at GBs causes high local stress concentration, which plays a dominant role in the observed fatigue cracking. The crack opening angle indicates that the exact associated hydrogen embrittlement mechanism is the hydrogen-enhanced decohesion (HEDE) mechanism instead of the hydrogen-enhanced localized plasticity (HELP) mechanism. Among different GB types, annealing twin boundary shows the highest immunity to hydrogen-related fatigue cracking.

Combined deformation twinning and short-range ordering causes serrated flow in high-manganese steels

The isolated impact of deformation twinning and short-range ordering on the serrated flow phenomena in two high-manganese twinning-induced plasticity steels, an X60Mn18 and an X30MnAl17-1 alloy, was studied via quasistatic tensile tests at room temperature and at 150 °C. In order to identify the effect of prior deformation twins, both alloys were investigated in recrystallized and non-recrystallized states. The experimental data suggest that the inhibition of the movement of trailing partial dislocations by short-range ordered clusters results in a promotion of deformation twinning. A multi-scale model is introduced to connect short-range ordering theory, deformation twinning, the formation and propagation of deformation bands and the occurrence of serrated flow.

Hot Deformation Behavior of V Micro-Alloyed TWIP Steel During Hot Compression

High manganese twinning induced plasticity (TWIP) steel is an attractive material for automotive applications as its use could result in an improved vehicle fuel efficiency and a superior passenger safety. Due to the limited research on the hot deformation behaviour of High Mn steel, the selection of suitable operating conditions for the hot rolling process is challenging. The present contribution focusses on the hot deformation behaviour and the dynamic recrystallization kinetics of V micro-alloyed high manganese TWIP steel, by means of single-hit compression test in the temperature range of 850–1000 °C and the strain rate range of 0.1–10 s−1. The activation energy for hot deformation and the processing map of a V-free TWIP steel and a V-added TWIP steel were compared by analysing their stress–strain curves. The V-added TWIP steel exhibited a higher activation energy than the V-free TWIP steel, i.e. 383.4 kJ/mol versus 372.5 kJ/mol. Processing maps based on a dynamic material model indicated that the hot workability of TWIP steel was decreased by micro-alloying with V. The effect of V on the hot deformation behaviour of TWIP steels was also analysed by means of its effect on the microstructure using the SEM-EBSD technique. The V-added TWIP steel was characterized by a higher peak stress at a lower peak strain as compared to the V-free TWIP steel, indicating that the onset of dynamic recrystallization was accelerated by the addition of V. The rapid dynamic recrystallization kinetics resulted in a smaller recrystallized grain size in the hot deformed microstructure of the V-added TWIP steel.

Investigation of nanoscale twinning in an advanced high manganese twinning-induced plasticity steel

A twinning induced plasticity (TWIP) steel, developed for automotive applications, was characterized down to the nanoscale to investigate the nature of the strengthening mechanisms. Both tensile-deformed and non-deformed materials were studied by light optical microscopy, X-ray diffraction, electron backscatter diffraction, and transmission electron microscopy. The investigated TWIP steel showed extensive twinning upon deformation. With high-resolution transmission electron microscopy, nano-twins as small as 3 nm in width were observed, and large-angle convergent-beam electron diffraction identified the pole mechanism as one of the twinning mechanisms in this TWIP steel. This study emphasizes that a thoughtful combination of techniques is necessary to fully capture the microstructure of this TWIP steel and explain the origin of superior mechanical properties compared to other TWIP steel grades.

The Role of Grain Orientation and Grain Boundary Characteristics in the Mechanical Twinning Formation in a High Manganese Twinning-Induced Plasticity Steel

In the current study, the dependence of mechanical twinning on grain orientation and grain boundary characteristics was investigated using quasi in-situ tensile testing. The grains of three main orientations (i.e., 〈111〉, 〈110〉, and 〈100〉 parallel to the tensile axis (TA)) and certain characteristics of grain boundaries (i.e., the misorientation angle and the inclination angle between the grain boundary plane normal and the TA) were examined. Among the different orientations, 〈111〉 and 〈100〉 were the most and the least favored orientations for the formation of mechanical twins, respectively. The 〈110〉 orientation was intermediate for twinning. The annealing twin boundaries appeared to be the most favorable grain boundaries for the nucleation of mechanical twinning. No dependence was found for the inclination angle of annealing twin boundaries, but the orientation of grains on either side of the annealing twin boundary exhibited a pronounced effect on the propensity for mechanical twinning. Annealing twin boundaries adjacent to high Taylor factor grains exhibited a pronounced tendency for twinning regardless of their inclination angle. In general, grain orientation has a significant influence on twinning on a specific grain boundary.

Influence of rolling asymmetry on the microstructure, texture and mechanical behavior of high-manganese twinning-induced plasticity steel

In this work, a Fe-23Mn-1.5Al-0.3C twinning-induced plasticity steel was subjected to both symmetric (SR) and asymmetric rolling (ASR) to investigate the influence of an additional shear strain component introduced during asymmetric rolling on the microstructure and texture evolution as well as on the mechanical properties. ASR was performed using a roll diameter ratio of 2:1. Electron backscatter diffraction, transmission electron microscopy, and X-ray diffraction were utilized for careful characterization of the microstructures and textures in the top, middle, and bottom layers of the sheets. The overall refinement of the microstructure was found to be more developed in the asymmetrically rolled specimens as compared to the symmetrically rolled ones. Furthermore, the top layer (at the smaller roll) revealed a stronger deformation with higher density of deformation twins compared to middle and bottom layer. Additionally, secondary twinning was activated during ASR, whereas only primary twinning was observed during SR. With increasing rolling degree (up to 40% thickness reduction) a weak Brass-type texture dominated by α-fiber (〈110〉//ND) texture components and the {123}〈634〉 S texture component developed during rolling using both processing techniques. As a result of the additional shear strain introduced during ASR, the complete texture rotated around the transverse direction by ~ 6°. It was shown that additional simple shear introduced by ASR can affect the mechanical properties, microstructure and texture development.

Effect of Cr on mechanical properties and corrosion behaviors of Fe-Mn-C-Al-Cr-N TWIP steels

By using scanning electron microscopy (SEM) equipped with electron back-scattered diffraction (EBSD) system, transmission electron microscopy (TEM) and CorrTest4 electrochemical workstation, effects of chromium content (1.35wt%–3.95wt%) on the mechanical properties and anti-corrosion behaviours of high manganese Fe-Mn-C-Al-Cr-N twinning-induced plasticity (TWIP) steels were studied. The results show that Cr content has an obvious influence on the mechanical properties and fracture behaviors of the high manganese TWIP steels. The yield and ultimate tensile strengths of the steel sheets were improved with increasing Cr content while the elongation was reduced. In addition, with the increase of Cr content, the fracture mode changed from ductile fracture pattern with coarse dimples and tear ridges (Cr content≤2.35%) to intergranular fracture (when Cr content is 3.95%). Furthermore, Cr content has a tremendous effect on anti-corrosion behaviors of the high manganese TWIP steels. The increase of Cr content enhanced the corrosion resistance of the annealed steel sheets by improving the proportion of low-angle boundary.

Properties of Resistance Spot-Welded TWIP Steels

High manganese TWIP (twinning-induced plasticity) steels are particularly attractive for automotive applications because of their exceptional properties of strength combined with an excellent ductility. However, the microstructure and properties of TWIP steels are affected by excessive thermal cycles, such as welding and heat treatment. This paper deals with characterization and understanding the effect of welding current and time on the mechanical properties and microstructure of the resistance spot welded TWIP steel. For this purpose, weld nugget diameter was evaluated and the hardness, tensile shear strength of the weldment, and failure mode of samples were also determined. It has been found that the tensile shear strength of the samples increased with increasing welding current and welding time without expulsion, which reduces the strength of the weldment. Tensile shear samples failed by a partial interfacial fracture mode for low-heat input welds. The pullout fractures were observed with a sufficient heat input without expulsion.

Characterization of Evolution of Microscopic Stress and Strain in High-Manganese Twinning-Induced Plasticity Steel

Characterization of Evolution of Microscopic Stress and Strain in High-Manganese Twinning-Induced Plasticity Steel

Microstructure and Mechanical Properties of High Manganese TWIP Steel after Thermo-Forming Processes

In recent years, the leading scientific centres focus their research on improvement of mechanical properties of steels used for car manufacturing. These steels belong to a so-called 2nd generation of steels showing above-the-average plasticity while maintaining high strength. Thanks to these properties, they may be used successfully in automotive, armaments or railway industries for elements absorbing energy of a collision and ensuring high rigidity of a structure owing to their resistance to breaking. These steels are called TWIP (Twinning Induced Plasticity) steels based on their hardening mechanism. In this paper, results of studies on the influence of parameters of thermo-plastic deformation on selected properties and structure of an X45MnAl20-3V austenitic steel showing the TWIP effect are presented. Moreover, an analysis of influence of the deformation on the structure of the studied steel in tensile tests has been carried out. The studied steel was manufactured by classic casting to a concast mould, obtaining ingots with dimensions of 100×100 mm, then subjected to rolling in 4 roll passes to a final thickness of 12 mm and 3 mm. The finish-rolling temperature was 950°C and the sheets were cooled in 2 media, i.e. in air and in water. It was confirmed that the studied steel belongs to the TWIP group of steels, with mechanical twinning being the prevailing process of hardening during deformation.