Papers by Keyword: TWIP Steel

Abstract: High-Mn Twinning Induced Plasticity (TWIP) steels have superior mechanical properties, which make them promising materials in automotive industry to improve the passenger safety and the fuel consumption. The TWIP steels are characterized by high work hardening rates due to continuous mechanical twin formation during the deformation. Mechanical twinning is a unique deformation mode, which is highly governed by the stacking fault energy (SFE). The composition of steel alloy was Fe-18Mn-0.6C-1Al (wt.%) with SFE of about 25-30 mJ/m² at room temperature. The SFE ensures the mechanical twinning to be the main deformation mechanism at room temperature. The microstructure, mechanical properties, work hardening behaviour and SFE of the steel was studied at the temperature range of ambient≤T[°C]≤400^°C. The mechanical properties were determined using Instron tensile testing machine with 30kN load cell and strain rate of 10^-3s^-1 and the work hardening behaviour curves were generated using true stress and true strain data. The microstructure after deformation at different temperatures was examined using Zeiss Supra 55VP SEM. It was found that an increase in the deformation temperature raised the SFE resulting in the deterioration of the mechanical twinning that led to decrease not only in the strength but also in the total strain of the steel. A correlation between the temperature, the SFE, the mechanical twinning, the mechanical properties and the work hardening rate was also found.

Abstract: he softening and static recrystallization behavior of typical Fe-Mn-Si-Al series TWIP steel between high temperature deformation passes was investigated by two-pass hot compress deformation experiments on Gleeble-3500 thermal simulated test machine. The dynamic model of static recrystallization was built according to the experimental results. The investigation of the effects of deformation temperature, deformation rate and pre-deformation shows that deformation rate is the most effective parameter, and the increase of deformation and pre-deformation can also promote the static recrystallization. The calculated static recrystallization activation energy of TWIP steel is about 147kJ/mol through the dynamic equation built. The results of model predict conform to the experimental results.

Abstract: High-alloy cast CrMnNi steels exhibit depending on the chemical composition either transformation induced plasticity (TRIP-effect) or twinning induced plasticity (TWIP effect). Whereas the TRIP effect is caused by a martensitic phase transformation from the f.c.c austenitic phase into the b.c.c α-martensite phase via the formation of deformation bands with high stacking fault density the so-called ε-martensite, the TWIP effect is the result of mechanical twinning during plastic deformation. The occurrence of both effects as well as the underlying microstructural processes are strongly affected by the austenite stability, the stacking fault energy and/or the test temperature. However, the onset stress and the kinetics of these deformation processes are still unknown. The in-situ measurement of acoustic emission signals during the plastic deformation of materials is a powerful tool for the investigation of such dynamic microstructural processes and their kinetics. In the present study acoustic emission measurements were performed during tensile deformation at room temperature on a high-alloy cast TWIP steel. The AE investigations were completed by SEM investigations on the deformed specimens. The statistical and cluster analysis of acoustic emission signals reveals different patterns of acoustic emission signals, which are correlated with underlying microstructural processes.

Abstract: The aim of the present study was to investigate the role of deformation temperature on the active deformation mechanisms in a 0.6C-18Mn-1.5Al (wt%) TWIP steel. The tensile testing was performed at different temperatures, ranging from ambient to 400°C at a constant strain rate of 10^-3 s^-1. The microstructure characterization was carried out using a scanning electron microscopy. The deformation temperature revealed a significant effect on the active deformation mechanisms (i.e. slip versus twinning), resulting in different microstructure evolution and mechanical properties. At the room temperature, the mechanical twinning was the dominant deformation mechanism, enhancing both the strength and ductility. Dynamic strain aging (DSA) effect was observed at different deformation temperatures, though it was more pronounced at higher temperatures. The volume fraction of deformation twins significantly reduced with an increase in the deformation temperature, deteriorating the mechanical behavior. There was a transition temperature (~300°C), above which the mechanical twinning was hardly observed in the microstructure even at fracture, resulting in low ductility and strength. The current observation can be explained through the change in the stacking fault energy with the deformation temperature.

Abstract: In this study, advanced high manganese austenitic steel sheets were welded by resistance spot welding at different welding parameters. The effects of welding current, clamping force, number of the current impulse, and duration of each current impulse were examined. Based on Taguchis method, an L-27(3¹³) orthogonal array was employed for carrying out resistance spot welding tests. The welded sheets were subjected to tensile-shear tests in order to determine the strength of the welded joints. Basically, the results showed that tensile-shear strength increase with clamping force at the medium and high effective welding time (>400 ms). However, the occurrence of micro cracks within the welded joints may justify the scattering of tensile-shear strength values.

Abstract: The highly interesting properties of high manganese steels can be further improved by microalloying. The introduction of carbon-nitride precipitates improves the yield strength and the microstructural control during the production process. Due to the high manganese content in these austenitic steels significant changes in the precipitation behaviour have to be expected in comparison to conventional carbon-manganese steels. However, although crucial for steel design, this has not been systematically described before. Preliminary results showing the effect of Nb and V are presented. Namely the softening behaviour is related to the precipitation state. In summary this allows describing the precipitation-time-temperature evolution and provides the necessary background for the alloy and process design.

Abstract: A study has been carried out on the evolution of microstructure, grain boundary character and mechanical properties in a Twinning Induced Plasticity steel heavily cold rolled and subsequently annealed.The cold rolled mcrostructures showed fine lamellar boundaries with many shear bands.With progress of annealing, numerous numbers of recrystallized grains were generated.The fully recrystallized steel showed equi-axed nanocrystalline grains with a mean grain size of 400 nm that enhanced the yield strength significantly while retaining tensile ductility.

Abstract: Texture evolution during static primary recrystallization of an austenitic Fe-28Mn-0.28C TWIP steel was analyzed. The cold-rolled material, which showed a Brass-type texture at medium (30% and 50%), and additionally a γ-fiber at high (80%) deformation degrees, was subjected to isothermal annealing at 700°C. The influence of rolling degree/starting texture on the development of particular texture components was studied. After recrystallization a weakened, retained rolling texture was observed for the examined reduction levels. In addition to the deformation components, Brass and Goss, further α-fiber components were formed mainly by annealing twinning leading to the development of this fiber.

Abstract: In this study, the constitutive flow behavior and static recrystallization characteristics of a Nb-microalloyed TWIP (Fe-20Mn-1.5Al-0.1Nb) steel under hot deformation conditions have been determined and results compared with those of Fe-25Mn-Al TWIP steels. Investigations using compression testing in a Gleeble simulator, including the double-hit technique, enabled the acquisition of flow stress and recrystallization data. These were analyzed to determine the powers of strain and strain rate as well as the activation energies of deformation and recrystallization (Qdef and Qrex). For given deformation and grain size parameters, the time for 50% recrystallization (t50) of the 0.1% Nb TWIP steel was significantly longer than for the Nb-free TWIP steel: it was comparable to that of Nb-microalloyed carbon steel. Qrex was higher than that of Type 304 stainless steel that has nevertheless much longer t50 times.

Abstract: The increasing demand by the automotive industry has resulted in a search for materials of increasingly high mechanical properties and, at the same time, plastic deformability. These requirements are met by AHSS (Advanced High-Strength Steels) multiphase steels. The group of AHSS type steels may include: diphase (DP), TRIP-effect, hot formed (HF) martensitic, plastic formed heat treated (PFHT), and TWIP-effect steels.