Papers by Keyword: TWIP Steel

Abstract: The aim of this paper is to determine the high-manganese austenite propensity to twinning induced by the cold working and its effect on structure and mechanical properties, and especially the strain energy per unit volume of new-developed high-manganese Fe – Mn – (Al, Si) investigated steel, containing about 24,5 % of manganese, 1% of silicon, 3 % of aluminium and microadditions Nb and Ti with various structures after their heat- and thermo-mechanical treatments. The new-developed high-manganese Fe – Mn – (Al, Si) steel provide an extensive potential for automotive industries through exhibiting the twinning induced plasticity (TWIP) mechanisms. TWIP steel not only show excellent strength, but also have excellent formability due to twinning, thereby leading to excellent combination of strength, ductility, and formability over conventional dual phase steels or transformation induced plasticity (TRIP) steels. Results obtained for high-manganese austenitic steel with the properly formed structure and properties in the thermo-mechanical processes indicate the possibility and purposefulness of their employment for constructional elements of vehicles, especially of the passenger cars to take advantage of the significant growth of their strain energy per unit volume which guarantee reserve of plasticity in the zones of controlled energy absorption during possible collision resulting from activation of twinning induced by the cold working as the fracture counteraction factor, which may result in significant growth of the passive safety of these vehicles' passengers.

Abstract: Austenitic FeMnCr steels have high strength, high toughness and formability because of the stress-and strain-induced γ→α and γ→ε martensitic phase transformations. These are the so-called TRIP (Transformation Induced Plasticity) and TWIP (Twining induced Plasticity) effects. TWIP steels deform by both glide of individual dislocations and mechanical twinning [1]. The type and mechanism of the austenite→martensite transformation depends on the composition, deformation rate and temperature. The ratio and quantity of the resulting phases determine the properties of the product. It is known that austenitic steels can transform into α and/or ε martensite phases during plastic deformation The characteristics of the martensitic transformations induced by uni-axial tensile tests between room temperature and 200°C in a FeMnCr steel with 2,26 w% Cr content were examined. Mechanical properties as, yield stress were determined from tensile tests. Metallographic examinations, quantitative and qualitative phase analysis by X-ray diffraction were carried out on the uniformly elongated part of the samples (cross, longitudinal sections).

Abstract: Dynamic mechanical properties and microstructures of pure iron and Fe-30Mn-3Si-4Al TWIP (TWinning Induced Plasticity) steel were conducted by SHPB (Split-Hopkinson Pressure Bar), OM (Optical Microscopy) and TEM (Transmission Electron Microscope), at the strain rate ranging from 10² to 10⁵ s^-1 and at room temperature. The effect of high strain rate on the mechanical responses of pure iron and Fe-30Mn-3Si-4Al TWIP steel belonging to BCC (Body Centered Cubic) and FCC (Face Centered Cubic) structures respectively was evaluated. The comparison of deformation mechanism was analyzed between them and it concluded that dislocation gliding is a major deformation mechanism in pure iron with BCC structure and deformation twinning plays a significant role in Fe-30Mn-3Si-4Al TWIP steel with FCC structure.

Abstract: The aim of this paper is to determine the high-manganese austenite propensity to twinning induced by the cold working and its effect on structure and mechanical properties, and especially the strain energy per unit volume of new-developed high-manganese Fe – Mn – (Al, Si) investigated steel with various structures after their thermo-mechanical treatments. The new-developed high-manganese steel provides an extensive potential for automotive industries through exhibiting the twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) mechanisms. TWIP steels not only show excellent strength, but also have excellent formability due to twinning, thereby leading to excellent combination of strength, ductility, and formability over conventional dual phase steels or transformation induced plasticity TRIP steels. The microstructure evolution in successive stages of deformation was determined in metallographic investigations using light, scanning and transmission electron microscopies as well as X-ray diffraction methods.

Abstract: We developed a plane-strain crystal plasticity finite element (CPFE) numerical model to predict the tensile behavior of twinning-induced plasticity (TWIP) steel with both slip and mechanical twinning as the main deformation modes. Our CPFE model may not only predict well the tensile stress versus strain (S-S) curve but also capture the variation in the volume fraction of twins with a reasonable accuracy. The nucleation of mechanical twin is obviously controlled by the stress concentration. At the same time, the growth of twin may either lead to a stress relaxation in the matrix or cause a local stress concentration around twin, which depends on the deformation condition.

Abstract: We investigate the non uniform plastic deformation of a TWIP FeMnC steel by diffraction of high energy synchrotron X-rays. In particular, we observe the propagation of bands of plastic strain localisation. Debye-Scherrer rings are recorded in situ during tensile tests at two different strain rates. Discontinuous initial rings characteristic of unstrained polycristals with no texture become rapidly continuous after several percents of plastic strain due to strain gradients within the grains and a strong texture develops. The crystallographic dependence of the Young’s modulus is estimated and is consistent with the elastic anisotropy of a cubic crystal. A delay between the serrations on the macroscopic tensile curve and the stepwise variations of the diffracted peak’s position and width are consistent with propagating bands nucleating outside the X-ray beam. Slower and thinner bands are observed at the lowest strain rate. A tensile test interrupted to perform a few minute relaxation leads to a displacement of the nucleation site of the bands from one end to the middle of the gauge part.

Abstract: A directionally solidified TWIP steel (Fe-25Mn-2.5Al-2.5Si) was prepared by liquid metal cooling technology. The microstructure and mechanical behavior were examined and compared with usually solidified samples. The directionally solidified TWIP steel shows a typical columnar grain structure, and the maximum true stress and true strain along the longitudinal direction of the sample are 1060MPa and 71% respectively. As a comparison, the usually solidified samples shows an equiaxed grain microstructure with the maximum true stress and true strain of only 994MPa and 58%, respectively. Moreover, the two solidification modes also lead to very different strain hardening behavior, particularly in the changes of strain hardening rate with strain. This suggests that the grain boundary plays a key role in the mechanical properties of TWIP steels, and changing the grain boundaries can be effective to improve the comprehensive mechanical properties of TWIP steels.

Abstract: The Visco-Plastic (VPSC) and Elasto-Plastic (EPSC) Self-Consistent models were applied to simulate the macroscopic stress-strain response and the evolution of crystallographic texture during the tensile loading of a fully annealed Fe-24Mn-3Al-2Si-1Ni-0.06C TWIP steel. Bulk texture measurements acquired by X-ray diffraction were used to validate the modelling results. Various modelling constructs were employed to assess the contribution of twinning and latent hardening to the texture evolution. The simulations revealed the dominant role of perfect slip and the limited effect of twinning on texture development as well as a possible role for latent hardening. Comparison between the texture predictions of the VPSC and EPSC models underscored the correlation between the latent hardening effects and the employed grain interaction scheme.

Abstract: Effect of cold rolling on the microstructure and mechanical properties of a Fe-23Mn-0.3C-1.5Al (in wt. %) TWIP steel with an initial grain size of 24 μm was studied. Extensive deformation twinning occurred upon reduction by rolling. The volume fraction of the deformation twins attained about 0.2 at a reduction of 20%. Then, the intensity of deformation twinning gradually decreased with increasing the total rolling reduction. The average twin thickness of about 20 nm remained unchanged, although the distance between twins progressively reduced with increasing strain. The deformation banding was observed after a reduction of 60%. The thickness and volume fraction of microshear bands increased with increasing rolling reduction. The cold rolling led to significant strengthening of the steel that is accompanied by a drop on ductility. The yield stress (YS) increased from 235 MPa in the initial state to 1400 MPa after cold rolling with a reduction of 80%, whereas the elongation to failure decreased from 96% to 4%, respectively.

Abstract: A recently introduced processing route consisting of cold rolling and recovery annealing allows the production of TWIP steels with high yield strength along with appreciable uniform elongation due to the thermal stability of mechanically induced nanoscale twins. A wide range of strength-ductility combinations was obtained using recovery and recrystallization annealing of 30%, 40%, and 50% cold-rolled Fe-23Mn-1.5Al-0.3C TWIP steel. Texture measurement during cold rolling and annealing was proven to be a suitable tool to determine the optimal deformation degree and annealing time for this processing method. As a consequence, texture analysis can be used to predict the final materials properties.