Papers by Keyword: TWIP Steel

Abstract: The influence of recrystallization annealing on the microstructure, mechanical properties and corrosion behaviour of an austenitic TWIP steel bearing medium Mn and V alloying was investigated. The steel undergone a heavy cold deformation of 65 % reduction. Subsequently, recrystallization annealing at 1000 °C and 1100 °C for 15 min was conducted to achieve different grain structures. The microstructural evolution was studied using optical microscopy and electron backscatter diffraction technique. Mechanical properties were determined using tensile tests at room temperature. Corrosion behaviour was measured by cyclic potentiodynamic polarization at 3.5 pct NaCl. For comparison, austenitic stainless-steel Type 201 was used in this study. It was observed that at 1000 °C, a non-uniform austenitic grain structure with vanadium carbides distributed throughout the matrix was obtained. However, a coarse grain structure without carbides was induced at 1100 °C. The fine-grained structure enhanced at 1000 °C exhibited higher strength and good ductility. Contrary to this, the corrosion results showed that a significant deterioration in the corrosion resistance could be observed in sodium chloride solution for the achieved structure at 1000 °C compared to 201 stainless steel.

Abstract: The mechanical properties of Fe-28%Mn-1.5%Al and Fe-0.6%C-18%Mn-1.5%Al-0.07%Nb (all in wt.%) steels subjected to hot plate rolling at a temperature of 1423 K with a total reduction of 60% were studied. The steels exhibited quite different mechanical properties in spite of almost the same original microstructures and similar stacking fault energies. The yield strength and total elongation of the Fe-28%Mn-1.5%Al steel are about 260 MPa and 45%, respectively, whereas those properties in the Fe-0.6%C-18%Mn-1.5%Al-0.07%Nb steel comprise 350 MPa and 53%, respectively. The tensile flow stress vs strain curves of the hot rolled steel samples can be described by Ludwigson-type relations with parameters being dependent on the strengthening mechanisms. Frequent deformation twinning in the Fe-0.6%C-18%Mn-1.5%Al-0.07%Nb steel promoted the strain hardening and improved the strength and ductility.

Abstract: The bullet-resistant vest (bullet proof vest) is an important accessory to absorb impact energy and stop bullets from penetrating the body. In the present work a sandwich composite structure was designed from different sequential layers of, twinning induced plastic (TWIP) steel, polypropylene – polyethylene (PP-PE) polymer and water for bullet proof vest application. Owing to the difficulty in experimentally testing materials for ballistic impact application, a finite element – smoothed particle hydrodynamic (FE-SPH) coupled simulation was applied for analyzing the impact characteristics of the proposed composite structure. Different structural layers of the composite are simulated to select the most effective thickness of steel/polymer/water layers in energy absorption and penetration prevention. The simulation results displayed that the optimum thickness of the layers are 2 mm steel/20 mm water/2 mm steel , which is able to stop a 9 mm bullet travelling at 360 m/s with less than 10 mm displacement of the inner surface of the composite. This composite is promising and has a great potential in fabrication of effective and light weight bullet proof vest with less expensive materials.

Abstract: The present study shows that warmly forged and low-temperature annealed twinning induced plasticity (TWIP) steel exhibited very high dislocation density and apparent yield-point phenomenon in addition to very high yield strength. The initial density of dislocations significantly affected the evolution of dislocations during the subsequent tensile deformation. Original high dense dislocations prompted the rapid increase of dislocations, and intensified the complexity of dislocation configurations. All these effects made the twinning deformation weakened but the dislocation deformation enhanced, leading to increased strength but decreased plasticity.

Abstract: TWIP steel has the characteristics of high elongation and high tensile strength at room temperature, and therefore it is widely used in the automobile manufacturing industry. Since the grain size has a very important influence on its mechanical properties and corrosion resistance it is significant to study the average grain size of TWIP steel. In this experiment, the specimens were heated to 700~1000°C and preserved for one hour, and the recrystallized structures with different average grain sizes were obtained. All the samples were tested by laser ultrasonic experimental platform and the standard sample was selected. Then the attenuation coefficients of remaining samples were compared with the standard samples by conventional methods. The results showed that the larger the difference between the average grain size of the test sample and the standard sample is, the greater the error of the test will be. The new evaluation model was established by the improved method, and the prediction error was reduced to 5.44~11.61%, which proved the effectiveness of the new method. In addition, laser ultrasonic testing has the advantages of high efficiency and no damage compared with traditional methods.

Abstract: Metal matrix composites with ceramic reinforcements such as particles or fibers have come into focus during the past decades due to rising requirements on engineering materials. In this work, composite materials out of high-alloy CrMnNi-steel matrices with varying Ni-contents (3 wt.% and 9 wt.%) and 10 vol.% Mg-PSZ were processed by hot-pressing. The variation in Ni-content resulted in a change in stacking fault energy (SFE) which significantly influenced the deformation mechanisms. The mechanical behavior of the developed composites was investigated in a wide strain rate range between 0.0004 s^-1 and 2300 s^-1 under compressive loading. This was done by a servohydraulic testing system, a drop weight tower, and a Split-Hopkinson Pressure Bar for the high strain rates. To study the influence on the deformation mechanisms such as martensitic transformations and/or twinning, interrupted tests were also carried out at 25 % compressive strain. Subsequent microstructural examinations were done by a magnetic balance to measure the quantity of α’-martensite as well as by scanning electron microscopy (SEM). The results show an increase of strength and strain hardening with decreasing SFE of the matrix due to increased α’-martensite formation. The addition of the Mg-PSZ particles resulted in further strengthening over almost the entire deformation range for all investigated composites. At high strain rates quasi-adiabatic heating suppressed the martensite transformation and reduced the strain hardening capacity of the matrix. Nonetheless the particle reinforcement retains its strengthening effect.

Abstract: The crystallographic orientation relations of phases forming during the martensitic transformation determine the properties of alloys. In TRIP/TWIP steels, the circumstances of thermomechanical treatment (e.g. temperature, deformation) define the forming of martensites of different origins. Due to the thermomechanical treatment, thermally induced martensite (ε_TH), strain induced martensite (ε_D) and α’ martensite phases are present in the samples besides the austenite. The proportion of martensites in the sample is defined by the parameters of treatment. The thermally and strain induced martensites which are simultaneously present in the alloy at room temperature can be differentiated by the orientation relations.The martensitic transformations were followed by different methods in FeMn alloys with different Cr content. The macroscopic crystallographic anisotropy was measured by X-ray diffraction method; the microscopic one was examined by EBSD. The cognition of phenomenon observed in the texture image in different scales helps determine the possible origin of martensites.

Abstract: The need for steel materials with increasing strength is constantly growing. The main application of such advanced high strength steels (AHSS) is the automobile industry, therefore the welding process of different types of AHSSs in dissimilar welding joint was investigated. To simulate the mass production of thin steel sheet constructions (such as car bodies) automated metal inert gas (MIG) welding process was used to weld the TWIP (twinning induced plasticity) and TRIP (transformation induced plasticity) steel sheets together. The welding parameters were successfully optimized for butt welded joints. The joints were investigated by visual examination, tensile testing, quantitative metallography and hardness measurements. The TRIP steel side of the joints showed increased microhardness up to (450-500 HV0.1) through increased fraction of bainite and martensite. Macroscopically the tensile specimen showed ductile behaviour, they broke in the austenitic weld material.

Abstract: The regularities of static recrystallization in an Fe-0.3C-17Mn-1.5Al TWIP steel subjected to cold rolling and annealing were studied. The cold rolling led to noticeable increase in the dislocation density, extensive mechanical twinning and shear banding. The subsequent annealing resulted in the development of recovered or recrystallized microstructure depending on the rolling reduction and the annealing temperature. An increase in the rolling reduction promoted the recrystallization development, which led to ultrafine-grained microstructure with a grain size below 10 μm. The developed ultrafine-grained steel samples are characterized by beneficial mechanical properties.

Abstract: Fatigue crack growth (FCG) behavior of three high manganese austenitic twin-induced plasticity (TWIP) steels with different stacking fault energy (SFE) was investigated, aiming at studying the correlation between the FCG resistance and the SFE of the steels. FCG tests were performed using three-point bending specimens at room temperature at stress ratio of 0.1 under the control of stress intensity factor range. Test results showed that the fatigue threshold values of these steels decrease with increasing the SFE. However, in the Paris regime, the crack growth rates of the steels do not appear to correlate directly with SFE. These results are discussed according to the degree of fatigue crack closure and the deformation mode of crack tip zone.