Papers by Keyword: Transformation Induced Plasticity (TRIP)

Abstract: In this work a new kind of thermal treatment, called Quenching and Bainite (Q&B), was proposed and studied. A rather standard Fe-C-Mn-Si composition was used for this study. Annealing trials were performed using a combination of salt pots on relatively big samples allowing to perform the standard tensile and hole expansion tests. The obtained results were compared with the properties obtained using more known Q&P annealing. Generated microstructures were also compared. Characterization was done using optical and Scanning Electron Microscopy as well as magnetization saturation method for measuring retained austenite fractions. The Q&B heat treatment provides an alternative way to obtain 3G AHSS with promising strength-ductility-formability compromise.

Abstract: Impact of first annealing on the formation and evolution of the microstructure during second annealing and on the final mechanical properties was investigated. Simple Fe-C-Mn-Si steel was used in this study. Dilatometry tests coupled with metallographic examinations were carried out to monitor the evolution of phase transformations and associated microstructure. Difference in the austenite evolution between simple and double annealing was highlighted. Based on the obtained results, conditions were selected for the annealing trials on bigger sample. Mechanical properties of heat-treated steels were assessed through the standard tensile tests. Double annealing treatment resulted in a better strength-ductility balance and in a good stability against soaking and quenching temperature variation. Complex ultra-fine multiphase microstructure containing at least 5 different microstructural constituents was revealed and observed using Scanning Electron Microscopy. As well, retained austenite fraction was estimated through magnetization saturation method.

Abstract: Annealed Martensite Matrix (AMM) concept was studied on two steel grades with low alloyed base composition of Fe-C-Mn-Si and two levels of Nb. Conditions for the thermal treatments were selected based on the experimental dilatometry tests and thermodynamic calculations. Annealing trials with short austempering holding were performed in the laboratory salt pots. Mechanical properties of heat treated steels have been investigated by tensile tests. Associated microstructures have been analyzed using Scanning Electron Microscopy as well as magnetization saturation method for measuring retained austenite fractions. Excellent strength-ductility balance was obtained due to the ultra-fine multiphase structure and high amount of stable retained austenite.

Abstract: Double annealing of low carbon medium Mn steel was studied. The second intercritical annealing was done at 650°C within a range of holding time: 3min to 30h. Tensile properties of the steel were measured as a function of holding time and the relation between microstructure and mechanical behavior was analyzed. Furthermore, a model, based on the mixture law combined with the considerations of equivalent increment of work in each microstructural constituent during mechanical loading, was proposed. The individual mechanical behavior of each considered microstructural constituent was described with the approaches existing in the literature. The complete model shows a very good agreement with the experimental stress-strain curves and predicts well the optimum strength-ductility balance after 2h holding.

Abstract: Based on cold rolled TRIP 780 and HSLA 340 steel sheet, the stamping of U-beam and its springback was studied experimentally and numerically. Collision analysis model of closed hat section beam was established on the basis of simulation analysis of U-beam stamping and springback. Mapping was used to transfer results of thickness, stress and strain from U-beam forming part to impact structural part. Collision processes of closed hat section were analyzed under various operating conditions considering the influence of forming. The results show that thickness variation, residual stress, and work hardening of the material after forming affect collision processes directly. The current paper provides reference for improving precision of simulation for vehicle collision.

Abstract: Tensile tests of single crystals of Gum Metal (Ti-36Nb-2Ta-3Zr-0.3O (wt %)) showed, anomalously, that (1) extensive, stress-induced (bcc)”(orthorhombic) transformation occurred in a crystal pulled in the <110> direction, but no transformation was observed in crystals pulled in the <100> or <111> directions and (2) little or no transformation occurred in tensile tests of severely worked rods, which are polycrystals with very strong <110> texture. Analysis of the energetics of the ” transformation offers straightforward explanations for these results. (1) An ” precipitate has very low elastic energy if it forms as a thin plate with a habit near {11√2}. A <110> tensile load significantly decreases the energy of this plate, promoting the transformation; loading along <100> or <111> is much less effective. (2) While cold-swaged rods of Gum Metal have a strong <110> axial texture, their perpendicular planes are severely distorted, increasing the elastic energy of ” and inhibiting the transformation.

Abstract: In this study a C-Mn High Strength Low Alloy steel (HSLAs) was processed by quenching and austenite reverted transformation during annealing (ART-annealing), which results in an ultrafine grained duplex microstructure characterized by scanning electron microscopy equipped with electron back scattered diffraction, transmission electron microscopy and x-rays diffraction (SEM/EBSD, TEM and XRD). Microstructural observation revealed that the full hard martensitic microstucture gradually transformed into ultrafine grained duplex structure with austenite volume fraction up to 30% at specific annealing conditions. Mechanical properties of this processed steel measured by uniaxial tensile testing demonstrated that an excellent combination of strength (Rm~1GPa) and total elongation (A5~40%) at 30% metastable austenite condition in studied C-Mn-HSLAs. This substantially improved strength and ductility were attributed to the strain induced phase transformation of retained austenite dispersed throughout the ultrafine grained microstructure. At last it is proposed that ART-annealing is a promising way to produce high strength and high ductility steel products.

Abstract: The effect of composition and processing schedule on the microstructure of C-Mn-Si-Mo-(Al)-(Nb) steels containing nano-bainite was studied using transmission electron microscopy (TEM) and atom probe tomography (APT). The major phase formed in all steels was nano-bainite. However, the steels with lower carbon and alloying addition content subjected to TMP had better mechanical properties than high alloyed steel after isothermal treatment. The presence of ferrite in the microstructure can improve not only ductility but lead to the formation of retained austenite with optimum chemical stability.

Abstract: In this study Quenching and Partitioning (Q&P) as proposed by Speer was applied to improve the ductility of C-Mn high strength Low Alloy steel (HSLAs). Microstructural observations revealed a multiphase microstructure including first martensite, fresh martensite and retained austenite in the Q&P processed steel. During tensile process, the austenite volume fraction gradually decreased with strain increasing, suggesting the phase transformation induced plasticity for the Q&P processed steel. Ultrahigh strength about 1300-1800MPa and tensile elongation about 20% were obtained after Q&P processing at specific conditions, which is significant higher than that of ~10% of conventional martensitic steel. The the product of tensile strength to total elongation increased from 25 to 35GPa% with increasing carbon content in studied steel. This improved mechanical properties were related to the ductility contribution from TRIP effects of the retained austenite and strength contribution from the hard martensitic matrix. At last it was turned out that the Q&P process is a promising way to produce ultrahigh strength steel with relative high ductility under tailored heat treatment conditions for different micro-alloyed carbon steel.

Abstract: The relationship between the microstructure and mechanical properties on the ternary ZrCoNi alloys has been investigated. Zr50Co50-xNix alloys undergo martensitic transformation from the B2 to B33 structure by the substitution of Ni for Co. The tensile ductility at room temperature increases remarkably without the loss of yield strength by substituting Ni. Zr50Co39Ni11 alloy has the extremely high plastic elongation of 21%. There are many {021}B33 deformation twins in the B2 parent phase just near the tensile fractured area. These twins are the strain induced martensitic phase due to the tensile deformation. Consequently, the remarkable improvement of the ductility of ZrCoNi alloys is due to the transformation induced plasticity.