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Microscopy and microanalysis experiments on two cast alloys, designed on the basis of equilibrium to contain substantial amounts of d-ferrite, reveal zero or much reduced fractions of this phase in the solidified condition. It app...
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Microscopy and microanalysis experiments on two cast alloys, designed on the basis of equilibrium to contain substantial amounts of d-ferrite, reveal zero or much reduced fractions of this phase in the solidified condition. It appears that the solid state transformation of δ-ferrite into austenite occurs without the required partitioning of solutes and that this is responsible for the development of non-equilibrium microstructures. This conclusion is supported by microanalytical data and through calculations of limiting phase diagrams based on paraequilibrium rather than equilibrium. Kinetic simulations confirm that this interpretation is consistent with the majority of austenite growing in the solid state without the partitioning of the substitutional solutes.
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The influence of intercritical deformation on the microstructure development of a TRIP steel has been investigated by the use of deformation dilatometry, X-ray diffraction, and metallography. The austenite decomposition into ferri...
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The influence of intercritical deformation on the microstructure development of a TRIP steel has been investigated by the use of deformation dilatometry, X-ray diffraction, and metallography. The austenite decomposition into ferrite and bainite during cooling from the intercritical annealing and the bainitic annealing have been investigated. A very distinct effect of plastic deformation on the microstructural development has been observed. Since the plastic deformation enhances the ferrite formation during cooling, the degree of deformation can be used to control the balance between allotriomorphic ferrite and bainitic ferrite in the eventual structure. In this paper a semi-quantitative phase map has been developed, showing the microstructural composition of the material as a function of the degree of plastic deformation. Using this map the influence of the intercritical deformation on the microstructure development can be followed.
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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 Plasti...
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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. 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 a and/or e martensite phases during plastic deformation The characteristics of the martensitic transformations induced by uni-axial.tensile tests between room temperature and 200℃ 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).
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The thermomechanical response of low-alloyed multiphase steels assisted by transformation-induced plasticity (TRIP steels) is analyzed taking into account the coupling between the thermal and mechanical fields. The thermomechanica...
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The thermomechanical response of low-alloyed multiphase steels assisted by transformation-induced plasticity (TRIP steels) is analyzed taking into account the coupling between the thermal and mechanical fields. The thermomechanical coupling is particularly relevant since in TRIP steels the phase transformation that occurs during mechanical loading is accompanied by the release of a considerable amount of energy (latent heat) that, in turn, affects the mechanical response of the material. The internal generation of heat associated with the martensitic phase transformation and the plastic deformation are modeled explicitly in the balance of energy. The momentum and energy equations are solved simultaneously by using a fully-implicit numerical scheme. The simulations are conducted using a micromechanica! formulation for single crystals of austenite and ferrite. The characteristics of the model are illustrated by means of simulations for a single crystal of austenite and an aggregate of austenitic and ferritic grains. For a single crystal of austenite, it is found that the increase in local temperature due to transformation actually hinders further transformation and, instead, promotes plastic deformation. However, for an aggregate of austenitic and ferritic grains in a multiphase steel, the increase in temperature due to transformation is limited since the heat generated in the austenite is conducted to the ferritic matrix, effectively lowering the temperature in the austenitic phase.
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With the aim of investigating a laser-welded dissimilar joint of TWIP and TRIP steel sheets, the microstructure was characterized by means of OM, SEM, and EBSD to differentiate the fusion zone, heat-affected zone, and the base mat...
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With the aim of investigating a laser-welded dissimilar joint of TWIP and TRIP steel sheets, the microstructure was characterized by means of OM, SEM, and EBSD to differentiate the fusion zone, heat-affected zone, and the base material. OIM was used to differentiate between ferritic, bainitic, and martensitic structures. Compositions were measured by means of optical emission spectrometry and EDX to evaluate the effect of manganese segregation. Microhardness measurements and tensile tests were performed to evaluate the mechanical properties of the joint. Residual stresses and XRD phase quantification were used to characterize the weld. Grain coarsening and martensitic areas were found in the fusion zone, and they had significant effects on the mechanical properties of the weld. The heat-affected zone of the TRIP steel and the corresponding base material showed considerable differences in the microstructure and properties.
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A TRIP-assisted steel, with a conventional composition containing 0.11 /100 C, 1.53 /100 Mn and 1.26 Si, and a hot band microstructure composed of ferrite, martensite and carbide particles, was cold rolled with a reduction of 70 ...
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A TRIP-assisted steel, with a conventional composition containing 0.11 /100 C, 1.53 /100 Mn and 1.26 Si, and a hot band microstructure composed of ferrite, martensite and carbide particles, was cold rolled with a reduction of 70 /100. Partially recrystallizaed samples were obtained by water quenching the cold rolled sheets which were reheated at a constant rate of 10deg.C/s to temperatures in the range between 525 deg.C (A_c3). Lt was demonstrated that the recovery and recrystallization behaviour was critically controlled by the carbide formation and growth during the initial stages f the annealing treatment.
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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 i...
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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.
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Stretch-flangeability measures the ability of a material to form into a complex shape. The parameter is often related to simple properties derived from tensile tests. An attempt is made here to discover the best way to exploit ten...
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Stretch-flangeability measures the ability of a material to form into a complex shape. The parameter is often related to simple properties derived from tensile tests. An attempt is made here to discover the best way to exploit tensile test data to indicate flangeabilty. It is found that the ultimate tensile strength of steel is the single most important criterion that correlates with stretch-flangeability.
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Low carbon low alloyed high strength steel with the chemical composition suitably designed to support the stabilization of retained austenite was used in this work. The steel was processed by conventionalannealing for a reference ...
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Low carbon low alloyed high strength steel with the chemical composition suitably designed to support the stabilization of retained austenite was used in this work. The steel was processed by conventionalannealing for a reference and several different heat and thermo-mechanical treatments were further proposedto test typical TRIP (transformation induced plasticity), DP (dual phase) steel and QP (quenchingand partitioning) processing routes. All the processing methods used the same soaking temperature of1050 °C. Processed samples were subjected to metallographic analysis, hardness measurement and tensiletest to characterise resulting microstructures. While simple annealing reached tensile strength of 861MPa with 25% of total elongation, the best combination of the highest tensile strength of 903 MPa and atotal elongation of 32% was obtained after processing typical for TRIP steel. QP treatment resulted inthe highest tensile strength of 1289 MPa with a total elongation of 19%
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A comprehensive experimental and numerical investigation of TRIP effect on mechanical behaviors, strain partitioning and fracture is carried out on a low-alloyed TRIP steel with and without subzero treatment. Load responses of ind...
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A comprehensive experimental and numerical investigation of TRIP effect on mechanical behaviors, strain partitioning and fracture is carried out on a low-alloyed TRIP steel with and without subzero treatment. Load responses of individual phases and macroscopic mechanical properties are investigated through nanoinden-tation and uniaxial tensile tests. The increase of martensite volume fraction caused by subzero treatment improves tensile strength but appears detrimental to ductility. Two types of martensite transformation are observed: a twin-relationship variant selection to accommodate strain interior unstable austenite grains, and a variant selection based on the same crystal orientation as adjacent coarse ferrite grain reducing interfacial energy between ferrite and newly transformed martensite. A microstructure-based simulation model considering martensite transformation is developed with phenomenologically calibrated crystal plasticity parameters. The shift of strain localization from unstable austenite grain to neighboring ferrite grain is clarified to reduce the strain partitioning at interfaces. This effect is argued to be the origin of ductility enhancement in low-alloyed TRIP steel.
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