Papers by Keyword: Dual Phase Steel

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Authors: Guang Can Jin, Shu Ying Chen, Mei Zhao, Qing Chun Li, Xu Dong Yue, Guo Wei Chang
Abstract: The tensile process of 800MPa grade dual-phase steel was observed in-situ by SEM. The initiation and propagation of crack in dual-phase steel, the initiation of micro-pores, the deformation behavior of ferrite and martensite were analyzed in this paper. The results show that the main crack goes across the soft ferrite and around the hard martensite during its propagation in the beginning. When the applied stress is small, the micro-pores are easy to form inside the ferrite grain or on the interface of ferrite and martensite. The micro-pores can also form in the fracture site of martensite with the increase of applied stress. The fracture morphology of the dual-phase steel is dimple and the formation of fracture belongs to plastic fracture.
Authors: Maria Giuseppina Mecozzi, C. Bos, Jilt Sietsma
Abstract: A three-dimensional cellular automata (CA) model is developed for the kinetic and microstructural modelling of the relevant metallurgical mechanisms occurring in the annealing stage of low–alloy steels: recrystallisation, pearlite–to–austenite transformation and ferrite–to–austenite transformation on heating and austenite–to–ferrite transformation on cooling. In this model the austenite–to–ferrite transformation is described by a mixed–mode approach, which implies that the transformation kinetics is controlled by both the interface mobility and the diffusivity of the partitioning elements. This approach also allows incorporation of the ferrite nucleation occurring on structural defects. The developed CA algorithm, in which the transformation rules for the grain boundary and interface cells are controlled by the growth kinetics of the forming phase, allows three-dimensional systems to be treated within relatively short simulation times. The simulated microstructure reproduces quite well the microstructure observed in experimental samples. A good agreement is obtained between the experimental and simulated ferrite recrystallisation and ferrite and austenite transformation kinetics. The present approach also models the development of the carbon concentration profile in the austenite, which is, for instance, essential for subsequent martensite formation.
Authors: Yoshitaka Adachi, Mayumi Ojima, Naoko Sato, Yuan Tsung Wang
Abstract: The features present in 3D structure have geometric properties that fall into two broad categories: topological and metric. Metric properties are generally the more familiar; these include volume, surface area, line length and curvature. Equally or even more important in some applications are the topological properties of features. The two principal topological properties are number per unit volume and connectivity. In the present study, a change in morphology of pearlite and dual phase microstructures was examined from differential geometry and topology viewpoint. 3D images of eutectoid pearlite and dual phase steels were obtained by reconstructing serial sectioning images. Their metric and topological features were evaluated using The Euler Poincare formula and The Gauss-Bonnet Theorem. In addition, newly developed fully-automated serial sectioning 3D microscope “Genus_3D” will be also introduced.
Authors: Anthony J. DeArdo, J.E. Garcia, Ming Jian Hua, C. Isaac Garcia
Abstract: TRIP steels containing Mn, Si, Al, Mo, and Nb have been examined using a laboratory simulation of a continuous hot dipped galvanizing line. The evolution of microstructure has been studied as the steel passes through the various stages of CG line processing. Tensile strengths approaching 800 MPa and ductilities approaching 30% have been achieved in the 1.5Mn-0.5Si- 1.0Al-0.015Mo-0.03Nb system.
Authors: Richard G. Thiessen, Jilt Sietsma, I.M. Richardson
Abstract: This work presents a unique approach for the modelling of the austenitisation of martensite in dual-phase steels within the phase-field method. Driving forces for nucleation and growth are derived from thermodynamic databases. Routines for nucleation are based on a discretisation of the classical nucleation theory. Validation is given via dilatometric experiments.
Authors: M.S. Niazi, V. Timo Meinders, H.H. Wisselink, C.H.L.J. ten Horn, Gerrit Klaseboer, A.H. van den Boogaard
Abstract: The global fuel crisis and increasing public safety concerns are driving the automotive industry to design high strength and low weight vehicles. The development of Dual Phase (DP) steels has been a big step forward in achieving this goal. DP steels are used in many automotive body-in-white structural components such as A and B pillar reinforcements, longitudinal members and crash structure parts. DP steels are also used in other industrial sectors such as precision tubes, train seats and Liquid Petroleum Gas (LPG) cylinders. Although the ductility of DP steel is higher than classical high strength steels, it is lower than that of classical deep drawing steels it has to replace. The low ductility of DP steels is attributed to damage development. Damage not only weakens the material but also reduces the ductility by formation of meso-cracks due to interacting micro defects. Damage in a material usually refers to presence of micro defects in the material. It is a known fact that plastic deformation induces damage in DP steels. Therefore damage development in these steels have to be included in the simulation of the forming process. In ductile metals, damage leads to crack initiation. A crack is anisotropic which makes damage anisotropic in nature. However, most researchers assume damage to be an isotropic phenomenon. For correct and accurate simulation results, damage shall be considered as anisotropic, especially if the results are used to determine the crack propagation direction. This paper presents an efficient plasticity induced anisotropic damage model to simulate complex failure mechanisms and accurately predict failure in macro-scale sheet forming processes. Anisotropy in damage can be categorized based on the cause which induces the anisotropy, i.e. the loading state and the material microstructure. According to the Load Induced Anisotropic Damage (LIAD) model, if the material is deformed in one direction then damage will be higher in this direction compared to the other two orthogonal directions, irrespective of the microstructure of the material. According to Material Induced Anisotropic Damage (MIAD) model, if there is an anisotropy in shape or distribution of the particles responsible for damage (hard second phase particles, inclusions or impurities) then the material will have different damage characteristics for different orientations in the sheet material. The LIAD part of the damage model is a modification of Lemaitre’s (ML) anisotropic damage model. Modifications are made for damage development under compression state and influence of strain rate on damage, and are presented in this paper. Viscoplastic regularization is used to avoid pathological mesh dependency. The MIAD part of the model is an extension of the LIAD model. Experimental evidence is given of the MIAD phenomenon in DP600 steel. The experimental analysis is carried out using tensile tests, optical strain measurement system (ARAMIS) and scanning electron microscopy. The extension to incorporate MIAD in the ML anisotropic damage model is presented in this paper as well. The paper concludes with a validation of the anisotropic damage model for different applications. The MIAD part of the model is validated by experimental cylindrical cup drawing wheras the LIAD part of the model is validated by the cross die drawing process.
Authors: Irina Pushkareva, Abdelkrim Redjaïmia, Antoine Moulin, Nathalie Valle
Abstract: A detailed analysis of the evolution of industrial Dual Phase (DP) steel microstructures is carried out as a function of various annealing and tempering conditions. Advanced characterization techniques such as Parallel Electron Energy Loss Spectroscopy (PEELS) in the TEM and high spatial resolution Secondary Ion Mass Spectrometry (NanoSIMS) are employed in order to provide qualitative and quantitative measurements of local carbon concentration in the martensite. For certain annealing and tempering conditions, it is observed that local variations in carbon levels have occurred inside the individual martensite islands. These carbon variations strongly influence the damage behaviour of the steel. During tensile tests, a clear dependence of the damage mode on the local martensite carbon content is observed. Better knowledge of the relationship between the microstructure evolution at the sub-grain level and the damage behaviour can facilitate the design of DP steels with improved damage resistance.
Authors: Tarja Jäppinen, Seppo Kivivuori
Abstract: In steel wire processing it is difficult to reach a homogenous structure throughout the cross-section of the wire particularly in greater diameters. One alternative for producing a homogenous structure is to find a cooling path with a wide transformation temperature range. Fully austenite steel wire rolled at high temperatures can be decomposed into ferritic-martensitic dual phase structure using relatively slow cooling rates. Test materials were low alloyed low carbon steels with variations in alloying elements. Gleeble-1500 thermomechanical simulator was utilised to study the effect of cooling rate on decomposition of austenite after deformation. The microstructures were studied with an optical microscope. In certain low alloyed steels slow cooling rates eliminate the bainite transformation and instead martensite is formed. The final microstructure depends mainly on the carbon content but also on the amount of other alloying elements and their effects on the austenite phase.
Authors: D.V. Edmonds
Abstract: Recent decades have witnessed some remarkable advances in engineering steels driven by the need to respond to challenges posed, for example, by recovery and transmission of oil and gas, or enhanced vehicle safety and fuel economy. Foremost amongst these must surely be the extended application of carbon steels, achieved principally through ferrite grain refinement by the practice of microalloying coupled with controlled thermomechanical processing. Limitations to strengthening ferrite/pearlite structures further by grain refinement or precipitation, however, has focused attention back to acicular forms of microstructure. One of the most interesting advances in this area has been the development of bainitic steels, which have been almost dormant since the mid-20th century. This resurgence may partly be attributed to a better appreciation of the bainite transformation mechanism, and the experimental work for this which unexpectedly spawned some interesting bainitic microstructures which have seen further development and application. These are the so-called ‘carbide-free’ bainites, which employ alloying to replace carbides, principally cementite, with carbon-stabilized retained austenite. Particularly noteworthy has been the emergence of the transformation induced plasticity (TRIP) sheet steels with enhanced properties principally targeted for automotive use. It is worth mentioning also that a parallel development has produced similar microstructure in austempered ductile irons (ADI), another important ferrous alloy which has seen recent expanding interest in its application. Even more recently, as we proceed into the 21st century, the concept of employing steel microstructures containing carbon-enriched retained austenite, has been developed further by combining both alloying and novel heat treatment procedures to exchange ‘bainitic’ ferrite with ‘martensitic’ ferrite. Interestingly, this non-equilibrium ‘quenching and partitioning’ process route also offers the possibility to increase the retained austenite carbon concentration to very high levels, potentially revealing new and previously unobtainable properties.
Authors: Shuang Kuang, Xiu Mei Qi, Yun Han
Abstract: The microstructures and mechanical properties of a high carbon DP steel and a low carbon Nb microalloying DP steel were investigated. The two types of DP steels have both qualified to meet European standard performance. But the high carbon content DP steel exhibits relatively low elongation and low hole expansion rate as well as poor bending performance. The martensite island in high carbon DP steel appears obvious band structure, and the size of martensite islands is big. Contrary, the matensite islands in low carbon and Nb microalloying DP steel are dispersed and fine, which lead to perfect comprehensive performance.
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