Papers by Author: Yvan Houbaert

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Authors: Pablo Rodriguez-Calvillo, Roumen H. Petrov, Yvan Houbaert, Leo Kestens
Abstract: Electrical steels, in particular Fe-Si alloys, are used as magnetic flux carrier in transformers and motors because of their excellent magnetic properties. They owe these magnetic properties in part to the presence of specific texture components such as the Goss ({110} <001>) or the cube components ({001} <010>), but also to the chemical composition which is optimum with 6.5 wt. % Si. This high silicon content provides a stable BCC lattice structure to the alloy over the entire solid state domain, but also renders the material more brittle. This embrittlement, which is induced by ordering phenomena, makes it impossible to produce the alloy in a conventional rolling process unless a specific thermomechanical route at high temperature is applied. In order to examine the working behaviour of high Si electrical steels, a series of room temperature plane strain compression tests was carried out on a Fe-3%Si alloy in hot band condition. The samples were compressed with a constant strain rate of 20 s-1 to a reduction of 10, 35 and 70% and subsequently annealed for different times at 800 and 900°C in an electrical furnace without protecting atmosphere. The hot rolled microstructure displayed an average grain size of 195 7m and the texture showed on the cube component ({001} <010>) of maximum 5x random levels. After plane strain compression the samples developed the conventional α (<110> // RD) / γ (<111> // ND) fibre texture by plastic shear which was also accommodated, in part, by mechanical twinning. With regard to the annealed material, it was observed that the recrystallisation started in grains with the higher stored energy and within the shear bands. After a reduction of 70% the samples that were annealed at 800°C for 4 hours displayed an average grain size of 27 7m and a relative maximum of 4x random on the cube component. Also other less intense components such as the rotated cube ({001} <110>) and the Goss ({110} <001>) were present in the annealing texture. The samples that were annealed at 900°C, after a reduction of 70%, were characterized by an average grain size of 36 7m and by the appearance of the {111} <121> γ fibre component with an intensity of 4.7.
Authors: Frans Leysen, Jan Penning, Yvan Houbaert
Abstract: As already intensive studies related to the processing of hot rolled dual phase steels have been reported in the past, the aim of this particular paper is to present some details within a processing strategy, that can be considered as useful for the processing of other special steel types. It will be shown that based on fundamental dilatometric measurements performed in the laboratory, a narrow window of processing parameters can be focused at. Moreover, practical dilatometric curves are shown, taking into account the transformation heat release in the arrested cooling zone (dry section), as to be considered regarding the industrial practice. Furthermore, by introducing a slab insert technique within the development schedule, not only time consuming research can be limited, especially main objectives can be achieved. Some of these objectives are: a) a simultaneous study of the influences of the chemical composition of the steels, b) a close control of the thermomechanical parameters encountered in the industrial environment, such as rolling reductions, strain rate and inter pass time conditions, c) a check of the homogeneity of mechanical properties versus the coil length.
Authors: Yvan Houbaert, Rafael Colás, José Barros, Daniel Ruch, Robert E. Vandenberghe, Marc De Wulf, Tanya Ros-Yáñez
Authors: Pablo Rodriguez-Calvillo, Lucia Suarez, Yvan Houbaert
Abstract: Steels alloyed with Si and Al are used as core material in flux carrying machines, they are commonly called electrical steels, divided into grain oriented and non-oriented when a material without magnetic anisotropy or not is desired and used in transformer and electrical motors, respectively. The appearance of brittle ordered structures when Si+Al content in steel is above 4 m.-% does not always make its industrial production easy. Therefore hot dipping in a Al-Si bath followed by a diffusion annealing was found to be a productive way of steels with high Si and/or Al concentration and to overcome the creation of fragile structures during deformation processes, such as rolling. The formation of different layered Al-(Si)-Fe intermetallics on the steel substrate depends on diverse processing parameters such as bath temperature and composition, immersion time, preheating of the steel substrate and its composition and cooling down to room temperature. This contribution reports the diffusion kinetics of Fe2Al5 products obtained during the hot dipping process in an Al iron saturated and a hypoeutectic Al – 5 m.-% Si baths of ultra low carbon steel and Fe-substrates with 3 m.-% Si, annealed and cold rolled to different thicknesses. The preheating of the samples and bath temperatures were varied between 670 to750°C. Dipping times between 1 to 600 sec. were applied. The different layers and compounds formed were characterized by Scanning Electron Microscopy (SEM), using Back Scattered Electron (BSE) detector and Energy Dispersive Spectroscopy (EDS). The influence of the substrate and bath chemical composition on the growth kinetics of the Fe2Al5 intermetallics was investigated assuming a parabolic law. Si addition retards the growth kinetics and, as result, raises the activation energy from 71.3 to 159.8 kJmol-1, the obtained results are in agreement with the literature.
Authors: Yvan Houbaert, T. Ros Yáñez, Marta Prado
Authors: Pablo Rodriguez-Calvillo, Juergen Schneider, Yvan Houbaert
Abstract: Steel containing a high Si-content is mainly used as electrical steel in flux carrying electrical machines. These materials are divided in the categories: grain oriented and non oriented electrical steels, mainly used in transformers and electrical motors, respectively. Their industrial production is normally limited to silicon contents lower than 3.5 m.-%, due to the generation of brittle ordered structures if the Si content is increased beyond this value. The paper reports on microstructure and texture evolution during processing by rolling of electrical steel in the high Si-range. The materials studied are two industrial electrical steels with a silicon content of 2.4 and 3.2 m.-%, their situation was as-received after hot rolling and industrial annealing. The different processing parameters, as rolling temperatures and cooling conditions have a strong influence on the final microstructures and textures. The importance of hot rolling and intermediate annealing processes is enhanced since above 2 m.-% Si these steels do not experience the usual α-γ-α phase transformation, because they present a bcc crystal structure over the entire solidus domain. Consequently, their microstructures and textures are strongly inherited from the earlier processing steps into the final product. The as-received materials were cold rolled with a nominal reduction of 75%. Their microstructures and textures were analysed by EBSD. The results obtained were compared with those of the industrial hot band. The textures were studied by the interpretation of the most important crystallographic fibre textures, extracted from the ODF’s of φ2 = 45o section of the Euler space. Special attention was given to the evolution of the most important magnetic textural components. Although in terms of grain shape, IQ, texture and normalised thickness position or ‘s’-parameter the microstructures obtained before and after cold rolling are totally different, the overall crystallographic textures seem not to differ very much.
Authors: Maribel de la Garza, Mayra Moreno, Martha Patrizia Guerrero-Mata, Patricia del C. Zambrano, Rafael Colás, Yvan Houbaert
Abstract: Galvannealed coatings are obtained by heat treating galvanized steel strips and are widely used in the automotive industry due to their improved properties. The variables influencing coating formation on high strength low alloy steel (HSLA) were analyzed. The study was carried out on samples obtained from an industrial plant and on samples coated in a hot dip process simulator. The mechanical properties of the steel substrate were evaluated, and the morphology and Fe content in the various phases of the coating and its characteristics were analyzed and compared. It was found that the Fe-Zn phases can be controlled and performed varying the temperature and holding time during the annealing cycles, avoiding the formation of undesirable phases.
Authors: I. Infante Danzo, Kim Verbeken, Yvan Houbaert
Abstract: In order to improve the magnetic properties of electrical steels, it may be desirable to increase the Si and/or Al content of the steel. A possible and alternative route to realize this is through the application of an Al-Si-rich coating on the steel substrate using a hot dipping process, followed by a diffusion annealing treatment. Previously, a series of compositions were used for dipping, namely: pure Al, Al + 10wt% Si (hypo-eutectic composition) and Al + 25wt% Si (hypereutectic composition). After these dipping experiments, and the subsequent evaluation of the coating and its formed intermetallic phases, the use of a hypo-eutectic Al-Si-bath was recommended for further investigation, because of certain advantages: i.e. hypo-eutectic concentrations allow lower dipping temperatures and reduce the formation of ordered Fe-Si-structures that cause brittleness in the coating and substrate. The present work reports on the results obtained on materials that were hot dipped in a hypo-eutectic Al-Si bath. An Al + 1wt%Si bath was used to coat electrical steel substrates with different silicon contents with dipping times, varying between 0 to 20 seconds, after a preheating of the samples to a temperature of 700°C. A thorough characterization of the formed intermetallics was made by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD). Three different compounds were identified as Fe2Al5, FeAl3 and a nearly pure Al phase.
Authors: José Barros, Tanya Ros-Yáñez, Yvan Houbaert
Abstract: The chemical and physical interaction between Fe-Si alloys in the range 0-3.8 Si wt% and a molten Al-(Si 25wt%) alloy at 800 °C has been studied for different reaction times (from 0.1 to 200s) by hot dipping tests. Several intermetallic phases have been identified, Fe2Al5, τ1-Al3Fe3Si2, τ2-Al12Fe6Si5, τ3- Al2FeSi and τ4- Al3FeSi2, which already were reported in the literature dealing with the interaction between iron and molten Al-Si alloys. In addition an ordered phase Fe3Si (D03) appears in contact with the Fe-Si substrate. Diffusion reaction and solidification phenomena appear to be involved in the developing of the coating. The growth kinetic has been studied and diffusion appears as the step controlling the intermetallic compounds growth. Special attention was paid to the effect of the microstructure of the dipped sheet on the interaction with the molten alloy. The higher deformed structures react faster; this effect can be explained by the faster diffusion through high diffusivity paths like grain boundaries and dislocations.
Authors: Elisabete Pinto da Silva, Wei Xu, Cecilia Föjer, Yvan Houbaert, Jilt Sietsma, Roumen H. Petrov
Abstract: Recent studies have shown the possibility to induce time-dependent phase transformations during isothermal treatment between the martensite start (MS) temperature and martensite finish (Mf,) temperature, i.e. after initial martensite formation. Such treatments result in specific complex microstructures consisting of bainite, martensite and retained austenite, depending on the holding temperature and time. However, the nature of the isothermal transformations below MS is not completely understood and issues like isothermal formation of martensite and bainite formation are still under discussion. The purpose of this study is to investigate the phase transformations from austenite, subsequent to initial martensite formation, during isothermal treatments at different temperatures of HSLA steel. The microstructure development was monitored by means of dilatometry and microstructural characterization of the transformation products by Optical Microscopy, Scanning Electron Microscope, Electron Backscatter Diffraction and X-ray diffraction. The phase transformations and complex competition and interactions between the different transformation mechanisms are discussed.
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