Papers by Author: Yvan Houbaert

Abstract: An oxide scale layer always forms at the strip surface during the hot rolling process. As a consequence, de-scaling and pickling operations must be performed prior or after hot rolling. Many surface defects caused by hot rolling are related to oxidation in the reheating furnace. One of these is the melting of eutectic FeO/Fe2SiO4 during reheating over 1170°C giving as a result red scale defects in Si-added steel. On the other hand, steel strip surface oxidation during hot rolling causes an industrial and environmental problem: secondary oxide is removed after roughing, but tertiary oxide scales already start to form before entering the finishing stands. Their properties affect the final steel surface quality and its response to further processing. Furthermore, the addition of alloying elements has an important impact on scale properties. In particular the alloying of silicon effects the region between scale and substrate. It causes peculiar surface properties inherited from its specific oxidation characteristics. Conventional oxidation experiments in air of silicon steels are a valuable tool to study the influence of Si on steel oxidation. After oxidation in air in the temperature range of 900-1250°C it has been observed that Si enhance markedly scale adhesion, especially above 1177°C (the eutectic temperature of FeO-Fe2SiO4 ) and also at lower temperatures. Special attention has been paid on the investigation of the effects of alloying Si on the high-temperature oxidation of steel, for a better understanding of the behaviour of modern steels during hot rolling.

Abstract: Steel is still the main construction material for automobiles, general equipment and industrial machinery. Hot dipping has been proven to be an excellent method of corrosion protection of steels for a wide range of applications worldwide. Coatings of Zn-Al alloys on steel sheet have high corrosion resistance due to the corrosion prevention ability from Zn and the passivation of Al Bath composition, immersion velocity/time and substrate composition are the hot dipping parameters that more influence on the thickness and corrosion resistance of the deposited coating. In order to study their influence small amounts of magnesium were added. Experiments were performed in a hot dipping simulator using different substrates, bath compositions and hot dipping parameters. Surface layers were characterised by: Scanning Electron Microscopy (SEM) and Energy dispersive X-Ray spectroscopy (EDX or EDS). Cyclic corrosion tests were performed in order to observe the corrosion resistance for different Zn-Al-Mg coatings. Results show that the microstructure and composition of the substrate strongly affect the desired coating properties. Nevertheless, the influence of the magnesium on coating thickness is relevant, increasing when added in small quantities in a molten bath of Zn-5wt %Al. The quality and microstructure of the coating is affected by the amount of Mg in the bath. Cyclic corrosion tests results show that the quality of the coating is affected by the amount of Mg in the bath.

Abstract: Non oriented electrical steels are soft magnetic materials used in the core of electrical motors. No preferential anisotropy of the electrical texture in the rolling plane is desired. Nowadays these special steels are mainly alloyed with Si, Al and some additives to improve the magnetic properties and to reach a good of formability. For (Si, Al)-concentrations higher than 2 wt.% the α- γ-α phase transformation is suppressed, resulting in a bcc crystalline structure from liquidus to room temperature. These electrical steels, which will be discussed in the paper, exhibit the lowest values of the magnetic losses. Hot rolling of FeSi electrical steels has been found to be one of the fundamental steps in producing these materials with optimum properties. The resulting properties, as well known, are determined by the type of magnetic textures and the structural inhomogeneities. Electron Backscattered Diffraction (EBSD) is a reliable tool for microstructural and texture characterization of different materials. Two compositions of electrical steel are studied by optical microscopy and EBSD, with special attention paid to characterize the grain morphology and its texture through thickness.

Abstract: The addition of aluminium (and of silicon) to steel increases its electrical resistivity and reduces therefore the power losses in electrical devices. There is also a favourable effect on magnetostriction. Nevertheless, these additions make the steel extremely brittle and very difficult to process through a conventional thermomechanical route. The authors developed an innovative processing route, avoiding the rolling of a brittle steel sheet. The used process consists of the hot dipping of a steel substrate in a pure aluminium bath, followed by a diffusion annealing treatment. In order to study the reaction of the aluminium with the substrates and the diffusion process during further annealing, two substrates (ultra low carbon steel (ULC) and a Fe + 3.4 m.-% Si steel) were used for immersion in a pure aluminium bath. Dipping times and temperatures were varied in the range of 700 to 750 °C and 5 to 1000 sec., respectively. The different surface layers formed during dipping and after annealing were characterised with an Elcometer, by Scanning Electron Microscopy (SEM) and by Energy Dispersive Spectroscopy (EDS). The results show that the chemical composition of the layers obtained is strongly dependant on the initial substrate composition. Diffusion gradients of Al and Si in the steel after hot dipping and diffusion annealing are shown and discussed. Samples with a concentration gradient of Si and Al over the thickness have been produced. There is only a light reduction of the power losses for the substrate with 3.4 m.-% Si. The ultra low carbon substrate presents worse power losses after the processing. Further improvement of the processing is still required.

Abstract: Electrical steels are used in flux carrying machines, divided in grain oriented and non oriented electrical steels mainly used in transformers and electrical motors, respectively. Their industrial production is not always easy due to the alloying elements which produce brittle order structures in the steel. Therefore hot dipping was found to be an alternative way of producing electrical steel with a high concentration of Al and/or Si: in a first series of experiments different steel substrates were coated by immersion in an Al + 23 m.-% Si hypereutectic alloy, followed by a high temperature diffusion annealing. The present contribution reports on the growth kinetics of Al-Si-Fe intermetallics formed during the dipping process in a hypoeutectic Al – 5 m.-% Si bath of Fe-substrates with 3 m.-% Si, previously cold rolled to different thickness. This bath composition allows a liquid phase at temperatures lower than the hypereutectic one with 23 m.-% Si and also less amount of eutectic formation. No Na-addition was made to the bath (the occurrence of a needle-like morphology of the Al-Si eutectic was not relevant for these experiments), furthermore this element might lower the magnetic properties of the steel. The preheating of samples and bath temperatures were not varied and set to 670°C. Short dipping times of 1 to 60 sec. were applied. The different layers formed were characterised by Scanning Electron Microscopy (SEM), using the Back Scattered Electron (BSE) detector and Energy Dispersive Spectroscopy (EDS).

Abstract: A good combination of strength and toughness in HSLA steels can be achieved by the addition of microalloying elements such as Nb. Nb can retard the static recrystallization of austenite at low temperatures by either a solute drag or by a precipitation pinning (when bonded to C or N) effect. Both mechanisms result in improved mechanical properties due to grain size refinement of the transformed ferrite. In this study, 3 Nb-microalloyed model alloys were designed to investigate the solute drag and the precipitation effect separately. The first alloy, containing a stoechiometric ratio of Nb and C, was designed to study the retarding effect of NbC on the recrystallization behavior. A second alloy, containing Nb and only few ppm C, was casted in order to study the effect of Nb in solid solution. The two alloys were compared with a C-Mn reference alloy. The recrystallization behavior of the three alloys were compared by multi-hit torsion tests and double hit compression tests. The Nb-C and the Nb-very low C showed small differences in recrystallization behavior. These results show that Nb delays the recrystallization by a solute drag effect or by the formation of a very small amount of precipitates.

Abstract: High silicon steel is used for electrical applications because its electrical resistivity is increased and the magnetostriction is reduced. A silicon content up to 6.5 wt.-% gives excellent magnetic properties. The improvement of the magnetic properties stays in contrast with the lack of ductility of these alloys, making their thermo-mechanical processing difficult. The optimum final microstructure and texture depends on the final application of the material: extremely big grains with a Goss orientation ({110} <001>) are desired in transformers and grains with an average size of 100 -m and cube component ({100} <001>) are used in electrical motors. A series of plane strain compression (PSC) tests were performed on 3 electrical steels, with a silicon content from 1.8 to 4.1 wt.-%, in a temperature range of 800 to 1100°C, strain rates between of 0.5 and 5 s-1. Reductions and time between deformation and quenching were also varied in order to study the recrystallisation progress. Apparent activation energies for hot working, calculated using the hyperbolic sine equation, was in good agreement with literature and higher than the activation energy for self diffusion in iron. These values increase with the silicon content. The high temperature texture evolution was investigated by means of electron back scattering Diffraction (EBSD) technique, which allows the quantification of important texture components in function of the thermo-mechanical parameters applied during hot rolling and the plane strain compression tests. The hot rolled microstructures have shown an average grain size of 140 -m and a texture with a maximum on the cube fibre ({001} <-1-10>). The conventional α (<110> // RD) / γ (<111> // ND) fibre texture was developed after plane strain compression and their intensities depend on the deformation temperature and reduction. A similar tendency was observed for the fraction of static recrystallised grains.

Abstract: The microstructural anisotropy together with the crystallographic texture of an industrial grade of X70 pipeline steel is studied by means of the 3D-EBSD technique known also as EBS3 which was recently developed by FEI. Samples of size 8x10x3mm³ were cut from the middle thickness of an industrial rolled plate and after special sample preparation have been studied in a Nova 600 dual beam scanning electron microscope equipped with a field emission gun and HKL Channel 6 EBSD data collection software for crystallographic orientation, which allows multiple sectioning of the sample in automatic mode and, afterwards reconstruction of both the 3D microstructure and texture of the examined volume. Three scanned zones of different volumes that varied between 15x10x27 4m³ and 16x14x6 4m³ have been examined and the results for the crystallographic orientation, grain shape and grain shape orientation are discussed together with the data for the anisotropy of the Charpy impact toughness of the material.

Abstract: Thin tertiary scale layers have been grown on ULC steel specimens under controlled conditions. After heating under a protective atmosphere (nitrogen), the samples have been oxidised in air for various oxidation times at 1050°C. These experiments are considered a quantitatively and qualitatively reasonable simulation of the scale formation and growth occurring before hot rolling. Immediately after controlled oxidation, some of the samples were subjected to plane strain compression, in order to simulate the finishing hot rolling process. This approach provided a better insight into the deformation behaviour of the tertiary oxide layer in the first hot rolling pass. The layers produced were examined under the SEM using the EBSD technique for texture characterisation and phase morphology determination. The texture of the deformed oxide scales, originally grown on ULC steel at 1050°C, was determined in order to achieve a better understanding of their complex deformation behaviour. This paper gives a first approach of the study of deformed oxides by EBSD. Strongly textured wustite grains with a clearly pronounced columnar structure were observed after oxidation at 1050°C. As the substrate deformation probably affects the oxide layer, orientation relationships between scale layer and substrate were observed. The detailed EBSD study reveals that the oxide layer can accommodate a significant amount of deformation. The oxide layers exhibit good adhesion to the substrate and remain homogeneous over the thickness after compression.

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.