Papers by Author: Pablo Rodriguez-Calvillo

Abstract: FeSi steels with and without addition of Al are widely used as electrical steels. To improve the knowledge of the effects by the addition of Si and Al on the hardening and softening under hot rolling conditions, the behaviour of the flow curves in a wide range of temperatures and deformation velocities have been studied.

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.

Abstract: Low alloy transformation-induced plasticity aided (TRIP) steels have attracted much interest over the last years. TRIP steels were initially developed for automotive applications as they offer an excellent combination of strength and ductility at reasonable costs. These excellent mechanical properties mainly arise from a complex multiphase microstructure of a ferrite matrix and a dispersion of multiphase grains of bainite, martensite and metastable retained austenite. The relevant influence of microstructure on physical and mechanical properties makes metallographic study essential for an appropriate understanding and improvement of the mechanical behavior. An accurate microstructural characterization and quantification of the amount of the different constituents is indispensable to know how the stresses and strains are distributed within the different microstructural constituents. Among the different characterization methods commonly used electron backscatter diffraction (EBSD) appears to be the unique technique able to observe retained austenite grains often no larger than 1 μm. The present work shows the evolution of retained austenite while straining. Microstructural and textural evolution after different strains was examined by optical microscopy OM, EBSD and XRD techniques on TRIP800 steel. EBSD technique appears as a powerful tool for characterizing the complex multiphase steel microstructure and provides an accurate evaluation of the local crystallographic texture. It allows to measure orientation gradients within individual grains of each different phase. The distinction between some phases is observed.

Abstract: The steel production from scrap using continuous cast technology has increased in last decades. Sometimes, steels processed via this route display poor ductility at high temperature. This feature is associated to cooling conditions and chemical composition, which in turns affect the segregation pattern and vary the transformation temperatures and the phase transformation kinetics. The material under study was a C40 steel with a dendrite solidification microstructure coming from an industrial continuous casting plant. The high temperature ductility was evaluated by means of tensile tests up to fracture at strain rate of 0.001 s-1 in a temperature range of 1100 to 710°C. The reduction in area at fracture as a function of temperature graphs show a clear reduction of the steel ductility in the intercritical region, but also after the pearlite transformation. Single deformation compression tests were also carried out on the steel in the austenitic temperature domain, 900 to 1100°C, and at strain rate of 0.001 to 1 s-1. A modification of the Garofalo hyperbolic sine equation has been employed to derive the peak and steady stresses of the flow curve. The work hardening, U, and dynamic recovery, Ω, parameters which describe the flow curve before dynamic recrystallization takes place and the k and t50 parameters, based on the JMAK model, to describe the recrystallization kinetics were also calculated for every test and expressed as a function of the Zener Hollomon parameter, Z.

Abstract: Oxide scales growing during hot rolling of steel represent an industrial and environmental problem. Tertiary oxide, which starts to form before entering the finishing stands, remains on the steel surface until the end of the process, affecting the final surface quality and the response to downstream processing. Characterizing scale layers and the scale/steel interface in terms of phase morphology, texture, grain structure and chemical composition is fundamental for a better understanding of their behaviour and the effect of thermomechanical cycles on the material response to further processing. Thin tertiary scale layers have been grown on ULC steel under controlled conditions in a laboratory device adequately positioned in a compression-testing machine, immediately before plane strain deformation. After heating under a protective atmosphere (nitrogen), the samples have been oxidized in air at 1050°C for a short oxidation time. Immediately after this controlled oxidation, some of the samples were subjected to plane strain compression (PSC) inside the experimental device, in order to simulate the finishing hot rolling process. Direct observations of oxide scale layers are impossible. The EBSD technique has been identified as a powerful tool that can be used to reveal the microstructure within the oxide scale and to distinguish between its constitutive phases, based on their distinct crystal lattices. The texture of the deformed oxide scales, originally grown on ULC steel, was determined in a SEM using the EBSD technique. This will help to achieve a better understanding of their complex deformation behaviour. Because the substrate deformation affects the oxide layer, orientation relationships between scale layer and substrate were measured and the crystallographic orientation between undeformed and deformed areas was determined. Strongly textured wustite grains with a clearly pronounced columnar structure were observed after oxidation at 1050°C. The detailed EBSD study reveals that the oxide layer is able to accommodate a significant amount of deformation.

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.

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: 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.