Papers by Author: Lucia Suarez

Abstract: Hot dip galvanizing has proven to provide excellent protection against corrosion of steel for a wide range of applications. Coatings of Zn-Al alloys on steel sheet give a high corrosion resistance due to the corrosion prevention by zinc and the passivation by Al. Many important industrial processing steps require a reliable procedure for process verification. Verification on production or pilot lines is neither economical nor efficient. Simulators for the HDP (Hot Dip Process) allow laboratory scale simulations of the (hot dip) coating and of the consequent annealing processes occurring in industrial production lines, serving for process and product improvement and development. To improve and further develop the production and the final coating properties, hot dipping experiments are performed in a HDP simulator using different substrates, bath compositions and hot dipping parameters. The results obtained by these simulations are transferable to the production process of real continuous galvanizing lines. Important industrial steps of the process can be simulated in the HDPS with a high variability of parameters.

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: 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: Exposure of metals and alloys to high temperatures leads to the formation of oxide scales, with a large impact on surface quality. The most important features of the oxide layer are its thickness, composition, structure, adherence and coherence. Temperature, time, gas atmosphere and chemical composition determine the growth of oxide layers. In this paper, the characteristics of the high temperature oxidation properties of Fe-Si alloys are discussed in terms of oxide growth mechanism, kinetics and phase morphology. The oxidation kinetics of different Fe-Si alloy steels in air, its scale structure and composition were investigated over the temperature range 900-1250°C. Oxidation experiments were performed in air, to analyse the oxidation process. Experiments were carried out in an electric furnace at temperatures ranging from 900 to 1250°C, for times between 16 and 7200s. Thus treated specimens were characterised by metallography and their scale thickness was measured by optical microscopy. Scale morphology was studied and scale composition confirmed by EDS (Energy Dispersive Spectroscopy) and EBSD (Electron Backscattered Diffraction) analysis. Results show that high temperature oxidation of Si- alloys presents the classic three layered oxide scale. On the grain boundaries in the scale, iron-silicate was found. Observations show a Sienrichment at the scale –metal interface. This enrichment is present in the form a mixed wustite-iron silicate (fayalite) phase, FeO-Fe2SiO4. A very rapid build-up of oxide occurs when a liquid phase, due to the wustite-fayalite eutectic, is present in the surface of the steel.

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: 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: An oxide scale layer always forms at the strip surface during the hot rolling process. Its properties have a large impact on surface quality. The most important features of the oxide layer are its thickness, composition, structure, adherence and coherence. Temperature, time and gas atmosphere determine the growth of oxide layers. In this paper, the high temperature oxidation properties of ultra low carbon steels are discussed in terms of oxide growth mechanism, kinetics and phase morphology. The oxidation kinetics of ultra-low carbon steel (ULC) in air, its scale structure and composition were investigated over the temperature range 923-1473K. Oxidation experiments were performed either under controlled atmosphere or in air, to analyse the oxidation process during strip production. A first series of experiments was carried out in an electric furnace at temperatures ranging from 923 to 1473K, for times between 16 and 7200s. A second series was carried out in a device especially designed to control the atmosphere. After heating under pure nitrogen, the samples were oxidised in air at temperatures between 923-1323K for various oxidation times. Thus treated specimens were characterised by metallography and their scale thickness was measured under the optical microscope. Scale morphology was studied and scale composition confirmed by EDS (Energy Dispersive Spectroscopy) and EBSD (Electron Backscattered Diffraction) analysis. Results show that scale growth under controlled atmosphere is significantly faster than under non controlled conditions, additionally the adherence of the scale formed in the laboratory device was significantly better than the other one. It is clear that scale thickness and constitution depend strongly on the oxidation potential of atmosphere. Computed parabolic activation energies (Ea) values are in good agreement with those found in the literature.