Lucia Suarez

Texture Evolution of Tertiary Oxide Scale during Steel Plate Finishing Hot Rolling Simulation Tests

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.

High Temperature Oxidation of Ultra-Low-Carbon Steel

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.

Tertiary Scale Behaviour during Finishing Hot Rolling of Steel Flat Products

Steel strip surface oxidation during hot mill processing represents 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. Controlling the oxide layer growth kinetics and mechanical properties can make pickling easier and improve downstream behaviour. A thin wustite-dominated scale layer (<20 μm) is created under controlled conditions in an original laboratory device adequately positioned in a compression test machine to investigate plane strain compression. A first series of oxidation tests were performed on a ULC steel grade to measure the kinetics of oxide scale growth. The samples were first heated up under a protective atmosphere (nitrogen), before being oxidised in air at different temperatures for various oxidation times. These experiments can be considered fair quantitative and qualitative simulations of scale growth as it occurs in a hot strip mill, insofar as the results thus obtained are in good agreement with the literature. After the oxide growth, plane strain compression (PSC) was performed immediately to simulate the hot rolling process. The oxide layers were characterised before and after compression tests by optical and secondary electron microscopy. As expected, the oxide is seen to deform during compression. The obtained oxide layers exhibit good adhesion to the substrate and homogeneity over the thickness, even after compression.

Al-Si-Fe Intermetallics on Fe-Substrates during Hot-Dipping

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.

Galvanized Coatings Produced in a Hot Dip Simulator (HDS)

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.

The Influence of Deformation on Microstructure Evolution of Low Alloy TRIP Steel

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.

Microstructure and Phase Identification of Tertiary Oxide Scale on Steel by EBSD

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.