Materials Science Forum Vols. 539-543

Abstract: In order to investigate the effect of heat treatment on the structure and morphology of spray formed structurally amorphous steel, several small samples were cut from one spray formed amorphous steel ingot (DAR – 35). The samples were heat treated at 700oC in a tube furnace under a constant flow of 99.9% argon for up to 120 hours. The samples were furnace cooled to 300oC. Later they were quenched in water. The structure was investigated using x-ray diffraction and the morphology of the polished and lightly etched samples was examined under both optical and scanning electron microscopes. The results indicate that the spray formed steels were amorphous. However, their microstructure is not homogeneous. The microstructure is composed of three distinctively different phase contrasts (viz. light gray, dark gray and bright white contrasts). The bright white phase contained maximum amount of tungsten and molybdenum, it had the least amount of iron. Both light and dark gray phases contained nearly the compositionally expected amount of iron, and more chromium than compositionally expected. The effect of heat treatment at 700oC had no effect on the three different phase contrasts. However, the heat treatment at 700oC showed some effect on the crystallization of the structure. The heat treatment for up to 24 hours has very little effect on the structure and morphology of the amorphous steel samples. However, if the samples were heat treated at 700oC for more than 24 and below 72 hours, it appears that a partial crystallization of the steel occurs. Significant crystallization of the spray formed amorphous steel occurs due to prolonged heat treatment above 72 hours.

Abstract: High Resolution Transmission Electron Microscope and Electron Energy Loss Spectroscopy and have been used to characterize the structure and chemical composition of niobium carbonitrides in the ferrite of a Fe-Nb-C-N model alloy at different precipitation stages. Experiments seem to indicate the coexistence of two types of precipitates: pure niobium nitrides and mixed sub-stoichiometric niobium carbonitrides. In order to predict the chemical composition of these precipitates, a thermodynamical formalism has been developed to evaluate (i) the nucleation and growth rates (classical nucleation theory) and (ii) the chemical composition of nuclei and existing precipitates. A model based on the numerical resolution of former equations, is used to compute precipitates size distribution evolution at a given temperature. The predicted compositions are in very good agreement with experimental results.

Abstract: Industry hot deformation processes such as hot rolling are complex in nature. Setting up a rolling mill requires precise knowledge of the loads needed to shape the metal. This in turn, demands the ability to predict the strength of the material when deformed to a value of strain and strain rate at a given temperature. On and off-line models need, however, to be fed with constitutive equations relating the stresses required to deform a certain metal under the usual process variables. This paper shows how a set of stress-strain curves can be modeled so that both hardening and softening mechanisms commonly present during hot deformation are taken into account. The model predictions are compared to a set of literature data in order to be validated. Reasonable agreement between published results and predicted values were obtained indicating how efficiently the model can assess values of stresses under hot working conditions.

Abstract: The present investigation evaluates the effect of static annealing variables on the grain structure and mechanical properties of a 0.04 %C-Al killed steel. The experimental variables selected were the heating rate and the initial heating temperature. The results showed that an increase in the initial heating temperature and heating rates inferior to 500°C and 100°C/h, respectively, do not have a major influence in the grain structure of the material. These annealing conditions lead to a full "pancake" type of microstructure of the recrystallized ferrite grain, and as consequence, the mechanical properties of the material are in the intervals required for deep drawing applications.

Abstract: This work explores the effect of Cr addition on the kinetics of strain ageing process. The strain ageing behaviour of low carbon steel with and without 0.7wt% Cr was investigated using transmission electron microscopy (TEM), atom probe tomography (APT) and tensile tests. The results have shown that Cr alloyed steel undergoes the same three stages of ageing as unalloyed low carbon steel. However, the alloying with Cr reduces the rate of strain ageing by ~2 times in comparison with non-alloyed low carbon steel. It especially delays the offset of stage III. This is explained by the reduction of carbon content in ferrite due to the enrichment of cementite with Cr leading to the reduction of its equilibrium solubility in ferrite.

Abstract: The present study aims to investigate the mechanism of the development of abnormal grain sizes in the through-thickness direction of hot rolled steel strips. For this purpose, industrially prepared steel strips were further hot rolled in a laboratory hot rolling mill, setting a variety of rolling parameters. As found, the deformation rate in the hot rolling practice exerts an important role in explaining the mechanism of abnormal grain growth, especially in the close vicinity of the strip surface. Furthermore, the influence of the cooling penetration depth, induced by the roll contact was examined closely, as this phenomenon might support abnormal grain growth mechanisms. Additional information was found in performing a texture analysis in the throughthickness direction of the steel strips, in accordance with the optical metallurgical survey of the microstructures. It will be shown that, the combination of particular hot rolling parameters provokes the occurrence of abnormal grain growth in the through-thickness direction of the ELC steel strips. These particular conditions were considered to be related to the finish hot rolling temperature and thus the roll cooling penetration depth imposed on the steel strip, the finishing reduction degree and especially the strain rate conditions. Moreover, the observed abnormal grain growth is sensitive to the coiling temperature applied. From the experiments, it can be concluded that the mechanism of the formation of a large grained ferrite band below the strip surface is strongly influenced by the development of a fine-grain ferrite layer at some distance below the strip surface. The existence of this layer of very small ferrite grains can be explained on the basis of texture analysis and calculations based on literature data. In this way, it was considered that dynamic recrystallisation of austenite at some depth below the steel strip surface is of most significance in supporting the development of abnormally large ferrite grains. In this paper, further considerations on the mechanism of the abnormal grain growth phenomenon will be dealt with.

Abstract: Experiments involving metal dusting in carbonaceous atmosphere and oxidation in air have been carried out at 593 and 704oC using T22, T91 and 800 alloy steels. The effect of Cr on these two processes can be understood because of their compositions. In general, the resistance to both metal dusting up to 1000 hours and oxidation up to 456 hours has been found to improve with increase in Cr content from 2.36 to 21.6%. However, mechanisms change from spalling to oxide scale in oxidation and from uniform metal dusting to pitting in metal dusting as the amount of Cr increases in these steels. Pitting corrosion shows higher C carbon concentration in the pits than at locations away from the pits. A detailed analysis has been performed to evaluate the scale and spalled materials in terms of oxide characterization using XRD, AES, SEM, EDS on SEM, optical microscopy and TEM.

Abstract: Arcelor produces « Bake-Hardening » steels for automotive outer panels, which present the advantages of a remarkable drawability combined with a significant hardening after stamping and paint baking by the car maker. This hardening enables to increase the dent resistance of those automotive parts. In order to give easy design criterion and support the development for new “bake-hardening” steels, a physically-based model for Bake-Hardening steels has been developed. It is suitable to predict: -the physical phenomenon of strain ageing based on Cottrell atmospheres formation. A detailed description of the strain ageing kinetics is given based on a generalized form of the Harper model taking into account the diffusion of carbon atoms in the stress field of a dislocation, progressive carbon depletion in the matrix and saturation of the available dislocation sites. -the plastic instabilities propagation during tensile testing according to Piobert–Lüders phenomenon using the finite element method. A local mechanical behaviour is introduced whose shape schematically describes the local dislocation behaviour. The effect of the grain size on the velocity of the Lüders’ band front is especially enlightened. -the effect of ageing process on dent resistance. To do so, a physical extension of the former approach to more complex loading paths is proposed.

Abstract: From the mechanical point of view, the successful cold drawing of wire-rods of low carbon steel requires a high ductility and a ratio between the yield stress and tensile strength as lowest as possible, both for diminish the fractures during the process. Small boron additions in the cast of this kind of steel produce an increase in the size of austenitic grain, and consequently the ferritic one, and a diminution of the deformation hardening due to the reduction in the nitrogen content in the solid solution. In this way, the goal of this work is to study the effect of small boron additions (67 to 117 ppm) in the mechanical properties of 5.5 mm diameter wire-rods of low carbon steel. The wire-rods were characterized by means of traction tests, metallographic analysis with optic and electronic microscopy, and the results were compared with the corresponding one for wire-rod of standard steel, i.e., without boron contents. The metallographic analysis confirms the increase in size of ferritic grain. The measured ductility present only slight increases in boron steel, no significant from the quality point of view, however, the ratio between yield stress and tensile strength in boron steels shown a clear improvement respect to the standard. Traction tests in samples obtained at the exit of each one of the matrixes used during the cold drawing process shows an improve in the behavior of low carbon steel with boron contents.