Frans Leysen

Effects of Mg Additions on Surface Morphology and Corrosion Resistance of Hot-Dipped Zn Coatings

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.

A View on the Strategy in the Processing of Hot Rolled Dual Phase Steels

As already intensive studies related to the processing of hot rolled dual phase steels have been reported in the past, the aim of this particular paper is to present some details within a processing strategy, that can be considered as useful for the processing of other special steel types. It will be shown that based on fundamental dilatometric measurements performed in the laboratory, a narrow window of processing parameters can be focused at. Moreover, practical dilatometric curves are shown, taking into account the transformation heat release in the arrested cooling zone (dry section), as to be considered regarding the industrial practice. Furthermore, by introducing a slab insert technique within the development schedule, not only time consuming research can be limited, especially main objectives can be achieved. Some of these objectives are: a) a simultaneous study of the influences of the chemical composition of the steels, b) a close control of the thermomechanical parameters encountered in the industrial environment, such as rolling reductions, strain rate and inter pass time conditions, c) a check of the homogeneity of mechanical properties versus the coil length.

Study of the Development of Heterogeneous Grain Size in the Through-Thickness Direction of Hot Rolled ELC Steels

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.