Jan Penning

High-temperature deformation properties of austenitic Fe-Mn alloys

The influence of the Mn content on the hot deformation properties of austenitic binary Fe-Mn alloys containing 1 to 20 mass pct Mn has been investigated for the first time. The influence of the Mn content on the constitutive equations was determined for the temperature range of 950 °C to 1250 °C and the strain rate range of 0.1 to 2 s−1. The activation energy for hot working increased with increasing Mn content, and dynamic recrystallization was observed for all the Fe-Mn binary alloys. The Mn was found to delay dynamic recrystallization. An increase in the Mn content resulted in an increase of both the peak stress σp and the corresponding peak strain εp, thus revealing the pronounced influence of Mn on the hot deformation process.

Recrystallization Behaviour of an Austenitic High Mn Steel

The recrystallization behaviour of a cold rolled austenitic Fe-Mn steel is studied to explain the fine grained final microstructure. Thorough investigation of the kinetics, the microstructure and the texture evolution during recrystallization showed that the fast recrystallization kinetics is responsible for the final microstructure, while an oriented nucleation mechanism determines the texture evolution. The reason for the fast recrystallization kinetics is the low amount of recovery prior to recrystallization, resulting in a high driving force for the latter.

Selective Oxidation during the Austenitic Annealing of a CMnSi Steel

High strength multiphase CMnSi steel is increasingly used in passenger cars. Si and Mn alloying levels are typically in the range of 1-2% in mass. While Si improves the mechanical properties, it considerably deteriorates the galvanisability of steel. Residual water vapour in the reducing gas atmosphere during the intercritical or austenitic annealing results in the selective oxidation of Si and Mn at the steel surface. Besides Mn and Si, C is oxidized as well at the steel surface, leading to the formation of CO gas and decarburisation of the steel surface. This decarburisation has a major influence on the phase composition in the steel surface region: it shifts the ferrite to austenite transformation to higher annealing temperatures, leading to differences in surface and bulk microstructure. The phase composition influences the solubility and diffusivity of all alloying elements near the surface. The evolution with temperature of the selective oxidation at the steel surface has been studied by interrupted annealing in a protective atmosphere containing residual water vapour. The influence of the annealing temperature on the selective oxidation of Mn and Si is characterized by XPS (X-ray Photo-electron Spectroscopy) analysis.

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.