Systematic Improvement of Steel Properties by means of Inclusion Metallurgy

Non-metallic inclusions distributed in the steel matrix generally are regarded to be harmful to steel properties depending on their number, size, morphology and composition. According to the application of steel, different requirements on the cleanness level and the steel properties (mainly strength and toughness) exist. The present project aims to utilize the presence of non-metallic inclusions for a positive impact on steel microstructure and therefore for an improvement of material properties. It is known that steels with a high volume fraction of acicular ferrite in the microstructure offer an excellent combination of strength and toughness and non-metallic inclusions can act as nuclei for the formation of acicular ferrite. So far, the most decisive mechanism for this effect and the related influencing parameters in dependence of the steel composition are not precisely known. The aim of the present project is the fundamental study of the relationship between inclusions and microstructure, especially considering the influence of different alloying elements and the impact of different inclusion types on the formation of acicular ferrite structure. Special attention should be paid on the effect of Titanium and the influence of multi- phase inclusions. Additionally, the potential of rare earth metals regarding this problem should be treated in detail. For this purpose the combination of several experimental and analytic methods as well as thermodynamic and kinetic calculations are necessary. Based on an extensive literature study and an adequate adaptation of analytic methods, alloys featuring different compositions and inclusion landscapes should be produced on laboratory scale providing the base material for further investigations and subsequent material tests. A major part of the project will be the in-situ observation of proceeding reactions (phase transformations, inclusion formation and modification, acicular ferrite formation) using a High-temperature Laser Scanning Confocal Microscope. For inclusion analyses primarily SEM/EDS measurements in combination with optical microscopy will be applied. The use of EBSD (Electron Backscatter Diffraction) and Atom Probe analysis will enable a detailed insight in the relationship between inclusions and microstructure through the determination of the crystallographic orientation of different structural constituents as well as the three-dimensional reconstruction of the distribution of different elements in the analyzed volume. In a second step, modifying the inclusion landscape through accurate metallurgical treatments in a way that enables the parallel use of the positive effects of inclusions on the one hand and minimizing the content of harmful inclusions on the other hand would offer a wide range of new perspectives regarding the appropriate design of steel.

The application field of steel is manifold. In order to ensure the required product quality and to avoid sudden material failure, material properties are essential. Another challenge is the demanded weight reduction at same strength values in automotive industry. This is necessary to keep steel competitive compared to aluminum or composite materials. Two criterions significantly influence steel properties: Steel cleanness and microstructure. Steel cleanness is characterized by the number, size and type of non-metallic inclusions. In general, the content and size of non-metallic inclusions distributed in the steel matrix should be as low as possible. However, inclusion formation cannot be avoided completely during steel processing. The project aimed at the specific adjustment of inclusion number, size and type provoking the formation of acicular ferrite a microstructure offering an excellent combination of strength and toughness. Through merging a tailored steel cleanness level with an accurate microstructure, steel properties can be improved significantly. Relevant mechanisms and various influencing parameters were investigated at laboratory scale for different steel grades. The developed systematic approach enables the formation of the desired microstructure without the addition of synthetic particles. This fact is highly important, since an artificial addition of particles decreases steel cleanness significantly. Only inclusions directly formed due to reactions in the melt were used as potential nuclei. Next to the influence of cooling rate and austenite grain size, special attention was paid on the role of divers alloying elements as well as the potential of different inclusion types acting as nuclei for the formation of acicular ferrite. A special method used within the project was the so called High-Temperature Laser Scanning Confocal Microscopy. The latter enables the in-situ study of acicular ferrite formation and the inclusion behaviour at very high temperatures. So, valuable conclusions regarding fundamental reactions could be obtained. The project results should now essentially contribute to the development of new alloying concepts for high quality steels.