The primary focus of this research group are divers questions concerning the cleanness of steels using different experimental and analytical methods as well as thermodynamic and kinetic calculations.

The cleanness of steels is an essential quality criterion for a wide variety of special steel applications. Non-metallic inclusions can initiate cracks or other material defects and can therefore be very harmful for material properties.

Due to the continuously increasing requirements on special steels, a reliable characterisation of the cleanness level is indispensable.

Furthermore, through a detailed analysis of inclusions over the process chain a better understanding of proceeding reactions can be gained. Based on this information, important indications regarding furhter process optimization are obtained.

Selected Publications

https://doi.org/10.1007/s11663-018-1271-2
https://doi.org/10.1002/srin.201800635
https://doi.org/10.2355/isijinternational.ISIJINT-2016-243
https://doi.org/10.1002/srin.201500102
https://doi.org/10.1016/j.matchar.2014.12.014
https://doi.org/10.1007/s11665-018-3468-6

The evolution of non-metallic inclusions during secondary metallurgy essentially influences the casting process as well as the final steel cleanness. Microsegregation, inclusion formation and modification from BOF to tundish are investigated by using different modeling approaches. The models are developed based on the concept of linking practical metallurgical models to thermodynamic databases. During the modeling process, FORTRAN is applied as program language; the thermodynamic­–library ChemApp is used to bridge the models to the databases offered by FactSage. Using the developed models, the formation thermodynamics and kinetics of the inclusions in the liquid steel and during the solidification process can be evaluated. The evolution of steel, slag and inclusions in the secondary steelmaking processes can be predicted and the influence of various metallurgical parameters can be studied. Combining the model calculations with laboratory and plant trials, a comprehensive knowledge of reaction and interactions can be obtained finally enabling a further optimization of specific process steps.

Non-metallic inclusions are generally known to primarily affect steel properties in a harmful way. Thus, rising demands on steel quality also increase the requirements regarding steel cleanness for a wide range of different steel applications. One example are highly stressed low alloyed steel grades which are widely used in the production of both mechanical engineering and automotive components tolerating demanding fatigue strength and high toughness at the same time. Based on thermodynamic and kinetic calculations, well-controllable laboratory experiments in a so-called Tammann-type furnace are used to create a specific inclusion landscape within a desired steel matrix composition. Different deoxidation practices and alloying procedures result in defined inclusion types, which are subsequently characterized with manual and automated SEM/EDS analyses. The produced samples can be used for further mechanical and deformation testing. This production of model alloys enables the direct study of the effect of defined inclusion types and morphologies on certain mechanical properties and thus allows conclusions regarding the impact of specific inclusion landscapes for defined applications and loading scenarios. Based on this knowledge, large-scale tests as well as plant trials to create a specific inclusion population are performed.

Clogging is a phenomenon which usually occurs in the flow control system during continuous casting. It describes the build-up of deposits at various positions in the continuous casting machine. The submerged entry nozzle (SEN) between tundish and mold is the area most frequently affected by the appearance of clogging. Certain steel grades, e.g. Ti-alloyed ULC steels, are especially known for their increased clogging tendency. Since the formation of non-metallic inclusions in the steel cannot be avoided completely, a deeper understanding of their development and behavior during secondary metallurgy and casting is required. Reactions in the tundish should possibly be used for inclusion removal through their separation from the liquid steel to the covering slag with a subsequent dissolution in the slag. Further, this includes the deposition of micro-inclusions to the steel/refractory interface in the submerged entry nozzle (SEN) between the tundish and the mold. For all these cases, interfacial properties of the system inclusion-steel-slag-refractory are believed to play a key role. Both, dissolution of inclusions in slag as well as adhesion of inclusion at the steel/refractory interface can be studied experimentally, primarily by means of HT-Laser Scanning Confocal Microscopy and Drop Shape Analysis. Additionally, a detailed model investigating fluid force-induced detachment criteria for inclusions adhered to a refractory/steel interface helps to define and investigate selective clogging sensitive cases. Finally, data (e.g. interfacial properties, viscosities) obtained in laboratory experiments should be used to provide metallurgical boundary conditions for numerical simulation approaches.

Although in general non-metallic inclusions are regarded to have a negative effect on steel properties, there are interesting aspects dealing with the positive influence of inclusions on steel microstructure influencing e.g. solidification or phase transformations. Typical examples are grain refinement or microstructure control through the specific adjustment of non-metallic inclusions in the steel matrix. One example is the production of high-alloyed steels. Since manufacturing processes like ingot and heavy steel casting do not offer the same possibilities for the improvement of solidification structure as continuous casting, the application of in-situ formed nucleation sites, which are independent of casting dimensions, size and process, are a promising research topic. In this context, the use and potential of rare-earths to improve the primary grain size and the columnar to equiaxed transition, is studied. Another interesting issue is that non-metallic inclusions can act as heterogeneous nuclei for acicular ferrite. Acicular Ferrite is a not equiaxed form of ferrite, which forms a chaotic microstructure of laths and plates nucleating intergranularly at non-metallic inclusions. This chaotic structure leads to an improvement of several steel properties, like toughness, transition temperature or fracture toughness.

A reliable characterization of inclusions is indispensable for all questions in terms of Inclusion Metallurgy. The continuous improvement of steel cleanness over the years, also increases challenges to characterization methods since macro cleanness is continuously replaced by meso and micro or even sub-micro cleanness. Due to the manifold appearance of inclusions and the related problems, also the world of characterization methods is wide. A main research focus lies on the automated SEM/EDS analysis of non-metallic inclusions, which offers a lot of information, but still faces some challenges. A main issue in identifying particles smaller than 1µm is the fact that the ratio of additional stimulated matrix volume and measured particle raises enormously, which essentially influences the measured EDS spectrum. Especially the detailed characterization of multiphase inclusions with a heterogeneous character gets more difficult. Second, since inclusions are 3D objects and inclusion analysis using SEM/EDS is done in cross sections of classical metallographic specimens, additional work in the interpretation of collected data regarding morphology and heterogeneity is necessary applying other methods like for example chemical or electrolytic extraction of particles.