Influence of Various Metallurgical Process Parameters on the Oxidic Meso- and Macroscopic Cleanness Level

This thesis is dedicated to the detailed investigation of non-metallic inclusions (NMIs) in steel, particularly in the mesoscopic (15–100 µm) and macroscopic size ranges (>100 µm). The focus is set on how different secondary metallurgical processes and casting conditions influence the formation, size and number of these NMIs. In addition, different methodological approaches were applied/developed and tested under real industrial conditions to reliably identify the origin and source of these NMIs.
A central element of this work is the application of a novel, non-destructive X-ray testing method for inclusion characterization, which was used industrially for the first time in this configuration. It enables the precise localization of NMIs in the transition range between mesoscopic and macroscopic sizes (>80 µm)—a range in which established methods reach their limits in terms of resolution or statistical significance.
The results show that the degree of cleanness of the steel in the mesoscopic/macroscopic range depends largely on the route chosen in secondary metallurgy. While simple treatment routes, such as heating and alloying in a ladle furnace without further measures, lead to a low inclusion amount, desulfurization or Ca treatments significantly increase the number and size of NMIs. These findings are particularly relevant for the integrated steelworks in Linz, which is facing a (partial) conversion from the blast furnace-converter route to electric arc furnace-based steel production. Due to the higher sulfur content in the input material, more intensive desulfurization measures will be necessary in the future, which could have a negative impact on the cleanness level of the final product.
In addition to the secondary metallurgy, the stability of the continuous casting process also has a significant influence on the steel’s cleanness in that size range. Particularly unsteady casting periods—which prevail during ladle changes, for example—lead to an increase in amount of exogenous macroscopic NMIs. Through the combined use of X-ray radiography and slag tracers, it was possible to determine both the temporal course of the degree of cleanness in the strand during this critical phase and the exogenous origin of individual inclusions.
To trace endogenous NMIs, rare earth metals (La, Ce) were added as tracers during Ti-IF steel production. The pre-existing endogenous NMI population was modified and marked using these rare earths. In addition to investigating NMI modification evolution, a focus was set on the separation behavior of the traced inclusions into the process slags and their contribution to the clogging formation within the submerged entry nozzle during casting.
Another industrial trial focused on a specific type of inclusions that was detected both under standard conditions and during tracer studies. Active tracer methods (La/Ce for marking endogenous NMIs and BaO for tracing the mold slag) and a passive method, known as the rare earth fingerprint, were used for this purpose. The latter compares the characteristic trace element profiles of NMIs with those of the input materials used, such as deoxidation agents and slag formers. The Ca-aluminate inclusion type investigated could be clearly traced back to entrapped mold slag. Differences in chemical composition compared to typical line defects, which are also attributed to this source, could be explained by different residence times in the liquid steel and were confirmed by thermodynamic model calculations.
Overall, the work provides new insights into the NMI landscape at the mesoscopic and macroscopic levels under industrial settings and offers innovative methods for determining NMI sources. The results provide a solid basis for optimizing metallurgical processes and developing sustainable strategies to produce clean steel, particularly regarding the challenges of decarbonization and the transition to the electric steelmaking route.