The Role of FeTi Addition to Micro-inclusions in the Production of ULC Steel Grades via the RH Process Route
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In the production of ULC steel grades via the RH process route, Al is added first for deoxidation after the end of decarburization and FeTi follows after a certain period of separation of alumina particles and the related reduction of the total oxygen content. The FeTi addition is well known to cause clogging problems in the following casting process.
The analysis of plant data by voestalpine Stahl in Linz indicates an increasing clogging tendency with a higher Ti/Al ratio in the steel. Automated SEM/EDS investigations on lollipop samples show the existence of a newly nucleated Ti-containing alumina particle population after the FeTi addition. These particles are comparably smaller than the alumina particles and, even if thermodynamically unstable, they still exist as a large population of small particles in samples taken from the tundish.
The addition of Al and FeTi into a molten steel sample with controlled initial oxygen activity for varying Ti/Al ratios was simulated in laboratory scale afterwards. Just like in the plant, a new population of small Ti-containing alumina particles nucleates, with the size and number depending on the Ti/Al ratio in the melt.
Laboratory experiments and plant observations are in consistence with each other and indicate—for the underlying process route and process parameters—some countermeasures for the better control of clogging.
KeywordsContinuous Casting Ti-ULC Clogging
Der Einfluss der FeTi-Zugabe auf die Bildung mikroskopischer, nichtmetallischer Einschlüsse in ULC Stählen
Die Produktion von Ti-stabilisierten ULC–Stählen sieht nach Tiefstentkohlung in der RH-Anlage die Desoxidation mit Aluminium und nach einer vorgegebenen Wartezeit, die Zugabe von FeTi vor, was nachfolgend zur Bildung von Anhaftungen im Gießsystem der Stranggießanlage („Clogging“) führen kann.
Die Auswertung von Betriebsdaten aus dem Stahlwerk der voestalpine in Linz zeigt eine deutlich höhere Clogging-Tendenz für zunehmendes Ti/Al-Verhältnis. REM/EDX-Untersuchungen an Lollipop-Proben nach der FeTi-Zugabe weisen auf die Existenz einer neu gebildeten Einschlusspopulation von kleinen Aluminiumoxiden mit geringem Ti-Gehalt hin. Obwohl thermodynamisch instabil, bleiben diese Partikel nahezu unverändert bis zum Vergießen erhalten und sind im Verteiler die bestimmende Einschlussklasse.
Die Zugabe von Al und FeTi zu einer Eisenschmelze mit kontrolliertem Anfangssauerstoffgehalt wurde anschließend im Labor simuliert. Auch unter diesen Bedingungen bilden sich Ti-haltige Aluminiumoxide, Zahl und Größe hängen von Anfangssauerstoffgehalt und dem Ti/Al-Verhältnis ab.
Aus den Untersuchungsergebnissen lassen sich qualitative Rückschlüsse auf mögliche Clogging-Gegenmaßnahmen ziehen.
SchlüsselwörterStranggießen von Stahl Ti-stabilisierter ULC Clogging
Ti-stabilized ULC and IF steel grades frequently cause the build-up of oxide layers or oxide networks in the flow control system of the casting machine. This process is known as clogging and has a negative impact on the productivity and product quality. According to the extensive literature, different mechanisms, like chemical reactions between steel and refractories, the solidification of steel inside the submerged entry nozzle (SEN) or the adhesion of nonmetallic inclusions (NMI) on the inner surface of the SEN may trigger clogging. Clogging incidences, however, can commonly not be assigned to a single mechanism. In addition, differences in plant configurations, Al deoxidation and FeTi addition practices as well as the design of the fluid flow control system of the casting machine also have a significant influence on clogging (eg. ). This complexity of possible influencing parameters may contribute to the widely spread and partly inconsistent research findings on the clogging of Ti-stabilized ULC steel; a comprehensive overview of the state of knowledge was recently published by Dorrer et al. . The present paper addresses only one of these aspects, namely the formation of a new inclusion population due to the FeTi addition after RH treatment as a possible cause for clogging.
2 Plant Observations
Voestalpine Stahl GmbH (vaS) in Linz produces ULC steel grades typically in heat sizes of 180 tons via the BOF-LF-RH-CC route. A general description of the production process can be found in . After tapping with appr. 500 ppm oxygen and a possible temperature adjustment in the ladle furnace, degassing in the RH degasser starts. After achieving the target value for carbon, Al is added depending on the actual oxygen activity and the Al target value. During the next minutes, most alumina inclusions are separated, the total oxygen content decreases to less than 30 ppm and finally FeTi is added. The slag still contains more than 15 wt.-% of FeO at this time, and to minimize reoxidation, the ladle is transferred to the casting machine after a few minutes.
For a specific analysis of the evolution of the number, size, and morphology of inclusions over the time between Al addition and casting, samples taken from the production were analyzed by means of automated SEM/EDS. Representative of a large number of investigated heats, the results for two of them will be discussed exemplarily: Samples were taken 2 min and 8 min after Al addition, immediately after FeTi addition and from the tundish after casting of half of the heat. The final Ti/Al ratio amounts to 1.25 for Heat A and to 1.65 for Heat B. Details on the sampling, metallographic/analytic investigations and classification of the inclusions can be found in .
The particle diameter statistics (ECD) for Classes Al2O3 and AT in tundish samples from Heats A and B show an aged population of Al2O3 particles with higher ECD (presumably resulting from deoxidation) and a population of small particles, pointing to the later nucleation, maybe due to the FeTi addition. For Class AT (a), all particles show a diameter of less than 3 µm, clearly indicating that they mostly nucleated after FeTi addition and are not the result of the modification of pre-existing oxides.
Former SEM/EDS investigations on clogged submerged entry nozzles after the casting of Ti-ULC steel grades indicate that the inclusions of Class AT are at least involved in the build-up of the clogging layer . Therefore, lab experiments were performed to investigate the formation of inclusions of Class AT.
3 Experimental Results
Adding Al and FeTi75 (5 min afterwards) to a steel melt with controlled initial oxygen content under Ar protection in Al2O3 crucibles without top slag to observe whether the population of AT inclusions forms due to the FeTi addition or not.
Variation of initial oxygen content and the Ti/Al ratio to observe the influence on number and volume fraction of newly nucleated particles.
Main parameters for the performed experiments according to 
Series I (5 experiments)
Initial oxygen: 140–170 ppm
Al: 0.043–0.062 wt.-%
Series II (3 experiments)
Initial oxygen: 127–176 ppm
Al: 0.016–0.033 wt.-%
Series III (3 experiments)
Initial oxygen: 62–85 ppm
Al: 0.030–0.035 wt.-%
Despite the exclusion of slag and air as oxygen sources, the FeTi addition results in the nucleation of a new population of Class AT.
Initial oxygen content and Ti/Al ratio are the main influencing factor for the final number and volume fraction of AT inclusions.
Even though the AT inclusions are thermodynamically unstable, they show no tendency to modify their shape or chemistry during the 8‑minute holding time after the FeTi addition.
The wetting behavior between ULC steel and AT inclusions is very similar to pure Al2O3. The high number of small AT particles should be attracted by the submerged entry nozzle and result in a higher tendency towards clogging [10, 11].
It should be noted that both the separation and agglomeration tendencies are rather small in the experiments as fluid flow in the melt only results from natural convection and in addition, thermal gradients are marginal. The separation of particles is limited to the adhesion at the crucible/steel interface. The significance of the results is limited to the above-listed conclusions and the results should not be interpreted with respect to the behavior of particles in a ladle.
Plant observations and laboratory experiments suggest the nucleation of a new inclusion population of Al2O3 particles with traces of Ti after FeTi addition. According to the understanding of the interfacial physics between steel and refractory in the fluid flow control system of casting machines, these particles will be attracted by the interface and as they are small and their number is high, they should increase the risk of clogging. It should not be concluded that a higher Ti/Al ratio is the only decisive parameter for clogging; nor is the adhesion of oxides by the interface the only clogging mechanism involved. Nevertheless, the control of O input by the added FeTi could be a beneficial measure for clogging prevention.
The authors gratefully acknowledge the funding support of K1-MET GmbH, metallurgical competence centre. The research program of the competence centre K1-MET is supported by COMET (Competence Centre for Excellent Technologies), the Austrian program for competence centres. COMET is funded by the Federal Ministry for Transport, Innovation and Technology, the Federal Ministry for Digital and Economic Affairs, the provinces of Upper Austria, Tyrol, and Styria, and the Styrian Business Promotion Agency. Beside public funding, the project is financed by the industrial partners voestalpine Stahl, voestalpine Stahl Donawitz and RHI Magnesita, Austria.
Open access funding provided by Montanuniversität Leoben.
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