In this study it was aimed to analyse refining processes such as decreasing reducible oxide content of ladle slag with affecting parameters in low carbon aluminum killed grades and for the research Erdemir low carbon steel grades 7112K and 7110K are selected. There was a negative correlation between reducible oxide amount in ladle slag and desulphurization capacity of ladle slag with metal-slag reaction and steel internal cleanliness. To refine these properties of slag, aluminium dross, which was aluminium production discard and has a metallic content around %30-35 was used and after ladle treatment operation, decrease in reducible oxides such as FetO, MnO, SiO2, P2O5 was analysed. After the study it was observed that 653 kg. of converter slag leaked during tapping of steel and SiO2 ve P2O5 content of ladle slag had negligible change after ladle treatment. According to the results, it is observed that initial %10-12 (FetO + MnO) content was reduced to % 4.5-5.0 (FetO + MnO) after ladle treatment with use of aluminium dross. Beside of this, in order to see the effect of this slag reduction on steel cleanliness, low carbon aluminium killed grades were compared with ultra-low carbon aluminium killed grades having (FetO + MnO) content of %16-17 in slag. It was seen that reoxidation of aluminium (loss of dissolved aluminium) during continuous casting for ULC (ultra-low carbon) gradesis 144 ppm but for LC grades it was 94 ppm and it was being expected that ULC steel group would have higher inclusion content after casting.

Abstract Composition Adjustment by Sealed argon bubbling – Oxygen Blowing (CAS-OB) process is a ladle treatment process that was developed for chemical heating and alloying of steel. The main stages of the process are heating, (possible) alloying and reduction of slag. The CAS-OB process aims for homogenization and control of the composition and temperature of steel. In this dissertation, a mathematical reaction model was developed for the slag reduction stage of the CAS-OB process. Slag reduction is carried out by applying vigorous bottom stirring from porous plugs at the bottom of the ladle. Due to the bottom stirring a circular steel flow is induced which disrupts top slag layer and due to shear stress at the steel-slag interface small slag droplets are detached. This induces an immense increase in the interfacial area between steel and slag which provides favourable conditions for the reduction reactions. In order to model reduction reactions, a sub-model for describing the interfacial area was needed. For this the slag droplet formation was studied using computational fluid dynamics (CFD) and based on these studies, a sub-model for droplet formation was developed. The model for the reduction stage of the CAS-OB process takes into account not only the reaction during the reduction but also the heat transfer between the phases, ladle and surroundings. The reduction stage model was validated using validation data obtained from the CAS-OB station of the SSAB Raahe steel plant in Finland. It was concluded that the model was able to predict steel and slag compositions as well as the steel temperature very well. / Tiivistelmä CAS-OB -prosessi on sulametallurgiassa käytettävä senkkakäsittelyprosessi, joka on kehitetty teräksen kemialliseen lämmittäseen ja seostukseen. CAS-OB-prosessin pääprosessivaiheet ovat lämmitysvaihe, mahdollinen seostusvaihe ja kuonan pelkistysvaihe. CAS-OB -prosessilla tavoitellaan teräksen koostumuksen homogenisointiin ja lämpötilan kontrollointiin. Tässä tutkimuksessa kehitettiin matemaattinen reaktiomalli CAS-OB -prosessin kuonan pelkistysvaiheen kuvaamiseen. Kuonan pelkistys tapahtuu senkan pohjassa olevien huuhtelutiilien avulla suoritettavan voimakkaan kaasuhuuhtelun avulla. Pohjahuuhtelu aiheuttaa kiertävän teräsvirtauksen senkassa. Teräsvirtaus irrottaa teräksen päällä olevasta kuonakerroksesta pisaroita ja kuonan ja teräksen välinen reaktiopinta-ala kasvaa voimakkaasti. Tämä tarjoaa suotuisat olosuhteet pelkistysreaktiolle senkassa. Pelkistysreaktioiden mallintamiseksi tässä työssä kehitettiin CFD-simulaatioiden avulla alimalli, jonka avulla voidaan kuvata teräksen ja kuonan välisen pinta-alan suuruutta. Pelkistysvaiheen mallissa huomioidaan reaktioiden lisäksi myös systeemissä tapahtuva lämmösiirto. Pelkistysmalli validoitiin mittausdatalla, joka hankittiin SSAB Raahen terässulaton CAS-OB -asemalla järjestetyssä validointikampanjassa. Tutkimuksessa havaittiin, että malli kykenee hyvin ennustamaan teräksen ja kuonan koostumuksen sekä teräksen lämpötilan.