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Study of formation behavior of CaO-Al2O3 inclusions in steel melts during an LF process
KTH.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Experiments were carried out during an LF process to investigate the formation behavior of CaO-Al2O3 inclusions in a steel melt. Samples were taken during the production of two different steel grades: a low-Al steel (Al=0.023%) and a high-Al steel (Al=1.1%). Liquid CaO-Al2O3 inclusions were detected in the low-Al steel samples. On the other hand, liquid CaO-Al2O3 inclusions were not detected in the high-Al steel samples. Instead, the inclusions consisted of MgO∙Al2O3 and Al2O3. The conditions of the LF operations, such as slag viscosities, temperatures, and gas flow rates, were almost the same in the two steel grades. This fact indicates that the detected liquid CaO-Al2O3 inclusions in the low-Al steel melt were not generated by slag entrapment, but by inclusion evolutions. In the low-Al steel, the thermodynamically stable phase of inclusion composition was calculated to be a liquid CaO-Al2O3 phase. On the other hand, the stable phase was found to be a CaO∙2Al2O3 phase in the high-Al steel. These differences in thermodynamic stable phases can influence the degree of inclusion evolution in the steel melts. However, in the high-Al steel, most inclusions have much lower CaO contents than that of the calculated phase (CaO∙2Al2O3). This large inconsistency between the detected and calculated inclusion compositions in the high-Al steel melt can be explained by two reasons: one is the low thermodynamic driving force of an Al2O3 modification and the other is the high removability of the inclusion types in the steel melt.

Keyword [en]
high Al, oxide inclusion, slag entrapment, inclusion evolution, thermodynamics.
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-227933OAI: oai:DiVA.org:kth-227933DiVA, id: diva2:1205573
Note

QC 20180522

Available from: 2018-05-14 Created: 2018-05-14 Last updated: 2018-05-22Bibliographically approved
In thesis
1. Parameters influencing inclusion compositions in Al-killed steel melts during a secondary refining process
Open this publication in new window or tab >>Parameters influencing inclusion compositions in Al-killed steel melts during a secondary refining process
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

     This study was carried out to clarify the factors influencing the evolution of inclusions in Al-killed steel melts during a secondary refining process. First, a case-hardening steel was the subject of study to understand the factors influencing the inclusion compositions in the steel melt. During the LF process, inclusions were transformed from the Al2O3 phase, which was the primary deoxidation product, to both MgO·Al2O3 and CaO-Al2O3-MgOliq phases simultaneously. This composition transition during the LF process occurred by composition evolutions toward thermodynamically stable phases. After the RH process, the inclusion compositions primarily consisted of the Al2O3 and CaO-Al2O3 phases. MgO·Al2O3 inclusions were removed, while the CaO-Al2O3 inclusions remained during the RH process. This behavior can be understood in terms of the interfacial properties of the oxide phases in a steel melt. The detected Al2O3 inclusions were considered to be generated by reoxidation during the RH treatment. Thus, it was confirmed that the equilibrium states, removal, and generation of inclusions determine the inclusion compositions in an Al-killed steel melt during an LF-RH refining process.

     Subsequently, the effect of high Al contents in a steel melt on the change in inclusion compositions during the LF-RH process was studied. Due to the high Al content in the steel melt, the thermodynamic driving forces for Al2O3 modification became lower than those in ordinary Al-killed steels. Therefore, the degree of inclusion evolution was restricted. This contributed to the low CaO contents in the inclusions. Due to the low CaO contents, the removability of the inclusions remained high throughout the LF-RH process. According to thermodynamic calculations, the low T.O contents in this steel grade are due to the low insoluble O contents. This can be explained by the fast removal of inclusions. Because the inclusions were removed smoothly, the CaO content in the inclusions was lower than that in the thermodynamically stable phase.

     In addition, a study was carried out to understand the formation and behavior of the CaS phase in an Al-killed high-S steel during the LF-RH process without Ca-treatment. In the initial stage of the LF process, a CaS phase was formed on the existing inclusions by a reaction between Ca and S. As the desulfurization of the steel melt progressed, the CaS phase started to be transformed into a CaO phase in the inclusions, which resulted in the formation of CaO-Al2O3-CaS inclusions. After desulfurization of the steel melt, the Al2O3 phase in the inclusions was transformed to the CaO-Al2O3liq phase without being hindered by a CaS phase. During the following RH process, the addition of FeS increased the activity of S, which then reacted with both CaO in the inclusions and with Ca, forming a CaS phase. Consequently, the majority of the inclusions consisted of the Al2O3-CaS phase. Thus, a CaS formation during the LF-RH process without Ca-treatment progresses under the thermodynamic driving forces of the following two reactions: the reaction between CaO in the inclusions and S and the reaction between Ca and S. Due to the formation of a CaS phase during the RH process, inclusions in the high S steel melt were covered by a CaS phase, which is difficult to remove from steel melts. Therefore, the castability of the high S steels can be deteriorated by the CaS inclusions, even without using Ca-treatment.

     In summary, it can be concluded that the removal of inclusions, generation, and composition evolution should be considered in order to control the inclusion compositions in Al-killed steel melts. In addition, steel components, such as Al and S, are important to monitor to control the inclusion evolution during secondary refining processes.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. p. 45
Series
TRITA-ITM-AVL ; 2018:32
Keyword
non-metallic inclusion, ladle furnace, RH, thermodynamics, contact angle, Al2O3, MgO•Al2O3 spinel, calcium aluminate, CaS.
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-227935 (URN)978-91-7729-778-9 (ISBN)
Public defence
2018-06-01, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2018-05-15 Created: 2018-05-14 Last updated: 2018-05-15Bibliographically approved

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Yoshioka, TakanoriKarasev, AndreyJönsson, Pär

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