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Studies on the decarburization of the commercial Submerged Entry Nozzles (SEN)
Dalarna University, School of Technology and Business Studies, Material Science.
Dalarna University, School of Technology and Business Studies, Material Science.
2011 (English)In: Steel Grips, ISSN 1866-8453, no 9, 121-127 p.Article in journal (Refereed) Published
Abstract [en]

Decarburization behaviours of Al2O3-C, ZrO2-C and MgO-C refractory materials constituting a commercial Submerged Entry Nozzle (SEN), have been investigated in different gas atmospheres consisting of CO2, O2 and Ar. The (CO2/ O2) ratio values were kept the same as it is in propane combustion flue gas at Air-Fuel-Ratio (AFR) values equal to 1.5 and 1 for both Air-Fuel and Oxygen-Fuel combustion systems. Laboratory experiments were carried out non-isothermally in the temperature range 873 K to 1473 K at 15 (K•min-1) followed by isothermal heating at 1473 K for 60 min. The decarburization ratio (a) values of the three refractory types were determined by measuring the real time weight losses of the samples. The results showed that the decarburization ratio (a) values of the MgO-C refractory became 3.1 times higher for Oxygen-Fuel combustion compared to Air-Fuel combustion at an AFR equal to 1.5 in the temperature range 873 K to 1473 K. The decarburization ratio (a) values for Al2O3-C samples were the same as for the isothermal heating at 1473 K and non-isothermal heating in the temperature range 1473 K to 1773 K with a 15 (K•min-1) heating rate. It substantiates the SEN preheating advantage at higher temperatures for shorter holding times instead of heating at lower temperatures for longer holding times. Diffusion models were proposed for estimating the decarburization rate of Al2O3-C refractory in the SEN. The activation energy for Al2O3-C samples heated at an AFR equal to 1.5, for Air-Fuel and Oxygen-Fuel combustions were found to be 81.8 (KJ•mol-1) and 88.8 (KJ•mol-1), respectively during non-isothermal heating in the temperature range 873 K to 1473 K.

Place, publisher, year, edition, pages
Bad Harzburg, Germany: Steel Grips , 2011. no 9, 121-127 p.
Keyword [en]
SEN; Al2O3-C; ZrO2-C; MgO-C; decarburization; reaction mechanism
Research subject
Stålformning och ytteknik, Studier angående bildningsmekanismer för igensättningar i gjutrör
Identifiers
URN: urn:nbn:se:du-5134OAI: oai:dalea.du.se:5134DiVA: diva2:520286
Available from: 2010-12-09 Created: 2010-12-09 Last updated: 2014-10-02Bibliographically approved
In thesis
1. An Experimental Study of Submerged Entry Nozzles (SEN) Focusing on Decarburization and Clogging
Open this publication in new window or tab >>An Experimental Study of Submerged Entry Nozzles (SEN) Focusing on Decarburization and Clogging
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The submerged entry nozzle (SEN) is used to transport the molten steel from a tundish to a mould. The main purpose of its usage is to prevent oxygen and nitrogen pick-up by molten steel from the gas. Furthermore, to achieve the desired flow conditions in the mould. Therefore, the SEN can be considered as a vital factor for a stable casting process and the steel quality. In addition, the steelmaking processes occur at high temperatures around 1873 K, so the interaction between the refractory materials of the SEN and molten steel is unavoidable. Therefore, the knowledge of the SEN behaviors during preheating and casting processes is necessary for the design of the steelmaking processes  The internal surfaces of modern SENs are coated with a glass/silicon powder layer to prevent the SEN graphite oxidation during preheating. The effects of the interaction between the coating layer and the SEN base refractory materials on clogging were studied. A large number of accretion samples formed inside alumina-graphite clogged SENs were examined using FEG-SEM-EDS and Feature analysis. The internal coated SENs were used for continuous casting of stainless steel grades alloyed with Rare Earth Metals (REM). The post-mortem study results clearly revealed the formation of a multi-layer accretion. A harmful effect of the SENs decarburization on the accretion thickness was also indicated. In addition, the results indicated a penetration of the formed alkaline-rich glaze into the alumina-graphite base refractory. More specifically, the alkaline-rich glaze reacts with graphite to form a carbon monoxide gas. Thereafter, dissociation of CO at the interface between SEN and molten metal takes place. This leads to reoxidation of dissolved alloying elements such as REM (Rare Earth Metal). This reoxidation forms the “In Situ” REM oxides at the interface between the SEN and the REM alloyed molten steel. Also, the interaction of the penetrated glaze with alumina in the SEN base refractory materials leads to the formation of a high-viscous alumina-rich glaze during the SEN preheating process. This, in turn, creates a very uneven surface at the SEN internal surface. Furthermore, these uneven areas react with dissolved REM in molten steel to form REM aluminates, REM silicates and REM alumina-silicates.

The formation of the large “in-situ” REM oxides and the reaction of the REM alloying elements with the previously mentioned SEN´s uneven areas may provide a large REM-rich surface in contact with the primary inclusions in molten steel. This may facilitate the attraction and agglomeration of the primary REM oxide inclusions on the SEN internal surface and thereafter the clogging. The study revealed the disadvantages of the glass/silicon powder coating applications and the SEN decarburization.

The decarburization behaviors of Al2O3-C, ZrO2-C and MgO-C refractory materials from a commercial Submerged Entry Nozzle (SEN), were also investigated for different gas atmospheres consisting of CO2, O2 and Ar. The gas ratio values were kept the same as it is in a propane combustion flue gas at different Air-Fuel-Ratio (AFR) values for both Air-Fuel and Oxygen-Fuel combustion systems. Laboratory experiments were carried out under nonisothermal conditions followed by isothermal heating. The decarburization ratio (α) values of all three refractory types were determined by measuring the real time weight losses of the samples. The results showed the higher decarburization ratio (α) values increasing for MgO-C refractory when changing the Air-Fuel combustion to Oxygen-Fuel combustion at the same AFR value. It substantiates the SEN preheating advantage at higher temperatures for shorter holding times compared to heating at lower temperatures during longer holding times for Al2O3-C samples. Diffusion models were proposed for estimation of the decarburization rate of an Al2O3-C refractory in the SEN.

Two different methods were studied to prevent the SEN decarburization during preheating: The effect of an ZrSi2 antioxidant and the coexistence of an antioxidant additive and a (4B2O3 ·BaO) glass powder on carbon oxidation for non-isothermal and isothermal heating conditions in a controlled atmosphere. The coexistence of 8 wt% ZrSi2 and 15 wt% (4B2O3 ·BaO) glass powder of the total alumina-graphite refractory base materials, presented the most effective resistance to carbon oxidation. The 121% volume expansion due to the Zircon formation during heating and filling up the open pores by a (4B2O3 ·BaO) glaze during the green body sintering led to an excellent carbon oxidation resistance.

The effects of the plasma spray-PVD coating of the Yttria Stabilized Zirconia (YSZ) powder on the carbon oxidation of the Al2O3-C coated samples were investigated. Trials were performed at non-isothermal heating conditions in a controlled atmosphere. Also, the applied temperature profile for the laboratory trials were defined based on the industrial preheating trials. The controlled atmospheres consisted of CO2, O2 and Ar. The thicknesses of the decarburized layers were measured and examined using light optic microscopy, FEG-SEM and EDS. A 250-290 μm YSZ coating is suggested to be an appropriate coating, as it provides both an even surface as well as prevention of the decarburization even during heating in air. In addition, the interactions between the YSZ coated alumina-graphite refractory base materials in contact with a cerium alloyed molten stainless steel were surveyed. The YSZ coating provided a total prevention of the alumina reduction by cerium. Therefore, the prevention of the first clogging product formed on the surface of the SEN refractory base materials. Therefore, the YSZ plasma-PVD coating can be recommended for coating of the hot surface of the commercial SENs.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. 87 p.
Keyword
Refractory, SEN, Clogging, REM, Coating, glaze, alkaline, Post- Mortem, industrial preheating, ZrSi2, Graphite, Alumina, Oxidation, Al2O3-C, ZrO2-C, MgO-C, decarburization, reaction mechanism, Yttria Stabilized Zirconia, plasma spray-PVD coating
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:du-11703 (URN)978-91-7501-0967-7 (ISBN)
Opponent
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Available from: 2013-01-16 Created: 2013-01-16 Last updated: 2015-12-08Bibliographically approved

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