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A study of slag corrosion of oxides and oxide-carbon refractories during steel refining
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
2010 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The use of ceramic material as refractories in the manufacturing industry is a common practice worldwide. During usage, for example in the production of steel, these materials do experience severe working conditions including high temperatures, low pressures and corrosive environments. This results in lowered service lives and high consumptions of these materials. This, in turn, affects the productivity of the whole steel plant and thereby the cost. In order to investigate how the service life can be improved, studies have been carried out for refractories used in the inner lining of the steel ladles. More specifically, from the slag zone, where the corrosion is most severe. By combining thermodynamic simulations, plant trails and post-mortem studies of the refractories after service, vital information about the behaviour of the slagline refractories during steel refining and the causes of the accelerated wear in this ladle area has been achieved. The results from these studies show that the wear of the slagline refractories of the ladle is initiated at the preheating station, through reduction-oxidation reactions. The degree of the decarburization process is mostly dependent on the preheating fuel or the environment. For refractories without antioxidants, refractory decarburization is slower when coal gas is used in ladle preheating than when a mixture of oil and air is used. In addition, ladle preheating of the refractories without antioxidants leads to direct wear of the slagline refractories. This is due to the total loss of the matrix strength, which results in a sand-like product. Thermal chemical changes that take place in the slagline refractories are due to the MgO-C reaction as well as the formation of liquid phases from impurity oxides. In addition, the decrease in the system pressure during steel refining makes the MgO-C reaction take place at the steel refining temperatures. This reduces the refractory’s resistance to corrosion. This is a serious problem for both the magnesia-carbon and dolomite-carbon refractories. The studies of the reactions between the slagline refractories and the different slag compositions showed that slags rich in iron oxide lead mostly to the oxidation of carbon/graphite in the carbon-containing refractories. This leads to an increased porosity and wettability and therefore an enhanced penetration of slag into the refractory structure. If the slag contains high contents of alumina and or silica (such as the steel refining slag), reactions between the slag components and the dolomite-carbon refractory are promoted. This leads to the formation of low-temperature melting phases such as calcium-aluminates and silicates. The state of these reaction products during steel refining leads to an accelerated wear of the dolomite-carbon refractory. The main products of the reactions between the magnesia-carbon refractory and the steel refining slag are MgAl2O4 spinels, and calcium-aluminates, and silicates. Due to the good refractory properties of MgAl2O4 spinels, the slag corrosion resistance of the magnesiacarbon refractory is promoted.

Place, publisher, year, edition, pages
Stockholm: US-AB , 2010. , x, 50 p.
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-25221ISBN: 978-91-7415-743-7 (print)OAI: oai:DiVA.org:kth-25221DiVA: diva2:356594
Presentation
2010-09-13, MAVE konferensrum, KTH, Brinellvägen 23, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20101013Available from: 2010-10-13 Created: 2010-10-13 Last updated: 2010-10-14Bibliographically approved
List of papers
1. Studies on decarburization of MgO-c refractories during ladle preheating
Open this publication in new window or tab >>Studies on decarburization of MgO-c refractories during ladle preheating
2010 (English)In: Steel GRIP JournalArticle in journal (Other academic) Accepted
Abstract [en]

The effect of ladle pre-heating conditions on the wear of MgO-C refractories at different steel plants has been investigated. The focus has been on refractories used in the slag zone, some including which contain anti-oxidants. This investigation shows that the wear /corrosion of the slagline refractories initiates already at the ladle pre-heating during real industrial conditions. The degree of the decarburization process is dependent both on the refractory composition, time and the pre-heating fuel or environment. For refractories without anti-oxidants, the refractory decarburization is 3.8 times slower when coal gas is used in ladle pre-heating than when a mixture of oil and air is used. Ladle pre-heating of the refractories without anti-oxidants leads to direct wear of the slagline refractories due to total loss of matrix strength, resulting in a sand-like product independent of the pre-heating method. When the refractories containing antioxidants are compared to each other, the decarburization rate was lower when coal gas was used in ladle perheating than when the refractories are pre-heated by electrical means. The cracking tendency of the refractories is enhanced by the increasing size of anti-oxidants. The refractory containing coarser grains of anti-oxidants (<3000 μm) cracked much early than those containing finer grains (<100 μm). For example after 6 weeks in storage, the crack thickness was 1.5 mm for  large-grained refractory whereas 0.3 mm for the fine-grained refractory.

Keyword
refractories, reduction-oxidation (redox) corrosion, anti-oxidants, pre-heating, decarburization, decarburization rate
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-25116 (URN)
Note
QC 20101008Available from: 2010-10-08 Created: 2010-10-08 Last updated: 2012-03-20Bibliographically approved
2. The use of thermodynamic computations to predict the phase transformation in MgO-C refractories during steel refining
Open this publication in new window or tab >>The use of thermodynamic computations to predict the phase transformation in MgO-C refractories during steel refining
2008 (English)In: Refractories Manual, 42-47 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:kth:diva-25118 (URN)
Note
QC 20101008Available from: 2010-10-08 Created: 2010-10-08 Last updated: 2010-10-13Bibliographically approved
3. Thermo-calc and SEM analysis of the dolomite lining during steel refining
Open this publication in new window or tab >>Thermo-calc and SEM analysis of the dolomite lining during steel refining
2008 (English)In: Proceedings for the Third Nordic Symposium for Young Scientists in Metallurgy, 2008, 46-50 p.Conference paper, Published paper (Refereed)
Abstract [en]

Dolomite refractories are widely used in the slag line of the steel refining vessels working in basic conditions. A Postmortem analysis (SEM investigation) has been carried-out on the dolomite refractory material after service in industrial conditions to identify its corrosion behaviour during steel refining. Results from the SEM investigation indicate that the high corrosion rate of this refractory could have been caused by the presence of a high content of iron oxide and silica in the slag, which possibly reacted with the refractory components at refining temperatures. High silicate slag lead to low temperature melting phases and therefore decreased corrosion resistance. The high content of iron elements and their distribution in the penetrated microstructure may indicate the oxidation of carbon by iron oxide in the slag with more slag penetration as a result. These results have then been combined with the thermodynamic calculation results by the Thermo-Calc program so that the behaviour of this refractory at refining conditions could be examined. Thermo-Calc results verified more graphite reduction with increasing iron oxide content in the slag. Other instability problems such as the MgO-C reaction and more increased low temperature melting phases such as tri-calcium silicates also weaken the refractory. This work is intended to increase the  understanding of the corrosion behaviour of the steel refining refractories, which is a key factor for prolonging ladle life.

Keyword
corrosion, dolomite, refractories, slag, slag penetration, reactions, corrosion behaviour
Identifiers
urn:nbn:se:kth:diva-25165 (URN)
Conference
TKK, Espoo, Finland
Note
QC 20101012Available from: 2010-10-12 Created: 2010-10-12 Last updated: 2010-10-13Bibliographically approved
4. Slagline refractory
Open this publication in new window or tab >>Slagline refractory
2008 (English)In: Proceedings for the SCANMET III-3rd International Conference on Process Development in Iron and Steel making, 2008, 377-384 p.Conference paper, Published paper (Refereed)
Abstract [en]

An important part to develop in existing ladle metallurgy processes is the ladle as a reaction vessel. The ladle refractory has several vital functions and must withstand chemical, thermal and mechanical wear for longer ladle cycle times to meet higher steelmaking productivity and lower production costs. Extensive refractory wear can be caused by complex united actions such as refractory manufacturing, ladle metallurgy practice, steel and slag composition. A special exposed area is the refractory slagline.

The presented results come from one collaborative project with academic work and industrial trials to identify causes of extensive wear in the slagline. The investigation has been carried out on the slagline MgO-refractories after service. The microscopy characterisation results have then been compared to those from thermodynamic calculations by Thermo-Calc software.

It was found that the refractory microstructure at the hot-face had been totally or partly penetrated and reacted by slag. The observed phases were magnesia-alumina spinel, slag, calcium silicates and dispersed metallic iron rich phases. Closer analyses of the slag phase showed the presence of calcium silicates and calcium aluminates and some few Mg-Al spinel phases. Further away from the hot-face towards the cold-face, the microstructure was composed mainly of magnesia, small calcium silicates phases and a carbon rich phase and therefore less affected.

The project shows that it is possible to combine the characterisation results from the ostmortem studies of the refractories after service with the thermodynamic calculations so that the information about the corrosion behaviour and the microstructure changes of the refractories at refining conditions can be achieved. Further work to test new refractory materials by industrial trials combined with academic work will be performed.

 

Keyword
refractory, ladle lining, microstructure, corrosion, hot-face, slagline
Identifiers
urn:nbn:se:kth:diva-25167 (URN)
Conference
MEFOS, Luleo, Sweden: 9-11 June
Note
QC 20101012Available from: 2010-10-12 Created: 2010-10-12 Last updated: 2010-10-13Bibliographically approved

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