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Some aspects of non-metallic inclusions during vacuum degassing in ladle treatment: with emphasize on liquid CaO-Al2O3 inclusions
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Micro-Modelling.
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The present thesis was to study non-metallic inclusions during vacuum degassing in ladle treatment. Emphasize was mostly given to liquid CaO-Al2O3 inclusions. A series of industrial experiments were carried out at Uddeholm Tooling AB, Hagfors, Sweden. To gain an insight into the industrial findings, laboratory investigations were also performed.

Large number of steel samples were collected and examined. Liquid calcium aluminate inclusions with low SiO2 and high SiO2 were often found with spinel inclusions before vacuum degassing. Laboratory experiments showed that spinel would react with the dissolved Ca in the liquid steel forming calcium aluminate inclusions. This laboratory results agreed with the industrial observation that spinel phase was quite often found in the center of the calcium aluminate phase.

After vacuum degassing, most of the inclusions were calcium aluminate liquid inclusions. When dissolved Al level was low, 2 types of liquid calcium aluminate inclusions with considerably different SiO2 contents were found to coexist even at the end of the process. In view of the lack of the thermodynamic data for SiO2 activities in the low silica region, thermodynamic measurements were conducted in the CaO-Al2O3-SiO2-MgO system. The experimental results could reasonably explain the coexistence of the two types of the liquid oxide inclusions.

While the total number of inclusions decreased during vacuum degassing, the number of bigger inclusions (>11.3 μm) increased generally in used ladles. This finding was in accordance with the previous studies, wherein, ladle glaze was found to be responsible for the supply of bigger inclusions.

The behaviors of several types of inclusions in liquid steel were examined using a laser scanning confocal microscope (LSCM). While alumina inclusions tended to impact on each other, agglomerate and grow very quickly, none of the other types of inclusions, such as spinel and calcium aluminate was observed to agglomerate. The results of LSCM study agreed well with the industrial observation. Examination on a huge number of inclusions did not show any indication of impact and physical growth of the inclusions, except the alumina inclusions.

The removal of inclusions around open-eye in a gas-stirred ladle was experimentally studied by a cold model with oil and water. Most of the simulated inclusions were brought up to the oil phase by gas-water plume. Inclusion removal into oil layer took place when the inclusions passed through the sphere-bed of the oil layer around the open-eye. A calculation showed that the contribution of metal-gas plume in inclusion removal was much larger than that of buoyancy mechanism.

The results of the industrial experiments revealed that the inclusions distribution strongly depended on stirring condition. When a ladle was stirred by both gas and induction, inclusion removal near slag layer was significant.

Place, publisher, year, edition, pages
Stockholm: KTH , 2007. , vii, 40 p.
Series
ISRN KTH/MSE, 2007:04
Keyword [en]
non-metallic inclusions, ladle refining, vacuum degassing, ladle glaze, spinel, SiO2 activity, agglomeration, cold model, open-eye, inclusion removal
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-4288ISBN: 978-91-7178-571-8 (print)OAI: oai:DiVA.org:kth-4288DiVA: diva2:11641
Public defence
2007-03-07, Sal D3, KTH, Lindstedtsvägen 5, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100803Available from: 2007-02-28 Created: 2007-02-28 Last updated: 2010-08-03Bibliographically approved
List of papers
1. Non-metallic inclusions and their distribution in the ladle before and after vacuum treatment of tool steel
Open this publication in new window or tab >>Non-metallic inclusions and their distribution in the ladle before and after vacuum treatment of tool steel
(English)In: Steel-GripsArticle in journal (Refereed) Accepted
Abstract [en]

The types of inclusions, inclusion population and distribution in tool steel before and after vacuum treatment were studied. Steel samples taken before and after vacuum degassing were analyzed. Contrary to the previous studies, liquid oxide inclusions with high SiO2 content were found even after vacuum treatment. The difference could be explained by the difference of dissolved aluminium. The number of inclusions > 11.3 μm were found to increase in used ladles during vacuum degassing. The steel samples taken at different locations in the ladles revealed that inclusions distributed uniformly in the melt before vacuum treatment. The inclusions, especially the bigger ones, were readily removed near slag layer during the vacuum treatment. Evidence for the movement of the inclusions with the stream lines of the flow was observed.

Keyword
non-metallic inclusion; ladle treatment; vacuum treatment; inclusion distribution; tool steel
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6818 (URN)
Note
QC 20100803Available from: 2007-02-28 Created: 2007-02-28 Last updated: 2010-08-03Bibliographically approved
2. Mechanism study on the formation of liquid calcium aluminate inclusion from MgO.Al(2)O(3) Spinel
Open this publication in new window or tab >>Mechanism study on the formation of liquid calcium aluminate inclusion from MgO.Al(2)O(3) Spinel
2006 (English)In: Steel Research International, ISSN 1611-3683, Vol. 77, no 11, 785-792 p.Article in journal (Refereed) Published
Abstract [en]

A study on the formation mechanism of liquid calcium aluminate inclusions from MgO center dot Al2O3 spinel was carried out. It was found that spinel reacted with dissolved Ca in liquid steel forming liquid calcium aluminate phase. Stability calculations showed good agreement with the experimental result. According to the thermodynamic calculation, liquid calcium aluminate inclusions would form from spinel even at very low level of dissolved Ca content at 1873 K. At lower temperatures, the compound CaO center dot 2Al(2)O(3) was found to be the stable phase at the spinel-metal interface. Potential sources of dissolved Ca during ladle processes were also discussed.

Keyword
Al deoxidation; Calcium aluminate; Ladle process; Liquid inclusion; Spinel; Dissolution; Interfaces (materials); Magnesium compounds; Thermal effects; Thermodynamics; Aluminum compounds
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6819 (URN)000242924200004 ()2-s2.0-33751395874 (Scopus ID)
Note

QC 20100803

Available from: 2007-02-28 Created: 2007-02-28 Last updated: 2016-12-21Bibliographically approved
3. Activities of SiO2 in some CaO-Al2O3-SiO2(-10%MgO) melts with low SiO2 contents at 1873K
Open this publication in new window or tab >>Activities of SiO2 in some CaO-Al2O3-SiO2(-10%MgO) melts with low SiO2 contents at 1873K
2007 (English)In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 47, no 6, 805-810 p.Article in journal (Refereed) Published
Abstract [en]

The activity of SiO2 in the CaO-Al2O3-SiO2(-10%MgO) systems was measured at the region of %SiO2<10%. The melts were equilibrated with a reference metal of Cu in a controlled oxygen partial pressure at 1 873 K. ISo-SiO2 activity contours were drawn from the measured values. For the CaO-Al2O3-SiO2 system, the measurement result showed positive deviation from the previous data. It was also found that the SiO2 activity decreased as CaO content increased. In the CaO-Al2O3-SiO2-10%MgO system, the composition dependence of the SiO2 activity seemed to be similar as the ternary system. On the other hand, the activity values were much lower than that of the ternary system. It could be attributed to the strong basic behaviour of MgO. Based on the result of the SiO2 activity measurement, the equilibrium between the inclusions and liquid steel during ladle treatment was discussed.

Keyword
Activity of SiO2; CaO-Al2O3-SiO2 system; CaO-Al2O3-SiO2-MgO system; Deoxidation equilibrium; Ladle process; Inclusions; Lime; Magnesia; Oxidation; Partial pressure; Phase equilibria; Controlled oxygen partial pressure; Deoxidation; Ladle treatment; Melts; Silica
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6820 (URN)10.2355/isijinternational.47.805 (DOI)000247549400004 ()2-s2.0-34548050414 (Scopus ID)
Note
QC 20100803. Uppdaterad från Submitted till Published 20100803.Available from: 2007-02-28 Created: 2007-02-28 Last updated: 2017-12-14Bibliographically approved
4. Some aspects of physical growth of non-metallic inclusion in ladle treatment
Open this publication in new window or tab >>Some aspects of physical growth of non-metallic inclusion in ladle treatment
2007 (English)Conference paper, Published paper (Refereed)
Abstract [en]

The behaviors of several types of inclusions at high temperature were examined using a laser scanning confocal microscope (1LM21H). While alumina inclusions tended to impact on each other, agglomerate and grow very quickly, none of the other types of inclusions, such as spinel and calcium aluminate was observed to agglomerate.

The results of confocal microscope study were compared with the industrial investigation. For this purpose, a huge number of steel samples were taken at different stages of ladle treatment. The samples were analyzed by scanning and light optical microscopes. About 50.000 inclusions of different types were examined. Only alumina inclusions were found to attach with each other and agglomerate. No attachment was observed in the case of the other types of inclusions. The conformity of the industrial data and the in situ observation by laser microscope indicates that while the growth of alumina inclusions needs consideration in ladle treatment, the attachment of the other types of inclusions seems not requiring special attention in process modeling and optimization.

Keyword
non-metallic inclusion; alumina; spinel agglomeration; confocal scanning; laser microscope
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6821 (URN)
Conference
METEC InSteelCon 2007, 2007, June, Dusseldorf, Germany
Note
QC 20100803Available from: 2007-02-28 Created: 2007-02-28 Last updated: 2012-03-21Bibliographically approved
5. Mechanism Study of inclusion removal around open-eye in ladle treatment
Open this publication in new window or tab >>Mechanism Study of inclusion removal around open-eye in ladle treatment
2007 (English)In: Ironmaking & steelmaking, ISSN 0301-9233, E-ISSN 1743-2812, Vol. 34, no 3, 253-261 p.Article in journal (Refereed) Published
Abstract [en]

In the present study, the inclusion removal mechanism by rising bubbles around the open eye in a gas stirred ladle was investigated using a cold model. Water and silicone oils were employed to simulate liquid metal and slag respectively, Both fine silicone oil droplets and charcoal particles were used to simulate inclusions. Around the open eye, the oil layer exhibited a sphere bed structure when a critical gas flowrate was reached. The sphere bed was composed of many oil droplets, each of which was coated with a thin water film. The injected inclusions mostly followed the stream of water flow and were brought up to the open eye by the gas-water plume. A fraction of the inclusions were captured by the sphere bed when passing through the openings between the oil droplets, while the rest of the inclusions came back to the water bath and joined the stream of the flow again. The sphere bed was found to function as a filter to the tiny particles or oil droplets. On the basis of this mechanism, a preliminary comparison of the contribution of inclusion removal by metal-gas plume in the open eye region with the contribution of buoyancy was made.

Keyword
Cold model; Gas injection; Inclusion removal; Ladle refining; Metal-gas plume; Buoyancy; Charcoal; Inclusions; Liquid metals; Silicones; Slags; Water
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6822 (URN)10.1179/174328107X168101 (DOI)000246963400011 ()2-s2.0-35348993737 (Scopus ID)
Note
QC 20100803. Uppdaterad från In press till Published 20100803.Available from: 2007-02-28 Created: 2007-02-28 Last updated: 2017-12-14Bibliographically approved

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