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Non-metallic inclusions and their distribution in the ladle before and after vacuum treatment of tool steel
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
Uddeholm Tooling, Hagfors.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
(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 [en]
non-metallic inclusion; ladle treatment; vacuum treatment; inclusion distribution; tool steel
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-6818OAI: oai:DiVA.org:kth-6818DiVA: diva2:11636
Note
QC 20100803Available from: 2007-02-28 Created: 2007-02-28 Last updated: 2010-08-03Bibliographically approved
In thesis
1. Some aspects of non-metallic inclusions during vacuum degassing in ladle treatment: with emphasize on liquid CaO-Al2O3 inclusions
Open this publication in new window or tab >>Some aspects of non-metallic inclusions during vacuum degassing in ladle treatment: with emphasize on liquid CaO-Al2O3 inclusions
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
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:nbn:se:kth:diva-4288 (URN)978-91-7178-571-8 (ISBN)
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

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