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Nitrogen and Hydrogen Refining During Vacuum Treatment of Liquid Steel
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Micro-Modelling.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Micro-Modelling.ORCID iD: 0000-0003-0856-3417
2011 (English)In: PR-258-024, 2011, no 6Conference paper, Published paper (Refereed)
Abstract [en]

Nitrogen and hydrogen strongly affects mechanical properties of steel. This is why control of liquid steel nitrogen and hydrogen contents is of the utmost importance. In this paper, the nitrogen and hydrogen refining during vacuum treatment and the variation of nitrogen and hydrogen content in the tundish, have been investigated. The results clearly show that the nitrogen refining depends on the gas pressure inside the tank as well as steel sulfur content and vacuum treatment time, while the hydrogen refining seems to reach equilibrium within the varied process parameters. It is further shown that the nitrogen and hydrogen contents vary during casting. A model predicting this variation as function of time is also presented.

Place, publisher, year, edition, pages
2011. no 6
Keyword [en]
Steel, vacuum, degassing, nitrogen, hydrogen, refining, tundish
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-172464OAI: oai:DiVA.org:kth-172464DiVA: diva2:848461
Conference
Intl. Symposium on Recent Developments in Plate Steels
Note

QC 20150825

Available from: 2015-08-25 Created: 2015-08-25 Last updated: 2015-08-25Bibliographically approved
In thesis
1. Solubility of hydrogen in slags and its impact on ladle refining
Open this publication in new window or tab >>Solubility of hydrogen in slags and its impact on ladle refining
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In the present thesis a study of the mechanisms of hydrogen control pickup during ladle treatment was undertaken. Previous studies showed that the presence of hydroxyl ions in the ladle slag resulteds in hydrogen transfer from the slag back into the steel bath. The main focus of the present work was therefore to gain deeper knowledge of the ladle slag, its properties and impact on hydrogen concentration in the liquid steel. For this purpose a number of slag compositions were examined in order to clarify whether these slags were single liquids at 1858 K. 14 out of 27 compositions in the Al2O3-CaO-MgO-SiO2 system werewas completely melted, while the rest had solid shape present . These results were in disagreement with the existing phase diagram.

 

Water solubility measurements were carried out by employing a thermo gravimetric technique. The temperature was found to have negligible effect in the water solubilities. The experimental results showed that the water capacity values varied between 1·103 and 2·103 in the majority of the composition range. However, for compositions close to CaO saturation the water capacity value could reach higher than 3·103. The experimental determined water capacity was further used to develop a water capacity model for the quaternary slag system Al2O3-CaO-MgO-SiO2. The model was constructed by considering the effects of the binary interactions between the cations in the slag on the capacity of capturing hydroxyl ions. The model calculations agreed reasonably well with the experimental results as well as with the literature data.

 

The water capacity model was used in the last part of the present thesis in order to determine the major source for hydrogen pick-up of the steel after vacuum degassing but before casting. For this purpose, samples of slag and metal were taken at different stages ofduring ladle treatment at SSAB. Hydrogen increase after vacuum treatment was observed. Moisture contents of the industrial slag were analysed and the water capacities of the slags were calculated. It could be seen that the hydrogen increase was correlated to the amount of moisture in the slag and the water capacity. The study showed that the slag containing most water was also the heat having the largest hydrogen increase. The slag with most water had the highest water capacity. It could be concluded that the major source for hydrogen coming back into the steel was due to the slag-metal reaction.

A tentative process model to predict the final contents of hydrogen and nitrogen after tundish process was attempted. More work is needed to improve the model.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. viii, 44 p.
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-172462 (URN)978-91-7595-594-0 (ISBN)
Public defence
2015-09-04, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20150825

Available from: 2015-08-25 Created: 2015-08-25 Last updated: 2015-08-25Bibliographically approved

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Hurtig, Jenny

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