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Growth and removal of inclusions during ladle refining
KTH, Superseded Departments, Materials Science and Engineering.
2004 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The overall purpose of this thesis work has been to further our understanding of the growth and removal of inclusions in gas- and induction-stirred ladles. The primary focus has been on alumina inclusions.

Growth mechanisms were studied using data from fundamental mathematical models of gas- and induction-stirred ladles. The results showed the turbulence mechanism to be the most dominant in alumina inclusion growth. The dynamic growth and removal of inclusions in a gas-stirred ladle was studied using mathematical modelling. The model results showed concentration gradients of inclusions. The effect was most obvious in the steel flow past the removal sites: top slag, ladle refractory, and gas plume (bubble flotation). A new removal model was developed for large spherical caps bubbles.

In order to verify the predicted concentration gradients for the size population of inclusions, three experiments were carried out in production. The sampling equipment enabled sampling at five different positions and different locations at the same time. The results showed that concentration gradients of inclusions do exist both in induction-stirred and gas-stirred ladles. A theoretical analysis showed that the drag force on the inclusions to be the dominating force and that therefore inclusions follow the fluid flow.

The cluster behaviour of alumina inclusions were examined on steel samples taken in an industrial-scale deoxidation experiment in a ladle. The samples were examined by microscope and the results used to study cluster growth. It was found that there was rapid cluster growth due to collision during stirring and that at the end of the deoxidation experiment a majority of the small inclusions were bound in clusters. The cluster growth data determined using the microscopic results were compared with predicted cluster-growth data. A method was developed for converting the experimental data observed per unit area into data given per unit volume and vice versa. An expression for the collision diameter of the cluster was also developed. The results showed that the predicted cluster growth agreed well with the microscopic observations for the assumptions made in the growth model.

Place, publisher, year, edition, pages
Stockholm: Materialvetenskap , 2004.
Keyword [en]
Meteorology, inclusions, steel, ladle, cluster growth, modelling
Keyword [sv]
Meteorologi
National Category
Meteorology and Atmospheric Sciences
Identifiers
URN: urn:nbn:se:kth:diva-45ISBN: 91-7283-886-8 (print)OAI: oai:DiVA.org:kth-45DiVA: diva2:12565
Public defence
2004-11-15, B3, KTH, Brinellvägen 23, Stockholm, 10:00
Opponent
Supervisors
Available from: 2004-11-15 Created: 2004-11-15 Last updated: 2014-03-20
List of papers
1. Most relevant mechanisms of inclusion growth in an induction-stirred ladle
Open this publication in new window or tab >>Most relevant mechanisms of inclusion growth in an induction-stirred ladle
2002 (English)In: Scandinavian journal of metallurgy, ISSN 0371-0459, E-ISSN 1600-0692, Vol. 31, no 3, 210-220 p.Article in journal (Refereed) Published
Abstract [en]

The purpose of this study has been to theoretically evaluate which inclusion growth mechanisms are important in an inductively stirred ladle. This has been done using data from a computational fluid dynamics model of a real ladle. The data was utilized as the input to the different collision equations. It was concluded that diffusion of oxygen and deoxidant to the inclusion surface, diffusion coalescence and Brownian motion collisions contribute very little to growth of inclusions during the stirring period in the ladle. This was in accordance with earlier findings in the literature. This study also showed that laminar shear collisions could be excluded from a growth model. The major growth mechanism is turbulent collisions. It was also shown that a 700-A stirring current leads to more growth due to turbulent collisions than a 500-A stirring current. The importance of a correct value of the collision efficiency coefficient for turbulent collisions was pointed out. If a large difference in inclusion size exists then collisions due to difference in buoyancy (Stokes collisions) also need to be considered in a growth model. Finally, it is concluded that the variation in fluid flow and turbulent properties in different parts of the ladle should be incorporated into a growth model, since it can affect average turbulent collision volumes by 25-30%.

Keyword
CFD, Inclusion growth, Induction stirring, Ladle
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-7511 (URN)10.1034/j.1600-0692.2002.310308.x (DOI)000176797200008 ()
Note
QC 20100923Available from: 2004-11-15 Created: 2004-11-15 Last updated: 2017-12-14Bibliographically approved
2. Modeling micro-inclusion growth and separation in gas-stirred ladles
Open this publication in new window or tab >>Modeling micro-inclusion growth and separation in gas-stirred ladles
2002 (English)In: Scandinavian journal of metallurgy, ISSN 0371-0459, E-ISSN 1600-0692, Vol. 31, no 2, 134-147 p.Article in journal (Refereed) Published
Abstract [en]

Mathematical models of inclusion behavior in stirred ladles are useful both for increasing our fundamental understanding of the growth and removal of inclusions as well as for future use in process control. This study reports on some efforts to use both static and dynamic modeling to better understand inclusion behavior in gas-stirred ladles. A computational-fluid-dynamics mathematical model of a gas-stirred ladle was developed earlier. In the investigation covered in this report, instantaneous fluid-flow results from the model were used in combination with inclusion growth and removal theories in order to study the importance of bubbles on inclusion flotation. The study results proved to be highly dependent on the theory used to describe bubble flotation. The model of the gas-stirred ladle was also used together with the inclusion theories to study the transient behavior of inclusions during growth and removal. The dynamic simulation results indicated that inclusion concentration gradients exist. The most important research task in the near future is to verify static and dynamic modeling results of inclusion behavior during stirring with experimental data. Here, the authors feel that carefully performed plant trials could provide useful information.

Keyword
gas-stirring, inclusions, ladle, modeling, secondary refining
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-7512 (URN)10.1034/j.1600-0692.2002.310208.x (DOI)000175885700008 ()
Note

QC 20110921

Available from: 2004-11-15 Created: 2004-11-15 Last updated: 2017-12-14Bibliographically approved
3. Inclusion growth and removal in gas stirred ladles
Open this publication in new window or tab >>Inclusion growth and removal in gas stirred ladles
2004 (English)In: Steel research, ISSN 0177-4832, Vol. 75, no 2, 128-138 p.Article in journal (Refereed) Published
Abstract [en]

A static modelling approach was used to study the growth and removal of inclusions during gas stirring in a ladle. A mathematical model of a gas-stirred ladle was used to predict the data necessary to calculate growth and removal of inclusions. Results indicated that inclusion growth resulting from laminar shear collisions is negligible in comparison with growth from turbulent and Stokes collisions. Furthermore, the need for a model describing inclusion flotation by spherical-cap bubbles was identified. Since the existing models presented in the literature are only valid for spherical bubbles, a model for the removal of inclusions by spherical-cap bubbles was developed. Inclusion removal to the slag, refractory and by bubble flotation was compared. The mechanism determined to be responsible for the removal of the majority of inclusions larger than 25 μm was Stokes flotation and for the majority of the smaller inclusions, bubble flotation by spherical-cap bubbles (assuming plane contact between the inclusion and the bubble).

Keyword
Bubble flotation, CFD model, Gas stirring, Inclusion growth, Inclusion removal
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-7513 (URN)2-s2.0-17644435372 (Scopus ID)
Note

QC 20100917

Available from: 2004-11-15 Created: 2004-11-15 Last updated: 2017-12-14Bibliographically approved
4. Size distribution gradients of inclusions during stirring
Open this publication in new window or tab >>Size distribution gradients of inclusions during stirring
2002 (English)In: Proceedings of the 6th International Conference on Clean Steel, 2002, 48-57 p.Conference paper, Published paper (Refereed)
Abstract [en]

Mathematical CFD simulations of inclusion growth and removal in stirred ladles have shown that size distribution gradients exist during different parts of stirring. Therefore, special plant experiments has been designed and carried out at Ovako Steel AB to confirm this. A special sampler was built allowing for sampling with five samples at the same time and at two depths. The size distribution and composition of the inclusions in the different sampling positions were compared.

National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-7514 (URN)000177135000006 ()
Conference
6th International Conference on Clean Steel, June 2002, IISI, Balatonfured, Hungary
Available from: 2004-11-15 Created: 2004-11-15 Last updated: 2011-09-21Bibliographically approved
5. An experimental study of number concentration gradients of inclusion during deoxidation using inductive and gas stirring
Open this publication in new window or tab >>An experimental study of number concentration gradients of inclusion during deoxidation using inductive and gas stirring
2005 (English)In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 76, no 7, 481-490 p.Article in journal (Refereed) Published
Abstract [en]

 Concentration gradients of inclusions during inductive and gas stirring of AISI H13 tool steel in a ladle have been studied using a unique experimental technique at Uddeholm Tooling AB in Sweden. A specially designed sampling equipment, which allows five samples to be taken at the same time and at different depths was used. The aim was to experimentally quantify the existence of inclusion size population gradients in different positions of the steel melt. One experiment was carried out using induction stirring and one using gas stirring. The main findings from the induction stirring experiment was that i) concentration gradients of inclusions exist, ii) the number of inclusions decreases with stirring time in all positions, and iii) that the superficial positions show a more rapid decrease than the deeper ones. The main findings from the gas stirring experiment are that i) concentration gradients of inclusions exist, ii) the superficial positions are mostly affected by the gas plumes, and show a very pronounced decrease of the number of inclusions with stirring time, and iii) the deeper positions and also one of the superficial position (no "3") shows no decreasing trend.

Keyword
ladle, inclusions, deoxidation, stirring, concentration, gradient
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-7515 (URN)000230676400002 ()2-s2.0-22744436228 (Scopus ID)
Note

QC 20100830

Available from: 2004-11-15 Created: 2004-11-15 Last updated: 2017-12-14Bibliographically approved
6. Growth of Small Alumina Clusters during Ladle Deoxidation
Open this publication in new window or tab >>Growth of Small Alumina Clusters during Ladle Deoxidation
2009 (English)In: High Temperature Materials and Processes, ISSN 0334-6455, E-ISSN 2191-0324, Vol. 28, no 6, 347-368 p.Article in journal (Refereed) Published
Abstract [en]

For the first time, the size distribution of small clusters was determined in a systematic manner from samples taken during deoxidation with aluminium in an induction-stirred ladle. Chemical and microscopic analyses of samples were performed. The results indicated rapid cluster growth throughout the experiment and at the end of the experiment a majority of the small inclusions were found to be bound in clusters. The cluster growth was concluded to be by collision. Growth-model calculations were carried out to verify the experimental findings. In order to perform these calculations, a method first needed to be developed for conversion of the experimental data determined per unit area to data given per unit volume. Next, the cluster collision diameter was modelled using a theory for fractal aggregates. The results from the growth-model calculations, simulating deoxidation with aluminium, were found to agree well with the experimental results.

Keyword
Deoxidation, Alumina clusters, Growth, Collisions, Inclusions, Ladle
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-32679 (URN)10.1515/HTMP.2009.28.6.347 (DOI)000279749700002 ()2-s2.0-77954720742 (Scopus ID)
Note

QC 20110418. Previous title: Growth of small alumina clusters during deoxidation in a ladle

Previous title:

Available from: 2011-04-18 Created: 2011-04-18 Last updated: 2017-12-11Bibliographically approved

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