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  • Some aspects of non-metallic inclusions during vacuum degassing in ladle treatment- with emphasize on liquid CaO-Al2O3 inclusions

    YoungJo Kang

    Doctoral Thesis

    Department of Materials Science and Engineering Division of Micromodeling

    Royal Institute of Technology SE-100 44 Stockholm

    Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskloan i Stockholm, framlägges för offentlig granskning för avläggande av Teknologie doktorsexamen, onsdagen den 7 mars 2007, kl. 10.00 i D3, Lindstedsvägen 5, Kungliga Tekniska Högskloan, Stockholm

    ISRN KTH/MSE--07/04--SE+MICROMODMETU/AVH ISBN 978-91-7178-571-8

  • ABSTRACT 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. Key words: non-metallic inclusions, ladle refining, vacuum degassing, ladle glaze, spinel, SiO2 activity, agglomeration, cold model, open-eye, inclusion removal

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  • ACKNOLEDGEMENTS First of all, I would like to express my special thanks to my principal supervisor, Professor Du Sichen, for his excellent guidance and encouragements throughout this work. I also convey my special thanks to my co-supervisor, Professor Kazuki Morita, for his valuable advices and supervision during this work. I am truly grateful to Alf Sandberg, Dr. Mselly Nzotta and Tech. Lic. Karin Steneholm, for fruitful discussions and valuable helps in industrial experiments. Warm kindness of many colleagues in Uddeholm Tooling AB is also appreciated. I also thank to Professor Piotr R. Scheller, for his helpful suggestions and comments as well as supports during my work in Frieberg. I am thankful to Professor Seshadri Seetharaman and Professor Pär Jönsson, for constant support during my study. I owe my gratitude to Fan Li, Liang Yu and Dr. Bahman Sahebakar for their essential contributions and assistances. Financial supports for this work from Uddeholm Tooling AB are gratefully acknowledged. I also thank to Professor Min Dong Joon, for his encouraging advice and comments. Thanks to all my friends in Division of Micromodeling and my colleagues and all faculties in Department of Material Science and Engineering for their friendship and support. I specially thank to all members of KOSAS and Korean friends around me, MY, KB, SS, JY, HS, JS, SW, DH, DY, SY, HN, YN, for numerous helps and kindness, which made me stay in Sweden happily. Finally, I would like send my sincere gratitude to my family in Korea for continuous trust and encouragement in my whole life. Stockholm, January 2007

    YoungJo Kang

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    SUPPLEMENTS The thesis is based on the following papers: Supplement 1: “Non-metallic inclusions and their distribution in the ladle before and

    after vacuum treatment of tool steel” YoungJo Kang, Mselly Nzotta and Du Sichen Steel-Grips, accepted for publication, 2006

    Supplement 2: “Mechanism Study on the Formation of Liquid Calcium Aluminate

    Inclusions from MgO·Al2O3 spinel” YoungJo Kang, Fan Li, Kazuki Morita and Du Sichen Steel Research International, Volume 77, Nov., 2006, pp. 785-792

    Supplement 3: “Activities of SiO2 in some CaO-Al2O3-SiO2(-10%MgO) melts with

    low SiO2 contents at 1873K” YoungJo Kang, Du Sichen and Kazuki Morita Sent to ISIJ International for publication

    Supplement 4: “Some aspects of physical growth of non-metallic inclusion in ladle

    treatment” YoungJo Kang, Bahman Sahebkar, Piotr R. Scheller and Du Sichen METEC InSteelCon 2007, 2007, June, Dusseldorf, Germany

    Supplement 5: “Mechanism Study of inclusion removal around open-eye in ladle

    treatment” YoungJo Kang, Liang Yu and Du Sichen Ironmaking and Steelmaking, in press, 2007

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    Parts of this work have been accepted at the following conferences: 1. “Activity of SiO2 in CaO-Al2O3-SiO2(-10%MgO) melts with low SiO2 content at

    1873K” YoungJo Kang, Du Sichen and Kazuki Morita 153rd ISIJ Spring Meeting, 2007, March, Chiba, Japan

    2. “Some aspects of non-metallic inclusions in vacuum treatment”

    YoungJo Kang and Du Sichen 7th International Conference on Clean Steel, 2007, June, Balatonfured, Hungary

  • TABLE OF CONTENTS

    ABSTRACT i

    ACKNOWLEGDEMENTS iii

    SUPPLEMENTS v

    1 INTRODUCTION 1

    2 EXPERIMENTAL 2 2.1 Analysis on industrial samples 2 2.2 Study on the formation of liquid CaO-Al2O3 phase 3 2.3 Measurement of SiO2 activity in CaO-Al2O3 melts with low SiO2 4

    content

    2.4 In-situ observation on the interactions of non-metallic inclusions 5 2.5 Cold model for the inclusions removal by bubble floatation 6

    3 RESULTS 8 3.1 Industrial study 8

    3.1.1 Compositions of steel and slag before and after vacuum degassing 8 3.1.2 Types of inclusions before and after vacuum degassing 8 3.1.3 Populations of inclusions before and after vacuum degassing 9 3.1.4 Distribution of inclusions before and after vacuum degassing 10

    3.2 Formation of CaO-Al2O3 phase in the presence of MgO·Al2O3 in 12 liquid steel

    3.3 Activities of SiO2 in low SiO2 region of the CaO-Al2O3-SiO2(-MgO) 14 system

    3.3.1 Activity of SiO2 in the CaO-Al2O3-SiO2 system 14 3.3.2 Activity of SiO2 in the CaO-Al2O3-SiO2-MgO system 15

    3.4 Interactions and behavior of various inclusions in liquid steel 17 3.4.1 Attractive interaction of alumina inclusions 17 3.4.2 Behaviors of inclusions other than alumina 18

    3.5 Behaviors of inclusions around open eye 20 3.5.1 Phenomena around open-eye and inclusion movements in the bath 20 3.5.2 Attachment of inclusions with different sizes 21

    4 Discussion 22 4.1 Formation of liquid inclusions before and after vacuum degassing 22

    4.1.1 Formation of liquid CaO-Al2O3 inclusions from spinel 22

  • 4.1.2 Equilibrium between liquid inclusions and liquid steel after vacuum 24 degassing

    4.2 Change of inclusions during vacuum degassing 26 4.2.1 Number of bigger inclusions during vacuum degassing 26 4.2.2 Physical growth of various inclusions during vacuum degassing 26

    4.3 Removal of inclusions during vacuum degassing 31 4.3.1 Mechanism of inclusion removal around open-eye 31 4.3.2 Contribution of the open-eye in inclusion removal 32 4.3.3 Distribution of inclusions in a ladle 35

    5 SUMMARY 37

    6 FUTURE WORKS 38

    7 REFERENCES 39

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