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  • I

    Formation of non-metallic inclusions and the

    possibility of their removal during ingot

    casting

    Lars Ragnarsson

    Doctoral Thesis

    Materials Processing

    Department of Material Science and Engineering

    School of Industrial Engineering and Management

    Royal Institute of Technology (KTH)

    SE-10044 Stockholm, Sweden

    Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan framlägges till

    offentlig granskning för avläggande av Teknologie Doktorsexamen, fredag den 30 april,

    2010, kl 10:00 i sal F3, Lindstedtsvägen 26, Kungliga Teknkiska högskolan, Stockholm

    ISRN KTH/MSE--10/12--SE+MICROMODMETU/AVH ISBN 978-91-7415-620-1

  • II

    Lars Ragnarsson, Formation of non-metallic inclusions and the

    possibility of their removal during ingot casting

    Division of Micro-Modeling

    Department of Materials Science and Engineering

    School of Industrial Engineering and Management

    Royal Institute of Technology (KTH)

    SE-100 44 Stockholm, Sweden

    ISRN KTH/MSE--10/12--SE+MICROMODMETU/AVH ISSN-1104-7127

    ISBN 978-91-7415-620-1

    © Lars Ragnarsson, March 2010

  • III

    “What we can´t imagine within ourselves, will likely never take place outside our mind.”

  • IV

    ABSTRACT

    The present study was carried out to investigate the formation and evolution of non-metallic

    inclusions during ingot casting. Emphasize have been on understanding the types of

    inclusions formed and developed through the casting process and on the development of

    already existing inclusions carried over from the ladle during casting. Industrial experiments

    carried on at Uddeholm Tooling together with laboratory work and Computational Fluid

    Dynamics (CFD) simulations.

    Ingots of 5.8 tons have been sampled and the types of inclusions together with their

    distribution within the ingot have been characterized. Two new types of inclusions have

    been found. Type C1 is found originated from casting powder and in the size from a few μm

    to 30 μm. Type C2 is of macro inclusion type sizing up to 70 μm. The presence of C2

    inclusions are few but very detrimental for the quality of the steel. Both types, C1 and C2

    consist of alumina, indicating that reoxidation is the main reason for their existence.

    The protecting argon shroud has been studied by the use of a 1:1 scaled 2D model. Both

    flow pattern and oxygen measurement have been carried out. CFD has also been used as an

    auxiliary tool. It has been found that the oxygen pickup through argon gas shroud depends

    mostly on the distance between the ladle and the collar placed on top of runner. Further

    increase of gas flow rate above 2.5 m 3 .h

    -1 had very little effect on the oxygen distribution

    since both the flushing effect and the entraining effect with respect to oxygen are enhanced

    by further increase of inert gas flow rate. In the case of dual gas inlet, the flow in the shroud

    was found much less diffused compared with either vertical or horizontal injection system.

    The oxygen content in this arrangement was also greatly reduced.

    Studies of the runner after casting revealed a sparse non-metallic network structure around

    the periphery of the steel rod remained in the runner. The surface of the refractory had been

    severely attacked by the mechanical force from the streaming steel. The erosions of the

    centre stone and the end stone were on the other hand negligible. CFD calculations showed

    that the flow at those locations is almost stagnant. The surface of the refractory in contact

    with the steel was found to have an increased content of alumina. The source for the alumina

    could come from either exchange reaction of dissolved aluminium replaces the silica or

    reoxidation products origin from oxygen pick up during the transfer from the ladle to the

    vertical runner. Inclusions were also found entrapped in the steel refractory interface.

    It was also found that a formation of a liquid slag film as early as possible during casting

    would increase the possibility to remove inclusions and especially inclusions generated by

    the casting powder.

    Key words; Ingot casting, tool steel, inclusion, runner, refractory, erosion, casting powder,

    cold model, argon shroud, reoxidation, oxygen measurement, CFD, PIV.

  • V

    ACKNOWLEDGEMENT

    I would like to express my gratitude to Professor Du Sichen. It has been a very interesting

    journey through tough decisions and challenges. The guidance to see beyond the horizon of

    experimental results and conclusions will always be on my mind and in my heart.

    Thanks to the people at Uddeholm Tooling AB, who have given me a good insight in both

    research and production of world class quality tool steel.

    I am grateful for the support from Alf Sandberg and his truly passion for finding new ways

    to improve the steel making process. Without your support and dedication we wouldn´t go

    far.

    Thanks to all my foreign friends who are working everyday in close contact with customers

    relaying on the quality of the steel.

    I also want to thank Dr. Mårten Görnerup at Metsol AB for fruitful discussion about the

    industrial perspectives of steel making.

    Jernkontoret and its staff, especially Lars-Henrik Österholm for the strive to build bridges

    between Swedish steel companies and form platforms for scientific discussion and joint

    projects together with researchers from institutes and the academic world.

    Financial support for this work provided by Uddeholm Tooling AB and Jernkontoret (for the

    last months) is gratefully acknowledged.

    I also want to thank Professor Yonglin Ma and Wei Song at Inner Mongolia University of

    Technology and of course all the teachers and students who made my stay and work joyful

    and interesting.

    Thanks are also due to Professor Pär Jönsson and all the colleagues at the department of

    Materials Science and Engineering for many interesting discussions and social activities.

    Thanks to all my friends who put up with my, from time to time, pressed schedule and your

    encouragement to take breaks and enjoy the life.

    And of course I want to a specially thank my beloved family who are always helpful and

    there for me when it is needed.

    Stockholm, 23 March 2010

    Lars Ragnarsson

  • VI

    SUPPLEMENTS

    This thesis is based on following papers:

    Supplement 1: “Inclusion generated during ingot casting of tool steel”

    Lars Ragnarsson and Du Sichen

    Steel Research International, Volume 81, 2010, No. 1. pp. 40-47.

    Supplement 2: “Erosion of runners in ingot casting”

    Lars Ragnarsson and Du Sichen

    AISTech 2009 Proceedings - Volume I, March, 2009, Saint Loius,

    USA, pp. 1077-1087.

    Supplement 3: “Physical and mathematical modelling of argon shroud for the

    protection of steel stream during casting”

    Lars Ragnarsson, Wei Song, Yonglin Ma and Du Sichen

    Sent to International Journal of Cast Metals Research for publication.

    Supplement 4: “Oxygen penetration in the protection shroud in steel casting”

    Lars Ragnarsson, Wei Song, Yonglin Ma and Du Sichen

    Sent to Steel Research International for publication.

    Supplement 5: “Flow pattern in the ingot during mould filling and its impact on

    inclusion removal”

    Lars Ragnarsson, Mattias Ek, Anders Eliasson and Du Sichen

    Ironmaking and Steelmaking, accepted for publication, 2010-01-19.

  • VII

    TABLE OF CONTENTS

    Content

    1. INTRODUCTION ................................................................................................................ 1

    2. EXPERIMENTAL ............................................................................................................... 3

    2.1. Industrial Process ........................................................................................................... 3

    2.2. Industrial Sampling and Analysis .................................................................................. 4

    2.2.1. Ingot ........................................................................................................................ 4

    2.2.2. Runner ..................................................................................................................... 5

    2.3. Modeling of Argon Shroud ............................................................................................ 6

    2.3.1. Argon flow in shroud using PIV technique – Physical model ................................ 7

    2.3.2. Oxygen measurement within argon shroud – Physical model ................................ 8

    2.4. Cold model for Inclusion Removal by Slag film in Ingot ............................................. 8

    3. CFD MODELING .............................................................................................................. 10

    3.1. Modeling of Mould Filling ......................................................................