An Experimental Study to Improve the Casting Performance ...1077205/FULLTEXT01.pdfTundish during...

KTH Industrial Engineering

and Management

An Experimental Study to Improve the Casting

Performance of Steel Grades Sensitive for Clogging

Jennie Katarina Sofia Svensson

Doctorial Thesis

Stockholm 2017

KTH Royal Institute of Technology

School of Industrial Engineering and Management

Department of Material Science and Engineering

Division of Processes

SE-100 44 Stockholm

Sweden

Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan I Stockholm, framlägges för

offentlig granskning för avläggande av teknologie doktorsexamen, tisdagen den 28 mars 2017, kl 10.00 i

Kollegiesalen, Brinellvägen 8, Kungliga Tekniska Högskolan, Stockholm.

ISBN 978-91-7729-275-3

Jennie Katarina Sofia Svensson: An Experimental Study to Improve the

Casting Performance of Steel Grades

Sensitive for Clogging

KTH School of Industrial Engineering and Management

Division of Processes

Royal Institute of Technology

SE-100 44 Stockholm

Sweden

ISBN 978-91-7729-275-3

Copyright © Jennie Svensson, 2017

Print: Universitetsservice US-AB, Stockholm 2017

“Thermodynamics is a

funny subject. The first time you go

through it, you don't understand it at all.

The second time you go through it, you

think you understand it, except for one or

two points. The third time you go through

it, you know you don't understand it, but

by that time you are so used to the

subject, it doesn't bother you anymore” Arnold Sommerfeld

i

Abstract

In this study, the goal is to optimize the process and to reduce the clogging

tendency during the continuous casting process. The focus is on clogging

when the refractory base material (RBM) in the SEN is in contact with the

liquid steel. It is difficult or impossible to avoid non-metallic inclusions in

the liquid steel, but by a selection of a good RBM in the SEN clogging can

be reduced.

Different process steps were evaluated during the casting process in order

to reduce the clogging tendency. First, the preheating of the SEN was

studied. The results showed that the SEN can be decarburized during the

preheating process. In addition, decarburization of SEN causes a larger risk

for clogging. Two types of plasma coatings were implemented to protect

the RBM, to prevent reactions with the RBM, and to reduce the clogging

tendency. Calcium titanate (CaTiO3) mixed with yttria stabilized zirconia

(YSZ) plasma coatings were tested in laboratory and pilot plant trials, for

casting of aluminium-killed low-carbon steels. For casting of cerium

alloyed stainless steels, YSZ plasma coatings were tested in laboratory,

pilot plant and industrial trials. The results showed that the clogging

tendency was reduced when implementing both coating materials.

It is also of importance to produce clean steel in order to reduce clogging.

Therefore, the steel cleanliness in the tundish was studied experimentally.

The result showed that inclusions originated from the slag, deoxidation

products and tundish refractory and that they were present in the tundish

as well as in the final steel product.

Key words: Preheating, Decarburization, RBM, Clogging, SEN, Plasma

sprayed coating, CaTiO3, YSZ, Continuous casting, Pilot plant trials,

Industrial plant trials, Clean steel, Tundish, Non-metallic inclusions, MISS

iii

Sammanfattning

Denna studie handlar om att optimera stränggjutningsprocessen och

reducera igensättning av gjutrör. Fokus är på igensättning, där keramiken i

gjutröret är i kontakt med flytande stål vid stränggjutning av stål. Det är

svårt att helt undvika icke metalliska inneslutningar i det flytande stålet

men genom bra materialval i gjutrör kan igensättningen reduceras.

Flera processteg i gjutprocessen har studerats, för att reducera

igensättningen. Först studerades förvärmningsprocessen av gjutrör.

Resultaten visade att gjutrören kan avkolas under förvärmningsprocessen.

Dessutom så resulterar avkolningen i en större igensättningsgrad. Därefter

undersöktes 2 plasma beläggningar för att kunna reducera

igensättningsgraden. En beläggning av kalcium titanat (CaTiO3) blandat

med yttria stabiliserad zirconia (YSZ) användes för gjutning av

aluminiumtätat stål. Beläggningen testades i laboratorieexperiment och

pilotskaleförsök. Den andra beläggningen bestod av YSZ och användes för

gjutning av ceriumlegerat rostfritt stål.

Beläggningen undersöktes i laboratorieexperiment, pilotskaleförsök och

industriella verksförsök. Resultaten visade på att igensättningsgraden

reducerades vid tillämpning av båda beläggningsmaterialen.

En till aspekt att ta hänsyn till för att reducera igensättningsgraden är att

tillverka ett rent stål. Därför kartlades stålet renhet i gjutlådan

experimentellt. Resultatet av kartläggningen visade att det förekom

inneslutningar från slaggen, desoxidations produkter och gjutlådans

infodringsmaterial. Inneslutningarna observerades både i gjutlådan och i

slutprodukten.

Nyckelord: Förvärmning, Avkolning, Infodringsmaterial, Igensättning,

Gjutrör, Plasma sprutad beläggning, CaTiO3, YSZ, Stränggjutning,

Pilotskaleförsök, Industriella verksförsök, Rent stål, Gjutlådan, Icke

metalliska inneslutningar, MISS

v

Acknowledgement

First of all both my supervisors Professor Voicu Brabie and Professor Pär

G. Jönsson are greatly acknowledged for their support, discussions,

guidance and encouragement throughout this time. A sincerely thank you

to both my professors for this opportunity and for staying with me until the

end. This work would not have been accomplished without their support.

My supervisor Erik Roos, at SSAB Special Steels in Oxelösund, is greatly

acknowledged for his support, guidance and help during industrial trials.

This work has been performed within the Steel Industry's Graduate School

with financial support from SSAB Special Steels in Oxelösund, Regional

Development Council of Dalarna, Regional Development Council of

Gävleborg, County Administrative Board of Gävleborg, Jernkontoret - The

Swedish Steel Producers' Association, Sandviken City and Dalarna

University. Additionally, VINNOVA and Jernkontoret committee

TO23052 are greatly acknowledged for their thrust and financial support

in the beginning of the project.

SSAB EMEA AB in Oxelösund and Luleå, Sweden, Outokumpu Abp in

Avesta, Sweden, Sandvik Materials Technology AB in Sandviken,

Sweden, and ComdiCast AB in Fagersta, Sweden, are all acknowledged

for their industrial support.

A special thanks to Arashk Memarpour, at SMT, Sandviken, for his

support and guidance; Olle Sundqvist at SMT, Sandviken, is

acknowledged for his guidance of writing and presenting experimental

results; Fredrik Larsson at Outokumpu Abp, Avesta, is acknowledged for

industrial support during experiments; Patrik Wikström, SSAB EMEA

AB, Luleå, is acknowledged for industrial support during experiments;

Sven Ekerot at ComdiCast AB, Fagersta, is acknowledged for his help with

the pilot plant trials and his guidance; Andrey Karasev at KTH, Stockholm,

is acknowledge for his help with the electrolytic extractions experiments.

Many thanks to my colleges for all the laughs and support during this time:

at both the Department of Materials Science, Dalarna University, and at

the Department of Materials Science and Engineering, KTH. Thank you

all for creating an enjoyable work environment.

vi

Last but not least my family and friends are greatly acknowledged for their

love, and for always supporting and believing in me. Thank you for making

it possible for me to accomplish this work.

Jennie Svensson

Stockholm, February 2017

vii

Supplements

The following supplements have been the basis for the thesis:

Supplement 1: “Studies of the decarburisation phenomena during

preheating of submerged entry nozzles (SEN) in

continuous casting processes”, Jennie K.S. SVENSSON,

Arashk MEMARPOUR, Voicu BRABIE and Pär G.

JÖNSSON, Ironmaking and Steelmaking, Vol. 44, No. 2,

pp. 108-116, 2017.

DOI:10.1080/03019233.2016.1156900

Supplement 2: “Studies of New Coating Materials to Prevent Clogging of

the Submerged Entry Nozzle (SEN) during Continuous

Casting of Al-killed low Carbon Steel”, Jennie K.S.

SVENSSON, Arashk MEMARPOUR, Sven EKEROT,

Voicu BRABIE and Pär G. JÖNSSON, Ironmaking and

Steelmaking, Vol. 44, No. 2, pp 117-127, 2017.

DOI: 10.1179/1743281215Y.0000000065

Supplement 3: “Implementation of an YSZ coating material to prevent

clogging of the submerged entry nozzle (SEN) during

continuous casting of Ce-treated steels”, Jennie K.S.

SVENSSON, Arashk MEMARPOUR, Sven EKEROT,

Voicu BRABIE and Pär G. JÖNSSON (Published online

in Ironmaking and Steelmaking, DOI:

10.1080/03019233.2016.1245916, 2016-11-01)

Supplement 4: “Post-mortem Studies of Submerged Entry Nozzle (SEN)

Coated with Yttria Stabilized Zirconia (YSZ)”, Jennie K.S.

SVENSSON, Fredrik LARSSON, Arashk

MEMARPOUR, Voicu BRABIE and Pär G. JÖNSSON

(Peer-reviewed article presented and published in the

ICS2015 proceedings of the 6th International Congress on

the Science and Technology of Steelmaking, Solidification

and Continuous Casting, Beijing, China, ISBN 978-7-111-

50125-1, Vol. 1, pp. 469-472, May 12-14, 2015)

ix

Supplement 5: “Experimental Study of the Slag/Steel Interface in the

Tundish during Continuous Casting of Steel”, Jennie K.S.

SVENSSON, Erik ROOS, Anders LAGERSTEDT,

Andrey KARASEV, Voicu BRABIE and Pär G.

JÖNSSON, Manuscript.

The contributions by the author of this thesis to the above supplement are

the following:

Supplement 1: Performed all of the literature survey, the experimental work at

steel plant SP3, observations and analyses of the FEG-SEM work at steel plant

SP3 and major part of writing.

Supplement 2: Performed all of the literature survey, most part of the experimental

work, observations and analyses of the FEG-SEM work and major part of writing.

Supplement 3: Performed all of the literature survey, most part of the experimental

work observations and analyses of the FEG-SEM work and major part of writing.

Supplement 4: Performed all of the literature survey, the experimental work,

observations and analyses of the FEG-SEM work and major part of writing.

Supplement 5: Performed all of the literature survey, major part of the

experimental work, major part of the FEG-SEM observations, majority of

inclusion data analysis and major part of writing.

Other relevant publications not included in the thesis:

• J. K.S. Svensson, A. Memarpour, V. Brabie: “Decarburization during

Preheating of the Submerged Entry Nozzle”, Technical Report, Limited

distribution, Jernkontorets forskning, TO 23-149, 2013.

• J. K.S. Svensson, A. Memarpour, V. Brabie, S. Ekerot: “Studies of

Clogging Phenomena with Plasma Coated Nozzles in Pilot Plant Trials

ix

at Comdicast AB”, Technical Report, Limited distribution, Jernkontorets

forskning, TO 23-150, 2013.

• J. K.S. Svensson, A. Memarpour, V. Brabie, S. Ekerot: “Studies of

Clogging Phenomena with Yttrium Stabilized Zirconia Coating”,

Technical Report, Limited distribution, Jernkontorets forskning, TO 23-

151, 2013.

• J. Alexis, T. Jonsson, V. Brabie, J. K.S. Svensson, A. Memarpour, E.

Roos, A. Tilliander, O. Sundqvist: “Improved Processing Techniques for

Casting of Steel Sensitive for Clogging”, Technical Report, Limited

distribution, Jernkontorets forskning, D 848, 2013.

xi

List of Tables

• Table 1. Overview of the 5 supplements

• Table 2. The chemical composition of the glass/silicon powder coating

[12]

• Table 3. The glass/silicon powder coating [12]

• Table 4. Overview of the results and applications of the supplements

xiii

List of Figures

• Figure 1. Longitudinal section showing the commercial SEN, where the

inlet consists of MgO-C, the bulk consists of Al2O3-C, and where the outlet

of the SEN slag line is reinforced with ZrO2-C.

• Figure 2. The CaTiO3-Al2O3 phase diagram and the eutectic point is at a

41 wt-% Al2O3 content and a 1580°C temperature [18].

• Figure 3. Presentation of how the 5 supplement are connected to each

other.

• Figure 4. Preheating set up with oxy-propane torches placed in the tundish

lid and at the SEN outlet. In total, 6 channels were drilled for inserting

thermocouples into the SEN. Channels 1 to 5 were placed at a 80 mm

distance apart. Thermocouples of type S were used in channels 1 to 5. In

channel 6, thermocouples of type K were used.

• Figure 5. Schematic presentation of the high-temperature graphite furnace

with CaTiO3 powder in an Al2O3 crucible and protected with argon

atmosphere.

• Figure 6. Longitudinal section showing the nozzle’s geometric and

dimension. About 25 mm of the nozzle’s internal wall could be plasma

sprayed with a 200 to 400 µm thick coating.

• Figure 7. Setup of the plasma spraying equipment utilized for coating of

the nozzles [23].

• Figure 8. The pilot plant experimental set-up of the induction furnace

teeming molten steel through the nozzles into a mould placed on a scale

[24]. The teemed steel mass was continuously measured and logged. The

temperature was measured and controlled with thermocouples.

• Figure 9. The SEN and stopper rod from the industrial plant trial: on the

left – the uncoated SEN and stopper rod are marked with yellow where the

powder coating was removed before coated with YSZ; on the right – the

SEN and stopper rod after a completed YSZ coating.

• Figure 10. The laboratory set-up of the equipment for electrolytic

extraction of steel samples using a 2% TEA solution.

• Figure 11. The MISS sampler [28] and MISS sample after removal from

the sampler with markings of where the analyzed sample was cut out.

xiv

• Figure 12. Presentation of the temperature profile inside the SEN over

time during the preheating process, performed at steel plant SP3. The

temperature in channel 1 was measured at the bottom of the SEN at a

distance of 8 cm apart from the other channels 2 to 5. The temperature in

channel 6 was measured at the inlet of the SEN.

• Figure 13. Crucibles after heating of CaTiO3 powder in Al2O3 crucibles:

(a) C1 – 12 minutes at 1600°C; (b) C2 – 60 minutes at 1600°C; (c) C3 –

60 minutes at 1575°C; (d) C4 – 60 minutes at 1565°C; (e) C5 – 60 minutes

at 1550°C; (f) C2 – A reaction between the powder and crucible was visible

by the change in colour. Also, CaTiO3 powder was found to be smeared

onto the crucible wall. (A) A lighter blue colour on the crucible’s surface.

(B) Reaction area into the crucible’s wall. (C) Powder attached to the

crucible’s wall, which have changed colour from white to grey. (D) The

reaction surface close to the powder had a deep blue colour.

• Figure 14. The actual teemed steel mass for all the 4 nozzles compared to

the theoretical teemed steel mass through the nozzles.

• Figure 15. Comparison between the theoretical and experimental steel

mass teemed through nozzles. Nozzle N3 and N4 were plasma coated with

an YSZ plasma coating. The reference nozzles N1 and N2 were cast

without using any coating materials.

• Figure 16. Movement of the stopper rod position for the industrial trials

(S1-S3) compared to the reference trial SR. The values have been modified

so that the stopper rods have the same starting positions at zero, when being

closed in the beginning. When a stopper rod is moved upwards it is a sign

of that the steel flow into the SEN had to be increased and it can be

interpreted as clogging. If a stopper rod is moved downwards it is a sign of

erosion. After approximately 53% of the teeming operation the difference

in movement between S1 and SR start to show. Overall, the biggest

difference was 13 mm in height.

• Figure 17. Mapping of the accretion and remaining part of the coating

material in sample A. The results from the mapping of element Zr shows

how much of the coating that remained after the casting. The thickness was

measured to have values of about 30 to 100 µm. In addition, a thin zone of

up to about 20 µm of Mg was observed between the coating material and

the accretion.

• Figure 18. The Al2O3-CaO-MgO ternary phase diagrams for DM sized

inclusions, from heat 1. Samples are collected from: (a) VTD; (b) MISS samples

xv

from position 1 and 2 at time A; (c) MISS samples from position 1 and 2 at time

B; (d) MISS samples from position 1 and 2 at time C; (e) slab.

• Figure 19. The Al2O3-CaO-MgO ternary phase diagrams for DL sized




















xvii

List of Symbols

C – Constant

P – Pressure

P0 – Pressure of the liquid steel at the free surface in the

induction furnace

P1 – Pressure at the nozzle outlet

ρ – Molten steel density

g – Acceleration due to gravity

z – Vertical distance

v – Velocity

v0 – Velocity of the molten steel, at the free surface of the steel

melt in the induction furnace

v1 – Velocity of the molten steel, at the nozzle outlet

Qh – Theoretical mass teeming rate

AN – Nozzle outlet cross-sectional area

h – Vertical distance between the nozzle and upper surface of

the liquid bath

d – Nozzle height

Lt – Mean value of the noise reduced measured steel mass, in

the pilot plant trials

t – Time

Qt – Noise reduced mean value of the measured steel mass

teeming rate, in the pilot plant trials

ƞ – Clogging factor

xix

List of Abbreviations

SEN – Submerged entry nozzle

RBM – Refractory base material

REM – Rare earth metal

Rh-Pt – Rhodium-Platinum alloy

O2 – Oxygen

C – Graphite

CO – Carbon monoxide

CO2 – Carbon dioxide

Al2O3/A – Aluminium oxide or alumina

Al – Aluminium

MgO – Magnesium oxide or magnesia

CaO – Calcium oxide

CaTiO3/CT – Calcium titanium oxide calcium titanate

CA2 – Monoalcium dialuminate (CaO·2Al2O3)

CA6 – Monoalcium hexa-aluminate (CaO·6Al2O3)

L – Liquid

Y2O – Yttrium oxide or yttria

ZrO2 – Zirconium oxide or zirconia

YSZ – Yttria stabilized zirconia

Ce – Cerium

La – Lanthanum

FeSi – Ferrosilicon

SP1/2/3 – Preheating trial at steel plant 1/2/3

C1/2/3/4/5 – Crucible 1/2/3/4/5 heated in laboratory experiments

N1/2/3/4 – Nozzle 1/2/3 in pilot plant trials

SEN1/2 – Industrial trial with submerged entry nozzle ½ with

corresponding stopper rod; both plasma sprayed with YSZ coating

REF – Industrial trial with reference SEN and stopper rod

without plasma coating

MISS – Momentary interfacial solidification sampling

MA1/2 – MISS sample, collected at time A, in position 1/2

MB1/2 – MISS sample, collected at time B, in position 1/2

MC1/2 – MISS sample, collected at time C, in position 1/2

EDC – Equivalent circle diameter

xx

DM – Diameter size of DM inclusion is: ECD ≥5.7 µm and

<11.3 µm

DL – Diameter size of DL inclusion is: ECD ≥11.3 µm

FEG-SEM – Field emission gun scanning electron microscope

EDS – Energy dispersive X-ray spectrometry

OES – Optical emission spectroscopy

XRF – X-ray fluorescence spectrometry

EXTR – Melting extraction technique

HFIR – High frequency melting infrared detection

PDA – Pulse determination analysis

VTD – Vacuum tank degassing station

xxi

CONTENTS

Abstract ...................................................................................................... i

Sammanfattning ....................................................................................... iii

Acknowledgement .................................................................................... v

Supplements ............................................................................................ vii

List of Tables ........................................................................................... xi

List of Figures ........................................................................................ xiii

List of Symbols ..................................................................................... xvii

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

1.3. PRESENT WORK ......................................................................... 4

1.4. OBJECTIVES OF THE WORK .................................................... 5

2. EXPERIMENTAL METHODS ............................................................ 9

2.1. PREHEATING TRIALS ................................................................ 9

2.1.1 STEEL PLANT DESCRIPTION ........................................... 11

2.1.2. POST-MORTEM STUDIES OF SEN .................................. 12

2.2. TEMPERING OF CaTiO3-POWDER ......................................... 12

2.3. PILOT PLANT TRIALS.............................................................. 13

2.3.1 THE NOZZLES ..................................................................... 13

2.3.2. SET-UP OF THE PILOT PLANT ........................................ 15

2.3.3 PILOT PLANT DESCRIPTION ............................................ 16

2.3.4. STEEL FLOW RATES DURING TEEMING ..................... 16

2.3.5. POST-MORTEM STUDIES OF THE NOZZLES ............... 18

2.4. INDUSTRIAL IMPLEMENTATION OF THE YSZ COATING

MATERIAL ........................................................................................ 18

2.4 STEEL PLANT DESCRIPTION .............................................. 19

2.5. ELECTROLYTIC EXTRACTIONS ........................................... 20

2.6. MAPPING OF THE TUNDISH .................................................. 21

xxii

2.6.1. STEEL PLANT DESCRIPTION .......................................... 23

2.6.2. ANALYSIS OF THE INCLUSIONS .................................... 23

3. RESULTS AND DISCUSSION .......................................................... 25

3.1. DECARBURIZATION ................................................................ 25

3.2. POSSIBILITIES TO USE CALCIUMTITANATE AS A

COATING MATERIAL FOR SENS .................................................. 28

3.2.1. FORMATION OF LIQUID CALCIUMALUMINATES ..... 28

3.2.2. IMPLEMATION OF CaTiO3 COATINGS IN PILOT PLANT

TRIALS ........................................................................................... 30

3.3. YSZ AS A COATING MATERIAL ............................................ 31

3.3.1. IMPLEMENTATION OF YSZ COATINGS IN PILOT

PLANT TRIALS ............................................................................. 31

3.3.3. MICROSCOPIC EVALUATIONS OF YSZ AS A COATING

MATERIAL .................................................................................... 34

3.4. REACTIONS IN THE TUNDISH DURING CASTING ............ 37

4. CONCLUDING DISCUSSION .......................................................... 47

5. CONCLUSIONS ................................................................................. 51

6. FUTURE WORK ................................................................................ 55

7. REFERENCES .................................................................................... 57

1

1. INTRODUCTION

In today’s steel production it is vital to manufacture clean steel in order to

produce high performance products without any castings defect [1-3]. In

the majority of the cases, it is beneficial to decrease the amount of

inclusions to produce clean steel. One reason for this is that if the

separation of non metallic inclusions to the slag is deficient, inclusions can

agglomerate and clog the Submerged Entry Nozzle (SEN) [4]. Thus,

clogging is the single biggest operational problem within continuous

casting of steel. The consequence is a shorter sequence length, which

results in a lower yield and a higher production costs. In addition, cancelled

castings results in productivity losses, large revenue losses and reduced

qualities that can lead to complaints from customers as well as image losses

[5-7].

During continuous casting the molten steel is transported from the tundish

through the SEN to the mould. The SEN provides optimal flow conditions

and protects the molten steel from oxygen and nitrogen pick up [8-9].

During casting of sensitive steel grades, non-metallic inclusions in the

molten steel can accumulate in the SEN at the wall and disturb or

completely prevent the steel flow [10-12]. Note, that it is difficult or

impossible to avoid the non-metallic inclusions in the liquid steel.

However, with a good material in the SEN clogging can be reduced.

The SENs consist of different Refractory Base Material (RBM): Al2O3-C,

MgO-C and ZrO2-C, (see Figure 1). Since the steelmaking process takes

place at temperatures around 1565°C, interaction between molten steel and

the RBM is unavoidable [8]. In addition, the SEN is preheated before

casting, both to prevent thermal shook and to prevent that the molten steel

freezes in the SEN. Since the RBM contains graphite there is a high risk

for the SEN to be decarburized due to an increased oxygen activity near

the inner wall [9, 13-15]. It is of high importance to avoid decarburization

of the SEN, since this will increase the clogging rate [7]. Clogging occurs

due to different factors such as a poor steel cleanliness, low steel

temperature, nozzle shape, steel flow speed, purity and stability of the

RBM [7, 16]. Gaseous transfer through the RBM will be greater in the

decarburized surface. The decarburization will result in a formation of CO

gas, and by an oxidation of Al this will lead to the formation of Al2O3

2

inclusions [6, 9, 11, 12]. Once the first deposited layer of non-metallic

inclusions has been formed, the SEN wall’s surface becomes rough and

therefore further clusters can easily be buildup [6, 7].

Figure 1. Longitudinal section showing the commercial SEN, where the inlet

consists of MgO-C, the bulk consists of Al2O3-C, and where the outlet of the SEN

slag line is reinforced with ZrO2-C.

Graphite gives the RBM good properties such as a high thermal shock

resistance and a high non-wettability by slag and molten steel [8, 17].

Different steel grades react differently with the RBM. During casting of

low-carbon Al killed steel grades accretions of Al clusters attach at the

SEN internal wall [7, 11]. By implementing a coating containing CaTiO3,

reactions with the Al2O3 inclusions in the molten steel can reduce the

clogging by a formation of liquid calcium aluminates [11, 16]. In the phase

diagram of the CaTiO3 and Al2O3 system it can be seen that the eutectic

reaction is possible at T<1600°C, see Figure 2 [18].

3

Figure 2. The CaTiO3-Al2O3 phase diagram and the eutectic point is at a 41 wt-%

Al2O3 content and a 1580°C temperature [18].

However, clogging of the SEN occurs during casting of Ce alloyed

stainless steel grades when the Rare Earth Metal (REM) are oxidized by an

increased oxygen activity and by elements in the RBM [9, 13-15,19].

Besides from gaseous transfer through the RBM, decarburisation of the

RBM will result in an increased oxygen activity [6, 9, 11, 12]. In addition,

the oxidation of REM results in a transformation layer consisting of Ce2O3

and La2O3 onto the SEN wall where molten steel containing REM

accumulates [19]. Also, it has been found that the rough and dense surface

will influence the turbulent flow of the steel in the nozzle during casting

[8, 12, 20].

To obtain an optimal prevention of clogging the plasma coating material

should be combined with a high steel cleanliness. Since the reaction

mechanisms at the steel/RBM interface will lead to the formation of more

Al2O3 inclusions in the molten steel [7]. Also, if the harmful Al2O3

inclusions have been separated from the molten steel the formation of

Ce2O3 inclusions can be decreased [21, 22]. The last process step for the

purification of the steel during casting is the tundish. Therefore, is of high

importance to understand which reactions that take place in the tundish in

order to produce high quality steel products.

4

1.3. PRESENT WORK

The experimental background in this work has been carried out based on

laboratory trials, pilot plant trials and industrial plant trials. In additions,

observations and evaluations of the chemical composition were performed

using SEM. This thesis consists of 5 supplements and the focus is on the

following issues:

• An evaluation and mapping of industrial preheating trials at different

steel plants for different preheating conditions and different parameters

i.e. placement of torches, temperature distribution in the SEN,

chemical gas composition during preheating (supplement 1).

• An evaluation of the interaction between calcium titanate and alumina

during tempering at 1550-1600°C (supplement 2).

• An evaluation of the use of plasma sprayed YSZ and CaTiO3 coatings

as possible materials to reduce clogging during casting of aluminium-

killed low-carbon steels (supplement 2).

• A post-mortem study of the interaction between plasma sprayed

coatings, containing a mixture of YSZ and calcium titanate, and molten

aluminium-killed low-carbon steel by using FEG-SEM (supplement

2).

• An evaluation of the plasma sprayed YSZ coating as a possible

material in order to reduce clogging during casting of Ce alloyed

stainless steels (supplement 3).

• A post-mortem study of interaction between plasma sprayed YSZ

coatings and molten Ce alloyed stainless steel by using FEG-SEM

(supplement 3).

• Industrial trials using YSZ plasma sprayed SENs and stopper rods in

order to reduce the clogging tendency (supplement 3).

• Electrolytic extraction to further evaluate the YSZ coating material

(supplement 4).

• Sampling in the tundish during one casting sequence to evaluate the

interaction between the steel/slag and steel/refractory interfaces

(supplement 5).

• Mapping of the inclusions present in the tundish and their origin

(supplement 5).

5

1.4. OBJECTIVES OF THE WORK

The supplements in this thesis focus on the continuous casting process and

includes the SEN’s behaviour, the clogging phenomena the steel

cleanliness in the tundish. The objective of this work has been to provide

information about these process parameters. By doing so the overall intent

was to reduce clogging in the SEN, and thusly enable longer casting

sequences. In Figure 3 a schematic overview of how the supplements are

connected is presented.

A possible prevention of decarburization in supplement 1 was further

investigated in supplements 2-4 by implementations of different plasma

coating materials. In order to decrease the clogging tendency it is also

important to produce clean steel. Therefore the tundish was mapped in

supplement 5. Methods implemented in this study are laboratory

experiments, pilot plant trials, industrial steel plant trials and post-mortem

evaluations. These different methods complement each other. In Table 1

an overview of the objectives, approaches and parameters for the

supplements is presented.

6

Steel grades sensitive to clogging during continuous casting

↓

Methods to reduce the clogging tendency:

↓ ↓ ↓ Interaction between the RBM & preheating gas resulting in decarburization. Supplement 1

Protecting the RBM from reactions by implementing plasma coating materials. Supplement 2 - 4

Produce clean steel with good separation of non-metallic inclusions to the covering slag in the tundish. Supplement 5

↓ ↓ ↓

Decarburized RBM will increase the oxygen potential at the SEN wall and increase the clogging tendency.

→

The plasma coating material can provide a smooth surface where non-metallic inclusions will not easily adhere. Also, the coating material can protect the RBM during preheating.

←

Non-metallic inclusions that are not separated to the tundish cover slag can accumulate at the SEN inlet and cause clogging of the SEN.

Figure 3. Presentation of how the 5 supplement are connected to each other.

7

Table 1. Overview of the 5 supplements

Study: Objective: Approach: Parameters:

1

Preheating process of SEN in the industry

Method to analyse the preheating process in industry

Analysis of the preheating gas during industrial trials and the RBM

Data from industrial trials and SEM data.

2

Implementation of CaTiO3 plasma sprayed coating in the nozzle inlet during pilot plant casting trials

Method to reduce the clogging tendency with the CaTiO3 coating material

Laboratory analysis of the CaTiO3 as a coating material & pilot plant trials

Data from the laboratory experiments and pilot plant trials. SEM data of etched samples from the steel/nozzles interface after pilot plant trials.

3

Implementation of YSZ plasma sprayed coating in the nozzle inlet during pilot plant casting trials & during industrial trials

Method to reduce the clogging tendency with the YSZ coating material

the YSZ coating material in pilot plant & industrial trials

Data from the pilot plant and industrial trials. SEM data of etched samples from the steel/nozzles interface after pilot plant trials.

4

In depth study of the YSZ plasma coating material

Method to analysis & evaluate the YSZ coating material after casting

FEG-SEM analysis of electrolytic extracted samples containing the steel/YSZ-interface

SEM data of the samples after electrolytic extraction.

5

In depth study of inclusions in the tundish during continuous casting

Sampling in the tundish with MISS sampler

FEG-SEM & INCAFeature analysis of samples from VTD, tundish and slab

MISS samples from the tundish, inclusions data from INCA Feature, data & analysis from FEG-SEM

9

2. EXPERIMENTAL METHODS

Different experimental methods have been used in the thesis. In

supplement 1, preheating trials at industrial steel plants as well as FEG-

SEM observations were conducted in order to evaluate the degree of

decarburization of the SEN. Also, plasma coating materials for the SEN

have been evaluated by laboratory experiments (supplement 2 and 4), pilot

plants trials (supplement 2 and 3) and industrial trials (supplement 3). The

results have been obtained by FEG-SEM observations to evaluate the

influence of a plasma coating material to reduce the clogging tendency. In

addition, the inclusions in the tundish have been studied by collecting steel

samples before, during and after casting (supplement 5). The experimental

methods are partly described in the following sections; detailed

information is given in the supplements.

2.1. PREHEATING TRIALS

Preheating trials were performed at three steel plants (referred to as SP1,

SP2 and SP3), in supplement 1. The trials were performed according to the

respective steel plant standard preheating process conditions using propane

torches and the standard equipment. The SENs consisted of the RBM

Al2O3-C, MgO-C and ZrO2-C in different parts of the SEN (see Figure 1).

The chemical compositions of the SEN’s RBM and of the protective

glass/silicon powder are presented in Table 2 [12] and Table 3 [12],

respectively.

During the preheating process the temperature distribution inside the SENs

was measured as well as the flue gas content (the CO, CO2 and O2 contents)

in contact with the RBM, at the internal RBM surface. In total, 6

thermocouples were placed in channels in the RBM. In Figure 4,

schematic setup for SP1 is presented, including the placement of

thermocouples and the gas intake. The intake for the gas analyses was

placed at the same position at all steel plants.

The experiments at SP1 and SP2 were performed by Memarpour et al. [12],

but the results are included in order to compare the data with

10

the current results. The preheating trials at SP3 were performed to obtain

additional information as well as to compare our results with the previous

preheating results [12].

Table 2. The chemical

composition of the glass/silicon

powder coating [12]

Table 3. The SEN’s chemical

composition [12]

Chemical

composition Weight percent

Al2O3 20.07

SiO2 55.25

CaO 0.99

Na2O 19.04

K2O 4.64

Chemical

composition Weight percent

Al2O3 55.1

SiO2 10.2

TiO2 0.8

CaO 0.2

MgO 0.3

Na2O 0.9

K2O 0.1

ZrO2 + HfO2 0.6

Fe2O3 0.2

C 31.6

At steel plant SP2, the tundish was equipped with 3 SENs. Therefore, two

preheating trials were performed; one trial was performed for each of the

SEN on the outer strand (SEN B1 and SEN B3). Also, at SP3 two

preheating trials were performed. During the first trial, interference from

the steel plant disturbed the temperature measurements. Since the gas

analysing equipment did not work during the second trial at company SP3

the flue gas analysis was taken from trial 1 and the temperature

measurement was taken from trial 2.

11

Figure 4. Preheating set up with oxy-propane torches placed in the tundish lid and

at the SEN outlet. In total, 6 channels were drilled for inserting thermocouples into

the SEN. Channels 1 to 5 were placed at a 80 mm distance apart. Thermocouples

of type S were used in channels 1 to 5. In channel 6, thermocouples of type K were

used.

2.1.1 STEEL PLANT DESCRIPTION

The setup for the preheating trials at the 3 steel plants consisted of:

• SP1: one tundish with a lid and one SEN placed within the tundish.

Also, propane torches were placed both at the SEN’s outlet and in the

lid.

• SP2: one tundish with a lid and 3 SENs placed within the tundish. Also,

propane torches were placed both at the SEN’s outlet and in the tundish

lid. Measurements were conducted during two trials. In the first trial the

SEN was placed at strand 1 (SEN B1) and in the second trial the SEN

was placed at strand 3 (SEN B3).

• SP3: one tundish with one SEN placed within the tundish. Also,

propane torches were placed at the SEN’s outlet.

12

2.1.2. POST-MORTEM STUDIES OF SEN

The chemically composition and microscopic evaluation of samples were

analyzed using an Ultra 55 field emission gun scanning electron

microscope (FEG-SEM; Carl Zeiss, Jena, Germany, equipped with an Inca

Penta FETX3, Oxford Instrument, Abingdon, UK) equipped with an

energy dispersive X-ray spectrometry (EDS). Samples were collected from

the same level as where the thermocouples were placed during the

preheating operation.

2.2. TEMPERING OF CaTiO3-POWDER

During the laboratory experiments in supplement 2, ~13 g CaTiO3 powder

(99 wt-% pure, -325 mesh) were tempered in alumina crucibles (h: 40 mm,

Ø: 30 mm, thickness: 3 mm). The alumina crucible was placed into a

graphite crucible and heated at a rate of 10°C/minute in a graphite furnace

with a protective argon atmosphere. The argon flow rate was

approximately 4 l/min and the furnace temperature accuracy was +/- 3°C.

In Figure 5, a schematic illustration of the furnace is presented.

In total, 5 different samples were prepared. Crucible C1 had a 12 min long

dwell time at 1600°C. Crucibles C2 to C5 had a 60 min long dwell time at

the temperatures 1600°C, 1575°C, 1565°C and 1550°C, respectively. In

addition, the samples were cooled down over a 2-3 hour period.

13

Figure 5. Schematic presentation of the high-temperature graphite furnace with

CaTiO3 powder in an Al2O3 crucible and protected with argon atmosphere.

2.3. PILOT PLANT TRIALS

2.3.1 THE NOZZLES

During the pilot plant trials, 4 ZrO2 RBM nozzles were used to simulate

the continuous casting process. The dimensions of the nozzles are

presented in Figure 6. The nozzles that were teemed as reference nozzles

did not have any coating. In supplement 2 the CaTiO3 powder (5 or 10 g)

was mixed with 100 g YSZ powder and in supplement 3 an YSZ powder

was used. The powder was fed into a plasma gas stream and deposited on

the nozzle wall, as illustrated in Figure 7 [23]. The nozzles were plasma

sprayed in an air atmosphere after being preheated up to 300°C. The

coating thickness was 200-400 µm and 25-35 g of the powder mixture was

consumed for each nozzle.

14

Figure 6. Longitudinal section showing the nozzle’s geometric and dimension.

About 25 mm of the nozzle’s internal wall could be plasma sprayed with a 200 to

400 µm thick coating.

Figure 7. Setup of the plasma spraying equipment utilized for coating of the

nozzles [23].

15

2.3.2. SET-UP OF THE PILOT PLANT

The aims of the experiments in supplements 2 and 3 were to evaluate if the

plasma coating materials could reduce the clogging tendency. The steel

was teemed into a mould placed on a scale, which weight was logged

continuously. The teeming of molten steel through the nozzles simulated

the gap in the industrial scale with an axisymmetric outlet. In Figure 8,

the set up for the pilot plant trials is presented [24].

Figure 8. The pilot plant experimental set-up of the induction furnace teeming

molten steel through the nozzles into a mould placed on a scale [24]. The teemed

steel mass was continuously measured and logged. The temperature was measured

and controlled with thermocouples.

During the experiments, the temperature in the nozzles and the steel melt

were continuously monitored with 4 Rh-Pt-thermocouples. In order to

prevent clogging due to freezing, the temperatures in the nozzles were kept

higher than in the steel melt in the furnace during the teeming operation.

Thus, if clogging did occur the clogging zone should consist of non-

metallic inclusions and not be due to steel freezing on the nozzle wall. In

addition, dual thickness samples were collected, both before and during the

casting from the furnace and during each trial. These were used to

determine the chemical composition of the steel and inclusions by using

16

optical emission spectroscopy (OES), X-ray fluorescence spectrometry

(XRF), melting extraction technique (EXTR), high frequency melting

infrared detection (HFIR) and pulse determination analysis (PDA).

2.3.3 PILOT PLANT DESCRIPTION

The pilot plant trials were performed with a 600 Hz induction furnace with

a 600 kg nominal melts size and an Al2O3-lining. The melt was protected

with liquid argon. During the trials with the CaTiO3 plasma coating, steel

scrap was melted and pieces of FeSi and Al were added to deoxidise the

molten steel (supplement 2). During the trials with the YSZ plasma

coating, REM was added instead of Al (supplement 3).

2.3.4. STEEL FLOW RATES DURING TEEMING

The clogging tendency was evaluated during the casting by measuring the

teemed steel mass. Due to the clogging phenomenon, the nozzle area will

decrease during the teeming and hence the steel flow through the nozzle

will decrease. The deviation of the measured steel flow rates were

compared with the theoretical flow rates calculated by using Bernoulli’s

equation, see Supplements 2 and 3. Bernoulli’s equation can in a general

form be expressed as:

𝐶 = 𝑃 + 𝜌𝑔𝑧 + 𝜌𝑣2

2 (1)

where C is a constant, P is the pressure, ρ is the molten steel density, g is

the acceleration due to gravity, z is the vertical distance and v is the molten

steel velocity.

Then the liquid velocity at the nozzle outlet was calculated by applying

equation (1) onto the pilot plant system by considering two points in the

flowing steel melt. One point was at the free surface of the steel melt in the

induction furnace (point 0) and the other at the nozzle outlet (point 1).

Thereby, the following relation can be obtained:

𝜌𝑣02

2+ 𝜌𝑔𝑧 + 𝑃0 = 𝜌

𝑣12

2+ 𝑃1 (2)

where P0 is the pressure in the liquid steel at the free surface in the

induction furnace, P1 is the pressure in the liquid steel at the nozzle outlet,

v0 is the molten steel velocity at the free surface of the steel melt in the

17

induction furnace, v1 is the molten steel velocity at the nozzle outlet and

this is valid for laminar steel flow.

In comparison to the free surface of the steel melt in the induction furnace

the nozzle area is insignificant. Therefore, the velocity of the free surface

of the steel melt in induction furnace (v0) can be neglected in comparison

with the outflow velocity (v1) and it is assumed that v0 = 0. Also, P is the

atmospheric pressure and it is as assumed that P=P0 = P1. The vertical

distance is z=h+d, where h is the vertical distance between the nozzle and

the liquid bath’s upper surface and d is the nozzle height. By inserting these

data into equation (2) the outflow velocity from the nozzle ca be calculated

as follows:

𝑣1 = [2𝑔(ℎ + 𝑑)]1 2⁄ (3)

The principle of continuity applied to incompressible liquid states that no

fluid appears or disappears during the flow. Therefore, equation (4) is valid

at every height position for the change of the theoretical mass teeming rate:

𝑄ℎ = 𝜌𝐴𝑣 = 𝜌𝐴𝑁[2𝑔(ℎ + 𝑑)]1 2⁄ (4)

where Qh is the theoretical mass teeming rate and AN is the nozzle outlet

cross-sectional area.

In order to extract interpretable data the registered signal from teemed steel

mass was noise reduced as follows:

𝐿𝑡 =1

121∑ 𝐿𝑡−60+𝑖120𝑖=0 (5)

where Lt is the steel mass logged on the scale (given in kg) at time t (given

in s). In addition, the measured mean mass teeming rate Qt (given in kg s-

1) for every minute:

𝑄𝑡 = 𝐿𝑡 (6)

The mass teeming rate from equation (6) was compared to the theoretical

mass teeming rate from equation (4). If the nozzle will start to clog a

deviation between data from equation (6) and (4) will appear. From a

hydrodynamic point of view, the teeming rate will in an ideal manner not

18

deviate. During the pilot plant experiments, the theoretical and

experimental teeming rates were compared. The amount of clogging was

compared by using the clogging factor ƞ suggested by Kojola et al [25]:

𝜂 =(𝑄ℎ−𝑄𝑡)

𝑄ℎ (7)

The clogging factor is proportional to the clogging area reduction fraction

and 0 ≤ η ≤ 1, where 0 represents no clogging and 1 represents a completely

clogged nozzle.

2.3.5. POST-MORTEM STUDIES OF THE NOZZLES

The nozzles were first cut into two pieces, in the vertical direction. The

nozzles were then further cut into three pieces (bottom, middle and top).

The solidified steel was baked in Bakelite, grinded, polished and etched in

acid (supplement 2: natal - mix of alcohol and nitric acid; supplement 3:

aqua regis – HNO3 and HCl).

The main focus in the microscopic studies was on the solidified steel at the

nozzle inlet. This since clogging of the upper part of the nozzle is flow

limiting and due to that accretions inside SEN do not affects the flowrate

until the inner diameter becomes flow limiting [24]. Therefore, accretions

at the nozzle inlet are of interest. In addition, the nozzle geometry made it

possible to only plasma spray the inlet part of the nozzles. The reactions

between non-metallic inclusions and the coating material were also of

interest to study. Thus, the focus was also on remaining parts of the coating

in the samples, after an experiment.

The chemical composition and microscopic evaluation of samples were

analyzed using an Ultra 55 field emission gun scanning electron

microscope (FEG-SEM; Carl Zeiss, Jena, Germany, equipped with an Inca

Penta FETX3, Oxford Instrument, Abingdon, UK) equipped with an

energy dispersive X-ray spectrometry (EDS).

2.4. INDUSTRIAL IMPLEMENTATION OF THE YSZ COATING MATERIAL

The existing glass/silicon powder coating in the SEN was first removed

before the SENs were plasma coated. This was done at a distance 6 to 7

19

cm of the inlet as well as the tip of the stopper rod. During the plasma

coating process the substrate was locally heated for a short time.

Thereafter, it was sprayed in a normal room atmosphere at a rate of 300

rpm. The coating thickness was approximately 210 µm. In Figure 9, the

SEN and stopper rod is presented before and after a coating process.

Figure 9. The SEN and stopper rod from the industrial plant trial: on the left – the

uncoated SEN and stopper rod are marked with yellow where the powder coating

was removed before coated with YSZ; on the right – the SEN and stopper rod after

a completed YSZ coating.

In order to evaluate the clogging tendency, the stopper rod position was

measured during the castings. The system of the steel flowrate between the

stopper rod and the seat is a steady state system where the volume flowrate

is controlled by the tundish bath height and the SEN’s cross sectional area

[25]. During clogging in the seat, the stopper rod will be moved upwards.

In addition, the steel composition was analysed from lollipop samples

collected from the tundish.

2.4 STEEL PLANT DESCRIPTION

The industrial plant trials were performed at Outokumpu Stainless AB in

Avesta, Sweden. The steel company produces a wide selection of

austenitic, duplex, ferritic and martensitic stainless steels with special

requirements as well as with a high focus on performance. In the steel

production, steel scrap is the main raw material. In the current work, a

REM alloyed austenite stainless steel grade 253MA (21Cr-11Ni-1.7Si-

0.09C-0.17N-0.05Ce, wt-%) was studied.

20

• Three trials were performed with YSZ plasma coated stopper rods

and SENs.

• The first trial is referred to as S1, the second as S2, and the third

as S3.

• The data from the industrial trials were compared to data from a

reference casting (SR) of the same steel grade without using a

plasma coated SEN and stopper rod.

• The data from the castings were modified to have the same starting

position for the stopper rods.

2.5. ELECTROLYTIC EXTRACTIONS

The 210 µm thick YSZ plasma coating was implemented in one industrial

trial during casting of a 253 MA stainless steel grade (21Cr-11Ni-1.7Si-

0.09C-0.17N-0.05Ce, wt-%). The SEN and stopper rod were plasma

sprayed in air atmosphere at room temperature, at a rate of 300 rpm, and

locally heated during the plasma spraying.

After the casting trial, two samples from the upper part of the SEN’s seat

were analyzed by using the electrolytic extraction method. The samples

were collected from two areas in the seat and the samples contained the

solidified steel from the steel/RBM interface. Sample A - was taken from

the first 40 mm vertical section of the seat and from the second area.

Sample B - was taken from the following 40 mm vertical section of the

seat. The samples dimensions were approximately 15x10x5 mm.

The electrolyte used for preparing the samples was a 2% TEA (2 v/v%

triethanol amine – 1 w/v% tetramethylammonium chloride – methanol)

solution. The current density was set to 50mA/cm2 and 300 coulombs

during the extraction. The dissolved weight from the samples during the

electrolytic extraction was 0.0598 g and 0.0672 g for sample A and B,

respectively. In Figure 10, the experimental setup is shown.

The composition of the samples was determined by using an Ultra 55 field

emission gun scanning electron microscope (FEG-SEM; Carl Zeiss, Jena,

Germany, equipped with an Inca Penta FETX3, Oxford Instrument,

Abingdon, UK) equipped with an energy dispersive X-ray spectrometry

(EDS).

21

Figure 10. The laboratory set-up of the equipment for electrolytic extraction of

steel samples using a 2% TEA solution.

2.6. MAPPING OF THE TUNDISH

The Momentary Interfacial Solidification Sampling (MISS) [26-28]

sampler was implemented in the tundish to evaluate interactions between

slag and molten steel as well as the tundish refractory lining and molten

steel. The MISS-sampler was made of 12 mm thick steel plates, which were

welded together as a mould (120×100mm) with an 80×8 mm column. In

Figure 11, the collected sample plate and MISS sampler is presented. From

the sample plate, one MISS sample was cut out from the middle column at

the top to be able to analyze the steel/slag interface.

22

(a) (b)

Figure 11. The MISS sampler and MISS sample after removal from the sampler

with markings of where the analyzed sample was cut out.

The MISS-sampler was insulated with super wool in order to not heat up

the sample. Also, the inside of the sample was etched to achieve good

wetting conditions. In total, 6 MISS-samples were collected from the

slag/steel interface in the surface region in the tundish during one heat. The

samples were collected from the tundish in the beginning of teeming

(samples MA1 and MA2), after teeming of half the ladle (samples MB1

and MB2), and at the end of teeming before changing to the next ladle

(samples MC1 and MC2). The MISS-sampler was lowered into the tundish

and held in the bath for 5 seconds. Two parallel sampling positions were

chosen, which easily could be accessed in the tundish, to compare

interactions at different locations in the tundish. Sampling position 1 was

chosen close to the wall (~110-150 mm from the tundish wall) to study if

particles from the refractory lining could be found. Sampling position 2

was chosen far away from the refractory wall near the center to study

mainly the interactions between the tundish powder and the steel.

Three heats were performed during teeming of the first casting sequence.

Before the heat, one lollipop sample (12 mm thickness) was collected at

the Vacuum Tank Degassing (VTD) station after a finished treatment

before casting. Also, one 30*20 mm sample was cut out from the center of

the slab at one quarter depth from the surface

40 mm

23

2.6.1. STEEL PLANT DESCRIPTION

The heats were performed at SSAB Special Steels in Oxelösund, Sweden.

In all heats a structural steel (0.165C-0.055Al-1.25Mn-0.22Si-0.6Mo-

0.2Cr, wt-%) was studied. The tundish capacity was 30 tonnes and slabs

were casted with the dimension of 220x1680 mm.

2.6.2. ANALYSIS OF THE INCLUSIONS

The samples were grinded, polished and then analyzed by using an Field

Emission Gun Scanning Electron Microscope (FEG-SEM), (Zeiss Merlin

equipped with Oxford Instruments INCA Feature for Windows 7)

combined with an Energy Dispersive Spectrometer (EDS), X-MAX 50

mm². Thus, determine the non-metallic inclusions composition,

morphology and Equivalent Circle Diameter (ECD). The INCA Feature

study was set to detect inclusions with an ECD value lager than 5.7 µm.

Thereafter, each inclusion was manually studied to eliminate errors, i.e.

pores or dust. In addition, the steel composition of all samples was analysed

by using optical emission spectrometry (OES).

The obtained data from the INCA Feature studies were normalized with

respect to Al2O3, CaO and MgO oxides. This was done since these

elements were the dominating elements observed in the inclusions

chemical compositions. The amount and size of the inclusions observed

differed to a great extent between the heats as well as the sampling

positions. Also, the inclusions were divided into two groups of non-

metallic inclusions; DM inclusions (ECD ≥5.7 µm and <11.3 µm) and DL

inclusions (ECD ≥11.3 µm).

25

3. RESULTS AND DISCUSSION

3.1. DECARBURIZATION

The preheating processes of the SEN at different steel plants (SP1, SP2 and

SP3) have been studied. Samples were collected from the SENs after

preheating and thereafter they were evaluated by using FEG-SEM. The

most important parameters were the temperature and flue gas

measurements during the preheating trials. All three steel plants had

different preheating setups, which provided information about the impact

of the different setups. The main difference was that steel plants SP1 and

SP2 used a tundish lid equipped with torches. At steel plant SP2, three

strands were used in the tundish whereas at the other steel plants the

tundish only was equipped with one SEN.

In total, 6 holes were drilled for thermocouples in each evaluated SEN,

respectively. The measured temperature profile inside the SEN showed an

uneven temperature distribution at all steel plants. In all SENs, the

temperature distribution varied by up to 560°C. At steel plant SP3, where

no lid was used during the preheating, a clear temperature difference of

~550°C was detected between the seat (channel 6) compared to the rest of

the SEN. The use of torches in the tundish lid helped to maintain a more

even temperature distribution inside the SEN.

Some assumptions had to be made during the measurement from steel plant

SP3 due to interference from the surroundings in the steel plant that

disturbed the measurement equipment. Thus, two trials were performed. In

the first trial, the temperature could not be measured. Also, in the second

trial the flue gas could not be measured. During the first trial the overall

measured temperature profile increased steadily in 4 out of 6

thermocouples. However, only the temperature measured in channel 6

varied in a similar way as during the second trial. The temperature

measurement varied severely during the measurements and some of the

temperatures were too low in comparison to the other corresponding

temperatures. In some channels, negative temperatures were also

measured. The temperature in channel 6 was about 1080°C during the first

trial and 920°C in the second trial, at the same time. Also, the heating rates

were about 4.0°C/min and 4.5°C/min during the first and second trials,

respectively.

26

A thermocouple of type K was used in channel 6 instead of type S

thermocouples, which were used in all the other channels in the SEN. Thus,

the two trials were assumed not to differ significantly with respect to the

temperatures and flue gas compositions. Since the preheating was

performed with the same setup, calculations of the Gibbs free energy were

done by using values from both trials at steel plant SP3. Hence, the results

from SP3 were compared with the results from steel plants SP1 and SP2.

The SENs consisted of three RBM zones, made of Al2O3-C, ZrO2-C and

MgO-C. Moreover, the SENs came from different manufacturers.

However, all SENs consisted of the same RBM type. Recommendations

for preheating the SEN, in order to decrease decarburisation, are to heat the

SEN in a fast manner without exceeding 800°C [9, 29]. This is due to that

when the SEN has been heated to 550°C decarburisation is possible based

on thermodynamics [9]. Also, in order for the glass/silicon powder coating

to form a dense and protective glaze layer, the SEN has to be heated to a

temperature above 1100°C [12]. Thus, after heating the SEN to 550°C it is

vital to use a high heating rate until a glaze is formed [9]. At all steel plants,

the temperature distribution in the SEN as well as the heating rate in the

different channels varied to a great extent. Higher temperatures were

achieved faster in the lower part of the SENs in comparison to the upper

part. In addition, a temperature of 550°C was reached after 5 to 15 min into

the preheating operation, at steel plants SP1 and SP2. At steel plant SP3,

the heating was so rapid that the decarburization only was possible during

about 0.5 min in channels 1 to 5. In channel 6, the decarburization was

possible after 1 min and during the entire preheating process. In Figure 12,

measurements of the temperature profile from trial 2 at steel plant SP3 is

presented. It can also be seen in Figure 12 that the heating rate at steel

plant SP3 was excessive, which can be explained by that the torches were

only heating from the SEN’s outlet. Implementation of a tundish lid with

torches could lower the contribution from the torches placed in the SEN

and generate a controlled preheating process. It is of most importance to

have a controlled preheating process.

27

Figure 12. Presentation of the temperature profile inside the SEN over time during

the preheating process, performed at steel plant SP3. The temperature in channel

1 was measured at the bottom of the SEN at a distance of 8 cm apart from the other

channels 2 to 5. The temperature in channel 6 was measured at the inlet of the

SEN.

At all steel plants, at least one channel inside the SEN did not reach a

temperature of 1100°C. Thus, the protective glaze could not be formed

inside the whole SEN. In one channel at steel plant SP2 the temperature

reached a value of 1100°C. Due to the long preheating time it was possible

for a semi-dense and protective layer to form when this temperature was

reached. However, since decarburization is possible already at lower

temperatures the SEN had already started to decarburize before the

protective layer was formed. From the FEG-SEM analyses a glaze was

found and it had penetrated into the RBM. Analyses of samples from SP1

also revealed that a penetration of the glaze into the RBM had taken place.

At steel plant SP3, the temperature in channel 6 reached a value of 1080°C

and a glaze was observed after the preheating. Thus, the glaze forms at a

lower temperature when using a long dwell time. However, the FEG-SEM

analyses of samples from SP3 showed no penetration of the glaze. The

SEN is bound to reach temperatures where decarburisation is possible

during the preheating operation. In order to prevent decarburization of the

28

SEN’s internal surface an even heating rate inside the SEN is of

importance. Then, the glass/silicon powder will have time to form the

glaze. The mapping of the preheating process has been the basis for the

development of new coating materials to protect the SEN from

decarburization.

3.2. POSSIBILITIES TO USE CALCIUMTITANATE AS A COATING MATERIAL FOR SENS

3.2.1. FORMATION OF LIQUID CALCIUMALUMINATES

The possibility of a formation of a liquid phase was evaluated by using

laboratory experiments with heating of CaTiO3 powders in Al2O3

crucibles. In total, 5 crucibles (named C1 to C5) were heated. The

laboratory experiments were performed at the temperatures 1600, 1575,

1565 and 1550°C, respectively. The dwell times were 12 minutes for

crucible C1 and 60 minutes for crucibles C2 to C5.

The reaction between CaTiO3 and Al2O3 was visibly observed by the

change in colour from white to blue and grey of the reaction products. This

was confirmed by FEG-SEM observations. Samples which had changed in

colour were cut out and gold coated. In total, the chemical composition in

59 points were studied by using FEG-SEM. Out of these, 19 points were

found to contain solid phases and 40 points were found to contain liquid

phases. In Figure 13, the crucibles C1 to C5 are presented after heating in

the furnace.

From the FEG-SEM studies it was found that the blue and grey coloured

areas contained Ca, Ti and Al. The darker colour the more Ca and/or Ti the

sample contained. Also, with higher heating temperatures and longer

dwelling times, the more intense was the change in colour. Thus, the FEG-

SEM studies indicated stronger reactions between CaTiO3 and Al2O3 as

well as a high tendency for liquid phase formations at higher temperatures.

The FEG-SEM analyses showed that the chemical composition correlated

with the liquid phase in the phase diagram for crucibles C1 to C5. See

supplement 2 for further discussion of the results from heating of CaTiO3

powders in Al2O3 crucibles.

29

Figure 13. Crucibles after heating of CaTiO3 powder in Al2O3 crucibles: (a) C1 –

12 minutes at 1600°C; (b) C2 – 60 minutes at 1600°C; (c) C3 – 60 minutes at

1575°C; (d) C4 – 60 minutes at 1565°C; (e) C5 – 60 minutes at 1550°C; (f) C2 –

A reaction between the powder and crucible was visible by the change in colour.

Also, CaTiO3 powder was found to be smeared onto the crucible wall. (A) A

lighter blue colour on the crucible’s surface. (B) Reaction area into the crucible’s

wall. (C) Powder attached to the crucible’s wall, which have changed colour from

white to grey. (D) The reaction surface close to the powder had a deep blue colour.

30

3.2.2. IMPLEMATION OF CaTiO3 COATINGS IN PILOT

PLANT TRIALS

An evaluation of CaTiO3 as a coating material was further done by

performing pilot plant trials. The results showed that the clogging tendency

is reduced when liquid accretions of Al2O3-cluster are washed off by the

steel flow. During an implementation of the plasma coating materials the

clogging tendency was reduced for both coatings containing 9.0% (N2 and

N3) and 4.8% CaTiO3 (N4). This was observed by the reduced steel flow

out from the nozzles that indicated that clogging had taken place. In Figure

14, the deviations of the measured steel masses from the casting from the

theoretical steel mass were compared. As can be seen, the casting curve for

the reference nozzle N1 is distinguished from all the other nozzles. After 2

minutes of teeming, the casting curve for nozzle N1 started to deviate from

the ideal teeming rate. The steel mass teemed through nozzle N1 deviated

32% from the ideal teeming rate after 2 min. The corresponding deviation

at the same time for nozzles N2, N3 and N4 was 7%, 4% and 8%,

respectively. Moreover, the teeming in N1 was terminated after 10 minutes

due to that the steel flow was reduced and due to that the molten steel

started to drip out of the nozzle. At this time the deviation of the teemed

steel mass for nozzles N1 to N4 from the ideal teeming rate was 70%, 25%,

30% and 28%, respectively. When each teeming operation was finished the

total deviation of the teemed steel mass from the ideal teeming rate for

nozzles N1 to N4 was 70%, 14%, 10% and 14%, respectively.

31

Figure 14. The actual teemed steel mass for all the 4 nozzles compared to the

theoretical teemed steel mass through the nozzles.

From the FEG-SEM studies, it was found that high concentrations of Al2O3

networks were observed close to the nozzle wall. Thus, accretions of Al-

clusters had not fully been prevented. After the teeming, the original

coating thickness of 200-400 µm had decreased to a value of about 50-70

µm. The chemical composition of the remaining coating consisted mostly

of ZrO2. Also, fractions originating from the coating were found in the

solidified steel in the middle part of the nozzle; where the nozzles had not

been coated.

3.3. YSZ AS A COATING MATERIAL

3.3.1. IMPLEMENTATION OF YSZ COATINGS IN PILOT

PLANT TRIALS

The YSZ coating material was studied in pilot plant trials to find if the

clogging tendency could be reduced during casting. The coating material

should prevent oxidation of REM in the molten steel, protect the RBM

against decarburization and provide a smooth surface in the SEN. From the

pilot plant trials, it was found that the clogging tendency was reduced when

32

using plasma YSZ coatings (N3 and N4) and when using a longer reaction

time after the REM additions before casting (N1). This was observed by a

reduced steel flow out from the nozzles, which indicated clogging. In

Figure 15, the deviations of the measured steel masses from the casting

from the theoretical steel mass were compared. During teeming trough

nozzle N1, there was no signs of that clogging of the nozzle had occurred.

Therefore, the casting was stopped after 11 minutes, and the teeming rate

deviated by 20% from the ideal teeming rate. The casting of nozzle N2 was

stopped after 4 minutes. At this time, the teeming rate deviated 44% from

the ideal teeming rate. Correspondingly, at the same time the other nozzles

(N1, N3 and N4) deviated by 3%, 15% and 26% from the ideal teeming

rate, respectively. After that each teeming operation was finished the total

deviation of the teemed steel mass from the ideal teeming rate for nozzles

N1 to N4 was 20%, 44%, 37% and 47%, respectively. However,

implementation of nozzle N3 and N4 also resulted in a long casting time

and thus it was possible to cast more steel. The best result was from the

reference casting with a longer separation time for nozzle N1. These results

show how important it is to produce clean steel for an efficient steel making

process.

The time for removal of inclusions from the steel after REM additions to

the melt and before the start of teeming were 10 minutes for nozzle N1 and

5 minutes for the other nozzles. Previous researchers have reported the

influence of a long time for removal of inclusions from the steel on the

clogging tendency to reduce clogging [25]. Thus, implementation of a

plasma coating is only one of several steps to reduce the clogging tendency

during continuous casting. Besides from the use of a plasma coating it is

essential to obtain a clean steel melt and it is important with a long enough

time for removal of non-metallic inclusions from the steel melt. In this

study, the goal was to study the clogging phenomena and clogging of

nozzles.

The FEG-SEM results of the nozzles showed the presence of accumulation

of Ce accretions on the nozzles’ walls. The original 200-400 µm thick YSZ

coatings were observed to have values between 70-80 µm thick after

casting. Thus, the coatings had been reduced during the casting.

33

Figure 15. Comparison between the theoretical and experimental steel mass

teemed through nozzles. Nozzle N3 and N4 were plasma coated with an YSZ

plasma coating. The reference nozzles N1 and N2 were cast without using any

coating materials.

3.3.2. IMPLEMENTATION OF YSZ IN INDUSTRIAL TRIALS

The movements of the YSZ plasma sprayed stopper rods and SEN was

monitored and compared to those of a reference casting (SR), see Figure

16. To keep the steel flow constant, the stopper rod was moved upwards to

compensate for when the SEN’s cross section was decreased due to

clogging. If the stopper rod instead was lowered, the cross sectional area

was increased. Thus, indicating that the RBM in the SEN or the stopper

rod had eroded [30].

Since the clogging tendency was reduced the casting rate was higher.

Therefore, the casting times were about 7%, 10% and 15% shorter when

implementing the YSZ plasma coating compared to when not using any

coating. However, for one heat there was a high risk for a breakout to

occur. Thus, at the end of the casting the operation had to be manually

controlled until the casting was finished.

34

The calculated Ce/Al ratio, from steel samples collected in the tundish, was

3.5, 7.6 and 5.0 for the heats with YSZ coatings. The variations in the

Ce/Al ratio correlate with the results in Figure 16; the Ce/Al ratios were

lowest for the heats where the clogging tendency was reduced to a greater

extent. Since a higher amount of Ce will increase the possibility for the

formation of more Ce clusters that can accumulate at the SEN wall [22].

Figure 16. Movement of the stopper rod position for the industrial trials (S1-S3)

compared to the reference trial SR. The values have been modified so that the

stopper rods have the same starting positions at zero, when being closed in the

beginning. When a stopper rod is moved upwards it is a sign of that the steel flow

into the SEN had to be increased and it can be interpreted as clogging. If a stopper

rod is moved downwards it is a sign of erosion. After approximately 53% of the

teeming operation the difference in movement between S1 and SR start to show.

Overall, the biggest difference was 13 mm in height.

3.3.3. MICROSCOPIC EVALUATIONS OF YSZ AS A

COATING MATERIAL

In the two samples that were analyzed after electrolytic extraction, the

coating could be observed in one sample, while only fragments of the

coating could be observed in the other sample. The samples were cut out

from the SEN inlet; sample A was taken from the first 40 mm of he vertical

35

section of the inlet and sample B was taken from the following 40 mm

vertical section of the inlet. In sample A, four layers were observed. They

consisted of YSZ coating materials, MgO, compact dendrites and smaller

accretions in the steel matrix. Also, the REM particles were found to have

agglomerated together and to have adhered to the SEN wall [22]. The

results also showed that the dendrites built up onto the SEN wall became

coarser into the center of the SEN. The measured thickness of the coating

varied between 30 to 100 µm. Thus, the coating thickness had been reduced

by up to 17% to 75%. The coating after casting as well as the reduced

coating thickness from sample A has been mapped in Figure 17.

The stopper rod movement controls the steel flow through the

hydrodynamically oversized SEN. Therefore, clogging of the upper part of

the SEN will become the flow limiting factor [24]. If the buildup onto the

SEN wall would have been due to freezing of the steel the microscopic

studies would have identified steel and not inclusions at the wall but this

was not the case. Specifically, the casting results showed that the clogging

tendency was reduced, there was buildup of REM oxides on the wall.

In the pilot plant trials, it was observed that the clogging tendency was

reduced for longer times after additions of REM alloys before the casting.

However, prolonging the time that the molten steel is kept in the tundish

before casting might not be a realistic idea. This is due to that the steel

plants i) often have several castings waiting in queue, ii) they want several

sequences to be teemed, and iii) they want a stable temperature in the

tundish. Also, from the industrial trials the Ce/Al ratio indicated the

importance of having clean steel in order to reduce the clogging tendency.

Therefore, in addition to implement a new plasma coating material it is

important to produce clean steel to reduce the overall clogging tendency

during casting.

36

Electron Image Zr

Mg O

Ce La

Figure 17. Mapping of the accretion and remaining part of the coating material

in sample A. The results from the mapping of element Zr shows how much of the

coating that remained after the casting. The thickness was measured to have values

of about 30 to 100 µm. In addition, a thin zone of up to about 20 µm of Mg was

observed between the coating material and the accretion.

37

3.4. REACTIONS IN THE TUNDISH DURING CASTING

It was difficult to observe a slightly convex slag surface on the MISS

samples, which previous researchers have reported [27]. A lollipop sample

collected from the VTD, a sample from the slab and the MISS samples

were compared with respect to the inclusion composition. It was found that

the amount and size of the inclusions found from the INCAFeature studies

varied amongst the samples. The inclusions chemical composition showed

that the amount of 3 oxide components were significantly higher compared

to the amounts of the other elements. Therefore, based on the chemical

composition of the analysed inclusions, the data was normalised with

respect to the oxides Al2O3, CaO and MgO.

The ternary phase diagrams of the system Al2O3-CaO-MgO with the

normalized inclusion compositions in different samples are presented, see

Figures 18-23. The data are plotted for each of the heats 1 to 3 with the

inclusion size groups DM and DL. The ternary phase diagrams are plotted

for samples collected from: (a) a lollipop sample from the VDT; (b) MISS

samples from position 1 (MA1 - red rings) and 2 (MA2 - blue triangles) in

the tundish at the beginning of casting (time A); (c) MISS samples from

position 1 (MB1 - red rings) and 2 (MB2 - blue triangles) in the tundish

after teeming of approximate half the ladle (time B); (d) MISS samples

from position 1 (MC1 - red rings) and 2 (MC2 - blue triangles) the tundish

at the end of teeming (time C); (e) a sample from the centre of the slab. For

each MISS sample the sampling time was as follows: (A) - in the beginning

of casting; (B) - in the middle of casting; (C) - at the end of casting. In the

phase diagrams the different sampling positions for the MISS samples are

1 (close to the tundish wall) and 2 (position in the middle of the tundish).

The inclusions in the ternary phase diagram originated from three main

groups: slag (I), deoxidation products (II), and refractory (III).

Inclusions in between these groups have been modified and inclusions

could also have reacted with the molten steel [31]. The origins of those

inclusions are difficult to determine and they were therefore classified as

the group: other (IV).

The inclusions chemical compositions of the inclusions from heats 1 and 3

are similar. In Figure 18-19 the DM and DL sized inclusions from heat 1

are presented, respectively. The observed inclusions in Figure 18

originated from the groups; I – most; II – some; III – low. The observed

38

inclusions in Figure 19 originated from the groups; I – most; II – low; III

– low. In Figure 22-23 the DM and DL sized inclusions from heat 3 are

presented, respectively. The observed inclusions in Figure 22 originated

from the groups; I – most; II – some; III – very low. The observed

inclusions in Figure 23 originated from the groups; I – most; II – low; III

– very low. In both heats 1 and 3, the inclusions mainly originate from slag

(group I). For the DM inclusions, 70 to 80 % of the inclusions in these

heats originate from group I in the slab sample. However, for the larger

inclusion size DL the origins of some inclusions are also from group III

and IV. The amount of DL inclusions from all heats, where low and it is

difficult to statistically ensure the origin of the DL inclusions. By

comparing the results in the ternary phase diagrams to the DM inclusions,

it can be seen that the chemical composition is similar for the same

sampling position. In Figure 20, it is also obvious that the DM sized

inclusions from heat 2 mainly originated from group II and I. The observed

inclusions in Figure 20 originated from the groups; I – most; II – some; III

– some. The larger sized inclusions, DL, as seen in Figure 21, mainly

originated from group I and II. The observed inclusions in Figure 21

originated from the groups; I – low; II – some; III – most. However,

significant amount of inclusions (up to 10~17% on average) originated

from refractory (group III). This result differed from the results from the

other heats, where much smaller number of inclusions originated from the

deoxidation products.

The steel grade in the heats was deoxidised with aluminium and thereafter

calcium treated. This will result in an increased amount of deoxidation

products (Al2O3) and slag (calcium aluminate) inclusions in the steel melt,

since inclusions originating from the slag have a similar composition [31].

Inclusions originating from the slag are found in the liquid region, where

the casting temperatures were between 1522-1541°C. They have low MgO

contents (<10 wt-%). The amount of inclusion originating from the slag

has been reported to increase due to an influence of previous castings, since

slag remains on the ladle walls [31].

Aluminium that reacts with O will form Al2O3 inclusions. These inclusions

originate from the deoxidation products and have high Al2O3 contents (60-

100 wt-%). Also, it is of highest importance to separate the Al2O3

inclusions from the steel to the tundish slag since they are harmful in the

final steel product [3]. These inclusions can also cause clogging of the

39

submerged entry nozzle. For this reason, a calcium treatment of the steel

in the ladle is important, since a reaction between CaO and Al2O3 will form

liquid calcium aluminate inclusions at steelmaking temperatures. Thus, the

risk for clogging of the SEN will be decreased when they have a smaller

tendency to form clusters that accumulate at the inner wall [4].

The inclusions in group III have high MgO contents (≥35 wt-%) and

originate from the tundish refractory lining material. This is due to that the

steel flow will cause a dispersion of both slag particles from the tundish

slag layer into the tundish as well as fragments from the refractory lining

into the steel [1, 27, 32, 33]. Inclusions originating from refractory material

were found in the VTD samples in all heats for the DM sized inclusion (see

Figures 18, 20, 22) and in heat 1 for the DL sized inclusions (see Figure

19). Moreover, the DM and DL inclusions from group III were observed

in some steel samples taken from tundish and slab in all heats.

40

(a)

(b)

(c)

(d)

(e)

Figure 18. The Al2O3-CaO-MgO ternary phase diagrams for DM sized inclusions,

from heat 1. Samples are collected from: (a) VTD; (b) MISS samples from position

1 and 2 at time A; (c) MISS samples from position 1 and 2 at time B; (d) MISS

samples from position 1 and 2 at time C; (e) slab.

1-L

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

1-MA11-MA2

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

1-MB11-MB2

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

1-MC11-MC2

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

1-S

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

41

(a)

(b)

(c)

(d)

(e)

Figure 19. The Al2O3-CaO-MgO ternary phase diagrams for DL sized inclusions,




1-L

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

1-MA1

1-MA2

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

1-MB1

1-MB2

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

1-MC1

1-MC2

1-S

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

42

(a)

(b)

(c)

(d)

(e)





2-L

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

2-MA12-MA2

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

2-MB12-MB2

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

2-MC12-MC2

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

2-S

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

43

(a)

(b)

(c)

(d)

(e)





2-L

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

2-MA1

2-MA2

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

2-MB1

2-MB2

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

2-MC1

2-MC2

2-S

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

44

(a)

(b)

(c)

(d)

(e)


from heat 3 Samples are collected from: (a) VTD; (b) MISS samples from position



3-L

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

3-MA13-MA2

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

3-MB13-MB2

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

3-MC13-MC2

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

3-S

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

45

(a)

(b)

(c)

(d)

(e)





3-L

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

3-MA1

3-MA2

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

3-MB1

3-MB2

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

3-MC1

3-MC2

3-S

40

20

60

80

20 40 60 80

80

60

40

20

Al2O

3

MgOCaO

47

4. CONCLUDING DISCUSSION

The thesis focuses on the clogging phenomena of the SEN during the

continuous casting process; with an emphasis on the tundish and SEN. In

the continuous casting process, the SEN is very important to control the

steel flow and to protect the molten steel during transportation from the

tundish to the mould. Clogging during casting is difficult to fully prevent

and it is therefore important to look at several aspects that affect the

clogging tendency. The thesis consists of 5 supplements which focus on

studying the inclusions in the tundish as well as the behaviour in the SEN

before and during teeming. The results and applications of the studies are

summarized in Table 4. However, in the respective supplement more

detailed descriptions of the results can be found.

Table 4. Overview of the results and applications of the supplements

Study: Results: Application:

1

Preheating process of SEN in the industry

An overview of the industrial preheating conditions showed decarburization during the preheating process

A new coating material needs to be implemented in the SEN

2

Implementation of CaTiO3 plasma sprayed coating in the nozzle inlet during pilot plant casting trials

Reduced clogging tendency for implementation of CaTiO3 plasma coatings during continuous casting of low-carbon Al-killed steel

Plasma coating materials have been analysed & implemented for reduction of the clogging tendency

3

Implementation of YSZ plasma sprayed coating in the nozzle inlet during pilot plant casting trials and during industrial trials

Reduced clogging tendency for implementation of YSZ plasma coatings during continuous casting of Ce alloyed stainless steel

4

In depth study of the YSZ plasma coating material

5

In depth study of inclusions in the tundish by MISS sampling during continuous casting

Distribution of the inclusion composition in the tundish

The chemical composition of inclusions indicates which origin is of most danger for the steel quality

48

In order to reduce the clogging tendency during casting, the preheating

processes at three different steel plants were first studied in supplement 1.

The SEN was preheated to minimize the thermal shock and without

preheating there is a risk of freezing of steel inside the SEN. The flue gas

(CO, CO2 and O2) and the temperature distribution in SEN were measured

during the preheating operation. Thereafter, the Gibbs free energy was

calculated. The result showed that decarburization of the SEN was possible

at all studied steel plants. Also, the protective glass/silicon powder was

found to form a glaze that reacted with the RBM, which resulted in an

uneven surface. This increases the risk of an inclusions entrapment at the

SEN’s inner wall. For this reason, it was concluded that a new coating

material for the internal SEN’s wall was needed.

The coating materials that have been suggested were, firstly a CaTiO3

plasma coating, for casting of low-carbon Al-killed steel grades

(supplement 2). The results show that an accumulation of Al-clusters on

the SEN’s internal wall will start to clog the SEN and to disturb or prevent

the steel flow. A reaction between CaTiO3 and Al2O3 was found to take

place at temperatures between 1550-1600°C in the laboratory experiments.

Furthermore, the CaTiO3 coating reacted in situ with the Al2O3 clusters and

formed liquid inclusions during casting in the pilot plant trials. Also, the

coating material was consumed during the trials which indicated that

reactions products from the Al2O3 and the coating followed the steel flow.

In addition, fragments of the coating were observed to be transported to

lower parts of the nozzles, which had not been coated.

Later in the studies, an YSZ plasma coating for casting Ce alloyed stainless

steel grades was implemented. This was first done in pilot plant trials and

then in industrial trials (supplements 3). The results from the pilot plant

trials showed that the clogging tendency was reduced when implementing

the YSZ plasma coating. However, the reduced clogging tendency was not

as clear in the industrial trials. From the Ce/Al ratio it was showed that

when the ratio was lower the clogging tendency was reduced by the YSZ

plasma coating. The YSZ coating material from the industrial trials was

further evaluated as a suitable coating material by performing electrolytic

extraction on samples taken from the solidified steel at the steel/YSZ

interface (supplement 4). The FEG-SEM studies of the steel/RBM

interface showed that four layers could be observed; the YSZ coating, a

49

thin layer of MgO, dendrites containing REM, and REM clusters in a steel

matrix.

In addition to implementation of a new coating material in the SEN, to

reduce the risk of decarburisation and reactions at the steel/RBM interface

resulting in clogging, it is also vital to produce clean steel in order to reduce

clogging during casting. The last process step to separate non-metallic

inclusions from the melt to the tundish cover slag is the tundish. The

reactions taking place in the tundish was mapped by using the MISS

sampler in the tundish (supplement 5). The molten steel can react with the

tundish refractory lining or the tundish covering slag and form new non-

metallic inclusions. Thus, the origin of the non-metallic inclusions could

be identified before, during, and after casting. Knowing the main origin of

the non-metallic inclusions, then the focus can be on preventing the most

harmful inclusions. For all heats, the inclusions in the slab sample

contained inclusions originating from group I (slag). For heat 1 and 3, the

inclusion mainly originated from the slag. Heat 2 differed and the inclusion

mainly originated from groups I and II (deoxidation products). In addition,

the inclusion also originated from the refractory material, both from the

ladle and the tundish.

51

5. CONCLUSIONS

The aim has been to study how it is possible to decrease the clogging

tendency in the continuous casting process. Several aspects have been

considered in order to decrease clogging: the RBM in the SEN,

implementations of new materials in the SEN and reactions in the tundish.

Specifically, the industrial preheating trials in supplement 1 first led to a

more profound understanding of the preheating processes. Overall, the

investigated steel plants all had different processes and parameters for the

preheating and casting processes. The conducted preheating study

(supplement 1) showed that the SENs’ surface is decarburized. These

results lead to a suggestion of using a new plasma coating materials for the

SEN (supplements 2-4). The formation of liquid inclusion due to reactions

between CaTiO3 and Al2O3 was studied in supplement 2 based on

laboratory heating experiments and pilot plant trials with plasma coated

nozzles. The protection of the reaction between the RBM and REM in the

molten steel, as well as a smooth surface where inclusions could not as

easily adhere by an YSZ plasma coating, was studied in supplement 3,

based on pilot plant and industrial trials. In supplement 4, the YSZ coating

was studied further after the industrial trials. The reactions in the tundish

with a focus on inclusions origin was studied in supplement 5 by

evaluating of the steel samples taken before, during and after casting. From

the results of the supplements the following have been concluded:

• Decarburisation of the RBM in the SEN is thermodynamically possible

both during and after the preheating process. After preheating, an

increasing porosity was observed in the RBM. In order to minimise the

decarburisation, it is necessary to control the preheating temperature

and the preheating rate. However, most important is to develop a new

coating material, which will protect the inner surface of the SEN

against decarburisation during the preheating operation.

• In the laboratory experiments, a reaction between CaTiO3 and Al2O3

was observed in the temperature interval between 1550 to 1600°C.

FEG-SEM analyses of the chemical composition showed that the

reaction between CaTiO3 and Al2O3 resulted in a formation of a liquid

phase. As the temperature was increased the stronger the reaction

became.

52

• Implementations of CaTiO3 coated nozzles in the pilot plant trials were

found to reduce the clogging tendency. Moreover, this was found for

the coatings containing both 4.8% and 9.1% CaTiO3. From the SEM

studies, accretions of Al2O3 inclusions were observed to have

accumulated at the nozzle wall. The clogging was not eliminated when

using the coated nozzles, but more steel could be teemed in comparison

to the reference nozzle. In total, the deviation of the teemed steel mass

for nozzles N1, N2, N3 and N4 from the ideal teeming rate after

respective teeming operation was 70%, 14%, 10% and 14%,

respectively. It was also observed that the coating material had been

consumed during the teeming operation. The thickness of the coating

varied and it was measured to be up to ~70 mm in thickness after a

completed teeming operation.

• Implementation of the YSZ plasma coatings on nozzles in the pilot

plant trials increased the total steel mass that could be teemed through

the nozzles. Also, a longer time for removal of inclusions before

casting increased the total teemed steel mass. Overall, the clogging of

the nozzles was not eliminated but the clogging tendency was reduced

for the YSZ plasma-coated nozzles compared to non-coated nozzles.

In total, the deviation of the teemed steel mass for nozzles N1, N2, N3

and N4 from the ideal teeming rate after respective teeming operation

was 20%, 44%, 37% and 47%, respectively. During the experiment

with nozzle N1 the reaction time after the REM additions before

teeming was longer. This experiment had the lowest deviation from the

ideal teeming rate during the whole teeming operation.

• The results from the SEM analyses of the YSZ plasma coated nozzles

showed that the plasma coatings were still present after the pilot plant

trials. The coating thickness had been reduced by 17 to 40% during the

pilot plant trials. Also, SEM studies of the uncoated reference nozzle

showed that accretions of Ce clusters onto the RBM had taken place.

• An implementation of the YSZ plasma coating on SENs in industrial

trials showed a correlation between the Al and Ce contents in the

molten steel. When the Ce/Al ratio was lower, a positive effect of

reducing the clogging tendency was observed. In the most promising

trial, the Ce/Al ratio was the lowest (3.5) of all three industrial trials,

and a reduced clogging tendency was achieved. The YSZ plasma

53

coating did not entirely stop the clogging tendency during the

industrial plant trials. However, a decreased stopper rod movement

showed a decreased clogging tendency for the coated SEN in

comparison to the non-coated SEN.

• After electrolytic extraction of samples from the steel/RBM interface,

the following FEG-SEM studies indicated the presence of four layers

in the microstructure. These were: the YSZ coating, a thin layer of

MgO, dendrites containing REM, and REM clusters in a steel matrix.

Also, traces of the original YSZ coating were observed in the sample

from the upper part of the inlet. That remaining coating had a thickness

of about 30 to 100 μm.

• During casting of Al-killed low-carbon steel inclusions originating

from the slag, deoxidation product and the tundish refractory lining

were observed in samples from the tundish. The inclusion composition

from samples in the slab showed that most inclusions originated from

the slag in all three heats. In one of the heats, a major part of the

inclusions was also found to originate from the deoxidation products.

55

6. FUTURE WORK

A cost-efficient steel production demands longer casting sequences

without production stops due to clogging. Moreover, clogging has been a

difficult problem for a long time and will not be solved overnight.

However, so far the research community has learned that several factors

are important when approaching the problem. Firstly, the cleanness of the

molten steel is vital before any other measures can be effective. Also, the

temperature during the casting process is around 1600°C and the risk of

reaction between the SEN’s RBM and the molten steel is high. Therefore,

a coating of the internal surface can prolong the SEN’s life and also protect

the SEN from decarburization. This decarburization is caused by a high

oxygen activity near the inner wall of the SEN. After this has happened, a

deposition of a first layer mostly containing Al2O3 particles is easily

formed onto the decarburized surface. Therefore, a suitable coating

material, that prevents both decarburization and accretions of Al-clusters,

is vital to develop. On this basis, the following suggestions for future work

are proposed:

• In order to prevent decarburization of the SENs internal surface, an

even heating rate inside the SEN is necessary. The preheating process

is performed on the basis to minimize the thermal shock during

teeming. Therefore, the preheating cannot be too excessive in the

beginning. In addition, from the preheating trials it was found that

coatings on the SENs inner wall need to be investigated to minimize

penetration of SiO2 and alkalines into the RBM. For this reason, it is

necessary to develop a new coating material which will protect the

SENs inner surface from decarburization. Also, the coating material

should not react with the RBM.

• In the pilot plant trials the nozzles’ RBM consisted of ZrO2. Therefore,

the reactivity between the coating materials and the SEN’s RBM needs

to be evaluated after teeming. Moreover, since all coating materials

were consumed the use of a larger thickness than a 200-400 µm coating

needs to be studied.

• The CaTiO3 coating materials need to be examined and evaluated in

industrial plant trials. Thus, experiments with plasma sprayed SENs

56

and stopper rods need to be performed. For industrial trials with both

CaTiO3 and YSZ coating materials statistical verifications from

industrial trials needs to be performed before the coatings can fully be

implemented as a standard method to reduce clogging. Also, for the

coatings to be utilized in the industry a control of the coating thickness

needs to be established.

• Findings from mapping of the tundish showed three main sources of

inclusions in the tundish. First, the focus should be on improving

conditions with the slag in both the ladle and in the tundish in order to

produce cleaner steels during the continuous casting process. One

parameter to study further is the influence of the slag carry over from

previous heats in the ladle on the clogging tendency.

57

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