Solubility of hydrogen in slags and its impact on ladle refining

Jenny Brandberg Hurtig

Doctoral Thesis in Micro Modelling

School of Industrial Engineering and Management Division of Micro Modelling Royal Institute of Technology

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, 4/9-2015, Kungliga Tekniska Högskolan, Stockholm

ISRN 978-91-7595-594-0

Jenny Brandberg Hurtig Solubility of hydrogen in slags and its impact on ladle refining KTH School of Industrial Engineering and Management Division of Micro Modelling Royal Institute of Technology SE-100 44 Stockholm Sweden ISRN 978-91-7595-594-0 © The Author

i

ABSTRACT

In the present thesis a study of the mechanisms of hydrogen pickup during ladle

treatment was undertaken. Previous studies showed that the presence of hydroxyl

ions in the ladle slag resulted in hydrogen transfer from the slag back into the steel

bath. The main focus of the present work was therefore to gain deeper knowledge

of the ladle slag, its properties and impact on hydrogen concentration in the liquid

steel. For this purpose a number of slag compositions were examined in order to

clarify whether these slags were single liquids at 1858 K. 14 out of 27 compositions

in the Al2O3-CaO-MgO-SiO2 system were completely melted, while the rest had

solid shape present These results were in disagreement with the existing phase

diagram.

Water solubility measurements were carried out by employing a thermo gravimetric

technique. The temperature was found to have negligible effect in the water

solubilities. The experimental results showed that the water capacity varied between

1·103 and 2·103 in the majority of the composition range. However, for

compositions close to CaO saturation the water capacity value could reach higher

than 3·103. The experimental determined water capacity was further used to

develop a water capacity model for the quaternary slag system Al2O3-CaO-MgO-

SiO2. The model was constructed by considering the effects of the binary

interactions between the cations in the slag on the capacity of capturing hydroxyl

ions. The model calculations agreed reasonably well with the experimental results as

well as with the literature data.

The water capacity model was used in the last part of the present thesis in order to

determine the major source for hydrogen pick-up of the steel after vacuum

degassing but before casting. For this purpose, samples of slag and metal were

taken at different stages of ladle treatment at SSAB. Hydrogen increase after

vacuum treatment was observed. Moisture contents of the industrial slag were

analysed and the water capacities of the slags were calculated. It could be seen that

the hydrogen increase was correlated to the amount of moisture in the slag and the

water capacity. The study showed that the slag containing most water was also the

heat having the largest hydrogen increase. The slag with most water had the highest

water capacity. It could be concluded that the major source for hydrogen coming

back into the steel was due to the slag-metal reaction.

Introduction

ii

A tentative process model to predict the final contents of hydrogen and nitrogen

after tundish process was attempted. More work is needed to improve the model.

iii

ACKNOWLEDGMENTS

I will start by acknowledge my supervisor Professor Du Sichen. Thank you for your

encouragement and excellent guidance. I appreciate your patience, support and

your honest feedback.

I would like to express a special gratitude to my mentor Christer Offerman who

pushed me and made it possible for me to finish this thesis on a personal level.

Thank you for taking me under your wings making me grow to a new level.

Thanks to Professor Seshadri Seetharaman for his fruitful discussions and valuable

comments.

Furthermore, I would like to thank Professor Pär Jönsson always giving

encouragements and valuable advice during this period.

I am grateful to SSAB for providing the time, the production facilities and the test

equipment during this work.

Jernkontoret and Brinell centre are gratefully acknowledged for their financial

support.

I choose to express a special thank you to everyone who has contributed to this

work in one way or another. Thanks to all the anonymous reviewers who

contributed with valuable advice and for their vital comments and encouraging

support.

A very special thanks is owed to my birth family for your endless love and support.

But also for always reminding me I was the youngest, pushing me to perform a

little bit more. Without you I would not be here.

Finally, I would like to thank my better half, Hans, who with his patience, support

and sense of humor helped me through all the difficult times. Not least, I am really

grateful to our four children – a never ending source for joy of life.

v

The thesis is based on following supplements:

I. Characterization of melting of some slags in the Al2O3-CaO-MgO-SiO2 quaternary

system

F. Dahl, J. Brandberg, Du Sichen

ISIJ International, vol. 46, no. 4, pp. 614-616, April 2006.

II. Water vapour solubility in ladle-refining slags in the Al2O3-CaO-MgO-SiO2

quaternary slag system

J. Brandberg, Du Sichen

Metallurgical and Materials Transactions B, vol. 37B, no. 3, pp. 389-393,

June 2006.

III. Water capacity model of Al2O3-CaO-MgO-SiO2 quaternary slag system

J. Brandberg, L. Yu, Du Sichen

Steel Research, vol. 78, 2007, no. 6, pp. 460-464,2007.

IV. Hydrogen pick up after vacuum degassing J. Brandberg Hurtig, Du Sichen

Accepted for publication in Ironmaking and Steelmaking May 2014.

V. Nitrogen and hydrogen refining during vacuum treatment of liquid steel

N. P. Kojola, J. A. E. Hurtig

2011 International Symposium on the Recent Developments in Plate

Steels.

The contribution by the author to the different supplements of the thesis:

I. Part of literature survey, assisted during experimentation and major part

of the writing.

II. Literature survey, experimentation and major part of writing.

III. Literature survey, assisted during experimentation, major part of

modeling and major part of the writing.

IV. Literature survey, sampling and major part of writing.

V. Literature survey, contributed in experiment and writing.

The following work has been carried out by the author but are not a part of the thesis

Hydrogen Pick-up in an Argon Bubble J. Brandberg, E. Johansson, M. Magnelöf, T. Niemi, E. Rutqvist, D, Sichen, U. Sjöström. Steel Grip Journal of Steel and Related Materials, in press.

Nyköping June 2015

The Author

vii

CONTENTS

1 Introduction 1 1.1 Previous work 2

1.2 The objectives of the present work 2

2 Experimental Work 5 2.1 Laboratory work 5

Materials and sample preparation 5

Experimental set-up and procedure 5

Characterization of melting of slags 5

Water vapour solubility experiments 7

2.2 Industrial data 9

Plant description 9

Sampling 9

3 Modeling work 11 3.1 Hydroxyl capacity model 11

Construction of the model 11

Hydrogen capacity data 13

3.2 Process model 13

Metal-gas reaction 14

Metal-slag reaction 15

Model calculation 16

4 Results 17 4.1 Laboratory work 17

Characterization of melting of slags 17

Water vapour solubility 18

4.2 Industrial sampling 20

Introduction

viii

4.3 Modeling work 21

Hydroxyl capacity model 21

Process model 23

5 Discussion 25 5.1 Laboratory work 25

Characterization of melting of slags 25

Water vapour solubility 26

5.2 Industrial work 29

5.3 Modeling work 29

Hydroxyl capacity model 29

Process model 30

6 Conclusions 33 6.1 Proposal for future work 34

Bibliography 35

1

Chapter 1

INTRODUCTION

Steel is the worlds most produced metallic material and has been a very important construction material for a long period of time. It is relatively cheap to produce and the properties of the steel can easily be varied by the addition of alloys. From the beginning the ladle was used only as a transport vessel between the melting furnace and casting. The demands of cleaner steel led to development in steel refining. Today, ladle metallurgy has become more and more crucial for various kinds of refining operations. Hydrogen is a non-metallic element that causes embrittlement, pipes and reduction of mechanical properties in the final steel products. Even the presence of a few parts per million of hydrogen in molten steel can have serious effect upon the end material. A study has shown that a hydrogen concentration as low as 1-2 ppm would have detrimental impact on the mechanical properties of tool steels.1 Although most hydrogen can be removed as vapour, low pressure is required to bring down the hydrogen concentrations below 1 ppm. An efficient dehydrogenation process is necessary in order to meet the demand of cleaner steel. The most common industrial process for dehydrogenation is the vacuum degassing treatment. In many industrial practices, synthetic ladle slags are based on the Al2O3-CaO-MgO-SiO2 quaternary system. The slag composition plays an important role in ladle refining. The primary source of hydrogen pick-up has been discussed in a numerous publications.2-8 While both the level of vacuum and argon flow rate are of great importance in determining the rate of hydrogen removal, the partition of hydrogen between slag and molten steel plays another crucial role.

Introduction

2

1.1 Previous work

Konolov et al. showed that a two-stage neutralization of the slag reduced the hydrogen content by 12% in the steel.9 Further Jauhiainen et al. investigated four different stirring methods concluding that combined gas and induction stirring with downwards stirring was beneficial for dehydrogenation.10 It was investigated in the literature whether the hydrogen control should be started from the BOF.11 These authors reported that ladle slag played an important role and it should be beneficial to have a slag with low basicity11 and high viscosity12 in order to reach a high dehydrogenation and low resupply of hydrogen from the slag to the liquid steel. Dor et. al. have reported that the water content in the slag after vacuum treatment could be 200 times higher than the value predicted from the metal-gas-slag equilibrium.12 This implies that the dehydrogenation kinetics in the steel is much faster than it is in the slag.13 The hydrogen solubilities of binary, ternary and quaternary slag system have been carried out by different research groups.13-18 Researchers have found the solubility of water to be proportional to the square root of the water pressure.13,19-22 This finding supports that the water vapour is present as free hydroxyl ions in basic slags. By increasing the basicity, the solubility of water is increased.13-14,20,23 The temperature on the other hand has been found to have negligible effect on the solubility of water vapour in metallurgical slags.20-21,24 This can be attributed to low heat of solution of water vapour in metallurgical slags.21,25 Some effort has been put forward to describe the hydrogen capacity as a function of temperature and slag composition. Sosinsky et al.20 relates hydroxyl activities in CaO-MgO-SiO2 melts to the activity of silica and I. D. Sommerville tries to relate hydroxyl capacities to basicities of slags.26 Ban-ya et al.27 used the regular solution model of Lumsden on binary and ternary slag systems in order to predict the hydroxyl capacities. Jo and Kim13 extended this model to the quaternary system Al2O3-CaO-MgO-SiO2. To the authors knowledge it has been limited number of publications describing the increase of hydrogen levels during the trimming step of the ladle treatment. However, Dor et al published an article describing the slag phase as a source for the scatter of hydrogen concentration during and after vacuum degassing.12

Since hydrogen has a larger diffusion coefficient than nitrogen in molten steel, nitrogen refining is more complex and is limited to 10-30%.28 Some effort has therefore been put forward by different researches in order to find slag systems

Chapter 1

3

that can remove nitrogen more effectively. Studies have shown that a slag with high nitride capacity have strong ability to reduce nitrides in molten steel. Several studies have shown that denitrogenation is promoted by high slag basicity and nitride forming agents as BaO, B2O3, TiO2 and Al2O3.

29-33 Also the steel composition has an influence upon the efficiency of the nitrogen refining process. J. Fu et al34 have shown that the nitrogen removal is controlled by nitrogen transferring in the liquid diffusion layer when sulfur content is 0.005%. Further, it has been shown that the nitride distribution increases with decreasing aluminum content in the steel.35 Another research study argues that the kinetics of the interfacial reaction is affected by surface active elements like sulfur and oxygen.30

1.2 The objective of the present work

The major oxide components of a ladle slag are usually Al2O3, CaO, MgO and SiO2. The phase diagram information from slag Atlas36 of the Al2O3-CaO-MgO-SiO2 quaternary system involves considerable uncertainties since many liquidus temperatures are given as dotted lines. Disagreements between the phase diagram and the reality have been found by both the present research laboratory and the steel industry37. One aim with this thesis is therefore to study a number of slag compositions to examine whether these slags are single liquids at 1858 K. Water vapour dissolves in liquid slag in a form of hydroxyl ion or hydroxyl radical. The presence of hydroxyl ions in the ladle slag will result in hydrogen transfer from the slag back into the steel bath.19 Hence, the initial concentration of hydroxyl ions in ladle slag would seriously affect the kinetics of the dehydrogenation process and therefore the hydrogen content in the final product. In order to optimize the dehydrogenation process, a good knowledge of the hydrogen solubility of the ladle slag is essential. The slags studied in the Al2O3-CaO-MgO-SiO2 system are mostly in the composition region having relatively higher SiO2 contents.13 On the other hand, many ladle processes operate with slags having less than 10 mass% SiO2 nowadays. To meet the demand of process optimization of ladle treatment, the present work focuses on the determination of hydrogen solubilities of some Al2O3-CaO-MgO-SiO2 quaternary slags having lower SiO2 concentrations. In order to produce cleaner steel, an efficient dehydrogenation process is required. In ladle refining, slag plays a crucial role in desulphurization, dephosphorization and inclusion removal. Optimization and the development of the ladle treatment necessitates good knowledge of the ladle slag with respect to, among other properties, its capacities of taking up sulphur, phosphorus and hydrogen. Very often, descriptions of these capacities as a function of temperature and slag composition are required, since the slag composition varies during refining. Jo and

Introduction

4

Kim13 described the hydrogen capacity for the quaternary Al2O3-CaO-MgO-SiO2 system as a function of temperature and slag composition. However, the model by Jo and Kim13 predicts much lower values when compared to the experimental data in the high basicity region. Therefore, this thesis aims at developing a water capacity model for quaternary ladle slags. For many steel producers a high basicity slag is of importance to reach low sulphur concentration and having to high viscosity in a slag creates practical problems during the production. Therefore, the increase of hydrogen after vacuum treatment is investigated trying to separate the slag contribution from other sources such as heating, alloying and refractory. For this purpose industrial sampling after vacuum treatment is made at SSAB in Oxelösund, Sweden. Also, numerous process data from the same industrial plant is used in order to investigate crucial relations for removing nitrogen and hydrogen during the degassing process. Further in order to be able to predict the concentration for hydrogen and nitrogen throughout the casting this thesis aims at describing the variation as a function of time in the tundish.

5

Chapter 2

EXPERIMENTAL WORK

2.1 Laboratory work

Materials and sample preparation

In many industrial practices, synthetic slags are based on the Al2O3-CaO-MgO-SiO2 quaternary system. These four oxide compounds were therefore used in the present work. The oxides were calcined separately for 24 hours at 1073 K. After calcinations, the oxides were ground and thoroughly mixed in predetermined proportions. The mixtures were pressed into pellets and platinum crucibles were used as sample container during the experiments. The pellets were kept in a sealed glass bottle that was kept in a desiccator before the experiment.

Experimental set-up and procedure

Characterization of melting of slags

A number of ladle slag compositions were studied to examine whether these slags are single liquids or multiphase mixtures at 1858 K. For this purpose a quenching technique was employed. A horizontal furnace with an alumina reaction tube was employed for most of the experiments. The experimental setup is schematically shown in Figure 1. The furnace had KANTHAL SUPER 1800 heating elements controlled by a regulator using a Pt-30 pct Rh/Pt-6 pct Rh thermocouple. An alumina holder was used to support two crucibles containing the samples. In a typical run, the samples were

Experimental Work

6

carefully introduced into the reaction chamber and placed in the even temperature zone. A constant argon gas flow was passed through the reaction tube at low flow rate. The samples were heated to 1858 K and kept for more than 5 hours. The sample holder was rapidly pulled out of the furnace. After a quick visual examination, to see if the sample was completely molten, the samples were quenched in liquid nitrogen. In order to confirm the reproducibility of the experimental method, each slag composition was investigated four times.

1 12 3 4 5 6 7 8

Figure 1: Schematic figure of the horizontal furnace. 1. silicone rubber stopper; 2. alumina reaction tube; 3. furnace; 4. gas inlet; 5. thermocouple; 6. alumina crucible holder; 7. platinum crucible containing the sample; 8. gas outlet.

In many ladle treatments, slags of high basicities are employed. In order to examine whether these high basicity slags were still single liquid at lower temperatures that might be encountered in ladle refining, these slags were quenched from 1793 K. To avoid the slight possibility of solidification of the sample during quenching, a vertical furnace was employed. The vertical furnace also had KANTHAL SUPER 1800 heating elements. As shown in Figure 2, an alumina reaction tube was used as the reaction chamber. The slag sample was placed in a platinum crucible welded in both ends. The platinum crucible was hanged by a piece of platinum wire in the even temperature zone of the furnace. The sample was heated and kept at the experimental temperature, 1793 K for 7 hours. The platinum wire was cut off and the sample was quenched in liquid nitrogen. The samples were embedded in bakelite under vacuum for microscopic examination.

Chapter 2

7

1

2

4

3

5

6 7 8

Figure 2: Schematic figure of the vertical furnace. 1 gas outlet; 2 silicone rubber stopper; 3 platinum wire; 4 samples; 5 alumina reaction tube; 6 silicone rubber stopper; 7 thermocouple; 8 gas inlet.

Water vapour solubility experiments

The thermo gravimetric technique was used for determination of the hydrogen solubilities of some CaO-SiO2 binary slags and Al2O3-CaO-MgO-SiO2 quaternary slags.

Figure 3: Schematic diagram of the experimental setup.

The experimental setup used in this study is schematically illustrated in Figure 3. The setup consists of three major components, the water column, the balance

Experimental Work

8

chamber and the furnace. The water column has heating elements and is thereby acting as a boiler. Argon gas is introduced at the bottom of the water column. The whole column is filled with small glass balls in order to provide good contact between water and argon gas. By adjusting the temperature of the water column, the water pressure in the H2O-Ar mixture can be controlled on the basis of the thermodynamic data.

The balance (METTLER TOLEDO AG285) is placed in an air tight box and it is connected to a computer, which follows the weight change in situ. An alumina transducer sitting on the balance is used to hold the sample in the even temperature zone of the furnace. The furnace is a high temperature furnace with an alumina tube employed as the reaction chamber. A pair of B type (Pt-30 pct Rh/Pt-6 pct Rh) thermocouple is used to control the furnace temperature. Another pair of B type thermocouple is placed just above the sample to measure the temperature. H2O-Ar gas mixture is introduced into the reaction chamber by a stainless steel tube connected to an alumina tube, the tip of which is about 5 cm below the sample. In order to prevent water condensation from the moist gas, the inlet stainless steel tube is always kept at 363 K. In a general run, a sample along with the platinum crucible was placed on the top of the alumina transducer before the reaction tube was sealed. An argon flow of 0.072 Nl/min was introduced into the reaction chamber. The furnace was heated up to the experimental temperature at a heating rate of 5 K/min. After a period of stabilization, H2O-Ar gas mixture was introduced into the reaction chamber. In most of the experiments, the water column was kept at 327 K. Repetitions of measurements of some slag compositions were carried out at different water pressure. The reproducibility was also checked using higher flow rate of argon flow. Calibration was made by making experiment using platinum crucible containing a piece of pure re-crystallized alumina under exactly identical conditions. To confirm the reliability of the experimental data, two slag samples were equilibrated in a horizontal furnace using the same water column. After equilibrating the slag samples with the H2O-Ar mixture they were quenched in liquid nitrogen. The samples were weighed before the experiment and just after quenching. The weight change would give the solubility of H2O or hydroxyl ions of the slag at the experimental temperature.

2.2 Industrial data

Plant description

Chapter 2

9

Process description

Industrial trials were carried out in an ore based steel plant of SSAB in Oxelösund, Sweden. The main production route is presented schematically in Figure 4. The hot metal is converted to steel in a BOF converter. Crude steel is tapped into a ladle of 200 tons. During tapping, main alloying is carried out. Thereafter, the steel is transferred to a TN station, where it is deoxidized and desulphurised. After TN treatment the slag is removed and the ladle is transferred to an integrated LMF-VTD station. The ladle has two porous plugs at its bottom for introducing purging argon gas. After securing the function of the porous plugs, slag builders are added. The slag builders consist of lime and a mixture of mostly Al2O3 and MgO. The oxides are kept dry until they are put into the alloying bin. Samples are taken once a day to ensure low moisture content.

BOF

VD 1

VD 2

LRFStirring station

CC1

CC2

Deslagging Deslagging

Figure 4: Schematic diagram of the production route.

At the LMF position, fine adjustments of temperature and composition are carried out. The liquid steel is transferred to the vacuum tank degasser for vacuum treatment. During degassing, additional alloying can be performed depending on steel grade. The flow rate of the purging gas is controlled and also monitored using a camera placed in the LRF/VD roof. After the degassing process, if necessary, the ladle returns to the LMF station for trimming temperature and composition before departure to the continuous casting machine. Sampling

The ladle treatment of the industrial trials was divided into three process steps, viz. before vacuum treatment, vacuum treatment and after vacuum treatment. The last possibility for sampling at the secondary metallurgy station was before departure of the steel to the casting. Steel and slag samples were taken along with hydrogen

Experimental Work

10

measurements at different stages during the ladle treatment. The hydrogen content was measured manually with Hydris. Seven heats were studied, all from the same steel grade. Additional heats were also studied, where hydrogen analyses were taken immediately after vacuum degassing and before departure. In order to clarify whether the hydrogen pick up was from the slag, heating or additives, the hydrogen content was measured before vacuum degassing, [H]BV, directly after vacuum treatment, [H](AV-1) and again 5-10 minutes after the vacuum treatment and before any heating or additions had been made[H](AV-2). The final sampling was done just before the departure, after final adjustments of temperature and composition of the steel, to the caster, denoted as [H]BD. It was ensured that the stirring intensity was low without any open eye formation during this period. In this way, any other source of hydrogen except slag was minimized. These samples could also indicate how fast the transfer of hydrogen from the slag back into the metal took place after the degassing process. The steel samples and temperature measurements were taken with an automatic sampler. The slag sample was taken with a sampling scope. The slag was cooled down fast and sealed immediately in glass bottle. The moisture content in the slag was determined by a thermo gravimetric method. The slag sample was heated to 873 K before holding it for 2 hours in an argon atmosphere. The weight loss was assumed to correspond to the moisture content in the slag. For verification of the tundish prediction model steel samples and hydrogen content were measured at the caster. The steel samples and temperature measurements were taken manually. The hydrogen concentration was also measured manually with Hydris. 66 heats were sampled at different casting lengths. More frequent samples were taken at the first heat in the sequence and at ladle switch. The guide line for sampling at the caster was every 5-10 meters casting length and the measuring started once the tundish was filled with liquid steel.

11

Chapter 3

MODELING WORK

The existing model for the quaternary system Al2O3-CaO-MgO-SiO2 developed by Jo and Kim13 predicts much lower values when compared to the experimental data in the present study. This disagreement could be attributed to the fact that the experimental measurements made by Jo and Kim13 in the quaternary system are mostly in the high silica content region. The measurements in the present work are in the same system but mainly in the high basicity region. A hydroxyl capacity model for quaternary ladle slags has therefore been developed.

3.1 Hydroxyl capacity model

Construction of the model

The hydroxyl capacity is introduced on the basis of the following slag-gas reaction,

)slag(OH2)slag(O)gas(OH 2

2

(1)

The hydroxyl capacity of a slag can be expressed as:

OH

Oo

1

OH f

a

RT

G5.0expC

2

(2)

Modeling work

12

where o

1G is the standard Gibbs energy of reaction (1), R is the gas constant and T

is the temperature in K. OHf and 2O

a stand for the activity coefficient of OH- and

the activity of O2- respectively.

The exponential term in (2) is system independent, while 2Oa and OH

f depend on

the slag composition and temperature. It should be mentioned that the standard states for ion activities are difficult to define. Hence, eq. (2) would have very little

practical use. On the other hand, both 2Oa and OH

f would relate to their partial

Gibbs energies exponentially. It is reasonable to expect that

RT

ψ'exp

f

a

OH

O2

(3)

where ψ' is a function of composition and temperature. Inserting eq. (3) into eq. (2)

leads to

RTexp

RT

ψ'exp

RT

G5.0expC

o

2

OH

(4)

in eq. (4) is a new function, which depends on the slag composition and

temperature. In line with the activity model38 and sulphide capacity model39 developed in the present department, the slag could be considered to have two sub-groupings. A silicate melt containing m oxides, C1c1Oa1, C2c2Oa2,....CiciOai,....CcmOam can be expressed as:

q2

y

vm

y

vi

y

2v

y

1v OCm,,Ci,,2C,1Ccmci2c1c

(5)

where Cvi stands for cations, the superscript vi denotes the electrical charge. Even Si4+ ion is included in the cation group. q in eq. (5) are stoichiometric coefficient. The different ions would have effects on each other with respect to the capacity to accommodate the OH- ions. Hence, in eq. (4) can be formulated as a function of

the interactions between different cations in the presence of O2-. In a multi-component system, is expressed by:

mix

m

1

iiX (6)

Chapter 3

13

where Xi is the mole fraction of the oxide component i. i iX in (6) describes

the linear variation of from the pure components in the absence of the interaction between different species.

mix arises from the mutual interaction

between the different cations in the presence of O2-. mix can be described by:

1m

1

m

1i

CCCCmix jiiiDyy (7)

jiCCD is the parameter for the binary interaction between cations Ci and Cj. It is

expressed as polynomials,

...yyTDDTDDD CjC2

1

1

1

2

o

1

o

CC iji (8)

where yi stands for the ionic fraction of cation within the cation grouping. Previous studies20-21,24 have shown that hydroxyl capacities of slags have very little dependence on temperature. Hence, the terms related to temperature can be omitted. The interaction parameters can be optimized by using the experimentally determined OH

C .

Hydrogen capacity data

The present hydroxyl capacity model is semi-empirical. The use of reliable experimental data in the optimization would be crucial for the validation of the model. Hence, a careful evaluation of the literature data is needed in order to optimize the model parameters. In the binary system CaO-SiO2 three publications was found.16-18 Unfortunately, considerable discrepancy is noticed. While SiO2 is a network former, CaO is a basic oxide. It is expected that the interaction between Ca2+ and Si4+ in the presence of oxygen would have strong effect on the hydroxyl capacity of the slag. New experimental measurements were made in order to decide if any of the documented data should be incorporated in the model. The literature data13-15,17-18,20,23 in the other systems were found to be in good agreement, and were therefore incorporated in the model calculation. Totally, 171 documented data were used for the optimization.

Modeling work

14

3.2 Predicting nitrogen and hydrogen variation in the tundish

A model to understand and predict the behavior of nitrogen and hydrogen in the tundish as a function of time has been developed with production data. The assumption made for the theoretical model is that tundish act as an ideal tank reactor and that the transient pickup of nitrogen and hydrogen only occur at the initiation of the casting process that is, until the tundish have been filled with liquid steel. The equation below describes behavior of an ideal tank reactor.

0exp i

E

ii Ctm

FCC (9)

Where, Ci is the concentration of a given element [ppm], E

iC is the initial excess

concentration of element “i” [ppm], F is the steel flow rate through the tundish

[kg/s], m is the mass of the liquid steel in the tundish [kg], t is time [s] and 0

iC is

the concentration of element ”i” as the steel entering the tundish from the ladle

[ppm]. Element “i” was either nitrogen or hydrogen. E

iC and 0

iC was calculated

with least square fit adjusted to the measured data. In total data from 66 heats were used in order to verify the theoretical prediction of nitrogen and hydrogen concentration in the tundish during casting.

15

Chapter 4

RESULTS

4.1 Laboratory work

Characterization of melting of slags

The compositions of the investigated slags along with the results of the experiments are presented in Table I.

Results

16

Table I: Compositions of the samples and the experimental results

As can be seen in Table I, 14 samples were completely melted. The four compositions that were investigated further with a fast quenching method were all found to be completely melted.

Water vapour solubility

Three slag compositions in the binary CaO-SiO2 system were studied. These compositions are listed in Table II. The experimentally determined H2O pick-up

Slag no. mass%Al2O3 mass%CaO mass%MgO mass%SiO2 Result

1858 K 1793 K

1 30 50 13 7 multi phase

2 45 35 13 7 multi phase

3 25 55 13 7 multi phase

4 40 40 13 7 multi phase

5 35 45 13 7 multi phase

6 35 45 10 10 multi phase

7 40 40 10 10 liquid

8 37 43 10 10 liquid

9 30 54 7 9 liquid liquid

10 10 40 10 40 liquid

11 35 50 5 10 liquid

12 20 30 5 45 liquid

13 25 45 10 20 liquid

14 30 45 10 15 liquid

15 35 50 10 5 multi phase

16 40 45 10 5 multi phase

17 20 47 10 23 multi phase

18 38 47 10 5 liquid

19 33 47 10 10 liquid

20 30 55 5 10 liquid liquid

21 35 55 5 5 multi phase

22 34 54 5 7 multi phase

23 35 52 7 6 multi phase

24 40 50 5 5 liquid liquid

25 38 50 7 5 liquid liquid

26 35 45 7 13 liquid

27 25 45 13 17 multi phase

Chapter 4

17

along with the evaluated water capacities and hydroxyl capacities of the slags are also presented in the table.

Table II: The experimental conditions along with the results and the calculated water and hydroxyl capacities.

Slag no. CaO:SiO2

(mass-%)

Temperature

(K)

PH2O

(kPa)

ppm H2O

Water capacity

(mass-%/atm1/2

)

Hydroxyl capacity

(mass-ppm/atm1/2

)

1a 38.05:61.95 1773 16.5 220 545 0.10

1b 38.05:61.95 1823 16.5 240 595 0.11

2 44.80:55.20 1853 16.5 290 720 0.14

3a 55.51:44.49 1823 9.1 240 795 0.15

3b 55.51:44.49 1823 12.3 285 815 0.15

3c 55.51:44.49 1823 16.5 345 855 0.16

In the CaO-Al2O3-MgO-SiO2 quaternary slag system, 12 slag compositions were investigated. The slag compositions along with the experimental conditions are presented in Table III. The mass percentages of H2O and the water and hydroxyl capacities in the slags evaluated from the gravimetric measurements are also listed in Table III.

Results

18

Table III: Water solubilities, water capacities and hydroxyl capacities obtained in the

present study.

Some of the experiments were carried out through both heating and cooling cycles. Table III shows that the water solubilities obtained in heating cycle and cooling cycle agree very well. In Tables II and III it can be seen that the reproducibility is good, irrespective of the argon flow rate or water pressure.

Slag no.

%Al2O3 %CaO %MgO %SiO2 TH2O (K) Tsample (K) CH2Ox10-

3

COH %H2O

1A 40 40 10 10 327 1747 1.9 0.36 0.07 Heating

1827 2.1 0.40 0.08 Heating

1B 327 1843 2.0 0.38 0.08 Quenching

2A 37 43 10 10 327 1747 2.6 0.49 0.10 Heating

1827 2.0 0.38 0.08 Heating

2B 1747 2.1 0.40 0.08 Cooling

3 30 54 7 9 327 1747 2.9 0.55 0.11 Heating

4A 10 40 10 40 327 1747 1.4 0.26 0.05 Heating

1827 1.3 0.25 0.05 Heating

4B 1843 1.0 0.19 0.04 Quenching

5 35 50 5 10 327 1747 1.7 0.32 0.07 Heating

1827 2.0 0.38 0.08 Heating

1747 1.7 0.32 0.07 Cooling

6 20 30 5 45 327 1747 1.1 0.21 0.04 Heating

1827 1.1 0.21 0.04 Heating

7 25 45 10 20 327 1747 1.1 0.21 0.04 Heating

8 30 45 10 15 327 1747 1.2 0.23 0.05 Heating

1827 1.2 0.23 0.05 Heating

1747 1.2 0.23 0.05 Cooling

9A 38 47 10 5 327 1747 1.8 0.34 0.07 Heating

1827 1.9 0.36 0.07 Heating

1747 1.7 0.32 0.07 Cooling

9B 1747 1.8 0.34 0.07 Heating

1827 1.8 0.34 0.07 Heating

1747 1.9 0.36 0.07 Cooling

9C 1747 1.8 0.34 0.07 Heating

1827 1.8 0.34 0.07 Heating

1747 1.8 0.34 0.07 Cooling

10 33 47 10 10 327 1747 2.2 0.42 0.08 Heating

1827 2.4 0.45 0.09 Heating

1747 2.4 0.45 0.09 Cooling

11 30 55 5 10 327 1747 3.8 0.72 0.15 Heating

1827 4.6 0.87 0.18 Heating

1747 4.5 0.85 0.17 Cooling

12 40 50 5 5 327 1747 3.6 0.68 0.14 Heating

1827 3.7 0.70 0.14 Heating

1747 3.3 0.62 0.13 Cooling

349 1747 2.9 0.55 0.11 Cooling

Chapter 4

19

For slag compositions 1 and 4, the water solubilities were measured by both gravimetric technique and quenching technique. The good agreement of the experimental results using the two techniques shown in Table III indicates again the reliability of the gravimetric technique. It is seen in Table III that irrespective of the slag compositions, the effect of temperature on the water solubilities is negligible. This result is in accordance with the observation reported in the literature.20-21,24

4.2 Industrial sampling

In this study it was shown that the hydrogen increase was faster just after the vacuum degassing process. This is illustrated in Figure 5 where the hydrogen concentration is plotted as a function of time after vacuum degassing for heat A-F.

Figure 5: Hydrogen concentration as a function of time after vacuum treatment.

Even though the hydrogen values after vacuum treatment is varying, Figure 6 shows that there is no clear correlation between the hydrogen concentration before vacuum degassing and the concentration after vacuum degassing. Although the importance of hydrogen control from BOF have been discussed in previous reports11 the results in this experimental study indicates that there are other more significant factors involved.

Results

20

Figure 6: The hydrogen concentration [H]AV_1 as a function of the hydrogen concentration before vacuum treatment.

It could be expected that the absorption of hydrogen should be higher when the concentration after degassing is low. Therefore, hydrogen resupply after vacuum degassing is plotted as a function of the hydrogen content measured directly after vacuum treatment in Figure 7.

Chapter 4

21

Figure 7: The hydrogen pickup after vacuum treatment as a function of the hydrogen content just after vacuum treatment.

The hydrogen resupply, denoted as ∆H1, is defined as the difference between the second and the first hydrogen sample taken after vacuum treatment according to eq. (10).

121 AVAV HHH (10)

Figure 7 shows no clear relationship between the hydrogen resupply and the hydrogen concentration after vacuum treatment. It had been expected that the hydrogen increase would be higher when 1AVH is low, since the driving force for

the liquid steel to absorb hydrogen is larger. However, this expectation is ruled out by the results shown in Figure 7. The hydrogen supply seems to have a superior relevance. The hydrogen resupply between the second sampling after degassing and steel departure is defined as

12 AVBD HHH (11)

In Figures 8a-8d, 2H is plotted as functions of ladle life, heating time, alloy

additions and time to departure, respectively.

Results

22

Figure 8 a: The hydrogen increase as a function of ladle life.

Figure 8 b: The hydrogen increase as a function of heating time after vacuum treatment.

Chapter 4

23

Figure 8 c: The hydrogen increase as a function of amount of alloy addition after vacuum treatment.

Figure 8 d: The hydrogen increase as a function of time from the end of vacuum treatment to departure.

Note that the number of data points for alloying with more than 300 kg is limited. There is no clear trend in any of the figures. This has also been reported previously. Since the alloys are added as one batch, it is impossible to separate the contribution of each alloy. Nevertheless, it can be concluded that in total there is no significant correlation of hydrogen resupply because of alloy addition.

Results

24

The moisture contents of the slags taken from the 7 heats are listed in Table IV along with the slag compositions. It can be seen that the moisture content in the slag varies considerably. Table IV: Slag compositions and moisture data of the sampled heats (mass%).

Heat Al2O3 CaO MgO SiO2 H2O in slag

A 40.9 49.8 6.9 2.4 0.28

B 36.3 54.9 6.3 2.5 0.18

C 37.5 53.7 6.5 2.3 0.05

D 32.3 41.1 17.0 9.5 0.42

E 33.5 47.6 9.2 9.7 0.05

F 39.9 50.9 6.7 2.5 0.1

G 39.5 50.6 7.2 2.6 0.34

In order to investigate the correlation between the moisture content in the slag and the hydrogen absorption in the liquid steel the relative hydrogen resupply after vacuum treatment is plotted as a function of the moisture content in the slag in Figure 9.

Figure 9: The relative hydrogen pickup of the steel after vacuum treatment as a function of the moisture content of the slag.

The relative hydrogen increase, ∆H3R, is defined as:

Chapter 4

25

100

1

123

AV

AVAVR

H

HHH

(12) A strong correlation between the hydrogen increase and the amount of moisture in the slag is brought out by the experimental observation shown in Figure 9. The hydrogen absorption increases with the moisture content of the slag. The slags containing most water are from the same heats that have the highest hydrogen increase. In the case of nitrogen solubility, a large number of production data were employed in order to have a better statistic. From this study it was shown that the nitrogen refining during vacuum is dependent on the nitrogen content when the vacuum treatment is initialized. A linear method of least square was used to predict the nitrogen refining during vacuum as function of the initial nitrogen content just before vacuum treatment. The “N-refining deviation” was defined as the difference between the actual nitrogen refining and the predicted refining. This factor is illustrated in Figure 10.

Figure 10: Nitrogen refining during vacuum treatment shown as function of pre-vacuum nitrogen content.

In the figure the variable y is predicted nitrogen refining and x is the initial nitrogen content. A positive refining indicates a nitrogen refining efficiency lower than average and if negative indicates a more efficient nitrogen refining than average. The refining deviation was evaluated versus several process variables for a large data set. However, the correlation between the variables and the refining was weak. Further and more detailed studies need to be performed in order to draw any conclusions.

Results

26

4.3 Modeling work

Hydroxyl capacity model

Only binary interactions are considered for the model optimization. The optimized results are summarized in eq. (13).

10100yyyy101030yyyy10166

yyyy10796yyyy1077

yy10232yy1043yy10331

yy1023yy10166yy103X1042

X1067X1097X103410lnTRClnTR

3

MgCaMgCa

3

SiAlSiAl

3

MgAlMgAl

3

CaAlCaAl

3

SiMg

3

SiCa

3

MgCa

3

iSAl

3

MgAl

3

CaAl

3

SiO

3

MgO

3

CaO

3

OAl

34

OH

2

322

(13)

The calculated iso-lines of water capacities at 1873 K in the sections of 5 mass% MgO and 10 mass% MgO are presented in Figure 11a and 11b, respectively. The experimental data available in these two sections are also included in the figures. Data of the slags containing 4 and 7 mass% MgO are also included in Figure 11a. It is seen that the model predictions are in reasonable agreement with most of the experimental data. Only a few values differ somewhat from the model predictions.

SiO2

Al2O3CaO90

90

90

80

80

80

70

70

70

60

60

60

50

5050

40

40

40 30

30

30

20

20

20

10

10

10

0

0

0

0.2x10 3

0.4x10 3

0.6x1031.0x10 3

1.3x103

1.6x10 3

2.3x10 33.4x10 3

A

BC

D/K

EF

GH

I

J

L

M

N

O

A13 1.0x103

B13 1.1x103

C13 1.1x103

D13 3.0x103 (7% MgO)

E13 2.1x103 (7% MgO)

F13 1.9x103 (7% MgO)

G13 1.8x103 (4% MgO)

H13 1.5x103 (4% MgO)

I13 1.3x103 (4% MgO)

J13 1.1x103 (4% MgO)

K 2.9x103 (7% MgO)

L 1.7x103

M 1.1x103

N 3.8x103

O 2.9x103

Figure 11a: Iso-lines of water capacity at 1873 K in the section of 5 mass% MgO.

The experimental results available in the section are also included.

Chapter 4

27

90

90

90

80

80

80

70

70

60

60

60

70

50

50

50

40

40

40

30

30

30

20

20

20

10

10

10

0

0

0

0.3x10 3

0.4x103

0.5x10 3

0.6x103

0.2x10 3

0.9x10 31.3x10 3

1.5x10 3

1.9x10 3

2.2x103

SiO2

Al2O3CaO

PQ

RS/V

UT

X

Y

P13 1.0x103

Q13 1.0x103

R13 1.2x103

S13 0.9x103

T 1.9x103

U 2.0x103

V 1.0x103

X 1.2x103

Y 1.7x103

Z 2.2x103

Z

Figure 11b: Iso-lines of water capacity at 1873 K in the section of 10 mass% MgO.

The experimental results available in the section are also included.

28

Chapter 5

DISCUSSION

5.1 Laboratory work

Characterization of melting of slags

In many industrial practices synthetic ladle slags are based on the Al2O3-CaO-MgO-SiO2 quaternary system. Usually the compositions of the slags are targeted on the single liquid region of the phase diagram. A comparison of the present results with the phase diagrams suggested by Slag Atlas36 shows that there is considerable disagreement. As an example, Figure 12 compares the present results with the section having 10 % MgO reproduced from Slag Atlas.36 According to the phase diagram all of the investigated samples should have been melted at the experimental temperature. However, the quenched samples 5, 15, 16 and 17 are all multi-phase mixtures. Likewise, samples 2 and 21-23 should have been melted according to the phase diagram in Slag Atlas36, but according to the present investigation these samples consist of multi-phase mixtures. From an industrial point of view this is of importance for instance when evaluating the slag performance with respect to refining capacity and ladle wear also to consider the possibility of having a multi-phase slag.

Chapter 5

29

Figure 12: Investigated compositions in the quaternary phase diagram Al2O3-CaO-MgO-SiO2

with 10 mass% MgO. The closed squares represent compositions of single liquids and the open ones represent the compositions of multi-phase mixtures.

Water vapour solubility

Knowledge of the slag with respect to liquidus temperature is one of the fundamental properties when designing a ladle slag. Nevertheless, its capacities to pick up non-metallic elements, such as hydrogen, as a function of composition and temperature are also of great importance in order to produce clean steel. As described earlier water vapor solubility measurements was performed in order to verify literature data but also for complementary measurements to generate hydroxyl capacity data lacking in the literature. The experimental results for the binary CaO-SiO2 system are included in Figure 13 for comparison with literature data. It is seen in the figure that the present results agree well with the data of Ende et. al.17 Note that even most of the data reported by Iguchi18 follow the same trends as that of Ende et. al.17 and the present study. On the other hand, the values by Wahlster et. al.16 is in controversy with the rest of the other researchers.17-18 Hence, the present experimental results along with values reported by Ende et. Al17 was employed in the optimization of the model parameters.

Discussion

30

Figure 13: Experimental data for the CaO-SiO2 system.

The only experimental data for the water solubilities of CaO-Al2O3-MgO-SiO2 quaternary slags were reported by Jo and Kim13. They determined the water solubilities of a number of slags at 1873 K using an inert gas fusion technique with thermal conductivity detection. The slag compositions of these authors are mostly in the region of relatively high silica contents. Two of the slag compositions of the present work are comparable with the slags studied by Jo and Kim13. A comparison between the water capacities reported by Jo and Kim13 and the present work in the sections of 5 mass% MgO and 10 mass% MgO are presented in Figure 14a and 14b along with the present experimental values. Even the experimental results in the section of 7 mass% MgO are included in Figure 14a for comparison. The values obtained by the two research groups appear to follow the same trends. From the figures there is a larger increase when the slag composition

approaches CaO saturation. A value of 33

OH 102to101C2

prevails in the majority

of the composition range. On the other hand, OH2C can reach a value higher than

3103 , when the slag compositions are close to CaO saturation.

Chapter 5

31

10

20

30

40

50

60

70

80

90

20 10

90

90 304050607080

10

20

30

40

50

60

70

80

0

0

0

SiO2

CaO Al2O

3

Present work (1747-1843 K)

3 2.9x103

5 1.7x103

6 1.1x103

11 3.8x103

12 2.9x103

Jo and Kim (1847 K)

I 1.0x103

II 1.1x103

III 1.1x103

IV 1.9x103

V 2.1x103

VI 3.0x103

1773

1773

I

III

II

IVV

3/VI11

12

5

6

Figure 14a: Comparison of the experimental results between the present study and the study

carried out by Jo and Kim13 in the section of 5 mass% MgO. The values presented are the water capacities (10-3).

10

20

30

40

50

60

70

80

90

20 10

90

90 304050607080

10

20

30

40

50

60

70

80

0

0

0

SiO2

CaO Al2O

3

1773

1773

1773

1A2A

9A

10

8

7

IX

4B/XIIX

VII

Present work (1747-1843 K)

1A 1.9x103

4B 1.0x103

7 1.1x103

8 1.2x103

9A 1.7x103

10 2.2x103

Jo and Kim (1873 K)

VII 1.0x103

IIX 1.0x103

IX 1.2x103

X 0.9x103

Figure 14b: Comparison of the experimental results between the present study and the study

carried out by Jo and Kim13 in the section of 10 mass% MgO. The values presented are the water capacities (10-3).

Hydroxyl capacity model

Some of the experimental values differ somewhat from the model predictions. While the experimental uncertainty caused by the presence of small amount of solid phase could be the reason of the discrepancy, the limitation of the model when

Discussion

32

applied to the region very close to CaO saturation could be another plausible explanation. Nevertheless, the model prediction could be considered satisfactory. The water capacity increases with the increase of the CaO content irrespective of the MgO concentration. The model would be a useful aid to the optimization of ladle slag. As an example of the application, Figure 15 presents the variation of water capacity at 1773 and 1873 K when Al2O3 is replaced by SiO2 addition, assuming that the slag contains 50 mass% CaO and 10 mass% MgO. The plot shows that the ability of the slag to pick up water increases as SiO2 is replaced by Al2O3. The figure also indicates that the temperature has little effect on the water capacity.

0 10 20 30 40

1000

1200

1400

1600

1800

2000

2200

2400

2600

2800

3000

3200

3400

1773 K

1873 K

Cal

cula

ted C

H2O

(pp

mH

2O/

atm

1/

2 )

Mass% Al2O

3

Figure 15: Variation of water capacity at 1773 and 1873 K when Al2O3 is replaced by SiO2 with 50 mass% CaO and 10 mass% MgO.

5.2 Industrial work

The slag sampled from the industrial trials is assumed to be completely melted since the composition and temperature in the ladle corresponds well to the conditions during the laboratory experiments. Also the reduced pressure during vacuum degassing is beneficial for the melting of the slag.

Chapter 5

33

It is possible to calculate the hydroxyl capacity of the slag based on its composition using the model developed in this study for the quaternary slag system Al2O3-CaO-MgO-SiO2. The hydroxyl capacity or water capacity is defined based on reaction (14):

OH2OOH 2

2 (14)

Combining these reactions the water capacity can be defined as:

OH

OH

OH

OH

OHM

MC

p

OppmHC 2

2

2

104

22

(15)

where ppm H2O is the water content in the slag in ppm unite and pH2O is the partial pressure of water. The pick-up of hydrogen of the steel from the slag can be expressed by reaction (16).

OH2OOH2 2 (16)

The underline in the above equation indicates that the element is dissolved in the liquid steel. Note that the reaction takes place in the revised direction, from right to left. The hydrogen exchange between slag and metal is controlled by the partition coefficient (L), equation (15), and the mixing fraction between slag and metal. Since the stirring intensity was kept minimal during the industrial trials, this mixing fraction can be assumed to be approximately the same in different heats and thereby it is the partition coefficient that differs between the melts.

Hwt

OHwtL

%

)%( (17)

In Figure 16, the relative hydrogen increase after vacuum,

RH3 , is plotted as

function of the calculated water capacity.

Discussion

34

Figure 16: The relative hydrogen pick-up of the steel after vacuum treatment as a function of the calculated water capacity for the sampled heats.

It is seen that the hydrogen pick-up in the steel after vacuum treatment increases with the water capacity of the slag. The slag containing most water also is the slag with the highest water capacity. The heat with the highest increase of hydrogen concentration has the slag with the highest content of moisture and the highest water capacity. The heat that does not have any hydrogen increase having the slag with the lowest content of moisture and the lowest water capacity. This implies that the slag composition is a factor that has to be considered The result from this investigation shows that the supply of hydrogen from the slag is superior to the other investigated factors, viz. refractory, heating time after degassing as well as time from degassing to casting. The major hydrogen increase comes within 5-10 minutes which often is below the time for transportation to the caster station. Also the results from the industrial trials reveal that the hydrogen increases after vacuum correlates to the moisture content in the slag rather than to the absolute value after vacuum treatment. After vacuum treatment, the total pressure in the ladle is increased and the argon flow is considerably decreased. In this period, there is no longer a driving force for dehydrogenation. Instead, as long as OH- ions is present in the ladle slag, hydrogen is transferred from the slag back into the steel bath.19 Many ladle slags operates using synthetic slags close to CaO saturation.40-41 Jha et al.11 indicated that the slag basicity should be kept low in order to decrease

Chapter 5

35

hydrogen increase. To examine this aspect, the RH3 is plotted as function of

basicity in Figure 17.

Figure 17: The relative hydrogen pick up of the steel after vacuum treatment as a function of the basicity.

For the slag compositions studied in the trials, there is a week correlation between the hydrogen pick-up and basicity. While the variation of the basicity is small, the water capacity of the slag is significant. This implies that good sulphur refining slag that has high basicity does not have strong impact on hydrogen pick-up. Dor et al. have reported that the water content in the slag after vacuum treatment could be 200 times higher than the predicted value from the metal-slag equilibrium.12 The reason for this is that the dehydrogenation kinetics in the steel is much faster than it is in the slag.19 This study supports their conclusions. Hence, the concentration of hydroxyl ions in the ladle slag can be the major source for hydrogen transferring back into the liquid steel after vacuum degassing. During vacuum degassing, the hydrogen content in the steel is decreased due to high argon flow combined with low pressure. The good contact between the rising argon bubbles and the steel greatly enhances the removal of hydrogen from the liquid steel to the gas bubbles. In this period, an open-eye is formed. Since most of the gas escapes from the bath to the gas phase through the open-eye, the contact between gas and slag is limited. While hydrogen would be transferred from the slag to the liquid metal due to the low hydrogen content in liquid steel, the efficient removal of hydrogen by gas bubbles would still reduce the hydrogen content in the

Discussion

36

metal. Previous studies have shown that the slag phase does not affect the hydrogen removal kinetics during vacuum degassing treatment. Since both steel and slag is protected from the surrounding atmosphere during vacuum degassing, there is no water transferring from the atmosphere to the slag.42 However, after vacuum degassing the slag is not more protected from the surrounding atmosphere. There is a constant supply of hydrogen from the moisture in the air to the steel through the slag layer. If slag having high water capacity is employed, a greater gradient of OH- through the slag would be expected. The big gradient of OH- associated with high water capacity slag would in turn enhance the hydrogen transfer from the surrounding to the steel bath, thereby increasing the hydrogen pick-up of the steel. Further, it can be assumed that the hydrogen increase directly from atmosphere is limited but the basic slag layer acting as transferring agent. It seems that the increase is not substantial when CH2O < 1650. Above this water capacity, the increase becomes dramatic. Since the hydrogen increase after vacuum treatment indicates low correlation to slag basicity, it is possible to have slag beneficial for sulphur refining and still minimize hydrogen increase. When designing the slag composition water capacity should also be considered.

Chapter 5

37

39

Chapter 6

SUMMARY

The main focus of this work was to clarify the importance of hydroxyl ions in ladle slags. For this purpose a number of slag compositions in the quaternary Al2O3-CaO-MgO-SiO2 slag system were examined in order to investigate whether these slags are single liquids or multiphase mixtures at 1858 K. The investigated compositions should all have been liquid according to the phase diagram. Even so it was found that 8 out of 27 compositions consisted of multi-phase mixtures at 1858 K. The majority of the liquid slag compositions were used to experimentally determine the water solubility. It was concluded that the water vapor dissolves as free hydroxyl ions in basic slags. Further it was concluded that temperature have negligible effect on water solubility. Many ladle slags operates using synthetic slags close to CaO saturation. It was found in this work that the water solubility in that composition range increases up to 200 % compared to lower CaO concentrations. The experimental result together with literature data was used to develop a water capacity model. The model was constructed by considering the effects of the binary interactions between the cations in the slag on the capacity of capturing hydroxyl ions. The model was applied on slag samples taken during ladle treatment. During the same industrial trials, steel samples were taken at different stages during the ladle treatment. Further, the water capacity was correlated to moisture content of ladle slag. The slag with the highest concentration of water also had the highest water capacity and associated with the highest hydrogen increase of the steel after vacuum degassing treatment. From these samples, it was found that the increase of hydrogen in liquid steel was dependent on the moisture content in the slag phase. After vacuum degassing there is a constant supply of water to the slag layer. If a slag with high water capacity is used, it could raise the rate of hydrogen transferring

Conclusions

40

back into the liquid steel. It is possible to design a slag with high basicity and low hydrogen capacity. The industrial trials showed a week correlation between the initial hydrogen concentration and the concentration after vacuum degassing. On the other hand it seems that the end concentration of nitrogen is correlated to the initial nitrogen concentration. In this respect, hydrogen and nitrogen behave differently during vacuum degassing.

6.1 Proposal for future work

Investigate nitrogen capacity for ladle slags.

Developing a model for nitrogen capacity and sample industrial heats similar

as in this investigation but for nitrogen instead of hydrogen. Also sample the

tundish slag/mould and investigate the correlation between the hydrogen

increases in the steel to the water capacity of the tundish slag.

Find a optimal slag compositionwith sufficient sulphur refining capacity and

low water capacity and repeat the industrial trials. This can also be carried

out for nitrogen. In order to verify the results similar trials should be carried

out at other plants.

Expand the water capacity model to include other oxides. This means that

more experimental data must be measured on slags with more oxides such as

BaO, B2O3, TiO2 that has in literature reported to have beneficial effect

upon nitrogen refining. This also should be verified with industrial trials with

a special designed slag for maximizing sulphur and nitrogen refining as well

as minimizing hydrogen increase after degassing.

Investigate different alloying elements on hydrogen degassing and hydrogen

increase after vacuum degassing.

41

BIBLIOGRAPHY

1. A. Sandberg: Uddeholm Tooling AB, 683 33 Hagfors, Sweden, private communication, 2005.

2. N. Bannenberg: ‘Demands on refractory material for clean steel production’, Unitec ’95 congress proceedings: Global development of refractories. UNITECR Kyouto, Japan, pp. 36-59, 1995

3. S. Misra, R. P. Stone, M. Kan, R. J. Fruehan, ‘Hydrogen and nitrogen pickup from ladle additions’, Iron and steel technology,2006, 3 (3), pp. 236-245.

4. D. Nolle; U. Euelenberg, A. Jahns, H. Miska, ‘Application of ladle stand degassing in the compound LD Steelmaking and continuous casting process’, Stahl Eisen, 1989, 109, (19), pp. 107-110.

5. E. F. Mazurov, V. V. Shakhnovich, A. F. Kablukovskiy, S. A. Kiseleva, ‘Refining of steel in a 150 ton ASEA-SKF ladle furnace system’, Sov. Mater. Sci. Rev., 1987, 1, (2), pp. 187-191.

6. F. Steiner, ‘Ladle furnace Cum vacuum treatment process for production of High quality steel –some lessons’, Tool Alloy Steel, 1985, 19, (1), pp. 31-37, 26, 12.

7. S. Misra, R. J. Fruehan, ‘Hydrogen and nitrogen control in ladle and casting operations’, Iron&Steel Society international technology conference and exposition 2003; Indianapolis; USA; 27-30 A pr. 2003, pp. 873-882

42

8. M. Hallberg, T L I Jonsson, P G Jönsson, ‘A new approach to using modeling for on-line prediction of sulphur and hydrogen removal during ladle refining’, ISIJ International, 2004, 44, (8), pp. 1318-1327.

9. V. S. Konolov, Y. U. B. Kushnir, Y. U. G. Korinovskii, S. I. Lavrenko, L. M.

Bagaeva, ‘A process for the deoxidation of a wheel steel involving two-stage neutralization of the slag in Furnace and in Ladle’, Metall. Gornorudn. Prom-st. , 1982, 3, pp. 15.

10. A. Jauhiainen, L. Jonsson, P. Jönsson, S. Eriksson, ‘The influence of stirring method on hydrogen removal during ladle treatment’, Steel Research, 2002, 73, (3), pp. 82-90.

11. K. N. Jha, M. K. Sardar, N. N Jha and S. Chakraborty, ‘Hydrogen control during steel making for medium carbon wheels’, Scandinavian Journal of metallurgy, 2003; 32, pp.296-300.

12. P. Dor, B. Carrier, M. Nadif, and C. Gatellier, ‘Influence of slag on steel dehydrogenation under vacuum, Rev. Metall/Cah. Inf. Tech., 1988, 85 (4), pp. 307-16.

13. S-K Jo, S-H Kim: Steel research, 2000, vol. 71, no 1 and 2, pp. 15-21. 14. K. Schwerdtfeger, H. G. Schubert: Metall. Trans. B., 1978, vol. 9B, pp. 143-

144. 15. M. Watanabe, Y. Iguchi, S. Ban-Ya: Tetsu-to-Hagané, 1990, vol. 76, pp. 1672-

1679. 16. V. M. Wahlster, H-H. Reichel: Arch. Eisenhuttenwes., 1969, vol. 40, pp. 19-25. 17. V. H. v. Ende, K. Hagen, v. H. Trenkler: Arch. Eisenhuttenwes., 1969, vol. 40,

pp. 27-36. 18. Y. Iguchi, S. Ban-Ya, T. Fuwa: Trans. ISIJ, 1969. vol. 9, pp. 189-195. 19. T. Rosenqvist: Principles of extractive metallurgy, 2nd ed., McGraw-Hill Book Co.

New York, NY, 1983, pp. 305-306. 20. D. J. Sosinsky, M. Maeda, A. McLean: Metall. Trans. B, 1985, vol. 16B,

pp. 61-66. 21. D. J. Zuliani, M. Iwase, A. Maclean, T. R. Meadowcroft: Can. Metall. Q.,

1981, vol. 20 no.2, pp. 181-187.

43

22. G. Leekes, N. Nowack, F. Schlegelmilch: Steel Research, 1988, vol. 59,

pp. 61-66. 23. Y. Iguchi, T. Fuwa: Transactions ISIJ, 1970, vol. 10, pp. 29-35. 24. J. H. walsh, J. Chipman, T. B. King, N. J. grant: J. Metals. Trans. AIME,1956,

vol. 206, pp. 1568-1576. 25. Y. Yang, Z. A. Daya, I. D. Sommerville, A. MacLean: J. Iron Steel Research Intl.

(China), 1998, vol. 5, no. 2, pp. 7-14. 26. D. Sommerville: Scaninject IV, Part I, Luleå, Sweden, 11-13 June 1986,

MEFOS, 1986, pp. 1-21. 27. S. Ban-Ya, M. Hino, T. Nagasaka: ISIJ International, 1993, vol.33, no. 1, pp.

12-19. 28. Fan, Peng; Cho, W D: AISTech 2005 Volume I&II and ICS 2005

Conference Proceedings; 2005, pp. 23-24,. 29. Fruehan, R J; Scaninject VI, 6th International Cinference in Refining

Processes I; Luleå, Sweden; 1992, pp. 285-316, 30. Sau, R; Ray, A K; Jagannathan, K P; Steel (India), Vol. 21, no. 2, 1998, pp.

61-68. 31. Fan, Peng; Cho, W D; Metallurgical and Materials Transactions B; Vol. 38,

no. 4, 2007, pp. 713-717. 32. Wang, Shi-Jun; Zhou, Yun; Zhang, Hua-Bing; Wang, Hai-Chuan; Huang,

Xiao-Dong; Journal of Iron and Steel Research, Vol. 19, no. 8, 2007, pp. 11-13.

33. Wang, Shi-Jun; Huang, Xiao-Dong; Wang, Hai-Chuan; Wang, Ru-Meng, Li, Yuan; Journal of Anhui University of Technology, Vol. 23, no. 1, 2006, pp. 1-4.

34. Fu, J; Zhou, S; Wang, P; Di, L; Zhu, J; Journal of Materials Science & Technology, Vol. 17; 2001, pp. 233-236.

44

35. Wang, Min; Bao, Yan-Ping; Cui, Heng; Wu, Hua-Jie; Jian-Hua: Journal of University of Science and Technology Beijing. Vol. 32, no. 2, 2010, pp. 175-178.

36. Slag atlas: Verlag Stahleisen GmbH, Dusseldorf, 2nd edition, ed. by Verein

Deutscher Eisenhuttenleute, (1995), p. 156. 37. M. Nzotta: Uddeholm Tooling AB, 683 33 Hagfors, Sweden, private

communication, 2004. 38. J. Björkvall, High Temperature Materials and Processes, 2000, vol. 19, no. 1,

pp. 49-59. 39. M. M. Nzotta, R. Nilsson, D. Sichen, S. Seetharaman, Iron and Steelmaking,

1996, vol. 24, pp. 300-305.

40. K. Beskow, J. Jia, C. H. P. Lupis, D. Sichen: Ironmaking and Steelmaking, 2002, vol. 29, no. 6, pp. 427-435.

41. T. Nagendra, N. Mzelly, D. Sichen: Steel Grips 2, 2004, no. 1, pp. 40-47. 42. J. Brandberg, Tech.Lic, Process model for dehydrogenation during vacuum

degassing, 2006.