I

Wettability and Agglomeration Characteristics of

Non-Metallic Inclusions

Changji Xuan

Doctoral Thesis

Division of Processes

Department of Materials Science and Engineering

School of Industrial Engineering and Management

KTH Royal Institute of Technology

SE-100 44 Stockholm

Sweden

Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i Stockholm,

framlägge för offentlig granskning för avläggande av Teknologie Doktorsexamen.

Onsdagen den 30 Mars 2016, kl. 10.00 i Kollegiesalen, Brinellvägen 8,

Kungliga Tekniska Högskolan, Stockholm

ISBN 978-91-7595-867-5

II

Changji Xuan Wettability and Agglomeration Characteristics of Non-Metallic

Inclusions

Division of Processes

Department of Materials Science and Engineering

School of Industrial Engineering and Management

KTH Royal Institute of Technology

SE-100 44 Stockholm

Sweden

ISBN 978-91-7595-867-5

Copyright © Changji Xuan (玄昌吉), 2016

Tryck: Universitetsservice US AB

III

To my beloved parents and Baihui

送给我亲爱的爸爸妈妈和百慧

IV

Abstract

In this thesis, both the wettability and the agglomeration characteristics of non-metallic

inclusions in liquid iron/steel were studied by using both experimental results and

thermodynamic considerations. The mechanisms of the wettability of different types of

inclusions were discussed. Also, the agglomeration behaviors of the inclusions were analyzed.

Firstly, the wettability of different types of inclusions (including Al2O3, MgO, Ti2O3, TiO2

and TiN) in contact with the liquid iron/steel was studied. For the TiN case, there is no

reaction formation at the interface between TiN and pure iron/steel. In the case of pure Fe, the

oxygen increase is the main factor for a contact angle decrease. As for the steel case, a sharp

decrease of the contact angle is due to the effects of both an increased oxygen content in the

liquid steel and a formation of a Ti(N,C,O) phase at the interface. For the Al2O3 and MgO

cases, the formation of a FeAl2O4 and a MgO-FeO reaction layer at the interface, respectively,

lead to a contact angle decrease. In the case of the Ti2O3/pure Fe case, the reaction at the

interface cannot be identified. For the Ti2O3/steel case, the formation of an Al2TiO5 reaction

layer is the main reason for a steep decrease of the contact angle. In the TiO2 case, the melting

region appears at the temperature below the melting point of pure iron. This is due to the

strong formation of a solid solution TiOx-FeO. The main source of the oxygen for the solid

solution formation is due to a TiO2 substrate decomposition and a low partial pressure of

oxygen in the chamber.

Regarding to the non-metallic particle additions (TiO2 and TiN) into the molten steel, the

steel composition should be controlled to have a small Al content (< 0.005mass%) and a high

Ti content (> 0.035mass%), so as to get a high number of Ti-rich oxide inclusions with a

small size. This conclusion is supported from the view point of the van der Waals force,

liquid-capillary force and wettability.

Regarding the Ti/Al complex deoxidation in the melt, the “mainly occupied clustered

inclusions” with spherical shape is due to a TiOx-FeO liquid inclusion precipitation after an

addition of Ti as a pre-deoxidizer. The much lower cluster number in the Ti/Al case than that

in the Al case is mainly due to a coagulation of single TiOx-FeO liquid inclusions. Also, the

cluster formation in a complex Ti/Al deoxidation is started after an Al addition rather than

after a Ti addition.

Regarding the Al2O3 cluster formation in deoxidation, the cavity bridge force is larger than

the van der Waals force. However, the difference between them is smaller than 7 times. In the

V

reoxidation process, the influence of the cavity bridge force due to the wettability decreased,

and became similar to that of the liquid-capillary force.

Key words: wettability; non-metallic inclusion; agglomeration mechanism; attraction force;

complex deoxidation; particle addition.

VI

Acknowledgement

I am extremely grateful to my supervisor Professor Pär G. Jönsson for his endless support

and kind guidance on my Ph. D study. I appreciate all the research opportunities you created

for me. Your warm care really encourages me a lot.

I would like to say many thanks to my supervisor Professor Keiji Nakajima for his strict

and patient guidance on my research. Your critical altitude helps me develop a strong heart to

face the life. I will never forget the cherished moment when we were struggling on writing

articles all days and nights. I hope I can inherit your spirit about the research: Keep on

opening the scientific eye.

Thanks to my supervisor Docent Andrey V. Karasev. Thank you for bringing me to the

metallurgical research world. Your kind teaching and training are very helpful for inspiring

my self interest in this field.

Thanks to Professor Hiroyuki Shibata and Dr. Sohei Sukenaga at Tohoku University

(IMRAM) for the big support on my sessile drop measurements in Sendai, Japan. Thanks to

Professor Zhe Zhao (KTH) for teaching and helping me perform the Spark Plasma Sintering

experiments. Also thanks to all the team members in NAMOS project for the technical

support and effective discussions. Dr. Wangzhong Mu and Dr. Yanyan Bi are acknowledged

for the discussion on inclusions and thermodynamic calculations. Thanks to my dear friend

Arkadiy Davydenko for big encouragement and help in my daily life in Sweden.

Thanks to all my colleagues in TPM division and MSE department. It is enjoyable to meet

you in Sweden. Thank you for sharing nice memory with you all.

European Commission Research Fund for Coal and Steel (RFCS) and Jernkontoret are

acknowledged for the financial support.

Finally, I hope to say many thanks to my parents and my wife Baihui Sheng. Without your

endless love and accompany I will never perform the successful work.

Changji Xuan, Stockholm, December 2015.

VII

Supplements

Supplement I: Wettability of TiN by Liquid Iron and Steel

Changji Xuan, Hiroyuki Shibata, Zhe Zhao, Pär G. Jönsson, Keiji Nakajima, ISIJ

International, 55 (2015), No. 8, 1642.

Supplement II: Wettability of Al2O3, MgO and Ti2O3 by Liquid Iron and Steel

Changji Xuan, Hiroyuki Shibata, Sohei Sukenaga, Pär G. Jönsson, Keiji Nakajima, ISIJ

International, 55 (2015), No. 9, 1882.

Supplement III: Effect of the Ti, Al Contents on the Inclusion Characteristics in Steels with

TiO2 and TiN Particle Additions

Changji Xuan, Wangzhong Mu, Zuriñe I. Olano, Pär G. Jönsson, Keiji Nakajima, Steel

Research International, 86 (2015), DOI: 10.1002/sirn.201500267.

Supplement IV: Wetting Behavior of Single Crystal TiO2 by Liquid Iron

Changji Xuan, Andrey V. Karasev, Hiroyuki Shibata, Pär G. Jönsson, ISIJ International, 56

(2016), No. 5, accepted.

Supplement V: Evaluation of Agglomeration Mechanisms of Non-metallic Inclusions and

Cluster Characteristics Produced by Ti/Al Complex Deoxidation in Fe-10 mass%Ni Alloy

Changji Xuan, Andrey V. Karasev, Pär G. Jönsson, submitted to ISIJ International, 2016.

Supplement VI: Attraction Force Estimations of Al2O3 Agglomerations in the Melt

Changji Xuan, Andrey V. Karasev, Pär G. Jönsson, to be submitted to Steel Research

International, 2016.

Contribution statement

Supplement I: Literature survey, experimental work and major part of writing.

Supplement II: Literature survey, experimental work and major part of writing.

Supplement III: Literature survey, major part of experimental work and major part of writing.

Supplement IV: Literature survey, experimental work and major part of writing.

Supplement V: Literature survey, major part of experimental work and major part of writing.

Supplement VI: Literature survey, theoretical calculation and major part of writing.

VIII

Parts of the work presented at the following conferences

Effect of the Ti, Al Contents on the Inclusion Characteristics in Steels with TiO2 and TiN

Particle Additions

Changji Xuan, Wangzhong Mu, Zuriñe I. Olano, Pär G. Jönsson, Keiji Nakajima

The 9th International Conference on Clean Steel, Sep. 2015, Budapest, Hungary.

Assesment of Cluster Characteristics in Fe-10%Ni Alloy Deoxidized with M (M= Al, Zr and

Mg) and Ti/M

Andrey V. Karasev, Changji Xuan, Ryo Inoue, Pär G. Jönsson,

The 8th International Conference on Clean Steel, May 2012, Budapest, Hungary.

IX

Contents

Chapter 1 Introduction ......................................................................................................................... 1

1.1 Background ................................................................................................................................... 1

1.2 Objectives and overview of the work ............................................................................................ 3

Chapter 2 Methodology ........................................................................................................................ 5

2.1 Sample preparation ........................................................................................................................ 5

2.1.1 Ceramic substrates making by Spark Plasma Sintering (SPS) ............................................... 5

2.1.2 Preparation of steel samples ................................................................................................... 7

2.2 Contact angle measurement by using sessile drop method ......................................................... 10

2.3 Electrolytic extraction method .................................................................................................... 13

2.4 Theoretical works ........................................................................................................................ 13

2.4.1 Equilibrium calculation of inclusions precipitation .............................................................. 13

2.4.2 Hamaker constant estimation of liquid iron ......................................................................... 13

Chapter 3 Results and Discussion ...................................................................................................... 16

3.1 Wettability of Al2O3, MgO, Ti2O3, TiO2 and TiN ....................................................................... 16

3.1.1 Contact angle analysis .......................................................................................................... 16

3.1.2 Estimation of oxygen partial pressure using H2-O2-H2O system.......................................... 22

3.1.3 Wettability mechanism ......................................................................................................... 23

3.2 TiO2 and TiN particle additions into steel ................................................................................... 36

3.2.1 Typical inclusions characteristic .......................................................................................... 36

3.2.2 Particle size distribution ....................................................................................................... 38

3.2.3 Thermodynamic equilibrium calculations ............................................................................ 40

3.2.4 The effect of attraction forces on particle agglomeration in steel samples .......................... 41

3.3 Agglomeration behavior of inclusions after a complex Ti/Al deoxidation ................................. 45

3.3.1 Typical single inclusion/cluster morphologies ..................................................................... 45

3.3.2 Typical cluster composition in Ti/Al deoxidation ................................................................ 46

3.3.3 Number of clusters ............................................................................................................... 47

3.3.4 Thermodynamic consideration for TiOx-FeO formation in Ti pre-deoxidation ................... 48

3.3.5 Collision rate estimation in Al and Ti/Al deoxidation ......................................................... 51

3.4 Attraction force estimations between Al2O3 inclusions in the melt............................................. 55

3.4.1 van der Waals force of Al2O3 in the melt ............................................................................. 55

3.4.2 Cavity bridge force due to the un-wetting behavior ............................................................. 56

3.4.3 Comparison of different attraction forces for Al2O3 cluster formation ................................. 58

Chapter 4 Conclusions ........................................................................................................................ 60

X

Chapter 5 Future work ....................................................................................................................... 62

References ............................................................................................................................................ 63

1

Chapter 1 Introduction

1.1 Background

It is well known that a control of the non-metallic inclusions in the liquid steel is extremely

important for the steelmaking industry. On one hand, the cluster formation in steel is

considered to be detrimental for both the casting process (clogging) and the mechanical

properties of the final product. For complex deoxidation processes, several studies have been

presented before [1-5]. The particle size, number and particle composition based on different

element addition type, order and amount have systematically been analyzed. Moreover, the

thermodynamic calculations of the prediction of the stable inclusion precipitation have also

been widely studied [6-10].

On the other hand, by using the specific type of inclusions with small sizes, the nucleation

of the intragranular ferrite (IGF) can be favorable according to the oxide metallurgy concept

[11]. Several researches [13-15] have been performed this research by adding effective

particles into the molten steel. Even though the promotion of the effective nucleation has been

proved, a detailed analysis is still needed regarding to the inclusion characteristics after an

addition. It is due to that the controlling of the dispersion potency and the characteristics (e.g.

size, number and composition) of the inclusions can strongly determine the yield ratio of the

addition.

In both the above situations, the study of the inclusions agglomeration behavior in the melt

is the key point. According to the different influencing factors, the agglomeration of the

inclusions can generally be summarized as described below.

Collision-coalescence

The collision-coalescence of the inclusions in the melt can be promoted by Brawnian

collisions (random movements of inclusions in the melt), Stoke collisions (flotation of

inclusions due to the density difference between inclusion and melt) and turbulent collisions

(movement of inclusions along with the melt flow) [16-21]. When the two inclusions have a

small distance to each other (nan-size level), the collision behavior can also be promoted by

the van der Waals force [22-24]. The latter force can affect the agglomeration coefficient of

the inclusions in liquid steel [22]. However, the Hamaker constant of liquid iron needs to be

known so as to obtain van der Waals force. Mizoguchi et al. [22], Sasai et al. [23] and

Taniguchi et al. [25] used the solid iron Hamaker constant (= 21.2x10-20

J [26]) as an

approximation for a liquid iron medium. But it is unclear whether the Hamaker constant of

2

solid iron can represent that of liquid iron or not. Furthermore, Lin et al. [27] reported the

Hamaker constant of liquid iron caltulation by using Fowkes module [28]. However, in their

estimation only the adhesion work of Al2O3 in liquid iron and AAl2O3 were selected for the

calculation. Also, due to the scattered Hamaker constant values for different oxide inclusions

(e.g.: AAl2O3=15.5 [29], AMgO=10.6 [29]), the deviation of this estimation method is relatively

high. It means that a detailed study on the estimation of the Hamaker constant of liquid iron is

still needed.

Wettability

After the collisions of inclusions occurs, the wettability of the inclusions in contact with

the melt can affect the agglomeration degree. For a strong wetting case (a contact angle < 90

degrees), the agglomeration is not favorable even though the collision might take place.

However, for un-wetting case (a contact angle > 90 degrees), the agglomeration is favorable

due to the wettability attraction force. For an un-wetting case (contact angle > 90 degrees), the

agglomeration behavior is promoted by the cavity bridge force [23-24]. As one of the most

important factors for an agglomeration, the wettability results of oxide inclusions such as

Al2O3 and ZrO2 have already been represented by many researchers [30-37]. However few

experiments were performed for other inclusion types. Even though Shibata et al. [36] and

Ogino et al. [35] reported results for MgO, the mechanism of its wetting behavior has not

been clearly discussed. As for the other typical particles such as Ti2O3, TiO2 and TiN, very

few wettability experiments have been performed before. Meanwhile the explanations about

the wetting mechanism of these different materials have not been systematically discussed. It

seems that an establishment of a clear picture about the wetting behaviors and the

mechanisms of the wettability for different type of inclusions are quite necessary.

Solid phase sintering

When collision happens, the solid phase sintering behavior of the inclusions at the

contacting region can enhance the agglomeration process. The solid phase sintering can be

described by the apparent self-diffusion coefficient, Dv. Previously Coble et al. [38] and

Kingery et al. [39] have systematically represented the self-diffusion coefficient of Al2O3

inclusions. The Al2O3 medium of the system is a protection atmosphere. Moreover, Ooi et al.

[40] measured the Dv values for Al2O3 clusters in liquid iron. Also, Sasai et al. [23] estimated

the FeAl2O4 solid bridge sintering effect in a re-oxidation process for an Al2O3 cluster

formation case.

3

To sum up, it is clear that a systematical study on how these above mentioned factors

influence agglomeration is quite significant. Specifically, a quantitative study of the

agglomeration behavior and mechanism of the inclusions in liquid steel is desired.

1.2 Objectives and overview of the work

In this thesis, firstly the continuous wetting behaviors of different types of substrates were

measured. Based on both experimental results and thermodynamic considerations, the

mechanisms of the wettability of different type inclusions were discussed. Secondly,

according to the investigations of the inclusions characteristics (e.g. size, number,

composition and morphology), the agglomeration behaviors of the inclusions were analyzed.

The mechanisms of the agglomeration phenomenon were explained based on both

experimental studies and theoretical studies. The outline of the supplements in this thesis is

summarized as shown in Fig. 1-1.

Fig. 1-1 The outline of each supplement in the present thesis.

According to Fig. 1-1, it can be seen that the content of this thesis is divided into four parts

so as to study the agglomeration behavior of different types of inclusions in the liquid metal.

4

Part 1 Wettability

The first part corresponds to Supplement I [41], Supplement II [32] and Supplement IV[42].

The wettability of such different inclusions as Al2O3, MgO, Ti2O3, TiO2 and TiN were studied

experimentally. The wetting mechanisms were discussed based on both experimental results

and thermodynamic considerations.

Part 2 Particle addition into molten steel

The second part of this thesis corresponds to Supplement III [43]. The characteristics of the

inclusions (size, number and composition) after TiN and TiO2 particle addition into the steel

were analyzed. Regarding to the particle size distribution results in different steel samples, the

agglomeration degree of inclusions was discussed based on the van der Waals force, liquid-

capillary force and wettability.

Part 3 Deoxidation

The third part of this thesis, which corresponds to Supplement V [44], shows an analysis of

the agglomeration behaviors of the inclusions in Fe-10 mass%Ni alloy by a complex Ti/Al

deoxidation method. An explanation of the agglomeration mechanism in a complex Ti/Al

deoxidation was pointed out.

Part 4 Deoxidation

The fourth part is related to Supplement VI [45]. The main study is focused on the

theoretical estimation of attraction forces (van der Waals force, cavity bridge force) for Al2O3

agglomeration in the melt. The effect of different attraction forces on the Al2O3 agglomeration

was compared.

5

Chapter 2 Methodology

2.1 Sample preparation

2.1.1 Ceramic substrates making by Spark Plasma Sintering (SPS)

The SPS (Spark Plasma Sintering) apparatus (SPS-2050, Sumitomo Coal Mining Co. Ltd.,

Japan) used in the present work is shown in Fig. 2-1. It includes a uniaxial pressure device in

which the water-cooled punches also work as electrodes. Moreover, it includes a water-cooled

reaction chamber which can be evacuated, a pulsed DC generator, as well as a system

controlling the pressure, temperature and punch position.

The prepared powders (TiN and Ti2O3) with prescribed sizes had weights ranging between

2 and 3g. Firstly, the powder was filled into a graphite die (outside diameter, 30.2mm; inside

diameter, 15mm; height, 30.2mm). Afterwards, the graphite die was sintered in the SPS

equipment under a ~5Pa vacuum pressure. Here, the graphite die inner region was separated

by using as graphite paper with a thickness of 150µm.

The uniaxial pressure program for the sintering procedure is represented as is shown in Fig.

2-2. A prescribed uniaxial pressure was applied during the sintering procedure. Afterwards, at

the end of sintering procedure the pressure was released. The temperature program in the

sintering process is reported in Fig. 2-2 as well. Prescribed holding temperatures of 1573-

1873K and holding times of 5min were applied. The temperature of the die was monitored

and regulated by an optical pyrometer. The heating rate during the sintering is about

100K/min. When the sintering is finished, the pulse current was shut off and the pellet was

cooled with liquid N2. The relative densities of the substrates were measured by using the

Archimedes immersion method and using a 99.9% methanol solution. The surface roughness

was measured with a surface roughness apparatus (Talysurf Ultra precision measurement

system FTS PGI 800).

6

Fig. 2-1. Basic configuration of a SPS apparatus.

Type: SPS-2050 Max. Pressure: 210MPa Max. Temperature: 2473K Max. Current: 5000A

7

Fig. 2-2. Illustration of the SPS sintering conditions (a Uniaxial pressure program and a

temperature program).

2.1.2 Preparation of steel samples

2.1.2.1 Ingot making with TiN and TiO2 particle addition

Four types of 35kg ingots were prepared and they were denoted “Eref”, “ETiO2”, “ETiN”,

and “EwTiO2”. The raw materials were melted in a crucible and poured into a metal mold

using an induction furnace. For the “Eref” sample (without an addition of powder), the

holding and pouring temperatures were controlled to be 1843 K. For the “ETiO2” and “ETiN”

samples with TiO2 and TiN particle additions, respectively, the holding and pouring

temperatures were set as 1873 K. The sample preparation procedure is shown in Fig. 2-3 (a).

Overall, 20 pellets (100g+8mass% TiO2/TiN powder) were added into the crucible. In

addition, 2 more pellets were placed inside the mold. The pellet-packets addition was divided

into three times and the interval time for each addition was 4min. At a time of 7min after a

final pellet-packets addition, Al and FeS were added into the melt as well. Then, the melt was

poured into the mold.

As for the “EwTiO2” sample where TiO2 particles were added, the holding and pouring

temperatures were controlled to have a value of 1923 K, as is shown in Fig. 2-3 (b). It can be

seen that the wires (876g) with a 20mass% TiO2 powder were placed inside the crucible. The

wire-packets additions were divided into four times and the addition interval time was 4min.

Fu

rna

ce

co

ntr

ol te

mp

era

ture

[K

]

Pre

ssu

re f

or

pre

ssio

ng

po

wd

er

[MP

a]

0 1.5 2 4.5

Time [min]

3min

Liquid N2

cooling

Holding time:

Prescribed holding

100K/min

temperature

873K

Prescribed Pressure 89.2 MPa

1573-1873K

5min

8

Also, Al and FeS were added into the liquid steel at 7min after the final wire-packet addition

had been made. Finally, the ingot was forged and normalized in order to obtain a homogenous

composition. The chemical compositions of these four samples are shown in Table 2-1.

(a) Information on the Pellet-packets addition and the placement of the pellet-packets inside

the mold.

(b) Wire-packets addition

Fig. 2-3. Schematic diagram of the melting process including the different particle addition

times.

1st 2nd 3rd

Pellet-packets

(total: 20 pellets)Various

alloys

4min 4min 7min

Electriciron

Pellet-packets(total: 2 pellets)inside mold

15min

1873-1883K

Al, FeS

Pouring

1st 2nd 3rd

Wire-packetsVarious

alloys

Electriciron

1923K

Al, FeS

Pouring

4th

4min 4min 4min 7min

19min

9

Table 2-1. Chemical composition of samples from the four trials

Sample [mass%] [ppm]

C Mn Si P S Cr Ni Mo V Cu Al Ti O N

Eref 0.28 1.43 0.69 0.014 0.044 0.15 0.12 0.041 0.097 0.15 0.008 0.011 27 133

ETiO2 0.28 1.24 0.67 0.015 0.035 0.15 0.16 0.045 0.096 0.14 0.005 0.025 75 133

EwTiO2 0.32 1.32 0.72 0.016 0.046 0.11 0.13 0.038 0.093 0.16 0.008 0.026 61 146

ETiN 0.29 1.34 0.68 0.007 0.042 0.11 0.13 0.038 0.086 0.15 0.007 0.034 28 229

2.1.2.2 Deoxidation of Fe-10mass%Ni alloys by Ti, Al and Ti/Al

The deoxidation experiments were carried out by a charging Fe-10mass%Ni alloys (~160g)

into a high-frequency induction furnace under an Ar protection. A praphite susceptor (wall

thickness is equal to 10mm) was installed between the crucible and induction coil so as to

avoid an induction stirring of the melt. After a holding time of 20min at a temperature of

1873K, the composition of the melt became homogeneous. Also, high purity Al2O3 crucibles

were used in the present work. The sampling procedures for Ti, Al and Ti/Al deoxidations are

schematically shown in Fig. 2-4. In case of 0.03 mass%Ti deoxidation in Fig. 2-4 (a), the

melt was deoxidized with Ti. Thereafter, the melt was mechanically stirred for 10s by using

an Al2O3 rod. After that, the melt was sampled with a quartz tube (QT) after holding times of

1 and 5min. Then, in order to preserve the inclusion characteristics a water quenching was

performed for the quartz tube. As for 0.06 mass%Al deoxidation in Fig. 2-4 (b), the

procedures were almost same as that in the Ti deoxidation experiments. After a holding time

of 10 min, the melt was cooled down from 1873 to 1473 K followed by water quenching. The

ingot sample (IC) was cut off from a central vertical slice. For the complex Ti/Al deoxidation

in Fig. 2-4 (c), the melt was firstly pre-deoxidized with a 0.03%Ti addition followed by 10s of

stirring. After a holding time of 1min at a temperature 1873K, 0.06%Al was added and then

the melt was stirred for 10s. The samples were taken at holding times of 1, 5 and 10min. A

small piece of sample was also cut off from the ingot sample (IC). The basic conditions of the

different deoxidations are listed in Table 2-2.

(a)

10

Fig. 2-4. Schematic illustration of (a) Ti, (b) Al and (c) Ti/Al deoxidation experiments.

Table 2-2. Main conditions in the Ti, Al and Ti/Al deoxidation experiment

Exp. No.

Deoxidation

First

addition

[mass%]

Second

addition

[mass%]

Sampling

Holding

Time

[min]

1 Ti 0.03%Ti - QT-1 1

QT-5 5

2

Al

0.06%Al

-

QT-1 1

QT-5 5

IC-15 15

3

Ti/Al

0.03%Ti

0.06%Al

QT-1 1

QT-5 5

QT-10 10

IC-15 15

2.2 Contact angle measurement by using sessile drop method

The basic conditions of the substrates and the chemical compositions of the pure iron and

steel materials are shown in Tables 2-3 and 2-4. The metal sample was placed on the

substrate, which in turn as placed on a platinum plate, as shown in Fig. 2-5. The temperature

was measured by using a thermocouple welded on the platinum plate back. Afterwards, the

sample was heated up at a heating rate of 100K/min and using an Ar protection atmosphere.

The sample was maintained at around 1815K for a prescribed time and then it was cooled

(b)

(c)

11

using a cooling rate of 20K/min. The oxygen partial pressure of the inlet gas was maintained

at a value between 10-20

and 10-23

atm. A gas purification of the gas was done before it entered

the inlet. During an experiments, the contact angle was measured after the metal sample was

melted. Also, the temperature and oxygen partial pressures were continuously recorded by

using a digital data logger. After a temperature of 1623K was achieved, the digital image was

continuously captured each few seconds during the entire experiment.

Table 2-3. Chemical composition of the pure Fe and carbon steel samples.

Pure Iron /steel Composition

Pure Fe [mass ppm]

O N H S P Mn C Al

67 7 5 2 - - 8 -

Cr Cu Si Mo Cl Ni V Ti

- 14 5 - 40 - - -

Carbon steel [mass %]

O N H S P Mn C Al

0.0015 0.0126 - 0.042 0.012 1.43 0.28 0.013

Cr Cu Si Mo Cl Ni V Ti

0.18 - 0.65 0.037 - 0.13 0.084 0.022

Table 2-4. Basic conditions of the substrates used for the contact angle measurements

Substrate

type

Making method Size: dia. x height

[mm]

Surface roughness, Ra [nm]

Al2O3 SC 10 x 1 ~5

MgO (100) SC 10 x 1 ~5

Ti2O3 SPS 15 x 2 ~3µm

TiO2 (100) SC 10 x 1 ~5

TiN SPS 15 x 2 <200

*SC: Single crystal, SPS: Spark plasma sintering.

12

Fig. 2-5. Schematic diagram of the contact angle measurement apparatus.

13

2.3 Electrolytic extraction method

The sample was dissolved by using the potentiostatic electrolytic extraction (EE) method.

A 10%AA electrolyte (10v/v% acetylacetone-1w/v% tetramethylammonium chloride-

methanol) was selected for the extraction. Also, the extraction conditions were set as follows:

a 3-4v voltage, ~60mA current and a 500-1000 coulomb. The dissolved weight varied from

0.12 to 0.2g. After the extraction, a polycarbonate (PC) membrane film filter (open pore size:

0.05µm and 0.4µm) was used to collect the undissolved inclusions during a filtration.

Thereafter, the characteristics of the inclusions were investigated by using a scanning electron

microscope (SEM). The number of inclusions per volume, Nv, was determined as follows.

The metal volume, Vmetal, which corresponds to the region measured on a film filter is given

by Eq. (2-1) [5].

f

obs

Fe

metalA

AW

V (2-1)

where ∆W is the dissolved sample weight, ρFe is the metal density (0.0078gmm-3

), Af is the

area of a film filter (1411.242mm2: 42.4mm in diameter) and Aobs is the observation area. The

Nv value is expressed by using the total observed number of the inclusions, n, as shown in Eq.

(2-2) [5].

metal

VV

nN (2-2)

where Vmetal is the volume of the dissolved sample. The morphology (such as circularity) and

size of the inclusions were measured by using the image software WinROOF®.

2.4 Theoretical works

2.4.1 Equilibrium calculation of inclusions precipitation

Koseki et al. [46] and Ichikawa et al. [47] have verified the equilibrium by using Thermo-

Calc to predict the inclusions characteristics for a wide composition range. In the present

study, the Thermo-Calc.3.0.1 version and the TCFE7 database for the calculations.

2.4.2 Hamaker constant estimation of liquid iron

Based on Fowkes module [28], the interfacial tension between a solid and a liquid phase

can be described in Eq. (2-3), and Young’s equation is shown in Eq. (2-4):

𝛾𝑙𝑠 = 𝛾𝑙 + 𝛾𝑠 − 2√𝛾𝑙𝑑𝛾𝑠

𝑑 (2-3)

14

𝛾𝑙𝑠 = 𝛾𝑠 − 𝛾𝑙 ∙ 𝑐𝑜𝑠𝜃 (2-4)

where γls is the interfacial tension between the solid and liquid, γs and γl are the surface tension

of the solid and liquid, respectively. The parameters γsd and γl

d are the London dispersion

force contribution to the surface tension for the solid and liquid, respectively. Also, Ɵ is the

contact angle. By combining Eq. (2-3) and (2-4), γsd can be described as shown in Eq. (2-5).

𝛾𝑠𝑑 = 𝛾𝑙

2(1 + 𝑐𝑜𝑠𝜃)2/4𝛾𝑙𝑑 (2-5)

For solid particles in H2O (T=293 K) and for the liquid iron system (T=1873 K), Eq. (2-5) can

be expressed as shown in Eq. (2-6) and (2-7), respectively.

𝛾𝑠𝑑 = 𝛾H2O

2(1 + 𝑐𝑜𝑠𝜃𝐻2𝑂/𝑠)2/4𝛾H2O

𝑑 (2-6)

𝛾𝑠𝑑 = 𝛾Fe(l)

2(1 + 𝑐𝑜𝑠𝜃𝐹𝑒(𝑙)/𝑠)2/4𝛾Fe(l)

𝑑 (2-7)

where γH2O is the surface tension of H2O at room temperature (=0.0728 N/m [48]), and γFe(l) is

the surface tension of liquid iron (T=1873 K). The latter can be obtained by using a function

suggested by Takiuchi et al. [49] in Eq. (2-8):

𝛾Fe(l) = 1.90 − 0.327ln(1 + 96𝑎𝑂) [N/m] (2-8)

The London dispersion force contribution to the surface tension for H2O, γH2Od is equal to

0.0218 N/m [48]. The Hamaker constant of substance “1” in vacuum can be described by Eq.

(2-9) [28].

𝐴11 = 6𝜋2𝑑12𝛾1

𝑑 (2-9)

where, d1 is the interfacial separation of the atomic centers at contact. The value of d is equal

to 4.3x10-10

m [50] for H2O (T=293 K), 4.0x10-10

m [50] for inorganic materials (T=293 K),

and 2.58x10-10

m [51] for liquid iron (T=1873 K), respectively. Due to the high melting point

of the ceramic particles (e.g. Al2O3), the A11 value at room temperature can be equal to that at

high temperatures [27]. Then, the value of γsd (T=293 K) is equal to that of γs

d (T=1873 K)

according to Eq. (2-5). Thus, Eq. (2-6) and (2-7) can be combined as is shown in Eq. (2-10).

𝛾Fe(l)𝑑 =

𝛾𝐻2𝑂𝑑

𝛾𝐻2𝑂2 ∙

(1+𝑐𝑜𝑠𝜃𝐹𝑒(𝑙)/𝑠)2

(1+𝑐𝑜𝑠𝜃𝐻2𝑂/𝑠)2 𝛾Fe(l)

2 = 4.1133(1+𝑐𝑜𝑠𝜃𝐹𝑒(𝑙)/𝑠)

2

(1+𝑐𝑜𝑠𝜃𝐻2𝑂/𝑠)2 𝛾Fe(l)

2 (2-10)

It can be seen that in order to calculate γFe(l)d an approximate value of the term (1+cosƟFe(l)/s)

2

/ (1+ cosƟH2O/s) 2

is needed. Table 2-5 shows the contact angle results of different

oxide/nitride ceramic materials in contact with H2O (T=293 K) and liquid iron (T=1813-1873

K). According to the results in Table 2-5, the average value of the term (1+cosƟFe(l)/s)2 / (1+

15

cosƟH2O/s)2 is equal to 0.051±0.008. After combining Eq. (2-9) and (2-10), the Hamaker

constant of liquid iron, A11 at a 1873 K temperature can be written as shown in Eq. (2-11).

𝐴11 = (82.7 ± 12.7) ∙ 𝛾Fe(l)2 (2-11)

For Al-killed steels with an oxygen content of 60ppm, the liquid iron surface tension can be

determined to be 1.751 N/m [27] by using Eq. (2-8). Consequently, the Hamaker constant of

liquid iron can be determined to have a value of 25.3x10-19

J by using Eq. (2-11).

Table 2-5. Contact angle values of various oxide/nitride substrates in contact with H2O/liquid

iron.

Material

ƟFe(l)/s [degree] ƟH2O/s in room temperature [degree] (1+ cosƟ Fe(l)/s)2/

(1+cosƟH2O/s) 2 Aver T [K] Aver

MgO

130 30)

126.5

- 35 54)

32.5

0.048 128 37)

1823

30 55)

123 33)

1873

125 37)

1873

Al2O3 132 35)

132 1873 65.8 56)

51.4 0.042

132 34)

1873 37 57)

Ti2O3 127.9 32)

123.5 1823 18 58)

18 0.053

119 30)

1813-1873

ZrO2

119 37)

127

1823 56.8 59)

56

0.065 122 37)

1823 50 60)

140 52)

1873 50 61)

67 62)

TiN

132 53)

128.9

1823 30.7 63)

45.9

0.048 125 53)

1823

132.8 41)

1813-1873 61 64)

125.7 41)

1813-1873

ZrN 140 53)

140 1823 88.5 63)

88.5 0.052

16

Chapter 3 Results and Discussion

3.1 Wettability of Al2O3, MgO, Ti2O3, TiO2 and TiN in contact with iron/steel

3.1.1 Contact angle analysis

(1) Al2O3

Fig. 3-1 shows the contact angle of Al2O3 in contact with pure iron as a function of the

time. It was found that the contact angle decreases slightly from 105.1 to 103.6 degrees during

the first 90s after a full melting has been obtained. After the initial melting, the temperature

was stable up to 720s, with a corresponding contact angle value of 103 degrees. Thereafter, a

quenching was performed at 860s. The contact angle results in the present work were found to

closely fit the results of Shibata et al. [36]. Specifically, the difference is smaller than 2%.

Fig. 3-1. Contact angle of Al2O3/pure Fe as a function of time after full melting.

(2) MgO

In the MgO/pure Fe case, the contact angle first decreased from 99.2 to 90.0 degrees

during the first ~120s of the experiment, as is shown in Fig. 3-2. Thereafter, the value was

kept constant up to a time of 600s. During these experiments, the melt was quenched after

900s. The present result is consistent with the reporting data of Shibata et al. [36]. Specifically,

the difference is smaller than 4%.

0

50

100

150

0 200 400 600 800 1000

Data 1

Shibatat [1]-11-20Shibata [1]-1012Shibata [1]-0001oginiol[3]Al2O3-Angle

Con

tact a

ngle

[d

eg

ree

]

Time [s]

MarkAtmosphere(P inlet)

Liquid metalmelting

point [ C]

MeasuringTemp. [ C]

Ceramicmaterial

Experiments

H-1600Sinteredmaterial

Ogino et al.(1973) [3]

(0001)

-1600(1012)

(1120)

1536.51540Singlecrystal

This work trial412

Shibata et al.(2009) [2]

Singlecrystal

Ar (10-19

)

2

Ar (10-21

)

oo 2O

Quenchedt=860s

Present worktrial412

Exp.

Ogino [33]

Sub-

Sintered

SCShibata [36]

Trial 412

1873

Temp.

1873

1815 1809.5

point [K]Melting

H2

Atmo-

Ar

Mark

Ar

[K]strate

SC

sphere

17

In addition, it can be seen that the contact angle values (t=0s) of Ogino et al. [33]

obtained for a sintered substrate (132 degrees for Al2O3 and 121 degrees for MgO) are larger

than those found in the present work (105.1 degrees for Al2O3 and 99.2 degrees for MgO).

This difference is probably due to the different surface roughness of the substrates and the

different oxygen partial pressures in the chamber in the two experimental setups.

Fig. 3-2. Contact angle values of MgO/pure Fe as a function of time after a full melting has

been reached.

(3) Ti2O3

Fig. 3-3 shows the contact angle values for a Ti2O3/pure Fe system. It was found that the

values decreased from around 128 to 121.0 degrees during the experiment. Overall, the

contact angle value in the present work is similar to that of Humenik et al. [30], who reported

a value of 119 degrees (t=0s). In the steel case (Fig. 3-4), the contact angle value decreased

sharply from 143.8 to 95.4 degrees during the first 173s of the experiments after a full melting

had been reached. Afterwards, the contact angle had a constant value of 90 degrees up to a

time of 703s.

0

50

100

150

0 200 400 600 800 1000

Data 4 22:16:33 2014-01-10MgO-413Hument [6]Ogino [3]shibata [2]Shibata [1]

Time [s]

Con

tact a

ngle

[d

eg

ree

]

MarkAtmosphere(P inlet)

Liquid metalmelting

point [ C]

MeasuringTemp. [ C]

Ceramicmaterial

Experiments

--1550Sinteredmaterial

Humenik et al.(1953) [1]

H-1600Sinteredmaterial

Ogino et al.(1973) [3]

-1600Singlecrystal

Shibata et al.(100) (2003) [4]

-1600Singlecrystal

Shibata et al.(100) (2009) [2]

1540.81540Singlecrystal

Present work(100) trial413

Shibata [1](2009) Single

crystal

Ar (10-18

)

2

Ar (10-19

)

oo 2O

Ar (1.4 10-18

)

Quenchedt=900s

Exp.Sub- Temp. Melting

point [K]Atmo-

Mark

Humenik[30]

Ogino [33]

Shibata[65]

Shibata[36]

Trial 413

Sintered

Sintered

SC

1823

1873

1873

1873

1816 1813.8

Ar

H2

Ar

Ar

[K]strate sphere

SC

SC

18

Fig. 3-3. Contact angle values of Ti2O3/pure Fe as a function of time after a full melting has

been reached.

Fig. 3-4. Contact angle values of Ti2O3/steel a function of time after a full melting has been

reached.

0

50

100

150

0 200 400 600 800 1000

Data 30

Ti2O3-Fe-418Ti2O3-Fe-502Humen [6]

Time [s]

Con

tact a

ngle

[d

eg

ree

]

MarkAtmosphere(P inlet)

Liquid metalmelting

point [ C]

MeasuringTemp. [ C]

Ceramicmaterial

Experiments

Vacuum-1550Sinteredmaterial

Humenik et al.(1953) [1]

Ar (7.3 10-21

)1541.81540Trial418

Ar (6.6 10-21

)1537.21540Trial502

oo 2O

Sinteredmaterial

Presentwork

Quenched

1823K Vacuum

t=906s

t=860sQuenched

Exp.Sub- Temp. Melting

point [K]Atmo-

Mark

Trial 418

Trial 502

Humenik[30] Sintered 1823

18161815

1816.8

1810.2

Vacuum

Ar

(Trial 418)

(Trial 502)

Ar

[K]strate

Sintered

sphere

0

50

100

150

0 200 400 600 800 1000

Data 33

Ti2O3-steel-1

Con

tact a

ngle

[d

eg

ree

]

Time [s]

MarkAtmosphere(P inlet)

Liquid metalmelting

point [ C]

MeasuringTemp. [ C]

Ceramicmaterial

Experiments

Ar (3.3 10-21

)1482.51540Sinteredmaterial

Present work Trial512

oo 2O

Quenchedt=943s

Exp. Sub-[K]

Meltingpoint [K]

Atmo-Mark

Trial 512 Sintered 1815 1765 Ar

Temp.strate sphere

19

(4) TiO2

Table 3-1 shows the dynamic change of the wetting process for the pure iron cylinder in

contact with a single crystal TiO2 substrate. It was found that a small melting region started to

appear at a temperature of 1739 K. Therefore, this moment was selected as the initial time (t=

0 s) for the observation. When the temperature reached a value of 1813 K (t= 45 s), the iron

specimen was fully melted and had obtained a strong wetting when being in contact with the

substrate surface. During the following 30 seconds at a temperature of 1813 K, the droplet

was continuously spread on the substrate surface, as illustrated in Table 3-1.

Table 3-1. Dynamic observations of the wetting behavior of a pure Fe droplet on TiO2

substrate

T= 1739 K; t= 0s

T= 1769 K; t= 20s

T= 1791 K; t= 35s

T= 1813 K; t= 45s

T= 1813 K; t= 65s

T= 1813 K; t= 75s

The measured temperatures and contact angles are shown as a function of the holding time

in Fig. 3-5. The wetting contact angle decreased from 45 to 37 degrees during 45 s. When the

full melting stage was achieved at 1813 K, the value continuously decreased from 37 to 30.5

degrees within 30 s at the given temperature. The strong wetting behavior for the TiO2

substrate observed in the present work is consistent with that of Humenik et al. [30] and

Amondarain et al. [31]. Specifically, Amondarain et al. [31] reported that the value of the

contact angle between TiO2 and liquid iron can significantly decrease with an increasing

holding time at a constant temperature.

20

Fig. 3-5. Contact angle values of TiO2/pure Fe a function of time.

(5) TiN

In case of TiN/pure Fe (Fig. 3-6), different holding times (t=150s and 900s) were selected

in the present work. The contact angle has a constant value of ~130degrees during the first

150s. After that, the contact angle gradually decreases and reaches a value of 87.9 degrees at a

holding time of 900s. The contact angle results in the present work is quite close to these of

Chuchmarev et al. [53] using a NH3 atmosphere, who measured a value of 132 degrees. The

contact angle values reported by Amadeh et al. [66] (125120degrees for 1000s at 1823K;

11085degrees for 1000s at 1843K) partially corresponded to the present results.

For the TiN/steel case, the contact angle decreased sharply from a value of ~110 to 76

degrees during the first 50s, as seen in Fig. 3-7. Afterwards, the contact angle value decreases

gradually with time. Finally, the contact angle reaches a value of 50 degrees at a holding time

of 981s. These results were a little bit smaller than the value reported by Chuchmarev et al.

[53], who measured a value of 122 degrees. In addition the contact angle values reported by

Amadeh et al. [66] at a 1843 K temperature can fit the final stage of the present results.

0

10

20

30

40

50

60

70

80

90

1700

1750

1800

1850

0 10 20 30 40 50 60 70 80

TiO2-Fe-414

Temperature

Time [s]

Te

mp

era

ture

, T

[K

]

Con

tact

an

gle

, [d

egre

e] Contact angle

Quenched

Full melting

Temperature

21

Fig. 3-6. Contact angle values of TiN/pure Fe as a function of time after a full melting has

been reached.

Fig. 3-7. Contact angle values of TiN/steel as a function of time after a full melting has been

reached.

0

50

100

150

0 200 400 600 800 1000

Data 3

TiN-504FeTiN 507FeChuvu [5]

Time [s]

Con

tact a

ngle

[d

eg

ree

]

Quenched 1843K

1823K

Quenched

MarkAtmosphere(P inlet)

Liquid metalmelting

point [ C]

MeasuringTemp. [ C]

Ceramicmaterial

Experiments

NH-1500-1550Sinteredmaterial

Chuchmarev [5](1967)

Ar-1550, 1570Sinteredmaterial

Amadeh et al.(2001) [6]

Ar (5.4 10-21

)15401540Trial504

Ar (2.6 10-21

)15391540Trial507

Ar (10-18

)

3

Ar (10-19

)

oo 2O

Ar (1.4 10-18

)

Sinteredmaterial

t=900st=150s

Chuch- [53]

Atmo-Mark

Meltingpoint [K]

Sub-

Amadeh [66]

NH3

Ar

Ar

Ar1812

1813

1773-1823

Temp.

1823,1843

1815

1814

Trial 504

Trial 507

Exp. spherestrate [K]

Sintered

Sintered

Sintered

marev

MarkAtmosphere(P inlet)

Liquid metalmelting

point [ C]

MeasuringTemp. [ C]

Ceramicmaterial

Experiments

NH-1500-1550Sinteredmaterial

Chuchmarev [5](1967)

Ar-1550, 1570Sinteredmaterial

Amadeh et al. [6](2001)

Ar (5.6 10-21

)1467.61540Trial511

Ar (9.3 10-21

)1423.81540Trial513

0

50

100

150

0 200 400 600 800 1000

Data 6

TiN-511steelTiN-513steelChu [5]

Time [s]

Con

tact a

ngle

[d

eg

ree

] 3

oo 2O

Sinteredmaterial

1843K

Quenched

1823K

Quenchedt=298s

t=981s

marev

Exp. Sub- MarkAtmo-Melting

point [K]

NH3

Ar

Ar

Ar1697

1741

1773-1823

[K]Temp.

1823,1843

1816

1815

Trial 511

Trial 513

Sintered

Sintered

Sintered

sphereChuch- [53]

strate

Amadeh [66]

22

3.1.2 Estimation of the oxygen partial pressure using H2-O2-H2O system

Although the argon gas purity is as extremely high as 99.9996% it still contains small

amounts of impurities such as H2O and O2. The oxygen partial pressure in the chamber can be

estimated by using the H2-O2-H2O equilibrium system. The chemical reaction of the H2O

formation is shown below [67].

O(g)2H)g(O)g(2H 222

109.88T-493070G o (J/mol) (3-1)

22

2

22

2

2

2

2

2

KOH

OH

OH

OH

PP

P

aa

a

(3-2)

where T is the temperature. The working temperature of the inlet oxygen sensor is 923K. The

ratio of PH2O and PH2 at 923K can be assumed to be constant, and to be independent of the

temperature. Thus, the PO2 value at desired temperature can be obtained by using Eq. (3-3).

2

9232

2

2

1

KH

OH

T

OP

P

KP

(3-3)

where KT is the equilibrium constant at the desired temperature, PH2O is the partial pressure of

H2O (g), and PH2 is the partial pressure of H2 (g). The calculated results for all samples are

listed in Table 3-2. The obtained PO2 value can be the highest PO2 value that can be achieved

in the chamber at ~1815K. Based on this estimation, it is possible to discuss the

thermodynamic considerations for the possible chemical reactions which take place at the

interface.

Table 3-2. PO2 values at temperatures between 1813-1815 K, calculated based on the

(PH2O/PH2) ratio at 923K.

Sample

PO2 at 923K

(inlet) [atm]

PO2 at T=1813-1815K calculated from

(PH2O/PH2) at 923K [atm]

1) Al2O3/Fe 6.85x10-23

3.54 x10-9

2) MgO/Fe 7.52 x10-22

3.89 x10-8

3)Ti2O3/Fe 3.13-5.75 x10-21

1.62-2.97 x10-7

Ti2O3/Steel 3.98 x10-21

2.06 x10-7

4) TiO2/Fe 9.02x10-22

1.12x10-8

23

3.1.3 Wettability mechanisms

(1) Al2O3

Fig. 3-8 shows the spot analysis of an interfacial cross section between pure Fe and an

Al2O3 substrate. The reaction layer is identified as a FeAl2O4 phase and the thickness is about

4µm.

Fig. 3-8. Cross sectional analysis for a Al2O3/pure Fe sample.

In case of the Al2O3/pure Fe system, the following chemical reactions may occur at the

interface:

FeO(l))g(1/2OFe(l) 2

44.82T+-233023G o[J/mol] [68-69] (3-4)

)s(OFeAl)g(1/2O(s)OAlFe(l) 42232

82.044T+-328348G o [J/mol] [70] (3-5)

(s)OAl)g(3/2O)l(2Al 322

324.15T+-1682300G o[J/mol][69-71] (3-6)

24

Fig. 3-9 shows the equilibrium PO2 values for the different chemical reactions. The “gray

region” represents the highest PO2 range in the chamber, which was estimated in section 3.1.2.

Based on thermodynamics, it can be seen that for the Al2O3/pure Fe, a FeO and FeAl2O4

formation might take place. Furthermore, that the Al2O3 substrate is thermodynamically stable.

The results of Shibata et al. [36] fit this prediction due to that FeO particles inside of the iron

and a FeAl2O4 reaction layer are identified at the interface. Although no FeO particles were

observed in the present work, a FeAl2O4 reaction layer at the interface was identified, as is

shown in Fig. 3-8. The formation of the FeAl2O4 reaction layer at the interface leads to a

decrease of the contact angle value during the first 90s of the experiments. Afterwards, the

contact angle value is almost stable. This means that the reaction layer formation reached an

equilibrium state.

Fig. 3-9. Equlibrium PO2 values for different chemical reactions as a function of the

temperature.

10-23

10-21

10-19

10-17

10-15

10-13

10-11

10-9

10-7

10-5

1800 1850 1900 1950 2000 2050

Al2O3MgOTi2O3-418Ti2O3-502Ti2O3-512

PO

2 [

atm

]

T [K]

Fe(l)+1/2O2(g)=FeO(l)

Fe(l)+1/2O2(g)+Al2O3(s)=FeAl2O4(s)

2Al(l)+3/2O2(g)=Al2O3(s)2Ti(l)+3/2O2(g)=Ti2O3(s)

Ti2O3(s)+4Al+7/2O2(g)=2Al2TiO5(s)

25

(2) MgO

In the case of the MgO/pure Fe system, a reaction layer of a MgO-FeO solid solution was

identified. The thickness of the reaction layer was about 20µm, as is shown in Fig. 3-10.

Fig. 3-10. Cross sectional analysis for a MgO/pure Fe sample.

Previously Dan et al. [68] has reported that FeO(l) can be formed at the interface of MgO

in contact with liquid iron. Once FeO is formed, it will immediately be dissolved into the

MgO surface. Then, it will form a FeO-MgO solid solution. According to this point, the molar

fraction of FeO (in MgO) can be thermodynamically estimated. This was done by using a [O]

content value of 635ppm, which was measured with an oxygen/nitrogen/hydrogen combustion

analyzer.

FeO(l)OFe(l) (in magnesiowüstite)

47.69T-115855G o[J/mol] [68] (3-7)

OlFe

lFeOo

aa

aRTKRT

)(

)(lnlnG (3-8)

26

6.969exp )5.736-/13934.09(

)(

)(

T

OlFe

lFeO

aa

aK (T=1815K) (3-9)

where aFeO(l), aFe(l) and aO are the activities for FeO(l), Fe(l) and [O], respectively. The value

of aFe(l) can be assumed to be unity and aO is expressed by the following Eq. (3-10):

OOO xfa (3-10)

where ƒO is the activity coefficient of oxygen in Fe and xO is the mass percentage of oxygen

in liquid Fe (ƒO has been reported to be unity [72]). In this case, the activity of FeO can be

obtained by Eq. (3-11):

OO xa 6.969 (3-11)

When xO is 635ppm, aFe(l) can be determined to have a value of 0.443 (in MgO). Also, the

molar fraction of FeO, NFeO, in the MgO-FeO solid-solution can be estimated to be 0.211.

This can be done by using the relation between aFeO and the molar fraction of FeO in a MgO-

FeO solid-solution [73], as is shown in Fig. 3-11. This thermodynamic estimation show

similar results as the experimental observation results, which is shown in Fig. 3-10. The

decrease of the contact angle during the first 120s after a full melting is possible due to a

MgO-FeO solid-solution is formed at the interface. After that, the reaction layer formation has

reached an equilibrium state and the contact angle value is kept stable.

Fig. 3-11. Activity of wüstite as a function of NFeO in the system “FeO”-MgO.

0

0,2

0,4

0,6

0,8

1

0 0,2 0,4 0,6 0,8 1

Data 206 13:48:20 2014-12-02

11001300D

aF

eO

NFeO

0.9

0.7

0.3

0.5

0.1

0.1 0.3 0.5 0.7 0.9

1373K

1573K

0.211

0.443

MgO FeO

Activity curve for a solutionobeying Raoult's law

Estimationresult

27

(3) Ti2O3

In the case of the Ti2O3/pure Fe system, it can be seen that Ti2O3 does not react with pure

Fe at the interface. Instead, the following reaction takes place.

(s)OTi)g(3/2O)l(2Ti 322

267.2T+-1520750G o [J/mol] [74] (3-12)

According to the thermodynamic result in Fig. 3-12, the Ti2O3 substrate is stable for the

Ti2O3/pure Fe case at a temperature 1815K. The slight decrease of the contact angle with time

might be because of the increased oxygen content in the melt.

Fig. 3-12. Cross sectional analysis for a Ti2O3/pure Fe sample.

For the Ti2O3/steel case (in Fig. 3-13), it was found that a Al2TiO5 reaction layer with a

thickness of ~5µm was identified. The measured contents of Al, Ti and O cannot strictly fit

the Al2TiO5 stoichiometric content. This might be due to a mixing of Al2TiO5 with the Ti2O3

substrate surface.

28

Fig. 3-13. Cross sectional analysis of a Ti2O3/Steel sample.

Due to the high Al (= 0.013 mass%) content in steel, Al2TiO5 particles may form according

to the following reaction.

)s(TiOAl2(g)7/2OAl4)s(OTi 52232

533.11T+-2868076G o [J/mol] [69, 75-77] (3-13)

According to the equilibrium PO2 value estimated in Fig. 3-9, it can be seen that a

precipitation of Al2TiO5 is thermodynamically possible. This consideration is consistent with

the cross sectional observation. According Fig. 3-4, the reaction layer formation of Al2TiO5 at

the interface almost arrived at the equilibrium state during the first 173s after a full melting of

the steel. Afterwards, the contact angle has almost kept a constant value of 90 degrees.

(4) TiO2

In the case of the TiO2/pure Fe system, the cross sectional analysis is shown in Fig. 3-14

and Fig. 3-15. It can be seen that the reaction layer (melting region) is made up of a complex

Fe-Ti-O oxide phase. Base on the ratio of Fe/Ti, the reaction layer roughly included a mixture

of FeO·2TiO2 and FeO·TiO2. The vertical cross sectional analysis at the central part of the

interface between the pure Fe and TiO2 substrate was also investigated by using SEM-EDS, as

29

is shown in Fig. 3-16. The thickness of the Fe-Ti-O reaction layer is about 7 µm. Also the

composition of the complex reaction layer near the pure Fe side is close to the compound

FeO·TiOx. Furthermore, the reaction layer near the TiO2 substrate is close to FeO·2TiOx. In

addition, the ratio of the Ti/O (in at%) on the TiO2 substrate side is shown in Fig. 3-17. On

the surface of the substrate, the composition is close to pure TiO2. However, according to the

ratio of Ti/O, the composition of the substrate inside varied mostly between TiO2 and Ti2O3. It

means that a TiO2 decomposition has occurred at the given experimental condition.

Fig. 3-14. Cross section image of a solid pure Fe droplet on a TiO2 substrate.

Fig. 3-15. Element mapping of the melting region between a pure Fe droplet and a TiO2

substrate.

30

(a)

(b)

Fig. 3-16. Cross sectional analysis of the reaction layer of the interface at the central part of a

Fe/TiOx sample.

Fig. 3-17. Ti/O ratio in a TiO2 substrate and in a TiOx-FeO reaction layer.

-20

-15

-10

-5

0

5

10

15

0 20 40 60 80 100

Data 1 21:17:05 2014-10-18

Ti-disFe-disO-dis

Ver

tica

l d

ista

nce

[μ

m]

Content [atom%]

Ti

Fe

Ti=4.17 [mass%]

O

Fe

TiOx

-30

-20

-10

0

10

at%Ti / at%O

Dis

tan

ce

fro

m s

ub

stra

te s

urf

ace

[µm

]

Substrate surface

TiO

2

Ti 4

O7

Ti 3

O5

Ti 2

O3

0.2 0.4 0.6 0.8 1.0

TiO

Fe droplet

TiO2 substrate

TiO -FeOx

31

In order to explain the wetting behavior of the TiO2/pure Fe system, the formation

mechanism of the melting region needs to be considered from a thermodynamic point of view.

The standard Gibbs free energies of the different chemical reactions are summarized in Table

3-3. Regarding to these reactions, the relationship between the equilibrium partial pressure of

oxygen and temperature is shown in Fig. 3-18. The gray zone corresponds to the temperature

range during the wetting experiment at which the melting region was visually observed.

According to the analysis result in Fig. 3-17, a decomposition of the TiO2 substrate has taken

place. The critical equilibrium value of PO2 for the TiO2 decomposition at 1739 K is about

4x10-12

atm. The actual PO2 near the sample for the experiment of TiO2/pure Fe case might be

close to this value. In this case, it can be seen that at temperatures higher than ~1700 K, the

reactions of (1), (2), (7) and (8) might not take place thermodynamically. However, due to the

decomposition of the TiO2 substrate (reactions (9)-(11)), the produced O2 might support

reactions (1), (2), (7) and (8) and then form a Fe-Ti-O reaction layer.

Table 3-3. Standard Gibbs free energy changes for different reactions

No. Chemical reaction ΔGo

[J/mole]

T [K] Ref.

1 Fe(s) + 1/2 O2(g) = ”FeO”(s) -264002 + 64.591∙T 298-1650 [79]

2 Fe(l) + 1/2 O2(g) = FeO(l) -256061 + 53.681∙T 1644-2273 [76]

3 Ti(s) + O2(g) = TiO2(s) -940982 + 177.569∙T 298-1943 [76]

4 3Ti(s) + 5/2 O2(g) = Ti3O5(s) -2435080 + 420.492∙T 298-1943 [76]

5 2Ti(s) + 3/2 O2(g) = Ti2O3(s) -150256 + 258.069∙T 298-1943 [76]

6 7 Ti3O5(s) = 5 Ti4O7(s) + Ti(s) 127680 + 59.9∙T 1750-1950 [71]

7 Fe(s) + 1/2 O2(g) + TiO2(s) = FeTiO3(s) -271600 + 63.35∙T 1173-1373 [80]

8 Fe(s) + 1/2 O2(g) + FeTiO3(s) = Fe2TiO4(s) -282400 + 62.93∙T 1173-1373 [80]

9 4 TiO2(s) =Ti4O7(s) + 1/2 O2(g) 380352 – 109.607∙T 1750-1950 *

10 3 TiO2(s) =Ti3O5(s) + 1/2 O2(g) 387866 – 112.215∙T 298-1943 *

11 2 TiO2(s) =Ti2O3(s) + 1/2 O2(g) 379908 – 97.069∙T 298-1943 *

*: Reactions (9)-(11) representing a TiOx decomposition were obtained by using a

combination of the respective chemical reactions (3)-(6).

32

Fig. 3-18. Relation between the equilibrium PO2 value and the temperature for different

reactions.

(5) TiN

The solubility of TiN is thermodynamically estimated based on the following equations:

NiTsTiN )( (3-14)

KRTG lno (3-15)

NTi aaK (3-16)

where ΔGo is the standard Gibbs free energy of a TiN formation, R is the gas constant, K is

the equilibrium constant for reaction (3-14). Furthermore, aTi and aN are the activities of Ti

and N in the iron/steel melt, respectively. The solubility line of TiN for pure Fe and steel (Si =

0.65 mass%) content is shown in Fig.3-19. In addition, the O, N and Ti contents in the

iron/steel sample after the experiment are also plotted in Fig. 3-19. For the TiN/pure Fe case,

it was found that TiN will be dissolved into the liquid iron during the first 150s holding time.

After that, the TiN substrate will not be dissolved anymore. As for the TiN/steel case, the TiN

substrate will always be kept stable and a dissolution will not take place.

33

Fig. 3-19. Solubility diagram for TiN in contact with pure Fe and steel.

Fig. 3-20 shows the cross sectional analysis of the interface between pure Fe and a TiN

substrate. It can be seen that iron is partially saturated into the open pores near the interface

and some peelings can be identified. It was also found that the TiN substrate has been slightly

dissolved into the iron. However, there no chemical reaction could be found at the interface.

At the initial stage (t=150s), the TiN substrate was slightly dissolved into the liquid iron

until the melt composition approached the solubility product line (t=150s), as is shown in Fig.

3-19. This dissolution process corresponds to a remarkable increase of the Ti and N contents

from 0.0001 to 0.057mass% and 0.0008 to 0.0103mass%, respectively, as is shown in Fig. 3-

21. Therefore, the substrate surface became rough and it contained protuberances and peeled-

off particles, as is shown in Fig. 3-20. According to the Thermo-Calc calculation, a slight

precipitation of Ti(N0.988,O0.012)=0.036mass% and Ti2O3=0.778mass% is formed at the

interface (t=150s). According to Fig. 3-21, it can also be seen that the oxygen content in iron

is stable at a value of around 0.0066mass% during the first 150s. At this stage, the contact

angle is almost stable.

When the composition of the melt exceeds the solubility product line (t=900s), TiN is

thermodynamically stable in liquid iron. At the interface, a small amount of Ti(N,O) phase

(=0.068mass%) combined with a TiOx phase (=1.017mass%) is slightly precipitated. Since

the precipitation amount is quite small, the effect of this value on the contact angle value is

0,0001

0,001

0,01

0,1

1

10

0,0001 0,001 0,01 0,1 1 10

Ti-con-Fe

Ti-con-steelFe-error barSteel-error bar

[Ti] c

onte

nt [m

ass%

]

[N] content [mass%]

T=1815K

t=981s

t=298s

t=0s

t=0s

t=80s

t=150s

t=900s

Pure Fe

Steel

[81-82]

eSiTi=+2.1 [81]

Si=0.65 [mass%]

eSiTi=+1.43 [82]

Si=0.0008 [mass%]

??????

??

??????

??

Pure Fe

Steel

SEM-EDS

34

negligible. In addition, the O content was found to increase from 0.0066mass% (t=150s) to

0.0129mass% (t=900s). Consequently, it is possible that the slight decrease of the contact

angle in the TiN/pure Fe case is due to the oxygen increase in the iron.

Fig. 3-20. Cross sectional analysis for a TiN/pure Fe sample.

Fig. 3-21. Relation between the contents of O, N, Ti in the metal and the holding time.

0,0001

0,001

0,01

0,1

1

10

0 200 400 600 800 1000

Data 300

O-FeN-FeTi-FeO-steelN-steelTi-steel

Con

ten

t [m

ass%

]

time [s]

Pure Fe Steel

ON

Ti

35

In case of the TiN/steel system, the dissolution of a TiN substrate in steel case will not

happen according to the former discussion. Thus, the substrate surface is relatively smooth as

is shown in Fig.3-22. According to the Thermo-Calc calculations, a reaction layer at the

interface can precipitate in form of Ti(N0.456,C0.537,O0.006)=0.154mass%, Ti2O3=0.477mass%

and TiO=0.740mass% at 298s and Ti(N0.849,C0.143,O0.008)=0.351mass% and Ti2O3=3.005mass%

at 981s, respectively. These predictions fit with a gradual increase of the Ti (from 0.205 to

0.725mass%) and N (from 0.0153 to 0.0707mass%) contents, as is shown in Fig. 3-21. It has

been known that liquid iron has a strong wetting behavior in contact with TiC [83]. Thus, the

slight precipitation of Ti(N,C,O) at the interface can be a reason of the sharp decrease of the

contact angle. In addition, the oxygen increase from 0.0015 to 0.0106mass% in the steel

(t=981) can also lead to a decreased contact angle value.

Fig. 3-22. Cross sectional analysis results for a TiN/Steel sample.

36

3.2 TiO2 and TiN particle additions into steel

The analysis in this section will only concentrate on the “Oxide+MnS+TiN” group

inclusions, which has the potential to form intragranular ferrite (IGF). Some inclusions in the

“MnS+TiN” group were also detected in the present work, but they will not be discussed here.

3.2.1 Typical inclusions characteristics

The phases of the typical particles found in the samples were obtained through a phase

identifications by using SEM-EDS mappings and spot analyses, as is shown in Table 3-4.

TiO2 particle addition

1) Al = 0.008 mass% constant, Ti = 0.011 0.026 mass% increase:

When the Al content has a stable value of 0.008 mass% and the Ti content is increased

from 0.011 (Eref) to 0.026 mass% (EwTiO2), the particle has a Ti-poor/free oxide phase (an

Al2O3 phase and a MnAl2O4 phase). In addition, a MnS phase and/or a TiN phase are

precipitated around the oxide phase.

2) Ti ≈ 0.025 mass% constant, Al = 0.008 0.005 mass% decrease:

When the Ti content has a stable value of ~0.025 mass% and the Al content has decreased

from 0.008 (EwTiO2) to 0.005 mass% (ETiO2), the oxide phase in the particle changes from a

Ti-poor/free oxide phase (a MnAl2O4 phase and a “Liquid#1/#2” phase) to a Ti-rich oxide

phase (an Al2TiO5 phase and a Mn(Al,Ti)2O4 phase). Here, the “Liquids#1” and “Liquid#2”

phases represent an oxy-sulfide phase with a high S content and an oxide phase with a high Si

content, respectively. In addition, a MnS phase and/or a TiN phase are precipitated around

these oxide phases.

TiN particle addition

In the ETiN trial (Al = 0.007mass%, Ti = 0.034mass%), the typical phase includes a Ti-

poor/free oxide phase (Al2O3 phase, “Liquid#1” phase) covered with TiN and (Ti, Al)N

phases.

37

Table 3-4. SEM-EDS mappings and spot analyses of typical particles

Sample SEM micrograph EDS mapping and spot analysis

ETiO2

(Al = 0.005 mas%,

Ti = 0.025 mass%)

Equivalent circle diameter: 1.279µm,

Circularity: 0.370

Magnification: x15000

Equivalent circle diameter: 1.645µm,

Circularity: 0.765

(x15000 magnification)

Magnification: x15000

EwTiO2

(Al = 0.008 mas%,

Ti = 0.026 mass%)

Equivalent circle diameter: 1.791µm,

Circularity: 0.533

Magnification: x15000

Eref

(Al = 0.008 mas%,

Ti = 0.011 mass%)

Equivalent circle diameter: 1.727µm,

Circularity: 0.750 Magnification: x15000

ETiN

(Al = 0.007 mas%,

Ti = 0.034 mass%)

Equivalent circle diameter: 0.974µm,

Circularity: 0.625

Magnification: x20000

38

3.2.2 Particle size distribution

(1) TiO2 particle addition

1) Al = 0.008 mass% constant, Ti = 0.011 0.026 mass% increase:

When the Al content is stable at a value of 0.008 mass% and the Ti content increases from

0.011(Eref) to 0.026 mass% (EwTiO2), the total number of inclusions increases from

4.02x105

to 1.14x106 mm

-3 according to Fig. 3-23. Furthermore, the peak sizes decrease from

0.605 to 0.303 µm. Thus, it is clear that the TiO2 particle addition can strongly affect the

particle size distribution.

2) Ti ≈ 0.025 mass% constant, Al = 0.008 0.005 mass% decrease:

When the Ti content is stable at a value of ~0.025 mass% and the Al content decreases

from 0.008 (EwTiO2) to 0.005 mass% (ETiO2), the total particle numbers increase from

1.14x106 to 4.35x10

6 mm

-3 according to Fig. 3-23. Thus, it was found that the decrease in the

Al content affects the particle size distribution.

(2) TiN particle addition

In case of the ETiN trial (Al = 0.007mass%, Ti = 0.034mass%), the total particle number

Nv value is 7.81x105

mm-3

, as is shown in Fig. 3-23. This value is larger than the values in the

Eref trial, but smaller than those in the ETiO2 trial. The peak particle size value is 0.428 µm,

which is smaller than that found in the Eref trial but larger than that found in the ETiO2 trial.

In summary, according to the result of Grong et al. [84], the total number of particles Nv,

required for a high grain refining potential should be larger than 5.0x106 mm

-3. From the

former comparison, it can be seen that with the lowest Al content (=0.005 mass%), the total

number of particles, Nv, in the ETiO2 trial (4.35x106

mm-3

) is close to this suggested critical

value.

39

Fig. 3-23. A comparison of the particle size distributions in the ETiO2, EwTiO2, Eref and

ETiN samples.

1000

104

105

106

107

0,1 1 10 100

ETiO2EwTiO2ErefETiN

Particle size, d, [μm]Part

icle

num

ber

pe

r unit v

olu

me,

Nv,

[mm

-3]

ETiO2

ETiN

Eref

EwTiO2

103

ETiO2

EwTiO2

Eref

ETiN

4.35x106

1.14x106

4.02x105

7.81x105

Nv [mm-3

]

40

3.2.3 Thermodynamic equilibrium calculations

The equilibrium calculations of the inclusion precipitation were done by using Thermo-

Calc 3.0.1 and using the TCFE7 database. The chemical compositions used in the calculations

are listed in Table 3-5. The calculations for case 2 to case 5 correspond to the trials Eref,

EwTiO2, ETiO2 and ETiN trials, respectively. Also, the predicted precipitation phase at 1523

K is assumed to be the final inclusion phase. This is due to that the phase types are fixed and

will not change to a large extent with a decreasing temperature.

Table 3-5. Chemical composition of steels used in the equilibrium calculations using Thermo-

Calc.[mass.%]

No. Sample C Mn Si S Mo V Al Ti O N

(1) 0.20 0.90 0.49 0.050 0.039 0.082 0.008 0.005 0.0020 0.0123

(2) Eref 0.28 1.43 0.69 0.044 0.041 0.097 0.008 0.011 0.0027 0.0133

(3) EwTiO2 0.32 1.32 0.72 0.046 0.038 0.093 0.008 0.026 0.0061 0.0146

(4) ETiO2 0.28 1.24 0.67 0.035 0.045 0.096 0.005 0.025 0.0075 0.0133

(5) ETiN 0.29 1.34 0.68 0.042 0.038 0.086 0.007 0.034 0.0028 0.0229

(6)

0.28 1.24 0.67 0.010 0.045 0.096 0.003 0.035 0.0075 0.0133

(7) 0.28 1.24 0.67 0.035 0.045 0.096 0.001 0.035 0.0075 0.0133

(8) 0.28 1.24 0.67 0.035 0.045 0.096 0.001 0.050 0.0075 0.0133

The inclusion stability diagram of the Fe-Al-Ti-O system has previously been reported by

Matsuura et al. [9] and it is shown in Fig. 3-24. The present Thermo-Calc calculations were

also inserted into the diagram. Generally, it was found that the stability phase diagram can fit

the Thermo-Calc calculation result. In addition, the thermodynamic calculations can support

the experimental observation results reported in section 3.2.1.

41

Fig. 3-24. Calculated equilibrium oxide phase diagram for a Fe-Al-Ti-O melt at a 1873 K

temperaure.

3.2.4 The effect of attraction forces on particle agglomeration in steel samples

3.2.4.1 Van der Waals force

The van der Waals force Fv between the two spherical inclusions (material1) with the same

radius r in the molten steel (material 2) can be expressed as is shown in Eq. (3-17) [28].

)12zr/AF 2

121v （ (3-17)

22211121 A-AA

(3-18)

where, A121 is the Hamaker constant between the two spherical inclusions in the molten steel

(J), which can be calculated from Eq. (3-18). The parameter z is the distance between the

spherical inclusions (m). The parameters A11 and A22 are the Hamaker constants for the

inclusion and iron in vacuum (J). The Hamaker constants for Al2O3, TiO2, TiN and Fe (liquid)

are equal to 1.55x10-19

J [50], 2.26x10-19

J [50], 1.823x10-19

J [85] and 10.5x10-19

J [27]. Thus,

the following values of A121 can be obtained: 3.98x10-19

J for Al2O3, 3.02x10-19

J for TiO2 and

3.59x10-19

J for TiN. The z value can be assumed to be 10nm [22]. The van der Waals force

was calculated as a function of the inclusion radius, as is shown in Fig. 3-25. It was found that

the van der Waals force for the different inclusion types increased in the following order: i)

0,01

0,1

1

0,0001 0,001 0,01 0,1

Data 12 16:06:02 2014-01-17

TI

[mass.%

Ti]

[mass.% Al]

<Ti2O3>

<Al2O3>

<Ti3O5>

<Al2TiO5>(1)

(2)

(3)

(5)

(6)(4)

(6)(7)

(8)

Al2O3+TiN+MnSAl(-Ti) oxide

+Liquid#1+TiN+MnS

Ti(-Al) oxide+TiN+MnS

Ti(-Al) oxide+Liquid#1

+TiN+MnS

Ti-Al oxide+Liquid#1

+TiN+MnS

1873K

42

TiO2, ii) TiN and iii) Al2O3. In addition, the relation between the contact angle and the

Hamaker constant, A121 are also shown in Fig. 3-26.

3.2.4.2 Liquid-capillary Force

In case of Al2O3, its strong agglomeration behavior has already been reported by

Mizoguchi et al. [22]. They calculated the liquid-capillary force FL due to the FeO liquid

bridge, as is shown in Fig. 3-25.

In the present work, the wettability measurements for the TiO2 case show that a TiOx-FeO

solid solution was formed. For the liquid steel with a high oxygen content and a TiO2 particle

addition, the liquid TiOx-FeO might also provide a liquid-capillary force that promotes on

agglomeration of inclusions. However, this phenomenon was not detected in the steel sample

in the present work since the oxygen content in reference sample only was 27ppm. Also, the

wettability of the TiN/steel system shows that the strong wetting behavior is due to an

increased oxygen content and a small precipitation of Ti(N,C,O) at the interface. However, a

liquid phase precipitation was not identified.

3.2.4.3 Wettability

The contact angle values of TiO2, TiN and Al2O3 are summarized in Fig. 3-26. Even

though many researchers have presented contact angle values for Al2O3, the reported results

are quite scattered due to the formation of a FeAl2O4 reaction layer. In this work, a value of

132 degrees, reported by Ogino et al. [35] by using an extremely low PO2 value (=1.0x10-

17atm) in the chamber, was selected. It can be seen that for the un-wetting cases (TiN, Al2O3),

the contact angle values of TiN are smaller than that of Al2O3. It means that the agglomeration

of Al2O3 after collision is more favorable than that of TiN. As for the TiO2 case, the results of

Humenik et al. [30] are summarized in Fig. 3-26. According to the results in the present work,

the actual measured value for the TiO2/pure Fe system is the contact angle of the TiOx-

FeO/TiO2 system. Due to the reaction layer formation, which covered the pure Fe droplet, the

exact contact angle value for the TiO2/pure Fe system cannot be obtained. In the present work,

the obtained oxide inclusions after TiO2 particle addition into the steel are generally identified

as AlxTiyO inclusions. According to the contact angle measurements for the Ti2O3/steel

system, the final contact angle value is 90 degrees. This wetting contact angle value is

actually that of AlxTiyO/steel, due to the AlxTiyO reaction layer formation. In this case, the

agglomeration is not favorable even though collisions might happen. This is due to the strong

wettability of the AlxTiyO phase.

43

In summary, based on the Van der Waals force, liquid-capillary force and wettability, the

agglomeration degrees of the different inclusions is increased in the following order: TiO2

(AlxTiyO) < TiN < Al2O3.

3.2.4.4 Agglomeration behavior

According to the SEM-EDS observations, the typical phases of the inclusions in the ETiO2

sample contain Ti-rich and Ti-poor oxide phases. The phases in the ETiN sample include

(Ti,Al)N and a Ti-poor oxide phase. As for the EwTiO2 and Eref samples, only Ti-poor oxide

phases were identified. Furthermore, according to the particle size distribution, the total

particle number and peak size varies in the following order: 1) Total particle number: ETiO2 >

EwTiO2 > ETiN > Eref, and 2) Peak size: ETiO2 = EwTiO2 < ETiN < Eref. These

experimental results correspond to the theoretical studies mentioned above in section 3.2.4.1

to section 3.2.4.3.

Fig. 3-25. Attraction forces between Al2O3, TiN and TiO2 particles present in molten steel.

10-15

10-13

10-11

10-9

10-7

10-5

0,001

0,1

0,1 1 10

Fv-TiO2Fv-Al2O3Fv-TiNFL-0.1FL-0.5

Particle radius, r [μm]

TiO2

TiN

Al2O3

r1=0.1μm

r1=0.5μm

Liquid-capillary for Al2O3 [22]

van der Waals

Attra

ctive fo

rce,

F [N

] 10-3

10-1

44

Fig. 3-26. Relationship between the Hamaker constant (A121) at room temperature and the

contact angle of liquid Fe in contact with substrates made of TiO2, TiN and Al2O3.

0

50

100

150

3 3,5 4 4,5

Angle-TiO2Angle-TiNAngle-Al2O3Cu-TiNCu-Al2O3

Hamaker constant [x10-20

J]

Con

tact

an

gle

[d

eg

ree]

[39]

[33]

Oginol

[30]

[30]

[30]

[41][53]

[66]

Amadeh

Rutile

NaCl

Corrundum

[41][66]

TiO2

TiN

Al2O3

45

3.3 Agglomeration behavior of inclusions after a complex Ti/Al deoxidation

3.3.1 Typical single inclusion/cluster morphologies

Table 3-6 reports the typical inclusion/cluster morphologies found in the Ti, Ti/Al and Al

deoxidation trials. In case of Ti, a cluster formation cannot be identified. Instead, single

inclusions of TiOx-FeO with a spherical shape and with sizes smaller than ~6 µm were

detected. As for the Ti/Al and Al cases, a cluster formation was found. Fig. 3-27

quantitatively shows the circularity of the single inclusions/clusters for the Ti, Ti/Al and Al

deoxidations. It was found that the circularity of the single inclusions/clusters decreased in the

following order: i) Ti (0.858), ii) Ti/Al (0.578) and iii) Al (0.193). It means that the clusters

found after a Ti/Al deoxidation are much more compact than those found after an Al

deoxidation.

Table 3-6. Typical single inclusion/cluster morphologies found in the Ti, Ti/Al and Al

deoxiation experiments.

Ti Ti/Al Al

Fig. 3-27. Circularity of single inclusion/clusters after a deoxidation with Ti, Ti/Al and Al.

0

0,2

0,4

0,6

0,8

1

aver

Deoxidation method

Circula

rity

of sin

gle

inclu

sio

n / c

luste

r

Ti Ti / Al Al

Single

Inclusion

Cluster

Cluster

Spherical

Regular

46

Fig. 3-28 shows the frequency of clustered inclusions after a deoxidation with Ti, Ti/Al

and Al. Based on the morphology type, the clustered inclusions were divided into a spherical

and a regular type, as shown in Table 3-6. Because clusters cannot be found in the Ti case,

the single inclusions were used in the comparison. It can be seen that the frequency of the

spherical inclusions decreased in the following order: i) Ti (100%), ii) Ti/Al (69.5%) and iii)

Al (34%). As for the regular type frequency, it increased from 30.5% in the Ti/Al case to 66%

in the Al case.

Fig. 3-28. Frequency of different types of clustered inclusions found in steel samples after a

deoxidation with Ti, Ti/Al and Al.

3.3.2 Typical cluster composition in Ti/Al deoxidation

Table 4-7 shows the elemental mapping analysis of the clusters obtained after a complex

Ti/Al deoxidation after a holding time of 1min. Based on the 2D cross section investigation, it

is concluded that the TiOx-FeO inclusions formed in the Ti pre-deoxidation have been

completely reduced into Al2O3 inclusions within 1min after an Al addition was made.

0

20

40

60

80

100

0 0,5 1 1,5 2 2,5 3

B

Fre

qu

en

cy o

f clu

ste

red

in

clu

sio

ns [

%]

Deoxidation method

Regular

Spherical

AlTi / AlTi

47

Table 3-7. Elemental mapping of typical clusters found after a complex Ti/Al deoxidation

Observation

method

Typical image

(x8000 magnification)

Ti Al

2D

investigation

by using

cross section

Fe O

3.3.3 Number of clusters

Fig. 3-29 shows a comparison of the total number of clusters per unit volume, NV-C, after a

deoxidation with Ti/Al and Al. It shows that as the holding time is increased from 1 to 15min,

the total number of the clusters (550150 mm-3

) in the Ti/Al case is much smaller than that

(5700260 mm-3

) in the Al case. According to the above analysis, it is obvious that the

agglomeration behavior for a complex Ti/Al deoxidation is much weaker than that of an Al

deoxidation. As was described above, the initial TiOx-FeO inclusions in the Ti/Al deoxidation

had been completely reduced into pure Al2O3 phase within 1min after Al addition. Therefore,

the smaller number of clusters in the Ti/Al deoxidation in comparison to the Al deoxidation

cannot be explained by the density differences of the formed clusters. Also, the difference of

the cluster number in these two experiments was evaluated according to the collision rate of

inclusions and clusters in the melt.

48

Fig. 3-29. Total number of clusters found in the steel samples after an Al and a Ti/Al

deoxidation.

3.3.4 Thermodynamic consideration for a TiOx-FeO formation during a Ti pre-deoxidation

The experimental results of the equilibrium relation between [Ti] and [O] in liquid iron

saturated with different Ti-oxide types are summarized in Fig. 3-30. In case of the critical [Ti]

content between Ti3O5 and Ti2O3, Suzuki et al. [86] and Pak et al. [87] reported the same

value of 0.25 mass%, which corresponds to the recommended JSPS data [77]. Also, Cha et al.

[74] obtained a little larger value of 0.36 mass%. For the critical [Ti] content between TiOx-

FeO and Ti3O5, Suzuki et al. [86] reported a value of 0.013 mass%, which corresponds to the

recommended JSPS data [77]. The chemical reactions for Ti2O3/Ti3O5 is expressed in Eq. (3-

19). Moreover, the thermodynamic data are summarized in Table 3-8.

Table 3-8. Standard Gibbs free energy changes of different chemical reactions

Chemical reaction △G0

(J/mole) Temper. [K] Ref.

O)g(1/2O2 (in liquid iron) 3.39T-117110- - 77

(s)OTi)g(5/2O)s(3Ti 532 420.492T2435080- 298-1943

76 (s)OTi)g(3/2O)s(2Ti 322 258.069T1502056- 298-1943

100

1000

104

0 5 10 15

Ti/AlAl

Holding time [min]

Nv-c

[m

m-3

]

Al

Ti/Al

103

102

49

O)s(O3Ti)s(O2Ti 3253

70.167T-246882G o [J/mol] [76-77]…………………… ……….(3-19)

O

OTi

OOTio aRTa

aaRTKRT lnlnlnG

2

3

53

32

.......................(3-20)

]%[ Omassfa OO .........................................................(3-21)

]%[]%[]%[log TimasseOmasseimassef Ti

O

O

O

i

i

OO .....................(3-22)

where 𝑎𝑇𝑖3𝑂5, 𝑎𝑇𝑖2𝑂3 and 𝑎𝑂 are the activities of Ti3O5 (s), Ti2O3 (s) and [O] (in liquid iron),

respectively. Furthermore, 𝑓𝑂 is the activity coefficient for [O], 𝑒𝑜𝑜 (= 0.0344-701/Ti [9]) and

𝑒𝑜𝑇𝑖 (= 0.76-1750/T [77]) are the interaction parameters.

The boundary line for Ti3O5 /Ti2O3 was obtained as is shown in Fig. 3-30. It can be seen

that the boundary line of Ti3O5 /Ti2O3 can almost fit the critical point of [Ti] with a value of

0.25 mass%. The boundary line of TiOx-FeO/Ti3O5 can be estimated based on the critical [Ti]

content between TiOx-FeO and Ti3O5 (= 0.013mass% [77, 86]). In the Fe-Ti-O system, the

shape of the boundary line between different Ti-oxide phases is determined by the values of

𝑒𝑜𝑜and 𝑒𝑜

𝑇𝑖 values, which are constant at 1873 K. Thus, the boundary line of TiOx-FeO/Ti3O5

can be estimated as the “dot-line” shown in Fig. 3-30.

The compositions of the sample obtained after a Ti deoxidation (t=1, 5min) in the present

work have been inserted into Fig. 3-30. At a holding time of 1min, the composition is located

in the TiOx-FeO stable region. When the holding time increased to 5min, the composition

started to approach the equilibrium line.

50

Fig. 3-30. Equilibrium relation between the dissolved O and Ti contents at a 1873 K

temperature.

0,0001

0,001

0,01

0,1

1

0,0001 0,001 0,01 0,1 1

O-sol-ChaO-sol-SuzukiO-parktotal O-0.2TiTiO2-Ti3O5Ti3O5-Ti2O3FeO0.025

O c

onte

nt [m

ass%

]

Ti content [mass%]

Cha [74]

Suzuki [86]

Park [87]

Ti2O3

0.025%Ti0.2%Ti

0.013% 0.25%

TiOx-FeO

Ti3O5

T=1873K

Present work

0.03%Ti

1min5min

51

3.3.5 Collision rate estimation in Al and Ti/Al deoxidation

According to Fig. 3-31, the particle size distribution of the inclusions in the cluster is

needed to estimate the collision rate for an Al2O3 cluster formation. Here, we assume that the

Al2O3 cluster formation is mainly due to the collisions of particle-particle in the clusters (P-P),

particle (in the cluster)-cluster (P-C) and cluster-cluster (C-C). It is well known that the

collision-coalescence among the inclusions in the melt happens due to the combined effects of

Brownian collisions 𝛽𝑖𝑗𝐵, Stoke collisions 𝛽𝑖𝑗

𝑆 and turbulent collisions 𝛽𝑖𝑗𝑇 . Therefore, the total

collision volume 𝛽𝑖𝑗𝑇𝐶 can be expressed as follows [88-93]:

𝛽𝑖𝑗𝑇𝐶 = 𝛽𝑖𝑗

𝐵 + 𝛽𝑖𝑗𝑆 + 𝛽𝑖𝑗

𝑇 (3-23)

with

𝛽𝑖𝑗𝐵 =

2𝑘𝑇(𝑟𝑖+𝑟𝑗)2

3𝜇𝑟𝑖𝑟𝑗 (3-24)

𝛽𝑖𝑗𝑠 =

2𝑔𝜋(𝜌𝑓−𝜌𝑜𝑥)

9𝜇(𝑟𝑖 + 𝑟𝑗)

3|𝑟𝑖 − 𝑟𝑗| (3-25)

𝛽𝑖𝑗𝑇 = 1.3𝛼𝑡√𝜋𝜌𝑓𝜀/𝜇(𝑟𝑖 + 𝑟𝑗)

3 (3-26)

where k is the Boltzman constant (= 1.3807x10-23

J/K), T is the temperature (= 1873 K), µ

is the dynamic viscosity of steel (0.006 kg/m.s [94]), g is the gravitational acceleration (= 9.81

m/s2), ρf and ρox are the density of steel (= 7100 kg/m

3) and Al2O3 inclusions (= 3950 kg/m

3)

respectively. The parameter αt is the agglomeration coefficient, ε is the turbulent energy

dissipation rate (= 0.01 m2/s

3 [94]), ri and rj are the radii of the two colliding inclusions. The

agglomeration coefficient, αt, is needed to estimate the turbulent collision 𝛽𝑖𝑗𝑇 . According to

the inclusion radii ri, the agglomeration coefficient can be calculated as follows [95-96]:

/)/(

727.0

242.0

121

2/13

A

r fi

t (3-27)

where A121 is the Hamaker constant for Al2O3 in liquid iron (= 14.3 x10-19

[J] [45]). The

values of ri corresponds to 0.5 times the value of the peak sizes in each size distribution line in

Fig. 3-31. The collision rate of the inclusions in the melt can be expressed as follows[97]:

𝑑𝑛𝑖𝑗

𝑑𝑡= 𝛽𝑖𝑗

𝑇𝐶𝑛𝑖𝑛𝑗 (3-28)

where t is time [s], ni and nj are the number of the collision inclusions with the radii sizes ri

and rj, respectively.. Furthermore, the selected values of ri corresponds to the 0.5 time values

of the peak sizes represented in Fig. 3-31. Moreover, the value of rj corresponds to each size

52

step in the size distribution curve. The selected values for ni and nj correspond to the Nv-pc

value for the cluster sizes ri and rj in the size distribution. By using Eq. (3-23)-(3-28), the total

collision rate (=∑𝑑𝑛𝑖𝑗

𝑑𝑡) of the clustered inclusions-clustered inclusions, CR(P-P), was obtained

as is shown in Fig. 3-32. The total collision rates between particle-clustered, CR(P-C) and

cluster-cluster, CR(C-C) interactions were calculated in the similar manner, as is shown in Fig.

3-32. It can be seen that the formation and growth rates of clusters in the Al deoxidation

experiments are significantly larger in comparison to those in the Ti/Al deoxidation

experiments. This is due to the larger values of the total collision rates. The values of CR(P-P),

CR(P-C) and CR(C-C) in the Ti/Al case at a 1 min of holding time are about 1400, 170 and 70

times smaller than those in the Al case. However, the difference between these total collisions

rates in both experiments decrease with an increased holding time. As a result, the CR(Al) /

CR(Ti/Al) ratio at a 15 min of holding time decreases by up to about 180, 65 and 7 times,

respectively.

It is interesting to point out that the Brownian collisions have no practically effect on

formation and growth of clusters because the values of 𝛽𝑖𝑗𝐵 for inclusions and clusters in both

experiments varied mostly between 0.01 and 3.0% of the total collision volume. The turbulent

collision (𝛽𝑖𝑗𝑇 ~51-93%) and Stokes` collision (𝛽𝑖𝑗

𝑆 ~6-48%) are the main reasons for a

formation and growth of clusters in the Al and Ti/Al experiments. Moreover, it was found that

the 𝛽𝑖𝑗𝑆 values for the particle-cluster and cluster-cluster collisions are significantly larger in

the Al experiment than those in the Ti/Al experiment. It may be explained by the larger size

of clusters and, as a result, by the larger the Stokes` collisions in the Al experiment.

53

(a)

(b)

Fig. 3-31. Clustered inclusion size distribution after (a) an Al, and (b) a Ti/Al deoxidation.

1

10

100

1000

104

105

0,1 1 10

1min5min10min15min

Particle size, dv-pc [μm]

Nv-p

c [

mm

3]

1min

5min

15min

Al deoxidation

103

102

1

10

100

1000

104

105

0,1 1 10

1min5min10min15min

Particle size, dv-pc [μm]

Nv-p

c [m

m3]

1min

5min

10min

15min

Ti/Al complexdeoxidation

103

102

54

Fig. 3-32. Relation between the total cluster collision rate and the holding time after an Al and

a Ti/Al deoxidation.

10-11

10-10

10-9

10-8

10-7

10-6

10-5

0 5 10 15

Al-P-PTi/Al-P-PAl-C-CTi/Al-C-CAl-P-CTi/Al-P-C

Holding time [min]

To

tal co

llisio

n r

ate

, C

R

Al

Ti/Al

P-PP-CC-C

55

3.4 Attraction force estimations between Al2O3 inclusions in the melt

3.4.1 van der Waals force of Al2O3 in the melt

The van der Waals force Fv between the two solid particles with the same radius r (as the

simplified assumption) in the liquid iron is expressed as shown in Eq. (3-29) [22].

𝐹𝑣 = 𝐴121 ∙ 𝑟/(12𝑧2) (3-29)

where A121 is the Hamaker constant between the solid particles in the liquid (J), z is the

distance between solid particles including their surface roughness, given on an atomic scale

(m). The Hamaker constant A121 between the solid particles in the liquid can be expressed as

shown in Eq. (3-30) [28]:

𝐴121 = (√𝐴11 −√𝐴22)2 (3-30)

where, A11 and A22 are the Hamaker constants in vacuum for the solid and liquid, respectively

(J). For an Al-killed steel (O= 60 ppm), the Hamaker constant of liquid iron is equal to 25.3

x10-19

J. The Hamaker constant of the Al2O3, A11 is equal to 15.5x10-20

J [50]. By using Eq.

(3-30), the Hamaker constant A121 of Al2O3 in the liquid iron at temperature 1873 K can be

calculated as 14.3x10-19

J. Thus, the van der Waals force was obtained by substituting the

values of A121 into Eq. (3-29), as is shown in Fig. 3-33. Three different distances (with the

value as 0.4nm [98], 1nm and 10nm) between oxide particles were estimated. It can be seen

that the van der Waal force is increased with an increased particle radius and a decreased

distance between the particles. When the radius of the particle is equal to 1µm and the

distance between particles is equal to 0.4 nm (closest distance for estimation the maximum

van der Waals force) [98], the van der Waal force has a value of 0.74x10-6

N.

56

Fig. 3-33. Relationship between the van der Waals force and the particle radius for Al2O3

inclusions.

3.4.2 Cavity bridge force due to the un-wetting behavior

Fig. 3-34. A schematic illustration of the un-wetting particle attraction.

For such un-wetting inclusions as Al2O3, a void region is formed between the particles, as

is shown in Fig. 3-34. The parameters R and r are the radius of the void region and the radius

of the inclusion, respectively. The cavity bridge force Fc [N] can be described as the sum of

the pressure difference, ΔPFe (=3.86x103Pa [23]) between the void region and the liquid iron

and the surface tension, γFe of the liquid iron. It is described by the Fisher equation [99] as

follows:

F𝑐 = 𝜋𝑅2∆𝑃𝐹𝑒 + 2𝜋𝑅𝛾𝐹𝑒 (3-31)

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

0,1 1 10

Fv-CeO2-1

Fv-TiO2-1

Fv-Al2O3-1

Fv-Ti2O3-1

CeO2-10

TiO2-10

Al2O3-10

Ti2O3-10

0,4-CeO2

0,4-Al2O3

0,4-Ti2O3

0,4-TiO2

Particle radius, r [μm]

Att

raction f

orc

e [

N]

r1=0.5μm

van der Waals

Van der Waals force

Al2O3

z=1nm

z=10nm

z=0.4nm

57

The surface tension, γFe of the liquid iron at a temperature of 1873K is equal to 1.75 N/m

[49]. The value of R can be obtained by using the equation below, which was derived by Sasai

et. al. [23]:

R = [−3𝛾𝐹𝑒 + (9𝛾𝐹𝑒2 − 8𝛾𝐹𝑒 ∙ ∆𝑃𝐹𝑒 ∙ 𝑟 ∙ 𝑐𝑜𝑠𝜃]

0.5/2∆𝑃𝐹𝑒 (3-32)

where Ɵ is the contact angle.

3.4.2.1 Al deoxidation

In an Al deoxidation, Al2O3 inclusions will be formed. The contact angle of Al2O3 in

contact with the liquid iron is equal to 132 degrees [33-34]. By using Eq. (13) and (14), the

cavity bridge force is calculated as is shown in Fig. 3-35. It can be seen that as the particle

radii is increased from 0.1 to 10µm, the cavity bridge force is increased from 4.90x10-7

to

4.90x10-5

N.

3.4.2.2 Reoxidation

When reoxidation occurs, the oxygen content in the melt rises locally or temporarily to the

oxygen concentration in equilibrium with FeAl2O4. In this case, the contact angle of

FeAl2O4/pure Fe needs to be considered. A contact angle value of 103 degrees was suggested,

based on the results in the present work. By using Eq. (3-31) and (3-32), the cavity bridge

force in the reoxidation case is calculated as is shown in Fig. 3-35. It can be seen that as the

particle radii increased from 0.1 to 10µm, the cavity bridge force increased from 1.65x10-7

to

1.65x10-5

N.

58

Fig. 3-35. Relationship between the cavity bridge force and particle radius.

3.4.3 Comparison of different attraction forces for Al2O3 cluster formation

Table 3-9 shows the different attraction forces for an Al2O3 cluster formation (r=1µm). In

case of an Al deoxidation, the difference is smaller than seven times. This despite that the

cavity bridge force is larger than that of the van der Waals force. Also, the estimation of van

der Waals force in the present work (= 0.74x10-6

N) is much larger than that presented by

Mizoguchi et al. [22] (≈ 10-10

N) and Sasai et al. [23] (≈ 10-9

N). This difference is because

that different Hamaker constants of the liquid iron were used. In their work, the solid iron

Hamaker constant with a value of 21.2x10-20

J was selected for the calculation. In the present

work, the Hamaker constant value of liquid iron corresponding to 25.3x10-19

J was used. The

latter was derived as explained in section 2.4.

When the reoxidation process was promoted, the cavity bridge force decreased from

4.91x10-6

N to 1.18x10-6

N. In the reoxidation case, Sasai et al. [23] and Mizoguchi et al. [22]

recognized the strong effect of the FeO liquid-capillary force (2.92x10-6

N and 1x10-6

N).

However, according the analysis in the present work, it still has a similar level as that of the

FeO liquid-capillary force. However, the cavity bridge force decreased in reoxidation case. It

means that both of them have important roles in the Al2O3 agglomeration.

10-8

10-7

10-6

10-5

0,0001

0,1 1 10

FvFs-Al2O3Fs-FeAl2O4

Att

raction f

orc

e [

N]

Cavity bridge forceAl2O3

FeAl2O4 van der Waals force

Al2O3

Particle radius, r [μm]

10-4

59

Table 3-9. Comparison of different attraction forces in case of Al2O3 particles with a radius of

1µm.

Attraction force Mizoguchi et

al.22)

Sasai et al. 23-24)

Present work

Van der Waals force [N] ~10-10 22)

~10-9

23)

0.74x10-6

Cavity bridge force

[N]

Deoxidation

(Al2O3)

- 3.50x10-6 24)

4.91x10-6

Reoxidation

(FeAl2O4)

- Negligible 23)

1.18x10-6

FeO liquid-capillary force [N] 1x10-6

22)

2.92x10-6

23)

-

60

Chapter 4 Conclusions

Wettability (Supplement I, II and IV)

(1) TiN has a good resistance towards corrosion of liquid iron and steel. In the case of pure

Fe, the oxygen increase in the liquid iron is the main reason for a contact angle decrease

during an experiment. In the steel case, a sharp decrease of the contact angle with time was

found. It is due to both an increased oxygen content in the liquid steel and due to a slight

formation of a Ti(N,C,O) phase at the interface.

(2) For Al2O3 and MgO in contact with liquid iron, a formation of a FeAl2O4 and a MgO-

FeO reaction layer at the interface, respectively, lead to a contact angle decrease with time. In

the Ti2O3/pure Fe case, an interfacial reaction cannot occur. As for the Ti2O3/steel case, the

steep decrease of the contact angle is due to the formation of an Al2TiO5 reaction layer.

(3) For TiO2 in contact with liquid iron, the melting region appears at the temperature

below the melting point of the pure iron. This is due to the strong formation of a TiOx-FeO

solid solution. This formation is due to the direct reactions among the pure iron, TiO2 (s) and

the oxygen gas. The main source of the oxygen for the reactions is from the decomposition of

the TiO2 substrate and the low oxygen partial pressure in the chamber.

Particle addition into molten steel (Supplement III)

For the TiO2 and TiN particles addition, the steel composition should be controlled to have

a small Al content (<0.005mass%) and a high Ti content (> 0.035mass%) so as to obtain a

high number of fine particles with a Ti-rich oxide phase. This consideration can be supported

from the Van der Waals force, liquid-capillary force and wettability. The agglomeration

degree of the studied inclusions were found to be as follows: TiO2 (AlxTiyO) < TiN < Al2O3.

Complex deoxidation (Supplement V)

(1) The average circularity of clusters is about 3 times larger in Ti/Al deoxidation than

those in Al deoxidation. It means that the clusters in the Ti/Al case are more compacted than

that in the Al case. The number of clusters, NV-C in all samples of the Ti/Al case is much

smaller (around 11-2 times) than those in the Al case. However due to the cluster formation

and the flotation, the difference between them decreased significantly with an increased

holding time.

61

(2) The evaluated total collision rates between particle-particle in clusters, particle-cluster

and cluster-cluster in the Ti/Al case at 1min of the holding time are about 1400, 170 and 70

times smaller than those in the Al case. However, the difference sharply decreased to ~180,

65 and 7 times at a 15 min holding time. The turbulent and Stokes’ collision are the main

factors that result in a formation and growth of clusters in the Al and Ti/Al deoxidation

experiments. Their specific collision volumes of (𝛽𝑖𝑗𝑇 and 𝛽𝑖𝑗

𝑆 ) correrspond to ~51-93% and

~6-48% from the total collision volume, respectively.

Theoretical consideration (Supplement VI)

By using Fowkes module, the variation of the Hamaker constant of the liquid iron, A11,

with the surface tension of liquid iron was determined to be as follows:

𝐴11 = (82.7 ± 12.7) ∙ 𝛾Fe(l)2

For the agglomeration behavior of Al2O3, the cavity bridge force is larger than van der

Waals force in the deoxidation case. Specifically, the difference between them is smaller than

7 times. In the case of reoxidation, the influence of the cavity bridge force in liquid iron

decreased and became similar to that of the liquid-capillary force.

62

Chapter 5 Future work

Rare earth elements such as Ce are commonly used as alloying elements/deoxidizers in the

steel making industry. However, due to the cluster formation of CeOx, it might cause clogging

problems during the continuous casting process. Thus, a further study regarding the

agglomeration of CeOx in the liquid steel is important.

Today, wettability measurements of CeOx in contact with iron or steel are extremely rare.

For CeO2, only Amondarain et al. [31] reported a strong wetting behavior in an approximate

manner. However, the wetting mechanism of CeO2/steel has not been discussed yet. Also, a

systematical experiment with a low PO2 value in the chamber is quite necessary so as to

obtain more accurate results. As for Ce2O3 case, no available wettability data exist. However,

Ce2O3 has been recognized to have a strong agglomeration behavior. Thus, the wettability

measurement of Ce2O3 has a significant meaning. In the open market, neither CeO2 nor Ce2O3

substrates with high relative densities are available. However, CeO2 substrate can be prepared

by using cold press sintering of a CeO2 powder. For Ce2O3, firstly the Ce2O3 powder needs to

be prepared by using a reduction treatment of CeO2. Afterwards, the substrate can be prepared

by using cold press sintering as well. During Ce2O3 sintering, a protective atmosphere is

needed so as to avoid an oxidation reaction.

63

References

[1] A. Karasev and H. Suito: Meta. Mater. Trans. B., 30B (1999), 249.

[2] M.-K. Sun, I.-H. Jung and H.-G. Lee: Metal. Meta. Inter., 14 (2008), 791.

[3] A.V. Karasev and H. Suito: ISIJ Inter., 48 (2008), 1507.

[4] H. Suito, A.V. Karasev, M. Hamada, R. Inoue and K. Nakajima: ISIJ Inter., 51 (2011),

1151.

[5] A. V. Karasev and H. Suito: ISIJ Inter., 49 (2009), 229.

[6] I.-H. Jung, S. Decterov and A. D. Pelton: ISIJ Int., 44 (2004), 527.

[7] S. A. Decterov, I.-H. Jung, E. Jak, Y.-B. Kang, P. Hayes and A. D. Pelton: Proc. 7th Int.

Conf. Molten Slags, Fluxes and Salts, Café Town, South Africa (2004), 839.

[8] F. Ruby-Meyer, J. Lehmann and H. Gaye: Scand, J. Metall., 29 (2000), 206.

[9] H. Matsuura, C. Wang, G.-H. Wen and S. Sridhar: ISIJ Int., 47 (2007), 1265.

[10] W. Choi, H. Matsuura and F. Tsukihashi: ISIJ Int., 51 (2011), 1951.

[11] J. Takamura and S. Mizoguchi: Proc. Sixth Int. Iron and Steel Congress, ISIJ, Nagoya,

(1990), 591.

[13] W. Mu, P. G. Jönsson and K. Nakajima: ISIJ Int., 2014, 54, 2907.

[14] C. Chen, H.-T. Xue, H.-F. Peng, L. Yan, L. Zhi and S.-X. Wang: J. Nano., (2014).

[15] J. M. Gregg and H. K. D. H. Bhadeshia: Acta Mater., 45 (1997), 739.

[16] Y. Miki, Y. Shimada, B. G. Thomas and A. Denisor: Iron Steelmaker, 24 (1997), 31.

[17] L. Zhang, S. Taniguchi and K. Cai: Metall. Mater. Trans. B, 31B (2000), 253.

[18] H. Lei, G. Xu and J. He: Chem. Eng. Technol., 30 (2007), 1650.

[19] H. Lei and J. He: J. Non-Cryst. Solid, 352 (2006), 3772.

[20] L. Zhang and W. Pluschkell: Ironmaking Steelmaking, 30 (2003), 106.

[21] J. Zhang and H. Lee: ISIJ Int., 44 (2004), 1629.

[22] T. Mizoguchi, Y. Ueshima, M. Sigiyama and K. Mizukami: ISIJ Int., 53 (2013), 639.

[23] K. Sasai and Y. Mizukami: ISIJ Int., 41 (2001), 1331.

[24] K. Sasai: Tetsu-to-Hagané, 101 (2015), 275.

[25] S. Taniguchi, A. Kikuchi, T. Ise and N. Shoji: ISIJ Int., 36 (1996), S117.

[26] F. M. Fowkes, in J. J. Burke (Ed.), Surfaces and Interfaces, Vol. 1, Syracuse University

Press, New York, 1967, 199.

[27] W. Lin and K. Simme: Tetsu-to-Hagané, 84 (1998), 7.

[28] F. M. Fowkes: Indus. Eng. Chem., 56 (1964), 40.

64

[29] G. Böhme, H. Krupp and W. Schnabel, in Ed. Drauglis (Ed.), Molecular Processes at

Solid Surfaces, McGraw-Hill, New York, 1969, 611.

[30] M. Humenik, Jr., and W. D. Kingery: J. Am. Ceram. Soc., 37 (1953), 18.

[31] Z. Amondarain, L. Kolbeinsen and J. L. Arana: ISIJ Int., 51 (2011), 733.

[32] C.-J. Xuan, H. Shibata, S. Sukenaga, P.G. Jönsson and K. Nakajima: ISIJ Int., 55 (2015),

1882.

[33] K. Ogino, K. Nogi and Y. Koshida: Tetsu-to-Hagané, 59 (1973), 1380.

[34] E. Kapilashrami, A. Jakobsson, A. K. Lahiri and S. Seetharaman: Metal. Mat. Trans. B.,

34B (2003), 193.

[35] K. Ogino, A. Adachi and K. Nogi: Tetsu-to-Hagané, 59 (1973), 1237.

[36] H. Shibata, Y. Watanabe, K. Nakajima and S. Kitamura: ISIJ Int., 49 (2009), 985.

[37] A. W. Cramb and I. jimbo: Proc. Phillrook Memorial Conference, ISS of AIME, Toronto,

1988, 25.

[38] R. L. Coble: J. Amer. Cera. Soc., 41 (1958), 55.

[39] W. D. Kingery and M. Berg: J. Appl. Phys., 26 (1955), 1205.

[40] H. Ooi, M. Sekine and G. Kasai: Tetsu-to-Hagané, 59 (1973), 1078.

[41] C.-J. Xuan, H. Shibata, Z. Zhao, P. G. Jönsson and K. Nakajima: ISIJ Int., 55 (2015),

1642.

[42] C.-J. Xuan, A. V. Karasev and P. G. Jönsson: ISIJ Int., (to be submitted).

[43] C.-J. Xuan, W.-Z. Mu, Z. I. Olano, P. G. Jönsson and K. Nakajima: Steel Res. Int., 86

(2015) (DOI: 10.1002/sirn.201500267).

[44] C.-J. Xuan, A. V. Karasev and P. G. Jönsson: ISIJ Int., (to be submitted).

[45] C.-J. Xuan, A. V. Karasev and P. G. Jönsson: Steel Res. Int., (to be submitted).

[46] T. Koseki, S. Ohkita and N. Yurioka: Sci. Technol. Weld. Joining, 2 (1997), 65.

[47] K. Ichikawa, T. Koseki and M. Fuji: Sci. Technol. Weld. Joining, 2 (1997), 231.

[48] D. K. Owens and R. C. Wendt: J. Appl. Poly. Sci., 13 (1969), 1741.

[49] N. Takiuchi, T. Taniguchi, Y. Tanaka, N. Shinozaki and K. Mukai: J. Japan Inst. Metals,

55 (1991), 180.

[50] J. Visser: Advances Colloid Interface Sci., 3 (1972), 331.

[51] Y. Waseda, M. Tokuda and M. Ohtani: Tetsu-to-Hagané, 61 (1973), 54.

[52] V. A. Grigorian, A. J. Stomahin, A. G. Ponomarenko, L. N. Belyanchikov, Yu. I.

Utochikin, G. I. Kotenikov, and O. I. Ostrovsky: Physical-Chemical Calculation of Electric

Steelmaking Processes, Metallurgizdat, Moscow, 1989.

[53] S. K. Chuchmarev, O.A. Esin, V.M. Kamyshov : Izv. VUZ. Chern. Met., 1 (1967), 16.

65

[54] K. S. Nam, H. J. Lee, S. H. Lee, G. H. Lee, Y. S. Song and D. Y. Lee: Surf. Coat. Tech.

201 (2006), 2567.

[55] P. K. Song, M. Yamagishi, H. Odaka and Y. Shigesato: Jpn. J. Appl. Phys., 42 (2003),

L1529.

[56] R. Edy, X.-J. Huang, Y. Guo, J. Zhang and J.-J. Shi: Nano. Res. Let., 8 (2013), 1.

[57] F. P. Santos, E. Campos, M. Costa, F. C. L. Melo, R. Y. Honda and R. P. Mota: Mater.

Res., 6 (2003), 353.

[58] D. Kuscer, J. Kovac, M. Kosec, R. Andriesen: J. Eur. Cera. Soc., 28 (2008), 577.

[59] Y. Kurihara, T. Sawazumi and T. Takeuchi: Royal Soci. Chem., 2014, S1.

[60] A. Noro, M. Kaneko, I. Murata and M. Yoshinari: J. Bio. Mater. Res. B.: Appl. Bio., 00B

(2012), 1.

[61] A. Miko, A. L. Demirel and M. Somer: Mater. Sci. Eng., 12 (2010), 1.

[62] B. B. Lopes, A. P. A. Ayres, L. B. Lopes, W. M. Negreiros and M. Giannini: Appl. Ad.

Sci., 17 (2014), 1.

[63] C.-C. Sun, S.-C. Lee, W.-C. Hwang, J.-S. Hwang, I.-T. Tang and Y.-S. Fu: Mater.,

Trans., 47 (2006), 2533.

[64] A. P. Serro, C. Completo, R. Colaco, F. Santos, C. L. Silva, J. M. S. Cabral, H. Araujo, E.

Pires and B. Saramago: Sur. Coat. Tech., 203 (2009), 3701.

[65] H. Shibata, X. Jiang, M. Valdez and A. W. Cramb: Metal. Mater. Trans. B,35B (2004),

179.

[66] A. Amadeh, S. -H. Manesh, J. C. Labbe, A. Laimeche and P. Quintard: J. Eur. Ceram.,

21 (2001), 277.

[67] O. Knacke, O. Kubaschewski, and K. Hesselmann, Thermochemical Properties of

Inorganic Substances, 2nd ed. (Springer-Verlag, Berlin, 1991), 811.

[68] T. Dan, N. Aritomi, K. Ogawa, K. Honma and T. Kimura: National Res. Inst. Met.. 1

(1992), 53.

[69] Steelmaking Data Sourcebook: The Japan Soc. For The Promotion of Science, The 19t

Committee on Steelmaking (revised ed.), Gordon and Breach Science Publishers, New York,

1988.

[70] J. C. Chan, C. B. Alcock and K. T. Jacob: Can. Metall. Q., 12 (1973), 439.

[71] NIST-JANAF Thermochemical Tables, 4th

ed., ed. By M. W. Chase, Jr., J. Phys. Chem.

Ref. Data, (1998).

[72] S. Matoba and K. Gunji: Tetsu-to-Hgané, 42 (1955), 58.

[73] W. C. Hahn, Jr. and A. Muan: Tran. AIME, 224 (1962), 416.

66

[74] W.-Y. Cha, T. Miki, Y. Sasaki and M. Hino: ISIJ Int., 46 (2006), 987.

[75] W.-Y. Cha, T. Miki, Y. Sasaki and M. Hino: CAMP-ISIJ, 19(2006), 760.

[76] E. T. Turkdogan: Physical Chemistry of High Temperature Technology, Academmic

Press, New York, 5(1980), 22.

[77] The 19th

Committee on the Steelmaking, The Japan Society for the Promotion of Science

(ed): Steelmaking Data Sourcebook, Gordon and Breach Science Publishers, New York,

(1998), 279.

[78] J. B. Macchesney and A. Muan: Amer. Min., 46 (1961), 572.

[79] D. R. Stull and H. Prophet: JANAF Thermochemical Tables, 2nd

ed., NSRDS-NBS 37,

U.S. Department of Commerce,Washingtong, D. C., (1971).

[80] S. Itoh, O. Inoue and T. Azakami: Mater. Tran., JIM, 39 (1998), 391.

[81] G. I. Batalin and V. S. Sudavtsova: Sov. Prog. Chem., 9 (1976), 930.

[82] K.Morita, M. Hota, A. Yamada and M. Ito: ICS Proceedings-2005, AIST, Warrendale,

PA, (2005), 15.

[83] Y. Kawai and Y. Shiraishi: Handbook of Physico-chemical Properties at High

Temperatures, ISIJ., (1988), 178.

[84] O. Grong, L. Kolbeinsen, C. van der Ejik, G. Tranell: ISIJ Int., 46 (2006), 824.

[85] S. Eichenlaub, C. Chan and S. P. Beaudoin: J. Coll. Inter. Sci., 248 (2002), 389.

[86] K. Suzuki and K. Sanbongi: Trans. Iron Steel Inst. Jpn., 15 (1975), 618.

[87] J.-J. Pak, J.-O. Jo, S.-I. Kim, W.-Y. Kim, T.-I. Chung, S.-M. Seo, J.-H. Park and D.-S.

Kim: ISIJ Int., 47 (2007), 16.

[88] Y. Miki, Y. Shimada, B. G. Thomas and A. Denisor: Iron Steelmaker, 24 (1997), 31.

[89] L. Zhang, S. Taniguchi and K. Cai: Metall. Mater. Trans. B, 31B (2000), 253.

[90] H. Lei, G. Xu and J. He: Chem. Eng. Technol., 30 (2007), 1650.

[91] H. Lei and J. He: J. Non-Cryst. Solid, 352 (2006), 3772.

[92] L. Zhang and W. Pluschkell: Ironmaking Steelmaking, 30 (2003), 106.

[93] J. Zhang and H. Lee: ISIJ Int., 44 (2004), 1629.

[94] K. Nakanishi and J. Szekely: Trans. Iron Steel Inst. Jpn., 15 (1975), 522.

[95] T. Nakaoka, S. Taniguchi, K. Matsumoto and S. T. Johansen: ISIJ Int., 41 (2001), 1103.

[96] H. Lei, K. Nakajima and J.-C. He: ISIJ Int., 50 (2010), 1735.

[97] M. Nabeel, A. Karasev and P. G. Jönsson: ISIJ Int., 55 (2015), 2358.

[98] Editorial Committee on Fundamentals of Powder Engineering: Fundamentals of Powder

Engineering, Nikkan Kogyo Shimbun, Tokyo, 119 (1992), 206.

[99] R. A. Fisher: J. Agric. Sci., 16 (1926), 492.