Wu Chao Peng Paul 200911 MSc Thesis

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Inclusion Characterization in High Strength Low

Alloy Steel

by

Chao Peng Paul Wu

A thesis submitted in conformity with the requirements for the degree of

Master of Applied ScienceGraduate Department of Materials Science and Engineering

University of Toronto

© Copyright by Chao Peng Paul Wu, 2009



Inclusion Characterization in High Strength Low Alloy

Steel

by

Chao Peng Paul Wu

Master of Applied Science

Department of Materials Science and Engineering

University of Toronto

2009

ABSTRACT

The cleanliness of high strength low alloy (HSLA) steel was assessed qualitatively and

quantitatively. The determination of inclusion type and inclusion morphology werecarried out using Selective Potentiostatic Etching by Electrolytic Dissolution (SPEED)

method allowing in-situ examination of inclusion morphology by analytical techniques

such as SEM/EDS. Inclusion size analysis mainly involved a combination of an

analytical technique to provide images of the sample surface and an image analysis

system to accurately measure the inclusion size. Four analytical methods were compared

in order to evaluate their suitability for subsequent quantitative analysis. It was found that

images taken with backscattered electron imaging mode from the scanning electron

microscope provides the most accurate representation of inclusion distribution. The

various techniques were used to evaluate HSLA steel grades of similar chemistry

produced with and without gas shrouding. The results confirmed that with reoxidation

minimized by gas shrouding between ladle and tundish, the steel cleanliness was

significantly improved.

ii



ACKNOWLEDGEMENTS

I owe my deepest gratitude to my supervisor, Professor A. McLean, whose

encouragement, guidance, and support have motivated and inspired all his students,

myself included.

I would like to express my sincere thanks to my mentors, Dr. Y.D. Yang and Dr. H. Soda

for their constant assistance and encouragement during the term of this project. This

thesis would not have been possible without the helpful discussions. I admire Dr. Yang’s

passion for research and his meticulous approach to questions, which have taught me

much. I am indebted to Dr. Soda, who always manages to make himself available to give

counsel on all aspects.

I would like to extend my thanks to the research group members for the moral support

and friendship. Appreciation is also expressed to the staff of the MSE department for the

technical and administrative support during the course of the project.

Financial support from Natural Sciences and Engineering Council of Canada and

University of Toronto fellowship is greatly appreciated. I would also like to thank Gerdau

Ameristeel Inc. for providing me with the steel samples. I am especially grateful to Mr. S.

Paul at Gerdau Ameristeel Inc. for his helpful advice and assistance.

Finally, I would like to thank my family and friends for their immense support and

encouragement throughout my study.

iii



TABLE OF CONTENTS

ABSTRACT ........................................................................................................................ ii

ACKNOWLEDGEMENTS ............................................................................................... iiiTABLE OF CONTENTS ................................................................................................... iv

LIST OF FIGURES ........................................................................................................... vi

LIST OF TABLES ............................................................................................................. ix

NOMENCLATURE ........................................................................................................... x

CHAPTER ONE: INTRODUCTION ................................................................................. 1

1.1 Introduction and Background ............................................................................... 1

1.2 References ............................................................................................................. 3

CHAPTER TWO: LITERATURE REVIEW ..................................................................... 4

2.1 Melting and Casting Operations ........................................................................... 4

2.2 Steel Deoxidation .................................................................................................. 6

2.2.1 Thermodynamics of deoxidation ............................................................... 6

2.2.2 Single component deoxidation ................................................................... 8

2.2.3 Multi-component deoxidation .................................................................. 12

2.3 Manganese Oxide – Silicon Oxide – Aluminum Oxide System......................... 17

2.4 Classification of Non-Metallic Inclusions .......................................................... 19

2.4.1 Based on inclusion chemistry and composition ....................................... 19

2.4.2 Based on inclusion formation mechanism ............................................... 22

2.5 References ........................................................................................................... 23

CHAPTER THREE: EXPERIMENTAL ASPECTS ........................................................ 24

3.1 Overview ............................................................................................................. 24

3.1.1 Sample preparation .................................................................................. 25

3.2 Qualitative Assessment ....................................................................................... 26

3.2.1 Inclusion morphology examination (SPEED method[1]

) ......................... 26

3.2.2 Inclusion species analysis ........................................................................ 28

3.3 Quantitative Assessment ..................................................................................... 29

3.3.1 Image acquisition ..................................................................................... 29

3.3.2 Image analysis .......................................................................................... 31

iv



3.4 References ........................................................................................................... 34

CHAPTER FOUR: RESULTS AND DISCUSSION ....................................................... 35

4.1 Qualitative Assessment ....................................................................................... 35

4.1.1 Al2O3 (Alumina) ...................................................................................... 35

4.1.2 SiO2 (Silica) ............................................................................................. 41

4.1.3 MnO (Manganosite) ................................................................................. 46

4.1.4 MnO-SiO2 (Rhodonite) ............................................................................ 47

4.1.5 MnO-Al2O3 (Galaxite) ............................................................................. 50

4.1.6 CaO-Al2O3 (Calcium aluminate) ............................................................. 52

4.1.7 CaO-SiO2 (Calcium silicate) .................................................................... 59

4.1.8 CaO-Al2O3-SiO2 (Calcium aluminosilicate) ............................................ 61

4.1.9 MnS (Manganese sulphide) ..................................................................... 63

4.1.10 Development of inclusion species during steelmaking .......................... 69

4.2 Quantitative Assessment ..................................................................................... 72

4.2.1 Particle size distribution ........................................................................... 73

4.2.2 Maximum particle size ............................................................................. 76

4.2.3 Inclusion area fraction.............................................................................. 78

4.2.3 Backscattered electron image analysis (BSE-IA) in steel cleanliness study

......................................................................................................................... 80

4.3 References ........................................................................................................... 81

CHAPTER FIVE: CONCLUSIONS ................................................................................ 82

5.1 Conclusions ......................................................................................................... 82

5.2 Future Work ........................................................................................................ 83

APPENDICES .................................................................................................................. 84

Appendix A: Inclusion’s Effect on Fatigue Behaviour ............................................. 84

Appendix B: Inclusion Particle Size Distribution of 1018S Samples....................... 85

Appendix C: Inclusion Particle Size Distribution of A529 Samples ........................ 88

v



LIST OF FIGURES

Figure 2-1: Schematic of melting and casting operations in steelmaking[2]

....................... 5

Figure 2-2: Free energy of formation for various oxides. Dash-dot line indicates equal

oxygen pressure in unit of atmosphere[4]

.................................................................... 7

Figure 2-3: Deoxidizing power of various elements at 1600°C[5]

...................................... 7

Figure 2-4: a) As-polished (2-dimensional) steel sample showing Al2O3 dendrite b)

Partial slime extracted (3-dimensional) steel sample showing the same Al2O3

dendrite[1]

.................................................................................................................. 11

Figure 2-5: Equilibrium relations for manganese-silicon deoxidation of steel at various

temperatures[3]

........................................................................................................... 13

Figure 2-6: The effect of manganese content on stability of oxide phases resulting from

steel deoxidation at 1550ºC (m: mullite; l: liquid manganese silicate)[9]

................. 15

Figure 2-7: CaO-Al2O3 equilibrium phase diagram[10]

..................................................... 16

Figure 2-8: Schematic representation of MnO-SiO2-Al2O3 ternary phase diagram[6]

...... 17

Figure 2-9: Free energy of formation for various sulphides. Dash-dot line indicates equal

sulphur pressure in unit of atmosphere[4]

.................................................................. 21

Figure 3-1: Flow chart of the scheme of experiments ...................................................... 24

Figure 3-2: Sampling locations ......................................................................................... 25Figure 3-3: Schematic of SPEED apparatus

[1]................................................................ 27

Figure 3-4 Anode polarization curve ................................................................................ 28

Figure 3-5: Images acquired using (a) optical microscopy, (b) laser confocal microscopy,

(c) SEM (secondary electron mode) and (d) SEM (backscattered electron mode) .. 32

Figure 3-6: Photograph processed by image analysis showing detected area as inclusions

(a) laser confocal microscopy, (b) SEM (backscattered electron mode) .................. 33

Figure 4-1: Oxide inclusions found in 1018S ladle sample: alumina ............................... 36

Figure 4-2: Oxide inclusion found in A529 billet sample: alumina dendrites .................. 36

Figure 4-3: Oxide inclusions found in A529 ladle sample: a) alumina and galaxite (G) . 37

b) alumina cluster .............................................................................................................. 37

Figure 4-4: Glassy Al2O3 (globular) inclusions found in 1018S furnace tap sample ....... 39

Figure 4-5: Glassy Al2O3 (plate) inclusions found in 1018S ladle sample ....................... 39

vi



Figure 4-6: Oxide inclusions in steel: corundum in a) manganese aluminosilicate matrix

[1018S ladle tap sample] b) calcium aluminate matrix [A529 tundish sample] ....... 40

Figure 4-7: Oxide inclusion in A529 billet sample: cristobalite (K) in rhodonite (R) ..... 42

Figure 4-8: Oxide inclusion in A529 billet sample: rhodonite (R), low quartz (Q),

tridymite (T) and glassy silica (A) ............................................................................ 43

Figure 4-9: Oxide inclusion in A529 tundish sample: cristobalite (K) in rhodonite (R)

matrix, and glassy silica (A) ..................................................................................... 44

Figure 4-10: Oxide inclusion in 1018S tundish sample: low quartz (Q) and tridymite (T)

................................................................................................................................... 45

Figure 4-11: Oxide inclusion in 1018S billet sample: low quartz (Q) .............................. 45

Figure 4-12: Oxide inclusion in A529 billet sample: manganosite .................................. 46

Figure 4-13: Oxide inclusions found in A529 ladle tap sample: rhodonite ...................... 47

Figure 4-14: Oxide inclusions found in A529 billet sample: rhodonite (R) and cristobalite

(K) ............................................................................................................................. 48

Figure 4-15: Oxide inclusions found in A529 billet sample: rhodonite (after rolling) ..... 49

Figure 4-16: Oxide inclusions found in 1018S ladle sample: rhodonite .......................... 49

Figure 4-17: Oxide inclusions found in A529 furnace tap sample: galaxite (G) .............. 50

Figure 4-18: Oxide inclusions found in 1018S ladle tap sample: galaxite (G) and

chromium galaxite (Cr G) ......................................................................................... 51

Figure 4-19: Oxide inclusions found in 1018S ladle tap sample: calcium aluminate ...... 52

Figure 4-20: Oxide inclusions found in 1018S furnace tap sample: calcium aluminate

(CA) and galaxite (G) ............................................................................................... 54

Figure 4-21: Oxide inclusions found in A529 ladle sample: calcium aluminate (CA) .... 55

Figure 4-22: Oxide inclusions found in A529 furnace tap sample: calcium aluminate .... 56

Figure 4-23: Oxide inclusions found in 1018S ladle tap sample: calcium aluminate ...... 56

Figure 4-24: Oxide inclusions found in A529 tundish sample: calcium aluminate .......... 57

Figure 4-25: Oxide inclusions found in 1018S ladle sample: calcium aluminate ............ 57

Figure 4-26: Oxide inclusions found in 1018S ladle sample: calcium aluminate ............ 58

Figure 4-27: Oxide inclusions found in 1018S ladle sample: calcium aluminate with high

silica content ............................................................................................................. 58

Figure 4-28: Oxide inclusions found in 1018S billet sample: calcium silicate ................ 59

vii



Figure 4-29: Oxide inclusions found in 1018S furnace tap sample: calcium silicate (CS)

and corundum (C) ..................................................................................................... 60

Figure 4-30: Oxide inclusions found in A529 tundish sample: calcium silicate .............. 61

Figure 4-31: Oxide inclusions found in A529 ladle tap sample: calcium aluminosilicate 62

Figure 4-32: Oxide inclusions found in 1018S ladle sample: calcium aluminosilicate .... 63

Figure 4-33: Sulphide inclusions found in 1018S billet sample: manganese sulphide..... 64

Figure 4-34: Sulphide inclusions found in 1018S billet sample: manganese sulphide..... 65

Figure 4-35: Sulphide inclusions found in A529 billet sample: manganese sulphide ...... 66

Figure 4-36: Sulphide inclusions found in A529 billet sample: manganese sulphide ...... 67

Figure 4-37: Sulphide inclusions found in A529 billet sample: manganese sulphide –

additional morphologies ............................................................................................ 67

Figure 4-38: Sulphide inclusions found in 1018S billet sample: manganese sulphide scale

(S) around silicate matrix (M)................................................................................... 68


(S) around silicate matrix (M)................................................................................... 69

Figure 4-40: Inclusion size distribution of 1018S............................................................. 74

Figure 4-41: Inclusion size distribution of A529 .............................................................. 74

Figure 4-42: Comparison of total inclusion count ............................................................ 75

Figure 4-43: Maximum particle size plot of 1018S steel samples .................................... 76

Figure 4-44: Maximum particle size plot of A529 steel samples ..................................... 77

Figure 4-45: Comparison of maximum particle size ........................................................ 77

Figure 4-46: Inclusion area fraction of 1018S steel samples ............................................ 78

Figure 4-47: Inclusion area fraction of A529 steel samples ............................................. 79

Figure 4-48: Comparison of inclusion area fraction ......................................................... 79

Figure 5-1: Proposed levitation apparatus ........................................................................ 83

viii



LIST OF TABLES

Table 2-1: Cleanliness requirements for steel products[1]

................................................... 4

Table 2-2: Stoichiometric composition of reported inclusion phases in Figure 2-8

[6]

...... 18Table 2-3: Inclusion phases found in MnO-SiO2-Al2O3, FeO-SiO2-Al2O3, and MnO-SiO2-

Cr 2O3 systems[6]

........................................................................................................ 19

Table 3-1: Specimen chemical composition ..................................................................... 25

Table 4-1: Summary of inclusion types present in A529 steel ......................................... 70

Table 4-2: Summary of inclusion types present in 1018S steel ........................................ 70

Table A: Coefficient of thermal expansion of various inclusion types[2]

......................... 85

ix



NOMENCLATURE

Symbols Units

ax Activity of “x”

d Maximum particle size μm

ΔG° Free energy of formation kCal

K Equilibrium constant

T Temperature K

T[O] Total oxygen ppm

λ Wavelength μm

α Coefficient of thermal expansion K -1

Element Abbreviations

Al Aluminum

C Carbon

Ca Calcium

Cu Copper

Cr Chromium

Fe Iron

O Oxygen

P Phosphorus

Pt Platinum

Mg Magnesium

Mn Manganese N Nitrogen

Ni Nickel

S Sulphur

Si Silicon

x



Compound Abbreviations

Al2O3 Alumina

CaO Calcia

CaO•Al2O3 Calcium aluminate

CaO•Al2O3•SiO2 Calcium aluminosilicate

CaO •SiO2 Calcium silicate

CaS Calcium sulphide

FeO Wüstite

FeO•Al2O3 Hercynite

FeS Troilite

MgO Periclase

MnO Manganosite

MgO•Al2O3 Spinel

MnO•Al2O3 Galaxite

MnO•SiO2 Rhodonite

MnS Manganese sulphide

SiO2 Silica

Abbreviations

ASTM American Society for Testing and Materials

BSE Backscattered Electron

DIC Differential Interference Contrast

EAF Electric Arc Furnace

EDS Energy Dispersive Spectrometry

HSLA High Strength Low Alloy

IA Image Analysis

LCM Laser Confocal Microscope

LIMCA Liquid Metal Cleanliness Analyzer

MIDAS Mannesmann Inclusion Detection by Analysis Surfboards

xi



xii

OES Optical Emission Spectrometry

OM Light-Optical Microscope

PDA Pulse Discrimination Analysis

ppm parts per million

SE Secondary Electrons

SEN Submerged Entry Nozzle

SEM Scanning Electron Microscope

SPEED Selective Potentiostatic Etching by Electrolytic Dissolution

wt% weight percent



CHAPTER ONE: INTRODUCTION

1.1 Introduction and Background

The standards governing steel cleanliness vary among different applications of the final

product. With decreasing product cross-sectional dimension, such as thin strip steel, the

presence of critically sized inclusions has increasing effects on product performance

hence the rising cleanliness requirements for steel quality in recent decades. Inclusion

particle size distribution is frequently used to provide characteristic description of steel

cleanliness. A comprehensive cleanliness evaluation also requires complementary

information on non-metallic inclusions found such as quantity, spatial distribution, type,

and morphology.

With the growing demand for high quality materials, the cleanliness of different steel

types has become very important. High strength low alloy (HSLA) steels offer improved

weldability, and superior mechanical and corrosion-resistant properties compared to mild

or low carbon steel at a minor price premium.[1]

Load-bearing HSLA steel found in

structural applications is susceptible to fatigue failure initiated by non-metallic

inclusions. However, very little is known about the types and amounts of inclusions present in this class of steel.

The great challenge faced by steelmakers, at the present, is the lack of reliable cleanliness

assessment methods to quickly evaluate the quality of steel products at low cost. The

cleanliness assessments performed traditionally using metallurgical microscopy, chart

comparison, and visual inspection of semi-finished products no longer give adequate

feedback in regard to micro-cleanliness. Image analysis is a powerful tool in assessing

steel cleanliness with improved efficiency and accuracy. However, it has been reported[2]

that with automated image analysis using optical microscopy there exists a common

problem of potential erroneous detection of defects arising from inadequate sample

preparation.

1





• To examine the effect of melt protection on the cleanliness of steel products.

1.2 References

[1] W.D. Callister, Materials Science and Engineering – An Introduction, John Wiley &

Sons, Inc., New York, NY, 2003, 6th

edition, p.336

[2] S. Johansson, “Inclusion assessment in steel using the new Jernkontoret Inclusion

Chart II for quantitative measurements”, Clean Steel 3 Conference Proceedings, The

Institute of Metals (London), 1987, pp. 60-67

[3] S. Millman, “Clean steel – Basic features and operating practices”, IISI Study on

Clean Steel, International Iron and Steel Institute, Belgium, 2004, pp. 8-10

[4] D.M. Mainy, J.P. Nectoux and R. Blondeau, “Contribution of computer-aided

quantitative microprobe analysis to determination of nonmetallic inclusions in steels”,

Clean Steel 3 Conference Proceedings, The Institute of Metals (London), 1987, pp. 78-

84

3



CHAPTER TWO: LITERATURE REVIEW

Most non-metallic inclusions present in steel have detrimental effects on properties,

which will lead to poor formability of the product as well as problems associated with

fatigue life. In addition to improved formability, cleaner steel also benefits the coating

and corrosion resistant properties. Cleanliness requirements for steel products (Table 2-1)

are often measured in total oxygen (T[O]), and maximum particle size (d).

Table 2-1: Cleanliness requirements for steel products[1]

Product Cleanliness Notes

Automotive sheet T[O] < 20ppm

d < 100µm

Ultra deep drawing

Drawn and ironed cans T[O] < 20ppm

d < 20µm

Crack prevention during

hot-rolling

Lead frame for LSI d < 5µm Crack prevention in

punch forming

Shadow mask for CRT d < 5µm Prevention of photo

etching

Tire cord T[O] < 15ppm

d < 20µm non-plastic inclusions

Prevention of rupture in

wire drawing

Ball bearings T[O] < 10ppm

d < 15 µm

Increased fatigue life

Line pipe T[O] < 30ppm

d < 100µm oxide shape

Sour gas service

2.1 Melting and Casting Operations

The melting and casting operations, shown in Figure 2-1, are most crucial to cleanliness

of steel throughout the steelmaking process. Charge materials include steel scrap,

limestone, and metallurgical coke fed into an electric arc furnace, where the melting

operation takes place. Once molten, decarburization is then carried out by injecting

4



oxygen into the steel melt; lowering carbon content to the desired level governed by the

specific steel grade. The molten decarburized steel is then tapped into a ladle, which is

transported to a continuous casting station. Within the ladle vessel, trim alloying elements

and deoxidizers such as ferromanganese and ferrosilicon are added to the steel melt in

order to remove excessive oxygen carried over from the decarburization process.

Manganese and silicon form stable oxides with dissolved oxygen in the steel melt, where

most oxide particles are removed by floatation, assisted by inert gas stirring and

collection at the slag layer. However, the complete removal of oxide particles cannot be

done within reasonable amount of time. The remaining oxide particles exist as inclusions

in solid steel. These inclusions are often classified indigenous in origin (Section 2.4.2).

Figure 2-1: Schematic of melting and casting operations in steelmaking[2]

Another source of inclusions may arise from oxidation of steel during transfer between

ladle to tundish, and tundish to mold. If the stream of previously deoxidized steel is

allowed to come in contact with oxygen-containing atmosphere, the dissolved

deoxidizers and some alloying elements will readily form oxide particles which have less

opportunity to be removed prior to casting and are likely to remain as inclusions. This

process is also known as reoxidation. Inclusions of this origin are often classified as

exogenous inclusions (Section 2.4.2). Implementing refractory/inert gas shrouding

5



between ladle and tundish and submerged entry nozzles between tundish and mold can

minimize reoxidation.

2.2 Steel Deoxidation

Maximum solubility of oxygen in liquid iron at the eutectic of 1527°C is about 0.16%.[3]

The oxygen solubility in solid iron, at temperature slightly below its melting point,

approaches zero. Upon solidification, majority of dissolved oxygen will precipitate as

FeO inclusions. In steel, the presence of alloying elements such as carbon can influence

the dissolved oxygen content. Equation 2-1 describes carbon-oxygen relationship in iron

up to 0.6% carbon.

[wt%C] • [wt%O] = ~0.0023 (2-1)

In order to prevent blowhole (carbon monoxide gas) formation, porous cast product, or

precipitation of FeO inclusions in sizeable quantities, liquid steel must be deoxidized

prior to casting.

2.2.1 Thermodynamics of deoxidation

The role of deoxidation process is to lower the oxygen content in liquid steel.

Deoxidation is commonly carried out by additions of elements having greater affinity for

oxygen than iron, this method is also known as precipitation deoxidation. The oxygen

affinity of various elements can be compared with free energy of oxide formation. Figure

2-2 gives a plot of curves for common elements found in steelmaking.

While elements having free energy of oxide formation lower than FeO are potential

candidates as deoxidizers, it is also important to consider that activity of these elements

in solution with liquid steel deviates from that of the pure elements. Figure 2-3 depicts

the deoxidizing power of various elements at 1600°C.

6



Figure 2-2: Free energy of formation for various oxides. Dash-dot line indicates equal

oxygen pressure in unit of atmosphere[4]

Figure 2-3: Deoxidizing power of various elements at 1600°C[5]

7



2.2.2 Single component deoxidation

Four cost-effective deoxidizers are carbon, manganese, silicon, and aluminum. Carbon is

often considered an effective deoxidation element, forming gaseous deoxidation

products. Carbon deoxidation does not generate inclusions and therefore will not be

discussed further, however, during the casting process, carbon in liquid steel may reduce

oxide inclusions resulting in gas formation and pinhole porosity.[6]

A general deoxidation

reaction can be described using Equation 2-2, where x and y are stoichiometric terms, M

is the dissolved deoxidizer, O is oxygen.

x[M]steel + y[O]steel = (MxOy) (2-2)

Manganese deoxidation

Manganese, in pure form, is rarely utilized as a deoxidizer. Mn is often introduced to

steel in the form of low C or high C ferroalloy. Mn and Fe will both participate in the

deoxidation reaction forming MnO-FeO product in liquid or solid solutions. A detailed

study by Lismer and Pickering[7]

has revealed that Mn deoxidation products are typically

small and homogeneously distributed in the steel and the morphology of this inclusion

type is mostly influenced by the MnO-FeO ratio. For inclusions with MnO content of up

to 30%, the morphology was globular single-phase or sometimes dual-phase spheres.

These inclusions rich in FeO had solidified after the matrix steel was solid. On the other

hand, for steel containing more than 0.7%Mn, it was found that the deoxidation products

are mostly pure MnO. Nearly pure MnO inclusions, having higher melting temperature

than steel, would solidify before steel, and therefore are characterized by a dendritic

structure.[6]

The manganese deoxidation reaction,

[Mn] + [O] = (MnO) (s) (2-3)

8



and corresponding equilibrium constant equation,

]][%[% O Mn

aK MnO

O Mn =− (2-4)

33.512440

log −=−T

K O Mn (2-5)

For = 1, the value of the equilibrium constant for manganese deoxidation is MnOa

K Mn-O = [%Mn][%O] = 4.88 x 10-2

at 1600ºC (2-6)

Silicon deoxidation

It can be seen from Figure 2-3, silicon has a much-improved deoxidizing power

compared with manganese. Deoxidation with pure silicon will yield either liquid iron

silicates or solid silicon oxide as reaction products at steelmaking temperature. Iron

silicate inclusions, like many other silicates, are usually glassy in appearance and

globular in morphology. Silicon oxides within steel exist in several modifications as a

result of various possible spatial arrangements of the SiO2 tetrahedra molecules. Low

quartz, high-quartz, tridymite, and cristobalite are among the common modifications[6]

;

where tridymite and cristobalite are high temperature modifications of silica. Due to

similar structures, low quartz-high quartz transformation as well as tridymite-cristobalite

transformation are fast and can be easily reversed. However, the transformation between

quartz and tridymite or cristobalite is a much slower process as the energy associated

with breaking the tetrahedral bonds are greater. The given reaction time and temperature

during ladle treatment are inadequate for the transformation of quartz to tridymite or

cristobalite to reach completion. On the contrary, tridymite and cristobalite, often formed

as deoxidation product, do not transform to quartz within the time-frame of subsequent

cooling and casting of steel. Therefore, the type of modification and composition can be

utilized as indicators for assessing silica inclusion’s origin.

9



The silicon deoxidation reaction,

[Si] + 2[O] = SiO2 (s) (2-7)


2]][%[%

2

OSi

aK

SiO

OSi =− (2-8)

5.1130000

log −=−T

K OSi (2-9)

For = 1, the value of the equilibrium constant for silicon deoxidation is2SiOa

K Si-O = [%Si][%O]2= 2.26 x 10

-5at 1600ºC (2-10)

Aluminum deoxidation

From Figure 2-3, it is clear that Aluminum is one of the most effective deoxidizers usedfor steel deoxidation. In aluminum deoxidized steel, there are generally two species of

deoxidation products: solid hercynite (FeO-Al2O3 spinel) and solid corundum (Al2O3, α-

modification). Among the two deoxidation products, corundum is the dominant species

found in steel. Corundum phase is characterized by having unique faceted shapes and

relative smaller diameter as single particles. It has been reported by Rege et al[8]

that

Al2O3, during deoxidation, follows dendritic growth pattern as shown in Figure 2-4. For

steels deoxidized solely with aluminum, α-Al2O3 products are formed; clusters of these

particles tend to remain as inclusions in steel. Corundum inclusions, usually having the

particle size of 1 to 5 μm, have a tendency to agglomerate upon colliding with one

another in order to lower the overall contact area with molten steel and therefore

effectively stabilize the entire unit by minimizing the surface energy.[6]

10



Figure 2-4: a) As-polished (2-dimensional) steel sample showing Al2O3 dendrite b)

Partial slime extracted (3-dimensional) steel sample showing the same Al2O3 dendrite[1]

Solid deoxidation products are often associated with nozzle clogging during casting of

liquid steel. This phenomenon is mainly caused by solid alumina inclusions having high

contact angles with liquid steel; therefore, alumina inclusions will readily anchor onto

refractory surfaces followed by subsequent agglomeration of inclusions.

Indigenous inclusions from aluminum deoxidation may take on different morphology

depending on the generation mechanism. There are generally three Al2O3 inclusion

generation processes:

I. Nucleation by super-saturation:

Al2O3 inclusions nucleate homogeneously in the steel bath as a result of super-

saturation. The resulting inclusions are finely dispersed corundum clusters[6]

II. Nucleation and growth on existing nuclei:

The existing nuclei can be both indigenous and exogenous in nature. Manganese and

silicon deoxidation products as well as emulsified furnace slag and eroded

refractories can serve as low-energy sites for Al2O3 inclusions to nucleate without

reaching super-saturation in the bath.

III. Reaction between aluminum metal and oxygen:

Excess aluminum addition or poor homogenization of the bath can lead to local high

concentration of aluminum metal reacting with dissolved oxygen. Reactions that

11



occur under localized superheat may reach the melting point of Al2O3; therefore the

products are partly molten Al2O3 inclusions sometimes having glassy appearance.

The aluminum deoxidation reaction,

2[Al] + 3[O] = Al2O3 (s) (2-11)


32 ][%][%

32

O Al

aK

O Al

O Al =− (2-12)

5.2062780

log −=−T

K O Al (2-13)

For = 1, the value of the equilibrium constant for aluminum deoxidation is32O Ala

K Al-O = [%Al]2[%O]

3= 9.58 x 10

-14at 1600ºC (2-14)

2.2.3 Multi-component deoxidation

In conventional ladle deoxidation, a combination of deoxidizers are utilized to achieve

improved deoxidation result, giving much lower residual oxygen in the bath. It is a

common practice to perform partial deoxidation while filling the tap ladle followed by

final killing of steel with aluminum at the ladle furnace station. This practice has many

advantages: (1) promotes the formation of low-melting-point deoxidation products with

ease of removal from the melt; (2) improves the solubility of elements having relative

high vapor pressure such as calcium and magnesium; (3) minimizes nitrogen pick-up

during furnace tapping[4]

.

12



Silicon-manganese partial deoxidation

Figure 2-5: Equilibrium relations for manganese-silicon deoxidation of steel at various

temperatures[3]

The practice of tap ladle deoxidation can effectively improve the extent of deoxidation

and at the same time minimize aluminum deoxidizer additions. Two general types of

deoxidation products may result from Si-Mn deoxidation: solid silica and liquid

manganese silicate at the steelmaking temperature. Under the influence of increasing

manganese content, the activity of silica is lowered. As the activity of silica decreases,

deoxidation products deviate from pure silica to molten manganese silicate. It was

suggested by Turkdogan[3]

that there exist critical ratios of [%Si]/[%Mn]2

at a given

temperature, which govern the type of deoxidation products formed. As shown in Figure

2-5, for steel compositions left of the curve, the deoxidation products will be solid silica

which indicates the absence of manganese participation in the reaction. On the other hand, for liquid steel containing higher manganese content (right of the curve) the

primary deoxidation products are likely to be liquid manganese silicate.

13



The equilibrium reaction governing Mn/Si deoxidation,

[Si] + 2MnO = 2[Mn] + SiO2 (2-15)


2

2

][%

][%2

MnO

SiO

Si MnaSi

a MnK

⋅

⋅=− (2-16)

27.11510

log +=−T

K Si Mn (2-17)

The Mn/Si deoxidation products are typically found to be globular and glassy in

appearance along with silica or rhodonite precipitation within the matrix of manganese

silicate. To facilitate the removal of deoxidation products, manganese is added as an

inclusion modifier yield liquid manganese silicates for improved coalescence and

flotation to the slag layer.

Manganese-silicon-aluminum deoxidation

In modern practice, it is common to charge deoxidizers into the tapping ladle during ladle

filling. The charge deoxidizers often consist of all three deoxidizers; manganese and

silicon in the form of ferromanganese, ferrosilicon, or silicomanganese, as well as

aluminum. The phases of resulting deoxidation products depend heavily on steel

chemistry and reaction temperature as illustrated in Figure 2-6. In the absence of

manganese, only solid phases such as silica, alumina and mullite are possible. On the

other hand, with manganese participating in steel deoxidation, the fourth phase - liquidmanganese silicate becomes stable; the stability range of liquid manganese silicate also

increases with increasing manganese content.

14



Figure 2-6: The effect of manganese content on stability of oxide phases resulting fromsteel deoxidation at 1550ºC (m: mullite; l: liquid manganese silicate)

[9]

Liquid silicates, in this deoxidation process, are characterized by an aluminum-rich core

and a shell of gradual increase in MnO-SiO2 content towards steel-inclusion interface.

The outer glassy MnO-Al2O3-SiO2 matrix, in metastable condition, was often found to

precipitate phases such as mullite, galaxite, and corundum lathes upon cooling in solid

state. These precipitates can nucleate easily on small steel particles or solidified slag

droplets within the inclusion.

Calcium modification

From Figure 2-2, it can be seen that calcium has a strong affinity to oxygen and could

potentially be utilized as steel deoxidizer. The challenge, however, lies in the following

properties of calcium: low boiling point (1439ºC), limited solubility in steel (0.032% Ca

at 1600ºC), and high vapor pressure at 1600ºC (1.81atm).[10]

Due to these reasons, it is

rather difficult to introduce calcium to molten steel in its metallic form, and it is usually

added as various iron-containing Ca-Si alloys. The primary deoxidation products are

therefore calcium silicates, which may also contain other oxides. When combinations of

Ca and Al or Mn/Si deoxidation are carried out, the primary deoxidation products can be

modified to oxides with lower activity and hence improve the removal of dissolved

oxygen. By converting the solid alumina inclusions to liquid calcium aluminates, the

15



extent of deoxidation can be improved from 8-10ppm O to 1ppm O in Al-killed steel

(0.05% Al).[9]

With a CaO:Al2O3 ratio of 12:7, calcium treated Al2O3 can reach a melting

point of 1360ºC at the CaO-Al2O3 eutectic (Figure 2-7) and therefore exists in the liquid

state at steelmaking temperatures. Moreover, there exist five modifications of calcium

aluminates as indicated in Figure 2-7; 12CaO•7Al2O3, 3CaO•Al2O3 and CaO•Al2O3 are

liquid, while CaO•2Al2O3 and CaO•6Al2O3 are solid at steelmaking temperatures.

Figure 2-7: CaO-Al2O3 equilibrium phase diagram[10]

Instead of agglomerating, in alumina inclusions, liquid calcium aluminates will coalesce

upon contact due to better wetting with liquid steel and will not easily attach onto

refractory surfaces. Hence, solid deoxidation products can also be calcium treated so that

the steel casting process is clogging-free.

16



2.3 Manganese Oxide – Silicon Oxide – Aluminum Oxide System

The MnO-SiO2-Al2O3 system effectively covers most of relevant inclusion phases that

result from combination of Mn, Si, and Al deoxidation. Figure 2-8 summarizes many

complex inclusions having compositions made up of various SiO2, MnO, and Al2O3

primary oxide contents. It is important to note that each inclusion species will have its

own homogeneity range in addition to stoichiometric compositions listed in Table 2-2.

Figure 2-8: Schematic representation of MnO-SiO2-Al2O3 ternary phase diagram[6]

Other inclusion systems such as FeO-SiO2-Al2O3 and MnO-SiO2-Cr 2O3 share many

similarities with the MnO-SiO2-Al2O3 system. Considerable numbers of MnO-SiO2-

Al2O3 inclusion phases exist with complete or part substitution of MnO with FeO due to

wide range of solid solubility; with the exception of FeO-SiO2 (counterpart to MnO-

SiO2), which has yet to be reported as an inclusion phase in the literature. According to

Figure 2-2, manganese has a stronger affinity for oxygen than iron and therefore it is also

common to find MnO among inclusions belonging to the FeO-SiO2-Al2O3 system. On the

17



other hand, Al2O3 and Cr 2O3 are interchangeable at elevated temperatures due to their

structural resemblance. Corresponding inclusion phases were often reported in both

MnO-SiO2-Al2O3 and MnO-SiO2-Cr 2O3 with notable difference in the absence of ternary

phases in the MnO-SiO2-Cr 2O3 system.[5]

Corresponding phases relating to MnO-SiO2-

Al2O3, FeO-SiO2-Al2O3, and MnO-SiO2-Cr 2O3 systems are summarized in Table 2-3.

Table 2-2: Stoichiometric composition of reported inclusion phases in Figure 2-8[6]

Mineral

classification

Chemical

formula

Stoichiometric composition (wt%)

MnO SiO2 Al2O3

Corundum Al2O3 -- -- 100

Cristobalite SiO2 -- 100 --

Tridymite SiO2 -- 100 --

Quartz SiO2 -- 100 --

Manganosite MnO 100 -- --

Galaxite MnO · Al2O3 41 -- 59

Mullite 3Al2O3 · 2SiO2 -- 28 72

Rhodonite MnO · SiO2 54 46 --

Tephroite 2MnO · SiO2 70 30 --

Spessartite 3MnO · Al2O3 ·

3SiO2

43 36 21

Mn-Anorthite MnO · Al2O3 ·

2SiO2

24 41 35

Mn-Cordierite 2MnO · 2Al2O3

· 5SiO2

22 46 32

18



Table 2-3: Inclusion phases found in MnO-SiO2-Al2O3, FeO-SiO2-Al2O3, and MnO-SiO2-

Cr 2O3 systems[6]

MnO-SiO2-Al2O3 FeO-SiO2-Al2O3 MnO-SiO2-Cr2O3

Mineral

classification

Chemical

formula

Mineral

classification

Chemical

formula

Mineral

classification

Chemical

formula

Corundum Al2O3 Corundum Al2O3 Escolaite Cr 2O3

Cristobalite SiO2 Cristobalite SiO2 Cristobalite SiO2

Tridymite SiO2 Tridymite SiO2 Tridymite SiO2

Quartz SiO2 Quartz SiO2 Quartz SiO2

Manganosite MnO Wüstite FeO Manganosite MnO

Galaxite MnO · Al2O3 Hercynite MnO ·

Al2O3

ChromiumGalaxite

MnO · Cr 2O3

Mullite3Al2O3 ·

2SiO2 Mullite

3Al2O3 ·

2SiO2 -- --

Rhodonite MnO · SiO2 -- -- Rhodonite MnO · SiO2

Tephroite 2MnO · SiO2 Fayalite2FeO ·

SiO2 Tephroite 2MnO · SiO2

Spessartite

3MnO ·

Al2O3 ·

3SiO2

Almandine

3FeO ·

Al2O3 ·

3SiO2

-- --

2.4 Classification of Non-Metallic Inclusions

2.4.1 Based on inclusion chemistry and composition

Oxides

In general, oxide inclusions can be classified into:

• Single oxides; some common examples: FeO, Fe2O3, MnO, SiO2, Al2O3, Cr 2O3,

TiO2

19



• Complex oxides, often takes the general form of AO•B2O3, where metal A has +2

oxidation number and metal B has +3 oxidation number. Some common examples

are FeO•Al2O3, MnO•Al2O3, MgO•Al2O3, FeO•Cr 2O3, MnO•Cr 2O3

Complex oxide inclusions are sometimes known as spinel type (MgO•Al2O3) inclusions

for their similarity in structures. Spinel type inclusions are characterized by faceted

structure and high melting temperature, usually higher than steelmaking temperature of

1873K. Spinel inclusions are especially harmful during steel processing as they do not

deform during hot rolling and often cause poor surface finish.

Calcium aluminate (CaO•Al2O3) type inclusions are also considered complex oxide

inclusions. Calcium and barium, have +2 oxidation number, but do not form spinel

structures due to their relatively large ionic radius. With common calcium treatment

practice, the usual Al2O3 inclusions are modified to calcium aluminates, which

effectively lower the melting temperature of inclusions from 2293K to around 1700K.

Sulphides

Sulphide inclusions are important to consider since it is common to have steel with

oxygen content less than 0.02% while having sulphur content at around 0.03%. Liquidsteel has a high solubility of sulphur where solid steel usually has significantly lower

sulphur solubility. As liquid steel cools, sulphur segregates and forms FeS with melting

point of 1460K. FeS often causes embrittlement of steel during heat treatment. Therefore

it has become a common practice to add sufficient amount of Mn, due to manganese’s

stronger affinity for sulphur, to form MnS (Tm = 1870K). Types of sulphide inclusions

will also depend on manganese to sulphur ratio. Examples of common sulphide

inclusions include MnS, FeS, (Mn, Fe)S and CaS. The sulphur affinity of various

elements can be compared with free energy of sulphide formation. Figure 2-9 gives a plot

of curves for common elements found in steelmaking.

20



Figure 2-9: Free energy of formation for various sulphides. Dash-dot line indicates equal

sulphur pressure in unit of atmosphere[4]

Two morphologies are frequently observed:• Globular: Both simple sulphides and oxysulphides, where the latter consists of

sulphides and oxides coexisting in one inclusion. This type of morphology is

generally present in silicon killed or semi-killed steel using aluminum, titanium,

or calcium.

• Faceted: Often appears in steel heavily deoxidized with aluminum.

Nitrides

In the presence of elements having high affinity for nitrogen, nitrides such as AlN, TiN,

ZrN, VN, BN, etc. can form as a result of molten steel contacting with air atmosphere

during unprotected vessel transfer. Like carbides, nitride inclusion contents in steel are

significantly less than that of oxides and sulphides.

21



2.4.2 Based on inclusion formation mechanism

There are generally two sources of inclusions in steel: exogenous, indigenous.

• Exogenous inclusions, usually larger in size, are results of reoxidation, slag

entrainment and refractory erosion. Although exogenous inclusions are generally

more harmful than indigenous inclusions, simple detection methods (due to larger

size) as well as fewer occurrences have reduced the concern for exogenous

inclusions significantly. In addition, with careful control of stirring and flowrate

monitoring, the amount of exogenous inclusions can be minimized.

• Indigenous inclusions, such as deoxidation products, are generated by chemical

reactions between dissolved species in the steel bath and are generally smaller in

size. Deoxidation products originate from the reaction between dissolved oxygen

and added deoxidants and can be both solid and liquid at steelmaking

temperatures. The presence of a few large indigenous inclusions has a strong

effect on the properties of steel products.

Indigenous inclusions often go through a series of transformations as the steel cools from

1600°C to room temperature. While trying to maintain equilibrium with the surroundings,inclusions may be undercooled during some steps of the treatment and result in

amorphous phases, or solidify and take the form of supersaturated solid solution.

Indigenous inclusions can therefore be categorized into formation steps, as summarized

below:

I. Primary inclusions: generated during deoxidation reaction

II. Secondary inclusions: generated due to equilibrium shift as temperature decreases

during vessel transfer, such as tapping and teeming operations

III. Tertiary inclusions: generated during the process of solidification, usually

characterized by rapid cooling

IV. Quaternary inclusions: generated during solid state phase transformation, which

causes changes in solubility limits of various constituents

22



2.5 References

[1] L. Zhang and B.G. Thomas, “State of the Art in Evaluation and Control of Steel

Cleanliness – Review”, ISIJ International, 2003, vol. 43, no. 3, pp. 271–291

[2] http://www.matter.org.uk/steelmatter/casting/5_1_5_2_7.htm, “Entrapment of non-

metallic inclusions”, Corus Corp. and Matter, date accessed: June 16, 2009

[3] E.T. Turkdogan, Fundamentals of Steelmaking, The Institute of Materials (London),

1996, pp. 111-113

[4] A. Muan and E.F. Osborn, Phase Equilibria Among Oxides in Steelmaking, Addison-

Wesley, Reading, Mass., USA, 1965, p. 4

[5] H.A. Sloman and E.L. Evans, JISI , 1951, vol. 169, pp. 145-152

[6] R. Kiessling and N. Lange, Non-Metallic Inclusions in Steel, The Institute of

Materials (London), 1978, vol. 2, pp. 13-50

[7] R.E. Lismer and F.B. Pickering: JISI , 1952, vol. 170, pp. 48-50

[8] R.A. Rege, E.S. Szekeres and W.D. Forgeng, "Three-Dimensional View of Alumina

Clusters in Aluminum-Killed Low-Carbon Steel", Met. Trans., AIME, 1970, vol. 1, no. 9,

pp. 2652-2653

[9] S. Millman, “Clean steel – Basic features and operating practices”, IISI Study on

Clean Steel, International Iron and Steel Institute, Belgium, 2004, pp. 39-60

[10] T. Ototani, Calcium Clean Steel, Springer-Verlag, New York, 1986, pp. 2-9

23

http://www.matter.org.uk/steelmatter/casting/5_1_5_2_7.htm

http://www.matter.org.uk/steelmatter/casting/5_1_5_2_7.htm



CHAPTER THREE: EXPERIMENTAL ASPECTS

3.1 Overview

The main purpose of this study was to characterize the non-metallic inclusions found in

high strength low alloy steel for structural applications and to track the development of

inclusions throughout the melting and casting operations. To do this, the experimental

approach was divided into two parts: qualitative and quantitative aspects. Qualitative

assessment involves inclusion morphology examination and inclusion type determination

by combining electrolytic dissolution technique for sample preparation and analytical

techniques such as scanning electron microscope (SEM) and energy dispersive x-ray

spectroscopy (EDS). Quantitative assessment involves the inclusion detection and size

determination, which ultimately leads to the construction of inclusion particle size

distribution by image analysis method. The experimental approaches are summarized in

Figure 3-1.

Qualitative Assessment Quantitative Assessment

Dissolution of matrix by SPEED

method

Image acquisition by SEM –

Backscattered electron mode

Inclusion species and morphology

stud b SEM and EDS

Sample preparation (grinding and polishing of specimen)

Inclusion Analysis

Inclusion counting by image

anal sis

Figure 3-1: Flow chart of the scheme of experiments

24



3.1.1 Sample preparation

Steel grades involved in this study are ASTM 529 grade 50 and ASTM 1018S High

Strength Low Alloy steels for structural applications, both of which are silicon-killed and

provided by Gerdau Ameristeel at Whitby, Ontario. Two metal samples were taken from

each stage of the melting and casting operations. The chemical analysis results

corresponding to each sampling location are summarized in Table 3-1. The sampling

locations include furnace tap, ladle, ladle tap, tundish and billet as depicted in Figure 3-2.

Table 3-1: Specimen chemical composition

Grade description C Mn P S Si Cu Ni Cr

529 (50)Furnace tap 0.1 0.64 0.01 0.05 0.13 0.31 0.1 0.04

Ladle tap 0.16 0.68 0.01 0.04 0.19 0.31 0.1 0.04

Tundish 0.17 0.74 0.01 0.03 0.17 0.33 0.09 0.05

Billet 0.18 0.8 0.01 0.03 0.18 0.33 0.1 0.05

Figure 3-2: Sampling locations

Grade description C Mn P S Si Cu Ni Cr

1018S Furnace tap 0.05 0.67 0.008 0.035 0.16 0.20 0.051 0.049

Ladle tap 0.16 0.66 0.008 0.025 0.17 0.19 0.046 0.060

Tundish 0.167 0.68 0.009 0.028 0.19 0.19 0.054 0.060

Billet 0.160 0.68 0.010 0.027 0.18 0.19 0.054 0.056

25



Specimens were ground to grit 1200 on wet silicon carbide papers followed by 1 μm

alumina suspended solution and then 0.3 μm alumina suspended solution for final

polishing on velvet cloth. It is important to note that contamination of abrasive particles

from either silicon carbide paper or alumina-polishing agent may take place during

sample preparation.

3.2 Qualitative Assessment

Both inclusion morphology and inclusion species have a dominant effect on steel

properties. An electrolytic dissolution technique was used to selectively dissolve the steel

matrix, thereby allowing examination of inclusion morphology by SEM and subsequent

inclusion type determination with EDS.

3.2.1 Inclusion morphology examination (SPEED method[1]

)

The Selective Potentiostatic Etching by Electrolytic Dissolution (SPEED) method is a

selective etching technique developed for in-situ observation and analysis of inclusions,

precipitates and grain orientations in steel samples. By varying control parameters such

as the composition of electrolyte, applied potential, current density and temperature, thematrix material can be selectively dissolved into the electrolytic solution and leave

behind the phases of interest, such as inclusions and precipitates. A schematic of the

SPEED apparatus is shown in Figure 3-3.

26



Calomel reference electrode

Cathode (Pt)

Electrolyte

Power

control unit

Anode (sample)

Filter clamp

Filter

Draining vessel

Figure 3-3: Schematic of SPEED apparatus [1]

In this method, non-aqueous solutions are employed as electrolyte in which the samples

are etched at predetermined constant electrical potential. The electrolyte used consists of

10% acetylacetone [CH3COCH2COCH3] – 1% tetramethylammonium chloride

[(CH3)4 NCl] – balance methyl alcohol [CH3OH].[1]

The optimum applied electrical

potential, which varies with different materials, was determined by conducting a

preliminary etching procedure on a sample material. The preliminary etching procedurestarts with applying –500 mV to the system and increases by increments of 50 mV every

5 minutes until 800 mV applied potential is reached. The measured current was recorded

following each applied potential change. Gathered data were used to construct a current-

potential curve, also known as an anode polarization curve, where the optimum applied

potential at 400 mV was indicated by the first peak on the curve, shown in Figure 3-4.

Etching of samples reported in this thesis was performed at 400 mV for 20 minutes.

27



Optimum

potential

Figure 3-4 Anode polarization curve

Upon completion of the etching process, the passive inclusion phase can either be found

on the matrix or in the electrolytic solution as a result of extraction. Extracted inclusions

and etched samples were analyzed and observed using analytical methods such as

SEM/EDS. SEM images, taken using acceleration voltage of 20kV, provide three-

dimensional views and yield information on the shape and morphology of inclusions,

which is rather difficult to obtain from conventional polished samples.

3.2.2 Inclusion species analysis

The chemical composition of different observed inclusions was determined by EDS

analysis from the same SEM where the images were taken. As oxygen content cannot be

accurately measured using EDS technique, the analysis of inclusion composition faces

certain challenge especially when oxide inclusions are in majority. However, the said

difficulty is not too severe knowing that no other light elements are present in comparable

concentrations at the same time. Calculation of inclusion chemistry was made possible

with general knowledge about inclusion types as well as relevant elemental valencies

28



such as Al, Si, Mn, Fe and other metals. It is widely accepted that analysis having total

oxides within 100 ± 5% is satisfactory.[2]

Inclusion types reported in this thesis are mainly classified based on respective EDS

spectra and morphologies. Therefore chemical formulae, instead of common mineral

classification, are utilized in inclusion type description. However, inclusions with

characteristic appearances, which can be easily identified, will also be given a mineral

classification. A more precise classification of non-metallic inclusions involves the

determination of phases as each chemical formula may include different phases, which

give rise to different properties and different formation conditions are often required.

Kiessling and Lange[2]

gave detailed reports on inclusion phases determined using

electron probe analysis.

3.3 Quantitative Assessment

A complete assessment of steel cleanliness not only consists of qualitative information,

but also quantitative information such as inclusion length, inclusion width, number of

inclusion per unit area, volume fraction, mean free path, etc. Using as-polished metal

samples, quantitative assessment involves a combination of a microscopic technique to provide images of the sample surface (image acquisition) and an image analysis system

to accurately measure the inclusion size.

3.3.1 Image acquisition

Image acquisition is a crucial part in the process of quantitative analysis. The ideal

technique for providing images of the sample surface must offer accurate representation

of inclusion distribution. Analytical instruments involved in this research project consist

of the following:

1. Light optical microscope

2. Laser confocal microscope

29



3. Scanning electron microscope

Light optical microscope:

Prior to the advent of electron microscopy, light-optical microscopy was used to quantify

and characterize inclusions based on morphology. The best-possible spatial resolution of

a light-optical microscope, which is approximately mμ 3.0 , is limited by the fixed

wavelength of light ( mμ λ 5.0≈ ).[3]

As the magnification increases, the light intensity

decreases, which results in darker image. Therefore it becomes rather difficult to utilize

the best-possible resolution of light in a conventional light-optical microscope.

Laser confocal microscope:

The laser confocal microscope (LCM) distinguishes itself from conventional optical

microscope and SEM in the following way:

• Laser confocal microscope is able to provide height information accurate to 0.01

μm. Once the height information is obtained, quantitative surface area and volume

measurement can then be calculated using the operating software. This technique is

especially important for particle analysis of metallurgical samples such as isolated

inclusions, etc.

• With DIC (differential interference contrast), laser confocal microscope provides 3-

dimensional images comparable to that of SEM, but without the issues of charging

in non-metallic areas of interest such as inclusions.

LCM utilizes blue laser as the transmitting medium, which has a wavelength of 473nm.

Therefore, when compared to light optical microscope, LCM offers a slightly improved

lateral spatial resolution at approximately 200nm.

Scanning electron microscope:

SEM and EDS are among the most employed methods of inclusion investigation mainly

due to the following advantages: high resolution, high sensitivity, quantifiability, minimal

sample preparation and ease of operation. The secondary electron mode of a SEM

provides an improved spatial resolution of 5~20 nm.[3]

30



The three modes used are secondary electron (SE), backscattered electron (BSE) and

EDS modes. Using the SE mode, the images formed are topographical representations of

the specimen. Since secondary electrons have a very small escape depth, the signals

received will reflect the surface structures of the specimen. However, using SE mode to

locate inclusions in a polished sample, given the topography of the specimen is flat, will

be rather difficult when inclusion size is small. The BSE mode, on the other hand, utilizes

backscattered electrons to create images showing elemental contrast, thereby revealing

the locations of non-ferrous inclusions in the ferrous matrix. BSE images are also able to

provide information on the homogeneity of inclusions.

In the current investigation, SE mode was used to image inclusions on polished and

SPEED etched surfaces for inclusion morphology study. Inclusion type determination

was performed by EDS mode simultaneously. For inclusion quantification, the BSE

mode was used in conjunction with image analysis software.

3.3.2 Image analysis

Detection and discrimination of inclusions utilize the difference in gray level intensity

between each inclusion species and the unetched matrix steel.[4]

Measurements are made

based on counting the number of picture point elements (termed pixels) that satisfy the

user-defined gray level threshold. The dimension of each image pixel is dependent on

both microscope magnification setting and image resolution. The images for the purpose

of quantitative analysis in this study are taken with the following parameters:

Magnification: 100X

Image resolution: 512 X 676 pixel

Dimension of each pixel: 1.742 μm/pixel

Figure 3-5 shows images taken of the same sample area, using four image acquisition

techniques: optical microscopy, laser confocal microscopy, SEM (SE mode) and SEM

(BSE mode). Figure 3-5 (a)-(b) are examples where surface defects such as voids and gas

31



holes due to solidification shrinkage, or limited hot ductility may be detected as oxide

inclusions in optical microscopy and LCM images; because their gray level range is

comparable to that of oxides.[4]

Other surface defects may also result from improper

polishing techniques, creating excessive relief pits, voids and deep scratches. Figure 3-5

(c) (SE mode), although reduced in number of surface defects, proved to be difficult in

image analysis processing due to lack of contrast between inclusion and matrix steel.

(a) (b)

ScratchVoids

MnS

inclusions

(c) (d)

Figure 3-5: Images acquired using (a) optical microscopy, (b) laser confocal microscopy,

(c) SEM (secondary electron mode) and (d) SEM (backscattered electron mode)

The presence of defects in acquired images shown in Figure 3-5 (a) and (b) can greatly

affect the reliability of subsequent inclusion detection and measurement represented in

32



Figure 3-6 (a), where the voids and scratches were identified as inclusions by the image

analysis software. However, complete elimination or minimization of these defects at the

image acquisition stage can be achieved using SEM under BSE imaging mode as shown

in Figure 3-5 (d) and its respective image analysis result in Figure 3-6 (b). Thus, SEM-

BSE is chosen as the most suitable image acquisition technique for the quantitative

analysis of inclusions.

(a) (b)

Figure 3-6: Photograph processed by image analysis showing detected area as inclusions

(a) laser confocal microscopy, (b) SEM (backscattered electron mode)

Two specimens were studied for each steelmaking operation, where the cleanliness of

each operation is indicated by an averaged result. Each specimen has a surveyed area of

at least 15mm by 15mm. Within the surveyed area, 40 fields of view were taken as data

images for subsequent image analysis. The field areas were aligned contiguously over a

rectangular area of 10 X 4 fields. Data images are then compiled and processed using

Discover Essentials software. The analysis of an image begins at setting the gray level

threshold that corresponds to the inclusions, which are showing as clusters of pixels on

the image. The next step is to define the classifications by creating bins that will hold

data of various ranges of values. The bins in this investigation are set to hold

measurements of inclusion particle size and area fraction. The defined range of each

particle size bin are: 0-10, 10-30, >30 in micrometer units. Lastly, prior to identification,

33



specific recognition parameters are imperative for the software to correctly identify an

inclusion based on the geometry. Parameters such as diameter inner maximum, diameter

inner minimum, area, aspect ratio, number of edges, etc. have optional upper or lower

limit filters to ensure accurate inclusion recognition.

3.4 References

[1] K. Takimoto, I. Taguchi, and R. Matsumoto, “Extraction and Determination of

Precipitates in Steel by Potentiostatic Electrolysis with Non-aqueous Electrolyte”, J.

Japan Institute of Metals, 1976, vol. 40, no. 8, pp. 834-838

[2] R. Kiessling and N. Lange, Non-metallic inclusions in steel, The Institute of Materials

(London), 1978, pp. 5-10

[3] R. Egerton, Physical Principles of Electron Microscopy: An Introduction to TEM,

SEM, and AEM, Springer, New York, 2005, p. 6

[4] ASTM International, “E45 Test Methods for Determining the Inclusion Content of

Steel”, Annual Book of ASTM Standards, ASTM, Philadelphia, USA, 2003, vol. 03

34





crystalline or amorphous, are almost pure; containing small amount of other metal oxides

including MnO and ferrous oxides. It is also important to keep in mind that, due to

electron interaction volume and X-ray generation, “Fe” signals in EDS spectra can be an

artefact reading from the steel matrix. For electron accelerating voltage of 20kV, this

phenomenon is much pronounced for target inclusion having particle size less than 2 μm.

Al2O3

Figure 4-1: Oxide inclusions found in 1018S ladle sample: alumina

Figure 4-2: Oxide inclusion found in A529 billet sample: alumina dendrites

36



Many of the observed corundum particles are partly held together by Al2O3 dendrites

(Figure 4-2) upon collision, forming alumina clusters. It should then be realized that these

alumina clusters are 3-dimensional units, as depicted in Figure 4-3, and will grow in

dimension as further agglomeration takes place. This Al2O3 inclusion generation

behaviour is also referred to as type I nucleation (see Section 2.2.2).

G

Al2O3

a b

Al2O3

Figure 4-3: Oxide inclusions found in A529 ladle sample: a) alumina and galaxite (G)

b) alumina cluster

37



Single-phase glassy alumina inclusions are frequently found to have globular shape as

shown in Figure 4-4. Globular morphology was an indication of the alumina inclusion

being partly molten when formed; mainly caused by localized superheat and poor

homogenization of the bath, as described by type III formation process (see Section

2.2.2). Glassy alumina inclusions are sometimes observed as having plate morphology

with 5 μm or less in thickness. Figure 4-5 shows alumina laths as an example of such

plate morphology in cross-section.

Corundum can also be found in multiphase inclusions as a result of type II generation

process (see Section 2.2.2). Micrographs in Figure 4-6, showing multiphase oxide

inclusions, are examples of corundum nucleation in molten manganese aluminosilicate

(Figure 4-6a) and calcium aluminate (Figure 4-6b). It is of importance to note that

corundum is microscopically similar to galaxite and spinel inclusions (see Section 4.1.5);

in this case, inclusions shown in Figure 4-6 may in fact include galaxite and spinel

inclusions.

Alumina inclusions found in the current samples bear little resemblance to the faceted

morphology reported in the literature. Faceted alumina inclusions are often the result of

deoxidation by adding excess aluminum to ensure low residual oxygen in the bath.However, the steel samples in the current investigation were primarily deoxidized by

Mn/Si partial deoxidation followed by trim additions of aluminum for final killing. Most

of the alumina inclusions examined appear to be spherical on a polished surface (Figure

4-1). The difference in morphology is probably attributed to manganese and silicon

reaction with primary corundum during cooling of the liquid steel. The precipitation of

MnO and SiO2 may cause a shift in overall inclusion composition and in turn, promote

initial crystallization of galaxite phase.

38



Figure 4-4: Glassy Al2O3 (globular) inclusions found in 1018S furnace tap sample

Figure 4-5: Glassy Al2O3 (plate) inclusions found in 1018S ladle sample

39



a) M C

C

M

C

b)M C

M

Figure 4-6: Oxide inclusions in steel: corundum in a) manganese aluminosilicate matrix

[1018S ladle tap sample] b) calcium aluminate matrix [A529 tundish sample]

40



4.1.2 SiO2 (Silica)

Silica inclusions were frequently found throughout silicon-killed steel as indigenous

deoxidation products, and exogenous slag entrapment. In this study, three silica

modifications have been identified: cristobalite, low quartz and tridymite.

Cristobalite is a high temperature modification of silica[1]

. Transformation is usually not

fast enough during cooling of the steel, and therefore cristobalite often remains

metastable at room temperature. It was found, in the current investigation, that

cristobalite phase only crystallizes as dendrites within a single- or multi-phase silicate

matrix. However, literature[2]

has indicated the possibility for cristobalite phase to also be

present as inclusion matrix in ferroalloys. Micrographs of polished surface will show

these dendrites taking on flower-like shape (Figures 4-7 and 4-9). Unlike other silica

modifications, cristobalite phase has extended solid solubility of other metallic oxides.

Depending on the species of dissolved oxides present, the inclusion’s origin and its

formation process can be deduced. An example is given in Figure 4-9. The inclusion in

Figure 4-9 shows fine and evenly distributed dendrites containing several percent of

Al2O3 and MgO, which often serve as an indicator of exogenous origin. Alumina and

magnesia particles acted as nucleation sites for cristobalite dendrites. The source of

alumina and magnesia in silicon-killed steel is likely from reaction between refractory

lining and liquid deoxidation product such as manganese silicate or manganese

aluminosilicates. In comparison with Figure 4-7, the large and well-defined dendrites

contain 100% SiO2 thus indicating that the inclusion particle is indigenous.

Quartz has both low and high temperature modifications, hence the low-quartz and high

quartz convention, where transformation often occurs at 573°C[1]

. As mentioned in

Section 2.2.2, the one-way transformation from quartz to cristobalite or tridymite is

possible but often incomplete in steelmaking due to inadequate reaction time and

temperature. Therefore it is clear that quartz particles (Figure 4-8), which frequently

crystallize from molten amorphous silica at steelmaking temperature, are conceivably

exogenous in nature.

41



K

R

Figure 4-7: Oxide inclusion in A529 billet sample: cristobalite (K) in rhodonite (R)

42



TA

Q

R

Figure 4-8: Oxide inclusion in A529 billet sample: rhodonite (R), low quartz (Q),

tridymite (T) and glassy silica (A)

The large inclusion particle shown in Figure 4-8 consists of two distinct components:

manganese silicate in the form of rhodonite and silica in the form of low quartz and

glassy silica. The intermediate phase, being darker than quartz and lighter than glassy

silica, is an example of incomplete transformation of quartz to cristobalite or tridymite

during cooling of the steel. In the presence of low quartz and glassy silica, it is reasonable

to conclude that the inclusion type shown in Figure 4-8 has an exogenous origin with

possible source being silica sand. Moreover, the quartz particle may also act as a

preferential site for other particle nucleation. The rhodonite component in Figure 4-8 is

probably a result of existing manganese silicate particle growth by dissolution and partial

43



consumption of the quartz component. This observation also holds true for inclusion

particle in Figure 4-9, where the glassy silica has been attacked by rhodonite matrix.

R

AK

Figure 4-9: Oxide inclusion in A529 tundish sample: cristobalite (K) in rhodonite (R)

matrix, and glassy silica (A)

Other common low quartz inclusion morphologies include cubic and bar as shown in

Figures 4-10 and 4-11. It can be seen that the thin and plate-like dark phase, which is

characteristic of tridymite, exists on the outer layer of the quartz particle shown in Figure

4-10. However, the exact composition cannot be determined due to limited resolution of

the technique used. Follow-up analysis with Auger Electron Spectroscopy will be helpful.

44



Q

T

Figure 4-10: Oxide inclusion in 1018S tundish sample: low quartz (Q) and tridymite (T)

Q

Figure 4-11: Oxide inclusion in 1018S billet sample: low quartz (Q)

45



4.1.3 MnO (Manganosite)

Throughout the investigation, no pure form of manganosite is observed in current steel

samples. As a reaction product of a relatively weak deoxidizer, manganosite tends to be

reduced by stronger deoxidizers upon formation or simply form solid solution with other

oxides, thereby lowering the melting point of the product thus improving the extent of

deoxidation. However, it has been repeatedly reported in the literature[2-3]

that

manganosite exists as an inclusion phase in steel.

There exists a wide range of solid solubility between MnO and FeO. It was mentioned in

Section 2.2.2 that iron participation in manganese deoxidation gives rise to MnO-FeO as

deoxidation products. The mechanism and thermodynamics of formation is given in

detail by Turkdogan[4]

. Figure 4-12 provides examples of MnO-FeO indigenous

inclusions containing about 35% MnO and 65% FeO.

Figure 4-12: Oxide inclusion in A529 billet sample: manganosite

46



4.1.4 MnO-SiO2 (Rhodonite)

Manganese silicate, rhodonite, is a common inclusion phase resulting from Si-Mn

deoxidation. By plotting the manganese and silicon contents of steel samples given in

Table 3-1 on the deoxidation product stability diagram shown in Figure 2-5, it is clear

that molten manganese silicate will likely form as the deoxidation product at 1600°C. In

the current investigation two morphologies of rhodonite phase were found, single-phase

inclusions and the matrix component of a multi-phase inclusion. Rhodonite single-phase

inclusions often exhibit a round globular shape on the polished surface, shown in Figures

4-13 and 4-14 (left), and are frequently observed as spherical droplets on a SPEED

etched surface, Figure 4-16.

Figure 4-13: Oxide inclusions found in A529 ladle tap sample: rhodonite

Liquid manganese silicate has a high solubility of silica at steelmaking temperatures.

During subsequent cooling, excess silica in manganese silicate is precipitated as

cristobalite while manganese silicate crystallizes as a rhodonite matrix. (Figures 4-7 and

47



4-14, right) Depending on the rate of cooling and length of later heat treatment, the

complete transformation of glassy manganese silicate to rhodonite may not be achieved.

Single-phase rhodonite inclusions are often considered to be indigenous; however, as

illustrated in Section 4.1.2, hints to the origin of rhodonite in multi-phase inclusions lie

within the inclusion morphology and the trace elements present. It is common practice to

have a high Mn-Si ratio in steel hence it is important to note[1]

that the interaction

between dissolved manganese in liquid steel and silica in the refractory material is a

major source of silicate inclusions such as rhodonite. The composition of the resulting

silicate product is also partly dependent on the Mn-Si ratio of the steel.

K

R

R

Figure 4-14: Oxide inclusions found in A529 billet sample: rhodonite (R) and cristobalite

(K)

Substitution of MnO by FeO and CaO was observed for rhodonite inclusions. The

rhodonite inclusion shown in Figure 4-15 is an example of CaO substitution for MnO.

Like type I sulphide inclusions, rhodonite may also undergo plastic deformation forming

elongated stringers along the steel rolling direction. (Figure 4-15)

48



Figure 4-15: Oxide inclusions found in A529 billet sample: rhodonite (after rolling)

Figure 4-16: Oxide inclusions found in 1018S ladle sample: rhodonite

49







matrix phase. In addition, literature[1]

has reported the observation of galaxite

precipitation in liquid manganese aluminosilicate matrix.

4.1.6 CaO-Al2O3 (Calcium aluminate)

Calcium metal has commonly been utilized in inclusion shape control. In aluminum-

killed steel, calcium addition can readily convert the solid alumina inclusions to liquid

calcium aluminates. The driving force for this phenomenon can be illustrated in the

CaO-Al2O3 phase diagram shown in Figure 2-7. Small amount of CaO dissolution can

significantly lower the melting point of Al2O3, which favours the merger of the two

oxides. In the present work, calcium-containing inclusions were observed to be binary

oxides and ternary oxides; no separate oxide in the form of calcia (CaO) was found. An

example of typical single-phase calcium aluminate inclusion is given in Figure 4-19.

Calcium aluminate inclusion in spherical shape usually indicates molten state in the steel

bath and retains its shape upon solidification.

Figure 4-19: Oxide inclusions found in 1018S ladle tap sample: calcium aluminate

52



The behaviour of calcium in molten steel at 1600°C is still uncertain and it remains

unclear whether calcium operates as a direct deoxidizer, which can be described by gas-

liquid interaction models, or it simply decreases the activity of silica by forming a

complex oxide, thereby improving the overall deoxidizing effect.

Most of the CaO-containing inclusions were deemed to be exogenous in origin due to 1)

calcium’s insolubility in steel thus making direct indigenous deoxidation unlikely to

occur, 2) frequent reactions between steel and slag as well as steel and refractory

materials, and 3) physical erosion of refractory materials and entrainment of slag. In

addition to Ca-Si charges, refractory materials and metallurgical slags serve as exogenous

sources of CaO. Refractory materials, such as dolomite, used in steelmaking vessels often

contain both CaO and Al2O3. Lime, CaO, is a component of most slag used in steel

refining and melt covers. It is, however, possible that a CaO-containing inclusion can

form by starting with exogenous nuclei follow by indigenous growth. On the contrary,

inclusions resulting from coalescence of indigenous inclusions with exogenous CaO

sources such as calcium aluminate slag are also highly probable. Though rare, calcium

aluminates free of MgO and containing trace amounts of MnO or SiO2 are likely products

of the deoxidation process and indigenous in nature (Figure 4-19 and 4-20).[5]

In order to establish the formation mechanism of specific calcia-rich inclusions, detailed

identification of phases and elements present is often necessary. Determination of

calcium aluminate’s origin has been particularly challenging in the present investigation,

given inclusion morphology and EDS spectra being the sole data.

Throughout the calcium aluminate inclusions found in the samples, the effective calcium

content has little influence on particle morphology. Morphology of calcium aluminate

inclusion is mostly globular. Four modifications, according to stoichiometric

composition, of calcium aluminate inclusions were observed:

• 12CaO•7Al2O3 (Figure 4-20 and 4-26)

• CaO•Al2O3 (Figure 4-21)

• CaO•2Al2O3 (Figure 4-22, 4-24, and 4-25)

53



• CaO•6Al2O3 (Figure 4-23)

G CA

CA

G

Figure 4-20: Oxide inclusions found in 1018S furnace tap sample: calcium aluminate

(CA) and galaxite (G)

The two examples of 12CaO•

7Al2O3 inclusions differ by the presence of SiO2 in theinclusion shown in Figure 4-26 whereas the inclusion shown in Figure 4-20 is free of

SiO2. Here a distinction can be made between SiO2-containing CaO-Al2O3 inclusions and

those free of SiO2. It would seem that the morphology of those CaO-Al2O3 inclusions

with SiO2 tends to deviate from spherical appearance towards irregular shape (Figure 4-

24 and 4-25) and dendrites (Figure 4-26). CaO-Al2O3 with high SiO2 content, shown in

Figure 4-27, is often observed as a single-phase glassy particle of diameter greater than

30 μm. Inclusions without SiO2 and having an Al2O3 content greater than 60 wt% are

characterized by higher hardness[5]

. They usually remain spherical upon rolling or follow

brittle response by breaking up into smaller particles if severe deformation of the

surrounding steel occurs[1]

.

54



It is conceivable that the porous structure in the calcium aluminate inclusion, shown in

Figure 4-21, is a result of gas evolution followed by rapid cooling. The calcium aluminate

inclusion in Figure 4-25, on the other hand, underwent solidification shrinkage as

indicated by the cracks within the void.

voids

CA

Figure 4-21: Oxide inclusions found in A529 ladle sample: calcium aluminate (CA)

CaO•2Al2O3 phase was found to crystallize as thin lathes in an aluminate matrix having

similar composition corresponding to CaO•2Al2O3. (Figure 4-22, 4-24, and 4-25) This

observation is in good agreement with literature[1]

. Presence of MgO in calcium

aluminate inclusions (Figure 4.22, 4-23 and 4-24) often indicates exogenous sources

since calcium containing deoxidizers are generally magnesium free. Calcium inclusions

with high MgO content most likely originate from magnesia refractories by erosion or by

reaction between slag and refractories. However, the raw material charged into the EAF

for making the current steel samples is mainly scrap steel containing trace MgO,

therefore this may contribute to the trace MgO detected in the inclusions. The strong

influence of refractory composition on calcium aluminate inclusions emphasizes the

55





Figure 4-24: Oxide inclusions found in A529 tundish sample: calcium aluminate

Figure 4-25: Oxide inclusions found in 1018S ladle sample: calcium aluminate

57





4.1.7 CaO-SiO2 (Calcium silicate)

In the study involving calcium-containing inclusions, the practical objective is to

distinguish calcium-aluminates from calcium-silicates as their difference in response to

deformation can significantly affect the cleanliness of steel. Such differentiation is also

important in establishing the inclusion’s origin since most of the calcium silicates belong

to common Ca-Si deoxidation products while majority of calcium aluminates tend to

follow the scheme of indigenous growth on exogenous nuclei.[5]

Partial substitution of CaO by MnO and FeO was common to all calcium silicate

inclusions found. (Figure 4-28) This type of inclusion shows little to no solubility of

MgO. Calcium aluminates, on the other hand, have been found to replace CaO with MgO

up to several percent. However, the characteristic substitution of CaO with MnO in

calcium silicates is absent in calcium aluminates. Therefore, detection of noticeable MgO

content or considerable amount of MnO can be used to differentiate calcium silicate and

calcium aluminate inclusions.

Figure 4-28: Oxide inclusions found in 1018S billet sample: calcium silicate

59



Differentiation among aluminates and silicates can also be made by the study of rolled

metal samples. Unlike calcium aluminates, calcium silicate inclusions exhibit similar

deformation behaviour as the manganese silicate inclusion shown in Figure 4-15. As for

calcium aluminate inclusions, the deformation pattern has been described by Kiessling[1]

as strings of crystals. It was also mentioned in his report that work-induced crystallization

of calcium aluminates is highly probable, whereas calcium silicates typically remain

glassy.

Calcium silicate inclusions observed in the current work mostly consist of homogenous

monophase, often glassy in appearance, spherical droplets. During the investigation,

some heterogeneous calcium silicates were found to be holding together corundum,

galaxite and other spinel type inclusions as shown in Figure 4-29. Such agglomerates

(Figures 4-6, 4-20 and 4-29) are more harmful to the material’s mechanical properties

than in the form of their individual constituents.

C

CS

Figure 4-29: Oxide inclusions found in 1018S furnace tap sample: calcium silicate (CS)

and corundum (C)

60



In general, CaO-content in inclusions was found to be higher in the ladle samples and

saw a decrease during tapping. However, calcium-containing inclusions identified in the

tundish samples were often high in CaO, possibly due to ladle calcia-rich slag

entrapment. An example of such silicate inclusion is shown in Figure 4-30.

Figure 4-30: Oxide inclusions found in A529 tundish sample: calcium silicate

4.1.8 CaO-Al2O3-SiO2 (Calcium aluminosilicate)

The ternary calcium aluminosilicate inclusions are also common products of Ca-Si

deoxidation. CaO-Al2O3-SiO2 phase differs from binary calcia phases (CaO-Al2O3 and

CaO-SiO2) in that solubilities of MnO, FeO, and MgO have been observed in the low-

melting point ternary oxide phase. They exist as metastable monophase inclusions in

solid steel since the cooling rate associated with common steelmaking practices is often

too fast for secondary phase precipitation to occur. Clues to establishing the origin of

calcium aluminosilicate inclusions lie with the amount of CaO and MgO present as well

as the CaO:MgO ratio. Inclusion’s origin assessment is often based largely on the

steelmaking practice associated with the specimen, especially for ternary inclusion phase

61



like calcium aluminosilicate since there is no crystalline phase that can be utilized as an

indicator.

Inclusion particle shown in Figure 4-31 is low in CaO and high in silica, alumina and

manganosite. This composition is typical for deoxidation products that likely have

indigenous origin; which is reinforced by the absence of MgO, implying the lack of

refractory participation while low CaO-content effectively indicated inclusion’s

minimum interaction with the slag.

Figure 4-31: Oxide inclusions found in A529 ladle tap sample: calcium aluminosilicate

On the contrary, the composition of calcium aluminosilicate inclusion shown in Figure 4-

32 suggests exogenous origin from refining slag. High CaO-content inclusion with

relatively less silica, alumina and manganosite is usually associated with turbulent slag

mixing during argon stirring. This inclusion being a Ca-Si deoxidation product, an

alternative origin, is less probable, mainly due to the SiO2:CaO ratio being less than

unity. No solid sulphide solubility has been reported for calcium aluminosilicate

62



inclusions. The sulphide signal is believed to have resulted from the MnS scale (see

Section 4.1.9.) on the inclusion’s surface.

Figure 4-32: Oxide inclusions found in 1018S ladle sample: calcium aluminosilicate

4.1.9 MnS (Manganese sulphide)

Sulphur is soluble in iron at steelmaking temperature, therefore no formation of common

metal sulphides are expected throughout the molten steel operations. As the liquid steel

solidifies, solubility of sulphur decreases to about 0.05 wt% S at 1365°C and further

cooling to 988°C decreases the solubility limit to 0.012 wt% S.[4]

During steel

solidification, insoluble sulphur segregates and forms metal sulphides with iron and

manganese. Stability of the two sulphides are comparable at steelmaking temperature, butMnS is the more stable phase at room temperature.

[1]

Although smaller in particle diameter, sulphide inclusions were found in greater quantity

than oxide inclusions. Sulphide precipitates were frequently observed in all specimens as

a result of decrease in solubility upon cooling, which may not truly reflect the condition

63



at liquid steel temperature. Therefore only sulphides found in billet samples are discussed

in this section.

Unlike oxide inclusions in previous sections, MnS inclusion is often more difficult to

locate due to smaller particle size and lack of contrast in polished sections, as illustrated

in Figure 4-33. Three-dimensional morphology of MnS inclusions, taken from the same

area, is depicted in Figure 4-34. This type of MnS inclusion, showing globular form, is

often referred to as type I sulphide.

Figure 4-33: Sulphide inclusions found in 1018S billet sample: manganese sulphide

At room temperature range, type I sulphides are generally regarded as highly deformable

inclusions; behave much like glassy silicates at elevated temperatures. (See Sections 4.1.4

and 4.1.7) Therefore type I sulphides generally have minimum impact on steel’s fatigue

properties. In contrast, type II sulphides follow dendritic crystallization at the grain

boundaries in steel commonly produced by ingot casting. Type III sulphides are

monophase particles characterized by random distribution in steel and faceted

morphology. No type II or type III sulphides were observed in the present work.

64



Figure 4-34: Sulphide inclusions found in 1018S billet sample: manganese sulphide

It has been reported by Kiessling and Westman[7]

that MnS phase has extended solubility

of vanadium, chromium, iron and cobalt as well as small amount of titanium and nickel.

In this study, a number of MnS inclusions, shown in Figures 4-34 and 4-35, have been

identified to show small amount of copper solubility.

No FeS phase has been identified in this investigation due to high manganese content in

the steel samples. The condition for FeS formation requires the steel to be free of

manganese and rich in sulphur. MnS inclusions observed in the specimen were repeatedly

found to contain iron in solid solution. The amount of iron present in solution with MnS

can impact the deformation response, which in turn affects the machinability of the

steel.[7]

The amount of iron present in MnS is mostly dependent on the sulphur content in

steel, sulphur solubility in liquid steel and the cooling rate associated with casting

operation. The control of sulphur content in steel as well as the Mn:S ratio can influence

the resultant sulphide composition. Evidently, the iron content in MnS inclusion increases

with increasing sulphur content in steel.[8]

The solubility of sulphur, however, is

65



dependent on oxygen content in the steel; where sulphur solubility is lowered under the

presence of higher residual oxygen. It is possible to have iron content greater than

equilibrium value if the inclusion is cooled rapidly from liquid steel temperature.

Figure 4-35: Sulphide inclusions found in A529 billet sample: manganese sulphide

Good sulphide inclusion retention on sample’s surface was achieved by SPEED etching

technique. The variations of sulphide morphology, spherical and cubic, are shown in

Figures 4-34, 4-35, 4-36, and 4-37; where the images were taken from SPEED etched

samples. It is otherwise difficult to present the details of surface feature on inclusions,

shown in Figures 4-36 and 4-37. Referring to the work by Liu et al[8]

, the dark fine

precipitates observed on the surface of MnS inclusions were believed to be copper

sulphides hence the trace copper signal shown in the EDS spectra.

66



Figure 4-36: Sulphide inclusions found in A529 billet sample: manganese sulphide

Figure 4-37: Sulphide inclusions found in A529 billet sample: manganese sulphide –

additional morphologies

67



Duplex inclusions, shown in Figure 4-38, are common type I sulphide variations;

characterized by sulphide scales/shell on oxides. Inclusions shown in Figure 4-39 are

three-dimensional representation of duplex inclusions, illustrating sulphide patches on

oxides. Two different formation mechanisms are generally accepted.[1]

Upon steel

solidification, existing oxides (M in Figures 4-38 and 4-39) such as aluminates and

silicates, are low-energy nucleation sites for sulphide precipitation (S in Figures 4-38 and

4-39). Alternatively, the sulphide scale may precipitate from sulphur-rich molten silicate

during cooling.

Studies[4]

have shown that when brittle oxides are coated by sulphides, the negative effect

of oxide inclusions on fatigue properties is minimized. The overall inclusion’s thermal

expansion coefficient can achieve a comparable value to that of steel’s thus minimizing

interfacial stress.

SS

MM

M

S

M S


(S) around silicate matrix (M)

68



S

M

M

M

S


(S) around silicate matrix (M)

4.1.10 Development of inclusion species during steelmaking

The steel cleanliness depends much on the melting and casting practices where a varied

operation often results in notable differences in total oxygen, total amount of inclusions

indicated by total number of inclusions or area fraction of inclusions, particle size

distribution as well as the types of inclusion present. A comparison of the inclusion types

present in steel grades A529 and 1018S is given in this section. Both EAF steels were

produced with identical deoxidation practice and were similar in overall composition as

shown in Table 3-1; however, the steelmaking operations associated with the two steel

grades differ in melt protection during vessel transfer. Refractory nozzle wasimplemented for the production of A529 steel samples whereas the 1018S steel samples

were produced with both refractory nozzle and inert gas shrouding. The inclusion types

present in A529 and 1018S steel are summarized in Table 4-1 and Table 4-2 respectively.

69



Table 4-1: Summary of inclusion types present in A529 steel

●●CaO-SiO 2

●●●(Fe, Mn)O

Steelmak ing S tep

Inclusion S pecies

Furnace

tap Ladle Ladle tap Tundish Billet

A l2O 3 ● ● ● ● ●

S iO 2 ● ●

MnO-SiO 2 ● ● ●

M n O - A l2O 3 ● ● ●

C a O - A l2O 3 ● ● ● ●

C a O - A l2O 3 -S iO 2● ● ●

MnS, (Fe, Mn)S ● ●

Table 4-2: Summary of inclusion types present in 1018S steel

●●CaO-SiO2

●●(Fe, Mn)O

Steelmak ing Step

Inclus io n Sp ec ies

Furnace

tap L adle Ladle tap Tundish Bil let

A l2O 3 ● ● ● ● ●

S iO2

● ●

MnO -SiO2

● ● ●

MnO-Al2O

3● ● ●

CaO-Al2O

3● ● ●

CaO-Al2O 3-S iO 2● ● ● ●

MnS, (Fe, M n)S ● ●

70



It is important to keep in mind the effects of cooling rate on inclusion species. The varied

degree of supercooling can retain some non-equilibrium inclusion phases while

suppressing the nucleation of other phases. Lollypop samples taken at furnace tap, ladle,

ladle tap and tundish experienced higher cooling rate due to smaller volume. Quenching

of these steel samples contribute to preserving the inclusion types present at steelmaking

temperature.

Primary steelmaking

The inclusions species present in the furnace tap samples are similar between the steel

operations. The presence of CaO-SiO2 in 1018S furnace tap samples implied furnace slag

entrainment due to excess turbulence at slag-metal interface. The glassy silicate

inclusions found in this process stage is often characterized with low calcium as shown in

Figure 4-29.

Secondary steelmaking

Addition of deoxidizers including ferromanganese, ferrosilicon, silicomanganese,

calcium-silicon, and small amount of aluminum bars were made at the tap ladle. The

changes in composition of inclusion species present in the ladle samples can be derived

from alloy addition and deoxidization products. One of the main objectives of the

refining period in ladle operation is the removal of detrimental inclusions, often marked

by large particle size and specific inclusion types. The detrimental inclusions types

named by various steelmakers[9]

, in the order of increasing harmfulness, are CaO-SiO2

system, CaO-Al2O3 system and Al2O3. However, complete removal of these harmful

inclusions by flotation is not feasible given limited ladle holding time. Therefore, it is

evident that ladle operation also aims at the conversion of binary oxides to low melting

point glassy oxides such as CaO-Al2O3-SiO2 and MnO-Al2O3-SiO2. Ternary oxides were

first found in the ladle samples of 1018S steel and in the ladle tap samples of A529 steel

as shown in Tables 4-1 and 4-2.

71



Continuous casting

In order to minimize ladle slag carry over into tundish, the bottom-pouring ladles were

equipped with electromagnetic-type slag detection system. Straight refractory nozzle was

used for protecting molten steel stream against reoxidation. For 1018S steel samples

nitrogen gas mixture was used in conjunction to provide an improved shrouding system

thus prevent reoxidation. Submerged entry nozzle (SEN) and sliding nozzle combination

were used to facilitate steel flow rate and to protect liquid steel from potential reoxidation

during tundish to mold transfer.

As liquid steel approaches to the continuous caster, calcium aluminates and most of the

calcium silicates were converted to ternary glassy (Ca, Mn)O-Al2O3-SiO2 at tundish

(1018s steel)) and at billet (A529 steel). It is clear that throughout the melting and casting

operations, inclusion species tend to develop from simple primary oxides to complex

binary and ternary oxides.

Trace sulphide inclusions were consistently found in both tundish samples from A529

and 1018S steel grades. Sulphide nucleation is facilitated by low oxygen potential in the

liquid steel, which matches the tundish condition.

4.2 Quantitative Assessment

Two specimens were studied for each steelmaking operation from each of the two steel

grades, for a total of 20 samples. For each sample, 40 data images were systematically

taken, where images are 1.5mm apart from one another, using backscattered electron

imaging mode. Therefore, the cleanliness result of each steelmaking operation is based on

the analysis of 80 data images. Three cleanliness measures are discussed in this section:

• Particle size distribution

• Maximum particle size

• Inclusion area fraction

Particle size distribution provides an overview of steel sample cleanliness. Maximum

particle size is no doubt an important requirement considering product performance is

72



mostly affected by macro-cleanliness of the steel. Inclusion area fraction is also a

common cleanliness measure that offers complementary information to size distribution.

4.2.1 Particle size distribution

Results of inclusion particle size distribution of the two steel grades are summarized in

Figures 4-40 and 4-41. Detailed particle size distributions for each of the five sampling

locations are given in Appendix B (1018S steel) and Appendix C (A529 steel).

Compared to samples taken at furnace tap, the inclusion count in ladle tap was reduced

by 79% and 45% for 1018S and A529 steels respectively. The reduction of inclusion

count in the ladle and ladle tap samples reflects well-controlled argon bubbling hence the

effective inclusion removal by flotation.

Depending on sampling location, deoxidation inclusions with particle size less than 10

μm account for 66-92% and 77-92% of total inclusion count in 1018S steel and in A529

steel respectively. 9% of total inclusions found in the 1018 ladle samples have particle

size greater than 30 μm, which is deemed detrimental to product properties by many

steelmakers

[9]

. However, the same 9% of the total inclusion count, or 79 inclusions/cm

2

,embodies 93% of the total inclusion area. Moreover, the single largest inclusion found in

the 1018S ladle samples is responsible for 35% of the total inclusion area, whereas 66%

of total inclusion count, having particle size less than 10 μm, would only represent about

4.5% of the total inclusion area. It is therefore clear that the control and removal of

macro-inclusions is vital to steel cleanliness.

According to the size distribution plots, reoxidation is common to both steel grades.

Reoxidation of tundish melt can arise from air entrainment, tundish slag reoxidation, and

tundish refractory reoxidation. During tundish filling, negative pressure develops in the

refractory nozzle connecting ladle and tundish. Air entrainment may take place if proper

sealing or inert gas flushing was not applied. Traditional tundish slag former often

consists of silica powder and limestone. Tundish melt is prone to reoxidation if the silica

73



0

500

1000

1500

2000

2500

3000

Furnace Tap Ladle Ladle Tap Tundish Billet

N u m b e r o f i n c l u s i o n s ( / c m ^ 2

)

<10 um 10-30 um >30 um

Figure 4-40: Inclusion size distribution of 1018S

0

500

1000

1500

2000

2500

3000

3500

4000


N u m b e r o f i n c l u s i o n s ( / c m ^ 2 )

<10 um 10-30 um >30 um

Figure 4-41: Inclusion size distribution of A529

74



content is too high in the slag. Reoxidation of tundish melt can also take place at the

metal-refractory interface. If olivine content (FeO-containing component) is high in the

refractory material, reduction of FeO at the tundish lining can oxidize the tundish melt.

For 1018S steel, inert gas flushing was employed during melt transfer between ladle and

tundish. The major contribution to the reoxidation of 1018S tundish melt is therefore

likely from refractory reoxidation as well as possible slag entrainment during tundish

filling. In addition to potential slag and refractory reactions, it is speculated that

reoxidation in the A529 tundish melt is mostly a result of primary oxide nucleation due to

ingress air, since inert gas shrouding was not applied. The above hypothesis is in good

agreement with the size distribution shown in Appendices B and C.

The total inclusion counts for billet samples are 1352/cm2

(1018S) and 3846/cm2

(A529);

showing 65% reduction in samples with inert gas protection. (Figure4-42) The enormous

increase in intermediate-sized inclusion count (10-30 μm) found in the A529 billet

samples is likely a result of melt reoxidation by uncontrolled atmosphere in the tundish.

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000


N u m b e r o f i n c l u s i o n s ( / c m ^ 2

1018S A529

Figure 4-42: Comparison of total inclusion count

75





0

40

80

120

160

200

240

280


M a x i m u m p

a r

t i c l e s i z e ( u m )

Max Particle Size (μm )

Furnace Tap 57±11

Ladle 54±9

Ladle Tap 51±5Tundish 196±39

Billet 204±44

Figure 4-44: Maximum particle size plot of A529 steel samples

0

100

200

300

400

500

600

700


M a x i m u m p

a r t i c l e s i z e ( u m )

1018S A529

Figure 4-45: Comparison of maximum particle size

77



4.2.3 Inclusion area fraction

Inclusion area fraction provides complementary information to total oxygen analysis and

inclusion particle size distribution.

After ladle refining, inclusion area fraction is reduced from 2494 ppm to 90 ppm for the

1018S steel samples and 284 ppm to 213 ppm for the A529 steel samples. (Figures 4-46-

47) This result is in good agreement with inclusion particle size distribution shown in

Figures 4-40 and 4-41.

Figure 4-48 is a comparison between the two melt-protection practices, which illustrates

the effects of casting speed on steel cleanliness. The inclusion area fraction found in the

A529 billet sample is approximately twice that found in the 1018S billet samples.

0

500

1000

1500

2000

2500

3000

3500


I n c l u s i o n a r e

a f r a c t i o n ( p p m )

Area Fraction (ppm )

Furnace Tap 548±119

Ladle 2494±

554Ladle Tap 90±17

Tundish 528±128

Billet 512±113

Figure 4-46: Inclusion area fraction of 1018S steel samples

78



0

200

400

600

800

1000

1200

1400

Furnace T ap Ladle Ladle Tap Tundish Billet

I n c l u s i o n a r e a f r a c t i o n ( p p m )

Area Fraction (ppm )

Furnace Tap 327±36

Ladle 284±28

Ladle Tap 213±17

Tundish 602±61

Billet 1047±108

Figure 4-47: Inclusion area fraction of A529 steel samples

0

500

1000

1500

2000

2500

3000

3500


I n c l u s i o n a r e a f r a c t i o n ( p p m )

1018S A529

Figure 4-48: Comparison of inclusion area fraction

79





• The quality of sample polishing required by BSE-IA method is less stringent than

that of OM-IA.

• BSE-IA method provides superior resolution and contrast compare to OM-IA.

4.3 References

[1] R. Kiessling and N. Lange, Non-metallic inclusions in steel, The Institute of Materials

(London), 1978, pp. 45-79

[2] C. Benedicks and H. Lofquist, Non-metallic inclusions in iron and steel, Chapman

and Hall, London, 1930

[3] T.R. Allmand, Microscopic identification of inclusions in steel, BISRA, London,

1962


1996, pp. 181-182

[5] T. Ototani, Calcium Clean Steel, Springer-Verlag, New York, 1986, pp. 64-69

[6] H. Jacobi and K.Wunnenberg, “Methods for determination of oxide cleanness in

steel”, IISI Study on Clean Steel, International Iron and Steel Institute, Belgium, 2004,

pp. 307-309

[7] R. Kiessling and C. Westman, “Sulfide Inclusions and Synthetic Sulfides of the(Mn,Me)S-type”, JISI , 1966, no. 204, pp. 377-379

[8] Z. Liu, Y. Kobayashi, F. Yin, M. Kuwabara, and K. Nagai, “Nucleation of Acicular

Ferrite on Sulfide Inclusion during Rapid Solidification of Low Carbon Steel”, ISIJ

International, 2007, vol. 47, no. 12, pp. 1781-1788

[9] M. Sumida, M. Nadif, M. Burty and V. Tusset, “Industrial practice worldwide – ULC

and LC steel grades”, IISI Study on Clean Steel, International Iron and Steel Institute,

Belgium, 2004, pp. 365-406

[10] L. Zhang, B.G. Thomas, X. Wang, and K. Cai, “Evaluation and Control of Steel

Cleanliness – Review”, 85th Steelmaking Conference Proceedings, ISS-AIME,

Warrendale, PA, 2002, pp. 431-452

81



CHAPTER FIVE: CONCLUSIONS

5.1 Conclusions

1. A novel procedure SPEED-SEM-IA for steel cleanliness assessment was carried out

on 1018S and A529 steels. The qualitative aspect consists of morphology and

inclusion phase characterization, which was achieved with SPEED etching and SEM

observation. The quantitative aspect evaluates inclusion size distribution, macro-

cleanliness and amount of inclusions. The analysis method involved the combination

of image acquisition (SEM-BSE) and image analysis (IA).

2. Four different image acquisition techniques were evaluated for the quantitative

analysis of inclusions and it was found that SEM-backscattered electron imaging

mode is the most suitable choice.

3. For inclusion morphological observation, good three-dimensional images of

sulphides, alumina, and galaxite phases were obtained using the SPEED-SEM

technique. The following inclusion types were frequently observed in this study using

SEM and compositions were analyzed using EDS: Al2O3, SiO2, (Fe, Mn)O, MnO-SiO2, MnO-Al2O3, CaO-Al2O3, CaO-SiO2, CaO-Al2O3-SiO2, MnS and (Fe, Mn)S.

4. Throughout the melting and casting operations, inclusion species tend to develop

from simple primary oxides to complex binary and ternary oxides. With reoxidation

minimized by gas shrouding between ladle and tundish, three steel cleanliness

improvements were achieved: I) reduction in total inclusion count by 65%, II)

reduction of maximum particle size by up to 66%, III) reduction in inclusion area

fraction by over 50%.

82





APPENDICES

Appendix A: Inclusion’s Effect on Fatigue Behaviour

Both A529 and 1018S high strength alloy steel end products are often utilized in

structural applications. Fatigue failure, frequently caused by non-metallic inclusions,

should be taken as the primary consideration when evaluating steel cleanliness for load

bearing applications.

The types of inclusions having detrimental effects on fatigue properties are, in the order

of increasing severity, sulphides, silicates, non-deformable spherical inclusion (simple

oxides, nitrides, and aluminates), alumina. In terms of fatigue properties, the presence of

silicate inclusions is more harmful than sulphide inclusions. At the service temperature,

silicates usually have limited ductility.[1]

As matrix steel deforms under cyclic load,

silicates do not deform, thereby producing microcracks at the steel-inclusion interface.

Studies[2-3]

also show that thermal expansion coefficient (α) of inclusions and the type of

steel plays a role in determining which inclusion types are more harmful than others.

Alumina, nitrides, spinel, silicates and calcium aluminates have lower α values than that

of common structural steels. The difference in α value can lead to internal stress

(negative deviation) or microvoid formation (positive deviation).

Reference:

[1] S. Millman and K.Y. Lee, “Industrial practice – Linepipe grades”, IISI Study on Clean

Steel, International Iron and Steel Institute, Belgium, 2004, pp. 409-442

[2] D. Brooksbank and K.W. Andrews, “Stress fields around inclusions and their relation

to mechanical properties”, Journal of the Iron and Steel Institute, UK, 1972, Vol. 210,

pp.246-255


1996, pp. 291-293

84



Table A: Coefficient of thermal expansion of various inclusion types[2]

Inclusion type Coefficient of thermal expansion, α,

0-800°C, (10-16

K -1

)

Deviation from 1% C-Cr

bearing steel (10-16

K -1

)

Al2O3 and MnO•Al2O3 7.95 - 4.55

MnO•SiO2 9.55 - 2.95

12CaO•7Al2O3 7.73 - 4.77

CaO•Al2O3 6.59 -5.91

CaO•2Al2O3 5.00 - 7.5

CaO•6Al2O3 8.86 - 3.64

CaO•SiO2 10.91 - 1.59

MnS 18.07 5.57

Appendix B: Inclusion Particle Size Distribution of 1018S Samples

Inclusion Size Distribution: 1018S - Furnace Tap

1

10

100

1000

10000

<5 5--10 10--15 15--20 20--25 25--30 30--35 35--40 40--45 45--50 >50 >100 >200

Inclusion particle size (um)


85



Inclusion Size Distribution: 1018S - Ladle

1

10

100

1000

10000

<5 5--10 10--15 15--20 20--25 25--30 30--35 35--40 40--45 45--50 >50 >100 >200



Inclusion Size Distribution: 1018S - Ladle Tap

1

10

100

1000

10000

<5 5--10 10--15 15--20 20--25 25--30 30--35 35--40 40--45 45--50 >50 >100 >200



86



Inclusion Size Distribution: 1018S - Tundish

1

10

100

1000

10000

<5 5--10 10--15 15--20 20--25 25--30 30--35 35--40 40--45 45--50 >50 >100 >200



Inclusion Size Distribution: 1018S - Billet

1

10

100

1000

10000

<5 5--10 10--15 15--20 20--25 25--30 30--35 35--40 40--45 45--50 >50 >100 >200



87



Appendix C: Inclusion Particle Size Distribution of A529 Samples

Inclusion Size Distribution: A529 - Furnace Tap

1

10

100

1000

10000

<5 5--10 10--15 15--20 20--25 25--30 30--35 35--40 40--45 45--50 >50 >100 >200


N u m

b e r o f i n c l u s i o n s ( / c m ^ 2 )

Inclusion Size Distribution: A529 - Ladle

1

10

100

1000

10000

100000

<5 5--10 10--15 15--20 20--25 25--30 30--35 35--40 40--45 45--50 >50 >100 >200



88



Inclusion Size Distribution: A529 - Ladle Tap

1

10

100

1000

10000

<5 5--10 10--15 15--20 20--25 25--30 30--35 35--40 40--45 45--50 >50 >100 >200



Inclusion Size Distribution: A529 - Tundish

1

10

100

1000

10000

<5 5--10 10--15 15--20 20--25 25--30 30--35 35--40 40--45 45--50 >50 >100 >200



89



Inclusion Size Distribution: A529 - Billet

1

10

100

1000

10000

<5 5--10 10--15 15--20 20--25 25--30 30--35 35--40 40--45 45--50 >50 >100 >200



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Wu Chao Peng Paul 200911 MSc Thesis