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2008

Effect of titanium additions to low carbon, low manganese steels on Effect of titanium additions to low carbon, low manganese steels on

sulphide precipitation sulphide precipitation

Sima Aminorroaya-Yamini University of Wollongong, sima@uow.edu.au

Follow this and additional works at: https://ro.uow.edu.au/theses

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Recommended Citation Recommended Citation Aminorroaya-Yamini, Sima, Effect of titanium additions to low carbon, low manganese steels on sulphide precipitation, PhD thesis, Engineering Materials Institute, University of Wollongong, 2008. http://ro.uow.edu.au/theses/403

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Effect of titanium additions to low carbon, low manganese steels on

sulphide precipitation

A thesis submitted in fulfillment of the requirements for the award of the degree of

Doctor of Philosophy

From

University of Wollongong

By

SIMA AMINORROAYA-YAMINI BSc.(Mat), MSc. (Mat)

Engineering Materials Institute

2008

II

Certification

I, Sima Aminorroaya, declare that this thesis, submitted in fulfillment of the

requirement for the award of Doctor of Philosophy, in the Engineering Materials

Institute, University of Wollongong, is wholly my own work unless otherwise

referenced or acknowledged. The document has not been submitted for qualifications

at any other academic institution.

Sima Aminorroaya-Yamini

2008

III

ABSTRACT

Recent developments of high strength line pipe steel have seen a decrease in carbon

content from 0.25 wt% to less than 0.05 wt% and manganese to less than 1 wt% in an

attempt to reduce centreline segregation. The sulphur content should be less than

0.008 wt% as well. However, recent papers argued that low manganese levels in

pipeline steels have the potential to improve weld line toughness and allow a higher

tolerance for sulphur. Should this be true it would be possible to eliminate the usual

desulphurising treatments thereby decreasing the production cost. The current study

was designed to explore the effect of titanium on sulphide precipitation in steels with

higher sulphur contents than currently used in the pipeline industry.

The mechanical properties of steels are influenced by the type, shape and size of

sulphide precipitates. Titanium additions to low carbon steel influence the

precipitation behavior and composition of sulphide precipitates and could provide

grain refinement, precipitation strengthening and sulphide shape control. In the

present study, titanium was added to a low carbon, low manganese steel which

contained approximately 0.01 wt% sulphur in order to study the effect of titanium on

sulphide formation in the absence of other alloying elements.

Microscopic assessment of centreline sulphide precipitates revealed that iron-

titanium-sulphide co-exists with (Mn,Fe)S. An increase of the titanium content from

0.008 wt% to 0.024 wt%, results in an increase in the titanium content of iron-

titanium-sulphide phases and a decrease in the iron content of the (Mn,Fe)S phases.

In contrast to reports in the literature where it has been suggested that titanium can

dissolve in MnS and even that it is possible for TiS to replace MnS. This study has

shown that Ti replaces iron in FeS but does not dissolve in manganese sulphides. The

presence of FeTiS2 (trigonal CdI2-type structure with a P3m1 space group and lattice

parameters of a = 0.34 nm and c = 0.57 nm) precipitates has been verified for the first

time in steel.

IV

A concentric solidification technique in a laser-scanning confocal microscope was

employed to observe in situ, solidification and the precipitation of sulphides

following segregation of alloying elements into the remaining liquid. Microstructural

development of low carbon steel upon solidification has been observed in situ,

alloying element distributions were measured experimentally and sulphide

precipitates have been compared to the precipitates at the centreline of continuously

cast steel slabs. Microstructural development was similar in the two cases and some

aspects of alloying element segregation upon solidification of a slab can be simulated

experimentally by the use of the concentric solidification technique. There is a

remarkable similarity between the sequence of events decreased in the in-situ

observations and that occurring in industrially-cast slabs reported in the literature.

Sulphide precipitates at the centreline of concentrically solidified specimens are

similar in morphology and composition to precipitates in continuously cast steel slabs

and TEM analysis confirmed that FeTiS2 as well as MnS (containing iron) can form

on prior austenite grain boundaries in both cases.

The progression of the solid/liquid interface in plain Fe-C and Fe-Ni alloys was

simulated by using Diffusion Controlled TRAnsformation (DICTRA) software which

operates in conjunction with Thermo-Calc. Good correlation was found between

experimental results and computations. Solute build up in the liquid at the solid/liquid

interface was determined experimentally in Fe-Ni alloys and there was good

agreement between the calculated concentration gradient of nickel and experimental

measurements using an EPMA technique. Therefore, it is possible to determine

comparatively the centreline segregation with fairly elementary experimental

procedures and mathematical simulation at various steel compositions if DICTRA

and Thermo-Calc are provided with sufficiently accurate thermodynamic information.

Precipitates which formed in the solid state were assessed by carbon replication

techniques, transmission electron microscope studies and atom probe tomography.

Manganese and copper sulphides were the dominant sulphide precipitates. Copper

sulphides nucleated on manganese sulphides and formed a shell around rod-like and

V

globular manganese sulphides. Iron did not dissolve in manganese sulphides and

therefore the manganese sulphides which formed in solid state have higher melting

points than sulphides formed on prior austenite grain boundaries at the centreline of

slab. Titanium nitrides were observed in various sizes and distributions. TiN acts as

nucleation sites for the precipitation of sulphides. An increase of the titanium content

from 0.008 wt% to 0.024 wt%, results in precipitation of a few micrometer TiN and a

decrease in the number of small cubic TiN precipitates.

Small particles (~ 10nm length and 3 nm thickness in size) that contain Ti, C, S and N

have been identified for the first time by the use of three-dimensional atom probe

tomography.

VI

Acknowledgements

I would like to express my deep gratitude to my supervisor professor Rian Dippenaar

for his encouragement, guidance and continuous support throughout all these years.

Many thanks to Dr. Dominic Phelan for his constructive comments on the modelling.

I owe sincere debt of gratitude to Mr. Mark Reid for training me to use the Laser-

Scanning Confocal Microscope, his assistance with experiments and his valuable

counsel in the course of this study.

I wish to acknowledge the following individuals from whom I have learned much: Dr.

David Wexler who gave me valuable insight into transmission electron microscopy,

Dr. Charlie Kong at the University of New South Wales for training me in Focused

Ion Beam, Dual Beam techniques and Mr. Alexander La Fontaine from the

University of Sydney for training me in three dimensional atom probe techniques.

I would like to acknowledge Mr. Keith Dilts from Nucor Steel and the courtesy of Dr

Kobus Geldenhuis for performing automated inclusion analysis on my samples.

I wish to thank the staff of the Faculty of Engineering, especially to Mr. Greg Tillman,

Mr. Nick Mackie and Dr. Zhixin Chen for their help and assistance. Many thanks to

the BlueScope Steel team especially Mr. Les Moore for conducting Electron Probe

Microanalyses.

I wish to express my heartfelt thanks to my family for their love and encouragement

that make me strong and free. Many thanks to all of my friends in this and other part

of the world for being always present and creating the special moments.

Finally I would like to thank BlueScope Steel and the University of Wollongong for

providing finantial support to the project.

VII

Table of contents

CHAPTER 1: Introduction

1-1 Recent developments in linepipe steels 1

1-2 Classification of precipitates in continuously cast slab 3

1-3 Simulation of macrosegregation in continuously cast slab 4

1-4 Chapters contents 5

CHAPTER 2: Literature survey

2-1 Introduction 7

2-2 Titanium microalloyed steel 8

2-3 Development of low carbon microalloyed linepipe steels 9

2-4 The Fe-Ti-Al-O system 12

2-5 The Fe-Ti-C-N system 14

2-6 Sulphide inclusions in steels 18

2-6-1 Manganese sulphide classification in steels 20

2-6-2 Deformation of sulphide inclusions during hot rolling 25

2-6-3 Sulphide inclusions in titanium containing steels 28

2-7 Segregation in Continuous casting 33

2.7.1 Microsegregation 34

2.7.2 Macrosegregation 34

2.7.3 White band 36

2-8 In-situ observation of sulphide precipitation in steel 36

2-8-1 In-situ observation of macrosegregation in steel 38

CHAPTER 3: Experimental instruments and methodology

3-1 Scanning Electron Microscopy (SEM) 39

3-1-1 Sample preparation for optical and scanning electron microscopy 39

3-1-2 High Resolution Scanning Electron Microscopy (HR-SEM) 39

VIII

3-2 Electron Probe Micro Analysing (EPMA) 40

3-3 Focused Ion Beam- Dual Beam (FIB) 40

3-3-1 Dual Beam 40

3-3-2 Three Dimensional Imaging and Reconstruction 41

3-4 High Resolution Transmission Electron Microscopy (HR-TEM) 42

3-4-1 Sample preparation for TEM analysis 42

3-4-1-1 Carbon replica 42

3-4-1-2 Selective Potentiostatic Etching by Electrolytic Dissolution 43

3-4-1-3 Focused Ion Beam 45

3-4-2 TEM analysis interpretation by Java Electron Microscopy

Software 46

3-5 High Temperature Laser Scanning Confocal Microscopy (HT-LSCM) 47

3-5-1 Concentric Solidification technique (CST) 49

3-5-2 Sample preparation for Laser Scanning Confocal Microscopy 50

3-6 Three dimensional local electrode Atom Probe 50

3-6-1 Sample preparation for Atom Probe investigations 51

3-6-2 Principles of three-Dimensional Atom Probe 52

3-6-3 Data analysis 53

3-7 Computational thermodynamics and kinetics 54

3-7-1 Thermo-Calc 54

3-7-2 DICTRA 55

3-7-2-1 DICTRA and diffusivities 56

3-7-2-1-1 Relations between diffusion coefficients and atomic

mobilities 56

3-7-2-1-2 Modelling of the atomic mobility 58

CHAPTER 4: Effect of titanium on the centerline sulphide precipitates of slabs

4-1 Introduction 60

4-2 Experimental technique 62

4-3 Inclusion analysis 63

4-4- Three dimensional imaging of precipitates 68

4-5 SEM evaluation of sulphide precipitates at the centerline of slabs 69

4-6 TEM evaluation of sulphides precipitates at the centerline of Slab A 72

4-7 TEM evaluation of sulphide precipitation at the centerline of Slab B 85

IX

4-8 Comparison of the sulphides extracted from the centerline of Slabs A and B 90

CHAPTER 5: Characterisation of precipitates in low carbon, low manganese,

titanium added continuously cast steel

5-1 Introduction 98

5-2 Experimental method 99

5-3 TEM analyses of precipitates formed at the edge of Slab A 102

5-4 TEM analyses of precipitates formed at the edge of Slab B 106

5-5 TEM analysis of precipitates formed at the centerline of Slab A 108

5-6 TEM analyses of precipitates formed in slab centre of Slab B 112

5-7 Comparison of precipitation in different steels 114

5-8 Three dimensional Atom probe analysis 117

5-9- Summary 124

CHAPTER 6: Initial experiments on segregation study

6-1 Introduction 126

6-2 Experimental method 127

6-3 Results and discussion 129

CHAPTER 7: A novel technique to study segregation at the centerline of slabs

using Laser-Scanning Confocal Microscopy

7-1 Introduction 140

7-2 Experimental techniques 141

7-3 Study of segregation in Laser-Scanning Confocal Microscopy 141

7-4 TEM investigation of sulphides precipitated in segregated area of concentrically

solidified samples in LSCM 157

7-4-1 TEM analysis of sulphides precipitated in the segregated area of a

concentrically solidified specimen in LSCM prepared from steel A 157

7-4-2 TEM analysis of sulphide precipitates in the segregated area of

concentrically solidified specimen in the LSCM prepared from steel B 162

7-5 Comparison of sulphides in centerline of slabs with concentrically solidified

specimens 165

CHAPTER 8: Modeling of the solid/liquid interface progression in the concentric

X

solidification technique

8-1. Introduction 167

8-2 Experimental data collection 168

8-3 Fe-0.1%C peritectic alloy Simulation 170

8-4 Heat transfer in concentrically solidified specimen 185

8-5 Fe-4.2%Ni peritectic alloy simulation 188

8-6 Conclusion 192

CHAPTER 9: Summary of findings, conclusions and recommendations

9-1 Summary of findings 194

9-2 Conclusion 198

9-3 Recommendations for future work 200

References 202

Appendices 213

Publications list 218

XI

List of Tables

Table (2-1): Chemical composition of invented steel 11

Table (2-2): Summary of observed precipitates in various titanium alloyed steel 31

Table (4-1): Bulk chemical composition of steel (percentage by mass) 62

Table (4-2): Minimum measured average diameter of inclusion at various positions of

slabs 67

Table (4-3): Comparison of angles calculated from indexed patterns and those

measured experimentally in the TEM 76

Table (4-4): Comparison between the angles measured experimentally in the TEM,

between the orientations shown in Figure (4-27) and calculated values 84

Table (4-5): Comparison of calculated angles between orientations in the indexed

patterns and the angles measured experimentally in the TEM 88

Table (4-6): Comparison of angles between orientations of indexed patterns in Figure

(4-33) and the experimentally measured values 90

Table (4-7): Composition of manganese sulphide and iron-titanium sulphide in Steels

A and B determined by EDS analysis in thin foils (weight percent) 91

Table (4-8): Atomic and ionic radii of elements which form iron-titanium sulphide 93

Table (5-1): Chemical compositions of the steels (weight percent) 99

Table (5-2): The composition, morphology and size of precipitates in Steels A and B 116

Table (6-1): Bulk chemical composition of steel (percentage by mass) 127

Table (7-1): d-spacing of the sulphide particle from three diffraction patterns

(nanometer) 160

Table (7-2): d-space of a particle from two diffraction patterns (Angstrom) 161

Table (8-1): Chemical composition of δ-ferrite in equilibrium with liquid in different

temperatures 171

Table (8-2): Summary of peritectic transformation temperatures for experimental and

simulated concentric solidification 181

XII

List of Figures

Figure (2-1): Timeline development for high strength pipeline steels 10

Figure (2-2): Influence of manganese content on Charpy energy (at room temperature)

of low carbon steels (0.07-0.10 wt %) at varying sulphur levels 12

Figure (2-3): Oxygen saturation and domains of stability of oxides in the system Fe-

Al-Ti-O (weight percent) at 1600 C 14

Figure (2-4): Oxygen saturation and domains of stability of oxides in the system Fe-

Al-Ti-O (weight percent) at 1550 C 14

Figure (2-5): Calculated precipitation of nitrides, nitrogen rich carbonitrides and

carbides in 0.01 wt%Ti steel at various nitrogen contents 15

Figure (2-6): Solubility data for TiN 16

Figure (2-7): Calculated precipitation of MnS, Ti4C2S2 and TiN in low manganese

steel 17

Figure (2-8): Primary spherical MnS in Fe-2.5 wt%/.Mn-1.3 wt%/.S-0.3 wt%C, (a)

optical micrograph, (b) scanning electron micrograph 21

Figure (2-9): Dendritic MnS (a) Optical, (b) Scanning electron micrograph 22

Figure (2-10): Angular MnS (a) Optical micrograph, (b) Scanning electron micrograph 22

Figure (2-11): Monotectic MnS (a) Optical micrograph, (b) Scanning electron

micrograph 23

Figure (2-12): Rod like eutectic MnS (a) Optical micrograph, b) scanning electron

micrograph 23

Figure (2-13): Irregular eutectic MnS a) Optical micrograph b) Scanning electron

micrograph 24

Figure (2-14): Changes in phase equilibria in the Fe-MnS pseudo-binary system by the

addition of C and Si 25

Figure (2-15): The effect of titanium on the relative plasticity of the sulphides in low

carbon steels The effect of titanium on the relative plasticity of the sulphides in low

carbon steels 29

Figure (2-16): Microhardness values for the different (Mn, Me)S solid solutions for

different Me contents 30

Figure (2-17): Schematic phase diagram of the FeS-TiS pseudo-binary system 31

Figure (2-18): Typical examples of segregation in continuously cast slabs following

XIII

Electro Magnetic Stirring (EMS) 33

Figure (2-19): microsegregation in low alloy steel: a) schematic drawing showing the

longitudinal and transverse cross-sections of dendrites; (b) solute distribution in the

transverse cross-section of the dendrite 34

Figure (2-20): Schematic diagram of concentric solidification 38

Figure (3-1): Column layout of the Dual Beam, FIB 41

Figure (3-2): Schematic of electrolytic cell Schematic of electrolytic cell 44

Figure (3-3): Differences of potential vs. current density curves between steels and

carbides in 10% AA type electrolyte 44

Figure (3-4): Differences of potential vs. current density curves between steels and

nitrides in 10% AA type electrolyte 44

Figure (3-5): Procedure of TEM sample preparation in FIB 46

Figure (3-6): Schematic representation of the confocal microscope 47

Figure (3-7): Confocal nature of optic 48

Figure (3-8): Schematic of laser-scanning confocal microscope chamber and sample

holder 48

Figure (3-9): Schematic diagram of concentric solidification 49

Figure (3-10): Schematic diagram of stage 1 electro-polishing 51

Figure (3-11): Micro-polishing in which the needle shaped specimen pierces a drop of

electrolyte suspended in a wire loop. The operation is monitored with an optical

microscope 52

Figure (3-12): Schematic diagram of Three-Dimensional Atom Probe 53

Figure (4-1): Examples of the appearance of centerline segregation 61

Figure (4-2): Schematic illustration of samples positions in slab 63

Figure (4-3): A light optical micrograph of sulphide precipitates at the centreline of an

industrially cast slab (unetched sample) 63

Figure (4-4): Chemical composition of inclusions on ternary phase diagrams, (a) Ti-

Al-Mn+S system of the edge of Slab B, (b) S-Al-Mn system of the edge of Slab B, (c)

Ti-Al-Mn+S system of the edge of Slab A, (b) S-Al-Mn system of the edge of Slab A 64

Figure (4-5): Chemical composition of inclusions on ternary phase diagrams, (a) S-Al-

Mn system of the centre of Slab B, (b) Ti-Al-Mn+S system of the centre of Slab B, (c)

S-Al-Mn system of the centre of Slab A, (b) Ti-Al-Mn+S system of the edge of Slab A 65

Figure (4-6): (a) Area fraction and (b) relative frequency of various inclusions at the

edge of Slabs A and B, (c) Area fraction and (d) relative frequency of various

XIV

inclusions at the centerline of Slabs A and B (sulphides at the edge refer to manganese

sulphides) 66

Figure (4-7): Position of detected inclusions at the centreline of Slabs A and B 67

Figure (4-8): (a) Schematic FIB-SEM 3D imaging method indicating the beam

directions, the sectioning and SEM imaging plane xy [103], (b) Pt layer deposited on

the surface of the selected particle 68

Figure (4-9): Reconstructed images of the selected sulphide precipitate 69

Figure (4-10): Concentration mapping (CM) of precipitates at the centreline of a Slab

A (EPMA) 70

Figure (4-11) SEM image and X-ray mapping of a precipitate in Steel B prepared by

HR-SEM 71

Figure (4-12): X-ray mapping of a precipitate in Steel A prepared by SEM 71

Figure (4-13): (a) Bright field image, (b) EDS trace of precipitate, (c-d) Selected area

diffraction patterns of the precipitate at the centreline of Slab A 72

Figure (4-14): (a) and (b) Bright field images of a precipitate from Steel A, (c)

Schematic illustration of the respective phases 73

Figure (4-15): (a) Bright field image, (b) EDS trace of elements, (c-e) Selected area

diffraction patterns of phase C1 in Figure (4-14) 74

Figure (4-16): (a) Bright field image, (b) EDS trace of elements of phase C2 in Figure

(4-14) 74

Figure (4-17): Selected area diffraction patterns of particle in three different simple

planes of a precipitate formed at the centreline of Steel A (Figure (4-14-a)) 75

Figure (4-18): (a) Three-dimensional view of FeTiS2 , (b) Schematic crystal structure 76

Figure (4-19): (a) Schematic diagram showing the Ewald sphere construction. O is the

origin of the reciprocal lattice, P a point on the Ewald sphere which is centred at X. (b)

Diagrams illustrating the relation between the Ewald sphere construction and the

geometry associated with the formation of TEM pattern where the effective camera

length, which is a function of the post-objective lens settings, is termed L 77

Figure (4-20): (a) and (b) Bright field image of a precipitate from Steel A, (c)

Schematic illustration of the constituent phases 79

Figure (4-21): (a) Bright field image, (b) EDS trace of elements, (c and d) Selected

area diffraction patterns of phase Q1 in Figure (4-20) 79

Figure (4-22): (a) Bright field image, (b) EDS trace of elements, (c-e) Selected area

diffraction patterns of phase Q2 in Figure (4-20) 80

XV

Figure (4-23): (a) TEM sample prepared from a precipitate at the centreline of a slab

from Steel A by the FIB technique, (b) Bright field TEM image of the same particle 81

Figure (4-24): (a) Bright field image of a precipitate at the centreline of Steel A, (b)

Schematic illustration of constituent phases 81

Figure (4-25): (a) A bright field TEM image, (b) EDS trace and selected area

diffraction pattern of phase P1 in Figure (4-24) 82

Figure (4-26): (a) Bright field TEM image, (b) EDS trace, (c, d) Selected area

diffraction patterns of phase P2 in Figure (4-24) 83

Figure (4-27): (a) Bright field TEM image, (b) EDS trace, (c-e) Three selected area

diffraction patterns of phase P3 in Figure (4-24) 84

Figure (4-28): (a) Bright field image, (b) EDS trace of the precipitate, (c-d) Selected

area diffraction patterns of the precipitate extracted from the centreline of Steel B 86

Figure (4-29): (a) Bright field image, (b-d) Selected area diffraction patterns of a

particle extracted from centreline of Steel B 86

Figure (4-30): (a) Bright field image of a precipitate extracted from Steel A, (b)

Schematic illustration of the two phases 87

Figure (4-31): (a) EDS analysis pattern, (b-d) Selected area diffraction patterns

(SADP) of phase R1 in Figure 6 in three different tilts 88

Figure (4-32): X-ray map of the area containing phases R1 and R2 shown in Figure (4-

26) 89

Figure (4-33): (a) Bright field image, (b) Dark field image, (c-d) Selected Area

Diffraction Patterns (SADP) of phase R2 in Figure 6 in three low-index planes 90

Figure (4-34): Changes in the unit cell parameters a and c of solid solutions in the

FexTi(1-x)S system, plotted as a function of composition 94

Figure (4-35): Phase diagram of FeS-MnS 96

Figure (4-36): Modified area fraction and relative frequency of various inclusions at

the centreline of slabs A and B 97

Figure (5-1): Schematic illustration of samples positions in slab 100

Figure (5-2): Pseudo-binary Fe-C phase diagrams predicted by Thermo-Calc: (a) Fe-

C- 0.3% Mn- 0.01% S phase diagram, (b) Fe-C- 0.3% Mn- 0.01% S- 0.008% Ti phase

diagram, (c) Fe-C- 0.3% Mn- 0.01% S- 0.024% Ti phase diagram, (d) Fe-C- 0.3%

Mn- 0.006% S- 0.024% Ti phase diagram 101

Figure (5-3): Thermo-Calc prediction of the Fe- S- phase diagram when the iron

contains 0.1% C, 0.3% Mn and 0.024% Ti 102

XVI

Figure (5-4): (a-d) Bright field images of precipitates extracted from the edge of Slab

A by carbon replica extraction, e) EDS analysis of rectangular precipitates 103

Figure (5-5): (a) A bright field TEM image of precipitates and (b) corresponding EDS

analysis 104

Figure (5-6): A bright field TEM image and corresponding Selected Area Diffraction

Pattern (SADP) of a precipitate formed at the edge of Slab A 105

Figure (5-7): A bright field TEM image of a thin foil prepared from the edge of Slab A 105

Figure (5-8): Bright field TEM images of precipitates formed at the edge of Slab A 106

Figure (5-9): Bright field TEM images of precipitates formed at the edge of Slab B 107

Figure (5-10): (a) Bright field TEM images of rod-like precipitates formed at the edge

of Slab B, (b) Bright field TEM images of precipitate in Figure (5-10-a) at higher

magnification, (c) EDS analysis 107

Figure (5-11): Bright field TEM images of sulphides extracted form the edge of Slab

B attached to TiN 108

Figure (5-12): HR-SEM images of precipitates formed within austenite grains at the

centreline of Slab A 109

Figure (5-13): Bright field TEM images of precipitates formed within austenite grains

at the centreline of Slab A 109

Figure (5-14): (a) Bright field TEM images of precipitates formed at the centreline of

Slab A, (b) EDS analysis of particle 1, (c) EDS analysis of particle 2, (d) EDS analysis

of particle 3 110

Figure (5-15): Bright field TEM images of precipitates formed at the centreline Slab A 111

Figure (5-16): Bright field TEM image of TiN formed at the centreline Slab A

together with corresponding Selected Area Diffraction Pattern (SADP) 111

Figure (5-17): Bright field TEM images of copper sulphide attached to TiN at the

centreline of Slab A at different magnifications 112

Figure (5-18): HR-SEM image of precipitates formed at the centreline of Slab B 112

Figure (5-19): (a) Bright field TEM image of a precipitate formed in centre of slab in

Steel B, (b) Selected area diffraction pattern of the precipitate, (c) EDS analysis of the

precipitate 113

Figure (5-20): Bright field TEM image of precipitates formed at the centre of Slab B 113

Figure (5-21): (a) Bright field TEM image of cubic precipitates formed at the centre of

Slab B together with (b) Selected area diffraction pattern of the marked precipitate 114

Figure (5-22): Temperature dependence of the solubility products of TiS and Ti4C2S2

XVII

reported in earlier research 115

Figure (5-23): Titanium and carbon atom map and corresponding 1.5 at% C iso-

concentration surface of a specimen prepared from the edge of Slab A 118

Figure (5-24): Concentration profile of elements across the precipitates in Figure (5-

23) 118

Figure (5-25): Titanium, sulphur, nitrogen and carbon atom map of a specimen which

was prepared from the centreline of Slab A 120

Figure (5-26): The 2 at% C iso-concentration surface of a specimen which was

prepared from the centreline of Slab A 121

Figure (5-27): The position of selected ‘box’ perpendicular to precipitate 1 as well as

distribution of elements within the precipitate shown in Figure (5-26) 122

Figure (5-28): Concentration profile of elements across the precipitate 1 in Figure (5-

26) 122

Figure (5-29): Atomic map and concentration profile of elements across the precipitate

prepared by IVAS 123

Figure (5-30): Concentration profile of elements across the precipitate 2 in Figure (6-

23) 123

Figure (6-1): Schematic illustration of samples positions in slab prepared for

concentric solidification technique 128

Figure (6-2): Schematic illustration of solidification in continuously cast slab and

concentrically solidified specimen 128

Figure (6-3): Floated particles on the surface of liquid steel during heating in LSCM 129

Figure (6-4): Solidification progress and peritectic transformation observed in LSCM 130

Figure (6-5): δ-ferrite dendrites that formed in the molten pool, observed by LSCM 130

Figure (6-6): In-situ observation of sulphide precipitation on the surface of the LSCM

sample 131

Figure (6-7): Schematic illustration of solidification and precipitation by concentric

solidification technique 132

Figure (6-8): SEM photograph of a colony of precipitates 132

Figure (6-9): X-ray mapping of precipitates 133

Figure (6-10): Presence of liquid steel as well as two different solid phases (LSCM

sample) 134

Figure (6-11): Solubility product [Ti][S] for (TiS)s precipitation as a function of

temperature 135

XVIII

Figure (6-12): Cross section of particles observed on the surface of concentrically

solidified specimen in the LSCM, (a) to (d) illustrate the cross section at different

magnifications 135

Figure (6-13): TEM sample prepared by FIB from the particles observed in the LSCM,

(a) Selected area, (b) Pt coating of interested area, (c) Prepared TEM sample, (d)

Thined prepared TEM sample in higher magnification 136

Figure (6-14): A bright field TEM image of layers in the sample shown in Figure (6-

13) 137

Figure (6-15): X-ray map prepared from TEM sample 138

Figure (6-16): Manganese sulphide precipitate surrounded by iron sulphide 139

Figure (7-1): Schematic view of a cross section (ABCD) of the concentrically

solidified specimen in LSCM 142

Figure (7-2): Cross section of a concentrically solidified sample 143

Figure (7-3): Fe–C phase diagram 144

Figure (7-4): (a) Typical microstructure of continuously cast slab in an Al-killed steel

with 0.15 wt% C, 1.4 wt% Mn, 0.46 wt% Si and 0.003 wt% S, (b) An optical

micrograph of the centerline segregation in 0.1 wt% C, 0.3 wt% Mn and 0.001 wt% S 145

Figure (7-5): Relative concentrations of Mn, Ti and S along a 7-mm length at the

centre of a concentrically solidified specimen (from steel A) at 20° C/min cooling rate

using line-scanning in electron probe microanalyses 146

Figure (7-6): Segregation profiles of C, P an Nb at the centreline of a continuously

cast slab 147

Figure (7-7): (a) Progression of δ-ferrite into the liquid pool (b) Progression of

austenite into the liquid pool following the peritectic reaction 148

Figure (7-8): Formation of dendrites ahead of the advancing solid/liquid interface in

the last remaining liquid 149

Figure (7-9): Relative concentrations of Mn along a 9.4-mm length at the centre of

concentrically solidified specimens (from Steel A) at different cooling rates using line-

scanning in EPMA analyses 150

Figure (7-10): Relative concentrations of Mn, Ti and S along a 9.4-mm length at the

centre of concentrically solidified specimens (from Steel A) at 5 and 80°C/min

cooling rates using line-scanning in electron probe microanalyses 151

Figure (7-11): The segregated area of a concentrically solidified sample in LSCM,

prepared from steel A in optical microscope. Sulphide precipitates formed on prior

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austenite grain boundaries followed by primary ferrite formation on further cooling.

(a) As polished (b) Etched in 2% Nital 153

Figure (7-12): Sulphide precipitates and schematic clarification on prior austenite

grain boundaries observed at higher magnification a) Centreline of a slab (steel A), b)

Segregated area of a concentrically solidified specimen prepared from steel A 153

Figure (7-13): High resolution scanning electron microscopy images of precipitates in

segregated area 154

Figure (7-14): Concentration mapping (CM) of precipitates at the centreline of a slab

of steel A (EPMA) 155

Figure (7-15): Concentration mapping (CM) of precipitates in the segregated area of a

concentrically solidified specimen from Steel A: (a) EPMA results (b) x-ray mapping

by SEM 156

Figure (7-16): (a) Bright field image and EDS trace of precipitate formed in the

segregated area of a concentrically solidified specimen from steel A, (b) Selected area

diffraction patterns of the precipitate from two low-index planes 158

Figure (7-17): Bright field image and EDS spectrum of a particle formed on grain

boundary of a concentrically solidified specimen prepared from Steel A 158

Figure (7-18): Selected area diffraction patterns of the particle in three different low-

index planes of a precipitate formed in the segregated area of a concentrically

solidified specimen from steel A 159

Figure (7-19): Selected area diffraction pattern of the ferrite phase in contact with the

sulphide particle (Figure (7-17)) 159

Figure (7-20): Bright field image and EDS spectrum of the particle in segregated area

of the segregated area of a concentrically solidified specimen from Steel A 161

Figure (7-21): Selected area diffraction pattern of the particle in Figure (7-20) 161

Figure (7-22): (a, b) Bright field TEM images (c) EDS spectrum of the bright area (A)

(d) EDS trace of the dark area of the particle in the segregated area of concentrically

solidified specimen of steel B 162

Figure (7-23): (a) A bright field image of a particle in segregated area of a

concentrically solidified specimen in LSCM from steel B, (b) A schematic graph of

different phases in the particle 163

Figure (7-24): (a, b) Selected area diffraction patterns of iron sulphide in Figure (7-

23), (c) Selected area diffraction patterns of manganese sulphide in Figure (7-23) 163

Figure (7-25): (a) Bright field image, b) EDS trace of a precipitate prepared from the

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segregated area of a concentrically solidified specimen from Steel B; (b) A schematic

illustration of the particle 164

Figure (7-26): Selected area diffraction patterns of the precipitate in Figure (7-25)

which was prepared from the segregated area of a concentrically solidified specimen

from Steel B

165

Figure (8-1): ): Fe-C Phase diagram showing local equilibrium at the solid/liquid

interface in the liquid plus delta-ferrite two phase region of an Fe-0.18%C alloy at the

temperature shown 169

Figure (8-2): δ-ferrite growth and peritectic reaction in Fe-0.18 pct C alloy in a

concentrically solidified specimen prepared by LSCM at cooling rate of 10 °C/min:

(a) Stable liquid pool at the initiation of solidification, (b) δ-ferrite growth into the

liquid, (c) Peritectic reaction, (d) Austenite growth into the liquid and transformation

of δ-ferrite to austenite 169

Figure (8-3): Interface position versus time for Case A at 5 °C/min cooling rate 172

Figure (8-4): Schematic illustration of the concentric solidification technique with

essential temperatures shown (T3 > T2 >T1) 173

Figure (8-5): Interface position versus temperature, Case A with adjusted experimental

data at 5, 10 and 20°C/min cooling rates 174

Figure (8-6): Weight percent of carbon versus distance at default diffusion coefficient

of carbon, 10, 100 and 1000 times faster carbon mobility in liquid, (a): 300 seconds

after the initiation of solidification at 5 °C/min cooling rate, (b): 700 seconds after the

initiation of solidification at 5 °C/min cooling 176

Figure (8-7): Confocal sample volume variation versus radius at constant thickness 176

Figure (8-8): Interface position versus temperature for Case A simulation system at 5,

10 and 20°C/min cooling rate and higher carbon diffusivity in liquid 177

Figure (8-9): Comparison of experimental data with simulation results at various

carbon diffusivity in the liquid 178

Figure (8-10): Interface position versus temperature for Case B at 5, 10 and 20°C/min

cooling rates 179

Figure (8-11): Interface position versus temperature for Case C simulations set at 5, 10

and 20°C/min cooling rates 182

Figure (8-12): a) Carbon profile at 1 and 50 seconds after start of solidification at

20 °C/min cooling rate for Case B set of simulations, b) Carbon profile at 30, 80 and

120 seconds at 20 °C/min cooling rate for Case C set of simulations 184

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Figure (8-13): Schematic of the concentric solidification set-up, and corresponding

temperature distribution 187

Figure (8-14): Pseudo-binary phase diagram of Fe-Ni 188

Figure (8-15): Sequence of events happened during solidification of concentrically

solidified specimen at 5 °C/min cooling rate with corresponded nickel profile which

measured experimentally and calculated by simulation 189

Figure (8-16): Comparison of nickel profile of a concentrically solidified specimen at

40 °C/min cooling rate with nickel profile calculated by Case B set of simulations 190

Figure (8-17): One-dimensional diffusive model for banding in peritectic alloys

showing the mechanism and banding window 191