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

Transcription

1 University of Wollongong Research Online University of Wollongong Thesis Collection University of Wollongong Thesis Collections 2008 Effect of titanium additions to low carbon, low manganese steels on sulphide precipitation Sima Aminorroaya-Yamini University of Wollongong, 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, Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library:

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3 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

4 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 II

5 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 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 irontitanium-sulphide co-exists with (Mn,Fe)S. An increase of the titanium content from wt% to wt%, results in an increase in the titanium content of irontitanium-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 FeTiS 2 (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. III

6 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 FeTiS 2 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 IV

7 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 wt% to 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. V

8 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. VI

9 Table of contents CHAPTER 1: Introduction 1-1 Recent developments in linepipe steels Classification of precipitates in continuously cast slab Simulation of macrosegregation in continuously cast slab Chapters contents 5 CHAPTER 2: Literature survey 2-1 Introduction Titanium microalloyed steel Development of low carbon microalloyed linepipe steels The Fe-Ti-Al-O system The Fe-Ti-C-N system Sulphide inclusions in steels Manganese sulphide classification in steels Deformation of sulphide inclusions during hot rolling Sulphide inclusions in titanium containing steels Segregation in Continuous casting Microsegregation Macrosegregation White band In-situ observation of sulphide precipitation in steel In-situ observation of macrosegregation in steel 38 CHAPTER 3: Experimental instruments and methodology 3-1 Scanning Electron Microscopy (SEM) Sample preparation for optical and scanning electron microscopy High Resolution Scanning Electron Microscopy (HR-SEM) 39 VII

10 3-2 Electron Probe Micro Analysing (EPMA) Focused Ion Beam- Dual Beam (FIB) Dual Beam Three Dimensional Imaging and Reconstruction High Resolution Transmission Electron Microscopy (HR-TEM) Sample preparation for TEM analysis Carbon replica Selective Potentiostatic Etching by Electrolytic Dissolution Focused Ion Beam TEM analysis interpretation by Java Electron Microscopy Software High Temperature Laser Scanning Confocal Microscopy (HT-LSCM) Concentric Solidification technique (CST) Sample preparation for Laser Scanning Confocal Microscopy Three dimensional local electrode Atom Probe Sample preparation for Atom Probe investigations Principles of three-dimensional Atom Probe Data analysis Computational thermodynamics and kinetics Thermo-Calc DICTRA DICTRA and diffusivities Relations between diffusion coefficients and atomic mobilities Modelling of the atomic mobility 58 CHAPTER 4: Effect of titanium on the centerline sulphide precipitates of slabs 4-1 Introduction Experimental technique Inclusion analysis Three dimensional imaging of precipitates SEM evaluation of sulphide precipitates at the centerline of slabs TEM evaluation of sulphides precipitates at the centerline of Slab A TEM evaluation of sulphide precipitation at the centerline of Slab B 85 VIII

11 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 Experimental method TEM analyses of precipitates formed at the edge of Slab A TEM analyses of precipitates formed at the edge of Slab B TEM analysis of precipitates formed at the centerline of Slab A TEM analyses of precipitates formed in slab centre of Slab B Comparison of precipitation in different steels Three dimensional Atom probe analysis Summary 124 CHAPTER 6: Initial experiments on segregation study 6-1 Introduction Experimental method 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 Experimental techniques Study of segregation in Laser-Scanning Confocal Microscopy TEM investigation of sulphides precipitated in segregated area of concentrically solidified samples in LSCM TEM analysis of sulphides precipitated in the segregated area of a concentrically solidified specimen in LSCM prepared from steel A TEM analysis of sulphide precipitates in the segregated area of concentrically solidified specimen in the LSCM prepared from steel B Comparison of sulphides in centerline of slabs with concentrically solidified specimens 165 CHAPTER 8: Modeling of the solid/liquid interface progression in the concentric IX

12 solidification technique 8-1. Introduction Experimental data collection Fe-0.1%C peritectic alloy Simulation Heat transfer in concentrically solidified specimen Fe-4.2%Ni peritectic alloy simulation Conclusion 192 CHAPTER 9: Summary of findings, conclusions and recommendations 9-1 Summary of findings Conclusion Recommendations for future work 200 References 202 Appendices 213 Publications list 218 X

13 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 XI

14 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 ( 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, Ti 4 C 2 S 2 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 XII

15 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 XIII

16 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 FeTiS 2, (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 XIV

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

18 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 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 Ti 4 C 2 S 2 XVI

19 reported in earlier research 115 Figure (5-23): Titanium and carbon atom map and corresponding 1.5 at% C isoconcentration 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 XVII

20 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 wt% S, (b) An optical micrograph of the centerline segregation in 0.1 wt% C, 0.3 wt% Mn and 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 linescanning 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 XVIII

21 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 lowindex 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 XIX

22 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 165 from Steel B 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 (T 3 > T 2 >T 1 ) 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 XX

23 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 XXI