Formation and Behaviour of Mn-containing Oxysulphide Inclusions during Desulphurisation, Deoxidation and Alloying (Vorming en gedrag van Mn-houdende oxisulfide inclusies gedurende ontzwaveling, deoxidatie en legering)

Manganese-containing oxysulphides can be used to minimise the detrimental effects of oxide inclusions and to improve mechanical properties of steel products, like ductility, fatigue strength and machinability. The effective utilization of Mn-containing oxysulphide inclusions requires an accurate control on their characteristics, e.g. chemistry, microstructure, size distribution and morphology. Therefore a deep understanding of their formation and behaviour during desulphurisation, deoxidation and alloying is necessary for a complete manipulation of Mn-containing oxysulphide inclusions. In the first part of the research, the desulphurisation and inclusion behaviour of stainless steel refining were investigated by using CaO-Al2O3 based top slags. Although a relatively lower sulphide capacity was obtained for an optimised CaO-Al2O3 slag (50 % CaO, 40.4 % Al2O3, 4.3 % SiO2, 2.1 % Fe2O3, 1.7 % TiO2 and 0.6 % MgO) compared to current CaO-SiO2-CaF2 slag, an equivalent desulphurisation efficiency and an improved steel cleanliness were obtained with this optimised CaO-Al2O3 slag. Based on these results, the substitution of current CaO-CaF2 based slag with CaO-Al2O3 based slag for stainless steel refining was discussed and evaluated. In the second part of the work, the interfacial reaction between molten steel and Mn/FeMnSi alloys at the early stages of deoxidation/alloying were studied with a metal suction method. A layer of Fe solidified around the cold alloy immediately after alloy addition. While enclosed by this Fe shell, the alloy was partially molten due to its low melting temperature. The oxysulphide inclusions were formed at the Mn-rich side of the interface, while an inclusion free zone was detected in the Fe-rich side close to the interface. Based on experimental findings and theoretical calculations, the mechanisms governing the Fe/Mn (or Fe/FeMnSi) interdiffusion and the inclusion free zone formation were proposed. Steel alloying, such as Mn, Al, FeSi and FeMn additions to the liquid steel with different oxygen and sulphur content was also simulated with a confocal scanning laser microscope combined with a special addition device. The effects of the sulphur and oxygen content in liquid iron, as well as that of the deoxidant type on the formation of oxides/oxysulphides were investigated. The inclusion behaviour on the liquid iron surface, i.e. the interaction after inclusion formation, the dissolution during the high temperature iso-thermal holding and growth during the cooling were observed. This part of the work contributes to the optimisation (or manipulation) of deoxidation/alloying operations to control the formation and characteristics of Mn-containing inclusions.Contents Foreword i Abstract iii Samenvatting v Symbols and abbreviations vii Contents ix PART I Chapter 1 General introduction 3 1.1 Project background 3 1.2 Research objectives 4 1.3 Outline of the thesis 4 Chapter 2 Literature study on ladle refining and steel cleanliness 7 2.1 Ladle refining and steel cleanliness 7 2.1.1 Ladle refining 7 2.1.2 Deoxidation 8 2.1.3 Desulphurisation 11 2.1.4 Alloying 16 2.2 Steel cleanliness and inclusion engineering 17 2.2.1 Steel cleanliness 17 2.2.2 Inclusion engineering 22 2.3 Challenges and tasks 26 2.3.1 On-set and in-situ deoxidation and alloying 26 2.3.2 Desulphurisation and steel cleanliness 27 PART II Chapter 3 Sulphide capacity and mineralogy of BaO and B2O3 modified CaO-Al2O3 top slag 33 Abstract 33 3.1 Introduction 34 3.2 Experimental 35 3.2.1 Materials preparation 35 3.2.2 Sulphide capacity measurement 36 3.2.3 Sample analysis and characterisation 38 3.3 Results and discussion 38 3.3.1 Sulphide capacity and its significant factors 38 3.3.2 Slag mineralogy 43 3.4 Conclusions 46 Chapter 4 Desulphurisation of stainless steel by using CaO-Al2O3 based slags during secondary metallurgy 49 Abstract 49 4.1 Introduction 50 4.2 Experimental 51 4.2.1 Materials 51 4.2.2 Experimental Procedure 52 4.2.3 Sample Analysis 53 4.3 Results and discussion 53 4.3.1 Desulphurisation results 53 4.3.2 Thermodynamic consideration on the desulphurisation 56 4.3.3 Desulphurisation kinetics 61 4.4 Conclusions 65 Chapter 5 Effect of the CaO-Al2O3 based top slag on the cleanliness of stainless steel during secondary metallurgy 67 Abstract 67 5.1 Introduction 68 5.2 Experimental 69 5.2.1 Materials 69 5.2.2 Experimental procedure 70 5.2.3 Compositional analysis and inclusion characterisation 72 5.3 Experimental results 72 5.3.1 Inclusion population 72 5.3.2 Inclusion composition 75 5.4 Thermodynamic and kinetic considerations 79 5.4.1 Slag-steel interaction 79 5.4.2 Slag-Steel-inclusions interaction 84 5.5 Conclusions 89 Chapter 6 Desulphurisation and inclusion behaviour of stainless steel refining by using CaO-Al2O3 based slag at low sulphur levels 93 Abstract 93 6.1 Introduction 94 6.2 Experimental 95 6.2.1 Materials 95 6.2.2 Experimental Procedure 95 6.2.3 Sample analysis 97 6.3 Results and discussion 97 6.3.1 Desulphurisation behaviour 97 6.3.2 Steel cleanliness 103 6.4 Conclusions 110 PART III Chapter 7 Interfacial reactions at early stages of Mn addition to liquid Fe 115 Abstract 115 7.1 Introduction 116 7.2 Experimental 117 7.3 Results and discussion 119 7.3.1 Overview of the Fe-Mn diffusion couple 119 7.3.2 The reaction zone development and the diffusion process 119 7.3.3 Effect of oxygen and sulphur contents 125 7.4 Comparison with previous studies 129 7.5 Conclusions 130 Chapter 8 Interfacial reaction and inclusion formation at early stages of FeMnSi addition to liquid Fe 133 Abstract 133 8.1 Introduction 134 8.2 Experimental 135 8.3 Results and discussion 137 8.3.1 Overview of the diffusion couple 137 8.3.2 The reaction zone development and the interfacial reaction 139 8.3.3 Inclusion formation in reaction zone 142 8.4 Conclusions 145 Chapter 9 In-situ observation of the formation and interaction behaviour of the oxide/oxysulphide inclusions on a liquid iron surface 147 Abstract 147 9.1 Introduction 148 9.2 Experimental 149 9.2.1 Experimental procedure 149 9.2.2 Materials 150 9.3 Results and discussion 150 9.3.1 Inclusion formation behaviour and its characteristics 150 9.3.2 Inclusion dissolution and/or growth during the deoxidation 156 9.4 Conclusions 160 PART IV Chapter 10 General conclusions and future perspectives 165 10.1 General conclusions 165 10.1.1 Desulphurisation and steel cleanliness of stainless steel refining by using CaO-Al2O3 based slag 165 10.1.2 Interfacial reaction and inclusion behaviour at early stages of alloying and deoxidation 166 10.2 Future perspectives 167 List of publications 169 Curriculum vitae 171nrpages: 191status: publishe