Steel Making [3rd printing ed.] 8120330501, 9788120330504

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A.K. CHAKRABARTI Visiting Professor Papua New Guinea University of Technology, Lae Morobe Province Papua New Guinea Formerly Professor Metallurgical and Materials Engineering Department Indian Institute of Technology Kharagpur

PH I Learning [;)u1[RlmO@ [bowJ□O@ctJ New Delhi-110001 2012

STEEL MAKING A.K. Chakrabarti

© 2007 by PHI Learning Private Limited, New Delhi. All rights reserved. No part of this book may be reproduced in any form, by mimeograph or any other means, without permission in writing from the publisher. ISBN-978-81-203-3050-4 The export rights of this book are vested solely with the publisher. Third Printing

May, 2012

Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus, New Delhi-110001 and Printed by Meenakshi Art Printers, Delhi-110006.

To the memory of my teacher PROFESSOR A.K. SEAL

Contents

Preface

xi

Acknowledgements

xiii

1. HISTORICAL PERSPECTIVE AND CURRENT STATUS OF STEEL MAKING Review Questions 4 References 5

1-5

2. PHYSICOCHEMICAL PRINCIPLES

6-21

2.1 2.2 2.3

Introduction 6 Structure of Silicate Slags 7 Reactions of Importance in Steel Making 2.3.1 Carbon-Oxygen Equilibrium 11 2.3 .2 Manganese Reaction 12 2.3 .3 Phosphorus Reaction 13 2.3.4 Sulphur Removal 16 2.3.5 Silicon Reactions 17 2.3.6 Nitrogen and Hydrogen 18 Review Questions 20 References 21

10

3. 22-35

REVIEW OF THE OLDER STEEL MAKING PROCESSES A. Bessemer Converter Process 3.1 3.2 3.3 3.4

Introduction 22 Design of the Converter and Operational Practice Chemistry of Refining 24 Decline of the Bessemer Process 26 V

23

vi

Contents

B. Open Hearth Furnace Steel Making Process 3.5 3.6 3.7

Introduction 27 Construction of the Open Hearth Furnace 28 Operation of the Basic Open Hearth Furnace 30 3.7.1 Charging 30 3.7.2 Melting Down 31 3.7.3 Oxidation and Refining 31 3. 7.4 Finishing 32 3.8 Developments in Open Hearth Furnace Practice 32 3.9 Reasons for Decline of the Open Hearth Process 34 Review Questions 34 References 35

4. TOP-BLOWN BASIC OXYGEN CONVERTER PROCESSES 4.1 Introduction 36 4.2 LD Converter Steel Making Practice 37 4.3 Oxygen Jet 39 4.4 Refractory Practice 43 4.5 Lime for LD Steel Making 45 4.6 Reactions in LD Converter 46 4. 7 Mechanism of Refining 49 4.8 Bath Agitation Process (BAP)-Combined Blowing 4.9 LDAC/OLP Process 55 4.10 KALDO Process 56 4.11 Rotor Process 57 4.12 Hybrid Processes 58 Review Questions 60 References 60

36-61

54

5. BOTTOM-BLOWN BASIC OXYGEN CONVERTER PROCESS (Q-BOP/OBM/LWS) 5.1 5.2

Introduction 62 Basic Metallurgical Characteristics of the Q-BOP Process 64 5.3 Mechanism of Refining 65 5.3.1 Decarburization 65 5.3.2 Phosphorus and Manganese Behaviour 67 5.3.3 Desulphurization 68 5.3.4 Hydrogen Removal 68 5.3.5 Nitrogenation 69 5.3.6 Argon Degassing of Steel 69 5.3.7 Oxygen Removal 70 Summary 70 Review Questions 70 References 71

62-71

Contents

vii

&. 72-103

ELECTRIC FURNACE STEEL MAKING 6.1 6.2

Introduction 72 Constructional Features 73 6.2.1 Hearth 73 6.2.2 Side Walls 75 6.2.3 Roof 75 6.3 Steel Melting 76 6.3.1 Charging 77 6.3.2 Melting Down 77 6.3.3 Refining and Finishing 78 6.3.4 Induction Stirring 80 6.4 Developments in EAF Technology 80 6.4.1 Developments in Furnace Design 81 6.5 Developments in Operational Features 86 6.5.1 Oxyfuel Burners and Oxygen-Lancing 86 6.5.2 Foamy Slag Practice 87 6.5.3 Preheating of Scrap and Waste Heat Recovery 6.5.4 Use of Sponge Iron as a Charge Material 90 6.5.5 Use of Hot Metal and Iron Carbide as Charge Materials 95 6.6 Stainless Steel Making 95 6.6.1 Physicochemical Principles 95 6.6.2 AOD Process 98 Review Questions 101 References 102

88

7. SECONDARY STEEL MAKING 7.1 7.2

7.3 7.4 7.5

7.6

7.7

Introduction 104 Inclusions in Steel 104 7.2.1 Oxide Inclusions 104 7.2.2 Sulphide Inclusions 107 Objectives of Secondary Steel Refining 108 Injection Ladle Metallurgy 109 Physicochemical Principles of Refining in the Ladle by Synthetic Slags 112 7.5.1 Characteristic of Synthetic Slags 112 Turbulence and Agitation of the Bath in the Ladle 116 7.6.1 Argon Stirred Ladles 116 7.6.2 Stirring Energy Calculation 119 7.6.3 Mixing Time Calculation 120 Vacuum Degassing Process 120 7.7.1 Applications of Vacuum Metallurgy in Steel Melting 120 7.7.2 Vacuum-Carbon Deoxidation 121 7.7.3 Fundamental Principles of Reactions under Vacuum 121

104-138

viii

Contents

7. 7 .4 Vacuum Tank Degasser 123 7. 7 .5 Stream Degassing 123 7.7.6 Recirculation Degassing 124 7.8 Refining by Remelting 129 7.8.1 CEVAM Process 129 7.8.2 Electroslag Remelting (ESR) 130 7.9 Ladle Metallurgy 132 7.9.1 Vacuum Arc Degassing (VAD) 132 7.9.2 Process Capabilities of Ladle Furnace Review Questions 136 References 137

134

8.

INGOT CASTING PRACTICE Introduction 139 Tapping the Liquid Steel 139 8.2.1 Ladle 140 8.2.2 Slide Gate Stopper 141 8.3 Teeming Practice 142 8.3.1 Top Pouring 142 8.3.2 Bottom Pouring 143 8.4 Types of Ingots 144 8.5 Ingot Defects 149 8.5.1 Piping 150 8.5.2 Hydrogen Embrittlement 8.5.3 Segregation 150 8.5.4 Surface Defects 151 8.5.5 Rough Surface 151 8.5.6 Cracks 151 8.5.7 Inclusions 152 Review Questions 152 References 153

139-153

8.1 8.2

150

9.

CONTINUOUS CASTING OF STEEL 9.1 9.2 9.3 9.4

9.5

Introduction 154 Tundish 155 Continuous Casting Machines 157 Recent Developments 159 9.4.1 Breakout Detection System 159 9.4.2 Ladle Slag Detection System 160 9.4.3 Submerged Entry Nozzle (SEN) 160 9.4.4 Mould Powders 160 9.4.5 Electromagnetic Stirring 161 9.4.6 Electromagnetic Brakes 162 Quality Control in Continuous Casting 163 9.5.1 Mid-face Longitudinal Cracking 163 9.5.2 Sticker Breakout 164 9.5.3 Oscillation Marks and Transverse Cracking

154-174

164

Contents

ix

9.5.4 9.5.5 9.5.6 9.5.7 9.5.8 9.5.9 9.5.10

Star Cracking 164 Angular Cracking 165 Centreline Cracking 165 Gas and Slag Entrapment 165 Segregation 165 Inclusion Accumulation 166 Operational Parameters Affecting Quality of Surface and Internal Cracks 166 9.5.11 Strand Breakout 167 9.6 Thin Slab Casting 167 9.6.1 Liquid Core Reduction 168 9.6.2 Reheating Furnace 169 9.6.3 Compact Strip Process (CSP) 169 9.6.4 In-line Strip Processing (ISP) 170 9.7 Thin Strip Casting 170 9.7.1 Twin Drum Strip Caster 171 9.7.2 Metallurgical Characteristics 172 9.7.3 Defects 172 Review Questions 172 References 173

10. TRANSPORT PROCESSES, DIMENSIONAL ANALYSIS AND PHYSICAL SIMULATION IN STEEL MAKING A. Transport Phenomena 10.1 Introduction 175 10.2 Flux Calculation Through Turbulent Fluid Medium 10.3 Some Examples 179 10.3.1 Heat Balance 179 10.3.2 Mass Balance 181 10.3.3 Force Balance 181

175-197

176

B. Dimensional Analysis and Physical Simulation in Steel Making 10.4 Introduction 183 10.5 Physical Simulation 184 10.6 Design of a Cold Model Experiment to Simulate the Slag-Metal Reaction in Gas Stirred Ladle 192 10.6.1 Design of Experimental Vessel 192 10.6.2 Selection of Low Temperature Analogues for Steel, Slag and Impurity 193 10.6.3 Analysis of Cold Model Experimental Data 195 10. 7 Exercise 195 10.7.1 Killing Steel with Aluminium Bullets 195 Review Questions 196 References 196

Contents

X

11. FERROALLOY TECHNOLOGY 11.1 Introduction 198 11.2 Principles of Ferroalloy Making 198 11.3 Reserves of Minerals/Ores in India 199 11.4 Production of Ferromanganese 200 11.5 Production of Ferrosilicon 204 11.6 Production of Silicomanganese 207 11. 7 Production of Calcium Silicide 208 11.8 Production of Ferrochromium 208 11.9 Production of Low Carbon and Speciality Ferroalloys Review Questions 213 References 214

198-214

210

12. WASTE MANAGEMENT AND ENERGY CONSERVATION

215-222

12.1 Introduction 215 12.2 Voluntary Action Programme of the Japanese Steel Industry 216 12.3 Collection and Utilization of Dust Generated from the Electric Arc Furnace 217 12.4 Management of Toxic Gases Evolving from Sintering Plants and Furnaces 219 Review Questions 221 References 222

INDEX

223-226

Preface

A

course on Steel Making is usually an essential component of the Metallurgical Engineering curriculum in all Indian Universities. The present text is primarily focussed towards the needs of undergraduate Metallurgical Engineering students and candidates for Associate Membership Examinations of Professional Societies like AMIIM, AMIE etc. However, some of the chapters are relatively more elaborate and may also serve as reference materials for more advanced students and professional engineers. The writeup on transport processes, dimensional analysis and physical simulation in steel making has been contributed by my colleague Prof. G.G. Roy (Chapter 10). The book consists of twelve chapters. After a brief introduction to the historical perspective and current status of steel making in Chapter 1, the physicochemical principles involved in steel making are discussed in Chapter 2. In Chapter 3, the principles and operational practices of the older steel making processes (e.g., Bessemer converter and open hearth processes) are briefly highlighted. The reasons for decline of these processes are also analyzed. Top- and bottom-blown basic oxygen converter processes (LD, Q-BOP etc.) are discussed in Chapters 4 and 5 respectively. An attempt has been made to cover both theory and practice of these processes. Recent developments in refractory practice, lance desigu and blowing strategy, and advances in understanding of the essential reaction mechanisms have been elucidated. xi

xii

Preface

Electric furnace steel making has been elaborated in Chapter 6. After discussing the basic constructional features and operational principles and practices of electric furnace steel making, current developments in electric furnace design and developments in charge preparation, melting and operational practices have been highlighted. Chapter 7 is a summary of the developments in secondary refining of steels. The basic principles of injection metallurgy vacuum treatment, treatment under synthetic slags and fluxes etc. have been brought out briefly. Various secondary refining practices have also been discussed. Chapters 8 and 9 discuss respectively the principles and practices of ingot casting and continuous casting of steels. Major developments in continuous casting practice, such as thin slab and thin strip casting technology have also been briefly discussed. Defects arise in both ingots and concast products. Hence a familiarity with the causes of these defects and the remedial measures is important for process and quality control. These topics have, therefore, been covered in the text. Techniques of physical simulation are discussed briefly in Chapter 10. Analytical methods of interpretation of the results of simulation and cold model studies are also highlighted. Ferroalloy technology and water management, and energy conservation are discussed in Chapters 11 and 12 respectively. The book has evolved largely from the author's experience of teaching steel making to metallurgy students. Inputs from Prof. G.G. Roy are intended to further illustrate the current trends in steel making research. The author will deem his labour rewarded if the book is considered useful by the metallurgical student community. A.K. CHAKRABARTI

Acknowledgements

T

he author thanks the CSIR, New Delhi, for granting him financial assistance in the form of an Emeritus Scientist Award tenable at IIT Kharagpur, during preparation of this manuscript. The author has also received full infrastructural support from the Metallurgical and Materials Engineering Department and the Central Library of IIT Kharagpur. This is gratefully acknowledged. The typed version of the manuscript has been mainly prepared by Shri P. Mitra, who must be thanked for his patience and sincerity. Secretarial help received from Shri T.K. Chakrabarti and Shri M. Dandasi is also thankfully acknowledged. Thanks are also due to Shri S.P. Hazra for preparing the line drawings. The author has received support and cooperation from all his colleagues in the Metallurgical and Materials Engineering Department, IIT Kharagpur, during the course of preparation of this manuscript. He is grateful to all of them.

A.K. CHAKRABARTI

xiii

1 Historical Perspective and Current Status of Steel Making

T

he history of human civilization is closely linked to the developments in the field of metals and alloys. The advent of the Copper Age nearly six thousand years ago, marked the beginning of human civilization. It was quickly followed by the Bronze Age, and then came the Iron Age. In the Iron Age, mankind learnt the art of iron making by solid state reduction of iron ore by charcoal. The product, a bloom, in most cases consisted of pure iron, iron oxide, carbon particles and slag. A further progress was made when the old craftsman learnt the technique of squeezing out slag from the bloom to produce wrought iron. India was a pioneer in the production of a variety of high carbon steel, known as 'wootz'. It was produced for nearly one thousand years in closely guarded monopoly. Wootz was exported from India to different countries. The famous 'Damascus' sword used to be made from this steel. India and China made far-reaching contributions in the field of iron and steel foundry practice as well. China developed the process of iron casting sometime around 800 to 700 B.C. Chinese technicians developed shaft-type furnaces for smelting iron ore with charcoal. India developed the crucible process of casting steel. Wrought bars were annealed in charcoal fire until a type of steel, 'blister-steel', was obtained. This was then converted into liquid steel by means of intense heating in a clay crucible. Unfortunately, the process steps were never documented. Moreover, further progress stopped in both the countries around 1000 A.D. 1 1

2

Steel Making

The history of the modem steel making really started form 1760, when Huntsman invented in England the crucible process for making the steel. The process was gradually upgraded to produce alloy and tool steels. But the quantity produced by the crucible process was small. Large-scale steel production became possible only after Henry Bessemer developed the Bessemer converter process of steel making in 1856. A series of developments have taken place in the last one hundred and fifty years. China has emerged as the largest steel producer in the world. India is also struggling hard to restore its pristine glory in the field of steel making. At the time of Indian independence, there were only three integrated steel plants in the country. Their total production was of the order of 2.5 million tons per annum. Medium phosphorus high silicon hot metal from blast furnaces used to be processed through the acid Bessemer-basic open hearth duplex steel making route. A part of the steel was also made directly in basic open hearth furnaces from steel scrap and hot metal mixed charge. Electric arc furnaces made only a very minor contribution to the total output. From the second five-year plan period and onwards, public sector integrated steel plants were set up in different parts of India. The establishment of these steel plants with foreign technical assistance not only boosted up Indian steel production, but also contributed to the training of a new breed of steel technologists. The availability of technical manpower and the demand for steel subsequently attracted large-scale private investment in the steel sector. As a result, a large number of electric arc furnace-based mini steel plants also came up in the country. Steel scrap and sponge iron are the basic feedstocks in these plants. Apart from blast fumace-LD converter (Top-blown basic oxygen converter) route, current Indian practices include sponge iron + scrap - EAF, sponge iron + scrap + hot metal - EAF (Electric Arc Furnace) and smelting reduction (COREX Process) - basic oxygen converter steel making route. Because of the relatively high cost of electricity in the country, the EAF-based steel industries face periodic slowdown. These problems will continue to haunt the Indian steel makers for quite some time. However, in the context of rising internal and external demands of steel, ambitious plans have been drawn up to raise the steel production from around 40.0 million tons in 2005-2006 by almost hundred per cent in the next five years. Several new integrated steel plants are in the pipeline. Future expansion programmes will have to be taken into account measures for reducing dependence on hard coke, especially the imported low-ash variety because a global shortage of this item is likely in the near future. 2

Historical Perspective and Current Status of Steel Making

3

In the past, Indian steel plants used to concentrate mainly on the production of structural items. As a country progresses, the proportion of flat products in the total output also gradually increases. A similar trend is being observed in India as well. In modern steel plants, the emphasis is simultaneously on quantity and quality. In steel industry, quality improvement calls for massive investment in the form of secondary refining facilities. The Indian steel industry has already made significant strides in this respect, but obviously much more needs to be done. A very interesting aspect of Indian steel industry is the growth of small induction furnace-based plants. The total installed capacity of the induction-based units is of the order of four million tons. These plants operate with small induction furnaces, often as small as one ton capacity. Their usual products are pencil ingots, which are rolled into bars and rods, primarily for the construction sector. Energy consumption per ton of steel production is often considered a benchmark of the operational efficiency. Indian plants are far off from the most efficient plants in other countries like Japan and Korea. Sustained effort is needed in this direction. Steel production necessarily involves production of million of tons of slag as well as waste gases containing harmful constituents like carbon dioxide, dioxin, furans, etc. Safe disposal of the slag and elimination of atmospheric pollution by the waste gases are now matters of serious concern in all steel producing countries. The advanced countries are now laying tremendous emphasis on (a) reduction of energy consumption per ton of steel produced, and (b) sustainable environmental management. Various measures adopted since the oil crisis of early seventies have already brought down their energy consumption appreciably. However, the most recent thrust is on more integrated green manufacturing and more intensive waste recycling for sustainable development. Efforts are on for economic use of all types steel plant wastes, such as slag, dust, flue gases, etc. Serious attempts are being made to reduce CO 2 burden through (a) more efficient operation of iron and steel making furnaces so as to reduce the generation of CO 2 , and (b) absorbtion of CO 2 in suitable media. A new concept being mooted is to utilise the waste from one industry as a resource in another industry, a practical example being the injection of waste plastics into the blast furnace. In addition to CO 2 , the harmful effects of other volatile species in waste gases such as dioxins, chlorinated tricyclic aeromatic compounds, etc. have also been identified. Active trials are on to prevent the release of such harmful effluents into the atmosphere. The level of consciousness about the need for energy saving and pollution control has increased considerably in India as

4

Steel Making

well. Gone are the days when bright reddish-yellow fumes evolving from Bessemer converters could be seen from a distance. Modem LD converters are equipped with devices for waste gas management. However, Indian preparedness in this respect need to be strengthened further. 3- 5 Total global production of steel has crossed one billion ton mark (in year 2005) and it is set to rise further. In the western countries, the proportion of steel made by remelting recycled steel scrap in electric arc furnaces account for almost half of their total steel production. It is expected to level off around 60 per cent. Eighty to one hundred tons capacity ultra-high power electric arc furnaces are quite common in those countries. In a developing country like India, electricity cost is high and steel scrap is in short supply. Hence, Indian growth-rate of the electric arc furnace-based steel industry will continue to be comparatively sluggish in the near future. The main thrust in expansion programmes in India will be based on the BF (Blast Fumace)-LD converter route of steel making. However, the present trends in industrial growth-rate suggest that Indian steel industry is certainly poised for a bright future.

REVIEW QUESTIONS 1. Fill up the blank space:

(i) A variety of high carbon steel known as .............. was made in India in ancient times. (ii) The era of modern steel making started with the development of the .............. process. (iii) Large scale steel production became possible only after the invention of the .............. . (iv) Acid Bessemer - Basic open heart duplex steel making was suitable for .............. phosphorus .............. silicon hot metal. (v) In most Indian integrated steel plants .............. process of steel making is the major steel making route. 2. What are the strategies adopted for reduction of CO2 burden in steel making?

3. EAF steel making accounts for the production of a larger proportion of steel in western industrialized countries than that in India. Why?

Historical Perspective and Current Status of Steel Making

5

4. Write true or false: (i) Induction furnace steel melting for ingot casting is not commercially viable in India. (ii) Energy consumption per ton of steel produced is higher in India than that in Korea. (iii) The demand for flat products decreases with increasing degree of industrialization. (iv) Granulated waste plastics may be injected into the blast furnace. (v) Japan is the largest steel producing country in the world.

REFERENCES 1. B.K. Basak, Indian Foundry Jr., 51(7), 2005, 7. 2. T.K. Roy, 14th A.K. Seal Memorial Lecture, IIM, Kolkata, 2005. 3. P. Tan and D. Nenschutz, Met. and Mat. Trans. B, 30B, 2004, 983-994. 4. T. Futatsuka, K. Shitogiden, T. Miky, T. Nasaka and M. Hino, ISIJ Int., 44(4), 2004, 753-761. 5. K. Nagahiro, T. Okazaki and M. Nishino, Iron Making and Steel Making, 32(3), 2005, 227-234. 6. T. Emi and D.J. Min, Iron Making and Steel Making, 32(3), 2005, 242-244.

2 Physicochem ical Pri nci pies

2.1 INTRODUCTION Steel is an alloy of iron and carbon. Although theoretically the carbon content in steel may range upto 1. 7%, in practice most of the steels produced are of hypoeutectoid composition. Eutectoid (0.8% C) and hypereutectoid (>0.8% C upto 1.1 % C) steels are produced only for special purposes. Commercial steels, however, routinely contain other elements like manganese and silicon, and traces of impurities like sulphur and phosphorus. In nature iron exists only in the form of oxides. Oxides of iron are reduced in the blast furnace to produce pig iron. Molten pig iron as tapped from the blast furnace is usually known as hot metal in steel plant parlance. Molten iron, however, dissolves carbon, silicon, manganese, sulphur and phosphorus in large quantities. A typical Indian hot metal contains 3.5-4.0% C, 0.4-0.5% Mn, 1.0-1.4% Si, 0.22-0.30% P, and 0.06-0.07% S. It is, therefore, necessary to remove the excess carbon, silicon, manganese and phosphorus from the hot metal to produce steel of the desired composition. This task is accomplished during steel making, which is essentially a process of oxidation. The rationale of such operation may be appreciated from a scrutiny of the Ellingham Diagram (Figure 2.1) which indicates that the free energies of formation of SiO 2 , MnO, CO and P 2O5 are lower than those for the formation of various oxides of iron. It is, therefore, apparent that these impurities are oxidized away in preference to iron in an oxidizing atmosphere. The oxides of silicon, 6

7

Physicochemical Principles

.J'v

---


I

1 Atmosphere X 5 Atmosph ere A 10 Atm osphere

0

1 0.15

I

I

•

~

- A

"

0.4

-

I\

K/1

0.8

' 1.2 Carbon (wt.%)

-

-1.6

u

2.0

2.4

The relation between carbon and oxygen contents in molten iron in equilibrium with carbon monoxide at various pressures.5

12

2.3.2

Steel Making

Manganoso Roaction

Since the oxidation of manganese is a slag metal reaction, the equation for the reaction and the equilibrium constant can be written as: [Mn] + [FeO] = (MnO) + [Fe] (in slag) KMn

= (MnO)/(FeO) [% Mn]

where the slag terms are written as mol fractions and the manganese as wt pct. The influence of temperature on the manganese equilibrium was determined by Chipman to be: log

KMn

= 6440/r - 2.95

This agrees with the operating experience that a higher manganese residual in the metal is obtained with higher temperature. The factors which favour a low residual manganese content in the bath are: (a) A low manganese content in the charge (b) High slag volume (c) High FeO in the slag (d) Low temperature (e) Semibasic slag with lime-silica ratio under 2.2 (f) Partial or complete flushing of the slag. During vigorous carbon boil depletion of the slag (FeO) content may result in a reversion of manganese from slag to metal in basic oxygen converters. When decarbonization kinetics slows down slag (FeO) content may again build up resulting in reoxidation of manganese in the bath. Slag in the electric arc furnace or converter is usually flushed off completely before addition of any deoxidizer to avoid any reversion of Mn and P. (MnO) and (FeO) are miscible in each other. When ferromanganese is added to the steel bath the product is usually a (FeO • MnO) slag which may be solid or liquid at l,600°C, depending on the relative proportions of FeO and MnO (Figure 2.5). If the (FeO) content is large, it is expected that the slag phase will be liquid and float up. But more often the slag phase is solid and remains in the steel as nonmetallic inclusions. This aspect will be further elaborated in the next section. 6

Physicochemical Principles

13

0.20 Liquid oxide

#_

I

0.15

C:

Cl>

0)

>,

0

0.10

1,550 0.05 Solid oxide 0 .___

0

FIGURE 2.5

2.3.3

-

-.. ----- ---

---

_..,_ ___.___ __.__ __,___~

0.2

0 .4 0 .6 Manganese (wt.%)

0.8

1.0

Limiting oxygen contents of iron-manganese alloys in equilibrium with the mixed oxides of iron and manganese.6

Phosphorus Roaction

The slag-metal reactions involving oxidation of phosphorus may be written as: 2[P] + 5[0] ~ (P2O5) (P2O 5) + 4(CaO) ~ 4(CaO · P 2O 5) The factors influencing phosphorus transfer from metal to slag are: 1. 2. 3. 4. 5.

High (FeO) content in the slag High slag basically-preferably a CaO/SiO 2, ratio over 2.2 Use of relatively low temperatures A high slag volume of good fluidity Removal of the oxide slag prior to deoxidation, thereby preventing reversion. However, for production of low phosphorus steel (0.01 % P or less), it is necessary to charge low phosphorus raw materials. Secondly, the silica content (SiO 2) in the slag should be low in order to maintain a high basicity (CaO%/SiO 2%) of the slag. The silica content in hot metal has been progressively brought down to less than 1.0% by innovative blast furnace operation and also by external desiliconization practice. Now-a-days it is possible to routinely achieve slag basicity of the order of 3.0-3.5 in both L.D. converter and electric arc furnace.

14

Steel Making

Phosphorus partition ratio, i.e., the ratio of phosphorus content in the slag to that in the metal, as influenced by (FeO) content in the slag and slag basicity are reported in Figures 2.6(a) and 2.6(b).

400

CL

300

-/!-.

I

"'0"' Q.

200

I

'if!..

o"'

I

i;j

0

.~

100

~"ca.OI

I

0

)
-

100 0

0 ~-~-~--~-~-~--·~-~

4

FIGURE 2.6(b)

8

12

16 20 (wt.% FeO)

24

28

32

Dependence of the distribution ratio (wt% PO)/(wt.% P), on the iron oxide content of slags, at 1,600°C.7

15

Physicochemical Principles

The joint influence of basicity and slag (FeO) content is shown in Figure 2. 7. It may be noted from these plots that a minimum slag basicity of 2.2 is desirable for effective dephosphorization. Laboratory experiments on dephosphorisation have further demonstrated that the optimum (FeO) concentration in the slag for dephosphorization is around 14-16%. The data in Figure 2.6 were collected by equilibrating melts with slag containing 1-5 wt% MnO and 2.5 wt% Al 20 3 at l,685°C. Any increase in the concentration of these oxides will reduce phosphorus distribution. 7 • 8 40 (4CaO · P2Os)/(%P)2 at 1,600°C (2 ,910°F)

cCl> 0

Q) a.

Cl> 20

"O

·x 0

C:

_g

10 0 1.5

2.0

2.5 3.0 %CaO/%SiO 2

3.5

4.0

FIGURE 2.7 Distribution of phosphorus between slag and metal at 1,600°C (2,910°F) showing the joint influence of the iron oxide content and the basicity of the slag .8

Basic steel making processes can eliminate major part of the phosphorus from the charge. The sensitivity of the phosphorus reaction to temperature is a matter of great concern in oxygen steel making. The free energy change involved in the oxidation of common elements during steel making is illustrated in Figure 2.1. The reactants and their products are assumed to be in their standard states. The free energy change in formation of {CO} through the reactions [2C + 0 2 ~ 2CO] at the steel making temperature of l,600°C is less that that of P 2 0 5 . This obviously means that phosphorus removal cannot occur until carbon in the bath is oxidized away. However, in basic oxygen steel making, a slag of high basicity may form quite early in the process. The activity of (P 20 5) is greatly reduced in a basic slag. This is illustrated by the clockwise rotation of the phosphorus oxidation line in the free energy-temperature diagram (Figure 2.1). The free energy changes involved in the formation of {CO} and (P 20 5) becomes nearly equal at l,600°C under the influence of a basic slag. Hence simultaneous removal of carbon and phosphorus becomes possible. This principle has been utilized in the basic oxygen steel making process.

16

Steel Making

2.3.4

Sulphur Romoval

The desulphurization of steel is favoured by: (a) High slag basicity (b) Low (FeO) content in the slag (c) Low-dissolved oxygen content in the bath (d) High temperature (e) Good fluidity (f) Intimate mixing of slag and metal. In basic oxygen and electric arc furnace steel making prcesses, conditions (a), (d) and (e) are easily maintained. But the dissolved oxygen content in the slag and metal is usually high in basic oxygen process. In electric arc furnace steel making, a second reducing slag with very low (FeO) content can be prepared. This enables efficient desulphurisation in the production of carbon and alloy steels. The effect of slag (FeO) content and basicity on the sulphur partition coefficient(% S) was investigated by Chipman et al. 9 Their observations are documented in Figure 2.8. The curves show that the sulphur partitioning in electric furnace reducing slag steeply increases with a drop in the oxygen content in the slag. Sulphur partitioning is almost independent of the slag (FeO) content when the (FeO) concentration is around 5-10 mol%. This is typical of the open hearth furnace slags. Only about one-third of the sulphur can be eliminated from the metal in basic steel making. The distribution ratio of sulphur between the basic slag and the metal will usually be about 4 to 6 and will seldom exceed a ratio of 8 to 10. Thus, in _!{ 50 U)

t

X

f

Cl>

"O

.!:

10

C:

.Q

-;

5

3.6

N

"§ .s::

a.

"S

8l

"O "O

-2! 0

0.5

~

0

(.)

0.01

0.05 0.1

0.5 1 Mol, percent (FeO)

FIGURE 2.8 Effect of oxygen content and basicity of slags on the sulphur distribution .9

Physicochemical Principles

17

a basic open hearth furnace in which the slag weight is about onetenth that of the metal, it is not unusual to find that more amount of sulphur is retained in the metal than that absorbed by the slag. The most positive control of sulphur is accomplished only by avoiding the use of high-sulphur charge materials. The generally accepted mode of transport of sulphur from the metal to the slag is shown by the equations. (CaO) + S = (CaS) + [O] K = (CaS)[O]/(CaO)[S] in slag in slag (MnO) + S = (MnS) + [O] K = (MnS)[O]/(MnO)[S] in slag in slag (MgO) + S = (MgS) + [O] K = (MgS)[O]/(MgO)[S] in slag in slag These reactions show that desulphurization is favoured by a high concentration of basic oxides in the slag and a low oxygen concentration in the metal. Distribution ratio of the order of 12 to 14 may be achieved in LD converters. In open hearth furnace, the (S)/[S] ratio seldom exceeds 8 to 10. The extreme turbulence and good mixing of slag and metal and the case with which highly basic slags can be produced in basic oxygen furnaces permits better sulphur transfer from metal to slag. Some sulphur is also eliminated from the L.D. converter bath by oxidation as SO2 and SO3 . 9

2.3.5

Silicon Roactions

The reaction between silicon and oxygen in dilute solution in liquid iron may be written as: [Si] + 2[0] = (SiO2 ) The equilibrium constant

K = asiOi[Asil x [Ao] where Asi and A0 represent henrian activities asiO 2 in silica saturated slag = 1 Assuming dilute solution behaviour, the equation simplifies to: [wt% Si] [wt% 0] 2 = 1/K or log [wt% Si] + 2[log wt% O] = log [1/K] The experimental plots of log [wt% Si] vs. log [wt% O] as shown in Figure 2. 9 also demonstrate a linear relationship. This confirms that substitution of henrian activities of silicon and oxygen by their respective wt.% does not involve much error. 10

18

Steel Making

0.2 0.1 0.05

~

0.02

C

0.01

I

Q)

Ol

>-

0.005

X

0

0.002 0.001 0.0005 0.01 0.02 FIGURE 2.9

0.05 0.1

0.2 0.5 1 Silicon (wt.%)

2

5

10

Influence of silicon on the oxygen content of iron in equilibrium with solid silica .10

Silicon is a much more powerful deoxidizer than manganese. However, when Mn and Si are used together in appropriate ratios for deoxidation purpose, the reaction product may be liquid 2Mn0 · Si0 2 . The liquid droplets coalesce easily, forming bigger droplets which may rise to the top of the bath and separate out as a slag. The rate of flotation is governed by the Stokes law which states that: V ~ = 2/9g (~ - ~')/µ where v = terminal velocity, ~ = density of droplet, and ~, = density of the liquid steel.

2.3.6

Nitrogon and Hydrogon

Both nitrogen and hydrogen are diatomic gases which are soluble in liquid and solid iron. The reaction between the nitrogen in the atmosphere and the bath is: l/2Nig) = N(wt%)(wt% N)/pN¥2 Some of the nitrogen molecules are also dissociated in the high temperature of arc in an arc furnace as Nig) = N(%) According to Sievert's law, the solubility of a diatomic gas in liquid metal is proportional to the square root of its partial pressure.

Physicochemical Principles

19

Thus, (wt% N) = kN · pN 2 112 Similarly, [wt% H] = kH-pH 2 The principle is utilized in vacuum degassing of liquid steel. According to Sievert's law, the solubility of nitrogen in liquid steel is proportional to the square root of the partial pressure of nitrogen which, at l,600°C and under a pressure of 1 atm of nitrogen, corresponds to a nitrogen content of the steel equal to 0.040 pct. The solubility of nitrogen in steel also increases with temperature. Therefore, it is absolutely necessary to use almost pure oxygen in basic electric furnaces in order to avoid the pick-up of nitrogen by the steel bath in the unusually high temperature reaction zone immediately under the oxygen lance. Both oxygen and sulphur are surface-active elements. The adsorbed surface layer of these elements inhibits transfer of oxygen from metal to gas and tends to reduce the efficiency of carbon boil for flushing nitrogen. 10 Alloying elements which form stable nitrides increase the solubility of nitrogen in liquid iron. Manganese, vanadium, niobium and chromium fall in this group. However, elements like C, Si, Ni and Cu reduce the solubility of nitrogen (Figure 2.10) shows the solubility of nitrogen in liquid iron alloys at l,600°C and 1 atm. pressure of N~ 1 . 0.20

...,~

Al

0.0:4

S

~

0.18 0.16

0 0

0.25 0.5 j,w/o

0.14

t!-.

0.12

c

0.10

~ Q)

Ol

g

z

0.08 0.06 Se Sn W Cu

(0.045) 0.04 0.02 0 0

FIGURE 2.10

2

4 6 8 10 12 Alloying element, j, w/o

14

Solubility of nitrogen in liquid iron alloys at 1,600°C, 1 atmosphere pressure of N2 (g) .11

20

Steel Making

Most of the common alloying elements however reduce the solubility of hydrogen in liquid iron. Sievert's law is not obeyed in the alloys of iron with Nb, Ti and Ta. The solubility of hydrogen in liquid iron increases in the presence of these alloying elements and this suggests a large interaction of these elements and hydrogen (Figure 2.11). 32 Ti 30

I

28

"LS]

E 25 c. 24 C.

I

Ta

I

34

p

S , , ___

32

23

0

0.5

1.0

>,

.Q "iij

26

0 28

Ea.

24

C

22

Ni

_e, Q)

Co

"I

>,

26

20

22

Ge 18

- 20

~

I~

"'E 0

cQ) O'l

e

"I

>,

Al

16

0:::-

24

O'l

e

E

O'l

0 0

18 16

14

14 2

FIGURE 2.11

4 6 8 10 12 Alloying element, j, w/o

14

Solubility of hydrogen in liquid iron alloys at 1,600°C , 1 atmosphere pressure of H2 (g) .11

REVIEW QUESTIONS 1. Represent schematically the structure of a silicate slag. How

does a basic oxide (e.g., CaO) reduce the viscosity of silicate slag? 2. What is the effect of CaF2 on the viscosity of basic slag? 3. What are the favourable physicochemical conditions for (a) dephosphorization (b) desulphurization Support your answer with data of laboratory experiments.

Physicochemical Principles

21

4. Prove that logarithms of silicon and oxygen contents in molten steel maintain a straight-line relationship at all temperature. 5. What is the effect of Pco on the carbon-oxygen equilibrium in liquid steel? 6. Which alloying elements in steel increase nitrogen solubility? How is nitrogen solubility influenced by carbon and silicon? 7. State Sievert's law. Why does vacuum treatment reduce the solubility of hydrogen in steel? Explain in terms of Sievert's law. How does argon bubbling reduce the solubility of hydrogen in steel?

REFERENCES 1. R.G. Ward, An Introduction to the Physical Chemistry of Iron and Steel Making, ELBS Edn., Edward Arnold, 1965. 2. F.D. Richardson, "Oxide Slags-A survey of our present knowledge" in Physical Chemistry of Steel Making, Ed. J.F. Elliot, MIT Press, 1958, 55-62. 3. R. Book, ''The Action of Calcium Fluoride in Slags", ibid, 84-86. 4. D.L. McBridge, J.O.M., July 1960, 531-537. 5. S. Marshall and J. Chipman, Trans. ASM, 30, 1942, 695-741. 6. R. Rocca, N.J. Grant and J. Chipman, Trans. AIME, 191, 1951, 319-326. 7. K. Balajiva, A.G. Quarrel and P. Vajragupta, J.I.S.I., 153, 1946, 111-145. 8. J.E. Gantz, "Electric Arc Furnace Practice", in Electric Furnace Steel Making, Ed. C.R. Taylor, Iron and Steel Society, 1985, 131. 9. J. Chipman, J.B. Gero and T.B. Winkler, Trans. A.I.M.E, 188, 1950, 341-345. 10. J. Chipman, Physical Chemistry of Liquid Steel, Basic Open Hearth Steel Making, A.I.M.E., New York, Chapter 16. 11. J.F. Elliot, ''The Physical Chemistry of Liquid Steel" in Electric Furnace Steel Making, Ed. C.R. Taylor, Iron and Steel Society, 1985, 291-319.

3 Review of the Older Steel Making Processes

A

BESSEMER CONVERTER PROCESS

3.1 INTRODUCTION The Bessemer converter process was first developed in 1856. Invented by Sir Henry Bessemer, the original Bessemer process was suitable only for refining low phosphorus iron, as it used an acidic refractory lining. The basic Bessemer process, suitable for refining high phosphorus hot metal was developed 20 years later by Thomas Gilchrist. The basic Bessemer process is also known as Thomas process. In this process, the vessel is lined with basic refractories. Hence, basic slag can be formed to remove phosphorus and sulphur. The Bessemer converter process has now been completely replaced by basic oxygen converters all over the world. However, the development of all subsequent basic oxygen converter processes followed from the Bessemer process. In fact, Henry Bessemer himself had mentioned in his patent that it would be better to blow pure oxygen, which was not available commercially in his time. The commercial availability of oxygen gas at affordable cost revolutionized the steel making industry. But Bessemer will continue to be remembered for his invention because it laid the foundation for all subsequent processes of converter steel making. The principle of the Bessemer converter process needs to be understood from this perspective. 22

Review of the Older Steel Making Processes

23

3.2 DESIGN OF THE CONVERTER AND OPERATIONAL PRACTICE The Bessemer converter is a pear-shaped vessel with a detachable bottom. It is mounted on a trunnion and can be rotated through 360°. After charging hot metal into the converter, blowing is started through the tuyeres located at the bottom of the converter as shown in Figure 3.1. The converter is gradually tilted into the upright blowing position. At the end of the blow, which usually lasts for 20-25 minutes for a 20-25 ton vessel, the converter is again tilted upside down, and metal and slag are tapped out.

Steel shell

Trunnion

FIGURE 3.1

Section of a Bessemer converter, the refractory lining may be acidic or basic.

The acid Bessemer converter is lined with acidic materials. Silica brick lining or a rammed lining of acidic refractory materials (e.g., ganister, mixtures of crushed quartz, old bricks and fireclay, etc.) is used. The converter shell can be detached from the trunnion for rebricking or relining. The bottom section, which is detachable, houses a large number of tuyeres (usually upto 30 nos.). These tuyeres are tapered hollow cylindrical fireclay bricks and are supported by a bottom plate. Acidic refractory material is rammed in the annular space between the tuyeres. Usually the bottom section has to be replaced after 10-25 heats. Hence, spare bottoms are always kept ready. The bottom section has to be dried and fired in special ovens. In the case of basic Bessemer converter, the converter shell has a monolithic tarred dolomite refractory lining. The bottom section is also built up from tarred dolomite rammed into position. The converter lining can be used upto about 200 heats, but the bottom needs replacement after every 25-30 heats.

24

Steel Making

The Bessemer converter process is completely autogeneous, i.e., no external heat supply is needed. The energy requirement for steel making is provided completely by the exothermic chemical reactions in the bath. In the case of acid Bessemer process, oxidation of silicon is the principal source of heat supply. In the basic Bessemer process, considerable heat is evolved during phosphorus oxidation, i.e., in the after-blow period. In addition, exothermic oxidation of carbon and manganese also contributes to the thermal requirement of the process. Silicon oxidation occurs first. Carbon oxidation peaks only after silicon and manganese are oxidized. In the basic process, phosphorus oxidation occurs after the carbon blow. The compositions of the hot metal for the acidic and basic processes are given in the following table: Wt%

Element Carbon Silicon Sulphur Phosphorus Manganese

Acidic process

Basic process

3.5-4.0 2.0-2.5 0.04 max. 0.04 max. 0.75-1.0

3.0-3.6 0.6-1.0 0.08-0.10 1.8-2.5 1.0-2.5

Carbon oxidation is indicated by the appearance of long flame over the mouth of the converter. The flame lengthens as the rate of carbon-oxygen reaction increases. It drops after the completion of the carbon-oxygen reaction. Phosphorus oxidation continues for another three to four minutes. This period is known as the afterblow period. In both the acidic and basic processes, the blown metal is deoxidized with ferromanganese and ferrosilicon. In the basic process, however, deoxidation must be carried out after slag-free tapping of the metal. This is necessary to avoid the reversion of phosphorus from slag to metal.

3.3 CHEMISTRY OF REFINING As soon as the blast is turned on, the oxygen in the blast starts reacting with the metal. Even though the free energy change involved in the formation of oxides of iron is much higher than that for Si02 , MnO and CO, iron is oxidized first due to its much larger mass. The reaction may be written as: Fe + 0 2 = 2[Fe0]

Review of the Older Steel Making Processes

25

Higher oxides of iron (e.g., Fe2 O3 ) may also form, but for the sake of simplicity, the state of oxidation is taken as FeO. The FeO, however, reacts immediately with silicon in the metal. 2[FeO] + [Si] µ (SiO 2 ) In both acidic and basic processes, silicon oxidation occurs first. After silicon oxidation, manganese reaction starts. [Mn] + (FeO) µ (MnO) + Fe The (SiO2 ) reacts with excess [FeO] and [MnO] forming a silicate slag (SiO 2 ) + 2(FeO) µ (2FeO · SiO2 ) (SiO 2 ) + (2MnO) µ (2MnO · SiO2 ) The carbon reaction starts only after the silicon and manganese have been oxidized away. CO + (FeO) µ {CO} + Fe The carbon-oxygen reaction in the Bessemer converter is a heterogeneous process. The nitrogen in the air escapes through the bath in the form of bubbles. The surfaces of these nitrogen bubbles are sites for carbon-oxygen reaction. The carbon monoxide diffuses into the nitrogen bubble, escapes from the converter and bums at the mouth of the converter, producing a long flame. As the reaction becomes more vigorous, the flame also becomes longer. When the carbon reaction approaches end point, the flame gradually drops. At this stage, the converter is titled and ferromanganese and ferrosilicon are added for deoxidation and adjustment of the composition. The carbon content in the bath may also be raised by addition of carbon in suitable form. Some variations occur in the basic process. Lime is added into the hot vessel before start of the blow. The phosphorus oxidation takes place after the carbon flame has dropped. Blowing has to be continued for another 3-4 minutes. This period is called the "afterblow" period. In the after-blow period, sufficient heat is available to dissolve lime (CaO) in the slag. The following reactions occur: (2FeO · SiO2 ) + (CaO) µ (2CaO · SiO2 ) + 2(FeO) (2MnO · SiO2 ) + 2(CaO) µ (2CaO · SiO2 ) + (MnO) The basic slag also reacts with the sulphur in the metal to some extent. [FeS] + (CaO) µ (CaS) + (FeO) [Mn] + [FeS] µ (MnS) + Fe

26

Steel Making

In the after-blow period, phosphorus is oxidized to P 2 O5 , which reacts with the CaO in the slag in the following manner: (P2 O5) + (2CaO) µ 4CaO · P 2 O5 The dephosphorization reaction, therefore, needs a high oxygen potential of the slag, which is normally available as the blowing is continued. The slag analysis varies considerably in case of the acidic and basic processes. This will be evident from the data presented below: Slag analysis wt%

Acidic process

Basic process

SiO2 FeO MnO Al2Os Fe 2O3 CaO MgO

63.00-68.00 12.00-18.00 12.00-18.00 2.00-4.00 < 1.0 Trace Trace

9.0-10.0 30.00 Trace 10.00 5.0-6.00 25.00-28.00 15.00-16.00

It is apparent from the above analysis that the basic slag has a much higher oxygen potential.

3.4 DECLINE OF THE BESSEMER PROCESS The Bessemer converter process served the steel industry for more than one hundred years. In India, the last Bessemer converter was shut down in late eighties. In spite of its basic simplicity, the Bessemer steel suffered from several limitations, which are as follows: (a) A large part of the heat generated through exothermic reactions is lost in the form of the sensible heat of the nitrogen gas. Hence the scrap melting ability of the acid and basic Bessemer processes is very limited. (b) The nitrogen content in the Bessemer steel is high-of the order of 0.012%. This is not compatible with the extremely low level of nitrogen (50-60 ppm) desired in most commercial steels. (c) The need to change the bottom section frequently was a severe irritant.

Review of the Older Steel Making Processes

27

(d) The Bessemer process could not refine in one single stage high silicon medium phosphorus iron. In Indian context, therefore, a duplex or triplex process became necessary to treat high silicon medium phosphorus (0.2-0.27% P) hot metal. The advent of the top blown basic oxygen steel making process (LD process) hastened the demise of the Bessemer process. The LD converter's capability to refine all kinds of pig iron, including Indian medium phosphorus iron, low nitrogen content in the steel, freedom from the bottom erosion problem, higher scrap melting capability and ability to produce both low and medium carbon steel at fast rates, proved too attractive. However, Bessemer's contribution will always be remembered because he was the first person to develop the technology for large scale steel production. Even today's oxygen converters basically follow from his initial desigu.

B. OPEN HEARTH FURNACE STEEL MAKING PROCESS 3.5 INTRODUCTION The open hearth furnace steel making process accounted for the major proportion of steel production in India and also in many other countries upto late eighties. The process was originally developed by Siemens in Germany and Martin in France in late nineteenth century. However, the open hearth process has also been largely replaced by the LD converter process of steel making all over the world. A few basic open hearth furnaces are still in operation in some of the older steel plants. The ability of open hearth furnaces to melt both light and heavy scrap to produce liquid steel of any carbon content and any desired chemical composition has saved this process till date from total obsolescence. However, open hearth steel making is a slow process. The thermal requirements of the process has to be met by combustion of gaseous or liquid fuel supplied from external sources. The thermal efficiency is usually low. Efforts have been made to accelerate the process of refming in open hearth furnace and also to improve its thermal efficiency. Although these efforts yielded positive results, still the open hearth process could not compete with the L.D. converter steel making process either in terms of thermal efficiency or productivity. The process has,

28

Steel Making

therefore, lost its preeminence throughout the world. Since the process is still being practised, albeit in a limited scale, there is a need to study the essential features of the process. The emphasis will be on basic open hearth steel making which is suitable for refining medium phosphorus hot metal.

3.6 CONSTRUCTION OF THE OPEN HEARTH FURNACE The capacity of an open hearth furnace may range upto 500 tons per heat. It is basically a reverberatory furnace in which hot metal and molten steel scrap are refined in a shallow basic lined hearth. The hearth can be seen after raising the charging door. A typical section of an open hearth furnace and its hearth are shown in Figure 3. 2. Combustion air and gaseous fuel are admitted through ports on one Hearth

Backwall

Waste gas downtakes

Air uptakes

- - - - -r, Monkey wall

l :·

·- - ,· I

Front wall

I

,

I

I •

J.

Burner

Induced draft fan

Stack

FIGURE 3.2

Schematic diagram of an open hearth furnace (Source: United States Steel Co) .

Review of the Older Steel Making Processes

29

side of the furnace. The fuel is burnt inside the furnace. The heat is transferred to the bath through the intermediate slag layer. The exhaust gases are allowed to escape through ports located in the opposite end. The hearth is in the form of a pan fabricated from steel plates. The metal plates are covered with asbestos sheets, which in tum are covered with a layer of porous fireclay bricks. Thereafter, a course of firebrick lining is provided. The first layer of basic lining of maguesia or stabilized doloma bricks is built up on the fireclay brick backing lining. The working lining is usually made by ramming maguesia refractory. The lining usually slopes towards the taphole. The back walls of a basic furnace are lined with magnesite bricks. The top layers of the back wall are necessarily made of chromemaguesite bricks to avoid acid-base reaction with the silica brick roof. The roof is usually of the sprung arch type and is made of silica bricks. When oxygen lancing into the bath is a routine practice, basic bricks are more suitable for roof construction. However, basic bricks are heavier than silica bricks. Therefore, these bricks are suspended by hangers. Gas and air are admitted into the furnace through ports. Fuel oil or tar are introduced into furnace chamber directly through burners. When the ports are used for admitting preheated air/gas, these are called uptakes. Downtakes carry away the products of combustion. Alternatively, the functions of uptakes and downtakes are reversed. The exhaust gases escaping through the ports first enter a slag pocket through a duct called fantail. The dust and slag particles carried with the gas drop off in the slag pocket. The ports are usually lined with silica bricks. The exhaust gas then passes through a regenerator. The facing lining of the chambers is made up of fireclay bricks. Checker works of refractory bricks are constructed inside these chambers. Usually, checkers are built of fireclay bricks to one half to two thirds of their height; silica or high alumina bricks are used for the upper courses. The cooled exhaust gas ultimately escapes through a chimney. The directions of flow of air and fuel gas, as well as that of the exhaust gas are alternately reversed by means of dampers. When the air and fuel gas are preheated, allowing it to flow through one set of preheated regenerators on one side of the furnace, the hot exhaust gas is conducted through another set of regenerators on the other side of the furnace. By alternately reversing the directions of flow of the gases, optimum preheating of air + fuel and maximum possible extraction of the sensible heat of the exhaust gas are arranged.

30

Steel Making

Slag pockets, regenerative chambers, flue systems and reversing valves constitute the lower part of an open hearth furnace. The capacity of slag pockets is desigued in such a manner that the entire amount of slag, dirt and dust accumulated during a furnace run can be retained. Slag pocket walls, floors and the vertical flues are lined with chrome magnesite bricks in basic furnaces.

3.7 OPERATION OF THE BASIC OPEN HEARTH FURNACE There is usually a wide choice of raw materials for steel making in the basic open hearth furnace. In Indian plants, usually steel scrap + hot metal mix constitutes the basic charge materials. The silicon content in the hot metal is maintained as low as possible, usually around 1. 0% Si or lower, in order to ensure optimum basicity of slag. Typical phosphorus content in the hot metal ranges between 0.22-0.30%. The formation of basic oxidizing slag in the process requires addition of iron ore and limestone. In addition, all modem furnaces have oxygen lancing facility. The slag in an open hearth furnace has dual functions. It is not only a receptacle of the impurities but it is also a refining medium. In integrated steel plants, the usual practice is to charge steel scrap, lime and iron ore first. The charge is heated up to a state to incipient fusion. Then hot metal is charged. Some operators prefer to add limestone instead of lime in order to take advantage of bath agitation due to evolution of CO2 through decomposition of limestone. The phenomenon is often referred to as lime boil. Steel melting in a basic open hearth furnace consists of the following stages: (a) Charging (b) Melting down (c) Oxidation and refining (d) Finishing

3.7 .1

Charging

Steel scrap, iron ore and lime/line stone are usually placed in charging buckets. The charging buckets are introduced into the furnace through the charging door by means of mechanical chargers. When the buckets are titled, the charge is dumped on the furnace floor. Hot metal is charged directly from ladle through spouts introduced through the charging door.

Review of the Older Steel Making Processes

3.7 .2

31

Molting Down

During the melting down period, the burners are turned on in full. As the charge melts, silicon and manganese are oxidized according to the following reaction: Si + 2FeO = SiO 2 + 2Fe Mn+ FeO = MnO + Fe The charge composition is so adjusted that the opening carbon content in the bath is 0.2-0.3% higher than that of the final targeted carbon content. When the proportion of the steel scrap in the charge is very high, it may be necessary to add extra carbon (in the form of petroleum coke/graphite block/anthracite coke), usually below the layer of the steel scrap charge. Evolution of carbon monoxide due to oxidation of carbon agitates the bath. During the melting down period, however, carbon oxidation does not usually occur because the free energy of formation of silicon dioxide is more negative than that of carbon monoxide. As the melting proceeds, lime floats up and reacts with the silica, forming a basic slag. MnO is also a basic oxide and it helps in slag formation.

3.7 .3

Oxidation and Rofining

Steel making is essentially an oxidation process. Therefore, a steady supply of oxygen is necessary during this period. This is ensured by adding iron ore. However, Fe 2 O3 dissociation is an endothermic process. In open hearth steel making, heat transfer from the furnace atmosphere to the bath occurs through the slag layer. It is usually a slow process. Only about 20% of the heat available in the furnace atmosphere is actually transferred to the bath. The rate of endothermic dissociation of iron ore (Fe 2 O3 ) is limited by heat supply to the slag metal interface. In order to accelerate the supply of oxygen to the metal bath, direct oxygen lancing into the bath below the slag level is considered a better practice. Evolution of carbon monoxide through oxidation of carbon agitates the bath vigorously. The phenomenon is usually known as carbon boil. This type of agitation assists in (a) heat transfer, (b) slag metal interaction, and (c) flotation of inclusions. As the temperature rises, more lime dissolves in the slag, raising its basicity. The slag viscosity is reduced by judicious addition of fluorspar (CaF2 ). The principal slag metal reactions are dephosphorization and desulphurization. Dephosphorization is favoured by (a) oxidizing slag, (b) high basicity

32

Steel Making

of slag, and (c) low temperature. Desulphurization is favoured by (a) a reducing slag, (b) high basicity of slag, and (c) high temperature. In reality, (FeO) content of the basic open hearth furnace slag is quite high and the slag is oxidizing in character. Hence dephosphorization is favoured in open hearth furnace. The extent of desulphurization is small. The phosphorus reaction proceeds as follows and reaches equilibrium: 2[P] + 5(Fe0) = (P20 5) + 5Fe 4(Ca0) + (P20 5) = (4Ca0 · P 205)

3.7 .4

Finishing

In the finishing period, excess oxygen in the bath is removed by the addition of deoxidizers. However, the bath is only partially deoxidized within the furnace by the addition of ferrosilicon and ferromanganese in order to avoid phosphorus reversion. Usually, the partition coefficient (FeO)/[FeO] = K is a constant at a particular temperature. A reduction of (FeO) content in the bath automatically leads to a reduction in the (FeO) content in the slag. However, the ability of the basic slag to hold (P20 5) in solution depends on its oxygen potential. Hence, too much reduction of (FeO) content of the slag leads to the reversion reaction: (P205) = 2[P] + 5(0) In order to avoid such a possibility, the final deoxidization is carried out in the ladle by the addition of aluminium as the molten steel is tapped into the ladle. Ferroalloys may also be added in the ladle for composition adjustment. 1- 5

3.8 DEVELOPMENTS IN OPEN HEARTH FURNACE PRACTICE Oxygen is used in the open hearth furnace to reduce fuel consumption and to improve productivity. Fuel saving is effected by (a) assisting melting of scrap, and (b) direct injection of oxygen into the bath. For assisted melting, oxygen is added either at a burner through a separate orifice below the fuel nozzles or to the combustion air before it enters the checker system. Oxygen may be injected into the metal bath either through consumable lances inserted at the slag metal interface or through water-cooled lances introduced through

Review of the Older Steel Making Processes

33

the roof and positioned above the slag surface. The oxygen stream is directed at high velocity through multi-hole nozzles so that it impinges upon the metal bath through the slag layer. In the submerged injection process, water cooled lances are introduced through the hearth (Figure 3.3).

FIGURE 3.3 Submerged injection process (SIP) : A process for refining pig iron and ferrous scrap into steel by blowing oxygen and a shielding hydrocarbon through tuyeres located at the bottom of an open-hearth containing the molten metal and scrap. 6 (Reproduced from J.O.M. with permission)

The use of oxygen reduces total heat time and fuel consumption. However, basic roof practice is necessary for oxygen injection into the bath. A further modification of the conventional open hearth process is the development of the twin-hearth process. In this process, two basic lined hearths are connected by an opening for the transfer of the products of consumption from one hearth to the other as shown in Figure 3.4. Regenerators are eliminated. Intensive oxygen lancing is carried out. The fundamental principle involves utilization of the 2

3

A

8

8

FIGURE 3.4 Schematic diagram of twin-bath furnace . 1-working chamber, 2 and 3-oxyfuel burners, 4-special lances for blowing powdered materials, 5-slag, 6-metal, 7-scrap, and 8-slag pocket. 7

34

Steel Making

physical and chemical heat from the gases formed during blowing for directly heating the solid charge materials. Only a small quantity of external fuel is required for thinning the slag prior to tapping and maintaining the heat of the furnace during tapping and fettling. The heat time is reduced to half of that of the conventional open hearth fumace. 6- 7

3.9 REASONS FOR DECLINE OF THE OPEN HEARTH PROCESS 1. Open hearth steel making is a very slow process. It cannot

match the productivity of modem basic oxygen converters where the tap to tap time is of the order of 40 to 60 minutes. 2. The dependence on external fuel supply is a serious constraint of the open hearth process. 3. Construction and maintenance of the roof and substructure of the open hearth furnace is more difficult than the overall maintenance of a basic oxygen converter.

REVIEW QUESTIONS 1. Explain the following:

(a) The activity of (FeO) in basic Bessemer converter slag is higher than that in the acid converter slag, although the total mol% FeO in the acid slag is higher, why? (b) In basic Bessemer converter, dephosphorization occurs after the carbon-oxygen reaction is practically over. What is the thermodynamic explanation? (c) Although the nitrogen bubbles do not directly participate in any refining reaction in a Bessemer converter, they do assist the carbon-oxygen reaction. How? (d) Direct oxygen blowing through the tuyeres is not possible in case of a Bessemer converter. Why? (e) Both acid and basic Bessemer converter steel making are autogenous processes. Explain. 2. (a) Although phosphorus and manganese reactions reach near equilibrium in open hearth furnace steel making, the carbon-oxygen reaction never reaches equilibrium. Why? (b) Although adequate heat is generated in the open hearth furnace atmosphere by burning fuel, the actual heat input into the bath is much less. How do you explain it?

Review of the Older Steel Making Processes

35

(c) In twin-hearth operation regenerators are not required. How is the heat balance maintained? (d) When oxidation of the open hearth bath is carried out by ore feeding, the kinetics of the carbon-oxygen reaction is controlled by the availability of the heat of iron oxide dissociation at the slag metal interface. Explain the kinetic considerations involved. (e) How can oxygen lancing contribute to enhanced productivity of the basic open hearth furnace? 3. (a) Discuss how Bessemer's invention of converter steel making influenced the developments in steel making technology? (b) Why did the original acid and basic converter steel making processes suffered decline? (c) What were the limitations of the open hearth steel making process?

REFERENCES 1. John L. Bray, Ferrous Processes Metallurgy, John Wiley & Sons, 1954. 2. G.R. Bashforth, The Manufacture of Iron and Steel, Vol. II, Indian edition, 1967. 3. The Making, Shaping and Treating of Steel, Ed. H.E. McGannan, U.S. Steel, 8th ed., 1964. 4. G. Oiks, Converter and Open Hearth Steel Manufacture, Mir Publishers (Moscow), 1977. 5. AK. Gupta, TISCO Jr. 16(3), 1969, 101-108. 6. J.O.M., March 1973, 39. 7. R.S. Shrotiya, TISCO Jr., 25(7), 1978, 101.

4 Top-Blown Basic Oxygen Converter Processes

4.1 INTRODUCTION The growth of the top-blown basic oxygen converter steel making process, originally known as the LD process, has revolutionized steel making technology in the world. The acid and basic Bessemer steel making processes had served the industry for a long period. But the high nitrogen content of the Bessemer steel proved a handicap for many applications. The basic open hearth process-the other major steel making process-could not match the high productivity of the converter process. Moreover, the open hearth process is not autogeneous. It needs continuous heat supply from external sources. Attempts were made to reduce the nitrogen content in Bessemer steel by oxygen enrichment of the air-blast blown through the bottom of the converter. But this could not progress beyond a point, because the oxygen enriched blast proved disastrous for the life of the tuyeres, which eroded rapidly. Attempts were, therefore, made to lance oxygen onto the bath through the throat of the converter. The easy availability of oxygen gas in the post Second World War period facilitated research on oxygen lancing. As a result of this endeavour, the top-blown basic oxygen converter process, popularly known as the LD process, was developed. LD stands for Linz and Donawitz, towns in Austria, where the developmental work was carried out. In course of time the process also came to be known as BOF (basic oxygen furnace) process or simply BOP (Basic oxygen process). The development of the LD process revolutionized steel making 36

Top-Blown Basic Oxygen Converter Processes

37

technology. Because of its flexibility, it can refine hot metal of varying compositions to produce low carbon, high carbon and low alloy steels. The LD process was introduced in India in the Rourkela Steel Plant in late fifties. Later, almost all the major integrated steel plants in India adopted this steel making technology.

4.2 LO CONVERTER STEEL MAKING PRACTICE The process steps involved in LD converter steel making are illustrated in Figure 4. l(a-f). Oxygen lance (water-cooled)

(a) Charging scrap

(b) Charging hot metal

Steel ladle (d) Sampling

FIGURE 4.1(a-f)

(c) Main blow

0

(e) Tapping

(I) Slag got

Schematic representation of the process steps in LD converter steel making (Source: Internet).

A basic oxygen converter is a pear-like vessel with a concentrically positioned oxygen lance. The steel shell is suitably lined with basic refractories. Hot metal, scrap, fluxes and ferroalloys are charged into the converter through the throat. Oxygen (99.9% pure) is blown through a water-cooled lance fitted with a copper nozzle of laval desigu. The position of the lance with respect to the bath and the flow-rate of oxygen are automatically controlled. The capacity of a modem LD converters may range from 100 tons to 400 tons. While desiguing a new converter, the ratio of its diameter to the depth of the bath is taken from 3. 0 to 3. 6 and the unit surface area of the bath from 0.13 to 0.16 m 2 per ton of steel.

38

Steel Making

The charge for an oxygen converter melt is composed of hot metal, steel scrap, lime, fluorspar, etc. The proportion of blast furnace hot metal may range from 70-100%. The silicon content in the hot metal should be low, otherwise more lime will be necessary to neutralize the silicon and slag volume will become large. In some plants, blast furnace hot metal is first desiliconized by oxygen lancing and then the low silicon hot metal is charged into the converter. Major part of the lime is added before starting the blow. Fluorspar is used to accelerate the dissolution of lime and ensure the required fluidity of the slag. Steel scrap is used to chill the bath. The pieces of steel scrap to be charged into the vessel should be suitably sized to ensure quick melting and also to avoid deflecting the oxygen jet. The proportion of steel scrap in the charge may be upto 30%. Optionally, iron ore and mill scale may also be used in limited quantities to chill overheated bath. However, the gangue content of the iron ore should be low. High carbon steels like rail steels (0.65%-0.74% C, 0.6%-1.0% Mn, 0.27-0.30% Si), ball-bearing steel (1.0% C, 1.2% Cr), etc. are also manufactured in the LD converter by the catch carbon technique. In this technique, dephosphorization is accelerated and completed before decarburization. Extralime and fluorspar are charged and the lance is raised to a higher position for maintaining a soft blow condition till phosphorus removal is completed. Thereafter, decarbonization is continued by a harder blow till the bath carbon content drops to the desired level. Alternatively, blowing may be continued to complete both dephosphorization and decarbonization. Required amount of carburizer is then added to the low carbon steel bath to raise the carbon content to the desired level. However, this method involves a risk of increasing the inclusion and nitrogen contents in the steel. These are picked up from the carburizer (e.g., petroleum coke or graphite). For production of low alloy steel, the alloying elements are usually added in the ladle during tapping the steel. The sulphur content in Indian blast furnace is usually high at 0.06-0.07%. In most cases, external desulphurization of hot metal is the best route to bring down the final sulphur content in steel to around 0.02%. Calcium carbide-based and magnesium-based reagents are generally suitable. Coinjection of both these types of reagents yield even better results. Typical constituents in industrial mixtures based on calcium carbide are carbide, limestone, lime and carbon. Magnesium-based reagents also contain carbide and carbon in addition to magnesium. Passivated maguesium granules have also been occasionally used for desulphurization of hot metal. After completion of injection, complete removal of slag is essential. The

Top-Blown Basic Oxygen Converter Processes

39

selection of desulphurization technique and reagent in a particular plant depends on factors like raw materials used, the process route, design of existing plant, etc. The blown metal needs to be deoxidized. Ferromanganese, ferrosilicon and aluminium are added in the order indicated into the ladle during tapping the steel. As the liquid steel is usually tapped through a tap hole located in the nose of the converter [as shown in Figure 4.l(f)], the slag continues to float on the bath. Every precaution is taken to prevent flow of slag into the ladle along with the metal during the last stage of tapping. Alloying elements are also added to the bath in the ladle. Since high-purity oxygen is lanced, the final nitrogen content in LD steel is of the order of 0.003-0.005%. This compares favourably with 0.004-0.0075% nitrogen in open hearth steel.

4.3 OXYGEN JET In the LD process, oxygen is blown at pressure of 8-10 atm through a convergent-divergent nozzle. The oxygen jet is supersonic and has a speed between 1.5 and 2.2 times the speed of sound. A supersonic jet is characterized by a supersonic core in which the jet velocity is higher than the speed of sound. As the jet travels away from the nozzle, it is retarded by the converter atmosphere so that the supersonic core shrinks radially and the axial velocity gradually decreases until at some distance away from the nozzle, the jet becomes fully subsonic. This point marks the end of the supersonic core. The main factors affecting the length of the supersonic core are the blowing speed and the ratio of the densities of the jet gas and the ambient medium. In LD steel making, this ratio would vary depending on the flow rate, the lance height and how far the blow has actually progressed. The oxygen jet becomes subsonic either when it strikes the bath or is very close to the bath surface. As the jet passes through the converter atmosphere, it carries some of the ambient medium along with it. This entrainment is much less in the supersonic portion of the jet but, nevertheless, the jet impinging on the bath surface will be a mixture of oxygen, carbon monoxide and carbon dioxide. Preliminary calculations in the case of a 100 ton vessel have shown that the oxygen content on the axis of the L.D. jet at the steel bath level could be as low as 60% when relatively high lance height (1.8 m) and soft blowing (1.8-2 m 3 N/min/ton of steel) are used. Furthermore, experimental measurements have shown that at any given distance from the nozzle, the concentration of oxygen decreases and the concentration

40

Steel Making

of carbon monoxide increases. The amount of carbon dioxide present in the jet appears to reach a maximum some distance away from the axis, decreasing on either side of this position. In the beginning of the blow, the jet strikes the metal surface directly and the kinetic energy of the jet is partially transferred to the bath. As a result, the bath starts to circulate and the oxidation of carbon and other metalloids begins. The oxidation reactions and the simultaneous stirring of the bath help in dissolving the scrap charged together with the hot metal at the beginning of the blow. The rate of scrap dissolution depends primarily on the size and amount of scrap charged, the blowing conditions and the hot-metal temperature. However, recently it has been conclusively proved that the bath circulates upwards on the vessel axis and radially outwards on the surface. As the lance is raised and the pressure of the oxygen jet is reduced, the zone of contact of the oxygen jet with the metal increases and correspondingly the depth of penetration of oxygen into metal decreases. Under this condition, FeO content of the slag increases, which favours dissolution of lime. As the jet impacts the liquid steel surface, a distinct crater is formed at the centre and the peripheral liquid is considerably splashed. This is schematically illustrated in Figure 4. 2. The important parameters are jet momentum, jet height and fluid properties.

FIGURE 4.2

Interaction of top-blown oxygen jet with liquid metal. 3

The depth of penetration of a gas jet into the melt can be described by the formula

Top-Blown Basic Oxygen Converter Processes

41

where h = depth of jet penetration (in metres), Pc & pz = density of gas and liquid respectively (in kg/m 3), w = velocity of gas along the axis of the jet at the bath surfaces, (in mis), and g = acceleration due to gravity (in m/s 2 ). 3 Splashing commences when the depth of depression reaches a critical value. The critical depth of depression is, however, influenced only slightly by lance height and is almost solely dependent on the liquid properties like viscosity, density and surface tension. Increasing the jet momentum or decreasing the lance height upto a limit raises the amount of liquid metal splashed. Under vigorous splashing conditions, mass transfer from the gas to the metal phase may play an important role. By properly selecting the height of the lance above the bath and adjusting the pressure or speed of the blow, deeper penetration of jet and more disintegration of the liquid bath into small droplets may be ensured. In modern LD converters, multihole lances (upto 8 holes) are used. The main advantage of increasing the number of outlets on a lance is to allow the total oxygen throughput to be increased without effectively increasing the pressure exerted by the jets impinging on the bath surface. It is this pressure which determines the depth of the depression formed and consequently the amount of liquid splashed from the bath. By increasing the number of nozzle openings, it is possible to increase the rate of oxygen supply without simultaneously decreasing the metal yield or increasing the risk of burning the converter bottom because of too deep a penetration bath. The limitation on the increase in the number of openings lies purely in the difficulty of accommodating these holes within the available space without having to decrease either the size or the inclination angle of each opening. The total energy of the jet issuing from the multihole lance is distributed over a larger surface area of the bath. This has been explained by theoretically calculating the jet momentum. 5 By definition, M= mv and where M = m = Pc = V = A =

m= Pc·A-V = Pc·Q jet momentum (in dynes), mass flow-rate of gas (in g/s), density (in g/cm3 ), gas velocity (in emfs), and cross-sectional area of nozzle opening (in cm3).

42

Steel Making

For a lance with n holes and total volume flow-rate Q, the flowrate through each nozzle * Q Q(n) =

n

Therefore, mass flow-rate through each nozzle Q·pg

* m(n)=

-n-

and flow velocity through each nozzle

v,* (n) -

Q

n-,id2 /4

where

n = number of holes in nozzle, Pc = density of the gas phase, m(n) = mass flow-rate through each nozzle in a multihole lance, and d = throat diameter of each nozzle opening (in cm). When throat diameter, d, and total flow-rate, Q, are constant *

M

>-

X 0

[CJ+ (FeO) =

60

coi

+ [Fe]

ni

:§

0

40

Q)

O> ct!

c

Q)

20

0

Q) CL

0 4 .0

3.0

2.0

1.0

0

Carbon content of bath (%)

FIGURE 4.9

Typical contribution of gaseous refining and emulsion refining during an LD blow. 19

During dephosphorization, only the metal/slag reaction interface is involved. It is most likely that dephosphorization actually takes place at the interfaces of the tiny metal droplets and the slag phase.

Top-Blown Basic Oxygen Converter Processes

53

The tiny droplets have a large surface area per unit volume which is important in enhancing the kinetics of dephosphorization. The phosphorous content of the metal droplets is about one-tenth that of the bath. The metal droplets are extensively dephosphorized during their passage through the slag. Where the blow is soft, phosphorus is preferentially removed. Under harder blowing conditions, carbon is removed first. 3 •16- 19 Fruehan et al. investigated the foaming characteristics of LD converter slag. The physicochemical properties of the slag such as viscosity, surface tension, film elasticity, electrical double layer repulsion, bubble size, critical thickness and rupture of bubble film can directly influence the stability mechanisms operating in the foam. The volume of foam formed at steady state is proportional to the gas flow rate, Qg. Fruehan et al. measured the foamibility of a slag in terms of a foaming index.

where Hr= measured change in foam height (m) after reaching a steady

v;

state; = superficial gas velocity (ms-1 )

Again,

vs= Qg

c A where Qg = gas flow rate (m3 s-1 ) A = uniform cross sectional area of the sample (m2 )

Physically, L represents the residence time of gas in the foam layer and can be measured from the slope of a plot of foam height versus superficial gas velocity. Fruehan studied the foamability of the CaO-SiO2-FeO slags. He observed that the foam index decreases with increasing FeO upto about 20% FeO and is almost constant from 20 to 32% FeO. Above 25% FeO, the viscosity is nearly constant. The foam index increases as basicity rises above 1.4, due to precipitates such as 2CaO • SiO2 or Fe, MgO which stabilizes the foam. It decreases with increasing temperature in range 1,673 to 1,873 K. 20-21

54

Steel Making

4.8 BATH AGITATION PROCESS (BAP)-COMBINED BLOWING Despite considerable advances in oxygen-lance technology, temperature and composition stratification inside the bath still occur, particularly in the presence of network of solid scrap. Thus, at the end of the early blowing period, mixing of high temperature over-oxidized slag with high carbon metal can result in slopping and metallic yield loss. Inefficient mixing within the bath leads to higher oxygen content of the metal phase for a given carbon content at tum down in top-blown steel making. These factors have promoted the combined blown steel making process. In this process, additional gas is introduced through tuyeres judiciously located in the bottom of the converter. The objects of tuyere injection of a relatively inert stirring gas during conventional top blowing with oxygen are: (i) Better mixing of slag and metal at all stages of the blow, thus preventing excessive composition and temperature stratification, and consequent slag and metal ejections. (ii) Smoother and more predictable carbon-removal trajectories during the main decarburization period with lower peak levels of waste gas evolution. (iii) Better mixing of slag and metal at the end of the blow, resulting in lower slag iron oxide levels at all carbon levels, thus increasing Fe yield. (iv) Smoother and more consistent refining over a wide range of hot-metal silicon contents. The experience gained in production plants suggest that the bath agitation process (BAP) offers many benefits. The principal benefits are: 1. Slopping and iron oxide levels in the slag are reduced compared with the conventional top-blown process. A higher silicon content in the hot metal may be tolerated. 2. Closer control of bath composition and better recovery of Mn and ferroalloys may be ensued. 3. The smoother and more predictable decarburization trajectory offers possibilities of improved process control. 4. It is possible to operate the process close to equilibrium, which results in improved rates of dephosphorization and desulphurization and makes the process more suitable for the production of low carbon steels.

Top-Blown Basic Oxygen Converter Processes

55

5. The stirring gases increase bath cooling and assimilation of post-blow additions of fluxes and coolants. But in blow cooling, effects reduce overall coolant consumption. 22- 24

4.9 LDAC/OLP PROCESS In many European countries, the phosphorus content in the hot metal may be as high as 1.8%. The LD process has been modified to refine such high phosphorus hot metal. The modified process is known as LDAC or OLP process. Charge materials are the same as in LD process except that only one-third of the total lime requirement is charged in the converter as lumpy lime prior to pouring the metal. The remaining lime is injected in powder form during the blow. The blowing procedure follows exactly the same patterns as that in LD process. After pouring the high phosphorus pig iron into the converter, the lance is lowered and oxygen and lime powder are blown. Two slag operations are necessary if the initial phosphorus content in the hot metal is high. The rate of lime injection is an important factor in the physical and chemical control of the slag. The high specific surface area of the powder ensures efficient use of the lime in slag forming. The control of the foaming by adjustment of lance height, oxygen flow rate and lime flow-rate is an essential component in the control of the process. The coolant (iron ore) is added in small proportions from the beginning of the blow. Steel scrap may also be charged as a coolant. Powdered lime enters directly into the reaction zone and guarantees easy formation of a reactive slag leading to rapid dephosphorization, efficient desulphurization and good protection of the bath. The dephosphorization reactions involved are as follows: 2[P] + 5(FeO) µ (P2 O5 ) + 5Fe 4(CaO) + (P2 O5) µ (4CaO · P 2 O5) At the end of the first stage, the metallic bath still contains from 1 to 1.5 pct C (starting with 3.8 pct), whereas the phosphorus content 0.2 to 0.4 pct (starting with 1.8 pct). The temperature at this moment is 1,600° to 1,650°C. At this stage such a slag contains about 60 pct CaO, 20 to 25 pct P 2 O5 and only 3 to 5 pct Fe in the form of oxide. The slag, which is not foamy, is partially drained off Thus, part of the phosphorus is also removed from the total system (metal+ slag). This first basic and low-iron slag has a very high desulphurizing power for a metallic bath containing more that 1 pct C, and sulphur partition coefficients of the order of 15 to 20 may be achieved.

56

Steel Making

After elimination of part of the first slag, the blowing of oxygen and lime is started again, with an addition of ore or scrap determined by the temperature in the first phase. At the end of this second phase, the metallic bath, which is around 1,600°C, contains about 0.5 pct C and less than 0.1 pct P, while the corresponding slag has about 52 pct CaO, 15 to 20 pct P 2 0 5 and 7 to 10 pct Fe in the form of oxide. The elimination of part of this slag removes further amount of phosphorus and sulphur. The blowing of oxygen and powdered lime is again resumed and continued until the desired carbon content is obtained. The total blowing time in the second and third stages is determined by the analysis of carbon at the end of the first phase. The final slag, if low carbon steel has been made, is rich in iron (about 20 pct) but its quantity is not large because of the two partial slag removals made during the operation. For continuous industrial operations, the final slag could be retained in the vessel by means of a lime dam, thus permitting the recovery of its iron and lime by the following charge. Two partial eliminations of slag during the refining, carry away a large part of the phosphorus initially in the pig iron. This refining process also gives very high efficiencies of desulphurization. The nitrogen content in the final steel is usually of the order of 0.001 to 0.002 pct only, with oxygen of 99.5 pct purity. 25- 26

4.10 KALDO PROCESS In Sweden, a rotating converter was developed to produce open hearth quality steel from high phosphorus hot metal. The process developed by Prof. Bo-Kalling is known as the KALDO process. A typical KALDO converter is illustrated in Figure 4.10. In this process high phosphorus iron containing about 1.8% P may be refined with high thermal efficiency. The main part of the phosphorus is oxidized before the iron reaches 1.5 pct C. The large excess of heat, which is obtained in the process is utilized for reduction of iron directly from ore. It can also be used for melting scrap. Most of the ore and lime additions are made before charging the hot metal. Further additions are made after each flush out of high phosphorus slag. The first slag-off is at 0.2 to 0.3 pct P. At this stage there is only 3 pct total iron in the slag. The second, smaller slag-off, is done at about 1 pct C. The phosphorus in the bath at this stage is usually less that 0.1 pct. The slag contains on an average, 16 to 18 pct P 2 0 5 and 5 pct Fe and is again flushed off.

Top-Blown Basic Oxygen Converter Processes

57

Cooling water

FIGURE 4.10 The KALDO converter with the positions for charging , blowing and tapping indicated 27 (Reproduced from J.O.M. with permission) .

There is usually one additional stop for temperature and carbon control. The tapping temperature can be kept within very narrow limits. Due to its characteristic features like high yield and outstanding heat economy, the KALDO process is also economical in the use of low phosphorus iron. Both low carbon and high carbon grades of steel may be prepared. The furnace body which rotates at a maximum speed of 30 rpm rests on four wheels within a frame that can be turned on trunions for different positions of charging, blowing and tapping. In blowing position, the furnace is inclined at an angle of 15°. The oxygen is blown into the furnace above the bath surface to refine high phosphorus iron (about 1.8% P) to steel with low S, P and N. Carbon is oxidized first to CO which again burnt inside the vessel to CO 2 . 27

4.11 ROTOR PROCESS The rotor process is also a basic oxygen furnace steel making process. It has found very limited application for treating high phosphorus pig iron. The furnace is a cylindrical vessel which is rotated around its horizontal axis at a speed of 0.1-4.0 rpm. The vessel, however, can be tilted for charging and tapping through openings provided at both ends. Oxygen for refming is blown into the bath through a lance known as the primary lance. Another lance, called a secondary lance, is positioned above the bath. Pure oxygen or oxygen + air

58

Steel Making

mixture is supplied through it to burn the carbon monoxide evolving from the bath into carbon dioxide. The heat generated is transferred into the bath by radiation as well as by conduction from the heated refractory lining, when the exposed part of the lining passes under the molten bath due en route for rotation of the vessel. During refming a basic oxidizing slag is formed. The first high phosphorus slag is flushed off when the phosphorus content in the bath drops to 0.1-0.12%. Fresh slag is then made and refining is continued. Finally the heat is finished in the same way as in LD and LDAC processes 3 •4 (Figure 4.11).

FIGURE 4.11

A rotor furnace : 1-Primary oxygen lance, 2-secondary oxygen lance, 3-gas outlet, 4-tap hole.

4.12 HYBRID PROCESSES A large number of hybrid processes based on simultaneous top and bottom blowing were developed from time to time. The basic principles of refining all these processes are also similar. These processes are not discussed separately because the salient metallurgical principles of combined blowing have already been elaborated. Starting with a hot metal containing up to 0.60% P, steels with less 0.015% P have been produced. The dephosphorization reactions involved are as follows: 2[P] + 5(FeO)

~

(P 2O5) + 5Fe

4(CaO) + (P 2 O5)

~

(4CaO · P 2O 5)

After pouring the high-pig iron the vessel, the lance to furnace and oxygen and powdered lime. In the course of the first phase, about two-thirds of the oxygen and powdered lime necessary for the complete refming are blown in. Ore or scrap is likewise added, depending upon the composition and temperature of the iron. At the end of the first stage, the metallic bath still contains from 1 to

Top-Blown Basic Oxygen Converter Processes

59

1.5 pct C (starting with 3.8 pct), whereas the phosphorus content is no higher than 0.2 to 0.4 pct (starting with 1.8 pct). The temperature at this moment is 1,600° to l,650°C in order to obtain a slag which is liquid, basic and poor in iron oxide; such a slag contains about 60 pct CaO, 20 to 25 pct P 2 0 5 and only 3 to 5 pct Fe in the form of oxide. That slag, which is not foamy, is then poured in part; this has the advantage of removing a part of the phosphorus originally contained in the pig iron from the total system (metal+ slag). The same reasoning applies to sulphur because this first basic and lowiron slag has a very high desulphurizing power for a metallic bath containing more that 1 pct C, and one frequently attains partition coefficients, pct S in slag/pct S in metal of the order of 15 to 20. After elimination of part of the first slag, the blowing of oxygen and lime is started again, with an addition of ore or scrap determined by the temperature in the first phase. At the end of this second phase, the metallic bath, which is around l,600°C, contains about 0.5 pct C and less than 0.1 pct P, while the corresponding slag has about 52 to 5 pct CaO, 15 to 20 pct P 2 0 5 and 7 to 10 pct Fe in the form of oxide. The elimination of part of this slag again removes from the total system (metal + slag) a part of the phosphorus which it contains, as well as part of the sulphur. For the third and last phase, the blowing of oxygen and powdered lime is continued until the desired carbon content is obtained. The total blowing time in the second and third stages is determined by the analysis of carbon at the end of the first phase. The final slag, if low carbon steel has been made, is rich in iron (about 20 pct) but its quantity is not large because of the two partial slag removals made during the operation. For continuous industrial operations, the final slag could be retained in the vessel by means of a lime dam, thus permitting the recovery of its iron and lime by the following charge. The production of steel with very low phosphorus contents is made easy and systematic. Two partial eliminations of slag during the refining, carry away a large part of the phosphorus initially in the pig iron. The easy dephosphorization even in the presence of carbon, permits one to obtain directly semihard or hard steels with a low phosphorus content by stopping the refining at the desired carbon content. This new refining process gives very high efficiencies of desulphurization, permitting the systematic attainment of very low sulphur contents in the final steel. This result is obtained principally by the removal from the high-carbon bath of basic intermediate slags with high partition coefficients (15-20 for the first slag, 10 for the second) sulphur.

60

Steel Making

The nitrogen content in the final steel usually lies between 0.001 and 0.002 pct, with 99.5 pct purity oxygen.

REVIEW QUESTIONS 1. Discuss the merits of using maguesia-carbon refractory lining

2. 3. 4. 5. 6.

7. 8.

9.

on a LD converter. What is slag splashing technology? How is it practised? What are the merits? How does an oxygen jet interact with the bath in LD steel making? Illustrate with schematic diagrams. How does the pattern of jet metal interaction change with an increase in the numbers of holes on the lance? Calculate the overall jet momentum of a multihole lance. What do you mean by lime reactivity? Which factors influence lime reactivity? How can you control the relative rates of decarburization and dephosphorization in LD steel making? How does a slag metal emulsion form in basic oxygen steel making? How does it account for the fast rate of decarburization in a basic oxygen converter? Explain the course of manganese reaction in a basic oxygen converter.

10. Why is it necessary to ensure slag-free tapping from the LD vessel? How do you actually ensure it? 11. Carbon and phosphorus can be simultaneously oxidized from the bath in a LD converter. Explain the phenomenon with reference to the Ellingham diagram. 12. What is the combined blowing? How does it improve the operation of a basic oxygen converter?

REFERENCES 1. A. Chatterjee and A.V. Bradshaw, J.I.S.I., 210(3), 1972, 179-187.

2. A. Chatterjee, N-0 Lindfors and J.A. Westor, Iron Making and Steel Making, 3(1), 1976, 21-32. 3. G. Oiks, Converter and Open Hearth Steel Manufacture, Mir Publishers (Moscow), 1977. 4. Making, Shaping and Treating of Steel, Ed. H.E. McGamnon, U.S. Steel, 8th Edn., 1964.

Top-Blown Basic Oxygen Converter Processes

61

5. A. Chatterjee, TISCO Jr., 20(2), 1973, 35-44. 6. U.K. Chaturvedi, C.C. Ojha, A.S.K. Ajmani, S. Sharma, H.M. Nerurhar, A. Chatterjee and T. Mukherjee, Tata Search, 1995, 43-49. 7. K. Koga, Y. Ohkita, M. Mizutani and A-kawni, Iron Making and Steel Making, 3(3), 1976, 146-162. 8. S.K. Mitra, B.N. Ghosh, B. Rao and K.S. Swaminthan, TISCO Jr., 23(1), 1976, 39-47. 9. M. Peatfield and D.R.F. Spencer, Iron Making and Steel Making, 6(5), 1979, 221-234. 10. M.J. Strelbisky and J. Manning, "Current slag splashing practices in selected mills", Internet, 2005. 11. R.J. Fruehan, Iron Making and Steel Making, 32(1), 2005 (Bessemer Lecture). 12. P. Nilles, E. Denis, P. Dauby and N. Bach, J.O.M., 19(1), 1967, 18-23. 13. A. Kumar, H.J. Billimoria and K.S. Mathews, Tata Search, 1995, 50-51. 14. S. Pathak, S. Kumar, S. Das, S.K. Roy and S.K. Mahapatra, Tata Search, 2003, 101-108. 15. S.K. Chowdhury, S.N. Lenka and A. Ghosh, Tata Search, (1) 2005, 137-144. 16. S. Jha Ajit, TISCO Jr., 42(5), 1975, 119-125. 17. H.W. Meyer, J.I.S.I., 207(6), 1969, 781-789. 18. J. Schoop, W. Resech and G. Mahn, Iron Making and Steel Making, 5(2), 1978, 72-80. 19. A. Chatterjee, TISCO Jr., 43(1), 1976, 22-28. 20. Sung-Mo Jung and R.J. Fruehan, ISIJ International, 40(4), 2000, 348-355. 21. C. Xexhip, S. Sun and S. Jahanshahi, Int. Mat. Reviews, 49(5) 2004, 286-298. 22. A. Balcer, A.S. Normanton, C.D. Spenceley and R. Atkinsen, Iron Making and Steel Making, 7(5), 1980, 227-238. 23. A. Chatterjee, C. Marique and P. Nilles, Iron Making and Steel Making, 11(3), 1984, 117-131. 24. S.K. Ajmani, AK. Das, P.K. Ghose and U.K. Chaturvedi, Tata Tech, 39, 1-5. 25. B. Trentini and A. Allond, J.O.M., 10(10), 1958, 466-470. 26. J. Jones, E. Parsons and N. Alorris, J.O.M., 15(8), 1963, 577-580. 27. Folke Johansson, J.O.M., 9(7), 1957, 972-975.

5 Bottom-Blown Basic Oxygen Converter Process (Q-BOP/OBM/LWS)

5.1 INTRODUCTION The bottom-blown oxygen converter process was developed in Europe in the late sixties. Essentially, there are two versions of the process based on the same principle. These are: (a) OBM-Oxygen-bolden blasen (bottom-blown) Maxhuette process developed in West Germany. The same process was renamed as Q-BOP (Quiet-basic oxygen process) in the United States. (b) LWS process, developed in France. A sketch of a bottom-blown converter (Q-BOP converter) is shown in Figure 5.1. A schematic drawing of a LWS converter is presented in Figure 5.2. When pure oxygen is blown through the bottom of the converter, intense heat is generated which burns off the tuyeres. It is, therefore, essential to simultaneously cool the reaction zone. This is done by using a shrouding hydrocarbon gas in the OBM/Q-BOP process, and fuel oil in the LWS process. On entry into the steel bath, the hydrocarbon gas/oil undergoes endothermic decomposition and absorbs heat from the reaction zone. The tuyeres in the bottom of the converter are, therefore, made up of two concentric tubes. The inner tube carries pure oxygen while the protective gas/fuel oil is injected through the annular space between the outer and inner tubes. The converter itself is a basic lined cylindroconical vessel 62

Bottom-Blown Basic Oxygen Converter Process (Q-BOP/OBM/LWS)

63

-~~ . ,..l/.;' ji' ~ \

,,•.... : ';. ' i ·,.' \.

,,' . ·:_.,_ 11., \,,'· ...

1

'

·,_:}~·.·:::_:.':((:.: ~ . ,° :

\

I

I

\

\ • •.

\

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Shielding gases

FIGURE 5.1 OBM , Q-BOP process 1 (Reproduced from J.O.M. with permission) .

FIGURE 5.2 Principle of LWS method . A-hollow trunnion ; B-ball and socket; C-bearing ; D-oxygen ; E-protective fluid ; F-2 tubes; G-oxygen flow ; H-6 tuyere bottom , only one tuyere shown 1 (Reproduced from J.O.M. with permission) .

fitted with a special bottom through which the tuyeres are inserted. The hydrogen liberated by the decomposition of hydrocarbon/fuel oil dissolves in the liquid steel. Before tapping, nitrogen is bubbled through the bath to remove the hydrogen. In order to promote turbulence the tuyeres are arranged on only one side of the converter. The tuyeres are usually made of stainless steel and are inserted in magnesia-lined bottom. 1- 2

64

Steel Making

5.2 BASIC METALLURGICAL CHARACTERISTICS OF THE O-BOP PROCESS The unique features of bottom-blown steel making are: (i) More efficient agitation of the bath. As a result there are no temperature and concentration gradients that can cause slopping. The oxygen utilization is so efficient that very little oxygen is available for post-combustion above the bath. (ii) Since there is no emulsification of slag and metal, it is necessary to inject lime as a fine powder along with the oxygen. The reaction mechanism is different from that in the LD converter. In these processes the reactions in fact proceed more efficiently. (iii) Operation of the process is very close to equilibrium. The specific advantages of a bottom-blown steel making converter are: (i) Better iron yield due to lower iron oxide loss in the slag, less fume dust generation and less slopping. (ii) Better phosphorus and sulphur partition coefficients because of the use of powdered lime and improved turbulence. (iii) Higher manganese content and lower oxygen content in the bath and hence better ferroalloy and aluminium recovery. (iv) The bottom-blown converter can produce ultra-low carbon steels without risk of over-oxidation of metal and slag. But it has limited ability to melt scrap although scrap of much larger size can be melted. (v) The product range includes rails and bars, plates and structures, and sheet and tin products. Bottom-blowing has certain inherent features. These are: (i) A highly-coupled gas-solid injection apparatus is required for bottom-blowing lime and oxygen together. (ii) The heat balance around each tuyere is critical and too much or too little coolant compared with the oxygen blown at any instant can prove to be detrimental to the tuyeres (14-22 in number). (iii) The life of the bottom tuyere area constitutes the heart of the process.

Bottom-Blown Basic Oxygen Converter Process (Q-BOP/OBM/LWS)

65

Disadvantages of the OBM/0-BOP processes 1. Both the manufacture and the maintenance of the OBM bottom is an extremely specialized job. This is a matter of serious concern. 2. The nitrogen content of the finished steel is relatively high. It is not suitable for manufacture of unkilled deep drawing varieties of steels.

5.3 MECHANISM OF REFINING The mechanisms of the refining reactions are discussed in the subsequent section. 2- 7

5.3.1

Decarburization

The progress of the metallurgical reactions during a blow is shown in Figure 5. 3. The carbon content drops almost linearly with the rate of oxygen injection. During reaction with carbon in the bath, the oxygen is converted to both CO and CO 2 . As the gas bubbles rise in the melt, the rate of oxidation of carbon is controlled, at high carbon levels, by diffusion of CO 2 in the gas stream to the gas bubble/ metal interface. This transport process is fast, and consequently there is essentially complete utilization of the oxygen for carbon

4

Carbon

0.3

?fl. C:

::? ci5

~

0.2

2

a:

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FIGURE 5.3 Oxidation of carbon, phosphorus, silicon , and manganese during blowing of a 251 Q-BOP heat 2 (Reproduced with permission of the Institute of Materials, Minerals and Mining) .

66

Steel Making

oxidation at carbon levels above 0.2%. In metal containing less than 0.2% carbon, blown oxygen is not utilized fully in the oxidation of carbon. The rate is now controlled by diffusion of carbon in liquid steel to the surface of gas bubbles and not by the rate of oxygen supply. If oxygen concentration in the bottom gas supply are decreased by dilution with inert gas to conform to the limitations of the decarburization rate at these lower carbon levels, the oxygen requirements for the oxidation of other components of the bath (such as iron or manganese) and the final oxygen content of the steel will decrease. More effective utilization of the oxygen can be obtained through the use of more tuyeres of smaller diameter. This practice increases the gas bubble/liquid metal surface area, and therefore, delays to a lower bath carbon content the transition of the ratecontrolling mechanism from CO 2 diffusion within the bubble to mass transfer of carbon within the bath to the oxygen-rich bubble (Figure 5.4). 1.0

~

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6

FIGURE 5.4 Decarburization for 25 t experimental Q-BOP using 81 m3 N/min oxygen and 5.4 m3 N/min natural gas 7 (Reproduced with permission of the Institute of Materials, Minerals and Mining) .

The high rates of liquid and gas phase mass transfer associated with the Q-BOP are further confirmed by the lower oxygen requirement for the Q-BOP in achieving an identical bath carbon content than that in the BOP. Iron oxidation is less in the OBM/Q-BOP process. In general, the FeO content in Q-BOP slag remains below 5% down to carbon levels of about 0.1 %. Decaburization in the low-carbon range proceeds easily in a OBM/Q-BOP converter. Even at the end of low carbon blow (0.03% carbon), the FeO content does not normally rise above 13%. The relationship between end point carbon and total iron in slag is shown in Figure 5. 5.

Bottom-Blown Basic Oxygen Converter Process (Q-BOP/OBM/LWS) o 250t LD

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20 40 60 80 Sponge iron in charge, percent

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Effect of gangue in sponge iron on slag weight. 6

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Electric Furnace Steel Making

91

With less than 92% metallisation, heat time increases considerably. (%

Metallization = Free Fetrotal Fe x 100)

Low gangue content ensures better productivity. In most cases, the oxygen remaining in freshly-metallized material is primarily in the form of FeO. This can be dealt with in one of the two ways: (i) The furnace may be operated in such a way as to allow much of the wiistite to be lost in the slag. (ii) An attempt can be made to recover as much as possible of the iron contained in the wiistite according to the reaction (FeO) + Q +heat= Fe+ {CO}. However, this increases both the melting energy consumption and the heat time. In most cases, economics and metallurgy favour the latter option. Reduced levels of metallization result in increased power consumption and consequently longer heat times. A 1% change in metallization results in a change in melting energy consumption of about 9-12 kWh/ton of steel tapped. This includes not only the energy to drive the reduction reaction, but also the additional heat losses resulting from prolonged heat times. In larger furnaces which have lower heat losses per unit of capacity, the observed rise in energy consumption would be somewhat less. In certain cases, the residual oxygen may be in the form of magnetite or haematite, rather than wiistite. The energy required to recover iron from these oxides is substantially greater than that for wiistite. For such cases, additional energy consumption might exceed 20 kWh/ton tapped per 1% change in metallization. Such materials may produce highly detrimental effects on steel making economics even at reasonably high levels of metallization. The correlation among percentage of sponge iron, melting time and productivity is illustrated in Figures 6.11 and 6.12. The effect of metallization on melting energy consumption is shown in Fignre 6.13. Batch-charging of sponge iron is feasible only upto 25-30% sponge iron in the charge. Batch-charging should be practised only in cases where sponge iron is used in limited quantities or sporadically, so that the additional cost of installing continuous charging facilities is not justified. Batchwise charging is also the only recommended procedure for 5 ton furnaces. In the case of continuous charging for furnaces of 10 ton or more capacity, upto 60% sponge can be used but the optimum sponge iron percentage appears to be between 40 and 45 under Indian conditions. These limitations are imposed by the relatively low-powered and

92

Steel Making

r ~-

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Percentage of sponge iron

FIGURE 6.11

Correlation between percentage of sponge iron melting time and productivity.23

0% gangue content

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9