NON Metallic Inclusions in steel

“STUDY OF THE NON METALLIC INCLUSIONS AND THEIR EFFECT ON THE PROPERTIES OF STEEL” A Thesis submitted to

CHHATTISGARH SWAMI VIVEKANAND TECHNICAL UNIVERSITY Bhilai (C.G.), India For the Award of Degree of

Master of Technology In Metallurgy Engineering (Specialization in Steel Technology) By

DEEPAK PATEL

(QUROOPHQW1R$'

Under the Guidance of Dr. VARSHA CHAURASIA Sr. Associate Professor H.O.D. METALLURGY U.P.U.G.P.D.

University Teaching Department Chhattisgarh Swami Vivekananda Technical Unversity …………………Bhilai

Session 2015 - 2016

DECLARATION BY THE CANDIDATE

I the undersigned solemnly declare that the report of the thesis work entitled “Study of the nonmetallic inclusions and their effect on the properties of steel” is based on my own work carried out during the course of my study under the supervision of Dr. Varsha Chauraisa, Sr. Associate Professor and H.O.D., Department of Metallurgy Engineering, U.P.U. Govt. Polytechnic, Durg, (C.G.).

I assert that the statement made and conclusions drawn are an outcome of the project work. I further declare that to the best of my knowledge and belief that the report does not contain any part

of

any

work

which

has

been

submitted

for

the

award

of

any

other

degree/diploma/certificate in this university/deemed university of India or any other country. All help received and citations used for the preparation of the thesis have been duly acknowledged.

_____________________ (CANDIDATE) Deepak Patel Roll No. 5005612005 Enroll. No. AD2437 ___________________ (SUPERVISOR) Dr.V. Chaurasia Sr. Associate Professor H.O.D. Department of Metallurgy Engineering, U.P.U. Govt. Poly. Durg C.G.

II

CERTIFICATE BY THE SUPERVISOR This is to certify that the report of the thesis entitled “Study of the non-metallic inclusions and their effect on the properties of steel” is a record of research work carried out by Deepak Patel bearing Roll No.: 5005612005 & Enrollment No.: AD2437 under my guidance and supervision for the award of Degree of Master of Technology in the faculty of Metallurgy Engineering with specialization in Steel Technology of Chhattisgarh Swami Vivekanand Technical University, Bhilai (C.G.), India. To the best of my knowledge and belief the thesis i)

Embodies the work of the candidate herself/himself

ii)

Has duly been completed

iii)

Fulfills the requirement of the Ordinance relating to the M. Tech Degree of the

University and iv)

Is up-to the standard in respect of both contents and language for being referred to the

examiners.

___________________ (SUPERVISOR) Dr.V. Chaurasia Sr. Associate Professor H.O.D. Department of Metallurgy Engineering, U.P.U. Govt. Poly. Durg C.G.

Forwarded to Chhattisgarh Swami Vivekanand Technical University, Bhilai C.G.

_____________________________________________ REGISTRAR CHHATTISGARH SWAMI VIVEKANAND TECHNICAL UNIVERSITY NORTH AVENUE SEC – 8, BHILAI, CHHATTISGARH

III

CERTIFICATE BY THE EXAMINERS

The Thesis entitled “Study of the non-metallic inclusions and their effect on the properties of steel” submitted by Deepak Patel, Roll No.: 5005612005 & Enrollment No.: AD2437 has been examined by the undersigned as a part of the examination and is hereby recommended for the award of the degree of Master of Technology in the faculty of Metallurgy Engineering with specialization in Steel Technology of Chhattisgarh Swami Vivekanand Technical University, Bhilai (C. G.).

____________________

____________________

Internal Examiner

External Examiner

Date:

Date:

IV

ACKNOWLEDGEMENT First, I would like to express my special gratitude to my main supervisor, Dr. Varsha Chaurasia, for her advices and encouragements during these years of studies. Her excellent guidance to see the mind of a researcher will always be in my heart. I am truly grateful to Dr. Ashok Srivastava, H.O.D. Met. OPJU, fo r his constant support and valuable discussions throughout this work. His boundless energy and positive attitude were very impressive to me for completing my work. I am thankful for the support from JINDAL STEEL & POWER LIMITED, @ RAIGARH C.G. regarding help with the industrial visits. I also thanks to the VP & H.O.D. of Technical Services Department & Qua lity Control Shri B. Lax minarsimham and his team including one of my college friend Ms. Neelam Sharma, for their valuable help throughout all industrial studies. They have given me a great insight in both research and production process of world-class quality steel. I specially would like to thank Professor A.K. Verma, for his encouraging advice and comments. I sincerely respect his passion for the study and research. Thanks to all my friends and c olleagues at the Department of Meta llurgy Engineering U.P.U.G.P.D. for their friendship and kindness. Finally, I would like to express my respect and gratitude to my parents for their continuous trust and love. Deepak Patel June 2015

V

ABSTRACT Non-metallic inclusions are a major issue during the production clean steels, as they influence the microstructure and structural properties effectively. They are often considered as harmful to the final product quality and to the steel processing productivity; therefore many industrial efforts are directed towards improving inclusion removal. Another way is to use non-metallic inclusions to produce steels with enhanced properties. In both cases, the key issue is to control the characteristics of the inclusion population in the liquid steel, such as qu antity/limit, composition, physical appearance or morphology, shape, size and distribution. The application of new secondary refining techniques and non-metallic inclusion reduction techniques in steel production processes has greatly reduced the size and amount of nonmetallic inclusions remaining in molten steels and steel products due to which inspection of inclusions is very difficult. The influences of inclusions on the p roperties of steels are dis cussed. As inclusions have influence on several properties of steel, such as formability, toughness, and machinability and corrosion resistan ce. In general, the less severe the inclusions, the higher quality of steel. This is the reason for, analysing and assessment of non-metallic inclusions is important for quality control. The main part of this work has been a literature survey, reviewing the main methods used for the characterization of inclusions in clean steels, experimental reports for information on how steel cleanness is evaluated today, and how the steel cleanness is related to the performance of clean steels as a product.

VI

LIST OF ABBREVIATIONS Symbols

Units

d

Maximum particle size

ΔG°

Free energy of formation

K

Equilibrium constant

T

Temperature

K

T[O]

Total oxygen

ppm

λ

Wavelength

μm

α

Coefficient of thermal expansion

μm kCal

K-1

Element Abbreviations

Compound Abbreviations

Al C Ca Cu Cr Fe O P Pt Mg Mn N Ni S Si

Al2O3 CaO CaO•Al2O3

Alumina Calcia Calcium aluminate

CaO •SiO2

Calcium silicate

CaS FeO FeO•Al2O3 FeS MgO MnO MgO•Al2O3 MnO•Al2O3

Calcium sulphide Wüstite Hercynite Troilite Periclase Manganosite Spinel

MnO•SiO2

Rhodonite

MnS SiO2

Manganese sulphide Silica

Aluminum Carbon Calcium Copper Chromium Iron Oxygen Phosphorus Platinum Magnesium Manganese Nitrogen Nickel Sulphur Silicon

Galaxite

Abbreviations

ASTM

American Society for Testing and Materials

BSE

Backscattered Electron

DIC

Differential Interference Contrast

EAF

Electric Arc Furnace

VII

JSPL

Jindal Steel and Power Limited

IA

Image Analysis

LCM

Laser Confocal Microscope

OES

Optical Emission Spectrometry

OM

Light-Optical Microscope

ppm

parts per million

SE

Secondary Electrons

SEN

Submerged Entry Nozzle

SEM

Scanning Electron Microscope

wt%

weight percentage

IS

Indian Standards

NMI

Non Metallic Inclusion

VIII

LIST OF FIGURES Figure 1.1. A schematic diagram of the process route in SMS at JSPL Figure 2.1: Sources of inclusions in liquid steel Figure 2.2: Free energy of formation for various sulphides. Dash-dot line indicates equal sulphur pressure in unit of atmosphere. Figure 2.3: Free energy of formation for various oxides. Dash-dot line indicates equal oxygen pressure in unit of atm. Figure 2.4: Deoxidizing power of various elements at 1600 0C Figure 2.5: a) As-polished (2-dimensional) steel sample showing Al2O3 dendrite b) Partial slime extracted (3-dimensional) steel sample showing the same Al2O3 dendrite Figure 2.6: Equilibrium relations for manganese-silicon deoxidation of steel at various temperatures Figure 2.7: The effect of manganese content on stability of oxide phases resulting from steel deoxidation at 1550ºC (m: mullite; l: liquid manganese silicate) Figure 2.8: CaO-Al2O3 equilibrium phase diagram. Figure 2.9: Schematic representation of MnO-SiO2-Al2O3 ternary phase diagram Figure 2.10: Morphology of NMI’s occurred in steel Figure 2.11: Schematic representation of mold powder entrapment Figure 2.12. Schematic drawing of Slab caster tundish furniture Figure 3.1: Flow chart of scheme of experiments IX

Figure 3.2: Light Optical Microscope @ JSPL Figure 3.3: Scanning Electron Microscope @ JSPL, Raigarh Figure 3.4: Image analyser attached with optical microscope Figure 3.5: Images acquired using (a) optical microscopy, (b) laser confocal microscopy, (c) SEM (secondary electron mode) and (d) SEM (backscattered electron mode) Figure 3.6: Photograph processed by image analysis showing detected area as inclusions (a) Laser confocal microscopy, (b) SEM (backscattered electron mode) Figure 4.1: Force applied by a Wheel on Rail Figure 4.2: Sample images taken @ TSD,JSPL for inclusion rating Figure 4.3: (a) SEM image of inclusion in Heat ID 1 at 799X magnification (b)EDS spectrum of point 3 shown in image Figure 4.4: Spectral imaging of inclusion in Heat ID 2 at 3210X magnification

X

LIST OF TABLES Table 2-1: Possible Sources of Inclusion Table 2-2: Stoichiometric composition of reported inclusion phases Table 2-3: Inclusion distribution characteristics in solidified slab samples collected from original and modified design tundish operations Table 4-1: The importance of clean steel with respect to mechanical properties of the product Table 4-2: Chemical composition of Grade 880 rails as per IRS T-12 2009 specifications Table 4-3 Inclusion Rating Results

XI

TABLE OF CONTENT:

CHAPTER 1

INTRODUCTION

1

1.1.

Need for the Work

2

1.2.

Clean steel

3

1.3.

Non-Metallic Inclusions, definition & role

4

CHAPTER 2 2.1 )

2.2 )

LITERATURE REVIEW

5

Non-metallic inclusions in steel 2.1.1 Classification & Sources of nonmetallic inclusions

6-9

2.1.2 Formation of nonmetallic inclusions

10-23

2.1.3 Morphology of nonmetallic inclusions

24

2.1.4 Influence of inclusions on the properties of steel

25

2.1.5 Non-metallic inclusions during industrial practice and their control

27

Clean steel

31

2.2.1 Role of secondary refining on steel cleanliness

31

2.2.2 Role of Tapping addition on steel cleanliness

32

2.2.3 Salient steps adopted during secondary refining for Steel Cleanliness 32 2.2.4 Salient steps adopted during Vacuum Degassing for steel cleanliness 33 2.2.5 Role of continuous casting CHAPTER 3

EXPERIMENTAL ASPECTS AND METHODOLOGY

34 35

3.1)

Overview

36

3.2)

Quantitative Assessment

37

3.2.1 Image Acquisition

38

3.2.2 Image Analysis

40

CHAPTER 4

RESULT AND DISCUSSION

42

4.1)

Introduction

43

4.2)

Experimental procedure

46

4.3)

Result

48

XII

CHAPTER 5

CONCLUSION AND SCOPE OF FURTHER WORK

REFRENCES

50 53

ANNEXURE ATTACHED EXTENDED SUMMARY IS 4163 2004

XIII

CHAPTER – 1 INTRODUCTION

1

1.1 NEED FOR THE WORK In the global steel scenario aimed at superior properties, control and cleanliness of steel turn out to be more and more dynamic. Challenges like tweaking the chemical composition and the homogeneity have been replaced by troubles triggered by the presence of non-metallic inclusions. Mainly the presence of aluminium oxide inclusions is considered as detrimental both for the production process itself and for the steel properties [A. GHOSH et al., 2000][19]. These inclusions take shape during deoxidation of the steel, which is basic for continuous casting. Thus non-metallic inclusions vary from the precipitates that are already present in the liquid steel, though precipitates that are formed at stage of solidification. Partial elimination of the non-metallic inclusions during secondary metallurgy and reoxidation of the steel melt stimulates nozzle clogging at the SEN in continuous casting. The accretion of clogged material constitutes significant clusters of NMI. Its thickness is linked to the volume of steel cast along with the cleanliness of the steel. Nozzle clogging lead to a declined production, due to slower casting rate (since the decreasing diameter) and due certain simultaneous casting disruptions [R. Dekker’s et al. 2002]. [21]. In the course of rolling, dendrites and aggregates fractures, frequently next to the necks and subgrains by virtue of which elongated strings of fragmented particles forms. At high strains often voids are detected amongst these fragmented particles, causing fatigue of the steel [S.K. Choudhary, 2011]. [16] As a generalization, inclusions have been found to be harmful to the mechanical properties and corrosion resistance of steel. This is more so for high-strength steels for critical applications. As a result, there is a move to produce clean steel. However, no steel can be totally free from inclusions. The number of inclusions has been variously estimated to range between 1010 and 1015 per ton of steel. Again, the yardstick for cleanliness depends on how one assesses it. For example, most of the inclusions are submicroscopic. Therefore, a microscopic examination cannot faithfully assess cleanliness. [A. GHOSH et al., 2000]. [19] In this thesis, Chapter 2 deals with the literature survey, including, inclusion classification, sources, morphology and formation, followed by chapter 3 dealing with the experimental aspects containing Quantitative analysis on NMI through SEM, EDS and Microscope, chapter 4 is about results and discussion, at last chapter 5 deals with conclusion and future scope.

2

1.2 CLEAN STEEL The word “clean steels” is uncertain in class and commonly indicates steel with very low contents of phosphorus, sulfur, oxygen, nitrogen, and hydrogen and non-metallic inclusions. Steel cleanliness is used to refer relative freedom from the entrapped nonmetallic particles of solid ingot. In some steels this is the most important criteria in judging their quality. The fact that it is nonmetallic and, therefore, incongruent with the metal lattice, has often been considered prima facie evidence of its undesirability. [R.H. Tupkary, 2012][12] The inclusions are the source of many defects. Several applications limits the maximum size of inclusions therefore size distribution of the inclusions is significant. Steel cleanliness is optimized by an extensive choice of operating practices right through the steelmaking practices. These consist of the phase and position of deoxidation and alloy additions, the pros and cons of secondary metallurgy refining, stirring and transfer means, covering systems, tundish geometry and casting methods. The steel making process route of JSPL is schematically shown in Fig. 2.

Fig. 1.1. A schematic diagram of the process route in SMS at JSPL

3

1.3 NONMETALLIC INCLUSIONS DEFINITION AND ROLE WHAT ARE NON-METALLIC INCLUSIONS? “Compounds of metals (Fe, Mn, Si) with nonmetals (Oxygen, Sulphur, Nitrogen, Hydrogen, Phosphorus), which may be present in steel, are termed non-metallic inclusions.”

ROLE IN STEEL MAKING Non-metallic inclusions are naturally occurring and typically undesired products that are formed into various types depending on their favorable thermodynamic conditions in almost all treatment practices involving molten steels.[A. GHOSH et al., 2000][19] Apart from some applications where inclusions are supposed to be demanded, like sulphides for improving machinability (that could be argued with recently available cutting machines and tools), they usually deteriorate mechanical properties and surface quality of steel products and could cause nozzle clogging and disruption of steelmaking and forming processes. It is widely believed that due to the presence of sulphide and oxide inclusions some of the mechanical properties of steels like ductility, toughness, anisotropy, and formability might be negatively affected. The harmful effects of non-metallic inclusions on fatigue properties of steel parts are because they can act as potential sites of stress concentration that can initiate cracks under cyclic loadings. [Kiessling & N. Lange et al., 1978] [14] COMMENTS ON NMI’S: 

Non-metallic inclusions in steel normally have a negative contribution to the

mechanical properties of steel, since they can initiate ductile and brittle facture. 

The type and appearance of these non-metallic inclusions depends on factors such

as grade of steel, melting process, secondary metallurgy treatments and casting of steel. 

Only 1 ppm each of oxygen and sulphide will still contains 109 -1012 non-metallic

inclusions per ton. 

A beneficial effect on steels properties by nucleating acicular ferrite during the

austenite to ferrite phase transformation especially in low carbon steels. 4

CHAPTER – 2 LITERATURE REVIEW

5

2.1 NON METALLIC INCLUSIONS IN STEEL 2.1.1 CLASSIFICATIONS OF NON-METALLIC INCLUSIONS:Traditionally non-metallic inclusions have been divided into four types (type A: Sulphides, type B: Aluminates, type C: Silicates, and type D: globular Oxides) [18] [WD CALLISTER et al. 2003]. Based on the sources of inclusion they can be either indigenous or exogenous. Indigenous inclusions are deoxidation products or inclusions that precipitate during cooling and solidification. Deoxidation products cause the majority of indigenous inclusions in steel, such as alumina inclusions in low-carbon Aluminium killed steel and silica inclusions in Silicon killed steel. They are generated by the reaction between the dissolved oxygen and the added deoxidant, such as aluminium and silicon. Exogenous inclusions arise from unintended chemical and mechanical interaction of liquid steel with its surroundings. [R.H. Tupkary, 2012][12] They generally have the most deleterious effect on machinability, surface quality, and mechanical properties because of their large size and location near the surface. In machining, they produce chatter, causing pits and gouges on the surface of machined sections, frequent breakage, and excessive tool wear. Exogenous inclusions come mainly from reoxidation, entrained slag, lining erosion, and chemical reactions. Because they are usually entrapped accidently during teeming and solidification, exogenous inclusions are sporadic. They easily float out, so they only concentrate in regions of the steel that solidify rapidly or where their escape by fluid transport and flotation is hampered. Consequently, they are often found near the ingot surface

Fig 2.1: Sources of inclusions in liquid steel [E.T. Turkdogan, 1996][3]

6

Table 2-1 Possible Sources of Inclusion. [A. GHOSH et al., 2000][19]

7

CLASSIFICATION BASED ON INCLUSION CHEMISTRY AND COMPOSITION:Oxides In general, oxide inclusions can be classified into: • Single oxides; some common examples: FeO, Fe2O3, MnO, SiO2, Al2O3, Cr2O3, TiO2 • Complex oxides; often takes the general form of AO•B2O3, where metal A has +2 oxidation number and metal B has +3 oxidation number. Some common examples are FeO•Al2O3, MnO•Al2O3, MgO•Al2O3, FeO•Cr2O3, MnO•Cr2O3 [Kiessling and Lange et al. 1978][6]. Complex oxide inclusions are sometimes known as spinel type (MgO•Al2O3) inclusions for their similarity in structures. Spinel type inclusions are characterized by faceted structure and high melting temperature, usually higher than steelmaking temperature of 1873K. Spinel inclusions are especially harmful during steel processing as they do not deform during hot rolling and often cause poor surface finish. Calcium aluminate (CaO•Al2O3) type inclusions are also considered complex oxide inclusions. Calcium and barium, have +2 oxidation number, but do not form spinel structures due to their relatively large ionic radius. With common calcium treatment practice, the usual Al2O3

inclusions are modified to calcium

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

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

oxygen content less than 0.02% while having sulphur content at around 0.03%. Liquid steel has a high solubility of sulphur where solid steel usually has significantly lower sulphur solubility. As liquid steel cools, sulphur segregates and forms FeS with melting point of 1460K. FeS often causes embrittlement of steel during heat treatment. Therefore it has become a common practice to add sufficient amount of Mn, due to manganese’s stronger affinity for sulphur, to form MnS (Tm = 1870K). Types of sulphide inclusions will a l s o d e p e n d o n m a n g a n e s e t o S u l p h u r r a t i o . Examples of common sulphide inclusions include MnS, FeS, (Mn, Fe)S and CaS. The Sulphur affinity of various elements can be compared with

8

free energy of sulphide formation. Figure 2.1.2 gives a plot of curves for common elements found in steelmaking. 

Figure 2.2: Free energy of formation for various sulphides. Dash-dot line indicates equal sulphur pressure in unit of atmosphere [4]. Two morphologies are frequently observed: •

Globular: Both simple sulphides and oxysulphides, where the latter consists of

sulphides and oxides coexisting in one inclusion. This type of morphology is generally present in silicon killed or semi-killed steel using aluminum, titanium, or calcium. •

Faceted: Often appears in steel heavily deoxidized with aluminum.

Nitrides In the presence of elements having high affinity for nitrogen, nitrides such as AlN, TiN, ZrN, VN, BN, etc. [R.H. Tupkary , 2012][12] can form as a result of molten steel contacting with air atmosphere during unprotected vessel transfer. Like carbides, nitride inclusion contents in steel are significantly less than that of oxides and sulphides. 9

2.1.2 FORMATION OF INCLUSIONS DURING SOLIDIFICATION Inclusions form during solidification by chemical reactions. Oxides, sulfides, and some oxysulfides are typical products. Even nitrides and carbides have been found to form. The driving force is supersaturation of solutes leading to precipitation of reaction products. The cause of supersaturation in a ladle is the addition of deoxidizers to the bath. However, that is not the situation in the mold. Here, the supersaturation arises for the following reasons: 1. The decrease in the temperature of liquid steel in the mold during freezing shifts the reaction equilibria in favor of the formation of oxides and sulfides. This can be generally understood from the Ellingham diagrams. We may consider the specific case of deoxidation of steel by aluminum, viz. 2 Al + 3 O = Al2O3 (s) 2. Solid metals and alloys have lower solubilities for solutes as compared to those for liquids. This causes rejection of solutes by the solidifying material into the melt at the solidliquid interface and leads to nonuniform chemical composition in the cast material. The phenomenon is known as segregation, which is one of the casting defects. 3. Some oxygen is invariably picked up during teeming. Also, the occasional addition of deoxidizers, such as aluminum shots, into the mold is practiced. As far as the kinetics of inclusion formation is concerned, most experimental observations indicate that an abundance of nonmetallic particles are always present, and subsequent reactions during solidification occur on them. As a consequence, nucleation is not required, and the growth of inclusions occurs without the need for appreciable supersaturation. This assumption constitutes the basis for thermodynamic analysis of inclusion formation.

10

STEEL DEOXIDATION Maximum solubility of oxygen in liquid iron at the eutectic of 1527ºC is about 0.16% [E.T. Turkdogan, 1996] [3]. The oxygen solubility in solid iron, at temperature slightly below its melting point, approaches zero. Upon solidification, majority of dissolved oxygen will precipitate as FeO inclusions. In steel, the presence of alloying elements such as carbon can influence the dissolved oxygen content. Equation 2-1 describes carbon-oxygen relationship in iron up to 0.6% carbon. [wt%C] • [wt%O] = ~0.0023

[2-1]

In order to prevent blowhole (carbon monoxide gas) formation, porous cast product, or precipitation of FeO inclusions in sizeable quantities, liquid steel must be deoxidized prior to casting [12]. THERMODYNAMICS OF DEOXIDATION The role of deoxidation process is to lower the oxygen content in liquid steel. Deoxidation is commonly carried out by additions of elements having greater affinity for oxygen than iron, this method is also known as precipitation deoxidation [17]. The oxygen affinity of various elements can be compared with free energy of oxide formation. Figure: 2.1.2 gives a plot of curves for common elements found in steelmaking. While elements having free energy of oxide formation lower than FeO are potential candidates as deoxidizers, it is also important to consider that activity of these elements in solution with liquid steel deviates from that of the pure elements. Figure 2.1.4 depicts the deoxidizing power of various elements at 1600 ‘C

11

Figure 2.3: Free energy of formation for various oxides. Dash-dot line indicates equal oxygen pressure in unit of atm [4].

Figure 2.4: Deoxidizing power of various elements at 1600 ‘C [5] 12

SINGLE COMPONENT DEOXIDATION

Four cost-effective deoxidizers are carbon, manganese, silicon, and aluminum. Carbon is often c o n s i d e r e d a n e f f e c t i v e d e o x i d a t i o n e l e m e n t , f o r m i n g g a s e o u s d e o x i d a t i o n products. Carbon deoxidation does not generate inclusions and therefore will not be discussed further, however, during the casting process, carbon in liquid steel may reduce oxide inclusions resulting in gas formation and pinhole porosity [Kiessling and Lange et al. 1978][6].. A general deoxidation reaction can be described using Equation 2-2, where x and y are stoichiometric terms, M is the dissolved deoxidizer, O is oxygen. x[M]steel + y[O]steel = (MxOy)

[2-2]

MANGANESE DEOXIDATION Manganese, in pure form, is rarely utilized as a deoxidizer. Mn is often introduced to steel in the form of low C or high C ferroalloy. Mn and Fe will both participate in the deoxidation reaction forming MnO-FeO product in liquid or solid solutions. A detailed study by [Lismer and Pickering] [7] has revealed that Mn deoxidation products are typically small and homogeneously distributed in the steel and the morphology of this inclusion type is mostly influenced by the MnO-FeO ratio. For inclusions with MnO content of up to 30%, the morphology was globular single-phase or sometimes dual-phase spheres. These inclusions rich in FeO had solidified after the matrix steel was solid. On the other hand, for steel containing more than 0.7%Mn, it was found that the deoxidation products are mostly pure MnO. Nearly pure MnO inclusions, having higher melting temperature than steel, would solidify before steel, and therefore are characterized by a dendritic structure. The manganese deoxidation reaction, [Mn] + [O] = (MnO)

[2-3] 13

and corresponding equilibrium constant equation,

%Mn %O 12440

For

5.33

[2-4]

[2-5]

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

%Mn %O

= 4.88 x 10-2 at 1600ºC

[2-6]

SILICON DEOXIDATION It can be seen from Figure 2.1.4, silicon has a much-improved deoxidizing power compared with manganese. Deoxidation with pure silicon will yield either liquid iron silicates or solid silicon oxide as reaction products at steelmaking temperature. Iron silicate inclusions, like many other silicates, are usually glassy in appearance and globular in morphology. Silicon oxides within steel exist in several modifications as a result of various possible spatial arrangements of the SiO2 tetrahedral molecules. Low quartz, high-quartz, tridymite, and cristobalite are among the common modifications [Kiessling and Lange et al. 1978]

[6]

.where tridymite and cristobalite are high temperature

modifications of silica. Due to similar structures, low quartz-high quartz transformation as well as tridymite-cristobalite transformation are fast and can be easily reversed. However, the transformation between quartz and tridymite or cristobalite is a much slower process as the energy associated with breaking the tetrahedral bonds are greater. The given reaction time and temperature during ladle treatment are inadequate for the transformation of quartz to tridymite or cristobalite to reach completion. On the contrary, tridymite and cristobalite, often formed as deoxidation product, do not transform to quartz within the time-frame of subsequent cooling and casting of steel. Therefore, the type of modification and composition can be utilized as indicators for assessing silica inclusion’s origin. 14

The silicon deoxidation reaction, [2-7]

[Si] + 2[O] = SiO2 (s) and corresponding equilibrium constant equation,

[2-8]

%

%

30000 T For

2

11.5

[2-9]

= 1, the value of the equilibrium constant for silicon deoxidation is =

%

%

= 2.26 x 10-5 at 1600ºC

[2-10]

ALUMINUM DEOXIDATION From Figure 2.1.4, it is clear that Aluminum is one of the most effective deoxidizers used for steel deoxidation. In aluminum deoxidized steel, there are generally two species of deoxidation products: solid hercynite (FeO-Al2O3 spinel) and solid corundum (Al2O3, Imodification). Among the two deoxidation products, corundum is the dominant species found in steel. Corundum phase is characterized by having unique faceted shapes and relative smaller diameter as single particles. It has been reported by [ Rege et al]. [8] that Al2O3, during deoxidation, follows dendritic growth pattern as shown in Figure 2-4. For steels deoxidized solely with aluminum, Į-Al2O3 products are formed; clusters of these particles tend to remain as inclusions in steel. Corundum inclusions, usually having the particle size of 1 to 5 Pm, have a tendency to agglomerate upon colliding with one another in order to lower the overall contact area with molten steel and therefore effectively stabilize the entire unit by minimizing the surface energy. [Kiessling and Lange et al. 1978] [6]

15

Figure 2.5: a) As-polished (2-dimensional) steel sample showing Al2O3 dendrite b) Partial slime extracted (3-dimensional) steel sample showing the same Al2O3 dendrite [1] Solid deoxidation products are often associated with nozzle clogging during casting of liquid steel. This phenomenon is mainly caused by solid alumina inclusions having high contact angles with liquid steel; therefore, alumina inclusions will readily anchor onto refractory surfaces followed by subsequent agglomeration of inclusions. Indigenous inclusions from aluminum deoxidation may take on different morphology depending on the generation mechanism. There are generally three Al2O3 inclusion generation processes: I.

Nucleation by super-saturation:

Al2O3 inclusions nucleate homogeneously in the steel bath as a result of super- saturation. The resulting inclusions are finely dispersed corundum clusters [Kiessling and Lange et al. 1978] [6]

II.

Nucleation and growth on existing nuclei:

The existing nuclei can be both indigenous and exogenous in nature. Manganese and silicon deoxidation products as well as emulsified furnace slag and eroded refractories can serve as low-energy sites for Al2O3 inclusions to nucleate without reaching super-saturation in the bath. 16

III. R e a c t i o n between aluminum metal and oxygen: Excess aluminum addition or poor homogenization of the bath can lead to local high concentration of aluminum metal reacting with dissolved oxygen. Reactions that occur under localized superheat may reach the melting point of Al2O3; therefore the products are partly molten Al2O3 inclusions sometimes having glassy appearance. The aluminum deoxidation reaction, 2[Al] + 3[O] = Al2O3 (s)

[2-11]

and corresponding equilibrium constant equation,

%

62780 T For

2 3

[2-12]

% 20.5

[2-13]

= 1, the value of the equilibrium constant for aluminum deoxidation is =

%

%

= 9.58 x 10-14 at 1600ºC

[2-14]

MULTI-COMPONENT DEOXIDATION In conventional ladle deoxidation, a combination of deoxidizers are utilized to achieve improved deoxidation result, giving much lower residual oxygen in the bath. It is a common practice to perform partial deoxidation while filling the tap ladle followed by final killing of steel with aluminum at the ladle furnace station. This practice has many advantages: (1) promotes the formation of low-melting-point deoxidation products with ease of removal from the melt; (2) improves the solubility of elements having relative high vapor pressure such as calcium and magnesium; (3) minimizes nitrogen pick-up during furnace tapping[4]. 17

SILICON-MANGANESE PARTIAL DEOXIDATION

Figure 2.6: Equilibrium relations for manganese-silicon deoxidation of steel at various temperatures [3] The practice of tap ladle deoxidation can effectively improve the extent of deoxidation and at the same time minimize aluminum deoxidizer additions. Two general types of deoxidation products may result from Si-Mn deoxidation: solid silica and liquid manganese silicate at the steelmaking temperature. Under the influence of increasing manganese content, the activity of silica is lowered. As the activity of silica decreases, deoxidation products deviate from pure silica to molten manganese silicate. It was suggested by [ Turkdogan, 1996][3] that there exist critical ratios of [%Si]/[%Mn]2 at a given temperature, which govern the type of deoxidation products formed. As shown in Figure 2.1.6, for steel compositions left of the curve, the deoxidation products will be solid silica which indicates the absence of manganese participation in the reaction. On the other hand, for liquid steel containing higher manganese content (right of the curve) the primary deoxidation products are likely to be liquid manganese silicate. 18

The equilibrium reaction governing Mn/Si deoxidation,

[Si] + 2MnO = 2[Mn] + SiO2

[2-15]

and corresponding equilibrium constant equation,

% %

1510 T

.

[2-16]

1.27

[2-17]

The Mn/Si deoxidation products are typically found to be globular and glassy in appearance along with silica or rhodonite precipitation within the matrix of manganese silicate. To facilitate the removal of deoxidation products, manganese is added as an inclusion modifier yield liquid manganese silicates for improved coalescence and flotation to the slag layer.

MANGANESE-SILICON-ALUMINUM DEOXIDATION In modern practice, it is common to charge deoxidizers into the tapping ladle during ladle filling. The charge deoxidizers often consist of all three deoxidizers; manganese and silicon in the form of ferromanganese, ferrosilicon, or silicomanganese, as well as aluminum. The p h a s e s o f r e s u l t i n g d e o x i d a t i o n p r o d u c t s d e p e n d h e a v i l y o n s t e e l chemistry and reaction temperature as illustrated in Figure 2.1.7. In the absence of manganese, only solid phases such as silica, alumina and mullite are possible. On the other hand, with manganese participating in steel deoxidation, the fourth phase - liquid manganese silicate becomes stable; the stability range of liquid manganese silicate also increases with increasing manganese content.

19

Figure 2.7: The effect of manganese content on stability of oxide phases resulting from steel deoxidation at 1550ºC (m: mullite; l: liquid manganese silicate) [9] Liquid silicates, in this deoxidation process, are characterized by an aluminum-rich core and a shell of gradual increase in MnO-SiO2 content towards steel-inclusion interface. The outer glassy MnO-Al2O3-SiO2 matrix, in metastable condition, was often found to precipitate phases such as mullite, galaxite, and corundum lathes upon cooling in solid state. These precipitates can nucleate easily on small steel particles or solidified slag droplets within the inclusion. CALCIUM MODIFICATION From Figure 2.1.3, it can be seen that calcium has a strong affinity to oxygen and could potentially be utilized as steel deoxidizer. The challenge, however, lies in the following properties of calcium: low boiling point (1439ºC), limited solubility in steel (0.032% Ca at 1600ºC), and high vapor pressure at 1600ºC (1.81atm) [OTOTANI et al. 1986] [10]. Due to these reasons, it is rather difficult to introduce calcium to molten steel in its metallic form, and it is usually added as various iron-containing Ca-Si alloys. The primary deoxidation products are therefore calcium silicates, which may also contain other oxides. When combinations of Ca and Al or Mn/Si deoxidation are carried out, the primary 20

deoxidation products can be modified to oxides with lower activity and hence improve the removal of dissolved oxygen. By converting the solid alumina inclusions to liquid calcium aluminates, the extent of deoxidation can be improved from 8-10ppm O to 1ppm O in Alkilled steel (0.05% Al)[S MILLMAN, 2004] [9]. With a CaO:Al2O3 ratio of 12:7, calcium treated Al2O3 can reach a melting point of 1360ºC at the CaO-Al2O3 eutectic (Figure 2.1.8) and therefore exists in the liquid state at steelmaking temperatures. Moreover, there exist five modifications of calcium aluminates as indicated in Figure 2.1.8; 12CaOx7Al2O3, 3CaOxAl2O3 and CaOxAl2O3 are liquid, while CaOx2Al2O3 and CaOx6Al2O3 are solid at steelmaking temperatures.

Figure 2.8: CaO-Al2O3 equilibrium phase diagram. [19] Instead of agglomerating, in alumina inclusions, liquid calcium aluminates will coalesce upon contact due to better wetting with liquid steel and will not easily attach onto refractory surfaces. Hence, solid deoxidation products can also be calcium treated so that the steel casting process is clogging-free.

21

MANGANESE OXIDE – SILICON OXIDE – ALUMINUM OXIDE SYSTEM The MnO-SiO2-Al2O3 system effectively covers most of relevant inclusion phases that result from combination of Mn, Si, and Al deoxidation. Figure 2.1.9 summarizes many complex inclusions having compositions made up of various SiO2, MnO, and Al2O3 primary oxide contents. It is important to note that each inclusion species will have its own homogeneity range in addition to stoichiometric compositions listed in Table 2-1.

Figure 2.9: Schematic representation of MnO-SiO2-Al2O3 ternary phase diagram[6] Other inclusion systems such as FeO-SiO2-Al2O3 and MnO-SiO2-Cr2O3 share many similarities with the MnO-SiO2-Al2O3 system. Considerable numbers of MnO-SiO2Al2O3 inclusion phases exist with complete or part substitution of MnO with FeO due to wide range of solid solubility; with the exception of FeO-SiO2 (counterpart to MnO22

SiO2), which has yet to be reported as an inclusion phase in the literature. According to Figure 2.1.3, manganese has a stronger affinity for oxygen than iron and therefore it is also common to find MnO among inclusions belonging to the FeO-SiO2-Al2O3 system. On the other hand, Al2O3 and Cr2O3 are interchangeable at elevated temperatures due to their structural resemblance. Corresponding inclusion phases were often reported in both MnOSiO2-Al2O3 and MnO-SiO2-Cr2O3 with notable difference in the absence of ternary phases in the MnO-SiO2-Cr2O3 system[SOLMAN AND EVANS, 1951][5]. Corresponding phases relating to MnO-SiO2-Al2O3, FeO-SiO2-Al2O3, and MnO-SiO2-Cr2O3.

Table 2-2: Stoichiometric composition of reported inclusion phases. [Kiessling and Lange et al. 1978][6] Mineral classification

Chemical formula

Stoichiometric composition (wt%) MnO

SiO2

Al2O3

Corundum

Al2O3

--

--

100

Cristobalite

SiO2

--

100

--

Tridymite

SiO2

--

100

--

Quartz

SiO2

--

100

--

Manganosite

MnO

100

--

--

Galaxite

MnO.Al2O3

41

--

59

Mullite

3Al2O3. SiO2

--

28

72

Rhodonite

MnO.SiO2

54

46

--

Tephroite

2MnO.SiO2

70

30

--

23

2.1.3 MORPHOLOGY OF NON-METALLIC INCLUSIONS:-



Globular shape of inclusions is preferable since their effect on the mechanical properties

of steel is moderate. Spherical shape of globular inclusions is a result of their formation in liquid state at low content of aluminum. Examples of globular inclusions are manganese sulfides and oxysulfides formed during solidification in the spaces between the dendrite arms, iron aluminates and silicates. 

Platelet shaped inclusions. Steels deoxidized by aluminum contain manganese sulfides

and oxysulfides in form of thin films (platelets) located along the steel grain boundaries. Such inclusions are formed as a result of eutectic transformation during solidification. Platelet shaped inclusions are most undesirable. They considerably weaken the grain boundaries and exert adverse effect on the mechanical properties particularly in hot state (hot shortness). 

Dendrite shaped inclusions. Excessive amount of strong deoxidizer (aluminum) results

in formation of dendrite shaped oxide and sulfide inclusions (separate and aggregated). These inclusions have melting point higher than that of steel. Sharp edges and corners of the dendrite shaped inclusions may cause local concentration of internal stress, which considerably decrease of ductility, toughness and fatigue strength of the steel part. 

Polyhedral inclusions. Morphology of dendrite shaped inclusions may be improved by

addition (after deep deoxidation by aluminum) of small amounts of rare earth (Ce,La) or alkaline earth (Ca, Mg) elements. Due to their more globular shape polyhedral inclusions exert less effect on the steel properties than dendrite shape inclusions.

24

Fig. 2.10: Morphology of NMI’s occurred in steel [Kiessling and Lange et al. 1978][6]. 2.1.4 INFLUENCE OF INCLUSIONS ON THE PROPERTIES OF STEEL The properties that are adversely affected are fracture toughness, impact properties, fatigue strength, and hot workability. The factors responsible for these may be classified as follows: 1. Geometrical factors: size, shape (may be designated as the ratio of major axis to minor axis), size distribution, and total volume fraction of inclusions. 2. Property factors: deformability and modulus of elasticity at various temperatures, coefficient of thermal expansion From a fundamental point of view, an inclusion/matrix interface has a mismatch. This causes local stress concentration around it. Application of external forces during working or service can augment it. If the local stress becomes high, then microcracks develop. The propagation of microcracks leads to fracture. Investigations have established that only large inclusions are capable of doing this kind of damage, and this led Kiessling [6] to develop the idea of critical size. In practice, it is customary to divide inclusions by size into macroinclusions and 25

microinclusions. Macroinclusions ought to be eliminated because of their harmful effects. However, the presence of microinclusions can be tolerated, since they do not necessarily have a harmful effect on the properties of steel and can even be beneficial. They can, for example, restrict grain growth, increase yield strength and hardness, and act as nuclei for the precipitation of carbides, nitrides, etc. The critical inclusion size is not fixed but depends on many factors, including service requirements. Broadly speaking, it is in the range of 5 to 500 μm (5 × 10–3 to 0.5 mm). [19] It decreases with an increase in yield stress. In high-strength steels, its size will be very small. Kiessling advocated the use of fracture mechanics concepts for theoretical estimation of the critical size for a specific situation. The objective, therefore, should be to produce steel that does not contain any macroinclusion (i.e., above the critical size). Technologically, this is difficult to achieve without escalating the cost to a high level. Therefore, we have to put up with some macroinclusions, and in this context we have to determine how to reduce their harmful effects by controlling their size, shape, and properties. This is known as inclusion modification, and to carry it out, we first have to know how various factors connected with inclusions affect the properties of steel. To sum up the effects, the following statements may be made: 1. Impact properties are adversely affected with an increase in volume fraction as well as inclusion length; spherical inclusions are better. Brittle inclusions or inclusions that have low bond strength with the matrix break up early during straining, with the initiation of voids at the inclusion/matrix interface. 2. The fatigue strength of high-strength steel is reduced by surface and subsurface inclusions, especially those that have lower coefficients of thermal expansion than steel. These set up stresses in the matrix and are primarily responsible for fatigue failure. 3. The hot workability of steel is affected by the low deformability of inclusions (i.e., more brittleness at hot working temperatures). 4. Anisotropy of a property is caused by orientation of elongated inclusions along the direction of working or the elongation of inclusions during working. 5. Macroinclusions of sulfides are desirable for better steel machining properties.

26

2.1.5 NON-METALLIC INCLUSIONS DURING INDUSTRIAL PRACTICE AND THEIR CONTROL @ JSPL There are generally two sources of inclusions in steel: exogenous, indigenous. •

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

entrainment and refractory erosion. Although exogenous inclusions are generally more harmful than indigenous inclusions, simple detection methods (due to larger size) as well as fewer occurrences have reduced the concern for exogenous inclusions significantly [12]. In addition, with careful control of stirring and flowrate monitoring, the amount of exogenous inclusions can be minimized. •

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

reactions between dissolved species in the steel bath and are generally smaller in size [12]. Deoxidation products originate from the reaction between dissolved oxygen and added deoxidant and can be both solid and liquid at steelmaking temperatures. The presence of a few large indigenous inclusions has a strong effect on the properties of steel products. Indigenous inclusions often go through a series of transformations as the steel cools from 1600°C to room temperature.[19] While trying to maintain equilibrium with the surroundings, inclusions may be undercooled during some steps of the treatment and result in amorphous phases, or solidify and take the form of supersaturated solid solution. Indigenous inclusions can therefore be categorized into formation steps, as summarized below: I. Primary inclusions: generated during deoxidation reaction II. Secondary inclusions: generated due to equilibrium shift as temperature decreases during vessel transfer, such as tapping and teeming operations III. Tertiary inclusions: generated during the process of solidification, usually characterized by rapid cooling IV. Quaternary inclusions: generated during solid state phase transformation, which causes changes in solubility limits of various constituent.

27

Exogenous inclusions are the real cause of concern during continuous casting, arise primarily from the incidental chemical (re-oxidation) and mechanical interaction of liquid steel with its surroundings (slag entrainment and erosion of lining refractory) [TURKDOGAN, 1996][3]. Air is the most common source of re-oxidation, which comes into contact with molten metal during casting when it is poured from ladle to tundish and tundish to mold.

SOURCES OF EXOGENOUS INCLUSIONS For continuous casting process, the following factors affect slag entrainment into the molten steel:  Vortexing effect in tundish during end of casting results slag entrainment into the solidified strand.  Emulsification and slag entrainment at the top surface especially under gas stirring above a critical gas flow rate.  Turbulence at the meniscus in the mold. Severe mould level fluctuation also leads to mould powder entrapment into solidified strands. The process of mould Slag entrapment due to level fluctuation illustrated in Fig 2.1.11  Erosion of refractories, including well block sand, loose dirt, broken refractory brickwork and ceramic lining particles, is a very common source of large exogenous inclusions which are typically solid and heavier in nature. These particles flushed out with liquid metal and got entrapped into the solidified strands.

Fig. 2.11: Schematic representation of mold powder entrapment [3] 28

To avoid such occurrences following steps are adopted during continuous casting: Metal in ladle is fully covered with ladle covering compound like Radex and the ladle is also covered with lid during casting to minimize heat loss and gaseous entrapment.  Starting of the casting is considered to be the most unsteady state of casting. Tundish level also gone down, if the next ladle in sequence could not open without free opening. Without Free opening cases are the most vital sources of inclusion due to re-oxidation of steel due to use of oxygen to open the ladle nozzles and casting without ladle shroud. Free opening of the ladles are also monitored regularly to avoid such occurrences. Special type of pre-heated Zirconia based Nozzlex powders are used to ensure Free Opening of the ladle [16].  Al2O3 base ladle shroud is used with Argon shrouding between ladles to tundish. Shroud submergence depth is ensured >150mm to avoid opening of eye during Ar shrouding.[16] Shroud straightness and Argon flow rate are important parameters, which are monitored continuously to avoid air ingression from joints and slag eye formation. Hydraulic shroud manipulator assembly is installed in shrouding system for tight sealing of the shrouds and it helps to minimize nitrogen pick up during casting. The study reveals average pick up of 4.05.0 ppm nitrogen from ladle to final steel, which is an indicator of minimal re-oxidation of steel. Special gaskets are also being used at the joints of shrouding to avoid air ingression. Any abnormal conditions results excessive re-oxidation followed by formation of large indigenous inclusions and nitrogen pick up in final steel.  Auto Mould Level Controllers are in place in all the casters to take care of mould level fluctuations during casting operation to avoid mould slag entrainment.  Tundish levels are also maintained at a constant level throughout the casting duration to avoid vortexing of slag. Even at the end of the casting and during sequencing the efforts are made to keep the tundish level constant.

29

 Flow control in the tundish is the key to the production of clean steel. Different types of flow modifiers are used in the tundish after doing mathematical modeling and water modeling of the tundish a. Pouring box b.Expendables and permanent dams c. Weirs and d.Slotted dams.  Different combinations of pouring boxes and permanent dams are used for different tundish at JSPL. These flow modifiers are invariably employed to protect excessive weir of tundish refractory, dampen turbulence in the shrouding areas and to provide directional flow of metal in order to provide nearly identical residence time to all strands in multi-strand tundish. Pouring boxes helps in upward directional flow supports inclusion floatation and assimilation into tundish slag. A rigorous Water Modeling study and mathematical modeling was conducted for slab caster and Combination caster tundish to improve yield and cleanliness of steel. Fig 2.1.12 illustrated the modified design of the slab caster tundish with use of different type of furnitures for flow modification.

Fig 2.12. Schematic drawing of Slab caster tundish furniture

30

The inclusion rating of the collected samples from tundish before and after modification clearly shows improvement in steel cleanliness after incorporation of the pouring box in the slab caster tundish. Table 2-2 illustrated the inclusion level before and after modification of slab caster tundish.

Table 2-3: Inclusion distribution characteristics in solidified slab samples collected from original and modified design tundish operations

2.2 CLEAN STEEL 2.2.1 Clean Steel: Role of Secondary Refining The cleanliness of steel depends right from selection of charge mix, primary refining process, killing practices and subsequently on secondary refining process [19]. Secondary refining alone cannot be the process, which can helps in producing Clean Steel. It is a combination of all the processes with stringent quality standards and SOP’s at every stages of steel making, right from selection of input raw material to end of casting decide the final quality of the steel. Steel cleanliness is a widely spread area and secondary refining only plays a part of the entire process for production of clean steel. 31

2.2.2 ROLE OF TAPPING ADDITION ON STEEL CLEANLINESS At JSPL, first the SOP’s are made for all the areas from EAF to caster for finalization of the procedure to be followed for the production of steel. For clean steel, selective charge mix are designed for the Electric Arc Furnace. The quantity of coal based DRI is reduced by design and % of Hot metal and HBI is increased proportionately. The in-built Eccentric Bottom tapping facility in EAF helps in 100% slag free tapping. The grade specific tapping additions are designed for initial killing of the bath. It is planned for 80% additions of the major ferroalloys must be completed during tapping itself. In addition to this freshly prepared lime also added during tapping for initial slag formation and for effective desulphurization. During tapping Si-Mn, Al ingots, pre-conditioned Synthetic slag and lime is added. To give a preferential Carbon boil 100 kg of CPC also added at the bottom of the ladle just before tapping. Mild purging with Argon after tapping carried out to ensure minimum air entrapment. The basic objective of controlled tapping addition is to lower down oxygen potential at opening of secondary refining for ensuring effective desulphurization and to reduce total processing time.

2.2.3 SALIENT STEPS ADOPTED DURING SECONDARY REFINING FOR STEEL CLEANLINESS The tapping additions are designed in such a fashion that during secondary refining only trimming additions are required to achieve the aim chemistry. Trimming additions were carried out in the initial period of processing along with vigorous purging for effective desulphurization Addition of lime is restricted to 2-3 kg/ton during secondary refining to avoid unwanted Hydrogen pick up in steel. The opening Aluminum is maintained around 32

0.04-0.06% during start of secondary processing to avoid further additions Aluminum in subsequent process. Calcium Silicide treatment is carried out at the end of processing to achieve a minimum Ca/Al ratio of 0.08 which ensure formation and subsequent floatation of Calcium aluminates. Mild Argon rinsing without opening of slag eye for minimum three minutes at the end of processing is ensured after Calcium silicide treatment. This helps in effective slag metal interaction for removal of inclusion from steel. To increase the inclusion absorption capacity of slag, (FeO + MnO) % is monitored in slag and it is maintained below 1.0%. For effective desulphurization the slag basicity also is maintained at 2.5 - 4.0 at end of secondary refining. Oxygen potential in final steel is considered to be an indirect measure of steel cleanliness.

2.2.4 SALIENT STEPS ADOPTED DURING VACUUM DEGASSING FOR STEEL CLEANLINESS The steel cleanliness is largely depends on inclusion level in final steel and final gaseous content in final steel is considered to be an indirect measure of steel cleanliness. JSPL is having the facilities of Vacuum Tank degasser and RH degasser both in steel manufacturing units. Depending on customer requirements and based on the end application of the steel process route is decided. For critical applications like wire drawing, Forging, Line pipes, Seamless pipes, Boiler grades, Fasteners grades and Automobile grades are routed through vacuum degassing. The steel is hold under vacuum level at < 1.0 mbar for min 10 minutes to achieve the favorable gaseous level and inclusion level in steel [17]. For Vacuum degassed heats after degassing Calcium silicide treatment is carried out followed by mild rinsing for three minutes for effective floatation of the inclusion [19]. During mild rinsing it is ensured that slag eye should not be opened. Slag basicity is maintained 3.0-4.0 and (FeO+MnO) % 33

in slag positively maintained below 1.0%. The Celox reading for dissolved oxygen for vacuum degassed heats aimed at 4.0 ppm max.

2.2.5

CLEAN STEEL: ROLE OF CONTINUOUS CASTING

Non-metallic inclusions are the most significant cause of concern in cast steels which can lead to field failures. Mechanical behavior of steel is controlled to a large extent by the volume fraction, size distribution, composition and morphology of inclusions and precipitates, which act as stress raisers. The inclusion size distribution is particularly important, because large macro-inclusions are the most harmful to mechanical properties though the large inclusions are far outnumbered by the small ones, their total volume fraction may be larger [19]. Ductility & impact toughness is appreciably decreased by increasing amounts of oxide or sulphide inclusions. Inclusions also lower resistance to Hydrogen Induced Cracks. The source of most fatigue problems in bearing steel are hard and brittle oxides, especially large alumina particles over 30μm

[18]

. The rest of this report is an

extensive review on sources of inclusions during continuous casting, their morphology, and sources of gaseous ingression in steel during casting. This also describes in detail about various measures adopted during Continuous Casting to avoid the occurrences of the above problems.

34

CHAPTER – 3 EXPERIMENTAL ASPECTS AND METHODOLOGY

35

3.1 OVERVIEW

The main purpose of this study was to characterize the non-metallic inclusions found in high strength low alloy steel for structural applications and to track the development of inclusions throughout the melting and casting operations. To do this, the experimental approach was divided into two parts: qualitative and quantitative aspects. Qualitative assessment involves inclusion morphology examination and inclusion type determination by sample preparation and analytical techniques such as scanning electron microscope (SEM) and energy dispersive x-ray spectroscopy (EDS). Quantitative assessment involves the inclusion detection and size determination, which ultimately leads to the construction of inclusion particle size distribution by image analysis method. The experimental approaches are summarized in fig 3-1

36

3.2 QUANTITATIVE ASSESSMENT A complete assessment of steel cleanliness not only consists of qualitative information, but also quantitative information such as inclusion length, inclusion width, number of inclusion per unit area, volume fraction, mean free path, etc. Using as-polished metal samples, quantitative assessment involves a combination of a microscopic technique to provide images of the sample surface (image acquisition) and an image analysis system to accurately measure the inclusion size. 3.2.1 IMAGE ACQUISITION Image acquisition is a crucial part in the process of quantitative analysis. The ideal technique for providing images of the sample surface must offer accurate representation of inclusion distribution. Analytical instruments involved in this research project consist of the following: 1. Light optical microscope 2. Laser confocal microscope 3. Scanning electron microscope

Figure 3.2: Light Optical Microscope @ JSPL 37

Light optical microscope: Prior to the advent of electron microscopy, light-optical microscopy was used to quantify and characterize inclusions based on morphology. The best-possible spatial resolution of a light-optical microscope, which is approximately 0.3μm, is limited by the fixed wavelength of light (λ ≈ 0.5μm) [ASTM, 2003][13]. As the magnification increases, the light intensity decreases, which results in darker image. Therefore it becomes rather difficult to utilize the best-possible resolution of light in a conventional light-optical microscope. Laser confocal microscope: The laser confocal microscope (LCM) distinguishes itself from conventional optical microscope and SEM in the following way: •

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

μm. Once the height information is obtained, quantitative surface area and volume measurement can then be calculated using the operating software. This technique is especially important for particle analysis of metallurgical samples such as isolated inclusions, etc. •

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

dimensional images comparable to that of SEM, but without the issues of charging in non-metallic areas of interest such as inclusions. LCM utilizes blue laser as the transmitting medium, which has a wavelength of 473nm. Therefore, when compared to light optical microscope, LCM offers a slightly improved lateral spatial resolution at approximately 200nm. Scanning electron microscope: SEM and EDS are among the most employed methods of inclusion investigation mainly due to the following advantages: high resolution, high sensitivity, quantifiability, minimal sample preparation and ease of operation. The secondary electron mode of a SEM provides an improved spatial resolution of 5~20 nm [15]. 38

Fig 3-3 Scanning Electron Microscope @ JSPL, Raigarh The three modes used are secondary electron (SE), backscattered electron (BSE) and EDS modes. Using the SE mode, the images formed are topographical representations of the specimen. Since secondary electrons have a very small escape depth, the signals received will reflect the surface structures of the specimen. However, using SE mode to locate inclusions in a polished sample, given the topography of the specimen is flat, will be rather difficult when inclusion size is small. The BSE mode, on the other hand, utilizes backscattered electrons to create images showing elemental contrast, thereby revealing the locations of non-ferrous inclusions in the ferrous matrix. BSE images are also able to provide information on the homogeneity of inclusions. In the current investigation, SE mode was used to image inclusions on polished and SPEED etched surfaces for inclusion morphology study. Inclusion type determination was performed by EDS mode simultaneously. For inclusion quantification, the BSE mode was used in conjunction with image analysis software.

39

3.2.2 IMAGE ANALYSIS

Figure 3.4 Image analyser attached with optical microscope Detection and discrimination of inclusions utilize the difference in gray level intensity between each inclusion species and the unetched matrix steel. Measurements are made based on counting the number of picture point elements (termed pixels) that satisfy the user-defined gray level threshold. The dimension of each image pixel is dependent on both microscope magnification setting and image resolution. The images for the purpose of quantitative analysis in this study are taken with the following parameters [13]: Magnification: 100X Image resolution: 512 X 676 pixel Dimension of each pixel: 1.742 μm/pixel Figure 3-4 shows images taken of the same sample area, using four image acquisition techniques: optical microscopy, laser confocal microscopy, SEM (SE mode) and SEM (BSE mode). Figure 3-4 (a)-(b) are examples where surface defects such as voids and gas holes due to solidification shrinkage, or limited hot ductility may be detected as oxide inclusions in optical microscopy and LCM images; because their gray level range is comparable to that of oxides. Other surface defects may also result from improper polishing techniques, creating excessive relief pits, voids and deep scratches. Figure 3-4 (SE mode), although reduced in number of surface defects, proved to be difficult in image analysis processing due to lack of contrast between inclusion and matrix steel. 40

Figure 3-5: Images acquired using (a) optical microscopy, (b) laser confocal microscopy, (c) SEM (secondary electron mode) and (d) SEM (backscattered electron mode) The presence of defects in acquired images shown in Figure 3-5 (a) and (b) can greatly affect the reliability of subsequent inclusion detection and measurement represented in Figure 3-6 (a), where the voids and scratches were identified as inclusions by the image analysis software. However, complete elimination or minimization of these defects at the image acquisition stage can be achieved using SEM under BSE imaging mode as shown in Figure 3-5 (d) and its respective image analysis result in Figure 3-6 (b). Thus, SEM- BSE is chosen as the most suitable image acquisition technique for the quantitative analysis of inclusions.

Figure 3-6: Photograph processed by image analysis showing detected area as inclusions (a) Laser confocal microscopy, (b) SEM (backscattered electron mode)

41

CHAPTER – 4 RESULT AND DISSCUSSION

42

4.1 INTRODUCTION STEEL CLEANLINESS OF RAILS: In order to obtain the satisfactory cleanliness of steel it is necessary to control and improve a wide range of operating practices throughout the steelmaking processes like deoxidant- and alloy additions, secondary metallurgy treatments, shrouding systems and casting practice. Table 4-1: The importance of clean steel with respect to mechanical properties of the product [12] Element Form S, O Sulfide and oxide inclusions

Mechanical Properties Affected



Ductility, Charpy impact value, anisotropy



Formability (elongation, reduction of area and bendability)



Cold forgeability, Drawability



Low temperature toughness

 C, N

Solid solution



Solid solubility (enhanced), hardenability

Settled dislocation



Strain aging (enhanced), ductility and toughness (lowered)

Pearlite and cementite



Dispersion (enhanced), ductility and toughness (lowered)

Carbide and nitride precipitates



Precipitation, grain refining (enhanced), toughness (enhanced)

 P

Fatigue strength

Solid solution

Embrittlement by intergranular precipitation



Solid solubility (enhanced), hardenability (enhanced)



Temper brittleness



Separation, secondary work embrittlement

Rail steel needs to conform to stringent quality standards described in the standards owing to its critical nature of its application. Chemical composition range of Grade 880, which is a common rail grade as per IRS-T12, is shown in Table 4-2.

43

Table 4-2: Chemical composition of Grade 880 rails as per IRS T-12 2009 specifications Grade

%C

%Mn

%Si

%S

%P

%Al

Grade

0.60-

0.80-

0.10-

0.03

0.03

0.015

880

0.80

1.30

0.50

max

max

max

%Nb -

H in ppm 1.6 max

Hydrogen in rail is restricted to a maximum of 1.6 ppm which makes degassing necessary. As far as inclusions are concerned, it is well known that they are detrimental to rails. IRS T12 2009 specifies that the inclusion rating level of rails, when examined as per IS: 4163, shall not be worse than 2.5 A, B, C, D thin or 2.0 A, B, C, D thick. EFFECT OF INCLUSIONS TO THE PHYSICAL CONTINUITY OF RAILS: Inclusions act as the barrier to the physical continuity of metal. The area in the vicinity of inclusion develops a local residual stress field; so that the initiation & propagation of crack gets driven. Fatigue is the result of progressive initiation & subsequent propagation of crack. Initiation is typically accepted to involve crack development- microcracks (size ranging from micrometer to millimetre) transforming into macro cracks (greater than millimetre, & up to as long as sizeable fraction of a metre). The really important crack dimension, which determines fatigue life, is penetration into the load bearing area. Initiation is dependent on slip processes, governed by cyclic shear stresses. Propagation is generally governed by cyclic tensile stresses & is caused by repeated plastic stretches & blunting at the crack tip. The classic explanation is that, when a flat crack is open by tensile stresses, stretching occurs normal to the crack tip, thereby advancing its position. In a generally compressive field, such as that under a wheel contact, early growth by shear is the only possible mechanism available to advance the crack. Later, under the influence of bulk bending stresses in the body of rail, the crack grows by tensile opening & closing. The extremely high contact stresses & the enormous power density (i.e the power passing through per unit) concentrated at the contact under the vertical loads, are enhanced by lateral (curving) longitudinal (traction & braking) loads. In these

circumstances, the initiation of crack is almost inevitable [21].

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Fig. 4.1 Force applied by a Wheel on Rail A wide variety of inclusion always exists in the rail steels of the composition shown in Table 2. The most common of which includes those of MnS, Al2O3 and SiO2. Large inelastic inclusions, such as those comprising of Ca, Al, Si and O tends to act as a nucleation site for crack growth below the surface of the rail head. These inclusions which are themselves brittle in nature; under the influence of stresses can shear in a brittle manner; thus leading to loss of serviceability. Rail industry has been constantly working in this regard to lower down the size & amount of inclusion prevailing. MnS inclusions can become crack initiators as they deform in a non-uniform manner to produce long thin inclusions. Studies reveal that MnS inclusions, present in the material are considerably elongated by the loading of the rail in service and contribute to spontaneous cracking, subsequently resulting in failure. [14] This study assesses the level and type of inclusions in rail steels produced at JSPL and tries to minimise the inclusion level by carrying out appropriate modifications in steel making & simultaneously carrying out the comparative study between VD & RH processed heat.

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4.2 EXPERIMENTAL PROCEDURE SAMPLE PREPARATION 4.2.1 A 20mmX20mmX10mm sample is cut from the standard location of the 60-100mm long rail sample, as per IS: 4163 by using Abrasive Cutter Machine. The polished area of the specimen shall be approximately 200mm2. It shall be parallel to the longitudinal axis of the product. It shall be located halfway between the outer surface and the center. 4.2.2 Rough filing is done on the surface to be polished by using stone grinder to remove the cut marks. 4.2.3 The specimen is polished by using coarse emery papers of size 240, 320, 400 to get the surface free from scratches. 4.2.4 Again it is polished by using fine emery papers of size 1/0, 2/0, 3/0 and 4/0 to get further smooth and scratch free surface. 4.2.5 Fine polishing of the rail sample is done by using Cloth Polishing Machine where the polishing media is Alumina powder to get mirror surface. Then it is washed with water and dried by using blower.

Fig 4-2 Sample images taken @ TSD, JSPL for inclusion rating

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DETERMINATION OF CONTENT OF INCLUSION 4.2.6

Inclusion content determination is done by using Optical Microscope at 100 magnification.

4.2.7

The following types of inclusion are determined in this method.

 Group A (Sulphide Type) – highly malleable, individual grey particles and generally rounded ends.  Group B Alumina - Numerous and non-deformable, angular, black or bluish particles (at least 3) aligned in the deformation direction.  Group C Silicate - highly malleable, individual black or dark grey particles and generally sharp ends.  Group D Globular Oxide – non deformable, angular or circular, black or bluish randomly distributed particle. 4.2.8

The image is projected on the ground glass and a clear plastic overlay is placed over the ground glass projection screen.

4.2.9

The image within the test square is compared with the standard chart diagrams of IS: 4163 Specification.

4.2.10 The entire polished surface is examined. Randomly any ten numbers of worst fields are chosen and each field is compared with the standard chart for each type of inclusion. 4.2.11 In each worst field, for each type of inclusion, total length of the inclusion is measured and corresponding severity number is noted down from the comparison chart of IS: 4163 specification

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4.2 RESULT Table 4-3 Inclusion Rating Results Heat ID

A type Thin

Thick

B type Thin

Thick

C type Thin

D type

Thick

Thin

1

1.5

0.5

1.0

2

1.0

-

0.5

Group A (SULPHIDE) (Thin) 1.5

Group B (ALUMINA) (Thin) 0.5

Group C (SILICATE) (Thin) -

Thick

Group D (OXIDE) (Thin) 1.0

To confirm that the inclusions are of sulphide type, SEM-EDS analysis was also carried out.

(a)

(b)

  Fig. 4.3: (a) SEM image of inclusion in Heat ID 1 at 799X magnification (b)EDS spectrum of point 3 shown in image 48

Fig. 4.3: Spectral imaging of inclusion in Heat ID 2 at 3210X magnification

SEM-EDS analysis confirms the results of inclusion rating and reveals that the inclusions are Manganese Sulphide (MnS) stringers. The control of sulphur and its associated level of sulphide inclusions in rail steel is a challenge in spite of RH-degassing. This can be attributed to the silicon killing practice adopted in rail steels and RH-degasser’s limitations for desulphurization understanding the effect of secondary refining parameters on desulphurization and inclusion removal.

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CONCLUSION AND FUTURE WORK

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CONCLUSION

 Presence of non-metallic inclusion can negatively affect both properties of product and subsequent processing.  Inclusions can come into steel from various sources main are deoxidation and refractory.  Inclusions can be classified depending on Source, Shape & their chemistry.  Oxides and sulphide are more detrimental for steel. In case of Al killed steel Al2O3 is major headache.  For Evaluation of steel Cleanliness it is necessary to combine several methods together.  Calcium Treatment is major tool for inclusion modification and flotation  Argon stirring improves floatation of inclusion.  Tundish metallurgy has big importance in steel cleanliness.  Mold is the last refining step where inclusions can be safely removed..  Inclusion size has the major effect on the fatigue properties.  The effect of an inclusion on the fatigue properties depends on its size, shape, thermal and elastic properties and its adhesion to the matrix.  Differences in the thermal expansion coefficients of the inclusion and the matrix can generate internal stresses around inclusions.  Four different image acquisition techniques were evaluated for the quantitative analysis of inclusions and it was found that SEM-backscattered electron imaging mode is the most suitable choice

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 Throughout the melting and casting operations, inclusion species tend to develop from simple primary oxides to complex binary and ternary oxides. With reoxidation minimized by gas shrouding between ladle and tundish, steel cleanliness improvements were achieved.

FUTURE WORK  Correlate the development of inclusion composition and count in the furnace, ladle, tundish and mold slags with inclusions found at each respective steelmaking vessel.  Aluminium oxide precipitates are formed during fast cooling of the liquid steel. The question arises whether these precipitates may act as nuclei for iron solidification and thus enable control of the steel microstructure in certain (future) conditions.  Development of automatic/online inclusion behavior and assessment technology during processing and production of steel

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REFERENCES

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REFERENCES [1]

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

Cleanliness – Review”, ISIJ International, 2003, vol. 43, no. 3, pp. 271–291 [2]

http://www.matter.org.uk/steelmatter/casting.htm, “Entrapment of non-

metallic inclusions”, Corus Corp. and Matter, date accessed: June 16, 2009 [3]

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

1996, pp. 111-113

[4]

A. Muan and E.F. Osborn, Phase Equilibria Among Oxides in Steelmaking,

Addison- Wesley, Reading, Mass., USA, 1965, p. 4 [5]

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

[6]

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

Materials (London), 1978, vol. 2, pp. 13-50 [7]

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

[8]

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

Clusters in Aluminum-Killed Low-Carbon Steel", Met. Trans., AIME, 1970, vol. 1, no. 9, pp. 2652-2653 [9]

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

Clean Steel, International Iron and Steel Institute, Belgium, 2004, pp. 39-60 T. [10]

Ototani, Calcium Clean Steel, Springer-Verlag, New York, 1986, pp. 2-9.

[11]

Tupkary R.H.: “Introduction To Modern Steel Making” Khanna Publisher, Delhi 7th

Edition 4th reprint 2012, Pp. 63-69, [12] R. Kiessling And N. Lange, Non-Metallic Inclusions In Steel, The Institute Of Materials (London), 1978 Pp. 100-105 [13] Astm International, “E45 Test Methods For Determining The Inclusion Content Of Steel”, Annual Book Of Astm Standards, Astm, Philadelphia, Usa, 2003, Vol. 03

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[14] http://www.matter.org.uk/steelmatter/casting.htm, “Entrapment of Non- Metallic Inclusions”, Corus Corp. And Matter, Date Accessed: June 16, 2015 [15] W.D. Callister, Materials Science And Engineering – An Introduction, John Wiley & Sons, Inc., New York, Ny, 2003, 6th Edition, P.336 [16] S. K. Choudhary, “Influence of Modified Casting Practice on Steel Cleanliness,” ISIJ International, vol. 51, no. 4, pp. 557–565, 2011 [17] J. Björklund, Thermodynamic Aspects on Inclusion Composition and Oxygen Activity during Ladle Treatment, no. April. 2008, Pp. 56-64 [18]

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

Chart II for quantitative measurements”, Clean Steel 3 Conference Proceedings, The Institute of Metals (London), 1987, pp. 60-67 [19] A. Ghosh: Secondary steelmaking: principles and applications, CRC Press, Boka Raton, FL, (2000). Pp. 248-290 [20] R. Kiessling and N. Lange, Non-Metallic Inclusions in Steel, The Institute of Materials (London), 1978, vol. 2 Pp. 40-85 [21] R. Dekkers: Non-metallic inclusions in liquid steel, Ph. D. Thesis, Katholieke Universiteit Leuven, Leuven, (2002).

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