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

II

DECLARATION BY THE CANDIDATE

I the undersigned solemnly declare that the report of the thesis work entitled Study of the non-

metallic 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.

III

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.

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

_____________________________________________

REGISTRAR

CHHATTISGARH SWAMI VIVEKANAND TECHNICAL UNIVERSITY

NORTH AVENUE SEC 8, BHILAI, CHHATTISGARH

___________________

(SUPERVISOR)

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

IV

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:

V

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

VI

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.

VII

LIST OF ABBREVIATIONS

Symbols Units

d Maximum particle size m G Free energy of formation kCal K Equilibrium constant

T Temperature K

T[O] Total oxygen ppm

Wavelength m Coefficient of thermal expansion K-1

Element Abbreviations

Al Aluminum C Carbon Ca Calcium Cu Copper Cr Chromium Fe Iron O Oxygen P Phosphorus Pt Platinum Mg Magnesium Mn Manganese N Nitrogen Ni Nickel S Sulphur Si Silicon

Abbreviations

ASTM American Society for Testing and Materials BSE Backscattered Electron DIC Differential Interference Contrast EAF Electric Arc Furnace EDS Energy Dispersive Spectrometry

Compound Abbreviations Al2O3 Alumina CaO Calcia CaOAl2O3 Calcium aluminate CaO SiO2 Calcium silicate CaS Calcium sulphide FeO Wstite FeOAl2O3 Hercynite FeS Troilite MgO Periclase MnO Manganosite MgOAl2O3 Spinel MnOAl2O3 Galaxite MnOSiO2 Rhodonite MnS Manganese sulphide SiO2 Silica

VIII

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

IX

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 1550C (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 NMIs 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

X

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

XI

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

XII

TABLE OF CONTENT:

CHAPTER 1 INTRODUCTION

1.1. Need for the Work 1.2. Clean steel 1.3. Non-Metallic Inclusions, definition & role

CHAPTER 2 LITERATURE REVIEW

2.1 ) Non-metallic inclusions in steel 2.1.1 Classification & Sources of nonmetallic inclusions 2.1.2 Formation of nonmetallic inclusions 2.1.3 Morphology of nonmetallic inclusions 2.1.4 Influence of inclusions on the properties of steel 2.1.5 Non-metallic inclusions during industrial practice and their control

2.2 ) Clean steel 2.2.1 Role of secondary refining on steel cleanliness 2.2.2 Role of Tapping addition on steel cleanliness 2.2.3 Salient steps adopted during secondary refining for Steel Cleanliness 2.2.4 Salient steps adopted during Vacuum Degassing for steel cleanliness 2.2.5 Role of continuous casting

CHAPTER 3 EXPERIMENTAL ASPECTS AND METHODOLOGY

3.1) Overview

3.2) Quantitative Assessment

3.2.1 Image Acquisition

3.2.2 Image Analysis

CHAPTER 4 RESULT AND DISCUSSION

4.1) Introduction

4.2) Experimental procedure

4.3) Result

1

2

3

4

5

6 - 9

10-23

24

25

27

31

31

32

32

33

34

35

36

37

38

40

42

43

46

48

XIII

CHAPTER 5 CONCLUSION AND SCOPE OF FURTHER WORK

REFRENCES

ANNEXURE ATTACHED -

EXTENDED SUMMARY

IS 4163 2004

50

53

1

CHAPTER 1

INTRODUCTION

2

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. Dekkers 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.

3

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

4

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 NMIS: - 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.

5

CHAPTER 2

LITERATURE REVIEW

6

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]

7

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

8

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 AOB2O3, where metal A has +2 oxidation

number and metal B has +3 oxidation number. Some common examples are FeOAl2O3,

MnOAl2O3, MgOAl2O3, FeOCr2O3, MnOCr2O3 [Kiessling and Lange et al. 1978][6].

Complex oxide inclusions are sometimes known as spinel type (MgOAl2O3) 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 (CaOAl2O3) 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 manganeses stronger affinity for sulphur, to form MnS (Tm = 1870K). Types of sulphide inclusions will a l s o de pen d on

ma n g a n es e t o S u l p h u r r a t i o . Examples of common sulphideinclusions include MnS, FeS, (Mn, Fe)S and CaS. The Sulphur affinity of various elements can be compared with

9

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.

10

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 solid-liquid 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.

11

STEEL DEOXIDATION

Maximum solubility of oxygen in liquid iron at the eutectic of 1527C 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

12

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]

13

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]

14

and corresponding equilibrium constant equation,

%Mn%O

12440 5.33 For = 1, the value of the equilibrium constant for manganese deoxidation is

= %Mn%O= 4.88 x 10-2 at 1600C

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 inclusions origin.

[2-4] [2-5] [2-6]

15

The silicon deoxidation reaction,

[Si] + 2[O] = SiO2 (s)

and corresponding equilibrium constant equation,

%%

30000T 11.5 For 2 = 1, the value of the equilibrium constant for silicon deoxidation is

= %%= 2.26 x 10-5 at 1600C

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, I-

modification). 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, -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]

[2-7] [2-8] [2-9] [2-10]

16

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.

17

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)

and corresponding equilibrium constant equation,

%%

62780T 20.5 For23 = 1, the value of the equilibrium constant for aluminum deoxidation is

= %%= 9.58 x 10-14 at 1600C

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].

[2-11] [2-12] [2-13] [2-14]

18

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.

19

The equilibrium reaction governing Mn/Si deoxidation,

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

and corresponding equilibrium constant equation,

%.

%

1510T 1.27

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

mo d i f i e r y i e l d l i q u i d man g a n e s e s i l i c a t e s f o r imp r o v e d c o a l e s c e n c e a n d

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 ep en d h e a v i ly 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.

[2-15] [2-16] [2-17]

20

Figure 2.7: The effect of manganese content on stability of oxide phases resulting from steel

deoxidation at 1550C (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 (1439C), limited solubility in steel (0.032% Ca at

1600C), and high vapor pressure at 1600C (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

21

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 Al-

killed 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 1360C 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.

22

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-SiO2-

Al2O3 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 MnO-

23

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 --

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 MnO-

SiO2-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]

24

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.

25

Fig. 2.10: Morphology of NMIs 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

26

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

m (5 103 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.

27

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

1600C 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.

28

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]

29

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 4.0-

5.0 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.

30

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

31

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 SOPs 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.

32

2.2.2 ROLE OF TAPPING ADDITION ON STEEL CLEANLINESS

At JSPL, first the SOPs 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 ferro-

alloys 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

33

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) %

34

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 30m [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.

35

CHAPTER 3

EXPERIMENTAL ASPECTS AND

METHODOLOGY

36

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

37

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

38

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 0.3m, is limited by the fixed wavelength of light ( 0.5m) [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 m. 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].

39

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.

40

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 m/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.

41

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)

42

CHAPTER 4

RESULT AND DISSCUSSION

43

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 Mechanical Properties Affected S, O Sulfide and oxide inclusions Ductility, Charpy impact value, anisotropy

Formability (elongation, reduction of area and bendability)

Cold forgeability, Drawability Low temperature toughness Fatigue strength

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)

Embrittlement by intergranular precipitation P Solid solution 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.

44

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

Grade %C %Mn %Si %S %P %Al %Nb H in ppm

Grade 880

0.60-0.80

0.80-1.30

0.10-0.50

0.03 max

0.03 max

0.015 max

- 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 T-12 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].

45

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.

46

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

47

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

48

4.2 RESULT

Table 4-3 Inclusion Rating Results

Heat ID A type B type C type D type

Thin Thick Thin Thick Thin Thick Thin Thick1 1.5 0.5 1.0 2 1.0 - 0.5

Group A (SULPHIDE)

(Thin)

Group B (ALUMINA)

(Thin)

Group C (SILICATE)

(Thin)

Group D (OXIDE)

(Thin) 1.5 0.5 - 1.0

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

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

of point 3 shown in image

(a)

(b)

49

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-degassers limitations for desulphurization understanding the

effect of secondary refining parameters on desulphurization and inclusion removal.

50

CONCLUSION AND FUTURE

WORK

51

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

52

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

53

REFERENCES

54

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