Steel Making Fundamentals

The Lecture Contains:

Attributes

Types of Steels

Effect of impurity elements

Historical Perspectives

Present Status of Steel Industry

Attributes:

Steel belongs to iron carbon system. This system has a unique feature to alloy with several elements of the periodic table to produce materials for diversified applications.

Iron-Carbon system is capable of creating any desired property by altering the microstructure through surface hardening, heat treatment and deformation processing.

Steel is recyclable and hence is a “green material”.

The above attributes make steel to be the most important engineering material. Around 2500 different grades are produced to cater the need of several industries ranging from structural to aero-space.

Types of steels: Below are given some applications. Details can be looked into references given at the end of the lecture.

Broadly we have either plain carbon (carbon is the principle alloying element) or alloy (in addition to carbon there are other alloying elements like Nb, V, W, Cr, Ni etc) steel. Plain carbon steels are the following types:

Properties Low carbon Medium carbon High carbon

Carbon Lower than 0.25 weightPercent

In between 0.25 and 0.6 weight percent

In between 0.6 and 1.4 weight percent

Some properties

Excellent ductility and toughness.Weldableand machinableNot amenable to Martensite transformation

Low hardenabilty.

These steel grades can be heat treated

Hardest,strongest and Least ductile

Module 1: Steelmaking FundamentalsLecture 1: Types of steels, History of modern steelmaking and Indian scenario

Some applications

Common products like Nuts, bolts, sheets etc.

For higher strength suchas in machinery,

Automobiles and agric-cultural parts (gears,axels, connecting rods) etc.

Used where strength,hardness and wear resistanceis required. Cutting tools, cable,Musical wires etc.

The alloy steels are classified as low (less than 5 weight% alloying elements), medium (in between 5 to 10 weight percent alloying elements) and high alloy steels (more than 10 weight percent alloying elements).

Note: Whether plain carbon or alloyed ones, all steels contain impurities like sulphur, phosphorus, hydrogen, nitrogen, oxygen, silicon and manganese, tramp elements like copper, tin, antimony, and non-metallic inclusions. These impurities are to be controlled during steelmaking

Effect of impurity elements on steel properties (some effects are given; details can be seen in the references given at the end of this lecture)

Carbon imparts strength to iron. It reduces ductility and impact strength. But presence of carbon allows heat treatment procedures.

Sulphur segregates during solidification (segregation coefficient is 0.02). Sulphur causes hot shortness due to formation of FeS formed during solidification of steel. Sulphide inclusions lower weldability and corrosion resistance. Presence of sulphur may also lead to development of tear and cracks on reheating the steel.

Phosphorus segregates during solidification (segregation coefficient is 0.02). Presence of phosphorus impairs plastic properties.

Silicon and manganese: Silicon reduces the drawing capacity of steel. Manganese is beneficial; it increases strength without affecting ductility and sharply reduces hot shortness.

Gases: Nitrogen impairs plastic properties and increases embrittlement at lower temperatures. Hydrogen causes defects such as flakes, fish-scale fracture.

Inclusions: Presence of inclusions at the grain boundary weakens intra-granular bonds. Inclusions also act as stress concentrators. Some type of inclusions is brittle.

Tramp elements: Tramp elements like copper, zinc, tin, antimony etc create problems during reheating of steels because their melting points are much lower than steel reheat temperature.

Historical Perspectives:

YearDevelopments

1856 Henry Bessemer developed a process for bulk steel production. He blew air in an acid lined pear shaped vessel. The process is termed Acid Bessemer

Process. No heat was supplied from outside. It did not become possible for him to remove S and P. Moreover oxygen content of steel was high. Hot shortness was a problem during rolling.

1878 S.G.Thomas and Gilchrist developed basic Bessemer process. They lined the vessel with basic refractory. High nitrogen content of steel, no usage of scrap and plugging of bottom blown tuyeres were the problems.

1868 Siemens’s and Martins developed Open Hearth Process. In this process thermal energy was supplied through combustion of gaseous and liquid fuels thus enabling them to use steel scrap in addition to other charge materials. Open Hearth Process for steelmaking has dominated the steel production for over approximately a century.

1900 Paul Heroult showed use of electricity for steel production. The quality of steel was better than open hearth process. The process was mainly used to produce alloy and special steels from scrap.

1950 Oxygen was used to produce steel at Linz and Donawitz and process was termed LD Converter steelmaking. Oxygen was supplied through a consumable single hole lance from top of a pear shaped vessel.

Present Status of Steel Industry:

Plain carbon steels are produced principally by the following routes:

1) Blast furnace→ Basic oxygen furnace →Ladle treatments→continuous casting→Rolling →flat or long products. Adopted by Integrated Steel Plants

2) Electric Arc Furnace→ Ladle treatments→Continuous casting→Rolling →Mostly long products but occasionally flat products. Adopted by Mini Steel Plants

Alloy and special steels are produced by route 2. Some plants employ Argon-Oxygen-Decarburization process instead of Electric Arc Furnace

Top steel producers in the world in the year 2010-2011

Rank Plant Production (Million (million tons)

1 ArcelorMittal, Luxembourg 103.3

2 Nippon Steel, Japan 37.5

3 Baosteel Group china 35.4

4 Posco, South Korea 34.7

8 Tata Steel India 24.4

10 United States Steel Corporation 23.2

20 Sumitomo Steel Industries, Japan 14.1

21 SAIL, India 13.7

Steelmaking in India

The first attempt to revive steel industry in India was made in 1874 when Bengal Iron Works cam into being at Kulti near Asansol in west Bengal. In 1907 Tata Iron and Steel Company was formed and produced steel in 1908-1909. In 1953 an integrated steel plant in public sector in Rourkela was signed with German Company. Then more integrated steel plants were added.

Indian steel industry is organised in three sectors as shown in the following:

Integrated steel plants

Mini steel plants Induction furnaces

Public sector Private Sector

Rourkela

Bhilai

Durgapur

Bokaro

Salem

Alloy steel plant Durgapur

Indian iron and steel company

Visvesyary a iron and steel

RINL, Vishakhapatnam

TISCO

ESSAR

ISPAT

JSW

Uttam steels

Kalyani steels

Lloyd steel

Usha martin

Tata Metalics

Mukand ltd.

(Reader may add more).

Dispersed

In vzrious

Parts of thecountry

Ghosh and A chatterjee: Ironmaking and steelmaking List of steel producers, wilkepedia www.steelads.com /…/ largeststeel/TOP30-Worlds- Largest- steel- Companies.html

Concept

The concept of modern

steelmaking is to make use of the steelmaking vessels like converter, ladle and tundish of a continuous caster. In all these vessels molten steel is handled for one or the other purpose. For examples ladles are used to transfer the molten steel either to ingot casting or continuous casting. Tundish of a continuous caster is used to transfer molten steel to the continuous casting mould. In all these vessels the residence time of molten steel is sufficiently long so as to carry out some refining operations like composition adjustment, removal of gases, control of S, removal of inclusions etc. in ladle and tundish. This has led into the development of ladles, tundishes for some refining operations like deoxidation, inclusion modification, desulphurization etc. and other operations like composition adjustment, inclusion removal etc. The basic idea of employing ladles and tundishes for either refining or composition adjustment or for producing clean steels is to use the steelmaking units like converter and electric furnace for producing steels without much bothering for final chemistry. Modern steelmaking comprises of hot metal / scrap to finished products through the following

a) Primary steelmaking

b) Secondary steelmaking

c)Continuous casting

d) Finishing operations

Principle chemical reactions

Hot metal contains and . Oxygen is blown from top and the following reactions occur:

123456

Lecture 2: Modern steelmaking


Concept

Primary steelmaking

Secondary steelmaking

Continuous casting and thin strip casting

Final finishing operations

7Note the following:

No heat is supplied from outside. The heat produced due to chemical reactions is sufficient enough to

raise the temperature of hot metal from around to molten steel tapping temperature

of .

Except carbon which is removed as a gaseous phase rest all other elements form slag. Slag formation of desired chemistry and physico-chemical properties is vital for the successful operation of converter

In electric arc furnace steelmaking scrap + hot metal + directly reduced iron is used to produce plain carbon steel

Electric energy is the principle source of thermal energy. Graphite electrodes are used to supply the current (see figure 2.2). The AC electric arc furnaces are very popular. EAF can be either normal power or ultra high power (UHP) with single or twin shell, with or without bottom stirring or post combustion. EAF generates a considerable noise. Now a days EAF has occupied a unique position in the steel industry: EAF can be switched over easily to produce plain C or alloy steel depending on the market requirements. The details are given in lectures 15,16 and 17.

Figure 2.2: Electric arc furnace

Secondary steelmaking

The objective of secondary steelmaking is to make the steel of desired chemistry and cleanliness by performing the following treatments in “Ladle”:

a) To stir the molten steel by purging inert gas through the bottom of the ladle.

b) To inject slag forming powder either through a lance for further refining

c) To produce clean steel either by removing inclusions or modify them by suitable injecting materials

d) To carry-out deoxidation and degassing.

Secondary steelmaking in ladles has become an integral part of steelmaking. Ladles have additional heating facility and are called Ladle furnaces (LF).

Continuous casting and strip casting

Molten steel is being cast continuously in to billets, blooms and slabs depending on the desired product i.e. whether long or flat products. In continuous casting, tundish, mold and secondary cooling sprays are arranged such that steel is poured continuously from the tundish and the solidified cast product is withdrawn continuously.

The arrangement of the tundish, mold and spray is shown in the figure 2.3.

Figure 2.3: continuous casting process

The original continuous casting machines were of vertical types. Now most of the continuous casters have either curved mould (Figure 2.3a) or vertical mold with bending rolls.

In the continuous casting, tundish is the important refractory lined vessel. It feeds the molten steel into the molds placed beneath the tundish through a submerged nozzle. Tundish also acts as reservoir of molten steel during ladle change-over periods and sequence casting. Modern tundishes are equipped with furniture like dams, weirs, slotted dams etc. to modify the molten steel flowing in the tundish during the process of continuous casting. Modern developments include thin slab caster, liquid core reduction. Thin slab casters are connected to the strip mill. The objective is to integrate the casting and rolling in order to save reheating cost.

Strip casting is ( Figure 2.3b) also becoming popular in steel plants. Here molten steel is cast directly into the strip. Lectures 33 and 34 describe the process of continuous casting.

Final finishing operations:

It has been considered appropriate to include final finishing operations in steelmaking course to appreciate integration between chemistry and cleanliness of steel and the final finishing operations. It is thought that the reader can appreciate the role of steelmaking in the product development and failure. The following finishing operations are dealt with in lectures 35,36 and 37.

Deformation processing technologies like forging rolling etc. Heat treatment to produce the finished product. Heat treatment consists

of heating the steel products to a temperature in the austenitic region and then cooling.

Surface hardening treatment

References:

Preamble

In

steelmaking, the impurities like carbon, silicon, manganese, phosphorus and sulphur are removed from hot metal through a combination of gas/metal, gas/slag and gas/metal/slag reactions so as to produce steel of desired chemistry and cleanliness (cleanliness refers to the inclusions). Science of steelmaking involves equilibrium concentration of an impurity between the phases for a given set of temperature and pressure,and the rate of transfer of an impurity from the hot metal.

Equilibrium between the phases:

The phases in steelmaking are hot metal, molten slag and gas. Hot metal is a multi-component solution in which impurities like carbon, silicon, manganese, phosphorus and sulphur are dissolved in very low amount (total concentration of all the impurities is approximately 5% to 6%) in iron. Slag is a solution of predominantly oxides with small amounts of sulphides, phosphides, silicates etc. Composition of the solutions in steelmaking is conveniently expressed either as weight% (Wt%) or mole fraction(N). The mole fraction of the ith component in a solution of n components is

(1)

Lecture 3: Science base of Steelmaking


Preamble

Equilibrium between phases

Activity of solution

Raoult’s law

Henry’s law

Interaction parameter

where is the number of moles of ith component. The equilibrium of a component between the liquid phases is expressed in terms of integral molar free energy. Integral molar free energy of solution

(2)

represents free energy of solution and is the free energy of pure components before entering into the

solution. The quantity is the partial molar free energy of mixing of component i and represent the change of energy or work which a mole of pure component i can make available.

Chemical potential is a useful concept to describe chemical equilibrium between liquid phases. At chemical equilibrium the chemical potential of any component is identical in all phases. Knowledge of chemical potential is important in steelmaking because an impurity can transfer in the gaseous or slag phase only when its chemical potential is lower than in hot metal.

The criterion for equilibrium at constant temperature and pressure is the change in the integral molar free

energy of the solution, i.e.

for an infinitesimal process and (3)

for a finite process

Where is change in integral molar free energy

At constant temperature and pressure when , a process occurs spontaneously. For an isothermal

chemical reaction say , where J is activity quotient and is the standard free energy change. At equilibrium

, where K is equilibrium constant.

(4)

Activity of solution

In dealing with chemical reactions in solution it is important to define the activity of a component. Activity of a component denotes its effective concentration. It is related to fugacity as

is the fugacity of component i in solution and is the fugacity of a component in its standard state (standard state could be either

pure element or compound at 1 atmospheric pressure) So at standard state activity equals 1. In an ideal gas activity of a

component i is equal to its partial pressure.

Raoults’s Law

An ideal solution obeys Raoult’s law, in which activity of a

component ai equals to its mole fraction

Real solutions exhibit either positive or negative deviation from Raoult’s law for a binary solution. Deviation from Raoult’ law is

taken care by activity coefficient

The Fe-Mn forms an ideal solution, whereas the Fe-Cu exhibits strong positive deviation and the Fe-Si strong negative from

Raoult’s law. Physically it implies that in Fe-Cu solution copper has a strong tendency to segregate, and in Fe-Si solution silicon

has a strong tendency to form chemical compound with iron.

In binary liquid oxides, FeO-MnO behaves ideally, whereas most

binary silicates i.e. show negative deviation from Raoult’s law.

Henry’s law

Liquid steel, and to a reasonable extent hot metal primarily fall in the category of dilute solution. In a dilute binary solution activity of a solute obeys Henry’s law, which is stated as

(8)

where is a constant (activity coefficient for the solute in dilute binary) and is the mole fraction of the specie i. Solutes in all infinite dilute solutions obey Henry’s law. Deviation from Henry’s law occurs when the solute concentration increases.

In steelmaking the concentration of solute in molten steel is expressed in weight percent. It is frequently most convenient to choose the infinitely dilute solution expressed in terms of weight percent as the standard state. This is defined as

(9)

For weight percent i other than zero

(10)

Interaction parameter

Molten steel contains several dissolved solutes in dilute scale. For example, molten steel contains C, S, P, Si, Mn etc. This steel is a multi-component solution. In multi-component solution solutes interact with one another and

thus influence activities of other solutes. If Fe is the solvent, and 1, 2….k are solutes in dilute state, then

(11)

The term is known as interaction parameter describing the influence of solute j on the activity coefficient of solute i. The value of interaction parameter can be found in any book on thermodynamics.

The concept of interaction parameter is very important in estimating the activity of a solute element in presence of other solute elements. For example we want to calculate the activity of sulphur in hot metal of composition C =

4%, Si = 1.5%, Mn =1% and S = 0.04% .By assuming infinite dilute solution as the standard state, the activity of sulphur is given by

Substituting the value of and

we get and activity of sulphur is 0.43.

ReferencesA.Ghosh and A. Chatterjee: Ironmaking and steel making


Preamble

The role of slag in steelmaking

Structure of pure oxides

Structure of pure silica

Network former and breaker oxides

Structure of slag

Preamble

Slag plays a very important role in steelmaking to the extent that it is said that “make a slag and slag makes steel”. Slag is a generic name and in steelmaking it is mostly a solution of oxides and sulphides in the molten state and the multi-crystalline phases in the solid state.Slag is a separate phase because

It is lighter than molten steel and

It is immiscible in steel

Slag is formed during refining of hot metal in which Si oxidizes to , Mn to MnO, Fe to FeO, and P to

, and addition of oxides such as CaO, MgO, iron oxide, and others. The addition of oxides is done to obtain desired physico-chemical properties of slag like melting point, basicity, viscosity etc. All these oxides float on the surface of the molten steel. Synthetic slag is also used to absorb inclusions to produce clean steel for certain applications.

The role of slag in steelmaking:

It acts as a sink for impurities during refining of steel It controls oxidizing and reducing potential during refining through FeO content. Higher FeO makes the

slag oxidizing and lower FeO reducing It prevents passage of nitrogen and hydrogen from atmosphere to the molten steel It absorbs oxide/sulphide inclusions It acts as a thermal barrier to prevent heat transfer from molten steel to the surrounding. It protects steel against re-oxidation It emulsifies hot metal and promotes carbon oxidation. In electric steelmaking slag prevents the radiation of heat of arc to the walls of the furnace and roof

The above functions require that slag should possess certain physical (density, melting point, viscosity) and chemical properties (basicity, oxidation potential). Both physical and chemical properties are controlled by composition and structure of slag. In steelmaking slag is predominantly a mixture of oxides with small amounts of sulphides and phosphides. The oxides are either acidic or basic in nature. We will first consider the structure of pure oxides and then we discuss what happens on addition of one type of oxide to the other.

Structure of pure oxides

In pure oxides

Metallic cations are surrounded by oxygen ions in a three dimensional crystalline network Each cation is surrounded by the maximum number of anions in a closed packed structure, and this

number is called coordination number

Cations of basic oxides such as CaO, MgO, FeO ( etc. have radii smaller than that of

cations of Structure of an oxide depends on the ratio of radii of cations/anions as shown in the following table

Structure CN Cation/anion Examples

Cubic 8 1 – 0.732

Octahedral 6 0.732 – 0.414 CaO, MgO, MnO, FeO etc.

Tetrahedral 4 0.414 – 0.225 SiO2, P2O5

Triangular 3 0.225 – 0.133

CN = Coordination number

As can be seen in the table the basic oxides have octahedral and acidic oxides tetrahedral structure.

Structure of pure silica

In silica, each atom of silicon is bonded with four oxygen atoms and each atom of oxygen is bonded with two silicon atoms. The elemental tetrahedral of silica are joined at the vertices to give the hexagonal network in three dimensions. The structure of pure solid and molten silica is shown in the figure

Figure 4.1: Structure of silica (a) solid and (b) moltenAs seen in the figure 4.1a, each tetrahedron is joined at the vertex so as to obtain the three dimensional hexagonal network. During melting the crystalline network of silica is broken by thermal agitation as shown on

figure 4.1b. Only at very high temperatures, molten silica consists of equal number of and ions.

Network former and breaker oxides

It must also be understood that the bonding between cations and anions in acidic oxides like SiO2 and is

strong, and these simple ions group to form complex ions as and . In slags, these tend to form hexagonal network. These oxides are, therefore, called network formers or acids. These acidic oxides can accept one or several oxygen ions.

Basic oxides like CaO, MgO, FeO dissociate and form simple ions like and . All basic oxides are

donors of oxygen ions. These oxides are called network breakers, since they destroy the hexagonal network of silica by reacting with it.

Structure of slag

Most slags are silicates. Pure silica has very high viscosity at the melting point. Addition of basic oxides Each mole of CaO introduces one mole of oxygen ions in the hexagonal network of silica and can break two

vertices of the hexagonal structure of silica. By adding 2 moles of for every mole of silica all the four

vertices are broken and we simply have and as shown below

Note that can combine with two tetrahedrons

The reaction between alkaline base oxides, e.g. Na2O and SiO2 is as follows:

and

Since Na has one charge, each tetrahedron of silica will have Na ion attached to oxygen ion. As a result one should expect more decrease in viscosity of silica on addition of alkaline base oxides as compared with basic oxides.

The number of vertices destroyed depends on the fraction of basic oxide, i.e. the ratio of O/Si as shown in the table

O:Si Formula Structure Equivalent silicate ions

2:1 SiO2 All corners of tetrahedron are (Si6O15)6− or (Si8O20)8−

shared

5:2 MO.2SiO2 One broken link per tetrahedron

(Si3O9)6− or (Si4O12)8−

3:2 MO.SiO2 Two broken link per tetrahedron

(Si3O9)6−

7:2 3MO.2SiO2 Three broken link per tetrahedron

(Si2O7)6−

4:1 2MO.SiO2 All link are broken (SiO4)4−

is viscosity, A is an empirical constant, E is activation energy, T is temperature and R is gas constant. For a given temperature, addition of basic oxides decreases rapidly the viscosity of a slag which

contains

and . The

decrease in viscosity is greater with alkaline oxides like and fluorides like as compared with CaO and MgO for the reasons discussed in lecture 4.

Alumina acts as a network breaker in an acidic slag and network former in a basic slag.

Presence of solid particles in slag increases the viscosity of slag as shown in the following expression:

(2)

Lecture 5: Physico-chemical properties of slag


Introduction

Viscosity

Basicity

Oxidation and reduction potential of slag

Slag foaming

Operational advantages

Quantification of slag foaming

Where is volume fraction of solids in slag

If volume fraction of the solid is in between 5% to 10%, viscosity of slag increases by 114% to 130%.

Basicity:

Basicity can be understood either from ionic or from molecular nature of slag. The ionic nature of slag assumes

slag to consist of ions. In slags, acidic oxides can accept one or several ions, whereas a basic slag is a

donor of ions. For example, 1 mole of can accept 2 moles ions so that each tetrahedron in

hexagonal structure becomes independent of each other. Similarly each mole of can accept 3 moles of ions. Thus

(3)

Amphoteric oxides behave as bases in presence of acid or as acids in presence of a base:

(4)

Bases can supply ions

In a neutral slag enough oxygen ions will be present to ensure that each tetrahedron remains independent of

each other. In binary , slag will become neutral when CaO is 66.7%, which corresponds to the

formation of Slag will be basic only when CaO content is more than 66.7%. Basicity can be

expressed in terms ions which are in excess than that required, thus satisfying the requirements of acidic oxides. In 100 g of slag

(5)

In industrial practice ionic definition of basicity is not useful; the molecular approach is more useful. The molecular approach assumes slag to consist of chemical compounds. The basicity of slag is

(6)

In presence of different basic oxides, the different strength of the basic oxides should be considered. In a slag

which contains CaO, MgO, and , the basicity is

(7)

In slag/metal reactions which involve desulphurization and dephosphorization, the concept of free lime in slag

is useful. Free lime in CaO, and slag is that amount which is available after the formation of neutral

compound like 2CaO. , 3CaO.

(8) For 100 ton hot metal with 1% silicon and 0.2% P the calculation shows that free CaO in slag would be available when CaO content exceeds 4540Kg.

Oxidation and reduction potential of slag

It refers to the capability of slag to transfer oxygen to and from the molten steel bath. FeO content of slag determines the oxidation potential of slag. Thus activity of FeO in slag is an important parameter. The equilibrium between FeO of slag and oxygen of steel is

(9)

The activity of oxygen in metal is proportional to the activity of FeO in slag.

Slag foaming:

Foam is a dispersion of gas bubbles in a liquid. A liquid is said to be foaming when gas bubbles could not escape through the liquid and as a result height of the liquid increases. In steelmaking, slag foaming can occur due to the following reactions:

This reaction occurs within the slag. The other reaction

This reaction occurs at the gas/metal interface. In both the cases when the CO gas bubbles are unable to escape through the slag, the slag is said to be foaming. If the reaction between carbon and oxygen occurs deep into the bath i.e. reaction 2 then gas bubbles have enough time to grow in size and can easily escape through the slag layer as compared to when the gas bubbles are produced by reaction 2. The reaction 2 occurs within the slag

Is slag foaming desirable? Yes to the extent that slag should not flow out of the reactor. Slag foaming enhances the reaction area. In electric steelmaking foamy slag practice prevents the transfer of heat of the arch to the refractory lining.

Operational advantages: A foaming slag

Shields molten steel against atmospheric oxidation Acts as a thermal barrier to prevent heat losses Shields the refractory lining particularly in electric arc furnace Control heat transfer from the post combustion flame Quantification of slag foaming:

Foaming index = Foam layer thickness/ average gas velocity

Low foaming index means easy escape of gas bubbles which can be obtained either by smaller gas bubbles or higher gas velocities. Foam life is directly proportional to foaming index

Increase in slag viscosity increases foaming index. Presence of solid particles and surface active agents increases the foaming index. Addition of calcium fluoride decreases the foaming index by decreasing the viscosity of slag. Foaming index (FI) can be calculated from the physical properties of slag and size of the gas bubble:

(10)

Calculate the foaming index slag of composition and 5% at 1773 K slag from the following data:

and and 0.01m.Substituting the value of the variables in eq. 10 we get

for and 485 for

If the volume fraction of solid particles in slag is 0.1

s for and 631 s for

Note that foaming index increases to 1.3 times due to presence of solid particles in slag.

Consider a slag of composition and 1% Ca F2 at 1873 K whose

and and

This slag would have foaming index 9s. Foaming tendency decreases drastically due to production of Ca F2 in slag.

Reference to lectures 3 and 4

A.Ghosh and A.Chatterjee:: Ironmaking and steelmakingZhang and Fruehan: Metallurgical and Materials Trans. B, 26(8), 1995

Introduction

In steelmaking the impurities in hot metal like carbon, silicon, manganese, phosphorus and sulphur are removed through oxidation and slag formation so as to produce steel of desired chemistry and cleanliness. For this purpose oxygen is supplied and slag of desired chemistry is formed. When oxygen is supplied, oxidation of all impurities of hot metal including iron begins simultaneously.

To understand the conditions favourable for the removal of an impurity, we will first consider oxidation of an individual impurity. We will be using principles of thermodynamics to obtain the optimum conditions for the removal of an impurity. Note the following

Carbon can oxidize to CO and CO2 but at high temperature carbon oxidation to CO is highly probable. We will consider oxidation of C to CO.

In expressing activity of solutes in molten steel, Henry’s law is used by using 1 weight % standard state. Raoult’s law is used to express activity of solutes in slag.

Since impurities are dissolved in molten metal, reactions between impurity and oxygen occur with dissolved oxygen.

Square brackets [ ] in a reaction denote impurity in metal, round brackets () in slag and curly {} in gas.

Lecture 6: Steel Making Reactions: Oxidation of Iron and Silicon


Introduction

What are oxidation reactions?

Iron oxidation

Oxidation of silicon

What are the oxidation reactions?

The principle reactions in steelmaking comprise of oxidation of impurity elements by oxygen dissolved in hot metal or FeO content of slag.

(1)(2)(3)(4)(5)

All reactions are exothermic. C is removed as gas. Except C, all other impurities are removed as oxides and all these oxides float on the surface of the

molten metal during refining of hot metal to steel. Iron oxidation is unavoidable. Oxidation of Fe is loss in productivity; hence its oxidation must be

controlled. Oxygen must be dissolved to remove an impurity from the hot metal.

We begin with considering oxidation of an individual element and evolve the optimum conditions using thermodynamic principles.

Iron Oxidation:

Oxidation of iron i.e. reaction 1 is the most important since it controls

FeO content of slag and oxygen content of steel Loss of iron in slag and hence affects productivity Oxidation potential of slag

In addition to the above FeO also helps in dissolution of lime in slag.Consider the reaction

The equilibrium constant is

(6)

h is henrian and a is raoultian activity. Since Fe in steel is pure; , and

(7)

(8)

In equation 8, T is in K. By equations 6, 7 and 8 we get

The equation 9 can be used to determine wt% O in steel at any temperature T, when in slag is known.

When pure FeO is in contact with Fe; . We can determine at saturation for different temperatures:

T (K)

1873 0.233

1923 0.285

We note that increase in temperature increases oxygen dissolved in molten iron.

The above values of dissolved oxygen correspond when pure FeO is in contact with Fe pure. In steelmaking FeO is present along with other oxides like calcium oxide, magnesium oxide, silica, manganese oxide etc, hence

activity of FeO is influenced by other solute oxides. Thus

where is activity coefficient and mole fraction of FeO in slag,. depends on slag composition. In

CaO- -FeO system, as ratio increases increases; physically it means that CaO replaces FeO

from FeO. . The following expression is used to express :

(10)

Consider a slag with = 0.5 ; according to equation 10 is 0.31; [wt% O] in steel would be 0.072 as calculated by equation 9.

Few other equations are available; i.e.

(11)

(12)

The calculations are made on [wt% O] by equations 9, 11 and 12 at different temperatures using .

T (K)

1873 0.233 0.229 0.220

1923 0.285 0.280 0.268

There is a slight difference in the values of dissolved oxygen content in steel. But all equations suggest that increase in temperature increases dissolved oxygen in iron which is in contact with pure FeO. This calculation indicates that control of temperature is important to limit the dissolution of oxygen in molten iron.

Oxidation of Silicon:

Consider reaction 2

(13)

(14)

Different sources give the following expression for

(15)

(16)

Both equations predict that decrease in temperature increases . There is a slight difference in values of .

Equation 15 predicts 15-17% higher than equation 16.

Conditions favourable for silicon oxidation are

Low temperature

Low in slag. A basic slag favours silicon oxidation.

In a basic slag, silicon oxidation occurs practically to a very low value since reacts with CaO and decreases activity of silica in slag.

Another important feature of silicon reaction is very high affinity of silicon with oxygen, silicon can be used as a deoxidizing agent.

By equation 14

(17)

by equation 16 and using , we get

(18)

Behavoiur of manganese in iron-carbon melt:

Mn is soluble in iron in any proportion Mn forms ideal solutions in iron

Carbon lowers the activity of Mn in Fe-Mn-C system by forming .

Oxidation of Manganese:

Lecture 7: Oxidation of Manganese and Carbon


Behaviour of manganese

Oxidation of manganese

Reduction of manganese

Oxidation of carbon

Rimming reaction

Illustration

Mn is oxidized readily at relatively low temperatures and can form oxides like , , etc. But MnO is stable at high temperature.

(1)(2)

The reaction 1 occurs with dissolved oxygen in metal, whereas reaction 2 is a slag/metal reaction. Both reactions are exothermic. Lower temperature favours oxidation of Mn from metal to slag; whereas higher temperature favours reduction of MnO of slag and there occurs reversal of Mn into steel. Reduction of MnO in slag is important; we consider reaction 2

(3)

Replacing activity by mole fraction and using , we get,

(4)

Grouping all activity coefficient terms and putting We get,

(5)

Where is an equilibrium quotient and it depends on composition of slag. Distribution of Mn between slag and metal can be written as

(6)

(7)

According to equation 7 increases with decrease in temperature( and 20.33 at temperatures 1923K, 1873K and 1773K respectively)

Condition for oxidation of Mn according to equation 6

High activity of in slag which means an oxidizing slag Decrease in temperature increases K* according to equation 7.

Reduction of Mn in slag

Conditions for reduction of MnO, that is reversal of reaction 2 is important. The reduction of MnO in slag transfers Mn from slag to metal and increases the concentration of manganese. The following are the

conditions for the reduction of MnO in slag

Low activity of FeO in slag which means a reducing slag

High temperature which decreases

Illustration:Consider a slag of basicity 1.8. At this basicity the activity coefficient of MnO in slag is 1.6. The mole fraction of FeO and MnO in slag is 0.25 and 0.05 respectively. Determine the equilibrium content of Mn and O in steel at 1873K. Given

Using equilibrium constant definition, we can write

(8)

Substituting the values, we get at 1873K

Oxidation of CarbonIt is important to note that amongst all steelmaking reactions, oxidation of carbon is the reaction whose product is gas i.e. CO. Therefore this reaction is of very much significance during steelmaking because

CO gas during escape from the molten bath can induce stirring in metal and slag phases during steelmaking.

CO gas can cause slag to foam which leads to increase in surface area. CO gas has a high calorific value and combustion of CO in steelmaking can contribute to energy

efficiency.

Carbon oxidation is also known as “decarburizing” reaction

(9)

(10)

(11)

If we assume that is at low concentration of carbon and oxygen in molten metal then

(12)

According to eq. 12, the product at a given temperature depends only on partial pressure of

CO in equilibrium with melt. It is important to note that depends on the location of nucleation of CO in steel

melt. If CO nucleates deep into the bath then will be great

Let us calculate equilibrium content of carbon and oxygen at 1873K for 1.2 atm and 1.5 atm

The value of is calculated from

[wt% C] [wt% O ]

0.05 0.0405 0.0486 0.0608

0.1 0.0202 0.0242 0.0303

0.5 0.0040 0.0048 0.0060

1.0 0.0020 0.0024 0.0030

From the table we note that

Decrease in carbon content increases the oxygen dissolved in steel. This is important in connection with production or ultra low carbon steel for certain applications. Production of ultra low carbon steels will be accompanied with dissolved oxygen if precautions are not taken during steelmaking.

Increase in increases [wt% O] in steel

er than atmospheric pressure.

Let us consider the evolution of CO gas. According to equation 9,

12 Kg C produces 22.4 CO (1 atm and 273K)

1 Kg C produces 1.87 CO (1 atm and 273K) which is equivalent to 12.83 CO (1 atm and 1873 K)

Now for 1000 Kg hot metal and 0.2% carbon in steel

CO production would be 488 (1atm, 1873 K) / ton of hot metal. This volume of CO will evolve no doubt over a period of time but at any time large amount of CO will be escaping the system. Escaping of this gas will agitate the bath and contribute to enhanced rates of mass transfer reactions. Also care must be taken for the easy and unhindered escape of CO gas from the vessel failing which foaming and eventually expulsion of slag may occur.

Rimming reaction

Other aspect of carbon reaction is the evolution of CO during solidification of steel. As the temperature of

molten steel decreases from 1873K to 1773K, increases from 494 to 532 which results in decrease

in as steel cools. This will lead to CO evolution during solidification and is called rimming reaction. Rimming reaction induces stirring in the solidifying liquid steel and minimizes segregation of solutes.

Reference

A. Ghosh and A. Chatterjee; Ironmaking and steelmaking R. Tupkary et.al. Modern methods of steelmaking A.K.Chakrabarti: Steelmaking


Preamble

Equilibrium considerations

How low should be?

Effect of FeO and CaO on dephosphorization

Illustration

Conditions for dephosphorization

Conditions for simultaneous removal of C and P

Preamble:

Phosphorus removal from hot metal is the most important refining reaction. Phosphorus has atomic number 15 and it can give up all 5 electrons from its outermost shell to become

or accept 3 electrons to become to attain stable configuration. This means that phosphorus can be removed both under oxidizing as well as reducing conditions.But removal of phosphorus under reducing conditions is not practical since its removal is highly hazardous. Thus P removal is practised mostly under oxidizing conditions..

Equilibrium Considerations:

Phosphorus removal reaction

(1)

At T > 1382K, becomes positive which results in decomposition of to

P and O.Thus removal of phosphrous requires that must be reduced.

(2)Now

and (3)

(4)

By equation 2 and 3 and replacing by using Raoult’s Law and after rearrangement

(5)

is activity coefficient of in slag. The LHS of equation 5 is index of dephosphorization and denotes distribution of phosphorus between slag and

metal. Higher value of LHS demands low in a slag of a given composition.

How low should be?:

Consider dephosphorization in a slag of at 1773K. Initial %P in metal is 0.1 and mole fraction of

in slag = 0.01. Let us calculate which will allow dephosphorization.

(6)

At 1773 K,

can be determined by equation 3 and 4. We substitute the values in equation 5. We

get . Now the question before us is how to attain such a low value of in a slag of

given composition? Such a low value of can be attained when we use basic oxides which have a very strong tendency to form a stable chemical compound. The different basic oxides have different ability to lower

. The following expression describes the relative effects of basic oxides on .

(7)

Alkaline oxides and BaO are stronger than CaO but they are corrosive to the refractory lining and hence not used.

Consider a slag

We calculate at different temperatures

T (K)

1773

1823

Effect of FeO and CaO on dephosphorization:

Figure 8.1 shows the variation of dephosphorization index as a function of wt%FeO for

CaO-FeO- slag at different basicities. The dephosphorization ratio increases with

Figure 8.1:Dephosphorization index as a function of weight percent FeO content of slag for different basicities.

increase in FeO content of slag and becomes maximum in between 15-16% FeO at all basicities. Further increase in FeO beyond 15-16%, dephosphorization decreases. The above behaviour can be observed at all basicities of slag.

The above behaviour is due to the dual role of FeO. FeO is the source of oxygen for oxidation of P according to the following reaction

(8) For a given basicity of slag, as FeO content of slag increases oxidizing power of slag increases and phosphorus

oxidation according to reaction 8 will be favoured because CaO of slag decreases the activity of by forming a stable compound. Beyond the optimum value of FeO in slag FeO replaces CaO and may either

combine with CaO or with . FeO is a weak base compared with CaO as a result of which the dephosphorization ratio decreases with addition of FeO beyond an optimum value.

The maximum dephosphorization ratio increases with the increase in the basicity of slag as can be seen in the figure 7.1. Higher basicity requires higher amount of CaO dissolved in slag. Any undissolved CaO will not be effective for dephosphorization. Optimum value of FeO is more or less independent of the basicity of slag. Thus control of FeO in slag is important for efficient dephhosphorization. Conditions for dephosphorization:

Dephosphorization requires oxidizing and basic slag:

(11)

in slag should be high. This means slag should have free dissolved lime.High basicity of slag is required.

in slag should be high; slag should be oxidizing. However for efficient dephosphorization the FeO content of slag should be in between 15 to 16%.

Low temperature favours high

Conditions for simultaneous removal of C and P

Removal of C and P both require oxidizing conditions but P removal is possible only when a basic and limy slag is formed. Consider the following reactions occurring simultaneously

(13)

It is assumed in eq. 14 and 15 that henrian activity is equal to (wt %). Both reactions 12 and 13 require oxygen but reaction 13 requires a slag which is basic in nature in addition to oxygen. Thus, if carbon and phosphorus are to be removed simulataneously,

an important requirement is the availability of slag which acts as a sink for ( ).

Thermodynamically slag is required in which activity coefficient of is very

(9)

(10)

(12)

(14)

(15)

low.

The question is how low activity of should be?. This value can be determined by equations 14 and 15 Replacing [wt% O] in equation 14 and 15, and after rearrangement,

From equation 16 one can determine the value of activity coefficient of which can lead to simultaneous removal of carbon and phosphorus.

Let us calculate the when molten metal contains 2% C and 0.15% P and

temperature T=1773K. The mole fraction of in slag is 0.1.

We can calculate and from equations 15 and 17. Substituting all the values

into equation 16 we get . We can also calculate at temperatures 1673K and 1873K. The results are given below

T(K)

1673

1773

1873

The calculations show:

Both decarburization and dephosphorization are possible simultaneously in presence of slag in

which has extremely low value.

Low temperature requires in slag to be higher than at high temperature. Thus low temperature is favourable.

References

(16)

(17)

A. Ghosh and A. Chatterjee; Ironmaking and steelmaking R. Tupkary et.al. Modern methods of steelmaking A.K.Chakrabarti: Steelmaking

Role of refractory

Refractory materials have a crucial impact on the cost and quality of steel products. The diversification on steel products and their cleanliness requirement in recent years have increased the demand for high quality refractory. Steelmaking requires high temperatures of the order of 1600 degree centigrade. In addition steelmaking handles high temperature phases like molten steel, slag and hot gases. These phases are chemically reactive; refractory materials are required to produces steels. High quality refractory at a cheaper cost is the main requirement because cost of refractory adds into the cost of product.

What is a refractory?

Refractories are inorganic nonmetallic material which can withstand high temperature without undergoing physico – chemical changes while remaining in contact with molten slag, metal and gases. It is necessary to produce range of refractory materials with different properties to meet range of processing conditions.

The refractory range incorporates fired, chemically and carbon bonded materials that are made in different combinations and shapes for diversified applications.

Lecture 9: Refractory Materials


Role of refractory

What is a refractory?

Why required?

Refractory requirements

Melting point of some pure compounds used to manufacture refractory

Properties required in a refractory

Types of refractory materials

Insulating materials

Why required?

To minimize heat losses from the reaction chamber To allow thermal energy dependent conversion of chemically reactive reactants into products because

metallic vessels are not suitable.

In steelmaking, the physico- chemical properties of the following phases are important:

Slag: Mixture of acidic and basic inorganic oxides like etc.; temperature

varies in between .

Molten steel: Iron containing carbon, silicon, manganese, phosphorous, tramp elements, non metallic inclusions, dissolved gases like nitrogen, oxygen and hydrogen and different alloying elements

like etc.; temperature

Gases: containing solidparticles of etc.; temperature

.

The above phases are continuously and constantly in contact with each other and are in turbulent motion

Compounds

Melting point

MgO (pure sintered) 2800

CaO(limit) 2571

Si C pure 2248

MgO (90-95%) 2193

Cr2O3 2138

Al2O3(pure sintered) 2050

Fireclay 1871

SiO2 1715

Kaolin (Al2O3. SiO2) 1816

Chromite (FeO. Cr2O3) 2182

.

Properties required in a refractory

The diversified applications of refractory materials in several different types of industries require diversified properties to meet the physico-chemical and thermal requirements of different phases. In some industrial units more than one phase are present e.g. in steel-making vessels slag /metal /gases are simultaneously present in

the vessel at high temperatures. In the heat treating furnaces solid/reducing or oxidizing gases are simultaneously present. Below are briefly described the properties of the refractory materials:

Refractoriness Refractoriness is a property at which a refractory will deform under its own load. The refractoriness is indicated by PCE (Pyrometric cone equivalent). It should be higher than the application temperatures. Refractoriness decreases when refractory is under load. Therefore more important is refractoriness under load (RUL) rather than refractoriness.

Porosity and Slag permeability Porosity affects chemical attack by molten slag, metal and gases. Decrease in porosity increases strength and

Spalling Spalling relates to fracture of refractory brick which may occur due to the following reasons:

A temperature gradient in the brick which is caused by sudden heating or cooling.

Compression in a structure of refractory due to expansion

Variation in coefficient of thermal expansion between the surface layer and the body of the brick

Variation in coefficient of thermal expansion between the surface layer and the body of the brick is due to slag penetration or due to structural change.

On sudden heating

On sudden cooling

Permanent Linear change (PLC) on reheatingIn materials certain permanent changes occur during heating and these changes may be due to

Change in the allotropic form Chemical reaction Liquid phase formative Sintering reactions

These changes determine the volume stability and expansion and shrinkage of the refractory at high temperatures.

Thermal conductivity Thermal conductivity of the bricks determines heat losses. Increase in porosity decreases thermal conductivity but at the same time decreases strength also.

Bulk density: Decrease in bulk density increases volume stability, heat capacity.

Types of refractory materials

This can be discussed in several ways, for example chemical composition of refractory or use of refractory or method of manufacture or in terms of physical shape. Below is given type of refractory depending on its chemical composition and physical shape.

A) Chemical composition

Refractories are composed of either single or multi-component in organic compounds with non metallic elements.

Acid refractory

The main raw materials used are and alumino- silicate. They are used where slag and atmosphere are acidic. They cannot be used under basic conditions. Typical refractories are fireclay, quartz and silica.

Basic refractory

Raw materials used are dolomite and chrome-magnesite. Basic refractories are produced from a

composition of dead burnt magnesite, dolomite, chrome ore.

a) Magnesite: Chrome combinations have good resistance to chemical action of basic slag and mechanical strength and volume stability at high temperatures.

b) Magnesite: Carbon refractory with varying amount of carbon has excellent resistance to chemical attack by steelmaking slags.

c) Chromite- Magnesite refractory: used in inner lining of BOF and side walls of soaking pits.( basic refractory)

d) Magnesite: Basic refractory in nature. Magnesite bricks cannot resist thermal stock, loose strength at

B) Physical form

Broadly speaking refractory materials are either bricks or monolithic.

Shaped refractories are in the form the bricks of some standard dimensions. These refractories are machine pressed and have uniform properties. Special shapes with required dimensions are hand molded and are used for particular kilns and furnaces. Different types are:

1. Ramming refractory material is in loose dry form with graded particle size. They are mixed with water for use. Wet ramming masses are used immediately on opening.

2. Castables refractory materials contain binder such as aluminate cement which imparts hydraulic setting properties when mixed with water. These materials are installed by casting and are also known as refractory concretes.

3. Mortars are finely ground refractory materials, which become plastic when mixed with water. These are used to fill the gap created by a deformed shell, and to make wall gas tight to prevent slag penetration. Bricks are joined with mortars to provide a structure.

4. Plastic refractories are packed in moisture proof packing and pickings are opened at the time of use. Plastic refractories have high resistance to corrosion.

Monolithic refractories

Monolithic refractories are replacing conventional brick refractories in steelmaking and other metal extraction industries. Monolithic refractories are loose materials which can be used to form joint free lining. The main advantages of monolithic linings are

Grater volume stability

Better spalling tendency Elimination of joint compared with brick lining Can be installed in hot standby mode Transportion is easier

Monolithic refractories can be installed by casting, spraying etc.

Ramming masses are used mostly in cold condition so that desired shapes can be obtained with accuracy.

Insulating materials

The role of insulating materials is to minimize heat losses from the high temperature reactors. These materials have low thermal conductivity while their heat capacity depends on the bulk density and specific heat. Insulating materials are porous in structure; excessive heat affects all insulating materials. Choice of insulating materials would depend upon its effectiveness to resist heat conductivity and upon temperature. High alumina

with thermal conductivity and silica with thermal conductivity etc are amongst others, used as insulating materials.

Ceramic fibres are important insulating materials and are produced from molten silica, titania, Zirconia etc in the form of wool, short fibres and long fibres. They have excellent insulation efficiency. They are long weight.

References:O.P.Gupta: Fuels, Furnace and refractory

Preamble

In steelmaking, refractory materials are used in converter, electric furnace, ladle, tundish, and reheating furnaces. In converter, electric furnace, ladle and tundish, molten steel is in contact with slag, whereas in reheating furnaces steel in the solid form is reheated for deformation processing, heat treatment and surface hardening methods.

BOF Refractories

Converter is lined with a permanent lining and above it there is a wear lining. Permanent lining thickness may vary from 100mm to 120mm and is made of chrome-magnesite permanent lining which is given on the full height of the converter.

Above the permanent lining, wear lining is constructed. The cylindrical portion of the converter (barrel) is lined with the ramming mass of tar dolomite and tar dolomite bricks. The detachable bottom is constructed by using mica, fireclay, chrome-magnesite and Mag-chrome bricks.

refractory materials with 15% high purity graphite have been found to provide increased corrosion resistance.

In duplex blowing (hybrid blowing or combined blowing) bricks are commonly employed for the bottom tuyeres and around them, since these areas severely worn.

The slag and metal penetration between the refractory grains, chemical attack by slag, mechanical erosion by molten steel movement contribute to the wear of the lining materials. Some developments to counteract this lining wear are

i) Dolomite is added to create a slag of about which is close to saturation level of slag.

ii) Critical wear zones (impact and top pads, slag tapping and trunion areas) are lined in furnaces with high quality bricks.

iii) Slag splashing in which the residual slag is splashed by high speed has resulted into high lining life (refer lecture 14)

iv) Lowering levels in slag and shorter oxygen-off to charge intervals have reduced refractory wear.

Refractory for secondary steelmaking

There are many operation and process in secondary steelmaking like vacuum degassing, ladle refining etc. Refractories are used in unique combinations of various bricks to meet diversified requirements. Following condition may be noted:

Lecture 10: Refractory in steelmaking


Preamble

BOF refractories

Refractory for secondary steelmaking

Refractory for continuous casting

Refractory for circulation degassing

Refractory for high temperature furnaces

Emerging trends

Refractory maintenance

Future issues

i) High temperature and long holding times of steel in ladle.

ii) Wide variation in slag composition

iii) Many types of vacuum treatment.

iv) Large thermal changes.

v) Molten steel agitation causes attack by motion of liquid steel.

In all ladle refining processes such as ladle furnace, ASE-SKF, VAD process, bricks are used at areas, where slag is in contact with steel. For general wall, high alumina bricks are widely used. For bottom zircon

bricks are used to prevent molten steel penetration into brick joint. In certain cases bricks

and castables are used in impact areas. bricks with addition of a couple of metals provides high hot strength, and are excellent in oxidation resistance.

Refractory for continuous casting

Tundish is a refractory lined vessel in continuous casting. It contains molten steel with minimum heat losses. Selection of refractory is critical due to longer casting sequence, faster tundish turnaround, higher campaign life and cleanliness of steel. Fireclay bricks are used. High alumina bricks are considered to be good for tundish hot rotation. Basic coating material is used over the lining. The coating installation method is gunning. Typically

mixture is used as a coating material.

Tundishes are equipped with dams and weirs. There are made of boards or alumina bricks.

Molten steel from tundish to mold is fed by nozzle submerged into molten steel in mold. Submerged nozzles must be resistant to corrosion and spalling, nozzle clogging is also important. Isostatic pressed submerged nozzle with alumina- graphite-fused silica are being used.

In recirculation degassing steel is made to flow from the ladle into a separate degassing chamber. In RH process, a refractory lined vessel equipped with two legs (snorkels) is used. These snorkels are immersed into molten steel. The refractory materials must have adequate spalling and abrasion resistance, volume stability and corrosion resistance at high temperature and in vacuum. Direct bonded magnesia- chrome bricks, semi rebonded magnesia chrome bricks are used in the lower vessel and snorkels. Extra high temperature burned magnesia –chrome bricks posses excellent corrosion and abrasion resistance and are preferred lining material.

Refractory lining for high temperature furnaces

Furnaces are used for heating steel within the temperature range to for heat treatment and deformation processing. Many different types of furnaces are used namely soaking pits (batch type) and continuous furnaces. Fireclay and high alumina refractories are used. Most of the continuous furnaces are lined with fireclay bricks. Plastic chrome ore ramming mixture and hard burnt chrome magnesite bricks are used to line the hearth to provide resistance to scale.

Emerging trends

Refractory has undergone many changes to meet the diversified requirements of the industry particularly steel industry. The main objective is to increase the lining life at reduced cost by developing

High quality refractory for critical applications in steel making at e.g. slag line, impact area of molten steel stream, bottom tuyere refractory in hybrid blowing, immersion nozzles in continuous casting etc. In this connection mention may be made of some refractory like MgO-C, Al2O3 – Si C – C, MgO– Ca O – C, Al,Mg and Al-Si alloy stabilized MgO – C brick, zircon based refractory, and Al2O3 – C

Repairing methods like slag splashing, slag coating, hot patching, gunning (flame gunning involves melting and spraying on hot surface).

Monolithic refractory

Monolithic refractory

Furnace refractory maintenance

The following methods are commonly practiced.

Slag splashing

Slag splashing is done in steelmaking vessels. After steel tapping, some amount of slag is retained. Composition of slag with respect to FeO and MgO is adjusted. FeO makes the slag adhesive on the lining and MgO makes the lining high temperature resistant. Nitrogen is blown from top to splash the slag. The splashed slag gets coated on the lining. To reduce excessive slag build up in the bottom, excess slag is then poured before charging.

In case of hybrid blowing practice formation of skull may result in a failure of the bottom stirring elements.

Slag coating and slag washing

The small amount of liquid slag is retained in the vessel after tapping. Slag is enriched with dolomite or raw dolomite to cool the slag and to increase its adhesive properties. Vessel is rocked several times to coat the

bottom and bottom joint with a slag.

Hot patching

Self flowing refractory mixtures enable precise maintenance of the scrap impact zone, tapping pad and bottom joint.

Gunning

By gunning, i.e. maintenance of pre- worn areas with special gunning mixtures, vessel lining life can be extended.

Flame gunning involves simultaneous melting of a refractory powder and gunning at the hot surface. Since the gunned repair material is dense and fused directly on the hot surface excellent results on life of lining is obtained in LD converter.

Future issues of Refractory technology

a) Durability of refractory for pairing nozzles and side dams determines the success of strip casting.

b) Technology of mass melting of scrap in converter by using post combustion requires super- high temperature refractories.

c) Super fine powder processing technology to produce refractory.

d) Use of monolithic refractory in steel making and refining furnaces require automating brick lying and intelligent repair.

e) Nano tech refractory is thermal shock and corrosion resistant The nano-particles act in two ways

They consist of mono spheres and improve properties like elasticity and strength Control of molecular structure as the particles have many small pores of several hundred

nanometers.

Reference: P.Mullinger and B. Jenkins: Industrial and process furnacesKenneth C. Mills et.al.: A review of slag splashing, ISIJ Intern. 45(2005), No. 5, PP 619-633Y.Naruse: Trends of steelmaking refractories

Steel Making Fundamentals

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