11 Modern Steelmaking Processes 1. Charge Materials

Topics to discuss…

1. Metallics

2. Auxiliary charges

3. Oxidants

4. Fuels

5. Refractories

Principal raw materials used for steel manufacture

• metallics (metallic charge, metallic additions),

• auxiliary charge (fluxes, deoxidisers),

• oxidants,

• fuels, and

• refractories

Raw Materials

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1. Metallics

1.1 Metallic Charge

The primary source contains

(a) pig iron (liquid or solid), and

(b) solid products of direct conversion of iron

from iron ores (a.k.a. sponge iron or DRI)

The secondary source

(a) steel scrap (also include pig iron scrap in some cases).

Ferro-alloys

(a) ferrosilicon, ferromanganese, ferrochrome, ferromolybdenum, etc.

(used primarily for deoxidation and alloy additions)

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On average, 1130-1140 kg metallic charge used per ton

of steel produced

The proportion pig iron and scrap used for making steel

depends on the type of steelmaking process

• In electric steelmaking, the use of pig iron is very low (less than 5%)

and the metallic charge consists mainly of scrap or a semi-product pre-

melted in a separate plant.

• In open-hearth processes, the charge is roughly 55% pig iron

(the balance is scrap)

• In converter steelmaking, 70-85% pig iron (the balance is scrap).

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merchant pig iron

Pig iron

In an integrated steelmaking plant, liquid iron,

rather pig iron, is used

The composition of liquid/solid iron produced is

always controlled to suit the steelmaking process.

‘Merchant’ pig iron (in solid form, 15-45 kg/pc)

• Acid, ‘hematite’ or ‘Swedish iron’ – (0.05% max. P) used in acid steelmaking processes

• Basic iron (0.2-0.4% P) – general purpose pig iron used in basic steelmaking processes

• Thomas iron (1.5% min. P) – used in special basic steelmaking processes

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Steel Scrap

Steel scrap consists of discarded

steel or steel products

Generally segregated by

composition and size or ‘grade’

suitable for melting.

• A scrap is off grade if it fails to meet

(i) applicable size limitations,

(ii) applicable requirements for the type

of scrap, and

(iii) applicable requirement with respect

to the scrap quality.

Three main types of scrap :

(i) internal scrap / home scrap

30-45% of total scrap generated

spillage, sheared ends, rejected materials

this scrap of known composition is mainly

used at the same plant as metallic charge

(ii) prompt scrap / process scrap

18-20% of total scrap

chips, trimming, forging and stamping wastes

(iii) dormant scrap / obsolete scrap

30% of the total scrap

used or worn-off machines, rails, domestic

appliances

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Hydraulically compressed

‘#1 bundle scrap’

Hydraulically compressed

‘#2 bundle scrap’

#1 Heavy melting scrap

#2 Heavy melting scrap

Shredded steel scrap

Busheling

Different types of steel scrap 8/28

Cast iron scrap

Rail scrap

Steel turnings

Ship scrap

Internal re-melt scrap (home scrap)

Internal re-melt scrap (ladle scrap)

Different types of steel scrap 9/28

Collection of scraps is carried out by

specialised organisations.

• returned to steel works either assorted or in

classified forms.

• often composition is not known exactly.

• often contaminated with (1) sulphur-containing

lubricating oils (2) non-ferrous metals (lead,

aluminium, tin, copper, etc.), and (3) soil, rust,

plastics, etc.

• some of these impurities are harmful for the

working personnel, steelmaking plants and

some influence the melting practice and steel

quality steel quality.

An appreciable percentage of scrap,

is rather inconvenient for charging into

steelmaking furnaces (low density, low

loose mass, inconvenient size,

contamination with soil, etc.).

Proper preparation of scrap (sorting,

removing dirt and plastics, bundling

loose scraps, etc.) can

(1) raise productivity of steelmaking shop

(by reducing the time of charging),

(2) increase yield of steel

(by lowering oxidation losses), and

(3) improve quality of finished product

(by reducing inclusions).

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Conventional

processes

Bessemer Up to 8%

Bessemer (with modification) Up to 12%

Open-hearth* Up to 75%

Electric* Up to 100%

Oxygen

steelmaking

processes

LD Up to 25%

LD (with modification) Up to 45%

Kaldo and Rotor Up to 45%

Proportion of scrap consumption in various steelmaking processes

*The open-hearth and the electric processes were developed chiefly to remelt the available scrap.

Until the advent of these two processes, there was no way to melt steel scrap because of the

limitations of the furnace not being capable of attaining steelmaking temperatures i.e.150-1600 °C.

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Sponge iron, or DRI

• The products of direct reduction of iron ores containing

90-98% iron is usually used in steelmaking plants,

mainly to dilute the impurities, as it contains the lowest

residual impurities.

• Specifications of metallised materials, also known as

direct reduced iron (DRI) or hot briquetted iron (HBI), or

sponge iron to be used in steelmaking should satisfy the

following requirements:

Metallisation degree, % > 90

Iron content, % > 90

Oxygen content (as FeO), % 2

Sulphur content, % < 0.01

Phosphorus content, % < 0.045

Gangue, % 4

Some special features of DRI

• practically free from impurity, which are common in steel scrap

• contains 5-8% gangue (SiO2) – more slag, more CaO required

• porous and low density – needs briquetting

• contain 1-2.5% C

• contains ~2% FeO – needs reducing

• porous, oxidises easily and self-ignites – to be careful in storing

1.2 Metallic Additions or, Ferro Alloys

Added principally to improve properties like tensile strength, ductility,

fatigue strength and corrosion resistance.

Additionally, there can be several other tasks for ferroalloys

• refining

• deoxidation

• control of non-metallic inclusions and precipitates

added as iron alloys to the bath to

• lower cost, and

• increase melting and dissolution rate (by reducing melting point of the alloy)

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Ferro alloy Grade Chemical Composition, wt%

C Si Mn Cr Other Fe

Ferro silicon 50-90% Si 1-2 50-90 -- -- -- Balance

Ferro manganese Standard HC 7 -- 78-82 -- -- Balance

Low carbon <0.50 6.5 85-90 -- -- Balance

Silico manganese EM <0.08 28-32 56-61 -- -- Balance

Standard 1 22 66 -- -- Balance

Ferro chrome

High carbon 7 max. 3 max. -- 58-65 -- Balance

Low carbon <0.75 2 max. -- 67-73 -- Balance

LC 65/5 <0.5 5 max. -- 64-68 -- Balance

Simplex LC <0.02 2 max. -- 68-71 -- Balance

Ferro moly 0.5 1.5 -- -- Mo 60-67 Balance

Ferro vanadium 0.1 1.25 -- -- V 50 Balance

Ferro niobium 0.25 4 max. 2 max. -- Nb 63.5 Balance

Ferro titanium 0.1 2 max. -- -- Ti 30-40 Balance

analysis of some common ferro alloys of standard grades

• Besides chemical composition and residual elements, size of ferroalloys is also important 14/28

2. Auxiliary Materials

Materials include:

• limestone

• lime

• bauxite,

• fluorspar,

• crushed fireclay, and

• mixtures and briquettes of one or more of

those previously mentioned (e.g., mixture

of lime and fluorspar or lime and bauxite).

Used as fluxes to

• bring down the softening point of gangue

materials and reduce viscosity of slag, and

• increase slag basicity, and decrease activity

of some components to make it stable in the

slag phase.

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Material Typical Composition, wt% Principal Activity

Limestone 50-54 CaO, 0.5-3.0 MgO, 0.6-1.0 SiO2, 0.01 S Increase slag basicity

Calcines dolomite 55 CaO, 34-38 MgO, 3-3 SiO2, 0.10 S Increase slag basicity

Lime 90-95 CaO, 2-3 MgO, 1.5 SiO2, 0.1-0.2 S Increase slag basicity

Ganister 0.5 CaO, 0.1 MgO, 94 SiO2 Increase slag acidity

Bauxite 54-56 Al2O3, 11-14 Fe2O3, 1-2 SiO2, 1-2 TiO2 Decrease slag viscosity

Fluorite 75-95 CaF2, 10 max. SiO2, 0.8 max. S Decrease slag viscosity

Crushed fireclay 35 Al2O3, 60 SiO2 Quickly decrease slag

viscosity of high-basic slag

Mixtures and

briquettes Mixture of lime-dolomite, lime-bauxite, etc. Accelerate slag formation

• SiO2 in dolomite or lime should be kept to a minimum

since it decreases the available base (%CaO – %SiO x Slag basicity).

• Presence of MgO in limestone is also detrimental to its quality

since MgO is not as effective as CaO in retaining phosphorus and sulphur in slag.

Typical characteristics of auxiliary materials

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3. Oxidants

Added to the bath to accelerate the

oxidation of carbon and other impurities.

Used either

• in the solid state (iron ore, sinter, ore pellets,

rolling scale, fluxed sinter, briquetted ore fines)

• or, in the gaseous state (compressed air,

oxygen, various mixtures including oxygen,

steam, carbon dioxide, etc.).

Solid oxidant should have

• a high concentration of iron oxide

and the least content of silica

• a high density where possible.

Gaseous oxidants should be

• clean and have as low nitrogen (<0.5%)

as possible to ensures proper conditions

for making steels free from nitrogen.

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4. Refractories

Inorganic non-metallic material, capable to withstand high

temperature (~1600 °C) without undergoing physico – chemical

changes while remaining in contact with chemically reactive

molten slag, metal and gases.

Have a crucial impact on the cost and quality of steel products.

The diversification on steel products and their cleanliness

requirement in recent years caused an increased demand for

high quality refractory.

• It becomes necessary to produce range of refractory materials with different

properties to meet range of processing conditions.

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Refractory materials are required to withstand:

1. A wide range of temperature, up to 2200 °C.

2. Sudden changes in temperature to cause thermal shock, resulting crack/fracturing.

3. Compressive stresses at both high and low temperatures.

4. Abrasive forces at both high and low temperatures.

5. The corrosive action of slags, ranging from acidic to basic in character.

6. great pressures and buoyant forces of molten metal

7. The corrosive action of gasses/volatile oxides/salts of metals.

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.

Why required?

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Steelmaking furnaces are lines with suitable refractory

materials, which erode during steel making and hence the

material of lining is also required as a recurring consumable

raw material.

The lining of furnace is made either by laying bricks (or

blocks) or by shaping the required contour in situ using a

refractory mix.

Often, the lining is repaired after certain number of heat to

maintain it in a proper ‘shape’ and ‘state’.

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Common manufacturing/installation methods

• Brick

• Castable – precast/vibrated/self-flow

• Shortcrete (wet gunning)

• Gunning

• Plastic

• Ramming material

• Mortar

• Insulation board/blanket

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Refractory materials and products are classified by several features

• chemical composition (acid, basic or normal)

• physical form (brick or monolithic/ ramming mass/castable/mortar/plastic mass)

• refractoriness (high (>2000 °C)/medium (1770-2000 °C)/low (1580-1770 °C))

• porosity and slag permeability

• strength

• density

• spalling resistance

• thermal expansion and thermal conductivity

Refractory Classification

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A. Based on chemical composition

2. Basic refractory

• raw materials: CaO, MgO, dolomite, chrome-magnesite.

• produced from dead burnt limestone, dolomite, magnesite,

chrome ore.

a) Magnesite cannot resist thermal stock, loose strength

at high temperature and are not resistant to abrasion.

b) Chrome-magnesite good resistance to chemical

action of basic slag and mechanical strength and volume

stability at high temperatures.

c) Magnesite-carbon excellent resistance to chemical

attack by steelmaking slags

1. Acid refractory

• raw materials: SiO2, ZrO2 and

alumino-silicate

• Typical refractories are fireclay,

quartz and silica.

3. Neutral refractory

• chemically stable to both acids

and bases

• manufactured from alumina,

chromia, and carbon

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B. Physical Form

1. Shaped refractories (bricks)

• have standard dimensions.

• machine pressed and have uniform properties.

• Special shapes are hand molded

• different types are:

(i) Ramming refractory material (dry/wet)

(ii) Castables

(iii) Mortars

(iv) Plastic mass

2. Monolithic refractories

(using ramming mass)

• loose materials used to form joint free lining

• can be installed by casting, spraying etc.

• used mostly in cold condition so that desired

shapes can be obtained with accuracy.

• main advantages

grater volume stability

better spalling tendency

eliminating joint

can be installed in hot standby mode

easier transportation

3. Insulating materials

• common insulating materials: ceramic fibres

(produced from molten SiO2, TiO2, ZrO2, etc)

in the form of wool, short fibres and long fibres

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Property Requirements

• Refractoriness

• Porosity and slag permeability

• Strength

• Specific gravity

• Resistance to spalling

• Permanent linear change on heating

• Thermal conductivity

• Bulk density

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Refractory

Major

Composition,

wt%

Refractori-

ness, %

Porosity,

%

Thermal

Resistance

at 1600 °C

Abrasion

Resistance

Resistance

Against Acidic

Slag and

Fluxes

Resistance

Against

Basic Slag

and Fluxes

Silica 93-96 SiO2 1700 16-17 Good Good Fair Poor

Low Alumina 40-45 SiO2

35-55 Al2O3 1700 20-23 Good Good Good Medium

High Alumina 1750 20-29 Good Good Good Poor

Magnesite 80-95 MgO <2000 20-23 Poor Medium Poor Good

Dolomite 58 CaO

40 MgO 1650 20 Poor Medium Poor Good

Carbon 85-90 C <2000 22-32 Good Poor Good Fair

Chromite

15-23 Al2O3

15-30 Cr2O3

10-17 Fe2O3

14-20 MgO

1800-1950 18-25 Fair Medium Fair Good

Carborundum 89-91 SiC <2000 17-20 Good Good Good Poor

Zirconia 67 ZrO2 2500 - Good Good Good Poor

Properties of some selected refractories

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The melting and casting of steel require an enormous quantity

(nearly 30 kg per ton of steel) of refractory materials.

Life of the working lining usually ranges from 20-100 heats.

Stability of refractories determines not only their consumption, but

also the productivity of steelmaking plant (the frequency and extent

of repairs), and the quality of steel produced.

Modern techniques of the off-furnace treatment of metal (inert-gas

blowing, vacuum treatment, etc.), which are associated with intense

metal stirring, require refractories of especially high quality.

Refractory Consumption

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Measures to decrease refractories consumption:

1. using better quality refractory;

2. collecting and reusing refractory waste left from furnace repair;

3. running the heats properly according to the operating possibilities of

the refractories;

4. replacing, fully or partially, the refractory lining by cooled (say, water-

cooled) elements; and

5. repairing the working layer of refractory lining periodically

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