Ppt_on_steel_making.ppt

Raw Materials for Production
 Iron Ore
 Limestone ----------
 Coke

Iron Ore
 Abundant, makes up 5% of earth’s crust
 Is not found in ‘free state’, must be found in rocks
and oxides, hence Iron ore.
 After mining, the ore is crushed and the iron is
separated, then made into pellets, balls or
briquettes using binders, such as water.
 The pellets are typically 65% iron, and about 1” in
diameter.

Coke
 Coke is formed by heating coal to 2100*F (1150 C),
then cooling it in quenching towers.
You need more than Iron? Why coke is
used…
1. Generates high heat, needed in order for chemical
reactions in ironmaking to take place.
2. Produces CO (carbon monoxide) which reduces
iron-oxide to Iron.

Limestone
 Limestone (calcium carbonate) is used to remove
impurities.
 When the metal is melted, limestone combines
with impurities and floats to the top of the metal,
forming slag. The slag can then be removed,
purifying the iron.

 Coking Coal and iron ore are the basic raw materials
for the production of pig iron, which will further be
refined to steel in the steel works.
 In the coke oven plant coal is processed to coke. Coke
is essential in the lower part of the blast furnace to
maintain the gas permeability. Due to the high weight
load of the burden column simple coal would break into
small particles and block the gas stream.
 As a byproduct of the coking process coke oven gas
mainly consisting of hydrogen and methane is
generated, which is used mainly for generation of
electric power in a thermal power plant.

 Most of the iron ores and additves are traded as
fines and need some pretreatment before being
processed in the blast furnace.
 This preprocess is called sintering, where, by
means of heat
 (approx. 1300°C) the fines are somehow baked to
particles of between 20 and 50 mm mean diameter.
These particles, called sinter, are also permeable
for the reduction gases.

 In the blast furnace the reduction of the iron
ores (Fe2O3) to metallic iron and the melting
of the iron and by products take place.
 Both, reduction and melting need high
temperature, which is generated by
combustion of coke using hot blast, blown in
via tuyeres located around the furnace.
 The reduction gas leaving the furnace is
called top gas and after a cleaning process
used to preheat the air in the hot blast
stoves.

 Hot metal and slag are further tapped
from the hearth of the blast furnace, by
drilling a so called tap hole, which will be
closed after aproximately one to one and a
half hours again.
 The separation of hot metal and slag takes
place in a so called pool runner outside the
furnace, using the density difference.

What is HBI?
 Product of a Direct Reduction process
that has been briquetted at a
temperature of greater than 650 C (1200
F) to a minimum density of 5.0g/cc
 Weight each from 0.5 to 1.5 kg, 1 to 3 ½
lb
 Size
 L=90 to 130 mm, 3 ½ to 5 in
 W=80 to 100 mm, 3 to 4 in
 T=20 to 50 mm, ¾ to 1 ½ in

Definitions
 DR: Direct Reduction. Reduce iron oxide
to metallic iron without melting.
Unreduced ore compounds remain as
undesirable oxides
 DRI: Direct Reduced Iron. Iron oxide
feedstock exits in same form as entered
(pellets in, pellets out; lumps in, lumps
out)

 HBI: Hot Briquetted Iron: DRI that has been
hot (1200 F, 650 C) briquetted to a high
density pillow shaped briquette
 Hot Metal: Molten iron in liquid form, above
2500 F, 1370 C
 Pig Iron: Solid product of the iron blast
furnace
 Residuals: Undesirable elements such as
copper, nickel, chromium, tin, sulfur
molybdenum, phosphorous

 Gangue: Rock minerals in the iron ore such
as silica (SiO2), alumina (Al2O3), calcia
(CaO), magnesia (MgO).
 These remain in the oxide form in DR
processes
 Reduction:
◦ Fe2O3 + 3CO = 2 Fe + 3CO2
◦ 2Fe2O3 + 3H2 = 2Fe + 3H2O
◦ (Fe2O3 > Fe3O4 > FeO > Fe )

Iron Ore (DR Pellet)
Chemistry
Fe 67.00 SiO2 1.60
Al2O3 0.45 CaO 0.60
MgO 0.45 Mn 0.04
P 0.030 S 0.003
Cu 0.003 Pb 0.02
Zn 0.008 V 0.005

HBI Chemistry (Typical)
Total Iron (Fe) 90.00 to 93.00
Acid Gangue 1.95 to 5.10
P 0.015 to 0.060
S 0.005 to 0.020
C 1.07 to 1.60

Stock House
Hot Blast Stoves
Gas Cleaning
Scrubber
Ironmaking in the Blast Furnace Plant
Bell Less Top
Blower
Combustion Air Slag, Hot Metal
Gasholder
Enrichment Gas
Combustion Gas
Sinter Coke

Raw Materials  Pig Iron
 The three raw materials are dumped into a blast
furnace.
 Hot air (2000*F) is blasted into the furnace, which
helps drive the chemical reaction.
 The coke forms CO and the CO reduces the iron
oxide to iron.
 The slag floats to the top and the metal is
transferred to molds and cools. IT IS NOW PIG
IRON, ready for more iron work or steelmaking.

Blast
Furnace
Tuyeres
(Same height as a 10 story building)

Blast
Furnace
•The purpose of a blast furnace
is to reduce and convert iron
oxides into liquid iron called "hot
metal".
•The blast furnace is a huge,
steel stack lined with refractory
brick.
•Iron ore, coke and limestone
are put into the top, and
preheated air is blown into the
bottom.

Why does Iron have to be extracted in a Blast Furnace?
• Iron can be
extracted by the
blast furnace
because it can be
displaced by carbon.
• This is more efficient
method than
electrolysis because
it is more cost
effective

Several reactions take place before the iron is finally produced...
• Oxygen in the air reacts with coke to give carbon
dioxide:
 C(s) + O 2(g)  CO2(g)
• The limestone breaks down to form carbon dioxide:
 CaCO3(s)  CO2 (g) + CaO(s)
• Carbon dioxide produced in 1 + 2 react with more
coke to produce carbon monoxide:
 CO2(g) + C(s)  2CO(g)

• The carbon monoxide reduces the iron in
the ore to give molten iron:
 3CO(g) + Fe2O3(s)  2Fe(l) + 3CO2(g)
• The limestone from 2, reacts with the sand
to form slag (calcium silicate):
 CaO(s) + SiO(s)  CaSiO3(l)

• Both the slag and iron are drained from the
bottom of the furnace.
• The slag is mainly used to build roads.
• The iron whilst molten is poured into moulds
and left to solidify - this is called cast iron and is
used to make railings and storage tanks.
• The rest of the iron is used to make steel.

Steelmaking
 Steelmaking is the second step in producing
steel from iron ore.
 In this stage, impurities such as S, P and
excess carbon are removed from the raw
iron, and alloying elements such as Mn, Ni,
Cr and vanadium are added to produce the
exact steel required.

Steel making processes
 Bessemer Converter Process.
 Open Hearth Process.
 Basic Oxygen Converter Processes.
(1) Bath Agitation Process.
(2) LDAC/OLP Process.
(3) KALDO Process.
 Electric Arc Furnaces.
(1) AOD process.
 Secondary Steel Making.
(1) Vacuum Degassing process (VOD).
(2) Refining by re-melting.
- Electroslag Remelting (ESR).
(3) Ladle Metallurgy.
- Process Capabilities of Ladle Furnace.

Bessemer Process
•In the Bessemer process
compressed air or oxygen is blown
into the bottom of a converter, a
furnace shaped like a cement
mixer, containing molten pig iron.
• The excess carbon in the iron
burns out, other impurities form a
slag, and the furnace is emptied
by tilting. The proportion of scrap
consumption is upto 8%.

Cont….
 In this process molten pig iron is held in a vessel
with perforated bottom called a converter.
 Cold air or oxygen enriched blast is forced through
the metal. This is a autogeneous process, i.e., no
external heat is needed, it is the exothermic chemical
reaction.

Cont….
 Refining is complete in about 15-20 minutes taking
into account the time required for charging, tapping,
teeming, etc.. A tap to tap time of about 30-35
minutes are required.
 It was capable of removing only silicon, carbon,
phosphorous (less than 0.05%) and manganese as
impurities from pig iron, i.e., it was an acid process.
 The large amount of phosphorous (more than 0.05%)
was eliminated by basic process.

 It is heated by either liquid or liquid and
gaseous fuels using the heat generation
principle so as to attain steel making
temperatures of about 1700ºC.
 A tap-to-tap time of about 6-10 hours.
 It takes much longer time for refining than
bessemer does and hence the heat losses by
radiation.
 The proportion of scrap consumption is 75%.

Basic Oxygen Converter Processes.
(LD, Lintz and Donawitz, Converter)
Basic oxygen furnace showing
BOF vessel during processing of a
heat. The proportion of scrap
consumption is upto 25 %.

Basic-Oxygen
Furnace(BOF)
Oxygen Blowing
Hot Metal Charge
Tap-Out & Transfer
to Ladle Metallurgy
Facility
Scrap Charge
BOF sequence : (1) charging of scrap and (2) pig iron, (3) blowing, (4)
tapping the molten steel, & pouring off the slag.

 The basic oxygen furnace converts iron from the blast
furnace into steel at 1260 to 1300ºC. This is achieved
by blowing oxygen through the molten iron in the
BOF vessel, where it combines with and removes
carbon as carbon monoxide and carbon dioxide.
Unwanted silicon, phosphorus and other elements are
also driven off, while added fluxes (typically lime)
combine with other impurities to be removed as slag.
 Alternative names for this plant/process are steel
converter, BOS (basic oxygen steelmaking), basic
oxygen process (BOP), and LD process.

 BOFs, which can refine a heat (batch) of steel in less than 45
minutes, the latter required five to six hours to process the
metal. The BOF’s rapid operation, lower cost, and ease of
control give it a distinct advantage over previous methods.
 Scrap is dumped into the furnace vessel, followed by the hot
metal from the blast furnace. A lance is lowered from above,
through which blows a high-pressure stream of oxygen to
cause chemical reactions that separate impurities as fumes or
slag. Once refined, the liquid steel and slag are poured into
separate containers.
 However this method involves a risk of increasing the
inclusions and nitrogen contents in the steel due to the oxygen
jet is supersonic and has a speed between 1.5 and 2.2 times the
speed of sound.
 40-50% sulphur is removed and 90% phosphorous is removed.

BATH AGITATION PROCESS
(combined blowing)
 In this process additional gas is introduced through
tuyeres judiciously located in the bottom and (oxygen +
lime) from the top of the converter.
 Better mixing of slag and metal, preventing excessive
compositions.
 Smoother and more predictable carbon-removal and
refining process.
 For production of low carbon steel.

LDAC/OLP PROCESS
 The modified process is known as LDAC process. The
phosphorous is removed from the total system.
 In this process where lime powder is introduced along
with oxygen through the lance to refine high
phosphorous.
 After pouring the high phosphorous (1.8%) pig iron into
the converter, two slag operations are necessary if the
initial phosphorous content in the hot metal is high.

KALDO PROCESS
 In this process high phosphorous iron containing about
1.8% P may be refined with thermal efficiency.
 In blowing position, the furnace is inclined at an angle
of 15 degree.
 The oxygen is blown into the furnace above the bath
surface to refine high phosphorous iron (1.8%) to steel
with low S, P and N.

Electric Furnace
 Steel-making furnace
where scrap is generally
100% of the charge.
 Heat is supplied from
electricity that arcs from
the graphite electrodes to
the metal bath.

 The EAF is a refractory-lined vessel with a retractable
cover through which large graphite electrodes are
lowered once the scrap has been charged and the
furnace top closed. EAFs are usually of 60-150t
capacity per melt, but occasionally larger. However,
they are usually much smaller than BOFs.

 Melting occurs due to the energy released by
arcing between electrode and scrap. There are
normally three electrodes, but only one with
direct current furnaces.
 Much effort has been directed at minimising the
time from scrap charging to steel pouring (tap-to-
tap time). It is now standard practice to transfer
steel to a separate furnace for alloying
modifications (secondary metallurgy) to free-up
the EAF for the next charge. Scrap pre-heating
and oxygen injection also raise productivity and
reduce energy use.

Furnace operation
 The electric arc furnace operates as a
batch melting process producing
batches of molten steel known
"heats". The electric arc furnace
operating cycle is called the tap-to-tap
cycle and is made up of the following
operations:
 Furnace charging.
 Melting.
 Refining.
 De-slagging.
 Tapping.
 Furnace turn-around.

AOD
 AOD stands for argon oxygen decarburization, A process
for further refinement of stainless steel through reduction
of carbon content.
 Most stainless steel is initially produced in an electric arc
furnace before being transferred to a separate ladle
furnace for refining to achieve the precise metallurgical
content required – a process known as secondary
metallurgy or secondary refining.
 The amount of carbon in stainless steel must be lower
than that in carbon steel or lower alloy steel. While
electric arc furnaces (EAF) are the conventional means of
melting and refining stainless steel.

 Molten, unrefined steel is transferred from the
EAF into a separate vessel. A mixture of argon
and oxygen is blown from the bottom of the
vessel through the melted steel.
 Cleaning agents are added to the vessel along
with these gases to eliminate impurities, while
the oxygen combines with carbon in the
unrefined steel to reduce the carbon level. The
presence of argon enhances the affinity of
carbon for oxygen and thus facilitates the
removal of carbon.

SECONDARY STEEL
 Forging grade ingots for critical applications now
requires a close control over the alloy composition,
inclusion and gas content.
 The carbon content in some new grades of flat
products have been brought down to levels which
cannot be economically achieved in primary steel
making processes like EAF or BOF.

 All these developments have spurred the growth of a
whole range of steel refining process. These processes
are called secondary steel making processes, where
the final process of refining, degassing, alloying and
temperature adjustment are carried out in suitably-
equipped vessels between tapping and teeming.
 Modern steel making is therefore carried out in two
stages- the first stage uses primary steel making in
BOF and EAF for the production of a raw steel, which
is then further refined in the second stage by various
process. Key operations can include deoxidation,
desulphurisation and dephosphorisation.

VACUUM DEGASSING PROCESS
 Vacuum degassing (VD) is used following steel
making to reduce the carbon, nitrogen,
hydrogen and sulphur content of molten steel.
Phosphorus can also be reduced.
 The process takes place under vacuum in a ladle
furnace where it is heated and stirred by an
electrical current while oxygen enters from the
top of the vessel , and is frequently employed
by both volume and special steels producers.

 When dealing with high-chromium steels, VD
allows very low carbon content to be achieved
without heavy chromium losses from the melt.
 Vacuum degassing has become widespread as
demand for higher quality steels has grown in
sectors like automotive, construction, offshore,
pipe making and rails.
 In alloy steel products like bearings VD steels
improve fatigue life, while in flat products, very
low carbon VD steels are well suited to
demanding processing and fabrication.

ELECTRO-SLAG RE-MELTING
 The consumable electrode is dipped into a
pool of slag in a water cooled mold.
 An electric current passes through the
slag, between the electrode and the ingot
being formed and superheats the slag so
that drops of metal are melted from the
electrode.
 They travel through the slag to the bottom
of the water cooled mold where they
solidify.
 The slag pool is carried upward as the
ingot forms. The new ingot of refined
material builds up slowly from the bottom
of the mold.

Ingots
 While steel is still molten, it is poured into a mold. The
mold may be a square, rectangle or round. The metal
becomes an “ingot” in the mold.
 They can weigh 100 lbs to 40 tons.
 The ingot will be removed from the mold and heated
uniformly to be rolled or formed into a final product.
 HOWEVER – While the molten metal cools, or
solidifies, gasses evolve and can affect the quality of
the steel. This leads to three types of steel: Killed
Steel, Semi-Killed Steel, and Rimmed Steel.

Killed – Semi-Killed – Rimmed Steel
 Killed Steel – This is a fully deoxidized steel, and thus,
has no porosity.
 This is accomplished by using elements like
aluminum to de-oxidize the metal. The impurities
rise and mix with the slag.
 It is called killed because when the metal is poured it
has no bubbles, it is quiet.
 Because it is so solid, not porous, the ingot shrinks
considerably when it cools, and a “pipe” or
“shrinkage cavity” forms. This must be cut off and
scrapped.

 Semi-Killed Steel: This is practically the same as killed
steel, with some minor differences.

 It is only partially de-oxidized, and therefore, is a
little more porous than killed steel.
 Semi-Killed does not shrink as much as it cools, so
the pipe is much smaller and scrap is reduced.
 It is much more economical and efficient to produce.

 Rimmed Steel:
 This is produced by adding elements like aluminum to
the molten metal to remove unwanted gases. The
gasses then form blowholes around the rim.
 Results in little or no piping.
 HOWEVER, impurities also tend to collect in the
center of the ingot, so products or rimmed steel need
to be inspected and tested.

Continuous Casting
-Molten metal skips ingot step,
and goes directly the furnace to
a “tundish”
-Metal solidifies in the mold
-The metal descends @ about
1”/sec
-The solidified metal then goes
through
‘pinch rollers’ that determine the
final
form.

Benefits of Continuous Casting
 Costs less to produce final product
 Metal has more uniform composition and properties
than ingot processing.

Ppt_on_steel_making.ppt

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