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Existing Process
Rolled-products- TMT Bars/ CTD Bars
/ Wire Rods (Rolling Mill)
The process involves converting
the shape stock, viz. ingots, billets
to the desired finished section
in the hot condition by way of passing
the material between a pair of grooved
rolls and providing suitable drafts
at various stages. The whole operation
is conducted at a particular temperature
range and within a limited time
span. The stages of rolling operation
are comprised of heating of feed
stock to rollable temperature, rolling
the feeding stock in different mill
stands, cropping the hot bar during
the process of rolling between mill
stands as applicable and subsequently
finishing in form of hot rolled
deformed bar in straight length.
The hot bar coming out of the last
pass is then conveyed through TMT
line and collecting in a cool bed
after shearing. The bars at almost
ambient temperature are sheared
to commercial length stored and
kept ready for dispatch.
The manufacturing
process of the rolling mills products
can be depicted as follows.
In TMT process
hot bars are subjected to quenching
by means of an intense cooling installation
(cooling installation specially
designed spray system). This step
hardens the surface layer to marten
site while the core structure remains
austensite. When the bar is free
of water chamber heat flows from
core to surface and surface gets
tempered to structure called martensite.
In the cooling bed due to atmosphere
cooling, the hardened zone is tempered
by temperature homogenization in
the cross section and the austerite
core is transferred to ductile-ferrite-pearlite
core.
In case of CTD Bars and wire rods
the thinner sections of hot bar
coming out of the last pass is coiled
through a coiler, whereas the thicker
sections of CTD bars are collected
on a cooling bed. The strength to
the CTD bar is given by twisting
it on the twisting machines and
not by the quenching process as
in the case of TMT bars.
Diagrammatic presentation
of the existing material flow -
1. For Alloy Steel &
Stainless Steel Billets
Graded Steel scrap is melted with
the help of electrical energy in
the Induction Furnace. The molten
metal is poured into the Ladle and
transferred to AOD Converter. Alloying
additions (Ferro Alloys) are made
in the AOD Converter to get the
desired grade of the stainless steel.
AOD Converter gets the required
energy from Oxygen, Nitrogen and
Argon gases blown continuously.
Samples for the chemistry are checked
at regular interval in the Lab with
the help of Spectrometer. Once the
desired chemistry and temperature
are achieved, the liquid metal is
poured into the Ladle and sent to
the Continuous Casting machine.
Here the liquid metal is poured
into water cooled Copper Mould Tubes
to solidify the liquid metal into
the desired size and shape. The
solid metal known as “Billet”
cut into the desired length and
after cooling on the Cooling Platform,
shifted to stockyard with the help
of EOT Cranes.
All the billets cast out of the
individual liquid metal ladle, are
given an identification mark/grade.
The billets are inspected for surface
and inner cracks/other surface defects
and sent for surface conditioning,
if required.
The process flow diagram is as follows:
2.For Stainless
Steel Flats & Rounds/ Wire Rods:
The quality-passed billets
of value added alloys steel including
stainless steel are rolled into
flats of desired dimensions on medium
section rolling mill and/or rolled
into wire rods of desired diameters,
on wire rod mill equipped with No
Twist Block. Medium section rolling
mill is also used to produce medium
and heavy rounds. During rolling,
finished/intermediate sections are
checked at regular intervals for
any rolling defects. The rolled
products are shifted to the finished
goods stockyard and marked lot-wise
with the same identification mark/grade
of the billets out of which the
same are rolled. The rolled products
after physical inspection are cleared
for dispatch. The process flow diagram
is as follows:
3.For Sponge Iron (DRI Plant)
One ported rotary kiln is provided
for reduction of iron ore into DRI
using non-coking coal as reductant.
The rotary kiln will be supported
on 4 piers. A slope of about 2.5%
will be provided. The two main drives
will be by two DC motors with Thyristor
control. The auxiliary drive of
kiln will be by two AC motors. The
speed of the kilns will be in the
range 0.25-0.75 rpm. Each kiln will
have 4 plain riding rings and one
thrust riding ring and will be provided
with 4 sets of support rollers at
the 4 piers and one set of hydraulic
thrust rollers with bush bearings.
A start up burner using fuel oil
will also be provided for initial
heating.
The kiln feed from the charging-end,
will consist of screened iron ore,
coal and limestone/dolomite. Air
will be supplied to the kiln through
ports provided on kiln periphery
over almost 2/3rd length of the
kiln. This ensures a controlled
combustion resulting in a very even
temperature profile. A part of required
coal shall be thrown from kiln discharge-end.
The slinger coal will be withdrawn
from the bin and pneumatically injected
into the kiln. Necessary rotary
feeder, compressor, piping and valves
will be provided.
In the kiln, the Iron Ore will be
dried and heated to the reduction
temperature of about 1000 deg. C.
The Iron oxide of the Ore will be
reduced to metallic iron by carbon
monoxide generated in the kiln from
Coal. The heat required for the
reduction process will also be supplied
by the combustion of Coal. Thermocouples
will be installed along the length
of the kiln shell for measurement
of thermal profile of the kiln.
The temperature will be controlled
by regulating the amount of combustion
air admitted into the kiln through
ports with the help of fans mounted
on the kiln shell and by controlled
coal slinging. The DC main drives
provided to rotate the kiln will
have variable speed. Auxiliary drive
is provided for slow single speed
rotation.
The reduced material from the kiln
will be cooled indirectly in a rotary
cooler by water spray. The rotary
cooler will be of 4.0m dia (ID)
and 50m length and will be supported
on two piers with a slope of about
2.5%. The main drive of the cooler
will be by one DC motor and the
auxiliary drive will be by one AC
motor. The speed range of the main
drive will be from 0.5-1.5 rpm.
The cooler will be provided with
one plain riding ring and one thrust
riding ring along with two sets
of supporting rollers with bush
bearings at two piers. The cooler
is provided with mechanical guide
roller and lifters in eight rows
along the length. About 1.5m length
at the discharge end of the cooler
will act as a screening section
which separates all the accretions
larger than 50mm from the reduced
material. These lumps will be discharged
separately via lump gate. Rest of
the material will be discharged
on to a conveyor via double flap
valve.
The cooler will be lined with castable
refractory for about 4m length from
the feed end. Bypass arrangement
will be provided at the discharge
end of the cooler for emergency
discharge of materials. The cooled
product will be conveyed to the
product processing building by a
system of belt conveyors.
4.Process of Steel Billets
(Steel Melting Shop)
Purchased scrap processed into appropriate
sizes, and return scrap will be
charged into the crucibles to ensure
optimum packing. Necessary Carbon
in the form of petroleum coke, breeze
coke, etc. will be added into the
crucibles to ensure the availability
of necessary carbon in the bath.
Once the liquid bath has
been formed and the minimum temperature
of the bath has been achieved, sponge
iron will be charged in small batches
and the slag formed will be removed
as and when necessary. After the
completion of charging of sponge
iron a sample will be drawn to determine
the composition of the bath and
adjust the carbon content of the
bath. Next, the temperature will
be raised to the tapping temperature.
The final content of C,
Mn, Si shall be adjusted in the
LRF by adding requisite amount of
Ferro Alloys, Burnt Lime, etc. The
final composition of Steel shall
be determined by drawing a sample
of liquid Steel from the ladle.
The continuous casting process shall
be used for casting the liquid steel
into billets.
5.Electric Power
Waste Heat Recovery Boiler (WHRB):
Waste Heat recovery Boiler is in
line with the ABC of the Sponge
Iron Kiln. The Hot gasses from the
Sponge Iron Kiln pass through the
ABC (After Burning Chamber) of the
kiln, where the remaining Carbon
Mono Oxide is burnt by injecting
air. The gas volume increases and
also the total heat, which is required
to generate more steam.
The hot gasses
are passed through the WHR Boiler.
The boiler is in three parts: Radiation
Zone, Superheating Zone and Economizer/
evaporator zone. As the gasses pass
through these zones, its temperature
falls and the heat is absorbed by
the water and also by steam.
Radiation Zone
is made of water walls where gasses
have maximum temperature. The water
absorbs heat by radiation method.
These walls are connected with the
water side of boiler drum. Gasses
enter the second pass where, first,
it comes in contact with the super
heater zone. In the super heater
zone, the steam absorbs the heat
to become superheated and than taken
to the common steam header for utilizing
in the Turbine. Upper stage of second
pass has Evaporator. Evaporator
is connected with the steam compartment
of boiler drum. Here the evaporated
steam absorbs heat to become saturated
i.e. all the water content gets
evaporated and becomes steam.
The much cooler
gasses pass through third pass where
some coils are of evaporators and
rest of economizer. In the economizer,
the feed water absorbs heat from
still hot gasses to increase the
efficiency of the boiler. The water
enters in the economizer from boiler
feed pump at 120 oC. In the super
heater zone, one stage of attemperator
is also added. The use of attemperator
is to control the temperature of
the steam leaving boiler.
Thus, in WHRB the water becomes
superheated steam at desired pressure
and temperature by absorbing heat
from hot gasses.
Atmospheric Fluidized Bed Combustion
(AFBC) Boiler:
AFBC boiler is
to combust the fuels like char and
coal to generate heat which is absorbed
by the water to become superheated
steam at desired temperature and
pressure. In the combustion zone,
fluidization process is used to
optimize the heat absorption from
low-calorie fuels for good efficiency.
The construction
of boiler is same as that of WHRB
except that it has fluidized bed
for combustion of fuel and some
super heater coils are embedded
in the fluidized bed to keep the
temperature of the bed material
and fuel under control.
The gasses are
passed in the furnace zone, where
water walls are there to absorb
heat by convection method. Other
stages of super heater, evaporator
and economizers are same as described
under WHRB. However, this boiler
has forced draft fan to give air
for fuel injection and also for
combustion inside the fluidized
bed zone. This FD fan air is pre-heated
in an air pre-heater where it absorbs
heat from hot gasses coming out
of the third pass of the boiler.
The fuel (Char
and Coal) are pneumatically pushed
in different zones of the fluidized
bed, where it combust due to heat
and presence of sufficient oxygen
in the air.
Turbo- generator:
Steam Turbo-generator is the machine
which coverts heat energy of the
steam into Electrical Power. It
consists of Steam Turbine, Generator
and brush-less Exciter. The steam,
when enters the turbine, rotates
it converting heat energy into mechanical
power. The rotation of the turbine
has to be kept fixed at designed
speed by Hydraulic governor. The
turbine is coupled with the generator.
The generator rotor has dc field
creating magnetic field of desired
strength. The magnetic field rotates
cutting the armature coil and generating
voltage in it. The level of voltage
is regulated by controlling the
excitation of field coils. This
is done automatically by AVR (Automatic
Voltage Regulator).
In the Turbine
outlet, steam comes out at very
low pressure and temperature. It
is condensed in the condenser (sort
of heat exchanger) and resend to
boiler through deaerator and boiler
feed pump. The electricity, so generated
is synchronized with available power
in the switchyard and used to operate
various section of the plant and
Steel Melt shop to manufacture steel
billets.
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