STEEL MAKING DEPARTMENT (Pakistan Steel)
STEEL MAKING DEPARTMENT
(
Pakistan Steel )
Steel industry is the most important mother industry that
assures the future of downstream manufacturing opportunities to drive economic
growth of a country.
An integrated
Iron and Steel manufacturing plant assures the sovereignty and progress of a nation endowed with absolute
technological control through production of customary steel grades to high
performance steel grades needed for specific applications.
The
Steel Making Department of Pakistan Steel (formerly known as Steel Making Plant
Complex) is comprised of a 1300 ton Mixer (a storage vessel of B.F molten pig
iron), two Linz. Donawitz Steel converters. Each Steel converter (Basic Oxygen
Furnace) has a production capacity of 130 tons of steel for which there was initially
one Bloom caster and two Slab casters were installed. A Billet continuous caster
was added later in 1989 to Steel Making Department to further enhance the
production facilities of long products
.
The gigantic erection and installation work of Steel Making Department
(Pakistan Steel).
The earthquake proof gigantic erection and
construction work of Steelmaking department is elite of its kind in Pakistan
and has a life of more than hundred years. Under the direction of Soviet
experts, a team of devoted and dedicated Pakistani engineers and technicians
(of which I myself was a part of team) had accomplished the task within
pertinent schedule of time.
The scheme of Steel Structure erection of
main building of Steel Making Department (Pakistan Steel) also illustrates the
scheme of installation of 130 tons LD-Converters and Waste Heat OKг-Boilers.
Steel
Structure erection of the Main Building of Converter Shop was carried out with a
distinctive crane BK- 1000 (Soviet make).
The
BK-160 crane’s tracks were installed on
erected roof cross bars ( encompass X- Ray welded joints ) of Continuous
casting span in order to expedite the erection work that houses the Waste heat OKг-Boilers
, steam drums of boilers, Gas Cleaning system, Smoke exhaust pipes, Oxygen lances, Conveyors
and storage bins for Bulk materials( lime, iron ore, CaF2, coke) supply system of
each 130 tons LD-Converter between the elevations of 40 – 73
meters.
A side view of
Steel Making Department (Pakistan Steel) illustrating the premises of main
equipments and stockyard adjoining the rolling mills.
The
above layout is the most efficient arrangement for the flow of the products
from steelmaking department to rolling mills and has plenty of room for further
modification as per present requirements.
For
instance, with modest amendments, the Bloom caster can be converted into Beam Blank
caster in conjunction to Billet rolling mills in order to make the steel
production more efficient and economical. Beam Blanks are the cast shapes close
to the shapes of rolled products.
A view of Steel Making Department illustrating
the Oxygen supply pipelines from Tonnage Oxygen Plant to Steel Making Converters.
The
LD-Converter is the key component in the steel production process in
an integrated steel plant like Pakistan Steel.
Above
is the group of my counterpart Soviet senior engineers that has helped in establishing
the steel production process and trained Pakistani engineers and technician’s of
steel making department Pakistan steel.
The Raw Materials for LD-Converter (B.O.F).
The
raw material charged into the LD-Converter consists mainly of hot metal (molten
pig iron), Steel scrap, high Calcium burnt lime , dolomite, fluorspar and Iron
Ore. The charged percentage of these materials is dependent upon a number of
operating parameters.
1300 Tons Mixer (Hot Metal storage vessel) at SMD.
Molten
pig iron (Hot metal):
The quantity, composition and temperature of
molten pig iron as tapped from Blast Furnaces may vary from one tapping to
another. Besides, the tapping schedule of a Blast furnace is usually not
matched or coincides with the time of charging of steel Converters of Steel
Making Department. In order to keep the pace of production smooth at Steel
Making Department, a 1300 tons capacity Mixer (approximate volume 285 m³) has been provided as per design which acts
as a reservoir of molten pig iron at SMD.
The
function of 1300 ton Mixer is to help the LD-Converter operators to set
operating parameters for steady production cycle.
Technical specifications of Hot metal for steel production at
Pakistan Steel.
The average Physical temperature of hot metal poured out of Mixer is kept 1320 ºC and the Chemical
composition is controlled within the limits as under:
-
Carbon not less than 4%
-
Si :
0.4 – 0.8%
-
Mn : 0.5 – 1.0%
-
P :
0.2% max
-
S :
0.04% max
LD
steel converter can principally operate on molten pig irons of widely varying
chemical composition, although the above limitations are chosen for an easier
run of the steelmaking process.
.
The content of Silicon:
As
Silicon is oxidized, it forms SiO2 and produces heat. SiO2 accelerates
dissolution of lime in slag and the process of slag formation as a whole, while
the liberated heat is utilized for melting scrap. With an excessively high
concentration of silicon, however, the quantity of SiO2 formed is too high and
necessitates an increased addition of lime. Besides, a large volume of slag produced
with high concentration of SiO2 is harmful for converter refractory lining and
creates slag handling problems.
The content of Manganese:
Manganese
liberates heat on oxidation and accelerates slag formation. Manganese is burned
off almost fully during converter blowing and promotes desulphurization of
metal.
The content of Phosphorus and Sulphur:
Phosphorus
impairs the plastic properties of steel by forming high- phosphorus brittle
streaks ( interlayer’s) between metal grains during solidification of a casting
that results in cold shortness of steels that have a tendency to break or
crumble during service, which is especially noticeable at low temperatures
(blue brittleness).The harmful effect of phosphorous is more pronounced in
steels with elevated carbon content.
An elevated concentration of
Sulphur in steel may often result in what is called ‘hot cracking’, the defect
being found, in particular, in continuously cast steel.
Sulphur is characterized by
unlimited solubility in liquid iron, and limited solubility in solid iron.
During solidification of steel, iron sulphides, which are last to solidify,
precipitate on grain boundaries. Iron and iron sulphide form a low-fusible
eutectic ( melting point 988 ºC ) which melts at even lower temperatures if oxygen is
present in the melt ( due to formation of oxy- sulphides).
The red-shortness effect is especially pronounce in
cast steel, since sulphides and oxysulphides are concentrated in that case on
boundaries of primary grains. Intergranular layers of the high-sulphur phase
are softened when the metal is preheated for rolling or forging, and the steel loses
its properties and may fracture (red shortness).
The
concentration of these harmful elements in molten pig iron must be kept as low as
possible.
On
the basis of above hot metal analysis and temperature, the ratio of the amounts
of liquid pig iron and scrap is adjusted for each batch (heat).
Steel Scrap.
Good quality scrap is continually becoming more scarce and more expensive, which is the reason that economic conditions are decisive in the selection of the steel production route. However the BF-LD-Converter route will remain the future process leader for production of high grade steels, especially for flat products.
The
LD-Converter (Basic Oxygen Furnace) can consume up to 25% scrap of the total
charge that comprises of most of the circulating scrap of the steel mill. Different
kinds of scrap need different processing due to size, density, thickness, composition
of the material and purity requirements of end use.
The rejected sheets and strips from hot and cold rolling mills.
The
rejected sheets and strips from hot and cold rolling mills are pressed in
Baling Press Section of S.M.D. that is sited near Rolling Mills area to form
Bales delivered to the stocking Pit of steel making department through trucks
for recycling in Converter.
Apart
from the pressed bales, other scrap of rejected sheared ends/ trimmings is
supplied to steelmaking department from various sources.
Two
magnetic cranes unload the scrap bearing trucks in the stocking pit of Steel Making
Department that is located at
the north far end of Converters charging aisle.
Appearance of 5 tons magnetic over head crane in the aisle of
130 tons LD-Converter S.M.D.
Subsequently these overhead magnetic cranes at
the scrap pit are employed for blending and preparing the weighed scrap trays for
charging into 130 tons LD-Converters at Steel Making Department.
Scrap stocking and blending pit is sited north
side of Converter charging aisle of steel making department. The scrap formed
at an integrated iron and steel plant (trimmed ends of rolled products,
rejected cast bloom/slabs end cuts scrap, etc) is heavy scrap of known chemical
composition and is mainly recycled in converter as metallic charge.
The scrap obtained from sources outside the
plant like machine building works (chips, trimming, forging and stamping waste,
etc) or dormant scrap (used or worn-off machines, rails, domestic appliances,
etc), its composition is often not known exactly.
Scrap is sometimes contaminated with
sulphur-containing lubricants (chips), non-ferrous metals ( lead, Zinc, aluminium,
tin, copper, etc). Some of these impurities, in particular lead, Zinc, and tin,
are harmful for the working personnel, steelmaking furnaces, boiler and steel
quality. For instance, Zinc vaporizes on heating and settles as Zinc oxide on
furnace refractory lining, elements of Waste heat boilers etc. Lead penetrates
even the finest pores of furnace brickwork can cause severe damages. Tin can
sharply impair the steel strength at elevated temperatures.
Residual
elements in the scrap, such as copper, tin, chromium, and nickel, can reduce
the sheet steel ductility in the final product. For demanding applications
especially in automotive industry, a higher-quality (more expensive) scrap
needs to be used as feedstock to minimize the amount of these unwanted
elements.
Another
alternative is to use direct reduced iron or hot briquetted iron as substitute of shortage of good quality
scrap to minimize the amount of the unwanted elements especially for production
of deep drawn cold rolled steel sheets for automotive industry.
Scrap Charging Chute or tray for LD-Converter.
The
physical characteristics of steel scrap have an important influence on melting,
both technically and economically, an unspecified scrap size can lead to a loss
of melting time, a greater risk of furnace refractories damage, excessive slag production
and reduced iron yield.
Bulk Materials are made available through main Conveyor K-3.
The Calcined / burnt Lime, Fluorspar, Iron
Ore, Dolomite and Coke are supplied to the hoppers /storage bins through Bulk
Material supplying main conveyor belt known as K-3. Each of the converter has
six storage bins, equipped with weighing and vibrating feeders that supply the
required amount of materials to each of the converter via sub- tracking
conveyors k-4 and k-5.
Appearance of set of hoppers /storage bins of Lime, Fluorspar,
Iron Ore, Dolomite at elevations for each LD- Steel Converter.
FRESHLY CALCINED LIME.
Fresh
burned lime with not less than 90% Cao ,0.5-3% Mgo, less than 3% Sio2. Sulphur
less than 0.1% Sulphur. The principle requirement to lime is that it should be
free from moisture. After few hours storage of calcined lime in air, its
hydrogen content begins to increase which is not permitted in making quality
steel and also most of the slaked/ powdered lime is carried off with the
exhaust gases instead of entering into the metal.
FLUOR SPAR.
Fluorspar
is used to accelerate the dissolution of lime in a basic slag and to maintain
the required slag fluidity during steelmaking process. The main constituent of
fluorspar is CaF2 (90-95%). The SiO2 content should be less than 5%,so as not
to adversely effect the basicity of slag.
IRON ORE.
The
temperature of the metal is regulated by adding iron ore during course of
oxygen blowing.
DOLOMITE.
Dolomitic lime (calcined dolomite) increases the Mgo content and increases the
viscosity of the slag and produces a less erosive slag to minimize wearing of
refractory lining of BOF ( it should not
exceed 25% of the total lime charged to maintain the required fluidity of slag).
The facility
for supplying calcined dolomite to converter shop has not been provided by the
designer(Tazpromexport ), a connecting conveyor for supplying calcined
dolomitic lime (CaO.MgO) to SMD from Refractory & Lime Production Shop can be
installed with slight modifications.
Presently raw dolomite (CaCO3.MgCO3) in small
quantity is used at S.M.D. as coolant.
Production Process of LD-Converter (BOF).
The
Basic Oxygen Furnace ( Steel converter) is the key component in the steel
production process in an integrated steel
plant. The productivity of a steel plant is dependent to a large extent
on the availability and lifetime of the Converter.
Based on the statistical calculations of
previous heats, a weighed amount of hot metal is poured out from 1300 tons Mixer
for every heat as per demand of Converter distributer/ operator. The Mixer
Section has arrangements for weighing hot metal and taking samples for hot
metal analysis.
The
hot metal composition and temperature are determined before charging a heat
into Basic Oxygen Furnace (LD-Converter). Based on the hot metal analysis and
temperature, the amounts of Bulk materials, Coolants and volume of Oxygen is
adjusted by the Converter distributor operator.
A view
of charging aisle of a modified LD-
Converter shop with dust exhausting system similar to Steel Making Department
Pakistan Steel.
Above
is view of LD-Converter’s charging aisle, showing 2nd control pulpits underneath Converters, from where
electrically driven steel ladle cars can be transported on rail tracks to
Continuous casting aisle and slag cars to Slag dump yard at Steel Making
Department Pakistan Steel.
LD-Converter receiving Steel Scrap.
The LD-Converter (BOF) is mounted on trunnion
ring, capable of rotating through an angle of 360°. The Converter is tilted for
charging weighed amount of steel scrape up to 25% of the total metallic charge via 30 tons overhead crane.
LD-Converter receiving hot metal.
For
each batch (heat), weighed amount of hot metal (75%
- 85% of total metallic charge) is poured via a lip-pour spout hot metal ladle into
LD-Converter. The
crane operator of 180 tons crane and the Converter distributer
operator from Converter control room simultaneously control the hot metal
pouring operation as per signals of converter floor- man, standing at elevation
+ 8 meter.
In
order to avoid any unforeseen breakdown/ accident, managing timely preventive
maintenance and repair of six of the above kind of production responsive hot
metal handling 180
tons cranes (three sited in converters charging aisle and three in ladle
preparation and continuous casting aisle) with an undertaking of minimum
interruption in production activities, is of utmost importance besides
scheduling the maintenance and repairs of overall 32 overhead cranes of steel making
department.
Waste heat exchange Boiler & Oxygen blowing water cooled lances.
Oxygen
lance handling hoist is comprised of a trolley with vertically driven carriages
that clamp and support the oxygen blowing lances with water and oxygen hoses capable
of moving up & down as a single unit into the LD-Converter. Each
LD-Converter is facilitated with two water cooled oxygen blowing lances (one
working and the other as standby).
Production
responsive oxygen blowing lance head.
The
optimal design of oxygen
lance head (with inside passage of cooling water and certain axes of nozzles as
per explicit internal dimensions of LD-Converter) is of utmost importance to
manage the desired quality of steels and production process faster, smooth and
to prevent the excessive wearing of refractory lining of LD-Converter from
attack of vigorous slag forming reactions.
The waste heat exchange boiler consists of a
vertically oriented elongated body, made up of ring shaped water circulating
seamless tubes. The process exhaust gas( at temperatures up to 17000C)
enters through the opening of fumes hood and passes upward through the U shape
waste heat exchange boiler which generates steam inside the water circulating
tubes while cooling the exhaust gas.
Waste
Heat OKг-Boiler.
The
composition of the gas varies from start to the end of the blow and this is a function
of the blow time. A typical composition of LD gas comprises of by volume -
Carbon Monoxide-58.4%, Carbon dioxide- 15.8%, Oxygen-0.2% and rest Nitrogen.
The
oxygen reacts with carbon dissolved in molten pig iron to generate huge volume
of process gas is very rich in energy. This
energy rich converter’s off- gas, after cooling and cleaning should be captured
for use in power generation thereby preventing atmospheric pollution. In
Pakistan Steel, presently this valuable converter gas after cooling and
cleaning is wasted into the atmosphere via huge exhauster (large blower) and Chimney.
LD-Converter’s
Control Pulpit.
Converter
pulpit is equipped with a large variety of measuring instruments that measure
the quantity, pressure and temperature of fluids (air, oxygen, steam, etc)
which flow in unit time; consumption, pressure and temperature of water that
cools the oxygen lance in top blowing process etc.
Operational control room of 130 Tons LD- Converter at Steel
Making Department Pakistan Steel.
The instruments for bulk materials (lime, dolomite, iron ore,
CaF2 etc.) supplying bunkers, weighing vibrators and tracking conveyors are maneuvered
from converter control pulpit as per required grade of steel.
The
steelmaking process in LD-Converter takes place at such faster rates that any enormous
change in inputs can cause difficulty to follow the course of blowing and stop
the process manually by observing the available instruments and change in flame
exuberance at the end point.
Modern
Converter shops are equipped with gas analyzer facilities that displays the temperature
and quantity of converter gases (CO & CO2) passing through the exhaust
system and helps the operating personnel to optimize the process.
After
charging the scrap and hot metal the converter is brought to an upright
position and a water cooled oxygen lance is lowered from the top; oxygen is
blown into the bath at supersonic speeds causing rapid mixing and generating
heat from the oxidation of iron and impurities.
A view
of Oxygen blowing LD-Converter in operation.
The
LD-Converter steelmaking process is an autogenous or self-sufficient in energy.
The primary raw materials for the LD-Converter are 70-80% liquid hot metal from
the blast furnace and the balance is steel scrap.
LD-Converter process differs from the Electric Arc Furnace in
that it does not need energy for melting or refining. Slightly over half the energy for the process
is supplied by the molten pig iron charged, having physical temperature about 1320 °C. The enduring energy is supplied by the
oxidation reactions of carbon and silicon in the hot metal that are the most
important sources of energy for the process.
High purity oxygen (99.5%) is blown through the molten bath
to lower the carbon, silicon, manganese and phosphorous content of the iron,
while various fluxes are used to reduce the sulfur and phosphorous levels.
The
oxygen burns out the carbon as carbon mono oxide CO and carbon dioxide CO2 gases
which are collected in the chimney stack after cleaning off its dust ( Fe2O3 and
lime dust particles,etc). The elements Mn, Si, P and S are oxidized and combine
with lime(CaO) and FeO formed by the oxidation of Fe to form a molten slag.
Because these oxidation reactions are highly exothermic, the process needs coolant
in order to control the temperature of the melt. This cooling is done by charging
scrap (recycled plant and mill scrap) and by adding iron ore during the blowing
process.
Modern LD-Converter shops are worldwide employing
computerized dynamic process control model to calculate the accurate amount of oxygen
for blowing and end point control and productivity.
Pakistan
steel is still using the manual procedure of an LD Converter steelmaking (heat)
that is statistically planned and recorded by hand in a production log book.
During the process an experienced operator dynamically interferes with the
process to make corrections in order to achieve the required chemical analysis
and tapping temperature that leads to loss of both, production and quality due
to human error.
After
completion of blowing, the oxygen is shut off and the oxygen lance is raised to
permit rotation of the vessel to sampling and temperature measuring position. Converter
floor men then take samples for a chemical analysis and measure the temperature
via a thermocouple.
Samples
are dispatched to Express laboratory situated in the same building of Converter
section.
Spark Optical Emission spectroscope.
Sample
is dispatched to express laboratory, located in the aisle of Converter section, where it is analyzed in an spark
optical emission spectrometer shown below.
An electrical arc is passed through a sample, under an argon
atmosphere, heating and exiting the atoms within the sample of the material.
While discharging energy the atoms give off a characistic wavelength of light
that is detected and measured to obtain an accurate weight percentage of the
elements like Carbon, Silicon, Manganese, Phosphorous and Sulpher etc. present
in the steel sample.
The sample’s result (amounts percent of
various elements in the ready steel like (C, Si. Mn,S & P )is displayed on
digital screen at Converter shop floor. If the results are
not as per targeted steel grade or the temperature is too low or the carbon is
too high, the vessel is again turned to vertical blowing position for
additional short reblow for correction.
In
cases when the temperature is too low or the carbon is too high, the vessel is
truned to vertical blowing position again for additional short re-blow for
correction.
After
finishing the (oxygen re- blowing if required), sampling and temperature
measurement at 130 ton LD-Converter, the
vessel is tilted at tapping position which is opposite to sampling position and
the steel is tapped through a tap hole into a giant refractory lined steel
ladle.
The
above shown process is called tapping the steel.
The purpose of the tap hole is to permit the tapping of steel only,
leaving slag in the furnace which after tapping, is discharged into slag pot
through Converter’s mouth by rotating the vessel to the charging side.
During
tapping of heat, weighed amount of deoxidizers and alloying elements ( like Aluminum, Ferro Silicon, Ferro Manganese,
Ferro Titanium, Ferro Vanadium, Ferro Niobium and others ) are added in ready
steel to achieve the desired composition of a steel grade. A typical weight of
heat of molten steel from a steel converter (BOF) of Steel Making Department
Pakistan Steel is about 120 tons.
A view of LD-Converter
steel tapping from ground floor.
After
every 45-55 minutes, a batch of molten steel (heat) is tapped from
the LD-Converter into a Steel ladle i.e. known as tap-to-tap time.
In
an integrated steel production shop, the heat size and the tap-to-tap cycle of
steelmaking furnace is designed to match and synchronize with the speed of
scheduled Continuous Casting machine to keep the process continued. Synchronization of LD-Converter’s tap-to tap cycle with the speed of
Continuous Caster is the key to control the rate of production/ productivity in
Steel Making Shop.
After tapping a batch of steel (heat) from the LD Converter, the
ladle is transported via an electrically driven Ladle car to the continuous
casting bay where it is lifted by 180 Tons overhead crane to carry it to the ladle
treatment station for fine tuning of chemical composition and adjustment of
temperature as per requisite grade of steel
Secondary
Metallurgy.
Ladle Treatment Station Steel Making Dept. Pakistan Steel.
The
ladle treatment station (Voestalpine Austria make) is provided in the aisle of Ladle
preparation section and Continuous Casting Bay, which is facilitated with three
overhead 180 tons cranes.
The undersupplied ladle treatment facilities in steelmaking department
are meager/ awfully inadequate and one of the bottlenecks not only to enhance the
future quality production but to cop up the present production cycle failures.
Appearance
of Ladle treatment station in operation at S.M.D. with the available Ladle addition facilities.
Need to enhance the Secondary Metallurgy
facilities at Steel Making Department Pakistan Steel.
In modern high productivity Steel making shops, Converter’s
tap-to-tap cycle of a heat is typically designed closest to 35 minutes by carrying
out fine-tuning of molten steel at ladle treatment station, facilitated with
Ladle Furnace and Vacuum De-gasser.
Presently the most common worldwide de-oxidation and alloying
practice is a split practice.
The major part of de-oxidation and alloys
addition is carried out during steel tapping from converter where as fine
tuning of chemical composition and temperature is performed at ladle treatment
station.
Ladle Furnace acts as a buffer between LD-Converter and
continuous casting machine and is used to :
-
- adjust the chemical
composition (fine- tuning) of molten metal exactly as per required steel grade.
- produce a wider
range of steel grades.
- de-sulphurise
steel.
- hold the molten
steel for casting operation.
- raise the
temperature of steel (superheat) in ladle required to maintain casting
operation.
After fine tuning/adjustment of chemical composition and
temperature of steel, the ladle is dispatched to the scheduled continuous
casting machine (Billet, Bloom or Slab Continuous Caster).
Continuous Casting Section
(Steel Making Department Pakistan Steel).
Synchronization of LD-Converter’s tap-to tap
cycle with the speed of Continuous Caster is the key to control the rate of
production/ productivity in Steel Making Department. In an integrated steel
production shop, the heat size and the tap-to-tap cycle of steelmaking furnace
is premeditated to match and synchronized with the planed sizes of moulds and
speed of scheduled Continuous Casting machine to keep the process continued.
BLOOM AND BILLET CONTINUOUS CASTERS
(Steel Making Department Pakistan Steel)
Slab
Continuous Casters (Steelmaking
Department PAKISTAN STEEL).
There
are two Slab casters provided in steelmaking department where as the foundations
for third slab caster are available for enhancing the production to 2.2 million
tons.
A view of Slab Caster’s operating posts at casting floor before
start of heat.
Continuous
casting machines are now required to cast a wide range of steel grades, particular
slab casters must cast steels ranging from ULC and low carbon grades to high
carbon and high quality pipeline grades. This must be achieved while maximizing
production output. In today's world, over 90% of steel
is made using the continuous casting process, whether carbon, alloy or
stainless steel grades.
The
bottomless copper or copper alloyed moulds are used to continuously cast steel
slabs. The four mould walls are nickel-coated, 30-5mm thick copper plates. The
plates are bolted to steel “water boxes”, which contain the cooling water that
circulates to remove heat during casting process.
The
entire assembly of mould oscillates up and down during casting, in order to
prevent sticking of the solidifying steel shell to the mould walls.
The
support rolls below the mould prevent the thin, weak solidifying steel shell
from bulging and bursting.
Sliding
the narrow faces allows casting of different strand widths, whereas adjusting
the wide faces east or west allows casting of different thicknesses.
Each
slab caster is comprised of two strands have the provision to cast Slabs of
dimensions shown below.
Thickness:
150 – 200 mm
Width:
700 – 1550 mm.
Dummy Bar serves for
the initial casting of the strand. Each dummy bar is provided with a dummy bar
head for sealing the mould during initial casting. The dummy bar head is
re-usable. The dummy bar consists of chain links connected by pins. The chain
links are of unilaterally flexible design. The material is wear-resistant.
To start the process of the continuous
casting, the dummy bar attached with dummy bar head ( which is connected to an
external mechanical withdrawal system) is inserted into the mould from the top
and positioned into the mould so that top of the dummy bar head closes the
bottom of the mould.
Flowchart of a Slab Caster:
.
Oxygen
Lancing a Steel Ladle:
Whenever new filled steel ladle is brought to casting
position, there is a chance of chocking of the ladle slide gate metal pouring
hole that is required to be opened with oxygen lancing as shown below
.
Subsequent to clearing the pouring hole of slide gate, a
refractory tube called shroud is installed to prevent re-oxidation of metal to
obtain clean steel. Metal surface in tundish is covered with rice husk to
prevent re-oxidation from atmosphere.
Once
the required level of metal is achieved in tundish, the metal is allowed to
flow in controlled amount in the water circulating copper moulds through mono
block stoppers and metering nozzles of the tundish via emersion nozzles. The metal
splashes over side walls of the mould are removed by means of cutting torch to
avoid any solidifying metal adherence to the mould walls. The metal surface in
the moulds is kept covered with slag forming compound (Flux Powder).
When
the liquid steel level in the mould reaches a predetermined position,
withdrawal of the dummy bar is initiated. Drive rolls lower in the continuous
casting machine continuously withdraw the dummy bar from the mould. The
withdrawal speed of the dummy bar is preset
based on the casting speed required or the liquid steel flow rate from
tundish. When the dummy bar head, which is now attached to the solidified shape
being cast, reaches a certain position in the withdrawal system, it is
mechanically disconnected and the dummy bar is removed and lifted by roller
drum wire rope system to get it parked till the next series of casting is
initiated.
The complex phenomena occurring inside a continuous caster mould
influence initial solidification in the meniscus region. However
continuous casting of steel can produce surface and sub-surface defects in
the slab. These defects arise during the casting process.
The formation of surface defects in continuous casting of
slab are caused by production technology – particularly by chemical composition
of steel, by technical condition of the mould and by technological parameters
of casting, casting temperature, casting speed, condition of secondary cooling
and the rolls and segment alignment. Here are some typical surface defects
which occur on continuously cast steel slabs.
·
Longitudinal and transverse surface or corner cracks.
·
Longitudinal and transverse depressions.
·
Deep oscillation marks with solute segregation.
·
Bleeding
·
Clusters of inclusions.
·
Slag patches or entrapped slag/ scums, gas holes, including blowholes
and pinholes.
Solidification of liquid steel, which is started in the water
cooled mould, continues progressively as the steel strand moves through the CC
machine. Solidification begins at the liquid steel meniscus level in the mould forming
a steel shell in contact with the walls of the mould. The distance from the
meniscus level to the point of complete solidification within machine is called
the metallurgical length. The point of complete solidification occurs ahead of
the gas cutting flying torches.
Scheme of rolls of different sections of Slab
Continuous Caster.
As the slab gradually cools,
it is guided through a 90-degree curvature where there are high temperatures
and very heavy Ferro-static forces from the liquid slab core.
Slab Caster process performance is directly
linked to roll line performance.
Traditionally, central grease distribution systems
cycle several times an hour to pump grease through the bearings of the rolls.
In continuous casting of slabs the secondary cooling system
has an important influence on the quality of the cast products.
A
secondary cooling zone in association with a containment section positioned
below the mould, i.e. 5 roll and 10 roll set through which the steel strand
(still mostly liquid) passes and is sprayed with water or a mix of water and
air (air mist) for further solidifying of the steel strand.
The moulds and secondary cooling sections are refurbished
after casting recommended series of heats to maintain their functional
requirements. Moulds are repaired in mould repair machine shop associated with
steel making department whereas set of secondary cooling sections and rolls of
continuous casters are overhauled in Repair shop complex (Central Maintenance Department) of Pakistan
Steel.
Cooling in the secondary zone is of vital importance in
determining the quality of slabs because uneven cooling may cause the formation
of cracks, bulging& scale formation. Any
defects on slabs are amplified in the rolling mill if not
removed first via scarfing. As a rule, a longitudinal defect on a slab will be
300 times longer after rolling. This would result in a reject coil.
Subsequent
to radial section comes a section for the unbending and straightening of steel
strand.
A
cutting section consisting of flying torches control the length of the Slabs
and let casting process continued uninterrupted as shown above.
The
run out table trolleys are employed to shift the slab to transportation rollers
line that have a slab Scarfing machine for removal of surface defects if any is
revealed.
Slab stockyard
Finally
the hot slabs are transported to cooling beds in slab stockyard to a product
transfer area from where they are shifted to rolling mills. The slabs of
150-200 mm thickness
and 700-1550 mm width
sizes are produced at the Steel Making Department Pakistan Steel to feed to the
hot strip mill where they are rolled into stripes, sheets and coils.
Hot rolled carbon steel coils, sheets, strips
and plates are used to form Seam welded pipes for Gas/Water/Oil, Storage Tanks,
Vessels, Containers and fabricated Sections/Structures for civil
construction.
The ultra-thick and wide
slabs, for instance, form unique initial products for Ships/Barges, Launches & Floating Structures and pipeline sheets.
Cold rolled sheets are used in forming Automobiles
body, manufacture of buckets, utensils, cans, Desert coolers, Air conditioners,
Water coolers and domestic appliances.
Hot and
cold rolled steel Grades SAE1008, SAE1010 are used in trailer construction, rail
coaches, wagons and tubing for bus bodies.
