Saturday, December 31, 2016



Pak Steel Executive.png

 ( 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.

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.

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.

The temperature of the metal is regulated by adding iron ore during course of oxygen blowing.

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.

(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.

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