Bloom & Billets Continuous Casting Facilities (S.M.D) Pakistan Steel.

Aqil Khan

Bloom & Billets Continuous Casting Facilities (S.M.D) Pakistan Steel.  

CONTINUOUS CASTING FACILITIES FOR LONG PRODUCTS (Blooms & Billets).

Steel Making Department Pakistan Steel.

It is essential to go through the characteristics of available production facilities of an integrated steel mill, not only to get optimum benefits out of it but also to put forward further improvement plan of production and productivity in integration.

Blooms and Billets are used to make long products. Before the advent of continuous casting machines, the individually cast ingots in stationary moulds were used to produce rolled Blooms and Billets. Now large size cast ingots are mainly used to obtain forged parts.

Cast Bloom :  A semi-finished long product of continuously cast steel of greater than 150 mm square or rectangular cross-section. Blooms are used primarily as feedstock for hot rolling to produce long heavy sections, rolled billets, piling, beams and railway rails. A Bloom is a bigger sized Billet.

Cast Billet :  A semi-finished long product of continuously cast steel up to 150 mm square or rectangular cross-section with round corners. A Billet is a smaller sized Bloom. Billets are used primarily as feedstock for hot rolling or other processes to produce sections, rods, seamless tubes, bars and wire products. Rolled bars can have cross-section in the shape of squares, rectangles, circles, hexagons and angles.

After tapping a batch of steel (heat) from LD Converter into a refractory lined ladle, it is dispatched via an electrically driven Ladle car to continuous casting bay/ aisle from where it is lifted by 180 Tonne overhead crane to get it transported to the ladle treatment station for fine tuning of chemical composition and adjustment of temperature as per required grade of steel and then onward to the scheduled continuous casting machine ( Billet , Bloom or Slab ) .

The ladle treatment station at Pakistan steel provided by Voestalpine Austria has facilities of small ladle additions, cooling and nitrogen purging from top where only high temperature heats can be treated and does not have any temperature raising facility in case if it is lowered down due to prolong holding .

4 - Strand Bloom Continuous Caster.

Bloom casters have been more widely installed by integrated plants with higher production rate. Because the casting rate for the larger Bloom cross-section is higher than for small billet sizes. Consequently larger heat sizes of 100 – 400 tons can be cast with relatively fewer strands and higher production rate in large integrated mills. Blooms sizes can vary from 260x260mm to 320x 360 mm or even bigger sizes.

            Continuous caster’s speed  varies and depends mainly on following factors:

-          Cross-sectional dimensions of moulds.

-          Chemical composition and temperature of Steel grade.

-          Rate of heat transfer in the mould.

-          Strength of the solidified casting crust.

       -     Methods of maintaining the track strand secondary cooling.

As per project, Pakistan steel is provided with a  4- Strand Bloom Continuous Caster capable of casting 260 x 260 mm cross section Blooms which are cut to length for subsequent reheating and rolling into various sizes of Billets in 800 mm reversible Billet Mill.

A Butterfly type ladle turret holds the steel teeming ladles, weigh up to 130 tons. By means of the ladle turret, the steel teeming ladles are alternately rotated backward and forward into casting and charging position. This function ensures the uninterrupted operation of the continuous casting plant. While one ladle is emptied, a full ladle is provided on the other side. The ladle change time is only about one minute.

Butterfly type turret has lifting and lowering lever arms on each side driven by two hydraulic cylinders that can facilitate adjustment of ladle height over tundish  independently on each side for better control of temperature in tundish.

A view of Butter Fly Type Ladle Turret of Bloom Continuous Casting Machine under maintenance. 

Continuous Casting Process:

Machine Preparation Before Commencement of Casting Operation:

To initiate the continuous casting process the dummy bars are drawn from dummy bar parking positions and are inserted through the secondary cooling sections into the bottom of moulds to a recommended height and the water cooling system of the continuous casting machine is turned on.

Dummy Bars.

Before the start of casting process, the motors for the dummy bars storage unit are actuated to lower the dummy bars down into the with drawer-straightening unit and are inserted, passing through the secondary cooling sections into the bottomless moulds to a recommended height with a detachable dummy bar head.  At the end of dummy bar withdrawal operation, the dummy bar head is removed and the dummy bar returns to the storage position and park there till next casting operation.

The heat finishing time or ladle emptying time depends on cross sectional area of the moulds, the   number of strands , temperature and speed of casting and  steel grade.

The filled steel ladle is transported via over head crane from ladle treatment station onto the ladle turret of the casting machine and brought to the teeming position above the tundish. The hydraulically operated slidegate of ladle is opened to dispense the steel into the tundish.

A view of ladle slidegate dispensing the metal into tundish at the commencement of continuous casting process. 

The tundish distributes precisely the right amount of liquid steel via monoblock stoppers and metering nozzles to water circulated copper moulds. One of the key functions of tundish is to hold buffer stock of metal to supply a continuous flow of liquid steel to the moulds during ladle exchange.

The amount of super-heat contained in the liquid steel has a profound effect upon the internal metallurgical quality of the cast product (Blooms or Billets). The liquid steel temperature in the tundish is therefore maintained within the range liquidus plus 15⁰C.

IRON- CARBON DIAGRAME

Also the effect of other alloying additions on liquidus temperature is taken into consideration that varies as per required steel grade.

During continuous casting process the refractories of steel casting ladle &  tundish are exposed to  heavy erosion and abrasion action of molten steel and slag.

Also the oxygen pick up by molten metal from atmosphere is a common phenomenon.

Therefore the quality of steel that comes out of tundish does not remain the same as that poured from teeming ladle. The inclusions of eroded refractory materials and the re-oxidation products are always there which deteriorates the quality of cast steel in the mould.

Inclusions Preventive Measures in Tundish :

-To prevent re-oxidation of metal from atmosphere and to avoid loss of temperature in tundish, the metal surface is covered with rice husk.

- A ladle to tundish refractory tube ( known as shroud) is used to further avoid reoxidation of metal from atmosphere and to obtain clean steel.

- The metal flow pattern inside tundish plays an important role for removal of  deoxidation/ reoxidation products through floatation, wall adhesion and agglomeration phenomenon, which in turn is a function of tundish design / weirs or dams building in the tundish.

The flow of molten steel between the tundish and moulds is controlled through monoblock stoppers ( Use of slide gates for metal dispensing from multi nozzle tundish profoundly raises repair and maintenance cost ).

Appearance of a 4 strand Bloom Tundish at the commencement of continuous casting process.

One operating panel box for casting is suspended beside each mould.

Submerged Entry Nozzles (SEN).

After attainlng smooth dispense of metal from metering nozzles of a newly started tundish, the Submerged Entry Nozzles (SEN) are sited to prevent metal re-oxidation from atmospheric oxygen to obtain clean steel. An important function of Submerged Entry Nozzles is also to prevent metal splash over mould walls and improve steel flow in the mould, avoid hot faces on mould walls surface.

The submerged entry nozzle are preheated before putting to casting process to eliminate the risk of freezing of metal in the nozzle at the beginning of casting.

Mould Powder / Flux:

The surface of molten steel in the mould is covered by continuous feeding mould Powder (synthetic slag) from the top of moulds.

The mould flux plays essential role in continuous casting process and its main functions are to provide:

-          Thermal insulation of metal surface to prevent heat loss and avoid premature solidification of liquid steel in the meniscus zone.

-          A lubricating film to prevent adhesion of solidifying steel to the mould walls and thus facilitates strand withdrawal.

-          Protection of the molten steel from reacting with atmospheric oxygen.

-          Uniform control of heat transfer between solidifying steel and mould to avoid surface defects.

-          Absorption and dissolution of non-metallic inclusions from the liquid steel, promotes the cleanliness of finished steel products.

Mould powder is constituted by a complex mix of Oxides mainly SiO₂, CaO, Na₂O, Al₂O₃, MgO, fluorspar (CaF₂) and carbon. The chemical composition varies according to steel grades and operational parameters of casting to achieve the required properties of uniform melting temperature range, thermal conductivity, viscosity and surface tension. Pakistan Steel produces it’s mould powders (slag forming compounds) for internal use in steelmaking department.

Water Cooled Moulds:

The main function of the mould is to establish a solid shell of cast metal sufficient in strength to contain its liquid core upon entry into the secondary spray cooling zone.

As per projects the appearance of copper moulds for Bloom Casting machine, placed over oscillating stand was as under. 

  

The mould is basically an open-ended /bottomless box, containing a water-cooled inner jacket made-up of a high purity copper (99.99%) or copper alloy plates. Freezing begins at the liquid steel meniscus level in the mould, forming a shell in contact with the walls of water circulated copper mold.  

Rate of cooling controls the solidification of metal and determines the microstructures in casting sections, controls the size and shape of the grains, segregation,and the distribution of inclusions and porosity. Solidification is also critical to the hot-cracking behavior of metal.

Appearance of Bloom Caster's Mould after revamping in the year1991.

Mould Oscillation:

The moulds are oscillated up and down vertically via motor driven cams installed under casting floor to employ a stroke pattern ( called “ negative strip” ), in which the downward stroke of the cycle enables the mould to move down faster than the strand withdrawal speed which prevents metal skin adhering (sticking) to mould walls in its upper part, ( called meniscus area) and facilitates solidifying metal strand withdrawal from mould. The oscillation is auto-adjusted and varies in frequency as per the speed of continuous casting process.

Secondary Cooling Section:

Just below the moulds, the four solidifying faces of Cast Blooms are typically supported with rollers of secondary cooling section through which the partially solidified metal is to be passed. A secondary cooling section is made up of several supporting roller frames and a series of nozzles sections which spray air mist water at a predetermined rate to further assist in solidifying the core of the strand.

Appearance of secondary cooling sections for  Bloom caster.

Choosing the right combination of casting speed and secondary cooling rate for a specific steel grade is of the utmost importance. This choice influences many different parameters during casting and is one of the key choices for getting a good quality cast.

The operator controls the casting speed by potentiometer. The oscillation frequency, controlled by PLC, is changed based on the casting speed. The electro-optical sensor captures the casting speed signal and sends it to the PLC system. Then the PLC system controls the pneumatic valve to open or close the secondary cooling water.

The water cooled moulds and secondary cooling sections are encased in a bunker/ chamber from where the process generated steam is expelled out throught exhaust ducts and blowers.

Bloom withdrawing guide rollers of radial portion of 4 –Strand Bloom Casting Machine  at Steel Making Department Pakistan Steel.

The solidifying blooms are withdrawn from the moulds and secondary cooling zone by a set of rolls which guide the steel strands through a radial portion (arc) until the strand is horizontal.

Withdrawing and straightening guide roller’s sections are the key parts of continuous casting machine which are used to insert the dummy bar into the bottomless moulds to initiate casting process, withdraw hot strands from the moulds, radial section and straighten them with the dummy bars.

The rolls are positioned close enough together to avoid bulging of the thin metal shell. The bloom casting machine of Pakistan Steel is a mechanically strong machine.

The rollers of Bloom Continuous casting machine are fitted with slide bearings that require extensive lubrication. Improper lubrication leads to bearings failures, frequent break-outs, machine damage and loss of production time. A central greasing system is provided in the design to accomplish the task. 

A view of straightening rollers of Bloom Casting Machine Steel Making Department Pakistan Steel.

Solidification begins at the liquid steel meniscus level in the mould forming a shell in contact with the walls of the water circulated copper mould and progressively continues as the strand moves down through the casting machine in the secondary cooling zone.

Choosing the right combination of casting speed and secondary cooling rate is of the utmost importance. This choice  influences many different parameters during casting and is one of the key choices for getting a good quality cast. One parameter that is directly influenced by this choice is the metallurgical length.

The distance from the meniscus level to the point of complete solidification within the machine is called the metallurgical length.

Solidification of cast blooms is gradually completed as they reach to Dummy bar separating pinch rollers. Rate of cooling controls the solidification of metal and determines the microstructures in casting sections, controls the size and shape of the grains, segregation, and the distribution of inclusions and porosity. Solidification is also critical to the hot-cracking behavior of metal.

Pinch rollers for Dummy Bars separation and Dummy Bars parking posts of 4-Strand Bloom Caster at steel making department Pakistan Steel.

Pinch rollers separate the dummy bars from initially cast blooms and direct the dummy bars towards the parking posts and permit continuously cast stream of the solidified blooms towards the gas cutting flying torches.

Another view of dummy bars parking posts and Gas cutting flying torches of 4-Strand Bloom Continuous Casting Machine at Steel Making Department Pakistan Steel.

Flying torches control the length of the blooms, synchronously cut the bloom's strands into a pre-determined length and lets the casting process continued for one hour to several hours.

A hot scarfing machine (French made) on the production line is used for treating the surface defects such as flaws, de-carburized layers, sharp corners and slag removal from the steel surface of continuously cast blooms for satisfying the quality requirements before dispatching to stock yard.

The Blooms are transported via electrically driven roller lines and trolley cars to the stockyard, where the blooms are stacked to their respective stocking area of Billet mill for their subsequent rolling into rolled billets.

In the designs of integrated steel mills especially of the epoch of 1970-80, the Bloom continuous caster is typically decoupled with the rolling mills, meaning that the Blooms are allowed to cool and are put into storage/stockyard.

When an order of appropriate-size of billets is received, the blooms are reheated before rolling to reduce the cross-section to required smaller sizes Billet in Billet Rolling Mill.

6 – Strand Billet Continuous Caster.

(Steel Making Department )  Pakistan Steel.

 A six stand Billet Continuous Caster (Voestalpine  Austria made ) was added to Steel Making Department in 1989 to maximize the efficiency of production capacity of long products, including through the identification of possible synergies and to optimize the production chain, such as in the holding across the production chain of steel making department and long rolled products.

The continuous Billet caster casts square strands of smaller cross-section that are used to produce materials for wire rod, bars, shapes and pipe etc.

The cross- sectional sizes of continuously cast Billets are as under:

150 x 150 mm.

125 x 125 mm.

100 x 100 mm.

80 x 80 mm.

The ladle turret holds the steel teeming ladles, each of which weigh up to 130 tons. The Ladle turret rotates backward and forward into casting and charging position and provides smooth rotation for accurate positioning, starts and stops ladle over tundish to reduce tundish metal level fluctuation and temperature drop.

One operating panel box is suspended beside each mould. To initiate casting process, the electrically driven tundish car is moved to align the metering nozzles of the tundish with the openings of water cooled copper moulds.

A view of casting moulds and operating panel boxes of 6- strand Billet caster ( steel making department) Pakistan Steel.

The parabolic tubular moulds (consist of a one‑piece copper lining) are employed in billet continuous caster that are capable of intensive heat extraction and high heat transfer efficiency.

The copper mould tubs work under high temperature condition so their material must have high thermal conductivity, high strength, sufficient wear resistance and hardness to obtain good life.

The moulds are moved up and down (oscillated) to stop the metal shell from adhering to the moulds.

  

The parabolic tubular moulds are crucially important for obtaining minimal clearance between the mould tube wall and continuous billets crust, which while moving inside the copper tubes and under intensive cooling, undergoes constant natural shrinkage and reduction of sections.

The molten steel is poured out of steel ladle through slidegate into the tundish and after maintaining the required level of metal in tundish, the molten steel is allowed to dispense into the moulds via metering nozzles of the tundish.

An open metering nozzles system is employed in billet caster tundish (distinct to mono block stoppers – metering nozzles system used at Bloom continuous caster), producing silicon‑killed steel grades.

Metal discharge rate is controlled by the bore of the nozzle and the ferro static pressure (metal height in the tundish) above the nozzle. Different bores of metering nozzles are selected depending on the section size cast and casting speed required.

The casting speed can be controlled within 0-4m/min according to different steel grades and cross-sections of moulds. With highly efficient cooling, steel billets with outer solid shell formed move out of the moulds.

 Subsequently underneath of moulds, the continuously cast billets are passed through the secondary cooling zone and radial portion of the machine to attain complete solidification. 

Then the solidified billet strands pass into guides that move them to the horizontal plane for leaving the stand.

 

The dummy bars complete their task after extracting the cast strands at the start of casting process, depart to the dummy bar storing posts and park there till next series of casting operation is initiated.

 A view of secondary cooling water piping, dummy bar parking devices and gas cutting torches of six strands Billet Caster at ( S.M.D ) Pakistan Steel.

The billets are cut to the required length by flame cutting torches and are transferred to the billet cooling bed by tilting gear or lifting devices.

 Appearance of flying gas cutting torches of six strand Billet Continuous Caster in operation.

 

After cooling the cast billets are hoisted to the appointed billet storing place in the premises of steel making department Pakistan Steel for onward direct supply to rolling mills.

 The rolled and cast billets are supplied to rolling mills to get them rolled in vast range of different shapes and sizes. Bars can have cross-sectional shapes of squares, rectangles, circles, hexagons, angles. They can be re-rolled in cross-sections shaped like an H or I (called joists, beams and columns), a U (channels) or a T. These types of steel “sections” are used in modern civil construction.

Construction is one of the world’s largest and most vital industries. From houses to skyscrapers, schools, hospitals, ware-houses, factories, shopping centers,construction also involves engineering projects including highways, bridges, dams, dredging and nuclear power Plants.