Rolling Mills Pakistan Steel

Rolling Mills Pakistan Steel:

Rolling Mills Pakistan Steel:

Billet Rolling Mill Pakistan Steel.

Cast Bloom of cross section 260 x 260 mm are supplied to 800 mm Reversible Stand Billet Mill to get them rolled in vast range of different shapes and sizes.

When order of specific grade of steel is accredited, the Blooms of that batch are transferred from Bloom stockyard to the premises of Billet mill for charging into the reheating furnace.

Pushers for feeding the cast Blooms into Reheating Furnace prior to hot rolling.

On all types of rolling mill, the semi-finished products first undergo to a roughing stand.

 A stand is a collection of steel rolls (or drums) on which pressure (hydraulic) is applied to squeeze the hot steel passing through them, and arranged so as to form the steel into the semi finished shape

At the roughing stand the severe thickness reduction must occur at an elevated temperature (1050 -1150 deg. Centigrade) because of the high forces required, as well as the need to control the micro structural phases within the steel.
After that the elongated bars are subjected to finish stands to acquire the final sizes.

 Hot-rolled prime steel Billets

Mechanical pushers are employed for handling the rolled billets on cooling bed for onward dispatch to stockyard through railway wagons.

Mechanical Pushers

The reason for the popularity of this material may be noted by its high strength characteristics and long life.
Dimensions: 80x80, 100x100, 125x125, 150x150 mm

 Hot rolled billets are preferably used in manufacturing of seamless pipes and machine components, forging and stamping and manufacturing of spring steel flat bars.

Besides, hot rolled billets are mainly used for manufacturing steel sections vise, rails, angles, joints, channels, squares, flat bars, rods, wire rods, bailing hoops, tees and chains etc.

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

Special Alloyed steel Billets are used for rolling of any special grade like Earthquake resistant TMT Re-Bars and for special grade structural steel products.

Hot Rolling Mill.

The 1700 hot strip mill operates on slabs produced at the steel making plant. It produces hot rolled sheets, coils and strips suitable for ship building and the manufacture of welded pipes of small, medium and large diameter, bodies of cars, buses and other vehicles, railway wagons, transformers, boilers, big tanks, machinery and formed sections. The hot rolled sheets are also utilized for the production of cold rolled sheets. It has a designed capacity of 445,000 tons.

Walking Beam Slab Reheating Furnaces at Hot Rolling Mill.

A view of a walking beam slab reheating furnace of Pakistan Steel during Capital Repair.

When an appropriate-size order is received, the slabs of specific batch are reheated into a reheat furnace to achieve the uniform rolling temperature (around 1200^o C), before hot rolling.

The interior of the furnace is divided into zones for temperature control: preheat, top-and-bottom; heating, top-and-bottom; and soak, top-and-bottom, east-and-west. The preheat and heating zones combust a mixture of natural gas and preheated combustion air with massive burners on the side walls of the furnace, both above and below the skids, to heat the slab.

The heated slabs are transferred to roughing stands via series of rollers transportation line

The two rolls touching the strip are called work rolls, and the larger rolls applying uniform pressure to the work rolls are called back-up rolls. The stands are synchronized to compensate for ever-increasing speeds as the material is elongated and reduced in gage.

After leaving the roughing stand, the slab passes continuously through a series of finishing stands which progressively squeeze the steel to make it thinner.

The finishing mills roll the same piece of steel in tandem, meaning same piece of steel will be rolled through all finishing stands at once.

As the steel becomes thinner, it also of course becomes longer, and starts moving faster. And because the single piece of steel will be a whole range of different thicknesses along its length as each section of it passes through a different stand, different parts of the same piece of steel are travelling at different speeds.

Series of finishing stands of hot rolling mill Pakistan steel.

This requires very close control of the speeds at which each individual stand rolls; and the entire process is controlled by workstation. By the time it reaches the end of the mill, the steel is travelling at about 40 miles per hour.

Careful control of the finishing temperature at the last rolling stand and the coiling temperature is necessary to achieve optimum properties in the final product.

Hot Rolling Coiler Pakistan Steel

Hot rolled strip is a flat product which has been coiled to make storage and handling easier. It is a lot thinner than plate, typically a few millimeters thick, although it can be as thin as 1mm. Its width can vary from 150mm to nearly 2 meters.

It frequently goes through further stages of processing such as cold rolling and is also used to make welded tubes (smaller tubes than those made from plates).

Cold Rolling Mill:

The production of a specific thickness of cold rolled sheet to meet an end user’s requirements involves sophisticated processing from the melting shop stage forward. Steel chemistry, hot strip mill processing variables, pickling practices, cold-rolling mill practices, annealing practices, and finally, temper rolling practices all have a role in achieving the manufacture of top quality cold-rolled sheet products.  This product is manufactured for a large number of applications that require a variety of properties from fairly simple to very sophisticated. 

Cold rolling occurs when the steel is at a temperature below it’s recrystallization temperature. This increases the steel's strength via strain hardening, or hardening that is produced via dislocation movements of atoms within a material's crystal structure.

The prime feed to cold rolling mill, are the coils from hot strip mill which are first routed through pickling process.

At the pickling process, the oxidized scale is removed from the surface of the coils that were produced by the hot rolling mill.

Pickling. 

As the hot rolled coil cools, an oxidized scale forms on the surface. This must be removed in an acid bath (pickling) before it is rolled down further to the specified thickness in the cold mill, known as a tandem mill.

Following the pickling process, the coils are then sent to the cold rolling process,

 Where they are rolled still thinner at cold (room temperature) conditions until they reach the thickness required for their final applications.

The primary feature of cold reduction is to reduce the thickness of hot-rolled coils into thinner thicknesses that are not generally attainable in the hot rolled state. The cold reduction operation induces very high strains (work hardening) into the sheet; thus, the sheet not only becomes thinner, but also becomes much harder, less ductile, and very difficult to form.  However, after the cold-reduced product is annealed (heated to high annealing temperatures), it becomes very soft and formable. 

The cold rolling process finalizes the strip thickness accuracy and flatness. Because the steel is rolled under cold conditions, there is no oxidation of the surface, yielding an attractive metal skin.

The cold rolling reduces the steel thickness by about 70 percent, using work rolls similar to those used in the hot strip mill, but not at such elevated temperatures. If the final thickness is targeted for 0.030 in., the starting hot band must be 0.100 in. As with hot rolling, width and thickness changes must be gradual. The tandem mill is equipped with automatic gauge control, x-ray gauge, data logging and thyristorisation.
 

Coming out of the tandem mill, the cold-reduced coil is at the specified thickness, but it is extremely hard and brittle.

Annealing process. 

The annealing process, used to relieve internal stresses and regain the ductility of the steel coil, involves heating the cold band to 650-700 degrees Centigrade in a protective atmosphere of typically either pure hydrogen or a controlled mixture of hydrogen and nitrogen. Pakistan steel has batch annealing process.

Batch Annealing furnace.

In a batch anneal process, critical variables that determine the steel properties are the rate at which the temperature is ramped up, the soak temperature and time, and the rate at which the coils are cooled.

The specific values are a function of chemistry and the desired coil properties, and as with everything else, only similar products are processed together.

Due to increased demand of cold rolled sheets in automotive industry, modern plants have invested in adding up a modified continuous annealing process on the production line that has increased the production several times. These mills can process a coil in 15 to 30 minutes rather than the three days it takes in a batch annealing process, during which three coils are stacked “eye to the sky” under a bell furnace

Following the cold rolling process, the cold coils become the final products after undergoing an annealing process or coating process according to their intended uses.

Hot Dip Galvanized Sheets.

In this section the coils are uncoiled, heated and then galvanized in the liquid zinc.

In a hot dipped continuous galvanizing process, the cold rolled sheets are dipped in a molten bath of Zinc. The formation of galvanized coating on the steel surface is process of metalurgically bonding of iron and zinc, thereby forming a uniform zinc-iron alloy’s layer, protecting the steel from corrosion

    

The formation of galvanized coating on the steel surface is process of metalurgically bonding of iron and zinc, thereby forming a uniform zinc-iron alloy layer, protecting the steel from corrosion.

Worldwide Auto manufacturers currently use the galvanized steel sheets that are pushing an increase demand of this product.

 The finished galvanized steel most common is thin sheet steel rolled into large coils. 

 

Galvanized sheets are used for making cars, vehicle and bus bodies, containers, trunks, boxes, packets, steel shuttering, desert coolers, construction and roofing, ducting equipment, appliances, paneling, utensils, air-conditioners, water heaters and fresh water tanks.