Billet export is fine, but what is China making with these billets?
How are they adding value? What is the final product and how much more expensive is it compared to the local processing and value added?
Could we add value at home for us, using machining/forming/extrusion (for our own use) instead of exporting low value billet for others to add value?
This is similar to Pakistan exporting cotton to other countries. They are missing a chance to add value by further processing.
Huge wasted opportunity for us by exporting billets when we could do more value-addition at home.
Here are some examples of steel billet value addition. Making railroad rail, steel structural beams, flat iron "tin" roofing sheets and making spiral welded and seamless tubes are only a few examples.
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Shapes
These are long products with irregular cross sections, such as beams, channels, angles, and rails. Rolling starts with blooms that may be 150 millimetres by 200 millimetres by 5 metres long. The blooms are received, either cold or hot, directly from the blooming mill or continuous caster. They are charged into a pusher or walking-beam continuous furnace and heated for up to three hours to 1,200° C. (Sometimes, three batch-type furnaces are used instead.)
Most shapes are formed by grooved rolls with mating projections that form together a window in their gap. This window becomes progressively smaller and more like the desired shape, pass after pass, until at the end, in the final pass, the specified cross section is obtained. D in the figure shows only 5 progressive passes out of about 11 in the rolling of a rail. Rolling shapes usually takes a total of 9 to 15 passes, with an area reduction of about 25 percent at the initial passes and only 7 percent at the last pass.
The rolling of structural shapes.
Encyclopædia Britannica, Inc.
Roll and pass design is critical for this rolling technology. There are usually three to five stands arranged in various ways, each taking one to five passes. Only one pass is made through the finishing stand, which controls the final dimension and surface. Sometimes two-high reversing mills are used at the beginning in a fashion similar to blooming mills, with manipulators on run-out roller tables. In other cases, two or three three-high, nonreversing stands are arranged as an open train; in this arrangement, lifting roller tables move the workpiece between the upper and lower pass lines, and the workpiece is in only one roll gap at a time. Mills that produce medium and small shapes often have stands in tandem arrangement, rolling one workpiece simultaneously in several stands and using a controlled loop between stands. Wide-flange I-beams and H-pilings are usually rolled on universal mills using vertical edgers, as indicated in E in the figure. Blooms with a dog-bone cross section are often supplied to these structural-shape mills by beam-blank continuous casters.
Rolling temperatures are carefully controlled for metallurgical reasons. Heavy-walled, wide-flange I-beams are sometimes heat-treated in-line by computer-controlled water quenching and by tempering with their own retained heat. The heads of rails are often heat-treated in-line to improve wear and impact resistance. Rails are also slow-cooled under an insulated cover, directly after rolling, for at least 10 hours to diffuse hydrogen out of the steel.
After rolling, a hot saw cuts the shapes into lengths that can be handled by the cooling bed. Each shop conducts large-size finishing operations such as straightening, cold-cutting to ordered length, marking, and inspection.
Tubes
Tubular products are manufactured according to two basic technologies. One is the
welding of tubes from strip, and the other is the production of seamless tube from rounds or blooms.
Welded tubes
The most widely used welding system, the
electric-resistance welding (ERW) line, starts with a descaled hot-rolled strip that is first slit into coils of a specific width to fit a desired tube diameter. In the entry section is an uncoiler, a welder that joins the ends of coils for continuous operation, and a looping pit, which permits constant welding rates of, typically, three metres per minute. Several consecutive forming rolls then shape the strip into a tube with a longitudinal seam on top, as shown schematically in A in the figure. Two squeeze rolls press the seam together, while two electrode rolls or sliding contacts feed the
electric power to the seam for resistance heating and welding. A cutting
tool removes the flash created during welding, and, after a preliminary inspection, the tube is cut into cooling-bed length by a saw that moves with the tube.
Production of welded tubes.
Encyclopædia Britannica, Inc.
Tubes up to 500 millimetres in diameter with walls 10 millimetres thick are produced on ERW lines. Larger-diameter
pipes are often produced by forming the strip into an endless spiral, as shown schematically in
B in the figure. Forming is followed by continuous welding of the seam, often by automatic
arc welding. Pipes up to 1.5 metres in diameter and with a 12-millimetre wall thickness are sometimes produced by this spiral welding process. Still larger pipes are produced from plates by a U-ing and O-ing process, which applies heavy presses to form plates into a U and then an
O. The longitudinal seam (or seams) are then welded by automatic arc-welding equipment.
Seamless tubes
Seamless tube rolling always begins by piercing a round or bloom to generate a hollow. In roll piercing, an oval round is preheated to about 1,200°
C and is cross-rolled slowly between two short, large-diameter rolls that rotate in the same direction (shown schematically in C in the figure). The round also revolves and is pulled into the roll gap in a spiraling motion, because the rolls have a converging-diverging shape and are installed relative to each other at an angle of about 20°.
This revolving, continuous plastic working of an oval
cross section between the two rolls creates tensile stresses in the long axes of the oval, which rupture the centre and create a cavity. At this point the cavity meets the piercer, which is a projectile-shaped rotating cone held in place by a bar and a thrust bearing. The piercer acts like a third roll in the centre and produces the inside of the tube.
Production of seamless tubes.
Encyclopædia Britannica, Inc.
The cross or helical rolling action of roll piercing demands excellent hot formability of the prerolled round. Another process, push piercing, does not have such exacting requirements. This usually takes continuously cast square blooms and forms them into hollow rounds by the action of a heavy hydraulic pusher, which pushes them into the gap of two large-diameter
contoured rolls that form together a circular pass line. In the roll gap the bloom is met by a heavy piercer, which forms the hollow, as shown in D in the figure. This mill can form a 250-millimetre-square, 3-metre-long bloom into a tube with an outside diameter of 300 millimetres and an inside diameter of 150 millimetres. Since there are only compression forces acting on the steel in this process, the workpiece is practically not elongated at all.
A number of rolling technologies are used to form the pierced hollows into tubes with specific dimensions and tolerances. Often, the hollow is reheated and then sent through another cross-roll piercer mill, called the elongator; this reduces the wall thickness by 30 to 60 percent. In a subsequent step, a long, preheated, lubricated cylinder called a
mandrel may be inserted into the tube. The tube would then be rolled, with the mandrel inside, in a continuous close-coupled, seven-stand, two-high mill, usually with the rolls arranged at a 45° angle and in an alternating pattern like the horizontal and vertical rolls.
A very uniform wall thickness can be formed by this process. Smaller diameter tubes are often formed from larger tubes in a continuous three-roll, close-coupled stretch-reduction mill (E in the figure). These mills sometimes have 20 sets of rolls arranged in tandem.
Open-die forging
Heavy ingots, some weighing up to 300 tons, are sometimes formed at steel plants by huge hydraulic presses with a forging force of up to 10,000 tons. These make such large products as rotors for power-generating units or large sleeves for rolls or pressure vessels. Careful, uniform heating of the ingots to forging temperature may take 60 hours, and, before completion of the forging process, the workpiece may be reheated six times. The forging is accomplished by flat-, vee-, or swage-shaped dies, depending on the shape of the final product. Saddles and mandrels are used for forging rings and sleeves. The workpiece is connected to a long bar, which helps to move and turn it by a crane or manipulator. Large heat-treating furnaces are available in these forging shops to improve microstructure and to release internal stresses caused by the forging operation.
