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STEEL MANUFACTURE

In most of the world, steel is manufactured by integrated steel facilities that produce steel
from basic raw materials, i.e., iron ore, coke, and limestone. However, the fastest growing
segment of the steel industry is the ‘‘minimill’’ that melts steel scrap as the raw material.
Both types of facilities produce a wide variety of steel forms, including sheet, plate, structural,
railroad rail, and bar products.
Ironmaking. When making steel from iron ore, a blast furnace chemically reduces the ore
(iron oxide) with carbon in the form of coke. Coke is a spongelike carbon mass that is
produced from coal by heating the coal to expel the organic matter and gasses. Limestone
(calcium carbonate) is added as a flux for easier melting and slag formation. The slag, which
floats atop the molten iron, absorbs many of the unwanted impurities. The blast furnace is
essentially a tall hollow cylindrical structure with a steel outer shell lined on the inside with
special refractory and graphite brick. The crushed or pelletized ore, coke, and limestone are
added as layers through an opening at the top of the furnace, and chemical reduction takes
place with the aid of a blast of preheated air entering near the bottom of the furnace (an
area called the bosh). The air is blown into the furnace through a number of water-cooled
copper nozzles called tuyeres. The reduced liquid iron fills the bottom of the furnace and is
tapped from the furnace at specified intervals of time. The product of the furnace is called
pig iron because in the early days the molten iron was drawn from the furnace and cast
directly into branched mold configurations on the cast house floor. The central branch of
iron leading from the furnace was called the ‘‘sow’’ and the side branches were called ‘‘pigs.’’
Today the vast majority of pig iron is poured directly from the furnace into a refractorylined
vessel (submarine car) and transported in liquid form to a basic oxygen furnace (BOF)
for refinement into steel.
Steelmaking. In the BOF, liquid pig iron comprises the main charge. Steel scrap is added to
dilute the carbon and other impurities in the pig iron. Oxygen gas is blown into the vessel
by means of a top lance submerged below the liquid surface. The oxygen interacts with the
molten pig iron to oxidize undesirable elements. These elements include excess carbon (because
of the coke used in the blast furnace, pig iron contains over 2% carbon), manganese,
and silicon from the ore and limestone and other impurities like sulfur and phosphorus.
While in the BOF, the liquid metal is chemically analyzed to determine the level of carbon
and impurity removal. When ready, the BOF is tilted and the liquid steel is poured into a
refractory-lined ladle. While in the ladle, certain alloying elements can be added to the steel
to produce the desired chemical composition. This process takes place in a ladle treatment
station or ladle furnace where the steel is maintained at a particular temperature by external
heat from electrodes in the lid placed on the ladle. After the desired chemical composition
is achieved, the ladle can be placed in a vacuum chamber to remove undesirable gases such
as hydrogen and oxygen. This process is called degassing and is used for higher quality steel
products such as railroad rail, sheet, plate, bar, and forged products. Stainless steel grades
are usually produced in an induction or electric arc furnace, sometimes under vacuum. To
refine stainless steel, the argon–oxygen decarburization (AOD) process is used. In the AOD,

an argon–oxygen gas mixture is injected through the molten steel to remove carbon without
a substantial loss of chromium (the main element in stainless steel).
Continuous Casting. Today, most steel is cast into solid form in acontinuous-casting (also
called strand casting) machine. Here, the liquid begins solidification in a water-cooled copper
partially solidified shape is continuously withdrawn from the machine and cut to length for
further processing. The continuous-casting process can proceed for days or weeks as ladle
after ladle of molten steel feeds the casting machine. Some steels are not continuously cast
but are poured into individual cast-iron molds to form an ingot that is later reduced in size
by forging or a rolling process to some other shape. Since the continuous-casting process
offers substantial economic and quality advantages over ingot casting, most steel in the world
is produced by continuous casting.
Rolling/Forging. Once cast into billet, slab, or bloom form, the steel is hot rolled through
a series of rolling mills or squeezed/hammered by forging to produce the final shape. To
form hot-rolled sheet, a 50–300-mm-thick slab is reduced to final thickness, e.g., 2 mm, in
one or more roughing stands followed by a series of six or seven finishing stands. To obtain
thinner steel sheet, e.g., 0.5 mm, the hot-rolled sheet must be pickled in acid to remove the
iron oxide scale and further cold rolled in a series of rolling stands called a tandem mill.
Because the cold-rolling process produces a hard sheet with little ductility, it is annealed
either by batch annealing or continuous annealing. New casting technology is emerging
where thin sheet (under 1 mm) can be directly cast from the liquid through water-cooled,
rotating rolls that act as a mold as in continuous casting. This new process eliminates many
of the steps in conventional hot-rolled sheet processing. Plate steels are produced by hot
rolling a slab in a reversing roughing mill and a reversing finishing mill. Steel for railway
rails is hot rolled from a bloom in a blooming mill, a roughing mill, and one or more finishing
mills. Steel bars are produced from a heated billet that is hot rolled in a series of roughing
and finishing mills. Forged steels are produced from an ingot that is heated to forging temperature
and squeezed or hammered in a hydraulic press or drop forge. The processing
sequence in all these deformation processes can vary depending on the design, layout, and
age of the steel plant.

Mechanical Engineers’
Handbook: Materials and Mechanical Design,
Volume 1, Third Edition.
Edited by Myer Kutz

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