Steelmaking is the process for producing steel from iron ore and scrap. In steelmaking, impurities such as nitrogen, silicon, phosphorus, sulfur and excess carbon are removed from the raw
iron, and alloying elements such as manganese, nickel, chromium and vanadium are added to produce different grades
of steel. Limiting dissolved gases such as nitrogen and oxygen, and entrained impurities (termed
"inclusions") in the steel is also important to ensure the quality of
the products cast from the liquid
steel.
Steelmaking has existed
for millennia, but it was not commercialized on a massive scale until the 19th
century. The ancient craft process of steelmaking was the crucible
process.
In the 1850s and 1860s,
the Bessemer process and
the Siemens-Martin process turned steelmaking into a heavy
industry.
Today there are two
major commercial processes for making steel, namely basic oxygen steelmaking, which has liquid pig-iron from the blast furnace and scrap steel as the
main feed materials, and electric arc furnace (EAF) steelmaking, which uses scrap steel or direct reduced iron (DRI) as the main feed materials.
Oxygen steelmaking is
fueled predominantly by the exothermic nature of the reactions inside the
vessel where as in EAF steelmaking, electrical energy is used to melt the solid
scrap and/or DRI materials. In recent times, EAF steelmaking technology has
evolved closer to oxygen steelmaking as more chemical energy is introduced into the process.
Bethlehem Steel, Bethlehem,
Pennsylvania facility was one of the world's largest
manufacturers of steel before its closure in 2003.
Steelmaking has played a
crucial role in the development of ancient, medieval, and modern technological
societies. Early processes of steel making were made during the classical era
in Ancient Iran, Ancient
China, India, and Rome, but
the process of ancient steelmaking was lost in the West due
to the fall of
the Western Roman Empire in the 5th century CE.
Cast
iron is a hard brittle material that is difficult to
work, whereas steel is malleable, relatively easily formed and a versatile
material. For much of human history, steel has only been made in small
quantities.
Since the invention of
the Bessemer process in
the 19th century and subsequent technological developments in injection
technology and process
control, mass production of steel has become an
integral part of the global economy and a key indicator of modern technological
development. The earliest means of producing steel was in a bloomery.
Early modern methods of producing
steel were often labor-intensive and highly skilled arts. See: finery forge, in
which the German
finery process could be managed
to produce steel. blister steel and crucible steel.
An important aspect of
the Industrial Revolution was the development of large-scale methods of producing forgeable
metal (bar
iron or steel). The puddling
furnace was initially a means of
producing wrought iron, but
was later applied to steel production.
The real revolution in
modern steelmaking only began at the end of the 1850s when the Bessemer process
became the first successful method of steelmaking in quantity, followed by
the open-hearth furnace.
Modern steelmaking
processes can be divided into two categories: Primary and Secondary
steelmaking.
Primary steelmaking
involves converting liquid iron from a blast
furnace and steel scrap into steel via basic oxygen steelmaking, or melting scrap steel or direct reduced iron (DRI) in an electric arc furnace.
Secondary steelmaking
involves refining of the crude steel before casting and the various operations
are normally carried out in ladles. In secondary metallurgy, alloying agents
are added, dissolved gases in the steel are lowered, and inclusions are removed
or altered chemically to ensure that high-quality steel is produced after
casting.
Basic oxygen steelmaking is a method of primary steelmaking in which carbon-rich molten pig
iron is made into steel. Blowing oxygen through molten
pig iron lowers the carbon content of the alloy and changes it into steel. The process is known as basic due
to the chemical nature of the refractories—calcium oxide and magnesium
oxide—that line the vessel to withstand the high temperature
and corrosive nature of the molten metal and slag in the vessel. The slag chemistry of the process is also controlled to ensure that impurities such as
silicon and phosphorus are removed from the metal.
The process was
developed in 1948 by Robert
Durrer and commercialized in 1952–53 by
Austrian VOEST and ÖAMG. The
LD converter, named after the Austrian towns of Linz and Donawitz (a district of Leoben) is a refined version of the Bessemer converter where
blowing of air is replaced with blowing oxygen. It reduced capital cost of the plants, time of smelting, and increased
labor productivity.
Between 1920 and 2000,
labor requirements in the industry decreased by a factor of 1,000, from more
than 3 worker-hours per ton to just 0.003.
The vast majority of
steel manufactured in the world is produced using the basic oxygen furnace; in
2011, it accounted for 70% of global steel output. Modern furnaces will take a
charge of iron of up to 350 tons and convert it into steel in less than 40
minutes, compared to 10–12 hours in an open hearth furnace.
Electric arc furnace steelmaking is the manufacture of steel from scrap or direct reduced
iron melted by electric arcs. In
an electric arc furnace, a batch of steel ("heat") may be started by
loading scrap or direct reduced iron into the furnace, sometimes with a
"hot heel" (molten steel from a previous heat). Gas burners may be
used to assist with the melt down of the scrap pile in the furnace. As in basic
oxygen steelmaking, fluxes are also added to protect the lining of the vessel
and help improve the removal of impurities. Electric arc furnace steelmaking
typically uses furnaces of capacity around 100 tons that produce steel every 40
to 50 minutes for further processing.
By-product gases from
the steel making process can be used to generate electricity through the use of
reciprocating gas engines.
Secondary steelmaking is
most commonly performed in ladles and often referred to as ladle (metallurgy).
Some of the operations performed in ladles include de-oxidation (or
"killing"), vacuum degassing, alloy addition, inclusion removal,
inclusion chemistry modification, de-sulphurisation and homogenisation. It is
now common to perform ladle metallurgical operations in gas stirred ladles with
electric arc heating in the lid of the furnace. Tight control of ladle
metallurgy is associated with producing high grades of steel in which the
tolerances in chemistry and consistency are narrow.
The HIsarna ironmaking process is a
process for iron making in
which iron ore is
processed almost directly into liquid iron or hot metal. The
process is based around a new type of blast furnace called a Cyclone
Converter Furnace, which makes it possible to skip the process of
manufacturing pig iron pellets that is necessary for the basic oxygen steelmaking process. Without the necessity for this preparatory step the HIsarna
process is more energy-efficient and has a lower carbon
footprint than traditional steelmaking
processes.
Norb
Leahy, Dunwoody GA Tea Party Leader
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