When men could produce enough heat
to melt iron ore, the Bronze Age gave way to Iron
Age. The cost factor played a key role in this change.
The borderline between iron and
steel was defined during the Industrial Revolution,
with the development of furnaces which allowed not
only to correct impurities in the iron, but also add
thereto properties such as resistance to weariness,
impact, corrosion, etc. Because of these properties
and its low cost steel came to represent around 90%
of all metals consumed by the industrial civilization.
Basically, steel is a carbon and
iron alloy. Iron is found throughout the surface of
the planet, strongly associated to oxygen and silica.
Iron ore is an iron oxide, mixed with thin sand.
Carbon is also relatively abundant
in nature and may be found under various different
forms. In the steel industry, mineral coal and, sometimes,
vegetable coal are used.
Coal plays a double role in the
production of steel. As fuel, it allows reaching high
temperatures (around 1,500ºC) necessary to melt
the ore. As a reducer, it combines with the oxygen
from the material at high temperatures, freeing the
iron from it. The process of removing oxygen from
iron to combine it with carbon is called reduction
and takes place inside a piece of equipment called
a blast furnace.
Before being put in the blast furnace,
the ore and coal are previously prepared so as to
improve the performance and cost-efficiency of the
process. The ore is transformed into pellets and the
ore is distilled to obtain coke, obtaining also carbochemical
byproducts.
During the reduction process, iron
becomes liquid and is called pig iron or first-melting
iron. Impurities such as limestone, silica, etc. Form
the scum, which is the raw material for the production
of cement.
The next stage is refining. The
pig iron is taken to the steel shop, still liquid,
to be turned into steel, by burning impurities and
additions. The refining of steel is made in oxygen
or electrical furnaces.
Finally, the third classic stage
in the process of producing steel is lamination. Steel,
during the process of solidification, is mechanically
conformed and transformed into steel products used
by the transformation industry, such as heavy and
thin plates, coils, rods, wire, drawn products, bars,
etc.
As technology evolved, the stages
of reduction, refining and lamination are being shortened,
ensuring higher production speeds.
Steel mills worldwide, according to their production process, are classified as:
