1. The Production of Steel
The production
of steel is indeed a complex process which if not handled carefully can lead to
accidents and the failure of receiving a good end product. The process of
manufacturing steel is a long and tedious one that undergoes various procedures
in order for the end product to transform into a fine and quality driven
output. Provided below are the various steps explained briefly, about the
manufacture of steel and its by-products.
i.
The
Raw Materials and the Coke Oven The raw materials
used for the manufacture of steel are Iron Ore Pellets (i.e. small pebbles of
raw Iron Deposits), Coal and Limestone (and Dolime in many cases). The coal is
then converted to coke (tiny particles of coal) in the coke ovens. The
impurities removed from refining the coal are usually sold in the market place
as by-products. The newly refined coal is called ‘metallurgical coke’ or pure
carbon. This metallurgical coke is used in the blast furnace while the
limestone reserves are used to remove impurities.
ii.
Blast
Furnace
This is
considered a very essential part of steelmaking. The use of blast furnaces has
been around for centuries.
Due to
the drastic advancement in technology, the process of manufacturing steel has
changed into a more sophisticated one. Many companies have different methods
and techniques of manufacturing steel. After the iron ore is converted into
molten metal by the Blast Furnace the output then undergoes a process called
the “Basic Oxygen Furnace”. The basic oxygen furnace is a 300 tonnes oxygen
converter. “70% iron and 30% recycled steel are mixed with other alloys in the
300-ton basic oxygen steelmaking furnace to create the metallurgical qualities
which suit the customer’s end requirements” (METSOC, 2011). After the molten
metal is converted into molten steel by the basic oxygen furnace it is then
sent to the Steel Refining Facility.
iii.
Steel
Refining Facility
The molten steel
is sent in to the steel refining facility where the molten steel is refined
into a fine and pure form of steel. This newly reformed and pure state of steel
is then sent to the continuous casting plant to be converted into slabs of
steel.
iv.
Continuous
Casting
The
refined molten steel is poured into a casting machine to be converted into
solid slabs of steel.
2. A Brief study on the production of Steel in
ArcelorMittal S.A.
ArcelorMittal is
the largest manufacturers of steel in the world with 10% of the worlds steel
production being produced by them (Fortune Global 500, 2010). Provided below is
a brief study on the production of stainless steel in ArcelorMittal’s plant
located in Paris, France.
a. Stage 1 – The Melt Shop
The use
of raw materials, mostly recycled stainless steel and ferro-alloys is converted
into molten metal using an electric arc furnace with a melting capacity of up
to 160 tonnes. The molten is then transferred to a converter. Alloying elements
are added into the converted to obtain the chemical composition thus allowing
the production of standard grades.
The ladle metallurgy consists of the addition of some alloying elements in order to obtain the similar chemical composition and the correct temperature before the continuous casting process. The produced molten steel is then poured, cooled and then solidified when undergoing the continuous casting process. During this stage, the maximum width of the steel is determined. The solid steel I cut with an oxygen flame into steel slabs that are up to 30 tonnes in weight, up to 12 metres in length, 200 mm thick and up to 2 metres wide. Some of the steel slabs are then ground in order to remove surface defects before being shipped to the hot rolling mill.
b. Stage 2 – Hot Rolling Mill
This process
takes place in the ArcelorMittal Chatelet and involves the reheating of the
steel slabs in a furnace at a temperature of 1250 °C.
The first reduction of the steel slabs with regards to thickness is from
200 mm to 40 mm after several passes on the roughing mill. The thermal tunnel
is responsible for maintaining the temperature of the steel slabs that are
reduced in thickness.
The slab
then is passed through a series of 7 rolling stands to reach a reduced size of
approximately 3 mm in thickness. The metal is then cooled and wound to form
coils or also known as “black coils” due to their oxidized nature and then
shipped to ArcelorMittal’s annealing and pickling plant. ArcelorMittal also
sells black coil to customers upon request.
Fig
2.1 Conversion of Steel Slabs into Black Coils
c. Stage 3 – Annealing and Pickling
The Black coils
are then heated in the annealing furnace. This step allows the recovery of the
stainless steel. The oxides of the surface of the steel are broken down by shot
blasting.
The pickling
process in the acid baths followed by water cleaning and air drying results in
the transformation of the black coils into white coils with no surface oxide;
this is called the HRAP finish. At this stage, the white coils or HRAP coils
can be sold in the market without cold rolling or further surface finishing.
d. Stage 4 – Cold Rolling and Final Annealing
This stage
allows the reduction of the thickness of the HRAP coil strips down to minimum
of 0.3 mm to meet the customer’s final requirements for stainless steel. A
final annealing provides the required properties for the use of the customer.
The cold rolling process helps produce the flatness of the stainless steel
together with an enhanced surface finish suitable for household appliances
(e.g. Stainless-steel cutleries).
e. Stage 5 – Metal Finishing
The end product
is then slit to width and cut in to length also allowing for some specific
surface finishes to be made. These metal finishing operations are carried out
in ArcelorMittals’s production plants and through their network of service
centres. The end products, according to ArcelorMittal, are 100% recyclable at
the end of their life.
METSOC.
(2011). Steel Production Flow-Sheet. Available: http://www.metsoc.org/virtualtour/processes/steel.asp.
Last accessed 11th February, 2012
Cable News
Network (CNN). (2010). Fortune Global 500 2010: The World's Biggest Companies -
ArcelorMittal - MT. Available: http://money.cnn.com/magazines/fortune/global500/2010/snapshots/11208.html.
Last accessed 10th February, 2012.
ArcelorMittal
. (2009). What is Stainless Steel? Manufacturing Process. ArcelorMittal Paris
Stainless Europe. 1 (1-5), p1-5.
