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In 2006, 6.3 million tonnes of ferrochromium were produced
with more than 90% going to the stainless steel industry.
It is chromium that confers upon stainless steel its remarkable
corrosion resistance. Other elements such as nickel and,
in the top grades of stainless steel, titanium and molybdenum
can also be added to improve technical performance in specific
environments.
Stainless Steels
The term 'stainless steel' covers a family of steels that
contain chromium. To be 'stainless', the steel must contain
at least 11-12%Cr but some steels contain up to 30%Cr. For
example, one of the best known stainless steels is the Series
304 steel, often referred to as 18/8 stainless where 18 is the
percentage of chromium and 8 is the nickel percentage. There
are standard compositions for the different grades of stainless
steel and designation numbers which are now universally used
by the producers and consumers. These are:
| Series 300 steels: |
austenitic nickel-containing grades |
| Series 400 steels: |
chromium-containing ferritic and martensitic
grades with no or very little nickel |
| Series 200 steels: |
where manganese replaces some of the nickel
in the 300 Series |
| Series 600 steels: |
precipitation hardening grades |
The Series 300 steels have excellent corrosion resistance,
are easily fabricated, can handle very low temperatures (cryogenic)
and also high temperatures (over 900º C). In some cases,
extra low carbon grades are produced for added corrosion resistance
and are designated with an L; e.g. Series 304L. The addition
of small amounts of Mo,Ti and Nb also improves corrosion resistance,
particularly in welded areas. The Series 300 steels are easily
cleanable and thus are used in environments where hygiene is
important. They have very wide applications such as in architecture,
food processing, hospitals, and in the chemical, petrochemical,
mineral processing and paper industries.
The Series 400 steels can be martensitic, such as Series 410,
420 and 431, with moderate corrosion resistance and are used,
for example, for cutlery, knife blades and surgical instruments;
or they can be ferritic, such as Series 409 and 430, with good
corrosion resistance and are used in domestic equipment, architectural
trims, chain conveyors, and dust and fume extractors.
Stainless steel is made by melting various amounts of carbon
steel and stainless steel scrap in an electric arc furnace.
To this melt, controlled additions of high-carbon ferrochromium
or charge chrome are made to achieve the correct chromium levels.
The proportions of ferrochromium and stainless scrap used depend
partly upon scrap availability and partly upon scrap prices
which themselves fluctuate with availability and with the nickel
contained in the scrap.
Secondary refining is carried out in AOD or VOD vessels in order
to lower the carbon content of the melt without excessive oxidation
losses of the chromium.
Finally, additions of nickel, and sometimes minor amounts of
elements such as Mo, Ti, and Nb, are made and also trimming
additions of chromium as low-carbon ferro-chromium to achieve
the required grade of stainless steel.
The liquid stainless steel is then cast into ingots, billets
or slabs for hot- and cold-working to other products such as
plate and coil.
Apart from the stainless steels, ferrochromium alloys are used,
but in much smaller quantities, in other ferrous materials such
as carbon steels, full alloy steels, bearing and high speed
steels, high-strength low-alloy steels and tool steels, and
in some cast irons, superalloys and welding materials. Alloy
cast irons use chromium to achieve hardness and resistance to
corrosion, abrasion and impact, and are used for pumps, valves,
pipes, rolls and wear plates. 
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