DEFINITION OF TECHNICAL TERMS
Sinter An iron rich
clinker formed by heating iron ore fines and coke in a sinter
of molten iron in a blast furnace from iron ore, sinter, coke
and limestone. The furnace generates great heat using blasts of
super heated air. The molten metal is about 95 per cent pure iron
with 5 per cent carbon and other elements and is known as hot
Steelmaking The conversion
of hot metal to steel in a converter using a process known as
basic oxygen steel making (BOS). A water-cooled lance is lowered
into the vessel and high purity oxygen is blown onto the metal
to remove carbon and other unwanted elements. Fluxing agents are
added during the blow and alloying elements are added, in various
forms of secondary treatment, to adjust the chemistry of the steel.
Slab Casting The production
of slabs from liquid steel by a continuous casting process. Continuous
casting is a technique in which the molten steel is channelled
through a water-cooled mould. The metal that has begun to solidify
on the outside is led through water sprays and between rolls to
be cut into lengths of slabs. Generally a slab thickness of 250
mm is used.
Hot Rolling The hot
mill comprises four stages:
(i) a slab re-heating furnace is used to
bring the slabs up to about 1300 degrees C.
(ii) the slabs are descaled and passed through
a roughing mill to reduce the thickness to about 25 mm.
(iii) the partially finished strip goes straight
onto the finishing section of the hot mill where it passes through
series of rolls (seven sets in tandem) for further reduction in
thickness generally between two and 12 mm although greater thicknesses
are produced for some applications
(iv) the strip then passes through water
sprays and onto a coiler.
Pickling Before coils
can be further processed, the scale formed during hot rolling
is removed by treatment with dilute sulphuric or hydrochloric
acid. This is done in a continuous line with the hot rolled strip
being uncoiled and then passed in succession through acid baths,
water tanks and drying ovens.
Cold Rolling The pickled
coil passes into a cold rolling mill where the gauge is further
reduced to generally between 0.15-2.0 mm. The cold mills typically
have five stands in tandem.
Annealing Cold rolling
makes the metal hard and springing and this hardness must be removed.
Annealing is carried out by heating the coils to about 700 degrees
C, keeping them at this temperature for a prescribed time and
then slow cooling. To prevent oxidation control of the atmosphere
in the annealing furnaces is necessary. Two types of annealing
are commoncontinuous annealing and batch annealing.
Temper Rolling A small
reduction of up to 1 per cent is necessary to impart mechanical
properties on the final cold roll strip. Using rolls of appropriate
surface roughness also give the required surface finish at this
Metallic Coating A
metallic coating is put onto steel to improve corrosion resistance.
Two processes are involved; either hot dipped coatings which are
generally zinc or aluminium, or electrolytic coatings which are
usually tin, zinc or a nickel zinc alloy.
Hot Dipped Galvanised (HDG) Coating The
hot dipped process involves a continuous line which processes
cold rolled unannealed strip by heating it to about 700 degrees
C, passing it through a bath of liquid zinc followed by a controlled
cooling process. An induction heating process after coating can
be employed to produce a galvannealed or iron zinc (IZ) product
used principally in the automotive sector.
Aluminised Strip Similar
to HDG strip but using a bath of liquid aluminium.
Tinplate Low carbon
cold rolled and annealed strip coated with a thin layer of tin
by electrolytic deposition in a tinning line.
Electro Chrome Coated Steel (ECCS) Also
called TFS (Tin Free Steel), low carbon mild steel coated on both
surfaces with metallic chromium and chromium oxide applied by
Ferrolite Very thin
cold rolled strip (usually ECCS) laminated with an organic film
Electrogalvanised Coating Annealed
cold rolled strip is coated by electrolytic deposition on a continuous
line which cleans the strip, coats it by passing through a number
of electrolytic cells before the final strip is dried and coiled.
Electrogalvanised coatings are either pure zinc or can involve
an alloy of nickel and zinc.
Prepainted or Prefinished Strip Two
principal products are available. Colorcoat coatings are applied
in liquid paint form whereas Stelvetite is a film coating laminated
to the steel using an adhesive. A variety of coatings are available
using different paint types and are generally applied to HDG coils
but can also be applied to uncoated CR or electrogalvanised coils.