Steel Rolling Mills
1.
Trucks
or trailers carrying billets from billet making industries enter the billet
yard.
2.
Billets
are unloaded in the billet yard by an overhead crane.
3.
Unloaded
billets are either moved to the platforms of shearing machines Pneumatically
operated shearing machines shear the billets into smaller sizes.
4.
Sheared
billets are piled over one another manually and these piles are picked by
overhead crane that lifts it to the raised platform of pusher. Billets are
placed manually in front of pusher. Pusher then pushes the billets into the
furnace.
5.
In
furnace billets are heated to over 1200 degree Celsius. When billets reach the
other end of the furnace they are hot enough to be rolled.
6.
Red-hot
billets are then pulled from the furnace on to the conveyor, which transports
it to the rolling
stand. Rolling process is composed of stage-wise rolling of steel. After
passing through rolling stands several times (depending upon the type of bar
required) they are taken to the cooling bed. Steel rolled in to bars by 12”
mill is transported to the cooling bed by conveyor while bars are hurled on to
the cooling bed by laborers in case of 8” mill.
7.
Bars
are air-cooled while on cooling bed. Chains carry the bars and transport them
at snail’s pace to the conveyor. Slow speed of chains allows sufficient time
for bars to cool down. Cooling bed adjacent to 8” mill drops the bars on to a
conveyor which takes them to the bar yard. The other cooling bed drops them
directly in to the bar yard without employing any conveyor.
8.
If
TOR bars are being manufactured, they have to be twisted. The twisting machines
carry out twisting.
9.
Finally
the ends of twisted bars (and the untwisted ones) are sheared off by a shearing
machine. Ends are sheared because they are often cracked.
10.
The
finished product is then loaded on to trucks and weighed on weighbridge.
11.
After
taking clearance, trucks leave the premises with ready-to-use bars.
FURNACE
Furnace
is the heart of this factory. Rolling is impossible without heating the
billets. Billets are heated so as to make them soft and suitable for rolling.
Moreover heating also prevents residual stresses. Furnace has three parts:
walls, roof and hearth (floor). Furnace is lined with several layers of
refractory bricks. It is insulated by glasswool. Heating zone of the furnace
has temperature of 1000 degree Celsius. Low Alumina bricks are used for this
zone. Soaking zone has temperature in excess of 1200 degree Celsius. High Alumina
bricks are used for this zone.
FOUNDRY
There’s
a small foundry beside the billet yard. This foundry is used for the casting of
keys and couplings. Keys and couplings wear out and loose their metal in
powdery form over time and therefore have to be replaced. The worn-out keys and
couplings are melted and this molten
metal is reused for casting. About 60% of the metal is recovered this way.
Keys
and bearings are made from brass. This particular alloy is used for their
manufacturing to prevent their fusion welding with the shaft.
SHEARING MACHINES
Shearing
machines are used to cut billets in to pieces. There are two shearing machines.
One is used to shear heavy billets while other shears light billets. Shearing
machines have pneumatically operated clutches. A compressor generating pressure
of 6 bars supply air to the shearing machine.
MAINTENANCE
Every
friday maintenance is carried out. Following are the few operations that are
carried out for maintenance:
The
grooves of rolls wear out within a week. In order to remake the TOR and
deformed profiles these rolls are replaced.
Furnace
is also cleaned. Billets melt partially while in furnace. This molten metal
sticks to the hearth. This metal has to be removed with a long chisel and
hammer.
Oxidized
steel flakes deposit on roller stands. These form a sludge-like substance When
they mixed with water (cooling water).
Water
is used as coolant for rolls. Flood application of water causes rolls to
corrode rapidly. Bolts also corrode. They don’t come off easily and are removed
by melting them.
Corroded
rolls are taken to the workshop where welder deposits layers of metal on them.
These layers are then machined by lathe machines to produce desired profiles.
Black
grease is applied generously wherever required.
COOLING BED
Cooling
bed, as the name suggests, is where finished bars are placed for cooling.
Cooling bed is simply a raised platform with chains ‘wrapped’ round it. Chains
roll on sprockets. These sprockets are mounted on a shaft, which rotates very
slowly. Slow speed is maintained to allow time for bars to cool down. These
chains carry the bars from one end to the other, from where it is dropped on to
a conveyor. Cooled bars are then transported to a yard where they are cold
worked, if required.
RECUPERATOR
For
efficient combustion of natural gas air is also blown in to the furnace. In
order to have a more efficient combustion air should be heated so that reaction
(combustion) starts instantly and no heat is lost in raising the temperature of
air to the required level. This heating is carried out by letting them to flow
closely (without mixing) in a device called recuperator. Heat is transferred
from hot exhaust gases to the air going into the furnace. These gases do not
mix. Air passes through the tube bank that is enclosed by an outer shell
through which exhaust gases pass.
HOISTS
Hoists
are used to lift heavy machine elements and move them horizontally. Hoists are
mounted on wide-flange beams (or I-beams). They carry heavy objects with the
help of wire rope having hook attached to its end.
Rolls
need to be changed often. Each weighs around ½ - 1 ton. Laborers cannot lift
them. Therefore hoists are used. At this facility, hoists are mounted over both
mills, over machine shop floor.
GRADES OF STEEL AND CLASSIFICATION OF BARS
Specifications
of billets include steel grade, heat number and size.e.g. a billet may have a
steel grade of SR24 and heat number 20392.[Here 24 in SR24 represents diameter
of bar. And heat number is a number embossed on billets in order to keep track
of the billets just in case anything goes wrong]
In
this factory following bars were produced:
1.
Deformed
bars: Grade 60, 40
2.
TOR
bars: SR24
Bars
are classified by the method by which they are rolled e.g. DF 12mm or TOR 25mm.
Deformed bars are dispatched without twisting while TOR bar has to be twisted.
Bars are twisted because of structural requirements. TOR bars are cold worked
later after leaving the mill. Helix angle of TOR bar is about 25 degrees.
SAMPLE COLLECTION (CHIPS FROM BILLETS)
Billet
samples are taken from the billet yard. Samples are taken to the drill press
where it is drilled. Resulting chips are collected and then sent to a testing
lab for chemical examination in order to make sure that different elements are
within prescribed limits.
TENSILE AND BENDING TEST
1.
Small
pieces from the final product are cut and taken to the testing lab for
determining several properties like UTS, yield stress, percentage elongation,
and weight per unit length.
2.
Weight
and the length of the piece are measured and weight/length is found out.
3.
Marks
are hammered on the piece at 4” intervals.
4.
The
piece is then fixed into the jaws of Universal Testing Machine and load is
applied till the specimen fails.
5.
Percentage
elongation is also measured.
6.
Some
bars exhibit cup and cone fracture.
7.
Samples
are also checked for sufficient ductility in bending. No readings are taken in
this test. It is checked whether the bar is ductile enough that it does not
fracture suddenly during bending.
1.12 8” ROLLING MILL’S CONFIGURATION
8”
rolling mill has 1 roughing and 6 finishing stands. Roughing stands have plain
rolls while finishing stands have grooved patterns on them.
Iron Rolling / Mill Rolling
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