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From Steel to Tube
An overview of welded tube manufacture (continued) |
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Strip Rolling
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A
diagram of the strip rolling mill is shown above
The slabs enter a reheat furnace where a computer calculates the
heat content of each slab in the furnace, and compares this with the desired
heat content of the slab in that position. Positive or negative errors are
summed for the whole furnace and used to control the speed of slabs through
the furnace. On leaving the furnace the slabs enter a reversing mill, which
reduces the thickness in 7.9 passes from 229mm to 30mm.
The slabs then enter a 6 strand-finishing mill fitted with
automatic set up and automatic gauge control. A computer system adjusts the
roll gaps to produce the correct gauge at the particular finishing
temperature taking into account all the slab variables. Information from
each strand is fed on to the next one to ensure the desired finishing
conditions are obtained.
Gauge performance is measured by an x-ray technique along the
centre line of the strip and is consistently within ±. 10mm. Information
from the width meter is fed back to the reversing mill where the width can
be adjusted by altering the amount of side spread of the slab. Laminar
cooling is used to control coiling temperatures. |
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Slitting |
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Each coil is labelled with
steel grade, dimensions and cast and coil numbers. In addition a computer
printout giving the details of all coils including the cast analysis is
produced. From these records it is possible to trace back an individual coil
to the position of its slab on the concast strand and hence its precise
steelmaking and casting details. The grade of the coils is regularly checked
with a portable optical emission spectrometer prior to slitting. The leading
end of each coil is cropped to present a square end to the slitting blades
and the coil edges are slit or milled off to obtain a precise and consistent
width. The coils are generally slit into three narrower coils of a
width suitable for feeding directly on to EW Mills to make pipes 139.7 –
193.7mm diameter. Pipes of smaller diameter and thin walled 139.7mm diameter
are made by stretch reducing 168.3mm o.d. pipes. Pipes of larger
diameter up to 508mm are made from strip rolled either from slit or full
width concast slab. |
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Making The Pipe |
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The coils are received at the Electric
Weld (EW) Mill from the slitting line in widths which are governed by the
wall thickness and the outside diameter of the finished pipe. Figure 6
diagrammatically shows the various stages through the EW Mill from the coil
to the welded pipe. The coil is first uncoiled and fed through a strip
leveller, which flattens the strip and prepares it for forming. The front
and back of each coil is sheared by the strip end shears to present a clean
square edge to the flash welder. At the flash welder, the ends of the coil
are joined by flash butt welding and the upset cleaned off. This allows the
mill to run continuously. The flash welded section is cut out at a later
stage and discarded. The looping pits form a strip accumulator between the
flash welder and the Forming Mill which allows the EW Mill to continue
welding, and producing pipes while the flash welding operation between coils
is taking place.
On EW Mills which produce quality welded pipes, it is essential to
present consistent edge conditions to the mill to ensure that optimum
welding conditions can be maintained and this is achieved using the Edge
Scarfing Unit. The Edge Scarfing Unit removes the shear plain edge and
presents a square machine edged surface to the mill. The forming mill
progressively shapes the strip from the flat ingoing material to the closed
oval. Careful design of this section avoids the creation of unnecessary
residual stresses within the final product. Regular checks are made on mill
alignment and precise setting of the mill is carried out to ensure that the
strip travels up the centre of the roll train so that on reaching the end of
the forming mill, there is no tendency to twist thus avoiding uneven working
of strip edges. The last stands within the forming mill are generally
referred to as the fin passes which work the edge of the strip to present a
consistent edge profile to the welding vee. The forming mill is adjusted to
standard settings and the actual values recorded for each size and gauge.
Discipline in the control of the mill set up contributes to the assurance of
consistent product quality and weld integrity.
The strip edges are heated to a welding temperature by the high
frequency welder. The welder is a large radio frequency oscillator producing
and alternating current of 200.400 kHz which resistance heats the strip
edges. Due to the two phenomena associated with radio frequency electric
current (i.e. The skin effect and the proximity effect) this current
concentrates in the surface of the strip edge.
At the weld head the two heated edges are brought together and
pressure applied to form a forged weld. All previously liquid metal is
expelled together with any oxides and the plastic areas behind the heated
edges upset. The geometry of the weld area is very important and the ingoing
and outgoing circumferences are measured to assess the amount of metal,
which has been pushed out. The symmetry and dimensions of the heat pattern
are regularly checked by cutting a sample from the tube, polishing and
etching a cross section of the weld and examining the microstructure under a
microscope.
Subsequent heat treatment completely removes the heat pattern
resulting in a uniform structure in the weld region. After welding,
the internal and external weld flash or bead is planed from the tube. The
strip edges are thickened in the fin passes before welding which allows the
internal and external bead planning equipment to marginally cut into the
parent metal without reducing the tube
wall below the nominal thickness, (Figure 14). The weld bead is regularly
checked for uniformity and integrity.
The weld line is then water cooled to lower the temperature of the
pipe as it enters the sizing mill. The sizing mill rounds up the pipe and
marginally reduces its diameter to give the required finished dimensions.
Within the sizing mill the weld line is inspected continuously using
ultrasonic shear wave and surface wave techniques together with eddy current
testing. The sizing mill is also used to produce a straight pipe by
adjusting the final restraining roll pass at the end of the sizing section.
The rotary cut-off is used to cut the continuous pipe into the required
lengths unless stretch reduction is required when the pipes are left in
lengths of up to 122 metres.
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