|
From Steel to Tube
An overview of welded tube manufacture |
|
Steelmaking
High quality strip is produced by the Basic Oxygen Steelmaking
process from high grade materials with low residual elements. The steel is
continuously cast under carefully controlled conditions and rolled on a wide
strip mill to tight dimensional control limit.
The cleanness of the steel is very important in strip for EW pipes
particularly the avoidance of inclusion stringers mainly present as oxides
and sulphides which can divert near the weld line. Inclusions as small as
200 microns can give rise to ultrasonic signals. Process routes have
therefore been developed to minimise the incidence of non metallic
inclusions and to eliminate stringer types by controlling the shape of the
inclusions in the rolled product.
The volume fraction of inclusions present is influenced by the
overall steelmaking operation. Vessel practice is designed to minimise the oxygen content of the bath when tapped. When the steel is in the ladle
special steps are taken to remove a high proportion of solid deoxidation
products and to prevent the formation of new inclusions formed by
re-oxidation or erosion/chemical attack on refractories. |
|
Alternatively, oxygen may be
removed as a gas by use of vacuum treatment. In spite of these precautions
some oxide inclusions will remain in the steel. These are rendered harmless
by deep injection of suitable modifying agents such as calcium silicide into
the steel ladle to produce non-clustering inclusions, which do not deform
into stringers during hot rolling.
Low sulphur steels down to .002% can be supplied which improve weld
and body impact values, through thickness ductility and resistance to
hydrogen cracking. |
Calcium Silicide Injection Station
|
|
Casting
The steel is transferred to the tundishes on the casting machine
through a refractory shroud tube, which prevent reoxidation occurring. The
liquid steel is continuously cast through two tundishes into moulds
producing slab sizes of 229mm thick x 1.1 to 1.8 metres wide referred to as
concast strands. A diagram of the casting operation is shown below. |
|
|
| |
|
The large tundishes provide a further
opportunity for inclusions to float out and the liquid steel is covered with
an insulating powder to preserve temperature and prevent contact with a the
atmosphere. The steel passing from the tundishes to the moulds is again
shrouded with refractory tubes to prevent reoxidation. The moulds are fitted
with automatic mould level control to prevent sub surface entrapment of
casting powder which could lead to surface and internal defects.
The casting conditions are very carefully controlled to minimise segregation
and the formation of surface cracks. Segregation is reduced by using a low
superheat and a slow casting speed adjusted to suit the grade of steel and
slab dimensions. Machine geometry of the concast strand is very important,
and roll gaps have to be adjusted to within fine limits.
Surface slivers on the strip are the result of surface cracks on
the concast slab which form when the concast curved strand is straightened
out. Control of the cooling water flow reduces the tendency for cracks to
form. Sulphur prints on transverse sections flame cut from the concast
strand are taken regularly to ensure freedom from severe segregation and
inclusion clusters. |
|
|
The continuous cast strand is flame cut into slabs 7.5 metres
long which are cooled in stacks.
When cool, the surfaces of the slab are inspected and any unacceptable
defects observed dressed out by hot scarfing. The slabs are then ready for
rolling to strip.
|
|
|
|
Next Page |
|
|
|
Products Chinese News
Company Ultrasonics
Links |
