Note: Descriptions are shown in the official language in which they were submitted.
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CONTINUOUS TUBE ROLLING METHOD AND MANDREL
ASSEMBLY FOR THE IMPLEMENTATION THEREOF
FIELD OF THE INVENTION
The present invention relates to tube production, in particular to
the production of tubes on a continuous tube rolling mill and the
design of a mandrel assembly.
BACKGROUND OF THE INVENTION
Continuous tube rolling mills with a floating long cylindrical
mandrel have been known since 1885 (patent of Canada No. 24956,
published on the 13th of September, 1886). For rolling tubes on the
continuous mills, a floating mandrel is used, it is inserted into a
tubular billet (hollow shell), and when rolling the mandrel moves with
rolled material beyond the continuous mill, then the mandrel is
removed from the rolled material using a chain-type mandrel
extractor. In this case, the mandrel is made monolithic, with a guide
cone at the front end, a cylindrical part and a tail part in the form of an
annular groove, for which a mandrel extractor grip is hooked.
Based on the necessity to expand the range of rolled tubes
through increasing outer diameter and reducing mandrels weight,
continuous tube-rolling mills with a retained mandrel were developed,
they became widespread since the 60th of last century (patent of Great
Britain No. 1092718, IPC B21B 17/04, B21B 21/00, published on the
11th of November, 1967).
A mandrel assembly with controlled movement during rolling is
used for rolling tubular billets (hollow shell) on continuous tube
rolling mills of two- and three-roll type (MPM, PQF, FQM). The
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retention rate of the mandrel assembly during rolling may be in the
range from 500 to 2000 mm / s depending on rolled tube grades,
equipment design features and selected parameters for rolling.
Extracting the mandrel from the rolled material is carried out with a
mill-extractor, which is located on the same line with the continuous
mill. The mandrel assembly consists of a cylindrical mandrel with a
guiding taper section at the front end, which opposite end is connected
to an extension element through a nipple connection, and in the same
way the extension element is connected to a shank end (the rear end
part of a mandrel assembly) to retain the mandrel in a gripper during
rolling (patent of the USA No. 3120139, B21B 25 / 04, published on
the 4th of February, 1964).
When rolling with a mandrel assembly, hollow shells are
deformed by a cylindrical mandrel, other elements of the mandrel
IS assembly relate to a machine-tool attachment and may include one or
more extension elements, a shank end, a plug fitting, various inserts,
bushings and fixtures.
The working part of the mandrel assembly is operated under
complex temperature conditions during the production process and is
subjected to constant cyclic alternating loads, which intensify its wear
from level of which the quality of finished tubes is depended. A
significant problem in the tube production on units with a continuous
rolling mill is a short service life and a high cost of mandrels, which
are mostly imported.
The critical value of mandrel wear is regarded as the mismatch
of geometric parameters and the state of a mandrel surface to specified
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requirements. The critical value of mandrel wear is specified in
technological instructions of tube manufacturers. The mandrels are
taken out of service when exceeding the wear critical value that
includes various parameters, for example: occurrence of sections
where the wear value at an outer diameter is above the specified one,
occurrence of the variety of surface inconsistencies in the form of
"comets" on the mandrel surface, mechanical damage, rough cracks
formed as a result of temperature influence, as well as when mandrel
surface roughness exceeds the specified critical values.
io When the critical value of the mandrel wear is reached, the
average specific mandrel consumption ranges from 0.1 to 4.0 kg / ton.
The specific consumption is regarded as the difference between
weight of a worn mandrel before regrinding and mandrel weight after
regrinding, referred to the number of rolled tons of products.
The disadvantage of the method of producing tubes on a
continuous mill with the use of a mandrel assembly of built-up
construction is that during rolling the mandrel wear is occurred at 200
¨ 7500 mm from the front end, whereas the opposite part of the
mandrel practically does not wear out. However, when there is the
critical value of mandrel wear, the mandrel is sent to regrinding,
during which the metal is removed over the entire mandrel surface,
which significantly reduces its operating life.
The most similar technical solution considered as the prototype
for the method and the mandrel assembly is the production of tubes on
a continuous tube-rolling mill using a mandrel assembly (patent of the
Russian Federation No. 2486976, IPC B21B 25/00, published on the
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7th of July, 2013), in which a cylindrical mandrel is used. At the ends
of a mandrel cylindrical part there are blind openings for connecting
the cylindrical part alternately to the extension element and the conical
part of the mandrel.
However, when producing tubes, as a result of deformation,
rolled metal flows into the gap between the conical and the cylindrical
mandrel parts. and various types of defects are formed on the inner
tube surface. In addition, it is possible that emergencies occur on a
continuous mandrel mill and a extractor-mill, for example, stopping
io the rolling process because of equipment breakage, failure of
protecting devices, full contact of all the mandrel with the rolled
metal, the formation of unfinished product. Also the prototype does
not regulate the value of the mandrel wear, which requires the
necessary replacement of the end connection of the mandrel with
mandrel assembly attachment, which can cause pre-schedule mandrel
shutdown.
BRIEF SUMMARY OF THE INVENTION
The technical problem solved in the Invention is to extend the
service life of the mandrel through increasing the number of rolled
tubes on the same mandrel, improving tubes surface quality and
eliminating emergency situations during rolling.
The specified problem is solved due to that the continuous tube
rolling method includes the deformation of the tubular hollow shell
using a cylindrical mandrel with identical coaxial blind openings at
the ends, control of mandrel dimensions and mandrel regrinding. The
mandrel is connected with the mandrel assembly attachment
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alternately to deform tubular hollow shell by the working sections
from different ends of the mandrel. According to the Invention, the
deformation of the tubular hollow shell is carried out until the wear
value is at least 25% of the critical value for one end of the mandrel
5 working section. However, the mandrel regrinding is carried out after
changing the end connection of the mandrel with the mandrel
assembly attachment, after subsequent deformation of the tubular
hollow shells and the wear of the working section on the other
mandrel end is at least 25% of the critical value.
io The specified problem is also solved due to that the mandrel
assembly of a continuous tube rolling mill, comprising a cylindrical
mandrel that is made with a beveled side surface at one end, has
identical coaxial blind thread openings at the mandrel ends for the
alternate connection through joining the mandrel to the mandrel
IS assembly attachment according to the Invention, the opposite mandrel
end is made with a beveled side surface, the angle of the beveled
surfaces generatrix with the longitudinal mandrel axis at the both ends
is the same and is in the range from 10 to 70 degrees, whereas the
opening at the front mandrel end is plugged at the working position.
20 BRIEF DESCRIPTION OF THE DRAWINGS
The invention is illustrated by the drawing, which schematically
shows a mandrel assembly for the implementation of the method of
continuous tube rolling. The mandrel assembly includes a cylindrical
mandrel 1 with a beveled side surface 2 at both ends and a machine-
25 tool attachment in the form of an extension element 3 and a shank
end
4. At both ends of the cylindrical mandrel 1 identical blind thread
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openings 5 are made for connecting the mandrel to the extension
element 3 alternately by means of, for example, a nipple connection.
At the working position, the opening at the mandrel front end is
plugged with the plug fitting 6 to protect from external influence and
prevent from clogging of scale and lubricant materials during rolling.
The mandrel assembly can be connected with the shank end 4 also by
means of a nipple connection.
DETAILED DESCRIPTION OF THE INVENTION
During producing tubes on a continuous tube rolling mill, the
io wear of a section
of the mandrel cylindrical part occurs mainly closer
to the front mandrel end, whereas the mandrel section near to the
opposite mandrel end is not almost subjected to the wear. The
deformation of the billets (hollow shells) is carried out until the wear
value of the working mandrel section on the one end is at least 25% of
IS its critical
value. When the wear value does not exceed 25% of the
critical value, satisfactory condition of the mandrel surface and
operational life is maintained, but when such mandrels are removed
from a mandrels circulation system of the continuous rolling mill and
when they are reground to a smaller diameter, a significant amount of
20 metal is grinded and the mandrels average specific consumption
increases from 2 to 40 times. In addition, the pre-schedule removal of
mandrels from operation significantly increases tool cost and
adversely affects the performance of tube-rolling equipment as a
whole.
25 After the working
mandrel section is worn away at one end,
other mandrel end is connected to the machine-tool attachment, the
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plug fitting is inserted into an opening at the front mandrel end and
hollow shells are deformed. Furthermore, during the next operation
cycle rolling is carried out on the unworn working mandrel section
until the wear value reaches at least 25% of the critical value at the
section on the other mandrel end. After wearing of the cylindrical
mandrel is at least 25% of critical value almost along its entire length,
the mandrel can be reground to other outer diameter along its entire
length, which favor to increase the service mandrel life, taking into
account the regulation of its wear value and improve the quality of
to finished tubes.
The mandrels wear value is regulated in the technological
documentation of the enterprise and depends, in particular, on
deformation modes on a continuous rolling mill, on power parameters
of the process, on the range and steel grade of rolled tubes, on the
temperature of rolled metal and rolling tools, on lubricants properties
and antioxidants for different tubes grades.
The mandrel is designed in such a way that the angle of the
beveled surfaces 2 generatrix with the longitudinal mandrel axis at the
both ends is the same and is in the range from 10 to 70 degrees, which
ensures to insert the mandrel easily into a hollow shell before rolling
in a continuous rolling mill and favors the reduction of the number of
defects on the inner tubes surface during the insertion and removal of
the mandrel, as well as during rolling. If the generatrix angle is less
than 10 degrees, there can be difficulties when the mandrel is inserted
into a hollow shell and when the extractor takes a mother tube out, in
addition defects on the inner surface of the rolled material are formed
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in the form of scratches and imprints. If the generatrix angle is more
than 70 degrees, the possibility of cracks formation increases at the
mandrel ends and in areas of blind openings, designed to connect with
the machine-tool attachment, which reduces the reliability of the
connection. In addition, after regrinding of worn mandrels outer
diameter to a smaller size, the angle of the bevelled surfaces
generatrix remains in the specified range, difficulties do not occur
when the mandrel is inserted into a hollow shell and there is
practically no formation of defects on the inner surface of a hollow
lo shell.
At the working position, the opening at the front mandrel end is
plugged with the plug fitting 6 to protect from external influences and
prevent clogging with scale and lubricant materials during rolling.
Thread connections of all the elements of the mandrel assembly are
IS made comparable to the connection of casing tubes and have a tapered
trapezoidal thread or, for example, a Buttress thread. Before screwing,
a lubricant is put on threads. Such constructive design of the mandrel
assembly allows increasing the mandrels service life, the quality of the
inner tube surface and preventing accidents during rolling.
20 EMBODIMENT OF THE INVENTION
The proposed method for the continuous tubes rolling with the
use of the mandrel assembly is as follows. In the rolling process, the
mandrel assembly is inserted into a hollow shell and then, together
with the hollow shell, is set into a continuous mill. The hollow shell is
25 deformed into a mother tube strictly on the cylindrical mandrel part,
located between two sections with beveled side surfaces. The wear of
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the cylindrical part section of the mandrel is formed mainly closer to
the front end, and the section of the mandrel near to the opposite end
practically does not wear out. After the front section of the cylindrical
mandrel is worn at least 25% of the critical wear value, the plug fitting
is removed from the opening at the front end of the mandrel, the
mandrel is disconnected from the attachment, connected by the other
end with the attachment and the plug fitting is inserted into the
opening at the other mandrel end. The operation of connecting and
disconnecting components of the mandrel assembly can be carried out
on a machine for screwing and unscrewing mandrels. After that,
during the next cycle, the rolling process is carried out on the unworn
section of the mandrel until it reaches a critical wear value at least of
25%. After wear of the cylindrical mandrel almost the entire length of
at least 25% of the critical, the mandrel can be reground along its
IS entire length to another outer diameter, which increases the service
life of the mandrel and improves the quality of the finished tube.
When implementing this method for rolling tubes, the mandrel is
easily inserted into a hollow shell and removed, which leads to a
significant reduction in the number of defects on the inner surface of
zo the rolled material.
When the pre-schedule removal of mandrels from operation, for
example, when the wear value does not exceed 25% of the critical
value, and its subsequent regrinding to another outer diameter, the
operational mandrels life is significantly reduced, since in this case the
25 average specific mandrels consumption increases from 2 to 40 times.
Furthermore, the service life increases due to the possibility of using a
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plug fitting made without a thread, which reduces thread wear in the
thread openings due to a smaller number of screwing and unscrewing
operations.
EXAMPLE OF THE EMBODIMENT OF THE INVENTION
5 The proposed
method for the tubes production was tested on a
PQF continuous tube mill. During industrial rolling processes,
mandrels with the diameter of 149.20 179.55 mm and with an angle
of 25 degrees for the beveled surfaces generatrix at the both ends were
used. Tubes were rolled using the 190 mm PQF mill calibration
10 system. The most significant mandrels wear occurred at the section
located at a distance of up to 5000 mm from the front mandrel end.
The mandrel section, located at a distance from 5,000 and up to
11.500 mm (rear end) from the front end, was almost not subjected to
wear. After the front mandrel section was worn by a value equal to
75% of the critical value, the plug fitting was removed from the
opening at the mandrel front end, the mandrel was joined to the
extension element by another end, in particular, on an equipment for
the mandrel screwing on and unscrewing, then, the mandrel was put
into operation again. Thereafter, the mandrel was operated until its
other end working section was worn at 75% of the critical value.
During the mandrel operation, the wear occurred along the entire
length of the working sections; both rolling cycles was characterized
by stable operation of the mandrel assembly, without damage to the
mandrel and threaded connections. On the tubes inner surface, the
depth of single defects of various types did not exceed critical values,
the tubes quality was satisfactory. As a result of the performed
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industrial rolling processes, the mandrels service life was increased by
an average of 45.6%, emergency situations did not occur on a
continuous rolling mill and mill-extractor.
The use of the proposed method for continuous tube rolling
with the use of the mandrel assembly reduces tool cost by increasing
its service life, reducing the formation of various types of defects on
tubes inner surface, and eliminating the risks associated with
emergency situations during the rolling process.
