TECHNICAL FIELD
[0001] The present invention relates to a method for producing a seamless steel tube by
the Mannesmann tube-making process, and a production facility for a seamless steel
tube suitable for carrying out the production method.
BACKGROUND ART
[0002] A seamless steel tube can be used for oil well tubes for which high strength and
toughness are required and can be produced by the Mannesmann tube-making process.
This tube-making process consists of the following steps:
- (1) piercing-rolling a billet, which is heated to a predetermined temperature, by
a piercing machine (piercer) to form the billet into a hollow blank (hollow shell);
- (2) elongation-rolling the hollow blank by an elongation-rolling mill (for example,
a mandrel mill);
- (3) diameter-adjusting rolling the hollow blank, which was subjected to elongation-rolling,
to have a predetermined outer diameter and a wall thickness by a diameter-adjusting
rolling mill (for example: a sizer and a stretch reducer); and
- (4) air cooling the seamless steel tube obtained by the diameter-adjusting rolling
by a cooling bed, or otherwise subjecting the seamless steel tube to quenching and
tempering.
[0003] Conventionally, in a production facility that adopts the Mannesmann tube-making process,
a tube blank that was subjected to elongation-rolling is heated by a reheating furnace
and is subjected to diameter-adjusting rolling. Moreover, when performing the quenching
of a steel tube that underwent diameter-adjusting rolling, the steel tube is again
heated by a quenching furnace and is quenched.
[0004] Recently, in order to promote the energy saving and improve the production efficiency,
a reheating furnace in the diameter-adjusting rolling process and a quenching furnace
in the quenching process are eliminated, and a series of processes from piercing-rolling
to diameter-adjusting rolling, and in some cases to quenching, may be arranged online
as an online production facility. In such a production facility where the reheating
furnace and the quenching furnace are eliminated, reheating will not be performed
at all even in diameter-adjusting rolling and quenching processes once the workpiece
is heated for piercing-rolling. When such a production facility is adopted, it is
likely that a steel tube after the diameter-adjusting rolling tends to have non-uniform
temperature distribution along a longitudinal direction such that the top part (front
end part) of the tube has a lower temperature and the bottom part (rear end part)
has a higher temperature. Such phenomenon occurs for the following reason.
[0005] Since the heated billet is pierced by a plug from its top part to the bottom part
during piercing-rolling, more heat is dissipated in the top part, which is pierced
first and is formed into a tube. As a result, in a hollow blank after piercing-rolling,
the top part tends to have a lower temperature and the bottom part tends to have a
higher temperature. Since the reheating of the workpiece is inhibited, such non-uniformity
in longitudinal temperature distribution also occurs in a similar fashion in the hollow
blank after elongation- rolling, and further in a steel tube after diameter-adjusting
rolling in a similar fashion. Thus, in a steel tube after diameter-adjusting rolling,
non-uniformity of temperature distribution along a longitudinal direction occurs.
[0006] When the temperature distribution along a longitudinal direction is non-uniform in
a steel tube after diameter-adjusting rolling, the amount of heat shrinkage of the
steel tube varies along a longitudinal direction while cooling, so that the diameter
of the steel tube gets non-uniform along a longitudinal direction after cooling. Further,
when a steel tube is subjected to quenching after diameter-adjusting rolling, the
temperature distribution along a longitudinal direction is non-uniform and causes
the level of quenching to vary along a longitudinal direction, whereby the mechanical
properties of the steel tube becomes non-uniform along a longitudinal direction after
quenching.
[0007] In the prior art regarding the temperature control of the workpiece in the production
of a steel tube, the followings can be listed.
[0008] Patent Literature 1 discloses a technology to prevent the temperature of the workpiece
from being excessively lowered while undergoing a continuous mill in a multi-stand
continuous mill for use in the production of seamless steel tubes. In the technology
disclosed in the above mentioned literature, a reheating furnace is disposed at both
the entrance and the intermediate position of the continuous mill, and thermometers
are disposed at both the entrance and the exit of the intermediate reheating furnace,
which is disposed at the intermediate position of the continuous mill, and further
at the exit of the continuous mill so that the temperature control of the intermediate
reheating furnace is performed based on the temperature of steel tube measured by
each thermometer.
[0009] Patent Literature 2 discloses a technology of preventing the occurrence of wall thickness
deviation caused by temperature decrease along a circumferential direction of a steel
tube in a stretch reducer used in the production of electric resistance welded steel
tubes. In the technology disclosed in the above mentioned literature, a plurality
of induction heating coils are disposed in series at the entrance of the stretch reducer,
and a thermometer is disposed at both the exit side of the induction heating coil
and the exit side of the stretch reducer, so that the electric power supply to the
induction heating coil is adjusted based on the temperature values measured by the
thermometer along a circumferential direction of the steel tube.
[0010] Patent Literature 3 discloses a technology for preventing the occurrence of bends
when a rectangular steel tube or a round steel tube which has undergone hot forming
is s cooled. In the technology disclosed in the same literature, a reheating furnace
is disposed at the preceding stage of the forming means for hot forming a steel tube
into a predetermined geometry, and water discharge means is disposed at the subsequent
stage of the forming means so that water is sprayed onto the steel tube along its
entire circumferential direction, that has undergone the forming means to thereby
uniformly cool the steel tube.
[0011] In the technologies disclosed in the above described Patent Literatures 1 to 3, the
steel tube is reheated before diameter-adjusting rolling so as to ensure uniform temperature
distribution, and therefore there will be no situation where the longitudinal temperature
distribution becomes non-uniform in a steel tube after diameter-adjusting rolling.
In any of the technologies disclosed in the above described Patent Literatures 1 to
3, however, since a reheating furnace and an induction heating coil are indispensable
at the preceding stage of the diameter-adjusting rolling mill, a significant amount
of fuel and/or electric power are consumed and therefore any of those technologies
can not provide an energy saving measure.
CITATION LIST
PATENT LITERATURE
[0012]
Patent Literature 1: Japanese Patent Application Publication No. 2004-58128
Patent Literature 2: Japanese Patent Application Publication No. 2005-7452
Patent Literature 3: Japanese Patent Application Publication No. 2007-301574
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0013] It is an object of the present invention to provide a method for producing a seamless
steel tube and a production facility therefor, which have the following features:
- (1) preventing non-uniform longitudinal temperature distribution from occurring in
a steel tube after diameter-adjusting rolling; and
- (2) enabling energy saving to be accomplished
SOLUTION TO PROBLEM
[0014] Key aspects of the present invention are as follows.
[0015] (I) A method for producing a seamless steel tube by piercing-rolling a heated billet
by a piercing machine to form the billet into a hollow blank, and without reheating
the hollow blank, by successively subjecting the hollow blank to elongation-rolling
with an elongation-rolling mill and diameter-adjusting rolling with a diameter-adjusting
rolling mill, the method including the steps of:
(step 1) measuring a temperature of the hollow blank along a longitudinal direction
at the exit of the elongation-rolling mill; and
(step 2) spraying water onto the hollow blank to cool the hollow blank at the entrance
of the diameter-adjusting rolling mill according to a measured longitudinal temperature
distribution of the hollow blank, whereby the longitudinal temperature distribution
of the hollow blank becomes uniform.
[0016] The production method of the above described (I) may be configured such that quenching
is performed without reheating subsequently to the diameter-adjusting rolling.
[0017] These production methods are preferably configured such that the amount of water
to be sprayed onto the hollow blank in the above described step 2 is adjusted for
every plurality of regions of the hollow blank sectioned in the longitudinal direction.
[0018] (II) A production facility for a seamless steel tube including a piercing machine
for piercing-rolling a heated billet to form the billet into a hollow blank, and an
elongation-rolling mill for elongation-rolling the hollow blank and a diameter-adjusting
rolling mill for diameter-adjusting rolling the hollow blank without reheating the
hollow blank, the production facility including:
a thermometer disposed at the exit of the elongation-rolling mill and for measuring
a temperature of the hollow blank along a longitudinal direction; and
a water-cooling apparatus disposed at the entrance of the diameter-adjusting rolling
mill, and for spraying water onto the hollow blank to cool the hollow blank according
to a longitudinal temperature distribution of the hollow blank measured by the thermometer
such that the longitudinal temperature distribution of the hollow blank becomes uniform.
ADVANTAGEOUS EFFECTS OF INVENTION
[0019] The method for producing a seamless steel tube of the present invention has the following
remarkable effects of:
- (1) preventing non-uniform longitudinal temperature distribution from occurring in
a steel tube after diameter-adjusting rolling; and
- (2) enabling energy saving to be accomplished.
[0020] The excellent effects of the production method of the present invention can be fully
exerted by the production facility for a seamless steel tube of the present invention.
BRIEF DESCRIPTION OF DRAWINGS
[0021]
[FIG. 1] FIG. 1 is a block diagram to illustrate a configuration example of a production
facility for a seamless steel tube of the present invention.
[FIG. 2] FIG. 2 is a diagram to illustrate a configuration example of a water-cooling
apparatus in the production facility for a seamless steel tube of the present invention,
in which FIG. 2 (a) shows a side sectional view along a traveling direction of a hollow
blank, and FIG. 2 (b) shows a front view, respectively.
[FIG. 3] FIG. 3 is a diagram to illustrate the correlation between the water amount
to be sprayed per one meter in a longitudinal direction of the hollow blank and the
amount of temperature reduction.
DESCRIPTION OF EMBODIMENTS
[0022] In order to achieve the above described object, the present inventors have made an
arduous study on a method for preventing non-uniformity of the longitudinal temperature
distribution from occurring in a steel tube after diameter-adjusting rolling, based
on the precondition that reheating is not performed all the way through diameter-adjusting
rolling and quenching, once the workpiece is heated for piercing-rolling. As a result,
the following findings (a) and (b) have been obtained.
[0023]
- (a) By making the longitudinal temperature distribution uniform in the hollow blank
before diameter-adjusting rolling, it is possible to prevent non-uniformity of the
longitudinal temperature distribution in the steel tube after diameter-adjusting rolling.
- (b) To make the longitudinal temperature distribution uniform in the hollow blank
before diameter-adjusting rolling, it is effective to cool the hollow blank by spraying
water onto the hollow blank according to the longitudinal temperature distribution
of the hollow blank.
[0024] The present invention has been completed based on the above described findings (a)
and (b). Hereafter, preferable embodiments of the method for producing a seamless
steel tube and the production facility therefor of the present invention will be described.
1. Production facility for seamless steel tube
[0025] FIG. 1 is a block diagram to illustrate a configuration example of a production facility
for a seamless steel tube of the present invention. As shown in the same figure, a
production facility 1 includes, as a series of equipments in an online facility, a
heating apparatus 2, a piercing machine 3 (piercer), an elongation-rolling mill 4
(for example, a mandrel mill), a diameter-adjusting rolling mill 5 (for example, a
sizer and a stretch reducer), and a cooling bed 6. Further, the production facility
1 includes a thermometer 7 disposed at the exit of the elongation-rolling mill 4,
a water-cooling apparatus 8 disposed at the entrance of the following diameter-adjusting
rolling mill 5, and a control apparatus 9, which is connected to the thermometer 7
and the water-cooling apparatus 8.
[0026] The heating apparatus 2 heats a billet as a workpiece to a predetermined temperature
suitable for piercing-rolling. The piercing machine 3 pierces and rolls the heated
billet to form it into a hollow blank. The elongation-rolling mill 4 elongates and
rolls the hollow blank without reheating it. The diameter-adjusting rolling mill 5
performs diameter-adjusting rolling of the hollow blank that underwent elongation-rolling,
without reheating it, thereby finishing a steel tube having a predetermined outer
diameter and a wall thickness. The steel tube that underwent diameter-adjusting rolling
is air-cooled in the cooling bed 6.
[0027] In this production facility 1, when performing diameter-adjusting rolling by the
diameter-adjusting rolling mill 5, the temperature of the hollow blank which underwent
elongation-rolling by the elongation-rolling mill 4 is measured by the thermometer
7 along a longitudinal direction. The control apparatus 9 successively receives signals
of measured temperature from the thermometer 7 and calculates the temperature distribution
along a longitudinal direction of the hollow blank to send action signals according
to the temperature distribution to the water-cooling apparatus 8. The water-cooling
apparatus 8 sprays an appropriate amount of water onto the hollow blank based on the
action signals from the control apparatus 9 to cool the hollow blank so that the longitudinal
temperature distribution of the hollow blank becomes uniform. The cooled hollow blank
is subjected to diameter-adjusting rolling by the diameter-adjusting rolling mill
5.
[0028] In this production facility 1, the hollow blank is transported in its longitudinal
direction by a roller conveyor from the piercing machine 3 to the elongation-rolling
mill 4, and from the elongation-rolling mill 4 to the diameter-adjusting rolling mill
5 through the water-cooling apparatus 8.
[0029] FIG. 2 is a diagram to illustrate a configuration example of a water-cooling apparatus
in the production facility for a seamless steel tube of the present invention, in
which FIG. 2 (a) shows a side sectional view along the traveling direction of a hollow
blank, and FIG. 2 (b) shows a front view, respectively. In FIG. 2 (a), the traveling
direction of the hollow blank is shown by a bold arrow.
[0030] As shown in the same figure, the cooling apparatus 8 includes an annular ring made
of pipe 11 wherein the center of the ring is positioned on the traveling path of the
hollow blank P that passes through the ring. The annular ring of pipe 11 is connected
with a water supply tube 12, which is connected with a water supply pump 13. The water
supply pump 13 can be actuated based on the action signal from the control apparatus
9 shown in FIG. 1 described above so as to enable to adjust the amount of water to
be supplied.
[0031] In the inner periphery of the annular ring of pipe 11, a plurality of nozzles 14
are disposed at constant intervals along a circumferential direction. Each nozzle
14 sprays out water that is supplied to the annular ring of pipe 11 through the water
supply tube 12 when the water supply pump 13 is actuated. As a result, the hollow
blank P which is traveled in a longitudinal direction is cooled uniformly along a
circumferential direction every time it passes through the annular ring of pipe 11.
[0032] The number of nozzles 14 is, though not particularly limited to, preferably about
4 to 24. This is because, if it is less than 4, uniform cooling may be insufficient
along a circumferential direction of the hollow blank P, and if it is more than 24,
it becomes redundant since sufficient uniform cooling should be achieved by specified
nozzles.
[0033] Each nozzle 14 is preferably slightly inclined toward the opposite direction relative
to the traveling direction of the hollow blank P (or the direction toward the bottom
part side of the hollow blank). That is to prevent water from entering the inside
of the hollow blank P, which has passed the annular ring of pipe 11, from its rear
end.
[0034] The production facility 1 shown in FIG. 1 described above may install the water-cooling
apparatuses 8 of such configuration at multiple stages along the traveling path of
the hollow blank P, but the installation of the water-cooling apparatus 8 at a single
stage fares well. A radiation thermometer may be adopted as the thermometer 7.
[0035] The production facility 1 shown in FIG. 1 described above may include a rapid cooling
apparatus in place of the cooling bed 6, or in parallel with the cooling bed 6 without
a quenching furnace being disposed, to perform quenching of the steel tube after diameter-adjusting
rolling. As the rapid cooling apparatus, one of water bath dipping type or one of
laminar water downflowing type may be adopted. A tempering furnace may be disposed
at the subsequent stage of the rapid cooling apparatus in order to perform tempering
of the steel tube after quenching.
2. Method for producing seamless steel tube
[0036] Referring to FIG. 1 described above, the method for producing a seamless steel tube
of the present invention will be described. In the production method of the present
invention, a heated billet is subjected to piercing-rolling by the piercing machine
3 to be formed into a hollow blank, and is successively subjected to elongation-rolling
by the elongation-rolling mill 4 and diameter-adjusting rolling by the diameter-adjusting
rolling mill 5 without reheating the hollow blank at all.
[0037] In this process, there is a risk that the hollow blank after piercing-rolling tends
to have non-uniform temperature distribution along its longitudinal direction caused
by a significant amount of heat dissipation in its top part during piercing-rolling:
therefore, the hollow blank after elongation-rolling should also have non-uniform
temperature distribution along its longitudinal direction in a similar fashion.
[0038] In this regard, in the production method of the present invention, when performing
diameter-adjusting rolling by the diameter-adjusting rolling mill 5, the temperature
of the hollow blank is measured along a longitudinal direction by the thermometer
7 at the exit of the elongation-rolling mill 4. Then, water is sprayed onto the hollow
blank at the entrance of the diameter-adjusting rolling mill 5 by the water-cooling
apparatus 8 to cool the hollow blank according to the measured longitudinal temperature
distribution of the hollow blank thereby making the longitudinal temperature distribution
of the hollow blank uniform.
[0039] To be specific, the control apparatus 9 connected to the thermometer 7 determines
the temperature for each of plural regions that are virtually allotted along the length
of the hollow blank in a longitudinal direction, and selects a minimum temperature
reading among the temperatures of allotted regions to determine temperature difference
values between the minimum temperature and each region temperature. Then, based on
the temperature differences, the amount of water to be sprayed onto the hollow blank
from the water-cooling apparatus 8 is calculated for every region, and an action signal
corresponding to the amount of water is transmitted to the water-cooling apparatus
8. Thereby, the traveling hollow blank is cooled by being sprayed with an appropriate
amount of water for each allotted region from the water-cooling apparatus 8, thereby
having a uniform longitudinal temperature distribution.
[0040] Here, there is a correlation between the amount of water to be sprayed to the hollow
blank, and the amount of temperature reduction. Therefore, the amount of water to
be sprayed onto the hollow blank can be calculated, for example, from the correlation
with the amount of temperature reduction of the hollow blank shown in FIG. 3 described
below, based on the above described temperature difference for each allotted region
of the hollow blank.
[0041] FIG. 3 is a diagram to illustrate the correlation between the water amount to be
sprayed per one meter of the hollow blank in a longitudinal direction and the amount
of temperature reduction. The same figure shows the result of the investigation of
the amount of temperature reduction of the area where water is sprayed by performing
testing in which hollow blanks with varied outer diameters and wall thicknesses are
used, and water is sprayed onto each hollow blank heated to 1100 °C by varying the
amount of water to be sprayed per one meter in its longitudinal direction.
[0042] As shown in the same figure, there is a correlation: ΔT = 160 × Q between the amount
of water Q [m
3] to be sprayed onto the hollow blank and the amount of temperature reduction ΔT [°C]
regardless of dimensions of the outer diameter and wall thickness of the hollow blank.
From this correlation expression, it is possible to calculate the amount of water
Q to be sprayed onto the hollow blank with the above described temperature difference
being ΔT for an allotted region of the hollow blank.
[0043] According to the method for producing a seamless steel tube of the present invention,
since it is possible to spray water onto the hollow blank thereby making the longitudinal
temperature distribution uniform when performing diameter-adjusting rolling, it never
happens that the longitudinal temperature distribution becomes non-uniform in a steel
tube after diameter-adjusting rolling. As a result, in the steel tube after diameter-adjusting
rolling, the amount of heat shrinkage associated with cooling will not vary along
a longitudinal direction, so that the outer diameter of the steel tube will become
uniform over the entire region in a longitudinal direction after cooling. Moreover,
even when the steel tube is subjected to quenching after diameter-adjusting rolling,
the level of quenching will not vary along a longitudinal direction, and the mechanical
properties of the steel tube become uniform over the entire region in a longitudinal
direction after quenching.
[0044] Moreover, according to the method for producing a seamless steel tube of the present
invention, since reheating will not be performed all the way through diameter-adjusting
rolling and quenching once the heating of the workpiece is performed for piercing-rolling,
a significant amount of fuel and electric power will not be consumed, thereby enabling
energy saving to be accomplished.
[0045] The effects of the method for producing a seamless steel tube of the present invention
can be fully exerted by the production facility for a seamless steel tube of the present
invention.
EXAMPLES
(Example 1)
[0046] In order to confirm the effects of the present invention, a full scale testing for
producing a seamless steel tube of the specification described as below was carried
out by performing piercing-rolling, elongation-rolling, and diameter-adjusting rolling
by using the production facility shown in FIG. 1 described above.
Dimensions: Outer diameter 406.4 mm, wall thickness 30.7 mm, and length 12 m
Material grade: Low carbon steel (C: 0.6 wt%)
[0047] When performing diameter-adjusting rolling, the temperature of the hollow blank
after elongation-rolling was measured and water was sprayed by the amount shown in
Table 1 shown below on the hollow blank according to its longitudinal temperature
distribution. Moreover, testing was carried out without spraying water for the comparison
purpose.
[0048] [Table 1]

[0049] The temperature of the seamless steel tube was measured immediately after diameter-adjusting
rolling and, after further cooling the steel tube, the outer diameter of the steel
tube was measured over the entire region in a longitudinal direction. The result thereof
is also shown together in Table 1. In Table 1, the results for a top part region which
is within 1 to 3 m from the front end of the steel tube, a middle part region which
is in the range of 2 m across the length-wise middle of the steel tube, and a bottom
part region which is within 1 to 3 m from the rear end of the steel tube are shown.
[0050] The results shown in Table 1 have revealed the followings.
[0051] In Inventive Examples of the present invention, as a result of spraying water onto
the hollow blank before diameter-adjusting rolling, the temperature of the steel tube
immediately after diameter-adjusting rolling was made uniform in a longitudinal direction.
As a result, in the steel tube after cooling, the outer diameter became uniform in
the longitudinal direction.
[0052] In Comparative Examples, since water was not sprayed onto the hollow blank before
diameter-adjusting rolling, the temperature of the steel tube immediately after diameter-adjusting
rolling became non-uniform with a deviation of about 40 °C in a longitudinal direction.
As a result, in the steel tube after cooling, the outer diameter became non-uniform
with a deviation of about 0.4 mm in the longitudinal direction.
(Example 2)
[0053] A full scale testing for producing a seamless steel tube of the specification described
as below was carried out by performing piercing-rolling, elongation-rolling, and diameter-adjusting
rolling by using the production facility shown in FIG. 1 described above, thereby
confirming the effects of water spraying onto the hollow blank on the mechanical properties
of the steel tube.
Dimension: Outer diameter 406 mm, wall thickness 14 mm, and length 12 m
Material grade: Low carbon steel with the chemical composition shown in Table 2 shown
below.
Mechanical property: API standard grade X65
[0054] [Table 2]
Table 2
Unit: mass% |
C |
Si |
Mn |
P |
S |
Cu |
Cr |
Ni |
Mo |
0.05 ∼ 0.07 |
0.20∼ 0.30 |
1.47 ∼ 1.55 |
0.013 or less |
0.0014 or less |
0.02 ∼ 0.05 |
0.25 ∼ 0.30 |
0.02 ∼ 0.05 |
0.26 ∼ 0.28 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Ti |
V |
Nb |
|
|
|
|
|
|
0.017 ∼ 0.022 |
0.01 or less |
0.023 ∼ 0.033 |
[0055] When performing diameter-adjusting rolling, the temperature of the hollow blank after
elongation-rolling was measured and, according to its longitudinal temperature distribution,
water was sprayed onto the hollow blank by the amount of water shown in Table 3 described
below. Moreover, for the purpose of comparison, testing was carried out without spraying
water.
[0056] [Table 3]

[0057] The temperature of the seamless steel tube was measured immediately before quenching
and after diameter-adjusting rolling and, after subjecting the steel tube to quenching
and tempering, a specimen was collected from each part along a longitudinal direction
to measure the grain size and the yield strength (YS). The testing method for measuring
the grain size and the yield strength conforms to ASTM testing standards. The results
thereof are shown together in Table 3. Similarly to Example 1 described above, results
for the top part region, the middle part region, and the bottom part region are shown
in Table 3 as well.
[0058] The results shown in Table 3 have revealed the followings.
[0059] In Inventive Examples of the present invention, as a result of spraying water onto
the hollow blank before diameter-adjusting rolling, the temperature of the steel tube
immediately before quenching was made uniform in a longitudinal direction. As a result,
in the steel tube after quenching and tempering, both the grain size and the yield
strength became uniform in a longitudinal direction.
[0060] In Comparative Examples, since water was not sprayed onto the hollow blank before
diameter-adjusting rolling, the temperature of the steel tube immediately before quenching
became non-uniform with a deviation of about 50 °C in a longitudinal direction. As
a result, the grain size of the steel tube after quenching and tempering became fine
showing a grain size number of 11 in the top part region and coarse showing a grain
size number of 5 in the bottom part region, thus exhibiting non-uniformity in a longitudinal
direction. The reason why the grain size became coarse in the bottom part region is
that the temperature until quenching was higher in the bottom part region than in
the top part region, and thereby the crystal grains grew coarse in the bottom part
region. Moreover, the yield strength of the steel tube after quenching and tempering
became non-uniform with a deviation of about 60 MPa in a longitudinal direction.
INDUSTRIAL APPLICABILITY
[0061] The present invention can be effectively utilized in the production of a seamless
steel tube by the Mannesmann tube-making process.
REFERENCE SIGNS LIST
[0062] 1: Production facility, 2: heating apparatus, 3: piercing machine, 4: elongation-rolling
mill, 5: diameter-adjusting rolling mill, 6: cooling bed, 7: thermometer, 8: water-cooling
apparatus, 9: control apparatus, 11: annular ring of pipe, 12: water supply tube,
13: water supply pump, 14: nozzle, P: hollow blank