U.S. patent application number 14/395245 was filed with the patent office on 2015-05-14 for method of producing plug for piercing-rolling.
The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Yasuyoshi Hidaka, Yasuto Higashida.
Application Number | 20150132501 14/395245 |
Document ID | / |
Family ID | 49482533 |
Filed Date | 2015-05-14 |
United States Patent
Application |
20150132501 |
Kind Code |
A1 |
Higashida; Yasuto ; et
al. |
May 14, 2015 |
METHOD OF PRODUCING PLUG FOR PIERCING-ROLLING
Abstract
A method for producing a plug for use in a piercing rolling mill
for producing a seamless steel tube/pipe includes an arc-spraying
step of melting iron wires, and spraying molten material thereof
onto a surface of a base metal of a plug by use of an arc-spray
gun, so as to form a film containing oxide and Fe on the surface of
the base metal of the plug. In the arc-spraying step, the surface
of the base metal of the plug is divided into plural sections along
an axial direction of the plug, and in turn, the arc-spraying is
separately carried out in each of the plural sections while an
intersection angle between the center line of a spraying stream
from the arc-spray gun and the surface of the plug base metal is
maintained within a range of 35 degrees to 90 degrees.
Inventors: |
Higashida; Yasuto; (Tokyo,
JP) ; Hidaka; Yasuyoshi; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
49482533 |
Appl. No.: |
14/395245 |
Filed: |
March 19, 2013 |
PCT Filed: |
March 19, 2013 |
PCT NO: |
PCT/JP2013/001858 |
371 Date: |
October 17, 2014 |
Current U.S.
Class: |
427/449 |
Current CPC
Class: |
C23C 4/06 20130101; C23C
4/131 20160101; B21B 19/04 20130101; B21B 25/00 20130101 |
Class at
Publication: |
427/449 |
International
Class: |
C23C 4/12 20060101
C23C004/12; C23C 4/06 20060101 C23C004/06; B21B 25/00 20060101
B21B025/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 24, 2012 |
JP |
2012-095448 |
Claims
1. A method for producing a plug for use in a piercing-rolling mill
for producing a seamless steel tube/pipe, the method comprising: an
arc-spraying step of melting iron wires, and spraying molten
material thereof onto a surface of a base metal of a plug by use of
an arc-spray gun, so as to form a film containing oxide and Fe on
the surface of the base metal of the plug, wherein in the
arc-spraying step, the surface of the base metal of the plug is
divided into plural sections along an axial direction of the plug,
and in turn, the arc-spraying is separately carried out in each of
the plural sections while an intersection angle between the center
line of a spraying stream from the arc-spray gun and the surface of
the plug base metal is maintained within a range of 35 degrees to
90 degrees.
2. The method for producing a plug for piercing-rolling according
to claim 1, wherein the plug has a bullet shape, and includes a
body portion and a tip end portion, while the plural sections
comprise a region of the body portion and a region of the tip end
portion.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for producing a
plug for piercing-rolling for use in a piercing-rolling mill
(hereinafter, also referred to simply as a "piercer") that produces
a seamless steel tube/pipe, particularly to a method for producing
a plug for piercing-rolling having a film formed by performing
arc-spraying of iron wires on a surface of a plug base metal.
BACKGROUND ART
[0002] A seamless steel tube/pipe is produced by the Mannesmann
tube-making process. The Mannesmann tube-making process includes
the following steps:
[0003] (1) piercing-rolling a starting material (round billet)
heated at a predetermined temperature into a hollow shell by using
a piercer;
[0004] (2) elongation-rolling the hollow shell by an elongation
rolling mill (e.g. mandrel mill); and
[0005] (3) carrying out diameter adjusting rolling on the
elongation-rolled hollow shell to have a predetermined outer
diameter and wall thickness by using a diameter adjusting rolling
mill (e.g. a stretch reducer).
[0006] In the piercing-rolling by using the piercer, a plug is used
as a piercing tool. This plug is mounted to a front end of a
mandrel so as to pierce a billet heated at a high temperature of
approximately 1200.degree. C.; thus the plug is subjected to a
hostile environment with a high surficial pressure and a high
temperature. In general, the plug includes a base metal made of hot
working tool steel, and a film of oxide scale is formed on a
surface of the base metal through a heating process in advance for
the purpose of protection of the base metal, and thereafter the
plug is used in the piercing-rolling. During the piercing-rolling,
the scale film on the surface of the plug insulates heat transfer
from the billet to the base metal of the plug, and also prevents
seizing between the billet and the plug.
[0007] Repetitive piercing-rolling using such a plug having the
scale film causes a gradual abrasion of the scale film. The
abrasion of the scale film deteriorates thermal insulation effect
of the film, resulting in increase in temperature of the plug
during the piercing, so that melting-incurred metal loss and
deformation by heat are likely to be caused to the plug base metal.
If the scale film is exhausted, and the plug base metal comes into
direct contact with the billet, seizing is caused, so as to
generate flaws on an internal surface of a steel tube/pipe.
Consequently, the plug becomes unusable at the moment when the film
is exhausted, and its durability life is expired.
[0008] Particularly in production of a seamless steel tube/pipe
made of high alloy steel such as high Cr steel containing Cr of 9%
or more, Ni-based alloy, and stainless steel, significant abrasion
of the scale film on the surface of the plug is generated during
the piercing-rolling, so that the durability life of the plug
becomes significantly reduced. For example, in the case of piercing
stainless steel, the scale film on the surface of the plug becomes
worn away through two or three passes (the number of times of
continuous piercing rolling), and the durability life of this plug
is expired. This requires a frequent replacement of the plug, which
deteriorates the production efficiency. In production of a seamless
steel tube/pipe of high alloy steel, it is required to enhance the
durability life of the plug during the piercing-rolling, thereby
enhancing the production efficiency of the steel tube/pipe.
[0009] To satisfy such a requirement, as an example of the film
formed on the surface of the plug base metal, Patent Literature 1
discloses such a plug having a film containing oxide and Fe formed
on the surface of the plug base metal by performing arc-spraying of
iron wires, instead of using the scale film formed through heat
treatment. Since the plug having the arc-sprayed film has a film
containing oxide and Fe on the surface of the plug, this plug is
excellent in thermal insulation performance and seizing prevention,
so that enhancement of the durability life of the plug is likely to
be achieved.
[0010] Patent Literature 1 discloses an equipment system of
producing (reproducing) a plug having an arc-sprayed film by
forming the film containing oxide and Fe on a surface of a base
metal of the plug in such a manner that, after shotblasting is
applied onto the surface of the plug, molten material is sprayed
from arc-spray guns onto the surface of the plug base metal while a
turntable on which the plug is mounted is being rotated. In this
equipment system, the spray guns are so disposed as to face a tip
end portion, a front-half of the body portion, and a rear-half of
the body portion of the surface of the plug base metal, and forms
the arc-sprayed film by operating all the spray guns at the same
time, thereby reducing time required for forming the film compared
to the case of using a single spray gun to form the arc-sprayed
film across the entire surface of the plug base metal, which
results in enhancement of production efficiency of the plug.
[0011] Unfortunately, even in the plug having the arc-sprayed film
formed by using the conventional equipment system disclosed in
Patent Literature 1, there occurs separation of the film if a
billet length to be pierced is long, or if a billet having high
elevated-temperature strength is used. This is due to the fact that
the adhesiveness of the film is unstable. In this regard, there is
still room for further improvement in securing the steadily
enhanced durability life of the plug.
CITATION LIST
Patent Literature
[0012] Patent Literature 1: Japanese Patent No. 4279350
SUMMARY OF INVENTION
Technical Problem
[0013] An object of the present invention, which has been made in
order to solve the problems according to the conventional art, is
to provide a method for producing a plug for piercing-rolling
having a film containing oxide and Fe formed on a surface of the
plug base metal by performing arc-spraying of iron wires, and the
method has the following features of:
[0014] (1) securing firm adhesiveness of the film formed on the
surface of the plug; and
[0015] (2) securing steady enhancement of the durability life of
the plug even if a billet length to be pierced is long, or even if
a billet having high elevated-temperature strength is used.
Solution to Problem
[0016] The summary of the present invention is as follows.
[0017] Provided is a method for producing a plug for use in a
piercing-rolling mill for producing a seamless steel tube/pipe,
and
[0018] the method for producing the plug for piercing-rolling
comprises
[0019] an arc-spraying step of melting iron wires, and spraying
molten material thereof onto a surface of a base metal of a plug by
use of an arc-spray gun, so as to form a film containing oxide and
Fe on the surface of the base metal of the plug.
[0020] In the arc-spraying step, the surface of the base metal of
the plug is divided into plural sections along an axial direction
of the plug, and in turn, the arc-spraying is separately carried
out in each of the plural sections while an intersection angle
between the center line of a spraying stream from the arc-spray gun
and the surface of the plug base metal is maintained within a range
of 35 degrees to 90 degrees.
[0021] In this method of producing the plug, it is preferable that
the plug has a bullet shape, and includes a body portion and a tip
end portion, while the plural sections comprise a region of the
body portion and the tip end portion.
Advantageous Effects of Invention
[0022] The method for producing a plug for piercing-rolling
according to the present invention achieves the following
remarkable effects of:
[0023] (1) securing firm adhesiveness of the arc-sprayed film
formed on the surface of the plug; and
[0024] (2) securing steady enhancement of the durability life of
the plug even if a billet length to be pierced is long, or even if
a billet having high elevated-temperature strength is used.
BRIEF DESCRIPTION OF DRAWINGS
[0025] FIG. 1 is a schematic illustration showing the state of the
arc-spraying conducted in basic tests for investigating
adhesiveness of the arc-sprayed film.
[0026] FIG. 2 is an illustration showing dependency on the
intersection angle between the center line of a spraying stream of
an arc-spray gun and the surface of the base metal of the plug as a
result of the basic tests on the adhesiveness of the arc-sprayed
film.
[0027] FIG. 3 is an illustration showing microscopic observation
photographs of the cross sections of each film depending on the
intersection angle between the center line of the spraying stream
of the arc-spray gun and the surface of the base metal of the plug
as the result of the basic tests on the adhesiveness of the
arc-sprayed film.
[0028] FIG. 4 is a schematic illustration explaining a conventional
film forming method through the arc-spraying, and an operation
condition of the arc-spraying of the Comparative Example 4.
[0029] FIG. 5 is a schematic illustration explaining the reason why
enhancement of the durability life of the plug cannot be achieved
by forming the arc-sprayed film in the conventional method shown in
FIG. 4.
[0030] FIG. 6 is a schematic illustration showing the steps of the
film formation through the arc-spraying in the production method of
the plug according to the first embodiment of the present
invention.
[0031] FIG. 7 is a schematic illustration showing the steps of the
film formation through the arc-spraying in the production method of
the plug according to the second embodiment of the present
invention.
[0032] FIG. 8 is a schematic illustration explaining an operation
condition of the arc-spraying of the Comparative Example 1.
[0033] FIG. 9 is a schematic illustration explaining an operation
condition of the arc-spraying of the Comparative Example 2.
[0034] FIG. 10 is a schematic illustration explaining an operation
condition of the arc-spraying of the Comparative Example 3.
[0035] FIG. 11 is a schematic illustration explaining an operation
condition of the arc-spraying of the Inventive Example 1.
[0036] FIG. 12 is a schematic illustration explaining an operation
condition of the arc-spraying of the Inventive Example 2.
[0037] FIG. 13 is a schematic illustration explaining an operation
condition of the arc-spraying of the Inventive Example 3.
DESCRIPTION OF EMBODIMENTS
[0038] In order to achieve the above object, the present inventors
conducted various tests and intensive studies on forming a film
containing Fe oxide and Fe on a surface of a plug base metal by
performing arc-spraying of iron wires onto the surface of the plug
base metal, focusing attention on the adhesiveness of the
arc-sprayed film in particular. As a result, the present inventors
have obtained the following findings.
[0039] The arc-spraying is a technique, for example, to use an
arc-spray gun, and generates arcs between front ends of two spray
wires serving as electrodes, so as to melt the spray wires, and at
the same time, supply a compressed air jet or a nitrogen gas jet
between the front ends of the spray wires so that the molten
material is sprayed, thereby spraying the molten material onto a
target object to form a film thereon. In the arc-spraying in which
iron wires are applied as the spraying wires, and a plug is used as
the target object, a film formed on the surface of the plug base
metal contains Fe oxide (iron oxide) and Fe. The Fe oxide contained
in the film results from molten material (molten iron) that is
sprayed from the arc-spray gun, and is oxidized in the air before
the molten material reaches the surface of the plug base metal. Fe
contained in the film results from the molten material that has
reached the surface of the plug base metal before being oxidized in
the air.
[0040] FIG. 1 is a schematic illustration showing the state of the
arc-spraying conducted in the basic tests for investigating the
adhesiveness of the arc-sprayed film. As shown in this
illustration, in the basic tests for investigating the adhesiveness
of the film, molten material resulted from the iron wires is
sprayed from the arc-spray gun 4 while the plug base metal 2 is
being rotated around the central axis Pc of the plug 1, so as to
form the film on the surface of the plug 1. At this time, various
films were formed by varying an intersection angle .theta. defined
by the center line Ac of a spraying stream from the arc-spray gun 4
and the surface of the base metal 2 of the plug 1. As an evaluation
procedure of the adhesiveness of the film, a peel stress in the
shear direction of the film (hereinafter referred to as
"adhesiveness") was measured for each of the plugs 1 having
different intersection angles, referred to as .theta.. The film
adhesiveness for the plug in case of the intersection angle .theta.
of 90 degrees was defined as a reference "1", and the evaluation of
film adhesiveness was conducted based on the ratio of the film
adhesiveness (adhesiveness ratio) of each plug having a different
intersection angle .theta. relative to this reference. The
microscopic observation of the cross section of the film of each
plug was also conducted.
[0041] FIG. 2 is an illustration showing the dependency on the
intersection angle between the center line of the spraying stream
of the arc-spray gun and the surface of the base metal of the plug
as a result of the basic tests on the adhesiveness of the
arc-sprayed film. FIG. 3 is, as the result of the basic tests, an
illustration showing microscopic observation photographs of the
cross sections of plugs in case of different intersection angles,
each defined by the center line of the spraying stream of the
arc-spray gun and the surface of the base metal of the plug.
[0042] As shown in FIG. 2, the adhesiveness ratio of the film
depends on the intersection angle .theta. between the center line
of the spraying stream of the arc-spray gun and the surface of the
base metal of the plug. Specifically, if the intersection angle
.theta. is smaller than 35 degrees, the adhesiveness ratio is apt
to significantly decrease. To the contrary, if the intersection
angle .theta. is 60 degree or more, there is no sign of the
decrease in the adhesiveness ratio.
[0043] As shown in FIG. 3, the reason for the deterioration of the
adhesiveness in case of a smaller intersection angle .theta. is
because the film might ununiformly adhere onto the surface of the
plug base metal, which results in increase of the percentage of
porosity in the film.
[0044] Here, the arc-spraying is generally used in the repair of a
tapping port of a metal refining vessel formed of a refractory
material, or in coating on an internal surface of a cylinder bore
of an engine. In this case, the target of the arc-spraying is the
internal surface of a cylindrical member, and is carried out such
that a spray gun is inserted in a cylindrical member that is
immobilized, so that the distance between the spray gun to the
target surface on which the film is to be formed, that is, the
spraying distance is approximately 50 mm, or approximately 150 mm
at most, which is small. In such a general arc-spraying, it is not
preferable to set the intersection angle between the center line of
the spraying stream of the spray gun and the target surface for the
film formation to be a large angle. If the intersection angle is
large, molten material sprayed from the spray gun splashes back
from the target surface for the film formation, and is then
returned to the spray gun; therefore damages are caused to the
spray gun, or the molten material splashed back from the target
surface for the film formation is inadvertently re-sprayed onto the
target surface for the film formation, which deteriorates the
adhesiveness of the film; thus a larger intersection angle is not
preferable in light of prevention of the above undesirable
incidents.
[0045] According to this theory, there might be a risk that, in the
arc-spraying for the plug as a target surface of the film
formation, a larger intersection angle between the center line of
spraying stream of the spray gun and the surface of the plug base
metal may also reduce the adhesiveness of the film. As described
above, in the arc-spraying for the plug, however, a larger
intersection angle .theta. rather secures enhancement of the
adhesiveness of the film. The reason for this is as follows.
[0046] In a case of the arc-spraying with iron wires to form the
film containing oxide and Fe on the surface of the plug base metal,
it is required to secure sufficient time for oxidizing the molten
material sprayed from the spray gun in the air; thus the spraying
distance from the spray gun to the surface of the plug base metal,
i.e., standoff distance from the surface is approximately 200 to
1000 mm, which is relatively large. Accordingly, even if the
intersection angle is set to be large, the splash back of the
molten material hardly occurs from the surface of the plug base
metal.
[0047] In the formation of the arc-sprayed film on the surface of
the plug base metal, the arc-spraying is carried out while the plug
is being rotated, the molten material that should have splashed
back from the surface of the plug base metal is tremendously
flicked off by the rotation of the plug, so that the molten
material is prevented from inadvertently adhering to the surface of
the plug base metal.
[0048] Based on the above basic tests, it is recognized that, in
order to secure the adhesiveness of the film formed on the surface
of the plug as well as allow this adhesiveness to have sufficient
strength, it is preferable to maintain the intersection angle
.theta. between the center line of spraying stream from the
arc-spray gun and the surface of the base metal of the plug within
the range of 35 degrees to 90 degrees while the arc-spraying is
being carried out to form the arc-sprayed film on the surface of
the plug base metal. It is more preferable to set the intersection
angle .theta. within the range of 60 degrees to 90 degrees.
[0049] An example of a method for carrying out the arc-spraying
with the intersection angle .theta. within the preferable range may
include the following method.
[0050] FIG. 4 is a schematic illustration explaining a conventional
film forming method through the arc-spraying. As shown in this
illustration, in the conventional method, the plug 1 has a bullet
shape, and the film 3 is formed on the surface of the plug base
metal 2 through the arc-spraying in such a manner that the spray
gun 4 is reciprocatingly moved from the rear end to the tip end
along the surface of the plug base metal 2 while the plug base
metal 2 is being rotated about the central axis P.sub.c of the plug
1. In this manner, the film 3 is formed from the tip end portion
1a, the front-half 1ba of the body portion 1b, to the rear-half 1bb
of the body portion 1b (reeling portion) across the entire surface
of the plug 1. At this time, the spray gun 4 is mounted to an
articulated arm that is operated by programming, and the motion and
posture of the spray gun 4 is controlled by the programming
[0051] In a case of forming the film in the conventional method
shown in FIG. 4, as verified in Example section described later,
the durability life of the plug cannot be enhanced as much as
expected. The reason for this is as follows.
[0052] FIG. 5 is a schematic illustration showing the reason why
enhancement of the durability life of the plug cannot be achieved
when the arc-sprayed film is formed by the conventional method
shown in FIG. 4. In the conventional method shown in FIG. 4, the
spray gun 4 is configured to move in a wide range from the rear end
to the tip end of the surface of the plug base metal 2, and thus it
is extremely complicated to control the motion and the posture of
the spray gun 4. Consequently, as shown in FIG. 5, if a slight
deviation occurs in the position adjustment or the posture
adjustment of the spray gun 4 relative to the plug base metal 2,
the intersection angle .theta. between the center line Ac of
spraying stream from the arc-spray gun 4 and the surface of the
plug base metal 2 may be deviated out of the above preferable range
(encircled portion in FIG. 5). Due to this, the adhesiveness of the
film becomes partially reduced.
[0053] Contrary to the conventional method, as verified in the
Example section described later, it is found that the durability
life of the plug can be significantly enhanced by employing a
method that divides the surface of the plug base metal into plural
sections along an axial direction of the plug, and in turn,
separately forms part of the arc-sprayed film in each section.
[0054] Patent Literature 1 describes a technique of forming the
arc-sprayed film such that disposes spray guns are disposed
opposite to the tip end portion, the front-half of the body
portion, and the rear-half of the body portion of the plug,
respectively, and operates all the spray guns to form the
arc-sprayed film at the same time in order to reduce time required
for the film formation through the arc-spraying. Even in this
technique, the durability life of the plug cannot also be enhanced
as much as expected, similarly to the conventional method shown in
FIG. 4, and the reason for this is not identified yet.
[0055] The present invention has been made based on the above
findings. Hereinafter, provided is the description on the
preferable embodiments of the method for producing the plug of the
present invention.
First Embodiment
[0056] FIG. 6 is a schematic illustration showing the steps of the
film formation through the arc-spraying in the production method of
the plug according to the first embodiment of the present
invention, FIG. 6(a) shows the state of forming the film at the
body portion of the plug as step 1, and FIG. 6(b) shows the state
of forming the film at the tip end portion of the plug as step 2.
The method of the first embodiment shown in this illustration is
based on the configuration of the conventional method shown in FIG.
4, and a duplicate description will be omitted when
appropriate.
[0057] In the method of the first embodiment, as shown in FIG. 6,
the plug 1 has a bullet shape, and the film 3 is formed on the
surface of the plug base metal 2 through the arc-spraying while the
plug base metal 2 set on a turntable (not shown) or the like is
being rotated about the central axis P.sub.c of the plug 1.
Specifically, the surface of the plug base metal 2 is divided into
two sections along an axial direction of the plug 1. FIG. 6 shows
an example of the plug 1 divided into the tip end portion 1a and
the body portion 1b. The body portion 1b is a portion of 80 to 98%
of the overall length from the rear end (lower end in the
illustration) of the plug 1 in an axial direction (vertical
direction in the illustration) of the plug 1.
[0058] Based on the above configuration, in step 1 as shown in FIG.
6(a), the arc-spraying is carried out such that the spray gun 4 is
disposed opposite to a region of the body portion 1b of the plug
among the surface of the plug base metal 2, and is reciprocatingly
moved along the surface of this region only. In this manner, the
film 3 is formed on the body portion 1b other than the tip end
portion 1a on the surface of the plug base metal 2.
[0059] Subsequently to the above step, in step 2 as shown in FIG.
6(b) the arc-spraying is carried out such that the spray gun 4 is
disposed opposite to a region of the tip end portion 1a of the plug
among the surface of the plug base metal 2, and is reciprocatingly
moved for a short distance along the surface of this region only.
In this manner, the film 3 is formed at the tip end portion 1a on
the surface of the plug base metal 2. Accordingly, the film 3 is
formed across the entire surface of the plug 1.
[0060] At this time, in both of the steps 1, 2 shown in FIG. 6(a)
and FIG. 6(b), the motion and the posture of the spray gun 4 are
controlled in such a manner that the intersection angle .theta.
between the center line A.sub.c of spraying stream from the spray
gun 4 and the surface of the plug base metal 2 is within the
preferable range that is found based on the result of the above
described basic tests, that is, within the range of 35 degrees to
90 degrees, more preferably of 60 degrees to 90 degrees.
[0061] According to the method of the present embodiment, it is
configured that the surface of the plug base metal 2 is divided
into two sections (the tip end portion 1a and the body portion 1b)
in an axial direction of the plug 1, and in turn, the arc-spraying
is carried out separately in each of two sections with the
intersection angle .theta. between the center line A.sub.c of
spraying stream from the spray gun 4 and the surface of the plug
base metal 2 maintained within the preferable range, so as to form
the film 3 through the arc-spraying, thereby producing the plug
having the arc-sprayed film with firm adhesiveness between the plug
base metal and the film as well as significantly enhanced
durability life of the plug. In addition, it is possible to reduce
the operational range of the spray gun 4 at the time of the
arc-spraying in each divided section; thus the above intersection
angle .theta. can be securely maintained within the preferable
range without controlling the motion and the posture of the spray
gun 4 in a complicated manner. Accordingly, the adhesiveness of the
film becomes stable across the entire surface of the plug, thereby
realizing stable durability life of the plug, as well.
Second Embodiment
[0062] FIG. 7 is a schematic illustration showing the steps of the
film formation through the arc-spraying in the production method of
the plug according to the second embodiment of the present
invention; and FIG. 7(a) shows the state of forming the film at the
rear-half of the body portion of the plug as step 1, FIG. 7(b)
shows the state of forming the film at the front-half of the body
portion of the plug as step 2, and FIG. 7(c) shows the state of
forming the film at the tip end of the plug as step 3. The method
of the second embodiment is based on the configuration of the above
first embodiment, while divided sections of the surface of the plug
base metal 2 are further increased. In the second embodiment, the
surface of the plug base metal 2 is divided into three sections.
FIG. 7 shows an example of the plug 1 divided into the tip end
portion 1a, the front-half 1ba of the body portion 1b, and the
rear-half 1bb of the body portion 1b of the plug 1.
[0063] In the second embodiment, in step 1 as shown in FIG. 7(a),
the arc-spraying is carried out such that the spray gun 4 is
disposed opposite to the region of the rear-half 1bb of the body
portion 1b of the plug among the surface of the plug base metal 2.
In this manner, the film 3 is formed on the rear-half 1bb of the
body portion other than those of the tip end portion 1a and the
front-half 1ba of the body portion on the surface of the plug base
metal 2.
[0064] Subsequently, in step 2 as shown in FIG. 7(b), the
arc-spraying is carried out such that the spray gun 4 is disposed
opposite to the region of the front-half 1ba of the body portion 1b
of the plug among the surface of the plug base metal 2. In this
manner, the film 3 is formed on the front-half 1ba of the body
portion on the surface of the plug base metal 2.
[0065] Subsequently, in step 3 as shown in FIG. 7(c), the
arc-spraying is carried out such that the spray gun 4 is disposed
opposite to the region of the tip end portion 1a of the plug among
the surface of the plug base metal 2. In this manner, the film 3 is
formed on the tip end portion 1a on the surface of the plug base
metal 2. Accordingly, the film 3 is formed across the entire
surface of the plug 1.
[0066] In all steps 1 to 3 shown in FIG. 7(a) to FIG. 7(c), the
motion and the posture of the spray gun 4 are controlled to carry
out the arc-spraying in such a manner that the intersection angle
.theta. between the center line A.sub.c of spraying stream from the
spray gun 4 and the surface of the plug base metal 2 is within the
preferable range that is found based on the above basic tests.
[0067] The method of the second embodiment can also achieve similar
effects as those by the method of the first embodiment.
[0068] The number of the divided sections of the surface of the
plug base metal may be any number more than one, and the number of
the divided sections may be determined depending on a taper angle
and/or the curvature of the surface of the plug.
[0069] The plug provided by the above method of each embodiment can
be reproduced after its durability life is expired through
repetitive piercing-rolling by re-forming the arc-spayed film on
the surface of the plug using the same method. Immediately before
the re-forming of the film, a jet air injection or the shotblasting
is preferably applied onto the surface of the plug, so as to remove
the film remaining on the surface of the plug in as-is condition
after the repetitive piercing-rolling.
[0070] It is preferable to always apply an air jet injection or the
shotblasting onto a target region for the arc-spraying of each
section immediately before the arc-spraying is applied to this
section of the divided surface of the base metal. If molten
material of the arc-spraying that is carried out to a different
section inadvertently adheres onto the target region for the
arc-spraying, it may happen at an inappropriate intersection angle,
which may cause ununiformity in the adhesiveness of the film
[0071] A shield plate may be disposed so as to cover a region other
than a target region for the film formation when the arc-spraying
is applied to each section of the divided surface of the base
metal. Specifically, in the above first embodiment, the shield
plate is so disposed as to cover the tip end portion at the time of
applying the arc-spraying to the body portion, and the shield plate
is so disposed as to cover the body portion at the time of applying
the arc-spraying to the tip end portion. In the second embodiment,
similarly to the first embodiment, the shield plate is so disposed
as to cover the front-half of the body portion and the tip end
portion at the time of applying the arc-spraying to the rear-half
of the body portion. The shield plate is so disposed as to cover
the rear-half of the body portion and the tip end portion at the
time of applying the arc-spraying to the front-half of the body
portion, and the shield plate is so disposed as to cover the
front-half of the body portion and the rear-half of the body
portion at the time of applying the arc-spraying to the tip end
portion of the plug. This is to prevent adhesion of the molten
material sprayed from the sprayer onto an unexpected region at an
unfavorable intersection angle, thereby preventing deterioration of
the adhesiveness between the plug base metal and the film. Hence,
the shield plate may be so disposed as to cover at least a region
where no sprayed film is formed yet, and unnecessary to be disposed
at a region where the sprayed film is already formed.
[0072] In the method of each embodiment, the film formed through
the arc-spraying may have a uniform thickness across the entire
surface of the plug, or may have a heavier thickness at the tip end
portion than at the body portion of the plug. The film having a
heavier thickness at the tip end portion of the plug is useful in
light of securing enhanced thermal insulation performance and wear
resistance of the film at the tip end portion of the plug where the
surficial pressure becomes high and the temperature is increased
during the piercing-rolling, so that further enhancement of the
durability life of the plug can be expected.
Example
[0073] For the purpose of verifying the effects of the present
invention, a piercing-rolling test was conducted in such a manner
that plugs for piercing-rolling were produced, and each of the
produced plugs was mounted to a piercer so as to carry out the
piercing-rolling. The test condition was as follows.
[Test Method]
[0074] (1) Production of Plug
[0075] A number of bullet-shaped plugs, each having a maximum
diameter of 57 mm, were produced using hot-working tool steel
specified by the JIS standard as the base metal. Plugs having the
arc-sprayed film were produced in such a manner that the
arc-spraying was carried out by using iron wires under various
operation conditions of the arc-spraying, so as to form a film on
the surface of the base metal of each plug. In the formation of the
arc-sprayed film, the arc-spraying was conducted for each plug with
the spraying distance from the spray gun to the surface of the plug
base metal initially fixed at 200 mm, and the arc-spraying was
carried out while the spray gun was gradually distanced away from
the surface of the plug base metal until the spraying distance
finally became 1000 mm. The thickness of the arc-sprayed film was
set to 400 .mu.m at the body portion of the plug, and 1200 .mu.m at
the tip end of the plug.
[0076] As a reference for the evaluation, plugs having scale films
were produced by forming an oxide scale film on the surface of the
base metal of each plug through a heat treat furnace. The thickness
of the scale film was set to 600 .mu.m.
[0077] Each operation condition of the arc-spraying was as
follows.
Comparative Example 1
[0078] As shown in FIG. 8, with the center line A, of the spraying
stream from the spray gun 4 always maintained to make a right angle
with the central axis P.sub.c of the plug metal base 2, the
arc-spraying is carried out while the spray gun 4 is being moved
across the entire region from the rear end to the tip end of the
surface of the plug base metal 2. In this example, the intersection
angle .theta. between the center line A, of the spraying stream
from the spray gun 4 and the surface of the plug base metal 2 is
deviated from the preferable range at the tip end portion 1a of the
plug.
Comparative Example 2
[0079] As shown in FIG. 9, with the center line A, of the spraying
stream from the spray gun 4 always maintained to be parallel to the
central axis P.sub.c of the plug metal base 2, the arc-spraying is
carried out while the spray gun 4 is being moved across the entire
regions of the body portion 1b and of the tip end portion 1a of the
plug. In this example, the intersection angle .theta. between the
center line A, of the spraying stream from the spray gun 4 and the
surface of the plug base metal 2 is deviated from the preferable
range at the body portion 1b of the plug.
Comparative Example 3
[0080] In step 1 as shown in FIG. 10(a), with the center line A, of
the spraying stream from the spray gun 4 always maintained to be
parallel to the central axis P.sub.c of the plug metal base 2, the
arc-spraying is carried out to the body portion 1b of the plug
while the sprayer 4 is being moved along only the region of the
body portion 1b of the plug. Thereafter, in step 2 as shown in FIG.
10(b), with the intersection angle .theta. between the center line
A, of the spraying stream from the spray gun 4 and the surface of
the plug base metal 2 always being set to 25 degrees, the
arc-spraying is carried out to the tip end portion 1a of the plug
while the spray gun 4 is being moved along only the region of the
tip end portion 1a of the plug. In this example, the intersection
angle .theta. between the center line A, of the spraying stream
from the spray gun 4 and the surface of the plug base metal 2 is
deviated from the preferable range across the entire regions of the
tip end portion 1a and of the body portion 1b of the plug.
Comparative Example 4
[0081] As shown in FIG. 4, the posture of the spray gun 4 is
controlled so as to carry out the arc-spraying in such a manner
that the center line A, of the spraying stream from the spray gun 4
intersects at right angle with the central axis P.sub.c of the plug
metal base 2 at the rear end of the plug base metal 2, and the
center line k of the spraying stream from the spray gun 4 becomes
parallel to the central axis P, of the plug metal base 2 at the tip
end of the plug base metal 2 while the spray gun 4 is being moved
across the entire region from the rear end to the tip end of the
surface of the plug base metal 2. In this example, the intersection
angle .theta. between the center line A.sub.c of the spraying
stream from the spray gun 4 and the surface of the plug base metal
2 is deviated from the preferable range in vicinity of the boundary
between the tip end portion 1a and the body portion 1b of the
plug.
Inventive Example of Present Invention 1
[0082] In step 1 as shown in FIG. 11(a), the arc-spraying is
carried out to the body portion 1b of the plug by controlling the
posture of the spray gun 4 in such a manner that the spray gun 4 is
moved along only the region of the body portion 1b of the plug
while the intersection angle .theta. between the center line A, of
the spraying stream from the spray gun 4 and the surface of the
plug base metal 2 is set to 90 degrees on the rear end side of the
body portion 1b of the plug, and is set to more than 35 degrees on
the tip end side of the body portion 1b of the plug. In step 2 as
shown in FIG. 11(b), the arc-spraying is carried out to the tip end
portion 1a of the plug in such a manner that the spray gun 4 is
moved along only the region of the tip end portion 1a of the plug
while the intersection angle .theta. between the center line A, of
the spraying stream from the spray gun 4 and the surface of the
plug base metal 2 is always maintained at 90 degrees. At this time,
the intersection angle .theta. between the center line A, of the
spraying stream from the spray gun 4 and the surface of the plug
base metal 2 is within the preferable range across the entire
regions of the tip end portion 1a and of the body portion 1b of the
plug.
Inventive Example of Present Invention 2
[0083] In step 1 as shown in FIG. 12(a), the arc-spraying is
carried out to the body portion 1b of the plug by controlling the
posture of the spray gun 4 in such a manner that the spray gun 4 is
moved along only the region of the body portion 1b of the plug
while the intersection angle .theta. between the center line
A.sub.c of the spraying stream from the spray gun 4 and the surface
of the plug base metal 2 is always set to 90 degrees. In step 2 as
shown in FIG. 12(b), the arc-spraying is carried out to the tip end
portion 1a of the plug in such a manner that the spray gun 4 is
moved along only the region of the tip end portion 1a of the plug
while the intersection angle .theta. between the center line
A.sub.c of the spraying stream from the spray gun 4 and the surface
of the plug base metal 2 is always maintained at 45 degrees. In
this example, the intersection angle .theta. between the center
line A.sub.c of the spraying stream from the spray gun 4 and the
surface of the plug base metal 2 is within the preferable range
across the entire regions of the tip end portion 1a and of the body
portion 1b of the plug.
Inventive Example of Present Invention 3
[0084] In step 1 as shown in FIG. 13(a), the arc-spraying is
carried out to the body portion 1b of the plug by controlling the
posture of the spray gun 4 in such a manner that the spray gun 4 is
moved along only the region of the body portion 1b of the plug
while the intersection angle .theta. between the center line
A.sub.c of the spraying stream from the spray gun 4 and the surface
of the plug base metal 2 is always set to 90 degrees. In step 2 as
shown in FIG. 13(b), the arc-spraying is carried out to the tip end
portion 1a of the plug in such a manner that the spray gun 4 is
moved along only the region of the tip end portion 1a of the plug
while the intersection angle .theta. between the center line
A.sub.c of the spraying stream from the spray gun 4 and the surface
of the plug base metal 2 is always maintained at 90 degrees. In
this example, the intersection angle .theta. between the center
line A.sub.c of the spraying stream from the spray gun 4 and the
surface of the plug base metal 2 is within the preferable range
across the entire regions of the tip end portion 1a and of the body
portion 1b of the plug.
[0085] (2) Piercing-Rolling
[0086] Using the above various plugs, the following hollow shells
were produced by repetitively piercing-rolling the following
workpieces (materials) heated at 1200.degree. C. [0087] Workpiece
size: round billet of 70 mm in diameter and 600 mm in length [0088]
Workpiece material grade: SUS304 [0089] Hollow shell: 73 mm in
outer diameter, 6.0 mm in wall thickness, 1800 mm in length.
[0090] [Evaluating Method]
[0091] Inspection was conducted on the appearance of each plug
every time the piercing-rolling was completed. For each plug, the
number of times of rolling pass until the plug became unusable due
to the peel-off of the film, or melting-incurred metal loss,
seizing or deformation was generated at the tip end of the plug was
investigated, in other words, the number of billets that
successfully got through the continuous piercing-rolling (number of
times of continuous piercing-rolling) was counted. The durability
life of the plug was evaluated based on the ratio of the durability
life of each plug relative to a reference "1.0" (hereinafter
referred to as "plug durability life ratio"), where the plug has
the scale film and its durability life is defined as the reference
"1.0".
[0092] [Test Result]
[0093] Test result is shown in Table 1.
TABLE-US-00001 TABLE 1 Classification Satisfiability on Preferable
Plug Plug Surface (Reference Range of Intersection Angle Durability
Condition after No Illustration) .theta. (35.degree. to 90.degree.
) Life Ratio Piercing-rolling 1 Comparative Deviated at Plug Tip
End 0.5 Seizing at Tip Example 1 Portion End Portion (FIG. 8) 2
Comparative Deviated at Plug Body 1.0 Film Peel-off at Example 2
Portion Body Portion (FIG. 9) 3 Comparative Deviated across Entire
Plug 0.5 Film Peel-off Example 3 from Tip End (FIG. 10) Portion to
Body Portion 4 Comparative Deviated in Vicinity of 2.5 Seizing at
Tip Example 4 Boundary Between Tip End End Portion (FIG. 4) Portion
and Body Portion of Plug 5 Inventive Within Preferable Range 6.0
Wrinkle at Body Example 1 across Entire Plug Portion (FIG. 11) 6
Inventive Within Preferable Range 5.0 Seizing at Tip Example 2
across Entire Plug End Portion (FIG. 12) 7 Inventive Within
Preferable Range 7.0 Wrinkle at Body Example 3 across Entire Plug
Portion (FIG. 13) Note: The durability life of each plug is
represented by using its ratio relative to the reference "1.0",
where the durability life of the plug having the scale film is
defined as the reference "1",
[0094] The result of Table 1 reveals the following. In the
Comparative Examples 1 to 3 shown in Classification Nos. 1 to 3,
since most of the films formed through the arc-spraying were
produced under the condition in which the intersection angle
.theta. was out of the preferable range (35 degrees to 90 degrees),
Peel-off of the film, melting loss or seizing of the plug was
caused at an early stage, and the durability life of the plug
became 1.0 or less; thus no enhancement of the durability life of
the plug could be confirmed. The Comparative Example 4 shown in
Classification No. 4 exhibited some enhancement of the durability
life of the plug, but the durability life ratio of the plug was 2.5
at most because the arc-spraying was carried out across the surface
of the plug base metal at a time without dividing the surface of
the plug base metal.
[0095] To the contrary, in Inventive Examples 1 to 3 shown in
Classification Nos. 5 to 7, the surface of the plug base metal was
divided into plural sections, and in turn, the arc-spraying was
separately carried out in each section under the condition of
satisfying the preferable range of the intersection angle .theta.
(35 degrees to 90 degrees) specified by the present invention.
Accordingly, it was confirmed that the adhesiveness of the film was
significantly enhanced, and the durability life ratio of each plug
became 5.0 or more, which reveals significant enhancement of the
durability life of the plug.
INDUSTRIAL APPLICABILITY
[0096] The present invention can be effectively used in production
of seamless steel tube/pipe of high alloy steel.
REFERENCE SIGNS LIST
[0097] 1: Plug, 1a: Tip end portion of plug, 1b: Body portion of
plug, [0098] 1ba: Front-half of body portion of plug, 1bb:
Rear-half of body portion of plug, [0099] 2: Plug base metal, 3:
Arc-sprayed film, 4: Arc-spray gun [0100] Pc: Central axis of plug,
Ac: Center line of spraying stream of arc-spray gun, [0101]
.theta.: Intersection angle
* * * * *