Note: Descriptions are shown in the official language in which they were submitted.
CA 02750014 2011-07-18
7
DESCRIPTION
METHOD FOR MANUFACTURING METAL PIPE
TECHNICAL FIELD
[0001]
The present invention relates to a method for manufacturing a metal pipe
having a
scale layer at least on the inner surface thereof. More particularly, the
present invention
relates to a method for manufacturing a metal pipe excellent in high-
temperature strength
and corrosion resistance. The metal pipe obtained by the present invention is
suitably
used as a pipe used in a carburizing gas atmosphere containing hydrocarbon
gas, CO gas,
and the like, such as a pyrolytic furnace pipe, a reforming furnace pipe, a
heating furnace
pipe, and a heat exchanger pipe in an oil refining plant, a petrochemical
plant, and the like.
BACKGROUND ART
[0002]
In recent years, a metal pipe containing 20 to 35 mass % of Cr and 20 to 70
mass %
of Ni has been used as a pyrolytic furnace pipe, a reforming furnace pipe, a
heating furnace
pipe, a heat exchanger pipe, and the like used in a carburizing gas atmosphere
containing
hydrocarbon gas, CO gas, and the like in, for example, an oil refining plant
or a
petrochemical plantmass %mass %. The reason is that this metal pipe is
excellent in
high-temperature strength and corrosion resistance.
[0003]
The inner surface of the metal pipe is exposed to a carburizing atmosphere.
Therefore, an oxide scale layer consisting mainly of Cr is preferably formed
on the inner
surface of the metal pipe in order to prevent carburization. The oxide scale
layer
consisting mainly of Cr is highly dense, and has an effect of shielding the
intrusion of
carbon into the metal pipe. The oxide scale layer consisting mainly of Cr has
a weak
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1 ,
catalytic action against coking (deposit of carbon). Therefore, the oxide
scale layer
consisting mainly of Cr also has an effect of restraining coking on the
surface of metal pipe.
As a result, the thermal conductivity to a fluid introduced into the metal
pipe can be kept
for a long period of time. Therefore, for example, in the case where such a
metal pipe is
used as a decomposition reaction tube, the yield of a reaction product such as
olefin is
stabilized. This oxide scale layer consisting mainly of Cr is also formed in
an
environment in which the metal pipe is used. However, because carbon intrudes
into the
metal pipe simultaneously as described above, it is difficult to form the
oxide scale layer
consisting mainly of Cr uniformly on the inner surface of metal pipe. For this
reason, it is
effective to form the oxide scale layer consisting mainly of Cr in advance on
the inner
surface of metal pipe.
[0004]
Patent Document 1 discloses a method in which when a stainless steel pipe
containing 12 to 20 mass % of Cr and 40 mass % or less of Ni is used in a high
temperature and pressure water environment, the steel pipe is subjected to
heat treatment of
being heated to 800 to 1100 C under an inert gas atmosphere containing 0.01 to
0.5 vol%
of oxygen and being held at that temperature for 2 to 20 minutes to form a
scale layer on
the surface of the steel pipe in order to prevent the Ni release from the
steel pipe.
Patent Document 2 discloses an invention in which an austenitic stainless
steel containing
14 mass % or less of Cr is heat treated at a temperature not lower than 1100 C
while the
CO concentration in a barrel furnace is controlled to at least 150 ppm to
prevent
unevenness of scale caused by abnormal oxidation of the steel surface.
[0005]
Patent Document 3 discloses an invention relating to a stainless steel used in
a
carburizing gas atmosphere, the stainless steel having an oxide scale layer
consisting
mainly of Cr, in which the Cr concentration in a Cr depleted zone is at least
10% by mass,
on the surface of a base metal containing 20 to 55 mass % of Cr, and further
having an
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CA 02750014 2011-07-18
oxide scale layer consisting mainly of Cr, in which the Cr content is at least
50% by mass,
on the outside thereof
[0006]
Patent Document 4 relates to a method for manufacturing an ethylene pyrolytic
furnace pipe excellent in coking resistance, and discloses an invention in
which a pipe
containing 15 to 30 mass % of Cr and 15 to 50 mass % of Ni is subjected to
cold working
of at least 50 1..tm depth from the surface, and then the pipe is heated to a
temperature not
lower than 11000C in an atmosphere containing less than 5 vol% of oxygen and
at least 20
vol% of nitrogen.
[0007]
[Patent Document 1]: JP2-47249A
[Patent Document 2]: JP3-197617A
[Patent Document 3]: JP2005-48284A
[Patent Document 4]: JP2-263895A
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0008]
In the inventions described in Patent Documents 1 and 2, because the Cr
content is
as low as 20 mass % or less, it is difficult to form the oxide scale layer
consisting mainly
of Cr.
[0009]
The stainless steel having the oxide scale layer as described in Patent
Document 3 is
excellent in carburization resistance and coking resistance.
However, in actual
manufacturing, it is difficult to uniformly form the oxide scale layer
consisting mainly of
Cr over the entire inner surface of pipe.
[0010]
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The invention described in Patent Document 4 describes that a fine-grained
layer of
not less than No. 7 in the austenitic crystal grain size can be created to a
depth of at least
30 lam from the surface with cold working and nitrogen permeating heat
treatment, so that
the stability of Cr203 oxide film produced during the use under an actual
operation
condition of 750 to 11000C can be improved. In this method, the oxide scale
produced in
the nitrogen permeating heat treatment is removed, and a stable Cr203 oxide
film is formed
on the fine-grained layer in the actual operation. However, the formation of
oxide film
during the actual operation requires a long period of time. In this method,
therefore,
carburization or coking may occur before the stable oxide film is formed.
[0011]
The present invention has been made to solve the above problems with prior
arts,
and accordingly an objective thereof is to provide a method for manufacturing
a metal pipe
having excellent resistance to carburization or coking caused by a carburizing
gas by
forming a uniform oxide scale layer consisting mainly of Cr on the inner
surface of metal
pipe.
MEANS FOR SOLVING THE PROBLEMS
[0012]
The present inventors earnestly conducted studies on the method for uniformly
forming the oxide scale layer consisting mainly of Cr having carburization
resistance and
coking resistance over the entire inner surface of metal pipe, and resultantly
obtained the
findings concerning the cause for the formation of nonuniform scale and the
method for
preventing the formation of nonuniform scale, as described below.
[0013]
(A) Various studies were carried out on the oxide scale layer formed on the
inner
surface of metal pipe, such as observation using an optical microscope and a
scanning
electron microscope (SEM), and quantitative analysis of elements using energy
dispersive
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X-ray spectrometry (EDX). As the result, on the surface of nonuniform scale,
either one
or both of alkali metal and alkaline earth metal were detected.
[0014]
(B) According to the results of studies by the present inventors, it was
revealed that
these elements are derived from a lubricant used at the time of cold working,
and the
lubricant remaining on the surface of metal pipe is a cause that hinders the
formation of the
oxide scale layer consisting mainly of Cr.
[0015]
(C) After cold working, an attempt was made to remove the lubricant sticking
to the
inner surface of pipe by carrying out degreasing, cleaning, and the like
method. With
these methods, however, in some cases, the lubricant could not be removed
sufficiently
throughout the overall length of metal pipe. Accordingly, various methods were
tested
for removing the lubricant. As the result, it was found that by subjecting the
inner surface
of metal pipe to mechanical treatment such as blasting, the lubricant on the
inner surface of
metal pipe can be removed uniformly throughout the overall length of metal
pipe.
[0016]
The present invention was completed on the basis of the above-described
findings,
and the gist thereof is methods for manufacturing a metal pipe given in the
items (1) to (4)
listed below.
[0017]
(1) A method for manufacturing a metal pipe containing, by mass percent, 20 to
55% of Cr and 20 to 70% of Ni, wherein
the inner surface of the metal pipe is subjected to mechanical treatment;
the metal pipe is subjected to heat treatment such as to be held in a
temperature
range of 1050 to 1270 C for 0.5 to 60 minutes; and thereby
an oxide scale layer consisting mainly of Cr is formed on at least the inner
surface
of the metal pipe.
[0018]
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(2) The method for manufacturing a metal pipe according to the above item (1),
wherein the metal pipe has a chemical composition consisting of, by mass
percent, C: 0.01
to 0.6%, Si: 0.1 to 5%, Mn: 0.1 to 10%, P: 0.08% or less, S: 0.05% or less,
Cr: 20 to 55%,
Ni: 20 to 70%, N: 0.001 to 0.25%, 0 (oxygen): 0.02% or less, and the balance
being Fe
and impurities.
[0019]
(3) The method for manufacturing a metal pipe according to the above item (2),
wherein
the metal pipe further contains at least one selected from the elements, by
mass
percent, given in the following items (a) to (g):
(a) Cu: 5% or less
(b) Co: 5% or less
(c) At least one selected from Mo: 3% or less, W: 6% or less, and Ta: 6% or
less
(d) One or two selected from Ti: 1% or less and Nb: 2% or less
(e) At least one selected from B: 0.1% or less, Zr: 0.1% or less, and Hf: 0.5%
or less
(f) At least one selected from Mg: 0.1% or less, Ca: 0.1% or less, and Al: 1%
or less
(g) At least one selected from Y: 0.15% or less and Ln group elements: 0.15%
or less.
[0020]
(4) The method for manufacturing a metal pipe according to any one of the
above
items (1) to (3), wherein the metal pipe has a rib-shaped protrusion on the
inner surface of
pipe.
EFFECT OF THE INVENTION
[0021]
According to the present invention, a metal pipe having an oxide scale layer
consisting mainly of Cr formed uniformly on the inner surface of the metal
pipe can be
manufactured. The metal pipe obtained by the manufacturing method of the
present
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invention is excellent in carburization resistance and coking resistance in a
carburizing gas
environment.
MODE FOR CARRYING OUT THE INVENTION
[0022]
The present invention provides a method for manufacturing a metal pipe,
wherein
the inner surface of a metal pipe having a predetermined chemical composition
is subjected
to mechanical treatment; the metal pipe is subjected to heat treatment such as
to be held in
a temperature range of 1050 to 1270 C for 0.5 to 60 minutes; and thereby an
oxide scale
layer consisting mainly of Cr is formed on at least the inner surface of the
metal pipe.
Hereunder, the chemical composition of the metal pipe obtained by the
manufacturing
method of the present invention, and the mechanical treatment and heat
treatment to which
the metal pipe is subjected are described. In the description below, "%"
relating to the
content of each element means "mass %".
[0023]
1. Chemical composition of metal pipe
The metal pipe obtained by the manufacturing method of the present invention
must
contain 20 to 55% of Cr and 20 to 70% of Ni.
[0024]
Cr: 20 to 55%
Cr (Chromium) must be contained in an amount of at least 20%. The reason is
that the oxide scale layer consisting mainly of Cr is formed stably on at
least the inner
surface of the metal pipe. However, if Cr is contained excessively, it is
difficult to
manufacture the metal pipe, and the micro-structure may become unstable during
the use at
high temperature. Therefore, the upper limit of Cr content is set to 55%. To
ensure the
workability and to prevent the structural stability from deteriorating, the
upper limit of Cr
content is preferably set to 35%. The further preferable range of Cr content
is 22 to 33%.
[0025]
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Ni: 20 to 70%
Ni (Nickel) is an element necessary for obtaining a stable austenitic
structure. Ni
should be contained in an appropriate amount depending on the Cr content. Ni
has an
effect of reducing the intrusion rate of carbon into the metal material. This
effect is
achieved in the case where the Ni content is set to at least 20%. However,
even if Ni is
added excessively, the effect saturates, and the manufacturing cost is
increased.
Excessive Ni makes the manufacture of pipe difficult. Therefore, the Ni
content is set to
20 to 70%. The lower limit of Ni content is preferably set to 23%, and the
upper limit
thereof is preferably set to 60%, further preferably 50%.
[0026]
The starting material for a metal pipe for manufacturing ethylene (ethylene
cracking
tube) preferably contains Cr: 20 to 35% and Ni: 20 to 60%.
[0027]
The metal pipe obtained by the manufacturing method of the present invention
has
the above-described chemical composition, and other components are not
limited.
However, the metal pipe preferably has a chemical composition consisting of C:
0.01 to
0.6%, Si: 0.1 to 5%, Mn: 0.1 to 10%, P: 0.08% or less, S: 0.05% or less, Cr:
20 to 55%,
Ni: 20 to 70%, N: 0.001 to 0.25%, 0 (oxygen): 0.02% or less, the balance being
Fe and
impurities. Hereunder, the reasons for restricting the content of each element
are
described.
[0028]
The impurities are components that mixedly enter from raw ore, scrap, and the
like
when the metal pipe is manufactured on an industrial basis, and are permitted
as far as the
content range does not adversely affect the present invention.
[0029]
C: 0.01 to 0.6%
C (Carbon) is an element effective in ensuring the high-temperature strength.
This
effect is remarkable when at least 0.01% of C is contained. If the C content
exceeds 0.6%,
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the toughness may be deteriorated extremely. Therefore, the C content is
preferably set to
0.01 to 0.6%. The lower limit of C content is further preferably set to 0.02%,
and the
upper limit thereof is further preferably set to 0.45%, still further
preferably 0.3%.
[0030]
Si: 0.1 to 5%
Si (Silicon) has an effect of assisting the uniform formation of the oxide
scale layer
consisting mainly of Cr because the affinity of Si for oxygen is high. This
effect is
remarkable when at least 0.1% of Si is contained. However, if the Si content
exceeds 5%,
the weldability is deteriorated, and the micro-structure may become unstable.
Therefore,
the Si content is preferably set to 0.1 to 5%. The upper limit of Si content
is preferably
set to 3%, further preferably 2%, and the lower limit thereof is preferably
set to 0.3%.
[0031]
Mn: 0.1 to 10%
Mn (Manganese) is an element effective for deoxidation and in improving the
workability. Also, because Mn is an austenite producing element, some of Ni
can be
replaced with Mn. To achieve these effects, at least 0.1% of Mn is preferably
contained.
However, if Mn is contained excessively, the formation of the oxide scale
layer consisting
mainly of Cr may be hindered. Therefore, the Mn content is preferably set to
0.1 to 10%.
The upper limit of Mn content is preferably set to 5%, further preferably 2%.
[0032]
P: 0.08% or less
S: 0.05% or less
P (Phosphorus) and S (Sulfur) are preferably reduced in amount as far as
possible
because these elements segregate at the crystal grain boundary and deteriorate
the hot
workability. However, because the excessive reduction leads to an increase in
cost, the P
content is preferably 0.08% or less, and the S content is preferably 0.05% or
less. The
P content is further preferably set to 0.05% or less, and the S content is
further preferably
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set to 0.03% or less. The P content is still further preferably set to 0.04%
or less, and
the S content is still further preferably set to 0.015% or less.
[0033]
N: 0.001 to 0.25%
N (Nitrogen) is an element effective in improving the high-temperature
strength.
This effect is remarkable when at least 0.001 4 of N is contained. However,
the excessive
addition of N may hinder the workability greatly. Therefore, the N content is
preferably
set to 0.001 to 0.25%. The upper limit of N content is preferably set to 0.2%.
[0034]
0 (Oxygen): 0.02% or less
0 (Oxygen) is an element existing as an impurity. If the 0 content exceeds
0.02%,
the oxide-base inclusions in the metal material precipitate in large amounts,
which
decrease the workability, so that the inclusions are a cause for the surface
defects of pipe.
Therefore, the 0 content is preferably set to 0.02% or less.
[0035]
The above-described metal pipe may further contain one or more elements
selected
from the elements given in the items (a) to (g) listed below.
[0036]
(a) Cu: 5% or less
Cu (Copper) is an element for stabilizing the austenitic phase. Cu is also an
element effective in improving the high-temperature strength. Therefore, Cu
may be
contained in the above-described metal pipe. However, if the Cu content is
excessive, the
hot workability may be decreased. Therefore, if Cu is contained, the content
thereof is
preferably set to 5% or less. The upper limit of the Cu content is further
preferably set
to 3%. The above-described effects are remarkable when 0.1% or less of Cu is
contained.
[0037]
(b) Co: 5% or less
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Co (Cobalt) is an element for stabilizing the austenitic phase. If Co is
contained,
some of Ni can be replaced with Co. Therefore, Co may be contained in the
above-
described metal pipe. However, if the Co content is excessive, the hot
workability may
be decreased. Therefore, if Co is contained, the content thereof is preferably
set to 5%
or less. The upper limit of the Co content is further preferably set to 3%.
The above-
described effect is remarkable when 0.1% or less of Co is contained.
[0038]
(c) At least one selected from Mo: 3% or less, W: 6% or less, and Ta: 6% or
less
Mo (Molybdenum), W (Tungsten) and Ta (Tantalum) are elements contributing to
solid-solution strengthening and effective in improving the high-temperature
strength.
Therefore, at least one selected from these elements may be contained in the
above-
described metal pipe. However, if the contents of these elements are
excessive, the
workability is deteriorated, and the structural stability may be hindered.
Therefore, if at
least one of these elements is contained, the Mo content is preferably set to
3% or less,
and the W and Ta contents each are preferably set to 6% or less. The upper
limit of
each of these elements is further preferably set to 2.5%, still further
preferably 2% or less.
For each of these elements, the above-described effects are remarkable when at
least
0.01% of each of these elements is contained. When these elements are
contained
compositely, the upper limit of the total amount is preferably set to 10%.
[0039]
(d) One or two selected from Ti: 1% or less and Nb: 2% or less
Ti (Titanium) and Nb (Niobium) have great effects of improving the high-
temperature strength, ductility, and toughness even if minute amounts of them
are
contained. Therefore, one or two selected from these elements may be contained
in the
above-described metal pipe. However, if the contents of these elements are
excessive, the
workability and weldability may be deteriorated. Therefore, if one or two of
these
elements are contained, the Ti content is preferably set to 1% or less, and
the Nb content
is preferably set to 2% or less. For each of these elements, the above-
described effects
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CA 02750014 2011-07-18
are remarkable when at least 0.01% of each of these elements is contained.
When these
elements are contained compositely, the upper limit of the total amount is
preferably set to
2%.
[0040]
(e) At least one selected from B: 0.1% or less, Zr: 0.1% or less, and Hf: 0.5%
or less
B (Boron), Zr (Zirconium) and Hf (Hafnium) are elements effective in
strengthening the grain boundary and improving the hot workability and high-
temperature
strength. Therefore, at least one selected from these elements may be
contained in the
above-described metal pipe. However, if the contents of these elements are
excessive, the
weldability may be deteriorated. Therefore, if at least one of these elements
is contained,
the B and Zr contents each are preferably set to 0.1% or less, and the Hf
content is
preferably set to 0.5% or less. For each of these elements, the above-
described effects
are remarkable when at least 0.001% of each of these elements is contained.
When these
elements are contained compositely, the upper limit of the total amount is
preferably set to
0.3%.
[0041]
(0 At least one selected from Mg: 0.1% or less, Ca: 0.1% or less, and Al: 1%
or less
Mg (Magnesium), Ca (Calcium) and Al (Aluminum) are elements effective in
improving the hot workability. Therefore, at least one selected from these
elements may
be contained in the above-described metal pipe. However, if the contents of
these
elements are excessive, the weldability may be deteriorated. Therefore, if at
least one of
these elements is contained, the Mg content is preferably set to 0.1% or less,
the Ca
content is preferably set to 0.1% or less, and the Al content is preferably
set to 1% or
less. The upper limits of the Mg content and the Ca content each are further
preferably
set to 0.05%, and the upper limit of the Al content is further preferably set
to 0.6%. The
above-described effect is remarkable when at least 0.001% of each of Mg and Ca
is
contained and when at least 0.01% of Al is contained. The lower limits of the
Mg content
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and the Ca content each are preferably set to 0.002%. When these elements are
contained
compositely, the upper limit of the total amount is preferably set to 0.5%.
[0042]
(g) At least one selected from Y: 0.15% or less and Ln group elements: 0.15%
or less
Y (Yttrium) and Ln (Lanthanide) group elements are elements effective in
improving the oxidation resistance. Therefore, at least one selected from
these elements
may be contained in the above-described metal pipe. However, if the contents
of these
elements are excessive, the workability is deteriorated. Therefore, if at
least one of these
elements is contained, the content of each element is preferably set to 0.15%
or less.
The above-described effect is remarkable when at least 0.0005% of each of
these elements
is contained. The upper limit of the content of each of these elements is
further
preferably set to 0.10%. When these elements are contained compositely, the
upper limit
of the total amount is preferably set to 0.15%. The Ln group elements are
elements of La,
which is element number 57, through Lu, which is element number 71. Among the
Ln
group elements, at least one of La, Ce and Nd is preferably used.
[0043]
2. Mechanical treatment
When the metal pipe is worked, a lubricant is used to reduce the friction
between
the metal pipe and a working tool. The lubricant is usually removed by
degreasing and
cleaning after working. However, some of the lubricant remains on the inner
surface of
pipe. As described above, the lubricant remaining on the surface of the metal
pipe
hinders the formation of the oxide scale layer consisting mainly of Cr. In the
present
invention, therefore, mechanical treatment is performed to remove the
remaining lubricant.
In some cases, in addition to the lubricant, the oxide scale produced at the
time of hot pipe-
making, dirt, and the like are adhered and remain on the surface of the metal
pipe. Such
remainder is preferably removed because it hinders the uniform formation of
the oxide
scale layer consisting mainly of Cr.
[0044]
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The mechanical treatment is treatment for enhancing the cleanliness of surface
by
physically removing deposits such as the lubricant remaining on the surface of
metal pipe,
dirt, and oxide scale. The mechanical treatment includes, for example,
blasting treatment,
grinding treatment (or friction treatment) for removing the deposits by
bringing an abrasive
into direct contact with the inner surface of metal pipe and rubbing the inner
surface
thereof with the abrasive, and a process for removing the deposits by spraying
high-
pressure water without the use of abrasive. As the blasting treatment, for
example, there
are available air-blasting in which blast media are propelled by compressed
air,
sandblasting (one kind of air-blasting) in which sand is used as the blast
media,
shotblasting in which blast media are propelled by the centrifugal force of an
impeller
made of an abrasion-resistant alloy, shotpeening (one kind of shotblasting)
mainly used for
giving strain to the metal surface, wet blasting, and the like. In the
shotpeening, deposits
on the surface can be removed simultaneously with the giving of strain. Wet
blasting in
which blast media are propelled together with high-pressure water can also be
applied.
[0045]
Although the abrasive used for the mechanical treatment is not limited, a
nonmetal
such as silica sand (Si02), alumina (A1203), zirconia (Zr02), boron nitride
(BN), or silicon
carbide (SiC), a mixture of these nonmetals or an abrasive containing these
nonmetals as
principal components is suitably used. Also, an abrasive consisting of a metal
such as
cast steel, stainless steel, metallic glass (amorphous), or Cr may be used. A
nonwoven
fabric or the like to which the abrasive is stuck may also be used. The shape
of the
abrasive is not limited, and the abrasive can take any shape such as a
granular shape, a grit
shape, or a powder shape. The size of the abrasive is not limited. However, in
the case
where the surface roughness is restrained to enhance the coking resistance,
the average
grain size (the average of the major axis and the minor axis) is preferably
300 pm or less,
further preferably 150 ilm or less.
[0046]
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In the case where the above-described abrasive is shot blasted onto the inner
surface
of pipe at a high speed, the abrasive may be shot blasted from one end or both
ends of the
metal pipe, or may be shot blasted while a blasting nozzle is inserted into
the metal pipe
and is moved in the pipe. Alternatively, the abrasive or the nonwoven fabric
to which the
abrasive is stuck may be brought into direct contact with the inner surface of
metal pipe in
a state of being dry or being wetted by a liquid and may be moved while
rubbing the inner
surface of metal pipe.
[0047]
3. Heat treatment
The metal pipe is subjected to mechanical treatment, subsequently heat
treatment,
and thereby an oxide scale layer consisting mainly of Cr is formed on the
inner surface of
the metal pipe. If the heat treatment temperature is lower than 1050 C, the
oxide scale
layer formed on the surface of metal pipe is thin, so that the shielding
property against the
intrusion of carbon into metal material is insufficient. If the heat treatment
temperature
exceeds 1270 C, pores or cracks are occurred in the oxide scale layer, and the
denseness is
decreased, which results in a decrease in carburization resistance. Therefore,
the heat
treatment is performed in the temperature range of 1050 to 1270 C. The lower
limit of
heat treatment temperature is preferably 1120 C, further preferably 1160 C.
[0048]
If the holding time of the heat treatment is shorter than 0.5 minute, the
oxide scale
layer consisting mainly of Cr excellent in carburization resistance cannot be
formed
uniformly. Even if the holding time exceeds 60 minutes, the thickness of the
oxide scale
layer merely increases, which leads to a decrease in productivity and an
increase in energy
cost. Moreover, there also arises a problem of decreased denseness of the
oxide scale
layer. Therefore, the holding time in the above-described temperature range is
set to 0.5
to 60 minutes. The lower limit of the holding time is preferably set to 2
minutes, further
preferably 5 minutes. The upper limit of the holding time is preferably set to
30 minutes,
further preferably 15 minutes.
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,
[0049]
It is preferable to encourage degreasing, cleaning, pickling, and other
treatment
after the mechanical treatment. Even if heat treatment is performed after
these kinds of
treatment, the uniform formation of the oxide scale layer consisting mainly of
Cr is not
hindered. These kinds of treatment are especially effective in the case where
there is a
concern about decrease in cleanliness caused by the abrasive remaining on the
inner
surface of pipe. The gas atmosphere in the heat treatment may be any
atmosphere in
which the oxide scale layer consisting mainly of Cr can be formed. For
example, the
atmosphere of atmospheric gas or a gas obtained by burning a hydrocarbon fuel
(LNG,
butane, etc.) and air may be used. Also, the atmosphere of DX gas, NX gas, RX
gas,
COG (C gas), or hydrogen gas whose dew point is controlled may be used. The
atmosphere of a gas obtained by mixing these gases in an arbitrary ratio may
also be used.
[0050]
4. Oxide scale layer consisting mainly of Cr
The oxide scale layer consisting mainly of Cr is very important from the
viewpoints
of carburization resistance and coking resistance. In particular, the oxide
scale layer
containing at least 50% of Cr has a high denseness and is excellent in
shielding property
against the intrusion of carbon into metal material. The oxide scale layer
consisting
mainly of Cr restrains coking on the surface of metal material because the
catalytic action
thereof against coking is weak. As a result, the thermal conductivity to a
fluid in the pipe
is kept for a long period of time. For example, in the case where the metal
pipe is used as
a decomposition reaction tube, the yield of a reaction product such as olefin
is stabilized.
[0051]
The Cr content in the oxide scale layer is preferably at least 80%. The oxide
scale
layer having a high Cr content is denser and achieves a great effect of
shielding the
intrusion of carbon into the metal material. The content of element in the
oxide scale
layer can be measured by EDX. The measurement should be made from the surface
of
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CA 02750014 2011-07-18
the oxide scale layer. The determination of element is made by the fraction of
the
detected element excluding C (carbon), 0 (oxygen), and the like.
[0052]
5. Others
The present invention is especially useful in manufacture of a metal pipe
having a
rib-shaped protrusion on the inner surface thereof. Usually, in the case of
such a metal
pipe having a rib-shaped protrusion on the inner surface thereof, it is
thought that the metal
pipe is liable to be attacked by carburizing gas, and the oxide scale is
liable to peel off.
According to the present invention, however, a metal pipe having high
carburization
resistance on the inner surface of the pipe and high repairability of the film
can be obtained.
A pipe having a protrusion on the inner surface thereof, a pipe having fins,
and the like are
cited as the pipe having the rib-shaped protrusion. The protrusion, the fin,
and the like
may be formed integrally with the pipe itself or may be formed by welding or
the like
means.
EMBODIMENTS
[0053]
The present invention is explained below more specifically by way of example.
The present invention is not limited to the example.
[0054]
Metal materials having the chemical composition given in Table 1 were melted
by
using an electric furnace or a vacuum furnace to form billets. The obtained
billets were
hot forged and cold rolled to produce metal pipes having an outside diameter
of 56 mm and
a wall thickness of 6 mm. The metal pipes of specimen Nos. 1 to 10 were
subjected to
mechanical treatment of the conditions given in Table 2. For some metal pipes,
the
mechanical treatment was omitted. Then, the metal pipes were subjected to heat
treatment under the conditions given in Table 2 to form oxide scale. Some
metal pipes
were subjected to alumina blasting as the mechanical treatment, and were not
subjected to
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CA 02750014 2013-05-08
heat treatment. To evaluate uniform carburization resistance and coking
resistance in the
metal pipe, each of the metal pipes was cut at a total of five places at a 2-m
pitch along the
pipe longitudinal direction to sample ring-shaped specimens each having a
width of 50 mm
and specimens for observation of oxide scale (20 x 20 mm square), described
later.
[0055]
[Table 1]
TABLE 1
Specimen Base metal chemical composition (mass %)
No. C Si Mn P S Cr Ni N O Others
1 0.21 0.36 0.42 0.020 <0.001 25.8 24.5 0.04 0.01 0.5Ti
2 0.11 1.67 0.28 0.017 <0.001 25.3 38.3 0.02 0.01
1.2Mo, 0.5Ti
3 0.08 0.35 1.20 0.025 <0.001 20.7 30.5 0.02 0.01
4 0.11 1.85 3.20 0.022 0.001 28.9 42.5 0.05 <0.01 1.3Nb, 0.003Ca
0.01 0.12 0.15 0.018 <0.001 31.2 60.8 0.01 0.01 0.004B, 0.029La
6 0.12 1.54 0.32 0.022 <0.001 25.6 38.3 0.02 0.01 1.0W, 1.1Cu,
0.01Y
7 0.11 1.48 0.36 0.020 <0.001 24.8 38.5 0.02 0.01 0.004Mg,
0.55Co
8 0.12 1.61 0.34 0.017 <0.001 25.7 38.5 0.02 <0.01 0.04A1, 0.02Zr
9 0.11 1.55 0.64 0.025 <0.001 25.2 38.1 0.01 <0.01 1.3Ta, 0.2Hf
0.11 0.46 1.31 0.025 0.001 *18.6 25.5 0.03 0.01
* indicates a value deviating from the range defined in the present invention.
[0056]
EDX analysis was made from the surface of the specimen for observation, and
the
Cr content (mass %) in the oxide scale layer produced on the metal pipe was
determined
from the average of three measurements. On the other hand, carburization and
coking
tests were conducted by holding the ring-shaped specimen at 1000 C for 300
hours in the
gas atmosphere of 15%C1-14-3%CO2-82%H2 by a volume ratio. Concerning the
coking
resistance, the mass of specimen was measured before and after the test to
determine the
increase amount due to coke deposit, and deposited coke amount per unit area
(mg/cm2)
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CA 02750014 2011-07-18
was determined. Concerning the carburization resistance, the amount of C
intruding into
the base metal was evaluated. That is, metal chips were sampled at a 0.5-mm
pitch in the
depth direction from the surface of the specimen having been tested, and the
amount of C
(mass %) at a depth of 0.5 to 1.0 mm and the amount of C (mass %) at a depth
of 1.0 to 1.5
mm were measured by chemical analysis. After the amount of C (mass %) of base
metal
before testing was subtracted, the average value of both the amounts of C was
defined as
the amount of intruding C (mass %) at a depth of 1 mm.
[0057]
The metal pipe that meets the following criteria for all items was accepted:
(1) Oxide scale layer consisting mainly of Cr:
Cr concentration in EDX analysis 50 mass %
(2) Carburization resistance:
Amount of intruding C at 1-mm depth 1.5 mass %
(3) Coking resistance:
Amount of deposited C 3 mg/cm2.
These results are summarized in Table 2.
[0058]
[Table 2]
- 19 -
TABLE 2
Blasting treatment Heat treatment Oxide scale
Specimen la)./er
Carburization Coking
No.
Classification
No. Abrasive Treatment Temperature Gas consisting
resistance resistance
method /time atmosphere mainly of Cr
_
_
1 Silica sand Blasting 1180
C/15min LNG burning 5/5 5/5 5/5 Present invention example
1
2 Cast steel Blasting 1270 C/
3min LNG burning 5/5 5/5 5/5 Present invention example
_
3 Silica sand Blasting 1200 C/ 8min Air
5/5 5/5 5/5 Present invention example
_
4 Alumina Blasting 1230 C/ 6min Air
5/5 5/5 5/5 Present invention example
Zirconia Blasting 1200 C/10min Air 5/5
5/5 5/5 Present invention example
6 2 Cast steel (grit) Blasting 1200 C/15min Air
5/5 5/5 5/5 Present invention example
7 Not performed 1230 C/ 6min Air *3/5
*3/5 *3/5 Comparative example
8 Alumina Blasting 1000 C/15min Air
*2/5 *3/5 *3/5 Comparative example
9 Alumina Blasting None *None
*3/5 *1/5 Comparative example n
Alumina-zirconiaBlasting 1200 C/ 5min Butane 5/5
5/5 5/5 Present invention example 0
mixture burning
N)
-,1
1 1 SiC Grinding 1120 C/30min Butane
5/5 5/5 5/5 Present invention example in
0
burning
0
H
12 3 Metal amorphous Blasting 1060
C/60min Butane 5/5 5/5 5/5 Present
invention example a,
burning
I.)
-
0
13 Cast steel Peening 1170 C/10min Butane
5/5 5/5 5/5 Present invention example H
H
burning _
,
_
0
14 Not performed 1160 C/10min Butane
*2/5 *2/5 *2/5 Comparative example
1
burning
. H
CO
4 BN Blasting 1220 C/ 8min
LNG burning 5/5 5/5 5/5 Present invention example
"
16 5 Alumina Blasting 1170
C/12min LNG burning 5/5 5/5 5/5 Present invention example
_
'
17 6 SiC Wet 1210 C/10min LNG burning 5/5
5/5 5/5 Present invention example
grinding
18 7 SiC-alumina stuck
Friction
nonwoven fabric 8min 1220r/ LNG burning 5/5 5/5 5/5
Present invention example
_ .
19 8 SUS Blasting 1230r/ 10min
LNG burning 5/5 5/5 5/5 Present invention example
_ .
9 Cr Blasting 1230r/
12min
Air 5/5
5/5 5/5 Present invention example
_
21 Silica sand Blasting 1200
C/20min LNG burning *0/5 *0/5 *0/5 Comparative example
22 10 Alumina Blasting 1200
C/10min LNG burning *0/5 *0/5 *0/5 Comparative example
* indicates that performance at which present invention aims is not obtained.
- 20 -
CA 02750014 2011-07-18
[0059]
The numeral in Table 2 denotes the number of specimens meeting the criteria of
the
above items (1), (2) and (3) per five specimens. For example, 3/5 denotes that
three of
five are acceptable. The present invention aims at excellent carburization
resistance and
coking resistance throughout the overall length of the inner surface of metal
pipe.
Therefore, it was determined that each of the criteria of the present
invention is met when
all of the five specimens are acceptable.
[0060]
As shown in Table 2, in examples of Nos. 21 and 22 in which specimen No. 10
that
did not meet the conditions of chemical composition defined in the present
invention was
used, although mechanical treatment was performed, the oxide scale layer
consisting
mainly of Cr could not be obtained, and both of the carburization resistance
and coking
resistance were poor. Of examples using specimen Nos. 2 and 3, in examples of
Nos. 7
and 14 in which mechanical treatment was omitted, some of five specimens did
not meet
the criteria, and the carburization resistance and coking resistance in the
pipe longitudinal
direction were nonuniform. Of examples using specimen No. 2, in examples of
No. 8 in
which the heat treatment temperature was low and No. 9 in which heat treatment
was not
performed, carburization and coking occurred in some of the specimens.
[0061]
On the other hand, for all of the specimens that used the metal pipes of
specimen
Nos. 1 to 9 meeting the conditions of chemical composition defined in the
present
invention, were subjected to mechanical treatment, and were subjected to heat
treatment
under the conditions defined in the present invention, all of the criteria of
the above items
(1), (2) and (3) were met, and the carburization resistance and coking
resistance were
excellent throughout the overall length in the metal pipe longitudinal
direction.
INDUSTRIAL APPLICABILITY
[0062]
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CA 02750014 2011-07-18
I =
According to the present invention, a metal pipe having an oxide scale layer
consisting mainly of Cr formed uniformly on the inner surface of the metal
pipe can be
manufactured, so that the metal pipe is excellent in carburization resistance
and coking
resistance in a carburizing gas environment. For this reason, the metal pipe
obtained by
the present invention is suitably used especially as a pipe used in a
carburizing gas
atmosphere containing hydrocarbon gas, CO gas, and the like, such as a
pyrolytic furnace
pipe, a reforming furnace pipe, a heating furnace pipe, and a heat exchanger
pipe in an oil
refining plant, a petrochemical plant, and the like.
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