U.S. patent application number 13/207535 was filed with the patent office on 2011-12-22 for method for manufacturing metal pipe.
This patent application is currently assigned to SUMITOMO METAL INDUSTRIES, LTD.. Invention is credited to Yoshitaka NISHIYAMA, Masatoshi TOYODA, Masaki UEYAMA.
Application Number | 20110308669 13/207535 |
Document ID | / |
Family ID | 42561875 |
Filed Date | 2011-12-22 |
United States Patent
Application |
20110308669 |
Kind Code |
A1 |
NISHIYAMA; Yoshitaka ; et
al. |
December 22, 2011 |
METHOD FOR MANUFACTURING METAL PIPE
Abstract
In this method, the inner surface of a metal pipe containing, by
mass percent, 20 to 55% of Cr and 20 to 70% of Ni 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.degree.
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. A metal pipe excellent in carburization resistance
and coking resistance in a carburizing gas environment can be
obtained by the method.
Inventors: |
NISHIYAMA; Yoshitaka;
(Hyogo, JP) ; UEYAMA; Masaki; (Hyogo, JP) ;
TOYODA; Masatoshi; (Hyogo, JP) |
Assignee: |
SUMITOMO METAL INDUSTRIES,
LTD.
Osaka
JP
|
Family ID: |
42561875 |
Appl. No.: |
13/207535 |
Filed: |
August 11, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2010/052165 |
Feb 15, 2010 |
|
|
|
13207535 |
|
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Current U.S.
Class: |
148/281 ;
148/282; 148/286 |
Current CPC
Class: |
C22C 19/05 20130101;
C22C 38/02 20130101; C23C 8/10 20130101; C22C 19/052 20130101; C21D
9/08 20130101; C23C 8/02 20130101; C21D 9/14 20130101; C22C 19/055
20130101; C22F 1/10 20130101; C10G 75/00 20130101; C22C 19/053
20130101; C22C 38/40 20130101; C22C 38/04 20130101; C22C 19/058
20130101; C21D 8/105 20130101; C22C 38/001 20130101 |
Class at
Publication: |
148/281 ;
148/286; 148/282 |
International
Class: |
C23C 8/00 20060101
C23C008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 16, 2009 |
JP |
2009-032975 |
Claims
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.degree. 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.
2. The method for manufacturing a metal pipe according to claim 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%, O (oxygen): 0.02% or less, the balance being Fe and
impurities.
3. The method for manufacturing a metal pipe according to claim 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.
4. The method for manufacturing a metal pipe according to claim 1,
wherein the metal pipe has a rib-shaped protrusion on the inner
surface of pipe.
5. The method for manufacturing a metal pipe according to claim 2,
wherein the metal pipe has a rib-shaped protrusion on the inner
surface of pipe.
6. The method for manufacturing a metal pipe according to claim 3,
wherein the metal pipe has a rib-shaped protrusion on the inner
surface of pipe.
Description
[0001] The disclosure of International Application No.
PCT/JP2010/052165 filed Feb. 15, 2010 including specification,
drawings and claims is incorporated herein by reference in its
entirety.
TECHNICAL FIELD
[0002] 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
[0003] 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.
[0004] 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 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.
[0005] 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.degree. 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.degree. 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.
[0006] 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 oxide scale layer consisting mainly of Cr, in
which the Cr content is at least 50% by mass, on the outside
thereof.
[0007] 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 .mu.m depth from the surface, and then the pipe is heated
to a temperature not lower than 1100.degree. C. in an atmosphere
containing less than 5 vol % of oxygen and at least 20 vol % of
nitrogen.
[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] 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 .mu.m
from the surface with cold working and nitrogen permeating heat
treatment, so that the stability of Cr.sub.2O.sub.3 oxide film
produced during the use under an actual operation condition of 750
to 1100.degree. C. can be improved. In this method, the oxide scale
produced in the nitrogen permeating heat treatment is removed, and
a stable Cr.sub.2O.sub.3 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 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
[0018] the inner surface of the metal pipe is subjected to
mechanical treatment;
[0019] the metal pipe is subjected to heat treatment such as to be
held in a temperature range of 1050 to 1270.degree. C. for 0.5 to
60 minutes; and thereby [0020] an oxide scale layer consisting
mainly of Cr is formed on at least the inner surface of the metal
pipe.
[0021] (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%, O (oxygen): 0.02% or
less, and the balance being Fe and impurities.
[0022] (3) The method for manufacturing a metal pipe according to
the above item (2), wherein
[0023] the metal pipe further contains at least one selected from
the elements, by mass percent, given in the following items (a) to
(g): [0024] (a) Cu: 5% or less [0025] (b) Co: 5% or less [0026] (c)
At least one selected from Mo: 3% or less, W: 6% or less, and Ta:
6% or less [0027] (d) One or two selected from Ti: 1% or less and
Nb: 2% or less [0028] (e) At least one selected from B: 0.1% or
less, Zr: 0.1% or less, and Hf: 0.5% or less [0029] (f) At least
one selected from Mg: 0.1% or less, Ca: 0.1% or less, and Al: 1% or
less [0030] (g) At least one selected from Y: 0.15% or less and Ln
group elements: 0.15% or less.
[0031] (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
[0032] 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 invention is
excellent in carburization resistance and coking resistance in a
carburizing gas environment.
MODE FOR CARRYING OUT THE INVENTION
[0033] 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.degree. 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 %".
1. Chemical Composition of Metal Pipe
[0034] 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.
Cr: 20 to 55%
[0035] 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%.
Ni: 20 to 70%
[0036] 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%.
[0037] The starting material for a metal pipe for manufacturing
ethylene (ethylene cracking tube) preferably contains Cr: 20 to 35%
and Ni: 20 to 60%.
[0038] 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%, O (oxygen):
0.02% or less, the balance being Fe and impurities. Hereunder, the
reasons for restricting the content of each element are
described.
[0039] 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.
C: 0.01 to 0.6%
[0040] 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%, 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%.
Si: 0.1 to 5%
[0041] 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%.
Mn: 0.1 to 10%
[0042] 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%.
P: 0.08% or less S: 0.05% or less
[0043] 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 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.
N: 0.001 to 0.25%
[0044] N (Nitrogen) is an element effective in improving the
high-temperature strength. This effect is remarkable when at least
0.001% 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%.
O (Oxygen): 0.02% or less
[0045] O (Oxygen) is an element existing as an impurity. If the O
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 O content is preferably set to
0.02% or less.
[0046] 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.
(a) Cu: 5% or less
[0047] 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.
(b) Co: 5% or less
[0048] 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.
(c) At least one selected from Mo: 3% or less, W: 6% or less, and
Ta: 6% or less
[0049] 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%.
(d) One or two selected from Ti: 1% or less and Nb: 2% or less
[0050] 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 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%.
(e) At least one selected from B: 0.1% or less, Zr: 0.1% or less,
and Hf: 0.5% or less
[0051] 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%.
(f) At least one selected from Mg: 0.1% or less, Ca: 0.1% or less,
and Al: 1% or less
[0052] 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 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%.
(g) At least one selected from Y: 0.15% or less and Ln group
elements: 0.15% or less
[0053] 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.
2. Mechanical Treatment
[0054] 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.
[0055] 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.
[0056] Although the abrasive used for the mechanical treatment is
not limited, a nonmetal such as silica sand (SiO.sub.2), alumina
(Al.sub.2O.sub.3), zirconia (ZrO.sub.2), 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 .mu.m or less, further
preferably 150 .mu.m or less.
[0057] 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.
3. Heat Treatment
[0058] 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.degree.
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.degree. 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.degree. C. The lower limit of heat treatment temperature is
preferably 1120.degree. C., further preferably 1160.degree. C.
[0059] 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.
[0060] 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.
4. Oxide Scale Layer Consisting Mainly of Cr
[0061] 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.
[0062] 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 the oxide scale layer. The determination of element
is made by the fraction of the detected element excluding C
(carbon), O (oxygen), and the like.
5. Others
[0063] 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
[0064] The present invention is explained below more specifically
by way of example. The present invention is not limited to the
example.
[0065] 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 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.times.20 mm square), described
later.
[Table 1]
[0066] 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.degree.
C. for 300 hours in the gas atmosphere of
15%CH.sub.4-3%CO.sub.2-82%H.sub.2 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/cm.sup.2) 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.
[0067] The metal pipe that meets the following criteria for all
items was accepted: [0068] (1) Oxide scale layer consisting mainly
of Cr: [0069] Cr concentration in EDX analysis .gtoreq.50 mass %
[0070] (2) Carburization resistance: [0071] Amount of intruding C
at 1-mm depth .ltoreq.1.5 mass % [0072] (3) Coking resistance:
[0073] Amount of deposited C.ltoreq.3 mg/cm.sup.2.
[0074] These results are summarized in Table 2.
[Table 2]
[0075] 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.
[0076] 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.
[0077] 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.
[0078] Although only some exemplary embodiments of this invention
have been described in detail above, those skilled in the art will
readily appreciate that many modifications are possible in the
exemplary embodiments without materially departing from the novel
teachings and advantages of this invention. Accordingly, all such
modifications are intended to be included within the scope of this
invention.
INDUSTRIAL APPLICABILITY
[0079] 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.
* * * * *