ORIGINAL
DESCRIPTION
AUSTENITIC STAINLESS STEEL TUBE
TECHNICAL FIELD
[oooll
The present invention relates to an austenitic stainless steel tube
excellent in corrosion resistance and, more particularly, to an austenitic
stainless steel tube for power-generating plants, which is suitable as a starting
material used for boilers, pipes, various types of heat exchangers, and the like
for power generating equipment and is excellent in corrosion resistance.
BACKGROUND ART
[00021
In recent years, in various technological fields, the global environment
problem including global warming has become a matter of great concern.
Under the situation, in a power-generating plant, it has become urgent to
reduce the total emissions of carbon dioxide gas, and a newly built plant has
been strongly required to have a facility capable of generating power with high
efficiency. For example, in a thermal power generation boiler, a technique for
increasing the temperature and pressure of steam to generate power with high
efficiency has been adopted as an effective measure. The increase in
temperature and pressure of steam leads to a rise in the tube wall temperature
of superheater tube and reheater tube of boiler, so that the steel tube used for
the boiler is required to have not only high-temperature strength but also
resistance to high-temperature oxidation caused by steam. As a method for
preventing steam oxidation of steel tube, various proposals have been made so
far as described below.
[00031
(A) Technique of performing solution heat treatment after formation of
worked layer
Patent Document 1 discloses an invention relating to a method for
producing a surface fine-grained stainless steel tube, in which method, after an
austenitic stainless steel tube has been subjected to solution treatment, cold
working such as shot blading, grinding, and polishing is performed on the
tube surface, and then predetermined re-solution treatment is performed.
Patent Document 2 discloses an invention relating to a method for thermomechanical
treatment, in which an austenitic stainless steel tube is subjected
to cold working at a working ratio of 20% or higher, and then is subjected to
solid solution heat treatment at a temperature rise rate of 2.9OCle or lower.
[0004]
Patent Document 3 discloses an invention in which a he-grained layer
having a thickness of 30 pm or larger is formed on the inner surface of an
austenitic iron alloy tube by using grains finer than the grain size number of
No. 7, and thereafter cold working at a working ratio of 20% or higher is
performed and recrystallization treatment is performed. Patent Document 4
discloses an invention in which cold working is performed so that the hardness
at a position 20 pm deep from the inner surface of an austenitic stainless steel
tube is Hv320 or higher, and solution treatment is performed.
[00051
(B) Technique of increasing contents of C and N to form fine-grained
micro-structure even after solution heat treatment
Patent Document 5 discloses an invention relating to an austenitic
stainless steel tube which has a fine-grained micro-structure having the grain
size number of No. 7 or higher on the inner surface side of steel tube and
whose fine-grained layer portion has a (C + N) content of 0.15% or more.
(C) Technique of peening to form cold-worked layer
Patent Document 6 discloses an invention in which, after an austenitic
stainless steel tube has been subjected to final heat treatment, peening is
performed by blowing particles onto the inner surface of the steel tube.
Patent Document 7 discloses an invention in which an austenitic stainless
steel tube is subjected to peening treatment under predetermined conditions to
form a worked layer having a thickness of 10 pm or larger. Patent Document
8 discloses an invention in which a tube body taken out of the existing boiler is
subjected to chemical cleaning to descale the inner surface after heat
treatment, and thereafter the inner surface of the tube body is subjected to
shot blasting to form a cold-worked layer.
[00071
(D) Technique for improving adhesiveness of scale
Patent Document 9 discloses an invention relating to a boiler steel tube
excellent in steam oxidation resistance, which is produced by subjecting an
austenitic stainless steel tube containing a rare earth metal to solution
treatment, and has a peened layer formed by blowing particles onto the inner
surface of the steel tube. Patent Document 10 discloses an invention relating
to a steel tube which contains 9 to 28 mass% of Cr, and in which the maximum
height of profile on the inner surface after cold working is 15 pm or larger, and
further the difference in Vickers hardness between the inner surface layer and
the wall thickness central portion of the tube is 100 or more.
[00081
(E) Technique for subjecting steel tube to cold working at high working
ratio
Patent Document 11 discloses an invention in which u l t r a s o n i c ~ a c t
treatment is performed on the inner surface of a heat resistant ferritic steel
tube or heat resistant austenitic steel tube containing 5 to 30 mass% of Cr.
Patent Document 12 diecloses an invention relating to an austenitic stainless
steel tube which contains 16 to 20 wt% of Cr and whose inner surface is cold.
worked, and more particularly relating to an austenitic stainlese steel tube for
boilers, in which the Cr concentration at a position near the inner surface of
the steel tube is 14 wt% or higher, and the hardness at a position 100 pm deep
from the inner surface of the steel tube ia 1.5 times or more of the average
hardness of base metal or Hv300 or higher. Patent Document 13 discloses an
invention relating to a steel tube excellent in steam oxidation resistance,
which has a worked layer having a high hardness of a steel tube containing 8
to 28 mass% of Cr.
[0009]
(F) Technique for improving steam oxidation resistance of heat resistant
ferritic steel
Patent Document 14 discloses an invention relating to a method for
working a heat resistant ferritic steel, in which method a steel containing 9.5
to 15% of Cr is normalized and tempered to make the crystal grains and the
micro-structure uniform, and thereafter a shot-blasted layer is formed by
blowing particles onto the surface.
LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENT
roo 101
Patent Document 1: JP53-114722A
Patent Document 2: JP54-138814.A
Patent Document 3: JP55-58329A
Patent Document 4: JP58-39733A
Patent Document 5: JP58- 133352A
Patent Document 6: JP49-135822A
Patent Document 7: JP62-8930A
Patent Document 8: JP63-54598A
Patent Document 9: JP6-322489A
Patent Document 10: JP2006-307313A
Patent Document 11: JP2004- 132437A
Patent Document 12: WO 20081023410
Patent Document 13: JP2009-68079A
Patent Document 14: JP2002-286236A
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
Loo 111
In the technique of item 0, solution heat treatment is performed at a
high temperature to prevent the decrease in creep rupture strength and to
suppress stress corrosion cracking. However, solution heat treatment, when
performed, removes a strain introduced into the crystal grains on account of
working, and causes recrystallization. Depending on the chemical
composition of steel tube, the crystal grains generated due to solution
treatment grow too large, and it becomes difficult to stably maintain the finegrained
layer on the surface of steel. As a result, the steam oxidation
resistance improved by cold working treatment may be degraded.
Loo 121
In the technique of item (B), although the resistance to steam oxidation
of tube can be somewhat improved, during the usage of boiler, the tube inner
surface layer is sensitized extremely, so that stress corrosion cracking may
occur as the operation stopped. Also, if the metal micro-structure of the whole
steel tube is made too fine, the creep rupture strength decreases, and there
arises a problem of failure to achieve the property of high-temperature
strength, which is inherently important as a high-temperature member.
[00131
Thus, both of the techniques of iteme (A) and (B) involve many problems
in practical use.
[0014]
The technique of item (C) is one of effective techniques used ae steam
oxidation preventive measures in boilers for business in current use, that is,
boilers in which the steam temperature is 566°C (1050°F), and has been
applied to some of boiler steel tubes made of austenitic stainless steel.
However, at a steam temperature of high-efficiency boiler, which is used in
new plants, of for example, 621°C (1150°F), the tube wall temperature of
superheater tube and reheater tube is 50 to 100°C higher than that of the
boiler in current use. There have been studied boilers having operation
conditions such that the steam temperature is further increased to 650°C or
700°C. In such a high temperature region, it is difficult to maintain the
suppressive effect brought about by the technique of item (C) on steam
oxidation for a long period of time.
[00151
According to the technique of item (D), although the protectability of
scale can be extended and maintained, protectability of scale may not be
sufficient considering the service time period of steel tube. In the technique of
item (El, a steel tube can be subjected to cold working at a high working ratio.
However, though being effective in forming initial scale, this technique still
has problems to be solved in maintaining scale for a long period of time. Also,
in the case of ultrasonic impact treatment, this technique has problems
concerning introduction and operation costs of equipment.
[OO16 1
In the technique of item (F), a protection film having high adhesiveness
can be formed on a heat resistant ferritic steel, on which it is difficult to form a
Cr oxide film, so that some improvement in steam oxidation resistance due to
the protection h can be anticipated. However, this technique does not
arrive at a fundamental solution considering long-term use. Also, this
technique does not relate to a heat resistant austenitic steel.
[00171
The present invention has been made to solve the above-described
problems with prior arts, and accordingly an objective thereof is to provide a
steel tube having remarkably excellent steam oxidation resistance.
MEANS FOR SOLVING THE PROBLEMS
[0018l
The present inventors conducted studies earnestly to resolve the
problems inherent in austenitic stainless steel tube for power-generating
plants, and resultantly obtained the findings described below.
[OO19 1
(a) When the surface of a material not subjected to working after
solution heat treatment, that is, a material remaining solution heat-treated is
brought into contact with high-temperature steam, thick steam oxidation scale
called two-layer scale is formed on the surface. In contrast, it is generally
said that when a material whose outer layer only has been subjected to
working after solution heat treatment is brought into contact with hightemperature
steam, extremely thin Cr oxide scale such as Crz03, which is low
in the growth rate, is formed on the surface of steel.
[00201
(b) However, it was found that as a result of performing working on
various steel tubes and observation of oxide scale after long-term steam
oxidation test, even for a steel tube subjected to working after solution heat
treatment, desired Cr oxide scale may not be maintained on the surface of the
steel tube. In a location where Cr oxide scale is not maintained, breakaway
oxidation occurs with this location being a starting point, and the steam
oxidation resistance is deteriorated remarkably.
[00211
(c) For various steel tubes, the micro-structure in the vicinity of outer
layer of the inner surface thereof was analyzed in detail, and resultantly, it
was found that for only the steel tube having a micro-structure involving highdensity
dislocation in the vicinity of outer layer, Cr oxide scale is formed
uniformly, and is also maintained for a long period of time.
[00221
(6) When dislocation is introduced further, the dislocation tends to
migrate such that the elastic energy is decreased. Therefore, when the
working rate is high, the micro-structure is divided into a portion where the
dislocation density is high and a portion where the dislocation density is low
(conversion of dislocation into cells). A high-density dislocation structure is
formed by performing working further thereon and by repeating the
introduction and the recovery of dislocation.
loo231
(e) When the steel tube is held at a temperature of 550°C or higher,
which is the operation temperature of power-generating plant, especially at a
temperature of 600°C or higher, the high-density dislocation structure
disappears, and changes to fine crystal grains having no internal strain, that
is, to a so-called recrystallization structure. Since the recrystallization
structure consists of fine grains of submicron to about several micrometers, the
Cr flux that is necessary for stably maintaining uniformly formed Cr oxide
scale can be assured through the grain boundary diffusion of fine recrystallized
-
9 grains. On the other hand, even if working is performed on the surface, in the
case where the dislocation density is low, a recovery structure, in which only
the rearrangement of dislocation occurs at the operation temperature, is
formed. In this case, the structure grain size at the time of solution
treatment of steel tube is maintained, so that the number of grain boundaries
is small, and the grain boundary diffusion of Cr is insacient. As a result,
the Cr flux becomes insufficient, the Cr oxide scale cannot be maintained, and
breakaway oxidation occurs. That is, in order to maintain Cr oxides, which
are effective for steam oxidation resistance, for a long period of time, it is
necessary to uniformly form a metal micro-structure in which dislocations
exist at a high density, which is to be called a "strongly worked layer" or
nsuper-strongly worked layer", in the outer layer of steel tube.
[0024]
@ Since the driving force of recrystallization is determined by the
dislocation density, the relationship between dislocation density and
recrystallization and the steam oxidation behavior were studied in detail. As
a result, it was found that in judging the metal micro-structure for
maintaining Cr oxides for a long period of time, it is most efficient to use the
average dislocation density (the sum total of dislocation lengths contained per
unit volume), which is obtained by XRD measurement using a Co tube, as an
indicator.
[0026]
(g) Especially in the case where the steel tube is used in a hightemperature
environment in which the temperature reaches about 700°C, it
may be necessary to supply Cr from the deep portion of base metal beyond the
recrystallization structure of the near-surface portion. In this case as well, in
order to assure the Cr flux, it is preferable that the crystal grains of metal
micro-structure, which has not been subjected to working, be made fine in
advance.
[0026]
The present invention was made based on the above-described findings,
and includes austenitic stainless steel tubes described in the following items
(1) to (3).
Loo271
(1) An austenitic stainless steel tube containing, by mass percent, 14 to
28% of Cr and 6 to 30% of Ni, wherein the steel tube has a metal microstructure,
in which an average dislocation density, which is determined by
XRD measurement using a Co tube, is 3.0 x 10'4 lm2 or higher, on the inner
surface of the steel tube.
100281
(2) The austenitic stainless steel tube according to (I), wherein the grain
size of the steel tube is 50 pm or smaller.
[00291
(3) The austenitic stainless steel tube according to (1) or (2), wherein the
steel tube is used as a member for a power-generating plant.
ADVANTAGEOUS EFFECTS OF THE INVENTION
[OO~O]
According to the present invention, even in the case where a steel tube is
used in a high-temperature environment in which the temperature reaches
about 750°C, scale that is excellent in protectability against steam oxidation
can be formed uniformly on the surface of the steel tube, and further, by stably
maintaining the protective scale, the occurrence of breakaway oxidation can be
prevented. Therefore, the austenitic stainless steel tube in accordance with
the present invention is suitable as a steel tube used in power-generating
plants having an operation temperature of 560°C or higher, especially 600°C or
higher.
MODE FOR CARRYING OUT THE INVENTION
[00311
In order to obtain a steel tube in which scale excellent in protectability is
formed uniformly on the inner surface of the tube and is maintained for a long
period of time, it is necessary to form high-density dislocations on the inner
surface of the steel tube. Since the dislocation is introduced @to the inner
grains, strong working is performed by controlling the working conditions. In
the steel tube with high-density dislocations formed on the surface, the highdensity
dislocation structure is recrystallized during the operation of powergenerating
plant, thereby forming fine structure, and many grain boundary
diffusion paths are formed on the surface, thereby assuring Cr flux. As a
result, the Cr-based oxide scale excellent in protectability is maintained for a
long period of time.
Loo321
Usually, the dislocation density of metal micro-structure after solution
heat treatment is about 1012 lm2, whereas the dislocation density of metal
micro-structure in the near-surface portion is raised by performing working.
The steel tube of the present invention has a metal micro-structure, in which
the average dislocation density determined by XRD measurement using a Co
tube is 3.0 x 1014 Im2 or higher, on the inner surface. In particular, in order to
maintain the steam oxidation resistance in a high-temperature environment
for a long period of time, it is preferable that the steel tube have a metal microstructure
having the average dislocation density of 3.5 x 1014 Im2 or higher,
and it is further preferable that the steel tube have a metal micro-structure
having the average dislocation density of 4.0 x 1014 Im2 or higher. Although a
higher average dislocation density is effective, the hardness rises with an
increase in dislocation density. If the hardness is too high, the starting point
of a crack is generated, and the mechanical properties of material may be
degraded. Therefore, the upper limit of the average dislocation density
determined by XRD measurement using a Co tube is 1.0 x 1016 Im2.
100331
The reason why the steel tube of the present invention is defined as
described above will be explained below.
lo0341
First, the method for measuring dislocation density by XRD
measurement using a Co tube in the present invention is explained.
[00351
The 8-28 measurement is made from the specimen surface. From the
obtained X-ray diffraction data, the angle and half-width of diffraction peak
and the half-width of diffraction strength are determined by the Lorentz
function approximation of (ill), (2001, (220) and (311) planes, and the
dislocation density is calculated from the Modified Williams-Hall equation and
the Moaed Warren-Averbach equation. At this time, as anisotropic elastic
constants necessary for contrast factor, the already-known values (C11 = 1.98,
Ciz = 1.25, and CM = 1.22) of Fe-18%Cr-14%Ni steel are used, and the average
contrast factor (Caoo = 0.345) is calculated. The Burgers vector at this time is
0.249 nm from the lattice constant.
E00361
At the mass absorption coefficient and density 7.94 gIcm3 of Fe-18%Cr-
9%Ni-3%Cu steel, the X-rays of a Co tube intrude into a position of about 11
pm at maximum from the surface, so that the average dislocation density down
to about 11 pm in depth from the surface can be obtained.
[0037]
0 When working is performed h m the outer layer, the dislocation density
becomes the highest on the surface. Since the X-rays of a Co tube intrude into
a position of about 11 pm at maximum from the surface of steel tube as
described above, the average dislocation density determined by the Co tube is
the mean value of dislocation densities down to 11 pn from the surface of steel
tube. Since the dislocation density decreases in a generally gradual manner
with the depth, on the assumption that the dislocation density decreases
linearly, if the average dislocation density determined by XRD measurement
using a Co tube is 3.0 x 1014 /m2 or higher, the dislocation density in the region
down to a depth of about 5 ym from the surface is 3.0 x 1014 /m2 or higher. If
the region having a high dislocation density, that is, the strongly worked layer
is formed down to a depth of about 5 pn from the inner surface of steel tube as
described above, the Cr oxide scale is formed uniformly, and the steam
oxidation resistance can be achieved saciently.
[00381
In the case where the usage environment of steel tube reaches a hightemperature
environment exceeding 700°C, it is important to stably maintain
the Cr oxide scale. In order to stably maintain the Cr oxide scale, it is
necessary to make the crystal grain size of base metal in a proper range to
assure Cr flux from the base metal. For this reason, it is preferable that the
grain size of base metal be 50 pm or smaller on the average. With decreasing
grain size, the number of diffusion paths through the grain boundary increases.
Therefore, the crystal grain size of base metal is more preferably 30 pm or
smaller on the average. However, if the crystal grain size is decreased
excessively, the creep properties of steel tube decrease. For this reason, the
crystal grain size is preferably 10 pm or larger on the average. Since the
crystal grain size of base metal is approximately uniform in a region not
subjected to working, the grain size should be measured in the central portion
0 of the wall thickness of steel tube. The crystal grain size can be measured by
using an optical microscope or the like.
[00391
The tube that is the object of the present invention is a heat resistant
austenitic steel tube or the like. Since the scale formed on the inner surface
of tube must consist mainly of Cr oxides, the material of tube is made an
austenitic stainless steel containing 14 to 28 mass% of Cr and 6 to 30 mass% of
Ni.
[OO~O]
As the examples of materials of tube that is the object of the present
invention, austenitic stainless steels such as SUS304, SUS309, SUS310,
SUS316, SUS321, and SUS347 specified in JIS Standard, and steels
equivalent to these steels can be cited. An example of the chemical
composition of the applicable steel type is as described below. In the following
description, the symbol "%" concerning the content of each element means
"percent by mass".
[00411
An austenitic stainless steel consisting of C: 0.2% or less, Si: 2.0% or less,
Mn: 0.1 to 3.0%, Cr: 14 to 28%, and Ni: 6 to 30%, the balance being Fe and
impurities. In lieu of a part of Fe, this steel may contain one or more types of
elements belonging to any group of the fist to fourth groups described below.
First group: Mo: 5% or less, W: 10% or less, Cu: 5% or less, and Ta: 5% or less
Second group: N: 0.3% or less
Third group: V: 1.0% or less, Nb: 1.5% or less, and Ti: 0.5% or less
Fourth group: Ca: 0.02% or less, Mg: 0.02% or less, Al: 0.3% or less, Zr: 0.5% or
less, B: 0.02% or less, and REM: 0.1% or less
[00421
The "impurities" in this description mean components that mixedly
enter on account of various factors in the production process, including raw
materials such as ore or scrap, when a stainless steel is produced on an
industrial scale, the components being allowed to exist in the range such that
they do not exert an adverse influence on the present invention.
Loo431
The advantageous effects of each component of the steel types and the
reasons for restricting the content of each component are described below.
[0044l
C: 0.2% or less
C (carbod is an element effective in assuring the strength and the creep
strength. However, if the C content exceeds 0.2%, undissolved carbides
remain in a solution treated state, and in some cases, C does not contribute to
the improvement in high-temperature strength. Also, C may exert an adverse
influence on the mechanical properties such as toughness. Therefore, the C
content is preferably 0.2% or less. From the viewpoint of deterioration in hot
workability and toughness, the C content is more preferably 0.12% or less. In
order to achieve the above-described effect, 0.01% or more of C is preferably
contained.
Loo451
Si: 2.0% or less
Si (silicon) is an element used as a deoxidizer, and moreover is an
element effective in improving the steam oxidation resistance. However, if
the Si content becomes high, the weldability or hot workability is deteriorated.
Therefore, the Si content is preferably 2.0% or less, and is more preferably
0.8% or less. The above-described effects become remarkable if 0.1% or more
of Si is contained.
lo0461
Mn: 0.1 to 3.0%
Mn (manganese) is, like Si, effective as a deoxidizer. Also, Mn has an
action for suppressing the deterioration in hot workability caused by S
contained as an impurity. In order to achieve the deoxidizing effect and to
improve the hot workability, it is preferable that 0.1% or more of Mn be
contained. However, since the excessive containing of Mn leads to
embrittlement, the upper limit of Mn content is preferably 3.0%, and is more
preferably 2.0%.
[00471
Cr: 14 to 28%
Cr (chromium) is an element effective in contributing to hightemperature
strength, and improving the oxidation resistance and corrosion
resistance by the formation of scale consisting mainly of Cr oxides on the inner
surface of steel tube. In order to achieve these effects, 14% or more of Cr
must be contained. However, if Cr is contained excessively, the toughness
and hot workability may be deteriorated. Therefore, the upper limit of Cr
content is 28%. The preferable lower limit of Cr content is 15%, and the
preferable upper limit thereof is 26%. Also, in the case where the acid
resistance is improved, the lower limit of Cr content is more preferably 16%.
[0048]
Ni: 6 to 30%
Ni (nickel) is an element necessary for stabilizing the austenitic
structure and improving the creep strength. Therefore, 6% or more of Ni
must be contained. However, even if Ni is contained in large amounts, the
effects are saturated, and merely the cost increases. Therefore, the upper
limit of Ni content is 30%. The preferable lower limit thereof is 7%. The
upper limit thereof is preferably 25%, more preferably 21%.
[00491
First group:
Mo: 5% or less
W: 10% or less
Cu: 5% or less
Ta: 5% or less
Mo (molybdenum), W (tungsten), Cu (copper), and Ta (tantalum) may be
contained because they enhance the high-temperature strength of steel.
However, if these elements are contained in large amounts, the weldability
and workability are degraded. If these elements are contained, the upper
limits of Mo content, Cu content, and Ta content are 5%, respectively, and the
upper limit of W content is 10%. The above-described effects become
remarkable if 0.1% or more of at least any one type of these elements is
contained.
[0050]
Second group:
N: 0.3% or less
N (nitrogen) contributes to solid-solution strengthening of steel, and has
an effect of strengthening a steel by precipitation strengthening action in
combination with other elements. However, if the N content is excessive, the
ductility and weldability may be deteriorated. Therefore, if N is contained,
the content thereof is 0.3% or less. In the case where it is desired to achieve
the above-described effects, 0.005% or more of N should be contained.
[00511
Third group:
V: 1.0% or less
Nb: 1.5% or less
Ti: 0.5% or less
Any of V (vanadium), Nb (niobium) and Ti (titanium) is an element that
combines with carbon and nitrogen to form carbo-nitrides, thereby
contributing to precipitation strengthening, so that these elements can be
contained as necessary. However, if the contents of these elements are
excessive, the workability of steel may be degraded. Therefore, it is
preferable that the V content is 1.0% or less, the Nb content is 1.5% or less,
and the Ti content is 0.5% or less. In the case where it is desired to achieve
the above-described effect, 0.01% or more of one or more types selected from
these elements is preferably contained.
[00621
Fourth group:
Ca: 0.02% or less
Mg: 0.02% or less
Al: 0.3% or less
Zr: 0.5% or less
B: 0.02% or less
REM: 0.1% or less
Any of Ca (calcium), Mg (magnesium), A1 (aluminum), Zr (zirconium), B
(boron) and REM (rare-earth metal, La, Ce, Y, Pr, Nd, etc.) has an effect of
improving the strength, workability, and steam oxidation resistance, so that
these elements can be contained as necessary. However, if the contents of
these elements are excessive, the workability or weldability may be degraded.
Therefore, it is preferable that the Ca content is 0.02% or less, the Mg content
is 0.02% or less, the A1 content is 0.3% or less, the Zr content is 0.5% or less,
the B content is 0.02% or less, and the REM content is 0.1% or less, and also it
is preferable that the total content of these elements is 0.8% or less. In the
case where it is desired to achieve the above-described effects, 0.0001% or more
of one or more types selected from these elements is preferably contained.
[00531
REM is the general term of a total of 17 elements consisting of Sc
(scandium), Y (yttrium), and lanthanoids, and the REM content means the
total content of these 17 elements.
lo0541
The method for producing the steel tube according to the present
invention is not especially limited, and a usual melting process, casting process,
and tube-making process can be employed. That is, for example, a steel
having the above-described chemical composition is melted and cast, thereafter
being turned to a material tube by various hot rolling process (extrusion tubemaking
process, drawing tube-making process, Mannesmann tube-making
process, etc.), and this material tube is subjected to softening heat treatment
as necessary. The hot material tube is turned to a tube having a desired
shape by one of various cold working processes such as a cold rolling process or
a cold drawing process, and thereafter a worked layer is formed on the inner
surface of steel tube. Also, after the tube has been made by cold working, to
homogenize the crystal grains, a strongly worked layer may be formed on the
inner surface of steel tube after the tube has been subjected to solution heat
treatment.
[00561
The method for forming the strongly worked layer on the inner surface
of steel tube is not especially limited as far as controlled blows or impacts are
given to the inner surface of steel tube. For example, publicly known
processes such as shot peening, shot blast, shot working, sandblast, sand
working, air blast, water jet, ultrasonic waves can be employed by controlling
the energy of blow or impact. The formation of strongly worked layer can be
realized by controlling the pressure of compressed air at the time of injection,
the injection amount of particles, and the shape of injection nozzle.
[0056]
The material quality, shape, size, and the like of blowing particles are
not limited. As the material quality, for example, steel, cast steel, stainless
steel, glass, quartz sand, alumina, amorphous alloy, zirconia, or the like can be
used. Also, as the shape, for example, spherical shape, cut wire, round cut
wire, grid, or the like can be used. Concerning the size, the optimum size
should be adopted to enhance the blow or impact energy because the size
exerts an influence on the energy. The particles may be blown by using
compressed air, centrifugal force created by an impeller, high-pressure water,
ultrasonic waves, or the like. When the particles are not used, the metalmade
member may be directly struck with ultrasonic waves or the like.
[00571
In the case of shot working, shot working only needs be performed by
selecting, as appropriate, the shot conditions and nozzle shape such that the
inner surface undergoes working stronger than before. The shot conditions
are not especially limited but, for example, the injection speed of shot particles
is preferably 50 m/s or higher. Also, with a decrease in shot particle size, the
injection speed can be increased, and the region near the inner surface can
undergo great working. In particular, if particles having a grain size of 0.5
mm or smaller on the average are blown under control, the structure of higherdensity
dislocation can be formed uniformly over the overall length in the
longitudinal direction of the inner surface of steel tube.
[0058]
By using these methods, a metal micro-structure satisfying the abovedescribed
conditions only needs to be formed on the inner surface of steel tube
by controlling various conditions.
EXAMPLE 1
[00591
Steel tubes each having the chemical composition given in Table 1 were
prepared under various conditions, and the crystal grain size of the steel tube
itself and the average dislocation density determined by XRD using a Co tube
on the inner surface side of the steel tube were measured by the methods
described below. Further, a steam oxidation test was conducted.
[0060]
[Table 11
Table 1 I Steel 1 Chemical composition (in mass%, balance: Fe and impurities) I
[006 11
For steel Nos. 1 to 10, a 180-kg ingot was prepared by vacuum melting
in a laboratory, and a material tube (outside diameter: 110 mm, wall
thickness: 12 mm) was produced by hot forging and hot extrusion. For steel
No. 1, surface scale was removed after hot extrusion, and thereafter solution
heat treatment was performed. For steel Nos. 2 to 10, a steel tube (outside
diameter: 50.8 mm, wall thickness: 8 mm) was produced by cold rolling, and
thereafter solution heat treatment was performed. For steel No. 2, steel tubes
having four kinds of crystal grain size were prepared by controlling the
temperature and time of solution heat treatment. On the inner surfaces of
No.
1
2
3
4
6
6
7
8
9
10
* indicates that conditions do not satisfy those defined by the present invention.
c
0.09
0.08
0.07
0.02
0.07
0.04
0.06
0.07
0.04
0.07
Si
0.4
0.2
0.4
0.3
0.3
0.2
0.5
1.2
0.5
0.1
Mn
1.6
0.8
1.2
1.6
1.2
1.1
0.2
1.5
0.6
1.0
Cr
18.3
18.6
26.0
18.6
13.6 *
19.1
19.8
18.1
18.3
20.5
Ni
11.4
9.0
20.0
12.8
9.6
9.2
11.6
8.7
9.6
28.6
Nb
0.9
0.5
0.6
0.7
Cu:2.9, N:0.1
N: 0.24
V:O.3, Mo:2.1, B:0.002, N:0.25
M0:O.l
W: 1.5, Ca : 0.002
Ti: 0.12, Mg: 0.003, Nd: 0.021
Al: 0.05, B: 0.002, Zr : 0.02
Ta :0.66, La: 0.008, Ce: 0.024
these steel tubes, surface treatment was performed under the conditions given
in Table 2 to prepare specimens. In the case of shot working, the depth of the
strongly worked layer was regulated by changing the injection pressure,
injection amount, injection angle, nozzle shape, and the like.
[00621
[Table 21
Table 2
indicates that conditions do not satisfy those defined by cliam 1 of the preaent invention.
** indicates that conditions do not satisfy those defined by cliam 2 of the present invention.
#1 indicates a crystal grain size of steel pipe.
#2 indicates a result of steam oxidation test at each temperatures for 1000 hours.
100631
[Crystal grain size of steel tube]
A small-piece test specimen was cut out of each specimen, a central
portion of steel tube wall thickness of the surface corresponding to the steel
tube cross section of each specimen was observed by using an optical
micrometer in four visual fields, and the crystal grain size of the steel tube
itself (base metal) was measured. The mean value of the measured crystal
grain sizes is given in Table 2.
[006dl
Dislocation density]
A small-piece test specimen was cut out of each specimen, and XRD
measurement using a Co tube was made from the inner surface side of each
teet specimen. Based on the obtained *action strength, the average
dislocation density was determined. The determined value is given in Table 2.
[0065]
[Steam oxidation test]
A strip-shaped test specimen with thickness of 2 mm, width of 10 mm,
and length of 25 mm was cut out of each specimen so that the tube inner
surface forms a part of test specimen surface. This test specimen was held in
a form of being hung down from a jig, being inserted in a horizontal tube
heating furnace, and an oxidation test was conducted in a steam atmosphere of
dissolved oxygen amount of 100 ppb at temperatures of 660°C and 750°C for
1000 hours. The test specimen taken out after being furnace-cooled was
embedded in a resin, and the cross section thereof was cut and mirror-polished.
Thereafter, the cross section of oxide scale formed on the inner surface of steel
tube was observed under an optical microscope over the overall length of 8 mm
of the test specimen width of 10 mm excluding 1 mm at each end. A location
where the scale thickness exceeded 10 ym was defined as breakaway oxidation,
.I
a the sum total of lengths of breakaway oxidation occurrence was determined,
and the value divided by the measurement length of 8 mm was determined as
a breakaway oxidation coverage (%). This value is given in Table 2. The test
specimen with a breakaway oxidation coverage of 15% or lese passed an
inspection.
[00661
Referring to Table 2, for the steels of test Nos. 1 to 3, 6 to 11, and 13 to
17, since the requirements of the invention according to claim 1 was satisfied,
those steels had the breakaway oxidation film with the coverage of 15% or less
in the steam oxidation test at both temperatures of 650°C and 750°C and
showed the property of excellent steam oxidation resistance. In these test
numbers, the steels of test Nos. 6 and 7, in which the base metal crystal grain
size was 50 pm or larger, falling outside the defined range of the invention
according to claim 2, had the breakaway oxidation film with the coverage of
relatively high as compared with that of test No. 3 that used the same steel No.
2 and satisfied the requirement of the invention according to claim 2, although
the coverage is within the acceptable range. On the other hand, the steel of
test No. 4 in which the dislocation density was 3.0 x 1014 Im2 or lower, falling
outside the defined range of the present invention, had the breakaway
oxidation film with the coverage of more than 16%, which is unacceptable.
Also, the steel of test No. 12 which used steel No. 5 in which the Cr content
was lower than the defined lower limit, had the breakaway oxidation film with
the coverage of loo%, which is unacceptable, in the steam oxidation test at
both temperatures of 650°C and 750°C.
INDUSTRIAL APPLICABILITY
[00671
According to the present invention, even in the case where a steel tube is
used in a high-temperature environment in which the temperature reaches
about 750°C, scale that is excellent in protectability against steam oxidation
can be formed uniformly on the surface of the steel tube, and further, by stably
maintaining the protective scale, the occurrence of breakaway oxidation can be
prevented. Therefore, the austenitic stainless steel tube according to the
present invention is suitable as a steel tube used in powergenerating plants
having an operation temperature of 660°C or higher, especially 600°C or

We claim:
1. An austenitic stainless steel tube containing, by mass percent, 14 to
28% of Cr and 6 to 30% of Ni, wherein the steel tube has a metal microstructure,
in which an average dislocation density, which is determined by
XRD measurement using a Co tube, is 3.0 x 1014 Im2 or higher, on the inner
surface of the steel tube.
2. The austenitic stainless steel tube according to claim 1, wherein the
grain size of the steel tube is 50 p or smaller.
3. The austenitic stainless steel tube according to claim 1 or 2, wherein
the steel tube is used as a member for a power-generating plant.
Dated this 6' day of December, 201 3.
Ni on Steel & S itomo Metal Corporation % fl
LJ Q/v/&%h
(Dev Robinson)
of Amarchand & Mangaldas &
Suresh A. Shroff & Co.
Attorneys for the Applicant