U.S. patent application number 14/115570 was filed with the patent office on 2014-06-05 for heat-resistant austenitic stainless steel having excellent cyclic oxidation resistance.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). The applicant listed for this patent is Kazuki Furuya, Takeo Miyamura, Shigenobu Namba. Invention is credited to Kazuki Furuya, Takeo Miyamura, Shigenobu Namba.
Application Number | 20140154128 14/115570 |
Document ID | / |
Family ID | 47139289 |
Filed Date | 2014-06-05 |
United States Patent
Application |
20140154128 |
Kind Code |
A1 |
Miyamura; Takeo ; et
al. |
June 5, 2014 |
HEAT-RESISTANT AUSTENITIC STAINLESS STEEL HAVING EXCELLENT CYCLIC
OXIDATION RESISTANCE
Abstract
A heat-resistant austenitic stainless steel comprising C: 0.05
to 0.2%, Si: 0.1 to 1%, Mn: 0.1 to 2.5%, Cu: 1 to 4%, Ni: 7 to 12%,
Cr: 16 to 20%, Nb: 0.1 to 0.6%, Zr: 0.05 to 0.4%, Ce: 0.005 to
0.1%, Ti: 0.1 to 0.6%, B: 0.0005 to 0.005%, N: 0.001 to 0.15%, S:
0.005% or less (not including 0%), and P: 0.05% or less (not
including 0%), with the balance of iron and unavoidable
impurities.
Inventors: |
Miyamura; Takeo; (Kobe-shi,
JP) ; Namba; Shigenobu; (Kobe-shi, JP) ;
Furuya; Kazuki; (Kobe-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Miyamura; Takeo
Namba; Shigenobu
Furuya; Kazuki |
Kobe-shi
Kobe-shi
Kobe-shi |
|
JP
JP
JP |
|
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi, Hyogo
JP
|
Family ID: |
47139289 |
Appl. No.: |
14/115570 |
Filed: |
May 10, 2012 |
PCT Filed: |
May 10, 2012 |
PCT NO: |
PCT/JP2012/062039 |
371 Date: |
January 29, 2014 |
Current U.S.
Class: |
420/40 |
Current CPC
Class: |
C22C 38/04 20130101;
C22C 38/02 20130101; C22C 38/58 20130101; C22C 38/44 20130101; C22C
38/005 20130101; C22C 38/001 20130101; C21D 6/004 20130101; C22C
38/50 20130101; C22C 38/002 20130101; C22C 38/48 20130101; C22C
38/54 20130101; C22C 38/42 20130101 |
Class at
Publication: |
420/40 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/50 20060101 C22C038/50; C22C 38/48 20060101
C22C038/48; C22C 38/00 20060101 C22C038/00; C22C 38/42 20060101
C22C038/42; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/54 20060101 C22C038/54; C22C 38/44 20060101
C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
May 11, 2011 |
JP |
2011-106588 |
Sep 16, 2011 |
JP |
2011-203604 |
Mar 5, 2012 |
JP |
2012-048357 |
Claims
1. A heat-resistant austenitic stainless steel, comprising: C in a
content by mass of from 0.05 to 0.2%, Si in a content by mass of
from 0.1 to 1%, Mn in a content by mass of from 0.1 to 2.5%, Cu in
a content by mass of from 1 to 4%, Ni in a content by mass of from
7 to 12%, Cr in a content by mass of from 16 to 20%, Nb in a
content by mass of from 0.1 to 0.6%, Zr in a content by mass of
from 0.05 to 0.4%, Ce in a content by mass of from 0.005 to 0.1%,
Ti in a content by mass of from 0.1 to 0.6%, B in a content by mass
of from 0.0005 to 0.005%, N in a content by mass of from 0.001 to
0.15%, S in a content by mass of greater than 0% and no more than
0.005%, P in a content by mass of greater than 0% and no more than
0.05%, and iron.
2. The heat-resistant austenitic stainless steel according to claim
1, further comprising: at least one element selected from the group
consisting of; Mo in a content by mass of greater than 0% and no
more than 3%, W in a content by mass of greater than 0% and no more
than 5%, Ca in a content by mass of greater than 0% and no more
than 0.005%, and Mg in a content by mass of greater than 0% and no
more than 0.005%.
3. The heat-resistant austenitic stainless steel according to claim
1, wherein a crystal grain size number of a microstructure is 6 or
more and less than 12 in terms of ASTM grain size number.
4. The heat-resistant austenitic stainless steel according to claim
2, wherein a crystal grain size number of a microstructure is 6 or
more and less than 12 in terms of ASTM grain size number.
Description
TECHNICAL FIELD
[0001] The present invention concerns a heat-resistant austenitic
stainless steel used preferably as materials of heat transfer tubes
such as for boilers and it particularly relates to a heat-resistant
austenitic stainless steel having excellent cyclic oxidation
resistance.
BACKGROUND ART
[0002] In recent years, for suppressing emission of carbon dioxide
as greenhouse gas, improvement in the efficiency of coal-fired
thermal power generation plants has been proceeded. For improving
the power generation efficiency, it is effective to increase the
temperature and the pressure of steam in boilers and, as the
materials for heat transfer pipes of boilers, those having
excellent high temperature strength and oxidation resistance are
used. Further, austenitic stainless steels are used generally as
such materials having excellent properties.
[0003] The oxidation resistance required for the materials of heat
transfer tubes include cyclic oxidation resistance. Since boilers
are started and stopped repeatedly, oxides formed on the surface of
the steel tubes (heat transfer tubes) are exposed to cyclic
oxidation circumstance undergoing high temperature circumstance and
low temperature circumstance alternately. In such a circumstance,
oxides are peeled due to the difference of thermal expansion
coefficient to the matrix to result in a problem of insufficiency
of strength caused by further development of oxidation and weight
loss (thinning) due to peeling off scale. The property less causing
such phenomenon (referred to as "cyclic oxidation resistance" in
the invention) even under such circumstance is required.
[0004] As heat-resistant materials having excellent oxidation
resistance in a wide sense including those properties other than
the cyclic oxidation resistance, 25Cr-20Ni austenitic stainless
steel (SUS310S) has been known. However, since the stainless steel
contains a great amount of expensive Ni, it takes a high cost. In
view of the above, it is an important factor for the material of
the heat transfer tubes of the boilers to use 18Cr-8Ni austenitic
stainless steel (SUS304) containing lower amount of Ni content and
having high temperature strength and satisfactory corrosion
resistance as a basic component.
[0005] SUS321 system has been known as the composition similar to
that of 18Cr-8Ni austenitic stainless steel and KA-SUS321J2HTB has
been known as the stainless steel for boilers having a
specification for thermal power station according to SUS321 system
has been known. As a technology for improving the oxidation
resistance in a wide sense includes (1) surface treatment such as
shot peening or mechanical polishing, (2) addition of Al, Si and
REM (rare earth metal) including Ce and La which are alloying
elements for improving the corrosion resistance, and (3) refining
of crystal grains. Technologies relating to austenitic stainless
steels using Ti compounds as precipitation hardening mechanism have
been proposed, for example, in Patent Literatures 1 and 2.
[0006] Among the technologies described above, the Patent
Literature 1 discloses improvement of the oxidation resistance by
the addition of Al that contributes to the improvement of the
corrosion resistance and by the promotion of the formation of a
Cr.sub.2O.sub.3 layer by surface polishing. Further, as a
substitute for obtaining the same effect as the surface polishing
treatment, the literature shows that the oxidation resistance can
be improved also by increasing the total amount of Al and Si to 4%
or more and, in addition, adding REM such as Ce, Y, and La, or
Ca.
[0007] However, while the effect of retarding the growing rate of
oxides formed on the surface of steel tubes can be expected, for
example, by the addition of Al and Si or formation of the
Cr.sub.2O.sub.3 layer, formation of the oxides cannot be prevented
completely and provision of satisfactory cyclic oxidation
resistance cannot be expected. Moreover, the steel containing Al
also has a problem that surface defects tend to be caused during
manufacture of tubes.
[0008] While the Patent Literature 2 discloses addition of Ce, La,
and Hf for improving the oxidation resistance, it is expected that
the cyclic oxidation resistance is low in the same manner as the
technologies described above and the technology is not based on the
recognition of the improvement for the cyclic oxidation
resistance.
[0009] As the technology of improving the cyclic oxidation
resistance, a technology as in the Patent Literature 3 has also
been proposed. However, since a great amount of Al and Si is
contained in this technology, this involves a problem of resulting
surface defects of steel tubes or resulting in embrittlement after
heat treatment for long time. In addition, while the literature
shows that the addition of REM such as La and Ce including Y
exhibits an effect of improving the scale adhesion, this effect is
not enough and the technology is not intended for recognition for
the improvement of the cyclic oxidation resistance.
[0010] On the other hand, as a technology of improving the
oxidation resistance of the austenitic stainless steel for boilers,
a technology as in the Patent Literature 4 has also been proposed.
This technology concerns "KA-SUS304J1HTB" component systems
containing Nb and N for precipitation and solution hardening. Also
in this technology, about 0.002 to 0.05% of Ti is added with an aim
of forming oxide type inclusions. However, in the steel using
precipitation of a Ti compound as the hardening mechanism such as
KA-SUS321J2HTB, it is expected that high temperature strength
cannot be ensured unless Ti is added by about 0.1 to 0.25%.
Further, this technology is not intended for the improvement of the
cyclic oxidation resistance and it is expected that the cyclic
oxidation resistance is low.
[0011] In the technology of Patent Literature 5, oxidation
resistance is improved by addition of REM and peening of blowing
particles. However, peening results in another problem of
increasing the cost due to increase in the manufacturing process
and since the technology is not intended for the improvement of the
cyclic oxidation resistance, it is expected that the cyclic
oxidation resistance is low.
CITATION LIST
Patent Literature
[0012] [Patent Literature 1] JP-A No. 2004-43903 [0013] [Patent
Literature 2] JP-A No. Hei 9 (1997)-165655 [0014] [Patent
Literature 3] JP-A No. Hei 8 (1996)-337850 [0015] [Patent
Literature 4] JP-A No. 2003-268503 [0016] [Patent Literature 2]
JP-A No. Hei 6 (1994)-322489
SUMMARY OF THE INVENTION
Technical Problem
[0017] The present invention has been accomplished in view of such
a situation and it intends to provide a heat-resistant austenitic
stainless steel having excellent cyclic oxidation resistance,
having a chemical composition comparable with that of 18Cr-8Ni
austenitic stainless steels in view of Ni and Cr content, not
depending on the addition of Al or Si and surface treatment, with
less peeling off surface oxides in cyclic oxidation circumstance,
and causing less weight loss.
Solution to Problem
[0018] A heat-resistant austenitic stainless steel of the invention
capable of solving the problem described above comprises; C: 0.05
to 0.2% (means mass % for chemical composition here and
hereinafter), Si: 0.1 to 1%, Mn: 0.1 to 2.5%, Cu: 1 to 4%, Ni: 7 to
12%, Cr: 16 to 20%, Nb: 0.1 to 0.6%, Zr: 0.05 to 0.4%, Ce: 0.005 to
0.1%, Ti: 0.1 to 0.6%, B: 0.0005 to 0.005%, N: 0.001 to 0.15%, S:
0.005% or less (not including 0%) and P: 0.05% or less (not
including 0%) respectively, with the balance of iron and
unavoidable impurities.
[0019] It is also useful that the heat-resistant austenitic
stainless steel of the invention further includes optionally, Mo:
3% or less (not including 0%) and/or W: 5% or less (not including
0%), and the high temperature strength is further improved by
containing of such elements.
[0020] In the heat-resistant austenitic stainless steel of the
invention, the yield of Ce can be improved and the toughness can be
improved by further addition optionally, Ca: 0.005% or less (not
including 0%) and/or Mg: 0.005% or less (not including 0%).
[0021] The heat-resistant austenitic stainless steel improved the
cyclic oxidation resistance can be obtained by controlling the
chemical composition as described above. Further, higher cyclic
oxidation resistance can be obtained and, in addition, the property
can be provided stably by refining the crystal grain size of a
metal structure to 6 or more and less than 12 in terms of the ASTM
grain size number.
Advantageous Effects of Invention
[0022] In the heat-resistant austenitic stainless steel of the
invention, since progress of oxidation due to peeling off scale and
accompanying weight loss of the steel are less caused even in a
cyclic oxidation circumstance, the power generation efficiency due
to increase in the steam temperature can be improved by using the
material as the heat transfer tube for coal-fired power plants and
the service life of the heat transfer tube can be made longer
compared with conventional materials, to reduce the maintenance
cost. Further, when the material is used as the heat transfer tube,
since less scale is peeled off, scattering of the scale in the
inside can be suppressed to decrease damages of the turbine.
DESCRIPTION OF EMBODIMENTS
[0023] The present inventors have made studies from various
approaches in order to realize an austenitic stainless steel
improved for cyclic oxidation resistance while maintaining
necessary high temperature strength. As a result, it has been found
that an outstandingly excellent cyclic oxidation resistance can be
provided by containing of a predetermined amount of Zr and Ce to
stainless steel having a chemical composition comparable with that
of 18Cr-8Ni austenitic stainless steel, in view of the content of
Ni and Cr to accomplish the present invention.
[0024] The heat-resistant austenitic stainless steel of the
invention has a feature of containing a predetermined amount of Zr
and Ce to the chemical composition comparable with that of the
18Cr-8Ni austenitic stainless steel in view of the content of Ni
and Cr, and the reason for defining the range of the content of Zr
and Ce is as described below.
[0025] Zr and Ce exhibit an effect of suppressing peeling off
oxides due to a synergistic effect of them. For providing such an
effect, 0.05% or more of Zr has to be contained. However, if the Zr
content is excessive, since coarse inclusions are formed to worsen
the surface property and the toughness of the steel material (or
steel tube), it is necessary to define the upper limit to 0.4% or
less. Further, Ce has to be contained 0.005% or more for providing
the effect. If the Ce content is excessive to exceed 0.1%, this
increases cost from an economical point of view.
[0026] Since the addition of Zr and Ce increases the cost of the
steel material, an appropriate content may be set while considering
the balance between the effect obtained by containment and increase
of the cost. From such a view point, a preferred lower limit of the
Zr content is 0.10% or more (more preferably, 0.15% or more) and a
preferred upper limit is 0.3% or less (more preferably, 0.25% or
less). Further, a preferred lower limit of the Ce content is 0.01%
or more (more preferably, 0.015% or more) and a preferred upper
limit is 0.05% or less (more preferably 0.03% or less).
[0027] While pure Ce may be added as the Ce source, necessary Ce
content can be added also by using a Ce-containing master alloy or
a Ce-containing misch metal prepared. If La, Nd, Pr, etc. to be
contained in the misch metal are contained in the steel at a
concentration lower than that of Ce in the steel, they cause no
problem, and handling during melting operation can be simplified by
using the master alloy or misch metal compared with easily
oxidizable pure Ce.
[0028] Among the prior art, the Patent Literatures 1, 3, and 5
disclose that adhesion of the oxides is improved by the addition of
REM including Y, La, and Ce but each of such disclosures is based
on the assumption of sole addition of REM and they do not disclose
at all the synergistic effect obtained by addition of Ce together
with Zr.
[0029] Further, the Patent Literature 2 also discloses that Zr and
Ce can be contained in combination. However, each of them is not an
essential alloy component in this technology and added optionally
also including the case with no addition. Particularly, Zr is
contained by a content less than the range defined in the invention
while intending to strengthen the grain boundary and improve the
creep ductility.
[0030] The heat-resistant austenitic stainless steel of the
invention has a chemical composition comparable with that of
18Cr-8Ni austenitic stainless steel in view of the content of Ni
and Cr. The chemical composition for each of the elements other
than Zr and Ce (C, Si, Mn, Cu, Ni, Cr. Nb, Ti, B, N, S, and P)
should also be controlled appropriately. The effect and the reason
for defining the range of such elements are as described below.
[C: 0.05 to 0.2%]
[0031] C is an element of forming carbides in a high temperature
service circumstance and having an effect of improving high
temperature strength and creep strength necessary for the heat
transfer tube, and it should be contained 0.05% or more in order to
ensure the amount of carbide precipitates that works as hardening
particles. However, when C is added excessively and its content is
more than 0.2%, it goes beyond the solid solubility limit to form
coarse carbides and no further hardening can be obtained. A
preferred lower limit of the C content is 0.07% or more (more
preferably, 0.09% or more) and a preferred upper limit is 0.18% or
less (more preferably, 0.15% or less).
[Si: 0.1 to 1%]
[0032] Si is an element having a deoxidation effect in molten
steels. Further, it acts effectively for the improvement of the
oxidation resistance if it is contained even in a small amount. For
providing such effects, it is necessary that the Si content is 0.1%
or more. However, if Si is added excessively and its content is
more than 1%, this results in formation of a phase to embrittle the
steel (a embrittlement). A preferred lower limit of the Si content
is 0.2% or more (more preferably, 0.3% or more) and a preferred
upper limit is 0.9% or less (more preferably, 0.8% or less).
[Mn: 0.1 to 2.5%]
[0033] Mn is an element having a deoxidation effect in molten
steels in the same manner as Si. Further, it has an effect of
stabilizing austenite. For providing such effects, it is necessary
that the Mn content is 0.1% or more. However, if Mn is added
excessively and its content is more than 2.5%, this deteriorates
hot workability. A preferred lower limit of the Mn content is 0.2%
or more (more preferably, 0.3% or more) and a preferred upper limit
is 2.0% or less (more preferably, 1.8% or less).
[Cu: 1 to 4%]
[0034] Cu is an element of forming coherent precipitates
(precipitates in which the atomic arrangement is continuous with
that of matrix) in steels and remarkably improving high temperature
creep strength which is one of principal hardening mechanisms in
stainless steels. In order to provide the effect, it is necessary
that Cu content is 1% or more. However, if Cu is added excessively
and its content is more than 4%, the effect is saturated. A
preferred lower limit of the Cu content is 2.0% or more (more
preferably, 2.5% or more) and a preferred upper limit is 3.7% or
less (more preferably, 3.5% or less).
[Ni: 7 to 12%]
[0035] Ni has an effect of stabilizing austenite and it is
necessary to be contained 7% or more in order to maintain an
austenitic phase. However, if Ni is added excessively and its
content is more than 12%, this increases the cost. A preferred
lower limit of the Ni content is 7.5% or more (more preferably,
8.0% or more) and a preferred upper limit is 11.5% or less (more
preferably, 11.0% or less).
[Cr: 16 to 20%]
[0036] Cr is an essential element for providing corrosion
resistance as a stainless steel. For providing such an effect, it
is necessary that Cr is contained 16% or more. However, if Cr is
added excessively and its content is more than 20%, a ferrite phase
which lowers the high temperature strength increases. A preferred
lower limit of the Cr content is 16.5% or more (more preferably,
17.0% or more) and a preferred upper limit is 19.5% or less (more
preferably, 19.0% or less).
[Nb: 0.1 to 0.6%]
[0037] Nb is an effective element to the improvement of the high
temperature strength by precipitation of carbonitrides (carbides,
nitrides, or carbonitrides) and, further, provides an effect of
improving the corrosion resistance as a subsidiary effect by
suppressing growing of the crystal grains and promoting diffusion
of Cr by means of precipitates. In order to ensure a required
precipitation amount, it is necessary that Nb is contained 0.1% or
more. However, if Nb is added excessively and its content is more
than 0.6%, precipitates become coarser to lower the toughness. A
preferred limit of the Nb content is 0.12% or more (more
preferably, 0.15% or more) and a preferred upper limit is 0.5% or
less (more preferably, 0.3% or less).
[Ti: 0.1 to 0.6%]
[0038] Ti also provides the same effect as Nb and, when it is added
with Nb and Zr, precipitates are further stabilized, which is also
effective for maintaining high temperature strength for a long
time. In order to provide such an effect effectively, it is
necessary that the Ti content is 0.1% or more. However, if the Ti
content becomes excessive, precipitates become coarser to lower the
toughness in the same manner as Nb, so that the Ti content should
be 0.6% or less. A preferred lower limit of the Ti content is 0.12%
or more (more preferably, 0.15% or more) and a preferred upper
limit is 0.5% or less (more preferably, 0.3% or less).
[B: 0.0005 to 0.005%]
[0039] B has an effect of promoting formation of M.sub.23C.sub.6
type carbides (M is carbide-forming elements) as one of principal
hardening mechanisms by being solved into steel. In order to
provide such an effect efficiently, it is necessary that the B
content is 0.0005% or more. However, if the B content is excessive,
since this deteriorates the hot workability and the weldability, it
should be 0.005% or less. A preferred lower limit of the B content
is 0.001% or more (more preferably, 0.0012% or more) and a
preferred upper limit is 0.004% or less (more preferably, 0.003% or
less).
[N: 0.001 to 0.15%]
[0040] N is an element having an effect of improving the high
temperature strength through solid-solution hardening by being
solved into steel, which is also effective for the improvement of
the high temperature strength by forming nitrides with Cr or Nb
under load at high temperature for a long time. In order to
efficiently provide the effect, it is necessary that the N content
is 0.001% or more. However, if N is added excessively and its
content is more than 0.15%, this results in formation of coarse Ti
nitrides or Nb nitrides to deteriorate the toughness. A preferred
lower limit of the N content is 0.002% or more (more preferably,
0.003% or more) and a preferred upper limit is 0.10% or less (more
preferably, 0.08% or less, and, further preferably, 0.02% or
less).
[S: 0.005% or Less (not Including 0%)]
[0041] S is an unavoidable impurity and, since hot workability is
deteriorated as the content increases, it is necessary that the
content is 0.005% or less. Further, since S fixes Ce as sulfides to
decrease the effect obtained by the addition of Ce, it is
preferably restricted to 0.002% or less (more preferably, 0.001% or
less).
[Fs: 0.05% or Less (not Including 0%)]
[0042] P is an unavoidable impurity and, since the weldability is
deteriorated as the content increases, it should be 0.05% or less.
Preferably, it is restricted to 0.04% or less (more preferably,
0.03% or less)
[0043] The contained elements defined in the invention are
described above and the balance is iron and unavoidable impurities.
In addition to La, Nd, Pr, etc. which are contained at a
concentration lower than Ce, respectively when adding a misch metal
as a Ce source, intrusion of elements which are introduced
depending on the raw materials, alloying source, and situations of
production facilities, etc. are permissible. However, since
impurity elements having low melting point such as Sn, Pb, Sb, As,
and Zn derived from scrap materials lower the grain boundary
strength during hot working and use at high temperature
circumstance, it is preferred that they are kept to a low
concentration in order to improve the hot workability and
embrittlement cracks in long time use. Further, in the steel of the
invention, Mo, W, Ca, and Mg, etc. may also be optionally contained
and the properties of the steel are further improved in accordance
with the kind of the elements to be contained.
[Mo: 3% or Less (not Including 0%) and/or W: 5% or Less (not
Including 0%)]
[0044] Mo and W have an effect of improving the high temperature
strength by solid solution hardening and can further increase the
high temperature strength by optionally adding them. However, since
the hot workability is deteriorated when the Mo content is
excessive, it is preferably 3% or less. More preferably, it is 2.5%
or less (further preferably, 2.0% or less). Further, since
excessive W content forms coarse intermetallic compounds to lower
the high temperature ductility, it is preferably less than 5% or
less. More preferably, it is 4.5% or less (further preferably, 4.0%
or less). A preferred lower limit for providing the effect
efficiently described above is 0.1% or more (more preferably, 0.5%
or more) for Mo and 0.1% or more (more preferably, 1.0% or more)
for W. However, while the effect as described above can be provided
by addition of such elements, since this increases the cost on the
other hand, the content may be determined in accordance with the
necessary hardening amount and an allowable cost.
[Ca: 0.005% or Less (not Including 0) and/or Mg: 0.005% or Less
(not Including 0)
[0045] Since Ca and Mg act as desulfurizing and deoxidizing
elements, they can suppress formation of Ce sulfides and Ce oxides
to improve the yield of Ce and suppress lowering of the toughness
due to formation of inclusions. A preferred lower limit for
providing such effect effectively is 0.0002% or more and, more
preferably, 0.0005% or more for each of them. However, if the
contents become excessive, since they impose restriction in view of
operation such as occurrence of bumping of molten steel during
melting operation, each of the upper limits is defined to 0.005% or
less. More preferably, the content of each of them is 0.002% or
less.
[0046] In the heat-resistant austenitic stainless steel of the
invention, cyclic oxidation resistance can be improved by addition
of a predetermined amount of Zr and Ce. For improving the property
further, it is effective to control the crystal grain size of
microstructure. From such a view point, the crystal grain size of
the microstructure of the heat-resistant austenitic stainless steel
is preferably defined as a fine structure of 6 or more and less
than 12 in terms of the ASTM (American Society for Testing and
Materials) grain size number. The grain size number (crystal grain
size number) is defined by ASTM and means a grain size number
calculated by a counting method (Planimetric method).
[0047] When the crystal grain size of the microstructure is less
than 6 in terms of the ASTM grain size number, while the effect of
improving the cyclic oxidation resistance per se by the addition of
Zr and Ce can be obtained, the improving effect cannot be increased
sufficiently. The grain size number is preferably 7 or more and,
more preferably, 9 or more. On the other hand, in the tube
production process by hot and cold working and heat treatment,
since an extremely fine crystal grain structure cannot be
manufactured substantially, an upper limit of the crystal grain
size is preferably less than 12. In view of the manufacturing cost
and the productivity, the upper limit is more preferably 10 or
less.
[0048] The range of the crystal grain size as described above can
be obtained by controlling the addition amount of the elements
contributing to the pinning at the crystal grain boundary,
conditions for hot and cold working such as drawing and extrusion
in the tube production process, and heat treatment. The optimal
condition for each of them changes depending on the three factors
and, in order to refine the crystal grain size, it is necessary to
increase the addition amount of the precipitating elements, make
the degree of strain higher, and lower the heat treatment
temperature. Cold and hot working are applied for controlling the
tube thickness and introducing strains and conditioning the crystal
grain structure by heat treatment after working and usually
performed at a reduction ratio of 30% or more. Further, the heat
treatment is applied for removing strains and performed in a
temperature range generally at 1,000.degree. C. or higher and lower
than 1,300.degree. C. For example, at the reduction ratio of about
35%, the defined range of the grain size can be obtained by setting
the heat treatment temperature to 1,250.degree. C. or lower and,
preferably, 1,225.degree. C. or lower and, particularly preferably,
1,150.degree. C. or lower, but the condition is not restricted
depending on the balance for precipitating elements, working, and
heat treatment.
[0049] When the heat transfer tubes of boilers are formed by using
the heat-resistant austenitic stainless steel described above, they
provide an excellent property under a cyclic oxidation
circumstance.
[0050] The present invention is to be described more specifically
with reference to examples. The invention is not restricted by the
following examples and it is of course possible to practice the
invention with appropriate modification within a range that can
conform to the purport described above and to be described later,
and each of them is included in the technical range of the
invention.
EXAMPLE
Example 1
[0051] 20 kg ingots prepared by melting various kinds of steels
comprising chemical compositions shown in the following Table 1 in
a vacuum melting furnace (VIF) were hot-forged each to 120 mm
width.times.20 mm thickness, applied with a heat treatment at
1250.degree. C. and processed by cold rolling to 13 mm thickness.
Subsequently, a heat treatment at 1150.degree. C. for 5 min was
performed again to provide a master material. A steel material of
20 mm.times.30 mm.times.2 mm was cut out from the master material
by machining and the surface of the steel material was smoothed and
mirror-finished by polishing using emery paper and by buff
polishing using diamond abrasive grains to prepare specimens.
[0052] Among the steels shown in the following Table 1, specimens
Nos. 1 to 10 are steels that satisfy the requirements defined in
the invention (steel of the invention), and specimens Nos. 11 to 16
are steels out of the requirements defined in the invention
(comparative steels), in which the specimens Nos. 14, 15, and 16
are "steels corresponding to KA-SUS304J1HTB", "steels corresponding
to SUS304L", and "steels corresponding to SUS310S" which are
conventional steels respectively. Further, the specimens Nos. 7 and
8 are steels with addition of Ce by using a misch metal and contain
La, Pr, Nd, etc. as impurities. The specimens Nos. 9 and 10 are
steels with addition of Mg and Ca respectively.
[0053] "Steel corresponding to KA-SUS304J1HTB" (specimen No. 14)
described above belongs to 18Cr-8Ni austenitic stainless steel
which is steel species used successfully as heat transfer tubes of
boilers (for example, in "MATERIA", vol. 46, No. 2, 2007, pp.
99-101). Further, steel corresponding to SUS310S (specimen No. 16)
belongs to 25Cr-20Ni austenitic stainless steel. While this is
expensive since it contains more Ni than 18Cr-8Ni austenitic
stainless steel, this is steel species more excellent in the
corrosion resistance than 18Cr-8Ni austenitic stainless steel
essentially in view of the chemical composition.
TABLE-US-00001 TABLE 1 Specimen Chemical composition* (mass %) No.
C Si Mn P S Ni Cr Cu Mo Nb Ti Zr Ce B N Others (remarks) 1 0.09
0.30 1.58 0.026 0.002 9.7 18.4 3.0 -- 0.19 0.20 0.19 0.015 0.0020
0.009 2 0.10 0.30 1.60 0.018 0.002 9.5 18.3 3.0 -- 0.18 0.14 0.25
0.092 0.0020 0.005 3 0.18 0.89 0.21 0.025 0.001 9.8 16.7 2.1 --
0.13 0.40 0.38 0.020 0.0048 0.130 4 0.10 0.15 1.80 0.032 0.004 9.2
18.1 3.1 -- 0.21 0.22 0.09 0.008 0.0021 0.004 5 0.10 0.32 1.26
0.029 0.003 9.5 17.9 1.3 0.8 0.18 0.19 0.20 0.017 0.0019 0.080 6
0.07 0.75 0.77 0.045 0.002 8.2 19.7 3.8 -- 0.56 0.12 0.35 0.034
0.0005 0.008 7 0.11 0.54 1.87 0.025 0.001 9.8 18.1 3.0 -- 0.18 0.26
0.19 0.023 0.0018 0.010 Ce added in the form of misch metal 8 0.12
0.76 1.14 0.018 0.003 11.3 18.4 2.8 -- 0.19 0.15 0.11 0.041 0.0019
0.019 Ce added in the form of misch metal 9 0.11 0.55 1.45 0.019
0.002 9.6 18.7 3.0 -- 0.18 0.26 0.16 0.025 0.0018 0.003 Mg: 0.0015
10 0.10 0.42 1.48 0.022 0.001 9.8 17.9 3.2 -- 0.17 0.25 0.10 0.013
0.0021 0.005 Ca: 0.0022 11 0.09 0.19 1.60 0.030 0.003 9.2 17.9 3.0
-- 0.27 0.23 0.02 0.019 0.0018 0.050 12 0.06 0.25 1.50 0.031 0.002
9.3 18.1 3.1 -- 0.19 0.16 0.15 <0.001 0.0022 0.015 13 0.11 0.29
1.48 0.031 0.002 9.3 18.1 3.0 -- 0.21 0.19 0.01 0.003 0.0022 0.023
14 0.10 0.19 0.73 0.030 0.003 9.2 18.0 3.1 -- 0.38 -- -- -- 0.0018
0.110 15 0.05 0.40 1.82 0.032 0.002 8.4 18.5 0.3 0.21 -- -- -- --
-- 0.059 16 0.05 1.46 1.80 0.030 0.001 19.52 24.2 0.08 0.15 -- --
-- -- 0.0012 0.058 *Balance: Iron and unavoidable impurities other
than P and S
[0054] Each of the specimens obtained as described above was used
and repeating oxidation tests were performed for evaluating a
weight loss. In the cyclic oxidation test, specimens were carried
into and out of a furnace at 1100.degree. C. in air at a cycle of
furnace heating for 25 min and cooling for 5 min in air, and
heating and cooling were repeated up to 20 cycles. After the cyclic
oxidation test, weight change of the specimen was measured by an
electronic balance and the weight loss (mgcm.sup.-2) of the
specimens was calculated. Further the surface roughness of the
specimen after the cyclic oxidation test was observed visually.
[0055] The result of the measurement (weight loss, surface
roughness) is shown in the following Table 2.
TABLE-US-00002 TABLE 2 Specimen Weight loss No. (mg cm.sup.-2)
Surface roughness 1 10.8 smooth 2 7.6 smooth 3 8.5 smooth 4 33.2
smooth 5 11.6 smooth 6 20.4 smooth 7 9.2 smooth 8 7.9 smooth 9 8.1
smooth 10 8.7 smooth 11 73.4 rough 12 76.9 rough 13 93.1 rough 14
80.5 rough 15 140.1 rough 16 0.4 smooth
[0056] In view of the result, it can be considered as below. The
weight loss is decreased in the steels that satisfy the chemical
composition defined in the invention (steel of invention: specimens
Nos. 1 to 10) compared with conventional steels (specimens Nos. 14,
15) and comparative steels that are out of the chemical
compositions defined in the invention (specimen Nos. 11 to 13), and
it can be seen that less scales are peeled and the weight loss can
be suppressed by compound addition of Zr and Ce.
[0057] Further, it can be seen that since the roughness at the
scale surface is smoothed in the steel of the invention scale are
not formed and peeled off. Further, the steel of the invention
provides properties comparable with those of steels corresponding
to conventional steels SUS310S of 25Cr-20Ni which contain higher Ni
content and are considered to be excellent in the corrosion
resistance (specimen No. 16), and the cyclic oxidation resistance
can be improved to a level comparable with that of 25Cr-20Ni
austenitic stainless steel although this is a 18Cr-8Ni austenitic
stainless steel and inexpensive.
Example 2
[0058] For the steels of the invention of specimens Nos. 1 to 6 and
the comparative steel of specimen No. 14 shown in Tables 1 and 2,
the heat treatment temperature was changed in temperature range of
1125 to 1275.degree. C. after cold working at 35% reduction ratio
to prepare specimens of the respective steels with crystal grain
size numbers of 4.5 to 10.0. In the cyclic oxidation test,
specimens were carried into and out of a furnace at 1100.degree. C.
in air at a temperature cycle including furnace heating for 25 min
and cooling for 5 min in air, and weight loss (reduction in
thickness: mgcm.sup.-2) was determined by comparing the mass of the
specimen after 40 cycles with the mass of the specimen in the
initial state.
[0059] For the number of cycles, since the weight loss was improved
greatly in some steels with addition of Zr and Ce and the weight
loss after 20 cycles was about at a level of an allowable error
depending on the grain size, heating and cooling were repeated till
40 cycles. The crystal grain size number was calculated by
observation for three view fields per one steel species.
[0060] The result of the measurement described above (weight loss)
are shown together with the crystal grain size number in the
following Table 3.
TABLE-US-00003 TABLE 3 Specimen No. 1 2 3 4 5 6 14 Heat Weight
Weight Weight Weight Weight Weight Weight treatment loss Crystal
loss Crystal loss Crystal loss Crystal loss Crystal loss Crystal
loss temperature Crystal (mg grain (mg grain (mg grain (mg grain
(mg grain (mg grain (mg (.degree. C.) grain size cm.sup.-2) size
cm.sup.-2) size cm.sup.-2) size cm.sup.-2) size cm.sup.-2) size
cm.sup.-2) size cm.sup.-2) 1125 9.8 9.6 9.2 20.4 10.0 15.6 9.6 73.8
10.0 13.5 9.8 25.2 9.8 311.7 1150 8.8 22.2 9.1 15.6 9.6 17.4 8.9
68.1 8.8 23.7 10.0 42.0 9.4 295.8 1200 8.0 63.9 7.7 44.7 8.7 52.2
7.9 108.3 8.1 80.4 8.9 84.6 8.4 312.6 1225 6.5 101.1 6.1 67.2 6.9
85.8 6.4 125.7 6.3 100.5 7.0 101.4 6.7 303.6 1275 5.0 107.1 5.1
71.1 5.3 99.6 4.9 130.5 5.1 108.3 5.1 104.7 4.5 282.6
[0061] Based on the result, it can be considered as below.
Specimens with a crystal grain size number of 6 or more are
examples of the invention that satisfy the definition in the
invention for the crystal grain size in addition to the chemical
composition and specimens with the number of less than 6 are
examples of the invention that satisfy the chemical composition but
do not satisfy the crystal grain size (grain size numbers are
underlined). As shown by the result of the comparative steel of the
specimen No. 14, it can be seen that in the steel out of the
chemical composition of the invention, weight loss does not change
substantially even when the crystal grain size changes but, in the
steel of the invention of specimens Nos. 1 to 6, the weight loss
tends to be decreased as the crystal grain size number is larger.
Further, since any of the steels of the invention of different
crystal grain size can decrease the weight loss more than the
conventional steel of specimen No. 14, it can be seen that the
cyclic oxidation resistance is improved by the addition of Zr and
Ce per se and that the property is further improved as the crystal
grain size is smaller even when the chemical composition is within
a range defined by the invention.
[0062] Referring to the grain size dependence of Nos. 1 to 6 as the
steels of the invention, it can be seen that while there is a
difference in the property in terms of the absolute value due to
the content of Zr and Ce for each of the steel species, the cyclic
oxidation resistance is higher when the crystal grain size number
is 6 or more compared with the cases of less than 6 in any of the
steel species and a remarkable improving effect is obtained,
particularly, in the case of the grain size number of 7 or more
and, further, 9 or more. That is, the cyclic oxidation resistance
can be improved in the steels that satisfy the range of composition
of the invention, and the effect can be increased further by
controlling the crystal grain size, and excellent cyclic oxidation
resistance can be obtained stably.
[0063] While the present invention has been described specifically
with reference to the specific embodiments, it will be apparent to
those skilled in the art that various modifications or changes can
be adopted without departing the gist and the range of the
invention.
[0064] The present application is based on Japanese patent
application filed on May 11, 2011 (Japanese Patent Application No.
2011-106588), Japanese patent application filed on Sep. 16, 2011
(Japanese Patent Application No. 2011-203604), and Japanese patent
application filed on Mar. 5, 2012 (Japanese Patent Application No.
2012-048357), the content of which is incorporated herein for
reference.
INDUSTRIAL APPLICABILITY
[0065] The heat-resistant austenitic stainless steel of the
invention can be used suitably as the material for heat transfer
tubes of boilers, etc.
* * * * *