U.S. patent application number 13/931197 was filed with the patent office on 2013-11-07 for heat-resistant steel.
The applicant listed for this patent is Babcock-Hitachi Kabushiki Kaisha, Toshio Fujita. Invention is credited to Toshio FUJITA, Takashi SATO, Kohji TAMURA.
Application Number | 20130294959 13/931197 |
Document ID | / |
Family ID | 38345132 |
Filed Date | 2013-11-07 |
United States Patent
Application |
20130294959 |
Kind Code |
A1 |
FUJITA; Toshio ; et
al. |
November 7, 2013 |
HEAT-RESISTANT STEEL
Abstract
Disclosed is a ferritic heat-resistant steel which has the
following chemical composition (by weight): C: from 0.01% to less
than 0.08%; Si: 0.30-1.0%; P: 0.02 or less; S: 0.010% or less; Mn:
0.2-1.2%; Ni: 0.3% or less; Cr: 8.0-11.0%; Mo: 0.1-1.2%; W:
1.0-2.5%; V: 0.10-0.30%; Nb: 0.02-0.12%; Co: 0.01-4.0%; N:
0.01-0.08%; B: not less than 0.001% and less than 0.010%; Cu: 0.3%
or less; and Al: 0.010% or less, provided that the chemical
composition satisfies the following equations:
Mo(%)+0.5.times.W(%)=1.0-1.6, and C(%)+N(%)=0.02-0.15%, and which
comprises a tempered martensite single-phase tissue produced by
thermal refining and contains 30% by weight or less of 5
ferrite.
Inventors: |
FUJITA; Toshio; (Tokyo,
JP) ; SATO; Takashi; (Hiroshima, JP) ; TAMURA;
Kohji; (Hiroshima, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Babcock-Hitachi Kabushiki Kaisha
Toshio Fujita |
Tokyo
Tokyo |
|
JP
JP |
|
|
Family ID: |
38345132 |
Appl. No.: |
13/931197 |
Filed: |
June 28, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12901221 |
Oct 8, 2010 |
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13931197 |
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12278306 |
Aug 5, 2008 |
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PCT/JP2007/051968 |
Feb 6, 2007 |
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12901221 |
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Current U.S.
Class: |
420/38 ;
420/91 |
Current CPC
Class: |
C22C 38/52 20130101;
F22B 37/10 20130101; C22C 38/22 20130101; C22C 38/30 20130101; C22C
38/46 20130101; F22B 37/04 20130101; C22C 38/02 20130101; C22C
38/24 20130101; C21D 8/105 20130101; C21D 9/085 20130101; C22C
38/001 20130101; C22C 38/26 20130101; C22C 38/48 20130101; C21D
9/08 20130101; C22C 38/06 20130101; C21D 2211/005 20130101; C22C
38/42 20130101; C22C 38/54 20130101; C22C 38/04 20130101; C21D
2211/008 20130101; C22C 38/44 20130101 |
Class at
Publication: |
420/38 ;
420/91 |
International
Class: |
C22C 38/54 20060101
C22C038/54; C22C 38/48 20060101 C22C038/48; C22C 38/46 20060101
C22C038/46; C22C 38/00 20060101 C22C038/00; C22C 38/42 20060101
C22C038/42; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/52 20060101
C22C038/52; C22C 38/44 20060101 C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 6, 2006 |
JP |
2006-029009 |
Claims
1. A heat-resistant steel which has the following chemical
composition (by weight): carbon (C): 0.01-0.08%; silicon (Si):
0.30-1.0%; phosphorus (P): 0.020% or less; sulfur (S): 0.010% or
less; manganese (Mn): 0.2-1.2%; nickel (Ni): 0.3% or less; chromium
(Cr): 8.0-11.0%, molybdenum (Mo): 0.1-1.2%; tungsten (W):
1.0-2.02%; vanadium (V): 0.10-0.30%; niobium (Nb): 0.02-0.12%;
cobalt (Co): 0.01-4.0%; nitrogen (N): 0.01-0.08%; boron (B): not
less than 0.001% and not more than 0.010%; copper (Cu): 0.3% or
less; aluminum (Al): 0.010% or less; provided that the amount of
(Mo %+0.5W %) is 1.0-1.6, and the amount of (C %+N %) is
0.02-0.15%, and which comprises a tempered martensite single-phase
tissue produced by thermal refining.
2. The heat-resistant steel according to claim 1, having the
chemical composition in which the amount of (Al %+0.1.times.Ni %)
is 0.02% by weight or less.
3. The heat-resistant steel according to claim 1, comprising from
0.01% to 0.075% carbon.
4. The heat-resistant steel according to claim 2, comprising from
0.01% to 0.075% carbon.
5. The heat-resistant steel according to claim 1, comprising
0.40-0.75% by weight of Si.
6. The heat-resistant steel according to claim 2, comprising
0.40-0.75% by weight of Si.
7. The heat-resistant steel according to claim 3, comprising
0.40-0.75% by weight of Si.
8. The heat-resistant steel according to claim 4, comprising
0.40-0.75% by weight of Si.
9. The heat-resistant steel according to claim 1, further
comprising (by weight) at least one of <0.2% Ta, <0.1% Ti,
<0.2% Zr, <0.1% La, <0.1% Ce, <0.2% Pd, <0.5% Re,
<0.3% Hf.
10. The heat-resistant steel according to claim 2, further
comprising (by weight) at least one of <0.2% Ta, <0.1% Ti,
<0.2% Zr, <0.1% La, <0.1% Ce, <0.2% Pd, <0.5% Re,
<0.3% Hf.
11. The heat-resistant steel according to claim 3, further
comprising (by weight) at least one of <0.2% Ta, <0.1% Ti,
<0.2% Zr, <0.1% La, <0.1% Ce, <0.2% Pd, <0.5% Re,
<0.3% Hf.
12. The heat-resistant steel according to claim 4, further
comprising (by weight) at least one of <0.2% Ta, <0.1% Ti,
<0.2% Zr, <0.1% La, <0.1% Ce, <0.2% Pd, <0.5% Re,
<0.3% Hf.
13. The heat-resistant steel according to claim 5, further
comprising (by weight) at least one of <0.2% Ta, <0.1% Ti,
<0.2% Zr, <0.1% La, <0.1% Ce, <0.2% Pd, <0.5% Re,
<0.3% Hf.
14. The heat-resistant steel according to claim 6, further
comprising (by weight) at least one of <0.2% Ta, <0.1% Ti,
<0.2% Zr, <0.1% La, <0.1% Ce, <0.2% Pd, <0.5% Re,
<0.3% Hf.
15. The heat-resistant steel according to claim 7, further
comprising (by weight) at least one of <0.2% Ta, <0.1% Ti,
<0.2% Zr, <0.1% La, <0.1% Ce, <0.2% Pd, <0.5% Re,
<0.3% Hf.
16. The heat-resistant steel according to claim 8, further
comprising (by weight) at least one of <0.2% Ta, <0.1% Ti,
<0.2% Zr, <0.1% La, <0.1% Ce, <0.2% Pd, <0.5% Re,
<0.3% Hf.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 12/901,221, filed Oct. 8, 2010, which is a continuation of U.S.
application Ser. No. 12/278,306, abandoned, which is a National
Stage of International Application No. PCT/JP2007/051968, filed
Feb. 6, 2007, which claims priority under 35 U.S.C. 119 of Japanese
Patent Application 2006-029009, filed Feb. 6, 2006. The disclosures
of U.S. applications Ser. Nos. 12/901,221 and 12/278,306 are
incorporated by reference herein in their entireties.
TECHNICAL FIELD
[0002] The present invention relates to a ferritic heat-resistant
steel, and more specifically, to a high-strength steel for boiler
steel pipes suitable for an ultra supercritical pressure thermal
power plant with improved power generation efficiency.
BACKGROUND ART
[0003] Recently, in thermal power plants, the temperature and
pressure of steam conditions have been raised for improvement in
plant efficiency in view of global environmental issues such as
CO.sub.2 emissions reduction. Now, plants which can raise the steam
temperature from the current maximum main steam temperature around
600.degree. C. to 650.degree. C., ultimately, to 700.degree. C.
have been developed and studied domestically and internationally.
According to such a steam temperature rise, a heat-resistant steel
with high creep rupture strength is required at high-temperature
and pressure resistant portions of a boiler. Therefore, for heat
transfer pipes of a boiler, an austenitic heat-resistant steel
having excellent corrosion resistance and creep rupture strength
has been increasingly used.
[0004] On the other hand, in the case of a thick pipe with a large
diameter like a header or piping, when this austenitic
heat-resistant steel is used, the linear expansion coefficient
becomes higher and the heat transfer coefficient becomes smaller
than those of the ferritic heat-resistant steel. Therefore, when
the plant starts or stops, a great thermal stress is applied to
these header and piping and the header and piping are easily
damaged due to thermal fatigue. In addition, the increase in
material cost and process cost causes an economic problem.
Therefore, development of new ferritic heat-resistant steel with
high creep rupture strength and excellent corrosion resistance has
been demanded. As an example of such ferritic heat-resistant steel,
a material obtained by increasing the proportion of Cr in a
conventional steel of 9% Cr and 1% MoNbV and adding alloy elements
of W and Co, etc., has been proposed (for example, Japanese Patent
No. 2528767).
[0005] However, for example, when a ferritic heat-resistant steel
is used for a boiler that will reach a steam temperature around
650.degree. C., the ferritic heat-resistant steel contains many W
and other alloy elements, so that during long-term use, aggregation
and coarsening of fragile intermetallic compounds or carbide occur.
Therefore, it has been found that the creep rupture strength lowers
during long-term use for several tens of thousands of hours or
longer. Particularly, at a high temperature around 650.degree. C.
much higher than 600.degree. C., so-called fluting that involves
sudden lowering in creep strength after about several tens of
thousands of hours is a great obstacle in the development of a
high-Cr steel (for example, Non-Patent Document 1).
[0006] These header and piping are exposed for a long time to
high-temperature steam that is an internal fluid of a boiler steel
pipe. When the high-Cr ferrite steel is used at a high temperature
around 650.degree. C., production of oxidized scale due to steam
becomes pronounced (for example, Proceedings of Workshop, pp. 153
described above), so that the scale growth and exfoliation, and
scale scattering to the downstream also come into question. To
solve these problems, a special method by addition of noble metals
(Non-Patent Document 2) has also been proposed, however, this
greatly increases the material cost, so that it has not been made
practicable.
[0007] According to Non-Patent Document 3, improvement in creep
rupture strength by adding components of V, Nb, N, Mo, W, and B,
etc., to high-chromium steel containing 9 to 12% of chromium is
described. Some of the high-chromium steels of the Non-Patent
Document 3 realize creep rupture strength at a temperature around
600.degree. C. during long-term use, however, the creep rupture
strength at a temperature around 650.degree. C. is much lower than
that at 600.degree. C.
[0008] The inventors previously developed a ferritic heat-resistant
steel with high creep rupture strength at a temperature around
650.degree. C. and applied for a patent (Japanese Laid-Open Patent
Publication No. 2005-23378). [0009] Patent Document 1: Japanese
Patent Publication No. 2528767 [0010] Patent Document 2: Japanese
Laid-Open Patent Publication No. 2005-23378 [0011] Non-Patent
Document 1: R. Viswanathan et al., "Materials for
Ultrasupercritical Coal-fired Power Plant Boilers" (p. 146-157); R.
Blum et al., "Materials Development in Thermie Project for
700.degree. C. USC Plant" (pp. 158-176), Committee of the 8th
Ultra-Steel Workshop, Steel Research Center, National Institute for
Materials Science, Jul. 22, 2004 [0012] Non-Patent Document 2:
Haruyama et al., "Influence of Pd addition on the steam oxidation
behavior of 9Cr ferrite steel," Material and Process, The Iron and
Steel Institute of Japan, March 2003, Vol. 16, No. 3, p. 648 [0013]
Non-Patent Document 3: Gabrel J et al., "Status of development of
the VM12 steel for tubular applications in advanced power plants,"
Proceedings of the 8th Liege Conference Part II, Materials for
Advanced Power Engineering 2006, Forschungszentrum Julich GmbH,
September 2006, p. 1065-1075
DISCLOSURE OF THE INVENTION
Problem to be Solved by the Invention
[0014] The ferritic heat-resistant steel described in Patent
Document 2, developed earlier by the present inventors, has high
creep rupture strength at a temperature around 650.degree. C.,
however, its long-term strength is insufficient, and there is room
for improvement in steam oxidation resistance (improvement in
property of oxidation due to steam inside the pipe). Thus, the
conventionally proposed alloys are insufficient in property as a
material to be used at a temperature around 650.degree. C. Further,
a ferritic heat-resistant steel which has high creep rupture
strength at a temperature around 650.degree. C., and maintains
stably the strength for a long period of time, and is excellent in
steam oxidation resistance has been demanded.
[0015] An object of the present invention is to provide a
high-strength ferritic heat-resistant steel which is excellent in
long-term creep rupture strength and steam oxidation in the case
where it is used at a temperature around 650.degree. C. in
comparison to conventional materials.
Means for Solving the Problem
[0016] The object of the present invention is achieved by the
following solution means.
[0017] A first aspect of the invention provides a ferritic
heat-resistant steel which has the following chemical composition
(by weight): carbon (C): 0.01-0.10%; silicon (Si): 0.30-1.0%;
phosphorus (P): 0.020% or less; sulfur (S): 0.010% or less,
manganese (Mn): 0.2-1.2%; nickel (Ni): 0.3% or less; chromium (Cr):
8.0-11.0%, molybdenum (Mo): 0.1-1.2%; tungsten (W): 1.0-2.5%;
vanadium (V): 0.10-0.30%; niobium (Nb): 0.02-0.12%; cobalt (Co):
0.01-4.0%; nitrogen (N): 0.01-0.08%; boron (B): not less than
0.001% and not more than 0.010%; copper (Cu): 0.3% or less;
aluminum (Al): 0.010% or less, provided that the amount of (Mo
%+0.5.times.W %) is limited to 1.0-1.6, and the amount of (C %+N %)
is limited to 0.02-0.15%, and which comprises a tempered martensite
single-phase tissue produced by thermal refining.
[0018] A second aspect of the invention provides the ferritic
heat-resistant steel according to the first aspect, which has the
chemical composition in which the amount of (Al %+0.1.times.Ni %)
is limited to 0.02% by weight or less.
[0019] A third aspect of the invention provides the ferritic
heat-resistant steel according to the first or second aspect,
containing 30% by weight or less of .delta. ferrite.
[0020] Carbon (C) is an element important for forming carbide
(M23C6, M6C, M7C3, etc.) that contributes to strengthening of a
high-Cr ferritic heat-resistant steel. Conventional practical
steels require about 0.1-0.12% of carbon, however, if the carbon
content exceeds 0.10%, it promotes aggregation and coarsening and
lowers the creep rupture strength, so that in the present
invention, the carbon content is set to 0.10% or less to stabilize
long-term creep strength. The lower the C content, the higher the
creep rupture strength, however, if the C content is less than
0.01%, the toughness deteriorates, so that the C content is set to
0.01-0.10% as a practical steel. Fine control of the C content
requires advanced techniques in steelmaking, however, in
particular, by reducing the C content from about 0.1% of a
conventional steel to 0.08% or less, the Ac1 point (transformation
point) is greatly raised, and the long-term creep rupture strength
can be further improved.
[0021] Silicon (Si) was an element necessary as a deoxidizing agent
for producing steel. However, recently, vacuum deoxidation has
become possible, and it becomes possible to obtain a low-Si steel
by vacuum deoxidation, and vacuum deoxidation has also been used
for obtaining a high-Cr heat-resistant steel. Si is an element that
improves the oxidation resistance, and to obtain steam oxidation
resistance necessary as a 600.degree. C.-class boiler material, at
least 0.30% of Si is necessary. To obtain steam oxidation
resistance sufficient as a 650.degree. C.-class boiler material, it
is generally preferable that the scale thickness is 200 micrometers
or less.
[0022] On the other hand, if a large amount of Si over 1.0% is
added, production of Laves phase of tungsten (W), etc. are
promoted, and ductibility is lowered due to grain boundary
segregation, etc. Therefore, when attaching importance to the creep
strength, the Si content tends to be made low, and this is an
obstacle in using the steel at a temperature around 650.degree.
C.
[0023] However, in the present invention, it was found that
according to the effect of reducing M23C6 carbide aggregation and
coarsening by aluminum (Al) described later, high creep strength
could be obtained even if the Si content was increased. Therefore,
to obtain steam oxidation resistance sufficient as a 650.degree.
C.-class boiler material, the Si content is set to 0.30-1.0%. When
emphasizing the ductibility of steel, the Si content is set to
0.30-0.80% (FIG. 1) since a high Si content lowers the
ductibility.
[0024] 0.2% or more of manganese (Mn) is necessary as a deoxidizing
agent, and at the same time, manganese is useful as an austenite
forming element for restraining production of .delta. ferrite.
However, if it is added by an amount over 1.2%, the
Ac1transformation point lowers, and the creep strength lowers.
Therefore, the Mn content is limited to 0.2-1.2%.
[0025] Phosphorus (P) and sulfur (S) are low-melting point
elements, so that if their contents are high, they harmfully
influence the creep rupture strength, so that the lower the
contents of these the better. However, it is difficult to
completely remove P and S, and if their contents are extremely low,
it causes an increase in material cost, so that it is not necessary
to make the contents extremely low, and P is limited to 0.020% or
less and S is limited to 0.010% or less. Chromium (Cr) is an
important element that provides a steel with oxidation resistance
and steam oxidation resistance, however, if its content is more
than 11%, it forms .delta. ferrite and lowers the toughness, and
precipitation of M23C6 carbide, etc., and growth coarsening due to
the precipitation become pronounced and lower the long-term creep
strength. Therefore, the Cr content must be set to 11% or less. In
the present invention, the steam oxidation resistance is improved
by adding a large amount of Si, however, its effect is insufficient
if the Cr content is less than 8%, so that the Cr content is set to
8 to 11%.
[0026] Molybdenum (Mo) is an effective element for increasing the
creep rupture strength by fine precipitation of carbide. Therefore,
the molybdenum content necessary for strengthening. by
precipitation of carbide is at least 0.1% or more, however, if
molybdenum over 1.2% is added and 0.1% or more of W is contained,
.delta. ferrite is produced, and aggregation and coarsening of
M23C6 carbide containing Mo occur and lead to creep strength
lowering. Therefore, the Mo content is set to 0.1 to 1.2%.
[0027] Tungsten (W) is an element most important for increasing the
creep rupture strength of this steel by precipitation strengthening
of carbide and solution hardening in base metal. It had been
conventionally said that addition of about 3% of W is effective,
and if 4% or more is added, M23C6 carbide and Laves phase (Fe2W)
including W aggregate and coarsen and lower the creep rupture
strength. However, it was found that even an addition of about 3%
lowered the long-term creep rupture strength, so that the content
is reduced to be low and is set to 2.5% or less with which creep
rupture strength of 100 N/mm.sup.2 or more at 650.degree. C. after
20,000 hours is obtained (FIG. 2). If the content is less than
1.0%, from the data shown in FIG. 2, it is presumed that the creep
rupture strength cannot be improved, so that the content is set to
1.0 to 2.5%. W influences the creep strength in combination with
Mo, so that the content of W only is regulated, and the value of
(Mo %+0.5.times.W %) is set to 1.0 to 1.6% effective for
improvement in creep rupture strength.
[0028] Cobalt (Co) is an austenite forming element, and prevents
production of .delta. ferrite without greatly lowering the Ac1
transformation point, so that it is an important element. The
amount of .delta. ferrite changes depending on quantities of other
alloy elements to be added, however, by adding at least 0.01%, and
4.0% at most of cobalt (Co), production of .delta. ferrite can be
sufficiently prevented.
[0029] Vanadium (V) is an element that forms carbide of V and
effectively increases the creep rupture strength at a relatively
small amount, and at least 0.10% must be added for forming carbide
of V. However, if the content is more than 0.30%, the carbide of V
aggregates and coarsens and lowers the creep rupture strength, so
that the content is set to 0.10 to 0.30%.
[0030] Niobium (Nb) forms Nb (C, N) (carbonitride of niobium) that
is stable carbonitride and can increase the creep rupture strength
even with supply of a small amount, however, if the content is more
than 0.12%, it is not good for long-term strength although
short-term strength is improved. If it is less than 0.02%,
precipitated Nb (C, N) is in short supply and strengthening is
insufficient, so that the content is set to 0.02 to 0.12%.
[0031] Nitrogen (N) increases the creep rupture strength by
precipitation strengthening by forming nitride of V and solution
hardening of itself. However, nitrogen over 0.08% forms too much
nitride and causes aggregation and coarsening, and lowers the creep
rupture strength, so that the nitrogen content is set to 0.08% or
less. If the nitrogen content is less than 0.01%, the effect of
increasing the creep rupture strength is insufficient, so that the
nitrogen content is set to 0.01 to 0.08%. The creep rupture
strength based on N has a close correlation with the amount of C,
and when (C %+N %) is limited to 0.02 to 0.15%, the highest creep
rupture strength is obtained.
[0032] Nickel (Ni) is an element effective for improving the
toughness and restraining production of .delta. ferrite, and in a
conventional boiler steel, it is added by about 0.5% without
special limitation. However, it was found that addition of nickel
greatly lowered the Ac1 transformation point and harmfully
influenced the long-term creep strength, so that in view of the
creep rupture strength, it is preferable that the nickel content is
lowered to be not more than 0.1%. However, for this, at the time of
steelmaking, the amount of nickel to be mixed from scrap steel, the
furnace wall, and ladle, etc., must be minimized, and this
increases the steelmaking technical limitations, so that the upper
limit of the nickel content is set to 0.3% as a practical steel. Ni
may not be contained.
[0033] Aluminum (Al) is conventionally added as a deoxidizing agent
and a crystal grain refining agent. However, excessive addition of
0.010% or more of Al captures nitrogen that is effective for
improving the creep strength as Al nitride, and is incrassated on
the surface of M23C6 carbide and promotes diffusion of Cr, and
accelerates aggregation and coarsening of M23C6 carbide. The
present inventors also found that when the Al content exceeded a
predetermined value, even if it was a very small amount, it greatly
lowered the long-term creep strength after several tens of
thousands of hours at a temperature around 650.degree. C. In a
conventional practical steel, the toughness is improved by adding
Al up to about 0.03%, however, in the heat-resistant steel of the
present invention, by reducing the Al content to 0.01% or less, the
long-term creep strength at 650.degree. C. is greatly
increased.
[0034] It was conventionally very difficult to reduce the Al
content to such an extremely low level. However, recently, it
became possible to manufacture an extremely-low level Al steel by
means of vacuum carbon deoxidation method. The increase in Si
content in the steel of the present invention is also one of the
features, and even if the deoxidation effect of Al is lost, the
deoxidation effect of Si can be used, so that the amount of oxygen
in the steel can be reduced. Al may not be contained.
[0035] FIG. 3 shows the creep rupture strength at 650.degree. C. in
materials with variously changed Al quantities and Ni quantities on
the vertical axis, and the Al quantities on the horizontal axis. As
creep rupture strength at 650.degree. C. after 100,000 hours, to
obtain about 100 N/mm.sup.2, strength over 100 N/mm.sup.2 after
20,000 hours is required. Therefore, for example, when the Ni
amount is about 0.3%, the Al amount is reduced to 0.005% or less.
When the Ni amount is 0.1% or less, the allowable range of the Al
amount can be widened.
[0036] The steel of the present invention has a feature in that Al
and Ni that are elements especially harmful for stabilizing the
creep strength are reduced, and reduction of these is essential.
The same test results as in FIG. 3 are shown in FIG. 4 by rewriting
the Al amount on the horizontal axis into Al amount +(0.1.times.Ni
amount). From these test results, as a range of the Al amount and
Ni amount with which creep rupture strength over 100 N/mm.sup.2 is
reliably obtained, (Al %+0.1.times.Ni %) is limited to 0.02% or
less.
[0037] Copper (Cu) has an effect of restraining production of
.delta. ferrite when mixed as an impurity into copper as in the
case of Co. However, mixture of Cu may lower the long-term creep
rupture strength at a temperature of 600.degree. C. or more, so
that it is limited to 0.3% or less. Cu may not be contained.
[0038] Boron (B) is a grain boundary strengthening element (element
that strengthens crystal grain boundaries), and remarkably
increases the creep rupture strength even at a small amount. It
dissolves into M23C6 carbide and restrains aggregation and
coarsening of M23C6 carbide to increase the creep rupture strength,
so that at least 0.001% of B is added. However, if 0.010% or more
of B is added, weldability of the steel is greatly deteriorated, so
that the additive amount of B is set to less than 0.10%.
[0039] The major chemical component ranges of the ferritic
heat-resistant steel of the present invention are as described
above, however, it may contain amounts of the following elements
less than the contents described below in units of percent by
weight as impurities.
[0040] Ta<0.2%, Ti<0.1%, Zr<0.2%, La<0.1%, Ce<0.1%,
Pd<0.2%, Re<0.5%, Hf<0.3%
These elements also have the effect of increasing the strength as
follows: [0041] Ta: forms TaC and strengthens the base metal.
[0042] Ti: forms TiC and strengthens the base metal. [0043] Zr:
forms ZrC and strengthens the base metal. [0044] La, Ce: lowers the
proportion of oxygen in the steel and increases the creep rupture
strength. [0045] Pd: improves the creep rupture strength and
oxidation resistance (steam oxidation resistance). [0046] Re:
strengths the base metal. [0047] Hf: forms HfC and strengthens the
base metal.
[0048] The ferritic heat-resistant steel of the present invention
is normalized at a temperature of 1,050 to 1,100.degree. C. and
tempered at a temperature of 750 to 800.degree. C. after molten and
forged, and then used as a tempered martensite tissue. From the
standpoint of obtaining toughness, a single phase of tempered
martensite tissue is desirable. However, when it is used as a
high-temperature boiler member, in the case where some degree of
lowering in toughness is permitted, it is allowed that the amounts
of ferrite forming elements such as Cr and Si, etc., are set to be
comparatively large in the limited ranges described above to
precipitate .delta. ferrite. In view of toughness and creep rupture
strength, it is known that a volume ratio of .delta. ferrite over
35% lowers the strength and toughness, so that the volume ratio is
limited to 30% or less.
[0049] The steel of the present invention is characterized by
containing C reduced to the half, lowered Al and Ni, and increased
Si in comparison to the idea of the conventional high-Cr
heat-resistant steel. By the compound effect of these, the
stability of the creep strength is improved for the first time, and
at the same time, the oxidation resistance (steam oxidation
resistance) is improved, whereby a high-Cr ferritic heat-resistant
steel usable up to 650.degree. C. is achieved. It is possible to
employ various manufacturing methods according to the purpose of
use of the steel, and the steel can be used not only as a steel
pipe but also as a steel sheet.
[0050] In addition, the ferritic heat-resistant steel of the
present invention has creep rupture strength remarkably improved in
comparison to the conventional ferritic heat-resistant steel, and
has strength and ductibility stable even during long-term use.
Therefore, by applying this steel to a high-temperature pressure
resistant portion of an ultra supercritical pressure boiler, the
steam temperature can be raised to about 650.degree. C., and the
plant efficiency of a thermal power plant can be improved. Further,
growth and exfoliation of steam oxidized scale and damage of
devices due to scattering of steam oxidized scale can be reduced.
Therefore, the durability of the plant is also improved, and a
remarkable effect in fuel consumption reduction such as coal and
CO.sub.2 emissions reduction in the thermal power plant can be
obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] FIG. 1 is a diagram showing results of the oxidation test
using steam when the Si amount was changed in 9CrWCo-based steel of
the present invention;
[0052] FIG. 2 is a diagram showing results of the creep rupture
test when the W amount was changed in the 9CrWCo-based steel of the
present invention;
[0053] FIG. 3 is a diagram showing results of the creep rupture
test by showing the Al amount on the horizontal axis when the Al
and Ni amounts were changed in the 9CrWCo-based steel of the
present invention; and
[0054] FIG. 4 is a diagram of the same test results as of FIG. 3 by
rewriting the Al amount on the horizontal axis into
(Al+0.1.times.Ni) amount.
BEST MODE FOR CARRYING OUT THE INVENTION
[0055] Hereinafter, examples/embodiments of the present invention
will be described by using actual examples.
[0056] The heat-resistant steels having the chemical compositions
shown in Table 1 of the present examples and comparative steels
were molten in a vacuum induction melting furnace, and forged into
50 kg ingots, respectively. The comparative steel A is nominal
9Cr1MoNbV steel, comparative steels B and C are nominal
9Cr0.5Mo1.8WNbV steels, and all of these have been made practicable
as boiler steels. After forming steel sheets with a thickness of 20
millimeters by hot forging, the steel sheets were normalized at
1,050.degree. C. for 60 minutes and tempered at 780.degree. C. for
60 minutes, and then subjected to a creep rupture test. In
addition, small-sized sheet-like test specimens were processed from
the steel sheets, and subjected to an oxidation test by using steam
at 650.degree. C.
TABLE-US-00001 TABLE 1 (wt %) C Si Mn P S Cr Mo W V Nb Ni Steel A
of the present invention 0.043 0.51 0.50 0.003 0.001 8.9 0.45 1.80
0.19 0.051 0.005 Steel B of the present invention 0.048 0.43 0.49
0.002 0.001 9.0 0.47 1.71 0.20 0.057 0.008 Steel C of the present
invention 0.075 0.40 0.48 0.003 0.001 9.2 0.50 1.90 0.18 0.060
0.007 Steel D of the present invention 0.019 0.60 0.52 0.003 0.001
9.5 0.45 1.81 0.21 0.059 0.010 Steel E of the present invension
0.012 0.75 0.51 0.002 0.001 8.8 0.40 2.00 0.18 0.050 0.015 Steel F
of the present invention 0.035 0.55 0.50 0.002 0.001 9.0 0.30 2.02
0.22 0.055 0.010 Comparative steel A 0.100 0.40 0.40 0.008 0.004
8.9 1.02 -- 0.21 0.058 0.28 Comparative steel B 0.110 0.30 0.35
0.010 0.006 9.0 0.48 1.81 0.20 0.062 0.22 Comparative steel C 0.082
0.07 0.42 0.008 0.005 9.1 0.52 1.76 0.19 0.053 0.20 Co N B Al Cu Mo
+ 0.5W C + N 0.1Ni + Al Steel A of the present invention 1.81 0.046
0.006 0.0010 0.005 1.350 0.089 0.0015 Steel B of the present
invention 1.91 0.043 0.005 0.0008 0.005 1.325 0.091 0.0016 Steel C
of the present invension 1.70 0.05 0.007 0.0012 0.005 1.450 0.125
0.0019 Steel D of the present invention 1.75 0.062 0.006 0.0009
0.004 1.355 0.081 0.0019 Steel E of the present invention 1.82
0.070 0.005 0.0010 0.005 1.400 0.082 0.0025 Steel F of the present
invention 1.75 0.055 0.006 0.0009 0.004 1.310 0.090 0.0019
Comparative steel A 0.00 0.048 0.000 0.007 0.03 1.02 0.15 0.0350
Comparative steel B 0.00 0.051 0.006 0.005 0.05 1.39 0.16 0.0270
Comparative steel C 0.00 0.043 0.006 0.006 0.04 1.40 0.13
0.0260
[0057] 100,000-hour creep rupture strengths at 600.degree. C. and
650.degree. C. estimated by plotting the results of the creep
rupture test of the steels of the examples of the present invention
and the comparative steels by temperature as a stress-rupture time
diagram and extrapolating these on the long-term side were shown in
Table 2.
TABLE-US-00002 TABLE 2 10.sup.5 h creep rupture strength
(N/mm.sup.2) 600.degree. C. 650.degree. C. Steel A of the present
invention 182 105 Steel B of the present invention 185 103 Steel C
of the present invention 188 107 Steel D of the present invention
185 105 Steel E of the present invention 180 102 Steel F of the
present invention 185 106 Comparative steel A 92 48 Comparative
steel B 135 68 Comparative steel C 140 70
[0058] The steels A and B of the present invention have creep
rupture strengths at 650.degree. C. after 100,000 hours which are
about twice the creep rupture strength of the comparative steel A
that was used as a conventional boiler heat-resistant steel for
many years, and are about 1.5 times the creep rupture strength of
high-strength comparative steels B and C, so that A and B have
revolutionary strengths. The results of the oxidation test using
steam conducted for the steels of the examples and comparative
steels are shown in Table 3, and the results show that the growth
of oxidized scale due to steam is restrained with respect to the
comparative steels, so that it is considered possible that the
steels of the present invention are sufficiently usable at a steam
temperature of 650.degree. C.
TABLE-US-00003 TABLE 3 Water vapor oxidized scale thickness (.mu.m)
(650.degree. C., 2000 h) Steel A of the present invention 120 Steel
B of the present invention 150 Steel C of the present invention 150
Steel D of the present invention 115 Steel E of the present
invention 80 Steel F of the present invention 120 Comparative steel
A 160 Comparative steel B 200 Comparative steel C 300
[0059] In the present invention, the experiments were performed by
setting the normalizing temperature to 1,050.degree. C., however, a
higher creep rupture strength can be obtained by raising the
normalizing temperature. However, at the same time, the toughness
lowers, so that the normalizing temperature is preferably in the
temperature range up to 1,100.degree. C.
[0060] The ferritic heat-resistant steel of the present invention
is suitable, in particular, as a material of a header and a main
steam pipe of a superheater of an ultra supercritical pressure
boiler with a steam temperature around 650.degree. C. The ferritic
heat-resistant steel of the present invention can be used not only
as a thick and large-diameter pipe material but also as a
small-diameter heat transfer pipe material.
INDUSTRIAL APPLICABILITY
[0061] The ferritic heat-resistant steel of the present invention
is highly industrially applicable not only as a material of a
header and a main steam pipe of a superheater and a thick and
large-diameter pipe material of an ultra supercritical pressure
boiler with a steam temperature around 650.degree. C. but also as a
small-diameter heat transfer pipe material.
* * * * *