U.S. patent number 10,385,438 [Application Number 15/486,595] was granted by the patent office on 2019-08-20 for heat resistant ferritic steel and method for producing the same.
This patent grant is currently assigned to NIPPON STEEL CORPORATION. The grantee listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Yoshitaka Nishiyama, Shunichi Otsuka.
United States Patent |
10,385,438 |
Nishiyama , et al. |
August 20, 2019 |
Heat resistant ferritic steel and method for producing the same
Abstract
There is provided a heat resistant ferritic steel including a
base material including, by mass percent, C: 0.01 to 0.3%, Si: 0.01
to 2%, Mn: 0.01 to 2%, P: at most 0.10%, 5: at most 0.03%, Cr: 7.5
to 14.0%, sol.Al: at most 0.3%, and N: 0.005 to 0.15%, the balance
being Fe and impurities, and an oxide film that is formed on the
base material and contains 25 to 97% of Fe and 3 to 75% of Cr. This
heat resistant ferritic steel is excellent in photoselective
absorptivity and oxidation resistance.
Inventors: |
Nishiyama; Yoshitaka (Tokyo,
JP), Otsuka; Shunichi (Tokyo, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
N/A |
JP |
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Assignee: |
NIPPON STEEL CORPORATION
(Tokyo, JP)
|
Family
ID: |
48469805 |
Appl.
No.: |
15/486,595 |
Filed: |
April 13, 2017 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20170218496 A1 |
Aug 3, 2017 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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14359735 |
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9657383 |
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PCT/JP2012/080198 |
Nov 21, 2012 |
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Foreign Application Priority Data
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Nov 22, 2011 [JP] |
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2011-255461 |
Dec 16, 2011 [JP] |
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2011-275725 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D
6/008 (20130101); C23C 8/14 (20130101); C22C
38/14 (20130101); C22C 38/24 (20130101); C22C
38/30 (20130101); C21D 6/004 (20130101); C23C
4/134 (20160101); C22C 38/06 (20130101); C22C
38/002 (20130101); C22C 38/20 (20130101); C22C
38/26 (20130101); C22C 38/44 (20130101); F24S
70/25 (20180501); C22C 38/005 (20130101); C22C
38/58 (20130101); C22C 38/42 (20130101); C23C
8/18 (20130101); C21D 6/002 (20130101); C22C
38/32 (20130101); C22C 38/38 (20130101); C22C
38/22 (20130101); C21D 6/005 (20130101); C21D
6/007 (20130101); F24S 70/225 (20180501); C22C
38/28 (20130101); C22C 38/001 (20130101); C22C
38/04 (20130101); C23C 4/11 (20160101); C22C
38/18 (20130101); C22C 38/02 (20130101); Y02E
10/40 (20130101); F24S 20/20 (20180501); C21D
2211/005 (20130101) |
Current International
Class: |
C22C
38/00 (20060101); C22C 38/30 (20060101); C21D
6/00 (20060101); C23C 8/10 (20060101); C22C
38/18 (20060101); C23C 8/14 (20060101); C23C
8/18 (20060101); C22C 38/38 (20060101); C22C
38/58 (20060101); C22C 38/02 (20060101); C22C
38/04 (20060101); C22C 38/20 (20060101); C22C
38/22 (20060101); C22C 38/24 (20060101); C22C
38/26 (20060101); F24S 70/225 (20180101); C22C
38/14 (20060101); C23C 4/11 (20160101); C23C
4/134 (20160101); C22C 38/06 (20060101); C22C
38/28 (20060101); C22C 38/32 (20060101); C22C
38/42 (20060101); C22C 38/44 (20060101); F24S
70/25 (20180101); F24S 20/20 (20180101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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52-126434 |
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Oct 1977 |
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JP |
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53-75132 |
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Jul 1978 |
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JP |
|
55-77667 |
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Jun 1980 |
|
JP |
|
58-195746 |
|
Nov 1983 |
|
JP |
|
60-57157 |
|
Apr 1985 |
|
JP |
|
62-182553 |
|
Aug 1987 |
|
JP |
|
64-47880 |
|
Feb 1989 |
|
JP |
|
04-52252 |
|
Feb 1992 |
|
JP |
|
07-325212 |
|
Dec 1995 |
|
JP |
|
2006-131945 |
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May 2006 |
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JP |
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2006-218595 |
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Aug 2006 |
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JP |
|
2008-101240 |
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May 2008 |
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JP |
|
2008-223128 |
|
Sep 2008 |
|
JP |
|
2010-78336 |
|
Apr 2010 |
|
JP |
|
2010-159487 |
|
Jul 2010 |
|
JP |
|
2011-190478 |
|
Sep 2011 |
|
JP |
|
1993/021356 |
|
Oct 1993 |
|
WO |
|
2011/026460 |
|
Mar 2011 |
|
WO |
|
Other References
Zhang et al., "Study of High . . . Ferritic Stainless Steel",
Proceedings of the Fifth Youth Academic Annual Conference of the
Chinese Society for Metals, Dec. 21, 2010. cited by applicant .
Shmakov, et al., J. Appl. Cryst. (1995), 28, 141-145. cited by
applicant.
|
Primary Examiner: Zheng; Lois L
Attorney, Agent or Firm: Clark & Brody
Parent Case Text
This application is a Divisional of U.S. Ser. No. 14/359,735 filed
on May 21, 2014, which is a national phase of PCT/JP2012/080198
filed on Nov. 21, 2012.
Claims
The invention claimed is:
1. A method for producing a heat resistant ferritic steel,
comprising the steps of: preparing a base material comprising, by
mass percent, C: 0.01 to 0.3%, Si: 0.01 to 2%, Mn: 0.01 to 2%, P:
at most 0.10%, S: at most 0.03%, Cr: 7.5 to 14.0%, sol. Al: at most
0.3%, and N: 0.005 to 0.15%, the balance being Fe and impurities;
and forming an oxide film on the base material by oxidizing the
base material at a temperature of 500 to 1150.degree. C. in a gas
atmosphere in which an oxygen partial pressure Po.sub.2 (atm)
satisfies Formula (3):
Po.sub.2.ltoreq.2.76.times.10.sup.15.times.exp
{-493.6.times.10.sup.3/(RT)} (3) where R is a gas constant whose
unit is JK.sup.-1mol.sup.-1, and T is a temperature whose unit is
K.
2. The method for producing the heat resistant ferritic steel
according to claim 1, wherein the base material further comprising
one or more elements selected from first to fourth groups in lieu
of some of Fe first group: Cu: at most 5%, Ni: at most 5%, and Co:
at most 5% second group: Ti: at most 1.0%, V: at most 1.0%, Nb: at
most 1.0%, Zr: at most 1.0%, and Hf: at most 1.0% third group: Mo:
at most 5%, Ta: at most 5%, W: at most 5%, and Re: at most 5%
fourth group: Ca: at most 0.1%, Mg: at most 0.1%, B: at most 0.1%,
and rare earth metal (REM): at most 0.1%.
Description
TECHNICAL FIELD
The present invention relates to a heat resistant steel and a
method for producing the steel and, more particularly, to a heat
resistant ferritic steel and a method for producing the steel.
BACKGROUND ART
In recent years, to achieve energy saving, the development of
highly efficient boilers has been advanced. For example, an ultra
supercritical pressure boiler uses higher temperature and pressure
of steam than those in a conventional boiler to enhance the energy
efficiency. Also, a boiler using wastes or biomass as a fuel other
than fossil fuels has been developed. Further, there has been
advanced the development of an electric power plant boiler
utilizing solar heat has been developed. In particular, a solar
thermal power plant boiler has attracted attention from the
viewpoints of energy saving and environmental preservation. As a
steel material of heat exchangers and the like for these boilers, a
heat resistant ferritic steel may be used. The boiler steam
temperature is high, and reaches a temperature close to 600.degree.
C. in some cases. The heat resistant ferritic steel used in such an
application is required to have excellent photoselective
absorptivity.
The photoselective absorptivity is a property such that
absorptivity changes in different wavelength regions. The term of
"excellent photoselective absorptivity" means that, for light
(electromagnetic wave) of visual to near-infrared region
(wavelength: 0.3 to 1 .mu.m, hereinafter referred to as "low
wavelength side"), the absorptivity is high, and for light
(electromagnetic wave) of medium- to far-infrared region
(wavelength: 2.5 to 25 .mu.m, hereinafter referred to as "high
wavelength side"), the radioactivity is low. In other words, the
photoselective absorptivity means that the reflectance of light on
the low wavelength side is low, and the reflectance of light on the
high wavelength side is high.
To attain excellent photoselective absorptivity, various methods
have been proposed so far. JP52-126434A (Patent Document 1) and
JP58-195746A (Patent Document 2) disclose methods in which the
photoselective absorptivity is enhanced by forming an organic
coated film on the surface of steel material. The paint disclosed
in Patent Document I consists of semiconductor particles having an
energy band width of 0.4 to 1.5 eV, a polyvinyl butyral organic
binder, and a solvent. The paint for photoselective absorbing film
disclosed in Patent Document 2 contains carboxylic acid amide
copolymer, oxides, and solvent-based paint.
JP53-75132A (Patent Document 3), JP60-57157A (Patent Document 4),
and JP62-182553A (Patent Document 5) disclose methods in which, to
attain the photoselective absorptivity, triiron tetraoxide
(Fe.sub.3O.sub.4: magnetite) is formed on the surface of steel by
chemical treatment or the like. Specifically, in Patent Document 3,
a selective absorbing film consisting of magnetite is formed by
immersing a base material consisting mainly of iron in a
high-temperature alkaline solution. In Patent Document 4, a
selective absorbing film consisting of magnetite is formed by
electrooxidizing a base material consisting mainly of iron in an
acidic solution. In Patent Document 5, a selective absorbing film
consisting of magnetite is formed by electrooxidizing a base
material consisting mainly of iron in an acidic solution after the
surface of base material has been iron-plated.
JP55-77667A (Patent Document 6) discloses a method in which an
oxide film consisting mainly of Fe that has a film thickness of 500
to 2000 angstroms and contains 11.00 to 26.00 wt % of Cr is formed
by a chemical treatment method or the like method, and the surface
of oxide film is mirror-polished. Patent Document 6 describes that
the photoselective absorptivity is enhanced by this method.
JP7-325212A (Patent Document 7) discloses a method in which a film
consisting of iron oxide is formed on the surface of steel by
spraying. Patent Document 7 describes that the photoselective
absorptivity is enhanced by this method.
DISCLOSURE OF THE INVENTION
In recent years, to increase power generation efficiency, the
boiler steam temperature in solar power generation is as high as
500 to 600.degree. C., and in the future, it is expected that the
boiler steam temperature will become much higher. In such a
high-temperature environment, it is difficult to maintain the
photoselective absorptivity. Since the coated film described in
Patent Documents 1 and 2 is organic, the coated film is less
applicable in the above-described high-temperature environment. The
oxide film described in Patent Documents 3 to 5 consists of
magnetite. Therefore, the radioactivity at high temperatures, that
is, the radioactivity on the high wavelength side is high, and the
photoselective absorptivity is poor. The oxide film described in
Patent Document 6 may have low photoselective absorptivity at high
temperatures. The oxide film described in Patent Document 7 may
have high radioactivity especially at high temperatures, that is,
high radioactivity on the high wavelength side.
An objective of the present invention is to provide a heat
resistant ferritic steel excellent in photoselective
absorptivity.
The heat resistant ferritic steel in accordance with the present
invention includes a base material comprising, by mass percent, C:
0.01 to 0.3%, Si: 0.01 to 2%, Mn: 0.01 to 2%, P: at most 0.10%, S:
at most 0.03%, Cr: 7.5 to 14.0%, sol.Al: at most 0.3%, and N: 0.005
to 0.15%, the balance being Fe and impurities, and an oxide film
which is formed on the base material and whose chemical composition
excluding oxygen and carbon contains 25 to 97% of Fe and 3 to 75%
of Cr. The oxide film contains spinel-type oxides and
Cr.sub.2O.sub.3.
The heat resistant ferritic steel in accordance with the present
invention is excellent in photoselective absorptivity.
BEST MODE FOR CARRYING CUT THE INVENTION
An embodiment of the present invention will now be described in
detail. The symbol "%" for the content of each element means "% by
mass" unless otherwise noted.
The present inventors conducted researches and studies on the heat
resistant ferritic steel excellent in photoselective absorptivity.
As a result, the present inventors obtained the following
findings.
(1) Among various oxides, triiron tetraoxide (hereinafter, referred
to as magnetite), which is an Fe-based oxide, exhibits excellent
absorptivity for light (electromagnetic wave) of visual to
near-infrared region (wavelength: 0.3 to 1 .mu.m, "low wavelength
side"). However, for light (electromagnetic wave) of medium- to
far-infrared region (wavelength: 2.5 to 25 .mu.m, "high wavelength
side"), magnetite has a high radioactivity. That is to say, in a
high-temperature environment of 500 to 600.degree. C., magnetite is
liable to radiate heat.
(2) If the magnetite is made thin, the radioactivity for the light
(electromagnetic wave) on the high wavelength side decreases.
However, even if the oxide film consisting of very thin magnetite
is formed, in high-temperature environments, Fe diffuses from the
base material to the oxide film, and the oxide film grows and
becomes thick. If the oxide film becomes thick, the photoselective
absorptivity decreases.
(3) In the case where the oxide film contains Fe-based oxides and
Cr-based oxides, or in the case where the Fe-based oxides
themselves in the oxide film contain Cr, the radioactivity on the
high wavelength side can be restrained. Chromium in the oxide film
further restrains the growth of oxide film in high-temperature
environments. Therefore, Cr can maintain the photoselective
absorptivity of the oxide film for a long period of time.
(4) If the chemical composition of oxide film contains 25 to 97% of
Fe and 3 to 75% of Cr, a heat resistant ferritic steel excellent in
photoselective absorptivity can be obtained.
(5) Preferably, the oxide film contains spinel-type oxides and
Cr.sub.2O.sub.3 (chromia). In this description, the spinel-type
oxides include magnetite as well. The spinel-type oxides other than
magnetite are oxides that contain, for example, Fe and Cr, and have
spinel-type structures.
Chromia (Cr.sub.2O.sub.3) enhances the reflectance on the high
wavelength side, and restrains the heat radiation of steel.
Further, Cr.sub.2O.sub.3 enhances the oxidation resistance.
Therefore, if the oxide film contains not only spinel-type oxides
but also Cr.sub.2O.sub.3, the heat resistant ferritic steel having
that oxide film is excellent in oxidation resistance, and also is
excellent in photoselective absorptivity. Specifically, the
reflectance of light (electromagnetic wave) on the low wavelength
side is low, and the reflectance of light on the high wavelength
side is high.
(6) Further preferably, in the case where the maximum diffraction
peak intensity of spinel-type oxides obtained by X-ray surface
analysis (XRD) is defined as Is, and the maximum diffraction peak
intensity of Cr.sub.2O.sub.3 is defined as Ic, if Formula (1) is
satisfied, the heat resistant ferritic steel attains excellent
photoselective absorptivity. This is because, if Formula (1) is
satisfied, Cr.sub.2O.sub.3 of an amount sufficient to enhance the
reflectance on the high wavelength side is contained in the oxide
film. 0.010.ltoreq.Ic/Is.ltoreq.10 (1)
(7) Still further preferably, Fe.sub.2O.sub.3 (hematite) contained
in the oxide film is restrained. If a large amount of
Fe.sub.2O.sub.3 is contained in the oxide film, the reflectance of
light (electromagnetic wave) on the low wavelength side of the
oxide film is high, and the reflectance of light on the high
wavelength side is low. As a result, the photoselective
absorptivity decreases. Therefore, the amount of Fe.sub.2O.sub.3
(hematite) in the oxide film is preferably smaller.
More specifically, in the case where the maximum diffraction peak
intensity of Fe.sub.2O.sub.3 is defined as Ih, Formula (2) is
preferably satisfied. If the oxide film of the produced heat
resistant ferritic steel satisfies Formula (2), since
Cr.sub.2O.sub.3 of an amount sufficient to enhance the reflectance
on the high wavelength side is contained in the oxide film with
respect to the content of Fe.sub.2O.sub.3, excellent photoselective
absorptivity can be attained. Ih/(Is+Ic).ltoreq.0.05 (2)
(8) The oxide film of the heat resistant ferritic steel is formed
by oxidation treatment. In the oxidation treatment, if the oxygen
partial pressure Po.sub.2 (atm) in a gas atmosphere satisfies
Formula (3), Fe.sub.2O.sub.3 is restrained effectively. More
specifically, if the oxygen partial pressure Po.sub.2 (atm)
satisfies Formula (3), the formed oxide film satisfies Formula (2):
Po.sub.2.ltoreq.2.76.times.10.sup.15.times.exp{-493.6.times.10.sup.3/(RT)-
} (3) where R is a gas constant whose unit is JK.sup.-1mol.sup.-1,
and T is a temperature whose unit is K.
The heat resistant ferritic steel in accordance with this
embodiment, completed on the basis of the above findings, and the
method for producing the steel are as described below.
The heat resistant ferritic steel includes a base material and an
oxide film. The base material comprises, by mass percent, C: 0.01
to 0.3%, Si: 0.01 to 2%, Mn: 0.01 to 2%, P: at most 0.10%, S: at
most 0.03%, Cr: 7.5 to 14.0%, sol.Al: at most 0.3%, and N: 0.005 to
0.15%, the balance being Fe and impurities. The oxide film is
formed on the base material and has a chemical composition,
excluding oxygen and carbon in the oxide film, containing 25 to 97%
of Fe and 3 to 75% of Cr. The oxide film contains spinel-type
oxides and Cr.sub.2O.sub.3.
In this case, the heat resistant ferritic steel has excellent
photoselective absorptivity.
Preferably, in the case where the maximum diffraction peak
intensity of Cr.sub.2O.sub.3 obtained by X-ray diffraction is
defined as Ic, and the maximum diffraction peak intensity of
spinel-type oxides obtained by the X-ray diffraction is defined as
Is, the following Formula (1) is satisfied.
0.010.ltoreq.Ic/Is.ltoreq.10 (1)
In this case, excellent photoselective absorptivity can be
attained.
The above-described base material of the heat resistant ferritic
steel may further comprises one or more elements selected from
first to fourth groups in lieu of some of Fe.
First group: Cu: at most 5%, Ni: at most 5%, and Co: at most 5%
Second group: Ti: at most 1.0%, V: at most 1.0%, Nb: at most 1.0%,
Zr: at most 1.0%, and Hf: at most 1.0%
Third group: Mo: at most 5%, Ta: at most 5%, W: at most 5%, and Re:
at most 5%
Fourth group: Ca: at most 0.1%, Mg: at most 0.1%, B: at most 0.1%,
and rare earth metal (REM): at most 0.1%
The method for producing the heat resistant steel in accordance
with this embodiment includes a step of preparing the base material
having the above-described chemical composition, and a step of
forming an oxide film on the base material by oxidizing the base
material at a temperature of 500 to 1150.degree. C. in a gas
atmosphere in which the oxygen partial pressure Po.sub.2 (atm)
satisfies Formula (3):
Po.sub.2.ltoreq.2.76.times.10.sup.15.times.exp{-493.6.times.10.sup.3/(RT)-
} (3) where R is a gas constant whose unit is JK.sup.-1mol.sup.-1,
and T is a temperature whose unit is K.
The heat resistant ferritic steel produced by this production
method has excellent photoselective absorptivity.
In the following, the details of the heat resistant ferritic steel
in accordance with this embodiment are explained.
[Configuration of Heat Resistant Ferritic Steel]
The heat resistant ferritic steel in accordance with this
embodiment includes a base material and an oxide film formed on the
base material.
[Configuration of Base Material]
The base material has the following chemical composition.
C: 0.01 to 0.3%
Carbon (C) is an austenite stabilizing element for making the base
material martensitic. Furthermore, C enhances the high-temperature
strength of steel by forming carbides. On the other hand, if the C
content is too high, carbides precipitate excessively, and
therefore the workability and weldability of steel are decreased.
Therefore, the C content is set to 0.01 to 0.3%. The lower limit of
C content is preferably higher than 0.01%, further preferably
0.03%. The upper limit of C content is preferably lower than 0.3%,
further preferably 0.15%.
Si: 0.01 to 2%
Silicon (Si) deoxidizes the steel. Furthermore, Si enhances the
steam oxidation resistance of steel. On the other hand, if the Si
content is too high, the toughness of steel is decreased. Further,
since the oxide film contains Si, if the Si content is too high,
the steel becomes liable to dissipate heat, and the photoselective
absorptivity decreases. Therefore, the Si content is set to 0.01 to
2%. The lower limit of Si content is preferably higher than 0.01%,
further preferably 0.05%, and still further preferably 0.1%. The
upper limit of Si content is preferably lower than 2%, further
preferably 1.0%, and still further preferably 0.5%.
Mn: 0.01 to 2%
Manganese (Mn) deoxidizes the steel. Furthermore, Mn forms MnS by
combining with S in the base material, and therefore enhances the
hot workability of steel. On the other hand, if the Mn content is
too high, the steel is embrittled, and also the high-temperature
strength of steel is decreased. Therefore, the Mn content is set to
0.01 to 2%. The lower limit of Mn content is preferably higher than
0.01%, further preferably 0.05%, and still further preferably 0.1%.
The upper limit of Mn content is preferably lower than 2%, further
preferably 1.0%, and still further preferably 0.8%.
P: at most 0.10%
S: at most 0.03%
Phosphorus (P) and sulfur (5) are impurities. P and S segregate at
crystal grain boundaries in the base material, and decrease the hot
workability of steel. Furthermore, P and S concentrate at the
interface between the oxide film and the base material, and
decrease the adhesiveness of oxide film. Therefore, the P content
and the S content are preferably as low as possible. The P content
is set to at most 0.10%, and the S content is set to at most 0.03%.
The P content is preferably at most 0.03%, and the S content is
preferably at most 0.015%.
Cr: 7.5 to 14.0%
Chromium (Cr) enhances the oxidation resistance of steel.
Furthermore, Cr is contained in the oxide film, and enhances the
photoselective absorptivity of steel. In particular, Cr enhances
the reflectance on the high wavelength side, and contributes to the
suppression of heat radiation of steel. Furthermore, Cr enhances
the adhesiveness of steel relative to the oxide film. On the other
hand, if the Cr content is too high, the amount of delta ferrite is
increased, and therefore the strength and toughness of steel are
decreased. Furthermore, much Cr.sub.2O.sub.3 is contained in the
oxide film on the base material by oxidation treatment, and in
particular, the light absorption on the low wavelength side is
decreased. Therefore, the Cr content is set to 7.5 to 14.0%. The
lower limit of Cr content is preferably higher than 7.5%, further
preferably 7.7%, and still further preferably 8.0%. The upper limit
of Cr content is preferably lower than 14.0%, further preferably
12.0%, and still further preferably 10.0%.
sol.Al: at most 0.3%
Aluminum (Al) deoxidizes the steel. On the other hand, if the Al
content is too high, the cleanliness of steel is decreased, and the
hot workability of steel is decreased. Therefore, the sol.Al
content is set to at most 0.3%. The lower limit of sol.Al content
is preferably 0.001%. The upper limit of sol.Al content is
preferably lower than 0.3%, further preferably 0.1%. The sol.Al
means acid soluble Al.
N: 0.005 to 0.15%
Nitrogen (N) solid-solution strengthens the steel. Furthermore, N
forms nitrides and/or carbo-nitrides, and therefore precipitation
strengthens the steel. On the other hand, if the N content is too
high, the nitrides and carbo-nitrides are coarsened, and the
toughness of steel is decreased. Therefore, the N content is set to
0.005 to 0.15%. The lower limit of N content is preferably higher
than 0.005%, further preferably 0.01%. The upper limit of N content
is preferably lower than 0.15%, further preferably 0.10%.
The balance of the base material of the heat resistant ferritic
steel in accordance with this embodiment consists of Fe and
impurities. The term "impurities" so referred to in this
description indicates the elements that are mixed on account of ore
or scrap used as a raw material of steel, environments in the
process of production, and the like. An impurity is, for example,
oxygen (O).
Furthermore, the base material of the heat resistant ferritic steel
in accordance with this embodiment may contain one or more elements
selected from the following first to fourth groups in lieu of some
of Fe.
First group: Cu: at most 5%, Ni: at most 5%, and Co: at most 5%
Second group: Ti: at most 1.0%, V: at most 1.0%, Nb: at most 1.0%,
Zr: at most 1.0%, and Hf: at most 1.0%
Third group: Mo: at most 5%, Ta: at most 5%, W: at most 5%, and Re:
at most 5%
Fourth group: Ca: at most 0.1%, Mg: at most 0.1%, B: at most 0.1%,
and rare earth metal (REM): at most 0.1%
First group: Cu: at most 5%, Ni: at most 5%, and Co: at most 5%
All of copper (Cu), nickel (Ni), and cobalt (Co) are selective
elements. These elements are austenite stabilizing elements, and
restrain the formation of delta ferrite. If at least one of these
elements is contained even a little, the above-described effect can
be achieved. On the other hand, if the contents of these elements
are too high, the creep strength on the long time side is
decreased. Therefore, the Cu content is set to at most 5%, the Ni
content is set to at most 5%, and the Co content is set to at most
5%. The lower limit of the content of each of these elements is
preferably 0.005%. The upper limit of each of these elements is
preferably lower than 5%, further preferably 3%, and still further
preferably 1%.
Second group: Ti: at most 1.0%, V: at most 1.0%, Nb: at most 1.0%,
Zr: at most 1.0%, and Hf: at most 1.0%
All of titanium (Ti), vanadium (V), niobium (Nb), zirconium (Zr),
and Hafnium (Hf) are selective elements. These elements form
carbides, nitrides, and carbo-nitrides, and precipitation
strengthen the steel. If at least one of these elements is
contained even a little, the above-described effect can be
achieved. On the other hand, if the contents of these elements are
too high, the workability of steel is decreased. Therefore, the Ti
content is set to at most 1.0%, the V content is set to at most
1.0%, the Nb content is set to at most 1.0%, the Zr content is set
to at most 1.0%, and the Hf content is set to at most 1.0%. The
lower limit of the content of each of these elements is preferably
0.01%. The upper limit of the content of each of these elements is
preferably lower than 1.0%, further preferably 0.8%, and still
further preferably 0.41.
Third group: Mo: at most 5%, Ta: at most 5%, W: at most 5%, and Re:
at most 5%
All of molybdenum (Mo), tantalum (Ta), tungsten (W), and rhenium
(Re) are selective elements. All of these elements enhance the
strength of steel. If at least one of these elements is contained
even a little, the above-described effect can be achieved. On the
other hand, if the contents of these elements are too high, the
toughness, ductility, and workability of steel are decreased.
Therefore, the Mo content is set to at most 5%, the Ta content is
set to at most 5%, the W content is set to at most 5%, and the Re
content is set to at most 5%. The lower limit of the content of
each of these elements is preferably 0.01%, further preferably
0.1%. The upper limit of the content of each of these elements is
preferably lower than 5%, further preferably 4%, and still further
preferably 3%.
Fourth group: Ca: at most 0.1%, Mg: at most 0.1%, B: at most 0.1%,
and rare earth metal (REM): at most 0.1%
All of calcium (Ca), magnesium (Mg), boron (B), and rare earth
metal (REM) are selective elements. All of these elements enhance
the strength, workability, and oxidation resistance of steel. If at
least one of these elements is contained even a little, the
above-described effects can be achieved. On the other hand, if the
contents of these elements are too high, the toughness and
weldability of steel are decreased. Therefore, the Ca content is
set to at most 0.1%, the Mg content is set to at most 0.1%, the B
content is set to at most 0.1%, and the REM content is set to at
most 0.1%. The lower limit of the content of each of these elements
is preferably 0.0015%. The upper limit of the content of each of
these elements is preferably lower than 0.1%, further preferably
0.05%. The "REM" is the general term of seventeen elements in which
yttrium (Y) and scandium (Sc) are added to the elements ranging
from lanthanum (La) of atomic number 57 to lutetium (Lu) of atomic
number 71 in the periodic table.
[Oxide Film]
The oxide film of the heat resistant ferritic steel in accordance
with this embodiment is formed on the base material. The heat
resistant ferritic steel in accordance with this embodiment has
excellent photoselective absorptivity because of having the oxide
film explained below.
[Chemical Composition of Oxide Film]
The oxide film consists of oxides. The chemical composition of
oxide film contains 25 to 97% of Fe and 3 to 75% of Cr. The content
of chemical composition of oxide film described here is a content
excluding oxygen (O) and carbon (C). Other than Fe and Cr, about 5%
or less of an element of Al, Si, Ti, Mn, Nb or the like having a
high affinity to oxygen may be contained. The heat resistant
ferritic steel can attain excellent oxidation resistance and
photoselective absorptivity because the oxide film has the
above-described chemical composition, especially because the Cr
content meets the condition of the above-described content
range.
The chemical composition of oxide film can be measured by
subjecting the base material having the oxide film to EDX (energy
dispersive X-ray spectroscopy) from the surface thereof. The
chemical composition is determined from the detected elements
excluding oxygen (O) and carbon (C) as described above.
The preferable chemical composition contains 50 to 95% of Fe and 5
to 50% of Cr. The further preferable chemical composition contains
70 to 95% of Fe and 5 to 30% of Cr.
[Structure of Oxide Film]
The oxide film contains a plurality of oxides. Preferably, the
oxide film mainly contains spinel-type oxides and Cr.sub.2O.sub.3.
The term "mainly" described here means that, in the case where the
cross section in the thickness direction of oxide film is
microscopically observed, the area ratio of the spinel-type oxides
and Cr.sub.2O.sub.3 is 60% or more of the whole oxide film.
The oxide film may contain oxides containing Al, Si, Ti, Mn, and Nb
in addition to spinel-type oxides and Cr.sub.2O.sub.3. If the oxide
film contains spinel-type oxides and Cr.sub.2O.sub.3, the heat
resistant ferritic steel can have excellent photoselective
absorptivity. More specifically, by causing the oxide film to
contain Cr.sub.2O.sub.3, the reflectance on the high wavelength
side is further enhanced, and the radiation of heat in
high-temperature environments is restrained.
The oxides in the oxide film are identified by XRD (X-ray
diffractometry) in which X-rays are applied to the surface of the
base material having the oxide film (heat resistant ferritic
steel). In the XRD, a Co bulb may be used as an X-ray bulb, or any
other bulbs may be used.
Preferably, the heat resistant ferritic steel satisfies Formula
(1): 0.010.ltoreq.Ic/Is.ltoreq.10 (1) where Is means the maximum
diffraction peak intensity of spinel-type oxides in the oxide film,
which is obtained by XRD. The symbol Ic means the maximum
diffraction peak intensity of Cr.sub.2O.sub.3 in the oxide film.
The maximum diffraction peak intensity so referred to in this
description corresponds, for spinel-type oxides, to the intensity
on the (311) plane, and corresponds, for Cr.sub.2O.sub.3, to the
intensity on the (104) plane. Generally, the volume ratio of each
of oxides is determined from the integration of peak intensities.
However, as described above, if the oxide film satisfies Formula
(1) defined by the maximum diffraction peak intensity ratio, the
heat resistant ferritic steel exhibits excellent photoselective
absorptivity.
It is defined that IRl=Ic/Is. If IRl is less than 0.010, the ratio
of Cr.sub.2O.sub.3 in the oxide film is excessively low. Therefore,
the photoselective absorptivity decreases. In particular, the
reflectance on the high wavelength side decreases. Furthermore, the
oxidation resistance of the heat resistant ferritic steel
decreases.
On the other hand, if IRl exceeds 10, the ratio of Cr.sub.2O.sub.3
in the oxide film is excessively high. In this case, although the
oxidation resistance of the heat resistant ferritic steel
increases, the photoselective absorptivity decreases
remarkably.
If IRl satisfies Formula (1), the heat resistant ferritic steel is
liable to absorb light, and is less liable to dissipate heat.
Specifically, the reflectance on the low wavelength side decreases,
and the reflectance on the high wavelength side increases. The
lower limit of IRl is preferably higher than 0.010, further
preferably 0.020, and still further preferably 0.050. The upper
limit of IRl is preferably lower than 10, further preferably 7, and
still further preferably 5.
For the oxide film in accordance with this embodiment, the content
of Fe.sub.2O.sub.3 is preferably lower. If the content of
Fe.sub.2O.sub.3 is high, the reflectance of light (electromagnetic
wave) on the low wavelength side of oxide film increases, and the
reflectance of light on the high wavelength side decreases. That is
to say, the photoselective absorptivity of oxide film decreases.
Therefore, the content of Fe.sub.2O.sub.3 is preferably lower.
More specifically, the oxide film of the heat resistant ferritic
steel preferably satisfies Formula (2): Ih/(Is+Ic).ltoreq.0.05 (2)
where Ih means the maximum diffraction peak intensity of
Fe.sub.2O.sub.3 in the oxide film. The maximum diffraction peak
intensity so referred to in this description corresponds, for
Fe.sub.2O.sub.3, to the intensity on the (104) plane. Generally,
the volume ratio of each of oxides is determined from the
integration of peak intensities. However, as described above, if
the oxide film satisfies Formula (2) defined by the maximum
diffraction peak intensity ratio, the heat resistant ferritic steel
exhibits quite excellent photoselective absorptivity.
It is defined that IRh=Ih/(Is+Ic). If IRh is 0.05 or less, the
ratio of Fe.sub.2O.sub.3 in the oxide film is sufficiently low.
Therefore, the heat resistant ferritic steel is liable to absorb
light, and less liable to dissipate heat. Specifically, the
reflectance on the low wavelength side decreases, and the
reflectance on the high wavelength side increases. The lower limit
of IRh is preferably lower than 0.05, further preferably 0.010, and
still further preferably 0.005.
The oxide film in accordance with this embodiment may contain FeO
(wustite). Wustite is less liable to appear on the surface of oxide
film because it is formed on the base material side as compared
with magnetite, which is a spinel-type oxide. That is to say,
wustite is less liable to be formed in the outermost layer of oxide
film. Therefore, wustite does not substantially exert an influence
on the photoselective absorptivity. Therefore, the oxide film may
contain or need not contain wustite.
[Production Method]
There is explained one example of a method for producing the heat
resistant ferritic steel in accordance with this embodiment.
The method for producing the heat resistant ferritic steel in
accordance with this embodiment includes a step of preparing the
base material (base material preparing step) and a step of
oxidizing the prepared base material to form the oxide film on the
base material (oxidizing step). In the following, the base material
preparing step and the oxidizing step are described in detail.
[Ease Material Preparing Step]
A starting material having the above-described chemical composition
is prepared. The starting material may be a slab, bloom, or billet
produced by the continuous casting process (including the round
continuous casting). Also, the starting material may be a billet
produced by hot-working an ingot produced by the ingot-making
process, or may be a billet produced by hot-working a slab or
bloom.
The prepared starting material is charged into a heating furnace or
a soaking pit, and is heated. The heated starting material is
hot-worked to produce the base material. For example, as the hot
working, the Mannesmann process is carried out. Specifically, the
starting material is piercing-rolled by using a piercing machine to
foam a material pipe. Successively, the starting material is
elongation-rolled and sized by using a mandrel mill and a sizing
mill to produce the base material as a seamless steel pipe. As the
hot working, the hot-extrusion process or the hot forging process
may be carried out to produce the base material. As necessary, the
base material produced by hot working may be subjected to heat
treatment, or may be subjected to cold working. The cold working
is, for example, cold rolling or cold drawing. By the
above-described step, the base material as a seamless pipe is
produced.
The base material may be a steel plate. In this case, the base
material used as a steel plate is produced by hot-working the
starting material. Also, the base material used as a bar steel may
be produced by hot working. Further, the base material used as a
welded steel pipe may be produced by welding a steel plate.
[Oxidizing Step]
Successively, the oxide film is formed on the produced base
material. The oxide film is produced, for example, by the method
described below.
The base material is subjected to oxidation treatment. The
oxidation treatment is performed in a gas atmosphere of, for
example, mixed gas or combustion gas. The preferable oxidation
treatment temperature is 1150.degree. C. or lower, and the
preferable oxidation treat time is 3 hours or shorter.
If the oxidation treatment temperature is too high, the ratio of
the spinel-type oxides in the oxide film increases excessively, and
the ratio of Cr.sub.2O.sub.3decreases excessively. If the oxidation
treatment temperature is too low, the oxide film is formed unevenly
on the base material, and in some cases, the oxide film cannot
cover the base material. For this reason, the photoselective
absorptivity decreases. Therefore, the preferable oxidation
treatment temperature is 500.degree. C. to 1150.degree. C.
Preferably, by controlling the gas atmosphere of oxidation
treatment, and by changing the structure of oxide film, an oxide
film satisfying Formula (2) can be obtained. More specifically, it
is preferable that the oxygen partial pressure Po.sub.2 (atm) in
the gas atmosphere of oxidation treatment satisfy Formula (3).
Po.sub.2.ltoreq.2.76.times.10.sup.15.times.exp{(-493.6.times.10.sup.3/(RT-
)} (3)
If Po.sub.2 satisfies Formula (3), the oxygen partial pressure in
the gas atmosphere thermodynamically becomes lower than the oxygen
partial pressure necessary for steady formation of Fe.sub.2O.sub.3.
Therefore, the formation of Fe.sub.2O.sub.3 is restricted. In the
case where composition fluctuations caused by a gas flow in the gas
atmosphere and composition fluctuations on account of combustion
state are considered, further preferably, the oxygen partial
pressure Po.sub.2 satisfies Formula (4).
Po.sub.2.ltoreq.1.00.times.10.sup.14.times.exp{-493.6.times.10.sup.3/(RT)-
} (4)
Concerning the gas atmosphere of oxidation treatment, for example,
the air-fuel ratio of combustion gas may be controlled.
Specifically, if the air-fuel ratio is controlled, the gas
composition in the gas atmosphere changes. Based on the gas
composition in the oxidation treatment gas atmosphere, the oxygen
partial pressure is determined. Based on the gas composition, the
oxygen partial pressure can be calculated by using, for example,
the thermodynamic computation software "MALT-2 for WIN".
As a fuel, natural gas, methane, propane, butane, or the like may
be used. Also, a mixed gas such as H.sub.2--H.sub.2O or
CO--CO.sub.2 may be used. Further, an oxidation treatment gas
atmosphere in which these gases are mixed may be used.
Oxidation treatment that doubles as normalizing treatment
(normalizing) may be performed. In this case, the cold-rolled base
material is normalized. The preferable oxidation treatment
temperature in this case is 900.degree. C., or higher. The
oxidation treatment time is preferably 30 minutes or shorter,
further preferably 20 minutes or shorter. If the oxidation
treatment temperature is too high and if the oxidation treatment
time is too long, the oxide film becomes excessively thick. In this
case, the adhesiveness between oxide film and base material
decreases, and the oxide film sometimes peels off. For this reason,
the photosensitive absorptivity of the heat resistant ferritic
steel decreases.
Oxidation treatment that doubles as tempering treatment
(low-temperature annealing) may be performed. In this case, the
normalized base material is subjected to the oxidation treatment
that doubles as tempering treatment. In this case, the preferable
oxidation treatment temperature is 650 to 850.degree. C., and the
preferable oxidation treatment time is 2 hours or shorter.
The oxidation treatment may be performed after the normalizing
treatment and tempering treatment. In this case, it is preferable
that the base material structure formed by the normalizing
treatment and tempering treatment be not changed in property. For
this reason, the preferable oxidation treatment temperature is not
higher than the tempering treatment temperature. Since the
oxidation treatment temperature is as low as not higher than the
tempering treatment temperature, the oxidation rate is low.
Therefore, the oxidation treatment time may be long. However,
considering the productivity, the preferable oxidation treatment
time is 3 hours or shorter.
The oxide film may be formed on the whole surface of base material.
However, the oxide film may be formed only on the surface required
to be excellent in photoselective absorptivity, such as the outer
peripheral surface of a pipe, which is the base material.
The above-described oxidation treatment may be performed one time
or a plurality of times. After each step of normalizing treatment,
tempering treatment, and oxidation treatment, straightening or the
like may be performed mechanically. In the case where oil or dirt
sticks to the surface of oxide film formed on the base material,
even if the treatment of degreasing or cleaning is performed, the
properties of oxide film are not changed.
In the above-described oxidation treatment, the composition of
oxide film can be changed by controlling the concentration of
combustion gas. By following the above-described steps, the heat
resistant ferritic steel having the base material and the oxide
film of this embodiment can be produced.
In the above-described oxidation treatment step, if Fe.sub.2O.sub.3
(hematite) is foamed on the outermost layer of oxide film as the
result that the oxygen partial pressure Po.sub.2 in the gas
atmosphere of oxidation treatment does not satisfy Formula (3), the
Fe.sub.2O.sub.3 (hematite) may be removed by shotblasting
treatment. Even in this case, the oxide film containing magnetite,
spinel-type oxides, and Cr.sub.2O.sub.3 of this embodiment is
formed.
EXAMPLE 1
Heat resistant ferritic steels having various chemical compositions
were produced, and the photoselective absorptivity thereof was
examined.
[Examination Method]
Heat resistant ferritic steels of steel Nos. 1 to 9 having the
chemical compositions given in Table 1 were melted to produce
ingots.
TABLE-US-00001 TABLE 1 Steel Chemical composition (mass %, balance
being Fe and impurities) No. C Si Mn P S Cr sol.Al N Others 1 0.11
0.32 0.41 0.011 0.003 9.4 0.01 0.04 -- 2 0.11 0.25 0.4 0.015
<0.001 8.9 <0.01 0.04 0.2V, 0.9Mo 3 0.04 0.34 0.54 0.011
0.002 9.2 <0.01 0.05 2Co, 2.4W 4 0.08 0.21 0.56 0.023 0.002 9.1
0.03 0.01 0.02Nd, 2.9W, 0.2Ni 5 0.04 1.12 0.14 0.029 0.001 13.2
0.08 0.02 0.6Nb 6 0.1 0.24 0.35 0.016 <0.001 9.2 <0.01 0.05
0.2V, 0.4Mo, 1.8W 7 0.18 0.35 1.08 0.008 0.005 8.3 0.03 0.01 2.5Re,
0.14Ti 8 0.11 0.16 0.45 0.013 0.001 15.4* 0.02 0.02 1.6Cu, 1.5Mo,
1.3Ni 9 0.09 0.21 0.42 0.015 0.001 7.0* 0.04 0.01 2.1W, 0.15V
*indicates deviation from range specified in present invention.
Referring to Table 1, for steels of steel Nos. 1 to 7, the chemical
composition of base material was within the range of chemical
composition of the present invention. On the other hand, for steels
of steel Nos. 8 and 9, the chemical composition of base material
was out of the range of chemical composition of the present
invention. Specifically, the Cr content of steel No. 8 exceeded the
upper limit of Cr content of the base material of the present
invention. The Cr content of steel No. 9 was lower than the lower
limit of Cr content of the base material of the present
invention.
Each of the produced ingots was hot-rolled and cold-rolled to
produce a base material. In this example, the base material was a
steel plate. The produced base material was subjected to oxidation
treatment under various conditions to form an oxide film on the
surface of base material. Table 2 gives steel No. used in each of
test Nos. and oxidation treatment conditions.
TABLE-US-00002 TABLE 2 Oxidation treatment Oxide film Maximum
diffraction peak Reflectance Test Steel Temperature .times.
Chemical intensity ratio (%) No. No. time composition (%) Oxides
IR1 = Ic/Is IRh = Ih/(Is + Ic) 0.5 .mu.m 10 .mu.m 1 1 1050.degree.
C. .times. 10 min Fe: 50, Cr: 46 Spine1, Cr.sub.2O.sub.3 3.333
<0.005 8 60 2 1 1050.degree. C. .times. 10 min Fe: 95, Cr: 3
Spine1, Cr.sub.2O.sub.3 0.008 <0.005 2 33 3 2 1050.degree. C.
.times. 10 min Fe: 72, Cr: 25 Spine1, Cr.sub.2O.sub.3 1.111
<0.005 4 65 4 2 750.degree. C. .times. 60 min Fe: 88, Cr: 8
Spine1, Cr.sub.2O.sub.3 0.025 <0.005 3 46 5 2 600.degree. C.
.times. 60 min Fe: 76, Cr: 22 Spine1, Cr.sub.2O.sub.3 0.526
<0.005 6 62 6 2 1050.degree. C. .times. 30 min Fe: 80, Cr: 16
Spine1, Cr.sub.2O.sub.3 0.074 <0.005 4 58 7 3 1050.degree. C.
.times. 10 min Fe: 82, Cr: 17 Spine1, Cr.sub.2O.sub.3 0.119
<0.005 6 55 8 4 780.degree. C. .times. 45 min Fe: 72, Cr: 25
Spine1, Cr.sub.2O.sub.3 0.064 <0.005 4 48 9 5 1120.degree. C.
.times. 5 min Fe: 78, Cr: 20 Spine1, Cr.sub.2O.sub.3 0.526
<0.005 6 46 10 7 730.degree. C. .times. 30 min Fe: 27, Cr: 70
Spine1, Cr.sub.2O.sub.3 6.667 <0.005 9 65 11 4 1070.degree. C.
.times. 10 min Fe: 96, Cr: 4 Spine1, Cr.sub.2O.sub.3, 0.015 0.079 8
35 Fe.sub.2O.sub.3 12 6 1060.degree. C. .times. 10 min Fe: 96, Cr:
3 Spine1, Cr.sub.2O.sub.3, 0.013 0.058 8 37 Fe.sub.2O.sub.3 13 6
1160.degree. C. .times. 15 min Fe: 98, Cr: 1* Spine1,
Cr.sub.2O.sub.3 0.007 <0.005 4 14* 14 8* 1060.degree. C. .times.
10 min Fe: 10, Cr: 88* Spine1, Cr.sub.2O.sub.3 25.000 <0.005 19*
20* 15 9* 1060.degree. C. .times. 10 min Fe: 98, Cr: <0.1*
Spine1, <0.001 <0.005 4 10* *indicates deviation from range
specified in present invention.
Referring to Table 2, for test Nos. 1 to 3, 6, 7, 9, and 11 to 15,
oxidation treatment that doubles as normalizing treatment was
performed. Specifically, the oxidation treatment was performed
under the conditions of oxidation treatment temperature of 900 to
1160.degree. C. and oxidation treatment time of 30 minutes or
shorter.
The oxygen partial pressure Po.sub.2 in a gas atmosphere in the
oxidation treatment of each test No. did not satisfy Formula (3).
Therefore, concerning test Nos. 1 to 3, 6, 7, 9, and 13 to 15,
after the oxidation treatment, the surface of oxide film was ground
thin by shotblasting to remove hematite. On the other hand, for
test Nos. 11 and 12, shotblasting was not performed.
For test Nos. 4, 8, and 10, oxidation treatment that doubles as
tempering treatment was performed. Specifically, the oxidation
treatment was performed under the conditions of oxidation treatment
temperature of 650 to 850.degree. C. and oxidation treatment time
of 2 hours or shorter. For test No. 5, the oxidation treatment was
performed assuming the oxidation treatment after normalizing
treatment and tempering treatment. Specifically, the oxidation
treatment was performed under the conditions of oxidation treatment
temperature of 600.degree. C. and oxidation treatment time of 60
minutes.
As described above, the oxygen partial pressure Po.sub.2 in a gas
atmosphere in the oxidation treatment of each test No. did not
satisfy Formula (3). Therefore, for test Nos. 4, 5, 8 and 10 as
well, after the oxidation treatment, shotblasting was performed to
grind the surface of oxide film thin, whereby hematite was
removed.
[Analysis of Chemical Composition of Oxide Film]
The chemical composition of oxide film of each test No. was
measured by the method described below. From the steel plate of
each test No., a test specimen including the oxide film was
sampled. Of the test specimen, on the surface of oxide film, the
chemical composition of oxide film was analyzed by EDX. Then, the
compositions of detected elements excluding oxygen and carbon were
determined. Table 2 gives the chemical composition of oxide film
produced for each test No.
[Identification of Oxides Forming Oxide Film]
The structure of oxide film of each test No. was identified by the
method described below. From the steel plate of each test No., a
test specimen including the oxide film was sampled. On the surface
on which the oxide film is formed, the oxides forming the oxide
film were identified by XRD. In the XRD, a Co bulb was used. The
identified oxides are given in Table 2.
Furthermore, from the obtained X-ray diffraction spectrum, the
maximum diffraction peak intensity Is of spinel-type oxides, the
maximum diffraction peak intensity Ic of Cr.sub.2O.sub.3, and the
maximum diffraction peak intensity Ih of hematite were measured. By
using the obtained Is, Ic, and Ih, IRl (=Ic/Is) and IRh=Ih/(Is+Ic)
were determined. The determined IRl and IRh are given in Table
2.
[Photoselective Absorptivity Evaluation Test]
The photoselective absorptivity of the steel plate of each test No.
was evaluated by the evaluation test described below. The
reflectance on the low wavelength side (wavelength: 0.3 to 1.0
.mu.m, visual to near-infrared region) of each test No. and the
reflectance on the high wavelength side (wavelength: 2.5 to 25
.mu.m, medium- to far-infrared region) were measured. Specifically,
for the reflectance on the low wavelength side, the reflectance of
light having a wavelength of 0.5 .mu.m was measured by using a
UV-Vis device (Cary 4000 spectrophotometer manufactured by VARIAN
Inc.). For the reflectance on the high wavelength side, the
reflectance of light having a wavelength of 10 .mu.m was measured
by using an FT-IR device (Varian 670-IR spectrometer manufactured
by VARIAN Inc.).
For light on the low wavelength side, high absorptivity is
required. Therefore, on the low wavelength side, lower reflectance
is preferable. On the other hand, for light on the high wavelength
side, low radioactivity is required. Therefore, on the high
wavelength side, higher reflectance is preferable. In this example,
in the case where the reflectance on the low wavelength side (light
having a wavelength of 0.5 .mu.m) is 10% or lower, and the
reflectance on the high wavelength side (light having a wavelength
of 10 .mu.m) is 30% or higher, it was evaluated that "the
photoselective absorptivity is high". On the other hand, in the
case where either the reflectance on the high wavelength side or
the reflectance on the low wavelength side does not meet the
above-described condition, it was evaluated that "the
photoselective absorptivity is low".
[Test Results]
Table 2 gives test results. The term "spinel" in the "oxides"
column in Table 2 means that the oxide film of the corresponding
test No. contains spinel-type oxides. The terms "spinel,
Cr.sub.2O.sub.3" mean that the oxide film contains spinel-type
oxides and Cr.sub.2O.sub.3. The terms "spinel, Cr.sub.2O.sub.3,
Fe.sub.2O.sub.3" mean that the oxide film contains spinel-type
oxides, Cr.sub.2O.sub.3, and Fe.sub.2O.sub.3.
Referring to Table 2, for test Nos. 1 to 12, the chemical
composition of base material was within the range of the present
invention, and the chemical composition of oxide film was within
the range of the present invention. Also, the oxide film contained
spinel-type oxides and the like and Cr.sub.2O.sub.3. Therefore, the
reflectance on the low wavelength side was 10% or lower, the
reflectance on the high wavelength side was 30% or higher, and
excellent photoselective absorptivity was exhibited.
Furthermore, test Nos. 1 and 3 to 12 satisfied Formula (1).
Therefore, the photoselective absorptivity, especially, the
reflectance on the high wavelength side was as high as 35% or more.
On the other hand, test No. 2 did not satisfy Formula (1), so that
the reflectance on the high wavelength side was 35% or lower. For
test No. 2, the spinel-type oxides contained much Cr, and the ratio
of Cr.sub.2O.sub.3 was low. Therefore, it is thought that the
reflectance on the high wavelength side was low as compared with
test Nos. 1 and 3 to 12.
Furthermore, test Nos. 1 and 3 to 10 satisfied Formula (2).
Therefore, the photoselective absorptivity, especially, the
reflectance on the high wavelength side was as high as 40% or more.
On the other hand, test Nos. 11 and 12 did not satisfy Formula (2),
so that the reflectance on the high wavelength side was lower than
40%. In the oxide films of test Nos. 11 and 12, the ratio of the
content of Fe.sub.2O.sub.3 to the contents of spinel-type oxides
and Cr.sub.2O.sub.3 was high. Therefore, it is thought that the
reflectance on the high wavelength side was low as compared with
test Nos. 1 and 3 to 10.
For test No. 13, the chemical composition of base material was
within the range of the present invention. However, the Cr content
of oxide film was lower than the lower limit of the present
invention, and IRl did not satisfy Formula (1). As a result, the
reflectance on the high wavelength side was low, and the
photoselective absorptivity was low. For test No. 13, the oxidation
treatment temperature was high. Therefore, it is thought that the
ratio of Cr.sub.2O.sub.3 in the oxide film was low, and
resultantly, the photoselective absorptivity was low.
For test No. 14, the Cr content of base material exceeded the upper
limit of the present invention. Therefore, the chemical composition
of oxide film was out of the range of the present invention.
Specifically, the content of Fe in the oxide film was lower than
the lower limit of the present invention, and the content of Cr
exceeded the upper limit of the present invention. Furthermore, IRl
exceeded the upper limit of Formula (1). Therefore, the reflectance
on the low wavelength side exceeded 10%, the reflectance on the
high wavelength side was lower than 30%, and the photoselective
absorptivity was low. It is thought that the photoselective
absorptivity was low because the ratio of Cr.sub.2O.sub.3 in the
oxide film was too high.
For test No. 15, the Cr content of base material was lower than the
lower limit of the present invention. Therefore, the content of Fe
in the oxide film exceeded the upper limit of the present
invention, and the content of Cr was lower than the lower limit of
the present invention. Furthermore, IRl was less than the lower
limit of Formula (1). Therefore, the reflectance on the high
wavelength side was lower than 30%, and the photoselective
absorptivity was low. It is thought that the steel was liable to
dissipate heat because the ratio of Cr.sub.2O.sub.3 in the oxide
film was too low.
EXAMPLE 2
Heat resistant ferritic steels having various chemical compositions
were produced, and the photoselective absorptivity thereof was
examined.
[Examination Method]
Heat resistant ferritic steels of steel Nos. 10 to 20 having the
chemical compositions given in Table 3 were melted to produce
ingots.
TABLE-US-00003 TABLE 3 Steel Chemical composition (mass %, balance
being Fe and impurities) No. C Si Mn P S Cr sol.Al N Others 10 0.11
0.32 0.41 0.011 0.003 9.4 0.01 0.04 -- 11 0.11 0.25 0.40 0.015
<0.001 8.9 <0.01 0.04 0.2V, 0.9Mo 12 0.08 0.15 0.48 0.009
0.002 9.0 0.02 0.03 0.5Mo, 1.9W, 0.06Nb, 0.004B 13 0.08 0.25 0.49
0.011 0.004 9.1 0.02 0.01 2.8Ta, 2.6Co, 0.05Zr, 0.02Ca 14 0.06 0.14
0.32 0.011 0.001 8.6 0.01 0.09 0.03Nd 15 0.03 0.75 0.14 0.029 0.001
9.5 0.08 0.02 0.2Ti, 0.5Nb 16 0.08 0.27 0.51 0.013 <0.001 9.1
<0.01 0.08 2.2Re, 0.02Mg 17 0.18 0.35 1.08 0.008 0.005 8.3 0.03
0.01 2.5W, 0.3Hf 18 0.11 0.19 0.65 0.012 0.001 11.6 0.02 0.03
0.3Ni, 1.5Cu 19 0.11 0.16 0.45 0.013 0.001 15.4* 0.02 0.02 1.6Cu,
1.5Mo, 1.3Ni 20 0.15 2.12* 1.12 0.012 0.002 8.4 0.01 0.01 0.9W,
0.18V *indicates deviation from range specified in present
invention.
Referring to Table 3, for steels of steel Nos. 10 to 18, the
chemical composition of base material was within the range of
chemical composition of the present invention. On the other hand,
for steels of steel Nos. 19 and 20, the chemical composition of
base material was out of the range of chemical composition of the
present invention. Specifically, the Cr content of steel No. 10
exceeded the upper limit of Cr content of the base material of the
present invention. The Si content of steel No. 11 exceeded the
upper limit of Si content of the base material of the present
invention.
From each of the produced ingots, a base material (steel plate) was
produced as in Example 1. The produced base material was subjected
to oxidation treatment under various conditions to form an oxide
film on the surface of base material. Table 4 gives steel No. used
in each of test Nos. and oxidation treatment conditions.
TABLE-US-00004 TABLE 4 Maximum diffraction Oxidation treatment
Oxide film peak intensity ratio Reflectance Test Steel Temperature
.times. Chemical IRh = (%) No. No. time Fc.sub.2 (atm) Pref (atm)
composition Oxides IR1 = Ic/Is Ih/(Is + Ic) 0.5 .mu.m 10 .mu.m 21
10 1020.degree. C. .times. 30 min 2.5 .times. 10.sup.-12 3.2
.times. 10.sup.-5 Fe: 70, Cr: 28 Spine1, Cr.sub.2O.sub.3 1.111
<0.005 6 62 22 11 1050.degree. C. .times. 10 min 6.5 .times.
10.sup.-12 9.0 .times. 10.sup.-5 Fe: 71, Cr: 26 Spine1,
Cr.sub.2O.sub.3 1.053 <0.005 5 64 23 11 1050.degree. C. .times.
10 min 3.5 .times. 10.sup.-7 9.0 .times. 10.sup.-5 Fe: 88, Cr: 10
Spine1, Cr.sub.2O.sub.3, 0.222 0.02 9 50 Fe.sub.2O.sub.3 24 11
1050.degree. C. .times. 10 min 5.8 .times. 10.sup.-1 9.0 .times.
10.sup.-5 Fe: 94, Cr: 4 Spine1, Cr.sub.2O.sub.3, 0.111 0.25 9 35
Fe.sub.2O.sub.3 25 12 1060.degree. C. .times. 10 min 4.6 .times.
10.sup.-6 1.3 .times. 10.sup.-4 Fe: 75, Cr: 21 Spine1,
Cr.sub.2O.sub.3 0.526 <0.005 6 62 26 12 720.degree. C. .times.
60 min 3.9 .times. 10.sup.-16 3.0 .times. 10.sup.-11 Fe: 82, Cr: 17
Spine1, Cr.sub.2O.sub.3, 0.083 0.008 4 58 Fe.sub.2O.sub.3 27 13
1050.degree. C. .times. 10 min 8.6 .times. 10.sup.-12 9.0 .times.
10.sup.-5 Fe: 85, Cr: 12 Spine1, Cr.sub.2O.sub.3 0.040 <0.005 6
55 28 13 620.degree. C. .times. 100 min 4.2 .times. 10.sup.-16 3.7
.times. 10.sup.-14 Fe: 93, Cr: 3 Spine1, Cr.sub.2O.sub.3, 0.167
0.06 10 38 Fe.sub.2O.sub.3 29 13 620.degree. C. .times. 100 min 6.5
.times. 10.sup.-21 3.7 .times. 10.sup.-14 Fe: 80, Cr: 16 Spine1,
Cr.sub.2O.sub.3 0.077 <0.005 6 60 30 14 980.degree. C. .times. 5
min 3.2 .times. 10.sup.-5 7.3 .times. 10.sup.-6 Fe: 81, Cr: 17
Spine1, Cr.sub.2O.sub.3 0.053 <0.005 6 57 31 15 1150.degree. C.
.times. 3 min 1.2 .times. 10.sup.-11 2.1 .times. 10.sup.-5 Fe: 88,
Cr: 8 Spine1, Cr.sub.2O.sub.3 0.033 <0.005 7 51 32 16
1080.degree. C. .times. 15 min 8.4 .times. 10.sup.-11 2.4 .times.
10.sup.-4 Fe: 77, Cr: 20 Spine1, Cr.sub.2O.sub.3 0.250 <0.005 6
62 33 17 1050.degree. C. .times. 10 min 2.5 .times. 10.sup.-12 9.0
.times. 10.sup.-5 Fe: 67, Cr: 28 Spine1, Cr.sub.2O.sub.3 1.500
<0.005 5 64 34 18 1100.degree. C. .times. 8 min 3.4 .times.
10.sup.-11 4.6 .times. 10.sup.-4 Fe: 80, Cr: 17 Spine1,
Cr.sub.2O.sub.3 0.074 <0.005 5 58 35 19* 1050.degree. C. .times.
15 min 2.5 .times. 10.sup.-12 9.0 .times. 10.sup.-5 Fe: 17, Cr: 80*
Spine1, Cr.sub.2O.sub.3 6.250 <0.005 16* 28* 36 20* 1100.degree.
C. .times. 10 min 3.4 .times. 10.sup.-14 4.6 .times. 10.sup.-4 Fe:
20, Cr: 62* Spine1, Cr.sub.2O.sub.3 2.250 <0.005 9 25*
*indicates deviation from range specified in present invention.
Referring to Table 4, for test Nos. 21 to 25, 27, 30, and 32 to 36,
oxidation treatment that doubles as normalizing treatment was
performed. Specifically, the oxidation treatment was performed
under the conditions of oxidation treatment temperature of 980 to
1100.degree. C. and oxidation treatment time of 30 minutes or
shorter. For test No. 31, oxidation treatment that doubles as
annealing treatment was performed. Specifically, the oxidation
treatment was performed under the conditions of oxidation treatment
temperature of 1150.degree. C. and oxidation treatment time of 3
minutes.
For test No. 26, oxidation treatment that doubles as tempering
treatment was performed. Specifically, the oxidation treatment was
performed under the conditions of oxidation treatment temperature
of 720.degree. C. and oxidation treatment time of 60 minutes. For
test Nos. 28 and 29, the oxidation treatment at low temperatures
was performed assuming the oxidation treatment after normalizing
treatment and tempering treatment. Specifically, the oxidation
treatment was performed under the conditions of oxidation treatment
temperature of 620.degree. C. and oxidation treatment time of 100
minutes.
The oxygen partial pressure Po.sub.2 in the gas atmosphere of the
oxidation treatment of each test No. was determined by using the
thermodynamic computation software "MALT-2 for WIN" based on each
gas composition obtained by gas analysis. Furthermore, it was
defined that
P.sub.ref=2.76.times.10.sup.15.times.exp{-493.6.times.10.sup.3/(RT)},
and P.sub.ref
for each test No. was determined. The determined Po.sub.2 and
P.sub.ref are given in Table 4.
For test Nos. 21 to 23, 25 to 27, and 29 to 36, the oxygen partial
pressure Po.sub.2 was lower than P.sub.ref, and Formula (1) was
satisfied. On the other hand, for test Nos. 24 and 28, the oxygen
partial pressure Po.sub.2 was higher than P.sub.ref, and Formula
(1) was not satisfied.
[Identification of Oxides Forming Oxide Film]
The structure of oxide film of each test No. was identified by the
same method as that of Example 1 (XRD). Furthermore, from the
obtained X-ray diffraction spectrum, the maximum diffraction peak
intensity Is of spinel-type oxides, the maximum diffraction peak
intensity Ic of Cr.sub.2O.sub.3, and the maximum diffraction peak
intensity Ih of Fe.sub.2O.sub.3 were measured, and IRl and IRh were
determined. The determined IRl and IRh are given in Table 4.
[Photoselective Absorptivity Evaluation Test]
The photoselective absorptivity of the steel plate of each test No.
was evaluated by the same evaluation test as that of Example 1.
[Test Results]
Table 4 gives test results.
Referring to Table 4, for test Nos. 21 to 34, the chemical
composition of base material and the chemical composition of oxide
film were within the range of the present invention, and the oxide
film contained spinel-type oxides and Cr.sub.2O.sub.3. Furthermore,
for these test Nos., IRl satisfied Formula (1). Therefore, the
reflectance on the low wavelength side was 10% or lower, the
reflectance on the high wavelength side was 35% or higher, and
excellent photoselective absorptivity was exhibited.
Furthermore, for test Nos. 21 to 23, 25 to 27, and 29 to 34, the
oxygen partial pressure Po.sub.2 in the gas atmosphere at the time
of oxidation treatment satisfied Formula (3). Therefore, for the
oxide films of these test Nos, IRh satisfied Formula (2), and the
reflectance on the high wavelength side was further higher, being
40% or higher. On the other hand, for test Nos. 24 and 28, the
oxygen partial pressure Po.sub.2 did not satisfy Formula (3).
Therefore, the reflectance on the low wavelength side was high, and
the reflectance on the high wavelength side was low as compared
with test Nos. 21 to 23, 25 to 27, and 29 to 34. In particular, the
reflectance on the high wavelength side was lower than 40%.
For test No. 35, the Cr content of base material exceeded the upper
limit of the present invention. Therefore, the reflectance on the
low wavelength side exceeded 10%, the reflectance on the high
wavelength side was lower than 40%, and the photoselective
absorptivity was low. It is thought that the photoselective
absorptivity was low because the ratio of Cr.sub.2O in the oxide
film was too high.
For test No. 36, the Si content of base material exceeded the upper
limit of the present invention. Therefore, the reflectance on the
high wavelength side was lower than 30%, and the photoselective
absorptivity was low. It is thought that the steel was liable to
dissipate heat because an oxide film containing much Si was
formed.
The above is the explanation of the embodiment of the present
invention. The above-described embodiment is merely an illustration
for carrying out the present invention. Therefore, the present
invention is not limited to the above-described embodiment, and the
above-described embodiment can be carried out by being modified as
appropriate without departing from the spirit and scope of the
present invention.
INDUSTRIAL APPLICABILITY
The heat resistant ferritic steel in accordance with this
embodiment can be applied widely to applications in which
photoselective absorptivity is required. In particular, the steel
is suitable as a steel material for solar thermal power plant
boilers.
* * * * *