CA 03032772 2019-02-01
DESCRIPTION
AUSTENITIC STAINLESS STEEL
TECHNICAL FIELD
[0001]
The present invention relates to an austenitic stainless steel, in particular
to an
austenitic stainless steel excellent in acid resistance.
[0002]
Oil and coal, so-called "fossil fuels", used as boiler fuels for thermal power
generation and industries contain sulfur (S). The fossil fuels therefore
produce sulfur
oxides (SOO in exhaust gas while burning. As the temperature of exhaust gas
drops,
SO, reacts with moisture in the gas to form sulfuric acid, forming dew on a
component
surface at a temperature lower than a dew-point temperature. This may be a
cause of
sulfuric acid dew point corrosion.
[0003]
Similarly, also in flue gas desulfurization facilities used in various
industrial
fields, when gas containing SO, flows, the sulfuric acid dew point corrosion
occurs as the
temperature of the gas drops. Hereinafter, explanation will proceed with such
gas
containing SO, as exhaust gas for simplicity.
[0004]
In view of the phenomenon described above, in a heat exchanger or other device
used in an exhaust gas system, the exhaust gas temperature is kept at as high
as 150 C or
a higher temperature so that sulfuric acid does not form dew on a component
surface.
[0005]
Due to a recent increase in demand for energy and from the viewpoint of
effective use of energy, however, there is a trend toward lowering the
temperature of
exhaust gas from a heat exchanger, for example, below the dew point of the
sulfuric acid
to collect thermal energy as effective as possible, which has led to a demand
for materials
having a resistance against sulfuric acid.
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[0006]
If the exhaust gas temperature is not kept at 150 C or higher, about 80% high-
concentration sulfuric acid forms dew on a component surface in a temperature
range of
about 140 C from exhaust gas of a typical composition. In such an environment,
a so-
called "low alloy steel" has been used as a steel for various components. This
is because
the low alloy steel has a higher corrosion resistance than a conventional
stainless steel
against such a high-temperature, high-concentration sulfuric acid.
[0007]
In contrast, as described in Non-Patent Document 1, an amount of sulfuric acid
that forms dew reaches the maximum at a temperature within a range lower than
a dew
point of the sulfuric acid by 20 to 60 C, and thus corrosion caused by the
sulfuric acid
becomes predominant. For this reason, if the exhaust gas temperature is not
kept at
150 C or higher, a temperature range near 100 C is in general a range in which
a corrosion
resistance is required most, where a concentration of the sulfuric acid
reaches about 70%.
In this range, however, not only a conventional stainless steel but also even
the low alloy
steel cannot be used because of a large corrosion amount.
[0008]
There has been a proposition that a specific corrosion resistant material can
be
used for a component in a sulfuric acid environment, and for example, Patent
Document
1 discloses a sulfuric acid dew point corrosion resistant stainless steel that
is excellent in
hot workability.
[0009]
In addition, Patent Document 2 discloses an austenitic stainless steel that
has an
excellent resistance to sulfuric acid corrosion as well as an excellent
workability.
LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENT
[0010]
Patent Document 1: JP4-346638A
Patent Document 2: JP3294282B
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NON PATENT DOCUMENT
[0011]
Non-Patent Document 1: Hiroo Nagano, "Sulfur Dewpoint Corrosion",
Corrosion engineering, 1977, vol. 26, issue 12, p.731-740
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0012]
A stainless steel described in Patent Document 1 is intended to stabilize an
austenitic micro structure and to ensure a corrosion resistance by containing
0.05 wt% or
more ofN (nitrogen). However, when 0.05 wt% or more ofN is contained, the
austenitic
stainless steel with Cu, Cr, and Mo added in combination rather decreases in
corrosion
resistance against sulfuric acid. In addition, in a case where a content of N
is 0.05 wt%
or more, increasing a content of Cu to enhance corrosion resistance against
sulfuric acid
raises a problem in that hot workability in a temperature range below 1000 C
significantly
decreases.
[0013]
The austenitic stainless steel described in Patent Document 2 has excellent
corrosion resistance against sulfuric acid and workability. The austenitic
stainless steel
is however susceptible to improvement in corrosion resistance against sulfuric
acid.
[0014]
An objective of the present invention is to provide an austenitic stainless
steel
with which the problem described above is solved and that has an excellent
acid resistance
in the environment where high-concentration sulfuric acid condenses.
[0015]
Note that, in the following description, the "environment where high-
concentration sulfuric acid condenses" refers to an environment where sulfuric
acid at a
concentration of 40 to 70% forms dew at a temperature of 50 to 100 C.
SOLUTION TO PROBLEM
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[0016]
The present invention has been made to solve the problems described above, and
the gist of the present invention is the following austenitic stainless steel.
[0017]
(1) An austenitic stainless steel comprising base metal and a coating film
formed
on at least part of a surface of the base metal,
a chemical composition of the base metal consisting of, in mass percent:
C: 0.05% or less;
Si: 1.0% or less;
Mn: 2.0% or less;
P: 0.040% or less;
S: 0.010% or less;
0: 0.020% or less;
N: less than 0.050%;
Ni: 12.0 to 27.0%;
Cr: 15.0% or more to less than 20.0%;
Cu: more than 3.5% to 8.0% or less;
Mo: more than 2.0% to 5.0% or less;
Co: 0.05% or less;
Sn: 0.05% or less;
V: 0 to 0.5%;
Nb: 0 to 1.0%;
Ti: 0 to 0.5%;
W: 0 to 5.0%;
Zr: 0 to 1.0%;
Al: 0 to 0.5%;
Ca: 0 to 0.01%;
B: 0 to 0.01%;
REM: 0 to 0.01%, and
the balance: Fe and impurities, wherein
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a chemical composition at a maximum-Cr depth where a concentration of Cr in
the coating film reaches a maximum satisfies a following formula (i):
(Cr + Ni + Cu + Mo) / Fe 1.0 ... (i)
where each symbol of element in the formula denotes a content (at%) of a
corresponding element.
[0018]
(2) The austenitic stainless steel according to the above (1), wherein
the chemical composition of the base metal containing one or more elements
selected from, in mass percent:
V: 0.01 to 0.5%;
Nb: 0.02 to 1.0%;
Ti: 0.01 to 0.5%;
W: 0.1 to 5.0%;
Zr: 0.02 to 1.0%;
Al: 0.01 to 0.5%;
Ca: 0.0005 to 0.01%;
B: 0.0005 to 0.01%; and
REM: 0.0005 to 0.01%.
[0019]
(3) The austenitic stainless steel according to the above (1) or (2), wherein
a minimum-Cr depth where the concentration of Cr in the coating film falls to
a
minimum lies closer to the base metal than the maximum-Cr depth, and
a chemical composition at the maximum-Cr depth satisfies a following formula
(ii), and a chemical composition at the minimum-Cr depth satisfies a following
formula
(iii):
Cr! (Ni + Cu + Mo) 1.0 ... (ii)
Cr! (Ni + Cu + Mo) < 1.0 ... (iii)
where each symbol of element in the formulas denotes a content (at%) of a
corresponding element.
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ADVANTAGEOUS EFFECTS OF INVENTION
[0020]
According to the present invention, an austenitic stainless steel having an
excellent acid resistance in the environment where high-concentration sulfuric
acid
condenses can be obtained.
DESCRIPTION OF EMBODIMENTS
[0021]
The present inventors conducted intensive studies on methods for enhancing
corrosion resistance against sulfuric acid, based on the austenitic stainless
steel according
to Patent Document 2, and as a result obtained the following findings.
[0022]
To enhance the corrosion resistance against sulfuric acid, a composition of a
coating film formed on a surface of base metal, which is to be in contact with
a high-
concentration sulfuric acid, is important. In the coating film, by making a
total content
of Cr, Ni, Cu, and Mo relatively higher than that of Fe, acid resistance is
greatly improved.
[0023]
In addition, the present inventors found that Cr, Ni, Cu, and Mo can be
concentrated in a coating film by subjecting a steel to heat treatment under
predetermined
conditions to form an oxide film mainly containing Fe on its surface and then
to acid
treatment to dissolve an Fe component preferentially.
[0024]
The present invention has been made based on the findings described above.
Requirements of the present invention will be described below in detail.
[0025]
1. Structure
An austenitic stainless steel according to the present invention includes base
metal and a coating film formed on at least part of a surface of the base
metal. The base
metal and the coating film will be each described below in detail.
[0026]
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2. Base Metal
A chemical composition of base metal will be described in detail. The reasons
for limiting contents of elements are as described below. In the following
description,
the symbol "%" for contents means "percent by mass".
[0027]
C: 0.05% or less
C (carbon) is an element that has a function of enhancing strength. However,
C combines with Cr to form Cr carbide in grain boundaries, resulting in
deterioration in
intergranular corrosion resistance. Accordingly, a content of C is set at
0.05% or less.
When there is a need to enhance strength, C is preferably contained at more
than 0.03%.
In contrast, when a priority is given to ensuring corrosion resistance, the
content of C
should be made as low as possible and is preferably set at 0.03% or less.
Although no
particular lower limit needs to be provided to the content of C, the content
of C is
preferably set at 0.01% or more to obtain the effect described above.
[0028]
Si: 1.0% or less
Si (silicon) is an element that has deoxidation action. However, a content of
Si
more than 1.0% contributes to deterioration in hot workability, and combined
with an
increase in a Cu content, Si at such a content makes it very difficult to work
the base
material into a product on an industrial scale. Accordingly, a content of Si
is set at 1.0%
or less. The content of Si is preferably 0.6% or less. Although there is no
particular
lower limit of the content of Si provided because Si is not necessarily
contained, the
content of Si is preferably set at 0.05% or more to obtain the effect
described above. In
a case where the content of Si is set to be extremely low for enhancement of
hot
workability, it is preferable to contain 0.1% or more of Si to have Si exert
its deoxidation
action sufficiently.
[0029]
Mn: 2.0% or less
Mn (manganese) has an action of immobilizing S to enhance hot workability as
well as of stabilizing an austenite phase. Containing Mn at a content more
than 2.0%,
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however, saturates its effect, resulting only in higher costs. Accordingly, a
content of
Mn is set at 2.0% or less. The content of Mn is preferably 1.5% or less.
Although
there is no particular lower limit of the content of Mn provided because Mn is
not
necessarily contained, the content of Mn is preferably set at 0.1% or more to
obtain the
effect described above.
[0030]
P: 0.040% or less
P (phosphorus) is contained in steel as an impurity and degrades hot
workability
and corrosion resistance, and therefore a content of P is preferably made as
low as possible.
In particular, when the content of P is more than 0.040%, P markedly degrades
corrosion
resistance in an environment where high-concentration sulfuric acid condenses.
Accordingly, the content of P is set at 0.040% or less.
[0031]
S: 0.010% or less
S (sulfur) is contained in steel as an impurity and degrades hot workability,
and
therefore a content of S is preferably made as low as possible. In particular,
when the
content of S is more than 0.010%, S markedly degrades hot workability.
Accordingly,
the content of S is set at 0.010% or less.
[0032]
0: 0.020% or less
0 (oxygen) is contained in steel as an impurity and decreases hot workability
and ductility, and therefore a content of 0 is preferably made as low as
possible. In
particular, when the content of 0 is more than 0.020%, 0 markedly decreases
hot
workability and ductility, and therefore the content of 0 is set at 0.020% or
less.
[0033]
N: less than 0.050%
In conventional practices, N (nitrogen) has been positively added for a
purpose
of stabilizing an austenitic structure or enhancing a resistance to local
corrosion such as
pitting and crevice corrosion. However, in the environment where high-
concentration
sulfuric acid condenses, an N content of 0.050% or more rather results in
deterioration in
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corrosion resistance of an austenitic stainless steel containing more than
3.5% of Cu, more
than 2.0% of Mo, and 15.0% or more to less than 20.0% of Cr. Furthermore, even
in a
case where upper limits of Cu and Mo are set at 8.0% and 5.0%, respectively,
an N content
of 0.050% or more results in deterioration in hot workability. To give an
austenitic
stainless steel corrosion resistance and hot workability in the environment
where high-
concentration sulfuric acid condenses, an N content is set less than 0.050%.
The lower
the content of N is, the more preferable it is, and the content of N is
preferably 0.045% or
less.
[0034]
Ni: 12.0 to 27.0%
Ni (nickel) has an action of stabilizing an austenite phase, as well as of
enhancing
corrosion resistance in the environment where high-concentration sulfuric acid
condenses.
To ensure such an effect sufficiently, it is necessary to contain Ni in an
amount of 12.0%
or more. Containing more than 27.0% of Ni however saturates its effect. In
addition,
being an expensive element, Ni leads to an extremely high cost and is thus
uneconomical
to use. Accordingly, a content of Ni is set at 12.0 to 27.0%. To ensure a
sufficient
corrosion resistance in the environment where high-concentration sulfuric acid
condenses,
Ni is preferably contained in an amount more than 15.0%, more preferably more
than
20.0%.
[0035]
Cr: 15.0% or more to less than 20.0%
Cr (chromium) is an element effective to ensure the corrosion resistance of an
austenitic stainless steel. In particular, in a case of an austenitic
stainless steel with N
restricted to the content described above, containing 15.0% or more of Cr,
preferably
16.0% or more of Cr, with Cu and Mo in amounts to be described later enables a
good
corrosion resistance to be ensured in the environment where high-concentration
sulfuric
acid condenses. However, containing an excessive amount of Cr rather degrades
the
corrosion resistance in the environment even in a case of an austenitic
stainless steel with
a low N content and with Cu and Mo added in combination, and in addition, the
content
also causes deterioration in workability. In particular, a content of Cr of
20.0% or more
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results in a significant degradation in the corrosion resistance of an
austenitic stainless
steel in the above environment. In addition, by setting a content of Cr at
less than 20.0%,
it is possible to increase the hot workability of the austenitic stainless
steel with Cu and
Mo added in combination, making it easy to work the base metal into a product
on an
industrial scale. Accordingly, the content of Cr is set at 15.0% or more to
less than
20.0%.
[0036]
Cu: more than 3.5% to 8.0% or less
Cu (copper) is an element indispensable for ensuring corrosion resistance in a
sulfuric acid environment. Containing more than 3.5% of Cu together with Cr in
the
amount described above and Mo in an amount to be described later, an
austenitic stainless
steel with an N content set at the content described above can be provided
with a good
corrosion resistance in the environment where high-concentration sulfuric acid
condenses.
A larger content of Cu with Cu and Mo added in combination produces a greater
advantageous effect of improving corrosion resistance, and thus a content of
Cu is
preferably set at a content of more than 4.0%. Note that increasing the
content of Cu
enables the improvement of the corrosion resistance in the environment but
causes
deterioration of hot workability, and in particular, a content of Cu more than
8.0% causes
a significant degradation in hot workability even when the N content is set at
the content
described above. Accordingly, the content of Cu is set at more than 3.5% to
8.0% or
less.
[0037]
Mo: more than 2.0% to 5.0% or less
Mo (molybdenum) is an element effective to ensure the corrosion resistance of
an austenitic stainless steel. Containing Mo at a content of more than 2.0%
together
with Cr and Cu at the contents described above, an austenitic stainless steel
with an N
content set at a content described above can be provided with a good corrosion
resistance
in the environment where high-concentration sulfuric acid condenses. However,
containing an excessive amount of Mo leads to deterioration in hot
workability, and in
particular, a content of Mo more than 5.0% causes a significant deterioration
in hot
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workability even with the content of N described above. Accordingly, a content
of Mo
is set at more than 2.0% to 5.0% or less. To ensure a sufficient corrosion
resistance in
the environment where high-concentration sulfuric acid condenses, Mo is
preferably
contained at a content more than 3.0%.
[0038]
Co: 0.05% or less
Co (cobalt) is an element contained in steel as an impurity. Co is an element
effective in enhancing toughness of steel but need not be added positively
because Co is
an expensive element. Accordingly, a content of Co is set at 0.05% or less.
[0039]
Sn: 0.05% or less
Sn (tin) is contained in steel as an impurity and degrades hot workability,
and
therefore a content of Sn is preferably made as low as possible. In
particular, the content
of Sn being more than 0.05% leads to a marked deterioration in hot
workability.
Accordingly, a content of Sn is set at 0.05% or less.
[0040]
V: 0.5% or less
V (vanadium) has an action of immobilizing C to enhance corrosion resistance,
especially intergranular corrosion resistance, and therefore V may be
contained as needed.
However, if its content is more than 0.5%, its nitride forms even with the
content of N set
at the content described above, which rather results in deterioration in
corrosion resistance,
and its content being more than 0.5% also leads to deterioration in hot
workability.
Accordingly, a content of V is set at 0.5% or less. To obtain the above
effect, it is
preferable to set the content of V at 0.01% or more.
[0041]
Nb: 0 to 1.0%
Nb (niobium) has an action of immobilizing C to enhance corrosion resistance,
especially intergranular corrosion resistance, and therefore Nb may be
contained as
needed. However, if its content is more than 1.0%, its nitride forms even with
the
content of N set at the content described above, which rather results in
deterioration in
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corrosion resistance, and its content being more than 0.5% also leads to
deterioration in
hot workability. Accordingly, a content of Nb is set at 1.0% or less. To
obtain the
above effect, it is preferable to set the content of Nb at 0.02% or more.
[0042]
Ti: 0 to 0.5%
As with Nb, Ti (titanium) has an action of immobilizing C to enhance corrosion
resistance, especially intergranular corrosion resistance, and therefore Ti
may be
contained as needed. However, if its content is more than 0.5%, its nitride
forms even
with the content of N set at the content described above, which rather results
in
deterioration in corrosion resistance, and its content being more than 0.5%
also leads to
deterioration in hot workability. Accordingly, a content of Ti is set at 0.5%
or less. To
obtain the above effect, it is preferable to set the content of Ti at 0.01% or
more.
[0043]
W: 0 to 5.0%
W (tungsten) has an action of enhancing corrosion resistance in the
environment
where high-concentration sulfuric acid condenses, and therefore W may be
contained as
needed. However, if its content is more than 5.0%, the above effect is
saturated,
resulting only in higher costs. Accordingly, a content of W is set at 5.0% or
less. To
obtain the above effect, it is preferable to set the content of W at 0.1% or
more.
[0044]
Zr: 0 to 1.0%
Zr (zirconium) has an action of enhancing corrosion resistance in the
environment where high-concentration sulfuric acid condenses, and therefore Zr
may be
contained as needed. However, if its content is more than 1.0%, the above
effect is
saturated, resulting only in higher costs. Accordingly, a content of Zr is set
at 1.0% or
less. To obtain the above effect, it is preferable to set the content of Zr at
0.02% or more.
[0045]
Al: 0 to 0.5%
Al has a deoxidation action, and therefore in a case where the content of Si
is set
to be extremely low, Al may be contained. However, when its content is more
than 0.5%,
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even an austenitic stainless steel with N at the content described above
deteriorates in its
hot workability. Accordingly, a content of Al is set at 0.5% or less. A lower
limit of
the content of Al is not particularly limited and may be within an impurity
range. In the
case where the content of Si is set to be extremely low, however, it is
preferable to add Al
positively to make Al contained at 0.02% or more, so as to exert the
deoxidation action
sufficiently. Even with 0.05% or more of Si contained, it is preferable to set
the content
of Al at 0.01% or more so as to exert the deoxidation action sufficiently.
[0046]
Ca: 0 to 0.01%
Combining with S, Ca (calcium) has an effect of preventing deterioration in
hot
workability, and therefore Ca may be contained as needed. However, a Ca
content more
than 0.01% results in deterioration in cleanliness of the steel, causing a
defect to occur in
production performed as a hot processing. Accordingly, a content of Ca is set
at 0.01%
or less. To obtain the above effect, the content of Ca is preferably set at
0.0005% or
more, more preferably 0.001% or more.
[0047]
B: 0 to 0.01%
B (boron) has an effect of improving hot workability, and therefore B may be
contained as needed. However, adding B in an excessive amount promotes
precipitation
of Cr-B compound in grain boundaries, leading to deterioration in corrosion
resistance.
In particular, a content of B more than 0.01% results in a significant
deterioration in
corrosion resistance. Accordingly, a content of B is set at 0.01% or less. To
obtain the
above effect, the content of B is preferably set at 0.0005% or more, more
preferably
0.001% or more.
[0048]
REM: 0 to 0.01%
REMs (rare earth metals) have an action of enhancing hot workability, and
therefore REM may be contained as needed. However, an REM content more than
0.01% results in deterioration in cleanliness of the steel, causing a defect
to occur in
production performed as a hot processing. Accordingly, a content of REM is set
at
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0.01% or less. To obtain the above effect, it is preferable to set the content
of REM at
0.0005% or more.
[0049]
Here, REM refers to Sc (scandium), Y (yttrium), and lanthanoids, 17 elements
in total, and the content of REM refers to the total content of these
elements.
[0050]
In the chemical composition of the base metal of the austenitic stainless
steel
according to the present invention, the balance is Fe and impurities. The term
"impurities÷ used herein means components that are mixed in steel in producing
the steel
industrially, owing to various factors including raw materials such as ores
and scraps, and
a producing process, and are allowed to be mixed in the steel within ranges in
which the
impurities have no adverse effect on the present invention.
[0051]
3. Coating Film
As described above, on at least part of a surface of the base metal, a coating
film
is formed. In the coating film, by making a total content of Cr, Ni, Cu, and
Mo relatively
higher than that of Fe, acid resistance is greatly improved.
[0052]
Specifically, the coating film includes a maximum-Cr depth where a
concentration of Cr reaches its maximum, and at the maximum-Cr depth, the
chemical
composition needs to satisfy the following formula (i). Note that a position
of the
maximum-Cr depth is not particularly limited, and the maximum-Cr depth may lie
in an
outermost layer of the coating film.
(Cr + Ni + Cu + Mo) / Fe 1.0 ... (i)
Each symbol of element in the above formula denotes a content (at%) of a
corresponding
element in the surface of the steel.
[0053]
The coating film according to the present invention generally has a structure
including a layer on an outer layer side in which Cr is relatively
concentrated, and a layer
on a base metal side in which Ni and the like are relatively concentrated.
That is, on a
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base metal side of the maximum-Cr depth, there is a minimum-Cr depth where the
concentration of Cr reaches its minimum.
[0054]
In addition, at the maximum-Cr depth, the chemical composition preferably
satisfies the following formula (ii), and at the minimum-Cr depth, the
chemical
composition preferably satisfies the following formula (iii):
Cr / (Ni + Cu + Mo) 1.0 ... (ii)
Cr / (Ni + Cu + Mo) < 1.0 ... (Iii)
where each symbol of element in the formulas denotes a content (at%) of a
corresponding
element.
[0055]
A thickness of the coating film is not particularly limited but is preferably
within
a range of, for example, from 2 to 10 nm. This is because if the thickness of
the coating
film is less than 2 nm, there is a risk that corrosion resistance against
sulfuric acid cannot
be obtained sufficiently, and at the same time, if the thickness of the
coating film is more
than 10 nm, there is a risk that a nonuniform composition of the coating film
and peeling
of the coating film is likely to occur.
[0056]
In the present invention, the chemical composition of the coating film is
assumed
to be measured by depth profile analysis with the X-ray photoelectron
spectroscopy
(XPS). By the depth profile analysis, a concentration profile of each element
is derived
in a form of a ratio (at%) to components except for 0, C, and N. In addition,
after
identifying the maximum-Cr depth and the minimum-Cr depth, concentrations of
the
elements at the depths are determined, and from values of the concentrations,
the above
formulas (i) to (iii) are calculated.
[0057]
The thickness of the coating film is determined from the concentration profile
of
0 (oxygen). Specifically, a position where a concentration of 0 is 1/3 of its
maximum
is determined as a boundary portion between the coating film and the base
metal, and a
length from a surface of the coating film to the above boundary portion is
determined as
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the thickness of the coating film. It is desirable to make measurements of the
composition and the thickness of the coating film at a plurality of points and
to adopt
average values of the measurements.
[0058]
4. Producing Method
Although there is no particular limit to conditions for producing the
austenitic
stainless steel according to the present invention, but the austenitic
stainless steel can be
produced by, for example, performing heat treatment and acid treatment on a
starting steel
material having the chemical composition described above under the following
conditions.
[0059]
<Heat Treatment Process>
The starting steel material is subjected to heat treatment in which the
starting
steel material is held at a temperature within a range of from 1060 to 1140 C,
for 60 to
600 seconds. On a surface of the starting steel material, an oxide film mainly
containing
Fe is thereby formed. If the temperature of the heat treatment is less than
1060 C, the
formation of the Fe oxide film becomes insufficient. In contrast, if the
temperature of
the heat treatment is more than 1140 C, grains in the base metal become
coarse, causing
Fe to less diffuse, which makes the Fe oxide film uneven and further causes
coating film
to easily separate. Consequently, the concentration of Cr, Ni, Cu, and Mo
hardly occurs
in any of the above cases.
[0060]
<Acid Treatment Process>
Subsequently to the heat treatment described above, the starting steel
material is
subjected to acid treatment. In an acid treatment process, Cr, Ni, Cu, and Mo
can be
concentrated in the surface of the steel by melting an Fe component
preferentially. To
melt the Fe component preferentially, it is preferable to immerse the start
steel material
in hydrofluoric and nitric acid containing 5 to 8 vol% of HNO3 and 5 to 8 vol%
of HF at
30 to 50 C, for 1 to 5 hours.
[0061]
Hereunder, the present invention is more specifically described with reference
to
16
= CA 03032772 2019-02-01
Examples, but the present invention is not limited to these Examples.
EXAMPLES
[0062]
Steels having chemical compositions shown in Table 1 (steel Nos. 1 to 11) were
melted in a 3.5 t VIM melting furnace, and subjected to hot forging, hot
extrusion, and
cold drawing using normal methods, so as to be produced into steel pipe
starting materials
each having an outer diameter of 75 mm and a wall thickness of 3 mm.
Thereafter, for
test Nos. 1 to 17 and 19 to 28, the steel pipe starting materials were
subjected to the heat
treatment and the acid treatment under conditions shown in Table 2, thereby
formed into
austenitic stainless steel pipes. For a test No. 18, the steel pipe starting
material was
subjected to the heat treatment and the acid treatment under the same
conditions as those
for a test No. 3, and its surface was polished.
[0063]
[Table 1]
17
I
I
I
I
. .
,
Table 1 '
. .
,
Steel
Chemical composition (in mass%, balance: Fe and impurities)
,
No. C Si Mn P S 0 N Ni Cr Cu Mo Co Sn V Nb Ti W Zr Al Ca B REM
'
1 0.02 0.48 1.01 0.006 0.001 0.003 0.037 15.51 17.97 423 3.23 0.02 0.002 0.02
0.012 0.008 - - 0.022 - 0.0035 -
- _
2 0.02 0.51 1.15 0.012 0.001 0.003 0.030 15.42 18.01 4.21 3.20 0.02 0.01 -
- - - - - - - -
_
_
3 0.02 0.46 a98 0.010 0.001 0.005 0.032 15.68 17.94 4.20 3.26 ace 0.01 0.02
0.030 - - - - - - -
_ _
_
4 0.02 0.45 1.05 0.011 0.001 0.006 0.038 15.67 17.95 431 3.27 0.02 0.01 0.02 -
0.020 - - - - - -
_
_ .
0.02 0.52 1.06 0.013 0.001 0.005 0.025 15.32 18.12 4.24 3.19 0.02 0.01 0.02 -
- 0.20 - - - , - -
. 6 0.02 0.49 1.12 0.012 0.001 0.008 0.031 15.79 18.06 4.18 3.21 0.02 0.01
0.02 - - - 0.03 - - - - :
_ _
7 0.02 0.55 0.96 0.015 0.001 0.010 0.039 15.61 18.09 4.17 3.27 0.02 0.01 0.02 -
- - - 0.030 - - -
_ _
_ .
8 0.02 0.53 0.95 0.014 0.001 0.008 0.034 15.55 17.99 4.23 3.26 0.02 0.01 0.02 -
- - - - 0.002 - -
_
--
- =
9 0.02 0.57 1.01,0.016 0.001 0.011 0.032 15.43 17.96 4.19 3.24 0.02 0.01 0.02 -
- - - - - 0.0030 - P
-
.
0.02 0.48 1.08 0.012 0.001 0.005 0.030 15.38 17.90 4.25 3.22 0.02 0.01 0.02 -
- - - 0.005
0
_ _
_ - - - . ,.,
1.,
11 0.01 0.26 1.57 <0.002 0.001 0.006 0.095* 14.51 17.41
0.08* 3.73 ..- - 0.30 - - - 0.014 - 0.0015
- ...3 i
...3
1., 1
0-- * indicates that
conditions do not satisfy those defined by the present invention.
oo
0
1-
1
1
0
1.,
1
0
1-
i
1
;
i
1
,
,
CA 03032772 2019-02-01
,
,
[0064]
[Table 2]
Table 2
Heat treatment conditions Acid treatment conditions
Test Steel
No No. Temperature Time HNO3 HF Temperature Time
.
( C) (s) (vol%) (vo1%) ( C) (h)
. 1 1 1040 60 5 5 30 1
2 1 1040 180 5 5 30 1
3 1 1060 60 5 5 30 1
4 1 1060 180 5 5 30 1
1 , 1080 60 5 5 30 1
6 1 1080 180 5 5 30 1
7 1 1100 60 5 5 30 1
8 1 1100 180 5 5 30 1
9 1 1100 600 5 5 = 30 1
1 1120 60 5 5 30 1
11 1 1120 180 5 5 30 1
12 1 1140 60 5 5 30 1
13 1 1140 180 5 5 30 1
14 1 1160 180 5 5 30 1
1 1100 180 10 10 40 2
16 1 1100 180 5 5 60 2
17 1 1100 180 5 5 40 0.5
18 1 Polished surface
19 2 1100 180 5 . 5 40 2
3 1100 180 5 5 ao = 2
21 4 1100 180 5 5 40 2
22 5 1100 180 5 5 40 2
23 6 1100 180 5 5 40 2
24 7 1100 180 5 5 40 2
8 1100 180 5 5 40 2
26 9 1100 180 5 5 ao 2
27 10 1100 180 5 5 40 2
28 11 1100 180 5 5 40 2 .
[0065]
Next, a chemical composition and a thickness of a coating film formed on a
surface of each steel pipe were measured by the depth profile analysis using
the XPS.
19
CA 03032772 2019-02-01
Specifically, a concentration profile of each element was derived in a form of
a ratio (at%)
to components except for 0, C, and N, the maximum-Cr depth and the minimum-Cr
depth
were identified, and then a concentration of each element at the depths was
determined.
Then, from values of the concentration, the formulas (i) to (iii) were
calculated. In the
present Example, the maximum-Cr depth laid in an outermost layer of the
coating film in
all of the Examples except for the test No. 18, and the minimum-Cr depth laid
closer to
the base metal side than the maximum-Cr depth in all of the Examples.
[0066]
The thickness of the coating film was determined from the concentration
profile
of 0 (oxygen). Specifically, a position where a concentration of 0 was 1/3 of
its
maximum was determined as a boundary portion between the coating film and the
base
metal, and a length from a surface of the coating film to the above boundary
portion was
determined as the thickness of the coating film.
[0067]
In addition, to evaluate the corrosion resistance against sulfuric acid, a
corrosion
test in a sulfuric acid environment was conducted. The corrosion test was
conducted by
immersing each steel pipe in a solution with a sulfuric acid concentration of
70% at a
temperature of 100 C. Then, a corrosion loss after the immersion for 8 hours
was
measured, and a corrosion rate per unit area was calculated. In the present
invention,
cases where the above corrosion rate was equal to or less than 1.00 g/(m2 h)
were
determined to be excellent in the corrosion resistance against sulfuric acid.
[0068]
The results are shown in Table 3 altogether.
[0069]
[Table 3]
_
C) Table 3
CD
---.1
C) Maximurn-Cr depth Minimum-Cr depth
Thickness =
Corrosion
Test Steel Chemical composition (at%) Left side Left
side Chemical composition (at%) Left side of coating
rate
No. No. value of value of value
of film (g/m241)
Cr Fe Ni Cu Mo formula (i) ill formula (ift
le Cr Fe Ni Cu Mo formula 00#3 (nm)
= ..
1 1 28.2 52.0 14.5 2.0 3.2 0.9 * 1.4 12.7
63.3 18.0 3.1 2.9 0.5 2.6 1.02 Comparative
2 1 28.9 51.5 14.5 1.7 3.4 _ 0.9 * 1.5 12.8
63.7 L 17.8 2.8 2.9 , 0.5 3.0 1.05 example
,
3 1 29.3 - 49.6 15.7 L. 1.7 3.7 1.0 1.4 13.3 6a7
20.1 2.8 3.1 0.5 3.4 0.92 ,
_
4 1 29.1 49.4 15.9 2.4 3.2 1.0 1.4 13.2 60.6
20.1 3.2 2.9 0.5 3.1 0.90 ,
1 32.2 L 46.4 15.3 2.3 3.8 1.2 1.5 14.9 58.8 19.8
3.3 3.2 0.6 2.8 0.71 .
6 I 33.3 46.1 14.0 2.3 _ 43 1.2 1.6 14.5
59.5 19.4 3.2 3.4 0.6 3.7 0.76 . =
,
7 1 28.4 49.8 15.6 2.5 3.7 1.0 1.3 12.9 60.6
20.3 3.2 3.0 0.5 3.4 0.82 ,
Inventive
8 1 31.7 45.8 15.3 2.4 4.8 1.2 1.4 13.0 60.0
18.5 5.0 3.5 0.5 3.5 0.85
' example
,
9 1 , 33.3 - 46.7 14.2 2.0 3.7 1.1 1.7 14.7 60.2
18.2 3.5 3.4 0.6 3.9 0.93
, 1 33.6 45.1 15.3 2.1 _ 3.8 1.2 1.6 15.4 58.6
18.7 3.8 3.5 0.6 4.1 0.65 P ,
11 1 33.9 L 45.7 14.7 2.0 3.7 , 1.2 1.7 16.0
58.0 19.0 4.0 3.0 0.6 3.8 0.75
0
12 1 34.8 L 45.8 13.6 2.2 3.7 1.2 1.8 16.5 58.5
18.1 3.7 3.2 0.7 4.2 0.81
1.,
....,
13 1 33.2 47.3 13.7 2.1 3.8 1.1 1.7 15.3 58.1
19.6 3.9 3.1 0.6 3.7 0.87 ....,
1.,
L 14 1 29.9 52.1 13.0 1.9 3.1 0.9 = 1.7 12.4
63.2 19.0 2.8 2.6 0.5 3.3 1.06
0
1 27.4 54.8 - 13.5 1.5 2.8 0.8 = 1.5 12.8 62.5 18.7
2.9 3.1 0.5 3.4 1.15 1-
. .
Comparative
1 i
16 1 24.5 L 57.0 13.7 1.8 3.0 _ 0.8 * 1.3 13.4
62.2 18.5 3.1 2.8 0.5 3.2 1.21
1.,
example
,
17 1 23.7 L 58.2 12.7 2.1 _ 3.3 0.7 * 1.3 14.6 L,
61.0 18.9 3.1 2.4 0.6 3.1 1.42 0
18 1 27.5 L 58.4 , 9.6 2.4 2.1 0.7 * 2.0 14.0 _ 63.5
18.0 2.0 2.5 0.6 2.8 1.50 ,
19 2 29.8 L 49.3 15.4 1.8 3.7 1.0 1.4 14.4 58.8
20.8 3.4 2.6 0.5 3.0 0.68 ,
1
3 29.1 49.5 16.0 2.2 3.2 1.0 1.4 13.2 60.1 20.7
3.6 2.4 0.5 2.7 0.72 .
21 4 32.3 L 46.2 15.5 2.1 3.9 L 1.2 1.5 14.3
58.5 20.8 3.3 3.1 0.5 2.8 0.91 .
,
,
,
22 5 33.5 46.1 13.9 2.2 4.3 1.2 1.6 15.3 , 57.7
20.7 3.7 2.6 0.6 3.1 0.83 ,
Inventive
,
23 6 31.3 L 48.8 14.4 2.2 3.3 1.0 1.6 15.1 58.4
20.7 3.6 2.2 0.6 3.3 0.67 .
example
.
24 7 31.8 L 45.9 15.2 2.3 4.8 , 1.2 1.4 13.8
58.9 20.4 3.7 , 3.2 0.5 3.9 0.84 .
8 33.6 45.1 15.3 2.3 3.7 1.2 1.6 15.8 58.1 20.1
3.8 2.2 0.6 4.5 0.79
,
26 9 34.1 45.7 14.7 1.9 3.6 - 1.2 1.7 17.6
56.4 19.6 4.0 2.4 0.7 4.0 0.64 ,
,
27 10 32_6 49.3 _ 13.3 1.8 3.0 1.0 1.8 L,L 16.4
57.9 18.6 4.0 , 3.1 0.6 3.8 0.91
28 11 * 29.2 51.3 15.7 0.1 3.7 0.9 ' _ 1.5 14.9
61.3 21.6 0.1 2.1 0.6 3.5 25.64 Comp. ex.
'
' indicates that conditions do not satisfy those defined by the present
invention.
in (Cr+Ni+Cu+Mo)/Fe.1.0 ...(i)
112 Cr/(N1+Cu+Mo)1.0 ...(ii)
113 Cr/(Ni+Cu+Mo)<1.0 ...(iii)
-
= CA 03032772 2019-02-01
As seen from Table 3, as to test Nos. 1, 2, and 14 to 17, and the test No. 18
with
the polished surface, in which the production conditions were inappropriate,
their
corrosion rates were high because the concentration of Cr, Ni, Cu, and Mo did
not occur
in their coating films, resulting in their poor corrosion resistances against
sulfuric acid.
Similarly, as to a test No. 28, in which a content of Cu in its base metal
fell out of the
regulation in the present invention, acid resistance brought by Cu was not
obtained, and
in addition, the concentration of Cr, Ni, Cu, and Mo in its coating film was
insufficient,
resulting in its poor corrosion resistance against sulfuric acid.
[0071]
In contrast to them, as to test Nos. 3 to 13 and 19 to 27, which satisfied the
regulations in the present invention, and in which Cr, Ni, Cu, and Mo were
concentrated
in their coating films, their corrosion rates were 1.00 g/(m2 h) or less,
resulting in their
excellent corrosion resistances against sulfuric acid.
INDUSTRIAL APPLICABILITY
[0072]
According to the present invention, an austenitic stainless steel having an
excellent acid resistance in the environment where high-concentration sulfuric
acid
condenses can be obtained. Therefore, the austenitic stainless steel according
to the
present invention can be applied to various members such as a heat exchanger
used in
thermal power generation or industrial boilers, members for a flue gas
desulfurization
facility used in a smoke flue, chimney, and various industrial fields, and
structure
members used in a facility operating in a sulfuric acid environment.
22
