U.S. patent application number 14/382923 was filed with the patent office on 2015-10-22 for acid dew point corrosion-resistant steel and exhaust gas flow path constituent member.
This patent application is currently assigned to NISSHIN STEEL CO., LTD.. The applicant listed for this patent is Susumu FUJIWARA, Yukio KATAGIRI, Shigeru MORIKAWA. Invention is credited to Susumu FUJIWARA, Yukio KATAGIRI, Shigeru MORIKAWA.
Application Number | 20150299829 14/382923 |
Document ID | / |
Family ID | 49221983 |
Filed Date | 2015-10-22 |
United States Patent
Application |
20150299829 |
Kind Code |
A1 |
KATAGIRI; Yukio ; et
al. |
October 22, 2015 |
ACID DEW POINT CORROSION-RESISTANT STEEL AND EXHAUST GAS FLOW PATH
CONSTITUENT MEMBER
Abstract
A steel has improved sulfuric acid dew point corrosion
resistance without relying upon the addition of Sb and can have
improved hydrochloric acid dew point corrosion resistance. The acid
dew point corrosion-resistant steel includes from 0.005 to 0.200%
of C, from 0.20 to 0.80% of Si, from 0.05 to 1.50% of Mn, from
0.002 to 0.020% of P, from 0.005 to 0.015% of S, from 0.10 to 0.50%
of Cu, from 0.05 to 0.30% of Ni, from 0.005 to 0.100% of Al, and 0
or more and less than 0.010% of Mo in terms of % by mass, with the
balance being Fe and impurities. In the case of attaching great
importance to hydrochloric acid dew point corrosion resistance, it
is desirable to control the Mo content in the foregoing steel to
from 0.005 to 0.030% by mass.
Inventors: |
KATAGIRI; Yukio; (Hiroshima,
JP) ; MORIKAWA; Shigeru; (Hiroshima, JP) ;
FUJIWARA; Susumu; (Horishima, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KATAGIRI; Yukio
MORIKAWA; Shigeru
FUJIWARA; Susumu |
Hiroshima
Hiroshima
Horishima |
|
JP
JP
JP |
|
|
Assignee: |
NISSHIN STEEL CO., LTD.
Tokyo
JP
|
Family ID: |
49221983 |
Appl. No.: |
14/382923 |
Filed: |
March 19, 2012 |
PCT Filed: |
March 19, 2012 |
PCT NO: |
PCT/JP2012/056962 |
371 Date: |
July 1, 2015 |
Current U.S.
Class: |
420/92 |
Current CPC
Class: |
F28F 21/082 20130101;
C22C 38/16 20130101; C22C 38/02 20130101; C22C 38/06 20130101; C22C
38/004 20130101; C22C 38/08 20130101; C22C 38/12 20130101; C22C
38/04 20130101; C22C 38/002 20130101 |
International
Class: |
C22C 38/16 20060101
C22C038/16; C22C 38/04 20060101 C22C038/04; C22C 38/00 20060101
C22C038/00; C22C 38/08 20060101 C22C038/08; C22C 38/12 20060101
C22C038/12; C22C 38/02 20060101 C22C038/02; C22C 38/06 20060101
C22C038/06 |
Claims
1. An acid dew point corrosion-resistant steel comprising from
0.005 to 0.200% of C, from 0.20 to 0.80% of Si, from 0.05 to 1.50%
of Mn, from 0.002 to 0.020% of P, from 0.005 to 0.015% of S, from
0.10 to 0.50% of Cu, from 0.05 to 0.30% of Ni, from 0.005 to 0.100%
of Al, and 0 or more and less than 0.010% of Mo in terms of % by
mass, with the balance being Fe and impurities.
2. The acid dew point corrosion-resistant steel according to claim
1, wherein the content of Mo is from 0.005 to 0.030%.
3. An exhaust gas flow path constituent member that is a member
using a steel plate comprising the steel according to claim 1 and
which in a flow path of a combustion exhaust gas in a coal-burning
thermal power plant, constitutes a site at which condensation is
generated on a surface thereof upon being exposed to the exhaust
gas.
Description
TECHNICAL FIELD
[0001] On a surface of a member coming into contact with a gas
containing a sulfur oxide or hydrogen chloride, so-called "sulfuric
acid condensation" is generated in a lower temperature state than a
dew point of the gas. In the case where the instant member is a
metal, there is a concern that corrosion proceeds due to condensed
water containing sulfuric acid, resulting in a problem. In the
present description, such corrosion to be caused due to an acid in
condensed water is called "sulfuric acid dew point corrosion". The
present invention relates to a steel having a resistance against
the sulfuric acid dew point corrosion given thereto and an exhaust
gas flow path constituent member using the same.
BACKGROUND ART
[0002] A combustion exhaust gas in a thermal power plant is chiefly
constituted of moisture, a sulfur oxide (e.g., sulfur dioxide or
sulfur trioxide), hydrogen chloride, a nitrogen oxide, carbon
dioxide, nitrogen, oxygen, and the like. In particular, when sulfur
trioxide of even 1 ppm is contained in the exhaust gas, a dew point
of the exhaust gas often reaches 100.degree. C. or higher, and
sulfuric acid condensation is easily generated. For metal members
constituting a flow path of such an exhaust gas (for example,
members constituting a flue duct wall or chimney, dust collector
members, heat exchange members for utilizing heat of an exhaust
gas, etc.), it is necessary to apply a material having excellent
sulfuric acid dew point corrosion resistance.
PRIOR ART
Patent Document
[0003] Patent Document 1: JP-B-43-14585
[0004] Patent Document 2: JP-A-2003-213367
SUMMARY OF THE INVENTION
Problem to be Solved by the Invention
[0005] As steels having improved sulfuric acid dew point corrosion
resistance, Sb-added steels are known (Patent Documents 1 and 2).
However, Sb is an expensive element and becomes a factor to bring
about a cost increase of steel materials, and also, in the case
where a large amount of Sb is consumed as a steel material raw
material, there is uncertainty about raw material procurement. In
addition, hot workability of a steel is lowered by the addition of
Sb. Furthermore, the toxicity level of Sb on a human body has not
necessarily been elucidated yet, and when elution of metal elements
due to corrosion is taken into consideration, it is desirable from
the standpoint of safety to avoid the use of Sb as far as
possible.
[0006] Meanwhile, though a stainless steel is generally good in
acid resistance, there may be the case where corrosion of the
stainless steel proceeds more easily depending upon the
concentration of the acid or the temperature as compared with
Sb-added steels. That is, not only the stainless steel is
expensive, but it may not be said that the stainless steel is a
perfect material against the sulfuric acid dew point corrosion.
[0007] In view of such present circumstances, in a steel made of
ordinary steel as a basis, an object of the present invention is to
improve sulfuric acid dew point corrosion resistance without
relying upon the addition of Sb, and desirably to further improve
corrosion resistance to hydrochloric acid contained in condensed
water (hydrochloric acid dew point corrosion resistance).
Means for Solving the Problem
[0008] As a result of detailed investigations, the present
inventors have found that in a steel having Cu added thereto, when
contents of P and S as impurity elements are strictly controlled to
specified narrow ranges, the sulfuric acid dew point corrosion
resistance can be improved. In addition, it has been noted that in
the case of incorporating a trace amount of Mo, corrosion
resistance to hydrochloric acid contained in condensed water
(hydrochloric acid dew point corrosion resistance) can also be
improved without impairing the sulfuric acid dew point corrosion
resistance. That is, in a steel composed of general steel component
elements which does not contain a special element such as Sb, it
has become clear that a "solution" of the component composition
range in which the above-described object can be achieved exists.
The present invention has been accomplished on a basis of such
novel findings.
[0009] In order to achieve the above-described object, the present
invention provides an acid dew point corrosion-resistant steel
comprising from 0.005 to 0.200% of C, from 0.20 to 0.80% of Si,
from 0.05 to 1.50% of Mn, from 0.002 to 0.020% of P, from 0.005 to
0.015% of S, from 0.10 to 0.50% of Cu, from 0.05 to 0.30% of Ni,
from 0.005 to 0.100% of Al, and 0 or more and less than 0.010% of
Mo in terms of % by mass, with the balance being Fe and impurities.
In particular, in the case of attaching great importance to the
hydrochloric acid dew point corrosion resistance, it is desirable
to control the Mo content in the above-described steel to from
0.005 to 0.030% by mass.
[0010] In addition, the present invention provides an exhaust gas
flow path constituent member that is a member using a steel plate
composed of the above-described steel and which in a flow path of a
combustion exhaust gas in a coal-burning thermal power plant,
constitutes a site at which condensation is generated on a surface
thereof upon being exposed to the above-described exhaust gas.
[0011] The exhaust gas flow path constituent member as referred to
herein means a member constituting a structure of the exhaust gas
flow path (for example, a duct, a chimney, etc.) and a member which
is disposed within the exhaust gas flow path (for example, a member
of a dust collector or heat exchanger). Examples of the member of a
heat exchanger include a "cooling fin" installed in a pipe through
which a fluid receiving heat flows.
Effect of the Invention
[0012] According to the present invention, it has become possible
to provide a steel which is improved in the sulfuric acid dew point
corrosion resistance, or further in the hydrochloric acid dew point
corrosion resistance, without adding Sb. This steel is composed of
only generally used steel component elements but does not contain a
special element, and therefore, its raw material cost is
inexpensive. In addition, a lowering of hot workability to be
caused due to the addition of a special element is avoided, too.
Furthermore, since Sb which is causing concern about its toxicity
on a human body is not used, the present invention is also
advantageous from the standpoint of safety. In consequence, the
present invention is especially useful for construction of
combustion exhaust gas flow path in a coal-burning thermal power
plant.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a graph illustrating influences of a P content on
a corrosion rate in a sulfuric acid aqueous solution.
[0014] FIG. 2 is a graph illustrating influences of an S content on
a corrosion rate in a sulfuric acid aqueous solution.
[0015] FIG. 3 is a graph illustrating influences of an Mo content
on a corrosion rate in a sulfuric acid aqueous solution.
[0016] FIG. 4 is a graph illustrating influences of an Mo content
on a corrosion rate in a hydrochloric acid aqueous solution.
MODES FOR CARRYING OUT THE INVENTION
[0017] According to detailed investigations made by the present
inventors, the sulfuric acid dew point corrosion resistance can be
enhanced by strictly controlling contents of P and S that are
impurity elements in a Cu-added steel. In addition, by
incorporating a trace amount of Mo, the hydrochloric acid dew point
corrosion resistance can be further enhanced, too. Although a
mechanism of such enhancement of the sulfuric acid dew point
corrosion resistance or hydrochloric acid dew point corrosion
resistance has not necessarily been elucidated yet, the following
findings are obtained at present.
[0018] (1) Cu is effective for formation of a hardly soluble CuS
film, and this film increases especially a resistance against
sulfuric acid.
[0019] (2) Since a decrease of P cleans the ferrite and primary
austenite grain boundaries, corrosion of the grain boundaries is
suppressed.
[0020] (3) Since the quantity of sulfide-based inclusions in the
steel is decreased due to a decrease of S, a boundary surface
between an easily corrosive inclusion and base iron decreases, and
a corrosion rate decreases. However, if the S content is too small,
the CuS film is hardly formed, and a corrosion weight loss
increases conversely.
[0021] (4) When a content of Mo increases, sulfuric acid resistance
is lowered. However, the sulfuric acid dew point corrosion
resistance is most improved in a range in which a trace amount of
Mo is added.
[0022] (5) Meanwhile, a corrosion potential is shifted to a noble
side by incorporation of Mo, and hydrochloric acid resistance is
enhanced. A content range of Mo in which in addition to the
sulfuric acid resistance, the hydrochloric acid can also be
improved exists.
[Sulfuric Acid Dew Point Corrosion Resistance]
[0023] In FIGS. 1, 2 and 3, influences of the P content, the S
content, and the Mo content on the corrosion rate in a sulfuric
acid aqueous solution are illustrated, respectively. This immersion
test is one adopting conditions of a sulfuric acid concentration of
40% by mass, a temperature of 60.degree. C., and an immersion time
of 6 hours as very severe conditions assuming a combustion gas of
heavy fuel oil (coal). As for the steels used, those in FIG. 1
contain from 0.008 to 0.010% by mass of S; those in FIG. 2 contain
from 0.010 to 0.012% by mass of P; and those in FIG. 3 contain from
0.010 to 0.012% by mass of P and from 0.008 to 0.010% by mass of S,
and in all of them, the contents of the balance elements other than
P, S and Mo all fall within the ranges specified in the present
invention.
[0024] Under the above-described sulfuric acid immersion test
conditions, the corrosion rate of conventional acid dew point
corrosion-resistant steels containing Sb, Cu and Mo approximately
falls within the range of from 10 to 20 mg/cm.sup.2/h. As is clear
from FIGS. 1, 2 and 3, excellent sulfuric acid dew point corrosion
resistance comparable to the conventional Sb-added steels is
obtained in the composition range where the P content is not more
than 0.020% by mass, the S content is from 0.005 to 0.015% by mass,
and the Mo content is from 0 or more to 0.030% by mass.
[Hydrochloric Acid Dew Point Corrosion Resistance]
[0025] In FIG. 4, influences of the Mo content on the corrosion
rate in a hydrochloric acid aqueous solution are illustrated. Test
conditions are adopted such that a hydrochloric acid concentration
is 1% by mass, a temperature is 80.degree. C., and an immersion
time is 6 hours. As is clear from FIG. 4, the hydrochloric acid
resistance is abruptly improved by the addition of a trace amount
of Mo, and when the Mo content is 0.005% by mass or more, the
hydrochloric acid resistance becomes good comparably to the
conventional Sb-added steels. In consequence, in an application of
attaching great importance to simultaneous improvements of the
sulfuric acid dew point corrosion resistance and the hydrochloric
acid dew point corrosion resistance, taking into consideration the
results of FIG. 3 in combination, the Mo content may be controlled
to the range of from 0.005 to 0.030.
[Component Elements]
[0026] The component elements of the steel of the present invention
are described. The term "%" regarding the component elements means
% by mass.
[0027] C is small in influences on the sulfuric acid dew point
corrosion resistance, and in order to ensure the strength as a
general structural material, its content is controlled to from
0.005 to 0.200%.
[0028] Since Si has an action to enhance the sulfuric acid
corrosion resistance, a content of 0.20% or more is ensured.
However, the excessive addition of Si lowers descaling properties
at the time of hot rolling and brings about an increase of scale
defects. Furthermore, Si also becomes a factor to lower welding
properties. As a result of various investigations, the Si content
is limited to not more than 0.80%.
[0029] Since Mn is effective for controlling the strength of steel
and also has an action to prevent hot brittleness due to S, a
content of 0.05% or more is ensured. It is more effective to make
the Mn content to 0.30% or more, and the Mn content may also be
controlled to 0.50% or more. However, incorporation of a large
quantity of Mn may possibly become a factor to lower the corrosion
resistance. The Mn content is tolerated to be up to 1.50%, and it
may be controlled to the range of not more than 1.20% or not more
than 1.00%.
[0030] Since P deteriorates the corrosion resistance, hot
workability, or welding properties, its content is limited to not
more than 0.020%, and more preferably not more than 0.018%. In
order to much more enhance the sulfuric acid corrosion resistance,
a decrease of the P content is effective. However, since the
excessive decrease of the P content increases the work load in
steelmaking and becomes a factor to push costs up, the P content
may be controlled to 0.002% or more.
[0031] Since S deteriorates the corrosion resistance or hot
workability, its content is limited to not more than 0.015%.
However, with respect to the sulfuric acid dew point corrosion
resistance, it was noted that when the S content is decreased, the
corrosion rate conversely turns to an increase (FIG. 2). This may
be assumed to be caused due to the matter that in the case of the
Cr-free steel that is objective in the present invention, a
contribution of the CuS film to an enhancement of the sulfuric acid
resistance is considered to be large, and when the S content
becomes small, the formation of this CuS film becomes insufficient.
As a result of various investigations, it is extremely effective to
control the S content to 0.005% or more.
[0032] Cu is effective for enhancing the sulfuric acid corrosion
resistance, and it is necessary to ensure its content of 0.10% or
more. But, since excessive incorporation of Cu becomes a factor to
lower the hot workability, the Cu content is limited to not more
than 0.50%.
[0033] Since Ni has an action to suppress a lowering of the hot
workability to be caused due to the addition of Cu, a content of
0.05% or more is ensured. It is more effective to make the Ni
content to 0.10% or more. However, since Ni becomes a factor to
deteriorate the sulfuric acid corrosion resistance, the Ni content
is limited to not more than 0.30%.
[0034] Al is an element necessary for deoxidation at the time of
steelmaking, and its content is controlled to 0.005% or more. It is
more effective to control the Al content to 0.010% or more.
However, since Al becomes a factor to lower the hot workability,
the Al content is limited to not more than 0.100%.
[0035] As described above, Mo is an extremely effective element for
enhancing the hydrochloric acid resistance, and therefore, Mo may
be added as the need arises in the case of attaching great
importance to the hydrochloric acid dew point corrosion resistance.
In order to sufficiently exhibit an action to enhance the
hydrochloric acid resistance, it is effective to ensure
incorporation of Mo of 0.005% or more (FIG. 4). However, since an
increase of the Mo content brings about a lowering of the sulfuric
acid dew point corrosion resistance, in the case of adding Mo, the
addition is conducted within the range of not more than 0.030%.
Meanwhile, in order to stably realize particularly excellent
sulfuric acid dew point corrosion resistance, it is preferable to
control the Mo content to the range of 0 or more and less than
0.010% by mass.
Examples
[0036] Steels shown in Table 1 were melted, and hot rolled steel
plates (sample materials) having a plate thickness of 2.0 mm were
fabricated by a customary method. Using a test specimen cut out
from each of the sample materials, a sulfuric acid immersion test
and a hydrochloric acid immersion test were conducted under the
same conditions (described above) as those in the case of obtaining
the plots of FIGS. 1, 2, 3 and 4. In the evaluation of sulfuric
acid dew point corrosion resistance, the case where the corrosion
rate in the sulfuric acid immersion test is not more than 20
mg/cm.sup.2/h was decided as ".largecircle." (good), and other
cases were decided as "X" (bad). In addition, in the evaluation of
hydrochloric acid dew point corrosion resistance, the case where
the corrosion rate in the hydrochloric acid immersion test is not
more than 4 mg/cm.sup.2/h was decided as ".circleincircle."
(excellent), the corrosion rate being more than 4 and not more than
20 mg/cm.sup.2/h was decided as ".largecircle." (good), and other
cases were decided as "X" (bad).
[0037] In addition, a JIS #135 test specimen was fabricated from a
cast slab of each of the steels shown in Table 1 and subjected to a
high-temperature tensile test at temperatures of three levels of
850.degree. C., 900.degree. C., and 950.degree. C. in conformity
with JIS G0567. The test was conducted in the following manner.
That is, the whole of a parallel part of the test specimen was
heated in the air using an infrared heating furnace; after reaching
a prescribed temperature, the test specimen was kept for 10
minutes; and a tensile load was then given at a tensile rate of 5
mm/min, thereby fracturing the test specimen. A temperature of the
test specimen was measured with a thermocouple connected to
substantially the center of the parallel part and controlled within
the range of a prescribed temperature .+-.10.degree. C.
[0038] The case where the fractured surface was ductile at all
temperatures of the 3 levels described above was decided as
".largecircle." (hot workability: good), and the case where a
brittle fractured surface was perceived at any one of the
temperatures was decided as ".DELTA." (hot workability: slightly
bad).
[0039] These results are shown in Table 2.
TABLE-US-00001 TABLE 1 Chemical composition (% by mass) No. C Si Mn
P S Cu Ni Al Mo Others Classification 1 0.230 0.02 0.08 0.010 0.008
0.01 0.02 0.018 -- -- Comparative example 2 0.044 0.28 0.87 0.011
0.008 0.28 0.12 0.022 -- -- Inventive example 3 0.041 0.55 0.86
0.009 0.010 0.27 0.14 0.029 -- -- Inventive example 4 0.041 0.38
0.84 0.004 0.014 0.29 0.15 0.026 -- -- Inventive example 5 0.044
0.31 0.86 0.007 0.007 0.27 0.14 0.021 -- -- Inventive example 6
0.046 0.16 0.85 0.013 0.008 0.28 0.15 0.025 -- -- Comparative
example 7 0.160 0.58 0.45 0.010 0.010 0.30 0.16 0.028 -- --
Inventive example 8 0.008 0.45 0.95 0.012 0.007 0.26 0.14 0.038 --
-- Inventive example 9 0.041 0.55 0.86 0.009 0.009 0.27 0.14 0.029
-- -- Inventive example 10 0.043 0.51 0.84 0.002 0.009 0.29 0.15
0.025 -- -- Inventive example 11 0.054 0.62 0.90 0.028 0.010 0.29
0.15 0.023 -- -- Comparative example 12 0.049 0.49 0.86 0.005 0.008
0.33 0.11 0.030 -- -- Inventive example 13 0.043 0.55 0.87 0.013
0.009 0.31 0.15 0.035 -- -- Inventive example 14 0.041 0.53 0.88
0.020 0.009 0.29 0.15 0.033 -- -- Inventive example 15 0.039 0.58
0.85 0.011 0.002 0.28 0.13 0.022 -- -- Comparative example 16 0.041
0.52 0.86 0.011 0.025 0.30 0.14 0.026 -- -- Comparative example 17
0.035 0.48 0.91 0.010 0.009 0.30 0.16 0.022 -- -- Inventive example
18 0.043 0.42 0.86 0.011 0.015 0.27 0.14 0.027 -- -- Inventive
example 19 0.052 0.43 0.89 0.014 0.005 0.31 0.14 0.024 -- --
Inventive example 20 0.043 0.53 0.86 0.012 0.003 0.27 0.14 0.019 --
-- Comparative example 21 0.031 0.62 0.95 0.011 0.010 0.26 0.14
0.038 0.005 -- Inventive example 22 0.041 0.49 0.88 0.010 0.009
0.28 0.15 0.044 0.018 -- Inventive example 23 0.026 0.73 0.78 0.010
0.009 0.28 0.13 0.031 0.029 -- Inventive example 24 0.028 0.69 0.74
0.011 0.009 0.29 0.15 0.033 0.052 -- Comparative example 25 0.033
0.47 0.82 0.011 0.009 0.31 0.15 0.028 0.088 -- Comparative example
26 0.040 0.58 0.88 0.011 0.010 0.03 0.13 0.024 -- -- Comparative
example 27 0.048 0.32 0.87 0.011 0.005 0.31 0.02 0.036 -- --
Comparative example 28 0.032 0.59 0.92 0.011 0.009 0.29 0.36 0.026
-- -- Comparative example 29 0.045 0.29 0.87 0.010 0.011 0.29 0.14
0.025 0.050 Sb: 0.05 Comparative example Underline: out of the
range specified by the present invention.
TABLE-US-00002 TABLE 2 Sulfuric acid Hydrochloric acid immersion
test immersion test Corrosion rate Corrosion rate Hot No.
(mg/cm.sup.2/h) Evaluation (mg/cm.sup.2/h) Evaluation workability
Classification 1 91.0 X 15.3 .largecircle. .largecircle.
Comparative example 2 15.5 .largecircle. 10.5 .largecircle.
.largecircle. Inventive example 3 14.5 .largecircle. 10.1
.largecircle. .largecircle. Inventive example 4 16.1 .largecircle.
9.9 .largecircle. .largecircle. Inventive example 5 14.6
.largecircle. 8.8 .largecircle. .largecircle. Inventive example 6
21.2 X 8.6 .largecircle. .largecircle. Comparative example 7 19.5
.largecircle. 9.3 .largecircle. .largecircle. Inventive example 8
16.6 .largecircle. 8.6 .largecircle. .largecircle. Inventive
example 9 15.5 .largecircle. 9.7 .largecircle. .largecircle.
Inventive example 10 10.2 .largecircle. 9.2 .largecircle.
.largecircle. Inventive example 11 23.8 X 11.6 .largecircle.
.largecircle. Comparative example 12 13.5 .largecircle. 9.3
.largecircle. .largecircle. Inventive example 13 17.1 .largecircle.
9.8 .largecircle. .largecircle. Inventive example 14 19.8
.largecircle. 8.7 .largecircle. .largecircle. Inventive example 15
43.0 X 7.9 .largecircle. .largecircle. Comparative example 16 23.5
X 13.4 .largecircle. .largecircle. Comparative example 17 17.2
.largecircle. 9.2 .largecircle. .largecircle. Inventive example 18
19.5 .largecircle. 9.5 .largecircle. .largecircle. Inventive
example 19 17.3 .largecircle. 9.6 .largecircle. .largecircle.
Inventive example 20 32.0 X 9.1 .largecircle. .largecircle.
Comparative example 21 15.3 .largecircle. 3.8 .largecircle.
Inventive example 22 15.8 .largecircle. 3.0 .largecircle. Inventive
example 23 19.4 .largecircle. 2.7 .largecircle. Inventive example
24 23.2 X 2.5 .largecircle. Comparative example 25 25.8 X 2.4
.largecircle. Comparative example 26 86.5 X 10.2 .largecircle.
.largecircle. Comparative example 27 15.8 .largecircle. 9.6
.largecircle. .DELTA. Comparative example 28 25.2 X 9.3
.largecircle. .largecircle. Comparative example 29 14.2
.largecircle. 2.9 .DELTA. Comparative example
[0040] As is clear from Tables 1 and 2, the steels having a
composition specified in the present invention were good in the
sulfuric acid dew point corrosion resistance; Nos. 21, 22 and 23
each further containing an appropriate amount of No were also good
in the hydrochloric acid dew point corrosion resistance; and all of
these were not problematic in the hot workability, too.
[0041] On the other hand, though No. 29 containing Sb, Cu, and Mo
(corresponding to the conventional acid dew point
corrosion-resistant steel) was good in the sulfuric acid dew point
corrosion resistance, it was inferior in the hot workability. It is
to be noted that No. 27 was inferior in the hot workability because
the addition amount of Ni was small.
* * * * *