DESCRIPTION
Title of Invention: Highly Corrosion-Resistant Austenite
Stainless Steel Well-Suited to Brazing
Technical Field
The present invention relates to austenitic
stainless steel which is used for a structure which is
joined by a nickel braze filler metal, copper braze
filler metal, or other braze filler metal. In particular,
the present invention relates to austenitic stainless
steel which is not only excellent in brazeability, but is
also excellent in corrosion resistance in an environment
in which condensation of combustion exhaust gas causes
formation of condensed water which contains nitric acid
ions and sulfuric acid ions and which is low in pH and in
corrosion resistance in an environment of an aqueous
solution which contains chloride ions.
Background Art
[0002] Brazing is the art of joining materials by
using a braze filler metal with a lower melting point
than a structural material and performing heat treatment
at a temperature somewhat higher than the melting point
of the braze filler metal. Brazing is a joining method
which is widely used even for stainless steel. The braze
filler metal which is used for brazing stainless steel is
an alloy of nickel or copper.
In brazing of stainless steel, the passivation
film of the stainless steel impedes the brazeability.
Therefore, brazing is performed in a vacuum or a hydrogen
environment so as to remove the passivation film by
reduction. The temperature of brazing is, for example,
about 1100°C when using the above nickel braze filler
metal.
[0004] In brazing, it is important that the braze
filler metal fully fill the clearances between joined
materials and that the strength of the joint be secured.
Therefore, the wettability of the braze filler metal to
the joined material of stainless steel becomes important.
On the other hand, if the braze filler metal is too good
in wettability, the braze filler metal flows out from the
clearances between joined materials, the clearances
cannot be filled by the braze filler metal, and the joint
strength is lowered. Therefore, as stainless steel which
is excellent in brazeability, having suitable wettability
becomes important.
[0005] As the stainless steel which is brazed,
austenitic stainless steel is generally used. Further, as
the austenitic stainless steel, a SUS304 material and a
SUS316 material of JIS (Japan Industrial Standard)
(below, referred to as a "SUS304 material" and a "SUS316
material") are being broadly used. The SUS304 material
and SUS316 material have not only workability, but also
the characteristic of being excellent in corrosion
resistance in a general environment. However, a SUS316-
based material and SUS316-based material have the problem
of being inferior in stress corrosion cracking
resistance.
Stress corrosion cracking occurs when tensile
stress remains in a material which is exposed to an
environment where corrosion occurs and which is highly
susceptible to stress corrosion cracking. When brazing
austenitic stainless steel, even if tensile stress
remains in the joined material at the stage before
brazing, there is no concern over stress corrosion
cracking. This is because austenitic stainless steel is
brazed at a temperature where austenitic stainless steel
is annealed and because the residual stress is removed
during brazing. This is because, for example, when using
a nickel braze filler metal, as explained above, the
brazing is performed at about 1100°C.
[0007] However, depending on the part, sometimes parts
are assembled by welding or screwing with other parts
after brazing. In this case, tensile stress occurs at the
parts after assembly and stress corrosion cracking is
liable to be caused. For this reason, austenitic
stainless steel which is brazed has to have stress
corrosion resistance.
[0008] As the environments in which the braze filler
metal of austenitic stainless steel is used, for example,
there are exhaust system parts of automobiles and
secondary heat exchangers of hot water heaters which are
equipped with latent heat recovery devices. These
materials are all used in an environment in which
condensation of combustion exhaust gas forms condensed
water which contains nitric acid ions and sulfuric acid
ions and which is low in pH. This is because the air
which is taken in for combustion contains a large amount
of nitrogen and the fuel or the scented substances which
are added to fuel contain sulfur compounds. Under such an
environment, copper is corroded. Therefore, as materials
which form the exhaust system parts of automobiles and
secondary heat exchangers of hot water heaters which are
equipped with latent heat recovery devices, copper cannot
be used. Austenitic stainless steel becomes essential.
Accordingly, it is important that the
austenitic stainless steel which is used for such a
member achieve both corrosion resistance and brazeability
even in an environment where condensed water which
contains nitric acid ions and sulfuric acid ions and
which is low in pH is formed.
Regarding the brazeability of stainless steel,
PLT 1 proposes a braze filler metal-precoated metal sheet
material which is obtained by spray coating a nickel-
based braze filler metal which is suspended together with
an organic binder on the surface of a sheet of stainless
steel, then heating it. Further, PLT 2 proposes a method
of production of nickel braze filler metal-coated
stainless steel sheet which is excellent in self
brazeability obtained by coating, by plasma spraying, a
stainless steel sheet adjusted in surface roughness with
a nickel-based braze filler metal. However, both PLTs 1
and 2 only study the conventional SUS304 materials and
SUS316 materials as austenitic stainless steel materials
to be coated with a braze filler metal.
[0011] PLT 3 proposes stainless steel which is reduced
in Al and Ti and which is excellent in brazeability.
Further, PLT 4 proposes stainless steel which has been
adjusted to an M value, which is shown by M=-
0.22T+34.5Ni+ 10.5Mn+13.5Cu-17.3Cr-17.3Si-18Mo+475.5, of
1 to 25. However, both PLT's 3 and 4 are studies of
ferritic stainless steel. Austenitic stainless steel is
not studied.
PLT 5 proposes an austenitic stainless steel
material which has stress corrosion cracking resistance
and crevice corrosion resistance. However, the steel
sheet which is proposed in PLT 5 is applied for use in
fuel system members of automobiles. The stress corrosion
cracking resistance was studied but the brazeability is
not described.
[0013] Further, when used for exhaust system parts of
automobiles or secondary heat exchangers of hot water
heaters which are equipped with latent heat recovery
devices, since the atmosphere which is taken in includes
chlorides, in particular when used in high salt damage
regions near the coast, the corrosion resistance in
environments which contain chloride ions also becomes an
issue.
Citations List
Patent Literature
[0014] PLT 1: Japanese Patent Publication No. 1-
249294A
PLT 2: Japanese Patent Publication No. 2001-26855A
PLT 3: Japanese Patent Publication No. 2009-174046A
PLT 4: Japanese Patent Publication No. 2010-65278A
PLT 5: Japanese Patent Publication No. 2007-9314A
[0014a] In this specification where reference has been
made to patent specifications, other external documents,
or other sources of information, this is generally for
the purpose of providing a context for discussing the
features of the invention. Unless specifically stated
otherwise, reference to such external documents is not to
be construed as an admission that such documents, or such
sources of information, in any jurisdiction, are prior
art, or form part of the common general knowledge in the
art.
Summary of Invention
Technical Problem
The present invention has as its object the
provision of austenitic stainless steel which is not only
excellent in brazeability, but is also excellent in
corrosion resistance in an environment where condensation
of combustion exhaust gas causes formation of condensed
water which contains nitric acid ions or sulfuric acid
ions and which is low in pH or corrosion resistance in an
environment of an aqueous solution which contains
chloride ions; and/or at least the provision of a useful
choice to the public.
Solution to Problem
The inventors engaged in intensive studies to
obtain austenitic stainless steel which achieves both
brazeability and corrosion resistance and as a result
discovered the following:
(a) In the case of austenitic stainless steel,
if Si and Cu are added in certain amounts or more, the
wettability becomes excessively good and braze filler
metal ends up flowing out from the clearances in the
joined materials, so the joint becomes insufficient. To
prevent this, it is important to prescribe not only the
upper limits of the contents of Cu and Si but also the
upper limit of the value of [Cu]×[Si]. Note that, in the
following explanation, [Cu] and [Si] are made the
contents of Cu and Si expressed by mass%.
(b) Brazed austenitic stainless steel
suppresses stress corrosion cracking due to the
synergistic effect of Cu and Si which are expressed by
the value of [Cu]×[Si].
(c) The corrosion resistance in an environment
where condensation of combustion exhaust gas causes the
formation of condensed water which contains nitric acid
ions and sulfuric acid ions and which is low in pH and
further the corrosion resistance in an environment of an
aqueous solution which contains chloride ions are
improved by making the value of 2[N]+[Mo] a certain value
or more. Note that, in the following explanation, [N] and
[Mo] are the contents of N and Mo expressed by mass%.
The present invention was made based on the
above findings and has as its gist the following:
(1) Austenitic stainless steel which is
excellent in corrosion resistance and brazeability
characterized by containing, by mass%, C: 0.080% or less,
Si: 1.2 to 3.0%, Mn: 0.4 to 2.0%, P: 0.03% or less, S:
0.003% or less, Ni: 6.0 to 12.0%, Cr: 16.0 to 20.0%, Cu:
0.2% to 3.0%, Al: 0.002 to 0.10%, N: 0.030 to 0.150%, and
Mo: 0.1 to 1.0%, having a balance of Fe and unavoidable
impurities, and satisfying the following Formula (A) and
Formula (B):
Formula (A): 1.6≤[Cu]×[Si]<4.0
Formula (B): 0.16≤2[N]+[Mo]≤1.0
where, [Cu], [Si], [N], and [Mo] are contents of elements
expressed by mass%.
(2) Austenitic stainless steel which is excellent in
corrosion resistance and brazeability characterized by
containing, by mass%, C: 0.080% or less, Si: 1.2 to 3.0%,
Mn: 0.4 to 2.0%, P: 0.03% or less, S: 0.003% or less, Ni:
6.0 to 12.0%, Cr: 16.0 to 20.0%, Cu: 0.2% to 3.0%, Al:
0.002 to 0.10%, N: 0.030 to 0.150%, and Mo: 0.1 to 1.0%,
having a balance of Fe and unavoidable impurities, and
satisfying the following Formula (A) and Formula (B):
Formula (A): 1.6≤[Cu]×[Si]≤3.77
Formula (B): 0.16≤2[N]+[Mo]≤1.0
where, [Cu], [Si], [N], and [Mo] are contents of elements
expressed by mass%.
(3) Austenitic stainless steel which is
excellent in corrosion resistance and brazeability as set
forth in (1) or (2) characterized by further containing,
by mass%, one or more of Nb: 0.1 to 0.7%, Ti: 0.1 to
0.5%, V: 0.1 to 3.0%, and B: 0.0002% to 0.003%.
[0022a] In the description in this specification
reference may be made to subject matter which is not
within the scope of the appended claims. That subject
matter should be readily identifiable by a person skilled
in the art and may assist in putting into practice the
invention as defined in the appended claims.
Advantageous Effects of Invention
[0023] According to the present invention, by
establishing a suitable content of Cu and content of Si
in austenitic stainless steel and controlling the content
of N and content of Mo, it is possible to provide
austenitic stainless steel which is excellent in
corrosion resistance and brazeability.
Further, according to the present invention, it
is possible to improve the corrosion resistance of heat
recovery devices of combustion exhaust gas which is
fueled by gasoline, LNG, LPG, oil, and other hydrocarbons
and other heat exchangers and other structures which are
obtained by brazing.
is a view which shows the relationship
between the contents of Cu and Si and the brazeability
and corrosion resistance.
is a view which shows the relationship between the
value of 2[N]+[Mo] and the maximum depth of corrosion.
is a view which shows the relationship between the
value of 2[N]+[Mo] and value of [Cu]×[Si] and the
brazeability and corrosion resistance.
Description of Embodiments
The present invention will be explained in
detail. In the following explanation, the % relating to
the chemical composition means the mass% unless otherwise
indicated.
First, tests run for obtaining a chemical
composition realizing both brazeability and corrosion
resistance and their results will be explained.
Austenitic stainless steel changed in Si, Cu, Mo, and N
were produced by vacuum melting. The other elements at
this time were made ones in the range of chemical
compositions of JIS SUS316.
[0028] Each of these austenitic stainless steels was
hot rolled and heat treated at 1150°C×1 minute, then was
descaled by grinding off the scale and further was cold
rolled to produce cold rolled sheet. This cold rolled
sheet was heat treated under conditions of 1050 to
1150°C×1 minute based on the recrystallization behavior,
then was dipped and pickled in a nitric hydrofluoric acid
aqueous solution until the scale was completely removed
to thereby obtain a material for brazing. This material
for brazing use was used to evaluate the brazeability and
stress corrosion cracking.
(Evaluation of Brazeability)
The material for brazing was cut to 40×50 mm and 25×30 mm
for use as a test material for evaluation of
brazeability. The thickness of the test material for
evaluation of the brazeability was 1 mm. The thus
prepared test material was brazed by using a silver braze
filler metal. The brazing was performed by superposing
two test materials, inserting at the superposed portions
0.3 g of a braze filler metal comprised of a JIS BNi5
nickel braze filler metal in which an organic binder was
mixed, and brazing. The brazing was performed using a
hydrogen reduction furnace and in a 1100°C, hydrogen 100%
atmosphere. The brazeability was evaluated by cutting the
brazed test material and observing the cross-section
visually.
The results of evaluation are shown in
At the cross-section of the brazed test material, the
cases where the braze filler metal material is completely
filled in the clearances are shown by the white circles
or black circles, while the cases where clearances remain
are shown by the × marks. Here, the white circles and the
black circles show the results of evaluation of the
stress corrosion cracking explained later differentiated.
The good cases where no stress corrosion cracking occurs
are shown by the white circles, while the defective cases
where stress corrosion cracking occurs are shown by the
black circles. Further, among the two curves which are
shown in the lower curve shows the case where the
value of [Cu]×[Si] is 1.6, while the upper curve shows
the case where the value of [Cu]×[Si] is 4.4. Note that,
in "SCC" means stress corrosion cracking. The
same is true for
As clear from if Si is over 3.0%, Cu is
over 3.0%, or the value of [Cu]×[Si] is over 4.4,
clearances are formed at the cross-section of the brazed
test materials. In the case of austenitic stainless
steel, the addition of Si and Cu causes the wettability
of the braze filler metal to become better. However, if
Si and Cu are added in certain amounts or more, the
wettability becomes excessively good and braze filler
metal ends up flowing out from the clearances between the
joint materials, so the joint becomes insufficient.
Therefore, the upper limit of the value of [Cu]×[Si] is
made 4.4. The more preferable upper limit is 4.0.
(Evaluation of Stress Corrosion Cracking)
Materials for brazing use were heated under the same
conditions as when brazing but without brazing, that is,
using a hydrogen reduction furnace, in a 1100°C, hydrogen
100% atmosphere. After this heating, the material for
brazing use was cut to 30×30 mm and 15×15 mm sizes and
polished at the cross-sectional end faces. Two materials
of different sizes were superposed and spot welded at the
center to impart clearance to the two materials and
obtain a test material for evaluation of stress corrosion
cracking. The test material for evaluation of stress
corrosion cracking was dipped in an aqueous solution
containing 200 ppm of Cl and held there at 100°C for
seven days. After the elapse of seven days, the spot
welded part was drilled through to separate the materials
and the presence of any cracks at the inside clearance
surfaces was evaluated. The presence of any cracks was
checked for by the dye penetration test detection test
(color check test).
The results of evaluation are shown together at
Case where stress corrosion cracking did not
occur are shown by white circles, while cases where
stress corrosion cracking occurred are shown by black
circles. In if investigating the test material
where no stress corrosion cracking occurs, the value of
[Cu]×[Si] was 1.6 or more. On the other hand, test
materials with values of [Cu]×[Si] of less than 1.6
suffered from stress corrosion cracking. In general,
there is the finding that for improvement of the stress
corrosion cracking resistance of austenitic stainless
steel, addition of Si and Cu is effective. In the present
invention, it is clear that in brazed austenitic
stainless steel as well, the effect of suppression of
stress corrosion cracking is obtained by the synergistic
effects of Cu and Si as expressed by the value of
[Cu]×[Si]. Therefore, the lower limit of the value of
[Cu]×[Si] is made 1.6. It is more preferably made 2.0.
[0034] Next, the method of evaluation of the corrosion
resistance against the condensed water which is formed
from the combustion exhaust gas and the results will be
explained. As explained above, the brazed structures are
used as exhaust system parts of an automobile or
secondary heat exchangers of hot water heaters which are
equipped with latent heat recovery devices etc.
Therefore, it is not enough that the austenitic stainless
steel forming a brazed structure be excellent in
brazeability and stress corrosion cracking resistance.
(Evaluation of Corrosion Resistance Against
Condensed Water Formed by Combustion Exhaust Gas)
The material which is used as the test material is
austenitic stainless steel which is excellent in
brazeability and stress corrosion resistance, that is,
has a value of [Cu]×[Si] of 1.6 to 4.4 in range. The test
solution was made one imitating the composition of
condensed water which is formed by the combustion of
general LNG or petroleum. Specifically, the test solution
was made a composition which is adjusted to nitric acid
ions: 100 ppm, sulfuric acid ions: 10 ppm, and pH2.5 and
which has chloride ions added to in an amount of Cl of
100 ppm to accelerate the corrosion.
A material of austenitic stainless steel with a
value of the above [Cu]×[Si] of 1.6 to 4.4 in range was
heated under conditions the same as when brazing but
without brazing, that is, using a hydrogen reduction
furnace in a 100% hydrogen atmosphere. The heated
material was cut to a size of 15×100 mm for use as a test
material for evaluation of the corrosion resistance. This
test material for evaluation of the corrosion resistance
was immersed exactly half in the test solution in a test
tube. Note that, the test solution in the test tube was
made 10 ml. Further, this test tube was immersed in 80°C
warm water and held there for several hours until
completely drying at the inside. After drying, another
test tube was newly filled with test solution and the
sample held there until completely drying. The repeated
drying and wetting test was performed for 14 cycles. The
corrosion resistance against condensed water formed by
the combustion exhaust gas was evaluated by measuring the
maximum depth of corrosion at the cross-section of the
test material for evaluation of the corrosion resistance
after the test.
The cases where the maximum depth of corrosion
is less than 100 μm are shown by the white circles, while
the cases where the depth is 100 μm or more are shown by
the × marks. As clear from when the value of
2[N]+[Mo] is 0.16 or more, the maximum depth of corrosion
becomes less than 100 μm. This is believed to be because
even in a low pH solution which contains Cl , an effect of
improvement of the pitting corrosion resistance by Mo and
N is obtained.
Note that, in even when the value of
2[N]+[Mo] is 0.16 or more, sometimes the maximum pitting
depth is 100 μm or more. If investigating the test
materials in this case, the content of Cu was outside the
range explained later. This is because Cu dissolves and
forms ions at the time of corrosion in a repeated wet and
dry corrosive environment which contains an oxidizing
agent such as nitric acid ions. Further, it was believed
that in such an environment, Cu ions act as an oxidizing
agent inside and outside pitting, so the depth of
corrosion increases.
Further, along with an increase in the value of
2[N]+[Mo], the maximum depth of corrosion is reduced, but
the drop in depth of corrosion becomes saturated at a
certain value or more. This believed to be because if the
contents of N and Mo exceed certain values, the effects
of elements other than N and Mo on the depth of corrosion
can no longer be ignored. In particular, if Cu is
present, the Cu ions promote the corrosion. For the above
such reason, the upper limit of 2[N]+[Mo] is made 1.0 or
less. The preferable upper limit is 0.77, while the more
preferable upper limit is 0.74. Note that, the lower
limit of 2[N]+[Mo], as explained above, is 0.16, while
the preferable lower limit is 0.20.
[0040] Summarizing the results which are shown in the
above-mentioned and by the relationship of
the value of [Cu]×[Si] and the value of 2[N]+[Mo], the
relationship which is shown in is obtained. As
clear from a test material with a value of
[Cu]×[Si] of 1.6 to 4.4 in range and a value of 2[N]+[Mo]
of 0.16 to 1.0 in range achieves both brazeability and
corrosion resistance. Note that, in the present
invention, the "corrosion resistance" means the stress
corrosion cracking resistance, the corrosion resistance
in an environment where condensation of combustion
exhaust gas causes the formation of condensed water which
contains nitric acid ions and sulfuric acid ions and
which is low in pH, and the corrosion resistance in an
environment of an aqueous solution which contains
chloride ions.
[0041] The austenitic stainless steel of the present
invention has to satisfy the following formula (A) and
formula (B) for Cu, Si, Mo, and N.
Formula (A): 1.6≤[Cu]×[Si]<4.0
Formula (B): 0.16≤2[N]+[Mo]≤1.0
[0042] Next, the reasons for limitation of the
elements which are contained in the austenitic stainless
steel of the present invention alone will be explained.
C causes the intergranular corrosion resistance
and the workability to fall, so the content has to be
reduced, therefore the upper limit has to be made 0.080%.
However, excessive reduction of the C content causes the
refining costs to deteriorate. Therefore, the preferable
C content is 0.005 to 0.060% in range.
Si, as explained above, like Cu, is added to
improve the wettability and prevent stress corrosion
cracking. If the Si content is less than 1.2%, these
effects are not manifested. On the other hand, if the Si
content exceeds 3.0%, the wettability is excessively
increased and the brazeability falls. Therefore, the Si
content has to be 1.2 to 3.0% in range. Preferably, it is
1.4 to 2.5% in range.
Mn is an element which is important as a
deoxidizing element, but if added in excess, it easily
forms MnS which acts as starting points of corrosion.
Therefore, the content of Mn has to be 0.4 to 2.0% in
range. More preferably, it is 0.5 to 1.2% in range.
P not only causes the weldability and
workability to drop, but also facilitates intergranular
corrosion, so must be kept as low as possible. For this
reason, the upper limit of the content of P has to be
0.03%. The preferable content of P is 0.001 to 0.025% in
range.
S causes the formation of the above-mentioned
MnS and other aqueous inclusions which form starting
points of corrosion, so have to be reduced as much as
possible. For this reason, the S content is made 0.003%
or less. However, excessive reduction of S is costly, so
the S content is preferably 0.0002 to 0.002% in range.
Ni has no effect on stress corrosion cracking
resistance in the amount prescribed in JIS SUS316L.
However, in an environment in which the steel is exposed
to the exhaust gas when LNG or oil burns, there is a
concern that the stress corrosion cracking resistance
will fall. Further, it is necessary to maintain the
austenite phase and also secure the workability.
Therefore, the Ni content has to be 6.0 to 12.0% in
range. Preferably, it is 6.5 to 11.0% in range.
Cr is the most important element in securing
the corrosion resistance of stainless steel. Therefore,
the lower limit of Cr content is made 16.0%. However, if
increasing the Cr, the corrosion resistance is also
improved, but the workability and other manufacturing
properties are lowered, so the upper limit of the content
of Cr is made 20.0%. The preferable content of Cr is 16.5
to 19.0% in range.
[0050] Cu, along with Si, causes a drop in
brazeability due to its addition, but acts to suppress
stress corrosion cracking. On the other hand, excessive
addition of Cu causes a drop in the corrosion resistance
in a solution which contains nitric acid ions. Therefore,
the content of Cu has to be 0.2 to 3.0% in range.
Preferably, it is 0.5 to 2.5% in range.
[0051] Al is important as a deoxidizing element.
Further, it controls the composition of the nonmetallic
inclusions and refines the structure. However, if
excessively added, it invites coarsening of the
nonmetallic inclusions and is liable to form starting
points for the formation of defects in the product.
Therefore, the content of Al has to be 0.002 to 0.10% in
range. Preferably it is 0.005 to 0.08% in range.
N improves the pitting corrosion resistance,
but excessive addition, like C, causes a drop in the
intergranular corrosion resistance and workability.
Therefore, the content of N has to be 0.030 to 0.150% in
range. Preferably, it is 0.037 to 0.10% in range.
Mo has an effect in repairing the passivation
film and is an element which is extremely effective for
improving the corrosion resistance. Furthermore, in an
environment which contains nitric acid ions and chloride
ions, in combination with N, it has an effect in
improving the pitting corrosion resistance. Therefore, Mo
has to be included in at least an amount of 0.1%. On the
other hand, if increasing Mo, the corrosion resistance is
improved, but excessive addition causes a drop in
workability and invites a rise in costs. Accordingly, the
upper limit of the content of Mo has to be made 1.0%. The
preferable content of Mo is 0.2 to 0.8% in range.
[0054] In the present invention, in addition to the
essential elements which have been explained up to here,
in accordance with need, it is possible to include one or
more elements from Nb, Ti, V, and B.
Nb, by addition, forms carbonitrides and
suppresses the sensitization near the weld zone so has
the effect of increasing the high temperature strength,
therefore can be added in accordance with need. However,
excessive addition invites a rise in cost. Therefore, the
content of Nb is preferably 0.1 to 0.7% in range.
Ti has effects similar to Nb, but excessive
addition invites an increase in surface defects due to
nitrides of titanium. Therefore, the content of Ti is
preferably made 0.1 to 0.5% in range.
V improves the corrosion resistance and crevice
corrosion resistance, so if keeping down the use of Cr
and Mo and adding V, excellent workability can be
secured. Therefore, V can be added in accordance with
need. However, excessive addition invites a drop in
workability. Accordingly, the content of V is preferably
0.1 to 3.0% in range.
B is a grain boundary strengthening element
which is effective for improvement of the hot
workability, so can be added in accordance with need.
However, excessive addition becomes a cause of a drop in
workability. Therefore, the lower limit of the content of
B is preferably 0.0002% and the upper limit is preferably
0.003%.
Examples
Next, the present invention will be further
explained by examples, but the conditions in the examples
are just illustrations which have been employed for
confirming the workability and effects of the present
invention. The present invention is not limited to these
illustrations. The present invention can employ various
conditions so long as not deviating from the gist of the
present invention and achieving the object of the present
invention.
Steel of each of the chemical compositions
which are shown in Table 1 was produced by the method of
production of usual austenitic stainless steel. First,
the steel was vacuum smelted, then cast into a 40 mm
thick ingot. This was hot rolled to a 4.0 mm thickness.
After this, the steel was heat treated at 1150°C×1 minute,
then was descaled by grinding off the scale and further
was cold rolled to produce a 1.0 mm thick steel sheet.
This was heat treated under conditions of 1050 to 1150°C×1
minute based on various recrystallization behaviors, then
was dipped and pickled in a nitric hydrofluoric acid
aqueous solution until the scale was completely removed.
This was used for the following three tests.
Table 1
(mass%)
Max.
Stress
2[N]+ Braze- corrosion
[Cu]×
No. C Si Mn P S Ni Cr Mo Cu Nb Ti Al V B N corrosion Remarks
depth
[Mo] [Si] ability
cracking
(μm)
1 0.064 1.3 0.7 0.028 0.0007 7.9 18.0 0.10 2.9 - - 0.048 - - 0.041 0.18 3.77 A A 99 Inv. ex.
2 0.055 1.6 1.1 0.028 0.0007 6.4 17.1 0.15 2.1 - - 0.025 - - 0.075 0.30 3.26 A A 85 Inv. ex.
3 0.033 2.9 1.3 0.020 0.0010 11.2 19.3 0.36 0.6 - - 0.020 - - 0.051 0.46 1.68 A A 55 Inv. ex.
4 0.040 1.8 0.6 0.022 0.0006 10.2 17.9 0.60 1.8 - - 0.045 - - 0.065 0.73 3.24 A A 49 Inv. ex.
0.066 2.6 0.4 0.019 0.0002 8.0 19.5 0.31 0.7 - - 0.034 - - 0.101 0.51 1.81 A A 35 Inv. ex.
6 0.039 1.5 0.6 0.020 0.0003 9.5 17.9 0.52 1.5 - - 0.036 - - 0.051 0.62 2.26 A A 58 Inv. ex.
7 0.044 1.9 0.7 0.022 0.0006 9.1 18.0 0.89 2.3 - - 0.013 - - 0.038 0.97 4.30 A A 51 Inv. ex.
8 0.035 2.0 0.5 0.028 0.0007 7.5 18.9 0.50 0.9 0.54 - 0.018 - - 0.049 0.60 1.82 A A 39 Inv. ex.
9 0.051 2.7 0.5 0.020 0.0005 7.0 16.9 0.62 1.5 - 0.25 0.032 - - 0.075 0.77 4.05 A A 40 Inv. ex.
0.047 1.3 0.4 0.027 0.0005 7.4 18.0 0.30 2.2 - - 0.038 0.41 - 0.075 0.45 2.86 A A 78 Inv. ex.
11 0.061 2.2 0.9 0.024 0.0009 8.8 19.5 0.35 1.4 - - 0.055 - 0.0006 0.035 0.42 3.08 A A 40 Inv. ex.
12 0.015 1.3 0.6 0.028 0.0003 7.0 16.4 0.13 1.4 - - 0.042 0.18 0.0003 0.039 0.21 1.75 A A 88 Inv. ex.
14 0.036 1.5 0.6 0.002 0.0007 10.5 18.4 0.19 3.5 - - 0.040 - - 0.063 0.32 5.25 B A 187 Comp. ex.
0.040 1.9 0.5 0.020 0.0010 7.9 17.6 0.28 3.2 0.50 - 0.061 - - 0.030 0.34 6.08 B A 174 Comp. ex.
16 0.054 2.1 1.5 0.018 0.0007 5.5 17.1 0.41 4.2 - 0.31 0.015 - - 0.048 0.51 8.82 B A 199 Comp. ex.
17 0.020 3.0 0.4 0.020 0.0010 11.0 16.1 0.70 3.1 - - 0.005 0.21 - 0.030 0.76 9.30 B A 120 Comp. ex.
18 0.027 2.2 0.5 0.022 0.0006 10.2 17.9 0.63 3.3 - - 0.100 - 0.0003 0.038 0.71 5.72 B A 104 Comp. ex.
19 0.025 1.4 0.9 0.002 0.0014 7.2 16.5 0.40 0.8 - - 0.003 - - 0.010 0.42 1.12 A B 94 Comp. ex.
0.025 1.8 0.9 0.002 0.0014 7.2 16.2 0.12 1.5 - - 0.003 - - 0.010 0.14 2.70 A B 121 Comp. ex.
21 0.015 3.2 0.3 0.023 0.0009 6.3 17.0 0.00 1.5 - - 0.054 - - 0.041 0.08 4.80 B A 168 Comp. ex.
22 0.051 1.4 1.5 0.022 0.0030 7.8 15.1 0.90 2.8 - - 0.005 - - 0.040 0.98 3.92 A A 110 Comp. ex.
23 0.015 0.8 0.6 0.023 0.0014 6.8 16.6 0.10 1.5 - - 0.003 - - 0.005 0.11 1.20 A B 189 Comp. ex.
24 0.045 0.6 0.8 0.020 0.0012 8.1 18.1 0.05 0.1 - - 0.003 - - 0.034 0.12 0.06 A B 151 Comp. ex.
0.010 0.2 0.2 0.020 0.0006 12.1 17.3 2.10 0.2 - - 0.002 - - 0.011 2.12 0.03 A B 30 Comp. ex.
Note) Underlines indicate outside range of present invention. "-" mean not contained.
(Brazeability Test)
Various types of stainless steel with a thickness of 1 mm
were cut into 40×50 mm and 25×30 mm pieces which were wet
polished over their entire surfaces using No. #600
waterproof emery paper (waterproof polishing paper) for
use as test materials. These were subjected to a
brazeability test using silver braze filler metal.
The brazing was performed by superposing two
test materials by the same method as explained above.
Specifically, the superposed parts of the test materials
were filled with 0.3 g of silver braze filler metal of
JIS BNi5 mixed with an organic binder and then brazed.
The brazing was performed using a hydrogen reduction
furnace in a 1100°C, hydrogen 100% atmosphere. In the
method of evaluation, the cases where visual observation
showed that the clearances in the cross-sections of the
brazed test materials were completely filled were judged
as good, while the cases where clearances remained were
judged as poor.
[0064] (Corrosion Resistance Test)
Next, the method of a repeated drying and wetting test
which is performed in a test solution imitating the
condensed water which is formed by combustion of LNG or
oil will be explained. For the test material, various
types of stainless steel were heated under the same
conditions as when brazing but without brazing, that is,
using a hydrogen reduction furnace in a 1100°C, hydrogen
100% atmosphere. After this, the steel was cut into a
×100 mm size and tested. Note that, the thickness of
the test material was 1 mm. The composition of the test
solution, as explained above, imitated the composition of
the condensed water which is formed by general LNG or
oil. It was adjusted to nitric acid ions: 100 ppm and
sulfuric acid ions: 10 ppm and pH2.5, imitated the
concentration of the salt content, and was given 100 ppm
of chloride ions. 10 ml of this test solution was placed
in a test tube. The test material was immersed half into
this and the test tube was placed in a 80°C warm bath. The
test tube was held until the test solution completely
dried. After drying, the sample was transferred to a new
test tube filled with the test solution and again dried.
This drying was performed 14 times, then the maximum
depth of corrosion after the test was measured.
(Stress Corrosion Cracking Evaluation Test)
The stress corrosion cracking evaluation test was
performed by heating a material the same as that used for
the brazeability test under the same conditions as when
brazing but without brazing, that is, using a hydrogen
reduction furnace, in a 1100°C, hydrogen 100% atmosphere.
This material was cut into 30×30 mm and 15×15 mm sizes and
was wet polished over its entire surface, then two sheets
were superposed and spot welded to impart clearances. The
test material given clearances in this way was immersed
in distilled water containing 200 ppm of Cl and was
treated continuously at 100°C for seven days. The spot
weld of the treated test material was removed by a drill
and the material separated, then the presence of any
cracks was checked for by the dye penetration test
detection test (color check test). Here, cases where no
cracks occurred were judged as "good" while cases where
cracks occurred were judged as "poor".
These test results are listed together in Table
1. Note that, in the results of the brazeability test and
the results of the stress corrosion cracking evaluation
test, good is indicated as "A" and poor as "B".
[0067] As clear from Table 1, it was confirmed that
the invention examples of Nos. 1 to 12 were excellent in
all of the brazeability test, the maximum depth of
corrosion in the corrosion resistance test (repeated
drying and wetting test), and the test for evaluation of
the stress corrosion cracking.
As opposed to this, it was confirmed that Nos.
14 to 18, and 21 with values of [Cu]×[Si] of over 4.4 did
not give sufficient brazeability. Further, it was
confirmed that Nos. 19, 23, 24, and 25 with values of
[Cu]×[Si] of less than 1.6 were excellent in
brazeability, but cracked in the stress corrosion
cracking evaluation test. Furthermore, Nos. 20, 21, 23,
and 24 with values of 2[N]+[Mo] below the lower limit of
the present invention had maximum depths of pitting of
100 μm or more in the corrosion resistance test (repeated
drying and wetting test). No. 22 has a value of [Cu]×[Si]
and a value of 2[N]+[Mo] in the range of the present
invention, but has Cr below the lower limit of the range
of the present invention, so had a maximum depth of
corrosion of over 100 μm in the corrosion resistance test
(repeated drying and wetting test). Note that, in Nos. 14
to 18, even if the values of 2[N]+[Mo] were in the range
of the present invention, the maximum depths of corrosion
exceeded 100 μm in the corrosion resistance test
(repeated drying and wetting test) because the Cu was
outside the present invention in range, so it was judged
that the effect of acceleration of corrosion due to the
eluted Cu ions was in action.
From the above, it could be confirmed that the
austenitic stainless steel of the present invention is
excellent in brazeability and did not suffer from stress
corrosion cracking even in an environment inside of a
heat exchanger which is exposed to combustion gas of a
hydrocarbon fuel. Further, simultaneously with this, it
was confirmed that the austenitic stainless steel of the
present invention is excellent in corrosion resistance in
an environment in which condensed water which contains
nitric acid ions and sulfuric acid ions and is low in pH
is formed and in an environment of an aqueous solution
which contains chloride ions.
Industrial Applicability
The present invention can be applied in
structures obtained by brazing austenitic stainless steel
in all applications requiring corrosion resistance in an
environment where condensed water which contains nitric
acid ions and sulfuric acid ions and which is low in pH
and corrosion resistance in an aqueous solution which
contains chloride ions. Specifically, the austenitic
stainless steel of the present invention is particularly
suitable when used as a material for heat exchanger use,
in particular a material for use for a secondary heat
exchanger of a latent heat type water heater fueled by
kerosene or LNG. In this case, the austenitic stainless
steel of the present invention may be applied to not only
heat exchanger pipes, but also cases, partition plates,
and other materials. Further, the austenitic stainless
steel of the present invention is similarly suitable even
if used as a part for recovery of heat from exhaust gas
such as EGR which is installed in an automobile which has
a gasoline or diesel engine.
In addition, the austenitic stainless steel of
the present invention is particularly suitable when used
in an environment of repeated drying and wetting in which
the steel is exposed to a solution which contains nitric
acid ions and sulfuric acid ions and which is low in pH.
Specifically, this includes outdoor panels, building
materials, roofing materials, outdoor equipment, etc.
which are envisioned as being exposed to an acid rain
environment. Further, the austenitic stainless steel of
the present invention is suitable when used as equipment
which is generally used around water and thereby stress
corrosion cracking is feared, specifically a cold water
or hot water storage tank, household electric appliance,
bath tub, kitchen equipment, and other outdoor and indoor
equipment. In this way, the present invention has a high
value of utilization in industry.