Note: Descriptions are shown in the official language in which they were submitted.
~143434
NSC-B859/PCT
- DESCRIPTION
Steel Having Excellent Corrosion Resistance and
Steel Having Excellent Corrosion Resistance and
Workability
TECHNICAL FIELD
The present invention relates to a steel having
excellent corrosion resistance and a steel having
excellent corrosion resistance and workability. More
particularly, the present invention relates to a steel
having excellent corrosion resistance in exhaust systems
of, for example, internal combustion engines in
automobiles and ships and a steel which is excellent in
corrosion resistance as well as in workability required
for working the steel into components.
PRIOR ART
A steel comprising a common steel plated with
aluminum or zinc for the purpose of avoiding internal or
external corrosion has hitherto been used in an exhaust
system of internal combustion engines including those of
automobiles from the viewpoint of preventing the steel
from being internally or externally corroded. In order
to prevent environmental pollution, however, a catalyst
or the like has been provided for exhaust gas
purification purposes in an exhaust system, rendering the
corrosion resistance of the above plated steel product
unsatisfactory. Japanese Unexamined Patent Publication
(Kokai) Nos. 63-143240, 63-143241, and 2-156048 and the
like disclose steels containing 3 to 12% of Cr for
improving the corrosion resistance of a steel substrate
in an exhaust system. Prolongation of the period of
service and the term of guarantee of vehicles in recent
years has led to extensive use of a high grade stainless
steel containing Cr in an amount up to about 18~ and/or
Mo in an exhaust system. Even such a stainless steel
oftèn undergoes pitting type local corrosion, and, hence,
;
~143434
the corrosion-resistance thereof is not always
satisfactory. Further, since the above stainless steel
contains large amounts of Cr and Mo, they have poor
workability, making it very difficult to produce members
having a complicate shape, for examples, those for an
exhaust system. This complicates the production process,
entailing increased working cost. Furthermore, the above
stainless steel cannot be worked into some shapes and, at
the same time, brings about increased material cost.
A steel incorporating a certain amount of Cr, which
is a representative example of a steel used in the above
exhaust system, is likely to unfavorably undergo local
corrosion when exposed to an aggressive environment. In
order to solve this problem, it is common practice to
increase the Cr or Mo content to improving the corrosion
resistance.
DISCLOSURE OF THE INVENTION
In view of the above problems, the present invention
has been made to provide a steel which has high
resistance to aggressive environments in exhaust systems
of internal combustion engines and the like and is cost
effective, or a steel which has high resistance to
aggressive environments in exhaust systems of internal
combustion engines and the like and, at the same time,
excellent workability and cost effectiveness.
In order to solve the above problems, the present
inventors have studied from various viewpoints steels
having excellent corrosion resistance under aggressive
environments including those of exhaust systems. At the
outset, the present inventors studied aggressive
environments in exhaust systems and, as a result,-faund
that the corrosion of the exhaust system in the internal
combustion engines occurs in an environment in which
chlorides, sulfate ions, and the like contained in an
exhaust gas are heated to 80 to 150C. Further, they
conducted various studies on means to improve the
corrosion resistance of the steel under aggressive
21~3~3~
environments and, as a result, found that, in contrast to
conventional stainless steel, when the Cr content is
reduced to 2.5 to 9.9% and Al is added in an amount of
more than 3.0 to 8.0%, the resultant steel has excellent
corrosion resistance under aggressive environments
including those in exhaust systems.
In order to develop a better steel, the present
inventors made further studies. As a result, they found
that, in the above steel, a reduction in the C and N
contents and, at the same time, the addition of Nb, V,
Ti, Zr, Ta, and Hf in amounts meeting a particular
requirement result in improved corrosion resistance and
improved workability. Further, they found that better
corrosion resistance can be provided by adding to the
above steel at least one member selected from Cu, Mo, Sb,
Ni and W and at least one member selected from REM and Ca
and, further, Si and Mn are proper as a deoxidizing and
strengthening element.
The present invention has been made mainly on the
above finding, and the subject matter of the first
invention resides in a steel having excellent corrosion
resistance, characterized by comprising by weight
Si: not less than 0.01 to less than 1.2%,
Mn: 0.1 to 1.5%,
Cr: 2.5 to 9.9%, and
Al: more than 3.0 to 8.0%, and, other elements with
the following upper limits:
C: not more than 0.02%,
P: not more than 0.03%,
S: not more than 0.01%, and
N: not more than 0.02%,
with the balance consisting of Fe and unavoidable
impurities.
The subject matter of the second invention resides
in a steel comprising the same ingredients as those
constituting the steel of the first invention and as an
~113~3~
.
additional ingredient at least one member selected from,
by weight,
Cu: 0.05 to 3.0%,
Mo: 0.05 to 2.0%,
Sb: 0.01 to 0.5%,
Ni: 0.01 to 2.0%, and
W: 0.05 to 3.0%.
The subject matter of the third invention resides in
a steel comprising the same ingredients as those
constituting the steel of the first or second invention
and as an additional ingredient at least one member
selected from, by weight,
rare earth element: 0.001 to 0.1%, and
Ca: 0.0005 to 0.03%.
The subject matter of the fourth invention resides
in a steel having excellent corrosion resistance and
workability, characterized by comprising by weight
Si: not less than 0.01 to less than 1.2%,
Mn: 0.1 to 1.5%,
Cr: 2.5 to 9.9%, and
Al: more than 3.0 to 8.0%, and, other elements with
the following upper limits:
C: not more than 0.02%,
P: not more than 0.03%,
S: not more than 0.01%, and
N: not more than 0.02%, and
0.01 to 0.5% in total of at least one element
selected from Nb, V, Ti, Zr, Ta, and Hf, provided that a
requirement represented by the following formula is met:
Nb V Ti Z r Ta H~ C N
-- + -- +-- + -- + -- + -- - 0.8 X [--+ --] 2
93 51 48 91 181 179 12 14
o
with the balance consisting of Fe and unavoidable
impurities.
The subject matter of the fifth invention resides in
a steel comprising the same ingredients as those
constituting the steel of the fourth invention and as an
~lq3434
additional ingredient at least one member selected from,
by weight,
Cu: 0.05 to 3.0%,
Mo: 0.05 to 2.0%,
Sb: 0.01 to 0.5%,
Ni: 0.01 to 2.0%, and
W: 0.05 to 3.0%.
The subject matter of the sixth invention resides in
a steel comprising the same ingredients as those
constituting the steel of the fourth or fifth invention
and as an additional ingredient at least one member
selected from, by weight,
rare earth element: 0.001 to 0.1%, and
Ca: 0.0005 to 0.03~.
Best Mode for Carrying Out the Invention
The present invention provides a steel member having
sufficiently high corrosion resistance to cope with use
under a harsh corrosive environment recently found in the
above automobile exhaust gas system and a steel member
having excellent corrosion resistance and workability.
The reason for the limitation of chemical ingredients,
which are technical features of the present invention,
will now be described in detail.
si
Si, when added to a steel having a Cr content of not
less than 2.5%, effectively serves as a deoxidizer and a
strengthening element. However, when the Si content is
less than 0.01%, the deoxidization effect is
unsatisfactory. On the other hand, when it is not less
than 1.2%, the effect is saturated and, at the same time,
the workability is deteriorated. For this reason, the Si
content is limited to not less than 0.01 to less than
1.2%.
Mn:
Mn is necessary as a deoxidizer for steèl and should
be contained in an amount of not less than 0.1%.
However, when the Mn content exceeds 2.0%, the effect is
2143434
saturated and; at the same time, the presence of
excessive Mn deteriorates the workability of the steel.
For this reason, the upper limit of the Mn content is
1.5%.
Cr:
Cr should be incorporated in an amount of not less
than 2.5% for ensuring the corrosion resistance of the
steel. The incorporation of Cr in an amount exceeding
9.9% uselessly incurs an increase in cost and, at the
same time, deteriorates the workability of the steel.
For this reason, the upper content of Cr is 9.9%.
Al:
Al, as with Cr, is an element important to the
present invention from the viewpoint of ensuring the
corrosion resistance. As described above, when the Al
content is not more than 3.0%, the effect of preventing
the pitting corrosion is unsatisfactory. On the other
hand, when the amount of Al added exceeds 8.0%, the above
effect is saturated and, at the same time, the
workability of the steel is deteriorated. For this
reason, the Al content is limited to more than 3.0 to not
more than 8.0%.
C, N:
C and N deteriorate the workability of the steel
sheet. Further, C combines with Cr to form a carbide
which deteriorates the corrosion resistance of the steel.
Further, N deteriorates the toughness of the steel. For
this reason, the lower the C and N contents, the better
the results, and the upper limits of the C and N contents
are both 0.02%.
P:
P, when present in a large amount, deteriorates the
toughness. Therefore, the lower the P content, the
better the results, and the upper limit of the P content
is 0.03%.
S :
~143434
S too deteriorates the pitting corrosion resistance
when it is present in a large amount. Therefore, the
lower the S content, the better the results, and the
upper limit of the S content is 0.01%.
Nb, V, Ti, zr, Ta, Hf, Nb, V, Ti, Zr, Ta, and Hf
serve to fix, as a carbide, C and N contained in a high
Cr steel, thereby significantly improving the corrosion
resistance and the workability. They may be added alone
or in combination. However, for the addition of these
elements alone or in combination, no effect can be
attained when the total amount of the elements added is
less than 0.01%. When the total amount exceeds 0.5%, the
cost is uselessly increased and, at the same time, a flaw
or the like is likely to occur during rolling. For this
reason, the upper limit of these element is 0.5%.
Further, in order to effectively improve the workability,
the total amount of the Nb, V, Ti, Zr, Ta, and Hf added
should satisfy a requirement represented by the following
formula:
Nb V Ti Zr Ta Hf C N
- + - + - + - + + - - 0.8 x [ - + - ] 2
93 51 48 91 181 179 12 14
o
The above elements are fundamental ingredients of
the steel having excellent corrosion resistance or the
steel having excellent corrosion resistance and
workability contemplated in the present invention.
Further, steels with the following elements being
optionally added for the purpose of further improving the
properties are also contemplated in the present
invention.
Cu:
Cu, when added in an amount of not less than 0.05%
to a steel having a Cr content of not less than 2.5% and
an Pl content exceeding 3.0%, has the effect of improving
the resistance to general corrosion. However, when the
Cu content exceeds 3.0%, the contemplated effect is
saturated and, at the same time, the hot workability of
;
214343~
the steel is deteriorated. For this reason, the upper
content of Cu is 3.0%.
Mo:
Mo, when added in an amount of not less than 0.05%
to a steel having a Cr content of not less than 2.5% and
an Al content exceeding 3.0%, has the effect of
inhibiting the occurrence and growth of pitting.
However, when the Mo content exceeds 1.5%, the
contemplated effect is saturated and, at the same time,
the workability of the steel is deteriorated. For this
reason, the upper content of Mo is 1.5%.
Sb:
Sb, when added in an amount of not less than 0.01%
to a steel having a Cr content of not less than 2.5% and
an Al content exceeding 3.0%, has the effect of improving
the resistance to pitting corrosion and general
corrosion. However, when the Sb content exceeds 0.5%,
the hot workability of the steel is deteriorated. For
this reason, the upper content of Sb is 0.5%.
Ni:
Ni, when added in an amount of not less than 0.01%
to a steel having a Cr content of not less than 2.5% and
an Al content exceeding 3.0%, has the effect of
preventing the pitting corrosion. However, when the Ni
content exceeds 2.0%, the contemplated effect is
saturated and, at the same time, the hot workability of
the steel is deteriorated. For this reason, the upper
content of Ni is 2.0%.
W:
W, when added, in an amount of not less than 0.05%,
in combination with ~ther additive elements, to a steel
having a Cr content of not less than 2.5% and an Al
content exceeding 3.0%, has the effect of significantly
inhibiting the occurrence and growth of pitting.
However, when the--W content exceeds 3.0%, the
contemplated effect is saturated and, at the same time,
2143434
the workability of the steel is deteriorated. For this
reason, the upper content of W is 3.0%.
Rare earth elements (REM), Ca:
Rare earth elements and Ca are elements having the
effect of improving the hot workability and the pitting
corrosion resistance. No satisfactory effect can be
attained when the content is less than 0.001% for the
rare earth element and less than 0.0005% for Ca. On the
other hand, when the content exceeds 0.1% for the rare
earth element and 0.03% for Ca, coarse nonmetallic
inclusions are formed to unfavorably deteriorate the hot
workability and the pitting corrosion resistance. For
this reason, the upper limit of the content is 0.1% for
the rare earth element and 0.03% for Ca. In the present
invention, the term ~rare earth element" is intended to
mean elements with atomic numbers 57 to 71 (lanthanoids),
atomic numbers 89 to 103 (actinoids), and atomic number
39 (Y).
The steel of the present invention, when used in an
exhaust system of internal combustion engines, may be
first produced in a steel sheet form, formed by means of
a press or the like into a predetermined shape, and
further worked and welded to provide a product.
Alternatively, the steel sheet may be first formed into a
steel pipe, for example, a seam welded steel pipe, and
then fabricated, welded, or subjected to other steps to
provide a product. All the steels having a composition
and a combination of elements, including the process,
specified in the present invention, are contemplated in
the present invention. Further, it is also possible to
select the optimal production process by taking into
consideration the cost, the production equipment
restrictions, and the like, but if another production
process is selected, the selected process should not
deviate from the scope of the claims of the present
invention. Furthermore, the steel of the present
invention can be applied to, in addition to an exhaust
2143434
- 10
system of internal combustion engines, various other
corrosive environments, such as an environment wherein an
aqueous solution containing chlorides, sulfate ions, or
the like is exposed to high temperatures or an
environment wherein heating and cooling are repeated.
The present invention will now be described in more
detail with reference to the following example and
comparative examples.
~ MPLE
Steels comprising ingredients specified in Tables 1
to 9 were prepared by the melt process and subjected to
conventional steel sheet production steps, such as hot
rolling and cold rolling, to provide 1 mm-thick steel
sheets which were then annealed at 850C. Test pieces
having a width of 50 mm and a length of 70 mm were
prepared from these steel sheets and applied to a
corrosion test. The corrosion test was carried out by
immersing a test piece to half the height thereof in an
aqueous solution (50 cm3) containing 100 ppm of a sulfate
ion, 100 ppm of a chloride ion, and 500 ppm of a
bicarbonate ion in the form of an ammonium salt, holding
the whole testing container in an atmosphere of 130C to
completely evaporate and volatilize the testing solution,
and repeating the above procedure 20 times. This test is
a simulation of corrosive conditions in an automobile
exhaust system.
In the results of the corrosion test given in Tables
1, 2, 3, 5, 7, and 9, ~3 represents that the maximum
corrosion depth was not more than o.lo mm, O represents
that the maximum corrosion depth was not more than 0.15
mm, and X represents that the maximum corrosion depth
exceeded 0.15 mm.
The workability was evaluated based on whether or
not cracking occurred in a cup reduction test with a
reduction ratio of 1.8.
The test results are also shown in Tables 5, 7, and
9. In the test results for workability given in these
214343~
tables, O repEesents that the results of the cup
reduction test were good, and x represents that cracking
occurred in the cup reduction test.
As is apparent from Tables 1, 2, 3, 5, 7, and 9,
steel Nos. 1 to 36 of the present invention listed in
Tables 1 and 2 and steel Nos. 50 to 86 of the present
invention listed in Tables 4, 5, 6, and 7 had good
corrosion resistance even in an exhaust gas environment
which was a very harsh corrosive environment. Further,
steel Nos. 50 to 86 of the present invention listed in
Tables 4, 5, 6, and 7 were excellent also in workability.
By contrast, steel Nos. 37 to 49 as comparative steels
listed in Table 3 had poor corrosion resistance, and
steel Nos. 87 to 98 as comparative steels listed in
Tables 8 and 9 were poor in corrosion resistance as well
as in workability.
Table 1
Ingredients, wt.% Max.
corrosion
No. C Si Mn P S Al Cr W Cu MO Sb Ni Ca REM N depth
1 0.011 0.12 0.21 0.015 0.005 4.15 2.6 0.013
2 0.008 1.02 0.41 0.017 0.007 4.08 2.6 2.99 2.28 1.7 0.29 1.l 0.019 0.017
3 0.003 0.60 0.31 0.015 0.010 3.16 2.7 1.59 0.007 O
4 0.014 1.14 0.23 0.010 0.008 7.40 3.0 0.9 0.015
0.008 0.39 1.41 0.010 0.008 6.37 3.0 1.31 1.6 0.008
6 0.005 1.18 1.26 0.019 0.006 3.48 3.3 1.0 0.006 O
7 0.009 0.98 0.81 0.023 0.006 3.53 3.4 0.42 2.05 0.1 0.026 0.015
Steel 8 0.008 0.65 1.46 0.014 0.006 4.07 3.5 0.37 0.28 0.058 0.009 O
of 9 0.005 0.31 0.81 0.010 0.005 6.11 3.6 0.5 0.009 0.008
inven-10 0.020 0.67 1.49 0.019 0.004 3.17 3.6 0.80 2.01 0.7 0.18 0.002 0.016
tion 11 0.005 0.93 1.47 0.026 0.010 5.99 3.8 1.48 1.10 0.010 0.011
12 0.014 0.52 1.13 0.020 0.004 3.51 4.0 1.35 0.009 ~ r~
13 0.003 0.67 1.01 0.023 0.007 3.06 4.1 2.88 2.30 1.1 0.017 ~ ~,
14 0.012 0.42 0.87 0.025 0.008 5.65 4.2 0.17 0.014 ~ ~C-
0.018 0.63 1.19 0.024 0.003 4.06 4.4 0.55 0.9 0.006 0.004 ~ C~
16 0.013 0.27 0.30 0.018 0.009 4.33 4.6 0.47 0.048 0.016
~i 17 0.014 0.62 0.34 0.028 0.004 3.914.7 2.66 1.70 1.4 0.03 0.4 0.027 0.041 0.003
18 0.006 0.28 0.57 0.018 0.009 3.824.9 0.6 1.5 o.oog
Table 2
Ingredients, wt.% Max
No. C Si Mn P S Al Cr W Cu Mo Sb Ni Ca REM N depth
19 0.007 0.92 1.14 0.016 0.008 6.39 5.3 0.019 0.058 0.011
20 0.006 0.44 0.64 0.022 0.008 6.09 5.5 0.83 2.0 0.07 O.g 0.012
21 0.010 0.72 0.33 0.010 0.007 4.27 5.7 1.3 0.08 0.018
22 0.017 0.14 0.31 0.020 0.006 5.58 5.9 0.013
23 0.016 1.02 0.21 0.027 0.004 3.62 6.4 1.35 1.5 0.44 0.013
24 0.008 0.63 1.19 0.025 0.003 4.09 6.8 2.84 1.5 0.077 0.0150.014 0.28 1.21 0.028 0.004 7.72 7.3 1.18 0.7 0.35 0.076 0.016
Steel 26 0.013 0.57 0.65 0.015 0.008 6.94 7.4 0.15 0.003 0.008
Of 27 0.011 0.99 0.62 0.018 0.008 4.12 7.6 2.05 0.5 0.6 0.017
inven- 28 0.012 0.51 1.13 0.013 0.007 5.50 7.7 0.06 1.0 0.016
tion 29 0.003 0.88 1.49 0.022 0.010 5.39 7.8 0.012 O
0.016 0.56 1.06 0.020 0.009 7.08 8.4 0.014
31 0.018 0.27 1.12 0.012 0.009 4.77 8.5 0.99 0.014 0.014 ~ ~a~
32 0.018 0.81 0.90 0.024 0.009 7.82 8.6 1.9 0.028 0.006
33 0.009 0.50 0.38 0.022 0.005 4.18 9.0 1.48 0.92 0.4 0.10 1.2 0.092 0.011 ~ C~
34 0.012 0.13 0.97 0.016 0.008 4.62 9.4 1.3 0.098 0.010
`i 35 0.006 0.15 0.92 0.021 0.009 4.70 9.4 0.54 0.013 0.013
36 0.013 0.35 1.23 0.020 0.004 5.36 9.8 2.55 1.77 0.19 0.007
Table 3
Ingredients, wt.% Max
No. C Si Mn P S AlCr W Cu Mo Sb Ni Ca REM N depth
37 0.009 0.49 0.64 0.028 0.005 0.04 7.5 0.011 X
38 0.005 0.45 1.09 0.026 0.003 0.25 7.3 0.08 0.003 X
39 0.016 0.49 0.73 0.028 0.004 0.21 7.5 0.15 2.5 0.008 X
40 0.016 0.13 0.50 0.035 0.005 0.22 9.3 0.013 X
Compar- 41 0.020 0.15 0.41 0.032 0.007 0.29 5.2 0.032 0.008 X
ative 42 0.014 0.14 1.21 0.031 0.005 0.03 9.8 0.003 0.011 X
steel 43 0.010 0.18 0.68 0.038 0.004 0.29 9.3 2.1 0.005 X
44 0.045 0.21 1.18 0.037 0.006 0.04 8.6 0.32 0.006 X
45 0.013 0.29 1.27 0.032 0.005 0.22 9.2 1.10 0.007 X
46 0.013 0.18 0.69 0.039 0.003 0.03 9.8 1.4 0.006 X
47 0.020 0.45 0.70 0.037 0.006 0.2111.5 1.40 0.006 X
48 0.017 0.47 0.87 0.045 0.003 0.3010.5 0.80 1.20 1.1 0.010 0.010 X
49 0.010 0.51 1.04 0.037 0.005 0.0511.8 1.10 0.41 1.5 0.05 0.4 0.011 X
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Table 5(Continuation of Table 4)
Ingredients, wt.%X value Work- Max.
x 10000 abily corrosion
NO. Hf CU Mo Sb Ni CaREM N depth
1.78 0.0580.0075.97
510.22 0.47 0.0000.014 9.61 O
52 1.20 1.60 0.015 6.55 O O
53 0.008 0.0730.005 6.86 O O
54 0.13 0.0350.013 7.73 O O
1.21 0.016 12.75 O O
56 0.0100.0145.24 O
Steel 570.09 1.05 1.06 0.017 26.73 0
of 580.07 1.84 0.011 0.015 1.80
inven- 59 2.97 1.35 0.21 0.016 0.013 15.12
tion 60 1.44 0.20 0.391.630.010 0.009 20.81 O
61 0.14 1.25 0.020 4.90 O O
620.17 0.49 0.44 0.010 13.71 0 0
63 1.22 0.0650.01918.55 0 ~ ~
64 1.24 0.00519.23 O ~ W
0.0036.85 0 0 C~
660.02 2.19 0.24 0.011 0.004 8.94 O O
670.07 1.75 0.023 0.020 29.62
680.05 0.50 0.09 0.22 0.081 0.016 10.74 O O
Note: X value in the table was calculated by the following formula:
Nb V Ti Zr Ta Hf C N
- + - + - + - + + - 0.8 X [ - + - ] 2 0
93 51 48 91 181179 12 14
214343~
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[Industrial Applicability]
As is apparent from the above example, the present
invention provides a steel having excellent corrosion
resistance in an exhaust system of an internal combustion
S engine in automobiles and the like or a steel having
excellent corrosion resistance and workability at low
cost and, hence, can greatly contribute to the
development of industries.
