Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Utility Patent
1999-11-01
2001-01-02
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
C420S104000
Utility Patent
active
06168756
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a highly corrosion resistant chromium-containing steel having corrosion resistance and oxidation resistance comparable or superior to those of low chromium (Cr) stainless steels and further having excellent intergranular corrosion resistance not offered by existing chromium containing steels, and despite having such a low Cr content as not to be included in the category of stainless steels.
2. Description of the Related Art
Typical corrosion resistant stainless steels contain 11% by weight or more of chromium. However, since Cr is expensive, there is a need for a steel composition that maintains its desirable characteristics even when the amount of chromium is decreased. Techniques for adding Si to compensate for a decrease in corrosion resistance of steels with lower chromium have been proposed. For example, Japanese Unexamined Patent Publication No. 58-224148 proposed chromium steels as components of automobile exhaust systems having a Cr content of from more than 5.0% by weight to less than 10.0% by weight with addition of more than 1.5% to less than 3.0% by weight of Si and 0.3% by weight or less of Ti. Furthermore, Japanese Unexamined Patent Publication No. 5-279791 proposed steels for internal combustion engine exhaust systems of excellent wet corrosion resistance in which from 0.01% to less than 1.2% by weight of Si is added to steels with Cr content of 5.5% to 9.9% by weight.
However, those chromium steels had no effective countermeasure against sensitization that is caused when Cr forms compounds with C or N and forms Cr depletion layers at the periphery adjacent those compounds. For example, if the steels are used as exhaust system components in automobiles, they cannot prevent sensitization caused by welding at the time of manufacture or by heating at the high exhaust gas temperatures encountered in use. As a result, Cr is deposited as carbides or nitrides, which causes intergranular corrosion at regions with lowered Cr concentration, thereby accelerating corrosion and even causing breakage of the eroded portion in a worst case.
OBJECT OF THE INVENTION
In view of the above, it is an object of this invention to overcome the foregoing problems and provide a highly corrosion resistant chromium-containing steel having corrosion resistance and oxidation resistance comparable or superior to those of low Cr stainless steels (Cr content: 11-13 wt %) and further having excellent intergranular corrosion resistance not offered by existing chromium containing steels, despite having such a low Cr content as not even to be classified as a stainless steel, through appropriate development of the steel composition.
SUMMARY OF THE INVENTION
For obtaining the foregoing object, the present inventors have made an intense study for the effect of various additive elements on the corrosion resistance, the oxidation resistance and the intergranular corrosion resistance of chromium-containing steels. As a result, it has been found that a steel having corrosion resistance, oxidation resistance and intergranular corrosion resistance comparable to or superior to those of low Cr-stainless steels can be achieved by adding Si and Ti and, optionally, Mo and, further, setting the Ti/(C+N) ratio to a predetermined minimum value in accordance with the Cr content.
The present inventors have found that it is effective to add Si, preferably in excess of a predetermined amount, and to add elements selected from Mo, Cu, Co, Ca, Nb and B for improving the corrosion resistance and the oxidation resistance. It has also been found that the intergranular corrosion resistance can be improved by first improving the corrosion resistance of the matrix by setting the addition amount of, for example, Si and Mo to appropriate values and, further, adding a sufficient amount of Ti while keeping the ratio of the Ti content [Ti] to the sum of the C content [C] and the N content [N]: [Ti]/([C]+[N]) in excess of a predetermined value in accordance with the Cr content [Cr].
The present inventors have conducted a intergranular corrosion test for chromium-containing steel sheets which are based on Fe—9 wt % Cr—1.2 wt % Si and in which the contents of Ti, C and N are varied, and have investigated the relationship of the Cr content [Cr] to the ratio of the Ti content [Ti] to the sum of the C content [C] and the N content [N]: [Ti]/([C]+[N]) in the Cr-containing steels. The results are shown in
FIG. 1
In the intergranular corrosion test, a test piece prepared by butt welding two sheets of test specimens by GTA (Gas Tungsten Arc) welding was immersed in a boiling solution of sulfuric acid+copper sulfate for 16 hours, and subjected to a bending test. The presence or absence of intergranular corrosion cracks was confirmed by observing the outer surface of the bend with a magnifying glass. In
FIG. 1
, ◯ represents the absence of intergranular corrosion cracks while &Circlesolid; represents the presence of intergranular corrosion cracks.
As shown in
FIG. 1
, it has been found that excellent intergranular corrosion resistance can be obtained for 9 wt % Cr—1.2 wt % Si steels by setting the Ti addition amount to more than 0.30% by weight and the value for the relation: [Ti]/([C]+[N]) to 28 or greater. As a result of further study, it was found that a higher value for [Ti]/([C]+[N]) is needed as [Cr] is decreased, in order to obtain a steel having excellent intergranular corrosion resistance. Specifically, a value of (64−4×[Cr]) or greater is necessary for the [Ti]/([C]+[N]) ratio at a Si content of more than 1.0% by weight in accordance with the Cr content [Cr], to best accomplish the objects of the present invention.
A steel composition according to the invention preferably has the following composition, on a weight % basis:
C: about 0.015% or less
Si: from more than about 1.0% to about 2.0%
Mn: about 0.5% or less
P: about 0.05% or less
S: about 0.01% or less
Ni: about 1.0% or less
Cr: from about 5.0% to about 10.4%
Al: about 0.1% or less
N: about 0.015% or less
sum of the C content and the N content (C+N): about 0.020% or less
Ti: from more than about 0.30% to about 0.50%
wherein the contents for Cr, Ti, C and N, that is, [Cr], [Ti], [C] and [N] satisfy the following relation:
[Ti]/([C]+[N])≧64−4×[Cr], and
the balance Fe and incidental impurities.
A preferred embodiment of this invention provides a highly corrosion resistant chromium-containing steel comprising, on a weight % basis, in addition to the ingredients described above, at least one element selected from:
Mo: from about 0.02% to about 2.0%,
Cu: from about 0.02% to about 2.0%, and
Co: from about 0.02% to about 2.0%.
Another preferred embodiment of this invention provides a highly corrosion resistant chromium-containing steel comprising, on a weight % basis, in addition to the ingredients described above at least one element selected from:
Ca: from about 0.0005% to about 0.0030%,
Nb: from about 0.001% to about 0.030%, and
B: from about 0.0002% to about 0.0050%.
REFERENCES:
patent: 3770394 (1973-11-01), Bressanelli
patent: 5049210 (1991-09-01), Miyasaka et al.
patent: 0 145 471 (1985-06-01), None
patent: 55-54550 (1980-04-01), None
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patent: 58-224148 (1983-12-01), None
patent: 60-13057 (1985-01-01), None
patent: 6-248394 (1994-09-01), None
patent: 8-035010 (1996-02-01), None
Hirasawa Junichiro
Ishii Kazuhide
Ishizuka Haruhiko
Miyazaki Atsushi
Satoh Susumu
Kawasaki Steel Corporation
Yee Deborah
Young & Thompson
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