Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Reexamination Certificate
2001-05-31
2002-07-16
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
C420S036000, C420S069000, C420S109000
Reexamination Certificate
active
06419878
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns a Fe—Cr alloy having excellent initial rust resistance, workability and weldability and, more particularly, a Fe—Cr alloy suitable for use in civil engineering and building structural materials requiring initial rust resistance, bending workability and toughness for weld zone.
DESCRIPTION OF THE RELATED ART
As civil engineering and building structural materials, carbon steels such as SS 400 (JIS G 3101,JIS is Japanese Industrial Standard,here in after JIS) and high tensile steels such as SM 490(JIS G 3106) and such steel materials applied with painting or plating have mainly been used.
However, as designs for the materials have been varied, use of various kinds of materials have been studied in recent years.
Among them, since Fe—Cr alloys which are excellent in corrosion resistance and aesthetic appearance scarcely require maintenance cost for rusting, they can be said to be highly attractive materials in view of life cycle cost (LCC).
Particularly, buildings constructed in coastal districts involve problems of short life and increased maintenance cost for suppressing corrosion. Further, also in view of the propagation for the water front development, the Fe—Cr alloys have been greatly expected as corrosion resistant functional materials for use in civil engineering and building structures excellent in corrosion resistance, weldability and, particularly, initial rusting resistance.
Fe—Cr alloys are generally classified in view of the metal structures into ferritic stainless steels represented by SUS 430 steels (JIS G 4304), martensitic stainless steels represented by SUS 410 steels (JIS G 4304) austenitic stainless steels represented by SUS 304(JIS G 4304), 2-phase stainless steels represented by SUS 329 steels (JIS G 4304) and precipitation hardened steels represented by SUS 630 (JIS G 4304).
Among various kinds of Fe—Cr alloys described above, austenitic stainless steels which have been actually used most frequently having material strength, corrosion resistance, easy weldability, toughness of weld zone and general applicability have been studied, particularly, so far as the materials for use in civil engineering and building structures.
Such austenitic stainless steels have characteristics fully satisfying the characteristics required for civil engineering and building materials such as strength, corrosion resistance, fire resistance and toughness of weld zone.
However, such austenitic stainless steels
(1) contain a great amount of alloying elements such as Ni and Cr and, accordingly, are very expensive compared with carbon steels,
(2) cause stress corrosion cracking, and
(3) show greater heat expansion coefficient and relatively low heat conductivity compared with carbon steels, so that heat-affected strains upon welding tend to be accumulated and they are difficult to be used to materials requiring high accuracy. In view of the above, they involve a problem that it is difficult to apply them to the use of a general purpose structural materials in which carbon steels or carbon steels applied with painting or plating are used and their application range is restricted.
In view of the above, low Cr content alloy steels with the Cr content of 15 mass % or less have been studied recently for the application use to civil engineering and building materials as substitutes for plated or painted carbon steels. Application of the martensitic stainless steels in the field of the civil engineering and building materials is an example.
Since the Fe—Cr alloys with the Cr content of 15 mass % or less have less Cr content and, further, less Ni content compared with Ni-containing Fe—Cr—Ni alloys as described above, they have a feature of being outstandingly inexpensive and having low heat expansion coefficient and high heat conductivity, as well as excellent in corrosion resistance and high yield strength compared with carbon steels.
Further, the martensitic stainless steels are also advantageous in that they are free from the worry of &sgr; embrittlement and 475° C. embrittlement that are the problem in high Cr alloys containing 15 mass % or more of Cr and, further, free from the worry of stress corrosion cracking in chloride containing circumstances that gives a problem in austenitic stainless steels.
However, since the martensitic stainless steels represented by SUS 410 steels have C content as high as about 0.1 mass %, they are poor in the toughness of weld zone and the workability of the weld zone and require pre-heating upon welding to deteriorate the welding operationability, they still leave a problem in the application use to those materials requiring welding.
As a countermeasure for the problems described above, Japanese Patent Publication No. 13463/1976 for example, proposes a martensitic stainless steel for use in welding structures, containing 10 to 18 mass % of Cr, 0.1 to 3.4 mass % of Ni, 1.0 mass % or less of Si and 4.0 mass % or less of Mn in which C is reduced to 0.030 mass % or less and N is reduced to 0.020 mass % or less and massive martensitic structure is formed in the heat-affect zone, thereby improving the performance of the weld zone.
Further, Japanese Patent Publication No. 28738/1982 proposes a martensitic stainless steel excellent in the toughness of weld zone and workability, requiring neither pre-heating nor post heating before and after the welding by incorporating 10 to 13.5 mass % of Cr, 0.5 mass % or less of Si and 1.0 to 3.5 mass % of Mn, reducing C to 0.020 mass % or less and N to 0.020 mass % or less and, further, strictly restricting Ni to less than 0.1 mass %.
However, the techniques disclosed in Japanese Patent Publication Nos. 13463/1976 and 28738/1982 involve a problem that no countermeasure is taken for the problem inherent to the civil engineering and building structural materials as shown below.
When considering the application use to the civil engineering and building structures, those members such as pillars or beams are not exposed to severe circumstances after the completion of structures as outer wall materials. However, they are sometimes left in the outdoor in a short period of time of about several months after worked into structural members such as steel pipes or steel shapes with various sections in factories and shipping therefrom till the completion of the constructing operation for the structures. Accordingly, it is important to improve the initial rust resistance of the steel materials for suppressing occurrence of initial rust caused during construction period after shipping in view of the appearance, as well as in view of the durability of the structures after completion.
Further, when they are used as the civil engineering and building structural materials, since the requirement for the surface property is not so strict, it is desirable with an economical point of view that they can be used as hot rolled or hot rolled and annealed in a state where scales are not removed from the surface of steel plates.
Further, considering fabrication, for example, to steel shapes having various sections, there are great demand for the improvement of the toughness of the steel plate, particularly, elongation and bending workability in base steel plates or weld zones.
In view of the problems described above, Japanese Patent Laid-Open No. 302796/1999 proposes a hot rolled stainless steel sheet for use in building structures of excellent corrosion resistance, as well as a manufacturing method thereof, the steel having compositional ingredients comprising:
C: 0.005 to 0.1 mass %,
Si: 0.05 to 1.5 mass %,
Mn: 0.05 to 1.5 mass %,
P: 0.04 mass % or less,
S: 0.05 mass % or less,
Cr: 10 to 15 mass % and
N: 0.055 mass % or less, reducing (C+N) to 0.1 mass % or less and containing one or two of Ni and Cu within a range from 0.1 mass % or more and less than 1.0 mass %, with the balance of Fe and inevitable impurities.
Further, Japanese Patent Laid-Open No. 302797/1999 proposes a hot rolled stainless steel sheet for use in building structures, of excellent corrosion resistance, as well as a manufacturing m
Ota Hiroki
Satoh Susumu
Ujiro Takumi
Yazawa Yoshihiro
Kawasaki Steel Corporation
Yee Deborah
Young & Thompson
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