Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Reexamination Certificate
2002-09-09
2004-03-02
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
C148S546000
Reexamination Certificate
active
06699338
ABSTRACT:
TECHNICAL FIELD
This invention concerns weathering resistant steel materials and, it relates to a flow rust reducing weathering resistant steel materials capable of effectively reducing occurrence of flow rust in relatively less salty circumstances such as mountain districts, rural districts and industrial districts, as well as steel materials excellent in earthquake proofness and coast weathering resistance applicable as steel structures such as bridges used in salty circumstances such as coast districts. The weathering resistance referred to in the invention means weathering resistance in a case of use in atmospheric air of coast districts.
BACKGROUND ART
1) Less Salty Circumstance
Weathering resistant steels with improved weathering resistance in atmospheric air with addition of alloying elements such as P, Cu, Cr and Ni in the steels have been used generally for structures such as bridges. The weathering resistant steels form, in several years, rust referred to as stable rust less permeating oxygen and water causing corrosion and suppress subsequent corrosion. Accordingly, the weathering resistant steels require no coating of anti-rust paints and they are highly corrosion resistant material which can be used in a so-called naked state.
However, since as long as several years are required till the stable rust is formed during which flow rust occurs in the weathering resistant steels, they involve problems of deteriorating scenes and causing environmental contamination.
In view of the problems described above, Japanese Patent Laid-Open No. 136557/1994, for example, proposes a surface treating method for steel materials of coating an aqueous solution of chromium sulfate or an aqueous solution of copper sulfate and further applying organic resin coating after drying the water content. Further, Japanese Patent Laid-Open No. 13158/1996 proposes a surface treating method of steel materials of coating an aqueous solution containing aluminum ions and further forming an organic resin film after drying of the water content.
However, in the techniques described in Japanese Patent Laid-Open No. 136557/1994 and Japanese Patent Laid-Open No. 13158/1996, while stable rust is grown in a short period of time, they still leave problems such that steps are complicated and the surface treating agents used are expensive, and development of weathering resistant steel materials not requiring surface treatment have been demanded.
In view of the above, for coping with such a demand, the invention intends to provide flow rust reducing weathering resistant steel materials capable of reducing occurrence of flow rust in the course of forming stable rust in weathering resistant steels used in a naked state.
2) Salty Circumstance such as Coast Districts
Steel structures such as bridge girders are generally applied with corrosion preventive means such as coating since their service life is long. However, the coating films are degraded to gradually reduce the corrosion preventive effect by chalking due to UV-rays, or expansion of rust by the corrosion under coating films. Accordingly, re-coating has been obliged on every certain periods. However, shortening of coating operators and increase in the personal expense in recent years makes the re-coating operation difficult. In view of the situations, weathering resistant steels requiring no coating of anti-rust paints and usable in a naked state have been applied more and more in steel structures.
The weathering resistant steels are those steel materials with addition of P, Cu, Ni and Cr in which stable rusts as protective films are formed in several years on the surface of steels in an atmospheric circumstance. Since the stable rust suppresses further development of corrosion, corrosion of the steel materials can be minimized. Accordingly, most of them are used with no coating.
However, in salty circumstances such as coast districts, no stable salt is formed after lapse of several years even in weathering resistant steels and steel materials are attacked violently.
In recent years, application guideline for weathering resistant steels have been issued from Minister of Construction (Joint Research Report Regarding Application of Weather Resistant Steel Material to Bridges (XX), March 1993, published from Civil Engineering Institute of Minister of Construction, KOZAI CLUB Co. and Nippon Kyoryo Kensetsu Kyokai), in which it is specified that existent weathering resistant steels (JIS G 3114: weathering resistant hot rolled steel materials for welding structure) can not be used with no coating in the district where atmospheric salt content is 0.05 mg/dm
2
/day or more, that is, in coast districts.
Accordingly, in salty circumstances such as coast districts, countermeasure has been adopted by applying coating such as of phthalic acid resin, chlorinated rubber or tar epoxy resin to ordinary steel materials. However, since bridges constructed in coast districts near the estuaries are often long and large and the corrosion is violent because of the use in the coast districts, the re-coating operation is extremely difficult and, accordingly, there is a strong demand for the steel materials that can be used with no coating.
Regarding this Japanese Patent Laid-Open No. 136557/1994, for example, leaves problems as described above.
Further, Japanese Patent No. 2572447, Japanese Patent Laid-Open NO. 51668/1993 and Japanese Patent Laid-Open No. 134587/1996 propose methods of improving the coast weathering resistance by adding a great amount of alloying elements such as P, Cu, Ni and Mo to steel materials.
However, referring to the bridge, the corrosive circumstance for steel materials are not always identical depending on the places to be used. Considering, for example, four main beam bridge, while outside of the beams are exposed to rainfall, water of condensation and sunshine, inside of the beam are exposed only to water of condensation but not suffer from rainfall. Generally, in a clean circumstance with no atmospheric salt content, it is said that the extent of corrosion is less in the inside of the beams when compared between the inside and the outside of the beams. On the other hand, in the circumstance with high atmospheric salt content, it is said that the extent of corrosion is rather greater in the inside of the beam than the outside of the beam. This reversal phenomenon occurs at a certain content of the atmospheric salt content as a boundary but the content can not be specified.
However, since outer beams, main beams and webs are exposed to two circumstances (with or without exposure to rainfalls) simultaneously (rear face and surface of plates), it is necessary for the steel materials to be used in steel structures such as bridges to maintain high weathering resistance in both of the circumstances.
However, in the existent techniques, evaluation was applied only under one circumstance (with rainfall or without rainfall), and development for steel materials having excellent coast weathering resistance simultaneously under two circumstances has been demanded.
3) Earthquake Proofness
On the other hand, the structural steel materials of this type utilized, for example, in bridge beams, have been (demanded to have an absorbed energy of 47J or more at −5° C. in a Charpy impact test in the rolling direction (L direction) and a cross direction (C direction) to the rolling direction of the steel materials in view of the safety. However, it has been found that high stresses may possibly exert in the direction of the plate thickness of the material to be used (Z direction) depending on the structure and the portions of the structures in large scale earthquakes such as Hanshin-Awaji disaster, so that it has been demanded for the steel materials for use in structures to improve the toughness in the direction of the plate thickness (Z direction) including the weld heat affect zone in order to further increase the earthquake proofness of steel materials after the Hanshin-Awaji disaster.
From the view points (1)-(3) above, the invention intends to provide a steel materi
Amano Keniti
Kawabata Fumimaru
Kimura Tatsumi
Shiotani Kazuhiko
JFE Steel Corporation
Yee Deborah
Young & Thompson
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