Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Patent
1995-06-06
1997-03-11
Yee, Deborah
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
420103, 420104, C22C 3806
Patent
active
056098189
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a steel that is excellent in corrosion resistance and processability and more particularly to a steel that exhibits excellent corrosion resistance in the exhaust system of an internal combustion engine such as of an automobile, ship or the like and which also exhibits excellent processability during processing into components.
BACKGROUND ART
In the exhaust systems of internal combustion engines, particularly those used in automobiles, corrosion from the internal and external surfaces has conventionally been suppressed by the use of ordinary steel plated with aluminum or zinc. However, since exhaust systems are now equipped with catalytic converters or the like for purifying the exhaust gas to reduce environmental pollution, the corrosion resistance provided by such plated steels has become inadequate. As taught, for example, by Japanese Patent Public Disclosures No. Sho 63-143240 and 63-143241, therefore, steels containing 5-10% of Cr were developed for increasing the corrosion resistance of the steel base material. In response to recent increases in the service life and warranty period of automobiles, however, large amounts of high-grade stainless steels containing around 18% of Cr, with or without addition of Mo, are now being used in exhaust systems. Even such high-grade stainless steels have been found to be insufficient in corrosion resistance and subject to localized corrosion (pitting) and the like. Moreover, since the large amounts of Cr and Mo contained in such high-grade stainless steels degrades their processability, they are extremely difficult to fabricate into complexly shaped components such as those used in exhaust systems, and the highly complicated manufacturing processes required increases the processing cost. They may even be completely unusable for some shapes and, at any rate, are expensive as a raw material.
When a steel with a given Cr content becomes more susceptible to pitting owing to a more severe operating environment, as is typical in exhaust systems, the problem is generally coped with by further increasing the Cr or Mo content so as to strengthen the resistance to corrosion.
In light of these circumstances, the object of the present invention is to provide a steel which exhibits strong resistance to a corrosive environment such as present in the exhaust system of an internal combustion engine, exhibits excellent processability, and is low in cost.
DISCLOSURE OF THE INVENTION
For achieving this object, the inventors conducted studies from various points of view in order to develop a steel with excellent corrosion resistance in corrosive environments, particularly in the corrosive environment of an exhaust system. Beginning their studies with the corrosive environment of exhaust systems, they discovered that corrosion of an internal combustion engine exhaust system occurs in an environment of chloride ions, sulfate ions and the like heated to 80.degree.-150.degree. C. They next examined various means for increasing corrosion resistance in such a corrosive environment and discovered that, exactly opposite from in prior art stainless steels, a steel with a reduced Cr content of 5.5-9.9% and added with 1.0-3.0% of Al exhibits extraordinarily good corrosion resistance in exhaust systems and other such corrosive environments. Additional studies directed to providing a further improved steel revealed that when the foregoing steel is reduced in C and N content and added with Nb, V, Ti, Zr, Ta and Hf to meet a prescribed condition, its corrosion resistance is increased and its processability improved, that when the foregoing steel is added with Cu, Mo, Sb, Ni and W, independently or in combination, its corrosion resistance is improved, and that Si and Mn are suitable deoxidation and strengthening elements for the steel.
The present invention was accomplished on the basis of the foregoing knowledge. In its first aspect, the invention resides in a steel excellent in corrosion resistance and processability which, in weight per cent, contai
Kato Kenji
Miyasaka Akihiro
Nippon Steel Corporation
Yee Deborah
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