Metal treatment – Stock – Ferrous
Patent
1997-04-17
1998-10-13
Yee, Deborah
Metal treatment
Stock
Ferrous
420 61, C22C 3842, C22C 3844
Patent
active
058206990
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a martensitic stainless steel having excellent resistance to corrosion by CO.sub.2 and sulfide stress cracking and good hot workability.
BACKGROUND ART
In recent years, the development of gas wells for producing gases containing a large amount of CO.sub.2, as well as Co.sub.2 injection has become made extensively. Due to severe corrosion, 13% Cr martensitic stainless steels exemplified by AISI420 having excellent resistance to corrosion by CO.sub.2 have been used in such environments. Even these corrosion-resistant steels suffer severe corrosion when the service temperature is increased and exceeds 120.degree. C.
The above environments, in some cases, contain H.sub.2 S. The above-mentioned corrosion-resistant steels have Japanese Patent Laid-Open Pub. Nos. 054063/1987 and 243740/1990 propose steels which could reduce these problems. These steels, however, cannot satisfactorily withstand a corrosive environment at temperatures above 150.degree. C. For this reason, the development of steels serviceable at higher temperatures has been desired in the art.
Martensitic stainless steels having improved corrosion resistance and corrosion fatigue strength in the corrosive environment of a suction roll for paper-manufacturing, a sea water pump, are described in Japanese Patent Laid-Open Pub. No. 19445/1990. These steels are intended for use in the manufacture by centrifugal casting, and do not have hot workability which is required for the manufacture of a seamless pipe for gas.
Steels having improved sulfide cracking resistance and corrosion resistance are described in Japanese Patent Publication No. 15977/1984 and Japanese Patent Laid-Open Pub. No. 174859/1985 and the like. For these martensitic stainless steels, in order to improve the corrosion resistance, the amounts of C and N added are markedly lowered, or several percent Mo is added with the C content being lowered. This poses a problem that a .delta. ferrite phase which deteriorates the hot workability is formed in an austenitic matrix upon heating of an ingot of the steel. Therefore, under severe working conditions such as used in seamless rolling, cracks or flaws occur, which unavoidably lowers the yield, increasing the cost. This makes it very difficult to manufacture a seamless pipe having high corrosion resistance using the above constituent system.
The present inventors previously developed a martensitic stainless steel having excellent resistance to corrosion by CO.sub.2 and, at the same time, sulfide stress cracking resistance and hot workability and already filed a patent application (Japanese Patent Laid-Open No. 263138/1993). In this steel, the contemplated properties, i.e. resistance to corrosion by CO.sub.2, sulfide stress cracking resistance, and hot workability, were realized by the following techniques. (1) The resistance to corrosion by CO.sub.2 was realized by reducing the C content and adding a necessary amount of Cr. (2) The sulfide stress cracking resistance was realized by regulating the structure. (3) The hot workability was realized by reducing the contents of P, S and the like to limit the form of inclusions and, at the same time, regulating the amounts of C and N added and further adding Ni to regulate the phase fraction and form of dissimilar phases having different deformation resistance.
Thereafter, the present inventors have made many studies and, as a result, have succeeded in improving the techniques described in Japanese Patent Laid-Open No. 263138/1993 to further improve the sulfide stress cracking resistance and the hot workability necessary for the manufacture of oil-well pipes as a major application for steels.
An object of the present invention is to provide, through the regulation of particular constituents, a martensitic stainless steel which can resist corrosion by CO.sub.2 at high temperatures above 150.degree. C. and has excellent sulfide stress cracking resistance and particularly excellent hot workability.
DISCLOSURE OF INVENTION
The martensitic stainless st
REFERENCES:
patent: 5716465 (1998-02-01), Hara et al.
Key To Steel, 10 Edition 1974, Germany.
Asahi Hitoshi
Hara Takuya
Kawakami Akira
Nippon Steel Corp.
Yee Deborah
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