Metal treatment – Stock – Ferrous
Patent
1996-08-27
1998-02-10
Yee, Deborah
Metal treatment
Stock
Ferrous
148608, 148610, C22C 3842, C21D 802
Patent
active
057164657
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 weldability.
BACKGROUND ART
In recent years, the development of gas wells for producing petroleum and natural gas containing a large amount of carbon dioxide gas (CO.sub.2) and CO.sub.2 injection, where CO.sub.2 is introduced into an oil well or a gas well to recover petroleum, have become extensively used in the art. Due to severe corrosion, 13% Cr martensitic stainless steels exemplified by AISI420 steel having excellent resistance to corrosion by CO.sub.2 have been used as an oil well pipe in such environments. Since line pipes emerged on the ground surface are joined to each other by welding, materials having excellent weldability are required of the line pipes. Since, however, these steels have a high C content, joining thereof by welding creates a weld which is very hard and has poor impact toughness. For this reason, line pipes of a higher-grade, duplex stainless steel have been reluctantly used. Further, since these line pipes are used in cold districts, the impact toughness of heat-affected zone is often specified to -20.degree. C. or below in terms of the ductile-brittle transition temperature.
In order to improve the weldability, it is generally necessary to lower the C content. Martensitic materials wherein the C content has been lowered to improve the weldability are disclosed, for example, in Japanese Patent Laid-Open Nos. 99127/1992 and 99128/1992. These steels, however, are still unsatisfactory in weldability and hot workability, making it difficult to actually produce such steels, or further have unsatisfactory sulfide stress cracking resistance (SSC resistance). Therefore, the quality of the steels is not yet on a level high enough to be usable as an alternative for the duplex stainless steel.
An object of the present invention is to provide a martensitic stainless steel having CO.sub.2 corrosion resistance high enough to withstand the maximum service temperature of the line pipe, excellent sulfide stress cracking resistance (SSC resistance), and good toughness of welding heat-affected zone by regulating specific constituents.
DISCLOSURE OF INVENTION
The high-corrosion-resistant martensitic stainless steel having excellent weldability of the present invention is characterized by comprising steel constituents satisfying by weight C: 0.005 to 0.035%, Si: not more than 0.50%, Mn: 0.1 to 1.0%, P: not more than 0.03%, S: not more than 0.005%, Cr: 10.0 to 13.5%, Cu: 1.0 to 4.0%, Ni: 1.5 to 5.0%, Al: not more than 0.06%, and N: not more than 0.01%, consisting of Ti: 0.005 to 0.1%, Zr: 0.01 to 0.2%, Ca: 0.001 to 0.02%, and REM: 0.003 to 0.4%, with the balance consisting essentially of Fe.
Further, the martensitic stainless steel having excellent weldability and SSC resistance according to the present invention is characterized by comprising steel constituents satisfying by weight C: 0.005 to 0.035%, Si: not more than 0.50%, Mn: 0.1 to 1.0%, P: not more than 0.03%, S: not more than 0.005%, Mo: 1.0 to 3.0%, Cu: 1.0 to 4.0%, Ni: 1.5 to 5.0%, Al: not more than 0.06%, N: not more than 0.01%, and Cr satisfying a requirement represented by the formula 13>Cr+1.6Mo.gtoreq.8, consisting of Ti: 0.05 to 0.1%, Zr: 0.01 to 0.2%, Ca: 0.001 to 0.02%, and REM: 0.003 to 0.4%, with the balance consisting essentially of Fe.
The process for producing a high-corrosion-resistant martensitic stainless steel according to the present invention is characterized by comprising the steps of: subjecting a steel plate, produced by hot-rolling a stainless steel slab having the above composition, to austenitization at a temperature of Ac.sub.3 point to 1000.degree. C.; subjecting the hardened steel plate to final tempering at a temperature of 550.degree. C. to Ac.sub.1 point; and cold-rolling the steel plate to prepare a steel pipe.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a diagram showing the influence of alloying elements on the resistance to corros
REFERENCES:
patent: 5089067 (1992-02-01), Schumacher
Hara Takuya
Hitoshi Asahi
Kawakami Akira
Muraki Taro
Tamehiro Hiroshi
Nippon Steel Corporation
Yee Deborah
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