JPS648684B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention provides a method for producing high-strength duplex stainless steel with excellent corrosion resistance, which exhibits excellent corrosion resistance particularly in a carbon dioxide corrosion environment or a stress corrosion environment, and It also has high strength and is suitable for oil country tubular goods.
The present invention relates to a method for producing a Cu-containing austenitic-ferritic duplex stainless steel. <Conventional technology> In recent years, oil wells and natural gas wells have become increasingly deep, and therefore there is a strong demand for improved strength of oil country tubular goods. Development efforts have begun to focus on oil and gas wells that contain corrosive substances such as carbon dioxide (CO ₂ ), hydrogen sulfide (H ₂ S), and chloride ions (Cl ^- ). Corrosion countermeasures are also becoming increasingly important. Conventionally, the most common method to prevent corrosion of oil country tubular goods has been to inject corrosion inhibitors into wells, but this method is often not expected to produce sufficient results. Another problem was that it could not be used effectively for offshore oil wells, gas wells, etc. In addition, attempts have been made to prevent corrosion by applying protective coatings to oil country tubular goods, but these efforts have not yielded satisfactory results. In view of these circumstances, there has recently been _a trend to use even higher-grade corrosion-resistant materials for oil country tubular goods than in the past. In order to deal with severe corrosion phenomena, attempts have been made to use 13Cr steel such as AISI410 steel and 420 steel, but even so, they cannot maintain resistance to CO ₂ for a long time in high-temperature environments exceeding 150℃. That was the current situation. Therefore, in high-temperature environments exceeding 150°C, the use of austenitic-ferritic duplex stainless steels with even higher Cr content, such as 22Cr steel and 25Cr steel, is attracting attention. However, this austenite-ferrite system 2
When phase stainless steel is solution treated, which is the usual method for producing steel materials, it is difficult to obtain a yield strength (0.2% yield strength) of 65 to 80 Ksi (45.7 to 56.2 Kgf/mm ² ). It did not have sufficient strength as oil country tubular goods for deep wells. Therefore, when applying austenitic-ferritic duplex stainless steel, the current practice is to perform further cold working after solution treatment to provide the high strength required for oil country tubular goods for deep wells. be. By the way, detailed experiments and research by the present inventors have revealed that duplex stainless steel, which has excellent corrosion resistance in a humid carbon dioxide environment at high temperatures of 150 to 250 degrees Celsius, If CO ₂ -H _{2 S- is contaminated with H 2 S} or Cl ^- , its corrosion resistance will deteriorate significantly.
The main type of corrosion in oil and gas well environments containing Cl ^- is stress corrosion cracking (hereinafter abbreviated as "SCC"), but SCC in this case behaves completely differently from normal corrosion. In addition to the presence of Cl ^- , the influence of H ₂ S is even greater. The H ₂ S partial pressure of
If the pressure exceeds 10 atm, SCC will occur, and if the material is strengthened by cold working, SCC will occur even if it contains about 1 atm of H ₂ S. The presence of Cl ^- promotes the occurrence of SCC. Moreover, in the duplex stainless steel under the CO ₂ - _{H 2} S - Cl ^- environment, even if SCC does not occur, the ferrite region is selectively dissolved due to the influence of H ₂ S, which is the so-called "selection". It was found that cold working has a considerable negative effect on this selective corrosion. As described above, duplex stainless steel has the problem that if it is subjected to severe cold working to compensate for its lack of strength, its corrosion resistance will deteriorate significantly in an environment where H ₂ S exists, and it is difficult to maintain the desired strength. Since this requires a large amount of processing (reduction amount), there are also restrictions from an equipment standpoint. <Objective of the invention> The main object of the invention is to solve the various problems mentioned above, and to eliminate the need for cold working which adversely affects resistance to SCC and selective corrosion in an H ₂ S coexistence environment. , discovered a means _of strengthening austenitic-ferritic duplex stainless steel, which exhibits excellent durability even in oil and gas well environments where the H ₂ S partial pressure is less than 10 atm, or at least less than 5 atm. The objective is to provide an oil country tubular material that can be used in an environment that contains a small amount of H ₂ S with a partial pressure of about 1 atm or less. Another object of the present invention is to provide a means for strengthening even with a small rolling mill or the like, that is, a means for achieving desired strengthening while minimizing deterioration of corrosion resistance with a small amount of cold working. <Structure of the Invention> From the above-mentioned viewpoint, the present inventors have conducted repeated research through trial and error in order to achieve the above-mentioned objective, and as a result, have obtained the knowledge shown in (a) to (d) below. It was. (a) Among austenitic-ferritic duplex stainless steels, those containing Cu are directly quenched (direct solution treatment) after hot working, followed by aging. If this treatment is carried out, the strength will be greatly improved due to the combined effect of frozen hot working strain and Cu precipitation, and it will also show good corrosion resistance. (b) In order to improve SCC resistance, it is necessary to It is preferable to keep the C content as low as possible, especially 0.03% by weight or less, but C, which is one of the components that ensures the strength of steel,
Even if such a material has a low content, if it contains Cu as a component, the above-mentioned "direct solution treatment" and subsequent "aging treatment"
(c) When the temperature of the "direct solution treatment" is set to 800°C or higher, the corrosion resistance of the resulting material is particularly good. (d) Furthermore, if mild cold working is performed between the direct solution treatment and the subsequent aging treatment, or after the direct solution treatment and the aging treatment, a small amount of work (reduction amount) can be achieved. ), it is possible to impart greater strength to the material, and the corrosion resistance is also better compared to conventional methods. This invention was made based on the above knowledge, and involves hot working austenitic-ferritic duplex stainless steel containing Cu by heating it to a temperature of 1000°C or higher, and then directly heating it to a temperature of 800°C or higher. quenching and then subjecting to aging treatment, or between the above "quenching" and "aging treatment" or "aging treatment"
After that, by further performing cold working, it is possible to obtain a duplex stainless steel that has excellent corrosion resistance such as carbon dioxide corrosion resistance and SCC resistance, and has sufficiently high strength. More specifically, this invention was achieved by comprehensively and organically combining the following technical means and the actions and effects brought about by them. . That is, ○…

Claims (1)

[Claims] 1. Hot working of austenitic-ferritic duplex stainless steel containing Cu is heated to 1000°C or higher, followed by rapid cooling from a temperature of 800°C or higher, followed by aging treatment. A method for producing high-strength duplex stainless steel with excellent corrosion resistance. 2 Cu-containing austenitic-ferritic duplex stainless steel is heated to 1000°C or higher and hot worked, then rapidly cooled from 800°C or higher, cold worked, and further aged. A method for producing high-strength duplex stainless steel with excellent corrosion resistance. 3 Cu-containing austenitic-ferritic duplex stainless steel is heated to 1000°C or higher and hot worked, then rapidly cooled from a temperature of 800°C or higher, aged, and further cold worked. A method for producing high-strength duplex stainless steel with excellent corrosion resistance.