STEEL FOR OIL WELL PIPES WITH HIGH WET CARBON DIOXIDE GAS CORROSION RESISTANCE AND HIGH SEAWATER CORROSION RESISTANCE, AND SEAMLESS OIL WELL PIPE

Abstract: A steel for oil well pipe, which has excellent resistance to localized corrosion in CO 2 environments and corrosion in seawater, and a seamless pipe made of said steel are provided. The steel consists of, by mass %, more than 0.10 % to 0.30 % of C, 0.10 % to 1.0 % of Si, 0.1 % to 3.0% of Mn, 2.0 % to 9.0 % of Cr and 0.01 % to 0.10% of Al and optionally 0.05 % to 0.5 % of Cu, and the balance of Fe and incidental impurities. P as the impurity is not more than 0.03 % and S as the impurity is not more than 0.01%. The steel has a substantially single martensite structure as quenched or as normalized condition, and not lower than 552 MPa yield strength as quenched-tempered or as normalized-tempered condition.

Description: FIELD OF THE INVENTION In these years, so-called sweet oil wells containing carbon dioxide (referred to as CO 2 hereafter) have been exploited because of increasing energy demand and a shortage of high quality oil resources that can be easily exploited. In addition, exploitation of rather small-scale oil wells, which have a short production life up to about 10 years because of relatively small reserves, is increasing. When the production efficiency of an oil well decreases, deaired (degassed) seawater is injected into the pipe, which is ordinary used for oil pipe production, in order to recover the oil production efficiency. In the situation as mentioned above, an oil well pipe having high corrosion resistance to both CO 2 and seawater, which contains small amounts of dissolved oxygen of about 500 ppb, is required. The seawater containing small amount of dissolved oxygen as mentioned above, is referred to as "seawater" in this specification. Conventionally an inhibitor is used to suppress corrosion of carbon steel pipes, when the pipe is used for both oil production and seawater injection. The inhibitor, however, not only increases production cost but also induces pollution. Therefore, steel for an oil well pipe, which has sufficient corrosion resistance to eliminate the inhibitor, is desired. It is already known, by the following references 1 and 2, that the corrosion rate of steel in CO 2 environments decreases and resistance to general corrosion is improved, according to an increase of Cr content. In fact, the JIS SUS 410 series steels, which contain 12 to 13 % of Cr ("%" for content of alloy elements means "mass %" in this specification), have already been utilized for the oil well pipe. Reference 1: A. Ikeda, M. Ueda and S. Mukai "Corrosion/83" NACE Houston, Paper No. 45, 1983 Reference 2: Masaki Ueda and A. Ikeda "Corrosion/96" NACE Houston, Paper No. 13, 1996 However, the SUS 410 series steels are expensive because of the high Cr content thereof. In addition, such high Cr steels have a disadvantage to suffer localized corrosion (pitting) in the seawater containing little dissolved oxygen. A steel, which contains smaller amounts of Cr and is cheaper than the 12 to 13 % Cr steel, is preferable for the oil well pipe for such short life wells as described above. Furthermore, considering said seawater injection, a steel having resistance to localized and general corrosion in seawater, i.e., a seawater resistant steel, is necessary. Tokuko Sho 53-38687 (reference 3) discloses a low alloy seawater resistant steel containing 1.0 - 6.0 % Cr and 0.1 to 3.0 % Al. However, this steel is not for an oil well pipe, and the CO 2 corrosion resistance thereof is not known. Tokukai Sho 57-5846 (reference 4) discloses a steel containing 0.5 - 5 % Cr and having resistance to sweet corrosion. While reference 4 states that this steel has a good corrosion resistance in seawater containing CO 2 , the resistance is merely the general corrosion resistance, which has been estimated by corrosion weight loss. In addition, the microstructure thereof is not clear because the producing method of the steel is not disclosed. Tokuko Sho 57-37667 (reference 5) proposes a wet CO 2 resistant steel for line pipes, which contains more than 3.0 % to 12.0 % Cr. This steel's resistance against localized corrosion is improved in specific areas such as the welded portion, where the heat treatment history is different from other areas. The steel, however, can not have a single martensite microstructure because of its low C content. Therefore, its tensile strength is low and its resistance to localized corrosion of the pipe made of it is not sufficient. Tokukai Hei 5-112844 (reference 6) discloses a steel pipe, which has good CO 2 corrosion resistance and can be used for oil well pipes. However, the Cr content of this steel pipe is as low as 0.25 - 1.0 %, since this pipe was not invented to improve the seawater corrosion resistance. In addition, the CO 2 corrosion resistance of this pipe is improved mainly by a decarburized layer of more than 100 µm thickness, which is formed in the inner surface of the pipe. Further, JP-A-6 0238418 discloses the manufacture of a corrosion resistant sucker rod for an oil well. The composition ranges in weight-% are 0.1-0.3% C, 0.1-0.8% Si, 0.3-1.2% Mn, ≤0.025% P, ≤0.01% S, 8-15% Cr and 0.001-0.05% Al. The steel is quenched after heating in a carburizing atmosphere at 900-1000°C and then tempered at 600-700°C. Example B features the composition: 0.13% C, 0.43% Si, 0.95% Mn, 0.013% P, 0.006% S, <0.01% Cu, 0.02% Ni, 8.7% Cr, 0.01% Mo and 0.014% Al and a tensile strength value of 116-119 ksi. As mentioned above, it is already well known that increasing of the Cr content improves the general corrosion resistance of the steel in CO 2 environments. However, it is uneconomical to use the steel of more than 10 % of Cr content for the short life oil wells such as 10 years or less. In addition, the steel containing such a high content of Cr has the disadvantage of localized corrosion (pitting) in the seawater of low dissolved oxygen. The oil well pipe becomes useless after suffering localized corrosion, which passes through the pipe wall, even if it has good general corrosion resistance. It means that not only general corrosion resistance but also localized corrosion resistance is remarkably important in the steel for an oil well pipe. DISCLOSURE OF THE INVENTION An objective of the present invention is to provide a steel that has all of the following properties: 1) Yield strength not less than 552 MPa (yield strength of API 80 grade or more) in a heat-treated condition by quenching-tempering or normalizing-tempering. 2) Superior resistance to localized corrosion in wet CO 2 environments and seawater of low dissolved oxygen. 3) Superior resistance to general corrosion in seawater of low dissolved oxygen. Another objective of the present invention is to provide a comparatively cheap seamless oil well pipe made of the above mentioned steel. The inventors have investigated the means to improve the resistance of steel for an oil well pipe to localized corrosion in CO 2 environments and corrosion in seawater. The inventors thereby have found the fact that the resistance not only to localized corrosion in CO 2 environments, but also to the corrosion in seawater can be remarkably improved by making the microstructure substantially of single martensite in a condition as quenched or as normalized. It is known that localized corrosion resistance to wet CO 2 environments of Cr-free carbon steel depends on the microstructure, and it is also known that the ferrite - pearlite duplex (dual-phase) structure is better than the single homogeneous martensite structure for localized corrosion resistance. However, according to the investigation by the present inventors, in steel containing Cr, the single martensite structure has superior resistance to localized corrosion in wet CO 2 environments. This invention provides, on the basis of said finding, a steel for an oil well pipe, which has all of the following characteristics. (a) Chemical Composition: The steel consists of, by mass %, more than 0.10 % to 0.30 % of C, 0.10 % to 1.0 % ofSi, 0.1 % to 3.0% of Mn, 2.0 % to 7.0 % of Cr and 0.01 % to 0.10% of Al, and the balance of Fe and incidental impurities; P as the impurity is not more than 0.03 % and S as the impurity is not more than 0.01%. Furthermore, 0.05 % to 0.5 % of Cu, as an alloy element, may also be contained. (b) Microstructure: The microstructure is substantially single martensite structure as quenched or as normalized condition. Said "substantially single martensite structure" means the structure in which about 95 % or more, in the cross-sectional area ratio, is martensite. In addition to martensite, existence of less than about 5 %, in all total, of ferrite, bainite and/or pearlite can be allowed. (c) Strength: The yield strength is not lower than 552 MPa after heat treatment of "quenching-tempering" or "normalizing-tempering". The present invention also provides a seamless oil well pipe, which is made of the above-mentioned steel and has excellent resistance to wet CO 2 corrosion and seawater corrosion. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a table showing chemical compositions of the tested steels and test results in the following Example. Figure 2 is a graph showing the relationship between Cr contents and martensite area ratio, and localized corrosion resistance in wet CO 2 environments and artificial seawater. Figure 3 is graph showing the relationship between Cr contents of 2.0 - 9.0 % Cr steel according to the present invention and corrosion rate in artificial seawater. THE BEST MODE FOR CARRYING OUT THE INVENTION The steel for oil well pipe of this invention has all the characteristics from (a) to (c) as mentioned above. Each of these characteristics will be described hereafter. 1. Chemical Composition of the Steel First, the reasons for selecting the above mentioned alloy elements and amounts thereof will be described. C: C is necessary to improve hardenability of the steel and to make its structure substantially single martensite, and thereby to confirm corrosion resistance and the strength of the steel. If the amount of C is no more than 0.10 %, the hardenability is not enough to obtain said structure and neither its corrosion resistance nor strength is sufficient. On the other hand, more than 0.30 % C induces quenching cracks, which makes production of the seamless pipe difficult. Therefore, the amount of C is selected in the range of more than 0.10 % to 0.30 %. More preferable the range is more than 0.10 % and to 0.25 %. Si: Si is used as a deoxidizing agent of the steel, and its content of not less than 0.10 % is necessary. More than 1.0 % of Si, however, has an unfavorable effect on the workability and the toughness of the steel. Mn: Not less than 0.1 % of Mn is necessary to improve the strength and the toughness of the steel. However, more than 3.0 % of Mn decreases resistance to CO 2 corrosion. The proper range of Mn content, therefore, is 0.1 - 3.0 %. Cr: Cr improves hardenability of the steel to increase strength and corrosion resistance in a wet CO 2 environment and also in seawater, which contains a small amount of dissolved oxygen. If the Cr content is less than 2.0 %, said effect is not sufficient. On the other hand, addition of large amounts of Cr makes the steel expensive. Further, in the steel containing more than 9.0 % Cr, localized corrosion occurs easily in said seawater and toughness decreases. Therefore, the proper range of Cr content is 2.0-9.0 %. From the viewpoint of balance of steel cost and properties, the most preferable range is 3.0 - 7.0 %. Al: Al is used as a deoxidizing agent of the steel. If its content is less than 0.01 %, there is a possibility of insufficient deoxidization. On the other hand, more than 0.10 % of Al deteriorates mechanical properties, such as toughness. Cu: Although Cu is not an indispensable element, it can optionally be contained in the steel because it is effective in order to improve seawater corrosion resistance. Such effect is insufficient when its content is lower than 0.05 %. On the other hand, more than 0.5 % of Cu deteriorates hot workability of the steel. Therefore, the Cu amount should be in the range 0.05 - 0.5 % when it is added. The steel of this invention consists of the above-mentioned elements and the balance Fe and incidental impurities. Among the impurities, particularly P and S should be limited as follows. P: P is inevitably contained in the steel. Since more than 0.03 % of P segregates on grain boundaries and decreases the toughness of the steel, it is limited to not more than 0.03 %. S: S also is inevitably contained in the steel and combines with Mn to form MnS and deteriorates toughness of the steel. Therefore, its content is limited to not more than 0.01 %. 2. Microstructure One of the remarkable characteristics of this steel according to this invention is its microstructure that is substantially single martensite. Steel pipes made of the steel of this invention are utilized as tempered after quenching or after normalizing. Therefore, the final structure becomes substantially single tempered martensite. Depending on the above mentioned chemical composition and microstructure, the steel of this invention has resistance to localized corrosion in wet CO 2 environments, resistance to seawater corrosion and sufficient strength. As previously described, "substantially single martensite" means the structure consisting of, in area % (measured by microscopic inspection), of about 95 % or more of martensite. It is preferable that the martensite is not less than 98 %. The reason for improvement of localized corrosion resistance in wet CO 2 environments and seawater by the microstructure consisting of substantially single martensite has not yet become clear. However, it can be considered as described bellow. Localized corrosion does not proceed while corrosion product, which is formed in corrosive environments, uniformly covers the surface of the steel. The structure of the corrosion product depends on the steel structure. Therefore, if the structure of the steel is single martensite, localized corrosion does not occur because the corrosion product uniformly covers the surface of the steel. If any structures, other than martensite, exist in amounts of about 5 % or more, the corrosion product on those structures becomes different from the corrosion product on the martensite. Said difference of the corrosion product or partial peeling off of the corrosion product induces the localized corrosion. The above mentioned structure can be obtained by heat treatment, conditions for which are properly determined depending on the chemical composition of the steel. For example, a substantially single martensite structure can be formed in a process, wherein the steel is heated in a range of 900 - 1100 °C and cooled with a controlled cooling rate in water cooling (quenching) or air cooling (normalizing). Tempering temperature can be selected in a range of 450 - 700 °C. 3. Strength of the Steel The steel of this invention has the yield strength of 552 MPa or more, in the condition as quenched-tempered or normalized-tempered as mentioned above. This yield strength corresponds to those of oil well pipes of Grade 80 (minimum yield strength is 80,000 psi) or higher, standardized in API (American Petroleum Institute). Therefore, the oil well pipe made of the steel of this invention can be utilized as high strength oil well pipes of the Grade 80 or higher. Although the above mentioned steel of this invention may be used for welded oil well pipe, it is more suitable for seamless oil well pipes. Those pipes can be manufactured in the conventional method. The seamless pipe can be manufactured in the Mannesmann process, the hot-extruding process etc. After manufacturing, the pipe should be heat treated in order to make the structure substantially single tempered martensite. EXAMPLE Steels having chemical compositions shown in Fig.1 were produced in a vacuum furnace and cast into ingots of 550 mm diameter. Then these ingots were hot forged into billets of 150 mm diameter at 1200 °C. Seamless pipes of 188 mm outer diameter and 12 mm thickness were manufactured from the billets in the Mannesmann pipe making process. The pipes were heated at 900 - 1100 °C and quenched or normalized to make the structure martensite of 83 - 99 area %. The area % of martensite was varied by controlling the heating temperature in said range and cooling rate in a range 5 - 40 °C/sec, depending on the chemical compositions of the steels. Test specimens for microscopic inspection were cut out of said pipes as quenched or as normalized, in order to examine the martensite area %. Thereafter, the pipes were tempered in a temperature range of 500 - 650°C to make pipes, which have a yield strength of API Grade 80 (yield strength: 552 - 655 MPa). Using said pipes, hardness test, tensile test and corrosion test, as mentioned hereinafter, were carried out. (A) Hardness Test HRC hardness was measured on cross sections vertical to the longitudinal direction of the sample pipes (pipes tempered after quenched or normalized). (B) Tensile Test Test specimens, having 4.0 mm diameter and 20 mm length of parallel portion, were cut out of the sample pipes. Tests were carried out at room temperature, and yield strength at 0.5 % total elongation and tensile strength were measured. Ratios of said yield strength to tensile strength (yield ratio, YR) were also calculated. (C) Martensite Area Ratio Ten visual fields of each cross section, vertical to the longitudinal direction of the pipes as quenched or normalized, were inspected with an optical microscope of 100 magnifications. Martensite area ratios were measured thereby, and averages of them were calculated. (D) Localized Corrosion Test in Wet CO 2 Environments Test specimens of 22 mm width, 3 mm thickness and 76 mm length were cut out of the sample pipes. The specimens were tested, after being polished with No. 600 emery paper, degreased and dried, by immersing for 720 hours in the following test solution. Weight losses of the specimens, after removing the corrosion product, were measured and existence of localized corrosion was visually investigated. Test Solution: 5 % NaCl solution saturated with 3 bar CO 2 Agitated at flow rate of 2.5 mm/s Solution temperature, 80 °C. (E) Sea Water Corrosion Test Test specimens of 22 mm width, 3 mm thickness and 76 mm length, cut out of the sample pipes, polished with No. 600 emery paper, degreased and dried, were used. Said specimens were immersed in artificial seawater with 500 ppb dissolved oxygen (according to ASTM D 1141-52 standard) for 72 hours. Thereafter, the corrosion product on the specimens was removed and weight losses thereof were measured. Existence of localized corrosion was also investigated by visual inspection. Test results are shown in Table 1, wherein "○" means no localized corrosion in the wet CO 2 corrosion test or the artificial seawater corrosion test and " × "means existence of localized corrosion in those tests. Fig.2 1 is a graph, which shows the relationship between Cr content, martensite ratio, and resistance to localized corrosion in CO 2 environments and artificial seawater, shown in Table 1. Fig.2 is a graph, which shows the relationship between Cr content of the steels according to this invention and corrosion rate in the artificial seawater shown in Table 1. Numbers Fig. 1 and Fig.2 are the same as those in Table 1. It is apparent from the test results in Table 1, Fig.1, and Fig.2 that the steels of this invention (Nos.1, 2 and 6-9), which have more than 95 area % martensite as quenched or normalized, never suffered localized corrosion in either CO 2 environments or artificial seawater. These steels have good resistance to general corrosion in the artificial seawater and high strength such as yield strength of not lower than 552 MPa at 0.5 % total elongation. Steels of Nos.6 - 9 are Cu containing steels according to this invention. The corrosion rates of these steels are much smaller. Steels of Nos.11 - 16 are comparative steels. Among them steels 11 and 12 are inferior in resistance to general corrosion in seawater and also suffer localized corrosion because of not enough Cr content. Steels of Nos.13 - 16 have the chemical compositions according to this invention, however, martensite ratios are small. Therefore, all of them suffer localized corrosion in seawater and wet CO 2 environments, although some of them (steels 14 - 16) show good resistance to general corrosion in seawater. It is apparent, from said facts, that not only selection of the proper chemical composition but also the substantially single martensite structure is necessary to prevent localized corrosion. INDUSTRIAL APPLICABILITY The steel of the present invention is excellent in resistance to localized corrosion in both wet CO 2 environments and seawater as well as resistance to general corrosion in seawater. In addition, the steel of the present invention has yield strength of not lower than 552 MPa, in quenched - tempered or normalized - tempered condition. Since steel pipes made of the steel of this invention are relatively cheap, they can be utilized, as oil well pipes for environments of coexistence of CO 2 and seawater, even in short life oil wells.

Claims: A steel excellent in resistance to wet CO 2 corrosion and seawater corrosion, characterized by consisting of, by mass %, more than 0.10 % to 0.30 % of C, 0.10 % to 1.0 % of Si, 0.1 % to 3.0 % ofMn, 2.0 % to 7.0 % of Cr and 0.01 % to 0.10 % of Al, and the balance of Fe and incidental impurities; P as the impurity is not more than 0.03 % and S as the impurity is not more than 0.01 %; and also characterized by a martensite structure, in which about 95 % or more in the cross-sectional area ratio is martensite as quenched or as normalized condition, and not lower than 552 MPa yield strength, as quenched-tempered or normalized-tempered condition. A steel excellent in resistance to wet CO 2 corrosion and seawater corrosion, characterized by consisting of, by mass %, more than 0.10 % to 0.30 % of C., 0.10 % to 1.0 % of Si, 0.1 % to 3.0 % of Mn, 2.0 % to 7.0 % of Cr and 0.01 % to 0.10 % of Al, 0.05 % to 0.5 % of Cu, and the balance of Fe and incidental impurities; P as the impurity is not more than 0.03 % and S as the impurity is not more than 0.01 %; and also characterized by a martensite structure, in which about 95 % or more in the cross-sectional area ratio is martensite as quenched or as normalized condition, and not lower than 552 MPa yield strength, as quenched-tempered or normalized-tempered condition. A seamless oil well pipe that is made of the steel according to Claim 1 or Claim 2.

Inventors: TAKABE HIDEKI, UEDA MASAKATSU, TAKABE, HIDEKI, UEDA, MASAKATSU, TAKABE, Hideki Sumitomo Metal Industries, Ltd., UEDA, Masakatsu Sumitomo Metal Industries, Ltd.
Applicants: SUMITOMO METAL IND, SUMITOMO METAL INDUSTRIES LIMITED
Application year: 1998
Main classification: C22C38
IPC codes: C21D1, C22C38, C21D6