Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum

Definitions

• This disclosure relates to a martensitic stainless steel seamless tube for oil country tubular goods, more particularly, to a seamless steel tube for OCTG which possesses both of high strength of 95 ksi (655 MPa) in terms of yield strength (YS) and excellent low-temperature toughness, and a manufacturing method thereof.
• a steel tube for an oil well used in such a severe environment is required to be made of a material which possesses high strength, excellent corrosion resistance and excellent toughness.
• JP-A-2002-363708 proposes martensitic stainless steel which contains 0.01 to 0.1% of C, 9 to 15% of Cr and 0.1% or less of Ni. Although the stainless steel exhibits relatively high C content so that the stainless steel possesses high strength, the stainless steel possesses high toughness. Accordingly, the stainless steel is preferably used for manufacturing oil country tubular goods.
• JP-A-2002-3637008 it is estimated that by reducing a quantity of carbide present in a prior austenite grain boundary to 0.5 volume % or less, by setting a maximum minor axis of carbide to 10 to 200 nm and by setting a ratio between average Cr concentration and average Fe concentration in carbide to 0.4 or less, the precipitation of M 23 C 6 -type carbide is suppressed and the precipitation of M 3 C-type carbide is accelerated, thus largely improving toughness.
• the stainless steel is subject to tempering at a temperature of 450° C. or below in such a manner that the stainless steel is subject to air cooling (standing to cool) after hot working, is held in air cooling (standing to cool) after solution treatment, or is subject to air cooling (standing to cool) after solution treatment.
• Hot straitening of our stainless steel tube can be realized. Accordingly, even when straitening is performed, the increase of yield strength is small so that it is possible to manufacture easily and stably a seamless steel tube for OCTG which possesses both of high strength of yield strength YS of 95 ksi grade (655 to 758 MPa) or more and excellent low-temperature toughness where a fracture transition temperature vTrs is ⁇ 40° C. or below, thus acquiring remarkable industrial advantageous effects.
• the seamless steel tube for OCTG is a martensitic stainless steel seamless tube which adopts the composition which contains 0.020% or less C, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance as the basic composition.
• the martensitic stainless steel seamless tube may adopt the composition which contains 0.020% or less C, 1.0% or less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S, 0.10% or less Al, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance as the basic composition.
• C is an important element relating to strength of martensitic stainless steel. Although it is desirable that the stainless steel contains 0.003% or more of C to ensure desired high strength, when a content of C exceeds 0.020%, toughness and also corrosion resistance are liable to be lowered. Accordingly, the content of C is limited to 0.020% or less. From a view point of stable assuring of strength and toughness, the content of C is preferably limited to a value which falls within a range from 0.003% to 0.015%.
• Cr is an element which enhances corrosion resistance due to the formation of a protective film, and is an element which effectively contributes to the enhancement of CO 2 corrosion resistance and CO 2 stress corrosion cracking resistance.
• the stainless steel contains 10% or more of Cr, the stainless steel can ensure required corrosion resistance as oil country tubular goods. Hence, 10% is set to a lower limit of Cr content.
• the stainless steel contains a large quantity of Cr exceeding 14%, ferrite is easily formed so that the addition of a large quantity of expensive austenite forming element becomes necessary for ensuring stability of a martensite phase or for preventing the lowering of hot workability whereby the content of Cr exceeding 14% is economically disadvantageous.
• the content of Cr is limited to a value which falls within a range from 10 to 14%. From a view point of ensuring the more stable structure and the more stable hot workability, the content of Cr is preferably limited to a value which falls within a range from 10.5 to 11.5%.
• Ni is an element which has a function of strengthening a protective film, and enhances corrosion resistance such as CO 2 corrosion resistance. Although it is desirable that the stainless steel tube contains 0.1% or more of Ni to acquire such an advantageous effect, when the content of Ni exceeds 3%, this leads to only a sharp rise of a manufacturing cost. Accordingly, the content of Ni is limited to a value which falls within a range not more than 3%. The content of Ni is preferably limited to a value which falls within a range from 1.5 to 2.5%.
• N is an element which remarkably enhances pitting corrosion resistance, and such an advantageous effect becomes outstanding when the content of N becomes 0.003% or more.
• the content of N exceeds 0.05%, various nitrides are formed thus lowering toughness. Accordingly, the content of N is limited to 0.05% or less.
• the content of N is preferably limited to a value which falls within a range from 0.01 to 0.02%.
• Nb is an important element.
• Nb is an element which forms a carbide, and increases strength of steel through precipitation strengthening by Nb carbide. Further, Nb plays an important role for preventing the grain boundary precipitation of M 23 C 6 type Cr carbide, thus enhancing toughness.
• it is necessary to set the content of Nb to 0.03% or more, and more preferably to a value which exceeds 0.03%. Further, from a view point of acquiring higher strengthening and higher toughness, it is preferable to set the content of Nb to 0.06% or more.
• the content of Nb exceeds 0.2%, toughness is lowered. Accordingly, the content of Nb is limited to a value which falls within a range from 0.03 to 0.2%.
• the content of Nb is preferably limited to a value which falls within a range from 0.03% to 0.15%.
• the content of Nb is more preferably limited to a value which falls within a range from 0.06 to 0.15%.
• the above-mentioned components are basic components of the stainless steel, it is preferable to adopt the composition which contains 1.0% or less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S, and 0.10% or less Al in addition to these basic components as the basic composition.
• Si is an element which functions as deoxidizing agent in a usual steel making process. Although it is desirable to set the content of Si to 0.1% or more, when the content of Si exceeds 1.0%, toughness is lowered and cold workability property is also lowered. Accordingly, the content of Si is limited to 1.0% or less. The content of Si is preferably limited to a value which falls within a range from 0.1 to 0.3%.
• Mn is an element which increases strength of the stainless steel. Although it is desirable that the content of Mn is 0.1% or more to allow the stainless steel to ensure strength necessary for a steel tube for oil country tubular goods, when the content of Mn exceeds 2.0%, toughness is adversely influenced by Mn. Accordingly, the content of Mn is limited to a value which falls within a range from 0.1 to 2.0%. The content of Mn is preferably limited to a value which falls within a range from 0.5 to 1.5%.
• P is an element which deteriorates corrosion resistance such as CO 2 corrosion resistance. Hence, it is desirable to reduce the content of P as much as possible. However, the extreme reduction of the content of P pushes up a manufacturing cost.
• the content P is limited to 0.020% or less.
• the content of P is preferably limited to 0.015% or less.
• S is an element which remarkably deteriorates hot workability in a tube manufacturing step. Although it is desirable to decrease the content of S as much as possible, the tube can be manufactured in a usual step by decreasing the content of 5 to 0.010% or less. Hence, the content of S is limited to 0.010% or less. The content of S is preferably limited to 0.003% or less.
• Al is an element which possesses a strong deoxidizing action. To acquire such an advantageous effect, it is desirable that the stainless steel contains 0.001% or more of Al. However, when the content of Al exceeds 0.10%, Al adversely influences toughness. Accordingly, the content of Al is limited to 0.10% or less. The content of Al is preferably limited to 0.05% or less.
• the content of Nb falls within the above-mentioned content range and, further, a following formula (I) is satisfied in view of the relationship between the content Nb and the contents of C, Al and N:
• the stainless steel cannot possess both of desired high strength (yield strength: 95 ksi or more) and high toughness (fracture transition temperature vTrs in a Charpy impact test is ⁇ 40° C. or below).
• the stainless steel may include one or two kinds of components in a group A and a group B described hereinafter:
• Both Cu and Mo are elements which have a function of enhancing corrosion resistance and the stainless steel may selectively contain these elements when necessary.
• Cu is an element which has a function of strengthening a protective film thus enhancing pitting resistance, and it is desirable to set the content of Cu to 0.2% or more to acquire such an advantageous effect.
• the content of Cu exceeds 2.0%, Cu or a part of Cu compound precipitates thus lowering toughness.
• the content of Cu is preferably limited to 2.0% or less.
• the content of Cu is more preferably limited to a value which falls within a range from 0.2 to 1.0%.
• Mo is an element which has a function of increasing resistance against pitting by Cl ⁇ , and it is desirable to set the content of Mo to 0.2% or more to acquire such an advantageous effect.
• the content of Mo exceeds 2.0%, the strength of the stainless steel is lowered and, at the same time, a manufacturing cost sharply rises. Accordingly, the content of Mo is preferably limited to 2.0% or less.
• the content of Mo is more preferably limited to a value which falls within a range from 0.2 to 1.0%.
• Group B one kind or two or more kinds selected from 0.20% or less V, 0.10% or less Ti, 0.005% or less B.
• V, Ti and B are elements which increase strength of the stainless steel, and the stainless steel may selectively contain one kind or two or more kinds of these elements when necessary.
• the stainless steel contains 0.02 or more V, 0.02% or more Ti, 0.0015% or more B.
• V exceeds, 0.20%
• Ti exceeds 0.10%
• B exceeds 0.005%
• toughness is lowered.
• the stainless steel contains these elements, it is desirable to set the content of V to 0.20 or less, the content of Ti to 0.10% or less, and the content of B to 0.005% or less. It is more preferable to set the content of V to 0.02 to 0.10%, the content of Ti to 0.02 to 0.05%, and the content of B to 0.0015 to 0.0040%.
• a balance of the stainless steel except for the above-mentioned components is formed of Fe and unavoidable impurities.
• unavoidable impurities 0.010% or less of 0 is allowable.
• the seamless steel tube for OCTG has the structure which is mainly in a tempered martensite phase and in which precipitated Nb is dispersed.
• the structure may contain 5 volume % or less of a delta ferrite and 5 volume % or less of austenite respectively.
• the content of delta ferrite may preferably be set to 2 volume % or less.
• the content of austenite may preferably be set to 2 volume % or less.
• a precipitated Nb quantity is set to 0.020 mass % or more in terms of Nb.
• the precipitated Nb quantity is preferably set to 0.025 mass % or more in terms of Nb.
• the seamless steel tube for OCTG does not contain M 3 c type Cr type carbide.
• the precipitated Nb quantity is determined such that an electrolytic residue obtained by electrolytic extraction using an electrolytic extraction method is subject to a chemical analysis, thus obtaining a quantity of Nb contained in the electrolytic residue, and the obtained quantity of Nb is used as the precipitated Nb quantity contained in a sample.
• the precipitated Nb is mainly formed on Nb carbide or Nb carbonitride.
• the precipitated Nb is a precipitated material having a spherical shape with an average particle size of 3 nm to 15 nm.
• molten steel having the above-mentioned composition is produced by a usually known melting method such as a steel converter, an electric furnace, a vacuum melting furnace, a molten state is formed into a steel tube raw material such as billets by a usual method such as continuous casting, ingot casting or blooming method. Then, these steel tube material is heated and is formed into a seamless steel tube having a desired size by hot working using manufacturing steps of a usual Mannesmann-plug mill method or a usual Mannesmann-mandrel mill method, and the seamless steel tube is used as a starting raw material.
• a seamless steel tube may be manufactured by hot extruding using a press method. Further, after forming the seamless steel tube, it is desirable to cool the seamless steel tube to a room temperature at a cooling rate of air cooling or more.
• the starting material (seamless steel tube) is firstly subject to quenching.
• Quenching implies treatment in which the seamless steel tube is reheated to a quenching temperature of A 3 transformation temperature or above and, thereafter, the seamless steel tube is cooled to a temperature zone of 100° C. or below from the quenching temperature at a cooling rate of air cooling or more. Due to such quenching, it is possible to form the structure of the starting material into the fine martensitic structure.
• the quenching heating temperature is below the A c3 transformation temperature, the temperature of the seamless steel tube cannot be heated at a austenite single phase region so that the sufficient martensitic structure cannot be formed by cooling after heating. Hence, the seamless steel tube cannot ensure desired strength (yield strength: 95 ksi or more). Accordingly, the heating temperature of quenching is limited to an A c3 transformation temperature or above. The heating temperature is preferably set to 1000° C. or below.
• cooling from the quenching heating temperature is performed until a temperature zone of 100° C. or below at a cooling rate of air cooling or more.
• the starting material possesses the high quenching property.
• the seamless steel tube can acquire the sufficient quenching structure (martensitic structure).
• the seamless steel tube to which quenching is applied is subsequently subject to tempering.
• Tempering is an important treatment for ensuring excellent low-temperature toughness.
• the seamless steel tube is heated to a tempering temperature which is 550° C. or more, and preferably A c1 transformation temperature or below, the heating is preferably held for 30 minutes or more and, thereafter, the seamless steel tube is cooled down preferably to a room temperature preferably at a cooling rate of air cooling or more. Due to such tempering, it is possible to produce the seamless steel tube which possesses both high strength of YS of 95 ksi or more and the excellent low-temperature toughness of vTrs of ⁇ 40° C. or below.
• the temperature at straightening is set to a tempering temperature or above, the structure is changed.
• straightening may be applied to the seamless steel tube for straightening the deformed steel tube shape following the tempering. It is preferable to perform straightening in a temperature zone of 450° C. or above. When the temperature at the straightening is less than 450° C., a working strain is locally generated in the steel tube at the time of performing straightening so that irregularities are liable to occur in mechanical properties, and particularly yield strength YS. Accordingly, when straightening is performed, the straightening is performed within a temperature zone of 450° C. or above.
• the desired irregularities ( ⁇ YS) of yield strength YS is 15 MPa or less.
• the seamless steel tube manufactured by the above-mentioned manufacturing method is formed into the martensitic stainless steel seamless tube which has the above-mentioned composition and structure, and possesses high strength of yield strength of 95 ksi or more (655 MPa or more) and the excellent low-temperature toughness of fracture transition temperature vTrs of ⁇ 40° C. or below in a Charpy impact test, and further possesses the sufficient corrosion resistance as the oil country tubular goods.
• the molten steel having the composition shown in Table 1 is degassed and, thereafter, billets (size: 207 mm ⁇ ) are formed by casting by a continuous casting method and are used as steel tube materials. These steel tube materials are heated, and are formed into tubes by hot working through Mannesmann-method manufacturing steps and, thereafter, the tubes are air-cooled so as to form seamless steel tubes (outer diameter of 177.8 mm ⁇ wall thickness of 12.65 mm).
• Specimens are sampled from the obtained seamless steel tubes, and quenching and tempering are applied and optional straightening is further applied to the specimens (steel tubes) under conditions shown in Table 2 and Table 3.
• Electrolytic extraction specimens are sampled from the specimens (steel tubes) to which quenching and tempering are applied and optional straightening is further applied. Using the sampled electrolytic extraction specimen, a quantity of Nb contained in an obtained electrolytic residue is obtained using an electrolytic extraction method and is determined as a quantity of precipitated Nb contained in the specimen.
• a strip specimen specified by API standard 5CT is sampled, and a tensile test is carried out on the strip specimen thus obtaining tensile characteristics (yield strength YS, tensile strength TS) of the strip specimen.
• tensile characteristics yield strength YS, tensile strength TS
• an increment ⁇ YS of YS by straightening is obtained.
• the tensile test is carried out on the non-straightened steel tubes manufactured under the same condition except for straightening, thus obtaining tensile characteristics (yield strength YS, tensile strength TS) of the strip specimens.
• An increment ⁇ YS of YS by straightening is calculated by a following formula:
• ⁇ YS ( YS of straightened steel tube) ⁇ ( YS of non-straightened steel tube).
• V-notch specimens (thickness: 10 mm) are sampled from specimens to which quenching and tempering are applied and optional straightening is further applied in accordance with the stipulation of JIS Z 2242.
• the V-notch specimens are subject to a Charpy impact test where a fracture transition temperature vTrs is obtained and toughness is evaluated.
• a corrosion specimen having a thickness of 3 mm, a width of 30 mm and a length of 40 mm is prepared by machining from the specimens, and a corrosion test is carried on the corrosion specimen.
• the corrosion test is carried out in such a manner that the corrosion specimen is immersed into a test solution: 20% NaCl aqueous solution (temperature of the test solution: 80° C., under CO 2 gas atmosphere of 30 atmospheric pressure) held in an autoclave, and an test duration is 1 week (168 h).
• a corrosion rate is calculated based on the reduction of weight before and after the corrosion test.
• All of our examples are martensitic stainless steel seamless tubes which exhibit the sufficient corrosion resistance, possess both of high strength of YS of 95 ksi or more (655 MPa or more) and excellent low-temperature toughness of vTrs of ⁇ 40° C. or less, and allows hot straightening at a temperature of 450° C. or more so that an increment of yield strength is small and the difference ⁇ YS of the average YS is small (15 MPa or less) even when straightening is applied.
• YS 95 ksi or more
• vTrs low-temperature toughness of vTrs

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• 2008
  • 2008-12-24 EP EP08876878.3A patent/EP2322679B1/en active Active
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