WO2020071219A1 - サワー環境での使用に適した継目無鋼管 - Google Patents
サワー環境での使用に適した継目無鋼管Info
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- WO2020071219A1 WO2020071219A1 PCT/JP2019/037758 JP2019037758W WO2020071219A1 WO 2020071219 A1 WO2020071219 A1 WO 2020071219A1 JP 2019037758 W JP2019037758 W JP 2019037758W WO 2020071219 A1 WO2020071219 A1 WO 2020071219A1
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- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
- C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
- C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
- C21D9/085—Cooling or quenching
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
Definitions
- the present invention relates to a steel pipe, and more particularly, to a seamless steel pipe.
- oil wells Due to the deepening of oil wells and gas wells (hereinafter, oil and gas wells are simply referred to as “oil wells”), higher strength steel pipes for oil wells are required. Specifically, steel pipes for oil wells of 80 ksi class (yield strength is less than 80 to 95 ksi, ie, less than 552 to 655 MPa) and 95 ksi class (yield strength is less than 95 to 110 ksi, ie, less than 655 to 758 MPa) are widely used. Recently, a steel pipe for an oil well of 110 ksi class (yield strength of 110 to 125 ksi, that is, 758 to 862 MPa) has been required.
- sour environment means an environment that contains hydrogen sulfide and is acidified. Note that the sour environment may contain carbon dioxide. Oil well steel pipes used in such a sour environment are required to have not only high strength but also sulfide stress cracking resistance (Sulfide / Stress / Cracking resistance: hereinafter referred to as SSC resistance).
- SSC resistance sulfide stress cracking resistance
- Patent Document 1 JP-A-2000-256784
- Patent Document 2 JP-A-2000-297344
- Patent Document 3 JP-A-2005-350754
- Patent Document 4 Japanese Patent Application Laid-Open No. 2012-26030
- Patent Document 5 International Publication No. 2010/150915
- the steel for a high-strength oil well disclosed in Patent Document 1 is, by weight%, C: 0.2 to 0.35%, Cr: 0.2 to 0.7%, Mo: 0.1 to 0.5%. , V: 0.1 to 0.3%.
- the total amount of precipitated carbide is 2 to 5% by weight, of which the proportion of MC type carbide is 8 to 40% by weight, and the prior austenite particle size is 11 or more in the particle size number specified by ASTM.
- Patent Document 1 describes that the high-strength oil well steel is excellent in toughness and sulfide stress corrosion cracking resistance.
- the oil well steel disclosed in Patent Document 2 is, by mass%, C: 0.15 to 0.3%, Cr: 0.2 to 1.5%, Mo: 0.1 to 1%, and V: 0 It is made of a low alloy steel containing 0.05 to 0.3% and Nb: 0.003 to 0.1%.
- the total amount of precipitated carbide is 1.5 to 4% by mass, and the proportion of MC type carbide to the total amount of carbide is 5 to 45% by mass, and the proportion of M 23 C 6 type carbide is t (Mm) and (200 / t) mass% or less.
- Patent Document 2 describes that the oil well steel is excellent in toughness and sulfide stress corrosion cracking resistance.
- the low alloy steel for oil country tubular goods disclosed in Patent Document 3 is, by mass%, C: 0.20 to 0.35%, Si: 0.05 to 0.5%, Mn: 0.05 to 1.0%. , P: 0.025% or less, S: 0.010% or less, Al: 0.005 to 0.10%, Cr: 0.1 to 1.0%, Mo: 0.5 to 1.0%, Ti: 0.002 to 0.05%, V: 0.05 to 0.3%, B: 0.0001 to 0.005%, N: 0.01% or less, O (oxygen): 0.01% Contains: The half width H and the hydrogen diffusion coefficient D (10 ⁇ 6 cm 2 / s) satisfy the expression (30H + D ⁇ 19.5). Patent Document 3 describes that the low-alloy oil country tubular steel has excellent SSC resistance even when it has a high yield strength (YS) of 861 MPa or more.
- YS yield strength
- the oil well steel pipe disclosed in Patent Document 4 is, by mass%, C: 0.18 to 0.25%, Si: 0.1 to 0.3%, Mn: 0.4 to 0.8%, P : 0.015% or less, S: 0.005% or less, Al: 0.01 to 0.1%, Cr: 0.3 to 0.8%, Mo: 0.5 to 1.0%, Nb: The composition contains 0.003 to 0.015%, Ti: 0.002 to 0.05%, and B: 0.003% or less, with the balance being Fe and unavoidable impurities.
- the microstructure of the steel pipe for an oil well described above has a tempered martensite phase as a main phase, and an M 3 C or M 2 having a major axis of 300 nm or more when the aspect ratio is 3 or less and the carbide shape is an ellipse included in a 20 ⁇ m ⁇ 20 ⁇ m region.
- the number of C is 10 or less
- M 23 C 6 is less than 1% by mass%
- needle-like M 2 C is precipitated in the grains
- Nb is precipitated as carbide having a size of 1 ⁇ m or more.
- the amount is less than 0.005% by mass.
- Patent Document 4 describes that the oil well steel pipe has excellent sulfide stress cracking resistance even if the yield strength is 862 MPa or more.
- the seamless steel pipe for oil wells disclosed in Patent Document 5 is 0.15 to 0.50% C, 0.1 to 1.0% Si, and 0.3 to 1.0% Mn in mass%.
- P 0.015% or less
- S 0.005% or less
- Al 0.01 to 0.1%
- N 0.01% or less
- Cr 0.1 to 1.7%
- Mo 0 0.4 to 1.1%
- V 0.01 to 0.12%
- Nb 0.01 to 0.08%
- B 0.0005 to 0.003%
- a solid solution of Mo Mo is contained in an amount of 0.40% or more, with the balance being Fe and unavoidable impurities.
- the microstructure of the above-mentioned oil well steel pipe has a tempered martensite phase as a main phase, a prior austenite grain size of 8.5 or more in particle size, and a substantially particulate M 2 C type precipitate of 0.06 mass% or more.
- a dispersed organization Patent Document 5 describes that the above-mentioned seamless steel pipe for oil wells has both high strength of 110 ksi class and excellent sulfide stress cracking resistance.
- Patent Documents 1 to 5 propose a steel pipe for an oil well which is adjusted to a desired yield strength and has excellent SSC resistance.
- HIC hydrogen-induced cracking
- SSC hydrogen-induced cracking
- HIC may occur in seamless steel pipes being used as oil well steel pipes.
- seamless steel pipes having a yield strength of 110 ksi class (758 to 862 MPa) almost no studies have been made on the HIC resistance.
- An object of the present disclosure is to provide a seamless steel pipe having a yield strength of 758 to 862 MPa (110 to 125 ksi, 110 ksi class) and having excellent HIC resistance.
- the seamless steel pipe according to the present disclosure is, by mass%, C: 0.15 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030. %, S: 0.0050% or less, Al: 0.005 to 0.070%, Cr: 0.30 to 1.50%, Mo: 0.25 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, rare earth element: 0.0001 to 0.0015%, Ca: 0.0001 to 0.0100%, N: 0.0100% or less, O: 0.0020% or less, V: 0 to 0.30%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Co: 0 to 1.00 %, W: 0 to 1.00%, Ni: 0 to 0.50%, and Cu: 0 to 0.50%, with the balance being Fe and impurities It made, having a chemical composition satisfying the formula (1).
- the maximum major axis of the inclusions in the seamless steel pipe predicted by the extreme value statistical processing is 150 ⁇ m or less.
- the seamless steel pipe according to the present disclosure has a yield strength of 758 to 862 MPa. (Ca / O + Ca / S + 0.285 ⁇ REM / O + 0.285 ⁇ REM / S) ⁇ (Al / Ca) ⁇ 40.0 (1)
- the content (% by mass) of the corresponding element is substituted for the element symbol in the formula (1).
- the seamless steel pipe according to the present disclosure has a yield strength of 758 to 862 MPa (110 ksi class) and has excellent HIC resistance.
- FIG. 1 is a diagram showing the relationship between the predicted maximum major axis of inclusions and HIC resistance.
- FIG. 2 is a schematic diagram showing the distribution of inclusions in the observation visual field when obtaining the predicted maximum major axis of the inclusions according to the present embodiment.
- the present inventors have investigated and examined the HIC resistance of a seamless steel pipe which is supposed to be used in a sour environment and has a yield strength (Yield Strength) of 758 to 862 MPa (110 ksi class), and has obtained the following knowledge.
- the present inventors first found that C: 0.15 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0. 030% or less, S: 0.0050% or less, Al: 0.005 to 0.070%, Cr: 0.30 to 1.50%, Mo: 0.25 to 2.00%, Ti: 0.002 0.020%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, rare earth element: 0.0001 to 0.0015%, Ca: 0.0001 to 0.0100% , N: 0.0100% or less, O: 0.0020% or less, V: 0 to 0.30%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Co: 0 to 1.0.
- the present inventors have studied in detail the relationship between coarse inclusions and HIC resistance. As a result, the following findings were obtained. If coarse inclusions are present in the seamless steel pipe, stress concentration is likely to occur at the interface between the inclusions and the base material. In this case, HIC is generated starting from the inclusion. Furthermore, among the coarse inclusions, those having a long major axis tend to cause stress concentration particularly at the interface with the base material. Therefore, in the case where an inclusion having a long major axis exists in the seamless steel pipe, the HIC resistance of the seamless steel pipe decreases. That is, in order to increase the HIC resistance of the seamless steel pipe, it is not necessary to simply reduce coarse inclusions, but to reduce inclusions having a long major axis.
- the particle diameter for example, the equivalent circle diameter or the square root of the area
- the long diameter of the inclusion obtained by microscopic observation have been used.
- the conventional microscope observation it is possible to observe the inclusions in the seamless steel pipe, but only to observe the average distribution of the inclusions such as the number density in several fields of view.
- it is determined whether or not there is an inclusion having a long major axis by a conventional microscope observation it is necessary to increase the number of fields of the microscopic observation and widen the visual field area.
- the number of visual fields for microscopic observation is easily increased, the time and cost required for performing microscopic observation increase.
- the present inventors thought that the major axis of the inclusions in the seamless steel pipe could be predicted using statistical processing.
- the extreme value statistical processing is a method of acquiring an extreme value (for example, the maximum major axis of an inclusion) in each visual field and estimating a probability distribution in a plurality of visual fields.
- an extreme value for example, the maximum major axis of an inclusion
- the present inventors have determined the relationship between the maximum major axis diameter of inclusions in a seamless steel pipe (hereinafter, also simply referred to as “predicted maximum major axis diameter of inclusions”) and the HIC resistance predicted by the extreme value statistical processing. investigated.
- FIG. 1 is a diagram showing the relationship between the predicted maximum major axis of inclusions and HIC resistance.
- FIG. 1 shows a predicted maximum major axis diameter Dmax ( ⁇ m) of inclusions obtained by a method described below for a seamless steel pipe having the above-described chemical composition and having a yield strength of 110 ksi class among the examples described below; It was prepared using the crack area ratio CAR (%) obtained by the HIC test described later.
- the yield strength of the seamless steel pipe shown in FIG. 1 was adjusted by adjusting the tempering temperature. Further, regarding the HIC resistance, when the crack area ratio CAR was less than 3.0%, it was determined that the HIC resistance was good.
- the down arrow in FIG. 1 means that the crack area ratio CAR is lower than the plot position shown.
- the seamless steel pipe according to the present embodiment satisfies the above-described chemical composition, has a yield strength of the 110 ksi class, and further has a predicted maximum major diameter Dmax of the inclusion of 150 ⁇ m or less.
- the seamless steel pipe according to the present embodiment has a crack area ratio CAR of less than 3.0% and exhibits excellent HIC resistance.
- the seamless steel pipe according to the present embodiment completed on the basis of the above findings has, in mass%, C: 0.15 to 0.45%, Si: 0.05 to 1.00%, and Mn: 0.01 to 1 0.000%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.070%, Cr: 0.30 to 1.50%, Mo: 0.25 to 2.
- the maximum major axis of the inclusions in the seamless steel pipe predicted by the extreme value statistical processing is 150 ⁇ m or less.
- the seamless steel pipe according to the present embodiment has a yield strength of 758 to 862 MPa. (Ca / O + Ca / S + 0.285 ⁇ REM / O + 0.285 ⁇ REM / S) ⁇ (Al / Ca) ⁇ 40.0 (1)
- the content (% by mass) of the corresponding element is substituted for the element symbol in the formula (1).
- the above chemical composition may contain V: 0.01 to 0.30%.
- the chemical composition may contain one or more kinds selected from the group consisting of Mg: 0.0001 to 0.0100% and Zr: 0.0001 to 0.0100%.
- the chemical composition may contain one or more selected from the group consisting of Co: 0.02 to 1.00% and W: 0.02 to 1.00%.
- the chemical composition may include one or more selected from the group consisting of Ni: 0.01 to 0.50% and Cu: 0.01 to 0.50%.
- the seamless steel pipe may be an oil well steel pipe.
- the oil country tubular goods may be oil country tubular goods.
- the oil country tubular goods are, for example, steel pipes used for casing and tubing applications.
- the seamless steel pipe according to the present embodiment is an oil well steel pipe, it has a yield strength of 758 to 862 MPa (110 ksi class) even when the wall thickness is 15 mm or more, and has excellent HIC resistance in a sour environment. Have.
- ⁇ ⁇ Excellent HIC resistance in the sour environment can be evaluated by a method based on NACETM 0284-2011. Specifically, it can be evaluated by the following method. A mixed aqueous solution of 5.0 mass% sodium chloride and 0.5 mass% acetic acid (NACE solution A) is used as a test solution.
- test piece made from a seamless steel pipe is immersed in a test solution at 24 ° C. After degassing the test solution, 1 atm of H 2 S is sealed to form a test bath. After holding the test bath for 96 hours with stirring, the test piece is taken out. The test piece taken out is subjected to an ultrasonic flaw detection test (C scan), and the area of the indication portion (HIC generation portion) is determined.
- C scan ultrasonic flaw detection test
- CAR (%) (area of indication portion / projected area) ⁇ 100 (2)
- the seamless steel pipe according to the present embodiment has a crack area ratio CAR (%) of less than 3.0% after 96 hours in the HIC resistance test.
- the chemical composition of the seamless steel pipe according to the present embodiment contains the following elements.
- Carbon (C) enhances the hardenability of the steel material and increases the yield strength of the steel material. C further promotes spheroidization of carbides during tempering during the manufacturing process, and further increases the yield strength of the steel material. If the C content is too low, these effects cannot be obtained. On the other hand, if the C content is too high, the toughness of the steel material decreases, and quenching tends to occur. Therefore, the C content is 0.15 to 0.45%.
- a preferred lower limit of the C content is 0.18%, more preferably 0.20%, further preferably 0.22%, and further preferably 0.24%.
- a preferred upper limit of the C content is 0.40%, more preferably 0.35%, further preferably 0.33%, and still more preferably 0.30%.
- Si 0.05-1.00% Silicon (Si) deoxidizes steel. If the Si content is too low, this effect cannot be obtained. On the other hand, if the Si content is too high, the SSC resistance of the steel material decreases. Therefore, the Si content is 0.05 to 1.00%.
- the lower limit of the preferred Si content is 0.15%, more preferably 0.20%.
- the preferable upper limit of the Si content is 0.85%, more preferably 0.70%, further preferably 0.60%, further preferably 0.50%, and still more preferably 0.45%. %, More preferably 0.40%.
- Mn 0.01-1.00%
- Manganese (Mn) deoxidizes steel. Mn further enhances the hardenability of the steel material and increases the yield strength of the steel material. If the Mn content is too low, these effects cannot be obtained. On the other hand, if the Mn content is too high, Mn segregates at the grain boundaries together with impurities such as P and S. As a result, the HIC resistance of the steel material decreases. If the Mn content is too high, MnS, which is an inclusion that is easily stretched, further increases. As a result, the predicted maximum major axis length of the inclusion becomes longer, and the HIC resistance of the steel material decreases. Therefore, the Mn content is 0.01 to 1.00%.
- a preferred lower limit of the Mn content is 0.02%, more preferably 0.03%.
- a preferred upper limit of the Mn content is 0.90%, more preferably 0.80%, further preferably 0.70%, further preferably 0.60%, and still more preferably 0.55%. %, More preferably 0.50%.
- Phosphorus (P) is an impurity. That is, the P content is more than 0%. P segregates at the grain boundary and embrittles the steel material. As a result, the HIC resistance of the steel material decreases. Therefore, the P content is 0.030% or less.
- the preferable upper limit of the P content is 0.025%, and more preferably 0.020%.
- the P content is preferably as low as possible. However, an extreme decrease in the P content greatly increases the manufacturing cost. Therefore, in consideration of industrial production, the lower limit of the P content is preferably 0.0001%, more preferably 0.0003%, further preferably 0.001%, and still more preferably 0.002%. It is.
- S 0.0050% or less Sulfur (S) is an impurity. That is, the S content is more than 0%. S segregates at the grain boundaries and embrittles the steel. As a result, the HIC resistance of the steel material decreases. S further combines with Mn to form MnS. MnS is an inclusion that is easily stretched, and as MnS increases, the predicted maximum major diameter of the inclusion increases. As a result, the HIC resistance of the steel material decreases. Therefore, the S content is 0.0050% or less. A preferable upper limit of the S content is 0.0045%, more preferably 0.0035%, further preferably 0.0030%, and further preferably 0.0025%. The S content is preferably as low as possible. However, an extreme reduction in the S content greatly increases the manufacturing cost. Therefore, in consideration of industrial production, a preferable lower limit of the S content is 0.0001%, and more preferably 0.0003%.
- Al 0.005 to 0.070%
- Aluminum (Al) deoxidizes steel. If the Al content is too low, this effect cannot be obtained. On the other hand, if the Al content is too high, coarse inclusions are formed in the steel material, and the predicted maximum major diameter of the inclusions increases. As a result, the HIC resistance of the steel material decreases. Therefore, the Al content is 0.005 to 0.070%.
- a preferred lower limit of the Al content is 0.010%, more preferably 0.015%.
- the preferable upper limit of the Al content is 0.060%, more preferably 0.050%, further preferably 0.045%, further preferably 0.040%, and still more preferably 0.035%. %.
- the “Al” content referred to in the present specification means “acid-soluble Al”, that is, the content of “sol. Al”.
- Chromium (Cr) enhances the hardenability of the steel material and increases the yield strength of the steel material. If the Cr content is too low, this effect cannot be obtained. On the other hand, if the Cr content is too high, coarse carbides are generated in the steel material, and the SSC resistance of the steel material decreases. Therefore, the Cr content is 0.30 to 1.50%.
- a preferable lower limit of the Cr content is 0.32%, more preferably 0.35%, further preferably 0.40%, further preferably 0.45%, and still more preferably 0.50%. %.
- a preferable upper limit of the Cr content is 1.40%, more preferably 1.30%, further preferably 1.25%, and further preferably 1.10%.
- Mo 0.25 to 2.00% Molybdenum (Mo) enhances the hardenability of the steel material and increases the yield strength of the steel material. If the Mo content is too low, this effect cannot be obtained. On the other hand, if the Mo content is too high, the above effect is saturated. Therefore, the Mo content is 0.25 to 2.00%.
- a preferable lower limit of the Mo content is 0.30%, more preferably 0.40%, further preferably 0.45%, further preferably 0.50%, and further preferably 0.55%. %, More preferably 0.60%.
- the preferable upper limit of the Mo content is 1.70%, more preferably 1.50%, further preferably 1.40%, and still more preferably 1.30%.
- Titanium (Ti) combines with N to form fine nitrides and refines crystal grains by a pinning effect. As a result, the yield strength of the steel material increases. If the Ti content is too low, this effect cannot be obtained. On the other hand, if the Ti content is too high, coarse Ti nitrides are formed in the steel material, and the HIC resistance of the steel material decreases. Therefore, the Ti content is 0.002 to 0.020%.
- a preferred lower limit of the Ti content is 0.003%, more preferably 0.004%.
- the preferable upper limit of the Ti content is 0.018%, more preferably 0.015%, further preferably 0.012%, and further preferably 0.010%.
- Niobium (Nb) combines with C to form fine carbides. As a result, the yield strength of the steel material increases. If the Nb content is too low, this effect cannot be obtained. On the other hand, if the Nb content is too high, carbides, nitrides, or carbonitrides (hereinafter, referred to as “carbonitrides”) may be excessively formed. In this case, the HIC resistance of the steel material decreases. Therefore, the Nb content is 0.002 to 0.100%.
- a preferred lower limit of the Nb content is 0.003%, more preferably 0.007%, further preferably 0.010%, further preferably 0.015%, and still more preferably 0.020%. %.
- the preferable upper limit of the Nb content is 0.080%, more preferably 0.050%, further preferably 0.040%, and further preferably 0.030%.
- B 0.0005 to 0.0040% Boron (B) forms a solid solution in steel to enhance the hardenability of the steel material and increase the yield strength of the steel material. If the B content is too low, this effect cannot be obtained. On the other hand, if the B content is too high, coarse B nitrides are formed, and the HIC resistance of the steel material decreases. Therefore, the B content is 0.0005 to 0.0040%.
- a preferred lower limit of the B content is 0.0008%, and more preferably 0.0010%.
- the preferable upper limit of the B content is 0.0030%, more preferably 0.0025%, further preferably 0.0020%, further preferably 0.0018%, and still more preferably 0.0015%. %.
- Rare earth element 0.0001 to 0.0015%
- Rare earth elements (REM) reduce FeO.
- cluster formation of Al 2 O 3 is suppressed, and Al 2 O 3 , X 2 O 3 and X 2 OS (X is REM) are formed.
- REM further combines with P in the steel material to suppress segregation of P at the grain boundaries.
- the HIC resistance of the steel material increases. If the REM content is too low, these effects cannot be obtained. On the other hand, if the REM content is too high, coarse inclusions are formed in the steel material, and the predicted maximum major axis length of the inclusions increases.
- the REM content is 0.0001 to 0.0015%.
- a preferred lower limit of the REM content is 0.0002%, more preferably 0.0003%, further preferably 0.0004%, further preferably 0.0005%, and still more preferably 0.0006%. %.
- a preferable upper limit of the REM content is 0.0012%, more preferably 0.0011%, further preferably 0.0010%, and further preferably 0.0009%.
- REM refers to scandium (Sc) having an atomic number of 21; yttrium (Y) having an atomic number of 39; and lanthanide, a lanthanum having an atomic number of 57 (La) to an atomic number of 71. At least one element selected from the group consisting of lutetium (Lu). Further, the REM content in this specification is the total content of these elements.
- Ca 0.0001 to 0.0100%
- Calcium (Ca) spheroidizes the inclusions in the steel and reduces the predicted maximum major diameter of the inclusions. As a result, the HIC resistance of the steel material increases. If the Ca content is too low, this effect cannot be obtained. On the other hand, if the Ca content is too high, coarse oxide-based inclusions are formed in the steel material, and the HIC resistance of the steel material decreases. Therefore, the Ca content is 0.0001 to 0.0100%.
- a preferable lower limit of the Ca content is 0.0002%, more preferably 0.0003%, further preferably 0.0005%, further preferably 0.0006%, and still more preferably 0.0008. %, More preferably 0.0010%.
- a preferred upper limit of the Ca content is 0.0040%, more preferably 0.0030%, further preferably 0.0025%, further preferably 0.0020%, and still more preferably 0.0017%. %, And more preferably 0.0015%.
- N 0.0100% or less Nitrogen (N) is inevitably contained. That is, the N content is more than 0%. N combines with Ti to form fine nitrides and refines crystal grains by a pinning effect. As a result, the yield strength of the steel material increases. On the other hand, if the N content is too high, coarse Ti nitrides are formed in the steel material, and the HIC resistance of the steel material decreases. Therefore, the N content is 0.0100% or less.
- the preferable upper limit of the N content is 0.0050%, more preferably 0.0045%.
- a preferable lower limit of the N content for more effectively obtaining the above effects is 0.0015%, more preferably 0.0020%, further preferably 0.0025%, and further preferably 0.0030%. It is.
- Oxygen (O) is an impurity. That is, the O content is more than 0%. O forms coarse oxide-based inclusions and increases the predicted maximum major diameter of the inclusions. As a result, the HIC resistance of the steel material decreases. Therefore, the O content is 0.0020% or less.
- the preferable upper limit of the O content is 0.0019%, more preferably 0.0018%, further preferably 0.0016%, and further preferably 0.0015%.
- the O content is preferably as low as possible. However, an extreme decrease in the O content greatly increases the production cost. Therefore, in consideration of industrial production, a preferable lower limit of the O content is 0.0001%, and more preferably 0.0003%.
- the balance of the chemical composition of the steel material according to the present embodiment consists of Fe and impurities.
- the impurities are ores as raw materials, scrap, or are mixed from the production environment or the like when industrially producing the steel material, and are in a range that does not adversely affect the steel material according to the present embodiment. Means acceptable.
- the chemical composition of the steel material described above may further contain V instead of part of Fe.
- V Vanadium
- V is an optional element and may not be contained. That is, the V content may be 0%. When contained, V forms fine carbides during tempering and increases the yield strength of the steel material. This effect can be obtained to some extent if V is contained at all. However, if the V content is too high, the toughness of the steel material decreases. Therefore, the V content is 0 to 0.30%.
- a preferred lower limit of the V content is more than 0%, more preferably 0.01%, further preferably 0.02%, further preferably 0.04%, and still more preferably 0.06%. And more preferably 0.08%.
- the preferable upper limit of the V content is 0.25%, more preferably 0.20%, further preferably 0.15%, and further preferably 0.12%.
- the chemical composition of the above-mentioned steel material may further contain one or more selected from the group consisting of Mg and Zr instead of a part of Fe. All of these elements are optional elements and enhance the HIC resistance of the steel material.
- Mg 0 to 0.0100%
- Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg refines sulfide-based inclusions in the steel material and shortens the predicted maximum major diameter of the inclusions. As a result, the HIC resistance of the steel material increases. This effect can be obtained to some extent if Mg is contained at all. However, if the Mg content is too high, coarse inclusions are formed in the steel material, and the predicted maximum major axis length of the inclusions increases. As a result, the HIC resistance of the steel material decreases. Therefore, the Mg content is 0 to 0.0100%.
- a preferable lower limit of the Mg content is more than 0%, more preferably 0.0001%, further preferably 0.0003%, further preferably 0.0006%, and still more preferably 0.0010%. It is.
- the preferable upper limit of the Mg content is 0.0040%, more preferably 0.0030%, further preferably 0.0025%, and further preferably 0.0020%.
- Zr Zirconium
- Zr Zirconium
- the Zr content may be 0%.
- Zr refines sulfide-based inclusions in the steel material and shortens the predicted maximum major diameter of the inclusions.
- the HIC resistance of the steel material increases. This effect can be obtained to some extent if Zr is contained at all.
- the Zr content is too high, coarse inclusions are formed in the steel material, and the predicted maximum major axis length of the inclusions increases. As a result, the HIC resistance of the steel material decreases. Therefore, the Zr content is 0 to 0.0100%.
- a preferred lower limit of the Zr content is more than 0%, more preferably 0.0001%, further preferably 0.0003%, further preferably 0.0006%, and still more preferably 0.0010%. It is.
- the preferable upper limit of the Zr content is 0.0040%, more preferably 0.0030%, further preferably 0.0025%, and further preferably 0.0020%.
- the chemical composition of the above-mentioned steel material may further contain one or more selected from the group consisting of Co and W instead of a part of Fe.
- Each of these elements is an optional element and forms a protective corrosion film in a sour environment to suppress hydrogen intrusion. Thereby, these elements enhance the HIC resistance of the steel material.
- Co 0 to 1.00%
- Co is an optional element and need not be contained. That is, the Co content may be 0%.
- Co forms a protective corrosion coating in the sour environment and inhibits hydrogen ingress.
- the HIC resistance of the steel material increases. This effect can be obtained to some extent if Co is contained even a little.
- the Co content is 0 to 1.00%.
- a preferable lower limit of the Co content is more than 0%, more preferably 0.02%, further preferably 0.03%, and further preferably 0.05%.
- a preferred upper limit of the Co content is 0.90%, and more preferably 0.80%.
- W 0-1.00% Tungsten (W) is an optional element and need not be contained. That is, the W content may be 0%. When included, W forms a protective corrosion coating in the sour environment and inhibits hydrogen ingress. As a result, the HIC resistance of the steel material increases. This effect can be obtained to some extent if W is contained at all. However, if the W content is too high, coarse carbides are generated in the steel material, and the steel material is embrittled. As a result, the HIC resistance of the steel material decreases. Therefore, the W content is 0 to 1.00%. A preferable lower limit of the W content is more than 0%, more preferably 0.02%, further preferably 0.03%, and further preferably 0.05%. A preferred upper limit of the W content is 0.90%, more preferably 0.80%.
- the chemical composition of the above-mentioned steel material may further contain one or more kinds selected from the group consisting of Ni and Cu instead of part of Fe.
- Each of these elements is an optional element and enhances the hardenability of the steel material and increases the yield strength of the steel material.
- Nickel (Ni) is an optional element and need not be contained. That is, the Ni content may be 0%. When contained, Ni enhances the hardenability of the steel material and increases the yield strength of the steel material. This effect can be obtained to some extent if Ni is contained at all. However, if the Ni content is too high, local corrosion is promoted, and the SSC resistance of the steel material decreases. Therefore, the Ni content is 0 to 0.50%.
- a preferred lower limit of the Ni content is more than 0%, more preferably 0.01%, and still more preferably 0.02%.
- the preferable upper limit of the Ni content is 0.10%, more preferably 0.08%, and further preferably 0.06%.
- Cu 0 to 0.50% Copper (Cu) is an optional element and need not be contained. That is, the Cu content may be 0%. When contained, Cu enhances the hardenability of the steel material and increases the yield strength of the steel material. This effect can be obtained to some extent as long as Cu is contained. However, if the Cu content is too high, the hardenability of the steel material becomes too high, and the toughness of the steel material decreases. Therefore, the Cu content is 0 to 0.50%.
- a preferable lower limit of the Cu content is more than 0%, more preferably 0.01%, further preferably 0.02%, and further preferably 0.05%.
- a preferred upper limit of the Cu content is 0.35%, more preferably 0.25%.
- Fn1 (Ca / O + Ca / S + 0.285 ⁇ REM / O + 0.285 ⁇ REM / S) ⁇ (Al / Ca)
- "0.285" of Fn1 is a coefficient when the REM content is roughly converted to the Ca content.
- “Ca / O + Ca / S + 0.285 ⁇ REM / O + 0.285 ⁇ REM / S” of Fn1 is the sum of the ratio of the Ca content to O and S obtained by converting the REM content to the Ca content.
- Al / Ca” of Fn1 is an index of the melting point of inclusions.
- Fn1 is 40.0 or more.
- a preferred lower limit of Fn1 is 41.0, more preferably 42.0.
- a preferred upper limit of Fn1 is 140.0, and more preferably 130.0.
- the maximum major axis diameter Dmax of inclusions (predicted maximum major axis diameter of inclusions) in the seamless steel pipe, which is predicted by the extreme value statistical processing, is 150 ⁇ m or less. If the predicted maximum major diameter Dmax of the inclusions exceeds 150 ⁇ m, the CAR of the seamless steel pipe becomes 3.0% or more, and the HIC resistance of the seamless steel pipe decreases. Therefore, the predicted maximum major axis diameter Dmax of the inclusion is 150 ⁇ m or less.
- the preferred upper limit of the predicted maximum major axis diameter Dmax of the inclusion is 148 ⁇ m, more preferably 145 ⁇ m. It is preferable that the predicted maximum major axis diameter Dmax of the inclusion is as small as possible.
- the predicted maximum major axis diameter Dmax of the inclusion can be obtained by the following method. From the center of the thickness of the seamless steel pipe according to the present embodiment, a test piece having an observation surface of 10 mm in the pipe axis direction and 10 mm in the pipe diameter direction is cut out. When the thickness of the seamless steel pipe is less than 10 mm, a test piece having an observation surface of the thickness of the steel pipe in the pipe axis direction of 10 mm and the pipe diameter direction is cut out. After the observation surface of the test piece is polished to a mirror surface, an n-field (n is a natural number) is observed with a secondary electron image using a scanning electron microscope (SEM: Scanning Electron Microscope).
- SEM Scanning Electron Microscope
- the observation field number n is 20 or more.
- the observation field number n is, for example, 108.
- the reference area S0 is 20 mm 2 or more.
- the reference area S0 is, for example, 196.5 mm 2 .
- the maximum major axis Lmax of the inclusion in each field of view can be obtained by image analysis of the observed image.
- the shortest distance between the plurality of inclusions is 40 ⁇ m or less in the tube axis direction and 15 ⁇ m or less in the tube diameter direction, these inclusions are regarded as one individual. Specifically, this point will be described with reference to the drawings.
- FIG. 2 is a schematic diagram showing the distribution of the inclusions in the observation field of view 1 when obtaining the predicted maximum major axis of the inclusions according to the present embodiment.
- FIG. 2 is a diagram for explaining whether two inclusions are regarded as one individual.
- the vertical direction in FIG. 2 corresponds to the tube axis direction. 2 corresponds to the tube radial direction.
- Reference numeral 10 in FIG. 2 indicates an inclusion in the observation visual field 1. Referring to FIG. 2, the shortest distance in the tube axis direction between the inclusions 10 and d L, the pipe diameter direction of the shortest distance between inclusions 10 and d T.
- the plurality of inclusions 10 whose shortest distance d L in the tube axis direction is 40 ⁇ m or less and whose shortest distance d T in the tube diameter direction is 15 ⁇ m or less are regarded as one individual.
- the plurality of inclusions 10 whose distance d L in the tube axis direction exceeds 40 ⁇ m are each regarded as one individual.
- each of the plurality of inclusions 10 whose distance d T in the pipe diameter direction exceeds 15 ⁇ m is also regarded as one individual.
- an approximate straight line (maximum inclusion distribution straight line) is created by the least squares method.
- the created approximate straight line can be represented by the following equation (5).
- yj c ⁇ Lmaxj + d (5)
- c and d are linear coefficients obtained by the least square method.
- T (S + S0) / S0 (6)
- S means the virtual surface area (mm 2 ) at the center of the thickness of the seamless steel pipe.
- S can be obtained by the following equation (7).
- S (Rt) ⁇ ⁇ ⁇ L (7)
- R means the outer diameter (mm) of the seamless steel pipe
- t means the wall thickness (mm) of the seamless steel pipe
- L means the axial length (mm) of the seamless steel pipe.
- L Lmax in the predicted standardized variable y is calculated from the obtained predicted standardized variable y and Expression (5).
- the obtained Lmax is defined as a predicted maximum major axis diameter Dmax ( ⁇ m) of the inclusion.
- the microstructure of the seamless steel pipe according to the present embodiment mainly includes tempered martensite and tempered bainite. Specifically, the microstructure has a total volume fraction of tempered martensite and tempered bainite of 90% or more. The balance of the microstructure is, for example, ferrite or pearlite. Provided that the microstructure of the seamless steel pipe having the above-described chemical composition contains at least 90% by volume of tempered martensite and tempered bainite, the seamless steel pipe is provided on the condition that it satisfies the other provisions of the present embodiment. Has a yield strength of 758 to 862 MPa (110 ksi class), and the yield ratio of the seamless steel pipe is 90.0% or more.
- the total volume ratio of tempered martensite and tempered bainite can be determined by microstructure observation. From the center of the thickness of the seamless steel pipe according to the present embodiment, a test piece having an observation surface of 10 mm in the pipe axis direction and 10 mm in the pipe diameter direction is cut out. When the thickness of the seamless steel pipe is less than 10 mm, a test piece having an observation surface of the thickness of the steel pipe in the pipe axis direction of 10 mm and the pipe diameter direction is cut out. After the observation surface is polished to a mirror surface, the structure is immersed in a 2% nital etching solution for about 10 seconds to reveal the structure by etching. The etched observation surface is observed with a secondary electron image in 10 visual fields using a scanning electron microscope (SEM). The viewing area is 400 ⁇ m 2 (magnification 5000 times).
- SEM scanning electron microscope
- the tempered martensite and tempered bainite can be distinguished from other phases (ferrite or pearlite) by contrast. Therefore, tempered martensite and tempered bainite are specified in each field of view.
- the total area ratio of the specified tempered martensite and tempered bainite is determined.
- the arithmetic average of the sum of the area ratios of the tempered martensite and the tempered bainite, which is obtained from all visual fields, is defined as the volume ratio of the tempered martensite and the tempered bainite.
- the preferred wall thickness is 9 to 60 mm. More preferably, the seamless steel pipe according to the present embodiment is suitable for use as a thick-walled oil well steel pipe. More specifically, even if the seamless steel pipe according to the present embodiment is a steel pipe for oil wells having a thickness of 15 mm or more, and even 20 mm or more, the yield strength of 758 to 862 MPa (110 ksi class) and the excellent HIC resistance And
- the yield strength of the seamless steel pipe according to the present embodiment is 758 to 862 MPa (110 ksi class).
- the yield strength referred to in the present specification means a stress at the time of 0.7% total elongation (0.7% proof stress) obtained in a tensile test.
- the yield strength of the seamless steel pipe according to the present embodiment is of the order of 110 ksi.
- the seamless steel pipe according to the present embodiment further has a yield ratio (YR) of 90.0% or more.
- YS yield strength
- TS tensile strength
- tempered martensite and tempered bainite are 90% by volume in the microstructure. That is all.
- the seamless steel pipe according to the present embodiment can achieve both a 110 ksi-class yield strength and excellent HIC resistance.
- the yield strength and the yield ratio of the seamless steel pipe according to the present embodiment can be obtained by the following method.
- a tensile test is performed by a method according to ASTM E8 / E8M (2013).
- a round bar test piece is collected from the center of the thickness of the seamless steel pipe according to the present embodiment.
- the size of the round bar test piece is, for example, a parallel part diameter of 8.9 mm and a parallel part length of 35.6 mm.
- the axial direction of the round bar test piece is parallel to the tube axis direction of the seamless steel pipe.
- a tensile test is performed at room temperature (25 ° C.) in the atmosphere using a round bar test piece.
- the resulting stress at 0.7% total elongation is defined as the yield strength (MPa).
- the obtained maximum stress during uniform elongation is defined as tensile strength (MPa).
- the yield ratio YR (%) is defined as the ratio of the yield strength YS to the tensile strength TS (Y
- the HIC resistance of the seamless steel pipe according to the present embodiment can be implemented by a method based on NACE TM0284-2011.
- a test piece for an HIC resistance test is produced. Specifically, an arc-shaped member is sampled from the seamless steel pipe according to the present embodiment in the circumferential direction of the pipe. Machine processing is performed so that the two curved surfaces (corresponding to the outer surface and the inner surface of the seamless steel pipe, respectively) of the sampled member become parallel planes. At this time, the thickness of the member is set to a thickness of ⁇ 2 mm of the seamless steel pipe.
- test piece having a rectangular cross section, having a width of 20 mm, a thickness of a seamless steel pipe of ⁇ 2 mm, and a length of 100 mm.
- the length direction of the test piece is parallel to the pipe axis direction of the seamless steel pipe, and the thickness direction of the test piece is parallel to the pipe diameter direction of the seamless steel pipe.
- test solution a mixed aqueous solution of 5.0% by mass of sodium chloride and 0.5% by mass of acetic acid (NACE solution A) is used.
- NACE solution A acetic acid
- the prepared test piece is immersed in a test solution at 24 ° C. N 2 gas is blown into the test solution for 3 hours to degas the test solution. 1 atm of H 2 S is blown into the degassed test solution to form a corrosive environment, which is used as a test bath. Hold the test bath with stirring for 96 hours. Remove the test specimen from the test bath after holding for 96 hours.
- the test piece taken out is subjected to an ultrasonic flaw detection test (C scan), and the area of the indication portion (HIC generation portion) is determined.
- C scan ultrasonic flaw detection test
- the crack area ratio CAR (%) can be obtained from the following equation (2).
- the projection area is, for example, 20 mm ⁇ 100 mm.
- CAR (%) (area of indication portion / projected area) ⁇ 100 (2)
- the seamless steel pipe according to the present embodiment has a crack area ratio CAR (%) of less than 3.0% after 96 hours in the HIC resistance test.
- a method for manufacturing a seamless steel pipe according to the present embodiment will be described.
- the manufacturing method described below is an example of the method for manufacturing a seamless steel pipe according to the present embodiment. That is, the method for manufacturing the seamless steel pipe according to the present embodiment is not limited to the manufacturing method described below.
- One example of the manufacturing method is a steelmaking process of refining and casting molten steel to produce a material (a slab, a steel ingot, or a steel slab), and a hot working process of hot working a material to produce a raw tube. And a quenching step of quenching the raw pipe and a tempering step of tempering the quenched raw pipe.
- Step making process In the steelmaking process, first, refining (primary refining) in a converter is performed on hot metal manufactured by a known method. The secondary refining is performed on the primary refined molten steel. In the secondary refining, a molten steel satisfying the above-described chemical composition is produced by adding an alloy element for adjusting the composition.
- deoxidization is performed on the molten steel discharged from the converter.
- the deoxidizing treatment may be performed by an element other than REM and Ca, and is not particularly limited.
- the deoxidizing treatment according to the present embodiment is performed by adding Al.
- Al is added in the deoxidation treatment, the oxygen concentration in the molten steel can be efficiently reduced. Therefore, in the steelmaking process according to the present embodiment, Al is added in the deoxidizing treatment.
- a residue removing treatment is performed. After the slag removal process, secondary refining is performed.
- RH Rasterstahl-Hausen vacuum degassing
- VAD Vauum Arc Degassing
- alloy components other than REM and Ca are adjusted to have the above-mentioned chemical composition. Then, after adding at least one or more elements of the REM, Ca is added to adjust the alloy components in the molten steel to have the above-described chemical composition.
- REM is added to molten steel, a simple substance of a metal belonging to REM may be used, or a misch metal may be used.
- REM reduces FeO, thereby suppressing Al 2 O 3 cluster formation.
- inclusions Al 2 O 3 , X 2 O 3 and X 2 OS (X is REM) are formed in the molten steel.
- Ca is added to molten steel after these inclusions are formed, fine inclusions, XCaAlOS (X is REM), are formed.
- Calcium aluminate may also be formed immediately after adding REM to molten steel, even when Ca is added. Specifically, if the time during which the molten steel is retained after the addition of REM to the addition of Ca (hereinafter, also referred to as “the molten steel retention time”) is less than 15 seconds, a large number of calcium aluminates are formed, and XCaAlOS (X is REM) formation is prevented. As a result, the maximum major diameter Dmax of the inclusions in the seamless steel pipe predicted by the extreme value statistical processing exceeds 150 ⁇ m, and the HIC resistance of the seamless steel pipe according to the present embodiment decreases.
- the modification of inclusions may not proceed. Specifically, if the molten steel holding time exceeds 600 seconds, the maximum major diameter Dmax of the inclusions in the seamless steel pipe exceeds 150 ⁇ m, and the HIC resistance of the seamless steel pipe according to the present embodiment decreases. Although the detailed reason is not clear, if the molten steel holding time is too long, the inclusions X 2 O 3 and X 2 OS (X is REM) in the molten steel are reduced, and XCaAlOS (X is REM) is formed. It is thought that it is difficult to be done.
- the molten steel holding time is set to 15 to 600 seconds. If the molten steel holding time is 15 to 600 seconds, the formation of calcium aluminate is suppressed, and the formation of fine inclusions XCaAlOS (X is REM) is promoted. As a result, the maximum major diameter Dmax of the inclusions in the seamless steel pipe predicted by the extreme value statistical processing can be set to 150 ⁇ m or less.
- the raw material is manufactured using the molten steel manufactured by the above method. Specifically, a slab (slab, bloom, or billet) is manufactured by continuous casting using molten steel. A steel ingot (ingot) may be manufactured by ingot-making method using molten steel. If necessary, slabs, blooms or ingots may be slab-rolled to produce billets. The raw material (slab, bloom, ingot, or billet) is manufactured through the above steps.
- the prepared material is hot worked to produce a raw tube.
- the billet is heated in a heating furnace.
- the heating temperature is not particularly limited, but is, for example, 1100 to 1300 ° C. Hot working is performed on the billet extracted from the heating furnace to produce a raw tube.
- the Mannesmann method is performed as hot working to produce a raw tube.
- the round billet is pierced and rolled by a piercing machine.
- the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0.
- the pierced and rolled round billet is further hot-rolled by a mandrel mill, a reducer, a sizing mill or the like to form a raw tube.
- the cumulative area reduction rate in the hot working step is, for example, 20 to 70%.
- the raw pipe may be manufactured from the billet by another hot working method.
- a raw tube may be manufactured by forging such as the Erhardt method.
- a raw pipe is manufactured by the above steps.
- the wall thickness of the raw tube is not particularly limited, but is, for example, 9 to 60 mm.
- the tube manufactured by hot working may be air-cooled (As-Rolled).
- the raw tube manufactured by hot working may also be directly quenched after hot working without cooling to room temperature, and after quenching after supplementary heating (reheating) after hot working. Is also good.
- quenching is performed after direct quenching or after supplementary heat, it is preferable to stop cooling during quenching or perform gentle cooling for the purpose of suppressing quenching cracks.
- SR treatment stress relief annealing
- quenching In the quenching step, quenching is performed on the raw tube manufactured by hot working. As used herein, “quenching” means to quench the A 3 point or more base pipe. Quenching may be performed by a known method, and is not particularly limited.
- the quenching temperature is, for example, 800 to 1000 ° C.
- the quenching temperature when direct quenching is performed after hot working, corresponds to the surface temperature of the raw tube measured by a thermometer installed on the outlet side of the final hot working device.
- the quenching temperature corresponds to the temperature of the auxiliary heating furnace or the heat treatment furnace when quenching is performed using the auxiliary heating furnace or the heat treatment furnace after the hot working.
- the raw tube is continuously cooled from the quenching start temperature and the temperature of the raw tube is continuously lowered.
- the method of the continuous cooling treatment is not particularly limited, and may be a known method.
- the method of the continuous cooling treatment is, for example, a method of immersing the element tube in a water tank to cool the element, or a method of accelerated cooling the element tube by shower water cooling or mist cooling.
- the raw pipe is rapidly cooled during quenching.
- the average cooling rate in the range of 800 to 500 ° C. of the raw tube during quenching is defined as a quenching cooling rate CR 800-500 (° C./sec). More specifically, the cooling rate during quenching CR 800-500 is the slowest cooled part in the cross section of the quenched tube (for example, when forcibly cooling both the outer surface and the inner surface of the tube, (At the center of the wall thickness).
- the preferred quenching cooling rate CR 800-500 is 8 ° C./sec or more. In this case, the microstructure of the quenched tube becomes mainly martensite and bainite.
- a more preferred lower limit of the quenching cooling rate CR 800-500 is 10 ° C./sec.
- the preferred upper limit of the cooling rate during quenching CR 800-500 is 500 ° C./sec.
- quenching is performed after heating the raw tube in the austenite region a plurality of times.
- the austenite grains before quenching are refined, the SSC resistance and low-temperature toughness of the seamless steel pipe increase.
- heating in the austenite region may be repeated a plurality of times, or by performing normalizing and quenching, heating in the austenite region may be repeated a plurality of times.
- tempering is performed on the quenched tube.
- tempering means that the quenched raw tube is reheated and maintained at a temperature of A c1 or lower.
- the tempering temperature is appropriately adjusted according to the chemical composition of the seamless steel pipe and the yield strength to be obtained. That is, the yield strength of the seamless steel pipe is adjusted to 758 to 862 MPa (110 ksi class) by adjusting the tempering temperature for the raw pipe having the chemical composition of the present embodiment.
- Tempering temperature corresponds to the temperature of the furnace when the quenched tube is heated and held.
- a preferable tempering temperature is 650 to 720 ° C.
- a more preferred lower limit of the tempering temperature is 655 ° C, and further preferably 660 ° C.
- a more preferred upper limit of the tempering temperature is 715 ° C, and further preferably 710 ° C.
- Tempering time means the time from when the tube is inserted into the furnace at the time of heating and holding the quenched tube until it is taken out. If the tempering time is too short, a microstructure mainly composed of tempered martensite and tempered bainite may not be obtained. On the other hand, if the tempering time is too long, the above effect is saturated. Therefore, in the tempering step of the present embodiment, the tempering time is preferably set to 10 to 180 minutes. A more preferred lower limit of the tempering time is 15 minutes. A more preferable upper limit of the tempering time is 120 minutes, and further preferably 90 minutes.
- the seamless steel pipe according to the present embodiment can be manufactured by the above manufacturing method.
- the above-mentioned manufacturing method is an example, and may be manufactured by another manufacturing method.
- Table 1 ⁇ Molten steel having the chemical composition shown in Table 1 was produced. Further, Table 2 shows Fn1 obtained from the chemical composition shown in Table 1 and the above formula (1). When Fn1 did not contain a corresponding element, “0” was substituted for the element symbol.
- the molten steel of each test number was manufactured by the following method. Primary refining in a converter was performed under the same conditions for hot metal manufactured by a known method. After removing the steel from the converter, Al was added to perform a deoxidizing treatment, and then a slag removing treatment was performed. Subsequently, after performing the RH vacuum degassing treatment, the components of alloy elements other than REM and Ca in the molten steel were adjusted. Subsequently, component adjustment was performed by adding REM to the molten steel and then adding Ca to the molten steel.
- Table 2 shows the time from the addition of REM to the addition of Ca (retention time of molten steel) for each test number.
- “A” Appropriate
- “S” Short
- “L” Long
- “Long) means that the molten steel holding time exceeded 600 seconds.
- a billet having a cross-sectional diameter of 310 mm was manufactured by continuous casting using molten steel of each test number.
- the manufactured billet was subjected to hot rolling to produce a raw tube (seamless steel tube) having an outer diameter of 244.48 mm, a wall thickness of 13.84 mm, and a length of 12000 mm.
- the raw tubes of each of the manufactured test numbers were allowed to cool, and the surface temperature of the raw tubes was set to normal temperature (25 ° C.).
- Quenching was performed on the raw tubes of each test number. Specifically, the tube of each test number after the above-mentioned cooling was kept in a quenching furnace at 920 ° C. for 10 minutes. The tube of each test number after holding was immersed in a water bath and cooled with water. At this time, the quenching cooling rate CR 800-500 was at least 300 ° C./min or more.
- the raw tubes of each test number were tempered to produce a seamless steel pipe of each test number.
- the tempering temperature of the raw tube of each test number was adjusted so as to be an API standard of 110 ksi class (yield strength: 758 to 862 MPa).
- Table 2 shows the tempering temperature (° C.) and the tempering time (minute) performed on the raw tubes of each test number.
- the stress at the time of 0.7% full elongation obtained in the tensile test was defined as the yield strength YS of each test number.
- the maximum stress during uniform elongation obtained in the tensile test was defined as the tensile strength TS of each test number.
- the ratio (YS / TS) between the obtained yield strength YS and tensile strength TS was defined as the yield ratio YR (%).
- Table 2 shows the obtained yield strength YS (MPa), tensile strength TS (MPa), and yield ratio YR (%).
- the yield strength of each test number was 758 to 862 MPa (110 ksi class). Further, the yield ratio of each test number was 90.0% or more. That is, in each of the microstructures of the seamless steel pipes of each test number, tempered martensite and tempered bainite were at least 90% by volume.
- the HIC resistance evaluation test was performed by the method described above. Specifically, the method was performed by a method based on NACE TM0284-2011. From the seamless steel pipe of each test number, a test piece having a rectangular cross section having a width of 20 mm, a thickness of the seamless steel pipe of ⁇ 2 mm, and a length of 100 mm was prepared. The length direction of the test piece was parallel to the pipe axis direction of the seamless steel pipe, and the thickness direction of the test piece was parallel to the pipe diameter direction of the seamless steel pipe.
- test solution a mixed aqueous solution (NACE solution A) of 5.0% by mass of sodium chloride and 0.5% by mass of acetic acid was used.
- NACE solution A a mixed aqueous solution
- the test pieces of each of the prepared test numbers were immersed in a test solution at 24 ° C., respectively. N 2 gas was blown into the test solution for 3 hours to degas the test solution of each test number.
- CAR (%) (area of indication portion / projected area) ⁇ 100 (2)
- Fn1 was less than 40.0. Therefore, the predicted maximum major axis diameter Dmax of the inclusion exceeded 150 ⁇ m. As a result, in the HIC resistance test, excellent HIC resistance was not shown.
- the seamless steel pipe according to the present invention is widely applicable to seamless steel pipes used in harsh environments such as polar regions, preferably, it can be used as a seamless steel pipe used in oil well environments, and more preferably, a casing. It can be used as an oil well tube for tubing.
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Abstract
Description
(Ca/O+Ca/S+0.285×REM/O+0.285×REM/S)×(Al/Ca)≧40.0 (1)
ここで、式(1)中の元素記号には、対応する元素の含有量(質量%)が代入される。
(Ca/O+Ca/S+0.285×REM/O+0.285×REM/S)×(Al/Ca)≧40.0 (1)
ここで、式(1)中の元素記号には、対応する元素の含有量(質量%)が代入される。
CAR(%)=(インディケーション部分の面積/投影面積)×100 (2)
本実施形態による継目無鋼管の化学組成は、次の元素を含有する。
炭素(C)は、鋼材の焼入れ性を高め、鋼材の降伏強度を高める。Cはさらに、製造工程中の焼戻し時において、炭化物の球状化を促進し、鋼材の降伏強度をさらに高める。C含有量が低すぎれば、これらの効果が得られない。一方、C含有量が高すぎれば、鋼材の靭性が低下し、焼割れが発生しやすくなる。したがって、C含有量は0.15~0.45%である。C含有量の好ましい下限は0.18%であり、より好ましくは0.20%であり、さらに好ましくは0.22%であり、さらに好ましくは0.24%である。C含有量の好ましい上限は0.40%であり、より好ましくは0.35%であり、さらに好ましくは0.33%であり、さらに好ましくは0.30%である。
シリコン(Si)は、鋼を脱酸する。Si含有量が低すぎれば、この効果が得られない。一方、Si含有量が高すぎれば、鋼材の耐SSC性が低下する。したがって、Si含有量は0.05~1.00%である。好ましいSi含有量の下限は0.15%であり、より好ましくは0.20%である。Si含有量の好ましい上限は0.85%であり、より好ましくは0.70%であり、さらに好ましくは0.60%であり、さらに好ましくは0.50%であり、さらに好ましくは0.45%であり、さらに好ましくは0.40%である。
マンガン(Mn)は、鋼を脱酸する。Mnはさらに、鋼材の焼入れ性を高め、鋼材の降伏強度を高める。Mn含有量が低すぎれば、これらの効果が得られない。一方、Mn含有量が高すぎれば、Mnは、P及びS等の不純物とともに、粒界に偏析する。その結果、鋼材の耐HIC性が低下する。Mn含有量が高すぎればさらに、延伸しやすい介在物であるMnSが増加する。その結果、介在物の予測最大長径が長くなり、鋼材の耐HIC性が低下する。したがって、Mn含有量は0.01~1.00%である。Mn含有量の好ましい下限は0.02%であり、より好ましくは0.03%である。Mn含有量の好ましい上限は0.90%であり、より好ましくは0.80%であり、さらに好ましくは0.70%であり、さらに好ましくは0.60%であり、さらに好ましくは0.55%であり、さらに好ましくは0.50%である。
燐(P)は不純物である。すなわち、P含有量は0%超である。Pは、粒界に偏析して、鋼材を脆化させる。その結果、鋼材の耐HIC性が低下する。したがって、P含有量は0.030%以下である。P含有量の好ましい上限は0.025%であり、より好ましくは0.020%である。P含有量はなるべく低い方が好ましい。ただし、P含有量の極端な低減は、製造コストを大幅に高める。したがって、工業生産を考慮した場合、P含有量の好ましい下限は0.0001%であり、より好ましくは0.0003%であり、さらに好ましくは0.001%であり、さらに好ましくは0.002%である。
硫黄(S)は不純物である。すなわち、S含有量は0%超である。Sは、粒界に偏析して、鋼材を脆化させる。その結果、鋼材の耐HIC性が低下する。Sはさらに、Mnと結合してMnSを形成する。MnSは延伸しやすい介在物であり、MnSが増加すれば、介在物の予測最大長径が長くなる。その結果、鋼材の耐HIC性が低下する。したがって、S含有量は0.0050%以下である。S含有量の好ましい上限は0.0045%であり、より好ましくは0.0035%であり、さらに好ましくは0.0030%であり、さらに好ましくは0.0025%である。S含有量はなるべく低い方が好ましい。ただし、S含有量の極端な低減は、製造コストを大幅に高める。したがって、工業生産を考慮した場合、S含有量の好ましい下限は0.0001%であり、より好ましくは0.0003%である。
アルミニウム(Al)は、鋼を脱酸する。Al含有量が低すぎれば、この効果が得られない。一方、Al含有量が高すぎれば、鋼材中に粗大な介在物が形成され、介在物の予測最大長径が長くなる。その結果、鋼材の耐HIC性が低下する。したがって、Al含有量は0.005~0.070%である。Al含有量の好ましい下限は0.010%であり、より好ましくは0.015%である。Al含有量の好ましい上限は0.060%であり、より好ましくは0.050%であり、さらに好ましくは0.045%であり、さらに好ましくは0.040%であり、さらに好ましくは0.035%である。本明細書にいう「Al」含有量は「酸可溶Al」、つまり、「sol.Al」の含有量を意味する。
クロム(Cr)は、鋼材の焼入れ性を高め、鋼材の降伏強度を高める。Cr含有量が低すぎれば、この効果が得られない。一方、Cr含有量が高すぎれば、鋼材中に粗大な炭化物が生成し、鋼材の耐SSC性が低下する。したがって、Cr含有量は0.30~1.50%である。Cr含有量の好ましい下限は0.32%であり、より好ましくは0.35%であり、さらに好ましくは0.40%であり、さらに好ましくは0.45%であり、さらに好ましくは0.50%である。Cr含有量の好ましい上限は1.40%であり、より好ましくは1.30%であり、さらに好ましくは1.25%であり、さらに好ましくは1.10%である。
モリブデン(Mo)は、鋼材の焼入れ性を高め、鋼材の降伏強度を高める。Mo含有量が低すぎれば、この効果が得られない。一方、Mo含有量が高すぎれば、上記効果が飽和する。したがって、Mo含有量は0.25~2.00%である。Mo含有量の好ましい下限は0.30%であり、より好ましくは0.40%であり、さらに好ましくは0.45%であり、さらに好ましくは0.50%であり、さらに好ましくは0.55%であり、さらに好ましくは0.60%である。Mo含有量の好ましい上限は1.70%であり、より好ましくは1.50%であり、さらに好ましくは1.40%であり、さらに好ましくは1.30%である。
チタン(Ti)は、Nと結合して微細な窒化物を形成し、ピンニング効果により、結晶粒を微細化する。その結果、鋼材の降伏強度が高まる。Ti含有量が低すぎれば、この効果が得られない。一方、Ti含有量が高すぎれば、鋼材中に粗大なTi窒化物が形成され、鋼材の耐HIC性が低下する。したがって、Ti含有量は0.002~0.020%である。Ti含有量の好ましい下限は0.003%であり、より好ましくは0.004%である。Ti含有量の好ましい上限は0.018%であり、より好ましくは0.015%であり、さらに好ましくは0.012%であり、さらに好ましくは0.010%である。
ニオブ(Nb)は、Cと結合して微細な炭化物を形成する。その結果、鋼材の降伏強度が高まる。Nb含有量が低すぎれば、この効果が得られない。一方、Nb含有量が高すぎれば、炭化物、窒化物又は炭窒化物(以下、「炭窒化物等」という)が過剰に形成される場合がある。この場合、鋼材の耐HIC性が低下する。したがって、Nb含有量は0.002~0.100%である。Nb含有量の好ましい下限は0.003%であり、より好ましくは0.007%であり、さらに好ましくは0.010%であり、さらに好ましくは0.015%であり、さらに好ましくは0.020%である。Nb含有量の好ましい上限は0.080%であり、より好ましくは0.050%であり、さらに好ましくは0.040%であり、さらに好ましくは0.030%である。
ホウ素(B)は、鋼に固溶して鋼材の焼入れ性を高め、鋼材の降伏強度を高める。B含有量が低すぎれば、この効果が得られない。一方、B含有量が高すぎれば、粗大なB窒化物が形成され、鋼材の耐HIC性が低下する。したがって、B含有量は0.0005~0.0040%である。B含有量の好ましい下限は0.0008%であり、より好ましくは0.0010%である。B含有量の好ましい上限は0.0030%であり、より好ましくは0.0025%であり、さらに好ましくは0.0020%であり、さらに好ましくは0.0018%であり、さらに好ましくは0.0015%である。
希土類元素(REM)は、FeOを還元する。その結果、Al2O3のクラスタ形成を抑制し、Al2O3、X2O3及びX2OS(XはREM)を形成する。その結果、介在物の予測最大長径が低下し、鋼材の耐HIC性が高まる。REMはさらに、鋼材中のPと結合して、結晶粒界におけるPの偏析を抑制する。その結果、鋼材の耐HIC性が高まる。REM含有量が低すぎれば、これらの効果が得られない。一方、REM含有量が高すぎれば、鋼材中に粗大な介在物が形成され、介在物の予測最大長径が長くなる。その結果、鋼材の耐HIC性が低下する。したがって、REM含有量は0.0001~0.0015%である。REM含有量の好ましい下限は0.0002%であり、より好ましくは0.0003%であり、さらに好ましくは0.0004%であり、さらに好ましくは0.0005%であり、さらに好ましくは0.0006%である。REM含有量の好ましい上限は0.0012%であり、より好ましくは0.0011%であり、さらに好ましくは0.0010%であり、さらに好ましくは0.0009%である。
カルシウム(Ca)は、鋼材中の介在物を球状化し、介在物の予測最大長径を低下させる。その結果、鋼材の耐HIC性が高まる。Ca含有量が低すぎれば、この効果が得られない。一方、Ca含有量が高すぎれば、鋼材中に粗大な酸化物系介在物が形成され、鋼材の耐HIC性が低下する。したがって、Ca含有量は0.0001~0.0100%である。Ca含有量の好ましい下限は0.0002%であり、より好ましくは0.0003%であり、さらに好ましくは0.0005%であり、さらに好ましくは0.0006%であり、さらに好ましくは0.0008%であり、さらに好ましくは0.0010%である。Ca含有量の好ましい上限は0.0040%であり、より好ましくは0.0030%であり、さらに好ましくは0.0025%であり、さらに好ましくは0.0020%であり、さらに好ましくは0.0017%であり、さらに好ましくは0.0015%である。
窒素(N)は、不可避に含有される。すなわち、N含有量は0%超である。Nは、Tiと結合して微細な窒化物を形成し、ピンニング効果により、結晶粒を微細化する。その結果、鋼材の降伏強度が高まる。一方、N含有量が高すぎれば、鋼材中に粗大なTi窒化物が形成され、鋼材の耐HIC性が低下する。したがって、N含有量は0.0100%以下である。N含有量の好ましい上限は0.0050%であり、より好ましくは0.0045%である。上記効果をより有効に得るためのN含有量の好ましい下限は0.0015%であり、より好ましくは0.0020%であり、さらに好ましくは0.0025%であり、さらに好ましくは0.0030%である。
酸素(O)は不純物である。すなわち、O含有量は0%超である。Oは、粗大な酸化物系介在物を形成し、介在物の予測最大長径を長くする。その結果、鋼材の耐HIC性が低下する。したがって、O含有量は0.0020%以下である。O含有量の好ましい上限は0.0019%であり、より好ましくは0.0018%であり、さらに好ましくは0.0016%であり、さらに好ましくは0.0015%である。O含有量はなるべく低い方が好ましい。ただし、O含有量の極端な低減は、製造コストを大幅に高める。したがって、工業生産を考慮した場合、O含有量の好ましい下限は0.0001%であり、より好ましくは0.0003%である。
上述の鋼材の化学組成はさらに、Feの一部に代えて、Vを含有してもよい。
バナジウム(V)は任意元素であり、含有されなくてもよい。すなわち、V含有量は0%であってもよい。含有される場合、Vは焼戻し時に微細な炭化物を形成し、鋼材の降伏強度を高める。Vが少しでも含有されれば、この効果がある程度得られる。しかしながら、V含有量が高すぎれば、鋼材の靭性が低下する。したがって、V含有量は0~0.30%である。V含有量の好ましい下限は0%超であり、より好ましくは0.01%であり、さらに好ましくは0.02%であり、さらに好ましくは0.04%であり、さらに好ましくは0.06%であり、さらに好ましくは0.08%である。V含有量の好ましい上限は0.25%であり、より好ましくは0.20%であり、さらに好ましくは0.15%であり、さらに好ましくは0.12%である。
マグネシウム(Mg)は任意元素であり、含有されなくてもよい。すなわち、Mg含有量は0%であってもよい。含有される場合、Mgは鋼材中の硫化物系介在物を微細化し、介在物の予測最大長径を短くする。その結果、鋼材の耐HIC性が高まる。Mgが少しでも含有されれば、この効果がある程度得られる。しかしながら、Mg含有量が高すぎれば、鋼材中に粗大な介在物が形成され、介在物の予測最大長径が長くなる。その結果、鋼材の耐HIC性が低下する。したがって、Mg含有量は0~0.0100%である。Mg含有量の好ましい下限は0%超であり、より好ましくは0.0001%であり、さらに好ましくは0.0003%であり、さらに好ましくは0.0006%であり、さらに好ましくは0.0010%である。Mg含有量の好ましい上限は0.0040%であり、より好ましくは0.0030%であり、さらに好ましくは0.0025%であり、さらに好ましくは0.0020%である。
ジルコニウム(Zr)は任意元素であり、含有されなくてもよい。すなわち、Zr含有量は0%であってもよい。含有される場合、Zrは鋼材中の硫化物系介在物を微細化し、介在物の予測最大長径を短くする。その結果、鋼材の耐HIC性が高まる。Zrが少しでも含有されれば、この効果がある程度得られる。しかしながら、Zr含有量が高すぎれば、鋼材中に粗大な介在物が形成され、介在物の予測最大長径が長くなる。その結果、鋼材の耐HIC性が低下する。したがって、Zr含有量は0~0.0100%である。Zr含有量の好ましい下限は0%超であり、より好ましくは0.0001%であり、さらに好ましくは0.0003%であり、さらに好ましくは0.0006%であり、さらに好ましくは0.0010%である。Zr含有量の好ましい上限は0.0040%であり、より好ましくは0.0030%であり、さらに好ましくは0.0025%であり、さらに好ましくは0.0020%である。
コバルト(Co)は任意元素であり、含有されなくてもよい。すなわち、Co含有量は0%であってもよい。含有される場合、Coはサワー環境中で保護性の腐食被膜を形成し、水素侵入を抑制する。その結果、鋼材の耐HIC性が高まる。Coが少しでも含有されれば、この効果がある程度得られる。しかしながら、Co含有量が高すぎれば、鋼材の焼入れ性が低下して、鋼材の降伏強度が低下する。したがって、Co含有量は0~1.00%である。Co含有量の好ましい下限は0%超であり、より好ましくは0.02%であり、さらに好ましくは0.03%であり、さらに好ましくは0.05%である。Co含有量の好ましい上限は0.90%であり、より好ましくは0.80%である。
タングステン(W)は任意元素であり、含有されなくてもよい。すなわち、W含有量は0%であってもよい。含有される場合、Wはサワー環境中で保護性の腐食被膜を形成し、水素侵入を抑制する。その結果、鋼材の耐HIC性が高まる。Wが少しでも含有されれば、この効果がある程度得られる。しかしながら、W含有量が高すぎれば、鋼材中に粗大な炭化物が生成して、鋼材が脆化する。その結果、鋼材の耐HIC性が低下する。したがって、W含有量は0~1.00%である。W含有量の好ましい下限は0%超であり、より好ましくは0.02%であり、さらに好ましくは0.03%であり、さらに好ましくは0.05%である。W含有量の好ましい上限は0.90%であり、より好ましくは0.80%である。
ニッケル(Ni)は任意元素であり、含有されなくてもよい。すなわち、Ni含有量は0%であってもよい。含有される場合、Niは鋼材の焼入れ性を高め、鋼材の降伏強度を高める。Niが少しでも含有されれば、この効果がある程度得られる。しかしながら、Ni含有量が高すぎれば、局部的な腐食を促進させ、鋼材の耐SSC性が低下する。したがって、Ni含有量は0~0.50%である。Ni含有量の好ましい下限は0%超であり、より好ましくは0.01%であり、さらに好ましくは0.02%である。Ni含有量の好ましい上限は0.10%であり、より好ましくは0.08%であり、さらに好ましくは0.06%である。
銅(Cu)は任意元素であり、含有されなくてもよい。すなわち、Cu含有量は0%であってもよい。含有される場合、Cuは鋼材の焼入れ性を高め、鋼材の降伏強度を高める。Cuが少しでも含有されれば、この効果がある程度得られる。しかしながら、Cu含有量が高すぎれば、鋼材の焼入れ性が高くなりすぎ、鋼材の靭性が低下する。したがって、Cu含有量は0~0.50%である。Cu含有量の好ましい下限は0%超であり、より好ましくは0.01%であり、さらに好ましくは0.02%であり、さらに好ましくは0.05%である。Cu含有量の好ましい上限は0.35%であり、より好ましくは0.25%である。
本実施形態による継目無鋼管の化学組成はさらに、式(1)を満たす。
(Ca/O+Ca/S+0.285×REM/O+0.285×REM/S)×(Al/Ca)≧40.0 (1)
ここで、式(1)中の元素記号には、対応する元素の含有量(質量%)が代入される。
本実施形態による継目無鋼管では、極値統計処理によって予測される、継目無鋼管中の介在物の最大長径(介在物の予測最大長径)Dmaxが150μm以下である。介在物の予測最大長径Dmaxが150μmを超えれば、継目無鋼管のCARが3.0%以上となり、継目無鋼管の耐HIC性が低下する。したがって、介在物の予測最大長径Dmaxは、150μm以下である。
Fj=j/(n+1) (3)
yj=-ln{-ln(Fj)} (4)
なお、式(4)における「ln」は、自然対数を意味する。
yj=c×Lmaxj+d (5)
ここで、c及びdは、最小二乗法によって求められる直線の係数である。
T=(S+S0)/S0 (6)
ここで、Sは継目無鋼管の肉厚中央部における、仮想表面積(mm2)を意味する。具体的に、Sは、次の式(7)で求めることができる。
S=(R-t)×π×L (7)
ここで、Rは継目無鋼管の外径(mm)、tは継目無鋼管の肉厚(mm)、Lは継目無鋼管の軸方向長さ(mm)を意味する。
y=-ln{-ln((T-1)/T)} (8)
なお、式(8)における「ln」は、式(4)と同様に、自然対数を意味する。
本実施形態による継目無鋼管のミクロ組織は、主として焼戻しマルテンサイト及び焼戻しベイナイトからなる。具体的に、ミクロ組織は、焼戻しマルテンサイト及び焼戻しベイナイトの体積率の合計が90%以上である。ミクロ組織の残部はたとえば、フェライト又はパーライトである。上述の化学組成を有する継目無鋼管のミクロ組織が、焼戻しマルテンサイト及び焼戻しベイナイトを体積率の合計で90%以上含有すれば、本実施形態の他の規定を満たすことを条件に、継目無鋼管の降伏強度が758~862MPa(110ksi級)となり、さらに、継目無鋼管の降伏比が90.0%以上となる。
本実施形態による継目無鋼管が油井用鋼管である場合、好ましい肉厚は9~60mmである。より好ましくは、本実施形態による継目無鋼管は、厚肉の油井用鋼管としての使用に適する。より具体的には、本実施形態による継目無鋼管が15mm以上、さらに、20mm以上の厚肉の油井用鋼管であっても、758~862MPa(110ksi級)の降伏強度と、優れた耐HIC性とを示す。
本実施形態による継目無鋼管の降伏強度は758~862MPa(110ksi級)である。本明細書でいう降伏強度は、引張試験で得られた0.7%全伸び時の応力(0.7%耐力)を意味する。要するに、本実施形態による継目無鋼管の降伏強度は110ksi級である。
本実施形態による継目無鋼管の耐HIC性は、NACE TM0284-2011に準拠した方法で実施できる。本実施形態による継目無鋼管から、耐HIC性試験用の試験片を作製する。具体的に、本実施形態による継目無鋼管から管周方向に円弧状の部材を採取する。採取した部材の2つの曲面(それぞれ継目無鋼管の外表面と内表面とに相当する)が、平行な平面になるように機械加工を行う。このとき、部材の厚さを、継目無鋼管の肉厚-2mmにする。このようにして、幅が20mm、厚さが継目無鋼管の肉厚-2mm、長さが100mmの、矩形断面を有する試験片を作製する。なお、試験片の長さ方向は継目無鋼管の管軸方向と平行であり、試験片の厚さ方向は継目無鋼管の管径方向と平行である。
CAR(%)=(インディケーション部分の面積/投影面積)×100 (2)
本実施形態による継目無鋼管の製造方法を説明する。以下に説明する製造方法は、本実施形態による継目無鋼管の製造方法の一例である。すなわち、本実施形態による継目無鋼管の製造方法は、以下に説明する製造方法に限定されない。
製鋼工程では、初めに、周知の方法で製造された溶銑に対して、転炉での精錬(一次精錬)を実施する。一次精錬された溶鋼に対して、二次精錬を実施する。二次精錬において、成分調整の合金元素の添加を実施して、上述の化学組成を満たす溶鋼を製造する。
熱間加工工程では、準備された素材を熱間加工して素管を製造する。始めに、ビレットを加熱炉で加熱する。加熱温度は特に限定されないが、たとえば、1100~1300℃である。加熱炉から抽出されたビレットに対して熱間加工を実施して、素管を製造する。
焼入れ工程では、熱間加工によって製造された素管に対して、焼入れを実施する。本明細書において、「焼入れ」とは、A3点以上の素管を急冷することを意味する。焼入れは、周知の方法で実施されればよく、特に限定されない。焼入れ温度は、たとえば、800~1000℃である。
焼戻し工程では、焼入れを実施された素管に対して、焼戻しを実施する。本明細書において、「焼戻し」とは、焼入れ後の素管をAc1点以下で再加熱して、保持することを意味する。焼戻し温度は、継目無鋼管の化学組成、及び、得ようとする降伏強度に応じて適宜調整する。つまり、本実施形態の化学組成を有する素管に対して、焼戻し温度を調整して、継目無鋼管の降伏強度を758~862MPa(110ksi級)に調整する。
上記の焼戻し後の各試験番号の継目無鋼管に対して、以下に説明する引張試験、介在物の予測最大長径測定試験、及び、耐HIC性評価試験を実施した。
引張試験はASTM E8/E8M(2013)に準拠して行った。各試験番号の継目無鋼管の板厚中央部から、平行部直径8.9mm、平行部長さ35.6mmの丸棒試験片を作製した。丸棒試験片の軸方向は、継目無鋼管の軸方向と平行であった。各丸棒試験片を用いて、常温(25℃)、大気中にて引張試験を実施して、各試験番号の継目無鋼管の降伏強度(MPa)、引張強度(MPa)、及び、降伏比(%)を得た。なお、本実施例では、引張試験で得られた0.7%全伸び時の応力を、各試験番号の降伏強度YSと定義した。同様に、引張試験で得られた一様伸び中の最大応力を、各試験番号の引張強度TSと定義した。求めた降伏強度YSと引張強度TSとの比(YS/TS)を、降伏比YR(%)とした。得られた降伏強度YS(MPa)、引張強度TS(MPa)、及び、降伏比YR(%)を表2に示す。
各試験番号の継目無鋼管について、上述の方法で、介在物の予測最大長径Dmax(μm)を求めた。なお、観察視野数nは108、基準面積S0は196.5mm2とした。さらに、継目無鋼管の肉厚中央部における仮想表面積Sは8.69×106mm2であった。
各試験番号の継目無鋼管について、上述の方法で、耐HIC性評価試験を実施した。具体的に、NACE TM0284-2011に準拠した方法で実施した。各試験番号の継目無鋼管から、幅が20mm、厚さが継目無鋼管の肉厚-2mm、長さが100mmの、矩形断面を有する試験片を作製した。また、試験片の長さ方向は継目無鋼管の管軸方向と平行であり、試験片の厚さ方向は継目無鋼管の管径方向と平行であった。
CAR(%)=(インディケーション部分の面積/投影面積)×100 (2)
表2に試験結果を示す。
Claims (6)
- 継目無鋼管であって、
質量%で、
C:0.15~0.45%、
Si:0.05~1.00%、
Mn:0.01~1.00%、
P:0.030%以下、
S:0.0050%以下、
Al:0.005~0.070%、
Cr:0.30~1.50%、
Mo:0.25~2.00%、
Ti:0.002~0.020%、
Nb:0.002~0.100%、
B:0.0005~0.0040%、
希土類元素:0.0001~0.0015%、
Ca:0.0001~0.0100%、
N:0.0100%以下、
O:0.0020%以下、
V:0~0.30%、
Mg:0~0.0100%、
Zr:0~0.0100%、
Co:0~1.00%、
W:0~1.00%、
Ni:0~0.50%、及び、
Cu:0~0.50%を含有し、残部がFe及び不純物からなり、式(1)を満たす化学組成を有し、
極値統計処理によって予測される、前記継目無鋼管中の介在物の最大長径が150μm以下であり、
降伏強度が758~862MPaである、継目無鋼管。
(Ca/O+Ca/S+0.285×REM/O+0.285×REM/S)×(Al/Ca)≧40.0 (1)
ここで、式(1)中の元素記号には、対応する元素の含有量(質量%)が代入される。 - 請求項1に記載の継目無鋼管であって、
前記化学組成は、
V:0.01~0.30%を含有する、継目無鋼管。 - 請求項1又は請求項2に記載の継目無鋼管であって、
前記化学組成は、
Mg:0.0001~0.0100%、及び、
Zr:0.0001~0.0100%からなる群から選択される1種以上を含有する、継目無鋼管。 - 請求項1~請求項3のいずれか1項に記載の継目無鋼管であって、
前記化学組成は、
Co:0.02~1.00%、及び、
W:0.02~1.00%からなる群から選択される1種以上を含有する、継目無鋼管。 - 請求項1~請求項4のいずれか1項に記載の継目無鋼管であって、
前記化学組成は、
Ni:0.01~0.50%、及び、
Cu:0.01~0.50%からなる群から選択される1種以上を含有する、継目無鋼管。 - 請求項1~請求項5のいずれか1項に記載の継目無鋼管であって、
前記継目無鋼管は油井用鋼管である、継目無鋼管。
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- 2019-09-26 US US17/259,651 patent/US11905580B2/en active Active
- 2019-09-26 WO PCT/JP2019/037758 patent/WO2020071219A1/ja unknown
- 2019-09-26 EP EP19869324.4A patent/EP3862454A4/en active Pending
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Also Published As
Publication number | Publication date |
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EP3862454A4 (en) | 2022-07-06 |
MX2021003195A (es) | 2021-05-27 |
US20210317553A1 (en) | 2021-10-14 |
JP6996641B2 (ja) | 2022-02-04 |
US11905580B2 (en) | 2024-02-20 |
EP3862454A1 (en) | 2021-08-11 |
JPWO2020071219A1 (ja) | 2021-09-02 |
AR116532A1 (es) | 2021-05-19 |
BR112021001353A2 (pt) | 2021-04-20 |
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