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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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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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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
  • C21D9/085—Cooling or quenching
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
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  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22C—ALLOYS
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  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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  • 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
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a stainless steel pipe which can preferably be used for oil wells and gas wells for crude oil and natural gas (hereafter, simply referred to as "oil wells") and geothermal wells and to a method for manufacturing the stainless steel pipe.
• a 13Cr martensitic stainless steel pipe has generally been used as an oil well pipe used for drilling in oil fields and gas fields in an environment containing carbon dioxide gas (CO 2 ), chloride ions (Cl - ), and the like.
• CO 2 carbon dioxide gas
• Cl - chloride ions
• oil wells in a corrosive environment having a notably higher temperature are being developed, and, in such an environment, there has been a case of insufficient corrosion resistance in the case of the 13Cr martensitic stainless steel pipe. Therefore, there is a demand for a steel pipe for oil wells which have excellent corrosion resistance and which can be used even in such an environment.
• geothermal wells for drilling steam for geothermal power generation geothermal wells deeper than ever before are being developed.
• Patent Literature 1 describes a high-strength stainless steel pipe for oil wells having improved high corrosion resistance.
• the high-strength stainless steel pipe for oil wells has a chemical composition containing, by mass%, C: 0.005% to 0.05%, Si: 0.05% to 0.5%, Mn: 0.2% to 1.8%, P: 0.03% or less, S: 0.005% or less, Cr: 15.5% to 18%, Ni: 1.5% to 5%, Mo: 1% to 3.5%, V: 0.02% to 0.2%, N: 0.01% to 0.15%, and O: 0.006% or less, in which Cr, Ni, Mo, Cu, and C satisfy a specified relational expression, and in which Cr, Mo, Si, C, Mn, Ni, Cu, and N satisfy a specified relational expression, and a microstructure including a martensitic phase as a base phase, in which the volume fraction of
• Patent Literature 1 states that, with this, it is possible to stably manufacture a stainless steel pipe for oil wells having sufficient corrosion resistance even in a harsh corrosive environment having a high temperature of up to 230°C and containing CO 2 and Cl - , high strength represented by a yield strength of more than 654 MPa (95 ksi), and high toughness.
• Patent Literature 2 describes a high-strength stainless steel pipe for oil wells having high toughness and improved corrosion resistance.
• the steel pipe has a chemical composition containing, by mass%, C: 0.04% or less, Si: 0.50% or less, Mn: 0.20% to 1.80%, P: 0.03% or less, S: 0.005% or less, Cr: 15.5% to 17.5%, Ni: 2.5% to 5.5%, V: 0.20% or less, Mo: 1.5% to 3.5%, W: 0.50% to 3.0%, Al: 0.05% or less, N: 0.15% or less, and O: 0.006% or less, in which Cr, Mo, W, and C satisfy a specified relationship, in which Cr, Mo, W, Si, C, Mn, Cu, Ni, and N satisfy a specified relationship, and in which Mo and W satisfy a specified relationship, and a microstructure including a martensitic phase as a base phase, in which the volume fraction of a ferrite phase is 10% to 50%.
• Patent Literature 2 states that, with this, it is possible to stably manufacture a high-strength stainless steel pipe for oil wells having high strength represented by a yield strength of more than 654 MPa (95 ksi) and sufficient corrosion resistance even in a high-temperature harsh corrosive environment containing CO 2 , Cl - , and furthermore H 2 S.
• Patent Literature 3 describes a high-strength stainless steel pipe having improved sulfide stress cracking resistance and high-temperature carbon dioxide corrosion resistance.
• a chemical composition containing, by mass%, C: 0.05% or less, Si: 1.0% or less, P: 0.05% or less, S: less than 0.002%, Cr: 16% (not inclusive) to 18%, Mo: 2% (not inclusive) to 3%, Cu: 1% to 3.5%, Ni: 3% to 5% (not inclusive), and Al: 0.001% to 0.1%, in which Mn and N satisfy a specified relationship under the condition in which Mn: 1% or less and N: 0.05% or less, the steel pipe having a microstructure including a martensitic phase as a base phase, in which the volume fraction of a ferrite phase is 10% to 40%, and in which the volume fraction of a retained austenite (y) phase is 10% or less, is obtained.
• Patent Literature 3 states that, with this, it is possible to obtain a high-strength stainless steel pipe having high strength represented by a yield strength of 758 MPa (110 ksi) or higher and improved corrosion resistance, that is, sufficient corrosion resistance even in a carbon dioxide gas environment having a high temperature of 200°C and sufficient sulfide stress cracking resistance even when there is a decrease in the temperature of the environmental gas.
• Patent Literature 4 describes a stainless steel pipe for oil wells.
• the stainless steel pipe for oil wells has a chemical composition containing, by mass%, C: 0.05% or less, Si: 0.5% or less, Mn: 0.01% to 0.5%, P: 0.04% or less, S: 0.01% or less, Cr: 16.0% (not inclusive) to 18.0%, Ni: 4.0% (not inclusive) to 5.6%, Mo: 1.6% to 4.0%, Cu: 1.5% to 3.0%, Al: 0.001% to 0.10%, and N: 0.050% or less, in which Cr, Cu, Ni, and Mo satisfy a specified relationship, and in which (C + N), Mn, Ni, Cu, and (Cr + Mo) satisfy a specified relationship, a microstructure including a martensitic phase and, in terms of volume fraction, 10% to 40% of a ferrite phase, in which ferrite phase grains intersect at a ratio of more than 85% with virtual line segments having a length of 50 ⁇ m in the wall thickness direction
• Patent Literature 5 describes a high-strength stainless steel pipe for oil wells having high toughness and improved corrosion resistance.
• the steel pipe has a chemical composition containing, by mass%, C: 0.04% or less, Si: 0.50% or less, Mn: 0.20% to 1.80%, P: 0.03% or less, S: 0.005% or less, Cr: 15.5% to 17.5%, Ni: 2.5% to 5.5%, V: 0.20% or less, Mo: 1.5% to 3.5%, W: 0.50% to 3.0%, Al: 0.05% or less, N: 0.15% or less, and O: 0.006% or less, in which Cr, Mo, W, and C satisfy a specified relationship, in which Cr, Mo, W, Si, C, Mn, Cu, Ni, and N satisfy a specified relationship, and in which Mo and W satisfy a specified relationship, and a microstructure, in which, in the largest crystal grain, the distance between two randomly selected points is 200 ⁇ m or less.
• Patent Literature 5 states that this steel pipe has high strength represented by a yield strength of more than 654 MPa (95 ksi), sufficient toughness, and sufficient corrosion resistance even in a high-temperature corrosive environment having a temperature of 170°C or higher and containing CO 2 , Cl - , and furthermore H 2 S.
• Patent Literature 6 describes a high-strength seamless martensitic stainless steel pipe for oil wells.
• the seamless steel pipe has a chemical composition containing, by mass%, C: 0.01% or less, Si: 0.5% or less, Mn: 0.1% to 2.0%, P: 0.03% or less, S: 0.005% or less, Cr: 15.5% (not inclusive) to 17.5%, Ni: 2.5% to 5.5%, Mo: 1.8% to 3.5%, Cu: 0.3% to 3.5%, V: 0.20% or less, Al: 0.05% or less, and N: 0.06% or less and, preferably, a microstructure including, in terms of volume fraction, 15% or more of a ferrite phase, 25% or less of a retained austenite phase, and a balance of a tempered martensitic phase.
• Patent Literature 6 states that the chemical composition may further contain W: 0.25% to 2.0% and/or Nb: 0.20% or less in addition to the constituents described above.
• Patent Literature 6 states that, with this, it is possible to stably manufacture a high-strength seamless martensitic stainless steel pipe for oil wells having satisfactory tensile properties, that is, high strength represented by a yield strength of 655 MPa or higher and 862 MPa or lower and a yield ratio of 0.90 or more, and sufficient corrosion resistance (carbon dioxide corrosion resistance and sulfide stress cracking resistance) even in a high-temperature harsh corrosive environment having a temperature of 170°C or higher and containing not only CO 2 , Cl - , and the like but also H 2 S.
• Patent Literature 7 describes a stainless steel pipe for oil wells.
• the stainless steel pipe has a chemical composition containing, by mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.01% to 1.0%, P: 0.05% or less, S: less than 0.002%, Cr: 16% to 18%, Mo: 1.8% to 3%, Cu: 1.0% to 3.5%, Ni: 3.0% to 5.5%, Co: 0.01% to 1.0%, Al: 0.001% to 0.1%, O: 0.05% or less, and N: 0.05% or less, in which Cr, Ni, Mo, and Cu satisfy a specified relationship, and, preferably, a microstructure including, in terms of volume fraction, 10% to 60% (not inclusive) of a ferrite phase, 10% or less of a retained austenite phase, and 40% or more of a martensitic phase.
• Patent Literature 7 states that, with this, it is possible to stably obtain a stainless steel pipe for oil wells having high strength represented by
• Patent Literature 8 describes a stainless steel material.
• the stainless steel material has a chemical composition containing, by mass%, C: 0.040% or less, Si: 0.05% to 1.0%, Mn: 0.010% to 0.30%, Cr: 18.0% (not inclusive) to 21.0%, Cu: 1.5% to 4.0%, Ni: 3.0% to 6.0%, sol.Al: 0.001% to 0.100%, Mo: 0% to 0.60%, W: 0% to 2.0%, Co: 0% to 0.30%, Ti: 0% to 0.10%, V: 0% to 0.15%, Zr: 0% to 0.10%, Nb: 0% to 0.10%, Ca: 0% to 0.010%, Mg: 0% to 0.010%, REM: 0% to 0.05%, B: 0% to 0.005%, and a balance of Fe and impurities, in which, regarding the impurities described above, that is, regarding the contents of P, S, O, and N, P: 0.050% or less, S
• An object of the present invention is to solve the related art problems described above and to thereby provide a stainless steel pipe having high strength represented by a yield strength of 758 MPa or higher, excellent low-temperature toughness, and excellent corrosion resistance and a method for manufacturing the steel pipe.
• excellent corrosion resistance in the present invention denotes a case of excellent carbon dioxide corrosion resistance and sulfide stress cracking resistance.
• excellent carbon dioxide corrosion resistance in the present invention denotes a case where, when a corrosion test is performed by immersing a test specimen in a testing solution, that is, a 25 mass% NaCl aqueous solution (having a temperature of 250°C in a CO 2 gas environment under a pressure of 30 atm), which is contained in an autoclave, for an immersion time of 336 hours, the corrosion rate is 0.125 mm/y or lower and no pitting corrosion is found in the test specimen after the corrosion test, and, when a corrosion test is performed by immersing a test specimen in a testing solution, that is, a 0.01 mol/L H 2 SO 4 aqueous solution (having a temperature of 250°C in a CO 2 gas environment under a pressure of 30 atm), for an immersion time of 336 hours, the corrosion rate is 0.125 mm/y or lower and no pitting corrosion is found in the test specimen after the corrosion test.
• a testing solution that is, a 25 mass% NaCl aqueous solution
• excellent sulfide stress cracking resistance in the present invention denotes a case of low sulfide stress cracking sensitivity in a sulfide stress cracking test (SSC test), in which the cracking sensitivity of a test specimen is evaluated under stress in a corrosive environment containing H 2 S.
• aqueous solution which has been prepared by adding acetic acid and sodium acetate to a testing solution, that is, a 5 mass% NaCl aqueous solution (having a temperature of 25°C in an atmosphere containing CO 2 gas corresponding to a pressure of 0.95 atm and H 2 S corresponding to a pressure of 0.05 atm) so that the pH is 3.5, under a loading stress equal to 90% of its yield stress for an immersion time of 720 hours, no cracking is found in the test specimen after the test has been performed.
• a testing solution that is, a 5 mass% NaCl aqueous solution (having a temperature of 25°C in an atmosphere containing CO 2 gas corresponding to a pressure of 0.95 atm and H 2 S corresponding to a pressure of 0.05 atm) so that the pH is 3.5, under a loading stress equal to 90% of its yield stress for an immersion time of 720 hours
• excellent low-temperature toughness (high toughness) in the present invention denotes a case of an absorbed energy vE -10 of 40 J or more when a Charpy impact test is performed in accordance with the prescription in JIS Z 2242 (2016) at a testing temperature of -10°C. It is preferable that the absorbed energy vE -10 described above be 250 J or less.
• the present inventors diligently conducted investigations regarding factors having effects on corrosion resistance (carbon dioxide corrosion resistance and sulfide stress cracking resistance) by using seamless steel pipes having chemical compositions of stainless steel, a yield strength of 758 MPa or more, and high toughness as stainless steel pipes.
• each of Cr, Mo, W, Cu, Ni, and C denotes the content (mass%) of the corresponding element and is assigned a value of zero when the corresponding element is not added.
• the present invention has been completed on the basis of the knowledge described above and additional investigations. That is, the subjective matter of the present invention is as follows.
• the present invention it is possible to provide a stainless steel pipe having high strength represented by a yield strength (YS) of 758 MPa or higher, satisfactory low-temperature toughness at a temperature of - 10°C, and excellent corrosion resistance even in a harsh corrosive environment having a high temperature of 250°C or higher and containing CO 2 and Cl - and a method for manufacturing the steel pipe.
• the stainless steel pipe according to the present invention can preferably be used as a seamless stainless steel pipe for oil wells.
• the C content is an important element which increases the strength of martensitic stainless steel.
• the C content be 0.003% or more.
• the C content is set to be 0.05% or less. It is preferable that the C content be 0.005% or more. It is preferable that the C content be 0.040% or less or more preferably 0.035% or less.
• Si is an element which functions as a deoxidizing agent, and, to obtain such an effect, it is preferable that the Si content be 0.005% or more.
• the Si content is set to be 1.0% or less. It is preferable that the Si content be 0.1% or more or more preferably 0.25% or more. It is preferable that the Si content be 0.6% or less.
• Mn is an element which increases the strength of martensitic stainless steel, and, to achieve the strength intended by the present invention, it is necessary that the Mn content be 0.10% or more.
• the Mn content is set to be 0.10% to 2.0%. It is preferable that the Mn content be 0.15% or more or more preferably 0.20% or more. It is preferable that the Mn content be 0.5% or less.
• the P content be as small as possible in the present invention. It is acceptable that the P content be 0.05% or less. Therefore, the P content is set to be 0.05% or less. It is preferable that the P content be 0.02% or less. Here, there is no particular limitation on the lower limit of the P content. However, since an excessive decrease in the P content causes an increase in manufacturing costs, it is preferable that the P content be 0.005% or more.
• the S content be as small as possible in the present invention.
• the S content is set to be less than 0.005%.
• the S content be 0.0015% or less or more preferably 0.0010% or less.
• the S content be 0.0003% or more.
• Cr is an element which contributes to improving corrosion resistance by forming a protective film on the surface of a steel pipe.
• the Cr content is 16.0% or less, it is not possible to achieve the corrosion resistance intended by the present invention. Therefore, it is necessary that the Cr content be more than 16.0%.
• the Cr content is set to be 16.0% (not inclusive) to 20.0%. It is preferable that the Cr content be 17.0% or more. It is preferable that the Cr content be 19.0% or less.
• Mo is an element which increases resistance to pitting corrosion caused by Cl - or a low pH by stabilizing a protective film on the surface of a steel pipe, thereby improving sulfide stress cracking resistance. To obtain such an effect, it is necessary that the Mo content be more than 0.6%. On the other hand, in the case where the Mo content is 1.4% or more when the Cr content is more than 16.0%, since there is an increase in the fraction of a ferrite phase, there is a deterioration in low-temperature toughness. Therefore, the Mo content is set to be 0.6% (not inclusive) to 1.4% (not inclusive). It is preferable that the Mo content be 0.7% or more. It is preferable that the Mo content be 1.2% or less or more preferably 1.1% or less.
• Ni is an element which contributes to improving corrosion resistance by strengthening a protective film on the surface of a steel pipe. Such an effect becomes noticeable in the case where the Ni content is 3.0% or more.
• the Ni content is set to be 3.0% to 5.0% (not inclusive). It is preferable that the Ni content be 3.5% or more. It is preferable that the Ni content be 4.5% or less.
• Al is an element which functions as a deoxidizing agent. To obtain such an effect, it is necessary that the Al content be 0.001% or more. On the other hand, in the case where the Al content is more than 0.10%, since there is a deterioration in cleanliness due to an increase in the amounts of oxides, there is a deterioration in low-temperature toughness. Therefore, the Al content is set to be 0.001% to 0.10%. It is preferable that the Al content be 0.01% or more or more preferably 0.02% or more. It is preferable that the Al content be 0.07% or less or more preferably 0.040% or less.
• N is an element which improves pitting corrosion resistance. To obtain such an effect, the N content is set to be 0.010% or more. On the other hand, in the case where the N content is more than 0.100%, since nitrides are formed, there is a deterioration in low-temperature toughness. Therefore, the N content is set to be 0.010% to 0.100%. It is preferable that the N content be 0.02% or more. It is preferable that the N content be 0.06% or less.
• the O content be as small as possible.
• the O content is set to be 0.01% or less. It is preferable that the O content be 0.0050% or less. It is preferable that the O content be 0.0010% or more or more preferably 0.0025% or more.
• the Cu is effective for inhibiting hydrogen entry into steel by strengthening a protective film on the surface of a steel pipe, thereby improving sulfide stress cracking resistance. To obtain such an effect, it is necessary that the Cu content be 0.3% or more. On the other hand, in the case where the Cu content is more than 3.5%, since CuS is precipitated at grain boundaries, there is a deterioration in hot workability. Therefore, the Cu content is set to be 0.3% to 3.5%. It is preferable that the Cu content be 0.5% or more, more preferably 1.0% or more, or even more preferably 1.5% or more. It is preferable that the Cu content be 3.0% or less.
• the contents of Cr, Ni, Mo, W, Cu, and C are controlled to be within the ranges described above and adjusted so that relational expression (1) is satisfied.
• each of Cr, Ni, Mo, W, Cu, and C denotes the content (mass%) of the corresponding element and is assigned a value of zero when the corresponding element is not added.
• the left-hand side value of relational expression (1) is calculated with the corresponding atomic symbol being assigned a value of 0 (zero). It is preferable that the left-hand side value of relational expression (1) be 22.0 or more.
• the left-hand side value of relational expression (1) there is no particular limitation on the upper limit of the left-hand side value of relational expression (1). From the viewpoint of inhibiting an increase in cost and of inhibiting a deterioration in strength caused by an excessive amounts of alloy elements added, it is preferable that the left-hand side value of relational expression (1) be 26.0 or less or more preferably 24.0 or less.
• the contents of Cr, Mo, W, and C are controlled to be within the ranges described above and adjusted so that relational expression (2) is satisfied.
• each of Cr, Mo, W, and C denotes the content (mass%) of the corresponding element and is assigned a value of zero when the corresponding element is not added.
• the left-hand side value of relational expression (2) there is no particular limitation on the upper limit of the left-hand side value of relational expression (2). From the viewpoint of such an effect becoming saturated, it is preferable that the left-hand side value of relational expression (2) be 28.0 or less or more preferably 25.0 or less.
• the remainder of the chemical composition which differs from the constituents described above is Fe and incidental impurities.
• the constituents described above are the basic constituents.
• the selective elements described below may be added as needed in addition to the basic constituents described above.
• the elements described below that is, Ti, Nb, V, Ta, B, Ca, REM, Mg, Zr, Sn, Sb, Co, and W, may be added as needed, the content of each of these constituents may be 0%.
• Ti 0.3% or less
• Nb 0.5% or less
• V 0.5% or less
• Ta 0.5% or less
• Ti, Nb, V, and Ta are elements which all increase strength, and one, two, or more of Ti, Nb, V, and Ta may be selectively added as needed.
• Ti, Nb, V, and Ta are effective for improving sulfide stress cracking resistance by trapping hydrogen when hydrogen generated by corrosion enters steel.
• Ta is an element which has the same effect as Nb, some of Nb may be replaced with Ta.
• each of Ti: 0.02% or more, Nb: 0.02% or more, V: 0.03% or more, and Ta: 0.03% or more be added. It is more preferable that each of Ti: 0.2% or less, Nb: 0.3% or less, V: 0.2% or less, and Ta: 0.2% or less be added.
• B 0.0050% or less
• Ca 0.0050% or less
• REM 0.010% or less
• B is an element which improves hot workability by improving grain-boundary strength
• B may be added as needed.
• the B content be 0.0010% or more.
• the B content in the case where the B content is more than 0.0050%, since nitrides are formed at grain boundaries, there is a deterioration in sulfide stress cracking resistance. Therefore, in the case where B is added, it is preferable that the B content be 0.0050% or less. It is more preferable that the B content be 0.0020% or more. It is more preferable that the B content be 0.0040% or less.
• Ca and REM are elements both of which contribute to improving sulfide stress cracking resistance through the morphological control of sulfides
• one or both of Ca and REM may be added as needed.
• each of Ca: 0.0001% or more and REM: 0.001% or more be added.
• each of Ca: 0.0050% or less and REM: 0.010% be added.
• Mg and Zr are elements both of which improve corrosion resistance through the morphological control of inclusions
• one or both of Mg and Zr may be selectively added as needed.
• each of Mg: 0.002% or more and Zr: 0.01% or more be added.
• each of Mg: 0.010% or less and Zr: 0.2% or less be added.
• each of Mg: 0.003% or more and Zr: 0.02% or more be added.
• Sn and Sb are elements both of which improve corrosion resistance by inhibiting active dissolution and by promoting passivation
• one or both of Sn and Sb may be selectively added as needed.
• each of Sn: 0.01% or more and Sb: 0.01% or more be added.
• Sn: 0.20% or less and Sb: 0.20% or less be added.
• Sn: 0.02% or more and Sb: 0.02% or more be added.
• Co 1.0% or less and W: 3.0% or less
• Co is an element which decreases the fraction of a retained austenite phase by increasing the Ms temperature, thereby improving strength and sulfide stress cracking resistance
• Co may be selectively added.
• the Co content be 0.01% or more.
• the Co content is set to be 1.0% or less. It is preferable that the Co content be 0.01% or more, more preferably 0.05% or more, or even more preferably 0.07% or more. It is preferable that the Co content be 0.15% or less or more preferably 0.09% or less.
• W is an element which contributes to increasing the strength of steel and which can improve sulfide stress cracking resistance by stabilizing a protective film on the surface of a steel pipe
• W may be added as needed.
• W in combination with Mo there is a particularly marked improvement in sulfide stress cracking resistance.
• the W content be 0.1% or more.
• the W content is set to be 3.0% or less. It is preferable that the W content be 0.1% or more, more preferably 0.5% or more, or even more preferably 0.8% or more. It is preferable that the W content be 2.0% or less.
• the stainless steel pipe according to the present invention has the chemical composition described above and a microstructure including, in terms of volume fraction, 45% or more of a tempered martensitic phase, 20% to 40% of a ferrite phase, and 5% to 25% of a retained austenite phase.
• Tempered martensitic phase 45% or more in terms of volume fraction
• the microstructure of the stainless steel pipe according to the present invention is controlled to include a tempered martensitic phase as a main phase to achieve the desired strength.
• the expression "main phase” denotes a phase constituting 45% or more in terms of volume fraction with respect to the entire steel pipe. It is preferable that the volume fraction of a tempered martensitic phase be 50% or more or more preferably 55% or more. It is preferable that the volume fraction of a tempered martensitic phase be 75% or less or more preferably 70% or less.
• a ferrite phase is precipitated as a second phase at least in an amount of 20% or more in terms of volume fraction with respect to the entire steel pipe. Consequently, since strain applied when hot rolling is performed is concentrated in a soft ferrite phase, it is possible to prevent flaws from occurring. In addition, by precipitating a ferrite phase in an amount of 20% or more in terms of volume fraction, since a ferrite phase functions as crack propagation resistance, it is possible to inhibit the propagation of sulfide stress cracking, which results in the corrosion resistance intended by the present invention being achieved.
• the volume fraction of a ferrite phase is set to be 20% to 40%. It is preferable that the volume fraction of a ferrite phase be 23% or more or more preferably 26% or more. It is preferable that the volume fraction of a ferrite phase be 37% or less or more preferably 34% or less.
• Retained austenite phase 5% to 25% in terms of volume fraction
• an austenite phase (retained austenite phase) is precipitated as a second phase in addition to a ferrite phase.
• a retained austenite phase which is excellent in terms of ductility and low-temperature toughness, there is an improvement in the ductility and low-temperature toughness of the entire steel.
• a retained austenite phase is precipitated in an amount of 5% or more in terms of volume fraction with respect to the entire steel pipe.
• the volume fraction of a retained austenite phase is set to be 5% or more and 25% or less. It is preferable that the volume fraction of a retained austenite phase be more than 10%. It is preferable that the volume fraction of a retained austenite phase be 20% or less or more preferably 15% or less.
• a test specimen for microstructure observation is taken from the central portion in the wall thickness direction of a cross section perpendicular to the pipe axis direction, the test specimen is etched in Vilella's reagent (reagent obtained by mixing picric acid, hydrochloric acid, and ethanol in the proportion of 2 g of picric acid and 10 ml of hydrochloric acid to 100 ml of ethanol), the photograph of a microstructure is taken by using a scanning electron microscope (at a magnification of 1000 times), the fraction (area fraction (%)) of a ferrite phase is calculated by using an image analyzer, and the obtained area fraction is regarded as the volume fraction (%).
• Vilella's reagent reagent obtained by mixing picric acid, hydrochloric acid, and ethanol in the proportion of 2 g of picric acid and 10 ml of hydrochloric acid to 100 ml of ethanol
• the photograph of a microstructure is taken by using a scanning electron microscope (at a magnification of 1000 times)
• a test specimen for X-ray diffraction is taken so that a cross section perpendicular to the pipe axis direction (C-cross section) is a measurement surface, the measurement surface is ground and polished, and the amount of retained austenite (y) is measured by performing X-ray diffraction analysis.
• the amount of retained austenite is calculated by using the following equation after the integral X-ray diffraction intensities of the (220) plane of ⁇ and the (211) plane of ⁇ (ferrite) have been measured.
• ⁇ volume fraction 100 / 1 + I ⁇ R ⁇ / I ⁇ R ⁇
• I ⁇ integral intensity of ⁇
• R ⁇ theoretically calculated value of ⁇ on the basis of crystallography
• Iy integral intensity of ⁇
• R ⁇ theoretically calculated value of y on the basis of crystallography
• fraction (volume fraction) of a tempered martensitic phase is defined as that of the remainder which differs from a ferrite phase and a retained y phase.
• a specified chemical composition in which the contents of the constituents are within the ranges described above, and in which relational expression (1) and relational expression (2) are satisfied, and a microstructure including, in terms of volume fraction, 45% or more of a tempered martensitic phase, 20% to 40% of a ferrite phase, and 5% to 25% of a retained austenite phase, it is possible to achieve the above-described properties intended by the present invention.
• a material for a steel pipe having the chemical composition described above is used as a starting material.
• the method for manufacturing the material for a steel pipe that is, the starting material.
• the material for a steel pipe be manufactured by preparing molten steel having the chemical composition described above with a steelmaking method such as one utilizing a converter and by making a cast piece such as a billet with a casting method such as a continuous casting method or an ingot casting-blooming method.
• the method for manufacturing a material for a steel pipe is not limited to this method.
• a steel piece which is made by further performing hot rolling on the cast piece to obtain desired dimensions may be used as the material for a steel pipe.
• this material for a steel pipe is subjected to a heat treatment (heating process).
• the material for a steel pipe (for example, billet) is heated to a heating temperature of 1100°C to 1350°C.
• the heating temperature in the heating process is set to be 1100°C to 1350°C. It is preferable that the heating temperature be 1150°C or higher. It is preferable that the heating temperature be 1300°C or lower.
• a "deterioration in hot-workability" in the present invention denotes, as described in EXAMPLES below, a case where, when a round-bar test specimen having a round bar shape and a parallel portion diameter of 10 mm, which has been taken from a billet, is heated to a temperature of 1250°C in a Gleeble testing machine, held for 100 seconds, cooled to a temperature of 1000°C at a cooling rate of 1°C/sec, held for 10 seconds, and thereafter subjected to tension until the test specimen is broken, and when a cross-sectional area reduction ratio (%) is measured, the cross-sectional area reduction ratio is less than 70%.
• the heated material for a steel pipe is subjected to hot working in a hot pipe making process so as to be made into a seamless steel pipe having a predetermined shape.
• the hot pipe making process be a Mannesmann-plug mill process or a Mannesmann-mandrel mill process.
• a seamless steel pipe may be manufactured by using a hot extrusion process involving a pressing process.
• There is no particular limitation on the conditions applied for the hot pipe making process as long as it is possible to manufacture a seamless steel pipe having a predetermined shape.
• the obtained seamless steel pipe may be subjected to a cooling treatment (cooling process).
• a cooling treatment cooling process
• the cooling process there is no particular limitation on the cooling process.
• the chemical composition according to the present invention by cooling the steel pipe to room temperature at a cooling rate corresponding to natural cooling after hot working has been performed in the hot pipe making process, it is possible to control the microstructure of a steel pipe to be one including a martensitic phase as a main phase.
• the seamless steel pipe is subjected to a heat treatment involving a quenching treatment and a tempering treatment.
• the quenching treatment is a treatment in which, after the seamless steel pipe has been reheated to a temperature (heating temperature) of 850°C to 1150°C and has been held for a predetermined time, the held steel pipe is cooled at a cooling rate equal to or higher than that corresponding to natural cooling to a temperature (cooling stop temperature) of 50°C to 0°C (not inclusive) in terms of the surface temperature of the seamless steel pipe.
• the heating temperature for a quenching treatment is set to be 850°C to 1150°C. It is preferable that the heating temperature be 900°C or higher. It is preferable that the heating temperature be 1000°C or lower.
• the seamless steel pipe which has been heated to the heating temperature described above, is held for a predetermined time. It is preferable that the soaking time be 5 min to 40 min to prevent a variation in material properties by providing uniform temperature distribution in the wall thickness direction of the seamless steel pipe. It is more preferable that the soaking time be 10 min or more.
• the cooling stop temperature in cooling for a quenching treatment is set to be 50°C to 0°C (not inclusive). It is preferable that the cooling stop temperature be 10°C or higher. It is preferable that the cooling stop temperature be 40°C or lower.
• a "cooling rate equal to or higher than that corresponding to natural cooling” denotes an average cooling rate of 0.01°C/sec or higher.
• the tempering treatment is a treatment in which, after the seamless steel pipe has been heated to a temperature (tempering temperature) of 500°C to 650°C and has been held for a predetermined time, the held seamless steel pipe is allowed to be naturally cooled. Natural cooling is air cooling.
• the tempering temperature is set to be 500°C to 650°C. It is preferable that the tempering temperature be 520°C or higher or more preferably 550°C or higher. It is preferable that the tempering temperature be 630°C or lower or more preferably 600°C or lower.
• the seamless steel pipe which has been heated to the tempering temperature described above, is held for a predetermined time. It is preferable that the soaking time (holding time) be 5 min to 90 min to prevent a variation in material properties by providing uniform temperature distribution in the wall thickness direction of the seamless steel pipe. It is more preferable that the soaking time (holding time) be 15 min or more. It is more preferable that the soaking time (holding time) be 60 min or less.
• the microstructure of the obtained steel pipe includes a tempered martensitic phase as a main phase and, in addition, a ferrite phase and a retained austenite phase as described above. Consequently, it is possible to obtain a high-strength seamless stainless steel pipe for oil wells having high strength, high toughness, and excellent corrosion resistance intended by the present invention at the same time.
• the stainless steel pipe obtained by using the present invention has a yield strength (YS) of 758 MPa or more, excellent low-temperature toughness, and excellent corrosion resistance. It is preferable that the yield strength be 800 MPa or higher. It is preferable that the yield strength be 1034 MPa or lower.
• Molten steels having the chemical compositions given in Table 1 were prepared by performing vacuum melting, and the obtained materials for a steel pipe (cast pieces) were subjected to a heating process in which heating was performed under the heating temperatures given in Table 2.
• the heated materials for a steel pipe were subjected to hot working utilizing a seamless rolling mill so as to be made into seamless steel pipes (having an outer diameter of 297 mm ⁇ and a wall thickness of 34 mm), and the obtained seamless steel pipes were cooled by air to room temperature (25°C).
• a test material was taken from the obtained seamless steel pipe, and the test material was subjected to a heat treatment (quenching treatment and tempering treatment) under the conditions given in Table 2.
• the test material was taken so that the longitudinal direction of the test material was the pipe axis direction.
• the average cooling rate when cooling for quenching treatment was performed was 11°C/sec, and the average cooling rate when air cooling (natural cooling) for a tempering treatment was performed was 0.04°C/sec.
• the sign "-" in the columns for "Chemical Composition” in Table 1 denotes a case where the corresponding element was not purposely added, that is, not only a case where the content of the corresponding element was 0% but also a case where the corresponding element was incidentally contained.
• test specimens were taken from the obtained test material, which had been subjected to a heating treatment, and each of the test specimens was used for respective microstructure observation, a tensile test, an impact test, and corrosion resistance tests. The methods for the tests and the like are described below.
• a test specimen for microstructure observation was taken so that a cross section in the pipe axis direction was an observation surface.
• the obtained test specimen for microstructure observation was etched in Vilella's reagent (reagent obtained by mixing picric acid, hydrochloric acid, and ethanol in the proportion of 2 g of picric acid and 10 ml of hydrochloric acid to 100 ml of ethanol), the photograph of a microstructure was taken by using a scanning electron microscope (at a magnification of 1000 times), and the fraction (area fraction (%)) of a ferrite phase was calculated by using an image analyzer. The obtained area fraction was regarded as the volume fraction (%).
• a test specimen for X-ray diffraction was taken so that a cross section perpendicular to the pipe axis direction (C-cross section) was a measurement surface from the obtained test material, which had been subjected to a heat treatment, the measurement surface was ground and polished, and the amount of retained austenite (y) was measured by performing X-ray diffraction analysis.
• the amount of a retained austenite was calculated by using the following equation after the integral X-ray diffraction intensities of the (220) plane of y and the (211) plane of ⁇ (ferrite) had been measured.
• ⁇ volume fraction 100 / 1 + I ⁇ R ⁇ / I ⁇ R ⁇
• I ⁇ integral intensity of ⁇
• R ⁇ theoretically calculated value of ⁇ on the basis of crystallography
• Iy integral intensity of y
• Ry theoretically calculated value of y on the basis of crystallography.
• the fraction (volume fraction) of a tempered martensite phase was that of the remainder which differs from a ferrite phase and a retained y phase.
• a case of a yield strength (YS) of 758 MPa or higher was judged as a case of high strength and as satisfactory.
• a case of a yield strength of lower than 758 MPa was judged as unsatisfactory.
• a V-notch test specimen (having a thickness of 10 mm) was taken so that longitudinal direction of the test specimen was the pipe axis direction in accordance with the prescription in JIS Z 2242 (2016) from the obtained test material, which had been subjected to a heat treatment, and a Charpy impact test was performed.
• the test temperature was -10°C
• the absorbed energy at a temperature of - 10°C, that is vE -10 was determined to evaluate low-temperature toughness.
• the number of the test specimens was three, and the arithmetic average of the determined values was defined as the absorbed energy (J) of the related steel pipe.
• a case of an absorbed energy at a temperature of -10°C, that is vE -10 , of 40 J or more was judged as a case of high toughness (excellent low-temperature toughness) and as satisfactory.
• a case of a vE -10 of less than 40 J was judged as unsatisfactory.
• each of the taken test specimens was used for respective corrosion tests as described below to evaluate carbon dioxide corrosion resistance and sulfide stress cracking resistance.
• a corrosion test specimen having a thickness of 3 mm, a width of 30 mm, and a length of 40 mm was taken by performing machining from the obtained test material, which had been subjected to a heat treatment. Each of the taken test specimens was used for respective corrosion tests as described below.
• a corrosion test was performed by immersing the test specimen described above in a testing solution, that is, a 25 mass% NaCl aqueous solution (having a temperature of 250°C in a CO 2 gas environment under a pressure of 30 atm), which was contained in an autoclave, for an immersion time of 14 days (336 hours).
• a testing solution that is, a 25 mass% NaCl aqueous solution (having a temperature of 250°C in a CO 2 gas environment under a pressure of 30 atm), which was contained in an autoclave, for an immersion time of 14 days (336 hours).
• the weight of the test specimen after the corrosion test had been performed was measured, and the corrosion rate was calculated from the difference in weight before and after the corrosion test.
• the surface of the test specimen after the corrosion test had been performed was observed by using a loupe at a magnification of 10 times to determine whether or not pitting corrosion occurred.
• the expression "with pitting corrosion” denotes a case where pitting corrosion having a diameter of 0.2 mm or more was observed.
• the expression "without pitting corrosion” denotes a case where no pitting corrosion was observed or only pitting corrosion having a diameter of less than 0.2 mm was observed.
• a case without pitting corrosion was judged as satisfactory, and a case with pitting corrosion was judged as unsatisfactory.
• a corrosion test was performed by immersing the test specimen described above in a testing solution, that is, a 0.01 mol/L H 2 SO 4 aqueous solution (having a temperature of 250°C in a CO 2 gas environment under a pressure of 30 atm), which was contained in an autoclave, for an immersion time of 14 days (336 hours).
• a testing solution that is, a 0.01 mol/L H 2 SO 4 aqueous solution (having a temperature of 250°C in a CO 2 gas environment under a pressure of 30 atm), which was contained in an autoclave, for an immersion time of 14 days (336 hours).
• the weight of the test specimen after the corrosion test had been performed was measured, and the corrosion rate was calculated from the difference in weight before and after the corrosion test.
• the surface of the test specimen after the corrosion test had been performed was observed by using a loupe at a magnification of 10 times to determine whether or not pitting corrosion occurred.
• the expression "with pitting corrosion” denotes a case where pitting corrosion having a diameter of 0.2 mm or more was observed.
• the expression "without pitting corrosion” denotes a case where no pitting corrosion was observed or only pitting corrosion having a diameter of less than 0.2 mm was observed.
• a case without pitting corrosion was judged as satisfactory, and a case with pitting corrosion was judged as unsatisfactory.
• a round bar-shaped test specimen (having a diameter of 6.4 mm ⁇ ) was taken by performing machining in accordance with Method A in NACE (National Association of Corrosion and Engineers) TM0177 from the obtained test material, which had been subjected to a heat treatment.
• a sulfide stress cracking resistance test (SSC resistance test) was performed on the taken round bar-shaped test specimen as described below.
• An SSC resistance test was performed by immersing the test specimen described above in an aqueous solution, which had been prepared by adding acetic acid and sodium acetate to a testing solution, that is, a 5 mass% NaCl aqueous solution (having a temperature of 25°C in an atmosphere containing CO 2 gas corresponding to a pressure of 0.95 atm and H 2 S corresponding to a pressure of 0.05 atm) so that the pH was 3.5, under a loading stress equal to 90% of its yield stress for an immersion time of 720 hours.
• the test specimen after the SSC resistance test had been performed was observed to determine whether or not cracking occurred.
• a round-bar test specimen having a round bar shape and a parallel portion diameter of 10 mm, which had been taken from a billet was used.
• the obtained round-bar test specimen was heated to a temperature of 1250°C in a Gleeble testing machine, held for 100 seconds, cooled to a temperature of 1000°C at an average cooling rate of 1°C /sec, held for 10 seconds, and thereafter subjected to tension until the test specimen was broken to determine a cross-sectional area reduction ratio (%).
• All of the examples of the present invention had a high strength represented by a yield strength (YS) of 758 MPa or more, excellent low-temperature toughness, and excellent corrosion resistance (excellent carbon dioxide corrosion resistance and excellent sulfide stress cracking resistance) in a harsh corrosive environment having a high temperature of 250°C or higher and containing CO 2 and Cl - .
• Yield strength 758 MPa or more
• excellent low-temperature toughness excellent corrosion resistance (excellent carbon dioxide corrosion resistance and excellent sulfide stress cracking resistance) in a harsh corrosive environment having a high temperature of 250°C or higher and containing CO 2 and Cl - .
• the characteristic value intended by the present invention was not achieved regarding at least one of yield strength, low-temperature toughness, carbon dioxide corrosion resistance, or sulfide stress cracking resistance.

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