Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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")
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—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
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a stainless steel seamless pipe suitably used in oil wells and gas wells (hereinafter referred to simply as oil wells). More particularly, the invention relates to a stainless steel seamless pipe having improved corrosion resistance in a severe high-temperature corrosive environment containing carbon dioxide (CO 2 ) and chlorine ions (Cl - ), an environment containing hydrogen sulfide (H 2 S), etc.
• CO 2 carbon dioxide
• Cl - chlorine ions
• H 2 S hydrogen sulfide
• Stainless steel seamless pipes are widely used for applications such as oil well steel pipes.
• the oil well steel pipes are required to have high yield strength.
• oil wells are being actively developed in severe corrosive environments that have not received much attention such as deep oil fields, environments containing carbon dioxide gas, and so-called sour environments containing hydrogen sulfide. Therefore, oil well steel pipes are also required to have high corrosion resistance.
• Examples of oil well steel pipes used for extraction in oil fields and gas fields in an environment containing CO 2 , Cl - , etc. include 13Cr martensitic stainless steel pipes.
• 13Cr martensitic stainless steel pipes In some corrosive environments, the corrosion resistance of the 13Cr martensitic stainless steel pipes is insufficient. There is therefore a need for oil well steel pipes having higher corrosion resistance and usable in such corrosive environments.
• CCS carbon capture and storage
• Patent Literature 1 to Patent Literature 4 propose the following techniques.
• Patent Literature 1 proposes stainless steel for oil wells that has a chemical composition containing, in 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: more than 16.0 to 18.0%, Ni: more than 4.0 to 5.6%, Mo: 1.6 to 4.0%, Cu: 1.5 to 3.0%, Al: 0.001 to 0.10%, and N: 0.05% or less, wherein Cr, Ni, Mo, Cu, C, N, and Mn satisfy a specific relation.
• Patent Literature 2 proposes a high-strength stainless steel seamless pipe for oil wells that has a chemical composition containing, in mass %, C: 0.005 to 0.06%, 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 18.0%, Ni: 1.5 to 5.0%, V: 0.02 to 0.2%, Al: 0.002 to 0.05%, N: 0.01 to 0.15%, and O: 0.006% or less and further containing one or two or more selected from Mo: 1.0 to 3.5%, W: 3.0% or less, and Cu: 3.5% or less, wherein Cr, Ni, Mo, W, Cu, C, Si, Mn, and N satisfy a specific relation.
• Patent Literature 3 proposes stainless steel having a chemical composition containing, in mass %, C: 0.001 to 0.06%, Si: 0.05 to 0.5%, Mn: 0.01 to 2.0%, P: 0.03% or less, S: less than 0.005%, Cr: 15.5 to 18.0%, Ni: 2.5 to 6.0%, V: 0.005 to 0.25%, Al: 0.05% or less, N: 0.06% or less, O: 0.01% or less, Cu: 0 to 3.5%, Co: 0 to 1.5%, Nb: 0 to 0.25%, Ti: 0 to 0.25%, Zr: 0 to 0.25%, Ta: 0 to 0.25%, B: 0 to 0.005%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, and REMs: 0 to 0.05%, wherein one or two selected from the group consisting of Mo: 0 to 3.5% and W: 0 to 3.5% satisfy a specific relation.
• Patent Literature 4 proposes a seamless steel pipe containing, in mass %, C: 0.050% or less, Si: 0.50% or less, Mn: 0.01 to 0.20%, P: 0.025% or less, S: 0.0150% or less, Cu: 0.09 to 3.00%, Cr: 15.00 to 18.00%, Ni: 4.00 to 9.00%, Mo: 1.50 to 4.00%, Al: 0.040% or less, N: 0.0150% or less, Ca: 0.0010 to 0.0040%, Ti: 0.020% or less, Nb: 0.020% or less, V: 0 to 0.20%, Co: 0 to 0.30%, and W: 0 to 2.00%.
• the "good stress corrosion cracking resistance" is defined as follows. A test specimen subjected to a stress corresponding to its yield stress at 200°C by four-point bending is immersed in a 20% by mass aqueous NaCl solution (the temperature of the aqueous solution: 200°C) in an autoclave for 720 hours.
• the aqueous NaCl solution has a pH of 4.5 adjusted by adding sodium hydrogencarbonate and is in contact with a 50 atm CO 2 -0.01 atm H 2 S gas in the autoclave.
• the stress corrosion cracking resistance is rated good when no cracking is found in the test specimen after the test and the corrosion rate determined by a weight loss method after removal of corrosion products is 0.10 mm/year or less.
• the present inventors have conducted extensive studies on various factors influencing the properties of stainless steel, particularly its stress corrosion cracking resistance. Then the inventors have noticed that, although elements such as Cr, Mo, and Cu generally known as corrosion resistant elements are effective for stress corrosion cracking resistance, the amount of a martensite phase that contributes to strength cannot be obtained stably in some cases when the amounts of these elements added are excessively large. Accordingly, the inventors have come up with an entirely new approach. Specifically, the piercing conditions in the manufacture of a seamless steel pipe are optimized and control the microstructural form of the steel. With this approach, good stress corrosion cracking resistance can be obtained without adding large amounts of corrosion resistant elements.
• the microstructural form of ferrite contained in the steel is controlled as follows.
• an optical micrograph obtained by capturing an image of a microstructure of a steel pipe in a plane including the longitudinal and wall thickness directions of the steel pipe at a magnification of 400X, ferrite is extracted by image analysis in an area of 300 ⁇ m in the longitudinal direction ⁇ 200 ⁇ m in the wall thickness direction in actual size.
• the inventors have found that good stress corrosion cracking resistance can be obtained by controlling the microstructural form such that the average ferrite filling factor is 0.80 or less.
• the "average ferrite filling factor" will be described later, and its description is omitted here.
• the "plane including the longitudinal and wall thickness directions of the steel pipe” in the invention is a cross section in the wall thickness direction that includes the pipe axis.
• the image analysis is performed in the "area of 300 ⁇ m in the longitudinal direction ⁇ 200 ⁇ m in the wall thickness direction in actual size" in any one point in the cross section.
• the "filling factor” is an indicator that can be computed using, for example, image analysis free software ImageJ used in Examples in the invention.
• image analysis free software ImageJ used in Examples in the invention.
• One of the main points of the invention is that the inventors have found that a microstructural form with good stress corrosion cracking resistance can be represented by this indicator.
• Patent Literature 1 focuses on an improvement in stress corrosion cracking resistance and describes a method for manufacturing stainless steel in which a reduction in area of a steel material at 850°C to 1250°C is 50% or more.
• the inventors manufactured a steel pipe using the manufacturing method in Patent Literature 1.
• the average ferrite filling factor could not be adjusted to 0.80 or less using only the control described in Patent Literature 1, and the target stress corrosion cracking resistance of the invention was not achieved.
• the inventors studied the relation between the manufacturing conditions of a seamless steel pipe and the average ferrite filling factor. The results are shown in Fig. 1 .
• the inventors have found that, when piercing in a piercing step is performed under the condition that a piercing speed specified according to the length of a hollow piece immediately after completion of the piercing is 3.3 m/second or less, the average ferrite filling factor can be stably 0.80 or less, as shown in Fig. 1 .
• the mechanism may be as follows.
• the amount of shear strain in the circumferential direction of the hollow piece increases, and a microstructure including ferrite and martensite entangled in a complex manner is formed. This may be the reason that the ferrite filling factor in the plane including the longitudinal and wall thickness directions of the steel pipe tends to be 0.80 or less.
• the present invention has been completed by conducting further studies on the basis of the above findings.
• the present invention is summarized as follows.
• a stainless steel seamless pipe having high strength in terms of a yield strength of 758 MPa or more and also having good stress corrosion cracking resistance can be obtained.
• the stainless steel seamless pipe having the above properties can be manufactured by optimizing the piercing conditions in the piercing step.
• Fig. 1 is a graph showing the relation between a manufacturing condition of a seamless steel pipe and an average ferrite filling factor.
• the stainless steel seamless pipe of the invention has the chemical composition described above. Frist, the reasons for the limitations on the chemical composition will be described. In the following description, “% by mass” is denoted simply as “%” unless otherwise specified.
• C is an element that is inevitably contained in the steel making process. If C is contained in an amount of 0.06% or more, corrosion resistance deteriorates. Therefore, the content of C is set to 0.06% or less.
• the content of C is preferably 0.05% or less, more preferably 0.04% or less, and still more preferably 0.03% or less. From the viewpoint of the corrosion resistance, the lower the content of C, the better. Therefore, no particular limitation is imposed on the lower limit of the content of C. However, from the viewpoint of the cost of decarbonization, the content of C is preferably 0.002% or more, more preferably 0.003% or more, and still more preferably 0.005% or more.
• carbon dioxide gas corrosion resistance carbon dioxide gas corrosion resistance
• SSC resistance sulfide stress cracking resistance
• SCC resistance stress corrosion cracking resistance
• Si is an element that functions as a deoxidizing agent. However, if Si is contained in an amount of more than 1.0%, the hot workability and the corrosion resistance deteriorate. Therefore, the content of Si is set to 1.0% or less.
• the content of Si is preferably 0.7% or less, more preferably 0.5% or less, and still more preferably 0.4% or less. No particular limitation is imposed on the lower limit of the content of Si. However, from the viewpoint of increasing the deoxidization effect, the content of Si is preferably 0.03% or more, more preferably 0.05% or more, and still more preferably 0.10% or more.
• Mn is an element that functions as a deoxidizing agent and a desulfurizing agent and improves the hot workability.
• the content of Mn is set to 0.01% or more.
• the content of Mn is preferably 0.03% or more, more preferably 0.05% or more, and still more preferably 0.10% or more.
• the content of Mn is 1.0% or less.
• the content of Mn is preferably 0.8% or less, more preferably 0.6% or less, and still more preferably 0.4% or less.
• the content of P is an element that reduces the carbon dioxide gas corrosion resistance and the SSC resistance.
• the content of P is set to 0.05% or less.
• the content of P is preferably 0.04% or less and more preferably 0.03% or less.
• S is an element that significantly reduces the hot workability and inhibits stable operation in a hot pipe making process.
• S is present in the steel as sulfide-based inclusions and reduces the corrosion resistance. Therefore, the content of S is set to 0.005% or less.
• the content of S is preferably 0.004% or less, more preferably 0.003% or less, and still more preferably 0.002% or less.
• the lower the content of S the better. Therefore, no particular limitation is imposed on the lower limit of the content of S, and the content of S may be 0%. From the viewpoint of the manufacturing cost, the content of S is more preferably 0.0005% or more.
• Cr is an element that forms a protective coating on the surface of the steel pipe and contributes to an improvement in the corrosion resistance. If the content of Cr is less than 15.2%, the desired stress corrosion cracking resistance cannot be obtained. Moreover, the carbon dioxide gas corrosion resistance deteriorates. Although Cr is an element that stabilizes the ferrite phase, the fraction of the ferrite phase is small if the content of Cr is less than 15.2%, so that the steel obtained does not have the desired phase fractions. Therefore, the content of Cr is 15.2% or more.
• the content of Cr is preferably 15.5% or more, more preferably 16.0% or more, and still more preferably 16.3% or more.
• the content of Cr is 18.0% or less.
• the content of Cr is preferably 17.5% or less, more preferably 17.2% or less, and still more preferably 17.0% or less.
• Mo stabilizes the protective coating on the surface of the steel pipe, increases resistance to pitting corrosion caused by Cl - or low pH, and thereby increases the corrosion resistance.
• the content of Mo is set to 1.5% or more.
• Mo is an element that stabilizes the ferrite phase, the fraction of the ferrite phase becomes small if the content of Mo is less than 1.5%, so that the steel obtained cannot have the desired phase fractions.
• the content of Mo is preferably 1.8% or more, more preferably 2.0% or more, and still more preferably 2.3% or more. If the content of Mo exceeds 4.3%, the fraction of the ferrite phase and the fraction of the retained austenite phase become excessively high, so that the desired strength cannot be obtained. Therefore, the content of Mo is 4.3% or less.
• the content of Mo is preferably 4.0% or less, more preferably 3.5% or less, and still more preferably 3.0% or less.
• Cu has the effect of strengthening the protective coating on the surface of the steel pipe to thereby increase the corrosion resistance, particularly the carbon dioxide gas corrosion resistance.
• the content of Cu is set to 0.5% or more.
• the content of Cu is preferably 0.8% or more, more preferably 1.5% or more, and still more preferably 2.0% or more. If the content of Cu is excessively large, the hot workability of the steel deteriorates, and outer surface flaws are formed during pipe making, so that the desired stress corrosion cracking resistance cannot be obtained.
• Cu is an element that stabilizes the austenite phase, the fraction of the ferrite phase decreases if an excessively large amount of Cu is added, and the steel obtained cannot have the desired phase fractions. Therefore, the content of Cu is 3.5% or less.
• the content of Cu is preferably 3.2% or less, more preferably 3.0% or less, and still more preferably 2.7% or less.
• Ni maintains the fraction of the austenite phase at high temperature and allows the required amount of the martensite phase in the invention to be obtained, thereby contributing to the strength enhancement.
• the content of Ni is set to 3.5% or more.
• Ni is an element that stabilizes the austenite phase, the fraction of the austenite phase at high temperature becomes small if the content of Ni is less than 3.5%, so that the desired phase fraction of the martensite phase transformed from the austenite is not obtained.
• the content of Ni is preferably 3.8% or more, more preferably 4.0% or more, and still more preferably 4.3% or more. If Ni is contained in an amount of more than 5.2%, the fraction of the austenite phase becomes excessively large.
• the hot workability of the steel deteriorates, and flaws are likely to be caused during hot rolling, so that the desired stress corrosion cracking resistance may not always be obtained.
• the austenite forming ability becomes high, the fraction of the ferrite phase decreases accordingly, so that the steel obtained cannot have the desired phase fractions. Therefore, the content of Ni is 5.2% or less.
• the content of Ni is preferably 5.0% or less.
• Al is an element that functions as a deoxidizing agent. However, if Al is contained in an amount of more than 0.10%, the corrosion resistance deteriorates. Therefore, the content of Al is set to 0.10% or less.
• the content of Al is preferably 0.07% or less and more preferably 0.05% or less. No particular limitation is imposed on the lower limit of the content of Al. However, from the viewpoint of increasing the deoxidization effect, the content of Al is preferably 0.005% or more, more preferably 0.010% or more, and still more preferably 0.015% or more.
• N is an element inevitably contained during the steel making process but is an element that increases the strength of the steel.
• the content of N is 0.10% or less.
• the content of N is preferably 0.07% or less, more preferably 0.05% or less, and still more preferably 0.03% or less. No particular limitation is imposed on the lower limit of the content of N.
• the content of N is preferably 0.002% or more, more preferably 0.003% or more, and still more preferably 0.005% or more.
• O oxygen
• the content of O is set to 0.010% or less.
• the content of O may be 0%. Since an excessive reduction in the content of O causes an increase in the cost of steel making, the content of O is more preferably 0.0005% or more.
• the stainless steel seamless pipe of the invention has the chemical composition containing the above-described components with the balance being Fe and incidental impurities.
• the components described above are basic components, and these basic components allow the stainless steel seamless pipe of the invention to have the intended properties.
• one or two or more selected from the group consisting of Nb, Ti, W, Co, B, Ta, Zr, Ca, REMs, Mg, Sn, and Sb may be optionally contained. Since Nb, Ti, W, Co, B, Ta, Zr, Ca, REMs, Mg, Sn, and Sb are optional steel components, their contents may be 0%.
• Nb is an element that forms carbonitrides, improves the strength and corrosion resistance, and may be optionally contained. However, the Nb carbonitrides tend to cause a deterioration in low-temperature toughness. Therefore, when Nb is added, the content of Nb is 0.3% or less. The content of Nb is preferably 0.2% or less and more preferably 0.1% or less. The content of Nb is more preferably 0.01% or more.
• Ti is an element that increases the strength and corrosion resistance and may be optionally contained. However, if Ti is contained in an amount of more than 0.3%, the low-temperature toughness deteriorates. Therefore, when Ti is added, the content of Ti is 0.3% or less.
• the content of Ti is preferably 0.2% or less and more preferably 0.1% or less.
• the content of Ti is preferably 0.001% or more and more preferably 0.01% or more.
• W is an element that contributes to an increase in the strength of the steel, stabilizes the protective coating on the surface of the steel pipe to thereby increase the corrosion resistance, and may be optionally contained. However, if W is contained in an amount of more than 2.0%, the fraction of the ferrite phase becomes excessively high, and the desired strength cannot be obtained. Therefore, when W is added, the content of W is 2.0% or less.
• the content of W is preferably 1.5% or less and more preferably 1.2% or less.
• the content of W is more preferably 0.3% or more and still more preferably 0.5% or more.
• Co is an element that improves the corrosion resistance and may be optionally contained. However, even if Co is contained in an amount of more than 1.0%, its effect is saturated. Therefore, when Co is added, the content of Co is 1.0% or less.
• the content of Co is preferably 0.5% or less, more preferably 0.3% or less, and still more preferably 0.1% or less.
• the content of Co is more preferably 0.01% or more.
• B is an element that contributes to an improvement in hot workability and has the effect of reducing the occurrence of cracking and splitting in the pipe making process and may be optionally contained. However, if B is contained in an amount of more than 0.010%, the low-temperature toughness deteriorates. Therefore, when B is added, the content of B is 0.010% or less.
• the content of B is preferably 0.007% or less and more preferably 0.005% or less.
• the content of B is more preferably 0.0005% or more and still more preferably 0.0010% or more.
• Ta is an element that has the effects of increasing the strength and improving the corrosion resistance and may be optionally contained. However, if Ta is contained in an amount of more than 0.3%, its effects are saturated. Therefore, when Ta is added, the content of Ta is 0.3% or less. The content of Ta is more preferably 0.001% or more.
• Zr is an element that increases the strength and may be optionally added. Zr also has the effect of improving the SSC resistance. However, even if Zr is contained in an amount of more than 0.3%, its effect is saturated. Therefore, when Zr is added, the content of Zr is 0.3% or less. The content of Zr is preferably 0.0005% or more.
• Ca is an element that improves the hot workability through shape control of its sulfide and has the effect of reducing the occurrence of cracking and chipping in the pipe making process and may be optionally contained.
• the content of Ca is set to 0.001% or more.
• the content of Ca is preferably 0.002% or more, more preferably 0.003% or more, and still more preferably 0.005% or more.
• the content of Ca is 0.010% or less.
• the content of Ca is preferably 0.008% or less and more preferably 0.007% or less.
• REMs are elements that contribute to an improvement in the stress corrosion cracking resistance through shape control of their sulfides and may be optionally contained. However, even if REMs are contained in an amount of more than 0.3%, their effects are saturated and are not expected to be commensurate with the content. Therefore, when REMs are added, the content of REMs is 0.3% or less. The content of REMs is preferably 0.0005% or more.
• the REMs in the invention are elements including scandium (Sc) with an atomic number of 21, yttrium (Y) with an atomic number of 39, and lanthanoid elements from lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71.
• the chemical composition of the stainless steel seamless pipe of the invention may optionally contain at least one of the REMs. Therefore, the content of REM in the invention is the total content of the above elements.
• Mg is an element that improves the corrosion resistance and may be optionally contained. However, even if Mg is contained in an amount of more than 0.01%, its effect is saturated and is not expected to be commensurate with the content. Therefore, when Mg is added, the content of Mg is 0.01% or less.
• the content of Mg is preferably 0.0005% or more.
• Sn is an element that improves the corrosion resistance and may be optionally contained. However, even if Sn is contained in an amount of more than 1.0%, its effect is saturated and is not expected to be commensurate with the content. Therefore, when Sn is added, the content of Sn is 1.0% or less. The content of Sn is preferably 0.001% or more.
• Sb is an element that improves the corrosion resistance and may be optionally contained. However, even if Sb is contained in an amount of more than 1.0%, its effect is saturated and is not expected to be commensurate with the content. Therefore, when Sb is added, the content of Sb is 1.0% or less. The content of Sb is preferably 0.001% or more.
• the microstructure of a stainless steel seamless pipe in one embodiment of the invention includes a martensite phase at a volume fraction of 40% or more, a ferrite phase at a volume fraction of 15 to 55%, and a retained austenite phase at a volume fraction of 40% or less, and the average ferrite filling factor in a plane including the longitudinal and wall thickness directions of the steel pipe described later is 0.80 or less.
• the volume fraction of the martensite phase is set to 40% or more.
• the volume fraction of the martensite phase is preferably 45% or more, more preferably 50% or more, and still more preferably 60% or more. No particular limitation is imposed on the upper limit of the volume fraction of the martensite phase, but the volume fraction of the martensite phase is preferably 90% or less and more preferably 85% or less.
• Ferrite phase 15 to 55% or less in terms of volume fraction
• the volume fraction of the ferrite phase is set to 55% or less.
• the volume fraction of the ferrite phase is preferably 50% or less, more preferably 45% or less, and still more preferably 40% or less. If the volume fraction of the ferrite phase is less than 15%, the desired stress corrosion cracking resistance is not obtained. Therefore, the volume fraction of the ferrite phase is 15% or more.
• the volume fraction of the ferrite phase is preferably 20% or more and more preferably 25% or more.
• Retained austenite phase 40% or less in terms of volume fraction
• the volume fraction of the retained austenite phase is set to 40% or less.
• the volume fraction of the retained austenite phase is preferably 30% or less and more preferably 25% or less. No particular limitation is imposed on the lower limit of the volume fraction of the retained austenite phase, but the volume fraction of the retained austenite phase is preferably 3% or more and more preferably 5% or more.
• volume fractions of the above phases can be measured by the following method.
• a steel piece is cut from a plane including the longitudinal and wall thickness directions of the stainless steel seamless pipe.
• the steel piece is embedded in a resin and mirror-polished to produce a specimen for microstructure observation.
• the observation surface is electrolytically etched with a KOH solution (i.e., a solution mixture of 35 g of KOH and 100 g of pure water) at a current density of 3 A/cm 2 for 35 seconds and then etched with Vilella's reagent (i.e., a reagent mixture of 2 g of picric acid, 5 mL of hydrochloric acid, and 50 mL of ethanol) for 30 seconds.
• a KOH solution i.e., a solution mixture of 35 g of KOH and 100 g of pure water
• Vilella's reagent i.e., a reagent mixture of 2 g of picric acid, 5 mL of hydrochloric acid, and 50 mL of ethanol
• an image of the microstructure of the test specimen for microstructure observation is taken using an optical microscope at a magnification of 400X.
• an image is cut from one selected position within an area of 300 ⁇ m in the longitudinal direction of the steel pipe ⁇ 200 ⁇ m in the wall thickness direction of the steel pipe in actual size and then analyzed using image analysis software (ImageJ 1.52p, National Institute of Health) to compute the microstructure fraction (area fraction (%)) of the ferrite phase.
• image analysis software ImageJ 1.52p, National Institute of Health
• a Weka Trainable Segmentation function is used for the optical micrograph. Three bright ferrite portions and three dark martensite portions are used as training data. The Segmentation function is used for other portions to automatically classify these portions, and the ferrite phase can thereby be extracted.
• the area fraction of the extracted ferrite phase is defined as the volume fraction (%) of the ferrite phase.
• a test specimen for X-ray diffraction cut from the stainless steel seamless pipe is ground and polished such that a cross section perpendicular to the pipe axis direction (i.e., a C cross section) serves as a measurement surface, and the microstructure fraction of the retained austenite ( ⁇ ) phase is measured by the X-ray diffraction method.
• the volume fraction of the retained austenite phase is computed from the integrated intensity of the austenite (220) plane and the integrated intensity of the ferrite (211) plane using the following formula.
• V ⁇ % 100 / 1 + I ⁇ R ⁇ / I ⁇ R ⁇
• the remaining portion other than the ferrite phase and the retained ⁇ phase determined by the above measurement method is used as the fraction of the martensite phase.
• the method for observing the microstructure will also be described in detail later in Examples.
• the microstructure of the stainless steel seamless pipe of the invention includes the martensite phase, the ferrite phase, and the retained austenite phase.
• Average ferrite filling factor 0.80 or less
• the stainless steel seamless pipe of the invention has a microstructural form in which the average ferrite filling factor is 0.80 or less.
• the average ferrite filling factor is determined by the following method. First, in the image subjected to the Segmentation and used to determine the microstructure fraction of the ferrite phase (i.e., the image at the "one selected position"), an Analyze Particle function is used for the ferrite positions to thereby obtain the number of ferrite grains and the feature of the ferrite grains.
• the Solidity outputted from the ImageJ corresponds to the filling factor of each ferrite grain.
• the average of the Solidity values of the ferrite grains is defined as the average ferrite filling factor in the invention.
• the filling factor is close to 1. However, when the convex hull has a shape with the martensite phase present thereinside, the filling factor is small.
• the ferrite phase which is richer in corrosion resistant elements such as Cr and Mo, acts as a barrier to occurrence of pitting corrosion serving as the starting point of stress corrosion cracking.
• the filling factor is large, i.e., when the ratio of the ferrite phase in the convex hulls surrounding the ferrite grains is high, pitting corrosion is unlikely to occur in the ferrite portions.
• the martensite phase and the ferrite phase are not entangled in a complex manner. Specifically, in this microstructure, the martensite phase is densely present. Therefore, pitting corrosion is likely to occur and grow in the martensite portions, and these martensite portions are disadvantageous for stress corrosion cracking.
• the filling factor is small, i.e., when the ratio of the ferrite phase in the convex hulls surrounding the ferrite grains is small, the ferrite phase and the martensite phase are entangled in a complex manner.
• the martensite phase which is disadvantageous for pitting corrosion, is surrounded by the ferrite phase. Therefore, even when slight pitting corrosion occurs, the pitting corrosion is unlikely to grow, and therefore good stress corrosion cracking resistance is obtained.
• the average ferrite filling factor be adjusted to 0.80 or less.
• the average filling factor is preferably 0.75 or less and more preferably 0.70 or less.
• the lower limit of the average ferrite filling factor is preferably 0.2 or more.
• the average ferrite filling factor is more preferably 0.4 or more.
• the stainless steel seamless pipe of the invention has a yield strength of 758 MPa or more. No particular limitation is imposed on the upper limit of the yield strength, but the yield strength is preferably 1034 MPa or less. The yield strength can be measured by a method described later in Examples.
• the stainless steel seamless pipe of the invention has a yield strength of 862 MPa or more.
• the volume fractions of the above-described phases are as follows.
• the volume fraction of the martensite phase is 45% or more; the volume fraction of the ferrite phase is 15 to 55%; and the volume fraction of the retained austenite phase is 30% or less.
• the application of the stainless steel seamless pipe of the invention is not limited, and the stainless steel seamless pipe can be used for any application.
• the stainless steel seamless pipe of the invention is extremely suitable for oil well applications and can also be used preferably as a CO 2 injection pipe for the CCS described above.
• the stainless steel seamless pipe of the invention can be manufactured by subjecting a steel material to pipe making to thereby obtain a seamless steel pipe and subjecting the obtained seamless steel pipe to quenchingtempering treatment under specific conditions.
• the steel material used may be a billet, and the steel material used has the chemical composition described above.
• any method may be used to manufacture the steel material.
• a steelmaking method using, for example, a converter is used to prepare molten steel having the chemical composition described above, and then a round billet-shaped steel material is formed using, for example, a continuous casting method or an ingot making-blooming method.
• the steel may be cast into a cylindrical shape to directly produce a round billet-shaped steel material.
• the above-obtained steel material is subjected to pipe making to obtain a seamless steel pipe.
• the pipe making is performed by hot working. Specifically, in the hot working, the steel material is heated, and the heated steel material is formed into a hollow piece (i.e., a hollow pipe) using a piercer. Then the hollow piece is subjected to rolling such as forming to thereby obtain a seamless steel pipe having the desired dimensions.
• a Mannesmann-plug mill method or a Mannesmannmandrel mill method may be used to obtain the seamless steel pipe.
• the heating temperature is preferably 1100 to 1350°C. After the heating, a piercing step of piercing the steel material is performed.
• the temperature (unit: °C) is the surface temperature of the steel pipe material or the steel pipe (i.e., the seamless steel pipe after the pipe making), unless otherwise specified.
• the surface temperature can be measured using, for example, a radiation thermometer.
• the piercing step is controlled such that the piercing speed specified based on the length of the hollow piece immediately after completion of the piercing step, i.e., after the piercing step but before a subsequent rolling step, is 3.3 m/second or less. This is because, by appropriately controlling the piercing step such that the piercing speed satisfies the above range, the average ferrite filling factor can be adjusted to 0.80 or less.
• the piercing speed is determined by dividing the length of the hollow piece immediately after the piercing by the piercing time.
• the "length of the hollow piece immediately after the piercing” is the total length (unit: m) of the hollow piece in the longitudinal direction immediately after completion of the piercing step.
• the "piercing time” is the time (unit: second) required from when a rolling load by the piercer is applied to the steel material until the rolling load is no longer applied to the pierced steel material. In the piercing mill, the rolling load varies with time.
• the state in which the rolling load is applied means a state in which the rolling load differs from that when the steel material is not in contact with the rolls.
• the state in which no rolling load is applied means a state in which the rolling load, which varies with time, returns to the level when the steel material is not in contact with the rolls.
• an indicator such as the displacement of a roll or a torque value that varies when the steel material is in contact with the rolls may be used instead of the rolling load.
• the piercing speed is preferably 2.0 m/second or less, more preferably 1.0 m/second or less, and still more preferably 0.5 m/second or less. It is considered that, as the piercing speed decreases, the average ferrite filling factor decreases. Therefore, the lower limit of the piercing speed is not specified. However, if the piercing speed is extremely small, the manufacturing efficiency deteriorates. Therefore, the piercing speed is preferably 0.05 m/second or more, more preferably 0.10 m/second or more, and still more preferably 0.2 m/second or more.
• cooling treatment may be performed after the pipe making.
• the cooling treatment may be performed under any conditions.
• the cooling treatment includes, after the hot working, cooling the steel pipe until the surface temperature of the steel pipe reaches room temperature at an average cooling rate equal to or more than that of natural cooling.
• the average cooling rate equal to or more than that of natural cooling means 0.01°C/second or more.
• the seamless steel pipe obtained is subjected to heat treatment including quenching treatment and tempering treatment under specific conditions.
• the quenching treatment conditions and the tempering treatment conditions will next be described.
• the seamless steel pipe is heated to a quenching temperature of 850 to 1150°C, and the heated seamless steel pipe is cooled to a cooling stop temperature of 50°C or lower at an average cooling rate of 0.01°C/second or more.
• the quenching temperature in the quenching treatment is lower than 850°C, reverse transformation from martensite to austenite does not occur, and transformation from austenite to martensite does not occur during cooling, so that the desired strength cannot be obtained. Therefore, the quenching temperature is set to 850°C or higher.
• the quenching temperature is preferably 900°C or higher. If the quenching temperature is higher than 1150°C, the crystal grains coarsen, and the low-temperature toughness thereby deteriorates. Therefore, the quenching temperature is 1150°C or lower.
• the quenching temperature is preferably 1100°C or lower.
• the soaking treatment in which the seamless steel pipe is held at the quenching temperature may be performed.
• the temperature of the seamless steel pipe can be made uniform in the wall thickness direction, and variations in quality can be reduced.
• the time for which the seamless steel pipe is held at the quenching temperature i.e., the soaking time
• the soaking time is preferably 5 to 30 minutes.
• Average cooling rate 0.01°C/second or more
• the average cooling rate in the quenching treatment is less than 0.01°C/second, the desired microstructure cannot be obtained. Therefore, the average cooling rate is set to 0.01°C/second or more.
• the average cooling rate is preferably 1.0°C/second or more, more preferably 5.0°C/second or more, and still more preferably 10.0°C/second or more.
• the cooling is performed preferably by at least one of natural cooling and water cooling and performed more preferably by water cooling.
• Cooling stop temperature 50°C or lower
• the cooling stop temperature in the quenching treatment is set to 50°C or lower.
• the cooling stop temperature is the surface temperature of the seamless steel pipe.
• the tempering treatment is performed in which the seamless steel pipe subjected to the quenching treatment is heated to a tempering temperature of 500 to 650°C.
• Tempering temperature 500 to 650°C
• the tempering temperature is set to 500°C or higher.
• the tempering temperature is preferably 520°C or higher. If the tempering temperature is higher than 650°C, large amounts of intermetallic compounds are precipitated, and good low-temperature toughness is not obtained. Therefore, the tempering temperature is set to 650°C or lower.
• the tempering temperature is preferably 630°C or lower.
• the seamless steel pipe is heated to the tempering temperature and then may be held at the tempering temperature.
• the time for which the seamless steel pipe is held at the tempering temperature i.e., the holding time.
• the holding time is preferably 5 to 90 minutes.
• the seamless steel pipe may be left to cool.
• the stainless steel seamless pipe obtained can have the strength described above and also have good stress corrosion cracking resistance. Moreover, the stainless steel seamless pipe of the invention can have good low-temperature toughness.
• a steel material was produced by casting using molten steel having a chemical composition shown in Table 1. Then the steel material is heated and subjected to hot working using a model seamless rolling mill to produce a seamless steel pipe having an outer diameter of 177.8 mm ⁇ a wall thickness of 16.0 mm, and the seamless steel pipe was cooled by natural cooling. In this case, the heating temperature of the steel material before the hot working was set to 1250°C. A piercing speed shown in Table 2 was used.
• the seamless steel pipe obtained was subjected to the quenching treatment and the tempering treatment under the following conditions to obtain a stainless steel seamless pipe.
• the seamless steel pipe obtained was subjected to the quenching treatment under conditions shown in Table 2. Specifically, the seamless steel pipe was heated to a quenching temperature shown in Table 2 and held at the quenching temperature for a soaking time shown in Table 2. Next, the seamless steel pipe was cooled to a cooling stop temperature of 5°C. The cooling was performed by water cooling. In the water cooling, the average cooling rate from the time when the seamless steel pipe was put into water until the temperature of the seamless steel pipe reached 50°C or lower was 20°C/second.
• the cooled seamless steel pipe was heated to a tempering temperature shown in Table 2 and held at the tempering temperature for a holding time shown in Table 2. Then the seamless steel pipe was cooled by natural cooling (i.e., left to cool). The average cooling rate in the natural cooling was 0.04°C/second.
• test specimen was cut from the obtained stainless steel seamless pipe and subjected to (1) microstructure observation, (2) a tensile test, and (3) a stress corrosion cracking test by methods described below.
• the stainless steel seamless pipe obtained was used to measure the volume fraction of each phase by the method described above.
• the amount of the martensite phase was determined as "100% - the volume fraction (%) of the ferrite phase - the volume fraction (%) of the retained austenite phase.”
• the ferrite phase is denoted as "F”
• the martensite phase is denoted as "M”
• the retained austenite phase is denoted as " ⁇ .”
• the average ferrite filling factor was computed using the method described above.
• An arc-shaped test piece for a tensile test was cut from the obtained stainless steel seamless pipe according to the specifications of API (American Petroleum Institute)-5CT such that the pipe axis direction coincided with a tensile direction, and the tensile test was performed to determine the yield strength (YS).
• YS yield strength
• the yield strength YS was 758 MPa or more
• the stainless steel seamless pipe was considered to have high strength and rated pass.
• the yield strength YS was less than 758 MPa, the stainless steel seamless pipe was rated fail.
• the stainless steel seamless pipe obtained was machined to produce a test specimen having a thickness of 5 mm ⁇ a width of 15 mm ⁇ and a length of 115 mm, and then a four-point bending test was performed.
• the four-point bending test was performed as follows. NaHCO 3 was added to a 20% by mass aqueous NaCl solution such that the pH was 4.5, and the prepared solution was held in an autoclave (solution temperature: 200°C, a 50 atm CO 2 -0.01 atm H 2 S gas atmosphere). The specimen was immersed in the solution, and the immersion time was set to 30 days (i.e., 720 hours). The load stress was set equal to the yield stress at 200°C.
• the weight of the corrosion test specimen with the corrosion products removed was measured, and the weight of the test specimen measured prior to the corrosion test was subtracted to determine the amount of reduction in weight by the corrosion test.
• the amount of reduction in weight was divided by the surface area of the test specimen used and the immersion time to thereby obtain the amount of reduction in weight per unit time and unit area.
• the amount of reduction in weight per unit time and unit area was divided by the density of the steel to convert it to the depth of corrosion per unit time and unit area.
• the thus-obtained depth of corrosion per unit time and unit area (mm / year) was used as the rate of corrosion.
• the corrosion rate was 0.10 mm / year or less, the stainless steel seamless pipe was rated pass.
• the corrosion rate was more than 0.10 mm / year, the stainless steel seamless pipe was rated fail.

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