US20180355451A1 - Seamless steel pipe and method of manufacturing the same - Google Patents

Seamless steel pipe and method of manufacturing the same Download PDF

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US20180355451A1
US20180355451A1 US15/775,409 US201615775409A US2018355451A1 US 20180355451 A1 US20180355451 A1 US 20180355451A1 US 201615775409 A US201615775409 A US 201615775409A US 2018355451 A1 US2018355451 A1 US 2018355451A1
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steel pipe
seamless steel
quenching
equation
tempering
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Keiichi Kondo
Taro OE
Yuji Arai
Yusuke SENDAI
Hiroki KAMITANI
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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Assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION reassignment NIPPON STEEL & SUMITOMO METAL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: ARAI, YUJI, KONDO, KEIICHI, KAMITANI, Hiroki, OE, TARO, SENDAI, YUSUKE
Publication of US20180355451A1 publication Critical patent/US20180355451A1/en
Assigned to NIPPON STEEL CORPORATION reassignment NIPPON STEEL CORPORATION CHANGE OF NAME (SEE DOCUMENT FOR DETAILS). Assignors: NIPPON STEEL & SUMITOMO METAL CORPORATION
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    • 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
    • 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
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    • 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
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • 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 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
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—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
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • 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
    • C21D2211/002—Bainite
    • 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
    • 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
    • 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

Definitions

  • the present invention relates to a seamless steel pipe and a method of manufacturing the same, and, more particularly, to a seamless steel pipe suitable for line pipe and a method of manufacturing the same.
  • a flow line is a line pipe laid along the topography of the surface of the earth or the seabed.
  • a riser is a line pipe disposed to rise from the seabed toward the platform (i.e. upward).
  • the inner side of a steel pipe forming part of a flow line laid in the deep sea is subject to a high interior fluid pressure having a pressure from deep strata added thereto and, when the operation is halted, is also affected by seawater pressures of the deep sea.
  • a steel pipe forming part of a riser is further affected by repeated distortions by ocean waves. Accordingly, it is desirable that steel pipes used for such applications have high strength and high toughness.
  • oil and gas wells are being developed in sour environments, which are harsher than conditions for conventional wells, such as deep seas and cold regions. Offshore pipe lines laid in such harsh sour environments are required to have a higher strength (i.e. pressure resistance) and toughness than conventional ones, and are further required to have hydrogen-induced cracking resistance (HIC resistance) and sulfide stress corrosion cracking resistance (SSC resistance).
  • HIC resistance hydrogen-induced cracking resistance
  • SSC resistance sulfide stress corrosion cracking resistance
  • Patent Document 1 discloses a seamless steel pipe with a large wall thickness for line pipe having high strength and good toughness, containing C: 0.03 to 0.08%, Si: 0.15 or less, Mn: 0.3 to 2.5%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo: 0.02 to 1.2%, Ti: 0.004 to 0.010%, N: 0.002 to 0.008% and one or more of Ca, Mg and REM: 0.0002 to 0.005% in total, the balance being Fe and impurities, where P in the impurities: 0.05% or less, S: 0.005% or less, and the wall thickness is 30 to 50 mm.
  • Patent Document 2 discloses a high-strength seamless steel pipe with a large wall thickness that is made by quenching and tempering and having a yield strength higher than 450 MPa for line pipe with good sour resistance where the Vickers hardness HV5 measurable at an outermost or innermost position of the pipe with an applied load of 5 kgf (with a force in the test of 49 N) is 250 HV5 or lower.
  • Patent Document 3 discloses a seamless steel pipe for line pipe containing, in mass %, C: 0.02 to 0.10%, Si: 0.5% or less, Mn: 0.5 to 2.0%, Al: 0.01 to 0.1%, Ca: 0.005% or less, and N: 0.007% or less, and one or more selected from the group consisting of Ti: 0.008% or less, V: less than 0.06% and Nb: 0.05% or less, the balance being Fe and impurities, where the total content of Ti, V and Nb is smaller than 0.06%, the carbon equivalent Ceq defined by the following equation is 0.38% or more, and the size of carbonitride particles containing one or more of Ti, V, Nb and Al is 200 nm or less.
  • Patent Document 4 discloses a seamless steel pipe with a chemical composition of, in mass %, C: 0.02 to 0.10%, Si: 0.05 to 0.5%, Mn: 1.0 to 2.0%, Mo: 0.5 to 1.0%, Cr: 0.1 to 1.0%, Al: 0.01 to 0.10%, P: 0.03% or less, S: 0.005% or less, Ca: 0.0005 to 0.005%, V: 0.010 to 0.040%, and N: 0.002 to 0.007% and one or more selected from the group consisting of Ti: 0.001 to 0.008% and Nb: 0.02 to 0.05%, the balance being Fe and impurities, where the carbon equivalent Ceq is 0.50 to 0.58%, the pipe containing a specified carbide.
  • Patent Document 3 WO 2011/152240
  • a seamless steel pipe having a strength of ⁇ 80 grade or higher as defined by the American Petroleum Institute (API) standards i.e. a lower limit yield strength of 555 MPa or higher
  • API American Petroleum Institute
  • the content of alloy elements such as carbon may be increased to increase hardenability.
  • the strength (i.e. hardness) of the surface of the steel pipe increases.
  • the surface layer is cooled at a high rate during quenching and can easily be hardened, increasing the hardness, while the in-the-wall portions have low hardness. This tendency may remain after tempering.
  • a surface layer hardness may exceed 250 Hv, which is the higher limit required in the sour resistance grade according to the API 5 L standards.
  • Patent Document 1 states that the hardness of the surface layer of a steel pipe can be controlled to be 250 HV5 or lower; however, it appears to require a special manufacturing process.
  • Patent Document 3 provides some considerations about SSC resistance; however, after hot forming, it is necessary to perform direct quenching or in-line quenching and then reheating-and-quenching.
  • Patent Document 4 provides some considerations about the hardness of the surface layer of a steel pipe and HIC resistance; however, a reheating-and-quenching step is necessary and, after hot forming, direct quenching or in-line quenching is used as necessary, which means manufacturing costs that are not very reasonable.
  • An object of the present invention is to provide a seamless steel pipe that can be manufactured by a relatively reasonable manufacturing process and that provides a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner.
  • a seamless steel in an embodiment of the present invention has a chemical composition of, in mass %, C: 0.02 to 0.15% Si: 0.05 to 0.5%; Mn: 0.30 to 2.5%; P: up to 0.03%; S: up to 0.006%; O: up to 0.004%; Al: 0.01 to 0.10%; Ti: 0.001 to 0.010%; N: up to 0.007%; Cr: 0.05 to 1.0%; Mo: not less than 0.02% and less than 0.5%; Ni: 0.03 to 1.0%; Cu: 0.02 to 1.0%; V: 0.020 to 0.20%; Ca: 0.0005 to 0.005%; and Nb: 0 to 0.05%, the balance being Fe and impurities, where a carbon equivalent Ceq as defined by equation (1) below is not less than 0.430% and less than 0.500%, a main phase of a microstructure from a surface layer to an in-the-wall portion is tempered martensite or tempered bainite, a size of prior austenite grains in the microstructure is lower than
  • a method of manufacturing a seamless steel pipe in an embodiment of the present invention includes: preparing a raw material having a chemical composition of, in mass %, C: 0.02 to 0.15%; Si: 0.05 to 0.5%; Mn: 0.30 to 2.5%; P: up to 0.03%; S: up to 0.006%; O: up to 0.004%; Al: 0.01 to 0.10%; Ti: 0.001 to 0.010%; N: up to 0.007%; Cr: 0.05 to 1.0%; Mo: not less than 0.02% and less than 0.5%; Ni: 0.03 to 1.0%; Cu: 0.02 to 1.0%; V: 0.020 to 0.20%; Ca: 0.0005 to 0.005%; and Nb: 0 to 0.05%, the balance being Fe and impurities; hot working the raw material to produce a hollow shell; quenching the hollow shell by direct quenching or in-line quenching; and tempering the quenched hollow shell.
  • a carbon equivalent Ceq as defined by equation (3) below is not less than 0.430% and less than 0.500%
  • a Larson-Miller parameter PL as defined by equation (4) below is not less than 18800
  • T is a tempering temperature
  • t is a holding period for this temperature.
  • T is in ° C.
  • t is in hours.
  • the present invention provides a seamless steel pipe that can be manufactured by a relatively reasonable manufacturing process and that provides a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner.
  • FIG. 1 is a block diagram illustrating an example of a manufacturing line.
  • FIG. 2 is a flow chart illustrating a process for manufacturing the seamless steel pipe.
  • FIG. 3 shows changes in the surface temperature of a workpiece during a manufacture versus time.
  • FIG. 4 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and yield strength YS for steel.
  • FIG. 5 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and yield strength YS for steel A.
  • FIG. 6 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and hardness at an outer surface, an in-the-wall portion and an inner surface for steel B.
  • FIG. 7 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and hardness at an outer surface, an in-the-wall portion and an inner surface for steel A.
  • FIG. 8 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and maximum difference in hardness for steel B.
  • FIG. 9 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and maximum difference in hardness for steel A.
  • the present inventors did research to find a method of providing a seamless steel pipe that ensures a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner. They found out that limiting the carbon equivalent of a steel to an appropriate range and reducing the difference between the hardness of the surface layer and the hardness of the in-the-wall portions of the seamless steel pipe ensures a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner, where only direct quenching or in-line quenching is performed after hot forming and no reheating-and-quenching is performed.
  • the surface layer of a seamless steel pipe is cooled at high rate and can easily be hardened.
  • the surface layer of the seamless steel pipe tends to be hard and may exceed the values of hardness specified by the API 5 L standards or DNV-OS-F101 standards.
  • the portions located in the middle in the wall thickness of the seamless steel pipe is cooled at a lower rate and cannot easily be hardened such that non-quenched structures such as ferrite may be included.
  • the carbon equivalent As defined by equation (2) below is 18800 or higher.
  • T is a tempering temperature (in ° C.) and t is a holding time (in hours) for that temperature.
  • the seamless steel pipe in the present embodiment has the chemical composition described below.
  • “%” for the content of an element means mass %.
  • Carbon (C) increases the strength of the steel. If the C content is lower than 0.02%, this effect cannot be sufficiently achieved. If the C content is higher than 0.15%, the toughness of the steel decreases. In view of this, the C content should be in the range of 0.02 to 0.15%.
  • the C content is preferably higher than 0.02%, and more preferably 0.04% or higher.
  • the C content is preferably 0.10% or lower, and more preferably 0.08% or lower.
  • Si deoxidizes steel. This effect can be clearly achieved if the Si content is 0.05% or higher. However, if the Si content is higher than 0.5%, the toughness of the steel decreases. In view of this, the Si content should be in the range of 0.05 to 0.5%.
  • the Si content is preferably higher than 0.05%, and more preferably 0.08% or higher, and still more preferably 0.10% or higher.
  • the Si content is preferably lower than 0.5%, and more preferably 0.25% or lower, and still more preferably 0.20% or lower.
  • Manganese (Mn) increases the hardenability of steel to increase the strength of the steel. These effects cannot be sufficiently achieved if the Mn content is lower than 0.30%. If the Mn content is higher than 2.5%, Mn segregates in the steel, decreasing the toughness of the steel. In view of this, the Mn content should be in the range of 0.30 to 2.5%.
  • the Mn content is preferably higher than 0.30%, and more preferably 1.0% or higher, and still more preferably 1.3% or higher.
  • the Mn content is preferably lower than 2.5%, and more preferably 2.0% or lower, and still more preferably 1.8% or lower.
  • Phosphorus (P) is an impurity. P decreases the toughness of steel. Thus, lower P contents are preferable. In view of this, the P content should be 0.03% or lower.
  • the P content is preferably lower than 0.03%, and more preferably 0.015% or lower, and still more preferably 0.012% or lower.
  • S Sulphur
  • MnS particles Sulphur (S) is an impurity. S bonds with Mn to form coarse MnS particles and thus decreases the toughness and HIC resistance of the steel. Thus, lower S contents are preferable. In view of this, the S content should be 0.006% or lower.
  • the S content is preferably lower than 0.006%, and more preferably 0.003% or lower, and still more preferably 0.002% or lower.
  • Oxygen (O) is an impurity. O forms coarse oxide particles or clusters of oxide particles, decreasing the toughness of the steel. Thus, lower O contents are preferable. In view of this, the O content should be 0.004% or lower. The O content is preferably 0.003% or lower, and more preferably 0.002% or lower.
  • Al Aluminum (Al) bonds with N to form fine nitride particles, increasing the toughness of the steel. This effect cannot be sufficiently achieved if the Al content is lower than 0.01%. If the Al content is higher than 0.10%, coarse Al nitride particles result, decreasing the toughness of the steel. In view of this, the Al content should be in the range of 0.01 to 0.10%.
  • the Al content is preferably higher than 0.01%, and more preferably 0.02% or higher.
  • the Al content is preferably lower than 0.10%, and more preferably 0.08% or lower, and still more preferably 0.06% or lower.
  • Al content means the content of acid-soluble Al (i.e. so-called “sol. Al”).
  • N Nitrogen bonds with Al and forms fine Al nitride particles, increasing the toughness of the steel.
  • the N content should be 0.007% or lower.
  • the N content is preferably lower than 0.007%, and more preferably 0.006% or lower, and still more preferably 0.005% or lower.
  • the N content is preferably 0.002% or higher.
  • Chromium (Cr) increases the hardenability of steel and increases the strength of the steel. Cr further increases the temper softening resistance of the steel. These effects cannot be sufficiently achieved if the Cr content is lower than 0.05%. If the Cr content is higher than 1.0%, the toughness of the steel decreases. In view of this, the Cr content should be in the range of 0.05 to 1.0%. The Cr content is preferably higher than 0.05%, and more preferably 0.2% or higher. The Cr content is preferably lower than 1.0%, and more preferably 0.8% or lower.
  • Molybdenum (Mo) improves the strength of steel by transformation toughening and solute strengthening. This effect cannot be sufficiently achieved if the Mo content is lower than 0.02%. If the Mo content is higher than 0.5%, the toughness of the steel decreases. In view of this, the Mo content should be not lower than 0.02% and lower than 0.5%.
  • the Mo content is preferably higher than 0.02%, and more preferably 0.05% or higher, and still more preferably 0.1% or higher.
  • the Mo content is preferably 0.4% or lower, and more preferably 0.3% or lower.
  • Nickel (Ni) increases the hardenability of steel and increases the strength of the steel. Further, Ni has the effect of improving the adherence of scales formed on the surface of the steel during the heating step for quenching, and also the effect of reducing the increase in the hardness of the surface layer of the steel since the scales reduce the cooling rate at the surface of the steel during the cooling step for quenching. These effects cannot be sufficiently achieved if the Ni content is lower than 0.03%. If the Ni content is higher than 1.0%, the SSC resistance decreases. In view of this, the Ni content should be in the range of 0.03 to 1.0%. The Ni content is preferably 0.05% or higher, and more preferably 0.08% or higher, and still more preferably 0.10% or higher. The Ni content is preferably lower than 1.0%, and more preferably 0.7% or lower, and still more preferably 0.5% or lower.
  • Copper (Cu) increases the hardenability of steel and increases the strength of the steel. Further, Cu has the effect of improving the adherence of scales formed on the surface of the steel during the heating step for quenching, and also the effect of reducing the increase in the hardness of the surface layer of the steel since the scales reduce the cooling rate at the surface of the steel during the cooling step for quenching. These effects cannot be sufficiently achieved if the Cu content is lower than 0.02%. If the Cu content is higher than 1.0%, the weldability of the steel decreases. Further, if the Cu content is too high, the grain boundary strength of the steel at high temperatures decreases, decreasing the hot workability of the steel. In view of this, the Cu content should be in the range of 0.02 to 1.0%. The Cu content is preferably 0.05% or higher, and more preferably 0.08% or higher, and still more preferably 0.10% or higher. The Cu content is preferably lower than 1.0%, and more preferably 0.7% or lower, and still more preferably 0.5% or lower.
  • V 0.020 to 0.20%
  • V Vanadium bonds with C in a steel and forms a V carbide to increase the strength of the steel. Further, V is dissolved in an Mo carbide to form a carbide. A carbide containing V is less likely to form coarse particles. These effects cannot be effectively achieved if the V content is lower than 0.020%. If the V content is higher than 0.20%, coarse carbide particles result. In view of this, the V content should be in the range of 0.020 to 0.20%. The V content is preferably higher than 0.020%, and more preferably 0.04% or higher. The V content is preferably lower than 0.16%.
  • Ca Calcium bonds with S in steel to form CaS.
  • CaS As CaS is formed, the formation of MnS is suppressed.
  • Ca increases the toughness and HIC resistance of the steel.
  • the Ca content should be in the range of 0.0005 to 0.005%.
  • the Ca content is preferably higher than 0.0005%, and more preferably 0.0008% or higher, and still more preferably 0.001% or higher.
  • the Ca content is preferably lower than 0.005%, and more preferably 0.003% or lower, and still more preferably 0.002% or lower.
  • the balance of the chemical composition of the seamless steel pipe in the present embodiment is made of Fe and impurities.
  • Impurity in this context means an element originating from ore or scraps used as a raw material of steel or an element that has entered from the environment or the like during the manufacturing process.
  • the chemical composition of the seamless steel pipe in the present embodiment may contain Nb in lieu of some of Fe.
  • Niobium (Nb) is an optional element. Nb bonds with C and/or N in steel and forms fine Nb carbide and/or carbonitride particles to increase the toughness of the steel. Further, Nb is dissolved in an Mo carbide and forms a specified carbide, thereby preventing coarse particles of a specified carbide from being produced. On the other hand, if the Nb content is higher than 0.05%, coarse carbide particles result. In view of this, the Nb content should be in the range of 0 to 0.05%. The above effects can be clearly achieved if the Nb content is 0.010% or higher. The Nb content is preferably 0.015% or higher, and more preferably 0.020% or higher. The Nb content is preferably 0.040% or lower, and more preferably 0.035% or lower.
  • a carbon equivalent Ceq as defined by equation (1) is not less than 0.430% and less than 0.500%.
  • the carbon equivalent Ceq is lower than 0.430%, it is difficult to ensure a certain strength of a seamless steel pipe. If the carbon equivalent Ceq is 0.500 or higher, it is difficult to reduce the Vickers hardness of the surface layer to 250 Hv or lower with a manufacturing process in which the quenching after hot forming is only one step of direct quenching or in-line quenching.
  • the main phase from the surface layer to the in-the-wall portions is tempered martensite or tempered bainite.
  • the seamless steel pipe in the present embodiment contains no recrystallized ferrite at least in a region deeper than a position 1 mm deep relative to the surface. Recrystallized ferrite extremely reduces the hardness of a portion at 1 mm from the surface layer of the seamless steel pipe.
  • the main phase being tempered martensite or tempered bainite generally means a microstructure in which the volume fraction of tempered martensite is 50% or higher, a microstructure in which the volume fraction of tempered bainite is 50% or higher, or a microstructure in which the sum of the volume fraction of tempered martensite and the volume fraction of tempered bainite is 50% or higher.
  • the above phrase means a microstructure in which the volume fraction of a structure that is neither tempered martensite nor tempered bainite (for example, ferrite) is lower than 50%.
  • the size of the prior austenite grains is lower than 6.0 in crystal grain size number, as defined in ASTM E112-10.
  • the prior austenite grain size number may be measured in accordance with ASTM E112-10 by cutting out a test specimen from each steel pipe preferably before tempering and after quenching, such that a cross section perpendicular to the length of the steel pipe (i.e. pipe forming direction) forms the observed surface, and imbedding the test specimen into a resin and then using the Bechet-Beaujard method where it is corroded by a picric acid saturated aqueous solution to let prior austenite grain boundaries appear.
  • the ASTM grain size number of prior austenite crystal grains of the tempered steel pipe may be determined by using methods such as electron beam backward scattering diffraction (EBSD) based on the orientation relationship of crystals.
  • EBSD electron beam backward scattering diffraction
  • the metal microstructure of a steel pipe after tempering is observed by EBSD in the following manner: A sample is obtained from the middle in the wall thickness in a cross section of a tempered seamless steel pipe (i.e.
  • the obtained sample is used to perform crystal orientation analysis by EBSD for an observed area of 500 ⁇ 500 pmt, and lines are drawn where a prior austenite grain boundary is defined as the boundary of grains in a misorientation angle in the range of 15 to 51° and, based on the resulting drawing, the crystal grain size number is calculated in accordance with ASTM E112-10.
  • the prior austenite grain size after quenching and before tempering is the same as the prior austenite grain size after tempering.
  • the prior austenite grain size determined by EBSD after tempering is substantially equal to the value obtained by observing crystal grains that were caused to appear by the Bechet-Beaujard method after quenching and before tempering, with an error of about ⁇ 0.2 in grain size number.
  • “the size of the prior austenite grains is lower than 6.0 in crystal grain size number, as defined in ASTM E112-10” as in the present invention means that, if the crystal grain size after quenching is not known, at least, a crystal grain size number determined by EBSD after tempering being lower than 5.8 is in the scope of the present invention.
  • prior austenite grain size is a value obtained by the Bechet-Beaujard method for a test specimen after quenching and before tempering.
  • prior austenite grains are fine grains with a crystal grain size number of 6.0 or higher, sufficient hardenability cannot be achieved in a material with a low carbon equivalent Ceq, as in the present embodiment. Thus, a predetermined strength may not be obtained. Further, it is difficult to produce a microstructure with such fine grains with a manufacturing process in which the quenching after hot forming is only one step of direct quenching or in-line quenching.
  • the crystal grain size number of prior austenite grains is preferably 5.5 or lower, and more preferably, 5.0 or lower.
  • a portion between a position at 1 mm from the inner surface and a position at 1 mm from the outer surface has a Vickers hardness of 250 Hv or lower. More specifically, in the seamless steel pipe in the present embodiment, the Vickers hardness measured in compliance with JIS Z 2244 at any position between a position at 1 mm from the inner surface and a position at 1 mm from the outer surface is 250 Hv or lower.
  • the seamless steel pipe of the present invention has smaller variations in hardness along the wall thickness direction. More specifically, the difference between the Vickers hardness of a portion at 1 mm from the inner surface and that of a portion in the middle in the wall thickness, the difference between the Vickers hardness of a portion at 1 mm from the outer surface and that of a portion in the middle in the wall thickness, and the difference between the Vickers hardness of a portion at 1 mm from the inner surface and that of a portion at 1 mm from the outer surface is 25 Hv or lower.
  • the seamless steel pipe in the present embodiment has a yield strength of ⁇ 80 grade or higher (i.e. 555 MPa or higher) according to the API standards.
  • the seamless steel pipe in the present embodiment may be suitably used as, although not limited thereto, a seamless steel pipe with a wall thickness of 25 to 55 mm. More preferably, to rationalize the use of alloys, the wall thickness of a seamless steel pipe is in the range of 25 to 40 mm.
  • FIG. 1 is a block diagram illustrating an example of a manufacturing line.
  • the manufacturing line includes a heating furnace 1 , a piercing machine 2 , an elongation rolling mill 3 , a sizing rolling mill 4 , a supplementary heating furnace 5 , a water-cooling apparatus 6 , and a tempering apparatus 7 .
  • a plurality of transport rollers 10 are disposed between these apparatuses.
  • FIG. 2 is a flow chart illustrating a process for manufacturing the seamless steel pipe in the present embodiment.
  • FIG. 3 shows changes in the surface temperature of a workpiece (i.e. a steel raw material, hollow shell or seamless steel pipe) during a manufacture versus time.
  • A1 indicates the Ac 1 point when considering a workpiece being heated, and indicates the Ar 1 point when considering a workpiece being cooled.
  • A3 indicates the Ac 3 point when considering a workpiece being heated, and indicates the Ar 3 point when considering a workpiece being cooled.
  • the manufacturing process involves first heating a steel raw material using the heating furnace 1 (heating step: S 1 ).
  • the steel raw material may be a round billet, for example.
  • the steel raw material may be produced by a continuous casting system such as round CC.
  • the steel raw material may be produced by hot working (e.g. forging or blooming) an ingot or slab. A case with a steel raw material that is a round billet will be described below.
  • the heated round billet is hot-worked to produce a seamless steel pipe (S 2 and S 3 ). More specifically, the round billet is piercing-rolled by the piercing machine 2 to produce a hollow shell (piercing-rolling step: S 2 ). Further, the hollow shell is rolled by the elongation rolling mill 3 and sizing rolling mill 4 to produce a seamless steel pipe (elongation rolling step and sizing rolling step S 3 ).
  • the seamless steel pipe produced by the hot working is heated to a predetermined temperature by the supplementary heating furnace 5 as necessary (supplementary heating step: S 4 ).
  • the seamless steel pipe produced by the hot working or the heated seamless steel pipe is quenched by the water-cooling apparatus 6 (quenching step: S 5 ). In either case, the seamless steel pipe produced by the hot working is quenched without being cooled to lower than Ar 3 temperature.
  • the quenched seamless steel pipe is tempered by the tempering apparatus 7 (tempering step S 6 ).
  • quenching is performed promptly after the hot working is finished. More specifically, after hot working, quenching is performed before the temperature of the seamless steel pipe is left to cool to decrease to around room temperature.
  • a heat treatment where a seamless steel pipe after hot working is rapidly cooled before the surface temperature becomes lower than the Ar 3 point will be hereinafter referred to as “direct quenching”, and a heat treatment where a seamless steel pipe after hot working is supplementarily heated at a temperature not lower than the Ac 3 point and then rapidly cooled will be hereinafter referred to as “in-line quenching”.
  • reheating-and-quenching makes the grains of the microstructure coarser than with a heat treatment in which a pipe is cooled after its production and then rapidly cooled. More specifically, the crystal grain size number after quenching is smaller than 6.0. This improves the hardenability of a microstructure compared with the reheating-and-quenching, and thus ensures a high strength even when a steel material with a low carbon equivalent Ceq is used.
  • a round billet is heated in the heating furnace 1 .
  • the heating temperature is preferably in the range of 1100 to 1300° C. Heating the round billet to this temperature range causes the carbonitride in the steel to dissolve. If a round billet is to be produced from a slab or ingot by hot working, it is only required that the slab or ingot be heated to a temperature of 1100 to 1300° C., and the temperature to which the round billet is heated by the heating furnace 1 does not have to be in the range of 1100 to 1300° C., because the carbonitride in the steel dissolves when the ingot or slab is being heated.
  • the heating furnace 1 may be a walking-beam furnace or a rotary furnace, for example.
  • the round billet is removed from the heating furnace 1 and the heated round billet is piercing-rolled by the piercing machine 2 to produce a hollow shell.
  • the piercing machine 2 includes a plurality of skewed rolls and a plug. The plug is disposed between the skewed rolls.
  • the piercing machine 2 is a cross-type piercer. A cross-type piercer is preferable because it can do piercing at high pipe expansion rate.
  • the hollow shell is rolled. More specifically, the hollow shell is elongation-rolled by the elongation rolling mill 3 .
  • the elongation rolling mill 3 includes a plurality of roll stands disposed in series.
  • the elongation rolling mill 3 may be a mandrel mill, for example.
  • the hollow shell that has been subjected to elongation rolling is subjected to reduction rolling by the sizing rolling mill 4 to produce a seamless steel pipe.
  • the sizing rolling mill 4 includes a plurality of roll stands disposed in series.
  • the sizing rolling mill 4 may be a sizer or stretch reducer, for example.
  • the elongation rolling step and sizing rolling step together may be referred to simply as rolling step.
  • the supplementary heating step (S 4 ) is performed as necessary. That is, the manufacturing method in the present embodiment need not include the supplementary heating step (S 4 ). More specifically, the supplementary heating step (S 4 ) is performed in such a way that the temperature of the seamless steel pipe is at a predetermined level that is equal to or higher than the Acs point directly before the water cooling of the quenching step (S 5 ). If the supplementary heating step (S 4 ) is not performed, the method in FIG. 2 proceeds from step S 3 to step S 5 . If the supplementary heating step (S 4 ) is not performed, the supplementary heating furnace 5 in FIG. 1 may not be provided.
  • the finishing temperature of the rolling step i.e. surface temperature of the seamless steel pipe directly after the rolling step is finished
  • the supplementary heating step (S 4 ) the seamless steel pipe is inserted into the supplementary heating furnace 5 and heated.
  • the heating temperature in the supplementary heating furnace 5 is preferably in the range of 900 to 1100° C.
  • the soaking time is preferably 30 minutes or less. If the soaking time is too long, carbonitrides made of Ti, Nb, C and N, i.e. (Ti, Nb) and (C, N), may precipitate and form coarse particles.
  • the supplementary heating furnace 5 may be replaced by an induction heating apparatus.
  • the seamless steel pipe is water-cooled in the water-cooling apparatus 6 .
  • the temperature (i.e. surface temperature) of the seamless steel pipe directly before water cooling is equal to or higher than the Ac 3 point, and preferably equal to or higher than 800° C.
  • the cooling rate for the temperature range of the seamless steel pipe from 800° C. to 500° C. is equal to or higher than 5° C./sec (300° C./min). This provides a uniform quenched microstructure.
  • the cooling is stopped at a temperature that is equal to or lower than the Ar 1 point.
  • the temperature at which cooling is stopped is preferably 450° C. or lower, and the cooling may be done down to room temperature.
  • the quenching step (S 5 ) changes the structure of the matrix to a structure mainly composed of martensite or bainite.
  • the water-cooling aperture 6 used for the quenching step (S 5 ) may have the following construction:
  • the water-cooling apparatus 6 includes a plurality of rotating rollers, laminar water flow device, and a jet water flow device.
  • the rotating rollers are disposed in two rows, and the seamless steel pipe is positioned between the two rows of rotating rollers. At this time, the rotating rollers in the two rows are in contact with bottom portions of the outer surface of the seamless steel pipe.
  • the laminar water flow device is located above the rotating rollers and pours water from above the seamless steel pipe. At this time, the water poured toward the seamless steel pipe forms a laminar water flow.
  • the jet water flow device is located near an end of the seamless steel pipe positioned on the rotating rollers.
  • the jet water flow device emits a jet water flow from the end of the seamless steel pipe toward the interior of the steel pipe.
  • the laminar and jet water flow devices cool the outer and inner surfaces of the seamless steel pipe at the same time.
  • a water-cooling device 6 with such a construction is suitable for accelerated cooling for a seamless steel pipe with a large wall thickness of 25 mm or larger.
  • the water-cooling device 6 may be a device other than the one including rotating rollers, laminar water flow device and jet water flow device discussed above.
  • the water cooling device 6 may be a water tank, for example. In such implementations, the seamless steel pipe is immersed in the water tank and thus subjected to accelerated cooling.
  • the water-cooling device 6 may include a laminar water flow device only. To sum up, the cooling device 6 is not limited to a specific type.
  • the quenched seamless steel pipe is subjected to tempering. More specifically, the quenched seamless steel pipe is heated to a predetermined tempering temperature that is lower than the Ac1 point, and is held at this temperature for a predetermined period of time in such a way that the Larson-Millar parameter PL as defined by equation (2) below is 18800 or higher:
  • T is a tempering temperature (° C.)
  • t is a holding time (in hours) for that temperature.
  • Log (t) is the logarithm of t whose base is 10.
  • PL is lower than 18800, the reduction in surface hardness is insufficient and some portions may have a Vickers hardness exceeding 250 Hv.
  • PL is preferably 18900 or higher.
  • PL is preferably 20000 or lower, and more preferably 19500 or lower.
  • the lower limit of tempering temperature is preferably 600° C., and more preferably 630° C., and still more preferably 650° C.
  • the upper limit of tempering temperature is preferably 700° C., and more preferably 680° C.
  • the lower limit of holding time is preferably one hour, and more preferably two hours, and still more preferably three hours.
  • the upper limit of holding time is preferably six hours, and more preferably five hours, and still more preferably four hours.
  • the above manufacturing process provides a seamless steel pipe with a wall thickness that is as large as 25 mm or more having good strength, toughness and HIC resistance.
  • the above manufacturing method is particularly suitable for a seamless steel pipe with a wall thickness of 25 mm or larger, and can even be used for a seamless steel pipe with a wall thickness of 40 mm or larger.
  • the upper limit of wall thickness is not limited to a specific value, but is typically 60 mm or lower.
  • the seamless steel pipe in one embodiment of the present invention and the method of manufacturing the same have been described.
  • the present embodiment provides a seamless steel pipe that can be manufactured by a relatively reasonable manufacturing process and that provides a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner.
  • a plurality of seamless steel pipes with various chemical compositions were produced and their yield strength, tensile strength, surface hardness and sour resistance were investigated.
  • Steels A, C, D1, D2 and J in Table 1 are steels in which the chemical composition or the value of Ceq does not meet the requirements of the present invention.
  • the round billets produced were heated by the heating furnace to a temperature in the range of 1100 to 1300° C. Subsequently, the round billets were piercing-rolled by the piercing machine to produce hollow shells. Subsequently, the mandrel mill was used to elongation-roll the hollow shells. Subsequently, the sizer was used to reduction-roll (i.e. sizing-roll) the hollow shells to produce seamless steel pipes having the outer diameters and wall thicknesses shown in Tables 2 and 3.
  • the seamless steel pipes that had been subjected to sizing rolling were heated by the supplementary heating furnace to 950° C., and quenching was then performed by the water-cooling apparatus where the pipes were cooled to room temperature at a cooling rate of 5° C./sec or higher.
  • the seamless steel pipes were tempered at the soaking temperatures and holding times shown in Tables 2 and 3. However, during the production of the steel of No. 62, after the above quenching was performed, before tempering, quenching was performed where the steel was reheated off-line to 950° C. and soaked for 20 minutes and then water-cooled.
  • the yield strength of the seamless steel pipe of each number was investigated. More specifically, a No. 12 test specimen (with a width of 25 mm and a gauge length of 50 mm) as specified in JIS Z 2241 was taken out so that the longitudinal direction of the specimen for tensile testing was parallel to the longitudinal direction of the steel pipe (i.e. L direction). The test specimen that had been taken out was used to conduct a tensile test in compliance with JIS Z 2241 in the atmosphere at room temperature (25° C.), and the yield strength (YS) and tensile strength (TS) were determined. The yield strength was determined using a 0.5% total elongation method. The determined yield strength (in MPa) and tensile strength (in MPa) are shown in Tables 2 and 3. The columns labeled “YS” in Tables 2 and 3 have yield strength and the columns labeled “TS” have tensile strengths determined for the test specimens of the various test numbers.
  • test specimens were taken from the seamless steel pipe of each number, the specimens being displaced from each other by 90° along the pipe's circumference, and a Vickers hardness test in compliance with JIS Z 2244 was conducted on arbitrary three points on a transverse cross-section (i.e. cross-section perpendicular to the center axis) of each test specimen, the points being at 1 mm inwardly in the wall thickness direction from the inner surface.
  • the force in the Vickers hardness tests, F was 10 kgf (i.e. 98.07 N). The maximum among the values for the 12 points that had been obtained was used as the value of hardness “at 1 mm from the inner surface”.
  • a Vickers hardness test was conducted on arbitrary three points of each of the four test specimens of the seamless steel pipe of each test number, the points being at 1 mm inwardly in the wall thickness direction from the outer surface, and the maximum among the values of the 12 points that had been obtained was used as the value of hardness “at 1 mm from the outer surface”. Further, a Vickers hardness test was conducted on arbitrary three points of each of the four test specimens of the seamless steel pipe of each test number, the points being near the middle in the wall thickness, and the maximum among the values of the 12 points that had been obtained was used as the value of hardness “in the wall”.
  • a sample was taken from the seamless steel pipe of each number, the sample containing the inner surface, outer surface and middle in the wall thickness, and the microstructure was observed. More specifically, each sample was etched by a nital etching solution to cause the microstructure to appear, which was observed using optical microscopy.
  • the seamless steel pipe of each number had a microstructure having a main phase of tempered martensite or tempered bainite.
  • recrystallization of ferrite had occurred in a region of a depth of 1 mm or deeper from the surface. Whether recrystallization of ferrite occurred in a region of a depth of 1 mm or deeper from the surface is shown in the column labeled “Ferrite Recrystallization” in Tables 2 and 3.
  • the crystal grain size number of the prior austenite grains of the microstructure was measured by the following method: First, a test specimen was cut out from each steel pipe such that a cross section perpendicular to the length of the steel pipe as quenched (i.e. pipe forming direction) forms the observed surface, and was imbedded into a resin; then the Bechet-Beaujard method was used where it is corroded by a picric acid saturated aqueous solution to let prior austenite grain boundaries appear, which were observed by optical microscopy (with a magnification of 200 times), and the prior austenite grain size number was measured in accordance with ASTM E112-10. Such grain size numbers are shown in the column “AsQ Prior ⁇ grain size No.” in Tables 2 and 3.
  • EBSD was performed by cutting out a test specimen such that a cross section perpendicular to the length of a tempered steel pipe forms the observed surface, finishing the observed surface by mirror polishing and electrolysis polishing, and an area of 500 ⁇ 500 pmt in the middle in the thickness of the steel pipe was observed.
  • the seamless steel pipes of Nos. 19 to 33 and 52 to 60 had a chemical composition falling in the scope of the present invention and had a carbon equivalent Ceq not lower than 0.430% and lower than 0.500%.
  • recrystallization of ferrite did not occur in a region of a depth of 1 mm or deeper from the surface, and a structure was present having a main phase of tempered martensite or tempered bainite from the surface layer to the in-the-wall portions, and the crystal grain size number of the prior austenite grains was lower than 6.0.
  • these seamless steel pipes had Vickers hardness values “at 1 mm from the outer surface”, “at 1 mm from the inner surface” and “in the wall” that were not higher than 250 Hv and had a yield strength of 555 MPa or higher. These seamless steel pipes had a maximum difference in hardness of 25 Hv or lower.
  • the seamless steel pipes of Nos. 1 to 17 had a yield strength lower than 555 MPa. This is presumably because the carbon equivalent Ceq of steel A was too low.
  • the seamless steel pipes of Nos. 34 to 42 and 47 to 51 had a Vickers hardness value “at 1 mm from the outer surface”, “at 1 mm from the inner surface” or “in the wall” that was higher than 250 Hv. Further, these seamless steel pipes had a maximum difference in hardness higher than 25 By. This is presumably because the Larson-Miller parameters PL of the seamless steel pipes of Nos. 34 to 42 and 47 to 51 were too low.
  • the seamless steel pipes of Nos. 43 and 44 had a Vickers hardness “at 1 mm from the inner surface” higher than 250 Hv. This is presumably because the carbon equivalent Ceq of steel C was too high.
  • the seamless steel pipes of Nos. 45 and 46 had yield strengths lower than 555 MPa. This is presumably because the carbon equivalents Ceq of steels D1 and D2 were too low.
  • the seamless steel pipe of No. 62 had a yield strength lower than 555 MPa. This is presumably because both in-line quenching and reheating-and-quenching were used, which produced too fine prior austenite grains, reducing hardenability and thus leading to insufficient strength.
  • FIG. 4 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and yield strength YS for steel B. As shown in FIG. 4 , the yield strength YS tended to decrease as the Larson-Miller parameter PL increased. Steel B provided a yield strength of 555 MPa or larger except for the seamless steel pipe of No. 18, in which the recrystallization of ferrite progressed.
  • FIG. 5 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and yield strength YS for steel A.
  • Steel A did not provide a yield strength not lower than 555 MPa even though tempering conditions were adjusted. This is presumably because the carbon equivalent Ceq of steel A was too low.
  • FIG. 6 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and hardness at an outer surface, an in-the-wall portion and an inner surface for steel B.
  • the hardnesses at the outer surface, in-the-wall portion and inner surface tended to decrease as the Larson-Miller parameter PL increased.
  • the Larson-Miller parameter PL was 18800 or higher, the hardnesses at the outer surface, in-the-wall portion and inner surface were 250 Hv or lower.
  • the Larson-Miller parameter PL was lower than 18800, the hardness at the outer surface, in-the-wall portion or inner surface was higher than 250 Hv.
  • FIG. 7 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and hardness at an outer surface, an in-the-wall portion and an inner surface for steel A.
  • steel A similar to steel B, the hardnesses at the outer surface, in-the-wall portion and inner surface tended to decrease as the Larson-Miller parameter increased.
  • FIG. 8 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and maximum difference in hardness for steel B. As shown in FIG. 8 , when the Larson-Miller parameter PL was 18800 or higher, the maximum difference in hardness was not more than 25 Hv. The seamless steel pipe of No. 18 had a large maximum difference in hardness presumably because the recrystallization of ferrite progressed in a region of a depth of 1 mm or deeper from the surface.
  • FIG. 9 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and maximum difference in hardness for steel A. As shown in FIG. 9 , the relationship between Larson-Miller parameter PL and maximum difference in hardness in steel A exhibited similar tendencies.
  • the seamless steel pipe of No. 3 had a large maximum difference in hardness presumably because the recrystallization of ferrite progressed in a region of a depth of 1 mm or deeper from the surface.
  • a sour resistance evaluation as described below (i.e. HIC resistance test, four-point bending test) was conducted on the seamless steel pipes of some of the numbers.
  • test specimen containing the inner surface From each seamless steel pipe were taken out a test specimen containing the inner surface, a test specimen containing the middle in the wall thickness, and a test specimen containing the outer specimen.
  • Each test specimen had a thickness of 20 mm and a width (along the circumference) of 20 mm, and a length of 100 mm.
  • the MC resistance of each test specimen was evaluated in accordance with NACE (National Association of Corrosion Engineers) TM 0284-2011.
  • the testing bath in which the test specimens were immersed was a 5% salt+0.5% acetic acid aqueous solution saturated with hydrogen sulfide gas at 1 atm at a temperature of 24° C.
  • each test specimen after being tested had a blister (i.e. a swollen part due to a crack near the surface), and the number of blisters produced on the test specimen was counted. The maximum among the numbers of blisters on the test specimen taken from each steel pipe was used as the number of blisters for this test number.
  • a stress of 95% of the actual yield strength (i.e. yield strength of the seamless steel pipe of each number) was applied to a test specimen containing the middle in the wall thickness of this seamless steel pipe using a four-point bending jig in accordance with ASTM G39.
  • the test specimens to which stresses were applied were placed in a test bath.
  • the test bath was a 5% salt+0.5% acetic acid aqueous solution saturated with hydrogen sulfide gas at 1 atm at a temperature of 24° C. After 720 hours, it was visually determined whether there was a crack in the test specimens. If a plate material had no crack, it was determined that this material had good SSC resistance.

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CA3013287A1 (fr) 2017-08-24
BR112018007744A2 (pt) 2018-10-23
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MX2018005240A (es) 2018-08-01
EP3418410A1 (fr) 2018-12-26
AU2016393486A1 (en) 2018-04-26
RU2706257C1 (ru) 2019-11-15
JPWO2017141341A1 (ja) 2018-02-22
BR112018007744B1 (pt) 2021-09-21
WO2017141341A1 (fr) 2017-08-24
CA3013287C (fr) 2019-12-31
JP6112267B1 (ja) 2017-04-12
CN108699644A (zh) 2018-10-23
AU2016393486B2 (en) 2019-07-18
EP3418410B1 (fr) 2021-04-07

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