US20220098712A1 - Steel material suitable for use in sour environment - Google Patents

Steel material suitable for use in sour environment Download PDF

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US20220098712A1
US20220098712A1 US17/422,870 US202017422870A US2022098712A1 US 20220098712 A1 US20220098712 A1 US 20220098712A1 US 202017422870 A US202017422870 A US 202017422870A US 2022098712 A1 US2022098712 A1 US 2022098712A1
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Prior art keywords
steel material
content
tempering
test
steel
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Inventor
Hiroki KAMITANI
Yohei Otome
Atsushi Soma
Taro OE
Nobuaki Komatsubara
Shinji Yoshida
Yuji Arai
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Nippon Steel Corp
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Nippon Steel Corp
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Assigned to NIPPON STEEL CORPORATION reassignment NIPPON STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: ARAI, YUJI, KAMITANI, Hiroki, KOMATSUBARA, Nobuaki, OE, TARO, OTOME, YOHEI, SOMA, ATSUSHI, YOSHIDA, SHINJI
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Definitions

  • the present disclosure relates to a steel material, and more particularly to a steel material suitable for use in a sour environment.
  • oil wells and gas wells Due to the deepening of oil wells and gas wells (hereunder, oil wells and gas wells are collectively referred to as “oil wells”), there is a demand to enhance the strength of oil-well steel materials represented by oil-well steel pipes.
  • 80 ksi grade yield strength is 80 to less than 95 ksi, that is, 552 to less than 655 MPa
  • 95 ksi grade yield strength is 95 to less than 110 ksi, that is, 655 to less than 758 MPa
  • oil-well steel pipes are being widely utilized, and recently requests are also starting to be made for 110 ksi grade (yield strength is 110 to 125 ksi, that is, 758 to 862 MPa) oil-well steel pipes.
  • a sour environment means an environment which contains hydrogen sulfide, and which is acidified.
  • a sour environment may contain carbon dioxide.
  • Oil-well steel pipes used in such a sour environment are required to have not only high strength but also sulfide stress cracking resistance (hereinafter, referred to as “SSC resistance”).
  • Patent Literature 1 Japanese Patent Application Publication No. 62-253720
  • Patent Literature 2 Japanese Patent Application Publication No. 59-232220
  • Patent Literature 3 Japanese Patent Application Publication No. 06-322478
  • Patent Literature 4 Japanese Patent Application Publication No. 08-311551
  • Patent Literature 5 Japanese Patent Application Publication No. 2000-256783
  • Patent Literature 6 Japanese Patent Application Publication No. 2000-297344
  • Patent Literature 7 Japanese Patent Application Publication No. 2005-350754
  • Patent Literature 8 National Publication of International Patent Application No. 2012-519238
  • Patent Literature 9 Japanese Patent Application Publication No. 2012-26030
  • Patent Literature 1 proposes a method for increasing SSC resistance of steel for oil well by reducing impurities, such as Mn and P.
  • Patent Literature 2 proposes a method for increasing SSC resistance of steel by performing quenching two times to make grain fine.
  • Patent Literature 3 proposes a method for increasing SSC resistance of a steel material having 125 ksi grade by making the micro-structure of steel fine by induction heat treatment.
  • Patent Literature 4 proposes a method for increasing SSC resistance of a steel pipe having 110 to 140 ksi grade by increasing the hardenability of steel by utilizing direct quenching process and also by increasing a tempering temperature.
  • Patent Literature 5 and Patent Literature 6 propose a method for increasing SSC resistance of steel for low alloy oil country tubular goods having 110 to 140 ksi grade by controlling the morphology of carbide.
  • Patent Literature 7 proposes a method for increasing SSC resistance of a steel material having 125 ksi grade or more by controlling dislocation density and a hydrogen diffusion coefficient to predetermined values.
  • Patent Literature 8 proposes a method for increasing SSC resistance of steel having 125 ksi grade by performing quenching a plurality of times on low alloy steel which contains C of 0.3 to 0.5%.
  • Patent Literature 9 proposes a method for controlling the morphology and the number of carbide by adopting a tempering process of two-stage heat treatment. More specifically, in Patent Literature 9, a number density of large-sized M 3 C or M 2 C is suppressed to increase SSC resistance of steel having 125 ksi grade.
  • Patent Literature 1 Japanese Patent Application Publication No. 62-253720
  • Patent Literature 2 Japanese Patent Application Publication No. 59-232220
  • Patent Literature 3 Japanese Patent Application Publication No. 06-322478
  • Patent Literature 4 Japanese Patent Application Publication No. 08-311551
  • Patent Literature 5 Japanese Patent Application Publication No. 2000-256783
  • Patent Literature 6 Japanese Patent Application Publication No. 2000-297344
  • Patent Literature 7 Japanese Patent Application Publication No. 2005-350754
  • Patent Literature 9 Japanese Patent Application Publication No. 2012-26030
  • a steel material according to the present disclosure has a chemical composition consisting of, in mass %: C: 0.20 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.60 to 1.50%, Mo: more than 1.00 to 2.00%, Ti: 0.002 to 0.020%, V: 0.05 to 0.30%, Nb: 0.005 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: less than 0.0020%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, and W: 0 to 0.50%, with the balance being Fe and impurities, and satisfying Formula (1).
  • a grain diameter of a prior-austenite grain is 11.0 ⁇ m or less.
  • An average area of precipitate which is precipitated in a prior-austenite grain boundary is 10.0 ⁇ 10 ⁇ 3 ⁇ m 2 or less in the steel material.
  • a yield strength of the steel material is 758 to 862 MPa.
  • the steel material according to the present disclosure has yield strength of 758 to 862 MPa (110 ksi grade), and also has excellent SSC resistance in a sour environment.
  • FIG. 1 is a view showing the relationship between Mo content and prior ⁇ grain diameter.
  • the present inventors have conducted investigations and studies regarding a method for obtaining excellent SSC resistance of a steel material which is expected to be used in a sour environment while yield strength of 758 to 862 MPa (110 ksi grade) is maintained. As a result, the following findings are obtained.
  • dislocation density in the steel material increases yield strength YS of the steel material. Meanwhile, there is a possibility that dislocations in the steel material occlude hydrogen. Therefore, when dislocation density in the steel material is increased, the amount of hydrogen occluded by the steel material may be increased. When hydrogen concentration in the steel material is increased as a result of an increase in dislocation density, high strength may be obtained, but SSC resistance of the steel material is reduced. Accordingly, to achieve both yield strength of 110 ksi grade and excellent SSC resistance, it is not preferable to increase strength by making use of dislocation density.
  • the present inventors considered that when yield strength of a steel material is increased using a method different from an increase in dislocation density of the steel material, excellent SSC resistance may be obtained even if yield strength of the steel material is increased to 110 ksi grade.
  • a steel material having the chemical composition including, in mass %: C: 0.20 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.60 to 1.50%, Ti: 0.002 to 0.020%, V: 0.05 to 0.30%, Nb: 0.005 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: less than 0.0020%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, and W: 0 to 0.50%, may achieve both yield strength of 110 ksi grade and SSC resistance.
  • the present inventors further considered that when Mo is contained in addition to the aforementioned chemical composition, alloy carbide is formed and hence, yield strength may be increased without increasing dislocation density excessively. Accordingly, the present inventors produced various steel materials where Mo is added to the aforementioned chemical composition, and investigated characteristics of the steel materials. As a result, the present inventors have newly found that, in the steel material having the aforementioned chemical composition, Mo content and the grain diameter of prior-austenite grain (hereinafter, also referred to as “prior ⁇ grain”) have dependencies.
  • FIG. 1 is a view showing the relationship between Mo content and a prior ⁇ grain diameter.
  • FIG. 1 is formed using Mo contents (mass %) and prior ⁇ grain diameters ( ⁇ m) acquired by microstructure observation described later with respect to steel materials which have the chemical composition other than Mo content satisfying the range of the aforementioned chemical composition, and which are produced by a preferred production method described later in an example which will be described later.
  • “prior ⁇ grain diameter” means the grain diameter of a prior ⁇ grain obtained by a method conforming to a comparison method defined in ASTM E112-10.
  • the prior ⁇ grain diameter is dramatically reduced. It became apparent that, in the steel material having the aforementioned chemical composition, when the Mo content becomes more than 1.00%, notable advantageous effect of reducing a prior ⁇ grain diameter to 11.0 ⁇ m or less is obtained. Further, when a prior ⁇ grain is fine, the steel material can increase both the yield strength and the SSC resistance. Accordingly, the chemical composition of the steel material according to the present embodiment contains Mo of more than 1.00 to 2.00% in addition to the aforementioned chemical composition. In this case, the prior ⁇ grain diameter in the steel material becomes 11.0 ⁇ m or less.
  • the present inventors consider the reason as follows.
  • the steel material having the aforementioned chemical composition contains Mo of more than 1.00 to 2.00%
  • Mo dissolved in the steel material segregates in austenite grain boundaries during heating in a quenching process.
  • dissolved Mo segregated in austenite grain boundaries suppresses the movement of grain boundaries.
  • austenite grain is prevented from being easily coarsened during heating in a quenching process and hence, it is considered that prior ⁇ grain on which tempering is performed is made fine.
  • F1 is defined as 2.7 ⁇ C+0.4 ⁇ Si+Mn+0.45 ⁇ Ni+0.45 ⁇ Cu+0.8 ⁇ Cr+2 ⁇ Mo.
  • F1 is the index of the hardenability of the steel material.
  • the steel material according to the present embodiment has the aforementioned chemical composition, and further has F1 of 3.90 or more.
  • the steel material according to the present embodiment has a chemical composition consisting of, in mass %: C: 0.20 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.60 to 1.50%, Mo: more than 1.00 to 2.00%, Ti: 0.002 to 0.020%, V: 0.05 to 0.30%, Nb: 0.005 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: less than 0.0020%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, W: 0 to 0.50%, and the balance being Fe and impurities, and the aforementioned F1 is 3.90
  • the present inventors have found that, of the coarse carbide, particularly, coarse carbide which is precipitated in prior ⁇ grain boundaries may cause a reduction in SSC resistance of a steel material. That is, the present inventors have found that SSC resistance of a steel material can be increased not by simply reducing coarse carbide but by reducing coarse carbide which is precipitated in the prior ⁇ grain boundaries.
  • the steel material according to the present embodiment has the aforementioned chemical composition and a prior ⁇ grain diameter of 11.0 ⁇ m or less and, further, reduces coarse carbide which is precipitated in the prior ⁇ grain boundaries.
  • the steel material according to the present embodiment has the aforementioned chemical composition and the prior ⁇ grain diameter of 11.0 ⁇ m or less.
  • the average area of the precipitates in the prior ⁇ grain boundary is 10.0 ⁇ 10 ⁇ 3 ⁇ m 2 or less.
  • the steel material according to the present embodiment can achieve both yield strength of 758 to 862 MPa (110 ksi grade) and excellent SSC resistance in a sour environment.
  • the steel material according to the present embodiment completed based on the aforementioned findings has the chemical composition consisting of, by mass %, C: 0.20 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.60 to 1.50%, Mo: more than 1.00 to 2.00%, Ti: 0.002 to 0.020%, V: 0.05 to 0.30%, Nb: 0.005 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: less than 0.0020%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, and W: 0 to 0.50%, with the balance being Fe and impurities, and satisfying
  • the grain diameter of a prior-austenite grain is 11.0 ⁇ m or less.
  • the average area of precipitates which are precipitated in the prior-austenite grain boundary is 10.0 ⁇ 10 ⁇ 3 ⁇ m 2 or less.
  • a yield strength of the steel material is 758 to 862 MPa.
  • the steel material is not particularly limited.
  • the steel material may be a steel pipe or a steel plate, for example.
  • the steel material according to the present embodiment exhibits yield strength of 758 to 862 MPa (110 ksi grade) and excellent SSC resistance.
  • the aforementioned chemical composition may contain one or more types of element selected from the group consisting of Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, and rare earth metal: 0.0001 to 0.0100%.
  • the aforementioned chemical composition may contain one or more types of element selected from the group consisting of Cu: 0.02 to 0.50%, and Ni: 0.02 to 0.50%.
  • the aforementioned chemical composition may contain one or more types of element selected from the group consisting of Co: 0.02 to 0.50%, and W: 0.02 to 0.50%.
  • the aforementioned steel material may be an oil-well steel pipe.
  • the oil-well steel pipe may be a steel pipe that is used for a line pipe or may be a steel pipe used for oil country tubular goods (OCTG).
  • OCTG oil country tubular goods
  • the shape of the oil-well steel pipe is not limited, and for example, the oil-well steel pipe may be a seamless steel pipe or may be a welded steel pipe.
  • the oil country tubular goods are, for example, steel pipes that are used for use in casing or tubing.
  • the aforementioned steel material may be a seamless steel pipe.
  • the steel material according to the present embodiment is a seamless steel pipe, even if a wall thickness is 15 mm or more, the steel material has yield strength of 758 to 862 MPa (110 ksi grade), and also has more stable SSC resistance in a sour environment.
  • the aforementioned excellent SSC resistance can be evaluated specifically by a method in accordance with “Method A” specified in NACE TM0177-2005 and a four-point bending test.
  • a mixed aqueous solution containing 5.0 mass % of sodium chloride and 0.5 mass % of acetic acid (NACE solution A) at 4° C. is employed as a test bath.
  • NACE solution A acetic acid
  • a stress equivalent to 90% of the actual yield stress is applied to the test specimen that is taken from the steel material according to the present embodiment, and the test specimen is immersed into the test bath.
  • the test bath is degassed, thereafter, H 2 S gas at 1 atm is blown into the test bath to cause saturation of the H 2 S gas.
  • the test bath where saturation of the H 2 S gas is caused is held for 720 hours at 4° C.
  • test specimen taken from the steel material by four-point bending in accordance with ASTM G39-99 (2011) such that stress applied to the test specimen is set to 90% of actual yield stress of the steel material.
  • 5.0 mass % sodium chloride aqueous solution at 24° C. is employed as a test bath.
  • the test specimen to which stress is applied is immersed into the test bath in the autoclave.
  • the test bath is degassed, thereafter, H 2 S gas at 20 atm is pressure-sealed into the autoclave. After the autoclave is sealed, the test bath is stirred for 720 hours at 24° C.
  • % in relation to an element means mass %.
  • the chemical composition of the steel material according to the present embodiment contains the following elements.
  • Carbon (C) increases the hardenability of the steel material, thus increasing the yield strength of the steel material. Further, C promotes spheroidization of carbides during tempering in a production process, thus the SSC resistance of the steel material is increased. When the carbides are dispersed, the yield strength of the steel material is further increased. When the C content is too low, these advantageous effects cannot be obtained. On the other hand, when the C content is too high, toughness of a steel material is reduced so that quenching cracks are liable to occur. Accordingly, the C content is within the range of 0.20 to 0.45%. A preferable lower limit of the C content is 0.21%, more preferably is 0.22%, and further preferably is 0.25%. A preferable upper limit of the C content is 0.40%, more preferably is 0.38%, and further preferably is 0.35%.
  • Si deoxidizes the steel.
  • the Si content is within a range of 0.05 to 1.00%.
  • a preferable lower limit of the Si content is 0.10%, and more preferably is 0.15%.
  • a preferable upper limit of the Si content is 0.85%, more preferably is 0.70%, and further preferably is 0.60%.
  • Mn Manganese deoxidizes the steel. Mn also enhances the hardenability of a steel material, thus the yield strength of the steel material is increased. When the Mn content is too low, these advantageous effects cannot be obtained. On the other hand, when the Mn content is too high, Mn segregates in grain boundaries together with impurities, such as P and S. In this case, the SSC resistance of the steel material is reduced. Accordingly, the Mn content is within a range of 0.01 to 1.00%. A preferable lower limit of the Mn content is 0.02%, more preferably is 0.03%, and further preferably is 0.10%. A preferable upper limit of the Mn content is 0.80%, more preferably is 0.70%, further preferably is 0.65%, further preferably is less than 0.60%, and further preferably is 0.55%.
  • Phosphorus (P) is an impurity. That is, the P content is more than 0%. P segregates at the grain boundaries, and reduces the SSC resistance of a steel material. Accordingly, the P content is 0.030% or less. A preferable upper limit of the P content is 0.025%, and more preferably is 0.020%. Preferably, the P content is as low as possible. However, when the P content is excessively reduced, the production cost increases significantly. Accordingly, in consideration of industrial production, a preferable lower limit of the P content is 0.0001%, more preferably is 0.0003%, further preferably is 0.001%, and further preferably is 0.002%.
  • S is an impurity. That is, the S content is more than 0%. S segregates at the grain boundaries, and reduces the SSC resistance of a steel material. Accordingly, the S content is 0.0050% or less. A preferable upper limit of the S content is 0.0040%, more preferably is 0.0030%, and further preferably is 0.0020%. Preferably, the S content is as low as possible. However, when the S content is excessively reduced, the production cost increases significantly. Accordingly, in consideration of industrial production, a preferable lower limit of the P content is 0.0001%, and more preferably is 0.0003%.
  • the Al content is within a range of 0.005 to 0.100%.
  • a preferable lower limit of the Al content is 0.015%, and more preferably is 0.020%.
  • a preferable upper limit of the Al content is 0.080%, and more preferably is 0.060%.
  • the “Al” content means content of “acid-soluble Al”, that is, the content of “sol. Al”.
  • Chromium (Cr) increases the hardenability of the steel material, and increasing the yield strength of the steel material. Further, Cr increases temper softening resistance, and enabling high temperature tempering. As a result, the SSC resistance of the steel material is increased. When the Cr content is too low, these advantageous effects cannot be obtained. On the other hand, when the Cr content is too high, coarse carbides are formed in prior ⁇ grain boundaries in the steel material. In this case, the SSC resistance of the steel material is reduced. Accordingly, the Cr content is within a range of 0.60 to 1.50%.
  • a preferable lower limit of the Cr content is 0.62%, more preferably is 0.64%, further preferably is 0.65%, further preferably is 0.67%, and further preferably is 0.70%.
  • a preferable upper limit of the Cr content is 1.40%, more preferably is 1.30%, further preferably is 1.20%, further preferably is 1.10%, further preferably is less than 1.00%, and further preferably is 0.95%.
  • Molybdenum (Mo) increases the hardenability of the steel material, and increasing the yield strength of the steel material. Further, Mo is dissolved in the steel material, and a part of the dissolved Mo segregates in austenite grain boundaries during heating in a quenching process. As a result, the prior ⁇ grain diameter in the steel material on which tempering is performed is reduced by a pinning effect. In this case, the SSC resistance of the steel material is increased. When the Mo content is too low, these advantageous effects cannot be obtained. On the other hand, when the Mo content is too high, coarse carbides are formed in prior ⁇ grain boundaries in the steel material. In this case, the SSC resistance of the steel material is reduced.
  • the Mo content is within a range of more than 1.00 to 2.00%.
  • a preferable lower limit of the Mo content is 1.01%, more preferably is 1.05%, further preferably is 1.10%, further preferably is 1.15%, and further preferably is 1.20%.
  • a preferable upper limit of the Mo content is 1.90%, more preferably is 1.80%, further preferably is 1.75%, further preferably is 1.70%, and further preferably is 1.65%.
  • the Mo content is less than 2.00 times as large as the Cr content.
  • the Mo content is too high with respect to the Cr content, there may be a case where prior ⁇ grain of the steel material is coarsened. The reason for such a phenomenon is not yet clear.
  • the prior ⁇ grain diameter in the steel material can be stably set to 11.0 ⁇ m or less. Accordingly, in the chemical composition of the steel material according to the present embodiment, it is preferable that the Mo content is less than 2.00 times as large as the Cr content.
  • a preferable upper limit of the ratio of the Mo content to the Cr content is 1.98, more preferably is 1.95, and further preferably is 1.90.
  • a preferable lower limit of the Mo/Cr ratio is not particularly limited. However, in the chemical composition of the steel material according to the present embodiment, the lower limit of the Mo/Cr ratio is substantially 0.67 or more.
  • Titanium (Ti) forms nitride, and refines the microstructure of the steel material by the pinning effect. As a result, the SSC resistance of the steel material is increased. When the Ti content is too low, this advantageous effect cannot be obtained. On the other hand, when the Ti content is too high, a large amount of Ti nitride is formed. As a result, the SSC resistance of the steel material is reduced. Accordingly, the Ti content is within a range of 0.002 to 0.020%. A preferable lower limit of the Ti content is 0.003%, and more preferably is 0.004%. A preferable upper limit of the Ti content is 0.018%, and more preferably is 0.015%.
  • V 0.05 to 0.30%
  • Vanadium (V) combines with C and/or N to form carbides, nitrides, or carbo-nitrides (hereinafter, referred to as “carbo-nitrides and the like”).
  • Carbo-nitrides and the like refines the microstructure of the steel material by the pinning effect. As a result, the SSC resistance of the steel material is increased.
  • V also combines with C to form fine carbides. As a result, the yield strength of the steel material is increased. When the V content is too low, these advantageous effects cannot be obtained.
  • carbo-nitrides and the like is excessively formed, and the SSC resistance of the steel material is reduced.
  • the V content is within a range of 0.05 to 0.30%.
  • a preferable lower limit of the V content is more than 0.05%, more preferably is 0.06%, further preferably is 0.07%, and further preferably is 0.09%.
  • a preferable upper limit of the V content is 0.25%, more preferably is 0.20%, and further preferably is 0.15%.
  • Niobium combines with C and/or N to form carbo-nitrides and the like.
  • Carbo-nitrides and the like refines the microstructure of the steel material by the pinning effect. As a result, the SSC resistance of the steel material is increased.
  • Nb also combines with C to form fine carbides. As a result, the yield strength of the steel material is increased. When the Nb content is too low, these advantageous effects cannot be obtained.
  • carbo-nitrides and the like is excessively formed, and the SSC resistance of the steel material is reduced. Accordingly, the Nb content is within a range of 0.005 to 0.100%.
  • a preferable lower limit of the Nb content is 0.007%, more preferably is 0.010%, further preferably is 0.012%, and further preferably is 0.015%.
  • a preferable upper limit of the Nb content is 0.080%, more preferably is 0.060%, further preferably is 0.050%, and further preferably is 0.030%.
  • B 0.0005 to 0.0040% Boron (B) dissolves in the steel, increasing the hardenability of the steel material and increases the yield strength of the steel material.
  • B content is too low, this advantageous effect cannot be obtained.
  • the B content is too high, coarse nitrides are formed, and the SSC resistance of the steel material is reduced. Accordingly, the B content is within a range of 0.0005 to 0.0040%.
  • a preferable lower limit of the B content is 0.0007%, more preferably is 0.0010%, and further preferably is 0.0012%.
  • a preferable upper limit of the B content is 0.0035%, more preferably is 0.0030%, and further preferably is 0.0025%.
  • N Nitrogen
  • the N content is more than 0%.
  • N combines with Ti to form fine nitrides and thereby refines the microstructure of the steel material by a pinning effect. As a result, the SSC resistance of the steel material is increased.
  • the N content is too high, coarse nitrides are formed, and the SSC resistance of the steel material is reduced. Accordingly, the N content is 0.0100% or less.
  • a preferable upper limit of the N content is 0.0080%, and more preferably is 0.0070%.
  • a preferable lower limit of the N content for effectively obtaining the aforementioned advantageous effects is 0.0020%, more preferably 0.0025%, further preferably is 0.0030%, further preferably is 0.0035%, and further preferably is 0.0040%.
  • Oxygen (O) is an impurity. That is, the O content is more than 0%. O forms coarse oxides and reduces the SSC resistance of the steel material. Accordingly, the O content is less than 0.0020%. A preferable upper limit of the O content is 0.0018%, and more preferably is 0.0015%. Preferably, the O content is as low as possible. However, when the O content is excessively reduced, the production cost increases significantly. Accordingly, in consideration of industrial production, a preferable lower limit of the O content is 0.0001%, and more preferably is 0.0003%.
  • the balance of the chemical composition of the steel material according to the present embodiment is Fe and impurities.
  • impurities mean materials which are mixed into the steel material from ore or scrap as a raw material, a production environment or the like in industrially producing the steel material, and which are allowed within a range where the impurities do not adversely affect the steel material of the present embodiment.
  • the chemical composition of the aforementioned steel material may further contain one or more types of element selected from the group consisting of Ca, Mg, Zr, and rare earth metal (REM) in lieu of a part of Fe.
  • element selected from the group consisting of Ca, Mg, Zr, and rare earth metal (REM) in lieu of a part of Fe.
  • REM rare earth metal
  • Ca is an optional element, and may not be contained. That is, the Ca content may be 0%.
  • Ca renders S in the steel material harmless by forming sulfides, and thereby increases the SSC resistance of the steel material. If even a small amount of Ca is contained, it is possible to obtain this advantageous effect to some extent.
  • the Ca content is within a range of 0 to 0.0100%.
  • a preferable lower limit of the Ca content is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, further preferably is 0.0006%, and further preferably is 0.0010%.
  • a preferable upper limit of the Ca content is 0.0040%, more preferably is 0.0030%, further preferably is 0.0025%, and further preferably is 0.0020%.
  • Magnesium (Mg) is an optional element, and may not be contained. That is, the Mg content may be 0%.
  • Mg renders S in the steel material harmless by forming sulfides, and thereby increases the SSC resistance of the steel material. If even a small amount of Mg is contained, it is possible to obtain this advantageous effect to some extent.
  • the Mg content is within a range of 0 to 0.0100%.
  • a preferable lower limit of the Mg content is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, further preferably is 0.0006%, and further preferably is 0.0010%.
  • a preferable upper limit of the Mg content is 0.0040%, more preferably is 0.0030%, further preferably is 0.0025%, and further preferably is 0.0020%.
  • Zirconium (Zr) is an optional element, and may not be contained. That is, the Zr content may be 0%.
  • Zr renders S in the steel material harmless by forming sulfides, and thereby increases the SSC resistance of the steel material. If even a small amount of Zr is contained, it is possible to obtain this advantageous effect to some extent.
  • the Zr content is within a range of 0 to 0.0100%.
  • a preferable lower limit of the Zr content is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, further preferably is 0.0006%, and further preferably is 0.0010%.
  • a preferable upper limit of the Zr content is 0.0040%, more preferably is 0.0030%, further preferably is 0.0025%, and further preferably is 0.0020%.
  • Rare earth metal is an optional element, and may not be contained. That is, the REM content may be 0%.
  • REM renders S in the steel material harmless by forming sulfides, and thereby increases the SSC resistance of the steel material.
  • REM also combines with P in the steel material and suppresses segregation of P at the grain boundaries. Therefore, a reduction in the low temperature toughness and the SSC resistance of the steel material that is attributable to segregation of P is suppressed. If even a small amount of REM is contained, it is possible to obtain these advantageous effects to some extent. However, when the REM content is too high, oxides in the steel material coarsen, and the low temperature toughness and the SSC resistance of the steel material are reduced.
  • the REM content is within a range of 0 to 0.0100%.
  • a preferable lower limit of the REM content is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, further preferably is 0.0006%, and further preferably is 0.0010%.
  • a preferable upper limit of the REM content is 0.0040%, more preferably is 0.0030%, further preferably is 0.0025%, and further preferably is 0.0020%.
  • REM refers to one or more types of element selected from a group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 that are lanthanoids.
  • Sc scandium
  • Y yttrium
  • REM content refers to the total content of these elements.
  • the chemical composition of the aforementioned steel material may further contain one or more types of element selected from the group consisting of Cu and Ni in lieu of a part of Fe. Each of these elements is an optional element, and increases the hardenability of the steel material.
  • Copper (Cu) is an optional element, and may not be contained. That is, the Cu content may be 0%.
  • Cu increases the hardenability of the steel material, and thereby increasing the yield strength of the steel material. If even a small amount of Cu is contained, it is possible to obtain this advantageous effect to some extent.
  • the Cu content is within a range of 0 to 0.50%.
  • a preferable lower limit of the Cu content is more than 0%, more preferably is 0.02%, further preferably is 0.03%, and further preferably is 0.05%.
  • a preferable upper limit of the Cu content is 0.35%, and more preferably is 0.25%.
  • Nickel (Ni) is an optional element, and may not be contained. That is, the Ni content may be 0%. When Ni is contained, Ni increases the hardenability of the steel material, and thereby increasing the yield strength of the steel material. If even a small amount of Ni is contained, it is possible to obtain this advantageous effect to some extent. However, when the Ni content is too high, corrosion is locally promoted, and thereby the SSC resistance of the steel material is reduced. Accordingly, the Ni content is within a range of 0 to 0.50%. A preferable lower limit of the Ni content is more than 0%, more preferably is 0.02%, further preferably is 0.03%, and further preferably is 0.05%. A preferable upper limit of the Ni content is 0.35%, and more preferably is 0.25%.
  • the chemical composition of the aforementioned steel material may further contain one or more types of element selected from the group consisting of Co and W in lieu of a part of Fe.
  • element selected from the group consisting of Co and W in lieu of a part of Fe.
  • Each of these elements is an optional element, that forms corrosion coating having protectability in a hydrogen sulfide environment, and thereby suppressing hydrogen penetration. With such a configuration, these elements increase the SSC resistance of the steel material.
  • Co Co
  • the Co content may be 0%.
  • Co forms corrosion coating having protectability in a hydrogen sulfide environment, and thereby suppressing hydrogen penetration.
  • the SSC resistance of the steel material is increased. If even a small amount of Co is contained, it is possible to obtain this advantageous effect to some extent.
  • the Co content is within a range of 0 to 0.50%.
  • a preferable lower limit of the Co content is more than 0%, more preferably is 0.02%, further preferably is 0.03%, and further preferably is 0.05%.
  • a preferable upper limit of the Co content is 0.45%, and more preferably is 0.40%.
  • Tungsten (W) is an optional element, and may not be contained. That is, the W content may be 0%.
  • W forms corrosion coating having protectability in a hydrogen sulfide environment, and thereby suppressing hydrogen penetration.
  • the SSC resistance of the steel material is increased. If even a small amount of W is contained, it is possible to obtain this advantageous effect to some extent.
  • the W content is within a range of 0 to 0.50%.
  • a preferable lower limit of the W content is more than 0%, more preferably is 0.02%, further preferably is 0.03%, and further preferably is 0.05%.
  • a preferable upper limit of the W content is 0.45%, and more preferably is 0.40%.
  • the chemical composition of the steel material according to the present embodiment also satisfies Formula (1).
  • F1 is less than 3.90, sufficient hardenability cannot be obtained, and the yield strength of the steel material cannot be obtained. Accordingly, the steel material according to the present embodiment has F1 of 3.90 or more.
  • a preferable lower limit of F1 is 3.93, and more preferably is 4.00.
  • a preferable upper limit of F1 is not particularly limited. However, in the steel material according to the present embodiment having the aforementioned chemical composition, the upper limit of F1 may be 8.27, for example.
  • a preferable upper limit of F1 is 8.20, more preferably is 8.10, and further preferably is 8.00.
  • the prior-austenite grain diameter is 11.0 ⁇ m or less.
  • the grain diameter of prior-austenite grain means the grain diameter of prior-austenite grain obtained in accordance with a comparison method of ASTM E112-10.
  • the steel material contains Mo of more than 1.00% to make prior ⁇ grain of the steel material fine.
  • a preferable upper limit of the prior ⁇ grain diameter in the steel material according to the present embodiment is 10.5 ⁇ m, and more preferably is 10.0 ⁇ m.
  • a preferable lower limit of the prior ⁇ grain diameter in the steel material according to the present embodiment is not particularly limited. However, the lower limit of the prior ⁇ grain diameter in the steel material according to the present embodiment may be 4.5 ⁇ m, for example.
  • the prior ⁇ grain diameter can be obtained in accordance with a comparison method of ASTM E112-10. More specifically, the prior ⁇ grain diameter can be acquired by the following method.
  • a test specimen having an observation surface perpendicular to the rolling direction is cut out from the center portion of the thickness.
  • a test specimen having an observation surface perpendicular to the axial direction of the steel pipe is cut out from the center portion of the wall thickness. The observation surface is polished into a mirror surface and, thereafter, is embedded into a resin. Then, the test specimen is immersed into a 2% nital etching reagent for approximately 10 seconds to develop prior ⁇ grain boundaries by etching.
  • the etched observation surface is subjected to 10 field observation in a secondary electron image using a Scanning Electron Microscope (SEM) to form a photographic image.
  • the observation magnification is ⁇ 200, for example.
  • SEM Scanning Electron Microscope
  • the grain size number is evaluated.
  • the average grain diameter of prior ⁇ grain in each visual field is acquired from the evaluated grain size number.
  • the arithmetic average value of the average grain diameters of prior ⁇ grains acquired in 10 visual field is defined as the grain diameter of the prior ⁇ grain (prior ⁇ grain diameter) ( ⁇ m).
  • the average area of precipitates which are precipitated in the prior-austenite grain boundaries is 10.0 ⁇ 10 ⁇ 3 ⁇ m 2 or less.
  • precipitates which are precipitated in the prior ⁇ grain boundaries are also referred to as “specific precipitates”.
  • the average area of specific precipitates is 10.0 ⁇ 10 ⁇ 3 ⁇ m 2 or less, both yield strength of 110 ksi grade and excellent SSC resistance can be achieved provided that the other specifications of the steel material according to the present embodiment are satisfied.
  • the average area of precipitates (specific precipitates) which are precipitated in the prior ⁇ grain boundaries is set to 10.0 ⁇ 10 ⁇ 3 ⁇ m 2 or less.
  • the average area of the specific precipitates is more than 10.0 ⁇ 10 ⁇ 3 ⁇ m 2 , there may be a case where SSC resistance of a steel material be reduced.
  • the average area of the specific precipitates is more than 10.0 ⁇ 10 ⁇ 3 ⁇ m 2 , there may be also a case where yield strength of 758 to 862 MPa (110 ksi grade) cannot be obtained.
  • the average area of precipitates which are precipitated in the prior ⁇ grain boundaries is 10.0 ⁇ 10 ⁇ 3 ⁇ m 2 or less.
  • a preferable upper limit of the average area of the specific precipitates is 9.9 ⁇ 10 ⁇ 3 ⁇ m 2 , and more preferably is 9.7 ⁇ 10 ⁇ 3 ⁇ m.
  • the lower limit of the average area of the specific precipitates is not particularly limited, and may be 0.0 ⁇ 10 ⁇ 3 ⁇ m 2 .
  • the lower limit of the average area of the specific precipitates may be 3.0 ⁇ 10 ⁇ 3 ⁇ m 2 , for example.
  • the average area of the specific precipitates can be acquired by the following method.
  • a test specimen is cut out from the steel material in a similar manner of the aforementioned determined method of the prior ⁇ grain diameter. Specifically, in the case where the steel material is a steel plate, a test specimen having an observation surface perpendicular to the rolling direction is cut out from the center portion of the thickness. In the case where the steel material is a steel pipe, a test specimen having an observation surface perpendicular to the axial direction of the steel pipe is cut out from the center portion of the wall thickness. The observation surface is polished into a mirror surface and, thereafter, is embedded into a resin.
  • test specimen is immersed into a 2% vital etching reagent for approximately 10 seconds to develop prior ⁇ grain boundaries by etching.
  • the etched observation surface is subjected to 10 field observation in a secondary electron image using a SEM to form a photographic image.
  • the observation magnification is ⁇ 10000 (ten thousand), for example.
  • the prior ⁇ grain boundaries are specified from the formed photographic image based on the contrast.
  • the precipitates are also specified from the formed photographic image based on the contrast. Note that, as described above, the observation magnification is ⁇ 10000, for example.
  • precipitates can be identified based on contrast when the precipitates have the equivalent circular diameter is 50 nm or more.
  • the upper limit of equivalent circular diameter of the identified precipitates is not particularly limited. In the steel material having the aforementioned chemical composition, the upper limit of the equivalent circular diameter of the identified precipitates is 1000 nm, for example. Therefore, in the present embodiment, the equivalent circular diameter of the identified precipitates is within a range of 50 to 1000 nm, for example.
  • Precipitates which overlap with the specified prior ⁇ grain boundaries and/or which come into contact with the specified prior ⁇ grain boundaries are specified as “specific precipitates”. That is, the specific precipitates (precipitates which are precipitated in the prior ⁇ grain boundaries) mean precipitates which partially overlap and/or come into contact with the prior ⁇ grain boundary.
  • the average area ( ⁇ m 2 ) of the specified specific precipitates is acquired by performing an image analysis.
  • the microstructure of the steel material according to the present embodiment is principally composed of tempered martensite and tempered bainite. More specifically, in the microstructure, the sum of the volume ratio of tempered martensite and the volume ratio of tempered bainite is 90% or more.
  • the balance of microstructure consists of ferrite or pearlite, for example.
  • the steel material having the aforementioned chemical composition contains tempered martensite and tempered bainite such that the sum of the volume ratio of tempered martensite and the volume ratio of tempered bainite is 90% or more, the steel material has yield strength of 758 to 862 MPa (110 ksi grade) provided that the other specifications of the present embodiment are satisfied.
  • the sum of the volume ratio of tempered martensite and the volume ratio of tempered bainite can be acquired by performing the microstructure observation.
  • the aforementioned photographic image formed at the time of acquiring the prior ⁇ grain diameter is used.
  • tempered martensite and tempered bainite can be distinguished from other phases (ferrite or pearlite, for example) based on the contrast. Accordingly, in each visual field, tempered martensite and tempered bainite are specified based on the contrast.
  • the sum of the area fraction of the specified tempered martensite and the area fraction of the specified tempered bainite is acquired.
  • the arithmetic average value of the sums of the area fraction of tempered martensite and the area fraction of tempered bainite, which are acquired in all visual fields, is assumed as the volume ratio of tempered martensite and tempered bainite.
  • Yield strength of the steel material according to the present embodiment is 758 to 862 MPa (110 ksi grade). Yield strength in the present description means stress at 0.7% elongation (0.7% yield stress) acquired in a tensile test. Even if yield strength of the steel material according to the present embodiment is 110 ksi grade, the steel material according to the present embodiment has excellent SSC resistance provided that the aforementioned chemical composition, prior ⁇ grain diameter, and average area of the specific precipitates are satisfied.
  • Yield strength of the steel material according to the present embodiment can be acquired by the following method.
  • the tensile test is performed by a method conforming to ASTM E8/E8M (2013).
  • a round bar test specimen is taken from the steel material according to the present embodiment.
  • a round bar test specimen is taken from a center portion of the thickness.
  • a steel material is a steel pipe
  • a round bar test specimen is taken from a center portion of the wall thickness.
  • the size of the round bar test specimen is such that the diameter of a parallel portion is 8.9 mm and the length of the parallel portion is 35.6 mm, for example.
  • the axial direction of the round bar test specimen is parallel to the rolling direction of the steel material.
  • the tensile test is performed using the round bar test specimen in the atmosphere at the normal temperature (25° C.) and the acquired stress at 0.7% elongation is defined as yield strength (MPa).
  • SSC resistance of the steel material according to the present embodiment can be evaluated by a method in accordance with “Method A” specified in NACE TM0177-2005, and a four-point bending test.
  • a round bar test specimen is taken from the steel material according to the present embodiment.
  • a round bar test specimen is taken from a center portion of the thickness.
  • a round bar test specimen is taken from the center portion of the wall thickness.
  • the size of the round bar test specimen is such that a diameter is 6.35 mm, and the length of a parallel portion is 25.4 mm, for example.
  • the axial direction of the round bar test specimen is parallel to the rolling direction of the steel material.
  • a mixed aqueous solution containing 5.0 mass % of sodium chloride and 0.5 mass % of acetic acid (NACE solution A) at 4° C. is employed as a test solution.
  • a stress equivalent to 90% of the actual yield stress is applied to the round bar test specimen.
  • the test solution at 4° C. is poured into a test vessel so that the round bar test specimen to which the stress has been applied is immersed therein, and this is adopted as a test bath. After degassing the test bath, H 2 S gas at 1 atm pressure is blown into the test bath and is caused to saturate in the test bath.
  • the test bath where saturation of the H 2 S gas is caused is held for 720 hours at 4° C.
  • a test specimen is taken from the steel material according to the present embodiment.
  • the test specimen is taken from a center portion of the thickness.
  • the test specimen is taken from the center portion of the wall thickness.
  • the size of the test specimen is such that the thickness is 2 mm, a width is 10 mm, and a length is 75 mm, for example.
  • the length direction of the test specimen is parallel to the rolling direction of the steel material.
  • test solution An aqueous solution containing 5.0 mass % of sodium chloride at 24° C. is employed as the test solution.
  • stress is applied to the test specimens by four-point bending so that the stress applied to each test specimen becomes 90% of the actual yield stress.
  • the test specimen to which stress has been applied is enclosed in an autoclave, together with the test jig.
  • the test solution is poured into the autoclave in a manner so as to leave a vapor phase portion, and adopted as the test bath. After the test bath is degassed, 20 atm H 2 S gas is sealed under pressure in the autoclave, and the test bath is stirred to cause the H 2 S gas to saturate. After sealing the autoclave, the test bath is stirred for 720 hours at 24° C.
  • cracking is not confirmed after 720 hours elapses in both the method in accordance with “Method A” and the four-point bending test.
  • the term “cracking is not confirmed” means that cracking is not confirmed in a test specimen in a case where the test specimen after the test was observed by the naked eye.
  • the shape of the steel material according to the present embodiment is not particularly limited.
  • the steel material may be a steel pipe or a steel plate, for example.
  • a preferable wall thickness is 9 to 60 mm.
  • the steel material according to the present embodiment is suitable for use as a heavy-wall seamless steel pipe. More specifically, even when the steel material according to the present embodiment is a seamless steel pipe having a wall thickness of 15 mm or more or, furthermore, 20 mm or more, the steel material exhibits the yield strength of 110 ksi grade and excellent SSC resistance.
  • a method for producing the steel material according to the present embodiment will be described.
  • the production method described hereinafter is a method for producing a seamless steel pipe, which is one example of the steel material according to the present embodiment. Note that the method for producing the steel material according to the present embodiment is not limited to the production method which will be described hereinafter.
  • an intermediate steel material having the aforementioned chemical composition is prepared.
  • a method for producing the intermediate steel material is not particularly limited.
  • the intermediate steel material in the case where an end product is a steel plate, the intermediate steel material is a plate-shaped steel material. Meanwhile in the case where the end product is a steel pipe, the intermediate steel material is a hollow shell.
  • the preparing process may preferably include a process of preparing a starting material (starting material preparing process), and a process of producing an intermediate steel material by performing hot working on the starting material (hot working process).
  • starting material preparing process a process of preparing a starting material
  • hot working process a process of producing an intermediate steel material by performing hot working on the starting material
  • a starting material is produced using molten steel having the aforementioned chemical composition.
  • a cast piece (slab, bloom, or billet) is produced by a continuous casting process using molten steel.
  • An ingot may be produced by an ingot-making process using molten steel.
  • a billet may be produced by blooming a slab, bloom or ingot when necessary.
  • the starting material (slab, bloom, or billet) is produced via the aforementioned processes.
  • the hot working process hot working is performed on the prepared starting material, thus producing an intermediate steel material.
  • the steel material is a steel pipe
  • the intermediate steel material corresponds to a hollow shell.
  • a billet is heated in a heating furnace.
  • the heating temperature is not particularly limited, for example, the heating temperature may be 1100 to 1300° C. Hot working is performed on the billet extracted from the heating furnace to produce a hollow shell (seamless steel pipe).
  • the Mannesmann process may be performed for hot working to produce a hollow shell.
  • a round billet is subject to piercing-rolling by a piercing machine.
  • a piercing ratio is not particularly limited, for example, the piercing ratio may be 1.0 to 4.0.
  • the round billet on which piercing-rolling is performed is further subject to hot rolling by a mandrel mill, a reducer, a sizing mill or the like, thus forming a hollow shell.
  • the cumulative reduction of area in the hot working process is, for example, 20 to 70%.
  • a hollow shell may be produced from a billet by another hot working method.
  • a hollow shell may be produced by performing forging by the Ehrhardt method or the like.
  • the hollow shell is produced via the aforementioned processes.
  • the wall thickness of a hollow shell to be produced is not particularly limited, for example, the wall thickness may be 9 to 60 mm.
  • the hollow shell produced by hot working may be air-cooled (As-Rolled).
  • the hollow shell produced by hot working may be subjected to direct quenching after hot working without being cooled to normal temperature, or may be subjected to quenching after undergoing supplementary heating (reheating) after hot working.
  • SR treatment stress relief treatment
  • the intermediate steel material is prepared in the preparing process.
  • the intermediate steel material may be produced by the aforementioned preferred processes, or may be an intermediate steel material produced by a third party, or an intermediate steel material that was produced in another factory other than the factory where a quenching process and a tempering process described later are performed, or at a different works.
  • heat treatment is performed on the prepared intermediate steel material.
  • quenching and tempering are performed on the prepared intermediate steel material.
  • quenching means to rapidly cool an intermediate steel material at the temperature of the A 3 point or more.
  • tempering means to reheat and hold the quenched intermediate steel material at the temperature of the A c1 point or less.
  • quenching and tempering it is preferable to perform quenching and tempering a plurality of times. Specifically, it is preferable to perform each of quenching and tempering two or more times. More specifically, it is preferable that quenching is performed and, thereafter, tempering is performed on the prepared intermediate steel material. Further, quenching is performed and, then, tempering is performed on the prepared intermediate steel material.
  • quenching and tempering may be performed three or more times. However, even if quenching and tempering are repeatedly performed four or more times, the advantageous effects obtained by performing the heat treatment saturates. Accordingly, in the heat treatment process according to the present embodiment, it is preferable to perform quenching and tempering two or three times. Hereinafter, quenching and tempering will be described in detail.
  • quenching temperature corresponds to the surface temperature of the intermediate steel material measured by a thermometer installed on the exit side of an apparatus which performs final hot working in the case where direct quenching is performed after hot working is performed.
  • the quenching temperature also corresponds to a temperature of a supplementary heating furnace or a heat treatment furnace in the case where quenching is performed using the holding furnace or the heat treatment furnace after hot working is performed.
  • quenching may be performed by rapidly cooling the intermediate steel material at 800 to 1000° C. after hot working is performed. Quenching may be performed such that the intermediate steel material on which hot working is performed is heated to 800 to 1000° C. using the supplementary heating furnace or the heat treatment furnace and, then, is rapidly cooled. Alternatively, quenching may be performed such that the intermediate steel material on which tempering is performed is heated to 800 to 1000° C. using the heat treatment furnace and, then, is rapidly cooled.
  • quenching temperature is preferably set to 800 to 1000° C.
  • a more preferable upper limit of the quenching temperature is 950° C.
  • a preferred quenching time is 5 to 20 minutes.
  • quenching time means a time from a point of time when an intermediate steel material is charged into the supplementary heating furnace or the heat treatment furnace to a point of time when the intermediate steel material is taken out.
  • quenching time is too long, prior ⁇ grain may be coarsened after last tempering is performed. Accordingly, in the case where quenching is performed using the supplementary heating furnace or the heat treatment furnace after hot working is performed in the heat treatment process according to the present embodiment, it is preferable to set the quenching time to 5 to 20 minutes.
  • a quenching method may be adopted where a hollow shell is continuously cooled from a temperature at which quenching is started to continuously reduce the temperature of the hollow shell.
  • the method for a continuous cooling process is not particularly limited, and a well-known method may be adopted.
  • the method for the continuous cooling process may be a method where a hollow shell is immersed into a water tank to cool, or a method where a hollow shell is cooled by shower water or is cooled by mist to perform accelerated cooling.
  • an intermediate steel material (hollow shell) is rapidly cooled during quenching.
  • an average cooling speed when the temperature of the intermediate steel material (hollow shell) during quenching falls within a range of 800 to 500° C. is defined as a cooling speed during quenching CR 800-500 (° C./sec). More specifically, the cooling speed during quenching CR 800-500 is decided from a temperature measured on the surface of the quenched intermediate steel material.
  • a preferred cooling speed during quenching CR 800-500 is 8° C./sec or more.
  • the microstructure of an intermediate steel material (hollow shell) on which quenching is performed is principally composed of martensite and bainite in a stable manner.
  • a preferable lower limit of the cooling speed during quenching CR 800-500 is 10° C./sec.
  • a preferable upper limit of the cooling speed during quenching CR 800-500 is 500° C./sec.
  • Tempering is performed on the intermediate steel material on which the aforementioned quenching is performed.
  • a tempering temperature and a tempering time are adjusted according to the chemical composition of the steel material and yield strength which is expected to be obtained. In this case, only last tempering is controlled and, conventionally, it is considered sufficient to set a tempering temperature to A c1 point or less during tempering other than last tempering.
  • prior ⁇ grain is made fine by increasing Mo content.
  • Mo is liable to form M 2 C carbide in the steel material having the aforementioned chemical composition.
  • M 2 C carbide is liable to be precipitated during tempering.
  • the sufficient amount of Mo is dissolved in a steel material on which second last tempering is performed.
  • the tempering parameter TMP 2 during the second last tempering when the tempering parameter TMP 2 during the second last tempering is 15000 to 19000, it is possible to make the prior ⁇ grain diameter in the steel material on which last tempering is performed fine.
  • the tempering parameter TMP 2 during the second last tempering is less than 15000, there may be a case where advantageous effects of tempering cannot be sufficiently obtained so that quenching cracks or season cracks occur in the steel material.
  • the tempering parameter TMP 2 during the second last tempering when the tempering parameter TMP 2 during the second last tempering is more than 19000, there may be a case where the sufficient amount of dissolved Mo cannot be obtained during heating in the last quenching so that a prior ⁇ grain on which last tempering is performed is coarsened.
  • a preferable tempering parameter TMP 2 during the second last tempering is 15000 to 19000.
  • a more preferable lower limit of the tempering parameter TMP 2 during the second last tempering is 15500, and further preferably is 16000.
  • a more preferable upper limit of the tempering parameter TMP 2 during second last tempering is 18500, and further preferably is 18000.
  • a preferable tempering temperature is 500 to less than 700° C.
  • a more preferable tempering time is 10 to 60 minutes. That is, in the present embodiment, in the second last tempering, the tempering temperature is set to 500 to less than 700° C., and the tempering time is set to 10 to 60 minutes, and further, the tempering parameter TMP 2 is set to 15000 to 19000.
  • tempering temperature in the present description corresponds to a temperature of a heat treatment furnace at the time of heating and holding an intermediate steel material on which quenching is performed.
  • a tempering time holding time means a time from a point of time when the intermediate steel material is charged into the heat treatment furnace for heating and holding the intermediate steel material on which quenching is performed to a point of time when the intermediate steel material is taken out.
  • second last tempering means tempering performed before last quenching and tempering. That is, in the case where each of quenching and tempering is performed two times in the heat treatment process, second last tempering means the first tempering. In the case where each of quenching and tempering is performed three times in the heat treatment process, second last tempering means the second tempering.
  • the tempering parameter TMP 1 during the last tempering is 19100 to 19600
  • coarse specific precipitates can be reduced in the steel material on which last tempering is performed.
  • the tempering parameter TMP 1 during last tempering is less than 19100, there may be a case where advantageous effects of tempering cannot be sufficiently obtained, and yield strength of a steel material on which tempering is performed becomes too high.
  • the tempering parameter TMP 1 during last tempering is less than 19100, there may be also a case where a large amount of coarse specific precipitates is precipitated.
  • the tempering parameter TMP 1 during last tempering is more than 19600, there may be a case where yield strength of a steel material on which tempering is performed becomes too low.
  • the tempering parameter TMP 1 during last tempering is more than 19600, there may be also a case where a large amount of coarse specific precipitates is precipitated.
  • a preferable tempering parameter TMP 1 during the last tempering is 19100 to 19600.
  • a more preferable lower limit of the tempering parameter TMP 1 during last tempering is 19200, and further preferably is 19300.
  • a more preferable upper limit of the tempering parameter TMP 1 during last tempering is 19570, and further preferably is 19500.
  • a preferable tempering temperature is 650 to 730° C.
  • a preferable tempering time (holding time) is 10 to 90 minutes. That is, in the present embodiment, in the last tempering, the tempering temperature is set to 650 to 730° C., and the tempering time is set to 10 to 90 minutes and further, the tempering parameter TMP 1 is set to 19100 to 19600.
  • a preferable tempering time is 15 to 90 minutes. It is sufficiently possible for those skilled in the art to set yield strength to 758 to 862 MPa (110 ksi grade) by appropriately adjusting the aforementioned tempering temperature and the aforementioned tempering time of the steel material having the chemical composition of the present embodiment.
  • the steel material according to the present embodiment can be produced by the aforementioned production method.
  • the method for producing a seamless steel pipe has been described as one example.
  • the steel material according to the present embodiment may be a steel plate, or may have another shape.
  • the method for producing a steel plate or a product having another shape also includes a preparing process and a heat treatment process, for example.
  • the aforementioned production method merely forms one example, and the steel material may be produced by another production method.
  • Billets were produced using the aforementioned molten steels by a continuous casting process.
  • the produced billets of respective test numbers were held for one hour at 1250° C., and thereafter hot rolling (hot working) was performed on the billets by the Mannesmann-mandrel method to produce hollow shells (seamless steel pipes) of respective test numbers.
  • Heat treatment was performed two times on each of the hollow shells of respective test numbers on which hot working was performed. Specifically, heat treatment was performed on the hollow shells of respective test numbers by the following method.
  • the hollow shells of respective test numbers produced by performing hot working were held for 5 minutes in a supplementary heating furnace at 950° C., and thereafter direct quenching (that is, first quenching) was performed. All of cooling speeds during quenching CR 800-500 in first quenching for respective test numbers were within a range of 8 to 500° C./sec. Note that the cooling speed during quenching CR 800-500 was acquired by measuring the surface temperature of the hollow shell of each test number.
  • first tempering that is, second last tempering was performed on the hollow shells of respective test numbers.
  • tempering was performed where each hollow shell is held at the tempering temperature (° C.) for the tempering time (min) described in the column of “second last tempering” in Table 2.
  • Second quenching that is, last quenching was performed on the hollow shells of respective test numbers on which the aforementioned first tempering was performed. Specifically, the hollow shell of each test number was held at the quenching temperature (° C.) for the quenching time (min) described in the column of “last quenching” in Table 2 and, thereafter, quenching was performed on the hollow shell. All cooling speed during quenching CR 800-500 in second quenching for respective test numbers were within a range of 8 to 500° C./sec.
  • second tempering that is, last tempering was performed on the hollow shells of respective test numbers on which last quenching was performed. Specifically, on the hollow shell of each test number, tempering was performed where each hollow shell was held at the tempering temperature (° C.) for the tempering time (min) described in the column of “last tempering” in Table 2.
  • the temperature of the supplementary heating furnace or the heat treatment furnace used for heating in quenching corresponded to “quenching temperature (° C.)”.
  • the temperature of the heat treatment furnace used in tempering corresponded to “tempering temperature (° C.)”.
  • a time from a point of time when the hollow shell is charged into the holding furnace or the heat treatment furnace at the time of heating the hollow shell in a quenching process to a point of time when the hollow shell is taken out corresponded to “quenching time (min)”.
  • a time from a point of time when the hollow shell is charged into the heat treatment furnace at the time of performing tempering to a point of time when the hollow shell is taken out corresponded to “tempering time (min)”.
  • a prior ⁇ grain diameter in the seamless steel pipe of each test number was measured by the aforementioned method.
  • the prior ⁇ grain diameters ( ⁇ m) of the seamless steel pipes of respective test numbers are shown in Table 2.
  • the average area of precipitates which was precipitated in prior ⁇ grain boundaries (specific precipitates) was also acquired by the aforementioned method.
  • the average areas of the specific precipitates ( ⁇ 10 ⁇ 3 ⁇ m 2 ) in the seamless steel pipes of respective test numbers are shown in Table 2.
  • Yield strength of the seamless steel pipe of each test number was measured by the aforementioned method. Specifically, a tensile test was performed in accordance with ASTM E8/E8M (2013). More specifically, a round bar tensile specimen having a parallel portion with a diameter of 8.9 mm and a length of 35.6 mm was prepared from the center portion of the wall thickness of the seamless steel pipe of each test number. The axial direction of the round bar tensile specimen was parallel to the axial direction of the seamless steel pipe.
  • a tensile test was performed using the round bar test specimen of each test number in the atmosphere at normal temperature (25° C.) to acquire yield strength (MPa) of the seamless steel pipe of each test number. Note that, in the present example, stress at 0.7% elongation acquired in the tensile test was defined as yield strength of each test number.
  • the acquired yield strength YS (MPa) and tensile strength TS (MPa) are shown in Table 2.
  • Three round bar test specimens each of which has a diameter of 6.35 mm and a parallel portion with a length of 25.4 mm were taken from the center portion of the wall thickness of the seamless steel pipe of each test number.
  • Each round bar test specimen was taken such that the axial direction of the round bar test specimen is parallel to the axial direction of the seamless steel pipe.
  • Tensile stress in the axial direction of the round bar test specimen was applied to the round bar test specimen of each test number. At this point of operation, adjustment was performed such that stress to be applied is 90% of actual yield stress of the seamless steel pipe of each test number.
  • a mixed aqueous solution containing 5.0 mass % of sodium chloride and 0.5 mass % of acetic acid (NACE solution A) was used as the test solution.
  • the test solution at 4° C. was poured into three test vessels, and these were adopted as test baths.
  • the three round bar test specimens to which the stress was applied were immersed individually in mutually different test vessels as the test baths.
  • H 2 S gas at 1 atm was blown into the respective test baths and caused to saturate.
  • the test baths in which the H 2 S gas at 1 atm was saturated were held at 4° C. for 720 hours.
  • the four-point bending test was performed by the following method. Three test specimens each of which has a thickness of 2 mm, a width of 10 mm, and a length of 75 mm, were taken from the center portion of the seamless steel pipe of each test numbers of the wall thickness. The test specimen was taken such that the longitudinal direction of the test specimen is parallel to the axial direction of the seamless steel pipe. Stress was applied to the test specimens of each test number by four-point bending in accordance with ASTM G39-99 (2011) such that stress applied to each test specimen is 90% of actual yield stress of the seamless steel pipe of each test number. The test specimen to which stress was applied was sealed into an autoclave together with a test jig.
  • test solution An aqueous solution containing 5.0 mass % of sodium chloride was used as the test solution.
  • the test solution was poured into the autoclave while maintaining a gas phase portion, thus preparing test bath. After the test bath was degassed, H 2 S gas at 20 atm was pressure-sealed, and the test bath was stirred to cause saturation of H 2 S gas in the test bath. After the autoclave was sealed, the test bath was stirred for 720 hours at 24° C.
  • test specimens of respective test numbers after being held for 720 hours were observed with respect to presence or absence of the occurrence of sulfide stress cracks (SSC). Specifically, the test specimens which were held for 720 hours were observed with the naked eyes. As a result of observation, a test number for which cracking was not confirmed in all of the test specimens was determined as “E” (Excellent). On the other hand, a test number for which cracking was confirmed in at least one test specimen was determined as “NA” (Not Acceptable).
  • Table 2 shows the test results. With respect to the SSC resistance test, the results of the test in accordance with “Method A” specified in NACE TM0177-2005 are shown in the column of “1 atm H 2 S”, and the results of the four-point bending test are shown in the column of “20 atm H 2 S”.
  • the tempering parameter TMP 1 during last tempering was too high. Therefore, the average area of the specific precipitates was more than 10.0 ⁇ 10 ⁇ 3 ⁇ m 2 . As a result, yield strength was less than 758 MPa so that yield strength of 110 ksi grade was not obtained.
  • the Cr content was too low.
  • the Mo content was too low.
  • F1 was too low.
  • the tempering parameter TMP 1 during last tempering was too low. Therefore, the prior ⁇ grain diameter was more than 11.0 ⁇ m. Accordingly, the average area of the specific precipitates was also more than 10.0 ⁇ 10 ⁇ 3 ⁇ m 2 . As a result, excellent SSC resistance was not shown in either the test in accordance with “Method A” specified in NACE TM0177-2005 or the four-point bending test.
  • the steel material according to the present invention is widely applicable for steel materials utilized in a severe environment, such as a polar region. It is preferable that the steel material according to the present invention can be used as a steel material utilized in an oil well environment. It is more preferable that the steel material according to the present invention can be used as a steel material, such as a casing pipe, a tubing pipe, or a line pipe.
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