WO2024071356A1 - Line pipe steel material having excellent hydrogen embrittlement resistance, manufacturing method therefor, line pipe steel tube having excellent hydrogen embrittlement resistance, and manufacturing method therefor - Google Patents

Line pipe steel material having excellent hydrogen embrittlement resistance, manufacturing method therefor, line pipe steel tube having excellent hydrogen embrittlement resistance, and manufacturing method therefor Download PDF

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WO2024071356A1
WO2024071356A1 PCT/JP2023/035558 JP2023035558W WO2024071356A1 WO 2024071356 A1 WO2024071356 A1 WO 2024071356A1 JP 2023035558 W JP2023035558 W JP 2023035558W WO 2024071356 A1 WO2024071356 A1 WO 2024071356A1
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steel
temperature
less
hydrogen
content
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PCT/JP2023/035558
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French (fr)
Japanese (ja)
Inventor
拓史 岡野
佳宏 西原
奈穂 井上
大地 泉
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Ｊｆｅスチール株式会社
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Publication of WO2024071356A1 publication Critical patent/WO2024071356A1/en

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    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • 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
    • 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to a steel material for line pipes that has excellent hydrogen embrittlement resistance and is suitable for applications such as line pipes for transporting hydrogen gas, a manufacturing method thereof, a steel pipe for line pipes, and a manufacturing method thereof.
  • Line pipes for transporting natural gas exist as an existing energy infrastructure. These steel materials have been required to suppress the occurrence of hydrogen-induced cracking in sour environments. Meanwhile, in recent years, hydrogen has been attracting a great deal of attention worldwide as a clean energy source for building a decarbonized society. For this reason, in order to transport large amounts of hydrogen gas, the construction of a hydrogen gas transportation network that uses natural gas mixed with some hydrogen in natural gas line pipes and pressurized hydrogen gas as an alternative is being considered. The transportation pressure during operation of these pipelines is expected to be high pressure of 1 to 40 MPa, and the line pipes will be placed in a high-pressure hydrogen gas exposure environment.
  • Austenitic stainless steels such as SUS316L, which are less susceptible to hydrogen embrittlement than low-alloy steels, have traditionally been used for steel structures used in high-pressure hydrogen gas environments.
  • austenitic stainless steels such as SUS316L are expensive and have low strength, so when designed to withstand high hydrogen pressures, the wall thickness becomes thick, and the price of the hydrogen structure itself becomes expensive. For this reason, there has been a strong demand for low-cost low-alloy steels for hydrogen steel structures that can withstand high-pressure hydrogen gas environments.
  • the steel for high-pressure hydrogen environments described in Patent Document 1 is a steel used in high-pressure hydrogen environments, and by making the Ca/S ratio less than 1.5 or 11 or more, it is said that the diffusible hydrogen concentration ratio is reduced and embrittlement due to diffusible hydrogen is suppressed.
  • Patent Document 2 describes a technology that uses low-alloy high-strength steel adjusted to a specific composition, which has been found to have greater reduction in area and elongation values in a 45 MPa hydrogen atmosphere than JIS G3128SHY685NS in the air tensile strength range of 900 to 950 MPa, and is superior in resistance to embrittlement in a high-pressure hydrogen environment.
  • Patent Document 3 also describes a Cr-Mo high-strength low-alloy steel that is tempered at a relatively high temperature of 560-580°C, with a grain size of 8.4 or more after tempering, and tensile strength adjusted to an extremely narrow range of 900-950 MPa, which results in a low-alloy high-strength steel that exhibits excellent elongation and drawing characteristics even in a 45 MPa hydrogen atmosphere and has excellent resistance to embrittlement in a high-pressure hydrogen environment.
  • the low-alloy steel for high-pressure hydrogen gas environments proposed in Patent Document 4 adds V, increases the Mo content compared to existing steels, raises the tempering temperature, and utilizes V-Mo carbides, improving the carbide morphology at the grain boundaries and significantly improving resistance to embrittlement in hydrogen environments.
  • Patent Document 5 also proposes a steel for high-pressure hydrogen gas storage containers with excellent hydrogen resistance. According to the technology described in Patent Document 5, when manufacturing steel plates, long-term stress relief annealing is performed after normalizing treatment, which causes fine, dense dispersion precipitation of MC-based carbides (Mo, V)C, improving the hydrogen resistance of the steel, including its resistance to hydrogen embrittlement.
  • MC-based carbides Mo, V
  • Patent Document 6 also proposes a steel material in which the metal structure is mainly composed of bainite with an area fraction of 90% or more, and in which cementite with an average grain size of 50 nm or less and an average aspect ratio of 3 or less is dispersed and precipitated in the bainite.
  • Non-Patent Document 1 lists the fatigue strength values of low alloy steels.
  • Non-Patent Document 1 it is known that the fatigue life of materials decreases in a high-pressure hydrogen environment. In other words, if a line pipe material is designed based on a conventional line pipe for natural gas, the service life of the line pipe material will decrease.
  • the above-mentioned conventional technology can suppress the occurrence of hydrogen-induced cracking in a sour environment, it is unable to sufficiently increase the fatigue strength in hydrogen gas. In other words, there is a problem in that it is difficult to simultaneously suppress the occurrence of hydrogen-induced cracking in a sour environment and achieve high fatigue strength in hydrogen gas.
  • the present invention aims to provide a line pipe steel material that has high strength and excellent resistance to hydrogen embrittlement in a high-pressure hydrogen gas environment, suitable for steel structures used in a high-pressure hydrogen gas environment, such as line pipes for 100% hydrogen gas or natural gas containing hydrogen at a partial pressure of 1 MPa or more (natural gas is a gas whose main components are hydrocarbons such as methane and ethane), a manufacturing method thereof, and a line pipe steel pipe and a manufacturing method thereof.
  • ASTM E466, Fatigue Testing at a frequency of 1 Hz, a repetitive waveform of sine wave, a control method of load control, loading conditions of uniaxial tension and compression, and
  • the fatigue limit stress in hydrogen under the above environment is 200 MPa or more
  • the fatigue limit stress in hydrogen of the steel under the above environment/fatigue limit stress in an inert gas environment is 0.90 or more
  • steel materials here includes thin steel plates, thick steel plates, seamless steel pipes, electric resistance welded steel pipes, steel sections, steel bars, etc.
  • the inventors have conducted extensive research into the conditions that must be satisfied by steel materials to obtain steel plates and steel pipes for line pipes that have excellent resistance to hydrogen embrittlement, and have come up with the invention of new high-strength steel plates and steel pipes for line pipes. Furthermore, the steel materials and steel pipes of the present invention have high strength, and in the present invention, high strength refers to a tensile strength of 520 MPa or more.
  • the gist of the present invention is as follows. [1] In mass%, C: 0.02 to 0.15%, Si: 0.01 to 2.0%, Mn: 0.5 to 1.5%, P: 0.0001 to 0.015%, S: 0.0002 to 0.0015%, Al: 0.005 to 0.15%, O: 0.01% or less, N: 0.010% or less, H: 0.0010% or less; Or even more so: Nb: 0 to 0.10%, Ca: 0 to 0.005%, Ti: 0 to 0.1%, Ni: 0 to 2.0%, Cu: 0 to 1.0%, Cr: 0 to 1.0%, Mo: 0 to 0.60%, W: 0 to 1.0%, V: 0 to 0.10%, Zr: 0 to 0.050%, REM: 0 to 0.050%, Mg: 0 to 0.050%, B: 0 to 0.0020%, Hf: 0 to 0.2%, Ta: 0 to 0.2%, Re: 0 to 0.005%, Sn: 0
  • the chemical composition, in mass%, is Nb: 0.001 to 0.10%, Ca: 0.0001 to 0.005%, Ti: 0.005 to 0.1%, Ni: 0.01 to 2.0%, Cu: 0.01 to 1.0%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.60%, W: 0.01 to 1.0%, V: 0.01 to 0.10%, Zr: 0.0001 to 0.050%, REM: 0.0001 to 0.050%, Mg: 0.0001 to 0.050%, B: 0.0001 to 0.0020%, Hf: 0.0001 to 0.2%, Ta: 0.0001 to 0.2%, Re: 0.0001 to 0.005%, Sn: 0.0001 to 0.3%,
  • the method for producing a steel material for line pipes comprising the steps of: [4] In a steel pipe for line pipe, In mass percent, C: 0.02 to 0.15%, Si: 0.01 to 2.0%, Mn: 0.5 to 1.5%, P: 0.0001 to 0.015%, S: 0.0002 to 0.0015%, Al: 0.005 to 0.15%, O: 0.01% or less, N: 0.010% or less, H: 0.0010% or less; Or even more so: Nb: 0 to 0.10%, Ca: 0 to 0.005%, Ti: 0 to 0.1%, Ni: 0 to 2.0%, Cu: 0 to 1.0%, Cr: 0 to 1.0%, Mo: 0 to 0.60%, W: 0 to
  • the chemical composition, in mass%, is Nb: 0.001 to 0.10%, Ca: 0.0001 to 0.005%, Ti: 0.005 to 0.1%, Ni: 0.01 to 2.0%, Cu: 0.01 to 1.0%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.60%, W: 0.01 to 1.0%, V: 0.01 to 0.10%, Zr: 0.0001 to 0.050%, REM: 0.0001 to 0.050%, Mg: 0.0001 to 0.050%, B: 0.0001 to 0.0020%, Hf: 0.0001 to 0.2%, Ta: 0.0001 to 0.2%, Re: 0.0001 to 0.005%, Sn: 0.0001 to 0.3%, A steel pipe for line pipe having excellent hydrogen embrittlement resistance according to [4], wherein Sb is 0.0001 to 0.3%.
  • the present invention makes it possible to easily and simply manufacture steel materials with extremely improved resistance to hydrogen embrittlement in a high-pressure hydrogen gas environment, which is of great industrial benefit.
  • the present invention also has the effect of significantly improving the hydrogen embrittlement resistance of steel structures such as line pipes for high-pressure hydrogen gas, improving fatigue resistance, and greatly contributing to extending the life of steel structures.
  • the C content 0.02 to 0.15%
  • the C content is set to 0.02% or more.
  • the C content is 0.03% or more.
  • the weldability decreases. Therefore, the C content is limited to 0.15% or less.
  • the C content is 0.13% or less.
  • the C content is 0.08% or less. More preferably, the C content is 0.05% or less.
  • Si 0.01 to 2.0% Si is added for deoxidation, but if the content is less than 0.01%, the deoxidation effect is insufficient. Therefore, the Si content is 0.01% or more.
  • the Si content is preferably 0.08% or more, and more preferably 0.1% or more.
  • the Si content exceeds 2.0%, the effect is saturated, so the Si content is 2.0% or less.
  • the Si content is preferably 1.8% or less, and more preferably 1.0% or less.
  • the Si content exceeds 0.5%, the toughness and weldability are deteriorated, so the Si content is even more preferably 0.5% or less.
  • Mn 0.5 to 1.5% Mn effectively contributes to improving strength and toughness, but if the content is less than 0.5%, the effect of adding it is poor. Therefore, the Mn content is set to 0.5% or more.
  • the Mn content is preferably 0.6% or more, more preferably 0.7% or more, and even more preferably 0.8% or more.
  • the Mn content is limited to 1.5% or less.
  • the Mn content is preferably 1.4% or less, and even more preferably 1.3% or less.
  • P 0.0001 to 0.015%
  • P is an inevitable impurity element that deteriorates weldability and increases the hardness of the central segregation, thereby deteriorating HIC resistance. This tendency becomes significant when the content exceeds 0.015%, so the upper limit of the P content is set at 0.015%.
  • the P content is preferably 0.010% or less, and more preferably, the P content is 0.008% or less. The lower the content, the better, but from the viewpoint of refining costs, the P content is set to 0.0001% or more.
  • S 0.0002 to 0.0015%
  • S is an inevitable impurity element, and since it becomes MnS inclusions in steel and deteriorates HIC resistance, the less the better, but up to 0.0015% is permissible. For this reason, the S content is set to 0.0015% or less.
  • the S content is preferably 0.0010% or less, and more preferably 0.0008% or less. The lower the content, the better, but from the viewpoint of refining costs, the S content is set to 0.0002% or more.
  • Al 0.005 to 0.15%
  • Al is added as a deoxidizer, but if it is less than 0.005%, it has no effect. Therefore, the Al content is set to 0.005% or more.
  • the Al content is preferably 0.01% or more, and more preferably 0.03% or more.
  • the Al content is limited to 0.15% or less.
  • the Al content is preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.05% or less.
  • O 0.01% or less
  • O is a cause of oxide inclusions, so the less the better, but there is no problem if the O content is 0.01% or less. For this reason, the O content is set to 0.01% or less.
  • the O content is preferably 0.005% or less. More preferably, the O content is less than 0.003%. There is no particular lower limit, but since making oxygen 0% increases costs, the O content is preferably 0.001% or more.
  • N 0.010% or less
  • the effect of N on the fatigue properties of steel is small, and if the N content is 0.010% or less, the effect of the present invention is not impaired from the viewpoint of toughness. Therefore, the N content is set to 0.010% or less.
  • the N content is preferably set to 0.008% or less, and more preferably set to 0.006% or less.
  • the N content is further preferably set to 0.004% or less.
  • the N content is preferably set to 0.00001% or more.
  • the N content is preferably set to 0.001% or more.
  • H 0.0010% or less H may be introduced into steel materials in various processes during manufacturing, and if the amount of H introduced is large, the risk of cracking after solidification increases and fatigue crack propagation accelerates. In addition, when the amount of H introduced is large, the fatigue stress limit decreases, so it is important to reduce the amount of hydrogen in the steel material. These effects do not become a problem if the H content is 0.0010% or less, so the H content is set to 0.0010% or less.
  • the H content is preferably 0.0005% or less, more preferably 0.0003% or less, and even more preferably 0.0001% or less. On the other hand, if the H content is less than 0.00001%, it will be a factor of cost increase, so the H content is preferably 0.00001% or more.
  • the amount of hydrogen is the amount of hydrogen remaining after forming of steel materials, steel pipes, UOE, etc.
  • composition of the disclosed steel sheet may contain one or more elements selected from Nb, Ca, Ti, Ni, Cu, Cr, Mo, W, V, Zr, REM, Mg, B, Hf, Ta, Re, Sn, and Sb in the ranges below to further improve the strength and toughness of the steel sheet.
  • Nb 0 to 0.10%
  • Nb is an element effective for increasing the strength and toughness of steel, but if it exceeds 0.10%, the toughness of the weld deteriorates, so if it is contained, the Nb content is 0.10% or less.
  • the Nb content is preferably 0.08% or less. It is more preferable that the Nb content is 0.06% or less.
  • the Nb content may be 0% or more, but if the Nb content is less than 0.001%, it is difficult to obtain the effect of containing Nb, so if it is contained, it is preferably 0.001% or more. It is more preferable that the Nb content is 0.01% or more.
  • Ca 0 to 0.005% Ca is an element effective in improving HIC resistance by controlling the morphology of sulfide-based inclusions, but not only does the effect saturate, but it also deteriorates HIC resistance by reducing the cleanliness of the steel, so if Ca is contained, the amount is limited to 0.005% or less.
  • the Ca content is preferably 0.003% or less.
  • the Ca content is more preferably 0.002% or less.
  • the Ca content may be 0% or more, but if it is less than 0.0001%, the effect of adding it is difficult to obtain, so if it is contained, it is preferably 0.0001% or more.
  • the Ca content is more preferably 0.001% or more.
  • Ti 0 to 0.1%
  • Ti is an element effective for increasing the strength and toughness of steel material, but if it exceeds 0.1%, the toughness of the welded part deteriorates, so if Ti is contained, the Ti content is 0.1% or less.
  • the Ti content is preferably 0.05% or less.
  • the Ti content is more preferably 0.03% or less, and further preferably 0.02% or less.
  • the Ti content may be 0% or more, but if the Ti content is less than 0.005%, the effect of containing Ti is difficult to obtain, so if Ti is contained, it is preferably 0.005% or more.
  • the Ti content is more preferably 0.008% or more.
  • Ni 0 to 2.0%
  • Ni is an element effective in improving toughness and increasing strength, but in order to suppress costs, the Ni content is set to 2.0% or less.
  • the Ni content is preferably 1.5% or less.
  • the Ni content is more preferably 1.2% or less, and even more preferably 1.0% or less.
  • the Ni content may be 0% or more, but in order to obtain the above effects, it is preferable to contain Ni at 0.01% or more.
  • Cu 0 to 1.0%
  • Cu is an element effective in improving toughness and increasing strength, but if the content is too high, weldability deteriorates, so when Cu is contained, it is set to 1.0% or less.
  • the Cu content is preferably 0.5% or less.
  • the Cu content is more preferably 0.3% or less, and further preferably 0.2% or less.
  • the Cu content may be 0% or more, but it is preferable to contain 0.01% or more to obtain the above effect.
  • Cr 0 to 1.0%
  • Cr is an effective element for obtaining sufficient strength even with low C content, but if the content is too high, the hardenability becomes excessive, and the SSCC resistance deteriorates. In addition, weldability also deteriorates.
  • the Cr content is set to 1.0% or less.
  • the Cr content is preferably 0.8% or less.
  • the Cr content is more preferably 0.5% or less, and further preferably 0.1% or less.
  • the Cr amount may be 0% or more, but to obtain this effect, it is preferable to contain Cr at 0.01% or more.
  • the Cr content is more preferably 0.02% or more.
  • Mo 0 to 0.60%
  • Mo is an element effective in improving toughness and increasing strength, and is an element effective in improving SSCC resistance regardless of hydrogen sulfide partial pressure, but if the content is too high, the hardenability becomes excessive, and the SSCC resistance deteriorates. In addition, weldability also deteriorates.
  • the Mo content is 0.60% or less. More preferably, it is 0.50% or less, and further preferably, it is 0.40% or less. Most preferably, the Mo content is 0.03% or less.
  • the Mo content may be 0% or more, but in order to obtain the above effect, it is preferable to contain Mo at 0.005% or more. It is more preferable to contain Mo at 0.01% or more.
  • W 0 to 1.0% W contributes to increasing the strength of steel pipes, but if the W content exceeds 1.0%, the effect saturates and becomes a factor in increasing costs, so if W is contained, the W content is set to 1.0% or less.
  • the W content is preferably set to 0.8% or less. To further reduce costs, the W content is more preferably set to 0.5% or less.
  • the W content is further preferably set to 0.03% or less.
  • the W content may be 0% or more, but in order to obtain the above-mentioned effects, the content is preferably set to 0.01% or more.
  • V 0 to 0.10%
  • V is an element that can be optionally contained to increase the strength and toughness of the steel material, but if the V content exceeds 0.10%, the toughness of the weld deteriorates, so if it is contained, it is set to 0.10% or less.
  • the V content is preferably set to 0.08% or less.
  • the V content is more preferably set to 0.06% or less, and even more preferably set to 0.03% or less.
  • the V content may be 0% or more, but if the content is less than 0.01%, the effect of the content is difficult to obtain, so it is preferably set to 0.01% or more.
  • Zr 0 to 0.050%
  • REM 0 to 0.050%
  • Mg 0 to 0.050%
  • Zr, REM, and Mg are elements that can be optionally contained in order to increase toughness through grain refinement or to increase crack resistance through control of inclusion properties.
  • the content exceeds 0.050%, the effect is saturated, so if any of them is contained, it is set to 0.050% or less. That is, if contained, the Zr content is set to 0.050% or less. It is preferable that the Zr content is set to 0.040% or less. It is more preferable that the Zr content is set to 0.030% or less.
  • the Zr content is set to 0.010% or less, and it is most preferable that the Zr content is set to 0.005% or less.
  • the REM content is set to 0.050% or less. It is preferable that the REM content is set to 0.040% or less. It is more preferable that the REM content is set to 0.030% or less.
  • the Mg content is set to 0.050% or less. It is preferable that the Mg content is set to 0.040% or less. It is more preferable that the Mg content is set to 0.030% or less.
  • the contents of these elements may be 0% or more, but since the effect of the inclusion is difficult to obtain when the contents are less than 0.0001%, it is preferable to set the contents to 0.0001% or more. That is, the Zr content is preferably 0.0001% or more. The Zr content is more preferably 0.0005% or more. The REM content is preferably 0.0001% or more. The REM content is more preferably 0.0005% or more. The Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more.
  • B 0 to 0.0020%
  • B is an element that improves hardenability, contributes to increasing the strength of the steel pipe, inhibits the coarsening of prior austenite grains, and improves various properties of the material.
  • the B content is set to 0.0020% or less.
  • the B content is preferably set to 0.0015% or less.
  • the B content is more preferably set to 0.0012% or less. In order to suppress costs, it is even more preferable to set the B content to 0.0010% or less.
  • the B content may be 0% or more, but in order to obtain the above effect, the content is preferably set to 0.0001% or more. More preferably, the B content is 0.0005% or more.
  • Hf 0 to 0.2%
  • Ta 0 to 0.2%
  • the Hf and Ta contents may be 0% or more, but in order to obtain the above-mentioned effect, it is preferable that the contents are 0.0001% or more. That is, the Hf content is preferably 0.0001% or more. More preferably, the Hf content is 0.0010% or more. Also, the Ta content is preferably 0.0001% or more. More preferably, the Ta content is 0.0010% or more.
  • Re 0 to 0.005% Re contributes to increasing the strength of steel materials, but if the content exceeds 0.005%, the effect is saturated and becomes a factor of increasing costs, so if Re is contained, it is set to 0.005% or less.
  • the Re content is preferably set to 0.003% or less.
  • the Re content is more preferably set to 0.002% or less.
  • the Re content may be 0% or more, but in order to obtain the above effect, the content is preferably set to 0.0001% or more. More preferably, it is set to 0.001% or more.
  • Sn 0 to 0.3%
  • Sb 0 to 0.3%
  • the Sb content is set to 0.1% or less. In order to suppress costs, it is even more preferable that the Sb content is set to 0.01% or less.
  • the contents of Sn and Sb may be 0% or more, but in order to obtain the above effects, it is preferable that the contents are 0.0001% or more. That is, it is preferable that the Sn content is set to 0.0001% or more. More preferably, the Sn content is 0.0010% or more.
  • the Sb content is preferably 0.0001% or more, and more preferably 0.0010% or more.
  • the remainder other than the above-mentioned components consists of Fe and unavoidable impurity elements.
  • the metal structure of the steel material of the present invention is described below.
  • the fatigue crack growth rate is reduced by making the retained austenite 3% or less. Reducing the retained ⁇ suppresses the occurrence of fatigue cracks in a hydrogen environment, thereby suppressing the decrease in the fatigue limit stress in hydrogen. For this reason, the retained austenite is set to 3% or less.
  • the retained austenite is preferably 2% or less.
  • the retained austenite is more preferably 1% or less.
  • the retained austenite may be 0%.
  • Bainite has an area fraction of 90% or more at the 1/4 position of the plate thickness
  • the steel structure needs to be a bainite structure.
  • the bainite structure includes bainitic ferrite or granular bainite that transforms during or after controlled cooling, which contributes to transformation strengthening, and also includes tempered bainite. If heterogeneous structures such as ferrite, martensite, pearlite, island martensite, and retained austenite are mixed in the bainite structure, a decrease in strength and a deterioration in toughness will occur, so the smaller the volume fraction of structures other than the bainite phase, the better.
  • the area fraction of bainite is set to 90% or more. It is preferable that the area fraction of bainite is 92% or more. It is more preferable that the area fraction of bainite is 95% or more, and even more preferable that it is 98% or more.
  • the area fraction of bainite may be 100%. Furthermore, since fatigue cracks occur from the inner surface of the steel pipe, the uniformity of the inner surface structure of the steel pipe is important. Therefore, the metal structure is specified at 1/4 of the wall thickness from the inner surface of the steel pipe, and for steel materials, the metal structure is specified at 1/4 of the wall thickness in order to obtain the above effect regardless of which surface is on the inner surface of the steel pipe.
  • the fatigue limit stress in hydrogen of 1 MPa or more is 200 MPa or more, and the fatigue limit stress in hydrogen of 1 MPa or more/inert gas environment is 0.90 or more. If the fatigue limit stress in hydrogen of 1 MPa or more is less than 200 MPa, and the fatigue limit stress in hydrogen of 1 MPa or more/inert gas environment is less than 0.90, it is significantly different from the design conditions of conventional pipelines, and it becomes necessary to increase the steel thickness (steel pipe thickness in the case of steel pipes). For this reason, the fatigue limit stress in hydrogen of 1 MPa or more is 200 MPa or more, and the fatigue limit stress in hydrogen of 1 MPa or more/inert gas environment is 0.90 or more.
  • the fatigue limit stress in hydrogen of 1 MPa or more is 220 MPa or more. It is more preferable that the fatigue limit stress in hydrogen of 1 MPa or more is 250 MPa or more, and even more preferable that it is 270 MPa or more. Although there is no particular upper limit, it is preferable that the fatigue limit stress in hydrogen of 1 MPa or more is 500 MPa or less. In addition, the fatigue limit stress in hydrogen of 1 MPa or more/fatigue limit stress in an inert gas environment is preferably 0.92 or more. The fatigue limit stress in hydrogen of 1 MPa or more/fatigue limit stress in an inert gas environment is more preferably 0.94 or more, and even more preferably 0.96 or more.
  • the inert gas referred to here includes the six elements in Group 0 of the periodic table, helium, neon, argon, krypton, xenon, and radon, as well as air, and an inert gas environment refers to an environment that includes any of the above.
  • the present invention has the above-mentioned chemical components and metal structure, which improves the fatigue limit stress in a high-pressure hydrogen atmosphere and suppresses the decrease in the fatigue limit stress in hydrogen/inert gas, while achieving a tensile strength of 520 MPa or more, making it applicable to hydrogen line pipes.
  • tensile strength There is no particular upper limit for the tensile strength, but it is preferably 950 MPa or less.
  • the thickness of the steel plate is not particularly limited, but is preferably 5 mm or more. It is preferable that the thickness be 30 mm or less.
  • the steel material of the present invention can be produced by sequentially carrying out a heating step, a hot rolling step, a controlled cooling step, and a dehydrogenation step of a steel material (slab).
  • the temperature is the temperature at the center of the thickness of the steel material or the steel pipe.
  • the average cooling rate means the temperature at 1/4 of the thickness from the inner surface of the steel pipe.
  • the temperature at the center of the thickness and the temperature at 1/4 of the thickness from the inner surface of the steel pipe are temperatures estimated from the steel pipe surface temperature measured with a radiation thermometer using heat transfer calculations that take into account the heat transfer coefficient of the steel material.
  • Heating process Heating temperature of steel material 1000-1250°C If the heating temperature of steel material such as billets or slabs is less than 1000°C, the diffusion of micro-segregated impurity elements such as C, P, and S is insufficient, and a homogeneous material cannot be obtained. For this reason, the heating temperature of the steel material is set to 1000°C or higher. On the other hand, if the heating temperature exceeds 1250°C, the crystal grains become too coarse and the toughness deteriorates. Therefore, the heating temperature of the steel material is set to 1250°C or lower. The heating temperature is preferably set to 1200°C or lower. The heating temperature is more preferably set to 1180°C or lower.
  • Hot rolling process Hot rolling end temperature Ar 3 point or more After reheating the steel material, hot rolling is performed to the desired tube thickness or plate thickness, but the end temperature of hot rolling is Ar 3 point or more, which is the ferrite formation temperature. If the temperature is less than Ar 3 point, in the case of a process in which cooling is performed immediately after hot rolling, the strength will decrease due to the formation of a soft ferrite phase.
  • the end temperature of hot rolling is preferably Ar 3 + 30 ° C or more. It is more preferable that the end temperature of hot rolling is Ar 3 + 50 ° C or more. In addition, if the temperature exceeds 1250 ° C, the crystal grains become too coarse and the toughness deteriorates, so the upper limit is preferably 1250 ° C or less. It is more preferable that the end temperature of hot rolling is 1200 ° C or less, and even more preferable that it is 1150 ° C or less.
  • Ar3 point varies depending on the alloying components of the steel, it may be determined by measuring the transformation temperature through experiments for each steel, but it may also be determined from the component composition using the following formula.
  • Ar3 (°C) 910 - 310C (%) - 80Mn (%) - 20Cu (%) - 15Cr (%) - 55Ni (%) - 80Mo (%)
  • the content of each alloying element is expressed as mass %.
  • Cooling start temperature of controlled cooling steel plate surface temperature is Ar 3 point or more If the steel plate surface temperature at the start of cooling is less than Ar 3 point, ferrite is generated before controlled cooling, and strength reduction increases. Therefore, the steel plate surface temperature at the start of cooling is Ar 3 point or more.
  • the steel plate surface temperature at the start of cooling is preferably Ar 3 + 30°C or more. It is more preferable to set it to Ar 3 + 50°C or more. If the cooling start temperature is too high, the crystal grain size becomes too large and the toughness decreases, so the steel plate surface temperature at the start of cooling is preferably less than 1250°C.
  • the steel plate surface temperature at the start of cooling is more preferably 1200°C or less, and even more preferably 1150°C or less.
  • the steel plate surface temperature at the start of cooling is the temperature of the tail end of the steel plate where the cooling start temperature is the lowest.
  • Cooling start time difference between the front end and the tail end of the steel plate in the controlled cooling within 50 seconds If the time difference between the front end and the tail end in the rolling direction of the steel plate at the start of cooling exceeds 50 seconds, the temperature difference between the front end and the tail end at the start of cooling becomes large, so the temperature variation at the time of stopping cooling becomes large, and the variation in Vickers hardness at 0.25 mm below the surface of the steel plate becomes large and the HISC resistance deteriorates. For this reason, the cooling start time difference between the front end and the tail end of the steel plate is within 50 seconds, preferably within 45 seconds. More preferably, it is within 40 seconds.
  • the lower limit is not particularly limited and may be more than 0 seconds.
  • Average cooling rate from 750 ° C to 550 ° C at the center of plate thickness 15 to 50 ° C / s If the average cooling rate from 750°C to 550°C at the center of the plate thickness is less than 15°C/s, the bainite structure is not obtained, and strength is reduced. Therefore, the average cooling rate at the center of the plate thickness is set to 15°C/s or more. From the viewpoint of suppressing the variation in the structure, the average cooling rate at the center of the plate thickness is preferably set to 17°C/s or more. The average cooling rate at the center of the plate thickness is more preferably set to 20°C/s or more, and even more preferably set to 25°C/s or more.
  • the average cooling rate at the center of the plate thickness is set to 50°C/s or less.
  • the average cooling rate at the center of the plate thickness is preferably set to 45°C/s or less.
  • the average cooling rate at the center of the plate thickness is more preferably set to 40°C/s or less.
  • the cooling at the steel plate temperature at the center of the plate thickness of 550°C or less is not particularly limited, but from the viewpoint of suppressing the variation in the structure and grain size, for example, the average cooling rate from 550°C to 300°C is preferably set to 15°C/s or more.
  • the average cooling rate from 550° C. to 300° C. is preferably 50° C./s or less.
  • Cooling stop temperature 250 to 650°C If the cooling stop temperature after hot rolling exceeds 650°C, the bainite transformation becomes incomplete, and the material strength is greatly reduced. For this reason, the cooling stop temperature is set to 650°C or less.
  • the cooling stop temperature is preferably set to 625°C or less.
  • the cooling stop temperature is more preferably set to 600°C or less.
  • the cooling stop temperature is set to 250°C or more. In order to suppress the amount of hydrogen in the steel, the cooling stop temperature needs to be set to a predetermined temperature or more.
  • the cooling stop temperature needs to be set to 250°C or more in order to reduce the amount of hydrogen in the steel.
  • the cooling stop temperature is preferably set to 270°C or more.
  • Dehydrogenation process When hydrogen is present in steel, the acceleration of fatigue crack growth is increased, and the fatigue life and fatigue stress limit in hydrogen are reduced. Therefore, dehydrogenation may be used to release the hydrogen remaining after manufacturing. Dehydrogenation can reduce the amount of hydrogen in the steel by holding the product at high temperature for a certain period of time before use, and it is possible to obtain a steel plate with excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.
  • the holding time R (sec) is preferably determined by the following formula (A) based on the plate thickness and pipe thickness t (mm) of the steel pipe, and the hydrogen diffusion coefficient D (mm ⁇ sec ⁇ 1 ) in steel at room temperature.
  • the hydrogen diffusion coefficient varies depending on the contained components and metal structure, but may be, for example, 1 ⁇ 10 ⁇ 5 to 5 ⁇ 10 ⁇ 3 mm 2 /s, and more preferably 5 ⁇ 10 ⁇ 4 mm 2 /s or less.
  • the dehydrogenation process is carried out before pipe making or welding to connect steel pipes. It is preferable to perform dehydrogenation at a high temperature because the hydrogen diffusion coefficient D at high temperatures becomes small and hydrogen escapes quickly. In the case of high temperatures, the diffusion coefficient D' (diffusion coefficient at each temperature) at which the value of D in the above formula (A) is maintained may be used for calculation. On the other hand, if the temperature T of the dehydrogenation process is too high, the material strength decreases significantly, so the dehydrogenation temperature T is set to 550°C or less. It is preferable that the dehydrogenation temperature T is set to 500°C or less.
  • the dehydrogenation temperature T is set to 400°C or less, and even more preferable that it is set to 300°C or less.
  • the dehydrogenation temperature T is set to room temperature or higher. It is preferable that the dehydrogenation temperature T is set to 50°C or higher. It is more preferable that the dehydrogenation temperature T is set to 100°C or higher, and even more preferable that it is set to 150°C or higher. It is noted that room temperature means 20 ⁇ 10°C.
  • the time and temperature of the dehydrogenation process may include the temperature and time applied when heating in the pipe-making process for electric resistance welded pipes, UOE, etc., as described below.
  • scale on the steel surface inhibits dehydrogenation, it is preferable to remove the scale before carrying out the dehydrogenation process.
  • the removal method There are no limitations on the removal method, but it may be physical cleaning using a high-pressure cleaner, or a chemical method using a scale remover. The effect of scale removal can be obtained if a thickness of about 100 ⁇ m is removed.
  • a UOE steel pipe which is an example of a high-strength steel pipe for line pipe, can be obtained by limiting the manufacturing conditions shown below, and the manufacturing method and conditions will be specifically described.
  • the component composition, metal structure, fatigue limit stress in hydrogen of 1 MPa or more, and fatigue limit stress in hydrogen of 1 MPa or more/fatigue limit stress in an inert gas environment of the UOE steel pipe are the same as those described for the steel material of the first embodiment, and as for the manufacturing method, the heating process, hot rolling process, controlled cooling process after hot rolling, and dehydrogenation process are performed in the same manner as those described for the steel material.
  • the pipe-making process after rolling will be specifically described below.
  • UOE steel pipes are manufactured by bending hot-rolled steel sheets, specifically by groove-cutting the ends of the hot-rolled steel sheets and forming them into a steel pipe shape using a C press, U press, or O press, then seam-welding the butt joints by internal and external welding, and, if necessary, by a pipe expansion process.
  • Any welding method may be used as long as it provides sufficient joint strength and joint toughness, but submerged arc welding is preferred from the viewpoint of excellent welding quality and manufacturing efficiency.
  • Pipe expansion can also be performed on steel pipes that have been formed into a pipe shape by press bending and then have their butt joints seam-welded.
  • an example of the high-strength steel pipe for line pipe according to the present invention is an electric resistance welded steel pipe, which can be obtained by limiting the manufacturing conditions shown below, and the manufacturing method and conditions will be specifically described below.
  • the steel material's component composition, metal structure, fatigue limit stress in hydrogen of 1 MPa or more, and fatigue limit stress in hydrogen of 1 MPa or more/fatigue limit stress in an inert gas environment are the same as those described for the steel material of the first embodiment, and as for the manufacturing method, the steps other than the cooling step after rolling and the pipe making step (heating step, hot rolling step, dehydrogenation treatment step) are carried out in the same manner as those described for the steel material.
  • Cooling process after rolling (controlled cooling process)
  • the cooling start temperature and the average cooling rate of the controlled cooling are the same as those described in the first embodiment.
  • Cooling stop temperature 250 to 650°C If the cooling stop temperature after hot rolling exceeds 650°C, the bainite transformation becomes incomplete, and the material strength is greatly reduced. For this reason, the cooling stop temperature is set to 650°C or less.
  • the cooling stop temperature is preferably set to 620°C or less. It is more preferable that the cooling stop temperature is set to 580°C or less.
  • the cooling stop temperature is set to 250°C or more. In order to suppress the amount of hydrogen in the steel, the cooling stop temperature needs to be set to a predetermined temperature or more.
  • the cooling stop temperature needs to be set to 250°C or more in order to reduce the amount of hydrogen in the steel.
  • the cooling stop temperature is preferably 390°C or more. More preferably, the cooling stop temperature is 450°C or more.
  • the cooling stop temperature is more preferably 480° C. or higher.
  • the material may be allowed to cool naturally, but in order to promote the formation of bainite, it is more preferable to slowly cool the material until the temperature drops by about 50° C. from the cooling stop temperature.
  • the cooling stop temperature here refers to the temperature at the center of the plate thickness.
  • the winding temperature is preferably 650°C or less. Also, the winding temperature is preferably 250°C or more.
  • the electric resistance welded steel pipe given as an example of the present invention is manufactured by forming the steel pipe into a cylindrical shape by cold roll forming, and then butting and welding both circumferential ends of the cylindrical shape together. Furthermore, the electric resistance welded steel pipe may be manufactured by forming the steel pipe into an electric resistance welded steel pipe material (electric resistance welded steel pipe) using a sizing roll that satisfies the following formula (1) (sizing process), and applying an internal pressure p (MPa) that satisfies the following formula (2) to the inner surface of the electric resistance welded steel pipe material (internal pressure application process).
  • the cylindrical shape means that the circumferential cross section of the tube is in a "C" shape.
  • the plate thickness of the hot-rolled steel plate means the plate thickness of the hot-rolled steel plate before the sizing process is performed.
  • X thickness of electric welded steel pipe material (mm) / radius of electric welded steel pipe material (mm)) ⁇ yield strength of electric welded steel pipe material (MPa)
  • MPa yield strength of electric welded steel pipe material
  • the wall thickness of the electric welded steel pipe material given as an example of the steel pipe of the present invention is preferably 5 mm or more, and preferably 30 mm or less.
  • the radius of the electric welded steel pipe material is preferably 400 mm or less.
  • the radius of the electric welded pipe material is preferably 200 mm or more.
  • the yield strength of the electric welded steel pipe material is preferably 480 MPa or more in order to withstand the gas pressure of pipeline operation, and 500 MPa or more is more preferable.
  • the yield strength is preferably 560 MPa or less.
  • the diameter of the sizing roll is set to satisfy the above formula (1) in order to reduce the absolute value of the residual stress in the axial direction of the tube. If the diameter of the sizing roll is less than the right side of the formula (1), the intended residual shear stress of the present invention cannot be obtained.
  • the diameter of the sizing roll is preferably 2000 mm or less.
  • the electric resistance welded steel pipe material is expanded to generate tensile stress in the circumferential direction of the pipe, thereby reducing the absolute value of the residual stress in the circumferential direction of the pipe.
  • the greater the internal pressure p (MPa) in the internal pressure loading step the smaller the absolute value of the residual stress in the circumferential direction of the pipe.
  • the tensile stress generated in the circumferential direction of the pipe becomes higher as the radius of the steel pipe becomes larger and the wall thickness of the steel pipe becomes smaller.
  • the left side (X) of the above equation (2) corresponds to the internal pressure p when the tensile stress generated in the circumferential direction of the pipe is equal to the yield stress of the electric resistance welded steel pipe material.
  • the internal pressure p is set to a value greater than the left side (X) of equation (2) and the electric resistance welded steel pipe material is expanded to the plastic region.
  • the steel material disclosed in the present invention is formed into a tube by press bending, roll forming, UOE forming, etc., and then the butt joint is welded to produce high-strength steel pipe for sour-resistant line pipe (UOE steel pipe, electric resistance welded steel pipe, spiral steel pipe, etc.) with excellent material uniformity within the steel plate, suitable for transporting crude oil and natural gas.
  • UOE steel pipe electric resistance welded steel pipe
  • spiral steel pipe etc.
  • the steel plate disclosed in the present invention for steel pipe it is possible to produce a steel pipe with excellent HISC resistance, even if there is a high hardness region in the weld.
  • billets were produced with the component compositions shown in Tables 1-1, 1-2, and 1-3.
  • the casting speed was 0.05 to 0.2 m/min.
  • the billets were heated to 1000 to 1100°C.
  • hot rolling was performed at 1000°C ⁇ 50°C.
  • the time difference between the head and tail of hot rolling was 30 to 45 seconds, and the steel plate was manufactured with a target thickness of 20 mmt. Controlled cooling was started when the surface temperature reached Ar 3 + 50°C.
  • steel materials were manufactured under the conditions shown in Tables 2-1, 2-2, and 2-3. For some steel materials (steel materials Nos.
  • the hot-rolled steel sheet was bent and the two ends were butted together and welded to form a pipe.
  • the hot-rolled steel sheet was formed into a cylindrical shape by cold roll forming, and the two circumferential ends of the cylindrical shape were butted together and welded to form a pipe.
  • Steel pipes Nos. 1 to 14, 16 to 30, and 92, and Nos. 15, 31 to 55, and 93 to 98 were obtained.
  • the dehydrogenation treatment was performed in the range of room temperature to 550°C.
  • the dehydrogenation treatment temperature Y shown in Table 2 was performed in the range of room temperature to 550°C, and N was performed at a dehydrogenation treatment temperature of over 550°C. After it was confirmed that the center temperature Tc reached room temperature, which was the target temperature, the temperature was maintained for R (sec) so as to satisfy the above formula (A).
  • billets having the composition shown in Steel No. 15 in Table 1-1 and Steel No. 56 in Table 1-2 were produced at various casting speeds shown in Table 3, and the billets were heated to 1000 to 1100°C. Then, hot rolling was performed at 1000 ⁇ 50°C. The time difference between the head and tail of hot rolling was 30 to 45 seconds, and the steel plate was produced with a target thickness of 20 mmt. The cooling start temperature was controlled cooling when the surface temperature reached Ar 3 +50°C. Then, steel materials and steel pipes were produced under the conditions shown in Table 3. Steel Nos. 15-1 to 3 and 56-1 to 3 were as-is steel materials, and steel pipes Nos.
  • 15-11, 15-12, 56-11, and 56-12 were produced by a pipe-making process in which hot-rolled steel plates were bent and both ends were butt-welded, and steel pipes Nos. Nos. 15-13 and 56-13 were obtained by a controlled cooling process, followed by a pipe-making process in which the hot-rolled steel sheet was formed into a cylindrical shape by cold rolling, and both circumferential ends of the cylindrical shape were butted together and electric resistance welded.
  • the metal structure and mechanical properties were evaluated using the following methods: The tempering temperature was adjusted so that the tensile strength of the material was in the range of 520 MPa to 700 MPa.
  • the metal structure and properties of the obtained steel material and steel pipe were evaluated, and the results are shown in Tables 2-1, 2-2, 2-3, and 3.
  • the evaluation methods were as follows.
  • Temperature-programmed hydrogen analysis The amount of hydrogen remaining in the steel was measured using a temperature-programmed desorption analysis method, using a low-temperature temperature-programmed hydrogen analyzer (gas chromatograph type) (JTF-20AL). Temperature-programmed desorption analysis was performed in the temperature range from room temperature to 400°C at a heating rate of 200°C/h, and the sum of the results was taken as the amount of hydrogen.
  • the test specimens were cylindrical, 30 mm long in the longitudinal direction of the steel pipe, at a 1/4 position of the plate thickness of the steel plate and a 1/4 position from the inner surface of the steel pipe, and had a diameter of 7 ⁇ . This amount of hydrogen was measured before the steel was subjected to the high-pressure hydrogen fatigue test described in the aging section below, and is the amount of H shown in Tables 1-1, 1-2, and 1-3.
  • the stress at which the specimen did not break after 2 million repetitions was defined as the fatigue limit stress in hydrogen. It was determined that the specimen passed the test if the fatigue limit stress in hydrogen obtained in this test was 200 MPa or more and the ratio of the fatigue limit strength in hydrogen to the fatigue limit stress in inert gas environment was 0.90 or more.
  • All of the examples of the present invention had excellent hydrogen embrittlement resistance, with a fatigue limit stress in hydrogen of 200 MPa or more, and a ratio of the fatigue limit strength in an inert gas atmosphere to the fatigue limit stress in hydrogen/fatigue limit stress in an inert gas environment of 0.90 or more. Furthermore, the tensile strength was 520 MPa or more.
  • steel pipes were manufactured under the following manufacturing conditions, and their characteristics were evaluated. Using steel types No. 1, 15, and 56 shown in Tables 1-1 and 1-2, they were manufactured under the same conditions as steel materials No. 1, 15, 56, 15-12, and 56-12 shown in Tables 2-1, 2-2, and 3 up until the controlled cooling process, and characteristics were evaluated when the dehydrogenation treatment conditions were changed. Steel pipe forming was performed in the same manner as in Example 1. The results are shown in Table 4.
  • the dehydrogenation temperature T (atmosphere temperature) of steel pipe and steel material No. 1A, 15A, 56A, 15-12A, and 56-12A was set to 50°C, and the holding time tc after the plate thickness center temperature Tc reached 50°C was set to satisfy the formula (A).
  • the dehydrogenation temperature T (atmosphere temperature) of steel pipe and steel material No. 1B, 15B, 56B, 15-12B, and 56-12B was set to 50°C, and the holding time tc after the plate thickness center temperature Tc reached 50°C was set to satisfy the formula (A) described above, but the holding time tc after the plate thickness center temperature Tc reached 50°C did not satisfy the formula (A) described above.
  • the dehydrogenation temperature T (atmosphere temperature) is 50° C., but neither the atmospheric temperature holding time t nor the holding time tc after the plate thickness center temperature Tc reaches 50° C. satisfies the above-mentioned formula (A).
  • dehydrogenation holding time t is Y
  • dehydrogenation treatment temperature T ambient temperature
  • holding time t satisfies formula (A)
  • dehydrogenation holding time t is N
  • dehydrogenation treatment temperature T ambient temperature
  • holding time tc at steel center temperature Tc is Y means that the holding time tc after the plate thickness center temperature Tc reaches 50°C satisfies formula (A), while “holding time tc at steel center temperature Tc is N” means that the plate thickness center temperature Tc reaches 50°C, but the holding time tc after Tc reaches 50°C does not satisfy formula (A).
  • All of the inventive examples of the present invention had a fatigue limit stress in hydrogen of 200 MPa or more, and the ratio of the fatigue limit strength in an inert gas atmosphere to the fatigue limit stress in hydrogen/fatigue limit stress in an inert gas environment was 0.90 or more. Furthermore, the tensile strength was 520 MPa or more. Among these, the fatigue properties were superior when the dehydrogenation treatment was performed under more favorable conditions.

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Abstract

The purpose of the present invention is to provide a line pipe steel material that is suitable for use in a steel structure used in a high-pressure hydrogen gas environment, such as a line pipe for 100% hydrogen gas or a natural gas (natural gas being a gas having a hydrocarbon such as methane or ethane as a primary component) containing hydrogen having a hydrogen partial pressure of at least 1 MPa, and that has high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. The purpose of the present invention is also to provide a manufacturing method therefor, as well as a line pipe steel tube and a manufacturing method therefor.　Provided is a line pipe steel material having excellent hydrogen embrittlement resistance that has a specific chemical composition and a specific structure, wherein the fatigue limit stress in hydrogen that is at 1 MPa or higher is at least 200 MPa, and the ratio of the fatigue limit stress in hydrogen that is at 1 MPa or higher to the fatigue limit stress in an inert gas environment is at least 0.90.

Description

耐水素脆化特性に優れたラインパイプ用鋼材、その製造方法、耐水素脆化特性に優れたラインパイプ用鋼管およびその製造方法Steel material for line pipes with excellent hydrogen embrittlement resistance, manufacturing method thereof, steel pipe for line pipes with excellent hydrogen embrittlement resistance and manufacturing method thereof
　本発明は、水素ガスの輸送用ラインパイプ等の用途に好適な耐水素脆化特性に優れたラインパイプ用鋼材、その製造方法、ラインパイプ用鋼管およびその製造方法に関する。 The present invention relates to a steel material for line pipes that has excellent hydrogen embrittlement resistance and is suitable for applications such as line pipes for transporting hydrogen gas, a manufacturing method thereof, a steel pipe for line pipes, and a manufacturing method thereof.
　既存のエネルギーインフラとして、天然ガス輸送用ラインパイプが存在する。これらの鋼材にはサワー環境における水素誘起割れの発生の抑制が求められてきた。一方、近年では脱炭素社会構築のためのクリーンなエネルギー源として、世界的に水素が大きく注目されている。そのため、水素ガスを大量に輸送することを目的として、天然ガスラインパイプに一部水素を混合した天然ガスや、水素ガスを代替として圧送する水素ガス輸送網の構築が検討されている。これらのパイプライン運転時の輸送圧力は、１～４０ＭＰａの高圧力が想定されており、ラインパイプは、高圧力の水素ガス曝露環境に置かれることになる。このような環境で使用される鋼材には、水素が鋼中に侵入し、特性が劣化する、「水素脆化」の発生が懸念される。そのため、従来のラインパイプに要求される高靭性、耐サワー性のみならず、水素ガス環境で要求される、水素脆化への耐性を兼ね備える必要がある。 　Line pipes for transporting natural gas exist as an existing energy infrastructure. These steel materials have been required to suppress the occurrence of hydrogen-induced cracking in sour environments. Meanwhile, in recent years, hydrogen has been attracting a great deal of attention worldwide as a clean energy source for building a decarbonized society. For this reason, in order to transport large amounts of hydrogen gas, the construction of a hydrogen gas transportation network that uses natural gas mixed with some hydrogen in natural gas line pipes and pressurized hydrogen gas as an alternative is being considered. The transportation pressure during operation of these pipelines is expected to be high pressure of 1 to 40 MPa, and the line pipes will be placed in a high-pressure hydrogen gas exposure environment. There is a concern that steel materials used in such environments will suffer from "hydrogen embrittlement," in which hydrogen penetrates into the steel and the properties deteriorate. Therefore, it is necessary for the steel materials to have not only the high toughness and sour resistance required of conventional line pipes, but also the resistance to hydrogen embrittlement required in hydrogen gas environments.
　高圧水素ガス環境下で使用される鋼構造物には、従来から、低合金鋼より水素脆化し難い、ＳＵＳ３１６Ｌ等のオーステナイト系ステンレス鋼が利用されてきた。しかし、ＳＵＳ３１６Ｌ等のオーステナイト系ステンレス鋼は鋼材のコストが高いことに加えて、強度が低いため、高い水素圧に耐えうるように設計すると、肉厚が厚くなり、水素用構造物自体の価格も高価となる。そのため、水素用鋼構造物向けとして、より低コストで、かつ高圧水素ガス環境にも耐えうる低合金系鋼材が強く要望されてきた。 　Austenitic stainless steels such as SUS316L, which are less susceptible to hydrogen embrittlement than low-alloy steels, have traditionally been used for steel structures used in high-pressure hydrogen gas environments. However, austenitic stainless steels such as SUS316L are expensive and have low strength, so when designed to withstand high hydrogen pressures, the wall thickness becomes thick, and the price of the hydrogen structure itself becomes expensive. For this reason, there has been a strong demand for low-cost low-alloy steels for hydrogen steel structures that can withstand high-pressure hydrogen gas environments.
　このような要望に対し、例えば、特許文献１に記載された高圧水素環境用鋼は、高圧水素環境下で使用される鋼であって、Ｃａ／Ｓ：１．５未満または１１以上とすることで、拡散性水素濃度比を低減し拡散性水素による脆化を抑制する、としている。 In response to such demands, for example, the steel for high-pressure hydrogen environments described in Patent Document 1 is a steel used in high-pressure hydrogen environments, and by making the Ca/S ratio less than 1.5 or 11 or more, it is said that the diffusible hydrogen concentration ratio is reduced and embrittlement due to diffusible hydrogen is suppressed.
　特許文献２には、特定の成分組成に調整した低合金高強度鋼を用いることで、９００～９５０ＭＰａの大気中引張強度範囲において、ＪＩＳ　Ｇ３１２８ＳＨＹ６８５ＮＳよりも４５ＭＰａ水素雰囲気中での絞りおよび伸び値の値が大きく、耐高圧水素環境脆化特性に優れるといった知見を見出した技術である。 Patent Document 2 describes a technology that uses low-alloy high-strength steel adjusted to a specific composition, which has been found to have greater reduction in area and elongation values in a 45 MPa hydrogen atmosphere than JIS G3128SHY685NS in the air tensile strength range of 900 to 950 MPa, and is superior in resistance to embrittlement in a high-pressure hydrogen environment.
　また、特許文献３には、Ｃｒ－Ｍｏ系高強度低合金鋼であり、５６０～５８０℃という比較的高い温度で焼戻処理を行い、調質後の結晶粒度番号が８．４以上の粒度で、引張強さ：９００～９５０ＭＰａの極めて狭い範囲に調整することで、４５ＭＰａ水素雰囲気中でも、優れた伸び、絞り特性を示す、耐高圧水素環境脆化特性に優れた低合金高強度鋼となるとしている。 Patent Document 3 also describes a Cr-Mo high-strength low-alloy steel that is tempered at a relatively high temperature of 560-580°C, with a grain size of 8.4 or more after tempering, and tensile strength adjusted to an extremely narrow range of 900-950 MPa, which results in a low-alloy high-strength steel that exhibits excellent elongation and drawing characteristics even in a 45 MPa hydrogen atmosphere and has excellent resistance to embrittlement in a high-pressure hydrogen environment.
　また、特許文献４に提案されている高圧水素ガス環境用低合金鋼は、Ｖを添加し、さらに既存の鋼よりもＭｏ含有量を増加させ、焼戻温度を高めて、Ｖ－Ｍｏ系炭化物を活用することで、粒界の炭化物形態が改善され、耐水素環境脆化特性が大きく向上するとしている。 Furthermore, the low-alloy steel for high-pressure hydrogen gas environments proposed in Patent Document 4 adds V, increases the Mo content compared to existing steels, raises the tempering temperature, and utilizes V-Mo carbides, improving the carbide morphology at the grain boundaries and significantly improving resistance to embrittlement in hydrogen environments.
　また、特許文献５には、耐水素性に優れた高圧水素ガス貯蔵容器用鋼が提案されている。特許文献５に記載された技術によれば、鋼板製造時に、焼準処理の後に長時間の応力除去焼鈍を施すことで、ＭＣ系炭化物（Ｍｏ、Ｖ）Ｃが微細かつ高密度に分散析出し、鋼の耐水素脆化特性等の耐水素性が向上するとしている。 Patent Document 5 also proposes a steel for high-pressure hydrogen gas storage containers with excellent hydrogen resistance. According to the technology described in Patent Document 5, when manufacturing steel plates, long-term stress relief annealing is performed after normalizing treatment, which causes fine, dense dispersion precipitation of MC-based carbides (Mo, V)C, improving the hydrogen resistance of the steel, including its resistance to hydrogen embrittlement.
　また、特許文献６には、金属組織が面積分率９０％以上のベイナイト主体組織で、ベイナイト中に平均粒径５０ｎｍ以下で、平均アスペクト比３以下のセメンタイトが分散析出している鋼材が提案されている。 Patent Document 6 also proposes a steel material in which the metal structure is mainly composed of bainite with an area fraction of 90% or more, and in which cementite with an average grain size of 50 nm or less and an average aspect ratio of 3 or less is dispersed and precipitated in the bainite.
　なお、非特許文献１には、低合金鋼の疲労強度の値が記載されている。 Non-Patent Document 1 lists the fatigue strength values of low alloy steels.
特開２００５－２３８６号公報JP 2005-2386 A 特開２００９－４６７３７号公報JP 2009-46737 A 特開２００９－２７５２４９号公報JP 2009-275249 A 特開２００９－７４１２２号公報JP 2009-74122 A 特開２０１０－３７６５５号公報JP 2010-37655 A 特開２０１２－１０７３３２号公報JP 2012-107332 A
　ラインパイプ内の圧力は、操業時の変動や定期的なシャットダウンを行うため、構造物に繰返し応力が負荷される。そのため、ラインパイプのような鋼構造物を設計する際には、疲労破壊を考慮することが必須となる。しかし、非特許文献１に示すように高圧水素環境下では材料の疲労寿命は低下することが知られている。すなわち、従来の天然ガス用ラインパイプを基準としたラインパイプ材の設計を行った場合、ラインパイプ材の使用寿命は低下することを意味する。しかしながら、上記した従来技術では、サワー環境における水素誘起割れの発生を抑制できるが、水素ガス中の疲労強度を充分に高くすることができない、つまり、サワー環境における水素誘起割れの発生の抑制と水素ガス中の高い疲労強度の両立は困難であるという問題があった。 The pressure inside the line pipe fluctuates during operation and undergoes periodic shutdowns, so the structure is subjected to repeated stress. For this reason, it is essential to consider fatigue fracture when designing steel structures such as line pipes. However, as shown in Non-Patent Document 1, it is known that the fatigue life of materials decreases in a high-pressure hydrogen environment. In other words, if a line pipe material is designed based on a conventional line pipe for natural gas, the service life of the line pipe material will decrease. However, while the above-mentioned conventional technology can suppress the occurrence of hydrogen-induced cracking in a sour environment, it is unable to sufficiently increase the fatigue strength in hydrogen gas. In other words, there is a problem in that it is difficult to simultaneously suppress the occurrence of hydrogen-induced cracking in a sour environment and achieve high fatigue strength in hydrogen gas.
　本発明は、上記した従来技術の問題に鑑み、１００％水素ガスまたは水素分圧が１ＭＰａ以上の水素を含む天然ガス（天然ガスはメタン、エタンなどの炭化水素を主な成分とするガス）用ラインパイプ等の、高圧水素ガス環境下で使用される鋼構造物用として好適な、高強度かつ高圧水素ガス環境下における耐水素脆化特性に優れたラインパイプ用鋼材、その製造方法、ラインパイプ用鋼管およびその製造方法を提供することを目的とする。 In view of the problems with the conventional technology described above, the present invention aims to provide a line pipe steel material that has high strength and excellent resistance to hydrogen embrittlement in a high-pressure hydrogen gas environment, suitable for steel structures used in a high-pressure hydrogen gas environment, such as line pipes for 100% hydrogen gas or natural gas containing hydrogen at a partial pressure of 1 MPa or more (natural gas is a gas whose main components are hydrocarbons such as methane and ethane), a manufacturing method thereof, and a line pipe steel pipe and a manufacturing method thereof.
　なお、ここでいう「高圧水素ガス環境下における耐水素脆化特性に優れた」とは、室温（２０±１０℃）、圧力：１ＭＰａ以上の水素ガス、または水素分圧として１ＭＰａ以上の水素を含む天然ガス（主成分はメタン、エタンなどの炭化水素）混合雰囲気の両環境下で、ＡＳＴＭ　Ｅ４６６、Ｆａｔｉｇｕｅ　Ｔｅｓｔｉｎｇに準拠して周波数：１Ｈｚ、繰返し波形：正弦波、制御方法：荷重制御、荷重条件：単軸引張圧縮、応力比：Ｒ＝－１．０で疲労試験を実施して求めた、繰り返し数２００万回で未破断となる応力である水素中疲労限応力が２００ＭＰａ以上かつ、前記水素中疲労限応力／不活性ガス環境下の疲労限応力が０．９０以上である場合をいうものとする。 Note that "excellent resistance to hydrogen embrittlement in a high-pressure hydrogen gas environment" here refers to a fatigue test performed in accordance with ASTM E466, Fatigue Testing, at a frequency of 1 Hz, a repetitive waveform of sine wave, a control method of load control, loading conditions of uniaxial tension and compression, and a stress ratio of R = -1.0, in both environments of room temperature (20±10°C), hydrogen gas at a pressure of 1 MPa or more, or a mixed atmosphere of natural gas (mainly hydrocarbons such as methane and ethane) containing hydrogen at a partial pressure of 1 MPa or more, where the fatigue limit stress in hydrogen, which is the stress at which the material does not break after 2 million repetitions, is 200 MPa or more, and the fatigue limit stress in hydrogen/fatigue limit stress in an inert gas environment is 0.90 or more.
　なお、上記環境下における水素中疲労限応力が２００ＭＰａ以上かつ、上記環境下における鋼材の水素中疲労限応力／不活性ガス環境下の疲労限応力が０．９０以上であれば、継目無鋼管やＵＯＥなどの鋼管を製造するプロセスで製造可能な板厚範囲で、長寿命のラインパイプなどの水素用鋼構造物の設計を行うことが可能になる。 Furthermore, if the fatigue limit stress in hydrogen under the above environment is 200 MPa or more, and the fatigue limit stress in hydrogen of the steel under the above environment/fatigue limit stress in an inert gas environment is 0.90 or more, it will be possible to design long-life steel structures for hydrogen use, such as line pipes, within the range of plate thicknesses that can be manufactured using processes for manufacturing seamless steel pipes, UOE steel pipes, and other steel pipes.
　また、ここでいう「鋼材」には、薄鋼板、厚鋼板、継目無鋼管、電縫鋼管、形鋼、棒鋼等が含まれる。 In addition, "steel materials" here includes thin steel plates, thick steel plates, seamless steel pipes, electric resistance welded steel pipes, steel sections, steel bars, etc.
　本発明者らは、耐水素脆化特性に優れたラインパイプ用鋼板及びラインパイプ用鋼管を得るための鋼材が満足すべき条件について鋭意研究を行い、新しい高強度ラインパイプ用鋼板及びラインパイプ用鋼管を発明するに至った。また、本発明の鋼材と鋼管は高強度を有しており、本発明において高強度とは５２０ＭＰａ以上の引張強さを指すものとする。 The inventors have conducted extensive research into the conditions that must be satisfied by steel materials to obtain steel plates and steel pipes for line pipes that have excellent resistance to hydrogen embrittlement, and have come up with the invention of new high-strength steel plates and steel pipes for line pipes. Furthermore, the steel materials and steel pipes of the present invention have high strength, and in the present invention, high strength refers to a tensile strength of 520 MPa or more.
　本発明の要旨は以下のとおりである。
［１］　質量％で、
Ｃ：０．０２～０．１５％、
Ｓｉ：０．０１～２．０％、
Ｍｎ：０．５～１．５％、
Ｐ：０．０００１～０．０１５％、
Ｓ：０．０００２～０．００１５％、
Ａｌ：０．００５～０．１５％、
Ｏ：０．０１％以下、
Ｎ：０．０１０％以下、
Ｈ：０．００１０％以下を含み、
あるいはさらに、
Ｎｂ：０～０．１０％、
Ｃａ：０～０．００５％、
Ｔｉ：０～０．１％、
Ｎｉ：０～２．０％、
Ｃｕ：０～１．０％、
Ｃｒ：０～１．０％、
Ｍｏ：０～０．６０％、
Ｗ：０～１．０％、
Ｖ：０～０．１０％、
Ｚｒ：０～０．０５０％、
ＲＥＭ：０～０．０５０％、
Ｍｇ：０～０．０５０％、
Ｂ：０～０．００２０％、
Ｈｆ：０～０．２％、
Ｔａ：０～０．２％、
Ｒｅ：０～０．００５％、
Ｓｎ：０～０．３％、
Ｓｂ：０～０．３％から選択される１種以上を含み、
残部がＦｅおよび不可避的不純物元素である、化学組成を有し、
残留オーステナイトが面積分率で０～３％であり、板厚１／４位置において、ベイナイトが面積分率で９０％以上であって、１ＭＰａ以上の水素中疲労限応力が２００ＭＰａ以上であり、かつ、１ＭＰａ以上の水素中疲労限応力／不活性ガス環境下の疲労限応力が０．９０以上である耐水素脆化特性に優れたラインパイプ用鋼材。
［２］　さらに、質量％で、前記化学組成が、
Ｎｂ：０．００１～０．１０％、
Ｃａ：０．０００１～０．００５％、
Ｔｉ：０．００５～０．１％、
Ｎｉ：０．０１～２．０％、
Ｃｕ：０．０１～１．０％、
Ｃｒ：０．０１～１．０％、
Ｍｏ：０．０１～０．６０％、
Ｗ：０．０１～１．０％、
Ｖ：０．０１～０．１０％、
Ｚｒ：０．０００１～０．０５０％、
ＲＥＭ：０．０００１～０．０５０％、
Ｍｇ：０．０００１～０．０５０％、
Ｂ：０．０００１～０．００２０％、
Ｈｆ：０．０００１～０．２％、
Ｔａ：０．０００１～０．２％、
Ｒｅ：０．０００１～０．００５％、
Ｓｎ：０．０００１～０．３％、
Ｓｂ：０．０００１～０．３％である［１］に記載の耐水素脆化特性に優れたラインパイプ用鋼材。
［３］　前記［１］または［２］に記載の化学組成を有する鋼素材を１０００～１２５０℃で加熱する加熱工程と、
前記加熱工程で加熱された鋼素材を、圧延終了温度：Ａｒ３点以上の条件で圧延する熱間圧延工程と、
前記熱間圧延工程で得られた熱延鋼板を、冷却開始温度が鋼板表面温度でＡｒ３点以上、前記熱延鋼板の先端と尾端の冷却開始時間差が５０秒以内、７５０℃から５５０℃までの平均冷却速度が板厚中央の温度で１５～５０℃／ｓ、冷却停止温度が２５０～６５０℃である条件で冷却する制御冷却工程と、
前記制御冷却工程で得られた鋼板を室温～５５０℃の範囲で保持する脱水素処理工程と、
を有するラインパイプ用鋼材の製造方法。
［４］　ラインパイプ用鋼管において、
　質量％で、
Ｃ：０．０２～０．１５％、
Ｓｉ：０．０１～２．０％、
Ｍｎ：０．５～１．５％、
Ｐ：０．０００１～０．０１５％、
Ｓ：０．０００２～０．００１５％、
Ａｌ：０．００５～０．１５％、
Ｏ：０．０１％以下、
Ｎ：０．０１０％以下、
Ｈ：０．００１０％以下を含み、
あるいはさらに、
Ｎｂ：０～０．１０％、
Ｃａ：０～０．００５％、
Ｔｉ：０～０．１％、
Ｎｉ：０～２．０％、
Ｃｕ：０～１．０％、
Ｃｒ：０～１．０％、
Ｍｏ：０～０．６０％、
Ｗ：０～１．０％、
Ｖ：０～０．１０％、
Ｚｒ：０～０．０５０％、
ＲＥＭ：０～０．０５０％、
Ｍｇ：０～０．０５０％、
Ｂ：０～０．００２０％、
Ｈｆ：０～０．２％、
Ｔａ：０～０．２％、
Ｒｅ：０～０．００５％、
Ｓｎ：０～０．３％、
Ｓｂ：０～０．３％から選択される１種以上を含み、
残部がＦｅおよび不可避的不純物元素である、化学組成を有し、残留オーステナイトが面積分率で０～３％であり、鋼管内面からの肉厚１／４位置において、ベイナイトが面積分率で９０％以上であって、１ＭＰａ以上の水素中疲労限応力が２００ＭＰａ以上であり、かつ、１ＭＰａ以上の水素中疲労限応力／不活性ガス環境下の疲労限応力が０．９０以上である耐水素脆化特性に優れたラインパイプ用鋼管。
［５］　さらに、質量％で、前記化学組成が、
Ｎｂ：０．００１～０．１０％、
Ｃａ：０．０００１～０．００５％、
Ｔｉ：０．００５～０．１％、
Ｎｉ：０．０１～２．０％、
Ｃｕ：０．０１～１．０％、
Ｃｒ：０．０１～１．０％、
Ｍｏ：０．０１～０．６０％、
Ｗ：０．０１～１．０％、
Ｖ：０．０１～０．１０％、
Ｚｒ：０．０００１～０．０５０％、
ＲＥＭ：０．０００１～０．０５０％、
Ｍｇ：０．０００１～０．０５０％、
Ｂ：０．０００１～０．００２０％、
Ｈｆ：０．０００１～０．２％、
Ｔａ：０．０００１～０．２％、
Ｒｅ：０．０００１～０．００５％、
Ｓｎ：０．０００１～０．３％、
Ｓｂ：０．０００１～０．３％である［４］に記載の耐水素脆化特性に優れたラインパイプ用鋼管。
［６］　前記［４］または［５］に記載の化学組成を有する鋼素材を１０００～１２５０℃で加熱する加熱工程と、
前記加熱工程で加熱された鋼素材を、圧延終了温度：Ａｒ３点以上の条件で圧延する熱間圧延工程と、
前記熱間圧延工程で得られた熱延鋼板を、冷却開始温度が鋼板表面温度でＡｒ３点以上、前記熱延鋼板の先端と尾端の冷却開始時間差が５０秒以内、７５０℃から５５０℃までの平均冷却速度が板厚中央の温度で１５～５０℃／ｓ、冷却停止温度が２５０～６５０℃である条件で冷却する制御冷却工程と、
前記制御冷却工程後、前記熱延鋼板を曲げ加工し、両端部を突合せて溶接する造管工程、前記制御冷却工程後、前記熱延鋼板を冷間ロール成形により円筒状に成形し、前記円筒状の周方向両端部を突合せて電縫溶接する造管工程のうちどちらか一方の造管工程と、
造管工程で得られた鋼管を室温～５５０℃の範囲で保持する脱水素処理工程と、
を有するラインパイプ用鋼管の製造方法。 The gist of the present invention is as follows.
[1] In mass%,
C: 0.02 to 0.15%,
Si: 0.01 to 2.0%,
Mn: 0.5 to 1.5%,
P: 0.0001 to 0.015%,
S: 0.0002 to 0.0015%,
Al: 0.005 to 0.15%,
O: 0.01% or less,
N: 0.010% or less,
H: 0.0010% or less;
Or even more so:
Nb: 0 to 0.10%,
Ca: 0 to 0.005%,
Ti: 0 to 0.1%,
Ni: 0 to 2.0%,
Cu: 0 to 1.0%,
Cr: 0 to 1.0%,
Mo: 0 to 0.60%,
W: 0 to 1.0%,
V: 0 to 0.10%,
Zr: 0 to 0.050%,
REM: 0 to 0.050%,
Mg: 0 to 0.050%,
B: 0 to 0.0020%,
Hf: 0 to 0.2%,
Ta: 0 to 0.2%,
Re: 0 to 0.005%,
Sn: 0 to 0.3%,
Sb: one or more selected from 0 to 0.3%,
The balance is Fe and unavoidable impurity elements,
A steel material for line pipes having excellent hydrogen embrittlement resistance, the steel having an area fraction of 0-3% retained austenite, an area fraction of bainite of 90% or more at the 1/4 plate thickness position, a fatigue limit stress in hydrogen of 1 MPa or more of 200 MPa or more, and a fatigue limit stress in hydrogen of 1 MPa or more/fatigue limit stress in an inert gas environment of 0.90 or more.
[2] Furthermore, the chemical composition, in mass%, is
Nb: 0.001 to 0.10%,
Ca: 0.0001 to 0.005%,
Ti: 0.005 to 0.1%,
Ni: 0.01 to 2.0%,
Cu: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Mo: 0.01 to 0.60%,
W: 0.01 to 1.0%,
V: 0.01 to 0.10%,
Zr: 0.0001 to 0.050%,
REM: 0.0001 to 0.050%,
Mg: 0.0001 to 0.050%,
B: 0.0001 to 0.0020%,
Hf: 0.0001 to 0.2%,
Ta: 0.0001 to 0.2%,
Re: 0.0001 to 0.005%,
Sn: 0.0001 to 0.3%,
The steel material for line pipes having excellent hydrogen embrittlement resistance according to [1], wherein Sb is 0.0001 to 0.3%.
[3] A heating step of heating a steel material having the chemical composition according to [1] or [2] at 1000 to 1250 ° C.;
A hot rolling process in which the steel material heated in the heating process is rolled under a rolling end temperature of Ar 3 points or more;
A controlled cooling process in which the hot-rolled steel sheet obtained in the hot rolling process is cooled under the conditions that the cooling start temperature is Ar 3 point or more at the surface temperature of the steel sheet, the cooling start time difference between the front end and the tail end of the hot-rolled steel sheet is within 50 seconds, the average cooling rate from 750 ° C. to 550 ° C. is 15 to 50 ° C./s at the temperature at the center of the sheet thickness, and the cooling stop temperature is 250 to 650 ° C.;
A dehydrogenation treatment step of holding the steel sheet obtained in the controlled cooling step at a temperature in the range of room temperature to 550°C;
The method for producing a steel material for line pipes comprising the steps of:
[4] In a steel pipe for line pipe,
In mass percent,
C: 0.02 to 0.15%,
Si: 0.01 to 2.0%,
Mn: 0.5 to 1.5%,
P: 0.0001 to 0.015%,
S: 0.0002 to 0.0015%,
Al: 0.005 to 0.15%,
O: 0.01% or less,
N: 0.010% or less,
H: 0.0010% or less;
Or even more so:
Nb: 0 to 0.10%,
Ca: 0 to 0.005%,
Ti: 0 to 0.1%,
Ni: 0 to 2.0%,
Cu: 0 to 1.0%,
Cr: 0 to 1.0%,
Mo: 0 to 0.60%,
W: 0 to 1.0%,
V: 0 to 0.10%,
Zr: 0 to 0.050%,
REM: 0 to 0.050%,
Mg: 0 to 0.050%,
B: 0 to 0.0020%,
Hf: 0 to 0.2%,
Ta: 0 to 0.2%,
Re: 0 to 0.005%,
Sn: 0 to 0.3%,
Sb: one or more selected from 0 to 0.3%,
A steel pipe for line pipe having excellent hydrogen embrittlement resistance, which has a chemical composition with the balance being Fe and unavoidable impurity elements, has an area fraction of 0 to 3% retained austenite, and at a position 1/4 of the way through the wall from the inner surface of the steel pipe, has an area fraction of bainite of 90% or more, has a fatigue limit stress in hydrogen of 1 MPa or more of 200 MPa or more, and has a fatigue limit stress in hydrogen of 1 MPa or more/fatigue limit stress in an inert gas environment of 0.90 or more.
[5] Furthermore, the chemical composition, in mass%, is
Nb: 0.001 to 0.10%,
Ca: 0.0001 to 0.005%,
Ti: 0.005 to 0.1%,
Ni: 0.01 to 2.0%,
Cu: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Mo: 0.01 to 0.60%,
W: 0.01 to 1.0%,
V: 0.01 to 0.10%,
Zr: 0.0001 to 0.050%,
REM: 0.0001 to 0.050%,
Mg: 0.0001 to 0.050%,
B: 0.0001 to 0.0020%,
Hf: 0.0001 to 0.2%,
Ta: 0.0001 to 0.2%,
Re: 0.0001 to 0.005%,
Sn: 0.0001 to 0.3%,
A steel pipe for line pipe having excellent hydrogen embrittlement resistance according to [4], wherein Sb is 0.0001 to 0.3%.
[6] A heating step of heating a steel material having the chemical composition according to [4] or [5] at 1000 to 1250 ° C.;
A hot rolling process in which the steel material heated in the heating process is rolled under a rolling end temperature of Ar 3 points or more;
A controlled cooling process in which the hot-rolled steel sheet obtained in the hot rolling process is cooled under the conditions that the cooling start temperature is Ar 3 point or more at the surface temperature of the steel sheet, the cooling start time difference between the front end and the tail end of the hot-rolled steel sheet is within 50 seconds, the average cooling rate from 750 ° C. to 550 ° C. is 15 to 50 ° C./s at the temperature at the center of the sheet thickness, and the cooling stop temperature is 250 to 650 ° C.;
a pipe-making process in which, after the controlled cooling process, the hot-rolled steel sheet is bent and both ends are butted together and welded, or a pipe-making process in which, after the controlled cooling process, the hot-rolled steel sheet is formed into a cylindrical shape by cold roll forming and both circumferential ends of the cylindrical shape are butted together and electric resistance welded;
a dehydrogenation treatment process in which the steel pipe obtained in the pipe-making process is kept at a temperature in the range of room temperature to 550°C;
A method for producing a steel pipe for line pipe having the above structure.
　本発明によれば高圧水素ガス環境下での耐水素脆化特性が極めて向上した鋼材を、容易にかつ簡便に製造でき、産業上格段の効果を奏する。また、本発明によれば、高圧水素ガス用ラインパイプ等の鋼構造物の耐水素脆化特性を顕著に向上でき、耐疲労特性が向上して、鋼構造物の寿命延長に大きく寄与するという効果もある。 The present invention makes it possible to easily and simply manufacture steel materials with extremely improved resistance to hydrogen embrittlement in a high-pressure hydrogen gas environment, which is of great industrial benefit. In addition, the present invention also has the effect of significantly improving the hydrogen embrittlement resistance of steel structures such as line pipes for high-pressure hydrogen gas, improving fatigue resistance, and greatly contributing to extending the life of steel structures.
　次に、本発明を実施する方法について具体的に説明する。なお、以下の説明は、本発明の好適な実施態様を示すものであり、本発明は以下の説明によって何ら限定されるものではない。第１実施形態として鋼材を具体的に説明し、続いて第２実施形態として本発明の鋼管の一例であるＵＯＥ鋼管を具体的に説明し、第３実施形態として本発明の鋼管の一例である電縫鋼管を具体的に説明する。 Next, a method for implementing the present invention will be specifically described. Note that the following description shows a preferred embodiment of the present invention, and the present invention is in no way limited by the following description. As the first embodiment, a steel material will be specifically described, followed by a UOE steel pipe, which is an example of a steel pipe of the present invention, as the second embodiment, and an electric resistance welded steel pipe, which is an example of a steel pipe of the present invention, as the third embodiment.
　第１実施形態
　［成分組成］
　本発明の鋼材の成分組成（化学組成）について、その限定理由を以下に説明する。なお、以下の説明における「％」は、特に断らない限り「質量％」を表すものとする。 First embodiment [Component composition]
The reasons for limiting the chemical composition of the steel material of the present invention will be described below. In the following description, "%" refers to "mass %" unless otherwise specified.
　Ｃ：０．０２～０．１５％
　Ｃは、強度の向上に有効に寄与するが、含有量が０．０２％未満では十分な強度や疲労限応力が確保できない。このため、Ｃ含有量は０．０２％以上とする。好ましくは、Ｃ含有量は０．０３％以上である。一方、０．１５％を超えると溶接性が低下する。このため、Ｃ量は０．１５％以下に限定する。好ましくは、Ｃ含有量は０．１３％以下である。さらに０．０８％を超えると、制御冷却時に表層部や中心偏析部の硬さが上昇するため、耐ＳＳＣＣ（耐硫化物応力腐食割れ）性および耐ＨＩＣ（水素誘起割れ）性が劣化する。また、靭性も劣化する。このため、より好ましくは、Ｃ含有量は０．０８％以下である。さらに好ましくは、Ｃ含有量は０．０５％以下である。 C: 0.02 to 0.15%
Although C effectively contributes to improving strength, if the content is less than 0.02%, sufficient strength and fatigue stress limit cannot be ensured. Therefore, the C content is set to 0.02% or more. Preferably, the C content is 0.03% or more. On the other hand, if it exceeds 0.15%, the weldability decreases. Therefore, the C content is limited to 0.15% or less. Preferably, the C content is 0.13% or less. Furthermore, if it exceeds 0.08%, the hardness of the surface layer and the central segregation increases during controlled cooling, so that the SSCC (sulfide stress corrosion cracking) resistance and HIC (hydrogen induced cracking) resistance deteriorate. In addition, the toughness also deteriorates. Therefore, more preferably, the C content is 0.08% or less. More preferably, the C content is 0.05% or less.
　Ｓｉ：０．０１～２．０％
　Ｓｉは、脱酸のため添加するが、含有量が０．０１％未満では脱酸効果が十分でない。このため、Ｓｉ含有量は０．０１％以上である。Ｓｉ含有量は、０．０８％以上が好ましく、０．１％以上がより好ましい。一方、２．０％超えではその効果は飽和するため、Ｓｉ含有量は２．０％以下である。Ｓｉ含有量は１．８％以下が好ましく、１．０％以下がより好ましい。さらに０．５％を超えると靭性や溶接性を劣化させるため、Ｓｉ含有量は０．５％以下がさらに好ましい。 Si: 0.01 to 2.0%
Si is added for deoxidation, but if the content is less than 0.01%, the deoxidation effect is insufficient. Therefore, the Si content is 0.01% or more. The Si content is preferably 0.08% or more, and more preferably 0.1% or more. On the other hand, if the Si content exceeds 2.0%, the effect is saturated, so the Si content is 2.0% or less. The Si content is preferably 1.8% or less, and more preferably 1.0% or less. Furthermore, if the Si content exceeds 0.5%, the toughness and weldability are deteriorated, so the Si content is even more preferably 0.5% or less.
　Ｍｎ：０．５～１．５％
　Ｍｎは、強度、靭性の向上に有効に寄与するが、含有量が０．５％未満ではその添加効果に乏しい。このため、Ｍｎ含有量は０．５％以上とする。Ｍｎ含有量は０．６％以上が好ましく、０．７％以上がより好ましく、０．８％以上がさらに好ましい。一方１．５％を超えると制御冷却時に表層部や中心偏析部の硬さが上昇するため、耐ＳＳＣＣ（耐硫化物応力腐食割れ）性および耐ＨＩＣ（水素誘起割れ）性が劣化する。また、溶接性も劣化する。このため、Ｍｎ量は１．５％以下に限定する。Ｍｎ含有量は１．４％以下が好ましく、１．３％以下がさらに好ましい。 Mn: 0.5 to 1.5%
Mn effectively contributes to improving strength and toughness, but if the content is less than 0.5%, the effect of adding it is poor. Therefore, the Mn content is set to 0.5% or more. The Mn content is preferably 0.6% or more, more preferably 0.7% or more, and even more preferably 0.8% or more. On the other hand, if it exceeds 1.5%, the hardness of the surface layer and the central segregation increases during controlled cooling, so that the SSCC (resistance to sulfide stress corrosion cracking) and HIC (hydrogen induced cracking) resistance deteriorate. In addition, weldability also deteriorates. Therefore, the Mn content is limited to 1.5% or less. The Mn content is preferably 1.4% or less, and even more preferably 1.3% or less.
　Ｐ：０．０００１～０．０１５％
　Ｐは、不可避不純物元素であり、溶接性を劣化させるとともに、中心偏析部の硬さを上昇させることで耐ＨＩＣ性を劣化させる。０．０１５％を超えるとその傾向が顕著となるため、Ｐ含有量の上限を０．０１５％に規定する。Ｐ含有量は０．０１０％以下が好ましく、より好ましくは、Ｐ含有量は０．００８％以下である。含有量は低いほどよいが、精錬コストの観点からＰ含有量は０．０００１％以上とする。 P: 0.0001 to 0.015%
P is an inevitable impurity element that deteriorates weldability and increases the hardness of the central segregation, thereby deteriorating HIC resistance. This tendency becomes significant when the content exceeds 0.015%, so the upper limit of the P content is set at 0.015%. The P content is preferably 0.010% or less, and more preferably, the P content is 0.008% or less. The lower the content, the better, but from the viewpoint of refining costs, the P content is set to 0.0001% or more.
　Ｓ：０．０００２～０．００１５％
　Ｓは、不可避不純物元素であり、鋼中においてはＭｎＳ介在物となり耐ＨＩＣ性を劣化させるため少ないほうが好ましいが、０．００１５％までは許容される。このため、Ｓ含有量は０．００１５％以下とする。Ｓ含有量は０．００１０％以下が好ましく、０．０００８％以下がより好ましい。含有量は低いほどよいが、精錬コストの観点からＳ含有量は０．０００２％以上とする。 S: 0.0002 to 0.0015%
S is an inevitable impurity element, and since it becomes MnS inclusions in steel and deteriorates HIC resistance, the less the better, but up to 0.0015% is permissible. For this reason, the S content is set to 0.0015% or less. The S content is preferably 0.0010% or less, and more preferably 0.0008% or less. The lower the content, the better, but from the viewpoint of refining costs, the S content is set to 0.0002% or more.
　Ａｌ：０．００５～０．１５％
　Ａｌは、脱酸剤として添加するが、０．００５％未満では添加効果がない。このため、Ａｌ含有量は０．００５％以上とする。Ａｌ含有量は、０．０１％以上が好ましく、０．０３％以上がより好ましい。一方、０．１５％を超えると鋼の清浄度が低下し、靱性が劣化するため、Ａｌ含有量は０．１５％以下に限定する。Ａｌ含有量は、０．１０％以下が好ましく、０．０８％以下がより好ましく、０．０５％以下がさらに好ましい。 Al: 0.005 to 0.15%
Al is added as a deoxidizer, but if it is less than 0.005%, it has no effect. Therefore, the Al content is set to 0.005% or more. The Al content is preferably 0.01% or more, and more preferably 0.03% or more. On the other hand, if it exceeds 0.15%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is limited to 0.15% or less. The Al content is preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.05% or less.
　Ｏ：０．０１％以下
　Ｏは、酸化物系介在物を生成する原因となるため少ないほど好ましいが、Ｏ含有量が０．０１％以下であれば問題とならない。このため、Ｏ含有量は０．０１％以下とする。Ｏ含有量は、好ましくは０．００５％以下である。より好ましくは、Ｏ含有量は０．００３％未満である。下限は特に限定されるものでは無いが、酸素を０％にするのはコスト増大の要因となるのでＯ含有量は０．００１％以上が好ましい。 O: 0.01% or less O is a cause of oxide inclusions, so the less the better, but there is no problem if the O content is 0.01% or less. For this reason, the O content is set to 0.01% or less. The O content is preferably 0.005% or less. More preferably, the O content is less than 0.003%. There is no particular lower limit, but since making oxygen 0% increases costs, the O content is preferably 0.001% or more.
　Ｎ：０．０１０％以下
　鋼材の疲労特性に及ぼすＮの影響は小さく、Ｎ含有量が０．０１０％以下であれば靭性の観点から本発明の効果を損なわない。よって、Ｎ含有量は０．０１０％以下とする。Ｎ含有量は０．００８％以下とすることが好ましく、Ｎ含有量は０．００６％以下とすることがより好ましい。Ｎ含有量は０．００４％以下とすることがさらに好ましい。一方、じん性向上の観点からは、Ｎ含有量が少ないことが望ましいが、過度の低減は製鋼上のコストを増大させるので、Ｎ含有量は０．００００１％以上とすることが好ましい。Ｎ含有量は０．００１％以上とすることが好ましい。 N: 0.010% or less The effect of N on the fatigue properties of steel is small, and if the N content is 0.010% or less, the effect of the present invention is not impaired from the viewpoint of toughness. Therefore, the N content is set to 0.010% or less. The N content is preferably set to 0.008% or less, and more preferably set to 0.006% or less. The N content is further preferably set to 0.004% or less. On the other hand, from the viewpoint of improving toughness, it is desirable for the N content to be small, but an excessive reduction increases the cost of steelmaking, so the N content is preferably set to 0.00001% or more. The N content is preferably set to 0.001% or more.
　Ｈ：０．００１０％以下
　Ｈは、製造中の種々の工程で鋼材中に導入される場合があり、導入量が多いと凝固後の割れ発生リスクが高まるとともに、疲労き裂進展を加速させる。また、導入量が多い状態では疲労限応力を低下させるため、鋼材中の水素量を低下させることが重要である。これらの影響はＨ含有量が０．００１０％以下であれば問題とならないため、Ｈ含有量は０．００１０％以下とする。Ｈ含有量は、好ましくは０．０００５％以下であり、より好ましくは０．０００３％以下であり、さらに好ましくは、０．０００１％以下である。一方、０．００００１％未満とするコスト増の要因となるため、Ｈ含有量は０．００００１％以上とすることが好ましい。なお、水素量は鋼材、鋼管、ＵＯＥ等の成形後の残存水素量である。 H: 0.0010% or less H may be introduced into steel materials in various processes during manufacturing, and if the amount of H introduced is large, the risk of cracking after solidification increases and fatigue crack propagation accelerates. In addition, when the amount of H introduced is large, the fatigue stress limit decreases, so it is important to reduce the amount of hydrogen in the steel material. These effects do not become a problem if the H content is 0.0010% or less, so the H content is set to 0.0010% or less. The H content is preferably 0.0005% or less, more preferably 0.0003% or less, and even more preferably 0.0001% or less. On the other hand, if the H content is less than 0.00001%, it will be a factor of cost increase, so the H content is preferably 0.00001% or more. The amount of hydrogen is the amount of hydrogen remaining after forming of steel materials, steel pipes, UOE, etc.
　本開示の成分組成は、鋼板の強度や靱性の一層の改善のために、Ｎｂ、Ｃａ、Ｔｉ、Ｎｉ、Ｃｕ、Ｃｒ、Ｍｏ、Ｗ、Ｖ、Ｚｒ、ＲＥＭ、Ｍｇ、Ｂ、Ｈｆ、Ｔａ、Ｒｅ、Ｓｎ、Ｓｂのうちから選んだ１種以上を、以下の範囲で任意に含有させることができる。 The composition of the disclosed steel sheet may contain one or more elements selected from Nb, Ca, Ti, Ni, Cu, Cr, Mo, W, V, Zr, REM, Mg, B, Hf, Ta, Re, Sn, and Sb in the ranges below to further improve the strength and toughness of the steel sheet.
　Ｎｂ：０～０．１０％
　Ｎｂは鋼材の強度および靭性を高めるために有効な元素であるが、０．１０％を超えると溶接部の靭性が劣化するため、含有する場合にはＮｂ含有量は０．１０％以下とする。Ｎｂ含有量は０．０８％以下とすることが好ましい。Ｎｂ含有量は０．０６％以下とすることがすることがさらに好ましい。Ｎｂ含有量は０％以上であってよいが、Ｎｂ含有量が０．００１％未満ではその含有効果が得られにくいため、含有する場合は０．００１％以上とすることが好ましい。Ｎｂ含有量は０．０１％以上とすることがより好ましい。 Nb: 0 to 0.10%
Nb is an element effective for increasing the strength and toughness of steel, but if it exceeds 0.10%, the toughness of the weld deteriorates, so if it is contained, the Nb content is 0.10% or less. The Nb content is preferably 0.08% or less. It is more preferable that the Nb content is 0.06% or less. The Nb content may be 0% or more, but if the Nb content is less than 0.001%, it is difficult to obtain the effect of containing Nb, so if it is contained, it is preferably 0.001% or more. It is more preferable that the Nb content is 0.01% or more.
　Ｃａ：０～０．００５％
　Ｃａは、硫化物系介在物の形態制御による耐ＨＩＣ性向上に有効な元素であるが、効果が飽和するだけでなく、鋼の清浄度の低下により耐ＨＩＣ性を劣化させるので、含有する場合にはＣａ量は０．００５％以下に限定する。Ｃａ含有量は０．００３％以下が好ましい。Ｃａ含有量は０．００２％以下がさらに好ましい。Ｃａ量は０％以上であってよいが、０．０００１％未満ではその添加効果が得られにくいため、含有する場合は０．０００１％以上とすることが好ましい。Ｃａ含有量は０．００１％以上がより好ましい。 Ca: 0 to 0.005%
Ca is an element effective in improving HIC resistance by controlling the morphology of sulfide-based inclusions, but not only does the effect saturate, but it also deteriorates HIC resistance by reducing the cleanliness of the steel, so if Ca is contained, the amount is limited to 0.005% or less. The Ca content is preferably 0.003% or less. The Ca content is more preferably 0.002% or less. The Ca content may be 0% or more, but if it is less than 0.0001%, the effect of adding it is difficult to obtain, so if it is contained, it is preferably 0.0001% or more. The Ca content is more preferably 0.001% or more.
　Ｔｉ：０～０．１％
　Ｔｉは、鋼材の強度および靭性を高めるために有効な元素であるが、０．１％を超えると溶接部の靭性が劣化するため、Ｔｉを含有する場合には、Ｔｉ含有量は０．１％以下とする。Ｔｉ含有量は０．０５％以下が好ましい。Ｔｉ含有量は０．０３％以下がより好ましく、０．０２％以下がさらに好ましい。Ｔｉ含有量は０％以上であってよいが、Ｔｉ含有量が０．００５％未満ではその含有効果得られにくいため、含有する場合は０．００５％以上とすることが好ましい。Ｔｉ含有量は０．００８％以上がより好ましい。 Ti: 0 to 0.1%
Ti is an element effective for increasing the strength and toughness of steel material, but if it exceeds 0.1%, the toughness of the welded part deteriorates, so if Ti is contained, the Ti content is 0.1% or less. The Ti content is preferably 0.05% or less. The Ti content is more preferably 0.03% or less, and further preferably 0.02% or less. The Ti content may be 0% or more, but if the Ti content is less than 0.005%, the effect of containing Ti is difficult to obtain, so if Ti is contained, it is preferably 0.005% or more. The Ti content is more preferably 0.008% or more.
　Ｎｉ：０～２．０％
　Ｎｉは、靭性の改善と強度の上昇に有効な元素であるが、コスト抑制のためには含有する場合には２．０％以下とする。Ｎｉ含有量は１．５％以下が好ましい。Ｎｉ含有量は１．２％以下がより好ましく、１．０％以下がさらに好ましい。Ｎｉ含有量は０％以上であってよいが、上記効果を得るにはＮｉを０．０１％以上含有することが好ましい。 Ni: 0 to 2.0%
Ni is an element effective in improving toughness and increasing strength, but in order to suppress costs, the Ni content is set to 2.0% or less. The Ni content is preferably 1.5% or less. The Ni content is more preferably 1.2% or less, and even more preferably 1.0% or less. The Ni content may be 0% or more, but in order to obtain the above effects, it is preferable to contain Ni at 0.01% or more.
　Ｃｕ：０～１．０％
　Ｃｕは、靭性の改善と強度の上昇に有効な元素であるが、含有量が多すぎると溶接性が劣化するため、Ｃｕを含有する場合は１．０％以下とする。Ｃｕ含有量は０．５％以下が好ましい。Ｃｕ含有量は０．３％以下がより好ましく、０．２％以下がさらに好ましい。Ｃｕ含有量は０％以上であってよいが、上記効果を得るには０．０１％以上を含有することが好ましい。 Cu: 0 to 1.0%
Cu is an element effective in improving toughness and increasing strength, but if the content is too high, weldability deteriorates, so when Cu is contained, it is set to 1.0% or less. The Cu content is preferably 0.5% or less. The Cu content is more preferably 0.3% or less, and further preferably 0.2% or less. The Cu content may be 0% or more, but it is preferable to contain 0.01% or more to obtain the above effect.
　Ｃｒ：０～１．０％
　Ｃｒは、Ｍｎと同様、低Ｃでも十分な強度を得るために有効な元素であるが、含有量が多すぎると、焼入れ性が過剰になるため、耐ＳＳＣＣ性が劣化する。また、溶接性も劣化する。このため、Ｃｒを含有する場合は１．０％以下とする。Ｃｒ含有量は０．８％以下が好ましい。Ｃｒ含有量は０．５％以下がより好ましく、０．１％以下がさらに好ましい。Ｃｒ量は０％以上であってよいが、この効果を得るにはＣｒを０．０１％以上含有することが好ましい。Ｃｒ含有量は０．０２％以上がより好ましい。 Cr: 0 to 1.0%
Like Mn, Cr is an effective element for obtaining sufficient strength even with low C content, but if the content is too high, the hardenability becomes excessive, and the SSCC resistance deteriorates. In addition, weldability also deteriorates. For this reason, when Cr is contained, the Cr content is set to 1.0% or less. The Cr content is preferably 0.8% or less. The Cr content is more preferably 0.5% or less, and further preferably 0.1% or less. The Cr amount may be 0% or more, but to obtain this effect, it is preferable to contain Cr at 0.01% or more. The Cr content is more preferably 0.02% or more.
　Ｍｏ：０～０．６０％
　Ｍｏは、靭性の改善と強度の上昇に有効な元素であり、硫化水素分圧によらず耐ＳＳＣＣ性の向上に有効な元素であるが、含有量が多すぎると、焼入れ性が過剰になるため、耐ＳＳＣＣ性が劣化する。また、溶接性も劣化する。このため、Ｍｏを含有する場合には、Ｍｏ含有量は０．６０％以下とする。より好ましくは０．５０％以下として、さらに好ましくは０．４０％以下とする。もっとも好ましくは、Ｍｏ含有量は０．０３％以下とする。Ｍｏ含有量は０％以上であってよいが、上記効果を得るにはＭｏを０．００５％以上含有することが好ましい。Ｍｏを０．０１％以上含有することがより好ましい。 Mo: 0 to 0.60%
Mo is an element effective in improving toughness and increasing strength, and is an element effective in improving SSCC resistance regardless of hydrogen sulfide partial pressure, but if the content is too high, the hardenability becomes excessive, and the SSCC resistance deteriorates. In addition, weldability also deteriorates. For this reason, when Mo is contained, the Mo content is 0.60% or less. More preferably, it is 0.50% or less, and further preferably, it is 0.40% or less. Most preferably, the Mo content is 0.03% or less. The Mo content may be 0% or more, but in order to obtain the above effect, it is preferable to contain Mo at 0.005% or more. It is more preferable to contain Mo at 0.01% or more.
　Ｗ：０～１．０％
　Ｗは、鋼管の強度上昇に寄与するが、Ｗ含有量が１．０％を越えると効果が飽和し、コストアップの要因となるため、含有する場合には、Ｗ含有量は１．０％以下とする。Ｗ含有量は０．８％以下とすることが好ましい。更なるコスト抑制のためには、Ｗ含有量は０．５％以下とすることがより好ましい。Ｗ含有量は０．０３％以下とすることがさらに好ましい。Ｗ含有量は０％以上であってよいが、前記効果を得るために、含有量を０．０１％以上とすることが好ましい。 W: 0 to 1.0%
W contributes to increasing the strength of steel pipes, but if the W content exceeds 1.0%, the effect saturates and becomes a factor in increasing costs, so if W is contained, the W content is set to 1.0% or less. The W content is preferably set to 0.8% or less. To further reduce costs, the W content is more preferably set to 0.5% or less. The W content is further preferably set to 0.03% or less. The W content may be 0% or more, but in order to obtain the above-mentioned effects, the content is preferably set to 0.01% or more.
　Ｖ：０～０．１０％
　Ｖは、鋼材の強度および靭性を高めるために任意に含有することができる元素であるが、Ｖ含有量が０．１０％を超えると溶接部の靭性が劣化するので、含有する場合は０．１０％以下とする。Ｖ含有量が０．０８％以下とすることが好ましい。Ｖ含有量が０．０６％以下とすることがより好ましく、０．０３％以下とすることがさらに好ましい。Ｖ含有量は０％以上であってよいが、含有量が０．０１％未満ではその含有効果が得られにくいため、０．０１％以上とすることが好ましい。 V: 0 to 0.10%
V is an element that can be optionally contained to increase the strength and toughness of the steel material, but if the V content exceeds 0.10%, the toughness of the weld deteriorates, so if it is contained, it is set to 0.10% or less. The V content is preferably set to 0.08% or less. The V content is more preferably set to 0.06% or less, and even more preferably set to 0.03% or less. The V content may be 0% or more, but if the content is less than 0.01%, the effect of the content is difficult to obtain, so it is preferably set to 0.01% or more.
　Ｚｒ：０～０．０５０％、ＲＥＭ：０～０．０５０％、Ｍｇ：０～０．０５０％
　Ｚｒ、ＲＥＭ、Ｍｇは、結晶粒微細化を通じて靭性を高めたり、介在物性状のコントロールを通して耐割れ性を高めたりするために任意に含有することができる元素である。一方０．０５０％を超えるとその効果が飽和するので、含有する場合はいずれも０．０５０％以下とする。すなわち、含有する場合には、Ｚｒ含有量は０．０５０％以下とする。Ｚｒ含有量は０．０４０％以下とすることが好ましい。Ｚｒ含有量は０．０３０％以下とすることがより好ましい。Ｚｒ含有量は０．０１０％以下とすることがさらに好ましく、０．００５％以下とすることがもっとも好ましい。また、含有する場合には、ＲＥＭ含有量は０．０５０％以下とする。ＲＥＭ含有量は０．０４０％以下とすることが好ましい。ＲＥＭ含有量は０．０３０％以下とすることがより好ましい。また、含有する場合には、Ｍｇ含有量は０．０５０％以下とする。Ｍｇ含有量は０．０４０％以下とすることが好ましい。Ｍｇ含有量は０．０３０％以下とすることがより好ましい。これらの元素は、その含有量は０％以上であってよいが、いずれも、含有量が０．０００１％未満ではその含有効果が得られにくいため、０．０００１％以上とすることが好ましい。すなわち、Ｚｒ含有量は０．０００１％以上とすることが好ましい。Ｚｒ含有量は０．０００５％以上とすることがより好ましい。また、ＲＥＭ含有量は０．０００１％以上とすることが好ましい。ＲＥＭ含有量は０．０００５％以上とすることがより好ましい。Ｍｇ含有量は０．０００１％以上とすることが好ましい。Ｍｇ含有量は０．０００５％以上とすることがより好ましい。 Zr: 0 to 0.050%, REM: 0 to 0.050%, Mg: 0 to 0.050%
Zr, REM, and Mg are elements that can be optionally contained in order to increase toughness through grain refinement or to increase crack resistance through control of inclusion properties. On the other hand, if the content exceeds 0.050%, the effect is saturated, so if any of them is contained, it is set to 0.050% or less. That is, if contained, the Zr content is set to 0.050% or less. It is preferable that the Zr content is set to 0.040% or less. It is more preferable that the Zr content is set to 0.030% or less. It is further preferable that the Zr content is set to 0.010% or less, and it is most preferable that the Zr content is set to 0.005% or less. Also, if contained, the REM content is set to 0.050% or less. It is preferable that the REM content is set to 0.040% or less. It is more preferable that the REM content is set to 0.030% or less. Also, if contained, the Mg content is set to 0.050% or less. It is preferable that the Mg content is set to 0.040% or less. It is more preferable that the Mg content is set to 0.030% or less. The contents of these elements may be 0% or more, but since the effect of the inclusion is difficult to obtain when the contents are less than 0.0001%, it is preferable to set the contents to 0.0001% or more. That is, the Zr content is preferably 0.0001% or more. The Zr content is more preferably 0.0005% or more. The REM content is preferably 0.0001% or more. The REM content is more preferably 0.0005% or more. The Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more.
　Ｂ：０～０．００２０％
　Ｂは、焼き入れ性を向上させる元素であり、鋼管の強度上昇に寄与するとともに、旧オーステナイト粒の粗大化を抑制し、素材の各種特性を向上させる。一方、Ｂ含有量が０．００２０％を越えると効果が飽和し、コストアップの要因となるため、含有する場合にはＢ含有量は０．００２０％以下とする。Ｂ含有量は０．００１５％以下とすることが好ましい。Ｂ含有量は０．００１２％以下とすることがより好ましい。コスト抑制のためには、０．００１０％以下とすることがさらに好ましい。Ｂ含有量は０％以上であってよいが、前記効果を得るために、含有量を０．０００１％以上とすることが好ましい。より好ましくは、Ｂ含有量は０．０００５％以上である。 B: 0 to 0.0020%
B is an element that improves hardenability, contributes to increasing the strength of the steel pipe, inhibits the coarsening of prior austenite grains, and improves various properties of the material. On the other hand, if the B content exceeds 0.0020%, the effect is saturated and causes an increase in cost, so if B is contained, the B content is set to 0.0020% or less. The B content is preferably set to 0.0015% or less. The B content is more preferably set to 0.0012% or less. In order to suppress costs, it is even more preferable to set the B content to 0.0010% or less. The B content may be 0% or more, but in order to obtain the above effect, the content is preferably set to 0.0001% or more. More preferably, the B content is 0.0005% or more.
　Ｈｆ：０～０．２％、Ｔａ：０～０．２％
　これらの元素は、鋼材の強度上昇に寄与するが、含有量が０．２％を越えると効果が飽和し、コストアップの要因となるため、含有する場合には０．２％以下とする。すなわち、含有する場合には、Ｈｆは０．２％以下とする。Ｈｆは０．１％以下とすることが好ましい。Ｈｆは０．０５％以下とすることがより好ましい。また、含有する場合には、Ｔａは０．２％以下とする。Ｔａは０．１％以下とすることが好ましい。Ｔａは０．０５％以下とすることがより好ましい。Ｈｆ、Ｔａ含有量は０％以上であってよいが、前記効果を得るために、含有量を０．０００１％以上とすることが好ましい。すなわち、Ｈｆ含有量は、０．０００１％以上が好ましい。より好ましくは、Ｈｆ含有量は、０．００１０％以上である。また、Ｔａ含有量は、０．０００１％以上が好ましい。より好ましくは、Ｔａ含有量は、０．００１０％以上である。 Hf: 0 to 0.2%, Ta: 0 to 0.2%
These elements contribute to increasing the strength of the steel material, but if the content exceeds 0.2%, the effect is saturated and becomes a factor of increasing costs, so if they are contained, they are set to 0.2% or less. That is, if they are contained, Hf is set to 0.2% or less. It is preferable that Hf is set to 0.1% or less. It is more preferable that Hf is set to 0.05% or less. Also, if they are contained, Ta is set to 0.2% or less. It is preferable that Ta is set to 0.1% or less. It is more preferable that Ta is set to 0.05% or less. The Hf and Ta contents may be 0% or more, but in order to obtain the above-mentioned effect, it is preferable that the contents are 0.0001% or more. That is, the Hf content is preferably 0.0001% or more. More preferably, the Hf content is 0.0010% or more. Also, the Ta content is preferably 0.0001% or more. More preferably, the Ta content is 0.0010% or more.
　Ｒｅ：０～０．００５％
　Ｒｅは、鋼材の強度上昇に寄与するが、含有量が０．００５％を越えると効果が飽和し、コストアップの要因となるため、含有する場合には０．００５％以下とする。Ｒｅ含有量は０．００３％以下とすることが好ましい。Ｒｅ含有量は０．００２％以下とすることがより好ましい。Ｒｅの含有量は０％以上であってよいが、前記効果を得るために、含有量を０．０００１％以上とすることが好ましい。より好ましくは、０．００１％以上である。 Re: 0 to 0.005%
Re contributes to increasing the strength of steel materials, but if the content exceeds 0.005%, the effect is saturated and becomes a factor of increasing costs, so if Re is contained, it is set to 0.005% or less. The Re content is preferably set to 0.003% or less. The Re content is more preferably set to 0.002% or less. The Re content may be 0% or more, but in order to obtain the above effect, the content is preferably set to 0.0001% or more. More preferably, it is set to 0.001% or more.
　Ｓｎ：０～０．３％、Ｓｂ：０～０．３％
　これらの元素は、鋼材の強度上昇と焼入れ性向上に寄与するが、含有量が０．３％を越えると効果が飽和し、コストアップの要因となるため、含有する場合には０．３％以下とする。すなわち、Ｓｎ含有量は０．３％以下とする。Ｓｎ含有量は０．２％以下とすることが好ましい。Ｓｎ含有量は０．１％以下とすることがより好ましい。コスト抑制のためには、Ｓｎ含有量は０．０１％以下とすることさらに好ましい。また、Ｓｂ含有量は０．３％以下とする。Ｓｂ含有量は０．２％以下とすることが好ましい。Ｓｂ含有量は０．１％以下とすることがより好ましい。コスト抑制のためには、Ｓｂ含有量は０．０１％以下とすることがさらに好ましい。Ｓｎ、Ｓｂの含有量は０％以上であってよいが、前記効果を得るために、含有量を０．０００１％以上とすることが好ましい。すなわち、Ｓｎ含有量は０．０００１％以上とすることが好ましい。より好ましくは、Ｓｎ含有量は０．００１０％以上である。また、Ｓｂ含有量は０．０００１％以上とすることが好ましい。より好ましくは、Ｓｂ含有量は０．００１０％以上である。 Sn: 0 to 0.3%, Sb: 0 to 0.3%
These elements contribute to increasing the strength and hardenability of steel, but if the content exceeds 0.3%, the effect is saturated and becomes a factor of increasing costs, so if they are contained, they are set to 0.3% or less. That is, the Sn content is set to 0.3% or less. It is preferable that the Sn content is set to 0.2% or less. It is more preferable that the Sn content is set to 0.1% or less. In order to suppress costs, it is even more preferable that the Sn content is set to 0.01% or less. Furthermore, the Sb content is set to 0.3% or less. It is preferable that the Sb content is set to 0.2% or less. It is more preferable that the Sb content is set to 0.1% or less. In order to suppress costs, it is even more preferable that the Sb content is set to 0.01% or less. The contents of Sn and Sb may be 0% or more, but in order to obtain the above effects, it is preferable that the contents are 0.0001% or more. That is, it is preferable that the Sn content is set to 0.0001% or more. More preferably, the Sn content is 0.0010% or more. The Sb content is preferably 0.0001% or more, and more preferably 0.0010% or more.
　鋼板および鋼管の成分組成において、上述した成分（元素）以外の残部は、Ｆｅおよび不可避的不純物元素からなる。 In the composition of steel plates and steel pipes, the remainder other than the above-mentioned components (elements) consists of Fe and unavoidable impurity elements.
　以下、本発明の鋼材の金属組織について述べる。 The metal structure of the steel material of the present invention is described below.
　金属組織
　残留オーステナイトが０～３％
　オーステナイトが鋼材中に残存することにより、鋼中の水素量が増加し、水素脆化感受性を増大させる場合がある。さらに、使用中の応力負荷によりオーステナイトがマルテンサイトに変態した場合、マルテンサイトが非常に硬質なため水素割れしやすく、マルテンサイト部分からき裂が発生する場合がある。本発明においては、残留オーステナイトを３％以下とすることで、疲労き裂進展速度を低減した。残留γの低減は水素環境中での疲労き裂の発生を抑制することで、水素中の疲労限応力の低下を抑制できる。このため、残留オーステナイトは３％以下とする。残留オーステナイトは好ましくは２％以下である。残留オーステナイトはより好ましくは１％以下である。残留オーステナイトは０％であってもよい。 Metal structure: 0-3% retained austenite
When austenite remains in the steel material, the amount of hydrogen in the steel increases, which may increase the hydrogen embrittlement susceptibility. Furthermore, when austenite is transformed into martensite due to stress load during use, the martensite is very hard and prone to hydrogen cracking, and cracks may occur from the martensite portion. In the present invention, the fatigue crack growth rate is reduced by making the retained austenite 3% or less. Reducing the retained γ suppresses the occurrence of fatigue cracks in a hydrogen environment, thereby suppressing the decrease in the fatigue limit stress in hydrogen. For this reason, the retained austenite is set to 3% or less. The retained austenite is preferably 2% or less. The retained austenite is more preferably 1% or less. The retained austenite may be 0%.
　板厚１／４位置においてベイナイトが面積分率で９０％以上
　引張強さが５２０ＭＰａ以上の高強度化を図るために、鋼組織は、ベイナイト組織とする必要がある。ここで、ベイナイト組織は、変態強化に寄与する制御冷却時あるいは制御冷却後に変態するベイニティックフェライトまたはグラニュラーベイナイトを含み、かつ、焼き戻しベイナイトを含むものとする。ベイナイト組織中に、フェライトや、マルテンサイト、パーライト、島状マルテンサイト、残留オーステナイトなどの異種組織が混在すると、強度の低下や靭性の劣化が生じるため、ベイナイト相以外の組織の体積分率は少ないほど良い。 Bainite has an area fraction of 90% or more at the 1/4 position of the plate thickness In order to achieve high strength with a tensile strength of 520 MPa or more, the steel structure needs to be a bainite structure. Here, the bainite structure includes bainitic ferrite or granular bainite that transforms during or after controlled cooling, which contributes to transformation strengthening, and also includes tempered bainite. If heterogeneous structures such as ferrite, martensite, pearlite, island martensite, and retained austenite are mixed in the bainite structure, a decrease in strength and a deterioration in toughness will occur, so the smaller the volume fraction of structures other than the bainite phase, the better.
　また疲労き裂の発生は、鋼材中に軟質相と硬質相が混在する場合は、疲労損傷が軟質相に優先的に蓄積され、き裂発生が生じやすくなることで疲労限応力が低下する。水素環境下においては、局所変形が助長されるため、軟質相への疲労損傷がより加速され、さらに水素中の疲労限応力が低下する。その結果、疲労限応力／不活性ガス環境中の疲労限応力が０．９０を下回る。これを改善するためには、相対的な軟質相の割合を低減する必要があり、ベイナイトの面積分率を９０％以上とした。ベイナイトは面積分率で９２％以上とすることが好ましい。ベイナイトは面積分率で９５％以上とすることがより好ましく、９８％以上とすることがさらに好ましい。上限は特に限定されるものではなく、ベイナイトは面積分率で１００％であってもよい。さらに、疲労き裂は鋼管内面から発生するため、鋼管内面組織の均一性が重要である。したがって、鋼管内面からの肉厚１／４位置における金属組織を規定し、鋼材については、どちらの表面が鋼管の内面側となっても上記効果を得るために、板厚１／４位置における金属組織を規定した。 Furthermore, when soft and hard phases are mixed in steel, fatigue damage accumulates preferentially in the soft phase, making it easier for cracks to occur, and thus lowering the fatigue limit stress. In a hydrogen environment, local deformation is promoted, so fatigue damage to the soft phase is accelerated, and the fatigue limit stress in hydrogen is further lowered. As a result, the fatigue limit stress/fatigue limit stress in an inert gas environment falls below 0.90. To improve this, it is necessary to reduce the relative proportion of the soft phase, and the area fraction of bainite is set to 90% or more. It is preferable that the area fraction of bainite is 92% or more. It is more preferable that the area fraction of bainite is 95% or more, and even more preferable that it is 98% or more. There is no particular upper limit, and the area fraction of bainite may be 100%. Furthermore, since fatigue cracks occur from the inner surface of the steel pipe, the uniformity of the inner surface structure of the steel pipe is important. Therefore, the metal structure is specified at 1/4 of the wall thickness from the inner surface of the steel pipe, and for steel materials, the metal structure is specified at 1/4 of the wall thickness in order to obtain the above effect regardless of which surface is on the inner surface of the steel pipe.
　１ＭＰａ以上の水素中疲労限応力が２００ＭＰａ以上かつ、１ＭＰａ以上の水素中疲労限応力／不活性ガス環境下の疲労限応力が０．９０以上
　１ＭＰａ以上の水素中疲労限応力が２００ＭＰａ未満、かつ１ＭＰａ以上の水素中疲労限応力／不活性ガス環境下の疲労限応力が０．９０未満となると、従来のパイプラインの設計条件と大きく異なるため、鋼材厚さ（鋼管の場合には鋼管厚さ）を大きくする必要が出てくる。このため、１ＭＰａ以上の水素中疲労限応力が２００ＭＰａ以上、かつ１ＭＰａ以上の水素中疲労限応力／不活性ガス環境下の疲労限応力が０．９０以上とする。１ＭＰａ以上の水素中疲労限応力が２２０ＭＰａ以上とすることが好ましい。１ＭＰａ以上の水素中疲労限応力が２５０ＭＰａ以上とすることがより好ましく、２７０ＭＰａ以上とすることがさらに好ましい。特に上限は限定されるものではないが、１ＭＰａ以上の水素中疲労限応力が５００ＭＰａ以下とすることが好ましい。また、１ＭＰａ以上の水素中疲労限応力／不活性ガス環境下の疲労限応力が０．９２以上とすることが好ましい。１ＭＰａ以上の水素中疲労限応力／不活性ガス環境下の疲労限応力が０．９４以上とすることがより好ましく、０．９６以上とすることがさらに好ましい。特に上限は限定されるものではないが、１ＭＰａ以上の水素中疲労限応力／不活性ガス環境下の疲労限応力が１．１以下であってよい。
なお、ここでいう不活性ガスとは、周期律表の０族の６元素ヘリウム、ネオン、アルゴン、クリプトン、キセノン、ラドンに加えて大気を含むものであり、不活性ガス環境下とは上記のいずれかを含む環境中のことである。 The fatigue limit stress in hydrogen of 1 MPa or more is 200 MPa or more, and the fatigue limit stress in hydrogen of 1 MPa or more/inert gas environment is 0.90 or more. If the fatigue limit stress in hydrogen of 1 MPa or more is less than 200 MPa, and the fatigue limit stress in hydrogen of 1 MPa or more/inert gas environment is less than 0.90, it is significantly different from the design conditions of conventional pipelines, and it becomes necessary to increase the steel thickness (steel pipe thickness in the case of steel pipes). For this reason, the fatigue limit stress in hydrogen of 1 MPa or more is 200 MPa or more, and the fatigue limit stress in hydrogen of 1 MPa or more/inert gas environment is 0.90 or more. It is preferable that the fatigue limit stress in hydrogen of 1 MPa or more is 220 MPa or more. It is more preferable that the fatigue limit stress in hydrogen of 1 MPa or more is 250 MPa or more, and even more preferable that it is 270 MPa or more. Although there is no particular upper limit, it is preferable that the fatigue limit stress in hydrogen of 1 MPa or more is 500 MPa or less. In addition, the fatigue limit stress in hydrogen of 1 MPa or more/fatigue limit stress in an inert gas environment is preferably 0.92 or more. The fatigue limit stress in hydrogen of 1 MPa or more/fatigue limit stress in an inert gas environment is more preferably 0.94 or more, and even more preferably 0.96 or more. There is no particular upper limit, but the fatigue limit stress in hydrogen of 1 MPa or more/fatigue limit stress in an inert gas environment may be 1.1 or less.
The inert gas referred to here includes the six elements in Group 0 of the periodic table, helium, neon, argon, krypton, xenon, and radon, as well as air, and an inert gas environment refers to an environment that includes any of the above.
　本発明は、上述した化学成分と、金属組織を有することで、高圧水素雰囲気下での疲労限応力の向上ならびに水素中疲労限応力／不活性ガス中疲労限応力の低下が抑制されるとともに、引張強度５２０ＭＰａ以上が得られ、水素ラインパイプへの適用が可能となる。引張強度の上限は特に限定されるものではないが、９５０ＭＰａ以下とすることが好ましい。 The present invention has the above-mentioned chemical components and metal structure, which improves the fatigue limit stress in a high-pressure hydrogen atmosphere and suppresses the decrease in the fatigue limit stress in hydrogen/inert gas, while achieving a tensile strength of 520 MPa or more, making it applicable to hydrogen line pipes. There is no particular upper limit for the tensile strength, but it is preferably 950 MPa or less.
　また、鋼材の板厚は特に限定されるものではないが、５ｍｍ以上とすることが好ましい。板厚は３０ｍｍ以下とすることが好ましい。 The thickness of the steel plate is not particularly limited, but is preferably 5 mm or more. It is preferable that the thickness be 30 mm or less.
　次に本発明の鋼材の製造方法について説明する。本発明の鋼材は、鋼素材（スラブ）の加熱工程、熱間圧延工程、制御冷却工程、および脱水素処理工程を順次行うことによって製造できる。
なお、以下の説明における温度は、特に断らない限り、鋼素材または鋼管の板厚中央の温度とする。平均冷却速度は、鋼管の内面からの肉厚１／４位置温度を意味する。なお、板厚中央の温度と鋼管の内面からの肉厚１／４位置の温度は、放射温度計で測定した鋼管表面温度から鋼材の熱伝達係数を考慮した伝熱計算等を用いて上記温度を推定した温度である。 Next, a method for producing the steel material of the present invention will be described. The steel material of the present invention can be produced by sequentially carrying out a heating step, a hot rolling step, a controlled cooling step, and a dehydrogenation step of a steel material (slab).
In the following description, unless otherwise specified, the temperature is the temperature at the center of the thickness of the steel material or the steel pipe. The average cooling rate means the temperature at 1/4 of the thickness from the inner surface of the steel pipe. The temperature at the center of the thickness and the temperature at 1/4 of the thickness from the inner surface of the steel pipe are temperatures estimated from the steel pipe surface temperature measured with a radiation thermometer using heat transfer calculations that take into account the heat transfer coefficient of the steel material.
　加熱工程
　鋼素材の加熱温度：１０００～１２５０℃
　ビレットやスラブ等の鋼素材加熱温度は、１０００℃未満ではミクロ偏析しているＣやＰ、Ｓ等の不純物元素の拡散が不十分で均質な材質が得られない。このため、鋼素材の加熱温度は１０００℃以上とする。一方、１２５０℃を超えると、結晶粒が粗大化しすぎ靱性が劣化する。従って、鋼素材の加熱温度は１２５０℃以下とする。加熱温度は１２００℃以下とすることが好ましい。加熱温度は、１１８０℃以下とすることがより好ましい。 Heating process Heating temperature of steel material: 1000-1250℃
If the heating temperature of steel material such as billets or slabs is less than 1000°C, the diffusion of micro-segregated impurity elements such as C, P, and S is insufficient, and a homogeneous material cannot be obtained. For this reason, the heating temperature of the steel material is set to 1000°C or higher. On the other hand, if the heating temperature exceeds 1250°C, the crystal grains become too coarse and the toughness deteriorates. Therefore, the heating temperature of the steel material is set to 1250°C or lower. The heating temperature is preferably set to 1200°C or lower. The heating temperature is more preferably set to 1180°C or lower.
　熱間圧延工程
　熱間圧延終了温度：Ａｒ３点以上
　鋼素材を再加熱した後、所望の管厚または板厚まで熱間で圧延を行うが、熱間圧延の終了温度は、フェライト生成温度であるＡｒ３点以上とする。Ａｒ３点未満では熱間圧延後に直ちに冷却を行うプロセスの場合、軟質なフェライト相の生成により強度低下を招くためである。熱間圧延の終了温度は、Ａｒ３＋３０℃以上とすることが好ましい。熱間圧延の終了温度はＡｒ３＋５０℃以上とすることがより好ましい。また、１２５０℃を超えると、結晶粒が粗大化しすぎ靱性が劣化するため上限は１２５０℃以下とすることが好ましい。熱間圧延の終了温度は、１２００℃以下とすることがより好ましく、１１５０℃以下とすることがさらに好ましい。 Hot rolling process Hot rolling end temperature: Ar 3 point or more After reheating the steel material, hot rolling is performed to the desired tube thickness or plate thickness, but the end temperature of hot rolling is Ar 3 point or more, which is the ferrite formation temperature. If the temperature is less than Ar 3 point, in the case of a process in which cooling is performed immediately after hot rolling, the strength will decrease due to the formation of a soft ferrite phase. The end temperature of hot rolling is preferably Ar 3 + 30 ° C or more. It is more preferable that the end temperature of hot rolling is Ar 3 + 50 ° C or more. In addition, if the temperature exceeds 1250 ° C, the crystal grains become too coarse and the toughness deteriorates, so the upper limit is preferably 1250 ° C or less. It is more preferable that the end temperature of hot rolling is 1200 ° C or less, and even more preferable that it is 1150 ° C or less.
　Ａｒ３点は鋼の合金成分によって変化するため、それぞれの鋼で実験によって変態温度を測定して求めてもよいが、成分組成から下式で求めることもできる。
Ａｒ３（℃）＝９１０－３１０Ｃ（％）－８０Ｍｎ（％）－２０Ｃｕ（％）－１５Ｃｒ（％）－５５Ｎｉ（％）－８０Ｍｏ（％）
各合金元素は含有量（質量％）とする。 Since the Ar3 point varies depending on the alloying components of the steel, it may be determined by measuring the transformation temperature through experiments for each steel, but it may also be determined from the component composition using the following formula.
Ar3 (°C) = 910 - 310C (%) - 80Mn (%) - 20Cu (%) - 15Cr (%) - 55Ni (%) - 80Mo (%)
The content of each alloying element is expressed as mass %.
　制御冷却工程
　制御冷却の冷却開始温度：鋼板表面温度でＡｒ３点以上
　冷却開始時の鋼板表面温度がＡｒ３点未満の場合、制御冷却前にフェライトが生成して、強度低下が大きくなる。このため、冷却開始時の鋼板表面温度はＡｒ３点以上とする。冷却開始時の鋼板表面温度はＡｒ３＋３０℃以上とすることが好ましい。Ａｒ３＋５０℃以上とすることがより好ましい。なお、冷却開始温度が高温過ぎると結晶粒径が大きくなりすぎて靭性が低下するため、冷却開始時の鋼板表面温度は１２５０℃未満であることが好ましい。冷却開始時の鋼板表面温度は１２００℃以下とすることがより好ましく、１１５０℃以下とすることがさらに好ましい。なお、冷却開始時の鋼板表面温度は、冷却開始温度が最も低くなる鋼板尾端部の温度である。 Controlled cooling process Cooling start temperature of controlled cooling: steel plate surface temperature is Ar 3 point or more If the steel plate surface temperature at the start of cooling is less than Ar 3 point, ferrite is generated before controlled cooling, and strength reduction increases. Therefore, the steel plate surface temperature at the start of cooling is Ar 3 point or more. The steel plate surface temperature at the start of cooling is preferably Ar 3 + 30°C or more. It is more preferable to set it to Ar 3 + 50°C or more. If the cooling start temperature is too high, the crystal grain size becomes too large and the toughness decreases, so the steel plate surface temperature at the start of cooling is preferably less than 1250°C. The steel plate surface temperature at the start of cooling is more preferably 1200°C or less, and even more preferably 1150°C or less. The steel plate surface temperature at the start of cooling is the temperature of the tail end of the steel plate where the cooling start temperature is the lowest.
　制御冷却の鋼板先端と鋼板尾端の冷却開始時間差：５０秒以内
　冷却開始時の鋼板圧延方向の先端と尾端の時間差が５０秒超えの場合、冷却開始時の先端と尾端の温度差が大きくなるため、冷却停止時の温度ばらつきが大きくなり、鋼板表面下０．２５ｍｍにおけるビッカース硬さのばらつきが大きくなると共に耐ＨＩＳＣ性が劣化する。このため、鋼板先端と鋼板尾端の冷却開始時間差は５０秒以内とし、好ましくは４５秒以内とする。より好ましくは４０秒以内とする。鋼板長が短くなることで冷却開始時間差を短くすることが可能であるが、製造性が低下するため、鋼板搬送速度を速くすることで冷却開始時間差を短くすることが好ましい。下限は特に限定されるものではなく、０秒超えであってよい。 Cooling start time difference between the front end and the tail end of the steel plate in the controlled cooling: within 50 seconds If the time difference between the front end and the tail end in the rolling direction of the steel plate at the start of cooling exceeds 50 seconds, the temperature difference between the front end and the tail end at the start of cooling becomes large, so the temperature variation at the time of stopping cooling becomes large, and the variation in Vickers hardness at 0.25 mm below the surface of the steel plate becomes large and the HISC resistance deteriorates. For this reason, the cooling start time difference between the front end and the tail end of the steel plate is within 50 seconds, preferably within 45 seconds. More preferably, it is within 40 seconds. Although it is possible to shorten the cooling start time difference by shortening the steel plate length, it is preferable to shorten the cooling start time difference by increasing the steel plate conveying speed, since this reduces manufacturability. The lower limit is not particularly limited and may be more than 0 seconds.
　板厚中央における７５０℃から５５０℃までの平均冷却速度：１５～５０℃／ｓ
　板厚中央における７５０℃から５５０℃までの平均冷却速度が１５℃／ｓ未満では、ベイナイト組織が得られず、強度低下が生じる。このため、板厚中央での平均冷却速度は１５℃／ｓ以上とする。組織のばらつき抑制の観点からは、板厚中央での平均冷却速度は１７℃／ｓ以上とすることが好ましい。板厚中央での平均冷却速度は２０℃／ｓ以上とすることがより好ましく、２５℃／ｓ以上とすることがさらに好ましい。一方、ベイナイトの粒径のばらつきを抑制するために、板厚中央での平均冷却速度は５０℃／ｓ以下とする。板厚中央での平均冷却速度は４５℃／ｓ以下とすることが好ましい。板厚中央での平均冷却速度は４０℃／ｓ以下とすることがより好ましい。なお、板厚中央における鋼板温度で５５０℃以下の冷却については、特に限定されないが、組織や粒径のばらつき抑制の観点から、例えば５５０℃から３００℃までにおける平均冷却速度は１５℃／ｓ以上とすることが好ましい。５５０℃から３００℃までにおける平均冷却速度は５０℃／ｓ以下とすることが好ましい。 Average cooling rate from 750 ° C to 550 ° C at the center of plate thickness: 15 to 50 ° C / s
If the average cooling rate from 750°C to 550°C at the center of the plate thickness is less than 15°C/s, the bainite structure is not obtained, and strength is reduced. Therefore, the average cooling rate at the center of the plate thickness is set to 15°C/s or more. From the viewpoint of suppressing the variation in the structure, the average cooling rate at the center of the plate thickness is preferably set to 17°C/s or more. The average cooling rate at the center of the plate thickness is more preferably set to 20°C/s or more, and even more preferably set to 25°C/s or more. On the other hand, in order to suppress the variation in the grain size of bainite, the average cooling rate at the center of the plate thickness is set to 50°C/s or less. The average cooling rate at the center of the plate thickness is preferably set to 45°C/s or less. The average cooling rate at the center of the plate thickness is more preferably set to 40°C/s or less. The cooling at the steel plate temperature at the center of the plate thickness of 550°C or less is not particularly limited, but from the viewpoint of suppressing the variation in the structure and grain size, for example, the average cooling rate from 550°C to 300°C is preferably set to 15°C/s or more. The average cooling rate from 550° C. to 300° C. is preferably 50° C./s or less.
　冷却停止温度：２５０～６５０℃
　熱間圧延後の冷却停止温度が６５０℃超えではベイナイト変態が不完全になり、材料強度が大きく低下する。このため、冷却停止温度は６５０℃以下とする。冷却停止温度は６２５℃以下することが好ましい。冷却停止温度は６００℃以下とすることがより好ましい。一方、冷却停止温度が２５０℃未満では、冷却時の焼割れが発生しやすくなる。また、均一なベイナイト組織を得るため、冷却停止温度を２５０℃以上とする。鋼中水素量を抑制するという点からも冷却停止温度は所定の温度以上とする必要がある。具体的に、冷却中に鋼中に存在した水素は徐々に抜けていき、高温程その効果は大きいが、冷却停止温度が低すぎる場合には過冷却となり、鋼中に水素が残存する。さらに、冷却停止温度を低くしすぎると、他の相と比較して多量に水素を急増する残留オーステナイトが形成されやすくなる。そのため、冷却停止温度は鋼中水素量を低減させるためにも、２５０℃以上とする必要がある。冷却停止温度は２７０℃以上とすることが好ましい。冷却停止後は放冷すればよいが、ベイナイトの生成を促進するために、冷却停止温度から５０℃程度温度が下がるまでは徐冷することがより好ましい。なお、ここでいう冷却停止温度は板厚中央の温度である。 Cooling stop temperature: 250 to 650°C
If the cooling stop temperature after hot rolling exceeds 650°C, the bainite transformation becomes incomplete, and the material strength is greatly reduced. For this reason, the cooling stop temperature is set to 650°C or less. The cooling stop temperature is preferably set to 625°C or less. The cooling stop temperature is more preferably set to 600°C or less. On the other hand, if the cooling stop temperature is less than 250°C, quench cracks are likely to occur during cooling. In addition, in order to obtain a uniform bainite structure, the cooling stop temperature is set to 250°C or more. In order to suppress the amount of hydrogen in the steel, the cooling stop temperature needs to be set to a predetermined temperature or more. Specifically, hydrogen present in the steel is gradually released during cooling, and the higher the temperature, the greater the effect, but if the cooling stop temperature is too low, the steel will be overcooled and hydrogen will remain in the steel. Furthermore, if the cooling stop temperature is set too low, retained austenite, which rapidly increases the amount of hydrogen compared to other phases, is likely to be formed. For this reason, the cooling stop temperature needs to be set to 250°C or more in order to reduce the amount of hydrogen in the steel. The cooling stop temperature is preferably set to 270°C or more. After cooling is stopped, the material may be allowed to cool naturally, but in order to promote the formation of bainite, it is more preferable to slowly cool the material until the temperature drops by about 50° C. from the cooling stop temperature. Note that the cooling stop temperature referred to here is the temperature at the center of the plate thickness.
　脱水素処理工程
　鋼材中にそもそも水素が存在する場合には疲労き裂進展の加速が増大され、疲労寿命および水素中疲労限応力が低下する。そのため、製造後に残存する水素を放出させるために、脱水素処理を用いてもよい。脱水素処理は、製品使用前に高温で一定時間保持することで鋼中水素量を低減させることができ、高圧水素ガス環境下における耐水素脆化特性に優れた鋼板を得ることができる。
保持時間Ｒ（ｓｅｃ）は、鋼管の板厚並びに管厚ｔ（ｍｍ）、および室温における鋼中の水素拡散係数Ｄ（ｍｍ・ｓｅｃ－１）から、以下の式（Ａ）とすることが好ましい。
Ｒ≧ｔ２／Ｄ・・・（Ａ）
水素拡散係数は含有している成分や金属組織によっても変わるが、例えば、水素拡散係数は１×１０－５～ ５×１０－３ｍｍ２／ｓを採用しても良い。より好ましくは ５×１０－４ｍｍ２／ｓ以下である。 Dehydrogenation process When hydrogen is present in steel, the acceleration of fatigue crack growth is increased, and the fatigue life and fatigue stress limit in hydrogen are reduced. Therefore, dehydrogenation may be used to release the hydrogen remaining after manufacturing. Dehydrogenation can reduce the amount of hydrogen in the steel by holding the product at high temperature for a certain period of time before use, and it is possible to obtain a steel plate with excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.
The holding time R (sec) is preferably determined by the following formula (A) based on the plate thickness and pipe thickness t (mm) of the steel pipe, and the hydrogen diffusion coefficient D (mm·sec −1 ) in steel at room temperature.
R≧ t2 /D (A)
The hydrogen diffusion coefficient varies depending on the contained components and metal structure, but may be, for example, 1×10 −5 to 5×10 −3 mm 2 /s, and more preferably 5×10 −4 mm 2 /s or less.
　脱水素処理工程は、造管または鋼管をつなげる溶接施工前に実施する。なお、脱水素処理は高温の水素拡散係数Ｄが小さくなり、早く水素が抜けるため高温である方が好ましい。高温の場合は上記（Ａ）式のＤの値を保持する温度の拡散係数Ｄ’（それぞれの温度における拡散係数）を用いて計算しても良い。一方、脱水素工程の温度Ｔが高すぎる場合には材料強度が著しく低下するため、脱水素処理温度Ｔは５５０℃以下とする。脱水素処理温度Ｔは５００℃以下とすることが好ましい。脱水素処理温度Ｔは４００℃以下とすることがより好ましく、３００℃以下とすることがさらに好ましい。また、室温よりも温度を低下させた脱水素処理は処理時間およびコスト増の要因であるという理由から脱水素処理温度Ｔは室温以上とする。脱水素処理温度Ｔは５０℃以上とすることが好ましい。脱水素処理温度Ｔは１００℃以上とすることがより好ましく、１５０℃以上とすることがさらに好ましい。なお、室温とは２０±１０℃であることをいう。 The dehydrogenation process is carried out before pipe making or welding to connect steel pipes. It is preferable to perform dehydrogenation at a high temperature because the hydrogen diffusion coefficient D at high temperatures becomes small and hydrogen escapes quickly. In the case of high temperatures, the diffusion coefficient D' (diffusion coefficient at each temperature) at which the value of D in the above formula (A) is maintained may be used for calculation. On the other hand, if the temperature T of the dehydrogenation process is too high, the material strength decreases significantly, so the dehydrogenation temperature T is set to 550°C or less. It is preferable that the dehydrogenation temperature T is set to 500°C or less. It is more preferable that the dehydrogenation temperature T is set to 400°C or less, and even more preferable that it is set to 300°C or less. In addition, since dehydrogenation at a temperature lower than room temperature increases the processing time and costs, the dehydrogenation temperature T is set to room temperature or higher. It is preferable that the dehydrogenation temperature T is set to 50°C or higher. It is more preferable that the dehydrogenation temperature T is set to 100°C or higher, and even more preferable that it is set to 150°C or higher. It is noted that room temperature means 20±10°C.
　特に、加熱する場合、鋼材および鋼管の板厚中央の温度Ｔｃが脱水素処理工程における雰囲気の温度（脱水素処理温度Ｔ）に到達するまでに時間を要するため、雰囲気温度において上記保持時間Ｒ（ｓｅｃ）を満たしていても、板厚中央が脱水素処理温度Ｔ（雰囲気温度）に達していない場合は脱水素処理が不十分となる可能性がある。そのため、板厚中央温度Ｔｃが目標とする脱水素処理温度Ｔに達してからＲ（ｓｅｃ）以上保持することが好ましい。さらに、所定の水素中疲労限応力、１ＭＰａ以上の水素中疲労限応力／不活性ガス環境下の疲労限応力を得るために、表層部と板厚中央の鋼材水素量を適切に調整する必要があり、そのために、脱水素処理温度Ｔで、（Ａ）式で規定されたＲ（ｓｅｃ）以上保持することが好ましく、さらに板厚中央温度Ｔｃが目標とする脱水素処理温度Ｔに達してからＲ（ｓｅｃ）以上保持することが好ましい。板厚中央温度Ｔｃは熱電対などをもちいて実測してもいいし、有限要素法などを用いて予測してもよい。 In particular, when heating, it takes time for the temperature Tc at the center of the thickness of the steel and steel pipe to reach the temperature of the atmosphere in the dehydrogenation process (dehydrogenation temperature T), so even if the above-mentioned holding time R (sec) is met at the atmospheric temperature, if the center of the thickness does not reach the dehydrogenation temperature T (atmospheric temperature), the dehydrogenation may be insufficient. Therefore, it is preferable to hold the temperature Tc for R (sec) or more after it reaches the target dehydrogenation temperature T. Furthermore, in order to obtain a specified fatigue limit stress in hydrogen, a fatigue limit stress in hydrogen of 1 MPa or more/fatigue limit stress in an inert gas environment, it is necessary to appropriately adjust the hydrogen content of the steel at the surface layer and the center of the thickness. For this reason, it is preferable to hold the temperature T for R (sec) or more as specified by formula (A), and furthermore, it is preferable to hold the temperature Tc for R (sec) or more after it reaches the target dehydrogenation temperature T. The temperature Tc may be measured using a thermocouple or the like, or it may be predicted using the finite element method or the like.
　なお、脱水素処理工程の時間と温度は、後述しているとおり電縫管やＵＯＥ等の造管工程で加熱する際に加えられた温度と時間が含まれても良い。さらに、鋼表面のスケールは脱水素を阻害するため、スケールを除去し脱水素処理行う方が好ましい。除去方法は限定されないが、例えば高圧洗浄による物理的な洗浄でもよいし、スケール除去剤を用いた化学的な手法を用いてもよい。厚みとして１００μｍ程度除去されればスケール除去の効果が得られる。 The time and temperature of the dehydrogenation process may include the temperature and time applied when heating in the pipe-making process for electric resistance welded pipes, UOE, etc., as described below. Furthermore, since scale on the steel surface inhibits dehydrogenation, it is preferable to remove the scale before carrying out the dehydrogenation process. There are no limitations on the removal method, but it may be physical cleaning using a high-pressure cleaner, or a chemical method using a scale remover. The effect of scale removal can be obtained if a thickness of about 100 μm is removed.
　第２実施形態
　さらに、ラインパイプ用高強度鋼管の一例として挙げられるＵＯＥ鋼管は下記に示す製造条件を限定することにより得ることができ、製造方法および条件を具体的に説明する。ＵＯＥ鋼管の成分組成、金属組織、１ＭＰａ以上の水素中疲労限応力、１ＭＰａ以上の水素中疲労限応力／不活性ガス環境下の疲労限応力は第１実施形態の鋼材で説明した内容と同様であり、製造方法についても加熱工程、熱間圧延工程、熱間圧延後の制御冷却工程、脱水素処理工程は鋼材で説明した内容と同等の内容で実施される。下記では、圧延後の造管工程を具体的に説明する。 Second embodiment Furthermore, a UOE steel pipe, which is an example of a high-strength steel pipe for line pipe, can be obtained by limiting the manufacturing conditions shown below, and the manufacturing method and conditions will be specifically described. The component composition, metal structure, fatigue limit stress in hydrogen of 1 MPa or more, and fatigue limit stress in hydrogen of 1 MPa or more/fatigue limit stress in an inert gas environment of the UOE steel pipe are the same as those described for the steel material of the first embodiment, and as for the manufacturing method, the heating process, hot rolling process, controlled cooling process after hot rolling, and dehydrogenation process are performed in the same manner as those described for the steel material. The pipe-making process after rolling will be specifically described below.
　造管工程
　ＵＯＥ鋼管は、熱延鋼板を曲げ加工、具体的にいうと熱延鋼板の端部を開先加工し、Ｃプレス、Ｕプレス、Ｏプレスで鋼管形状に成形する加工を施した後、内面溶接および外面溶接で突き合わせ部をシーム溶接し、さらに必要に応じて拡管工程を経て製造される。また、溶接方法は十分な継手強度と継手靭性が得られる方法であれば、いずれの方法でも良いが、優れた溶接品質と製造能率の観点から、サブマージアーク溶接を用いることが好ましい。また、プレスベンド成形により管状に成形した後、突き合せ部をシーム溶接した鋼管に対しても、拡管を実施することができる。 Pipe-making process UOE steel pipes are manufactured by bending hot-rolled steel sheets, specifically by groove-cutting the ends of the hot-rolled steel sheets and forming them into a steel pipe shape using a C press, U press, or O press, then seam-welding the butt joints by internal and external welding, and, if necessary, by a pipe expansion process. Any welding method may be used as long as it provides sufficient joint strength and joint toughness, but submerged arc welding is preferred from the viewpoint of excellent welding quality and manufacturing efficiency. Pipe expansion can also be performed on steel pipes that have been formed into a pipe shape by press bending and then have their butt joints seam-welded.
　第３実施形態
　さらに、本発明に係るラインパイプ用高強度鋼管には、一例として電縫鋼管が挙げられ、電縫鋼管は下記に示す製造条件を限定することにより得ることができ、製造方法および条件を具体的に説明する。鋼材の成分組成、金属組織、１ＭＰａ以上の水素中疲労限応力、１ＭＰａ以上の水素中疲労限応力／不活性ガス環境下の疲労限応力は第１実施形態の鋼材で説明した内容と同様であり、製造方法についても圧延後の冷却工程、造管工程以外の工程（加熱工程、熱間圧延工程、脱水素処理工程）は鋼材で説明した内容と同等の内容で実施される。 Third embodiment Furthermore, an example of the high-strength steel pipe for line pipe according to the present invention is an electric resistance welded steel pipe, which can be obtained by limiting the manufacturing conditions shown below, and the manufacturing method and conditions will be specifically described below. The steel material's component composition, metal structure, fatigue limit stress in hydrogen of 1 MPa or more, and fatigue limit stress in hydrogen of 1 MPa or more/fatigue limit stress in an inert gas environment are the same as those described for the steel material of the first embodiment, and as for the manufacturing method, the steps other than the cooling step after rolling and the pipe making step (heating step, hot rolling step, dehydrogenation treatment step) are carried out in the same manner as those described for the steel material.
　圧延後の冷却工程（制御冷却工程）
　制御冷却の冷却開始温度、制御冷却の平均冷却速度は第１実施形態で記載と同じ内容で実施される。 Cooling process after rolling (controlled cooling process)
The cooling start temperature and the average cooling rate of the controlled cooling are the same as those described in the first embodiment.
　冷却停止温度：２５０～６５０℃
　熱間圧延後の冷却停止温度が６５０℃超えではベイナイト変態が不完全になり、材料強度が大きく低下する。このため、冷却停止温度は６５０℃以下とする。冷却停止温度は６２０℃以下とすることが好ましい。冷却停止温度は５８０℃以下とすることがより好ましい。一方、冷却停止温度が２５０℃未満では、冷却時の焼割れが発生しやすくなる。また、均一なベイナイト組織を得るため、冷却停止温度を２５０℃以上とする。鋼中水素量を抑制するという点からも冷却停止温度は所定の温度以上とする必要がある。具体的に、冷却中に鋼中に存在した水素は徐々に抜けていき、高温程その効果は大きいが、冷却停止温度が低すぎる場合には過冷却となり、鋼中に水素が残存する。さらに、冷却停止温度を低くしすぎると、他の相と比較して多量に水素を急増する残留オーステナイトが形成されやすくなる。そのため、冷却停止温度は鋼中水素量を低減させるためにも、２５０℃以上とする必要がある。冷却停止温度は、好ましくは３９０℃以上である。より好ましくは、冷却停止温度は４５０℃以上である。冷却停止温度は、さらに好ましくは４８０℃以上である。冷却停止後は放冷すればよいが、ベイナイトの生成を促進するために、冷却停止温度から５０℃程度温度が下がるまでは徐冷することがより好ましい。なお、ここでいう冷却停止温度は板厚中央の温度である。 Cooling stop temperature: 250 to 650°C
If the cooling stop temperature after hot rolling exceeds 650°C, the bainite transformation becomes incomplete, and the material strength is greatly reduced. For this reason, the cooling stop temperature is set to 650°C or less. The cooling stop temperature is preferably set to 620°C or less. It is more preferable that the cooling stop temperature is set to 580°C or less. On the other hand, if the cooling stop temperature is less than 250°C, quench cracks are likely to occur during cooling. In addition, in order to obtain a uniform bainite structure, the cooling stop temperature is set to 250°C or more. In order to suppress the amount of hydrogen in the steel, the cooling stop temperature needs to be set to a predetermined temperature or more. Specifically, hydrogen present in the steel during cooling is gradually released, and the higher the temperature, the greater the effect, but if the cooling stop temperature is too low, the steel will be overcooled and hydrogen will remain in the steel. Furthermore, if the cooling stop temperature is set too low, retained austenite, which rapidly increases the amount of hydrogen compared to other phases, is likely to be formed. Therefore, the cooling stop temperature needs to be set to 250°C or more in order to reduce the amount of hydrogen in the steel. The cooling stop temperature is preferably 390°C or more. More preferably, the cooling stop temperature is 450°C or more. The cooling stop temperature is more preferably 480° C. or higher. After cooling is stopped, the material may be allowed to cool naturally, but in order to promote the formation of bainite, it is more preferable to slowly cool the material until the temperature drops by about 50° C. from the cooling stop temperature. The cooling stop temperature here refers to the temperature at the center of the plate thickness.
　その後、上記のようにして得られた熱延鋼板をコイル状に巻取る。巻取り温度は６５０℃以下とすることが好ましい。また、巻き取り温度は２５０℃以上とすることが好ましい。 Then, the hot-rolled steel sheet obtained as described above is wound into a coil. The winding temperature is preferably 650°C or less. Also, the winding temperature is preferably 250°C or more.
　造管工程
　本発明の一例として挙げている電縫鋼管は、冷間ロール成形により円筒状に成形し、前記円筒状の周方向両端部を突き合わせて溶接することによって製造される。さらに、以下の（１）式を満たすサイジングロールを用いて電縫鋼管素材（電縫鋼管）に成形し（サイジング工程）、前記電縫鋼管素材の内面に以下の（２）式を満たす内圧ｐ（ＭＰａ）を負荷する（内圧負荷工程）ことによって製造してもよい。
なお、前記円筒状とは、管周断面が「Ｃ」形状であることを指す。
サイジングロールの直径（ｍｍ）≧熱延鋼板の板厚（ｍｍ）／０．０２０　・・・（１）
熱延鋼板の板厚とは、サイジング工程を行う前の熱延鋼板の板厚のことである。
　Ｘ＜ｐ≦Ｘ×１．５　・・・（２）
なお、Ｘ＝（電縫鋼管素材の肉厚（ｍｍ）／電縫鋼管素材の半径（ｍｍ））×電縫鋼管素材の降伏強度（ＭＰａ）
前記した内圧の負荷は、例えば、ゴム素材のパッキンで管端を封じて管内部に水圧を負荷することにより実施することができる。また、形状を安定化させるために、必要に応じて外枠として所期した径の金型を使用することもできる。 Pipe-making process The electric resistance welded steel pipe given as an example of the present invention is manufactured by forming the steel pipe into a cylindrical shape by cold roll forming, and then butting and welding both circumferential ends of the cylindrical shape together. Furthermore, the electric resistance welded steel pipe may be manufactured by forming the steel pipe into an electric resistance welded steel pipe material (electric resistance welded steel pipe) using a sizing roll that satisfies the following formula (1) (sizing process), and applying an internal pressure p (MPa) that satisfies the following formula (2) to the inner surface of the electric resistance welded steel pipe material (internal pressure application process).
The cylindrical shape means that the circumferential cross section of the tube is in a "C" shape.
Diameter of sizing roll (mm) ≧ Thickness of hot-rolled steel sheet (mm)/0.020 (1)
The plate thickness of the hot-rolled steel plate means the plate thickness of the hot-rolled steel plate before the sizing process is performed.
X<p≦X×1.5 (2)
In addition, X = (thickness of electric welded steel pipe material (mm) / radius of electric welded steel pipe material (mm)) × yield strength of electric welded steel pipe material (MPa)
The above-mentioned internal pressure can be applied, for example, by sealing the end of the tube with a rubber packing and applying water pressure to the inside of the tube. In order to stabilize the shape, a mold of a desired diameter can be used as an outer frame as necessary.
　なお、本発明の鋼管の一例として挙げている電縫鋼管素材の肉厚は５ｍｍ以上が好ましく、３０ｍｍ以下が好ましい。電縫鋼管素材の半径について上限は特に規定しないが、大きくなると設備の負荷が増大するため、電縫鋼管素材の半径は４００ｍｍ以下が好ましい。また、電縫管素材の半径は、２００ｍｍ以上が好ましい。また、電縫鋼管素材の降伏強度はパイプライン操業ガス圧力に耐えるため、４８０ＭＰａ以上が好ましい。５００ＭＰａ以上がより好ましい。一方、水素脆化感受性増大を避けるために、降伏強度は５６０ＭＰａ以下が好ましい。 The wall thickness of the electric welded steel pipe material given as an example of the steel pipe of the present invention is preferably 5 mm or more, and preferably 30 mm or less. There is no particular upper limit for the radius of the electric welded steel pipe material, but since a larger radius increases the load on the equipment, the radius of the electric welded steel pipe material is preferably 400 mm or less. Furthermore, the radius of the electric welded pipe material is preferably 200 mm or more. Furthermore, the yield strength of the electric welded steel pipe material is preferably 480 MPa or more in order to withstand the gas pressure of pipeline operation, and 500 MPa or more is more preferable. On the other hand, in order to avoid increased susceptibility to hydrogen embrittlement, the yield strength is preferably 560 MPa or less.
　サイジング工程では、ロール通過時にロール形状に沿って管軸方向に曲げ変形が生じ、管軸方向の残留応力が発生する。前記曲げ変形における曲げひずみが大きいほど、管軸方向の残留応力の絶対値が大きくなる。前記曲げひずみは、サイジングロールの直径が小さいほど、また熱延鋼板の板厚が大きいほど大きくなる。
よって、本発明では、せん断残留応力を低くする観点から、管軸方向の残留応力の絶対値を小さくするため、サイジングロールの直径を前記（１）式満足させるものとする。
サイジングロールの直径が前記（１）式の右辺未満の場合、本発明で目的とするせん断残留応力が得られない。なお、特にサイジングロールの直径の上限は規定しないが、サイジングロールが大きくなると設備の負荷が増大するため、サイジングロールの直径は２０００ｍｍ以下とすることが好ましい。 In the sizing process, bending deformation occurs in the tube axial direction along the roll shape when the steel sheet passes through the rolls, and residual stress in the tube axial direction is generated. The larger the bending strain in the bending deformation, the larger the absolute value of the residual stress in the tube axial direction. The bending strain increases as the diameter of the sizing rolls decreases and as the thickness of the hot-rolled steel sheet increases.
Therefore, in the present invention, from the viewpoint of reducing the shear residual stress, the diameter of the sizing roll is set to satisfy the above formula (1) in order to reduce the absolute value of the residual stress in the axial direction of the tube.
If the diameter of the sizing roll is less than the right side of the formula (1), the intended residual shear stress of the present invention cannot be obtained. Although there is no particular upper limit to the diameter of the sizing roll, the larger the sizing roll, the greater the load on the equipment, so that the diameter of the sizing roll is preferably 2000 mm or less.
　内圧負荷工程では、電縫鋼管素材を拡管することにより、管周方向に引張応力を発生させて、管周方向の残留応力の絶対値を小さくする。
かかる内圧負荷工程の内圧ｐ（ＭＰａ）が大きいほど、管周方向の残留応力の絶対値が小さくなる。管周方向に発生する引張応力は、鋼管の半径が大きいほど、鋼管の肉厚が小さいほど、高くなる。 In the internal pressure application process, the electric resistance welded steel pipe material is expanded to generate tensile stress in the circumferential direction of the pipe, thereby reducing the absolute value of the residual stress in the circumferential direction of the pipe.
The greater the internal pressure p (MPa) in the internal pressure loading step, the smaller the absolute value of the residual stress in the circumferential direction of the pipe. The tensile stress generated in the circumferential direction of the pipe becomes higher as the radius of the steel pipe becomes larger and the wall thickness of the steel pipe becomes smaller.
　前記（２）式の左辺（Ｘ）は、管周方向に発生する引張応力が電縫鋼管素材の降伏応力に等しくなる場合の内圧ｐに対応する。
本発明では、せん断残留応力を低くする観点から、管軸方向の残留応力の絶対値を小さくするため、内圧ｐを（２）式の左辺（Ｘ）より大きい値とし、電縫鋼管素材を塑性域まで拡管させる。一方、内圧ｐが（２）式の右辺（Ｘ×１．５）超になると、管周方向の残留応力の絶対値は小さくなるが、拡管による加工硬化量が大きくなり過ぎて、管表面の転位密度が上昇し、水素中の耐疲労特性が低下する。 The left side (X) of the above equation (2) corresponds to the internal pressure p when the tensile stress generated in the circumferential direction of the pipe is equal to the yield stress of the electric resistance welded steel pipe material.
In the present invention, in order to reduce the absolute value of the residual stress in the pipe axial direction from the viewpoint of reducing the shear residual stress, the internal pressure p is set to a value greater than the left side (X) of equation (2) and the electric resistance welded steel pipe material is expanded to the plastic region. On the other hand, if the internal pressure p exceeds the right side (X × 1.5) of equation (2), the absolute value of the residual stress in the pipe circumferential direction becomes smaller, but the amount of work hardening due to pipe expansion becomes too large, the dislocation density on the pipe surface increases, and the fatigue resistance in hydrogen decreases.
　一部は上記で説明しているとおり、本発明の鋼管については、本発明で開示の鋼材を、プレスベンド成形、ロール成形、ＵＯＥ成形等で管状に成形した後、突き合わせ部を溶接することにより、原油や天然ガスの輸送に好適な鋼板内の材質均一性に優れた耐サワーラインパイプ用高強度鋼管（ＵＯＥ鋼管、電縫鋼管、スパイラル鋼管等）を製造することができる。また、本開示の鋼板を鋼管に用いることにより、溶接部の高硬度域が存在しても、耐ＨＩＳＣ性に優れる鋼管を製造することができる。 As partially explained above, for the steel pipe of the present invention, the steel material disclosed in the present invention is formed into a tube by press bending, roll forming, UOE forming, etc., and then the butt joint is welded to produce high-strength steel pipe for sour-resistant line pipe (UOE steel pipe, electric resistance welded steel pipe, spiral steel pipe, etc.) with excellent material uniformity within the steel plate, suitable for transporting crude oil and natural gas. In addition, by using the steel plate disclosed in the present invention for steel pipe, it is possible to produce a steel pipe with excellent HISC resistance, even if there is a high hardness region in the weld.
　次に、実施例に基づいて本発明をさらに具体的に説明する。以下の実施例は、本発明の好適な一例を示すものであり、本発明は、記載の実施例によって何ら限定されるものではない。 Next, the present invention will be described in more detail based on examples. The following examples show preferred examples of the present invention, and the present invention is not limited in any way to the described examples.
　まず、表１－１、１－２、１－３に示した成分組成のビレットを作製した。その際の鋳造速度は０．０５～０．２ｍ／ｍｉｎで実施した。前記ビレットを１０００～１１００℃に加熱した。その後、１０００℃±５０℃で熱間圧延を実施した。熱間圧延の先尾端の時間差は３０～４５秒間で実施し、鋼板の狙い厚さは２０ｍｍｔで製造した。冷却開始温度は表面温度でＡｒ３＋５０℃になったら制御冷却を開始した。その後、表２－１、２－２、２－３に示す条件で鋼材を製造し、一部の鋼材（鋼材Ｎｏ.１～１４、１６～３０、９２）については制御冷却工程後、熱延鋼板を曲げ加工し、両端部を突合せて溶接する造管工程を行い、また一部の鋼材（鋼材Ｎｏ.１５、３１～５５、９３～９８）については制御冷却工程後、熱延鋼板を冷間ロール成形により円筒状に成形し、前記円筒状の周方向両端部を突合せて電縫溶接する造管工程を行い、それぞれ鋼管Ｎｏ.１～１４、１６～３０、９２、とＮｏ．１５、３１～５５、９３～９８を得た。また、実施例１の脱水素処理では、脱水素処理を室温～５５０℃の範囲で行った。表２に記載の脱水素処理温度Ｙは、室温～５５０℃の範囲で脱水素処理を行っており、Ｎは脱水素処理温度が５５０℃超えで実施している。板厚中心温度Ｔｃが目標温度である室温に達することを確認した後に、上述の（Ａ）式を満足するようにＲ（ｓｅｃ）保持した。 First, billets were produced with the component compositions shown in Tables 1-1, 1-2, and 1-3. The casting speed was 0.05 to 0.2 m/min. The billets were heated to 1000 to 1100°C. Then, hot rolling was performed at 1000°C ± 50°C. The time difference between the head and tail of hot rolling was 30 to 45 seconds, and the steel plate was manufactured with a target thickness of 20 mmt. Controlled cooling was started when the surface temperature reached Ar 3 + 50°C. Thereafter, steel materials were manufactured under the conditions shown in Tables 2-1, 2-2, and 2-3. For some steel materials (steel materials Nos. 1 to 14, 16 to 30, and 92), after the controlled cooling process, the hot-rolled steel sheet was bent and the two ends were butted together and welded to form a pipe. For some steel materials (steel materials Nos. 15, 31 to 55, and 93 to 98), after the controlled cooling process, the hot-rolled steel sheet was formed into a cylindrical shape by cold roll forming, and the two circumferential ends of the cylindrical shape were butted together and welded to form a pipe. Steel pipes Nos. 1 to 14, 16 to 30, and 92, and Nos. 15, 31 to 55, and 93 to 98 were obtained. In the dehydrogenation treatment of Example 1, the dehydrogenation treatment was performed in the range of room temperature to 550°C. The dehydrogenation treatment temperature Y shown in Table 2 was performed in the range of room temperature to 550°C, and N was performed at a dehydrogenation treatment temperature of over 550°C. After it was confirmed that the center temperature Tc reached room temperature, which was the target temperature, the temperature was maintained for R (sec) so as to satisfy the above formula (A).
　さらに、表１－１の鋼種Ｎｏ．１５、表１－２の鋼種Ｎｏ．５６に示した成分組成のビレットを表３に示す種々の鋳造速度で作製し、前記ビレットを１０００～１１００℃に加熱した。その後、１０００±５０℃で熱間圧延を実施した。熱間圧延の先尾端の時間差は３０～４５秒間で実施し、鋼板の狙い厚さは２０ｍｍｔで製造した。冷却開始温度は表面温度でＡｒ３＋５０℃になったら制御冷却を開始した。その後、表３に示す条件で製造し鋼材および鋼管を得た。鋼材Ｎｏ．１５－１～３、５６－１～３は鋼材ままであり、鋼管Ｎｏ．１５－１１、１５－１２、５６－１１、５６－１２は熱延鋼板を曲げ加工し、両端部を突合せて溶接する造管工程を行って製造し、鋼管Ｎｏ．１５－１３、５６－１３は制御冷却工程後、熱延鋼板を冷間ロール成形により円筒状に成形し、前記円筒状の周方向両端部を突合せて電縫溶接する造管工程を行って得られた。
金属組織と機械的特性を評価した。評価方法は、以下の通りである。焼戻し温度は材料の引張強度が５２０ＭＰａ～７００ＭＰａの範囲となる様に任意に調整した。
得られた鋼材および鋼管のそれぞれについて、金属組織および材質を評価した結果を表２－１、２－２、２－３、３に示す。評価方法は、以下の通りである。 Furthermore, billets having the composition shown in Steel No. 15 in Table 1-1 and Steel No. 56 in Table 1-2 were produced at various casting speeds shown in Table 3, and the billets were heated to 1000 to 1100°C. Then, hot rolling was performed at 1000±50°C. The time difference between the head and tail of hot rolling was 30 to 45 seconds, and the steel plate was produced with a target thickness of 20 mmt. The cooling start temperature was controlled cooling when the surface temperature reached Ar 3 +50°C. Then, steel materials and steel pipes were produced under the conditions shown in Table 3. Steel Nos. 15-1 to 3 and 56-1 to 3 were as-is steel materials, and steel pipes Nos. 15-11, 15-12, 56-11, and 56-12 were produced by a pipe-making process in which hot-rolled steel plates were bent and both ends were butt-welded, and steel pipes Nos. Nos. 15-13 and 56-13 were obtained by a controlled cooling process, followed by a pipe-making process in which the hot-rolled steel sheet was formed into a cylindrical shape by cold rolling, and both circumferential ends of the cylindrical shape were butted together and electric resistance welded.
The metal structure and mechanical properties were evaluated using the following methods: The tempering temperature was adjusted so that the tensile strength of the material was in the range of 520 MPa to 700 MPa.
The metal structure and properties of the obtained steel material and steel pipe were evaluated, and the results are shown in Tables 2-1, 2-2, 2-3, and 3. The evaluation methods were as follows.
　残留オーステナイト測定
　上記に従って得られた鋼材および鋼管の長手方向中央部の板幅中央部より金属組織観察用サンプルを採取し、長手方向と平行な断面を観察対象面としてバフ研磨まで行い、その後、ピクリン酸エッチングにより表層を化学研磨により除去し、Ｘ線回折測定を用いて測定した。具体的に、入射Ｘ線にはＣｏ－Ｋα線源を用い、フェライトの（２００）、（２１１）、（２２０）面とオーステナイトの（２００）、（２２０）、（３１１）面の強度比から残留オーステナイトの面積分率を算出した。 Measurement of retained austenite Samples for metallographic observation were taken from the longitudinal center of the steel material and steel pipe obtained as described above, and the cross section parallel to the longitudinal direction was subjected to buffing and then the surface layer was removed by chemical polishing using picric acid etching, and the area fraction of retained austenite was calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.
　ベイナイトの面積分率測定
　鋼板の長手方向中央部、板厚１／４位置から採取した試験片、および鋼管の長手方向中央部、肉厚１／４から採取した試験片をバフ研磨まで行い、３ｖｏｌ％ナイタールを用いてエッチングをした。その後、光学顕微鏡を用いて倍率１００倍で３視野を観察し、また１０００～５０００倍間の適切な倍率で走査電子顕微鏡（ｓｃａｎｎｉｎｇ　ｅｌｅｃｔｒｏｎ　ｍｉｃｒｏｓｃｏｐｅ）写真を撮影し、ベイナイトを観察した。ベイナイトは、非特許文献２の組織写真と比較して目視で判断し、組織分率は、上記判断を基に光学顕微鏡写真またはＳＥＭ写真についてベイナイトとその他の領域を二値化した画像を用いて、画像解析（ｉｍａｇｅ　ａｎａｌｙｓｉｓ）により面積分率を求めた。光学顕微鏡写真またはＳＥＭ写真からそれぞれ求めた値の平均値をベイナイト面積率とした。 Measurement of bainite area fraction Test pieces taken from the longitudinal center of the steel plate and 1/4 of the plate thickness position, and test pieces taken from the longitudinal center of the steel pipe and 1/4 of the wall thickness position were buffed and etched with 3 vol% nital. Then, three fields of view were observed at a magnification of 100 times using an optical microscope, and scanning electron microscope photographs were taken at an appropriate magnification between 1000 and 5000 times to observe bainite. Bainite was judged visually by comparing with the structure photograph of Non-Patent Document 2, and the structure fraction was determined by image analysis using an image in which bainite and other regions were binarized in the optical microscope photograph or SEM photograph based on the above judgment. The average value of the values obtained from the optical microscope photograph or SEM photograph was taken as the bainite area fraction.
　引張強さ（ＴＳ）
　上記に従って得られた鋼材および鋼管から、ＪＩＳ　Ｚ　２２０１に準拠してＪＩＳ１４号比例試験片（平行部直径７ｍｍ、標点間距離３５ｍｍ）を採取し、引張強さを測定した。 Tensile strength (TS)
From the steel materials and steel pipes obtained as described above, JIS No. 14 proportional test pieces (parallel part diameter 7 mm, gauge length 35 mm) were taken in accordance with JIS Z 2201, and the tensile strength was measured.
　水素昇温分析
　鋼中に残存する水素量は昇温脱離分析法を用いて、低温型昇温式水素分析装置〈ガスクロマトグラフタイプ〉（ＪＴＦ－２０ＡＬ）を用いた。昇温脱離分析は２００℃／ｈの昇温速度で室温から４００℃までの温度範囲で行い、その総和を水素量とした。試験体は鋼板の板厚１／４位置および鋼管の内面から１／４位置で鋼管長手方向に３０ｍｍ長さで直径７Φの円柱形状である。なお、この水素量は後述している時効で説明する高圧水素疲労試験に供する前であり、表１－１、１－２、１－３に示すＨ量である。 Temperature-programmed hydrogen analysis The amount of hydrogen remaining in the steel was measured using a temperature-programmed desorption analysis method, using a low-temperature temperature-programmed hydrogen analyzer (gas chromatograph type) (JTF-20AL). Temperature-programmed desorption analysis was performed in the temperature range from room temperature to 400°C at a heating rate of 200°C/h, and the sum of the results was taken as the amount of hydrogen. The test specimens were cylindrical, 30 mm long in the longitudinal direction of the steel pipe, at a 1/4 position of the plate thickness of the steel plate and a 1/4 position from the inner surface of the steel pipe, and had a diameter of 7Φ. This amount of hydrogen was measured before the steel was subjected to the high-pressure hydrogen fatigue test described in the aging section below, and is the amount of H shown in Tables 1-1, 1-2, and 1-3.
　疲労試験
　室温（２０±１０℃）、大気中の高圧ガス混合雰囲気中で、ＡＳＴＭ　Ｅ４６６　、Ｆａｔｉｇｕｅ　Ｔｅｓｔｉｎｇに準拠して周波数：１～１５Ｈｚ、繰返し波形：正弦波、制御方法：荷重制御、荷重条件：単軸引張圧縮、応力比：Ｒ＝－１．０で疲労試験を実施して求めた。繰り返し数１０００万回で未破断となる応力を大気中の疲労限強度と定義した。 Fatigue test: A fatigue test was performed at room temperature (20±10°C) in a high-pressure gas mixture atmosphere in air in accordance with ASTM E466, Fatigue Testing, with a frequency of 1 to 15 Hz, a repetitive waveform of sine wave, a control method of load control, loading conditions of uniaxial tension and compression, and a stress ratio of R = -1.0. The stress at which the specimen did not break after 10 million repetitions was defined as the fatigue limit in air.
　高圧水素疲労試験
　室温（２０±１０℃）、圧力：４０ＭＰａの水素ガス（１００％ガス）または圧力１ＭＰａ以上の水素ガス、または水素分圧として１ＭＰａ以上の水素を含む天然ガス（主成分はメタン、エタンなどの炭化水素）混合雰囲気中で、ＡＳＴＭ　Ｅ４６６、Ｆａｔｉｇｕｅ　Ｔｅｓｔｉｎｇに準拠して周波数：１Ｈｚ、繰返し波形：正弦波、制御方法：荷重制御、荷重条件：単軸引張圧縮、応力比：Ｒ＝－１．０で疲労試験を実施して求めた。繰り返し数２００万回で未破断となる応力を水素中疲労限応力と定義した。なお、本試験で得られた水素中疲労限応力が２００ＭＰａ以上かつ、不活性ガス雰囲気で前述の疲労限強度との比である上記の水素中疲労限応力／不活性ガス環境中の疲労限応力が０．９０以上を満足した水準について合格したと判断した。 High pressure hydrogen fatigue test: A fatigue test was performed at room temperature (20±10°C), in a mixed atmosphere of hydrogen gas (100% gas) at a pressure of 40 MPa or hydrogen gas at a pressure of 1 MPa or more, or natural gas (mainly hydrocarbons such as methane and ethane) containing hydrogen at a partial pressure of 1 MPa or more, in accordance with ASTM E466, Fatigue Testing, with a frequency of 1 Hz, a repetitive waveform: sine wave, a control method: load control, a load condition: uniaxial tension and compression, and a stress ratio: R = -1.0. The stress at which the specimen did not break after 2 million repetitions was defined as the fatigue limit stress in hydrogen. It was determined that the specimen passed the test if the fatigue limit stress in hydrogen obtained in this test was 200 MPa or more and the ratio of the fatigue limit strength in hydrogen to the fatigue limit stress in inert gas environment was 0.90 or more.
　本発明の発明例では全て水素中疲労限応力が２００ＭＰａ以上かつ、不活性ガス雰囲気で前述の疲労限強度との比である水素中疲労限応力／不活性ガス環境中の疲労限応力が０．９０以上である優れた耐水素脆化特性を満足した。さらに引張強さは５２０ＭＰａ以上を満足した。 All of the examples of the present invention had excellent hydrogen embrittlement resistance, with a fatigue limit stress in hydrogen of 200 MPa or more, and a ratio of the fatigue limit strength in an inert gas atmosphere to the fatigue limit stress in hydrogen/fatigue limit stress in an inert gas environment of 0.90 or more. Furthermore, the tensile strength was 520 MPa or more.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000006
Figure JPOXMLDOC01-appb-T000006
Figure JPOXMLDOC01-appb-T000007
Figure JPOXMLDOC01-appb-T000007
　以下、本発明の効果を検証した実施例について、説明する。なお、以下の実施例において鋼管を以下の製造条件で製造し、特性評価を行った。表１－１、１－２に示す鋼種Ｎｏ．１、１５、５６を用いて、制御冷却工程までは表２－１、２－２、３で示す鋼材Ｎｏ．１、１５、５６、１５－１２、５６－１２と同一の条件で製造し、脱水素処理条件を変化させたときの特性評価を行った。鋼管成形は実施例１と同様の方法で実施している。上記結果を表４に示す。 Below, examples verifying the effects of the present invention are described. In the following examples, steel pipes were manufactured under the following manufacturing conditions, and their characteristics were evaluated. Using steel types No. 1, 15, and 56 shown in Tables 1-1 and 1-2, they were manufactured under the same conditions as steel materials No. 1, 15, 56, 15-12, and 56-12 shown in Tables 2-1, 2-2, and 3 up until the controlled cooling process, and characteristics were evaluated when the dehydrogenation treatment conditions were changed. Steel pipe forming was performed in the same manner as in Example 1. The results are shown in Table 4.
　本実施例では、鋼管および鋼材Ｎｏ．１Ａ、１５Ａ、５６Ａ、１５－１２Ａ、５６－１２Ａは脱水素処理温度Ｔ（雰囲気温度）を５０℃とし、板厚中心温度Ｔｃが５０℃に到達してからの保持時間ｔｃを（Ａ）式が満足するように実施した。鋼管および鋼材Ｎｏ．１Ｂ、１５Ｂ、５６Ｂ、１５－１２Ｂ、５６－１２Ｂは脱水素処理温度Ｔ（雰囲気温度）を５０℃とし、脱水素処理温度Ｔが５０℃で保持時間ｔｃが上述している（Ａ）式を満足するように行っているものの、板厚中央温度Ｔｃが５０℃に到達してからの保持時間ｔｃは上述している（Ａ）式を満足していない。
鋼管および鋼材Ｎｏ．１Ｃ、１５Ｃ、５６Ｃ、１５－１２Ｃ、５６－１２Ｃは、脱水素処理温度Ｔ（雰囲気温度）は５０℃であるが、雰囲気温度の保持時間ｔ、板厚中央温度Ｔｃが５０℃に到達してからの保持時間ｔｃがともに上述している（Ａ）式を満足していない。 In this embodiment, the dehydrogenation temperature T (atmosphere temperature) of steel pipe and steel material No. 1A, 15A, 56A, 15-12A, and 56-12A was set to 50°C, and the holding time tc after the plate thickness center temperature Tc reached 50°C was set to satisfy the formula (A). The dehydrogenation temperature T (atmosphere temperature) of steel pipe and steel material No. 1B, 15B, 56B, 15-12B, and 56-12B was set to 50°C, and the holding time tc after the plate thickness center temperature Tc reached 50°C was set to satisfy the formula (A) described above, but the holding time tc after the plate thickness center temperature Tc reached 50°C did not satisfy the formula (A) described above.
In steel pipes and steel materials Nos. 1C, 15C, 56C, 15-12C, and 56-12C, the dehydrogenation temperature T (atmosphere temperature) is 50° C., but neither the atmospheric temperature holding time t nor the holding time tc after the plate thickness center temperature Tc reaches 50° C. satisfies the above-mentioned formula (A).
　表４において、「脱水素保持時間ｔがＹ」は、脱水素処理温度Ｔ（雰囲気温度）は５０℃とし、保持時間ｔが（Ａ）式を満足しており、「脱水素保持時間ｔがＮ」は、脱水素処理温度Ｔ（雰囲気温度）は５０℃としているが、保持時間ｔが（Ａ）式を満足していない。また、「鋼材中心温度Ｔｃにおける保持時間ｔｃがＹ」は、板厚中央温度Ｔｃが５０℃に到達してからの保持時間ｔｃが（Ａ）式を満足しており、「鋼材中心温度Ｔｃにおける保持時間ｔｃがＮ」は、板厚中央温度Ｔｃが５０℃に到達するものの、Ｔｃが５０℃に到達してからの保持時間ｔｃが（Ａ）式を満足していない。 In Table 4, "dehydrogenation holding time t is Y" means that the dehydrogenation treatment temperature T (ambient temperature) is 50°C and the holding time t satisfies formula (A), while "dehydrogenation holding time t is N" means that the dehydrogenation treatment temperature T (ambient temperature) is 50°C, but the holding time t does not satisfy formula (A). Also, "holding time tc at steel center temperature Tc is Y" means that the holding time tc after the plate thickness center temperature Tc reaches 50°C satisfies formula (A), while "holding time tc at steel center temperature Tc is N" means that the plate thickness center temperature Tc reaches 50°C, but the holding time tc after Tc reaches 50°C does not satisfy formula (A).
　種々評価については実施例１に記載の方法で実施している。 Various evaluations were carried out using the methods described in Example 1.
　本発明の発明例は、すべて水素中疲労限応力が２００ＭＰａ以上かつ、不活性ガス雰囲気で前述の疲労限強度との比である上記の水素中疲労限応力／不活性ガス環境中の疲労限応力が０．９０以上を満足した。さらに引張強さは５２０ＭＰａ以上を満足した。そのなかでも，脱水素処理条件がより好適な条件で実施される方が、疲労特性は優れていた。 All of the inventive examples of the present invention had a fatigue limit stress in hydrogen of 200 MPa or more, and the ratio of the fatigue limit strength in an inert gas atmosphere to the fatigue limit stress in hydrogen/fatigue limit stress in an inert gas environment was 0.90 or more. Furthermore, the tensile strength was 520 MPa or more. Among these, the fatigue properties were superior when the dehydrogenation treatment was performed under more favorable conditions.
Figure JPOXMLDOC01-appb-T000008
  
Figure JPOXMLDOC01-appb-T000008
  

Claims (6)

  1. 　質量％で、
    Ｃ：０．０２～０．１５％、
    Ｓｉ：０．０１～２．０％、
    Ｍｎ：０．５～１．５％、
    Ｐ：０．０００１～０．０１５％、
    Ｓ：０．０００２～０．００１５％、
    Ａｌ：０．００５～０．１５％、
    Ｏ：０．０１％以下、
    Ｎ：０．０１０％以下、
    Ｈ：０．００１０％以下を含み、
    あるいはさらに、
    Ｎｂ：０～０．１０％、
    Ｃａ：０～０．００５％、
    Ｔｉ：０～０．１％、
    Ｎｉ：０～２．０％、
    Ｃｕ：０～１．０％、
    Ｃｒ：０～１．０％、
    Ｍｏ：０～０．６０％、
    Ｗ：０～１．０％、
    Ｖ：０～０．１０％、
    Ｚｒ：０～０．０５０％、
    ＲＥＭ：０～０．０５０％、
    Ｍｇ：０～０．０５０％、
    Ｂ：０～０．００２０％、
    Ｈｆ：０～０．２％、
    Ｔａ：０～０．２％、
    Ｒｅ：０～０．００５％、
    Ｓｎ：０～０．３％、
    Ｓｂ：０～０．３％から選択される１種以上を含み、
    残部がＦｅおよび不可避的不純物元素である、化学組成を有し、
    残留オーステナイトが面積分率で０～３％であり、板厚１／４位置において、ベイナイトが面積分率で９０％以上であって、１ＭＰａ以上の水素中疲労限応力が２００ＭＰａ以上であり、かつ、１ＭＰａ以上の水素中疲労限応力／不活性ガス環境下の疲労限応力が０．９０以上である耐水素脆化特性に優れたラインパイプ用鋼材。     In mass percent,
    C: 0.02 to 0.15%,
    Si: 0.01 to 2.0%,
    Mn: 0.5 to 1.5%,
    P: 0.0001 to 0.015%,
    S: 0.0002 to 0.0015%,
    Al: 0.005 to 0.15%,
    O: 0.01% or less,
    N: 0.010% or less,
    H: 0.0010% or less;
    Or even more so:
    Nb: 0 to 0.10%,
    Ca: 0 to 0.005%,
    Ti: 0 to 0.1%,
    Ni: 0 to 2.0%,
    Cu: 0 to 1.0%,
    Cr: 0 to 1.0%,
    Mo: 0 to 0.60%,
    W: 0 to 1.0%,
    V: 0 to 0.10%,
    Zr: 0 to 0.050%,
    REM: 0 to 0.050%,
    Mg: 0 to 0.050%,
    B: 0 to 0.0020%,
    Hf: 0 to 0.2%,
    Ta: 0 to 0.2%,
    Re: 0 to 0.005%,
    Sn: 0 to 0.3%,
    Sb: one or more selected from 0 to 0.3%,
    The balance is Fe and unavoidable impurity elements,
    A steel material for line pipes having excellent hydrogen embrittlement resistance, the steel having an area fraction of retained austenite of 0-3%, an area fraction of bainite of 90% or more at the 1/4 position of the plate thickness, a fatigue limit stress in hydrogen of 1 MPa or more of 200 MPa or more, and a fatigue limit stress in hydrogen of 1 MPa or more/fatigue limit stress in an inert gas environment of 0.90 or more.
  2. 　さらに、質量％で、前記化学組成が、
    Ｎｂ：０．００１～０．１０％、
    Ｃａ：０．０００１～０．００５％、
    Ｔｉ：０．００５～０．１％、
    Ｎｉ：０．０１～２．０％、
    Ｃｕ：０．０１～１．０％、
    Ｃｒ：０．０１～１．０％、
    Ｍｏ：０．０１～０．６０％、
    Ｗ：０．０１～１．０％、
    Ｖ：０．０１～０．１０％、
    Ｚｒ：０．０００１～０．０５０％、
    ＲＥＭ：０．０００１～０．０５０％、
    Ｍｇ：０．０００１～０．０５０％、
    Ｂ：０．０００１～０．００２０％、
    Ｈｆ：０．０００１～０．２％、
    Ｔａ：０．０００１～０．２％、
    Ｒｅ：０．０００１～０．００５％、
    Ｓｎ：０．０００１～０．３％、
    Ｓｂ：０．０００１～０．３％である請求項１に記載の耐水素脆化特性に優れたラインパイプ用鋼材。     Further, the chemical composition, in mass%, is
    Nb: 0.001 to 0.10%,
    Ca: 0.0001 to 0.005%,
    Ti: 0.005 to 0.1%,
    Ni: 0.01 to 2.0%,
    Cu: 0.01 to 1.0%,
    Cr: 0.01 to 1.0%,
    Mo: 0.01 to 0.60%,
    W: 0.01 to 1.0%,
    V: 0.01 to 0.10%,
    Zr: 0.0001 to 0.050%,
    REM: 0.0001 to 0.050%,
    Mg: 0.0001 to 0.050%,
    B: 0.0001 to 0.0020%,
    Hf: 0.0001 to 0.2%,
    Ta: 0.0001 to 0.2%,
    Re: 0.0001 to 0.005%,
    Sn: 0.0001 to 0.3%,
    2. A steel material for line pipes having excellent hydrogen embrittlement resistance according to claim 1, wherein Sb is 0.0001 to 0.3%.
  3. 　請求項１または２に記載の化学組成を有する鋼素材を１０００～１２５０℃で加熱する加熱工程と、
    前記加熱工程で加熱された鋼素材を、圧延終了温度：Ａｒ３点以上の条件で圧延する熱間圧延工程と、
    前記熱間圧延工程で得られた熱延鋼板を、冷却開始温度が鋼板表面温度でＡｒ３点以上、前記熱延鋼板の先端と尾端の冷却開始時間差が５０秒以内、７５０℃から５５０℃までの平均冷却速度が板厚中央の温度で１５～５０℃／ｓ、冷却停止温度が２５０～６５０℃である条件で冷却する制御冷却工程と、
    前記制御冷却工程で得られた鋼板を室温～５５０℃の範囲で保持する脱水素処理工程と、
    を有するラインパイプ用鋼材の製造方法。     A heating step of heating a steel material having the chemical composition according to claim 1 or 2 at 1000 to 1250 ° C.;
    A hot rolling process in which the steel material heated in the heating process is rolled under a rolling end temperature of Ar 3 points or more;
    A controlled cooling process in which the hot-rolled steel sheet obtained in the hot rolling process is cooled under the conditions that the cooling start temperature is Ar 3 point or more at the surface temperature of the steel sheet, the cooling start time difference between the front end and the tail end of the hot-rolled steel sheet is within 50 seconds, the average cooling rate from 750 ° C. to 550 ° C. is 15 to 50 ° C./s at the temperature at the center of the sheet thickness, and the cooling stop temperature is 250 to 650 ° C.;
    A dehydrogenation treatment step of holding the steel sheet obtained in the controlled cooling step at a temperature in the range of room temperature to 550°C;
    The method for producing a steel material for line pipes comprising the steps of:
  4. 　ラインパイプ用鋼管において、
    　質量％で、
    Ｃ：０．０２～０．１５％、
    Ｓｉ：０．０１～２．０％、
    Ｍｎ：０．５～１．５％、
    Ｐ：０．０００１～０．０１５％、
    Ｓ：０．０００２～０．００１５％、
    Ａｌ：０．００５～０．１５％、
    Ｏ：０．０１％以下、
    Ｎ：０．０１０％以下、
    Ｈ：０．００１０％以下を含み、
    あるいはさらに、
    Ｎｂ：０～０．１０％、
    Ｃａ：０～０．００５％、
    Ｔｉ：０～０．１％、
    Ｎｉ：０～２．０％、
    Ｃｕ：０～１．０％、
    Ｃｒ：０～１．０％、
    Ｍｏ：０～０．６０％、
    Ｗ：０～１．０％、
    Ｖ：０～０．１０％、
    Ｚｒ：０～０．０５０％、
    ＲＥＭ：０～０．０５０％、
    Ｍｇ：０～０．０５０％、
    Ｂ：０～０．００２０％、
    Ｈｆ：０～０．２％、
    Ｔａ：０～０．２％、
    Ｒｅ：０～０．００５％、
    Ｓｎ：０～０．３％、
    Ｓｂ：０～０．３％から選択される１種以上を含み、
    残部がＦｅおよび不可避的不純物元素である、化学組成を有し、残留オーステナイトが面積分率で０～３％であり、鋼管内面からの肉厚１／４位置において、ベイナイトが面積分率で９０％以上であって、１ＭＰａ以上の水素中疲労限応力が２００ＭＰａ以上であり、かつ、１ＭＰａ以上の水素中疲労限応力／不活性ガス環境下の疲労限応力が０．９０以上である耐水素脆化特性に優れたラインパイプ用鋼管。     In line pipe steel,
    In mass percent,
    C: 0.02 to 0.15%,
    Si: 0.01 to 2.0%,
    Mn: 0.5 to 1.5%,
    P: 0.0001 to 0.015%,
    S: 0.0002 to 0.0015%,
    Al: 0.005 to 0.15%,
    O: 0.01% or less,
    N: 0.010% or less,
    H: 0.0010% or less;
    Or even more so:
    Nb: 0 to 0.10%,
    Ca: 0 to 0.005%,
    Ti: 0 to 0.1%,
    Ni: 0 to 2.0%,
    Cu: 0 to 1.0%,
    Cr: 0 to 1.0%,
    Mo: 0 to 0.60%,
    W: 0 to 1.0%,
    V: 0 to 0.10%,
    Zr: 0 to 0.050%,
    REM: 0 to 0.050%,
    Mg: 0 to 0.050%,
    B: 0 to 0.0020%,
    Hf: 0 to 0.2%,
    Ta: 0 to 0.2%,
    Re: 0 to 0.005%,
    Sn: 0 to 0.3%,
    Sb: one or more selected from 0 to 0.3%,
    A steel pipe for line pipe having excellent hydrogen embrittlement resistance, which has a chemical composition with the balance being Fe and unavoidable impurity elements, has an area fraction of 0 to 3% retained austenite, and at a position 1/4 of the way through the wall from the inner surface of the steel pipe, has an area fraction of bainite of 90% or more, has a fatigue limit stress in hydrogen of 1 MPa or more of 200 MPa or more, and has a fatigue limit stress in hydrogen of 1 MPa or more/fatigue limit stress in an inert gas environment of 0.90 or more.
  5. 　さらに、質量％で、前記化学組成が、
    Ｎｂ：０．００１～０．１０％、
    Ｃａ：０．０００１～０．００５％、
    Ｔｉ：０．００５～０．１％、
    Ｎｉ：０．０１～２．０％、
    Ｃｕ：０．０１～１．０％、
    Ｃｒ：０．０１～１．０％、
    Ｍｏ：０．０１～０．６０％、
    Ｗ：０．０１～１．０％、
    Ｖ：０．０１～０．１０％、
    Ｚｒ：０．０００１～０．０５０％、
    ＲＥＭ：０．０００１～０．０５０％、
    Ｍｇ：０．０００１～０．０５０％、
    Ｂ：０．０００１～０．００２０％、
    Ｈｆ：０．０００１～０．２％、
    Ｔａ：０．０００１～０．２％、
    Ｒｅ：０．０００１～０．００５％、
    Ｓｎ：０．０００１～０．３％、
    Ｓｂ：０．０００１～０．３％である請求項４に記載の耐水素脆化特性に優れたラインパイプ用鋼管。     Further, the chemical composition, in mass%, is
    Nb: 0.001 to 0.10%,
    Ca: 0.0001 to 0.005%,
    Ti: 0.005 to 0.1%,
    Ni: 0.01 to 2.0%,
    Cu: 0.01 to 1.0%,
    Cr: 0.01 to 1.0%,
    Mo: 0.01 to 0.60%,
    W: 0.01 to 1.0%,
    V: 0.01 to 0.10%,
    Zr: 0.0001 to 0.050%,
    REM: 0.0001 to 0.050%,
    Mg: 0.0001 to 0.050%,
    B: 0.0001 to 0.0020%,
    Hf: 0.0001 to 0.2%,
    Ta: 0.0001 to 0.2%,
    Re: 0.0001 to 0.005%,
    Sn: 0.0001 to 0.3%,
    5. A steel pipe for line pipe having excellent hydrogen embrittlement resistance according to claim 4, wherein Sb is 0.0001 to 0.3%.
  6. 　請求項４または５に記載の化学組成を有する鋼素材を１０００～１２５０℃で加熱する加熱工程と、
    前記加熱工程で加熱された鋼素材を、圧延終了温度：Ａｒ３点以上の条件で圧延する熱間圧延工程と、
    前記熱間圧延工程で得られた熱延鋼板を、冷却開始温度が鋼板表面温度でＡｒ３点以上、前記熱延鋼板の先端と尾端の冷却開始時間差が５０秒以内、７５０℃から５５０℃までの平均冷却速度が板厚中央の温度で１５～５０℃／ｓ、冷却停止温度が２５０～６５０℃である条件で冷却する制御冷却工程と、
    前記制御冷却工程後、前記熱延鋼板を曲げ加工し、両端部を突合せて溶接する造管工程、前記制御冷却工程後、前記熱延鋼板を冷間ロール成形により円筒状に成形し、前記円筒状の周方向両端部を突合せて電縫溶接する造管工程のうちどちらか一方の造管工程と、
    造管工程で得られた鋼管を室温～５５０℃の範囲で保持する脱水素処理工程と、
    を有するラインパイプ用鋼管の製造方法。     A heating step of heating a steel material having the chemical composition according to claim 4 or 5 at 1000 to 1250 ° C.;
    A hot rolling process in which the steel material heated in the heating process is rolled under a rolling end temperature of Ar 3 points or more;
    A controlled cooling process in which the hot-rolled steel sheet obtained in the hot rolling process is cooled under the conditions that the cooling start temperature is Ar 3 point or more at the surface temperature of the steel sheet, the cooling start time difference between the front end and the tail end of the hot-rolled steel sheet is within 50 seconds, the average cooling rate from 750 ° C. to 550 ° C. is 15 to 50 ° C./s at the temperature at the center of the sheet thickness, and the cooling stop temperature is 250 to 650 ° C.;
    a pipe-making process in which, after the controlled cooling process, the hot-rolled steel sheet is bent and both ends are butted together and welded, or a pipe-making process in which, after the controlled cooling process, the hot-rolled steel sheet is formed into a cylindrical shape by cold roll forming and both circumferential ends of the cylindrical shape are butted together and electric resistance welded;
    a dehydrogenation treatment process in which the steel pipe obtained in the pipe-making process is kept at a temperature in the range of room temperature to 550°C;
    A method for producing a steel pipe for line pipe having the above structure.
PCT/JP2023/035558 2022-09-29 2023-09-28 Line pipe steel material having excellent hydrogen embrittlement resistance, manufacturing method therefor, line pipe steel tube having excellent hydrogen embrittlement resistance, and manufacturing method therefor WO2024071356A1 (en)

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JP2012122103A (en) * 2010-12-09 2012-06-28 Sumitomo Metal Ind Ltd Thick steel plate excellent in hydrogen-induced crack resistance, brittle crack propagation arrest characteristic, and corrosion resistance
WO2014156187A1 (en) * 2013-03-29 2014-10-02 Ｊｆｅスチール株式会社 Steel material and hydrogen container as well as manufacturing methods therefor
JP2018012855A (en) * 2016-07-20 2018-01-25 新日鐵住金株式会社 Low alloy steel material, low alloy steel tube and container and method for producing the container
WO2020137812A1 (en) * 2018-12-26 2020-07-02 Ｊｆｅスチール株式会社 Steel for high pressure hydrogen gas environments, steel structure for high pressure hydrogen gas environments, and method for producing steel for high pressure hydrogen gas environments
WO2022030818A1 (en) * 2020-08-07 2022-02-10 주식회사 포스코 Steel material having excellent hydrogen embrittlement resistance and impact toughness and method for manufacturing same
JP2022068942A (en) * 2020-10-23 2022-05-11 Ｊｆｅスチール株式会社 Steel material for high-pressure hydrogen gas environment, and method for manufacturing the same
WO2022209896A1 (en) * 2021-03-30 2022-10-06 Ｊｆｅスチール株式会社 Steel pipe for high-pressure hydrogen, container for high-pressure hydrogen, and method for manufacturing said steel pipe

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2012122103A (en) * 2010-12-09 2012-06-28 Sumitomo Metal Ind Ltd Thick steel plate excellent in hydrogen-induced crack resistance, brittle crack propagation arrest characteristic, and corrosion resistance
WO2014156187A1 (en) * 2013-03-29 2014-10-02 Ｊｆｅスチール株式会社 Steel material and hydrogen container as well as manufacturing methods therefor
JP2018012855A (en) * 2016-07-20 2018-01-25 新日鐵住金株式会社 Low alloy steel material, low alloy steel tube and container and method for producing the container
WO2020137812A1 (en) * 2018-12-26 2020-07-02 Ｊｆｅスチール株式会社 Steel for high pressure hydrogen gas environments, steel structure for high pressure hydrogen gas environments, and method for producing steel for high pressure hydrogen gas environments
WO2022030818A1 (en) * 2020-08-07 2022-02-10 주식회사 포스코 Steel material having excellent hydrogen embrittlement resistance and impact toughness and method for manufacturing same
JP2022068942A (en) * 2020-10-23 2022-05-11 Ｊｆｅスチール株式会社 Steel material for high-pressure hydrogen gas environment, and method for manufacturing the same
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