US20250305101A1 - Seamless steel pipe - Google Patents

Seamless steel pipe

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Publication number
US20250305101A1
US20250305101A1 US18/706,067 US202318706067A US2025305101A1 US 20250305101 A1 US20250305101 A1 US 20250305101A1 US 202318706067 A US202318706067 A US 202318706067A US 2025305101 A1 US2025305101 A1 US 2025305101A1
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steel pipe
seamless steel
content
less
toughness
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US18/706,067
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Kosuke AOKI
Hirofumi Nakamura
Tomohiko Omura
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Nippon Steel Corp
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Nippon Steel Corp
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Assigned to NIPPON STEEL CORPORATION reassignment NIPPON STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: AOKI, KOSUKE, OMURA, TOMOHIKO, NAKAMURA, HIROFUMI
Publication of US20250305101A1 publication Critical patent/US20250305101A1/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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C21D9/14—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes wear-resistant or pressure-resistant pipes
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/34—Methods of heating
    • C21D1/42—Induction heating
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/60—Aqueous agents
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C21D9/085—Cooling or quenching

Definitions

  • the present invention relates to a seamless steel pipe.
  • airbag systems have been installed, which inflate an airbag with gas or the like between an occupant and a steering wheel, an instrument panel, or the like at the time of a collision before the occupant impacts these objects, so as to absorb the kinetic energy of the occupant, thus reducing injuries of the occupant.
  • airbag systems of a type that uses an explosive chemical have been adopted to date, a system that uses high-pressure fill gas has been developed from the viewpoint of environmental recyclability, and the system is increasingly applied.
  • Patent Document 1 discloses a seamless steel pipe for an airbag accumulator that has a tensile strength of 850 MPa or more and resistance to burst at ⁇ 20° C. and can be produced only by normalizing heat treatment, without quenching and tempering.
  • Patent Document 2 discloses a seamless steel pipe for an airbag system having a tensile strength of 1000 MPa or more that is subjected to cold working followed by quenching+tempering and has excellent low-temperature resistance to burst when used as an airbag accumulator component with a shrunk portion.
  • Patent Document 3 discloses a process for producing a pipe for a high-strength, high-toughness airbag that enables simplification of a cold draw step and reduction in alloy cost.
  • the present inventors thus conducted studies about a method for increasing strength while maintaining low-temperature toughness and found that simply increasing strength of a pipe may result in significant decrease in hydrogen embrittlement resistance properties of the pipe.
  • restraint of embrittlement by hydrogen entering a pipe during a production step and in a usage environment is required even when high strength is given to the pipe.
  • the present invention is made to solve the above problems and has a gist of the following seamless steel pipe.
  • a seamless steel pipe that has high strength and excellent low-temperature toughness and further has excellent hydrogen embrittlement resistance properties can be provided.
  • FIG. 1 is a diagram for describing a shape of a specimen for toughness evaluation.
  • FIG. 2 is a diagram for describing a shape of an arc-shaped tensile test specimen used for measurement of critical hydrogen concentration.
  • the present inventors conducted diligent studies about a method for increasing strength of a seamless steel pipe with low-temperature toughness of the seamless steel pipe being maintained and further keeping hydrogen embrittlement resistance properties. As a result, the present inventors obtained the following findings.
  • Mn is also an element that enhances hardenability. However, Mn excessively contained segregates in grain boundaries to degrade low-temperature toughness. In addition to Mn, P is also an element that segregates in grain boundaries to degrade low-temperature toughness. In contrast, N precipitates in the form of nitrides, and if a content of N is excessive, the number of nitrides is increased to degrade low-temperature toughness.
  • Si is an element that has a deoxidation action and increases hardenability of steel to enhance strength of steel.
  • a content of Si is set to 0.05% or more.
  • the content of Si is set to 0.50% or less.
  • a range of the content of Si is preferably 0.10% or more to 0.40% or less, and more preferably 0.15% or more to 0.30% or less.
  • P phosphorus
  • steel is contained in steel as an impurity and leads to decrease in toughness and hydrogen embrittlement resistance properties due to grain-boundary segregation.
  • the content of P is set to 0.025% or less.
  • the content of P is preferably 0.020% or less, and more preferably 0.015% or less.
  • S sulfur
  • S is contained in steel as an impurity and decreases toughness particularly in a T direction of a pipe (a direction perpendicular to a pipe axis direction of the pipe). If a content of S is more than 0.020%, the decrease in toughness in the T direction of a pipe becomes significant. Therefore, the content of S is set to 0.020% or less. The content of S is preferably 0.010% or less.
  • Ni nickel
  • a content of Ni more than 0.50% leads to increase in alloy cost. Therefore, the content of Ni is set to 0.10 to 0.50%.
  • the content of Ni is preferably 0.15% or more, and more preferably 0.20% or more.
  • the content of Ni is preferably 0.45% or less, and more preferably 0.40% or less.
  • Cr chromium
  • a content of Cr more than 1.20% leads to increase in alloy cost. Therefore, the content of Cr is set to 0.10 to 1.20%.
  • the content of Cr is preferably 0.15% or more, and more preferably 0.20% or more.
  • the content of Cr is preferably 1.00% or less, and more preferably 0.90% or less.
  • Mo molybdenum
  • a content of Mo more than 0.50% leads to increase in alloy cost. If the content of Mo is excessively high, a resultant seamless steel pipe tends to increase in strength even in air cooling after hot rolling, which requires softening heat treatment before cold drawing work, leading to increase in production cost. Therefore, the content of Mo is set to 0.10 to 0.50%.
  • the content of Mo is preferably 0.15% or more, and more preferably 0.20% or more.
  • the content of Mo is preferably 0.45% or less, and more preferably 0.40% or less.
  • Ti titanium fixes N in steel, enhancing toughness.
  • Ti nitrides finely dispersed strongly pin grain boundaries to subject grains to grain refinement, enhancing toughness of steel.
  • 0.005% or more of Ti needs to be contained. However, if more than 0.050% of Ti is contained, its nitrides are coarsened, rather decreasing toughness. Therefore, a content of Ti is set to 0.005 to 0.050%.
  • the content of Ti is preferably 0.040% or less, and more preferably 0.030% or less.
  • Ca (calcium) fixes S that is present in steel as an unavoidable impurity in the form of its sulfide and improves anisotropy of toughness to increase toughness in a T direction of a pipe, thereby increasing resistance to burst.
  • Ca restrains production of MnS, thus contributing to enhancement in hydrogen embrittlement resistance properties.
  • the effect appears when 0.0005% or more of Ca is contained. However, if more than 0.0025% of Ca is contained, inclusions increase, rather decreasing toughness. Therefore, a content of Ca is set to 0.0005 to 0.0025%.
  • the Ca content is preferably 0.0010% or more, more preferably more than 0.0010%, further preferably 0.0012% or more, and further preferably 0.0015% or more.
  • Al is an element that has a deoxidation action and is effective in increasing toughness and workability. However, if more than 0.080% of Al is contained, occurrence of macro-streak-flaw becomes significant. Therefore, a content of Al is set to 0.080% or less.
  • the content of Al is preferably 0.060% or less, and more preferably 0.040% or less.
  • the content of Al may be on the level of impurity. Thus, a lower limit of the content of Al is not limited to a particular content. However, the content of Al is preferably set to 0.005% or more.
  • the content of Al in the present invention refers to a content of acid-soluble Al (what is called sol. Al).
  • N nitrogen
  • nitrogen forms fine nitrides, thereby strongly pinning grain boundaries to subject grains to grain refinement, thus enhancing toughness of steel.
  • a content of N is set to 0.0100% or less.
  • the content of N is preferably 0.0080% or less, and more preferably 0.0050% or less.
  • the content of N may be on the level of impurity.
  • a lower limit of the content of N is not limited to a particular content.
  • the content of N is preferably set to 0.0005% or more, and more preferably 0.0010% or more.
  • V vanadium
  • V vanadium
  • V vanadium
  • a content of V is set to 0.100% or less.
  • the content of V is preferably 0.050% or less, and more preferably 0.010% or less. Even a trace quantity of V enables the action of V to be recognized. However, to provide the effect sufficiently, 0.001% or more of V is preferably contained.
  • B (boron) is an element that segregates in grain boundaries in steel to enhance hardenability of steel significantly. Therefore, B may be contained as necessary. However, if more than 0.0050% of B is contained, there is a tendency for borides to precipitate coarsely in crystal grain boundaries, decreasing toughness. Therefore, in a case where B is contained, a content of B is set to 0.0050% or less. The content of B is preferably 0.0030% or less, and more preferably 0.0020% or less. Even a trace quantity of B enables the action of B to be recognized. However, to keep the effect sufficiently, 0.0001% or more of B is preferably contained, and 0.0005% or more of B is more preferably contained.
  • Mg manganesium
  • Mg is an element that fixes S present in steel as an unavoidable impurity in the form of its sulfide and improves anisotropy of toughness to increase toughness in a T direction of a pipe, thereby increasing resistance to burst. Therefore, Mg may be contained as necessary. However, if more than 0.0050% of Mg is contained, inclusions increase, rather decreasing toughness. Therefore, in a case where Mg is contained, a content of Mg is set to 0.0050% or less. The content of Mg is preferably 0.0040% or less, and more preferably 0.0030% or less. Even a trace quantity of Mg enables the action of Mg to be recognized. However, to keep the effect sufficiently, 0.0001% or more of Mg is preferably contained, and 0.0005% or more of Mg is more preferably contained.
  • REM rare earth metal
  • REM is one or more elements that fix S present in steel as an unavoidable impurity in the form of their sulfides and improve anisotropy of toughness to increase toughness in a T direction of a pipe, thereby increasing resistance to burst. Therefore, REM may be contained as necessary. However, if more than 0.0050% of REM is contained, inclusions increase, rather decreasing toughness. Therefore, in a case where REM is contained, a content of REM is set to 0.0050% or less. The content of REM is preferably 0.0040% or less, and more preferably 0.0030% or less. Even a trace quantity of REM enables the action of REM to be recognized. However, to keep the effect sufficiently, 0.0001% or more of REM is preferably contained, and 0.0005% or more of REM is more preferably contained.
  • REM refers to Sc (scandium), Y (yttrium), and lanthanoids, 17 elements in total, and in a case where REM includes one element, “the content of REM” refers to a content of the element, and in a case where REM includes two or more elements, “the content of REM” refers to a total content of the elements.
  • REM is supplied in the form of misch metal, which is an alloy of a plurality of types of REM. For this reason, REM may be contained in such a manner as to add one, or two or more separate elements of REM or may be added, for example, in the form of misch metal.
  • contents of elements further satisfy Formula (i) shown below on the precondition that the contents of elements fall within their respective ranges described above.
  • Formula (i) shown below on the precondition that the contents of elements fall within their respective ranges described above.
  • keeping sufficient contents of C, Mo, and Cr enhances hardenability, thus enabling the achievement of increase in strength of the seamless steel pipe.
  • the left side value of the Formula (i) below is preferably 1.20 or more, and more preferably 1.50 or more.
  • symbols of elements in the formula mean contents (mass %) of the elements in the steel, and when an element is not contained, zero will be set to the corresponding symbol.
  • the chemical composition of the seamless steel pipe according to the present embodiment satisfies Formula (ii) shown below in conjunction with a prior-austenite grain size number.
  • Formula (ii) shown below in conjunction with a prior-austenite grain size number.
  • symbols of elements in the formula mean contents (mass %) of the elements in the steel, and when an element is not contained, zero will be set to the corresponding symbol, and where GN means the prior-austenite grain size number.
  • the prior-austenite grain size number is measured in conformance with ASTM E112 (2013). Specifically, a specimen including the entire wall thickness of the seamless steel pipe is taken such that a surface of the seamless steel pipe including a pipe axis direction and a wall-thickness direction of the seamless steel pipe (hereinafter, referred to as a “longitudinal section”) serves as a test surface (hereinafter, referred to as an “observation surface”), and the observation surface is subjected to mirror polish. After the polish, prior-austenite crystal grain boundaries in the observation surface are made to appear with picral etchant.
  • a prior-austenite grain size number is determined by the comparison procedure specified in ASTM E112 (2013), and the average value of the visual fields is taken as a prior-austenite grain size number of the seamless steel pipe.
  • a base observation magnification is set to ⁇ 100, and an observation magnification is set to ⁇ 200 or ⁇ 400 in accordance with the grain size number.
  • correction is made in conformance with ASTM E112 (2013) using a correction factor Q defined by Formula (I) shown below.
  • prior-austenite grain size number is not limited to a particular number as long as the prior-austenite grain size number satisfies Formula (ii) and Formula (iii) described above.
  • the prior-austenite grain size number can be set to 10.0 or more or 11.0 or more.
  • the seamless steel pipe according to the present embodiment has high strength. Specifically, its tensile strength is 1200 MPa or more. When the tensile strength is 1200 MPa or more, the seamless steel pipe exerts excellent resistance to burst even in a case where the seamless steel pipe is used as an accumulator for high-pressure gas to which a stress is loaded at a high strain rate in an extremely short time.
  • the tensile strength is measured in conformance with JIS Z 2241:2011. Specifically, a tubular test specimen having a certain length is cut from the seamless steel pipe and fabricated into a No. 11 test coupon specified in JIS Z 2241:2011. Subsequently, the tubular tensile test specified in JIS Z 2241:2011 is performed on the No. 11 test coupon to measure the tensile strength.
  • the seamless steel pipe according to the present embodiment has excellent low-temperature toughness.
  • a lower limit temperature at which its ductile fracture percent is 100% (vTrs100), which is determined by the Charpy impact test specified in JIS Z 2242:2018, is preferably ⁇ 80° C. or less, and more preferably ⁇ 85° C. or less. More specifically, in the present embodiment, vTrs100 is determined by the following method.
  • a tubular test specimen of 10 mm in length ( FIG. 1 a ) is taken from a seamless steel pipe, cut in the axial direction of the pipe at room temperature into a C shape ( FIG. 1 b ), and thereafter spread into a plate shape ( Figure lc). Then, after cutting both ends in the longitudinal direction to obtain a rectangular test specimen with a length of 55 mm, a width of 10 mm, and a thickness of the original wall thickness d of the steel pipe, a V notch with notch angle of 45°, notch depth of 2 mm, and notch bottom radius of 0.25 mm is introduced in the longitudinal center of the test specimen, so that the notch bottom is parallel to the thickness direction of the test specimen ( FIG. 1 d ). Using the obtained test specimen, a Charpy impact test is performed in accordance with JIS Z 2242:2018 to determine vTrs100.
  • the seamless steel pipe according to the present embodiment has excellent hydrogen embrittlement resistance.
  • a critical hydrogen concentration is 2.5 ppm or more. This makes it possible to ensure high reliability when used as a steel pipe for an air bag or the like.
  • the critical hydrogen concentration is more preferably 2.7 ppm or more.
  • the critical hydrogen concentration is determined by the following method.
  • a plurality of arc-shaped tensile test specimens having a shape illustrated in FIG. 2 are taken from the seamless steel pipe.
  • the arc-shaped tensile test specimen is made by cutting an arc-shaped test specimen with a length of 120 mm, a width of 9.0 mm, and a thickness of the original wall thickness d of the steel pipe from the seamless steel pipe, thereafter, providing a reduced width portion in the central part of the arc-shaped test specimen in the longitudinal direction, while holding portions are left at both ends of the arc-shaped test specimen in the longitudinal direction, and further providing a U notch in the central part of the reduced width portion in the longitudinal direction.
  • Each of the holding portions has a length of 45 mm and a width of 9.0 mm
  • the reduced width portion has a length of 30 mm and a width of 2.0 mm. Both ends of the reduced width portion have curved surfaces having a radius of curvature of 5.0 mm and are connected to the holding portions.
  • the U notch has notch width of 0.20 mm, notch depth of 0.35 mm, and notch bottom radius of 0.10 mm.
  • the cathode charge constant load test with a potential within the range of ⁇ 0.9 to ⁇ 1.2 V is performed with the arc-shaped tensile test specimens being immersed in various types of aqueous solutions containing 3% NaCl and ammonium thiocyanate within the range of 0 to 30 g/L. At this time, a stress that is 90% of tensile strength of each seamless steel pipe is loaded.
  • the seamless steel pipe according to an embodiment of the present invention can be produced by the following method.
  • the cast ingots or cast pieces are subjected to blooming or hot forging.
  • This step is a step for producing starting materials used for final hot rolling (e.g., pipe-making by piercing, rolling, and elongating steps performed as hot processing, or pipe-making by hot extrusion press).
  • the cast pieces made to have the round billet shape by the “round continuous casting” can be directly finished into seamless steel pipes. Therefore, the blooming or the hot forging is not necessarily performed.
  • the starting materials to be used for final hot rolling produced by the blooming or the hot forging described above or the cast pieces made to have the round billet shape (hereinafter, these will be collectively referred to as “cast pieces”) are subjected to a hot rolling step, a cold working step, a quenching step, and a tempering step in this order, by which seamless steel pipes in the present embodiment are produced.
  • the hollow shells provided by the method described above are subjected to cold working for enhancement of dimensional accuracy.
  • a method for the cold working is not limited to a particular method as long as the method enables the hollow shells to be processed evenly.
  • a cold draw bench which is provided with a holed die and a plug
  • a cold rolling machine called a cold Pilger mill, or the like is industrially advantageous.
  • the hollow shells after the cold working are next subjected to an induction hardening process in which the hollow shells are subjected to high-frequency induction heating to a temperature of 900 to 1050° C. and to rapid cooling.
  • a heating temperature of the high-frequency induction heating of less than 900° C. may cause incompletion of austenitization, failing to impart high strength.
  • a heating temperature of the high-frequency induction heating of more than 1050° C. causes austenitic grains to coarsen through rapid growth, failing to impart excellent toughness.
  • the hollow shells subjected to the induction hardening are subjected to a tempering process in which the hollow shells are heated to 370 to 410° C. and then cooled to room temperature.
  • a heating temperature of the tempering is less than 370° C., strength can be kept but low-temperature toughness is decreased.
  • a heating temperature of the tempering is more than 410° C., excellent low-temperature toughness can be obtained but strength is decreased, failing to provide a tensile strength of 1200 MPa or more.
  • a retention time at the heating temperature is preferably set to 10 to 30 minutes, which however depends on the size of the hollow shells.
  • the heating temperature refers to a temperature at the outer surfaces of the hollow shells.
  • a cooling rate for the tempering is not limited to a particular cooling rate. Accordingly, cooling in accordance with facilities such as allowing cooling in the air, forced air cooling, mist cooling, oil cooling, and water cooling is to be performed.
  • preheating is performed so that the residence time in the temperature range of 250 to 350° C. is 5 minutes or longer. As described above, it is considered that this is because the temperature distribution in the thickness direction is eliminated and the metal structure becomes uniform by performing preheating.
  • the resultant seamless steel pipes were first subjected to measurement of prior-austenite grain size number.
  • the prior-austenite grain size number was measured in conformance with ASTM E112 (2013). Specifically, a specimen including the entire wall thickness of each seamless steel pipe was taken such that a longitudinal section of the seamless steel pipe serves as an observation surface, and the observation surface was subjected to mirror polish. After the polish, prior-austenite crystal grain boundaries in the observation surface were made to appear with picral etchant. Subsequently, five visual fields in the observation surface were observed under an optical microscope with a 1 ⁇ 4 position from an outer surface of the seamless steel pipe being centered in each visual field.
  • a prior-austenite grain size number was determined by the comparison procedure specified in ASTM E112 (2013), and the average value of the visual fields was taken as a prior-austenite grain size number of the seamless steel pipe.
  • a base observation magnification was set to ⁇ 100, and an observation magnification was set to ⁇ 200 or ⁇ 400 in accordance with the grain size number.
  • correction was made in conformance with ASTM E112 (2013) using a correction factor Q defined by Formula (I) shown below.
  • each seamless steel pipe was evaluated in terms of tensile strength, low-temperature toughness, and hydrogen embrittlement resistance properties by the following methods.
  • a tubular test specimen having a certain length is cut from each seamless steel pipe and fabricated into a No. 11 test coupon specified in JIS Z 2241:2011. Subsequently, the tubular tensile test specified in JIS Z 2241:2011 was performed on the No. 11 test coupon to measure the tensile strength.
  • a tubular test specimen having a length of 10 mm ( FIG. 1 a ) was taken from each seamless steel pipe, cut in its pipe axis direction at room temperature into a C shape ( FIG. 1 b ), and spread into a plate shape ( FIG. 1 c ). Then, after cutting both ends in the longitudinal direction to obtain a rectangular test specimen with a length of 55 mm, a width of 10 mm, and a thickness of the original wall thickness d of the steel pipe, a V notch with notch angle of 45°, notch depth of 2 mm, and notch bottom radius of 0.25 mm was introduced in the longitudinal center of the test specimen, so that the notch bottom is parallel to the thickness direction of the test specimen ( FIG. 1 d ).
  • vTrs100 was determined and taken as an index of low-temperature toughness.
  • a case where vTrs100 was ⁇ 80° C. or less was determined to be excellent in low-temperature toughness.
  • An arc-shaped tensile test specimen having a shape illustrated in FIG. 2 was taken from each seamless steel pipe and subjected to a cathode charge constant load test. Specifically, the cathode charge constant load test with a potential within the range of ⁇ 0.9 to ⁇ 1.2 V was performed with a plurality of arc-shaped tensile test specimens with holding portions and reduced width portions being immersed in various types of aqueous solutions containing 3% NaCl and ammonium thiocyanate within the range of 0 to 30 g/L. At this time, a stress that was 90% of tensile strength of each seamless steel pipe was loaded.
  • a seamless steel pipe that has high strength and excellent low-temperature toughness and further has excellent hydrogen embrittlement resistance properties can be provided. Accordingly, the seamless steel pipe according to the present invention is suitable for airbags.

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US20020033591A1 (en) * 2000-09-01 2002-03-21 Trw Inc. Method of producing a cold temperature high toughness structural steel tubing
JP3858615B2 (ja) * 2001-03-29 2006-12-20 住友金属工業株式会社 引張強度が９００ＭＰａ以上の高強度エアバッグ用継目無鋼管の製造方法
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