WO2018179169A1 - ラインパイプ用アズロール電縫鋼管 - Google Patents

ラインパイプ用アズロール電縫鋼管 Download PDF

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WO2018179169A1
WO2018179169A1 PCT/JP2017/013013 JP2017013013W WO2018179169A1 WO 2018179169 A1 WO2018179169 A1 WO 2018179169A1 JP 2017013013 W JP2017013013 W JP 2017013013W WO 2018179169 A1 WO2018179169 A1 WO 2018179169A1
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steel pipe
base material
content
vickers hardness
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PCT/JP2017/013013
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English (en)
French (fr)
Japanese (ja)
Inventor
健三 田島
坂本　真也
石塚　哲夫
孝聡 福士
朝日　均
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新日鐵住金株式会社
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Application filed by 新日鐵住金株式会社 filed Critical 新日鐵住金株式会社
Priority to JP2017549548A priority Critical patent/JP6288390B1/ja
Priority to EP17904175.1A priority patent/EP3546610B1/en
Priority to KR1020197016507A priority patent/KR20190084092A/ko
Priority to PCT/JP2017/013013 priority patent/WO2018179169A1/ja
Priority to CN201780077913.5A priority patent/CN110088317A/zh
Publication of WO2018179169A1 publication Critical patent/WO2018179169A1/ja

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    • 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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • 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
    • C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
    • 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
    • C22C38/04—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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • This disclosure relates to an as-roll ERW steel pipe for line pipes.
  • Crude oil or natural gas produced in recent years contains wet hydrogen sulfide (H 2 S).
  • An environment containing hydrogen sulfide is called a sour environment.
  • Pipelines carrying drilled crude oil or natural gas are exposed to such a sour environment. For this reason, the tolerance (sour resistance) with respect to a sour environment is calculated
  • JP2013-11005A Patent Document 1
  • Patent Document 1 describes a thick, high-strength hot-rolled steel sheet for line pipes having excellent sour resistance, in mass%, C: 0.01 to 0.07%, Si: 0.40% or less, Mn: 0.5 to 1.4%, P: 0.015% or less, S: 0.003% or less, Al: 0.1% or less, Nb: 0.01 to 0 .15%, V: 0.1% or less, Ti: 0.03% or less, N: 0.008% or less, and Nb, V, Ti satisfy Nb + V + Ti ⁇ 0.15, and Cm 0.12 or less, the composition comprising the balance Fe and inevitable impurities, and a structure containing 95% or more area ratio of bainite phase or bainitic ferrite phase, in the thickness direction A line having a maximum hardness of 220 HV or less and a yield strength of 450 MPa or more. Thick high-strength hot-rolled steel sheet is disclosed for the pipe.
  • Cm C + Si
  • Patent Document 1 Japanese Patent Application Laid-Open No. 2013-11005
  • sour resistance includes resistance to hydrogen induced cracking (hereinafter also referred to as “HIC”) that occurs mainly in the central portion of the steel pipe (hereinafter referred to as “HIC resistance”). And resistance to sulfide stress corrosion cracking (hereinafter also referred to as “SSC”) that occurs mainly from the inner peripheral surface of the steel pipe (hereinafter also referred to as “SSC resistance”). Is included.
  • HIC resistance hydrogen induced cracking
  • SSC resistance resistance to sulfide stress corrosion cracking
  • SSC resistance resistance to sulfide stress corrosion cracking
  • ERW steel pipes for line pipes require a certain degree of strength (for example, yield strength in the tube axis direction of 415 MPa or more and tensile strength in the tube axis direction of 461 MPa or more) from the viewpoint of improving transport efficiency and operational efficiency. Is done.
  • the strength should not be too high for ERW steel pipes for line pipes. (For example, the yield strength in the tube axis direction is 550 MPa or less, and the tensile strength in the tube axis direction is 625 MPa or less).
  • an object of the present disclosure is to provide an as-roll electric-welded steel pipe for line pipes having a yield strength in the tube axis direction of 415 to 550 MPa and a tensile strength in the tube axis direction of 461 to 625 MPa, which is excellent in SSC resistance. That is.
  • Means for solving the above problems include the following aspects. ⁇ 1> Including the base metal part and the ERW welded part,
  • the chemical composition of the base material part is mass%, C: 0.01 to 0.10%, Si: 0.01-0.40%, Mn: 0.50 to 2.00%, P: 0 to 0.030%, S: 0 to 0.0015%, Al: 0.010 to 0.050%, N: 0.0030 to 0.0080%, Nb: 0.010 to 0.050%, Ti: 0.005 to 0.020%, Ni: 0 to 0.20%, Mo: 0 to 0.20%, Ca: 0 to 0.0050%, Cr: 0 to 1.00%, V: 0 to 0.100%, Cu: 0 to 1.00%, Mg: 0 to 0.0050%, REM: 0 to 0.0100%, and the balance: Fe and impurities,
  • the area ratio of polygonal ferrite is 80% to 98%, and the balance is composed of at least one of bain
  • the chemical composition of the base material part is mass%, Ni: more than 0% and 0.20% or less, Mo: more than 0% and 0.20% or less, Ca: more than 0% and 0.0050% or less, Cr: more than 0% and 1.00% or less, V: more than 0% and 0.10% or less, Cu: more than 0% and 1.00% or less,
  • the as-rolled electric-welded steel pipe for line pipes according to ⁇ 1> containing one or more selected from the group consisting of Mg: more than 0% and 0.0050% or less, and REM: more than 0% and 0.0100% or less.
  • the chemical composition of the base material part is mass%, Ni: 0.001 to 0.20%, and Mo: An as-roll electric-welded steel pipe for line pipes according to ⁇ 1> or ⁇ 2>, containing at least one selected from the group consisting of 0.1 to 0.20%.
  • the chemical composition of the base material part is mass%, An as-roll electric-welded steel pipe for line pipes according to any one of ⁇ 1> to ⁇ 3>, containing Ca: 0.0005 to 0.0050%.
  • ⁇ 5> The as-roll electric-welded steel pipe for line pipes according to any one of ⁇ 1> to ⁇ 4>, wherein the wall thickness is 10 to 25 mm and the outer diameter is 114.3 mm to 660.4 mm.
  • an as-roll electric-welded steel pipe for line pipes having a yield strength in the tube axis direction of 415 to 550 MPa and a tensile strength in the tube axis direction of 461 to 625 MPa and excellent SSC resistance is provided.
  • FIG. 1 It is a scanning electron micrograph (500 times magnification) which shows an example of the metal structure of the base material part in this indication. It is the scanning electron micrograph which expanded a part of FIG. 1 (magnification 2000 times).
  • this indication it is a schematic front view of the tensile test piece used for a tensile test. It is a schematic perspective view which shows an example of the pipe making process for manufacturing the ERW steel pipe of this indication.
  • a numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.
  • “%” indicating the content of a component (element) means “% by mass”.
  • the content of C (carbon) in the base material part may be referred to as “C content”.
  • the content of other elements in the base material part may be described in the same manner.
  • the term “process” is not limited to an independent process, and is included in this term if the intended purpose of the process is achieved even when it cannot be clearly distinguished from other processes. It is.
  • azurole electric pipes for line pipes may be simply referred to as “electric rolls” or “azurole electric pipes”.
  • an as-rolled electric resistance welded steel pipe refers to an electric resistance steel pipe that has not been subjected to heat treatment other than seam heat treatment after pipe making.
  • pipe making refers to a process from roll forming a hot-rolled steel sheet into an open pipe, and the process of forming the electro-welded section by electro-welding the butt portion of the obtained open pipe. Point to.
  • roll forming refers to bending a hot-rolled steel sheet into an open tubular shape.
  • the electric resistance welded steel pipe of the present disclosure (that is, an as-roll electric resistance welded steel pipe for line pipes) includes a base metal part and an electric resistance welded part, and the chemical composition of the base metal part is C: 0.01-0. 10%, Si: 0.01-0.40%, Mn: 0.50-2.00%, P: 0-0.030%, S: 0-0.0015%, Al: 0.010-0 0.050%, N: 0.0030 to 0.0080%, Nb: 0.010 to 0.050%, Ti: 0.005 to 0.020%, Ni: 0 to 0.20%, Mo: 0 to 0.20%, Ca: 0 to 0.0050%, Cr: 0 to 1.00%, V: 0 to 0.100%, Cu: 0 to 1.00%, Mg: 0 to 0.0050%, REM: 0 to 0.0100%, balance: Fe and impurities, and the area ratio of polygonal ferrite is 80% to 98% in the metal structure of the base metal part.
  • the balance is at least one of bainite and pearlite, the yield strength in the tube axis direction (hereinafter also referred to as “YS”) is 415 to 550 MPa, and the tensile strength in the tube axis direction (hereinafter also referred to as “TS”) is 461.
  • the maximum Vickers hardness of the inner surface layer of the base metal part is 248 HV or less, and is 5 HV or less smaller than the maximum Vickers hardness of the outer surface layer of the base material part.
  • the base metal portion refers to a portion of the electric resistance welded pipe other than the electric resistance welded portion and the heat affected zone.
  • the heat affected zone (hereinafter also referred to as “HAZ”) is the influence of heat by electric resistance welding (however, when performing seam heat treatment after electric resistance welding, electric resistance welding and seam heat treatment). The part affected by the heat).
  • the highest Vickers hardness of the inner surface layer of the base material portion means a value measured as follows. First, as a measurement point of Vickers hardness, in the C cross section of the electric resistance welded pipe (that is, a cross section perpendicular to the pipe axis direction), the circumference having a depth from the inner peripheral surface of the electric resistance welded pipe is 0.1 mm. Nine points with a pitch of 1 mm centering on the 180 ° position of the base material (that is, a position shifted by 180 ° in the pipe circumferential direction from the ERW weld) are selected. A specimen including the nine measurement points selected above is taken from the ERW steel pipe.
  • the maximum value among the nine obtained measurement results is defined as the highest Vickers hardness of the inner surface layer of the base material. That is, the maximum Vickers hardness of the inner surface layer of the base material portion is roughly the maximum Vickers hardness in the vicinity of the inner peripheral surface of the base material portion.
  • the maximum Vickers hardness of the outer surface layer of the base material portion is the same as the maximum Vickers hardness of the inner surface of the base material portion except that “inner peripheral surface” is read as “outer peripheral surface”. Means the measured value. That is, the maximum Vickers hardness of the outer surface layer of the base material portion is roughly the maximum Vickers hardness in the vicinity of the outer peripheral surface of the base material portion.
  • the electric resistance welded steel pipe of the present disclosure has a certain degree of strength (that is, YS and TS in the above-described range) and is excellent in SSC resistance.
  • the conventional ERW steel pipe for line pipes (for example, the ERW steel pipe for line pipes described in Patent Document 1 described above) has taken HIC resistance as sour resistance into consideration.
  • SSC resistance as sour resistance was not considered.
  • the location where cracking occurs differs between HIC (hydrogen induced cracking) and SSC (sulfide stress corrosion cracking).
  • HIC occurs mainly at the thickness center portion of the electric resistance welded steel pipe
  • SSC occurs mainly from the inner peripheral surface of the electric resistance welded steel pipe.
  • a fluid containing wet hydrogen sulfide (specifically, crude oil or natural gas; hereinafter also referred to as “sour fluid”) contacts the inner peripheral surface of the ERW steel pipe for line pipes. In this state, it occurs starting from the inner peripheral surface. Therefore, even an ERW steel pipe having excellent HIC resistance may be inferior in SSC resistance.
  • the maximum Vickers hardness of the inner surface layer of the base material portion is 248 HV or less, and the highest Vickers hardness of the inner surface layer of the base material portion is the highest Vickers of the outer surface layer of the base material portion. It is 5HV or less smaller than the hardness.
  • SSC which is a crack starting from the inner peripheral surface, is suppressed (that is, the SSC resistance is improved) in a state where the sour fluid is in contact with the inner peripheral surface of the ERW steel pipe.
  • SSC tends to occur more easily as the strength of the ERW steel pipe increases.
  • YS is limited to 550 MPa or less and TS is limited to 625 MPa or less. Thereby, SSC resistance improves.
  • the highest Vickers hardness of the inner surface layer of the base material part is 5HV or more smaller than the highest Vickers hardness of the outer surface layer of the base material part, so The highest Vickers hardness of the outer surface layer is secured to some extent.
  • a certain degree of strength (specifically, YS 415 MPa or more and TS 461 MPa or more) is secured as a whole of the electric resistance welded steel pipe.
  • the maximum Vickers hardness of the inner surface layer of the base metal part and the maximum Vickers hardness of the outer surface layer of the base material part are substantially equal due to the following circumstances.
  • the condition that “the highest Vickers hardness of the inner surface layer of the base material part is 5 HV or more smaller than the highest Vickers hardness of the outer surface layer of the base material part” was not satisfied. That is, an electric resistance steel pipe is manufactured by using a hot coil made of a hot-rolled steel sheet as a raw material and pipe-making (that is, roll forming and electric-resistance welding) the hot-rolled steel sheet that has been unwound from the hot coil.
  • first surface is the outer surface of the ERW steel pipe
  • the second surface is also referred to as the inner surface of the ERW steel pipe.
  • the manufacturing process of a hot coil includes each step of hot rolling, cooling, and winding in this order. Conventionally, from the viewpoint of suppressing warpage of the hot-rolled steel sheet after cooling or from the viewpoint of productivity, this cooling is performed by water-cooling two surfaces of the hot-rolled steel sheet obtained by hot rolling at substantially the same cooling rate. Had gone by.
  • the maximum Vickers hardness of the inner surface layer of the base material portion and the maximum Vickers hardness of the outer surface layer of the base material portion are substantially equal (that is, “the base material portion of The maximum Vickers hardness of the inner surface layer was 5 HV or more smaller than the maximum Vickers hardness of the outer surface layer of the base material part ”.
  • the present inventors set a difference in the cooling rate for the two surfaces when cooling the two surfaces of the hot-rolled steel sheet obtained by hot rolling (specifically, By making the cooling rate of the second surface corresponding to the inner peripheral surface slower than the cooling rate of the first surface corresponding to the outer peripheral surface), the maximum Vickers hardness of the inner surface layer of the base material portion is set to It succeeded in making 5HV or more smaller than the highest Vickers hardness of the outer surface layer. Furthermore, the present inventors have found that since the cooled hot-rolled steel sheet is wound after that, the warp of the hot-rolled steel sheet after cooling is not a problem in practice. Based on the above knowledge of the present inventors, the electric resistance welded steel pipe of the present disclosure was completed.
  • the SSC resistance contributes not only to the highest Vickers hardness of the inner surface layer of the base material part, but also to the chemical composition of the base material part, the metal structure of the base material part, and the as-roll ERW steel pipe. . Further, the chemical composition of the base material part, the metal structure of the base material part, and the as-rolled electric resistance welded steel pipe also contribute to the achievement of YS in the above range and TS in the above range.
  • the chemical composition of the base material part and the metal structure of the base material part will be described.
  • C 0.01 to 0.10% C increases the strength of the steel. If the C content is too low, this effect cannot be obtained. Therefore, the C content is 0.01% or more.
  • the C content is preferably 0.03% or more, and more preferably 0.04% or more.
  • the C content is 0.10% or less.
  • the C content is preferably 0.09%, more preferably 0.08% or less.
  • Si 0.01-0.40% Si deoxidizes steel. If the Si content is too low, this effect cannot be obtained. Therefore, the Si content is 0.01% or more.
  • the Si content is preferably 0.02% or more, and more preferably 0.10% or more.
  • the Si content is 0.40% or less. Si content becomes like this. Preferably it is 0.38% or less, More preferably, it is 0.35% or less.
  • Mn 0.50 to 2.00% Mn increases the hardenability of the steel and increases the strength of the steel. If the Mn content is too low, this effect cannot be obtained. Therefore, the Mn content is 0.50% or more.
  • the Mn content is preferably 0.60% or more, and more preferably 0.80% or more.
  • the Mn content is 2.00% or less.
  • the Mn content is preferably 1.80% or less, more preferably 1.50% or less.
  • P 0 to 0.030%
  • P is an impurity. P segregates at the grain boundary and embrittles the grain boundary. Therefore, P decreases the toughness and SSC resistance of steel. Therefore, it is preferable that the P content is small. Specifically, the P content is 0.030% or less. The P content is preferably 0.021% or less, more preferably 0.015% or less, and still more preferably 0.010% or less. On the other hand, the P content may be 0%. From the viewpoint of reducing the dephosphorization cost, the P content may be more than 0% or 0.001% or more.
  • S 0 to 0.0015%
  • S is an impurity. S combines with Mn to form a Mn-based sulfide. Mn-based sulfides are easy to dissolve. Therefore, the toughness and SSC resistance of the steel are reduced. Accordingly, the S content is preferably as low as possible. Specifically, the S content is 0.0015% or less. The S content is preferably 0.0010% or less, and more preferably 0.0008% or less. On the other hand, the S content may be 0%. From the viewpoint of reducing the desulfurization cost, the S content may be more than 0%, 0.0001% or more, or 0.0003% or more.
  • Al 0.010 to 0.050% Al deoxidizes steel. If the Al content is too low, this effect cannot be obtained. Therefore, the Al content is 0.010% or more.
  • the Al content is preferably 0.012% or more, and more preferably 0.013% or more.
  • the Al content is 0.050% or less. Al content becomes like this. Preferably it is 0.040% or less, More preferably, it is 0.035% or less, More preferably, it is 0.030% or less.
  • Al content in this specification means content of all the Al in steel.
  • N 0.0030 to 0.0080% N increases the strength of the steel by solid solution strengthening. If the N content is too low, this effect cannot be obtained. Therefore, the N content is 0.0030% or more. On the other hand, if the N content is too high, the carbonitrides are coarsened and the SSC resistance is lowered. Accordingly, the N content is 0.0080% or less. N content becomes like this. Preferably it is 0.0070% or less, More preferably, it is 0.0060% or less, More preferably, it is 0.0040% or less.
  • Nb 0.010 to 0.050% Nb combines with C and N in steel to form fine Nb carbonitride. Fine Nb carbonitride increases the strength of the steel by dispersion strengthening. If the Nb content is too low, this effect cannot be obtained. Therefore, the Nb content is 0.010% or more.
  • the Nb content is preferably 0.020% or more, and more preferably 0.030% or more.
  • the Nb content is too high, the Nb carbonitride becomes coarse and the SSC resistance of the steel decreases. Furthermore, if the Nb content is too high, the toughness of the electric seam welded portion decreases. Therefore, the Nb content is 0.050% or less.
  • the Nb content is preferably 0.045% or less, more preferably 0.040% or less.
  • Ti 0.005 to 0.020%
  • Ti combines with N in the steel to form Ti nitride and / or Ti carbonitride.
  • Ti nitrides and / or Ti carbonitrides refine steel grains. If the Ti content is too low, this effect cannot be obtained. Accordingly, the Ti content is 0.005% or more.
  • the Ti content is preferably 0.007% or more, and more preferably 0.010% or more.
  • the Ti content is 0.020% or less.
  • the Ti content is preferably 0.018% or less, and more preferably 0.016% or less.
  • Ni is an optional element and may not be contained. That is, the Ni content may be 0%. When Ni is contained, Ni increases the strength of the steel by solid solution strengthening. Ni further increases the toughness of the steel. From the viewpoint of these effects, the Ni content is preferably more than 0%, more preferably 0.001% or more, more preferably 0.005% or more, and still more preferably 0.01%. % Or more, more preferably 0.05% or more. On the other hand, if the Ni content is too high, the weldability of the steel decreases. Therefore, the Ni content is 0.20% or less. The Ni content is preferably 0.18% or less, and more preferably 0.15% or less.
  • Mo 0 to 0.20%
  • Mo is an optional element and may not be contained. That is, the Mo content may be 0%.
  • Mo increases the hardenability of the steel and increases the strength of the steel. Furthermore, since micro segregation of Mo hardly occurs, the generation of HIC due to center segregation is suppressed. From the viewpoint of these effects, the Mo content is preferably more than 0%, more preferably 0.10% or more, and further preferably 0.12% or more.
  • Mo content since Mo is expensive, if Mo is excessively contained, the manufacturing cost increases. Therefore, the Mo content is 0.20% or less. Mo content becomes like this. Preferably it is 0.18% or less, More preferably, it is 0.15% or less.
  • Ca 0% to 0.0050% Ca is an optional element and may not be contained. That is, the Ca content may be 0%.
  • Ca makes the form of MnS, which is the starting point of SSC, spherical, and suppresses the generation of SSC.
  • Ca further forms CaS and suppresses the generation of MnS.
  • the Ca content is preferably more than 0%, more preferably 0.0005% or more, still more preferably 0.0010% or more, and further preferably 0.0020% or more. It is. On the other hand, if the Ca content is too high, the effect is saturated and the manufacturing cost increases. Therefore, the Ca content is 0.0050% or less.
  • the Ca content is preferably 0.0045% or less.
  • Cr 0 to 1.00% Cr is an arbitrary element and may not be contained. That is, the Cr content may be 0%. When Cr is contained, Cr contributes to improvement of hardenability. From the viewpoint of this effect, the Cr content is preferably more than 0%, more preferably 0.01% or more. On the other hand, if the Cr content is too high, the toughness of the ERW weld may deteriorate due to the Cr-based inclusions generated in the ERW weld. Therefore, the Cr content is 1.00% or less. The Cr content is preferably 0.50% or less, more preferably 0.30% or less, and still more preferably 0.20% or less.
  • V 0 to 0.100%
  • V is an arbitrary element and may not be contained. That is, the V content may be 0%.
  • V contributes to improvement of toughness. From the viewpoint of such effects, the V content is preferably more than 0%, more preferably 0.001% or more, and further preferably 0.005% or more.
  • the V content is 0.100% or less. V content becomes like this. Preferably it is 0.070% or less, More preferably, it is 0.050% or less, More preferably, it is 0.030% or less.
  • Cu 0 to 1.00%
  • Cu is an arbitrary element and may not be contained. That is, the Cu content may be 0%.
  • Cu contributes to improving the strength of the base material portion. From the viewpoint of this effect, the Cu content is preferably more than 0%, more preferably 0.01% or more, and further preferably 0.05% or more.
  • the Cu content is 1.00% or less. Cu content becomes like this. Preferably it is 0.70% or less, More preferably, it is 0.50% or less, More preferably, it is 0.30% or less.
  • Mg 0 to 0.0050%
  • Mg is an arbitrary element and may not be contained. That is, the Mg content may be 0%.
  • Mg functions as a deoxidizing agent and a desulfurizing agent. Moreover, a fine oxide is produced and it contributes also to the improvement of the toughness of HAZ.
  • the Mg content is preferably more than 0%, more preferably 0.0001% or more, and further preferably 0.0010% or more.
  • the Mg content is too high, the oxide tends to agglomerate or coarsen, and as a result, the HIC resistance may be lowered, or the toughness of the base material or HAZ may be lowered. Therefore, the Mg content is 0.0050% or less.
  • the Mg content is preferably 0.0030% or less.
  • REM 0 to 0.0100%
  • REM is an arbitrary element and may not be contained. That is, the REM amount may be 0%.
  • “REM” is a rare earth element, that is, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. It refers to at least one element selected.
  • REM functions as a deoxidizer and a desulfurizer. From the viewpoint of such effects, the REM content is preferably more than 0%, more preferably 0.0001% or more, and further preferably 0.0010% or more.
  • the REM content is 0.0100% or less.
  • the REM content is preferably 0.0070% or less, more preferably 0.0050% or less.
  • the chemical composition of the base metal part is Ni: more than 0% to 0.20% or less, Mo: more than 0% to 0.20% or less, Ca: more than 0% to 0.0050% or less, Cr: more than 0% to 1.00%
  • V more than 0% and 0.100% or less
  • Cu more than 0% and 1.00% or less
  • Mg more than 0% and 0.0050% or less
  • REM more than 0% and 0.0100% or less
  • One or more selected may be contained.
  • the more preferable amount of each arbitrary element is as described above.
  • the remainder excluding the above-described elements is Fe and impurities.
  • the impurity refers to a component contained in raw materials (for example, ore, scrap, etc.) or a component mixed in a manufacturing process and not intentionally contained in steel.
  • impurities include all elements other than the elements described above.
  • the element as the impurity may be only one type or two or more types.
  • the impurities include O, B, Sb, Sn, W, Co, As, Pb, Bi, and H.
  • O is preferably controlled so that the content is 0.006% or less.
  • Sb, Sn, W, Co, and As are usually mixed with a content of 0.1% or less, and Pb and Bi are mixed with a content of 0.005% or less.
  • Pb and Bi are mixed with a content of 0.005% or less.
  • the area ratio of polygonal ferrite (hereinafter also referred to as “ferrite fraction”) is 80 to 98% in the metal structure of the base material portion, and the balance is from at least one of bainite and pearlite.
  • ferrite fraction is 80 to 98% in the metal structure of the base material portion, and the balance is from at least one of bainite and pearlite.
  • the ferrite fraction is preferably 81% or more, more preferably 82% or more.
  • the ferrite fraction is 98% or less, YS415 MPa or more and TS461 MPa or more can be achieved.
  • the ferrite fraction is preferably 97% or less, more preferably 95% or less.
  • the remainder in the metal structure of the base metal part is made of at least one of bainite and pearlite.
  • SSC resistance improves compared with the case where a remainder contains a martensite, for example.
  • the concept of “bainite” in this specification includes bainitic ferrite, upper bainite, and lower bainite.
  • the concept of “perlite” in this specification includes pseudo pearlite.
  • the metal structure of the base material portion described above is related to the fact that the electric resistance welded steel pipe of the present disclosure is an as-roll electric resistance welded steel pipe (that is, heat treatment other than seam heat treatment is not performed after pipe forming). .
  • the electric resistance welded steel pipe of the present disclosure is an as-roll electric resistance welded steel pipe (that is, heat treatment other than seam heat treatment is not performed after pipe forming).
  • martensite is formed as the metal structure of the base metal part.
  • the ERW steel pipe is inferior in SSC resistance.
  • Measurement of the ferrite fraction in the metal structure of the base metal part and identification of the remaining part are performed as follows.
  • the metal structure at the center of the thickness in the L cross-section at the 180 ° position of the base metal is subjected to nital etching, and a photograph of the metal structure after the nital etching (hereinafter also referred to as “metal structure photograph”) is shown by a scanning electron microscope (SEM). ) And observing at a magnification of 500 times.
  • SEM scanning electron microscope
  • the metal structure photograph is taken for 10 fields of view with a magnification of 500 times (0.48 mm 2 minutes as the actual area of the cross section).
  • Image processing is performed on the photographed metal structure photograph to measure the ferrite fraction and specify the remainder.
  • the image processing is performed using, for example, a small general-purpose image analyzer LUZEX AP manufactured by Nireco Corporation.
  • FIG. 1 is a scanning electron micrograph (SEM photograph; magnification 500 times) showing an example of a metal structure of a base material part in the present disclosure
  • FIG. 2 is an SEM in which a part of the area in FIG. 1 is enlarged. It is a photograph (magnification 2000 times).
  • the SEM photograph (500 times) in FIG. 1 is one (one field of view) of the SEM photographs used for the measurement of the ferrite fraction and the identification of the remainder in test number 22 described later.
  • the metal structure according to this example is a metal structure mainly composed of polygonal ferrite, and the balance is pearlite.
  • the maximum Vickers hardness of the inner surface layer of the base material portion is 248 HV or less, and the Vickers hardness of the inner surface layer of the base material portion is higher than the maximum Vickers hardness of the outer surface layer of the base material portion. Is also smaller than 5HV.
  • the maximum Vickers hardness of the inner surface layer of the base material portion and the maximum Vickers hardness of the outer surface layer of the base material portion are as described above.
  • the difference obtained by subtracting the Vickers hardness of the inner surface layer of the base metal part from the maximum Vickers hardness of the outer surface layer of the base material part (that is, the highest Vickers hardness of the outer surface layer of the base material part ⁇ the inner surface layer of the base material part) Vickers hardness) is also called “outer / inner hardness difference”.
  • the Vickers hardness of the inner surface layer of the base material part is 5 HV or more smaller than the highest Vickers hardness of the outer surface layer of the base material part” is also referred to as “the inner / outer hardness difference is 5 HV or more”.
  • the maximum Vickers hardness of the inner surface layer of the base metal part exceeds 248 HV, the toughness of the steel is lowered and the SSC resistance of the ERW steel pipe is lowered. Therefore, the maximum Vickers hardness of the inner surface layer is 248 HV or less.
  • the maximum Vickers hardness of the inner surface layer is preferably 245 HV or less, and more preferably 220 HV or less.
  • the lower limit of the maximum Vickers hardness of the inner surface layer is not particularly limited. From the viewpoint of further improving the strength (that is, YS and TS) of the electric resistance welded steel pipe, the maximum Vickers hardness of the inner surface layer is preferably 175 HV or more, more preferably 180 HV or more, and further preferably 185 HV or more. .
  • the outer / inner hardness difference is 5 HV or more, preferably 6 HV or more.
  • the outer / internal hardness difference is preferably 20 HV or less, more preferably 15 HV or less, and even more preferably 10 HV or less, from the viewpoint of the suitability for manufacturing an ERW steel pipe.
  • the maximum Vickers hardness of the outer surface layer of the base material part is not particularly limited as long as it satisfies the maximum Vickers hardness of the inner surface layer of the base material part and the difference in outer and inner hardness.
  • the maximum Vickers hardness of the outer surface layer of the base material portion is preferably 180 MPa to 250 MPa, more preferably 210 MPa to 230 MPa.
  • the Vickers hardness of the inner surface layer of the base material portion is 5 HV or less smaller than the highest Vickers hardness of the outer surface layer of the base material portion.
  • the maximum Vickers hardness of the inner surface layer may be 5 HV or more lower than the maximum Vickers hardness of the outer surface layer not only in the base material portion but also in the ERW welded portion.
  • the maximum Vickers hardness of the inner surface layer may be lower by 5 HV or more than the maximum Vickers hardness of the outer surface layer even in the ERW weld.
  • the electric resistance welded steel pipe of the present disclosure has a yield strength (YS) in the pipe axis direction of 415 to 550 MPa.
  • YS yield strength
  • YS is 415 MPa or more
  • the strength as an electric resistance welded steel pipe for line pipes is ensured.
  • YS is preferably 430 MPa or more.
  • YS is 550 MPa or less (that is, YS is not too high)
  • YS is preferably 530 MPa or less.
  • the electric resistance welded steel pipe of the present disclosure has a tensile strength (TS) in the pipe axis direction of 461 to 625 MPa.
  • TS tensile strength
  • TS is preferably 500 MPa or more, and more preferably 510 MPa or more.
  • TS is 625 MPa or less (that is, TS is not too high)
  • TS is preferably 620 MPa or less.
  • FIG. 3 is a schematic front view of a tensile test piece used for the tensile test. The unit of the numerical values in FIG. 3 is mm. As shown in FIG.
  • the length of the parallel part of the tensile test piece is 50.8 mm, and the width of the parallel part is 38.1 mm.
  • a tensile test is performed at room temperature in accordance with the API standard 5CT. Based on the test results, YS and TS are obtained.
  • YR yield ratio
  • the wall thickness of the electric resistance welded steel pipe of the present disclosure is preferably 10 to 25 mm.
  • the wall thickness is more preferably 12 mm or more. If the wall thickness is 25 mm or less, it is advantageous in terms of the suitability for producing an electric resistance steel pipe (specifically, the formability when forming a hot-rolled steel sheet into a tubular shape).
  • the wall thickness is more preferably 20 mm or less.
  • the outer diameter of the electric resistance welded steel pipe of the present disclosure is preferably 114.3 to 660.4 mm (ie, 4.5 to 26 inches).
  • the outer diameter is preferably 152.4 mm (ie 6 inches) or more, more preferably 254 mm (ie 10 inches) or more.
  • the outer diameter is preferably 609.6 mm (ie, 24 inches) or less, more preferably 508 mm (ie, 20 inches) or less.
  • Process A is A preparation step of preparing a slab having the chemical composition described above; A hot rolling step of heating the prepared slab and hot rolling the heated slab to obtain a hot-rolled steel sheet; A cooling step of cooling the first surface of the hot-rolled steel sheet at a cooling rate V1 and cooling the second surface opposite to the first surface of the hot-rolled steel plate at a cooling rate V2 slower than the cooling rate V1.
  • the hot rolled steel sheet is unwound from the hot coil, and the unrolled hot rolled steel sheet is formed into an open pipe by roll forming in a direction in which the first surface becomes the outer peripheral surface and the second surface becomes the inner surface.
  • the step of preparing a slab is a step of preparing a slab having the above-described chemical composition.
  • the step of preparing the slab may be a step of manufacturing a slab, or a step of simply preparing a slab that has been manufactured in advance.
  • the molten steel which has the above-mentioned chemical composition is manufactured, for example, and a slab is manufactured using the manufactured molten steel.
  • the slab may be manufactured by a continuous casting method, or the ingot may be manufactured using molten steel, and the ingot may be subjected to partial rolling to manufacture the slab.
  • a hot rolling process is a process of heating the slab prepared above and hot rolling the heated slab to obtain a hot rolled steel sheet.
  • the heating temperature for heating the slab is preferably 1100 to 1250 ° C.
  • the heating temperature is 1100 ° C. or higher, crystal grain refinement during hot rolling and precipitation strengthening after hot rolling are facilitated, and as a result, the strength of the steel is easily improved.
  • the heating temperature is 1250 ° C. or lower, the austenite grains can be further prevented from being coarsened, so that the crystal grains can be easily refined, and as a result, the strength of the steel can be further improved.
  • the slab is heated by a heating furnace, for example.
  • the slab heated as described above is hot-rolled to obtain a hot-rolled steel sheet.
  • the hot rolling is preferably performed under the condition that the finish rolling finish temperature (hereinafter also referred to as “finish rolling temperature”) is 780 to 830 ° C.
  • Hot rolling is generally performed using a roughing mill and a finish rolling mill. Both the rough rolling mill and the finish rolling mill generally include a plurality of rolling stands arranged in a row, and each rolling stand includes a roll pair.
  • the finish rolling temperature ie, finish rolling end temperature
  • the finish rolling temperature is the surface temperature of the hot-rolled steel sheet on the exit side of the final stand of the finish rolling mill.
  • the finish rolling temperature is 780 ° C. or higher, the rolling resistance of the steel sheet can be reduced, and thus productivity is improved. Further, when the finish rolling temperature is 780 ° C. or higher, the phenomenon of rolling in a two-phase region of ferrite and austenite is suppressed, and the formation of a lamellar structure and the deterioration of mechanical properties accompanying this phenomenon can be suppressed. On the other hand, when the finish rolling temperature is 830 ° C. or lower, the phenomenon that the steel becomes too hard is suppressed, so that the phenomenon that the YS and / or TS of the obtained ERW steel pipe becomes too high is suppressed.
  • the rolling reduction in the austenite non-recrystallization temperature region is preferably 70 to 80%. In this case, the unrecrystallized structure is refined.
  • the first surface of the hot-rolled steel sheet is cooled at a cooling rate V1
  • the second surface opposite to the first surface of the hot-rolled steel sheet is cooled at a cooling rate V2 that is slower than the cooling rate V1.
  • the first surface may be an upper surface (a surface opposite to the direction of gravity; the same applies hereinafter)
  • the second surface may be a lower surface (a surface in the direction of gravity; the same applies hereinafter).
  • the surface may be the lower surface and the second surface may be the upper surface.
  • both the cooling of the first surface and the cooling of the second surface include water cooling.
  • the hot-rolled steel sheet may be water-cooled immediately after hot rolling, or the hot-rolled steel sheet immediately after hot rolling may be air-cooled and then water-cooled.
  • the cooling rate V1 and the cooling rate V2 satisfy the following formula (1). Thereby, it is easier to manufacture a hot-rolled steel sheet having a hardness of the second surface lower than that of the first surface, and as a result, the ERW steel pipe of the present disclosure having an outer-internal hardness difference of 5 HV or more is more manufactured. easy.
  • V1 represents the cooling rate V1 (° C./s)
  • V2 represents the cooling rate V2 (° C./s)
  • the cooling rate V1 is preferably 5 to 25 ° C./s.
  • the cooling rate V2 is not particularly limited. From the viewpoint of further increasing the strength (YS and TS) of the electric resistance welded steel pipe, the cooling rate V2 is preferably 0.5 ° C./s or more, more preferably 0.8 ° C./s or more.
  • the cooling rate V1 and the cooling rate V2 can be adjusted, for example, by adjusting the water flow density in a water cooling device for performing water cooling.
  • a water cooling device for performing water cooling.
  • the second surface side has a water flow density that satisfies the above formula (1).
  • the water flow density and the water flow density on the first surface side are adjusted independently.
  • Winding process is a process of obtaining the hot coil which consists of a hot-rolled steel plate by winding the hot-rolled steel plate cooled by the cooling process.
  • the surface temperature of the hot-rolled steel sheet at the start of winding (hereinafter also referred to as “winding temperature”) is preferably 620 ° C. or less, and more preferably 600 ° C. or less. Since the coarsening of a crystal grain can be suppressed more as winding temperature is 620 degrees C or less, the minimum of winding temperature which can improve the intensity
  • the tube forming process unwinds the hot-rolled steel sheet from the hot coil, and rolls the unrolled hot-rolled steel sheet in a direction in which the first surface becomes the outer peripheral surface and the second surface becomes the inner surface.
  • an open-welded steel pipe is obtained by forming an open-welded welded portion by electro-welding the butt portion of the open tube obtained.
  • the tube-forming step can be performed according to a known method except that roll forming is performed in such a direction that the first surface becomes the outer peripheral surface and the second surface becomes the inner surface.
  • FIG. 4 is a schematic perspective view showing an example of a pipe making process.
  • a hot-rolled steel sheet unwound from a hot coil is formed using a forming roll (not shown) in a direction in which the first surface becomes the outer peripheral surface 1 and the second surface becomes the inner peripheral surface 2.
  • Roll forming into an open tube is subjected to electric resistance welding using an electric power supply 60 and a welding roll 70 to obtain an electric resistance steel pipe 200.
  • the manufacturing method A may include other steps as necessary.
  • the other steps include a step of performing seam heat treatment on the ERW welded portion of the ERW steel pipe after the pipe making step, a step of adjusting the shape of the ERW steel pipe with a sizing roll after the pipe making step, and the like.
  • Test numbers 1 to 26 According to the manufacturing method A mentioned above, the ERW steel pipe of each test number was manufactured. Details are shown below.
  • the slab was heated in a heating furnace, and the heated slab was hot-rolled into a hot-rolled steel sheet using a plurality of hot-rolling mills, and the obtained hot-rolled steel sheet was air-cooled, then water-cooled and water-cooled.
  • a hot coil made of a hot-rolled steel sheet was obtained by winding the hot-rolled steel sheet.
  • the heating temperature when heating the slab, the finish rolling temperature in hot rolling, the cooling rate when cooling the hot-rolled steel sheet with water (V1 and V2), and the winding when winding the water-cooled hot-rolled steel sheet The temperatures are as shown in Table 2, respectively.
  • the upper surface of the hot-rolled steel sheet is the first surface
  • the cooling rate of the first surface is V1
  • the lower surface of the hot-rolled steel sheet is the second surface
  • the cooling rate of the second surface is Was V2.
  • Water cooling of the hot-rolled steel sheet was performed by spraying a water-cooled shower on the upper surface (namely, the first surface) and the lower surface (namely, the second surface) of the hot-rolled steel sheet.
  • the water flow density of the water-cooled shower with respect to the upper surface and the water flow density of the water-cooled shower with respect to the lower surface were adjusted to adjust V1 and V2 to the values shown in Table 2.
  • the conventional standard water-cooling conditions are the conditions of test number 12 (comparative example).
  • the hot-rolled steel sheet is unwound from the hot coil, and the unrolled hot-rolled steel sheet is opened by roll forming in a direction in which the first surface becomes the outer peripheral surface and the second surface becomes the inner peripheral surface of the pipe.
  • a butt portion of the obtained open tube was electro-welded to form an electro-welded welded portion, thereby obtaining an electric-welded steel tube (hereinafter also referred to as “an electric-sealed steel tube before shape adjustment”).
  • the ERW welded portion of the ERW steel pipe before the shape adjustment is subjected to seam heat treatment, and then the shape is adjusted by a sizing roll, whereby an ERW steel pipe having an outer diameter of 406.4 mm and a wall thickness of 15.9 mm (that is, an as-roll electric pipe). Sewn steel pipe).
  • test number 16 the heat-treated steel pipe after seam heat treatment (that is, the as-roll electric-welded steel pipe) was further subjected to heat treatment at a heating temperature of 760 ° C. for 30 minutes, and then water-cooled.
  • the above manufacturing process does not affect the chemical composition of steel. Therefore, it can be considered that the chemical composition of the base material part of the obtained ERW steel pipe is the same as the chemical composition of the molten steel as a raw material.
  • the ferrite fraction (hereinafter also referred to as “F fraction”) was measured by the method described above, and the type of the remainder was confirmed.
  • F fraction The ferrite fraction
  • B means bainite
  • P means pearlite
  • M means martensite
  • YS, TS Based on the measurement method described above, YS (MPa) and TS (MPa) in the tube axis direction of the ERW steel pipe were measured. In the tensile test in the tube axis direction in the measurement of YS and TS, yield elongation was observed in test number 16 (comparative example), but no yield elongation was observed in any other test numbers.
  • test piece having a total thickness of 120 mm (pipe circumferential direction) ⁇ 25 mm (pipe axis direction) was taken from the position of the base material 180 ° of the electric resistance steel pipe. EFC (European Federation of Corrosion Publications) No.
  • This test piece was immersed in the following test bath for 720 hours under a load corresponding to 90% of YS according to 16 Method B (4-point bending test).
  • As the test bath a liquid in which hydrogen sulfide gas was saturated in an aqueous solution containing 5% by mass of sodium chloride and 0.4% by mass of sodium acetate was used.
  • the temperature of the test bath at the time of immersion was normal temperature (23 ° C.).
  • the chemical composition and metal structure of the base material part in the present disclosure are satisfied, YS (that is, 415 to 550 MPa) and TS (that is, 461 to 625 MPa) in the present disclosure are satisfied,
  • YS that is, 415 to 550 MPa
  • TS that is, 461 to 625 MPa
  • the electric resistance welded steel pipes of the respective examples in which the maximum Vickers hardness of the inner surface layer of the base material part was 248 HV or less and the difference in outer / inner hardness was 5 HV or more were excellent in SSC resistance.
  • test number 12 the SSC resistance deteriorated.
  • the reason for this is considered that the maximum Vickers hardness of the inner surface layer exceeded the upper limit, TS and YS both exceeded the upper limit, and the difference in outer and inner hardness was less than 5 HV.
  • SSC resistance deteriorated also in the test number 16 (comparative example). The reason for this is considered to be that martensite was contained in the metal structure of the base metal part because tempering was performed after pipe making.
  • Test numbers 9, 10, and 15 are all comparative examples in which TS and YS exceeded the upper limit
  • test numbers 25 and 26 are comparative examples in which TS and YS were below the lower limit. In Test No.
  • the maximum Vickers hardness of the inner surface layer of the base material portion is 248 HV or less, but the difference in outer and inner hardness is less than 5 HV, which is excellent in SSC resistance, but TS and YS Fell below the lower limit.

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