EP3546610B1 - As-rolled electric resistance welded steel pipe for line pipe - Google Patents

As-rolled electric resistance welded steel pipe for line pipe Download PDF

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Publication number
EP3546610B1
EP3546610B1 EP17904175.1A EP17904175A EP3546610B1 EP 3546610 B1 EP3546610 B1 EP 3546610B1 EP 17904175 A EP17904175 A EP 17904175A EP 3546610 B1 EP3546610 B1 EP 3546610B1
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EP
European Patent Office
Prior art keywords
electric resistance
less
resistance welded
base metal
steel pipe
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EP17904175.1A
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German (de)
English (en)
French (fr)
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EP3546610A1 (en
EP3546610A4 (en
Inventor
Kenzo TASHIMA
Shinya Sakamoto
Tetsuo Ishitsuka
Takaaki Fukushi
Hitoshi Asahi
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Nippon Steel Corp
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Nippon Steel Corp
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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

  • the present disclosure relates to an as-rolled electric resistance welded steel pipe for a line pipe.
  • Crude oil or natural gas produced in recent years includes wet hydrogen sulfide (H 2 S).
  • An environment including hydrogen sulfide is referred to as a sour environment.
  • a pipeline for transporting drilled crude oil or natural gas is exposed to such a sour environment.
  • sour resistance resistance to a sour environment
  • Patent Document 1 discloses, as a thick-walled high-strength hot-rolled steel sheet for a line pipe, which has excellent sour resistance, a thick-walled high-strength hot-rolled steel sheet for a line pipe, which has a composition including, in terms of % by mass, from 0.01 to 0.07% of C, 0.40% or less of Si, from 0.5 to 1.4% of Mn, 0.015% or less of P, 0.003% or less of S, 0.1% or less of Al, from 0.01 to 0.15% of Nb, 0.1% or less of V.
  • Patent Document 2 discloses an electric resistance welded steel pipe comprises: a composition containing, in mass%, C: 0.04-0.15%, Si: 0.10-0.50%, Mn: 1.0-2.2%, P: 0.050% or less, S: 0.005% or less, Cr: 0.2-1.0%, Ti: 0.005-0.030%, Al: 0.010-0.050%, the balance being obtained from Fe and unavoidable impurities; and a structure having at least 70% volume fraction of polygonal ferrite, 3-20% volume fraction of retained austenite, the balance being obtained from one or more selected from martensite, bainite and pearlite.
  • Patent Document 1 JP-A No. 2013-11005
  • Patent Document 2 WO2016/143270
  • sour resistance includes resistance to hydrogen induced cracking (hereinafter also referred to as “HIC”) generated mainly in the central portion of the wall thickness of the steel pipe (hereinafter also referred to as “HIC resistance”) and resistance to sulfide stress cracking (hereinafter also referred to as "SSC”) generated mainly from the inner peripheral surface of the steel pipe as the initiating point (hereinafter also referred to as "SSC resistance”).
  • HIC hydrogen induced cracking
  • SSC sulfide stress cracking
  • Patent Document 1 only the HIC resistance is evaluated, and the SSC resistance is not evaluated as the sour resistance.
  • the high-strength hot-rolled steel sheet for a welded steel pipe for a line pipe of Patent Document 1 may have low SSC resistance.
  • a certain amount of high strength for example, a yield strength in a pipe axis direction of 415 MPa or more, and a tensile strength in the pipe axis direction of 461 MPa or more) is required.
  • not-too-high strength for example, the yield strength in the pipe axis direction of 550 MPa or less, and the tensile strength in the pipe axis direction of 625 MPa or less.
  • an object of the disclosure is to provide an as-rolled electric resistance welded steel pipe for a line pipe, which has a yield strength in a pipe axis direction of from 415 to 550 MPa, which has a tensile strength in the pipe axis direction of from 461 to 625 MPa, and which has excellent SSC resistance.
  • Means of solving the problem described above includes the following aspects.
  • an as-rolled electric resistance welded steel pipe for a line pipe which has a yield strength in a pipe axis direction of from 415 to 550 MPa, which has a tensile strength in the pipe axis direction of from 461 to 625 MPa, and which has excellent SSC resistance, is provided.
  • a numerical range expressed by "from x to y" herein includes the values of x and y in the range as the minimum and maximum values, respectively.
  • the content of a component (element) expressed by “%” herein means “% by mass”.
  • the content of C (carbon) in a base metal portion may be herein occasionally expressed as "C content”.
  • the content of another element in the base metal portion may be expressed similarly.
  • step herein encompasses not only an independent step but also a step of which the desired object is achieved even in a case in which the step is incapable of being definitely distinguished from another step.
  • an "as-rolled electric resistance welded steel pipe for a line pipe” may be simply referred to as an “electric resistance welded steel pipe” or an “as-rolled electric resistance welded steel pipe”.
  • the as-rolled electric resistance welded steel pipe refers to an electric resistance welded steel pipe which is not subjected to heat treatment other than seam heat treatment after pipe-making.
  • the "pipe-making” refers to a process of making an open pipe by roll-forming of a hot-rolled steel sheet and forming an electric resistance welded portion by electric resistance welding of abutting portions of the obtained open pipe.
  • roll-forming refers to forming of a hot-rolled steel sheet into an open pipe shape by bending work.
  • An electric resistance welded steel pipe (i.e., an as-rolled electric resistance welded steel pipe for a line pipe) of the disclosure includes a base metal portion and an electric resistance welded portion, wherein a chemical composition of the base metal portion consists of, in terms of% by mass: from 0.01 to 0.10% of C, from 0.01 to 0.40% of Si, from 0.50 to 2.00% of Mn, from 0 to 0.030% of P, from 0 to 0.0015% of S, from 0.010 to 0.050% of Al, from 0.0030 to 0.0080% of N, from 0.010 to 0.050% of Nb, from 0.005 to 0.020% of Ti, from 0 to 0.20% of Ni, from 0 to 0.20% of Mo, from 0 to 0.0050% of Ca, from 0 to 1.00% of Cr, from 0 to 0.100% of V, from 0 to 1.00% of Cu, from 0 to 0.0050% of Mg, from 0 to 0.0100% of REM, and the balance being Fe
  • the base metal portion refers to a portion other than the electric resistance welded portion and a heat affected zone in the electric resistance welded steel pipe.
  • the heat affected zone (hereinafter also referred to as "HAZ”) refers to a portion affected by heat caused by electric resistance welding (affected by heat caused by the electric resistance welding and seam heat treatment in a case in which the seam heat treatment is performed after the electric resistance welding).
  • the maximum Vickers hardness of the inner surface layer of the base metal portion means a value measured as follows.
  • the maximum value among the obtained nine measurement results is regarded as the maximum Vickers hardness of the inner surface layer of the base metal portion.
  • the maximum Vickers hardness of the inner surface layer of the base metal portion is, approximately speaking, a maximum Vickers hardness in the vicinity of the inner peripheral surface of the base metal portion.
  • the maximum Vickers hardness of the outer surface layer of the base metal portion means a value measured in the same way as the maximum Vickers hardness of the inner surface of the base metal portion described above except that the "inner peripheral surface” is read as the "outer peripheral surface”.
  • the maximum Vickers hardness of the outer surface layer of the base metal portion is, approximately speaking, a maximum Vickers hardness in the vicinity of the outer peripheral surface of the base metal portion.
  • the electric resistance welded steel pipe of the disclosure has a certain amount of strength (i.e., YS and TS in the ranges described above) and has excellent SSC resistance.
  • HIC hydrogen induced cracking
  • SSC sulfuride stress cracking
  • the electric resistance welded steel pipe may have poor SSC resistance.
  • the maximum Vickers hardness of the inner surface layer of the base metal portion is 248 HV or less, and the maximum Vickers hardness of the inner surface layer of the base metal portion is smaller than the maximum Vickers hardness of the outer surface layer of the base metal portion by 5 HV or more.
  • SSC tends to be easily generated as the strength of the electric resistance welded steel pipe becomes higher.
  • the YS is limited to 550 MPa or less, and the TS is limited to 625 MPa or less, respectively. As a result, the SSC resistance is improved.
  • the maximum Vickers hardness of the inner surface layer of the base metal portion is smaller than the maximum Vickers hardness of the outer surface layer of the base metal portion by 5 HV or more, so that the maximum Vickers hardness of the outer surface layer of the base metal portion is relatively secured to a certain degree.
  • a certain amount of high strength (specifically, YS of 415 MPa or more, and TS of 461 MPa or more) is secured as the entire electric resistance welded steel pipe.
  • the maximum Vickers hardness of the inner surface layer of the base metal portion was almost the same as the maximum Vickers hardness of the outer surface layer of the base metal portion, and the condition that "the maximum Vickers hardness of the inner surface layer of the base metal portion is smaller than the maximum Vickers hardness of the outer surface layer of the base metal portion by 5 HV or more" was not satisfied owing to the following circumstances.
  • the electric resistance welded steel pipe is produced by using a hot coil consisting of a hot-rolled steel sheet as a raw material and subjecting the hot-rolled steel sheet, uncoiled from the hot coil, to pipe-making (i.e., roll-forming and electric resistance welding).
  • a hot coil consisting of a hot-rolled steel sheet as a raw material and subjecting the hot-rolled steel sheet, uncoiled from the hot coil, to pipe-making (i.e., roll-forming and electric resistance welding).
  • first surface One of two surfaces of the hot-rolled steel sheet uncoiled from the hot coil
  • second surface becomes an inner surface of the electric resistance welded steel pipe.
  • a production process of the hot coil includes respective stages of hot-rolling, cooling, and coiling in this order.
  • this cooling was conventionally performed by water-cooling the two surfaces of the hot-rolled steel sheet obtained by hot-rolling at cooling rates which are almost the same.
  • the maximum Vickers hardness of the inner surface layer of the base metal portion was almost the same as the maximum Vickers hardness of the outer surface layer of the base metal portion (i.e., the condition that "the maximum Vickers hardness of the inner surface layer of the base metal portion is smaller than the maximum Vickers hardness of the outer surface layer of the base metal portion by 5 HV or more" was not satisfied).
  • the present inventors succeeded in making the maximum Vickers hardness of the inner surface layer of the base metal portion smaller than the maximum Vickers hardness of the outer surface layer of the base metal portion by 5 HV or more by providing a difference between the cooling rates for the two surfaces when the two surfaces of the hot-rolled steel sheet obtained by hot-rolling are cooled (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). Furthermore, the present inventors found that, virtually, the warpage of the hot-rolled steel sheet after cooling is not matter too much because the cooled hot-rolled steel sheet is subsequently coiled.
  • the chemical composition of the base metal portion, the metallographic microstructure of the base metal portion, and being the as-rolled electric resistance welded steel pipe also contribute to the achievement of the YS in the range described above and the TS in the range described above.
  • 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%, and still more preferably 0.08% or less.
  • Si from 0.01 to 0.40%
  • the Si deoxidizes steel. In a case in which a Si content is too low, the effect cannot be obtained. Accordingly, the Si content is 0.01% or more.
  • the Si content is preferably 0.02% or more, and still more preferably 0.10% or more.
  • the Si content is 0.40% or less.
  • the Si content is preferably 0.38% or less, and more preferably 0.35% or less.
  • Mn from 0.50 to 2.00%
  • Mn enhances the hardenability of steel and enhances the strength of steel. In a case in which a Mn content is too low, the effect cannot be obtained. Accordingly, 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, and more preferably 1.50% or less.
  • a P content is preferably 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.
  • the P content may be 0%. From the viewpoint of reducing a dephosphorization cost, the P content may be more than 0%, and may be 0.001% or more.
  • S is an impurity.
  • S binds to Mn to form a Mn-based sulfide.
  • the Mn-based sulfide is diffluent.
  • the toughness and SSC resistance of steel are decreased.
  • a 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.
  • the S content may be 0%.
  • the S content may be more than 0%, may be 0.0001% or more, and may be 0.0003% or more.
  • Al deoxidizes steel. In a case in which an Al content is too low, the effect cannot be obtained. Accordingly, 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.
  • the Al content is preferably 0.040% or less, more preferably 0.035% or less, and still more preferably 0.030% or less.
  • the Al content herein means the content of total Al in the steel.
  • N enhances the strength of steel by solid-solution strengthening. In a case in which a N content is too low, the effect cannot be obtained. Accordingly, the N content is 0.0030% or more.
  • the N content is 0.0080% or less.
  • the N content is preferably 0.0070% or less, more preferably 0.0060% or less, and still more preferably 0.0040% or less.
  • Nb from 0.010 to 0.050%
  • Nb binds to C and N in the steel to form a fine Nb carbonitride.
  • the fine Nb carbonitride enhances the strength of steel by dispersion strengthening. In a case in which a Nb content is too low, the effect cannot be obtained. Accordingly, 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 0.050% or less.
  • the Nb content is preferably 0.045% or less, and more preferably 0.040% or less.
  • Ti binds to N in the steel to form a Ti nitride and/or Ti carbonitride.
  • the Ti nitride and/or Ti carbonitride refines crystal grains of the steel. In a case in which a Ti content is too low, the 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 from 0 to 0.20%
  • Ni is an optional element and may not be contained. In other words, a Ni content may be 0%.
  • Ni enhances the strength of steel by solid-solution strengthening. Ni further enhances the toughness of steel. From the viewpoint of the effect, the Ni content is preferably more than 0%, more preferably 0.001% or more, more preferably 0.005% or more, still more preferably 0.01% or more, and still more preferably 0.05% or more.
  • the Ni content is 0.20% or less.
  • the Ni content is preferably 0.18% or less, and still more preferably 0.15% or less.
  • Mo is an optional element and may not be contained. In other words, a Mo content may be 0%.
  • Mo In a case in which Mo is contained, Mo enhances the hardenability of steel and enhances the strength of steel. Furthermore, since micro segregation of Mo is difficult to be generated, generation of HIC caused by center segregation is suppressed. From the viewpoint of the effect, the Mo content is preferably more than 0%, more preferably 0.10% or more, and still more preferably 0.12% or more.
  • the Mo content is 0.20% or less.
  • the Mo content is preferably 0.18% or less, and more preferably 0.15% or less.
  • Ca is an optional element and may not be contained. In other words, a Ca content may be 0%.
  • Ca makes the form of MnS that becomes a initiating point of generation of SSC into a spherical shape and suppresses the generation of SSC.
  • Ca further forms CaS and suppresses 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 still more preferably 0.0020% or more.
  • the Ca content is 0.0050% or less.
  • the Ca content is preferably 0.0045% or less.
  • Cr is an optional element and may not be contained. In other words, a Cr content may be 0%.
  • the Cr content is preferably more than 0%, and more preferably 0.01% or more.
  • 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 from 0 to 0.100%
  • V is an optional element and may not be contained. In other words, a V content may be 0%.
  • V contributes to improvement in toughness.
  • the V content is preferably more than 0%, more preferably 0.001% or more, and still more preferably 0.005% or more.
  • the V content is 0.100% or less.
  • the V content is preferably 0.070% or less, more preferably 0.050% or less, and still more preferably 0.030% or less.
  • Cu is an optional element and may not be contained. In other words, a Cu content may be 0%.
  • Cu contributes to improvement in the strength of the base metal portion.
  • the Cu content is preferably more than 0%, more preferably 0.01% or more, and still more preferably 0.05% or more.
  • the Cu content is 1.00% or less.
  • the Cu content is preferably 0.70% or less, more preferably 0.50% or less, and still more preferably 0.30% or less.
  • Mg is an optional element and may not be contained. In other words, a Mg content may be 0%.
  • Mg functions as a deoxidizer and a desulfurizer. Moreover, Mg forms a fine oxide and also contributes to improvement in the toughness of an HAZ. From the viewpoint of the effect, the Mg content is preferably more than 0%, more preferably 0.0001% or more, and still more preferably 0.0010% or more.
  • the Mg content is 0.0050% or less.
  • the Mg content is preferably 0.0030% or less.
  • REM is an optional element and may not be contained. In other words, an REM content may be 0%.
  • REM refers to a rare earth element, i.e., at least one element selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.
  • REM functions as a deoxidizer and a desulfurizer.
  • the REM content is preferably more than 0%, more preferably 0.0001% or more, and still more preferably 0.0010% or more.
  • the REM content is 0.0100% or less.
  • the REM content is preferably 0.0070% or less, and more preferably 0.0050% or less.
  • the chemical composition of the base metal portion may contain one or more selected from the group consisting of: more than 0% but equal to or less than 0.20% of Ni, more than 0% but equal to or less than 0.20% of Mo, more than 0% but equal to or less than 0.0050% of Ca, more than 0% but equal to or less than 1.00% of Cr, more than 0% but equal to or less than 0.100% of V, more than 0% but equal to or less than 1.00% of Cu, more than 0% but equal to or less than 0.0050% of Mg, and more than 0% but equal to or less than 0.0100% of REM.
  • the balance excluding each element described above is Fe and impurities.
  • the impurities refer to components which are contained in a raw material (for example, ore, scrap, and the like) or mixed into in a production step, and which are not intentionally incorporated into a steel.
  • impurities examples include any elements other than the elements described above. Elements as the impurities may be only one kind, or may be two or more kinds.
  • impurities examples include O, B, Sb, Sn, W, Co, As, Pb, Bi, and H.
  • O is preferably controlled to have a content of 0.006% or less.
  • Sb, Sn, W, Co, or As may be included in a content of 0.1% or less
  • Pb or Bi may be included in a content of 0.005% or less
  • B may be included in a content of 0.0003% or less
  • H may be included in a content of 0.0004% or less
  • the contents of the other elements need not particularly be controlled as long as being in a usual range.
  • an areal ratio of polygonal ferrite (hereinafter also referred to as "ferrite fraction") is from 80 to 98%, and the balance is composed of at least one of bainite or pearlite.
  • a YS of 550 MPa or less and a TS of 625 MPa or less can be achieved by allowing a ferrite fraction to be 80% or more.
  • the ferrite fraction is preferably 81% or more, and more preferably 82% or more.
  • a YS of 415 MPa or more and a TS of 461 MPa or more can be achieved by allowing a ferrite fraction to be 98% or less.
  • the ferrite fraction is preferably 97% or less, and more preferably 95% or less.
  • the balance in the metallographic microstructure of the base metal portion is composed of at least one of bainite or pearlite.
  • the SSC resistance is improved compared to a case in which the balance contains, for example, martensite.
  • pearlite herein includes pseudo-pearlite.
  • the above-described metallographic microstructure of the base metal portion relates to the electric resistance welded steel pipe of the disclosure being an as-rolled electric resistance welded steel pipe (i.e., not subjected to heat treatment other than seam heat treatment after pipe-making).
  • martensite may be formed as the metallographic microstructure of the base metal portion.
  • the electric resistance welded steel pipe in this case has poor SSC resistance.
  • the measurement of the ferrite fraction and the identification of the balance in the metallographic microstructure of the base metal portion are performed as follows.
  • a metallographic microstructure of the central portion of the wall thickness in an L cross-section at a base metal 180° position is nital-etched, and micrographs of the nital-etched metallographic microstructure (hereinafter also referred to as "metallographic micrographs") are observed with a scanning electron microscope (SEM) at a magnification of 500 times.
  • SEM scanning electron microscope
  • Metallographic micrographs corresponding to ten 500-times visual fields (corresponding to actual cross-sectional area of 0.48 mm 2 ) are taken.
  • the measurement of the ferrite fraction and the identification of the balance are performed by performing image processing of the metallographic micrographs that were taken.
  • the image processing is performed using, for example, a small-sized general-purpose image analysis apparatus LUZEX AP manufactured by NIRECO CORPORATION.
  • Fig. 1 is a scanning electron micrograph (SEM micrograph; a magnification of 500 times) showing an example of a metallographic microstructure of a base metal portion in the disclosure
  • Fig. 2 is a SEM micrograph (a magnification of 2,000 times) obtained by enlarging a region of Fig. 1 .
  • the SEM micrograph (500 times) in Fig. 1 is one (one visual field) of SEM micrographs used in the measurement of the ferrite fraction and the identification of the balance in Test Number 22 described later.
  • the metallographic microstructure according to this example is a metallographic microstructure which is mainly composed of polygonal ferrite and in which the balance is pearlite.
  • the metallographic microstructure is revealed to be a metallographic microstructure which is not subjected to heat treatment after pipe-making (i.e., a metallographic microstructure of an as-rolled electric resistance welded steel pipe).
  • a maximum Vickers hardness of an inner surface layer of the base metal portion is 248 HV or less, and the maximum Vickers hardness of the inner surface layer of the base metal portion is smaller than a maximum Vickers hardness of an outer surface layer of the base metal portion by 5 HV or more.
  • a difference obtained by subtracting the maximum Vickers hardness of the inner surface layer of the base metal portion from the maximum Vickers hardness of the outer surface layer of the base metal portion is hereinafter also referred to as an "outer-inner hardness difference".
  • the maximum Vickers hardness of the inner surface layer of the base metal portion is smaller than the maximum Vickers hardness of the outer surface layer of the base metal portion by 5 HV or more
  • the outer-inner hardness difference is 5 HV or more
  • the maximum Vickers hardness of the inner surface layer of the base metal portion exceeds 248 HV, the toughness of steel is decreased, and the SSC resistance of the electric resistance welded steel pipe is decreased. Accordingly, 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 more improving the strength of the electric resistance welded steel pipe (i.e., YS and TS), the maximum Vickers hardness of the inner surface layer is preferably 175 HV or more, more preferably 180 HV or more, and still more preferably 185 HV or more.
  • the outer-inner hardness difference is less than 5 HV, depending on the value of the maximum Vickers hardness of the inner surface layer of the base metal portion, at least one of the deterioration of the SSC resistance, the deficiency of the YS, or the deficiency of the TS occurs. Accordingly, the outer-inner hardness difference is 5 HV or more, and preferably 6 HV or more.
  • the upper limit of the outer-inner hardness difference is not particularly restricted. From the viewpoint of the production suitability of the electric resistance welded steel pipe, the outer-inner hardness difference is preferably 20 HV or less, more preferably 15 HV or less, and still more preferably 10 HV or less.
  • the maximum Vickers hardness of the outer surface layer of the base metal portion may satisfy the maximum Vickers hardness of the inner surface layer of the base metal portion and the outer-inner hardness difference described above, and others are not particularly restricted.
  • the maximum Vickers hardness of the outer surface layer of the base metal portion is preferably from 180 MPa to 250 MPa, and more preferably from 210 MPa to 230 MPa.
  • the maximum Vickers hardness of the inner surface layer of the base metal portion is smaller than the maximum Vickers hardness of the outer surface layer of the base metal portion by 5 HV or more.
  • the maximum Vickers hardness of the inner surface layer may be lower than the maximum Vickers hardness of the outer surface layer by 5 HV or more in not only the base metal portion but also the electric resistance welded portion.
  • the maximum Vickers hardness of the inner surface layer may be lower than the maximum Vickers hardness of the outer surface layer by 5 HV or more also in the electric resistance welded portion.
  • the electric resistance welded steel pipe of the disclosure has a yield strength in a pipe axis direction (YS) of from 415 to 550 MPa.
  • a YS of 415 MPa or more secures the strength as the electric resistance welded steel pipe for a line pipe.
  • the YS is preferably 430 MPa or more.
  • a YS of 550 MPa or less is advantageous in view of the improvement in the SSC resistance or a bending deformation property and the suppression of buckling in the case of laying a pipeline formed using the electric resistance welded steel pipe for a line pipe.
  • the YS is preferably 530 MPa or less.
  • the electric resistance welded steel pipe of the disclosure has a tensile strength in a pipe axis direction (TS) of from 461 to 625 MPa.
  • a TS of 461 MPa or more secures the strength as the electric resistance welded steel pipe for a line pipe.
  • the TS is preferably 500 MPa or more, and more preferably 510 MPa or more.
  • a TS of 625 MPa or less is advantageous in view of the improvement in the SSC resistance or a bending deformation property and the suppression of buckling in the case of laying a pipeline formed using the electric resistance welded steel pipe for a line pipe.
  • the TS is preferably 620 MPa or less.
  • the YS and the TS are measured by the following method.
  • a full thickness tensile test specimen is sampled from the base metal 90° position of the electric resistance welded steel pipe. Specifically, the tensile test specimen is sampled such that a longitudinal direction of the tensile test specimen is parallel to the pipe axis direction of the electric resistance welded steel pipe and the shape of a cross-section of the tensile test specimen (i.e., a cross-section parallel to a width direction and a thickness direction of the tensile test specimen) is an arcuate shape.
  • Fig. 3 is a schematic front view of a tensile test specimen used for a tensile test.
  • a unit of numerical values in Fig. 3 is mm.
  • the length of a parallel part of the tensile test specimen is set to be 50.8 mm, and the width of the parallel part is set to be 38.1 mm.
  • the tensile test is conducted using the tensile test specimen in conformity with standard API, specification 5CT at ordinary temperature.
  • the YS and the TS are determined based on the test result.
  • a YR of 95% or less is advantageous in view of the suppression of buckling in the case of laying a pipeline formed using the electric resistance welded steel pipe for a line pipe.
  • the wall thickness of the electric resistance welded steel pipe of the disclosure is preferably from 10 to 25 mm.
  • the wall thickness is more preferably 12 mm or more.
  • a wall thickness of 25 mm or less is advantageous in view of the production suitability of the electric resistance welded steel pipe (specifically, formability in formation of a hot-rolled steel sheet into a pipe shape).
  • the wall thickness is more preferably 20 mm or less.
  • the outer diameter of the electric resistance welded steel pipe of the disclosure is preferably from 114.3 to 660.4 mm (i.e., 4.5 to 26 inches).
  • the outer diameter is preferably 152.4 mm (i.e., 6 inches) or more, and more preferably 254 mm (i.e., 10 inches) or more.
  • the outer diameter is preferably 609.6 mm (i.e., 24 inches) or less, and more preferably 508 mm (i.e., 20 inches) or less.
  • One example of a method of producing the electric resistance welded steel pipe of the disclosure is the following production method A.
  • the production method A includes:
  • the electric resistance welded steel pipe in which the hardness of the inner peripheral surface is lower than the hardness of the outer peripheral surface is easily produced, and therefore, the electric resistance welded steel pipe of the disclosure having an outer-inner hardness difference of 5 HV or more is easily produced.
  • the step of preparing a slab is a step of preparing a slab having the chemical composition described above.
  • the step of preparing a slab may be a step of producing a slab or a step of simply preparing a slab produced in advance.
  • molten steel having the chemical composition described above is produced, and a slab is produced using the produced molten steel.
  • the slab may be produced by continuous casting, or the slab may be produced by producing an ingot using molten steel and breaking down the ingot.
  • the hot-rolling step is a step of heating the prepared slab described above and hot-rolling the heated slab, thereby obtaining a hot-rolled steel sheet.
  • the heating temperature in heating the slab is preferably from 1,100 to 1,250°C.
  • the heating temperature is 1,100°C or more
  • refining of crystal grains during hot-rolling and precipitation strengthening after hot-rolling easily proceed, and therefore, the strength of steel is easily improved.
  • the heating temperature is 1,250°C or less, since coarsening of austenite grains can be more suppressed, crystal grains are easily refined, and therefore, the strength of steel is easily improved.
  • the heating of the slab is performed by, for example, a heating furnace.
  • a hot-rolled steel sheet is obtained by hot-rolling the heated slab described above.
  • finish rolling temperature (hereinafter also referred to as “finish rolling temperature”) is from 780 to 830°C.
  • the hot-rolling is generally performed using a rough rolling mill and a finish rolling mill.
  • Both the rough rolling mill and the finish rolling mill generally include multiple rolling stands in a row, and each of the rolling stands includes a pair of rolls.
  • the finish rolling temperature i.e., finish rolling finishing temperature
  • the finish rolling temperature is a surface temperature of the hot-rolled steel sheet at the exit side of a final stand of the finish rolling mill.
  • the finish rolling temperature is 780°C or more, since the rolling resistance of the steel sheet can be reduced, the productivity is improved.
  • the finish rolling temperature is 780°C or more
  • a phenomenon in which rolling is performed in a two-phase region of ferrite and austenite is suppressed, and the formation of a banded structure and the decrease in mechanical properties associated with the phenomenon can be suppressed.
  • the rolling reduction in an austenite non-recrystallization temperature region is preferably from 70 to 80%. In this case, a non-recrystallization structure is refined.
  • the cooling step is a step of cooling a first surface of the hot-rolled steel sheet at a cooling rate V1 and cooling a second surface which is the opposite side of the first surface of the hot-rolled steel sheet at a cooling rate V2 which is slower than the cooling rate V1.
  • the first surface may be an upper surface (a surface on the opposite side with respect to the gravity direction, the same shall apply hereinafter) and the second surface may be a lower surface (a surface oriented in the gravity direction, the same shall apply hereinafter), or the first 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 preferably include water-cooling.
  • the hot-rolled steel sheet may be water-cooled immediately after the hot-rolling, or the hot-rolled steel sheet immediately after the hot-rolling may be first air-cooled and then water-cooled.
  • the cooling rate V1 and the cooling rate V2 preferably satisfy the following Formula (1).
  • the hot-rolled steel sheet in which the hardness of the second surface is lower than the hardness of the first surface is more easily produced, and therefore, the electric resistance welded steel pipe of the disclosure having an outer-inner hardness difference of 5 HV or more is more easily produced.
  • V1 represents the cooling rate V1 (°C/s)
  • V2 represents the cooling rate V2 (°C/s).
  • the cooling rate V1 is preferably from 5 to 25°C/s.
  • the cooling rate V2 is not particularly limited. From the viewpoint of more increasing the strength of the electric resistance welded steel pipe (YS and TS), the cooling rate V2 is preferably 0.5°C/s or more, and more preferably 0.8°C/s or more.
  • the cooling rate V1 and the cooling rate V2 can be adjusted by, for example, adjusting a water flow density in a water-cooling apparatus for performing water-cooling. For example, on the presupposition that the water flow density on the second surface side is made smaller than the water flow density on the first surface side (i.e., V2 ⁇ V1), in order to satisfy the above Formula (1), the water flow density on the second surface side and the water flow density on the first surface side are respectively independently adjusted.
  • the coiling step is a step of coiling the hot-rolled steel sheet cooled in the cooling step, thereby obtaining a hot coil consisting of the hot-rolled steel sheet.
  • the surface temperature of the hot-rolled steel sheet at the start of coiling (hereinafter also referred to as "coiling temperature”) is preferably 620°C or less, and more preferably 600°C or less.
  • the strength of steel can be more improved.
  • the lower limit of the coiling temperature is not particularly limited.
  • the coiling temperature is preferably 500°C or more, and more preferably 530°C or more.
  • the pipe-making step is a step of uncoiling the hot-rolled steel sheet from the hot coil, roll-forming the uncoiled hot-rolled steel sheet in a direction such that the first surface is an outer peripheral surface and the second surface is an inner surface to thereby make an open pipe, and subjecting abutting portions of the obtained open pipe to electric resistance welding to form an electric resistance welded portion, thereby obtaining an electric resistance welded steel pipe.
  • the pipe-making step can be performed in accordance with a known method except the roll-forming in the direction such that the first surface is an outer peripheral surface and the second surface is an inner surface.
  • Fig. 4 is a schematic perspective view showing an example of a pipe-making step.
  • a hot-rolled steel sheet uncoiled from a hot coil is roll-formed using a forming roll (not shown in the drawing) in a direction such that a first surface is an outer peripheral surface 1 and a second surface is an inner peripheral surface 2, thereby making an open pipe. Abutting portions 3 of the open pipe are subjected to electric resistance welding using a power feed terminal 60 and a welding roll 70, thereby obtaining an electric resistance welded steel pipe 200.
  • the production method A may include other steps, if necessary.
  • Examples of the other steps include a step of subjecting the electric resistance welded portion of the electric resistance welded steel pipe to seam heat treatment after the pipe-making step, and a step of adjusting the shape of the electric resistance welded steel pipe by a sizing roll after the pipe-making step.
  • Slabs were produced by continuous casting of molten steel having chemical compositions of Steel A to Steel O set forth in Table 1.
  • REM in Steel L is specifically Ce.
  • Each of the slabs described above was heated in a heating furnace, the heated slab was hot-rolled using multiple hot rolling mills to obtain a hot-rolled steel sheet, the obtained hot-rolled steel sheet was air-cooled and then water-cooled, and the water-cooled hot-rolled steel sheet was coiled, whereby a hot coil consisting of the hot-rolled steel sheet was obtained.
  • the heating temperature in heating the slab, the finish rolling temperature in the hot-rolling, the cooling rates in water-cooling the hot-rolled steel sheet (VI and V2), and the coiling temperature in coiling the water-cooled hot-rolled steel sheet are respectively set forth in Table 2.
  • the upper surface of the hot-rolled steel sheet was set as a first surface
  • the cooling rate of the first surface was set as V1
  • the lower surface of the hot-rolled steel sheet was set as a second surface
  • the cooling rate of the second surface was set as V2.
  • the water-cooling of the hot-rolled steel sheet was performed by spraying the upper surface (i.e., first surface) and the lower surface (i.e., second surface) of the hot-rolled steel sheet, respectively, with a water-cooling shower.
  • the water flow density of the water-cooling shower for the upper surface and the water flow density of the water-cooling shower for the lower surface were respectively adjusted, so that V1 and V2 were adjusted to be values set forth in Table 2.
  • a conventional standard condition of water-cooling is a condition of Test Number 12 (Comparative Example).
  • the hot-rolled steel sheet was uncoiled from the hot coil described above, the uncoiled hot-rolled steel sheet was roll-formed in a direction such that the first surface is an outer peripheral surface and the second surface is an inner peripheral surface of a pipe to thereby make an open pipe, and abutting portions of the obtained open pipe was subjected to electric resistance welding to form an electric resistance welded portion, thereby obtaining an electric resistance welded steel pipe (hereinafter also referred to as "electric resistance welded steel pipe before shape adjustment").
  • an electric resistance welded steel pipe i.e., as-rolled electric resistance welded steel pipe having an outer diameter of 406.4 mm and a wall thickness of 15.9 mm.
  • the above production step does not affect the chemical composition of a steel. Accordingly, the chemical composition of the base metal portion of the obtained electric resistance welded steel pipe can be considered to be the same as the chemical composition of the molten steel which is a raw material.
  • the ferrite fraction (hereinafter also referred to as "F fraction") was measured, and the kind of the balance was confirmed.
  • the maximum Vickers hardness of the inner surface layer of the base metal portion (HV) and the maximum Vickers hardness of the outer surface layer of the base metal portion (HV) were respectively measured based on the measurement method described above.
  • the outer-inner hardness difference was calculated based on the measurement result by the following Formula.
  • HV Outer-inner Hardness Difference
  • the YS (MPa) and the TS (MPa) in the pipe axis direction of the electric resistance welded steel pipe were respectively measured based on the measurement method described above.
  • a full thickness specimen having a size of 120 mm (pipe circumferential direction) ⁇ 25 mm (pipe axis direction) was sampled from the base metal 180° position of the electric resistance welded steel pipe.
  • the electric resistance welded steel pipe of each Example which satisfies the chemical composition and the metallographic microstructure of the base metal portion in the disclosure, satisfies the YS (i.e., from 415 to 550 MPa) and the TS (i.e., from 461 to 625 MPa) in the disclosure, has the maximum Vickers hardness of the inner surface layer of the base metal portion of 248 HV or less, and has the outer-inner hardness difference of 5 HV or more, had excellent SSC resistance.
  • Test Number 12 Comparative Example
  • the SSC resistance was deteriorated.
  • the reason thereof is considered that the maximum Vickers hardness of the inner surface layer exceeded the upper limit, both the TS and the YS exceeded the upper limit, and the outer-inner hardness difference was less than 5 HV.
  • Test Numbers 9, 10, and 15 are all Comparative Examples in which the TS and the YS exceeded the upper limit, and Test Numbers 25 and 26 are Comparative Examples in which the TS and the YS were lower than the lower limit.

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EP17904175.1A 2017-03-29 2017-03-29 As-rolled electric resistance welded steel pipe for line pipe Active EP3546610B1 (en)

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EP3546610A4 (en) 2020-04-29
CN110088317A (zh) 2019-08-02

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