EP3608434A1 - Widerstandsgeschweisstes stahlrohr im walzzustand für leitungsrohr und warmgewalztes stahlblech - Google Patents

Widerstandsgeschweisstes stahlrohr im walzzustand für leitungsrohr und warmgewalztes stahlblech Download PDF

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EP3608434A1
EP3608434A1 EP17914739.2A EP17914739A EP3608434A1 EP 3608434 A1 EP3608434 A1 EP 3608434A1 EP 17914739 A EP17914739 A EP 17914739A EP 3608434 A1 EP3608434 A1 EP 3608434A1
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electric resistance
hot
resistance welded
pipe
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French (fr)
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EP3608434B1 (de
EP3608434A4 (de
Inventor
Kenzo TASHIMA
Shinya Sakamoto
Shunichi Kobayashi
Hideto KAWANO
Takaaki Fukushi
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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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • 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
    • 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—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
    • 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
    • C21D2211/002—Bainite
    • 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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/001—Heat treatment of ferrous alloys containing Ni
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si

Definitions

  • the present disclosure relates to an as-rolled electric resistance welded steel pipe for a line pipe and a hot-rolled steel sheet.
  • Patent Document 1 discloses a hot-rolled steel sheet including, in terms of % by mass, from 0.02 to 0.08% of C, from 0.05 to 0.5% of Si, from 1 to 2% of Mn, from 0.03 to 0.12% of Nb, from 0.005 to 0.05% of Ti, and the balance being Fe and inevitable impurity elements, in which a pro-eutectoid ferrite fraction is from 3% to 20% and the others are a low-temperature transformation phase and pearlite of 1% or less in a microstructure at a depth of a half thickness of a wall thickness from a steel sheet surface, a number average crystal grain diameter of the whole of the microstructure is from 1 ⁇ m to 2.5 ⁇ m and an area average grain diameter is from 3 ⁇ m to 9 ⁇ m, a standard deviation of the area average grain diameter is from 0.8 ⁇ m to 2.3 ⁇ m, and
  • Patent Document 1 states that the hot-rolled steel sheet described therein can be used in the production of an electric resistance welded steel pipe or a spiral steel pipe.
  • Patent Document 1 WO 2012/002481
  • UOE steel pipes produced using heavy plates for example, heavy plates having a wall thickness of 30 mm or more
  • electric resistance welded steel pipes or spiral steel pipes produced using hot coils made of hot-rolled steel sheets are used.
  • low-temperature toughness evaluated by DWTT Drop Weight Tear Test
  • DWTT Drop Weight Tear Test
  • the low-temperature toughness tends to be required for the steel pipes for a line pipe, which have a thick wall thickness. This is because the wall thickness of the steel pipes for a line pipe being thick is advantageous in the strength but disadvantageous in the low-temperature toughness.
  • a heavy plate process for producing heavy plates as materials of the UOE steel pipes has a relatively high degree of freedom with respect to production conditions.
  • low-temperature rolling is easily performed, and, for cooling after the rolling, complex controlled cooling is easily performed.
  • complex controlled cooling is easily performed.
  • fine adjustment of a metallographic microstructure by the low-temperature rolling, the complex controlled cooling, and the like has been generally performed.
  • a hot-rolling process for producing hot coils (specifically, hot-rolled steel sheets in the form of hot coils) as materials of the electric resistance welded steel pipes has a lower degree of freedom with respect to production conditions compared to the heavy plate process due to limitations of equipment focusing on the productivity.
  • a hot-rolled steel sheet after rolling is cooled to a coiling temperature (CT) of, for example, about from 400 to 600°C, and then coiled into a coil shape.
  • CT coiling temperature
  • low-temperature rolling and complex controlled cooling after the rolling are more difficult to be performed compared to the heavy plate process due to the limitations.
  • a cooling rate during the air-cooling is relatively fast.
  • a cooling rate during the air-cooling is relatively slow.
  • the metallographic microstructure may be substantially tempered during the air-cooling in the form of a hot coil.
  • the low-temperature toughness has received attention in the field of the UOE steel pipes for a line pipe, but the low-temperature toughness has received little attention for the electric resistance welded steel pipes for a line pipe.
  • the low-temperature toughness is likely to be required for the electric resistance welded steel pipes for a line pipe because of a circumstance in which a laying environment of a pipeline becomes more severe, a circumstance in which the production of electric resistance welded steel pipes having a thick wall thickness becomes possible due to the progress of a production technology of electric resistance welded steel pipes, and the like.
  • Patent Document 1 described above is one of the few documents focusing on the low-temperature toughness of hot-rolled steel sheets which may be used in the production of electric resistance welded steel pipes.
  • the low-temperature toughness may be required to be further improved.
  • An object of the disclosure is to provide an as-rolled electric resistance welded steel pipe for a line pipe, which has excellent low-temperature toughness evaluated by DWTT, and a hot-rolled steel sheet suitable for the production of the as-rolled electric resistance welded steel pipe for a line pipe.
  • 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 excellent low-temperature toughness evaluated by DWTT, and a hot-rolled steel sheet suitable for the production of the as-rolled electric resistance welded steel pipe for a line pipe are 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 0.030 to 0.120% of C, from 0.05 to 0.30% of Si, from 0.50 to 2.00% of Mn, from 0 to 0.030% of P, from 0 to 0.0100% of S, from 0.010 to 0.035% of Al, from 0.0010 to 0.0080% of N, from 0.010 to 0.080% of Nb, from 0.005 to 0.030% of Ti, from 0.001 to 0.20% of Ni, from 0.10 to 0.20% of Mo, from 0 to 0.010% of V, from 0 to 0.0030% of O, from 0 to 0.0050% of Ca, from 0 to 0.30% of Cr, from 0 to 0.30% of Cu, from 0 to 0.0050% of Mg, from 0
  • 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 electric resistance welded steel pipe of the disclosure has excellent low-temperature toughness (i.e., low-temperature toughness evaluated by DWTT).
  • Such an effect is achieved by the chemical composition of the base metal portion described above (including F1 being from 0.300 to 0.350) and the metallographic microstructure of the base metal portion described above (approximately speaking, the metallographic microstructure in which crystal grains are refined).
  • the metallographic microstructure of the base metal portion is achieved by a chemical composition and a production condition of a hot-rolled steel sheet as a material.
  • the chemical composition of the base metal portion and the metallographic microstructure of the base metal portion, and a preferred production condition of the hot-rolled steel sheet will be described later.
  • the electric resistance welded steel pipe of the disclosure has excellent low-temperature toughness.
  • the electric resistance welded steel pipe of the disclosure is suitable as, for example, one member for forming a submarine pipeline which undergoes cyclic straining due to waves or one member for forming a line pipe for cold climates.
  • the electric resistance welded steel pipe of the disclosure has a yield ratio in a pipe axis direction of from 80 to 95%
  • a yield ratio of the electric resistance welded steel pipe of 95% or less secures a plastic deformation allowance required as a steel pipe for a line pipe.
  • a yield ratio of the electric resistance welded steel pipe of 95% or less more suppresses buckling in the case of laying a pipeline formed using the electric resistance welded steel pipe by a reeling method or the like.
  • a yield ratio of the electric resistance welded steel pipe of 80% or more has excellent production suitability of the electric resistance welded steel pipe.
  • the chemical composition of the base metal portion in the disclosure (including F1 being from 0.300 to 0.350) is referred to as the "chemical composition in the disclosure”.
  • the C content is 0.030% or more.
  • the C content is preferably 0.035% or more, and more preferably 0.045% or more.
  • the C content is 0.120% or less.
  • the C content is preferably 0.110% or less.
  • TS tensile strength
  • YS yield strength
  • Si from 0.05 to 0.30%
  • 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 0.05% or more.
  • the Si content is preferably 0.10% or more, and still more preferably 0.15% or more.
  • the Si content is 0.30% or less.
  • the Si content is preferably 0.25% or less, and more preferably 0.21% 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 0.80% or more, and more preferably 1.00% 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.
  • P is an impurity. P decreases the low-temperature toughness of steel. Accordingly, a P content is preferably small. Specifically, the P content is 0.030% or less. The P content is preferably 0.020% or less, and more preferably 0.015% or less.
  • the P content may be 0%.
  • the P content may be more than 0%, may be 0.001% or more, and may be 0.005% or more.
  • S is an impurity. S binds to Mn to form a Mn-based sulfide. Thus, in a case in which a S content is too high, the low-temperature toughness and the sour resistance of steel are decreased. Accordingly, the S content is 0.0100% or less.
  • the S content is preferably 0.0080% or less, and more preferably 0.0050% or less.
  • the S content may be 0%.
  • the S content may be more than 0%, may be 0.0001% or more, may be 0.0010% or more, and may be 0.0020% 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 0.015% or more, and more preferably 0.020% 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 forms a nitride to suppress coarsening of austenite grains in a heating step.
  • the austenite grains are refined in a rolling step, and crystal grains after transformation become fine. Therefore, the low-temperature toughness of steel is enhanced.
  • N further 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 0.0010% or more.
  • the N content is preferably 0.0020% or more, and more preferably 0.0025% 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 0.0050% or less.
  • Nb from 0.010 to 0.080%
  • Nb binds to C and N in the steel to form a fine Nb carbonitride.
  • the Nb carbonitride suppresses coarsening of crystal grains, and the average crystal grain diameter becomes small. Thus, the low-temperature toughness of steel is enhanced. Furthermore, 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 0.015% or more.
  • the Nb content is 0.050% or less.
  • the Nb content is preferably 0.040% or less, and more preferably 0.030% or less.
  • Ti binds to N in the steel to form a TiN and suppress a decrease in the low-temperature toughness of steel due to a solid solution of N. Furthermore, the dispersion precipitation of the fine TiN suppresses coarsening of crystal grains. As a result, the low-temperature toughness of steel is enhanced. 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 0.030% or less.
  • the Ti content is preferably 0.020% or less, and more preferably 0.017% or less.
  • Ni from 0.001 to 0.20%
  • Ni enhances the hardenability of steel and enhances the strength of steel. In a case in which a Ni content is too low, the effect cannot be obtained. Accordingly, the Ni content is 0.001% or more.
  • the Ni content is preferably 0.01% or more, more preferably 0.05% or more, and still more preferably 0.07% or more.
  • the Ni content is 0.20% or less.
  • the Ni content is preferably 0.15% or less.
  • Mo enhances the hardenability of steel and enhances the strength of steel. Mo further refines austenite grains and enhances the low-temperature toughness of steel. In a case in which a Mo content is too low, the effect cannot be obtained. Accordingly, the Mo content is 0.10% or more. The Mo content is preferably 0.15% or more.
  • the Mo content is 0.20% or less.
  • the Mo content is preferably 0.19% or less, and more preferably 0.18% or less.
  • V is an optional element. Accordingly, a V content may be 0%.
  • V binds to C and N in the steel in a coiling step to form a fine carbonitride and enhance the strength of steel.
  • the fine V carbonitride further suppresses coarsening of crystal grains and enhances the low-temperature toughness of steel.
  • the V content may be more than 0%, may be 0.001% or more, and may be 0.002% or more.
  • the V content is more than 0.010%, the low-temperature toughness is deteriorated by coarsening of the V carbonitride. Accordingly, the V content is 0.010% or less.
  • O is an impurity. O forms an oxide and decreases the hydrogen induced cracking resistance (hereinafter, also referred to as "HIC resistance") of steel. O further decreases the low-temperature toughness of steel. Accordingly, an O content is 0.0030% or less. The O content is preferably 0.0025% or less. The O content is preferably as low as possible.
  • the O content may be 0%.
  • the O content may be more than 0%, may be 0.0001% or more, may be 0.0010% or more, may be 0.0015% or more, and may be 0.0020% or more.
  • Ca is an optional element. Accordingly, a Ca content may be 0%.
  • the Ca content may be more than 0%, may be 0.0001% or more, may be 0.0010% or more, may be 0.0015% or more, and may be 0.0020% or more.
  • the Ca content is more than 0.0050%, a coarse oxide-based inclusion is formed. Accordingly, the Ca content is 0.0050% or less.
  • the Ca content is preferably 0.0045% or less.
  • a Cr content may be 0%.
  • the Cr is an element that improves the hardenability and enhances the strength of steel. From the viewpoint of such an effect, the Cr content may be more than 0%, and may be 0.01% or more.
  • the Cr content is 0.30% or less.
  • the Cr content is preferably 0.20% or less, more preferably 0.10% or less, and still more preferably 0.05% or less.
  • Cu is an optional element. Accordingly, a Cu content may be 0%.
  • the Cu enhances the hardenability of steel and enhances the strength of steel. From the viewpoint of such an effect, the Cu content may be more than 0%, may be 0.01% or more, may be 0.05% or more, and may be 0.10% or more.
  • the Cu content is 0.30% or less.
  • the Cu content is preferably 0.25% or less, and more preferably 0.20% 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 0.010% of V, more than 0% but equal to or less than 0.0030% of Ca, more than 0% but equal to or less than 0.30% of Cr, more than 0% but equal to or less than 0.30% 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 B, Sb, Sn, W, Co, As, Pb, Bi, and H.
  • 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.
  • F1 from 0.300 to 0.350
  • F1 defined by the following Formula (1) is from 0.300 to 0.350.
  • F 1 C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 (In Formula (1), each of C, Si, Mn, Ni, Cr, Mo, V, and Nb represents % by mass of a corresponding element.)
  • F1 is correlated to the metallographic microstructure of the base metal portion (in particular, crystal grain diameter).
  • F1 is less than 0.300
  • ferrite grains since polygonal ferrite grains (hereinafter, also simply referred to as “ferrite grains”) are coarsened, the average crystal grain diameter may become large, and moreover, since the metallographic microstructure becomes a mixed-grain microstructure, the coarse crystal grain ratio may become large. Therefore, the low-temperature toughness may be deteriorated.
  • F1 is 0.300 or more.
  • F1 is preferably 0.305 or more.
  • F1 is 0.350 or less.
  • F1 is preferably 0.345 or less, and more preferably 0.340 or less.
  • F2 defined by the following Formula (2) is preferably from 0.230 to 0.300, and more preferably from 0.230 to 0.290.
  • F1 0.300 or more is more easily achieved.
  • F2 Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 (In Formula (2), each of Si, Mn, Ni, Cr, Mo, V, and Nb represents mass% of a corresponding element.)
  • the metallographic microstructure of the wall thickness direction central portion of the base metal portion (hereinafter, also referred to as the "metallographic microstructure of the base metal portion") will be described below.
  • the polygonal ferrite fraction (hereinafter, also simply referred to as "ferrite fraction") is from 60 to 90%, the average crystal grain diameter is 15 ⁇ m or less, and the coarse crystal grain ratio, which is an areal ratio of crystal grains having a crystal grain diameter of 20 ⁇ m or more, is 20% or less.
  • the ferrite fraction i.e., polygonal ferrite fraction
  • the metallographic microstructure of the wall thickness direction central portion of the base metal portion is a metallographic microstructure which is mainly composed of ferrite (i.e., polygonal ferrite).
  • the ferrite fraction In a case in which the ferrite fraction is less than 60%, the average crystal grain diameter and/or the coarse crystal grain ratio becomes too large, and therefore, the low-temperature toughness may be deteriorated. In a case in which the ferrite fraction is 60% or more, the crystal grains are refined (specifically, the average crystal grain diameter and the coarse crystal grain ratio are decreased), and therefore, the low-temperature toughness is enhanced. Accordingly, the ferrite fraction is 60% or more.
  • the ferrite fraction is preferably 65% or more, and more preferably 70% or more.
  • a metallographic microstructure having a ferrite fraction of 90% or less is easily formed. Accordingly, the ferrite fraction in the metallographic microstructure of the wall thickness direction central portion of the base metal portion is 90% or less.
  • the ferrite fraction is preferably 85% or less.
  • Average Crystal Grain Diameter 15 ⁇ m or less
  • the average crystal grain diameter is 15 ⁇ m or less.
  • the average crystal grain diameter is more than 15 ⁇ m, the low-temperature toughness is deteriorated. Accordingly, the average crystal grain diameter is 15 ⁇ m or less, and preferably 12 ⁇ m or less.
  • the lower limit of the average crystal grain diameter is not particularly restricted.
  • the average crystal grain diameter is preferably 3 ⁇ m or more, more preferably 5 ⁇ m or more, and still more preferably 8 ⁇ m or more.
  • the coarse crystal grain ratio is 20% or less.
  • the coarse crystal grain ratio herein means an areal ratio of crystal grains having a crystal grain diameter of 20 ⁇ m or more.
  • the coarse crystal grain diameter ratio is more than 20%, the low-temperature toughness is deteriorated. Accordingly, the coarse crystal grain diameter ratio is 20%.
  • the coarse crystal grain diameter ratio is preferably 18% or less, and still more preferably 15% or less.
  • the lower limit of the coarse crystal grain diameter ratio is not particularly restricted.
  • the coarse crystal grain diameter ratio is preferably 3% or more, more preferably 5% or more, and still more preferably 8% or more.
  • the ferrite fraction herein means an areal ratio of ferrite (i.e., polygonal ferrite).
  • Confirmation of the metallographic microstructure of the wall thickness direction central portion of the base metal portion herein is performed by confirming the metallographic microstructure of the wall thickness direction central portion in an L cross-section at a base metal 90° position of the electric resistance welded steel pipe.
  • the base metal 90° position refers to a position shifted from the electric resistance welded portion by 90° in a pipe circumferential direction.
  • the L cross-section refers to a cross-section parallel to a pipe axis direction and a wall thickness direction.
  • the ferrite fraction is measured by the following method.
  • a sample for observing the wall thickness direction central portion in the L cross-section at the base metal 90° position is sampled from the electric resistance welded steel pipe.
  • An observation surface of the sampled sample is polished by colloidal silica polish for from 30 to 60 minutes.
  • the polished observation surface is analyzed using EBSD-OIM (trademark) (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy), and an areal ratio of polygonal ferrite in a visual field range of 200 ⁇ m (pipe axis direction) ⁇ 500 ⁇ m (wall thickness direction), centered at the wall thickness direction central portion in the L cross-section at the base metal 90° position, is determined as the ferrite fraction.
  • a visual field magnification (observation magnification) of EBSD-OIM is 400 times, and a measurement step is 0.3 ⁇ m.
  • the ferrite fraction is determined by KAM (Kernel Average Misorientation) method equipped in EBSD-OIM.
  • a visual field range is divided into regular hexagonal pixel units, and one regular hexagonal pixel in the visual field range is selected as the central pixel.
  • one regular hexagonal pixel in the visual field range is selected as the central pixel.
  • the average value of the obtained misorientations is determined as a KAM value of the central pixel.
  • a KAM value is determined for each pixel included in the visual field range. The calculating method of these KAM values is a method which is sometimes referred to as "third approximation".
  • a KAM map indicating the KAM values of the respective pixels included in the visual field range is produced based on the above result.
  • an areal fraction of pixels having a KAM value of 1° or less with respect to the total area of the visual field range is determined as the ferrite fraction.
  • a microstructure of pixels having a KAM value of 1° or less is polygonal ferrite, and a microstructure of pixels having a KAM value of more than 1° is at least one of bainite or pearlite.
  • Fig. 1 is a KAM map used in measurement of the ferrite fraction in the electric resistance welded steel pipe according to an example of the disclosure.
  • a KAM map is typically displayed by color.
  • black parts are polygonal ferrite.
  • an areal ratio of the black parts (polygonal ferrite) with respect to the whole of Fig. 1 (the whole of the metallographic microstructure) is the polygonal ferrite fraction.
  • the average crystal grain diameter and the coarse crystal grain ratio herein are measured as follows by EBSD-OIM method.
  • a sample for observing the wall thickness direction central portion in the L cross-section at the base metal 90° position is sampled from the electric resistance welded steel pipe, and an observation surface of the sampled sample is polished by colloidal silica polish for from 30 to 60 minutes.
  • the polished observation surface is analyzed using EBSD-OIM, and an area average grain diameter in a visual field range of 200 ⁇ m (pipe axis direction) ⁇ 500 ⁇ m (wall thickness direction), the range is centered at the wall thickness direction central portion in the L cross-section at the base metal 90° position, is determined as the average crystal grain diameter.
  • An areal ratio of crystal grains having a crystal grain diameter of 20 ⁇ m or more (i.e., coarse crystal grains) with respect to the whole of the visual field range is determined as the coarse crystal grain ratio.
  • a visual field magnification (observation magnification) of EBSD-OIM is 400 times, and a measurement step is 0.3 ⁇ m.
  • orientation measurement for each measurement step of 0.3 ⁇ m is performed, and a 15° large inclination grain boundary map in which a position where a misorientation between adjacent measurement points is more than 15° is regarded as a crystal grain boundary is produced.
  • 15° is a threshold value of a high angle grain boundary and is generally recognized as a crystal grain boundary.
  • a region surrounded by the crystal grain boundaries is regarded as a crystal grain, and a grain diameter and an area of each crystal grain are respectively determined.
  • the grain diameter of each crystal grain is an equivalent circle diameter of each crystal grain.
  • an area average grain diameter is determined as the average crystal grain diameter.
  • An areal ratio of crystal grains having a crystal grain diameter of 20 ⁇ m or more (i.e., coarse crystal grains) with respect to the whole of the visual field range is determined as the coarse crystal grain ratio.
  • Fig. 2 is a 15° high angle grain boundary map used in measurement of the average crystal grain diameter and the coarse crystal grain ratio in the electric resistance welded steel pipe according to an example of the disclosure.
  • Fig. 2 shows the metallographic microstructure at the same part as Fig. 1 .
  • fine (i.e, small area) crystal grains are ferrite grains, and large area crystal grains are bainite grains or pearlite grains.
  • the balance in the metallographic microstructure of the base metal portion is preferably composed of at least one of bainite or pearlite.
  • the balance contains, for example, martensite.
  • the concept of "bainite” herein includes bainitic ferrite, upper bainite, and lower bainite.
  • the concept of "bainite” herein further includes tempered bainite formed during air-cooling after coiling the hot-rolled steel sheet (i.e., during air-cooling in the form of a hot coil).
  • pearlite herein includes pseudo-pearlite.
  • the electric resistance welded steel pipe of the disclosure is an as-rolled electric resistance welded steel pipe (i.e., an electric resistance welded steel pipe which is not subjected to heat treatment other than seam heat treatment after pipe-making).
  • the balance easily becomes at least one of bainite or pearlite.
  • martensite may be formed as the metallographic microstructure of the base metal portion.
  • the electric resistance welded steel pipe in this case tends to have poor low-temperature toughness.
  • Fig. 3 is a scanning electron micrograph (SEM micrograph; a magnification of 500 times) showing an example of the metallographic microstructure of the base metal portion in the disclosure.
  • a test specimen for observing the wall thickness direction central portion in the L cross-section at the base metal 90° position was sampled from the electric resistance welded steel pipe according to an example of the disclosure.
  • the L cross-section in the sampled test specimen was nital-etched, and a micrograph of the nital-etched metallographic microstructure (hereinafter, also referred to as "metallographic micrograph") was taken with a scanning electron microscope (SEM) at a magnification of 500 times.
  • SEM scanning electron microscope
  • the metallographic microstructure according to this example is revealed to be a metallographic microstructure which is mainly composed of ferrite (i.e., polygonal ferrite).
  • the electric resistance welded steel pipe of the disclosure has preferably a yield strength in a pipe axis direction (YS) of from 450 to 540 MPa.
  • the YS is preferably 460 MPa or more, and more preferably 480 MPa or more.
  • a YS of 540 MPa or less is advantageous in view of a bending deformation property or 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, and more preferably 520 MPa or less.
  • the electric resistance welded steel pipe of the disclosure has preferably a tensile strength in a pipe axis direction (TS) of from 510 to 625 MPa.
  • a TS of 510 MPa or more easily satisfies the strength required as the electric resistance welded steel pipe for a line pipe.
  • the TS is preferably 530 MPa or more, more preferably 540 MPa or more, and still more preferably 545 MPa or more.
  • a TS of 625 MPa or less is advantageous in view of a bending deformation property or 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, more preferably 600 MPa or less, still more preferably 590 MPa or less, and still more preferably 575 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 wall thickness direction of the tensile test specimen) is an arcuate shape.
  • Fig. 4 is a schematic front view of the tensile test specimen used for a tensile test.
  • a unit of numerical values in Fig. 4 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 (i.e., pipe axis direction 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.
  • the YR is preferably 93% or less.
  • the YR is preferably 84% or more.
  • the wall thickness of the electric resistance welded steel pipe of the disclosure is preferably from 12 to 25 mm.
  • a wall thickness of the electric resistance welded steel pipe of the disclosure of 12 mm or more improves the strength of the electric resistance welded steel pipe.
  • the wall thickness becomes thicker, a brittle fracture becomes easy to occur (i.e., the toughness is decreased).
  • the wall thickness is 12 mm or more, excellent low-temperature toughness is exhibited.
  • both the strength and the low-temperature toughness are satisfied at a higher level.
  • the wall thickness of the electric resistance welded steel pipe of the disclosure is more preferably 14 mm or more, and still more preferably 16 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 roll-forming of a hot-rolled steel sheet as a material).
  • the wall thickness is preferably less than 25 mm, more preferably 22 mm or less, and still more preferably 20 mm or less.
  • the outer diameter of the electric resistance welded steel pipe of the disclosure is preferably from 304.8 to 660.4 mm (i.e., from 12 to 26 inches).
  • An outer diameter of 304.8 mm (i.e., 12 inches) or more has excellent transport efficiency of a fluid (for example, natural gas).
  • the outer diameter is preferably 355.6 mm (i.e., 14 inches) or more, and more preferably 406.4 mm (i.e., 16 inches) or more.
  • an outer diameter of 609.6 mm (i.e., 24 inches) or less has excellent production suitability of the electric resistance welded steel pipe.
  • the outer diameter is more preferably 508 mm (i.e., 20 inches) or less.
  • hot-rolled steel sheet of the disclosure As a material of the electric resistance welded steel pipe of the disclosure (hereinafter, also referred to as the "hot-rolled steel sheet of the disclosure") will be described.
  • the hot-rolled steel sheet of the disclosure has a chemical composition which is the above-described chemical composition in the disclosure, and, in a metallographic microstructure of a wall thickness direction central portion, has a polygonal ferrite fraction of from 60 to 90%, an average crystal grain diameter of 15 ⁇ m or less, and a coarse crystal grain ratio, which is an areal ratio of crystal grains having a crystal grain diameter of 20 ⁇ m or more, of 20% or less.
  • a preferred embodiment of the chemical composition in the hot-rolled steel sheet of the disclosure is the same as a preferred embodiment of the above-described chemical composition in the disclosure (i.e., the chemical composition in the base metal portion of the electric resistance welded steel pipe of the disclosure).
  • a preferred embodiment of each of the polygonal ferrite fraction, the average crystal grain diameter, and the coarse crystal grain ratio in the hot-rolled steel sheet of the disclosure is the same as a preferred embodiment of each of the polygonal ferrite fraction, the average crystal grain diameter, and the coarse crystal grain ratio in the electric resistance welded steel pipe of the disclosure.
  • the form of the hot-rolled steel sheet of the disclosure is preferably the form of a hot coil in which the sheet is coiled into a coil shape.
  • a preferred range of the wall thickness (i.e., sheet thickness) of the hot-rolled steel sheet of the disclosure is the same as a preferred range of the wall thickness of the electric resistance welded steel pipe of the disclosure.
  • the hot-rolled steel sheet of the disclosure has a yield strength in a rolling direction (YS) of from 450 to 500 MPa and a tensile strength in the rolling direction (TS) of from 510 to 580 MPa.
  • the rolling direction in the hot-rolled steel sheet corresponds to a longitudinal direction in the hot-rolled steel sheet uncoiled from the hot coil.
  • Measurement of the YS and the TS of the hot-rolled steel sheet is performed in the same way as the measurement of the TS and the YS of the electric resistance welded steel pipe.
  • the YS of the hot-rolled steel sheet is preferably from 465 to 495 MPa.
  • the TS of the hot-rolled steel sheet is preferably from 531 to 565 MPa.
  • the YR of the hot-rolled steel sheet is preferably from 82 to 92%.
  • the YS and the TS increase by roll-forming the hot-rolled steel sheet of the disclosure.
  • the production method A of the hot-rolled steel sheet includes:
  • the heating temperature of the slab means a surface temperature of the slab.
  • the temperature of the hot-rolled steel sheet means a surface temperature of the hot-rolled steel sheet.
  • the cooling rate (VI, V2) means a cooling rate in the wall thickness direction central portion.
  • the cooling rate (VI, V2) is determined by thermal conduction calculation.
  • the chemical composition of the hot-rolled steel sheet in the form of a hot coil produced by the production method A can be considered to be the same as the chemical composition of the slab which is a raw material. The reason is that each step in the production method A does not affect the chemical composition of a steel.
  • a metallographic microstructure mainly composed of ferrite and a metallographic microstructure in which crystal grains are refined can be formed.
  • the hot-rolled steel sheet of the disclosure can be produced, in which, in the metallographic microstructure of the wall thickness direction central portion, a ferrite fraction is from 60 to 90%, an average crystal grain diameter is 15 ⁇ m or less, and a coarse crystal grain ratio is 20% or less.
  • the heating temperature in the hot-rolling step is made to be 1200°C or less, so that coarsening of crystal grains (specifically, austenite grains in a heated stage) is suppressed.
  • the hot-rolled steel sheet formed in the hot-rolling step is strong-cooled at the cooling rate V1 of 5°C/s or more to the strong-cooling stop temperature T1 of from 580 to 680°C with a time from the end of the hot-rolling (specifically, the end of finish rolling) to the start of the strong-cooling being set to 20 seconds or less, so that numerous nucleation sites are generated in a non-recrystallization structure of the hot-rolled steel sheet.
  • the strong-cooled hot-rolled steel sheet is gradual-cooled under the above condition, and then coiled under the above condition, so that fine ferrite grains are generated from each of the numerous nucleation sites generated in the strong-cooling, and a metallographic microstructure mainly composed of polygonal ferrite is formed.
  • a metallographic microstructure mainly composed of ferrite and a metallographic microstructure in which crystal grains (specifically, ferrite grains) are refined can be formed.
  • the metallographic microstructure is mainly composed of bainite
  • laths elongated microstructure
  • orientations of these laths are aligned in each block, and each block substantially becomes one crystal grain.
  • the size of the crystal grains in the metallographic microstructure mainly composed of bainite depends on the size of the prior austenite grains.
  • the metallographic microstructure is mainly composed of bainite, the crystal grains are easily coarsened.
  • CCT diagram continuous cooling transformation diagram
  • Fig. 5 is the continuous cooling transformation diagram (CCT diagram) of the hot-rolled steel sheet in the production method A.
  • F indicates a ferrite region
  • P indicates a pearlite region
  • B indicates a bainite region
  • Ar 3 indicates an Ar 3 transformation temperature
  • Ms indicates a temperature at which martensite begins to be generated.
  • the ferrite region exists at a higher temperature position than the pearlite region and the bainite region.
  • a finish rolling temperature (i.e., finish rolling finishing temperature) is a temperature equal to or more than the Ar 3 transformation temperature.
  • the hot-rolled steel sheet after the finish rolling is cooled from a temperature equal to or more than the Ar 3 transformation temperature.
  • a dashed line C1 in Fig. 5 is a cooling curve in a case in which the hot-rolled steel sheet is cooled under a conventional cooling condition.
  • the conventional cooling condition passes through all of the ferrite region, the pearlite region and the bainite region.
  • the ferrite fraction in the metallographic microstructure is decreased.
  • the metallographic microstructure mainly composed of bainite is obtained.
  • the hot-rolled steel sheet is cooled along a cooling curve of a dashed line C2.
  • the hot-rolled steel sheet is strong-cooled at the cooling rate V1 of 5°C/s or more to the strong-cooling stop temperature T1 of from 580 to 680°C with the time from the end of the hot-rolling (specifically, the end of finish rolling) to the start of the strong-cooling being set to 20 seconds or less (S31 in Fig. 5 ).
  • the strong-cooling stop temperature T1 is located in the vicinity of a ferrite nose. In a case in which the steel is rapidly cooled by the strong-cooling, numerous strains are generated in the steel, and therefore, numerous nucleation sites are generated in a non-recrystallization structure.
  • the hot-rolled steel sheet is gradual-cooled to the gradual-cooling stop temperature T2 of from 550 to 670°C (satisfying T1 > T2) (S32 in Fig. 5 ).
  • T2 the gradual-cooling stop temperature
  • the temperature of the steel is maintained in the ferrite region of Fig. 5 .
  • fine ferrite grains are generated from each of the numerous nucleation sites generated in the strong-cooling.
  • a metallographic microstructure mainly composed of fine ferrite grains (specifically, a metallographic microstructure in which the ferrite fraction is high and crystal grains are refined) is formed.
  • F1 defined by the above Formula (1) affects a position of an S curve of each phase of ferrite, pearlite, and bainite in the CCT diagram.
  • F1 is from 0.300 to 0.350.
  • the S curve of each phase is arranged at an appropriate position in the CCT diagram.
  • the hot-rolled steel sheet is cooled mainly through the ferrite region as the cooling curve C2 in Fig. 5 .
  • the ferrite fraction in the microstructure is increased, and crystal grains (i.e., ferrite grains) are refined.
  • the S curve of each phase is shifted too much to the left side.
  • the temperature of the steel enters the ferrite region before the nucleation sites are sufficiently generated.
  • ferrite grains are coarsened, and the average crystal grain diameter becomes large.
  • the metallographic microstructure is easy to become a mixed-grain microstructure, and thus, the coarse crystal grain ratio becomes large.
  • the preparation step in the production method A is a step of preparing a slab having the chemical composition in the disclosure.
  • 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 chemical composition of the slab can be considered to be the same as the chemical composition of the molten steel which is a raw material. The reason is that the step of producing a slab does not affect the chemical composition of a steel.
  • the hot-rolling step in the production method A is a step of heating the slab to a temperature of from 1060 to 1200°C and hot-rolling the heated slab, thereby obtaining a hot-rolled steel sheet.
  • a temperature at which the slab is heated (hereinafter, also referred to as "heating temperature") of 1200°C or less can refine austenite grains.
  • the heating temperature is preferably 1180°C or less.
  • a heating temperature of 1060°C or more can realize refining of crystal grains during rolling.
  • a heating temperature of 1060°C or more can realize precipitation strengthening after rolling, and therefore, the strength of the hot-rolled steel sheet can also be improved.
  • the heating temperature is preferably 1100°C or more.
  • the heating temperature of the slab means a surface temperature of the slab.
  • the temperature of the hot-rolled steel sheet means a surface temperature of the hot-rolled steel sheet.
  • the cooling rate (V1, V2) means a cooling rate in the wall thickness direction central portion, which is determined by thermal conduction calculation.
  • the hot-rolling is performed by carrying out rough rolling and finish rolling in this order for the slab heated to the above heating temperature.
  • the rough rolling and the finish rolling are performed using a rough rolling mill and a finish rolling mill, respectively.
  • Both the rough rolling mill and the finish rolling mill include multiple rolling stands in a row, and each of the rolling stands includes a pair of rolls.
  • finish rolling temperature FT 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 FT (°C) is preferably the Ar 3 transformation temperature or more.
  • the finish rolling temperature (°C) is the Ar 3 transformation temperature 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 Ar 3 transformation temperature can be 750 or more.
  • the rolling reduction in an austenite non-recrystallization temperature region is preferably from 60 to 80%. In this case, a non-recrystallization structure is refined.
  • the cooling step in the production method A is a step of strong-cooling the hot-rolled steel sheet obtained in the hot-rolling step at a cooling rate V1 of 5°C/s or more to a strong-cooling stop temperature T1 of from 580 to 680°C with a time from the end of the hot-rolling (specifically, the end of finish rolling) to the start of the strong-cooling being set to 20 seconds or less, and then gradual-cooling the hot-rolled steel sheet to a gradual-cooling stop temperature T2 of from 550 to 670°C (satisfying T1 > T2).
  • the cooling step in the production method A is performed on a ROT (Run Out Table).
  • the cooling step in the production method A may be referred to as a "ROT cooling".
  • the surface temperature of the steel sheet before the strong-cooling is not particularly limited, and is preferably the Ar 3 transformation temperature or more. In a case in which the surface temperature of the steel sheet just before the strong-cooling is the Ar 3 transformation temperature or more, coarsening of crystal grains and a decrease in the strength caused thereby can be suppressed.
  • the strong-cooling is started within 20 seconds (more preferably within 10 seconds) from the end of the hot-rolling (specifically, the end of finish rolling).
  • the strong-cooling is performed at the cooling rate V1 of 5°C/s or more.
  • the cooling rate V1 is a cooling rate at the wall thickness direction central portion.
  • the cooling rate V1 is a value calculated with thermal conduction.
  • a cooling rate V1 of 5°C/s or more makes the degree of supercooling by the cooling sufficient, and therefore, nucleation sites of ferrite are sufficiently obtained.
  • the cooling rate V1 is preferably 7°C/s or more, and more preferably 8°C/s or more.
  • the strong-cooling is performed to the strong-cooling stop temperature T1 of from 580 to 680°C.
  • a strong-cooling stop temperature T1 of 580°C or more can suppress a phenomenon in which the temperature of the hot-rolled steel sheet passes through the ferrite region and reaches the pearlite region and/or the bainite region in the CCT diagram, so that a ferrite fraction of 60% or more is easily achieved.
  • the strong-cooling stop temperature T1 is preferably 600°C or more, and more preferably 610°C or more.
  • a strong-cooling stop temperature T1 of 680°C or less can suppress a phenomenon in which Nb precipitation which strengthens pro-eutectoid ferrite is overaged, and therefore, a decrease in the strength of the hot-rolled steel sheet can be suppressed.
  • the strong-cooling stop temperature T1 is preferably 670°C or less, and more preferably 655°C or less.
  • the strong-cooling is preferably performed by water-cooling.
  • the strong-cooling is performed using, for example, a water-cooling apparatus by making a water flow density in the water-cooling apparatus higher than a usual condition.
  • the strong-cooling stop temperature T1 is, in other words, a gradual-cooling start temperature.
  • the strong-cooled hot-rolled steel sheet is gradual-cooled to the gradual-cooling stop temperature T2 of from 550 to 670°C (satisfying T1 > T2).
  • the gradual-cooling is preferably performed at a cooling rate V2 of from 2 to 4°C/s.
  • the cooling rate V2 is 2°C/s or more, since the gradual-cooling stop temperature T2 and a coiling temperature CT can be made lower, coarsening of crystal grains can be suppressed.
  • the cooling rate V2 is 4°C/s or less, since a phenomenon in which the temperature of the hot-rolled steel sheet passes through the ferrite region and reaches the pearlite region and/or the bainite region in the CCT diagram can be suppressed, a ferrite fraction of 60% or more is easily achieved.
  • the gradual-cooling is performed to the gradual-cooling stop temperature T2 of from 550 to 670°C (satisfying T1 > T2).
  • the gradual-cooling stop temperature T2 is 550°C or more, since a phenomenon in which the temperature of the hot-rolled steel sheet passes through the ferrite region and reaches the pearlite region and/or the bainite region in the CCT diagram can be suppressed, a ferrite fraction of 60% or more is easily achieved.
  • the gradual-cooling stop temperature T2 is preferably 580°C or more, and more preferably 590°C or more.
  • the gradual-cooling stop temperature T2 is 670°C or less, coarsening of crystal grains can be suppressed.
  • the gradual-cooling stop temperature T2 is preferably 650°C or less, more preferably 635°C or less, and still more preferably 620°C or less.
  • the gradual-cooling is preferably performed by water-cooling.
  • the gradual-cooling is performed using, for example, a water-cooling apparatus by making a water flow density in the water-cooling apparatus lower than the water flow density in the strong-cooling.
  • the coiling step in the production method A is a step of coiling the hot-rolled steel sheet cooled in the cooling step at a coiling temperature CT of from 500 to 600°C (satisfying T2 > CT), thereby obtaining a hot-rolled steel sheet in the form of a hot coil.
  • a cooling rate in cooling from the gradual-cooling stop temperature T2 to the coiling temperature CT is preferably from 0.1 to 1.5°C/s, more preferably from 0.3 to 1.5°C/s, and still more preferably from 0.5 to 1.5°C/s.
  • the coiling temperature CT is from 500 to 600°C.
  • the coiling temperature CT is 500°C or more, since a phenomenon in which the temperature of the hot-rolled steel sheet passes through the ferrite region and reaches the pearlite region and/or the bainite region in the CCT diagram can be suppressed, a ferrite fraction of 60% or more is easily achieved. As a result, an average crystal grain diameter of 15 ⁇ m or less and a coarse crystal grain ratio of 20% or less are easily achieved.
  • the coiling temperature CT is preferably 510°C or more, and more preferably 520°C or more.
  • the coiling temperature CT is 580°C or less, coarsening of ferrite grains can be suppressed. As a result, an average crystal grain diameter of 15 ⁇ m or less and a coarse crystal grain ratio of 20% or less are easily achieved.
  • the coiling temperature CT is preferably 590°C or less, and more preferably 580°C or less.
  • the production method X of the electric resistance welded steel pipe includes:
  • the pipe-making step in the production method X does not affect the chemical composition, the polygonal ferrite fraction, the average crystal grain diameter, and the coarse crystal grain ratio. Accordingly, the electric resistance welded steel pipe of the disclosure is produced by the production method X using the hot-rolled steel sheet of the disclosure.
  • the hot-rolled steel sheet preparation step is preferably a step of preparing the hot-rolled steel sheet of the disclosure in the form of a hot coil.
  • the hot-rolled steel sheet of the disclosure is uncoiled from the hot coil, and the uncoiled hot-rolled steel sheet of the disclosure is roll-formed.
  • the hot-rolled steel sheet preparation step may be a step of producing the hot-rolled steel sheet of the disclosure (preferably, the hot-rolled steel sheet of the disclosure in the form of a hot coil) or a step of simply preparing the hot-rolled steel sheet of the disclosure (preferably, the hot-rolled steel sheet of the disclosure in the form of a hot coil) produced in advance.
  • the hot-rolled steel sheet of the disclosure in the form of a hot coil is preferably produced in accordance with the production method A described above.
  • Each operation in the pipe-making step is not particularly limited, and can be performed in accordance with a known method.
  • the production method X of the electric resistance welded steel pipe 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.
  • Each of the slabs described above was heated in a heating furnace.
  • the heating temperature (°C) of the slab was set forth in Table 2.
  • the slab after the heating was rolled using a rough rolling mill, and was cooled to 920°C.
  • finish rolling was performed by a finish rolling mill.
  • the rolling reduction in a non-recrystallization temperature region was from 60 to 80% in all Examples and Comparative Examples.
  • the finish rolling temperature was the Ar 3 or more (specifically, 750°C or more) in all Examples and Comparative Examples.
  • the ROT cooling i.e., cooling step
  • the ROT cooling was performed by sequentially carrying out the strong-cooling and the gradual-cooling.
  • Time from the end of the finish rolling to the start of the strong-cooling was 10 seconds or less.
  • Both the strong-cooling and the gradual-cooling were performed using a water-cooling apparatus. Both the cooling rate V1 in the strong-cooling and the cooling rate V2 in the gradual-cooling were adjusted by adjusting a water flow density in the water-cooling apparatus.
  • the cooling rate V1 (°C/s) in the strong-cooling, the strong-cooling stop temperature T1 (°C), and the gradual-cooling stop temperature T2 (°C) were set forth in Table 2.
  • the cooling rate V2 (°C/s) in the gradual-cooling was in the range of from 2 to 4°C/s in all examples.
  • the hot-rolled steel sheet after the ROT cooling was cooled, and coiled at the coiling temperature CT set forth in Table 2, thereby obtaining a hot coil (i.e., the hot-rolled steel sheet in the form of a hot coil).
  • the cooling rate in cooling from the gradual-cooling stop temperature T2 (°C) to the coiling temperature CT was estimated to be from 0.5 to 1.5°C/s in all Examples and Comparative Examples.
  • the hot-rolled steel sheet was uncoiled from the hot coil described above, the uncoiled hot-rolled steel sheet was roll-formed 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 17 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 YS in the rolling direction and the TS in the rolling direction were respectively measured. Furthermore, the YR (%) in the rolling direction was calculated based on the YS in the rolling direction and the TS in the rolling direction.
  • a full thickness tensile test specimen used in the measurement of the YS and the TS was sampled from a position where a distance from one end of the hot-rolled steel sheet in a sheet width direction is 1/4 of the sheet width (i.e., a position corresponding to the base metal 90° position in the electric resistance welded steel pipe).
  • the YS in the pipe axis direction and the TS in the pipe axis direction were measured.
  • the detailed measurement method has been described above.
  • the YR (%) in the rolling direction was calculated based on the YS in the pipe axis direction and the TS in the pipe axis direction.
  • the ferrite fraction, the average crystal grain diameter, and the coarse crystal grain ratio in the metallographic microstructure of the wall thickness direction central portion of the base metal portion were respectively measured using EBSD-OIM by the method described above.
  • TSL OIM Analysis 7 As analysis software in EBSD-OIM, "TSL OIM Analysis 7" manufactured by TSL Solutions Ltd. was used.
  • the kind of the balance i.e., microstructure other than polygonal ferrite
  • the kind of the balance i.e., microstructure other than polygonal ferrite
  • the average crystal grain diameter and the coarse crystal grain ratio of the wall thickness direction central portion of the base metal portion in the electric resistance welded steel pipe after the shape adjustment by a sizing roll were measured by the method described above.
  • Fig. 6 is a schematic front view of the obtained DWTT test specimen.
  • a unit of numerical values in Fig. 6 is mm.
  • the longitudinal direction of the DWTT test specimen (a direction of a length of 300 mm) corresponds to a pipe circumferential direction of the electric resistance welded steel pipe.
  • the central portion of the DWTT test specimen in the longitudinal direction corresponds to the base metal 90° position of the electric resistance welded steel pipe.
  • a notch having a depth of 5 mm was formed at the central portion in the longitudinal direction.
  • the DWTT test was performed using the DWTT test specimen in conformity with specification ASTM E 436, and a DWTT guarantee temperature which is the lowest value of a temperature at which a percent ductile fracture is 85% or more was determined.
  • the electric resistance welded steel pipe of each Example which satisfies the chemical composition of the base metal portion in the disclosure (including F1 being from 0.300 to 0.350), and, in the metallographic microstructure of the wall thickness direction central portion of the base metal portion, has a F fraction of from 60 to 90%, an average crystal grain diameter of 15 ⁇ m or less, and a coarse crystal grain ratio of 20% or less, had a low DWTT guarantee temperature and excellent low-temperature toughness.
  • the electric resistance welded steel pipe of each Example had a YR in the range of from 80 to 95%, and was confirmed to secure a plastic deformation allowance required as a steel pipe for a line pipe.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Manufacturing & Machinery (AREA)
  • Heat Treatment Of Steel (AREA)
EP17914739.2A 2017-06-22 2017-06-22 Widerstandsgeschweisstes stahlrohr im walzzustand für leitungsrohr und warmgewalztes stahlblech Active EP3608434B1 (de)

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EP4029962A4 (de) * 2019-11-20 2022-09-21 JFE Steel Corporation Warmgewalztes stahlblech für elektrogeschweisste stahlrohre und verfahren zu deren herstellung, elektrogeschweisste stahlrohre und verfahren zu deren herstellung, leitungsrohre und baustruktur
EP4095280A4 (de) * 2020-04-02 2022-12-28 JFE Steel Corporation Elektrogeschweisstes stahlrohr und verfahren zu seiner herstellung

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JP7206792B2 (ja) * 2018-10-22 2023-01-18 日本製鉄株式会社 ラインパイプ用鋼材
JP7206793B2 (ja) * 2018-10-22 2023-01-18 日本製鉄株式会社 ラインパイプ用電縫鋼管、及び、ラインパイプ用熱延鋼板
JP7159785B2 (ja) * 2018-10-22 2022-10-25 日本製鉄株式会社 ラインパイプ用鋼材
KR102236852B1 (ko) * 2018-11-30 2021-04-06 주식회사 포스코 우수한 저항복비 및 저온인성 특성을 가지는 구조용강 및 그 제조방법
KR102630980B1 (ko) * 2019-08-23 2024-01-30 닛폰세이테츠 가부시키가이샤 라인 파이프용 전봉 강관
JP7315834B2 (ja) * 2019-09-10 2023-07-27 日本製鉄株式会社 ラインパイプ用電縫鋼管、及び、ラインパイプ用熱延鋼板
US20220373108A1 (en) * 2019-10-31 2022-11-24 Jfe Steel Corporation Electric resistance welded steel pipe, method for producing the same, line pipe, and building structure
EP4066954A4 (de) * 2020-02-10 2023-07-05 Nippon Steel Corporation Elektrisch widerstandsgeschweisstes stahlrohr zur verwendung für leitungsrohre
US20240026999A1 (en) 2020-10-05 2024-01-25 Jfe Steel Corporation Electric resistance welded steel pipe and method for manufacturing the same
WO2023053837A1 (ja) * 2021-09-29 2023-04-06 Ｊｆｅスチール株式会社 角形鋼管およびその製造方法、熱延鋼板およびその製造方法、並びに建築構造物

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EP4095280A4 (de) * 2020-04-02 2022-12-28 JFE Steel Corporation Elektrogeschweisstes stahlrohr und verfahren zu seiner herstellung

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JPWO2018235244A1 (ja) 2019-06-27
WO2018235244A1 (ja) 2018-12-27
CN110546289A (zh) 2019-12-06
EP3608434A4 (de) 2020-09-02
JP6260757B1 (ja) 2018-01-17

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