EP3960891B1 - Widerstandsgeschweisstes stahlrohr für leitungsrohre - Google Patents

Widerstandsgeschweisstes stahlrohr für leitungsrohre Download PDF

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Publication number
EP3960891B1
EP3960891B1 EP19942655.2A EP19942655A EP3960891B1 EP 3960891 B1 EP3960891 B1 EP 3960891B1 EP 19942655 A EP19942655 A EP 19942655A EP 3960891 B1 EP3960891 B1 EP 3960891B1
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Prior art keywords
electric resistance
resistance welded
steel pipe
less
welded steel
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English (en)
French (fr)
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EP3960891A4 (de
EP3960891A1 (de
Inventor
Kensuke Nagai
Tatsuo Yokoi
Osamu Yoshida
Hideto KAWANO
Shunichi Kobayashi
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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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • 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
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
    • 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
    • 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
    • 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
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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

Definitions

  • the present disclosure relates to an electric resistance welded steel pipe for a linepipe.
  • pipelines which are one of the means for transporting mainly crude oil or natural gas, and linepipes used for forming these pipelines, is increasing more than ever.
  • Patent Document 1 discloses a technique in which a repeated strain is applied to a steel strip as a material, for example, by a bending-unbending process, before pipe-making, to induce Bauschinger effect, thereby achieving a decrease in the yield ratio in the pipe axis direction of the resulting electric resistance welded steel pipe.
  • Patent Document 2 discloses a technique in which a metallographic microstructure of a hot-rolled steel sheet for producing an electric resistance welded steel pipe is adjusted to a microstructure composed of a ferrite structure, and martensite having an area ratio of from 1 to 20%, thereby achieving a decrease in the yield ratio in the pipe axis direction of the electric resistance welded steel pipe.
  • Patent Document 3 discloses an electric resistance welded steel pipe for a linepipe which has certain degrees of tensile strength and yield strength, which has a decreased yield ratio, and whose base metal portion and electric resistance welded portion have excellent toughness.
  • the base metal portion has a chemical composition including, in terms of % by mass, from 0.080 to 0.120% of C, from 0.30 to 1.00% of Mn, from 0.005 to 0.050% of Ti, from 0.010 to 0.100% of Nb, from 0.001 to 0.020% of N, from 0.010 to 0.450% of Si, from 0.001 to 0.100% of Al, and the balance including Fe and impurities, wherein the value of the following CMeq is from 0.170 to 0.300, the ratio Mn/Si is 2.0 or more, and the value of the following LR is 0.210 or more; wherein the base metal portion contains ferrite in an area ratio of from 60 to 98%, and the balance including tempered bainite; where
  • Patent Document 4 discloses a thick-walled electric resistance welded steel pipe which has both a low yield ratio of 95% or less, preferably 92% or less, and a low temperature toughness, which is obtained by electric resistance welding a base steel sheet formed in the shape of a pipe, and which has a thickness /outer diameter ratio of from 4.0 to 7.0%.
  • the base steel sheet has a component composition including, in terms of % by mass: from 0.06 to 0.15% of C, from 1.00 to 1.65% of Mn, from 0.005 to 0.020% of Ti, from 0.005 to 0.030% of Nb, from 0.001 to 0.006% of N, P limited to 0.02% or less, S limited to 0.005% or less, 0.45% or less of Si, 0.08% or less of Al, less than 0.20% of Mo, 0.50% or less of Cu, 0.50% or less of Ni,1.00% or less of Cr, 0.10% or less of V, 0.0050% or less of Ca, 0.0050% or less of REM, and the balance consisting of Fe and unavoidable impurities, wherein the value of the following Ceq is from 0.32 to 0.43; wherein the base steel sheet has a metallographic microstructure which contains polygonal ferrite in an area ratio of from 50 to 92%, and the polygonal ferrite has an average particle
  • Patent Document 5 discloses an as-rolled electric resistance welded steel pipe for a linepipe which has an excellent low temperature toughness as evaluated by DWTT.
  • a base metal portion includes, 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 0.010 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, and the balance including Fe and impurities, wherein the value of the following F1 is from 0.300 to 0.350; wherein the metallographic microstructure of a central portion in a thickness direction of the base metal portion has a polygonal ferrite fraction of from 60 to 90%, an average crystal grain size of 15 ⁇ m or less, and a coarse crystal grain size, which is the area
  • Patent Document 2 there is a case in which a further improvement in the toughness of the base metal portion of the steel pipe is required.
  • Patent Document 4 describes that "there has been a finding that it is possible to reduce the precipitation of Nb carbonitrides and to form a multiphase structure, by reducing the content of Nb to a level lower than the conventional one, further optimizing hot rolling conditions and performing a two-stage accelerated cooling after the hot rolling, as a result of which a low Y/T can be ensured. " However, there is a case in which a decrease in the yield ratio of the electric resistance welded steel pipe as well as ensuring the toughness of the base metal portion and the electric resistance welded portion are required, without being limited to the technique (a reduction in Nb content) disclosed in Patent Document 4.
  • Patent Document 5 describes that "an average crystal grain size of 15 ⁇ m or less, and a coarse crystal grain size, which is the area ratio of crystal grains having a crystal grain size of 20 ⁇ m or more, of 20% or less” was achieved, by restricting the chemical composition of the base metal portion to a specific range and controlling the conditions in the hot rolling step, thereby improving the low temperature toughness as evaluated by DWTT.
  • a decrease in the yield ratio of the electric resistance welded steel pipe as well as ensuring the toughness of the base metal portion and the electric resistance welded portion are required, without being limited to the technique disclosed in Patent Document 5.
  • An object of the present disclosure is to provide an electric resistance welded steel pipe for a linepipe which has certain degrees of tensile strength and yield strength, which has a decreased yield ratio, and whose base metal portion and electric resistance welded portion have excellent toughness.
  • the present invention provides an electric resistance welded steel pipe for a linepipe which has certain degrees of tensile strength and yield strength, which has a decreased yield ratio, and whose base metal portion and electric resistance welded portion have excellent toughness.
  • a numerical range expressed by "from x to y" in the present disclosure includes the values of x and y in the range as the lower limit and upper limit values, respectively.
  • the content of a component (element) expressed by “%” in the present disclosure means “% by mass”.
  • C content may sometimes be expressed as "C content”.
  • the content of another element may be expressed similarly.
  • step in the present disclosure encompasses not only an independent step but also a step which is not clearly distinguishable from another step as long as the desired object of the step is achieved.
  • An electric resistance welded steel pipe for a linepipe according to the present invention (hereinafter, also simply referred to as “electric resistance welded steel pipe”) includes:
  • the chemical composition (including the fact that each of the value of Ceq, the ratio of the content of Mn with respect to the content of Si, and the value of LR, satisfy each requirement described above) of the base metal portion described above, is also referred to as the chemical composition in the present invention.
  • the electric resistance welded steel pipe according to the present invention includes a base metal portion and an electric resistance welded portion.
  • an electric resistance welded steel pipe is produced by: forming (hereinafter, also referred to as "roll forming") a hot-rolled steel sheet in the shape of a pipe to prepare an open pipe; subjecting the abutting portion of the thus prepared open pipe to electric resistance welding to form an electric resistance welded portion (the process up to this point is also referred to as “pipe-making”); and then performing a seam heat treatment on the electric resistance welded portion, if necessary.
  • 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.
  • heat affected zone refers to a zone which has been affected by heat due to electric resistance welding (in the case of performing a seam heat treatment after electric resistance welding, a zone affected by the electric resistance welding and the seam heat treatment).
  • a hot-rolled steel sheet which is the material of the electric resistance welded steel pipe, is produced using a hot strip mill. Specifically, a continuous hot-rolled steel sheet which has been wound in the form of a coil (hereinafter, also referred to as "hot coil”) is produced by a hot strip mill.
  • hot coil a continuous hot-rolled steel sheet which has been wound in the form of a coil
  • a hot-rolled steel sheet which is the material of the electric resistance welded steel pipe, is different from a steel plate, which is produced using a plate mill, in that the hot-rolled steel sheet is a continuous steel sheet.
  • the steel plate is not a continuous steel sheet, and thus cannot be used in roll forming, which is a continuous bending processing.
  • the electric resistance welded steel pipe is clearly distinguished from a welded steel pipe (such as a UOE steel pipe) which is produced using a steel plate in that the electric resistance welded steel pipe is produced using a hot-rolled steel sheet.
  • a welded steel pipe such as a UOE steel pipe
  • a yield elongation as measured in a tensile test in the pipe axis direction, is less than 0.2%.
  • the fact that the yield elongation is less than 0.2% means that the yield elongation is not substantially observed.
  • the electric resistance welded steel pipe according to the present invention is an electric resistance welded steel pipe which has not been subjected to any heat treatment other than the seam heat treatment, after pipe-making (namely, an electric resistance welded steel pipe as it is made (also referred to as "as-rolled electric resistance welded steel pipe”)).
  • the electric resistance welded steel pipe according to the present invention has certain degrees of tensile strength and yield strength (specifically, a yield strength in the pipe axis direction of from 360 to 600 MPa, and a tensile strength in the pipe axis direction of from 465 to 760 MPa), a decreased yield ratio (specifically, a yield ratio in the pipe axis direction of 0.90 or less), and an excellent toughness (specifically, each of the base metal portion and the electric resistance welded portion has a Charpy absorbed energy at 0°C of 100 J or more).
  • the tensile strength in the pipe axis direction is also referred to as "TS”
  • the yield strength in the pipe axis direction is also referred to as “YS”
  • the yield ratio in the pipe axis direction is also referred to as “YR”
  • decreasing the YR of the electric resistance welded steel pipe is also referred to as "achieving a decrease in YR”.
  • the electric resistance welded steel pipe according to the present invention is an electric resistance welded steel pipe as it is made, and has the above-described effects.
  • a YR of 0.90 or less contributes to achieving a decrease in YR (namely, a YR of 0.90 or less).
  • the reason for this is thought to be because an LR value of 0.25 or more allows an improvement in work hardening properties provided by C and an improvement in work hardening properties provided by Nb to be effectively achieved, as a result of which a decrease in YR is achieved.
  • the fact that the value obtained by subtracting the hardness of the first phase from the hardness of the second phase contributes to achieving a decrease in YR (namely, a YR of 0.90 or less).
  • difference in hardness a difference in hardness of 50 Hv or more causes the occurrence of non-uniform deformation due to processing strain during pipe-making, thereby exhibiting anisotropic hardening properties of the steel.
  • a decrease in YR namely, a YR of 0.90 or less
  • means for performing a heat treatment on an electric resistance welded steel pipe as it is made can possibly be used, as means for achieving a decrease in YR.
  • a decrease in YR is achieved, despite being an electric resistance welded steel pipe as it is made.
  • the difference in hardness is 100 Hv or less contributes to an improvement in the toughness of the base metal portion.
  • the reason for this is thought to be because a difference in hardness of 100 Hv or less results in a decrease in internal stress within the metallographic microstructure.
  • the metallographic microstructure of the base metal portion is mainly formed during the process of producing a hot-rolled steel sheet, which is the material of the electric resistance welded steel pipe.
  • a hot-rolled steel sheet which is the material of the electric resistance welded steel pipe.
  • the electric resistance welded steel pipe according to the present invention has a low YR, the effect of allowing a reduction in the occurrence of buckling of the electric resistance welded steel pipe is expected.
  • One example of the case in which a reduction in the occurrence of buckling of a steel pipe is required may be, for example, the case of laying a steel pipe for a submarine linepipe by reeling (hereinafter, referred to as "reel-laying").
  • reel-laying the steel pipe is produced on land in advance, and the produced steel pipe is wound on a spool on a barge ship. Thereafter, while unwinding the wound steel pipe on the sea, the steel pipe is laid on the sea bed.
  • plastic bending is applied to the steel pipe during the winding and unwinding of the steel pipe, and thus, there is a case in which the steel pipe buckles.
  • the pipe laying operation has to be stopped, which causes an enormous damage.
  • the buckling of the steel pipe can be reduced by decreasing the YR of the steel pipe.
  • the electric resistance welded steel pipe according to the present invention is expected to provide, for example, the effect of allowing a reduction in the occurrence of buckling during the reel-laying, when used as an electric resistance welded steel pipe for a submarine linepipe.
  • the electric resistance welded steel pipe according to the present invention is expected to provide an excellent effect of stopping crack propagation upon bursting, when used as an electric resistance welded steel pipe for a linepipe, because the base metal portion and the electric resistance welded portion thereof have an excellent toughness.
  • C is an element necessary for forming at least one of pearlite or bainite, and to improve the work hardening properties important for achieving a decrease in YR.
  • the C content is 0.03% or more, from the viewpoint of obtaining such an effect.
  • the area ratio of the ferrite structure may be excessively increased, failing to improve the work hardening properties. As a result, there is a case in which a decrease in YR cannot be achieved.
  • the C content is 0.10% or more
  • cementite may be formed in a large amount, possibly resulting in a decrease in the toughness of the base metal portion and the electric resistance welded portion. Accordingly, the C content is less than 0.10%.
  • the C content is preferably 0.09% or less, more preferably less than 0.08%, and still more preferably 0.07% or less.
  • Mn from 0.30% to 1.00%
  • Mn is an element which improves the hardenability of the steel. Further, Mn is also an element essential for detoxifying S. The Mn content is 0.30% or more, from the viewpoint of obtaining these effects.
  • an excessive Mn content may lead to a marked segregation at the central portion in the thickness direction, to cause the formation of MnS or the formation of a hardened phase of coarse martensite and/or bainite, possibly resulting in the impairment of the toughness of the base metal portion and the electric resistance welded portion.
  • an excessive Mn content may lead to a CNeq value of more than 0.25, and as a result, the strength may be excessively increased (specifically, it may result in a failure to achieve at least one of a YS of 600 MPa or less, or a TS of 760 MPa or less).
  • the Mn content is 1.00% or less.
  • the Mn content is preferably less than 1.00%, more preferably 0.90% or less, still more preferably 0.80% or less, and yet still more preferably 0.70% or less.
  • Ti is an element which contributes to the refinement of the crystal grain size by forming carbonitrides, and is an element necessary for ensuring the toughness of the base metal portion and the electric resistance welded portion.
  • the Ti content is 0.005% or more, from the viewpoint of obtaining such effects.
  • the Ti content is preferably 0.010% or more.
  • the Ti content is more than 0.050%, however, coarse TiN may be formed to cause a decrease in the toughness of the base metal portion and the electric resistance welded portion. Accordingly, the Ti content is 0.050% or less.
  • Nb has the effect of increasing the toughness by rolling in a non-recrystallization region at a high temperature. Further, Nb is also an element which improves the work hardening properties by precipitation strengthening (namely, an element which contributes to achieving a decrease in YR).
  • the Nb content is 0.010% or more, preferably 0.020% or more, and more preferably 0.030% or more, from the viewpoint of obtaining these effects.
  • the Nb content is more than 0.100%, however, coarse Nb carbides may be formed to cause a decrease in the toughness. Accordingly, the Nb content is 0.100% or less.
  • the Nb content is preferably 0.080% or less, and more preferably 0.060% or less.
  • N is an element which reduces the coarsening of crystal grains by forming metal nitrides, thereby improving the toughness of the base metal portion and the electric resistance welded portion.
  • the N content is 0.001% or more, from the viewpoint of obtaining such an effect.
  • the N content is more than 0.020%, however, the amount of alloy carbides formed may be increased, to cause a decrease in the toughness of the base metal portion and the electric resistance welded portion. Accordingly, the N content is 0.020% or less.
  • the N content is preferably 0.010% or less, and more preferably 0.006% or less.
  • Si from 0.010 to 0.500%
  • Si is an element which is used as a deoxidizing agent for the steel. Si reduces the formation of coarse oxides in the base metal portion and the electric resistance welded portion, thereby improving the toughness.
  • the Si content is 0.010% or more, from the viewpoint of obtaining such an effect.
  • the Si content is preferably 0.030% or more.
  • the Si content is 0.500% or less.
  • the Si content is preferably 0.400% or less, and more preferably 0.350% or less.
  • Al from 0.001 to 0.100%
  • Al is an element which is used as a deoxidizing agent, in the same manner as Si.
  • the Al content is 0.001% or more, from the viewpoint of improving the toughness of the base material and thus preventing the occurrence of cracks due to free oxygen.
  • the Al content is preferably 0.005% or more, and more preferably 0.010% or more.
  • the Al content is more than 0.100%, however, Al-based oxides may be formed during the electric resistance welding, along with which the toughness of the electric resistance welded portion may be decreased. Accordingly, the Al content is 0.100% or less.
  • the Al content is preferably 0.090% or less.
  • P is an element which can be present in the steel as impurities.
  • the P content is more than 0.030%, P may be segregated at grain boundaries, possibly resulting in the impairment of the toughness of the base metal portion and the electric resistance welded portion. Accordingly, the P content is 0.030% or less.
  • the P content may also be 0%.
  • the P content may be more than 0%, or 0.001% or more, from the viewpoint of reducing dephosphorization cost.
  • S is an element which can be present in the steel as impurities.
  • the S content is more than 0.030%, the toughness of the base metal portion and the electric resistance welded portion may be impaired. Accordingly, the S content is 0.030% or less.
  • the S content is preferably 0.020% or less, and more preferably 0.010% or less.
  • the S content may also be 0%.
  • the S content may be more than 0%, or 0.001% or more, from the viewpoint of reducing desulfurization cost.
  • Mo is an optional element.
  • the Mo content may be 0%, or more than 0%.
  • Mo is an element which has the effect of improving the hardenability of the steel, thereby improving the strength of the steel.
  • the Mo content may be 0.01% or more, from the viewpoint of obtaining such an effect.
  • the Mo content is more than 0.50%, however, Mo carbonitrides may be formed to cause a decrease in the toughness. Accordingly, the Mo content is 0.50% or less.
  • the Mo content may be 0.30% or less, or 0.10% or less.
  • Cu is an optional element.
  • the Cu content may be 0%, or more than 0%.
  • the Cu is an element effective for improving the strength of the base metal portion.
  • the Cu content may be 0.05% or more, from the viewpoint of obtaining such an effect.
  • the Cu content is more than 0.50%, however, fine Cu particles may be formed to cause a significant decrease in the toughness. Accordingly, the Cu content is 0.50% or less.
  • the Cu content may be 0.40% or less, or 0.30% or less.
  • Ni from 0 to 0.50%
  • Ni is an optional element.
  • the Ni content may be 0%, or more than 0%.
  • Ni is an element which contributes to improvements in the strength and the toughness.
  • the Ni content may be 0.05% or more, from the viewpoint of obtaining such an effect.
  • the strength may be excessively increased (specifically, it may result in a failure to achieve at least one of a YS of 600 MPa or less, or a TS of 760 MPa or less). Accordingly, the Ni content is 0.50% or less.
  • the Cr content may be 0%, or more than 0%.
  • the Cr is an element which improves the hardenability.
  • the Cr content may be 0.05% or more, from the viewpoint of obtaining such an effect.
  • the Cr content is more than 0.50%, however, Cr-based inclusions may be formed in the electric resistance welded portion, to cause a decrease in the toughness of the electric resistance welded portion. Accordingly, the Cr content is 0.50% or less.
  • the Cr content may be 0.40% or less.
  • V from 0 to 0.10%
  • V is an optional element.
  • the V content may be 0%, or more than 0%.
  • V has almost the same effects as Nb.
  • the V content may be 0.010% or more, from the viewpoint of obtaining such effects.
  • V content is more than 0.10%, however, V carbonitrides may be formed to cause a decrease in the toughness. Accordingly, the V content is 0.10% or less.
  • Ca is an optional element.
  • the Ca content may be 0%, or more than 0%.
  • Ca is an element which controls the form of sulfide-based inclusions, and thereby improving the low temperature toughness.
  • the Ca content may be 0.0001% or more, or 0.0002% or more, from the viewpoint of obtaining such an effect.
  • the Ca content is more than 0.0100%, however, CaO-CaS may form large-sized clusters or inclusions, and there is a risk of adversely affecting the toughness. Accordingly, the Ca content is 0.0100% or less.
  • the Ca content may be 0.0080% or less, or 0.0060% or less.
  • the REM is an optional element.
  • the REM content may be 0%.
  • REM refers to a rare earth element(s), namely, 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.
  • the REM has effects as a deoxidizing agent and a desulfurizing agent.
  • the REM content may be 0.0001% or more, from the viewpoint of obtaining such effects.
  • the REM content is more than 0.0100%, however, coarse oxides may be formed to cause a decrease in HIC resistance (resistance to hydrogen cracking during the electric resistance welding) as well as a decrease in the toughness of the base metal portion and HAZ. Accordingly, the REM content is 0.0100% or less.
  • the chemical composition of the base metal portion may contain at least one selected from the group consisting of more than 0% and equal to or less than 0.50% of Mo, more than 0% and equal to or less than 0.50% of Cu, more than 0% and equal to or less than 0.50% of Ni, more than 0% and equal to or less than 0.50% of Cr, more than 0% and equal to or less than 0.10% of V, more than 0% and equal to or less than 0.0100% of Ca, and more than 0% and equal to or less than 0.0100% of REM.
  • the balance excluding the respective elements described above is Fe and impurities.
  • impurities refers to components which are contained in raw materials (such as ores and scraps), or components which are mixed-in during production steps, and are not intentionally incorporated into the steel.
  • Examples of the impurities include all elements other than the elements described above. Only one kind, or two or more kinds of elements may be contained as the impurities.
  • impurities examples include B, Sb, Sn, W, Co, As, Pb, Bi, and H.
  • any of Sb, Sn, W, Co and As can be contained, for example, in a content of 0.1% or less, any of Pb and Bi can be contained, for example, in a content of 0.005% or less, B can be contained, for example, in a content of 0.0003% or less, and H can be contained, for example, in a content of 0.0004% or less, as the impurities.
  • the contents of other elements need not be particularly controlled, as long as the contents are within normal ranges.
  • CNeq represented by Formula (1) is from 0.12 to 0.25.
  • CNeq C + Mn / 6 + Cr / 5 + Ni + Cu / 15 + Nb + Mo + V
  • each element symbol represents the content of each element in % by mass.
  • the value of CNeq is less than 0.12, it may result in a failure to achieve at least one of a YS of 360 MPa or more or a TS of 465 MPa or more. Accordingly, the value of CNeq is 0.12 or more.
  • the value of CNeq is preferably 0.15 or more, from the viewpoint of improving at least one of YS or TS of the electric resistance welded steel pipe.
  • the strength may be excessively increased (specifically, it may result in a failure to achieve at least one of a YS of 600 MPa or less, or a TS of 760 MPa or less). Accordingly, the value of CNeq is 0.25 or less.
  • the ratio Mn/Si (namely, the ratio of the content of Mn with respect to the content of Si; hereinafter also referred to as "Mn/Si") is 1.8 or more.
  • the toughness of the electric resistance welded portion may be decreased, due to the formation of MnSi-based inclusions.
  • the ratio Mn/Si is preferably 1.9 or more, and more preferably 2.0 or more, from the viewpoint of further improving the toughness of the electric resistance welded portion.
  • the upper limit of the ratio Mn/Si is not particularly limited.
  • the ratio Mn/Si is preferably 50 or less, more preferably 30 or less, and still more preferably 20 or less, from the viewpoint of further improving the toughness of the base metal portion and the electric resistance welded portion.
  • the value of LR represented by Formula (2) is 0.25 or more.
  • LR 2.1 C + Nb / Mn [In Formula (2), each element symbol represents the content of each element in % by mass.]
  • C and Nb which improves the work hardening properties and thereby contributes to achieving a decrease in YR
  • Mn which has a risk of reducing the work hardening properties to cause an increase in YR
  • the upper limit of the value of LR is not particularly limited, and may be, for example, 0.90, 0.80 or the like.
  • the area ratio of the first phase composed of ferrite (hereinafter, also referred to as "ferrite fraction") is from 80 to 98%.
  • the ferrite fraction is less than 80%, the degree of carbon enrichment in the second phase may be insufficient. As a result, the difference in hardness between the first phase and the second phase may be excessively decreased, possibly leading to a difference in hardness of less than 50 Hv. Accordingly, the ferrite fraction is 80% or more, and preferably 82% or more.
  • the ferrite fraction is more than 98%, however, the degree of carbon enrichment in the second phase may be increased excessively. As a result, the hardness of the second phase is excessively increased, possibly leading to a difference in hardness of more than 100 Hv. Accordingly, the ferrite fraction is 98% or less, preferably 95% or less, and more preferably 90% or less.
  • the second phase which is the balance (namely, the balance excluding the first phase from the metallographic microstructure of the base metal portion), contains at least one of pearlite or bainite, and the area ratio of martensite with respect to the total area of the second phase is less than 1%.
  • a difference in hardness (namely, a value obtained by subtracting the hardness of the first phase from the hardness of the second phase) of 100 Hv or less is more easily achieved.
  • the area ratio of martensite with respect to the total area of the second phase is less than 1% means that the second phase does not substantially contain martensite.
  • the strength may be excessively increased (specifically, it may result in a failure to achieve at least one of a YS of 600 MPa or less, or a TS of 760 MPa or less).
  • bainite in the present disclosure encompasses bainitic ferrite, granular bainite, upper bainite and lower bainite.
  • pearlite in the present disclosure encompasses a pseudo-pearlite structure.
  • the measurement of the ferrite fraction and the identification of the second phase, in the metallographic microstructure of the base metal portion are carried out by performing nital etching on the metallographic microstructure of the central portion in the thickness direction in an L cross section at a base metal 90° position of the electric resistance welded steel pipe, and then observing a photograph of the metallographic microstructure (hereinafter, also referred to as "metallographic micrograph") after the nital etching, captured using a scanning electron microscope (SEM) at a magnification of 1,000 times. At this time, the metallographic micrograph is captured for a region corresponding to 10 fields of view in a field of view of 1,000 times (a region corresponding to 0.12 mm 2 of the actual area of the cross section).
  • SEM scanning electron microscope
  • the thus captured metallographic micrograph is subjected to image processing, to perform the measurement of the ferrite fraction and the identification of the second phase.
  • image processing is performed, for example, using a small, general purpose image analysis apparatus, LUZEX AP, manufactured by Nireco Corporation.
  • base metal 90° position refers to a position 90 degrees away from the electric resistance welded portion in the pipe circumferential direction
  • L cross section refers to a cross section parallel to the pipe axis direction and the thickness direction
  • the term “the central portion in the thickness direction” refers to a position corresponding to 1/2 of the thickness.
  • the difference in hardness (namely, the value obtained by subtracting the hardness of the first phase from the hardness of the second phase) is from 50 to 100 Hv.
  • the difference in hardness is 50 Hv or more, and preferably 52 Hv or more, from the viewpoint of achieving a decrease in YR.
  • the difference in hardness is 100 Hv, however, the internal stress of the metallographic microstructure may be increased excessively, to cause a decrease in the toughness of the base metal portion. Accordingly, the difference in hardness is 100 Hv or less, preferably 96 Hv or less, and more preferably 90Hv or less.
  • the difference in hardness is measured as follows.
  • the hardnesses of the first phase and the second phase in the metallographic microstructure of the central portion in the thickness direction are respectively measured, and the value obtained by subtracting the hardness of the first phase from the hardness of the second phase is taken as the difference in hardness.
  • the hardness of the first phase is determined as follows.
  • micro-Vickers hardness was measured at each of the thus selected 50 points, by a micro-Vickers hardness test under the condition of a load of 10 gf.
  • Each of the measurement points to be selected may span across the crystal grain boundaries. From the thus obtained 50 measured values, measured values which are obviously too high (specifically, measured values of more than 350 Hv) are excluded, and the arithmetic mean value of the selected remaining measured values is determined. The thus determined arithmetic mean value is taken as the hardness of the first phase.
  • the hardness of the second phase is determined in the same manner as the hardness of the first phase.
  • the electric resistance welded steel pipe according to the present invention has a yield strength (YS) in the pipe axis direction of from 360 to 600 MPa.
  • the YS is 360 MPa or more, the strength required as a steel pipe for a linepipe is ensured.
  • the YS is preferably 380 MPa or more, and more preferably 400 MPa or more.
  • the YS is 600 MPa or less, a bending deformability (namely, ease of bending) at the time of laying the electric resistance welded steel pipe for a linepipe can be ensured, and the buckling of the electric resistance welded steel pipe for a linepipe can be reduced.
  • the YS is preferably 590 MPa or less.
  • the “YS” in the electric resistance welded steel pipe according to the present disclosure refers to "0.5% under load proof stress”.
  • the electric resistance welded steel pipe according to the present invention has a tensile strength (TS) in the pipe axis direction of from 465 to 760 MPa.
  • the TS is 465 MPa or more, the strength required as a steel pipe for a linepipe is ensured.
  • the TS is preferably 470 MPa or more.
  • the TS is 760 MPa or less, the bending deformability (namely, ease of bending) at the time of laying the electric resistance welded steel pipe for a linepipe can be ensured, and the buckling of the electric resistance welded steel pipe for a linepipe can be reduced. Further, the deterioration in the toughness of the base metal portion is further reduced.
  • the TS is preferably 700 MPa or less, and more preferably 680 MPa or less.
  • the lower limit of the YR is not particularly limited, and may be, for example, 0.80, 0.82 or the like.
  • each of the TS, YS and YR refers to a value measured as described below.
  • test specimen for tensile test is obtained from the base metal 90° position of the electric resistance welded steel pipe, such that the test direction (tensile direction) in the tensile test is the pipe axis direction of the electric resistance welded steel pipe.
  • the test specimen as used above is formed in the shape of a flat plate which is in accordance with The American Petroleum Institute Standard, API 5L (hereinafter, simply referred to as "API 5L").
  • a tensile test in the pipe axis direction (namely, a tensile test in which the pipe axis direction of the electric resistance welded steel pipe is taken as the test direction) is carried out, at room temperature and in accordance with API 5L, to measure each of the TS and YS.
  • the YS as used above refers to the 0.5% under load proof stress, as described above.
  • a yield elongation as measured in a tensile test in the pipe axis direction, is less than 0.2% (namely, the yield elongation is not substantially observed).
  • the yield elongation described above is determined by the above-described tensile test in the pipe axis direction for determining the TS, YS and YR.
  • the fact that the yield elongation is less than 0.2% means that the electric resistance welded steel pipe according to the present invention is an electric resistance welded steel pipe as it is made.
  • a substantial yield elongation (namely, a yield elongation of 0.2% or more) is observed in a tensile test in the pipe axis direction, in an electric resistance welded steel pipe in which a heat treatment has been performed on the entire pipe, after pipe-making (such as the electric resistance welded steel pipe disclosed in Patent Document 3).
  • the toughness of the base metal portion and the electric resistance welded portion is ensured.
  • each of the base metal portion and the electric resistance welded portion has a Charpy absorbed energy at 0°C (hereinafter, also referred to as "vE") of 100 J or more.
  • vE Charpy absorbed energy at 0°C
  • the vE of the base metal portion is preferably 150 J or more, more preferably 200 J or more, and still more preferably 250 J or more.
  • the vE of the base metal portion is preferably 400 J or less.
  • the vE of the electric resistance welded portion is preferably 150 J or more, more preferably 190 J or more, and still more preferably 200 J or more.
  • the vE of the electric resistance welded portion is preferably 400 J or less, and more preferably 350 J or less.
  • the vE (namely, the Charpy absorbed energy at 0°C) of the base metal portion refers to a value measured as follows.
  • a full-size test specimen with a V-notch (a test specimen for Charpy impact test) is obtained from the base metal 90° position of the electric resistance welded steel pipe.
  • the full-size test specimen with a V-notch is obtained such that the test direction is the pipe circumferential direction (C direction).
  • the thus obtained full-size test specimen with a V-notch is subjected to a Charpy impact test, under the temperature condition of 0°C and in accordance with API 5L, to measure the vE.
  • the measurement as described above is carried out five times per one electric resistance welded steel pipe, and the mean value of the measured values of the five tests is taken as the vE of the base metal portion of the electric resistance welded steel pipe.
  • the vE of the electric resistance welded portion refers to a value measured as follows.
  • the electric resistance welded steel pipe according to the present invention preferably has a thickness of from 10 to 25.4 mm.
  • the thickness of the electric resistance welded steel pipe is 10 mm or more, it is advantageous in that the YR can be easily decreased utilizing the strain at the time of forming a hot-rolled steel sheet in the shape of a pipe.
  • the thickness is more preferably 12 mm or more.
  • the thickness is 25.4 mm or less, it is advantageous from the viewpoint of production suitability (specifically, formability at the time of forming a hot-rolled steel sheet in the shape of a pipe) of the electric resistance welded steel pipe.
  • the thickness is more preferably 20 mm or less.
  • the electric resistance welded steel pipe according to the present invention preferably has an outer diameter of from 254.0 to 660.4 mm (namely, from 10 to 26 inches).
  • the steel pipe is more suitable as an electric resistance welded steel pipe for a linepipe.
  • the outer diameter is preferably 304.8 mm (namely, 12 inches) or more.
  • the outer diameter is 660.4 mm (namely, 26 inches) or less, it is advantageous in that the YR can be easily decreased utilizing the strain at the time of forming a hot-rolled steel sheet in the shape of a pipe.
  • the outer diameter is more preferably 508.4 mm (namely, 20 inches).
  • production method A One example (hereinafter, referred to as "production method A") of the method of producing the electric resistance welded steel pipe according to the present disclosure will now be described.
  • the temperature and cooling rate refer to the temperature and cooling rate at the surface of a steel material (namely, a slab or a hot-rolled steel sheet), respectively, unless otherwise specified.
  • the production method A is the method of producing the electric resistance welded steel pipe used in Examples to be described later.
  • the production method A includes:
  • the production method A enables to produce the electric resistance welded steel pipe according to the present disclosure.
  • the slab preparation step in the production method A is a step of preparing a slab having the chemical composition in the present disclosure.
  • the step of preparing a slab may be a step of producing a slab, or may be a step of simply preparing a slab which has been produced in advance.
  • a molten steel having the chemical composition in the present disclosure is produced, and the thus produced molten steel is used to produce the slab.
  • the slab may be produced by a continuous casting method, or alternatively, the slab may be produced by forming an ingot using the molten steel, and subjecting the ingot to blooming.
  • the hot rolling step in the production method A is a step of heating the slab prepared above to a slab heating temperature of from 1100°C to 1350°C, rough rolling the heated slab, and hot rolling the rough-rolled slab under the conditions of a finish rolling start temperature of 950°C or lower, a finish rolling finishing temperature of 820°C or lower, and a cumulative rolling reduction ratio in the finish rolling of 2.5 or more, to obtain a hot-rolled steel sheet.
  • the slab heating temperature is 1350°C or lower, the coarsening of crystal grains is reduced, and as a result, the deterioration of toughness in the finally obtained electric resistance welded steel pipe can be reduced.
  • the finish rolling start temperature is 950°C or lower, the coarsening of crystal grains is reduced, and as a result, the deterioration of toughness in the finally obtained electric resistance welded steel pipe can be reduced.
  • the finish rolling finishing temperature is 820°C or lower, the coarsening of crystal grains is reduced, and as a result, the deterioration of toughness in the finally obtained electric resistance welded steel pipe can be reduced.
  • the cumulative rolling reduction ratio is 2.5 or more, the coarsening of crystal grains is reduced, and as a result, the deterioration of toughness in the finally obtained electric resistance welded steel pipe can be reduced.
  • the first cooling step in the production method A is a step of subjecting the hot-rolled steel sheet obtained in the hot rolling step described above to the first cooling, wherein the time until the start of the first cooling after the completion of the finish rolling is adjusted within 20 s (seconds), and wherein the first cooling is carried out at a first cooling rate of from 10°C/s to 80°C/s, until a first cooling finishing temperature of from 600°C to 700°C is reached.
  • the first cooling rate is 10°C/s or more
  • an excessive formation of ferrite is reduced, and an excessive C enrichment (carbon enrichment) in the second phase is reduced.
  • a ferrite fraction of 98% or less and a difference in hardness of 100 Hv or less can be achieved, in the finally obtained electric resistance welded steel pipe.
  • the formation of ferrite is accelerated, and the C enrichment (carbon enrichment) in the second phase proceeds to a certain degree.
  • a ferrite fraction of 80% or more and a difference in hardness of 50 Hv or more can be achieved, in the finally obtained electric resistance welded steel pipe.
  • the first cooling may be water cooling or air cooling.
  • the first cooling rate is controlled by adjusting the water flow density of cooling water.
  • the first cooling rate is controlled by adjusting the amount of cooling air.
  • the second cooling step in the production method A is a step of subjecting the hot-rolled steel sheet which has been subjected to the first cooling to a second cooling, wherein the second cooling is carried out at a second cooling rate of from 5°C/s to 30°C/s, until a coiling temperature (namely, a second cooling finishing temperature) of from 450°C to 700°C is reached.
  • the hardness of the second phase is increased to a certain degree, and as a result, a difference in hardness of 50 Hv or more can be achieved.
  • the second cooling may be water cooling or air cooling.
  • the second cooling rate is controlled by adjusting the water flow density of the cooling water.
  • the second cooling rate is controlled by adjusting the amount of cooling air.
  • the coiling step in the production method A is a step of coiling the hot-rolled steel sheet which has been subjected to the second cooling, at the coiling temperature, to obtain a hot coil composed of the hot-rolled steel sheet.
  • the coiling step is not particularly limited, and may be carried out under known conditions.
  • the pipe-making step is a step in which the hot-rolled steel sheet is uncoiled from the hot coil, the uncoiled hot-rolled steel sheet is roll-formed to prepare an open pipe, and the abutting portion in the thus prepared open pipe is subjected to electric resistance welding to form an electric resistance welded portion, thereby obtaining an electric resistance welded steel pipe.
  • the pipe-making step is not particularly limited, and may be carried out under known conditions.
  • the pipe-making step may include:
  • the electric resistance welded steel pipe is not subjected to any heat treatment (any heat treatment other than the seam heat treatment) after pipe-making.
  • the chemical composition of the base metal portion of the electric resistance welded steel pipe produced by the production method A can be regarded as the same as the chemical composition of the raw material (the molten steel or the slab).
  • No. 1 to No. 31 are Examples which are within the scope of the present invention, and No. 32 to No. 58 are Comparative Examples which are outside the scope of the present invention.
  • CNeq indicates the value of CNeq represented by Formula (1) described above
  • Mn/Si indicates the ratio of the Mn content with respect to the Si content
  • LR indicates the value of LR represented by Formula (2) described above.
  • Each of the slabs obtained as described above was heated to the slab heating temperature shown in Table 2, and the heated slab was subjected to hot rolling under the hot rolling conditions (specifically, the finish rolling start temperature, the finish rolling finishing temperature, and the rolling reduction ratio) shown in Table 2, to obtain a hot-rolled steel sheet (the hot rolling step).
  • the hot rolling conditions specifically, the finish rolling start temperature, the finish rolling finishing temperature, and the rolling reduction ratio
  • rollering reduction ratio refers to the “cumulative rolling reduction ratio” in the finish rolling.
  • the time (seconds) until the start of the cooling after the completion of the finish rolling was adjusted as shown in the column of the "Time until the start of cooling" in Table 2, and the first cooling was started at the first cooling rate shown in Table 2.
  • the first cooling was carried out until a first cooling finishing temperature of from 600°C to 700°C was achieved.
  • the second cooling was carried out at the second cooling rate shown in Table 2 until the coiling temperature (CT) shown in Table 2 was achieved, and then each cooled hot-rolled steel sheet was coiled at the coiling temperature, to obtain a hot coil composed of a hot-rolled steel sheet having a sheet thickness of 17.5 mm (the first cooling step, the second cooling step, and the coiling step).
  • CT coiling temperature
  • Each of the first cooling and the second cooling was carried out by water cooling, and each of the first cooling rate and the second cooling rate was adjusted by adjusting the water flow density of the cooling water.
  • the hot rolling step, the first cooling step, the second cooling step and the coiling step described above were carried out using a hot strip mill.
  • each hot-rolled steel sheet was uncoiled from each hot coil obtained as described above, the uncoiled hot-rolled steel sheet was roll-formed to prepare an open pipe. Thereafter, the abutting portion of each resulting open pipe was subjected to electric resistance welding to form an electric resistance welded portion. Then a seam heat treatment is performed on the electric resistance welded portion, and the shape of each pipe is adjusted using a sizer, to obtain an as-rolled electric resistance welded steel pipe having an outer diameter of 406 mm and a thickness of 17.5 mm (the pipe-making step).
  • the measurement of the ferrite fraction namely, the area ratio of the first phase with respect to the total area of the metallographic microstructure
  • the identification of the second phase type namely, the type of the second phase
  • the description "P, B” in the column of the second phase type indicates that the second phase contains at least one of pearlite or bainite, and does not substantially contain martensite (namely, the area ratio of martensite with respect to the total area of the second phase is less than 1%), and the description "B + M” therein indicates that the second phase is a mixed structure of bainite and martensite (namely, the area ratio of martensite with respect to the total area of the second phase is more than 1% or more).
  • the difference in hardness (namely, the value obtained by subtracting the hardness of the first phase from the hardness of the second phase) was measured by the method described above.
  • the TS, YS, YR and the yield elongation were identified by the method described above.
  • each electric resistance welded steel pipe of each of Examples satisfied a YS of from 360 to 600 MPa, a TS of from 465 to 760 MPa and a YR of 0.90 or less, and showed an excellent toughness of the base metal portion and the electric resistance welded portion, despite being an electric resistance welded steel pipe as it is made.
  • each electric resistance welded steel pipe had a toughness satisfying each of a vE of the base metal portion and a vE of the electric resistance welded portion of 100 J or more.

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Claims (3)

  1. Ein widerstandsgeschweißtes Stahlrohr für ein Leitungsrohr, wobei das Stahlrohr umfasst:
    einen Basismetall-Abschnitt; und
    einen widerstandsgeschweißten Abschnitt,
    wobei ein Basismetall-Abschnitt eine chemische Zusammensetzung aufweist, bestehend aus, bezogen auf Massen-%:
    0,03 % oder mehr und weniger als 0,10 % C,
    von 0,30 bis 1,00 % Mn,
    von 0,005 bis 0,050 % Ti,
    von 0,010 bis 0,100 % Nb,
    von 0,001 bis 0,020 % N,
    von 0,010 bis 0,500 % Si,
    von 0,001 bis 0,100 % Al,
    von 0 bis 0,030 % P,
    von 0 bis 0,010 % S,
    von 0 bis 0,50 % Mo,
    von 0 bis 0,50 % Cu,
    von 0 bis 0,50 % Ni,
    von 0 bis 0,50 % Cr,
    von 0 bis 0,10 % V,
    von 0 bis 0,0100 % Ca,
    von 0 bis 0,0100 % REM und
    einem Rest bestehend aus Fe und Verunreinigungen,
    wobei CNeq, dargestellt durch Formel (1), von 0,12 bis 0,25 beträgt, ein Verhältnis eines Gehalts an Mn, bezogen auf einen Gehalt an Si, 1,8 oder mehr beträgt und LR, dargestellt durch Formel (2), 0,25 oder mehr beträgt: CNeq = C + Mn / 6 + Cr / 5 + Ni + Cu / 15 + Nb + Mo + V
    Figure imgb0011
     LR = 2,1 C + Nb / Mn
    Figure imgb0012
     wobei in den Formeln (1) und (2) jedes Elementsymbol einen Gehalt jedes Elements in Massen-% darstellt,
    wobei der Basismetall-Abschnitt eine metallographische Mikrostruktur aufweist, bei der:
    eine erste Phase, bestehend aus Ferrit, ein Flächenverhältnis von 80 bis 98 % aufweist,
    eine zweite Phase, die ein Rest ist, mindestens eines aus Perlit oder Bainit enthält;
    ein Flächenverhältnis von Martensit, bezogen auf eine Gesamtfläche der zweiten Phase, weniger als 1 % beträgt; und
    ein Wert, erhalten durch Abziehen einer Härte der ersten Phase von einer Härte der zweiten Phase, von 50 bis 100 Hv beträgt,
    wobei das widerstandsgeschweißte Stahlrohr aufweist:
    eine Streckgrenze in einer Rohrachsenrichtung von 360 bis 600 MPa;
    eine Zugfestigkeit in der Rohrachsenrichtung von 465 bis 760 MPa; und
    ein Streckgrenzenverhältnis in der Rohrachsenrichtung von 0,90 oder weniger,
    wobei jeder des Basismetallabschnitts und des widerstandsgeschweißten Abschnitts bei 0 °C eine absorbierte Energie nach Charpy von 100 J oder mehr aufweist und
    wobei eine Streckdehnung des widerstandsgeschweißten Stahlrohrs, wie gemessen in einem Zugversuch in der Rohrachsenrichtung, weniger als 0,2 % beträgt, wobei die metallographische Mikrostruktur, Härte, Streckgrenze, Zugfestigkeit, Streckgrenzenverhältnis, absorbierte Energie nach Charpy und Streckdehnung gemäß der Beschreibung bestimmt werden.
  2. Das widerstandsgeschweißte Stahlrohr für ein Leitungsrohr gemäß Anspruch 1, wobei die chemische Zusammensetzung des Basismetall-Abschnitts umfasst, bezogen auf Massen-%, mindestens eines, ausgewählt aus der Gruppe bestehend aus:
    mehr als 0 % und gleich oder weniger als 0,50 % Mo,
    mehr als 0 % und gleich oder weniger als 0,50 % Cu,
    mehr als 0 % und gleich oder weniger als 0,50 % Ni,
    mehr als 0 % und gleich oder weniger als 0,50 % Cr,
    mehr als 0 % und gleich oder weniger als 0,10 % V,
    mehr als 0 % und gleich oder weniger als 0,0100 % Ca und
    mehr als 0 % und gleich oder weniger als 0,0100% REM.
  3. Das widerstandsgeschweißte Stahlrohr für ein Leitungsrohr gemäß Anspruch 1 oder 2, wobei das widerstandsgeschweißte Stahlrohr für ein Leitungsrohr eine Dicke von 10 bis 25,4 mm und einen Außendurchmesser von 254,0 bis 660,4 mm aufweist.
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